The Cu Stockwork and Massive Sulfide Ore of the Feitais Volcanic-Hosted Massive Sulfide Deposit, Aljustrel, Iberian Pyrite Belt, Portugal: A Mineralogical, Fluid Inclusion, and Isotopic Investigation

Abstract

The Variscan Feitais volcanic-hosted massive sulfide deposit in the Aljustrel district of the Iberian Pyrite Belt consists of 55 million metric tons of Zn-Pb-Cu massive sulfide overlying a Cu-rich stockwork. The massive ore is overlain by up to 30 m of feldspar-phyric, rhyolitic volcaniclastic rock and locally by a jasper and/or chert layer up to 15 m thick. The massive sulfide orebody consists dominantly of pyrite, sphalerite, galena, chalcopyrite, tetrahedrite-tennantite, arsenopyrite, and bournonite, together with minor quartz, chlorite, sericite, carbonate, and barite. The orebody is up to 100 m thick and is underlain by a tabular alteration zone of chloritedominated, locally silicified, felsic volcanic rock, the upper 30 to 60 m of which contains chalcopyrite-quartzchlorite-sericite-carbonate‐bearing stockwork vein(let)s that prior to deformation were at a shallow angle to the base of the massive orebody. Chloritized footwall rocks extend up to 20 m below the Cu stockwork zone and are underlain by up to 50 m of quartz-sericite-pyrite‐altered rhyolitic rock. The stockwork veins also contain pyrite, tetrahedrite-tennantite, sphalerite, and arsenopyrite. Pyrite, both in stockwork and massive ore, locally displays partly recrystallized framboidal, reniform, and cellular textures. Two generations of quartz, Q1 and Q2, and carbonate in the stockwork veins contain primary (in growth zones) and pseudosecondary fluid inclusions, with homogenization temperatures of 270° to 315°C and salinities of 2.2 to 8.1 wt percent NaCl equiv. The δ34S(CDT) values of massive and stockwork ores range from ‐15.4 to +4.7 (mean, ‐2.8) and ‐11.2 to +11.9 (mean, ‐0.4) per mil, respectively, the lowest values from colloformtextured pyrite. With no evidence of oxidation of sulfide sulfur during mineralization, the most negative values indicate an origin by biogenic reduction of seawater sulfate. The δ13C(PDB) values for carbonates, ‐7.5 to ‐13.7 and +9.3 to ‐14.3 per mil in massive and stockwork ore, respectively, indicate an origin mostly by oxidation of methane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmatic carbon. There are no significant lateral or vertical variations in S isotope values in sulfides or C-O isotope values in carbonates, either in massive or stockwork ore. The δ18O(SMOW) values for quartz in stockwork and massive sulfide are 11.6 to 13.9 and 16.7 to 17.9 per mil, respectively. Coexisting, and texturally contemporaneous, carbonate and quartz in stockwork veins are not in isotopic equilibrium, indicating that the C-O isotope values may have been reset. The δ18O values of fluid calculated to be in equilibrium with quartz at fluid inclusion homogenization temperatures are 4.2 to 5.2 per mil. Barite from the hanging wall and massive ore yields δ34S values (21.9‐27.9‰) equal to or slightly higher than those of coeval seawater; 87Sr/86Sr ratios (0.708438‐ 0.709063) are slightly more radiogenic than those of coeval seawater (0.7080‐0.7085), and much more radiogenic than those of coeval volcanic rocks (0.703304‐0.706642), probably representing mixtures between seawater Sr and radiogenic Sr in fluids sourced in the crustal pile. Deposition of the massive sulfide on the sea floor is suggested by its stratiform nature, the stronger alteration of footwall relative to hanging-wall rocks, the stockwork system terminating sharply at the base of the massive sulfide, the presence of sedimentary-like textures in the massive sulfide, the absence of replacement fronts, and the presence of framboidal and other sea-floor depositional textures indicative of fluid quenching. The sheetlike form, lack of rubble mounds and chimneys, scarcity of barite, reduced mineral assemblage, and metal zoning distinguish Feitais from Kuroko-type deposits. It shares most of the characteristics of those Iberian Pyrite Belt deposits for which a brine-pool origin has been proposed based on fluid inclusion data,