Iron oxide copper–gold (IOCG) deposits – A review (part 2): fluid, metal and sulfur sources and ore genesis

• Data on sources of fluids, metals and sulfur synthesised for IOCG deposits globally. • Evidence supports fluid mixing as main process in forming major IOCG deposits. • Precambrian and Mesozoic deposits mostly differ in the fluid carrying Cu-Au. • Ore fluid in most Precambrian deposits was evolved basinal brine that leached metals. • Second fluid was commonly of magmatic-hydrothermal origin, volatile-rich. Genetic models for the formation of iron oxide copper–gold (IOCG) deposits have been debated since recognition of the class, with proposed ore fluid origins ranging from non-magmatic basinal to magmatic-hydrothermal, with or without fluid mixing. This review reassesses previous models by synthesising and re-interpreting fluid geochemical and stable and radiogenic isotope data for IOCG deposits worldwide. These data are integrated with previously compiled information for 10 IOCG provinces to build a new and holistic model of IOCG deposit genesis. Although many gaps in data remain, previous disparate models can be reconciled and key features of most major IOCG deposits explained by mixing of fluids sourced from both (meta)sedimentary ± (meta)volcanic basins and from magmas. The roles of these fluids as carriers of Cu-Au differ, however, between IOCG deposits in Precambrian orogenic to post-orogenic versus Mesozoic magmatic arc settings. Relatively limited fluid geochemical and isotope data for Mesozoic arc-hosted IOCG deposits are consistent with formation of most ores from Cu-Au-bearing magmatic-hydrothermal fluids that mixed with Na-Ca-rich oxidised basin-derived brines. In contrast, and based on variably comprehensive datasets, the Cu-Au ore fluids forming most IOCG deposits in Precambrian orogenic and post-orogenic settings are interpreted to have been Ca-Fe-Ba-rich evolved basin-derived brines ± metamorphic fluids that leached metals from Na-Ca-altered and mafic rock sequences. These generally low-sulfur fluids mixed with a second fluid, commonly of magmatic-hydrothermal origin, that carried sulfur and other volatiles (CO 2 ± P ± B ± F) as well as Fe and K ± Mo, to form most major IOCG deposits.