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Abstract

Panasqueira is a world-class tungsten-vein deposit. Several paragenetic stages have been proposed 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 through combination of detailed petrography, microthermometric measurements and LA-ICP-MS analyses. We have identified the fluids related to several mineralizing stages in the system. Three fluid generations recorded in pseudo-secondary to secondary fluid inclusions have been identified at Panasqueira and their chemical composition has been characterized. The first fluid generation identified in quartz and wolframite 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. This stage (1) was likely trapped under high pressures and lithostatic conditions. Two lower-temperature CO2-absent fluid generations (2a and 2b) have been identified in quartz and are represented by secondary fluid inclusions postdating the main oxide-silicate stage. 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 suggest that generation (2a) is related to the main sulfide stage and generation (2b) is related to the late carbonate stage. The fluids at Panasqueira show a PTX evolution from an early magmatic-like chemical signature to a signature that seems to indicate an influx of non-magmatic fluids at least in the latest stages of mineralization (late carbonate stage).