LITHOSTRATIGRAPHIC AND RADIOMETRIC APPRAISAL OF DEEPER PARTS OF SRISAILAM AND PALNAD SUB-BASINS IN KOTTAPULLAREDDIPURAM–ACHCHAMMAGUNTA– RACHCHAMALLEPADU AREA, GUNTUR DISTRICT, ANDHRA PRADESH
Rahul Banerjee* , N.K. Bahukhandi, M. Rahman, K.K. Achar, P.V. Ramesh Babu, K. Umamaheswar and P.S. Parihar
Atomic Minerals Directorate for Exploration and Research,
Department of Atomic Energy,
Begumpet, Hyderabad
*Email: rahulbnrg@gmail.com
Abstract
In the northern part of Cuddapah Basin, Meso- to Neoproterozoic sediments are deposited in Srisailam and Palnad sub-basins over Neoarchaean to Palaeoproterozoic basement granitoids with younger felsic and mafic intrusives, and are affected by a number of lineaments, faults and fractures signifying reactivation in the area. The lihtostructural setup is akin to the classical unconformity type uranium mineralisation model as evident from significant uranium mineralisation in the marginal parts with <139m thick sedimentary column. It is an established fact that thick pile of cover sediments plays a vital role in enrichment of grade and thickness of such type of mineralisation. In view of this, the deeper parts of these sub-basins were explored by stratigraphic core drilling. In Kottapullareddipuram and Achchammagunta sectors of Srisailam Sub-basin a thick succession of quartzite has been established with shale–siltstone intercalations. These are unconformably deposited over basement granitoids with mafic intrusives. The unconformity depth in these sectors varies from 344.80m to 419.40m and 254.35m to 394.45m, respectively. It is observed that the sediment thickness increases due south and southeast of the explored blocks. Rachchamallepadu sector in Palnad Sub-basin, revealed thick sequence (117.15m to 173.25m) of Palnad sediments under lain by 113.85m to 181.05m thick column of Sr isailam sediments deposited over basement granitoids with unconformity depth var ying from 231m to 354.30m along E–W section line. Thinning of sedimentary column towards E is probably due to the presence of NNE–SSW trending fault. Palnad sediments are predominantly represented by alternate sequence of arenaceous and argillaceous rocks belonging to Banganapalle Formation. Radiometric examination of stratigraphic boreholes has indicated lean mineralisation ranging from 0.010 to 0.017%eU3O8 grade and 0.20m to 1.70m thickness in granitoids and mostly occurring 11–17m below the unconformity. Quartzite hosted lean mineralisation (0.010–0.011% eU3O8 x 0.20–0.40m), just above the unconformity, has also been recorded in Srisailam Sub-basin. This has opened up the deeper parts of Srisailam and Palnad sub-basins for detailed uranium exploration.
Keywords: Uranium mineralisation, Unconformity, Stratigraphic drilling, Srisailam and Palnad Sub-basins, Guntur district, Andhra Pradesh.