THE WITWATERSRAND LEGACY: WHAT DOES IT MEAN FOR EARLY EVOLUTION OF THE EARTH'S ATMOSPHERE?

‘Witwatersrand-style’ gold and uranium mineralization remains a cornerstone of the reducing Archean atmosphere hypothesis. In fact, the first serious suggestion that the early Earth’s atmosphere was reducing originated in response to the discovery of uraninite in the Witwatersrand orebodies in the early 1950s. Since the oxidation of pyrite and reduced uranium minerals, such as uraninite, occurs readily under modern atmospheric conditions, their persistence during detrital transport in an oxidized atmosphere is highly unlikely, although remotely possible, given localized modern occurrences. As a result, the close association of reduced uranium minerals, pyrite and gold in the Witwatersrand posed a clear challenge to the placer model for gold which dominated at the time. In response to this dilemma, several South African placerists postulated that the Earth’s atmosphere must have been ‘reducing’ during Witwatersrand deposition. The alternative of revising the origin of the mineralization was not considered.

There are several versions of what is meant by a ’reducing’ atmosphere, but the presence of ferric and ferrous iron, or just ferrous iron, in near-surface environments is critical to many geological processes including those forming major rock types. The same cannot be said for many of the trace elements and isotopic systems used to contribute to the early atmosphere debate.

One of the main recent developments has been the hydrothermal model for Witwatersrand mineralization. Evidence for hydrothermal alteration in orebodies is ubiquitous and together with textural studies of the ore minerals, casts considerable doubt on the origin of gold, pyrite and uraninite. In addition, studies of paleosols in the Witwatersrand and elsewhere that have been used to constrain atmospheric conditions are plagued by difficulties distinguishing primary chemical signatures through complex depositional and post-depositional chemical processes. Given these complications, the evidence from the Witwatersrand does not unambiguously support a reducing atmosphere during the Archean, and several features in fact suggest the atmosphere was not reducing, i.e. ferric iron was stable. The alternative hypothesis of an oxidizing Archean atmosphere has important implications for ore genesis and exploration.