Fertility of Late Archaean basement granite in the vicinity of U-mineralized Neoproterozoic Bhima basin, peninsular India

Late Archaean granitoids constituting the basement for the Neoproterozoic Bhima Group are exposed along the southern margin of the Bhima basin in southern India. These are rich in accessory minerals such as sphene, allanite, apatite and zircon, which are the main carriers of uranium and thorium. In situ gamma-ray spectrometric analysis reveals that these granitoids have higher abundances of Th, U and K (Th range 10-43 ppm, mean 26 ppm; U range 3-21 ppm, mean 8 ppm; K range 1.2-5.2%, mean 4.0%) relative to granitoids occurring farther away from the basin. Thus, they belong to the class of fertile granitoids from the point of view of uranium mineralization. The granitoids have been mylonitized and hydrothermally altered in the Gugi-Ukkinal fault zone, which constitutes the zone of uranium mineralization discovered recently along the southern margin of the Bhima basin. Uranium apparently derived from hydrother-mal leaching of basement granitoid rocks may have got deposited in the fault zone at the contact of carbonate rocks, which provided favourable geochemical environment (Eh-pH conditions) for uranium precipitation.