Uranium and Thorium Anomalies in the ~2.5 Ga Vendodu Leucogranite, Nellore Schist Belt, SE India and its Potential to Generate Uranium Deposits

Abstract

The Vendodu intrusive, emplaced at 2483±3 Ma within the Nellore schist belt, SE India, is a K-rich per-aluminous A-type leucogranite composed of quartz and alkali feldspar (essential minerals), biotite, zircon, allanite, titanite, magnetite and apatite (magmatic accessory minerals) and muscovite, haematite, fluorite and uraninite (hydrothermal accessory minerals); zircon and uraninite could be both magmatic and hydrothermal. The Vendodu leucogranite is characterized by high Rb, Zr, Nb, Th, U and REE, low Ca, Al, Ba and Sr abundances, and large negative Eu anomalies. U content in the Vendodu leucogranite averages 17.63 ppm and is 6 to 11 times higher than average concentration of U in Upper Continental Crust (UCC) and Archaean granitoids. Similarly, Th content averages 61.38 ppm and is 3 to 20 times higher than average concentration of Th in Archaean granitoids and UCC. The U distribution in the Vendodu leucogranite is influenced by both magmatic and high-temperature hydrothermal processes. Processes that have enriched U have also increased LREE, Nb and Cr contents in the leucogranites. Geochemical proxies including Th/U, Zr/U and V/Yb suggest both magmatic and high-T hydrothermal (deuteric) enrichment of U over a wide range of temperatures and oxygen fugacities. High Th/U ratios in the Vendodu leucogranite (1.62–9.76) hint that the hydrothermal (deuteric) fluids were possibly magmatic. Petrographic and geochemical evidences suggest that the Vendodu leucogranite experienced magmatic and deuteric U enrichment that has potential to form mineralized zones either in situ or in pegmatitic veins.