Hydrogeochemical exploration for unconformity-related uranium mineralization: example from Palnadu sub-basin, Cuddapah Basin, Andhra Pradesh, India

Abstract Hydrogeochemical exploration was undertaken over an area of 3100 km 2 in the northeastern part of Cuddapah Basin to identify target areas for exploring the unconformity-related U-mineralization. Water samples from 626 borewells drilled in the Kurnool Group of the Palnadu sub-basin, Nallamalai Group and basement rocks were collected. For the individual lithological domain, univariate and multivariate statistical techniques were applied to the analytical data on 20 variables, i.e. U, Na, K, Ca, Mg, Cl, SO 4 , HCO 3 , CO 3 , Si, Fe, Cu, Ba, Zn, F, Mn, conductivity, total dissolved salts (TDS), Eh and pH. The R-mode factor analysis establishes geochemical association between U, Na, K, Cl, SO 4 , HCO 3 , TDS and conductivity. Another important association is between Fe, Mn and Zn. Maps of factor-1 scores (with high loadings on U) and U-contours, clearly delineate the well-established U-mineralization at Koppunuru–Dwarkapuri in the Palnadu sub-basin. Interestingly, the same maps also display another prominent north–south trending anomaly zone extending over a linear stretch of 45 km, commencing in the basement, passing through the Nallamalai Group and continuing into the younger Kurnool Basin in the eastern part. The hydrogeochemical anomaly around Julakallu is significant for targeting unconformity-related U-mineralization as, in this area, the Kurnool Group sediments unconformably overly the most promising Cumbum Formation comprising low-grade metamorphosed carbonaceous pelites.