Beauvoir Granite Research Articles

Quantification of lithium using handheld instruments: application of LIBS and XRF spectroscopy to assay the lithium content of mineral processing products

Two lithium assay methods have been developed using handheld tools, offering fast and reliable results. These methods are crucial for inline optimization of processes, particularly in the extraction of lithium from hard rock deposits.

Characterization and Liberation Study of the Beauvoir Granite for Lithium Mica Recovery

A significant proportion of Europe’s lithium endowment is hosted by unconventional lithium resources such as rare-metal granites (RMG) of which the Beauvoir granite in France is a prime example. In such hard-rock deposits, where lithium is mostly hosted in micas (lepidolite, zinnwaldite), the ability to assess whether lithium can be extracted economically from the ore is essential and requires a comprehensive understanding of mineralogical properties and lithium deportment. Using three exploratory drill cores distributed along the North–South axis, a preliminary geometallurgical assessment of the granite has been conducted based on a combination of techniques including Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), Atomic Adsorption Spectroscopy (AAS), Electron Probe Microanalysis (EPMA), Scanning Electron Microscope (SEM) coupled with automated mineralogy software, X-Ray Diffraction (XRD), optical microscope and sieving. Lithium distribution appears to be variable, reflecting the evolution of the granite, with higher mica content in the southern area and higher Li grade towards the center of the orebody. The size of micas in the assessed sample does not vary significantly. The grindability and liberation size of micas varies in the different zones investigated, PERC S being the most difficult to grind. There is always more than 50 wt% of the micas that are liberated in the samples when crushed to 1 mm. Indirect estimation of Li content based on EPMA and SEM analysis suggests that the content of lithium inside mica crystals could vary. If this estimation is confirmed by direct Li measurement, it for sure makes the calculations of the Li deportment more challenging.

Experimental Crystallization of the Beauvoir Granite as a Model for the Evolution of Variscan Rare Metal Magmas

Abstract New experiments on the strongly peraluminous, Li-, P- and F-rich Beauvoir granite (Massif Central, France) provide a basis for understanding crystallization and chemical fractionation in Variscan rare metal magmas. Crystallization experiments were performed on two natural granite compositions under H2O-saturated conditions at 100, 200, and 300 MPa, from 540°C to 700°C and between ~NNO + 3 and NNO-1.4. Experimental charges were examined by SEM and their products (glasses and crystals) analyzed for major elements by EMPA. Trace element concentrations in selected glasses were determined by LA ICP-MS. Despite experimental durations commonly exceeding 1000 h and some up to 4000 h, kinetic problems were encountered in particular in the 100 MPa charges whereas, at 200 and 300 MPa, results consistent with previous melting experiments were obtained. Beauvoir melts crystallize quartz, plagioclase, K-feldspar and mica as major phases. At NNO-1.4, mica is a biotite, whereas it is a Li-mica between ~NNO+3 and NNO-1. Apatite, Fe-Ti oxides, either hematite or magnetite, topaz, amblygonite, cassiterite and columbite-tantalite appear as accessory phases between ~NNO + 3 and NNO + 1. Experimental plagioclases are albitic (An <4.5 mol%) and more Ca and K-rich than natural albites in the granite whereas experimental K-feldspars are more sodic (Ab <45 mol%) than the natural crystals. The less evolved starting melt crystallized Li phengites whereas the most evolved yielded Li-, F-rich micas near the polylithionite-zinnwaldite series, similar to natural micas in the granite. Equilibrium crystallization increases A/CNK, F and P and concentrates Li, Be, B, Rb, Cs, W, U in the melt. Nb and Ta are also enriched, their behavior being controlled by the solubility of columbite-tantalite in the melt. Other elements are either unchanged (Mn, Zn, Ti) or depleted (Sr, Pb) during magmatic fractionation. Sn is concentrated in Li-mica and hematite, and it behaves compatibly at high fO2. Beauvoir melts crystallize at very low temperatures, below 670°C for the two compositions studied and solidus temperatures, determined from previous melting experiments and confirmed by the new crystallization experiments, are near 550°C. The experiments demonstrate that most of the mineralogical and geochemical characteristics that make the Beauvoir granite distinctive result from magmatic rather than hydrothermal post-magmatic processes. Albitic plagioclase, Li-mica, topaz, and amblygonite are of magmatic origin. Glass major element compositions suggest that the two granite samples represent crystallized liquids. Trace element fractionations for most elements at Beauvoir can be accounted for by magmatic crystallization–differentiation processes. Implications for the mineralogy, fO2, volatile concentrations, crystallization and conditions of emplacement, fractionation mechanisms and origin of the Beauvoir granite are discussed.

Mica trace-element signatures: Highlighting superimposed W-Sn mineralizations and fluid sources

In the Echassières district of the French Massif Central, occur several outstanding magmatic/hydrothermal systems enriched in strategic metals, such as the Beauvoir rare-metal granite. In this contribution we propose a systematic approach, based on mica trace chemistry, to decipher the different events leading to mineralization. Twelve groups of micas were defined by their specific petrographic features and/or location in the district. Their trace element composition, obtained by LA-ICP-MS, varies widely from one mica group to another, although homogeneous signatures within groups could be distinguished. Some of the trace elements are remarkably enriched, such as W in igneous lepidolite and Sn in greisen muscovite, both of which occur in the Beauvoir granite. A statistical approach based on a set of multivariate analyses highlights that the trace chemistry of micas is inherited from their source, whether hydrothermal or igneous, thus providing a signature for their origin. This approach also shows that differences in major element composition (i.e., different mica species) impact only slightly the trace-element signature. For instance, muscovite and zinnwaldite from one granite have a coherent signature, but they contrast with same mica species in another granite or hydrothermal veins. It is thus possible to genetically link two different mica species from remote locations, or inversely, to recognize different origins for a same mica species in the same sample (e.g., superposed alterations). A second, important implication is that trace-element signatures of micas provide a record of metal remobilization and transport. In the Echassières district, greisen alteration of the Beauvoir granite caused dissolution of W-rich (ca. 290 ppm in average) lepidolite and cassiterite (SnO2). Newly-formed greisen muscovite incorporated most remobilized Sn (ca. 1000 ppm in average), while W precipitated in distal quartz veins as wolframite. As a consequence, Sn is concentrated in the granite, while W occurs outside of it. This is also underlined by the gradual Sn decrease and W increase recorded in micas from distal veins. Finally, wolframite-bearing veins do not contain cassiterite, validating mica trace-element chemistry as a powerful tool to decipher Sn-W ore forming hydrothermal processes.

Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS). The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite “a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration.

Contrasting fluid behavior during two styles of greisen alteration leading to distinct wolframite mineralizations: The Echassières district (Massif Central, France)

The Echassières district of central France hosts diverse magmatic and magmatic- hydrothermal deposits of rare metals, mostly related to the well-known Beauvoir granite. Tungsten mineralization crops out at three distinct wolframite occurrences, the two most important of which are related to two distinct magmatic bodies, emplaced ca. 335 and 310 Ma (Monnier et al., 2019). The mineralization occurred at 335 Ma formed during a hydrothermal episode marked by precipitation of topaz replacing quartz in a stockwork system and as veinlets in the surrounding schist. Fluid inclusions in topaz and quartz display similar features, i.e., all have low salinity, contain significant amount of LiCl, display constant liquid/vapor ratios, and homogenized within a narrow temperature range (Th ≈ 380 °C). No evidence for fluid pressure variations was observed, and temperature decrease is considered to be the main cause for wolframite deposition. The younger W mineralization is related to greisenizing fluids that altered the Beauvoir granite and generated several quartz (±topaz and apatite) veins. All greisen-related fluid inclusions display low salinity, however, Th are spread from ca. 190 to 400 °C, and several populations exhibit heterogeneous liquid/vapor ratios while others consist of only vapor-rich fluid inclusions. Respectively, these populations are interpreted to have been trapped during boiling or flashing (vaporization) of the fluid. In contrast with the other regional veins, flashing was particularly intense in the Mazet veins, which host the bulk of the last wolframite generation. Consequently, it is proposed that flashing is the key factor that triggered W precipitation.This work highlights the role of two physical parameters, pressure and temperature, whose variations played a preponderant role on wolframite mineralization. It documents, in depth, an example of greisen fluid evolution, providing critical information on W behavior in orthomagmatic fluids, and on greisen-related rare-metal deposits.

Multiple Generations of Wolframite Mineralization in the Echassieres District (Massif Central, France)

The Echassières district in central France contains complex rare-element ore deposits, whose formation is related to exotic igneous events and several hydrothermal episodes that are not entirely understood to date. Tungsten mineralization consists of three generations of wolframite, characterized by distinct Fe/Mn ratios (8.4; 3.5 and 0.3, for wolframite a, b and c, respectively), formed during three separate hydrothermal episodes related to the Variscan orogeny. Wolframite a occurs in quartz veins of the La Bosse stockwork where it crystallized before the Barrovian metamorphism that affected these veins and the host rock. After metamorphism, before intrusion of the Beauvoir and Colettes granites, wolframite b crystallized in the stockwork during massive topazification. High concentrations of wolframite c occur in the proximal quartz veins in the Mazet area, while only scant amounts are found in the La Bosse stockwork. In both settings, wolframite c precipitated from the fluid responsible for greisen alteration that massively affected the Beauvoir granite. In the La Bosse stockwork, greisen alteration is characterized by hydrothermal topaz that is texturally and chemically distinct from that precipitated during topazification. Supergene alteration responsible for kaolinization of Beauvoir and Colettes granites caused remobilization of a non-negligible amount of tungsten (W) during replacement of wolframite by W-rich goethite in all units of the Echassières district. This model for multiple W mineralizing events is novel and can prove essential in distinguishing potential economic deposits worldwide.

Quartz trace-element composition by LA-ICP-MS as proxy for granite differentiation, hydrothermal episodes, and related mineralization: The Beauvoir Granite (Echassières district), France

Despite quartz being a ubiquitous mineral in the Earth's crust, only little is known about its trace chemical composition and whether and how this is influenced by the medium from which it precipitated. Using in-situLA-ICP-MS analysis, we investigated the trace element composition of several types of quartz from a magmatic-hydrothermal system in the Echassières district of the French Massif Central. This system consists of the Colettes granite, the Beauvoir rare-element granite, a W-bearing stockwork and dispersed mineralized veins. All of the elements tested (Li, Be, B, Na, Al, Ti, Ge, Rb, Sr, Nb, Sn, Sb, Ta, W)were detected in quartz from at least one sample. A rigorous multivariate statistical approach permitted to assign a specific chemical signature to each of the quartz groups that were recognized by petrographic observations. Differences among these groups were interpreted to reflect different sources or genetic evolutionary trends within magmatic and hydrothermally derived quartz groups. For example, the evolution of Al, Rb, Ge, Li and Ti, could be related to genetic processes such as the degree of magma differentiation and crystallization temperatures. Quartz from the W-stockwork, anterior to the Beauvoir leucogranite, clearly stands out from that related to the Beauvoir mineralization system, i.e.,the greisen facies and proximal Ta-Nb-Sn-W mineralized veins. It was also possible to demonstrate a similarity in composition, notably the Sb contents, between quartz from the greisen and from distal stibnite-bearing veins. In addition, elevated values of Sb, Sn as well as Li and Rb in quartz correspond to similarly high values of these elements in the rock, indicating that quartz trace chemistry can provide useful information for the mineralization potential of its host, at least for these elements. On the other hand, Nb, Ta and W are less reliable, as they occur mostly at very low levels in quartz, the highest values corresponding to samples that were not particularly enriched in these metals. A comparison of our quartz data with those from a similar granitic complex, at Cínovec (Czech Republic, Breiter et al., 2017a), shows a similar trend for the Ge/Ti ratio, reflecting magmatic differentiation. Conversely, the Al/Ti ratio values differ in the two complexes, the Al content of quartz from the various Cínovec granites being systematically lower than in quartz from Echassières. Nevertheless, caution must be taken when analyzing quartz for its trace element chemistry, as processes occurring during (i.e.,trapping of fluid inclusions and formation of zoning) and after crystal growth (i.e.,diffusion and alteration), can introduce heterogeneities thus affecting the original chemistry.

Geochemical Signature of Magmatic-Hydrothermal Fluids Exsolved from the Beauvoir Rare-Metal Granite (Massif Central, France): Insights from LA-ICPMS Analysis of Primary Fluid Inclusions

The Beauvoir granite (Massif Central, France) represents an exceptional case in the European Variscan belt of a peraluminous rare-metal granite crosscutting an early W stockwork. The latter was strongly overprinted by rare-metal magmatic-hydrothermal fluids derived from the Beauvoir granite, resulting in a massive topazification of the quartz-ferberite vein system. This work presents a complete study of primary fluid inclusions hosted in quartz and topaz from the Beauvoir granite and the metasomatized stockwork, in order to characterize the geochemical composition of the magmatic fluids exsolved during the crystallization of this evolved rare-metal peraluminous granite. Microthermometric and Raman spectrometry data show that the earliest fluid (L1) is of high temperature (500 to >600°C), high salinity (17–28 wt.% NaCl eq), and Li-rich (Te<−70°C) with Na/Li ratios ~5. LA-ICPMS analyses of L1-type fluid inclusions reveal that the chemical composition of this magmatic-hydrothermal fluid is dominated by Na, K, Cs, and Rb, with significant concentrations (101–104 ppm) in rare-metals (W, Nb, Ta, Sn, and Li). This study demonstrates that primary fluid inclusions preserved the pristine signature of the magmatic-hydrothermal fluids in the Beauvoir granite but also in the metasomatized W stockwork, despite the distance from the granitic cupola (>100 m) and interaction with external fluids.

Modelling of melt segregation processes by high-temperature centrifuging of partially molten granites-II. Rayleigh-Taylor instability and sedimentation

SUMMARY The present experimental study deals with the laboratory modelling of two different mechanisms of gravitational percolation in partially melted rocks: ( 1) diapiric percolation of heavy material and (2) the sedimentation of heavy particles. These two mechanisms of mass transport in partially melted rocks result in different scales of the segregation process in the melt-crystal matrix. A centrifuge furnace was used to simulate the percolation of the heavy particle layer through the partially molten granite at temperatures of up to 1000 C. Samples of Beauvoir granite (Massif Central, France, grain size 0.16-0.5 mm with an initial degree of partial melting -45 per cent) were used as a matrix. A layer of Pt powder suspended in a melt of the same composition as the partially melted matrix was placed on the top of the granite sample. After centrifuging for various times (up to 2 x lo4 s), X-ray images of samples were obtained and the evolution of the percolation process of heavy suspension in the partially molten granite was monitored from the Pt particle distribution. The diapiric or finger regime of percolation starts when the growth rate of a Raleigh-Taylor instability of the heavy layer is faster than the Stokes sedimentation velocity of individual particles in the upper layer. This relationship is a complex function of the size and initial concentration of heavy particles, as well as the ratio of particle to crystal size, the permeability of the matrix, and the heterogeneity scale in the partially melted matrix. At small concentrations (several per cent) and at large concentrations (where close packing of heavy particles results in an anomalous viscosity increase in the upper heavy layer) Stokes sedimentation is dominant in the vertical percolation of the heavy material. The sinking velocity of the diapir decreases when the size of heavy particles in it becomes comparable with the size of crystals in the partially melted granite. In this situation the vertical sinking of the diapir is not stable and the horizontal instability of the vertical mass transport starts to become important. Mass transport via diapiric percolation results in more efficient crystal-melt segregation of partially melted rocks. The percolation of individual particles provides only local melt-crystal flow on a scale comparable with the heavy particle size. The diapiric percolation provides a much larger scale of partial melt segregation with a length-scale comparable with the diapir size.

Modelling of melt segregation processes by high-temperature centrifuging of partially molten granites-I. Melt extraction by compaction and deformation

The present study deals with the experimental modelling of two different mechanisms of crystal-melt segregation in crustal rocks: (1) the buoyancy-driven compaction of the crystal + melt matrix and (2) melt filtering in a partially crystalline matrix due to differential stresses. These two segregation mechanisms have differing relative efficiencies in the deformation of crustal rocks and result in different texture scales depending on melt fraction, melt viscosity and tectonic stresses. A centrifuge furnace has been used in the present study for the modelling of melt migration in partially molten granitic rocks. Samples of Beauvoir granite (Massif Central, France) with a grain size of 0.16–0.5 mm and dimensions of diameter ∼5 mm, length ∼16 mm were used. These samples had been pre-fused at temperatures of 1000–1075 °C, yielding an initial average melt fraction of ∼45–50 volume per cent. The centrifuging of partially melted samples during ∼6 hr at an acceleration of 1000g (g is gravity) results in a linear vertical distribution of melt over the length of the sample without the development of a compaction layer. The gradient of the melt fraction (melt migrates to the top of samples) correlates with temperature: 1075°C ∼7 volume per cent mm-1; 1050°C ∼4 volume per cent mm-1; 1000°C ∼1.5 volume per cent mm-1. The calculated rate of melt migration varies from 3x10-5 cm s-1 (1075°C) to 2x10-6 cm s-1 (1000°C). Differential stresses of ∼0.7–1.4 MPa have been generated in the centrifuge by putting a piston (weight ∼1.02–2.05 g, diameter ∼4.5 mm) on the top of the partially melted sample, which is then centrifuged at ∼1000g. The rate of melt squeezing from the sample in this case is about two orders of magnitude higher than that observed without the piston. After centrifuging for 6 hr, a compaction layer below the piston is formed with a thickness of ∼2.5 mm and a crystal fraction of ∼70–65 volume per cent. Further centrifuging (∼15 hr) does not result in any increase of the compaction-layer thickness or volume percentage of crystals in it. The comparison of the two segregation mechanisms confirms the much greater efficiency of differential-stress-induced melt segregation and accumulation in veins and pockets than the compaction mechanism.

Geochemical evidence for a multistage magmatic genesis of Ta-Sn-Li mineralization in the granite at Beauvoir, French Massif Central

The Beauvoir rare metal granite, at the southern edge of the Echassieres granite massif, French Massif Central, is mineralized by disseminated Ta, Sn, and Li. The granite body formed from an independent melt which was emplaced soon after the main granite facies of the massif. The geochemistry of the Beauvoir granite relates it to the phosphorus-rich type of Ta-bearing granites, and geochemical and mineralogical data establish its similarity to pegmatite melts. The Beauvoir granite results built up from two distinct intrusions. Both intrusions are cogenetic and were emplaced within a short time interval. A geochemical study of a deep drill hole bored down to 900 m under the surface to the floor of the granite sheet, with selected key zones observed in greater detail, has allowed the recreation step by step of a long process leading to the economic concentration of Ta, Sn, and Li. The comparison of major element, trace element, and isotopic geochemistry with the petrology makes it possible to unravel the numerous processes contributing to the ore formation. Local events need to be determined before main events can be identified. Constraints on the interpretation of the ore genesis are provided by a careful observation of data; interpretations of these observations are strongly backed up by experimental data (e.g., Keppler, 1993). The source rock was most likely enriched in Sn and W at about 10 ppm, precluding a slightly enriched granitic melt by partial melting; it is not known whether the melt was already enriched in Ta, but this is not a prerequisite. Subsequently, deep-seated fractional crystallization produced granitic magma similar in composition (Ta nearly equal 9 m) to the adjacent Colettes granite, and further fractionation processes led to a melt already strongly enriched in Ta (ca. 40 ppm), strongly peraluminous, and enriched in the depolymerizing elements F and P. The viscosity was therefore very low, allowing easy ascent. During transport, small-scale fluctuations in the viscosity were amplified by upward movement, leading to an F- and P-rich melt preceding the magma batch. This less viscous melt again underwent crystallization while moving through the upper..

Neutron porosity logging and core porosity measurements in the Beauvoir granite, Massif Central Range, France : C. Galle, Journal of Applied Geophysics, 32(2–3), 1994, pp 125–137

Neutron porosity logging and core porosity measurements in the Beauvoir granite, Massif Central Range, France : C. Galle, Journal of Applied Geophysics, 32(2–3), 1994, pp 125–137

Particle-induced-gamma-ray-emission spectrometry applied to the determination of light elements in individual grains of granite minerals

The quantitative determination of Li, Be, B, F is feasible using a sensitive and non-destructive method by the analysis of the prompt γ-rays induced with charged particles (PIGE). The irradiation source is a Van de Graaff accelerator, and γ-analysis has to be performed during the irradiation. Measurements were made at the CERI laboratory in Orleans with the PIGE method using a 100×300 μm2 focused beam of 3MeV α particles. Among the applications, micas along a drill core from the Beauvoir granite were analyzed. Bulk analyses of light elements in the drill core have previously been made. Coarse-grained micas were selected, individually irradiated and directly analyzed in polished thin sections from different elevations in the drill core. The sensitivity depends on the cross-sections of the nuclear reactions, on the particle current and on the duration of the irradiation. With a particle current irradiation of 25 nA during 1000 seconds, a 20 ppm limit of detection is reached for Be, 25 ppm for Li, 900 ppm for B and 450 ppm for F. A large variation of Be contents in the micas was found at equal drill hole elevations, and there was no strong correlation between whole-rock compositions and the mineral chemistry. The latter have been determined both by PIGE and by mineral separation and ICP analyses. On the other hand, a strong Li−F correlation was observed between the fluor-polylithionite and muscovite end-members, supporting previous analyses from equal elevation wells on sorted micas. Be−Li correlation is poorly defined, but the contents of the micas analyzed remained between the contents of bityite (Be−Li bearing mica) and lepidolites (Li-bearing micas) end-members. This suggests a solid-solution between the end-members, the extent of which is very narrow because the Be contents were not higher than 500 ppm. The detection limit is a function of the spatial resolution, because the current of the incident α (or p) particles decreases with the impact size. The results show that the analysis of small individual grains of minerals with PIGE can thus be complementary to other analytical methods such as the electronic microprobe.

Neutron porosity logging and core porosity measurements in the Beauvoir granite, Massif Central Range, France

A large suite of geophysical logs have been run in the Beauvoir granite. The drillhole (900 m deep), first target of the French Deep Geology programme, is located in the Hercynian bedrock of Echassières in central France (Massif Central Range). After geochemical and petrological studies, the batholith was used for experiments pertaining to the storage of radioactive wastes. With its low porosity, its weak fracturing and its high homogeneity, the Beauvoir granite was chosen for the analysis of the relationship between logged data and the properties measured in the core. The study focused on neutron porosity and core water porosity. The Beauvoir granite has a total free water porosity of around 2% (average value of 54 core samples of rock mass) and an average neutron porosity of around 10%. We show that the origin of this significant difference is related to the neutron matrix effect of the granite. This phenomenon is partly due to the slowing-down effect of the combined water of clays and micas but also to the neutronic capture effect linked with the relatively high lepidolite (lithium mica) content of the granite. The Li 2O content controls 85% of the granite macroscopic capture cross-section. These two factors represent around 75% of the global neutron porosity of the Beauvoir granite. They have to be taken into consideration to get representative water contents of a low-porosity igneous rock from a neutron porosity log. Further investigations also demonstrated the necessity of choosing a better adapted neutron tool calibration for crystalline rocks. Instead of a standard calibration with limestone blocks, a calibration in granite blocks was simulated in order to obtain a better evaluation of the global neutron response of the granite. Then, by correcting this new neutron porosity for the matrix effect, it has been possible to determine water contents in accordance with laboratory water porosity values measured on core samples (2% average porosity). This experimental study shows that geophysical logging must be adapted to the type of rock and that the geological context (mineralogy, geochemistry) has to be taken into account before running the tool in the borehole. The technique could help determine if neutron porosity data are revelant in other granitic or crystalline rocks.

The Beauvoir topaz-lepidolite albite granite (Massif Central, France); the disseminated magmatic Sn-Li-Ta-Nb-Be mineralization

The Beauvoir topaz lepidolite albite granite (French Massif Central) is the latest intrusion in a Variscan peraluminous granitic complex composed of three units successively emplaced: the concealed La Bosse granite, the Colettes two-mica granite, and the Beauvoir granite. The 900-m-deep drill hole of the Geologie Profonde de la France deep drilling program of the continental crust has provided a continuously cored section of the Beauvoir granite. The exposed section of the Beauvoir granite is presently mined for kaolin in the upper 200 m. It also represents a huge subeconomic disseminated Li, Sn, Ta, Be deposit sharing many characteristics with rare metal-bearing pegmatites. Compared to similar Li-F-rich igneous bodies, the Beauvoir granite is strongly enriched in Sn (200-1,400 ppm), Ta (20-400 ppm), and Be (20-300 ppm). Li is located in lepidolite and amblygonite, Ta and Nb mainly in columbo-tantalite and uranium-rich microlite, and Sn in cassiterite. Be is located in lepidolite crystals and in unidentified mineral(s). The major element composition of the Beauvoir granite is similar to that of Macusani volcanic glasses of Peru and ongonite subvolcanic rocks of Mongolia. The main structural, textural, mineralogical, and geochemical characteristics of the Beauvoir granite can be explained by extreme fractionation of F- and Li-rich magmas and associated fluid phases. Most trace elements show a strong upward enrichment. The high f (sub O 2 ) , low solidus temperature, and high fluorine content of the Beauvoir magma have considerably reduced Sn and W fractionation in the expelled magmatic fluid despite its high chlorine content (25-30 wt % NaCl equiv). However, F- and Li-enriched geochemical halos have developed in the enclosing mica schist in response to the boiling of the magmatic fluid. The relatively deep emplacement of the granite (3 km) has limited the development of hydraulic fracturing and subsequent hydrothermal circulation and mineralization. Thus, despite the very high specialization of the Beauvoir granite, no significant vein-type mineralization is directly related to it. The La Bosse quartz-ferberite stockwork was developed before the emplacement of the Colettes and Beauvoir granites and is related to the earlier concealed La Bosse granite.

Hydrothermal alterations in the Echassi�res granitic cupola (Massif central, france)

Detailed petrographic and mineralogic investigations of an albite-lepidolite granite at Echassieres (Massif Central, France; scientific deep drill program) shows the existence of hydrothermal stages which are closely related to the magmatic and structural history. According to fluid inclusion data, K-Ar datations and 18O/16O-D/H compositions of secondary minerals, two successive hydrothermal periods have been recognized. The early one (273–268 million years) produced a series of aluminous phyllosilicates: muscovite, pyrophyllite, donbassite, tosudite, kaolinite which are observed as vein deposits (<10 mm wide) and alteration products of primary minerals in wall-rocks. The vein system was sealed by monomineralic assemblages during a cooling period (400–150°C). This early hydrothermal alteration stage was controlled by interactions of rock with low salinity (1–10 wt% NaCl equivalent) fluids expelled from the granitic body during the cooling processes. The chemical properties of these fluids were the following: low pH, very low Mg and Fe and high Li, Na and K contents. Thermodynamic calculations show that the sequence pyrophyllite, Li-bearing donbassite, tosudite is mostly temperature dependent. From the chemical composition of secondary minerals and isotopic data it can be deduced that these fluids, which have a meteoric origin, have been expelled from the granite body during its cooling period and after interaction with it at high temperature. The late hydrothermal stage corresponds to deposits of fluorite and Fe-Mg rich illite (151 million years) in subvertical fractures. Temperature conditions did not exceed 250° C and fluids came through the surrounding metamorphic rocks into the granitic body. IIlite/smectite mixed-layer minerals have been identified in subvertical fractures which were opened during Tertiary periods. In the host micaschists, successive hydrothermal alterations took place during the cooling of the Beauvoir granite. Early magmatic fluids interacted with these micaschists. Locally, the metamorphic assemblage is replaced by a metasomatic one. Secondary topaz and (F, Li)-rich mica crystals were formed over a range of 450 of 150°C. Later hydrothermal fluids reacted with the country rocks to form phengite-biotite, chlorite-illite and kaolinite over a range of 300 to 150°C. Illite/smectite mixed-layer minerals crystallized in the roof micaschists and within the Beauvoir granite during the Tertiary alteration period. Meteoric water invaded open fractures producing supergene alteration mineral assemblages.

Near-solidus delta 18 O depletion in a Ta-Nb-bearing albite granite; the Beauvoir Granite, France

The Beauvoir albite-topaz-lepidolite granite has been intersected through an interval of 700 m in a fully cored drill hole at Echassieres, in the Massif Central (France). The Beauvoir granite is a southerly inclined sheet, bearing late Hercynian hololeucocratic Ta-Nb-Li-Be mineralization, that intrudes mica schists. The drill hole intersects the schists from the surface to a 100-m depth and again at 800 m at the bottom of the hole. Three facies of the granite have been defined on the basis of petrographic and geochemical studies--B1 (100-500 m), B2 (500-750 m), and B3 (750-790 m). Trace element and Sr isotope geochemistry indicate the cogenetic nature of the three facies. Ta contents increase fairly regularly from [asymp]30 to [asymp]250 ppm from the base to the top of the sheet.Previously analyzed samples from exploration drill holes in the apical varieties of the granite (B1, B2) gave scattered delta 18 O values ranging from 4.6 to 8.6 per mil. The delta 18 O value was measured in whole rocks and mineral separates from the deep drill hole samples. These samples were carefully selected to be unaffected by hydrothermal and low-temperature alteration. The B1 facies and the upper part of the B2 facies are characterized by the presence of two generations of quartz, an early globular form (Q g ) and a late interstitial quartz (Q i ). The results show a decrease in delta 18 O values from depth upward. In the B3 granite, the average values are normal for leucogranites: delta 18 O of interstitial quartz = 11.6 per mil and of mica = 9.3 per mil, whereas in the B1 granite they are 5.3 per mil for interstitial quartz and 4.5 per mil for mica. The values for the B2 granite are intermediate between those for the B1 and B3 facies. Oxygen isotope temperatures are 570 degrees + or - 50 degrees C, similar to experimentally obtained solidus temperatures. The delta D values of lepidolite and whole rock from fresh samples from the drill hole range, respectively, from -91 to -66 and -76 to -60 per mil compared with SMOW. The wt percent of H 2 O in whole rock ranges from 0.23 to 0.76. No correlation exists between the delta D values of lepidolite or whole rock and the delta 18 O values of minerals, or the depth or wt percent of H 2 O. These results are interpreted as evidence of interaction, in the liquidus-solidus interval, between the magmatic system and external water of meteoric origin. This water probably originated in the convective system of the hydrothermal fluids related to the penecontemporaneous emplacement of the nearby Colettes granite.Globular quartz from the B1 facies shows delta 18 O values similar to quartz in the lower part of the drill hole (delta 18 O = 8.5-11ppm). This similarity confirms that the three granitic facies are cogenetic and constrains the time of interaction with the external fluid to a period after the crystallization of the early globular quartz. Topaz is also an early crystallizing mineral that contains fluid inclusions of magmatic origin. A variation of 5.0 to 10.1 per mil in the delta 18 O values of topaz downward in the borehole agrees with a high-temperature isotopic equilibrium between quartz and lepidolite and provides evidence of early interaction between the upper part of the Beauvoir granite with external fluids during the magmatic and late magmatic stages. This early interaction shown by the isotopic characteristics of magmatic mineralogy and by the isotopic temperatures took place across the liquidus-solidus interval.

Lithium-Bearing Donbassite and Tosudite from Echassières, Massif Central, France

AbstractLithium-bearing donbassite and tosudite were found in veins in hydrothermally altered granite (Beauvoir granite) in the northern part of the Massif Central, France. The two minerals are characterized by their high Li contents and low Mg and Fe contents; their structural formulae are: $${\left( {S{i_{3.81}}A{l_{0.19}}} \right)_{\Sigma = 4.00}}{O_{10}}{\left( {A{l_{3.81}}L{i_{0.52}}Fe_{0.01}^{2 + }C{a_{0.02}}M{g_{0.01}}} \right)_{\Sigma - 4.38}}{\left( {OH} \right)_8}{\left( {N{a_{0.07}}{K_{0.04}}} \right)_{\Sigma = 0.11}}$$ for donbassite and $${\left( {S{i_{3.50}}A{l_{0.50}}} \right)_{\Sigma = 4.00}}{O_{10}}{\left( {A{l_{2.95}}L{i_{0.22}}Fe_{0.01}^{3 + }T{i_{0.01}}} \right)_{\Sigma = 3.19}}{\left( {OH} \right)_5}{\left( {C{a_{0.01}}N{a_{0.15}}{K_{0.18}}} \right)_{\Sigma = 0.34}}$$ for tosudite.These chemical compositions indicate that the donbassite is an intermediate member of the donbassite-cookeite solid solution series and that the tosudite consists of interstratified Li-donbassite and beidellite. Both Li-bearing minerals show thermal behavior distinct from those previously reported for dioctahedral chlorite and tosudite.Petrographie investigation of drill cuttings from the Echassières area indicates that the two minerals were formed in an intermediate stage of hydrothermal alteration following an early stage characterized by formation of muscovite (2M1) at &gt;350°C and before the latest stage characterized by deposition of kaolinite and randomly interstratified illite/smectite at &lt; 100°C. Moreover, tosudite occurs in the upper part of the granite, whereas donbassite is restricted to the lower part, suggesting the formation of tosudite at lower temperatures.