Gold-silver mineralization in submarine hydrothermal sulphides: INAA results after ten years of intense prospecting

During the last twenty years, IFREMER has organized many diving cruises to collect geological, mineralogical and geochemical data on submarine hydroth ermal mineralization. Gold and chalcophile trace elements have been determi ned by instrumental neutron activation analysis (INAA) in Pierre Sue Labora tory. Particularly for gold and silver determination, but also for In, As, Sb, Se, INAA is the most sensitive instrumental procedure. Typically, the E ast-Pacific Rise (EPR) deposits are gold-goer compared with the Mid-Atlanti c Ridge (MAR) and the Lau Basin deposits. Our INAA data plotted in a Au-Ag diagram support the difference between back-are basin mineralization and sp reading oceanic ridges. The most interesting samples for gold prospecting h ave been discovered in the Logatchev hydrothermal field at 14 degrees 45' N on the Mid-Atlantic Ridge: they shaw an exceptionally high mean content of gold, about 9 ppm, with a maximum of 56 ppm for one sample which contains significant amounts of native gold and electrum. It is the first conclusive evidence of a primary gold-copper association in seafloor sulphides. This may be compared with fossil hydrothermal deposits from the Oman ophiolitic complex.