Geological and mineralogical notes on placer occurrences in some conglomerates of the Pongola Sequence

Pyritic placer mineralization in conglomerates of the 3 000 Ma-old Pongola Sequence of the south-eastern Transvaal, nothern Zululand, and Swaziland, supported a number of small gold-mining operations, from 1890 to 1930. Recently, several attempts have been made to find uranium in the Pongola sediments, and some of the old gold workings are currently being reassessed. Mineralogical, geological, and geochemical findings, were gathered from four occurrence. Reworking of the conglomerates occurred only to a minor extent, as suggested by the predominance of matrix-supported conglomerates, by the poor concentration of heavy minerals, and by their often poor rounding. Well-developed mineralization, carrying gold contents in the range from 1 to 4 g/t are usually restricted to the inconsistent clast-supported conglomerate layers. The mineralogical composition of the ores resembles broadly that of the Witwatersrand reefs, and detrital, as well as a number of authigenic ore minerals, which formed as a result of diagenesis and mild metamorphic overprint, were found. Gamma- and alpha-spectrometric data prove the presence of radioactive disequilibria in the weathered conglomerate outcrops. The absence of uraninite, thus, may be due to recent removal of uranium by supergenic processes