## 3 Natural Gas Emissions and Origin of CO\({}_{2}\)  
A recent soil CO\({}_{2}\) flux survey [16] identified a high diffuse natural soil CO\({}_{2}\) flux in the AVGA area reaching 13,275 t/d in a surveyed area of 225 km\({}^{2}\) consisting of 2482 measurement points with a density of 12 stations per km\({}^{2}\). This is the widest and most detailed survey performed in the Italian geothermal areas. In the same paper, high and low soil flux areas were delimited and soil CO\({}_{2}\) degassing was demonstrated to be inhomogeneous, reflecting the geologic structures and geothermal systems related to the volcano feeding system, magma chamber/intrusions, and regional fault systems. Four main high soil flux areas were identified, Bagnore, Pancastagnaio, Bagni San Filippo, and Seggiano. Low and very low soil flux areas were delimited as coinciding with the slopes of the volcanic massif and with the Arcidosso area (see Figure 6 in [PERSON] et al. [16]).  
Many focused gas emissions are present and are mostly located outside of the volcanic massif. Gases are also discharged by cold springs and Ca-SO\({}_{4}\)-rich thermal springs [15; 34] and in correspondence with the tunnels and infrastructures of abandoned Hg mines [35; 36; 37; 38]. Several papers and databases report the position, chemical, and isotopic composition and flux rates of the gas emissions and thermal and cold springs (i.e., [14; 15; 35; 39]). AVGA gas emissions are composed mainly of CO\({}_{2}\) (85-93% vol.) followed by CH\({}_{4}\), N\({}_{2}\), H\({}_{2}\)S, H\({}_{2}\), Ar, CO, and He [14; 15]. The tectonic control on gas emissions, thermal springs, traverting deposition, and Hg mineralization in the Pietrineri-Bagni San Filippo area is connected to Quaternary faulting related to the Mt. Amiata and Campiglia d'Orcia transtensional fault systems and the associated releasing step-over zones [40]. Gas leaks are also identified in correspondence with other abandoned Hg mines and are widespread in the Mt. Amiata region, of which the Abbadia San Salvatore mine was the largest and most important one. Hg-rich mineralizations and mines in AVGA are mostly located in correspondence with Quaternary faults and fractures [9; 40; 41] (Figure 1).  
The origin of CO\({}_{2}\) in the Mt. Amiata region has been discussed and summarized in several papers, where both mantle and crustal origins were hypothesized [14; 15; 23; 42]. The anomalously high CO\({}_{2}\) degassing that affects the peri-Tyrrhenian margin of Italy is mainly related to the degassing of a CO\({}_{2}\)-rich mantle based on isotopic and thermodynamic models [31; 42]. Mt. Amiata is located inside the Tuscan Roman degassing structure, a belt in which an anomalously high flux of deep CO\({}_{2}\) mainly of mantle origin increases [34]. Consequently, the CO\({}_{2}\) degassing phenomenon is independent of the presence of the geothermal fields, extending over areas larger than those hosting geothermal fields in operation.  
However, CO\({}_{2}\) crustal production also derives from thermometamorphic reactions in the deep roots of geothermal fields occurring in the thermometamorphic halos of granitic intrusions, where graphitic phyllites, thermometamorphic carbonates, and marbles are present, furnishing CO\({}_{2}\) of crustal origin [23; 42; 43; 27; 44]. Continuous CO\({}_{2}\) production from the roots of the AVGA system results in constant CO\({}_{2}\) contents over time [42]. A decrease in the CO\({}_{2}\) content of geothermal fluids was recorded only in the early stages of geothermal exploitation in the Mt. Amiata region when the production was limited to the gas caps of the shallow reservoir [18]. In addition to the deeply derived CO\({}_{2}\), CO\({}_{2}\) is also produced in soils by heterotrophic and autotrophic reactions [45].  
Figure 1: Geological sketch map of the Mt. Amiata volcanic–geothermal area (AVGA) area (modified from [20]).