Contrasting fluid behavior during two styles of greisen alteration leading to distinct wolframite mineralizations: The Echassières district (Massif Central, France) Loïs Monnier, Stefano Salvi, Victor Jourdan, Souleymane Sall, Laurent Bailly, Jérémie Melleton, Didier Béziat

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Loïs Monnier, Stefano Salvi, Victor Jourdan, Souleymane Sall, Laurent Bailly, et al.. Contrast- ing fluid behavior during two styles of greisen alteration leading to distinct wolframite mineraliza- tions: The Echassières district (Massif Central, France). Ore Geology Reviews, Elsevier, 2020, 124, ￿10.1016/j.oregeorev.2020.103648￿. ￿hal-02989819￿

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1 Contrasting fluid behavior during two styles of greisen

2 alteration leading to distinct wolframite mineralizations: the

3 Echassières district (Massif Central, France)

4 Loïs Monniera, Stefano Salvia, Victor Jourdana, Souleymane Salla, Laurent Baillyb, Jérémie

5 Melletonb, Didier Béziata

6 a Géosciences Environnement Toulouse (GET), CNRS, IRD, UPS, CNES. Université de Toulouse, 14

7 avenue Edouard Belin, 31400 Toulouse, France

8 b Bureau de Recherches Géologiques et Minières (BRGM), 3 Avenue Claude Guillemin, 45000 Orléans,

9 France

10 Corresponding author: [email protected]

11 Highlights

12  Fluid cooling, without evidence of sharp pressure variations, is the main cause for one

13 wolframite mineralization episode.

14  Fluid flashing (vaporization), triggered by fracture-induced pressure drops, initiated

15 crystallization of a second type of wolframite mineralization.

16  Greisenization of the Beauvoir granite is a continuous process (ca. 400°C down to 190°C),

17 occurring at high temperature as pervasive alteration of the granite body, while at lower

18 temperature it is localized to a vein system.

19  Globally, greisen-forming fluids have a strong potential for transporting and precipitating W.

1

20  In greisen systems worldwide, regional veins distal to the altered causative magmatic body

21 can be mineralized by greisen-forming fluids.

22 Abstract

23 The Echassières district of central France hosts diverse magmatic and magmatic- hydrothermal

24 deposits of rare metals, mostly related to the well-known Beauvoir granite. Tungsten mineralization

25 crops out at three distinct wolframite occurrences, the two most important of which are related to

26 two distinct magmatic bodies, emplaced ca. 335 and 310 Ma (Monnier et al., 2019). The

27 mineralization occurred at 335 Ma formed during a hydrothermal episode marked by precipitation of

28 topaz replacing quartz in a stockwork system and as veinlets in the surrounding schist. Fluid

29 inclusions in topaz and quartz display similar features, i.e., all have low salinity, contain significant

30 amount of LiCl, display constant liquid/vapor ratios, and homogenized within a narrow temperature

31 range (Th ≈ 380°C). No evidence for fluid pressure variations was observed, and temperature

32 decrease is considered to be the main cause for wolframite deposition. The younger W

33 mineralization is related to greisenizing fluids that altered the Beauvoir granite and generated several

34 quartz (± topaz and apatite) veins. All greisen-related fluid inclusions display low salinity, however, Th

35 are spread from ca. 190 to 400°C, and several populations exhibit heterogeneous liquid/vapor ratios

36 while others consist of only vapor-rich fluid inclusions. Respectively, these populations are

37 interpreted to have been trapped during boiling or flashing (vaporization) of the fluid. In contrast

38 with the other regional veins, flashing was particularly intense in the Mazet veins, which host the

39 bulk of the last wolframite generation. Consequently, it is proposed that flashing is the key factor

40 that triggered W precipitation.

41 This work highlights the role of two physical parameters, pressure and temperature, whose

42 variations played a preponderant role on wolframite mineralization. It documents, in depth, an

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43 example of greisen fluid evolution, providing critical information on W behavior in orthomagmatic

44 fluids, and on greisen-related rare-metal deposits.

45 Introduction

46 An important proportion of felsic igneous bodies emplaced in the upper part of the crust, particularly

47 the more evolved, rare-metal enriched, and/or peraluminous varieties, exhibits greisen alteration to

48 some extent. Greisenization is characterized by replacement of igneous minerals by various

49 proportions of muscovite, quartz and topaz (Štemprok, 1987), ± tourmaline, apatite, HFSE-bearing

50 minerals, to mention the most common (Pirajno, 2009). It is well established that greisen alteration is

51 caused by magmatic-related, acidic fluids that trapped fluid inclusions (FI) showing a wide range of

52 homogenization temperatures (Th; 200 to 450°C; e.g., Burt, 1981; Štemprok, 1987; Halter et al.,

53 1998; Cui et al., 2019), and, in the case of rare metal granites, low salinity (<10 wt.% NaCl eq.;

54 Charoy, 1981 ; Cuney et al., 1992; Dobeš et al., 2005; Breiter et al., 2017a). Similarly, FI associated

55 with W-Sn ore deposits display for the most part low salinity and range in homogenization

56 temperature from 200 to 400°C (e.g., Naumov et al., 2011). Based on this evidence, as well as on

57 structural, petrographic, geochemical and geochronological data, numerous models propose that the

58 same fluids (with possible local evolution or mixing etc.) that form greisen alteration are also

59 responsible for the formation of the surrounding wolframite (± cassiterite)-bearing stockwork (e.g.,

60 Štemprok, 1987; Pirajno, 2009; Halter et al., 1998; Williamson et al., 1997; Yokart et al., 2003; Mao et

61 al., 2013; Zhao et al., 2017; Korges et al., 2018; Monnier et al., 2018; 2019; Zheng et al. 2018; Chen et

62 al., 2019). A remaining challenge to the understanding of greisen-related wolframite mineralization

63 are the different factors controlling wolframite precipitation, particularly its location, i.e., in quartz

64 veins that are quasi systematically in the host rock and not in the greisenized granite.

65 To provide an answer to this question, numerous studies have nourished a recent debate on W

66 transport/deposition mechanisms. Lecumberri-Sanchez et al. (2017) suggest alteration of the host

67 rock as a key factor for providing the Fe and Mn necessary for wolframite ((Fe,Mn)WO4)

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68 precipitation. However, Heinrich (1990) and Yang et al. (2019) argue that orthomagmatic fluids

69 contain sufficient Fe and Mn to permit wolframite precipitation. This affirmation is verified by LA-ICP-

70 MS analyses of magmatic-related FI, which record high amounts of Fe and Mn, in addition to W

71 (Audétat et al., 1998; Harlaux et al., 2017; Yang et al., 2019). Yokart et al. (2003), Legros et al. (2019)

72 and Liu et al. (2018) propose mixing between orthomagmatic and meteoric fluids as the cause for

73 wolframite deposition, while Pan et al. (2019) suggest an input of sedimentary fluids as instrumental

74 in precipitating wolframite. Based on microthermometric data, Korges et al. (2018) and Jiang et al.

75 (2019) propose that depressurization, triggering boiling of the fluid, is the main factor for formation

76 of greisen alteration and simultaneous wolframite-bearing veins. On the other hand, Ni et al.

77 (2015a), Li et al. (2018) and Chen et al. (2018) suggest that simple cooling during fluid transport is

78 sufficient to induce wolframite precipitation. The experimental data of Wood and Samson (2000)

79 confirm that, given a sufficient W concentration in the fluid, cooling and depressurization can control

80 wolframite precipitation, as also proposed by Yang et al. (2019) to explain wolframite precipitation as

81 an infill along fractures. Liu et al. (2018), based on numerical modelling, confirm the important role of

82 repeated depressurization episodes, caused by cyclic fracturing. Concerning the fluid chemistry,

83 Wang et al. (2019a) and Wang et al. (2020) show, respectively, that salinity and CO2 have only a

84 minor impact on W mobility, whereas the role of pH seems critical (see also Wood and Samson,

85 2000), notably the pH increase during interaction with graphite-rich schist host rock (O’Reilly et al.,

86 1997).

87 The Echassières district in central France is well suited to investigate the role that different

88 parameters might play on the origin of W mineralization. In this area, most of the W is in the form of

89 three wolframite generations related to three distinct hydrothermal events, separated in time

90 (Monnier et al., 2019). The earliest W episode occurred in the form of wolframite a, outcropping in a

91 stockwork vein system. The bulk of the W stock consists of a wolframite generation that precipitated

92 during a topazification event (wolframite b), attributed to percolation of a F-rich greisenizing fluid. A

93 last major mineralization event (wolframite c) took place during OH-rich greisen alteration following

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94 emplacement of the highly evolved Beauvoir granite. However, occurrence of wolframite c is uneven;

95 it is never found in greisenized granite, but occurs in large amounts in a swarm of mineralized

96 greisen-related quartz veins emplaced in the vicinity of the granite. Interestingly, other veins that are

97 interpreted to have the same origin are barren (Monnier et al., 2018). A previous study ruled out a

98 possible effect of contrasting composition of the greisen-forming fluid between granite and veins to

99 explain this peculiar mode of occurrence (Monnier et al., 2018). The mineral paragenesis of these

100 two wolframite generations suggest similar fluid properties (low-CO2, strong acidity and low salinity;

101 Monnier et al., 2019), indicative of greisen affinity (Štemprok, 1987).

102 Here, we report the results of a FI microthermometric study and investigate the role that two

103 intensive physical parameters, pressure and temperature, may have played on these mineralizations.

104 This study presents an important data collection of FI microthermometric features (more than 800

105 measures) in the Echassières district, and highlights the opposite behavior of fluids at the origin of

106 wolframite b and c. At a global scale, this work clearly evidences two physical constraints on

107 wolframite mineralization, and provides critical information on the greisen alteration processes.

108 In addition to W, in contrast with other well-known Variscan W districts in Europe (e.g., Panasqueira

109 district: Noronha et al., 1992; Erzgebirge mountains: Breiter, 2012; Cornwall district: Campbell and

110 Panter, 1990) it is possible that greisen alteration of the Beauvoir granite may be linked to distal Sb

111 mineralization in the Nades area. A similar metal association is commonly reported in some W

112 deposits in China (Hu et al., 2017; Wang et al., 2019b). This genetic relationship is supported by

113 quartz trace element composition (very similar signatures and high Sb content for quartz from

114 greisen and Nades vein; Monnier et al., 2018), notably the Sb content in greisen quartz which is >1

115 ppm with median value equal to ca. 5 ppm, corresponding only to quartz associated to Sb

116 mineralization according to data from literature (Rusk et al., 2011; Pacák et al., 2019). In this work,

117 the characteristics of the fluids associated with the stibnite-quartz veins cropping out at the south of

118 Beauvoir granite will be also discussed.

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119 Regional geology

120 Located in the northern part of the French Massif Central (Fig. 1), the Echassières district is hosted by

121 the Sioule metamorphic series. The district is bounded by the crustal-scale strike-slip Sillon Houiller

122 fault on the west, by the Saint-Gervais granite and a formation known as anthracite tuff on the south,

123 while the Tréban granite and Cenozoic sediments limit it on the North and East, respectively. The

124 Sioule series consists of three metamorphic units which form an inverted metamorphic sequence

125 structured in two major antiforms. Two granitic systems, the Beauvoir/Colettes plutonic pair and

126 Pouzol-Servant laccolith intrude the deeper para-autochtonous unit of the series, respectively, in the

127 northern and southern antiform (Fig. 1). The Sioule series records mostly the peak of barrovian

128 metamorphism (ca. 600°C and 7 kbar for the para-autochtonous unit; Schulz et al., 2001; Schulz,

129 2009) occurring at ca. 360 Ma (Do Couto et al., 2016). It is intruded by the Pouzol-Servant laccolith

130 (ca. 330 Ma; Pin, 1991), during the Visean peak of peraluminous magmatic activity, also recorded by

131 the resetting of the 40Ar/39Ar systematics in metamorphic micas (ca. 333 Ma, Faure et al., 2002; Do

132 Couto et al., 2016), caused by associated hydrothermal activity. The Beauvoir/Colettes granitic

133 system was emplaced in the series during the late extensional orogenic stage of the Variscan belt (ca.

134 310 Ma; Duthou and Pin, 1987; Cheilletz et al., 1992; Melleton et al., 2015).

135 Mineralization events

136 The Echassières district is a remarkable site involving a complex sequence of events, many of which

137 mineralized mainly in Sn, W, Sb, Li, Nb-Ta. The earliest occurrence is the La Bosse stockwork (Fig. 1.A-

138 B), a swarm of sub-horizontal quartz veins emplaced contemporaneously to multiple aplitic dykes

139 before regional metamorphism. The stockwork veins contain minor wolframite (ferberite/hubnerite

140 ratio ≈ 8.4; calculated using atomic proportions in Monnier et al., 2019), commonly altered, referred

141 to as “wolframite type a”. However, it was impossible to identify primary fluid inclusions linked to its

142 formation, notably due to extensive quartz recrystallization during metamorphism, and percolation

6

143 of several subsequent generations of fluids. Therefore, this generation was not considered further in

144 this study.

145 After metamorphism, during the Visean magmatism, a series of topaz veins crosscut the stockwork

146 and the surrounding schist, locally replacing partially dissolved quartz veins. This topazification event

147 was accompanied by pervasive F-rich alteration (topaz ± F-Li-rich micas; first greisen event) and

148 precipitation of abundant wolframite (type b; ferberite/hubnerite ratio ≈ 3.5) and lower amounts of

149 cassiterite in the quartz and the topaz veins as well as in the schist.

150 Followed the emplacement of the Beauvoir/Colettes granitic system (Fig. 1), during the Stephanian.

151 Colettes, the larger body, is a porphyritic two-micas granite while Beauvoir is a highly-evolved albite-

152 lepidolite-topaz equigranular granite, well known for its rare-metal content, rich in cassiterite,

153 colombo-tantalite and pyrochlore (Aubert, 1969; Wang et al., 1992). The apical part of the Beauvoir

154 granite shows remarkable enrichment in high-field strength elements (HFSE; ca. 50 ppm of W, 100

155 ppm of Nb, 150 ppm of Ta, and up to 1000 ppm of Sn), although Zr and Hf are strongly depleted

156 (Raimbault et al., 1995). In addition to Li, lepidolite is also enriched in Rb and F. Niobium and Ta are

157 concentrated in columbo-tantalite group minerals, Sn in cassiterite, and pyrochlore-group minerals

158 contain important quantities of U, Nb, Ta, W (Fonteille, 1987; Cuney et al., 1992; Wang et al., 1992).

159 The Beauvoir granite (in particular its apex) exhibits an important greisen alteration (Fig. 1.B) second

160 greisen event) consisting of replacement of igneous minerals by muscovite, quartz and apatite. At

161 Beauvoir, greisen alteration is not accompanied by rare-metal mineralization, and igneous cassiterite

162 is replaced by muscovite. However, in the host rocks next to the Beauvoir granite, cassiterite

163 precipitated together with topaz , during reactivation of the topaz veins formed during topazification.

164 This superimposed greisen alteration also caused minor cassiterite, colombo-tantalite and wolframite

165 (type c; hubnerite/ferberite ratio ≈ 0.3) precipitation in quartz veins in the vicinity of the Colettes and

166 Beauvoir granites (Fig. 1.A; known as ‘proximal veins’; Monnier et al., 2018), with the exception of

167 the Mazet veins, where wolframite c is very abundant. More distal from this intrusive system, occurs

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168 a set of quartz veins, characterized by the presence of sulphides. One of these occurrences consists

169 of the Nades stibnite veins, interpreted to have derived from the greisen fluids (Monnier et al.,

170 2018).

171 The last hydrothermal episode consists of late-stage kaolinization of the Beauvoir and Colettes

172 granites, mostly overprinting the greisen alteration (Charoy et al., 2003). Wolframite was also

173 altered, commonly replaced by W-rich goethite. Some mineralized occurrences in the Echassières

174 district, e.g., Sb veins of Pouzol-Servant granite and Cu-Sn sulphide veins of the Chaillat locality,

175 remain poorly genetically constrained.

176 Sampling and analytical methods

177 Most of the samples used in this study were collected in the field, from the Beauvoir open pit,

178 Colettes granite, and in the Suchot area (near the town of Echassières, France; Fig. 1). In addition,

179 several samples, representative of the different facies at depth, were taken from the GPF (Deep

180 Geology of France) drill-hole series collection (hole # 1) (Orléans, France). Finally, samples from the

181 Mazet wolframite mineralization were obtained from the French Geological Survey (BRGM, Orléans,

182 France) collection, because the mine site has since been rehabilitated and does not crop out any

183 longer.

184 The mineralogy and textural relationships were investigated using optical microscopy. Fluid inclusion

185 studies of quartz and topaz were done using double-polished 0.2 mm-thick wafers.

186 Microthermometric measurements were carried out using a Linkam THMGS 600 heating-freezing

187 stage, mounted on an Olympus BX-51 microscope. Measurements were performed at the GET

188 laboratory, following the procedures outlined by Roedder (1984) and Shepherd et al. (1985). The

189 stage was calibrated against pure H2O synthetic inclusions (0 and 374.1°C), supplied by SynFlinc, and

190 pure CO2-bearing natural inclusions (–56.6°C) from Camperio (Ticino, Switzerland). Measurements

191 near and below 0°C are accurate to 0.1°C and to 1°C at higher temperatures. Heating rates were

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192 0.2°C/min when phase transitions were approached. Cryogenic experiments were carried out before

193 heating experiments to avoid the risk of inclusions decrepitating. Salinity (S) of fluid inclusions,

194 expressed as wt.% eq. NaCl, was calculated based on the temperature of final ice melting (Tm) and

2 3 195 the equation of Bodnar (1993) (S = -1.78 Tm + 0.0442 Tm + 0.000557 Tm ). It has been suggested that

196 primary FI commonly occur as individual isolation or groups along growth zones of quartz or healed

197 micro-fractures, whereas secondary FI tend to occur in trails and go through quartz grain boundaries

198 (Goldstein and Reynolds, 1994). However, using the above criteria classifying fluid inclusions is not

199 always feasible because the crystal growth banding in quartz cannot be always observed in these

200 three deposits. Therefore, apart from the typical primary FI along quartz growth zones, other FI data

201 on quartz in this study were obtained from isolated FI that might be primary in origin according to

202 Roedder (1984) or FI assemblages which have similar heating-freezing behavior (Fall and Bodnar,

203 2018; and references therein).

204 Fluid inclusion petrography

205 A detailed description of magmatic and hydrothermal rocks of the Echassières district is available in

206 Monnier et al. (2018) and Monnier et al. (2019). Consequently, we focus on some aspects of

207 mineralogy and textural relationships that bear an impact on the petrography of fluid inclusions.

208 Unfortunately, identification of FI population coeval with wolframite a crystallization, and more

209 globally contemporaneous of the formation of quartz veins of the La Bosse stockwork is impossible,

210 considering the reset triggered by regional metamorphism and the subsequent hydrothermal

211 episodes (topazification, greisenization, and kaolinization) which affected the stockwork (Monnier et

212 al., 2019).

213 Beauvoir area

214 Greisen in the Beauvoir granite

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215 Greisen alteration is particularly strong at the apex of the Beauvoir granite (Fig. 2.A), where it is

216 expressed by pervasive replacement of magmatic minerals (feldspars ± lepidolite and quartz) by

217 hydrothermal quartz, muscovite ± apatite (Fig. 2.C), and formation of subvertical quartz vein (Fig.

218 2.A). Newly formed, disseminated quartz crystals are euhedral, contrarily to primary igneous quartz

219 which is commonly partly dissolved. Large greisen quartz crystals display, in their median part, a

220 characteristic growth band marked by the presence of FI (Fig. 2.E). These FI have irregular shapes,

221 vary in size from <1 to ca. 20 µm and are mostly composed of a vapor (V) phase (Fig. 2.F). In the core

222 of these crystals, surrounded by the V-rich FI band, there is a population of liquid-vapor (L-V) FI, 3 to

223 4 µm in size, consisting of 70 % liquid (L), of oval to rectangular shape. On the outer part of the V-rich

224 FI band, the rim of these quartz crystals contains disseminated FI that are similar to those found in

225 the quartz core but with higher proportions of L, i.e. ca. 80 % by volume. These three populations of

226 FI are considered as primary, as they are each restrained to a different growth zones. Greisen-related

227 quartz veins within the Beauvoir granite are composed of euhedral cm-sized crystal (Fig. 2.B). A well-

228 marked growth zoning is developed in these crystals, highlighted by a succession of alternating FI-

229 rich and FI-poor bands (Fig. 2.D). FI display irregular shapes and variable liquid/vapor ratios (Fig. 2.G).

230 These FI are very small (rarely > 3µm) and only a few FI of relatively larger size (~5 µm) were

231 monitored. The rare FI observed in the FI-poor bands display homogeneous liquid/vapor ratio, equal

232 to ca. 90 %.

233 La Bosse stockwork

234 - Quartz veins

235 Quartz veins of La Bosse stockwork are subhorizontal, commonly 10 to 20 cm wide (Fig. 3.A), and

236 contain topaz, wolframite and rare cassiterite and colombo-tantalite. Given that several

237 metamorphic and hydrothermal episodes were superposed to the La Bosse stockwork mineralization,

238 we are unable to discern FI potentially contemporaneous to its formation. The oldest FI population

239 that could be clearly identified is the one synchronous to the topazification episode that affected the

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240 stockwork after its formation (and before the Barrovian regional metamorphism). Topaz I, common

241 throughout the whole stockwork (Fig. 3.C), contains abundant FI forming a single population,

242 consistently so in each topaz crystal studied. These FI contain a liquid phase filling ca. 50 to 60 % of

243 the inclusion volume (Fig. 3.E), are regular in shape and vary in size from a few to ca. 50 µm. In these

244 quartz veins, occur small sub-veinlets of recrystallized quartz that are connected with crystals of

245 wolframite b (Fig 3.B), which we interpret to have formed during percolation of topazification fluid

246 (see also, Monnier et al., 2019). Formation of these veinlets caused obliteration of the FI already

247 present and trapping of a new FI population (Fig. 3.D). The latter FI show regular shape with varied

248 liquid/vapor ratios (ca 20 to 60 %; Fig. 3.F).

249 - Topaz veins

250 Topaz veins (corresponding to the first topaz generation, topaz I), commonly up to 8 cm wide and for

251 the most part subvertical, crosscut the horizontal quartz veins of the La Bosse stockwork (Fig. 4.A-B)

252 below and above the Beauvoir granite. In addition to topaz, they contain various amounts of

253 wolframite b and lepidolite to F-rich biotite micas series, plus minor rutile and cassiterite. Topaz

254 crystals trap important quantities of FI, which distributed in clusters (primary FI) or along plans which

255 affect only limited portions of a crystal (pseudosecondary FI). FI display tabular to irregular shapes

256 (Fig. 4.C), and the liquid phase occupies ca. 50 to 60 % of the inclusion volume, similarly to the FI

257 described above from disseminated topaz in quartz veins. Some of the topaz veins are reactivated,

258 and sealed by a second topaz generation (topaz II) and, in some cases, by an additional quartz

259 generation (polyphased topaz veins; Fig. 4.D). The second topaz generation contains regularly-shaped

260 primary FI with high proportion of liquid (70 %; Fig. 4.E). Lastly, primary FI occurring in quartz from

261 the cores of these veins exhibit slightly higher proportion of liquid (ca. 70 to 80 %). The majority of all

262 of the FI populations mentioned above from the polyphased topaz veins have regular shapes and

263 measure from ca. 5 to 10 µm.

264 - Aplitic dykes

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265 Contemporaneous to the La Bosse stockwork, aplitic dykes consist mostly of K-feldspar relicts (now

266 mostly clays + quartz) and, like the stockwork veins, are strongly affected by metamorphism and

267 subsequent topazification. In these dykes, topaz I is closely associated with wolframite b (Fig. 5.A-B)

268 and contains a homogeneous FI population of regular shape, small size (few µm), with L filling ca. 50-

269 60 % of the total volume.

270 - Stockwork enclave in the Beauvoir granite

271 A particularly interesting sample was collected from a drill core from the Beauvoir granite that

272 intersected an enclave of the La Bosse stockwork consisting of a stockwork vein with its host schist.

273 This sample records all of the hydrothermal episodes related to wolframite crystallization in the

274 Echassières district (Monnier et al., 2018; 2019).

275 The stockwork episode is expressed by quartz and wolframite a, the topazification episode by topaz I

276 and wolframite b, and the Beauvoir greisen event by blue apatite and wolframite c (Fig. 5.C-G). Of

277 these, only the greisen affects the Beauvoir granite (Fig. 5.D). FI in topaz I from the quartz vein share

278 the same features than the other FI already described in this mineral, i.e., a regular, tabular to

279 globular shape, and L filling ca. 50 to 60 % of the total volume (Fig. 5.F). FI interpreted to represent

280 the greisen fluid are found in quartz from muscovite-quartz veinlets altering igneous minerals in the

281 Beauvoir granite (Fig. 5.E), as well as in blue apatite replacing quartz in the stockwork vein (Fig. 5.G).

282 In some instances, grains of apatite show textural evidence of coprecipitation and apatite with

283 wolframite c (cf. Fig. 4.G in Monnier et al., 2019). Fluid inclusions in Beauvoir greisen quartz and

284 apatite are L-rich (70 to 80 % L); those in quartz show regular to rounded shapes, while those in

285 apatite are systematically elongated parallel to the C axis.

286 Regional veins

287 Proximal veins

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288 Proximal veins form a network of subvertical quartz veins located in the vicinity or within the

289 Colettes/Beauvoir granitic complex (Fig. 1).

290 - The Suchot vein

291 The Suchot vein measures from 1 to 2 m in width (Fig. 6.A), and consist essentially of two quartz

292 generations, plus minor muscovite and cassiterite. The vein selvages contain quartz and muscovite,

293 similarly to the Beauvoir greisen. In the vein, the first quartz generation is euhedral, up to cm in size,

294 with growth zones exhibiting sequential FI poor and FI rich areas (Fig. 6.B). Differently than for the

295 quartz veins at the interior of the Beauvoir granite, FI in these growth zones show homogeneous L/V

296 ratios, with L comprising ca. 80 % of total FI volume (Fig. 6.C). FI in the second quartz generation are

297 aligned along the direction of the quartz fibers (Fig. 6.B), and display variable L/V volume ratios (Fig.

298 6.D).

299 - Mazet veins

300 Quartz veins at Mazet share several characteristics with the Suchot vein, i.e., geometry (metric in

301 width, subvertical, oriented N/S), presence of two quartz generations. Also similar to that at Suchot,

302 the first quartz generation is euhedral and cm-sized, whereas the second quartz generation presents

303 a not-well crystallized, micro-quartz texture (post-recrystallized H2O-rich colloform silica). The micro-

304 quartz is accompanied by important quantity of wolframite c (Fig. 6.E). As in quartz from Suchot vein,

305 FI located in growth zones of the first quartz generation are L-rich (ca. 80 %; Fig. 6.F). On the other

306 hand, in the rare parts of the second quartz generation where FI could be observed, these display

307 globular shapes and contain only a V phase (Fig. 6.E).

308 Sb-bearing quartz veins

309 - Nades vein

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310 At the Nades locality, a large quartz vein consisting of a meter-sized main body and interconnected

311 cm- to mm-sized satellite veinlets (Fig. 7.B) forms a lode of several meters in width. Stibnite occurs in

312 this vein, and textural evidence suggests it is either synchronous with quartz, or later. Its abundance

313 is not related to the dimension of the veins, and locally one can observe veinlet composed entirely of

314 stibnite (Fig. 7.A). Quartz contains pseudosecondary trails of FI, accompanied by tiny acicular stibnite

315 crystals (< 5 µm), as well as FI clusters close to larger stibnite crystals (Fig. 7.C). These FI are

316 irregularly shaped, approximately 5 µm in size, and contain a large proportion of the L phase (ca. 90

317 %).

318 - Capitraux and Cros veins

319 In both localities occur a quartz vein of some 20 cm in width, displaying the same unusual feature,

320 i.e., a radial quartz texture, called “star quartz” in this study, initiated around a small ferrous oxide

321 nuclei. The star quartz crystals, roughly spherical, are about 5 mm in diameter, uncommonly up to 2

322 cm (Fig. 7.E). Sealing of these veins are commonly incomplete, and they can exhibit a strong porosity

323 (up to 50 %). Stibnite occurs in variable amount, and is found mainly filling the porosity between star

324 quartz crystals (Fig. 7.C,E) and, in lesser proportions, as disseminated tiny acicular crystals within the

325 outer rims of quartz. FI are very rare in the core of star quartz, whereas the rims contain clusters of

326 FI, accompanying the stibnite (Fig. 7.F). These FI are generally regular, ca. 5 µm in width, and contain

327 ca. 95 % L.

328 To resume, the FI populations described in the above paragraphs can be grouped into two main

329 groups, based on careful FI petrography and associated hydrothermal alteration parageneses. These

330 correspond to the two main hydrothermal episodes that affected these rocks, after metamorphism,

331 and that were at the origin of the two W mineralization events: 1) A first greisen-forming fluid, which

332 induced topazification, related to wolframite b and predating the granite intrusion; trapped FI in

333 topaz I and in recrystallized quartz of the La Bosse stockwork veins; 2) Greisen alteration related to

334 the Beauvoir granite, which induced muscovitization; trapped fluid inclusions in quartz from the

14

335 pervasive alteration and subvertical greisen veins within the granite, in blue apatite in the stockwork

336 enclave, in muscovite-quartz veinlets affecting the granite, in topaz II and quartz from the polyphase

337 topaz veins, in proximal veins (Suchot et Mazet) and, possibly, Nades veins (Capitraux and Cros are

338 not sufficiently constrained).

339 Microthermometric results

340 For each FI type investigated in this study, several occurrences of any given FI assemblage were

341 measured from different crystals, to confirm the data repeatability, unless otherwise specified. FI

342 that could be related petrographically to the topazification episode, whether in topaz I or in

343 stockwork quartz, form a rather homogeneous population, whereas the fluid associated with

344 formation of the OH-greisen is recorded in a wide variety of FI types in different hydrothermal

345 minerals. None of the IF monitored in this study show solid phases. Similarly, ice was the only phase

346 that formed upon cooling, confirming the relative low concentration of gases such as CO2. During the

347 heating process, all L-rich and V-rich FI homogenized, respectively, to the liquid and vapor phases.

348 For FI assemblages showing heterogeneous trapping of the L and V phases, only those displaying the

349 lowest vapor/liquid ratios were retained for monitoring.

350 Descriptive statistics of microthermometric data for each FI population are summarized in Table 1.

351 Equivalent salinity and homogenization temperature median values, plus sketches depicting the

352 petrography and the locality of the studied FI populations, are given in Figures 8-10.

353 Salinities

354 During sub-zero heating runs the first occurrence of ice melting was recorded to obtain an estimate

355 of the eutectic point of the system (Te). For all FI belonging to the topazification episode the first-

356 melting, when observed, occurred at temperatures just above or equal to -72°C (Fig. 4.F). Such low

357 values suggest that Li was likely an important component in the fluid (Monnin et al., 2002; Dubois et

358 al., 2010). Determination of eutectic temperature for the greisen fluid was challenging, as this phase

15

359 transition was difficult to observe due to the small size of most FI and to their low salinities (see

360 below). However, by applying the cycling technique described by Reynolds (1988), we could record a

361 sufficient number of measurements, which approached and were never lower than -21°C. These

362 measurements were confirmed by the occurrence of a few larger FI where a first melting at -21°C

363 could be observed. Such figure is consistent with NaCl dominating other salt species in the fluid (Te of

364 the H2O-NaCl system is -21.1°C; Bodnar, 1993). Nevertheless, these results are indicative, and it is

365 possible that other salts were present in addition to Na, particularly cations such as K+ and Mg2+,

366 given that their presence does not change significantly the eutectic temperature. The presence of Li+

367 in the topazification fluid is not surprising, as lepidolite is a common alteration mineral, whereas the

368 alteration paragenesis in the greisen is dominated by the presence of muscovite and quartz. The

369 temperature of final ice melting ranged from -10 to 0°C for all FI from both alteration episodes, with

370 a great majority of data above -4°C. These data indicate relatively low salinities, with 99% of values <

371 10 wt.% NaCl eq. and the majority of values < 4 wt.% NaCl eq. However, a weak trend toward

372 somewhat higher salinities can be distinguished for the greisen fluid, moving away from the Beauvoir

373 granite, e.g. the median salinity is 1 wt.% NaCl eq. for greisen in the granite, while it is 5 wt.% NaCl

374 eq. for quartz from Nades and Suchot veins (Quartz 1).

375 Homogenization Temperatures (Th)

376 All Th values, for each FI population, are detailed in the histograms depicted in Figure 11. FI trapped

377 during the topazification episode record a very narrow range of Th, comprised between 370 and

378 395°C (Fig. 11.A,F,H,I,J,M), except for a small peak at 250-270°C. The latter range, however, is well

379 marked for some FI assemblages in topaz I from the polyphased topaz vein and in recrystallized

380 quartz in the La Bosse stockwork (Figs. 9 and 11.A,H). Beauvoir greisen fluid is recorded in a wide

381 variety of FI populations, which display a range of Th, covering a large interval from ca. 190 to 400°C

382 (median values). Globally, FI trapped during greisen pervasive alteration of the Beauvoir granite

383 (hydrothermal quartz and apatite, micro-fractures and -veinlets in igneous minerals) recorded

16

384 elevated Th (260-400°C; Fig. 11.G,L,P) with lower values for FI trapped in quartz veins, with no

385 apparent correlation to the distance from the granite (190-260°C; Fig. 11,C,D,E,K,N). Zoned crystals of

386 the disseminated greisen type show markedly different Th for FI located in cores and in the rims. Data

387 from the core show a peak at around 300°C while for the growth band they range from 400 to 450°C.

388 However, FI in the outermost rim of these crystals have lower Th, with a peak at ca 250°C (Fig. 11.P).

389 Zoned quartz crystals of the Suchot veins display a similar trend although the values are lower, i.e.,

390 cores have a peak at 200°C while rims peak at about 230°C (Fig. 11.D). A smaller difference can be

391 observed between cores and rims of quartz from the Mazet veins (Fig. 11.N).

392 Discussion

393 P-T conditions

394 Most FI populations described above display homogeneous vapor/liquid ratio suggesting trapping of

395 a supercritical fluid. For these populations, Th represents the minimal estimate for the trapping

396 conditions. In some instances, the occurrence of coexisting L-rich and V-rich FI within the same

397 assemblage (e.g., group or growth zone; Fig. 2.F,G; Fig. 6.D) such as observed in greisen veins within

398 the Beauvoir granite and in Suchot vein quartz, indicates local boiling conditions. In this case, we

399 used the homogenization temperatures of the vapor- and liquid-rich end-members to estimate the

400 actual trapping temperature.

401 Because we do not have independent means for constraining the temperature or pressure of the

402 system, we can assume that the trapping conditions lie along an isochoric path defined by the

403 physical properties of the fluid inclusions, confined to conditions given by a reasonable geothermal

404 gradient for this system. If we consider a common average crust geothermal gradient (ca. 30°C/km),

405 the intersections with some isochores of Beauvoir greisen and topazification FI occur at

406 unrealistically elevated temperatures (more than 600°C and 1000°C, respectively; Fig. 12.A). Given

407 the presence of crystallizing intrusions, we suggest a gradient for our system at about 150°C/km (Fig.

17

408 12.A). Using this gradient, we obtain trapping conditions that fall within the range for FI populations

409 related to greisen alteration and wolframite mineralization described in the literature (e.g.,

410 Williamson et al., 1997; Naumov et al., 2011, Cui et al., 2019; Jiang et al., 2019). Such conditions are

411 coherent with the emplacement of a hot granitic magma body at shallow depth, the Beauvoir

412 granite, proposed to have occurred at ca. 3 km according to Cuney et al. (1992; based on stable

413 isotopic systematics on mineral pairs and on microthermometric results on orthomagmatic FI), and

414 suggest that circulation of at least the Beauvoir greisenizing fluid took place before the granite

415 cooled substantially. This gradient is also consistent with the elevated Th of the FI related to

416 topazification, which suggests that this fluid also originated from a shallow magmatic source, even

417 though such magmatic body does not crop out at the present surface in the Echassières district.

418 While the FI homogenization data for the topazification episode show a very well-defined peak,

419 bracketing the conditions of the topazification fluid to a narrow range (Fig. 12.B), the FI populations

420 related to the greisen-forming fluid record a wider scatter of Th, varying with different quartz

421 generations. Fluid inclusion data show that disseminated quartz in greisen, and quartz from Mazet

422 and Suchot veins, record a similar temperature pattern: in all cases the earliest quartz generation

423 trapped FI with lower Th than the following generation (Table 1; Fig. 8; Fig. 10; Fig. 11.D,N,P).

424 Monnier et al. (2018) have shown that all of these quartz generations have the same trace-element

425 signature. It is practically impossible for two different hydrothermal fluids to have exactly the same

426 trace element chemistry, therefore, we have to exclude the possibility that the second greisen quartz

427 generation precipitated from a different, higher-temperature fluid than the first generation. Instead,

428 this increase in Th can be explained by variations in physical parameters of the same fluid, such as a

429 rise in temperature or depressurization. The sharp textural differences in quartz (euhedral vs fibrous

430 and micro-quartz, both of which indicating rapid silica precipitation) and FI properties in overgrowth

431 and second quartz generations (variable vapor/liquid ratio or vapor-rich), favor the depressurization

432 scenario (Moncada et al., 2012). A possible cause for depressurization is fracturing, which would

433 result in connecting the fluid to the surface. Because fracturing is a punctual event, we can consider

18

434 temperature to remain constant during the fast crystallization of the quartz immediately after

435 fracturing (adiabatic depressurization; hence producing FI with lower Th but effectively same trapping

436 temperature, cf., point pairs [1] - [2] in Fig 12.C), with pressure dropping to a minimal value

437 constrained either by boiling (on the liquid-vapor curve) or flashing (below the liquid-vapor curve)

438 conditions. Flashing, or flash vaporization, consists of particularly intense boiling where the fluid is

439 instantly transformed to vapor. Evidence for this was observed in greisen disseminated hydrothermal

440 quartz in the Beauvoir granite as well as in the Mazet veins, suggested by the occurrence of

441 populations of FI consisting exclusively of vapor-only individuals. The presence of micro-quartz

442 textures indicating crystallization from amorphous silica, which is common in cases of quartz

443 formation from a vapor-only fluid (e.g., Moncada et al., 2012), confirms this interpretation. From

444 Figure 12.C one can estimate that between crystallization of quartz [1] and quartz [2] generations,

445 fracturing induced pressure drops of ca. 70 Mpa, 45 MPa and 20 Mpa for disseminated greisen,

446 Suchot and Mazet veins, respectively. Plotting the data from Cuney et al. (1992; see also Harlaux et

447 al., 2017) on Fig. 12.C (white arrow) for the orthomagmatic fluid, we note a pressure drop which

448 disconnects the initiation of the greisen process (roughly at lithostatic pressure) to the evolving

449 orthomagmatic fluid (which underwent boiling). It is thus probable that, after exsolution of this fluid,

450 important mineral precipitation or tectonic activity sealed off fractures and porosity, changing the

451 pressure from hydrostatic back to lithostatic (dashed white arrow in Fig. 12.C).

452 Main differences in fluid behavior during deposition of the two types of wolframite

453 For a geothermal gradient of 150°C/km, wolframite b crystallized in a temperature range between ca.

454 400 to 550°C while wolframite c from Mazet vein crystallized between ca. 250 to 260°C.

455 Crystallization temperature of wolframite c in the stockwork was apparently more elevated, as

456 indicated by high homogenization temperatures measured in FI from contemporaneous apatite (ca.

457 350°C; apatite a in Fig. 11.G). Given the relatively large temperature difference between

458 wolframite c from stockwork and Mazet, it is remarkable to see a similar chemical composition. This

19

459 also indicates that the Fe/Mn ratio variations in wolframite c are apparently not a function of fluid

460 temperature, as recently suggested as a general characteristic (Michaud and Pichavant, 2019).

461 As mentioned in the previous section, pressure variations induced by fracturing played a key role in

462 wolframite c deposition (Fig. 12.C), whereas FI that trapped the fluids that precipitated

463 wolframite b did not record evidence of boiling. Given that decrease in either pressure and

464 temperature is considered to be an efficient process for destabilizing W complexing in the fluid and

465 triggering wolframite precipitation (Wood and Samson, 2000), it is likely that, in the absence of

466 fracture-activated pressure drops, simple cooling could have triggered W saturation in the fluid and

467 crystallization of wolframite b, together with topaz I, during the topazification episode.

468 The salinity values obtained from FI ice melting temperatures for the two greisen generations are

469 generally low, varying from about 0 to 10 wt.% NaCl eq., with most data concentrating between ca. 2

470 and 5 wt.% (Fig. 13). The variations inherent to the Beauvoir greisen group show a slight increase in

471 median values and in overall range with transport distance from the source, while the fluid that

472 underwent unmixing is characterized by slightly lower values (with the exception of quartz 2 from

473 Suchot vein; median value of 5 wt.%). Most salinity values measured in topaz I from the first greisen

474 episode cluster around 3 wt.%, although the entire data population stretches from 0 to ca. 10 wt.%

475 (Fig. 13). Notably, topaz I from the La Bosse dyke displays uncommonly high salinity median values of

476 8 wt.% NaCl eq. Considering the similarities of the FI data for all populations, it is not possible to

477 distinguish between the two greisen episodes based on the fluids' ionic concentration.

478 Trace-element data permit to discriminate the two types of hydrothermal topaz in the La Bosse

479 Stockwork, and to assign them to topazification (topaz I) and Beauvoir greisen alteration (topaz II)

480 (Monnier et al., 2019). FI Th data also show separate clusters for the two topaz types, with median

481 values equals to ca. 270 and 380°C, respectively (Fig. 14.A), as well as slightly higher salinities for the

482 topaz-II generation (Fig. 14.B; see also Th vs salinity plot in Supplementary Material). Nonetheless,

483 there is an overlap in some of the data for the two topaz types in the 240-300°C range (Fig.

20

484 14.A), indicating dissolution/reprecipitation of the older generation, topaz I, during percolation of

485 greisen fluids and precipitation of topaz II. The Th values for topaz I coincide with data reported in

486 Harlaux et al. (2017; they did not recognize the topaz II generation, less abundant).

487 Lithium concentrations in the fluids responsible for the topazification and second greisen formation

488 seem to be significantly different, as suggested by the low eutectic temperature recorded only for FI

489 from the former (near -72°C). The nature of accompanying micas, i.e., F-Li-rich lepidolite to F-rich

490 biotite (unpublished data) for topazification alteration and OH-rich muscovite for the Beauvoir

491 greisen fluid (Fonteille, 1987), suggests that F and Li are enriched in the former case, but depleted in

492 the latter. Hence, topazification, responsible for wolframite b, share several characteristics with the

493 F-Li-rich greisen type (Štemprok, 1987), suggesting it may correspond to an exogreisen, given that it

494 is not observed within a granite. Sericitic alteration of the Beauvoir granite (wolframite c) is, on the

495 other hand, in accordance with the OH-rich greisen type. Monnier et al. (2019) demonstrated that it

496 was two different granites that sourced the hydrothermal fluids that led to topazification and

497 Beauvoir greisen alterations. The data presented here confirm the occurrence of two greisen

498 episodes and that they were characterized by different fluid chemistry and evolution, as illustrated

499 by their different P-T paths in Figure 12.

500 Mineralizing processes

501 Topazification (wolframite b episode)

502 Topazification clearly affected the La Bosse stockwork, host to wolframite a (Fig. 15.A,B). As

503 mentioned above, no evidence for boiling was observed in fluid inclusions related to topazification

504 (topaz I), suggesting that the hydrothermal process involved in the concomitant precipitation of

505 wolframite b did not involve changes in pressure. The limited occurrence of topaz veins, in addition

506 to very effective pervasive fluid percolation (i.e., rock permeability) detected in these rocks are

507 consistent with limited pressure fluctuations (Fig. 15.B). This can probably be explained by a high

21

508 quartz solubility in the HF-rich fluid (Ellis, 1973; Mitra and Rimstidt, 2009), which created connected

509 porosity inhibiting local fluid overpressure. Dissolution of quartz permitted to focus fluid circulation

510 in the stockwork quartz veins triggering an important concentration of topaz and wolframite b in the

511 vein. Wolframite b, as well as topaz, are also found in albite dykes. Preferential occurrence of W in

512 Mn-Fe-poor rocks (veins and dykes) rather than in the schist suggests that there was no need for

513 fluid interaction with the latter to provide the Fe and Mn needed for wolframite precipitation.

514 The strong density of FI Th values around 380°C for topaz I could stand to signify a threshold

515 temperature for the topazification episode. As mentioned above, in this case cooling is likely the

516 cause for W saturation and consequent wolframite b crystallization. Another factor that could

517 contribute to decrease wolframite solubility is decreasing of fluid acidity due to fluid-rock interaction

518 (Wood and Samson, 2000) and massive topaz crystallization (e.g., Halter et al., 1996).

519 The relatively high temperature of wolframite b crystallization compared to other deposits would

520 mean that the topazification fluid is strongly enriched in W, accordingly to Wang et al. (2020). Two

521 factors can explain this anomaly. A first one is an uncommon fluid chemistry, more acidic and F-rich

522 than typical fluids at the origin of W deposit; a second one is the potential remobilization of

523 wolframite a of the La Bosse stockwork (cf. Monnier et al., 2019), which increased the amount of W

524 in solution. Although wolframite b crystallizes contemporaneously with topaz, the role of fluoride

525 complexing of W in an aqueous phase is still not well understood (Wood and Samson, 2000).

526 The source of the topazification fluid also remains not well constrained (Monnier et al., 2019). A

527 greisen-like origin after boiling of a previous orthomagmatic fluid should be considered (Fig. 15.B),

528 given that the timing of wolframite precipitation (ca. 335 Ma; Harlaux et al., 2018) corresponds to

529 the Visean peak of peraluminous magmatism in the French Massif Central. This is consistent with the

530 elevated homogenization temperatures of the fluid inclusions.

22

531 Locally, in the La Bosse stockwork, small sub-veinlets of recrystallized quartz (Fig. 3.D) trapped FI with

532 two populations of FI. The highest liquid volume (70 to 80 %) displays lower Th (ca. 250 to 270°C)

533 than those with liquid volume ca. equal to 60 % (370 to 395°C). The latter are in the same range as

534 the primary FI found in topaz I, indicating that some quartz dissolution/reprecipitation occurred

535 during wolframite b precipitation and topazification. The low-Th FI population has characteristics

536 comparable to those observed in topaz II generation, so probably correspond to a later fluid

537 percolation, most likely the Beauvoir greisen-forming fluid.

538 Beauvoir greisen (wolframite c episode)

539 In a study of fluid inclusions from late magmatic quartz and topaz at Beauvoir, Cuney et al. (1992)

540 documented the exsolution of an orthomagmatic fluid that underwent boiling, to form coexisting

541 brine and a low-salinity fluid (Fig. 15.C), and considered that the latter corresponds to the

542 greisenizing fluid (L3 in their study) (Fig. 12.C). The low salinities and moderate temperatures that we

543 recorded in this study are consistent with this interpretation. The greisen episode related to the

544 Beauvoir granite is a relatively long-lived alteration episode that started at temperatures of ca. 400°C

545 and waned at about 190°C (Fig. 12.C), but which did not involve an evolution in fluid chemistry, as

546 shown by consistent quartz trace composition (c.f., Monnier et al., 2018). A duration stretched over

547 time agrees with a diachronic formation of the different quartz veins during the greisen

548 episode. Greisen alteration initiated at relatively high temperature (400°C) with pervasive alteration

549 which affected essentially the Beauvoir granite (recorded by the core of hydrothermal disseminated

550 quartz) under lithostatic pressure (Fig. 15.D). Subsequently, an important fracturing event affecting

551 the Beauvoir granite connected the greisen fluid to the surface. This produced flashing of the fluid

552 (transition between Fig. 15.D and Fig. 14.E). Rapidly, pressure steadied roughly at hydrostatic values,

553 as recorded by a marked peak of fluid inclusion data at ca. 250-280°C, indicating a period of intense

554 greisen activity. This consisted of pervasive alteration of the granite (rim of disseminated quartz), and

555 vein formation/reactivation in the host rock (e.g., Mazet veins, topaz II), due to local pressure

23

556 variations (Fig. 15.E). At lower temperature (≤ 230°C), pervasive alteration seemed to be ineffective

557 as indicated by the absence of FI with lower Th in the disseminated quartz oh the granite. Fluid flow

558 was concentrated in veins, inside (greisen veins) and outside the granite body (Suchot vein, Nades

559 vein; Fig. 15.F).

560 Wolframite c is not a common mineral in the stockwork, formed during the flashing episode

561 mentioned above, while mineralization was especially efficient in the Mazet area, likely triggered by

562 a second flashing episode (Fig. 12.C). Antimony mineralization in the Nades area occurred only during

563 the last peak of greisen activity at 200°C. During fluid cooling and development of greisen alteration,

564 salinity remained low (mainly < 5 wt.% NaCl eq.), as already observed in other system (Charoy, 1981;

565 Jiang et al., 2019).

566 Surprisingly, greisen alteration at Beauvoir is relatively poor in F and Li, despite the fact that this

567 granite is strongly enriched in these and other rare metals. Nevertheless, the exceptional P content

568 of the fresh granite (Raimbault et al., 1995) is reflected by the remarkably high abundance of apatite

569 in the greisen. In the Cínovec granite, roughly analogous to Beauvoir (Monnier et al 2018), greisen

570 alteration consists of quartz plus zinnwaldite. The latter is the most F, Li, and rare-metal enriched

571 mica found in these rocks, including igneous varieties. This evidence can be interpreted as indicating

572 an opposite behavior of these elements during greisen alteration of the two granites. At Beauvoir,

573 most of the F and Li are transported by greisen fluids to the surrounding host rocks, as suggested by

574 the low concentrations of these elements found in greisen-altered granite (Merceron et al., 1992;

575 Raimbault et al., 1995). At Cínovec, greisen alteration is mostly confined to the granitic body,

576 retaining these elements as well as ore metals such as Sn and W. We therefore suggest that it is

577 critical to apprehend the extent of mobility of a greisen-forming fluid to understand its role on ore

578 metal transport and deposition (compare with the Cínovec granite: Breiter et al., 2017a; 2017b;

579 2017c; 2019).

24

580 Greisen is developed only in zones displaying enhanced permeability, i.e. the apex and fractured

581 zones in the Beauvoir and Colettes granites, and fractures in the host schist, whereas dykes of

582 Beauvoir granite intruding the schist, or the La Bosse quartz veins, only show limited signs of greisen

583 alteration (they are mostly kaolinized, cf., Monnier et al., 2019). These results contradict a recent

584 fluid inclusion study, which proposes that FI in the stockwork represent the orthomagmatic fluids

585 derived from the Beauvoir granite (Harlaux et al., 2018). However, at the time of their writing, it had

586 not been recognized that the episode of topazification of the La Bosse stockwork predates the

587 emplacement of the Beauvoir granite (Monnier et al., 2019). Consequently, we suggest that the fluid

588 inclusions from the La Bosse stockwork studied by Harlaux et al. (2017) record the orthomagmatic

589 signature of the magma that sourced the topazification fluid, not that of the Beauvoir granite.

590 Suchot (barren) vs Mazet (W mineralized) proximal veins

591 The distribution of wolframite in the proximal veins raises the question as to why wolframite c is

592 massively localized in the Mazet area and not in other proximal veins, e.g., Suchot vein. A marked

593 difference that was observed in these two localities is that fluid flashing (evidenced by micro-quartz

594 texture and presence of a vapor-rich FI population) took place in the Mazet veins, whereas only

595 evidence for boiling could be recognized in FI from the Suchot veins. This distinction in fluid behavior

596 may explain the restriction of wolframite at Mazet. Indeed, during boiling of a fluid, W is known to

597 strongly fractionate into the liquid phase (Audétat et al., 1998; Harlaux et al., 2017), suggesting that

598 flashing triggered wolframite precipitation by causing vaporization of the totality of the liquid. In case

599 of only limited boiling on the other hand, W could fractionate into the liquid phase without

600 necessarily exceeding its saturation in W, which is likely what happened at Suchot, hence the

601 absence of W mineralization.

25

602 Cyclic behavior of Beauvoir greisen fluid

603 Before fracturing, greisen fluid circulation was restricted to the granite body, because of the low

604 permeability of the surrounding schist and the sealed stockwork system. When regional shear slip

605 constraints (Gagny and Jacquot, 1987), coupled with increasing fluid pressure, triggered vertical

606 fracturing (Fig. 16.A), the fluid was immediately connected to the surface. This was a very effective

607 drain, focusing the greisenizing fluid circulation though the proximal quartz veins, and limiting mixing

608 with the host schist , as also indicated by investigation of the quartz chemistry (Monnier et al., 2018).

609 During fracturing, sharp pressure drops drastically lowered mineral solubility in the greisen fluid,

610 triggering massive precipitation of new minerals, such as quartz and muscovite. This rapidly sealed

611 the porosity (Moncada et al., 2012; Launey et al., 2019) reducing permeability and increasing fluid

612 pressure, thus initiating a new fracture-seal cycle and wolframite mineralization, all along the cooling

613 path of the fluid (Fig. 12.C and Fig. 16.B; Bons et al., 2012). This process is particularly well recorded

614 by the FI populations in the different greisen quartz generations, from which we obtained large

615 variations in Th, and which provide evidence for several boiling/flashing episodes. Possibly, in

616 addition to hydraulic fracturing, seismic activity played a key role in fracturing during greisen

617 alteration, as suggested by the occurrence of a proximal vein transformed to cataclasite (fault core

618 zone) in the Colettes granite (Monnier et al., 2018).

619 On a much smaller scale, one single greisen vein within the Beauvoir granite provided a similar set of

620 evidence for several boiling cycles. Centimeter-size quartz crystals exhibit FI evidence for a

621 succession of boiling episodes (see above; Fig. 2.B,D,G), and the FI Th recorded from the first to the

622 last growth zones are practically the same. Such occurrence confirms the fact that numerous boiling

623 episodes took place during the greisen fluid evolution, of which only a few could be evidenced in this

624 study. Fluid boiling during greisen alteration plays a critical role in concentrating rare metals into the

625 fluid phase, despite the antagonistic effect of possible dilution by mixing with meteoric fluid when

626 the system becomes open to the surface. A combination of successive boiling (concentrating metals

26

627 in fluid) with subsequent flashing (initiating the precipitation) appears to be an effective mechanism

628 to form a rare metal deposit (less so for metals with an affinity for the vapor phase, e.g., Cu, cf.

629 Williams-Jones and Heinrich, 2005).

630 Stibnite mineralization

631 The two distal Sb deposits, “Nades veins” in the schist and “Capitraux vein” in the Pouzol Servant

632 granite (Fig. 1), show clearly different values of FI Th and different quartz textures, which probably

633 points to a different origin for their respective Sb mineralizations. Based on quartz chemistry,

634 Monnier et al. (2018) draw the attention to a possible genetic link between greisen fluid and Sb

635 mineralization at Nades. This hypothesis is not contradicted by the FI data from this study (features

636 of Th and FI petrography are similar for the Nades veins and Suchot-Beauvoir greisen veins), which

637 suggest Nades may correspond to late stages of greisen activity (Fig 15.F). Nevertheless, salinity is

638 slightly higher in Nades vein FI, which can probably be attributed to interaction of this fluid with the

639 schist. Also, the consequent spread of Th recorded by FI from the Nades veins would indicate

640 continuous Sb precipitation during fluid cooling, which contrasts with the Capitraux vein, where the

641 FI and quartz texture record a single, sharp episode.

642 Comparison with other W deposits

643 As far as we know, there is no equivalent of the entire multistage W mineralization such as occurred

644 in the Echassière district, worldwide. Nevertheless, each mineralizing event can be taken individually

645 as an analogue of other wolframite deposits. An example is the Maoping deposit in China (Legros et

646 al., 2019) where a topazification episode shows similar features to the first (F-rich) greisen episode

647 that affected the La Bosse stockwork, both in terms of mineral assemblage (topaz, Fe-Li-micas,

648 wolframite and cassiterite), and characteristics of topaz-hosted FI, i.e., relatively high Th (ca. 330-

649 340°C), low salinity (ca. 7 wt.% eq. NaCl), and no sign of boiling. However, formation of quartz veins

650 and evidence of fluid mixing with a meteoric fluid, contrast with the topazification alteration

27

651 observed at the Echassières district, which was pervasive and unconnected to the surface. The

652 features observed in FI from the Beauvoir greisen fluid correspond to more common wolframite vein

653 type deposit. As an example, fluids in quartz veins from the Shirenzhang, Meiziwo and Yaoling W

654 deposits displays similar properties (Th and salinity; Jiang et al., 2019) as those in proximal veins in

655 the Echassières district. They were interpreted as an orthomagmatic fluid which underwent boiling

656 and subsequently precipitated wolframite ± cassiterite in quartz veins outside of the granitic sources.

657 U-Pb dating of cassiterite from greisen and W-Sn-bearing quartz veins in the Maopping deposit (Chen

658 et al., 2019), provides proof for a coeval formation of greisen and distal veins and confirms that

659 greisen alteration is a major depository for W-Sn mineralization. Globally, the “five floor” model

660 (metal zoning in function of distance from the greisenized granite), developed for vein-type greisens

661 in China (Gu, 1982; see also Mao et al., 2013; Zhao et al., 2017; Wang, 2019b) to explain distal

662 wolframite-bearing quartz veins related to greisen alteration, is compatible with the processes that

663 triggered precipitation of wolframite c during the Beauvoir greisen episode. For instance, similarly to

664 formation of wolframite c, the role of extensional fracturing is emphasized for the multistage Xitian

665 W-Sn and Pb-Zn ore deposit, where several scheelite generations involved cyclic reactivation of fault

666 systems (Liu et al., 2019a) with repeated fluid boiling (Liu et al., 2019b).

667 Above, we highlighted that flashing was responsible for the precipitation of wolframite c, whereas

668 boiling was inefficient. To our knowledge, this is the first time that flashing is clearly identified as the

669 process generating W mineralization, while this mechanism is largely invoked in other metal deposit,

670 such as Au-bearing quartz lodes (Weatherley and Henley, 2013; Moncada et al., 2017) or porphyry Cu

671 (Yasami and Ghaderi, 2019). However, several studies on greisen systems do recognize one boiling

672 episode as the key factor to trigger W precipitation (e.g. Jiang et al., 2019; Li et al., 2018). It is

673 possible that at least in some of these occurrences a succession of boiling/flashing stages, as shown

674 at Echassieres, took place. A drawn-out mineralization sequence involving cyclic boiling is a

675 frequently recorded process in porphyry systems (Calagary, 2004; Ni et al., 2015b; Wang et al., 2017).

28

676 Several studies, particularly in recent years, suggest that FI in gangue minerals may not provide the

677 same data as those investigated in Sn-W ore minerals, particularly for quartz which shows

678 systematically lower FI Th (e.g., Campbell and Panter 1990; Moura et al., 2014; Ni et al., 2015a; Legros

679 et al., 2019). Nevertheless, in most of these studies, petrographic textures clearly show that the

680 gangue minerals postdate the ore. This is not the case for wolframite b and c at Echassières, where

681 extraordinary textural relationships between the second quartz generation and wolframite c in the

682 Mazet veins provide undisputable evidence of synchronous crystallization of the two minerals (Fig. 6;

683 see also Fig. 4 in Monnier et al., 2019). The timing of crystallization of wolframite b in the stockwork

684 compared to that of topaz I is not as obvious. However, wolframite b crystals locally occur as

685 inclusion in a single topaz crystal as well as in fractures that affected this mineral, implying that topaz

686 and wolframite are contemporaneous. Also, quartz recrystallization during wolframite b precipitation

687 (documented in Monnier et al., 2019) trapped the same FI population as topaz I. Simultaneous

688 crystallization of topaz and wolframite is also suggested for the Maoping deposit (China; Legros et al.,

689 2019), for which FI in wolframite and topaz display very similar Th.

690 In summary, this work confirms that, granted certain conditions such as very detailed petrographic

691 observations and a large set of FI data, studying FI in gangue minerals can provide valuable

692 information to reconstruct the behavior of the ore fluids. Different, likely complementary, constrains

693 are obtained from studies where only FI in ore minerals are considered. Obviously, studying FI from

694 both gangue and ore minerals permit the best comprehension of a system.

695 Conclusions

696 The following conclusions can be drawn from the fluid inclusion investigation on W mineralization in

697 the Echassières area.

698 1. Although, at Echassières, three distinct episodes of W mineralization have been recognized (cf.

699 Monnier et al., 2019), we could not find primary fluid inclusions coeval with the first generation

29

700 of wolframite (type a). The second generation, wolframite b, crystallized at ca. 400 to 550°C

701 while the great majority of wolframite c at ca. 250 to 260°C.

702 2. Cooling was the principal mechanism responsible for precipitation of wolframite b. Flashing of

703 the fluid during fracturing, recorded by fluid inclusions at the Mazet veins notably, was the main

704 cause for wolframite c crystallization. Therefore, this study confirms that both cooling and

705 depressurization are factors that can trigger formation of wolframite ore.

706 3. At Echassières, the greisen fluids was not constrained to the granites, but circulated to some

707 distance in the country rock inducing the formation of distal veins, a number of which carried W

708 mineralization.

709 4. Greisen alteration at Beauvoir, contrary to most occurrences worldwide, cannot be tied directly

710 to the exsolution of an orthomagmatic fluid but took place after boiling of such fluid, once

711 crystallization was complete. Nevertheless, boiling/flashing episodes continued throughout the

712 evolution of the greisen fluid.

713 5. Fluid inclusion data confirm the textural observations and trace chemical evidence (Monnier et

714 al., 2019) for the presence of two topaz generations in the La Bosse stockwork, topaz I and topaz

715 II, originated from two distinct hydrothermal episodes, respectively, early (topazification) and

716 later (Beauvoir) greisen alterations.

717 The critical observations obtained on the Beauvoir greisen highlighted above (greisen regional

718 influence; role of boiling and flashing; succession of boiling/flashing stages) can provide important

719 information on greisen formation processes and related mineralization, at the interior of a granite

720 body as well as in regional veins, which can potentially improve future ore exploration.

721 Acknowledgments

722 We would like to acknowledge the BRGM (French Geological Survey), the CNRS (French National

723 Center for Scientific Research) as well as the University of Toulouse for providing financial support to

30

724 this study. Two anonymous OGR reviewers provided thorough and constructive criticisms that helped

725 improving the final version of this manuscript.

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1012

43

1013 Table captions

1014 Table 1: Descriptive fluid inclusion data statistics for all populations studied in the Echassières

1015 district.

1016 Figure captions

1017 Figure 1: Simplified geology and location of the study area. Sampling sites are indicated by stars. [A]:

1018 Geology of the Echassières district. The segment a-b localizes the cross section depicted in B (the

1019 empty white star refers to samples in the cross-section in (B). [B]: A sketch of the Beauvoir granite

1020 along the a-b cross-section, constructed using surface outcrop and a GPF bore-hole projection (deep

1021 drilling project of France; located by the dashed line). Vein and dyke thicknesses are exaggerated for

1022 clarity.

1023 Figure 2: Petrography of FI observed in greisen alteration objects, i.e., in pervasive alteration of the

1024 Beauvoir granite and quartz veins. [A]: Outcrop image of strongly greisenized apex of the Beauvoir

1025 granite. Quartz veins related to greisen are mainly subvertical and 5 to 30 cm wide. Locally, the

1026 greisen is altered to kaolinite. [B]: High-resolution scan of a whole thick section from the greisen vein

1027 in (A) showing cm-sized zoned quartz. Zoning is marked by successive FI-rich and FI-poor growth

1028 bands. [C]: Crossed-polarized light (XPL) image of Beauvoir greisen, consisting of quartz and lepidolite

1029 relicts undergoing partial replacement by hydrothermal muscovite and quartz (greisen assemblage).

1030 [D]: Plane-polarized light (PPL) image of the sharp transition between FI-rich and FI-poor growth

1031 zones. Individual fluid inclusions are clearly aligned along the growth planes. [E]: PPL image showing

1032 the common texture of euhedral greisen quartz from Beauvoir. The three zones are characterized by

1033 different populations of FI, ie., scarce L-rich FI in the core, abundant V-rich FI in the first growth band,

1034 and a last population of L-rich FI in the outer rim. [F]: Detail in PPL of a V-rich FI band from

1035 disseminated greisen quartz. [G]: Detail in PPL of FI displaying heterogeneous L/V ratios in a growth

1036 zone of euhedral quartz from a greisen vein. Qtz: quartz.

44

1037 Figure 3: A series of images depicting, at different scales, the genetic relationships of FI trapped

1038 during topazification of stockwork quartz vein of La Bosse stockwork. [A]: Outcrop image showing

1039 subhorizontal quartz veins of the La Bosse stockwork. [B]: High resolution scan of a whole thick

1040 section showing a contact between a quartz vein and the schist host rock, as well as wolframite a and

1041 b. [C]: High resolution scan of a whole thick section exhibiting several crystals of wolframite b and

1042 topaz formed in a quartz vein during the topazification episode. [D]: Combined PPL (left) and XPL

1043 (right) photomicrographs of quartz recrystallized during the topazification episode. The

1044 recrystallization band is characterized by newly formed individual quartz crystal (visible in XPL) and

1045 resetting of ancient FI (visible in PPL). [E]: PPL image of a FI cluster located in a topaz crystal of a

1046 stockwork quartz vein. [F]: Detail in XPL of a FI cluster trapped during the quartz recrystallization. The

1047 bottom-right corner zooms on two FI from this cluster (PPL). Wf: wolframite.

1048 Figure 4: Genetic relationships of FI observed in a topaz vein occurring in the La Bosse stockwork

1049 area. [A]: Photograph of a core sample showing a vein consisting mainly of topaz and wolframite b

1050 that intersects a wolframite a bearing quartz vein. [B]: High resolution scan of a whole thick section

1051 from the sample in A. Wolframite b is coeval to post-dating topaz I. [C]: PPL image of a FI cluster

1052 located in a topaz crystal from the sample in B. [D]: High resolution scan of a whole thick section

1053 exhibiting a reactivated topaz vein. The first sealing episode consists of topaz I and wolframite b, the

1054 second episode crystallizes topaz II and cassiterite, and the last episode consists of barren quartz. [E]:

1055 PPL images of primary FI trapped in topaz I, topaz II and quartz. The different fluid inclusions display

1056 similar shapes and different L/V ratios. [F]: PPL images of an unusually large FI located in topaz I,

1057 taken during heating-freezing runs, at different temperatures.

1058 Figure 5: Genetic relationships of FI examined in altered La Bosse dyke (A-B), and in the stockwork

1059 enclave found at depth within the Beauvoir granite (C-F). [A]: High resolution scan of a polished

1060 section depicting the alteration of the La Bosse dyke during the topazification episode. [B]: PPL image

1061 of a FI cluster in topaz I. In this sample, wolframite (shown in the inset) occurs prior to

45

1062 contemporaneously to topaz. [C]: A photograph of part of the stockwork enclave (quartz vein +

1063 schist) in the Beauvoir granite. [D]: High resolution scan of a whole thick section exhibiting the sharp

1064 boundary between the Beauvoir granite and quartz vein enclave. Dashed lines highlight the

1065 fractures/veinlets mineralized with wolframite c. [E]: PPL image of a muscovite/quartz veinlet

1066 postdating igneous feldspar. Hydrothermal quartz contains abundant FI, detailed in inset. [F]: PPL

1067 image of FI located in topaz I close to the fracture/veinlet. [G]: PPL image of blue apatite from a

1068 greisen fracture/veinlet. The FI shown in the inset, is elongated along the C crystallographic axis of

1069 apatite.

1070 Figure 6: Genetic relationships of FI from the Suchot (A-D) and Mazet (E-F) proximal quartz veins. [A]:

1071 Outcrop image showing the metric-sized Suchot vein. [B]: High resolution scan of a whole thick

1072 section showing euhedral quartz 1 and plumose quartz 2. [C]: PPL image detailing the boundary

1073 between a FI-rich and a FI-poor growth zone in quartz 1. FI display irregular shapes but homogeneous

1074 L/V ratios. [D]: PPL image of a FI assemblage exhibiting heterogeneous L/V ratios. The FI are oriented

1075 parallel to the fibers of plumose quartz 2. [E]: High resolution scan of a whole thick section showing

1076 euhedral quartz 1 on the right side, and anhedral quartz 2 with wolframite c, on the left side. The

1077 inset details a cluster of V-rich FI in the second quartz generation. [F]: PPL image of successive cm-

1078 sized growth zones in quartz 1. The inset details FI from a FI-rich zone.

1079 Figure 7: Genetic relationships of FI from Sb-bearing quartz veins at Nades (A-C) and Capitraux (D-F).

1080 [A]: Photographs of a stibnite-rich vein crosscutting the host schist. [B]: High resolution scan of a

1081 whole thin section showing multiple stibnite and quartz veinlets (only the largest one is labelled). [C]:

1082 PPL image of a homogeneous two-phase FI assemblage in quartz next to a large stibnite crystal. [D]:

1083 Photograph of a pluricentimetric quartz vein crosscutting the Pouzol Servant granite, showing

1084 stibnite mineralization. [E]: High resolution scan of a whole thin section showing radial texture in

1085 quartz (star quartz). Stibnite occurs interstitially to the star quartz. [F]: PPL image of tiny stibnite rods

1086 and fluid inclusions localized in the external part of a star quartz crystal, indicating that the quartz

46

1087 crystallization ended only after the initiation of stibnite formation. The inset details two FI typical of

1088 the stibnite zone in the star quartz.

1089 Figure 8: A sketch depicting the principal alteration objects examined in the Beauvoir granite, their

1090 crosscutting relationships, and the main textural relationships of the FI populations within their

1091 trapping context. Th and salinity are also given.

1092 Figure 9: A sketch depicting the principal alteration objects examined in the La Bosse stockwork area,

1093 their crosscutting relationships and the main textural relationships of the FI populations within their

1094 trapping context. Th and salinity are also given.

1095 Figure 10: A sketch depicting the regional veins examined in this study, their crosscutting

1096 relationships and the main textural relationships of the FI populations within their trapping context.

1097 Th and salinity are also given.

1098 Figure 11: Histograms displaying the homogenization temperature (Th) data for the different FI

1099 populations investigated in this study (labelled for each diagram). All diagrams are displayed at the

1100 same scale. References are given in each diagram to the Figures illustrating the corresponding FI

1101 populations. In diagram [G], Th values for apatite a (contemporaneous with wolframite c) are

1102 distinguished from those of other apatite grains (b).

1103 Figure 12. Diagram in pressure-temperature space, depicting isochores for the different FI

1104 populations, lithostatic and hydrostatic gradients, and the salt-free L-V phase boundary (thick line in

1105 C; the black dot represents critical conditions). Because of the elevated number of FI monitored and

1106 the normal distribution of Th value for each FI population, we used the median Th and salinity values

1107 (notified in Figure 8, 9 and 10) for calculating isochores of each FI populations. Isochores related to

1108 greisen paragenesis are colored in orange while grey color is attributed to the topazification episode.

1109 For better visibility, only selected isochores are represented. [A]: Median isochores for the different

1110 fluid inclusion population (See Figure 8) from a: greisen quartz vein. b: Suchot vein (quartz 1). c:

47

1111 Suchot vein (quartz 2). d: Mazet vein (quartz 1). e: L-rich rim of disseminated greisen quartz. f: Quartz

1112 from polyphased topaz vein. g: population B of recrystallized quartz in stockwork. h: L-rich core of

1113 disseminated greisen quartz i: greisen apatite a. j: topaz vein in stockwork enclave of Unit 3. k: topaz

1114 in stockwork above Unit 1. l: population A of recrystallized quartz in stockwork. m: V-rich growth

1115 zone of disseminated greisen quartz. n: Mazet vein (V-rich quartz 2). Also shown are a strong

1116 geothermal gradient (150°C/km) and a more conservative crustal gradient (30°C/km). [B]:

1117 interpretation of the P-T conditions for the topazification fluid. [C]: Pressure-temperature path

1118 showing the evolution of the greisen fluid and mineralizing episodes, as constrained by isochores and

1119 other FI properties. For simplicity, the greisen fluid is represented as one single connected fluid,

1120 affected by all boiling/flashing episode evidenced in this study in the Echassières district. It is a

1121 generalization, greisen-forming fluids have in reality different behavior as a function of locality, with

1122 local boiling/flashing occurrence. Boiling and flashing episodes are represented by the corresponding

1123 star symbols (cf. legend in inset). [1] and [2] refer to several episodes of depressurization recorded

1124 throughout greisen fluid evolution, evidenced by quartz textures and boiling assemblages such as

1125 depicted in the left-hand-side inset. Diagram drawn using the software FLUIDS 1 (Bakker, 2003). The

1126 equation of state used for determining the freezing point depression of H2O-NaCl solutions is from

1127 Bodnar (1993), thermodynamic properties of the (H2O-NaCl) system - from Archer (1992), and

1128 equation of state for isochore calculation - from Bodnar and Vityk (1994). Because of the very low

1129 salinities (approaching zero) obtained from FI in quartz 2 from the Mazet vein, we monitored the

1130 latter assuming a salt-free system. The thermodynamic properties were taken from Wagner and

1131 Pruss (1993) and the equation of state for isochore calculation from Haar et al. (1984).

1132 Figure 13. Boxplot diagram displaying salinity values for the two greisen fluids. Data for the Beauvoir

1133 greisen fluid (second greisen event) is organized as a function to distance from the Beauvoir granite

1134 and type of fluid (homogeneous or unmixed).

48

1135 Figure 14. [A]: Histogram reporting Th for FI trapped in topaz I and II. The sharp peak around 380°C is

1136 marked by FI from topaz I. A more diffuse peak (ca. 250 to 300°C) is defined by FI from topaz II and

1137 secondary FI from topaz I. [B]: Histogram reporting salinities in NaCl wt. % eq. for the two topaz

1138 generations.

1139 Figure 15. A schematic model for the evolution of greisen alterations in the Beauvoir granite system.

1140 Fluid inclusion textural details and data are provided schematically for parts of this sketch in Figures

1141 8 to 10. [A]: Emplacement of the La Bosse stockwork quartz veins, accompanied by W mineralization

1142 (wolframite a). Fluid inclusions related to this event could not be found (more information about La

1143 Bosse stockwork in Monnier et al., 2019). [B]: First greisen alteration (F-rich), responsible for

1144 topazification of the stockwork and wolframite b mineralization. Fluid inclusion isochores are

1145 consistent with an elevated geothermal gradient, suggesting the presence of a concealed high-level

1146 intrusion. [C to F]: Emplacement of the Beauvoir granite and subsequent development of the second

1147 greisen event (HO-rich). Alteration is pervasive at high temperature while at lower temperature,

1148 during fracturing, vein development prevails. [C]: Orthomagmatic stage, predating the greisen

1149 alteration. The microthermometric study in Cuney et al. (1992) reveals the exsolution of magmatic

1150 brines (circles in the granitic cupola), fracturing and subsequent boiling which triggered fluid

1151 unmixing to form a brine and a low-salinity fluid. [D]: The latter fluid initiated pervasive

1152 greisenization of the Beauvoir granite. In this study, the FI with highest Th trapped in greisen indicate

1153 lithostatic pressures, suggesting sealing of the connections with the surface that had occurred during

1154 stage C. Only few fractures occur in the host rock and fluids remained essentially within the granitic

1155 cupola, at lithostatic pressure. At the end of this stage, intense fracturing connected for a second

1156 time the Beauvoir granite to the surface, causing important pressure drops and subsequent

1157 vaporization of the greisen fluid. [E]: Pervasive alteration persisted in the Beauvoir granite. Fracturing

1158 allows migration of the greisen fluid, reactivating some of the topaz veins and forming W-mineralized

1159 quartz veins (wolframite c) during local depressurization. [F]: At this stage, alteration only occured in

1160 quartz vein selvages. Fluid transport was very efficient with formation of numerous quartz veins

49

1161 within and outside the Beauvoir granite. In all panels, the presence of a chemical element denotes

1162 the timing of mineralization.

1163 Figure 16: Illustration of the role of fluid pressure (Pf) variation in fracturing and mineralization. [A]:

1164 Conceptual Mohr-Coulomb diagram. A Pf increase inhibits lithostatic constrains and shifts the Mohr

1165 circle to the left (effective stress is equal to σn-Pf), triggering subvertical fracturing in an extensive

1166 tectonic regime. τ: Shear stress. σn: Normal stress. α: angle of the plane fracture with the normal

1167 stress. Modified after Sibson and Scott (1998) and Bons et al. (2012). [B]: Variation of fluid pressure

1168 during fracturing/sealing cycles. Ore deposition is triggered by instantaneous Pf decrease. Modified

1169 after Sibson et al. (1988).

50