P-V-T-X-FO2 EVOLUTION FROM WOLFRAMITE TO SULFIDE DEPOSITIONAL STAGES IN INTRAGRANITIC W-VEINS - AN EXAMPLE FROM THE SPANISH CENTRAL SYSTEM

The relative P-T-X conditions of the deposition of wolframite and sulp hides in quartz veins has been investigated using representative W-pol ymetallic sulphide veins hosted by peraluminous granites in the centra l domain of the Spanish Central System. Ore-bearing quartz veins fall into two groups: W-veins (wolframite-bearing quartz veins with minor a mounts of sulphides) and sulphide (S) dominated veins (wolframite free ). The two vein types (S and W) have similar orientation, silicate and sulphide mineralogy. A multidisciplinary approach based on a detailed fluid-inclusion analysis in relation with a paragenetic reconstructio n of vein fillings suggests a similar genesis and fluid history, as fo llows: (i) an early fluid stage, lacking in S-veins, characterized by CO2-H2O-NaCl rich vapours is at the origin of a strong K-mica alterati on of the granite wall-rock, and of vein filling by milky quartz (QI) - phengite I - (wolframite), at around 130 +/- 20 MPa and 525 +/- 25-d egrees-C; (ii) a second stage characterized by H2O-CO2-CH4-NaCl fluids with a low volatile-phase density, at the origin of the deposition of : a) saccharoidal quartz (QII) (+ phengite II and minor amounts of sc heelite in W-veins), and then, b) a chlorite-sulphide (pyrrhotite, sph alerite and galena) assemblage. A progressive decrease of the CO2 cont ent in fluids is recorded, CH4 being the major volatile species in the latest inclusions spatially associated with sulphides. Pressure is ar ound 130 +/- 30 MPa, and temperature decreases from 380-degrees to 330 -degrees-C throughout the course of the fluid trapping. iii) later flu id inputs in the veins are shown by two types of secondary aqueous flu id inclusions in healed microfissures, which display moderate Th (160- degrees-220-degrees-C) and salinites. The two main stages (wolframite, then sulphides) are characterized by a nearly isobaric cooling and di lution of volatile rich fluids. The X-fO2 evolution indicates that flu id chemistry was probably controlled during the first stage by graphit e-fluid equilibrium, implying a fluid source external to the granite ( surrounding metamorphic series). The sulphide stage, which is ubiquito us in most W deposits, appears clearly in this example as the latest s tage of a primary hydrothermal cycle involving volatile bearing fluids , and is not disconnected from the W stages.