Hydrothermal mineral deposits are characterized by sub-concentric envelopes of alteration minerals surrounding the metallic profitable zone. These alteration halos are often used as exploration pathfinders, and their characterization can also be important for mine planning and geometallurgical purposes. This paper introduces a new approach to jointly model alteration halos and their geological controls. Indeed, the geometry of hydrothermally altered rocks is mainly controlled by geological features such as fault systems or unconformities and smaller-scale objects such as fracture networks. The proposed method defines a multicomponent structural skeleton as a support for modeling the boundaries of these halos. For this, an alteration potential field d(x) computed from this structural skeleton is calculated to extract alteration levels as isovalues. As alteration patterns also depend on the petrophysical properties of the host rock, this alteration potential field can be adapted to account for heterogeneous rock material. This process is integrated into a marked point process framework: the skeleton components are simulated by a Metropolis–Hastings sampler, using a dedicated likelihood term to compute the consistency between the current model parameters and the observations. The application of the proposed methodology on a two-dimensional outcrop in Utah, analogous to mineralized hydrothermal sites, demonstrates the ability of the methodology to jointly infer plausible geometries of alteration halos and of the associated geological structures.

The use of in situ Rb-Sr geochronology has boomed in recent years following its implementation using LA-ICP-MS/MS technology, which enables fast, in situ analyses at the micron scale on selected minerals. The Rb-Sr geochronometer applied to micas is now commonly used to date the crystallization or cooling of metamorphic and magmatic rocks, based on the assumptions of a closed isotope system after passing the closure temperature and of a homogeneous Sr isotopic composition at the time of crystallization. In situ Rb-Sr geochronology applied to micas and related alteration products in geological contexts involving hydrothermal fluid circulation affecting micas after crystallization could provide a new way to decipher the timing and duration of fluid circulation in various settings such as mountain belts or sedimentary basins. The behavior and applicability of the Rb-Sr system in such contexts are, however, poorly understood, as the system may be partially reopened with differential redistribution of Rb and Sr at the grain scale. To test this hypothesis, we selected a case study related to unconformity-related U deposits from the Athabasca Basin (Canada), which formed through intense hydrothermal fluid circulation at the interface between crystalline basement and siliciclastic sedimentary rocks and represent archetypes of unconformity-related metal deposits. Muscovite grains from metamorphic and magmatic rocks were targeted across a range of alteration states, from hydrothermally unaltered to strongly altered domains. We focused on a specific hydrothermal alteration linked to the formation of hydrothermal illite and sudoite at the expense of metamorphic or magmatic minerals. In unaltered zones, muscovite displays variable but high Rb / Sr ratios, whereas the 87Sr / 86Sr intercepts derived from Rb-Sr regressions are scattered and are not interpreted as meaningful initial isotopic compositions. The resulting ages range from ca. 1870 to ca. 1720 Ma and are consistent with the geological context. In distal-to-proximal alteration halos of U deposits, muscovite and related alteration products yield lower 87Rb / 86Sr ratios and highly variable regression intercepts. The mean age calculated across the different samples and investigated sites clusters around similar to 1640 Ma, a value previously obtained by Ar-Ar geochronology on illite and U-Pb geochronology on other hydrothermal phases and proposed to correspond to a major hydrothermal event linked to a geodynamic reorganization affecting the Canadian Shield at the circum-Laurentian scale. The similar to 1640 Ma age is geologically meaningful in the studied context and is interpreted as reflecting partial, micrometric-scale resetting of the Rb-Sr system in muscovite during this hydrothermal event. The wide range of regression intercept values commonly observed in disturbed Rb-Sr systems is interpreted as an apparent result of open-system behavior, reflecting partial system reopening and non-conservative redistribution of Rb and Sr at the grain scale, rather than as a physically meaningful initial isotopic composition. These results demonstrate that detailed analysis of Rb-Sr system perturbations in altered muscovite and related alteration products can constrain the timing of ancient hydrothermal activity and the spatial dynamics of fluid-rock interaction. This approach provides a valuable complement to conventional fluid-tracing methods and opens new perspectives for reconstructing paleo-hydrothermal systems in ancient basement terrains.

High-grade unconformity-related uranium (URU) deposits in the Athabasca Basin are spatially associated with graphitic-rich shear zones rooted in the basement and their propagation as brittle faulting and damage zones in the overlying sandstone. Investigating the tectonic phases associated with URU formation is particularly challenging due to its large polyphased history. This study is based on geological observations from 19 exploration drill cores in the Waterfound project (NE Athabasca Basin) and combines structural characterization with paleostress joint-inversions. Three main tectonic regimes were identified: (1) a pre-Athabasca Basin deposition NNE-SSW shortening affecting only the basement, (2) a syn-lithification NW-SE extension, and (3) a post-Athabasca NW-SE shortening. This polyphased tectonic history led to an increase in both the density and variety of fractures in the basin above a low-friction ENE-WSW graphitic shear zone in the basement. The extensional phase (2) developed with low horizontal differential stress, low fluid pressure and potentially relates to post-orogenic collapse or burial stress conditions only. The latest shortening stage (3), associated with uranium mineralization, locally reactivated a pre-existing fracture network in the sandstone and developed above an inherited ENE-WSW-trending graphitic shear zone in the basement. It requires tectonic stresses and fluid pressures estimated at 45-65 MPa to explain the observed structures. These results highlight the interplay between the mechanical strength of inherited structures, stress states, and fluid pressure in governing uranium-bearing fluid flow and provide new insights for Athabasca URU deposits exploration.

The Djengeldi uranium deposit, located in the Kyzylkum metallogenic province (Uzbekistan), is hosted by Maastrichtian sandstones cemented mainly by dolomite and calcite. These carbonates, which also occur in the underlying and overlying units, are unrelated to uranium mineralization and formed during distinct diagenetic stages, but strongly influence in-situ leaching techniques that make these deposits economic. In the roof and wall rocks, dolomite formed during eogenesis under oxidizing conditions and is respectively associated with sabkha-related evaporitic facies and with paleosols of an alluvial plain. In the shoreface reservoir sands, dolomite clearly predates uranium deposition, whereas calcite cementation postdates it. Carbon and oxygen isotopes data indicate that the fluids responsible for dolomite in the wall and the reservoir were meteoric, while dolomite in the roof reflects a mixture of meteoric and marine waters typical of sabkha settings. Combined evidences from reducing conditions during precipitation, stoichiometry (Ca/Mg ratio), and oxygen-isotopes thermal condition suggest that Maastrichtian dolomites formed during shallow burial. All calcites show unequivocal meteoric isotopic signatures, and UPb dating reveals that they crystallized during very recent fluid circulation. Altogether, these results indicate that carbonate formation in the Djengeldi deposit contradicts the deep, reduced-fluid model commonly invoked for other Uzbek uranium deposits. Because such fluids are also considered the main reductant for uranium precipitation, this metallogenic model is not supported for Djengeldi.

Geochemical footprints of unconformity-related uranium (URU) deposits in the Athabasca Basin (Saskatchewan, Canada) have been extensively studied at individual deposit scales, but no regional comparison across multiple deposits has been undertaken. Using a compilation of legacy exploration geochemical data covering 11 world-class deposits in the Eastern Athabasca Basin (95,739 samples, 4,251 drillholes), this study compares alteration footprints across deposits and evaluates their dependence on local geochemical background. A clear typology emerges between the dickite-dominated Northeast, where illitization produces a characteristic decrease in Al2O3/K2O towards deposits, and the illite-dominated Southeast, where dravitization, chloritization and kaolinization generate distinct signatures (increasing MgO/TiO2, decreasing Al2O3/MgO and Fe2O3/MgO, and increasing Al2O3/K2O). These footprints are only meaningful when defined relative to their local background, not a basin-wide average. Basement-hosted deposits display minimal geochemical variation from background in the overlying sandstone. These findings provide area-specific reference values and guide the selection of optimal geochemical ratios for uranium exploration targeting.

The Sioule region in the northern part of the French Massif Central of the Variscan belt hosts the Echassières complex, where a large W(-Sn) quartz vein system was intruded by the Beauvoir rare-metal granite (RMG) hosting disseminated Sn, Nb-Ta and Li mineralization. We combined whole-rock geochemical data, zircon U-Pb geochronology of migmatites and felsic igneous rocks of the Sioule area, along with U-Pb dating of cassiterite, wolframite, and apatite in the Echassières complex to reconstruct the regional magmatic and metallogenic evolution. Results reveal: (i) early W mineralization at 351 ± 9 Ma, coeval with peraluminous granite magmatism (Chantelle and St-Gervais massifs) and a N115-striking dextral shear zone; (ii) a second W mineralization phase at 329 ± 5 Ma, linked to rare-metal-rich rhyolitic dykes and the Colettes granite during NW-SE extension, synchronous with regional biotite microgranites (Pouzol-Servant Massif) and trachy-dacitic tuffs and lavas; and (iii) late-orogenic emplacement of the Beauvoir RMG with minor hydrothermal Sn and W at ca. 320-310 Ma, synchronous with biotite granites hosting quartz-tourmaline orbicules (Champs Massif) and pyroclastic flows. Our study reveals a 40 Myr-long metallogenic evolution with hydrothermal W mineralization preceding hydrothermal Sn and magmatic Li-Sn-Nb-Ta. Each mineralization period coincided with widespread crustal magmatism involving the anatexis of late-Ediacaran metasedimentary rocks and Cambrian-Ordovician metagranites, as shown by the dating of zircon in migmatites and inherited zircon in Variscan igneous rocks. However, rare-metal-enriched magmas remained spatially confined to the Echassières complex suggesting a structural corridor that repeatedly focused magmas and fluids from a localized fertile source.

Apatite is a widely distributed accessory mineral in igneous and hydrothermal systems. Here we integrate cathodoluminescence (CL) imaging, in situ REE geochemistry, and oxygen and strontium isotope analyses of apatite from 43 samples spanning 16 European deposits representative of diverse magmatic to magmatic–hydrothermal settings, including carbonatites, alkaline complexes, lamproites, layered intrusions, IOA–IOCG deposits, granite-pegmatite systems, and hydrothermal and metasomatic veins. The combined dataset reveals systematic chemical and isotopic trends that distinguish primary magmatic apatite from magmatic–hydrothermal transition and hydrothermal–metasomatic generations, while also providing diagnostic criteria for different classes of phosphate-bearing deposits. Primary magmatic apatite is commonly characterized by LREE-enriched patterns, low δ18O values (generally 3–9‰), relatively low to moderate radiogenic Sr(i) signatures (0.701–0.707), and oscillatory CL zoning. Progressive fluid interaction with primary magmatic apatite is typically expressed by partial REE depletion, increasing δ18O values, more radiogenic Sr(i) compositions, and the development of dissolution–reprecipitation textures. Granite–pegmatite systems are distinguished by strong intra-REE fractionation expressed by tetrad effects (> 1.1), radiogenic Sr isotopic compositions, and pronounced negative Eu anomalies. Apatite from hydrothermal and metasomatic deposits exhibits the strongest modification of primary signatures, including marked REE depletion, elevated δ18O values, radiogenic Sr(i) ratios, and commonly turbid or microcrystalline CL textures. This study therefore highlights the analytical and interpretative power of integrated in situ methodologies for resolving the evolution of economically and geologically significant P-rich systems.

I n situ mica Rb‐Sr geochronology lacks a diverse range of widely utilised reference materials. Here, we characterize and evaluate six natural mica reference materials to test their suitability for in situ Rb‐Sr geochronology, including one phlogopite (Mica‐Mg‐NP phlogopite), three biotite (Mica‐Fe biotite, Mount Dromedary biotite and La Posta biotite) and two muscovite candidates (Högsbo muscovite, WA1ms muscovite). For each of these reference materials, we evaluate chemical and microstructural heterogeneity and assess age and/or Rb‐Sr isotopic homogeneity through 40 Ar/ 39 Ar, solution Rb‐Sr and long‐term in situ LA‐ICP‐MS/MS Rb‐Sr geochronology. Mount Dromedary biotite (99.37 ± 0.20 Ma; 2 s , including all sources of uncertainty) and Mica‐Fe biotite (307.75 ± 0.80 Ma) are effective low and high Rb reference materials for pulse and analogue collector modes on single‐collector systems, respectively, combining a wide spread of Rb/Sr ratios within and between grains, and producing accurate Rb‐Sr ages across multiple laser ablation sessions. La Posta biotite (91.30 ± 0.25 Ma) and Högsbo muscovite (1029.7 ± 2.0 Ma) are better suited as secondary low and high Rb reference materials due to minor chloritisation and alteration, respectively. Mica‐Mg‐NP phlogopite shows significant isotopic and age heterogeneity, outside of the precision of in situ Rb‐Sr geochronology, as well as showing different down‐hole fractionation compared with natural micas. WA1ms muscovite may prove a useful reference material, with a published 40 Ar/ 39 Ar age of 2613.3 ± 4.4 Ma, but further testing is required as its in situ Rb‐Sr age is significantly younger. All new reference materials investigated here are isochronous, rather than isotopically homogeneous, necessitating new data reduction schemes for processing isochronous data with accurate propagation of uncertainties and down‐hole fractionation corrections. The authors may be contacted to obtain vials of mica concentrates.

Recent studies have shown the interest of automating the classification of rock images using deep learning architectures. However, a major challenge for practitioners when applying these methods to real-world data sets generated during mineral exploration is the long time required to create and label a data set. This study proposes a complete workflow to label and classify drill core photographs with minimal time required for labeling through five successive steps: (i) using exploration drill core photographs, rock cores are separated from wooden trays using morphological operators; (ii) feature descriptors are then extracted from rock images using color histograms for colorimetric information and Gabor filters for texture information; (iii) feature descriptors then serve as input data for self-organizing maps (SOM) to generate clusters that can be partially labeled by geologists for creating a labeled data set with limited efforts, yielding a data set made of labeled and unlabeled images; (iv) the partially labeled data set can then be used to train either fully supervised or semi-supervised deep learning architectures for generating classifications; (v) the classification model obtained can then be re-used on unseen data to automate logging process. This study presents this workflow separately for two different geological domains, a data set of sedimentary rocks classified according to the intensity of bleaching features and a data set of crystalline basement rocks classified by lithological domains. Software code and data set are made publicly available.

Assessing uncertainties is an essential step throughout the mineral resource development cycle, from exploration campaigns to development planning and remediation. These uncertainties reflect the scarcity of subsurface data, which need to be complemented by geological knowledge.This paper aims to assess the impact of conceptual variations on the geometry of uranium deposits. For this purpose, a case study was carried out on the interpretation of drilling data in a 2D section, in the context of an unconformity-related uranium deposit in the Athabasca Basin. Based on a reference section from this area, a blank cross-section with synthetic drillcores was produced and given to 32 interpreters. These 32 interpreters are from various backgrounds and are divided between experts in uranium geology (8) and non-experts (24). A set of fifty mathematical criteria is defined and correspond to five categories: mineralized zones, associated altered zones, structural network, geometric relationship between structures and mineralization, and annotations. Individual and group analyses of the defined criteria are performed.Results show that uranium experts and non-experts are difficult to discriminate by the metrics used in this study, but experts tend to filter out the most speculative conceptual models. A generalized parsimony of interpretations is observed for all survey respondents. What also emerges is that fault network connectivity is key to conceptual model discrimination in hydrothermal deposits.

Geochronological data on ore-bearing minerals can constrain the absolute ages and duration mineralising processes. Of the tungsten-bearing minerals, wolframite is the most promising geochronometer for these mineral systems, but high precision ages are hampered by common Pb and intra-grain heterogeneity. We present ID-TIMS and LA-ICP-MS U-Pb isotope data of wolframite , combined with milli scale (SEM-EDS) and microscale (LA-ICP-MS) element maps and nanoscale (FIB-TEM) chemical and structural images to elucidate the complex geochemical behaviour of this mineral and the implications for U-Pb geochronology.During this study, three new U-Pb reference materials have been developed with ages of ca 158 Ma, 289 Ma and 325 Ma; these are available to interested laboratories. The best precision obtained by ID-TIMS was 1.24%, whilst we estimate the best possible precision for LA-ICP-MS ages to be ca 1.8%. Apart for analytical uncertainties, the main contributor to age uncertainty is the poor dispersion in U-Pb data (for Discordia fitting) and unknown common Pb composition for ID-TIMS data, and micron-scale heterogeneity for LA-ICP-MS data.Microscale (LA-ICP-MS maps) to nanoscale (FIB-TEM) imaging techniques show large chemical and structural heterogeneity of wolframite related to the complex geological environments in which it is precipitated and altered. Trace element mapping highlights oscillatory and sector zoning not typically observed when using traditional SEM-based techniques. The variable distribution of the analysed elements (Fe, Mn, Sc, Nb, Ta, Y, Pb, Th and U for this study) can be explained both by coupled substitution and changing fluid chemistry recorded within a single wolframite crystal. The nano-scale structure of a strongly altered wolframite is characterised by rare ca 10x10 nm non-symmetric zones of amorphous crystal structure, and bands of elongate (ca 100 x 20 nm oval-shaped) low density zones that we consider representing porosity developed during rapid crystallisation of wolframite.Although no real intra-grain age dispersion is observed in the analysed samples, the precision of U-Pb ages is strongly affected by the local chemical and structural characteristics of the wolframite. Most notably, the concentration of 238U and 238U/204Pb can vary by an order of magnitude within a zone smaller than a typically laser ablation spot (e.g., 100 µm).

Although the Bohemian Massif hosts significant vein-type U-deposits associated with shear zones, their metallogenetic model in the frame of the Central European Variscides is poorly constrained due to the lack of geochronological data. This contribution presents the first SIMS U-Pb age of 270.8 +/- 7.5 Ma obtained so far for the U-mineralization from the Bohemian Massif. This age coincides with the main U mineralizing events in the entire Variscan Orogenic Belt, including large metallogenic provinces in the Massif Central, Armorican Massif, Black Forest, and Erzgebirge. The recognition of the Permian mineralizing event indicates that the metallogenesis of U-deposits in the Bohemian Massif was linked to the lithospheric extensional events responsible for the coeval exhumation of the metamorphic complexes and the formation of the Late Carboniferous to Lower Permian sedimentary basins that generated oxidizing basinal fluids percolating through and liberating U from the crystalline basement. The AFT thermochronology along with U-Pb apatite dating reflects the rapid Permian exhumation of the crystalline basement, which allowed the influx of oxidized basinal brines mixed with meteoritic waters, which are the most important ore-forming fluids in the entire European Variscan Belt.

Basement/sedimentary cover interfaces localize hydrothermal circulation, making them preferential areas for trapping and accumulating metals. Although these zones are well known for their potential to host mineralization, they have poor structural constraints. The Athabasca Basin (Saskatchewan, Canada) is a metallogenic province widely explored for Unconformity Related Uranium (URU) deposits. In this study, we focus on the basement/cover interface at five different sites located in the northeastern part of the basin, including unmineralized zones (Snake Lake, Pat Lake), small deposits (Waterfound, McClean South), and the giant Cigar Lake deposit. We combine drillcore logging and acoustic televiewer data to constrain the structural, mineralization and alteration patterns on both sides of the unconformity. Results show that the localization of clay alteration and uranium mineralization at the base of the basin and the top of the basement is mainly controlled by the brittle fracture network and lithological heterogeneities. Acoustic televiewer data show the presence of two main families of fractures: moderately to steeply dipping fractures (60°-80°) and fractures sub-parallel to the bedding (< 20°). This second fracture set is most important in uranium mineralized areas and its expression correlates with the size of the uranium deposit. In the giant deposit, the fractures do not show a preferential orientation suggesting they are related to breccia formation. These findings support the development of a revised genetic model for URU-type deposits, in which the magnitude of the hydrothermal mineralizing event is governed by the intensity of fluid circulation and the associated overpressure, which drives uranium precipitation and orebody development.

Cassiterite (SnO2) is one of the dominant ore phases in tin-tungsten bearing magmatic-hydrothermal deposits. It can contain high uranium contents and usually hosts low levels of common Pb, making it one of the best U-Pb geochronometers among ore minerals [1]. The widespread use of in-situ techniques to obtain crystallization ages for cassiterite, however, is limited by a paucity of accurately characterized reference materials (RMs). Such shortage is mostly caused by the difficulty of achieving closed-system acid decomposition of this mineral, which represents the foundation of isotope dilution techniques, necessary for accurate and precise determination of U-Pb isotopic composition using thermal ionization mass spectrometry (TIMS) techniques. In this contribution, we present a new set of U-Pb isotopic compositions of two cassiterite samples from the archetypal Variscan Sn-W greisen deposits of Panasqueira (Portugal) and Krasno (Czechia) obtained with an updated protocol of complete HBr decomposition of cassiterite in the presence of a U-Pb tracer, followed by U and Pb purification, and TIMS analyses. Previous to dissolution, the U-Pb isotopic compositions of the same cassiterite aliquots are characterized via laser-ablation-inductively coupled-mass spectrometry (LA-ICP-MS) and each cassiterite fragment is imaged with an ultra-fast washout laser ablation system to obtain high-resolution maps of the content and distribution of key trace elements (e.g. U, Pb, Fe, REE). These two samples show variable but high U concentrations (2-20 ppm) and produce U-Pb isochron ages with 1% precision and low dispersion. We compare these new materials with established RMs (Yankee [2]; AY-4 [3]) and discuss their usability as primary reference materials for microbeam applications. [1] Neymark, L. A., et al., Chemical Geology. 2018, 483, 410-425. [2] Carr, P.A., et al., Chemical Geology. 2020, 539, 119539. [3] Yuan, S., et al., Ore Geology Reviews. 2011, 43, 235–242.

The relationship between critical metal mineralisations (W, Sn, Nb, Ta, Li) and collisional orogens remains poorly understood. We combine U-Pb geochronology of metal-carrying minerals and geochemical modelling to constrain metal mobility in the Variscan Iberian collisional orogen. Two overlapping periods of mineralisation are identified: magmatic-hydrothermal Sn-W deposits from ca. 340 to 285 Ma and rare-metal granite (RMG) and pegmatite (RMP) Li-Sn-Nb-Ta deposits from ca. 315 to 295 Ma. We show that W-rich fluids can be produced from metapelite-derived anatectic melts without further differentiation, leading to their dominance over Sn (+Li-Nb-Ta) early in orogenesis. In contrast, RMG/RMP generation is favoured by transcurrent structures and melting of felsic meta-igneous protoliths. Both features become increasingly abundant towards the end of this collisional orogen. The progression from W- to Sn-dominant magmatic-hydrothermal mineralisation, followed by RMG/RMP, is a consequence of collisional orogen dynamics.

The unconformity-related uranium (URU) deposits in the Proterozoic Athabasca Basin are one of the most important U resources in the world. This type of U deposit can be divided into monometallic (U) and polymetallic (U-Ni-Co-As) subtypes. While it is generally agreed that the URU deposits formed from reaction between oxidizing, basinal brines carrying U and/or Ni-Co-As with reducing basement fluids or lithologies, it is debatable whether the polymetallic deposits formed from co-enrichment of U-Ni-Co-As or enrichment of U superimposed by a separate Ni-Co-As mineralization event. This study addresses this problem through mineralogical, geochemical and fluid inclusion investigation of the Midwest U-Ni-Co-As deposit. Petrographic studies indicate that the sequence of ore precipitation started with uraninite, followed by Ni-Co arsenides and sulfoarsenides and then Cu-Pb-Fe sulfides, and this sequence was repeated episodically. This observation suggests that the deposit did not form from two separate U and Ni-Co-As mineralization events, but rather multiple episodes of U-Ni-Co-As mineralization. Linear correlations between chemical ages and Si-Ca-Fe contents of the most pristine uraninite U1 suggest a maximum primary mineralization of ca. 1600 Ma, which is consistent with the inferred primary U mineralization age in the Athabasca Basin. Microthermometric and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of fluid inclusions in syn-mineralization drusy quartz indicate that the composition of the ore-forming fluids is characterized by the H2O-NaCl-CaCl2-KCl-MgCl2 system and comparable to those from both monometallic and polymetallic URU deposits. The relationship between U and Ni + Co in the fluid inclusions and its comparison with other URU deposits support a model in which U and Ni-Co were co-enriched in a unified mineralization process. The development of breccia structures in the ores and the dramatic fluid pressure fluctuation revealed by fluid inclusions suggest that the deposit formed from multiple episodes of fluid flow related to repeated reactivation of basement-rooted faults.

Isotopic dating is a valuable method to constrain the timing of lithospheric processes: geodynamic episodes, ore deposition and geothermal regimes. The K-Ar dating technique has the main advantage of being applied to ubiquitous K-bearing minerals that crystallize at various temperatures, from magmatic to low temperatures. Clays are of significant interest among all K-bearing minerals, as they crystallize during various hydrothermodynamic processes. Nonetheless, the dating of illites by the K-Ar method is not straightforward. K-Ar dates on illite usually rely on a mixed isotopic signal referring to various illitic populations that might have experienced isotopic resetting or recrystallization processes. Therefore, reliable K-Ar dates on illite depend on (1) the grain size separation of large numbers of clay fractions; (2) the study of the morphology, mineralogy and crystallography; (3) the determination of precise K-Ar dates on each clay size fraction; and (4) the meaningful interpretation of ages using either end-member ages or the illite age analysis (IAA) method. This paper describes the instrumentation and methods recently developed at the GeoRessources laboratory of the University of Lorraine to obtain valuable ages on illite mixtures.

Intense hydrothermal activity hosted by regional tectonic structures occurs in the Eastern part of the Pyrenees. Helium isotope ratios (3He/4He) in hot springs along the T & ecirc;t (0.033-0.099 Ra) and the Tech (0.171-0.375 Ra) faults indicate different signatures from purely crustal to slightly contaminated by magmatic helium. 3He/4He ratio increased towards the East, consistently with the observed thinning of the continental crust. These results suggest localised and not interconnected hydrothermal systems at fault scale. The origin of magmatic helium contamination is discussed in the light of the Gulf of Lion geodynamic evolution and data from comparable orogenic hydrothermal systems in the Alps.

The Fregeneda-Almendra pegmatite field of the Iberian Massif represents a typical expression of peraluminous rare-metal magmatism that occurred over western Europe at the end of the Variscan orogeny. It is the host for two main types of Li-mineralized intrusions, identified at the scale of the Variscan belt, including petalite- or spodumene-rich pegmatites, as well as Li-mica-rich pegmatites, for which the origin of mineralogical-chemical differences is not yet understood. Here, we provide cassiterite and columbite-group mineral (CGM) U-Pb ages along with oxide, mica and phosphate mineral compositions for Li-pegmatites from the Fregeneda-Almendra field in order to assess their petrogenesis and tectonic-magmatic context of emplacement. U-Pb geochronology indicates that petalite-rich and Li-mica-rich pegmatites were mostly emplaced sub-synchronously from 315 ± 6 to 308 ± 6 Ma, during strike-slip deformation and granitic magmatism within an anatectic dome bounding the pegmatite field. U-Pb data and pegmatite geographic zonation suggest that Li-pegmatites were sourced from buried equivalents of leucogranites and migmatites from the dome. Li-pegmatites experienced a complex crystallization including K-feldspar, petalite, topaz, Nb-Ta-Fe-Mn-rich cassiterite, amblygonite-group minerals (AGM) and CGM as early magmatic phases, followed by lepidolite for Li-mica-rich pegmatites. At the magmatic-hydrothermal transition, notably leading to the formation of Nb-Ta-Mn-Fe-poor cassiterite hosting CGM inclusions, earlier minerals were resorbed by muscovite and albite. A later F-rich hydrothermalism is locally reflected by zinnwaldite overgrowths on muscovite. Cassiterite, CGM and micas from petalite-rich pegmatites show lower Mn/Fe ratios and higher Ti contents, along with lower Zr-Ga contents for cassiterite, than that from Li-mica-rich pegmatites. Such behavior is consistent with a magmatic differentiation process whereby Ti content decreased and the degree of Mn-Fe geochemical fractionation and solubilities of Ga and Zr increased in the melts, possibly in relation with high fluorine activity. In Li-mica-rich pegmatites, AGM equilibrated with a melt with up to 2 wt% F, similar to that in equilibrium with lepidolite (1–3 wt%). In petalite-rich pegmatites, the relatively high F concentration of the melts equilibrated with AGM (≤ 1.5 wt% F) contrasts with the liquid equilibrated with muscovite (< 0.5 wt% F). This can be accounted for by muscovite crystallization after the exsolution of a F-rich aqueous phase at the magmatic-hydrothermal transition. Relatively similar F contents in the initial melts of petalite- and Li-mica-rich pegmatites support the hypothesis that the stability of lepidolite does not only involve high F but also a low H 2 O/F activity ratio. For the Fregeneda-Almendra Li-mica-rich pegmatites, this could be explained by a decrease of melt H 2 O solubility due to a relatively low pressure of emplacement.

Geochronological data on ore-bearing minerals can constrain the absolute ages and duration mineralising processes. Of the tungsten-bearing minerals, wolframite is the most promising geochronometer for these mineral systems, but high precision ages are hampered by common Pb and intra-grain heterogeneity. We present ID-TIMS and LA-ICP-MS U-Pb isotope data of wolframite , combined with milli scale (SEM-EDS) and microscale (LA-ICP-MS) element maps and nanoscale (FIB-TEM) chemical and structural images to elucidate the complex geochemical behaviour of this mineral and the implications for U-Pb geochronology. During this study, three new U-Pb reference materials have been developed with ages of ca 158 Ma, 289 Ma and 325 Ma; these are available to interested laboratories. The best precision obtained by ID-TIMS was 1.24%, whilst we estimate the best possible precision for LA-ICP-MS ages to be ca 1.8%. Apart for analytical uncertainties, the main contributor to age uncertainty is the poor dispersion in U-Pb data (for Discordia fitting) and unknown common Pb composition for ID-TIMS data, and micron-scale heterogeneity for LA-ICP-MS data. Microscale (LA-ICP-MS maps) to nanoscale (FIB-TEM) imaging techniques show large chemical and structural heterogeneity of wolframite related to the complex geological environments in which it is precipitated and altered. Trace element mapping highlights oscillatory and sector zoning not typically observed when using traditional SEM-based techniques. The variable distribution of the analysed elements (Fe, Mn, Sc, Nb, Ta, Y, Pb, Th and U for this study) can be explained both by coupled substitution and changing fluid chemistry recorded within a single wolframite crystal. The nano-scale structure of a strongly altered wolframite is characterised by rare ca 10x10 nm non-symmetric zones of amorphous crystal structure, and bands of elongate (ca 100 x 20 nm oval-shaped) low density zones that we consider representing porosity developed during rapid crystallisation of wolframite. Although no real intra-grain age dispersion is observed in the analysed samples, the precision of U-Pb ages is strongly affected by the local chemical and structural characteristics of the wolframite. Most notably, the concentration of 238U and 238U/204Pb can vary by an order of magnitude within a zone smaller than a typically laser ablation spot (e.g., 100 µm).