Sulphur isotope composition of Silurian shale-hosted PGE-Ag-Au-Zn-Cu mineralisations of the Prades Mountains (Catalonia, Spain)

Abstract

Several Silurian metamorphosed shale-hosted sulphide occurrences have been studied in the Prades Mountains, southern part of the Coastal Catalonian Ranges. Most of the sulphides are found as stratiform or shale-disseminated occurrences. Pyrrhotite is the most abundant sulphide mineral. Chalcopyrite and arsenopyrite are less common. Gold, Pd-bearing lollingite (partly replaced by arsenopyrite), sperrylite, hessite, clausthalite, altaite, galena, sphalerite, molybdenite, scheelite, and V-Cr oxides and silicates are minor components. Sulphur isotopic analyses were made on three outcrops (Roca de Ponent, Coma Fosca and Sant Miquel) and in the Silurian black shales of the Sant Bernat series, in order to determine the origin of the sulphur which formed these deposits. The Coma Fosca and Sant Miquel outcrops yield a narrow range of δ34S values (–11.3 to –4.6‰), whereas the Roca de Ponent and Sant Bernat series have a wider range (–19.9 to –7.6‰ in pyrrhotite, and up to +36‰ in pyrite from the top of the Sant Bernat series). In the Roca de Ponent outcrop the δ34S values decrease with increasing stratigraphic height. All the deposits have a mean δ34S composition close to –9‰. Annealing of pyrrhotite took place during the Hercynian metamorphism. Later, during the retrograde metamorphism, pyrite replaced pyrrhotite, and arsenopyrite replaced lollingite. Large-scale compositional inhomogeneities survived metamorphism (from pyroxene to amphibolite facies). A hydrothermal source of S has been postulated for the Coma Fosca and Sant Miquel outcrops. Sulphur in these deposits is from leaching of sulphides from the underlying rocks. In the Roca de Ponent outcrop and in the Sant Bernat series, sulphur was derived from a hydrothermal source and from biogenic reduction of seawater sulphate. The contribution of the latter source increased with time. Biogenic reduction of seawater sulphide in a closed or semi-closed system for sulphate is considered responsible of the high δ34S values of pyrite from the uppermost part of the Sant Bernat series. Based on textural and isotopic evidence, we propose an exhalative origin for the precious metals, cogenetic with the stratiform sulphides.