An International Journal of MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage DOI: 10.2451/2014PM0019 Periodico di Mineralogia (2014), 83, 3, 345-365 PERIODICO di MINERALOGIA established in 1930 An International Journal of MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage The effect of Alpine metamorphism on an oceanic Cu-Fe sulfide ore: the Herin deposit, Western Alps, Italy Irene Fantone 1,* , Giovanni Grieco 1 , Andrea Strini 2 and Alessandro Cavallo 3 1 Università degli Studi di Milano, Dip. di Scienze della Terra - Via S. Botticelli, 23 - 20133 Milano, Italy 2 Studio Geosfera - Via Rossini, 18-21100 Varese, Italy 3 Università degli Studi di Milano Bicocca - Dip. di Scienze Ambiente e Territorio e Scienze della Terra Piazza della Scienza, 1 - 20126 Milano, Italy * Corresponding author: irene.fantone@unimi.it Abstract Herin mine (Champdepraz, Aosta, Italy), located in Aosta Valley, approximately between 1600 and 1800 m a.s.l., was exploited for at least 250 years for its Cu-Fe sulfide ore. The deposit host rocks belong to the metaophiolitic Zermatt- Saas unit, the eclogitic lower portion of the Piedmont Western Alpine Nappe. The ore mineral association mainly comprises pyrite and chalcopyrite, along with other sulfides such as pyrrhotite, sphalerite, cubanite and oxides (magnetite, rutile, ilmenite). The deposit occurs within lenticular massive bodies and thin layers hosted in various greenschist-facies metamorphosed lithotypes. New data on geometric features, mineralogy, mineral chemistry, petrography, minerography were collected and compared with the existing models for massive-sulfide mineral deposits. We suggest a hydrothermal-volcanogenic primary origin of the mineralization with original characteristics largely obliterated by subsequent metamorphic history. On the basis of studies and our results, we identified two parameters as driving criteria for a comprehension of the multistage process that led to the present configuration of the ore: (a) textural characters of pyrite and (b) distribution of selected trace elements (Co, Ni, As) in sulfides. Spot analyses and atomic maps obtained by electron microprobe provided an integration of these two sets of data. Trace elements, in fact, show a zoned distribution, in particular in pyrite, that can be related to specific textural styles. We selected cobalt as a useful trace element, due to its high concentration and wide range in pyrite (270 -22200 ppm). We determined a critical concentration value for cobalt at 3160 ppm, useful as a discriminate between two generations of pyrite. This led to the delineation of a series of dissolution and crystallization events that describe the metamorphic history of the sulfide ore. Key words: pyrite; trace elements; massive sulfides; Western Alps; EMPA; metamorphism.