Antonin Richard | Université de Lorraine (original) (raw)

Papers by Antonin Richard

Lithium isotopes in fluid inclusions as tracers of crustal fluids: An exploratory study, 2018

Lithium isotopes are extensively used to trace fluid migration and fluid-rock interactions in the... more Lithium isotopes are extensively used to trace fluid migration and fluid-rock interactions in the shallow Earth's crust. Although, Li isotope systematics might give important information about deep crustal fluids, this stable isotope system remains surprisingly under exploited in fluid inclusions. Here, twenty-three samples from a variety of deep environments, from diagenetic, metamorphic to magmatic-hydrothermal conditions (~150 to 600 °C), have been investigated in order to provide the first exploratory overview of the range in Li isotopic composition of fluid inclusions. From 1 to 2 g of hydrothermal quartz or dolomite, cation analysis (Na, K, Ca, Mg, Sr, Li), Li fraction recovery and Li isotopic analysis of bulk fluid inclusion leachates were carried out by using an innovative approach coupling crush-leach extraction, high performance ion chromatography and MC-ICPMS analysis. Reconstructed Li concentrations and δ 7 Li values of these fluid inclusions (respectively 12 to 653 mg/l and −1.4 ± 0.2‰ to +41.3 ± 0.8‰) are compatible with previously proposed models for the origin of the fluids and fluid-rock interactions. δ 7 Li values are independent from other tracers (Br/Cl, I/Cl) and parameters such as temperature, salinity, and cation content. The results show that, in conjunction with other parameters, the Li isotopic compositions of fluid inclusions are potentially powerful source and process tracers of deep crustal fluids within a wide range of geological environments and related ore-forming systems, for example in from sedimentary basins, seafloor hydrothermal systems and subduction zones.

Advances in 3D imaging and volumetric reconstruction of fluid and melt inclusions by high resolution X-ray computed tomography

Fluid and melt inclusions are tiny pockets of fluid and melt trapped in natural and synthetic min... more Fluid and melt inclusions are tiny pockets of fluid and melt trapped in natural and synthetic minerals. Characterizing the 3D distribution of fluid and melt inclusions within minerals, their shape and the volume fraction of their different phases is crucial for determining the conditions of crystal growth and paleostress analysis. However, their relatively small size (typically 5 to 100 μm), complex shape, heterogeneous content, the opaque nature of some host minerals and projection bias frequently hamper accurate imaging and volumetric reconstruction using conventional microscopic techniques. High resolution X-ray computed tomography (HRXCT) is a non-destructive method which uses contrasts of X-ray attenuation in a series of contiguous radiographs with different view angles to reconstruct the 3D distribution of areas of different densities within a large variety of materials. In this work, we show the capabilities of HRXCT for: (i) imaging the 3D distribution of aqueous and hydrocarbon-bearing fluid inclusions and silicate melt inclusions in a crystal; (ii) characterizing the shape of fluid and melt inclusions and (iii) reconstructing the total volume and the volume of the different phases (liquid, glass, crystal, vapor) of fluid and melt inclusions. We have used a variety of hand specimens and chips of transparent and opaque minerals (olivine, quartz, feldspar, garnet, emerald, wolframite), that we analyzed using three different HRXCT setups. When a resolution of ~1 μm 3 /voxel is achieved, HRXCT allows identifying > 5 μm fluid inclusions, and the identification and volumetric reconstruction of the different phases can be carried out with reasonable confidence for relatively large (> 25 μm) inclusions. Density contrasts are high enough to properly identify: (i) a silicate melt inclusion, and its different phases (glass, vapor and crystals such as clin-opyroxene and spinel) in an olivine crystal; (ii) aqueous monophase (liquid) and two-phase (liquid + vapor) fluid inclusions in transparent and opaque minerals (quartz, garnet, emerald, wolframite). In the case of hydrocarbon bearing fluid inclusions containing a vapor phase and two liquid phases (oil and aqueous solution), the two liquid phases could not be distinguished from each other. Volumetric reconstruction of liquid and vapor phases of aqueous and hydrocarbon-bearing fluid inclusions show compatible results with independent calculations using known pressure, temperature, molar volume and composition (P-TV -x) conditions of trapping or imaging using confocal laser scanning microscopy respectively. Collectively, our results show that HRXCT is a promising tool for non-destructive characterization of fluid and melt inclusions.

The ore-forming magmatic-hydrothermal system of the Piaotang W-Sn deposit (Jiangxi, China) as seen from Li-mica geochemistry, 2018

Many studies have proved the usefulness of Li-mica and chlorite geochemistry as indicators of the... more Many studies have proved the usefulness of Li-mica and chlorite geochemistry as indicators of the chemical and thermal evolution of magmatic systems. This study highlights the suitability of Li-micas as tracers of hydrothermal mineralizing events in world-class W-Sn deposits associated with Jurassic (190–150 Ma) granites in China through the complex magmatic–hydrothermal evolution of the Piaotang deposit (South Jiangxi). A paragenetic sequence has been established for the Piaotang deposit comprising (1) a first " silicate-oxide " stage that hosts abundant W-Sn mineralization (wolframite and cassiterite), (2) a " calcic " stage with scheelite and wolframite, (3) a " base metal sulfides " stage with cassiterite and wolframite, and (4) a late " sulfide " stage, involving for the first time a polyphase emplacement of the mineralization. Li-micas from the underlying granite, greisen, and the different stages represented in the veins, were studied. The chemistry of the micas (characterized by intermediate compositions between phlogopite-zinnwaldite-muscovite poles) demonstrates the presence of end-members representing three different fluids that were involved in the emplacement of the Piaotang deposit. These end-members can be linked to previous fluid inclusion studies conducted on this deposit. The three fluids are identified to be magmatic, meteoric (as previously reported in the literature), and also metamorphic, and are shown to have mixed throughout the different stages. Moreover, it appears that the magmatic fluids could not have been derived from the Piaotang biotite granite but instead must have originated from a more evolved rare metal granite that is presently unidentified. These fluids were responsible for the greisenization. Finally, chlorite geochemistry reveals the occurrence of a heating process (from 200 °C in stage II to 300 °C in stage III) during the post-mineralizing stages, which was responsible for the precipitation of new generations of ore-bearing minerals (cassiterite and wolframite) concomitant with a continuous gain of metals during the emplacement of the Piaotang deposit.

Metamorphic brines and no surficial fluids trapped in the detachment footwall of a Metamorphic Core Complex (Nevado-Filábride units, Betics, Spain, 2018

The ductile-brittle transition zone in extensional regimes can play the role of a hydrogeological... more The ductile-brittle transition zone in extensional regimes can play the role of a hydrogeological barrier. Quartz veins developed within an orthogneiss body located in the detachment footwall of a Metamorphic Core Complex (MCC) in the Nevado-Filábride units (Betics, Spain). The detachment footwall is composed mainly of gneisses, schists and metacarbonates from the Bédar-Macael sub-unit. Schist and metacarbonate bodies show evidence of ductile deformation at the time the gneiss was already undergoing brittle deformation and vein opening during exhumation. The vein system provides the opportunity to investigate the origin, composition and PVTX conditions of the fluids that circulated in the detachment footwall while the footwall units were crossing the ductile-brittle transition. The analysis of fluid inclusions reveals the presence of a single type of fluid: 30–40 mass% NaCl > KCl > CaCl 2 > MgCl 2 brines, with trace amounts of CO 2 and N 2 and tens to thousands of ppm of metals such as Fe, Sr, Li, Zn, Ba, Pb and Cu. δD fluid values between −39.8 and −16.7‰ and δ 18 O fluid values between 4.4 and 11.7 ± 0.5‰ show that the brines have undergone protracted interaction with the host or-thogneissic body. Coupled salinity and Cl/Br ratios (200 to 4400) indicate that the brines originate from dissolution of Triassic metaevaporites by metamorphic fluids variably enriched in Br by interaction with graphitic schists. This study highlights the absence of any record of surficial fluids within the veins, despite the brittle deformation conditions prevailing in this orthogneiss body. The fact that fluids from the detachment footwall were isolated from surficial fluid reservoirs may result from the presence of overlying schists and metacarbonates that continued to be affected by ductile deformation during vein formation in the gneiss, preventing downward circulation of surface-derived fluids.

Fluid inclusions from quartz and dolomite veins from five unconformity-related uranium deposits (... more Fluid inclusions from quartz and dolomite veins from five unconformity-related uranium deposits (Athabasca basin, Canada) have been analyzed by Raman spectrometry in order to identify trace gases in their vapor phase at room temperature. About 80% of fluid inclusions have detectable gases. The most common gases are H 2 , O 2 , CO 2 , CH 4 , C 2 H 6 and N 2. So-called " NaCl-rich " and " CaCl 2-rich " brine inclusions have similar gas contents. U-bearing fluid inclusions have similar gas compositions when compared to U-absent fluid inclusions. Radiolysis (i.e. production of H 2 and O 2 from H 2 O when fluids have been in contact with previously deposited UO 2) and fluid-rock interactions are the most probable origins for H 2 , O 2 and CO 2 , CH 4 , C 2 H 6 and N 2 respectively. The relative abundance of radiolytic O 2 is related to distance to ore and could be used as vectors towards mineralization. Other trace gases may be indicative of ore-forming processes, specifically of fluid-rock interaction and UO 2 deposition.

Li-micas have been used as indicators of the evolution of granites. However, hydrothermal Li-mica... more Li-micas have been used as indicators of the evolution of granites. However, hydrothermal Li-micas are less documented. World-class W-Sn deposits associated with Early Yanshanian granites (South Jiangxi, China) show magmatic and hydrothermal Li-micas which could help unravelling the magmatic-hydrothermal evolution of rare metal deposits. Six types of Li-micas have been identified in the vein system of the Maoping W-Sn deposit through detailed petrography and EPMA and LA-ICP-MS analyses, by chronological order: (i) late-magmatic Li-micas in feldspar veins, associated with late crystallization of a peraluminous melt; (ii) hydrothermal Fe-Li micas (Fe-Li mica veins and selvages); (iii) hydrothermal Fe-Li micas in W-Sn veins; (iv) Fe-Li micas in later banded quartz veins; (v) Li-muscovite in the final stages; and finally (vi) micas associated with alteration at each stage. Based on oscillatory variations and trends in major elements composition, the chemical variations in Li-micas from the successive stages and in hydrothermal micas that crystallized in the veins are interpreted to reflect mixing between at least three fluids of possible magmatic, meteoric and metamorphic origins. The crystallization of zircons and REE minerals, combined with variations of major and trace element concentrations in the Li-micas, notably an enrichment of rare metals (W-Sn-Ta-Nb) in the Li-micas, implies emplacement of a hidden peralkaline REE-rich magma during the crystallization of the banded quartz veins, a source which was different to the pre-existing peraluminous granites. The possible involvement of both peraluminous and peralkaline intrusives suggests the existence of polyphase magmatic-hydrothermal systems in the Maoping deposit , during the Yanshanian event (190–80 Ma).

Analyses of halogen concentration and stable chlorine isotope composition of fluid inclusions fro... more Analyses of halogen concentration and stable chlorine isotope composition of fluid inclusions from hydrothermal quartz and carbonate veins spatially and temporally associated with giant unconformity-related uranium deposits from the Paleoproterozoic Athabasca Basin (Canada) were performed in order to determine the origin of chloride in the ore-forming brines. Microthermometric analyses show that samples contain variable amounts of a NaCl-rich brine (Cl concentration between 120,000 and 180,000 ppm) and a CaCl2-rich brine (Cl concentration between 160,000 and 220,000 ppm). Molar Cl/Br ratios of fluid inclusion leachates range from ∼100 to ∼900, with most values between 150 and 350. Cl/Br ratios below 650 (seawater value) indicate that the high salinities were acquired by evaporation of seawater. Most δ37Cl values are between −0.6‰ and 0‰ (seawater value) which is also compatible with a common evaporated seawater origin for both NaCl- and CaCl2-rich brines.Slight discrepancies between the Cl concentration, Cl/Br, δ37Cl data and seawater evaporation trends, indicate that the evaporated seawater underwent secondary minor modification of its composition by: (i) mixing with a minor amount of halite-dissolution brine or re-equilibration with halite during burial; (ii) dilution in a maximum of 30% of connate and/or formation waters during its migration towards the base of the Athabasca sandstones; (iii) leaching of chloride from biotites within basement rocks and (iv) water loss by hydration reactions in alteration haloes linked to uranium deposition.The chloride in uranium ore-forming brines of the Athabasca Basin has an unambiguous dominantly marine origin and has required large-scale seawater evaporation and evaporite deposition. Although the direct evidence for evaporative environments in the Athabasca Basin are lacking due to the erosion of ∼80% of the sedimentary pile, Cl/Br ratios and δ37Cl values of brines have behaved conservatively at the basin scale and throughout basin history.

The nature of uranium source rocks, transport conditions and deposition processes are still highl... more The nature of uranium source rocks, transport conditions and deposition processes are still highly controversial for world-class unconformity-related U deposits. This article presents the first detailed chemistry of brines associated with the giant McArthur River U deposit, Canada. LA-ICP-MS analysis of individual fluid inclusions suggests mixing between a Na–Ca–Mg–K–Sr–Ba brine and a Ca–Mg–Na–K–Sr–Ba brine. The brines share a common origin (evaporated seawater) and show evidence for contrasting interaction with basement rocks. The Na-rich brine lost Mg and K in alteration haloes around U ores, while the Ca-rich brine results from Na–Ca exchange and Sr–Ba gain. U concentrations (0.3–530 μg g−1) are anomalously high compared with usual basinal fluids, this indicating that U uptake occurred within basement rocks. The two brine end-members have mixed within the main U deposit area, which could be one of the major driving forces for U deposition.

The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted n... more The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U deposits in the world, are the result of massive basinal fluid migrations in the basement rocks.This study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths (hundred metres below the unconformity) and their chemical modification (salinity increase) by interaction with basement lithologies. These processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity-type mineralization.Microfracture networks, which opened during the basinal brine stage (ca. 1600–1400 Ma) are interpreted as sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing. The stress field at that stage (σ1 = N130–150 °E, subvertical) partly reopened the earlier microcrack networks (σ1 = N80–110 °E and N130–150 °E, subvertical) issued from the Trans-Hudson Orogeny late retrograde metamorphic stage (ca. 1795–1720 Ma). The circulation of the two types of fluids (carbonic and brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity-related uranium mineralization in the Athabasca Basin.

The Jiangxi province (South China) is currently the world's leading tungsten (W) producer. Wolfra... more The Jiangxi province (South China) is currently the world's leading tungsten (W) producer. Wolframite ((Fe,Mn)WO4) and cassiterite (SnO2) are hosted in quartz veins associated to polyphased granite intrusions. The respective implications of magmatic vs non-magmatic fluids in the ore-forming process is still controversial. This study focuses on (i) establishing a detailed paragenetic sequence for the Maoping deposit and (ii) acquiring fluid inclusion microthermometric and Raman data in transparent gangue minerals. Six successive veins filling stages have been identified. Stage I and II correspond to feldspath-only and quartz-zinnwaldite veins respectively. Stage III corresponds to mostly quartz-topaz-wolframite-cassiterite-zinnwaldite veins. Stage IV corresponds to metric banded quartz veins with zinnwaldite, feldspar and rare earth minerals underlining the banding. Stage V is mostly characterized by kaolinite, fluorite and various sulfides. Fluid inclusions in Stage III to V quartz, topaz and fluorite show no significant CO2 content, salinities of ca. 0.5-7 wt. % eq. NaCl and homogenization temperatures of ca. 200-350°C which do not favour a magmatic origin for the ore-forming fluids.

Spectacular alteration of monazite by diagenetic/hydrothermal brines is well documented in some P... more Spectacular alteration of monazite by diagenetic/hydrothermal brines is well documented in some Proterozoic sedimentary basins in close relationship with high-grade uranium (U) deposits. Hence, monazite has been proposed as a viable source for some U deposits. However, monazite alteration remains enigmatic with regard to its high stability in relatively low temperature hydrothermal conditions. Here, the results of batch experiments in which 10 mg of natural monazite grains were reacted with 15 mL of Na-Ca-Cl (6 molal Cl) solutions as well as in pure water at 150 °C and saturated vapor pressure (psat) for one and six months are reported. The influence of pH (pH = 1, 3, 7) and relative molar proportions of Na and Ca (Na/(Na + Ca) = 0, 0.5, 1), were tested. Discrete alteration features (etch pits and roughened surfaces) appear in a minority of the one month experiments and are more developed in the six months experiments, especially at pH = 1 and 3. Although spectacular alteration of monazite, as seen around U deposits, could not be reproduced here, this study shows that monazite is unstable in the presence of fluids analogous to acidic deep basinal brines.

The Paleoproterozoic Athabasca Basin (Canada) hosts numerous giant unconformity-related uranium d... more The Paleoproterozoic Athabasca Basin (Canada) hosts numerous giant unconformity-related uranium deposits. The scope of this study is to establish the pressure, temperature, and composition (P-T-X conditions) of the brines that circulated at the base of the Athabasca Basin and in its crystalline basement before, during and after UO2 deposition. These brines are commonly sampled as fluid inclusions in quartz- and dolomite-cementing veins and breccias associated with alteration and U mineralization. Microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data from five deposits (Rabbit Lake, P-Patch, Eagle Point, Millennium, and Shea Creek) complement previously published data for the McArthur River deposit. In all of the deposits investigated, fluid inclusion salinity is between 25 and 40 wt.% NaCl equiv., with compositions displaying a continuum between a “NaCl-rich brine” end-member (Cl > Na > Ca > Mg > K) and a “CaCl2-rich brine” end-member (Cl > Ca ≈ Mg > Na > K). The CaCl2-rich brine has the highest salinity and shows evidence for halite saturation at the time of trapping. The continuum of compositions between the NaCl-rich brine and the CaCl2-rich brine end-members combined with P-T reconstructions suggest anisothermal mixing of the two brines (NaCl-rich brine, 180 ± 30 °C and 800 ± 400 bars; CaCl2-rich brine, 120 ± 30 °C and 600 ± 300 bars) that occurred under fluctuating pressure conditions (hydrostatic to supra-hydrostatic). However, because the two brines were U bearing and therefore oxidized, brine mixing was probably not the driving force for UO2 deposition. Several scenarios are put forward to account for the Cl-Na-Ca-Mg-K composition of the brines, involving combinations of seawater evaporation, halite dissolution, mixing with a halite-dissolution brine, Mg/Ca exchange by dolomitization, Na/Ca exchange by albitization of plagioclase, Na/K exchange by albitization of K-feldspar, and Mg loss by Mg-rich alteration. Finally, the metal concentrations in the NaCl-rich and CaCl2-rich brines are among the highest recorded compared to present-day sedimentary formation waters and fluid inclusions from basin-hosted base metal deposits (up to 600 ppm U, 3000 ppm Mn, 4000 ppm Zn, 6000 ppm Cu, 8000 ppm Pb, and 10,000 ppm Fe). The CaCl2-rich brine carries up to one order of magnitude more metal than the NaCl-rich brine. Though the exact origin of major cations and metals of the two brines remains uncertain, their contrasting compositions indicate that the two brines had distinct flow paths and fluid-rock interactions. Large-scale circulation of the brines in the Athabasca Basin and Basement was therefore a key parameter for metal mobility (including U) and formation of unconformity-related U deposits.

Three configurations of inductively coupled plasma mass spectrometers (ICPMS), namely: a quadrupo... more Three configurations of inductively coupled plasma mass spectrometers (ICPMS), namely: a quadrupole (QMS) and a sector-field (SFMS), both equipped with a standard cylindrical ablation cell, and an orthogonal time-of-flight (TOFMS), equipped with a fast washout ablation cell, were coupled with the same 193 nm Excimer laser ablation system in order to evaluate their capabilities for measurement of multiple minor and trace elements in small quantities of liquids (pl to nl), such as fluid inclusions. Analyses were performed with different objects: (i) multi-element solutions sealed in silica capillaries of internal diameter of 20 µm serving as synthetic analogues of natural fluid inclusions; (ii) natural two-phase (liquid + vapour) fluid inclusions with low salinity (ca. 4.8 wt% NaCl eq) and homogeneous compositions, trapped in quartz crystals from the Alps; (iii) natural multi-phase (liquid + vapour + multiple solids) fluid inclusions with high salinity (ca. 13-15 wt% NaCl eq) and homogeneous compositions, trapped in quartz crystals from the Zambian Copperbelt. This study demonstrates that the SFMS and TOFMS provide improvements, particularly in term of limits of detection (LODs) and precision, compared to the QMS traditionally used for the measurement of fluid inclusions. SFMS leads on average to lower LODs within one order of magnitude compared to QMS and TOFMS, but precision and accuracy are lower due to longer acquisition cycle times. TOFMS presents both advantages of having rapid and quasi-simultaneous acquisition for all isotopes from 6Li to 238U in a very short cycle time down to 30 µs, with higher precisions and lower LODs than for QMS for isotopes with m/Q > 11. Its use, coupled to a fast washout cell, leads to (i) the improvement in the analysis of small-size (< 10 µm) and multi-phase fluid inclusions and (ii) detection of higher number of isotopes compared to QMS and SFMS, which are both limited by the number of measured isotopes from short transient signals of fluid inclusions. Consequently, the tested TOFMS, coupled with a fast washout ablation cell, appears to be a promising instrument for the analysis of natural fluid inclusions by LA-ICPMS, especially for small, multi-phase and/or low salinity fluid inclusions.

The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada... more The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada) involved basin-scale circulation of U-bearing brines during high-grade diagenesis (150-200 °C) at ∼1.6-1.5 Ga. The UO2 ores occur both sides of the unconformity and are associated with extensive brecciation and illite-sudoite-dravite alteration. Quartz and dolomite cementing veins and breccias are associated with alteration and mineralisation and contain a fairly uniform population of fluid inclusions characterised by variable Na:Ca and salinities of 25-35 wt.% salts and high U concentrations of up to 600 ppm U. In order to further constrain the origin of these U-rich brines, we analysed the naturally occurring isotopes of Ar, Kr and Xe, together with halogens (Cl, Br and I), K, Ca and U in irradiated quartz and dolomite samples containing representative fluid inclusions. This was achieved by the noble gas method for halogen measurement (extended 40Ar-39Ar methodology) using a combination of noble gas extraction techniques.
The fluid inclusions opened by crushing quartz and dolomite samples in vacuum have similar molar Br/Cl ratios of 5.8 × 10−3 to 10.4 × 10−3, and molar I/Cl ratios of 1.8 × 10−6 to 8.2 × 10−6. These compositions lie over the top half of the modern-day seawater evaporation trajectory, consistent with the fluids deriving the bulk of their salinity by subaerial evaporation of seawater, beyond the point of halite saturation. The I/Cl ratios are much lower than is typical of fluids that have interacted with I-rich organic matter present in many sedimentary basins or fluid inclusions found in Mississippi Valley type (MVT) Pb-Zn ore deposits. This is significant because provided the U-rich fluid inclusions are representative of the ore-stage fluids, the low I/Cl ratios of the fluid inclusions do not favour fluid interaction with organic matter (or hydrocarbons), as a major process for localizing U mineralisation.
The majority of samples contain fluid inclusions with age-corrected 40Ar/36Ar of between the modern atmospheric value of ∼300 and 450. These values are considered representative of the fluid's initial composition and are typical of upper crustal sedimentary formation waters. The fluid inclusions non-radiogenic 84Kr/36Ar and 129Xe/36Ar ratios are slightly enriched in 129Xe relative to air and the fluid inclusions are estimated to contain 0.5-17.3 × 10−10 mol g−136Ar which is up to twenty times the 36Ar concentration of air-saturated seawater. The data are interpreted to reflect acquisition of atmospheric noble gases from sedimentary rocks and suggest acquisition of radiogenic 40Ar within K-rich basement rocks, that would have been an important source of excess 40Ar, was limited by temperatures of less than 200 °C.
Taken together the halogen and noble gas composition of the U-bearing fluid inclusions are strongly controlled by subaerial evaporation and subsequent interaction with sedimentary rocks, showing that low temperature evaporitic brines dominated the mineralising system. Mineralisation is unlikely to have been triggered by fluid interaction with organic matter, or mixing with voluminous basement-derived fluids; however, the data do not completely preclude a role for volumetrically minor fluid or gas phases introduced by deep-seated basement faults preferentially located at the sites of mineralisation.

The Paleoproterozoic Athabasca Basin (Canada) hosts numerous world class unconformity-related ura... more The Paleoproterozoic Athabasca Basin (Canada) hosts numerous world class unconformity-related uranium deposits. Many aspects of their genesis remain debated. The objective here is to review recent fluid inclusion and stable isotope studies that have contributed to a better understanding of: (1) the origin and P-T-X conditions of the mineralizing fluids; (2) the conditions for transport of metals and in particular uranium; (3) the relationships between fluid chemistry and alteration features (fluid-rock interaction) and (4) some aspects of uranium deposition. As a whole, the Athabasca system was dominated by basinal brines of evaporated-seawater origin. The brines have percolated through reactivated basement structures, leached metals in basement rocks, and caused intensive Mg-B alteration. Furthermore, the brines were anomalously acidic and have transported the uranium at the highest concentration so far reported for crustal fluids. This feature alone could simply explain the spectacular alteration and also the tremendous grades and tonnages of the Athabasca uranium deposits.

In the Pan-African Lufilian belt (Western Zambian Copperbelt), uranium mineralizations, preferent... more In the Pan-African Lufilian belt (Western Zambian Copperbelt), uranium mineralizations, preferentially scattered in kyanite ± talc micaschists (metamorphosed evaporitic sediments) or concentrated along transposed quartz veins provide an opportunity to (1) understand the time/space relationship between the ore minerals and the deformation of the host rocks, (2) identify the different fluid events associated with specific stages of quartz deformation and (3) characterize the ore fluid geochemistry in terms of fluid origin and fluid/rock interactions. In the U occurrences studied in Lolwa and Mitukuluku (Domes region, Western Zambian Copperbelt), two mineralizing stages are described. The first generation of ore fluids (53–59 wt% CaCl2, 13–15 wt% NaCl; N2–H2 in the gas phase of fluid inclusions) circulated during the high-temperature quartz recrystallization, at 500–700 °C. This temperature is in agreement with the P–T conditions recorded during the crustal thickening related to continental collision at ca. 530 Ma. LA-ICPMS analyses show the presence of uranium within this fluid, with a concentration mode around 20 ppm. The second generation of ore fluid (21–32 wt% NaCl, 19–21 wt% CaCl2; CO2–CO in the gas phase of fluid inclusions) percolated at lower temperature conditions, at the brittle–ductile transition, between 200 and 300 °C. This temperature could be related to the exhumation of the high-grade metamorphic rocks at ca. 500 Ma. The formation of H2 and CO is interpreted as the result of radiolysis in the presence of dissolved uranium in the aqueous phase of these fluid inclusions. Finally, a late fluid (14–16 wt% NaClequiv) circulated in the brittle domain but seems unrelated to U (re-)mobilization event.

Fluid inclusions from two quartz samples of the McArthur River and Rabbit Lake unconformity-relat... more Fluid inclusions from two quartz samples of the McArthur River and Rabbit Lake unconformity-related uranium deposits (Athabasca Basin, Canada) were analysed by synchrotron X-ray fluorescence (SXRF) and X-ray absorption near-edge spectroscopy (XANES) to shed light on (i) the detailed chemistry of the fluids having transported the uranium and (ii) the speciation of uranium in these fluids. The analysed samples contain variable proportions of NaCl-rich or CaCl2-rich (2535 wt% salts) fluid inclusions that homogenise into the liquid phase between 120 and 200 degrees C. For unknown reason, all of the CaCl2-rich fluid inclusions decrepitate under X-ray beam after a few seconds, precluding any SXRF or XANES measurement. SXRF on 12 homogenised NaCl-rich fluid inclusions from the Rabbit Lake sample shows that the fluid inclusions contain appreciable amounts of Br, Fe, Sr, transition metals (Mn, Ni, Cu, Zn), Pb, U and rare earth elements (REE) (La, Ce), with concentrations being relatively homogeneous among fluid inclusions. Within the complex McArthur River sample (numerous fluid inclusions+randomly distributed solids), statistical analyses of hyperspectral SXRF images were performed using the non-negative matrix approximation (NNMA) method. This strategy allows distinguishing the three domains contributing to the overall signal: (i) the quartz matrix, which notably contains significant amounts of Cr, Cu and Pb; (ii) the fluid inclusions characterised by high amounts of Br, Fe, Sr and transition metals; and (iii) La-Ce +/- Fe solids. Part of the U and REE are spatially associated with distinct optically invisible solids within the quartz matrix. XANES on four McArthur River sample fluid inclusions at room temperature and at 150 degrees C (fluid inclusion trapping temperature) as well as in solid and liquid U(IV) and U(VI) standards, respectively, shows that the uranium has remained in the form of U(VI) from trapping to present in the fluid inclusion.

Fluid inclusions (FIs) from unconformity-related uranium deposits of the Athabasca Basin (Canada)... more Fluid inclusions (FIs) from unconformity-related uranium deposits of the Athabasca Basin (Canada) were analyzed by Synchrotron X-Ray Fluorescence (SXRF) and X-Ray Absorption Near-Edge Spectroscopy (XANES) in order to shed light on: (1) the origin of the fluids having transported and precipitated the U; (2) the pathways and associated fluid-rock interactions leading to their observed chemistry and (3) the U species associated to U transport. SXRF shows that the FIs are highly enriched in Br and Sr as well as transition metals (Cr, Fe, Ni, Cu, Pb). Part of the U and REEs are spatially associated to distinct optically invisible solids within the quartz matrix. XANES on FIs at room temperature and at 150°C (trapping temperature) as well as in synthetic solid and liquid U(IV) and U(VI) standards shows that the U has remained in the form of U(VI) from FI trapping to present.

Lithium isotopes in fluid inclusions as tracers of crustal fluids: An exploratory study, 2018

Lithium isotopes are extensively used to trace fluid migration and fluid-rock interactions in the... more Lithium isotopes are extensively used to trace fluid migration and fluid-rock interactions in the shallow Earth's crust. Although, Li isotope systematics might give important information about deep crustal fluids, this stable isotope system remains surprisingly under exploited in fluid inclusions. Here, twenty-three samples from a variety of deep environments, from diagenetic, metamorphic to magmatic-hydrothermal conditions (~150 to 600 °C), have been investigated in order to provide the first exploratory overview of the range in Li isotopic composition of fluid inclusions. From 1 to 2 g of hydrothermal quartz or dolomite, cation analysis (Na, K, Ca, Mg, Sr, Li), Li fraction recovery and Li isotopic analysis of bulk fluid inclusion leachates were carried out by using an innovative approach coupling crush-leach extraction, high performance ion chromatography and MC-ICPMS analysis. Reconstructed Li concentrations and δ 7 Li values of these fluid inclusions (respectively 12 to 653 mg/l and −1.4 ± 0.2‰ to +41.3 ± 0.8‰) are compatible with previously proposed models for the origin of the fluids and fluid-rock interactions. δ 7 Li values are independent from other tracers (Br/Cl, I/Cl) and parameters such as temperature, salinity, and cation content. The results show that, in conjunction with other parameters, the Li isotopic compositions of fluid inclusions are potentially powerful source and process tracers of deep crustal fluids within a wide range of geological environments and related ore-forming systems, for example in from sedimentary basins, seafloor hydrothermal systems and subduction zones.

Advances in 3D imaging and volumetric reconstruction of fluid and melt inclusions by high resolution X-ray computed tomography

Fluid and melt inclusions are tiny pockets of fluid and melt trapped in natural and synthetic min... more Fluid and melt inclusions are tiny pockets of fluid and melt trapped in natural and synthetic minerals. Characterizing the 3D distribution of fluid and melt inclusions within minerals, their shape and the volume fraction of their different phases is crucial for determining the conditions of crystal growth and paleostress analysis. However, their relatively small size (typically 5 to 100 μm), complex shape, heterogeneous content, the opaque nature of some host minerals and projection bias frequently hamper accurate imaging and volumetric reconstruction using conventional microscopic techniques. High resolution X-ray computed tomography (HRXCT) is a non-destructive method which uses contrasts of X-ray attenuation in a series of contiguous radiographs with different view angles to reconstruct the 3D distribution of areas of different densities within a large variety of materials. In this work, we show the capabilities of HRXCT for: (i) imaging the 3D distribution of aqueous and hydrocarbon-bearing fluid inclusions and silicate melt inclusions in a crystal; (ii) characterizing the shape of fluid and melt inclusions and (iii) reconstructing the total volume and the volume of the different phases (liquid, glass, crystal, vapor) of fluid and melt inclusions. We have used a variety of hand specimens and chips of transparent and opaque minerals (olivine, quartz, feldspar, garnet, emerald, wolframite), that we analyzed using three different HRXCT setups. When a resolution of ~1 μm 3 /voxel is achieved, HRXCT allows identifying > 5 μm fluid inclusions, and the identification and volumetric reconstruction of the different phases can be carried out with reasonable confidence for relatively large (> 25 μm) inclusions. Density contrasts are high enough to properly identify: (i) a silicate melt inclusion, and its different phases (glass, vapor and crystals such as clin-opyroxene and spinel) in an olivine crystal; (ii) aqueous monophase (liquid) and two-phase (liquid + vapor) fluid inclusions in transparent and opaque minerals (quartz, garnet, emerald, wolframite). In the case of hydrocarbon bearing fluid inclusions containing a vapor phase and two liquid phases (oil and aqueous solution), the two liquid phases could not be distinguished from each other. Volumetric reconstruction of liquid and vapor phases of aqueous and hydrocarbon-bearing fluid inclusions show compatible results with independent calculations using known pressure, temperature, molar volume and composition (P-TV -x) conditions of trapping or imaging using confocal laser scanning microscopy respectively. Collectively, our results show that HRXCT is a promising tool for non-destructive characterization of fluid and melt inclusions.

The ore-forming magmatic-hydrothermal system of the Piaotang W-Sn deposit (Jiangxi, China) as seen from Li-mica geochemistry, 2018

Many studies have proved the usefulness of Li-mica and chlorite geochemistry as indicators of the... more Many studies have proved the usefulness of Li-mica and chlorite geochemistry as indicators of the chemical and thermal evolution of magmatic systems. This study highlights the suitability of Li-micas as tracers of hydrothermal mineralizing events in world-class W-Sn deposits associated with Jurassic (190–150 Ma) granites in China through the complex magmatic–hydrothermal evolution of the Piaotang deposit (South Jiangxi). A paragenetic sequence has been established for the Piaotang deposit comprising (1) a first " silicate-oxide " stage that hosts abundant W-Sn mineralization (wolframite and cassiterite), (2) a " calcic " stage with scheelite and wolframite, (3) a " base metal sulfides " stage with cassiterite and wolframite, and (4) a late " sulfide " stage, involving for the first time a polyphase emplacement of the mineralization. Li-micas from the underlying granite, greisen, and the different stages represented in the veins, were studied. The chemistry of the micas (characterized by intermediate compositions between phlogopite-zinnwaldite-muscovite poles) demonstrates the presence of end-members representing three different fluids that were involved in the emplacement of the Piaotang deposit. These end-members can be linked to previous fluid inclusion studies conducted on this deposit. The three fluids are identified to be magmatic, meteoric (as previously reported in the literature), and also metamorphic, and are shown to have mixed throughout the different stages. Moreover, it appears that the magmatic fluids could not have been derived from the Piaotang biotite granite but instead must have originated from a more evolved rare metal granite that is presently unidentified. These fluids were responsible for the greisenization. Finally, chlorite geochemistry reveals the occurrence of a heating process (from 200 °C in stage II to 300 °C in stage III) during the post-mineralizing stages, which was responsible for the precipitation of new generations of ore-bearing minerals (cassiterite and wolframite) concomitant with a continuous gain of metals during the emplacement of the Piaotang deposit.

Metamorphic brines and no surficial fluids trapped in the detachment footwall of a Metamorphic Core Complex (Nevado-Filábride units, Betics, Spain, 2018

The ductile-brittle transition zone in extensional regimes can play the role of a hydrogeological... more The ductile-brittle transition zone in extensional regimes can play the role of a hydrogeological barrier. Quartz veins developed within an orthogneiss body located in the detachment footwall of a Metamorphic Core Complex (MCC) in the Nevado-Filábride units (Betics, Spain). The detachment footwall is composed mainly of gneisses, schists and metacarbonates from the Bédar-Macael sub-unit. Schist and metacarbonate bodies show evidence of ductile deformation at the time the gneiss was already undergoing brittle deformation and vein opening during exhumation. The vein system provides the opportunity to investigate the origin, composition and PVTX conditions of the fluids that circulated in the detachment footwall while the footwall units were crossing the ductile-brittle transition. The analysis of fluid inclusions reveals the presence of a single type of fluid: 30–40 mass% NaCl > KCl > CaCl 2 > MgCl 2 brines, with trace amounts of CO 2 and N 2 and tens to thousands of ppm of metals such as Fe, Sr, Li, Zn, Ba, Pb and Cu. δD fluid values between −39.8 and −16.7‰ and δ 18 O fluid values between 4.4 and 11.7 ± 0.5‰ show that the brines have undergone protracted interaction with the host or-thogneissic body. Coupled salinity and Cl/Br ratios (200 to 4400) indicate that the brines originate from dissolution of Triassic metaevaporites by metamorphic fluids variably enriched in Br by interaction with graphitic schists. This study highlights the absence of any record of surficial fluids within the veins, despite the brittle deformation conditions prevailing in this orthogneiss body. The fact that fluids from the detachment footwall were isolated from surficial fluid reservoirs may result from the presence of overlying schists and metacarbonates that continued to be affected by ductile deformation during vein formation in the gneiss, preventing downward circulation of surface-derived fluids.

Fluid inclusions from quartz and dolomite veins from five unconformity-related uranium deposits (... more Fluid inclusions from quartz and dolomite veins from five unconformity-related uranium deposits (Athabasca basin, Canada) have been analyzed by Raman spectrometry in order to identify trace gases in their vapor phase at room temperature. About 80% of fluid inclusions have detectable gases. The most common gases are H 2 , O 2 , CO 2 , CH 4 , C 2 H 6 and N 2. So-called " NaCl-rich " and " CaCl 2-rich " brine inclusions have similar gas contents. U-bearing fluid inclusions have similar gas compositions when compared to U-absent fluid inclusions. Radiolysis (i.e. production of H 2 and O 2 from H 2 O when fluids have been in contact with previously deposited UO 2) and fluid-rock interactions are the most probable origins for H 2 , O 2 and CO 2 , CH 4 , C 2 H 6 and N 2 respectively. The relative abundance of radiolytic O 2 is related to distance to ore and could be used as vectors towards mineralization. Other trace gases may be indicative of ore-forming processes, specifically of fluid-rock interaction and UO 2 deposition.

Li-micas have been used as indicators of the evolution of granites. However, hydrothermal Li-mica... more Li-micas have been used as indicators of the evolution of granites. However, hydrothermal Li-micas are less documented. World-class W-Sn deposits associated with Early Yanshanian granites (South Jiangxi, China) show magmatic and hydrothermal Li-micas which could help unravelling the magmatic-hydrothermal evolution of rare metal deposits. Six types of Li-micas have been identified in the vein system of the Maoping W-Sn deposit through detailed petrography and EPMA and LA-ICP-MS analyses, by chronological order: (i) late-magmatic Li-micas in feldspar veins, associated with late crystallization of a peraluminous melt; (ii) hydrothermal Fe-Li micas (Fe-Li mica veins and selvages); (iii) hydrothermal Fe-Li micas in W-Sn veins; (iv) Fe-Li micas in later banded quartz veins; (v) Li-muscovite in the final stages; and finally (vi) micas associated with alteration at each stage. Based on oscillatory variations and trends in major elements composition, the chemical variations in Li-micas from the successive stages and in hydrothermal micas that crystallized in the veins are interpreted to reflect mixing between at least three fluids of possible magmatic, meteoric and metamorphic origins. The crystallization of zircons and REE minerals, combined with variations of major and trace element concentrations in the Li-micas, notably an enrichment of rare metals (W-Sn-Ta-Nb) in the Li-micas, implies emplacement of a hidden peralkaline REE-rich magma during the crystallization of the banded quartz veins, a source which was different to the pre-existing peraluminous granites. The possible involvement of both peraluminous and peralkaline intrusives suggests the existence of polyphase magmatic-hydrothermal systems in the Maoping deposit , during the Yanshanian event (190–80 Ma).

Analyses of halogen concentration and stable chlorine isotope composition of fluid inclusions fro... more Analyses of halogen concentration and stable chlorine isotope composition of fluid inclusions from hydrothermal quartz and carbonate veins spatially and temporally associated with giant unconformity-related uranium deposits from the Paleoproterozoic Athabasca Basin (Canada) were performed in order to determine the origin of chloride in the ore-forming brines. Microthermometric analyses show that samples contain variable amounts of a NaCl-rich brine (Cl concentration between 120,000 and 180,000 ppm) and a CaCl2-rich brine (Cl concentration between 160,000 and 220,000 ppm). Molar Cl/Br ratios of fluid inclusion leachates range from ∼100 to ∼900, with most values between 150 and 350. Cl/Br ratios below 650 (seawater value) indicate that the high salinities were acquired by evaporation of seawater. Most δ37Cl values are between −0.6‰ and 0‰ (seawater value) which is also compatible with a common evaporated seawater origin for both NaCl- and CaCl2-rich brines.Slight discrepancies between the Cl concentration, Cl/Br, δ37Cl data and seawater evaporation trends, indicate that the evaporated seawater underwent secondary minor modification of its composition by: (i) mixing with a minor amount of halite-dissolution brine or re-equilibration with halite during burial; (ii) dilution in a maximum of 30% of connate and/or formation waters during its migration towards the base of the Athabasca sandstones; (iii) leaching of chloride from biotites within basement rocks and (iv) water loss by hydration reactions in alteration haloes linked to uranium deposition.The chloride in uranium ore-forming brines of the Athabasca Basin has an unambiguous dominantly marine origin and has required large-scale seawater evaporation and evaporite deposition. Although the direct evidence for evaporative environments in the Athabasca Basin are lacking due to the erosion of ∼80% of the sedimentary pile, Cl/Br ratios and δ37Cl values of brines have behaved conservatively at the basin scale and throughout basin history.

The nature of uranium source rocks, transport conditions and deposition processes are still highl... more The nature of uranium source rocks, transport conditions and deposition processes are still highly controversial for world-class unconformity-related U deposits. This article presents the first detailed chemistry of brines associated with the giant McArthur River U deposit, Canada. LA-ICP-MS analysis of individual fluid inclusions suggests mixing between a Na–Ca–Mg–K–Sr–Ba brine and a Ca–Mg–Na–K–Sr–Ba brine. The brines share a common origin (evaporated seawater) and show evidence for contrasting interaction with basement rocks. The Na-rich brine lost Mg and K in alteration haloes around U ores, while the Ca-rich brine results from Na–Ca exchange and Sr–Ba gain. U concentrations (0.3–530 μg g−1) are anomalously high compared with usual basinal fluids, this indicating that U uptake occurred within basement rocks. The two brine end-members have mixed within the main U deposit area, which could be one of the major driving forces for U deposition.

The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted n... more The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U deposits in the world, are the result of massive basinal fluid migrations in the basement rocks.This study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths (hundred metres below the unconformity) and their chemical modification (salinity increase) by interaction with basement lithologies. These processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity-type mineralization.Microfracture networks, which opened during the basinal brine stage (ca. 1600–1400 Ma) are interpreted as sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing. The stress field at that stage (σ1 = N130–150 °E, subvertical) partly reopened the earlier microcrack networks (σ1 = N80–110 °E and N130–150 °E, subvertical) issued from the Trans-Hudson Orogeny late retrograde metamorphic stage (ca. 1795–1720 Ma). The circulation of the two types of fluids (carbonic and brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity-related uranium mineralization in the Athabasca Basin.

The Jiangxi province (South China) is currently the world's leading tungsten (W) producer. Wolfra... more The Jiangxi province (South China) is currently the world's leading tungsten (W) producer. Wolframite ((Fe,Mn)WO4) and cassiterite (SnO2) are hosted in quartz veins associated to polyphased granite intrusions. The respective implications of magmatic vs non-magmatic fluids in the ore-forming process is still controversial. This study focuses on (i) establishing a detailed paragenetic sequence for the Maoping deposit and (ii) acquiring fluid inclusion microthermometric and Raman data in transparent gangue minerals. Six successive veins filling stages have been identified. Stage I and II correspond to feldspath-only and quartz-zinnwaldite veins respectively. Stage III corresponds to mostly quartz-topaz-wolframite-cassiterite-zinnwaldite veins. Stage IV corresponds to metric banded quartz veins with zinnwaldite, feldspar and rare earth minerals underlining the banding. Stage V is mostly characterized by kaolinite, fluorite and various sulfides. Fluid inclusions in Stage III to V quartz, topaz and fluorite show no significant CO2 content, salinities of ca. 0.5-7 wt. % eq. NaCl and homogenization temperatures of ca. 200-350°C which do not favour a magmatic origin for the ore-forming fluids.

Spectacular alteration of monazite by diagenetic/hydrothermal brines is well documented in some P... more Spectacular alteration of monazite by diagenetic/hydrothermal brines is well documented in some Proterozoic sedimentary basins in close relationship with high-grade uranium (U) deposits. Hence, monazite has been proposed as a viable source for some U deposits. However, monazite alteration remains enigmatic with regard to its high stability in relatively low temperature hydrothermal conditions. Here, the results of batch experiments in which 10 mg of natural monazite grains were reacted with 15 mL of Na-Ca-Cl (6 molal Cl) solutions as well as in pure water at 150 °C and saturated vapor pressure (psat) for one and six months are reported. The influence of pH (pH = 1, 3, 7) and relative molar proportions of Na and Ca (Na/(Na + Ca) = 0, 0.5, 1), were tested. Discrete alteration features (etch pits and roughened surfaces) appear in a minority of the one month experiments and are more developed in the six months experiments, especially at pH = 1 and 3. Although spectacular alteration of monazite, as seen around U deposits, could not be reproduced here, this study shows that monazite is unstable in the presence of fluids analogous to acidic deep basinal brines.

The Paleoproterozoic Athabasca Basin (Canada) hosts numerous giant unconformity-related uranium d... more The Paleoproterozoic Athabasca Basin (Canada) hosts numerous giant unconformity-related uranium deposits. The scope of this study is to establish the pressure, temperature, and composition (P-T-X conditions) of the brines that circulated at the base of the Athabasca Basin and in its crystalline basement before, during and after UO2 deposition. These brines are commonly sampled as fluid inclusions in quartz- and dolomite-cementing veins and breccias associated with alteration and U mineralization. Microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data from five deposits (Rabbit Lake, P-Patch, Eagle Point, Millennium, and Shea Creek) complement previously published data for the McArthur River deposit. In all of the deposits investigated, fluid inclusion salinity is between 25 and 40 wt.% NaCl equiv., with compositions displaying a continuum between a “NaCl-rich brine” end-member (Cl > Na > Ca > Mg > K) and a “CaCl2-rich brine” end-member (Cl > Ca ≈ Mg > Na > K). The CaCl2-rich brine has the highest salinity and shows evidence for halite saturation at the time of trapping. The continuum of compositions between the NaCl-rich brine and the CaCl2-rich brine end-members combined with P-T reconstructions suggest anisothermal mixing of the two brines (NaCl-rich brine, 180 ± 30 °C and 800 ± 400 bars; CaCl2-rich brine, 120 ± 30 °C and 600 ± 300 bars) that occurred under fluctuating pressure conditions (hydrostatic to supra-hydrostatic). However, because the two brines were U bearing and therefore oxidized, brine mixing was probably not the driving force for UO2 deposition. Several scenarios are put forward to account for the Cl-Na-Ca-Mg-K composition of the brines, involving combinations of seawater evaporation, halite dissolution, mixing with a halite-dissolution brine, Mg/Ca exchange by dolomitization, Na/Ca exchange by albitization of plagioclase, Na/K exchange by albitization of K-feldspar, and Mg loss by Mg-rich alteration. Finally, the metal concentrations in the NaCl-rich and CaCl2-rich brines are among the highest recorded compared to present-day sedimentary formation waters and fluid inclusions from basin-hosted base metal deposits (up to 600 ppm U, 3000 ppm Mn, 4000 ppm Zn, 6000 ppm Cu, 8000 ppm Pb, and 10,000 ppm Fe). The CaCl2-rich brine carries up to one order of magnitude more metal than the NaCl-rich brine. Though the exact origin of major cations and metals of the two brines remains uncertain, their contrasting compositions indicate that the two brines had distinct flow paths and fluid-rock interactions. Large-scale circulation of the brines in the Athabasca Basin and Basement was therefore a key parameter for metal mobility (including U) and formation of unconformity-related U deposits.

Three configurations of inductively coupled plasma mass spectrometers (ICPMS), namely: a quadrupo... more Three configurations of inductively coupled plasma mass spectrometers (ICPMS), namely: a quadrupole (QMS) and a sector-field (SFMS), both equipped with a standard cylindrical ablation cell, and an orthogonal time-of-flight (TOFMS), equipped with a fast washout ablation cell, were coupled with the same 193 nm Excimer laser ablation system in order to evaluate their capabilities for measurement of multiple minor and trace elements in small quantities of liquids (pl to nl), such as fluid inclusions. Analyses were performed with different objects: (i) multi-element solutions sealed in silica capillaries of internal diameter of 20 µm serving as synthetic analogues of natural fluid inclusions; (ii) natural two-phase (liquid + vapour) fluid inclusions with low salinity (ca. 4.8 wt% NaCl eq) and homogeneous compositions, trapped in quartz crystals from the Alps; (iii) natural multi-phase (liquid + vapour + multiple solids) fluid inclusions with high salinity (ca. 13-15 wt% NaCl eq) and homogeneous compositions, trapped in quartz crystals from the Zambian Copperbelt. This study demonstrates that the SFMS and TOFMS provide improvements, particularly in term of limits of detection (LODs) and precision, compared to the QMS traditionally used for the measurement of fluid inclusions. SFMS leads on average to lower LODs within one order of magnitude compared to QMS and TOFMS, but precision and accuracy are lower due to longer acquisition cycle times. TOFMS presents both advantages of having rapid and quasi-simultaneous acquisition for all isotopes from 6Li to 238U in a very short cycle time down to 30 µs, with higher precisions and lower LODs than for QMS for isotopes with m/Q > 11. Its use, coupled to a fast washout cell, leads to (i) the improvement in the analysis of small-size (< 10 µm) and multi-phase fluid inclusions and (ii) detection of higher number of isotopes compared to QMS and SFMS, which are both limited by the number of measured isotopes from short transient signals of fluid inclusions. Consequently, the tested TOFMS, coupled with a fast washout ablation cell, appears to be a promising instrument for the analysis of natural fluid inclusions by LA-ICPMS, especially for small, multi-phase and/or low salinity fluid inclusions.

The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada... more The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada) involved basin-scale circulation of U-bearing brines during high-grade diagenesis (150-200 °C) at ∼1.6-1.5 Ga. The UO2 ores occur both sides of the unconformity and are associated with extensive brecciation and illite-sudoite-dravite alteration. Quartz and dolomite cementing veins and breccias are associated with alteration and mineralisation and contain a fairly uniform population of fluid inclusions characterised by variable Na:Ca and salinities of 25-35 wt.% salts and high U concentrations of up to 600 ppm U. In order to further constrain the origin of these U-rich brines, we analysed the naturally occurring isotopes of Ar, Kr and Xe, together with halogens (Cl, Br and I), K, Ca and U in irradiated quartz and dolomite samples containing representative fluid inclusions. This was achieved by the noble gas method for halogen measurement (extended 40Ar-39Ar methodology) using a combination of noble gas extraction techniques.
The fluid inclusions opened by crushing quartz and dolomite samples in vacuum have similar molar Br/Cl ratios of 5.8 × 10−3 to 10.4 × 10−3, and molar I/Cl ratios of 1.8 × 10−6 to 8.2 × 10−6. These compositions lie over the top half of the modern-day seawater evaporation trajectory, consistent with the fluids deriving the bulk of their salinity by subaerial evaporation of seawater, beyond the point of halite saturation. The I/Cl ratios are much lower than is typical of fluids that have interacted with I-rich organic matter present in many sedimentary basins or fluid inclusions found in Mississippi Valley type (MVT) Pb-Zn ore deposits. This is significant because provided the U-rich fluid inclusions are representative of the ore-stage fluids, the low I/Cl ratios of the fluid inclusions do not favour fluid interaction with organic matter (or hydrocarbons), as a major process for localizing U mineralisation.
The majority of samples contain fluid inclusions with age-corrected 40Ar/36Ar of between the modern atmospheric value of ∼300 and 450. These values are considered representative of the fluid's initial composition and are typical of upper crustal sedimentary formation waters. The fluid inclusions non-radiogenic 84Kr/36Ar and 129Xe/36Ar ratios are slightly enriched in 129Xe relative to air and the fluid inclusions are estimated to contain 0.5-17.3 × 10−10 mol g−136Ar which is up to twenty times the 36Ar concentration of air-saturated seawater. The data are interpreted to reflect acquisition of atmospheric noble gases from sedimentary rocks and suggest acquisition of radiogenic 40Ar within K-rich basement rocks, that would have been an important source of excess 40Ar, was limited by temperatures of less than 200 °C.
Taken together the halogen and noble gas composition of the U-bearing fluid inclusions are strongly controlled by subaerial evaporation and subsequent interaction with sedimentary rocks, showing that low temperature evaporitic brines dominated the mineralising system. Mineralisation is unlikely to have been triggered by fluid interaction with organic matter, or mixing with voluminous basement-derived fluids; however, the data do not completely preclude a role for volumetrically minor fluid or gas phases introduced by deep-seated basement faults preferentially located at the sites of mineralisation.

The Paleoproterozoic Athabasca Basin (Canada) hosts numerous world class unconformity-related ura... more The Paleoproterozoic Athabasca Basin (Canada) hosts numerous world class unconformity-related uranium deposits. Many aspects of their genesis remain debated. The objective here is to review recent fluid inclusion and stable isotope studies that have contributed to a better understanding of: (1) the origin and P-T-X conditions of the mineralizing fluids; (2) the conditions for transport of metals and in particular uranium; (3) the relationships between fluid chemistry and alteration features (fluid-rock interaction) and (4) some aspects of uranium deposition. As a whole, the Athabasca system was dominated by basinal brines of evaporated-seawater origin. The brines have percolated through reactivated basement structures, leached metals in basement rocks, and caused intensive Mg-B alteration. Furthermore, the brines were anomalously acidic and have transported the uranium at the highest concentration so far reported for crustal fluids. This feature alone could simply explain the spectacular alteration and also the tremendous grades and tonnages of the Athabasca uranium deposits.

In the Pan-African Lufilian belt (Western Zambian Copperbelt), uranium mineralizations, preferent... more In the Pan-African Lufilian belt (Western Zambian Copperbelt), uranium mineralizations, preferentially scattered in kyanite ± talc micaschists (metamorphosed evaporitic sediments) or concentrated along transposed quartz veins provide an opportunity to (1) understand the time/space relationship between the ore minerals and the deformation of the host rocks, (2) identify the different fluid events associated with specific stages of quartz deformation and (3) characterize the ore fluid geochemistry in terms of fluid origin and fluid/rock interactions. In the U occurrences studied in Lolwa and Mitukuluku (Domes region, Western Zambian Copperbelt), two mineralizing stages are described. The first generation of ore fluids (53–59 wt% CaCl2, 13–15 wt% NaCl; N2–H2 in the gas phase of fluid inclusions) circulated during the high-temperature quartz recrystallization, at 500–700 °C. This temperature is in agreement with the P–T conditions recorded during the crustal thickening related to continental collision at ca. 530 Ma. LA-ICPMS analyses show the presence of uranium within this fluid, with a concentration mode around 20 ppm. The second generation of ore fluid (21–32 wt% NaCl, 19–21 wt% CaCl2; CO2–CO in the gas phase of fluid inclusions) percolated at lower temperature conditions, at the brittle–ductile transition, between 200 and 300 °C. This temperature could be related to the exhumation of the high-grade metamorphic rocks at ca. 500 Ma. The formation of H2 and CO is interpreted as the result of radiolysis in the presence of dissolved uranium in the aqueous phase of these fluid inclusions. Finally, a late fluid (14–16 wt% NaClequiv) circulated in the brittle domain but seems unrelated to U (re-)mobilization event.

Fluid inclusions from two quartz samples of the McArthur River and Rabbit Lake unconformity-relat... more Fluid inclusions from two quartz samples of the McArthur River and Rabbit Lake unconformity-related uranium deposits (Athabasca Basin, Canada) were analysed by synchrotron X-ray fluorescence (SXRF) and X-ray absorption near-edge spectroscopy (XANES) to shed light on (i) the detailed chemistry of the fluids having transported the uranium and (ii) the speciation of uranium in these fluids. The analysed samples contain variable proportions of NaCl-rich or CaCl2-rich (2535 wt% salts) fluid inclusions that homogenise into the liquid phase between 120 and 200 degrees C. For unknown reason, all of the CaCl2-rich fluid inclusions decrepitate under X-ray beam after a few seconds, precluding any SXRF or XANES measurement. SXRF on 12 homogenised NaCl-rich fluid inclusions from the Rabbit Lake sample shows that the fluid inclusions contain appreciable amounts of Br, Fe, Sr, transition metals (Mn, Ni, Cu, Zn), Pb, U and rare earth elements (REE) (La, Ce), with concentrations being relatively homogeneous among fluid inclusions. Within the complex McArthur River sample (numerous fluid inclusions+randomly distributed solids), statistical analyses of hyperspectral SXRF images were performed using the non-negative matrix approximation (NNMA) method. This strategy allows distinguishing the three domains contributing to the overall signal: (i) the quartz matrix, which notably contains significant amounts of Cr, Cu and Pb; (ii) the fluid inclusions characterised by high amounts of Br, Fe, Sr and transition metals; and (iii) La-Ce +/- Fe solids. Part of the U and REE are spatially associated with distinct optically invisible solids within the quartz matrix. XANES on four McArthur River sample fluid inclusions at room temperature and at 150 degrees C (fluid inclusion trapping temperature) as well as in solid and liquid U(IV) and U(VI) standards, respectively, shows that the uranium has remained in the form of U(VI) from trapping to present in the fluid inclusion.

Fluid inclusions (FIs) from unconformity-related uranium deposits of the Athabasca Basin (Canada)... more Fluid inclusions (FIs) from unconformity-related uranium deposits of the Athabasca Basin (Canada) were analyzed by Synchrotron X-Ray Fluorescence (SXRF) and X-Ray Absorption Near-Edge Spectroscopy (XANES) in order to shed light on: (1) the origin of the fluids having transported and precipitated the U; (2) the pathways and associated fluid-rock interactions leading to their observed chemistry and (3) the U species associated to U transport. SXRF shows that the FIs are highly enriched in Br and Sr as well as transition metals (Cr, Fe, Ni, Cu, Pb). Part of the U and REEs are spatially associated to distinct optically invisible solids within the quartz matrix. XANES on FIs at room temperature and at 150°C (trapping temperature) as well as in synthetic solid and liquid U(IV) and U(VI) standards shows that the U has remained in the form of U(VI) from FI trapping to present.