FORMATION OF LARGE SULFIDE MINERAL-DEPOSITS ALONG FAST SPREADING RIDGES - EXAMPLE FROM OFF-AXIAL DEPOSITS AT 12-DEGREES-43'N ON THE EAST PACIFIC RISE

Submersible investigations with Nautile near the 13 degrees N hydrothe rmal field on the EPR were made in 1992. We selected diving areas base d 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 forme d at a fast spreading ridge (12 cm/yr). Despite an overlap in composit ion between axial and off-axial deposits the general trend reflects re al differences in compositions. Off-axis deposit differs from axial de posits by the scarcity of zinc sulfides and cobalt and selenium enrich ment. Sulfur isotope variation is minimal for the off-axis seamounts a nd suggests that sulfides precipitated from unmodifed end-member hydro thermal solutions. Axial sulfides have a broader range due to reductio n of seawater sulfate at mixing fronts between hydrothermal and seawat er solutions along the axial graben fault system. Lead isotopic compos itions of sulfides are in two overlapping but distinct fields indicati ng different Pb sources and separate convective systems for the seamou nt and the axial deposits. Our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber unde r the seamount. This configuration is by far more efficient than unsta ble axial processes to create, during several hydrothermal episodes, m ajor 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 oc currences 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 de posits along fast- to medium-spreading ridges.