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Gold metallogeny of the northern Capricorn Orogen Imogen Fielding Simon Johnson Geological Survey of Western Australia Geological Survey of Western Australia 100 Plain Street, East Perth, WA, 6004 100 Plain Street, East Perth, WA, 6004 Imogen.Fielding@dmirs.wa.gov.au SimonPaul.JOHNSON@dmirs.wa.gov.au

INTRODUCTION

Gold is a significant economic driver of many developed and developing countries but despite continued high levels of funding for gold exploration worldwide, the rate of discovery has drastically fallen over the past decade (Jennings and Schodde 2016). The odds for exploration success can be vastly improved by understanding the large-scale controlling factors that affect the distribution and occurrences of gold within a region (Hronsky et al., 2012, McCuaig et al., 2010). Determining how gold mineralization relates to the regional-scale crustal architecture and tectonothermal evolution is fundamental for effective exploration targeting. This is particularly important in Proterozoic orogens and Archean cratons as they both commonly show complex geological histories derived from multiple overprinting tectonic events (Johnson et al., 2017a, Raimondo et al., 2010). Such areas commonly lack a detailed chronology for the timing of hydrothermal activity and gold mineralization because dating these events can be challenging as many common chronometers are reset during post-mineralization events (Chesley 1999).

The Capricorn Orogen of Western Australia (Figure. 1) has a long and well-documented geological history (Johnson et al., 2017a and references within). A recent deep crustal seismic reflection survey across the orogen, as well as numerous other geophysical surveys, have led to a growing understanding of the relationship between the deep crustal architecture and the location and setting of the ore deposits (Johnson et al., 2013). Gold mineralization in the Capricorn Orogen shows a close spatial relationship with pre-existing crustal-scale faults that show evidence for multiple reactivation and mineralizing events (Fielding et al., 2017a, 2019, under review, Johnson et al., 2013), making it an ideal area to assess the relationship between the timing of gold mineralization, crustal architecture and the fault reactivation history. Here we provide a summary of recent high-precision geochronology to provide a more holistic understanding of the timing and setting of gold mineralization and its relation to shear zone/fault movement and associated hydrothermal fluid flow across the northern Capricorn Orogen.

GEOLOGICAL SETTING The Capricorn Orogen of Western Australia is a ~1000 km long, 500 km wide region of variably deformed and metamorphosed igneous and sedimentary rocks located between the Pilbara and Yilgarn Cratons (Figure 1). It marks the Paleoproterozoic amalgamation of the West Australian Craton during the Ophthalmia and Glenburgh Orogenies. During the 2215–2145 Ma Ophthalmia Orogeny the Pilbara Craton collided with the Glenburgh Terrane which subsequently collided with the Yilgarn Craton during the 2005–1950 Ma Glenburgh Orogeny. Once amalgamated, the orogen experienced over a billion years of episodic intracratonic reworking and reactivation, including basin formation, metamorphism and magmatism. This includes the 1820–1770 Ma Capricorn Orogeny, which was originally interpreted as representing oblique convergence between the Pilbara and Yilgarn Cratons, the 1680–1620 Ma Mangaroon Orogeny, the 1321–1171 Ma Mutherbukin Tectonic Event, the 1030–955 Ma Edmundian Orogeny, the 931–749 Ma Kuparr Tectonic Event and the c. 570 Ma Mulka Tectonic Event. The orogen includes the deformed margins of the Pilbara and Yilgarn Cratons and associated continental margin rocks deposited in the Fortescue, Hamersley and Turee Creek Basins in the north; medium- to high-grade metamorphic rocks of the Gascoyne Province; and various low-grade metasedimentary rocks deposited in the Ashburton, Blair, Padbury, Bryah, Yerrida, Earaheedy, Edmund and Collier Basins, that overlie these tectonic units (Johnson et al., 2017a and references within).

SUMMARY The timing and distribution of gold mineralization in Proterozoic orogens is influenced by crustal architecture which is often established long before gold mineralization occurs. Gold occurrences in such settings are commonly associated with crustal-scale faults formed at cratonic margins. Once established, these faults provide critical pathways for hydrothermal and mineralizing fluids which during repeated fault reactivations can result in remobilization or introduction of new auriferous fluids resulting in overprinting gold events. Recently published geochronological data for the northern part of the Proterozoic Capricorn Orogen in Western Australia show it has experienced at least three episodes of gold mineralization occurring at c. 2400, 1770 and 1680 Ma. Many of the gold deposits are associated with intracratonic reworking during the 1820–1770 Ma Capricorn Orogeny and 1680–1620 Ma Mangaroon Orogeny. Intracratonic settings are not normally considered prospective for gold mineralization due to a lack of input of juvenile material. However, it appears that repeated hydrothermal fluid flow during intracratonic events, has the potential to upgrade gold mineralization or increase gold endowment throughout the orogen, either through gold remobilization or through introduction of new gold, increasing the potential for economic gold deposits. Key words: Crustal architecture, Capricorn Orogen, intracratonic orogen, geochronology, gold metallogeny.

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CRUSTAL ARCHITECTURE A deep crustal seismic reflection survey across the Capricorn Orogen (Figures 1a and 2) has defined the crustal architecture of the West Australian Craton (Johnson et al., 2013). The survey identified several discrete crustal blocks that are exposed at the surface, including the Pilbara and Yilgarn Cratons, and the Glenburgh Terrane, as well as several unexposed deep crustal terranes, including the Bandee and MacAdam Seismic Provinces (Figure 2; Johnson et al., 2013). These discrete tectonic blocks are sutured along three major crustal structures, the Cardilya, Lyons River and Baring Downs Faults (Figures 1a and 2), which most likely represent collisional suture zones associated with the assembly of the West Australian Craton. Other major faults which extend to the mantle include the Nanjilgardy and Talga Faults (Johnson et al., 2013). The location and orientation of these major structures appear to have fundamentally controlled all subsequent intraplate reworking events, including the style and orientation of deformation, as well as the location of magmatism, sedimentation and mineralization (Johnson et al., 2013, 2017b). GOLD MINERALIZATION IN THE NORTHERN

CAPRICORN OROGEN Gold deposits in the northern Capricorn Orogen are intimately associated with the major crustal-scale structures identified in the seismic reflection survey (Johnson et al., 2013). In particular, the Nanjilgardy and Baring Downs Faults appear to be the most important as they both extend to the mantle and show a close association with gold mineralization (Figure. 1 and 2; Fielding et al., 2017a, 2018, 2019, under review, Johnson et al., 2013). The largest gold deposits are associated with the Nanjilgardy Fault, and include orogenic gold mineralization at the Paulsens deposit, and Carlin-like gold mineralization at the Mount Olympus deposit, both with endowments of >1 Moz (Fielding et al., 2017a, 2019). Numerous smaller orogenic gold deposits occur throughout the Ashburton Basin and Wyloo Inlier (Figure 1; Thorne and Seymour 1991, Thorne and Trendall 2001) and include the Belvedere (Fielding et al., 2018) and Star of the West gold deposits (Fielding et al., under review). These deposits have formed parallel to the youngest regional fabric and are interpreted to have occurred during hydrothermal activity associated with the Mangaroon Orogeny. Paulsens The Paulsens gold deposit is an orogenic gold deposit situated in the Wyloo Inlier adjacent to the Hardey Fault, which is a second order structure to the Nanjilgardy Fault. A mineralized quartz-carbonate-sulfide vein is hosted within the Paulsens gabbro which is a 40 m-thick, folded and faulted gabbroic dyke, which was emplaced into sedimentary and volcanic rocks at the base of the Fortescue Group at c. 2700 Ma (Fielding et al., 2017a). Gold mineralization occurs as two distinctive lodes focused along either margin of the auriferous vein, locally termed the ‘upper zone’ and ‘lower zone’. Upper zone mineralization is characterised by massive to brecciated pyrite, with gold that occurs as small rounded inclusions within the pyrite, or along fractures and grain boundaries of pyrite crystals. Lower Zone mineralization is characterized by laminated quartz-carbonate-sulfide veins, abundant carbonaceous stylolites and inclusions of altered siltstone are common and are associated with abundant free gold. Ore-stage alteration assemblages of muscovite–quartz–ankerite ± chlorite ± leucoxene surround the auriferous veins (Fielding et al., 2017a, 2017b).

Figure 1. (A) simplified geological map of the Capricorn orogen. The thick grey lines define the boundaries of the Capricorn orogen. Abbreviations: GC – Gawler Craton; KC – Kimberley Craton; MI – Marymia inlier; NAC – North Australian Craton; PC – Pilbara Craton; SAC – South Australian Craton; WAC – West Australian Craton; YC – Yilgarn Craton; YGC – Yarlarweelor Gneiss Complex. (B) distribution of mineral occurrences within the orogen. First-order structures shown in dotted lines after (Johnson et al. 2017b). Monazite intergrown with ore-stage alteration minerals and euhedral xenotime crystals interlocking with auriferous pyrite provides an age for primary gold mineralization at c. 2400 Ma (Fielding et al., 2017a). This age is synchronous with a Pilbara-wide cryptic orogenic event where widespread hydrothermal activity is recorded by the growth of hydrothermal monazite throughout the western Pilbara (Rasmussen et al. 2005), hydrothermal xenotime growth at the nearby Belvedere prospect (Fielding et al., 2018), resetting of high-U zircons in the Hamersley Group (Pickard 2002), as well as extensive uplift and erosion of the southern Pilbara region (Takehara et al., 2010). The second mineralization event at Paulsens is recorded by the brecciation of former c. 2400 Ma-aged pyrite accompanied by new xenotime growth and deposition of gold

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along fractures and grain boundaries at c. 1680 Ma (Fielding et al., 2017a). This event was coeval with the onset of the intracratonic 1680–1620 Ma Mangaroon Orogeny in the Gascoyne Province farther south. Mount Olympus The Mount Olympus gold deposit is a Carlin-like gold deposit situated along a dextral transpressional fault, locally known as the Zoe Fault, which is a second order structure to the Nanjilgardy Fault (Figure 1; Fielding et al., 2019). Most of the gold occurs in strata-bound lodes where coarse-grained, interbedded sandstone and conglomerate units of the Mount McGrath Formation are separated from dolomitic mudstones of the Wooly Dolomite by the Zoe Fault. Alteration associated with the ore zones includes the addition of quartz, illite and sericite and the removal of carbonate minerals (Young et al., 2003). LA-ICPMS trace element mapping of arsenian pyrite crystals shows that gold forms as a solid solution in the arsenian pyrite crystals and has similar geochemical characteristics to the Carlin gold deposits in Nevada, with comparable relationships between Au–As, Au–Ni and Au–V for zones of pyrite enriched in Au–As, accompanied by elevated Au, As, Cu, Sb, Hg and Tl (Fielding et al., 2019).

Figure 3. Summary histograms showing age modes of gold mineralization at the Paulsens, Mount Olympus, Belvedere and Star of the West gold mines defined by xenotime. Known orogenic events are defined by blue shading = cryptic orogenic event at c. 2400, pink = collisional orogenies, grey = intracratonic orogenies. Mount Olympus (after Fielding et al., under review). In situ geochronology of xenotime crystals intergrown with auriferous arsenian pyrite and ore-stage alteration minerals provides an age for gold mineralization of c. 1770 Ma which is followed by two discrete hydrothermal events at c. 1730 and 1680 Ma (Figure 3; Fielding et al., 2019). Mineralization occurred during the final stages of the 1820–1770 Ma intracratonic Capricorn Orogeny during a change from dominantly compression to dextral strike-slip reactivation of

the Nanjilgardy Fault and its associated structures (Young et al., 2003). The younger two events represent periods of hydrothermal activity and possible fault reactivation. At c. 1730 Ma hydrothermal fluid movement is recorded along the Nanjilgardy Fault at both the Mount Olympus and Paulsens gold deposits (Fielding et al., 2017a, 2019), but does not appear to be related to gold formation (cf. Şener et al., 2005). Finally, the c. 1680 Ma event represents widespread hydrothermal activity and gold mineralization throughout the northern Capricorn Orogen and the southern Pilbara region associated with the early stages of the intracratonic 1680–1620 Ma Mangaroon Orogeny (Fielding et al., 2017a, 2018). Belvedere Belvedere is an orogenic, vein-hosted gold deposit located in the Wyloo Inlier approximately 6.5 km to the southeast of the Paulsens deposit adjacent to the Hardey Fault (Fielding et al., 2018). Mineralized quartz–carbonate–arsenopyrite veins occur in the c. 2082 Ma Belvedere dolerite (Fielding et al., 2018) which intrudes vesicular basalt, polymictic conglomerate, sandstone and siltstone of the c. 2775 Ma Mount Roe Basalt (Thorne and Trendall 2001). Mineralized veins are 2–12 m wide with gold forming as inclusions up to 150 µm within euhedral arsenopyrite crystals and are surrounded by ore-stage alteration assemblages of muscovite–quartz–carbonate ± rutile ± albite (Fielding et al., 2018). Hydrothermal xenotime intergrown with ore-stage alteration minerals and fully encased with arsenopyrite crystals provided an age for gold mineralization at the Belvedere deposit of c. 1680 Ma (Figure 3; Fielding et al., 2018) related to onset of the Mangaroon Orogeny, during reactivation of the Hardey Fault. Star of the West The Star of the West gold deposit is hosted within sedimentary rock of the Ashburton Formation of the Wyloo Group (Thorne and Seymour 1991) and is associated with a narrow fault that is subparallel to, and interpreted as a splay of, the Baring Downs Fault (Fielding et al., under review). Siltstones and fine-grained sandstones are cross-cut by 5–20 cm wide quartz veins and which have been pervasively altered by hydrothermal fluids producing an alteration assemblage of muscovite and large (>2 cm), euhedral pyrite crystals. The gold mineralization occurs as native gold inclusions in the pyrite crystals, and as microscopic inclusions in the altered matrix of the siliciclastic sedimentary rocks rather than in the quartz veins or fault itself. Xenotime intergrown with ore-stage muscovite from gold-bearing hydrothermal aureoles around one of these veins are dated at c. 1672 (Figure 3), interpreted as timing the emplacement of quartz veins and associated hydrothermal activity and gold mineralization. This age is synchronous with the timing of gold mineralization associated with the Nanjilgardy Fault at both the Paulsens and Belvedere gold deposits (Fielding et al., 2017a, 2018) which are implied to have formed during reactivation of the Nanjilgardy Fault and its major ancillary structures during the early stages of the intracratonic 1680–1620 Ma Mangaroon Orogeny.

DISCUSSION Ore deposits in many gold provinces throughout the world tend to cluster around crustal-scale faults and suture zones that commonly mark the boundaries between different tectonic blocks (Korsch and Doublier 2016). During the lead-up to the

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collisional or accretionary events that amalgamate these terranes, the margins of these tectonic blocks may have been the sites for emplacement of juvenile, mantle-derived material, either during initial rifting and breakup or in subsequent magmatic arcs, thus priming the crust and improving the potential for mineralization (Goldfarb et al., 2001). Considering that trans-crustal faults, such as former suture zones, provide critical pathways for the transport of fluid or magma from the mantle and lower crust to form ore deposits in the upper crust (Hronsky et al., 2012, McCuaig et al., 2010), it is probable that such a favourable priming and crustal architecture needs to be established before gold mineralization can take place (Korsch and Doublier 2016). Deep crustal seismic reflection imaging of the Capricorn Orogen and the associated cratonic margins, has significantly improved the understanding of the lithospheric architecture, demonstrating the link, at least in the northern part of the orogen, between major crustal-scale faults and the occurrence of gold mineralization (Johnson et al. 2013). In particular the gold deposits cluster around major crustal structures, the Baring Downs and Nanjilgardy Faults, near the Archean Pilbara and Yilgarn Craton margins (Figure 1) (Johnson et al., 2013, 2017b). Based on past production and remaining resource figures calculated in 2017 (Johnson et al., 2017b), rocks within or directly overlying the northern Yilgarn and southern Pilbara Craton margins host the majority (88% at 502.6 tonnes) of the known gold endowment of the orogen. The central part, which includes the Gascoyne Province and younger basins, hosts only 64.7 tonnes. Although only one of the two main gold-hosting crustal structures, the Baring Downs Fault, has been shown to be a major suture zone (Figure 2), the Nanjilgardy Fault which is significantly more endowed, is a major terrane-bounding structure which separates the 1840–1800 Ma Wyloo Group from the >2200 Ma Shingle Creek, 2629–2440 Ma Hamersley and 2775–2629 Ma Fortescue Groups (Johnson et al. 2013). It is possible that the long-lived nature of this structure could have played an important role during rifting and formation of the Fortescue, Hamersley and Turee Creek Basins, as well as during accretion and ocean closure during the Ophthalmia Orogeny. Understanding the processes of mineralization at a range of scales, the distribution of gold through both space and time can be better understood and hence improve the odds for exploitation success (McCuaig et al. 2010). In the northern Capricorn Orogen, a strong spatial relationship exists between gold occurrences and their proximity to steeply-dipping, crustal-scale faults, or their ancillary structures, which juxtapose different stratigraphic packages or tectonic blocks (Figure 2). These faults have a long lived history of fault (re)activation accompanied by hydrothermal fluid flow and gold mineralization with at least three episodes of hydrothermal gold mineralization occurring across the northern part of the orogen, at c. 2400, 1770 and 1680 Ma (Fielding et al. 2017a, 2018, 2019, under review). These events can be directly linked to discrete, regional-scale tectonothermal events recorded elsewhere in the orogen (Figure 3). At c. 2400 Ma orogenic gold mineralization at Paulsens is linked to the timing of a cryptic orogenic event, although increasing evidence suggests that this event may mark the onset of accretion associated with the Ophthalmia Orogeny. This is followed by deposition of Carlin-like gold mineralization at the Mount Olympus deposit at c. 1770 Ma, the timing of which can be linked the to the final stages of the intracratonic 1820–1770 Ma Capricorn Orogeny during which a change from dominantly compressional to dextral strike-slip reactivation of the Nanjilgardy Fault took

place. The youngest mineralization event occurred at c. 1680 Ma during the early stages of the 1680–1620 Ma Mangaroon Orogeny. This event is characterized by widespread hydrothermal activity and gold mineralization throughout the northern Capricorn Orogen and Pilbara region associated with fault reactivation (Fielding et al., 2017a, 2018, under review, Rasmussen et al., 2007). Apart from the c. 2400 Ma Paulsens orogenic gold deposit, all other gold occurrences in the northern Capricorn Orogen formed during periods of intracratonic reworking. These settings are not usually considered prospective for gold mineralization due to limited input of fertile material (Goldfarb et al., 2001). Although the Capricorn and Mangaroon Orogenies are associated with felsic magmatism (Johnson et al., 2017a), these intrusive rocks were emplaced much farther to the south in the Gascoyne Province and the westernmost parts of the Ashburton Basin (Figure 1) and have no influence on the distribution of gold in the northern part of the orogen. Many of the deposits however, show evidence for multiple hydrothermal or gold forming events (Fielding et al., 2017, 2019). As yet it is not apparent if the total gold endowment of the region was deposited during a single, orogenic gold-forming event at c. 2400 Ma, and that gold from these primary deposits was redistributed during successive reactivation events, or if there were multiple gold-endowment events. At the Paulsens deposit petrographic and geochronological evidence demonstrates that gold was locally mobilised and transported on a millimetre scale during the c. 1680 Ma event (Fielding et al., 2017). The fact that the majority of the younger (c. 1680 Ma) occurrences have limited gold endowments, and lie on the same major crustal structures, suggests that remobilization of pre-existing gold may have played a significant role in forming these deposits. This scenario would negate the need for the generation of regional-scale gold-fertile fluids during these intracratonic reworking events. Seismic imaging of the Capricorn Orogen has led to an improved understanding of the crustal architecture of the orogen. This information combined with recent geochronological information demonstrate an intrinsic link between fault (re)activation, hydrothermal fluid flow and gold mineralization and indicate that the major crustal faults and their splays provide a critical pathway for mineralizing hydrothermal fluids.

ACKNOWLEDGEMENTS This project was funded through an ARC linkage grant (LP130100922), the Exploration Incentive Scheme and an industry scholarship by Northern Star Resources as a part of a PhD by I.O.H. Fielding. I.O.H. Fielding and S.P. Johnson publish with the permission of the director of the Geological Survey of Western Australia.

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Fielding, I.O.H., Johnson, S.P., Zi, J.-W., Rasmussen, B., Muhling, J.R., Dunkley, D.J., Sheppard, S., Wingate, M.T.D., and Rogers, J.R., 2017a, Using in Situ Shrimp U-Pb Monazite and Xenotime Geochronology to Determine the Age of Orogenic Gold Mineralization: An Example from the Paulsens Mine, Southern Pilbara Craton. Economic Geology 112, 1205-30. Fielding, I.O.H., Johnson, S.P., Rogers, J.R., and Elliot, L.K., 2017b, Paulsens Gold Deposit. Australian Ore Deposits, 405-10. Fielding, I.O.H., Johnson, S.P., Zi, J.W., Sheppard, S., and Rasmussen, B., 2018, Neighbouring Orogenic Gold Deposits May Be the Products of Unrelated Mineralizing Events. Ore Geology Reviews 95, 593-603. Goldfarb, R.J., Groves, D.I., and Gardoll, S., 2001, Orogenic Gold and Geologic Time: A Global Synthesis. Ore Geology Reviews 18, 1-75. Hronsky, J.M., Groves, D.I., Loucks, R.R., and Begg, G.C., 2012, A Unified Model for Gold Mineralisation in Accretionary Orogens and Implications for Regional-Scale Exploration Targeting Methods. Mineralium Deposita 47, 339-58. Jennings, K. and Schodde, R., 2016, From Mineral Discovery to Project Delivery. SEG Newsletter 105, 20-25. Johnson, S.P., Korhonen, F.J., Kirkland, C.L., Cliff, J.B., Belousova, E.A., and Sheppard, S., 2017a, An Isotopic Perspective on Growth and Differentiation of Proterozoic Orogenic Crust: From Subduction Magmatism to Cratonization. LITHOS 268-271, 76-86. Johnson, S.P., Cutten, H.N., Korhonen, F.J., and Wyche, N.L., 2017b, Geology and Metallogeny of the Capricorn Orogen. in G. N. Phillips (ed.) Australian Ore Deposits, The Australian Institute of Mining and Metallurgy: Melbourne, 289-392. Johnson, S.P., Thorne, A.M., Tyler, I.M., Korsch, R.J., Kennett, B.L.N., Cutten, H.N., Goodwin, J., Blay, O., Blewett, R.S., Joly, A., Dentith, M.C., Aitken, A.R.A., Holzschuh, J., Salmon, M., Reading, A., Heinson, G., Boren, G., Ross, J., Costelloe, R.D., and Fomin, T., 2013, Crustal Architecture of the Capricorn Orogen, Western Australia and Associated Metallogeny. Australian Journal of Earth Sciences 60, 681-705. Korsch, R.J. and Doublier, M.P., 2016, Major Crustal Boundaries of Australia, and Their Significance in Mineral Systems Targeting. Ore Geology Reviews 76, 211-28.

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Figure 2. Time-space plot of the Capricorn Orogen showing the timing of hydrothermal activity and gold mineralization and how it relates to the host rocks (after Johnson et al., 2013).