LAUSR

Abstract The Okinawa Trough (OT), is a back-arc basin where rifting is in its initial stages, and where hydrothermal fields (HTFs) have developed above terrigenous sediments and volcaniclastics layers of variable thickness. The source of ore-forming materials within the hydrothermal systems of the OT is complex, and may include seawater, magmatic rock, magmatic fluid, and sediments. The contribution of magmatic fluids and sediments to the hydrothermal systems of the OT is still uncertain. At least 10 modern seafloor HTFs have been reported from the OT, including the Jade, Hakurei, Iheya North Knoll, CLAM, Tangyin, Yonaguni Knoll IV, Hatoma, and Irabu. Here, we report results of mineralogical, geochemical, and whole-rock Sr–Pb and in situ S isotopic analyses of hydrothermal precipitates from the Tangyin HTF in the southern OT. This field was discovered during the HOBAB3 cruise in 2014 and contains four types of hydrothermal precipitates: Fe–Zn- and Zn–Pb-rich massive sulfides, silica-rich precipitates, and native sulfur-rich samples. Sulfates in Zn–Pb-, Fe–Zn-, and silica-rich samples have 87Sr/86Sr ratios of 0.71068–0.71750, which are higher than those of modern seawater and magmatic rocks, and indicate that the Sr is derived mainly from seawater and sediments. The Zn–Pb-, Fe–Zn-, and silica-rich samples have 206Pb/204Pb (18.5129–18.5307), 207Pb/204Pb (15.6374–15.6489), and 208Pb/204Pb (38.8759–38.9113) ratios that are intermediate between sediments and magmatic rocks, suggesting the Pb is a mixture of sediment- and magmatic-rock-derived Pb. The in situ S isotopic data indicate that the ore-forming fluids associated with all sample types had low δ34S values, with the Fe–Zn- (mean δ34S = 1.27‰ ± 1.38‰; SD, n = 12) and Zn–Pb-rich (mean δ34S = 0.93‰ ± 0.32‰; n = 36) massive sulfides, and silica-rich precipitates (mean δ34S = 2.85‰ ± 1.14‰; n = 58) having lower δ34S values than those of native sulfur-rich samples (mean δ34S = 4.23‰ ± 0.72‰; n = 5). The Zn–Pb- and Fe–Zn-rich massive sulfides, and silica-rich precipitates represent the medium–high temperature (>120 °C) early–middle stages of mineralization, formed under reducing conditions at low sulfur fugacity (fS2), based on the occurrence of low-fS2 mineral assemblages (e.g. high-Fe sphalerite ± isocubanite ± pyrrhotite). The low δ34S values of these three types of samples, and of the Hakurei and CLAM HTFs with low-fS2 mineral assemblages and low concentrations of Cu, Bi, and Au, most likely reflect the incorporation of biological sulfur previously formed by MSR within the sediments. The native sulfur-rich samples of the Tangyin HTF formed at low temperatures (