# One hydrothermal system may hide another: Insights from a geochemical exploration of Chaudes-Aigues and nearby springs (French Massif Central)

Emmy Penhoët, Bernard Sanjuan, Laurent Guillou-Frottier, Manuel Moreira, Laurent Arbaret

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## To cite this version:

Emmy Penhoët, Bernard Sanjuan, Laurent Guillou-Frottier, Manuel Moreira, Laurent Arbaret. One hydrothermal system may hide another: Insights from a geochemical exploration of Chaudes-Aigues and nearby springs (French Massif Central). Geothermics, 2025, 133, pp.103476. 10.1016/j.geothermics.2025.103476. hal-05222615

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# One hydrothermal system may hide another: Insights from a geochemical exploration of Chaudes-Aigues and nearby springs (French Massif Central)

Emmy Penhoët<sup>a,b,*</sup>,<sup>†</sup> Bernard Sanjuan<sup>a,†</sup>, Laurent Guillou-Frottier<sup>a,b</sup>,<sup>†</sup> Manuel Moreira<sup>b</sup>,<sup>‡</sup> Laurent Arbaret<sup>b,‡</sup>

## ARTICLE INFO

### Keywords:

Chaudes-Aigues

French massif central

Granite

Thermal springs

Geochemistry

Isotopes

Numerical modelling

Multi reservoir

## ABSTRACT

Granitic-hosted hydrothermal systems offer valuable insight into deep fluid circulation mechanisms in amagmatic contexts. This study focuses on the Chaudes-Aigues hydrothermal system in the French Massif Central, which hosts one of the hottest natural springs in Europe (Par Spring, 80–82 °C). Our work aims to better constrain the functioning of the system, including reservoir temperature, recharge zone, and fluid pathways. A combined approach of water geochemistry (major elements, isotopes) and thermo-hydraulic numerical modeling was applied. Using 22 thermal springs distributed across the Margeride Granitic Complex (MGC) and the Cantal Volcanic Complex (CVC) the temperature of Chaudes-Aigues reservoir has been estimated and is thought to reach 190 ± 10 °C, validating and refining previous estimates. Isotopic and geochemical tracers point to a meteoric origin, with the recharge area most likely located along the southern edge of the MGC, connected to the emergence zone via a thick N150E fault zone. This study redefines the Chaudes-Aigues system as fully amagmatic and structurally controlled. Other geothermal reservoirs have also been identified within the Margeride Granitic Complex, with temperatures exceeding 100 °C, indicating a broader geothermal potential of the area. Furthermore, this study highlights the potential for identifying similar hidden geothermal systems in other fractured granites with limited surface manifestations, offering new perspectives for high-temperature geothermal exploration in Europe.

## 1. Introduction

Hydrothermal systems hosted in granitic context provide valuable insights into the interplay between heat transfer, fluid circulation, and structural controls in the Earth's crust. Unlike magmatic (recent volcanic) systems where heat is derived from active intrusions, these amagmatic geothermal systems rely on different mechanisms (e.g. Moeck, 2014; Jolie et al., 2021). They imply deep fluid circulation, where meteoric waters infiltrate along fault zones, are heated at depth before ascending through areas of high permeability and then emerge at low topography points (López and Smith, 1995; Sonney and Vuataz, 2009; Alt-Epping et al., 2021; Stober et al., 2022).

Granite-hosted hydrothermal systems are critically influenced by structural factors, particularly faults and fracture networks, which

control fluid circulation and pathway in the crust (Sibson, 1987; Hickman et al., 1995; Curewitz and Karson, 1997; Baietto et al. 2008; Yoshida et al., 2009; Koike et al., 2015; Haines et al., 2016). Faults act as primary conduits, enabling the deep penetration of surface waters and their subsequent ascent to finally emerge as thermal springs. The thermal and geochemical characteristics of these systems are controlled by lithological properties, the depth of fluid circulation, and the tectonic activity of fault zones.

This study is focused on exploring the hydro-geochemical processes occurring in geothermal systems in granitic settings, with the example of the Chaudes-Aigues hydrothermal system in the French Massif Central. Renowned for hosting one of the hottest natural springs in Europe (The Par Spring, 80–82 °C, Vasseur et al., 1997), the functioning of the hydrothermal system, which results in a total of 30 thermal springs,

Abbreviations: BRGM, Bureau de Recherches Géologiques et Minières; ISTO, Institut des Sciences de la Terre d'Orléans; LGU, Lower Gneiss Unit; UGU, Upper Gneiss Unit; MGC, Margeride Granite Complex; CVC, Cantal volcanic complex; TDS, Total Dissolved Solids; SM, Supplementary material.

* Corresponding author at: BRGM, F-45060 Orléans, France.

E-mail addresses: e.penhoet@brgm.fr (E. Penhoët), b.sanjuan@brgm.fr (B. Sanjuan), l.guillou-frottier@brgm.fr (L. Guillou-Frottier), manuel.moreira@cnrs-orleans.fr (M. Moreira), laurent.arbaret@univ-orleans.fr (L. Arbaret).

<footnote><sup>a</sup> BRGM, F-45060 Orléans, France</footnote>

<footnote><sup>b</sup> ISTO, UMR 7327, Université d'Orléans, CNRS, BRGM, F-45071 Orléans, France</footnote>
remains imprecise and incomplete. The waters have been identified as a
mixture between a meteoritic component and a hot deep reservoir
(Cailleaux et al., 1976; Vasseur et al., 1997). This phenomenon com-
plicates the estimation of reservoir temperature, which has been esti-
mated between 175 and 195 °C (Cailleaux et al., 1976; Michard et al.,
1981; Vasseur et al., 1997). Similarly, the impluvium (infiltration area)
has not really been identified, although the Aubrac plateau has been
suggested by Vasseur et al. (1997). More generally, dynamics of hy-
drothermal systems leading to the emergence of hot springs has been
rarely studied (see however Vasseur et al., 1997; Magri et al., 2016;
Taillefer et al., 2018; Dupuy, 2021; Carbajal-Martínez et al., 2024).

The objective of this work is to significantly improve the under-
standing of the functioning of the Chaudes-Aigues hydrothermal system
by determining the temperature of the deep reservoir and its di-
mensions. Additionally, it aims to resolve uncertainties regarding the
recharge area of the system. To answer all these questions, a combina-
tion of water geochemistry and thermo-hydraulic modelling was carried
out. First, 22 thermo-mineral springs were sampled (Fig. 1), including
springs located in the middle of the Margeride Granitic Complex (MGC)
and others on the southern side of the Cantal volcanic complex (CVC).
By studying the major components, we were able to isolate several
waters groups and then determine the temperature of the associated
deep reservoirs using geothermometric calculations and geochemical
modeling. Stable isotope analysis allowed to remove the uncertainty on
the location of the impluvium. To better constrain the temperature of the
reservoir feeding the Chaudes-Aigues springs, a parametric study was
carried out using simplified thermo-hydraulic numerical models

reproducing surface heat flow as well as spring temperatures.

2. Hydrogeology of the study area

2.1. *Geological settings*

At Chaudes-Aigues, the thirty thermal springs are located near the
northern edge of the Margeride Granitic Complex (MGC), central part of
the French Massif Central (Fig. 1). This area is characterized by a
complex geological history, including Variscan orogeny processes and
subsequent extensional tectonics during the Mesozoic and Cenozoic. The
MGC was emplaced into the Variscan basement, made of metamorphic
units (LGU and UGU), during the late Carboniferous (~300 Ma) (Talbot
et al., 2005). Its emplacement is associated with post-orogenic extension
and is accompanied by a network of fractures and faults that have been
reactivated during subsequent tectonic episodes (Talbot et al., 2005;
Penhoët et al., 2025). This geological context is further developed in
Penhoët et al. (2025).

Two volcanic edifices are present in the study area, the Cantal Vol-
canic Complex (CVC) to the North-West and the Aubrac plateau to the
South-West. The Cantal stratovolcano was mostly built between 13 and
2.9 Ma, i.e. from the Middle Miocene to the Upper Pliocene (Nehlig
et al., 2001). The slope visible in our area is made up of debris avalanche
deposits, overlain by supracantalian basalts, which correspond to the
last flows emitted (De Goer de Herve et al., 1994). The Aubrac plateau is
made of subalkaline to alkaline basalts or basanites (De Goer de Herve
et al., 1994; Couturié et al., 1979), these flows were produced entirely

<figure><img src="image_4.png" /><figcaption>Fig. 1. Location of sampling points on a simplified geological map. The coordinates correspond to the Lambert 93 projection system EPSG 2154.</figcaption></figure>
during the Upper Tortonian, between 8.7 and 7 Ma, with some Messi-
nian recurrences dated up to 6 Ma (De Goer de Herve et al., 1994). Its
thickness is lower than 100 m on the entire peripheral edge and is
estimated at 300 m below the three highest peaks (De Goer de Herve
et al., 1994).

## 2.2. Hydro and thermal settings

The 30 thermal waters of the Chaudes-Aigues area emerge at tem-
peratures between 50 and 80-82 °C, this latter value being measured
several times in the last decades and showing slight variations. The
emergence area, in the gneissic units, is characterized by a high density
of brittle structures (Fig. 1). The springs are located at the edges of major
NW-SE fault systems crosscut by 2 other Variscan fault zones (Gibert
et al., 1975, De Goer De Herve, 1991; Penhoët et al., 2025). These
NW-SE faults intersect the whole MGC, creating a favourable setting for
hydrothermal activity. These high-permeability areas, due to brittle
deformation fracturing, facilitate the infiltration of meteoric water and
its interaction with the granite at depth. Further south in the MGC (10
km), located in the same NW-SE fault zone, another thermal spring, the
"La Chaldette" spring emerges at 32.5 °C in the granite laccolith. Since
1995, the borehole (Les Granits), drilled at a depth close to 75 m, near
the thermal spring, and reaching a temperature of 34 °C, has replaced
this spring as the water supply for today's spa. Its link with the
Chaudes-Aigues hydrothermal system has never been clearly identified
in the literature (De Goer de Herve et al., 1994).

Numerous cold springs exist at the surface of the MGC (Fig. 1). The Mazel springs (Fontaine Basse, Fontaine Haute and le Ranc) are located in its southern part. The only available information is given by Négréel et al. (2000), who have determined, using isotopes analysis, that these waters originate from a deep circulation in the MGC. On the southern Cantal side, the Moulinges, Sainte-Marie and Fontanes cold springs have not been studied before. This large-scale sampling, including the cold springs far from Chaudes-Aigues, will enable the hydrothermal system lateral extension to be delineated.

From a thermal point of view, Chaudes-Aigues is a suitable case study of geothermal system in granite environment. Although the springs are surrounded by two volcanic systems, the CVC and Aubrac, it is difficult to assert that the heat source of the system comes from a magmatic context. In fact, no study has confirmed the existence of a present-day potential magma chamber in the CVC. The differentiated magmas are thought to come from more than 20 km below the surface (Nehlig et al., 2001). In the case of Aubrac plateau, made of basalts (De Goer de Herve et al., 1994), magma ascends via dykes without the need of a magma chamber. Chaudes-Aigues can therefore be considered as an amagmatic system. According to Vasseur et al. (1997), the thermal gradient is around 30-35 °C/km, and a regional heat flow of around 110 mW/m² has been estimated (Lucazeau et al., 1984). Heat production resulting from the radioactive decay of the sub-horizontal granitic laccolith (around 3.8 mW/m³, Lucazeau et al. 1984) release, for a thickness of 7 km, the amount of 27 mW/m², i.e. 24 % of the surface heat flow. As suggested by Lucazeau et al. (1984), mantle heat flow is anomalously high beneath this part of the French Massif Central, reaching values around 60 mW/m².

## 2.3. Past and present models of the Chaudes-Aigues hydrothermal system

Models describing the hydrothermal system at Chaudes-Aigues have evolved over time. De Launay (1906) proposed that the water came from the gneissic plateau near the town and seeped down to a depth of 2500 m before rising, under the effect of pressure, through the microgranite veins. Henou (1973) refined this theory by locating the reservoir at 2000 m depth, with the drains still being the microgranite veins. Subsequent geochemical studies carried out by Cailleaux et al. (1976), followed by Michard et al. (1981), suggested a mixture of deep warm water and colder surface water. The temperature of a deep reservoir has been

estimated at around 175–200 °C, on the basis of water chemistry data
and use of chemical geothermometers as well as geochemical modelling
of water–rock–gas interaction processes. Vasseur et al. (1997) proposed
the first simplified diagram of the hydrothermal system, including
infiltration zone, fluid pathways and emergence conditions. According
to their model, two distinct water circuits would coexist within the MGC:
a surficial, where cold waters infiltrate from the Aubrac and Margeride
mounts or the CVC and reach 2000 m depth. Another part of the water
would flow through a deep path reaching 5000 m depth. These two
circuits would mix in an intermediate reservoir at a depth of 2000 m,
with an estimated proportion of 40 % deep hot water from a geothermal
reservoir at 190 ± 10 °C and 60 % shallow cold water.

## 3. Sampling and analytical methods

The water and gas samples presented here were collected in two field campaigns. The first in 2011–2013 (unpublished data collected by BRGM) and the second in 2023 (this study). Same methods for sampling and analyses have been used for both.

### 3.1. Water samples

A total of 32 water samples was collected including 27 samples of thermo-mineral waters and 5 samples of surface waters (see Table S1 in Supplementary Material, SM, and Fig. 1). Collection of the fluid samples in the field was accompanied by appropriate on-site measurements such as fluid temperature, conductivity, pH, Redox potential, alkalinity and H₂S detection. The temperature, conductivity, pH and redox potential measurements were performed on the raw fluid samples using a WTW portable multiparameter measuring device and the corresponding electrodes, whereas alkalinity was analysed using HCl titration on fluid samples filtered at 0.45 µm. Absolute uncertainty concerning the pH measurements was 0.05 pH units and relative uncertainty concerning the other parameters varied from 5 % to 10 %, depending on the parameter and the range of measured values. For the parameters measured using the WTW portable device and the corresponding electrodes, absolute and relative uncertainties are given in their instruction manual.

Collection and conditioning of all the water samples followed the classical procedures recommended for each of the chemical and isotopic analyses to be performed. Thus:

- For the chemical analysis of major anions and some trace elements, such as Cl, SO₄, Br, F, NH₄ and PO₄, the water samples were filtered at 0.45 µm and collected in 100 ml polyethylene bottles.

- For the chemical analysis of major cations, the water samples were filtered at 0.45 µm, then acidified using Suprapur HNO₃ and collected in 100 ml polyethylene bottles.

- In order to avoid silica precipitation, the samples of hot water for dissolved silica analysis were collected in 50 ml polyethylene bottles and immediately diluted by a factor of 10 using Milli-Q water.

- For the chemical analysis of the other trace elements, such as B, Sr, Li, Ba, Mn, Fe, Al, Cs, Rb, Ge, As, Nd, Ag, Cd, Co, Cr, Cu, Ni, Pb and Zn, as well as for the isotopic Li and Sr analyses, the water samples were filtered at 0.1 µm, then acidified using Suprapur HNO₃ and collected in 100 ml polyethylene bottles.

- For the isotopic analysis of B, the water samples were filtered at 0.1 µm, then acidified using Suprapur HNO₃ and collected in 1 l polyethylene bottles.

- Untreated fluid samples for the isotopic analysis of D and ¹⁸O in the water and of ¹³C in the carbon dioxide were collected in 100 ml and 1 l polyethylene bottles, respectively.

- For the isotopic analysis of ¹⁸O and ³⁴S in the dissolved sulphate, cadmium acetate was added to the water samples collected in 2 l polyethylene bottles.
All the chemical analyses for both major and trace elements in the collected water samples were done in the BRGM laboratories using standard water analysis techniques such as Ion Chromatography (IC), Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), Flame Emission Spectrophotometry, and Colorimetry. Depending on the concentration of the species in solution, the relative analytical uncertainties vary from 2 to 5 % for major species and from 10 to 15 % for trace and infra-trace elements.

The isotopic analyses of the water samples (δ<sub>D</sub> and δ<sup>18</sup>O of the water, δ<sup>18</sup>O and δ<sup>34</sup>S of the dissolved sulphate, δ<sup>13</sup>C of the carbon dioxide, plus the δ<sup>7</sup>Li, δ<sup>11</sup>B and δ<sup>87</sup>Sr/δ<sup>86</sup>Sr) were also performed in the BRGM laboratories using Thermo Ionization Mass Spectrometry and Neptune Multi Collector ICP-MS. More details relative to the BRGM analytical procedures are given in Millot et al. (2010). Deuterium and oxygen-18 isotope analyses were systematically carried out on all sampled water points. Values for Li, <sup>11</sup>B and the δ<sup>87</sup>Sr/δ<sup>86</sup>Sr isotope ratio were determined at most sampling points. Values for <sup>34</sup>S and δ<sup>18</sup>O of dissolved sulphates and <sup>13</sup>C of dissolved CO<sub>2</sub> were measured on only a few selected samples.

The absolute uncertainty for the δ<sub>D</sub> and δ<sup>18</sup>O analyses of the water samples was ± 0.8 ‰ and ± 0.1 ‰, respectively. The absolute uncertainty for the δ<sup>18</sup>O and δ<sup>34</sup>S analyses of the dissolved sulphate was ± 0.1 ‰ and ± 0.3 ‰, respectively, while for the δ<sup>13</sup>C analyses of the carbon dioxide, it was ± 0.1 ‰. The external reproducibility of the δ<sup>7</sup>Li and δ<sup>11</sup>B analyses was estimated at around ± 0.5 ‰ and ± 0.3 ‰, respectively, and the in-run precision of the δ<sup>87</sup>Sr/δ<sup>86</sup>Sr ratio was generally better than ± 10 × 10<sup>-6</sup> (2 σm, where σm is the standard error of the mean).

## 3.2. Gas samples

Gas samples were collected in specially designed glass flasks. Only two samples of non-condensable gases associated with the thermo-

mineral waters of the Lavoir and Magnac have been sampled without significant atmospheric contamination. The corresponding chemical analyses (CO<sub>2</sub>, N<sub>2</sub>, O<sub>2</sub>, He, H<sub>2</sub>, H<sub>2</sub>S, Ar, CH<sub>4</sub>, ethane, propane, butane and pentane) were carried out by gas chromatography at the BRGM laboratories. These chemical analyses were also carried out on one gas sample extracted from the water of the Montchanson spring, after acidification with nitric acid. The relative analytical uncertainty of the measurements is 3 ‰, except for He, H<sub>2</sub> and H<sub>2</sub>S gases, where it is 5 %. In addition, mass spectrometry analyses of δ<sup>13</sup>C bound to free carbon dioxide were carried out on these three samples in BRGM laboratories. The absolute uncertainty on these measurements is ± 0.1 ‰.

Analyses of helium and neon abundances as well as of the isotopic ratio (R/R<sub>a</sub> = (³He/⁴He)<sub>sample</sub>/(³He/⁴He)<sub>atmospheric</sub>) were carried out in the noble gas laboratory of the Institut de Physique du Globe de Paris (IPGP) on the first two samples of non-condensable gases. These analyses were performed using the ARESIBO mass spectrometer. Absolute analytical uncertainties on R/R<sub>a</sub> isotope ratios range from ±0.007 to ±0.02, depending on the helium abundance. The R/R<sub>a</sub> isotope ratio was also measured on dissolved gases in the water from the Montchanson thermo-mineral spring. The absolute analytical uncertainty is ±0.02.

# 4. Analytical results

## 4.1. Water chemistry and isotopes

The analytical results obtained for major and trace species are reported in SM Tables S4 and S5, respectively. The ionic balance (I.B.) values, all below 5 ‰, suggest good analytical quality for major species.

The studied thermo-mineral waters are all carbo-gaseous, with emergence temperatures ranging from 2.2 °C (Mazel Spring in Margeride) to 80 °C (Par Spring in Chaudes-Aigues) and variable TDS values ranging from 0.4 g/l (Loubaresse Spring) to 4.8 g/l (Fontaine Basse 1, FB 1). In the Piper diagram shown in Fig. 2, most thermo-mineral waters

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Piper diagram. The thermal waters were grouped by geographical location: South Cantal Area (Sainte-Marie, Moulinges, Fontanes), Chaudes-Aigues area (Par 2012, Par 2023, Lavoir 2012, Lavoir 2023, Ban 2012, Ban 2023, Boucharinc, Vialard, Tourette, Poste, Condamines, MTC 2012, MTC 2023, Laval, Loubaresse), Est Margeride area (Mazel, FH, FB1, FB2, Ranc) and West Margeride area (LC 2012, LC 2023).</figcaption></figure>
have a sodium-bicarbonate facies. Springs of the Margeride Est group and some of the Chaudes-Aigues area (Loubaresse, Jenny and Laval) deviate from this trend and correspond to sodium chloride facies. The Condamines spring (Chaudes-Aigues area) is alone in the calcic and magnesian chloride sulphate facies.

According to Beaucaire et al. (1987), the major components of carbo-gaseous waters (Na and HCO₃) and soluble elements such as Cl, Br

and B are mainly acquired in the deep reservoir and are only subsequently affected by mixing processes with surface waters. Concentration ratios of these elements remain constant over large areas. These ratios can therefore be used to classify thermo-mineral waters into different groups, depending on the deep reservoir to which they belong.

Fig 3a and b represents, respectively, bromide concentrations and boron concentrations as a function of chloride concentrations. Fig. 3c

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a) Concentrations of bromide, (b) boron as a function of chloride ions. (c) Bicarbonate concentrations as a function of sodium (Na) concentrations. The mixing lines have enabled different reservoirs to be identified, highlighted by three different colours: Green for the Chaudes-Aigues, Orange for the East Margeride and Blue for South Cantal area.</figcaption></figure>
represents bicarbonate ions as a function of sodium. Three main groups of thermo-mineral waters belonging to different deep reservoirs can be identified. These generally correspond to the geographical location of the springs.

The first group includes the waters from the thermo-mineral springs of Chaudes-Aigues area, mainly located on the northern edge of the MGC (see Fig. 1), with Cl/Br mass ratios ranging from 361 to 515 (Fig. 3a). These waters also present values from 36 to 76 for the Cl/B mass ratio values (Fig. 3b) and similar HCO₃-Na ratios (Fig. 3c). If the thermal water of La Chaldette (LC 2012 and 2023) indicates a similar HCO₃/Na ratio to that of the Chaudes-Aigues waters, its Cl/Br and Cl/B mass ratio values (312–253 and 31–27, respectively) are relatively lower.

A second group of waters is made up of thermo-mineral springs of the South Cantal area (Fig. 1, Sainte-Marie, Fontanes and Moulinges), with Cl/Br and Cl/B mass ratio values ranging from 484–518 and 172–271, respectively (Fig. 3a and b). Their comparable HCO₃/Na ratio values are the highest ones of the 3 groups (Fig. 3c) and linked to their low measured pH values (5.76–6.19). They suggest relatively high-pressure of associated CO₂ gas. This could be due to their proximity with the CVC.

The third group gathers the East Margeride thermo-mineral springs (Fig. 1, Mazel, Fontaine-Haute, Fontaine-Basse 1, Fontaine-Basse 2, Ranc), with Cl/Br mass ratio values ranging from 171 to 323 (the lowest, Fig. 3a). Cl/B mass ratio values range from 94 to 157 (Fig. 3b). Their HCO₃-Na ratio values are similar and lie between those of the waters from the two other groups (Fig. 3c).

The existence of these three groups and linear correlations suggests processes of mixing between endmembers of deep, and therefore hotter waters, and surface cold waters, as these endmembers rise to the surface.

The isotopic analysis results are given in SM Table S6 and will be interpreted in the discussion section.

## 4.2. Gas chemistry and isotopes

All the obtained results are reported in the SM Table 5. As shown in this table, the non-condensable gases associated with the waters of the two thermo-mineral springs that were correctly sampled (Lavoir and Magnac springs) are mainly CO₂. The methane concentrations of 0.08 and 0.04 %, respectively, are relatively low. Slight contamination by air, most probably during sampling, which contributes small quantities of nitrogen, oxygen and argon, is observed for these gas samples.

# 5. Discussion

## 5.1. Origin of thermo-mineral waters

Stable isotopes in water provide information about the origin of water (Craig, 1963; Blavoux, 1995). Using the six surface water samples taken, a Local Meteoric Water Line (LMWL) was drawn, parallel to the Global Meteoric Water Line (GMWL; Craig, 1963). Its equation is shown in Fig. 4.

The different groups of waters are distributed around the GMWL, indicating their meteoric origin. Waters from the South Cantal group (blue dots, see Fig. 4) and the East Margeride (green dots, see Fig. 4) are located on the local meteoric line, with some points shifted slightly to the left, indicating a possible interaction with CO₂ (Ferrara et al., 1965; Ferronskii and Poliakov, 1982; Karolytė et al., 2017). Most of the waters from the Chaudes-Aigues group is located to the right of the GMWL. This shift may be due to interactions with relatively hot waters and/or low water-rock ration values (high water-rock interaction processes).

Fig 5a and b, representing O and D isotopic values as a function of Cl concentrations, mobile species, also suggest that most of the studied water samples are the result of mixing processes. Except Magnac spring, waters of the Chaudes-Aigues group would therefore be the result of a mixture of surface component, the composition of which would be close to that of present-day meteoric water (blue crosses) and an endmember of deep and hot water from the geothermal reservoir. The Montchanson water (MTC 2012) is the closest to the isotope composition and chloride concentration of this deep hot endmember. These results confirm the studies carried out by Vasseur et al. (1997). The Magnac water would be also the result of a mixing of this endmember and cold surface water, but with a lower isotope signature than that of the present-day meteoric waters (Fig. 5a and b). This could also be the case for the La Chaldette water, for which the contribution of the endmember of deep and hot water would, however, be much less significant than for the Magnac water. La Chaldette water would be mainly made up of surface waters.

The waters of the East Margeride group are also the result of a mixture of current surface waters (blue crosses), identical to those of the Chaudes-Aigues group, with a deep component, which is different this time.

For the South Cantal group, made up of only 3 springs, the Fontane spring has the most mineralized water (including the Cl concentration of 150 mg/l) and could have the isotope signature and chloride concentration closest to the deepest and hottest water endmember of this group.

Using mixing lines, we can estimate the composition of the reservoir. Here the deep poles would have a concentration of 180 mg/l and 56 mg/

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Deuterium (δ²H) values versus 18-oxygen (δ¹⁸O) values in sampled waters. Colors correspond to previously identified groups: Orange for Chaudes-Aigues, Green for Margeride and Blue for Sainte-Marie. Surface waters (streams, rivers) are symbolized by blue crosses. The local meteoric line is based on these points.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. (a) δ²H and (b) δ¹⁸O concentrations versus chloride concentrations in sampled water. The colours of the dots correspond to the groups identified above: green for the East Margeride group, orange for Chaudes-Aigues and blue for South Cantal. The blue crosses correspond to the rivers or streams sampled, the purple to a stream near Montchanson (Vasseur et al., 1997).</figcaption></figure>

l for the Chaudes-Aigues and Margeride groups respectively (Fig. 5a and b).

Chloride ions are mobile species that are involved in very few chemical reactions, so they can be used as tracers to identify dilutions. Using the deep compositions previously estimated, it is possible to calculate the mixing proportions of the studied springs. As mentioned above, the water from the Montchanson spring is the closest to the reservoir, representing on average 80 % of its composition (Table 1). Conversely, the rest of the springs in the Chaudes-Aigues group are more diluted, with an average of 60 % meteoric waters (Table 1). Water from the La Chaldette spring is mainly meteoric water, with less than 10 %

<caption>**Table 1**</caption>

Estimation of the percentages of deep pool and surface water originally constituting the water of the thermal springs sampled. The percentages are calculated using the chloride ion compositions estimated using Fig 5a and b, 180 mg/l and 56 mg/l respectively for the Chaudes-Aigues and East Margeride groups. For the South Cantal group, Fontanes is supposed to be the deep endmember.

<table><thead><tr><th></th><th>Springs</th><th>Deep Reservoir (%)</th><th>Meteoric water (%)</th></tr></thead><tbody><tr><td rowspan="3">Chaudes-Aigues</td><td>Par</td><td>42</td><td>58</td></tr><tr><td>Magnac</td><td>61</td><td>39</td></tr><tr><td>MTC</td><td>82</td><td>18</td></tr><tr><td rowspan="5">Margeride</td><td>Mazel</td><td>17</td><td>83</td></tr><tr><td>FH</td><td>52</td><td>48</td></tr><tr><td>FB 1</td><td>68</td><td>32</td></tr><tr><td>FB 2</td><td>58</td><td>42</td></tr><tr><td>Ranc</td><td>53</td><td>47</td></tr><tr><td rowspan="2">South Cantal</td><td>Sainte Marie</td><td>24</td><td>76</td></tr><tr><td>Moulinges</td><td>31</td><td>69</td></tr></tbody></table>

coming from a deep component (Table 1).

The group of East Margeride springs contains less diluted water, with an average of 60 % deep reservoir water (Table 1). Only the Mazel Spring is predominantly surface water (83 %).

In the case of the group of South Cantal springs, Fontanes water is the closest to the deep hot endmember.

Using oxygen isotopes, it is possible to determine the altitude at which water has infiltrated (Taillefer et al., 2018; Dupuy, 2021; Carbajal-Martínez et al., 2024). Using surface water samples, a local isotope gradient can be calculated (see Fig. 6a). Elevation values were calculated using a local gradient based on surface water samples. The equation is shown in Fig. 6a.

The springs in South Cantal group have the lowest altitudes (blue dots in Fig. 6a). Using the isotopes, water is supposed to come from 625 to 700 m a.s.l. The Chaudes-Aigues springs (orange dots in Fig. 6a), for their part, indicate infiltration altitudes ranging from 650 to 1300 m a.s. l. The dilution of thermal water has little effect on altitude, and the composition of the deep endmember (−9.6 ‰, see Fig. 5b) indicates an altitude of 1300 m a.s.l. The springs in the MGC (East Margeride group, green dots in Fig. 6a) come from the highest altitudes, 1000–1450 m a.s. l.

Using average values for the South Cantal, Chaudes-Aigues and East Margeride groups, we find: 675 m, 900 m and 1250 m respectively. These altitudes were compared with a digital elevation model to try and identify possible feeding areas for each group. As can be seen from Fig. 6b, the impluvium of the Sainte-Marie group may correspond to the immediate environment of the springs (grey area). The southern slopes of the Cantal and the right bank of the Truyère correspond to these altitudes. For the Chaudes-Aigues springs, the feed zone appears to be
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** (a) Inferred recharge elevation of thermos-mineral waters sampled using a regional gradient determined with modern surface water (streams or rivers). Chaudes-Aigues group is represented in orange, South Cantal blue and East Margeride green. (b) Topographic map showing the different supposed impluvium, blue areas correspond to the elevation lower than 850 m a.s.l, grey between 850 and 1000 m a.s.l, brown higher than 1000 m a.s.l, green above 1400 m. Thermo-minerals springs, here represented by yellow circles, correspond to those shown in Fig. 1. The MGC boundaries are shown in white dotted lines. The averages are calculated for all the spring values in each group.</figcaption></figure>

further away. In fact, the eastern or western part of the MGC or the CVC in its upper part may correspond to the altitudes mentioned above. For the Mazel springs, the even higher altitude reduces the surface area available. The eastern part of the MGC, close to the springs, has consistent altitudes. Further away, the Cantal and the south-western end of the MGC also reach 1400 m a.s.l.

Since altimetry cannot eliminate neither the CVC nor the Aubrac or the southern edge of the MGC, the hydrogeological characteristics of the region must be considered. The CVC is separated from the Chaudes-Aigues springs by a natural barrier: the Truyère river. It seems

unrealistic that the flows would be deep enough to avoid it. Furthermore, the structure of the Lower Gneiss Unit, located between the volcano and the springs, is unfavourable as their foliation is generally oriented towards the north (Penhoët et al., 2025). A final reason against this would be the sedimentary basin (Aurillac basin) beneath the volcano (Nehlig et al., 2001), which would be more likely to retain fluids.

Concerning the Aubrac, although it is made up of very deep drains (basalts upwellings), it does not seem to be the best candidate either. Due to its highly fractured nature and the presence of volcanic rock, the plateau is an area of high permeability and therefore high infiltration.
Eighty-nine springs (including seasonal ones) have been counted here (Bourlier et al., 2005). However, like the CVC, it remains at a significant distance (more than 20 km) from the Chaudes-Aigues springs. Although the vertical circulation of fluids seems to be confirmed, the geological map does not show any structures enabling fluids to be transported to Chaudes-Aigues. Finally, no evidence of basalt or other volcanic products was found in the water analyses. Consequently, the amagmatic characteristic of the Chaudes-Aigues hydrothermal system is strengthened.

## 5.2. Reservoir temperature

In hydrothermal contexts, it is common practice to use chemical and isotopic geothermometers to estimate the reservoir temperature of emerging thermal spring waters. These thermometric relationships using the chemical and isotope compositions of these waters are based on empirical, semi-empirical or experimental relations derived from known or unknown chemical equilibrium between water and minerals taking place in the geothermal reservoir. They assume that these waters have reached a thermodynamic equilibrium at depth and that this equilibrium has not been altered during their ascent to the surface.

Since 1965, numerous chemical and isotope geothermometers have been proposed in the literature. Among the most used are geo-thermometers such as Silica (Quartz or Chalcedony; Fournier, 1977; Michard, 1979), Na-K (Fournier and Potter, 1979; Arnórsson, 1983; Giggenbach, 1988; Michard, 1983; 1990), Na-K-Ca (Fournier and Truesdell, 1973), K-Mg (Giggenbach, 1988), Na-Li (Fouillac and Michard, 1981; Kharaka et al., 1982) and δ¹⁸O~H₂O~SO₄ (Lloyd, 1968; Mizutani and Rafter, 1969; Zheng, 1999; T. T. Boschetti et al., 2011).

### 5.2.1. Group of the Chaudes-Aigues thermal waters

Given that all waters in the Chaudes-Aigues group all seem to be the result of a mixture between an endmember of deep hot water from the geothermal reservoir and surface waters (Figs. 4a-c, 6a and c), we will only attempt to estimate the temperature of this endmember. The Par thermal spring, which indicates the highest temperature of discharge and has a relatively high flowrate, has probably the water for which the Cl and SiO₂ concentrations are the least affected other than by the mixing process (depletion or enrichment in dissolved silica by mineral precipitation or dissolution reactions, for example). As the Cl concentration of the deep hot endmember from the geothermal reservoir was previously determined (180 mg/l), we can therefore calculate its corresponding concentration of dissolved silica, using the following relationship:

$$ [\text{SiO}_2]_{\text{reservoir}} = [\text{SiO}_2]_{\text{par}} \frac{[\text{Cl}]_{\text{reservoir}}}{[\text{Cl}]_{\text{par}}} \quad (1) $$

This results in a concentration of dissolved silica of the order of 200–220 mg/l (see Fig. 7).

Using the Quartz-geothermometer (Fournier, 1977; Michard, 1979), the temperature of the deep hot endmember in the geothermal reservoir can be then estimated at 180–190 °C.

This estimation is very close to those using the geothermometers such as Na-K (Michard, 1983; Arnórsson, 1983) and Na-K-Ca (Fournier and Truesdell, 1973), which give values of 193–202 °C, 202–213 °C, and 192–196 °C, respectively, for the Par water samples (see SM Table S8 and S1).

The K-Mg geothermometer defined by Giggenbach (1988) gives a very underestimated temperature value (98 °C), but this is not surprising as it is known that, unlike the hot waters which are depleted in magnesium, the cold surface waters can be rich in this element (Michard and Roekens, 1983; Guo et al., 2019) and can disturb the chemical equilibrium of the hot waters with their deep reservoir, during the process of mixing between these waters.

Among the δ¹⁸O~H₂O~SO₄ thermometric relationships applied on the Par water sampled in 2011, only those determined by Zheng (1999) and Boschetti et al. (2011), give values of temperature of 164 °C and 171 °C, close to those estimated using silica-quartz, Na-K and Na-K-Ca geothermometers, but slightly lower. The other relationships determined by Mizutani and Rafter (1969) and Lloyd (1968) yield lower estimates (142 and 149 °C, respectively).

Despite the uncertainties on the chemical analyses as well as on the isotope values, these estimates of reservoir temperature for the Chaudes-Aigues geothermal system, taken as a whole, are very close to that made by Cailleaux et al. (1976) and Vasseur et al. (1997) of 190 ± 10 °C that can be considered as the most appropriate one.

During the ascent of gaseous hot waters to the surface, the concentrations of many chemical species can be considerably modified either by reactions of dissolution linked to the high cold aggressiveness of CO₂-rich waters, or by reactions of precipitation due to cooling and degassing of the solutions. Consequently, some variations in temperature estimates for each of the Chaudes-Aigues springs are to be expected, depending on the time taken to reach the surface, cooling processes, and the extent of mixing with surface waters, as shown in Table S8 (SM) and Fig. 8.

Geochemical modelling (see details in S1, SM) support the consistency of our geothermometric results and confirm that the chemical composition of the deep fluid from the Chaudes-Aigues geothermal reservoir, before mixing with cold surface waters (see Table 2), is

<figure><img src="image_10.png" /><figcaption>Fig. 7. (a) Silica concentration as a function of chloride concentration in sampled water. The green (Margeride group) and orange (Chaudes-Aigues group) lines correspond to the concentrations of chloride of the hot deep reservoir, previously estimated at 56 mg/l and 180 mg/l respectively. Temperature of the two hot deep reservoirs have been estimated, T₀z: Fournier (1977). (b) Zoom on the cluster of points corresponding to the Chaudes-Aigues thermal springs, The Par Spring (Par 2012 and 2023) used for the estimate is highlighted by red dots.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Estimated reservoir temperatures for the Chaudes-Aigues group using geothermometry. T<sub>Na/K</sub> (1) : Fournier (1979); T<sub>Na/K</sub> (2) : Michard (1979); T<sub>Na/K</sub> (3) : Arnórsson (1983) T < 250 °C; T<sub>Na/K</sub> (4) Giggenbach (1988); T<sub>Na/K/Ca</sub> : Fournier and Truesdell (1973). The dashed red line indicates the temperature (185 °C) of the Chaudes-Aigues reservoir, which has been estimated using numerical modelling (see S2, SM).</figcaption></figure>

<caption>**Table 2**</caption>

Estimated composition of the deep endmembers based on previous dilution coefficients. For the South Cantal, the deep endmember has the composition of Fontanes spring.

<table><thead><tr><th>Group</th><th>T</th><th>pH</th><th>Na</th><th>K</th><th>Ca</th><th>Mg</th><th>Cl</th><th>SO<sub>4</sub></th></tr></thead><tbody><tr><td>Chaudes-Aigues</td><td>°C<br>180–200</td><td>6.92–7.08</td><td>mg/l<br>610–653</td><td>mg/l<br>68–72</td><td>mg/l<br>50–54</td><td>mg/l<br>0.10–0.20</td><td>mg/l<br>175–185</td><td>mg/l<br>210–215</td></tr><tr><td>Margeride</td><td>80–120</td><td>6.00</td><td>1370–1380</td><td>50–60</td><td>220–230</td><td>15–20</td><td>50–60</td><td></td></tr></tbody></table>

controlled at 190 °C by an equilibrium reaction with respect to a mineralogical assemblage constituted, at least, of Quartz, K-Feldspar, Albite, and Illite (Muscovite). The CO₂ pressure values at 190 °C estimated using these pH values and HCO₃ concentration range from 2.3 to 3.4 bar (2.8 bar for pH = 7.0 ± 0.1).

At this temperature in the reservoir, the deep fluid is also in equilibrium with respect to Laumontite, which could control the calcium concentration if this element is only affected by the process of mixing with cold surface waters and no other process. As we have seen that magnesium was most probably enriched by mixing with surface waters, we can estimate its concentration in the reservoir water before mixing if we assume that it is controlled by the Chlorite-Albite-Muscovite equilibrium reaction at 190 °C and if we use the K-Mg thermometric relation proposed by Giggenbach (1988), which is based on this equilibrium reaction. In this case, the concentration of Mg would be of the order of 0.15 mg/l.

Reservoir temperature beneath the Chaudes-Aigues thermal waters has also been estimated by numerical simulations of hydrothermal convection within a permeable fault zone (see SM S2). Appropriate thermal boundary conditions allowed to reproduce spring temperature (80–82 °C) and surface heat flow (110 mW/m²) for a reservoir temperature of 185 °C at a depth of 5 km, thus confirming the previous estimated temperatures using geothermometers and geochemical modelling.

### 5.2.2. La Chaldette thermal water

The thermal water from the La Chaldette drillhole, which is located in the granite basement, can be considered as a particular case. Even if a small contribution of deep hot water from the Chaudes-Aigues geothermal system cannot be ruled out (of the order of 10 %) and most of the fluid is constituted of surface water, most of the values of reservoir temperature estimated using chemical and isotope geothermometers are relatively comparable and range from 85 to 140 °C

(see Table S8, SM and Fig. 8). This result suggests that this water is close to the full chemical equilibrium with respect to the reservoir rocks at a temperature of 110 ± 20 °C. Its pH value estimated at this temperature (7.33) is higher than for the Chaudes-Aigues thermal waters and its associated CO₂ pressure (0.093 atm) is much lower. Consequently, this fluid is less aggressive towards the rocks it encounters as it rises to the surface and presents far fewer risks of CO₂ outgassing and scaling deposits. Very probably, it retains almost all of its chemical composition resulting from equilibrium reactions at 110 °C with respect to the rocks of its geothermal reservoir, which is also colder and shallower than that of Chaudes-Aigues.

### 5.2.3. Group of the east Margeride waters

For the thermo-mineral waters in this group, the same approach as that applied to the waters from the Chaudes-Aigues springs can be used to determine the concentration of silica in the deep hot endmember, before mixing with surface waters, and at the same time estimate its temperature. A chloride concentration of around 56 mg/l corresponds to a silica concentration of 75 mg/l (Fig. 7). Here again, it can be estimated that there was no precipitation of silica before and after mixing of the waters, the concentration of dissolved silica being relatively low. In the Icelandic volcanic environment, Arnórsson (1975) showed that Quartz was the controlling phase above 170 °C, and by Chalcedony below 125 °C. In the waters from granitic environment studied by Michard (1990), the transition temperature is close to 100 °C. In general, waters with aquifer temperature of < 80 °C equilibrate with Chalcedony, but some low-temperature waters can also equilibrate with Quartz. In the 80–100 °C temperature range, silica concentrations are intermediate between Quartz and Chalcedony solubilities. Depending on whether the Chalcedony or the Quartz geothermometer (without vapour loss) is used, temperatures of 90–93 °C (Fournier, 1977; Arnórsson, 1983) to 122 °C (Fournier, 1977) are estimated for the deep endmember of the East Margeride waters.
This temperature range is close to the values estimated using the Na/
K thermometric relationships proposed by Michard (1990) and Arnórsson,
son, 1983, for thermal waters with temperature values ranging from 25
to 150 °C. Using the chemical composition of the deep endmember (see
Table 2), these estimates are 108 and 113 °C, respectively. The isotopic
compositions of the springs indicate a temperature between 68 and 79.5
°C (Table S8, SM). Consequently, for the East Margeride thermo-mineral
waters, the temperature of their deep reservoir can be estimated at 100
± 20 °C. If we assume that the Mg in the reservoir water before mixing is
controlled by the Chlorite-Albite-Muscovite equilibrium reaction at this
temperature and use the K-Mg thermometric relation proposed by
Giggenbach (1988), based on this equilibrium reaction, its concentra-
tion would be then close to 18 mg/l.

If we consider, like Sarazin et al. (1986), that the pH value of the Mazel water is the less affected by outgassing, because this water is the most diluted by the shallow waters before reaching the surface, we can estimate a pH value close to 6.00 for the deep reservoir water at 100 °C, using the pH value measured at the discharge of the Mazel spring (6.19 at 2.2 °C) and the previous approximate relationship proposed by Michard et al. (1981). The associated CO₂ pressure is then calculated to be close to 14.9 atm. This order of magnitude of CO₂ pressure, between 10 and 15 atm, is quite common in many places in the Massif central (Michard et al., 1987). Numerous geothermometers give either higher or lower temperature estimates (see Table S8, SM, and Fig. 9), which are probably due to secondary reactions taking place during the ascent of the aggressive and unstable CO₂-rich water of the deep geothermal reservoir towards the surface (probable reactions of mineral dissolution and scaling precipitation due to degassing of CO₂).

5.2.4. Group of the south Cantal thermal waters

For the Sainte-Marie, Fontanes and Moulinges thermo-mineral waters in this group, the chemical geothermometers give relatively different temperature estimates (see Table S8, SM and Fig. 10), which are probably due to the significant changes in a large part of the chemical composition of these acidic and aggressive CO₂-rich waters between the deep reservoir and the surface, which may also degas or mix with surface waters, particularly as regards species such as potassium, calcium and magnesium.

For these waters, the reservoir temperature is evaluated at 130 ± 15 °C, mainly based on the temperatures estimated for the Fontanes thermo-mineral water, using the silica-Quartz geothermometer (Fournier, 1977), the Na-K thermometric relationships proposed by Michard (1990) and Arnórsson (1983), and the δ¹⁸O₃H₂O₅SO₄ isotope thermometric relationships defined by Lloyd (1968), Boschetti et al. (2011) and Zheng (1999), which give relatively concordant values of

116 °C, 127 and 138 °C, and 129 and 120 °C, respectively. As mentioned
before (see Fig. 4 and 5), the Fontane water could be mainly constituted
of deep CO₂-rich fluid coming from the geothermal reservoir, which
cooled and partially interacted with neighbouring rocks as it rose to the
surface.

At the reservoir temperature of 125 °C, the pH value was calculated to be close to 5.80, using the lowest pH value (5.76) measured at the discharge of the three thermal springs. The associated CO₂ with the Fontanes water was then calculated to be around 8.7 atm in the reservoir conditions, confirming the high quantities of CO₂ accompanying these waters.

5.3. Isotopic signatures of the thermal waters

As the CO₂- rich deep thermo-mineral waters are aggressive due to
their low pH value, they can interact with the rocks they encounter as
they rise to the surface and be enriched in trace elements. They can also
degas at the surface, causing scaling deposits such calcite, which can
trap trace elements. In addition, these waters can mix with cold shallow
waters. Consequently, unlike bromide and boron, numerous trace ele-
ments such strontium and lithium have not always a conservative
behaviour, due to chemical re-equilibration processes, and it is difficult
to know what their concentration was in the deep water of the
geothermal reservoir. Frequently, the use of Sr, Li and B isotopes can
help to better understand the processes controlling these elements and
their behaviour. They can be useful tracers of water-rock interaction
processes and of mixing of waters from different origins.

5.3.1. Strontium and lithium signatures

Fig. 11a and b indicate that the group of East Margeride waters, which is completely located in the granitic complex, is very enriched in strontium and lithium, compared to the other groups, while the isotopic values of these two elements are tighter (0.712964–0.718803 and 1.4–5.5 ‰, respectively), except for the Par, Ban and Lavoir thermo-mineral waters from Chaudes-Aigues, which have higher values of ⁸⁷Sr/⁸⁶Sr ratio ranging from 0.721948 to 0.721996, and the Moulinges water from the South Cantal area, which indicates a lower value of 0.711970 (Table S5, SM).

In these two Figures, except the Ranc water, the East Margeride
fluids seem to confirm the mixing between a deep saline endmember
water and surface dilute waters. For the waters of the other groups, this
is much less obvious. The high amounts of Sr and Li observed in the East
Margeride waters compared to those of the other waters (Fig. 12a and b),
must be probably associated to changes of granite mineralogical facies,
given that their estimated reservoir temperature is the lowest one.

<figure><img src="image_12.png" /><figcaption>Fig. 9. Estimated reservoir temperatures The East Margeride group using geothermometry. T<sub>Na/K</sub> (1) : Fournier (1979); T<sub>Na/K</sub> (2) : Michard (1979); T<sub>Na/K</sub> (3) : Arnórsson (1983) T < 250 °C; T<sub>Na/K</sub> (4) Giggenbach (1988); T<sub>Na/K/Ca</sub> : Fournier and Truesdell (1973).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Estimated reservoir temperatures for the South Cantal group using geothermometry. TNa/K (1) : Fournier (1979); TNa/K (2) : Michard (1979); TNa/K (3) : Arnórsson (1983) T < 250 °C; TNa/K (4) Giggenbach (1988); TNa/K/Ca : Fournier and Truesdell (1973).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** (a) Concentrations of Sr, (b) Li as a function of chloride ions. Waters groups are highlighted by three different colours: Green for the East Margeride area, Orange for Chaudes-Aigues and Blue for South Cantal area. Surface waters (streams, rivers) are symbolized by blue crosses.</figcaption></figure>

In their study on thermal and mineral waters from Hercynian granites in Portugal, Marques et al. (2012) determined ⁸⁷Sr/⁸⁶Sr ratio values between 0.710599 and 0.731496, which represents a similar but larger range of values than that of our study. They noted that many of these values were close to the Sr isotopic ratios of the plagioclases of the local

granitic rocks (0.71261–0.72087 and 12b) and not to those of the whole granitic rocks (0.735697–0.789683).

These results seem to be supported by the study carried by Zuddas et al. (1995) relative to granite-fluid interaction processes, who found that the Sr concentration and the ⁸⁷Sr/⁸⁶Sr ratio values in the waters
<figure><img src="image_15.png" /><figcaption>Fig. 12. (a) $^{87}$Sr/$^{86}$Sr ratios as a function of Strontium concentration, (b) $^{87}$Sr/$^{86}$Sr ratios as a function of Lithium concentration in sampled waters. Isotopic values of specific minerals, used in the text, have been added (black arrows, their abscissas are not significant).</figcaption></figure>

depended on the dissolution of the minerals from the granite, such as plagioclase, biotite, K-feldspar. Based upon dissolution of feldspars under controlled laboratory conditions, Brantley et al. (1998) observed that plagioclase dissolved at relatively fast rates and could release significant Sr into solution, giving a Sr isotopic signature to the solution close to that of the plagioclase. Franklyn et al. (1991) had already concluded, in their study based on groundwater $^{87}$Sr/$^{86}$Sr ratios in the Eye-Dashwa Lake pluton, in Canada, that the Sr isotope ratios of the deep groundwaters were mainly the result of interactions with minerals such as plagioclase.

As noted by Marques et al. (2012), in open fluid systems, the time for reaction between plagioclase and water is short and, thus, the Sr isotopic signature of the fluids reflects the hydrolysis of plagioclase with low $^{87}$Sr/$^{86}$Sr ratios rather than dissolution of the whole rock (Franklyn et al., 1991; McNutt et al., 1987, 1990). However, radiogenic Sr from weathering of biotite and K-feldspars may be effectively removed from various granitic weathered systems (Blum et al., 1993; Zuddas et al., 1995) and usually, K-feldspars and biotite, which have high Rb contents, indicate higher $^{87}$Sr/$^{86}$Sr ratios.

The highest Sr isotopic values analysed in the Par, Ban and Lavoir waters could indicate that these processes of K-feldspar and biotite dissolution have a greater contribution in the Sr contents of these waters.

For the Moulinges water, which has the lowest $^{87}$Sr/$^{86}$Sr ratio, its location in the Cantal volcanic deposits, characterized by Sr isotope

signatures close to 0.703–0.705 in the host basalt environment (Fig. 12a), and their influence on the chemical composition of this water, could be put forward to explain this value (Stettler and Allègre, 1978).

In an experimental study of Li isotope fractionation between Li-mica and aqueous fluids from 300 to 500 °C, carried out by Wunder et al. (2007), the T-dependence of the Li-isotope fractionation was approximated by the following equation:

$$ \Delta^{7}\text{Li}_{\text{Li-mica}} = \delta^{7}\text{Li}_{\text{Li-mica}} - \delta^{7}\text{Li}_{\text{water}} = -4.52 \left[ \frac{1000}{T(\text{K})} \right] + 4.74 \quad (2) $$

We can also write:

$$ \delta^{7}\text{Li}_{\text{Li-mica}} = \delta^{7}\text{Li}_{\text{water}} - 4.52 \left[ \frac{1000}{T(\text{K})} \right] + 4.74 \quad (3) $$

If we apply this equation to the Par water at 190 °C, which is the estimated reservoir temperature, we find a value of –0.72 ‰ for the corresponding solid phase, using the $^{87}$Sr analysed in this water (4.3 ‰, Table S6, SM). For the La Chaldette water, this value is –1.56 ‰, at 110 °C. For the Mazel - Fontaine Basse 1 water, this value is –2.07 ‰, at 100 °C, and for the Fontanes water, this value is –4.51 ‰, at 125 °C. These values are very different from those that can be calculated using other rare equations relative to Li fractionation existing in the literature, like those between clinopyroxene (synthetic spodumene) and aqueous fluids from 500 to 900 °C (Wunder et al., 2006) and between basalt and diluted seawater from 25 to 250 °C (Millot et al., 2010). For the first equation,
we find respective values of -3.17 ‰ at 190 °C, -4.05 ‰ at 110 °C, -4.57 ‰ at 100 °C, and -7.00 ‰ at 125 °C. For the second equation, these values are: -4.55 ‰ at 190 °C, -6.89 ‰ at 110 °C, -7.64 ‰ at 100 °C, and -9.52 ‰ at 125 °C.

The δ⁷Li values calculated for the solid phases using the equation of Li isotope fractionation between Li-mica and aqueous fluids defined by Wunder et al. (2007) for the thermo-mineral of this study, at the estimated reservoir temperatures, seem to be those that are most in line with the literature data found for micas (δ⁷Li values from -6.0 to 7.7 ‰; Deveaud et al., 2015; Martin et al., 2015; Chen et al., 2020; Zhou et al., 2021; see Fig. 12b). These results suggest that micas are the main lithium-bearing minerals in the different geothermal reservoirs of this study and the temperatures estimated for these reservoirs are quite good. Except for the Fontanes water (Fig. 12b), these results are consistent with those obtained by Teng et al. (2004) on the average upper continental crust of 0 ± 2 ‰ and on granite matrices (-2.5 to 2.7 ‰; Deveaud et al., 2015). According to Deveaud et al. (2015), most pegmatite and granite δ⁷Li signature range from -2 to 2 ‰. For the Par water, the δ⁷Li value of -0.72 ‰ calculated for the solid phase suggests that Li-rich pegmatite (δ⁷Li values ranging from -1.0 to 10 ‰; Chen et al., 2020) could carry this element.

### 5.3.2. Boron signatures

Unlike lithium and strontium, boron often indicates a conservative behaviour in the thermal waters. The Cl-B diagram of the Fig. 3b enabled us to distinguish the different groups of thermo-mineral waters of this study. The d11B values vary greatly from one group to another, with only those in the South Cantal and East Margeride showing the highest values (positive values) in Fig. 13.

As for Li isotopes, a similar approach can be used for boron isotopes. Two equations of boron isotope fractionation have been proposed in the literature by Williams et al. (2001) relative to the illitization of Smectite at 300 and 350 °C, and by Wunder et al. (2005) for micas-clays melt

between 25 °C and 1000 °C. These respective equations are as follows:

$$ \Delta^{\mathrm{11}}\mathrm{B}_{\mathrm{water}}^{\mathrm{mineral}} = \delta^{\mathrm{11}}\mathrm{B}_{\mathrm{mineral}} - \delta^{\mathrm{11}}\mathrm{B}_{\mathrm{water}} = -10.12 \left[ \frac{1000}{T(\mathrm{K})} \right] + 2.44 \quad (4) $$

$$ \Delta^{\mathrm{11}}\mathrm{B}_{\mathrm{water}}^{\mathrm{mineral}} = \delta^{\mathrm{11}}\mathrm{Li}_{\mathrm{mineral}} - \delta^{\mathrm{11}}\mathrm{Li}_{\mathrm{water}} = -10.69 \left[ \frac{1000}{T(\mathrm{K})} \right] + 3.88 \quad (5) $$

We can also write:

$$ \delta^{\mathrm{11}}\mathrm{B}_{\mathrm{mineral}} = \delta^{\mathrm{11}}\mathrm{B}_{\mathrm{water}} - 10.22 \left[ \frac{1000}{T(\mathrm{K})} \right] + 2.44 \quad (6) $$

and:

$$ \delta^{\mathrm{11}}\mathrm{B}_{\mathrm{mineral}} = \delta^{\mathrm{11}}\mathrm{B}_{\mathrm{water}} - 10.69 \left[ \frac{1000}{T(\mathrm{K})} \right] + 3.88 \quad (7) $$

If we apply these two equations to the Par water at 190 °C, which is the estimated reservoir temperature, we find respective values of -22.94 and -22.73 ‰ for the corresponding solid phases, using the δ¹¹B analysed in this water (-3.53 ‰, see Table S6, SM). For the La Chaldette water, these values are -27.95 and -28.00 ‰, respectively, at 110 °C. For the Mazel - Fontaine Basse 1 water, these values are -11.01 and -11.10 ‰, at 100 °C, and for the Fontanes water, these values are -12.60 and -12.59 ‰, at 125 °C.

All these isotopic values are lower than those analysed by Martin et al. (2015) in micas and pyroxenes (δ¹¹B ≥ -9.9 ‰), but most of them are in the range observed in the continental crustal rocks (δ¹¹B from -30 to 22 ‰; Finley et al., 1962; Swihart and Moore, 1989; Chaussidon and Albarède, 1992).

The low δ¹¹B values calculated for the Par and La Chaldette waters could be explained by the relatively abundant presence of tourmaline described in the sub-alkaline Muscovite, Biotite and Tourmaline leucogranites. These granites are generally fine-grained rocks, often aplitic, but frequently with pegmatitic zones, and clearly posterior to the

<figure><img src="image_16.png" /><figcaption>Fig. 13. δ¹¹B values as a function of Boron concentration in the water sampled. Isotopic values of specific minerals, used in the text, have been added (black arrows, their abscissas are not significant).</figcaption></figure>
Margeride granite, which they cross, forming numerous generally sub-vertical veins no more than ten metres wide, like in the Chaudes-Aigues thermal area, or sub-horizontal sheets often several tens of metres thick (Couturié et al., 1979; Bayer, 1982). In the pegmatitic zones, we mainly find (Couturié et al., 1979):

- Xenomorphic Quartz.

- Albite and perthitic K-Feldspar, rarely self-morphing, which sometimes form masses of up to several cubic metres.

- Blades of Muscovite and, more rarely, Biotite.

- Very common black Tourmaline (Schorl), of needle packs arranged in parallel that can weigh several kilos.

- Automorphic Cordierite, entirely phyllitised, in stocky prisms 1 to 2 cm in diameter.

- Pink andalusite, in prisms 1 to 2 cm in diameter.

Presence of Tourmaline was also observed in the clear facies of the Margeride granite (Couturié et al., 1979). Tourmaline is the most abundant boron-bearing mineral in the crust (Barth, 1993). Chaussidon and Albarède (1992) have studied tourmalines by ion microprobe. They found a wide range in δ¹¹B values of -29.9 to -2.9 ‰ for Tourmaline minerals hosted in granites, pegmatites and aplites, which covers the entire range of the values calculated in our study. The most positive δ¹¹B values were found in Li-rich Tourmalines, while schorl was the most depleted in ¹¹B, which is concordant with the mineralogical observations in the leucogranites of our study, especially in the pegmatitic areas.

Chaussidon and Albarède (1992) determined their lowest δ¹¹B values (-29.9 to -20.5 ‰) in Tourmalines from Archean pegmatitic granites, in Swaziland, Southern Africa, and from Bouaflé granite, in Ivory Coast, Africa, which are comparable values with those calculated for the solid phases relative to the Par and La Chaldette waters. The presence of sub-vertical veins of leucogranite with pegmatite areas, enriched in Micas, in the Chaudes-Aigues area, could also explain the highest ⁸⁷Sr/⁸⁶Sr ratios observed in this study (see previous sub-section about strontium signature).

Relative to the Mazel - Fontaine Basse 1 and Fontanes waters, similar δ¹¹B values for solid phases were reported by Chaussidon and Albarède (1992) in the range of -16.3 to -10.3 ‰ in Tourmalines from Gangotri and Manasu leucogranites, in high Himalaya, from Soublaké granite, in Ivory Coast, Africa, from pegmatitic dike, in Niger, Africa, and from banded aplite in layered pegmatite, in Swaziland, Southern Africa. Li et al. (2025) determined B isotope compositions of tourmaline ranging from -14.8 to -12.6 ‰ in Li-poor pegmatites in the Mufushan granitic batholith, South China.

Given all these results, it could be concluded, in our study, that δ¹¹B values in the thermo-mineral waters mainly depend on temperature and the amount of Tourmaline in the granite rocks. However, the behaviour of the B isotope fractionation is probably more complex because the data relative to the Mazel - Fontaine Basse 1 and Fontanes waters can be also compatible with the B isotopic values reported by Lei et al. (2023) in their study on samples of two-mica granite and Biotite granite from the Huayang-Wulong granite suite (South Qinling, Central China). For Biotite in Biotite granite, these authors determined δ¹¹B values ranging from -12.7 to -9.6 ‰ and this value was -12.0 ‰ in two-mica granite. Note that Muscovite had systematically lower δ¹¹B values than Biotite by more 5-8 ‰. Feldspar + Quartz had δ¹¹B values of -13.1 to -9.6 ‰ for Biotite granite and -12.6 ‰ for two-mica granite, slightly lower than Biotite from the same granite samples.

Finally, Williams et al. (2007) proposed the following geo-thermometer based on intracrystalline boron isotope partitioning in Illite-Smectite:

$$T(^{\circ}\text{C}) = \frac{(\delta^{11}\text{B}_{\text{tetrahedral}} - \delta^{11}\text{B}_{\text{interlayer}} + 30)}{0.05} \qquad (8)$$

This approximate formula suggests that the interlayer sites of Illite-Smectite preserve the B isotopic composition of water at the

temperature that produced the authigenic Illite. If we apply this geo-thermometer using the B isotopic data of the Par Spring, we can then estimate a temperature of 212 °C, which is slightly higher than that given for the geothermal reservoir, using the other geothermometers (T = 190 ± 10 °C). For the other geothermal systems (La Chaldette, Mazel - Fontaine Basse 1 and Fontanes), using this same formula, we find the respective estimations of reservoir temperature of 121 °C, 106 °C and 140 °C, which are also relatively concordant with those previously estimated using the other geothermometers (110 °C, 100 °C and 130 °C).

### 5.4. Gas origin

The ³He/⁴He ratios measured on the non-condensable or dissolved gases give a rough estimate (considering possible kinetic fractionation during degassing) of the percentage of helium of mantellic origin, according to the formula of Clauser et al. (2002):

$$\% \text{He}_{\text{mantle}} = \left[ \left( \frac{R}{\text{Ra}} \right) * \frac{(\text{Ra} - R_{\text{crust}})}{(R_{\text{mantle}} - R_{\text{crust}})} \right] * 100 \qquad (9)$$

with R_a = 1.385 × 10⁻⁶ (Mamyrin and Tolstikhin, 1984), R_mantle = 1.2 × 10⁻⁵ (Craig and Lupton, 1981) for the MORB source and R_crust = 1.5 × 10⁻⁸ (Mamyrin and Tolstikhin, 1984). If we consider, according to Gautheron et al. (2005), that the mantle beneath the Massif Central has a different isotopic signature and that this signature of intra-plate volcanism is instead characterized by an ³He/⁴He ratio of the order of 6.3 ± 0.3 Ra, then we must take a value of 8.7 × 10⁻⁶ for the R_mantle isotope ratio.

The ³He/⁴He measured on the three gas samples associated with the Lavoir, Magnac and Montchanson thermo-mineral springs are, respectively: 0.828 ± 0.007, 1.119 ± 0.009 and 0.720 ± 0.02 (Table S7, SM), allowing us to estimate, for each source, helium percentages of mantle origin of the order of 13.0, 17.6 and 11.3 ‰, if we consider a mantle signature of 6.3 Ra. Using a MORB signature for the mantle component, the values are 9.4, 12.8 and 8.2 ‰, respectively.

These values can be compared with those compiled by (Matthews et al., 1987), who positioned thermo-mineral water and gas samples from the Massif Central in relation to major European geothermal zones (Fig. 11). As this work was carried out in 1987, the percentages of helium of mantle origin estimated by these authors are based on a MORB-type mantle helium ratio and are, in principle, somewhat underestimated in relation to the currently known context of the Massif Central. It should be noted that the contributions of mantle origin estimated in our study are relatively significant. In particular, they are much higher than those estimated for the Upper Rhine Graben (URG), where R/R_a values range from 0.1- to 0.2. The He contents associated with the Lavoir and Magnac thermal springs vary from 50 to 60 ppm. These He contents are low compared to non-condensable gases associated with waters originating from a known granitic reservoir such as Soultz, in the URG (He contents from 4500 to 22,000 ppm; Sanjuan et al., 2010). This suggests a short residence time of these gases in the granite. If we compare the results for the two water samples on which δ¹³C values were measured for both thermo-mineral water and gaseous CO₂ associated with thermo-mineral springs (Table S7, SM), we confirm that isotopic fractionation is greater at lower temperatures, since we obtain: Δ(CO₂)gas-(CO₂)solution = δ¹³C(CO₂)gas - δ¹³C(CO₂)solution = -6.4 ‰ for Lavoir spring water, which indicates an emergence temperature of 67.7 °C, and -10.5 ‰ for Magnac spring water, which has an emergence temperature of 10.5 °C.

From these results and the δ¹³C values measured on thermo-mineral waters (Table S7, SM), we can roughly estimate, the δ¹³C values of gaseous CO₂ associated with these waters. Thus, for gaseous CO₂ from the Montchanson spring, we find a similar δ¹³C value (-5.7 ‰) to that of gaseous CO₂ from the Magnac spring (-6.3 ‰), which is very close to a MORB-type signature of mantle origin.

For gaseous CO₂ from the Laval and Jenny thermal springs, δ¹³C
values would be between -8.0 and -10 ‰, suggesting a greater contribution of organic and atmospheric origin (mixing of the deep pole with a much more pronounced surface water input). The same type of remarks can be made for gaseous CO₂ from the waters of the Sainte-Marie, Fontanes and Moulinges spring group, for which we obtain δ¹³C values ranging from -8.0 to -11 ‰, or for the waters of the Margeride spring group for which δ¹³C values range from -9.5 to -13 ‰. For gaseous CO₂ from the La Chaldette borehole, the estimated value of δ¹³C is around -12 ‰, which is compatible with a relatively superficial water.

By way of comparison, although the Massif Central shows a signature of intraplate volcanism, volatile compounds from MORBs, in most cases, have an average CO₂/³He ratio value of 2 ± 1 10⁹ (Marty and Jambon, 1987). Values of these ratios higher than those of volatile MORB compounds are due to the mixing of a crustal compound rich in CO₂ and ⁴He with gases from the upper mantle (Fiebig et al., 2004).

The CO₂/³He ratio can be calculated according to the following formula of Giggenbach et al., (1994):

$$CO_2/³He = \frac{X_{CO2}}{\left( Ra*\left(\frac{R}{Ra}\right) * X_{He} \right)} \quad (10)$$

where XCO₂ and XHe are the proportions of gas by volume. For gas samples from the Lavoir, Magnac and Montchanson springs, the CO₂/³He ratio values give, respectively: 1.4 10¹⁰, 1.10 10¹⁰, > 4.4 10¹⁰.

These values, associated with an isotopic signature of intra-plate volcanism, are higher than those of the CO₂/³He ratio, of gases associated with mid-ocean ridge basalts (MORB). The values found for gases associated with the Lavoir and Magnac springs are comparable to those found on the Waitopu (9.7 10⁹) and Wairakei (1.4 10¹⁰) geothermal fields, characteristic of rift gas emissions. Furthermore, Bräuer et al. (2016) support this interpretation by highlighting similar elevated CO₂/³He ratios in continental rift zones, linking them to deep magmatic CO₂ contributions and low mantle helium dilution, consistent with tectonic settings characterized by lithospheric extension. The Massif Central is known for its large quantities of carbon dioxide emissions, which are particularly visible in groundwater (Ricci et al., 2024). Many springs in the Massif Central are rich in carbon dioxide, such as those at Vichy (Michard et al., 1976), Limagne (Michard et al., 1979), Chaudes-Aigues and others

## 5.5. Conceptual model of Chaudes-Aigues hydrothermal system

The work here has not only confirmed the hypotheses previously established but has also provided new information to clarify the functioning of Chaudes-Aigues hydrothermal system.

Consequently, in the light of our results, the southern edge of the granite (see white dots line, Fig. 6b) represents the best candidate to be the impluvium. The Chaudes-Aigues springs are spatially connected to this area by a N150E fault zone several kilometres thick (2.5 km according to the geological map, De Goer de Herve et al., 1994). Its southern part is covered by morainal sediments. Analyses have demonstrated that the surface water component of Chaudes-Aigues springs could correspond to that of La Chaldette. As the spring is located in the middle of the fault zone, this confirms that the water could infiltrate into the southern part of the MGC and could be then drained north-westwards towards the town. The meteoric component of the Chaudes-Aigues springs therefore comes from the MGC. Contrary to the model of Vasseur et al. (1997), no glacial origin is suggested here.

Once the water has reached the northern edge of the MGC, it could use the Couffour Granite as a vertical drain. This small granite is supposed to be younger (De Goer de Herve, 1991). This would explain the higher ⁸⁷Sr/⁸⁶Sr values compared to those of the East Margeride springs. Thanks to the topographic gradient and the numerous N50E faults, fluids flow through the valley carved out by the Remontalou and reach the

emergence area. These faults are also responsible for the upwelling of mantle gases into the reservoir. Fig. 14 summarises all the results for the Chaudes-Aigues hydrothermal system and the new reservoirs.

# 6. Conclusions and perspectives

Our study based on a “large-scale” sampling of thermo-mineral waters helped to characterise the hydrothermal system of Chaudes-Aigues but also revealed the presence of two others hydrothermal systems within the MGC.

Firstly, the largest reservoir could exist on the northern edge of the MGC. This would partly supply the springs of Chaudes-Aigues, Loubaresse, Laval, Magnac and Montchanson. The Montchanson spring is the closest in composition to this deep endmember. Our data enabled us to accurately estimate the temperature of this reservoir, which is 190 ± 10 °C, a value which is confirmed by geochemical and numerical models (Fig. 8 and SM, S2). This initial component is supplemented by another component of cold surface water. Due to the high density of faults and fractures around Chaudes-Aigues springs, fluids can rise in abundance and at high rates maintaining their high temperature. Conversely, the springs further East (Montchanson, Laval, Loubaresse, Magnac), with few faults, fluid ascent is limited, resulting in low flow rates and temperatures.

Further south, waters from La Chaldette spring are locally infiltrated and heat up to 110 ± 10 °C at depth without any mixing. This hydrothermal system could be associated with circulation in a horizon affected by both tectonics (fault zone) and supergene alteration (fluid circulation) such as the regolith.

Finally, the East Margeride group comes from a mixture of surface water from the Margeride Mounts, located just to the east, and waters from and a deep reservoir. Based on our data, the reservoir temperature is estimated at around 100 ± 10 °C. Their low surface temperature could be due to relatively low fracturing in the area.

Other fractured granites in France and Europe, such as the MGC, could host hydrothermal systems consisting of several reservoirs with multiple fluid pathways, which would be of interest for high-temperature geothermal exploration. A well-known European reference is the Soultz-sous-Forêts system, located in Alsace, France, in the Upper Rhine Graben (URG). Even if the geothermal system is developed within fractured Hercynian granite, it lacks sufficient natural permeability and active fluid circulation. In consequence, the system required hydraulic stimulation to artificially improve reservoir permeability (Genter et al., 2010). This highlights a key distinction: while Soultz-sous-Forêts is an Enhanced Geothermal System (EGS), Chaudes-Aigues represents a naturally functioning, amagmatic hydrothermal system, with no sedimentary cover, sustained by fluid flow through inherited fault and fracture networks without any artificial enhancement.

In terms of analogues, we can find systems with similar geological characteristics as our study area. In eastern France, Bains-les-Bains and Plombières-les-Bains are two towns known for their thermal springs, with temperatures ranging from 27 to 51 °C and 43 to 80 °C, respectively (Desprez et al., 1971). Very few geological studies have been carried out on these systems, and the only information available is that the springs are linked to grabens bordered by Variscan faults (Desprez et al., 1971), and that the water would flow in the granite (Lefort and Jutier, 1862). Other cold springs have been found further east of these two systems. This area is made up of a granite massif covered by a thin layer of sediment, represents a direct analogue to the MGC. Another example is the Pietrapola thermal station in south-east Corsica, with several springs ranging from 27 to 54 °C (Dupuy, 2021). Here, according to geochemical studies, after a long circulation through the Hercynian bedrock (monzogranite), waters spring up through faults (Dupuy, 2021). Similarly, the Karlovy Vary, formerly known as Karlsbad, springs in the Czech Republic, are also associated with a granitic basement (Vrba, 1996). The massif is rich in hot springs, indicating a potential geothermal reservoir.
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** 3D block diagram (not to scale) summarizing fluid circulation and the various geothermal reservoirs within the Margeride Granitic Complex. Couffour granite, near Chaudes-Aigues, is represented in orange. The South Cantal group was not represented.</figcaption></figure>

The common point between all these systems is that despite their renown as thermal centers, few geological or hydrogeological studies have been carried out. The identification of blind geothermal system, that have no surface manifestations (Hanson et al., 2014; Dobson, 2016; Craig et al., 2021), remains challenging and happens often by accident (Garg et al., 2010; Faulds and Hinz, 2015; Dobson, 2016). In our case, the systems are not blind but hidden by unstudied surface indicators. The chemical composition of cold springs is already a favourable surface indicator. This exploration would need to be extended around known hydrothermal systems to identify hidden ones. This represents a fantastic opportunity for high-temperature geothermal exploration.

## Funding

The financial support was provided by Bureau de Recherches Géologiques et Minières (BRGM), Institut des Sciences de la Terre d'Orléans (ISTO) and by the project « GERESFAULT » funded by the French Research Agency (ANR-19-CE05-0043).

## CRediT authorship contribution statement

**Emmy Penhoët:** Writing – original draft, Methodology, Investigation, Data curation, Conceptualization. **Bernard Sanjuan:** Writing – original draft, Validation, Supervision. **Laurent Guillou-Frottier:** Writing – original draft, Supervision, Conceptualization. **Manuel Moréira:** Writing – original draft, Validation, Data curation. **Laurent Arbaret:** Validation, Supervision, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Laurent Guillou-Frottier reports financial support was provided by French National Research Agency. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

Geochemical data from the 2012 field campaign were acquired by BRGM for a private company, which finally did not publish these data. We would like to thank both anonymous reviewers for their advice leading to a more concise and structured manuscript.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.geothermics.2025.103476.

## Data availability

Data will be made available on request.

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