Multiple episodes of hydrothermal alteration and uranium mineralization in the Singhbhum Shear Zone, eastern India: constraints from chemical and boron isotope composition of tourmaline

Abstract

In this study, the chemical and B-isotope composition of tourmalines that coprecipitated with uraninite in schistose rocks of the Mohuldih and Bagjata uranium deposits along the Singhbhum Shear Zone are used to constrain the sources and the nature of the mineralizing fluids, along with the mechanism of primary uraninite precipitation. Three generations of tourmaline are identified, which can be correlated with three generations of uraninite in the host rocks. These correspond to three major episodes of hydrothermal alteration of the shear zone rocks at 1.8-1.9 Ga, 1.66-1.56 Ga, and ca. 1.0 Ga, synchronous with metamorphism/granitoid activity. The texturally earliest tourmaline with δ11B of 8.8 ± 0.3 ‰ precipitated together with primary uraninite from externally-sourced high saline fluids derived from subducting slab during the 1.8–1.9 Ga hydrothermal alteration event. The precipitation of uraninite can be linked to the decreasing salinity of the hydrothermal fluid as a consequence of fluid-rock interaction. Dissolution of chlorite/biotite and the precipitation of more Mg-rich tourmalines resulted in decrease of fluid salinity, which destabilized Fe-chloride and U-chloride complexes leading to the precipitation of Fe-rich tourmaline together with primary uraninite. The texturally later second and third generation tourmalines (δ11B = 2.5 to 6.3 ‰) and uraninites formed from mixed fluids derived from the slab and dehydration of pelitic assemblages during renewed episodes of hydrothermal alteration synchronous with metamorphism at 1.66-1.56 Ga and ca. 1.0 Ga. The δ11B of the tourmalines from the schists of the Singhbhum Shear Zone correspond to the range of values (–10.4 ‰ to +6.0 ‰) reported for IOCG-type deposits associated with metamorphic slab-derived fluid sources.