P. Omenetto’s research while affiliated with University of Padua and other places

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.

The distribution of ore-deposit types in different sectors of the circum-Mediterranean realm that have been affected by subduction processes since the Cretaceous varies in space and time. Sectors involved in W-directed subduction (Sardinia, Apennines–Maghrebides, Internal Betics, Tyrrhenian, Western-Eastern Carpathians) are dominated by relatively low-sulphidation epithermal (±VMS) deposits. Orogens formed by NE-directed subduction (Dinarides–Hellenides–Pontides–Anatolides–Taurides; DHPAT) were initially dominated by pluton-related porphyry–skarn–high-sulphidation epithermal associations. These distinct metallogenic styles can be related to the systematic tectono-magmatic asymmetry of E-NE- and W-directed subduction systems and are analogous to the relationship observed in circum-Pacific belts. Exceptions to this simple pattern occurred in the DHPAT in the Cenozoic, when deposit associations typical of both E-directed and W-directed systems were formed. Such exceptions are interpreted to reflect superimposition of contrasting subduction trends and inheritance from earlier metallogenic stages (Apuseni) or the interference of subduction processes with subduction-unrelated extension (Hellenides, West Anatolia).This article is protected by copyright. All rights reserved.

The isotopic compositions of lead in copper-bearing hydrothermal sulphide deposits from the central-eastern Southalpine domain were analysed using a Multi-Collector-ICP-MS. The data were combined with existing lead isotope data (ore lead) for hydrothermal polymetallic deposits from the same area and compared with the isotopic compositions of potential lead sources. Copper and polymetallic pre-Variscan (Late Ordovician–Early Silurian) stratiform, post-Variscan (Permian to Triassic) vein, and stratabound sediment-hosted (Late Permian to Early Triassic) deposits, are characterised by highly variable ratios of radiogenic to non-radiogenic lead, but show very similar, high, time-integrated mu (=238U/204Pb) and W (=232Th/204Pb) values. A progressive relative increase in radiogenic lead is observed from (i) pre-Variscan deposits to (ii) post-Variscan sulphide-rich veins in the Variscan metamorphic basement and in the lower units of the Early Permian volcanic sequence, to (iii) post-Variscan sulphide-rich and fluorite-rich veins in the upper units of the Early Permian volcanic sequence, to (iv) post-Variscan fluorite-rich veins cutting the overlying Late Permian sediments and mid-Triassic mafic dikes. The dominant lead sources for all these deposits were Cambrian–Devonian (meta) sediments of the Variscan basement. Contributions from Permian and Triassic igneous rocks were of minor importance, if any, even for vein deposits which were evidently related to Permian magmatism. The isotopic compositions of some of the Permian vein deposits are consistent with, although they do not unequivocally prove, remobilization of metals from the pre-Variscan stratiform deposits. Stratabound deposits in the Late Permian sandstones and, possibly, those in the Early Triassic carbonates also received a major lead input from the Variscan metasediments, with a variable additional contribution from the host Permian sediments. Deposits in Triassic magmatic rocks are displaced to slightly lower m and W values, suggesting lead contribution from Triassic magmatism. The high m and W values of the deposits studied here are consistent with regional isotopic patterns of Pb–Zn-rich deposits in more northerly and easterly sectors of the Eastern Alps (Austroalpine, eastern Southalpine) and of several circum-Mediterranean Pb–Zn and polymetallic deposits of Paleozoic to Triassic age (Sardinia, Betic Cordillera) or derived from remobilisation of Paleozoic deposits (Tuscany). This isotopic uniformity suggests that an isotopic province characterized by the dominance of old (Early Proterozoic to Archean) detrital source material extended across a relatively wide portion of the former north-Gondwanan margin.

Ultramafic–mafic- and ultramafic-hosted Cu (Co, Ni, Au) volcanogenic massive sulfide (VMS) deposits from ophiolite complexes of the Main Uralian Fault, Southern Urals, are associated with island arc-type igneous rocks. Trace element analyses show that these rocks are geochemically analogous to Early Devonian boninitic and island arc tholeiitic rocks found at the base of the adjacent Magnitogorsk volcanic arc system, while they are distinguished both from earlier, pre-subduction volcanic rocks and from later volcanic products that were erupted in progressively more internal arc settings. The correlation between the sulfide host-rocks and the earliest volcanic units of the Magnitogorsk arc suggests a connection between VMS formation and infant subduction-driven intraoceanic magmatism.

The provenance of ore minerals used in prehistoric and historic times for copper smelting and extraction is one of the basic questions that archaeologists pose to modern analytical archaeometry [1]. To aid metal provenancing studies, a database of fully characterized Alpine copper mineralisations is being developed as the fundamental reference frame for metal extraction and diffusion in the past. In the early stages of the project, some of the most well known copper deposits in the Western Alps were selected and compared with very different minerogenetic deposits from the French Queyras (Saint Veran) and the Ligurian Apennines (Libiola, Monte Loreto). The fully characterized samples were then analysed by ICP-QMS (inductively coupled plasma-quadrupolar mass spectrometry). The abundances of about 60 minor and trace elements, including most transition metals and chalcophile elements, and the rare earths were measured in all samples. Furthermore, the feasibility of the routine reliable measurement of the 65Cu/63Cu isotope ratio [2] and its eventual use as a possible ore tracer was tested. Multicollector ICP-Mass Spectrometry was used to determine precise Pb isotopic ratios (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) and is being used for 65Cu/63Cu ratios as well. Advanced strategies based on multivariate analysis were then used to discriminate the ore mineral provenance. Data were treated with the chemometric software "The Unscrambler Version 9.5" (CAMO AS, Trondheim, Norway). Data pre-treatment, PCA [3] and PLS-DA [4,5] models were performed as implemented in the software. The availability of such unprecedented and complete amount of data of Alpine copper deposits also yields information relevant for the geochemical and minerogenetic intepretation of the deposits themselves. Application of PCA and PLS-DA to the geochemical and isotopic database proved to be a very powerful tool to discriminate the ore source areas with very little ambiguity. The applications to archaeometallurgical copper specimens from the Agordo area (Veneto) and the recently found prehistoric slags from Millan (Alto Adige) indicate that the approach is successful in provenance and trade route investigations. Future efforts are directed towards (1) completion of the mine database, (2) investigation of archaeological copper slags, (3) deeper interpretation of the geochemical tracers and their behaviour during the smelting processes. References. [1] Renfrew, C., Bahn, P. (2000): Archaeology: Theories, methods and practice. Thames & Hudson, London; [2] Ciceri, E., Dossi, C., Recchia, S., Angelini, I., Artioli, G., Colpani, F. (2005): Problematiche connesse con la determinazione del rapporto isotopico 63Cu/65Cu mediante ICP-QMS. Atti del XIX Congresso di Chimica Analitica, 11-15 settembre 2005. Università degli Studi di Cagliari; [3] Wold, S., Esbensen, K., Geladi, P. (1987): Chemometrics Intell. Lab. Syst. 2, 37; [4] Esbensen K, 2002. Multivariate Data Analysis - In Practice. ISBN 82-993330-3-2, CAMO Process AS, Oslo, 5th Edition; [5] Geladi P, Kowalski BR (1986): Partial least-squares regression: a tutorial. Anal. Chim. Acta 185, 1-17

Some Cu-rich, mafic–ultramafic- and ultramafic-hosted massive sulfide deposits from the southern segment of the Main Uralian Fault Zone (Ivanovka and Ishkinino deposits, southern Urals) show unusual characteristics. Their major features include: (i) relatively high Co (Ni, Au), very low Zn and negligible Pb grades; (ii) a pyrrhotite-dominated mineralization, locally characterized by the presence of open-latticework aggregates of lamellar pyrrhotite with Mg-saponite ± Mg-chlorite and carbonate matrix; (iii) hydrothermal alteration of ultramafic host rocks into talc ± carbonate ± quartz ± chlorite and of mafic host rocks into chloritites; (iv) the presence of clastic facies with reworked sulfide and ultramafic or mafic components; (v) the widespread occurrence of sulfide-associated chromite; (vi) the specific mineralogy of Co, Ni, Fe and As, including sulfoarsenides, mono- and diarsenides, and Co-rich pentlandite and pyrite; (vii) the supra-subduction-zone geochemical signature of the host serpentinites and volcanic rocks. Although some of these features have been separately reported in certain modern ocean-seafloor and ophiolite-hosted fossil deposits, a true equivalent has yet to be found. Based on recognized partial analogies with a few modern seafloor examples, the arc tholeiitic–boninitic geochemical signature of sulfide-associated volcanic rocks and the highly refractory compositions of sulfide-hosted chromite relicts, the studied deposits are believed to have formed by seafloor–subseafloor hydrothermal processes in an oceanic island arc setting. Possible tectonostratigraphic correlation of sulfide-associated units with infant, non-accretionary arc volcanic units of the adjacent Magnitogorsk oceanic island-arc system suggests formation of the studied deposits during the earliest stages of Devonian subduction-related volcanism.

Abstract Tracing the provenance of ore minerals used in prehistoric and historic times for copper smelting and extraction is a very hot topic in modern archaeometallurgy. To aid metal provenancing studies, a database of fully characterized Alpine copper mineralisations is being developed as the fundamental reference frame for metal extraction and diffusion in the past. A preliminary account of the protocols and scopes guiding the database development is presented, together with the advanced strategies of multivariate chemometric techniques presently used to interrogate the database and discriminate the ore mineral provenance. It is prospected that the availability of such unprecedented and complete amount of data of Alpine copper deposits may also yield interesting information concerning the geochemical and minerogenetic intepreta- tion of the deposits themselves. Examples of archaeometallurgical applications are provided concerning cop- per metal provenance from the Agordo area, Veneto, and the recently found prehistoric slags from Milland/Millan, South Tyrol/Alto Adige.

We have studied textural relationships and compositions of phyllosilicate minerals in the mafic–ultramafic-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 seafloor–subseafloor environment, characterized by the presence of abundant mafic–ultramafic breccias of tectonic and/or sedimentary origin.