Stable isotope geochemistry of carbonates from the mining district of Apuane Alps, Northern Tuscany, Italy

Rock, vein filling and ore-gangue carbonates associated with the main stratabound and largely stratiform barite-iron oxides-pyrite deposits of the Apuane Alps were analysed for carbon and oxygen isotope compositions. The orebodies occur within a metamorphic phyllitic complex with carbonate-bearing intercalations (Scisti di Fornovolasco), and at the contact with the overlying metamorphic dolostones (Grezzoni). Carbonates from the tectonically underlying Nucleo Metamorfico Apuano unit were also analysed. The δ⅝PDB and δ18OsMθw values of dolomite and calcite are in the ranges -0.9 to 2.8%o and 23.1 to 31.4%o, respectively, for country rocks, -1.9 to O.O%o and 22.2 to 26.0%o for vein carbonates, and -29.3 to -20.3%o and 16.6 to 19.4%o for gangue calcites. The main isotopic features are (1) the lowest δ180 values for carbonate rocks are found at the contacts with either schist or orebody, (2) vein dolomite is depleted in both C and l O relative to host rock dolomite, and their δ180 values very well correlate positively, (3) the δ13C of gangue calcites matches with that of carbonates associated to sulphate-reducing bacterial activity. The isotopic data and their spatial distribution can satisfactorily be interpreted either in terms of variable alteration of the protolith isotopic signatures by interaction with metamorphic fluids, or in terms of crystallization at metamorphic temperatures from fluids driven out of the schists. Water to rock mole ratios are tentatively estimated. The present results are compatible with a sedimentary-diagenetic origin for stratiform orebodies, followed by metamorphic recrystallization and mobilization processes.