Silurian black shales from Sousel-Barrancos Metallogenic Belt (Ossa-Morena Zone, Portugal): characterization and interplay to Cu deposits

Abstract

Black shales are commonly spatially associated with metal-rich deposits, wherein organic matter in these rocks may condition or influence the processes related with the concentration of metals and formation of ore deposits. In the Ossa-Morena Zone (South of Portugal), namely in the Sousel-Barrancos metallogenic belt, several Cu mineralized veins occurs, being generally hosted in Silurian metasedimentary units mainly composed of black and grey shales, with interbedded lyddites and black quartzites. This research aims to contribute to the study of the association between those Silurian black shales and the Cu ore deposition in the Sousel-Barrancos metallogenic belt. The petrographic, geochemical and mineralogical analysis of black shales was performed to characterize the organic and inorganic fractions, to investigate elements concentration and its mode of occurrence. The data allow to identify the spatial pattern of thermal maturation of these black shales related to the variability of metamorphic processes, and the studied samples collected in drill cores from Miguel Vacas deposit also allow to identify an irregular pattern of maturation associated to the pathway of hydrothermal fluids. A significant Cu enrichment in black shales in the supergene enrichment domains were identified and data seems to indicate that Cu concentration is not directly related with organic carbon content, probably indicating a distinct source for Cu content. The mobilization of hydrothermal fluids, possibly of magmatic source, contributed to the discrepant maturation of organic matter in black shales.