ABSTRACT
Hydrothermal activity is well documented from many fields on most mid-ocean ridges, but so far there is only one hydrothermally mineralized area known from the Central Indian Ocean. This mature massive sulfide deposit, referred to as the MESO mineral zone, is located near 23°S on the intermediate-spreading Central Indian Ridge. Hydrothermal precipitates occur at a water depth of about 2,850 m close to the rift axis on the top of an intrarift ridge. This neovolcanic ridge forms a 20-km-long topographic high in the central part of the 11-km-wide W-shaped axial valley.
A systematic survey of the mineralized zone with a deep-towed photosledge indicates that the MESO zone contains three different sites with evidence of former hydrothermal activity. Hydro-thermal mineralization appears as chimney relicts, sulfidic talus or sulfide-debris impregnated jasper, and as sediment discolorations. Chimney accumulations are structurally controlled by fissures and cracks mainly running both parallel and vertical to the general strike direction of the ridge (N153 °).
Chimney samples are composed of pyrite, marcasite, chalcopyrite, and secondary Cu-minerals and show high concentrations of base metals; remarkable are extremely high Cu values (up to 31 wt.%). The formation of late-stage Cu-sulfides such as bornite, digenite, and less frequently covellite accounts for this concentration of Cu. Au is also enriched in late-stage pyrite. The massive sulfide samples do not contain nonsulfide mineral phases except for minor amounts of amorphous silica. In contrast, the jasper breccia, formed by impregnation of Fe-hydroxide mud by silica-rich solutions, contains high amounts of barite; unlike the massive sulfides, the jasper contain a lot of sphalerite.
Chimney structures were formed by multiple hydrothermal pulses in a period of time between around 50 and 10 ka years BP. The youngest hydrothermal event produced the sphalerite-bearing jasper material, which is a low-temperature mineral assemblage. The MESO zone deposits appear to be typical of mid-ocean ridge massive sulfide occurrences but are disintegrating and about to be buried by sediment.
