WITWATERSRAND PALEOPLACER MINERALOGY AS AN INDICATOR OF THE ARCHEAN ATMOSPHERE

The 3.0 Ga age of uraninite grains in Witwatersrand conglomerates using U-Pb dating has established that they are older than the onset of sedimentation at 2.97 Ga. and therefore that they are detrital in origin. A precise Re-Os isochron age of 2.99 ± 0.11 Ga recently obtained for rounded pyrite grains associated with the uraninite has established that the pyrite is also detrital. This new information supports numerous other lines of evidence that have been used in the past to interpret the uranium and pyrite concentrates as Archean paleoplacers.

The sedimentary and stratigraphic history of the Witwatersrand succession documents that these minerals were part of the sub-aerial sediment load over a period of 150 million years. Sedimentation rates during the accumulation of the Central Rand Group is estimated to have been in the order of less than 20 meters per million years and individual deposits would have covered areas of up to 400 km². From Kaapvaal mining production figures available it is evident that collectively the amount of uraninite and pyrite exposed to the Archean atmosphere over a region exceeding 70 000 km² during this period was more than 1.5 and 243 million tons respectively. These conditions must also have been prevalent on the Sao Francisco Craton in Brazil and on the Canadian Shield where pyrite / uraninite paleoplacers also developed.

From the onset of Proterozoic red-bed sedimentation on the Kaapvaal craton there is no record of either detrital uraninite or pyrite being concentrated in paleoplacers. It is concluded that they were unable to survive the prevailing weathering conditions. This change in the preservation potential of uraninite and pyrite is considered to be related to an evolutionary change in the composition of the atmosphere that took place after 2642 Ma, which is the best age constraint available on the youngest uraninite and pyrite paleoplacers on the Kaapvaal craton. The huge scale and geographic distribution of the deposits involved in proportion to the extent of continental area present at that time leads to the conclusion that the conditions were worldwide.