Geochemical evolution of uranium mineralization in the Mohuldih deposit, Singhbhum Shear Zone, India: constraints from uraninite and brannerite chemistry

The study reports textures and compositions of uraninite and brannerite from the Mohuldih uranium deposit of the Singhbhum Shear Zone. Three types of texturally varied uraninite are identified: (i) pre-tectonic (UG-1), (ii) syn to post-tectonic (UG-2), and (iii) inclusions (UG-3). Based on UO2 and PbO composition, these are further divided into five subgroups (UG-1a, UG-2a, UG-1b, UG-2b, and UG-3). The negative correlation between the rare-earth elements including Y (Sigma REE2O3 + Y2O3) and UO2 of UG-1a and UG-2a uraninite indicates substitution of REE and Y for U4+ in the structure of these grains during primary crystallization, whereas UG-1b and UG-2b resulted from REE + Y + U-enrichment during secondary fluid-induced alteration. Compositional trends further reveal post-crystallization incorporation of Na, Si, K, Ca, Ti, and Fe, replacing radiogenic Pb in the structure of uraninite. The U-Th-Pb-Total dating of uraninite yields two clusters: older (similar to 1.71-1.64 Ga; preserved by UG-1a and UG-2a) and younger (similar to 0.93 Ga; UG-1b and UG-2b). Additionally, the extrapolated best-fit linear trend for uraninite in UG-3 population preserves chemical dates ranging from similar to 1.71 to similar to 1.64 Ga. The similar to 1.64 Ga represents a U + Y + HREE + Ca + Fe hydrothermal event, resulting in the precipitation of uraninite, followed by dissolution-reprecipitation of the mineral along the grain boundary and fractures at similar to 0.93 Ga. The oldest mesured date (similar to 1.71 Ga) is attributed to partial Pb-loss by older uraninite during the similar to 1.64 Ga hydrothermal event, with no evident geological significance in the area. The dissolution-reprecipitation of the early-formed uraninite also resulted in the formation of brannerite and ilmenite during the similar to 0.93 Ga event.