Geophysical signatures of uranium mineralization and its subsurface validation at Beldih, Purulia District, West Bengal, India: a case study

摘要

The Beldih open cast mine of the South Purulia Shear Zone in Eastern India is well known for apatite deposits associated with Nb-rare-earth-element-uranium mineralization within steeply dipping, altered ferruginous kaolinite and quartz-magnetite-apatite rocks with E-W strikes at the contact of altered mafic-ultramafic and granite/quartzite rocks. A detailed geophysical study using gravity, magnetic, and gradient resistivity profiling surveys has been carried out over approximate to 1 km(2) area surrounding the Beldih mine to investigate further the dip, depth, lateral extension, and associated geophysical signatures of the uranium mineralization in the environs of South Purulia Shear Zone. The high-to-low transition zone on the northern part and high-to-low anomaly patches on the southeastern and southwestern parts of the Bouguer, reduced-to-pole magnetic, and trend-surface-separated residual gravity-magnetic anomaly maps indicate the possibility of highly altered zone(s) on the northern, southeastern, and southwestern parts of the Beldih mine. The gradient resistivity survey on either side of the mine has also revealed the correlation of low-resistivity anomalies with low-gravity and moderately high magnetic anomalies. In particular, the anomalies and modeled subsurface features along profile P6 perfectly match with subsurface geology and uranium mineralization at depth. Two-dimensional and three-dimensional residual gravity models along P6 depict the presence of highly altered vertical sheet of low-density material up to a depth of approximate to 200 m. The drilling results along the same profile confirm the continuation of uranium mineralization zone for the low-density material. This not only validates the findings of the gravity model but also establishes the geophysical signatures for uranium mineralization as low-gravity, moderate-to-high magnetic, and low-resistivity values in this region. This study enhances the scope of further integrated geophysical investigations along the South Purulia Shear Zone to delineate suitable target areas for uranium exploration.