Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge
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Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge Sun Ki Choi 1,2 & Sang Joon Pak 3 & Jonguk Kim 2 & Jung-Woo Park 1 & Seung-Kyu Son 2 Received: 6 December 2019 / Accepted: 7 August 2020 # The Author(s) 2020
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 Snenriched 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. Keywords Seafloor hydrothermal . Gold . Tin . Ultramafic-hosted mineralisation . Central Indian Ridge
Introduction Editorial handling: T. Monecke Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00126-020-01012-5) contains supplementary material, which is available to authorized users. * Sang Joon Pak [email protected] 1
School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, South Korea
2
Deep-sea and Seabed Mineral Resources Research Center, Korea Institute of Ocean Science & Technology, Busan 49111, South Korea
3
Global Ocean Research Center, Korea Institute of Ocean Science & Technology, Busan 49111, South Korea
Seafloor massive sulphide (SMS) deposits distributed along mid-ocean ridg
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