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The sedimentary setting of Witwatersrand placer mineral deposits in an Archean atmosphere

By
Wyatt Ernest Lawrence Minter
Wyatt Ernest Lawrence Minter
1
Department of Geosciences, University of Cape Town, Rondebosch 7700, South Africa
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Published:
January 01, 2006

The 3.05 Ga U-Pb dating of uraninite grains in Dominion and Witwatersrand conglomerates has established that they were older than the onset of Witwatersrand sedimentation at 2.97 Ga. and therefore that they are detrital in origin. A precise Re-Os isochron age of 2.99 Ga obtained for rounded pyrite grains associated with the uraninite indicates that the pyrite is also detrital. Evidence of detrital forms in the gold has confirmed its placer origin prior to modification during metamorphism. Furthermore, rhenium-depletion ages ranging from 3.5 to 2.9 Ga for Witwatersrand gold support numerous other lines of evidence that have been used in the past to interpret the gold, uranium and pyrite concentrates as Archean placers.

The sedimentary and stratigraphic history of the Witwatersrand succession indicates that uraninite, pyrite, and gold were part of a sub-aerial sediment load over a period of 180 m.y. Net sedimentation rate during the accumulation of the Central Rand Group is estimated at less than 14 m per million years, reflecting stratigraphic losses due to repeated reworking. Individual paleoplacers would have been exposed over areas of up to 400 km2 but collectively they covered a region exceeding 2000 km2 and contained more than 243 million tons of pyrite, 1.5 million tons of uraninite, and 80,000 tons of gold.

Because there is no record of either detrital uraninite or pyrite in Proterozoic red-bed sediments on the Kaapvaal Craton, it is concluded that a change in the composition of the atmosphere took place there after 2.64 Ga.

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GSA Memoirs

Evolution of Early Earth's Atmosphere, Hydrosphere, and Biosphere - Constraints from Ore Deposits

Stephen E. Kesler
Stephen E. Kesler
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Hiroshi Ohmoto
Hiroshi Ohmoto
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Geological Society of America
Volume
198
ISBN print:
9780813711980
Publication date:
January 01, 2006

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