# The origin of fluorite-barite mineralization at the interface between the Paris Basin and its Variscan basement: insights from fluid inclusion chemistry and isotopic (O, H, Cl) composition

Louise Lenoir, Thomas Blaise, Diana Chourio-Camacho, Antonin Richard, Alexandre Tarantola, Pierre Agrinier, Thomas Rigaudier, Gaël Monvoisin, Gérard Bardoux, Benjamin Brigaud, et al.

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Louise Lenoir, Thomas Blaise, Diana Chourio-Camacho, Antonin Richard, Alexandre Tarantola, et al. The origin of fluorite-barite mineralization at the interface between the Paris Basin and its Variscan basement: insights from fluid inclusion chemistry and isotopic (O, H, Cl) composition. Mineralium Deposita, 2024, 59 (2), pp.397-417. 10.1007/s00126-023-01219-2. hal-04483761

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Distributed under a Creative Commons Attribution 4.0 International License
1 The origin of fluorite-barite mineralization at the interface between the Paris Basin

2 and its Variscan basement: Insights from fluid inclusion chemistry and isotopic (O, H, Cl) composition

4 Mineralium Deposita

5 Authors:

6 Louise Lenoir¹* (0000-0003-0954-2714)

7 Thomas Blaise¹ (0000-0003-1820-8152)

8 Diana Chourio-Camacho² (0000-0001-7094-9041)

9 Antonin Richard³ (0000-0002-0702-4764)

10 Alexandre Tarantola³ (0000-0002-5145-2344)

11 Pierre Agrinier⁴ (0000-0002-5119-2423)

12 Thomas Rigaudier⁵ (0000-0003-4947-3784)

13 Gaël Monvoisin¹ (0000-0001-9947-342X)

14 Gérard Bardoux⁴ (0000-0002-7398-1415)

15 Benjamin Brigaud¹ (0000-0001-6961-2177)

16 Jocelyn Barbarand¹ (0000-0003-2166-9232)

17

18 ¹Université Paris-Saclay, CNRS, GEOPS, 91405, Orsay, France

19 ²Université PSL, MINES ParisTech, Centre de Géosciences, 77300, Fontainebleau, France

20 ³Université de Lorraine, CNRS, GeoRessources Laboratory, B.P. 70239, F-54506 Vandœuvre-lès-Nancy, France

21 ⁴Université Paris Cité, Institut de physique du globe de Paris, CNRS, 75005 Paris, France

22 ⁵Université de Lorraine, Centre de Recherche Pétrographiques et Géochimiques, UMR 7358 CNRS, France
24 Abstract

We provide new constraints for the fluid flow system at the origin of two F-Ba deposits located at the unconformity
between the south of the Paris Basin and the northern edge of the French Massif Central. We used microthermometry
and bulk crush-leach analyses to determine isotope ratios of mineralizing fluids (δ¹⁸O, δD, δ³⁷Cl), together with cation
and anion composition of fluid inclusions hosted by fluorite. Chlorinity and Cl/Br molar ratios (212-521) indicate the
involvement of a brine, whose origin likely corresponds to Triassic evaporated seawater compatible with supratidal
dolomitic facies preserved nearby. Microthermometry reveals high Ca/Na ratios, suggesting that the brine composition
evolved from hydrothermal alteration of the Variscan basement and partial dissolution and replacement of the host
sedimentary rocks. δ³⁷Cl values are lower than the expected value of evaporated seawater, suggesting Cl isotope
fractionation by ion filtration in clay-rich horizons. Fluorite crystallized at minimum temperatures of 70 to 110 °C, 10-
40 °C warmer than the host Triassic sedimentary rocks. Ascending brines were expelled during the Early Cretaceous
and experienced a drop in pressure and temperature, together with possible mixing with the SO₄-rich pore water of the
sedimentary rocks, causing precipitation of silica, followed by fluorite and barite, forming a stratabound deposit
similar to those found in many areas in Western Europe.

38

39 **Keywords:** F-Ba deposit, fluid inclusion, chlorine isotope, granite hydrothermal alteration, Paris Basin

40

41 Introduction

42 In Western Europe, the unconformity between the Variscan crystalline basement and the Mesozoic sedimentary cover
43 hosts a large diversity of deposits of economically attractive elements such as U, Pb, Zn, Cu, Au, Bi, F and Ba (Banks
44 et al. 2002; Sanchez et al. 2009; Haschke et al. 2021). Fluorite-barite mineralization may occur as veins in the
45 basement (Cardellach et al. 1990; Galindo et al. 1994; Munoz et al. 1999; Fourcade et al. 2002; Baatartsogt et al. 2007;
46 De Graaf et al. 2019; Walter et al. 2018) as well as replacing Mesozoic sedimentary rocks (Sizaret 2003; Boiron et al. 2010; Gigoux et al. 2016). Among these deposits, fluorite-barite mineralization is well developed on the southern border of the Paris Basin. World-class deposits hosted in Late Triassic rocks are known at the interface between the Paris Basin and the Variscan basement (Soulé de Lafont and Lhégu, 1980; Gigoux et al., 2016). Significant fluorite deposits are hosted as stratabound deposits in Late Triassic silicified sedimentary rocks lying unconformably over Paleozoic granitic rocks. A syn-sedimentary origin was first proposed (Boirat et al. 1980; Davaine 1980; Soulé de Lafont and Lhégu 1980). Nigon (1988) and Gigoux et al. (2016) documented minimal trapping temperatures of fluid inclusions in fluorite in the range 80-100 °C. Sm-Nd geochronology on fluorite (Gigoux et al. 2015) revealed that precipitation occurred during the Early Cretaceous period. Driving mechanisms for mineralization remain unclear, as well as the origin of heat, fluids and elements.

56 Gigoux et al. (2016) highlighted thermal disequilibrium between mineralizing fluids and the host sedimentary rocks of about 40 °C during deposition (Uriarte 1997). Similar hydrothermal activity has been documented in the central and eastern parts of the Paris Basin (Brigaud et al. 2020; Mangenot et al. 2017), recording fluid ascent along deeply rooted faults. Boiron et al. (2002), Cathelineau et al. (2012) and Gigoux et al. (2016) suggested that precipitation resulted from the mixing of diluted cool water with dense sedimentary brines percolating downwards through the crystalline basement. Mixing between ascending fluids and formation water has been documented in several F-Ba-Pb-Zn-Cu stratabound and vein deposits across Western Europe (e.g., Trinkler et al. 2005; Baatartsogt et al. 2007; Fusswinkel et al. 2013; De Graaf et al. 2019).

64 Different mechanisms have been proposed to explain the origin of mineralizing brines. Using Cl/Br and Na/K ratios in fluorite, quartz or dolomite fluid inclusions, De Graaf et al. (2019) showed evaporation of seawater (primary brine) in Central Germany, whereas Banks et al. (2000) and Cathelineau et al. (2012) proposed evaporite dissolution (secondary brine) in Colombia and Western France, respectively. In the Schwarzwald district, SW Germany, Walter et al. (2017) used Cl/Br ratios to show that Jurassic-Cretaceous veins formed by the mixing of primary and secondary brines displaying similar salinities. Interaction with basement rocks is also likely considered, meteoric water percolated to
depth of at least 10 km (Munoz et al. 1999; Gleeson et al. 2003; Diamond et al. 2018). In such conditions, mineral
hydration reactions can consume H₂O and thus increase salinity of the interacting fluid (Stober and Bucher, 2004;
Burisch et al. 2016). In addition, the low δD values of fluid inclusions in fluorite, in mineralization developed in the
northeastern and southeastern borders of the French Massif Central (FMC), were attributed to fluids of meteoric origin
(Munoz et al. 1999; Sizaret et al. 2004, 2009).

Cl/Br ratios and chlorine stable isotope compositions in fluid inclusions can efficiently discriminate fluid origins (e.g.,
Frape et al. 1984; Richard et al. 2011) but, such data is scarce in F-Ba stratabound and vein deposits from the borders
of the FMC. The few data comes from Boiron et al. (2002) in the northwestern part of the FMC, where fluorite fluid
inclusion high Cl content, together with Cl/Br ratios lower than the average seawater value, were interpreted as
resulting from seawater evaporation during the Late Triassic to Early Jurassic (Boiron et al. 2002). Because multiple
fluids can be involved in mineral deposition at the basement-cover unconformity (Walter et al. 2018), a detailed study
of fluid inclusion physical-chemical characteristics is essential to provide robust genetic models (Burisch et al. 2017;
Walter et al. 2018).

This study builds on the works of Gigoux et al. (2015, 2016) by characterizing the chemical and isotopic compositions
of fluorite inclusion fluids coming from two massive stratabound F-Ba deposits in the south of the Paris Basin. We
focus on the origin of dissolved chlorine through the measurements of major dissolved ions and δ¹⁸O, δD and δ³⁷Cl
values in fluid inclusions using bulk crush-leach methods. Three fluorite generations were documented at Pierre-
Perthuis (Gigoux et al. 2016; Lenoir et al. 2021). The first and last fluorite stages correspond to translucent to purple
fluorite and were not considered for this research because the crystals are too small, sub millimetric in size, and devoid
of fluid inclusions. Therefore, only the second fluorite stage dated at 130 ± 15 Ma by Sm-Nd geochronology is
considered (Gigoux et al. 2016; Lenoir et al. 2021). This stage is represented by large yellow-honey fluorite crystals,
well-developed in geodes and veins and thus allowing for sampling for analytical methods used in this study. Our main
objectives are to identify the origin of mineralizing fluids and to clarify the sources of F, Ba, Si, S and the mechanisms
of mass transfer and mineral precipitation, then compare the physical, chemical and isotopic characteristics of paleo-
fluids trapped in fluorite inclusions with present-day pore water in deep aquifers of the Paris Basin and Rhine graben above the Paleozoic basement. From this comparison, we further discuss fluid sources and mixing between the basement and deep sedimentary reservoirs.

97

## 98 Geological context

Mesozoic sedimentary rocks of the south-eastern part of the intracratonic Paris Basin overlie unconformably Paleozoic rocks outcropping in the Morvan Massif (Vialette 1973; Lardeaux et al. 2014) at the northern edge of the FMC. The massif is mainly composed of granitic, metamorphic and volcano-sedimentary units that formed during the Early and Late Carboniferous (Fig. 1, Vialette 1973; Lardeaux et al. 2014). The FMC is known to contain massive stratabound fluorite deposits, some of which are of economic grade (Fig. 1b): Egreuil (0.4 Mt CaF₂), Marigny-sur-Yonne (0.5 Mt CaF₂), Pontaubert (0.6 Mt CaF₂), Courcelles-Frémoy (1 Mt CaF₂), Pierre-Perthuis (1.4 Mt CaF₂) and Antully (1.6 Mt CaF₂, Soulé de Lafont and Lhégu, 1980). The two fluorite deposits studied are at the unconformity between the southern Paris Basin and the north-western part of the Morvan Massif (Fig. 1a). Both are hosted in the Carnian-Norian Assise de Chitry Formation, above the crystalline basement. This formation is heterogeneous, being karstified or brecciated and of variable thickness (Lefavrais- Raymond et al., 1965). The primary mineralogy of this formation is controversial as it is now largely silicified. It reveals ghosts of evaporite, algal and stromatolitic veils as well as desiccation cracks (Bois, 1978). It was originally calcareous, but was extensively dolomitized and later replaced by chalcedony, fluorite and barite (Gigoux et al. 2016) and hence hosts most of the fluorite and barite resources.

The Pierre-Perthuis deposit hosts the main fluorite resources in France, estimated at 1.4 million tonnes of CaF₂ (De Launay 1913; Soulé de Lafont and Lhégu 1980). Most of this resource is stratabound in the Assise de Chitry Formation. The Pierre-Perthuis fluorite deposit is near a regional N-S normal fault with an offset estimated at 100 m (Soulé de Lafont and Lhégu, 1980).
116 Situated 35 km southeast of Pierre-Perthuis near another N-S normal fault, the La Collancelle deposit consists of
117 massive fluorite and barite crystals associated with minor sulfides (mainly galena and chalcopyrite) in the Assise de
118 Chitry Formation above Paleozoic rhyolitic basement (Scolari 1966).

119

120

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121

122 **Fig. 1** a) Regional settings. b) Geological map of the Morvan massif with the location of the fluorite deposits from Pierre-Perthuis and La Collancelle (modified according the 1:1,000,000 geological map, French Geological Survey).

123

124 AM : Armorican Massif, BF : Black Forest, FA : French Alps, FMC : French Massif Central, PB : Paris Basin, RG :
125 Rhine Graben, V : Vosges Massif.

126
127 Material and methods

128 Sampling strategy and fluid inclusion petrography

129 Centimetric fluorite crystals from Pierre-Perthuis and La Collancelle were collected in outcrops. At Pierre-Perthuis,
130 centimetric fluorite crystals filling cavities were sampled in three areas: (1) Bois Dampierre, (2) Bois de l'Epenay and
131 (3) La Roche Percée (Fig. 2a), (Fig. 2c).

132 At La Collancelle, F-Ba mineralization forms large veins that were excavated during the construction of the Nivernais
133 canal (Fig. 2b). These fluorite crystals are similar to the ones from Pierre-Perthuis, with zoning marked by growth
134 stages of a milky aspect rich in fluid inclusions alternating with more translucent layers (Fig. 2d). The age of fluorite
135 precipitation at La Collancelle is unknown. All samples are presented in electronic supplementary material (ESM1,
136 Table S1).

137 Twenty polished thin sections (30 µm) were prepared and studied using optical and cathodoluminescence microscopy.
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Fig. 2 Location of the mineralized outcrops at (a) Pierre-Perthuis and (b) La Collancelle. (c) Geodic fluorite sample from Pierre-Perthuis (Gigoux 2016, sample PP16). (d) Fluorite-barite from La Collancelle. (e) Mineralized sample from Pierre-Perthuis. The silicified host rock (circled in black) contains clasts of the underlying altered granite (outlined in white). The circular inset is a microphotograph (reflected light) of Ccp and Gn crystals included in barite. Microphotographs in (f) plane polarized light and (g) cathodoluminescence microscopy illustrating two fluorite growth stages. $Fl_{euh}$ is the first euhedral fluorite generation showing sector zoning, not investigated in this study. $Fl_{geo}$ is the second fluorite generation, showing homogeneous and dull cathodoluminescence, corresponding to the geodic crystals illustrated in (c) and (e). Fl: fluorite; Ba: Barite; Qz: Quartz; Mal: Malachite; Ccp: Chalcopyrite; Gn: Galena

147

## 148 Fluid inclusion microthermometry

Microthermometry was carried out on 10 double-polished 200-µm-thick sections using a semi-automatic gas-flow heating-cooling Linkam MDS 600 stage attached to a Leica DM2500 optical microscope. The stage was calibrated using two synthetic standards (pure water FI with critical density, which homogenizes at 374 °C and an in-house three-phase H₂O-CO₂ FI with a melting temperature of CO₂ calibrated at -58.1 °C. Applied corrections are documented in ESM1 (Table S2). The composition of two-phase aqueous fluid inclusions (liquid and vapor) was approximated in the NaCl-CaCl₂-H₂O system because (1) three phase transitions were observed at low temperatures (Steele MacInnis et al. 2016) and (2) first ice melting temperature was measured from -54 to -46 °C (Yanatieva 1946). Total salinities (NaCl + CaCl₂) are expressed as 'Swt' in weight percent equivalent (wt.%) NaCl + CaCl₂, from the dissolution temperature of hydrohalite and the melting temperature of ice ($Tm_{hh}$ and $Tm_{ice}$) using the Excel® spreadsheet of Steele-MacInnis et al. (2011). The first melting of a solid phase was difficult to observe and is given with a precision of ±2 °C, estimated from repeated measurements in the largest FI (Te*). The dissolution temperature of hydrohalite ($Tm_{hh}$) and melting temperature of ice ($Tm_{ice}$) are reported with an accuracy of 1 and 0.1 °C respectively, and homogenization temperature (Th) is ±1 °C. Each fluid inclusion was measured twice. Data that could not be replicated were discarded. The few
fluid inclusions belonging to the same assemblage having variable liquid-vapor ratios were excluded and considered to have been affected by post-trapping deformation (stretching, leakage) or necking-down (Goldstein and Reynolds 1994). In small fluid inclusions (< 10 µm), the melting temperature of antarcticite, hydrohalite and ice were not measured (Chu et al. 2016). A total of 171 fluid inclusions were observed and measured, 70 from Pierre-Perthuis and 101 from La Collancelle.

## 167 Fluid inclusion oxygen and hydrogen stable isotope composition

Oxygen and hydrogen stable isotope compositions were measured at the CRPG (Université de Lorraine - CNRS, Vandoeuvre-lès-Nancy, France) on eight samples from Pierre-Perthuis and eight samples from La Collancelle. Individual fluorite zones were extracted from the host rocks and reduced into pieces of 1 to 5 mm. Mineral impurities were removed manually under the binocular microscope. Each sample consists of 5 to 10 g of fluorite crystals. Samples were placed in stainless steel tubes and degassed overnight to remove adsorbed water at the crystal surfaces. Water trapped in fluid inclusions was extracted by mechanical crushing and vaporized by heating at a temperature of 250 °C. The liberated H₂O was collected in a liquid nitrogen cold trap at -170 °C and then separated from non-condensable gases. Water (70-285 µmol) was transferred to a micro-equilibration reactor into which 12 to 75 µmol CO₂ of known isotopic composition was added (ESM1, Table S3). Samples were placed at 25 °C for at least 24 h for a complete oxygen isotope equilibration between H₂O and CO₂. After this time, H₂O and CO₂ were separated cryogenically. The oxygen isotopic composition of equilibrated CO₂ was measured using a dual-inlet ThermoFisher MAT 253 mass spectrometer. The δ¹⁸O of water was calculated using the mass balance equation of Kishima and Sakai (1980). After separation from CO₂ the water was reduced to H₂ using a uranium furnace heated to 800 °C for four Pierre-Perthuis samples. For the rest of the samples, the water was collected into a silica tube containing about 400 mg of fine-grained chromium and reduced into H₂ by Cr at 1,000 °C for 6 min. For all samples, H₂ was then transferred and quantified into a calibrated volume using a Toepler pump. All D/H ratios were measured using a dual-inlet GV
185 Isoprime mass spectrometer. Values are reported in per mil relative to the Vienna Standard Mean Ocean Water (V-SMOW) for oxygen and hydrogen. External reproducibility of the internal reference material was ± 2.0 ‰ and ± 0.5 ‰ for δD and δ¹⁸O measurements respectively.

188

189 **Major dissolved ions in fluid inclusions**

190 Fluid inclusions were analyzed for their elemental composition using a bulk crush-leach method by mechanical crushing (Banks et al. 2000; Gleeson et al. 2003). For each sample, 1.5 to 50 g of fluorite crystals were separated from the host rocks. Mineral impurities were removed under the binocular tube. Crystals were washed twice using MilliQ water in an ultrasonic bath and then twice in boiling water for 10 min. Samples were then crushed in an agate mortar and inclusion fluids diluted by adding 10 to 12 mL of MilliQ water. After 12 hours of decantation, the electrolytic solutions were collected with single-use plastic syringes with a 0.2 µm cellulose acetate membrane filter and separated into two aliquots for cation and anion analyses. Some solutions were generated by merging several crush-leach solutions of about 15 g of fluorite crystals from the same sample to increase ion concentration (ESM1, Tables S4 and S5). Thirty-five samples were prepared from Pierre-Perthuis and thirty-two from La Collancelle (ESM1, Table S6).

199 The cation composition (Na, K) of fourteen samples from Pierre-Perthuis was measured using an atomic absorption spectrometer – AAS (240 FS Varian) in fast sequential mode at Géosciences Paris-Saclay (GEOPS). Thirteen other samples from Pierre-Perthuis and thirty-two from La Collancelle were analyzed using ion chromatography using a Dionex Aquion ion chromatography system with a Dionex IonPac CS-16 column and IonPac CG-16 guard column (Thermo Fischer Scientific, Waltham, USA) at GEOPS. These aliquots were acidified with 0.2 mL of ultrapure HNO₃ (AnalaR NORMAPUR®, down to pH=2) triply distilled using the DST 1000 Savillex system to minimize adsorption and reprecipitation of calcium (Gleeson et al. 2003, Köhler et al. 2009). The second aliquot from these samples were used to quantify the anions (Cl, Br) by ion chromatography using an ICS-1000 Dionex with a Dionex IonPac AS-14 column and IonPac AG-14 guard column at GEOPS.
208 Thirteen electrolytic solutions generated from Pierre-Perthuis samples and eighteen from La Collancelle samples were
209 analyzed at the BRGM by ion chromatography using a Dionex Aquion system (Thermo Fischer Scientific, Waltham,
210 USA) for chlorine concentration and by ICP-MS for bromine and lithium concentrations (Table S6).

211 Uncertainties on the anion and cation concentrations were determined by repeated measurements of standard solutions
212 and are given with a 95% confidence interval. Concentrations of fluid inclusions have been recalculated by applying a
213 correction factor obtained by comparing the chlorine concentration obtained by crush-leach analyses and the mean of
214 those calculated by microthermometry. These correction factors have been applied to all elements measured by crush-
215 leach solutions (Banks et al. 2000). The major cation and anion content in the fluid inclusion leachates is expressed in
216 ppm of solution and is discussed in the text in terms of molar ratios.

217

218 **Fluid inclusion chlorine stable isotope composition**

219 For each sample, 90 to 120 g of fluorite crystals were extracted and prepared following the dissolved ion crush-leach.
220 Five samples were analyzed from Pierre-Perthuis and three from La Collancelle. Unfractionated chloromethane of
221 sufficient purity for isotope measurements was produced in three steps: (1) precipitation of silver chloride by addition
222 of silver nitrate (AgNO3), (2) reaction of silver chloride with iodomethane (CH3I) and (3) separation by gas
223 chromatography. δ37Cl was then measured on a dual-inlet gas source mass-spectrometer Finnigan Delta plus XP
224 (Thermo Fisher Scientific) at the Institut de physique du globe de Paris (IPGP, Université Paris Cité) on gaseous
225 CH3Cl. The overall analytical procedure follows the methods developed by Kaufmann et al. (1984), detailed by
226 Eggenkamp (1994). Data is reported in per mil variations relative to Standard Mean Ocean Chlorine (SMOC, Godon et
227 al. 2004). The external reproducibility for the internal seawater standard was ±0.031 ‰ (1σ, n=4) for the first session
228 and ±0.005 ‰ (1σ, n=3) for the second (ESM1).

229
230 **Results**

231 **Data significance**

232 The significance of the fluid inclusions leachate analyses is examined by considering the (1) origin of fluid inclusions,
233 (2) potential contamination of the solutions and (3) analytical bias.

234 (1) Origin of fluid inclusions

235 Crystals selected for this research are homogenous under cathodoluminescence microscopy (Fig. 2f). Most of the fluid
236 inclusion assemblages were interpreted as primary or pseudo-secondary (Goldstein and Reynolds 1994). However, the
237 occurrence of secondary inclusions in our samples could not be excluded, since a significant proportion of fluid
238 inclusions could not be unambiguously attributed to well-defined fluid inclusion assemblage. Some liquid-only
239 aqueous inclusions were observed (Fig. 3c), interpreted here as in a metastable state (Roedder 1984; Diamond 2003).
240 The proportion of these single-phased fluid inclusions was estimated to be less than 5% of the total fluid inclusions
241 content. These single-phased fluid inclusions were assumed to have the same composition as the primary fluid
242 inclusions.

243 Finally, a large portion of fluid inclusions appeared to be opened and leaked in both deposits, not affecting the leachate
244 composition.

245 (2) Potential contamination

246 A series of tests was conducted to evaluate potential sources of contamination of the solutions generated by the bulk
247 crush-leach of fluorite crystals (Fig. 12 in Appendix, Table S5 in ESM1). Elemental ratio values (Cl/Br, Na/Br, Na/K)
248 are comparable from one leachate to another. Based on these results and the repeated analysis of blank solutions during
249 the crush-leach analysis (ESM1, Table S4), we conclude that there is minimal contamination in Cl, Br, Na and K in our
250 solutions.

251 (3) Analytical bias

252 Because Br content was less than 1 mg/L in almost all crush-leach solutions, two quantification methods were
253 employed: ion chromatography (Br detection limit =50 µg/L) and ICP-MS (Br detection limit =10 µg/L), yielding
comparable values (ESM1 Tables S6 and S7). Hence, we conclude that there is no significant analytical bias, and that
the Cl/Br and Na/Br ratios accurately represent those of the fluorite-mineralizing fluids.

256

257 Fluid inclusion petrography and microthermometry

Fluid inclusions were classified depending on geometric criteria relative to the host mineral as 1) isolated inclusions assumed primary in origin, 2) inclusions aligned or distributed in clusters along planes parallel to the mineral growth zones, also assumed primary in origin, 3) pseudo-secondary fluid inclusion assemblages that are not along planes parallel to the mineral growth zones, and that do not crosscut the growth zones of the mineral or 4) secondary fluid inclusion planes crosscutting mineral growth zones.

Complex fluid inclusion assemblages coexist in both deposits (Fig. 3a). In all samples, fluid inclusions are aqueous and contain mostly two phases (H₂O liquid and vapor). Most of the very large fluid inclusions (>50 µm) decrepitated and form clusters located along growth bands or cleavages (Fig. 3a, b).

The fluid inclusions in fluorite of Pierre-Perthuis are of two-types, either isolated and large (>20 µm; Fig. 3.d) or small (<20 µm) and in growth zones (Fig. 3f). In fluorite samples from La Collancelle, fluid inclusions appear as (1) primary, isolated, elongated and large (10 µm < FI size <40 µm) (Fig. 3e), (2) aligned along planes that do not intersect the mineral growth zones (Fig. 3f) and (3) randomly distributed in clusters according to the growth planes (>40 µm) (Fig. 3a, g).

Rare and small monophasic liquid inclusions (<10 µm) are observed either as metastable fluid inclusion in a plane of two-phase aqueous fluid inclusions or aligned along planes at the growth zones of crystals. The latter are considered negligible in comparison to the total volume of fluid inclusions (estimated <5%, Fig. 3h).

Some two-phase fluid inclusions (isolated or aligned along planes) cannot be assigned to a well-defined fluid inclusion assemblage. If the liquid/vapor ratio is the same as in primary fluid inclusions, these two-phase fluid inclusions were assumed to be of primary origin.
Evidence of post-trapping deformation and necking-down were observed in some primary fluid inclusion assemblages, showing variable L/V ratios. These fluid inclusions with inconsistent L/V ratios represent less than 5% of the inclusions in each fluid inclusion assemblage. These fluid inclusions were excluded for microthermometric measurements. Secondary fluid inclusion planes crosscutting mineral growth zones were not observed.

281

282
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283

284 **Fig. 3** Fluorite fluid inclusion petrography. (a) Schematic presentation of typical fluid inclusion assemblages in fluorite

285 from La Collancelle. (b) Clusters of large opened and leaked inclusions along sub-parallel fractures in a fluorite crystal

286 from Pierre-Perthuis. (c) Complex fluid inclusion assemblage in fluorite samples example from Pierre-Perthuis. (d) and

287 (e) Isolated fluid inclusion interpreted as primary from Pierre-Perthuis. Blue dashed line indicates a pseudo-secondary
plane. (f) Fluid inclusion assemblage of liquid-vapor aqueous inclusions aligned along a growth zone represented by the green dashed line from La Collancelle. (g) Cluster of fluid inclusions from La Collancelle, that includes leaked, one-phase and two-phases inclusions. (h) Fluid inclusion plane containing single-phased aqueous-liquid inclusions from Pierre-Perthuis

Fluorite crystals in both deposits contain abundant fluid inclusions that are almost devoid of solid inclusions, except for some rare and small co-precipitated hematite that does not significantly contribute to the bulk fluid characterization. Homogenization temperatures (Th) vary from 80 to 245 °C with 85% of the measurements between 80 and 110 °C at Pierre-Perthuis and from 67 to 231 °C with more than 75% of the values between 80 and 120 °C at La Collancelle (Fig. 4a). The primary and pseudo-secondary fluid inclusions in each deposit yield the same range of Th values.

In both fluorite deposits, the first apparent melting (Te*) occurred at -48 °C ± 5 (2 σ) consistent with the eutectic of the H₂O-NaCl-CaCl₂ system (Yanatieva 1946, Steele McInnis et al. 2011). Hydrohalite systematically dissolved before ice. Hydrohalite final dissolution temperatures range from -48 to -25 °C and from -36 to -21 °C at Pierre-Perthuis and La Collancelle, respectively. Ice melting temperatures were between -14 and -1 °C at Pierre-Perthuis and from -16 to 0 °C at La Collancelle. Total salinity (NaCl + CaCl₂) thus varies from 13 to 21 wt% in Pierre-Perthuis and from 0 to 19 wt% in La Collancelle. Fluid inclusions from Pierre-Perthuis have on average a higher total salinity than those of La Collancelle (14 vs. 11 wt%) and are more calcic (with average of 12.8 and 6.8 wt% CaCl₂= at Pierre-Perthuis and La Collancelle, respectively). Na/Ca ratios are highly variable, with a median value of 0.2 at Pierre-Perthuis (n=67) and 0.7 at La Collancelle (n=24). Microthermometry data are summarized in Table 1 and reported in ESM1, Table S2. The composition of fluid inclusion is plotted in a NaCl-CaCl₂-H₂O ternary diagram in Fig. 4b.
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316
317</figcaption></figure>

<caption>**Fig. 4** (a) Histogram of homogenization temperature (Th) according to the type of fluid inclusions, (b) Fluid inclusion diagram showing the composition in the H₂O-NaCl-CaCl₂ system, (c) homogenization temperature (Th) as a function of ice melting temperature (Tmice), and (d) Na/Ca vs. Th.</caption>
<table><thead><tr><th></th><th></th><th>Th</th><th>Tmice</th><th>Tmhh</th><th>Te*</th><th>Swt</th><th>wt% NaCl</th><th>wt% CaCl<sub>2</sub></th></tr></thead><tbody><tr><td rowspan="3">fluorite deposit</td><td rowspan="3">type of fluid inclusion</td><td>min max med n min</td><td>max med n min</td><td>max med n min</td><td>med n min</td><td>med n min</td><td>med n min</td><td>med n min</td></tr><tr><td>primary</td><td>+84+181 +97 # -19.3 -12.0 -14.4 9 -37.9 -34.4 -37.3 6 -50 -45 -48 4 16 21 17 6 1,4 3,7 2 6 ## 16,8 12,9 6</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>pseudo-secondary</td><td>+85+136+100 # -15.4 -14.1 -14.4 4 -44.6 -24.7 -32.1 3 - - - 0 18 18 18 3 1,4 10,7 6 3 7 16 11.5 3</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Pierre-Perthuis</td><td rowspan="3">All fluid inclusion</td><td></td><td>+84+181 +97 # -19.3 -12.0 -14.4 # -44.6 -24.7 -36.7 9 -50 -45 -48 4 16 21 18 9 1,4 10,7 2 9 7 16.8 14.6 9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>primary</td><td>+74+231 +99 # -17.4 0 -3.9 # -36.0 -20.9 -26.8 # -50 -43 -48 # 0 19 9,6 9 0 10,7 3 9 0 16 5.4 9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>pseudo-secondary</td><td>+67+223+101 # -16.4 -1.1 -6.5 # -34.3 -20.9 -28.6 # -54 -43 -49 # 2 18 10 # 0,9 7,4 4 # 1 14 6,4 15</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">La Collancell e</td><td rowspan="3">All fluid inclusion</td><td></td><td></td><td>+67+231+101 # -17.4 0 -6.2 # -36.0 -20.9 -28.1 # -54 -43 -48 # 0 19 10 # 0 10,7 4 # 0 16 6,2 24</td><td></td><td></td><td></td><td></td></tr><tr><td>primary</td><td>+74+231 +99 # -17.4 0 -3.9 # -36.0 -20.9 -26.8 # -50 -43 -48 # 0 19 9,6 9 0 10,7 3 9 0 16 5.4 9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>pseudo-secondary</td><td>+67+223+101 # -16.4 -1.1 -6.5 # -34.3 -20.9 -28.6 # -54 -43 -49 # 2 18 10 # 0,9 7,4 4 # 1 14 6,4 15</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
318

319 **Table 1** Summary of microthermometric data for fluid inclusions from Pierre-Perthuis and La Collancelle, according
to the type of fluid inclusions

321

322 **Oxygen, hydrogen and chlorine stable isotope composition of inclusion fluids**

323 The δ¹⁸O of inclusion water at Pierre-Perthuis varies from –7.8 to –5.0 ‰ whereas δD ranges from –51.2 to –37.0 ‰.

324 La Collancelle fluorite samples yield δ¹⁸O values between –7.8 and –4.2 ‰, while the δD values are between –40.1 and –23.2 ‰ (Table 2).

<table><thead><tr><th>Locality</th><th>Sample</th><th></th><th>δ<sup>18</sup>O<br>(‰v-SMOW)</th><th>δD<br>(‰v-SMOW)</th><th>δ<sup>37</sup>Cl<br>(‰SMOC)</th></tr></thead><tbody><tr><td rowspan="10">Pierre-Perthuis</td><td rowspan="4">PP16</td><td>min</td><td>-7.8</td><td>-51.2</td><td>-2.19</td></tr><tr><td>max</td><td>-5.0</td><td>-37.0</td><td>-1.88</td></tr><tr><td>med</td><td>-5.9</td><td>-40.5</td><td>-2.04</td></tr><tr><td>n</td><td>7</td><td>7</td><td>2</td></tr><tr><td>PP19-3</td><td>n=1</td><td>-7.9</td><td>-51.2</td><td>-</td></tr><tr><td>PP14</td><td>n=1</td><td>-</td><td>-</td><td>-1.76</td></tr><tr><td rowspan="4">PPFI</td><td>min</td><td>-</td><td>-</td><td>-2.10</td></tr><tr><td>max</td><td>-</td><td>-</td><td>-2.07</td></tr><tr><td>med</td><td>-</td><td>-</td><td>-2.09</td></tr><tr><td>n</td><td>-</td><td>-</td><td>2</td></tr><tr><td rowspan="4">La Collancelle</td><td rowspan="4">COL19</td><td>min</td><td>-7.8</td><td>-38.9</td><td>-1.95</td></tr><tr><td>max</td><td>-4.2</td><td>-27.1</td><td>-1.31</td></tr><tr><td>med</td><td>-5.8</td><td>-33.8</td><td>-1.92</td></tr><tr><td>n</td><td>8</td><td>8</td><td>3</td></tr></tbody></table>

326 **Table 2** Oxygen, hydrogen and chlorine isotopic compositions of fluid inclusions in fluorite from Pierre-Perthuis and
La Collancelle deposits

328
329 $\delta^{37}$Cl values of Pierre-Perthuis fluorite samples vary from -2.19 to -1.76 ‰ with an average of -2.00 ± 0.17 ‰ (Table 2). La Collancelle samples vary from -1.95 to -1.31 ‰ with an average of -1.73 ± 0.36 ‰ (Table 2). Both are significantly depleted in $^{37}$Cl relative to seawater ($\delta^{37}$Cl=0 ‰) and chlorides from evaporite minerals (-1< $\delta^{37}$Cl ‰ < 0.6; Eggenkamp 1994).

333

334 Ion composition

335 Pierre-Perthuis and La Collancelle fluid inclusions have average Cl concentrations estimated from microthermometry
336 of 100,000 ppm (66 values) and 69,900 ppm (24 values), respectively (ESM1, Table S7). The concentrations obtained
337 from crush-leach solutions and the calculated concentrations from the average salinity obtained from
338 microthermometric measurements are reported in ESM1 (Table S7). The molar Cl/Br ratio varies from 216 to 406 at
339 Pierre-Perthuis and from 260 to 521 at La Collancelle.

340
341

342 **Discussion**

343 **Origin and physical-chemical characteristics of mineralizing brines**

344 **Temperatures and salinities of fluids**

345 Nigon (1988) documented a minimum trapping temperature for fluorite deposits from Marigny-sur-Yonne (about 20 km from Pierre-Perthuis and 10 km from La Collancelle, Fig. 1b) between 75 and 125 °C. Here, minimal trapping temperatures for both fluorite deposits range from 85 to 115 °C, in agreement with Gigoux et al. (2016). This temperature range is consistent with the stability limit of the yellow coloration in fluorite (around 250 °C, Trinkler et al. 2005). Th higher than 115 °C are mostly measured in the largest inclusions, suggesting post-trapping deformation (Fig. 11). Variations in Th and salinity measured in primary fluid inclusions have commonly been observed in similar contexts (Richard et al. 2016; Walter et al. 2017; Scheffer et al. 2019) and may have been caused by natural fluctuations in the fluid temperature, the formation depth or variable fluid mixing ratios.

353 The geodic fluorite of Pierre-Perthuis was dated at 130 ± 15 Ma by Sm-Nd geochronology (Gigoux et al. 2015).
354 Barbarand et al. (2013) estimated a maximum thickness of sedimentary cover during the Early Cretaceous period of
355 about 1.3 kilometers in the northwestern part of the Morvan Massif and therefore a maximum burial temperature of
356 around 70 °C for Triassic rocks using a Cretaceous geothermal gradient of 40 °C/km and a mean surface temperature
357 of 25 °C. This estimation is consistent with the host rocks temperatures of about 60-65 °C during the Early Cretaceous
358 determined from the maturity of sedimentary organic matter (Uriarte 1997). Consequently, the mineralizing fluids
359 were at least 10 to 40 °C warmer than the host sedimentary rocks during fluorite crystallization, considering the
360 minimal trapping temperatures obtained from FI microthermometry from 85 to 115 °C. Assuming a hydrostatic
361 pressure gradient and a trapping pressure of fluid inclusions approximately at 100 bars, a correction of about + 5°C
362 should be applied to obtain the trapping temperatures (Fig. 13).

363 The total salinity of fluid inclusions in La Collancelle displays variations from 0 to 19 wt% that could be interpreted as
364 a result of a mixing between two fluids: a CaCl2-rich brine diluted by seawater and/or meteoric water. At Pierre-
Perthuis, the total salinity is more homogeneous, indicating either a single fluid source or fluids having been well-mixed before fluorite precipitation.

Fluid temperature was calculated using various Na/K and Na/Li geothermometers (Table 5) proposed in literature (Truesdell 1976; Fournier 1979; Tonani 1980; Arnórsson et al. 1983; Kharaka et al. 1982; Nieva and Nieva 1987; Giggenbach 1988; Verma and Santoyo 1997). These chemical geothermometers return temperatures from 150 to 250 °C, about a hundred degrees higher than the minimum trapping temperature of fluid inclusions given by the homogenization temperature. The Na/K ratios in geothermal fluids are essentially controlled by reactions with K-feldspar and albite (Arnórsson et al. 1983). At Pierre-Perthuis and La Collancelle, the feldspar reservoir is the underlying granitic basement. Consequently, the equilibrium temperatures provided by Na/K geothermometry tend to show the involvement of a basement brine that reached chemical equilibrium at a depth of about 4 to 8 km, considering a geothermal gradient of about 30 °C/km. The ascent of these solutions towards the basement-sedimentary cover unconformity would be accompanied by a temperature drop of about 100 °C. Such a difference in temperature from fluid inclusion microthermometry and the Na/K geothermometer was previously reported in fluorite veins from the Montroc and Le Burc deposits, south of the French Massif Central (Deloule 1982). In present-day deep aquifers of the Rhine graben, Na/K and Na/K/Ca geothermometers yield temperatures about 100 °C higher than measured bottom hole temperature (Pauwels et al. 1993; Aquilina et al. 1997; Sanjuan et al. 2016).

However, the temperatures provided by these chemical geothermometers may be overestimated in the case of evaporated seawater that reached halite saturation. In such a case, Na will be incorporated in halite during the evaporation stage, which will lower the Na/K and Na/Li ratios leading to overestimation of temperature.

<table><thead><tr><th>Localities</th><th>Values</th><th>Fournier (1979)</th><th>Truesdell (1976)</th><th>Giggenbach (1988)</th><th>Tonani (1980)</th><th>Nieva and Nieva (1987)</th><th>Amorsson (1983)</th><th>Verma and Santoyo (1997)</th><th>Verma and Santoyo (1997)</th><th>Kharaka et al. (1982)</th></tr></thead><tbody><tr><td>Geothermometers</td><td></td><td></td><td></td><td></td><td>Na/K</td><td></td><td></td><td></td><td>Na/Li</td><td></td></tr></tbody></table>
<table><caption>**Table 5** Equilibrium temperatures from various geothermometers</caption><tbody><tr><td rowspan="4">Pierre-Perthuis</td><td>mean</td><td>197</td><td>166</td><td>213</td><td>196</td><td>185</td><td>174</td><td>162</td><td>306</td><td>282</td></tr><tr><td>min</td><td>170</td><td>131</td><td>179</td><td>159</td><td>158</td><td>141</td><td>141</td><td>280</td><td>198</td></tr><tr><td>max</td><td>230</td><td>205</td><td>244</td><td>242</td><td>217</td><td>211</td><td>190</td><td>328</td><td>368</td></tr><tr><td>n</td><td>31</td><td>31</td><td>31</td><td>31</td><td>31</td><td>31</td><td>31</td><td>13</td><td>16</td></tr><tr><td rowspan="4">La Collancele</td><td>mean</td><td>187</td><td>151</td><td>204</td><td>183</td><td>174</td><td>160</td><td>151</td><td>337</td><td>207</td></tr><tr><td>min</td><td>159</td><td>118</td><td>178</td><td>145</td><td>147</td><td>128</td><td>133</td><td>322</td><td>200</td></tr><tr><td>max</td><td>335</td><td>349</td><td>340</td><td>407</td><td>319</td><td>344</td><td>194</td><td>360</td><td>219</td></tr><tr><td>n</td><td>27</td><td>27</td><td>27</td><td>27</td><td>27</td><td>27</td><td>27</td><td>14</td><td>14</td></tr></tbody></table>

387

388 **Focus on the source of fluids**

389 Yellow fluorite crystals were investigated by Bill and Calas (1978) and Bill (1982), who showed thermally unstable $O_3^-$ molecular ions, which substituted for two nearest-neighbor $F^-$ ions. Since no significant difference in the oxygen and hydrogen isotopic composition from different fluorite samples is documented in this study, we assume that oxygen substitution in fluorite should not bias the $\delta^{18}O$ values, which are hereafter interpreted as the oxygen isotope composition of the fluid from which it precipitated. 390 391 392 393

394 $\delta^{18}O$ and $\delta D$ values of fluid inclusions in fluorite plot close to the global meteoric water line (GMWL) (Fig. 5, Craig 1961). It is worth noting that some values plot above the GMWL suggesting isotopic shifts related to low-temperature fluid-rock interaction at depth in the basement, similar to those reported in the present-day groundwater in the FMC granite (Kloppmann et al. 2002). Values shifted to the right of the GMWL record the interaction with host carbonate rocks. 395 396 397

399 Low $\delta D$ in fluid inclusions were also measured in some fluorite deposits at the northeastern and southeastern boundaries of the FMC (Sizaret et al. 2009; Munoz et al. 1999, respectively), and were interpreted to contain a main contribution of meteoric waters, despite high salinities. In Pierre-Perthuis, considering the Early Cretaceous crystallization age (Gigoux et al. 2015), the paleo-latitude of the southern Paris Basin was about 30° (Thierry and Barrier 2000) and the paleo-altitude was probably low. In such a paleo-geographical situation, the Early Cretaceous meteoric water should have been much less depleted in $^{18}O$ and D compared to the isotopic composition of our fluid 400 401 402 403 404
inclusions. Furthermore, the measured isotopic composition is at odds with the high salinity of the mineralizing fluid (Fig. 4) that suggests evolved seawater, unless we consider either (1) an increase in salinity acquired solely by fluid-rock interactions in the granitic basement (Stober and Bucher 2004; Burisch et al. 2016) or (2) the dissolution of evaporite.

In samples from Pierre-Perthuis, the δ¹⁸O and δD values are in the range of the present-day meteoric and shallow ground waters (Delbart 2014). Thus, a plausible explanation would be to consider the isotopic values of fluid inclusions as reflecting the presence of present-day meteoric water in fluorite. It is known that water molecules are easily adsorbed on the surface of fluorite crystals (Foucaud et al. 2018). However, adsorbed water was removed during analysis. Since fluorite crystals were not washed before mechanical crushing, we also rule out any contamination during sample preparation. It is possible that the cavities generated by the leakage of fluid inclusions in fluorite crystals (Fig. 3b, c, g) were partially filled by recent meteoric waters and may have resisted desorption as the result of the high tortuosity of the porous network. The fractures may have formed from freeze-thaw in outcrop and may have been enhanced by the presence of clusters of large fluid inclusions constituting weak areas within the crystals.

At this stage, the O and H stable isotope composition cannot be reliably used to determine the origin of mineralizing fluids. The compositions may correspond to a mixing between Early Cretaceous meteoric water and recent meteoric water. The microthermometric data from fluid inclusions from La Collancelle shows some low salinity primary fluid inclusions, and fluctuations of the total salinity in several fluid inclusions belonging to the same fluid inclusion assemblage, suggesting mixing of a saline solution with diluted waters, likely meteoric in origin. Such low-salinity fluid inclusions were, however, not found in fluorite from Pierre-Perthuis.

Finally, it is worth mentioning that all fluid inclusions in barite crystals from both Pierre-Perthuis and La Collancelle have leaked. Hence, no measurements of their oxygen and hydrogen stable isotope composition were conducted. In our
428 case, the entire replacement of primary fluid inclusions by more recent meteoric water in barite crystals is likely, as
429 suspected by De Graff et al. (2019) in a similar F-Ba deposit in the Harz mountains in Germany.

430

<figure><img src="image_20.png" /></figure>

431

432 **Fig. 5** Hydrogen and oxygen isotope composition of water from fluid inclusions in fluorite crystals and from contemporaneous waters in the FMC basement and in Triassic aquifers from the Paris Basin and the Rhine graben.

434 Seawater evaporation trend is from Pierre (1982) and Global Meteoric Water Line is from Craig (1961)

435
436 Calcic-brine origin and sources of Si, F and Ba

437 Over the last few years, CaCl₂-rich brines have been detected in several fluid inclusion's studies carried on
438 unconformity-related deposits (Lüders and Möller 1992; Banks et al. 2002; Derome et al. 2005; Gleeson and Turner
439 2007; Piqué et al. 2008; Richard et al. 2016; Walter et al. 2017). Moreover, CaCl₂-rich fluid inclusions in fluorite are
440 common (Scheffer et al. 2019). Low Na/Ca saline solutions can be generated through several processes, including (1)
441 dissolution of gypsum or anhydrite, (2) dissolution of calcite and dolomite or dolomitization of calcite, (3) evaporation
442 of Cretaceous-like "CaCl₂ seas", (4) Na-Ca exchange reactions in silicate minerals.

443

444 (1) Dissolution of gypsum or anhydrite

445 The sulfate content in our fluorite fluid inclusions leachates is low (about a factor of 100 lower than Cl content).

446 Additionally, no sulfate accumulations are known in the sedimentary rocks from the southern edge of the Paris Basin.

447 We therefore excluded the dissolution of gypsum or anhydrite.

448

449 (2) Dissolution of calcite and dolomite or dolomitization of calcite

450 The first generation of fluorite in the Pierre-Perthuis deposit (Fl<sub>euh</sub>, Fig. 2f) formed from the dolomitization and
451 dissolution of the host carbonate rocks whereas the second generation, which is studied here, crystallized in cavities
452 (Fig. 2e). Although this second fluorite generation is devoid of carbonate inclusions, it is likely that that Ca is sourced
453 from dolomitization of the host carbonate rocks.

454

455 (3) Evaporation of Cretaceous-like "CaCl₂ seas"

456 Evaporation of Cretaceous-like seawater ("CaCl₂ seas") can generate brines with low Na/Ca. (Kovalevych et al. 2006;
457 Lowenstein et al. 2001; Lowenstein and Timofeeff 2008). In the Paris Basin, the chalk deposits are of Late Cretaceous
458 ages, i.e., post-dating the crystallization of fluorite at Pierre-Perthuis (Gigoux et al. 2015). Hence, the evaporation of
459 Cretaceous seawater is an unlikely source of Ca.
460

461 (4) Na-Ca exchange reactions in silicate minerals

462 Albitization concomitant with the replacement of biotite by chlorite is widespread in granitic rocks of the FMC (Carrat

463 1969; Parneix et al. 1985; Cathelineau et al. 1986). In a Cl/Br vs. Na/Br plot, fluid compositions are aligned along the

464 seawater evaporation trend (Fig. 6). If Ca was mainly sourced from albitization, the Na/Br ratios of the fluids should be

465 shifted to lower values, i.e., towards the left of the seawater evaporation trend. Since such a shift is not observed in our

466 data, we conclude that Ca is not predominantly sourced from albitization processes.

467
<figure><img src="image_21.png" /><figcaption>468
469 **Fig. 6** Cl/Br molar ratio as a function of Na/Br molar ratio (crush-leach solutions) from fluid inclusions in fluorite from Pierre-Perthuis and La Collancelle deposits</figcaption></figure>

471
472 **Sources of dissolved chlorine**

473 Cl/Br ratios measured in fluid inclusions are inconsistent with a brine derived solely from the dissolution of evaporite (Cl/Br molar ratios > 1000, Banks et al. 2000). They are also lower than the seawater value (Cl/Br<sub>seawater</sub> (molar) = 650, Fontes and Matray 1993; Fig. 7) but are consistent with seawater evaporation beyond halite saturation. As shown in a
Na/Br vs. Cl/Br plot (Fig. 6) seawater evaporation appears to be more advanced at Pierre-Perthuis than at La Collancelle. Chlorine concentration in fluid inclusions is lower than seawater undergoing halite precipitation, which indicates dilution by a low salinity fluid (Fig. 7; Scheffer et al. 2019). Our fluorite fluid inclusion compositions are comparable with those documented by Boiron et al. (2002) in the Poitou high at the western edge of the FMC (Figs. 7 and 8). They are also similar to the composition of present-day brines in the Keuper (Carnian-Norian) aquifer of the Paris Basin (Millot et al. 2011).

$\delta^{37}\text{Cl}$ values are lower than the expected value of seawater ($\delta^{37}\text{Cl} \approx 0 \text{ ‰}$, Eggenkamp et al. 2019b, Fig. 8). Evaporite chlorides that precipitate by evaporation of seawater have $\delta^{37}\text{Cl}$ values mostly above –0.5 ‰ (Eggenkamp et al. 2019).

Therefore, additional chlorine isotope fractionation processes must have occurred to produce the $^{37}\text{Cl}$-depletion in chlorides. This can be neither be through sequestration of $^{37}\text{Cl}$ in minerals because clay and other low-temperature minerals contain small amounts of Cl, nor through advection which is known not to fractionate $\text{Cl}^{-}$ isotopes (Bernachot et al. 2017). On the other hand, slow ion transport processes, such as diffusion and ion filtration, are capable of fractionating $\text{Cl}^{-}$ isotopes (Desaulniers et al. 1986; Eggenkamp 1994, 2014; Agrinier et al. 2021; Strydom et al. 2022).

However, diffusion was shown to generate $\delta^{37}\text{Cl}$ change larger than 1 ‰ only for downstream concentrations lower than 10 mM, unrealistic conditions in the aquifers of the Paris Basin, where $[\text{Cl}^{-}]$ is generally higher than 10 mM. Ion filtration could generate $^{37}\text{Cl}$-depletion of pore fluid forced through clay membranes as proposed by Agrinier et al. (2019, 2021). This has been suggested as an explanation for the strong $^{37}\text{Cl}$-depletion in clay-rich young sediment pore fluids. It is possible that Paris Basin evaporated seawater was fractionated by ion filtration across clay-rich horizons.

Finally, it is worth noting that present-day deep fluids in Triassic aquifers from the central and eastern Paris Basin are similarly depleted in $^{37}\text{Cl}$ (Fig. 8).

496
<figure><img src="image_22.png" /><figcaption>Fig. 7 Cl/Br molar ratio (crush-leach solutions) as a function of chlorine concentration (microthermometry) from fluid inclusions in fluorite from Pierre-Perthuis and La Collancelle deposits. Seawater evaporation trend according to Fontes and Matray (1993)</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. 8** δ³⁷Cl values from fluid inclusions in fluorite from Pierre-Perthuis and La Collancelle deposits, with data from the Triassic aquifers in the Paris Basin (PB). The range for chlorides in evaporite salts is from Eggenkamp et al. (2019).</figcaption></figure>

## Cause of fluorite precipitation

Richardson and Holland (1979a) showed that the solubility of fluorite in NaCl-CaCl₂ solutions decreases with decreasing temperature. In the Pierre-Perthuis and La Collancelle deposits, ascent of hot basement fluids through the fracture network is likely, and the associated temperature decrease may have been a driving fluorite crystallization. Since chlorine concentrations in fluorite fluid inclusions are lower than the expected values for seawater evaporated beyond halite saturation (Fontes and Matray 1993), dilution of the basinal calcic brine by meteoric water or seawater is likely (Fig. 7). Such a mixing process might explain the variations in Th measured in fluid inclusions (Fig. 4a). At La Collancelle, fluid inclusions in fluorite display a large range of salinities, likely caused by variable mixing ratios between a brine and meteoric or seawater (Fig. 4b).
Hence, fluorite crystallization in Pierre-Perthuis and La Collancelle was most likely triggered by the cooling of a F-rich basement brine and its mixing with less saline pore water when the solutions reached the sedimentary rocks. The partial dolomitization of the host sedimentary rock released the Ca required for fluorite to precipitate.

## Comparison with present-day pore water in the Paris Basin and the Rhine graben

Millot et al. (2011) analyzed groundwater in the Keuper argillaceous sandstones located at a depth of between 2.5 and 3 km, above the Paleozoic basement, with temperatures of approximately 90 °C and chlorine concentrations ranging from 72,000 to 102,000 mg/L, comparable to those found in fluorite fluid inclusions. Matray and Fontes (1990) measured the stable isotope composition of water in the Keuper sandstones and documented values from -4 to -3 ‰ for δ¹⁸O and from -40 to -25 ‰ for δD. These authors suggested that Keuper pore water originated from mixing of a sedimentary brine with meteoric waters. The nature of the brine endmember was approximated using Cl/Br ratios.

Millot et al. (2011) reported molar ratios between 240 and 260. In the eastern Paris Basin, where the lower Triassic sandstones are buried at a depth of approximately 2 km, Rebeix et al. (2011) measured δ¹⁸O and δD in pore water similar to the central part of the Basin, and a Cl/Br (molar) value of 167. Finally, the stable chlorine isotope composition was measured in Triassic pore waters in the eastern Paris Basin by Rebeix et al. (2014) and in the central Paris Basin by Eggenkamp (1994) who reported variable δ³⁷Cl values between -0.3 and -2.1 ‰. Hence, present-day deep Triassic fluids in the central and eastern Paris Basin display comparable chemical characteristics with paleo-fluids in Pierre-Perthuis and La Collancelle. Present-day and paleo-waters result from mixing between evaporated seawater and meteoric water. However, the main difference resides in the Na/Ca ratios of the fluids, which are much higher in Triassic aquifer water than in fluorite fluid inclusions (Fig. 9). This can be explained by the high degree of interactions with the host sedimentary rocks at Pierre-Perthuis and La Collancelle.

Basement fluids coming from a deep borehole (2.7 to 3.3 km depth) in the central Paris Basin are over-saturated with respect to CaF₂ at temperatures ranging from 100 to 120 °C (Boulègue et al. 1990). Na/Ca ratios in these basement fluids are lower than in the overlying Triassic aquifers (Fig. 9).
In the geothermal fields of the Rhine graben, the Cronenburg, Bruchsal and Bühl wells reached the Triassic and Permian reservoirs located above the granite basement, at depths ranging from about 2.5 to 2.8 km and temperatures between 115 and 140 °C (Sanjuan et al. 2016). Geothermal fluids have chlorinities ranging from 60,000 to 120,000 mg/L and Cl/Br molar ratios lower than the average seawater value (Fig. 7), which is interpreted as deriving from the evaporation of seawater (Pauwels et al. 1993). Na/Ca ratios are much higher than those measured at Pierre-Perthuis and La Collancelle, which can be explained by the shorter residence time of waters in the basement reservoir, estimated to be about 1 My (Sanjuan et al. 2016), together with a lower degree of interaction with carbonate rocks.

As a conclusion, contemporaneous waters flowing in the deep aquifers of the Paris Basin and the Rhine graben located above the crystalline basement show similar temperatures and chlorinities to the fluorite mineralizing fluids. Most of these formations waters are composed of evolved seawater diluted by meteoric water (Pauwels et al. 1993; Matray et al. 1994). The main difference resides in the Na/Ca ratios, much lower in fluorite mineralizing fluids likely as a result of dolomitization reactions.

553

<figure><img src="image_24.png" /></figure>

554
555

**Fig. 9** Comparison of Na/Ca ratios in present-day geothermal fluids in the Paris Basin and the Rhine Graben with paleo-fluids from Pierre-Perthuis and La Collancelle fluorite deposits

556
557

558 **Conceptual model for F-Ba ore deposition**

559 We have shown that fluorite mineralizations in the south of the Paris Basin were formed from evaporated seawater
560 infiltrated in the basement (Fig. 7). Evaporation of seawater may have occurred at the end of the Triassic. Massive
561 halite deposits are known to exist in the basin more than 100 km north of the study area. These halite deposits are
562 broadly contemporaneous to the Assise de Chitry Formation, host of F-Ba mineralization. According to Cl/Br and
563 Na/Br ratios, the mineralizing brine passed halite saturation (Fig. 10). The evaporated seawater interacted with clay-
564 rich horizons in the basin and was depleted in ³⁷Cl through ion filtration. The dense Triassic brine penetrated
565 downward in the fractured basement and was stored in this reservoir during several tens of million years. Such long
566 residence times are conceivable for basement fluids (Fehn and Snyder 2005). We propose that this brine was diluted by
567 meteoric water and/or seawater during the Cretaceous.

568 The first fluorite mineralization event is associated with dolomitization and massive silicification of the Assise de
569 Chitry Formation, before fluorite and barite crystallization (Gigoux et al. 2016). This first generation of euhedral
570 fluorite is major in volume and formed from the dissolution and recrystallization of the host carbonate rocks (Gigoux
571 et al. 2016). The source of silica, together with the timing of silicification remains unknown. Quartz dissolution is well
572 documented in the granite of the FMC (Parneix et al. 1985; Cathelineau 1986) and constitutes a source of Si, together
573 with feldspar dissolution (Fig. 10).

574 The simultaneous chloritization of biotite and feldspar dissolution and recrystallization enriched the basement fluids in
575 F, Ba, Pb, Zn and Cu. Mineral hydration reactions may have contributed to increase the salinity of the solution (Bons
576 et al. 2014). From the Lower Jurassic to the Lower Cretaceous, continental Europe was affected by the opening of the
577 Central Atlantic and Alpine Tethys rift (Stampfli and Borel 2004; Ziegler and Dèzes 2006). The transition to the
578 Arctic-North Atlantic rift system provoked major crustal extension and subsidence in continental Europe (Burisch et
579 al. 2022) with estimation of crustal thinning of about 20 to 40 kilometers (Willner et al. 1991; Ziegler et Dèzes 2006;
580 Etheve et al. 2018). The major event of the Early Cretaceous extensional regime provoked the rifting phase in the Bay
of Biscay and caused a local compressive regime in the north of the FMC and the Paris Basin, with evidence of fault reactivation (Tremolieres 1981; Benard et al. 1985). Guillocheau et al. (2000) and De Wever et al. (2002) documented two unconformities in the sedimentary cover of the Paris Basin during the Early Cretaceous which were interpreted as a consequence of a NE-SW to E-W compression. Basement fluids were likely expelled through the reactivation of Variscan faults caused by this local compression regime (Gigoux et al. 2015). This dense Triassic brine was likely Mg-rich and may have contributed to further dolomitization and porosity generation in the surrounding sedimentary rocks. Calcium may have been sourced from the local dolomitization of the host carbonate rocks (Banks et al. 1991). Metals would have been carried in solution through chloride and/or fluoride complexes. The stability of these complexes may have drastically decreased when basement fluids mixed with less saline, S-rich solutions. Fluorite and barite, together with minor sulfides (galena, sphalerite and chalcopyrite), crystallized from (1) the temperature drop resulting from ascending fluids and (2) mixing with shallower fluids (Fig. 10). F-Ba mineralizations, associated with Pb, Zn and Cu on the southern border of the Paris Basin, are therefore comparable to other "Mississippi Valley Type" mineralizations in western Europe and northern Africa formed under the Mesozoic extensional regime (Muchez et al. 2005; Burisch et al. 2022).
<figure><img src="image_25.png" /><figcaption>**Fig. 10** Conceptual model for Early Cretaceous F-Ba deposit on the northwestern edge of the French Massif Central</figcaption></figure>

**Conclusion**

Chemical and isotopic analysis of bulk fluid inclusions hosted in fluorite crystals from two F-Ba deposits located in the southern border of the Paris Basin provide new constraints on the origins of mineralizing fluids and the fluid-rock interactions responsible for F-Ba ores. Based on the Cl/Br ratio and chlorine concentration, evolved seawater migrated
deeply into the basement These brines were enriched in F, Ba, Pb, Zn, Cu due to biotite and feldspar dissolution and partial recrystallization, which occurred in the granitic basement. Episodic events of tectonic activity forced the brines upwards leading to ore deposition at shallower crustal levels. F-Ba mineralizations are interpreted as being the result of mixing F-Si-Cl-rich brines with SO₄-rich meteoric and/or seawater from the sedimentary basin, which decreased the solubility of fluorite. Overall, this study provides an example of granite hydrothermal alteration and fluid transfer across the crystalline basement and the overlying sedimentary reservoirs leading to unconformity-related mineralizations. The ore-forming paleofluids are analogous to present-day geothermal waters in Triassic reservoirs of the Paris Basin and the Rhine graben.

610

# 611 Appendix

612

# 613 Microthermometry

614
<figure><img src="image_26.png" /></figure>

615

616 **Fig. 11 Homogenization temperature vs. fluid inclusion length**

617

618 **Crush-leach protocol**

619 Blank solutions were generated to test all potential sources of contamination during aliquot preparation, such as: (1) single-use equipment (plastic tubes, syringe, filter), (2) purity of MilliQ water and (3) the agate mortar. Three sets of blank solutions have been prepared and detailed in Table S4, compiled with major ions composition. Concentrations in Cl, Br, Na and K in crush-leach solutions were always at least 4 times higher, and almost always 10 times higher than in the blank solutions.

624

625 Then, we measured major dissolved ions in aliquots prepared from (1) a variable sample mass (1.5 g<m<25.4 g) and a constant volume of MilliQ water (V=10 mL; samples PP16-1 to PP16-10) and (2) a constant sample mass (m=5 g) and
a variable volume of MilliQ water (4 mL<V<20 mL; samples PP16-11 to PP16-15). Concentrations of major dissolved anions and cations from these solutions are presented in Table S5.

For a given sample mass, as the dilution volume increases, the anion concentrations decrease. In the same way, for a given dilution volume, as the sample mass increases, the anion concentrations increase.

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

**Fig. 12** Variation of chlorine concentration as a function of a) dilution volume (PP16-11 to PP16-15) for a constant mass and b) sample mass (PP16-1 to PP15-10) for a constant volume of dilution

## Fluid inclusion pressure correction
637 Using the excel® spreadsheet from Bodnar (1993) and Steele MacInnis et al. (2012), the average slope of the dP/dT isochore is 25.1 bar/ °C at Pierre-Perthuis and 22.7 bar/ °C at La Collancelle (Fig. 13), for an average fluid density of 638 1.1 and 1.0 g.cm⁻³ respectively (simplified in the H₂O-NaCl system). 639

640 According to the present-day preserved sedimentary succession in the Paris Basin and Barbarand et al. (2013), the 641 maximum sedimentary thickness during the Early Cretaceous mineralization period is about 1 km. Fluorite 642 precipitation likely occurred under hydrostatic equilibrium at such a shallow burial depth, fluids being connected 643 through the fracture and pore networks developed in both basement and sedimentary rocks. Considering a hydrostatic 644 pressure gradient (Goldstein and Reynolds 1994), we therefore postulated a trapping pressure of 100 bars. 645 Consequently, the corrected trapping temperature for fluid inclusions is around 5 degrees higher than the minimal 646 trapping temperatures (Fig. 13). 647

<figure><img src="image_29.png" /></figure>

647
648 **Fig. 13** P-T plots in the H₂O-NaCl system illustrating the pressure correction applied on fluid inclusions (modified after Goldstein and Reynolds 1994)

650

651 **Average chlorine concentrations in fluid inclusions**

652 The average of chlorine concentration in fluid inclusions from Pierre-Perthuis and La Collancelle has been computed from microthermometric measurements (detailed in Table 3) using the following equation:

654

$$
655 \quad \mathrm{Cl} = \left( \frac{(\mathrm{wt}\%\mathrm{NaCl} \cdot 2\mathrm{MCl})}{\mathrm{MNa} + \mathrm{MCl}} + \frac{(\mathrm{wt}\%\mathrm{CaCl}_2 \cdot \mathrm{MCl})}{\mathrm{MCa} + 2\mathrm{MCl}} \right) \cdot 1000 \quad \text{Equation n}^{\circ}\text{1}
$$

656

657 With MCl=35, MNa=23 and MCa=40 (molar masses of chlorine, sodium and calcium, respectively, in g.mol⁻¹).

658

<table>
  <thead>
    <tr>
      <th rowspan="2">Locality</th>
      <th colspan="3">Average of salt concentrations from microthermometry</th>
      <th>Average Cl concentration (ppm)</th>
    </tr>
    <tr>
      <th>Nb of value</th>
      <th>NaCl (wt%)</th>
      <th>CaCl<sub>2</sub> (wt%)</th>
      <th>Cl</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Pierre-Perthuis</td>
      <td>66</td>
      <td>3.00</td>
      <td>12.80</td>
      <td>100,000</td>
    </tr>
    <tr>
      <td>La Collancelle</td>
      <td>24</td>
      <td>4.36</td>
      <td>6.85</td>
      <td>69,900</td>
    </tr>
  </tbody>
</table>

659 **Table 3** Average chlorine concentration in ppm of fluid inclusions computed from the average of microthermometric measurements

661

662 **Analytical details of Cl stable isotope composition measurements**

663 The volume of solutions and the associated chlorinities and δ³⁷Cl of each sample is reported in Table 4.

664
<table><caption>Table 4 Summary of δ<sup>37</sup>Cl (‰SMOC) values from fluorite fluid inclusions</caption><thead><tr><th>Locality</th><th>Sample</th><th>Volume (µL)</th><th>µmol Cl/mL</th><th>δ<sup>37</sup>Cl ‰SMOC</th></tr></thead><tbody><tr><td rowspan="6">Pierre-Perthuis</td><td>PPFl-1</td><td>17,000</td><td>0.92</td><td>-2.07</td></tr><tr><td>PP14</td><td>19,800</td><td>0.61</td><td>-1.76</td></tr><tr><td>PP16-1</td><td>17,500</td><td>0.44</td><td>-2.19</td></tr><tr><td>PPFl-2</td><td>17,500</td><td>0.87</td><td>-2.10</td></tr><tr><td>PP16-2</td><td>19,536</td><td>1.85</td><td>-1.88</td></tr><tr><td>COL19-1</td><td>20,153</td><td>1.02</td><td>-1.92</td></tr><tr><td rowspan="2">La Collancelle</td><td>COL19-2</td><td>16,442</td><td>2.08</td><td>-1.31</td></tr><tr><td>COL19-3</td><td>20,313</td><td>0.83</td><td>-1.95</td></tr></tbody></table>

666

## 667 Statements and Declarations

## 668 Conflict of interest and funding

669 The authors declare that they have no conflict of interest.

670

## 671 Author contributions

672 All authors contributed to the study conception and design. Antonin Richard, Diana Chourio-Camacho and Gaël Monvoisin were involved in the crush-leach analysis. Pierre Agrinier and Gérard Bardoux supervised the quantification of stable chlorine isotopes. Thomas Rigaudier supervised the measurements of fluid inclusions oxygen and hydrogen stable isotope composition. Antonin Richard, Alexandre Tarantola, Jocelyn Barbarand and Benjamin Brigaud were involved in data interpretation and the improvement of the paper. The first draft of the manuscript was written by Louise Lenoir and Thomas Blaise and all authors read, commented on previous versions of the manuscript and approved the final version.

679

## 680 Acknowledgments
681 The authors acknowledge S. Fernando for providing crush-leach solutions of fluorite and R. Millot, S. Andrieu and T. Conte from BRGM (Bureau des Recherches Géologiques et Minières) for technical advice on ICP-MS analyses. This work was supported by the Paris Ile-de-France Region - DIM "Matériaux anciens et patrimoniaux". We would like to thank Mathias Burisch and an anonymous reviewer for the constructive comments on our manuscript, as well as associate editor David Banks and editor Georges Beaudoin.

686

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