Boiling and fluid mixing in the chlorite zone of the Larderello geothermalsystem

The geochemical features of the geothermal fluids produced within the boili ng zone in the relatively shallow parts of the Larderello geothermal system (Italy) have been documented as a result of deep drilling which provided s amples from 1480 to 2500 m depth. Four wells (Monteverdi 1, Monteverdi 2A, Sasso 22 and Capannoli 2B) have been sampled in the intermediate parts of t he Larderello aquifer located in a metamorphic basement underlying the Tert iary nappe complex which constitutes the shallow aquifer at Larderello, Flu id inclusions from recrystallized quartz lenses and quartz veins in samples displaying a predominant quartz-chlorite-(epidote-adularia) paragenesis ha ve been studied by microthermometry and Raman spectroscopy. The inclusions are primary and pseudo-secondary in origin when formed in authigenic quartz , or of secondary origin, when located in fluid inclusion planes related to microfracturing of metamorphic quartz lenses. Several generations of fluid s are present and include: H2O-(CO2)-dominated vapours and liquids, and a s eries of aqueous liquids, for the most part of relatively low salinity. The T-m-ice of both early and late inclusions are mostly between 0.0 and -4.5 degrees C, indicating that the salinities of the hydrothermal fluid were ve ry low to moderate. However, rare fluid inclusions with lower T-m-ice (from -4.9 to -25.0 degrees C) were also observed. These inclusions may record t he occasional input of saline fluids, which may be derived from the interac tion of the hydrothermal waters with the evaporites present in the shallow part of the Larderello field. At Capannoli 2B, earliest H2O-(CO2) liquids w ere trapped under minimal pressures of 610-645 bars, which bracket the esti mated present-day lithostatic pressure (640 bars). Tn all other samples, th e main stage of quartz-chlorite crystallization occurs under boiling condit ions attested by the presence of liquid and vapour-rich inclusions, that, i n some instances, can be texturally interpreted to be coeval. Their trappin g conditions (350-375 degrees C, 160-215 bars) are higher than the present day temperatures at the same depth, Later fluid inclusions attest to a sign ificant cooling of the fluids down to temperatures similar to the present-d ay temperatures. During this time, pressures were close to hydrostatic cond itions. Most fluid inclusions were trapped within the liquid field, this in dicating that a significant pressure drop has since affected the main aquif ers or fractured zones which are, at present, under vaporstatic conditions. (C) 1999 Elsevier Science B.V. All rights reserved.