Geochemistry and chemical dating of uraninite in the Samarkiya area, central Rajasthan, northwestern India – Implication for geochemical and temporal evolution of uranium mineralization.

In the Samarkiya area, located at the central part of the Aravalli-Delhi Fold Belt (ADFB), uranium mineralization is hosted both by the basement Mangalwar Complex and the overlying supracrustal rocks of the Pur-Banera belt. The present study aims to appraise the geochemical and temporal evolution of uranium mineralization from the basement and the adjoining supracrustals in the Samarkiya area integrating textural features, geochemistry, and in situ U-Th-Pb Total dating of uraninite. Uraninite occurs as inclusions in the major rock forming minerals, viz. plagioclase, quartz, biotite, and chlorite. Based on the shape, location in the host mineral (well inside/at the grain boundary/along or connected to micro-cracks etc.) and association with other secondary uranium minerals, uraninites are classified into different groups, which are compositionally distinct, barring some exceptions. Integrating texture, geochemistry and in situ electron probe dating we propose that in addition to an old event at ∼1.88 Ga in the basement rocks, there are two major events of uraninite formation at ∼1.24–1.20 Ga and ∼1.01–0.96 Ga in both the basement and supracrustal rocks. Although none of the pristine, unaltered uraninites that formed during the above mentioned events contain significant intrinsic minor or rare earth elements, the basement uraninites are consistently much enriched in thorium compared to those from the supracrustal. Based on the compositions, we propose that the basement uraninites formed from a high temperature magmatic/metamorphic fluid, whereas those in the supracrustal rocks crystallized from a low temperature, presumably oxidized fluid. Back-scattered electron images, X-ray elemental mapping of selected elements and EPMA spot analysis of large uraninite grains (both from the basement and the supracrustals) collectively demonstrate that subsequent to the major mineralizing event at ∼1.24–1.20 Ga, the mineralized rocks were subjected to fluid-mediated alteration, which resulted in ∑REE + Y- and Si (Ca)-enrichment of existing ∼1.24–1.20 Ga uraninites in the basement and supracrustal rocks, respectively. We cannot constrain the exact timing of this alteration event. However, as this event altered the ∼1.24–1.20 Ga uraninites and as spot ages of the altered grains yield ages largely between ∼1.24 and 0.96 Ga, it is reasonable to place this event between the second and third stages of uranium mineralization/mobilization at ∼1.20 Ga and ∼1.01 Ga, respectively. The last event that took place at ∼1.01–0.96 Ga most likely represent an episode of recrystallization/alteration of existing uraninite leading to complete Pb-loss and resetting of the isotopic clock. However, we do not entirely reject the possibility of neo-mineralization. The discrete events deciphered from uraninite in the Samarkiya area can also be broadly linked to some major magmatic-metamorphic events, identified from other independent studies, in the ADFB. For example, the earliest ∼1.88 Ga event displayed by basement uraninite is most likely related to a pervasive magmatic-metamorphic event (∼1.86–1.82 Ga) that affected the basement, whereas the last/latest event ∼1.01–0.96 Ga can be linked to a pervasive metamorphic event that affected perhaps the entire ADFB. This last episode can also be linked to the tectono-thermal event related to the Rodinian amalgamation. The ∼1.24–1.20 Ga event appears to be somewhat enigmatic in the context of well-known geological events in the area. However, based on some very recently published data, we interpret this to be a post-peak metamorphic (∼1.37–1.35 Ga) hydrothermal event or even a new metamorphic event, hitherto unknown. [ABSTRACT FROM AUTHOR]