Isotopic and fluid inclusion study of hydrothermal and metamorphic carbonates in the Larderello geothermal field and surrounding areas, Italy

Fluid inclusions have been studied on six calcite veins from the shallow part (480 to 1515 m below ground level) of the Larderello geothermal field and outcropping in peripheral zones of the geothermal area. Oxygen and carbon isotopic analyses have been carried out on these carbonate veins, as well as on the dolostone layers found inside the Paleozoic metamorphic units of the deep part of the field (from 1939 to 3177 m below ground level). Fluid inclusion observations suggest that boiling processes probably occurred during calcite precipitation in most of the veins. The fluids that formed or interacted with the calcite veins below the uppermost reservoir (made up of Mesozoic marine carbonates), and with the calcite hydrothermal veins of Sassa, were characterised by an apparent salinity from 1.3 to 5.3 wt.% NaCl eq. and a homogenisation temperature from 137 to 245°C. The fluid inclusions related to the calcite veins hosted above the uppermost reservoir show a wide range of apparent salinity (from 1.7 to 22.2 wt.% NaCl eq.) and homogenisation temperatures from 224 to 296°C. Apparent salinity/homogenisation temperature covariations of the latter veins are interpreted as being the result of a mixing process between a low-temperature, high-salinity fluid and a higher-temperature, moderate-salinity fluid. The oxygen isotopic compositions of the calcite veins (δ¹⁸O from 10.34 to 11.45‰) located below the Mesozoic carbonates and in the outcrops (δ¹⁸O from 9.42 to 17.07‰) indicate that the vapour in equilibrium with these veins was isotopically similar to the present-day discharge steam. The aqueous fluids in equilibrium with these veins could be meteoric water that interacted with the Mesozoic carbonates of the upper reservoir. The δ¹³C values of the CO₂ produced at Larderello and the constant concentration of this gas over time are, however, indicative of a deep source inside the reservoir that is probably related to the decarbonation reaction within the metamorphic units that form the present-day deep reservoir. Fluid inclusion salinities (up to 22.2 wt.% NaCl eq.) and isotopic results (δ¹⁸O from 13.43 to 21.99‰, δ¹³C between −1.26 and −0.18‰) on the calcite veins hosted above the uppermost reservoir suggest that the water circulating in these veins has strongly interacted with Mesozoic carbonates or Neogene sediments containing evaporite layers. The isotopic values (δ¹⁸O from 14.09 to 19.91‰, δ¹³C from −4.09 to 1.90‰) of dolomite samples present in the Paleozoic metamorphic rocks indicate a reaction with fluid of variable temperatures under different water/rock ratios. The isotopic composition of one sample reveals equilibrium with present-day discharge fluids. This fact aside, the remaining data indicate that the Paleozoic dolomitic layers do not seem to contribute significantly to the production of CO₂.