Role of hydrothermal activity in uranium mineralisation in Palnad Sub-basin, Cuddapah Basin, India

• Hydrothermal signatures in an unconformity-related U deposit in Cuddapah Basin.
• Hydrothermal veins and associated alteration in granite and cover sediments.
• Ore controls of U mineralisation in cover sediments of Palnad Sub-basin.

Unconformity related uranium mineralisation occurs in Banganapalle Formation of Palnad Sub-basin, Cuddapah Basin. Several evidences of hydrothermal activity exist in both basement granite and the cover sediments in Koppunuru and Rallavagu Tanda (R.V. Tanda) uranium prospects of Palnad Sub-basin. Profuse development of fracture filled veins consisting of epidote–quartz, chlorite–quartz and quartz is observed at various depths above and below unconformity. Fluid–rock interaction during the formation of these veins has resulted in the alteration of feldspars and mafic minerals of granite and arkosic quartzite into a mineral assemblage consisting of various proportion of illite, chlorite, muscovite and pyrite, with the intensity of alterations being highest near to the unconformity. Pyrite is often associated with illite dominant alteration zone. We infer that circulation of basinal brine through basement granite and cover sediments was responsible for mobilising uranium from granite and its precipitation at favourable locations in cover sediments. Increase in pH of ore fluid due to illitisation and chloritisation of wallrock together with availability of carbonaceous matter and pyrite as reductant have controlled the localisation of uranium mineralisation in Banganapalle Formation.