Mineral chemistry of radioactive and associated phases from neoproterozoic unconformity related uranium deposits from Koppunuru, Palnad sub-basin, Guntur District, Andhra Pradesh, India

Unconformity proximal uranium mineralization at Koppunuru occurs in basement granitoids and the overlying Banganalapalle Formation of Kurnool Group in Palnad sub-basin. The U-mineralization transgresses the unconformity both above and below. Later remobilization of uranium is evident, as they are intermittently intercepted within the sediments overlying the unconformity. Subsurface exploration by drilling intercepted three mineralization bands, viz. Band A and B upto 80m above the unconformity in the overlying Banganapalle quartzite and Band C, mostly sub-parallel to the unconformity and confined to basal conglomerate/grit horizon except a few boreholes where it is transgressing to granitic horizon (<5m). The radioactive minerals include massive colloform pitchblende and scaly coffinite associated with carbonaceous matter, as well as veins, grain boundary/fracture fillings in granite and quartzite. The grit/granite radioactive samples, under microprobe, indicate presence of pitchblende, coffinite, and varying complexes like U-Ti-Si-AI, U-Si-Ti-AI, U-Si-AI, U-Si-AI-Pb, U-Si, U-Si-Ca-P and U-Si-Pb-AI. This suggests higher activity of alteration processes and remobilization of phases in an open chemical system. The alteration is closely associated with fine-grained zircons appear to be due to the incorporation of UO2 (upto 2.00 %), ThO2 (0.03 to 1.51 %) and RE2O3 (0.12 to 3.56 %). Such activities signify the processes of epigenetic fluid/gel related to U-concentration. At increasing depths, possibility of AI- bearing radioactive phases is also envisaged. The radioactive phases present in the samples reveal negligible to low thorium indicating low temperature phenomena. They are likely to be emplaced by the epigenetic solution/gel rich in U, Ti, Si, AI, Ca, P and Pb, preferably along available spaces as vein, cavity and grain boundary. U-associated sulphides occurring as veins and fracture fills, essentially comprise pyrite, pyrrhotite, chalcopyrite, pentlandite and galena. They have normal chemistry but for subtle variations in minor elements. The pyrite and pyrrhotite are invariably arseniferous and they dominate the sulphides. Thus, it is concluded that the area has potential for multi-episodic epigenetic U-mineralization