The major objective of this work is to document the evolution in the fluid chemistry and the P-T conditions prevailing from the volcano-sedimentary massive sulfide (VMS) deposition, the peak of metamorphism and the ductile/ brittle deformation in the Iberian pyrite belt. Thus, all VMS deposits underwent deformation and recrystallisation under very low-grade conditions (prehnite-pumpellyite facies: Munha, 1990) of metamorphism, it may be suspected that metamorphic fluids were trapped within the deposits, possibly overprinting the earlier, ore-depositing, fluids in some instances. Structural studies have documented the existence of two main tectonic events at the origin of two stages of deformation verging to the south, and then to the north.

Small quartz grains were selected for crush-leach (Banks & Yardley, 1992). The samples include deformed Q1 and chalcopyrite-bearing tension-gashes (Q2 & Q3). The measured compositions are similar in all samples, meaning that they are representative of late fluids: with Na/K ratio between 15 and 40, Na/Li ratio ~200, Cl/SO4 ~200 ratio and log(Br/Cl) between -3 and -2.7. These fluids differ significantly both from that of seawater and those of granite-related fluids. They are similar to evolved basinal fluids.

Oxygen isotopes were analysed in the same quartz separates that were used for crush-leach experiments. The results suggest a rather clear distinction between early Q1 (13.2-13.5‰) and the late Q2-Q3 quartz (17.4-18.4‰), with intermediate values (15.7 to 16.5‰) in more or less recrystallised Q1.

The isotopic features are not compatible with a single stage model for quartz crystallisation from a fluid of marine origin. The present data suggest the involvement of metamorphic fluids for the formation of quartz vein or re-crystallisation.