Deep hydrothermal circulation in a granite intrusion beneath Larderello geothermal area (Italy): constraints from mineralogy, fluid inclusions and stable isotopes

Abstract

A shallow (∼2 km) granite body has been intersected by the MONT-4 well in new producing areas of the Larderello geothermal field. The granite at 2.2–2.5 km and below 3 km underwent pervasive propylitic alteration that is dominated by the assemblage chlorite+adularia+quartz+illite+calcite+albite±epidote±sphene. Fluid inclusion studies indicate two stages of alteration. The first one is characterised by high-temperature fluids that, at 2218 m, were trapped under boiling conditions at about 340°C and 15 MPa, whereas at 3520 m evidence of boiling is not revealed and the temperature of the fluid was calculated to be about 350°C. The second alteration stage shows a cooling of the system, particularly evident in the shallower portion of the intrusion (∼235°C at 2218 m and ∼315°C at 3520 m). The temperature of homogenisation for subsequent cooler inclusions approximates present-day down-hole temperatures, and may be considered as representative of the fluid actually circulating in the granite in recent times. Calculated δ¹⁸O values of 0±1‰, for fluids in equilibrium at 340°C (2218 m) and 350°C (3520 m) with feldspar, epidote and chlorite, and δD values of −51 to −61‰, for fluids in equilibrium with chlorite, together with directly measured δD values of −33 to −61‰ for fluid inclusions in quartz, indicate derivation from meteoric waters. The original meteoric signature of the fluids has been altered through water–rock interaction, but remains clearly distinguished from a primary magmatic signature, for which no isotopic evidence was found. Active fracturing and the high structural relief of granite emplacement promoted hydrothermal circulation in the granitic body drilled by MONT-4 well, suggesting that such characteristics may be a pre-requisite for such circulation at Larderello.

Introduction

The crust in Southern Tuscany (Italy) is characterised by high heat flow due to the Neogene magmatism of the Tuscan Magmatic Province and crustal thinning (Marinelli, 1963, Marinelli et al., 1993, Serri et al., 1993, Baldi et al., 1995, Mongelli et al., 1998). Since the late Miocene, the development of widespread hydrothermal activity connected with this magmatism is indicated by the presence of hydrothermal ore deposits and active geothermal systems (Marinelli, 1963, Marinelli, 1969, Marinelli, 1983, Cataldi et al., 1978, Tanelli, 1983, Gianelli et al., 1988, Gianelli et al., 1997, Lattanzi, 1999). The vapour-dominated Larderello geothermal field is the most striking feature of this active hydrothermal activity, and has been extensively exploited for several decades for electrical energy production by ENEL (Italian Electricity Board). Two main productive horizons are exploited: (1) a shallow reservoir, located in Mesozoic carbonate and anhydritic units, and (2) a deep reservoir in permeable levels of the Palaeozoic basement which has been metamorphosed regionally and by contact with the Neogene intrusions. Geological and geophysical data indicate the presence of these shallow intrusive bodies beneath the Larderello geothermal field, belonging to a composite batholith distributed at different depths within the crust (Gianelli et al., 1997). Radiometric ages of 3.8–2.5 Ma were measured in granites and granite dykes from deep Larderello geothermal wells (Batini et al., 1985, Villa et al., 1997). As a consequence, these shallow intrusions cannot be considered responsible for the actual geothermal anomaly. However, geophysical data support the presence of a still partially molten magmatic body below Larderello (Gianelli et al., 1997). This is considered to be responsible for the very high positive thermal anomaly of the area (geothermal gradient up to >300°C·km⁻¹; Baldi et al., 1995). Aplitic dykes and two-mica granites have been penetrated by deep (3–4.5 km) geothermal wells, and associated high-grade contact metamorphic aureoles are common (Gianelli et al., 1997, Carella et al., 2000). Several studies revealed a complex evolution of the fluids circulated in the subterrane of Larderello (Cavarretta et al., 1982, Belkin et al., 1985, Valori et al., 1992, Cathelineau et al., 1994, Ruggieri et al., 1999, Ruggieri and Gianelli, 1999), from an early stage, dominated by magmatic and thermometamorphic fluids associated with the emplacement of granitic intrusions, to the present-day fluids, prevalently of meteoric origin.

In the last few years, research into the exploitation of high enthalpy fluids at Larderello has moved the exploration toward the southeastern part of the field (Barelli et al., 2000). In the Montieri zone (Fig. 1) a deep well (MONT-4) encountered a granitic body at relatively shallow depth (∼2000 m below ground level) and traversed through to the depth of 3721 m. This granite shows clear evidence of hydrothermal alteration throughout the well. Our work is focussed on the characterisation of the fluids involved in the hydrothermal alteration of this granite, through mineralogical, fluid inclusion and stable isotope (δ¹⁸O and δD) investigations. We show that the granitic intrusion was first affected by the circulation of meteoric-sourced fluids at around 340°C, and a more recent event – perhaps still on-going – associated with meteoric-sourced fluids at temperatures ranging between 235°C (2218 m) and 315°C (3520 m).