Mineralogy and geochemistry of phosphate minerals and brannerite from the Proterozoic Carbon Leader Reef gold and uranium placer deposit, Witwatersrand, South Africa

Yttrium-phosphate, the most common phosphate phase, is closely associated with detrital uraninite. It is assumed having formed by the reaction of mobile phosphate with Y and HREE liberated from the lattice of detrital uraninite, during diagenesis and/or metamorphism of the sediments. Authigenic brannerite (UTi2-3O6-8) is well-defined microscopically and geochemically. Textural relationships indicate that both titanium migrated to uraninite, forming brannerite, and mobile uranium caused the 'branneritization' of rutile/leucoxene. Microprobe analyses demonstrate that brannerite from the Carbon Leader Reef displays a distinct and small variation of UO2/TiO2 ratios, which lie close to the theoretical value of 1.117 for UTi3O8, in contrast to uraniferous leucoxene/brannerite from other Witwatersrand-type deposits, the latter showing a wide scatter of UO2/TiO2 ratios. (orig./HP)