A research team, including a scientist from the University of Michigan, discovered a new gold-sulfur complex which helps researchers understand how o deposits form
Gold contained in mineral deposits associated with volcanoes surrounding the Pacific Ring of Fire comes from the Earth's mantle and is transported by magma to the Earth's surface. However, how this gold reaches the surface has been debated. Now the research team used numerical models to reveal the specific conditions that lead to gold enrichment in magmas which rise from the Earth's mantle towards the surface.
Gold miles deep
The model reveals in particular the importance of a gold-trisulfide complex the existence of which has been hotly debated, according to Adam Simon, professor of earth and environmental sciences at UM and co-author of the study.
The presence of this gold-trisulfide complex under a very specific set of pressures and temperatures in the mantle, at a distance between 48 and 80 kilometers below active volcanoes, causes the transfer of gold from the mantle to magmas which then move towards the Earth's surface. The team's results are published in the journal Proceedings of the National Academy of Sciences.
“The thermodynamic model we published is the first to reveal the presence of the gold-trisulfide complex, which until now was not known to exist under these conditions,” said Simon. “It constitutes the most plausible explanation for the very high concentrations of gold present in certain mineral systems of subduction zones.”
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 beneath the Pacific Ocean) is sinking beneath surrounding continental plates. In these veins where the continental plates meet, magma from the Earth's mantle has the possibility of rising to the surface.
“On every continent surrounding the Pacific Ocean, from New Zealand to Indonesia, the Philippines, Japan, Russia, Alaska, the western United States and Canada, and up 'In Chile we have many active volcanoes,' Mr Simon said. “All of these active volcanoes form on or in a subduction zone. The processes that cause volcanic eruptions are the same as those that cause gold deposits. »
Gold is found in abundance in the Earth's mantle, above the subducting oceanic plate. However, when conditions are favorable for the addition of a fluid containing the triazufre ion from the subducting plate to the mantle, gold prefers to bind with the triazufre to form a gold-triazufre complex. This complex is very mobile in the magma.
Scientists already knew that gold forms complexes with various sulfur ions, but this study, which included scientists from China, Switzerland, Australia and France, is the first to present a robust thermodynamic model for the existence and importance of the gold-triazole complex.
Better understand the formation of gold deposits
To identify this new complex, researchers developed a thermodynamic model based on laboratory experiments in which researchers control the pressure and temperature of the experiment and then measure the results of the experiment. The researchers then developed a thermodynamic model that predicted the results of the experiment. This thermodynamic model can then be applied to real conditions.
“These results provide a better understanding of what makes certain subduction zones produce very gold-rich deposits,” said Mr. Simon. “By combining the results of this study with existing studies, We are improving our understanding of how gold deposits form, which can have a positive impact on exploration. »
Article reference:
Deng-Yang He et al, Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2404731121
