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

Abstract

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