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The first serious suggestion that the Archean atmosphere was reducing was based on the interpretation of round uraninite and pyrite grains in the Witwatersrand Basin in the early 1950s. It was then inferred that these minerals were detrital and that they reflected equilibrium with a reducing Archean atmosphere.

Over the past 20 years the understanding of the Witwatersrand Basin has changed dramatically with more integrated studies of the basin and the recognition of widespread alteration in close spatial association with the mineralization in every goldfield. Post-depositional mobility of gold, sulfur, and uranium during alteration is widespread and supports hydrothermal ore genesis, or at least substantial modification of the original mineral assemblage. Pseudomorphic replacements of pre-existing detrital minerals (e.g., pyrite after titano-magnetite), and precipitation and/or chemical rounding to generate round mineral shapes (e.g., uraninite in carbon seams) have all been documented.

The recognition that the carbon seams formed by the post-depositional introduction and maturation of migrated hydrocarbons is a dramatic departure from earlier models of coalified algal material deposited with the sediments. The enrichment of both gold and uraninite in carbon seams implies that these minerals are hydrothermal and that their shapes do not reflect detrital processes. Uranium mobility in basinal waters may in fact require a relatively oxidizing atmosphere.

None of the existing arguments for the Witwatersrand mineralization unambiguously support a placer or modified placer model for the mineralization. Consequently, round uraninite and pyrite of the Witwatersrand Basin do not provide support for a reducing Archean atmosphere.

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