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

Boyce, A.J., Fulignati, P. and Sbrana, A. (2003) Deep hydrothermal circulation in a granite intrusion beneath Larderello geothermal area (Italy): constraints from mineralogy, fluid inclusions and stable isotopes. Journal of Volcanology and Geothermal Research, 126(3-4), pp. 243-262. (doi: 10.1016/S0377-0273(03)00150-1)

Abstract

A shallow (similar to2 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 in, were trapped under boiling conditions at about 340degreesC and 15 Wa, whereas at 3520 in evidence of boiling is not revealed and the temperature of the fluid was calculated to be about 350degreesC. The second alteration stage shows a cooling of the system, particularly evident in the shallower portion of the intrusion (similar to235degreesC at 2218 in and similar to315degreesC at 3520 in). 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 delta(18)O values of 0 +/- 1parts per thousand, for fluids in equilibrium at 340degreesC (2218 in) and 350'C (3520 in) with feldspar, epidote and chlorite, and deltaD values of -51 to -61 parts per thousand, for fluids in equilibrium with chlorite, together with directly measured 6D values of -33 to -61 parts per thousand 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.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Boyce, Professor Adrian
• Authors: Boyce, A.J., Fulignati, P., and Sbrana, A.
• Subjects: Q Science > QE Geology
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Journal of Volcanology and Geothermal Research
• ISSN: 0377-0273