Lithos 124 (2011) 243–254

Contents lists available at ScienceDirect

Lithos

j ourna l homepage: www.e lsev ie r.com/ locate / l i thos

The Pb-rich sulfide veins in the Boccassuolo ophiolite: Implications for thegeochemical evolution of hydrothermal activity across the ocean-continenttransition 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, Austriab Societá Reggiana di Scienze Naturali, Reggio Emilia, Italyc Dipartimento di Scienze della Terra, Universitá di Parma, Parma, Italy

⁎ Corresponding author. Department Angewandte GeoMontanuniversität Leoben, Austria.

E-mail address: giorgio.garuti@unileoben.ac.at (G. G

0024-4937/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.lithos.2010.11.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 April 2010Accepted 12 November 2010Available online 25 November 2010

Keywords:GalenaHydrothermal sulfide veinsOphioliteExternal LiguridesItaly

Galena bearing sulfide veins have been discovered coexistingwith Fe–Cu–Zn dominated veins in the hydrothermalstockwork of the Boccassuolo ophiolite (External Ligurides, Northern Apennine, Italy). The galena-rich veins cutacross a volcanic pile composed of pillow lava flows, pillow breccia, and ophiolitic sandstone. Bulk-ore analysesindicate significant enrichment in Pb giving raise to mantle normalized Pb–Ag–Au–Zn–Cu patterns with unusualnegative slope, in contrast with the average flat pattern of most sulfide deposits in the Internal Liguride ophioliteswhich reflect the Fe–Cu–Zn assemblage of ophiolite-hosted Volcanic-associatedMassive Sulfide (VMS) deposits allover the world.A wide literature shows that, in contrast with the Internal Ligurides, plutonic and volcanic rocks of the ExternalLigurides display less depleted and even enriched geochemical characters, not consistent with common oceaniccrust at mid oceanic ridges (MOR), but probably originated in the ocean–continent transition of the Adriacontinental margin. In this geodynamic context, pillow basalts become locally enriched in Pb with high Pb/Curatios, and other crustal-compatible elements such as Mo and U. The Pb enrichment observed in the veinsBoccassuolo 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 evolvedfrom Cu–Zn rich in MOR-type assemblages of the Internal Ligurides, towards composition enriched in Pb in theExternal Liguride domain, representing the transition from the Ligurian ocean to the Adria continental margin.

wissenshaften und Geophysik,

aruti).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Variation of Pb with respect to its companion metals (Cu, Zn) hasbecome pivotal to the classification and geotectonic interpretation ofancient Volcanic-associated Massive Sulfide (VMS) deposits world-wide (see a summary of the classification criteria in Misra, 2000). Thebehavior of Pb in a global geochemical cycle indicates higherincompatibility during magmatic processes and preferential mobilityin hydrothermal fluids compared with Cu and Zn, thereby reachingcontinental–crust/primitive-mantle enrichment factors of 10 to 30times higher than the other chalcophile metals. Basaltic rocks as awhole may have Pb/Cu ratios in the range of 0.005–0.070 in contrastwith rhyolites and granites which are characterized by Pb/Cu ratios ashigh as 2.45 or more (Hofmann, 1988; Wedepohl, 1991). Since themetals in VMS deposits were essentially derived from leaching ofintrusive 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 geotectonicsetting of ore deposition (Barrie and Hannington, 1999). A majorimplication is that VMS deposits associated with mafic–ultramaficrocks in ancient and modern sub-oceanic crust from MOR, to islandarc and subduction-influenced settings, are generally Pb-poor andCu–Zn rich (Barrie and Hannington, 1999; Galley and Koski, 1999). Incontrast, VMS associated with transitional basalts and felsic volcanicrocks, in continental arcs and rifted continental margins, exhibitprogressive increase of the Pb content with respect to Cu and Zn.There is, however, at least one exception to this rule. In the ChileanRidge at 38°S, abundant galena is observed in sulfide-mineralizedquartz veins associated with a MORB dominated slow-spreadingsystem (Mühe et al., 1977). In this case, the anomalous Pb enrichmentis interpreted to be a result of the highermobility of Pb comparedwithCu and Zn, during hydrothermal over-leaching of the oceanic crust.The process removed larger-than-normal proportion of Pb that is nowconcentrated in the quartz veins, and left the flushed section ofoceanic crust, far from themineralized zone, more depleted in Pb thannormal.

In this paper, we present results of a detailed study of sulfidemineralized quartz veins cutting across pillow-basalt and basalt

244 G. Garuti et al. / Lithos 124 (2011) 243–254

breccia in the Boccassuolo ophiolite complex (Ligurian Tethys,Northern Apennine). The study has revealed that veins dominatedby the Cu–Zn assemblage typical of ophiolite-hosted VMS deposits,coexist with others carrying abundant galena in the sulfide assem-blage. The case of Boccassuolo apparently represents a furtherexception to the assumption that VMS deposits associated withmafic–ultramafic rocks in ophiolite complexes are depleted in Pb.However, the reasons for the observed Pb-anomaly, do not seem to berelated with over-leaching of Pb from the rock substrate, they ratherreflect a change in the chemistry of the hydrothermal activity relatedwith geochemical changes in the leached basalts across an ancientocean–continent transition zone. Evidences in support of thisinterpretation are discussed.

2. Geological setting and petrologic background

2.1. The ophiolite sequences of the Ligurian basin

The ophiolites exposed in the Northern Apennine (Fig. 1)represent fragments of the oceanic lithosphere that floored theLigurian basin in the western limb of the Jurassic Tethys (Piccardoet al., 2002, and references therein). The ophiolites and theirsedimentary cover are conventionally divided into Internal andExternal Ligurides. In the Internal Ligurides, ophiolites form the baseof deeply folded northeast-verging blocks bounded by southwest-dipping fault planes, in which stratigraphic relations between theplutonic basement and sedimentary cover are still preserved (Abbateet al., 1980; Barrett, 1982; Elter, 1975). In contrast, ophiolites of theExternal Ligurides occur as un-rooted and dismembered blocksfloating in a mélange of argillaceous rocks and disrupted fragmentsof carbonaceous flysch and siliciclastic turbidite. Noteworthy, the twodomains are believed to represent oceanic crust in two different

Fig. 1. Sketch structural map of the Internal and External Ligurides and location of themajor copper sulfide deposits in the Northern Apennine. RP=Reppia, BD=MonteBardeneto, BC=Monte Bianco, LB=Libiola, CP=Campegli, CS=Casali, VI=Vigonzano,GR=Groppallo, FE=Ferriere, CO=Corchia, MO=Montecreto, BO=Boccassuolo.

paleogeographic contexts: one in proximity of the rifting zone(Internal Ligurides), the other closer to the Adria continental margin(External Ligurides) (Piccardo et al., 2002, and references therein).Consistent with this view, mantle peridotites in External Ligurides aresub-continental spinel lherzolite having a less depleted charactercompared with those in Internal Ligurides (Rampone et al., 1995).Basaltic rocks have MORB-type REE-trace element geochemicalsignature in both Internal and External Ligurides, although the latterappear systematically less depleted displaying transitional-MORBaffinity in some cases (Ottonello et al, 1984; Venturelli et al., 1981).

In the Modena province, including the Boccassuolo ophiolite,basaltic rocks display variable compositions suggesting differentmagmatic source and mixing. Typical N-MORB basalts similar tothose of the Internal Ligurides are rare. Basalts commonly displayslightly depleted or even enriched bulk geochemistry, with high Zr/Hfratios (Capedri and Toscani, 2000). Zaccarini et al. (2008) haveshown that basalts from six localities of the External Ligurides displayN-MORB type distribution of the REE, but are variably enriched in Pb,and Mo, and depleted in Cu and Ni with respect to N-MORB (Table 1).These geochemical anomalies have been tentatively interpreted byCapedri and Toscani (2000) as possible evidence supporting the originof the basalts in a subduction-related, back-arc/inter-arc setting.Paleo-geographic evolution of the Ligurian Tethys indicates that thebasin was still expanding or in a steady state during late Jurassic–Cretaceous times. Intra-oceanic subduction and obduction of Ligurianunits onto the Adria continental margin occurred in the southernsegment of the oceanic basin corresponding to the Northen Apennine,when a compressive regime started to be active (in UpperCretaceous?). We do not exclude the possibility that some fragmentsof subduction-related assemblages might have been sampled by theobducting system. In the External Liguride ophiolites, however, uppermantle assemblages typical of subduction zones (harzburgite–dunite–chromitite) are absent, as well as high-Mg volcanics (i.e.boninite) that frequently accompany subduction-related volcanism(Crawford et al., 1989). Structural and geochemical lines of evidenceindicate that the External Liguride ophiolites mostly formed in apre-collisional stage (Marroni et al., 2001; Piccardo et al., 2002).The observed geochemical deviation from typical N-MORB is notnecessarily related to a subduction setting. The close spatialassociation with almost undepleted sub-continental mantle stronglysupports the conclusion that these hybrid MORB-type melts mighthave generated in the final stages of the extensional regime, in anocean–continent transition similar to the modern non-volcanic riftedmargins of Iberia (Marroni et al., 1998, 2001, 2002; Montanini et al.,2008; Rampone and Piccardo, 2000; Robertson, 2002; Tribuzio et al.,2000, 2002).

2.2. VMS deposits in the Internal and External Ligurides

TheNorthern Apennine ophiolites contain a number of hydrothermalsulfide deposits known as amajor source of copper since 3500 BC (Maggiand Pearce, 2005; Pearce, 2007). The deposits pertain to the VMS class(Volcanic-associated Massive Sulfide deposits), and because of theirgenetic relation with basaltic volcanics and a dominant Cu–Zn metalassemblage, they have been ascribed to the Mafic type of Barrie andHannington (1999), resembling VMS deposits of the Cyprus sub-typeassociatedwith Phanerozoic ophiolites worldwide (Garuti and Zaccarini,2005; Garuti et al. 2008; Zaccarini and Garuti, 2008).

The sulfide deposits of the Internal and External Ligurides displaydifferent structural styles varying from seafloor-stratiform andseafloor-stratabound massive ore bodies to sub-seafloor stockworkveins with high-grade disseminated sulfides (Ferrario and Garuti,1980; Garuti and Zaccarini, 2005; Garuti et al., 2008). The bestrepresentative examples of the Internal Ligurides concentrate in thelarge ophiolite complex of Val-di-Vara, in the inland of Sestri Levante(eastern Liguria) whereas the deposits associated with the External

Table 1Trace metal concentrations (ppm) in basalts from the External Ligurides compared with N-MORB.

RS CA BC CI CG VM VM Average N-MORB*

Ba 16.97 18.97 9.11 4.40 4.95 77.00 52.73 26.30 13.87Co 28.27 30.24 39.14 14.95 40.21 50.83 55.05 36.96 47.10Ni 60.06 57.59 94.34 27.48 111.44 77.37 63.30 70.23 149.50Cu 30.71 33.38 50.92 12.13 34.61 42.59 35.22 34.22 70.00Zn 51.61 81.55 62.52 13.24 65.39 72.44 96.00 63.25 84.00Ga 11.92 15.73 13.13 5.88 15.70 16.83 18.58 13.97 17.00Mo 1.26 1.01 1.96 1.21 1.40 4.19 4.56 2.23 0.31Sn 1.03 1.35 1.28 0.64 1.32 4.76 1.56 1.71 1.30Pb 1.44 0.62 0.49 0.89 0.74 1.52 0.58 0.90 0.35U 0.34 0.19 0.12 0.08 0.18 0.07 0.09 0.15 0.07Th 0.61 0.18 0.30 0.06 0.19 0.16 0.15 0.24 0.19Pb/Cu 0.047 0.019 0.010 0.073 0.021 0.036 0.016 0.026 0.005

Localities: RS: Rossenalla, CA: Castellaro, BC: Boccassuolo, CI: Cinghi, CG: Case Giannasi, VM: Valmozzola.ICP-MS data from Zaccarini et al. (2008). *) Barrie and Hannington (1999).

245G. Garuti et al. / Lithos 124 (2011) 243–254

Ligurides occur in isolated ophiolite blocks spread along the NorthernApennine ridge (Emilia Romagna), over a distance of more than150 km (Fig. 1). Structural types, host–rock association and repre-sentative composition of the sulfide ores are listed in Table 2.

The Northern Apennine sulfide deposits are characterized by amonotonous sulfide assemblage pyrite–chalcopyrite–sphalerite withaccessory pyrrhotite, in a gangue of quartz, calcite and chlorite(Bertolani, 1952, 1953, 1959, 1962; Brigo and Ferrario, 1974). However,both the depositional environment (sea-floor vs. sub-seafloor) andcountry rock lithology (basalt vs. serpentinite) appear to haveinfluenced the ore composition in terms of the metal ratios Co/Ni, Zn/Cu, Au/Ag, Cr/Mn, Mg/Al, the U content and S-isotopes geochemistry(Garuti and Zaccarini, 2005; Garuti et al., 2009; Zaccarini and Garuti,2008). The data indicate that sulfides deposited in contact with freshseawater (stratiformand strataboundores),with a fewexceptions, havehigh Au and U contents (Garuti and Zaccarini, 2005), and those whichunderwent long time exposure at the seafloor (stratiform massivesulfides) are characterized by low δ34S‰down to negative values due tobiogenic reworking of the ore (Garuti et al., 2009). Compared withbasalt-hosted sulfide deposits, the ores associated with serpentinite

Table 2Structural style, host–rock association, and average composition of VMS ores from the Ligu

S Pb Ag Au Zn Cu

% ppm ppm ppb % %

Internal Ligurides1) seafloor stratabound massive ore and stringer ore in pillow basalt

Libiola 24.0 27.0 10.0 284 0.47 7.302) seafloor straiform massive ore in serpentinite breccia covered with pillow basalt

Monte Bardeneto 19.4 0.91 1.9 637 0.17 1.76Monte Bianco 2 24.7 71 3.2 159 0.01 0.70Reppia I 17.1 59 0.83 287 0.03 2.84

3) seafloor straiform massive ore in pillow basalt covered with chertReppia II 19.0 1.1 792 0.01 0.83

4) sub-seafloor stockwork veins in pillow basalt or gabbroReppia III 10.9 2.6 75 0.06 10.1Monte Bianco 1 0.75 5.0 0.01 0.58Casali 1.71 0.70 0.04 1.61Campegli 4.60 7.8 21 0.10 2.96

External Ligurides1) seafloor stratiform massive ore in pillow basalt breccia covered with Palombini shale

Corchia (Py–Cp) 20.9 71 8.8 1458 0.15 4.26Corchia (Sph) 19.0 221 106 47 22 0.36

2) sub-seafloor stockwork veins in serpentinite and serpentinite brecciaVigonzano 17.2 1.4 4.3 38 0.02 1.35Groppallo 7.0 5.0 0.01 0.25Ferriere 10.0 5.3 80 0.06 8.70

3) sub-seafloor stockwork veins in pillow basalt and pillow brecciaMontecreto 4.7 2.7 0.40 7.0 0.01 0.39Boccassuolo (*) 5.4 24.5 3.1 8.5 1.5 2.10

Average compositions from the data base of Zaccarini and Garuti (2008). S-isotope data fro(Py–Cp)=massive pyrite and chalcopyrite; (Sph)=massive sphalerite. (*) Except galena-r

have higher Cu/Zn ratio, due to lowmodal proportion of sphalerite, andlow Co/Ni reflecting the composition of pyrite. They have higher Cr/Mnand Mg/Al ratios accounted for by compositional variation of chloriteand occurrence of detrital chromite incorporated in the sulfide ore(Zaccarini and Garuti, 2008). Serpentinite-hosted deposits also havelower δ34S‰ values compared with ores in basalt, due to leaching ofmantle-derived sulfur with predominant δ34S≈0.0‰ (Garuti et al.,2009). These variations are apparently independent on the paleogeo-graphic location of the deposits in the Internal or External Ligurides, andtheir association with MOR-type oceanic crust or regions closer to thecontinental margin.

2.3. Internal stratigraphy of the Boccassuolo ophiolite

The ophiolite complex of Boccassuolo is located to the east side ofthe External Ligurides (Fig. 1). It is the most extended of more than900 ophiolite blocks spread throughout the Cretaceous sedimentarymélange, in the provinces of Modena and Reggio Emilia (Bertolani andCapedri, 1966). According to Plesi et al. (2002), the Boccassuoloophiolite complex is comprised in the “Val Braganza ophiolitic unit”

rian ophiolites.

Co Ni Mg Al Cr Mn U δ34S

ppm ppm % % ppm ppm ppm ‰

220 45 1.97 1.65 76.8 380 6.17 +7.9

497 483 1.69 0.24 514 685 2.9 +1.5125 62 0.67 0.40 61 72

2275 693 2.20 0.44 459 304 1.1 +3.75

347 130 0.27 0.21 18.8 98.3 +4.25

281 36 0.71 0.60 11.0 19561 24 2.36 5.69 14.0 594 1.326 50 4.70 4.43 133 1640 +10.747 37 1.48 1.39 14.0 614 +8.65

3143 750 0.40 0.26 46 294 2.5 +1.11360 96 0.03 0.10 133 +5.15

340 374 0.78 0.39 171 133 +6.3115 434 3.17 1.35 408 821401 511 0.06 0.14 232 538 2.1 +4.65

47 277 2.0 0.93 376 394 +6.272 30 1.3 0.85 45.7 310 +8.81

m Garuti et al. (2009).ich samples.

246 G. Garuti et al. / Lithos 124 (2011) 243–254

which has not been precisely dated, however, relative ages can besometimes deduced from stratigraphic relationships. A LowerCretaceous age (Barremian–Albian) can be tentatively assumed forthe sedimentary cover (Palombini Shale) that constitutes largeportions of the sedimentary mélange in this region (Plesi et al.,2002). As a consequence formation of the ophiolitic rocks must beplaced at any time between Upper Jurassic and Lower Cretaceous.Despite of tectonic fragmentation and faulting, stratigraphic relation-ships between ophiolitic rocks and sedimentary cover are locallypreserved at Boccassuolo (Fig. 2). The complex consists of two majorblocks (Poggio Bianco Dragone and Cinghio del Corvo) separated by asub-vertical fault, with tectonic interposition of a zone of Palombinishale and ophiolitic breccias considered by Plesi et al. (2002) of“Debris flow type” (the Poggio Bianco Dragone breccia). The Poggio

Fig. 2. Simplified geological map of the Boccassuolo ophiolite modified after theGeological Map of Italy (Sheet 235, Pievepelago), with the contribution of originalunpublished mapping by A. Rossi. Abbreviations: PBD, Poggio Bianco Dragone; CC,Cinghio del Corvo; MC, Madonna del Calvario; PM, Poggio Medola; LL, Cascina Lame.Numbers in italic are elevation above sea level in meters. Sample location in circles: 1)Due Livelli mine, 2) Lumaca prospect, 3) Allagata mine, 4) Dolicopoda–Pipistrelloprospects, 5) Labirintica mine, La Pioppaccia dump, 6) Filone Omar outcrop, 7) Lameprospect A, 8) basaltic aggregate quarry of Cinghio del Corvo. The arrow indicates thedirection to the Boccassuolo village.

Bianco Dragone is bordered to the west by the tectonic contact withthe sedimentary mélange, and tectonically overlies strongly deformedand folded carbonate-argillaceous sediments poorly exposed at thefootwall. The lower part of the block consists of a sequence of pillowlava flows intercalated with horizons of ophiolitic breccia andophiolitic sandstone composed of clasts of basalt, chert, limestone,and rare granite (see Fig. 3A). These ophiolitic sediments show welldefined graded bedding and, although tilted with respect to theiroriginal position, display stratigraphic contacts with pillow lava flows.Close to the top of Poggio Bianco Dragone, at about 850 m (Fig. 2) athin horizon of siltitic limestone and clay, tilted about 20° to the W,occurs intercalated within pillows and pillow-breccia in apparentstratigraphic contact. We have ascribed this sedimentary intercalationto the Palombini Shale formation, although the analogy is restricted tosedimentological features not supported by the finding of micro-fossils. These observations indicate that episodes of debris flowaccumulation and pelagic sedimentation occurred at intervals, coevalwith basaltic volcanism.

Fig. 3. Boccassuolo ophiolite. A) View of pillow lava flows (P) alternated with layers ofophiolitic breccia (Br) composed of basalt, chert, micritic limestone and rare granite,Dragone creek, NW of PBD. B) Outcrop “Filone 101”: hydrothermal vein with sulfidemineralization (Hv) cutting across pillow basalt (P), vicinity of the Allagata mine,locality 3 (Fig. 2).

247G. Garuti et al. / Lithos 124 (2011) 243–254

The Cinghio del Corvo block is mainly composed of pillow basaltand pillow breccia well exposed all along the main fault and in theCinghi quarry. Large accumulations of Poggio Bianco Dragone brecciacrop out S of the Cinghio del Corvo peak (1079 m), however, theirgeometric relations with the rest of the ophiolitic block are unclear.Close to the top of the peak, pillows and pillow breccia are capped bysiliciclastic sediments containing thin layers of reddish, Mn-rich clay(Colombara lithofacies?). The contact with underlying volcanic rocksis locally exposed showing normal stratigraphic relationships,whereas the eastern contact with Upper Cretaceous–Cenozoic(Campanian–Maastrichtian–Paleocenic) turbiditic sediments is cov-ered, but probably tectonic.

2.4. Field relations of the Boccassuolo sulfide ores

The sulfide deposits of Boccassuolo are classified as sub-seafloorstockwork veins hosted in pillow basalt and basalt breccia (Garutiet al., 2008; Zaccarini and Garuti, 2008). Stockwork veins form sub-vertical pipes exposed in both ophiolite blocks. They consist ofreticulate quartz–calcite–chlorite–sulfide veins from a few centi-meters to more than one meter thick, emplaced within hydrother-mally altered pillow basalt and basalt breccia (Fig. 3B). These depositsare equivalent to those of the Internal Ligurides, at the localities ofCasali and Monte Loreto, where a thick pile of pillow basalt and basaltbreccia is cut across by quartz–calcite–chlorite hydrothermal veinsmineralized with Cu–Fe–Zn sulfide assemblage.

In the Poggio Bianco Dragone block, hydrothermal veins appear tobe deep seated. They start to be exposed along the Dragone creek at anelevation of about 600 m, in the Due Livelli mine (site no. 1 in Fig. 2),and stop at 750 m, above the Labirintica mine. No tectonic dislocationis observed, but the veins shrink and suddenly disappear topped bypillow lava flow. Conspicuously, above this level, volcanic andsedimentary rocks, including the intercalation of siltitic limestone at850 m, are barren. In general, the veins have sharp contact with thecountry rock pillow basalt (Fig. 3B). However, underground worksshow a permeability-controlled emplacement of the hydrothermalsolution along the intercalated breccia. Here, the mineral corensitefrequently appears in the clayey matrix of the breccia (Setti andLopez-Galindo personal communication 2007) indicating reaction ofthe clay minerals with hot hydrothermal solution. The ophioliticsandstone is only weakly mineralized showing diffuse infiltration ofminute carbonate–epidote–sulfide fissure filling and impregnation.

In the Cinghio del Corvo block, three major sulfide occurrences areobserved. One consists of a single vein (Filone Omar), intensivelymineralized but never exploited, emplaced vertically within pillowbasalt (site no. 6 in Fig. 2). The vein cuts across slightly deformedpillow and is only interrupted by the steep erosion surface. Othermineralized veins exposed in old mining assays (trenches andgalleries) occur NE of Cinghio del Corvo, between 1030 and 1060 m(site no. 7 in Fig. 2). The third occurrence is in the quarry for basalticaggregates (site no. 8 in Fig. 2) where the Pb-rich sulfide veins werediscovered. Here, a vertical section of the mineralized stockwork isexposed through several ten meters from level 875 m upwards.Towards the top, the stockwork gradually disappears topped bypillow lava. The Pb-rich veins were discovered about 50–80 m belowthe hanging wall contact with the Cretaceous siliciclastic sediments,and in no case they are seen to cut across the volcanic–sedimentinterface. Their stratigraphic position, however, suggests emplace-ment at amore shallow depth below the ancient seafloor, with respectto the veins in the Poggio Bianco Dragone block.

3. Sampling and methods

Three galena-rich samples were systematically investigated byoptical and electron microscopy. Compositions of sulfides (Table 3)and chlorite (Table 4) were determined by electron microprobe at the

Eugen StumpflMicroprobe Laboratory (University of Leoben), using aJeol Superprobe JXA-8200, operated at 15 kV accelerating voltage,10 nA beam current, ~1 μm beam diameter, and counting times of 20and 10 s for peak and backgrounds, respectively. The X-ray Kα lineswere used for Si, Mg, Al, Ti, Cr, Ca, Na, K, Mn, Fe, Co, Ni, Cu, Zn, S, the Lαline was used for Cd and As, and the Mα.for Pb. Synthetic NiAs, CoAsS,FeS2, CuFeS2, ZnS, CdS, and PbSwere used as referencematerials in theanalysis of sulfide minerals; natural silicates and chromian spinelwere the standards for chlorite.

About 1 kg of each sample composed of sulfide ore and ganguewaspowdered and analyzed in triplicate by XRF and AA, for Cu, Zn, Pb, andAg. Gold was pre-concentrated by Te-precipitation after hot dissolu-tion with aqua regia. Analytical results are reported in Table 5compared with compositions of normal sulfide veins (Zaccarini andGaruti, 2008) and pillow-basalts from Boccassuolo (Zaccarini et al.,2008), and the average N-MORB (Barrie and Hannington, 1999).

4. The Pb-rich, hydrothermal veins of Boccassuolo

4.1. Mineralogy and texture

The Pb-rich veins discovered in the Cinghi quarry show consistentassemblage pyrite–chalcopyrite–sphalerite–galena–(pyrrhotite).Similarly to the other veins, pyrite forms euhedral grains or lessregular aggregates, usually replaced by chalcopyrite and sphalerite,however, a late generation of pyrite is common, filling cracks andfissures in the other sulfides. Sphalerite grows over chalcopyrite andshows the “chalcopyrite-disease” texture typical of the sphaleritefrom Boccassuolo (Bertolani, 1953). Galena occurs as aggregates of up1mm, growing over or replacing sphalerite and chalcopyrite along themargins of the sulfide aggregates (Fig. 4). Pyrrhotite is very rare,occurring as small patches (b100 μm) inside pyrite suggesting late-stage crystallization. The Pb-rich veins have the same association ofgangue minerals as the other veins consisting of quartz, calcite andchlorite, with accessory epidote and titanite.

The hydrothermal veins at Boccassuolo generally have disruptedor cataclastic structure or consists of the close association of irregularanastomotic veins possibly formed by repeated opening and mineral-precipitation events, followed by tectonic deformation. Howeversome veins characterized by the Fe–Cu–Zn assemblage (i.e. FiloneOmar, samples BO97, BO98, Table 5), and those enriched in galena(samples BO91, BO92 and BO93) appear to be only weakly deformed,with well-preserved wall boundaries, and mineral banding symmet-rical about the center of the veins. Large sulfide aggregates occurintermixed with a palisade of quartz crystals growing perpendicularto the vein walls. Abundant calcite occurs concentrated in the core ofveins. Irregular patches of chlorite are spread in the quartz-calcitegangue sometimes forming vermicular and rosette-like aggregates.According to Zaccarini and Garuti (2008), this type of chlorite isauthigenic, generated by metasomatic reaction between the hydro-thermal fluid and the country rock basalt (Fig. 5A). Strong hydro-thermal alteration is also induced in pillow basalt surrounding thestockwork (Fig. 5B, C). The wall basalt is minutely infiltrated byquartz–calcite fissures and is diffusely spotted with idiomorphic andsub-idiomorphic pyrite crystals reaching up to 1 cm in size.

4.2. Mineral chemistry of sulfides and chlorite

Galena contains sporadic traces of Fe, Cu, and Zn (Table 3).Cadmiumwas systematically detected, although it possibly representsa spurious interference from lead that could not be corrected. Pyritecarries traces of Ni, Co, Mn and As (Table 3). The Co/Ni ratio issystematically higher than unity, that distinguishes the galena richveins from normal veins in which the Co/Ni ratio of pyrite varies fromb1 to N1 (Fig. 6A). Pyrrhotite corresponds to the monoclinic variety(Fe7S8)withminorNi, CoandMn(Table3). Sphalerite contains significant

Table 3Electron microprobe composition (wt.%) of galena and associated sulfides from the Cinghi quarry (Boccassuolo ophiolite).

DL Gn Gn Gn Gn Gn Gn Gn Gn Gn Gn Gn

91/30 91/31 91/34 91/35 91/36 91/38 91/40 93/25 93/26 93/27 93/28

Fe 0.07 0.07 0.07 0.08Ni 0.04Co 0.04Cu 0.12 0.14 0.12Zn 0.17 0.2 0.2Cd 0.05 0.14 0.14 0.14 0.16 0.13 0.15 0.14 0.13 0.16 0.15 0.15Mn 0.02S 0.11 12.9 13.31 13.36 13.15 13.23 12.96 13.23 13.11 13.16 13.2 12.84As 0.11Pb 0.05 85.67 85.73 85.37 86.43 85.58 85.67 87.22 85.75 85.67 85.25 85.79Total 98.71 99.25 98.87 100.15 98.94 98.78 100.79 98.99 98.99 98.8 98.78

DL Py Py Py Py Po Cp Cp Sph Sph Sph Sph

91/29 91/48 93/149 93/154 91/55 91/44 93/145 91/31 91/41 93/155 93/161

Fe 0.07 46.76 47.2 46.68 46.97 59.25 30.53 30.19 8.5 6.68 5.39 6.68Ni 0.04 0.04 0.04 0.07Co 0.04 0.07 0.04 0.14 0.06 0.17 0.1 0.05 0.05Cu 0.12 34.12 34.69 9.55 7.39 6.26 7.7Zn 0.17 0.22 0.27 48.06 52.46 54.71 51.89Cd 0.05 0.18 0.26 0.05 0.06Mn 0.02 0.05 0.25 0.21 0.06 0.19 0.07 0.03S 0.11 53 53.96 53.98 53.76 38.8 34.83 34.54 33.4 33.03 32.63 32.97As 0.11 0.37 0.14 0.31 0.19 0.15 0.11 0.42Pb 0.05Total 100.42 101.39 101.36 100.98 98.47 99.96 99.71 99.96 99.85 99.11 99.77

Gn, galena; Py, pyrite; Po, pyrrhotite; Cp, chalcopyrite; Sph, sphalerite. DL, average detection limits.Ag, Sn, and Mo were checked but not detected.

248 G. Garuti et al. / Lithos 124 (2011) 243–254

copper (up to 12.7 wt.% Cu) and/or iron (up to 14.3 wt.% Fe), and traceamounts of cadmium (b0.72 wt.% Cd), manganese (b0.34 wt.% Mn) andarsenic (b0.46 wt.% As). There are essentially two types of sphalerite inthe hydrothermal veins of Boccassuolo: one is characterized by Fesubstituting for Zn, without Cu, the other shows Fe and Cu (Fig. 6B). Asshown by Garuti et al. (2009), this feature is an analytical artifact causedby the “chalcopyrite disease” in sphalerite. Noteworthy, all sphalerite

Table 4Electron microprobe composition and structural formula of chlorite from galena rich veins

wt.% DL 91/4 91/5 91/6 91/12 93/1 93/2 93/3

SiO2 0.06 37.56 35.35 35.33 33.11 31.56 32.82 32.64TiO2 0.01 0.02 0.04 0.02Al2O3 0.03 13.89 11.57 12.47 11.63 16.57 15.98 16.25FeO 0.06 12.19 22.92 22.32 26.83 14.99 15.9 16MnO 0.06 0.33 0.3 0.48 0.5 0.22 0.3 0.34MgO 0.02 22.57 16.15 15.73 13.95 23.16 22.03 22.33CaO 0.01 0.48 0.43 0.5 0.41 0.19 0.2 0.1Na2O 0.02 0.1 0.05 0.02 0.05 0.03 0.02 0.02K2O 0.01 0.09 0.06 0.09 0.03Cr2O3 0.04 0.11 0.09 0.13 0.09Total 87.34 86.83 87.03 86.51 86.89 87.27 87.77

at.% on the basis of 28 oxygensSi 7.27 7.32 7.28 7.08 6.31 6.54 6.47Ti 0 0.01 0 0Al tot 3.17 2.82 3.03 2.93 3.9 3.75 3.8Al(IV) 0.73 0.68 0.72 0.92 1.69 1.46 1.53Al(VI) 2.44 2.15 2.3 2.01 2.21 2.29 2.27Fe 1.97 3.97 3.85 4.8 2.51 2.65 2.65Mn 0.05 0.05 0.08 0.09 0.04 0.05 0.06Mg 6.51 4.99 4.83 4.45 6.9 6.54 6.6Ca 0.1 0.1 0.11 0.09 0.04 0.04 0.02Na 0.04 0.02 0.01 0.02 0.01 0.01 0.01K 0.02 0.02 0.02 0.01Cr 0.02 0.01 0.02 0.01oct 11.16 11.29 11.22 11.47 11.73 11.59 11.63

DL, average detection limit; oct, octahedral occupancy.

analyses from the galena-rich samples plot along theCu/Fe=1 regressionline, at Fe=CuN5 at%.

Chlorites from the galena-rich veins of Boccassuolo (Table 4)contains sporadic traces of Ti, Cr, Na, K, Ca, and substantial amounts ofmanganese up to 0.53 wt.% MnO. They show the Fe–Mg exchange atalmost constant AlVI (Fig. 7) characteristic of chlorites from basalt-hosted veins of the Northern Apennine (Zaccarini and Garuti, 2008).

of the Boccassuolo ophiolite.

93/5 93/6 93/7 93/8 93/10 93/11 93/12 93/18

35.87 34.71 32.24 34.7 33.74 33.42 35.32 37.220.03 0.02 0.03 0.03

14.2 15.67 16.31 16 16.01 17.07 15.96 12.1512.41 12.69 13.96 12.85 13.13 14.17 12.83 18.260.37 0.53 0.37 0.41 0.28 0.53 0.39 0.32

22.52 23.85 22.17 22.68 22.56 22.5 22.27 18.760.33 0.27 0.18 0.28 0.24 0.17 0.25 0.640.04 0.03 0.06 0.03 0.02 0.05 0.010.03 0.05 0.02 0.060.14 0.01 0.12 0.2 0.04 0.01

85.94 87.78 85.28 87.12 86.22 87.92 87.11 87.42

7.09 6.75 6.51 6.79 6.69 6.54 6.89 7.440.01 0 0.01 0.013.31 3.59 3.88 3.69 3.74 3.94 3.67 2.860.91 1.25 1.49 1.21 1.31 1.46 1.11 0.572.39 2.34 2.4 2.48 2.43 2.48 2.56 2.32.05 2.06 2.36 2.1 2.18 2.32 2.09 3.050.06 0.09 0.06 0.07 0.05 0.09 0.07 0.056.63 6.91 6.68 6.62 6.67 6.56 6.48 5.590.07 0.06 0.04 0.06 0.05 0.04 0.05 0.140.01 0.01 0.03 0.01 0.01 0.02 0.010.01 0.01 0 0.020.02 0 0.02 0.03 0.01 0

11.26 11.46 11.55 11.37 11.42 11.49 11.28 11.14

Table 5Bulk-rock sulfur and metal concentrations in ore samples from Boccassuolo.

Locality and reference no. in Fig. 2 Sample S % Pb ppm Ag ppm Au ppb Zn % Cu % Type

Cinghi quarry (galena rich)* 8 BO91 2.09 1323 1.08 6.5 0.44 0.49 IIICinghi quarry (galena rich)* 8 BO92 4.82 1532 0.87 4.5 0.01 0.02 IIICinghi quarry (galena rich)* 8 BO93 3.07 906 0.63 4.1 0.18 0.6 IIICinghi quarry (normal) ** 8 BO11-12-14 1.06 16 0.3 2.0 0.10 0.08 IIPipistrello prospect ** 4 BO28 3.00 117 0.49 6.0 0.11 0.34 IIPipistrello prospect ** 4 BO38 1.57 3.43 0.3 2.0 0.01 0 IILa Pioppaccia ** 5 BO33 9.57 0.3 10.1 15 2.5 4.77 ILabirintica mine ** 5 BO3-4-5 6.23 0.3 2.9 11 1.92 2.09 ILabirintica mine ** 5 BO6 12.3 0.3 7.28 17 4.85 5.94 ILame prospect A ** 7 BO35-36 3.39 12 0.35 2.0 0.87 0.19 IIFilone Omar ** 6 BO97 8.73 21 3.8 13 2.31 3.89 IIFilone Omar ** 6 BO98 3.19 50 2.1 8.5 0.98 1.33 IIBoccassuolo basalt*** BC 0.49 0.0063 0.0051Cinghi quarry basalt*** CI 0.89 0.0013 0.0012N-MORB 0.35 0.03 0.87 0.0084 0.0070

* This work; ** Zaccarini and Garuti (2008);***Zaccarini et al. (2008). N-MORB from Barrie and Hannington (1999).Types I, II and III refer to profiles in Fig. 8.

249G. Garuti et al. / Lithos 124 (2011) 243–254

On average, chlorites from the galena-rich veins have slightly lowerAlIV compared with chlorite in the normal veins of Boccassuolo andother stockwork deposits of the Northern Apennine, possiblyreflecting lower temperature of crystallization (Fig. 7).

4.3. Bulk sulfide composition

Metal concentrations in 100% sulfide calculated from data inTable 5, are presented as mantle-normalized spider-diagrams in theorder Pb–Ag–Au–Zn–Cu (Fig. 8), corresponding to decreasing mag-matic compatibility, or decreasing solubility in hydrothermal fluidswith declining temperature (Lydon, 1988; Reed, 1983). The approx-imate bulk-sulfide composition was calculated by combining Cu, Zn,Pb and S concentrations with the average electron microprobe

Fig. 4. Textural relations of galena. A) BSE image of galena (white) associated with sphaleritethe square area in A (reflected light) showing the galena–sphalerite association and minutaggregate (samp. BO 91). D) the same as C (reflected light).

compositions of chalcopyrite, sphalerite and galena, respectively,and ascribing the remaining Fe and S to pyrite. Uncertainty in thiscalculation derives from variation in the Fe content of sphalerite, andthe presence of trace amount of pyrrhotite in the sulfide. Theapproach, however, does not affect metal ratios, and minimizes theeffects of ore-grade variations in comparing disseminated withmassive deposits.

The mantle-normalized patterns are characterized by similarcontents of Ag, Au, Zn and Cu, but distinguish for a threefold behaviorof Pb. Profile of type I was found in samples coming from locality 5(Fig. 2). It displays low Pb, enriched up to only a few ten times theprimitive mantle value, with an extremely low Pb/Cu normalized ratios(b0.0023), similar to basalt-hosted veins of the Internal Ligurides,although with sensibly higher Zn (Fig. 8A). Type-II is the most common

(Sph), chalcopyrite (Cp) and pyrite (Py) in quartz vein (samp. BO 91). B) Enlargement ofe “chalcopyrite disease” in sphalerite. C) Galena growing on the rim of a large sulfide

Fig. 5. Transmitted light microphotographs of chlorite. A) Small fragment of spilitizedbasalt (albite, chlorite, titanite, plus opaque minerals) engulfed in a quartz vein.Chlorite (Chl) forms after the basalt fragment and fades into vermicular and rosettechlorite (Chl) included in quartz (Qz). B) Weakly metasomatized basalt away fromhydrothermal stockwork veins: chlorite grows interstitial to acicular plagioclase,partially replacing orthopyroxene. C) Strongly spilitized basalt in contact withhydrothermal veins: chlorite forms small patches interstitial to a felt of acicular albitecrystals (no magmatic phase is preserved).

Fig. 6. A) Co–Ni distribution in pyrite, and B) Cu–Fe distribution in sphalerite fromthe hydrothermal veins of Boccassuolo; sphalerite with Cu/Fe=1 is characterized bysub-microscopic scale “chalcopyrite disease”. Open square: normal veins. Filled square:galena-bearing veins.

250 G. Garuti et al. / Lithos 124 (2011) 243–254

at Boccassuolo, It is characterized by Pb enrichment between 103 and104 times the primitive mantle, and Pb/Cu normalized ratios between0.09 and 18.4 (Fig. 8B). The appearance of abundantmodal galena in thesulfidemineral assemblage results in extremely high Pb, originating thetype-III metal pattern broadly concave shape, with negative slopePb–Ag–Au, and positiveAu–Zn–Cu (Fig. 8C). The normalized Pb/Cu ratiois 24.3 and 49.2 in samples BO93 andBO91, respectively, but reaches theexceptional value of 1226 in sample BO92, due to extreme depletion inZn andCu, that results in a negative normalized pattern similar to that ofthe bulk continental crust. In summary, there is a progressive increase inthe Pb content from type-I to type-III, with the type-II samples having atransitional character. Significantly, all of the profiles but one have

shapes comparable to the pattern of N-MORB, although with a morepronounced negative anomaly in Au.

5. Discussion and conclusions

5.1. Origin of the Pb-rich veins at Boccassuolo

Galena is a relatively rare mineral in the Northern Apennine VMSthat has been reported from only two sulfide deposits, Reppia II andCorchia, in the Internal and External Ligurides, respectively (Garutiand Zaccarini, 2005; Zaccarini and Garuti, 2008). At these localitiesgalena occurs as sub-microscopic grains (b5 μm) only visible with theuse of electronmicroscopy, and accounts for less than 0.007 wt.% Pb inthe bulk sulfide. It is unclear if galena precipitated directly from thehydrothermal solution, or formed in some secondary stage, bysubmarine reworking of the ore. In contrast, mineral assemblageand mode of occurrence of galena in the Boccassuolo veins indicatesthat the mineral is a major component of the sulfide assemblage,co-genetic with pyrite, chalcopyrite and sphalerite. Galena-bearingveins (type III) contain up to more than 6 wt.% Pb, in the bulk ore,suggesting crystallization from hydrothermal solutions enriched in Pbcompared with basalt-hosted veins of the Internal Ligurides. Type IIIveins coexist with many others characterized by high Pb, but withoutgalena (type II), and a few characterized by Pb-depleted composition(type I), thus indicating a chemical change in the hydrothermalsolutions of Boccassuolo. Relative timing of vein emplacement is

Fig. 7. Composition of chlorite from galena-rich quartz veins of Boccassuolo. A) Ternaryplot of Mg, Fe, and AlVI in the octahedral site. B) Reciprocal variation of AlIV in thetetrahedral site and Fe/(Fe+Mg). Open square: normal veins. Filled square: galena-bearing veins. Grey field: chlorite from basalt-hosted veins of the Northern ApennineVMS deposits according to Zaccarini and Garuti (2008).

Fig. 8. Primitive-mantle normalized diagrams for Pb, Ag, Au, Zn and Cu in different typesof hydrothermal veins from Boccassuolo. A) type I: samples BO33, BO 3-4-5, BO6;B) type II: samples BO11-12-14, BO28, BO38, BO35-36, BO97, BO98; C) type III: samplesBO91, BO92, BO93. Normalized patterns for the Internal Ligurides basalt-hosted veins isafter Zaccarini and Garuti (2008). Values for Primitive-mantle, N-MORB and bulkContinental crust are reported in Table 7 after Barrie and Hannington (1999).

251G. Garuti et al. / Lithos 124 (2011) 243–254

difficult to be established, in the absence of absolute dating. However,veins of type II and III appear to be less deformed and disrupted andpossibly reflect lower temperature of crystallization compared withtype I. This is not to imply a substantial difference in age (i.e.Cretaceous vs. Upper Jurassic), but may simply reflect solubility-temperature controlled fractionation of metals from a hydrothermalsolutionmoderately enriched in Pb. Under the circumstance, the mostsoluble Pb precipitates in the last stages after deposition of substantialpyrite, chalcopyrite and sphalerite. On the other hand, chemicalchanges in the hydrothermal solution might have been due toleaching of basalts with Pb-depleted or un-depleted composition,thus hinting that there were more than one hydrothermal episode atBoccassuolo.

5.2. Geodynamic significance of sulfide metal assemblages in VMSdeposits

The classification schemes proposed for the VMS deposits are basedon three major criteria: i) the ore composition in terms of Cu–Zn–Pbreciprocal proportion, ii) the host–rock lithology, and iii) the inferredtectonic setting (see a summary of the classification criteria in Misra,2000). Althoughwith some differences, the three schemes give broadlycoincident results.

Using the criterion of host–rock lithology, Barrie and Hannington(1999) have classified 878 VMS deposits worldwide into five majorcategories (Table 6). Three types (Mafic, Bimodal–Mafic, Mafic–Siliciclastic) are characterized by predominance of mafic volcanicrocks (mainly basaltic tholeiites), sometimes associated with mafic–

ultramafic intrusives. The others (Bimodal–Felsic, Bimodal–Siliciclastic)generally have predominance of felsic volcanic rocks (high-silica,transitional, and calc-alkalic rhyolites) in the host rock sequence. Thesiliciclastic component generally consists of turbiditic sediments withminor carbonate. The five categories correspond to a spectrum oftectonic settings that have taken place during geodynamic evolution ofthe Earth's crust from the Archean to present. They show mantlenormalized Pb–Ag–Au–Zn–Cupatterns varying fromnearly flat inMafictype VMS deposits formed at oceanic ridges and nascent arcs, tonegative sloping in Bimodal–Felsic and Bimodal–Siliciclastic typedeposits related with rifted continental margins and continental arcs.The global variation from the Mafic to the Felsic types involvingprogressive increase in “incompatible” metals (Pb) is interpreted as aneffect of the increasing influence of the sub-continental crust in themagmatic processes (Barrie and Hannington, 1999).

Prototypical composition of VMS deposits plotted in the Cu–Zn–Pbtriangle (Fig. 9) split into three groups: Cu–Zn VMS deposits of Mafictype (Cyprus); Cu–Zn–Pb VMS deposits of Bimodal–Mafic and Mafic–

Table 6Classification of VMS deposits based on host–rock composition (Barrie and Hannington, 1999).

VMS Type Host–rock association Geodynamic setting Ancient and modern examples

Mafic Tholeiitic basalts±ultramafics±siliciclastites. MOR, BAR, SSZ-nascent arc Phanerozoic ophiolites: Cyprus, Oman, Ergani, Urals.Cu-Zn Pre-Phanerozoic mafic-ultramafic complexes: Outukumpu, Abitibi

province, Coronation.Bimodal Mafic Tholeiitic basalts/rhyolites =N 3/1. Primitive volcanic arc. Archean-E.Proterozoic: Noranda, Mattabi, Mattagami, Flin Flon.Cu–Zn Rifted primitive volcanic arc.Mafic Siliciclastic Tholeiitic basalts+siliciclastites±carbonates. Rifted continental margin,

sedimented oceanic ridge.M.Proterozoic: Besshi, Windy Craggy.

Cu–Zn–(Pb) Modern: Guaymas, Middle Valley, Escabana trough, Red Sea.Bimodal Felsic Rhyolite/tholeiitic basaltN50%±siliciclastites

(b15%).Mature volcanic arc. Cenozoic: Kuroko.

Cu–Zn–Pb Rifted volcanic arc. E.Phanerozoic: Mt. Read.Archean: Izok Lake.

Bimodal Siliciclastic Rhyolite/tholeiitic basaltN50% Continental arc. Paleozoic Iberian Pyrite Belt:Zn–Pb–Cu Volcanics=siliciclastites. Rifted continental arc. Neves Corvo, Rio Tinto, La Zarza.

MOR: mid oceanic ridge; BAR: back arc rift; SSZ: supra-subduction zone.

252 G. Garuti et al. / Lithos 124 (2011) 243–254

Siliciclastic types (Noranda and Besshi); Zn–Pb–Cu VMS deposits ofBimodal–Felsic and Bimodal–Siliciclastic types (Kuroko and IberianPyrite Belt). Data for the VMS deposits of the Northern Apennineophiolites (Table 7) plot in the Cu–Zn rich field typical of ophioliticVMS deposits (Cyprus type), however, the galena-bearing samplesfrom Boccassuolo enter the Cu–Zn–Pb field typical of Bimodal–maficand Bimodal–felsic VMS type deposits.

The average of the mantle-normalized patterns for VMS deposits(excluding the Pb-rich veins) in the Internal and External Ligurides israther flat as expected for VMS deposits associated with predominantmafic rocks (Fig. 10A). However, a slight, but significant enrichment inPb and Zn, and a higher normalized Pb/Cu ratio is seen in the ores ofthe External Ligurides with respect to the Internal–Liguride ones. Theaverage patterns from the Boccassuolo veins are comparedwithmetalpatterns of the most primitive and the most evolved VMS types(Fig. 10B, C). As observed before, the average of type I and II broadlyreflects N-MORB pattern similar to deposits of the Mafic, BimodalMafic and Siliclastic type, whereas the type III is more consistent withVMS deposits with strong Pb enrichment, similar to the averageContinental Crust.

Fig. 9. Triangular plot of Cu–Zn–Pb for the hydrothermal veins of Boccassuolo (filledsquare) and VMS deposits of the Northern Apennine ophiolites (open square) afterZaccarini and Garuti (2008). Dark grey: field for Mafic-type VMS deposits (Cyprus)associated with ophiolites; Middle grey: field for Bimodal–Mafic and Mafi–Siliciclastictypes VMS deposits (Noranda and Besshi); Light grey: field for Bimodal–Felsic andBimodal–Siliciclastic types VMS deposits (Kuroko and Iberian Pyrite Belt). Fields areredrawn after Franklin (1993), Galley and Koski (1999) and Barrie and Hannington(1999). See Table 7 for a summary of used data.

5.3. Paleo-geographic significance of the Boccassuolo hydrothermal veins

Ophiolites in the External Liguride are believed to have formed in apre-collisional stage (Marroni et al., 2001; Piccardo et al., 2002).Geochemical anomalies suggest that basalts, such as those describedfrom the Modena province (including Boccassuolo) were notextruded at ocean ridge segments, neither correspond to suprasub-duction-zone magmatism. Proposed geodynamic settings indicateregions approaching to ocean–continent transition zones proximal tothe Adria continent (Marroni et al., 1998, 2002; Montanini et al.,2008).

Sulfide veins from Boccassuolo have S-isotope characters consis-tent with sub-seafloor stockwork deposits hosted in basalt of theInternal Ligurides (Table 2). The data support the conclusion thatsulfur was predominantly contributed by chemically modifiedseawater during convective circulation in the seafloor substrate(Garuti et al., 2009), while the required heath source was probablyprovided by diffuse uprising of hot basaltic magma (Garuti andZaccarini, 2005; Garuti et al., 2008; Zaccarini and Garuti, 2008). In theInternal Ligurides, hydrothermal activity lasted after basalt outflow,and led to deposition of sulfides at the top of the volcanic pile(Ferrario and Garuti, 1980; Garuti and Zaccarini, 2005), andmanganese ores in the overlaying cherts (Bonatti et al, 1976; Cabellaet al., 1998). The lack of VMS-type deposits in sediments overlayingthe cherts indicates that the hydrothermal cycle ceased beforesedimentation of Palombini shale and Calpionella limestone. Similar-ly, structural relations of the Boccassuolo stockwork veins supportthat the hydrothermal activity came to an end before the deposition ofBarremian–Albian (?) sediments. In summary, structural, geochem-ical and isotopic data indicate that the hydrothermal veins ofBoccassuolo formed in a sub-seafloor hydrothermal convectivesystem, during the outflow of pillow lava accompanied by repeatedaccumulation of ophiolitic debris, possibly in the Upper Jurassic (Plesiet al., 2002).

At Boccassuolo, only a small group of ore samples display mantle-normalized pattern (type I) similar to that of basalt-hosted veins inthe Internal Ligurides (Fig. 8A). This pattern broadly reflect the N-MORB metal distribution and resembles that of Mafic-type VMSdeposits, characterized by low Pb/Cu normalized ratios (Barrie andHannington, 1999). Type II veins are the most common. They stilldisplay N-MORB pattern, but a substantial enrichment in Pb is clearlyobserved. Type III metal pattern characterizes the galena-rich veins,with remarkable Pb-enrichment and anomalous increase of the Pb/Cunormalized ratio. One sample even reflects the Continental crustmetal pattern that is typical of VMS deposits of the Bimodal Felsic andBimodal Siliciclastic types (Kuroko, Iberian Pyrite Belt). The ExternalLigurides lack the rhyolitic volcanism typical of the Bimodal Felsic andBimodal Siliciclastic, therefore they must reflect a different geo-

Fig. 10. Average primitive-mantle normalized diagrams for Pb, Ag, Au, Zn and Cu in theBoccassuolo veins types 1–2 and 3 compared with VMS deposits of the NorthernApennine and different types of VMS deposits worldwide. N-MORB and bulkContinental crust (after Barrie and Hannington, 1999). A) I-L: average of Internal–Liguride stratiform, stratabound and stockwork VMS deposits (39 samples); E–L:average of External–Ligurides stratiform and stockwork VMS deposits (29 samples, Pb-rich veins excluded); B) BO1-2: average of Boccassuolo veins type 1 and 2, C) BO:average of galena-bearing veins of Boccassuolo, N: Noranda, C: Cyprus, B: Besshi; IPB:Iberian Pyrite Belt, K: Kuroko. Data source: Zaccarini and Garuti (2008), Barrie andHannington (1999), Galley and Koski (1999), Misra (2000), this work.

253G. Garuti et al. / Lithos 124 (2011) 243–254

dynamic setting of emplacement. We may also exclude similaritieswith the hydrothermal veins in the Chilean Ridge, where over-leaching of a MORB-type source has been invoked to explain Pbenrichment (Mühe et al., 1977).

The increasing Pb content in the veins of Boccassuolo has to berelated with the leaching of substrate volcanic rocks, the compositionof which varies from N-MORB to enriched basalts (transitional?). Wedo not know if this happened in one or more hydrothermal episodes,certainly it indicates a geochemical evolution of the hydrothermalactivity with respect to basalt-hosted veins of the Internal Ligurides.In agreement with the proposed geo-dynamic setting of the ExternalLiguride ophiolites, we suggest that the Pb-rich veins of Boccassuolowere probably emplaced in the ocean–continent transition zone,possibly reflecting increasing influence of the sub-continental crust inthe magmatic processes (Barrie and Hannington, 1999).

Acknowledgements

The University Centrum of Applied Geosciences of Austria (UCAG)is acknowledged for making available the facilities of the “Eugen F.Stumpfl” microprobe laboratory. Thanks are due to A. Rossi (Univer-sity of Modena) for having made available original unpublishedmapping of the Boccassuolo ophiolite. R. Baccarani is gratefullythanked for his help during exploration and sampling of theabandoned underground mines. Preliminary information on miner-alogy of clay materials has been kindly provided by M. Setti(University of Pavia) and A. Lopez-Galindo (University of Granada).We are grateful to two unknown referees and the guest editorialboard, for their constructive comments and criticisms that havesubstantially contributed to improve the original manuscript.

This paper is dedicated to the memory of Mario Bertolani, to recallthe relevance of his pioneer work in reflected-light microscopicanalysis of the sulfide mineralogy at Boccassuolo, and several otherophiolites of Liguria and Emilia Romagna.

References

Abbate, E., Bortolotti, V., Principi, G., 1980. Apennine ophiolites: a peculiar oceanic crust.In: Rocci, G. (Ed.), Tethyan ophiolites, Ofioliti Special Issue, 1, pp. 59–96.

Barrett, T.J., 1982. Review of stratigraphic aspects of the ophiolitic rocks and pelagicsediments of the Vara complex, North Apennines, Italy. Ofioliti 7 (1), 3–46.

Barrie, C.T., Hannington, M.D., 1999. Classification of Volcanic-Associated MassiveSulfide deposits based on host–rock composition. In: Barrie, C.T., Hannington, M.D.(Eds.), Volcanic-associated massive sulfide deposits: processes and examples inmodern and ancient settings: Review in Economic Geology, 8, pp. 1–11.

Bertolani, M., 1952. I giacimenti cupriferi nelle ofioliti di Sestri Levante. Periodico diMineralogia XXI 2/3 (3), 149–170.

Bertolani, M., 1953. I giacimenti cupriferi dell'Appennino Modenese. Ricerchemicroscopiche a luce riflessa. Atti della Societá dei Naturalisti e Matematici diModena 82, 3–10.

Bertolani, M., 1959. Ricerche sulle rocce prasinitiche e anfibolitiche e sul giacimentometallifero di Vigonzano Appennino piacentino. Atti della Societá dei Naturalisti eMatematici di Modena 89 (90), 3–31.

Bertolani, M., 1962. Linneite, calcopirrotina e strutture blenda-pirrotina nelle miner-alizzazioni metallifere del giacimento di Corchia Appennino Parmense. Periodico diMineralogia XXXI 2/3 (3), 287–301.

Bertolani, M., Capedri, S., 1966. Le ofioliti delle province di Modena e Reggio Emilia. Attidella Societá dei Naturalisti e Matematici di Modena 97, 3–52.

Bonatti, E., Zerbi, M., Kay, R., Rydell, H., 1976. Metalliferous deposits from the Apennineophiolites: Mesozoic equivalents of modern deposits from oceanic spreadingcenters. Bulletin of the Geological Society of America 87, 83–94.

Brigo, L., Ferrario, A., 1974. Le mineralizzazioni nelle ofioliti della Liguria orientale.Rendiconti della Societá Italiana di Mineralogia e Pettrografia 30, 305–316.

Cabella, R., Lucchetti, G., Marescotti, P., 1998. Mn-ores from Eastern Ligurian ophioliticsequences (“Diaspri di monte Alpe” formaion, northern apennines, Italy). Trends inMineralogy 2, 1–17.

Capedri, S., Toscani, L., 2000. Subduction-related ? ophiolitic metabasalts fromNorthernApennines Modena province, Italy. Chemie der Erde 60, 111–128.

Crawford, A.J., Falloon, T.J., Green, D.H., 1989. Classification, petrogenesis andtectonic setting of boninites. In: Crawford, A.J. (Ed.), boninites. Unwin Hyman,London, pp. 1–49.

Elter, P., 1975. L'ensemble ligure. Bulletin de la Société Géologique Française 17,984–997.

Ferrario, A., Garuti, G., 1980. Copper deposits in the basal breccias and volcano-sedimentary sequences of the Eastern Ligurian ophiolites (Italy). MineraliumDeposita 15, 291–303.

Franklin, J.M., 1993. Volcanic-associated massive sulfide deposits. Geological Associa-tion of Canada Special Paper 40, 315–334.

Galley, A.G., Koski, R.A., 1999. Setting and characteristic of ophiolite-hosted volcano-genic massive sulfide deposits. In: Barrie, C.T., Hannington, M.D. (Eds.), Review inEconomic Geology, 8, pp. 221–246.

Garuti, G., Zaccarini, F., 2005. Minerals of Au, Ag, and U in volcanic-rock-associatedmassive sulfide deposits of the Northern Apennine ophiolite, Italy. The CanadianMineralogist 43, 935–950.

Garuti, G., Bartoli, O., Scacchetti, M., Zaccarini, F., 2008. Geological setting and structuralstyles of Volcanic Massive Sulfide deposits in the Northern Apennines (Italy):evidence for seafloor and sub-seafloor hydrothermal activity in unconventionalophiolites of the Mesozoic Tethys. Boletin de la Sociedad Geologica Mexicana 60/1,121–145.

Garuti, G., Alfonso, P., Zaccarini, F., Proenza, J.A., 2009. Sulfur-isotope variations insulfide minerals from massive sulfide deposits of the Northern Apennineophiolites: inorganic and biogenic constraints. Ofioliti 34, 43–62.

Hofmann, A.W., 1988. Chemical differentiation of the Earth: the relationship betweenmantle, continental crust, and oceanic crust. Earth Planetary Science Letter 90,297–314.

254 G. Garuti et al. / Lithos 124 (2011) 243–254

Large, R.R., 1992. Australian volcanic-hosted massive sulfide deposits: features, stylesand genetic models. Economic Geology 87, 471–510.

Lydon, J.W., 1988. Volcanogenic massive sulphide deposits. 2. Genetic models.Geoscience Canada 15, 43–65.

Maggi, R., Pearce,M., 2005.Mid fourth-millennium coppermining in Liguria, north-westItaly: the earliest known copper mines in Western Europe. Antiquity 79, 66–77.

Marroni, M., Molli, G., Montanini, A., Tribuzio, R., 1998. The association of continentalcrust rocks with ophiolites (Northern Apennine, Italy): implications for thecontinent-ocean transition. Tectonophysics 292, 43–66.

Marroni, M., Molli, G., Ottria, G., Pandolfi, L., 2001. Tectono-sedimentary evolution of theExternal Liguride units (Northern Apennine, Italy): insights in the pre-collisionalhistory of a fossil ocean-continent transition zone. Geodinamica Acta 14, 307–320.

Marroni, M., Molli, G., Montanini, A., Ottria, G., Pandolfi, L., Tribuzio, R., 2002. TheExternal Ligurian units (Northern Apennine, Italy): from rifting to convergence of afossil ocean-continent transition zone. Ofioliti 27/2, 119–131.

Misra, K.C., 2000. Understanding mineral deposits. Kluwer Academic Publishers,Dordrecht, Boston, London. pp. 845.

Montanini, A., Tribuzio, R., Vernia, L., 2008. Petrogenesis of basalts and gabbros from anancient continent-ocean transition zone (External Liguride ophiolites, NorthernItaly). Lithos 101, 453–479.

Mühe, R., Peucker-Ehrenbrick, B., Devey, C.W., Garbe-Schönberg, D., 1977. On theredistribution of Pb in the oceanic crust during hydrothermal alteration. ChemicalGeology 137, 67–77.

Ottonello, G., Joron, J.L., Piccardo, G.B., 1984. Rare Earth and 3D transition elementgeochemistry of peridotitic rocks: II. Ligurian peridotites and associated basalts.Journal of Petrology 25, 373–393.

Pearce, M., 2007. Bright Blades and Red Metal, essays on north Italian prehistoricmetalwork. In: Herring, E., Whitehouse, R.D., Wilkins, J.B. (Eds.), Accordia SpecialistStudies on Italy, 14. Accordia Reserches Institute, University of London, pp. 53–144.

Piccardo, G.B., Rampone, E., Romairone, A., 2002. Formation and composition of theoceanic lithosphere of the Ligurian Tethys: inferences from the Ligurian ophiolites.Ofioliti 27 (2), 145–161.

Plesi, G., Daniele, G., Chicchi, S., Bettelli, G., Catanzariti, R., Feroni, C., De Nardo, M.T.,Martinelli, P., Ottria, G., Panini, F., 2002. Note illustrative della Carta Geologicad'Italia alla scala 1:50.000, foglio 235 Pievepelago. Presidenza del Consiglio deiMinistri, Servizio Geologico d'Italia, Regione Emilia Rmagna. pp. 138.

Rampone, E., Piccardo, G.B., 2000. The ophiolite-oceanic lithosphere analogue: newinsight from the Northern Apennine. In: Dilek, J., Moores, E., Elthon, D.,Nicolas, A. (Eds.), Ophiolites and oceanic crust: new insights from field studiesand Ocean Drilling Program, Boulder Colorado: Geol. Soc. Am. Spec. Paper, 349,pp. 21–34.

Rampone, E., Hofmann, A.W., Piccardo, G.B., Vannucci, R., Bottazzi, P., Ottolini, L., 1995.Petrology, mineral and isotope geochemistry of the External Ligurides peridotites(Northern Apennine, Italy). Journal of Petrology 36, 81–105.

Reed, M.H., 1983. Seawater-basalt reaction and origin of greenstones and related oredeposits. Economic Geology 78, 466–485.

Robertson, A.H.F., 2002. Overview of the genesis and emplacement of Mesozoicophiolites in the Eastern Mediterranean Tethyan region. Lithos 65, 1–67.

Tribuzio, R., Tiepolo, M., Vannucci, R., 2000. Evolution of gabbroic rocks from theNorthern Apennine ophiolites (Italy): Comparison with the lower oceanic crustfrom modern slow-spreading ridges. In: Dick, J., Moores, E., Elthon, D., Nicolas, A.(Eds.), Ophiolites and oceanic crust: New insights from field studies and OceanDrilling Program: Boulder Colorado: Geol. Soc. Am. Spec. Paper, 349, pp. 129–138.

Venturelli, G., Thorpe, R.S., Potts, P.J., 1981. Rare Earth and trace element characteristicsof ophiolitic metabasalts from the Alpine-Apennine belt. Earth Planetary ScienceLetters 53, 109–123.

Wedepohl, K.H., 1991. Chemical composition and fractionation of the continental crust.Geologische Rundschau 80 (2), 207–223.

Zaccarini, F., Garuti, G., 2008. Mineralogy and composition of VMS deposits of NorthernApennine ophiolites, Italy: evidence for the influence of country rock type on orecomposition. Mineralogy and Petrology 94, 61–83.

Zaccarini, F., Morales-Ruano, S., Scacchetti, M., Garuti, G., Heide, K., 2008. Investigationof datolite (CaB[SiO4/(OH)]) from basalts in the Northern Apennines ophiolites(Italy): Genetic implications. Chemie der Erde 68, 265–277.