GEOCHEMISTRY AND CHEMICAL DATING OF URANINITE IN THE JADUGUDA URANIUM DEPOSIT, SINGHBHUM SHEAR ZONE, INDIA--IMPLICATIONS FOR URANIUM MINERALIZATION AND GEOCHEMICAL EVOLUTION OF URANINITE

Abstract

The Jaduguda uranium deposit in the Singhbhum shear zone is the most important and best-known uranium deposit in India. Uranium mineralization is hosted in hydrothermally altered, metamorphosed, and deformed chlorite and biotite schists. Uraninite is the most abundant and ubiquitous U ore mineral in the shear zone, including the deposit at Jaduguda. The deposit is known to have undergone multiple events of hydrothermal fluid flux. Integrating chemical texture, geochemistry, and in situ electron microprobe U-Th-Pb chemical dating, we aim at deciphering the geochemical and temporal evolution of uranium mineralization in the Jaduguda deposit. Geochemistry of uraninite and X-ray mapping of selected elements demonstrate that, though compositions of small uraninite grains (~10 μ m) were mostly or completely modified by subsequent hydrothermal alteration, the larger grains (~100 μ m) locally retain the footprints of the earlier events and capture the imprints of subsequent events. In general, all analyzed uraninite grains are poor in Th (<2.5 wt % ThO2) and most grains contain detectable rare earth elements (REEs; ∑REE2O3: ~1.0–~12.5 wt %). Y and heavy rare earth elements (HREEs) are ubiquitous and most important in terms of concentrations. Based on the concentrations of U, Pb, and REEs, the Jaduguda uraninite can be classified into three compositional types: group-1a with low U, high Pb, and low ∑REEs; group-1b with low U and high Pb, but high ∑REEs; and group-2 with high U, low Pb, and moderate ∑REEs. These three compositional types represent three discrete hydrothermal events. Chemical textures of uraninite indicate that the earliest recognized hydrothermal event is manifested in the formation of nearly pure uraninite (group-1a), which is depleted in REEs and other minor elements. The second hydrothermal event is characterized by Y-HREE (+ Ca + Fe) metasomatism that altered the compositions of existing uraninite, shifting its composition toward that of group-1b. The third event further modified the compositions of the previous uraninite, mainly by removing part of the REEs, resulting in uraninite of group-2 composition. Using U-Th-Pb chemical ages (with a mean error of ~25 m.y.) and taking into account the effect of cation exchange on chemical age during alteration, we propose that the uranium mineralization (represented by group-1a uraninite) in the Jaduguda deposit is Paleoproterozoic in age and took place sometime circa 1.80 to 1.90 Ga. The timing of HREE metasomatism is not well constrained. However, published data and this study indicate a ~1.66 Ga age of HREE metasomatism. Based on a strong cluster of ages of group-2 uraninite, we place the latest hydrothermal event, which affected the Singhbhum shear zone in general and the Jaduguda deposit in particular, at ~1.0 Ga.