Microfracturing and fluid mixing in granites: W–(Sn) ore deposition at Vaulry (NW French Massif Central)

Tectonophysics 336 (2001) 43±61

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Microfracturing and ¯uid mixing in granites: W±(Sn) ore deposition at Vaulry (NW French Massif Central) Jean Vallance a,*, Michel Cathelineau a, Christian Marignac b, Marie-Christine Boiron a, Serge Fourcade c, FrancËois Martineau c, CeÂcile Fabre a a UMR 7566 and CREGU, BP 23, 54501 Vandoeuvre les Nancy Cedex, France Ecole des Mines and CRPG-CNRS BP20, 54501 Vandoeuvre-les-Nancy Cedex, France c UPR 4662 GeÂosciences Rennes, Univ. Rennes 1, 35042 Rennes Cedex, France

b

Abstract The Vaulry W±(Sn) mineralisation, located at the eastern boundary of the Blond rare metal leucogranite, is contained in a set of subvertical quartz veins, locally with muscovite and minor quartz selvages. The sequence of deposition was: (1) milky quartz, predominantly as fracture ®lling, generally affected by subsequent ductile deformation; (2) hyaline quartz±wolframite± cassiterite; (3) minor sulphides. Other sets of quartz veinlets, although generally barren are observed in the Blond massif. Fluid migration at the microscopic scale within the granite and in the vicinity of quartz fractures was constrained by studying the geometry of ¯uid-inclusion planes and ¯uid-inclusion chemistry in and outside the mineralised area. Three major sets of subvertical ¯uid-inclusion planes are recognised: a N0508±0608E set, mostly developed in the veins and in the immediate vicinity, a N1108±1308E set, regionally developed in the granite and a N140±1608E set of local extent. As a whole, the density of FIP decreases from the mineralised zones toward the barren part of the pluton, except for the N1408±1608E set. These are locally abundant around quartz veinlets with similar orientations that form a broad ªN±Sº band near the Blond locality. Mineralising ¯uids observed as primary inclusions in cassiterite and in undeformed hyaline quartz are mostly aqueous, with moderate salinity and a minor volatile component, at variance with many other W±(Sn) deposits in the Variscan belt. Ore deposition occurred around 3158C, at an estimated depth of 5.5 km, under hydrostatic to slightly suprahydrostatic pressures. It resulted from ¯uid mixing, in the central part of a large hydrothermal system, between two end-members: (i) a hot (425±4308C) moderately saline ¯uid, that contained a diluted volatile component and, although Na-dominated, minor amounts of Li and Ca. The estimated d 18O indicates that this ¯uid was completely equilibrated with the tectono-magmatic pile (pseudo-metamorphic ¯uid). (ii) a ªcoldº (2308C) low-salinity ¯uid (evolved meteoric water), that mixed with, and eventually overprinted, the early moderately saline ¯uid responsible for granite muscovitization at 425±4308C. Later, a second hydrothermal system was initiated by the percolation of heated meteoric water, with very low salinity. The system was by then at least 3.5 km deep and the ¯uid was heated to 3008C. These characteristics are reminiscent of the ca. 305 Ma episyenitic hydrothermal system known elsewhere in the N Limousin. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Blond massif; N Limousin; quartz±wolframite±cassiterite; muscovite

1. Introduction

* Corresponding author. E-mail address: [email protected] (J. Vallance).

The French Massif Central contains numerous granite-related quartz±wolframite and quartz±cassiterite mineralisation. Most are metasediment-hosted

0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0040-195 1(01)00093-2

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J. Vallance et al. / Tectonophysics 336 (2001) 43±61

close to granite cupolas and were associated with high-temperature C±O±H±N ¯uids of metamorphic derivation (review Marignac and Cuney (1999)). However, in N. Limousin, several showings and minor deposits are granite-hosted. For instance, the St-Goussaud Sn±W showings (eastern part of the St Sylvestre peraluminous pluton) were recently investigated (Alikouss, 1993) and were shown to have been related to moderately saline aqueous ¯uids devoid of any volatile components. A connection with late Carboniferous-specialized peraluminous leucogranites was suspected. The exhausted Vaulry W±(Sn) deposit, hosted in the ca. 310 Ma Li±F-rich Blond granite (Fig. 1), one the most typical peraluminous leucogranites, was chosen to further assess this possibility. The Vaulry W±(Sn) mineralisation, located at the

eastern boundary of the Blond rare metal leucogranite, consists of a set of subvertical N0108±0308E quartz veins (2±10 cm wide), with local muscovite and minor quartz selvages. The sequence of deposition indicates a multistage ¯uid percolation within the same structures. Milky quartz (Q1) is the predominant fracture-®lling generally affected by subsequent ductile deformation (Vallance, 1997). The ore assemblage (hyaline quartz (Q2)-wolframite±cassiterite) is formed later, generally after re-opening of the fracture walls. Other sets of generally barren quartz veinlets are observed in the Blond massif, but their relationship with the ore zones is unclear. The aim of the present study was thus: (i) to investigate ¯uid ¯ow generated in and around this rare-metal leucogranite, (ii) to determine the nature of the W±(Sn) mineralising

Fig. 1. (a) Location of the studied area in the Northern Limousin (French Massif Central, shown in the inset). (b) geological map of the Blond massif (modi®ed from Raimbault, 1998) with indication of the main granite facies and the sampling locations (open squares).

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

¯uids at Vaulry and to reconstruct the P±T conditions of ore deposition; (iii) to place the hydrothermal system in the Blond massif, within the context of a larger scale late Carboniferous crustal ¯uid ¯ow in N. Limousin, (iv) to compare the processes of quartz sealing in fracture sets from barren and ore zones within the same pluton, and to understand the geometry of ¯uid circulation in and outside (granite matrix) the major sets of fractures. An 8 km long E±W sampling pro®le (Fig. 1) in the northern Blond pluton, starting from the Vaulry deposit to the east was de®ned for detailed investigation of ¯uid types and host structure, both in barren granites and veins, and zones hosting the W±(Sn) quartz veins. The nature and geometry of ¯uid migration was constrained by studying: (i) the geometry of all microstructural markers, especially ¯uid- inclusion planes (FIP), (ii) the relative chronology of the quartz types and ore minerals was determined taking into account the host microstructures identi®ed previously, (iii) P±V±T±X properties of the ¯uids associated with each type of quartz was obtained by studying ¯uid inclusions, (iv) the possible source of ¯uids was determined through isotopic analyses of quartz, and investigation of paleo¯uid chemistry (cation ratios). 2. Geological setting The Blond pluton, which hosts the Vaulry W±(Sn) mineralisation (Fig. 1) is an important member of the rare metal peraluminous granites (RMG: Cuney et al., 1994). In the French Massif central, these Li±F-rich leucogranites occur as small isolated plutons, forming a small province comparable to the Erzgebirge RMG province (Marignac and Cuney, 1999). They are considered to correspond to the Late Carboniferous (ca. 310 Ma) granulitization event of the Variscan lower crust, probably related to a lithospheric delamination event (Marignac and Cuney, 1999). They are rather late in the magmatic history of the variscan belt, and constitute a third generation of peraluminous granites, after: (i) a ®rst generation of peraluminous biotite (^cordierite) granites (GueÂret type, Stussi, 1989) dated at ca. 350±360 Ma (Duthou, 1977) which form large plutons that seal the thrust contact between the Upper Gneiss Unit (UGU), made of amphibolites and migmatitized metagreywackes, and

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the Lower Gneiss Unit (LGU), consisting of metagranites (augen gneisses) and micaschists (Floc'h, 1983; Ledru et al., 1994). (ii) a second generation of peraluminous two-mica leucogranites (Limousin type, Stussi, 1989) dated at ca. 325 Ma (Duthou, 1977; Holliger et al., 1986), which intrude into the LGU and are related to the end of the Variscan collision (Friedrich et al., 1988). However, Faure (1989) suggests they could record the onset of the post-collisional gravitational collapse of the belt. The Blond leucogranite outcrops as an elongated body in an E±W direction, and the gravity data (AmeÂglio et al., 1998) point to a sheet-like morphology, with roots along the N1308E Oradour fault, along the SW boundary of the pluton (Fig. 1). This fault was initially a ductile shear-zone that evolved as a brittle dextral transcurrent fault (AmeÂglio et al., 1998). According to Sou® (1988), the intrusion of the Blond granites was related to the activity of the Oradour fault. To the north the Blond pluton intruded into the UGU (here, mainly composed of amphibolites), but the contacts were reworked by E±W faults, in particular to the east (in the area of the W±(Sn) mineralisation), where a mylonite is seen at the northern boundary of the pluton. To the east and the south the pluton intruded into the biotitic granite of the GueÂret family, the CieuxVaulry pluton and the La Glane pluton, north and south of the Oradour fault, respectively (Fig. 1). These granites were intruded into the LGU. According to AmeÂglio et al., 1998), the lower contact of the Blond pluton is with metamorphic rocks of the LGU. Thus, the Blond pluton should have been located at the boundary between the UGU and the LGU. The Blond pluton is composite (Fig. 1b) consisting of six main granite facies: an internal and external medium-grained facies with biotite phenocrysts, a ®ne-grained and a coarse-grained facies with concentric disposition (the ®ne-grained facies to the North and the coarse-grained facies to the South), a muscovite facies, sometimes with a topaz-rich zone and a ®ne-grained facies with small potassic-feldspar phenocrysts (Raimbault, 1998). The Vaulry deposit, at the ªLa Mineº locality, is hosted in a medium-grained external facies but some mineralised veins overprint mylonites. Sampling included: (i) a series from the W±(Sn) mine area located in the north-eastern part of the Blond granite; (ii) a pro®le 8 km long in the granite perpendicular to the main W-ore zone, e.g. from the

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J. Vallance et al. / Tectonophysics 336 (2001) 43±61

Fig. 2. Schematic distribution of microstructures (¯uid-inclusion planes, noted FIP) and quartz veinlets in the Vaulry ore zone. Insets show at distinct scales relationships among ¯uid inclusion types (noted Lw1, Lw2, Lw3) and host quartz (Q1, Q2, Q3). Q1: early milky quartz; Q2: grey hyaline quartz associated with Sn±W ores; Qm: magmatic quartz in the granite; W: wolframite.

mineralised zone (locality ªLa Mineº) and the Blond village to the west (Fig 1). Most of the samples were oriented (blocks of granite and quartz veins), the exceptions being some mineralised blocks from the deposit and from locality no. 12 (sample VAU 12). Un-oriented samples of mineralised quartz were obtained from blocks of former mine workings (now unaccessible) (CFM 8, 15, 16 and 17).

3. The quartz vein system 3.1. Geometry In the ore area, ªLa Mineº (Fig. 1), numerous thin quartz veins (1±2 cm wide, several tens of metres

long), with a subvertical dip, sporadically contain W±(Sn) mineralisation (Fig. 2). The veins have two sets of directions (N000±0108E and N020±0408E) branching one on the other, their geometrical relationship suggests that the N020±0408E direction corresponds to sinistral shear planes due to N000± 0108E shortening. The exhausted mineralised veins were dominantly along the N000±N108E direction, as seen from present day outcrops and old workings (trenches). They were signi®cantly wider (up to a metre wide). Around the Blond massif, there are few occurrences of barren quartz veins outside the mineralised area, mostly along the northern border of the pluton. They are usually a few centimetres wide and a few metres long, with a single stage in®lling and without associated alteration. They are sub-vertical and strike

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

mostly N±S. At the VAU 12 locality, 10 cm wide veins consist of quartz combs. 3.2. Mineralogy of W±(Sn) quartz veins and their host rocks The quartz-hosted ore veins are characterised by several episodes of deposition (Fig. 2): (1) The main quartz in®lling (Q1, white quartz) is barren. It has been weakly deformed. As a result, all primary inclusions have been destroyed, and appear as black decrepitated inclusions or as highly deformed inclusions (with squeezed shapes, and no identi®able vapour bubble). (2) Grey hyaline Q2 quartz overprinted the deformed quartz, as small fracture-in®llings, or local reopenings of the Q1 veins. Wolframite and cassiterite deposition was coeval with Q2, as demonstrated by vug-like in®llings of these oxides capping subhedral Q2 quartz. In addition, wolframite and cassiterite may also be found within clear Q2 quartz that overprints Q1 quartz, with a development of thin rims of hyaline recrystallised quartz around the ore. (3) Late microcracks were ®lled with loÈellingite, chalcopyrite and pyrite. Hydrothermal alteration in the granite wall rock is poorly expressed, and mostly consists of the development of muscovite, after feldspar, over a few centimetres up to a decimetre from the vein. In some instances, a nearly complete muscovitization of the granite (muscovitite) occurs through a nearly complete dissolution of magmatic quartz and a complete alteration of the feldspars and biotites into muscovite. A later silici®cation of the muscovitized granite resulted in pseudo-greisen assemblages (muscovite 1 minor quartz). Cassiterite is found in vugs of the pseudo-greisen, and may be either associated with this alteration event, or crystallised later. A few metres away from the ore fracture sets, it is still possible to observe the otherwise unaltered granite ®ssure-like muscovitized zones devoid of quartz veins. 4. Fluid characterisation In the host granite, ¯uid migration resulted in the formation of healed crack sets (FIP) which can be observed in magmatic quartz grains (Qm). Systematic

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measurements of FIP were carried out to relate different stages of ¯uid percolation to a regional succession of deformational and hydrothermal events. The geometry (strike, dip) and the chronology of FIP have been determined on oriented wafers using transmitted light microscopy and an interactive videographic analyser adapted to such studies (Lapique et al., 1988; Nogueira and Noronha, 1995). Most FIP were found to be subvertical (Fig. 3), allowing a rose diagram representation of the direction sets (Fig. 4). Fluid inclusions (FI) that represent examples of the percolating ¯uids have been systematically studied in healed FIP from the magmatic quartz in granite and from quartz veins and veinlets. Microthermometric characterisation of the ¯uids ( Roedder, 1962, 1984) was carried out on oriented wafers (200 mm), using a heating±freezing Chaix-Meca stage (Poty et al., 1976) either on primary inclusions in quartz veins and cassiterite, or in FIP from quartz veins and magmatic quartz grains. The FIP were characterised by the microthermometric data from individual FI, such as the melting temperature of ice (Tm ice) and the homogenization temperature (Th), which are related to the salinity and the density of the ¯uid respectively, and by microstructural parameters, such as the direction of the ¯uid-inclusion planes. All the studied inclusions are small (5±15 mm) aqueous two-phase inclusions homogenizing in the liquid phase; the formation of clathrates could not be detected. Nevertheless, the presence of volatile components in the FI was investigated using a Raman microprobe (Dubessy et al., 1989). The results are summarized in Tables 1 and 2. P±V±T±X features, as a function of the direction of ¯uid migration were estimated, and a partial ¯uid composition determined by in-situ laser ablation-optical emission spectroscopy (LA-OES, Boiron et al., 1991; Fabre et al., 1999), and stable isotope studies of hydrothermal quartz. 4.1. Fluid types associated with quartz veins Four types of ¯uid inclusions are observed (Fig. 5): (1) Lw1. This type was not found in veins, only in the muscovitized granite, on both sides of the veins. It occurs as FI in N1108E and N010±0208E planes in quartz from the muscovitized granite (e.g. VAU 3:

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J. Vallance et al. / Tectonophysics 336 (2001) 43±61

Fig. 3. Density pole stereograms for FIP sets observed in granite and quartz vein samples from the northern border of the Blond granite, including the La Mine area.

Fig. 4), either relict magmatic quartz or quartz related to late silici®cation (probably, Q1 quartz). Lw1 FI have Tm ice ranging from 24.7 to 248C, indicating a moderate salinity of 6.5±7.5 eq. wt% NaCl (using

the equation of Bodnar, 1993). The Th varies from 210 to 3108C. (2) Lw2. This type is characteristic of the ore ¯uid, and was found either in the hyaline quartz

Table 1 Summary of ¯uid inclusion types from Vaulry area with indication of micro thermometric data (Tm ice: melting temperature of ice; Th homogenization temperature; range and mode in parenthesis are given for each type of ¯uid; nomenclature for ¯uid inclusions is explained in the text) Tm ice (8C)

Th (8C)

Lw1

Muscovitized granite FIP FIP in granite

24/24.7 (24.3)

210±310 (260/270)

Lw2

Clear quartz Q2 Ð ore zone (W±Sn) Cassiterite N508E FIP in Q1 FIP in granite

22/23.5 (22.5/23) 22.5 23.7/22.1 (22.3/22.7) 23.7/22.1 (23.3/22.5)

150±320 (260/280) 280 150±320 (280/300) 150±300 (200/250)

Lw3

FIP post clear quartz Q2 FIP in granite

20.2/21.9 (21/21.5) 20.1/21.9 (20.8/21)

150±220 (170/180) 150±260 (230/250)

Lw4

Hydrothermal quartz- VAU 12 FIP in granite

20.1/20.3 (20.1) 20.1/20.5 (20.1)

150±200 (170/180) 160±320 (190/200)

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

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Fig. 4. Rose diagrams showing strike of FI sets in granites, and in quartz veinlets with reference to the main fracture direction.

Q2(1cassiterite), in the cassiterite itself, or in some FIP from Q1. In cassiterite, a few primary inclusions were measured, indicating a moderate salinity (Tm ice 22.5 to 22.48C, i.e. 4.0±4.2 wt% eq. NaCl) and a single Th at 2808C. In undeformed quartz Q2, riming ore minerals, primary inclusions, have a moderate salinity (Tm ice in the range 23.5 to 22.08C, i.e. 3.4±5.7 wt% eq. NaCl), and Th ranging from 150 to 3208C. In Q1 veinlets, numerous FIP striking N050± 0608 contain ¯uids very similar to those found in

primary inclusions from Q2 quartz (Tm ice ranging from 23.7 to 22.18C, i.e. 3.6±6.0 wt% eq. NaCl; and Th from 150 to 3208C). Some of the Lw2 FI from the altered granites and from Q2 close to wolframite or cassiterite are usually characterised by a distinctly darker vapour bubble. Although no evidence of volatiles was found (no observable clathrate), a Raman microprobe survey demonstrated the presence, in some Lw2 FI, of a lowdensity volatile component in the vapour phase, CH4

Table 2 Chemical compositions of the different ¯uid inclusion types from Vaulry area. Nomenclature for ¯uid inclusions is explained in the text Raman spectroscopy (mol%)

Lw1 Lw2 Lw3 Lw4

Bulk composition (mol%)

Cations (mol)

ZCO2

ZCH4

ZN2

XH2O

XCO2

XCH4

XN2

XNaCl

Na/Li

Na/Ca

0 0 0 0

0±41.7 0±53.9 0 0

0±58.3 0±95.5 0 0

96.8±100 95.4±100 99.45±100 98.4±100

0 0 0 0

0±0.8 0±1.3 0 0

0±1 0±4 0 0

1.1±1.4 0.6±1.3 0.02±0.55 0±1.6

6^1 10^6 8^2 Na

22^1 17^2 Na

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J. Vallance et al. / Tectonophysics 336 (2001) 43±61

Fig. 5. Tm ice Ð Th plot for ¯uid inclusions from the mineralised system (see text) and barren quartz veins (VAU12).

and N2, but no CO2. The salinity and the Th span a similar range to those in other FI from the mineralised system. In the volatile phase, CH4 ranges from 30 to 45 mol% and N2 from 55 to 70 mol%, and calculation of the bulk compositions show that these ¯uids are water dominated, with 95±98 mol% H2O (Table 2). (3) Lw3. Secondary inclusions in hyaline quartz Q2 (FIP, in a single set of directions) have lower salinities (Tm ice from 21.9 to 20.18C, i.e. 0.2±3.2 wt% eq. NaCl) and Th (150±2208C). In quartz Q1, some N508E FIP contain FI with lower salinity (Tm ice from 21.9 to 20.18C, i.e. 0.2±3.2 wt% eq. NaCl) and Th (from 150 to 2108C), similar to the secondary inclusions in Q2 quartz. (4) Lw4. Outside the ore zones, primary ¯uid inclusions were studied in barren quartz from the VAU 12 zone. In VAU 12 quartz, Lw4 inclusions have low Tm ice values around 20.3 to 20.18C (#0.5 wt% eq.

NaCl), and low Th (150±2008C). Fluid inclusions with similar low salinities were also found in FIP from granites, but these may have higher Th (up to 3208C). In this case, the Lw4 FIP with low to very low salinities (#0.9 wt% eq. NaCl) are distinctly younger, than other FIP, as depicted in Fig. 6 for the VAU 10 sample, where earlier FIP are clearly contaminated by the younger low Th ±low Tm ice FI. 4.2. Orientation and distribution of FIP from host granites 4.2.1. FIP in the ªLa Mineº granites In the ªLa Mineº area, far from the Q1 veins, magmatic quartz in the granite contains FIP sets with several well-expressed preferential orientations (e.g. VAU 6), including N1108±1308E, N010±0208E and N0508±0608E. Closer to the Q1 veins (e.g. VAU

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

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Fig. 6. Chronological relationships between a N0508E FIP (containing Lw2 ¯uid) and a N1608E FIP (containing Lw4 ¯uid) in the VAU 10 sample. The N1608E FIP is later as indicated by the re®lling of ¯uid inclusions in the N0508E FIP, close to the N1608E one.

3) the N0108±0208E direction becomes predominant, together with N1108E. In other instances (e.g. VAU 7), the N0508E direction is more preferred and again N1108±1308E. It can be noted again that within the Q1 veins (VAU4), the predominant direction is N0508E with a secondary N1408±1508E direction. Neither the N0008±N0108E nor the N1108±1308E directions are found in quartz veins. These ®ndings are summarized in Fig. 2 and suggest that the N1108±1308E direction was present in the granite prior to the formation of Q1 veins, whereas the N0508E and N1408±1508E directions came later. 4.2.2. FIP in the Blond granite outside theªLa Mineº area FIP in magmatic quartz of the Blond granite display basically the same orientation pattern as in the granite near the Vaulry deposit (e.g. VAU 16, VAU 10), with the exception of samples close to quartz veinlets (e.g. VAU 13, VAU 14), in which the dominant FIP directions mainly re¯ect those of the quartz veinlets, dominantly the N1508±1608E direction (Fig. 4). 4.2.3. Microthermometric data on FIP FIP in granites display as usual a rather complex record of the ¯uid stages in the different FIP sets

described above. The secondary FI trapped in the magmatic quartz display a large range of microthermometric values: Th values are from 100 to 4008C and Tm ice are between 0 and 2 4.78C (Table 1) covering most Tm ice Th pairs for Lw1 to Lw4 ¯uids. There is no regular relationships between a given set of FIP directions and the Th ±Tm ice ranges of the trapped ¯uids (Fig. 7), although the Lw4 FIP have low to very low salinity (#0.9 wt% eq. NaCl) are distinctly younger, in a given sample, than other FIP, as shown in Fig. 6. In addition, the FIP in the N1408±1608E direction trend to contain FI of low to very low salinity (Lw4), particularly those far from the mineralised system. Conversely, FIP in the N1108±1308E direction tend to contain FI with moderate salinities (Tm ice between 23.78 and 22.58C, i.e. 6.0±4.2 eq. wt% NaCl) (Fig. 7).

5. Fluid evolution 5.1. Fluids in the mineralised system Th ±Tm ice diagrams were constructed for ¯uids from the mineralised system (Fig. 5) which allow the reconstruction of the ¯uid evolution(s) based

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J. Vallance et al. / Tectonophysics 336 (2001) 43±61

Fig. 7. Tm ice Ð Th plot for ¯uid inclusions from FIP studied in granite samples with reference to the strike of the FIP.

on the assumption that each ¯uid-inclusion set (either cluster of primaries or individual FIP) records a speci®c batch of circulating ¯uid in space and time. From Fig. 5, we conclude that three speci®c ¯uid types successively occurred in the main hydrothermal channels related to the W±(Sn) mineralising system: (1) Moderately saline ¯uids (Lw1) were related to the muscovitization of the granite surrounding the Q1 veins. These ¯uids were earlier than those trapped in Q2 quartz. This is demonstrated by their absence in the N0508E FIP, in which the major mineralising ¯uids have ¯owed. Nevertheless, the Lw1 ¯uids were clearly related to the mineralising system, since they were trapped not only in N1108±1308E FIP (presumed to record an earlier deformation event) but also in N0108±0208E FIP, found only in the vicinity of Q1 veins (most probably as mode-I

tension joints related to the stress ®eld at the time of Q1 formation, see above and Fig. 2). Their minimal trapping temperatures are rather high at around 300± 3208C. (2) Slightly less saline ¯uids (Lw2), with similar to slightly lower Th than Lw1 were related to the crystallisation of later Q2 and to the W±(Sn) mineralisation (as demonstrated by the similar patterns in primary ¯uid inclusions from both the Q2 quartz and the cassiterite). The Lw2 ¯uids migrated in the NE±SW FIP network, as similar patterns of ¯uid inclusions are found in N0508E FIP from barren Q1 veins and in Q2. (3) Low-salinity ¯uids with low Th (Lw3) were trapped slightly later, as they overprint primary orerelated Lw2 inclusions. Nevertheless, these Lw3 ¯uids also circulated in the N508E system, as seen from the Lw3-bearing N050±0608E FIP found in

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

barren Q1 veins and thought to be coeval with the preceding ones. 5.2. A comparison with ¯uids in granites From Fig. 7, ¯uids with similar features (Th and salinities) to the Lw1, Lw2 and Lw3 ¯uid types found in the veins have also circulated in the Blond leucogranite, far from the mineralised system. However, two points are worth noting: (1) Lw1 ¯uids were only recorded close to the ªLa Mineº area (VAU 6), whereas Lw2 ¯uids were observed up to a few km to the West (VAU 7 and VAU 10) and Lw3 ¯uids are far from the mineralised vein system (VAU 16). Although the data are not suf®cient for a detailed reconstruction, it appears that the W±(Sn)-related hydrothermal system was large, with a ca. 10 km radius at the level of exposure, and a broad zonal distribution, with ªsalineº ¯uids in the Vaulry vein system and low-salinity ¯uids outwards. (2) As already stated, the relationship between the nature of the ¯uid and the direction of trapping FIP, is unclear in the granites (Fig. 7), because of the limited data set due to the lack of suitable inclusions. Most of the ¯uids in the W±(Sn) system were trapped in FIP with a N110±1308E direction, i.e., the early regional direction in the Blond granite (the dominantly N1408E direction trapping Lw1 ¯uids, in the VAU 6 sample, is a likely related direction). The only exception, far from Vaulry, is in the VAU 16 sample, where the N0508±0608E direction is the host for Lw3 ¯uids. These ®ndings indicate that, far from the N0508± 0608E central mineralising system at Vaulry, related ¯uid circulation occurred using previously formed FIP sets, that were re-opened (probably overprinting earlier ¯uid inclusions). This is consistent with the fact mentioned above that the FIP with NE±SW directions become rare when going outside the Vaulry area. Combining Figs. 5 and 7, it is clear that the overall ¯uid evolution of the W±(Sn) system is characterised by a strong mixing trend between two hydrothermal end-members, a ªhighº-salinity ¯uid ( ^ 7.5 wt% eq. NaCl), represented by the more saline Lw1 inclusions and a low-salinity ¯uid (ca. 1.5 wt% eq. NaCl), represented by the Lw3 inclusions. As the differences in Th between Lw1 and Lw3 inclusions cannot be simply accounted for by pressure ¯uctuations, the overall

53

trend must also be interpreted as a mixing trend between high-temperature and low-temperature ¯uids. Post-trapping modi®cations (such as neckingdown), although locally present, may be ruled out as a general explanation, because much care was taken during the measurements to avoid FIP obviously presenting such effects. The only possible exception concerns the set of Lw1 ¯uid inclusions in the muscovitized granite. It is possible that some of these were affected by partial decrepitation (stretching) (Vityk and Bodnar, 1995) during subsequent ¯uid circulation, explaining the large Th range recorded from these ¯uid inclusions. At Vaulry (ªLa Mineº area), the anisothermal trend is time-dependent, high-temperature Lw1 ¯uids circulating earlier and being responsible for the muscovite alteration, whereas further circulation was characterised by the progressive involvement of cooler Lw3 ¯uids, probably externally derived, based on the spatial distribution of ¯uids outlined previously. Eventually, the Lw3 ¯uids became dominant, leading to the extinction of the hydrothermal system. The intermediate Lw2 ¯uids characterise the main ore stage, suggesting that ore deposition was controlled by mixing. 5.3. The late hydrothermal system Fig. 7 shows that an hydrothermal system independent of the mineralising ¯uids was present in the Blond leucogranite. It is characterised by the circulation of very low-salinity ¯uids (,1.0 wt% eq. NaCl) (Lw4). There is evidence of cooling from temperatures similar to those attained in the mineralising system, although the decreasing Th trend at constant Tm ice in Fig. 7 may incorporate necking-down effects, (observed in many N1408±1608E FIP). The following facts are relevant: (a) The Lw4 ¯uids are mainly observed in FIP from the N1408±1608E set; combined with the observation that this orientation set is mostly found in the vicinity of quartz veinlets which have the same orientation (Fig. 4). This suggests that this direction was the most important for the circulation of Lw4 ¯uids. (b) As demonstrated by the relationships in Fig. 6, the Lw4 hydrothermal system is younger than the W±(Sn) mineralising system. This explains why FIP directions other than N1408±1608E may also contain

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J. Vallance et al. / Tectonophysics 336 (2001) 43±61

Fig. 8. P±T reconstruction of the ¯uid evolution based on representative isochores of the three ¯uid types, consideration of thermal gradients, and additional temperature estimates.

Lw4 ¯uids (Fig. 7). They were re-opened at the time of Lw4 circulation and the previous ¯uid inclusion content was overprinted. (c) Although Lw4 ¯uids may be found in the mineralised vein system (Fig. 5), their occurrences are scarce. In the Blond granite, these ¯uids are mostly found in a ªN±Sº band comprising the mesoscopic N1608E quartz veinlets (Fig. 4), suggesting that the Lw4 hydrothermal system had this band as a ¯uid conduit. 6. P±T reconstruction In order to reconstruct the P±T evolution during hydrothermal circulations, a series of isochores representative of the main ¯uid types (Fig. 8) have been calculated using the data from Zhang and Frantz (1987). As the differences in Th between Lw1 and Lw3

inclusions cannot simply be accounted for by pressure ¯uctuations, the overall trend must be considered as an anisothermal mixing trend. Therefore, salinity and temperature should simultaneously and continuously decrease with time during the existence of the hydrothermal system, at least in the mineralised Vaulry vein system. Nevertheless there are signi®cant Th variations recorded at a given salinity (around 508C and even more) (Figs. 5 and 7). A part, if not all, of these Th variations must be accounted for by pressure variations. The Th range at a given salinity may be interpreted as recording pressure variations, between values close to lithostatic pressure (higher Th) and those corresponding to lower pressures if temperature is assumed to be sub-constant (for one ¯uid type of a given salinity). On this basis, a consistent model of P±T variations may be constructed, assuming that the lowermost Th at a given salinity could re¯ect lithostatic values. In Figs.5 and 7, we give our best estimates of mixing lines for high and low Th. We then

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

55

Blond late hydrothermal system, the temperatures would have decreased from a maximum of 3308C down to 190±2208C, whatever the pressures, if they were not higher than those estimated for the preceding stage. The latter should be the temperature of deposition of the barren N1608E quartz veins, judging by the microthermometric results in the VAU 12 comb quartz vein.

7. Origin of the ¯uids 7.1. Bulk composition data: the mineralised system Fig. 9. Laser ablation spectra from Lw2 ¯uid inclusion containing Na, Li and Ca.

consider the extreme isochores for the three families of ¯uids (Lw1, Lw2, Lw3) (Fig. 8) and ®nd that the best estimate of the lithostatic±hydrostatic couple of pressures is found at a depth of 5.5 km (1.5 kbar of lithostatic pressure, assuming a 2.7 crustal density). Finally, taking into account the mean Th for each family, the Lw1 ¯uids have mainly circulated under lithostatic conditions, at temperatures as high as 425± 4308C, whereas the ore-forming Lw2 ¯uids mainly circulated under hydrostatic or slightly higher (up to 1.0 kbar) pressures, at temperatures between 310 and 3508C. The late stage of ¯uid circulation (Lw3) occurred under similar pressure conditions than during Lw2 stage, at lower temperatures (down to 2308C). The validity of this approach may be tested by considering the oxygen isotopic temperature calculated for the quartz±wolframite pair, using the calibration of Zheng (1992) and assuming that the two adjacent minerals are in isotopic equilibrium on the sub-millimetre scale. The calculated value is 3158C, which agrees with our estimates for the oreforming stage. Moreover, it is the same temperature as that given by the only ¯uid inclusion in cassiterite in which a Th measurement was possible (Th at 2808C, yielding a temperature of 3158C at 550 bars, Fig. 8). The cooling trend of very low-salinity Lw4 ¯uids, which is typical for the late hydrothermal system at Blond, is very reminiscent of the ªepisyenite trendº recorded in the nearby St-Sylvestre leucogranite at 305 Ma (e.g.. Lespinasse and Cathelineau, 1990; Alikouss, 1993; El Jarray et al., 1994). For the

Laser ablation-optical emission spectroscopy (LA-OES, Boiron et al., 1991; Fabre et al., 1999) is a destructive technique allowing in situ determination of the ratios between speci®ed cations in solution within a given ¯uid inclusion. The ef®ciency of the method is limited when inclusions are small (#10 mm, low quantity of analysed liquid) and/or of low chlorinity (rather low concentration of ions in solution). Consequently, in the Blond massif, the application of this technique was dif®cult due to the small size and low salinity of the inclusions. Nevertheless, a few inclusions yielded signi®cant results (Fig. 9); they are representative of the Lw1 to Lw3 ¯uids, in both the mineralised area and further away (VAU 16 sample). The ¯uids appear to be Na-dominated, as was already deduced from the high ®rst melting temperature recorded in ¯uid inclusions (ca. 2208C). In addition, Li is often present and Ca may occasionally be found. There is apparently no relation between the cationic content and either the salinity or the location of the studied inclusions. In the same way, from two of the measured inclusions also containing a volatile component, one is a Na-¯uid, the other a (Na 1 Li)¯uid. Most signi®cantly, the Na/Li and Na/Ca ratios are similar whatever the salinity of the inclusions, (Table 2). Since in the mixing model between Lw1 and Lw3, the salinity of the Lw3 end-member is taken at a rather low value (ca. 1.5% NaCl), the constancy of the catonic Na/Li and Na/Ca ratios in the mixed ¯uids (see above) is evidence for these ratios being characteristic of the ªsalineº Lw1 end-member. The Na/Li ratio is particularly interesting, since it is

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J. Vallance et al. / Tectonophysics 336 (2001) 43±61

Table 3 Oxygen isotope data Nature

Rock sample

Nature of the sample

d 18O% SMOW

Magmatic quartz

VAU 6

Wafer

111.78

Quartz related to the greisen

VAU 3-B

Wafer

112.09

VAU 2 VAU 3-A VAU 4 CFM 15 CFM 16-1A CFM 16-1 C CFM 16-2

Wafer Wafer Wafer Bulk Wafer Wafer Wafer

111.95 111.15 111.54 112.10 111.55 112.15 112.00

VAU 12-A VAU 12-B VAU 9 VAU 11

Wafer Wafer Bulk bulk

20.10 20.70 15.63 10.5

Wafer

15.36

Quartz veins (Ore zone) Barren quartz (Q1) Grey hyaline quartz (Q2)

Regional veins

Wolframite

signi®cantly higher than the values of this ratio in magmatic ¯uids issued from Li-enriched magmas, that are typically lower than ®ve (Boiron et al., 1997). The Na/Li values around six that characterise the Lw1 ¯uid are similar or lower than those found in ¯uids equilibrated with granites (e.g. ¯uids equilibrated within the Mont Blanc granite during alpine retrograde metamorphism: Boiron et al., 1997). This is not surprising, considering that the Vaulry system is hosted in a Li-rich leucogranite. However, the presence of Ca (at a Na/Ca ratio around 22) is evidence that the Lw2 ¯uid was equilibrated with other rocks, since the Blond granite is very poor in calcium. Possible candidates in the surroundings are the biotite granites of the GueÂret-type (La Glane or Cieux-Vaulry) or the amphibolites of the UGU just to the North of the Blond pluton. The volatile component which is occasionally found in the mixed ¯uids is best explained as coming from the low-temperature equilibration of water (ca. 4008C) with a C-rich rock and/or from the devolatilization of a C-rich rock. This points towards a metamorphic or pseudo-metamorphic origin of this component (under the term pseudo-metamorphic, we mean ¯uids of any origin, subsequently equilibrated at depth with a metamorphic±magmatic pile and having lost most, if not all, of their initial characteristics).

7.2. Stable isotope geochemistry of quartz and wolframite 7.2.1. d 18O data The oxygen isotopic composition was measured on separated quartz grains and quartz microsamples extracted from the proper ¯uid inclusion wafers in which FI were studied. Details of the analytical procedures are found in Barrat et al. (1998). A summary of the data is presented in Table 3. The selected quartz samples are representative of the main quartz in®llings: quartz from the ªpseudogreisenº; Q1 in the ore zone; selected microdomains, around 100 £ 100 mm in ¯uid inclusion wafers, providing samples of the mineralised grey hyaline quartz (Q2); and milky barren quartz in the granite far from the mineralised area. In addition, magmatic quartz from the unaltered Blond leucogranite at 500 m from the mine area was also selected; and a wolframite was analysed, from a sample already selected for Q2 quartz. Measured d 18O values on quartz from the mineralised system display a restricted range, with nearly similar mean values for bulk milky quartz from the muscovitized zones (112.1½), Q1 quartz (111.7½) and for quartz sample enriched in clear mineralised Q2 quartz related to W±(Sn) mineralisation (111.95½). These values compare with the

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

57

Fig. 10. Theoretical ¯uid mixing trend with recalculated isotopic values for ore ¯uids issued from the mixing of high-temperature moderately saline ¯uid (Lw1) and dilute, low-temperature ¯uid (Lw3), based on two distinct assumptions: (a) a mixing of about 50% based on salinity data, (b) a mixing of 15% based on isotopic data.Qtz: quartz, ¯: ¯uid.

magmatic quartz (111.8½). By contrast, the quartz from barren quartz veins coming from the northern granite contact displays d 18O values from 2 0.7/ 20.1 (VAU 12) to 15.6½ (VAU 9).The different isotopic compositions of two microsamples of quartz VAU 12 show that this type of quartz is isotopically heterogenous on the scale of a few millimetres. Finally, the wolframite sample yields a 15.4½ value. 7.2.2. Interpretation The interpretation of isotope data is based on the estimated temperatures (4308C for the hottest Lw1 ¯uids, 3158C for the wolframite deposition and 2308C for the Lw3 ¯uid end-member) and the model of ¯uid mixing, together with the use of the quartz± water O isotope fractionation curve of Zheng (1993). As discussed above, the ore-forming ¯uids (Lw2) resulted from a mixing between hot slightly saline ¯uids (Lw1 ¯uid type) and low-temperature, lowsalinity ¯uids (Lw3). The degree of mixing may be estimated: (i) either from the salinity data, or (ii) from the ¯uid d 18O (calculated, at the temperatures of interest, from that of corresponding quartz). The latter calculation supposes that the two end-members of the mixing may be isotopically identi®ed. The hot, saline Lw1 ¯uids have a calculated d 18O value of

ca. 17.7½. This could be taken as a magmatic value; however, it is signi®cantly lower than the 19.3½ value for the magmatic ¯uid issued from the Blond leucogranite (calculated at 6008C from the d 18O of the VAU 6 magmatic quartz). The estimated Lw1 value is also typical of rock-dominated systems, i.e., metamorphic or pseudo-metamorphic ¯uids, in good agreement with the bulk composition data. For the Lw3 end-member, we may tentatively use the same meteoric ¯uid as the Lw4 component involved in later hydrothermal circulations. The d 18O value of the ore-forming ¯uids being calculated at ca. 4.9½ (at 3158C), this assumption implies that 15% of the meteoric ¯uid mixed with the hot ¯uid. This solution is not consistent with the salinity constraints (see below). Conversely, if we inject the salinity constraints into the model, we may calculate the d 18O value of the Lw3 ¯uids which is required to match the 14.9½ value of the ore-forming ¯uids. According to the mixing model, the salinity of the ore-forming ¯uid at 3158C was around 4.3 wt% eq. NaCl (Fig. 10), which may be interpreted as the result of the mixing of about 50% of Lw3 ¯uids with 50% of Lw1. If a 50% Lw3 ¯uid contribution is considered, the

58

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

isotopic composition of the Lw3 ¯uid is estimated to be 12½, given the estimated value for Lw1. Of course, it is theoretically possible to consider that 213 ^ 2½ is the correct estimate for the Lw3 endmember and that the Lw1 value needs re-evaluation: however, this would lead to ¯uids with values around 113½ (again, different from the Blond magmatic reference), which would have deposited quartz with d 18O around 117½ Ð a value never encountered in the present study. Finally, we conclude that the ore deposition at Vaulry resulted from the mixing of two-end members: a hot moderately saline ¯uid (Lw1), with d 18O around 8½, and compositions consistent with equilibration with metamorphic and granitic rocks; and a lowsalinity ¯uid (Lw3), with d 18O values around 12½. Considering the geodynamic reconstructions of the Variscan orogen at that time, only meteoric water modi®ed by partial isotopic equilibration with basement lithologies is consistent with the estimated isotopic composition for Lw3, within uncertainties. In this case too, a dilution of hot ¯uids by surfacederived water is likely at the period of ore-deposition. We stress the point that the hot ¯uid could be either a true metamorphic ¯uid or a surface-derived ¯uid that was deeply in®ltrated and equilibrated with the granitic basement (ªpseudo-metamorphicº ¯uid). 7.2.3. The late hydrothermal system If we consider that the ¯uids which produced the hydrothermal quartz located on the northern edge of the Blond granite are reliable equivalents of the low-salinity Lw4-type ¯uids, which is quite likely in view of the above interpretations, the O isotopic composition of Lw4 may be estimated, using the quartz-water O isotope fractionation curve of Zheng (1993). As discussed above, the deposition temperature of the VAU 12 quartz (Lw4 ¯uid type) could not be higher than 2308C, most likely between 190 and 2208C. With these temperatures, the calculated d 18Owater value is 213 ^ 2½, which is clearly the isotopic signature of meteoric water. It is worth noting here that hydrothermal systems dominated by such low d 18O ¯uids are already known in Limousin. They were responsible, in the late Variscan, for the production of subsolidus alterations of granites, characterised by the entire dissolution of quartz (ªepisyenitesº: Leroy, 1978; Cathelineau, 1986) by

low density dilute waters (El Jarray et al., 1994) which have similar isotopic values to those in this study (Turpin et al., 1990). Such ¯uids are also identi®ed in northern and southern Limousin as producing late quartz and carbonate in®llings at temperatures lower than the Sb-mineralising stage, i.e. ,1608C (Fourcade et al., 2000). These ®ndings reinforce the above statement that the late N1608Econtrolled hydrothermal system in the Blond granite is comparable to the episyenite-producing hydrothermal systems documented elsewhere in the N Limousin. 8. Conclusions (1) Extended ¯uid percolation occurred in the northern border of the Blond pluton. Two main successive hydrothermal systems were active at the kilometre-scale. The geometry of the plumbing systems was determined by regional tectonics. The nature of the circulating ¯uids varied with time, but there is no evidence of a strong participation of magmatic-derived ¯uids at the level of exposure: no magmatic-hydrothermal system is identi®ed from the study of the W±(Sn) area. (2) The ®rst hydrothermal system involving two main ¯uid types resulted in W±(Sn) deposition at Vaulry. It was centred on the Vaulry area, ¯uid ¯ow being focused there by local faulting in response to a regional compressive stress ®eld. Early quartz veins (Q1) and granite alteration (pseudo-greisen) were formed under N0108 shortening and were the locus of further fracturing and quartz (Q2) deposition according to a N0508E shortening direction. W±(Sn) mineralising ¯uids ¯owed in these microfractures and wolframite and/or cassiterite deposition was coeval with Q2. They are interpreted as the result of the mixing between an earlier ¯uid Lw1 and a cooler less saline ¯uid Lw2. Outside the vein system, ¯uids migrated up to 10 km from the centre of the system, using earlier microfractures with N1108±1308E directions within the Blond leucogranite. (3) The mineralising system was 5.5 km deep (estimated), with initial temperatures up to 425±4308C in the mineralised area. These temperatures and depth are comparable with the characteristics of the deep parts of modern high enthalpy geothermal systems

J. Vallance et al. / Tectonophysics 336 (2001) 43±61

(e.g.. Larderello: Ruggieri et al., 1999). Progressive cooling, down to 2308C, occurred in the centre of the system. There is some evidence of a broad thermal zoning around the mineralised zone, the more external parts apparently not reacting more than 2308C, which would appear to represent the regional background temperature during the activity of the mineralising system. (4) While the hydrothermal system was active, there were permanent pressure variations: pressure ¯uctuated between lithostatic and infralithostatic values, occasionally as low as the estimated hydrostatic value. The pressure was mainly close to the 150 MPa, lithostatic value at the onset of ¯uid circulation, being later signi®cantly lowered (#100 MPa). This could mean that the overlying UGU, in which amphibolites are likely to have acted as an aquitard at the beginning, was progressively fractured while the hydrothermal mineralising system was active. (5) Cooling in the central part of the mineralising system was due to the progressive in¯ux of ªcoldº (2308C) low-salinity waters (evolved meteoric waters with salinity around 1.5 wt% eq. NaCl and d 18O at 2½), that mixed with, and eventually overprinted, the early moderately saline ¯uid responsible for granite muscovitization at 425±4308C. The latter ¯uid (up to 7.5 wt% eq. NaCl) contained a diluted volatile component (CH4 1 N2) and, although Na-dominated, contained minor Li and Ca. This composition, together with the estimated d 18O at 7.7½, is evidence that the early ¯uid was completely equilibrated with the tectono-magmatic pile (pseudometamorphic ¯uid), having lost its original characteristics. Nevertheless, considering the tectonic setting, it is a likely possibility that this ¯uid, too, was a surface-derived deeply in®ltrated ¯uid thoroughly re-equilibrated with the granites and the metamorphic rocks of the UGU and LGU. (6) Cassiterite and wolframite deposition resulted from the mixing process and occurred at about 3158C, the bulk salinity of the ¯uid being then around 4.3 wt% eq. NaCl. The source of the concentrated metals is not known with certainty, but it is most probable that they were leached from the W±(Sn) rich Blond leucogranite through the upward percolation of the hot pseudo-metamorphic ¯uids. Sulphides with traces of other metals (Au)

59

are minor and could correspond to the ultimate cooling stage of the system. (7) The second hydrothermal system in the Blond pluton recorded the percolation of heated meteoric water, with very low-salinity (less than 1 wt% eq. NaCl, often close to 0.2%) and a d 18O around 213½. The system was then at least of 3.5 km depth and the ¯uid was heated up to 3008C (with a regional background around 2308C). That the meteoric ¯uid was poorly exchanged with granites and metamorphic rocks under these conditions is evidence that the percolation rates were very high, suggesting that the system was controlled by extensional tectonics with opening along the N1608E direction which was the major conduit for this late system. These characteristics are reminiscent of the ca. 305 Ma episyenitic hydrothermal system known elsewhere in the N Limousin. (8) Since these two Late Carboniferous hydrothermal systems occurred while the Blond granite had already cooled (as indicated by the estimated background values of 230 and 1908C for the mineralising and late hydrothermal systems, respectively), it ensues that local thermal anomalies were responsible for ¯uid circulation. The high thermal gradients that were found in this study are typical of abnormal heat ¯ows produced by late intrusions. Thus, the existence of concealed Late Carboniferous intrusions may be envisaged under the present level of exposure within or underneath the Blond pluton. Such protracted magmatic activity in the N Limousin after the emplacement of the main plutons (Saint Sylvestre, Blond¼) is at variance with the thermal modeling of Scaillet and Cuney (1997), since these authors relate the abnormal temperature in the whole N Limousin between 315 and 305 Ma to crustal delamination and quick uplift. Acknowledgements This work has been carried out within the framework of the Geofrance 3D program (Publication no. 89), devoted to the space and time relationships between granitic magmatism, thermal and tectonic events and ¯uid circulation at the crustal scale in the variscan French Massif Central. The authors thank Fernando Noronha, David Banks, Jean-Louis

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