Mineral assemblages, stable isotopes and fluid inclusions in ore veins from the Macigno Formation at Calafuria (Livorno Mountains, northern Tuscany, Italy)

The present paper deals with an integrated geological, mineralogical and geochemical study on mineralized faults and fractures recognized in the Macigno Formation at Calafuria in the Livorno Mountains of northern Tuscany. In the field, two main different systems of faults were recognized: an earlier one related to a compressive phase with NE-SW and N-S trending faults, and a younger one related to a relaxation phase with NW-SE trending faults. They distinguish for specific mineral assemblages, consisting mainly of quartz in the former, and barite-marcasite-pyrite in the latter. Minerals were analyzed for fluid inclusions and isotopic composition. Data on fluid inclusions indicate that three main mineralizing episodes affected the fault systems. In the first episode, the fluids deposited quartz in the fault systems related to the compressive phase. The second episode was the main one, and led to the deposition of euhedral quartz in the NE-SW faults, and barite, maracasite and pyrite essentially in the N-S and NW-SE faults. The fluid evolution ended with the deposition of calcite, dolomite and a series of supergene minerals originated by alteration of Fe-sulfides. The sulfur isotopic composition of minerals (–6.3 to 25.1‰) would seem to exclude a magmatic origin of the sulfur, favoring instead a sedimentary origin. Moreover, a temperature of about 400°C is estimated by sulfur isotope thermometry for the second stage of mineralization. Owing to pressure effects, the homogenization temperature given by the fluid inclusions for this stage of mineralization is considerably lower that the isotopic one. A lithostatic pressure of 125 MPa is estimated for the barite and sulfide mineralization, this corresponding to a depth of 5 Km. Finally, a post-Middle Pliocene age is proposed for the ore veins at Calafuria, that may have formed during tectonic relaxation events from hydrothermal fluids rich in CO2 and H2S, as are observed in the Larderello geothermal area at present.