Sr-isotope composition of barite veins at Pollone deposit, Apuane Alps, Tuscany.

The barite (Pb,Zn,Ag) deposit of Pollone is hosted within a siliciclastic formation of preNorian age, belonging to the Apuane Alps basement, metamorphosed during the Apenninic orogeny (P=3-4 Kbar; T= 350-450°C). The deposit consists of both stratiform and vein orebodies. The former are of possible syngenetic origin, whereas the latter are ascribed to syn-metamorphic remobilization of pre-existing anomalies. In particular. a syn-metamorphic shear-zone extensively affecting the northern sector of the deposit, favored intense circulation and focusing of fluids, leading to the emplacement of a number of metamorphogenic barite ± polymetallic sulfide orebodies. The geochemical features of stratiform and vein orebodies and of their host rocks have been used to constrain the hydrology and possible provenance of mineralizing fluids. Syngenetic barites are systematically Sr87-depleted with respect to epigenetic barites, indicating that they did not act as a Sr-isotope buffer for syn-metamorphic mineralizing fluids. In addition. the Sr-isotope composition of host rocks does not reflect equilibrium with adjoining vein bodies. These results indicate that (I) interaction between mineralizing fluids, syngenetic barite and host rock was rather scarce, reflecting a channeled rather than pervasive fluid flow; (2) the source of radiogenic Sr was not represented by Pollone host rocks, but possibly by basement rocks located at depth below the barite orebodies. This conclusion is in accordance with previous data on Rb and Sr geochemistry.