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Study of fluid inclusions in minerals from sulfide ores sampled in the Ashadze and Logatchev hydrothermal fields during the Serpentine cruise of the R/V Pourquoi Pas? in 2007 gave an insight into the physicochemical conditions of ore-forming systems of “black smokers” associated with mantle ultrabasites in the Central Atlantic region. It was established that the studied hydrothermal systems differ in mineral formation temperatures, fluid salinity, and hydrothermal processes running within basaltic complexes in both low-spreading (Mid-Atlantic Ridge) and high-spreading (East Pacific Rise) mid-ocean ridges. It was shown for the Ashadze and Logatchev hydrothermal fields that the temperatures and salinity of inclusion solutions in the minerals vary over a wider range of values than those of fluids flowing out from the ridge vents onto the ocean floor. Study of fluid inclusions showed that the Ashadze and Logatchev sulfide structures resulted from the action of both high-temperature (up to 355 °C) and low-temperature (no lower than 170 °C) solutions. The salinity of fluid in the inclusions (mainly 8 wt.%) is more than twice as high as the salinity of seawater. The data obtained substantiate the phase separation of fluid in the deepest-water submarine hydrothermal Ashadze system and give an idea of the chemical composition and temperature of fluids forming sulfide deposits in the Ashadze and Logatchev hydrothermal fields in the Central Atlantic region.

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fides results in the development of ore zoning with a decreasing Cu/(Zn + Pb) ratio toward the roof and flanks of sulfide bodies (Borodaevskaya et al., 1979;Smirnov, 1982;Starostin and Ignatov, 1997;Franklin et al., 1981;Solomon and Walshe, 1979;Ohmoto and Skinner, 1983;Lydon, 1988;Large, 1992). Such models were based on the study of ancient massive sulfide deposits. The discovery of modern black smokers validated this hypothesis. However, the precipitation of sulfides from hydrothermal solutions directly entering seawater does not lead to the formation of large orebodies because 99% of metal transported by ore-forming fluid is dispersed through the water (Rona, 1984). The study of the structure of currently formed and ancient massive sulfide orebodies brought to the fore the suggestion that sulfides accumulate due to both precipitation on the seafloor and growth of the sulfide mound from below. This suggestion was stated as a concept of subseafloor ore formation. The screening seafloor surface hinders dispersion of metals and determines the dimensions of ore deposits. A separate deposit or an orebody fragment may be formed as a result of combined ore deposition above and below the seafloor. Several large deposits hosted in recent and ancient volcanic sequences were Abstract -The sequence of orebody formation at the Talgan massive sulfide deposit; morphology of sulfide orebodies; mineralogy, texture, and structure of ore; chemical composition of minerals; and fluid inclusions and relationships between stable isotopes (S, C, O) in sulfides from ores and carbonate rocks are discussed. The deposit is localized in the Uzel'ga ore field of the northern Magnitogorsk Megazone. The sulfide ore is hosted in the upper felsic sequence of the Middle Devonian Karamalytash Formation, composed of basalt, basaltic andesite, and rhyodacite. Orebodies are irregular lenses lying conformably with host rocks. Pyrite, chalcopyrite, sphalerite, and fahlore are the major ore minerals; galena, bornite, and hematite are of subordinate abundance. Sulfide mineralization bears attributes of deposition under subseafloor conditions. The carbonate and rhyolite interlayers at the roofs of orebodies and the supraore limestone sequence served as screens. Zoning typical of massive sulfide deposits was not established. The study of fluid inclusions has shown that the temperature of the hydrothermal solution varied from 375 to 110 ° C. δ 34 S ‰ ranges from -2.4 to +3.2‰ in pyrite, from -1.2 to +2.8‰ in chalcopyrite, and from -3.5 to +3.0‰ in sphalerite (CDT). These parameters correspond to an isotopic composition of magmatic sulfur without a notable percentage of sulfate sulfur. δ 13 C and δ 18 O of carbonates vary from -18.1 to +5.9‰ (PDB) and from +13.7 to +27.8‰ (SMOW), respectively. The carbon and oxygen isotopic compositions of carbonates from ores and host rocks markedly deviate from the field of marine carbonates; a deep source of carbon is suggested. The results obtained show that the main mass of polysulfide ore at the Talgan deposit was for...