Fluid sources and precipitation mechanisms of Pb–Zn–(Cu) sulphide–sulphate in the Iheya North Knoll hydrothermal field, Okinawa Trough: insights from fluid inclusions, He and Ar isotopes.

The Okinawa Trough is an extensive back-arc basin that was formed by the subduction of the Philippine Sea plate beneath the Eurasian continent. Instances of Zn–Pb–(Cu) mineralization, which are typically associated with volcanism, are commonly found in many of the hydrothermal fields throughout this trough. We analysed the fluid inclusions and helium isotopic compositions of samples collected from the sulphide–sulphate chimneys/mounds in the Iheya North Knoll hydrothermal field in the Middle Okinawa Trough. For fluid inclusions in sphalerite and anglesite, we found that the homogenization temperatures ranged from 288°C to 345°C and 242°C to 370°C, while the salinity values ranged from 5.45 to 10.55 wt.% NaCl equiv. and 6.45 to 8.55 wt.% NaCl equiv., respectively. Microthermometry results from these inclusions yielded temperatures of 330°C–350°C; at these temperatures, seawater may have been boiling during the ore formation process. These data, in addition to measurement of high hydrothermal fluid 3He/4He ratios (6.5 to 7.8 Ra), suggest that the fluid is mainly derived from the back-arc basin basalts (BABBs), and have been mixed with a small amount of seawater. Moreover, the 3He enrichment found in this hydrothermal system indicates that these fluids may be sourced from deeper basaltic magma, rather than strongly degassed rhyolitic magma. Based on these anglesite-hosted fluid inclusions and the results of IODP Expedition 331 in this hydrothermal field, we infer that multiple hydrothermal fluid reservoirs exist beneath the seafloor in this region; the accumulation and migration of various solutions in these reservoirs promote mineralization, and extensive alteration of the host rocks. Mantle-derived He enters the hydrothermal fluid system by basaltic magma degassing. After experiencing fluid mixing, phase separation, and cooling, Pb–Zn–(Cu) minerals ultimately precipitate out of this hydrothermal fluid. [ABSTRACT FROM AUTHOR]