Mechanisms Of Gold Enrichment in Seafloor Massive Sulfide Deposits: A Study on the HIGASHI-AOGASHIMA Hydrothermal Field

The study of Seafloor Massive Sulfides (SMS) deposit formation is vital for assessing both the economic potential and environmental risks posed by future marine mining projects. Metal-rich SMS deposits form in hydrothermal vent systems within tectonically active regions, such as back-arc basins, where faults enhance crustal permeability, enabling fluid migration driven by magmatic heat sources at depth in the crust. Currently our understanding of SMS formation processes is based on bulk-rock geochemical and isotopic data, lacking the necessary spatial and temporal resolution to understand ore forming processes. As a result, critical gaps remain in understanding the exact mechanisms responsible for metal enrichment in these systems, particularly the role of magmatic volatile influx as a source of sulfur and metals such as Au in some hydrothermal systems. Recent research conducted by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) revealed unusually high gold concentrations, reaching up to 275 ppm, in samples from the Higashi-Aogashima Knoll Caldera (HAKC), located on the Izu-Bonin arc, Japan. Three active vent sites have been identified within the caldera, namely the Central Cone, East, and Southeast sites, located at a water depth of ~730 meters below sea level, venting fluid ranging in temperature from 155 to 284°C. This study will investigate the underlying processes driving Au enrichment at HAKC, and SMS deposits more widely. Specifically, this study utilizes in situ sulfur isotope (δ34S) and trace metal geochemistry, to test the hypothesis that magmatic volatile influx is the dominant processes driving Au enrichment at HAKC. The mineralogy of samples varied systematically between the three vent sites and is unusual in a global context due to the occurrence of orpiment (As₂S₃) and realgar (As₂S2), which are exceptionally rare on the seafloor. Chimneys from the East site contain high amounts of barite, colloform and framboidal pyrite and dendritic mineral phases, indicative of high seawater influx. At the Southeast site, chalcopyrite is commonly observed by overprinting galena and sphalerite, indicative of active zone refining processes, which are well developed in samples from the Central Cone site. This transition in mineral assemblage and textures represents a progression in the maturity of chimneys across the three vent sites. Sulfur isotope (δ34S) analysis of sulfide minerals from HAKC, regardless of site and mineral analyzed average 2.7±2.7‰ (1σ, n=178), showing that sulfur is sourced predominately from the leaching of igneous host rocks and thermochemical sulfate reduction (TSR) of seawater. The large range in δ34S values -4.4 to 10.1‰ is attributed to microbial sulfate reduction and variable amounts of TSR, although minor magmatic volatile influx cannot be ruled out. Thus, the absence of δ34S values <0‰ does not support the hypothesis that magmatic volatile influx is responsible for Au enrichment at HAKC. The deportment of Au between vent sites and different mineral phases shows that Au occurs as both native grains associated with galena and chalcopyrite, and in the matrix of sulfide minerals. Chalcopyrite from the Southeast site and pyrite from the Central Cone host the highest concentrations of Au, whereas the East site contains comparatively lower concentrations, highlighting the potential for large amounts of Au to be contained within sulfide mineral grains. The results demonstrate that Au enrichment at HAKC is not primarily driven by magmatic volatile influx, as initially hypothesized. Instead, spatial variations in Au concentrations, sulfur isotopic signatures and mineral textures across the vent sites suggest that localized fluid processes, such as transient boiling and seawater mixing, play a more dominant role. This study exemplifies the complexity of Au enrichment in arc-hosted hydrothermal systems and the need to integrate various in-situ geochemical approaches to constrain the temporal and spatial evolution of metal transport and precipitation.