Role of volatiles and metasomatized subcontinental lithospheric mantle in the genesis of magmatic Ni–Cu–PGE mineralization: insights from in situ H,Li, B analyses of hydromagmatic phases from the Valmaggia ultramafic pipe,Ivrea‐Verbano Zone (NW Italy)

In situ trace element and isotopic data of hydromagmatic phases from the Valmaggia peridotite pipe provide insights into the origin of the metasomatic fluids that affected the Ivrea‐Verbano Zone (NW Italy) during the late Carboniferous and shed new light on genetic models for the formation of Ni–Cu–PGE deposits. Volatiles implicated in the formation of hydromagmatic phases are not hydrothermal and did not derive from crustal assimilation. Low boron concentrations exclude the implication of fluids derived from dehydration of a subducted slab and indicate an origin from mantle‐derived juvenile water. Metasomatism introduced elevated contents of alkalis, Cu, PGEs and S into the depleted mantle of the Ivrea‐Verbano Zone. Increased water activity caused the harzburgite to undergo partial melting, thus producing pockets of volatile‐rich sulphide‐bearing ultramafic magma that evolved to form independent intrusions that host Ni–Cu–PGE mineralization.