Iron oxide copper-gold (IOCG) deposits – a review (part 1): settings, mineralogy, ore geochemistry, and classification within the Cu-Au-Fe (± Co, REE) deposit family

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Roger Skirrow


Characteristics of ten of the world’s metallogenic provinces hosting iron oxide Cu-Au (IOCG) deposits have been critically assessed, including geochronology, 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 global family of Cu-Au-Fe (± Co, REE) or CGI deposits that also includes Fe-rich Cu-Au deposits lacking significant iron oxides. The CGI family is subdivided on the basis of combined tectonic setting, ore geochemistry, and oxidation state of ore-related mineralogy. Three subtypes of CGI deposits have elevated REE ± U, F and Ba and/or enrichments in Co (± Ni, Bi, Se, Te), with known deposits occurring in Precambrian syn- and post-orogenic settings. A fourth subtype of CGI deposits generally lacks significant enrichments in REE, U, F, Ba, Co, Ni, Bi, Se and Te and occur mainly in Phanerozoic Andean type magmatic arc settings. The REE- and/or Co-rich CGI deposit subtypes are geologically and geochemically distinct from porphyry Cu (-Au), skarn Fe-Cu and iron oxide-apatite (IOA) deposits, although there are some shared features. The low-REE-U-Co arc-hosted CGI deposits have more in common with other arc-hosted Cu-Au deposit types, and may differ genetically from the REE-U- and/or Co-rich CGI deposits.

Many of the world’s major IOCG-bearing metallogenic provinces host representatives of the syn-orogenic CGI deposit group. In some cases syn-orogenic settings evolved over a few 10s of millions of years to post-orogenic settings via a switch from compressional to extensional tectonic regimes. CGI deposits in arc-hosted settings differ from those in orogenic-related settings in forming during extension/transtension of continental margin magmatic arcs, mostly prior to inversion tectonics in the case of the Mesozoic Andean deposits. Metallogenic provinces hosting CGI deposits in all three settings are characterised by the coincidence in space and time between sedimentary ± volcanic basins and intrusive ± volcanic magmatism. Extended continental margin basins and later bimodal magmatism with A- and high-temperature I-type igneous rocks are characteristic of the syn- and post-orogenic settings, inboard of craton paleo-margins. In contrast, arc-hosted CGI deposits formed where back-arc basins were overprinted by syn-mineralisation calc-alkaline dominantly intermediate-composition magmatism.

Other features shared by the CGI including IOCG deposits in all three settings include the following.
• A metal association of both Cu and Au with highly elevated Fe (e.g. 10-60 wt % Fe), as abundant Fe oxides and/or Fe sulfides and/or Fe-rich silicates, with sufficient Cu-Au to be classed as a resource.
• A spectrum of oxidation states of ore-related hydrothermal mineralogy in many provinces, from reduced (e.g. pyrrhotite-chalcopyrite ± magnetite ± Fe-silicate-bearing) through intermediate-redox (e.g. magnetite-pyrite-chalcopyrite-bearing) to oxidised (e.g. hematite-pyrite-bornite ± chalcocite ± sulfate minerals).
• Minor elements (REE, U, F, Ba, Co, Ni, Mo, Bi, Se, Te) that are variably enriched in three of the four subtypes of CGI deposits, correlating with both redox state and geotectonic settings (reduced CGI-Co deposits in syn-orogenic settings; intermediate-redox CGI-Co-REE deposits in syn-orogenic settings; oxidised CGI-REE deposits in post-orogenic settings); deposits of the fourth, arc-hosted, subtype have generally low levels of these minor elements.
• Paragenetically early Na ± Ca-rich hydrothermal alteration (generally in regional-scale alteration zones) followed by combinations of Fe-, Ca- and K-rich minerals that accompanied or preceded Cu-Au mineralisation together with volatile-bearing minerals (e.g. carbonate (CO2), apatite (P), fluorite (F), barite (SO4), tourmaline (B)).
• A paucity of hydrothermal quartz and Mg-rich hydrothermal minerals in the IOCG members of the Cu-Au-Fe family, but abundant quartz in many of the Fe-oxide-poor members.

It is argued in this and a companion paper reviewing the fluids and ore genesis that it is the special combinations of basinal-derived and magmatic/igneous-derived inputs to the deposits that produces the distinctive range of characteristics of Cu-Au-Fe (± Co, REE) deposits.



Physical Sciences and Mathematics


copper, gold, iron oxide Cu-Au (IOCG) deposits, metallogeny, rare-earth elements, cobalt


Published: 2021-06-01 02:25


CC BY Attribution 4.0 International

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Data Availability (Reason not available):
review article, with all data sources cited

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