Gold and Tin Mineralization of Sulfide Chimneys from the Ultramafic-Hosted Cheoeum Vent Field, Central Indian Ridge

Abstract

The newly discovered ultramafic-hosted Cheoeum vent field is located in the middle part of the Central Indian Ridge. Here, we conducted detailed mineralogical and geochemical study on subsea chimneys that are highly enriched in Sn (up to 1,720 ppm) and Au (up to 17.8 ppm). The recognized ore-forming condition is characterized by the steep temperature gradient and substantial redox inversion of vent fluid during chimney growth; (1) low-temperature stage (<250℃) consisting largely of anhedral sphalerite and pyrite/marcasite, (2) high-temperature stage (250- 365℃) featured by pyrrhotite, isocubanite, chalcopyrite, Fe-rich sphalerite and electrum, (3) seawater alteration stage distinguished by the mineral assemblages of marcasite pseudomorph, Y-phase, covellite and amorphous silica. Gold mineralization intensively occurred during main mineralization stage, together with high fineness of electrum (801 to 909‰) that is indicative of the selective saturation of Au over Ag in the fluid during high-temperature mineralization. This differs from the gold mineralization in MORB-hosted hydrothermal systems on mid-ocean ridges. TEM and LA-ICP-MS analyses represent that tin is mainly incorporated into the structures of sphalerite, isocubanite and chalcopyrite as solid solution, and not as mineral inclusion. The continuous hydrothermal reworking of previously formed sulfides enables the redistribution of Sn to produce the significantly Sn-enriched replacement boundaries between isocubanite with chalcopyrite exsolution and sphalerite. We suggest that the effective Au-Sn mineralization could be favored by the low redox potential of ultramafic-hosted hydrothermal system such as in the CVF, which may affect gold solubility and transport capacity of tin during hydrothermal process.