Iron-oxide Cu-Au deposits: an Australian perspective on their unifying characteristics

The iron-oxide copper-gold or IOCG deposit class includes a broad range of mineralisation styles so broad that debate continues on which deposits are members of the class. These epigenetic magnetite- and/or hematite-rich deposits range in age from late Archaean (e.g., Salobo, Brazil), through Proterozoic (e.g., Olympic Dam, Ernest Henry, Tennant Creek district), to Mesozoic (e.g., Candelaria). Rather than focussing on the boundaries in deposit classification, let's consider the ‘constants’ in observed characteristics and processes that recur within regions of major IOCG deposits. This will allow an improved understanding of the local ‘variables’ that result in such diverse deposits. Four ‘constants’ are recognised in the major Australian IOCG districts. First, in each there is evidence for a major regional thermal event broadly coeval with IOCG formation, represented by low to medium grade metamorphism, and/or mafic intrusions, and/or I- or A-type granitoids. Coeval volcanics are preserved in some districts. However, a gap remains in our understanding of the role of magmas in IOCG genesis. Second, the host supracrustal sequences were relatively Fe-rich prior to IOCG-related hydrothermal activity, but contain only minor or no reduced carbon-bearing strata, at least at the crustal levels now exposed. Evaporites, basalts and regional-scale sodic-calcic alteration occur in some but not all major IOCG districts. Third, trans-crustal sutures are present in regions of the major IOCG deposits, linked to networks of brittle-ductile shears or brittle faults that were active during the regional thermal event(s). Although new studies are investigating the crustal architecture of some IOCG ore-forming systems (e.g., seismic surveys near Cloncurry and Olympic Dam), 3D models with predictive capability have yet to be developed. Fourth, two fundamentally different fluids are recognised in IOCG districts, whose variable interaction (mixing and fluid-rock reaction) arguably gives rise to many of the mineralogical and geochemical variations among IOCG deposits. One fluid was high salinity, ~350-500°C, intermediate oxidation state (magnetite-pyrite-stable) or was locally reduced, and transported Fe, K, Cu, Ba, Mn, REE, and by inference Au and H2S > SO42-. The second fluid was lower salinity, ~150-250°C, oxidized (hematite-pyrite-stable, SO42- > H2S) to very oxidised, possibly carried some Au, Cu and U, and is most evident where IOCG deposits developed at shallow crustal levels. Additionally, many IOCG mineralising systems contain CO2-rich fluids. The key variables giving rise to differences in IOCG deposits within and between districts are: crustal depths of ore formation (including extent of telescoping of shallow on deeper styles), and local host rock types which are very diverse. Variations in depth of formation lead to a spectrum of structural styles (e.g., breccia at shallow levels, shear-hosted styles at deeper levels), and alteration and ore mineral assemblages (e.g., hematite-bearing at shallower levels; magnetite-bearing at deeper levels).