AbstractFluids with compositions in the system CO2‐H2O‐NaCl were trapped in quartz veins enclosed in low‐grade metamorphic rocks (chlorite zone) on the southern flank of the Canigó Massif, eastern Pyrenees. The veins, which also contain arsenopyrite crystals, were formed contemporaneously with the main Hercynian foliation and metamorphism. Volumetric properties of the fluid and the results of arsenopyrite geothermometry suggestP‐Ttrapping conditions of 4.6–6 kbar and 450–530° C. This implies that an episode of metamorphism with an average geothermal gradient of 25° C km−1occurred during the main deformation event. This episode preceded the low‐P/high‐Tmetamorphism described around domes and to date considered as characteristic of the Hercynian oro