Temperature Dependent Textural and Chemical Variations in Uraninites from Diverse Geological Environments of India

Uraninite contains primarily UO₂ besides a few other oxides, viz radiogenic PbO, ThO₂ and minor amount of (RE)₂O₃in varying proportions. The environment and process of uraninite formation control its variation in chemical composition and optical properties. Uraninites generally form in high temperature magmatic granites and pegmatites, with repeated remobilization of uranium lead to its economic concentration, by hydrothermal/metasomatic activity, metamorphic, sedimentary phenomena or conjunction of one or more of these processes spread in space and time. Petrology and mineral chemistry of uraninites help in inferring the processes of uranium mineralisation, which have important bearing on exploration and ore beneficiation.

The present study of uraninites from some important uraniferous/thoriferous horizons in parts of India shows that they are controlled by temperature of formation. These uraninites may be classified as high- and low-temperature types. The high temperature type occurs in magmatic rocks, high temperature hydrothermal veins, migmatites and high temperature metasomatites. The typical high temperature magmatic/migmatic type uraninites occurring in granite, pegmatites and migmatites, are coarse, euhedral to subhedral grains, of high reflectivity and high micro hardness, and occur as inclusions within gangue minerals and also are associated with oxides, sulphides, rare earth and rare metal minerals. Chemically, such uraninites are marked by higher concentration of ThO₂, (RE)₂O₃, Y₂O₃ and lower content of SiO₂, CaO and TiO₂.

The low temperature type found in sedimentary uraninites is invariably fine grained, anhedral with lower reflectivity and hardness, distributed along weak planes and associated with sulphides with or without organic matter. In such uraninites, the concentration of ThO₂, (RE)₂O₃, Y₂O₃ is relatively lower and they have substantial amount of SiO₂ CaO and TiO₂.

The high temperature uraninites formed by fluid activity in veins and metasomatites are anhedral to euhedral grains, coarse to fine in grain size and are associated with both opaque oxides, sulphides and occasionally with organic matter. These uraninites contain variable amount of ThO₂, moderate to high (RE)₂O₃ and high Y₂O₃ and low SiO₂, CaO and TiO₂.