Fluid inclusion and sulfur isotope studies in probable modern analogue Kuroko-type ores from the JADE hydrothermal field (Central Okinawa Trough, Japan)

Fluid inclusions and sulfur isotopic compositions were studied in sulfides and barites from the JADE active hydrothermal field in the Central Okinawa Trough. The mineral assemblages in the JADE field strongly resemble ancient Kuroko-type deposits. The formation of massive sulfide mineralization is related to alteration of felsic volcanic rocks by deep penetrating heated seawater. Fluid inclusions in sphalerite from stockwork mineralization show homogenization temperatures that range between 270°C and 360°C and strong variations in salinity (2–15 equiv. wt.% NaCl). Varying salinities are attributed to two-phase separation that partially occurred at deeper levels. The resulting brine favored the transportation of other metals besides Pb, Zn, Fe, Cu, and led to a complex tetrahedrite/tennantite and enargite-bearing sulfide mineralization in the stockwork region. Fluid inclusions in enargite, therefore, show salinities more than three times higher than seawater. Even though the δ34S values of sulfide separates from stockwork mineralization suggest a common sulfur source, ore deposition occurred under non-equilibrium isotopic conditions. Barite in stockwork mineralization, smokers, and mounds on the seafloor precipitated via mixing of hydrothermal solution with seawater. Microthermometric data of barite-hosted fluid inclusions point to maximum formation temperatures of between 150°C and 200°C. Partial reduction of seawater sulfate is indicated by locally observed δ34S values in barite heavier than the ambient seawater.