Fluid inclusions in minerals from the geothermal fields of Tuscany, Italy

A reconnaissance study on fluid inclusions from the geothermal fields of Tuscany indicates that the hydrothermal minerals were formed by fluids which were, at least in part, boiling. Four types of aqueous inclusions were recognized: (A) two-phase (liquid + vapor) liquid rich, (B) two-phase (vapor + liquid) vapor rich, (C) polyphase hypersaline liquid rich and (D) three phase—H₂O liquid + CO₂ liquid + CO₂-rich vapor. Freezing and heating microthermometric determinations are reported for 230 inclusions from samples from six wells. It is suggested that boiling of an originally homogeneous, moderately saline, CO₂-bearing liquid phase produced a residual hypersaline brine and a CO₂-rich vapor phase. There are indications of a temperature decrease in the geothermal field of Larderello, especially in its peripheral zones.