Oil-triggered Gold Formation in the Witwatersrand, South Africa

English Abstract:

The Witwatersrand basin is the largest gold and uranium repository on Earth. The concentration of the metals in the basin has been debated for several decades. Opinion has been divided over whether they represent detrital mineralization that has been partly remobilized by hydrothermal fluids or if the mineralization is entirely hydrothermal. The highest concentration of gold and uraninite occurs in conglomeratic reefs spatially associated with pyrobitumen, suggesting that this constituent played a role in the ore-forming process. Oil inclusions preserved in quartz grains provide evidence that the pyrobitumen was derived from circulating liquid hydrocarbons. Fuchs et al. (2015) reported evidence that these liquids mobilized uranium and concentrated it as uraninite nanoparticles in the pyrobitumen.

Recent high-resolution SEM/TEM imaging of gold grains has furnished compelling evidence that the gold mineralization was triggered by a complex interplay between immiscible aqueous and hydrocarbon liquids. The gold crystals contain numerous rounded or elliptical inclusions filled with solidified hydrocarbons. These inclusions are interpreted to represent oil droplets, which matured and shrank to form pyrobitumen during burial. The thin layers of pyrobitumen contain nanocrystals of uraninite. We propose that the gold was dissolved in a hydrothermal fluid that mixed with liquid hydrocarbons forming an emulsion of the two liquids. According to this hypothesis, the gold was dissolved as a bisulphide complex and precipitated as the native metal around the hydrocarbon droplets due the reduction in fO₂ that accompanied interaction of the two fluids. The uraninite nanoparticles crystallized from the hydrocarbons during subsequent maturation of the latter.

Fuchs, S., Schumann, D., Williams-Jones, A.E., Vali, H., 2015. The growth and concentration of uranium and titanium minerals in hydrocarbons of the Carbon Leader Reef, Witwatersrand Supergroup, South Africa. Chem. Geol. 393–394, 55-66.