2.2 Porphyry-epithermal mineral system

Mineral deposits of the porphyry-epithermal mineral system are associated with magmatism that generally, although not exclusively, is associated with magmatic arcs within convergent geodynamic settings. This mineral system involves mainly magmatic-hydrothermal and meteoric fluids that form porphyry Cu-Au-Mo deposits¹, epithermal Au-Ag, Ag-Zn-Pb and Au-Cu deposits, and Cu-Au and Zn-Pb-Ag skarn deposits. More detailed descriptions of aspects of the porphyry-epithermal mineral system can be found in Buchanon (1981), White and Hedenquist (1995), Seedorff et al. (2005), Simmons et al. (2005) and Berger et al. (2008).

2.2.1 Geological setting

Mineral deposits that form in the porphyry-epithermal mineral system generally have a spatial and temporal association with intermediate to felsic sub-aerial volcanic rocks and related sub-volcanic intrusions. Although, geologically recent porphyry-epithermal mineral systems are mostly thought to form in magmatic arcs (both continental and oceanic) associated with convergent plate margins (Seedorff et al., 2005; Simmons et al., 2005), the geological setting of older examples is less clear, and some workers have suggested alternative settings that do not involve arcs (e.g., Hou et al., 2003). In all cases, deposits are thought to have formed at shallow crustal levels (<1.5 km for epithermal and <6 km for porphyry deposits: Seedorff et al., 2005; Simmons et al., 2005). This very shallow depth of emplacement and consequent low preservation potential account for the fact that geologically old (Paleozoic or older) deposits are uncommon (Seedorff et al., 2005; Simmons et al., 2005).

Figure 2.2.1 shows schematically the relationship of different deposits to geological features within the porphyry-epithermal mineral system. This diagram also distinguishes three types of epithermal deposits. Adularia-sericite Au-Ag (or low sulfidation²) and Ag-Zn-Pb (intermediate sulfidation) epithermal deposits are located schematically in the low temperature, periphery of the mineral system, with porphyry Cu-Au-Mo and pyrophyllite-kaolinite (high sulfidation or acid-sulfate) Au-Cu epithermal systems more proximal to intrusive bodies. Skarn deposits are also zoned in this mineral system, with Cu-rich skarns generally very close to the causative intrusion, but Zn-Pb-rich skarns more distal. An important feature of the porphyry-epithermal mineral system is the telescoping of different deposit types, for instance porphyry Cu-Au-Mo deposits and epithermal deposits of various types.

2.2.2 Sources of fluid, metals and energy

Although most workers concur that magmas were probably the energy source in the porphyry-epithermal mineral system, the evidence is not definitive as to the role of magmatic-hydrothermal fluids as sources of fluid, sulfur and metals. For deposit types more proximal to intrusive complexes, such as porphyry Cu-Au-Mo, pyrophyllite kaolinite Au-Cu epithermal and Cu-rich skarns, a direct magmatic-hydrothermal source for the fluids, metal and sulfur is likely (Arribas, 1995; Bodnar, 1995), but for the more distal adularia-sericite epithermal and Zn-rich skarn deposits, the evidence is less clear. Although Simmons et al. (2005) suggest a significant magmatic hydrothermal component for some adularia-sericite epithermal deposits, they also recognise that definitive evidence for such a component is not present in many deposits. In all cases, however, a significant meteoric fluid component is present (Simmons et al., 2005). In any case, the likely driver of fluid flow, whether magmatic-hydrothermal of heated meteoric, is probably magma emplacement.

2.2.3 Fluid pathways

In the porphyry-epithermal mineral system, pathways for fluids and their contained metals, ligands and sulfur include faults, stratigraphic aquifers and crystallising intrusive bodies. Crystallising intrusions are particularly important pathways for high-temperature, proximal deposits such as porphyry Cu-Au-Mo deposits and Cu-rich skarns. These deposits are commonly associated with phallic intrusions and apical zones of larger intrusions (Figure 2.2.1). The fluid pathways for epithermal and Zn-rich skarns include faults and stratigraphic aquifers. The volcanic-intrusive complexes generate geothermal systems which are focussed along these fluid pathways. Regional zones of propylitic alteration are indicative of convective fluid flow systems (Masterman et al., 2005).

2.2.4 Depositional processes

Mechanisms for ore deposition in the porphyry-epithermal mineral system are many and varied, with the main mechanisms being depressurisation and associated processes such as boiling, fluid mixing, cooling, and wall rock interaction. Mechanisms that form porphyry Cu-Au-Mo deposits include exsolution of a magmatic-hydrothermal fluid from the causative magma followed by cooling and mixing with meteoric fluids (Seedorff et al., 2005). Highly acidic ore fluids produced by disproportionation of magmatic SO₂ are neutralised by wall rock reaction or fluid mixing to produce pyrophyllite-kaolinite Au-Cu epithermal deposits (Simmons et al., 2005). In adularia sericite Au-Ag and Ag-Zn-Pb deposits, ore deposition is caused by boiling of the ore fluid as it approaches the paleosurface and/or by fluid mixing and cooling (Simmons et al., 2005). In skarn deposits, ore deposition is caused by acid neutralisation as the ore fluid interacts with carbonate rocks to form the skarn.

2.2.5 Australian examples

Because of the lack of major, geologically young, felsic to intermediate (i.e., Mesozoic and younger) magmatic belts, deposits of the porphyry-epithermal mineral system are not common in Australia. The most important Australian porphyry-epithermal mineral provinces are the Paleo- to Mesoarchean North Pilbara Terrane (Marshall, 1999), the Neoarchean Southwest Terrane (McCuaig et al., 2001), the Siluro-Ordovician Macquarie Arc and the Middle Carboniferous Drummond Basin (Champion et al., 2009). In addition, there are a number of smaller and/or emerging porphyry-epithermal mineral provinces elsewhere in Australia.

The North Pilbara Terrane contains a number of quartz veins with textures and mineralogies consistent with adularia-sericite deposits (Marshall, 1999), a significant porphyry Cu-Mo deposit and several pyrophyllite- and kaolinite-bearing alteration zones (Van Kranendonk and Pirajno, 2004). These porphyry-epithermal deposits range in age from ^∼3490 Ma to ^∼2760 Ma (Huston et al., 2002), with the Spinifex Ridge porphyry Cu-Mo deposit (Table 2.2.1) having an age of ^∼3300 Ma (Stein et al., 2007). The world-class Boddington deposit (total resources at 31 December 2010 of 1.53 Gt grading 0.102% Cu and 0.579 g/t Au), in the Saddleback Greenstone Belt of the Southwest Terrain of the Yilgarn Craton, is interpreted as a ^∼2707 Ma porphyry Cu-Au deposit that has been overprinted by a ^∼2629 Ma lode gold event (McCuaig et al., 2001: Stein et al., 2001).

The main porphyry-epithermal province in Australia is the Siluro-Ordovician Macquarie Arc in New South Wales, which contains major porphyry Cu-Au deposits at Cadia (global resource of 1.31 Gt at 0.31% Cu and 0.74 g/t Au: Cooke et al., 2007) and Northparkes (global resource of 153 Mt at 1.03% Cu and 0.46 g/t Au: Cooke et al., 2007). This province has many features of a magmatic arc and is interpreted as such by many workers (e.g., Crawford et al., 2007; Meffre et al., 2007). In addition to the major porphyry Cu-Au deposits, the Macquarie Arc also contains adularia-sericite Au-Ag and pyrophyllite-kaolinite Au-Cu epithermal deposits at Lake Cowal and Cargo, and at Peak Hill and Gidginbung, respectively (Cooke et al., 2007; Champion et al., 2009).

The fourth major Australian porphyry-epithermal province is the Drummond Basin in northeast Queensland, which contains several adularia-sericite Au-Ag deposits, the most important of which is Pajingo-Vera-Nancy (global resources of 43.5 t Au and 69.8 t Ag: Richards et al., 1998). Apart from these major provinces, the Golden Plateau adularia-sericite Au-Ag epithermal deposits are hosted by Lower Permian volcanic rocks along the western side of the Connors-Auburn Province near Cracow in south-eastern Queensland, and are related to the extensional event that formed the Bowen Basin. As of 31 December 2011, the economically demonstrated resources (EDR)³ for epithermal gold deposits in Queensland is 37.56 t gold (Geoscience Australia, 2013).

Smaller, sub-economic and emerging porphyry-epithermal provinces elsewhere in Australia include the Permo-Carboniferous Kennedy magmatic province of northermal Queensland, and the Mesoproterozoic (?) Peterlumbo Ag-Pb province (Paris deposit) in South Australia. In the Kennedy Magmatic Province, porphyry Cu-Au deposits are associated with the intermediate Almaden Supersuite (Champion et al., 2009), and include the Mount Turner, Ruddygore and, most importantly, the ^∼289 Ma (Mugalov et al., 2008) Mount Leyshon deposit (global resource 70 Mt grading 1.43 g/t Au: Morrison and Blevin, 2001).

One of the more significant greenfields Australian discoveries in 2011 was the Paris Ag prospect in the southern Gawler Province of South Australia. Subsequent exploration has identified several other similar prospects in the region, which has been termed the Peterlumbo mineral field (www.investigatorresources.com.au). The Paris prospect consists mainly of sheet-like zones and sub-vertical veins in dolomite just below the unconformity with Gawler Range Volcanics. The deposit is interpreted to be epithermal in origin and has some similarities to chimney- and manto-type deposits in Mexico and South America.

2.2.6 Associated critical commodities

Porphyry Cu-Au-Mo and epithermal deposits are geochemically zoned, both at the district and deposit scales (Buchanon, 1981; Berger et al., 2008). For example, at the Ann-Mason porphyry Cu-Mo deposit in Nevada (US), Cohen (2011) document a vertical zonation of Cu → Mo → W → Sn → As-Bi → Sb → Li → Tl from the ore zone over a distance of 3 km. Selenium enrichment extended throughout this vertical zonation, but Zn, Mn, Co, V and Sc were enriched 1–3 km laterally from the ore zone. Berger et al. (2008) found similar patterns more generally about porphyry Cu-Au-Mo deposits, although noting that Ag, Ba, Pb, Sb and Te can also be enriched lateral to ore. In some cases, Ag is also enriched also within ore zones. Of the elements enriched in and around porphyry Cu-Au-Mo deposits, only Cu, Mo, Au, Ag, Re and Se are recovered (see below); the other elements, including critical commodities, are not of sufficient grade or value to warrant recovery. Cu-rich and Zn-rich skarns have broadly similar geochemical associations to the proximal and lateral parts of the porphryry Cu-Au-Mo systems, respectively.

Buchanon (1981) documented a generalised vertical geochemical/mineralogical zonation in adularia-sericite Au-Ag epithermal deposits, as follows: Cu-As → Zn-Pb-Cu-Ag → Au-Ag-Sb-Ba-F → As-Sb → Hg, with Au and Ag enrichment beginning at the base of boiling zones. Adularia-sericite Ag-Zn-Pb epithermal deposits have a broadly similar pattern, albeit with enhanced Ag and base metal grades. Pyrophyllite-kaolinite epithermal deposits are not as well zoned, but are characterised by a metal assemblage of Cu-Au-Ag-As-Sb-Bi-Hg-Te-Sn-Pb-Mo (White and Hedenquist, 1995).

Of the critical commodities considered in this review, Mo is economically the most significant in the porphyry-epithermal mineral system and is a common by-product of porphyry Cu-Au-Mo deposits globally. Although Mo is not recovered (or reported) from porphyry Cu-Au deposits of the Macquarie arc, the Spinifex Ridge deposit contains a JORC-compliant ore reserve totalling 0.295 Mt Mo (and 0.520 Mt Cu: Table 2.2.1), the largest Mo resource in Australia.

Rhenium is recovered as a by-product during roasting of molybdenite concentrates, which in turn are by-products from porphyry Cu-Mo deposits (Magyar, 2003). Although molybdenite from porphyry Cu-Mo deposits can contain up to 0.4% Re, typical analyses are in the range 0.01-0.10% (McCandless et al., 1993). Based on a molybenite Re content of 33 ppm (average of two molybdenite separates), as determined during Re-Os age dating (Stein et al., 2007; Barley et al., 2008; M Barley, pers.comm., 2013), the Spinifex Ridge deposit contains on the order of 16 t Re⁴.

In addition to Mo, some porphyry Cu-Au-Mo deposits produce by-product Ag and Se (e.g., Erdenet (Mongolia): Maksimyuk et al., 2008). Selenium, Te and precious metals are commonly extracted from anode slimes produced during copper refining. Approximately 90% of Se is produced in this manner from copper concentrates in the United States (Butterman and Brown, 2004), presumably sourced mainly from porphyry Cu ores.

Although limited critical commodities are produced as by-products from the porphryry-epithermal mineral system, a number of critial commodities (e.g., Sb, Bi, Ga, In, Ge, Mn and Cd) potentially could be recovered from concentrates produced by mines of porphyry and epithermal deposits. Recovery of these commodities would depend upon recovery methods and critical commodity prices. Some critical commodities (e.g., As, Sb, Mn and Cd) currently can attract smelter penalties.

Endnotes

1 Porphyry Mo deposits have been included as part of the granite-hosted mineral system.

2 We have adopted names for deposit types based on observable features (e.g., characteristic alteration assemblages) as opposed more abstract features such as sulfidation state.

3 Economically demonstrated resources are resources that are presentaly though to be economically viable, and include JORC-compliant ore reserves and measured and indicated mineral resources.

4 Rhenium potential reported herein is indicative only and based upon very limited data