Preliminary studies on geochronology and geochemistry of uraniferous granites from Chitrial area, Nalgonda district, Andhra Pradesh

The crescent-shaped Cuddapah basin, covering an area of 44,500 Sq km is the second largest among the Purana basins in Peninsular India and has been considered highly promising for exploring unconformity-related uranium mineralization. The Atomic Minerals Directorate (AMD) for Exploration and Research had initiated reconnaissance radiometric survey in the Cuddapah basin and significant radioactive anomalies due to uranium were located in the basement granitoid close to the non-conformity contact with the Middle to Upper Proterozoic cover rocks of the Srisailam Formation, around Chitrial village, Nalgonda District, Andhra Pradesh. For geochronological studies, conventional isotope dilution techniques were used for the estimation of Rb, Sr and the isotopic ratios were measured on a VG 354 multi-collector mass spectrometer following the procedure described elsewhere. Rb and Sr abundances thus obtained range from 211 to 354 ppm and 15.9 to 136 ppm respectively and the values are in agreement with XRF data. The 87Rb/86Sr and 87Sr/86Sr ratios in these samples vary from 5.40 to 47.8 and 0.87089 to 1.6024 respectively. Seven out of the analyzed eight samples, when regressed together indicate an isochron age of 1234 ± 170 Ma (MSWD = 95) with initial 87Sr/86Sr ratio of 0.784 ± 0.025. Semi-quantitative U and Th data by geochemical (XRF) studies shows that the analyzed core samples have higher U/Th ratio in the range of 0.62-5.68 compared to average value of 0.26 in granitic rocks. It is therefore inferred that the Rb-Sr age of 1234 ± 170 Ma with very high initial 87Sr/86Sr of 0.784 is the time of the Sr isotopic re-homogenization during an episode of fluid interaction possibly related to process of U mineralization