FLUID INCLUSION EVIDENCE FOR P-V-T-X EVOLUTION OF HYDROTHERMAL SOLUTIONS IN LATE-ALPINE GOLD-QUARTZ VEINS AT BRUSSON, VAL-DAYAS, NORTHWEST ITALIAN ALPS

The auriferous veins at Brusson formed in active normal faults and dilatent joints during uplift and cooling of the Western Alps approx 32 Ma ago, some 4 to 8 Ma after the peak of local greenschist-facies metamorphism. Fluid inclusion petrography, microthermometry, laser Raman spectroscopy, and crush-leach analyses show that the main-stage vein minerals, precipitated from a homogeneous fluid composed of (in mol percent) 90.9 H2O, 5.9 CO2, 1.47 Cl, 1.47 Na, 0.15 N2 , 0.06 K, 0.02 S, 0.02 Ca, 0.003 Mg, 0.002 CH4. Thermodynamic calculations and fluid inclusion data indicate that these minerals precipitated between 300°C, 1300 bars and 240°C, 650 bars, in response to fluid-wallrock reactions and to P-T decreases imposed by regional uplift. The latest generation of fluid inclusions is made up of low-XCO2 and high-XCO2 assemblages that satisfy requirements of immiscibility in the system H2O-CO2-N2-CH4-NaCl. -from Author