URANIUM MINERALIZATION IN THE SOUTH-WESTERN PART OF CUDDAPAH BASIN - A PETROMINERALOGICAL AND GEOCHEMICAL STUDY

Two types of uranium mineralisation occur in the south-western part of Cuddapah basin, viz., stratabound type hosted by impure dolostone of the Vempalle Formation of the Papaghni Group and structurally-controll ed type hosted by deformed basement granitoid. The carbonate-hosted ty pe occurs over a 140 km-long belt from Reddipalle in the west to Maddi madugu in the east with promising mineralisation at Tummalapalle, Rach akuntapalle and Gadankipalle. It is sandwiched between a lower massive limestone and upper shale and cherty limestone. The mineralised carbo nate rock is a stromatolite-bearing 'siliceous, calcitic dolostone' (S CD+/-phosphate) associated with other impure dolostones, quartzite, ch ert, phyllite, conglomerate, and intrusive dolerite and basalt. Uraniu m mineralisation occurs along the bedding plane, carbonate-phosphate c ontact, micro-stylolites, grain boundaries of clasts and within pelloi ds, mainly in the form ultrafine pitchblende, in intimate association with pyrite and as disseminations in collophane-rich parts, besides as minor phases of coffinite and U-Ti complex. The associated ore minera ls include pyrite, molybdenite, chalcopyrite, bornite, digenite and co vellite. The fertile basement granite (s.1.) is the source of uranium and the mineralisation, which appears to be polygenetic (syn-, dia- an d epi-genetic), is controlled by impure nature of dolostone and organi c activity. The mineralisation in the southern part of the belt in Cud dapah district is typically molybdenum-rich, whereas the northern port ion, in parts of Anantapur district, is characterised by high content of copper. The mineralised SCD in the S/SE parts is also marked by hig h content of P, V and Pb, in addition to Mo. There is a good positive correlation of U with P and V (r : 0.98 and more). The structurally-co ntrolled uranium mineralisation occurs around Rayachoti (outside the S W margin of Cuddapah basin) along a number of fracture zones within th e basement granitoid, and is hosted by mylonites and cataclasites indi cative of intense dislocation metamorphism. Of many such zones, the on e between T. Sundupalle and Sanipaya is promising and has a strike ext ension of about 16 km. This mineralisation is epigenetic hydrothermal vein-type, mainly represented by coffinite, pitchblende, U-Ti complex and secondary uranium minerals.