From Flames to Fortune: The Fiery Journey of Gold to Earth’s Surface

The mechanisms that transport gold to the surface from the depths of Earth’s mantle depend on sulfur that rises from active volcanic areas.

Two recent studies have concurred that certain forms of sulfur create molecular bonds with gold, preventing it from remaining trapped in the mantle, thereby enabling the rich metal to ascend.

However, there is some disagreement regarding which type of sulfur plays the most significant role.

According to numerical simulations and research carried out by a group of geologists led by Deng-Yang He from the China University of Geosciences, trisulfur is essential. Conversely, findings from experiments by Stefan Farsang and Zoltán Zajacz from the University of Geneva highlight bisulfide as the pivotal element.

Both sets of findings are intriguing and merit further investigation – as understanding the formation of gold deposits can assist us in optimizing this exquisite, precious, and valuable asset.

Gold deposits are frequently linked with volcanic activity, particularly at locations where tectonic plates converge. At such sites, one tectonic plate may slide beneath another, which creates what is known as a subduction zone. The interactions between these plates give rise to a region marked by earthquakes and volcanic activity, such as the extensive volcanic arc called the Pacific Ring of Fire.

The gold in these deposits has its origins deep within Earth, in the mantle. Left undisturbed, this dense metal would remain there; however, it becomes incorporated into the magmas that ascend through volcanic processes to the surface, where it settles.

Experts believe that sulfur is central to its transportation. Sulfur has strong bonds with heavy metals, including gold. The ongoing debate among geoscientists centers on the form this sulfur takes to carry gold through the subduction zones of Earth.

Deng-Yang He and his team performed experiments at varying pressures and temperatures to create a thermodynamic model that could forecast real-world circumstances that facilitate gold transportation. Their findings indicated that, under very specific temperature and pressure conditions where water is heated and oxidized due to the sinking crust, gold and trisulfide bond to form a soluble compound with the formula Au(HS)S₃–.

This complex, according to their calculations, can transport gold concentrations of several grams per cubic meter of fluid – exceeding three orders of magnitude above the typical gold abundance in Earth’s mantle. It represents a highly effective means of extracting gold from the mantle and depositing it into the crust.

“This thermodynamic model we have just published is the first to unveil the existence of the gold-trisulfur complex that we were previously unaware of at these conditions,” states geologist Adam Simon from the University of Michigan.

“This presents the most logical explanation for the very high concentrations of gold found in some mineral systems within subduction zone environments.”

Nonetheless, it might not be the sole method of transportation. In the experiments carried out at the University of Geneva, Farsang and Zajacz discovered a means of adjusting the oxidation state of the sulfur, applying it to pressure conditions and temperatures at 875 degrees Celsius (1607 Fahrenheit), which corresponds with the temperatures of natural magmas.

Previous research, including a widely referenced 2011 study, had indicated that trisulfur was the factor responsible for transport. The recent results demonstrated that bisulfide, hydrogen sulfide, and sulfur dioxide were all prominently present at magmatic temperatures.

This is significant, as bisulfide contributes to metal transportation in hydrothermal fluids, which operate at lower temperatures. It was previously believed that bisulfide could not exist at magma temperatures; however, Farsang and Zajacz’s research established that it can.

“By meticulously selecting our laser wavelengths,” Farsang explains, “we also demonstrated that earlier studies had significantly overestimated the amount of sulfur radicals in geological fluids, showing that 2011 study outcomes were fundamentally based on a measurement error, thus resolving this debate.”

Those are fighting words. Your move, trisulfur.

The two studies have been published in the Proceedings of the National Academy of Sciences, and Nature Geoscience, respectively.