Fluid Evolution in the Deep Reservoir of the Mt. Amiata Geothermal Field, Italy

Abstract

The fluid-rock interaction processes occurring in deepseated reservoirs of the Mt. Amiata geothermal fields have been studied to outline a conceptual model of the evolution of the geothermal system. A metamorphic event pre-dating the geothermal activity is evident in the shallower part of the reservoir (above ~2.2 km). The geothermal fluids underwent boiling with degassing and/or mixing and evolved through a lowering of the salinity and the gas content, to the present-day geothermal fluid composition. The high CO2 amount, both in paleo- and present-day fluids, can explain the absence and/or rare occurrence of Ca-Al-silicates that represent a common hydrothermal mineral assemblage at these temperatures (i.e. 250-350 °C). Evidences of contact metamorphism has been observed below 3 km of depth, suggesting temperatures of approximately 500 °C. The chlorite geothermometry did not give reliable results, while the use of fluid inclusions has been a valid tool to estimate the reservoir temperatures when direct borehole measurements are missing or affected by uncertainty.