Crystallized gold in matrix.
A research team including a University of Michigan scientist has discovered a new mechanism that helps researchers understand how gold deposits are formed.
Gold is a surprisingly common metal (it is more common than lead if we consider the bulk composition of the Earth), but most is locked away deep within Earth’s mantle. On the surface, it is concentrated in volcanic or magmatic rocks. But how that gold is brought to the surface has been a subject of debate. Now, a research team has used numerical modeling to reveal the specific conditions that lead to the enrichment of gold-bearing magmas.
A specific kind of sulfur existing under a very specific set of pressures and temperatures as found at a depth of 50 to 80 kilometers (or 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.
Scientists have previously known that gold forms molecular bonds with two to three sulfur atoms, but this study, which includes researchers 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.
Pure gold is inert in Earth’s mantle and tends to stay there. But when a fluid containing sulfur is added, gold strongly prefers to bond with three sulfur ions (an ion is an atom or molecule with a net electrical charge) to form a gold-trisulfur complex. This complex is highly mobile in the molten sections of the mantle—the part that geologists call magma.
The researchers developed the new thermodynamic model based on lab experiments in which the researchers control pressure and temperature of the experiment to create artificial magma. This thermodynamic model can then be applied to real-world conditions.
Of great interest are subduction zones. Subduction zones are regions where a tectonic plate is diving under another plate. In these seams where the plates meet each other, magma from Earth’s mantle has the opportunity to rise to the surface. The subducting plate, melting as it sinks deeper into the mantle, also provides the sulfur-rich fluids needed to form the gold-bearing magmas.
“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,” explains Adam Simon, U-M professor of Earth and environmental sciences and co-author of the study.
“These results provide a really robust understanding of what causes certain subduction zones to produce very gold-rich ore deposits. 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,” concludes Simon.
The study, “Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones,” was published in the Proceedings of the National Academy of Sciences and can be found here.
Additional material and interviews provided by the University of Michigan.