Petrological and metallogenic outlines of the Valmaggia ultaramafic pipe (Ivrea zone), NW Alps, Italy

The Valmaggia pipe intrudes the uppermost part of the Main gabbro (Basic Complex, Ivrea-Verbano Zone, Italy). The ore body associated with the pipe consists of a stockwork and disseminated Fe-Ni-Cu sulphide mineralisation, with accessory Platinum Group Elements (PGE). The pipe consists of a roughly ellipsoidal main body, shortened along a WNW-ESE axis and elongated along a NNE-SSW axis, dipping steeply (70°-90°) towards WNW. Fe-Ni-Cu sulphide mineralisation is hosted in the internal margins of the pipe, from the contact with the host gabbro to a distance of 4-5 metres inside the pipe. The ore mineral paragenesis comprises sulfides (pyrrhotite, pentlandite, chalcopyrite, cubanite), tellurides (altaite, wehrlite, hessite) and PGM (merenskyte). Cross-sections of the pipe mapped from the drives, together with petrographic and metallogenic observations, show that the pipe is concentrically zoned, with a homogeneous core and a heterogeneous rim. On the basis of textural evidences and mineral chemistry we suggest that the pipe is primarily composed of a crystal mush comprising relatively low Fo olivine including buck-shot textured Fe-Ni sulphides and hercynite, which is cemented by products of the residual liquid. Crystallisation proceeded from the rim to the core, where residual melt was no more in equilibrium with cumulus phases, that were replaced extensively by late hydrous phases. PGE-sulphide mineralisation formed in the early stages of crystallisation and is limited to the rim portions of the pipe. It is possible, mainly on the basis of age relationships, to envisage a correlation between the restitisation process that occured in Finero ultramafic body and production of melt that formed Valmaggia and other pipes of Ivrea Verbano Zone. PGE chondri tic profiles of sulphidepoor and sulphide-rich portions are generally similar and reflect the primary PGE content of the magma.