Contact Metamorphism in the Larderello Geothermal System

Abstract

The potential reservoir of the steam-heated Larderello geothermal field consists of various metamorphic rocks and granite. These rocks have been found in the deepest part of the field in wells reaching depths close to 4.5 km. The intrusion of granite into the wall rocks, the circulation of magmatic fluids and their evolution to the present-day conditions of superheated steam involve complex processes which are reconstructed by fluid-inclusion and mineral studies. The results obtained through a study of core samples of granite and contact metamorphic rock indicate that: 1) the granitic rocks are highly differentiated, and their solidus temperature may be as low as 600?C; 2) the thermal metamorphism developed at temperatures of 400-600?C and pressures of approximately 100-120 MPa and the fluids permeating the contact aureole evolved from magmatic to meteoric; 3) the chemistry of the early magmatic stage is characterised by the presence of elements such as F, Cl, Li and B, derived from the anatectic magma and therefore of crustal origin; 4) at least part of the He and CO2, and probably other gases as well, have a deep origin, possibly from the Earthís mantle. These findings depict a complex scenario for the deepest part of the Larderello geothermal field. When deep exploration is successful, the well produces superheated steam. However, projects for deep exploration, in particular those in the vicinity of the major seismic reflector present throughout the geothermal region, should take into consideration the possibility of encountering very hot fluids, with pressures greater than hydrostatic and unusual and undesired chemistry. Moreover, the prospect of finding melted rock (possibly as dykes of differentiated granite) should also be considered.