HYDROGEOCHEMISTRY OF URANIUM LEACHING, TRANSPORTATION AND CALCRETE-HOSTED CARNOTITE PRECIPITATION IN SURFICIAL ENVIRONMENTS IN HOT, ARID AND DESERT REGIONS

Carnotite is a uranium–potassium vanadate [K₂(UO₂)₂(VO₄)₂.3(H₂O)], an ore of uranium, that is precipitated from ground waters in near-surface environments in calcrete bodies within palaeochannels in hot, arid and desert regions. Carnotite precipitation has been located at several places in Rajasthan, western India, particularly around Barr and Didwana, where the processes leading to carnotite mineralization are currently operating. The important rock types found in the area include granitic gneisses, pegmatites, mica schists, meta-carbonates and calc-silicates and amphibolites. The pegmatites and granitic gneisses showed greenish-lemon yellow surface staining, which is characteristic of uranium leaching from the constituent minerals (generally mica). Thus pegmatites and granites could be the sources of uranium, while the vanadium could be drawn from the surrounding amphibolites. Detailed sampling of ores, rocks and ground waters have been carried in the area, including the measurements of pH, Eh, conductivity and temperature. Further, detailed sampling of the ores, ground waters and host rocks is being carried out in and around the Langer Heinrich, Namibia, which is world's largest palaeochannel-hosted surifical uranium deposit. In this contribution we report the results and interpretations of the analyses of the aquifer rocks for U, V, K and for uranyl-complexating ligands (carbonate, sulfates and hydroxides), of the ground waters for uranyl-complexes, and of changes in Eh-pH conditions leading to precipitation of carnotite in the Barr and Didwana areas, western India and the Langer-Heinrich area, Namibia.