The Pb-rich sulfide veins in the Boccassuolo ophiolite: Implications for the geochemical evolution of hydrothermal activity across the ocean-continent transition in the Ligurian Tethys (Northern-Apennine, Italy) Giorgio Garuti a,b, ⁎, Federica Zaccarini a,b , Maurizio Scacchetti b , Omar Bartoli b,c a Department Angewandte Geowissenshaften und Geophysik, Montanuniversität Leoben, Austria b Societá Reggiana di Scienze Naturali, Reggio Emilia, Italy c Dipartimento di Scienze della Terra, Universitá di Parma, Parma, Italy abstract article info Article history: Received 1 April 2010 Accepted 12 November 2010 Available online 25 November 2010 Keywords: Galena Hydrothermal sulfide veins Ophiolite External Ligurides Italy Galena bearing sulfide veins have been discovered coexisting with Fe–Cu–Zn dominated veins in the hydrothermal stockwork of the Boccassuolo ophiolite (External Ligurides, Northern Apennine, Italy). The galena-rich veins cut across a volcanic pile composed of pillow lava flows, pillow breccia, and ophiolitic sandstone. Bulk-ore analyses indicate significant enrichment in Pb giving raise to mantle normalized Pb–Ag–Au–Zn–Cu patterns with unusual negative slope, in contrast with the average flat pattern of most sulfide deposits in the Internal Liguride ophiolites which reflect the Fe–Cu–Zn assemblage of ophiolite-hosted Volcanic-associated Massive Sulfide (VMS) deposits all over the world. A wide literature shows that, in contrast with the Internal Ligurides, plutonic and volcanic rocks of the External Ligurides display less depleted and even enriched geochemical characters, not consistent with common oceanic crust at mid oceanic ridges (MOR), but probably originated in the ocean–continent transition of the Adria continental margin. In this geodynamic context, pillow basalts become locally enriched in Pb with high Pb/Cu ratios, and other crustal-compatible elements such as Mo and U. The Pb enrichment observed in the veins Boccassuolo is interpreted to be a result of leaching of such anomalous volcanics forming the ophiolitic substrate. The case of Boccassuolo supports the conclusion that the geochemical character of hydrothermal activity evolved from Cu–Zn rich in MOR-type assemblages of the Internal Ligurides, towards composition enriched in Pb in the External Liguride domain, representing the transition from the Ligurian ocean to the Adria continental margin. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Variation of Pb with respect to its companion metals (Cu, Zn) has become pivotal to the classification and geotectonic interpretation of ancient Volcanic-associated Massive Sulfide (VMS) deposits world- wide (see a summary of the classification criteria in Misra, 2000). The behavior of Pb in a global geochemical cycle indicates higher incompatibility during magmatic processes and preferential mobility in hydrothermal fluids compared with Cu and Zn, thereby reaching continental–crust/primitive-mantle enrichment factors of 10 to 30 times higher than the other chalcophile metals. Basaltic rocks as a whole may have Pb/Cu ratios in the range of 0.005–0.070 in contrast with rhyolites and granites which are characterized by Pb/Cu ratios as high as 2.45 or more (Hofmann, 1988; Wedepohl, 1991). Since the metals in VMS deposits were essentially derived from leaching of intrusive and extrusive igneous rocks in the footwall substrate (Large, 1992; Reed, 1983), their relative abundance is a reflection of the type of magmatism, and indirectly, it indicates the large-scale geotectonic setting of ore deposition (Barrie and Hannington, 1999). A major implication is that VMS deposits associated with mafic–ultramafic rocks in ancient and modern sub-oceanic crust from MOR, to island arc and subduction-influenced settings, are generally Pb-poor and Cu–Zn rich (Barrie and Hannington, 1999; Galley and Koski, 1999). In contrast, VMS associated with transitional basalts and felsic volcanic rocks, in continental arcs and rifted continental margins, exhibit progressive increase of the Pb content with respect to Cu and Zn. There is, however, at least one exception to this rule. In the Chilean Ridge at 38°S, abundant galena is observed in sulfide-mineralized quartz veins associated with a MORB dominated slow-spreading system (Mühe et al., 1977). In this case, the anomalous Pb enrichment is interpreted to be a result of the higher mobility of Pb compared with Cu and Zn, during hydrothermal over-leaching of the oceanic crust. The process removed larger-than-normal proportion of Pb that is now concentrated in the quartz veins, and left the flushed section of oceanic crust, far from the mineralized zone, more depleted in Pb than normal. In this paper, we present results of a detailed study of sulfide mineralized quartz veins cutting across pillow-basalt and basalt Lithos 124 (2011) 243–254 ⁎ Corresponding author. Department Angewandte Geowissenshaften und Geophysik, Montanuniversität Leoben, Austria. E-mail address: giorgio.garuti@unileoben.ac.at (G. Garuti). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.11.006 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos