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 shall ow part (480 to 1515 m below ground level) of the Larderello geotherma l held and outcropping in peripheral zones of the geothermal area. Oxy gen and carbon isotopic analyses have been carried out on these carbon ate veins, as well as on the dolostone layers found inside the Paleozo ic metamorphic units of the deep part of the field (from 1939 to 3177 m below ground level), Fluid inclusion observations suggest that boili ng processes probably occurred during calcite precipitation in most of the veins. The fluids that formed or interacted with the calcite vein s below the uppermost reservoir (made up of Mesozoic marine carbonates ), and with the calcite hydrothermal veins of Sassa, were characterise d by an apparent salinity from 1.3 to 5.3 wt.% NaCl eq. and a homogeni sation temperature from 137 to 245 degrees C. The fluid inclusions rel ated 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 h omogenisation temperatures from 224 to 296 degrees C. Apparent salinit y/homogenisation temperature covariations of the latter veins are inte rpreted as being the result of a mixing process between a low-temperat ure, high-salinity fluid and a higher-temperature, moderate-salinity f luid. The oxygen isotopic compositions of the calcite veins (delta(18) O from 10.34 to 11.45 parts per thousand) located below the Mesozoic c arbonates and in the outcrops (delta(18)O from 9.42 to 17.07 parts per thousand) indicate that the vapour in equilibrium with these veins wa s isotopically similar to the present-day discharge steam. The aqueous fluids in equilibrium with these veins could be meteoric water that i nteracted with the Mesozoic carbonates of the upper reservoir, The del ta(13)C values of the CO2 produced at Larderello and the constant conc entration of this gas over time are, however, indicative of a deep sou rce 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 i sotopic results (delta(18)O from 13.43 to 21.99 parts per thousand, de lta(13)C between -1.26 and -0.18 parts per thousand) on the calcite ve ins hosted above the uppermost reservoir suggest that the water circul ating in these veins has strongly interacted with Mesozoic carbonates or Neogene sediments containing evaporite layers. The isotopic values (delta(18)O from 14.09 to 19.91 parts per thousand, delta(13)C from -4 .09 to 1.90 parts per thousand) of dolomite samples present in the Pal eozoic metamorphic rocks indicate a reaction with fluid of variable te mperatures 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 dolomi tic layers do not seem to contribute significantly to the production o f CO2. (C) 1997 CNR.