MINERAL DEPOSITS OF THE COBAR BASIN

Transcription

1 MINERAL DEPOSITS OF THE COBAR BASIN

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5 360 hard rock deposits in Cobar Basin Cu, Au, Ag, Pb, Zn combinations and quartz-au veins Cobar

6 23 major deposits in Cobar Basin

7 World class Endeavor mine World class CSA mine Nymagee, Hera Peak, New Cobar, New Occidental, Perseverance, Chesney DPI size classification Manuka Shuttleton Mt Hope Mallee Bull, May Day

8 Combined historical production and indicated ore reserves exceed (David 2005): 2.2 Mt Cu 2.4 Mt Pb 3.8 Mt Zn 4 500t Ag 200t Au A substantial metal endowment make this one of Australia s most desirable and high prospectivity exploration areas

9 WHAT IS THE COBAR BASIN? A Late Silurian - late Early Devonian, shallow to deep marine intracratonic pullapart basin within the Lachlan Orogen Cobar basin site Diagram courtesy of Ian Metcalfe)

10 WHAT IS THE COBAR BASIN? Formed as a result of extension in the back-arc, intracratonic basin. Two stages of basin development: 1. sag or syn-rifting phase (basin extension) - high energy turbidites and adjacent shallow water shelf sediments - felsic volcanics erupt along basin margins - subsidence along normal listric faults - movement along basement transform faults - many faults become growth faults

11 From David (2005)

12 WHAT IS THE COBAR BASIN? Two stages of basin development: 1. sag or syn-rifting phase (basin extension) - high energy turbidites - felsic volcanics erupt along basin margins - subsidence along normal listric faults - movement along basement transform faults - many faults become growth faults 2. post-rifting phase - low energy turbidites - progressive infilling of basin

13 BASIN INVERSION The onset of compressional tectonics reverses movement along extensional faults and the basin is folded, low grade regionally metamorphosed and uplifted Inversion took place during Middle Devonian Tabberabberan Orogeny Tectonic environment was oblique, left lateral, transpressional. Resulted in initial open folding and axial plane cleavage development, low angle thrusting and reactivation of some low angle normal listric faults as reverse faults Later during inversion tear faults developed along eastern margin of Cobar trough, forming complex fault arrays and tight folding in that area

14 High strain zone along eastern margin of Cobar Trough Progressive deformation during inversion From David (2005)

15 High strain zone

16 MINERALISATION Early and late mineralisation types: Early types associated with syn-rift stage of basin development i.e. Include: > carbonate-hosted Zn-Pb-Ag (e.g. Manuka/Wonawinta, Endeavor/Elura) > epithermal Au (e.g. McKinnons Tank, Mt Boppy?, Mineral Hill) > VMS Au-Pb-Zn-Cu-Ag (e.g. Mt Boppy?, Mt Hope) Late types: Syn-deformational, accompanying inversion; mesothermal: 1. structurally controlled within and adjacent to faults most deposits along the eastern edge of the Cobar Trough, and 2. small number are lithologically controlled within and adjacent to volcaniclastic debris flows - Mallee Bull, May Day, Shuttleton

17 EARLY MINERALISATION Carbonate-hosted Zn-Pb-Ag - Endeavor (Elura) Sub-vertical massive sulphide lenses linked by massive and vein/stringer style mineralisation. 700 m strike, 800 m depth Two mineralised systems: - turbidite-hosted mineralisation - massive sulphide pyrrhotite/pyrite core within a concentric siliceous pyrrhotite/pyrite envelope surrounded by vein and stringer mineralisation; strong magnetic signature - limestone-hosted semi massive pyrite-+ sphalerite + galena Alteration: pervasive siderite, silicification and carbonate spotting Origin inconclusive: either a partially remobilised syngenetic Irish-type (SEDEX-MVT hybrid), or a mesothermal replacement-type (inversion-related)

18 From David (2005) Endeavor section

19 EARLY MINERALISATION Epithermal (hybrid) McKinnons Gold mine Primary mineralisation early and late stage: structurally controlled, quartz vein and quartz breccia -hosted gold and local sphalerite and galena Alteration: quartz-sericite haloes around silicified faults Origin: Early-stage, base-metal mineralisation reflects structurally focussed inversion processes. Late-stage gold mineralisation is magmatically driven and of epithermal style. (Forster & Seccombe 1999)

20 EARLY MINERALISATION VMS Mount Hope Primary mineralisation: 1. Cu (Pb-Zn) - narrow, vein-like bodies with ore distributed in shoots 2. Cu - stockwork of narrow veins and veinlets Chalcopyrite, pyrrhotite (pyrite, sphalerite, galena) Alteration: chlorite, sericite, silica, ankerite Host rocks: felsic tuffs and lavas Origin: Exhalative mineralisation in shallow marine to subaerial environment; subsequent deformation and remobilisation during inversion

21 SYN-DEFORMATIONAL MINERALISATION Two classes of deposit style: high strain deposits, low-medium strain deposits High strain deposits in the eastern Cobar trough Cobar-type deposits Structurally controlled, highly discordant to stratigraphy. Occurs in lenses with short strike lengths (<300m). Narrow width (<30m) and steep dip. Strong vertical extent (+1km). Weak alteration halos. High Grade.

22 SYN-DEFORMATIONAL MINERALISATION High strain deposits in the eastern Cobar trough Cobar-type deposits CSA Cu, Pb, Zn, Ag Vertical, podiform lenses within steep shear zones Great Cobar Cu, Au Lenses in steeply dipping shear in hinge of anticline adjacent to transform fault New Cobar Cu-Au Steep lenses to about 700 m deep in splay fault associated with a jog in the Great Chesney fault. Peak Au, Cu, Pb, Zn, Ag Podiform lenses in dilational zones within the steeply dipping Peak Shear New Occidental Au (Pb-Zn) Subvertical lenses to about 1200 m deep in a dilational jog in steeply dipping Gt Chesney fault Hera Au, Cu, Pb, Zn (Ag) Steeply dipping lenses to 250 m deep in splay of Rookery Fault

23 From David (2005) 1500 m

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25 Hera mine section From David (2005)

26 SYN-DEFORMATIONAL MINERALISATION High strain deposits in the eastern Cobar trough Cobar-type deposits Mineralisation: Lenses of sulphides within zones of strong quartz veining Lens boundaries gradational Some lenses largely massive sulphide, whereas others are sulphides in quartz veins or quartz vein breccia Veins and lenses show complex geometry; veins occur in multiple overprinting sets, and early veins are strongly deformed. Evidence for multiple, episodic deformation associated with reverse faulting during basin inversion. Sulphides commonly chalcopyrite, sphalerite, galena, pyrite, pyrrhotite, cubanite and bismuthinite. Magnetite present in many Cu-Au and Au deposits.

27 Vein-style sphalerite, Endeavor Massive sulphides, Endeavor Massive sulphides, Hera

28 SYN-DEFORMATIONAL MINERALISATION High strain deposits in the eastern Cobar trough Alteration: Pervasive silicification, commonly producing chert-like altered siltstones Pervasive Fe-chlorite and local zones of Mg-chlorite Rare volcanics are sericite-altered

29 SYN-DEFORMATIONAL MINERALISATION Low-moderate strain deposits in the central Cobar trough Lithologically controlled, concordant with stratigraphy. Occurs in lenses with short strike lengths (<300m). Narrow width (<30m) and steep dip. Strong vertical extent (+800 m). Weak alteration halos. High Grade.

30 SYN-DEFORMATIONAL MINERALISATION Low strain deposits in the central Cobar trough Mallee Bull Cu-Ag-Zn-Pb-Au Steeply dipping, stratabound lenses of massive, disseminated and stringer-vein sulphides Mayday mine Zn-Pb-Cu-Ag-Au Steeply dipping, stratabound and shear-hosted lenses of massive and disseminated sulphides Shuttleton Cu-Pb-Zn-Ag Steeply dipping, stratabound lenses of massive, and stringer-vein sulphides Wirlong Cu-Zn-Pb-Ag Stratabound and shear-hosted massive, disseminated and stringer-vein sulphides

31 metres Mallee Bull section UPPER AMPHITHEATRE GROUP Mallee Bull formation - turbiditic silty sandstone to mudstone Mallee Bull formation allochthonous facies - gravelly to bouldery felsic volcanic detritus; lesser limestone and intermixed sandstone and mudstone 600 Mallee Bull formation - Keep It Dark Sandstone member - massive, well sorted fine-grained quartz sandstone LOWER AMPHITHEATRE GROUP Shume Formation - turbiditic quartz sandstone to mudstone MINERALISATION Massive to semi-massive pyrrhotite-pyrite 800 Massive to semi-massive sphalerite-galena Chalcopyrite-rich stringer zone, becoming sphalerite-galena - rich near-surface

32 Mineralisation envelope

33 SYN-DEFORMATIONAL MINERALISATION Low strain deposits in the central Cobar trough Mineralisation: Identical to Cobar-type, except mineralisation always intimately associated with felsic volcaniclastics, both within volcaniclastics and adjacent turbidites. Magnetite rare. Alteration: Identical to Cobar-type

34 Massive pyrite-pyrrhotite Mallee Bull Semi-massive and stringer chalcopyrite-pyrrhotite Mallee Bull

35 SYN-DEFORMATIONAL MINERALISATION Genetic model criteria: Isotopic data: S isotopes source in host basin sediments Pb isotopes basement Au and Cu and basinal Ag-Pb-Zn and Cu; age corresponds with basin formation and inversion O and H isotopes water was metamorphic followed by meteoric, with isotope exchange between hydrothermal fluids and local host rocks Ar-Ar Age dating Middle Devonian: Hera ± 2.2 Ma (Downes at al 2015). Elura 386.2± 2.0 to ± 1.6 Ma (Sun et al 2000). Peak ± 1.4 Ma (Sun et al 2000) Fluid chemistry: low-moderate T fluids interpreted as basinal, high T fluids interpreted as metamorphic

36 SYN-DEFORMATIONAL MINERALISATION Now, let s apply those criteria (eastern high strain zone): Deposits structurally controlled into dilational sites, at fault intersections and anticlinal hinges and aligned within or close to regional cleavage High discordance to stratigraphy Isotopic data indicates S source in host basin sediments, Pb derived from basement and basinal sources, O and H indicate metamorphic and meteoric sources Fluid chemistry indicate basinal and metamorphic fluid sources Age data correspond with inversion All criteria support a process of metamorphic fluids generated during basin inversion passing along major pre-existing faults and shears into dilational sites. The fluids were derived in part from the basement and basin, sourcing metals from multiple sites. On-going deformation during inversion resulted in complex vein relationships and remobilisation and deformation of sulphides.

37 Lawrie & Hinman (1998)

38 SYN-DEFORMATIONAL MINERALISATION And now for the central low-moderate strain zone: Deposits lithologically controlled by permeable volcaniclastic gravels with local alignment within or close to regional cleavage; some show additional shear control Close spatial association with major regional crustal fractures Stratabound, parallel or subparallel to stratigraphy Isotopic data identical to high strain deposits Age data correspond with inversion, or with basin sedimentation Previously regarded as VMS because of volcanic rock association. All criteria support a process similar to the high strain Cobar-type deposits. Fluid transport was achieved via major faults and shears, and by gravelly volcaniclastic aquifers. Deposits localised into sites similar to petroleum traps (e.g. anticlinal limbs, impermeability barriers), locally influenced by dilational sites such as jogs adjacent to major fault systems.

39 FINAL COMMENTS Enormous volume of research since 1970s but? A mature exploration area?

40 Thanks for listening!