Sedimentary facies analysis of Badami Group of sediments and its implication on uranium mineralization in western part of Kaladgi basin, Belagavi district, Karnataka, India

The discovery of uranium mineralization in the sediments of the Badami Group, especially at Deshnur, the Kaladgi basin has attracted geologists as one of the most favorable targets for uranium exploration. Three broad fining-upward sedimentary sequences were delineated within the Badami Group of sediments. Mesoscopically, sixteen different sedimentary facies have been identified and grouped into five genetically related facies associations (FA-1 to FA- 5). The lateral variation of the identified facies and their vertical stacking pattern indicates that the lower part of the succession is dominated by an alluvial fan to channel-fill depositional regime with periodic deposition of over bank sediments. The middle part of the succession is characterised by deposition in tide-dominated estuarine conditions, whereas, the facies motifs indicate deposition of the upper part of the succession in storm-dominated shallow shelf environments. Uranium mineralisation is mainly hosted by matrix-supported conglomerate (F1), clast- to matrix-supported conglomerate grading to mudstone (F2), crudely laminated mudstone (F9) that are parts of alluvial fan (FA-1) and fluvial channel (FA-2) facies associations. Mineralization is confined mainly along bedding planes, and the mineralized zones occur as 0.2- 6.0 m thick lenses. Uraninite and coffinite commonly occur along the grain boundaries of quartz and feldspar and within the interstitial pore spaces, invariably associated with pyrite, thus indicating epigenetic introduction. Geochemically, mineralized sandstone is characterized by relatively low SiO₂ and CaO, moderate Na₂O and high Al₂O₃, K₂O, whereas non-uraniferous sandstone contains high SiO₂, moderate Al₂O₃, K₂O and low Na₂O and CaO compared to average Proterozoic sandstone. Uraniferous facies has higher concentration of Cu, Ni, Zn and Pb contained in sulphides. In comparison to non-mineralised sandstone, mineralised sandstone show the lower values of the ratios SiO₂/Al₂O₃ and K₂O/Al₂O₃ (avg. 0.32) are related to silica dissolution and formation of clay minerals during diagenesis as well as alteration by mineralizing fluids. Petrographic features coupled with geochemical data indicate modification of primary porosity and permeability of immature fluviatile sediments during diagenesis. This played a significant role in facilitating the migration of uranium bearing solution and subsequent uranium mineralization. (author)