The recently explored Tianzuo hydrothermal field in serpentinized ultramafic rocks of the amagmatic segment of the ultraslow-spreading Southwest Indian Ridge displays high-temperature sulfide mineralization (isocubanite, sphalerite, and minor pyrrhotite) and low-temperature (pyrite and covellite) phases. Pyrite can be subdivided into pyrite-I and -II, with the former generally having a pseudomorphic texture after pyrrhotite and the latter typically growing around isocubanite, sphalerite, and pyrite-I or occurring as individual grains in quartz veinlets. The sulfide minerals have the greatest range of δ34S values (− 23.8 to 14.1‰), found so far among modern sediment-starved ridges, with distinct δ34S values for low- and high-temperature mineral phases. The high δ34S values of isocubanite (9.6 to 12.2‰) and sphalerite (9.1 to 14.1‰) suggest that sulfate, which precipitated from seawater during an early low-temperature phase of hydrothermal circulation, was the main sulfur source for these sulfides. Pyrite-II has the lowest and most variable δ34S values (− 23.8 to − 3.6‰), suggesting microbial sulfate reduction. Pyrite-I has variable and generally positive δ34S values (− 0.1 to 12.0‰), with sulfur being inherited from pyrrhotite from the original thermochemical reduction of sulfate, mixed with volcanogenic sulfur. Intermittent magmatism represented by gabbroic intrusions, and high permeability caused by well-developed fractures associated with detachment faults, contributed to the formation of sulfides in the Tianzuo hydrothermal field. These factors possibly control sulfide mineralization in amagmatic segments of ultraslow-spreading ridges.

中文翻译：

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西南印度洋脊沿线硫化物的硫同位素组成：对超基性岩成矿的影响

最近在西南印度洋脊超慢扩张岩浆段蛇纹岩化超基性岩中勘探的天左热液田显示高温硫化物矿化（异方辉石、闪锌矿和次要磁黄铁矿）和低温（黄铁矿和铜黄铁矿）相。黄铁矿可细分为黄铁矿-I 和-II，前者通常在磁黄铁矿之后具有假晶结构，而后者通常生长在异方石英、闪锌矿和黄铁矿-I 周围，或作为单个颗粒出现在石英细脉中。硫化物矿物的δ34S 值范围最大（- 23.8 至14.1‰），在现代沉积物匮乏的山脊中发现，低温和高温矿物相的δ34S 值不同。异方石英（9.6 至 12.2‰）和闪锌矿（9.1 至 14. 1‰)表明在热液循环早期低温阶段从海水中沉淀出的硫酸盐是这些硫化物的主要硫源。黄铁矿-II 的δ34S 值最低且变化最大（- 23.8 到- 3.6‰），表明微生物硫酸盐还原。黄铁矿-I 具有可变且通常为正的 δ34S 值（- 0.1 至 12.0‰），硫从硫酸盐的原始热化学还原的磁黄铁矿中继承，与火山生成的硫混合。以辉长岩侵入为代表的间歇性岩浆活动，以及与拆离断层相关的发育良好的裂缝导致的高渗透性，促成了天左热液田硫化物的形成。这些因素可能控制着超慢扩张脊岩浆段中的硫化物成矿作用。在热液循环的早期低温阶段从海水中沉淀出来的硫是这些硫化物的主要硫源。黄铁矿-II 的δ34S 值最低且变化最大（- 23.8 到- 3.6‰），表明微生物硫酸盐还原。黄铁矿-I 具有可变且通常为正的 δ34S 值（- 0.1 至 12.0‰），硫从硫酸盐的原始热化学还原的磁黄铁矿中继承，与火山生成的硫混合。以辉长岩侵入为代表的间歇性岩浆活动，以及与拆离断层相关的发育良好的裂缝导致的高渗透性，促成了天左热液田硫化物的形成。这些因素可能控制着超慢扩张脊岩浆段中的硫化物成矿作用。在热液循环的早期低温阶段从海水中沉淀出来的硫是这些硫化物的主要硫源。黄铁矿-II 的δ34S 值最低且变化最大（- 23.8 到- 3.6‰），表明微生物硫酸盐还原。黄铁矿-I 具有可变且通常为正的 δ34S 值（- 0.1 至 12.0‰），硫从硫酸盐的原始热化学还原的磁黄铁矿中继承，与火山生成的硫混合。以辉长岩侵入为代表的间歇性岩浆活动，以及与拆离断层相关的发育良好的裂缝导致的高渗透性，促成了天左热液田硫化物的形成。这些因素可能控制着超慢扩张脊岩浆段中的硫化物成矿作用。