Biotite chemistry and mineral association as an indicator of redox conditions in the iron oxide Cu-Au (IOCG) system: Constraints from the Khetri Copper Belt, western India

Key constraints related to the nature of alteration and mineralization in geologically complex economically important iron oxide copper–gold (IOCG) systems remain highly speculative, especially, physicochemical characteristics hydrothermal fluids. This study was conducted on samples from Khetri Copper Belt (KCB) western India that hosts several IOCG-type deposits. The aims constrain qualitative oxygen fugacity conditions during alteration-mineralization events using biotite uraninite chemistry. inferred redox are backed up by presence or absence redox-sensitive minerals including magnetite, ilmenite, graphite. Biotite is present association with REE ± Th U Cu-Co-U-REE mineralization, which has a genetic connection IOCG-style K-Fe-Mg alteration. first type mostly sulfide-free characterized REE-silicate, REE-phosphate, U-Th-silicate, U-oxides minor pyrite. other Cu-Co-Fe sulfides, U-oxides, REE-silicates -phosphates significant volume Fe3+ calculated electron probe data two different methods, concentrations were measured an microanalyzer (EPMA). Stability phases (magnetite, graphite), Fe3+-Fe2+-Mg content biotite, U/Th ratio collectively suggest essentially below haematite-magnetite buffer often reaching fayalite-quartz-magnetite buffer. We conclude studied assemblages represent deeper reduced facies IOCG system. chemistry can be used as indicator for alteration/mineralization where particularly magnetite absent.