The Pen?a do Seo W-Sn deposit, NW Iberia: Petrology, fluid inclusions and O-H-S isotopes

The Pen similar to a do Seo W-Sn ore deposit in NW Iberia consists mainly of quartz veins hosted in schists. Vein mineralogy comprises wolframite, cassiterite and minor molybdenite. Peraluminous S-type granites and minor metaluminous granitoid breccias crop out in the same location. Whole rock geochemistry, mineral characterization, fluid in-clusions and stable isotope geochemistry have been combined to infer hydrothermal mineralization conditions.A pre-ore stage (0) involving the alteration of host rocks, three stages of mineralization (I, II and III), and a supergene stage (IV) have been identified. Stage I (oxide-halide-sulfide stage) consisted of a cassiterite-pyrite-fluorite mineralization in granitoid breccias. Stage II (main oxide stage) involved wolframite-rich selvages and wolframite-cassiterite-(molybdenite) quartz veins. Stage III (main sulfide stage) consisted of a sulfide minerali-zation in quartz veins comprising pyrite and minor arsenopyrite and chalcopyrite.Three types of fluid inclusions were found in vein quartz: (1) aqueous two-phase inclusions, with homoge-nization temperatures (Th) between 445 degrees C and 280 degrees C and moderate salinities (9-14 wt% NaCl eq.), (2) aqueous-carbonic three-phase fluid inclusions, with Th from 340 degrees C to 260 degrees C and low salinities (2-7 wt% NaCl eq.), and (3) aqueous two-phase fluid inclusions, with Th from 270 degrees C to 155 degrees C and low salinities (0-6 wt% NaCl eq.). 818O values in quartz from mineralized veins range from +11.2%o to +13.4%o, and between +15.0%o and +15.4% in quartz from mineralized granitoid breccias. 834S values in sulfides (pyrite, arsenopyrite and chal-copyrite) range between +13.0 %o and +37.1 %o, thus suggesting a marine source of sulfate and possible equilibration with host-rock sulfides. 8D values in muscovite and chlorite from quartz veins range between-105.7 %o and-71.5 %o and between-69.4 %o and-67.1 %o, respectively, indicating a transition from magmatic to magmatic-metamorphic conditions.An aqueous (H2O-NaCl) magmatic-hydrothermal fluid led to the W-Sn mineralization, as deduced from the studied fluid inclusions and isotopic signatures. High W and Sn contents in the peraluminous granites indicate that the W-Sn mineralization in Pen similar to a do Seo could partially be related to the granites and granitoid breccias. Whereas the Sn (+/- W) likely derived from hydrothermal fluids exsolving from the crystallizing magmas, host quartz schists would have supplied other elements required for wolframite deposition such as Fe and Mn. Sulfide mineralization in stage III was probably driven by fluid dilution.