Hydrothermal polymetallic sulphides collected along the Mid-Atlantic Ridge and hosted in ultramafic rocks (Ashadze, Logatchev and Rainbow ore fields) are enriched in Sn compared to sulphides associated with volcanic rocks. At Logatchev, the averaged Sn ore concentration reaches 2000 ppm. The distribution of Sn among sulphides was studied using scanning electron microscopy and electron-microprobe analysis. The Sn concentration can reach up to 6 wt.% in sphalerite and 2 wt.% in chalcopyrite. Raman micro-spectroscopy investigation suggests that most of Sn is carried by stannite micro-inclusions in sulphides. According to the mineralogical and chemical studies, the following paragenetic sequence is proposed: (1) Sn (<1 wt.%) first precipitates as solid solution in low-temperature sphalerite; (2) at high temperature (300 °C) and low pH (~3) sphalerite is replaced by chalcopyrite; Sn, previously contained in the low-temperature sphalerite, contributes to the formation of stannite, as micro-inclusions in the replacement front; (3) eventually Sn is distributed within newly formed chalcopyrite (<1 wt.%). Tin enrichment of Zn-Cu ores appears as an indicator of hydrothermal reworking of ultramafic sulphide deposits.

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