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 reco gnized 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 degrees S, The mineralized area, referred to as 'MESO zone', covers an area of about 0.6 km(2), and i s 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 str ike direction of the CIR axis (152 degrees N). Sulfides of the MESO zone fo rmed over a period of at least 30,000 years; hydrothermal activity ceased 1 0,000 years ago. Detailed mineralogical and geochemical studies of sulfides establish the growth and decay of chimney structures, which are mainly com posed of pyrite, marcasite and chalcopyrite. Chimneys show high concentrati ons of base metals, particularly Cu and Fe (more than 40 wt.%) as well as h igh 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 o f the hydrothermal products. The Zn-content is generally low (< 1 wt.%), on ly selected samples show higher Zn-concentrations (up to 7.9 wt.%). Remnant s of chimney structures are characterized by the absence of sulfates, but t hey contain minor amounts of amorphous silica. (C) 1999 Elsevier Science B. V. All rights reserved.