Thermodynamic model identifies how gold reaches Earth’s surface

A research team including a University of Michigan scientist has discovered a new gold-sulfur complex that helps researchers understand how gold deposits are formed.

Gold in ore deposits associated with volcanoes around the Pacific Ring of Fire originates in Earth’s mantle and is transported by magma to its surface. But how that gold is brought to the surface has been a subject of debate. Now, the research team has used numerical modeling to reveal the specific conditions that lead to the enrichment of gold in magmas that rise from the Earth’s mantle to its surface.

Specifically, the model reveals the importance of a gold-trisulfur complex whose existence has been vigorously debated, according to Adam Simon, U-M professor of Earth and environmental sciences and co-author of the study.

The presence of this gold-trisulfur complex under a very specific set of pressures and temperatures in the mantle 30 to 50 miles beneath active volcanoes causes gold to be transferred from the mantle into magmas that eventually move to the Earth’s surface. The team’s results are published in the Proceedings of the National Academy of Sciences.

“This thermodynamic model that we’ve now published is the first to reveal the presence of the gold-trisulfur complex that we previously did not know existed at these conditions,” Simon said. “This offers the most plausible explanation for the very high concentrations of gold in some mineral systems in subduction zone environments.”

Gold deposits associated with volcanoes form in what are called subduction zones. Subduction zones are regions where a continental plate—the Pacific plate, which lies under the Pacific Ocean—is diving under the continental plates that surround it. In these seams where continental plates meet each other, magma from Earth’s mantle has the opportunity to rise to the surface.

“On all of the continents around the Pacific Ocean, from New Zealand to Indonesia, the Philippines, Japan, Russia, Alaska, the western United States and Canada, all the way down to Chile, we have lots of active volcanoes,” Simon said. “All of those active volcanoes form over or in a subduction zone environment. The same types of processes that result in volcanic eruptions are processes that form gold deposits.”

Gold is happy in Earth’s mantle above the subducting ocean plate. But when the conditions are just right that a fluid containing the trisulfur ion is added from the subducting plate to the mantle, gold strongly prefers to bond with trisulfur to form a gold-trisulfur complex. This complex is highly mobile in magma.

Scientists have previously known that gold complexes with various sulfur ions, but this study, which includes scientists from China, Switzerland, Australia and France, is the first to present a robust thermodynamic model for the existence and importance of the gold-trisulfur complex.

To identify this new complex, the researchers developed a thermodynamic model based on lab experiments in which the researchers control pressure and temperature of the experiment, then measure the results of the experiment. Then, the researchers developed a thermodynamic model that predicts the results of the experiment. This thermodynamic model can then be applied to real-world conditions.

“These results provide a really robust understanding of what causes certain subduction zones to produce very gold-rich ore deposits,” Simon said. “Combining the results of this study with existing studies ultimately improves our understanding of how gold deposits form and can have a positive impact on exploration.”