Investigating fossil hydrothermal systems by means of fluid inclusions and stable isotopes in banded travertine: an example from Castelnuovo dell’Abate (southern Tuscany, Italy)

Abstract

Southern Tuscany (Italy) hosts geothermal anomalies with associated widespread CO2 gas-rich manifestations and active travertine-deposing thermal springs. Geothermal anomalies have been active since the Late Miocene and have led to the formation of widespread Late Miocene–Pleistocene travertine deposits and meso- and epithermal mineralizations. This study investigates the travertine deposit exposed in the Castelnuovo dell’Abate area of southern Tuscany. Here, a fissure-ridge type travertine deposit and its feeding conduits, currently filled with banded calcite veins (i.e. banded travertine), represent a spectacular example of fossil hydrothermal circulation in the peripheral area of the exploited Monte Amiata geothermal field. The Castelnuovo dell’Abate travertine deposit and associated calcite veins were analysed to establish the characteristics of the parent hydrothermal fluids, and the age of this circulation. The focus of the study was on fluid inclusions, rarely considered in travertine studies, but able to provide direct information on the physico-chemical characteristics of the original fluid. Uranium–thorium geochronological data provided further constraints on the: (1) age of tectonic activity; (2) age of the hydrothermal circulation; and (3) evolution of the Monte Amiata geothermal anomaly. Results indicate that brittle deformation (NW- and SE-trending normal to oblique-slip faults) was active during at least the Middle Pleistocene and controlled a hydrothermal circulation mainly characterized by fluids of meteoric origin, and as old as 300–350 ka. This is the oldest circulation documented to date in the Monte Amiata area. The fluid chemical composition is comparable to that of fluids currently exploited in the shallow reservoir of the Monte Amiata geothermal field, therefore suggesting that fluid composition has not changed substantially over time. These fluids, however, have cooled by about 70 °C in the last 300–350 ka, corresponding to a cooling rate of the Monte Amiata geothermal area of about 20 °C 100 ka−1.