2024-12-24 ミシガン大学
＜関連情報＞
- https://news.umich.edu/study-identifies-how-gold-reaches-earths-surface/
- https://www.pnas.org/doi/10.1073/pnas.2404731121
硫黄によるマントルの酸化が、沈み込み帯における巨大な金鉱床の形成を促す Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones
Deng-Yang He, Kun-Feng Qiu , Adam C. Simon, +7, and Jun Deng
Proceedings of the National Academy of Sciences Published:December 19, 2024
DOI:https://doi.org/10.1073/pnas.2404731121
Significance
Upon subduction, the oceanic crust releases aqueous fluids that infiltrate the overlying mantle. Compared to primitive mantle, this metasomatized mantle is enriched in gold and other economic metals to provide a source for mineral deposits. However, the fundamental causes of metal enrichment remain enigmatic. We demonstrate that sulfur is the key agent causing Au enrichment in the fluid upon its reaction with the mantle, by forming the soluble Au(HS)S3– complex. This species concentrates in fluid up to 1,000 times more Au than its average mantle abundance. This gold enrichment in fluid is a key condition for forming Au-rich melts by fluid-present mantle melting. Our work provides a quantitative assessment of the behaviors of sulfur and gold during subduction-related processes.
Abstract
Oxidation of the sub-arc mantle driven by slab-derived fluids has been hypothesized to contribute to the formation of gold deposits in magmatic arc environments that host the majority of metal resources on Earth. However, the mechanism by which the infiltration of slab-derived fluids into the mantle wedge changes its oxidation state and affects Au enrichment remains poorly understood. Here, we present the results of a numerical model that demonstrates that slab-derived fluids introduce large amounts of sulfate (S6+) into the overlying mantle wedge that increase its oxygen fugacity by up to 3 to 4 log units relative to the pristine mantle. Our model predicts that as much as 1 wt.% of the total dissolved sulfur in slab-derived fluids reacting with mantle rocks is present as the trisulfur radical ion, S3–. This sulfur ligand stabilizes the aqueous Au(HS)S3– complex, which can transport Au concentrations of several grams per cubic meter of fluid. Such concentrations are more than three orders of magnitude higher than the average abundance of Au in the mantle. Our data thus demonstrate that an aqueous fluid phase can extract 10 to 100 times more Au than in a fluid-absent rock-melt system during mantle partial melting at redox conditions close to the sulfide-sulfate boundary. We conclude that oxidation by slab-derived fluids is the primary cause of Au mobility and enrichment in the mantle wedge and that aqueous fluid-assisted mantle melting is a prerequisite for formation of Au-rich magmatic hydrothermal and orogenic gold systems in subduction zone settings.
