Massive sulfides of the new hydrothermal sulfide cluster Semyenov (13°31' N), Mid-Atlantic Ridge

The paper describes the ores from the new hydrothermal sulfide cluster named Semenov (13°31´N, Mid-Atlantic Ridge) composed of several hydrothermal fields. Ores from the Semyenov-1, -3 and -4 hydrothermal fields show marcasite-pyrite composition with fine-grained, colloform, and clastic textures. Ores from the Semyenov-2 hydrothermal site are characterized by isocubanite-chalcopyrite-sphalerite-wurtzite mineral assemblage with fine-grained aggregates. They are uniquely enriched in Au (up to 188 ppm) and Ag (up to 1787 ppm) and conatin visible gold associated with opal, sphalerite and chalcopyrite. Ore textural analyses of ores from the Semenov hydrothermal sulfide cluster allowed us to reveal several ore facies (cf. Maslennikov and Zaykov, 2006): subseafloor hydrothermal, seafloor hydrothermal and clastic. The veinlet-disseminated ores from the Semyenov-2 and -4 hydrothermal fields belong to the subseafloor hydrothermal facies which is formed below the seafloor simultaneously with seafloor hydrothermal processes. Seafloor hydrothermal facies, formed on the seafloor surface near the hydrothermal vents, was found at the Semenov-1 and -2 hydrothermal fields and includes fine-grained and colloform ores. Clastic facies resulted from destruction of sulfide ores occurs at the Semenov-3 field as colluvial pyrite breccia. Textural and mineralogical features of ore facies from the Semenov hydrothermal cluster are comparable with those from massive sulfide deposits of the Urals.