The Zn-Pb deposits of Casario (Ligurian Alps, NW Italy): Late Palaeozoic sedimentary-exhalative bodies affected by the alpine metamorphism

摘要

In the Ligurian Alps (South-Western Italian Alps), Zn-Pb deposits occur within late Palaeozoic meta-sedimentary units belonging to the Brianconnais Zone near Casario (Tanaro valley). Different types of sulphide-rich, tens-shaped mineralizations are recognized: sphalerite-galena massive sulphide bodies, pyrite-rich lenses and sulphide-rich quartz-carbonate-chloritoid granofels. Sulphide lenses and host rocks are affected by at least three ductile deformation phases and by a polyphase alpine metamorphism, whose climax conditions are estimated, based on P-T pseudosection calculations, at T = 300-325 degrees C and P = 0.55-0.60 GPa. In all the mineralized lenses the ore minerals are represented, in variable amount, by Fe-poor sphalerite, galena, pyrite and arsenopyrite (+/- tetrahedrite, chalcopyrite and pyrrhotite); the gangue consists of quartz, carbonate (siderite-magnesite +/- rhodochrosite s.s.), Fe-chloritoid, muscovite-phengite and chlorite. The mineralizations are associated with chloritoid - carbonate micaschists displaying a finely bedded texture, with sharp between-bed compositional contrast, which suggests their exhalative origin. In spite of the tectono-metamorphic overprint, some pre-metamorphic features of the hydrothermal system are still recognized, like relics of the hydrothermal feeding system, primary growth textures and sulphide-rich microbreccias. These massive sulphide lenses, which share many characters with the SEDEX deposits, testify to the occurrence of an exhalative event of Upper Carboniferous age previously unrecognized in the Ligurian Brianconnais Unit.