Water–rock interaction in the active geothermal system of Pantelleria, Italy

The geothermal fluid sampled in a deep (990 m b.s.l.) well drilled into the peralkaline volcano of Pantelleria consists for the most part of seawater. The fluid is depleted in Ca and Mg and enriched in K, Rb, Cs and F due to interaction with rocks of trachytic composition. A forward geochemical model has been used to simulate the water–rock interaction processes occurring within the geothermal system in the southern part of the island. The simulation assumes different mixtures of seawater with groundwater and volcanic gas. The system model that emerges from the simulation is the following: the seawater flows through fractures and faults, reaching temperatures of over 300°C in the southern part of the island where an upflow zone of volcanic gas is present. Volcanic gases increase the amount of C and S in the system, lower the pH, and enhance the dissolution of trachyte. After reaction with trachyte the pH increases and the fluid reaches saturation conditions with respect to albite, quartz, saponite, K-feldspar and muscovite, in agreement with the natural hydrothermal mineral assemblages. The model confirms that the reservoir fluid is a mix between seawater and meteoric water, more saline fluids possibly existing in deeper levels. The fluid chemistry and the hydrothermal minerals are similar to those found in the Icelandic geothermal systems of Reykjanes and Svartsengi.