Lead isotope systematics in ophiolite-associated sulphide deposits from the Western Alps and Northern Apennine (Italy)

We report Multi-Collector-ICP-MS analyses of Pb isotopes for hydrothermal deposits in ophiolitic units of the Western Alps (WA) and Northern Apennine (NA). The deposits include (i) volcanogenic massive sulphides formed on the seafloor of the Mesozoic Piemonte–Liguria ocean, which were subjected to subduction- (blueschist to eclogite facies) and collision-related (greenschist facies) metamorphism during the Alpine orogenesis (WA) or escaped Alpine metamorphism (NA), and (ii) post-collision veins cutting the metamorphic oceanic units. The unmetamorphosed sulphides have a MORB-like Pb isotope signature. Sulphides that re-crystallised under eclogitic conditions incorporated an old continental Pb component, which was released from gangue minerals or neighbouring sediments by dehydration reactions at the blueschist–eclogite transition. Our data suggest a limited mobility of sulphide-hosted metals in the subducted oceanic crust up to eclogite-facies conditions. Sulphides in the blueschist-facies and, possibly, eclogite-facies units incorporated further continental Pb derived from oceanic metasedimentary host-rocks containing a continent-sourced terrigenous component during subsequent greenschist-facies metamorphism. Some of the post-collision veins show isotopic similarity with the massive sulphides contained in the same ophiolitic units, suggesting derivation of metals from similar sources (i.e., ophiolites and/or associated metasediments). In the Saint-Véran syn-metamorphic vein deposit, a complex Pb isotope pattern suggests mixing of fluids derived from local retrogressed blueschist-facies rocks with farther-travelled fluids discharged by or reacted with deeper, eclogitic units.