Diagenesis versus hydrothermalism and fluid–rock interaction within the Tuscan Nappe of the Monte Amiata CO₂‐rich geothermal area (Italy)

Abstract

In southern Tuscany (central Italy), deep to shallow geothermal systems were active since the Pleistocene and comprise fluids carrying variable amounts of CO2. The Monte Amiata geothermal fields include two main reservoirs: a deep one located in the Palaeozoic metamorphic succession (1300–3000 m deep) belonging to the Tuscan metamorphic ‘basement’ and a shallow one hosted in the Mesozoic sedimentary succession (500–1000 m deep) belonging to the Tuscan Nappe. Multiple sets of calcite veins were investigated in some outcrops of the Tuscan Nappe succession exposed in the Monte Amiata area and surroundings. Two main fluid systems were characterized: the former related to combined diagenetic syn‐tectonic processes from highly saline fluids after interaction with the Late Triassic evaporites (Burano Fm) and the latter related to a fossil geothermal system, Pleistocene in age, fed by low‐salinity meteoric fluids, carrying CO2, radiogenic Sr and heavy O isotopes after interaction with the subsurface rocks. Comparison with the present‐day hydrothermal fluids allowed to sketch the thermal evolution of the system from past to present. Geochemical data proved that portions of the vein–host rocks were reset after interaction with CO2‐rich fluids circulating in the past. Consequently, textural and mineralogical changes are expected to have occurred with respect to the undisturbed rock masses. Understanding these modifications, their location and extension, is a prerequisite to construct (i) geochemical models addressed to the comprehension of the diagenesis induced in sediments after long‐term CO2 storage; and (ii) geological models applied to the prediction of rock heterogeneity distribution within the upper reservoirs, characterizing the shallow (500–1500 m) geothermal systems of southern Tuscany.

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

“…Combined with previous studies on active and fossil neighbouring hydrothermal systems (Frondini et al 2009;Gasparrini et al 2013), these results provide new insights into the evolution of the shallow hydrothermal reservoir of the Monte Amiata region and its possible cooling rate during the last 300-400 ka.…”

“…After the stacking of the tectonic units, extensional tectonics (Carmignani et al 1994;Brogi et al 2005) and coeval magmatism (Peccerillo 2003;Dini et al 2005) affected the previously overthickened continental crust since the Early-Middle Miocene (Brunet et al 2000) and led to the exhumation of deeper tectonic units (Carmignani et al 1995), widespread geothermal anomalies (Della Vedova et al 2001) and hydrothermal circulation (Tanelli 1983). Around Monte Amiata, hydrothermal circulation was responsible for the world-class Hg and Sb deposits, extensively mined up to thirty years ago (Rimondi et al 2015), as well as to barren hydrothermal calcite veins (Gasparrini et al 2013). Hydrothermal fluid flow is still active in this area as manifested by the travertine deposition from thermal springs at Bagno Vignoni and Bagni San Filippo (Fig.…”

“…Hydrothermal circulation at these sites was favoured by the presence of Neogene-Quaternary brittle structures (Brogi et al 2010a(Brogi et al , b, 2012 that allowed the upwelling of large volumes of meteoric-derived aqueous solutions, heated by the regional high geothermal gradient and enriched in Sr and F through their circulation in Mesozoic carbonate and Triassic evaporite successions (e.g. host rocks of the shallower reservoir; Duchi et al 1992;Cortecci and Lupi 1994;Gasparrini et al 2013;Brogi et al 2015).…”

“…Following the data of Gasparrini et al (2013) on hydrothermal calcite veins sampled in Mesozoic carbonate rocks of the Monte Amiata surroundings, the Bagni San Filippo fossil hydrothermal system was characterized by temperatures of about 165 °C, relatively low salinities (generally comprised between 1.2 and 1.7 wt% NaCl eq. ), δ 13 C = -2.6 to +0.6 ‰ and δ 18 O = + 21.6 to +23.1 ‰ (Gasparrini et al 2013) (Fig.…”

“…The composition of Bagni San Filippo modern and fossil thermal springs and of rainwater of southern Tuscany is reported. *Data from Gasparrini et al (2013); **from Fancelli and Nuti (1975), DST (2010) 1 3…”