2.7 Orogenic mineral system

The orogenic mineral system (Figure 2.7.1) unites a diverse group of mineral deposits that form during orogenesis, and include lode gold and Cobar-type Cu-Au-Zn-Pb-Ag deposits. These deposits form in response to major orogenic events caused by accretion or changes in subduction direction or dip. In many cases, the deposit form during short periods of extension during overall contractional tectonostructural events (Blewett et al., 2010).

2.7.1 Geological setting

Both lode gold and Cobar-type Cu-Au-Zn-Pb-Ag deposits, the two main deposit types in the orogenic mineral system, are associated with continental margin accretionary (oceanic-continental) and collisional (continent-continent) orogens. They typically occur in granite-greenstone terranes or in terranes dominated by turbiditic (meta-sedimentary) rocks, and are commonly associated with second- and third-order faults and shear zones (Vearncombe et al., 1989; Lawrie and Hinman, 1998). Temporally, the deposits commonly form during the late stages of orogenesis. Although present in rocks that are characterised by a large range in metamorphic grade (pumpellyite-prehnite to granulite facies), these deposits are most common in low to mid-greenschist facies rocks (Groves et al., 1998). This observation on lode gold deposits led Groves et al. (1998) to propose a crustal continuum model for lode gold deposits, in which metal assemblages change with depth, with Hg and Sb enriched in high level systems (a trend also present in the porphyry-epithermal mineral system). As Cobar-type deposits are much less common and less studied, variations in deposit characteristics with crustal depth have not been documented, although Zn-Pb-rich deposits seem to be slightly younger than Cu-Au-rich deposits in the Cobar district (cf., Champion et al., 2009).

The relation between mineralisation in the orogenic mineral system and magmatism is fraught. In several major lode gold provinces, such as the Eastern Goldfields Superterrane and the Tanami province, there is a close temporal and, in some cases, spatial relationship between magmatism and mineralisation (Blewett et al., 2010; Huston et al., 2007), yet in other major gold events, such as the ˜440 Ma Benambran event in Victorian goldfields, magmatism has not been found to overlap gold mineralisation, despite extensive geochronologic studies of granites and lode gold deposits (Champion et al., 2009). In the Cobar Cu-Au-Zn-Pb-Ag district, magmatism is not known to overlap in time with mineralisation (e.g., van der Wielen and Glen, 2007; Champion et al., 2009).

2.7.2 Sources of fluid, metals and energy

One of the unifying characteristics of lode gold deposits through time and space are low- to moderate-salinity (generally <10 eq wt % NaCl), CO 2 -bearing ore fluids (Ridley and Diamond, 2000). The origin of these fluids is contentious, with some workers inferring a metamorphic origin (Phillips, 1993), but others prefering a magmatic-hydrothermal origin, at least in part (Mueller et al., 1991). Both origins have shortcomings (e.g., difficulties in producing sufficient metamorphic fluids to form some deposits, or the lack of syn-ore magmatism in some lode gold provinces), and the fluid characteristics are not sufficiently diagnostic to distinguish the alternatives.

The source of Au is also contentious, with magmatic, metamorphic and even mantle sources proposed (e.g., Groves et al., 2003; Morelli et al., 2007). As noted by Groves et al. (2003), the constraints are insufficient to resolve the source. The likely energy source is heat associated with orogenesis, in many, but not all, cases indicated by syn-ore magmatism.

Limited data from the Cobar district indicate that Cobar-type fluids have some similarities to the lode gold fluids. The ore fluids have low to moderate salinities (mostly <10 eq wt % NaCl), with higher salinities associated with higher base metals. The homogenisation temperatures are variable, mostly between 200 and 350°C (Stegman, 2001), and the ore fluids are characterised by the presence of methane and higher order hydrocarbons with variable CO₂ (Lawrie and Hinman, 1998). These fluids are thought to have a basin or metamorphic (basement) origin (Lawrie and Hinman, 1998).

As initially suggested by Lawrie and Hinman (1998) and supported with more precise data by Mernagh (2007), Pb isotope data suggest two Pb sources in the Cobar district, a less evolved, possibly slightly older basement Pb associated with Cu-Au mineralisation and a more evolved, possibly slightly younger basinal Pb associated with Zn-Pb-Ag mineralisation. These data indicate an evolution with time of the Pb source from a more juvenile source to a more evolved source.

Although the Cu-Au and Zn-Pb-Ag ore fluids are generally interpreted as metamorphic and/or basinal fluids (see above), Cleverly and Barnicoat (2007) suggested the possibility of early magmatic-hydrothermal fluids, citing the presence of a very early magnetite-scheelite-cassiterite-bearing vein assemblage (Stegman, 2001). A magmatic origin for main-stage Cu-Au and Zn-Pb-Ag mineralisation is problematic as the timing of mineralisation (401-384 Ma: Perkins et al., 1994) does not overlap periods of local known magmatism (435–420 Ma: van der Wielen and Glen, 2007). A possible origin for the early W-Sn-bearing assemblage could be ˜420 Ma magmatism, which is present as a high level (locally peperitic) rhyolitic intrusion at the Peak deposit. The Cobar district is along the northern extension of the Wagga-Omeo Sn belt province (Figure 1.5.2) (Section 2.2.2), which contains Sn deposits of this age. In this interpretation, the Sn-W-bearing assemblage is significantly older (15–20 million years or more) than the main stage Cu-Au and Zn-Pb-Ag mineralising events.

2.7.3 Fluid pathways

It has long been established that the distribution of lode gold deposits is strongly controlled by faults and shear zones, with deposits localised along second or third order structures (e.g., Vearncombe et al., 1989), associated with major regional structures that commonly form boundaries between or within major crustal blocks. In detail, the deposits are commonly associated with transcrustal contractional structures (Vearncombe et al., 1989; Blewett et al., 2010). The major regional structures are zones of regional fluid flow, with tapping of the major regional structures into lower order structures allowing more effective physical and chemical changes to ore fluids and ore deposition.

In the Cobar district, the distribution of deposits is also controlled by regional-scale structures, mostly along the eastern margin of the Cobar Basin (Stegman, 2001). In detail, ore lenses are controlled by areas of higher strain (Stegman, 2001) and anticlinal closures (Lawrie and Hinman, 1998). As with lode gold deposits, these lower-order structures tapped fluid from the regional structures and allowed water-rock reactions, fluid mixing and ore deposition.

2.7.4 Depositional processes

The tapping of ore fluids from regional structures into lower order structures allows for ore deposition through physicochemical reactions that desulfidise or reduce the ore fluid, causing gold deposition. Chemical processes that cause gold deposition commonly involve reaction with iron-rich (e.g., dolerite or banded iron formation) or reduced-carbon-rich (e.g., carbonaceous sediments) rocks. Gold deposition can also occur through phase separation resulting from fault-valve behaviour and fluid mixing. All four processes probably happen in lode gold systems, but fluid mixing is the main depositional mechanism in Cobar-type deposits.

In addition to Au, lode gold deposits typically are anomalous in Ag, As, Ba, oxidised C, K, Rb, S, Sb, Si, Te and W (Eilu and Groves, 2001) and can be anomalous in B, Bi, Cu, Mo and Pb. Greenschist-facies lode gold deposits typically have a proximal sericite-dominated alteration zone, surrounded by carbonate-rich zones (Eilu and Groves, 2001). Sulfide minerals in the ores typically include pyrite and may include arseonpyrite and pyrrhotite.

In addition to base and precious metals, Cobar-type deposits are anomalous in Bi, As, Sb, Ni, Cd, Hg and Ba, and are characterised by silica-chlorite-albite-sericite assemblages that grade outward to carbonate dominated assemblages. The dominant sulfide minerals in Cobar ores are chalcopyrite, sphalerite, galena, pyrrhotite, pyrite and cubanite. Minor magnetite is present locally (Stegman, 2001).

2.7.5 Australian examples

Australia has several world class lode gold provinces. The largest and most productive gold province in Australia is the Yilgarn Craton (including the Eastern Goldfields Superterrane) in Western Australia which has an EDR of 2396 t gold (Geoscience Australia, 2013). Lode gold deposits in the Eastern Goldfields Superterrane formed in the restricted time window of between 2660 and 2620 Ma (Vielreicher et al., 2003). Other major Australian lode gold provinces include the ˜450–440 Ma and ˜380 Ma Victorian goldfields (Philips et al., 2012), the ˜410–400 Ma Charters Towers district in northern Queensland (Kreuzer, 2005), and the ˜1810–1795 Ma Pine Creek and Tanami provinces in the Northern Territory (Cross, 2009). In addition to these major provinces, minor lode gold deposits are present in all Australian States and Territories. As of 31 December 2011, Australia's total EDR for lode gold was 3276 t gold (Geoscience Australia, 2013). The most important example of Cobar-type Cu-Au-Zn-Pb-Ag deposits is the 405-385 Ma Cobar district in New South Wales (Glen et al., 1992; Perkins et al., 1994), with the mined-out Woodcutters deposit in the Pine Creek mineral province another example.

2.7.6 Associated critical commodities

Although Au, Cu, Zn, Pb and Ag, in that order, are the main products of the orogenic mineral system, critical commodities, in particular Sb and W, are presently being recovered from deposits of this mineral system. Lode Sb-Au deposits form a small, but important, subset of lode gold deposits. Antimony-Au deposits form at relatively shallow depths in the continuum model of Groves et al. (1998). The majority of Australia's known antimony resources (˜65%) are contained in the Hillgrove Sb-Au deposit in New South Wales. With defined resources of 102 kt Sb (Table 2.2.1), it has the largest identified Sb resource outside China. Other Sb mines and prospects (Table 2.2.1) include the Bielsdown Project (Wild Cattle Creek) in New South Wales (20.5 kt Sb), Costerfield in Victoria (producing mine and not included in Table 2.2.1: 28.8 kt Sb combined resources and reserves at 9 July 1012), and the Blue Spec Shear Au-Sb project (7.8 kt Sb) in the Eastern Pilbara Granite-Greenstone Terrane of Western Australia. In addition to Sb and Au, many of these deposits contain or have produced significant W. For example, historically the Hillgrove district has produced 200 t of W. Unlike other subclasses of lode gold deposits, mining of lode Sb-Au deposits requires the production of a Sb concentrate, which is treated off site at specialised antimony smelters (Anderson, 2012). Other than Sb in Sb-rich lode gold deposits, critical commodities in lode gold deposits are unlikely to be recovered as they have relatively low concentrations and are unlikely to be recovered during heap leaching, which is the dominant method of gold recovery for this class of deposits.

Although geochemical studies indicate that some critical commodities are present in Cobar-type ores, publicly available data as to concentrations in the ores are not available. These deposits should be considered to have unknown potential for Bi, As, Sb, Ni, Cd and Hg.