Structural geology, tectonics and gold mineralisation of the southern Anakie Inlier. David G. Wood

Transcription

1 Structural geology, tectonics and gold mineralisation of the southern Anakie Inlier David G. Wood A thesis submitted for the degree of Doctor of Philosophy THE AUSTRALIAN NATIONAL UNIVERSITY December 2006

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3 Statement of authorship This thesis contains no material which has been accepted for the award of any other degree or diploma in any university. To the best of the authors knowledge and belief, it contains no material previously published or written by another person, except where due reference is made in the text. David G. Wood August 2, 2007 iii

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5 Acknowledgements Firstly I would like to thank Professor Gordon Lister for taking me on initially as an Honours student when I was in dire straits, and then guiding me through what has been perhaps the biggest learning experience of my life. My time as a student under Gordo has never had a dull moment, and I appreciate the opportunities he has afforded me. Roric Smith and AngloGold Ashanti are thanked for providing funding for this project, without which it would not have gone ahead. Mentoring and great company in the field was provided by Simon Richards, who gave this project direction at a critical time. For this I am particularly grateful. I am indebted for assistance provided by Jim Dunlap, Geoff Fraser, Sandra McLaren, Marnie Forster and Julien Célérier for help with theoretical and practical aspects of 40 Ar/ 39 Ar thermochronology. Trevor Ireland and Peter Holden are thanked for their guidance in matters of SHRIMP U-Pb geochronology. The work undertaken could not have occurred without technical assistance provided by John Mya, Shane Paxton, Chris McPherson, Ashley Norris, Tanya Ewing, Xiaodong Zhang, Ryan Ickert, Jörg Hermann and Harri Kokkonen. Ian Withnall of the Queensland Geologic Survey is acknowledged for an open exchange of information, as well as donation of samples for geochronology. Steven Micklethwaite, Caroline Forbes, Stephen Cox, Justin Freeman provided invaluable assistance, guidance and advice at important stages of this project The owners of Redrock near Clermont, Margaret and Elliot Finger are thanked for their hospitality, as well as the warm and welcoming way in which they treated me while staying there. All of the farmers in the Clermont region were in a dreadful time of drought, yet were v

6 always open and friendly. Gordon and Marion at the Clermont Caravan Park were great hosts, and always made going to Clermont feel like going home. Barry and Olga Dunn who run the Clermont Detectors store are thanked for insights into gold in the Clermont region, as well as enjoyable beers at happy hour. The students at the RSES are the ones who made life in Canberra a time to remember, and all are thanked for their support and great company. In particular Daniel, Courts, Joe, Bridget, Marco, Nick, Gisela, Jules, Chucky, Shaun, Kat, Meghan and Stewart are thanked for friendship and laughs. I would also like to pay my respect, thanks and well-wishes to Fred, Amos and Nicole who were always there for me. Special thanks goes to Courtney for being a very patient soul, and for keeping things in perspective. My grandfather, the late Reverend L.G. Wood was a catalyst for my interest in geology, and also sparked my desire for further study. He provided invaluable wisdom at pertinent times in my life, I only wish he could still be here. To Mum, Dad and Merran, I thank you the most for supporting me. I wouldn t have made it to here without your constant encouragement. vi

7 Abstract The Late Neoproterozoic-Early Palaeozoic geology of northeast Australia is not well known, and is restricted to studies of relatively small basement inliers in Queensland. The Anakie Inlier is one such area. Basement rocks of the Anakie Inlier comprise the Anakie Metamorphic Group, and provide a window into crust that potentially underlies a significant area of northeast Australia. A ductile flat-lying foliation is the dominant structural feature of the Anakie Metamorphic Group, and both extensional and shortening processes have previously been interpreted for its formation. A combination of structural, metamorphic, 40 Ar/ 39 Ar thermochronologic and SHRIMP U-Pb geochronologic studies were used to elucidate the nature of the flat-lying foliation, as well as provide tectonic constraints for the Late Neoproterozoic-Early Palaeozoic evolution of northeast Australia. Detailed structural mapping and microstructural analysis revealed a more complex deformation history than previously interpreted. A minimum of 6 distinct deformation events are interpreted, up from a previous total of 3. Early deformation of the Anakie Metamorphic Group is characterised by upright isoclinal folding coeval with mid-amphibolite facies metamorphism, which is overprinted by recumbent folding and a flat-lying foliation synchronous with (retrograde) greenschist facies metamorphism. The formation of shear bands, stretching mineral lineations and asymmetric folding during flat-lying foliation development indicates a component of simple shear during deformation. The contrast of upright folding followed by low angle shearing is interpreted to reflect a switch between shortening and extensional deformation. Younger deformation formed variably trending uprights folds that reoriented the flat-lying foliation, and resulted in complex outcrop patterns. The age of early deformation of the Anakie Metamorphic Group is constrained to between ca Ma from detrital zircon ages in a previous study, and from 40 Ar/ 39 Ar ages in this study. SHRIMP U-Pb analysis undertaken constrains the age of younger upright deformation to vii

8 between ca Ma, based on cross-sutting relationships between intrusives and structures in the metamorphic rocks. A U-Pb age of ± 6.2 Ma is interpreted to represent the absolute age of a regional (D4) deformation event. Gold in the Anakie Inlier occurs in a variety of settings. In the Clermont region, gold can be divided into two broad groups, the first is structurally controlled lode gold mostly in the Bathampton Metamorphics, the second is gold in a basal conglomerate horizon of Permian basins. Structurally controlled gold occurs in shear zones along the limbs of the Oaky Creek Antiform, and is concentrated at the intersection of the shear zones with areas of younger intense deformation. The earliest known gold mineralisation occurs in structures that are dated at ± 6.2 Ma. Gold in Permian conglomerate is enigmatic, and occurs as palaeoplacer nuggets and hydrothermal related deposits in close proximity to each other. Gold is concentrated in, and adjacent to, fractures that cut the unconformity between Permian sediments and the underlying Anakie Metamorphic Group. A model of fluid mixing along the unconformity interface best explains the presence of concentrated gold in this setting. Correlations between the Anakie Metamorphic Group and equivalent metamorphic rocks elsewhere in Queensland indicate that the Late Neoproterozoic - Early Palaeozoic evolution of northeast Australia was dominated by extensional tectonics, punctuated by short-lived episodes of lithospheric shortening. viii

9 Contents Acknowledgements vii Abstract ix 1 Introduction Project Aims and Objectives Thesis Structure Regional Geological Setting The Anakie Inlier Stratigraphy and Relationships Structural and Metamorphic History, Anakie Metamorphic Group Introduction Previous Work Structure ix

10 CONTENTS CONTENTS Metamorphism Structure and Metamorphism: this study The Oaky Creek Antiform Area: Structure Sequence diagrams: a method for structural analysis D1 Deformation D2 Deformation D3 Deformation D4 Deformation D5 Deformation D6 Deformation The Oaky Creek Antiform Area: Metamorphism M1 Metamorphism M2 Metamorphism M3 Metamorphism Comparison: The Eastern Creek area Comparison: The Miclere-Blair Athol area Regional Trends Rubyvale Region Mt Coolon Region x

11 CONTENTS CONTENTS 2.5 Discussion Early Deformation History of the Anakie Metamorphic Group Late-Stage Deformation History of the Anakie Metamorphic Group Metamorphic History of the Anakie Metamorphic Group Constraints on M3 Metamorphism from White Mica Composition M2 to M3 Metamorphism Summary and Conclusions Age of Early Deformation and Metamorphism of The Anakie Metamorphic Group: Constraints from 40 Ar/ 39 Ar Analysis Introduction Regional Geology Clermont Region Nebine Ridge Previous Geochronology Why 40 Ar/ 39 Ar Analysis? Methodology Sample Selection Mineral Separation and 40 Ar/ 39 Ar Analysis Results xi

12 CONTENTS CONTENTS 3.4 Discussion Ar/ 39 Ar Analysis of D2 and D3 Microstructures, Anakie Metamorphic Group Background The Anakie Metamorphic Group Microstructural Domain Analysis Summary Ar/ 39 Ar age constraints on D3 deformation, Anakie Metamorphic Group, Clermont region Ar/ 39 Ar correlations between the Anakie Metamorphic Group and the Nebine Ridge Conclusions Late-Stage Deformation of The Anakie Metamorphic Group: Constraints from U- Pb Geochronology Introduction Regional Geology, Rubyvale Region Bathampton Metamorphics Fork Lagoons Beds Gem Park Granite Mt Newsome Granodiorite Geochronology Previous Work xii

13 CONTENTS CONTENTS Geochronology: this study Results Gem Park Granite: Monazite Gem Park Granite: Zircon Mt Newsome Granodiorite: Zircon Discussion Structural Context Gem Park Granite Mt Newsome Granodiorite Zircon and Monazite Ages: Gem Park Granite Monazite - Inherited or Crystallisation Age? Zircon Rim Data Summary and Conclusions Gold in the Clermont Region Introduction Gold in the Clermont Region Previous Work Lode gold deposits in the Anakie Metamorphic Group Gold Occurrences Around the Oaky Creek Antiform xiii

14 CONTENTS CONTENTS Lode Gold Metallogenesis Permian Conglomerate Hosted Gold This Study Structurally Controlled Gold: The Oaky Creek Antiform area D4 Shear Zone Hosted Gold D5 Structural Corridors D5 Faults D6 Structural Corridor Permian Conglomerate Hosted Gold: New Observations Discussion Gold in the Oaky Creek Antiform area Age of Mineralisation Permian Conglomerate Hosted Gold Summary Conclusions Synthesis Introduction History of the Anakie Metamorphic Group Deposition of the proto- Anakie Metamorphic Group xiv

15 CONTENTS CONTENTS Inferred Deformation and Metamorphism Early Deformation and Metamorphism Late Deformation Neoproterozoic - Early Palaeozoic Tectonics Equivalent rocks of the Anakie Metamorphic Group in Queensland Cape River Metamorphics Argentine Metamorphics Running River Metamorphics Charters Towers Metamorphics Barnard Metamorphics Correlations ca Ma Upright Folding ca Ma Flat-Lying Foliation Development Structural Evolution Outcomes of Large-Scale Comparison Gold in the Clermont Region Structurally Controlled Gold D4 Shear Zone Hosted Gold Gold in D5 Structures xv

16 CONTENTS CONTENTS Gold in D6 Structures Relationships Gold in Permian Conglomerate Prior to Deposition - Palaeoplacer Gold After Deposition - Hydrothermal Gold Tectonic Model Neoproterozoic-Cambrian subduction in Queensland Roles of Extension and Shortening Tectonic Controls on Gold Mineralisation Tectonic Model Conclusions Appendices 255 A Geological Map of the Oaky Creek Antiform Area 255 A.1 See map insert on back cover xvi

17 List of Figures 1.1 Regions and provinces of the Tasman Fold Belt System in eastern Australia Regional geological setting of the Anakie Inlier in Queensland Gondwana Simplified geologic map of the southern Anakie Inlier Simplified geologic map of the Clermont region Cross section of the Clermont region Regions of the Anakie Inlier Synthesis sequence diagram - structure Overprinting relationships S2/S3 fabric relationships S2 cleavage in S3 microlithon Photomicrograph of S2 biotite altered to chlorite, and folded during D Photomicrograph of S2 dissected by D3 shear bands xvii

18 LIST OF FIGURES LIST OF FIGURES 2.8 Plot of poles to S2 in the Oaky Creek Antiform area Photomicrograph of S3 differentiated crenulation cleavage in pelite Photomicrograph of mylonitic S3 in quartzite Features of simple shear during D Effects of D3 on D2 quartz veins S3 foliation parallel to bedding Examples of F3 folding Refolded F2 fold Structural domains in the Oaky Creek Antiform area Plot of poles to S3 in the Oaky Creek Antiform area L 2 3 intersection lineation of S2 with S Mineral stretching lineation on an S3 surface Plot of L 2 3 intersection lineations and mineral stretching lineations on S3, Oaky Creek Antiform area Intensity distribution of D3 deformation in the Oaky Creek Antiform area Plot of poles to S4, Oaky Creek Antiform area Examples of F4 folding Examples of S4 crenulation cleavage Photomicrograph of S4 crenulation cleavage Oaky Creek Antiform cross sections xviii

19 LIST OF FIGURES LIST OF FIGURES 2.27 Images of D4 shear zones Outcrop scale analogue of the Oaky Creek Antiform Different structural trends north and south of the Grasstree Fault Plot of poles to S5, Oaky Creek Antiform area Folds associated with D5 deformation Features of D5 fault zones Features of D6 folding and shearing Plot of poles to S6, Oaky Creek Antiform area D6 quartz vein Sequence diagram - metamorphism Photomicrograph of M2 garnets Photomicrograph of M2 magnetite and biotite Photomicrograph of M2 aluminosilicate altered to white mica Distribution of M2 metamorphic minerals Distribution of M3 metamorphic minerals Change in metamorphic grade of S3 mineralogy Eastern Creek cross section Plot of poles to S3, Eastern Creek area Thin section images from the Eastern Creek area xix

20 LIST OF FIGURES LIST OF FIGURES 2.46 Differentiated S3 foliation in the east of the Miclere-Blair Athol area Differentiated S2 foliation in the west of the Miclere-Blair Athol area Miclere-Blair Athol area S2 and S3 mineralogy in the Miclere-Blair Athol area Plot of poles to S3, Miclere-Blair Athol area Recumbent F3 fold in the Miclere-Blair Athol area D5 kinks in the Miclere-Blair Athol area The Rubyvale region Interpreted cross section through the Rubyvale region F3 fold in amphibolite, Rubyvale region Gem Park Granite, Rubyvale region Synthesis sequence diagram - structure and metamorphism Inferred structural and metamorphic event history D4 deformation and the Oaky Creek Antiform Pop-up in the Bendigo Goldfield Interpreted P-T path of the Anakie Metamorphic Group Sample locations for Si p.f.u. content of S3 white mica Microprobe spot analyses locations Si p.f.u. content of S3 white mica xx

21 LIST OF FIGURES LIST OF FIGURES 3.1 Location of the Clermont region, Nebine Ridge and AOP ALBA Geologic map of the Clermont region Metamorphic grade changes across the Clermont region S2/S3 fabric relationships Paragenetic mineral sequence Microstructural separation process S2 mica in folded quartz vein Ages revealed by 40 Ar/ 39 Ar analysis, Clermont region Cross section through the Anakie Metamorphic Group, Clermont region showing 40 Ar/ 39 Ar ages Summary of 40 Ar/ 39 Ar age spectra and K/Ca ratios Plots of 40 Ar/ 39 Ar apparent age spectra with method of asymptotes and limits applied Ar/ 39 Ar step heating data Simplified geologic map of the Rubyvale region Cross section through the Rubyvale region Photomicrograph of the Gem Park Granite Outcrop of the Gem Park Granite Photomicrograph of the Mt Newsome Granodiorite, DW SHRIMP results for Gem Park Granite monazite xxi

22 LIST OF FIGURES LIST OF FIGURES 4.7 Cathodoluminescence image of zircon grains from the Gem Park Granite Zircon data from the Gem Park Granite Cathodoluminescence image of zircon grains from the Mt Newsome Granodiorite SHRIMP results for Mt Newsome Granodiorite zircons Location of the Clermont Goldfield, the Oaky Creek Antiform area and known gold occurrences Permian basins, Clermont goldfield Carbonate veins and siliceous alteration in a D4 shear zone Dioritic feldspar-hornblende pophyry dyke in a zone of siliceous alteration D5 quartz vein Permian conglomerate and unconformity at Black Ridge Permian unconformity surface at Miclere Reverse fault at Miclere Transfer fault/fracture at Miclere Styles of gold at Miclere Scanning electron microscope images, secondary gold growth on water-worn gold nugget Interpreted styles of gold that occurs at the base of Permian basins in the Clermont region Trace element patterns of mafic rocks xxii

23 LIST OF FIGURES LIST OF FIGURES 6.2 Summary of events to affect the southern Anakie Inlier Neoproterozic-Cambrian outcrop in Queensland Time-space plot of relevant metamorphic and other rocks in central and northern Queensland from late Neoproterozoic - Devonian Trend of upright structures formed at ca 510 Ma in central and northern Queensland Trend of flat-lying structures formed between ca 500 Ma Ma in central and northern Queensland Bouger gravity image of central and northern Queensland Temperature-time path of the Anakie Metamorphic Group and Cape River Metamorphics The Bendigo goldfield in southeast Australia Tectonic reconstruction, Neoproterozoic Ma Gondwana during the ca Ma Ross-Delamerian Orogeny xxiii

24 LIST OF FIGURES LIST OF FIGURES xxiv

25 List of Tables 2.1 White mica microprobe data Summary information of samples selected for 40 Ar/ 39 Ar analysis Summary of 40 Ar/ 39 Ar age data Results of SHRIMP U-Th-Pb isotopic analyses xxv

26 LIST OF TABLES LIST OF TABLES xxvi

27 Chapter 1 Introduction Recent tectonic studies have highlighted the interplay of extensional and shortening processes during continental growth, mostly in the back arc region of supra-subduction zone settings (Collins, 2002a; Busby, 2004; Pubelliera et al., 2004; Richards, 2004; Richards and Collins, 2004; Schellart and Lister, 2005). Interestingly, extensional processes are emerging as fundamental to accretion and cratonisation of new continental crust at ocean-continent margins. There is no doubt that shortening and thickening processes strongly affect continental development in subduction accretion settings, however extension appears to be the driving force for growth of new crust. Extension dominated processes have been recognised for parts of the southern Tasman Fold Belt System (Figure 1.1), in the eastern Lachlan Fold Belt of Australia (Collins, 2002b; Richards, 2004), however in the northern Tasman Fold Belt System, the dominant processes and tectonic setting during continental growth are mostly unconstrained. The Tasman Fold Belt System (TFBS, Figure 1.1) preserves the Palaeozoic tectonic evolution of eastern Australia. The early history of the TFBS is obscured by younger events and extensive surface cover, however basement inliers exposed at the surface provide insight into early Palaeozoic processes. The Anakie Inlier in central Queensland (Figure 1.1, 1.2) is one such inlier, and forms a rare area of basement rock in a region dominated by basin development. The northern part of the TFBS in Queensland is understudied relative to the southern part, in particular, very little is known about its tectonic setting and evolution during the Cambrian and Early Ordovician. A small number of inliers in the northern TFBS contain rocks that were deposited and deformed in the earliest Palaeozoic, and constitute a small area (< 5%) of Palaeozoic outcrop in Queensland. The Anakie Inlier is the largest of these inliers to be affected by 1

28 1.1 Project Aims and Objectives Introduction Cambrian-Ordovician processes, and thus provides a window into the tectonic evolution of the northern TFBS. 1.1 Project Aims and Objectives The purpose of this thesis is to provide a better understanding of tectonic processes in Queensland during the latest Neoproterozoic to Early Devonian, using data from the Anakie Inlier. Particular focus is given to the Ma interval, as very little is known about the tectonic setting and evolution of Queensland during this time. Structural controls on gold mineralisation in the Anakie Metamorphic Group are also explored, with the aim to provide a framework for patterns of gold occurrences in at least one area. The main objectives of this study are to: 1. Determine the structural geology of the Anakie Metamorphic Group, and the timing of deformation events. 2. Better understand the tectonic evolution of the continental margin in Queensland during the early Palaeozoic. 3. Outline the structural controls on gold mineralisation in the Anakie Metamorphic Group in the Clermont region. 1.2 Thesis Structure This thesis is divided into six chapters, four of which present new data from the Anakie Inlier. Chapters 2 through 5 are original work, and comprise structure and metamorphism, 40 Ar/ 39 Ar thermochronology, gold, and SHRIMP U-Pb geochronology respectively. A synthesis is presented in Chapter 6, including a model for the tectonic setting of Queensland from Ma. Each of the chapters with new work has been produced as a stand alone study, with the aim to modify them for future submission and publication. As a result there may be degree of overlap and some repetition of basic facts and geologic setting. 2

29 Introduction 1.2 Thesis Structure Hodgkinson Province 12 Broken Rover Province QUEENSLAND Tasman Line Anakie Inlier Figure 1.2 New England Fold Belt 20 Thomson Fold Belt 28 SOUTH AUSTRALIA NEW SOUTH WALES Lachlan Fold Belt VICTORIA 36 Delamerian Fold Belt N TASMANIA 500 km Figure 1.1: Regions and provinces of the Tasman Fold Belt System in eastern Australia, including location of the Anakie Inlier (modified from Foster et al. (1998) and Wellman (1995)). 3

30 Galilee Basin 1.2 Thesis Structure Introduction Charters Towers Province Drummond Basin Coral Sea Anakie Inlier 22 Bowen Basin N 24 Great Artesian Basin Figure Nebine Ridge 149 ophiolite Younger cover, includes Great Artesian Basin Permian-Triassic sediments New England Orogen Pre Devonian basement (Thomson Orogen) 500 km Sub-surface Drummond Basin Fault Gravity trend Figure 1.2: Regional geological setting of the Anakie Inlier in Queensland, modified from Henderson et al. (1998). 4

31 Introduction 1.2 Thesis Structure Chapter 1: Introduction Chapter 1 introduces the questions to be addressed in this dissertation, as well as the geologic setting of the Anakie Inlier. Details of the stratigraphy, structure, metamorphism and geochronology already known for the region will be presented and summarised. Information from previous studies pertaining to specific research covered in subsequent chapters will be included in the relevant chapter. Chapter 2: Structural and Metamorphic History, Anakie Metamorphic Group Chapter 2 contains the results of extensive structural mapping and observations in the Anakie Inlier, Clermont region, and provides a framework for thermochronology and geochronology undertaken in following chapters. Metamorphism is linked to specific deformation events, and is used to help understand the history of the Anakie Metamorphic Group. This chapter focusses on the nature and origin of a pervasive, flat-lying foliation in the southern Anakie Inlier, which is determined from structural and metamorphic evidence, white mica composition of distinct fabric elements, and regional geologic trends. Chapter 3: Age of Early Deformation and Metamorphism of The Anakie Metamorphic Group: Constraints from 40 Ar/ 39 Ar Analysis New 40 Ar/ 39 Ar ages of the Anakie Metamorphic Group in the Clermont region are presented in this chapter. A method is presented that attempts to directly date white mica grown in distinct microstructures from the same sample. 40 Ar/ 39 Ar ages from the Anakie Metamorphic Group are compared with an 40 Ar/ 39 Ar age obtained from a sample of the Nebine Ridge, which is thought to be a sub-surface continuation of the Anakie Inlier. The results of 40 Ar/ 39 Ar thermochronology are used to provide new constraints on the age of deformation in the Anakie Metamorphic Group. 5

32 1.3 Regional Geological Setting Introduction Chapter 4: Late-Stage Deformation of The Anakie Metamorphic Group: Constraints from U-Pb Geochronology Chapter 4 presents the results of structurally constrained SHRIMP U-Pb geochronology of two different intrusives. SHRIMP U-Pb dating of zircons from a syn-kinematic intrusion which intrudes a D4 shear zone in the Rubyvale region indicates D4 deformation ocurred at ca 443 Ma. This chapter also explores the response of granitoids of different composition to deformation, as well as differences in monazite and zircon ages from an S-type granite. Chapter 5: Structural Controls on Gold in The Anakie Inlier, Clermont Region This chapter presents a new model for the structural controls on gold mineralisation in the Clermont region, using relationships observed during detailed structural mapping. Previously unidentified structures and relationships related to gold mineralisation are reported, and can be used in future exploration and target identification. Chapter 6: Synthesis Chapter 6 contains a synthesis of events that occurred in the Anakie Inlier from ca Ma. Drawing on evidence from the Anakie Inlier and equivalent rocks in Queensland, a model for the tectonic evolution of Queensland during the latest Neoproterozoic - early Palaeozoic is also presented. The roles of shortening and extension during continental growth of Queensland are examined here, and their significance evaluated. 1.3 Regional Geological Setting The Anakie Inlier is situated in central east Queensland, and outcrops as a north-south oriented elongate body up to 60 km wide and 180 km long (Figure 1.2). It forms part of the Tasman Fold Belt System (TFBS) in eastern Australia (Coney et al., 1990; Vandenberg et al., 2000), which is the Australian section of the Terra Australis Orogen that stretched across the Gondwanan margins of Australia, Antarctica, South America and South Africa (Cawood, 2005, Figure 1.3). The TFBS is comprised mainly of Palaeozoic rocks, but has a history spanning back to the Neoproterozoic. Included in the TFBS are the Delamerian (or Kanmantoo), Lachlan, Thomson, New England and Hodgkinson-Broken River fold belts (Scheibner and Veevers, 2000), however 6

33 Introduction 1.3 Regional Geological Setting the relationships between these separate areas are largely obscured by sediment cover from the Eromanga, Galilee, Bowen and Great Artesian Basin systems. The TFBS formed in the back arc area of a major subduction zone along the southeastern edge of Gondwanaland during the Palaeozoic, as the hinge zone retreated oceanwards (Foster and Gray, 2000; Cawood, 2005). Subduction initiation along the proto-pacific margin in the Australian section is thought to have occurred around Ma (Cawood, 2005), prior to onset of the Delamerian Orogeny at 515 ± 5 Ma (Boger and Miller, 2004). Before subduction initiation, the eastern margin of Australia was a passive margin setting dominated by continental rifting between Ma (Betts et al., 2002). The type area affected by the Delamerian Orogeny is the Neoproterozoic Adelaidean succession in South Australia, termed the Delamerian or Kanmantoo Fold Belt (Flöttman et al., 1993, 1998). Elsewhere in eastern Australia, rocks affected by the Delamerian Orogeny between ca Ma include various metamorphic complexes in Tasmania (eg. Foster et al. (2005)), the Glenelg River Metamorphic Complex in western Victoria (Turner et al., 1993), and the Wonominta Block of western New South Wales (Crawford et al., 1997). In Queensland the Delamerian Orogen is interpreted to have affected the Anakie Metamorphic Group of the Anakie Inlier (Withnall et al., 1996), and other similar aged rocks of the Cape River, Argentine, Running River, Barnard and Charters Towers Metamorphics (Hutton et al., 1997b; Bultitude and Rees, 1997; Fergusson et al., 2005) The Anakie Inlier Basement rocks of the Anakie Inlier comprise polydeformed, greenschist to amphibolite facies, metasedimentary and metavolcanic rocks of the Anakie Metamorphic Group. These are interpreted to have been deposited in a continental passive margin setting (Withnall et al., 1995). Detrital zircon and monazite studies indicate deposition from the latest Neoproterozoic - Middle Cambrian for the lowest stratigraphic levels, and during the Middle Cambrian for the uppermost stratigraphic levels (Fergusson et al., 2001). Early metamorphism and deformation of the Anakie Metamorphic Group is dated at ca 500 Ma from K-Ar ages of muscovite and whole rock samples in the Clermont region (Withnall et al., 1996, Figure 1.4). The Anakie Metamorphic Group is intruded by Mid-Devonian granitoids of the Retreat Batholith, and overlain in places by the Mid Devonian Theresa Creek Volcanics and Devonian- Carboniferous Silver Hills Volcanics (Withnall et al., 1995; Blake et al., 1995, Figure 1.4). Contacts between volcanic stratigraphy and the metamorphics are unconformable, as are contacts 7

34 1.3 Regional Geological Setting Introduction Figure 1.3: The location of the Tasman Fold Belt System in Australia, as part of the Terra Australia Orogen (black) in the Cambrian. The system formed as a result of subduction along the Pacific margin of Gondwana during the Palaeozoic. Figure modified after Foster and Gray (2000), Cawood (2005), and Vos (2005). Reconstruction from Li and Powell (1993). 8

35 Introduction 1.3 Regional Geological Setting between discrete volcanic packages. The Theresa Creek Volcanics, located south of Clermont, are a package of basaltic and andesitic lavas intruded in parts by the Retreat Batholith. The Theresa Creek Volcanics and the Retreat Batholith are considered to be genetically related on geochemical grounds (Withnall et al., 1995). Strong calc-alkaline affinities are shown for the Retreat Batholith, Theresa Creek Volcanics and Silver Hills Volcanics, with samples from the Retreat Batholith plotting within the syn-collisional/volcanic-arc granitoids field of Pearce et al. (1984), similar to other granitoids of the Anakie Inlier (Withnall et al., 1996). Outcrop in the northern part of the Anakie Inlier is very poor, resulting in most work being conducted in the southern part of the inlier (Figure 1.4). Previous structural studies by Withnall et al. (1995) and Green et al. (1998) interpreted that there were three main periods of deformation in the Anakie Metamorphic Group. They found that an early bedding parallel, biotite defined fabric is strongly overprinted by a strong (regionally dominant) S2 foliation, the S2 foliation is usually gently dipping and is associated with mylonitisation and microlithon development. D3 is interpreted by Green et al. (1998) to incorporate all post D2 deformation, including dome and basin style folding and upright brittle faulting. The highest grade rocks in the Anakie Inlier occur in the Eastern Creek area (Green et al., 1998; Fergusson et al., 2001), and reached mid amphibolite facies conditions. Most of the metamorphic rocks west of Clermont have been to amphibolite facies, and were retrogressed to greenschist facies during D2 deformation. There is a metamorphic gradient in the Anakie Metamorphic Group, with increasing grade from northeast to southwest across the inlier (Withnall et al., 1995). The inlier is fault-bound to the west by the Devonian-Carboniferous Drummond Basin. Basal syn-rift volcanics of the Drummond Basin are juxtaposed against the Anakie Metamorphic Group as a result of basin inversion. The eastern margin of the Anakie Inlier is bound by Drummond Basin volcanic stratigraphy to the north, and Permain aged Bowen Basin sediments to the south. The contacts are largely obscured by Tertiary lavas and Cainozoic cover. Small Permian rift basins on the eastern edge of the Anakie Inlier in the Clermont region host rich coal and gold deposits. For more detail on the general geology and relationships, the reader is referred to Withnall et al. (1995), which provides in-depth descriptions. The Anakie Metamorphic Group is thought to be a window into the Thomson Orogen, which is previously known only from geophysical investigations and examination of drill core (Kirkegaard, 1974; Murray and Kirkegaard, 1978; Murray, 1990, 1994). The Anakie Inlier 9

36 1.3 Regional Geological Setting Introduction Cainozoic cover Tertiary basalt v Lucky Break Au v v v v Permian Bowen Basin sediments CARBONIFEROUS Withersfield Quartz Syenite N 22 30' Clermont Region Cz Belyando Au v v Miclere Blair Athol Fletchers Awl v v v v v v v Drummond Basin Sequence DEVONIAN Silver Hills Volcanics Greybank Volcanics Karin Granite Retreat and Taroborah Batholiths (I-type) Theresa Creek Volcanics SILURIAN Gem Park Granite (S-type) LATE ORDOVICIAN 23 00' v Redrock v v v v v Clermont Cz Fork Lagoons beds EARLY CAMBRIAN Mooramin Granite(?) (S-type) Anakie Metamorphic Group volcanic centre boundary fault structural trend Rubyvale Region v Rubyvale v Anakie Emerald Cz ' ' 0 10 km Figure 1.4: Simplified geologic map of the southern Anakie Inlier, modified from Withnall et al. (1995). This study focussed around the Clermont region, mostly west of Clermont township. 10

37 Introduction 1.3 Regional Geological Setting contains rocks that are significantly older than extensive surrounding basins, and may be representative of basement that underlies a large portion of Queensland. Studies and interpretation of structural and tectonic evolution of the Thomson Orogen are limited by a lack of outcrop, thus the Anakie Metamorphic Group may provide valuable information into processes that affect a significant part of the northern Tasman Fold Belt System Stratigraphy and Relationships This section covers the stratigraphy and relationships in the Clermont region, which is where most of the work was undertaken. The relationships discussed in this section are from Withnall et al. (1995), stratigraphy and relationships in other areas are discussed where relevant. Withnall et al. (1995) identified six units that make up the Anakie Metamorphic Group in the Clermont region, their distribution is shown in Figure 1.5, and are summarised below: Bathampton Metamorphics - heterogenous package of rocks comprised mainly of finegrained mica schist grading into phyllite with common white, foliated quartzite. Laminated greenstones or mafic schists form units up to several hundred metres thick, parallel to lithological layering. A more substantial package of greenstones forms the Yan Can Greenstone Member north of the Oaky Creek Antiform, and could be a pile of basaltic lavas. Antigorite serpentinite crops out with the greenstones, which have a tholeiitic basalt geochemistry. Rolfe Creek Schist - homogenous, dark grey and fine-grained mica schist grading into phyllite. Monteagle Quartzite - white, strongly foliated quartzite interlayered with mica schist and phyllite. Wynyard Metamorphics - psammopelitic unit comprising crystalline meta-arenite and coarse grained biotite schist. They contain local garnet porphyroblasts, as well as relict staurolite and andalusite now replaced by white mica aggregates. These are the highest grade rocks in the Anakie Metamorphic Group. Scurvy Creek Meta-arenite - Foliated, green, chloritic, labile meta-arenite interlayered with phyllite. Hurleys Metamorphics - Foliated quartzite with minor interlayered phyllite. 11

38 0 10 km Miclere Au Blair Athol boundary synform antiform fault dip and trend main foliation Black Ridge Au A v Cz Miclere - Blair Athol area N Clermont Cz v Cz Eastern Creek area A v E E v v Oaky Creek Antiform area v v v Peak Downs Cu Cainozoic cover Permian sedimentary rocks Drummond Basin Sequence Silver Hills Volcanics Theresa Creek Volcanics v Retreat Batholith and related rocks Wynyard Metamorphics Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Greenstone Member metasediments greenstone Bathampton Metamorphics ' Figure 1.5: Simplified geologic map of the Clermont region, showing main tectonostratigraphic units and structures. Modified from Withnall et al. (1995). Cross section A-A is shown in Figure 1.6, and the cross section E-E is shown in Figure See Appendix 1 for enlargement of the Oaky Creek Antiform area.

39 Overall metamorphic grade increase Rhyolite dykes Oaky Creek Antiform, western arm Section here is parallel to strike, see Oaky Creek Antiform sections for details SW NE A A -?? V V V S3 foliation 5 km Fault v Silver Hills Volcanics Wynyard Metamorphics Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Greenstone Member quartzite mica schist Bathampton Metamorphics greenstone Figure 1.6: Interpreted cross section A-A through the Clermont region, based on stratigraphy and dominant foliation. The middle of the section is parallel to strike, and can be seen in more detail in Figure 2.26

40 1.3 Regional Geological Setting Introduction The Bathampton Metamorphics outcrop prominently in the core of the Oaky Creek Antiform, which is the most prominent structural feature in the Clermont region (Figure 1.5, Appendix 1). They are structurally the lowest package of rocks in the Anakie Metamorphic Group (Withnall et al., 1995). The Bathampton Metamorphics are bound on their eastern margin by the Scurvy Creek Meta-arenite, the contact is marked by a mylonite zone up to 200 metres wide, and both units dip moderately to the west. Here the Scurvy Creek Meta-arenite structurally underlies the Bathampton Metamorphics, and the contact is interpreted as a thrust (Withnall et al., 1995). Outcrop in the Eastern Creek area (Figure 1.5) comprises the Wynyard Metamorphics and Monteagle Quartzite. Compositional layering and dominant foliation in both units dips moderately to steeply to the west, and there is an overall increase in metamorphic grade from east to west in the area (Fergusson et al., 2001). The Wynyard Metamorphics are bound on the west by the Mid Devonian Silver Hills Volcanics of the Drummond Basin (Figure 1.5), which have been thrust against the Anakie Inlier during basin inversion (Withnall et al., 1995; Henderson et al., 1998). Basement outcrop in the Miclere-Blair Athol area (Figure 1.5) is dominated by the Scurvy Creek Meta-arenite and Hurleys Metamorphics, they are the lowest grade metamorphic rocks in the Clermont region (Withnall et al., 1995). They are unconformably overlain by Permian sediments of the Miclere and Blair Athol basins, which formed as intracratonic graben and half graben structures (Dickins and Malone, 1973; Zhou et al., 1994). Bedding features are visible in the metamorphic rocks here, and they are not as strongly deformed compared to rocks to the west. The dominant foliation and compositional layering/bedding in this area variably dips from shallowly to the west to subhorizontal to shallowly to the east, which contrasts with the dominant westerly dip observed across most of the Clermont region. The Wynyard Metamorphics and Scurvy Creek Meta-arenite have been interpreted to represent the same unit on lithological grounds, so too the Monteagle Quartzite and Hurleys Metamorphics (Withnall et al., 1995). The Wynyard Metamorphics and Monteagle Quartzite are higher grade than the Scurvy Creek Meta-arenite and Hurley Metamorphics, with the two lower grade units preserving the earlier deformation history better than their higher grade equivalents, which have been strongly overprinted. The lithologies outlined by Withnall et al. (1995) and accompanying map were used as a base map during this project. Withnall et al. (1995) used a nomenclature that describes tectonostratigraphic units or lithodemes rather than lithostratigraphic units, as the nature of the contacts 14

41 Introduction 1.3 Regional Geological Setting between units may be partly or wholly tectonic. This is an appropriate way to describe metamorphic rocks such as these, where distinct map units are evident, but contacts within and between them may not be purely lithological. 15

42 1.3 Regional Geological Setting Introduction 16

43 Chapter 2 Structural and Metamorphic History, Anakie Metamorphic Group This chapter explores the structural and metamorphic history of the Anakie Metamorphic Group, with detailed mapping and observations focussed around the Oaky Creek Antiform area. Regional trends are considered in the interpretation, and observations from the Clermont region are compared to the Rubyvale and Mt Coolon regions. The structural and metamorphic history interpreted in this chapter will be used as a framework for thermochronology and geochronology studies (Chapters 3, 4), and will be used to better understand the timing and structural controls of gold mineralisation in the southern Anakie Inlier (Chapter 5). The interpretations and concepts explored in the Anakie Metamorphic Group were prompted by discussion with Professor Gordon Lister. Valuable discussion in the field was had with Caroline Forbes, and also with Simon Richards, who made a significant contribution towards an interpretation of the structural geology. Useful discussions with Daniel Viete concerning problems with linking structure and metamorphism are acknowledged. Technical support for the microprobe was provided by Ashley Norris, and discussions with Marco Beltrando and Jörg Hermann on the uses and limitations of white mica compositions in metamorphic rocks are also acknowledged. The concept of sequence diagrams that is introduced in this chapter is a new way of bridging the gap between field observations, and interpretation of a deformation history in an area. The use of sequence diagrams has been employed by members of the Structure and Tectonics Group at the Research School of Earth Sciences, and has been the subject of much discussion. Sequence diagrams had their inception in recent work in the Otago Schist of New Zealand by Marnie Forster and Gordon Lister, whom have essentially pioneered the technique. In this thesis the 17

44 2.1 Introduction Structure and Metamorphism term sequence diagrams refers to deformation history, not sequence stratigraphy. 2.1 Introduction This chapter documents the geodynamic setting and metamorphic conditions under which a flat-lying foliation within the Anakie Metamorphic Group developed. The flat-lying foliation is the dominant structural feature in the Clermont region, and has been compared to a similar foliation in the Cape River Metamorphics to the north (Fergusson et al., 2005). The flat-lying foliation has previously been interpreted as a result of shortening (Withnall et al., 1995; Green et al., 1998), and also extension (Wood, 2004) (Fergusson et al., 2005). Flat-lying foliation elsewhere is known to form in shortening tectonic settings (Ramsay, 1981), and extensional tectonic settings (Sandiford, 1989; Gibson, 1991), both of which existed during the Palaeozoic evolution of eastern Australia (Preiss, 1990; Vandenberg et al., 2000; Betts et al., 2002; Collins, 2002b; Henderson et al., 2004; Fergusson et al., 2005; Foster et al., 2005). Thus the origin of the flat-lying foliation in the Anakie Metamorphic Group has significant bearing for interpreting tectonic processes in Queensland during the early Palaeozoic. 2.2 Previous Work Previous structural studies in the Clermont region (Figure 1.5) have been conducted by Withnall et al. (1995), and a more detailed study was undertaken by Green et al. (1998). Withnall et al. (1995) and Green et al. (1998) cite three major periods of deformation, D1, D2, D3, with metamorphic mineral growth accompanying D1 and D Structure In previous work, D1 deformation is interpreted to have produced a well-defined, pervasive foliation that is parallel to relict bedding (Withnall et al., 1995). S1 is best observed in quartzite layers as parting planes spaced up to 2 mm, giving the rock a finely laminated appearance 18

45 Galilee Basin Structure and Metamorphism 2.2 Previous Work Charters Towers Province Mt Coolon Region Coral Sea Drummond Basin Anakie Inlier Clermont Region 22 Bowen Basin N 24 Great Artesian Basin Rubyvale Region Nebine Ridge 149 ophiolite Younger cover, includes Great Artesian Basin Permian-Triassic sediments New England Orogen Pre Devonian basement (Thomson Orogen) 500 km Sub-surface Drummond Basin Fault Gravity trend Figure 2.1: Regions of the Anakie Inlier used for interpretation of the structural and metamorphic evolution of the Anakie Metamorphic Group in this chapter. Most of the work focussed around the Clermont region (Figure 1.5), and to a lesser extent to the Rubyvale region (Figure 2.53). 19

46 2.2 Previous Work Structure and Metamorphism (Withnall et al., 1995). Green et al. (1998) described S1 as a differentiated cleavage defined by aligned biotite in alternating mica- and quartz-rich layers. However in higher grade areas to the east (Eastern Creek area, Figure 1.5), S1 is transposed parallel to S2 and dominated by aligned muscovite. Withnall et al. (1995) interpreted a shallow angle for the formation of D1 structures, however Green et al. (1998) inferred upright F1 folds based on macroscale double F2 closures, and S1 occurring at a high angle to S2 in F2 hinges. Previously interpreted D2 deformation is pervasive in the Anakie Metamorphic Group, and is characterised by a strong foliation that forms the main schistosity (Green et al., 1998). S2 is commonly a crenulation cleavage and microlithons are well developed, particularly in pelitic units (Green et al., 1998). The microlithons are up to 1 cm wide, separated by thinner micaceous layers defined by aligned muscovite, biotite and chlorite (Green et al., 1998). In quartzite units S2 is commonly mylonitic in character (Withnall et al., 1995; Green et al., 1998). D2 structures are generally flat-lying around the Blair Athol basin north of Clermont (Figure 1.5), but have been variably reoriented by F3 folds (Withnall et al., 1995; Green et al., 1998). F2 recumbent folds are prevalent at the microscopic and mesoscopic level and are generally tight to isoclinal (Withnall et al., 1995). Where D2 is intense, S1 is commonly obliterated (Green et al., 1998), but rootless isoclinal fold hinges are common (Green et al., 1998). An L 1 2 intersection lineation occurs on S2. It is commonly parallel to a mineral lineation L m that is defined by elongate biotite or actinolite, both plunge shallowly to moderately to the west/southwest (Green et al., 1998). Rare shear bands associated with D2 are spaced 2-10 cm apart, and sense of shear is top to the east to southeast (Green et al., 1998). Withnall et al. (1995) and Green et al. (1998) interpreted D2 deformation as the result of shallow thrusting in a mid-crustal shear zone. D3 deformation resulted in significant reorientation of S2 on a regional scale. Dome and basin interference patterns are interpreted to be the resulted of oblique fold sets formed during D3 (Withnall et al., 1995; Green et al., 1998). The Oaky Creek Antiform west of Clermont (Figure 1.5) is interpreted as a major dome formed during reorientation of D2 structures during D3, and has several complementary folds to the east and south (Withnall et al., 1995). East, north-east and south-east oriented crenulation cleavage trends are all interpreted as part of D3 deformation, as no overprinting relationships were observed by Green et al. (1998). Conjugate kink bands are common in the area (Green et al., 1998), especially near major faults which have northwest, southwest and east-west trends (Figure 1.5). The faulting is interpreted to be contemporaneous with D3 (Withnall et al., 1995; Green et al., 1998), hence, the conjugate kink bands are also attributed to D3. 20

47 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Metamorphism Previous metamorphic studies of the Anakie Metamorphic Group are limited to descriptions in Withnall et al. (1995), Green et al. (1998) and Fergusson et al. (2001). Based on the presence of biotite, muscovite and chlorite aligned along S1 and S2 cleavage planes, as well as andalusite, staurolite and garnet porphyryoblasts, they concluded that metamorphism accompanied D1 and D2 deformation, and was of the low-p/high-t type. To the west and northwest of Clermont (Figure 1.5) metapelites contain biotite, chlorite and abundant muscovite, the rocks in this area are greenschist facies (Withnall et al., 1995). Garnet occurs in schists at the northeastern end of the Oaky Creek Antiform, (Withnall et al., 1995). Around the Eastern Creek area (Figure 1.5), staurolite and andalusite occurs, but is mostly retrogressed to fine grained muscovite or coarse muscovite-biotite aggregates. The rocks in the eastern Creek area are amphibolite facies (Withnall et al., 1995; Fergusson et al., 2001). An overall metamorphic grade increase from northeast to southwest across the Clermont region (Figure 1.6) is interpreted based on biotite abundance (Withnall et al., 1995), and Fergusson et al. (2001) interpreted andalusite-in and staurolite-in isograds in the Eastern Creek area. 2.3 Structure and Metamorphism: this study Preservation of the structural and metamorphic history is highly variable in the Clermont region, reflecting effects of multiple overprinting events and prolonged surface exposure. The Anakie Metamorphic Group in the Oaky Creek Antiform area (Figure 1.5) was found to preserve all elements of deformation and metamorphism, and is where the majority of mapping was undertaken. Observations from the Oaky Creek Antiform area were compared with the Eastern Creek and Miclere-Blair Athol areas (Figure 1.5), although work in the other areas is restricted to a transect through the Eastern Creek area, and outcrop observations in the Miclere-Blair Athol area. Comparisons are focussed on developing an understanding of the potential changes in structural style and metamorphism associated with the early deformation events, specifically a flat-lying foliation. Regional changes in structural trends and metamorphism within the Anakie Metamorphic Group were examined using observations from the Rubyvale and Mt Coolon regions (Figure 2.1). Bedding (S0), although rarely observed, was documented, however most lithological contrasts are best described as compositional layering. Graded bedding and way-up was not able 21

48 2.3 Structure and Metamorphism: this study Structure and Metamorphism to be established. Adding to this complexity is parallelism between bedding and two variably developed foliations. In the hinge zone of recumbent folds, the subparallel foliations were able to be distinguished, however along the limbs of recumbent folds, the fabrics were transposed and formed a composite foliation The Oaky Creek Antiform Area: Structure Detailed structural analysis undertaken in this project was focussed in the Oaky Creek Antiform area to the west of Clermont (Figure 1.5, Appendix 1). The Oaky Creek Antiform is an eastwest to northeast-southwest trending structure that can be separated into two distinct structural zones based on the principal trend of the fold axis. The core of the antiform contains what are interpreted to be structurally the lowest rocks in the Anakie Metamorphic Group (Withnall et al., 1995), although the overall facing of the structure is unknown. Kilometre-scale, tight to isoclinal folds defined by quartzite units are evident on the southern limb of the Oaky Creek Antiform, where they help to define the structure in an area of pelite dominated schists. A Type 3 interference pattern (Ramsay, 1967) is interpreted to have formed by the interference between an early, kilometre-scale isoclinal fold set, and a later open fold generation (Appendix 1). Structurally controlled gold deposits are located along and adjacent to the Oaky Creek Antiform, which are discussed in detail in Chapter 5. Dioritic hornblende-feldspar bearing porphyritic dykes intrude into fault zones along the limbs, and in the hinge zone of the antiform. Outcrop in the Oaky Creek Antiform area is dominated by strata of the Bathampton Metamorphics, which includes a variety of rock types. A dominantly quartz-mica schist (Pb p ), which is the main rock type, is interbedded with a quartzite-rich unit (Pb q ) and a greenstone unit (Pb g ). Two small serpentinite dominated bodies (Ps p ), several hundred metres in diameter also outcrop in the area. Limestone beds up to 10 m thick are outlined in the west of the area (Appendix 1), and form part of the quartz-mica schist unit, but are not laterally extensive. A small area of Rolfe Creek Schist outcrops in the southernmost part of the map area, and adjacent to the Sunny Park Granodiorite. Small monzodiorite bodies up to 30 m wide appear to be spatially associated with basic dykes and faults that are also adjacent to the Sunny Park Granodiorite. 22

49 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Sequence diagrams: a method for structural analysis Structural analysis in this study was based on observations of structural patterns, styles and overprinting relationships, as well as relationships between structural and intrusive elements. At the individual outcrop level it was often possible to define sequences of events, based on overprinting relationships between structures. For example, a common sequence observed in outcrop was a differentiated foliation folded around recumbent fold axes, and overprinted by upright folds. The sequence diagram 1 for this observation would look like this: S S, F R, F U (see Figure 2.2 for key) By routinely developing such sequence diagrams, a tool emerges that allows for: (a) systematic documentation of observations and; (b) correlation of sequences between individual outcrops. This is particularly useful in polydeformed rocks, where reorientation and overprinting of earlier structures by later deformation can obscure elements of the structural history. A sequence diagram approach allows sequences of events to be determined without immediately locking into a numbering scheme of D1, D2, D3 and so on. Application of a numbering scheme to a deformation history is generally useful, but can become confusing when the identification number of a particular event suddenly changes due to new evidence. Sequence diagrams are more descriptive than a D1, D2, D3 scheme, and can include observations of metamorphism and features such as shear zones with respect to other structures. Sequence diagrams provide a link between observation and interpretation, and results in a deformation history being methodically constructed. They provide an alternative to the current practice, which can see a deformation history reported as a series of assertions and lacking justification for the interpreted order of events. Sequence diagrams produced in the field can be eventually correlated to produce a synthesis sequence diagram of an area (Figure 2.2). The events can be numbered, or labelled, thus enabling comparison with any earlier interpretations of deformation history (Figure 2.2). The key is that individual sequence diagrams can be independently recorded, and then later correlated using explicitly documented criteria. For example, in the Anakie Metamorphic Group, a pervasive foliation that was observed in every outcrop 1 Sequence diagrams refers to terminology that is being developed within the Structure and Tectonics Group, Research School of Earth Sciences. 23

50 2.3 Structure and Metamorphism: this study Structure and Metamorphism provided a frame of reference for correlation between sequence diagrams recorded over the entire region. One advantage of using a sequence diagram is the ability to distinguish between closely linked, yet distinctly different events. An example from the Anakie Metamorphic Group is recumbent folding and low-angle shearing that were previously considered as different manifestations of the same deformation event, termed D2 by Withnall et al. (1995) and Green et al. (1998). Observations within a sequence diagram framework separates the two as different structures, related to two potentially different events. The use of sequence diagrams resulted in specific conclusions as to the significance of the earliest recognisable structure in the Anakie Metamorphic Group. This is a differentiated cleavage, associated with upright isoclinal folding (Figure 2.2). This differentiated cleavage had previously been interpreted as S1, related to D1 (Withnall et al., 1995; Green et al., 1998). However, if we assume the mechanisms for formation of a differentiated cleavage (Passchier and Trouw, 1998, page 70), then this implies the presence (although not preservation) of an earlier fabric. The first observed fabric is interpreted as an S2 foliation. This implies a D1, D2, D3 scheme to explain the observed structures, but the lack of observational data for S1 is hidden by the D1, D2, D3 terminology. In this situation the sequence diagram approach is a more transparent method of interpretation, as it highlights any anomalies in the observed sequence of events. A sequence of events can only be derived from directly observing overprinting relationships, however it is rare to find a full deformation history preserved in any single outcrop. Correlation through the use of sequence diagrams addresses the issue raised by Potts and Reddy (1999), who noted the futility in attempting to determine overprinting relationships between each and every event. The method proposed by Potts and Reddy (1999) outlines the permutations and possible combinations of deformation histories from a series of incomplete overprinting relationships. Sequence diagrams achieve a similar end result in an intuitive, straight forward manner, in which a structural geologist is led progressively towards a logical synthesis. Individual outcrops in the Oaky Creek Antiform area usually characterised 2 to 4 events on the basis of distinct overprinting (eg, folding of a foliation, cross-cutting cleavage(s) etc.). In this study, in the instance of a fold with an axial planar cleavage being observed in relation to another structure, the fold was given priority over its axial planar cleavage for inclusion in the sequence diagram for that outcrop. After constructing sequence diagrams at many different outcrops, patterns began to emerge that allowed correlation between outcrops. 24

51 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Some sequence diagrams of outcrop observations contained patterns that occurred more frequently than others. These were: S S, F R, F U F R, SZ F, F U, S SZ F, S D, S (see Figure 2.2 for key) More rarely, sequence diagrams at individual outcrops comprised the following, where the two adjacent S elements are defined by distinct overprinting and differences in orientation at the outcrop scale: SZ F, S D, SZ U SZ F, S D, S, S (see Figure 2.2 for key) One outcrop preserved the most complete record of the structural evolution of the Anakie Metamorphic Group, and had clear overprinting relationship (Figure 2.3). The sequence diagram at this location comprised: S S, F R, SZ F, S D, S, S (see Figure 2.2 for key) Figure 2.2 compares the previously interpreted deformation history of the Anakie Metamorphic Group with a deformation history inferred from a sequence diagram. The sequence diagram has more detail, yet still allows direct correlation with the previous deformation history. The outcrop in Figure 2.3, along with the event history that emerged from a synthesis sequence diagram, are used as justification of the deformation history (D1 - D6) that is presented in this thesis for the Anakie Metamorphic Group. This numbering scheme is by no means set in stone, however it is underpinned by the logic of sequence diagrams, and provides the reader with a means of comparison within this study, and also with previous studies. 25

52 2.3 Structure and Metamorphism: this study Structure and Metamorphism Sequence Diagram (synthesis of structure only) Fabric Structure Inferred History Correlation With Previous History D 1 S S F UI S XPR F R S D S S SZ F SZ F U flt SZ U F U F U U D 2 D 3 D 4 D 5 D 6 D 1 D 2 D 3 Observations New Interpretation Previous Interpretation S S - differentiated foliation F UI - upright isoclinal folding S XPR - axial planar foliation to recumbent folds SZ F - low angle shear zone SZ U - upright shear zone S D - crenulation cleavage defined by dissolution S - crenulation cleavage, with overprinting F R - recumbent folding F U - upright folding, with overprinting flt - faulting D 2 - deformation event and number Figure 2.2: Synthesis sequence diagram - structure. By using observations of particular structures and fabrics, and correlating the patterns between them, a sequence of events can be constructed. The sequence can then be numbered or labelled for interpretation, and compared with previous interpretations. The previous interpretations in this instance are from Withnall et al. (1995) and Green et al. (1998). 26

53 Structure and Metamorphism 2.3 Structure and Metamorphism: this study S5 S4 S6 30 cm S5 (S) S4 S3 (SZ F ) S3 S6 (S) F3 (F R ) [folding S2 (S S )] S4 (S D ) S3 S2 S3 S4 Figure 2.3: Overprinting relationships for the various structures mapped around the Oaky Creek Antiform area. This locality preserves all of the fabric elements and overprinting relationships in one outcrop. 27

54 2.3 Structure and Metamorphism: this study Structure and Metamorphism D1 Deformation The key to understanding the nature of S1 is understanding the mechanisms associated with development of S2. In the Anakie Metamorphic Group, S2 is a well-developed differentiated cleavage. Passchier and Trouw (1998, page 70) show that such well developed fabrics require the presence of an earlier formed fabric, usually at a high angle to the second fabric. Based on these prerequisites, an S1 fabric is suggested to have developed prior to D2 deformation, therefore D1 deformation is inferred D2 Deformation D2 is commonly manifest as an S2 differentiated cleavage in high-angle microlithons of an S3 crenulation cleavage, and is most visible in D3 low strain zones or in the hinges of F3 folds (Figure 2.4, 2.5). The S2 foliation is defined by alternating domains of aligned biotite and quartz parallel to bedding, although biotite is often replaced by chlorite where S3 foliation is intense (Figure 2.6). S2 quartz domains are up to 5 mm wide, and biotite domains are usually thinner (Figure 2.4). In many places the S2 foliation has been transposed to form a composite S2/S3 fabric, and is often obliterated (Figure 2.7). S2 has an overall north-south strike, and poles to S2 have a best fit great circle with a beta pole that plunges gently to the south (Figure 2.8). Quartz veins occur as laminae parallel to S2, or as narrow veins in S2 cleavage domains. The quartz veins have been strongly deformed during D3, ranging from boudinaged porphyroclasts to tightly folded quartz layers forming rootless isoclinal folds (Figure 2.12). In many outcrops quartz veins are parallel to S2. F2 folding was rarely observed, however one location preserved an upright, tight to isoclinal F2 fold that had been overprinted by low angle structures (Figure 2.15) D3 Deformation D3 deformation of the Anakie Metamorphic Group produced dominantly sub-horizontal structures, however in the Oaky Creek Antiform area they are reoriented by later deformation. S3 is the pervasive structural feature of the Anakie Metamorphic Group in the Oaky Creek Antiform 28

55 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Differentiated S2 microlithons Subhorizontal S3 Figure 2.4: Differentiated S2 cleavage forming microlithons between S3 crenulation cleavage. S2 and S3 are more often observed at a low angle to each other, however this image is from the hinge zone of an F3 recumbent fold. 3mm S3 biotite chlorite quartz S2 Figure 2.5: Photomicrograph of differentiated S2 cleavage defined by biotite and quartz (subvertical orientation) in microlithons of S3 crenulation cleavage. Biotite parallel to S2 has altered to chlorite in some of the microlithons. Image is taken under plane polarised light. 29

56 2.3 Structure and Metamorphism: this study Structure and Metamorphism 3mm S2 microlithons, chlorite after biotite relict S2 biotite S2 S2=chlorite (after biotite)and quartz Figure 2.6: Photomicrograph of S2 cleavage defined by biotite (altered to chlorite) and quartz, folded around F3 fold axes. S2 is parallel to bedding (S0) and to S3. Abundant chlorite after S2 biotite in S3 microlithons occurs in the pelitic layer (green colour) above a more quartz rich layer. Image is taken under plane polarised light. 2mm biotite, muscovite chlorite S3 S2 quartz C Figure 2.7: Photomicrograph of S2 cleavage being transposed and drawn into parallelism with S3 (subhorizontal). Extensional crenulation cleavage (C ) bands formed during D3 dissect a quartz-biotite S2 foliation. Image is taken under plane polarised light. 30

57 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Figure 2.8: Plot of poles to S2 in the Oaky Creek Antiform area. area. In pelitic rocks, the S3 foliation is a penetrative, differentiated crenulation cleavage defined by elongate muscovite, biotite, chlorite and quartz (Figure 2.9). In quartzite units S3 is mylonitic in character, apparent from dynamic recrystallisation and formation of elongate quartz ribbons (Figure 2.10). S-C shear bands and extensional crenulation cleavage (C shear bands) occur late in the D3 deformation history, offsetting earlier formed S3 foliation (Figure 2.11). Asymmetric folding and kinematic indicators from S-C and C shear bands as well as rotated garnets that are wrapped by S3 suggests a component of simple shear during D3 deformation (Figure 2.37, 2.11), with top to the east/northeast sense of shear. Asymmetric pressure shadows around quartz porphyroclasts and folding of quartz veins also indicate top to the east/northeast sense of shear (Figure 2.12). The S3 fabric is generally sub-parallel to bedding/compositional layering where recognisable (Figure 2.13). Zones of apparent high strain during D3 resulted in transposition of earlier structural and lithological features into parallelism with the S3 foliation, obscuring earlier structural elements. F3 folds are characterised by tight to isoclinal recumbent folding of a differentiated S2 fabric at both the microscale and macroscale (Figure 2.14). Large-scale (>100 m) F3 folds have been identified in the Oaky Creek Antiform area, namely the kilometre-scale isoclinal and refolded folds in the southern part of the map area (Appendix 1). Outcrop-scale refolding of an F2 fold 31

58 2.3 Structure and Metamorphism: this study Structure and Metamorphism S2 microlithons S2 quartz muscovite-biotite-chlorite S3 2mm Figure 2.9: Photomicrograph of S3 differentiated crenulation cleavage in pelite. S2 microlithons are visible, and contain relict biotite, chlorite and quartz. Top image is taken under plane polarised light, bottom image is cross polarised light. 32

59 Structure and Metamorphism 2.3 Structure and Metamorphism: this study 2mm S3 dynamically recrystallised quartz ribbons, parallel to S3 Figure 2.10: Photomicrograph of mylonitic S3 in quartzite. The S3 foliation is characterised by dynamic recrystallisation and formation of elongate quartz ribbons, with interstitial mica (mostly muscovite). Image taken under cross polarised light. 33

60 2.3 Structure and Metamorphism: this study Structure and Metamorphism S C asymmetric fold S3 (a) C S3 C shear band in S3 foliation (b) 34

61 Structure and Metamorphism 2.3 Structure and Metamorphism: this study S3 D2 quartz vein rootless, asymmetric fold (c) Figure 2.11: This and previous page. (a) S-C fabrics formed in quartz vein-rich horizon. C planes truncate earlier formed S3 differentiated foliation. (b) C shear bands (extensional crenulation cleavage) truncating S3 foliation. C bands are more common than S-C bands. (c) Asymmetric folding of D2 quartz veins, tectonic transport direction is indicated from short limb-long limb relationships. Sense of shear in all images is towards the northeast. 35

62 2.3 Structure and Metamorphism: this study Structure and Metamorphism recrystallised pressure shadow fringes D2 quartz (a) boudinaged D2 quartz vein S3 (b) Figure 2.12: (a) Rotated D2 quartz porphyroclast with recrystallised quartz in pressure shadows. Sense of shear is top to the northeast. (b) D2 quartz vein stretched parallel to S3 to form boudins. 36

63 Structure and Metamorphism 2.3 Structure and Metamorphism: this study bedding/s0 quartzite S3 quartz-mica schist D2 quartz veins, now parallel to S3 Figure 2.13: S3 foliation parallel to bedding. D2 quartz veins are drawn parallel to S3. by flat-lying D3 structures was observed in metre thick bedded quartzite in the far southwest of the main map area (Figure 2.15, Appendix 1), and shows the overprinting relationship between D2 and D3 structures. There are two types of folds related to D3 deformation, symmetric and asymmetric. Early recumbent folding of S2 forms symmetric structures with an axial planar cleavage (eg. Figure 2.14(c)). A second type of folding comprises tight to isoclinal folds with distinct asymmetry, and are usually defined by quartz layers that occur proximal to shear bands. The later folds lack an axial planar cleavage (eg. Figure 2.11 (a), (c)). Components of pure shear and simple shear are represented by the fold styles and shear bands formed during D3 deformation. Early pure shear is represented by vertical thinning and formation of recumbent folds, followed by a top to the east simple shear component that formed shear bands and asymmetric folds. To best display structural measurements associated with D3 deformation in the Oaky Creek Antiform area, the map was separated into Domain 1 and Domain 2 (Figure 2.16). These domains are based on reorientation of S3 around younger structures, the dividing line coincides with the change in trend of the axial trace of the Oaky Creek Antiform. 37

64 2.3 Structure and Metamorphism: this study Structure and Metamorphism F3 axial trace S2 (a) quartzite pelite quartzite pelite S3 F3 axial trace S0 // S2 (b) 38

65 Structure and Metamorphism 2.3 Structure and Metamorphism: this study S2 F3 axial plane (c) Figure 2.14: (a) Tight, small scale F3 folding of an S2 differentiated foliation. (b) Outcrop scale F3 recumbent folding, here bedding (S0) is parallel to S2. An S3 axial planar cleavage is developed in pelite in the hinge zone. (c) Differentiated S2 foliation folded around F3 recumbent fold. 39

66 2.3 Structure and Metamorphism: this study Structure and Metamorphism quartzite S2 F2 axial trace pelite quartzite pelite late fault S0 recumbent F3 axial trace Figure 2.15: F2 fold refolded around F3 fold axes to give a Type 3 fold interference pattern. The formation of S2 originally at high angle to bedding (S0) suggests this location is in the hinge zone of an F2 fold. This locality demonstrates the structural development that leads to S0, S2 and S3 all subparallel. 40

67 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Domain 1 comprises the western part of the map area, here S3 strikes east-west to westnorthwest, however can be locally variable. S3 dips to the south overall (Figure 2.17), except for a small area adjacent to the Grasstree Fault where it dips to the north (Appendix 1). Local changes in the dip of S3 are related to east-west trending F4 folds, and local north to northwest trending F5 folds (Appendix 1). The far west of Domain 1 is dominated by north-south to north-northwest trending S3, and forms the western margin of a regional dome structure (Figure 2.16). Domain 2 is more complex, and contains a kilometre scale Type 3 fold interference pattern. The dominant trend of S3 is northeast, reflecting reorientation around D4 structures. Poles to S3 in Domain 2 have a best fit great circle with a beta pole plunging gently to the southwest (Figure 2.17). This orientation correlates with the trend of the Oaky Creek Antiform, around which S3 is folded. Local changes in the strike of S3 correlates with east and northwest oriented folding and faulting during D5 and D6 deformation, discussed in more detail further on. An intersection lineation between S2 and S3 (L 2 3 ) is common in the Oaky Creek Antiform area, and is variable in orientation (Figure 2.18, 2.20). Where L 2 3 and mesoscopic F3 folds are found in the same outcrop they are usually subparallel. A mineral stretching lineation on S3 (L m ) is defined by elongate streaks of muscovite, chlorite (after biotite) and quartz, or actinolite in greenstone units (Figure 2.20). The L m consistently plunges to the west/southwest (Figure 2.20), and where D3 deformation has been intense, the L m and L 2 3 are parallel. The L m is more strongly developed in quartzite units, the L 2 3 occurs more prominently in pelite. There is a gradient in the apparent intensity of D3 deformation. The core of the Oaky Creek Antiform is most strongly affected by D3 deformation, here the features of earlier deformation and metamorphism are mostly destroyed by intense S3 foliation. Away from the Oaky Creek Antiform, S3 decreases in intensity, and earlier structures and metamorphic minerals are better preserved (Figure 2.21). Asymmetric folding and shear band formation during D3 is restricted to regions where S3 is most intense D4 Deformation D4 deformation produced mainly northeast trending, gently plunging structures (Figure 2.22), characterised by the reorientation of originally flat-lying S3 foliation around steeply inclined fold hinges (Figure 2.23). Macroscale D4 structures include the Oaky Creek Antiform and the 41

68 2.3 Structure and Metamorphism: this study Structure and Metamorphism N 2 km OAKY CREEK ANTIFORM Domain 2 Domain 1 plunge of S3 Oaky Creek Antiform Domal Structure plunge of S3 InI Figure 2.16: Structural domains in the Oaky Creek Antiform area. The domain boundary coincides with a change in trend of S3 and the axial trace of the Oaky Creek Antiform. Dashed lines at bottom represent plunge and trend of S3 associated with the Oaky Creek Antiform Domal Structure. The trends are taken perpendicular away from the centre of the exposed Oaky Creek Antiform, but are not directly overlain. Blue dashed lines are trend and plunge of major D4 structures, see Appendix 1 for stratigraphy. 42

69 Structure and Metamorphism 2.3 Structure and Metamorphism: this study (a) (b) Figure 2.17: (a) Poles to S3 in Domain 1. (b) Poles to S3 in Domain 2. Oaky Creek Antiform area L2 3 intersection lineation S2 S3 Figure 2.18: L 2 3 intersection lineation of S2 with S3. 43

70 2.3 Structure and Metamorphism: this study Structure and Metamorphism Stretching mineral lineation on S3 plane defined by quartz and muscovite. Figure 2.19: Mineral stretching lineation on an S3 surface in quartzite. The lineation here is defined by elongate muscovite and quartz., although in other places is defined by biotite and actinolite. Quartzite horizons in the Oaky Creek Antiform area typically have a well developed L m. (a) (b) Figure 2.20: (a) L 2 3 intersection lineations. (b) Mineral stretching lineations on S3. Oaky Creek Antiform area 44

71 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Intense D3 deformation, S3 mostly a shear fabric. Greatest destruction and retrogression of D2 mineralogy D3 lessens in intensity N 2 km D3 lessens in intensity LessintenseD3. MoreD2structure and mineralogypreserved. D3 shearing not as strong InI Figure 2.21: Intensity distribution of D3 deformation in the Oaky Creek Antiform area. Rosevale Synform (Appendix 1), and mesoscale F4 folds are concentrated along the limbs of the Oaky Creek Antiform. Mesoscale and outcrop scale F4 folds have open to close geometries, and a weak to moderate axial planar crenulation cleavage (Figure 2.24). An S4 crenulation cleavage is prominent around the Oaky Creek Antiform, concentrated in areas of F4 folding (Figure 2.24), and proximal to D4 shear zones (see below). The S4 crenulation cleavage is most obvious in pelitic units, where it is defined by pressure solution seams (Figure 2.25). The Oaky Creek Antiform The Oaky Creek Antiform has a total length of approximately 19km, and is best defined by greenstone units interlayered with pelitic rocks of the Bathampton Metamorphics. The trend of the structure changes at a bend in the axial trace (Appendix 1). The northeast trending section of the structure is doubly plunging, with a steeply inclined hinge dipping to the south east. The outcrop pattern and cross sectional views of this part of the Oaky Creek Antiform show a welldefined hinge zone and axial trace with steep limbs on either side (Figure 2.26). The east-west trending section of the Oaky Creek Antiform plunges gently to the west, and has a broad, poorly defined hinge zone that cannot be delineated with a single axial trace (Appendix 1). Several macroscopic warps and folds form parasitic structures to steep limbs on either side of the hinge 45

72 2.3 Structure and Metamorphism: this study Structure and Metamorphism Figure 2.22: Plot of poles to S4, Oaky Creek Antiform area. zone (Figure 2.26). Steep D4 shear zones occur subparallel to the strike of the limbs of the Oaky Creek Antiform. The shear zones are 5 to 200 m wide and are subparallel to S3 and compositional layering in steeper limb zones. The shear zones usually occur close to the contact between greenstone and pelite, and locally crosscut compositional layering (Figure 2.26). The shear zones are defined by strongly cleaved areas of rock where significant grainsize reduction and total obliteration of earlier fabrics occurs (Figure 2.27). The closely spaced foliation within the shear zones has a steep mineral stretching lineation defined by quartz slickensides (fibres) (Figure 2.27). The S4 crenulation cleavage is parallel to, and most strongly developed adjacent to the shear zones, and decreases in intensity over 10 to 50 m away from their margins. The shear zones and S4 crenulation cleavage appear to have formed at the same time. Across the wider shear zones, significant displacement has occurred (Figure 2.26), as evidenced in the hinge zone of the Oaky Creek Antiform which has been displaced upwards relative to the limbs (Figure 2.26). The resulting geometry is best described as a pop-up. An analogue of the geometry of the Oaky Creek Antiform was found in the outcrop-scale hinge zone of a fold at the western end of the antiform (Figure 2.28). 46

73 Structure and Metamorphism 2.3 Structure and Metamorphism: this study S4 S3 (a) S3 S4 (b) Figure 2.23: Examples of F4 folding. (a) Hinge zone of F4 fold in the hinge zone of the Oaky Creek Antiform, the axial plane was originally steeper but has been rotated by downhill soil creep. (b) Outcrop scale F4 folding. 47

74 2.3 Structure and Metamorphism: this study Structure and Metamorphism S4 S3 compass (a) S3 S4 (b) Figure 2.24: Examples of S4 crenulation cleavage. (a) Widely spaced S4 crenulation cleavage in the hinge zone of the Oaky Creek Antiform. (b) Small spaced S4 crenulation cleavage adjacent to an outcrop-scale F4 fold hinge. 48

75 Structure and Metamorphism 2.3 Structure and Metamorphism: this study S4 2mm S3 Figure 2.25: Photomicrograph of S4 crenulation cleavage. S4 is developed in pelitic layers, and is defined by pressure solution seams (dark subvertical lines). 49

76 2.3 Structure and Metamorphism: this study Structure and Metamorphism Broad, poorly defined external hinge zone of F4 pop-up structure F3 fold pair F4 F4 F4 A A B B 2 km S3 fabric Fold hinge F4 Fold vergence quartzite S4 fabric Fault/shear zone mica schist greenstone Bathampton Metamorphics Extension of Grasstree Fault F4 Oaky Creek Antiform (F4) Well defined internal hinge zone of pop-up structure Basic dykes F4 F4 (a) Rosevale Synform (F4) C C F3 (b) Figure 2.26: (a) Cross section through the western arm of the Oaky Creek Antiform. This profile shows the broad and poorly defined geometry of the external part of the F4 hinge zone, no single axial trace can be identified here. (b) Cross section through the north-eastern arm of the Oaky Creek Antiform. This profile shows the well-defined closure in the internal part of the hinge zone. Vertical scale is 1:1 with horizontal scale. See Appendix 1 for cross section locations. 50

77 Structure and Metamorphism 2.3 Structure and Metamorphism: this study oxidised sulphides shear fabric S3 quartz-carbonate veining D4 SHEAR ZONE STEEP S3 (a) strongly cleaved rock, steep lineation on shear plane (b) Figure 2.27: (a) Creek cutting of D4 shear zone margin, S3 is at an angle to the shear fabric. The bright white is a strongly oxidised crust over a weathered sulphide rich zone. The shear zones are lighter in colour compared to surrounding rock, particularly around quartz-carbonate veins. (b) Closely spaced shear fabric within D4 shear zone. No earlier fabric elements are preserved. A steep lineation is present but not visible here. The shear zones typically formed recessive topography and subcrop. 51

78 2.3 Structure and Metamorphism: this study Structure and Metamorphism S3 Broad external hinge zone S4 D4 shear zone equivalents Well defined internal hinge zone Figure 2.28: Outcrop scale analogue of the Oaky Creek Antiform. 52

79 Structure and Metamorphism 2.3 Structure and Metamorphism: this study The boundaries of the shear zones are distinct. Rocks within the shear zones are typically altered and bleached compared to surrounding rocks, and contain quartz and carbonate veins (Figure 2.27). Where the shear zones intersect younger (D5 and D6) structures, strong alteration has occurred (Appendix 1), and is marked by an increase in the abundance of quartz veins. Where several D4 shear zones intersect a D5 structure in the western limb of the Oaky Creek Antiform, siliceous bodies 10 s of metres in diameter occur. The siliceous bodies contain visible sulphides that are now mostly oxidised, similar to gold-bearing structures described in Withnall et al. (1995). The potential for gold in these features is examined in Chapter 5. The bend in the axial trace of the Oaky Creek Antiform does not appear to be related to younger deformation. No overprinting structures have been observed that could result in the current geometry. On this basis, the bend in the axial trace is thought to have formed synchronously with D4 deformation. The Yan Can Greenstone Member to the north of the Oaky Creek Antiform (Figure 2.29) appears to be unaffected by D4 deformation, and is located in an area of rock that has a wedge-like geometry. The Oaky Creek Antiform is separated from the Yan Can Greenstone Member by the Grasstree Fault, and there is a break in structural trend either side of the Grasstree Fault. North of the fault the dominant structural grain trends northsouth, however south of the fault the structural grains trends east-west and northeast (Figure 2.29). The Grasstree Fault appears to have accomodated reverse and sinistral movement during northwest-southeast oriented shortening, preventing D4 structures from developing in the Yan Can Greenstone Member. The wedge-like indentor formed by the Yan Can Greenstone Member appears to have acted as a pinning point, around which the limbs of the Oaky Creek Antiform wrapped during D4 deformation D5 Deformation D5 deformation in the Oaky Creek Antiform is concentrated to within two northwest trending corridors that are up to 700 m wide (Appendix 1). Within these corridors a northwest to northnorthwest trending crenulation cleavage (S5) is axial planar to upright folds at both the outcrop and mesoscale (Figure 2.30, 2.31). Outside of the two structural corridors, S5 is manifest as an irregular, widely spaced crenulation or kink bands (Figure 2.31). On a larger scale, broad D5 warping and folding has resulted in the north to northwest strike trend of compositional layering and S3 in the Clermont region (Figure 1.5). 53

80 2.3 Structure and Metamorphism: this study Structure and Metamorphism N 2 km north-south trend YAN CAN GREENSTONE MEMBER GRASSTREE FAULT LEO GRANDE FAULT GRASSTREE FAULT northeast trend OAKY CREEK ANTIFORM east-west trend InI STRUCTURAL TREND STR Figure 2.29: Different structural trends north and south of the Grasstree Fault. North of the fault is dominated by north-south trending structural grain, south of the fault is dominated by east-west and northeast trending structural grain. Note the indentor geometry of the of the block that contains the Yan Can Greenstone Member. See Appendix 1 for key and stratigraphy. 54

81 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Figure 2.30: Plot of poles to S5, Oaky Creek Antiform area. Intersections between D5 structural corridors and D4 shear zones are characterised by silica and sulphide alteration of country rock (Appendix 1). In the D5 structural corridor at the western arm of the Oaky Creek Antiform, abundant sulphides in strongly altered rock were observed, but are now mostly oxidised (Appendix 1). A known copper deposit (Arsenic Ridge) occurs within the western structural corridor (Appendix 1). In the far east of the Oaky Creek Antiform area, two narrow D5 fault zones occur (Appendix 1), which if projected to the north, intersect at low angles. Fault 1 is north-northwest oriented and up to 70 m wide (Figure 2.32), it is characterised by a fine-grained and cleaved fault gouge that surrounds less deformed blocks of rock that contain S3 (Figure 2.32). The fault gouge was comprised of mainly clay sized particles that are bleached and oxidised (Figure 2.32 (b)). Fault 1 is intruded by a 15 m wide felsic dyke. Sense of movement along this fault was not able to be determined, as no shear sense indicators were found. The amount of offset was also unable to be determined, due to the lack of marker units. Relative movement either side of Fault 1 is not known due to a lack of outcrop. The along strike projection of Fault 1 to the north intersects the Talc Zone copper locality (Appendix 1). Fault 2 is also considered to have formed during D5 deformation, it is oriented northwest, and outcrops approximately 1 km east of Fault 1. It is characterised by a breccia-like appearance, and exhibits a foliation that is parallel to the fault boundaries. Abundant clasts of foliated 55

82 2.3 Structure and Metamorphism: this study Structure and Metamorphism S3 (a) S5 S3 D5 crenulation (b) Figure 2.31: (a) Outcrop scale, F5 upright folding of S3. (b) Single D5 crenulation. This is typical of D5 deformation outside the two structural corridors. 56

83 Structure and Metamorphism 2.3 Structure and Metamorphism: this study S3 preserved in coherent block unconformity S3 S5 exposed fault (a) clay rich fault gouge, strongly altered S5 S3 coherent blocks (b) 57

84 2.3 Structure and Metamorphism: this study Structure and Metamorphism fragmented schist block schist clasts S5 (c) Figure 2.32: (a) North-northwest D5 fault outcropping in creek cutting. The total width of this fault is 70 m. (b) Close up of north-northwest D5 fault zone. Coherent blocks of country rock are surrounded by a matrix of fine grained, clay rich fault gouge. (c) Outcrop of northwest D5 fault zone, clasts of schist are surrounded by a foliated matrix. Larger country rock blocks are fragmented. 58

85 Structure and Metamorphism 2.3 Structure and Metamorphism: this study schist and other country rock are cemented in a silica rich matrix, the clasts are subrounded (Figure 2.32 (c)). Adjacent clasts exhibit markedly different fabric orientations, suggesting disaggregation and reorientation of the country rock fragments during faulting. Larger blocks of country rock up to 20 cm were evident. The northwest projection of Fault 2 continues into the Leo Grande Fault (Appendix 1), suggesting they may be the same structure. The Leo Grande Fault is a known gold-bearing structure immediately north of the Oaky Creek Antiform (Figure 2.29) D6 Deformation D6 deformation produced east-west to west-northwest trending upright structures (Figure 2.34), including spaced, discontinuous crenulations and kinks. A zone of concentrated D6 deformation occurs in an east-west trending structural corridor east of the bend in the axial trace of the Oaky Creek Antiform (Appendix 1). The corridor is up to 400 m wide, and within this zone the intensity of D6 deformation increases from east to west. The eastern end of the corridor is characterised by tight folds several centimetres to 10 s of metres wide with an associated penetrative crenulation cleavage (Figure 2.33). The folds are defined by S3, which is reoriented around east-west fold axes. The western section of the structural corridor is sheared, characterised by an east-west trending, upright, brittle dominated fabric (Figure 2.33). A dextral sense of shear is indicated from shear bands, and from en echelon style quartz veins up to 15 metres long and 1.5 metres wide. The rocks here are strongly bleached and altered adjacent to the quartz veins and where the shear fabric is also most intense. The quartz veins have laminated margins up to 30 cm wide, but usually contain a single massive textured vein within the centre (Figure 2.35). The quartz veins are concentrated at the projected intersection of the D6 fault zone with a D4 shear zone (Appendix 1). A known gold locality K-2 occurs in the western end of the D6 structural corridor. The effects of D6 deformation outside of the the structural corridor are not well known. Any folding on a larger scale is not immediately obvious from outcrop patterns, although east-west trending widely spaced crenulations and kinks are locally developed throughout the Oaky Creek Antiform area. 59

86 2.3 Structure and Metamorphism: this study Structure and Metamorphism crenulation cleavage S6 S3 (a) S6 is a shear fabric here strongly altered and bleached country rock (b) Figure 2.33: (a) Spaced S6 crenulation cleavage in the eastern end of the east-west D6 structural corridor. (b) Sheared and altered country rock in the western end of the D5 structural corridor. Shear bands not visible on this image indicate a dextral sense of shear. 60

87 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Figure 2.34: Plot of poles to S6, Oaky Creek Antiform area. extent of quartz vein laminated margin massive centre Figure 2.35: Large quartz vein in the western end of the D6 structural corridor. Overall it has an en echelon geometry consistent with dextral shear. The vein has multiple growth events recorded, with laminated margins and a massive quartz centre. 61

88 2.3 Structure and Metamorphism: this study Structure and Metamorphism The Oaky Creek Antiform Area: Metamorphism Observations of metamorphic minerals that occurred together in stable assemblages, and those that replaced pre-existing minerals were placed in sequence diagrams. A synthesis sequence diagram of metamorphis is shown in Figure 2.36, and is used as a basis for the inferred metamorphic history. Sequence Diagram (synthesis of metamorphism only) Increasing Metamorphic Grade Inferred History M 1 BMQ MCQ GABMQ TBMCFQ SGBMDQ BMFCQ M 2 M 3 M 4 Observations R New Interpretation Correlation With Previous History D 1 D 2 Previous Interpretation BMQ - biotitemuscovitequartz GABMQ - garnetaluminosilicatebiotite muscovitequartz SGBMDQ - staurolitegarnetbiotitemuscovite decussate white mica (after andalusite) quartz MCQ - muscovitechlorite (after biotite) quartz TBMCFQ - magnetitebiotitemucovitechlorite (after biotite)fine white mica (after aluminosilicate)quartz BMFCQ - biotitemuscovitefine white mica (after aluminosilicate) chlorite (after staurolite)quartz R - biotitemuscovitequartz (Rubyvale region only) M 2 - metamorphic event and number D 2 - deformation event and number (previous study) Figure 2.36: Sequence diagram - metamorphism. A synthesis of metamorphic observations from the Anakie Metamorphic Group is presented along with inferred event history, and compared to previous history of Withnall et al. (1995) and Green et al. (1998). 62

89 Structure and Metamorphism 2.3 Structure and Metamorphism: this study M1 Metamorphism The earliest phase of mineral growth in the Anakie Metamorphic Group is characterised by aluminosilicate minerals that appear to be andalusite, however they are only preserved in the Eastern Creek area. Intense deformation during D3 has obscured the early metamorphic history in the Oaky Creek Antiform area, thus M1 metamorphism can only be inferred M2 Metamorphism M2 is characterised by the assemblage magnetite, garnet, biotite, muscovite and quartz (Figure 2.37, 2.38), as well as aluminosilicate phases thought to be andalusite or staurolite, now altered to white mica (Figure 2.38, 2.39), consistent with observations of Green et al. (1998) and Fergusson et al. (2001). Porphyroblastic growth of M2 minerals is restricted to the hinge zone of the Oaky Creek Antiform (Figure 2.40), although variable preservation of M2 mineralogy appears to be dependent on the apparent intensity of D3 deformation. Where D3 has resulted in particularly intense foliation development, M2 metamorphic minerals are either strongly retrogressed, or completely replaced compared to areas of less intense D3 (Figure 2.21, 2.40). The M2 mineral assemblage is consistent with medium-grade, low-p/moderate-t amphibolite facies metamorphism in the Oaky Creek Antiform area. Biotite and muscovite that grew during M2 define the S2 foliation (Figure 2.5, 2.6, 2.9), and in some altered aluminosilicate porphyroblasts, the secondary white mica has a preferred orientation, reflecting the orientation of S2 during crystal growth (Figure 2.39). Garnets have straight inclusion trails of quartz and ilmenite, indicating rapid syn- D2 or post D2 growth. The inclusion trails are usually at a high angle to surrounding S3, which wraps around the garnet crystals (Figure 2.37). Where several garnets occur close together, inclusion trail orientations differ between individual crystals, but remain oblique to surrounding S3 (Figure 2.37). The porphyroblast-foliation relationships of garnet with S2 and S3 suggest that M2 metamorphism occurred syn- late D2 deformation, and before D3 deformation. 63

90 2.3 Structure and Metamorphism: this study Structure and Metamorphism S3 S i biotite alteration on outer edges along fractures quartz ilmenite inclusion trail 2mm S3 gnt variably oriented inclusion trails (S i ) in garnet S2 biotite (chlorite altered) gnt gnt Figure 2.37: Photomicrograph of M2 garnets. The garnets have straight inclusion trails (S i ) of quartz and ilmenite that are at an angle to surrounding S3. The inclusion trails reflect the original orientation of S2. Secondary biotite alteration at garnet crystal edges and along fractures within the garnets has occurred. Biotite porphyroblasts that grew during M2 are present in microlithons, and are sometime altered to chlorite. Image is taken under plane polarised light. 64

91 Structure and Metamorphism 2.3 Structure and Metamorphism: this study differentiated S3 bt/musc - quartz andalusite or staurolite retrogressed to fine white mica S3 S2 btmuscquartz in microlithons magnetite 2mm Figure 2.38: Photomicrograph showing M2 biotite in microlithons of S3, and magnetite porphyroblasts. The magnetite is usually flattened subparallel to S3. Relict M2 aluminosilicate porphyroblasts thought to be andalusite or staurolite are now altered to very fine grained mats of white mica. Top image is taken under plane polarised light, bottom image is cross polarised light of same area. 65

92 2.3 Structure and Metamorphism: this study Structure and Metamorphism S3 0.5 mm S2 replacement mica regularly oriented andalusite or staurolite altered to fine white mica Figure 2.39: Photomicrograph showing M2 aluminosilicate now altered to fine grained white mica. The replacement mica has a preferred orientation, reflecting the original orientation of S2 in the porphyroblast. Image is taken under cross polarised light. 66

93 Structure and Metamorphism 2.3 Structure and Metamorphism: this study andalusite, staurolite, garnet, biotite muscovite biotite muscovite only N InI 2 km Figure 2.40: Distribution of M2 metamorphic minerals. Porphyroblastic growth is concentrated along the hinge zone of the Oaky Creek Antiform. See Appendix 1 for stratigraphy and key M3 Metamorphism M3 metamorphism is characterised by the assemblage biotite, chlorite, muscovite and quartz, and this assemblage of minerals all define the S3 foliation (Figure 2.7, 2.9, 2.37, 2.38). The M2 garnets described in Section are replaced by M3 biotite along crystal edges and internal fractures (Figure 2.37), and M2 aluminosilicates (andalusite, staurolite) are replaced by aggregates of fine white mica to form pseudomorphs (Figure 2.38, 2.39). Biotite that defines the S2 foliation is variably retrogressed to chlorite during M3 (Figure 2.5, 2.6, 2.37). The M3 mineral assemblage is consistent with mid-greenschist facies metamorphism (Figure 2.61), and is retrograde relative to M2 metamorphism. The M3 mineral assemblage that defines S3 appears to systematically vary across the Oaky Creek Antiform, and suggests a concomittant change in metamorphic grade (Figure 2.42). In the hinge zone of the Oaky Creek Antiform, the S3 foliation is intense, and is defined by abundant biotite and muscovite (Figure 2.21, 2.41). To the south of the hinge zone S3 is less intense, here the fabric is defined by muscovite and chlorite (no biotite, Figure 2.21, 2.41). The change in mineralogy that defines S3 correlates with a decrease in structural depth with respect to S3. 67

94 2.3 Structure and Metamorphism: this study Structure and Metamorphism S3 in the hinge zone of the Oaky Creek Antiform is structurally lower than S3 along the limbs (Figure 2.26), the lower structural depths correlate with higher intensity S3 foliation and an apparent increase in the metamorphic grade of the mineralogy that defines S3. coarse biotite muscovite increasing biotite biotite, muscovite, chlorite muscovite, chlorite N InI 2 km Figure 2.41: Distribution of M3 metamorphic minerals. The highest grade mineralogy is concentrated in the hinge of the Oaky Creek Antiform, and increases towards the west. See Appendix 1 for stratigraphy and key. 68

95 Structure and Metamorphism 2.3 Structure and Metamorphism: this study Increasing structural depth with respect to S3 Increasing structural depth with respect to S3 M2 garnetstaurolite±andalusite porphyroblasts S2 defined by biotitemuscovite restricted to the hinge zone of the Oaky Creek Antiform S3 defined by muscovitechlorite S3 defined by biotitemuscovite 2 km F4 A A B B F4 F4 F4 Fold vergence F3 Fold hinge 10 Possible P-T conditions of S3 mineralogy Possible P-T conditions of S3 mineralogy 10 S3 fabric S4 fabric Fault/shear zone 8 6 BIO-in 4 KY Biotite muscovite SIL Muscovite chlorite (no biotite) KY SIL BIO-in Pressure (kb) Pressure (kb) quartzite mica schist greenstone Bathampton Metamorphics 2 AND Temperature ( C) Temperature ( C) 2 AND Figure 2.42: This figure highlights the change in metamorphic grade of S3 mineralogy with respect to structural depth of S3 across the limbs of the Oaky Creek Antiform. The potential P-T fields of the different locations suggest an increase in metamorphic grade with respect to the biotite-in curve. The P-T fields outlined on the petrogenetic grids do not represent the absolute P-T fields of S3 mineralogy. Petrogenetic grids taken from Yardley (1989, page 86). Location of the cross section shown on Appendix 1. 69

96 2.3 Structure and Metamorphism: this study Structure and Metamorphism Comparison: The Eastern Creek area Structure The Eastern Creek area lies in the southwest part of the study area (Figure 1.5), and contains the highest grade metamorphic rocks in the Anakie Inlier. The main structural element is an S3 differentiated crenulation cleavage, defined by elongate biotite, muscovite and quartz (Figure 2.45). Remnants of S2 are preserved as relict equant biotite crystals in quartz rich domains of S3, and inclusion trails in garnet (Figure 2.45 (b), (e)). Where observed in outcrop (albeit rarely), S2 occurs at a low angle to S3. A transect across the Eastern Creek area revealed a consistent moderate to steep dip to the west for S3 (Figure 2.43, 2.44). Isoclinal folding of S2 around F3 fold axes was observed, however folding of any type was rare overall. The only upright folds observed were located in the far west of the transect, and comprised metre-scale open folds of S3 around steeply west dipping hinges, and northwest oriented axes (Figure 2.43). No shear bands were observed in the Eastern Creek area, and isoclinal folds were all symmetric. Rare kinks and crenulation cleavage related to D4 or younger deformation were observed, but could not be correlated between outcrops. Reorientation of garnet porphyroblasts has resulted in adjacent garnets often exhibiting oblique inclusion trails (Figure 2.45 (b), (e)). The inclusion trails are usually oblique to surrounding S3, suggesting that the inclusion are S2 or earlier (Figure 2.45 (b), (e)). Metamorphism The earliest phase of metamorphism in the Eastern Creek area is inferred to have involved growth of andalusite, based on pseudomorphs that contain white mica and which resemble metamorphic andalusite (Figure 2.45). The pseudomorphs are restricted to pelitic horizons, and are closely associated with quartz veins (Figure 2.43). The pseudomorphs are elongate parallel to S3, and have been flattened (Figure 2.45), they have diffuse margins and appear to be less rigid compared to adjacent staurolite and garnet, which is consistent with the appearance of andalusite in polydeformed and metamorphosed rocks. Inferred andalusite grew prior to growth of adjacent staurolite and garnet (Figure 2.45), and on this basis, andalusite is considered to represent M1 metamorphism in the Anakie Metamorphic Group. The inferred presence of andalusite is consistent with observations by Fergusson et al. (2001). 70

97 Structure and Metamorphism 2.3 Structure and Metamorphism: this study increasingly coarse schistosity v.coarse btmusc and andmuscbt gntstaand btmusc andmuscbt (first appearance of knotted and) basalt dyke muscbt muscbt muscbt E E WEST EAST 1 km no knotted and, sta Wynyard Metamorphics Quartz veins Monteagle Quartzite Psammite/quartzite horizon and - andalusite sta - staurolite musc - muscovite bt - biotite gnt - garnet Porphyroblasts present S3 Fault Figure 2.43: Cross section through the Eastern Creek area. Metamorphic grade increases from east to west, with increasing coarseness of schistosity and appearance of andalusite, staurolite and garnet. Porphyroblastic mineral growth is restricted to pelitic horizons, and associated with quartz veins. The location of the cross section is outlined in Figure

98 2.3 Structure and Metamorphism: this study Structure and Metamorphism Figure 2.44: Plot of poles to S3, Eastern Creek area. The second assemblage of metamorphic minerals (M2) in the Eastern Creek area is characterised by staurolite, garnet, biotite and muscovite. Idioblastic staurolite was observed in association with quartz veins, retrogressed andalusite and garnet (Figure 2.43, 2.45). Staurolite is also retrogressed mostly to fine grained white mica, and minor chlorite and biotite (Figure 2.45 (b), (d)). Some relict staurolite was observed (Figure 2.45 (b)). Decussate white mica comprises the inferred andalusite pseudomorphs, and forms blades up to 1 mm long and 0.5 mm wide. The decussate white mica exhibits resorption features in the form of embayments, mostly at grain boundaries (Figure 2.45 (b), (c)). The alignment of the pseudomorphs parallel to S3, along with the lack of alignment of decussate white mica and lack of randomly oriented white mica outside of the pseudomorphs suggests that this generation of white mica predates S3. Second generation biotite and muscovite define S2, which is preserved in pressure shadows between porphyroblasts, as well as in quartz-rich layers of a differentiated S3 foliation (Figure 2.45 (a)). Idioblastic garnet has inclusion-rich cores and inclusion-poor rims, secondary alteration to biotite has occurred along grain boundaries and internal fractures (Figure 2.45 (b), (e)). Inclusion trails in garnet are primarily straight and defined by 0.01 mm quartz grains (Figure 2.45 (b)). At porphyroblast margins, inclusion trails are not present, so the absolute relationship between S i (inclusion trails) and S e (enveloping foliation, S3) is difficult to reconcile. Geometric relationships between the straight S i and S e that wraps garnet, in conjunction with a lack of wrapping of S i into S e suggests that garnet growth occurred post-s2 (S i ) and pre-s3 (S e ). Therefore the inclusion trails in garnet are considered to be S2. 72

99 Structure and Metamorphism 2.3 Structure and Metamorphism: this study M3 metamorphism was contemporaneous with D3 deformation, and is characterised by the assemblage biotite, chlorite (after staurolite), muscovite and quartz. Biotite, muscovite and quartz all define the S3 foliation (Figure 2.45 (a)). Biotite replaces M2 garnet and staurolite (Figure 2.45 (b), (e)), and staurolite is also replaced by aggregates of fine-grained white mica and chlorite (Figure 2.45 (b), (d)). The fine-grained texture of M3 white mica aggregates that replace staurolite suggests rapid growth. Resorption features and embayments in decussate white mica that replaces andalusite appear to have formed during M3, and is texturally earlier than S3 muscovite (Figure 2.45 (a), (b), (c)). This suggests that deccusate white mica that grew during M2 was unstable under M3 conditions. Abundant growth of M3 biotite and a lack of chlorite replacing M2 biotite is considered to be evidence for a higher grade of M3 metamorphism in the Eastern Creek area compared to the Oaky Creek Antiform area. 3mm garnet S3 S3 (d) (e) (b) andalusite (replaced by muscovite) (c) staurolite (replaced by muscovite) elongate biotite, muscovite quartz relict S2 biotite (a) 73

100 2.3 Structure and Metamorphism: this study Structure and Metamorphism fine white mica and coarser biotite after staurolite bladed white mica after andalusite relict staurolite quartz inclusion trails S3 distinct margin and crystal faces on staurolite pseudomorph diffuse margins on andalusite pseudomorph (b) 200 um embayments and resorption features in white mica, altering to quartz 74(c)

101 Structure and Metamorphism 2.3 Structure and Metamorphism: this study chlorite after staurolite fine grained white mica 200 um (d) inclusion-poor rims S i 1mm quartz inclusions biotite after garnet S3 chlorite after staurolite S i (e) 75

102 2.3 Structure and Metamorphism: this study Structure and Metamorphism Figure 2.45: (a) Photomicrograph of high-grade metamorphic rock from the Eastern Creek area. Pseudomorphs of andalusite are replaced by decussate muscovite, and are parallel to S3. M2 porphyroblasts include garnet and staurolite, and M2 biotite and muscovite define S2. S3 is defined by elongate biotite, muscovite and quartz, and preserves M2 biotite in quartz-rich microlithons. Image is PPL. (b) Enlargement showing pseudomorphed andalusite and staurolite, as well as variably oriented inclusion trails in garnet. Note the difference in white mica replacement textures between andalusite and staurolite. Image is PPL. (c) Close up of decussate white mica replacing andalusite. The mica crystals have embayments and resorption features. Image is XPL. (d) Close up of white mica replacing staurolite. Note larger chlorite and biotite crystals also replacing staurolite. Image is XPL. (e) Enlargement showing inclusion-rich cores of garnet defined by regularly oriented quartz (S i ), and inclusion poor rims. Garnet is altered to biotite along margins and internal fractures. Image is PPL Comparison: The Miclere-Blair Athol area Structure Outcrop in the Miclere-Blair Athol area (Figure 1.5) is dominated by the Scurvy Creek Metaarenite and Hurleys Metamorphics. The dominant structural feature in the east of the Miclere- Blair Athol area is a shallow- to moderate-dipping S3 differentiated crenulation cleavage (Figure 1.6, 2.46), while in the far west of the area an S2 differentiated cleavage is dominant (Figure 2.47, 2.48). In the east, both S2 and S3 are defined by elongate muscovite, biotite, chlorite and quartz (Figure 2.49), although S2 has a higher abundance of chlorite that has replaced earlier biotite. S3 generally dips to west (Figure 1.6, 2.50), and in the highest structural level relative to S3 in the area (far west), both S2 and S3 are defined by muscovite and quartz only (no biotite, Figure 2.47). An increase in the apparent intensity of S3 correlates with increasing structural depth relative to S3 (Figure 2.48). No F2 folds were observed in the Miclere-Blair Athol area. F3 recumbent folds were observed near the Blair Athol mine (Figure 2.51), but are relatively uncommon when compared to the Oaky Creek Antiform area. No shear bands or asymmetric features associated with D3 deformation were observed, and the apparent intensity of D3 is not as high as in the hinge zone of the Oaky Creek Antiform. D4 structures were rare overall, but where observed they comprised spaced crenulations in outcrop. Abundant north-northwest oriented D5 crenulations and kinks were observed in road cuttings (Figure 2.52), and are axial planar to broad folds and warps (Figure 2.48). D6 structures were not encountered. 76

103 Structure and Metamorphism 2.3 Structure and Metamorphism: this study 2mm S3 differentiated S2,mostly muscovite and quartz S2 differentiated S3, muscovite domain Figure 2.46: Differentiated S3 foliation in the Miclere-Blair Athol area. Differentiated S2 is preserved as microlithons between muscovite-rich S3 domains. Image taken under cross-polarised light (XPL). weak S3 crenulation cleavage, defined by alignment of muscovite differentiated S2, defined by muscovite quartz Figure 2.47: Differentiated S2 foliation in the west of the Miclere-Blair Athol area. S2 is the dominant foliation here, overprinted by a weak S3 crenulation cleavage. Both foliations are defined by muscovite and quartz, no biotite occurs at this locality. 77

104 2.3 Structure and Metamorphism: this study Structure and Metamorphism 5 km Miclere Au N S3 dominant S2 dominant Blair Athol increasing structural depth Black Ridge Au Cainozoic cover Permian sedimentary rocks Scurvy Creek Meta-arenite Hurleys Metamorphics Yan Can Greenstone Member metasediments greenstone v v Bathampton Metamorphics boundary synform antiform fault dip and trend S3 Clermont Peak Downs Cu Figure 2.48: Miclere-Blair Athol area. There is an increase in structural depth with respect to S3 from southwest to northeast across the area. S2 is the dominant foliation at the highest structural level in the southwest, adjacent to a reverse fault that separates the Scurvy Creek v Meta-arenite from rocks of the Bathampton Metamorphics. S3 is the dominant foliation in the northeast around the Black Ridge Diggings. North-northwest oriented folds are D5 structures. 78 Cz Oaky Creek Antiform area (main map)

105 Structure and Metamorphism 2.3 Structure and Metamorphism: this study muscovite S3 domain S2 S3 relict biotite and chlorite defining S2 1mm Figure 2.49: S2 and S3 mineralogy in the Miclere-Blair Athol area, this sample is from lower structural levels exposed around the Black Ridge diggings in the west of the area. S2 contains relict biotite, muscovite and chlorite after biotite and quartz, S3 contains muscovite and quartz. Top image is PPL, bottom image is XPL. 79

106 2.3 Structure and Metamorphism: this study Structure and Metamorphism Figure 2.50: Plot of poles to S3, Miclere-Blair Athol area. S2 S3 Figure 2.51: Recumbent F3 fold in the Miclere-Blair Athol area. A differentiated S2 foliation is wrapped around flat-lying F3 fold axes. 80

107 Structure and Metamorphism 2.3 Structure and Metamorphism: this study S3 S5 Figure 2.52: D5 kinks in the Miclere-Blair Athol area. These kinks and associated cleavage are axial planar to kilometre scale open folds and warps (Figure 2.48). Metamorphism The earliest metamorphism is characterised by the assemblage biotite, muscovite and quartz, which all define S2 (Figure 2.49). Based on the fabric association of this assemblage, it is considered to represent regional M2 metamorphism in this area. A second assemblage is characterised by muscovite, chlorite (after biotite) and quartz, which all define S3 (Figure 2.46). Based on the fabric association of this assemblage, it is considered to represent regional M3 metamorphism in this area. The mineral assemblages for both M2 and M3 in the Miclere-Blair Athol area are consistent with greenschist facies metamorphism for both events, and it appears that both metamorphic events were contemporaneous with deformation. 81

108 2.4 Regional Trends Structure and Metamorphism 2.4 Regional Structural and Metamorphic Trends Regional differences and similarities in structure and metamorphism of the Anakie Metamorphic Group were examined. Comparisons were focussed around the southern part of the inlier, however, rocks in the Mt Coolon area in the far north (Figure 1.2) were briefly examined as well. Studies of the Anakie Metamorphic Group in areas other than the Clermont region were not as detailed, however they are still relevant for interpreting the structural and metamorphic history Rubyvale Region Structure Rocks of the Bathampton Metamorphics outcrop in the Rubyvale region (Figure 1.4) in a northeast trending belt 35 km long and up to 10 km wide (Figure 2.53). An S1 fabric is inferred from a strongly differentiated S2 foliation, which itself is reoriented around flat-lying F3 fold axes (Figure 2.55). A kilometre-scale, northeast trending F4 fold is defined by amphibolite layers north of Rubyvale (Figure 2.53), similar in appearance and orientation to the Oaky Creek Antiform in the Clermont region. D4 is characterised by a northeast trending S4 foliation that grades from a crenulation cleavage to a mylonite, and dips steeply to the northwest. Intense S4 is locally developed within, and adjacent to, a northeast-oriented fault contact between the Bathampton Metamorphics and the Ordovician Fork Lagoons Beds (Figure 2.53, 2.54). S4 is mylonitic in the Gem Park Granite adjacent to the contact (Figure 2.53, 2.54, 2.56), and has a mineral stretching lineation that plunges steeply to the northwest. S4 decreases in apparent intensity to the north and south away from the fault contact. The northwest dip of S4, which is also parallel to the fault contact, is consistent with the Bathampton Metamorphic occurring in the hanging wall of a northwest dipping fault. To the north of the fault contact, S4 is axial planar to the structure defined by folded amphibolite-rich layers north of Rubyvale (Figure 2.53). To the south of the contact, S4 is represented as axial planar cleavage to upright folds in the Fork Lagoons Beds. The structural relationships between the Bathampton Metamorphics and Fork Lagoons Beds are examined in detail in Chapter 4. No D5 or D6 structures were observed in the Rubyvale region. 82

109 Structure and Metamorphism 2.4 Regional Trends KETTLE CREEK FAULT Amphibolite defined F4 structure, S3 dominant 10 km N R zone of intense foliation development, S4 dominant 70 Rubyvale Cz boundary 55 Sapphire fault reverse fault R 85 strike and dip, S3 strike and dip, S Cz form surface line RUBY CREEK MYLONITE ZONE Anakie ' Zone of strong S4 foliation development Cz Cainozoic Cover Bowen Basin sequence Drummond Basin sequence Withersfield Quartz-Syenite serpentinite basalt sediments Fork Lagoons Beds Gem Park Granite (foliated) Unassigned Granodiorite amphibolite Taroborah Granodiorite Keilambete Tonalite Whitdale Granodiorite Mt Newsome Granodiorite Kilmarnock Granodiorite Retreat Batholith metasediments mylonite zone Bathampton Metamorphics Unnamed gabbro Figure 2.53: Simplified geologic map of the Rubyvale region. Modified from Withnall et al. (1995). 83

110 2.4 Regional Trends Structure and Metamorphism S3 parallel to compositional layering, S4 at angle to S3 mylonitised Gem Park Granite zone of intense S4 foliation continuation of Ruby Creek Mylonite Zone? R R - 1 km 1 km NNE SSW S3 foliation S4 foliation intense S4 foliation interpreted D4 fault (major) interpreted D4 fault (minor) interpreted D4 fault(strike-slip) - Figure 2.54: Interpreted cross section through the Rubyvale region, see Figure 2.53 for rock types. S2 S3 Figure 2.55: F3 fold in amphibolite, north of Rubyvale. 84

111 Structure and Metamorphism 2.4 Regional Trends elongate feldspar quartz and feldspar domains separated by mica rich seams S3 (a) elongate quartz ribbon fsp S3 fsp fsp 1mm (b) Figure 2.56: (a) Gem Park Granite outcrop. The rock is affected by a mylonitic S3 foliation, defined by 1 to 2 mm wide domains of quartz and feldspar separated by thin seams of mica. A strong mineral stretching lineation defined by biotite, muscovite and quartz occurs on the S3 plane, but is not visible in this image. (b) Photomicrograph of the Gem Park Granite showing dynamic recrystallisation of quartz that results in ribbon textures. Image is XPL, fsp = feldspar. 85

112 2.4 Regional Trends Structure and Metamorphism The early deformation history in the Rubyvale region is essentially the same as in the Clermont region, however later deformation (D5 and D6) is poorly represented. D4 produced characteristic northeast trending structures in both regions, and formed the two kilometre-scale folds defined by mafic units that feature prominently. The two regions are interpreted to have experienced a similar, if not the same deformation history. Metamorphism The Bathampton Metamorphics in the Rubyvale region are similar in metamorphic grade to the Clermont region. Biotite and muscovite is aligned along S2 and S3 (Withnall et al., 1995), however no porphyroblasts are observed. Biotite and muscovite define S4 in the Rubyvale region, which is not observed in the Clermont region. The mineral assemblage that defines S2 and S3 is similar to the Clermont region, and considered to reflect M2 and M3 metamorphism. No earlier mineral assemblage is preserved, in contrast to the Clermont region which preserves evidence for an earlier high-t/low-p metamorphic event (Section 2.3.3). Metamorphism during regional D4 occurs only in the Rubyvale region, the mineral assemblage of biotite and muscovite that defines S4 suggests greenschist facies conditions were reached. Local cordierite occurs in contact aureoles around granitoids of the Retreat Batholith (Withnall et al., 1995), and is younger than regional deformation and metamorphism. The pattern, mineralogy and timing of metamorphism in the Rubyvale region is similar to that of the Clermont region. The only differences are a lack of evidence for an early high-t/low- P event in the Rubyvale region, and also that D4 in the Rubyvale region was synchronous with greenschist facies metamorphism, which did not occur in the Clermont region Mt Coolon Region Rocks of the Anakie Metamorphic Group outcrop very poorly in the Mt Coolon area (Figure 1.4), and are characterised by mica schist and phyllite grading to sandstone and siltstone (Hutton et al., 1998). Cross bedding and ripple marks are visible in some beds, a feature never observed in metamorphic rocks of the Clermont region owing to intense deformation. Petrographically, most of the rocks are fine grained arenite to argillite, and contain 25-30% of a very fine-grained matrix. All of the rocks in this area have a matrix with moderately to strongly aligned mica (Hutton et al., 1998). Mafic igneous or volcanic layers are present (Hutton et al., 1998), but it is not known if they are sills or volcanic beds. 86

113 Structure and Metamorphism 2.4 Regional Trends The rocks are folded around upright, shallowly plunging, tight to isoclinal fold axes that strike north-south, and have a well developed axial planar cleavage defined by mica (Hutton et al., 1998). These folds are overprinted by sub-horizontal fold axes that become more prominent towards the Clermont region, where they completely reorient the early upright fold axes and associated cleavage (Hutton et al., 1998). The pattern of north-south trending, upright, isoclinal structures overprinted by flat-lying folds is identical to D2 and D3 overprinting relationships in the Clermont region. The rocks in this area are the lowest metamorphic grade, and least deformed of the Anakie Metamorphic Group. There are similarities in structural development between here and the Clermont region, although the dominant foliation here is related to upright folding and not flat-lying structures. If there is no major discontinuity between Mt Coolon and the Clermont region, then rocks in the Mt Coolon region may well be structurally and stratigraphically higher equivalents to those further south. This is consistent with the observed gradient in metamorphic grade increasing from northeast to southwest across the inlier (Withnall et al., 1995). 87

114 2.5 Discussion Structure and Metamorphism 2.5 Discussion This study has outlined a more complex structural and metamorphic history in the Anakie Metamorphic Group than previously interpreted by Withnall et al. (1995) and Green et al. (1998). Based on field observations in this study, a minimum of six deformation events are now interpreted. Structural overprinting observed resulted in previously interpreted D3 being separated into three distinct events (D4, D5, D6), and allows a better understanding of outcrop patterns related to late upright folding. This chapter is centered around understanding the structural and metamorphic conditions during development of a flat-lying foliation (S3), which will be the main focus of discussion. A summary of the structural and metamorphic events to affect the Anakie Metamorphic Group is presented in a synthesis sequence diagram in Figure The synthesis sequence diagram draws upon, and can be used to correlate between, observations from the different areas studied. One example is from the Oaky Creek Antiform area which preserved a more complete structural history compared to the Eastern Creek area. Intense deformation however, has resulted in poor preservation of the metamorphic history. Sequence diagrams allowed the structural history from the Oaky Creek Antiform to be correlated with that in the Eastern Creek area, which contained a more complete history of metamorphism. A summary of the inferred event history and associated structures that is a product of the synthesis sequence diagram is shown in Figure Early Deformation History of the Anakie Metamorphic Group D1 As outlined in Section , the key to understanding the nature of S1 is understanding the mechanisms associated with development of S2. In the Anakie Metamorphic Group, S2 is a well-developed differentiated cleavage. Passchier and Trouw (1998, page 70) show that such well developed fabrics require the presence of an earlier formed fabric, usually at a high angle to the second fabric. The steep attitude of S2 (Figure 2.8) is consistent with S1 having formed subhorizontally, and then subsequently transposed during D2 deformation. 88

115 Structure and Metamorphism 2.5 Discussion Synthesis Sequence Diagram Fabric Structure Metamorphism S S S XPR S D S S SZ F UI F F SZ R F U flt SZ U F U F U U R Observations Inferred History D 1 D 2 D 3 D 4 D 5 D 6 M 1 M 2 M 3 M 4 New Interpretation Correlation With Previous History D 1 D 2 D 3 Previous Interpretation S S - differentiated foliation F UI - upright isoclinal folding S XPR - axial planar foliation to recumbent folds SZ F - low angle shear zone SZ U - upright shear zone S D - crenulation cleavage defined by dissolution S - crenulation cleavage, with overprinting flt - faulting F R - recumbent folding F U - upright folding, with overprinting - metamorphic mineral growth, R refers to Rubyvale region only D 2 - deformation event and number M 4 - metamorphic event and number Figure 2.57: Synthesis sequence diagram of structural and metamorphic observations. Details of metamorphic observations are shown in Figure The previous event history of Withnall et al. (1995) and Green et al. (1998) is included for comparison. 89

116 2.5 Discussion Structure and Metamorphism Regional Event (this study) Regional Event (previous study) D1 D2 D3 D4 D5 D6? D2 qtz vein S5 S6 S4 S3 S2/S3 C S3? E-W E-W NW-SE SSW-NNE N-S NE-SW S3 Outcrop-microscale Structures Macro-mesoscale Structures Movement Orientation? M1 M2 M3 M4??? Rubyvale region only D1 M1 D2 M2 D3 inferred foliation S3 S2 Figure 2.58: Inferred structural and metamorphic event history for the Anakie Metamorphic Group, and = andalusite, sta = staurolite, gnt = garnet, bt = biotite, musc = muscovite, chl = chlorite, qtz = quartz. The previous event history of Withnall et al. (1995) and Green et al. (1998) is included for comparison. 90

117 Structure and Metamorphism 2.5 Discussion D2 Evidence from this study and from previous studies (eg., Green et al. 1998), indicates that D2 deformation produced north-south trending upright isoclinal folds, associated with an S2 differentiated cleavage (Section ). North-south trending upright D2 structures in the Clermont region probably correlate with north-south trending upright folds in the Mt Coolon region, which are interpreted to represent higher stratigraphic levels and therefore largely unaffected by younger flat-lying structures. D2 structures in the Clermont and Rubyvale regions are overprinted by recumbent F3 folds, and a flat-lying S3 differentiated crenulation cleavage (Section , 2.4.1). D3 Two stages of D3 deformation are inferred in the Clermont region (Figure 2.57). An early pure shear component produced a flat-lying crenulation cleavage, and a late simple shear or rotational component formed shear bands and asymmetric folds that is interpreted to deform the earlier formed S3. The presence of mylonitic S3 in quartzite units (Figure 2.10) also suggests deformation involving simple shear. S3 is interpreted in this study, and in previous studies (Withnall et al., 1995; Green et al., 1998), to have formed in a ductile shear zone. Fergusson et al. (2005) interpreted an extensional origin for the flat-lying foliation in the Anakie Metamorphic Group, based on non-rotational strain (i.e. coaxial shortening perpendicular to the foliation) indicated by symmetric pressure fringes on magnetite grains. Fergusson et al. (2005) proposed a model of ductile vertical thinning (i.e. extension), with imposed rotational deformation that approximated simple shearing. The results of this study support a model of vertical thinning and simple shear, however this is not unequivocal evidence for extensional deformation. The contrast in orientation of D2 and D3 structures suggests a change in structural setting, however, any interpretation of conditions during D3 must take into account that flat-lying foliations can form in both thrust settings (Ramsay, 1981) and extensional settings (Sandiford and Powell, 1986; Sandiford, 1989; Gibson, 1991; Argles et al., 1999). The most robust method for characterising the nature of a shear zone (ie. extensional or shortening related) is to link movement history (kinematics) with P-T-t data from rocks in the hangingwall and footwall. This is not possible in the Anakie Metamorphic Group, as no hangingwall or footwall can be clearly defined. 91

118 2.5 Discussion Structure and Metamorphism The patterns of metamorphism with respect to both D2 and D3 do not unequivocally support either an extensional or shortening setting during D3. M2 (prior to D3) mineralogy is best preserved in the Eastern Creek area, which is structurally higher than the Oaky Creek Antiform area. The presence of the highest grade rocks at the top of the structural pile was used by Green et al. (1998) to support a thrust setting. However, a young reverse fault that was previously unmapped is located between the areas (Figure 1.6), and is interpreted to have resulted in the current structural configuration. Even if the young fault was not present, a low-angle retrograde shear zone could result in high grade rocks being preserved at higher structural levels. Regional patterns and trends in deformation do not indicate an extensional or shortening setting for D3 deformation, although they do indicate an upper structural/stratigraphic limit for D3 deformation. The structural history and overprinting relationships in the Clermont and Rubyvale regions are the same, with an S3 crenulation cleavage forming the dominant structure in most rocks. Mt Coolon is different, the dominant structures are north-south oriented, upright isoclinal folds with associated axial planar cleavage. The folds in Mt Coolon are reoriented around flat-lying axes closer to Clermont to the south (Hutton et al., 1998). It is suggested that the Mt Coolon region is structurally and stratigraphically higher than the Clermont and Rubyvale regions, consistent with the overall increase in metamorphic grade from northeast to southwest across the inlier. Upright folding in the Mt Coolon region is interpreted to correlate with D2 deformation in the Clermont and Rubyvale regions, consistent with the north-south strike of D2 structures in the Clermont region. The top of what is essentially a D3 shear zone is interpreted to lie between the Clermont region and the Mt Coolon region, however a lack of outcrop in this area does not allow its exact location to be marked. An extensional origin for D3 deformation is favoured, based on the striking contrast in structural orientation and style between D2 and D3, and also because of vertical thinning (extension) that formed S3 crenulation cleavage in the early stages of D3. However, the structural geology of the Anakie Metamorphic Group alone is not able to unequivocally resolve the nature of D3 deformation. Changes in metamorphic conditions that are examined in Section , and larger scale relationships presented in Section are used to provide additional constraints for the nature and setting of the Anakie Metamorphic Group during D3 deformation. 92

119 Structure and Metamorphism 2.5 Discussion Late-Stage Deformation History of the Anakie Metamorphic Group A major feature of late stage deformation in the Clermont region is the Oaky Creek Antiform. Transects across the antiform indicate a pop-up geometry for the hinge zone (Figure 2.26), and outcrop scale F4 folds can have the same geometry (Figure 2.28). The bend in the axial trace of the Oaky Creek Antiform is interpreted to have also formed during D4 deformation. No evidence exists for younger structures having caused the bend. The Grasstree Fault that bounds the north side of the Oaky Creek Antiform is interpreted to be a major accommodating structure during northwest-southeast oriented shortening. North of this fault, the area of rock that contains the Yan Can Greenstone is thought to have acted as a rigid indentor, against which heterogenously layered (and more easily deformed) rocks to the south folded and wrapped to form the Oaky Creek Antiform (Figure 2.59). D4 shear zones are interpreted to have formed on the limbs of the Oaky Creek Antiform when folding could no longer accommodate shortening. Movement along the shear zones resulted in the hinge zone of the antiform being translated upwards relative to the limbs, creating the pop-up geometry (Figure 2.59). The formation of the Oaky Creek Antiform as a pop-up is similar to the geometry of gold-bearing anticlines in the Bendigo Goldfield of central Victoria (BMNL, 2001; Wood, 2001). In Bendigo, flexural slip during folding forms laminated veins at contacts between shales and sandstones, analogous to D4 shear zones formed between greenstone and pelite in the Oaky Creek Antiform. When flexural slip can no longer accommodate shortening, fold lock-up occurs, and the slip zones propagate upwards as reverse faults into higher layers creating offset (Figure 6.9). In Bendigo, the geometry of anticlinal hinge zones being dissected by faults and transported upwards relative to limbs is a pop-up. One major difference between the Oaky Creek Antiform and the Bendigo Goldfield is that the Oaky Creek Antiform represents a single fold structure adjacent to a major fault. The Victorian examples have many repeating fold structures, with several major reverse faults and a number of smaller subsidary faults in a fold and thrust system. Both D5 and D6 deformation episodes are are regarded as minor in terms of geological impact. They are manifest as crenulations and kinks, except in discrete structural corridors and fault zones (see Appendix 1). D5 deformation resulted in the dominant north-south to northnorthwest strike of S3 in the Clermont region, and formed similarly oriented faults. The fault separating the Bathampton Metamorphics from the Scurvy Creek Meta-arenite in the Miclere- Blair Athol area, and the fault between the Oaky Creek Antiform area and the Eastern Creek area (Figure 1.5, 1.6) are both interpreted to have formed during D5 deformation. The effects 93

120 2.5 Discussion Structure and Metamorphism GRASSTREE FAULT Rigid indentor during D4 deformation YAN CAN GREENSTONE MEMBER GRASSTREE FAULT OAKY CREEK ANTIFORM InI Heterogenous, layered rock south of the Grasstree Fault deformed against and wrapped around rigid indentor (a) D4 shortening pop-up in hinge zone shear zones offsetting layers (b) 94

121 Structure and Metamorphism 2.5 Discussion Figure 2.59: Proposed scenario for formation of the Oaky Creek Antiform during D4 deformation. (a) Northwest/southeast oriented shortening resulted in multiply layered rocks south of the Grasstree Fault wrapping around a rigid indentor formed by the Yan Can Greenstone Member north of the Grasstree Fault. The Oaky Creek Antiform developed as a pop-up structure during folding and thrusting along the Grasstree Fault. (b) Schematic block diagram of the Oaky Creek Antiform. hinge zone pop-up relative to limbs offset along reverse faults Figure 2.60: Example of a pop-up structure in the Sheepshead Anticline in the Bendigo goldfield. The zones outlined in red are gold-ore. Image from BMNL (2001). 95

122 2.5 Discussion Structure and Metamorphism of D6 deformation are restricted to the Oaky Creek Antiform area, mainly within an east-west trending structural corridor (Appendix 1) Metamorphic History of the Anakie Metamorphic Group An attempt is made to construct a P-T evolution relative to deformation in the Clermont region, using petrographic observations mainly from the Oaky Creek Antiform area and the Eastern Creek area. Figure 2.61 shows the interpreted P-T path for rocks in the Clermont region, based on stable mineral assemblages that developed during each of the metamorphic events GR KY QZ AN (*) Pressure (kb)? D4 D5 D6? ? 2 Si p.f.u. isopleths? Pressure conditions for M2 may be higher BIO-in PYP ALS QZ H 2 O ILM KY QZ GT-in KY AND ALM RT 3.3 CTD ALS 3.2 ST QZ H 2 O M3 3.1??? M1 ST QZ AB KF QZ H 2 O M2 CD ALS H 2 O Fe-ST QZ melt SIL AND ALM ALS H 2 O MS QZ KF ALS H 2 O ALM SIL QZ Fe-CD Temperature ( C) MS KF QZ H 2 O melt MS QZ KF ALS melt KY SIL Figure 2.61: Interpreted P-T path of the Anakie Metamorphic Group, see text for details. Petrogenetic grid is for pelites with P = P H2, except curve (*). Abbreviations used are: AB = albite; ALM = almandine; ALS O = Al-silicate; AN = anorthite; AND = andalusite; BIO = biotite; CD = cordierite; CTD = chloritoid; GR = grossular; ILM = ilmenite; KF = k-feldspar; KY = kyanite; MS = muscovite; PYP = pyrophyllite; QZ = quartz; RT = rutile; SIL = sillimanite; ST = staurolite. Stippled bands are approximate conditions of the biotite and garnet isograds. Figure modified from Yardley (1989, page 86), Si p.f.u. isopleths for phengite from Simpson et al. (2000). 96

123 Structure and Metamorphism 2.5 Discussion M1is preserved in the Eastern Creek area only, and comprises andalusite pseudomorphs now replaced by decussate white mica (Figure 2.45). The only constraint for M1 is that it occurred in the andalusite stability field, which ranges from approximately C and up to 4 kilobars (Figure 2.61). The mineral assemblage of garnetbiotitemuscovitestaurolite during M2 metamorphism constrains the P-T conditions to within the staurolite stability field, at temperatures of C (Figure 2.61). Higher temperatures than this were not reached, evidenced by a lack of melting despite the presence of abundant quartz veins. The pressure estimate of M2 metamorphism interpreted in Figure 2.61 is not well constrained. Minimum pressure constraints during M2 are provided by andalusite becoming unstable in the presence of staurolite, however there are no constraints on maximum pressure. The curve in Figure 2.61 indicates a path from the andalusite field (M1) into the staurolite field (M2), based on the mineral paragenesis observed in the Eastern Creek area (Section 2.3.3). Fergusson et al. (2001) outlined andalusite-in and staurolite-in isograds in the Eastern Creek area. However, the results of this study indicate that the two minerals are related to two separate metamorphic events, which suggests that the isograds are of little significance for interpreting patterns of metamorphism Constraints on M3 Metamorphism from White Mica Composition Phengites are intermediate members of the muscovite-celadonite, KAl 2 [AlSi 3 O 10 ](OH) 2 K(Mg, Fe 2 )(Fe 3, Al)[Si 4 O 10 ](OH) 2 solid solution series. Previous studies of natural phengites observed rising silica per formula unit (Si p.f.u.) content with increasing pressure and decreasing temperature (Ernst, 1963; Sassi, 1972; Guidotti, 1978, 1984). This has been confirmed and quantified experimentally for some reactions in the KMASH, KFASH and KFMASH systems (Velde, 1965; Massone and Schreyer, 1987, 1989; Massone and Szpurka, 1997; Simpson et al., 2000). Pure muscovite has an Si p.f.u. of 3, and phengites have recorded Si p.f.u. contents up to 3.8 at 20 kbar under experimental conditions (Massone and Schreyer, 1987). Studies of phengite composition as a tool for pressure estimates are usually conducted in low temperature, high pressure systems. As a result, silica isopleths in white mica have not been constrained for P-T conditions similar to M1 or M2 in the Anakie Metamorphic Group. Fortunately, isopleths have been constrained in greenschist and lower amphibolite facies 97

124 2.5 Discussion Structure and Metamorphism conditions, thus can be used to estimate P-T conditions during M3. Simpson et al. (2000) calculated silica isopleths for the mineral assemblage phengite chlorite biotite quartz H 2 O in the KMFASH system, and these are overlain on Figure Using the M3 mineral assemblage of biotite, muscovite (phengite), chlorite and quartz in conjunction with the Si p.f.u. content of the white mica, a tighter constraint for the P-T field of M3 (and by inference D3) can be gained than from mineral assemblage alone. Sample Selection Two samples of the Anakie Metamorphic Group from the Clermont region were selected for analysis, both samples were from the Bathampton Metamorphics. Sample DW04-03 is from drill core from near the Peak Downs Copper Mine south of Clermont, and sample DW is from the western end of the Oaky Creek Antiform (Figure 2.62). Both of these samples comprise mica schist with a pervasive S3 differentiated crenulation cleavage, and have S2 preserved in microlithons (Figure 2.63). White mica defines fabric elements for both S2 and S3 in these samples. Two slides from each sample (A B) that had clear S2/S3 overprinting relationships (Figure 2.63) were targeted, thus allowing confident analysis of S3 white mica. Results S3 white mica from both samples was analysed under a Cameca SX-100 electron microprobe at the Research School of Earth Sciences, Australian National University, using a 20nA beam accelerated to 15keV and focussed to 10µm diameter. Data were normalised to a charge balance of 22, and representative analyses from each sample are presented in Table 2.1. No compositional zoning was observed in the analysed micas. Figure 2.64 shows the Si p.f.u. content of S3 white micas in the Clermont region. The Si p.f.u. contents range from approximately 3.35 to just above 3.1, with the majority of analyses in the range Using the majority cluster of Si p.f.u. values ( ) in conjunction with the M3 mineral assemblage, a P-T field with temperatures of C and pressures of 3-6 kbar is indicated for M3 (Figure 2.61) M2 to M3 Metamorphism Replacement textures and overgrowths of the M2 mineral assemblage suggests M2 mineralogy was unstable under M3 conditions; staurolite was replaced by fine-grained aggregates of white mica, as well as chlorite and biotite; garnet was replaced by biotite along rims and cracks; and 98

125 Structure and Metamorphism 2.5 Discussion 0 10 km Miclere Au boundary synform antiform fault dip and trend main foliation Blair Athol v Cz N DW04-03 A B Clermont Cz Cz v v DW02110 A B v v v Peak Downs Cu Cainozoic cover Permian sedimentary rocks v v Drummond Basin Sequence Silver Hills Volcanics Theresa Creek Volcanics v Retreat Batholith and related rocks Wynyard Metamorphics Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Greenstone Member metasediments greenstone Bathampton Metamorphics ' sample locaility, Si p.f.u. content in S3 white mica Figure 2.62: Sample locations for Si p.f.u. content of S3 white mica. 99

126 2.5 Discussion Structure and Metamorphism decussate white mica that replaced M2 andalusite became unstable and formed embayments and resorption features (Figure 2.45). The difference in stability fields between M2 and M3 indicates a temperature decrease of up to 270 C, however changes in pressure between the two events are not well constrained. The location of M2 within the staurolite field is uncertain, however the maximum and minimum pressures of the staurolite field relative to the M3 P-T field are such that most paths between M2 and M3 have a negative slope (Figure 2.61). A negative slope along a P-T path can be associated with extension (Pitra and Guiraud, 1996; Sandiford and Powell, 1986; Sandiford, 1989; Wickham and Oxburgh, 1985), however is not absolute evidence for extensional processes. The data are more suited to a pressure decrease or isobaric cooling between M2 and M3, but cannot be used alone for interpreting processes. DW A DW B DW04-03 A DW04-03 B SiO TiO Al 2 O Fe MnO MgO CaO Na 2 O K 2 O Cr 2 O ClO Total Si Ti Al Fe Mn Mg Ca Na K Sum Cations Table 2.1: Representative microprobe analyses for S3 white mica in the Clermont region. 100

127 Structure and Metamorphism 2.5 Discussion DW A bt S2 musc S3 analysis qtz 0.25 mm (a) DW04-03 A S3 S2 analysis 0.25 mm (b) Figure 2.63: Examples of microprobe analyses locations of S3 white mica, S2 and S3 minerals can be clearly seen. 101

128 2.5 Discussion Structure and Metamorphism 3.35 S3 White Mica Si p.f.u. 3.3 DW04-03 A DW04-03 B DW A DW B 3.25 Si p.f.u Na/NaK Figure 2.64: Si p.f.u. content of S3 white mica from the Clermont region. 102

129 Structure and Metamorphism 2.6 Summary and Conclusions 2.6 Summary and Conclusions The results of structural and metamorphic analysis in this study have added significant knowledge to the Anakie Metamorphic Group. Evidence from structure and metamorphism alone cannot be used to confidently interpret the nature of the flat-lying foliation that formed during D3, although an extensional setting is favoured. If additional pressure constraints became available for M2, a more substantial conclusion about the nature and setting of D3 could be reached. Specific conclusions from structural and metamorphic analysis of the Anakie Metamorphic Group are: An extensional origin for the flat-lying foliation (S3) development in the Anakie Metamorphic Group is favoured, however the structural and metamorphic evidence is not conclusive. The conditions under which flat-lying S3 foliation formed are constrained to C and 3-6 kbar in the Oaky Creek Antiform area. A minimum of six deformation events are now inferred for the Anakie Metamorphic Group, up from a previously interpreted total of three events. Previously interpreted D1 correlates with D2 of this study, previously interpreted D2 correlates with D3 of this study, and previously interpreted D3 is now separated into three distinct events, D4, D5 and D6. A previously unrecognised deformation event (D1) is inferred to have occurred, based on the characteristics of the earliest recognisable foliation. A previously unrecognised metamorphic event (M1) is inferred to have occurred, based on relict mineral assemblages. 103

130 2.6 Summary and Conclusions Structure and Metamorphism 104

131 Chapter 3 Age of Early Deformation and Metamorphism of The Anakie Metamorphic Group: Constraints from 40 Ar/ 39 Ar Analysis This chapter has benefited greatly from technical assistance provided by Julien Célérier, Xiaodong Zhang, Marnie Forster and Shane Paxton. Valuable discussions with Geoff Fraser, Sandra McClaren, Gordon Lister and Jim Dunlap significantly improved the scientific content. All aspects of mineral separation, preparation and analysis were carried by David Wood, except for 2 samples analysed by Jim Dunlap. Jim is thanked in particular for his aid in all facets of 40 Ar/ 39 Ar thermochronology. Core samples from the Clermont region and the Nebine Ridge were generously provided by Ian Withnall at the Queensland Geologic Survey. Discussion about the use of sequence diagrams with respect to 40 Ar/ 39 Ar studies was undertaken with Gordon Lister. 105

132 3.1 Introduction 40 Ar/ 39 Ar ANALYSIS 3.1 Introduction 40 Ar/ 39 Ar analysis in this thesis was undertaken for two main reasons: To place age constraints on the timing of flat-lying foliation development in the Anakie Metamorphic Group, and at the same time determine if micas preserved in distinct microstructures could be dated directly. To compare 40 Ar/ 39 Ar ages from the Anakie Metamorphic Group, with basement samples from potential equivalent rocks of the Nebine Ridge. A complex history of deformation, metamorphism and overprinting has affected the Anakie Metamorphic Group in the Clermont region, evident from the sequence of events outlined in Chapter 2. The age of early deformation that formed ductile structures is of most interest, in particular low angle shearing during D3 deformation that formed the dominant flat-lying foliation. The age of the flat-lying foliation in the Anakie Inlier may be significant for tectonic setting and development of Palaeozoic north-eastern Australia, especially if it formed in an extensional environment. The Nebine Ridge (Figure 3.1) is a geophysical gravity feature that trends south-southwest from the Anakie Inlier. The feature is called the Anakie Gravity High, and has been interpreted as a sub-surface continuation of the Anakie Inlier (Lonsdale, 1965; Darby, 1969). Rocks of the Nebine Ridge are known from a study of basement core samples by Murray (1994). They are poly-deformed, and lithologically and petrologically similar to rocks of the Anakie Metamorphic Group Regional Geology Clermont Region Figure 3.2 is a map of the Clermont region, showing sample localities and location of a regional cross section. The regional geology of the Clermont region has already been described, however several points pertinent to this chapter are outlined here. Metamorphic grade of the Anakie Metamorphic Group increases from northeast to southwest across the Clermont region, from 106

133 40 Ar/ 39 Ar ANALYSIS 3.1 Introduction lower greenschist facies-muscovite grade conditions around Miclere-Blair Athol, to amphibolite facies/staurolite grade in the Eastern Creek area (Figure 3.3). The gradient in metamorphic grade is not linear. Differential preservation of early high grade metamorphism is the result of variable overprinting by low angle shearing (Section 2.21), with further complexity due to the later generations of upright folding. Figure 3.3 highlights metamorphic grade changes in the Clermont region Nebine Ridge Metamorphic rocks of the Nebine Ridge range from lower-greenschist to amphibolite facies, and comprise chlorite-muscovite phyllite, chlorite-biotite-muscovite phyllite, and garnet-muscovitebiotite schist (Murray, 1994). The highest grade rocks have an S2 crenulation cleavage defined by biotite (Murray, 1994). The rocks are poly-deformed, with a dominant foliation that is steeply to moderately dipping, and reoriented by small scale open folds in coarser grained schists. Lower grade phyllites preserve a more complex history with up to three episodes of deformation. Mica growth is contemporaneous with the two early fabrics in the lower grade rocks (Murray, 1994). Basement sample from drill hole AOP ALBA 1 (Figure 3.1) contained up to 30% chlorite as a retrograde mineral phase after biotite. Bladed muscovite crystals define an axial planar fabric to centimetre-scale F3 folds that reorient a biotite-quartz differentiated S2 foliation. The sructural overprinting and foliation mineralogy of S2 and S3 in rocks of the Nebine Ridge is similar to that outlined for the Anakie Metamorphic Group in the previous chapter. 107

134 Galilee Basin 3.1 Introduction 40 Ar/ 39 Ar ANALYSIS Charters Towers Province Coral Sea Drummond Basin Anakie Inlier Clermont Region 22 Bowen Basin N Great Artesian Basin AOP ALBA 1 Nebine Ridge 149 ophiolite Younger cover, includes Great Artesian Basin Permian-Triassic sediments New England Orogen Pre Devonian basement (Thomson Orogen) 500 km Sub-surface Drummond Basin Fault Gravity trend Figure 3.1: Location of the Clermont region and drill hole AOP Alba 1. The Nebine Ridge is a gravity high that continues south from the Anakie Inlier. See Figure 3.2 for an enlargement of the Clermont region. 108

135 40 Ar/ 39 Ar ANALYSIS 3.1 Introduction boundary 0 10 km Miclere Au synform antiform fault Black Ridge Au DW02-117(S3) A Blair Athol v Hurleys 5 N Cz Cz Clermont Cz A v DW02-110(S2) DW02-110(S3) v DW05-PEG(S2) v OAKY CREEK ANTIFORM DW02-121(S2) DW04-02(S3) DW04-02(S2) v v v v v Peak Downs Cu Cainozoic cover Permian sedimentary rocks Drummond Basin Sequence Silver Hills Volcanics Theresa Creek Volcanics Retreat Batholith and related rocks Wynyard Metamorphics Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Gnst Mbr metasediments greenstone Bathampton Metamorphics ' K-Ar sample, previous study Ar-Ar sample, this study Figure 3.2: Simplified geologic map of the Clermont region showing sample locations and the fabric element analysed. 109

136 Overall metamorphic grade increase SW S2=biotitemuscovitequartz S2 overlaps with growth of garnet S3=biotitemuscovitechloritequartz Section here is parallel to strike S2=biotitemuscovitequartz S3=muscovitechloritequartz S2=muscovitequartz S3=muscovitequartz S2=muscovitequartz S3=muscovitequartz Oaky Creek Antiform, western arm Rhyolite dykes A A -?? NE V V V S2=biotitemuscovitequartz S2 overlaps with growth of staurolite and garnet S3=biotitemuscovitechloritequartz both foliations defined by coarse schistosity S2=biotitemuscovitequartz S2 overlaps with growth of andalusite/staurolite and garnet S3=biotitemuscovitechloritequartz S2=biotitemuscovitequartz S2 overlaps with growth of garnet S3=biotitemuscovitechloritequartz S3 foliation Fault 5 km Silver Hills Volcanics Wynyard Metamorphics Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Greenstone Member quartzite mica schist Bathampton Metamorphics greenstone v Figure 3.3: Northeast-southwest cross section of structure and metamorphic grade changes with respect to D2 and D3 deformation across the Clermont region. Cross section location is shown on Figure 3.2.

137 40 Ar/ 39 Ar ANALYSIS 3.1 Introduction Previous Geochronology The age of early deformation and metamorphism of the Anakie Metamorphic Group in the Clermont region associated with mica growth (D2 and D3, this study) is interpreted by Withnall et al. (1996) to occur at ca 500 Ma based on whole-rock K/Ar ages. Withnall et al. (1996) analysed whole-rock drill core samples from the Peak Downs copper mine (Figure 3.2) in the Bathampton Metamorphics, and from Hurleys 5 (Figure 3.2), which was a drill hole in Hurleys Metamorphics above an electromagnetic anomaly (Bruvel and Dash, 1987) located several kilometres southwest of the Blair Athol coal mine. Withnall et al. (1996) reported a spread of K/Ar ages from ca 550 Ma to 469 Ma from 6 samples. The older ages up to ca 550 Ma were speculated to represent either a minimum age for earliest ductile deformation and metamorphism that was locally preserved (based on the lack of a second fabric in the sample), or possibly due to preservation of detrital Proterozoic muscovite. The younger ages down to ca 469 Ma were thought to be the result of thermal or fluid effects from either a proximal intrusive body or faulting in the hinge of a younger antiform. Withnall et al. (1996) also reported a K/Ar muscovite age of 466 Ma from the Anakie Metamorphic Group west of Clermont, recalculated from Webb and McDougall (1968). The location of the sample that yielded this age is constrained only to somewhere in the hinge of the Oaky Creek Antiform (Figure 3.2). Detrital zircon and monazite studies of the Bathampton and Wynyard Metamorphics in the Clermont region indicate a Neoproterozoic to Middle Cambrian age of deposition of the Anakie Metamorphic Group (Fergusson et al., 2001). Major components are in the age range Ma for the Bathampton Metamorphics, with the youngest detrital zircons in these rocks having ages of ca 580 Ma. A sample of the Wynyard Metamorphics had detrital monazite with age components of ca Ma and ca 540 Ma, and detrital zircon from another sample had a youngest age component between 510 Ma and 700 Ma (Fergusson et al., 2001). There are no intrusives in the Clermont region known to have formed prior to, or syn- D2 and D3 deformation. The Bathampton Metamorphics are intruded by Mid-Devonian granitoids of the Retreat Batholith (Withnall et al., 1995), providing a minimum age constraint for deformation and metamorphism. A maximum age of deformation and metamorphism is constrained by the youngest detrital zircon age of ca 510 Ma (Fergusson et al., 2001). Basement rock of the Nebine Ridge from AOP ALBA 1 yielded a biotite K/Ar age of 416 ± 2 Ma. This age was interpreted to be the age of regional metamorphism (Murray, 1986). 111

138 3.1 Introduction 40 Ar/ 39 Ar ANALYSIS Why 40 Ar/ 39 Ar Analysis? The 40 Ar/ 39 Ar technique can provide information about the temperature-time history of the mineral being analysed. The samples previously dated by K/Ar from the Anakie Metamorphic Group and the Nebine Ridge have been re-analysed to determine if additional information can be gained. Metamorphics in the Clermont region are particularly amenable to 40 Ar/ 39 Ar analysis, they are mica-rich, and samples were able to be structurally constrained through detailed mapping. Mica separates from samples in the Clermont region could also be constrained microstructurally, and this is addressed in detail below. One of the aims of this study was to target mica that had grown during each of the D2 and D3 events in the Anakie Metamorphic Group. Figure 3.4 shows a common relationship between D2 and D3 microstructural domains in the Anakie Metamorphic Group: an S3 crenulation cleavage that contains microlithons of an S2 crenulation cleavage. Both cleavages contain white mica, and can be microsampled. Micas that grew during D2 can potentially preserve an isotopic signature of the earlier event, however this is strongly dependant on the temperature-time (T-t) history between D2 and D3. Amphibolite facies M2 that is constrained to have occurred syn- to post D2, and prior to D3 (Section ), is overprinted by greenschist facies M3, contemporaneous with D3 (Section ). The T-t path between these two metamorphic events, and by inference the deformation events, is unknown. If the rocks have cooled in a linear path after M2, then micas formed during D2 are likely to record similar 40 Ar/ 39 Ar ages to micas formed during D3. Additionally, if the rocks cooled slowly after D3, then micas in both microstructures could again yield the same 40 Ar/ 39 Ar age. Figure 3.5 shows a paragenetic sequence of metamorphic minerals that grew during D2 and D3. The observation that biotite is stable during D2 and D3 suggests that micas were at temperatures above which significant amounts of 40 Ar 1 can be retained until after D3, however the time lapse and thermal history between D2 and D3 is unconstrained. The implications of mineral paragenesis and other concepts of argon behaviour in metamorphic rocks are explained in more detail in Sections , and Radiogenic argon produced by natural decay of 40 K 112

139 40 Ar/ 39 Ar ANALYSIS 3.1 Introduction S2 S3 10 mm (a) S2 white mica S3 white mica 3 mm (b) Figure 3.4: (a) S3 crenulation cleavage with microlithons of S2 crenulation cleavage. (b) Photomicrograph of S3 crenulation cleavage with microlithons of S2 crenulation cleavage. Image is taken under cross polarising light. 113

140 3.2 Methodology 40 Ar/ 39 Ar ANALYSIS D1 D2 D3 D3 biotite? muscovite? chlorite quartz? garnet staurolite Figure 3.5: Paragenetic mineral sequence for the Anakie Metamorphic Group. 3.2 Methodology Sample Selection In the Clermont region, a suite of samples was taken along an approximate northeast-southwest transect, corresponding with the overall gradient in regional metamorphic grade outlined in Figures 3.2 and 3.3. Samples selected for analysis were all schists that contained a pervasive S3 crenulation cleavage, and visible S2 microlithons. Most samples contained D2 quartz veinlets folded around F3 fold axes. Thin section analysis of all samples was undertaken to assist in the mineral separation process, and a summary of sample information is shown in Table 3.1. A total of 16 samples were collected in the Clermont region for 40 Ar/ 39 Ar analysis, however due to problems encountered during the mineral separation process, a total of only 7 separates was obtained from the Anakie Metamorphic Group, and one from the Nebine Ridge. These problems were mainly due to the small amount of material processed for each sample, and could not be avoided given the nature of the microstructure separation process described in Section Mica separates that could not reach >95% purity were not analysed. 114

141 40 Ar/ 39 Ar ANALYSIS 3.2 Methodology Sample Target Fabric Rock Type Unit Name UTM Coordinates DW02-110(S2) DW02-110(S3) S2 S3 Muscovite schist Bathampton Metamorphics DW04-02(S3) DW04-02(S2) S3 S2 Muscovite schist Bathampton Metamorphics DW02-121(S2) S2 Biotite schist DW02-117(S3) S3 Muscovite schist Monteagle Quartzite Hurleys Metamorphics DW04-01(WR) n/a Biotite-chloritemuscovite schist Nebine Ridge DW05-PEG(S2) S2 Muscoviteandalusite vein Wynyard Metamorphics Table 3.1: Summary information of samples selected for 40 Ar/ 39 Ar analysis. Sample DW01-117(S3) is from the Black Ridge locality (Figure 3.2), and is the lowest metamorphic grade sampled. Samples DW04-02(S3) and DW04-02(S2) were from drill core stored at the Queensland Geologic Survey, and taken from the same down-hole depth sampled by Withnall et al. (1996) for K/Ar analysis. The drill core samples are from the Bathampton Metamorphics at the Peak Downs copper mine site (Figure 3.2). Also obtained from drill core was sample DW04-01(WR), which is basement material of the Nebine Ridge. Samples DW02-110(S2) and DW02-110(S3) are from the hinge zone of the Oaky Creek Antiform (Figure 3.2), taken from highly deformed rocks in which S3 had destroyed almost all previous features. Sample DW02-121(S2) is a sample of biotite schist located 7 km away from an intrusion of the Retreat Batholith. Sample DW05-PEG(S2) is from the highest metamorphic grade rocks in the Clermont region (Eastern Creek area), taken from a D2 coarse-grained, muscovite-andalusite vein. This style of vein was observed in only one outcrop, located in meta-pelitic rocks that also contained abundant D2 quartz veins. Structurally, the veins are interpreted to be D2 based on local fabric relationships, however their mineralogy is not representative of regional metamorphic conditions during D2. The spread of samples taken in the Clermont region can be used to place minimum age constraints on D3 (and M3), as well as highlight any changes in 40 Ar/ 39 Ar age with respect to metamorphic grade. The sample from the Nebine Ridge can be used to compare 40 Ar/ 39 Ar ages 115

142 3.2 Methodology 40 Ar/ 39 Ar ANALYSIS with the Anakie Metamorphic Group as a potential correlative Mineral Separation and 40 Ar/ 39 Ar Analysis Samples were selected to obtain mica that had grown during one or both of the D2 and D3 events, now preserved in distinct distinct microstructures. White mica was the main target mineral to be separated, although a biotite separate was also obtained from one sample (DW02-121(S2)). Biotite was not targeted for two reasons, one is that it was not present as a fabric forming mineral in all samples, two is that biotite had a close association with chlorite as intergrowths or secondary alteration. Sample DW04-01(WR) from the Nebine Ridge was a drill core sample, thus the full deformation history of the basement rocks it was sourced from may not be represented by one specimen. Thin section analysis showed that the only muscovite present formed an axial planar fabric to locally defined D3 microfolds. Any younger deformation history was not obvious from thin section analysis. A whole-rock muscovite separate from this rock was considered to be representative of mica that grew during the youngest observable deformation event. Thin section analysis of sample DW02-121(S2) from the Clermont region showed that biotite had grown during D2, with no subsequent growth of biotite during younger deformation. A whole-rock biotite separate was considered to be a representative sample of D2 in the Anakie Metamorphic Group. The above two samples were the only two whole-rock separates, all other separates were obtained by microstructural separation that targeted distinct fabric elements. Samples were cut into slabs 7-8 mm thick, parallel to the mineral stretching lineation on S3, and perpendicular to the S3 foliation (Figure 3.6). Using a binocular microscope, heavy tweezers, and a scalpel blade, the slabs were split along the S3 plane, and then broken down further into slivers. S2 and S3 micas were able to be sampled directly from their microstructural site. This technique ensured that any mica collected was from the central part of the desired microstructural domain, thus avoiding composite sampling. Material collected underwent standard heavy liquid and magnetic separation techniques to obtain pure mica separates. Owing to the small grain size of mica in some samples (<75µm), centrifuging in the heavy liquid stage was also necessary. Sample DW02-110(S2) was separated following a similar method, however S2 mica oc- 116

143 40 Ar/ 39 Ar ANALYSIS 3.2 Methodology Mineral Stretching Lineation on S3 plane muscovite-biotite-quartz S2 microlithons muscovite-quartz S3 crenulation cleavage Slabs ~7mm thick cut parallel to mineral stretching lineation. Micas in the slabs were able to be targeted using preferential parting along cleavage planes to break up the sample. Figure 3.6: Illustration of how samples were cut into slabs so that S2 and S3 micas could be separately targeted and processed. curred in folded quartz veinlets and not in microlithons (Figure 3.7). Quartz veins 1cm to 0.5cm wide were separated from the cut rock slabs using a scalpel and tweezers, and the vein fragments crushed and processed by standard procedure. Sample DW02-110(S3) is a muscovite separate from the same sample, obtained from the surrounding wallrock which contains a pervasive S3 crenulation cleavage. DW02-110(S3) contained no vein material. Samples DW04-02(S3) and DW04-02(S2) were from the same hand specimen. DW04-02(S3) was obtained by separating several millimetre wide zones of micaceous material that defined the S3 fabric and shear bands. DW04-02(S2) was obtained from microlithons between the S3 fabric, these domains were up to 7 mm wide and preserved a differentiated S2 cleavage. Mica separates were irradiated together with flux monitor GA 1550 Biotite (Spell and Mc- Dougall, 2003, age = 98.5 ± 0.8 Ma) in the Lucas Heights reactor for a period of 12 days, except for samples DW02-110(S3) and DW05-PEG(S2) that were irradiated for 24 days in a separate period. Analyses were undertaken following the procedure of McDougall and Harrison (1999). Step heating was undertaken on approximately 3mg aliquots of mica, using a double-vacuum resistance furnace coupled to an ultra-high vacuum line. Samples were progressively heated from 550 C to 1350 C with a dwell time of 12 minutes for each step, the number of steps ranged between 16 and 22 dependant on outgassing behaviour. Gas released from each step was cleaned of 117

144 3.2 Methodology 40 Ar/ 39 Ar ANALYSIS S2 qtz-bt-musc S3 musc-qtz 3 mm Figure 3.7: Photomicrograph of a quartz vein containing S2 muscovite and biotite, folded around F3 fold axes. Surrounding the quartz vein is pervasive S3 crenulation cleavage that is axial planar to the F3 fold. The quartzose layers were separated from the surrounding wallrock, and mica from the quartz veins was analysed separately to mica from the surrounding foliation. 118

145 40 Ar/ 39 Ar ANALYSIS 3.3 Results active gas species using a Ti-Zr-Al SAES T M getter, prior to isotopic analysis with a VG Isotech MM 1200 gas source mass spectrometer. Sensitivity of the mass spectrometer with an electron multiplier attached was approximately 5 x mol/mv. Corrections were made for mass spectrometer discrimination, line blanks, background and for the decay of 37 Ar and 36 Ar during and after the irradiation. Corrections for neutron-induced isotopes have been made to the radiogenic ( 40 Ar ) argon to K-generated 39 Ar ratio ( 40 Ar / 39 Ar K ), and for 36 Ar generated from calcium and chlorine. Amounts of 39 Ar are derived from the measured sensitivity of the mass spectrometer, totals quoted are the % 39 Ar weighted means of the analyses. J value was calculated by interpolation of neutron flux measured from GA 1550 Biotite during irradiation (λ = x ). Correction for atmospheric argon was based on the atmospheric 40 Ar/ 36 Ar ratio of (Nier, 1950). Decay constants used were those recommended by the International Union of Geological Sciences Subcommission on Geochronology (Steiger and Jäger, 1977). 40 Ar/ 39 Ar age spectra were produced using the K/Ar Date data reduction software, and plateau ages were calculated using the program ISOPLOT (Ludwig, 2003). The criteria for defining a plateau age were: 4 or more continuous steps containing a minimum of 50% of the 39 Ar, probability of fit of the weighted mean age of the steps must be greater than 95%. This approach to define a plateau age is a variant of that defined by Dallmeyer and Lecorche (1990). After plateau ages were calculated, an error propagation of 0.3% from J was factored in for final age uncertainty. Uncertainties are quoted at 1σ level on all plateau ages. 3.3 Results A summary of the samples and ages obtained from the eight 40 Ar/ 39 Ar analyses is shown in Table 3.2, and a locality map of the samples and ages obtained in the Clermont region is shown in Figure 3.8. The location of 40 Ar/ 39 Ar ages with respect to structure and metamorphism are shown in Figure 3.9. Step heating age spectra with corresponding K/Ca ratios are shown in Figure 3.10, and data tables for analyses are shown in Figure Isochron plots were constructed for all samples, however no additional information was yielded. The isochron plot ages were in very close agreement with 40 Ar/ 39 Ar ages of all samples, with most data for each sample plotting close to the 39 Ar/ 40 Ar intercept. 119

146 3.3 Results 40 Ar/ 39 Ar ANALYSIS There is an overall increase in 40 Ar/ 39 Ar ages from north-east to south-west, parallel to cross section A-A (Figures 3.8, 3.9) across the Clermont region. An exception to this gradient is sample DW02-121(S2), which yielded a significantly younger age(s). The increase in 40 Ar/ 39 Ar ages across the Clermont region correlates with a decrease in metamorphic grade, and will be discussed in Section Six of the eight samples yielded plateau ages, however samples DW02-110(S3) and DW02-121(S2) did not satisfy the plateau criteria. The apparent age spectra for sample DW02-110(S3) appears plateau like, and is very similar to sample DW02-110(S2) from the same hand specimen, however a statistical plateau age was not yielded. Five of the six samples with plateau ages show remarkably flat age spectra (Figure 3.10), none of the samples showed evidence of excess argon. Sample Mineral Analysed Target Fabric Separate Type Amount Analysed (mg) Grainsize ( m) Total Fusion Age ± 1 Ma 40 Ar/ 39 Ar 'Plateau Age' ± 1 Ma DW02-110(S2) Muscovite S2 Muscovitebearing quartz vein 2.3 < ± ± 1.6 DW02-110(S3) Muscovite S3 Wall Rock only ± 1 n/a - see text DW04-02(S3) Muscovite S3 DW04-02(S2) Muscovite S2 Hand separated S3 fabric Hand separated S2 microlithons 2.8 < ± ± < ± ± 1.5 DW02-121(S2) Biotite S2 Whole rock ± 6 DW02-117(S3) Muscovite S3 Hand separated S3 fabric n/a - see text ± ± 1.6 DW04-01(WR) Muscovite n/a Whole rock ± ± 1.3 DW05-PEG(S2) Muscovite S2 Hand separated S2 vein material ± ± 1.4 Table 3.2: Summary of 40 Ar/ 39 Ar age data. 120

147 40 Ar/ 39 Ar ANALYSIS 3.3 Results Sample DW02-110(S2) yielded a plateau age of ± 1.6 Ma, however DW02-110(S3), a muscovite separate from the same hand specimen, didn t meet plateau criteria. An age of ± 3.7 Ma was calculated for DW02-110(S3) from thirteen steps that comprised the central part of the age spectra (Figure 3.10). Together these steps have the shallowest gradient for this sample and the probability of fit of the weighted mean age of the steps is greater than 95%. These two samples were derived from different microstructures in the same rock, yet still give ages that are within error of each other. Samples DW04-02(S3) and DW04-02(S2) yielded plateau ages of 468 ± 1.5 Ma and ± 1.5 Ma respectively. These two samples are from different microstructural domains in the same hand specimen, and are almost identical in age. Sample DW02-121(S2) was the only biotite separate analysed, and yielded a disturbed age spectrum (Figure 3.10 (e)). Apparent ages of individual heat steps increase abruptly during the first 40% of gas release, then remain similar between 40% and 80% of 39 Ar released, before decreasing over the final 20%. Mean ages were calculated from the two flatter sections in the age spectra, however are not plateau ages according to the criteria. An age of ± 6.2 Ma is yielded from the flatter section between the 40% to 80% gas release interval, and an age of 386 ± 1.4 Ma is yielded from the flatter section between approximately 85% and 95% of the gas release. The K/Ca plot mirrors the shape of the apparent age spectra, suggesting that compositionally different biotite was being sourced during step heating. Texturally the biotite contained in this sample formed during D2 deformation, however it yielded the youngest ages of all analyses in the Clermont region. 121

148 3.3 Results 40 Ar/ 39 Ar ANALYSIS boundary 0 10 km Miclere Au synform antiform fault S3-483 Ma A v Hurleys Ma Blair Athol Cz N Cz Clermont Cz A v v S2-458 Ma S3-458 Ma S2-459 Ma 412 Ma 386 Ma S2-468 Ma S3-469 Ma 500 Ma v v v v v Peak Downs Cu Cainozoic cover Permian sedimentary rocks Drummond Basin Sequence Silver Hills Volcanics Theresa Creek Volcanics v Retreat Batholith and related rocks Wynyard Metamorphics Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Gnst Mbr metasediments greenstone Bathampton Metamorphics ' 500 Ma K-Ar age, previous study 470 Ma Ar-Ar age, this study Figure 3.8: 40 Ar/ 39 Ar ages in the Clermont region. The increase in metamorphic grade from northeast to southwest corresponds with a decrease in 40 Ar/ 39 Ar ages. The cross section marked is shown in Figure

149 Overall metamorphic grade increase SW 459 Ma S2=biotitemuscovitequartz S2 overlaps with growth of garnet S3=biotitemuscovitechloritequartz 458 Ma Section here is parallel to strike S2=biotitemuscovitequartz S3=muscovitechloritequartz S2=muscovitequartz S3=muscovitequartz Oaky Creek Antiform, western arm Rhyolite dykes A A -?? 468 Ma S2=muscovitequartz S3=muscovitequartz NE 483 Ma V V V S2=biotitemuscovitequartz S2 overlaps with growth of staurolite and garnet S3=biotitemuscovitechloritequartz both foliations defined by coarse schistosity S2=biotitemuscovitequartz S2 overlaps with growth of andalusite/staurolite and garnet S3=biotitemuscovitechloritequartz Overall increase in 40 Ar/ 39 Ar age S2=biotitemuscovitequartz S2 overlaps with growth of garnet S3=biotitemuscovitechloritequartz S3 foliation Fault Silver Hills Volcanics Wynyard Metamorphics 5 km Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Greenstone Member quartzite mica schist Bathampton Metamorphics greenstone v Figure 3.9: Cross section through the Clermont region with 40 Ar/ 39 Ar ages included.

150 3.3 Results 40 Ar/ 39 Ar ANALYSIS Sample DW02-117(S3) yielded a plateau age of 483 ± 1.6 Ma, which is the oldest apparent age of all samples. This sample was taken from the easternmost extent of the Anakie Metamorphic Group in the Clermont region, and preserved the lowest metamorphic grade of all samples for both D2 and D3 deformation. This age is from near the highest structural level in the Clermont region. DW04-01(WR), from drill core into the Nebine Ridge, yielded a plateau age of ± 1.3 Ma that included 91% of the 39 Ar K gas. The K/Ca ratios for this sample are the lowest of all analyses, ranging from around 7 to 30. This is an order of magnitude lower on average than samples collected from the Anakie Metamorphic Group. Close inspection of the separate from this sample (after analysis had been performed) revealed a high percentage of chlorite (approximately 30%) that was almost colourless in plane polarised light. It is likely that a proportion of the analysed mineral separate was chlorite, decreasing overall potassium content. The 40 Ar/ 39 Ar age of this sample is younger than most samples from the Clermont region, and is similar to the previously calculated K/Ar age of 416 ± 2. Sample DW05-PEG(S2) yielded a plateau age of ± 1.4 Ma. The age of this sample is similar to DW02-110(S2) and DW02-110(S3) (458.2 ± 1.6 Ma and ± 3.7 Ma respectively), situated 15km to the north-east. 124

151 40 Ar/ 39 Ar ANALYSIS 3.3 Results DW02-110(S2) DW02-110(S3) Plateau age = ± 1.6 Ma (1 ) MSWD = 0.89 Probability of fit = Includes 58% of the 39 Ar Mean = ± 3.7 Ma 95% conf. data point error MSWD = 13 Probability of fit = K/Ca Fraction Ar39 released 0 0 (a) DW04-02(S2) (b) DW04-02(S3) Plateau age = ± 1.5 Ma (1 ) MSWD = 0.31 Probability of fit = 0.96 Includes 61% of the 39 Ar Plateau age = 468 ± 1.5 Ma (1 ) MSWD = 0.32 Probability of fit = 0.98 Includes 76.7% of the 39 Ar K/Ca K/Ca Fraction Ar39 released (c) Fraction Ar39 released (d)

152 3.3 Results 40 Ar/ 39 Ar ANALYSIS DW02-121(S2) Mean = ± 6.2 Ma 95% conf. data point error MSWD = 3.5 Probability of fit = DW02-117(S3) Mean = ± 1.4 Ma 95% conf. data point error MSWD = 0.91 Probability of fit = 0.44 Plateau age = 483 ± 1.6 Ma (1 ) MSWD = 0.54 Probability of fit = 0.81 Includes 58.4% of the 39 Ar K/Ca K/Ca Fraction Ar39 released (e) Fraction Ar39 released (f) DW04-01(WR) DW05-PEG(S2) Plateau age = 412.9± 1.3 Ma (1 ) MSWD = 1.7 Probability of fit = Includes 91.2% of the 39 Ar Plateau age = ± 1.4 Ma (1 ) MSWD = 1.8 Probability of fit = Includes 58.3% of the 39 Ar K/Ca Fraction Ar39 released 0 0 (g) 126 (h)

153 40 Ar/ 39 Ar ANALYSIS 3.3 Results Figure 3.10: (Previous two pages) Summary of 40 Ar/ 39 Ar age spectra with corresponding K/Ca ratios. The bounding bars indicate steps included for preferred or plateau like age calculations. Note that K/Ca plots are in log scale. The asymptotes and limits of the 40 Ar/ 39 Ar data were investigated following the method of Forster and Lister (2004). The method of asymptotes and limits may provide a means of evaluating any effects of argon gas mixing resulting from a multi-population mineral separate, or by partial recrystallisation and resetting of a mineral. The asymptotes and limits method is able to recognise frequently measured ages (FMA), including circumstances where there are several age populations within a sample. For apparent age spectra from the Anakie Metamorphic Group that were reasonably flat, yet did not satisfy the plateau age criteria (eg., DW02-110(S3)), the method of asymptotes and limits can provide a more systematic age determination than simply taking a mean of the central part of the apparent age spectra. Application of the method of asymptotes and limits revealed no significant deviation from either the estimated plateau ages, or mean ages that were calculated in the previous section (Figure 3.10, 3.11). Samples DW02-110(S3) and DW05-PEG(S2) show some evidence of gas mixing (Figure 3.11 (b), (h)), and have gaussian plots with distinct spikes adjacent to the maxima. However, the gaussian maxima ages for these two samples are within error of the plateau and mean age already calculated, thus no re-evalaution of these (or any other samples) is considered necessary. With the exception of sample DW02-121(S2), all other apparent age spectra have closely spaced limits that result in a single dominant gaussian fit. Some higher and lower limits are identified in all samples, however they represent gas released at the very start or very end of the apparent age spectra. 127

154 3.3 Results 40 Ar/ 39 Ar ANALYSIS DW02-110(S2) asymptotes and limits applied Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (a) DW02-110(S3) asymptotes and limits applied Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (b) Gaussian plot of asymptotes and limits 550 Gaussian plot of

155 40 Ar/ 39 Ar ANALYSIS 3.3 Results DW04-02(S2) asymptotes and limits Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (c) DW04-02(S3) asymptotes and limits applied Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (d) 129

156 3.3 Results 40 Ar/ 39 Ar ANALYSIS DW02-121(S2) asymptotes and limits applied Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (e) DW02-117(S3) asymptotes and limits applied Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (f) 130

157 40 Ar/ 39 Ar ANALYSIS 3.3 Results DW04-01(WR) asymptotes and limits applied Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (g) DW05-PEG(S2) asymptotes and limits applied Gaussian plot: asymptotes and limits Age Ma % 39 Ar % 39 Ar (h) 131

158 3.4 Discussion 40 Ar/ 39 Ar ANALYSIS Figure 3.11: Previous four pages - plots of 40 Ar/ 39 Ar apparent age spectra with method of asymptotes and limits applied. Dashed horizontal lines define the asymptotes and limits. 3.4 Discussion Ar/ 39 Ar Analysis of D2 and D3 Microstructures, Anakie Metamorphic Group. This study has used microstructural separation of micas, and the interpretation of 40 Ar/ 39 Ar ages from these to answer a specific geological question. It is not the aim here to change, or add to, current knowledge of argon behaviour in mica and any implications for 40 Ar/ 39 Ar studies Background The potential for micas from different microstructures in the same rock to yield 40 Ar/ 39 Ar ages of crystal growth depends on a number of parameters, the most important of which is the temperature-time (T-t) path between formation of the microstructures, and cooling rate of the rock after the youngest metamorphic event. Two other important parameters are diffusivity of argon in mica, and recrystallisation. Essentially, the problem lies in determining if an 40 Ar/ 39 Ar apparent age is recording the age of crystallisation of a mica, or a younger age related to postcrystallisation 40 Ar loss. External factors that control whether a crystallisation age, or a younger age is being recorded include; partial resetting by a short-lived thermal pulse (Lister and Baldwin, 1996); formation or deformation in the argon partial retention zone (Baldwin and Lister, 1998); recrystallisation (Dunlap, 1997); and rate of metamorphic cooling (Lister and Baldwin, 1996; Dunlap, 2000). Recrystallisation of micas in greenschist facies conditions has been interpreted to result in multi domain crystals, with 40 Ar gas being stored in reservoirs of different aged microstructures. (Wijbrans and McDougall, 1986; Baldwin and Lister, 1998). Ductile shear and recrystallisation of micas at crustal temperatures that were not sufficient to completely reset the argon isotope system gave rise to the term argon partial retention zone (Baldwin and Lister, 1998), and typically results in heterogenous 40 Ar/ 39 Ar age distributions. Lister and Baldwin (1996) and 132

159 40 Ar/ 39 Ar ANALYSIS 3.4 Discussion Baldwin and Lister (1998) highlighted the effects of short thermal pulses, as well as extended residence time at ambient temperatures that resulted in partial retention of 40 Ar in micas. The effects of cooling rate have been shown to affect the apparent 40 Ar/ 39 Ar age of biotite, where a long residence time at near isothermal conditions at temperatures as low as 180 C resulted in partial (but significant) resetting of the argon system (Dunlap, 2000). The effects of temperature and time on apparent age were modelled by Lister and Baldwin (1996), who determined theoretical maximum ages that could be recorded by a mineral, given specific diffusion, time and temperature paarameters. Muscovite crystals can record isotopic evidence for polymetamorphism (Hames and Cheney, 1997), with older ages able to be preserved even after minimum temperatures of 425 C during younger metamorphic events. Hames and Cheney (1997) studied millimetre-sized muscovite porphyroclasts that had cores overgrown by two generations of chemically and isotopically different rims. The cores and rims were able to be differentiated within the resolution of the 40 Ar/ 39 Ar method, but application is restricted to micas that are >1 mm in diameter The Anakie Metamorphic Group There is no evidence to suggest exhumation of the Anakie Metamorphic Group between D2 and D3 deformation. Structural and stratigraphic relationships indicate that the Anakie Metamorphic Group was exhumed post-d3. Micas that define S2 have not been recrystallised, therefore recrystallisation is not considered to be a factor for apparent ages yielded by those samples. Micas that define S3 do not have any compositional zoning (Section 2.5.3), thus are not considered to be affected by metamorphism after D3. Possible scenarios for 40 Ar/ 39 Ar apparent ages in the Anakie Metamorphic Group are; cooling only after D2; cooling after D2, with a return to heating during D3; or, slow cooling after D3. Samples DW and DW04-02 had micas separated from adjacent S2 and S3 microstructures. The paragenetic sequence of metamorphic minerals in Figure 3.5 reveals growth of biotite and muscovite continuous through D2 and D3 deformation. This indicates it was possible for recrystallisation of any earlier formed micas up to the end of D3 deformation, and predicts that the ages of mica from any microstructure in any one rock are likely to be the same. 133

160 3.4 Discussion 40 Ar/ 39 Ar ANALYSIS DW02-110(S2) and DW02-110(S3) These two samples were from the same hand specimen. DW02-110(S2) targeted S2 white mica contained within quartz veinlets that had been folded during D3, and DW02-110(S3) comprised micas that defined S3 in adjacent wallrock. In theory, these samples have the potential to preserve ages of separate deformation episodes, however this does not appear to have occurred. The age spectra presented in Figure 3.10 (a) and (b) for this rock are flat for both samples, with a plateau age of ± 1.6 Ma for DW02-110(S2), and a mean age of ± 3.7 Ma over the central heat steps in DW02-110(S3). These two ages are the same within error. DW04-02(S3) and DW04-02(S2) These two white mica samples are also from the same hand specimen. DW04-02(S3) was separated from shear bands and micaceous domains that defined a pervasive S3 crenulation cleavage, and DW04-02(S2) was from S2 microlithon domains between S3. Plateau ages of 468 ± 1.5 Ma and ± 1.5 Ma were yielded from DW04-02(S3) and DW04-02(S2) respectively. The age spectra for these samples are virtually identical, and cannot be visually differentiated. This specimen has experienced an identical structural history to samples DW02-110(S2) and DW02-110(S3), however the 40 Ar/ 39 Ar ages yielded are approximately 10 million years older. Implications Samples DW02-110(S2), DW02-110(S3) and samples DW04-02(S3), DW04-02(S2) have 40 Ar/ 39 Ar apparent ages from S2 and S3 micas that are essentially identical at each location. This suggests a cooling age common to both microstructures is being preserved at the two localities. Furthermore, the age difference of approximately 10 million years between these two localities, and the range of 40 Ar/ 39 Ar apparent ages yielded from S3 micas across the Clermont region suggests that 40 Ar/ 39 Ar cooling ages after D3 deformation are being recorded. Flat apparent age spectra are typically associated with either a history of rapid cooling, or crystallisation below temperatures at which argon is readily lost from a mineral (McDougall and Harrison, 1999). The common ages in S2 and S3 micas from adjacent microstructures are considered to reflect an age of cooling, not crystallisation. The shape of the apparent age spectra suggests that cooling was rapid. The range of ages for S3, and the fact that the highest grade rocks yielded the youngest age, is attributed to the higher grade rocks being kept above temperatures sufficient for significant loss of 40 Ar longer than the lower grade rocks. A perturbed geotherm during exhumation may have accentuated the age difference between the low and high 134

161 40 Ar/ 39 Ar ANALYSIS 3.4 Discussion grade rocks Microstructural Domain Analysis Summary The results of this study indicate that 40 Ar/ 39 Ar ages yielded from S2 and S3 muscovite in the Anakie Metamorphic Group reflect the timing of cooling after D3 deformation, not absolute ages of the D2 and D3 events. The data can be used to constrain minimum ages of D3 deformation, however the exact age of S3 fabric formation cannot be determined. The T-t path after D2 is interpreted to be cooling only, with no subsequent heating during D3 or younger events. There is no evidence of rapid cooling, or rapid heating, between D2 and D Ar/ 39 Ar age constraints on D3 deformation, Anakie Metamorphic Group, Clermont region. 40 Ar/ 39 Ar ages of S3 muscovite across the Clermont region range from ca Ma, and there is a correlation between increasing metamorphic grade and decreasing age from northeast to southwest across the region. The oldest age of ca 483 Ma is from Hurleys Metamorphics at the Black Ridge gold diggings, north of Clermont (DW02-117(S3)). This is near to the highest stratigraphic level defined by Withnall et al. (1995). Structurally, it is the highest rock sampled and also records the lowest metamorphic grades of both D2 and D3 mineralogy. The 40 Ar/ 39 Ar age of this sample is interpreted to be the oldest minimum age limit of low-angle shearing to affect the Anakie Metamorphic Group. Sample DW02-121(S2), that has a disturbed apparent age spectra, was taken from the Monteagle Quartzite near the Middle Devonian Sunny Park Granodiorite (Withnall et al., 1995). The disturbed spectra yielded by this sample (Figure 3.10 (e)), and corresponding changes in K/Ca ratios, may reflect the result of either partial resetting during intrusion of the Sunny Park Granodiorite, or perhaps reflect anaysis of several generations of biotite. The disturbed shape of the apparent age spectra for sample DW02-121(S2), along with the uncertainties of the source of the disturbance, are interpreted to be of litle use for providing additional age constraints for D3 deformation. The range of ages across the Clermont region may be the result of differential exhumation after D3 deformation, reorientation and juxtaposition of different rock units during younger 135

162 3.4 Discussion 40 Ar/ 39 Ar ANALYSIS deformation, or both. Abrupt changes in metamorphic grade across some faults (eg, between the Bathampton Metamorphics and the Scurvy Creek Meta-arenite, Figure 3.8, 3.9) indicates that post D3 deformation has probably affected the distribution of 40 Ar/ 39 Ar ages. A maximum age limit for D2 and D3 is provided by the youngest detrital zircon grains from the Wynyard Metamorphics at ca 510 Ma (Fergusson et al., 2001). This brackets D1, D2 and D3 deformation to between ca 510 Ma and ca 483 Ma. Post D3 metamorphic cooling persisted until at least ca 458 Ma. Withnall et al. (1996) interpreted K/Ar ages of ca 500 Ma from the Anakie Metamorphic Group to reflect metamorphism associated with the Delamerian Orogen that affected the Kanmantoo and Adelaide Fold Belts, the Willyama and Wonominta Blocks, parts of western Tasmania and the Transantarctic Mountains. This event is thought to affect much of the eastern edge of Gondwana, and marks a time of tectonic reorganisation manifest as orogenesis, uplift and denudation along eastern Australia and Antarctica (Boger and Miller, 2004). The age and style of D2 is similar to shortening deformation and metamorphism experienced by other regions along the Gondwana margin during the Delamerian Orogeny Ar/ 39 Ar correlations between the Anakie Metamorphic Group and the Nebine Ridge. Sample DW04-01(WR) from the Nebine Ridge yielded a muscovite 40 Ar/ 39 Ar plateau age of ± 1.3 Ma. This is in close agreement with a previously obtained biotite K/Ar age of 416 ± 2 Ma from the same sample (Murray, 1986). Both of these ages are considerably younger than the youngest cooling age of ca 458 Ma obtained from the Anakie Metamorphic Group in the Clermont region. The age from Murray (1986), and from this study are within error of each other, but are from structurally different fabrics. This suggests that the ages yielded are likely to be minimum cooling ages of regional metamorphism. These ages do not rule out a correlation of the Anakie Inlier with the Nebine Ridge, as the deformation and metamorphic history is similar in the two regions. Seismic reflection profiles suggest that the Nebine Ridge may be an uplifted section of the Thomson Fold Belt (Murray, 1994), although Harrington (1974) proposed it was a Palaeozoic volcanic arc split off from Precambrian craton to form a marginal sea. The second model is not favoured by Murray (1994) who found no volcanic rocks in basement cores from along the Nebine Ridge. Murray (1994) found no age or petrologic correlations between rocks of the Nebine Ridge, and adjacent low 136

163 40 Ar/ 39 Ar ANALYSIS 3.5 Conclusions grade Devonian metasediments of the Timbury Hills Formation to the east. Basement rocks west of the Nebine Ridge comprise low grade sediments intruded by Silurian granites, and are considered to be Palaeozoic in age, although they could be lower grade equivalents of the Nebine Ridge (Murray, 1994). If basement rocks of the Nebine Ridge were uplifted and eroded during the Silurian as suggested by Murray (1994), then it is difficult to correlate the younger 40 Ar/ 39 Ar cooling ages with Cambrian-Ordovician cooling ages from the Anakie Metamorphic Group. Detrital zircon provenance studies may provide a more direct basis for comparison, however this has not been undertaken on drill core from the Nebine Ridge. 3.5 Conclusions Conclusions from 40 Ar/ 39 Ar analysis of the Anakie Metamorphic Group in the Clermont region, and basement of the Nebine Ridge are summarised below: Micas from adjacent S2 and S3 microstructures preserve a common cooling age. The micas in both microstructures cooled from above a temperature sufficient for significant loss of 40 Ar together. The ages yielded provide minimum age constraints on low-angle shearing that occurred during D3 deformation. 40 Ar/ 39 Ar apparent ages range from ca Ma, and there is a correlation between decreasing age and increasing metamorphic grade from northeast to southwest across the Clermont region. The spread in ages and metamorphic grade is attributed to the effects of differential exhumation and cooling, further complicated by younger folding and faulting. The age of D2 and D3 deformation is bracketed to between ca 510 Ma and ca 483 Ma from detrital zircons and minimum cooling ages. D2 deformation most likely records shortening deformation related to the Delamerian Orogeny. 40 Ar/ 39 Ar ages from the Anakie Inlier do not directly correlate with those from the Nebine Ridge. Structurally and metamorphically, rocks from both of these areas are very similar, however based on the differences in cooling ages, any further correlation is tenuous. 137

164 3.5 Conclusions 40 Ar/ 39 Ar ANALYSIS DW02-110(S2) Muscovite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1 K/Ca (C) (mol) (mol) (mol) (mol) 39 Ar (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-01 Total 2.665E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (a) DW02-110(S3) Muscovite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1 K/Ca (C) (mol) (mol) (mol) (mol) 39 Ar (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E01 Total 2.268E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (b) 138

165 40 Ar/ 39 Ar ANALYSIS 3.5 Conclusions DW04-02(S3) Muscovite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1 K/Ca (C) (mol) (mol) (mol) (mol) Ar39 (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-02 Total 8.986E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (c) DW04-02(S2) Muscovite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1 K/Ca (C) (mol) (mol) (mol) (mol) Ar39 (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-01 Total 1.788E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (d) 139

166 3.5 Conclusions 40 Ar/ 39 Ar ANALYSIS DW02-121(S2) Biotite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1 K/Ca (C) (mol) (mol) (mol) (mol) Ar39 (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E00 Total 3.579E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (e) DW02-117(S3) Muscovite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1 K/Ca (C) (mol) (mol) (mol) (mol) Ar39 (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E03 Total 1.486E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (f) 140

167 40 Ar/ 39 Ar ANALYSIS 3.5 Conclusions DW04-01(WR) Muscovite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1σ K/Ca (C) (mol) (mol) (mol) (mol) Ar39 (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-01 Total 2.234E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (g) DW05-PEG(S2) Muscovite Temp 36 Ar 37 Ar 39 Ar 40 Ar % 40 Ar* 40 Ar*/ 39 Ar(K) Cumulative Calculated ± 1 K/Ca (C) (mol) (mol) (mol) (mol) 39 Ar (%) age (Ma) (Ma) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E04 Total 3.885E E E E Constants used in age calculationslambda K40 = 5.543E-10 J = (h) Figure 3.12: 40 Ar/ 39 Ar step heating data. Notes regarding age tables: Data corrected for mass spectrometer discrimination, line blanks, and for the decay of 37 Ar and 39 Ar during and after irradiation. Corrections for neutron-induced isotopes have been made to the radiogenic ( 40 Ar ) argon to K-generated 39 Ar ratio ( 40 Ar / 39 Ar K ), and for 36 Ar generate from calcium and chlorine. Amounts of 39 Ar are derived from the measured sensitivity of the mass spectrometer. Totals are the % 39 Ar weighted means of the analyses. Flux monitor: GA 1550 Biotite (98.5 ± 0.5 Ma, J calculated by interpolation from fluence monitors equally spaced along the irradiated can). Lambda (λ) = x

168 3.5 Conclusions 40 Ar/ 39 Ar ANALYSIS 142

169 Chapter 4 Late-Stage Deformation of The Anakie Metamorphic Group: Constraints from U-Pb Geochronology SHRIMP analysis in this study was undertaken by David Wood. Trevor Ireland and Peter Holden are thanked for technical supervision. Discussions concerning data processing, reduction and interpretation with Amos Aikman resulted in significant improvements in my understanding of SHRIMP U-Pb geochronology. The method used for quantitative evaluation of zircon data was developed by Amos. Ryan Ickert is also thanked for assistance in data processing and evaluation. John Mya is thanked for supervision of mineral separation. 4.1 Introduction This chapter aims to better understand the timing of late-stage regional deformation in the Anakie Metamorphic Group, focussing around the age of regional D4 deformation. The age of D4 is important as it resulted in formation of the Oaky Creek Antiform, synchronous with the earliest known gold deposits in the Anakie Inlier. The age of early ductile deformation (D2 and 143

170 4.2 Regional Geology, Rubyvale Region U-Pb Geochronology D3) is constrained to before ca 458 Ma in the Clermont region from 40 Ar/ 39 Ar ages (Chapter 3). Subsequent deformation reoriented the ductile structures(see Chapter 2 for details), however the timing of the younger event(s) is not constrained. Rocks of the Rubyvale region (Figure 4.1) preserve a northeast trending S4 foliation that formed during D4 deformation. S4 deforms intrusives of the Gem Park Granite and surrounding Bathampton Metamorphics. These rocks that contain the S4 foliation are truncated by the Mt Newsome Granodiorite. The ages of the Gem Park Granite and the Mt Newsome Granodiorite can therefore provide an age bracket for S4 foliation development. Sensitive High Resolution Ion Microprobe (SHRIMP) U-Pb dating of accessory minerals from the Gem Park Granite and the Mt Newsome Granodiorite from the Retreat Batholith was undertaken to obtain these constraints. 4.2 Regional Geology, Rubyvale Region Bathampton Metamorphics The Bathampton Metamorphics outcrop in the Rubyvale region in a belt 5-10 km wide and approximately 30 km long (Figure 4.1). Within the belt is a northeast trending elliptical structure defined by amphibolite, approximately 13 kilometres long by 8 kilometres wide. The ellipse is similar in style and dimension to the Oaky Creek Antiform in the Clermont region, which is defined by greenstone layers in the Bathampton Metamorphics. The Gem Park Granite intrudes the Bathampton Metamorphics southwest of Rubyvale as a series of strongly foliated elongate bodies (Figure 4.1). Both the Bathampton Metamorphics and Gem Park Granite are intruded by rocks of the Retreat Batholith, including the Kilmarnock and Mt Newsome Granodiorites and several smaller unnamed plutons. The deformation history of the Bathampton Metamorphics in the Rubyvale is identical to that in the Clermont region (Section 2.4.1), although the accompanying metamorphism is generally of a higher grade. A flat-lying S3 crenulation cleavage in the Rubyvale region is overprinted by a steeply dipping, northeast trending S4 foliation. S3 is the dominant foliation around the elliptical structure north of Rubyvale, and S4 is axial planar to the long axis of the ellipse. S4 becomes dominant closer to a fault zone separating the Bathampton Metamorphics from the Fork Lagoons Beds (Figure 4.2). Within the fault zone, S4 is mylonitic. 144

171 U-Pb Geochronology 4.2 Regional Geology, Rubyvale Region DW03-41 KETTLE CREEK FAULT Amphibolite defined F4 structure, S3 dominant 10 km N R zone of intense foliation development, S4 dominant DW Rubyvale Cz boundary 55 Sapphire fault reverse fault R 85 strike and dip, S3 strike and dip, S Cz form surface line RUBY CREEK MYLONITE ZONE Anakie ' Zone of strong S4 foliation development Cz Cainozoic Cover serpentinite Bowen Basin sequence Drummond Basin sequence basalt sediments Fork Lagoons Beds Withersfield Quartz-Syenite Gem Park Granite (foliated) Unassigned Granodiorite amphibolite Taroborah Granodiorite Keilambete Tonalite Whitdale Granodiorite Mt Newsome Granodiorite Kilmarnock Granodiorite Retreat Batholith metasediments mylonite zone Bathampton Metamorphics Sample site for U-Pb geochronology Unnamed gabbro Figure 4.1: Simplified geologic map of the Rubyvale region showing the two sample sites for geochronology. See Figure 4.2 for cross section. Modified from Withnall et al. (1995). 145

172 4.2 Regional Geology, Rubyvale Region U-Pb Geochronology S3 parallel to compositional layering, S4 at angle to S3 mylonitised Gem Park Granite zone of intense S4 foliation continuation of Ruby Creek Mylonite Zone? R R - 1 km 1 km NNE SSW S3 foliation S4 foliation intense S4 foliation interpreted D4 fault (major) interpreted D4 fault (minor) interpreted D4 fault(strike-slip) - Figure 4.2: Cross section through the Rubyvale region. See Figure 4.1 for location Fork Lagoons Beds The Ordovician Fork Lagoons Beds crop out in a belt up to 10 km wide and 35 km long, southeast of Rubyvale (Figure 4.1), and consist predominantly of quartz sandstone with minor siltstone and mudstone. The western margin of the Fork Lagoons Beds is a northeast trending fault contact with the Bathampton Metamorphics. The fault is up to 1 km wide, and is marked by a mylonite zone, an example of which is the Ruby Creek Mylonite Zone southwest of Rubyvale (Withnall et al., 1995). A single, differentiated foliation is developed axial planar to tight upright folds parallel to the northeast oriented fault contact with the Bathampton Metamorphics. The foliation is not as strongly developed in the Fork Lagoons Beds as the S4 foliation in the Bathampton Metamorphics adjacent to the fault. The age of deformation and foliation development in the Fork Lagoons Beds is constrained by their Ordovician deposition age based on fossil evidence (Anderson and Palmieri, 1977; Palmieri, 1978) and cross cutting relationships with granitoids of the Retreat Batholith, discussed below. 146

173 U-Pb Geochronology 4.2 Regional Geology, Rubyvale Region Gem Park Granite The Gem Park Granite comprises elongate bodies southwest of Rubyvale that are aligned parallel to S4 in surrounding rocks (Figure 4.1). The intrusive bodies are felsic muscovite, and muscovite-biotite granite. The mineralogy and geochemistry are consistent with the Gem Park Granite being an S-type granite (Withnall et al., 1995). Petrographically, the Gem Park Granite comprises recrystallised quartz (30-35%), with anhedral, elongate grains up to 4 mm comprised of subgrains (<1 mm). Recrystallised quartz forms ribbon textures (Figure 4.3) similar to those in quartz-mylonites described by Lister and Snoke (1984). Alkali feldspar (>35%) forms anhedral, perthitic and poikilitic grains, which are undulose and fractured (Withnall et al., 1995). Generally anhedral, albite-rich plagioclase (20-25%) is zoned, and deformed or fractured grains are common. Elongate biotite and muscovite (<5%) are up to 1 mm, partly- to fully aligned along foliation planes, and commonly deformed (Withnall et al., 1995). Mylonitic foliation in the Gem Park Granite (Figure 4.4) is parallel to S4 in the surrounding Bathampton Metamorphics, and has a strong mineral stretching lineation that plunges moderately to the north. The Gem Park Granite is intruded by parts of the Retreat Batholith, including the Mt Newsome Granodiorite, Whitdale Granodiorite, Keilambete Tonalite and unassigned granodiorite bodies (Withnall et al., 1995) Mt Newsome Granodiorite The Mt Newsome Granodiorite is part of the Retreat Batholith, and crops out over a large area in the southwest of the Anakie Inlier (Figure 4.1). It intrudes the Bathampton Metamorphics, the Gem Park Granite, and the Whitdale Granodiorite. Petrographically, the Mt Newsome Granodiorite comprises anhedral, undulose quartz (20-30%) up to 5 mm (Figure 4.5), with alkali feldspar (10-15%) forming mostly subhedral grains up to 3 mm (Withnall et al., 1995). Plagioclase (40-50%) is anhedral to euhedral, up to 4 mm, commonly with oscillatory zoning. Green-brown biotite and hornblende (15-20%) is anhedral to euhedral with biotite flakes up to 8 mm in diameter (Figure 4.5), and hornblende crystals up to 11 mm. Hornblende often contains inclusions of biotite and plagioclase (Withnall et al., 1995). Withnall et al. (1995) noted increasing foliation, recrystallisation and strain in the Mt Newsome Granodiorite towards the south and east. The foliation is defined by aligned and recrystallised quartz and biotite, deformed plagioclase, and 147

174 4.2 Regional Geology, Rubyvale Region U-Pb Geochronology elongate quartz ribbon fsp S3 fsp fsp 1mm Figure 4.3: Photomicrograph of the Gem Park Granite. Image taken under cross polarised light, fsp = feldspar, qtz = quartz. elongate feldspar quartz and feldspar domains separated by mica rich seams S3 Figure 4.4: Strongly foliated outcrop of the Gem Park Granite. 148

175 U-Pb Geochronology 4.3 Geochronology rare undulose hornblende (Withnall et al., 1995). 4.3 Geochronology Previous Work The Gem Park Granite was tentatively assigned a Cambrian-Ordovician age by Withnall et al. (1995), based on fabric relationships between the intrusive bodies and surrounding metamorphics. Withnall et al. (1995) reports an age of 375 Ma from a personal communication with P. Carr (1992) that was obtained from Rb/Sr dating of biotite from a small body near the Rubyvale- Reward road, however this was considered to reflect resetting during intrusion of the Retreat and Taroborah Batholiths. The Mt Newsome Granodiorite was dated by the K/Ar method on biotite by Webb and McDougall (1968), their age was recalculated by Withnall et al. (1995) using the decay constants of Steiger and Jäger (1977) to 370 Ma. A personal communication by P. Carr (1993) in Withnall et al. (1995) reported several Rb/Sr ages from whole rock-biotite pairs that range from 372 to 382 Ma. Other intrusive bodies related to the Retreat Batholith that truncate S4 in the Rubyvale region include: Kilmarnock Granodiorite; Webb et al. (1963) obtained two K/Ar biotite ages recalculated to 352 Ma and 355 Ma, and two K/Ar hornblende ages recalculated to 363 Ma and 365 Ma (Withnall et al., 1995). Webb and McDougall (1968) reported a K/Ar biotite age recalculated to 370 Ma, and a hornblende age recalculated to 360 Ma, and P. Carr in a personal communication (1993) reported an age of 375 Ma from a Rb/Sr whole rock biotite pair (Withnall et al., 1995). Taroborah Granodiorite; Webb et al. (1963) obtained K/Ar biotite and hornblende ages recalculated to 372 Ma and 374 Ma respectively (Withnall et al., 1995). Whitdale Granodiorite; Webb and McDougall (1968) obtained a K/Ar biotite age recalculated to 370 Ma, however this rock is intruded by the Mt Newsome Granodiorite and this age may reflect isotopic resetting (Withnall et al., 1995). 149

176 4.3 Geochronology U-Pb Geochronology qtz 2 mm kfsp bt bt fsp fsp pfsp Figure 4.5: Photomicrograph of sample DW03-41 from the Mt Newsome Granodiorite. Image taken under cross polarised light, kfsp = potassium feldspar, pfsp = plagioclase feldspar, bt = biotite, qtz = quartz Geochronology: this study Zircon separates were prepared from the Gem Park Granite (DW03-43) and the Mt Newsome Granodiorite (DW03-41), as well as a monazite separate from the Gem Park Granite (Figure 4.1). Zircon and monazite grain sizes ranged from 100 µm to 500 µm, with most 100 µm µm in diameter. Hand picked grains were mounted in an epoxy resin along with reference materials: zircon SL13 (Claoué-Long et al., 1995, 238 ppm U), FC1 zircon standard (Schmitz et al., 2003, 206 Pb/ 238 U age = 1099 Ma) and Wendall monazite standard (RSES in-house standard, 206 Pb/ 238 U age = ± 0.8% Ma). The mounts were documented by transmitted and reflected light photomicroscopy, as well as cathodoluminescence imaging of zircon on a Hitachi S-2250N SEM (Figure 4.7, 4.9). U-Th-Pb isotopic analyses were undertaken on the SHRIMP-II and SHRIMP-RG ion microprobes at the Research School of Earth Sciences, Australian National University, Canberra, using procedures similar to those described in Williams (1998). Analytical conditions were: 2-5 na O 2 primary beam accelerated to 10kV, µm diameter spot, positive secondary ions accelerated to 10 kv, mass resolution 5000 (1%), peak switching single electron multiplier. The primary beam was reduced slightly for monazite so that the ThO signal would not exceed 150

177 U-Pb Geochronology 4.3 Geochronology the maximum accepted by the ion counter. Isotopic compositions were measured directly, without correction for mass fractionation ( 0.25%/AMU). Fractionation of Pb relative to U and Th was corrected using the relationship Pb /U = A(UO /U ) 2 (Claoué-Long et al., 1995). Ages were calculated using the constants recommended in Steiger and Jäger (1977). Analytical uncertainties in tables and figures are 1σ precision estimates. Uncertainties in pooled ages are quoted at the 95% confidence level. Common lead corrections used 204 Pb assuming laboratory-derived common Pb composition of Broken Hill galena. Isotopic analyses and calculated ages are shown in Table 4.1. Zircon from the Gem Park Granite was analysed in three separate sessions, and uncertainties in the 206 Pb/ 238 U ages quoted from this sample include Pb/U calibration errors. Individual ages of zircon from the Gem Park Granite were calculated according to the following set of quantitative criteria: The best estimate for 206 Pb/ 238 U apparent ages were calculated by taking a weighted mean of the 204 Pb, 207 Pb and 208 Pb corrected values for each analysis. If the MSWD of these three values was found to be greater than a critical threshold of two (typically related to common Pb content and associated correction factor), the data point was flagged for further investigation. A final age was calculated by taking a weighted mean of the best estimate 206 Pb/ 238 U age, and the 207 Pb/ 206 Pb apparent age, however if the MSWD of these two ages exceeded a critical threshold of seven, the data point was again flagged for further investigation. A final age was taken as the best estimate 206 Pb/ 238 U age for data where the weighted mean of the 206 Pb/ 238 U, and 207 Pb/ 206 Pb apparent ages was less than 800 Ma, or the 207 Pb/ 206 Pb if greater than 800 Ma. In cases where the calculated 207 Pb/ 206 Pb apparent age is within error of zero (young samples with low radiogenic 207 Pb content), the 207 Pb/ 206 Pb apparent age is ignored. This method is useful for samples that record a large range of apparent ages, for example, detrital zircons or an intrusive with a high level of inheritance Results Gem Park Granite: Monazite Monazite from the Gem Park Granite are anhedral, honey-yellow grains that are mostly inclusion free, and display no visible signs of secondary alteration or resorption. Ion probe transects across several individual grains revealed no age difference from rim to core. Monazite age calculations were made using a 207 Pb correction. This was done for two reasons, one is due to the presence of 151

178 4.3 Geochronology U-Pb Geochronology an unidentified isobar at approximately amu (Stern and Sanborn, 1998) that complicates determination of 204 Pb, which is normally used for 206 Pb/ 238 U age corrections, and two is that the 204 Pb uncorrected data plot close to concordia and have low common lead contents (Table 4.1 (b). The monazite data have a 207 Pb corrected 206 Pb/ 238 U weighted mean age of ± 6.2 Ma (Figure 4.6(a)) DW03-43 Mz Gem Park Granite. Monazite analyses. n= Pb/ 206 Pb Weighted mean 207 Pb corrected 206 Pb/ 238 U age = ± 6.2, 95% conf. MSWD = 0.52, probability of equivalence = U/ 206 Pb Figure 4.6: (a) Tera-Wasserburg concordia plot of monazite U-Pb isotopic analyses from the Gem Park Granite. Plotted data is uncorrected for 204 Pb Gem Park Granite: Zircon A 6kg of sample of Gem Park Granite yielded only twenty eight zircon grains. Cathodoluminescence (CL) imaging of individual zircon grains revealed distinct detrital cores, with up to three overgrowth rim domains of variable appearance (Figure 4.7). The outermost rims are commonly CL-dark, with intermediate zones ranging from CL-bright to CL-cloudy. Two grains have rim zones with oscillatory zoning patterns. The presence of cloudy and unzoned rims along with oscillatory zoning in some grains suggests that secondary growth of zircon involved both metamorphic and magmatic processes (Zhao et al., 2002). 152

179 U-Pb Geochronology 4.3 Geochronology Results of the isotopic analyses are shown in Table 4.1(a), and ages quoted include standardisation errors of 1.19%, 1.02% and 1.70% over three analytical sessions. A concordia plot of all data (Figure 4.8(a)) reveals a large range of apparent 206 Pb/ 238 U ages. Using the scheme described in Section for calculating best estimate ages, some data with younger apparent ages, as well as several older apparent ages were flagged for further consideration. The flagged data all had elevated levels of common 206 Pb (measured as the percent of common 206 Pb of total 206 Pb; f 206 Pb), and above a threshold level of 2.48% (lowest measured content flagged), the data were slightly to moderately discordant and had large uncertainties. Owing to discordance and some uncertainty in spot locations, the data with elevated common 206 Pb are not considered to be reliable ages. A total of 16 analyses from a group of 73 was flagged using this method, and these 16 analyses are not considered further for the interpretation. A second plot of all zircon analyses from the Gem Park Granite, excluding the flagged data is shown in Figure 4.8(b) ± 34 Ma 2449 ± 26 Ma Elevated U and common 206 Pb 389 ± 5 Ma 407 ± 6 Ma 389 ± 6 Ma 937 ± 12 Ma Figure 4.7: Cathodoluminescence image of Gem Park Granite zircon grains with examples of several core-rim age differences. Note the outermost, dark rim domains that yielded high analytical uncertainties associated with common lead correction. 153

180 4.3 Geochronology U-Pb Geochronology 0.20 DW03-43 Gem Park Granite. All zircon analyses. n= Pb/ 206 Pb U/ 206 Pb (a) DW03-43 Gem Park Granite. All zircon analyses, excluding elevated 206 Pb. n= Pb/ 206 Pb Cores Rims U/ 206 Pb (b) 154

181 U-Pb Geochronology 4.3 Geochronology 25 DW03-43 Gem Park Granite. All Zircon Analyses, excluding elevated 206 Pb. n=57 20 Number Age (Ma) (c) DW03-43 Gem Park Granite Zircon excluding elevated 206 Pb. n= overlapping analyses 207 Pb/ 206 Pb single grain analyses Cores Rims 0.04 possible group? U/ 206 Pb (d) Figure 4.8: (a) Tera-Wasserburg concordia plot of all zircon data for the Gem Park Granite. (b) Tera- Wasserburg concordia plot of zircon data from the Gem Park Granite, excluding analyses with greater than 2.48% common 206 Pb. (c) Cumulative probability plot of all zircon analyses from the Gem Park Granite, excluding analyses with greater than 2.48% common 206 Pb. (d) Tera-Wasserburg concordia plot of zircon data from ca 600 Ma to ca 300 Ma from the Gem Park Granite, excluding analyses with greater than 2.48% common 206 Pb. All of the plotted data has been 204 Pb corrected. 155

182 4.3 Geochronology U-Pb Geochronology The data range in apparent ages from ca 2449 Ma to ca 302 Ma, with distinctively different core and rim populations. Some spots reported as cores are actually single grain analyses, and a second examination of spot location in some analyses revealed overlap of core and rim domains had occurred (Figure 4.8(d)). A cumulative probability plot of all data (excluding analyses with greater than 2.48% common 206 Pb) is shown in Figure 4.8(c), the large peak at ca 400 Ma reflects the high number of rim analyses of similar age. Older ages from detrital cores are consistent with ages from detrital zircon and monazite from the Anakie Metamorphic Group in the Clermont region (Fergusson et al., 2001), although a Ma zircon population, and a Ma monazite population that are locally significant components in the Clermont region are not well represented by detrital zircon cores from the Gem Park Granite. This is not unusual considering the relatively small number of analyses made on detrital cores from one sample of Gem Park Granite, compared to a suite of analyses on several samples from the Bathampton Metamorphics in the Clermont region performed by Fergusson et al. (2001). A plot of the younger apparent ages (Figure 4.8(d)) reveals a range of apparent rim ages from ca 312 Ma to ca 440 Ma, the clustered group outlined has a weighted mean age of 418 ± 15 Ma. Four rim analyses, all from cloudy homogenous overgrowths, have apparent ages from 366 ± 11 Ma to 389 ± 5 Ma (1σ). These younger ages are similar to a zircon concordia age of ± 3.3 Ma (1σ) from the intruding Mt Newsome Granodiorite (Section 4.3.4, this study) and are in close agreement with the range of Rb/Sr and K/Ar biotite ages from the Mt Newsome Granodiorite and other intrusives of the Retreat Batholith (Webb et al., 1963; Withnall et al., 1995) Mt Newsome Granodiorite: Zircon Twenty zircon grains from the Mt Newsome Granodiorite were analysed. The grains displayed oscillatory zoning under cathodoluminescence with no secondary overgrowths (Figure 4.9). A concordia plot of all analyses (Figure 4.10(a)) shows a cluster of 17 grains close to concordia, yielding a weighted mean 204 Pb corrected 206 Pb/ 238 U age of ± 10.2 Ma (Figure 4.10(b)). Two older grains with apparent 206 Pb/ 238 U ages of 1572 ± 37.5 Ma and 689 ± 28.1 Ma (1σ) respectively are most likely inherited grains, and similar in age to zircon core ages from the Gem Park Granite. The source of a single younger grain with an apparent 206 Pb/ 238 U age of ± 12.8 (1σ) is not known, it plots close to concordia, however may reflect radiogenic lead loss. No magmatic related rocks with an age of ca 338 Ma are known from the Rubyvale region. The zircon weighted mean age of ± 10.2 Ma from the Mt Newsome Granodiorite is similar 156

183 U-Pb Geochronology 4.3 Geochronology to some younger zircon apparent ages from the Gem Park Granite into which it intrudes. The zircon age is slightly older than Rb/Sr and K/Ar biotite ages from the same granitoid (382 Ma, 370 Ma respectively), as well as Rb/Sr and K/Ar biotite ages from other intrusives of the Retreat Batholith (Webb et al., 1963; Withnall et al., 1995, Ma). Figure 4.9: Cathodoluminescence image of zircon grains from the Mt Newsome Granodiorite. The grains show oscillatory zoning and prismatic crystal habit. Note a lack of secondary overgrowths compared to zircon from the Gem Park Granite. 157

184 4.3 Geochronology U-Pb Geochronology DW03-41 Mt Newsome Granodiorite. All Analyses. n= Pb/ 206 Pb U/ 206 Pb (a) DW03-41 Mt Newsome Granodiorite. n= Pb/ 206 Pb Weighted Mean 204 Pb corrected 206 U/ 238 U age = ± 10.2 Ma, 95% conf. MSWD = 0.78 Probability of equivalence = U/ 206 Pb (b) Figure 4.10: (a) Tera-Wasserburg concordia plot of all zircon analyses from the Mt Newsome Granodiorite zircon grains. (b) Tera-Wasserburg concordia plot of clustered analyses from the Mt Newsome Granodiorite. All of the plotted data is 204 Pb corrected. 158

185 U-Pb Geochronology 4.4 Discussion 4.4 Discussion The Gem Park Granite yielded a monazite weighted mean age of ± 6.2 Ma, and a range of zircon apparent ages. The range in zircon ages are interpreted to reflect analysis of detrital grains that have several overgrowth phases. The Mt Newsome Granodiorite yielded a weighted mean age of ± 10.2 Ma from a cluster of analyses, with two older grains reflecting inheritance and a younger age most likely the result of radiogenic lead loss. In order to understand what the pooled ages reflect, and what they can be used to constrain, it is necessary to understand the structural context of both intrusive suites Structural Context Gem Park Granite The smaller bodies of Gem Park Granite near Rubyvale are along strike from the Ruby Creek Mylonite Zone (Figure 4.1). The intrusives are elongate parallel to S4 in the surrounding rocks, and contain a mylonitic foliation which has the same orientation as S4. Mylonitic foliation within the Gem Park Granite is interpreted to be the same generation as S4 in the Bathampton Metamorphics. The occurrence of the Gem Park Granite bodies parallel to S4, containing a mylonitic foliation the same as surrounding rocks suggests it intruded pre- to syn-d4. There are several foliations in the Bathampton Metamorphics, and only a single foliation within the Fork Lagoons Beds. S4 in the Bathampton Metamorphics adjacent to the fault zone, and S1 in the Fork Lagoons Beds both dip to the northwest, parallel to the fault contact that separates them. Given the structural continuity observed, these are likely to be the same foliation formed during movement along the fault zone. It is interpreted that D4 caused faulting between the Bathampton Metamorphics and the Fork Lagoon Beds, forming S4 in the Bathampton Metamorphics and first generation folds in the Fork Lagoons Beds. The northwest dip of foliation (S4) associated with the fault, and location of higher grade basement rocks above lower grade sedimentary rocks in the footwall suggests reverse movement. 159

186 4.4 Discussion U-Pb Geochronology The relationship and geometry of the Gem Park Granite to the fault zone, and associated mylonitic foliation (S4) can be interpreted in two ways. One is that the Gem Park Granite intruded prior to fault zone activity, with subsequent deformation resulting in significant grainsize reduction and mylonitisation. The granite bodies could have been reoriented parallel to the fault zone during deformation, resulting in the elongate geometry. The second interpretation is that the Gem Park Granite was generated during D4 deformation and metamorphism at depth, and intruded along and parallel to the fault zone. The presence of fresh biotite crystals in foliation planes, and no evidence of retrogression during mylonitisation suggests that the granite bodies were deformed while they were still at high temperature. This suggests that the Gem Park Granite was either emplaced or deformed while still at high temperature in the fault zone during deformation, in which case its crystallisation age reflects an absolute age of regional D4 deformation Mt Newsome Granodiorite The Mt Newsome Granodiorite intrudes the Bathampton Metamorphics, the Gem Park Granite and the Whitdale Granodiorite. The relationship between foliation in the Mt Newsome Granodiorite and S4 in the Bathampton Metamorphics and the Gem Park Granite is not clear. Withnall et al. (1995) proposed two possible origins for foliation in the Mt Newsome Granodiorite; one is that it is a magmatic foliation; two is that it is related to deformation. Given the intensity of S4 in the Rubyvale region, if the two foliations are the same, then more regional fabric development in the Retreat Batholith might be expected. This was not observed to be the case. Thus, foliation in the Mt Newsome Granodiorite is more likely to be magmatic in origin. Magmatic foliations can form at the base of a magma chamber, where regular alignment and stacking of close proximity crystals occurs during settling. There is an increase in foliation development in the Mt Newsome Granodiorite towards the south-west (Withnall et al., 1995), and this coincides with a general increase in metamorphic grade of the Anakie Metamorphic Group from northeast to southwest across the inlier. Tilting of the Anakie Inlier after intrusion of the Retreat Batholith may have resulted in deeper levels of both metamorphic and intrusive rocks being exposed in the southwest part of the inlier. This would be closer to the base of the Mt Newsome Granodiorite, and is consistent with foliation intensity increase to the southwest in outcrop. 160

187 U-Pb Geochronology 4.4 Discussion The composition of a granitoid body can affect how it will deform, and controls foliation development during deformation. Vernon and Flood (1988) showed an example of contemporaneous I- and S-type granitoids in the eastern Lachlan Fold Belt, which had been subsequently deformed at the same time. Ratios of strong to weak (or brittle to ductile) minerals were interpreted as the controlling factor for foliation development, with S-type granites having a higher ratio of quartz and micas (weak/ductile) that facilitated prominent foliation development. The I-type granites had a higher ratio of feldspars (strong/brittle) that restricted foliation development (Vernon and Flood, 1988). The Mt Newsome Granodiorite and the Gem Park Granite have similar amounts of quartz, and the Mt Newsome Granodiorite has up to 20% biotite compared to <5% biotite and muscovite for the Gem Park Granite. The ratios of quartz and mica to feldspar predict that the Mt Newsome Granodiorite should form a foliation preferentially to the Gem Park Granite under the same deformation conditions, given the interpretation of Vernon and Flood (1988). Thus if D4 had occurred after intrusion of the Mt Newsome Granodiorite, it would form a foliation similar to, or more prominent than in the Gem Park Granite. This was not observed. It is interpreted that S4 foliation in the Gem Park Granite and surrounding metamorphics is unrelated to the foliation in the Mt Newsome Granodiorite, which is interpreted to be a magmatic foliation Zircon and Monazite Ages: Gem Park Granite U-Pb isotopic analyses of monazite and zircon rims from the Gem Park Granite reveal a difference in ages yielded from the two different minerals. The monazite data yield a weighted mean age of ± 6.2 Ma, whereas the zircon rim data have a spread of ages that range from ca 312 Ma to ca 440 Ma, and a clustered group with a weighted mean age of 418 ± 15 Ma (tσ), most plotting close to concordia. To determine if the weighted mean age from a cluster of either one of these minerals reflects the age of crystallisation and emplacement of the Gem Park Granite, two parameters need to be examined: Do the monazite data record an inherited age or a magma crystallisation age? What is the cause of the apparent spread in the zircon rim age data? 161

188 4.4 Discussion U-Pb Geochronology Monazite - Inherited or Crystallisation Age? The detrital zircon age pattern yielded from the Gem Park Granite is consistent with patterns observed in a study of detrital zircon and monazite by Fergusson et al. (2001) from the Anakie Metamorphic Group in the Clermont region. Therefore, it is reasonable to suggest that the Gem Park Granite was sourced from anatexis of the Anakie Metamorphic Group at depth during D4 deformation, given its S-type affinity, close relationship to faulting, and mylonitic S4 foliation. Dependent on the thermal history of the Anakie Metamorphic Group in the source region prior to magma genesis, it is possible that an inherited or detrital signature could be preserved in the monazite grains as well. The study of Fergusson et al. (2001) indicated the youngest age populations for zircon were between 510 and 700 Ma, and ca 540 Ma for monazite. These ages are considerably older than the monazite weighted mean age from the Gem Park Granite, and can be interpreted in two ways; one, the Gem Park Granite was sourced from rocks younger than the Anakie Metamorphic Group; and two, that the monazite weighted mean age is a direct age of magma crystallisation and emplacement. The latter is likely given the lack of age population spread and that the data all plot close to concordia. The behaviour of monazite in S-type granite generated during metamorphism and anatexis of surrounding country rock has been documented by Williams (2001) in the Cooma Complex of southeastern Australia. During high temperature-low pressure metamorphism, dissolution of detrital monazite began around the andalusite isograd, with new growth occurring in the K-feldspar isograd, and within migmatites associated with anatexis and generation of the S-type Cooma Granodiorite (Williams, 2001). Monazite that crystallised within the Cooma Granodiorite had a different composition to detrital monazite, and the U-Pb isotopic system was completely reset (Williams, 2001). The U-Pb system of detrital zircon grains in the Cooma Complex was unaffected by new zircon overgrowths, and the overgrowths yielded ages close to the ages yielded by the newly crystallised monazite (Williams, 2001). A similar situation to the Cooma Granodiorite is interpreted for the Gem Park Granite. Therefore, monazite U-Pb ages from the Gem Park Granite reflect isotopic resetting, and are interpreted to record the age of crystallisation of the intrusion at ± 6.2 Ma. 162

189 U-Pb Geochronology 4.4 Discussion Zircon Rim Data The zircon rim data from the Gem Park Granite yielded a spread of 206 Pb/ 238 U apparent ages from ca 312 Ma to ca 440 Ma for analyses that exclude elevated common 206 Pb. This spread of ages may be the result of several factors, including but not restricted to, analyses of flawed grains, overlapping analyses, zircon rim overgrowth prior to, during, and after inclusion into the granitic melt, and loss of radiogenic lead. Overlapping analyses have been identified, and typically recorded older ages than the main rim population. Flawed grain sites analysed by ion probe usually comprise cracks and similar defects, these can yield anomalous common lead and oxide content which can affect the data reduction process. The sorting of data by common 206 Pb content is considered to have eliminated most of the spurious analyses, however as no examination of spot locations under a scanning electron microscope was undertaken, there is a possibility that analyses of flawed sites were included in the final data set. The most likely reason for spread in the zircon rim ages is a combination of multiple rim overgrowths and loss of radiogenic lead. Up to three distinct overgrowth events were detected by CL imaging on some grains, and some display a combination of magmatic and metamorphic zircon overgrowths. Younger ages were yielded by metamorphic rim overgrowths. The rim zones are more susceptible to radiogenic lead loss than the detrital cores, as a result of a poorly crystalline internal structure evidenced from cloudy CL. The spread in ages is thus attributed to a combination of metamorphic growth during cooling, and radiogenic lead loss. On a regional scale the southern Anakie Inlier has been subject to a younger thermal event during extension and magmatism associated with formation of the adjacent Drummond Basin in the Late Devonian- Early Carboniferous (Johnson and Henderson, 1991; Henderson et al., 1998). This event could explain the young rim overgrowths. Given that the 206 Pb/ 238 U apparent ages from zircon rim overgrowths in the Gem Park Granite probably reflect a combination of cooling, radiogenic lead loss and younger thermal pertubations, then the weighted mean age of 418 ± 15 Ma from the clustered group in Figure 4.8(d) is probably not an accurate measure of the true age of crystallisation. The monazite weighted mean age of ± 6.2 Ma from the Gem Park Granite is more likely to reflect the age of emplacemtent/crystallisation. 163

190 4.4 Discussion U-Pb Geochronology Summary and Conclusions The age of S4 in the Anakie Metamorphic Group has been bracketed by the age of the Gem Park Granite and the Mt Newsome Granodiorite. A maximum age of ± 6.2 Ma from monazite in the Gem Park Granite, and a minimum age of ± 10.2 Ma from zircon in the Mt Newsome Granodiorite is indicated for the age of D4. The geometric relationship and fabric association of the Gem Park Granite with a northeast trending fault zone between the Bathampton Metamorphics and the Fork Lagoons Beds suggests it either intruded syn-kinematically with, or immediately prior to, regional D4 deformation. An absolute age of D4 deformation is therefore either slightly younger than, or the same as, the age of the Gem Park Granite. D4 produced northeast trending structures in both the Rubyvale and Clermont regions, and correlates with D1 in the Fork Lagoons Beds. D4 resulted in reverse movement along a fault separating the Bathampton Metamorphics from the Fork Lagoons Beds, and also formed the elliptical structure defined by amphibolite north of Rubyvale. In the Clermont region, the Oaky Creek Antiform is a major feature of D4, and its formation is related to the earliest known gold mineralisation in the area. The age of D4 is later than the youngest 40 Ar/ 39 Ar age of ca 460 Ma from the Clermont region, which is interpreted to reflect cooling after D3 (Chapter 3). Thus an age of ca 443 Ma for D4 is consistent with known ages and regional deformation history of the Anakie Metamorphic Group. 164

191 U-Pb Geochronology 4.4 Discussion Conclusions: The Gem Park Granite has a monazite 206 Pb/ 238 U weighted mean age of ± 6.2 Ma, interpreted to be an absolute age of regional D4 deformation. Zircon rim data from the Gem Park Granite yielded a spread of 206 Pb/ 238 U apparent ages. Metamorphic rim overgrowth ages reflect the effects of growth during cooling, possible younger thermal perturbations, and radiogenic lead loss. The Mt Newsome Granodiorite has a 206 Pb/ 238 U concordia age of ± 10.2 Ma from single grain zircon analyses. Foliation in the Mt Newsome Granodiorite is magmatic in origin, and unrelated to foliation formed during D4 in surrounding rocks. 165

192 4.4 Discussion U-Pb Geochronology Spot Name ppm U Gem Park Granite Zircon (sample DW03-43) ppm Th Th/U ƒ206 P b 238 U/ 206 Pb % err 207 Pb/ 206 Pb % err Age (Ma) ± (Ma) DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW (a) 166

193 U-Pb Geochronology 4.4 Discussion Gem Park Granite Monazite (sample DW03-43) Spot Name ƒ 206 Pb 232 Th/ 238 U 238 U/ 206 Pb % err 207 Pb/ 206 Pb % err Age (Ma) ± (Ma) DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m DW43m Spot Name (b) Mt Newsome Granodiorite Zircon (sample DW03-41) ppm U ppm Th Th/U ƒ 206 Pb 238 U/ 206 Pb % err 207 Pb/ 206 Pb % err Age (Ma) ± (Ma) DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW DW (c) Table 4.1: Results of SHRIMP U-Th-Pb isotopic analyses. (a) Gem Park Granite Zircon. (b) Gem Park Granite Monazite. (c) Mt Newsome Granodiorite. Analytical errors and age uncertainty are quoted at 1σ and include Pb/U calibration uncertainty. f 206 Pb = percent common lead as a fpercent of total 206 Pb. 167

194 4.4 Discussion U-Pb Geochronology 168

195 Chapter 5 Gold in the Clermont Region The work in this chapter was prompted by conversations with Roric Smith and Gordon Lister, who provided the impetus for initial interest in gold in the Anakie Inlier and surrounds. Discussion and assistance in the field was provided by Caroline Forbes and Simon Richards, Simon is acknowledged for observations that led to the final interpretations. Access to the underground site at Miclere was provided by Geoff and Regina Starr. Geoff Starr is the owner of the gold specimens presented. 5.1 Introduction Gold in the Clermont region occurs in a variety of deposit styles. The earliest known gold deposits are structurally controlled lodes, and occur in faults and shear zones in the Anakie Metamorphic Group (Figure 5.1). Little is known about the timing and relationship of lodegold deposits to regional deformation. The Clermont Goldfield (Figure 5.1) comprises mostly alluvial gold, concentrated in conglomerates at the base of Permian basins north of Clermont (Figure 5.2), and in Tertiary deep leads. Gold in Permian basins is enigmatic, its genesis has been a subject of debate ever since its discovery. This chapter aims to constrain the structural controls on lode gold mineralisation west of Clermont, and to better understand the nature of 169

196 5.1 Introduction Gold Permian conglomerate hosted gold Gold in the Clermont Region Up to 1993, the Clermont region produced over 12 t of gold, mostly from Permian and Tertiary deep leads, but also from quartz-sulphide veins in schist of the Anakie Metamorphic Group (Lam, 2005). From 1962 to 2005, more than 240 Exploration Permits for Mineral (EPM s) were granted to explore within the Clermont 1: Sheet area, most were granted in the 1980 s and early 1990 s (Lam, 2005). In 2005, the western part of the Clermont 1: Sheet was being explored by numerous EPM s, including the Oaky Creek Antiform area. Areas of the Bathampton Metamorphics that contain greenstones and mylonite zones have recently been recognised as gold bearing, and are the focus of several exploration programs (Lam, 2005). Lode-gold deposits and younger alluvial gold deposits in the Clermont region have been documented and summarised by Withnall et al. (1995), and an extensive review of mines, exploration and metalliferous mineralisation was undertaken by Lam (2005). Withnall et al. (1995) outlined 4 settings that gold occurred in within the Clermont region. These are: lode gold deposits, mostly as quartz veins in the Bathampton Metamorphics of the Anakie Metamorphic Group; gold in Permian conglomerates, either along the unconformity with the Anakie Metamorphic Group, or in false bottoms above the unconformity; Tertiary aged alluvium comprising linear belts of poorly consolidated sediments, now cut by the present drainage pattern; Quaternary alluvial deposits along the modern drainage systems 170

197 Gold 5.1 Introduction 0 10 km Miclere Au boundary synform antiform fault dip and trend main foliation Black Ridge Au N Blair Athol (coal) v CLERMONT GOLDFIELD Cz Clermont Cz v Cz v Peak Downs Cu v v v Cainozoic cover Permian sedimentary rocks Oaky Creek Antiform area v v Drummond Basin Sequence Silver Hills Volcanics Theresa Creek Volcanics v Retreat Batholith and related rocks Wynyard Metamorphics Scurvy Creek Meta-arenite Hurleys Metamorphics Monteagle Quartzite Rolfe Creek Schist Yan Can Greenstone Member metasediments greenstone Bathampton Metamorphics ' minesite/diggings gold occurrence Figure 5.1: Location of the Clermont Goldfield, the Oaky Creek Antiform area and known gold occurrences. 171

198 5.2 Previous Work Gold 5.2 Previous Work Lode gold deposits in the Anakie Metamorphic Group Lode gold deposits north and south of Clermont occur in the Bathampton Metamorphics (Figure 5.1). Most lodes are planar sheets in fracture-openings, and crosscut the main schistosity (Lam, 2005). Lodes are up to 2 m wide and have a pinch-swell geometry, they consist of discrete quartz veins to zones of schist with numerous narrow quartz veins (Withnall et al., 1995). Gold is concentrated in the narrower end of the lodes, and in quartz leaders within schist (Lam, 2005). Accessory minerals in the quartz veins comprise pyrite, arsenopyrite, sphalerite, galena, calcite and siderite (Withnall et al., 1995). The lodes range in strike from north, northwest, northeast and east, however the northwest trending deposits were worked more extensively (Lam, 2005). The east and northwest trending lodes occupy shear zones and faults in schist adjacent to, or at the contact with greenstone, however do not extend into the greenstone (Lam, 2005) Gold Occurrences Around the Oaky Creek Antiform West of Clermont, lode gold deposits and occurrences are located along, and north of, the Oaky Creek Antiform (Figure 5.1, Appendix 1). Deposits located along the Oaky Creek Antiform are summarised from previous work, and used as basis for correlation during this study. Southern Cross The Southern Cross deposit had an overall strike of 125 and dipped 50 to the southwest (Shepherd, 1952). The lode was up to 60 cm wide, and averaged 15 cm of quartz containing rich shoots up to 30 g/t gold (Reid, 1947). High gold values were mainly in massive grey quartz with disseminated galena and sphalerite, white quartz had low gold values (Withnall et al., 1995). The lodes are in east and southeast trending shear zones in mica schist, and mineralisation was correlated with a system of altered felsic and honblende-feldspar porphyry dykes (Withnall et al., 1995). 81 kg of gold from 1140 t of ore was yielded by the Southern Cross deposit (Withnall et al., 1995). 172

199 Gold 5.2 Previous Work Smeagols Smeagols workings were spread over three lodes in mica schist adjacent to a large serpentinite body (Withnall et al., 1995). The lodes occurred in a northwest trending fissure system up to 200 m long, and comprised networks of quartz veins up to 50 cm wide, enclosed in an unmineralised breccia formation (Withnall et al., 1995). Grades up to 60 g/t were reported from lenticular quartz veins carrying disseminated pyrite, arsenopyrite, galena and sphalerite (Withnall et al., 1995). Fig Tree Gold at Fig Tree was mined from quartz veins within an east-west trending shear zone in mica schist (Lam and Garrad, 1993). Keno The Keno prospect is a series of workings over a kilometre distance, along a structure striking 060 (Withnall et al., 1995). Gold grades up to 130 g/t were obtained from quartz vein samples in one of the old workings (Lam and Garrad, 1993). Rock types around the prospect include foliated and boudinaged quartz-mica schist, highly silicified chlorite schist with abundant quartz and calcite veinlets, and a transitional chloritic quartz-mica schist (Lam and Garrad, 1993). Midas Gold at Midas was associated with narrow quartz veinlets in broad east, southeast and northeast trending shear zones (Withnall et al., 1995). A zone of intense hydrothermal alteration contained gold mineralisation in narrow quartz veins, with accessory galena, arsenopyrite and pyrite (Lam and Garrad, 1993). Carbonate alteration associated with gold mineralisation was most intense at, and along, a shear zone through the prospect (Lam and Garrad, 1993). Goldfinger Gold mineralisation at Goldfinger occurred in brecciated quartz-muscovite-sericite schist containing up to 15% fine-grained, disseminated and weakly banded pyrite and arsenopyrite (Withnall et al., 1995). Low grade mineralisation occurred in flat-lying mineralised lenses in a broad east trending shear zone adjacent to a felsic dyke (Withnall et al., 1995). Native gold was 173

200 5.2 Previous Work Gold reported from a narrow quartz vein by Lam and Garrad (1993). Little Finger The Little Finger prospect is located within an arcuate chlorite schist unit 80 m wide (Lam and Garrad, 1993). Gold occurs in an anomalous zone comprising a 2 to 3 m wide banded iron formation that locally crosscuts the chlorite schist (Lam and Garrad, 1993). Wee Five The Wee Five prospect was a gold anomaly in siliceous rock, irregularly distributed within a northeast trending shear zone (Lam and Garrad, 1993). others Roulette, Hillview, un-named, and K-2 were all identified as anomalous areas in gold from reconnaissance pan-concentrate stream sediment and rock chip samples by Noranda Australia Ltd and Pioneer minerals Australia Ltd (Withnall et al., 1995). The Leo Grande Fault also had anomalous levels of gold reported (Withnall et al., 1995) Lode Gold Metallogenesis Lode gold in the Clermont region is structurally controlled, and occurs in either discrete shear zones or broad corridors (Withnall et al., 1995). The age of mineralisation has previously been interpreted as either Middle Devonian or Late Devonian-Carboniferous, based on spatial and temporal relationships with dykes that are possibly related to the Retreat Batholith, or younger magmatism (Withnall et al., 1995). Oxygen isotope studies by Wilson and Golding (1984) suggested a metamorphic origin for gold bearing quartz veins west of Clermont, and concluded that auriferous fluids were not locally derived or buffered. Mackay (1987) suggested that lode gold deposits north of Clermont were granite related, and proposed a model that involved a decaying magma system emplaced into shallow levels in the waning stages of metamorphism and deformation. Support for this model is found in shallow buried intrusions detected by magnetic surveys north of Clermont, that correlate with hornfelsed outcrop at the surface (Withnall et al., 1995). 174

201 Gold 5.2 Previous Work Mustard (1990) concluded that a deposit north of Clermont (Belyando) is best classified as orogenic in style. Even though quartz textures, mineralogy and nature of the gold are typical of the plutonic class, the fluid characteristics and timing of mineralisation with respect to metamorphism, deformation and magmatism support a metamorphic or modified magmatic fluid origin (Mustard, 1990) Permian Conglomerate Hosted Gold A series of northwest aligned Permian basins are situated on the eastern margin of the Anakie Inlier in the Clermont Goldfield (Figure 5.2). They are tectonically aligned with the Denison Trough in the adjacent Bowen Basin, and formed as intracratonic half graben features during Permian extension (Dickins and Malone, 1973; Zhou et al., 1994). Gold in these basins is concentrated in the lowest 50 cm of a basal conglomerate unit, directly above an unconformity with the Anakie Metamorphic Group. Gold is also found in the first 30 cm of metamorphics below the unconformity, and in false bottoms higher up in the Permian sequence. The conglomerate consists of cobble to boulder size (3 m) clasts, and is derived from schist and quartzite of the Anakie Metamorphic Group, with minor quartz and rare granitoid and volcanic rocks (Withnall et al., 1995). The Black Ridge deposit at the southern end of the Miclere basin (Figure 5.2) produced over 70% of the total gold won from the Permian rocks (Withnall et al., 1995). Gold occurs as mm-sized grains in, or adjacent to, siderite veinlets in association with marcasite in the basal conglomerate (Zhou, 1995). Gold is concentrated along vertical fractures and joints that transect the unconformity. The fractures vary from several cm wide, with visible diplacement of the unconformity, to hairline fractures with negligible displacement (Day, 1995). The Miclere deposit is located at the northern end of the Miclere Basin (Figure 5.2), and contains characteristic coarse nuggets, with some individual pieces several ounces in weight (Day, 1995). Gold in both of these deposits also formed as paint and sub-circular nuggets along fracture planes in conglomerate clasts (Day, 1995). A peculiar features of the Permian deposits is the presence of parent nuggets, that have thin wing-like overgrowths of gold on their surface. Dunstan (1902) noted that elevated gold grades at Black Ridge were associated manganese dioxide coated clasts of quartz and quartzite, commonly referred to as pilot stones. 175

202 5.2 Previous Work Gold Miclere Miclere Basin 5 km Black Johnsons Basin Black Ridge N Springs Basin Blair Athol Basin Wolfgang Basin Clermont Basin Clermont V V V V V Karin Basin Tertiary basalt/black soil Permian sediments Anakie Metamorphic Group Fault v Theresa Creek Volcanics (Devonian) Gold mining site Sunny Park Granodiorite (Devonian) Figure 5.2: Simplified geologic map showing the location of Permian basins in the Clermont goldfield. The basins are situated along a northwest axis that is tectonically aligned with the Denison Trough. After Zhou et al. (1994). 176

203 Gold 5.3 This Study Previous Ore Deposit Models Ball (1906) carried out the first detailed studies of gold in Permian basins, and concluded that gold was introduced after deposition of the basal conglomerate layer. Gold precipitated within a zone of mixing between rising auriferous fluids, and fluids circulating in the conglomerate close to the unconformity (Ball, 1906). Detailed studies of fluid geochemistry, alteration assemblages and vein systems were undertaken by Zhou et al. (1994) and Zhou (1995) at the Black Ridge deposit. Zhou (1995) recognized that elevated gold grades occur where steep northwest, and north-northeast trending faults cut the unconformity, bonanza grades occur where these faults sets intersect. Zhou (1995) concluded a hydrothermal origin for mineralisation at Black Ridge, related to igneous or tectonic activity after Permian sedimentation. Zhou (1995) suggested a fluid mixing model at the unconformity between auriferous hydrothermal fluid, and fluid percolating through the Permian sediments. Palaeoplacer models were proposed by Reid (1936), Veevers et al. (1964), Murray (1975) and I ons (1983), however Day (1995) noted that early miners had pursued and excavated classic alluvial traps, only to find the sedimentary structures completely devoid of gold. Day (1995) proposed a model in which extensive coal deposits in overlying sediments induced a chemical cell in fluids percolating through the stratigraphic pile. The presence of dissolved humic/fulvic acid, tannins, sulphur complexes and organic compounds in groundwater could allow remobilisation, transportation and concentration of gold over a long period of time. Day (1995) found that this model does not preclude an alluvial source for the gold, nor is it incompatible with hot ascending fluid from below. Day (1995) suggested that rising fluids may have established a convection cell in the solute-rich groundwater above. 5.3 This Study Structurally Controlled Gold: The Oaky Creek Antiform area The Oaky Creek Antiform west of Clermont (Figure 5.1, Appendix 1) formed as a pop-up structure during D4 deformation, and was overprinted by folds, faults and shear zones during D5 and D6 deformation (see Chapter 2 for details). There is a strong spatial correlation of gold mineralisation with greenstones and shear zones that occur along the limbs of the Oaky Creek 177

204 5.3 This Study Gold Antiform (Appendix 1). This section explores the links between gold bearing structures, and the deformation events that may have controlled their formation D4 Shear Zone Hosted Gold There is a strong correlation between lode gold deposits and D4 shear zones along the limbs of the Oaky Creek Antiform (Appendix 1). Lode gold in D4 shear zones occurs at bends in the shear zones, where they intersect greenstone units, and where they are cut by D5 and D6 structural corridors (Appendix 1). Structurally, the D4 shear zones host the earliest known gold mineralisation in the region. D4 shear zones have strike orientations of east and northeast. Previously identified lode gold deposits that have these orientation, and which occur along D4 shear zones are Wee Five and Gold Finger. Observations around the previously identified lode gold deposits were consistent with those of previous studies. D4 shear zones contain lode gold in siliceous bodies, hosted in strongly altered and silicified country rock. The siliceous bodies outcropped over 10 s of metres, however alteration of country rock persisted up to 100 m away. The siliceous bodies were strongly weathered, and negative crystal structures after sulphides were common. Proximal to siliceous bodies were carbonate veins up to 15 cm wide, and parallel to the D4 shear zone fabric (Figure 5.3). In one locality, a diorite dyke (feldspar-hornblende porphyry) intruded into country rock adjacent to a siliceous body (Figure 5.4). Mapping revealed no preferred structural height along the D4 shear zones for gold lodes to concentrate. Gold (and copper) is also concentrated along or proximal to the D4 shear zones where they intersect with high angle fault zones and structural corridors. These are discussed below D5 Structural Corridors In the Oaky Creek Antiform area, two southeast trending D5 structural corridors were identified (Appendix 1). The corridors are zones of concentrated deformation, characterised by F5 folding that grades to shearing and bleaching of the metamorphic rocks (Section ). Where the D5 corridors intersect D4 shear zones, quartz veins similar to gold bearing structures described by Lam and Garrad (1993) occur. The quartz veins were up to 1 m wide, and usually had an en echelon geometry (Figure 5.5). Wallrock adjacent to the quartz veins was bleached up to several metres away, and contained multiple smaller quartz and carbonate veinlets up to 178

205 Gold 5.3 This Study carbonate vein parallel to S4 siliceous and carbonate alteration S4 Figure 5.3: Carbonate veins and siliceous alteration in a D4 shear zone. 179

206 5.3 This Study Gold dioritic, feldspar-hornblende porphyry dyke 1 m (a) feldspar hornblende (b) Figure 5.4: (a) Dioritic hornblende-feldspar pophyry dyke in a zone of siliceous alteration. (b) Close up of the dyke. 180

207 Gold 5.3 This Study several centimetres wide. Strongly weathered sulphides were present within the large quartz veins, and concentrated within the laminated area at vein margins. Negative crystal imprints were up to 5 mm wide, however none could be properly identified. No previously identified lode gold deposits were located within D5 structural corridors, although the K-2 prospect is along strike of the eastern D5 corridor (Appendix 1). S4 en echelon quartz vein bleached and sheared greenstone Figure 5.5: En echelon style quartz vein at the intersection of a D4 shear zone with a D5 structural corridor D5 Faults Several discrete fault zones were identified in far east of the Oaky Creek Antiform area, adjacent to the contact between metamorphic rocks and the Sunny Park Granodiorite (Appendix 1). The two faults are described in Section The two faults are characterised by bleaching and alteration of the country rock, and strong grainsize reduction. Narrow quartz veinlets occurred parallel to a fine-grained shear fabric, in association with weathered sulphides. These features are similar to those described by Lam and Garrad (1993) for gold prospects nearby. The along strike projection of the two faults intersect with known mineralised structures. Projection of the north-northwest trending fault intersects the Talc Zone copper prospect (Appendix 1), and 181

208 5.3 This Study Gold projection of the northwest trending fault coincides with the Leo Grande Fault that contains the Leo Grande prospect (Appendix 1) D6 Structural Corridor A single D6 structural corridor was identified during mapping in the Oaky Creek Antiform area, and is described in Section The D6 corridor is an east-west feature located adjacent to the change in trend of the Oaky Creek Antiform (Appendix 1). This corridor contains a previously identified gold prospect (K-2), and is approximately along strike from several other gold localities located at the change in trend of the the Oaky Creek Antiform (Appendix 1). At the western end of the corridor, shearing and bleaching of the country rocks has occurred, as well as zones of silicification and quartz veining (Section ). The quartz veins observed were similar to those described by Lam and Garrad (1993) as gold bearing. The western part of the D6 corridor intersects with the along strike projection of a northeast trending D4 shear zone (Appendix 1) Permian Conglomerate Hosted Gold: New Observations A conglomerate that forms the basal unit to Permian basins north of Clermont, and known to be gold bearing, was observed at the Black Ridge and Miclere mine camps (Figure 5.1, 5.2). At Black Ridge, the basal conglomerate was up to 10 m thick, and comprised a cobble to boulder conglomerate (Figure 5.6) that is described in detail in Withnall et al. (1995). Haematite and manganese staining occurred in the bottom 50 cm of the conglomerate (Figure 5.6), and also as stratabound alteration in coarse conglomerate layers higher in the sequence. At Black Ridge, the unconformity between Permian sediments and the underlying metamorphics was at a low angle, and parallel to S3 in the underlying rocks (Figure 5.6). At Miclere, access to a small mine site was possible. Here the unconformity surface was variable in orientation, so too was the orientation of S3 in the underlying metamorphics (Figure 5.7). The mine was located on a series of northwest trending normal faults, and northeast trending transfer faults, both sets of faults transected the unconformity. Movement along the normal faults had resulted in wedge-shaped sedimentary infill in some down-thrown blocks. In some places the normal faults had been reactivated as reverse faults (Figure 5.8). Transfer faults formed discrete surfaces and fractures from several mm up to 30 cm wide (Figure 5.9). 182

209 Gold 5.3 This Study Haematite alteration occurred variably in the basal conglomerate, however no control on its distribution or intensity was observed. Numerous vertical fractures oriented parallel to transfer faults were present, most had little or no displacement. The fractures contained quartz-siderite veins and marcasite in association with gold, and were enveloped by oxidised carbonate alteration of surrounding wallrock rock (Figure 5.9). Quartz-siderite veins and quartz-gold specimens in the vertical fractures were concentrated in the first 50 cm of basal conglomerate, although the fractures extended for several metres above the unconformity. Current and historic mining at Miclere was focussed on the northeast trending vertical fractures. Several types of gold were observed in the Miclere locality, and included water-worn nuggets, hydrothermal gold-quartz specimens with accessory siderite and marcasite, and nodular gold intergrown with marcasite (Figure 5.10). All of the gold observed was from the basal conglomerate in the first 50 cm above the unconformity. The hydrothermal gold-quartz specimens were from a transfer fracture that cut both the conglomerate and the unconformity. A centimetre sized, water-worn gold nugget recovered from the basal conglomerate horizon at Miclere was able to be studied under a scanning electron microscope (SEM). The results of the analysis indicate secondary growth of gold associated with haematite on the eroded surface of the nugget (Figure 5.11). The overgrowths occur as clusters of small rounded nodules from 5 µm to 30 µm wide, their texture is similar to gold that is intergrown with marcasite (Figure 5.10 (c)). 183

210 5.3 This Study Gold haematite staining/alteration alteration front (a) unconformity S3 (b) Figure 5.6: (a) Basal Permian conglomerate immediately above the unconformity at Black Ridge. Haematite alteration is visible at the bottom of the image. (b) The Permian unconformity surface, S3 in the underlying metamorphics is subparallel to the unconformity. 184

211 Gold 5.3 This Study Permian sediments Anakie Metamorphic Group S3 Figure 5.7: Permian unconformity surface at Miclere. Anakie Metamorphic Group Permian sediments Figure 5.8: Reverse fault at Miclere. Rocks of the Anakie Metamorphic Group have been thrust over Permian sediments. 185

212 5.3 This Study Gold oxidised siderite alteration transfer fracture The fracture has negligible offset, and contains quartz- Figure 5.9: Transfer fault/fracture at Miclere. siderite(marcasite) veinlets. 186

213 Gold 5.3 This Study (a) marcasite angular gold `paint quartz siderite (b) 187

214 5.3 This Study Gold marcasite (grey) gold nodules (c) Figure 5.10: Styles of gold at Miclere. (a) Water-worn gold nugget from the basal conglomerate at Miclere. (b) Hydrothermal gold-quartz specimen from a transfer fracture transecting the unconformity. This specimen was found approximately 30 cm above the unconformity, in association with quartz-siderite veining and marcasite. (c) Intergrown marcasite and gold. 188

215 Gold 5.3 This Study (a) (b) Figure 5.11: (a) Scanning electron microscope image of a centimetre-size gold nugget from the basal Permian conglomerate, note the embayment containing dark material at the top of the nugget. (b) Close up of the embayment, secondary overgrowth of microscopic gold nodules is associated with haematite. 189

216 5.4 Discussion Gold 5.4 Discussion Gold in the Oaky Creek Antiform area The results of mapping around the Oaky Creek Antiform during this study indicate a strong structural control on early gold mineralisation in the Anakie Metamorphic Group. Observations suggest that the previously recognised gold occurrences are located within structures that formed during one or more regional deformation events. The exact timing of gold mineralisation with respect to formation of the host structures is unconstrained. However, the different styles of mineralisation in different generation structures suggests that gold was emplaced synchronously with deformation. The structural sites that contain gold in the Oaky Creek Antiform area west of Clermont are: Northeast and east-west trending D4 shear zones along the limbs of the Oaky Creek Antiform. Gold occurs in siliceous bodies, associated with carbonate veins at bends in the shear zones, and at intersections with greenstone units. Northwest trending D5 structural corridors that intersect the Oaky Creek Antiform at high angle. Gold occurs in quartz veins at their intersection with D4 shear zones. Northwest and north-northwest trending D5 faults that intersect the Oaky Creek Antiform at high angle. These faults correlate with known mineralised structures. An east-west trending D6 structural corridor. Gold occurs in quartz veins in a zone of shearing that could be an intersection with a D4 shear zone. The D5 fault zones outcrop poorly in the Oaky Creek Antiform area, and were not studied in any detail in the field. Thus discussion will focus on D4 shear zones and D5 and D6 structural corridors. A feature common to most occurrences of gold in the Oaky Creek Antiform area is the presence of, or intersection with, a D4 shear zone. The D4 shear zones themselves host gold in siliceous bodies, which is different to quartz vein hosted lode-gold in D5 and D6 structural corridors. The difference in mineralisation styles between D4 and D5/D6 sites may reflect a slight difference in metamorphic grade, D4 being higher. D4 formed an S4 crenulation cleavage defined by dissolution seams, while S5 and S6 foliations are defined by realignment of platy minerals only, perhaps suggesting lower grade conditions during D5 and D6. Alternatively, fluids present at the time of deformation may have had different characteristics. 190

217 Gold 5.4 Discussion The occurrence of gold along D4 shear zones, and at intersections with D5 and D6 structural corridors, suggests that D4 shear zones provide the primary control on gold distribution. A genetic relationship is suggested between gold formed in D4 shear zones, and gold formed in D5 and D6 sites. Gold in quartz veins in D5 and D6 structural corridors is likely to have been sourced from D4 shear zones, concentrating at the intersections of these structures. The D5 and D6 structures may have caused a degree of upgrade during remobilisation, evidenced by visible gold occurring in quartz veinlets (Lam and Garrad, 1993). Visible gold and quartz veins are not known to be associated with the siliceous bodies that are restricted to within D4 shear zones Age of Mineralisation D4 shear zones host the oldest known gold mineralisation in the Oaky Creek Antiform area. The age of D4 deformation in the southern Anakie Inlier has been constrained by SHRIMP U-Pb dating during this study at ca 443 Ma (Chapter 4). The age of D5 and D6 deformation is not exactly known, but is constrained to between ca 443 Ma and ca 392 Ma (Section 4.4.3) Permian Conglomerate Hosted Gold. The different textures and morphology, as well as the different mineral associations of gold in Permian conglomerate indicates that gold in this setting can be divided into two groups: gold that formed prior to deposition of the conglomerate, and gold that formed after deposition. Gold Prior to Deposition The water-worn texture of gold nuggets (Figure 5.10 (a)) in the conglomerate suggests that they were formed distal to the Miclere Basin, and deposited as alluvial placer gold. Gold of this form has also been reported from false bottoms in conglomerate horizons higher up in the sequence (Withnall et al., 1995). The prevalence of schist and quartzite clasts of the Anakie Metamorphic in the Permian conglomerate suggests that water-worn nuggets were sourced from the Anakie Metamorphic Group. The exact source of conglomerate is unknown, but is thought to have been shed from the nearby Drummond Range to the northwest (Withnall et al., 1995). It may be that structurally controlled lode gold in quartz veins was eroded and deposited in the Permian basins, implying a genetic relationship between the two styles of gold. 191

218 5.4 Discussion Gold Gold After Deposition The morphology and mineral associations of gold in gold-quartz specimens, and intergrown marcasite-gold pieces, suggests formation after deposition of the conglomerate. Gold-quartz specimens with accessory siderite and marcasite (Figure 5.10 (b)), occur in fractures that were formed during ongoing deformation of the Permian basins. This style of gold has been studied by Zhou et al. (1994) and Zhou (1995), who concluded that hydrothermal fluid rising from the metamorphics mixed with fluid in the Permian sediments, resulting in a four order magnitude drop in the solubility of gold, which mineralised close to the unconformity. Vertical fractures that transect the unconformity have formed conduits that focus fluid flow, resulting in rich pockets of gold mineralisation. A paragenesis of alteration minerals in the Miclere Basin (Zhou et al., 1994), shows that marcasite growth is restricted to the interval when gold mineralisation occurs. This suggests that the marcasite-gold intergrowths (Figure 5.10 (c)) and gold-quartz specimens with accessory siderite and marcasite were formed at the same time. However, the textural difference between the two styles of later gold suggests mineralisation under different conditions. The exact location of intergrown marcasite-gold mineralisation is unknown, but is constrained to have formed in the first 50 cm of conglomerate above the unconformity. Marcasite-gold intergrowths were not observed within fractures, and are interpreted to have formed in a static environment close to the unconformity. The habit and morphology of gold in fractures associated with quartzsideritemarcasite suggests a more dynamic environment, with flux of silica-carbonate rich fluids probably sourced from below the unconformity. The morphology of gold nodules intergrown with marcasite is similar to secondary gold nodules intergrown with haematite on water-worn gold nuggets. Growth of marcasite and haematite is restricted to the interval when gold mineralisation occurs in the Miclere Basin (Zhou et al., 1994). Therefore, secondary gold overgrowths on waterworn nuggest are interpreted to have formed coevally with marcasite-gold intergrowths, and with gold-quartz specimens. Ore Deposit Model An ore deposit model for gold in Permian conglomerate can be interpreted using observations from this and previous studies. Figure 5.12 shows the interpreted styles of gold that occurs at the base of Permian basins in the Clermont region. A classic palaeoplacer model (Reid, 1936; Veevers et al., 1964; Murray, 1975; I ons, 1983) can explain the presence of water-worn 192

219 Gold 5.5 Summary nuggets in the basal conglomerate horizon. A deformation-related, hydrothermal fluid mixing model interpreted by Zhou et al. (1994) and Zhou (1995) accounts for gold-quartz specimens in quartz-siderite (marcasite) veins in fractures that cut the unconformity. Fluid mixing at the unconformity may have caused a change in fluid chemistry, resulting in crystallisation of fracture hosted gold close to the unconformity. The model of Day (1995) with a chemical cell in percolating fluids driven by thick overlying coal deposits may be the cause of secondary gold concentrating along the unconformity. A strong redox gradient across the unconformity surface where ascending fluids meets basin fluids can explain concentrated mineralisation along the contact. Day (1995) also suggested that gold could be remobilised, transported and concentrated over a long period of time via dissolved humic/fulvic acid, tannins, sulphur complexes and organic compounds. Gold deposited from solution by these agents may have formed the marcasite-gold intergrowths and secondary gold overgrowths on water-worn nuggets, coevally with fracture hosted gold. 5.5 Summary The structural controls on lode gold deposits in the Oaky Creek Antiform area were previously not known. Previous workers (Ball, 1906; Lam and Garrad, 1993; Morton, 1933; Shepherd, 1952; Lam, 2005) described shear zone hosted lode gold deposits that have strike orientations ranging from east, northeast and northwest, however no interpretation was made as to the reason for these orientations. This study has provided a structural framework for controls on the earliest gold occurrences, and can be used for predictive discovery and potential future exploration in the Oaky Creek Antiform area. There is a spatial correlation between gold and the intersection of D4 shear zones with D5 and D6 structural corridors in the Oaky Creek Antiform area. It is suggested that gold in D5 and D6 structural corridors is remobilised from lodes in the D4 shear zones, with a possible upgrade in gold concentration between the younger and older structural sites. Appendix 1 has several localities marked that have potential for gold mineralisation, based on the structural relationships observed. These sites have been outlined from a combination of direct observations, as well as interpolation of structural features. Gold in Permian conglomerate occurs in a minimum of three different styles, all are concentrated within 50 cm above the unconformity with the Anakie Metamorphic Group. Water-worn 193

220 5.5 Summary Gold water-worn nuggets hydrothermal gold in quartz - siderite and marcasite carbonate alteration zone around fracture false bottom fluid circulating through Permian sediments Permian sediments basal Permian conglomerate ascending auriferous fluid Anakie Metamorphic Group marcasite intergrown with gold boudinaged quartz vein 50 cm concentrated fluid flow S3 Figure 5.12: Interpreted styles of gold that occurs at the base of Permian basins in the Clermont region. 194

221 Gold 5.6 Conclusions gold nuggets are deposited as placer gold in conglomerate, hydrothermal gold occurs in fractures across the unconformity, and intergrown marcasite and gold has grown in situ in the conglomerate. The placer style gold may have been sourced from eroded lode gold deposits in the Anakie Metamorphic Group. The source of the secondary and hydrothermal gold in conglomerate has not been addressed, but is likely to be from the Anakie Metamorphic Group as well. 5.6 Conclusions A minimum of five different styles of gold were identified in the Clermont region during this study, not including Tertiary and recent alluvial deposits. The styles of gold are summarised here in order from oldest to youngest: Siliceous lodes in D4 shear zones in the Oaky Creek Antiform area, with pyrite, arsenopyrite, galena and sphalerite as accessory minerals. The lodes form at intersections with greenstone layers in the Bathampton Metamorphics, and at jogs or bends in the shear zones. The age of mineralisation is interpreted to be ca 443 Ma, which is the age of regional D4 deformation. Gold in quartz veins in D5 and D6 structural corridors in the Oaky Creek Antiform area, concentrated at intersections with D4 shear zones. The age of mineralisation is interpreted to be between ca 443 Ma and ca 392 Ma. Water-worn gold nuggets in the basal conglomerate unit in Permian basins. Gold is concentrated in the first 20 to 50 cm above the unconformity, however has also been found in false bottoms within the sequence. Microscopic secondary overgrowths of younger gold associated with haematite formed on the surface of the water-worn nuggets. These nodules probably grew out of solution at the same time as hydrothermal gold was forming nearby. Hydrothermal gold associated with quartz-siderite veinlets and marcasite. The gold is in northeast trending transfer fault and fracture systems in conglomerate at the base of Permian basins. The transfers structures cut the unconformity, but usually have little to negligible offset. 195

222 5.6 Conclusions Gold Gold intergrown with marcasite occurs in the basal Permian conglomerate, close to the unconformity. Texturally the gold is similar to secondary overgrowths on water-worn nuggets, and its association with marcasite suggests it grew coevally with hydrothermal gold in transfer structures. 196

223 Chapter 6 Synthesis This chapter is a synthesis of results that were presented in previous chapters, and will focus primarily on the Anakie Metamorphic Group in the Clermont region. A tectonic model is interpreted for Queensland during the Neoproterozoic to mid-palaeozoic, based on work from this study, and from studies of relevant rocks elsewhere in Queensland and eastern Australia. Discussions on the tectonics and geochemical signatures of subduction margins with Simon Richards and Ivo Vos are acknowledged. 6.1 Introduction The work in this study was aimed at developing a better understanding of the tectonic processes that operated in Queensland during the late Neoproterozoic to Early Devonian, and focussed on the interval Ma. Chapter 2 examined the deformation and metamorphic history of the Anakie Metamorphic Group, and focussed on understanding the nature of a pervasive flatlying foliation in the Clermont region. Chapter 3 presented results of 40 Ar/ 39 Ar thermochronology, which constrained the timing of early deformation and metamorphism. SHRIMP U-Pb geochronology in Chapter 4 constrained the timing of regional D4 deformation, and the age of the earliest known gold mineralisation. Chapter 5 explored the different styles of gold that occur 197

224 6.2 History of the Anakie Metamorphic Group Synthesis in the Clermont region, and outlined structural controls on the earliest known gold mineralisation. In this chapter, the interpreted history of the Anakie Metamorphic Group will be compared with similar age and style metamorphic complexes elsewhere in Queensland. Using correlations between the different metamorphic complexes, along with direct age and other data from the Anakie Metamorphic Group, a structural evolution can be interpreted for the Late Neoproterozoic - Early Palaeozoic in Queensland. The results of this evolution will be used in conjunction with evidence from other relevant rocks in eastern Australia for the interpretation of a tectonic model. 6.2 History of the Anakie Metamorphic Group Deposition of the proto- Anakie Metamorphic Group The age of deposition of sediments that comprise the Anakie Metamorphic Group is constrained to the latest Neoproterozoic to Middle Cambrian (Figure 6.2) for the Bathampton Metamorphics (lowest stratigraphic level), and Middle Cambrian for the Wynyard Metamorphics (Fergusson et al., 2001). The youngest age for deposition is constrained by a 510 Ma detrital zircon in the Wynyard Metamorphics (Fergusson et al., 2001). The tectonic setting for deposition of the proto- Anakie Metamorphic Group is important for understanding the early deformation history, which occurs soon after deposition ceased (Section 1.3.1). The tectonic setting for deposition of sediments that comprise the Anakie Metamorphic Group has previously been interpreted as a passive margin (Withnall et al., 1995), however, evidence from the geochemistry of mafic rocks in the Bathampton Metamorphics suggests otherwise. In order to constrain the tectonic setting during deposition, geochemical trends from mafics in the Bathampton Metamorphic have been compared with those from known tectonic settings. 198

225 Synthesis 6.2 History of the Anakie Metamorphic Group Mafic Rocks as Indicators of Tectonic Setting: results from the Anakie Metamorphic Group Mafic rocks formed in different tectonic settings have distinct trace element patterns relative to normal mid-oceanic ridge basalt (N-MORB) (Pearce and Parkinson, 1993; Sun and McDonough, 1989), therefore they can be used in conjunction with other geological evidence to interpret the tectonic setting at the time of generation and emplacement. Generation of mafic magma is also known to occur under conditions of lithospheric extension (McKenzie and Bickle, 1988; Collins, 2002b), restricting their formation to specific environments, the exception being mafics produced by hotspot volcanism. The tectonic setting of actively depositing sediments of the Anakie Metamorphic Group can be interpreted by using the trace element patterns of intercalated mafic rocks. Figure 6.1 shows representative trace element patterns of mafic rocks formed in different tectonic settings, overlain with element fields from mafics in the Anakie Metamorphic Group. The laminated greenstone unit forms sills and volcanic horizons in the Bathampton Metamorphics, in the Oaky Creek Antiform area (Figure1.5, Appendix 1). The Yan Can Greenstone Member is a mafic lava from the Bathampton Metamorphics, immediately north of the Oaky Creek Antiform (Figure 1.5) Volcanic arc basalts (VAB) typically exhibit spiked patterns of high (enriched) but variable large-ion lithophile (LILE; Rb, Ba, Th, K, Sr) and rare earth elements (REE; La, Ce), but low (depleted) concentrations of high field strength elements (HFSE; Nb, Zr, Ti, Y) that are similar to N-MORB values (Collins, 2002b). The chemical decoupling of enriched and depleted components observed in VAB relative to N-MORB reflects the multi-component source origin for VAB. LILE and REE patterns reflect a slab-derived fluid source component, while the depleted HFSE reflects input from the asthenosphere (Collins, 2002b, and references therein). Trace element patterns of back arc basin basalts (BABB) are usually intermediate between N-MORB and VAB, also reflecting a multi-component source origin. Usually, BABB have lower LILE but higher HFSE than arc basalts, indicating a lower concentration of slab-derived fluid and melting of more fertile asthenoshphere compared to that below volcanic arcs (Collins, 2002b). The steepest trends come from basalts sourced from continental lithosphere, and Figure 6.1 gives an example from the Colorado Plateau (Thompson et al., 1990). Mafic rocks from a tectonic setting involving subduction commonly have patterns of selective LILE enrichment, as well as Nb depletion (Gamble et al., 1993; Pearce and Parkinson, 1993). Analysis of the laminated greenstone and the Yan Can Greenstone Member reveals se- 199

226 6.2 History of the Anakie Metamorphic Group Synthesis Sample/N-Type MORB MORB OIB CFB IAB 0.01 Rb Ba Th U K Nb La Ce Sr P Zr Ti Y Sc Cr Ni Colorado Plateau Western Lau Basin Laminated greenstone, Clermont region (a) Sample/N-Type MORB MORB OIB CFB IAB 0.01 Rb Ba Th U K Nb La Ce Sr P Zr Ti Y Sc Cr Ni Colorado Plateau Western Lau Basin Yan Can Greenstone Member, Clermont region (b) 200

227 Synthesis 6.2 History of the Anakie Metamorphic Group Figure 6.1: (Modified from (Collins, 2002b)). N-MORB normalised trace element patterns of mafic rocks from various tectonic settings, showing characteristic chemical trends. OIB = ocean island basalt, CFB = continental flood basalt, IAB = island arc basalts (equivalent to volcanic arc basalts, VAB). Colorado Plateau basalts represent melts from subcontinental lithospheric mantle, all others reflect different degrees of asthenospheric melting. N-MORB, OIB and CFB data from Sun and McDonough (1989), IAB data from Pearce et al. (1994), Western Lau Basin data from Hawkins (1994). (a) Comparison with trace element field of laminated greenstones from the Bathampton Metamorphics west of Clermont, sampled around the Oaky Creek Antiform (data from Withnall et al. (1995)), see text for details. (b) Comparison with trace element field of the Yan Can Greenstone Member from the Bathampton Metamorphics west of Clermont, sampled immediately north of the Oaky Creek Antiform (data from Withnall et al. (1995)), see text for details. lective enrichment in large ion lithophile elements (LILE), and a trough in Nb (Figure 6.1). Withnall et al. (1995) suggested these mafics were continental or ocean-island tholeiites, or possibly even back arc basalts, and interpreted crustal contamination as the cause for selective enrichment in large ion lithophile elements (LILE) and a trough in Nb. An alternative explanation for this pattern is enrichment of selected LILE from a slab derived fluid, with an asthenospheric melting component resulting in flat HFSE trends (Figure 6.1). Laminated greenstones from the Bathampton Metamorphics have relatively flat chemical patterns, and are very similar to BABB erupted in the Western Lau Basin Figure 6.1(a), which formed above thinned continental crust (Hawkins, 1994). The pattern of LILE enrichment suggests some component of slab derived fluid input, however, compared to VAB this enrichment is not as strong. Additionally, the pattern of LILE in the laminated greenstones is not as variable as that of VAB, suggesting the slab derived component is not as large. The Yan Can Greenstone Member shows a greater enrichment in LILE, and similar Nb depletion compared to the laminated greenstone. The chemical trends of the Yan Can Greenstone are more like VAB (Figure 6.1(b)), and may reflect a greater input of slab derived fluids during magma genesis. The Yan Can Greenstone Member is located stratigraphically above the laminated greenstone in the Bathampton Metamorphics, while the laminated greenstone units represent the lowest stratigraphic level in the Clermont region. The pattern of increasing LILE enrichment from the laminated greenstone to the Yan Can Greenstone Member suggests an increasing influence of slab derived fluids, consistent with an evolving subduction margin. Similar trends have been attributed to crustal contamination of the mafics in other areas (Saunders and Tarney, 1984; Cox and Hawkesworth, 1985), however the overall chemical trends are most like those found in continental back arcs (Hawkins, 1994; Collins, 2002b). The change in trace element patterns between the two types of mafic rocks in the Clermont region is interpreted to 201

228 6.2 History of the Anakie Metamorphic Group Synthesis reflect a change in tectonic setting during deposition, from a passive margin to continental back arc. More simply, the area of extending continental crust in a supra-subduction zone setting is progressively more affected by slab-derived components Inferred Deformation and Metamorphism D1 Deformation A D1 event is inferred to have affected the Anakie Metamorphic Group, based on the characteristics of S2, and evidence of early HT/LP metamorphsim. Sections and show that the characteristics of S2 imply existence of S1. Passchier and Trouw (1998, page 70) show that a well developed fabric (eg. S2) generally requires the presence of an earlier formed fabric (S1), usually at a high angle to the second fabric. Based on the observation that S2 has a consistent upright orientation (Figure 2.8), S1 is interpreted to have formed in a subhorizontal orientation. The available geochronology constrains D2 and D3 to between ca 510 Ma and ca 483 Ma (Chapter 3), and given the lack of any further age constraints, D1 may have also occurred in this time interval. M1 Metamorphism The growth of andalusite porphyroblasts (Section 2.5.3) is interpreted to be a result of high- T/low-P conditions during M1, and is constrained to pre- ca 510 Ma (Chapter 3, Figure 6.2). Evidence of M1 is found only in the Eastern Creek area, which contains the highest metamorphic grade rocks of the Anakie Metamorphic Group (Section 2.3.3). High-T/low-P metamorphism is typical of conditions in an extending lithosphere (Richards and Collins, 2004; Richards, 2004), and supports an extensional setting during deposition of the Anakie Metamorphic Group. 202

229 Synthesis 6.2 History of the Anakie Metamorphic Group CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN CARBONIFEROUS PERMIAN Age Ma Deposition Miclere and Blair Athol Basins Foss.? Zr U-Pb K-Ar Silver Hills Volcanics Theresa Ck Volcanics Fork Lagoons Beds Foss.? Mz, Zr Anakie U-Pb Metamorphic Group Magmatism Metamorphism Deformation Mineralisation K-Ar Ma Retreat Batholith Zr U-Pb 388.9±3.3 Ma Gem Park Granite Mz U-Pb 441.9±6.2 Ma Upright folding in Fork Lagoons Beds. Juxtaposition of Fork Lagoons Beds against Anakie Metamorphic Group K-Ar M4 Rubyvale region only Ar-Ar M3 M2? M1? N-S D6 upright folding, faulting NE-SW D5 upright folding, faulting NW-SE D4 upright folding, faulting NE-SW D3 E-W D2? D1? (flat-lying foliation?) alluvial Au hydrothermal Permian conglomerate hosted Au qtz vein Au qtz vein Au SZ Au Au Au Au Cu Cu flat-lying foliation, recumbent folding and shearing upright isoclinal folding, differentiated foliation deposition metamorphism cooling felsic magmatism intermediate magmatism mafic magmatism intrusion U-Pb Ar-Ar K-Ar Foss. M - monazite Zr - zircon Musc - muscovite Bt - biotite fossil age age constraints Au NW-SE mineralisation movement orientation extension shortening Figure 6.2: Summary of events to affect the southern Anakie Inlier. Geochronological constraints are sourced from this study, and from Henderson et al. (1998), Fergusson et al. (2001), Withnall et al. (1995) and Withnall et al. (1996). 203

230 6.2 History of the Anakie Metamorphic Group Synthesis Early Deformation and Metamorphism D2 and M2 D2 (and D3) is constrained to between ca 510 Ma and ca 483 Ma (Chapter 3, Figure 6.2). The available evidence suggests that D2 involved east-west shortening, forming upright isoclinal folds and a north-south oriented, differentiated S2 foliation in the Clermont region (Section ). Based on overprinting geometries (Section ), D2 is interpreted to have formed north-south trending upright folds (Section 2.5.1). M2 reached amphibolite facies conditions, and is characterised by the assemblage staurolitegarnetbiotitemuscovite (Section 2.5.3), with indicated temperatures of C (Figure 2.61). M2 overlaps with D2, and porphyroblast-fabric relationships indicates metamorphism occurred late syn- to post-d2 (Section 2.3.3, Figure 6.2)). Pressure estimates during M2 are poorly constrained, but potentially range between 3-16 kbar (Figure 2.61), however there is no evidence of such high pressures having been reached. An overall increase in pressure is interpreted during M1 to M2 (Figure 2.61), which is interpreted to reflect burial during D2. The timing, style and orientation of D2 deformation, and conditions of M2 metamorphism are similar to that in the Kanmantoo and Adelaide Fold Belts, the Willyama and Wonominta Blocks, parts of western Tasmania, and the Transantarctic Mountains (Withnall et al., 1996; Boger and Miller, 2004). All of these areas were affected by the Delamerian Orogeny at ca Ma along the eastern Gondwana margin. D3 and M3 Deformation during D3 resulted in the reorientation of upright D2 structures around flatlying axes, contemporaneous with retrograde M3 metamorphism (Section , ). D3 involved early flattening (pure shear), followed by top to the northeast shearing (simple shear, Section ). A pervasive flat-lying S3 crenulation cleavage is the dominant foliation in the Clermont region, and has been reoriented around younger structures (Section ). Conditions of M3 are constrained to C and pressures of 3-6 kbar, indicating that a drop in temperature of up to 270 C occurred between M2 and M3 (Section ). The slope of the path between M2 and M3 is not well constrained, however the relative positions of the two P-T fields results in most paths having a negative gradient (Figure 2.61), consistent with a drop in pressure. 204

231 Synthesis 6.2 History of the Anakie Metamorphic Group The contrast in structural geometry between D2 and D3 structures, along with the component of pure shear and drop in pressure between the two concomittant metamorphic events can be used as evidence for extension during D3. However, the structural and metamorphic relationships alone do not provide unequivocal evidence for extension. The nature and implications of the style of deformation during D2 and D3 are discussed in more detail in Section 6.3.2, with respect to larger scale relationships Late Deformation D4 D4 produced primarily northeast trending structures, including the major Oaky Creek Antiform in the Clermont region (Section 2.59). In the Rubyvale region, the Gem Park Granite is spatially associated with a fault that is interpreted to have formed during D4. The Gem Park Granite is elongate parallel to S4, and the growth and alignment of biotitemuscovitequartz along S4 planes in the Gem Park Granite suggests it was emplaced syn-d4 (Chapter 4). The Gem Park Granite has a monazite 206 Pb/ 238 U age of ± 6.2 Ma, which is interpreted to be an absolute age of regional D4 deformation. D4 also marked a change from ductile to more brittle conditions, and in the Clermont region is not associated with metamorphic mineral growth (Section 2.59). In the Rubyvale region however, D4 is associated with ductile deformation (Section ). D4 is associated with the first known gold mineralisation in the Clermont region, and will be discussed in more detail in Section 6.4. D5 D5 produced northwest to north-northwest trending structures, focussed in corridors of concentrated deformation around the Oaky Creek Antiform (Section , Appendix 1). D5 is interpreted to have resulted in the dominant north-northwest strike trend of S3 in the Clermont region. D5 is characterised by low-grade conditions, and did not have associated metamorphic mineral growth. Brittle faults formed during D5 add complexity to metamorphic and structural trends in the Clermont region. D5 faults are also host to lode gold, concentrated at their intersection with D4 shear zones (Section ). 205

232 6.3 Neoproterozoic - Early Palaeozoic Tectonics Synthesis D6 D6 is manifest as east-west trending crenulations across the Clermont region, although an east-west oriented structural corridor in the Oaky Creek Antiform area contains concentrated F6 folding, grading to shearing (Section , Appendix 1). Gold occurs where the D6 structural corridor is interpreted to intersect a D4 shear zone (Section ). The age of D5 and D6 is constrained to between ca 443 and ca 392 Ma (Section 4.4.3). 6.3 Neoproterozoic - Early Palaeozoic Tectonics The tectonics of north-eastern Australia in the late Neoproterozoic - early Palaeozoic are not well understood. This section uses large-scale relationships to provide tectonic context for results from the Anakie Inlier. Evidence from the Anakie Inlier, and from equivalent terranes elsewhere in Queensland (Figure 6.3) are used for the interpretation of tectonic processes operating at that time. A combination of structural, metamorphic, sedimentological, geochronologic and geochemical arguments are used to interpret a tectonic model of the late Neoproterozoic - early Palaeozoic evolution of the northeast Australian margin. The time frame discussed here spans from ca 560 Ma to ca 440 Ma, as this is when deposition, metamorphism and ductile deformation took place in the Anakie Metamorphic Group Equivalent rocks of the Anakie Metamorphic Group in Queensland Metamorphic rocks in Queensland that have a similar age of deposition, as well as similar structural and metamorphic histories to the Anakie Metamorphic Group include the Cape River Metamorphics, Argentine Metamorphics, Barnard Metamorphics, Charters Towers Metamorphics and Running River Metamorphics (Withnall et al., 1995, Figure 6.3). All of these rocks have been interpreted as being deposited on attenuated Proterozoic crust in a passive margin setting during the Neoproterozoic and Cambrian (Murray, 1990; Fergusson et al., 1998, 2001), similar to models proposed for the Neoproterozoic Kanmantoo Group in the Adelaide Fold Belt (Preiss, 1990; Flöttman et al., 1998). 206

233 Synthesis 6.3 Tectonics Palmerville Fault 145 E CRM AM BM RRM CTM Hodgkinson Province Lolworth Ravenswood Province Townsville Townsville 20'S Georgetown Inlier (Early-mid Proterozoic) Metamorphic rocks (probable early Palaeozoic) Neoproterozoic-Cambrian metamorphic terranes Marlborough Ophiolite (Neoproterozoic) 70 Mile Range Group (Cambro-Ordovician) Galilee Basin AMG Drummond Basin Anakie Inlier Bowen Basin CORAL SEA Broken River Province (Ordovician-Devonian) New England Fold Belt AMG - Anakie Metamorphic Group CRM - Cape River Metamophics RRM - Running River Metamorphics CTM - Charters Towers Metamorphics AM - Argentine Metamorphics BM - Barnard Metamorphics N Great Artesian Basin Sub-surface Drummond Basin Fault NEBINE RIDGE 200 km Gravity Trend Figure 6.3: Location map of relevant Neoproterozoic-Cambrian aged metamorphic and metasedimentary rocks in central and northern Queensland. 207

234 6.3 Tectonics Synthesis The Anakie Metamorphic Group and equivalent rocks in Queensland are relatively understudied compared to their southern Australia equivalents (eg. Adelaide Fold Belt, western Lachlan Fold Belt). Despite the limited amount of information available, the compilation of information used here is considered to be adequate for the scope of this section. Most of the summary information presented is from AGSO Bulletin 240/ Queensland Geology 9; North Queensland Geology (1997), edited by J.H.C. Bain and J.J Draper, and references therein. This volume is still the main source of information available for most of these areas Cape River Metamorphics The Cape River Metamorphics (Figure 6.3) are a northwest trending belt of mainly amphibolite grade, thick bedded feldspathic meta-arenite and fine-grained mica schist that variably grades into gneiss (Withnall, 1997). Provenance is thought to be a high-grade metamorphic or plutonrich source area, although a calcareous component is common in the arenites. A detrital zircon SHRIMP U-Pb age of 1145 ± 21 indicates a Late Mesoproterozoic maximum age for deposition of the Cape River Metamorphics (M.Fanning, unpublished report to QLD Geologic Survey, 1996). The Cape River Metamorphics are intruded by granitoids of the the Fat Hen Creek Complex, which are locally associated with high-grade rocks in metamorphic aureoles. Rare hornblende-rich mafic units are found in higher grade rocks, and have a similar geochemical signature to mafic rocks in the Anakie Metamorphic Group and Argentine Metamorphics (Withnall, 1997b). Structure Locally, a differentiated S1 foliation is preserved in S2 quartzose domains, and is more closely spaced than S2 (Withnall, 1997b). Relict S1 fabric elements in the hinge zone of F2 folds strike southeast-northwest and are steeply dipping. The dominant structural feature of the Cape River Metamorphics is a strong S2 foliation, manifest as a spaced differentiated foliation with quartzo-felspathic domains up to 5mm wide, separated by narrower biotite domains (Fergusson et al., 2005). The S2 foliation dips consistently to the southwest, and shallows in that direction. Isoclinal F2 fold hinges strike east-southeast and are gently plunging around northeast-southwest trending F3 fold axes (Fergusson et al., 2005). In the far southwest of the area, S2 is flat-lying, interpreted to reflect its original orientation based on a lack of younger overprinting (Fergusson et al., 2005). Stretching mineral lineations plot shallowly to moderately to the east-southeast and southeast, asymmetric feldspar porphyroclasts, fold trains and shear bands indicate a top to 208

235 Synthesis 6.3 Tectonics the west sense of shear (Hammond, 1986; Fergusson et al., 2005). Metamorphism Metamorphism of the Cape River Metamorphics reached upper amphibolite facies in places, with formation of gneiss and migmatites in the highest grade rocks (Fergusson et al., 2005). Biotite and muscovite are aligned along all of the foliations, and locally overgrow S3, indicating metamorphism occurred during and after deformation (Fergusson et al., 2005). S2 forms a gneissosity in the highest grade rocks, indicating that the formation of the flat-lying folaition was coeval with high grade metamorphism. Correlations The early structural history of the Cape River Metamorphics is similar to that interpreted for the Anakie Metamorphic Group in this study. Differentiated S1 in the Cape River Metamorphics is interpreted as the correlative of S2 in the Anakie Metamorphic Group, and both are overprinted by a pervasive flat-lying differentiated crenulation cleavage, which is also interpreted to be a correlative structure. The flat-lying foliation in both areas is associated with formation of isoclinal folds, shear bands and mineral stretching lineations. Shear sense on the flat-lying foliation in the Cape River Metamorphics is to the west, differing to a northeast sense of shear for the flat-lying foliation in the Anakie Metamorphic Group (Section ). The flat-lying foliation in both areas is reoriented around shallow-plunging upright folds. The first generation of folds that developed after the flat-lying foliation in the Cape River Metamorphics (F3) trend northwest, however the equivalent fold generation in the Anakie Metamorphic Group (F4) trends northeast. The Fat Hen Creek Complex has an age of ca Ma (Hutton et al. 1997), and is not affected by D1 deformation. The Fat Hen Creek Complex is affected in parts by S2, indicating D2 deformation overlapped with intrusion of the granitoids (Figure 6.4). Metamorphic cooling ages constrain deformation and metamorphism to pre- ca 423 Ma (Fergusson et al., 2005), therefore development of the flat-lying foliation (S2) and high grade metamorphism in the Cape River Metamorphics occurred between ca Ma. These ages overlap with the development of a flat-lying foliation and metamorphism in the Anakie Metamorphic Group (Figure 6.4). Withnall et al. (1997) correlate the age of S1 and S2 in the Cape River Province with deformation and metamorphism in the Anakie Metamorphic Group at ca 500 Ma, based on K-Ar mica ages from the Anakie Inlier (Withnall et al., 1996). The S2 foliation in the Cape River 209

236 6.3 Tectonics Synthesis Metamorphics is interpreted to reflect extension related to rollback of the palaeo-pacific subduction after the principally shortening processes that culminated in the Delamerian Orogeny at ca 500 Ma (Fergusson et al., 2004, 2005) Argentine Metamorphics The Argentine Metamorphics (Figure 6.3) are divided into higher and lower grade metamorphic packages in the north and south of the outcrop respectively (Withnall and McLennan, 1991). The lower grade package consists of mica schist and quartzite with subordinate intervals of laminated amphibolite, chlorite schist, calc-silicate rocks and minor impure marble. Metamorphic grade ranges from greenschist facies to middle amphibolite facies (Withnall, 1997a). Compositional layering is subparallel to S1, which is defined by the alignment of micas, elongate quartz and amphibole prisms. S1 is a differentiated fabric in more pelitic layers (Withnall, 1997a). A low angle S2 foliation overprints the S1 foliation and is associated with shearing, local isoclinal and sheath folding, and retrograde metamorphism. Intersection and mineral stretching lineations on S2 trend east-southeast, and shear sense is top to the west (Hammond, 1986). Fergusson et al. (2004) noted that structural development and overprinting in the Argentine and Cape River Metamorphics is identical. Both areas have a steeply dipping S1 foliation associated with medium-grade metamorphism, which is overprinted by a low angle S2 foliation associated with shearing and retrograde metamorphism. The original orientation of D1 structures in the Argentine Metamorphics is not well constrained, but are thought to be north-south trending (Hutton et al., 1997a). U-Pb zircon ages from granites in the Argentine Metamorphics indicate a mid-ordovician age for the low angle S2 foliation (Fergusson et al., 2004). Early deformation and metamorphism is thought to have occurred at ca 500 Ma, and has been correlated with deformation and metamorphism in the Anakie Metamorphic Group (Hutton et al., 1997a) Running River Metamorphics The Running River Metamorphics consist of biotite gneiss, migmatite, amphibolite, mica schist and quartzite and are considered equivalents of the high-grade rocks of the Argentine Metamorphics (Wyatt et al., 1970; Withnall and McLennan, 1991, Withnall, 1997). The whole area has been variably affected by contact metamorphism related to Carboniferous intrusives. The structure of the Running River Metamorphics has not been studied in detail. The dominant foliation 210

237 Synthesis 6.3 Tectonics is interpreted to be S2 (Withnall, 1997), it trends northeast, parallel to the edge of the Broken River Province (Figure 6.3). S2 has been reoriented by younger folding. Deformation, metamorphism and structural development is suggested to be similar to the Cape River and Argentine Metamorphics (Figure 6.4), and thought to have occurred at the same time (Hutton et al., 1997a) Charters Towers Metamorphics The Charters Towers Metamorphics consist of quartz-biotite-plagioclase schist, cordierite-quartzbiotite schist, quartzite and minor calc-silicate (Reid, 1917; Wyatt et al., 1970; Peters and Golding, 1987). The schists are fine-grained and interlayered with more massive quartzite, which forms pale grey layers comprising mainly quartz and plagioclase with minor biotite and local sillimanite (Peters and Golding, 1987). Cordierite aggregates up to 3 mm occur locally, and are retrogressed to sericite and green biotite. Banded calc-silicate layers comprise quartz-tremolite and clinozoisite with secondary feldspar, sphene and calcite (Baker, 1975). Rocks are generally metamorphosed to amphibolite grade, although some muscovite schists are within the upper greenschist facies (Peters and Golding, 1987). The Charters Towers Metamorphics are broadly correlated with the Cape River Metamorphics on lithological and structural grounds (Hutton et al., 1997a). The Charters Towers Metamorphics preserve only one foliation, which strikes northwest and dips steeply to the northeast. The foliation is associated with steep isoclinal folds, that plunge to the northwest (Peters and Golding, 1987). Steep isoclinal folding and metamorphism in the Charters Towers Metamorphics is correlated with D1 upright folding and metamorphism in the Cape River Metamorphics (Hutton et al., 1997a). D1 in the Cape River Metamorphics has been correlated with upright folding and metamorphism (D2) in the Anakie Metamorphic Group (Fergusson et al., 2005), which implies that upright folding and metamorphism occurred synchronously in all three areas. Limited geochronologic data exists for the Charters Towers Metamorphics. The age of deposition is loosely constrained by the age of the Bucklands Hill Diorite, which intrudes as a series of dykes parallel to lithological layering (Hutton and Crouch, 1993). The Buckland Hills Diorite has an age of 508 ± 7 Ma (Fanning, 1995, a). It exhibits low-k, MORB-type, arctholeiite chemical trends and has a primitive isotopic signature (Hutton et al., 1994). Dykes that make up the Buckland Hills Diorite have been deformed, and now form elongate pods, suggesting they were emplaced prior to deformation. Therefore, deposition of the Charters Towers 211

238 6.3 Tectonics Synthesis Metamorphics is considered to have occurred prior to ca 508 ± 7 Ma. The minimum age for deposition of the Charters towers Metamorphics is similar to the Anakie Metamorphic Group, based on a 510 Ma age from detrital zircon in the Clermont region (Fergusson et al., 2001). The timing of deposition with respect to upright folding and metamorphism in the Charters Towers Metamorphics matches the pattern observed in the Anakie Metamorphic Group (Figure 6.4), suggesting a common tectonic setting Barnard Metamorphics The Barnard Metamorphics consist of predominantly meta-arenite, quartzite, phyllite, greenstone, chlorite schist (sheared and altered greenstone), muscovite-chlorite and biotite-muscovite schist, with subordinate biotite gneiss, migmatitic gneiss and metagranite (Bultitude et al., 1997). Mineral assemblages preserved in high-grade rocks indicate that middle- to upper amphibolite, and occasionally granulite facies conditions were reached. Locally elevated temperatures are interpreted to be related to syn-metamorphic Ordovician granites (Bultitude et al., 1997). Three regional deformation events have been recognised in the Barnard Province (Bultitude et al., 1997). The first deformation produced a differentiated mylonite fabric in upper greenschist and higher grade rocks, and a bedding parallel-slaty cleavage in the lower grade rocks (Hammond, 1986). Cross-cutting relationships with an Ordovician granite indicate that D1 occurred before the early Ordovician (Bultitude et al., 1996; Bultitude and Garrad, 1997). An intense S2 crenulation cleavage transposes S1, and forms the dominant foliation in the metamorphics and granites. No orientation data is available for S1 and S2. A maximum age of ca 460 Ma is interpreted from syn-deformation intrusives (Bultitude et al., 1997). D3 produced mesoscopic upright folds and steep cleavage, which is more pronounced in lower grade rocks. D3 deformation occurred in the Early Permian (Bultitude et al., 1997). An age of ca 485 Ma from granite that intrudes the Barnard Metamorphics constrains a minimum age of deposition (Bultitude et al., 1997), although late Neoroterozoic and early Palaeozoic ages for deposition of some rocks has been suggested by Champion (1991) and Champion and Bultitude (1994). The structural history and timing of deformation of the Barnard Metamorphics is very similar to the Anakie Metamorphic Group, as well as the other terranes discussed. The Barnard Metamorphics are considered to have experienced a similar depositional, structural and metamorphic history to the other areas already discussed. 212

239 Synthesis 6.3 Tectonics Correlations A strikingly similar pattern and timing of deformation and metamorphism is revealed for rocks deposited during the Neoproterozoic to Early Cambrian in central and north Queensland (Figure 6.4). Evidence from the metamorphic complexes described in the previous section suggests that Neoproterozoic-Cambrian sedimentation was interrupted at ca 510 Ma, and that lithologies of the different metamorphic complexes are broadly similar. Major differences between the various complexes include the presence and volume of intrusives, and variations in metamorphic grade. For example, the Anakie Metamorphic Group contains no intrusives of Ordovician or older age, and has only reached middle amphibolite facies grade in a small area (Chapter 3). The Cape River Metamorphics contains abundant intrusives of Ordovician age, and most rocks are amphibolite grade or higher. The structural development and timing of deformation of the two areas is almost the same, perhaps suggesting that the two complexes have experienced similar histories in the same evolving system. Mafic rocks in the Cape River Metamorphics, Argentine Metamorphics and Running River Metamorphics have similar geochemical trends to mafic rocks in the Anakie Metamorphic Group (Withnall, 1997b), and are interpreted to have formed in a similar tectonic setting. 213

240 deposition metamorphism cooling shortening deformation felsic magmatism mafic magmatism volcanism V V V V V age constraints Ma Region Age Anakie Inlier Retreat Batholith Theresa Ck Volcanics V V V V V U-Pb Zr E-W Au NW-SE Au Gem Park Granite U-Pb M Au NE-SW Fork Lagoons Beds Foss. Ar-Ar Musc Bt? W over E shear sense NW-SE K-Ar Musc Cape River Metamorphics Hodgkinson/Broken River Province Running River Metamorphics 70 Mile Range Group Charters Towers Metamorphics Argentine Metamorphics U-Pb N-S extensional deformation flat lying structures M - monazite Zr - zircon Musc - muscovite Bt - biotite Ar-Ar K-Ar fossil age Foss. gold mineralisation Au NW-SE structure orientation U-Pb M Anakie Metamorphic Group? NE-SW E-W Ar-Ar Musc Bt Ar-Ar NW-SE? E over W shear sense Fat Hen Creek Complex U-Pb Zr? E over W shear sense? NW-SE? Cape River Metamorphics? Broken River Group Foss. Foss. Shield Ck Formation NE-SW Graveyard Ck Group Foss. Carriers Well Formation Foss. V V V V V Everetts Ck Volcanics? U-Pb Zr? Judea Formation??? Barnard Metamorphics??? E over W shear sense NNW-SSE Running River Metamorphics NE-SW Au Lolworth/ Ravenswood Batholiths E-W U-Pb Zr Ravenswood Batholith Thalanga Volcanics U-Pb Zr Lavery Ck Supersuite U-Pb Zr Au V V V V V U-Pb Zr?? 70 Mile Range Group Foss. Buckland Hills Diorite? NW-SE NNW-SSE? U-Pb Zr??? Charters Towers Metamorphics Argentine Metamorphics? REGIONAL TRENDS EXTENSION SHORTENING EXTENSION (LOCAL UPLIFT/ COOLING) SHORTENING EXTENSION (LOCAL UPLIFT/ COOLING) SHORTENING EXTENSION CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN inferred high-t/low-p metamorphism??

241 Synthesis 6.3 Tectonics Figure 6.4: Time-space plot of relevant metamorphic and other rocks in central and northern Queensland from the late Neoproterozoic - Devonian. Data collated from this study and from Withnall et al. (1995), Bain and Draper (1997, and references therein), Fergusson et al. (2001), Fergusson et al. (2005) ca Ma Upright Folding Figure 6.4 highlights cessation of sedimentation at ca 510 Ma, followed immediately by a period of upright folding and metamorphism from 510 Ma Ma. The trend of upright structures formed at ca 510 Ma is northwest to north-northwest in all of the metamorphic complexes (Figure 6.5), suggesting east-west shortening over a large area. The orientation of the upright structures is remarakably consistent, considering the extensive younger deformation history known to affect some areas (eg. Anakie Metamorphic Group). In most places, the upright structures and associated foliation are transposed by a flat-lying foliation between ca Ma (Figure 6.4). The only area that does not have a flat-lying foliation is the Charters Towers Metamorphics. A pervasive S2 crenulation cleavage in the Barnard Metamorphics is interpreted to correlate with flat lying foliation in the other areas, although there is no data to confirm its orientation. In most areas, the earliest upright folds have been interpreted as D1 structures, and the flatlying structures as D2. The exception is the Anakie Metamorphic Group, where the flat-lying foliation is interpreted to have formed during D3 deformation. Fergusson et al. (2005) noted an earlier fabric element in S1 microlithons of the Cape River Metamorphics, however it was nonpervasive and observed infrequently in pelitic rocks only. The nature of this fabric is the same as that interpreted for the earliest deformation in the Anakie Metamorphic Group. A similar structural development may well exist in the Argentine, Running River and Barnard Metamorphics, but has simply not been previously recognised. More importantly than event/deformation numbering is the observation that all of the rocks discussed in this section underwent upright folding and metamorphism between ca Ma, followed by development of pervasive flat-lying foliation between ca 500 Ma Ma ca Ma Flat-Lying Foliation Development The development of a flat-lying foliation in the metamorphic complexes is coeval with intrusion of the Fat Hen Creek Complex in the Cape River Metamorphics (Hutton et al., 1997a), intrusion 215

242 6.3 Tectonics Synthesis of the Lavery Creek Supersuite in the Charters Towers Metamorphics (Hutton et al., 1997a), intrusion of the Ravenswood Batholith (Hutton and Reinks, 1997), deposition of the Judea Formation, Everetts Creek Volcanics, and Carriers Well Formation in a back arc basin setting in the Hodgkinson/Broken River province (Withnall and Lang, 1993; Withnall, 1997c), deposition of the 70 Mile Range Group and Thalanga Volcanics in a back arc basin setting (Hutton et al., 1997a), and deposition of the Fork Lagoons Beds in a back arc basin setting (Withnall et al., 1995). The occurrence of back arc extension and deposition coevally with flat-lying foliation development and syn-deformation intrusives is interpreted to reflect an extensional setting in Queensland from ca Ma (Figure 6.4). Thus, D3 deformation in the Anakie Metamorphic Group that formed a flat-lying crenulation cleavage is interpreted to be the result of extensional processes. One difference between the metamorphic complexes is the sense of shear associated with development of the flat-lying foliation, and the orientation of the mineral stretching lineation on the foliation plane. The flat-lying foliation in the Anakie Metamorphic Group has a top to the northeast sense of shear, and a northeast trending mineral stretching lineation (Figure 6.6). However, the flat-lying foliation in the Cape River, Argentine and Running River Metamorphics has a top to the northwest sense of shear and northwest trending mineral stretching lineation (Figure 6.6). No data are available for the equivalent foliation in the Barnard Metamorphics. The age of development of the flat-lying foliation in the Anakie Metamorphic Group is constrained to between ca Ma, and to between ca Ma in the Cape River Metamorphics(Figure 6.4). These ages overlap by 10 million years, however the difference in kinematics and metamorphic grade between the two areas does not allow direct correlation. The flat-lying foliation in the two areas formed in the same broad time period, however may not have formed at exactly the same time. Diachronous development of flat lying foliation can be attributed to variety of reasons, including, but not restricted to: strain partitioning, changing stress field, locus of magmatism/volcanism, and pre-existing structures in the crust, any one of which could explain the observed age difference. It is beyond the scope of this study to investigate all of the potential causes for diachronous foliation development, however a model based on evidence from the Anakie Metamorphic Group, Cape River Metamorphics and regional geophysical features is presented below. The interpretation of flat-lying foliation development will focus on the Anakie Metamorphic Group and the Cape River Metamorphics, as these areas have more data available than the other metamorphic complexes. 216

243 Galilee Basin Synthesis 6.3 Tectonics Barnard Metamorphics? Argentine Metamorphics Palmerville Fault 145 E BM? Running River Metamorphics steep/vertical foliation strike and dip of foliation CRM RRM AM CTM 20'S 85 Charters Towers Metamorphics 85 CORAL SEA Cape River Metamorphics Drummond Basin Anakie Inlier Anakie Metamorphic Group Bowen Basin AMG Great Artesian Basin N NEBINE RIDGE 200 km Figure 6.5: Trend of upright structures formed at ca 510 Ma in central and northern Queensland. See Figure 6.3 for key. 217

244 Galilee Basin 6.3 Tectonics Synthesis Barnard Metamorphics? Argentine Metamorphics mineral stretching lineation on dominant (flat-lying) foliation plunge if known Palmerville Fault 145 E BM? Running River Metamorphics sense of shear associated with dominant (flat-lying) foliation CRM RRM AM CTM 20'S Charters Towers Metamorphics CORAL SEA Cape River Metamorphics Drummond Basin Anakie Inlier Anakie Metamorphic Group Bowen Basin AMG Great Artesian Basin N NEBINE RIDGE 200 km Figure 6.6: Trend of flat-lying structures formed between ca 500 Ma Ma in central and northern Queensland. No flat-lying foliation is developed in the Charters Towers Metamorphics. No data is available for the Barnard Metamorphics. See Figure 6.3 for key. 218

245 Synthesis 6.3 Tectonics Structural Evolution The consistent strike orientation of ca 510 Ma upright structures suggests that the different metamorphic complexes were probably deformed as part of the same block and that only limited amounts of displacement between, and rotation of, individual complexes has occurred since. With that in mind, the Anakie Metamorphic Group and the Cape River Metamorphics can be interpreted as having formed in the same section of crust, and having experienced a similar deformation history. Development of a flat-lying foliation in the two areas appears to have occurred diachronously in a similar geodynamic setting, as part of the same deformation event. Two factors to be considered when addressing the apparent diachroneity of flat-lying foliation development are the difference in metamorphic grade between the Anakie Metamorphic Group and the Cape River Metamorphics, and that both areas flank the same broad northnorthwest gravity high (Figure 6.7). The metamorphic grade of the flat-lying foliation in the Anakie Metamorphic Group is mid-greenschist facies, in contrast to the flat-lying foliation in the Cape River Metamorphics which is amphibolite facies. This is interpreted to reflect different positions in the crust during the flat-lying foliation development, with the Anakie Metamorphic Group situated higher relative to the Cape River Metamorphics. If these rocks were all exhumed at the same time during extensional deformation, then the higher-grade Cape River Metamorphics should preserve a younger 40 Ar/ 39 Ar cooling age compared to the Anakie Metamorphic Group, which matches the observed pattern of 40 Ar/ 39 Ar ages for these rocks. The difference in shear sense between the two metamorphic complexes could also be related to a north-northwest trending gravity high that features prominently in east-central Queensland (Figure 6.7). The age and origin of the gravity high is unknown, but may represent a long lived positive feature of the crust. Such a feature could have significant influence on structural development in higher crustal levels during east-west directed deformation. A model of basement controlled deformation in upper crustal levels is interpreted, whereby extensional faults that form on the flanks of a basement (gravity) high during lithospheric extension have opposing vergence, resulting in opposing shear sense either side of the high (Figure 6.3.2(c)). This model accounts for the difference in kinematics of the flat-lying foliation formed in the Anakie Metamorphic Group and the Cape River Metamorphics, and could also explain the differences in 40 Ar/ 39 Ar ages from the two regions. 219

246 6.4 Gold in the Clermont Region Synthesis Outcomes of Large-Scale Comparison The results of the large-scale comparison has provided a context for the timing and nature of the structural and metamorphic evolution documented in the Anakie Metamorphic Group. Correlation with equivalent metamorphic rocks elsewhere in Queensland, as well as the timing of deformation with respect to sedimentation, metamorphism and magmatism (Figure 6.4), has outlined the geodynamic conditions under which the flat-lying S3 foliation formed. The flat-lying foliation in the Anakie Metamorphic Group developed in an extensional setting, which was unable to be resolved from structural and metamorphic evidence in the Clermont region alone. The patterns of sedimentation, deformation, metamorphism and magmatism outlined in Figure 6.4 indicate that Queensland was dominated by extension between the Late Neoproterozoic to Early Palaeozoic. Episodes of shortening appear to be short-lived by comparison (eg. ca Ma), and are an exception in a setting characterised by extension-related geology. 6.4 Gold in the Clermont Region This study has outlined a minimum of five different styles of gold in the Clermont region (Section 6.4), not including Tertiary and younger deposits. Broadly, gold occurrences can be separated into two groups: structurally controlled lodes mostly in the Bathampton Metamorphics, and gold in conglomerate at the base of Permian basins north of Clermont (Section 5.5) Structurally Controlled Gold D4 Shear Zone Hosted Gold D4 shear zones that were identified along the limbs of the Oaky Creek Antiform (Section 2.59, Appendix 1) are host to the earliest known gold mineralisation in the Anakie Metamorphic Group (Section ). Gold is associated with sulphides in siliceous lenses up to 10 s of metres in size (Section ), and occurs at the intersection of the D4 shear zones with greenstone layers, as well as bends in the shear zones (Section ). There is a spatial correlation between the gold-bearing siliceous bodies and carbonate veining, as well as intrusion of diorite dykes (Section ). 220

247 Synthesis 6.4 Gold in the Clermont Region BM AM Townsville RRM CRM CTM AMG Gravity high = basement high? NEBINE RIDGE 200 km Figure 6.7: Bouger gravity image of central and northern Queensland overlain with relevant metamorphic rocks. Gravity trend lines approximate linear trends associted with the Anakie high. Geophysics from Geoscience Australia online GIS service. 221

248 6.4 Gold in the Clermont Region Synthesis The structural development and geometry of the Oaky Creek Creek Antiform that formed during D4 is identical to gold bearing structures in the Bendigo goldfield of central Victorian (Section 2.5.2). The age of gold mineralisation in D4 shear zones is interpreted to be close to the age of regional deformation during D4, which is interpreted to be ± 6.2 Ma (Chapter 4). This age is similar to that of the structurally controlled gold deposits formed at ca 440 Ma in central Victoria, including Bendigo (Foster et al., 1998; Bierlein et al., 1999; VandenBerg, 1999; Arne et al., 2001). The tectonic implications of the similarities in structure and timing of the two areas is discussed below Gold in D5 Structures Two D5 structural corridors were identified around the Oaky Creek Antiform, and contained quartz-gold veins that were concentrated at their intersection with D4 shear zones (Section , Appendix 1). The quartz veins usually had an en echelon geometry, and were associated with wallrock bleaching and carbonate veining (Section ). Two D5 faults up to 70 m wide were identified (Section ), the along strike projection of these faults intersected known gold (and copper) deposits that had been previously identified (Section , Appendix 1) Gold in D6 Structures A single D6 structural corridor was identified in the Oaky Creek Antiform area, and contained a previously known gold deposit (K-2, Section , Appendix 1). The K-2 deposit occurs at the intersection of the D6 structural corridor and the along-strike projection of a D4 shear zone (Appendix 1). Quartz veins within the D6 structural corridor are up to 1.5 m wide, and have laminated margins and a massive centre, as well as en echelon geometries (Section ) Relationships The spatial relationship of gold with D4 shear zones, including concentrated gold at the intersection of D4 shear zones with D5 and D6 structures (Appendix 1), suggests that the D4 shear zones provide the primary control on gold distribution in the Oaky Creek Antiform area 222

249 Synthesis 6.4 Gold in the Clermont Region (Section 5.4). Remobilisation and an upgrade of gold concentration is interpreted to have occurred at the intersection of the D4 shear zones and D5 and D6 structural corridors (Section 5.5) Gold in Permian Conglomerate Permian conglomerate hosted gold in the Clermont Goldfield formed by a number of different processes, and can be divided into gold that formed prior to deposition of the conglomerate, and gold that formed after (Section 5.4.2) Prior to Deposition - Palaeoplacer Gold Waterworn gold nuggets that formed prior to Permian basin formation are most likely sourced from the Anakie Metamorphic Group (Section 5.4.2), which comprises the conglomerate clasts. The waterworn nuggets have secondary overgrowth of gold nodules associated with haematite (Figure 5.11), interpreted to have formed during younger hydrothermal activity, although not necessarily as hydrothermal style mineralisation (Section 5.4.2) After Deposition - Hydrothermal Gold Gold that formed after deposition of Permian conglomerate consists of gold in association with quartzsideritemarcasite in fractures that cut the unconformity, restricted to the first 50 cm of conglomerate (Section 5.3.2, 5.4.2), as well as gold-marcasite intergrowths close to the unconformity (Section 5.4.2, Figure 5.12). Gold at Miclere is concentrated in, and proximal to, northeast trending transfer faults and parallel fractures, and at their intersection with northwest trending normal faults (Figure 5.2, Section 5.3.2). The transfer faults are interpreted to have formed conduits for ascending auriferous fluids during ongoing deformation, resulting in the extremely rich gold mineralisation observed (Figure 5.10, Section 5.4.2). A model of fluid mixing along the unconformity interface, coeval with deformation, accounts for the different mineralisation styles in close proximity to each other (Section 5.4.2, Figure 5.12). Targeting of transfer faults and adjacent areas at the base of Permian basins in the Clermont region has not been previously undertaken, and potentially hosts appreciable amounts of gold. 223

250 6.5 Tectonic Model Synthesis 6.5 Tectonic Model Using evidence from the Anakie Metamorphic Group in this study, as well as information compiled from other studies (eg. Figure 6.4), a tectonic model is interpreted for northeast Australia from the Late Neoproterozoic ( 560 Ma) to the Early Palaeozoic ( 440 Ma). However, before this can be achieved, three points need to be considered; one is the age of the onset of subduction under the east Australian margin; two is the relative roles of extension and shortening during continental margin evolution, and; three is the tectonic controls on gold mineralisation in the Anakie Inlier Neoproterozoic-Cambrian subduction in Queensland Subduction along the eastern Gondwana margin is thought to have initiated by ca 560 Ma (Goodge, 1997), but is not represented everywhere by underthrusting of oceanic lithosphere underneath the continental margin (Boger and Miller, 2004). Evidence for subduction along the northeast Australian margin by that time is limited, the main hindrance is a lack of outcrop, and of geochemical and geochronological data. The Marlborough Ophiolite (Figure 6.3) preserves the best evidence for subduction. It has an Sm/Nd age of 562 ± 22 Ma from five cogenetic mafic samples (Bruce et al., 2000), and a depleted MORB geochemical signature, suggesting formation at either an oceanic or back arc basin spreading centre (Cawood, 2005), and exposed during later deformation. Mixing of first-cycle magmatic arc and Gondwana craton derived detritus in the Anakie Inlier was used by Cawood (2005) as evidence for subduction initiation along the Gondwana margin, close to the ocean-continent interface. Cawood (2005) also inferred a range of convergent plate margin configurations resulting from an irregular continental margin, and suggested that the Anakie Inlier may have rafted off the craton to form a continental ribbon. Subduction initiation in southern Australia is interpreted to have occurred outboard of continental margin re-entrants, resulting in continental margin sedimentation in the Adelaide Fold Belt in South Australia until orogenesis at Ma (Cawood, 2005). The timing of sedimentation and deformation in the Anakie Metamorphic Group is very similar to that of the Adelaide Fold Belt, perhaps indicating a similar mode of formation. 224

251 Synthesis 6.5 Tectonic Model Roles of Extension and Shortening Evidence from this study and from previous studies (eg. Fergusson et al., 2005) suggest that the Anakie Metamorphic Group and equivalent metamorphic rocks in Queensland have experienced both extension and shortening. The patterns of structure, metamorphism, magmatism and sedimentation outlined in Figure 6.4 indicate that northeast Australia during the late Neoproterozoicearly Palaeozoic was dominated by extensional processes. Evolution of a continental margin as an extensional accretionary orogen is described by Collins (2002b) and Richards (2004) for the eastern Lachlan Fold Belt in southern Australia, as well as more recent examples from the Taupo Volcanic Zone in New Zealand, and the Cenozoic in south east China (Collins, 2002b). The early tectonic development of central and northern Queensland is considered to be similar to these, and the different metamorphic terranes discussed in Queensland are interpreted to represent different crustal locations in the same evolving margin. A fundamental part of an extensional accretionary orogen is that heat input to the crust occurs during lithospheric extension (Collins, 2002b; Richards, 2004). Lithosheric thinning in the back arc of a subduction zone results in mantle upwelling and decompression melting, enhanced by slab flux melting. The mafic melts produced migrate and induce metamorphism and anatexis of the crust, creating granitic melts synchronous with volcanic activity and deposition in rift basins. Richards (2004) demonstrated that shortening deformation occurs after peak (high- T/low-P) metamorphic conditions in an extensional accretionary orogen, although heating may continue and overlap with the onset of crustal thickening. Shortening deformation in an extensional accretionary orogen is concentrated in narrow belts, but is not associated with high pressure metamorphism or paired metamorphic belts (Collins, 2002b). Other features of extensional accretionary orogens include the presence of primitive mafic rocks throughout the orogenic history, sequential rift basins, lack of a well-defined arc chain, isobaric cooling associated with low-p/variable-t metamorphism, lack of old, high-grade basement and lack of evidence for substantial early crustal overthickening (Collins, 2002b). These features are similar to those described for the Neoproterozoic-Cambrian metamorphic complexes in Queensland. The flat-lying foliation in the Anakie Metamorphic Group is associated with a retrograde (low-grade) mineralogy. In contrast, the flat-lying foliations in the Cape River, Argentine and Running River Metamorphics developed contemporaneously with high- (pro)grade metamorphism. This can be explained by the effects of heating and cooling related to extension and shortening of the rocks, and relative position in the crust. Peak heat input to the Anakie 225

252 6.5 Tectonic Model Synthesis Metamorphic Group occurred during extension, evidenced by high-t/low-p metamorphism that produced andalusite (Section The timing of D1/M1 is constrained to between ca 510 Ma and ca 500 Ma (Section 3.4.2). The setting changed to east-west oriented shortening prior to ca 500 Ma (Section 3.4.2, Figure 6.4), evidenced by north-south trending upright isoclinal folds in the Anakie Metamorphic Group (Section 2.4.2). The rocks either continued to heat, or were isothermal at the onset of crustal thickening, but by the time extensional tectonism resumed at ca 500 Ma, the rocks had begun to cool (Figure 6.4, Figure 6.8). The flat-lying foliation in the Anakie Metamorphic Group is interpreted to preserve this switch from shortening to extension. Retrograde metamorphism and flat-lying foliation development in the Anakie Metamorphic Group is constrained to earlier than 483 Ma (oldest 40 Ar/ 39 Ar age), and is probably closer to ca 500 Ma (Chapter 3). The short lived nature of flat-lying foliation in the Anakie Metamorphic Group compared to the Cape River Metamorphics (Figure 6.8) is attributed to partitioning of ductile extensional deformation into lower crustal levels. Renewed heat input into the lower crust after the return to lithospheric extension at ca 500 Ma is interpreted to have focussed ductile deformation down, represented by the flat-lying foliation in the Cape River, Argentine and Running River Metamorphics that formed during the Ordovician. In these rocks, the flat-lying foliation is a high grade fabric relative to the Anakie Metamorphic Group. Syn-deformation intrusives of the Fat Hen Creek Complex in the Cape River Metamorphic suggests emplacement into lower crustal levels during extension. This is in contrast to the Anakie Metamorphic Group, which has a lower grade flat-lying foliation, and no Ordovician or earlier intrusives, the latter consistent with a higher crustal location during Cambro-Ordovician lithospheric extension. The relationship between the Anakie Metamorphic Group (and Charters Towers Metamorphics) and the other equivalent metamorphic complexes is interpreted to be one of a hangingwall-footwall, where the Anakie Metamorphic Group represents the hanging wall. Figure 6.8 shows the decoupling in temperature path of the Anakie Metamorphic Group and the Cape River Metamorphics after ca 500 Ma, interpreted to reflect ongoing heat input into the stratigraphically/structurally lower Cape River Metamorphics. A feature of the Charters Towers Metamorphics is a complete lack of flat-lying foliation development. This is attributed to similar relationships observed in the Anakie Inlier, where a low metamorphic grade and lack of a flat-lying foliation in the north of the inlier (Mt Coolon region) is interpreted to reflect higher stratigraphic and structural levels (Section 2.4.2). The Charters Towers Metamorphics are broadly along strike from the Anakie Inlier along a distinct gravity high (Figure 6.7). The Mt Coolon region and the Charters Towers Metamorphics are 226

253 Synthesis 6.5 Tectonic Model interpreted to represent an area sufficiently high in the crust to have been unaffected by post ca 500 Ma extensional deformation. Evidence from the Anakie Metamorphic Group and equivalent terranes in Queensland indicate a record of extension prior to crustal thickening at ca 510 Ma, followed by ongoing extension from ca 500 Ma to ca 440 Ma. Geochemical evidence suggests that the tectonic setting for these processes was a continental back arc. The timing and nature of deposition, metamorphism and structural development indicates the predominance of extensional processes during the Late Neoproterozoic - Early Palaeozoic continental evolution of Queensland Tectonic Controls on Gold Mineralisation Studies of gold in this thesis have focussed on understanding the map-scale controls on the different styles of deposits, however, the reason for gold fertility in the Anakie Inlier has not yet been examined. The spatial, temporal and mineral associations of the earliest known gold in the Clermont region, that is, gold in D4 shear zones, are features that are commonly used to describe orogenic gold (Groves et al., 1998). This classification implies an intimate relationship between mineralisation and subduction tectonics, which has been well-defined for similar age and style deposits in the Lachlan Fold Belt in central Victoria (Foster et al., 1998; Bierlein et al., 2002; Gray et al., 2002). The central Victorian deposits include Bendigo (Figure 6.9), where the gold bearing structures have a remarkably similar geometry and structural development to the Oaky Creek Antiform (Section 2.5.2). Given the parallels between the evolution of parts of the Lachlan Fold Belt and parts of east and north Queensland, it is not surprising that there are similarities in the timing and style of mineralisation. A model for formation of orogenic gold deposits during the Phanerozoic was proposed by Goldfarb et al. (2001), and involved several key parameters. A global synthesis of known orogenic gold deposits revealed that Phanerozoic deposits are restricted to narrow mobile belts that formed on the margins of older cratons, where subduction underneath the continental margin has occurred (Goldfarb et al., 2001). Orogenic gold deposits are interpreted by Goldfarb et al. (2001) to form preferentially above zones of elevated heat flow, as a result of processes that may include; crustal thickening above a magmatic arc; plume-slab interaction; lithospheric extension caused by subduction rollback; subduction of an oceanic ridge; erosion of mantle lithosphere; or delamination of mantle lithosphere. The interpreted heat flux is inferred to result in voluminous hydrothermal fluid activity, which liberates, transports and ultimately concentrates 227

254 6.5 Tectonic Model Synthesis EXTENSION EXTENSION AMG CRM SHORTENING crustal thickening Flat-lying foliation development and peak metamorphism in the CRM. Intrusion of Fat Hen Creek Complex from 493 Ma Ma. peak metamorphism FOOTWALL Ar-Ar cooling age SHORTENING crustal thickening Time Ma Peak metamorphic conditions at the onset of D2 in the AMG. N-S oriented, upright, isoclinal folding and differentiated foliation development in the AMG, CRM, AM, RRM and CTM. 200 Flat-lying foliation development and retrograde metamorphism of the AMG. 100 upper temp range = 405 C, based on 180 m mica from the CRM, and assuming a cooling rate of 300 /My? Eastern Creek HANGINGWALL Oaky Ck Antiform lower temp range = 336 C, based on 75 m mica from the AMG, and assuming a cooling rate of 10 /My Miclere? INCREASING STRUCTURAL DEPTH? D2 CRM D2 AMG D1 CRM D4 AMG D3 CRM D3 AMG D1/early fabric element development in the AMG and CRM. Onset of high grade metamorphism in the AMG. Sediments depositing in the highest stratigraphic levels of the AMG until 510 Ma. Temperature C (approx) 228

255 Synthesis 6.5 Tectonic Model Figure 6.8: Approximate temperature-time (T-t) path interpreted for rocks of the Anakie Metamorphic Group (AMG) and Cape River Metamorphics (CRM). The lower structural level of the CRM relative to the AMG is evident from peak metamorphism in the CRM, synchronous with the flat-lying foliation development, while at the same time cooling is occurring in the AMG. The link between extensional tectonism and heat input can be clearly seen, with shortening events (and cooling) episodically occurring. 40 Ar/ 39 Ar ages are from this study and from Fergusson et al. (2005). The temperature window for 40 Ar/ 39 Ar ages is a range below which argon loss is negligible. They were calculated using the diffusion parameters for white mica determined by Célérier et al. (2006), the mica grain sizes for the AMG ( 75µm, Table 3.1) and CRM ( 180µm, Fergusson et al. (2005), and upper and lower cooling rates of 300 C/My and 10 C/My respectively. These parameters were used in the bulk closure temperature equation for an infinite cylinder geometry (McDougall and Harrison, 1999). gold, typically in the greenschist facies metamorphic zone of the crust (Goldfarb et al., 2001). A key feature for generation of large orogenic gold deposits is the presence of crustal-scale faults along which the hydrothermal fluids at depth can ascend, however, gold is usually deposited in secondary or ternary structural sites and plumbing systems (Goldfarb et al., 2001). There is evidence that during deposition of the proto-anakie Metamorphic Group, subduction was actively occurring (Section 6.2.1). Additionally, the geodynamic setting of the Anakie Metamorphic Group is interpreted to have been dominated by extension during the early Palaeozoic (Figure 6.4). During the early Palaeozoic, eastern Australia was in the back arc of a west dipping subduction zone that was undergoing rollback to the east (Kleinschmidt and Tessensohn, 1987; Betts et al., 2002; Collins, 2002b; Richards and Collins, 2004; Cawood, 2005; Foster et al., 2005). In Queensland, this is evidenced by the geochemical signature of basalts erupted into back arc basins of the Hodgkinson-Broken River Province (Vos et al., 2006a,b), and more locally from basalts in the Ordovician Fork Lagoons Beds at the southern end of the Anakie Inlier (Withnall et al., 1995). The eruption of basalts during the early Palaeozoic in Queensland, as well as generation and emplacement of silicic magmas (eg. Fat Hen Creek Complex) indicates an elevated geotherm, probably related to lithospheric thinning during subduction rollback. The elevated lithospheric temperatures would have been favourable for hydrothermal fluid activity, leading to gold mineralisation in the Anakie Metamorphic Group during shortening deformation at ca 443 Ma. Considering that the tectonic setting of early Palaeozic eastern Australia was favourable for hydrothermal activity that leads to gold mineralisation, and that similar conditions existed along the eastern Australian margin at that time, there is a large difference in gold endowment of the Anakie Metamorphic Group compared to the similar age and style deposits in the Bendigo goldfield. The total documented amount of gold recovered from the Anakie Metamorphic 229

256 6.5 Tectonic Model Synthesis Group is approximately 12 t (Lam, 2005), compared to the Bendigo goldfield which has yielded approximately 700 t (Wilkinson, 1988). This is more than an order of magnitude difference. With respect to the above outlined pre-requisites for generation of Phanerozoic orogenic gold deposits, and in particular large deposits, there are two main differences between the geological setting of the Anakie Metamorphic Group and rocks that host mineralisation in the Bendigo goldfield. The first difference is crustal thickness. Sediments that host mineralisation in Bendigo were deposited onto oceanic crust, and the crustal thickness after an estimated 70% shortening is only 35 km (Finlayson et al., 1980; Foster and Gray, 2000). In contrast, sediments that comprise the Anakie Metamorphic Group were probably deposited onto attenuated continental crust (Murray, 1990, 1994; Fergusson et al., 2001), of which the total thickness including the overlying sediments in the vicinity of the Nebine Ridge is 44 km (Finlayson et al., 1990). The greater crustal thickness in the Queensland region may have resulted in a lesser magnitude thermal pertubation than in thinner crust in central Victoria for the same tectonic processes, resulting in less hydrothermal fluid activity, and therefore less gold. The second difference is the presence of large faults. The Bendigo goldfield is located within a fold and thrust system that is bound by crustal-scale reverse faults (eg. the Heathcote Fault Zone, Figure 6.9 (b)), and cut by subsidary reverse faults (eg. Whitelaw Fault, Figure 6.9 (c), (d)). The sites that host mineralisation in Bendigo are reverse faults and spurs through anticlinal hinge zones that represent 3rd and 4th order structures relative to the nearby crustal-scale faults. There are no known crustal-scale faults proximal to the Anakie Inlier, although any such fault(s) could be obscured by the extensive younger cover. The lack of any identifiable crustal-scale structure in the Anakie Metamorphic Group is interpreted to be another factor for limited gold mineralisation compared to the Bendigo region. Based on the large-scale relationships of gold mineralisation to tectonic setting and processes, as well as the influence of regional geology, gold in the Anakie Metamorphic Group is interpreted to be related to rollback driven hydrothermal activity in the early Palaeozoic, and deposited during shortening deformation at ca 443 Ma. The thickness of the crust and a lack of crustal-scale faults are interpreted as controlling features that resulted in lesser amounts of gold in the Anakie Metamorphic Group, compared to richly mineralised rocks in the Bendigo goldfield that have a similar age and style of mineralisation, and formed in the same tectonic setting. 230

257 Synthesis 6.5 Tectonic Model (c) (a) (b) 231

258 6.5 Tectonic Model Synthesis N A A (c) Bendigo A A (d) Figure 6.9: This and previous page. (a) Palaeozoic outcrop and the separate metallogenic provinces of Victoria, from Ramsay and Willman (1988). (b) Simplified schematic profile through Victoria at latitude 37.5 C showing major boundaries and geologic provinces. MFZ = Woorndoo-Moyston Fault Zone; SAFZ = Stawell-Ararat Fault Zone; AFZ = Avoca Fault Zone; HFZ = Heathcote Fault Zone; MWFZ = Mt Wellington Fault Zone; WF = Wonnangatta Fault; KFZ = Kiewa Fault Zone; IFZ = Indi Fault Zone, figure from Gray (1988). (c) Map showing the major faults, structural trends and outcrop patterns of Palaeozoic rock in central Victoria. AFZ = Avoca Fault Zone; WF = Whitelaw Fault; MF = Muckleford Fault, figure modified from Foster et al. (1998). (d) Cross section showing the location of the Bendigo goldfield in a fold and thrust system, and close to crustal-scale and subsidary faults, figure modified from Foster et al. (1998). 232

259 Synthesis 6.5 Tectonic Model Tectonic Model Using the constraints and relationships discussed, a tectonic model for the evolution of Queensland is interpreted from ca Ma (Figure 6.10). In this model, west directed subduction under the continental margin initiated by ca 560 Ma (Figure 6.10 (a)), synchronous with deposition of the Anakie Metamorphic Group and equivalent rocks in a back arc continental setting (Section 6.2.1). D1 deformation and M1 metamorphism in the Anakie Metamorphic Group are interpreted to have taken place shortly after, ca 510 Ma. The primitive Buckland Hills Diorite (508 ± 7 Ma, Section 6.3) is interpreted to have intruded the Charters Towers Metamorphics prior to the onset of crustal shortening (Figure 6.4), and may represent one of the only known pre-shortening intrusives in the area. The location of the future Marlborough Ophiolite (Figure 6.5) is not exactly known, but postulated to be east of the back arc spreading region, and maybe even part of the subducting slab (Section 6.5.1). After ca 510 Ma (and after D1/M1 in the Anakie Metamorphic Group), but prior to 500 Ma, the region of back arc continental extension is interpreted to have undergone an episode of east-west shortening and crustal thickening (Figure 6.10 (b)). Evidence for this is found in the cessation of Cambrian sedimentation (Figure 6.4), as well as north-south trending upright isoclinal folding preserved in the northern Anakie Inlier, and in the Charters Towers Metamorphics. In the other Neoproterozoic-Cambrian metamorphic complexes, and more deformed areas of the Anakie Metamorphic Group, the east-west shortening is preserved as a high-angle differentiated fabric in microlithons of a younger flat-lying foliation (Section 6.3). Heat flux from the lithosphere appears to have been interrupted at the onset of shortening, however heating via conduction overlaps with shortening deformation in the Anakie Metamorphic Group (Figure 6.8). Moderate-P and T type M2 metamorphism in the Anakie Metamorphic Group persists after shortening ceases, resulting in idioblastic growth of garnet and staurolite (Section 2.3.3). Extension dominated processes resumed after ca 500 Ma, evidenced by deposition of the 70 Mile Range Group, Fork Lagoons Beds and Judea Formation in a continental back arc setting (Figure 6.4, Section 6.3.2, Figure 6.10 (c)). A flat-lying foliation (S3) developed in the Anakie Metamorphic Group, and also in most of the other equivalent metamorphic complexes (Section 6.3.2). The flat-lying foliation in the Anakie Metamorphic Group is interpreted to record the transition from shortening to extension (Figure 6.4, Figure 6.8), as well as preserving the upper limits of where subhorizontal extensional deformation reorients upright structures in the metamorphic complexes (Section 6.3.2, Figure 6.10 (c)). Heat flux to the base of the lithosphere during extension is interpreted to have resulted in partitioning of ongoing flat-lying foliation 233

260 6.5 Tectonic Model Synthesis Late Neoproterozoic (~560 Ma) - ca 510 Ma crustal extension W Deposition of sediments, mafic lavas and volcanics in continental back arc. Formation of S1, coeval with high-t/low-p metamorphism in the Anakie Metamorphic Group. Intrusion of Buckland Hills Diorite into the proto-charter Towers Metamorphics. E 0 depth km MOHO oceanic crust 50 lithosphere asthenosphere HINGE RETREAT asthenosphere (a) 510 Ma Ma crustal thickening (transient) W Upright isoclinal folding. Heat input ceases, but conductive heating continues for a short time. Formation of early differentiated foliation (S2) in the AMG. E 0 depth km 50 MOHO NEBINE RIDGE oceanic crust lithosphere asthenosphere asthenosphere (b) 234

261 Synthesis 6.5 Tectonic Model 500 Ma Ma crustal extension 70 Mile Range Group, Judea Formation, Fork Lagoons Beds CRM AMG 0 depth km 50 Fat Hen Creek Complex MOHO NEBINE RIDGE oceanic crust lithosphere asthenosphere HINGE RETREAT asthenosphere Resumed extension and development of flat-lying foliation, synchronous with deposition of the 70 Mile Range Group, Fork Lagoons Beds and Judea Formation and intrusion of the Fat Hen Creek Complex. (c) Figure 6.10: Proposed tectonic setting during the Early Palaeozoic, section is oriented east-west. See text for detailed description. Red zones indicates areas of heating and metamorphism, AMG = Anakie Metamorphic Group, CRM = Cape River Metamorphics. Blow ups in (c) show oppposing shear sense in different regions as a result of basement controlled deformation, and highlights different crustal levels of the AMG and CRM. 235

262 6.5 Tectonic Model Synthesis development into deeper crustal levels during the Ordovician (Section 6.3.2). This is evident from ca Ma mica cooling ages of the flat-lying foliation in the Anakie Metamorphic Group, compared to ca Ma mica cooling ages in the Cape River Metamorphics (Section 6.3.2). Syn-deformation granitoid intrusives of the Fat Hen Creek Complex in the Cape River Metamorphics indicate ongoing flat-lying foliation development from ca Ma, synchronous with sedimentation and volcanism (Section 6.3). Basement controlled deformation is interpreted to have resulted in opposing kinematics of the flat-lying foliation in different areas during extension (Section 6.3.2). An episode of regional shortening deformation at ca 443 Ma is marked by a break in sedimentation and widespread upright folding of the flat-lying foliation (Figure 6.4). This episode of deformation in the Anakie Metamorphic Group coincides with the earliest known gold mineralisation, which forms in D4 structures (Chapter 5). Discussion The interpreted tectonic model accounts for the timing and features of deposition, structure and metamorphism in the relevant rocks of Queensland. However, a point of contention is the location and influence of the interpreted west dipping subduction zone. On the basis that subduction initiation preferentially occurs at ocean-continent boundaries (Cloos, 1993; Regenauer-Lieb et al., 2001), a fossil subduction trench may lie between between the Anakie Inlier and the Marlborough Ophiolite. If the Marlborough Ophiolite were to represent true oceanic crust adjacent to a continental passive margin, then subduction initiation (and associated trench) may have occurred within 300 km east of the Anakie Inlier. Eclogite in the Yarrol-Peel Fault system that forms the eastern margin of the Marlborough Ophiolite yielded zircon with a SHRIMP U-Pb age of 572 ± 22 Ma (Watanabe et al., 1998). This age is considered by Watanabe et al. (1998) to be the age of eclogite formation in a subduction zone, and may outline the location of the ancient subduction margin. The timing of shortening deformation and crustal thickening at ca 510 Ma broadly correlates with the Delamerian Orogeny of southern Australia and the Ross Orogeny of Antarctica (Flöttman et al., 1993; Withnall et al., 1996; Boger and Miller, 2004; Foster et al., 2005). The cause of shortening has been variously attributed to; island arc-continent collision in southern Australia and Northern Victoria Land (Kleinschmidt and Tessensohn, 1987; Foster et al., 2005); a shift in plate convergence kinematics in Southern Victoria Land (Encarnación and Grunow, 1996); and terminal suturing of Gondwana components resulting in a shift in relative plate mo- 236

263 Synthesis 6.5 Tectonic Model tion (Boger and Miller, 2004). Considering that there is no local evidence to suggest arc-collision at ca 510 Ma along northeastern Australia, and that shortening deformation is synchronous over thousands of kilometres along the same margin, a large scale event or change in plate behaviour similar to that proposed by Boger and Miller (2004) is the most likely cause of ca 510 Ma shortening (Figure 6.11). South America West Gondwana Africa K Mozambique suture D spcm Antarctica Indo - Antarctica India Kuunga suture P/Y Australia G Ross-Delamerian Orogeny Terminal suturing Neoproterozoic-Cambrian metamorphic complexes in Queensland Figure 6.11: Interpreted Gondwana configuration during the ca Ma Ross-Delamerian Orogeny. This reconstruction interprets that plate reorganisation related to terminal suturing along the Kuunga zone resulted in an abrupt change in relative plate motion. This resulted in shortening deformation along the southeast margin of Gondwana, manifest as the Ross-Delamerian Orogeny. D = Dwarhai craton; G = Gawler craton; P/Y = Pilbara/Yilgarn craton; K = Kalahari craton; spcm = southern Prince Charles Mountains. Figure modified from Boger and Miller (2004). Rocks of the western Lachlan Fold Belt in southeastern Australia preserve evidence for rollback of a west-dipping subduction zone, coevally with sedimentation in an extending back arc from ca 500 Ma to ca 460 Ma along the east-australian margin (Foster and Gray, 2000; Betts et al., 2002; Foster et al., 2005). The timing of back arc basin development along the eastern margin of Australia overlaps with flat-lying foliation development in the Anakie Metamorphic Group and equivalent rocks in Queensland, and probably occurs as part of the same suprasubduction zone setting. 237