Diverse Range of Mineralization Induced by Phase Separation of Hydrothermal Fluid: Case Study of the Yonaguni Knoll IV Hydrothermal Field in the Okinawa Trough Back‐Arc Basin
Diverse Range of Mineralization Induced by Phase Separation of Hydrothermal Fluid: Case Study of the Yonaguni Knoll IV Hydrothermal Field in the Okinawa Trough Back‐Arc Basin
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DOI：
10.1111/j.1751-3928.2008.00061.x
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发表时间：
2008-09
期刊：
影响因子：
1.4
通讯作者：
R. Suzuki;J. Ishibashi;Miwako Nakaseama;U. Konno;U. Tsunogai;K. Gena;H. Chiba
中科院分区：
文献类型：
--
作者：
R. Suzuki;J. Ishibashi;Miwako Nakaseama;U. Konno;U. Tsunogai;K. Gena;H. Chiba
与那国穹丘IV热液喷口区（24°51′N，122°42′E）位于冲绳海槽南部西缘附近水深1370-1385 m处。在YK 03 -05和YK 04 -05考察期间，使用潜水器Shinkai 6500，对目前从烟囱丘结构中排出的热液沉淀物（硫化物/硫酸盐/碳酸盐）和高温流体（Tmax = 328°C）进行了广泛采样。收集到的喷发流体具有广泛的化学性质（Cl浓度为376-635 mmol kg−1），这被认为是海底下相分离的证据。当富含Cl的烟黑色流体从热液中心的两个相邻的烟囱丘结构中排出时，在该区域的外围区域发现了有时含有CO2液滴的清澈透明的流体。这种分布模式可以用海底下相分离后富含蒸汽的热液在多孔沉积层内的迁移来解释。热液沉淀物显示出多种矿化类型，可分为五类：（i）富含硬石膏的烟囱状未成熟沉淀物，包括硬石膏中的硫化物浸染;（ii）块状Zn-Pb-Cu硫化物，由闪锌矿、纤锌矿、方铅矿、黄铜矿、黄铁矿和白铁矿组成;（三）钡砷烟囱，由重晶石和硫化物浸染组成，有时与雄黄和雌黄过度生长有关;（iv）富锰烟囱，由碳酸盐（方解石和菱镁矿）和硫化物（闪锌矿、方铅矿、黄铜矿、硫锑铅矿和少量砷铜矿和硫砷铜矿）组成;以及（v）路面，硅化沉积物，包括丰富的天然硫或重晶石。硫化物/硫酸盐矿化（组i-iii）被发现在烟囱丘结构与蒸汽损失（富氯）流体通风。相比之下，硫化物/碳酸盐矿化（第四组）特别发现于烟囱中，预计富含蒸汽（Cl-贫化）的流体排出，路面（第五组）与海底沉积物的扩散排出有关。这种对应关系强烈表明，海底相分离在与那国IV油田的各种矿化中起着重要作用。观察到的硫化物矿物组合是一致的硫逸度计算从闪锌矿/纤锌矿中的FeS含量和流体温度为每个网站，这表明，由于参与的挥发性物种在相分离过程中的硫逸度的移动是一个重要因素，以诱导不同的矿化。相比之下，碳酸盐成矿作用归因于富含蒸汽的热液流体和海水的显著混合。大陆边缘弧后盆地内的海底热液系统可被认为是在有利于各种矿化的地质环境中发育的，其中相对较浅的水深导致海底下热液流体的相分离，沉积物积累可促进富含蒸汽的热液流体的迁移。
The Yonaguni Knoll IV hydrothermal vent field (24°51′N, 122°42′E) is located at water depths of 1370–1385 m near the western edge of the southern Okinawa Trough. During the YK03–05 and YK04–05 expeditions using the submersible Shinkai 6500, both hydrothermal precipitates (sulfide/sulfate/carbonate) and high temperature fluids (Tmax = 328°C) presently venting from chimney‐mound structures were extensively sampled. The collected venting fluids had a wide range of chemistry (Cl concentration 376–635 mmol kg−1), which is considered as evidence for sub‐seafloor phase separation. While the Cl‐enriched smoky black fluids were venting from two adjacent chimney‐mound structures in the hydrothermal center, the clear transparent fluids sometimes containing CO2 droplet were found in the peripheral area of the field. This distribution pattern could be explained by migration of the vapor‐rich hydrothermal fluid within a porous sediment layer after the sub‐seafloor phase separation. The collected hydrothermal precipitates demonstrated a diverse range of mineralization, which can be classified into five groups: (i) anhydrite‐rich chimneys, immature precipitates including sulfide disseminations in anhydrite; (ii) massive Zn‐Pb‐Cu sulfides, consisting of sphalerite, wurtzite, galena, chalcopyrite, pyrite, and marcasite; (iii) Ba‐As chimneys, composed of barite with sulfide disseminations, sometimes associated with realgar and orpiment overgrowth; (iv) Mn‐rich chimneys, consisting of carbonates (calcite and magnesite) and sulfides (sphalerite, galena, chalcopyrite, alabandite, and minor amount of tennantite and enargite); and (v) pavement, silicified sediment including abundant native sulfur or barite. Sulfide/sulfate mineralization (groups i–iii) was found in the chimney–mound structure associated with vapor‐loss (Cl‐enriched) fluid venting. In contrast, the sulfide/carbonate mineralization (group iv) was specifically found in the chimneys where vapor‐rich (Cl‐depleted) fluid venting is expected, and the pavement (group v) was associated with diffusive venting from the seafloor sediment. This correspondence strongly suggests that the subseafloor phase separation plays an important role in the diverse range of mineralization in the Yonaguni IV field. The observed sulfide mineral assemblage was consistent with the sulfur fugacity calculated from the FeS content in sphalerite/wurtzite and the fluid temperature for each site, which suggests that the shift of the sulfur fugacity due to participation of volatile species during phase separation is an important factor to induce diverse mineralization. In contrast, carbonate mineralization is attributed to the significant mixing of vapor‐rich hydrothermal fluid and seawater. A submarine hydrothermal system within a back‐arc basin in the continental margin may be considered as developed in a geologic setting favorable to a diverse range of mineralization, where relatively shallow water depth induces sub‐seafloor phase separation of hydrothermal fluid, and sediment accumulation could enhance migration of the vapor‐rich hydrothermal fluid.