METAMORPHOGENIC BARITE-PYRITE (PB-ZN-AG) VEINS AT POLLONE, APUANE ALPS, TUSCANY - VEIN GEOMETRY, GEOTHERMOBAROMETRY, FLUID INCLUSIONS AND GEOCHEMISTRY

The barite-pyrite-(Pb-Zn-Ag) deposit of Pollone is located in the sout hernmost tip of the Apuane Alps metamorphic core complex, and is hoste d by a siliciclastic formation of pre-Norian age. The southern sector of the deposit mainly consists of stratiform, supposedly syngenetic, b arite-pyrite orebodies, whereas the northern area is characterized by a barite-pyrite-(Pb-Zn-Ag) vein system. Vein geometry in the northern area is controlled by a shear zone, developed during the greenschist f acies metamorphism which affected the Apuane Alps core complex between 27 and 8 Ma, that was responsible for fluid focusing and vein emplace ment. At Pollone, arsenopyrite and chlorite geothermometers show broad ly comparable results, and suggest local metamorphic peak temperatures between 320 and 350 degrees C. Phengite geobarometry indicates minimu m pressures of about 3.5 kbar. Fluid inclusion data and mineral equili bria suggest that the mineralizing fluids were initially hotter than t he country rocks (about 450 degrees C at 3.5-4.0 kbar). Rocks in direc t contact with the orebodies are depleted in Rb and enriched in Sr in comparison to similar rocks elsewhere in the area. This is attributed to the presence of Rb-poor muscovite and Sr-rich barite. Rb-depleted m uscovites suggest mineral-fluid interaction in a rock reservoir charac terized by a different (modal) mineralogical composition than the Poll one host rocks. The progressive decrease of Sr in barite with increasi ng distance from the orebodies may be explained with a temperature dec rease along the infiltration paths of mineralizing fluids (i.e., from the Vein into the wall rocks). The similar O-isotope composition of qu artz from veins and host rocks is explained with the overall homogeneo us O-isotope composition of the Alpi Apuane basement rocks. This indic ates a limited interaction between mineralizing fluids and the rocks e xposed at Pollone. Remobilization of syngenetic orebodies was conceiva bly of minor importance in the production of metamorphogenic veins. Fl uid cooling along a major tectonic lineament is thought to be responsi ble for barite deposition.