Mineralogy and paragenesis of the Meso-Proterozoic Rohil uranium deposit, North Delhi Fold Belt, Rajasthan, India

Abstract

The Rohil uranium deposit, located about 70 km south of the Khetri copper deposit in the North Delhi Fold Belt, Rajasthan, India, is albitite-hosted. This deposit is associated with Na-metasomatism, mainly affecting the metasedimentary wall rocks. Quartzite, quartz biotite schist, quartz amphibole schist, and graphite schist underwent variable degrees of albitization, as documented by a wide range of altered wall rocks, including weakly altered metasediments to almost pure albitites. These host rocks consist of metamorphic mineral assemblages such as Ca plagioclase – quartz – biotite – Ca amphibole – graphite – garnet – sillimanite – corundum – ilmenite. The hydrothermal alteration assemblages contain variable amounts of albite, chlorite, biotite, sericite, orthoclase, sphene, rutile, allanite, actinolite, and calcite. The textural relationships between primary host rock minerals and hydrothermal alteration products suggest four main stages of metasomatism: (1) an early pre-albitization stage, (2) a Na-metasomatic stage, that are followed by (3) a Na‐Ca-Mg- and (4) K- metasomatic stage. Albites are of two types: albite-1 is subhedral to euhedral and equidimensional, formed by the alteration of metamorphic plagioclase during the Na-metasomatic stage. Albite-2 is feathery and formed in fractures/cavities by precipitation of alkali-rich fluid during Na-Mg-Ca metasomatic stages. Albitization is pervasive on a regional scale. The hydrothermal metasomatic fluids utilized structures such as shears and breccias in the metasediments as conduits and as the loci of uranium mineralization. Major uranium minerals are uraninite, U-silicate, and brannerite. Uranium-bearing minerals occur as disseminations, in veins and as minute inclusions in biotite and titanite within albitized metasediments. The petrographic studies suggest that the metasomatic alteration of host rocks is characterized by albitization, actinolitization, carbonatization, chloritization, sericitization, biotitization, sulfidation, and silicification. The close association of U, REE-bearing minerals, and albite-2 indicates that the main U-REE mineralization stage was coeval with the Na-Ca-Mg and K-metasomatic stages.