WATER-ROCK INTERACTION IN THE LARDERELLO GEOTHERMAL-FIELD (SOUTHERN TUSCANY, ITALY) - AN O-18/O-16 AND D/H ISOTOPE STUDY

Stable isotope and fluid inclusions investigations were carried out on metamorphic rocks belonging to the Southern Tuscany Paleozoic basement that constitutes the main deep fluid reservoir ( > 1000 m below ground level) of the Larderello Geothermal Field. Whole-rock samples have deltaO-18 and deltaD values ranging from + 2.9 to + 11.6 parts per thousand and from - 65 to - 76. parts per thousand, respectively; most negative deltaO-18 and deltaD values have been obtained on rocks from highly fractured levels. Calculated deltaO-18 and deltaD values for the fluids in equilibrium with the rocks are from + 3 to + 8 parts per thousand and - 31 to - 43 parts per thousand respectively, temperatures of homogenization of fluid inclusions range from 245-degrees-C to 324-degrees-C and salinities from 1.0 to 12.8 wt.% NaCl eq. The combined data are consistent with mixing between a deep circulating (> 2500 m below ground level) meteoric-hydrothermal water and a deep thermal water of probable magmatic or metamorphic origin. After mixing and exchange the water/rock mass ratios are small, typically < 0.3. These O-18-rich hydrothermal fluids at T> 250-degrees-C in the deeper explored levels contrast with the much less strongly O-18-shifted meteoric-hydrothermal waters which circulate in the upper 1000 m or so of the system at T < 250-degrees-C. These new stable isotope data and other petrological, geophysical and geochemical data are all consistent with the existence of two apparently separate hydrothermal systems (and productive horizons) at different depths bearing fluids of different physico-chemical characteristics. The ''shallow'' upper meteoric system may not be continuosly connected to the lower meteoric-magmatic system because, at least locally, fluid pressures in the lower levels are higher than hydrostatic pressures.