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We report Multi-Collector-ICP-MS analyses of Pb isotopes for hydrothermal deposits in ophiolitic units of the Western Alps (hereafter, WA) and Northern Apennine (hereafter, 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.

We have studied textural relationships and compositions of phyllosilicate minerals in the maficultramafic-hosted massive-sulfide deposit of Ivanovka (Main Uralian Fault Zone, southern Urals). The main hydrothermal phyllosilicate minerals are Mg-rich chlorite, variably ferroan talc, (Mg, Si)-rich and (Ca, Na, K)-poor saponite (stevensite), and serpentine. These minerals occur both as alteration products after mafic volcanics and ultramafic protoliths and, except serpentine, as hydrothermal vein and seafloor mound-like precipitates associated with variable amounts of (Ca, Mg, Fe)-carbonates, quartz and Fe and Cu (Co, Ni) sulfides. Brecciated mafic lithologies underwent pervasive chloritization, while interlayered gabbro sills underwent partial alteration to chlorite + illite ± actinolite ± saponite ± talc-bearing assemblages and later localized deeper alteration to chlorite ± saponite. Ultramafic and mixed ultramafic-mafic breccias were altered to talc-rich rocks with variable amounts of chlorite, carbonate and quartz. Chloritization, locally accompanied by formation of disseminated sulfides, required a high contribution of Mg-rich seawater to the hydrothermal fluid, which could be achieved in a highly permeable, breccia-dominated seafloor. More evolved hydrothermal fluids produced addition of silica, carbonates and further sulfides, and led to local development of saponite after chlorite and widespread replacement of serpentine by talc. The Ivanovka deposit shows many similarities with active and fossil hydrothermal sites on some modern oceanic spreading centers characterized by highly permeable upflow zones. However, given the arc signature of the ore host rocks, the most probable setting for the observed alteration-mineralization patterns is in an early-arc or forearc seafloorsubseafloor environment, characterized by the presence of abundant mafic-ultramafic breccias of tectonic and/ or sedimentary origin.

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