Uranium exploration in the upper proterozoic Bhima Basin, Karnataka, India: A new target area

The unconformity between the Lower Proterozoic basement granite and overlying Middle-Upper Proterozoic sediments of Purana basin constitute the most important target for locating unconformity related uranium deposit in India. The Upper Proterozoic Bhima basin occurring in the northern part of Karnataka comprising predominantly calcareous and clastic sediments is presently one of the important targets for uranium exploration. Integrated exploration programme along with sub-surface drilling resulted in establishing a medium grade and low tonnage fracture controlled uranium mineralization associated with Shahabad Limestone of Bhima group and basement granite along the EW trending 30 km long Gogi - Kurlagere reverse fault in proximity to the unconformity at Gogi, Karnataka. Several other surface uranium occurrences have been identified in different lithic units viz., phosphatised limestone at Ukinal and Madnal, limestone/cherty breccia at Darshanapur and shale at Kasturpalli. The reverse faulting at the southern margin of the basin has resulted in folding of the sedimentary units and the brecciated - fractured axial zone host rich grade uranium mineralization, which occur as veins and veinlets at Gogi. The mineralized zones have steep to moderate dips proximal to the hangwall sediment-basement granite contact and attain near horizontal dips proximal to the footwall sediment - basement granite contact. The thick mineralized zone formed by the merger of footwall and hangwall lodes resembles a 'nose-like structure', which is probably formed due to reverse faulting. In the sheared basement granite, uranium mineralization occurs about 5 to 20m below the unconformity. Few fractures in hangwall granite are also mineralized but have restricted extent. Pitchblende and coffinite, the main uranium phases are essentially ultrafine (5 to 10 microns), localized in sulphides rich portions and also occurs as dispersed specks in brecciated calcareous and siliceous matter. Notable concentration of SiO2 in pitchblende, and appreciable amount of REE, Pb, Ag, Au in pyrites have been observed. Significant target areas have been delineated by hydrogeochemical survey and Airborne Gamma-ray Spectrometry. The Bhima basin is currently under active exploration involving an integrated approach to identify large tonnage, high-grade and low-cost uranium deposits. (author)