Mantle Oxidation by Sulfur Drives the Formation of Giant Gold Deposits in Subduction Zones

Academic Background

Most of Earth’s metal resources are concentrated in magmatic arc environments, and subduction zones are the primary regions for mass exchange between the mantle and crust. Fluids released from subducting slabs are believed to oxidize the overlying mantle wedge, thereby promoting the enrichment of metals such as gold. However, the mechanisms by which these fluids alter the mantle’s oxidation state and influence gold enrichment remain poorly understood. This study uses numerical modeling to reveal that slab-derived fluids introduce large amounts of sulfate (S6+) into the mantle wedge, significantly increasing its oxygen fugacity and facilitating the migration and enrichment of gold.

Source of the Paper

This paper, titled “Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones,” was authored by Deng-Yang He, Kun-Feng Qiu, Adam C. Simon, Gleb S. Pokrovski, Hao-Cheng Yu, James A. D. Connolly, Shan-Shan Li, Simon Turner, Qing-Fei Wang, Meng-Fan Yang, and Jun Deng. It was published on December 19, 2024, in PNAS (Proceedings of the National Academy of Sciences).

Research Process

1. Devolatilization of the Subducting Slab

The study first simulated the devolatilization process of the subducting slab under hot (1000°C, 2.4 GPa) and cold (1000°C, 3.3 GPa) subduction geothermal gradients. The results showed that the slab released approximately 60% and 90% of its initial sulfur under hot and cold subduction conditions, respectively. Under hot subduction, the sulfur species in the fluid transitioned from reduced forms (HS– and H2S) to oxidized forms (HSO4–, SO42–, KSO4–, HSO3–, and SO2), while under cold subduction, sulfate remained the dominant sulfur species.

2. Mantle Wedge Oxidation Model

The study further modeled the interaction between slab-derived fluids and mantle rocks. The results indicated that fluids containing 1-2 wt.% S6+ could increase the mantle’s oxygen fugacity by at least 2 log units. This oxidation process led to the formation of the trisulfur radical ion (S3–), which forms soluble complexes with gold, such as Au(HS)S3–, significantly enhancing gold’s mobility.

3. Gold Solubility and Speciation in Fluids

The study also calculated the solubility and speciation of gold under typical subduction zone conditions. The results showed that under oxidizing conditions, Au(HS)S3– is the primary carrier of gold, with solubility three orders of magnitude higher than that of Au(HS)2– under reducing conditions. This high solubility of gold complexes allows fluids to transport large amounts of gold, providing the material basis for gold deposit formation.

4. Efficiency of Gold Extraction During Mantle Partial Melting

The study simulated the efficiency of gold extraction during mantle partial melting under both oxidizing and reducing conditions. The results demonstrated that under oxidizing conditions, even with only 1% partial melting, gold in the mantle could be effectively extracted into fluids and melts. This efficient extraction process provides the necessary conditions for gold deposit formation.

Main Results

1. Devolatilization of the Subducting Slab: The slab released approximately 60% and 90% of its initial sulfur under hot and cold subduction conditions, respectively, with sulfur species transitioning from reduced to oxidized forms in the fluid.
2. Mantle Wedge Oxidation Model: Fluids containing 1-2 wt.% S6+ increased the mantle’s oxygen fugacity by at least 2 log units, leading to the formation of the trisulfur radical ion (S3–), which significantly enhanced gold mobility.
3. Gold Solubility and Speciation in Fluids: Under oxidizing conditions, Au(HS)S3– is the primary carrier of gold, with solubility three orders of magnitude higher than that of Au(HS)2– under reducing conditions.
4. Efficiency of Gold Extraction During Mantle Partial Melting: Under oxidizing conditions, even with only 1% partial melting, gold in the mantle could be effectively extracted into fluids and melts.

Conclusion

Through numerical modeling, this study reveals that sulfur-driven mantle oxidation in subduction zones is the primary cause of gold migration and enrichment. Slab-derived fluids introduce large amounts of sulfate into the mantle wedge, significantly increasing its oxygen fugacity and promoting gold migration and enrichment. This mechanism provides a new explanation for the formation of gold deposits in subduction zone environments, offering significant scientific and practical value.

Research Highlights

1. Key Discovery: Sulfur-driven mantle oxidation in subduction zones is the primary cause of gold migration and enrichment.
2. Methodological Innovation: The study employed numerical modeling to quantitatively predict the chemical properties of slab-derived fluids and their impact on mantle oxidation.
3. Scientific Value: The research elucidates the mechanisms of gold deposit formation in subduction zones, providing a theoretical basis for the exploration and development of metal resources.

Additional Valuable Information

The findings of this study are not only applicable to the formation of gold deposits but can also be extended to the study of other metal deposits. Additionally, the research highlights the critical role of sulfur in subduction zone mass cycling, offering new perspectives for understanding deep Earth material cycling.