Late Hercynian polymetallic vein-type base-metal mineralization in the Iberian Pyrite Belt: fluid-inclusion and stable-isotope geochemistry (S-O-H-Cl)

Abstract

Late Variscan vein-type mineralization in the Iberian Pyrite Belt, related to the rejuvenation of pre-existing fractures during late Variscan extensional tectonism, comprises pyrite-chalcopyrite, quartz-galena-sphalerite, quartz-stibnite-arsenopyrite, quartz-pyrite, quartz-cassiterite-scheelite, fluorite-galena-sphalerite-chalcopyrite, and quartz-manganese oxide mineral assemblages. Studies of fluid inclusions in quartz, stibnite, and barite as well as the sulfur isotopic compositions of stibnite, galena, and barite from three occurrences in the central part of the Iberian Pyrite Belt reveal compelling evidence for there having been different sources of sulfur and depositional conditions. Quartz-stibnite mineralization formed at temperatures of about 200 degreesC from fluids which had undergone two-phase separation during ascent. Antimony and sulfide are most probably derived by alteration of a deeper lying, volcanic-hosted massive sulfide mineralization, as indicated by delta(34)S signatures from -1.45 to -2.74parts per thousand. Sub-critical phase separation of the fluid caused extreme fractionation of chlorine isotopes (delta(37)Cl between -1.8 and 3.2parts per thousand), which correlates with a fractionation of the Cl/Br ratios. The source of another high-salinity fluid trapped in inclusions in late-stage quartz from quartz-stibnite veins remains unclear. By contrast, quartz-galena veins derived sulfide (and metals?) by alteration of a sedimentary source, most likely shale-hosted massive sulfides. The delta(34)S values in galena from the two study sites vary between -15.42 and -19.04parts per thousand. Barite which is associated with galena has significantly different delta(34)S values (-0.2 to 6.44parts per thousand) and is assumed to have formed by mixing of the ascending fluids with meteoric water.