28th December 2024, John C. Menzies: This post is a review of a paper on the impact of mantle oxidation and transport of gold rich fluids from the mantle to the near surface. Overall, this study demonstrates that mantle oxidation by S–bearing aqueous fluids is a key trigger mechanism for gold enrichment, with the trisulfur radical ion (S3-) playing a crucial role in gold mobilization, and fluid-assisted partial melting being vital for forming gold-rich magmas.
Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones, 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, PNAS December 2024. https://doi.org/10.1073/pnas.240473112

Summary: Mantle Oxidation by Sulfur Drives Formation of Giant Gold Deposits
This study investigates the role of sulfur in the formation of giant gold deposits in subduction zones, using numerical modeling to demonstrate that slab-derived fluids introduce sulfate (S6+) into the mantle wedge, causing oxidation and leading to gold enrichment. The model predicts that the infiltration of sulfate-rich fluids significantly increases the oxygen fugacity (⨍O) of the mantle wedge by 3 to 4 log units relative to the pristine mantle. This oxidation is essential for gold mobility, as it promotes the formation of the trisulfur radical ion (S3-), a key ligand for gold transport.
The research indicates that up to 1 wt.% of dissolved sulfur in slab-derived fluids is present as the trisulfur radical ion (S3-). This ion stabilizes the aqueous Au(HS)S3- complex, which can transport gold at concentrations several orders of magnitude higher than the average gold abundance in the mantle. The study shows that an aqueous fluid phase can extract 10 to 100 times more gold than a fluid-absent rock-melt system during mantle partial melting, making fluid-assisted melting a prerequisite for forming gold-rich magmatic hydrothermal and orogenic gold systems.
The oxidation process is driven by sulfate-bearing fluids that oxidize ferrous iron in silicate minerals. This reaction reduces the fluid and enriches it in the trisulfur radical ion (S3-), which forms soluble complexes with gold. The devolatilization of altered oceanic crust (AOC) releases fluids in two main stages, with the second stage at fore-arc to sub-arc depths releasing most of the water and sulfur, accompanied by the oxidation of pyrite to anhydrite. This oxidation introduces the major fraction of oxidized sulfur into the sub-arc mantle. The interaction of mantle rocks with these fluids significantly increases the mantle oxygen fugacity, highlighting the importance of sulfur in this oxidation process.
The dominant gold complex in the fluid shifts from Au(HS)2- to Au(HS)S3– at higher oxygen fugacities, with the highest abundance of Au(HS)S3- occurring near the pyrite-pyrrhotite-magnetite (PPM) buffer. Gold solubility is also enhanced in sulfur-rich fluids. Partial melting of the metasomatized mantle is critical, with low-degree melting of oxidized mantle leading to high gold concentrations in arc magmas. The presence of an aqueous fluid phase diminishes the stability of sulfide liquid, and this fluid can transport significantly more gold than silicate melt.

These findings provide a quantitative assessment of sulfur and gold behavior during subduction-related processes, identifying the trisulfur radical ion (S3-) as a key agent in gold transport within the mantle. This research emphasizes the importance of mantle oxidation and sulfur-rich fluids in the mobilization and concentration of gold, thereby providing both the source and transport conditions for the formation of gold-rich porphyry-epithermal and orogenic gold systems.
The model predicts that fluid-altered clinopyroxene and orthopyroxene in the mantle wedge will have elevated Fe3+/ΣFe ratios consistent with values found in metasomatic mantle peridotite. Overall, this study demonstrates that mantle oxidation by S–bearing aqueous fluids is a key trigger mechanism for gold enrichment, with the trisulfur radical ion (S3-) playing a crucial role in gold mobilization, and fluid-assisted partial melting being vital for forming gold-rich magmas.
Executive Summary: Mantle Oxidation by Sulfur Drives Formation of Giant Gold Deposits
This study investigates the critical role of sulfur in the formation of large gold deposits within subduction zones. Using numerical modeling, the research demonstrates that slab-derived fluids introduce sulfate (S6+) into the mantle wedge, which significantly increases its oxidation state, creating ideal conditions for gold mobilization and enrichment. The key findings and implications are summarized below:
- Mantle Oxidation: The infiltration of sulfate-rich fluids from subducting slabs elevates the oxygen fugacity (⨍O2) of the mantle wedge by 3 to 4 log units relative to the pristine mantle. This oxidation is essential for gold mobility, and it is driven by the introduction of sulfate (S6+).
- Trisulfur Radical Ion (S3-): The model predicts that up to 1 wt.% of dissolved sulfur in slab-derived fluids is present as the trisulfur radical ion, S3-. This ion is crucial because it stabilizes the aqueous Au(HS)S3- complex, which can transport gold at concentrations several orders of magnitude higher than the average gold abundance in the mantle.
- Gold Transport and Enrichment: The study shows that an aqueous fluid phase can extract 10 to 100 times more gold than a fluid-absent rock-melt system during mantle partial melting. This highlights that fluid-assisted mantle melting is a prerequisite for forming gold-rich magmatic hydrothermal and orogenic gold systems.
- Sulfur-Driven Redox: The study details that sulfate-bearing fluids oxidize ferrous iron in the silicate minerals of the mantle wedge. As a result, the fluid becomes reduced and enriched in the trisulfur radical ion (S3-), which forms soluble complexes with gold.
- Fluid Release from Subducting Slab: The devolatilization of altered oceanic crust (AOC) releases fluids in two dominant stages. The second stage, occurring at fore-arc to sub-arc depths, releases most of the water and sulfur, where pyrite is oxidized to anhydrite. This oxidation of pyrite to soluble sulfate is responsible for introducing the major fraction of oxidized sulfur into the sub-arc mantle.
- Mantle Metasomatism: The interaction of mantle rocks with sulfate-bearing fluids causes a significant increase in mantle oxygen fugacity. The model highlights the importance of sulfur in this oxidation process. As fluid infiltrates the mantle, the oxygen fugacity increases and then decreases with further infiltration.
- Gold Speciation and Solubility: The dominant gold complex in the fluid shifts from Au(HS)2- to Au(HS)S3- at higher oxygen fugacities. The highest abundance of Au(HS)S3- occurs near the pyrite-pyrrhotite-magnetite (PPM) buffer. Gold solubility is strongly enhanced in sulfur-rich fluids.
- Partial Melting and Gold Extraction: Partial melting of metasomatized mantle is critical for the formation of gold deposits. Low-degree melting of oxidized mantle can lead to high gold concentrations in arc magmas. The presence of an aqueous fluid phase diminishes the stability of sulfide liquid, and this fluid can transport significantly more gold than silicate melt under mantle-wedge conditions.
- Implications:
- This study provides a quantitative assessment of sulfur and gold behavior during subduction-related processes.
- It identifies the trisulfur radical ion (S3-) as a key agent in gold transport within the mantle.
- The findings emphasize the importance of mantle oxidation in the formation of gold deposits.
- The research suggests that sulfur-rich fluids, generated by subduction processes, are critical for the mobilization and concentration of gold in the mantle wedge, providing a source for gold deposits.
- The study also suggests that mantle oxidation is crucial for the efficient recycling of volatiles and metals in subduction zones.
- Model Predictions: The model predicts that fluid-altered clinopyroxene and orthopyroxene in the mantle wedge will have Fe3+/ΣFe ratios of 0.30 to 0.45 and 0.10 to 0.20, respectively, which is consistent with values found in metasomatic mantle peridotite.
In conclusion, this research demonstrates that mantle oxidation by S-bearing aqueous fluids is a key trigger mechanism for gold enrichment and release within the mantle wedge. The trisulfur radical ion (S3-) plays a crucial role in the mobilization of gold, and fluid-assisted partial melting is vital for the formation of gold-rich magmas and subsequent ore deposits.. This study provides a significant contribution to our understanding of the processes that lead to the formation of giant gold deposits in subduction zone settings.
Critical Comment
While this paper presents a strong, well-supported model for the role of sulfur in mantle oxidation and gold enrichment, it does have some limitations. The authors convincingly show that sulfate-rich fluids from subducting slabs oxidize the mantle, leading to the formation of the trisulfur radical ion (S3-) which is crucial for gold transport. This is a novel and significant finding. The paper’s strength lies in its quantitative approach, using thermodynamic simulations to model sulfur and gold behavior during subduction, which is further backed by geochemical data and experimental studies. The model for fluid release from altered oceanic crust is detailed, and the study shows how partial melting of the oxidized mantle leads to high gold concentrations.
However, some aspects could be improved:
- Model Simplifications: The models, while sophisticated, involve simplifications regarding the variability of slab and mantle compositions. Explicit acknowledgement of these limitations would be beneficial.
- Fluid Complexity: While the paper addresses sulfur’s importance in the fluid and its effect on gold solubility, other components like chlorine also affect gold complexation, especially in acidic conditions, which is not discussed in detail. The study assumes a H2O-CO2 binary solution, so exploring how other volatile components may affect the results could be useful.
- Equilibrium Assumption: The model assumes equilibrium conditions, but kinetics could play a role in the dynamic subduction zone environment. The paper could be strengthened by acknowledging these kinetic constraints.
- Limited Temporal and Spatial Scale Discussion: While the study notes that enriched mantle can be stored for very long periods before melting, there is not an in-depth discussion about the time scale of fluid migration and its effects, or on the spatial scale of metasomatism in the mantle wedge.
- Role of Slab Melts: The study focuses on aqueous fluids and only briefly discusses the role of slab melts. A more detailed analysis of the interplay between fluids and melts could enhance the paper.
Despite these points, the paper makes a major contribution to understanding gold deposit formation by emphasizing the importance of mantle metasomatism and the role of the trisulfur radical ion. The findings are well-supported and relevant to understanding the genesis of gold deposits in subduction zones.