Abstract
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.
Highlights
Mafic and ultramafic rocks collected with the chimney samples include harzburgite, microgabbro, pegmatitic gabbro, gabbro, and basalt, showing variable degrees of serpentinisation and alteration
It was during this stage that electrum largely formed, suggesting that Au mineralisation originated from a high-temperature, reducing fluid
Gold-rich and Sn-bearing hydrothermal chimneys hosted by ultramafic rocks were collected from the newly discovered Cheoeum vent field (CVF) on the slow-spreading middle part of the CIR (MCIR)
Summary
Compared with fast-spreading ridges, the deeply penetrating faults at slow to intermediate spreading ridges result in more heterogeneous substrates (i.e. lower crustal and upper mantle rocks) and allow the circulation of hydrothermal fluids down to much greater depths (McCaig et al 2007; Escartín et al 2008). These differences influence the composition of hydrothermal fluids expelled on the seafloor due to variable fluid–rock interactions (Allen and Seyfried 2003; Hannington et al 2005).
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