Trisulfur anion helps explain gold deposits on Earth

Native gold nugget. Credit: public domain

(—People have been excavating gold for thousands of years as a precious rare metal. Gold is in relatively low abundance in the Earth's crust likely coming from the metallic core and from meteorites. For reasons that are not entirely known, even though the average amount of gold throughout the Earth's crust is small, there are deposits containing gold in significantly greater concentrations than the crust's average abundance. These deposits tend to be in locations of geological activity and are likely due to the action of hydrothermal fluids. However, the concentration of gold in deposits is still higher than what would be expected from solutions carrying gold-chloride or gold-hydrogen sulfide.

A conglomerate of researchers hailing from the CNRS and several universities, and organizations in France has found a possible cause for the higher concentrations of gold. They propose that under hydrothermal conditions, the trisulfur ion (S3-) plays a key role in the deposition of gold. Their findings have implications for the way scientists predict the location of gold deposits and may lead to as-yet-discovered sites. Using , hydrothermal reactor measurements coupled with first-principle molecular dynamics, and thermodynamic modeling, they provide quantitative data for the effect of the trisulfur ion in hydrothermal conditions. Their work appears in the Proceedings of the National Academy of Sciences.

According to Pokrovski et al., existing gold speciation studies ignore the trisulfur anion, S­3-. Prior studies by this group and others have shown that the trisulfur ion exists in aqueous environments at high temperatures and pressures. For example, in previous research using in situ Raman spectroscopy, Pokrovski's group demonstrated that this form of sulfur is stable in aqueous solutions in a temperature range of at least 200oC to ~700oC and 30bar. Trisulfur had been difficult to identify because at cooler temperatures S­3- breaks down to sufate and sulfide in aqueous solutions. Their prior work provided an impetus for considering more than and chloride, the trisulfur anion in analyzing gold transport and deposition.

In an effort to quantify how S3- affects the concentration and precipitation of gold from aqueous hydrothermal fluids, Pokrovski et al. used model fluid systems comprised of gold metal and hydrogen sulfide, sulfate, and S3- and varied many of the system parameters such as temperature, pressure, redox potential, and acidity. Their data is from in situ X-ray absorption spectroscopy (XAS) and hydrothermal reactor measurements assisted by first-principles molecular dynamics (FPMD) and thermodynamic modeling.

Their study reveals that while Au(HS)2- is the most stable species in aqueous solutions at moderate temperatures (<250oC), as the concentration of S3- increases with increasing temperature, the measured gold concentrations became significantly greater than in hydrogen sulfide solutions alone. Of the possible S3- ion complexes, Au(HS)S3- was found to be the most stable. They point out that their estimates are likely lower than the reality because in addition to Au(HS)S3- other gold-trisulfur species may be stable beyond the temperature and pressure conditions of their experiments.

Their studies demonstrate three properties of S3- that could explain observations at certain gold deposit sites. These observations strongly argue for a change in the way ore deposition is modeled. First, the trisulfur ion serves to enhance gold extraction from deep seated magma or rocks. The trisulfur complex is stable and highly soluble, meaning that more gold can be found in a small amount of aqueous fluid compared to hydrogen sulfide and chloride complexes. Indeed, their data show gold concentration levels that coincide with some of the highest gold concentrations found in natural fluid inclusions trapped in minerals in gold ore deposit systems. These concentrations are much higher than what would be predicted with traditional modeling.

Additionally, the S3- ion is sensitive to temperature, pressure, redox potential, pH as well as sulfur concentration. Their data demonstrate that only small changes in any one of these variables can result in a significant amount of gold from Au(HS)S3- precipitating in a very short amount of time. This may account for certain isolated locations containing an unusually large gold grades.

Finally, S3- is highly selective for gold and, possibly, some other sulfur-loving metals (e.g., platinoids, molybdenum, rhenium). This kind of selectivity may account for the high concentration of in aqueous solutions as well as the metal signatures found in particular deposit sites. This may also account for oddities, such as the elevated molybdenum concentrations in porphyry systems below 500oC.

"Our findings show that one of the oldest metals, known from Antiquity, has yet to divulge all its secrets," says lead author Gleb Pakrovski. "We are just at the beginning of our understanding of geological fluids from the deep."

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More information: Gleb S. Pokrovski et al. Sulfur radical species form gold deposits on Earth, Proceedings of the National Academy of Sciences (2015). DOI: 10.1073/pnas.1506378112

Current models of the formation and distribution of gold deposits on Earth are based on the long-standing paradigm that hydrogen sulfide and chloride are the ligands responsible for gold mobilization and precipitation by fluids across the lithosphere. Here we challenge this view by demonstrating, using in situ X-ray absorption spectroscopy and solubility measurements, coupled with molecular dynamics and thermodynamic simulations, that sulfur radical species, such as the trisulfur ion S−3, form very stable and soluble complexes with Au+ in aqueous solution at elevated temperatures (>250 °C) and pressures (>100 bar). These species enable extraction, transport, and focused precipitation of gold by sulfur-rich fluids 10–100 times more efficiently than sulfide and chloride only. As a result, S−3 exerts an important control on the source, concentration, and distribution of gold in its major economic deposits from magmatic, hydrothermal, and metamorphic settings. The growth and decay of S−3 during the fluid generation and evolution is one of the key factors that determine the fate of gold in the lithosphere.

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