Fluid inclusion and stable isotope geochemistry of the Ernest Henry Fe oxide-Cu-Au deposit, Queensland, Australia

Ernest Henry is an iron oxide-copper-gold deposit characterized by magnetite≫hematite, chalcopyrite as the only significant hypogene copper mineral. It occurs within a large, zoned, medium to high temperature alteration system. Pre- to synmineralization quartz contains three broad populations of fluid inclusions. Type 1 (L-V-H±nS) inclusions decrepitate or homogenise at 200–500° C, have salinities of 32–55 wt% NaCl

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, display chemical variations that parallel the paragenetic sequence of alteration, and have variable Br/Cl mostly less than magmatic brines. Type 2 (L-V) inclusions homogenise at temperatures of 120–350°C have salinities up to 20 wt% NaClequiv and inferred to contain CaCl

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. Type 3 L-rich CO

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inclusions were entrapped at 130–370MPa. dD and dO of pre- to synmineralisation fluids are estimated to have been −23 to −66 and 8 to 11 per mil respectively. dS in main ore pyrite and chalcopyrite is −1.6 to +5.4 per mil. The data are compatible with a substantial magmatic contribution to Ernest Henry fluids but also suggest the system was complex and involved fluids and other components with different sources. Ernest Henry is distinguished from the giant Olympic Dam deposit by evidence for high temperature/salinity fluids and for no major involvement of surficial waters during mineralization.