Sorosite (η-Cu 6Sn 5)-bearing native tin and lead assemblage from the Mir zone (Mid-Atlantic Ridge, 26°N)

Abstract

A number of small, irregular-shaped and spherical shiny metallic particles have been found in the sediments from the Mir zone, Trans-Atlantic Geotraverse hydrothermal field (Mid-Atlantic Ridge, 26°N). The phase variety of the particles examined is represented by metallic tin, tin-rich lead, and 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 analysis. Tin–lead grains have the typical eutectic microtexture of the metal components. Tin–copper grains are formed from single crystals of sorosite, η-Cu 6Sn 5. The Sn–Pb–Cu complex grains are composed of fine stannoan lead (Pb 0.74Sn 0.26) and tin crystals as well as fine (or occasionally larger) sorosite (Cu 6.1Sn 4.9) idiomorphic crystals, in a tin–lead matrix forming eutectic microtexture. On the basis of data obtained, a natural origin is proposed for the examined Sn–Pb–Cu association, and its parent relations with the tectono-magmatic events in the rift zone. This association has probably been formed (1) during the hydrothermal seepings through the Trans-Atlantic Geotraverse sediment cover, or (2) during the evolution of ridge crest magmatic systems. Crystallisation sequence from an initial melt with falling temperature is: firstly sorosite (Cu 6Sn 5) ( T ≤ 380°C) → crystallisation of tin crystals ( T ≤ 227°C) → crystallisation of Sn–Pb eutectic mixture, composed of tin + stannoan lead ( T ≤ 183°C) → limited tin exsolutions in stannoan lead ( T << 183°C). 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 sediments.