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