2.4 Iron oxide copper-gold mineral system

Iron oxide copper-gold (IOCG) deposits (Hitzman et al., 1992) are a diverse family of mineral deposits characterised by the following features: (1) Cu with or without Au, as economic metals, (2) hydrothermal ore styles and strong structural controls, (3) abundant magnetite and/or hematite, (4) Fe oxides with Fe/Ti greater than those in most igneous rocks, and (5) no clear spatial associations with igneous intrusions as, for example, displayed by porphyry and skarn ore deposits (Williams et al., 2005). In addition, most IOCG deposits display a broad space-time association with batholithic granitoids, occur in crustal settings with very extensive and commonly pervasive alkali metasomatism, and many are enriched in a distinctive, geochemically diverse suite of minor elements including various combinations of U, REE, F, P, Mo, Ag, Ba, Co, Ni and As (Williams et al., 2005). Many of these minor elements are also critical commodities in the context of the present study.

Uranium-rich IOCG deposits in which U is an economic metal are an important yet uncommon subset of the IOCG family (Hitzman and Valenta, 2005). Currently the Olympic Dam deposit is the only IOCG deposit in which U is extracted as a major economic commodity. This deposit is the world's largest single resource of U (BHP Billiton, 2010 Annual Report, www.bhp.com). In a global context, most of the other IOCG deposits containing higher grades of U are found in the Gawler Craton and Curnamona Province of southern Australia (Hitzman and Valenta, 2005; Skirrow, 2011). Based on current knowledge of IOCG deposits globally, it would appear that the IOCG deposits with the highest grades of REE are also confined to southern Australia.

2.4.1 Geological setting

Figure 2.4.1 shows a generalised model for IOCG mineral systems. The geodynamic settings of IOCG deposits have been widely debated (Williams et al., 2005). Hayward and Skirrow (2010) briefly reviewed tectonic and geodynamic models, in particular for the Gawler Craton, and proposed a distal continental retro-arc environment where earlier subduction-related processes (possibly at ˜1850 Ma) led to metasomatism of the upper mantle. Melts derived from this enriched mantle, driven by a mantle plume or perhaps by removal of lithospheric mantle, resulted in extensive crustal melting and production of high-temperature A- and I-type magmas associated with K-rich mafic melts between ˜1595 Ma and ˜1575 Ma. In the Gawler Craton the felsic melts are represented by the Hiltaba Suite and Gawler Range Volcanics, which are temporally and spatially linked to IOCG deposits in the Olympic IOCG Province. In the Curnamona Province to the east of the Gawler Craton the Benagerie Volcanics and A-type granites on the northern Benagerie Ridge are probably the igneous equivalents of the Gawler Range Volcanics and Hiltaba Suite, respectively (Schofield, 2010).

2.4.2 Sources of fluids, metals and energy

High to extreme paleogeothermal gradients are considered to be a key driver (energy source) of regional-scale upper crustal fluid flow in major IOCG systems. Regional to crustal scale (hydro)thermal systems are necessary to explain the huge scale of the alteration systems and the masses of hydrothermal precipitates (e.g., 10⁷ to 10¹⁰ tonnes of ore rich in hydrothermal Fe oxides, sulfides and silicates) in individual IOCG deposits. In the Gawler Craton and possibly in other major IOCG provinces bimodal magmatism was roughly coeval with IOCG formation. The mafic igneous rocks may mark the foci of crustal-scale thermal anomalies, as well as providing a source of ore metals and/or sulfur in IOCG systems (Johnson and McCulloch, 1995; Skirrow et al., 2002, 2007). Additionally, high-temperature A- or I-type crustal (felsic) melts, emplaced at high levels in the crust or at surface, are considered to have augmented the thermal flux provided by mantle magmatism, and so their presence is considered important as an indicator of a favourable driver or energy source in the IOCG systems. These igneous associations and their tectonic context are shown schematically in Figure 2.4.1.

A distinctive feature of IOCG deposits is the presence of two distinct fluids during deposit formation: (1) a highly oxidised fluid (e.g., meteoric/ground waters), and (2) deep-sourced high-temperature brines (magmatic-hydrothermal fluids and/or fluids reacted with metamorphic rocks). In many IOCG systems there is also evidence of volatile-rich fluids during ore formation (e.g., CO₂-bearing; see review by Williams et al., 2005, and references therein). The sources of Cu, Au, S, Cl and CO₂ may be either coeval magmas (felsic and/or mafic) or sedimentary and igneous rocks that were leached by the ore fluids, as marked by the presence of Na-Ca regional alteration zones (Oreskes and Einaudi, 1992; Johnson and McCulloch, 1995; Haynes et al., 1995; Williams et al., 2005; Oliver et al., 2004; Skirrow et al., 2007). Uranium and REE were most likely leached from granitoid or felsic volcanic rocks. Alternatively, direct magmatic-hydrothermal sources of U and REE are possible in IOCG systems; there is currently insufficient evidence available to distinguish between these possible scenarios for the sources of U and REE.

Pre-IOCG basins hosting major IOCG provinces tend to lack major reduced sucessions (Haynes, 2000) and commonly show evidence for the (former) presence of evaporite minerals; rift basin sequences may supply some of the Fe, Cl, and S to IOCG deposits, particularly those of overall oxidised character with subaerial to shallow marine depositional settings. These include continental back-arc basins and foreland basins. Low metamorphic grade of these basins prior to IOCG formation is favourable because of potentially higher permeability and fluid content than basins metamorphosed to medium or high grade. In models involving non-magmatic fluids, exposure near/at the paleosurface of U- and REE-rich source rocks is favourable for sourcing highly oxidised surface-derived waters capable of transporting U and REE. Topographic depressions (e.g., calderas, grabens, maar complexes, etc) are conducive to mixing of shallow-crustal and deep-sourced fluids.

2.4.3 Fluid pathways

Terrane boundary zones initiated during earlier orogenic belt formation are believed to form part of the crustal-scale magma and fluid pathways for major IOCG systems. Such boundaries have been documented beneath the Olympic Dam deposit in the Gawler Craton (Lyons and Goleby, 2005; Heinson et al., 2006; Direen et al., 2007; Hayward and Skirrow, 2010). Groves et al. (2010) extended this concept to other major IOCG deposits globally. Major IOCG systems may preferentially occur in the hangingwall of boundary zones between crustal blocks, above zones of partial crustal melting and mafic underplating (Figure 2.4.1). Fluid flow is enhanced by juxtaposition of earlier rift basins with this high-temperature melt province. Pre-existing basinal structures and second-order cross structures (e.g., conjugate fault sets) localise dilational deformation, brecciation (at high crustal levels), and fluid flow. The intersections of second-order faults with crustal-scale terrane boundaries are favoured locations for IOCG systems.

Hydrothermal alteration zones mark the passage of fluids and hence map fluid flow pathways. In IOCG systems regional Na-Ca alteration zones may represent fluid flow paths in deeper and/or more distal parts of the systems, where some of the ore constituents were leached (e.g., Barton and Johnson, 1996; Oliver et al., 2004). On the other hand, magnetite-biotite, magnetite-K-feldspar, and hematite-rich alteration zones represent more proximal IOCG settings, and thus not only mark fluid flow paths but also represent the sites of physico-chemical gradients proximal to the sites of ore deposition (see below).

2.4.4 Depositional processes

Although it is widely accepted that two fluids were involved in the formation of most if not all of the major IOCG deposits, it remains unclear which of the two fluids carried the bulk of the copper, gold, uranium, REE and other metals. In the Olympic Dam district, the high temperature brines carried at least 300 ppm Cu in places but data for the oxidised lower temperature fluid are inconclusive (Bastrakov et al., 2007). Similar hypersaline brines in other IOCG provinces globally also carry elevated levels of Cu and other transition metals (e.g., Carajas province, Brazil: Xavier et al., 2010, and references therein). For IOCG systems of the eastern Gawler Craton a key Cu-Au-U depositional process appears to have been mixing of large volumes of oxidised groundwaters (or shallow basinal waters) with deep-sourced Fe-rich brines of intermediate redox state (Haynes et al., 1995). This process would have resulted in reduction and cooling of the oxidised fluids as well as possible changes in pH and ligand activity of both fluids, causing metal deposition. Additionally, reaction of the oxidised fluids with rocks rich in Fe 2+ -rich minerals such as magnetite, siderite and chlorite, or with reduced sulfur in sulfide minerals, or with reduced carbon, also may have resulted in Cu-Au-U deposition. Chemical modelling by Bastrakov et al. (2007) showed that higher grade Cu and Au mineralisation is expected in zones where hematite has replaced earlier magnetite.

The implication of these findings is that hematite-rich alteration zones in IOCG systems are more favourable for higher grade Cu-Au-U mineralisation in comparison to magnetite-rich zones. The hematite may occur above the magnetite (e.g., Olympic Dam) or laterally adjacent to the magnetite (e.g., Prominent Hill, Belperio et al., 2007). The U mineralisation may occur in overlapping and/or separate zones relative to Cu-Au mineralisation. The occurrence of the REE in IOCG systems is less clear but in the Olympic Dam deposit the barren hematite core is in fact significantly enriched in the light REEs (Reeve et al., 1990; Ehrig et al., 2013). Elevated light REE concentrations also occur throughout the Cu-Au mineralised zones.

2.4.5 Australian examples

There are two major IOCG provinces in Australia of global significance: the Olympic IOCG Province along the eastern margin of the Gawler Craton in South Australia, and the Cloncurry district in the eastern Mount Isa Inlier of northwest Queensland. In addition there are several other metallogenic provinces that contain or may contain medium sized or small IOCG deposits, including the Tennant Creek district (Northern Territory), Curnamona Province (South Australia and New South Wales), and the Aileron province, Northern Territory (Schofield, 2012).

The Olympic IOCG Province is defined by the distribution of known early Mesoproterozoic IOCG±U mineralisation and alteration, and encompasses three known districts (from north to south): Mt Woods Inlier which hosts the Prominent Hill deposit; Olympic Dam district hosting the Olympic Dam, Carrapateena and Wirrda Well deposits; and the historic Moonta-Wallaroo Cu-Au mining district with the recently discovered Hillside deposit. The Olympic IOCG Province is a metallogenic entity superimposed on older geological domains. The Olympic Dam deposit is currently the world's fourth largest Cu resource, fifth largest Au resource and the world's largest U resource by far, with all resources contained in a single deposit with an areal extent of less than 25 km² (BHP Billiton Annual Report 2012). The resource in 2012 stood at 9576 Mt at 0.82% Cu, 0.26 kg/t U₃O₈, 0.31 g/t Au and 1.39 g/t Ag (www.bhp.com).

The only other currently producing IOCG mine in the province is the Prominent Hill deposit, where production commenced in 2009, although Cu production also came from several small open pits and underground mines in the Moonta-Wallaroo IOCG district on the Yorke Peninsula. The Hillside deposit in this district has a resource of 330 Mt at 0.6% Cu and 0.15 g/t Au (February 2013), and is currently undergoing feasibility studies. The small Cairn Hill deposit in the Mt Woods Inlier produces iron ore (magnetite) with by-product copper. Carrapateena and Wirrda Well are significant deposits buried under relatively deep cover (>450 m). A resource was announced in 2011 by OzMinerals for the Carrapateena deposit: 203 Mt at 1.31% Cu (cutoff 0.7%), 0.56 g/t Au, and 270 ppm U (www.ozminerals.com).

The Olympic Dam deposit is hosted by the Olympic Dam Breccia Complex (ODBC) in the Roxby Downs Granite, a member of the Hiltaba Suite. The geology of the deposit is described in detail by Reeve et al. (1990), Oreskes and Einaudi (1992), Haynes et al. (1995), Reynolds (2000) and Ehrig et al. (2013). The zoned funnel-shaped ODBC comprises multi-phase heterolithic breccias ranging from granite-rich to hematite-rich. The core hematite-quartz breccias lacks major Cu mineralisation but have elevated concentrations of REE, Ba and locally U. The margins of the core zone contain native Au and Cu mineralisation with low temperature illite and local silicification, and grade outwards and downwards into hematite-granite breccias hosting the bulk of the Cu-U-Au mineralisation. The distribution of individual hematitic breccia bodies partly controls Cu grades. However grades are also controlled by an important deposit-wide sharp interface between bornite and chalcopyrite that is broadly funnel shaped and in detail highly convoluted. Bornite-chalcocite mineralisation above the interface commonly attains grades of 4–6% Cu, whereas chalcopyrite mineralisation below the interface rarely exceeds 3% Cu (Reeve et al., 1990).

The deeper and peripheral zones of the ODBC contain greater proportions of magnetite and chlorite relative to hematite and sericite, and siderite is locally abundant. Uranium mineralisation is present throughout the hematite-rich breccias broadly in association with Cu mineralisation, but higher grade zones occur at the upper margin of the bornite-chalcocite zone. Uraninite (as pitchblende) is the dominant U mineral, whereas minor coffinite and brannerite occur in upper/shallower and deeper/peripheral zones, respectively (Reynolds, 2000). Olympic Dam ores have average grades of 0.17% La and 0.25% Ce with the light REEs present mainly within basnasite and florencite; REEs are not extracted currently (Ehrig et al., 2013).

The Cloncurry district in the eastern Mt Isa Inlier hosts the large Ernest Henry IOCG deposit as well as several medium sized and small IOCG and affiliated deposits. IOCG mineralisation developed in the Mesoproterozoic, during two periods, around 1590 Ma and 1500–1530 Ma (see Williams et al., 2005 and references therein). The breccia-hosted Ernest Henry deposit differs from the generally hematite-rich IOCG deposits in the Olympic IOCG Province in that the major iron oxide is magnetite, with associated hydrothermal K-feldspar, biotite and carbonate (Williams et al., 2005). Unlike the Olympic Dam, Prominent Hill and Carrapateena deposits in which bornite and chalcocite are important ore minerals, Cu mineralisation at Ernest Henry is primarily as chalcopyrite. Uranium and REE concentrations at Ernest Henry, while highly anomalous, are much lower than in the major deposits of the Olympic IOCG Province. Molybdenite is rare at Olympic Dam (0.0015 weight percent, Ehrig et al., 2013) but is widespread as a minor phase in the Ernest Henry deposit (Williams et al., 2005). Notwithstanding these and other differences in mineralogy and geochemistry, the magnetite-rich Ernest Henry deposit and hematite-rich IOCG deposits of the Olympic IOCG Province constitute a spectrum of deposit styles within the IOCG family. From a mineral systems perspective they share features of geological setting, sources of fluids-metals-energy, fluid pathways, and ore depositional processes, as summarised above.

Other Cu-Au deposits in the Cloncurry district include Osborne, Eloise, Mt Elliot, Mt Dore, Monakoff, and many other small deposits and prospects. Among these deposits there is considerable variation in deposit geology, style, mineralogy and geochemistry (Williams and Skirrow, 2000). The abundances of minor elements are highly variable; for example some of the Cu-Au deposits are notable for their Co (Mt Elliot; Williams and Skirrow, 2000).

2.4.6 Associated critical commodities

IOCG deposits are characteristically diverse in their minor element compositions, and contain elevated concentrations of many critical commodities. Recently published data for the Olympic Dam deposit have revealed the presence of an extremely broad range of minerals and corresponding geochemical variation. More than 90 minerals have been identified, and in addition to Cu, U, Au and light REEs the deposit is enriched to strongly enriched in As, Ba, Bi, C, Cd, Co, Cr, F, Fe, In, Mo, Nb, Ni, P, Pb, S, Sb, Se, Sn, Sr, Te, V, W, Y, and Zn (Ehrig et al., 2013). It is interesting to note that almost all of these elements are included in the current study of critical commodities.

Alteration, mineralisation and elemental abundances are zoned within the Olympic Dam deposit. Ehrig et al. (2013) describe the zones from the periphery inward and upward from depth towards the deposit centre, as follows: (1) reduced Fe-oxide alteration (magnetite-apatite-siderite-chlorite-quartz) → oxidised Fe-oxide alteration (hematite-sericite-fluorite) → hematite-quartz-barite alteration, (2) siderite → fluorite → barite, (3) sphalerite → galena → pyrite → chalcopyrite → bornite → chalcocite → non-sulfide, and (4) distal or paragenetically early (?) base-metal poor (Mo-W-Sn-As-Sb) → base-metal rich (Cu-Pb-Zn) minerals → sulfide-barren hematite-quartz-barite breccia in the deposit centre.

As noted above, some IOCG deposits and affiliated(?) deposits in the Cloncurry district are enriched or highly enriched in combinations of Ba, Co, Ni, As, Mo, Re and other elements (Williams et al., 2005). The Merlin deposit is a high-grade Mo-Re resource (6.7 Mt at 1.4% Mo with 0.3% Mo cutoff, and 23 ppm Re, as of 2012) with minor Cu-Au mineralisation and insignificant iron oxides that occurs adjacent to the Mt Dore IOCG deposit, and with which Merlin mineralisation may be affiliated.

The Tennant Creek historic mining district in the central Northern Territory is best known for its high grade Proterozoic Au deposits associated with magnetite-hematite-rich ironstones (Wedekind et al., 1989; Huston et al., 1993; Skirrow and Walshe, 2002). It has produced 5.5 million ounces of Au at an average grade of 19.3 g/t Au, but also has produced significant Cu (488 000 t Cu at average grade of 2.9%) and 5000 t of Bi. The wide variety of Bi minerals includes sulfosalts with Se (Wedekind et al., 1985).