The Cheoeum vent field (CVF) is the first example of an inactive ultramafic-hosted seafloor massive sulphide (SMS) deposit identified in the middle part of the Central Indian Ridge. Here, we report on the detailed mineralogy and geochemistry of ultramafic-hosted sulphide sample atop a chimney, together with a few small fragments. Hydrothermal chimneys are characterised by high concentrations of Au (up to 17.8 ppm) and Sn (up to 1720 ppm). The sulphide mineralisation in the CVF shows (1) early precipitation of anhedral sphalerite and pyrite–marcasite aggregates under relatively low-temperature (< 250 °C) fluid conditions; (2) intensive deposition of subhedral pyrrhotite, isocubanite, chalcopyrite, Fe-rich sphalerite (Sp-III), and electrum from high-temperature (250–365 °C) and reduced fluids in the main mineralisation stage; and (3) a seawater alteration stage distinguished by the mineral assemblage of marcasite pseudomorphs, altered isocubanite phase, covellite, amorphous silica, and Fe-oxyhydroxides. Electrum (< 2 μm in size) is the principal form of Au mineralisation and is mainly associated with the main mineralisation stage. The consistently high fineness of electrum (801 to 909‰) is indicative of the selective saturation of Au over Ag in the fluid during high-temperature mineralisation, which differs from the Au mineralisation associated with typical basaltic-hosted hydrothermal systems on mid-ocean ridges. Tin is mainly substituted in structures of sphalerite, isocubanite, and chalcopyrite as a solid solution, and not as mineral inclusions. The continuously ascending hydrothermal fluids enable the early formed Sn-bearing sulphide to be dissolved and reprecipitated, producing significantly Sn-enriched replacement boundaries between isocubanite and Sp-III. This study suggests that Au–Sn mineralisation could be facilitated by the low redox potential of ultramafic-hosted hydrothermal systems such as in the CVF, which may be a common occurrence along slow-spreading mid-ocean ridges.

中文翻译：

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印度中脊超镁铁质 Cheoeum 喷口油田中的金和锡矿化

Cheoeum 喷口油田 (CVF) 是在印度洋中脊中部发现的非活性超镁铁质海底块状硫化物 (SMS) 矿床的第一个例子。在这里，我们报告了烟囱顶部的超镁铁质硫化物样品以及一些小碎片的详细矿物学和地球化学。热液烟囱的特点是含有高浓度的金（高达 17.8 ppm）和锡（高达 1720 ppm）。CVF 中的硫化物矿化显示 (1) 在相对低温 (< 250 °C) 流体条件下，早期的自形闪锌矿和黄铁矿-白铁矿聚集体沉淀；(2) 亚自形磁黄铁矿、异方铜矿、黄铜矿、富铁闪锌矿(Sp-III)和金黄铁矿在主要成矿阶段高温(250-365℃)和还原流体的密集沉积；(3) 海水蚀变阶段，以白铁矿假晶型、蚀变异方石英相、铜钴矿、无定形二氧化硅和氢氧化铁的矿物组合为特征。Electrum（尺寸< 2 μm）是Au 矿化的主要形式，主要与主要矿化阶段有关。金银矿的一贯高细度（801 至 909‰）表明高温成矿过程中流体中 Au 选择性饱和，这与大洋中脊典型玄武岩热液系统相关的 Au 矿化不同. 锡主要以固溶体的形式出现在闪锌矿、异方石英和黄铜矿的结构中，而不是作为矿物包裹体。不断上升的热液流体使早期形成的含锡硫化物溶解和再沉淀，在异立方岩和 Sp-III 之间产生显着的富锡置换边界。这项研究表明，诸如 CVF 中的超镁铁质热液系统的低氧化还原电位可以促进 Au-Sn 矿化，这可能是沿着缓慢扩张的洋中脊的常见现象。