Subduction zones are the most important sites of material exchange between the crust and mantle on Earth. They are also the locations where the largest deposits of strategic metals, such as copper, molybdenum, and gold, are found. These metals form in the upper crust at depths of a few kilometers (several miles). But how do they form and why are they concentrated there?
Figure:
(A) Diagram of a subduction zone illustrating the transfer of gold and sulfur from the subducting slab through the mantle to the upper crust hosting major metal deposits.
(B) Grain-scale schematic of fluid interaction with mantle minerals (olivine, pyroxene), leading to gold enrichment in the fluid.
© D-Y. He
It is believed that water, metals, and volatile compounds (SO
2, SO
3, CO
2) are released through dehydration of the subducting oceanic plate at great depths (between 80 and 120 km / 50-75 miles), then infiltrate the overlying mantle (see Figure). However, the mechanisms of metal transfer remain enigmatic. For a metal- and sulfur-bearing fluid, traversing the mantle is no simple task because, upon first contact, the reaction of Fe(II)-rich mantle minerals with the fluid should reduce all sulfur to sulfide S(-II) and precipitate iron sulfide (FeS)—the phase known to concentrate gold.
An international consortium of researchers from 5 countries (China, Australia, the United States, Switzerland, and France), involving a CNRS Earth & Universe laboratory (see box), developed a thermodynamic model that quantitatively explains this transfer phenomenon for the first time. The model accounts for the trisulfur ion, [S
3•]
-, a recently discovered but previously overlooked form of sulfur in fluids. The new model demonstrates that [S
3•]
- is the agent responsible for gold enrichment in the fluid during its reaction with the mantle, forming a highly soluble complex, [Au(HS)S
3]
-.
This complex concentrates up to 1000 times more gold in the fluid compared to its average abundance in the mantle, thereby counteracting the action of iron sulfide phases that attempt to sequester gold. This gold enrichment in the fluid is the necessary condition for generating a gold source in the mantle of subduction zones, followed by the subsequent transfer of the metal via fluids and magmas to the upper crust to form economic deposits (see Figure).
The model developed in this study provides a better understanding and predictive capability for economically viable concentrations of strategic metals on our planet.