Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge

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.

Trace-element distribution and ore-forming processes in Au–Ag-rich hydrothermal chimneys and mounds in the TA25 West vent field of the Tonga Arc

“…11). The HAADF-STEM images show that the discrete grains of sphalerite are aggregated, which is suggestive of overgrowth and/or recrystallization (Choi et al 2021;Fig. 11a, b, c).…”

“…Numerous studies have focused on the trace element compositions of sulfide minerals because the geochemistry of individual sulfide crystals from different types of hydrothermal vent fields can help to constrain the ore-forming processes associated with variable physicochemical conditions of hydrothermal fluids (Belissont et al 2014;Keith et al 2016;Maslennikov et al 2017;Meng et al 2020;Choi et al 2021). In addition, the S isotope composition of sulfide minerals is commonly used to define the source components, such as wall-rock leaching of metal from substrates and potential contributions of magmatic volatiles (de Ronde et al 2005(de Ronde et al , 2011Yeats et al 2014;Falkenberg et al 2021).…”

Characterization of Geochemistry in Hydrothermal Sediments From the Newly Discovered Onnuri Vent Field in the Middle Region of the Central Indian Ridge

“…In the Indian Ocean ridges with various speeding rates (<12-60 mm/yr in full spreading rate), the hydrothermal vent systems are expected to vary in accordance with diverse geological and tectonic features (German et al, 1998;Wang et al, 2011;Beaulieu et al, 2015); however, these ridges remain underexplored relative to the Pacific and Atlantic ridges, and thus available data regarding their hydrothermal vents is still very limited. In this respect, recent systematic ridge expeditions at the Central and Southwest Indian regions and their results have been receiving scientific attention (e.g., Son et al, 2014;Park et al, 2017;Suo et al, 2017;Liao et al, 2018aLiao et al, ,b, 2019Zhou et al, 2018;Choi et al, 2021).…”

A Review on Mineralogical and Geochemical Characteristics of Seafloor Massive Sulfide Deposits in Mid-Ocean Ridge and Volcanic Arc Settings: Water-Rock Interaction and Magmatic Contribution

“…따라서, 물-암석 상호작용과 마그마 영향은 주 요 금속 공급원으로서 이들의 기여도 차이는 다양한 지 구조에 배태되는 해저열수광상의 광물•지구화학적 특성 에 반영된다 (Hannington et al, 2005;Fouquet et al, 2010). (Haymon, 1983;Hannington et al, 2005;Fouquet et al, 2010;Toffolo et al, 2020;Choi et al, 2021a). 연구 (Schmidt et al, 2011;Berkenbosch et al, 2012;Pattern et al, 2016Pattern et al, , 2020.…”

“…이와는 대조적으로 배호에 형 성된 해저열수광상들은 인접한 화산호로부터의 거리에 따라 물-암석 작용과 마그마 영향을 복합적으로 다양하 게 받는다 (Herzig et al, 1993;Kim et al, 2004;Yeats et al, 2014) Hannington et al, 2011;Beaulieu et al, 2015). 최선기 정석(barite)의 광범위한 산출은 마그마 영향 때문임 (Melekestseva et al, 2014), (3) 중앙해령 Semenov-2 열 수대에서 확인되는 비정상적인 금의 부화(최대 188 ppm) 는 마그마 유체에 의해 상당량의 금이 공급된 결과임 (Melekestseva et al, 2017) (Fouquet et al, 2010;Toffolo et al, 2020;Choi et al, 2021a).…”