How Gold Reaches Earth’s Surface

 Scientists Explains How Gold Reaches Earth’s Surface         

A research team including a University of Michigan scientist has discovered a new mechanism that helps researchers understand how gold deposits are formed. A ground breaking thermodynamic model reveals how gold travels from deep within Earth’s mantle to its surface through volcanic activity, solving a longstanding mystery about the formation of major gold deposits around the Pacific Ring of Fire. Gold is a surprisingly common metal, 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 modelling to reveal the specific conditions which lead to the enrichment of gold-bearing magmas. 

Sub-duction Zone:- A region where one tectonic plate dives beneath another, creating conditions for volcanic activity

Mantle:- The layer of Earth between the crust and core where magma originates

Thermodynamic Model:- A mathematical framework that predicts how materials behave under different temperatures and pressures

An international research team has identified a crucial new gold-sulphur complex that helps explain how gold becomes concentrated in rich deposits around volcanically active regions. Their discovery illuminates the specific conditions deep within Earth which allow gold to be transported from the mantle to the surface through magma. A specific kind of sulphur existing under a very specific set of pressures and temperatures as found at a depth of 50 to 80 km's (or 30 to 50 miles) beneath active volcanoes causes gold to be transferred from the mantle into magmas which eventually move to the Earth's surface.

“This thermodynamic model that we’ve now published is the first to reveal the presence of the gold-trisulphur complex that we previously did not know existed at these conditions,” said Adam Simon, professor of earth and environmental sciences at the University of Michigan and co-author of the study. “This offers the most plausible explanation for the very high concentrations of gold in some mineral systems in sub-duction zone environments.” Scientists have previously known that gold complexes with various sulphur ions, 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-trisulphur complex.

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. Pure gold is inert in Earth's mantle and tends to stay there. But when a fluid containing the trisulphur ion is added, gold strongly prefers to bond with trisulphur to form a gold-trisulphur complex. This complex is highly mobile in the molten sections of the mantle, the part that geologists call magma.

The research focuses on sub-duction zones, regions where oceanic plates dive beneath continental plates around the Pacific Ocean. These geological seams create conditions where magma from Earth’s mantle can rise to the surface, carrying gold with it. The process occurs at depths of 30 to 50 miles beneath active volcanoes, where specific pressures and temperatures allow gold to bond with trisulphur to form a highly mobile complex. 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 sulphur-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 US and Canada, all the way down to Chile, we have lots of active volcanoes. All of those active volcanoes form over or in a sub-duction 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. The team’s model demonstrates that when fluids containing trisulphur ions from subducting plates interact with the mantle under precise conditions, gold strongly prefers to bond with trisulphur. This new complex can concentrate gold up to 1,000 times more than its average mantle abundance, explaining the formation of rich deposits near volcanic regions.

"These results provide a really robust understanding of what causes certain sub-duction 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. To develop their model, researchers combined laboratory experiments under controlled pressure and temperature conditions with theoretical predictions. The resulting thermodynamic framework provides real-world insights which could improve gold exploration efforts.