Peraluminous granites: potential source for uranium mineralization in Koppunuru area, Guntur district Andhra Pradesh

Atomic Minerals Directorate has established uranium mineralization in Koppunuru area lying to the western part of Palnad Sub basin. The granitic rocks with high intrinsic uranium content (27.88 ppm, N=968) forms the basement for Banganapalli and Narji Formations of Kurnool Group. Basement granites are potash rich, low calcic and peraluminous in nature. Geochemically, the granites (N=19) are characterized by high Al2O3 (13.31-24.23%), and moderate SiO2 (62.09-74.01%) and FeO (1.41-4.11%), Low MgO (0.03-1.63%) and CaO (0.11-1.84%) and very low MnO (0.01-0.04%) content. They are mainly low sodic with Na2O/K2O ratio varying from 0.09-0.95 (Av. 0.63, N=14). A few samples show high sodic nature with Na2O/K2O ratio varying from 1.00-1.68 (Av. 1.26, N= 5). Compositionally, they are granitic to granodioritic in nature and are peraluminous with A/CNK ratio > 1.1. Granites are mainly A-Type and characterized by high U/Th ratio (0.05-24.50, Av.= 4.41). Uranium content varies from < 5 to 59 ppm with an average 32 ppm (n=16) (excluding the mineralized samples) whereas thorium content varies from < 5 to 140 ppm in the study area. Petrographically, these granites are essentially composed of quartz, orthoclase, microcline, perthite, and plagioclase. Biotite and zircon are present as accessory minerals. Besides minor ore minerals like pyrite, chalcopyrite, galena, goethite and leucoxene are also observed. Mineral alterations are characterized by assemblage of sericite, muscovite and chlorite. Biotite is often squeezed along the intergranular spaces, highly bleached and altered to muscovite. Primary uranium minerals pitchblende and coffinite are found to occur as stringers and veins. Besides variable concentrations of uranium are associated with biotite and hydrated iron oxide along the basement fractures. These favorability features and high intrinsic content of uranium in the basement granite make it a potential source for uranium mineralization in the overlying Kurnool sediments in Koppunuru area. (author)