Mineralogy of massive sulfides from the Ashadze hydrothermal field, 13 degrees N, Mid-Atlantic Ridge

The ultramafic-rock-hosted and active Ashadze submarine hydrothermal field, located at a depth of 4100-4200 m at 13 degrees N along the Mid-Atlantic Ridge, was discovered and sampled by dredge and grab sampler at four stations in 2003, on the 22 (super nd) cruise of RV "Professor Logatchev". Recovered material included fragments of sulfide chimneys, massive and veinlet-disseminated ore assemblages, sulfide breccia and mineralized peridotite. Preliminary investigations of the field suggested a zonal structure, which was confirmed by further laboratory investigations. A mineralogical study was carried out, using EMPA and SEM/EMD (280 analyses). Data on the morphology and composition of the principal sulfides indicate non-equilibrium conditions of submarine formation of the minerals. The following main characteristics of the massive sulfides in the Ashadze field were established: 1) varied morphologies of the main sulfide minerals (pyrrhotite, sphalerite, chalcopyrite, isocubanite, pyrite and chalcocite). 2) Variations occur in the composition of the main sulfide minerals. Concentrations of minor elements are much higher than the amounts estimated experimentally in corresponding systems under equilibrium conditions, but may also depend on mineral associations. 3) Temporal and spatial transformations of minerals were observed and are reflected by pseudomorphs (most commonly after pyrrhotite grains) and exsolution phenomena in various solid-solutions. For the first time, an exsolution texture was found in phase Y (Cu (sub 2) FeS (sub 5) ) along with that in isocubanite and bornite. The latter transformations occur as a result of microfacies zoning of chimney walls.