Geochemical evolution and timing of uranium mineralization in the Khetri Copper Belt, western India

Abstract The Khetri Copper Belt (KCB), located in the northwestern part of the Aravalli-Delhi Fold Belt, Western India, is famous for Cu mineralization and known for Au ± Ag ± Co ± Fe ± REE ± U ± P occurrences. The present study conducted in and around the Madan-Kudan-Kolihan-Chandmari Cu-deposits of the KCB integrates the mode of occurrence, mineralogical association, textural relation, and chemistry of hydrothermal minerals, and in situ chemical dating of uraninite. We propose that the U mineralization, represented by Type 1 to Type 6 uraninites, has evolved through six successive stages: U-mineralization (Type 1 uraninite) of uncertain origin →U-mineralization (Type 2 uraninite) during Fe-Mg alteration →U-mineralization (Type 3 uraninite) during Ca-Na alteration →Cu-Fe-Co-REE-U mineralization (Type 4 uraninite) during Na-Ca-K alteration →U-mineralization (Type 5 uraninite) during chloritization (Fe-Mg alteration) →Cu-Co-REE-U mineralization (Type 6 uraninite) during K-Fe-Mg alteration. The chemical ages of uraninite suggest that the hydrothermal mineralization associated with Type 2 to Type 5 uraninites formed during a hitherto unknown older event (compared to previously reported age of mineralization in the KCB i.e. ∼850 Ma), which initiated during the second phase of metamorphism of ∼1.40-1.30 Ga (M2) and terminated well before the third phase of metamorphism of ∼985-920 Ma (M3). The K-Fe-Mg alteration and associated mineralization most likely is time-equivalent of the known alteration-mineralization event of ∼850 Ga. The chlorite and biotite thermometry in tandem with U/Th ratios of uraninite suggest that uraninite crystallized at high temperature (> 340 oC) in all the hydrothermal stages. The common presence of magnetite and ilmenite, the occasional presence of graphite, and the Fe3+/(Fe3++Fe2+) ratios of the co-genetic gangue minerals such as amphibole, biotite, and chlorite suggest reduced environment, below haematite-magnetite buffer, during the crystallization of uraninite in all the hydrothermal stages. Based on the composition of gangue minerals (apatite, amphibole, biotite, chlorite, and scapolite) in alteration assemblages/veins and physicochemical characters of the fluids, we discuss the possibility of transportation of U as U4+-chloride and -fluoride complexes. The high temperature and reduced nature of the fluids, high Th content and low U/Th ratios of most of the uraninites, similar age of mineralization (Type 2 to Type 5 uraninite) and M2 metamorphism, and the absence of concomitant magmatic activity collectively suggest that hydrothermal mineralization related to Type 2 to Type 5 uraninites took place from metamorphic fluids. We report, for the first time, a unique uraninite-graphite association in the KCB, which can be best explained by their co-precipitation from hydrothermal fluid during the earliest Fe-Mg alteration.