The transport of gold from the Earth’s mantle to the surface relies on sulfur from active volcanoes. Two recent studies agree that certain forms of sulfur bind to gold, allowing this precious element to rise to the surface. However, scientists do not agree on which form of sulfur is more important, reports ScienceAlert in a publication on Monday, December 30, 2024. Both hypotheses are worth further investigation, because understanding how gold deposits form could help us better exploit this valuable resource in the future.
Experiments by Deng-Yang He and his team at the University of Geosciences of China (published in PNAS) found that it was trisulfide – a molecule containing three sulfur atoms that binds to other elements – which plays a key role. But according to the work of Stefan Farsang and Zoltán Zajacz of the University of Geneva in Switzerland (published in Nature Geoscience), it is disulfide – a molecule composed of two sulfur atoms, often linked to metals like gold – which would be the deciding factor.
In Iceland, living with volcanoes
Trisulfide or bisulfide: which form of sulfur plays a key role?
Typically, gold deposits are associated with volcanic areas located at tectonic plate boundaries. One of the plates slides under the other, creating subduction zones where volcanoes and earthquakes form, like in the famous Pacific Ring of Fire. The gold contained in these deposits comes from the Earth’s mantle, at depths where it would remain trapped without volcanic activity. The gold is thus transported in the magma which rises to the surface and is deposited. Scientists know that sulfur is essential for this transport. This chemical element easily binds to heavy metals. Debate centers on which form of sulfur is responsible for transporting gold via subduction zones.
On the one hand, Deng-Yang He and his team developed a thermodynamic model aimed at predicting the conditions necessary for the transport of gold. Their research shows that at specific temperatures and pressures, gold binds to trisulfide, forming a soluble complex capable of transporting gold at concentrations much higher than those present in the mantle. This model suggests that trisulfide is an extremely efficient way to extract gold from the mantle and bring it to the Earth’s crust.
Adam Simon, a geologist at the University of Michigan, participated in the Chinese study. “This thermodynamic model, which we have just published, is the first to demonstrate the existence of the gold-trisulfide complex, which we were unaware of under these conditions”he observes in a press release. He believes that this model optimally justifies the high concentrations of gold observed in certain mineral systems of subduction zones.
At the same time, results from the University of Geneva suggest that it is disulfide that plays a key role. Stefan Farsang and Zoltán Zajacz changed the oxidation state of sulfur in their experiments, under temperatures reaching 875°C, similar to those of natural magmas. Unlike the experiments of their peers, who pointed to trisulfide as responsible for transport, their results attest that bisulfide, hydrogen sulfide and sulfur dioxide are all present at magmatic temperatures. An interesting discovery, because it was not known that disulfide could exist at these high temperatures.
A scientific debate that could redefine mining
The results of the two studies, whether they concern trisulphide or bisulphide, provide valuable insight into the mechanisms of formation of gold deposits in extreme geological conditions. This work paves the way for future research aimed at clarifying the exact chemical processes responsible for the transport of heavy metals in subduction zones, which could influence mining practices in these regions.
