MODERN SEAFLOOR HYDROTHERMAL DEPOSITS HOSTED IN ULTRAMAFIC ROCKS

Recent exploration has demonstrated that hydrothermal systems related to mantle outcrops are common in the modern ocean. Most sites are located along slow spreading ridges with a low magmatic budget. The preferential location is off axis at the end of volcanic segments were the magmatic budget is minimum. At this location occur typically an ultramafic dome like structure which upper surface is often interpreted as a detachment fault. Two processes can be considered for driving the hydrothermal cells: Cooling of a deep gabbroic intrusion and exothermic heat produced during serpentinisation. Along the Mid Atlantic Ridge four types of hydrothermal mineralisation can be considered: 1) High temperature (>350°C) sulfide mineralisation related to low pH fluids, ( ex. The Logatchev field at 14°45'N and the Rainbow field at 36°14 N ) - 2) Medium temperature carbonate chimneys ( <100°C) related to high pH fluids (ex. The Lost City site at 30°N ) (Kelley et al., 2001) - 3) Low temperature diffuse venting associated with extremely high methane discharge and pervasive alteration and silicification of both volcanic and ultramafic rocks (ex : The Saldanha field at 36°34 N and the Menez Hom field at 37°9'N) - 4) Stockwork mineralisation and quartz veins with sulfides related to gabbroic intrusions within the ultramafic rocks (15°N). When compared to mineralisation in basaltic environment sulfide deposits are enriched in copper, zinc and specific elements related to an ultramafic rock source such as Ni and Co. They also are significantly enriched in gold. At the Rainbow site the fluids (365°C, pH : 2.8) have a high chlorinity (750 mmol/kg) and low silica (6.9 mmol/kg) concentration. High hydrogen concentration (40% of the gas extracted from fluids, Charlou et al., in press) is due to the hydration of olivine and pyroxene with conversion of Fe(II) to Fe(III) in magnetite during serpentinization. Dissolved CO2 is transformed into reduced-C species including methane, ethane, propane, and straight chain hydrocarbons in ultramafic rocks through Fischer-Tropsch Type (FTT) reactions.