Formation of large sulfide mineral deposits along fast spreading ridges. Example from off-axial deposits at 12°43′N on the East Pacific Rise

Submersible investigations with Nautile near the 13°N hydrothermal field on the EPR were made in 1992. We selected diving areas based on deep towed side scan sonar imaging. Close to a pit-crater at the top of a young seamount 2 km east of the axial ridge we discovered one of the largest (70 m high and 200 m in diameter) sulfide mounds formed at a fast spreading ridge (12 cm/yr). Despite an overlap in composition between axial and off-axial deposits the general trend reflects real differences in compositions. Off-axis deposit differs from axial deposits by the scarcity of zinc sulfides and cobalt and selenium enrichment. Sulfur isotope variation is minimal for the off-axis seamounts and suggests that sulfides precipitated from unmodifed end-member hydrothermal solutions. Axial sulfides have a broader range due to reduction of seawater sulfate at mixing fronts between hydrothermal and seawater solutions along the axial graben fault system. Lead isotopic compositions of sulfides are in two overlapping but distinct fields indicating different Pb sources and separate convective systems for the seamount and the axial deposits. Our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber under the seamount. This configuration is by far more efficient than unstable axial processes to create, during several hydrothermal episodes, major sulfide deposits near a fast spreading ridge. Off-axis seamounts are localised areas with high magmatic budget able to drive and focus hydrothermal systems at the same place for a long time. Other known occurrences of sulfides related to seamounts are at less than 11 km from the ridge axis. Thus we suggest that off-axial volcanoes close to the ridge are first order targets to discover active or inactive large deposits along fast- to medium-spreading ridges.