GENESIS OF URANIUM MINERALIZATION IN SURFICIAL ENVIRONMENTS: INSIGHTS FROM ERONGO REGION (NAMIBIA) AND WESTERN RAJASTHAN (INDIA)

Carnotite is a uranium–potassium vanadate [K₂(UO₂)₂(VO₄)₂.3(H₂O)], an ore of uranium that is precipitated from oxidized uranium bearing ground waters in near-surface environments. Carnotite is typically hosted by calcrete bodies within palaeochannels and playa lakes in hot and arid, desert regions.

The palaeochannels of Erongo Region, western Namibia, contain significant accumulations of calcrete-hosted uranium mineralization including world’s largest surficial uranium deposit at Langer Heinrich. In this contribution, we describe a conceptual genetic model of surficial uranium systems in Erongo based on intensive field studies, proprietary exploration data and new geochemical data. The model reveals that the mineralization is mainly controlled by channel morphology including low hydraulic gradients, confluence of tributary channels, and the presence of basement highs on the channel floor and impervious rocks across the channels that lead to pooling of uranium bearing oxidized ground waters. The concentration of high-grade mineralization in the proximity of channel confluences indicate that fluid mixing could also have been partially responsible for breakdown of uranyl carbonate complexes.

Carnotite precipitation has been located at several places in the Rajasthan desert, western India, particularly around Barr and Didwana, where the processes leading to carnotite mineralization are presumably currently operating. The pegmatites and granitic gneisses present in the study area show greenish-lemon yellow surface staining, which is characteristic of uranium leaching from the constituent minerals and are the possible sources of uranium, while the vanadium could be drawn from the surrounding amphibolites. Extensive ICP MS analyses of ground waters from various aquifer systems in the area indicate anomalously high uranium contents up to 800 mg/litre that are strongly correlated with the pH and Eh of the waters. The concentrations of major anions (Cl^-, SO₄^2-, CO₃^- and HCO₃^- ) in the water samples were quantified in order to identify the ligand complexating with uranyl ions at a given pH-Eh. In this contribution we report the results and interpretations of the geochemical analyses of the aquifer rocks for U, V, K and groundwaters for U, V and uranyl-complexating ligands.