Roger Skirrow

Characteristics of ten of the world’s metallogenic provinces hosting iron oxide Cu-Au (IOCG) deposits have been critically assessed, including their geological and tectonothermal evolution, alteration-mineralisation parageneses, and ore geochemistry. A new classification framework is proposed in which IOCG deposits form the major part of a family of deposits within Cu-Au-Fe (CGI) mineral systems. Other family members include Fe-sulfide Cu-Au (ISCG) deposits lacking significant iron oxides. The classification combines three criteria: geological and tectonic settings, oxide-sulfide mineralogy, and ore geochemical characteristics. These criteria form the basis for defining deposit subtypes, and also distinguish deposits in CGI mineral systems from porphyry Cu (-Au), skarn Fe-Cu (-Au) and iron oxide-apatite (IOA) deposits, although there are some shared features.
CGI mineral systems with IOCG deposits occur in three geological and tectonic settings. Two settings are closely linked and are termed orogenic and post-orogenic settings. Syn-deformational IOCG and related deposits in orogenic settings formed during regional tectonothermal events at mid- to shallow-crustal levels at generally brittle-ductile conditions. Available data are consistent with a model in which provinces hosting orogenic IOCG deposits experienced tectonic switching from compression to extension, which was also commonly marked by regional bimodal magmatism. This review shows that many of world’s IOCG provinces are hosted in such orogenic settings. The rarer post-orogenic extensional setting, often previously described as ‘anorogenic’, hosts the super-giant Olympic Dam deposit. Andean-type continental margin magmatic arcs undergoing extension comprise the third important setting of IOCG provinces. All major IOCG metallogenic provinces are characterised by the coincidence in space and time between pre-IOCG sedimentary ± volcanic basins and syn-IOCG intrusive ± volcanic regional magmatism.
IOCG and ISCG deposits in CGI mineral systems are characterised by an association of Cu and Au with highly elevated Fe (e.g. 15-60 wt % Fe) in the form of abundant Fe oxides and/or Fe sulfides and/or Fe-rich silicates, and with sufficient Cu-Au to be classed as a resource. Deposits in CGI mineral systems exhibit distinctive enrichments of elements in the chalcophile-siderophile suite (Co, Ni, Bi, Se, Te) and/or elements in the LILE-HFSE suite (REE, U, F, Ba, Mo), the ratios of which vary greatly between deposits and define a continuum. A key finding is the correlation of these geochemical variations with the range of oxidation-reduction (redox) characteristics of the ore-related hydrothermal minerals, and also with the three geological-tectonic settings. The causes of these variations are likely fundamental in the formation of IOCG and related deposits.
CGI mineral systems are characterised by paragenetically early Na ± Ca-rich hydrothermal alteration (generally in regional-scale zones), followed by combinations of Fe-, Ca- and K-rich minerals that preceded or accompanied Cu-Au mineralisation. This review has shown that volatile-bearing minerals (e.g. carbonate (CO2), apatite (P), fluorite (F), barite (SO4), tourmaline (B)) were deposited with the Cu-Au mineralisation in almost all IOCG provinces, albeit less abundantly in the continental arc-hosted deposits.
It is argued that the special combinations of basinal-derived and magmatic/igneous-derived inputs to ore formation produces the distinctive range of characteristics of IOCG and related deposits in CGI mineral systems.