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 (U_G−1), (ii) syn to post-tectonic (U_G−2), and (iii) inclusions (U_G−3). Based on UO₂ and PbO composition, these are further divided into five subgroups (U_G−1a, U_G−2a, U_G−1b, U_G−2b, and U_G−3). The negative correlation between the rare-earth elements including Y (ΣREE₂O₃ + Y₂O₃) and UO₂ of U_G−1a and U_G−2a uraninite indicates substitution of REE and Y for U⁴⁺ in the structure of these grains during primary crystallization, whereas U_G−1b and U_G−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 (~ 1.71−1.64 Ga; preserved by U_G−1a and U_G−2a) and younger (~ 0.93 Ga; U_G−1b and U_G−2b). Additionally, the extrapolated best-fit linear trend for uraninite in U_G−3 population preserves chemical dates ranging from ~ 1.71 to ~ 1.64 Ga. The ~ 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 ~ 0.93 Ga. The oldest mesured date (~ 1.71 Ga) is attributed to partial Pb-loss by older uraninite during the ~ 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 ~ 0.93 Ga event.