Fluid Sources at the Panasqueira Tungsten-Vein Deposit

Panasqueira is a world-class tungsten-vein deposit. Several paragenetic stages have been proposed (Polya et al., 2000) including two pre-ore stages (crack-seal quartz-seam, and muscovite selvages) and four ore stages (main oxide-silicate stage, main sulfide stage, pyrrhotite alteration stage, and late carbonate stage). In this study, compositions of the mineralizing fluids at Panasqueira have been determined by a combination of detailed petrography, microthermometric measurements and LA-ICPMS analyses. We have characterized the fluids related to several mineralizing stages and determined the information they provide about the fluid sources in this system. Three fluid generations recorded in pseudosecondary to secondary fluid inclusions have been identified at Panasqueira. The first fluid generation identified consists of CO2-bearing fluid inclusions with homogenization temperatures ranging between 260 and 320 °C and salinities between 5 and 8 eq wt % NaCl. Petrographic constraints indicate that this first generation (1) is paragenetically related to the main oxide-silicate stage. Two lower-temperature CO2-absent fluid generations (2a and 2b) have been identified and are represented by secondary fluid inclusions postdating the main oxide-silicate stage. This stage was likely trapped under high pressures and lithostatic conditions (Jacques and Pascal, 2017). Generation (2a) consists of high-salinity (20-30 eq wt % NaCl) fluids with homogenization temperatures ranging between 180°C and 250°C. Generation (2b) consists of low-salinity (<2 wt %) low homogenization temperature (100-150°C) fluid inclusions. Conclusive petrographic evidence of the relationship between these two late-stage fluid generations and specific late mineral stages are scarce. However, fluid compositions suggests that generation (2a) is related to the main sulfide stage and generation (2b) is related to the late carbonate stage. The PTX evolution of fluids at Panasqueira indicate a transition from magmatic dominated fluids to a likely influx of non-magmatic fluids at least in the latest stages of mineralization (main sulfide stage and late carbonate stage) which is in good agreement with recent results from isotopic studies (Codeço et al., 2017).