ABSTRACT

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.

摘要

冲绳海槽是一个广阔的弧后盆地，由欧亚大陆下方的菲律宾海板块俯冲形成。Zn-Pb-(Cu) 矿化的实例通常与火山活动有关，通常在该槽的许多热液田中发现。我们分析了从冲绳海槽中部 Iheya North Knoll 热液场的硫化物-硫酸盐烟囱/土丘收集的样品的流体包裹体和氦同位素组成。对于闪锌矿和角铁矿中的流体包裹体，我们发现均质温度范围为 288°C 至 345°C 和 242°C 至 370°C，而盐度值范围为 5.45 至 10.55 wt.% NaCl 当量。和 6.45 至 8.55 wt.% NaCl 当量。这些夹杂物的显微测温结果得出的温度为 330°C–350°C；在这些温度下，海水可能在矿石形成过程中沸腾了。这些数据，除了测量高热液³ He/ ⁴ He 比值（6.5 至 7.8 Ra）表明流体主要来自弧后盆地玄武岩（BABBs），并与少量海水混合。此外，³在这个热液系统中发现的富集表明这些流体可能来自更深的玄武岩岩浆，而不是强烈脱气的流纹质岩浆。根据这些含角岩的流体包裹体以及IODP 331号远征在该热液区的结果，我们推断该区域海底下存在多个热液流体储层；这些储层中各种溶液的积累和迁移促进了矿化和主岩的广泛蚀变。幔源He通过玄武岩浆脱气进入热液系统。在经历流体混合、相分离和冷却后，Pb-Zn-(Cu) 矿物最终从这种热液流体中沉淀出来。