Ni-sulfides from the Bottino mine (Tuscany, Italy)

The Pb-Ag(Zn) mineralization at Bottino (Tuscany, Italy) include several late-stage Ni-sulphides, such as gersdorffite, ullmannite, pentlandite, and argentopentlandite (first report for Italy). The occurrence of Ni-enriched cores within earlier arsenopyrite crystals is also distinctive. Gersdorffite and ullmannite may be commonly observed both as isolated subhedral crystals and as rims around early-formed sulphides. Flames of pentlandite within pyrrhotite are suggestive of exsolution from Fe-Ni-S "mss" (monosulphide solid solution). Argentopentlandite is disposed along the margins of chalcopyrite aggregates. Its composition Fβ4 78Ni3 oγAgSg compares well with that commonly reported for argentopentlandites. It is not clear whether argentopentlandite at Bottino is due to exsolution from a Ag-bearing Cu-Fe-S "iss" or, alternatively, to replacement processes. The core-to-rim decrease in Ni contents within arsenopyrites, and the late-stage deposition of separate Niphases are interpreted that, as temperature decreased, Ni could be no longer accommodated into the lattices of phases such as arsenopyrite (and/or mss and/or iss). Thermochemical features of the Bottino Ni-sulphides are consistent with deposition at temperatures of about 350-400°C and relatively low sulfur fugacities.