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

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 the previously reported age of the 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 the 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 uraninites crystallized at high temperature (> 390 °C) 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 the 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 contents 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 a hydrothermal fluid during the earliest Fe-Mg alteration.