A Mineralogical-Geochemical Study of Mesoarchaean Quartz-Pebble Conglomerates from the Pongola Supergroup and Implications for U/Au Placer Deposits

The Witwatersrand Basin contains one of the largest gold reserves on Earth. The Pongola Supergroup is correlative to Wits Basin strata, but so far no metal enrichments of economic interest have been found. We studied the sedimentology, geochemistry and mineralogy of basal conglomerates of the Nsuze Group, lowermost group of the Pongola Supergroup, from two separate areas in KwaZulu-Natal, South Africa. The conglomerates form thin beds intercalated with coarse-grained sand-stones. They consist predominantly of vein quartz pebbles and are enriched in a suite of heavy minerals. Uranium is locally enriched (up to 780 ppm) and associated with secondary REE-rich phosphate phases, recording a hydrothermal overprint.