(Miniera di Prabornaz, Saint-Marcel, Valle d'Aosta, Italia)
The manganese concentrations of Praborna are restricted to a 4x8 m thick quartzite unit of diverse lithologies which represent the basal part of the sedimentary cover of an ophiolite suite. Together they constituted the Piedmont Nappe which was metamorphosed up to the eclogite facies in Cretaceous times and was retrogressed in the early Tertiary.
The quartzite unit is composed of a succession of siliceous levels characterized by Mn-bearing minerals reflecting differences in chemical composition and oxygen fugacity inherited from the original sediments. The parageneses evolved during the polyphasic Alpine metamorphism passing through a pre-eclogite blue-schist facies event, the main eclogite facies event, and the several post-eclogite stages terminating in the greenschist facies.
These Mn concentrations were subsequently penetrated by several networks of cross-cutting veins and late fractures which were responsible for the mobilization of certain elements, in particular K, Sr and Ba.
References:
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