Relation of stibnite mineralisation and geothermal fluids in southern Tuscany (central Italy): An isotope (C, O, H, S) and Rare Earth Element study

Southern Tuscany (central Italy) is a mining district with epithermal stibnite (Hg and Au) mineralisation, an area charac-terized by a marked geothermal anomaly with underlying magmatic bodies, geothermal fluids feeding geothermal wells and springs depositing travertine. This study deals with the actual to subrecent relation between high temperature geothermal fluids and epither-mal stibnite (gold and cinnabar) mineralisation in Southern Tuscany on the basis of chemical and stable isotope (C, O, H, S) data, of ore samples showing the paragenesis stibnite, calcite, fluorite, gypsum and of geothermal fluids tapped by the geothermal wells in the upper and lower aquifer of the Piancastagnaio geothermal field (Monte Amiata geothermal area). δ¹³C_PDB data between +5.6 to +7.1‰ and δ¹⁸O values ranging from 13.8 to 23‰, suggest that the CO₂ in the fluids that deposited the calcite gangue of the stibnite ores at 100 to 250 °C, predominantly at 120 to 160 °C, are produced by thermometamorphic fluid/rock interaction in the aquifer. Negative Eu and Ce anomaly in der REE distribution pattern of calcite, fluorite and gypsum are produced by a thermochemical reduction of Eu³⁺ to Eu²⁺ and the oxidation of Ce³⁺ to Ce⁴⁺, respectively, that prohibits the substitution of Eu and Ce in the calcite, fluorite and gypsum lattice. Oxygen and hydrogen isotope data of the fluids produced in the Piancastagnaio geothermal field reveal with δ¹⁸O_SMOW from +6.66 to +7.68 ‰, δ²H_SMOW from -41.4 to -34.3 ‰ and ³H from 0.7 to 1.2 TU that those fluids are a mixture of a dominating deep-seated uprising high-temperature fluid and a subordinate amount of recent shallow-seated cool groundwa-ter. The isotopic composition of sulphur of the geothermal fluids close to 0 ‰ suggests that most of the sulphur derives from the underlying magmatic intrusions or, alternatively, from the sulphides disseminated in the Paleozoic rocks of the Tuscan crystalline basement. The difference in δ¹⁸O between H₂O and SO₄^-2 in the deep-seated fluids of the Piancastagnaio geothermal field gives an equilibrium temperature of about 300 °C. This temperature corresponds to the measured bottom-hole temperature determined in the geothermal wells. High temperature, isotopic data and elevated Sb, As, Au, Na, K, Cl, B, ammonium, H₂S, sulphate and CO₂ load makes the high temperature geothermal fluid of the Piancastagnaio geothermal field a model for the fluid that deposited the stibnite mineralisation of southern Tuscany in a temperature range from 120 to 300 °C and the travertines at a temperature of about 120 °C. The migration of the geothermal fluids was driven by fracture zones produced by extensional tectonics and the consequent series of earthquakes that characterize until now the study area. The strata-bound character of the stibnite mineralisation is due to the preferential deposition of the stibnite ores in the very porous and fractured evaporitic strata of the Calcare Cavernoso formation.