Fingerprinting alteration and mineralization in the iron oxide Cu-Au (IOCG) system using biotite chemistry and monazite geochronology: constraints from the Khetri Copper Belt, western India

In this contribution, we compare textural relations and major-trace element-halogen chemistry of multiple generations of hydrothermal and metamorphic biotite from IOCG deposits located in the Khetri Copper Belt (KCB), western India. We also present U–Pb isotope monazite age and recalculate the chemical age of uraninite using published U and Th and new Pb concentration data. The textural and geochemical data are used to constrain the evolution of the alteration assemblages, the physicochemical characteristics of the fluids, and the timing of alteration/mineralization in the KCB. We compare biotite halogen data with those from other geologic settings including porphyry, Archean gold, and hydrothermal rare earth deposits for deposit type discrimination. In the KCB, biotite was formed by metamorphic processes as well as during multi-stage hydrothermal alteration events involving K ± (Fe + Mg) metasomatism. Among the five biotite groups studied, three hydrothermal types are genetically related to IOCG-style mineralization (REE (phosphate) ± U, LREE (silicate) ± Th ± U, and Cu (sulfide)-Co-U-REE mineralization) and two types (one metamorphic and one hydrothermal) are genetically unrelated to IOCG mineralization. Comparing monazite and uraninite ages with established ages of alteration and mineralization in the belt, we suggest that (1) IOCG mineralization in the KCB is recursive and took place at ~ 1.31 Ga and 0.85–0.82 Ga, (2) hydrothermal biotite unrelated to the IOCG alteration formed at ca. 1.37 Ga, predating both of the IOCG events, and (3) the IOCG biotite types formed during the younger event at 0.85–0.82 Ga. We propose that biotite groups genetically related to the 0.85–0.82 Ga IOCG event crystallized from high-temperature (average: 460 °C; Ti-in-biotite thermometry) and reduced fluids largely below the fayalite-quartz-magnetite buffer. We note compositional differences among the three biotite groups related to the IOCG mineralization and suggest that low concentrations of Li, Cs, Zn, V, Nb, and Ta in biotite may be an indicator of sulfide mineralization in the KCB. We show that crystal-chemical parameters (X Fe ) and halogen fugacity of fluids (log( f HCl/ f H 2 O) fluid ) exerted a significant control on the incorporation of Cl in biotite in the KCB. In contrast, trace element concentrations of biotite are largely controlled by temperature and (log( f HCl/ f H 2 O) fluid but do not appear to be controlled by crystal-chemical parameters. Compared to hydrothermal biotite from other mineralization settings (porphyry, magmatic-hydrothermal rare earth metal, Archean-Au deposits), biotite in the KCB and several other IOCG deposits precipitated from fluids with higher HCl activity, which we interpret to reflect the direct or indirect involvement of evaporite in their fluid source. The comparison of trace element data shows a significant difference in biotite chemistry in magmatic and hydrothermal settings and in different types of hydrothermal deposits such as IOCG, porphyry, and Archean Au deposits. We think that these elements may be useful for building robust discriminators in the future.