Abstract

The presence of detrital uraninite and pyrite in fluvial placers of the Witwatersrand basin, South Africa, has been used to infer low levels of atmospheric oxygen during the Archean (>2500 Ma). However, recent studies advocate a hydrothermal origin for these minerals, thereby limiting their value as constraints on the composition of the early atmosphere. In contrast, ca. 3250–2750 Ma fluvial siliciclastic sediments from the Pilbara Craton in Australia have never undergone significant hydrothermal alteration, and their heavy minerals are of unequivocal detrital origin. These heavy minerals include the redox-sensitive phases pyrite, uraninite, and gersdorffite, along with more inert zircon, rutile, chromite, and monazite. Locally, siderite is a major constituent (to 90%) of the heavy mineral population, with grains displaying evidence for several episodes of erosion, rounding, and subsequent authigenic overgrowth. Detrital siderite is very rare in post-Archean sandstones, largely due to its instability in oxidizing environments. However, its frequent survival of prolonged transport in well-mixed and therefore well-aerated Archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present. Moreover, concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization.

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