SR-ISOTOPE COMPOSITION OF BARITE VEINS AT POLLONE DEPOSIT, APUANE ALPS, TUSCANY (ITALY)

The barite (+/- Pb, Zn, Ag) deposit of Pollone is hosted within a sili ciclastic formation of pre-Norian age, belonging to the Apuane Alps (A A) basement, metamorphosed during the Apenninic orogeny (P = 3-4 Kbar; T = 350-450 degrees C). The deposit consist of both stratiform and ve in orebodies. The former are of possible syngenetic origin, whereas th e latter are ascribed to syn-metamorphic remobilization of pre-existin g anomalies. In particular, a syn-metamorphic shear-zone, extensively affecting the northern sector of the deposit, favored intense circulat ion and focusing of fluids, leading to the emplacement of a number of metamorphogenic barite +/- polymetallic sulfide orebodies. The geochem ical 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 Sr-87-deple ted with respect to epigenetic barites, indicating that they did not a ct as a Sr-isotope buffer for syn-metamorphic mineralizing fluids. In addition, the Sr-isotope composition of host rocks does not reflect eq uilibrium with adjoining vein bodies. These results indicate that (1) interaction between mineralizing fluids, syngenetic barite and host ro cks was rather scarce, reflecting a channeled rather than pervasive fl uid flow; (2) the source of radiogenic Sr was not represented by Pollo ne host rocks, but possibly by basement rocks located at depth below t he barite orebodies. This conclusion is in accordance with previous da ta on Rb and Sr geochemistry.