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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
To cite this version:
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�
Contrasting fluid behavior during two styles of greisen
1
alteration leading to distinct wolframite mineralizations: the
2
Echassières district (Massif Central, France)
3
Loïs Monniera, Stefano Salvia, Victor Jourdana, Souleymane Salla, Laurent Baillyb, Jérémie 4
Melletonb, Didier Béziata 5
a Géosciences Environnement Toulouse (GET), CNRS, IRD, UPS, CNES. Université de Toulouse, 14 6
avenue Edouard Belin, 31400 Toulouse, France 7
b Bureau de Recherches Géologiques et Minières (BRGM), 3 Avenue Claude Guillemin, 45000 Orléans, 8
France 9
Corresponding author: stefano.salvi@get.omp.eu 10
Highlights
11
Fluid cooling, without evidence of sharp pressure variations, is the main cause for one 12
wolframite mineralization episode.
13
Fluid flashing (vaporization), triggered by fracture-induced pressure drops, initiated 14
crystallization of a second type of wolframite mineralization.
15
Greisenization of the Beauvoir granite is a continuous process (ca. 400°C down to 190°C), 16
occurring at high temperature as pervasive alteration of the granite body, while at lower 17
temperature it is localized to a vein system.
18
Globally, greisen-forming fluids have a strong potential for transporting and precipitating W.
19
In greisen systems worldwide, regional veins distal to the altered causative magmatic body 20
can be mineralized by greisen-forming fluids.
21
Abstract
22
The Echassières district of central France hosts diverse magmatic and magmatic- hydrothermal 23
deposits of rare metals, mostly related to the well-known Beauvoir granite. Tungsten mineralization 24
crops out at three distinct wolframite occurrences, the two most important of which are related to 25
two distinct magmatic bodies, emplaced ca. 335 and 310 Ma (Monnier et al., 2019). The 26
mineralization occurred at 335 Ma formed during a hydrothermal episode marked by precipitation of 27
topaz replacing quartz in a stockwork system and as veinlets in the surrounding schist. Fluid 28
inclusions in topaz and quartz display similar features, i.e., all have low salinity, contain significant 29
amount of LiCl, display constant liquid/vapor ratios, and homogenized within a narrow temperature 30
range (Th ≈ 380°C). No evidence for fluid pressure variations was observed, and temperature 31
decrease is considered to be the main cause for wolframite deposition. The younger W 32
mineralization is related to greisenizing fluids that altered the Beauvoir granite and generated several 33
quartz (± topaz and apatite) veins. All greisen-related fluid inclusions display low salinity, however, Th
34
are spread from ca. 190 to 400°C, and several populations exhibit heterogeneous liquid/vapor ratios 35
while others consist of only vapor-rich fluid inclusions. Respectively, these populations are 36
interpreted to have been trapped during boiling or flashing (vaporization) of the fluid. In contrast 37
with the other regional veins, flashing was particularly intense in the Mazet veins, which host the 38
bulk of the last wolframite generation. Consequently, it is proposed that flashing is the key factor 39
that triggered W precipitation.
40
This work highlights the role of two physical parameters, pressure and temperature, whose 41
variations played a preponderant role on wolframite mineralization. It documents, in depth, an 42
example of greisen fluid evolution, providing critical information on W behavior in orthomagmatic 43
fluids, and on greisen-related rare-metal deposits.
44
Introduction
45
An important proportion of felsic igneous bodies emplaced in the upper part of the crust, particularly 46
the more evolved, rare-metal enriched, and/or peraluminous varieties, exhibits greisen alteration to 47
some extent. Greisenization is characterized by replacement of igneous minerals by various 48
proportions of muscovite, quartz and topaz (Štemprok, 1987), ± tourmaline, apatite, HFSE-bearing 49
minerals, to mention the most common (Pirajno, 2009). It is well established that greisen alteration is 50
caused by magmatic-related, acidic fluids that trapped fluid inclusions (FI) showing a wide range of 51
homogenization temperatures (Th; 200 to 450°C; e.g., Burt, 1981; Štemprok, 1987; Halter et al., 52
1998; Cui et al., 2019), and, in the case of rare metal granites, low salinity (<10 wt.% NaCl eq.;
53
Charoy, 1981 ; Cuney et al., 1992; Dobeš et al., 2005; Breiter et al., 2017a). Similarly, FI associated 54
with W-Sn ore deposits display for the most part low salinity and range in homogenization 55
temperature from 200 to 400°C (e.g., Naumov et al., 2011). Based on this evidence, as well as on 56
structural, petrographic, geochemical and geochronological data, numerous models propose that the 57
same fluids (with possible local evolution or mixing etc.) that form greisen alteration are also 58
responsible for the formation of the surrounding wolframite (± cassiterite)-bearing stockwork (e.g., 59
Štemprok, 1987; Pirajno, 2009; Halter et al., 1998; Williamson et al., 1997; Yokart et al., 2003; Mao et 60
al., 2013; Zhao et al., 2017; Korges et al., 2018; Monnier et al., 2018; 2019; Zheng et al. 2018; Chen et 61
al., 2019). A remaining challenge to the understanding of greisen-related wolframite mineralization 62
are the different factors controlling wolframite precipitation, particularly its location, i.e., in quartz 63
veins that are quasi systematically in the host rock and not in the greisenized granite.
64
To provide an answer to this question, numerous studies have nourished a recent debate on W 65
transport/deposition mechanisms. Lecumberri-Sanchez et al. (2017) suggest alteration of the host 66
rock as a key factor for providing the Fe and Mn necessary for wolframite ((Fe,Mn)WO4) 67
precipitation. However, Heinrich (1990) and Yang et al. (2019) argue that orthomagmatic fluids 68
contain sufficient Fe and Mn to permit wolframite precipitation. This affirmation is verified by LA-ICP- 69
MS analyses of magmatic-related FI, which record high amounts of Fe and Mn, in addition to W 70
(Audétat et al., 1998; Harlaux et al., 2017; Yang et al., 2019). Yokart et al. (2003), Legros et al. (2019) 71
and Liu et al. (2018) propose mixing between orthomagmatic and meteoric fluids as the cause for 72
wolframite deposition, while Pan et al. (2019) suggest an input of sedimentary fluids as instrumental 73
in precipitating wolframite. Based on microthermometric data, Korges et al. (2018) and Jiang et al.
74
(2019) propose that depressurization, triggering boiling of the fluid, is the main factor for formation 75
of greisen alteration and simultaneous wolframite-bearing veins. On the other hand, Ni et al.
76
(2015a), Li et al. (2018) and Chen et al. (2018) suggest that simple cooling during fluid transport is 77
sufficient to induce wolframite precipitation. The experimental data of Wood and Samson (2000) 78
confirm that, given a sufficient W concentration in the fluid, cooling and depressurization can control 79
wolframite precipitation, as also proposed by Yang et al. (2019) to explain wolframite precipitation as 80
an infill along fractures. Liu et al. (2018), based on numerical modelling, confirm the important role of 81
repeated depressurization episodes, caused by cyclic fracturing. Concerning the fluid chemistry, 82
Wang et al. (2019a) and Wang et al. (2020) show, respectively, that salinity and CO2 have only a 83
minor impact on W mobility, whereas the role of pH seems critical (see also Wood and Samson, 84
2000), notably the pH increase during interaction with graphite-rich schist host rock (O’Reilly et al., 85
1997).
86
The Echassières district in central France is well suited to investigate the role that different 87
parameters might play on the origin of W mineralization. In this area, most of the W is in the form of 88
three wolframite generations related to three distinct hydrothermal events, separated in time 89
(Monnier et al., 2019). The earliest W episode occurred in the form of wolframite a, outcropping in a 90
stockwork vein system. The bulk of the W stock consists of a wolframite generation that precipitated 91
during a topazification event (wolframite b), attributed to percolation of a F-rich greisenizing fluid. A 92
last major mineralization event (wolframite c) took place during OH-rich greisen alteration following 93
emplacement of the highly evolved Beauvoir granite. However, occurrence of wolframite c is uneven;
94
it is never found in greisenized granite, but occurs in large amounts in a swarm of mineralized 95
greisen-related quartz veins emplaced in the vicinity of the granite. Interestingly, other veins that are 96
interpreted to have the same origin are barren (Monnier et al., 2018). A previous study ruled out a 97
possible effect of contrasting composition of the greisen-forming fluid between granite and veins to 98
explain this peculiar mode of occurrence (Monnier et al., 2018). The mineral paragenesis of these 99
two wolframite generations suggest similar fluid properties (low-CO2, strong acidity and low salinity;
100
Monnier et al., 2019), indicative of greisen affinity (Štemprok, 1987).
101
Here, we report the results of a FI microthermometric study and investigate the role that two 102
intensive physical parameters, pressure and temperature, may have played on these mineralizations.
103
This study presents an important data collection of FI microthermometric features (more than 800 104
measures) in the Echassières district, and highlights the opposite behavior of fluids at the origin of 105
wolframite b and c. At a global scale, this work clearly evidences two physical constraints on 106
wolframite mineralization, and provides critical information on the greisen alteration processes.
107
In addition to W, in contrast with other well-known Variscan W districts in Europe (e.g., Panasqueira 108
district: Noronha et al., 1992; Erzgebirge mountains: Breiter, 2012; Cornwall district: Campbell and 109
Panter, 1990) it is possible that greisen alteration of the Beauvoir granite may be linked to distal Sb 110
mineralization in the Nades area. A similar metal association is commonly reported in some W 111
deposits in China (Hu et al., 2017; Wang et al., 2019b). This genetic relationship is supported by 112
quartz trace element composition (very similar signatures and high Sb content for quartz from 113
greisen and Nades vein; Monnier et al., 2018), notably the Sb content in greisen quartz which is >1 114
ppm with median value equal to ca. 5 ppm, corresponding only to quartz associated to Sb 115
mineralization according to data from literature (Rusk et al., 2011; Pacák et al., 2019). In this work, 116
the characteristics of the fluids associated with the stibnite-quartz veins cropping out at the south of 117
Beauvoir granite will be also discussed.
118
Regional geology
119
Located in the northern part of the French Massif Central (Fig. 1), the Echassières district is hosted by 120
the Sioule metamorphic series. The district is bounded by the crustal-scale strike-slip Sillon Houiller 121
fault on the west, by the Saint-Gervais granite and a formation known as anthracite tuff on the south, 122
while the Tréban granite and Cenozoic sediments limit it on the North and East, respectively. The 123
Sioule series consists of three metamorphic units which form an inverted metamorphic sequence 124
structured in two major antiforms. Two granitic systems, the Beauvoir/Colettes plutonic pair and 125
Pouzol-Servant laccolith intrude the deeper para-autochtonous unit of the series, respectively, in the 126
northern and southern antiform (Fig. 1). The Sioule series records mostly the peak of barrovian 127
metamorphism (ca. 600°C and 7 kbar for the para-autochtonous unit; Schulz et al., 2001; Schulz, 128
2009) occurring at ca. 360 Ma (Do Couto et al., 2016). It is intruded by the Pouzol-Servant laccolith 129
(ca. 330 Ma; Pin, 1991), during the Visean peak of peraluminous magmatic activity, also recorded by 130
the resetting of the 40Ar/39Ar systematics in metamorphic micas (ca. 333 Ma, Faure et al., 2002; Do 131
Couto et al., 2016), caused by associated hydrothermal activity. The Beauvoir/Colettes granitic 132
system was emplaced in the series during the late extensional orogenic stage of the Variscan belt (ca.
133
310 Ma; Duthou and Pin, 1987; Cheilletz et al., 1992; Melleton et al., 2015).
134
Mineralization events
135
The Echassières district is a remarkable site involving a complex sequence of events, many of which 136
mineralized mainly in Sn, W, Sb, Li, Nb-Ta. The earliest occurrence is the La Bosse stockwork (Fig. 1.A- 137
B), a swarm of sub-horizontal quartz veins emplaced contemporaneously to multiple aplitic dykes 138
before regional metamorphism. The stockwork veins contain minor wolframite (ferberite/hubnerite 139
ratio ≈ 8.4; calculated using atomic proportions in Monnier et al., 2019), commonly altered, referred 140
to as “wolframite type a”. However, it was impossible to identify primary fluid inclusions linked to its 141
formation, notably due to extensive quartz recrystallization during metamorphism, and percolation 142
of several subsequent generations of fluids. Therefore, this generation was not considered further in 143
this study.
144
After metamorphism, during the Visean magmatism, a series of topaz veins crosscut the stockwork 145
and the surrounding schist, locally replacing partially dissolved quartz veins. This topazification event 146
was accompanied by pervasive F-rich alteration (topaz ± F-Li-rich micas; first greisen event) and 147
precipitation of abundant wolframite (type b; ferberite/hubnerite ratio ≈ 3.5) and lower amounts of 148
cassiterite in the quartz and the topaz veins as well as in the schist.
149
Followed the emplacement of the Beauvoir/Colettes granitic system (Fig. 1), during the Stephanian.
150
Colettes, the larger body, is a porphyritic two-micas granite while Beauvoir is a highly-evolved albite- 151
lepidolite-topaz equigranular granite, well known for its rare-metal content, rich in cassiterite, 152
colombo-tantalite and pyrochlore (Aubert, 1969; Wang et al., 1992). The apical part of the Beauvoir 153
granite shows remarkable enrichment in high-field strength elements (HFSE; ca. 50 ppm of W, 100 154
ppm of Nb, 150 ppm of Ta, and up to 1000 ppm of Sn), although Zr and Hf are strongly depleted 155
(Raimbault et al., 1995). In addition to Li, lepidolite is also enriched in Rb and F. Niobium and Ta are 156
concentrated in columbo-tantalite group minerals, Sn in cassiterite, and pyrochlore-group minerals 157
contain important quantities of U, Nb, Ta, W (Fonteille, 1987; Cuney et al., 1992; Wang et al., 1992).
158
The Beauvoir granite (in particular its apex) exhibits an important greisen alteration (Fig. 1.B) second 159
greisen event) consisting of replacement of igneous minerals by muscovite, quartz and apatite. At 160
Beauvoir, greisen alteration is not accompanied by rare-metal mineralization, and igneous cassiterite 161
is replaced by muscovite. However, in the host rocks next to the Beauvoir granite, cassiterite 162
precipitated together with topaz , during reactivation of the topaz veins formed during topazification.
163
This superimposed greisen alteration also caused minor cassiterite, colombo-tantalite and wolframite 164
(type c; hubnerite/ferberite ratio ≈ 0.3) precipitation in quartz veins in the vicinity of the Colettes and 165
Beauvoir granites (Fig. 1.A; known as ‘proximal veins’; Monnier et al., 2018), with the exception of 166
the Mazet veins, where wolframite c is very abundant. More distal from this intrusive system, occurs 167
a set of quartz veins, characterized by the presence of sulphides. One of these occurrences consists 168
of the Nades stibnite veins, interpreted to have derived from the greisen fluids (Monnier et al., 169
2018).
170
The last hydrothermal episode consists of late-stage kaolinization of the Beauvoir and Colettes 171
granites, mostly overprinting the greisen alteration (Charoy et al., 2003). Wolframite was also 172
altered, commonly replaced by W-rich goethite. Some mineralized occurrences in the Echassières 173
district, e.g., Sb veins of Pouzol-Servant granite and Cu-Sn sulphide veins of the Chaillat locality, 174
remain poorly genetically constrained.
175
Sampling and analytical methods
176
Most of the samples used in this study were collected in the field, from the Beauvoir open pit, 177
Colettes granite, and in the Suchot area (near the town of Echassières, France; Fig. 1). In addition, 178
several samples, representative of the different facies at depth, were taken from the GPF (Deep 179
Geology of France) drill-hole series collection (hole # 1) (Orléans, France). Finally, samples from the 180
Mazet wolframite mineralization were obtained from the French Geological Survey (BRGM, Orléans, 181
France) collection, because the mine site has since been rehabilitated and does not crop out any 182
longer.
183
The mineralogy and textural relationships were investigated using optical microscopy. Fluid inclusion 184
studies of quartz and topaz were done using double-polished 0.2 mm-thick wafers.
185
Microthermometric measurements were carried out using a Linkam THMGS 600 heating-freezing 186
stage, mounted on an Olympus BX-51 microscope. Measurements were performed at the GET 187
laboratory, following the procedures outlined by Roedder (1984) and Shepherd et al. (1985). The 188
stage was calibrated against pure H2O synthetic inclusions (0 and 374.1°C), supplied by SynFlinc, and 189
pure CO2-bearing natural inclusions (–56.6°C) from Camperio (Ticino, Switzerland). Measurements 190
near and below 0°C are accurate to 0.1°C and to 1°C at higher temperatures. Heating rates were 191
0.2°C/min when phase transitions were approached. Cryogenic experiments were carried out before 192
heating experiments to avoid the risk of inclusions decrepitating. Salinity (S) of fluid inclusions, 193
expressed as wt.% eq. NaCl, was calculated based on the temperature of final ice melting (Tm) and 194
the equation of Bodnar (1993) (S = -1.78 Tm + 0.0442 Tm2
+ 0.000557 Tm3
). It has been suggested that 195
primary FI commonly occur as individual isolation or groups along growth zones of quartz or healed 196
micro-fractures, whereas secondary FI tend to occur in trails and go through quartz grain boundaries 197
(Goldstein and Reynolds, 1994). However, using the above criteria classifying fluid inclusions is not 198
always feasible because the crystal growth banding in quartz cannot be always observed in these 199
three deposits. Therefore, apart from the typical primary FI along quartz growth zones, other FI data 200
on quartz in this study were obtained from isolated FI that might be primary in origin according to 201
Roedder (1984) or FI assemblages which have similar heating-freezing behavior (Fall and Bodnar, 202
2018; and references therein).
203
Fluid inclusion petrography
204
A detailed description of magmatic and hydrothermal rocks of the Echassières district is available in 205
Monnier et al. (2018) and Monnier et al. (2019). Consequently, we focus on some aspects of 206
mineralogy and textural relationships that bear an impact on the petrography of fluid inclusions.
207
Unfortunately, identification of FI population coeval with wolframite a crystallization, and more 208
globally contemporaneous of the formation of quartz veins of the La Bosse stockwork is impossible, 209
considering the reset triggered by regional metamorphism and the subsequent hydrothermal 210
episodes (topazification, greisenization, and kaolinization) which affected the stockwork (Monnier et 211
al., 2019).
212
Beauvoir area 213
Greisen in the Beauvoir granite 214
Greisen alteration is particularly strong at the apex of the Beauvoir granite (Fig. 2.A), where it is 215
expressed by pervasive replacement of magmatic minerals (feldspars ± lepidolite and quartz) by 216
hydrothermal quartz, muscovite ± apatite (Fig. 2.C), and formation of subvertical quartz vein (Fig.
217
2.A). Newly formed, disseminated quartz crystals are euhedral, contrarily to primary igneous quartz 218
which is commonly partly dissolved. Large greisen quartz crystals display, in their median part, a 219
characteristic growth band marked by the presence of FI (Fig. 2.E). These FI have irregular shapes, 220
vary in size from <1 to ca. 20 µm and are mostly composed of a vapor (V) phase (Fig. 2.F). In the core 221
of these crystals, surrounded by the V-rich FI band, there is a population of liquid-vapor (L-V) FI, 3 to 222
4 µm in size, consisting of 70 % liquid (L), of oval to rectangular shape. On the outer part of the V-rich 223
FI band, the rim of these quartz crystals contains disseminated FI that are similar to those found in 224
the quartz core but with higher proportions of L, i.e. ca. 80 % by volume. These three populations of 225
FI are considered as primary, as they are each restrained to a different growth zones. Greisen-related 226
quartz veins within the Beauvoir granite are composed of euhedral cm-sized crystal (Fig. 2.B). A well- 227
marked growth zoning is developed in these crystals, highlighted by a succession of alternating FI- 228
rich and FI-poor bands (Fig. 2.D). FI display irregular shapes and variable liquid/vapor ratios (Fig. 2.G).
229
These FI are very small (rarely > 3µm) and only a few FI of relatively larger size (~5 µm) were 230
monitored. The rare FI observed in the FI-poor bands display homogeneous liquid/vapor ratio, equal 231
to ca. 90 %.
232
La Bosse stockwork 233
- Quartz veins 234
Quartz veins of La Bosse stockwork are subhorizontal, commonly 10 to 20 cm wide (Fig. 3.A), and 235
contain topaz, wolframite and rare cassiterite and colombo-tantalite. Given that several 236
metamorphic and hydrothermal episodes were superposed to the La Bosse stockwork mineralization, 237
we are unable to discern FI potentially contemporaneous to its formation. The oldest FI population 238
that could be clearly identified is the one synchronous to the topazification episode that affected the 239
stockwork after its formation (and before the Barrovian regional metamorphism). Topaz I, common 240
throughout the whole stockwork (Fig. 3.C), contains abundant FI forming a single population, 241
consistently so in each topaz crystal studied. These FI contain a liquid phase filling ca. 50 to 60 % of 242
the inclusion volume (Fig. 3.E), are regular in shape and vary in size from a few to ca. 50 µm. In these 243
quartz veins, occur small sub-veinlets of recrystallized quartz that are connected with crystals of 244
wolframite b (Fig 3.B), which we interpret to have formed during percolation of topazification fluid 245
(see also, Monnier et al., 2019). Formation of these veinlets caused obliteration of the FI already 246
present and trapping of a new FI population (Fig. 3.D). The latter FI show regular shape with varied 247
liquid/vapor ratios (ca 20 to 60 %; Fig. 3.F).
248
- Topaz veins 249
Topaz veins (corresponding to the first topaz generation, topaz I), commonly up to 8 cm wide and for 250
the most part subvertical, crosscut the horizontal quartz veins of the La Bosse stockwork (Fig. 4.A-B) 251
below and above the Beauvoir granite. In addition to topaz, they contain various amounts of 252
wolframite b and lepidolite to F-rich biotite micas series, plus minor rutile and cassiterite. Topaz 253
crystals trap important quantities of FI, which distributed in clusters (primary FI) or along plans which 254
affect only limited portions of a crystal (pseudosecondary FI). FI display tabular to irregular shapes 255
(Fig. 4.C), and the liquid phase occupies ca. 50 to 60 % of the inclusion volume, similarly to the FI 256
described above from disseminated topaz in quartz veins. Some of the topaz veins are reactivated, 257
and sealed by a second topaz generation (topaz II) and, in some cases, by an additional quartz 258
generation (polyphased topaz veins; Fig. 4.D). The second topaz generation contains regularly-shaped 259
primary FI with high proportion of liquid (70 %; Fig. 4.E). Lastly, primary FI occurring in quartz from 260
the cores of these veins exhibit slightly higher proportion of liquid (ca. 70 to 80 %). The majority of all 261
of the FI populations mentioned above from the polyphased topaz veins have regular shapes and 262
measure from ca. 5 to 10 µm.
263
- Aplitic dykes 264
Contemporaneous to the La Bosse stockwork, aplitic dykes consist mostly of K-feldspar relicts (now 265
mostly clays + quartz) and, like the stockwork veins, are strongly affected by metamorphism and 266
subsequent topazification. In these dykes, topaz I is closely associated with wolframite b (Fig. 5.A-B) 267
and contains a homogeneous FI population of regular shape, small size (few µm), with L filling ca. 50- 268
60 % of the total volume.
269
- Stockwork enclave in the Beauvoir granite 270
A particularly interesting sample was collected from a drill core from the Beauvoir granite that 271
intersected an enclave of the La Bosse stockwork consisting of a stockwork vein with its host schist.
272
This sample records all of the hydrothermal episodes related to wolframite crystallization in the 273
Echassières district (Monnier et al., 2018; 2019).
274
The stockwork episode is expressed by quartz and wolframite a, the topazification episode by topaz I 275
and wolframite b, and the Beauvoir greisen event by blue apatite and wolframite c (Fig. 5.C-G). Of 276
these, only the greisen affects the Beauvoir granite (Fig. 5.D). FI in topaz I from the quartz vein share 277
the same features than the other FI already described in this mineral, i.e., a regular, tabular to 278
globular shape, and L filling ca. 50 to 60 % of the total volume (Fig. 5.F). FI interpreted to represent 279
the greisen fluid are found in quartz from muscovite-quartz veinlets altering igneous minerals in the 280
Beauvoir granite (Fig. 5.E), as well as in blue apatite replacing quartz in the stockwork vein (Fig. 5.G).
281
In some instances, grains of apatite show textural evidence of coprecipitation and apatite with 282
wolframite c (cf. Fig. 4.G in Monnier et al., 2019). Fluid inclusions in Beauvoir greisen quartz and 283
apatite are L-rich (70 to 80 % L); those in quartz show regular to rounded shapes, while those in 284
apatite are systematically elongated parallel to the C axis.
285
Regional veins 286
Proximal veins 287
Proximal veins form a network of subvertical quartz veins located in the vicinity or within the 288
Colettes/Beauvoir granitic complex (Fig. 1).
289
- The Suchot vein 290
The Suchot vein measures from 1 to 2 m in width (Fig. 6.A), and consist essentially of two quartz 291
generations, plus minor muscovite and cassiterite. The vein selvages contain quartz and muscovite, 292
similarly to the Beauvoir greisen. In the vein, the first quartz generation is euhedral, up to cm in size, 293
with growth zones exhibiting sequential FI poor and FI rich areas (Fig. 6.B). Differently than for the 294
quartz veins at the interior of the Beauvoir granite, FI in these growth zones show homogeneous L/V 295
ratios, with L comprising ca. 80 % of total FI volume (Fig. 6.C). FI in the second quartz generation are 296
aligned along the direction of the quartz fibers (Fig. 6.B), and display variable L/V volume ratios (Fig.
297
6.D).
298
- Mazet veins 299
Quartz veins at Mazet share several characteristics with the Suchot vein, i.e., geometry (metric in 300
width, subvertical, oriented N/S), presence of two quartz generations. Also similar to that at Suchot, 301
the first quartz generation is euhedral and cm-sized, whereas the second quartz generation presents 302
a not-well crystallized, micro-quartz texture (post-recrystallized H2O-rich colloform silica). The micro- 303
quartz is accompanied by important quantity of wolframite c (Fig. 6.E). As in quartz from Suchot vein, 304
FI located in growth zones of the first quartz generation are L-rich (ca. 80 %; Fig. 6.F). On the other 305
hand, in the rare parts of the second quartz generation where FI could be observed, these display 306
globular shapes and contain only a V phase (Fig. 6.E).
307
Sb-bearing quartz veins 308
- Nades vein 309
At the Nades locality, a large quartz vein consisting of a meter-sized main body and interconnected 310
cm- to mm-sized satellite veinlets (Fig. 7.B) forms a lode of several meters in width. Stibnite occurs in 311
this vein, and textural evidence suggests it is either synchronous with quartz, or later. Its abundance 312
is not related to the dimension of the veins, and locally one can observe veinlet composed entirely of 313
stibnite (Fig. 7.A). Quartz contains pseudosecondary trails of FI, accompanied by tiny acicular stibnite 314
crystals (< 5 µm), as well as FI clusters close to larger stibnite crystals (Fig. 7.C). These FI are 315
irregularly shaped, approximately 5 µm in size, and contain a large proportion of the L phase (ca. 90 316
%).
317
- Capitraux and Cros veins 318
In both localities occur a quartz vein of some 20 cm in width, displaying the same unusual feature, 319
i.e., a radial quartz texture, called “star quartz” in this study, initiated around a small ferrous oxide 320
nuclei. The star quartz crystals, roughly spherical, are about 5 mm in diameter, uncommonly up to 2 321
cm (Fig. 7.E). Sealing of these veins are commonly incomplete, and they can exhibit a strong porosity 322
(up to 50 %). Stibnite occurs in variable amount, and is found mainly filling the porosity between star 323
quartz crystals (Fig. 7.C,E) and, in lesser proportions, as disseminated tiny acicular crystals within the 324
outer rims of quartz. FI are very rare in the core of star quartz, whereas the rims contain clusters of 325
FI, accompanying the stibnite (Fig. 7.F). These FI are generally regular, ca. 5 µm in width, and contain 326
ca. 95 % L.
327
To resume, the FI populations described in the above paragraphs can be grouped into two main 328
groups, based on careful FI petrography and associated hydrothermal alteration parageneses. These 329
correspond to the two main hydrothermal episodes that affected these rocks, after metamorphism, 330
and that were at the origin of the two W mineralization events: 1) A first greisen-forming fluid, which 331
induced topazification, related to wolframite b and predating the granite intrusion; trapped FI in 332
topaz I and in recrystallized quartz of the La Bosse stockwork veins; 2) Greisen alteration related to 333
the Beauvoir granite, which induced muscovitization; trapped fluid inclusions in quartz from the 334
pervasive alteration and subvertical greisen veins within the granite, in blue apatite in the stockwork 335
enclave, in muscovite-quartz veinlets affecting the granite, in topaz II and quartz from the polyphase 336
topaz veins, in proximal veins (Suchot et Mazet) and, possibly, Nades veins (Capitraux and Cros are 337
not sufficiently constrained).
338
Microthermometric results
339
For each FI type investigated in this study, several occurrences of any given FI assemblage were 340
measured from different crystals, to confirm the data repeatability, unless otherwise specified. FI 341
that could be related petrographically to the topazification episode, whether in topaz I or in 342
stockwork quartz, form a rather homogeneous population, whereas the fluid associated with 343
formation of the OH-greisen is recorded in a wide variety of FI types in different hydrothermal 344
minerals. None of the IF monitored in this study show solid phases. Similarly, ice was the only phase 345
that formed upon cooling, confirming the relative low concentration of gases such as CO2. During the 346
heating process, all L-rich and V-rich FI homogenized, respectively, to the liquid and vapor phases.
347
For FI assemblages showing heterogeneous trapping of the L and V phases, only those displaying the 348
lowest vapor/liquid ratios were retained for monitoring.
349
Descriptive statistics of microthermometric data for each FI population are summarized in Table 1.
350
Equivalent salinity and homogenization temperature median values, plus sketches depicting the 351
petrography and the locality of the studied FI populations, are given in Figures 8-10.
352
Salinities 353
During sub-zero heating runs the first occurrence of ice melting was recorded to obtain an estimate 354
of the eutectic point of the system (Te). For all FI belonging to the topazification episode the first- 355
melting, when observed, occurred at temperatures just above or equal to -72°C (Fig. 4.F). Such low 356
values suggest that Li was likely an important component in the fluid (Monnin et al., 2002; Dubois et 357
al., 2010). Determination of eutectic temperature for the greisen fluid was challenging, as this phase 358
transition was difficult to observe due to the small size of most FI and to their low salinities (see 359
below). However, by applying the cycling technique described by Reynolds (1988), we could record a 360
sufficient number of measurements, which approached and were never lower than -21°C. These 361
measurements were confirmed by the occurrence of a few larger FI where a first melting at -21°C 362
could be observed. Such figure is consistent with NaCl dominating other salt species in the fluid (Te of 363
the H2O-NaCl system is -21.1°C; Bodnar, 1993). Nevertheless, these results are indicative, and it is 364
possible that other salts were present in addition to Na, particularly cations such as K+ and Mg2+, 365
given that their presence does not change significantly the eutectic temperature. The presence of Li+ 366
in the topazification fluid is not surprising, as lepidolite is a common alteration mineral, whereas the 367
alteration paragenesis in the greisen is dominated by the presence of muscovite and quartz. The 368
temperature of final ice melting ranged from -10 to 0°C for all FI from both alteration episodes, with 369
a great majority of data above -4°C. These data indicate relatively low salinities, with 99% of values <
370
10 wt.% NaCl eq. and the majority of values < 4 wt.% NaCl eq. However, a weak trend toward 371
somewhat higher salinities can be distinguished for the greisen fluid, moving away from the Beauvoir 372
granite, e.g. the median salinity is 1 wt.% NaCl eq. for greisen in the granite, while it is 5 wt.% NaCl 373
eq. for quartz from Nades and Suchot veins (Quartz 1).
374
Homogenization Temperatures (Th) 375
All Th values, for each FI population, are detailed in the histograms depicted in Figure 11. FI trapped 376
during the topazification episode record a very narrow range of Th, comprised between 370 and 377
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 378
marked for some FI assemblages in topaz I from the polyphased topaz vein and in recrystallized 379
quartz in the La Bosse stockwork (Figs. 9 and 11.A,H). Beauvoir greisen fluid is recorded in a wide 380
variety of FI populations, which display a range of Th, covering a large interval from ca. 190 to 400°C 381
(median values). Globally, FI trapped during greisen pervasive alteration of the Beauvoir granite 382
(hydrothermal quartz and apatite, micro-fractures and -veinlets in igneous minerals) recorded 383
elevated Th (260-400°C; Fig. 11.G,L,P) with lower values for FI trapped in quartz veins, with no 384
apparent correlation to the distance from the granite (190-260°C; Fig. 11,C,D,E,K,N). Zoned crystals of 385
the disseminated greisen type show markedly different Th for FI located in cores and in the rims. Data 386
from the core show a peak at around 300°C while for the growth band they range from 400 to 450°C.
387
However, FI in the outermost rim of these crystals have lower Th, with a peak at ca 250°C (Fig. 11.P).
388
Zoned quartz crystals of the Suchot veins display a similar trend although the values are lower, i.e., 389
cores have a peak at 200°C while rims peak at about 230°C (Fig. 11.D). A smaller difference can be 390
observed between cores and rims of quartz from the Mazet veins (Fig. 11.N).
391
Discussion
392
P-T conditions 393
Most FI populations described above display homogeneous vapor/liquid ratio suggesting trapping of 394
a supercritical fluid. For these populations, Th represents the minimal estimate for the trapping 395
conditions. In some instances, the occurrence of coexisting L-rich and V-rich FI within the same 396
assemblage (e.g., group or growth zone; Fig. 2.F,G; Fig. 6.D) such as observed in greisen veins within 397
the Beauvoir granite and in Suchot vein quartz, indicates local boiling conditions. In this case, we 398
used the homogenization temperatures of the vapor- and liquid-rich end-members to estimate the 399
actual trapping temperature.
400
Because we do not have independent means for constraining the temperature or pressure of the 401
system, we can assume that the trapping conditions lie along an isochoric path defined by the 402
physical properties of the fluid inclusions, confined to conditions given by a reasonable geothermal 403
gradient for this system. If we consider a common average crust geothermal gradient (ca. 30°C/km), 404
the intersections with some isochores of Beauvoir greisen and topazification FI occur at 405
unrealistically elevated temperatures (more than 600°C and 1000°C, respectively; Fig. 12.A). Given 406
the presence of crystallizing intrusions, we suggest a gradient for our system at about 150°C/km (Fig.
407
12.A). Using this gradient, we obtain trapping conditions that fall within the range for FI populations 408
related to greisen alteration and wolframite mineralization described in the literature (e.g., 409
Williamson et al., 1997; Naumov et al., 2011, Cui et al., 2019; Jiang et al., 2019). Such conditions are 410
coherent with the emplacement of a hot granitic magma body at shallow depth, the Beauvoir 411
granite, proposed to have occurred at ca. 3 km according to Cuney et al. (1992; based on stable 412
isotopic systematics on mineral pairs and on microthermometric results on orthomagmatic FI), and 413
suggest that circulation of at least the Beauvoir greisenizing fluid took place before the granite 414
cooled substantially. This gradient is also consistent with the elevated Th of the FI related to 415
topazification, which suggests that this fluid also originated from a shallow magmatic source, even 416
though such magmatic body does not crop out at the present surface in the Echassières district.
417
While the FI homogenization data for the topazification episode show a very well-defined peak, 418
bracketing the conditions of the topazification fluid to a narrow range (Fig. 12.B), the FI populations 419
related to the greisen-forming fluid record a wider scatter of Th, varying with different quartz 420
generations. Fluid inclusion data show that disseminated quartz in greisen, and quartz from Mazet 421
and Suchot veins, record a similar temperature pattern: in all cases the earliest quartz generation 422
trapped FI with lower Th than the following generation (Table 1; Fig. 8; Fig. 10; Fig. 11.D,N,P).
423
Monnier et al. (2018) have shown that all of these quartz generations have the same trace-element 424
signature. It is practically impossible for two different hydrothermal fluids to have exactly the same 425
trace element chemistry, therefore, we have to exclude the possibility that the second greisen quartz 426
generation precipitated from a different, higher-temperature fluid than the first generation. Instead, 427
this increase in Th can be explained by variations in physical parameters of the same fluid, such as a 428
rise in temperature or depressurization. The sharp textural differences in quartz (euhedral vs fibrous 429
and micro-quartz, both of which indicating rapid silica precipitation) and FI properties in overgrowth 430
and second quartz generations (variable vapor/liquid ratio or vapor-rich), favor the depressurization 431
scenario (Moncada et al., 2012). A possible cause for depressurization is fracturing, which would 432
result in connecting the fluid to the surface. Because fracturing is a punctual event, we can consider 433
temperature to remain constant during the fast crystallization of the quartz immediately after 434
fracturing (adiabatic depressurization; hence producing FI with lower Th but effectively same trapping 435
temperature, cf., point pairs [1] - [2] in Fig 12.C), with pressure dropping to a minimal value 436
constrained either by boiling (on the liquid-vapor curve) or flashing (below the liquid-vapor curve) 437
conditions. Flashing, or flash vaporization, consists of particularly intense boiling where the fluid is 438
instantly transformed to vapor. Evidence for this was observed in greisen disseminated hydrothermal 439
quartz in the Beauvoir granite as well as in the Mazet veins, suggested by the occurrence of 440
populations of FI consisting exclusively of vapor-only individuals. The presence of micro-quartz 441
textures indicating crystallization from amorphous silica, which is common in cases of quartz 442
formation from a vapor-only fluid (e.g., Moncada et al., 2012), confirms this interpretation. From 443
Figure 12.C one can estimate that between crystallization of quartz [1] and quartz [2] generations, 444
fracturing induced pressure drops of ca. 70 Mpa, 45 MPa and 20 Mpa for disseminated greisen, 445
Suchot and Mazet veins, respectively. Plotting the data from Cuney et al. (1992; see also Harlaux et 446
al., 2017) on Fig. 12.C (white arrow) for the orthomagmatic fluid, we note a pressure drop which 447
disconnects the initiation of the greisen process (roughly at lithostatic pressure) to the evolving 448
orthomagmatic fluid (which underwent boiling). It is thus probable that, after exsolution of this fluid, 449
important mineral precipitation or tectonic activity sealed off fractures and porosity, changing the 450
pressure from hydrostatic back to lithostatic (dashed white arrow in Fig. 12.C).
451
Main differences in fluid behavior during deposition of the two types of wolframite 452
For a geothermal gradient of 150°C/km, wolframite b crystallized in a temperature range between ca.
453
400 to 550°C while wolframite c from Mazet vein crystallized between ca. 250 to 260°C.
454
Crystallization temperature of wolframite c in the stockwork was apparently more elevated, as 455
indicated by high homogenization temperatures measured in FI from contemporaneous apatite (ca.
456
350°C; apatite a in Fig. 11.G). Given the relatively large temperature difference between 457
wolframite c from stockwork and Mazet, it is remarkable to see a similar chemical composition. This 458
also indicates that the Fe/Mn ratio variations in wolframite c are apparently not a function of fluid 459
temperature, as recently suggested as a general characteristic (Michaud and Pichavant, 2019).
460
As mentioned in the previous section, pressure variations induced by fracturing played a key role in 461
wolframite c deposition (Fig. 12.C), whereas FI that trapped the fluids that precipitated 462
wolframite b did not record evidence of boiling. Given that decrease in either pressure and 463
temperature is considered to be an efficient process for destabilizing W complexing in the fluid and 464
triggering wolframite precipitation (Wood and Samson, 2000), it is likely that, in the absence of 465
fracture-activated pressure drops, simple cooling could have triggered W saturation in the fluid and 466
crystallization of wolframite b, together with topaz I, during the topazification episode.
467
The salinity values obtained from FI ice melting temperatures for the two greisen generations are 468
generally low, varying from about 0 to 10 wt.% NaCl eq., with most data concentrating between ca. 2 469
and 5 wt.% (Fig. 13). The variations inherent to the Beauvoir greisen group show a slight increase in 470
median values and in overall range with transport distance from the source, while the fluid that 471
underwent unmixing is characterized by slightly lower values (with the exception of quartz 2 from 472
Suchot vein; median value of 5 wt.%). Most salinity values measured in topaz I from the first greisen 473
episode cluster around 3 wt.%, although the entire data population stretches from 0 to ca. 10 wt.%
474
(Fig. 13). Notably, topaz I from the La Bosse dyke displays uncommonly high salinity median values of 475
8 wt.% NaCl eq. Considering the similarities of the FI data for all populations, it is not possible to 476
distinguish between the two greisen episodes based on the fluids' ionic concentration.
477
Trace-element data permit to discriminate the two types of hydrothermal topaz in the La Bosse 478
Stockwork, and to assign them to topazification (topaz I) and Beauvoir greisen alteration (topaz II) 479
(Monnier et al., 2019). FI Th data also show separate clusters for the two topaz types, with median 480
values equals to ca. 270 and 380°C, respectively (Fig. 14.A), as well as slightly higher salinities for the 481
topaz-II generation (Fig. 14.B; see also Th vs salinity plot in Supplementary Material). Nonetheless, 482
there is an overlap in some of the data for the two topaz types in the 240-300°C range (Fig.
483
14.A), indicating dissolution/reprecipitation of the older generation, topaz I, during percolation of 484
greisen fluids and precipitation of topaz II. The Th values for topaz I coincide with data reported in 485
Harlaux et al. (2017; they did not recognize the topaz II generation, less abundant).
486
Lithium concentrations in the fluids responsible for the topazification and second greisen formation 487
seem to be significantly different, as suggested by the low eutectic temperature recorded only for FI 488
from the former (near -72°C). The nature of accompanying micas, i.e., F-Li-rich lepidolite to F-rich 489
biotite (unpublished data) for topazification alteration and OH-rich muscovite for the Beauvoir 490
greisen fluid (Fonteille, 1987), suggests that F and Li are enriched in the former case, but depleted in 491
the latter. Hence, topazification, responsible for wolframite b, share several characteristics with the 492
F-Li-rich greisen type (Štemprok, 1987), suggesting it may correspond to an exogreisen, given that it 493
is not observed within a granite. Sericitic alteration of the Beauvoir granite (wolframite c) is, on the 494
other hand, in accordance with the OH-rich greisen type. Monnier et al. (2019) demonstrated that it 495
was two different granites that sourced the hydrothermal fluids that led to topazification and 496
Beauvoir greisen alterations. The data presented here confirm the occurrence of two greisen 497
episodes and that they were characterized by different fluid chemistry and evolution, as illustrated 498
by their different P-T paths in Figure 12.
499
Mineralizing processes 500
Topazification (wolframite b episode) 501
Topazification clearly affected the La Bosse stockwork, host to wolframite a (Fig. 15.A,B). As 502
mentioned above, no evidence for boiling was observed in fluid inclusions related to topazification 503
(topaz I), suggesting that the hydrothermal process involved in the concomitant precipitation of 504
wolframite b did not involve changes in pressure. The limited occurrence of topaz veins, in addition 505
to very effective pervasive fluid percolation (i.e., rock permeability) detected in these rocks are 506
consistent with limited pressure fluctuations (Fig. 15.B). This can probably be explained by a high 507
quartz solubility in the HF-rich fluid (Ellis, 1973; Mitra and Rimstidt, 2009), which created connected 508
porosity inhibiting local fluid overpressure. Dissolution of quartz permitted to focus fluid circulation 509
in the stockwork quartz veins triggering an important concentration of topaz and wolframite b in the 510
vein. Wolframite b, as well as topaz, are also found in albite dykes. Preferential occurrence of W in 511
Mn-Fe-poor rocks (veins and dykes) rather than in the schist suggests that there was no need for 512
fluid interaction with the latter to provide the Fe and Mn needed for wolframite precipitation.
513
The strong density of FI Th values around 380°C for topaz I could stand to signify a threshold 514
temperature for the topazification episode. As mentioned above, in this case cooling is likely the 515
cause for W saturation and consequent wolframite b crystallization. Another factor that could 516
contribute to decrease wolframite solubility is decreasing of fluid acidity due to fluid-rock interaction 517
(Wood and Samson, 2000) and massive topaz crystallization (e.g., Halter et al., 1996).
518
The relatively high temperature of wolframite b crystallization compared to other deposits would 519
mean that the topazification fluid is strongly enriched in W, accordingly to Wang et al. (2020). Two 520
factors can explain this anomaly. A first one is an uncommon fluid chemistry, more acidic and F-rich 521
than typical fluids at the origin of W deposit; a second one is the potential remobilization of 522
wolframite a of the La Bosse stockwork (cf. Monnier et al., 2019), which increased the amount of W 523
in solution. Although wolframite b crystallizes contemporaneously with topaz, the role of fluoride 524
complexing of W in an aqueous phase is still not well understood (Wood and Samson, 2000).
525
The source of the topazification fluid also remains not well constrained (Monnier et al., 2019). A 526
greisen-like origin after boiling of a previous orthomagmatic fluid should be considered (Fig. 15.B), 527
given that the timing of wolframite precipitation (ca. 335 Ma; Harlaux et al., 2018) corresponds to 528
the Visean peak of peraluminous magmatism in the French Massif Central. This is consistent with the 529
elevated homogenization temperatures of the fluid inclusions.
530
Locally, in the La Bosse stockwork, small sub-veinlets of recrystallized quartz (Fig. 3.D) trapped FI with 531
two populations of FI. The highest liquid volume (70 to 80 %) displays lower Th (ca. 250 to 270°C) 532
than those with liquid volume ca. equal to 60 % (370 to 395°C). The latter are in the same range as 533
the primary FI found in topaz I, indicating that some quartz dissolution/reprecipitation occurred 534
during wolframite b precipitation and topazification. The low-Th FI population has characteristics 535
comparable to those observed in topaz II generation, so probably correspond to a later fluid 536
percolation, most likely the Beauvoir greisen-forming fluid.
537
Beauvoir greisen (wolframite c episode) 538
In a study of fluid inclusions from late magmatic quartz and topaz at Beauvoir, Cuney et al. (1992) 539
documented the exsolution of an orthomagmatic fluid that underwent boiling, to form coexisting 540
brine and a low-salinity fluid (Fig. 15.C), and considered that the latter corresponds to the 541
greisenizing fluid (L3 in their study) (Fig. 12.C). The low salinities and moderate temperatures that we 542
recorded in this study are consistent with this interpretation. The greisen episode related to the 543
Beauvoir granite is a relatively long-lived alteration episode that started at temperatures of ca. 400°C 544
and waned at about 190°C (Fig. 12.C), but which did not involve an evolution in fluid chemistry, as 545
shown by consistent quartz trace composition (c.f., Monnier et al., 2018). A duration stretched over 546
time agrees with a diachronic formation of the different quartz veins during the greisen 547
episode. Greisen alteration initiated at relatively high temperature (400°C) with pervasive alteration 548
which affected essentially the Beauvoir granite (recorded by the core of hydrothermal disseminated 549
quartz) under lithostatic pressure (Fig. 15.D). Subsequently, an important fracturing event affecting 550
the Beauvoir granite connected the greisen fluid to the surface. This produced flashing of the fluid 551
(transition between Fig. 15.D and Fig. 14.E). Rapidly, pressure steadied roughly at hydrostatic values, 552
as recorded by a marked peak of fluid inclusion data at ca. 250-280°C, indicating a period of intense 553
greisen activity. This consisted of pervasive alteration of the granite (rim of disseminated quartz), and 554
vein formation/reactivation in the host rock (e.g., Mazet veins, topaz II), due to local pressure 555
variations (Fig. 15.E). At lower temperature (≤ 230°C), pervasive alteration seemed to be ineffective 556
as indicated by the absence of FI with lower Th in the disseminated quartz oh the granite. Fluid flow 557
was concentrated in veins, inside (greisen veins) and outside the granite body (Suchot vein, Nades 558
vein; Fig. 15.F).
559
Wolframite c is not a common mineral in the stockwork, formed during the flashing episode 560
mentioned above, while mineralization was especially efficient in the Mazet area, likely triggered by 561
a second flashing episode (Fig. 12.C). Antimony mineralization in the Nades area occurred only during 562
the last peak of greisen activity at 200°C. During fluid cooling and development of greisen alteration, 563
salinity remained low (mainly < 5 wt.% NaCl eq.), as already observed in other system (Charoy, 1981;
564
Jiang et al., 2019).
565
Surprisingly, greisen alteration at Beauvoir is relatively poor in F and Li, despite the fact that this 566
granite is strongly enriched in these and other rare metals. Nevertheless, the exceptional P content 567
of the fresh granite (Raimbault et al., 1995) is reflected by the remarkably high abundance of apatite 568
in the greisen. In the Cínovec granite, roughly analogous to Beauvoir (Monnier et al 2018), greisen 569
alteration consists of quartz plus zinnwaldite. The latter is the most F, Li, and rare-metal enriched 570
mica found in these rocks, including igneous varieties. This evidence can be interpreted as indicating 571
an opposite behavior of these elements during greisen alteration of the two granites. At Beauvoir, 572
most of the F and Li are transported by greisen fluids to the surrounding host rocks, as suggested by 573
the low concentrations of these elements found in greisen-altered granite (Merceron et al., 1992;
574
Raimbault et al., 1995). At Cínovec, greisen alteration is mostly confined to the granitic body, 575
retaining these elements as well as ore metals such as Sn and W. We therefore suggest that it is 576
critical to apprehend the extent of mobility of a greisen-forming fluid to understand its role on ore 577
metal transport and deposition (compare with the Cínovec granite: Breiter et al., 2017a; 2017b;
578
2017c; 2019).
579
Greisen is developed only in zones displaying enhanced permeability, i.e. the apex and fractured 580
zones in the Beauvoir and Colettes granites, and fractures in the host schist, whereas dykes of 581
Beauvoir granite intruding the schist, or the La Bosse quartz veins, only show limited signs of greisen 582
alteration (they are mostly kaolinized, cf., Monnier et al., 2019). These results contradict a recent 583
fluid inclusion study, which proposes that FI in the stockwork represent the orthomagmatic fluids 584
derived from the Beauvoir granite (Harlaux et al., 2018). However, at the time of their writing, it had 585
not been recognized that the episode of topazification of the La Bosse stockwork predates the 586
emplacement of the Beauvoir granite (Monnier et al., 2019). Consequently, we suggest that the fluid 587
inclusions from the La Bosse stockwork studied by Harlaux et al. (2017) record the orthomagmatic 588
signature of the magma that sourced the topazification fluid, not that of the Beauvoir granite.
589
Suchot (barren) vs Mazet (W mineralized) proximal veins 590
The distribution of wolframite in the proximal veins raises the question as to why wolframite c is 591
massively localized in the Mazet area and not in other proximal veins, e.g., Suchot vein. A marked 592
difference that was observed in these two localities is that fluid flashing (evidenced by micro-quartz 593
texture and presence of a vapor-rich FI population) took place in the Mazet veins, whereas only 594
evidence for boiling could be recognized in FI from the Suchot veins. This distinction in fluid behavior 595
may explain the restriction of wolframite at Mazet. Indeed, during boiling of a fluid, W is known to 596
strongly fractionate into the liquid phase (Audétat et al., 1998; Harlaux et al., 2017), suggesting that 597
flashing triggered wolframite precipitation by causing vaporization of the totality of the liquid. In case 598
of only limited boiling on the other hand, W could fractionate into the liquid phase without 599
necessarily exceeding its saturation in W, which is likely what happened at Suchot, hence the 600
absence of W mineralization.
601
Cyclic behavior of Beauvoir greisen fluid 602
Before fracturing, greisen fluid circulation was restricted to the granite body, because of the low 603
permeability of the surrounding schist and the sealed stockwork system. When regional shear slip 604
constraints (Gagny and Jacquot, 1987), coupled with increasing fluid pressure, triggered vertical 605
fracturing (Fig. 16.A), the fluid was immediately connected to the surface. This was a very effective 606
drain, focusing the greisenizing fluid circulation though the proximal quartz veins, and limiting mixing 607
with the host schist , as also indicated by investigation of the quartz chemistry (Monnier et al., 2018).
608
During fracturing, sharp pressure drops drastically lowered mineral solubility in the greisen fluid, 609
triggering massive precipitation of new minerals, such as quartz and muscovite. This rapidly sealed 610
the porosity (Moncada et al., 2012; Launey et al., 2019) reducing permeability and increasing fluid 611
pressure, thus initiating a new fracture-seal cycle and wolframite mineralization, all along the cooling 612
path of the fluid (Fig. 12.C and Fig. 16.B; Bons et al., 2012). This process is particularly well recorded 613
by the FI populations in the different greisen quartz generations, from which we obtained large 614
variations in Th, and which provide evidence for several boiling/flashing episodes. Possibly, in 615
addition to hydraulic fracturing, seismic activity played a key role in fracturing during greisen 616
alteration, as suggested by the occurrence of a proximal vein transformed to cataclasite (fault core 617
zone) in the Colettes granite (Monnier et al., 2018).
618
On a much smaller scale, one single greisen vein within the Beauvoir granite provided a similar set of 619
evidence for several boiling cycles. Centimeter-size quartz crystals exhibit FI evidence for a 620
succession of boiling episodes (see above; Fig. 2.B,D,G), and the FI Th recorded from the first to the 621
last growth zones are practically the same. Such occurrence confirms the fact that numerous boiling 622
episodes took place during the greisen fluid evolution, of which only a few could be evidenced in this 623
study. Fluid boiling during greisen alteration plays a critical role in concentrating rare metals into the 624
fluid phase, despite the antagonistic effect of possible dilution by mixing with meteoric fluid when 625
the system becomes open to the surface. A combination of successive boiling (concentrating metals 626
