Combined effect of magmatic fluid–seawater mixing and host rock alteration on the formation of gold-enriched massive sulfides in the Forecast vent field, southern Mariana Trough

Massive sulfides and sulfide-bearing breccia were collected from the Forecast vent field in the southern Mariana Trough. These samples exhibit varying degrees of hydrothermal alteration and/or mineralization, with the former type having significant enrichments in Au (up to 101 ppm). In this study, detailed analyses of mineralogy, geochemistry, S isotope, and fluid inclusion were conducted to understand the unusual Au-rich mineralization. Mineralogical investigations and geochemical analyses (LA–ICP–MS and EPMA) of dominant sulfide minerals indicate that electrums, the main phase of Au mineralization, are characterized by two different generations based on mineralogical relationships and fineness. Early-stage high fineness electrums (930–981 ‰) are associated with sphalerite under relatively high-temperature fluid conditions, whereas galena is main host mineral related to late-stage low fineness electrums (773–868 ‰). Sphalerite geothermometry (231–264 °C), the occurrence of colloform textured sulfides, and microthermometric characteristics of fluid inclusions (T_h = 220–304 °C; Salinity = 1.4–6.6 wt% NaCl equivalent; a general trend of decreasing salinity with decreasing T_h) indicate that significant decreases in H₂S activity, attributed to seawater dilution and rapid precipitation of sulfide minerals, facilitated the efficient destabilization and deposition of Au transported as sulfide complexes from hydrothermal fluids. In particular, the lower ³⁴S values (+0.2 ‰ to + 1.3 ‰) of sulfides compared to those of arc/back-arc lavas (δ³⁴S = +5‰ to + 11 ‰), along with the prevalence of CO₂ in the vapor phase of fluid inclusions, reflect that the significant contribution of magmatic volatile influx supplied most of the sulfur and metals (including Au) to the Forecast hydrothermal fluids. Additionally, the abundance of montmorillonite and varying proportions of sulfide and gangue minerals observed in hydrothermal samples suggest that the chemical buffering of fluids by associated substrates and/or sediments could be another significant factor in promoting Au-rich mineralization. Therefore we conclude that the combination of magmatic fluid–seawater mixing and host rock alteration plays an important role in the formation of Au-enriched massive sulfides in the Forecast vent field.