Mineralogical and geochemical features of sulfide chimneys from the MESO zone, Central Indian Ridge

Hydrothermal activity is fairly well documented from most mid-ocean ridges. However, despite various efforts there is only one hydrothermal field recognized so far in the Indian Ocean. Products of former hydrothermal activity were sampled in the fourth segment of the Central Indian Ridge (CIR) north of the Rodriguez Triple Junction (RTJ), near 23°S. The mineralized area, referred to as `MESO zoneʹ, covers an area of about 0.6 km2, and is located at a water depth of 2850 m, close to the height of a neovolcanic intrarift ridge. Hydrothermal mineralization is structurally controlled by fissures and cracks running both parallel and orthogonal to the general strike direction of the CIR axis (152°N). Sulfides of the MESO zone formed over a period of at least 30,000 years; hydrothermal activity ceased 10,000 years ago. Detailed mineralogical and geochemical studies of sulfides establish the growth and decay of chimney structures, which are mainly composed of pyrite, marcasite and chalcopyrite. Chimneys show high concentrations of base metals, particularly Cu and Fe (more than 40 wt.%) as well as high trace element contents like Co (up to 0.24 wt.%) and As (<1 wt.%). In contrast, sphalerite is rarely observed, which is attributed to replacement and refining processes during the weathering and physical disintegration of the hydrothermal products. The Zn-content is generally low (<1 wt.%), only selected samples show higher Zn-concentrations (up to 7.9 wt.%). Remnants of chimney structures are characterized by the absence of sulfates, but they contain minor amounts of amorphous silica.