An example of uraniferous leucogranites in the Rössing South-West deposit, Namibia

Abstract The southern Central Zone (sCZ) of the Neoproterozoic to early Palaeozoic Damara orogenic belt hosts significant uranium deposits in Namibia. Rossing South-West, a leucogranite-hosted uranium deposit in the sCZ is partitioned into two sections; north-easterly and south-westerly sections with the latter (termed mining licence 120, i.e. ML120) forming the locus of this investigation. The deposit lies within the vicinity and to the west of the Welwitschia lineament which served as a conduit for mineralized fluids along with the Rossing and Husab deposits. Mineralogy and major and trace element geochemistry of the uraniferous leucogranites in ML120 suggest they are S-type granites derived from a sedimentary source as evidenced by the presence of igneous phases of garnet and cordierite together. Microscope and electron microprobe results from 133 powdered, drill core and hand samples prove that the deposits host similar primary uranium mineralization (chiefly uraninite which mainly exist as independent uranium minerals) and similar secondary U mineralization. At Rossing-SW U minerals include uraninite, coffinite, pitchblende, betafite, uranophane, betauranophane, carnotite, gummite and might partially exist as Th isomorphs. Mineralization at all locales is associated with post-tectonic D and E-type sheeted leucogranites containing quartz, alkali-feldspar and variable amounts of plagioclase producing rocks of variable compositions from tonalite to alkali-feldspar granite. Total reserves calculated at a cut-off grade of 100 ppm were reported as 171 Mt of ore at 376 ppm for Rossing and 205 Mt of ore at 497 ppm for Husab (probable). The Rossing-SW orebody is open-ended however total inferred resources stand at 304.4 Mt of ore at 235 ppm while measured resources stand at 8830t ore at circa 231 ppm. A comparison of the geological parameters (uranium mineralogy, leucogranite petrology and geochemistry) suggests similar ore composition. Consequently, ore from Rossing-SW's ML120 area could be processed at existing metallurgical plants (such as at Rossing or Husab) with a probable increase in U3O8 yield.