Geological and geochemical appraisal of uranium bearing Gulcheru Formation of Cuddapah Supergroup, Gandi Area, Cuddapah District, Andhra Pradesh

摘要

Gulcheru Formation constitutes 200 m - 250 m thick siliciclastic sediments. Based on grain size texture and colour of the sediments, four distinct facies are demarcated. They are pink massive quartzite, cross bedded grey quartzite, purple siltstone and pitted quartzite. The ratios of Si/Al is higher (62.9 to 292.18) in pitted quartzite and lowest (2.122 18.247) in siltstone, where as grey quartzite has wide range of 3.78 - 8637.67. This difference is essentially due to higher quartz content in pitted quartzite and dominance of clays/shaly component in siltstone. The wide range of Si/Al ratio in grey quartzite is due to interlayering of siltstone/shale laminations/bands. Distribution of uranium indicates grey quartzite (av. U3O8 = 9.9 ppm) and siltstone (av. 15.42 ppm) have higher intrinsic uranium than that of pitted quartzite (average 5.6 ppm) and pink massive quartzite (average 7ppm) which is considered very high in comparison to normal sandstone (av = 0.5ppm, Taylor, 1965). Trace elements like Mo, V, Ni, Cu and Pb are enriched in mineralised grey quartzite. Pitchblende is the main uranium mineral in grey and pitted quartzite units. It occurs as fracture fillings and also as interstitial material, associated intimately with chlorite. Close association between pitchblende and sulphides like pyrite, chalcopyrite and galena is seen when pitchblende occurs as fracture fills. Uranium in siltstone, is generally in adsorbed form with ferruginous material, where as associated trace elements like Mo (r = 0.515), V (r = 0.806), Ni (r = 0.4) and even TiO2 (r = 0.87) are well correlated with Al2O3 suggesting their association with clays/phyllosilicates. U/Pb dating of uraniferous grey quartzite indicated ages of 1336 +/- 14 Ma and 446 +/- 29 Ma suggesting remobilisation and concentration of intrinsic uranium.