Sorosite (eta-Cu6Sn5)-bearing native tin and lead assemblage from the Mir zone (Mid-Atlantic Ridge, 26 degrees N)

A number of small, irregular-shaped and spherical shiny metallic particles have been found in the sediments from the Mir zone, Trans-Atlantic Geotrave rse hydrothermal held (Mid-Atlantic Ridge, 26 degreesN). The phase variety of the particles examined is represented by metallic tin: tin-rich lead, an d tin-copper phases. A detailed mineralogical study of these particles was carried out using optical microscopy, nuclear microscopy, scanning electron microscopy, electron microprobe, proton microprobe and X-ray diffraction a nalysis. Tin-lead grains have the typical eutectic microtexture of the meta l components. Tin-copper grains are formed from single crystals of sorosite , eta -Cu6Sn5. The Sn-Pb-Cu complex grains are composed of fine stannoan le ad (Pb0.74Sn0.26) and tin crystals as well as fine (or occasionally larger) sorosite (Cu6.1Sn4.9) idiomorphic crystals, in a tin-lead matrix forming e utectic microtexture. On the basis of data obtained, a natural origin is pr oposed for the examined Sn-Pb-Cu association, and its parent relations with the tectono-magmatic events in the rift zone. This association has probabl y been formed (1) during the hydrothermal seepings through the Trans-Atlant ic Geotraverse sediment cover, or (2) during the evolution of ridge crest m agmatic systems. Crystallisation sequence from an initial melt with falling temperature is: firstly sorosite (Cu6Sn5) (T less than or equal to 380 deg reesC) --> crystallisation of tin crystals (T less than or equal to 227 deg reesC) --> crystallisation of Sn-Pb eutectic mixture, composed of tin + sta nnoan lead (T less than or equal to 183 degreesC) --> limited tin exsolutio ns in stannoan lead (T < < 183 degreesC). Sn-Pb-Cu grains might be present as accessory minerals in the basic rocks of the east rift wall, which have undergone degradation, permitting their deposition into rift valley sedimen ts. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.