The Ophiolite-Hosted Cu–Fe–Zn Ores in Tuscany (Italy): VMS Deposits from a Jurassic Slow-Spreading Ridge

Small Cu–Fe–Zn VMS deposits are widespread in Tuscan ophiolites (Ligurian Units). Industrial production was small and definitively ceased from the 1960s. Locally, massive ore (chalcopyrite-bornite-chalcocite) having exceptionally high grade was found. The Montecatini Val di Cecina mine exploited the largest “bonanza” and, for few decades in nineteenth century, became one of the richest and most profitable copper mines in Europe. This study (PRIN-MUR-2017AK8C32 project) was performed in order to define the structural setting, the potential for precious-critical metals and the redox reactions involving sulfides and gangue silicates. Ophiolite-hosted Cu–Fe–Zn ores in Tuscany mostly occur in serpentinite crosscut by dykes-laccoliths of gabbro and dyke-swarms of basalt. Chalcopyrite stockworks are common but the largest deposits are hosted by cataclastic shear-zones in serpentinite-gabbro basement and at the contact with the overlying basalt. The early chalcopyrite stockworks were strongly mechanically chemically reworked with formation of chalcopyrite-bornite-chalcocite nodules embedded in a chlorite, Ca-rich amphibole, Fe-rich serpentine, quartz, andradite and xonotlite assemblage. The ores contain substantial amount of sphalerite and pyrite and ubiquitous grains of clausthalite, native tellurium and gold, in accordance with the significant Te–Se–Au content of these deposits. The overall characters of the Cu–Fe–Zn ores and host rocks suggest a formation in an active oceanic core complex along the Jurassic slow-spreading ridge of the Piedmont–Ligurian Ocean.