Sedimentological and mineralogical aspects of the newly discovered Witwatersrand placer deposit that reflect Proterozoic weathering, Welkom gold field, South Africa

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The Eldorado paleosurface marks the last major period of erosion in the Witwatersrand succession in the Welkom gold field. Fluvial and debris flow sediments on this surface contain placer concentrates that were ultimately derived by the repeated erosion of subcropping placers, which occur within a sequence of onlapping formations, separated by unconformities, near the southern margin of the basin. Autoradiographs of core recovered by drilling through the Eldorado paleosurface indicate that uranium mineralization is associated with detrital pyrite and gold accumulations. This mineralization occurs as fluvial bed-load concentrates that were deposited in shallow paleochannels and with conglomeratic diamictites. Comparison of heavy mineral assemblages in crushed concentrates of the subcropping placers with those in the placer alluvium on the Eldorado pediment indicates that the suite of heavy minerals is the same; all three varieties of pyrite show detrital rounding, including secondary pyrite which originally formed in situ in the older placers; kerogen particles appear to be rounded allogenic grains of broken columnar kerogen derived from erosion of older placers; uraninite is very sparse and inclusions found in the rounded kerogen grains have been largely altered to brannerite; the dominant uranium-bearing minerals are uraniferous leucoxene and brannerite; and various stages of alteration from uraninite to brannerite are evident. The rarity of uraninite indicates that most of the grains of that mineral were altered during reworking and prolonged exposure in a slightly oxidizing atmosphere.