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Mineral prospectivity mapping (MPM) aims at outlining areas with the highest mineralization hosting likelihood. The outcome of data-driven MPM approach highly depend on the spatial resolution and precision of the input cartographic datasets. Besides, the geological map has the inherent specificity of displaying 3D structures on a 2D surface map view; some structural configurations can usually lead to a misestimation of the relative proportion of the geological units, the faults or other features displayed in the map. From the Cell-Based Association (CBA), we develop here a new approach – the “Disc-Based Association” (DBA) – analyzing neighboring associations of cartographic features. First, disks entities are generated over a regular node grid covering the study area. Each disk discretize the overlapped data layers by integrating their information into a multivariate spectrum. Then, a Random Forest (RF) predictive model classify the multivariate spectrum from both mineralized nodes (e.g. whose associated disk contain a mineralization) and nonmineralized ones. As mineral concentration processes are inherently rare natural phenomena, we argue that all nodes except the mineralized one can be regarded as non-mineralized and thus used in the classification process. Predictivity scores are computed over the area based on the multivariate spectrum classification. This approach could allow the identification of several signature spectra associated to mineralization; which could be interpreted as several distinct metallotects / mineralization processes forming deposits in the study area. To test and evaluate its consistency, this new methodology is applied to prospectivity mapping of Sb along the Ibero-Armorican Arc (Western European Variscan Range). We infer this new data-driven approach applied to natural cases will improve prospectivity mapping and automatic recognition of new metallotects while giving new insights on the genetic processes forming deposits

International audience; Geological processes leading to lithium-rich rare metal (RM) pegmatites and granites remain poorly constrained. To acknowledge geological processes related to Li-rich melt generation, we applied complementary approaches in the Montagne Noire (MN) and in the Monts d’Ambazac (MA). In the MN, the mineralogy of pegmatite clusters depends of the nature of the migmatitic host rock. Geostatistical studies demonstrate that pegmatite clusters are scattered and do not appear randomly. Lithium isotopes tracing (7Li, ‰) show that the mineralized pegmatites are not fractionated and display likely a crustal signature. Rb/Sr dating on lepidolites shows that their emplacements are synchronous with regional partial melting. In MA, structural studies and 3D modelling suggest the presence of an anatectic dome masked by a granite. Geostatistic studies demonstrate that emplacement of pegmatite clusters are controlled by normal faulting. Lithium isotopes show that the mineralized pegmatites are not fractionated and have a crustal signature. Rb/Sr dating on lepidolites show that pegmatite emplacement postdates the granite emplacement.A compilation of RM intrusions in France has been conducted in the present work and shows that: i) they are usually located in or along antiformal structures; ii) the core of antiformal structures consists of migmatites, amphibolite-grade metasediments or anatectic granites; iii) RM granites are spatially associated with major faults; iv) Li-rich RM pegmatites are absent in the case of LP-HT partial melting. These results point to an anatectic origin of RM pegmatite fields with a strong tectonic control of the intrusions near migmatitic domes. Then, the presence of RM pegmatite indicates presence of an underlying partial melting zone which should be more distal in the case of a granite. Moreover, the absence of Li-rich intrusions in the case of LP melting conditions suggests that pressure has a key role in magma enrichment processes during partial melting.

International audience; This work presents the preliminary results of geochemistry of mafic intrusions (diabase dykes) and their relationship with antimony mineralization in the Central-Iberian Zone (Variscan Belt). Two different areas were studied, the Almadén (Al) and the San Antonio (SA) areas.Both macroscopic and microscopic observations show that mafic dykes are mainly composed by clinopyroxene, plagioclase, Fe-Ti oxides and to a lesser extent of calcite and sulphides (pyrite, chalcopyrite and pyrrhotite). These samples are altered presenting chlorite and epidote as alteration minerals. Pyroxene is sometimes altered to amphibole.Whole rock geochemistry analyses from 20 samples show a difference between SA and Al dolerites. The first fall into the classical basalt field whereas the second fall into the alkali basalt field according to the Zr/TiO2 vs Nb/Y diagram. The tectonic setting for the SA samples coincides with the volcanic arc setting whereas the samples from Al fall into the within plate magmatism. Primitive mantle normalized diagrams display high negative anomalies in Rb, K, with small negative anomalies in Nb and Ta for both SA and Al. High positive anomalies for both areas in Cs, Pb (especially for SA) and Li accompanied by small positive anomalies in P and Ti can be observed. Dolerites from Al are more enriched in Ba, Th, U, Nb, Ba, La, Ce, Sr, P, Nd, Sn, Zr, Hf than SA. All samples are depleted in HREE and enriched in LREE. Anomalies in Rb, Nb, Ta and Li may be related with crustal contamination. Pb anomalies could be associated with assimilation of country rocks, especially marine sediments, this anomaly is also related to subduction processes. Positive P and Ti anomalies of some samples is due to the apatite and ilmenite enrichment respectively. Negative anomalies in K could be associated with presence of phlogopite in the source. Rare Earth Elements contents are compatible with the presence of garnet in the source and low degree of partial melting, this is consistent with the correlation between La/Sm vs Gd/Yb and La/Sm vs Rb. Trace element ratios such as Th/La (0,10 for SA) and (0,09 for Al) suggest an enriched mantle source.Some of these mafic intrusions were collected near antimony mineralization whereas the other are located at distance but in the same swarm of mafic dykes. A spatial and genetic link between Sb mineralization and mafic magmatism has been proposed in other parts of the Variscan Belt, especially in the Armorican Massif.The source of these Sb mineralization could be related to an enriched mantle with crustal contamination. The geochemical link between mafic magmatism and Sb mineralization and their source in the Central Iberian Zone is still a matter of study.AcknowledgmentsThis work was funded by the ANR (ANR-19-MIN2-0002-01), the AEI (MICIU/AEI/REF.: PCI2019-103779) and author’s institutions in the framework of the ERA-MIN2 AUREOLE project, as well as by Project SBPLY/17/180501/000273, Consejería de Educación, Regional Government of Castilla-La Mancha, Spain.

International audience

article de vulgarisation en anglais; International audience

National audience

In Europe, exploration geochemistry is mainly restricted to classical soils geochemical analyses based on total or near total digestion of samples. Examples of biogeochemistry usage in continental Europe remain anecdotic. However, with the actual detection levels in analytical methods and the knowledge about metals accumulation reached in a large spectrum of plants, biogeochemistry has already been successfully tested in various environments and over distinct deposit types. Within the UpDeep project (European Institute of Innovation and Technology-Raw Materials) framework that aims to develop the geochemical expertise on deep buried exploration in Europe, a biogeochemical survey has been done on several known Sb deposit of the Vendée District (France). One of the challenging questions of exploration geochemistry in Western Europe is the long-term anthropogenic contamination related to past mining activities but also agricultural and forestry activities, which can strongly affect the primary geochemical anomalies especially in soil horizons and plants. In the studied area, intensive agriculture is widespread, and forestry has been conducted intensively. In such environment, hedges remain the only area where variety of plant species is important enough to chose a good media, in particular for trees. Within the scope of testing different plant species, oak bark and bramble leaves were sampled at around 110 sites. After powdering, the samples experienced a HNO 3 digestion followed by ICM-MS analyses. To identify the geochemical signatures related to Sb mineralizations, classical compositional data analyses have been carried out before response ratios and log ratios applications.

In order to face the challenges of mineral exploration in inhabited areas, portable XRF (pXRF) is gaining attention as a low-footprint exploration technique. To evaluate it, we revisited historic mining districts, simulating grassroots exploration with pXRF, within the UpDeep project under EIT-RM EU funding. A specific attention was given to strategic metals and mineral resources. A first study on a Sb district was presented at AIMS 2019 (Lemiere et al., 2019). The second study was led around an historic W and kaolin district (Echassières, central France) where modern Nb-Ta-Li exploration is going on in an agricultural and forested area. We performed top soil sampling (Ah and B horizons) along profiles to understand better the endogenic geochemical anomaly signals. The survey, limited to a fraction of the mining area, succeeded in recognising fault-controlled anomalies of several types (Cu-Sn-As, Fe-As) besides the cupola type W-Sn-As anomalies. Li and Be being beyond pXRF capabilities were not targeted. Despite of an expected bias with laboratory analyses, pXRF measurements effectively located the various mineralised structures, thanks to their multi-element capabilities. Composition data processing (CoDa) and horizon-selective sampling significantly improved the efficiency of the method. On-site measurements allow dynamic sampling and mapping, helping laboratory sample selection for further investigations. The very high As background extending beyond mineralised areas is actually controlled by a NNE-SSW faults system. It can be used as a guide to possible fertile structures, but also to barren ones. The main Sn-W anomalies are located along a known structure, but much beyond previous mining works. Cu-Sn anomalies along another structure seem to confirm the location of ancient mining works.

Lithium, cobalt and natural graphite are essential for energy storage technologies. Lithium and cobalt are used in rechargeable batteries. Natural graphite is used as refractory for steel production, but its consumption for batteries is growing significantly. Demand for these elements is expected to surge with the increasing electrification in the transport sector. Both natural graphite and cobalt are critical raw materials (CRM) for the EU, while lithium is above the supply risk threshold. As these elements are produced outside Europe, their supply for the European industry is potentially a threat. To address this issue, the FRAME project has been designed to research CRM in Europe that are essential for "green" energy supply technologies. An objective of FRAME is to produce predictive assessments of CRM based on GIS exploration tools at continental scale, in order to identify high potential mineral provinces and mining districts. In this contribution, we present mineral prospectivity maps of Europe for primary lithium, cobalt and graphite. They are based on CBA ("Cell Based Association") that is an alternative to GIS supported prospectivity methods, developed by BRGM to better manage uncertainties related to cartographic data (e.g., lithological contours or point locations). We present the approach, benefits for continental scale MPM and results. We have also applied a classical Weight of Evidence (WofE) method and hybrid fuzzy WofE model for mineral potential mapping that generates fuzzy predictor patterns based on (a) knowledge-based fuzzy membership values and (b) data-based conditional probabilities applied to a comparison of the results.

International audience; In Europe, exploration geochemistry is mainly restricted to classical soils geochemical analyses based on total digestion of samples, despites the good results obtained in other areas with approaches related to selective and partial leaches, in particular to reveal anomalies linked to deeply buried deposits. Similarly, examples of biogeochemistry usages in Europe remain anecdotic, but it is now very efficient to help in focusing exploration efforts with the actual level of element detection reached in a large spectrum of plants. Within the framework of the UpDeep project (EIT Raw Materials), which aims at developing the geochemical expertise on deep buried exploration into Europe, we performed a comparison of some of these new geochemical methods on the Vendée antimony district (France). One of the challenging questions of exploration geochemistry in Western Europe is the long-term anthropogenic contamination related to past mining activities but also agriculture and forestry, which can strongly affect primary geochemical anomalies. Within the scope of testing different well-documented approaches, we collected around 160 samples of soils on which we performed separately the following treatment: deionized water leach, sodium pyrophosphate leach, and Aqua Regia digestion on Ah horizon; and hot hydroxylamine leach, Ionic LeachTM (ALS) and Aqua Regia digestion on B horizon. Each soil samples have been analyzed by pXRF. We also collected 184 samples of oak and 94 samples of bramble twigs in order to compare their geochemical signal. In this Western European context, identification of a homogenously distributed vegetation media represents a complex issue.

International audience

International audience; There are, based on extracts from the national mineral resource databases of the EuroGeoSurvey members, 1195 occurrences of Li, Co and graphite in Europe, 17of these are active mines This compilation is part of work package 5 of the FRAME project (Forecasting and Assessing Europe's Strategic Raw Materials needs) a section within the framework of the GeoERA project. The data collected classify the occurrences according to their genetic type, their occurrence type and production status. We regard in this compilation all Co locations with a mean Co >100ppm as occurrences for Co. For the other commodities, Li bearing minerals or graphite must be positively identified or explored for to be included. Methodology We collected from European Geological Surveys basic geographical information, status (active/closed/not exploited), occurrence type (occurrence/prospect/deposit) and genetic type for each occurrence. We show the distribution of Li, Co and graphite occurrences in different EU countries (Fig. 1) and their locations and classification according to their genetic types, in Fig 2, 3 and 4.. Results: Lithium Spatial distribution The distribution of lithium in Europe shows a strong clustering (Fig. 2) highlighting the Li potential of the Variscan belt of south and central Europe. Deposit types High-grade Li deposits (Gourcerol et al., 2019; Gloaguen et al., 2018) are represented by: • Li-rich LCT pegmatites • Rare metal granites (Melleton et al., 2015) • Atypical stratiform deposits such as Jadar, encountered in intramontane lacustrine evaporate basins Medium-grade Li deposits (Gourcerol et al., 2019) are represented by: • hydrothermal deposits such as greisens • Li-bearing quartz veins associated with some peraluminous rare metal granites. Other types (Gloaguen et al., 2018) Li-rich (tosudite) hydrothermal alteration aureole around gold quartz veins; Cookeite/lithiophorite in black shales (Dauphiné: ave. 441 ppm Li 2 O, up to 1 847 ppm) or in bauxite deposits and Mn-(Fe) deposits. Li-rich clays (hectorite) are presently unknown in the EU. Lithium carrier Lithium is hosted by various minerals such as phosphates (amblygonite, montebrasite, triphylite), inosilicate (spodumene), phyllosilicates (lepidolite series, zinnwaldite, petalite) and scarce borosilicate (jadarite). Co-products are generally Ta, Nb, Sn, Be. Examples, grades & tonnages Jadar-type Li deposits occur in Serbia and Bosnia. In 2017, the total mineral resources report 135.7 Mt of ore at a grade of 1.86 % Li 2 O and 15.4 % B 2 O 3 (Rio Tinto, 2017) that represents a giant deposit of 2.524 Mt of Li 2 O. / Wolfsberg pegmatites, Austria (236 366.46t Li 2 O-grade 1.0%) / Sepeda pegmatites, Portugal (221 728.1t Li 2 O-grade 1.0%). / Beauvoir rare-metal granite, France (375 000t Li2O-grade 0.78%) / Cinovec greisen (Czech Republic, 5 652 990.2t Li 2 O-grade 0.4 Li 2 O). / Argemela quartz-amblygonite veins mine (Portugal, 80 400t Li 2 O-grade 0.4%).

International audience; Driven by a global move towards renewable energy through the last decade, lithium has become a strategic metal due to its physical and chemical properties (e.g., lightest solid element and excellent conductor of heat and electricity) making it an excellent candidate for electrification of transport and green technologies such as Li-ion batteries and other energy storage devices. In this context, European interest in lithium mining and exploration has risen sharply and many countries are currently assessing their own mineral resources and reserves in order to secure their supplies. From this perspective, a geographically-based compilation of the European lithium hard-rock occurrences and ore deposits with their corresponding features (e.g., deposit types, Li-bearing minerals, Li concentrations) is presented. Accordingly, it appears that lithium is not particularly rare and is well represented and distributed through various deposit types related to several orogenic cycles from Precambrian to Miocene ages. Thus, about thirty hard-rock deposits have been identified mostly resulting from endogenous processes such as lithium-cesium-tantalum (LCT) pegmatites (e.g., Sepeda in Portugal; Aclare in Ireland; Läntta in Finland), rare-metal granites (RMG; Beauvoir in France; Cinovec in Czech Republic) and greisen mineralisation (Cligga Head, Tregonning-Godolphin, Meldon in UK; Montebras in France). Local exogenous processes such as hydrothermal and erosion transport may be also related to significant secondary Li-endowments including jadarite precipitation in the Jadar Basin (Serbia) but are rarely related to economic grade of lithium (e.g., Mn-(Fe) deposits, bauxite). Moreover, a systematic assessment of metallogenic processes in the Li endowment has been conducted and several relevant parameters have been identified and may offer exploration guidance for mining companies and governments. These parameters are: 1) a pre-existing Li-rich source related either to paleoenvironmental sedimentation conditions or a crustal anomaly; 2) presence of lithospheric thickening, which may reflect a favourable process to concentrate Li; 3) a regional or local extensional regime; and 4) existence of fracture sets acting as channel ways for exogenous processes.

International audience; The French Massif Central (FMC) forms the hinterland of the west-European Variscan belt and was subject to numerous tectonic and metamorphic events from Devonian to Permian times. The accreted tectonic units belonging to Gondwana and Laurussia were variably involved in subduction, nappe stacking and syn-to post-orogenic extension. Besides, plutons and migmatites showing syn-collision crust melting are very common in the Variscan belt. Such orogenic evolution implies that the resulting finite structure likely reflects the interference between fabrics formed at different stages of the orogenic cycle with different degrees of overprinting, depending on e.g. the spatial strain gradients, lithologies, structural level or partial melting grade. We present a model of finite strainfield of the western FMC based of the analysis/interpolation of ductile and brittle fabrics, at regional to local scale, using geostatistics. Ductile fabrics within autochthonous units and syn-collisional plutons are poorly disturbed and reflect the transition from syn-thickening to extensional lateral flow of the partially molten mid-lower crust. In contrast, allochtonous units are affected by numerous perturbations including polyphased ductile shearing, folding and faulting inducing local bloc tilting and rotations. Taking into account the overprint of late tectonic events, we show that early fabrics related to High Pressure metamorphism or nappes' stacking have suffered intense reorientations. The finite trend of the related lineations L1 must be treated with extreme caution and may not reflect the original direction of stretching and shearing that occurred during the initial compressional stages. Taking into account i) the spatial distribution and amount of reorientation of these D1 fabrics and ii) the deconvolution of these disturbances, we propose that:-The high dispersion of lineation trend can be simply explained by passive rotations, during late isoclinal folding, rather than two superposed tectonic phases, as previously inferred.-The allochtonous units formed one single tectonic unit.-This D1 event looks related to the exhumation of the subducted continental crust due to partial melting-The formation of Variscan orogenic prism in western FMC only results of continuous south-directed thrusting rather than a switch from NE-SW to NW-SE thrusting.

International audience; Geological processes that lead to lithium-caesium-tantalum-rich pegmatites remains poorly constrained and understood. This type of rare-metal pegmatites, which sometimes constitutes economic Li±Sn±Ta±Be deposits, are generally encountered in the vicinity of large pegmatite fields frequently focused on migmatitic areas. Two distinct models for pegmatitic melt generation are proposed:1-products of the end-of-crystallization melts of fractionated granites; or2-products of low-rate partial melting of metasedimentary rocks.In order to constrain geological processes related to pegmatitic melt generation, we applied several approaches (field work, statistical spatial analyses, mineralogical, geochemical and isotopic studies) on two well-known areas hosting rare-metal and barren pegmatite fields: 1) the Montagne Noire migmatitic dome; and 2) the Ambazac Mounts.In the Montagne Noire migmatitic area, the mineralogy of pegmatite clusters mimics the host-rocks with biotite-bearing pegmatites mainly hosted by migmatitic paragneisses and muscovite-bearing pegmatites mainly hosted by migmatitic orthogneisses. statistical spatial analyses studies demonstrate that pegmatite clusters are scattered in the center of the dome and do not appear randomly. Lithium isotopes show that the mineralized pegmatites are not fractionated and have likely a crustal signature. Rb/Sr dating on lepidolites shows that pegmatite emplacements are synchronous with the regional partial melting. Secondly, in the Ambazac mounts area, structural studies and 3D modelling suggest that this area may correspond to an anatectic dome masked by a granite, where pegmatite clusters are hosted by the Saint-Sylvestre granite. This granite crops on the border of the dome. Statistical spatial analyses studies demonstrate that emplacement of pegmatite clusters are controlled by normal faulting. Lithium isotopes show that the mineralized pegmatites are not fractionated and have also a crustal signature. Rb/Sr dating on lepidolites shows that pegmatite emplacement postdates the granite emplacement and is coeval with a late regional partial melting event. Following this, a compilation of the geology of rare-metal pegmatites fields of France has been conducted. This compilation underlines several common features between the distinct pegmatites fields: i) they are usually located in or along antiformal structures; ii) the core of antiformal structures consists of migmatites if erosion level is sufficient, otherwise the core consists of metasedimentary rocks or is occupied by anatectic granites; iii) the highest density of pegmatites occurs mainly in migmatites; iv) rare-metal pegmatites seem to be absent in the case of LP-HT partial melting. Thus, these results point to an anatectic origin of pegmatite fields with a strong tectonic control of pegmatite cluster emplacements above migmatitic domes. Then, the presence of pegmatite indicates presence of an underlying partial melting zone. Moreover, the absence of mineralized pegmatites in the case of low-pressure melting conditions suggests that pressure has a key role in magma enrichment processes during partial melting.

International audience

International audience; Numerous Sb-Au deposits are known in the European Variscan belt and are often associated with the late Variscan hydrothermal events linked to postorogenic extension and strike-slip faults, through the whole belt. Central Brittany hosts several Sb-Au occurrences, such as the Le Semnon Sb-Au deposit or the Saint-Aubin-des-Châteaux base metal-Sb-Au occurrence. In order to provide new constraints on the Sb-Au mineralization framework of the Central Brittany, structural, geochronological and chemical analyses have been performed on the Le Semnon deposit and the Saint-Aubin-des-Châteaux occurrence. Main results show that Sb-Au trapping of these two occurrences occurred at ca. 360 Ma. This indicates an Early Carboniferous economic Sb-Au mineralizing peak in the eastern part of the Central Brittany. Moreover, a mafic magmatism is widespread in the region and can acted as the plumbing system for Sb-Au mineralizing fluids, such as the dolerite dyke of the Semnon Sb-Au deposit. Absolute dating of this mafic magmatism coupled with the early character of hydrothermal alteration associated with Sb-Au mineralizing fluids strongly suggest that the emplacement of Sb-Au mineralization and mafic magmatism was coeval. It appears that the emplacement of mafic magmatism represent a major trigger for this mineralizing system at shallow depths (less than 3 km). These new data and the ca. 360 Ma massive sulfides of south Iberia emphasize that the Early Carboniferous mafic magmatism has to be taken into account for the understanding of the genesis of mineralizing systems and the redistribution of metals in the crust, at the scale of the whole Variscan belt.

International audience; Numerous methods of mineral prospectivity mapping have been developed during the last decades and it is often difficult to evaluate their reliability and adequacy. Some objective a posteriori approaches allow calculating their performance such as, for instance, AUC values on ROC curves. However, we believe that field control is a necessary step, especially in the context of mineral targeting in greenfield exploration. Mineral prospectivity mapping generally relies on unequivocal associations between discrete data (e.g., known deposits) and polygons (e.g., lithology, geophysics, geochemistry, etc.). Consequently, the quality of results obtained from 'classical' methods (e.g., Weight of Evidence, Fuzzy Logic, Logistic Regression, Neural Network, etc.) strongly depends on the accuracy of input data (location of points and contours of polygons) which, in many cases, may be questionable. To address this issue of geographic inaccuracy of map elements, BRGM has developed the CBA (Cell Based Association) method. Its base principle is not to rank polygons in which known deposits are located, but to identify favorable association neighboring these deposits (i.e. in cells of a regular grid containing them). In order to assess the adequacy of CBA for mineral targeting in early exploration phases, we have done a first field validation campaign for Sb in the Vendée region (western France). 107 soil samples have been collected (auger drill) in both favorable and unfavorable cells (according to CBA results) and across know Sb-bearing veins. They have been analyzed at ultra-trace level using ICP-MS/ICP-AES (detection range of 0.05 to 10 000 ppm for Sb). In this contribution, we present an overview of the CBA method, its resulting prospectivity map for Sb in Vendée, the sampling and analyzing procedure, the preliminary results we have obtained and the first conclusion they allow.

International audience; The W-Sn veins of Panasqueira are connected with a massive quartz-muscovite greisen cupola. This greisen results from interactions between granitic rock and acid Sn-W hydrothermal fluid, which causes the destruction of feldspars of the Panasqueira granite. Greisenisation can constitute an important factor of control on fluid flow, and its role on metal transport remains to be established. Mineral reactions of greisenisation involve a decrease of the rock volume and thus an increase porosity (~6%). Consequently permeability increases significantly (10-20 to 10-17 m²) enhancing fluid flow in the greisen cupola and promoting further hydrothermal alteration and transport of metals. Finally a part of the mineralization is trapped in this porosity, with precipitation of cassiterite and sulfides. Therefore, Sn-W fluids are able to generate their own pathways in initial impermeable granite via greisenisation reactions.

National audience; Palaeozoic oolitic ironstones layers are widespread in Europeand North Africa, mainly in external parts of the Variscanorogeny where they have been preserved from intense deformationand metamorphism. These ironstones are mainly of two types,either composed of carbonates and silicates (siderite-chlorite), ormade of oxides and silicates (magnetite-chlorite). In the first case,ironstones are chemically very reactive to crustal fluid circulations.In the central domain of the Armorican Massif (Western France),it has been demonstrated (Gloaguen et al., 2007) that the ooliticiron ore from the Saint-Aubin-des-Châteaux quarry suffered anepigenetic transformation by hydrothermal fluids that led to thedissolution of the iron ore and its replacement by massive sulphidelenses. Such hydrothermal alteration is still localized atthe intersection between the ironstone layer and Variscan rightlateralstrike slip faults. Both, structural constraints, parageneticsequence and geochronological results (Gloaguen et al., 2007;Tartèse et al., 2015) underline 100 Ma of successive fluid flowsand polyphazed hydrothermal alteration of the ironstone. A verysimilar type of oolitic-hosted epigenetic gold deposit has beendescribed by Couto and Moëlo (2011) in the Banjas gold mine(Dúrico-Beirã district, Valongo, Portugal; Couto et al., 1990;Combes et al., 1992). This argues for a widespread distributionof such epigenetic mineralization through these two orogenic segmentsof the Ibero-Armorican arc at least. Since these Palaeozoicironstone layers are frequently hosted within, or close to, thicksandstone formations, outcrops are generally scarce. Moreover,because of the lack of magnetite and a thickness in a range 2to 6 m, these layers are not well targeted by geophysical approaches.Then, development of a specific targeting focused onintersections between faults and palaeozoic siderite-bearing ironstonelayers may lead to several significant new discoveries ofAu-Ag-Sb, Cu-Pb-Zn ore deposits.

International audience; Les appareils de fluorescence X portables (FPXRF) sont couramment utilisés lors des campagnes d'exploration.Le faible coût de ces analyses, leur rapidité d'acquisitionsur échantillons bruts, sur le terrain ou en laboratoire, ainsi que des gammes de teneurs analysables variant de quelques dizaines de ppm à plusieurs pourcents, sont des arguments forts pour une généralisation de cette technique en exploration.Cependant, de nombreux paramètres relatifs à la qualité des échantillons et conditions analytiques influent sur la pertinence et l'exploitabilité des résultats.De ce fait, la validité des analyses FPXRF par rapport aux analyses géochimiques classiques est encore largement débattue. Afin de comparer les analyses géochimiques ICP-AESet FPXRF, et les réponses des différents éléments métalliques, deux lots d'échantillons de type streamsédiments prélevés lors de l'inventaire minier français (années 1970-80s) ont été ré-analysés par FPXRF au moyen d'unNiton XL3t 600S GOLDD. Les échantillons testés correspondent à la fraction inférieure à 125 µm et présentent un taux d'humidité inférieur à 3%. Après plusieurs tests, le temps d'analyse a été fixé à 60s en mode sol et en mode mine. La Zone 1 est centrée sur l'amas sulfuré à Cu-Pb-Zn de Chizeuil. La Zone 2 comprend les indices à U de Sandelesse, les indices à Sb de la Marnière et de la Ramade, les indices à BaSO 4 de Saint-Merd-la-Breuille, les indices à As d'Allevergue et Foliat, et les indices à F de Veaumas, ainsi qu'une forte anomalie géochimique à W.

National audience; Les pegmatites sont des objets crustaux d’origine magmatiqueaux caractéristiques exceptionnelles : il s’agit toujours d’amasd’injections de très petits volumes enrichis à extrêmement enrichisen fluides et éléments incompatibles par rapport aux granitescrustaux. Leur variabilité géochimique et les différents contextesdans lesquels on peut les observer font des pegmatites des objetsd’intérêt tout particulier du point de vue du traçage de l’histoiredes circulations magmatiques et des fluides au travers de la croûte.Cependant, les processus de genèse, migration et mise en place deschamps de pegmatites restent mal connus, ceci étant illustré pardeux modèles diamétralement opposés : (i) les pegmatites dérivantd’un processus de fractionnement long à partir d’une chambremagmatique moins fractionnée et considérée être la source de cesliquides granitiques, (ii) les magmas formant les pegmatites directementextraits de sources de fusion partielle de bas taux dansla croûte continentale ; les mécanismes d’extraction, de remontéeet de mise en place des pegmatites devant alors fortement êtrecontrôlés par la localisation de la déformation dans la croûte continentale.Nous développons ici une approche d’analyse spatiale statistiquesur la répartition de pegmatites dans plusieurs champs de la ChaîneVarisque. En effet, alors que l’analyse des distributions spatialesd’objets en carte est souvent restreinte à une analyse purementvisuelle, des outils et méthodes de cartographie numérique permettentde quantifier les tendances de répartition ou d’associationspatiale entre objets. Dans le détail, les objets cartographiquesd’intérêt sont considérés comme jeu de données d’apprentissage.Différents calculs de paramètres sont alors possibles pour quantifierleur répartition cartographique et en particulier leur degré deregroupement/dispersion sur une gamme d’échelles continue.Un second objectif consiste à quantifier le degré de recouvrementou de proximité des pegmatites avec les granites et/ou les structures.Même si ces relations spatiales peuvent parfois paraîtreévidentes à la simple lecture d’une carte, les configurations cartographiquesne permettent pas toujours de bien appréhender lesassociations spatiales. Différents outils d’analyse spatiale peuventêtre appliqués afin de pouvoir formaliser la quantification du degréd’association entre les objets cartographiques.Les résultats montrent une caractéristique des pegmatites enrichiesen Li à former des regroupements à plusieurs échellescomme dans le champ de Barroso-Alvão (Portugal). En outre,la dispersion cartographique et l’alignement des corps pegmatitiquesminéralisés des Monts d’Ambazac (France) montrent uncontrôle clair de structures régionales.

International audience; The Armorican belt is part of the westernEuropean Variscan belt. Although the region was oneof the world leader producers of antimony at thebeginning of the 20th century, the geological controlsbehind these deposits remain badly constrained. Herewe present a spatial statistical analysis of the Sbmineralisation,together with geophysical andgeological data [1]. Results show that the Sbmineralisationis spatially associated with strongpositive gravity and magnetic anomalies which mustbe linked to the presence of mafic/ultramafic bodiesat depth. This spatial link is further supported by thenumerous outcropping occurrences of dolerite dykesand sills close to the Sb-deposits and sometimeshosting the mineralisation. This mafic magmatism,dated by in-situ U-Pb analyses on apatite at ca. 360Ma, appears as a regional-scale event at theDevonian-Carboniferous boundary. In addition, newdata suggests that mafic magmatism may haveprobably played a role in the early distribution orredistribution processes of metal stock in thesubsequent history of the antimony in the VariscanArmorican belt.[1] Pochon et al. (2016). Terra Nova, doi:10.1111/ter.12201

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International audience; This work describes the use of conformer-selective laser spectroscopy following supersonic expansion to probe the local folding proclivities of four-membered ring cyclic β-amino acid building blocks. Emphasis is placed on stereochemical effects as well as on the structural changes induced by the replacement of a carbon atom of the cycle by a nitrogen atom. The amide A IR spectra are obtained and interpreted with the help of quantum chemistry structure calculations. Results provide evidence that the building block with a trans-substituted cyclobutane ring has a predilection to form strong C8 hydrogen bonds. Nitrogen-atom substitution in the ring induces the formation of the hydrazino turn, with a related but distinct hydrogen-bonding network: the structure is best viewed as a bifurcated C8/C5 bond with the N heteroatom lone electron pair playing a significant acceptor role, which supports recent observations on the hydrazino turn structure in solution. Surprisingly, this study shows that the cis-substituted cyclobutane ring derivative also gives rise predominantly to a C8 hydrogen bond, although weaker than in the two former cases, a feature that is not often encountered for this building block.

International audience; In order to define new metallogenic guides for Li, Nb and Ta supply, three Variscan LCT pegmatite fields are investigated. A multidisciplinary approach is favoured to discuss both the genetic and emplacement models for LCT-type deposits. The spatial statistical analyses demonstrate the high clustering rate of Li-rich pegmatites and the spatial relationships between Li-pegmatites location and surrounding damage fractured zone. The Li-isotopes analyses on micas suggest the absence of genetic continuum between surrounding granites and pegmatites since δ 7 Li values (‰) are similar through pegmatite field and inside granite samples. To refine these previous results, preliminary numerical models are developed to constrain the ascent of these low viscosity pegmatite-forming melts and determine key criterion of their uncoupling from parental magma source. According to these results, the high permeability seems to play a crucial role in the pegmatite melts collection and during their ascent. The combination of geostatistical, geochemical and numerical tools allows suggesting an alternative model of direct crustal anatexis to the common parental granite model. These high permeability damage shear zones are considered to favour the low volume of pegmatite-forming melt pumping and their escape from their deeper crustal magma source.

International audience; The European Variscan belt, which is characterized by a huge amount of granitic intrusions, displays several significant rare-elements pegmatites and/or granites fields which could represent an important source of raw materials. An overview of these occurrences gave the possibility to discuss the processes at their origin and their common classification. Indeed, the spatial and temporal distributions of rare-element pegmatites in the European Variscan belt suggest that rare-elements magmatism would rather be related to local specific conditions (in particular sources, tectonic and thermal regime) than lower crustal processes like rare-elements enrichment subsequent to their release during granulite metamorphism. Actual classification of rare-elements pegmatites, based on their geochemistry and mineral composition, does notallow to discern pegmatites formed in different tectonic regimes and which exhibit various internal structures (e.g. the pegmatites of the lepidolite-subclass from Western Iberia and from the Moldanubian domain of the Bohemian Massif for example).

International audience; A new type of large ultramafic intrusion-hosted Fe-Ti-V deposit has been discovered by BRGM in the West-African Archean craton. The N’Guérédonké intrusive complex (7 x 0.9 km) is composed of a dunite core surrounded by rims of wehrlites and pyroxenites. This syntectonic complex is emplaced within a crustalscale shear zone. The disseminated mineralisation made of ilmenite and magnetite is mainly hosted by pyroxenite units along the northern and southern contacts. Surface and drilling exploration work (47 drill holes totalling 10124 m) demonstrate polyphased injection of successive ultramafic melts from core to rims and also crosscutting of dunite core by a network of successive pyroxenite generations. A 3D modeling constrained by drilling and joined inversion of magnetic and gravity data ascribe the thickness of the intrusion up to 1000 m. A first very conservative titanium resource assessment on a small part of the intrusion (1000 x 100 x 250 m) lead to definition of 141.38 Mt ore @ 10.52 % TiO2. The lack of typical features of Ural-Alaskan type intrusions and strong differences with classical Gabbro-Anorthositehosted Fe-Ti-V deposits demonstrate a new type of large low grade high tonnage Fe-Ti-V deposit occurring in Archean craton

International audience; High-phosphorus peraluminous rare-elements granites and rare-elements LCT (Lithium, Caesium, Tantalum) pegmatites are the most important sources of raw materials for some critical metals like tantalum (1,2)represent important economic storehouses for industrial minerals like feldspar, quartz, mica or kaolin. They principally emplace in orogenic settings (3). Afast overview of three mainEuropean Variscan districts, i.e. the Moldanubian domain of the Bohemian massif, the French Massif Central (FMC) and the NW Iberia provides a basis for questioning the origin of rare-elements magmatism and the actual classification of rare-elements pegmatites, in particular the LCT pegmatites. Granitic pegmatites are widespread in most of the Bohemian Massifbut LCT pegmatites are most common in the Moldanubian domain. In this area, their emplacements seem mainly controlled by migmatitic domes and shear zones and correspond to two events(4). The older at ~ 333 ± 3 Ma just follow HT-MP event of the end of the Moravo-Moldanubian phase and the younger at ~ 325 ± 4 Ma is contemporaneous with beginning of the Bavarian phase (U-Pb ages on colombite and tantalite). In the FMC, most of the actually known rare-elements magmatic bodies form a province in the North Limousin area, which represents the northwestern part of the FMC.U-Pb dating of columbite-group minerals from Beauvoir, Montebras and Chèdeville rare-elements magmatic bodies leads to emplacement ages at 317 ± 6 Ma, 314 ± 4 Ma and 309 ± 5 Ma respectively. The contemporaneous Marche fault system (5), which crosscuts in a general E-W trend all the northern part of the Limousin, seems to be a key-structure for the rare-elements magmatism of the area.

International audience; Rare-elements (Li, Cs, Ta, Be, Sn...) magmatism is well expressed in the European Variscan belt with mainly pegmatites and granites. These bodies are known in most of the different realms of the belt, but they are particularly abundants in localized parts of the Iberian massif, the French Massif Central and the Bohemian Massif. U-Pb dating of columbite-tantalite from selected Variscan rare-elements granites and pegmatites of these three massifs has been performed using laser ablation system connected to a single collector magnetic SectorField - Inductively Coupled Plasma - Mass Spectrometer (LA-SF-ICP-MS). The main aims of this study are to investigate the timing of this magmatism in the Variscan belt and the chronological relationship with the surrounding granitoid suites. Our results highlighted the existence of several emplacement episods : - in the Moldanubian domain of the Bohemian Massif, rare-elements pegmatites emplaced at around 340-330 Ma. - in the North of French Massif Central, the Montebras and Beauvoir granites and the Chèdeville pegmatite lead to emplacement age at 315-310 Ma. - in the North of Iberian massif, three different events have been recognized, with the emplacement of the Argemela granite at 326 ± 3 Ma, a first group of pegmatites emplaced at 310 ± 5 Ma, a second at 301 ± 3 Ma. In this context, rare-elements magmatism do not look to be related to lower crust processes (i.e. granulitic metamorphism) and rare metal magmatism event appears diachroneous and polyphased.

International audience; U-Pb geochronological data were collected on columbite-tantalite from rare-elementpegmatites and granites (Li, Ta, Nb, Be, Sn) from the Iberian Variscan Massif in orderto investigate timing of rare-element events during the late evolution of the Variscanbelt and the connection with granitoid suites. Our data set emphasises three events: (i)The Argemela rare-element granite, from the Central Iberian Zone (CIZ), emplaced at326±3 Ma and predates the three main late orogenic granitoid suites, (ii) most of thestudied rare-element pegmatites of the LCT family from the Galicia-Trás-Os-MontesZone (GTOMZ) gave an average age of 310±5 Ma, contemporaneous with an earlysyn-D3 granodiorite-monzogranite suite and the syn-D3 two-micas granites suite; and(iii) The youngest event is dated at around 301±3 Ma and is marked by emplacements ofrare-element pegmatites of the LCT family in both the CIZ and in the South of theGTOMZ. This event postdates the late to post-D3 bt±crd-granodiorite-monzogranitesuite emplacements, which are probably not genetically related to rare-elementpegmatites. Several pegmatites dated during this study do not display contemporaneousages with neighbouring supposed "parent" granites.Contemporaneous rare-element pegmatites fields seem to be located along belts as it isobserved for granitoids suites. Considering both GTOMZ and CIZ fields, emplacementages look to decrease toward the South. This is in good agreement with the knownpropagation of deformation, metamorphism and magmatism between the differentgeotectonic zones of the Iberian Massif.Lower crustal processes are not at the origin of rare-element granite genesis since in theIberian Massif, RM magmatism is polyphased with at least three distinct events.Hypothetical sources of RM melts are supposed to be set in a middle to upper crustposition, as it has been shown for granitoid magmatism.

International audience

International audience; For a big part of the planet, the majority of economic deposits located near the surface have been targeted. Then, new exploration projects must aim to discover "hidden" deposits for which "traditional" methods of exploration like stream sediments or soil geochemical prospecting are unfortunately less efficient. However, groundwater interacts extensively with these deposits in subsurface. For a limited number of mineralization types, several studies have demonstrated the existence of geochemical and isotopic tracers in groundwaters. Metals related to peraluminous granites-related deposits like W, Sn, Ta or Cs are crucial for the developments of some "green" and "high" technologies. The Beauvoir-La Bosse deposits of Echassières, located in the North of the Variscan French Massif Central belongs to at least two distinct metallogenic systems, a stockwork constituted by quartz-wolframite veins, crosscut by the Beauvoir rare-elements granite. Moreover, in the southern watershed, Sb and Cu occurrences have been documented by former prospections. It is located on the top of a hill, and past and actual operations on the deposits provided good conditions for water-mineralization interactions. Waters from surrounding streams and springs located on and around the different deposits were collected. In order to investigate the hydrogeochemical signatures related to these deposits, we performed major, trace elements and rare-earth elements analyses as weel as isotopic measurement (O, D, C, Li, S, Zn, Cu and Sr isotopes). According to these preliminary results, we are able to decipher the geochemical signature of shallow and surface groundwater to understand water migration through the deposits and water-rock-interactions. Moreover, it is possible to distinguish several geochemical types that could be relied on the composition of the deposits. These results give thus good indications on the applicability of hydrogeochemical prospecting for these kinds of deposits, with valuable results in complex systems such as overlapping of several metallogenic systems

International audience

International audience; Post collisional collapse commonly enhances crustal melting. Such melting typically produces peraluminous granitic magmas. In the French Massif Central, a mid-crustal segment of the western Variscan belt, two large granitic bodies were produced during the collapse of the Variscan Belt. The St Sylvestre Leucogranitic Complex (SSyL) in the western part of the Massif Central and the Velay Migmatitic Complex (VMC) in the Eastern part. Although these two complexes are formed in similar geodynamic context they present meaningful petrological and geochemical differences. The VMC (~305 Ma) is clearly intrusive in migmatitic terranes. The migmatitic host recorded two successive melting events M3 (720 °C and 5kb) dated between 335 and 315 Ma and M4 (850°C and 4 kb) dated at 305 Ma. The compositions of the VMC are strictly H2O-undersaturated and ranges from leucogranitic to granodioritic. Three main successive granite types have been distinguished (1) A heterogeneous banded biotite granite, (2) A main biotite-cordierite granite, where cordierite can be prismatic, as cockade or pseudomorphic (3) a late magmatic with large K-feldspar phenocryst and prismatic cordierite. The compositions of the VMC granites are quite similar to typical Australian S-type granites in the sense that they also show a positive correlation between ferromagnesian abundance and aluminosity. The SSyL (~320 Ma) is intrusive in upper greenschist facies to upper amphibolite migmatitic metasediment and orthogneiss (~3kb). The compositional variety observed in the SSyL suggests a continuous trend from a moderately mafic, peraluminous magma (cd- and sill- granite) to a H2O saturated granite ("two-mica" granite) facies and finally to an extremely felsic, H2O-saturated magma. Three granitic units have been recognized in the SSyL: (1) the western "Brame Unit" composed of the less evolved cd- and sill- granite facies (2) the central "St Sylvestre Unit", composed mainly by U-rich two-mica granite, intruded by two synchronously emplaced fine grained granites to its western margin: "Fanay" (biotite dominant) and "Sagne" (Li-muscovite only) (3) the eastern "St Goussaud Unit" mostly composed of a muscovite-dominant leucogranite surrounded by Sn-W mineralization. The compositions in the SSyL have a weak ferromagnesian character, negatively correlated with aluminosity and are quite similar in composition to High Himalayan syn-collisionnal peraluminous granite (e.g. Manaslu) Compositional trend of the VMC granite can be easily mimicked by the addition of peritectic material (i.e. produced during incongruent biotite melting) to experimental melts produced through the melting of metasediments (Dirty). Compositions of the SSyL are similar to experimental melt only (Tidy). Such differences in composition could be the results of different processes: 1) Different melting reactions as a consequence of different conditions of melting (e.g. different source composition, temperature). 2) The peritectic phases remained "trapped" in the source during melt extraction due to the structuration of the source or a lower melt viscosity (e.g. higher F, Li, H2O contents). Interestingly, while within dirty granites, peritectic phase entrainment controls compositional variability, Tidy granites display original melt compositional variability as well as the potential effects of late magmatic processes.

International audience; Rare-elements (Ta, Nb, Be, Cs, Li, Sn, etc.) are critical raw material for the European Union. One significant primary source of these metals is rare-element-enriched LCT pegmatites and high phosphorus peraluminous granites. This type of magmatism is particularly well expressed in the Variscan belt, mainly in localized parts of the Iberian massif, the Bohemian Massif and the French Massif Central (FMC). In the latter, an E-W rare-element magmatic belt (Cuney et al., 2002) is defined by three rare-element peraluminous granites (Beauvoir, Montebras) or volcanic equivalent (Richemont rhyolite) and the LCT pegmatite field of Chédeville. The main aims of this study are to investigate the timing of this magmatism and the chronological relationship with deformation, metamorphism and peraluminous magmatism in the FMC and to compare, at the scale of the Variscan belt, these results with our previous studies. U-Pb dating of columbite-group minerals from Beauvoir, Montebras and Chédeville rare-elements magmatic bodies has been performed using laser ablation system connected to a single collector magnetic Sector Field - Inductively Coupled Plasma - Mass Spectrometer (LA-SF-ICP-MS). The Beauvoir granite leads to an emplacement age of 317 ± 6 Ma which is significantly older than previous Ar/Ar on lepidolite published one (308 ± 2 Ma, Cheilletz et al., 1992). Conversely our U/Pb ages of 314 ± 4 Ma and 309 ± 5 Ma obtained on Montebras granite and Chèdeville pegmatites are identical, within the analytical uncertainties, from previous published Ar/Ar age (Cuney et al., 2002; Cheilletz et al., 1992) and also identical to the 313 ± 3 Ma Ar/Ar age of Richemont rhyolite (Cuney et al., 2002). In the north of the FMC, several granites are slightly early to coeval with this rare element magmatism; however, the spatial distribution of rare-element magmatic bodies is by far smallest than those of coeval granites. Migmatites sources of theses granites are unknown in the FMC since there is a lack of Bashkirian to Moscovian migmatites (ca. 318-307 Ma, e.g. Faure et al., 2010). However, a more discrete migmatitic event seems to occur since cordierite-bearing migmatitic gneisses has been recently dated at 316 ± 2 Ma in the North of the MCF (Gébelin et al., 2009). The Marche fault system seems to be a key-structure for the rare-element magmatism since it appears as an hyphen between all bodies. Moreover the Marche fault system is coeval with the rare-element magmatism as evidenced by Ar/Ar ages ranging from 316 ± 5 to 312 ± 2 Ma (Gébelin et al., 2007). We compare these results with our previous studies on the Moldanubian domain of the Bohemian Massif (Melleton et al., 2012) and the Iberian Massif (Melleton et al. submitted) where rare-element magmatic bodies display interesting relationships with metamorphic events and crustal-scale structures. In the light of these new results, rare-elements magmatic bodies of the MCF appear as a milestone in the polyphased rare-metal events of the Variscan belt which extend at least from 333 to 280 Ma.REFERENCESCHEILLETZ et al. (1992) Ages 40Ar/39Ar du leucogranite à topaze-lépidolite de Beauvoir et des pegmatites sodolithiques de Chédeville (Nord du Massif Central, France). Signification pétrologique et géodynamique. C. R. Acad. Sci. Paris 315, 329–336.CUNEY et al. (2002) The timing of W-Sn-rare metals mineral deposit formation in the Western Variscan chain in their orogenic setting: the case of the Limousin area (Massif Central, France), in: Blundell, D.J., Neubauer, F., von Quadt, A. (Eds.), The Timing and Location of Major Ore Deposits in an Evolving Orogen. The Geological Society of London, pp. 213–228.FAURE et al. (2010). Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central). Gondwana Research 18, 653–673.GEBELIN et al. (2007) Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and 40Ar/39Ar investigations. Tectonics 26, TC2008, doi:10.1029/2005TC001822, 2007.GEBELIN et al. (2009) Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France. Tectonophysics 477, 229–243.MELLETON et al. (2012) How Are the Emplacement of Rare-Element Pegmatites, Regional Metamorphism and Magmatism Interrelated in the Moldanubian Domain of the Variscan Bohemian Massif, Czech Republic? Can Mineral 50, 1751–1773.MELLETON et al. (submitted) T. Polyphased rare-element magmatism during late orogenic evolution : geochronological constraints from the NW Variscan Iberia. Contribution to Mineralogy and Petrology.

International audience

International audience; Le développement de nouvelles technologies et des pays émergents exerce une forte pression sur le marché d'un certain nombre d'éléments chimiques. Par exemple, la demande actuelle pour le lithium (Li) est en constante augmentation du fait du développement des batteries Li-ion, particulièrement pour les véhicules électriques ou hybrides ; le tantale (Ta) est très utilisé dans l'industrie électronique, notamment les systèmes de télécommunica-tions et informatiques, et le niobium (Nb) est très utilisé dans l'industrie aéronautique. Ces trois métaux sont principalement produits à l'échelle mondiale à partir de gisements de types différents. Le lithium provient pour environ 60 % des salars (Amérique du Sud principalement) et pour 40 % des pegmatites, le tantale est issu des pegmatites géantes à éléments rares de la famille Li-Cs-Ta (Australie, Canada, Brésil, Afrique et Chine) et le niobium provient de carbonatites (Brésil, Canada). Ces types de gisements sont peu communs ou absents en Europe, sauf les pegmatites et granites à éléments rares (PGER à Li-Cs-Ta, Scandinavie et chaîne varisque). Les PGER sont presque toutes exploitées pour l'industrie de la céramique (exemples : Beauvoir, Monts d'Ambazac en France, champs de pegmatites du Portugal, d'Espagne…). La valorisation des sous-produits de ces exploitations (Li, Cs, Ta, Nb, Sn…) pourrait permettre une certaine indépendance de l'Union européenne. De plus, même si ce type de ressource n'est pas aussi rentable que celle des salars, leur exploitation peut être utile du fait de la réactivité de production possible (par exemple, le Li 2 CO 3 peut être produit en 24 heures à partir d'une pegmatite contre un an à partir des salars) et des autres éléments valorisables en sous-produits.

National audience; Bien qu’il n’y ait plus de mine métallique en activité en France, la longue et complexe histoire géologique et minière de notre territoirenational, combinée aux vastes surfaces de socles et couvertures intrudées de granites, sont néanmoins de bons ingrédients pour observer sur leterrain différents exemples de minéralisations magmatiques et ou magmatiques-hydrothermales associées aux granites.Ces minéralisations couvrent unlarge spectre d’associations métalliques, de types morphologiques et de contextes géologiques. Bien que leur métallogénie puisse être extrêmementcomplexe et dépasse totalement l’objectif de cet article (pour plus dedétails, voir par exemple Jébrak et Marcoux, 2008), les minéralisations associées aux granites peuvent néanmoins être vues assez simplement. Elles sont principalement contrôlées par : – la géochimie des granites et de leurssources ;– la profondeur et la nature de la zone de mise en place des magmas ;– la composition des fluides impliqués dans les systèmes ;– les déformations affectant la zone minéralisée.Ces paramètres vont induire différents types principaux de minéralisations : les granites et pegmatites métaux rares, les minéralisations filoniennesspatialement associées aux granites, les skarns, les minéralisations de type porphyre cuprifère et les épithermaux. On peut aussi leur associer certains gisements de roches et minéraux industriels comme les gisements d’andalousite et de kaolin. Bien que tous ces types de gisements nesoient pas présents en France métropolitaine, nous vous proposons un tour de France d’un gîtologue à travers quelques exemples de terrain qui permettent une bonne introduction aux minéralisations associées aux granites et, avec un peu de chance, la collecte de magnifiques spécimens minéralogiques.

International audience; This paper documents relationships between deformation and magmatic activity that occurred in the central part of eastern Mongolia during late Mesozoic continental-scale NW-SE extension. Two coarse-grained, biotite-bearing, syn-tectonic intrusions are described. The Nartyn granite that extends over an area greater than 30 by 10 km was emplaced within low-grade metasediments and shows a weak pervasive, magmatic fabric reworked by solid-state deformation along its margins. The northwestern roof of the granite is marked by a normal shear zone, the Choyr Shear Zone, characterized by top-to-the-NW motions. The shear zone is overlain by the Choyr Basin, which is filled with unmetamorphosed continental sedimentary rocks of early Cretaceous ages. From structural and geochronological data, we propose that the Nartyn massif was emplaced as a flat laccolith-type intrusion at ca. 136-130 Ma during crustal thinning. The Altanshiree granite, located 140 km east of the Nartyn granite, is a syn-kinematic pluton of similar age (134-128 Ma), also emplaced during crustal thinning. In the Nartyn and Altanshiree areas, extension implies pervasive crustal thinning, combined with limited exhumation. These areas are different from classical metamorphic core complexes, where strong strain localization along detachments induces exhumation of hot middle to lower crust. Results also suggest that early Cretaceous syn-extensional intrusions are an important feature of the tectonic history of eastern Mongolia.

National audience; Pendant la sécheresse du printemps 2011, nous avons visité les anciennes carrières souterraines ouvertes dans la pegmatites lithique de Chèdeville, près d'Ambazac, et nous avons mis en évidence des cristaux de pétalite atteignant 30 centimètres, dans une zone habituellement noyée. Ceux-ci ont d'abord été comparés à des échantillons similaires provenant de pegmatites du nord du Portugal avant d'être analysés. Ils se sont révélés être pseudomorphosés en minéraux argileux et en feldspath potassique. Si ce rare silisate de lithium était dejà signalé sur ce gisement (Raimbault, 1998 entre autres), en petits cristaux situés dans une zone qu'il est observé en individus pluridécimétriques. Cette étude permet également de préciser le type de pegmatite présent à Chèdeville.

International audience; The late Carboniferous (330-300 Ma) Variscan evolution of the Armorican Massif (France) is characterized by the complex interaction of various processes, all linked to each other, namely deformation, magmatism, circulation of various types of fluids and related alteration and ore formation. In the Armorican Massif, the major structural feature is the lithospheric dextral South Armorican Shear Zone (SASZ). The SASZ is divided into two branches along which synkinematic peraluminous leucogranites were emplaced. The Lizio and the Questembert granites were emplaced at 315 _ 5 Ma and 315 _ 4 Ma (U-Pb on zircon) along the northern and southern branches, respectively. Supracrustal leucogranites such as the Questembert granite have been pervasively altered by fluids. The nature of the alteration and the fluids involved changed through time from a high temperature alteration by magmatic fluids expelled from the crystallizing magma to a late hydrothermal alteration triggered by a mix between metamorphic and meteoric fluids as observed in the French Massif Central 40Ar/39Ar dating of muscovite grains from the Questembert granite yielded plateau dates ranging from 319 Ma down to 303 Ma, interpreted as a 15 Ma time span of pervasive hydrothermal circulations. Highly strained mylonitic rocks sampled all along the SASZ recorded the same history, which demonstrates the regional extent of this pervasive alteration. In one sampled site, meteoric water infiltration, evidenced by an extremely low whole rock _18O has been dated at 299 _ 3 Ma (U-Pb on zircon and monazite). This pervasive circulation of various fluids within the crust was very likely one of the most prominent ore-forming processes in the region. Indeed, deformation and associated pervasive alteration along the SASZ were responsible for element mobility, including metals. Pervasive circulation of mixed fluids with an oxydizing meteoric component may have leached Uranium from the supracrustal leucogranites. This is the case for example for most of the U-bearing quartz veins in the Pontivy granite as well as for the polymetallic mineralisations in the Saint-Aubin-des-Châteaux quarry. Tectonic considerations provide some temporal constraints. Such orientations are compatible with a roughly N-S shortening direction, which corresponds to a well characterized late Carboniferous direction. Finally, this orientation around N160° is also found for a set of giant quartz veins interpreted as giant tension gashes, was formed by the precipitation of silica during the upward motion of fluids, which contained a significant proportion of surface-derived meteoric water. Therefore, during the orogenic collapse in the Armorican Massif around 300 Ma, it appears probable that a meteoric water convective cycle was established. The heat advection caused by the lower crust exhumation likely provided the driving force required to sustain this cycle. The mylonitized SASZ acted as a drain for the descending meteoric fluids while giant barren quartz veins are the witnesses of their upward motions. As most of the ore bodies were emplaced in structural traps orientated in the same direction than the giant quartz veins, we suggest that most of them are formed contemporaneously following similar processes.

The Bouillante geothermal field, located on the west coast of Guadeloupe (Lesser Antilles), was explored in the 1970s, developed in the 1980s, brought into production in 1986, and expanded in 2005 to currently produce up to 15 MWe. In addition to its electricity-producing role, the Bouillante field is an outstanding research laboratory for improving our knowledge of a reference high temperature (260°C) geothermal system in island-arc environment. A great deal of scientific research and exploration investigation have been carried out on the Bouillante geothermal field for 10 years, including geological investigation (volcanism), structural analysis of the fluid conduits, geophysical investigations both offshore (magnetics and high-resolution shallow seismics, see Calcagno et al.) and onshore (gravimetry, electrical resistivity tomography profile and passive seismic, see Gailler et al.), characterization of the surface and deep hydrothermal manifestations, numeric geological modelling of the developed field, fluid geochemistry and tracer tests (Sanjuan et al.) and hydrogeological modelling. This large range of multidisciplinary data has made it possible to develop an innovative conceptual model of this type of high-temperature geothermal system. The model highlights the major influence of structural control on the development of the geothermal activity at different scales and especially on the geometry of the reservoir. It also explores new ideas concerning the geothermal system in terms of heat source, fluid circulation and spatio-temporal scenario. This scenario takes into account all events from the early magmatic activity of the Bouillante volcanic Chain (~500,000 y) up to the beginning of the geothermal field some 250,000 years later, having probably started with a phreatic explosion in the bay of Bouillante (see Patrier-Mas et al., Verati et al.). This 2D geothermal resource conceptual model will be used as an exploration tool to reduce geological risks for the next exploration drilling campaign. This work is supported by the French Agency for Energy and Environment (ADEME).

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International audience

Hot orogens, as best exemplified by Archean and Proterozoic accretionnary orogens, show several specific characteristics that we review in this paper. At lithospheric scale, hot orogens combine horizontal flow of a thick and hot ductile crust with strain concentration along steeply dipping deformation zones along which pieces of upper crust are buried. These steeply dipping deformation zones may simply develop as transfer zones accommodating differential flow of lower crustal rock masses. They may also reflect overall transpressive boundary conditions to the considered orogen. Consistently, associated principal stretch directions may vary from dominantly down-dip to subhorizontal from one orogen to another. In contrast, lower crustal domains where horizontal flow dominates are characterized by flat-lying fabrics and principal stretch directions at low angle to the strike of the orogen and high-strain steep deformation zones. Field examples and analogue models outline that the spatial and temporal distribution of domains dominated by horizontal flow or vertical motions depends on the coupling between the viscous lower crust and the plastic upper crust. Coupling is achieved by an attachment layer that accommodates the contrasted kinematic responses of the upper plastic crust and the lower viscous crust to convergence, vertical mass transfers and longitudinal flow. Overall, hot orogens are marked by rather distributed strains compared to those involving cold lithospheres with a stiff and localising mantle lithosphere and thick brittle crust, allowing strong strain localisation in lithospheric-scale shear zones. At crustal scale, hot orogens are characterized by a thick partially molten lower crust subjected to very high temperatures, near isothermal conditions, extensive melting, and juvenile mantle magmatism. The viscous crust may overly a stiff granulitic lower crust, which itself overlies a mantle lid formed at the end of orogeny by the residue of melting and eclogitized mantle melts. The crystallization of such a mantle lid leads to the cratonization / quenching of the orogen. The formation of the stiff granulitic lower crust would be crucial in defining a transient orogenic flow mode by activation of an attachment layer between the mantle and the crust until the mantle lid ultimatly crystallizes. Distributed deformation at crustal scale has important implications on strain rates throughout hot orogens, their topography and relief, and specific surface mass transfers (erosion and sedimentation). Modern orogens that involve initially cold lithosphere may acquire, after maturation, comparable deformation patterns as a result of thickening- and/or mantle magmatism-induced crustal weakening. High plateaux in Cordilleran and collision belts would be Phanerozoic analogues of ancient exhumed hot orogens.