Gold found in ore deposits near volcanoes in the Pacific Ring of Fire originates deep within Earth's mantle. Magma carries it upward, but the exact mechanisms have long been debated. Using advanced numerical modeling, the team uncovered the conditions that enrich magmas with gold as they ascend from the mantle to the surface.
Key to this discovery is the role of a gold-trisulfur complex, whose existence has been controversial. Adam Simon, U-M professor of earth and environmental sciences and a study co-author, explained the findings: "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. This offers the most plausible explanation for the very high concentrations of gold in some mineral systems in subduction zone environments."
The complex forms under specific pressures and temperatures 30 to 50 miles beneath active volcanoes. These conditions enable gold to bind with trisulfur, becoming highly mobile in magma and facilitating its journey to the Earth's surface. The team's research appears in the Proceedings of the National Academy of Sciences.
Gold deposits near volcanoes are common in subduction zones, areas where a continental plate, such as the Pacific plate beneath the Pacific Ocean, dives under neighboring plates. These zones provide pathways for magma to ascend to the surface.
Simon noted, "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. 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."
Under normal conditions, gold remains stable in Earth's mantle. However, when fluids containing the trisulfur ion enter from the subducting plate into the mantle, gold preferentially bonds with trisulfur, forming a highly mobile gold-trisulfur complex. Previous studies have identified various gold-sulfur interactions, but this is the first to present a robust thermodynamic model confirming the gold-trisulfur complex's existence and its critical role in gold transport.
The team's thermodynamic model stems from controlled lab experiments simulating pressure and temperature conditions. Results from these experiments were used to develop a model that can predict similar outcomes in natural settings.
"These results provide a really robust understanding of what causes certain subduction zones to produce very gold-rich ore deposits," Simon added. "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."
Research Report:Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones
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