Fluid inclusion evidence for P-V-T-X evolution of hydrothermal solutions in late-Alpine gold-quartz veins at Brusson, Val d'Ayas, northwest Italian Alps

Abstract

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