The Witwatersrand Basin in South Africa holds the world’s largest gold deposits. But how did it get there?
Some 40 percent of the precious metal that has been found so far comes from this area, and hundreds of tons of gold deposits still lie beneath the earth. Across the 124 mile-long (200 km) swath, individual ore deposits occur in thin layers over areas up to 6 by 6 miles.
Geologists debate how these giant deposits formed. Christoph Heinrich, professor of mineral resources at ETH Zurich and the University of Zurich, recently published a new explanation in Nature Geoscience, in which he tries to reconcile the contradictions of two previously published theories.
The prevailing “placer gold” theory states that the gold at Witwatersrand was transported and concentrated through mechanical means as metallic particles in river sediment. Such a process has led to the gold-rich river gravels that gave rise to the Californian gold rush.
Here, nuggets of placer gold have accumulated locally in river gravels in the foothills of the Sierra Nevada, where primary gold-quartz veins provide a nearby source of the nuggets.
But no sufficiently large source exists in the immediate sub-surface of the Witwatersrand Basin. This is one of the main arguments of proponents of the “hydrothermal hypothesis,” according to which gold, chemically dissolved in hot fluid, passed into the sediment layers half a billion years after their deposition.
For this theory to work, a 6-mile-thick (10 km) blanket of later sediments would be required in order to create the required pressure and temperature. However, the hydrothermal theory is contradicted by geological evidence that the gold concentration must have taken place during the formation of host sediments on the Earth’s surface.
Hydrogen sulfide in the atmosphere
Heinrich believes the concentration of gold took place at the Earth’s surface, by flowing river water, but in chemically dissolved form. With such a process, the gold could be easily “collected” from a much larger catchment area of weathered, slightly gold-bearing rocks.
He examined the possibility of this middle way by calculating the chemical solubility of the precious metal in surface water under the prevailing atmospheric and climatic conditions.
Experimental data show that the chemical transport of gold was indeed possible in the early stages of Earth evolution. The prerequisite was that the rainwater had to be at least occasionally very rich in hydrogen sulfide. Hydrogen sulfide binds itself in the weathered soil with widely distributed traces of gold to form aqueous gold sulfide complexes, which significantly increases the solubility of the gold.
However, hydrogen sulfide in the atmosphere and sulfurous gold complexes in river water are stable only in the absence of free oxygen.
“Quite inhospitable environmental conditions must have dominated, which was possible only three billion years ago during the Archean eon,” says Heinrich. “It required an oxygen-free atmosphere that was temporarily very rich in hydrogen sulfide—the smell of rotten eggs.”
In today’s atmosphere, oxygen oxidizes all hydrogen sulfide, thus destroying gold’s sulfur complex in a short time, which is why gold is practically insoluble in today’s river water.
Volcanoes and bacteria
In order to increase the sulfur concentration of rainwater sufficiently in the Archean eon, basaltic volcanism of gigantic proportions was required at the same time. Indeed, in other regions of South Africa there is evidence of giant basaltic eruptions overlapping with the period of the gold concentration.
A third factor required for the formation of gold deposits at Witwatersrand is a suitable location for concentrated precipitation of the gold. The richest deposits of gold ore in the basin are found in carbon-rich layers, often just millimeters to centimeters thick, but which stretch for many kilometers.
These thin layers contain such high gold concentrations that mining tunnels scarcely a meter high, some three kilometers below the Earth’s surface, are still worthwhile.
The carbon probably stems from the growth of bacteria on the bottom of shallow lakes and it’s here that the dissolved gold precipitated chemically, according to Heinrich’s interpretation.
The nature of these life forms is not well known. “It’s possible that these primitive organisms actively adsorbed the gold,” Heinrich speculates. “But a simple chemical reduction of sulfur-complexed gold in water to elementary metal on an organic material is sufficient for a chemical ‘gilding’ of the bottom of the shallow lakes.”
The gold deposits in the Witwatersrand, which are unique worldwide, could have thus been formed only during a certain period of the Earth’s history: after the development of the first continental life forms in shallow lakes at least 3 billion years ago, but before the first emergence of free oxygen in the Earth’s atmosphere approximately 2.5 billion years ago.
Source: ETH Zurich