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Witwatersrand gold deposits formed by volcanic rain, anoxic rivers and Archaean life

Abstract

The Witwatersrand Basin in South Africa is one of the best-preserved records of fluvial sedimentation on an Archaean continent1. The basin hosts the worlds biggest gold resource in thin pebble beds, but the process for gold enrichment is debated2,3,4,5. Mechanical accumulation of gold particles from flowing river water is the prevailing hypothesis2,6, yet there is evidence for hydrothermal mobilization of gold by fluids invading the metasedimentary rocks after their burial3,4,7. Earth’s atmosphere three billion years ago was oxygen free8, but already sustained some of the oldest microbial life on land9. Here I use thermodynamic modelling and mass-balance calculations to show that these conditions could have led to the chemical transport and precipitation of gold in anoxic surface waters, reconciling the evidence for fluvial deposition with evidence for hydrothermal-like chemical reactions. I suggest that the release of sulphurous gases from large volcanic eruptions created acid rain that enabled the dissolution and transport of gold in surface waters as sulphur complexes. Precipitation of the richest gold deposits could have been triggered by chemical reduction of the dissolved gold onto organic material in shallow lakes and pools. I conclude that the Witwatersrand gold could have formed only during the Archaean, after the emergence of continental life but before the rise of oxygen in the Earth’s atmosphere.

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Figure 1: Rock slab of Carbon Leader Reef, with black carbon containing bonanza-grade gold as microscopic inclusions.
Figure 2: Gold solubility in Archaean surface water as a function of redox conditions and acidity.
Figure 3: Process cycle for the formation of giant carbon-leader gold reefs in the Witwatersrand basin.

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Acknowledgements

I thank N. Arndt, T. Driesner, B. M. Eglington, I. Halevy, G. Hall, B. Lehmann, A. Pather, T. Schlegel, T. Wagner, V. Wall and P. Weis for inspiration and help with field observations, and for critical comments on this paper since its first presentation at the 2013 Goldschmidt conference. Supported by ETH Zurich and Swiss National Science Foundation grants 200020-146681 and 200021-146651.

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Correspondence to Christoph A. Heinrich.

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Heinrich, C. Witwatersrand gold deposits formed by volcanic rain, anoxic rivers and Archaean life. Nature Geosci 8, 206–209 (2015). https://doi.org/10.1038/ngeo2344

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