GEOCHEMISTRY OF WATER AND GAS-DISCHARGES FROM THE MT. AMIATA SILICIC COMPLEX AND SURROUNDING AREAS (CENTRAL ITALY)

The Mt. Amiata volcano in central Italy is intimately related to the p ost-orogenic magmatic activity which started in Pliocene times. Major, trace elements, and isotopic composition of thermal and cold spring w aters and gas manifestations indicate the occurrence of three main res ervoir of the thermal and cold waters in the Mt. Amiata region. The de epest one is located in an extensive carbonate reservoir buried by thi ck sequences of low-permeability allochthonous and neo-autochthonous f ormations. Thermal spring waters discharging from this aquifer have a neutral Ca-SO4 composition due to the presence of anhydrite layers at the base of the carbonate series and, possibly, to absorption of deep- derived H2S with subsequent oxidation to SO42- in a system where pH is buffered by the calcite-anhydrite pair (Marini and Chiodini, 1994). I sotopic signature of these springs and N-2-rich composition of associa ted gas phases suggest a clear local meteoric origin of the feeding wa ters, and atmospheric O-2 may be responsible for the oxidation of H2S. The two shallower aquifers have different chemical features, One is C a-HCO3 in composition and located in several sedimentary formations ab ove the Mesozoic carbonates. The other one has a Na-Cl composition and is hosted in marine sediments filling many post-orogenic NW-SE-trendi ng basins. Strontium, Ba, F, and Br contents have been used to group w aters associated with each aquifer, Although circulating to some exten t in the same carbonate reservoir, the deep geothermal fluids at Later a and Mt. Amiata and thermal springs discharging from their outcroppin g areas have different composition: Na-Cl and Ca-SO4 type, respectivel y, Considering the high permeability of the reservoir rock, the meteor ic origin of thermal springs and the two different composition of the thermal waters, self-sealed barriers must be present at the boundaries of the geothermal systems. The complex hydrology of the reservoir roc ks greatly affects the reliability of geothermometers in liquid phase, which understimate the real temperatures of the discovered geothermal fields. More reliable temperatures are envisaged by using gas composi tion-based geothermometers. Bulk composition of the 67 gas samples stu died seems to be the result of a continuous mixing between a N-2-rich component of meteoric origin related to the Ca-SO4 aquifer and a deep CO2-rich component rising largely along the boundaries of the geotherm al systems. Nitrogen-rich gas samples have nearly atmospheric N-2/Ar ( = 83) and N-15/N-14 (delta = 0 parts per thousand) ratios whereas CO2- rich samples show anomalously high delta(15)N values (up to +6.13 part s per thousand), likely related to N-2 from metamorphic schists lying below the carbonate formations. On the basis of average C-13/C-12 isot opic ratio (delta(13)C around 0 parts per thousand), CO2 seems to orig inate mainly from thermometamorphic reactions in the carbonate reservo ir and/or in carbonate layers embedded in the underlying metamorphic b asement. Distribution of He-3/He-4 isotopic ratios indicates a radioge nic origin of helium in a tectonic environment that, in spite of the p resence of many post-orogenic basins and mantle-derived magmatics, can presently be considered in a compressive phase. (C) 1997 Elsevier Sci ence B.V.