Abstract

The Rössing Mine area in Namibia contains uranium mineralization primarily within crustally derived post-collisional sheeted leucogranites. Adjacent to the mine in the SH area, a small 1-km long granite cupola of coalesced sheets is also characterized by elevated U levels. In the Rössing pit, in the SJ area, U is hosted by uraninite and secondary U minerals including beta-uranophane. Conversely, in the SH area, U is hosted principally by the pyrochlore-group mineral, betafite. Manometric determination of the proportions of H2O, CO2 and non-condensables (mostly methane) has been carried out on inclusions within quartz from leucogranite samples from both the SH and SJ anomalies. This method has been able to distinguish between leucogranites of the SH and SJ areas which has not been possible with thermometric fluid inclusion studies or by major and trace element geochemical investigations. Fluid geochemistry of sheeted leucogranites from the SH area exhibit lower absolute H2O contents, lower H2O/CO2 ratios and low total fluids compared to that from the SJ area. Fluid geochemistry and large ion lithophile (LIL) element data suggest that the occurrence of the SJ anomaly as a major uranium deposit is linked to the high H2O and total fluid contents. This study illustrates the usefulness of this technique to distinguish between economic mineralization and the sub-economic SH anomaly.

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