A Giant Mesoarchean Crustal Gold-Enrichment Episode: Possible Causes and Consequences for Exploration

Comparison of conglomerate-hosted, Witwatersrand-type gold deposits and/or occurrences worldwide reveals that this deposit type is by no means unique to the Kaapvaal craton but common to most Archean and/or Paleoproterozoic cratons. The age of the variably mineralized fluvial to fluvio-deltaic conglomerates ranges from 3.1 to 1.9 Ga. They were deposited in tectonic settings ranging from continental rifts to passive margins and synorogenic foreland basins, and all of them are paleoplacers. Although several of them show evidence of local mobilization of ore components by postdepositional hydrothermal fluids, purely epigenetic hydrothermal models fail to explain the geometry of the orebodies as well as available lithogeochemical, mineral chemical, and isotope data. Conglomerates older than 2.4 Ga are characterized by an abundance of detrital (and second-ary) pyrite, and in most cases also detrital uraninite, whereas most of the younger examples (<2.2 Ga) contain Fe oxides instead. A common denominator of Witwatersrand-type deposits is the stratigraphic position above erosional unconformities adjacent to an Archean to Paleoproterozoic hinterland. The Witwatersrand deposits themselves differ from all other examples of this type by a gold endowment that is two to three orders of mag-nitude greater, an abundance of gold-rich “carbon ” seams that reflect former microbial mats, a scarcity of gold nuggets, and orders of magnitude higher Os contents in the gold