P-V-T-X -f02 evolution from wolframite to sulphide depositional stages in intragranitic W-veins. An example from the Spanish Central System

Abstract

The relative P-T-X conditions of the deposition of wolframite and sulphides in quartz veins has been investigated using representative W-polymetallic sulphide veins hosted by peraluminous granites in the central domain of the Spanish Central System. Ore-bearing quartz veins fall into two groups : W-veins (wolframite-bearing quartz veins with minor amounts 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 reconstruction of vein fillings suggests a similar genesis and fluid history, as follows : (i) an early fluid stage, lacking in S-veins, characterized by CO2-H20-NaCl rich vapours is at the origin of a strong K-mica alteration of the granite wall-rock, and of vein filling by milky quartz (QI) - phengite I - (wolframite), at around 130 ± 20 MPa and 525 ± 25°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 scheelite in W-veins), and then, b) a chlorite-sulphide (pyrrhotite, sphalerite and galena) assemblage. A progressive decrease of the CO2 content in fluids is recorded, CH4 being the major volatile species in the latest inclusions spatially associated with sulphides. Pressure is around 130 ± 30 MPa, and temperature decreases from 380° to 330°C throughout the course of the fluid trapping. iii) later fluid inputs in the veins are shown by two types of secondary aqueous fluid inclusions in healed microfissures, which display moderate Th (160°-220°C) and salinities. The two main stages (wolframite, then sulphides) are characterized by a nearly isobaric cooling and dilution of volatile rich fluids. The X-fO2 evolution indicates that fluid chemistry was probably controlled during the first stage by graphite-fluid equilibrium, implying a fluid source external to the granite (surrounding metamorphic series). The sulphide stage, which is ubiquitous in most W deposits, appears clearly in this example as the latest stage of a primary hydrothermal cycle involving volatile bearing fluids, and is not disconnected from the W stages.