Society of Economic Geologists Monash Student Chapter 2017 Field Trip

Transcription

1 Society of Economic Geologists Monash Student Chapter 2017 Field Trip The alkalic porphyry & epithermal deposits of the Ordovician June 13 th -19 th, 2017 Field guide prepared by: Christopher Voisey Bsc (Hons), G.I.T Photo references: Left) Py-Cpy rich breccia & Qtz vein, Mineral Deposits of the Cobar Basin, AUSIMM. Top Right) Monzonite porphyry from Cadia Hill, cut by bornite chalcocite vein, D. R. Cooke Bottom Right) Qtz-Mgn-Sulfide from Ridgeway alkalic prophyry, D. R. Cooke

2 ACKOWLEDGEMENTS We would like to sincerely thank the Society of Economic Geologists for their contributions via the Student Chapter Stewart R. Wallace Fund and the Monash School of Earth, Atmosphere and Environment for their financial support. Thanks to Joe Booth at Evolution Mining for accommodating our mine visit to Cowal. We are grateful to Mark Stander and Daniel Steven for hosting us at Peak gold mines in Cobar. Thanks to Jeneta Wellard and Brooke Lees for their hospitality in hosting our group at the Northparkes mine. Special thanks to Nathan Fox (CODES) for his insights and providing materials for this guidebook.

3 June 13th 19th, 2017 Itinerary June 13 th : Travel day! Melbourne Woolshed flat campground June 14 th : Travel day! Woolshed flat Cobar June 15 th : Peak gold mine visit. Cobar Parkes June 16 th : North Parkes mine visit. Parkes West Wyalong June 17 th : Lake Cowal deposit visit. West Wyalong Gundagai June 18 th : Travel day! Gundagai Mount Pilot National Park June 19 th : Return to Melbourne

4 CONTACT INFORMATION Accommodation Cobar Caravan Park Address: 101 Marshall Street, Cobar NSW 2835 Phone: (02) Newell Hwy Hotel & Caravan Park Address: Forbes St, Parkes NSW 2870 Phone: Country Lodge Motor Inn Address: 25 Main St, West Wyalong NSW 2671, Australia Phone: Gundagai Motel Address: 264 Sheridan St, Gundagai NSW 2722, Australia Phone: Mine Site Visits Daniel Steven Senior Geologist Peak Gold Mines Hillston Road, PO Box 328, Cobar NSW, Australia, 2835 T Jeneta Owens Exploration Team Leader Northparkes Mines, PO Box 995, Parkes, NSW 2870, Australia T: Joseph Booth Geology Superintendent Cowal Operations Lake Cowal Road, Lake Cowal, NSW, 2671 T:

5 PREFACE The alkalic porphyry & epithermal deposits of the Ordovician Overview The Society of Economic Geologists Monash Student Chapter 2017 field excursion comprises visits to some of the Australia s most significant porphyry deposits and their associated epithermal systems. These deposits are located within the fragmented Macquarie Arc and record over 50 million years of arc construction and geochemical evolution before being accreted to the margin of Gondwana. The emplacement of Au-rich alkalic magmas into the accreted terrane is responsible for the deposition of more than 50 million ounces of gold. This excursion will focus on the key characteristics of these world class alkalic porphyry and epithermal systems by using drill core at mine sites and outcrop exposures when accessible. Highlighting distinctive alteration and mineralisation styles will aid in understanding not only these ore forming systems, but also those which are analogous such as modern intraoceanic arcs (e.g., SW Pacific) as well as ancient accreted arcs (e.g., British Columbia). Geological Setting The tectonic evolution and mineralisation of the Lachlan Orogen is complex and research to understand the evolution is still ongoing because of its nature and economic mineral endowment (Gray and Foster, 2004; Squire and Miller, 2008; Glen, 2005). A particular component of interest is the Macquarie Arc (Fig. 1), an island arc that was formed on the northern end of a 900 km long trench off the Delamerian coast before being accreted to Gondwana. These authors (Glen et al., 2007; Glen, 2009; Moresi et al., 2014; Cayley et al., 2012; Cayley, 2013) suggest that the Macquarie Arc was accreted above a west-dipping subduction zone on the eastern margin of Gondwana as a linear belt. The collision of VanDieland micro-continent with the Eastern Gondwana margin during the Benambran Orogeny has played a critical role in the evolution and resultant post-silurian architecture (Cayley, 2012; Moresi et al., 2014; Kreuzer et al., 2014; Cayley, 2013).

6 Figure 1: Map of the Ordovician Macquarie Arc from Percival and Glen (2007).

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8 Integration of geochronology, stratigraphy and geochemistry confirms periodic evolution of the arc with four pulses of arc-related magmatism over 50 Million years (Fig. 2) (Cooke et al., 2007; Crawford et al., 2007; Ferguson et al., 2007). Originating as an intraoceanic arc in the Early Ordovician, the Macquarie Arc evolved from producing high-k calc-alkaline and shoshonitic composition melts to medium-k calc-alkaline magmatism (Glen et al., 2007a). Volcanism returned to high-k calc-alkaline and shoshonitic compositions in the Late Ordovician and these rocks are host to mineralization around Cadia and Northparkes (Crawford et al., 2007; Harris et al., 2014). The Macquarie Arc was then accreted to the margin of Gondwana during the Benambran Orogeny and post-orogenic extension allowed the emplacement of alkali auriferous magmas in the Silurian (Gllen et al., 2007b; Fox et al., 2015). Mineralisation The mineral systems in Macquarie Arc include the alkalic (e.g., Cardia and North parks district) to calc-alkaline (Copper Hill, Cargo) porphyry Cu-Au, skarn (Big Cadia), high sulfidation Au-Cu (Peak Hill) and carbonate base-metal epithermal deposits (Cowal) (Fig. 3) (Sillitoe, 1997; 2000; Holiday & Cooke, 2007). This variety of deposit types is typical for subduction-related magmatic arcs and their metallogeny is spatially and temporally related to the evolution of the arc over time. The alkali porphyry deposits are centred upon a post-subduction, oxidized porphyritic monzonite intrusive complex, and related to the early phase of Benambran arcrelated magmatism (Kreuzer et al., 2014). These deposits are the most economically significant. On the other hand, the calc-alkaline porphyry Cu-Au deposits hosted in ca. 450 to 445 Ma calc-alkaline intrusions emplaced during syn-subduction are small and remain poorly understood (Glen et al., 2009). Later deformation partially dismembered the arc thereby redistributing porphyry Cu-Au systems and their host rocks (Kreuzer et al., 2014; Huston et al., 2016). Table 1 contains a summary of the resources for significant deposits and prospects in the Macquarie Arc.

9 Figure 2: Tectonic evolution of the Macquarie Arc from Glen, Craworf & Cooke (2007).

10 Figure 3 Metallogeny of the Macquarie Arc. LTZ = Lachlan Transverse Zone. From Anothy Harris & Nathan Fox (2015)

11 Table 1: Total resources (indicated + inferred + measured) of porphyry and epithermal occurrences in the Macquarie Arc. From Anthony Harris and Nathan Fox (2015).

12 DAY 3: FIRST MINE SITE VISIT JUNE 15 TH Peak Gold Mine Cobar-style Au-mineralization, Cobar Basin. The Cobar mining district occurs along the eastern margin the regional tectonic-stratigraphic Early Devonian age Cobar Basin, which lies within the northern part of the Central Belt of the Lachlan Orogen. This polymetallic basin is host to 23 major economic mineral deposits that have historically produced 200 tonnes of Au and over 6.2 tonnes of base metals, making it one of Australia s most prospective exploration areas (David, 2014) The Cobar Superbasin System developed during the Silurian-Devonian as four deep-water troughs surrounded by shallow-water shelfs and was since inverted by combined thick and thin-skinned tectonics (Glen, 1990; Glen, 1995). The northern Cobar Basin consists of siliclastic sediments that are locally intruded by felsic volcanics (Fig. 4). The southern portion is dominated by bimodal volcanic and associated volcaniclastic rocks. During the Early Devonian, scattered carbonate reefs were deposited along the eastern margin of the basin and as rifting associated with the Tabberaberran Orogeny persisted these reefs broke down and collapsed into the superbasin depths. The overall structure of the Cobar Basin is NW-SE folding that is overprinted by NE-SW folding and eastwards oblique thrusting. The basin development was generally influenced by the emplacement of Silurian aged granitic intrusions that acted as structural weaknesses. The Cobar Superbasin System as a whole represents a mineralisation continuum whereby the deposits formed during formed during a syn-rift phase and then underwent structural overprinting and green-schist grade metamorphism during basin inversion. In the early mineralisation phase, intrusion related epithermal, VMS and Irish type deposits were formed in the syn-rift sedimentary sequences. Ultimately, basement architecture controls their occurrences and they are distributed along intersections of growth faults and major transform/transfer faults. Later mineralisation events formed the Cobar style deposits, quartz vein hosted deposits and MVT deposits. These occurrences are controlled by the structures associated with the inverted tectonic environment and include; deflected segments of strike slip faults, intersections of reactivated growth and transform/transfer faults, and at the junction of major faults. The Peak Gold Mines operation lies within a 10-kilometre section of the Rookery Fault system, referred to as the Peak Mine Corridor (Fig. 5). Five separate gold-copper deposits have been developed into underground mines and include: Perseverance, Peak, New Occidental, Chesney and New Cobar. Mineralization often occurs as polymetallic sulphides, which vary from gold-copper-lead-zinc assemblages at Peak and Perseverance, to simpler copper-gold at New Cobar, Chesney, and Great Cobar. Gold mineralization typically occurs as discrete lenses within envelopes of base metal mineralization (David, 2014).

13 Figure 4: Simplified geological map of the Cobar Basin.

14 Figure 5: Cobar district geological settings and mineralisation.

15 DAY 4: SECOND MINE SITE VISIT JUNE 16 TH Northparkes porphyry Au-Cu district and Peak Hill high-sulfidation epithermal Au, Junee-Narromine Volcanic Belt The Northparkes porphyry Au-Cu deposits are associated with a late Ordovician shoshonitic volcano-intrusive centre - the Goonumbla Volcanic Complex (Fig. 6). The Goonumbla Volcanic Complex is part of the Ordovician to Early Silurian Junee-Narromine Volcanic Belt, a remnant of the Macquarie Volcanic Arc. These shoshonitic lavas and associated volaniclastic rocks were deposited in a shallow marine to emergent environment (Simpson et al., 2000). The complex is divided into three parts (Lickfold et al., 2003), all products of arc volcanism, and include: Nelungaloo Volcanics, which include Early Ordovician andesitic lavas that are overlain by volcanic conglomerate and sandstone, as well as the sedimentary rocks of the Yarrimbag Formation. Goonumbla Volcanics, which comprise a Middle to Late Ordovician sequence of coherent basaltic andesitic to trachyandesitic volcanic rocks (lavas and shallow sills with peperitic margins), volcaniclastic rocks of similar composition, as well as minor intercalated limestones. Wombin Volcanics, which are typically dark red hematite-dusted glassy lavas, ignimbrites, polymictic volcanic breccias and other volcanic sediments that are of Late Ordovician in age. The Goonumbla and Wombin Volcanics are believed to have formed as an ancient subaqueous volcanic apron built on top of sedimentary and volcanic rocks along the flanks of a stratovolcano during the Mid to Late Ordovician (Heithersay and Walshe, 1995; Simpson et al., 200). The volcanic complex has been intruded by multiple monzonite and quartz monzonite bodies, including the quartz monzonite porphyry pipes associated with Au-Cu mineralisation in the district. Pre- and syn-mineralisation brittle structures are interpreted to have localised the emplacement of the porphyry intrusives, including the NNW-trending Endeavour Linear, as well as several other NW- and NE-trending fracture sets. Minor brittle movement on certain post quartz monzonite porphyry structures resulted in mostly small-scale disruption and some dislocation of the economic mineralisation (Lickfold et al., 2003).

16 Figure 6: Geological map of the Northparkes district and the Peak Hill mine, Junee-Narromine Volcanic Belt. From Lickfold et al. (2007)

17 DAY 5: THIRD MINE SITE VISIT JUNE 17 TH Lake Cowal low-sulfidation epithermal Au, Junee-Narromine Volcanic Belt The Lake Cowal Volcanic Complex lies within the southern end of the Junee Narromine Volcanic Belt of the Macquarie Arc (Fig. 7). This volcanic complex is characterised by Ordovician, subaqueous, volcano-sedimentary successions that have been intruded by diorite to granodiorite stocks and dykes (Crawford, Cooke & Fanning 2007). Most have low to medium K, calc-alkaline chemistry which is suggested to be consistent with subductionrelated magmatism. The Cowal Au-Cu district is host to several porphyry-style prospects, some of which also have epithermal characteristics (Fig. 7) (e.g., Logjam and Milly Milly). They are associated with monzodiorite, monzonite and granodiorite intrusions of predominantly medium-k calcalkaline affinity (Crawford, Cooke & Fanning 2007; Zukowski et al. 2014). Gold mineralisation has been dated at 440 Ma which is contemporaneous with early stages of the Benambran Orogeny, as well as the alkalic porphyry Cu Au mineralisation typical of Cadia and Northparkes (Lickfold et al. 2003; Forster et al. 2004; Wilson et al. 2007; Glen, Crawford & Cooke 2007; Glen 2013). Gold is associated with quartz-pyrite-sphalerite-carbonate veins and the distribution of these assemblages is controlled by rock type, structure and geometry of the mineralised zones. These systems transition from early high-temperature assemblages (e.g., actinolite-magnetite-albite-chlorite), typical of an alkali porphyry environment, to assemblages more characteristic of epithermal environments (e.g., quartzcarbonate-adularia-illite) The Lake Cowal Volcanic Complex is also host to large deposits that are not porphyry style but are: 1) gold dominant, with characteristics of epithermal gold deposits; 2) strongly structurally controlled by faults and shears; and 3) are associated with rocks of low medium-k calc-alkaline affinity (Blevin 2002; Crawford, Glen et al. 2007; Cooke & Fanning 2007; Forster et al., 2014). Several of these gold deposits lie along a north south trending, structurally deformed belt that is known as the gold corridor located along the western margin of the Cowal Volcanic Complex (Zukowski et al. 2014; Henry et al. 2014). They include E46, E42 and E41 and are controlled by faults and shears and are situated along the margin of a diorite sill (Fig. 7) (Miles & Brooker 1998). These deposits have been described as mesothermal (Miles & Brooker 1998) or as transitional between porphyry and lowsulfidation epithermal gold systems (Bywater et al. 2004; Henry et al. 2014).

18 Figure 7: Geology of the Lake Cowal district from Zukowski et al. (2014).