# Unconformity-related mineralization and fluid transfers in the northern Aquitaine Basin (France) revealed by fluid inclusions and S-Sr isotopes studies

Loïc Bouat, Pierre Strzerzynski, Véronique Gardien, Guillaume Barré, Yannick Branquet, Jérémie Melleton, Régis Mourgues

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Loïc Bouat, Pierre Strzerzynski, Véronique Gardien, Guillaume Barré, Yannick Branquet, et al. Unconformity-related mineralization and fluid transfers in the northern Aquitaine Basin (France) revealed by fluid inclusions and S-Sr isotopes studies. Journal of Geochemical Exploration, 2025, 272, pp.107713. 10.1016/j.gexplo.2025.107713. insu-04941840v2

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# Unconformity-related mineralization and fluid transfers in the northern Aquitaine Basin (France) revealed by fluid inclusions and S-Sr isotopes studies

Loïc Bouat<sup>a,*</sup>, Pierre Strzerzynski<sup>a</sup>, Véronique Gardien<sup>b</sup>, Guillaume Barré<sup>c</sup>, Yannick Branquet<sup>d,e</sup>, Jérémie Melleton<sup>f</sup>, Régis Mourgues<sup>a</sup>

## ARTICLE INFO

**Keywords:**

F-Ba-Pb-Zn(±Ag) deposits

Basement-cover interface

Fluid inclusions

S-Sr isotopic data

Fluid circulation

Extensive Mesozoic geodynamic

## ABSTRACT

In sedimentary basins, unconformity between basement and sediments is the ideal site where fluids from different sources can flow and mix, initiating the formation of ore deposits. In western Europe there are numerous F-Pb-Zn-Ba (±Ag, Ge) basin-hosted deposits located near the unconformity between Mesozoic Basins and the Variscan basement as for example the deposits of the Vendée Coast (France) containing fluorite, baryte, pyrite and quartz. Here, microthermometric data on the primary fluid inclusions of these minerals indicate salinity ranging from 1 to 20 wt% eq. NaCl and homogenization temperatures between 100 and 390 °C. We interpret these data as resulting of a fluids circulation in the ore deposit zone, with an early incursion of basin brines expelled by the leaching of Hettangian evaporites buried several tens of kilometers away, followed by an ascent of basement-derived fluids and, finally, a recharge of seawater. In other French deposits, the δ³⁴S isotopes of baryte are also consistent with a source of fluids from buried evaporites. The ⁸⁷Sr/⁸⁶Sr ratio of baryte demonstrates a crustal source of elements associated with brines-leached base metals and F—Ba. The process of buried-derived evaporites brines altering the basement along the unconformity is ubiquitous in all unconformity deposits in France.

With our results, we confirm that the basin deposits in the Southeastern Massif Central occur along structures formed during Tethys rifting at around 200 Ma, whereas in the Western part they form at ca. 145 Ma in link with the opening of the Bay of Biscay. This highlights that these basin-hosted deposits are preferentially formed during extensional activity in rifting settings, rather than in compressive settings.

## 1. Introduction

Hydrothermal deposits in sedimentary basins host most of the world's Pb—Zn reserves (Kyle and Li, 2002), such as the Broken Hill deposit (Wright et al., 1987), the Upper Silesia district (Heijlen et al., 2003), the Kupferschiefer deposits (Blundell et al., 2003; Hitzman et al., 2010), but also REE by-product such as El Hammam (Cheilletz et al., 2010) and Ba deposits in Jebilet (Valenza et al., 2000). Fluid mixing is the main suggested process for hydrothermal mineralization in sedimentary basins (e.g., Anderson, 1975; Corbella et al., 2004; Leach et al.,

2005; Muchez et al., 2005; Sverjensky, 1981). This involves the circulation of fluids from water sources as diverse as meteoric, marine and connate waters to fluids related to evaporites, hydrocarbons or magmas (e.g., Boiron et al., 2010; Bons et al., 2014; Lawrence and Cornford, 1995; Sizaret et al., 2009). The movement of these fluids is driven by temperature and/or pressure gradients resulting to subsidence or relief formation (e.g., Garven, 1985; Leach et al., 2010; Oliver et al., 2006; Sverjensky, 1986). The drains through which the fluids flow are the microporosity of the sediments (Boiron et al., 2002; Cathelineau et al., 2012), the macroporosity of rocks such as karsts (Corbella et al., 2006;

<footnote><sup>a</sup> Laboratoire de Planétologie et Géodynamique, UMR6112, CNRS, Le Mans Université, France</footnote>

<footnote><sup>b</sup> Laboratoire de Géologie de Lyon, UMR5276, CNRS, Université Lyon 1, France</footnote>

<footnote><sup>c</sup> Département de géologie et génie géologique, Centre E4M, Université Laval, Canada</footnote>

<footnote><sup>d</sup> Géosciences Rennes, UMR6118, CNRS Université de Rennes, France</footnote>

<footnote><sup>e</sup> Institut des Sciences de la Terre d'Orléans, UMR7237, CNRS, Université d'Orléans, France</footnote>

<footnote><sup>f</sup> Bureau de Recherches Géologiques et Minières (BRGM), Orléans, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: loic.bouat@univ-orleans.fr (L. Bouat).</footnote>
Rhodes et al., 1984; Valenza et al., 2000) and the fracture network including faults (Bons, 2001; Blundell et al., 2003).

Hydrothermal deposits in general and unconformity-related deposits in particular, result from unusual mixing of different fluids enabled by geodynamics (Ingebritsen and Appold, 2012). Due to different driving forces and drainages, fluid flows can be sequenced, as paragenetic and geochemical analysis show (Boiron et al., 2002; Munoz et al., 1994; Walter et al., 2018). To better understand how these deposits were formed, it is therefore necessary to properly define the fluid sources and their sequence of circulation.

In France, unconformity-related deposits have been studied to trace fluid sources, driving forces and drains (Boiron et al., 2002, 2010; Corbella et al., 2004; Heijlen et al., 2003; Hitzman et al., 2010; Leach et al., 2006; Muchez et al., 2005). The work of Valenza et al. (2000), Sizaret et al. (2009), Aquilina et al. (2011) and Laurent et al. (2020) have shown that baryte is a key mineral to track these different parameters during the formation of such deposits. Baryte associated with quartz and calcite is found in the ore deposits on the unconformity of the Vendée Coast (France). The study of quartz fluid inclusions (FIs) and calcite stable isotope analysis show that the ore deposits result from the mixing of seawater, brines and hot fluids (Cathelineau et al., 2012). Fluids movements takes place along the main structures, i.e. horizontally along the unconformity in porous basal sediments and vertically along normal fault planes cutting the basement (Bouat et al., 2023; Cathelineau et al., 2012; Strzerzynski et al., 2020). However, the chronology and timing of these fluid circulations and the geodynamic context associated with the formation of these ore deposits are not yet well constrained.

The aim of this study is to investigate the formation of these unconformity-related ore deposits. Our multidisciplinary approach combines petrographic observations, analysis of FIs in different minerals, and sulfur and strontium isotopic analyses of baryte. The objective is to highlight the nature, origin and succession of fluids at the origin of the deposits on the Vendée Coast. Sulfur and strontium analyses were extended to unconformity deposits from similar deposits in the Southern Armorican Massif and Massif Central. Previous studies have provided us with the regional geodynamic constraints that enable us to determine the exact timing of the circulations of fluids at the origin of the Western Europe deposits.

## 2. Geological setting

### 2.1. Geological setting of Vendée (France)

France's first order geology consists of a Variscan basement made of the Armorican, Central, Vosges and Ardennes massifs covered by the Mesozoic sediments of the Paris Basin, Southeast Basin, Grands-Causses Basin and Aquitaine Basin (Fig. 1a). The Variscan massifs dated from Silurian to Carboniferous ages are the witnesses of the collision between the continents Laurussia and Gondwana after the closure of the Rheic and Central oceans (Ballèvre et al., 2009; Faure et al., 2009; Matte, 1986; Ledru et al., 1989). From the southern domain of the Armorican Massif to the Cévennes (Fig. 1a), the massifs are structured into paragneiss and orthogneiss nappes containing ophiolitic relics (i.e. Lower and Upper Gneiss Units) intruded by late-orogenic calc-alkaline and peraluminous granites (Ballèvre et a., 2009; Faure et al., 2009). The Vendée Coast, south of the Armorican Massif (Fig. 1b), is mainly formed of migmatitic orthogneiss, gneissic micaschists and micaschists. Here, the late orogenic WNW-ESE extension characterized by ductile to brittle deformation (Turrillot et al., 2011) and associated with HT-LP metamorphism is well preserved (Cagnard et al., 2004; Goujou et al., 1994; Iglesias and Brun, 1976).

The intracontinental marine basins that opened on the Variscan basement during the Mesozoic (Barbarand et al., 2020; Brunet, 1984; Brunet and Le Pichon, 1982; François et al., 2020), are witness to the opening of the Tethys Rift in the Lower Jurassic for the southeastern basins (Handy et al., 2010; Lemoine et al., 1986; Stampfli and Borel, 2002) and of the Bay of Biscay in the west during the Lower Cretaceous (Jammes et al., 2009). Convergence between Iberian and Eurasian plates in the Upper Cretaceous generated compression that formed retroforeland basins in the southern Aquitaine Basin (Biteau et al., 2006; Ortiz et al., 2020) and the South-East Basin (Handy et al., 2010; Lemoine et al., 1986; Stampfli and Borel, 2002). Jurassic sediments deposited in the Aquitaine and the South-East Basins are continental detrital to marine carbonate platforms (Barbarand et al., 2020; Biteau et al., 2006; Curnelle and Dubois, 1986). On the Vendée Coast (Fig. 1b), Jurassic transgression is recorded with basal Hettangian detrital sediments (SU1-unit), Hettangian-Sinemurian dolomitic sediments (SU2-unit), Pliensbachian limestones with clay layers (SU3-unit), Toarcian-Aalenian shales (SU4-unit) and thick Bajocian-Callovian limestones (Fig. 1b,

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Location of unconformity-related ore deposits between Variscan massifs and Jurassic sediments (white area representing post-Jurassic sediments). a: Simplified geological map of western France with Ba-F-Pb-Zn deposits districts (the framed "b" corresponds to the location of the detailed Fig. 1b). N.A.S.Z. and S.A.S. Z.: North and South Armorican Shear Zone, respectively, Cévennes F.: Cévennes Fault. b: Detailed geological map of the Vendée Coast (modified from Bouat et al., 2023). Mineralized ore districts/deposits are labeled from 1 to 7 on (a) and from 8 to 13 for the Vendée coast on (b).</figcaption></figure>
SU5-unit; see sedimentary details in Bouat et al., 2023). An interstratum karst level in unit SU2 is reported in relation to a minor emersion dated to the Late Hettangian and Early Sinemurian (Fauré and Bohain, 2017). Upper Jurassic to Upper Cretaceous sediments are absent from the Vendée Coast due to Cretaceous and Cenozoic regressions that lead to emersions (Goujou et al., 1994).

## 2.2. Unconformity-related ore deposits

France hosts numerous unconformity-related deposits, grouped into districts, all located near the unconformity between Variscan massifs and Mesozoic sediments (Fig. 1). Bordering the Massif Central (Fig. 1a), we find the Morvan district (5.5Mt fluorite reserve; Gigoux et al., 2016) to the north, the Cévennes district (among the major Pb—Zn district in Europe; Muchez et al., 2005) to the south-east, the Grands-Causses (Pb-Zn-Ba deposits) and Albigeois (3.5Mt fluorite past-production; Munoz et al., 2005) districts to the south and the Périgord district (Pb-Zn-Ba deposits) to the west. Between the Armorican Massif and the Massif Central, lies the Poitou High district (Pb-Zn ± Ag deposits; Boiron et al., 2010; Cathelineau et al., 2012) and, south of the Armorican Massif, the Vendée Coast district (Ba-Pb-Zn ± Ag; Goujou et al., 1994).

These mineral deposits located on unconformity are made up of similar baryte, quartz, fluorite, galena and sphalerite parageneses dating from the Triassic to Lower Cretaceous (Bonhomme et al., 1987; Cathelineau et al., 2012; Gigoux et al., 2015; Munoz et al., 2005; Bouat et al., 2023). These deposits correspond either to veins hosted in the basement beneath the unconformity and/or in the sedimentary cover, or to stratiform and karst-fill mineralization. The fluids responsible for their formation are of shallow origin, with temperatures between 60 and 160 °C and salinities between 0 and 25 wt% eq. NaCl (Cathelineau et al., 2012; Gigoux et al., 2016; Laurent et al., 2020; Léost et al., 1999; Munoz et al., 1999; Sabouraud et al., 1980; Sizaret et al., 2004).

In the Vendée Coast districts, ore deposits consist of baryte, quartz, pyrite and a minor amount of Ag-rich galena and sphalerite attesting of hot fluid up to 280 °C (Cathelineau et al., 2012). Previous mining works have identified zones containing 0.36 % Pb, 0.19 % Zn and 45 g/t Ag (Goujou et al., 1994). Mineralization occurs in karsts and NW-SE basement fractures and in the units SU1 to SU3-unit (Bouat et al., 2023). Calcite-filled NW-SE fractures are dated by U—Pb at 145 Ma by Bouat et al. (2023). Associated with these deposits, the basement is affected by alteration below the unconformity (Cathelineau et al., 2012; Strzerezynski et al., 2020) and above, the cover is strongly silicified (Fig. 1b; Bouat et al., 2023; Cathelineau et al., 2012).

# 3. Methods

## 3.1. Sampling strategy

In this study, we analyzed 22 samples from the Vendée Coast. They are representative of all zones and are characterized either by dissemi-nated minerals hosted in the SU2-unit, or by NW-SE trending veins crosscutting all formations (the basement and SU2 to SU5-units) or, filling karsts in the SU2-unit (Fig. 1b; Table 1). Among these samples, 10 samples were studied for fluid inclusions microthermometry, 8 samples for strontium isotopes and 13 for sulfur isotopes.

For comparison, 8 samples of baryte from veins crosscutting the basement near the unconformity and the sedimentary cover (Fig. 1a; Table 1) from ore deposits in the Périgord (Guillot et al., 1979), the Grands-Causses (Briand et al., 1979, 1993; Legendre et al., 2009; Messnessier and Collomb, 1986; Roig et al., 2002) and the Cévennes districts (Alabouvette et al., 1988). These samples were part of the 1970s France mining inventory collection of the B.R.G.M. (French geological survey). In this study, these samples were studied for strontium and sulfur isotopes.

## 3.2. Fluid inclusions microthermometry

The paragenetic sequence of the deposits on the Vendée Coast was first obtained by observing the minerals in the field, then using a LEICA DM750P optical microscope. Microthermometric analyses were performed on a Linkam MDS600 heating/freezing stage connected to an Olympus BH2 binocular microscope at the LGL-TPE laboratory (Lyon). Calibration was based on phase transition temperatures measurements on FIs of synthetic samples of pure H₂O (melting temperature of ice, T_m pure H₂O = 0 °C, homogenization temperature, T_h pure H₂O = +374 °C) and H₂O-CO₂ (melting temperature of pure CO₂, T_m CO₂ = -56.6 °C, melting temperature of CO₂-clathrate, T_m CO₂-Clath = +10 °C). Salinity is calculated in weighted percent equivalent of NaCl (wt% eq. NaCl) from the ice melting temperature of the FIs using the Bodnar (1993) relationship for NaCl-H₂O systems. To avoid cracking through the twining and decompensation of FIs, the microthermometric measurements on fluorite and baryte followed a single heating-cooling cycle.

## 3.3. Isotopic ratio $^{87}$Sr/$^{86}$Sr analysis on baryte

Samples were collected using a Dremel® tool to drill holes to precisely separates the crystals from the host-rock and to make 50-500 mg of an extremely fine powder (<250 µm). The $^{87}$Sr/$^{86}$Sr isotopic ratio of baryte was performed at Laboratory Géosciences Rennes using a Finnigan MAT-262 seven-collector mass spectrometer. Each sample was partially dissolved in a mixture of hydrofluoric and nitric acids, then dried and taken up with concentrated hydrochloric acid. Sr separation was performed by cation exchange columns chromatography using BioRad AG50W-X8 resin. Calibration was carried out using Sr standard NBS-987 ($^{87}$Sr/$^{86}$Sr = 0.710250). Mass fractionation was checked using the value of 8.3752 for $^{88}$Sr/$^{86}$Sr. Blanks analyses gave values <100 pg for Sr and are therefore considered negligible.

## 3.4. Sulfur isotopic content on baryte and sulfide

The sulfur isotopic composition (δ³⁴S) of baryte and pyrite from the Vendée Coast and baryte from other unconformity-related ore deposits was measured online using a Thermo Scientific EA IsoLink IRMS System at CRPG laboratory (Vandoeuvre les Nancy, France). A quantity of 0.7 mg of each sample was enclosed in tin capsules to which approximately 0.5 mg of vanadium pentoxide was added. The tin capsules were then transferred to an autosampler and burned at 1020 °C in a combustion reactor consisting of a quartz tube filled with tungsten oxide and pure copper. The gases produced (N₂, CO₂ and SO₂) were separated on a chromatographic column, and the isotopic composition of the sulfur was measured using a Thermo Scientific Delta V Advantage continuous-flow isotope ratio mass spectrometer. The Sulfur isotopic composition (δ³⁴S) of the sulfates was obtained by comparison with three international standards regularly included in the analysis: (i) IAEA SO6 (δ³⁴S = -34.1 ‰), (ii) IAEA SO5 (δ³⁴S = +0.5 ‰) and (iii) NBS 127 (δ³⁴S = +20.3 ‰). The sulfur isotopic composition (δ³⁴S) of the sulfides was determined by comparison with three international standards regularly included in the analysis: (i) IAEA S1 (δ³⁴S = -0.3 ‰), (ii) IAEA S2 (δ³⁴S = +22.62 ‰) and (iii) IAEA S3 (δ³⁴S = -32.49 ‰). Values are quoted in the delta notation in ‰ versus VCDT and reproducibility was better than 0.3 ‰.

# 4. Results

## 4.1. Vendée Coast deposits paragenetic sequence

Based on field and microscopic observations, 6 stages of mineralization have been distinguished in 6 outcrops on the Vendée Coast (Fig. 1b and Fig. 2). Only the Anse de St-Nicolas site presents the entire paragenetic sequence (i.e. the 6 stages). Stage 1 and 2 correspond to pervasive mineralization disseminated in the sediments while stage 3 to
<caption>**Table 1**</caption>

Studied samples with localities, mineral association, morphologies, FIs data on samples from the Vendée Coast and isotope data on all samples. Qz: quartz, Py: pyrite, Brt: baryte, Fl: fluorite, Mrc: marcasite, Gn: galena, Sp: sphalerite, Cal: calcite, Ccp: chalcopyrite. Labeled minerals refer to the paragenetic stage (Fig. 2).

<table><thead><tr><th rowspan="2">District</th><th rowspan="2">Zone</th><th rowspan="2">Host-rock</th><th rowspan="2">Mineral</th><th rowspan="2">Morphology</th><th rowspan="2">Sample</th><th rowspan="2">Number</th><th colspan="2">Salinity (wt% NaCl)</th><th colspan="2">T<sub>h</sub> (°C)</th><th rowspan="2"><sup>87</sup>Sr/<sup>86</sup>Sr</th><th rowspan="2">δ<sup>34</sup>S (vCDT %)</th></tr><tr><th>Mean</th><th>Range</th><th>Mean</th><th>Range</th></tr></thead><tbody><tr><td rowspan="10">Vendée Coast</td><td>Anse aux Moines</td><td>Orthogneiss (near unconformity)</td><td>Qtz5</td><td>NW-SE vein</td><td>AM2</td><td>10</td><td>6.7 ± 1.4</td><td>4.9 to 12.5</td><td>249.2 ± 39.4</td><td>169.6 to 332.8</td><td></td><td></td></tr><tr><td>Cayola Bay</td><td>Silicified SU2</td><td>Qtz5</td><td>Karst</td><td>QCAY</td><td>3</td><td>6.4 ± 1.0</td><td>4.9 to 7.7</td><td>231.2 ± 45.2</td><td>163.4 to 274.4</td><td></td><td></td></tr><tr><td>Mine des Sards</td><td>Silicified SU2</td><td>Qtz5</td><td>NW-SE vein</td><td>MS11</td><td>14</td><td>9.2 ± 3.8</td><td>3.6 to 18.4</td><td>257.7 ± 70.7</td><td>160.2 to 394.1</td><td></td><td></td></tr><tr><td rowspan="3">Payré Estuary</td><td rowspan="3">SU2</td><td rowspan="3">Brt4</td><td rowspan="3">NW-SE vein</td><td>MS301</td><td></td><td></td><td></td><td></td><td></td><td></td><td>-23.0 ± 0.5</td></tr><tr><td>PPX</td><td></td><td></td><td></td><td></td><td></td><td>0.709692 ± 1.0E-5</td><td></td></tr><tr><td>PN201</td><td></td><td></td><td></td><td></td><td></td><td>0.709787 ± 0.9E-5</td><td>± 0.6</td></tr><tr><td>Anse de St-Nicolas</td><td>Micaschists (near unconformity)</td><td>Brt4</td><td>NW-SE vein</td><td>min4</td><td></td><td></td><td></td><td></td><td></td><td>0.710900 ± 1.0E-5</td><td>± 0.6</td></tr><tr><td rowspan="10">Jard-Sur-Mer</td><td rowspan="10">Neuil</td><td rowspan="10">Gneiss with plagioclase and mica (near unconformity)</td><td rowspan="10">Qtz5</td><td rowspan="10">Disseminated</td><td>Pt167a</td><td>4</td><td>17.5 ± 3.6</td><td>13.3 to 21.2</td><td>132.8 ± 21.9</td><td>110.4 to 173.1</td><td></td><td></td></tr><tr><td rowspan="3">Brt4</td><td rowspan="3">Karst</td><td>GA4</td><td>8</td><td>7.2 ± 1.7</td><td>3.8 to 10.6</td><td>304.9 ± 39.6</td><td>243.0 to 378.0</td><td></td><td></td></tr><tr><td>K1</td><td>2</td><td>6.9 ± 0.7</td><td>6.2 and 7.6</td><td>227.3 ± 32.1</td><td>195.2 and 259.4</td><td></td><td></td></tr><tr><td rowspan="3">Py5</td><td rowspan="3">NW-SE vein</td><td>21</td><td></td><td></td><td></td><td></td><td>0.710751 ± 1.0E-5</td><td>± 0.6</td></tr><tr><td>25</td><td></td><td></td><td></td><td></td><td>0.710363 ± 1.0E-5</td><td>± 0.6</td></tr><tr><td>PP9</td><td></td><td></td><td></td><td></td><td>0.710126 ± 1.0E-5</td><td>± 0.6</td></tr><tr><td rowspan="3">Py5</td><td rowspan="3">NW-SE vein</td><td>Pyrite-St-Nicolas</td><td></td><td></td><td></td><td></td><td>0.710093 ± 1.0E-5</td><td>± 0.6</td></tr><tr><td>GA3</td><td>13</td><td>6.8 ± 2.6</td><td>2.7 to 12.9</td><td>272.8 ± 61.6</td><td>166.0 to 384.8</td><td></td><td></td></tr><tr><td>QSN</td><td>5</td><td>4.6 ± 0.2</td><td>4.3 to 4.9</td><td>191.7 ± 24.5</td><td>145.8 to 230.2</td><td></td><td></td></tr><tr><td rowspan="3">Py5</td><td rowspan="3">NW-SE vein</td><td>20</td><td>5</td><td>3.6 ± 2.6</td><td>1.0 to 7.1</td><td>273.1 ± 56.3</td><td>140.9 to 366.2</td><td></td><td></td></tr><tr><td rowspan="3">Py5</td><td rowspan="3">NW-SE vein</td><td>Pt201b</td><td>4</td><td>6.9 ± 1.0</td><td>5.3 to 8.7</td><td>235.0 ± 15.4</td><td>204.3 to 260.1</td><td></td><td></td></tr><tr><td rowspan="2">II</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td>MR202</td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td rowspan="3">Py5</td><td rowspan="3">NW-SE vein</td><td>Brt4</td><td>NW-SE vein</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td>Mr</td><td>Nodule</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.6</td></tr><tr><td>MR201</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td rowspan="3">Py5</td><td rowspan="3">NW-SE vein</td><td>Brt4</td><td>NW-SE vein</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.6</td></tr><tr><td>Py5</td><td>NW-SE vein</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td>0.709581 ± 1.0E-5</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td rowspan="3">Périgord</td><td rowspan="3">Neuil</td><td rowspan="3">Gneiss with plagioclase and mica (near unconformity)</td><td rowspan="3">Qtz5</td><td rowspan="3">NW-SE veins</td><td>JM11</td><td></td><td></td><td></td><td></td><td></td><td>0.708969 ± 1.0E-5</td><td>± 0.6</td></tr><tr><td>Mr</td><td>Nodule</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.6</td></tr><tr><td>MR202</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td rowspan="3">Grands-Causses</td><td rowspan="3">Kaymard</td><td rowspan="3">Sericitic schists (near unconformity)</td><td rowspan="3">Qtz5</td><td rowspan="3">WNW-ESE veins</td><td>JM4</td><td></td><td></td><td></td><td></td><td></td><td>0.710136 ± 1.3E-5</td><td>± 0.6</td></tr><tr><td>Brt ± Gn</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td>Brt</td><td>NW-SE veins</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td rowspan="3">St-Geniez d'Olt Barjac</td><td rowspan="3">Micaschists (near unconformity)</td><td rowspan="3">Lower Jurassic dololimestones</td><td rowspan="3">Qtz5</td><td rowspan="3">NW-SE veins</td><td>JM2</td><td></td><td></td><td></td><td></td><td></td><td>0.710685 ± 1.1E-5</td><td>± 0.6</td></tr><tr><td>Brt</td><td>NW-SE veins</td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr><tr><td>JM8</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>± 0.5</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1 (continued)**</caption><thead><tr><th rowspan="2">District</th><th rowspan="2">Zone</th><th rowspan="2">Host-rock</th><th rowspan="2">Mineral</th><th rowspan="2">Morphology</th><th rowspan="2">Sample</th><th rowspan="2">Number</th><th colspan="2">Salinity (wt% NaCl)</th><th colspan="2">T<sub>h</sub> (°C)</th><th rowspan="2"><sup>87</sup>Sr/<sup>86</sup>Sr</th><th rowspan="2">δ<sup>34</sup>S (vCDT %)</th></tr><tr><th>Mean</th><th>Range</th><th>Mean</th><th>Range</th></tr></thead><tbody><tr><td rowspan="2"></td><td>Allenc</td><td>Granite and Lower Jurassic dololimestones</td><td>Qtz-Brt-Gn-SI</td><td>NW-SE veins</td><td>JM6</td><td></td><td></td><td></td><td></td><td></td><td>0.711998<br/>± 1.0E-5</td><td>+14.2<br/>± 0.6</td></tr><tr><td>Bleymard</td><td>Middle Jurassic limestones</td><td>Qtz-Cal-Brt-Gn-SI</td><td>Stratiform, NNW-SSE veins</td><td>JM7</td><td></td><td></td><td></td><td></td><td></td><td>0.710639<br/>± 1.0E-5</td><td>+16.9<br/>± 0.6</td></tr><tr><td rowspan="2">Cévennes</td><td>Millau</td><td>Lower Jurassic dololimestones</td><td>Brt-Gn-Cp</td><td>NW-SE veins</td><td>ML1b</td><td></td><td></td><td></td><td></td><td></td><td>0.710042<br/>± 0.9E-5</td><td>+14.0<br/>± 0.6</td></tr><tr><td>Malines</td><td>Cambrian dololimestones, Triassic shales and Middle Jurassic limestones</td><td>Sl-Gn-Brt ± Qtz</td><td>Karsts, ENE-WSW veins</td><td>JM9</td><td></td><td></td><td></td><td></td><td></td><td>0.710457<br/>± 1.0E-5</td><td>+13.9<br/>± 0.6</td></tr></tbody></table>

<table><caption>**Fig. 2.** Paragenetic sequence. Letters refer to mineralized zones from the Vendée Coast (Fig. 1b): M = Anse aux Moines, C=Cayola Bay, S = Mine des Sards, P=Payré Estuary, SN = Anse de St-Nicolas and J = Jard-sur-Mer.</caption><thead><tr><th>STAGE</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th></tr></thead><tbody><tr><td>FLUORITE</td><td>Fl1<br/>SN</td><td>Fl2<br/>SN</td><td>Brt2<br/>SN</td><td>Brt4<br/>C S SN P J</td><td></td><td></td></tr><tr><td>BARYTE</td><td></td><td></td><td>Qz2<br/>M C S SN</td><td>Qz3<br/>C S SN</td><td>Qz5<br/>M C S SN</td><td></td></tr><tr><td>QUARTZ</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>PYRITE</td><td></td><td></td><td></td><td></td><td>Py5<br/>M C S SN J</td><td></td></tr><tr><td>CALCITE</td><td></td><td></td><td></td><td></td><td></td><td>Cal6<br/>S SN P J</td></tr></tbody></table>

6 are fracture fillings (veins) and karsts (geodes).

The 6 paragenetic stages are defined as follows:

**Stage 1** corresponds to disseminated (sometimes clustered) fluorite (Fl1) in silicified SU2 (Fig. 3a). The fluorite crystals take the form of beige, euhedral cubic crystals 100 µm to 0.5 mm wide. Microscopically, the epigenized primary oolites are preserved within the Fl1-crystals (Fig. 3a) but the primary mineral (probably carbonates) is completely pseudomorphosed into fluorite.

**Stage 2** corresponds to a second generation of fluorite (Fl2-fluorite), associated with baryte (Brt2) and quartz (Qz2), the cryptocrystalline quartz is responsible for silicification (Fig. 3a-e-i). Stage 2 minerals replace previous oolitic structures and constitute up to 90 % of the rock (Fig. 3a). Fl2-fluorite is yellow and appears either as a growth rim around Fl1-fluorite or, as isolated and/or clustered cubic crystals 100-150 µm wide (Fig. 3a). Brt2-baryte crystals are white and needle-shape from 300 µm to more than a few centimeters, showing the epigenization of bivalve fossils.

**Stage 3** is characterized by quartz (Qz3) which consists of a thin overgrowth surrounding Qz2-quartz (Fig. 3a-i).

**Stage 4** corresponds to the filling stage of karsts and veins oriented WNW-ESE. The filling consists of rare 0.5 mm wide yellow euhedral cubic crystals of fluorite (Fl4) and baryte (Brt4-baryte). Brt4-baryte consists of white crystals aggregated in crest-like forms (or rosettes) 0.5 cm to 3 cm wide. In karst, it has a cockade morphology (Fig. 3d). In the Mine des Sards, isolated boxworks of crested aggregates in karsts have already been reported (Bouat et al., 2023). In the veins Brt4-baryte shows a syntaxial growth (Fig. 3b-c).

**Stage 5** corresponds to a second episode of veins and karsts filling. It is composed of euhedral crystals of pyrite (Py5-pyrite) and quartz (Qz5-quartz). In WNW-ESE oriented veins, Py5-pyrite forms thin layers (≤1 cm) overlying Brt4-baryte (Fig. 3b-d) or between Qz3-quartz and Qz5-quartz (Fig. 3e). In Anse de St-Nicolas, Py5-pyrite fills NNW-SSE fractures cutting WNW-ESE oriented Brt4-baryte veins (Fig. 3c). In SU5 unit,

WNW-ESE oriented lenticular Brt4-baryte and Py5-pyrite mineralization outcrops on the beach west of Jard-sur-Mer harbor. The Qz5-quartz consists of large euhedral crystals 100 µm to a few centimeters long (Fig. 3e-i) covering the Brt4-baryte in the karsts (Fig. 3d). It appears in single-mineral veins or in association with Brt4-baryte and Py5-pyrite (Fig. 3e).

**Stage 6** corresponds to euhedral calcites (Cal6) filling the veins (Fig. 3f). Calcite Cal6- is not observed in the basement, is rare in the silicified SU2-unit and ubiquitous in the fractures of non-silicified SU2, SU3 and SU5 units. U-Pb dating of Cal6-calcite gives two distinct ages of growth at 145 and 70-80 Ma respectively (Bouat et al., 2023).

## 4.2. Vendée Coast deposits fluid inclusions analysis

FIs analyzes were conducted on Fl2-fluorite, Brt4-baryte and Qz5-quartz. Microscopic observations on these minerals hosted in the basement and mainly in the silicified SU2, show that their primary FIs are biphasic, with a liquid and vapor phases (Fig. 3 g-h-j). Vapor volume is between 10 and 15 % in FIs of Fl2-fluorite, between 15 and 20 % in FIs of Brt4-baryte and around 20 % in FIs of Qz5-Quartz. No secondary FIs in trails or different FIs populations in the same sample were observed. Microthermometric analysis of the FIs of Fl2-fluorite, and Brt4-baryte samples analyzed are from the Anse de St-Nicolas deposit (Fig. 1b). Qz5-Quartz samples are from the Anse aux Moines, Cayola Bay, Mine des Sards and Anse de St-Nicolas deposits (Fig. 1b). All microthermometric data are available in supplementary materials (Table S1) with histograms (Fig. S1).

Translucent Fl2-fluorite crystals host irregularly shaped FIs 15 to 30 µm wide (Fig. 3 g). Brt4-baryte crystals form white crest-like span hosting numerous solid inclusions and FIs making them difficult to observe (Fig. 3 h). The FIs have an elongated shape (up to 40 µm long) and are preferably oriented along the crystal's c-axis. In Qz5-quartz crystals, FIs are rare, up to 25 µm wide and randomly scattered.

Microthermometric data are reported in Table 1 and Fig. 4. Ice melting temperatures (T<sub>m</sub>) range from -18.2 °C and -0.4 °C indicating salinities ranging between 21.2 and 1.0 wt% eq. NaCl, respectively. Homogenization temperatures (T<sub>h</sub>) in the liquid phase range from 110.4 °C to 394.1 °C depending on the host-mineral.

In Anse de St-Nicolas, FIs of Fl2-fluorite have the highest salinity (13.3 to 21.2 wt% eq. NaCl) and the lowest T<sub>h</sub> ranging between 110.4 °C and 173.1 °C (Table 1; Fig. 4a). Brt4-baryte FIs are the least saline (4.0 and 9.5 wt% eq. NaCl) but have the highest T<sub>h</sub> ranging from 195.2 °C to 378.0 °C (Table 1; Fig. 4a). In Qz5-quartz, FIs have highly variable salinities (between 1.0 and 12.9 wt% eq. NaCl) and T<sub>h</sub> value ranging between 140.9 and 384.8 °C (Table 1; Fig. 4a). Microthermometry data for FIs contained in a single Qz5-quartz crystal and positioned along the crystal's c-axis show a decrease of both salinities from 10.0 to 3.2 wt% eq. NaCl and T<sub>h</sub> from 378.9 to 215.7 °C, respectively (Fig. 3i-j). In Mine des Sards, FIs in Qz5-quartz, have the most dispersed salinity and T<sub>h</sub>,
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** a: Disseminated Fluorite in association with epigenized oolites (Oo) with Fl2 overgrowth, Fl2-fluorite crystals, Brt2 in needle-shape and cryptocrystalline Qz2 hosted in SU2. b: Basement-hosted WNW-ESE Brt4-baryte vein. c: WNW-ESE Brt4-baryte vein hosted in SU2- and crosscut by NNW-SSE Py5-pyrite vein. d: karst filled by Brt4-baryte, Py5-pyrite and Qz5-quartz hosted in SU2. e: Vein with silicified clasts (Qz2) with a rim of Qz3 on the wall and larger Qz5-quartz crystals with Py5-pyrite in the core. f: Vug in a Qz5-quartz vein filled by Cal6. g: Representative Fluorite in Fl2-fluorite. h: Representative FIs in Brt4-baryte. i-j: Representative FIs in Qz5-quartz. In (j), FIs salinity and Th values are reported, showing a decreasing of salinity and Th along the crystal growth direction.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4. Salinity vs $T_h$ diagram for FIs from (a) Anse de St-Nicolas, (b) Mine des Sards and (c) Anse aux Moines and Cayola Bay (Table 1).</figcaption></figure>

ranging from 3.6 to 18.4 wt% eq. NaCl and from 160.2 to 394.1 °C, respectively (Table 1; Fig. 4b). For the same generation of Qz5-quartz, at Anse aux Moines and Cayola Bay (Table 1; Fig. 4c), the FIs have a salinity between 4 and 12.5 wt% eq. NaCl and $T_h$ between 160 and 335 °C.

### 4.3. $^{87}$Sr/$^{86}$Sr$_{baryte}$ and $\delta^{34}$S$_{baryte-sulfide}$

$^{87}$Sr/$^{86}$Sr and $\delta^{34}$S analyses of Brt4-baryte and Py5-pyrite samples from the Vendée Coast district were carried out (Table1; Fig. 5a) and compared with analyses of baryte samples from unconformity-related around the Massif Central (Table 1; Fig. 1a). The $^{87}$Sr/$^{86}$Sr ratio and $\delta^{34}$S values of Brt4-baryte vary from 0.709581 to 0.710900 and from +23.5 to +43.6 ‰ respectively (Table 1; Fig. 5a). The $^{87}$Sr/$^{86}$Sr ratio vary according to the host-rock (Fig. 5a). Basement Brt4-baryte has the highest value (0.710900), while the lowest value of 0.709581 corresponds to the Brt4-baryte hosted in SU5- at the top of the stratigraphic sequence (Fig. 5a). The $^{87}$Sr/$^{86}$Sr ratio of Brt4-baryte from SU2 shows

<figure><img src="image_6.png" /><figcaption>Fig. 5. a: $^{87}$Sr/$^{86}$Sr vs $\delta^{34}$S plot for Brt4-baryte samples from the Vendée coast and baryte samples from other districts (see Table 1). Shaded area and boxes represent literature data. b: $\delta^{34}$S plot for baryte samples. Brt4-baryte samples correspond to the SU5 sample from Anse de St-Nicolas (see Table 1).</figcaption></figure>

mean values between 0.709692 and 0.709787 in unaltered SU2 and between 0.710093 and 0.710751 in silicified SU2 (Table 1; Fig. 5a). Brt4-baryte $\delta^{34}$S values of are clustered around +25 ‰ in basement embedded and SU2 samples and increase to +27.2 and +43.6 ‰ in SU5 (Table 1; Fig. 5a-b). A sedimentary marcasite nodule in SU5 shows highly depleted $\delta^{34}$S value at -36.9 ‰, while Py5-pyrite in SU2 and SU5 units is less depleted with values at -23.5 ‰ and -7.3 ‰, respectively (Table 1). $\delta^{34}$S values were obtained on two Brt4-baryte /Py5-pyrite pairs from the silicified SU2 of Anse de St-Nicolas (baryte = +28.6 ‰; pyrite = -23.5 ‰) and SU5 from Jard-Sur-Mer (baryte = +43.6 ‰; pyrite = -7.3 ‰; Table 1), leading to similar $\Delta^{34}$S$_{pyrite-baryte}$ at +52.1 ‰ and +50.9 ‰, respectively.

Baryte samples from other Massif Central districts have $^{87}$Sr/$^{86}$Sr and $\delta^{34}$S values between 0.708969 and 0.712843 and between +13.9 and +23.8 ‰, respectively (Table 1; Fig. 5a). $^{87}$Sr/$^{86}$Sr values differ from district to district and host rocks to host rock. The Périgord baryte
sample hosted in a gneiss (Fig. 1a) has the lowest $^{87}$Sr/$^{86}$Sr ratio with a value at 0.708969. In the Grands-Causses (Fig. 1a), baryte samples have $^{87}$Sr/$^{86}$Sr ratio ranging from 0.710136 to 0.710685 in the basement. In the overlaying Lower and Middle Jurassic sedimentary series, $^{87}$Sr/$^{86}$Sr ratios range from 0.710042 to 0.712843. The $^{87}$Sr/$^{86}$Sr ratio of the Cévennes baryte sample hosted in the Middle Jurassic has a mid-range value of 0.710457. For the $^{δ}$34S signature, there is no clear correlation between baryte and the host-rock or district. For example, in the Grands-Causses, $^{δ}$34S values vary from +14.0 to +23.8 ‰, between +20.0 and +23.8 ‰ in the basement, between +14.0 and +23.3 ‰ in the Lower Jurassic and stand at +16.9 ‰ in the Middle Jurassic. These Brt4-baryte isotopic values are consistent with those obtained previously (Aquilina et al., 2011; Fontes and Matray, 1993; Gigoux, 2015; Laurent et al., 2020; Leach et al., 2006; Sizaret et al., 2009) and reported in Fig. 5a-b.

# 5. Discussion

## 5.1. Interpretation of fluid inclusions data from the Vendée Coast deposits

Homogenization temperatures ($T_h$) obtained on all FIs contained in fluorite, baryte and quartz crystals range from 110.4 to 394.1 °C (Table 1; Fig. 4). These $T_h$ values are well above the 60 °C in the Vendée Variscan basement proposed by François et al. (2020) based on low-temperature thermochronology (LTT) models. Maximum $T_h$ are lower than the 440 °C corresponding to the boiling temperature in the H₂O-NaCl system for a salinity of 10 wt% eq. NaCl and a pressure of 390 bars (corresponding to a depth of 1.3 km according to François et al., 2020) based on simulations by Driesner and Heinrich (2007), which is coherent with observations of biphasic FIs (no demixing).

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** a. Fluid mixing model of the Vendée Coast deposits based on mineralogy and microthermometry data of the Anse de St-Nicolas zone (Table 1; Fig. 4a). The three sources of fluids are represented by striped box and numbered as the interpreted order of appearance (see text for details). b. Genetic model of the unconformity-related deposits of the Vendée Coast in a cross-section through the Aquitaine Basin. Yellow arrows represent brines flowing from buried evaporites along the unconformity in porous sediments confined by an impermeable layer above. The brines could leach the basement along the unconformity (orange area). The fluid from the basement (purple arrows) ascends along active faults during the extensional tectonic activity (black arrows) around 145 Ma up to the unconformity. The mixing with the brines enable mineral precipitation and dissolution of the hosted sediments (light purple area). Finally, seawater (blue arrows) percolates from the surface in the hydrothermal system and mix with other fluids. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
The paragenetic sequence (Fig. 2) shows the evolution of minerals through time and their relationship with the host rock. In Anse de St-Nicolas minerals are first disseminated in SU2-unit (stage 1 and 2), then in the basement, and finally in SU2 and SU5-units where fluorite-baryte and quartz-pyrite associations fill fractures and karsts (stage 4 to 5 respectively). As the formation temperature of the host minerals is close to that of the T<sub>h</sub> of the FIs in unconformity-related ore deposits (Cathelineau et al., 2012), we will consider them identical in the following discussion. FIs for stage 2 fluorite, stage 4 baryte and stage 5 quartz show variations in salinity and T<sub>h</sub> during successive mineral precipitation.

Stage 2 mineral growth at Anse de St-Nicolas is associated with relatively low temperature (T<sub>h</sub> between 110.4 °C and 173.1 °C) and high salinity (13.3 to 21.2 wt% eq. NaCl; Table 1; Fig. 4a and Fig. 6) fluid circulation. Stages 4 and 5 are associated with a fluid salinity of between 1.0 and 12.9 wt% eq. NaCl and temperatures of 140.9 °C to 384.8 °C (Fig. 4a). Overall, during quartz growth in stage 5, there is a joint decrease in salinity from 10 to 3 wt% eq. NaCl and in temperature from 350 to 400 °C to 140 °C (Fig. 3i-j, Fig. 4a and Fig. 6).

Mineral growth at stage 5 in the Mine des Sards, Anse aux Moines and Cayola Bay shows the same trend as that described for Anse de St-Nicolas (Table 1; Fig. 4 and Fig. 6) with, however, for stage 5 in Mine des Sards, a salinity starting at 14 wt% eq. NaCl (Fig. 4a-b). This suggests that the starting fluid in the Mine des Sards at stage 2 probably has higher salinity and lower temperatures than in Anse de St-Nicolas (Fig. 4a-b and Fig. 6). The microthermometric data for the Vendée Coast deposits show a trend of decreasing salinity associated with increasing temperature path, ending with cooling to about 140 °C (Fig. 6). Mine des Sards and Anse de St-Nicolas recorded the most complete salinity-temperature path, while Anse aux Moines and Cayola Bay only recorded the end.

Analysis of the FIs allows us to establish the fluids succession of within the paragenetic sequence (Fig. 6) as follows:

(1) Circulation of low temperature brines with salinities up to 22 wt % eq. NaCl (Babel and Schreiber, 2014) probably sourced from the leaching of evaporites buried in the basin.

(2) Around 145 Ma, rise of a hot fluid through the basement faults (Bouat et al., 2023). In Vendée, no magmatic activity being described at 145 Ma, this eliminates a magmatic source for the hot fluid. Some authors have invoked an abnormally high regional geothermal gradient (Boiron et al., 2002) or the expulsion of a deep hot fluid in thermal disequilibrium with the basement. The first hypothesis is contradicted by the regional context in Vendée and neighboring regions which are affected by a normal geothermal gradient (Barbarand et al., 2001, 2013, 2020; François et al., 2020). The hypothesis of hot fluids (T > 300 °C) released from the basement at 145 Ma therefore seems more probable. Previous works on the southern Armorican Massif have identified surface fluids trapped in the crust during the Variscan orogeny (Dusséaux et al., 2019, 2022; Le Hébel et al., 2007). Such a long-residence time of a surface fluids trapped in the basement- (Bons et al., 2014; Bons and Gomez-Rivas, 2020), could allow its heating then, its release into the overlying cover using inherited Variscan faults reactivated at 145 Ma.

(3) Seawater percolation along faults (Fig. 6) with low salinity (probably lower than 5 wt% eq. NaCl) and surface temperature (maximum 40 °C) as already mentioned by Cathelineau et al. (2012) from C and O isotopic analysis of calcite (last mineral of the paragenesis).

These three fluids are probably mixed sequentially during basement faults activities (Bouat et al., 2023). This is in good agreement with previous studies showing a mixing of three fluids leading to the hydrothermal deposits of the Vendée Coast (Cathelineau et al., 2012), although giving higher maximum temperatures (Fig. 4).

## 5.2. Sources of baryte from the Vendée Coast deposits

### 5.2.1. Source of strontium and barium

The 87Sr/86Sr ratios of the Vendée deposits are all higher than those of seawater or Triassic and Jurassic evaporites, which have isotopic ratios between 0.707 and 0.708 (Koepnick et al., 1990). This implies an enriched (i.e. radiogenic) source of Sr for these deposits. Basement rocks generally have a high 87Sr/86Sr ratio due to Rb-rich minerals whose radiogenic 87Rb decays to 87Sr (Canals and Cardellach, 1993; Valenza et al., 2000). The enrichment observed in baryte in SU5 at about 0.709581 and 0.710900 in the basement, could be explained by a significant addition of radiogenic 87Sr (Table 1; Fig. 5a). In the Vendée Coast and Poitou High deposits, basement feldspars and micas are altered beneath the unconformity (Boiron et al., 2002; Cathelineau et al., 2012; Fourcade et al., 2002; Strzerzynski et al., 2020), and are known to be a possible source of radiogenic 87Sr (McNutt et al., 1990). We therefore propose that the basement is the source not only of the barium but also very likely of the associated base metals, F and Si observed in the deposits of the Vendée Coast.

### 5.2.2. Source of sulfur

Sulfur isotopes (δ³⁴S) on baryte and pyrite from the Vendée Coast vary between +23.5 and + 43.6 ‰ and between -23.5 and - 7.3 ‰, respectively (Table 1; Fig. 5a-b). This enriched signature in baryte does not correspond to a magmatic signature (around 0 ‰), which is consistent with the absence of magmatic activity at this time. Sediments, in particular evaporites which consist of sulfates minerals could be the source of sulfur in the area. In France, evaporite deposits are known from Upper Triassic to Lower Jurassic (Curnelle and Dubois, 1986; Biteau et al., 2006) with δ³⁴S signature ranging from +12 to +21 ‰ (Crockford et al., 2019; Kampschulte and Strauss, 2004). Baryte values from the Vendée Coast are higher, implying a mechanism allowing an isotopic enrichment in the δ³⁴S value. Baryte and pyrite are observed in two samples from SU2 and SU5 (Samples Pt201 and MR201, Table 1) with similar isotopic fractionations between them with a Δ³⁴S around -50 ‰. Using the fractionation equations between aqueous SO₄²⁻ and H₂S (after Eldridge et al., 2016), we obtained a temperature of 70 °C which is far below the temperature obtained by microthermometry on baryte (T<sub>h</sub> between 195 °C 380 °C; Table 1). We concluded that sulfates-sulfide isotopic equilibrium was not been achieved in the samples from the Vendée Coast. Sulfate-sulfide disequilibrium is common in a context of low sulfidation (Hutchison et al., 2020), or as in sulfides from modern seafloor hydrothermal vent (Ono et al., 2007). Indeed, the rate of precipitation is faster than the rate of isotope exchange to reach isotopic equilibrium between aqueous sulfate and H₂S, especially during fluid mixing (Shelton and Rye, 1982; Zheng, 1991; Ohmoto and Lasaga, 1982). Baryte - which is slightly soluble (Djamali et al., 2016) - precipitate rapidly compared to pyrite, thus explaining the difficulty for this system to reach isotopic equilibrium.

In our case, the coexistence of baryte and pyrite, in such a hydrothermal context, may be due to a sulfate reduction process caused by bacterial (Bacterial Sulfate Reduction, BSR; Machel, 2001) or abiotic activities (Thermochemical Sulfate Reduction, TSR; Goldstein and Aizenshtat, 1994). BSR is ineffective above 80 °C, as sulfate-reducing bacteria cease to metabolize (Machel, 2001). In contrast, TSR is effective above 120 °C (Goldstein and Aizenshtat, 1994; Machel, 2001). Temperatures obtained on baryte FIs from the Vendée Coast are >150 °C, suggesting that TSR is the more likely process to explain the co-precipitation of baryte and pyrite. The catalyst for TSR is a reducing agent which may be organic matter, potentially present either in the sediments (Galbrun et al., 1994) or in the basement (Le Corre et al., 1991; Le Hébel et al., 2007).

North of the Aquitaine Basin, the largest source of sulfur in the Lower Jurassic sediments is the Hettangian Anhydrite Zone (Biteau et al., 2006; Curnelle and Dubois, 1986), whose δ³⁴S signature is +14.2 ‰ (Crockford et al., 2019). To explain the significant enrichment of some
baryte compared to Lower Jurassic anhydrites, we performed Rayleigh distillation calculations (e.g., *Seal*, 2006). The observed isotopic signature could result from a partial reduction of about 28 % in aqueous sulfates derived from the Lower Jurassic anhydrites for the pair hosted by SU2-unit and 58 % for the pair hosted by SU5-unit. We interpret this phenomenon as simultaneous precipitation during tectonic fracturing that extended from basement to cover. Sulfates-rich fluids derived from Lower Jurassic anhydrites probably circulated along the unconformity and mixed in the SU2-unit with ascending hot fluids (>150 °C), rich in base metals trapped in the basement, resulting in a ~ 28 % TSR reduction of the aqueous sulfates. Residual dissolved sulfates continue to be reduced by up to 58 % during fracture propagation in the Middle Jurassic formation (SU5).

Using sulfur isotopes (δ³⁴S) on baryte and pyrite it is deduced that the Vendée hydrothermal system sulfur is compatible with and may originate from the Lower Jurassic Anhydrite Zone of the Aquitaine Basin, with partial reduction of aqueous sulfates by TSR.

## 5.3. French unconformity-related ore deposits link with brines

### 5.3.1. Role of buried evaporites involvement in brines formation

In this section, we will highlight the role of buried evaporites in the formation of the unconformity-related french deposits based on our data, together with data from the literature (Tables 1 and 2; Fig. 1 and Fig. 8). Quartz FIs of from the Poitou High and Albigeois districts (Massif Central) yield T<sub>H</sub> values of 100 to 160 °C and 85 to 170 °C and salinity of 10 to 21 wt% eq. NaCl and 20–26 wt% eq. NaCl (Table 2; Fig. 7; Cathelineau et al., 2012; Munoz et al., 1999). In the southeast of the Massif Central, in the Cévennes, the T<sub>H</sub> of the FIs of the Largentière and Trèves deposits are below 200 °C with salinities between 10 and 25 wt% eq. NaCl (Léost et al., 1999; Sabouraud et al., 1980). At Trèves, the δ³⁴S of baryte varies from +12.2 ‰ to +19.2 ‰ (Table 2; Fig. 5b; Leach et al., 2006). In Largentière, baryte data vary from +6.8 to +24.7 ‰ (Table 2; Fig. 5a, Aquilina et al., 2011). To explain these variations, the authors conclude that there is a mixture between a brine from Paleozoic-Triassic sulfates and seawater. In the Lodève district, the δ³⁴S of the baryte varies little between +15.6 and + 18.5 ‰ and the T<sub>H</sub> varies from 50 °C to 220 °C for salinities between 1 and 21 wt% eq. NaCl (Table 2; Fig. 5a; Laurent et al., 2017, 2020).

North of the Massif Central, in the Chaillac deposit, fluorite FIs yield T<sub>H</sub> below 150 °C and salinity of 10 to 25 wt% eq. NaCl (Fig. 7; Sizaret et al., 2004). The δ³⁴S of the baryte ranges from +13.9 ‰ to +21.0 ‰ (Table 2; Fig. 5a; Sizaret et al., 2009). In the Morvan district, Gigoux et al. (2016) obtained a lower homogenization temperature (70–140 °C) and salinities up to 16.3 wt% eq. CaCl₂ from FIs from the Courcelles deposit. Values of the δ³⁴S on baryte are homogeneous between +17 and + 18.5 ‰ (Table 2; Fig. 5b; Gigoux, 2015).

In all basins around the Massif Central, low-temperatures and high salinity brines showing δ³⁴S values on baryte between +12 to +21 ‰, are consistent with buried evaporites from the Upper Triassic to Lower Jurassic (Fig. 5a; Kampschulte and Strauss, 2004). These evaporites present in all these basins (Fig. 8; Biteau et al., 2006; Curnelle and Dubois, 1986; Mégnien et al., 1980) are similar to the Vendée Coast deposits reinforcing the hypothesis that here the brines likely originate from similar buried evaporites corresponding to the main sulfur stock in all these deposits. Moreover, the δ³⁴S enrichment of the baryte in these basins, can also be explained by Rayleigh distillation as in the Vendée Coast deposits.

This may imply brines transfer along the unconformity, but over a length of a few kilometers for the Morvan and Cévennes districts and at least a hundred kilometers for the Grands-Causses and Albigeois districts. This mechanism of brine expulsion by leaching from buried-evaporites is already known from the Poitou High and Cévennes deposits (Boiron et al., 2002; Cathelineau et al., 2012; Ramboz and Charef, 1988) and from the Pine Point deposits in Canada (Rhodes et al., 1984).

### 5.3.2. Basement-brines interactions as a source for metals

In addition to very similar mineral associations (Cathelineau et al., 2012; Gigoux et al., 2016; Laurent et al., 2017; Leach et al., 2006; Sizaret et al., 2004) and evidence of evaporitic brines circulation, all these unconformity-related deposits give arguments in favor of silicification of the cover. Moreover, the high ⁸⁷Sr/⁸⁶Sr ratio between 0.707 and 0.708 is higher than that of evaporites (Koepnick et al., 1990), confirming the significant influence of basement derived fluids. In the western Massif Central, Coiteux (1983) reports silicification of sediments above the unconformity in the Melle deposit (Poitou High; Fig. 1a). This phenomenon is also reported in the Périgord (Guillot et al., 1979) and Grands-Causses districts (Legendre et al., 2009; Roig et al., 2001). Furthermore, in these deposits the ⁸⁷Sr/⁸⁶Sr ratio on baryte range between 0.708969 (Neuil zone, Périgord district) and 0.712843 (Barjac zone, Grands-Causses district). In the Albigeois district, silicification is not reported in sediments but in the fault zones (Munoz et al., 1999). To the southeast of the Massif Central, the Cévennes district host silicified sediments (Leach et al., 2006) with a wider range of ⁸⁷Sr/⁸⁶Sr ratios on baryte (Table 2; Fig. 5a; Aquilina et al., 2011). In Laurent et al. (2020) the ⁸⁷Sr/⁸⁶Sr ratio on baryte from the Lodève deposit is similar to that of the other deposits, with no silicification reported in the cover despite the propagation of silicification in fault zones as in the Albigeois district. In the northern Massif Central, sediment silicification is known as “sinter” in the Chaillac deposit (Sizaret et al., 2009) associated with baryte with an ⁸⁷Sr/⁸⁶Sr ratio comparable to the Lodève deposits (Table 2; Fig. 5a). In the Morvan district, pervasive silicification of sediments above the unconformity is described by Gigoux et al. (2016) in association with hydrothermal mineralizations.

All these data are consistent with brines-rock interactions between the Variscan basement and the sedimentary cover similar to that of the Vendée Coast deposits (see section 5.2.1). Alteration of feldspar and mica in these unconformity-related deposits is probably responsible for the high ⁸⁷Sr/⁸⁶Sr ratios, as well as high silica, base metals and F-Ba contents (Boiron et al., 2010; Burisch et al., 2016; Fourcade et al., 2002; Walter et al., 2019). Brines have the ability to leach cations from the basement and transport them via chloride complexes (Helgeson, 1964; Yardley, 2005). This could explain the similarity between Fl-Brt-Qz parageneses and sulfides associations with silicification in all these unconformity-related deposits (Table 1 and Table 2). Moreover, these different ranges of high ⁸⁷Sr/⁸⁶Sr ratios could be related to the diversity of lithologies in the basement from each district, or to different degree of mixing between the brines and basement-derived fluids.

## 5.4. French unconformity-related deposits and rifting settings with deep hot fluids

In order to establish the temporal specificity of the formation of French unconformity-related deposits, we will compare our results (Table 1) with available data on deposits emplaced in all Mesozoic basins of France (Table 2).

### 5.4.1. Bay of Biscay rifting

On the Vendée coast, previous studies have highlighted the link between mineral deposits and a regional N-S extension dated at 144.1 ± 26.2 Ma and characterized by NW-SE normal faults and fractures (Fig. 8, Bouat et al., 2023; Strzerzynski et al., 2020). Microthermometric data obtained on Brt4-baryte and Qz5-quartz FIs attest to the circulation of a low salinity (< 10 wt% eq. NaCl), high temperature (T° > 350 °C) fluid from the basement into these deposits (see section 5.1; Fig. 7). From Poitou High (Boiron et al., 2002) to Grand-Causses (Briand et al., 1979, 1993; Burg et al., 1992; Legendre et al., 2009; Mennessier and Collomb, 1986; Roig et al., 2001), via Périgord districts (Guillot et al., 1979), similar NW-SE trending vein deposits are observed. Further south, in the Albigeois, these are striking E-W (Munoz et al., 1999, 2005). The timing of mineral deposition is constrained between 191 Ma and 132 Ma by K–Ar and ⁴⁰Ar/³⁹Ar dating on adularia and associated clays (Table 2
<caption>**Table 2**</caption>

Literature data on French unconformity-related deposits reporting geological contexts, ages of the ore deposits, temperatures and salinities determined in FIs of different hosted-minerals and $^{87}$Sr/$^{86}$Sr-$\delta^{34}$S on baryte (See references for details). Qz: quartz, Brt: baryte, Fl: fluorite, Mrc: marcasite, Gn: galena, Sp: sphalerite, Cal: calcite, Anh: anhydrite, Dol: dolomite.

<table><thead><tr><th rowspan="2">District</th><th rowspan="2">Zone</th><th rowspan="2">Main minerals</th><th rowspan="2">Host-rock</th><th rowspan="2">Morphology</th><th rowspan="2">Age (Ma)</th><th colspan="3">Microthermometry</th><th colspan="2">Baryte</th><th rowspan="2">Reference</th></tr><tr><th>Host</th><th>Temperature (°C)</th><th>Salinity (wt %NaCl)</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th>δ<sup>34</sup>S (vCDT‰)</th></tr></thead><tbody><tr><td rowspan="4">Vendée Coast Poitou High</td><td>Anse de St-Nicolas Melle</td><td>Brt-Qtz-Cal</td><td>SU5-unit</td><td>NW-SE vein</td><td>144.1 ± 26.2 Ma</td><td>Qtz</td><td>100–160</td><td>10–21</td><td></td><td></td><td>Bouat et al. (2023)</td></tr><tr><td>Charroux-Civray Chaillac</td><td>Qtz-Gn<br>Qtz-Fl-Brt<br>Fl-Brt-Gn</td><td>Dolomitized arenite (Low. Jurassic)<br>Granitoid<br>Micaschists and Sandstones (Low. Jurassic)</td><td>Karst<br>NW-SE vein<br>Stratiform, NNE-SSW vein</td><td>140.5–154.3 (K–Ar & Ar–Ar)<br>201–199 (stratigraphic)<br>154.5 (K–Ar)</td><td>Fl</td><td>84–150</td><td>0.4–21.8</td><td>0.70903–0.71185</td><td>13.9–21.0</td><td>Cathelineau et al. (2012)</td></tr><tr><td>Morvan</td><td>Pierre-Perthuis</td><td>Silicified dolomite (Up. Triassic)</td><td>Karst</td><td>130 ± 15 (Sm–Nd)</td><td>Fl</td><td>76–245</td><td>1.2–10.7</td><td></td><td></td><td>Sizaret et al. (2004)</td></tr><tr><td>Courcelles</td><td>Fl-Brt-Qtz</td><td>Limestone (Low. Jurassic)</td><td>Stratiform</td><td></td><td>Fl</td><td>72–137</td><td>0.2–1.9</td><td></td><td></td><td>Cathelineau et al. (2012)<br>Sizaret et al. (2009)<br>Gigoux et al. (2015)</td></tr><tr><td rowspan="2">Grands-Causses</td><td>St-Geniez d'Olt</td><td>Brt</td><td>Micaschists</td><td>NW-SE vein</td><td>191–146 (K–Ar)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Bestex Montpestels Colombiès</td><td>Brt<br>Brt<br>Brt</td><td>Granit Granit Micaschists</td><td>NW-SE vein<br>NW-SE vein<br>NW-SE vein</td><td>169–146 (K–Ar)<br>163–149 (K–Ar)<br>132 (K–Ar)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cévennes</td><td>Largentière</td><td>Sl-Gn-Brt ± Anh ± Dol</td><td>Sandstone (Up. Triassic)</td><td>Stratiform, NE-SSW vein</td><td>217–210 (K–Ar)</td><td>Anh-Dol-Qtz</td><td>63–295</td><td>1–24</td><td></td><td></td><td>Bonhomme et al. (1987)</td></tr><tr><td rowspan="2">Lodève</td><td>Masdieu</td><td>Brt</td><td>Sandstone and dolomite (Low.-Mid. Triassic)</td><td>Stratiform</td><td>213–205 (K–Ar)</td><td></td><td></td><td></td><td></td><td></td><td>Bonhomme et al. (1987)</td></tr><tr><td>Trèves</td><td>Sl-Gn-Qtz ± Brt</td><td>Dolomitized carbonates (Low. Jurassic)</td><td>Stratiform, NNE-SSW vein</td><td></td><td>Qtz</td><td>60–130</td><td>15–24</td><td>0.708201–0.71561</td><td>6.8–24.7</td><td>Bonhomme et al. (1987)</td></tr><tr><td rowspan="2">Albigeois</td><td>Gatuzières</td><td>Gn</td><td>Sandstone and carbonates (Triassic-Low. Jurassic)</td><td>NW-SE vein</td><td>188 (K–Ar)</td><td></td><td></td><td></td><td></td><td></td><td>Sabouraud et al. (1980)</td></tr><tr><td>Malines</td><td>Sl-Gn-Brt ± Qtz</td><td>Cambrian dololimestones, Triassic shales and Middle Jurassic limestones</td><td>Karsts, ENE-WSW vein</td><td>271–211 (K–Ar)</td><td>Brt</td><td>130–380</td><td>3–12</td><td></td><td></td><td>Leach et al. (2006)<br>Bonhomme et al. (1987)</td></tr><tr><td rowspan="2"></td><td rowspan="2">Mont-Roc</td><td rowspan="2">Fl-Qtz</td><td rowspan="2">Detritic sediments (Permian)</td><td rowspan="2">ENE-WSW vein</td><td rowspan="2">182–174 (K–Ar)</td><td>Brt</td><td>75–222</td><td>1.7–21</td><td rowspan="2">0.70888–0.711913</td><td rowspan="2">15.6–18.5</td><td>Ramboz and Charef (1988)<br>Bonhomme et al. (1987)<br>Laurent et al. (2017)<br>Laurent et al. (2020)<br>Bonhomme et al. (1987)</td></tr><tr><td>Qtz</td><td>48–195</td><td>12.6–13.6</td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td><td>E-W vein</td><td>246–149 (K–Ar)</td><td>Qtz</td><td>85–170</td><td>20.0–26.0</td><td></td><td></td><td>Munoz et al. (2005)<br>Munoz et al. (1999)</td></tr></tbody></table>
<figure><img src="image_8.png" /><figcaption>Fig. 7. Salinity-Temperature diagram of FIs from ore deposits in French basin (See Table 2 for references).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Simplified geological map of France with basin-hosted ore deposits. See Table 2 for details and references about deposits age. The Upper Triassic and Lower Jurassic map of the buried evaporites from Curnelle and Dubois (1986) and Mégnien et al. (1980).</figcaption></figure>
and Fig. 8; Bonhomme et al., 1987, Cathelineau et al., 2012) and Sm—Nd dating on fluorite (Munoz et al., 2005). This wide age range is related to alteration processes over a long period of time but also to the presence of a fraction of white micas inherited from the older Variscan basement (Bonhomme et al., 1987; Cathelineau et al., 2012). We consider that the mineralized deposits were probably formed in the youngest part of these periods obtained for each deposit, i.e. around 155 and 140 Ma in the Poitou High (Table 2 and Fig. 8; Cathelineau et al., 2012) and around 149 Ma and 111 Ma in the Grands-Causses and Albigeois, respectively (Bonhomme et al., 1987; Munoz et al., 2005). West of the Massif Central, in the region including Vendée Coast, Poitou High, Périgord and Grands-Causses districts, unconformity-related deposits formed between the Upper Jurassic and the Lower Cretaceous. Their veins oriented NW-SE (Fig. 8) are consistent with the onset of rifting in the Bay of Biscay (Asti et al., 2022; Bouat et al., 2023; Constantin et al., 2002; Jammes et al., 2009), located west of the Variscan Massifs (Fig. 8). So, the rifting in the Bay of Biscay probably control the formation of the unconformity-related deposits located from the Vendée Coast to the Grands-Causses.

### 5.4.2. Alpine Tethys rifting

In the Cévennes (Alabouvette et al., 1988; Aquilina et al., 2011; Leach et al., 2006) and Lodève districts (Laurent et al., 2017), the veins show a more complex NNE-SSW to NE-SW orientation. Chronological data obtained on these veins range from 211 Ma to 174 Ma (Bonhomme et al., 1987). Microthermometric analyses of Les Malines baryte FIs by Ramboz and Charef (1988) indicate that these deposits are related to the circulation of a fluid of low salinity (<10 wt% eq. NaCl, Table 2; Fig. 7) and high temperature (up to 380 °C). In the same district, Léost et al. (1999) report a maximum fluid temperature of 295 °C with a salinity of <5 wt% eq. NaCl (Table 2).

Based on these results, we propose a model for ore deposits genesis that involves the ascent of deep hot fluids from the basement in an extensional context. The stress field and the ages of the deposits may vary locally. For example, the deposits of the southeastern Massif Central are dated from the Upper Triassic to the Lower Jurassic with the same NE-SW strike. However, at that time, the southeast Massif Central correspond to the western margin of the Alpine Tethys (Handy et al., 2010; Lemoine et al., 1986; Stampfli and Borel, 2002). Therefore, this rifting appears to control the Cévennes and Lodève basin-hosted ore deposits (Fig. 8). Thus, the southeast Massif Central unconformity-related deposits seem to form in the context of the rifting of the Alpine Tethys.

### 5.4.3. Multi-phase unconformity-related ore deposits

On the northern side of the Massif Central, the deposits are both stratiform in the sedimentary cover and in basement faults oriented NNE-SSW (Chaillac deposit; Sizaret et al., 2004) or NNW-SSE (Morvan district; Gigoux et al., 2015). In the Morvan district, basement-hosted fluorite deposits are dated to the Lower Jurassic (K—Ar on adularia; Gigoux et al., 2015; after Baubron et al., 1980 and Joseph et al., 1973) and to the Lower Cretaceous in the sedimentary cover (Sm—Nd on fluorite, Table 2 and Fig. 8; Gigoux et al., 2015). Preliminary results on U—Pb fluorite overgrowths yield ~40 Ma (Lenoir et al., 2021). At Chaillac, the deposits are dated indirectly to the Lower Jurassic by Sizaret et al. (2004) and to 155 Ma (K—Ar dating on adularia) by Cathelineau et al. (2012).

Gigoux et al. (2016) and Sizaret et al. (2004) have shown that the Morvan and Chaillac deposits were formed at relatively low temperatures. In the Antully fluorite deposits from Morvan, the temperature of the fluid ranges between 70 and 280 °C (Table 2; Fig. 7; Gigoux et al., 2016). Similarities between deposits on the Vendée Coast and Antully deposits could suggest the involvement of a basement-derived fluid for the later but further field investigations are required. These previous works highlight at least two mineralizing stages.

Our work on the ore deposits of the Vendée Coast complements

previous studies on deposits on a European scale (Muchez et al., 2005), demonstrating that the formation of base metals deposits is likely favored in a context of intracontinental rifting. In such a context, faults crossing the unconformity could allow significant fluids-rocks interactions involving brines or, in some cases, the potential release of trapped fluids in the basement, as in the Vendée Coast district and Les Malines deposits (Ramboz and Charef, 1988). In France, Mesozoic rifting took place in two major stages: the opening of the Bay of Biscay and the Tethys Rift. Both periods seem to be associated with at least two stages of mineralizations at the Triassic-Jurassic transition in the southeast of the Massif Central, and at the Jurassic-Cretaceous transition in the southwest and the west of the Massif Central.

## 6. Conclusion

On the Vendée Coast, petrographic observations coupled with the analysis of FIs allowed to propose the succession of three different fluids responsible for the formation of the deposits: i) brines deriving from the leaching of evaporites buried in the basin, ii) hot fluids rising from the basement and iii) the incursion of seawater from the surface. Isotopic data for baryte from the deposits on the Vendée Coast and French unconformity deposits seem to indicate fluids-rocks interactions involving sulfur-rich brines altering basement feldspars and micas along the unconformity.

On a global scale we show that in western Europe, brines originating from the leaching of buried evaporites could be at the origin of the formation of all unconformity-related deposits. Our study makes it possible to define the structural framework of the formation of these deposits. The prevailing extensional context corresponds to two successive stages of rifting: i) the rifting of the Alpine Tethys (southeast of the Massif Central) and the rifting of the Bay of Biscay (west of the Variscan massifs). Finally, our study open news opportunities for the exploration of such deposits. By combining a precise structural and microstructural study coupled with the analysis of fluids and their source, could lead to a better targeting of unconformity-related deposits.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gexplo.2025.107713.

### CRediT authorship contribution statement

**Loïc Bouat:** Writing – original draft, Visualization, Supervision, Methodology, Investigation, Conceptualization. **Pierre Strzerzynski:** Writing – original draft, Visualization, Supervision, Methodology, Investigation, Conceptualization. **Véronique Gardien:** Writing – original draft, Methodology, Investigation. **Guillaume Barré:** Writing – original draft, Methodology, Investigation. **Yannick Branquet:** Writing – original draft, Methodology, Investigation. **Jérémie Melleton:** Writing – original draft, Investigation. **Régis Mourgues:** Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Strzerzynski reports financial support was provided by National Centre for Scientific Research. Strzerzynski reports financial support was provided by Nantes Atlantic Observatory of the Sciences of the Universe. 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.

### Acknowledgements

This work was founded by the Tellus-Cessur program of CNRS and by the OSUNA (France). The authors are grateful of Laurent Lenta from Laboratoire de Planétologie et Géodynamique for sample preparation,
David Vilbert from Géosciences Rennes for the $^{87}$Sr/$^{86}$Sr isotopic ratio analysis of baryte samples and Thomas Rigaudier from CRPG for the sulfur isotopes analysis of baryte and pyrite samples presented in this study.

## Data availability

All the data used in this manuscript are inserted into the text and in the supplementary materials section.

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