3.2 Ga detrital uraninite in the Witwatersrand Basin, South Africa: Evidence of a reducing Archean atmosphere

The evolution of oxygen levels in the Archean atmosphere is one of the most important questions in the study of early Earth environments. The presence of redox-sensitive uraninite in the Archean Witwatersrand Basin, South Africa, has been cited as evidence for a reducing Archean atmosphere; however, whether the uraninite is detrital or hydrothermal in origin has been intensely debated. Here, we present, for the first time, in situ U-Pb ages of uraninite from the Witwatersrand Basin that are significantly older than the host sedimentary rocks (the 3074 +/- 6 Ma Dominion and 2872 +/- 6 Ma Vaal Reefs). Uraninite grains from the Dominion Reef are 3186 +/- 19 Ma and may have been eroded from 3174 +/- 9 Ma granites in the immediate hinterland, while uraninite grains from the Vaal Reef formed at 2994 +/- 48 Ma and 2918 +/- 20 Ma and are similar in age to detrital zircon populations within the Central Rand Group, which includes the Vaal Reef. These predepositional ages conclusively prove the detrital origin of the uraninite and represent arguably the best evidence to date that the Mesoarchean atmosphere was more reducing than the modern atmosphere. The detrital origin of the uraninite strongly supports the modified paleoplacer model for uranium mineralization within the Witwatersrand Basin over the hydrothermal models.