Mineralogy, geochemistry, and Sr-Pb and in situ S isotopic compositions of hydrothermal precipitates from the Tangyin hydrothermal field, southern Okinawa Trough:Evaluation of the contribution of magmatic fluids and sediments to hydrothermal systems

摘要

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 sea-floor 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 mineralogi-cal, 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 inZn-Pb-, Fe-Zn-, and silica-rich samples have ~(87)Sr/~(86)Sr 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 ~(206)Pb/~(204)Pb (18.5129-18.5307), ~(207)Pb/~(204)Pb (15.6374-15.6489), and ~(208)Pb/~(204)Pb (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 δ~(34)S values, with the Fe-Zn- (mean δ~(34)S = 1.21‰ ± 1.38‰; SD, n = 12) and Zn-Pb-rich (mean δ~(34)S = 0.93‰ ± 0.32‰; n = 36) massive sulfides, and silica-rich precipitates (mean δ~(34)S = 2.85‰ + 1.14‰; n = 58) having lower δ~(34)S values than those of native sulfur-rich samples (mean δ~(34)S = 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 (fS_2), based on the occurrence of low-fS2 mineral assemblages (e.g. high-Fe sphalerite + isocubanite + pyrrhotite). The low δ~(34)S values of these three types of samples, and of the Hakurei and CLAM HTFs with low-fS_2 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 (＜ 120 °C), and have slightly higher δ~(34)S values than the Fe-Zn-, Zn-Pb-, and silica-rich samples, may reflect the incorporation of biological sulfur formed mainly by MSR during hydrothermal flow in a seawater recharge zone. Some sulfates in the Jade and Iheya North Knoll HTFs have δ~(34)S values lower than that of modern seawater; this, in combination with high-fS_2 oxidizing hydrothermal fluids, and high concentrations of Cu and Au in sulfide samples, indicates a magmatic fluid contribution to these HTFs.