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In this study, data from thirteen laboratories from around the world are presented for a successful certification of uranium isotope ratios in uranium ore concentrate (UOC) certified reference materials from the National Research Council Canada. Different mass spectrometric measurement principles (including SF-ICP-MS, quadruple ICP-MS, TIMS, MC-ICP-MS, SIMS and AMS) and measurement procedures were employed. In general, data from all sources exhibit good consistency with TIMS and MC-ICP-MS showing superior performance. The three candidate UOC CRMs (UCLO-1, UCHI-1 and UPER-1) have natural uranium isotope ratios with certified values of 0.5304(7) × 10⁻⁴, 0.5475(2) × 10⁻⁴ and 0.5361(4) × 10⁻⁴ for n(²³⁴U)/n(²³⁸U) and 0.007 2563(13), 0.007 2563(10) and 0.007 2542(11) for n(²³⁵U)/n(²³⁸U), respectively, with expanded uncertainty (k = 2) applicable to the last digit of the value given in the parentheses. Information values for n(²³⁶U)/n(²³⁸U) in these three CRMs, measured by AMS, are also provided: 10 × 10⁻¹², 200 × 10⁻¹² and 22 × 10⁻¹². The uncertainties of the proposed certified values of uranium isotope ratios in UOC CRMs are superior to available reference materials, and the values of n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) show significant variation among the three CRMs.

CO2 + O2 in-situ leaching (ISL) of sandstone-type uranium ore represents the third generation of solution mining in China. In this study, reactive transport modeling of the interaction between hydrodynamic and geochemical reactions is performed to enable better prediction and regulation of the CO2 + O2 in-situ leaching process of uranium. Geochemical reactions between mining solutions and rock, and the kinetic uranium dissolution controlled by O2 (aq) and bicarbonate ( $${\text{ HCO}}_{3}^{ - }$$ ) are considered in the CO2 + O2 ISL reactive transport model of a typical sandstone-hosted uranium ore deposit in northern China. The reactive leaching of uranium is most sensitive to the spatial distribution of the mineralogical properties of the uranium deposit. Stochastic geostatistical models are used to represent the uncertainty on the spatial distribution of mineral grades. A Monte Carlo analysis was also performed to simulate the uranium production variability over an entire set of geostatistical realizations. The ISL stochastic simulation performed with the selected geostatistical realizations approximates the uranium production variability well. The simulation results of the ISL reactive transport model show that the extent of the uranium plume is highly dependent on mineralogical heterogeneity. The uncertainty analysis suggests the effect of uranium grade heterogeneity was found to be important to improve the accurate capture of the uncertainty. This study provides guidance for the accurate simulation and dynamic regulation of the CO2 + O2 leaching process of uranium at the scale of large mining areas.

Purpose is to increase uranium content in a PR solution while developing a technique varying oxidation-reduction potential of a leach solution with its oxygenation and identify changes in the oxygenation depending upon sulfuric acid concentration as well as transportation distance of the solution. Methods. A laboratory facility, involving solution tank, pump, Venturi tube, tank to install oxygen analyzer, and a dump tank, has been manufactured under the lab conditions to determine a leach solution oxygenation taking into consideration its delivery rate, sulfuric acid concentration, and temporal preservation of the concentration. Solution flow velocity; the deli-vered solution volume; sulfuric acid concentration; and distance from oxygenation point to a seam changed and varied during the study. Oxygenation was measured with the help of AZ 8403 oximeter; IT-1101 device was used to measure pH value as well as oxidation-reduction potential (ORP). Findings. A technique for a leach solution oxygenation and results of laboratory tests to identify influence of a sulfuric acid as well as transportation distance of a solution on oxygen concentration in the solution have been represented. It has been determined that Venturi tube helps oxygenize a leach solution; in this context, maximum oxygen concentration is achieved if a flow velocity is optimum one. It has been specified that a solution oxygenating depends upon a sulfuric acid concentration decreasing moderately with the increasing distance of the solution transportation. Originality.Following new dependencies have been determined: oxygen concentration in a solution upon a flow velocity and solution volume; and oxygen concentration in a solution upon distance from concentration place and sulfuric acid concentration. Practical implications.A leach solution oxygenation results in the increased oxidation-reduction potential and in the increased content of a useful component in the pregnant solution respectively. The proposed technique is notable for its low capital spending. Moreover, it is integrated easily into the available system being absolutely environmentally friendly.

This study integrates mineralogical and hydrogeochemical analysis of channel and playa uranium deposits to characterize aquifer evolution and the physico-chemical mechanisms that result in the accumulation of uranium into potentially economic deposits. This subset of surficial U deposits occur in Tertiary to Recent calcrete and dolomitic, clay-rich fluvial paleochannel and palustrine sediments, wherein uranium is largely bound in the potassium-uranyl-vanadate mineral carnotite [K2(UO2)2(VO4)2·3H2O]. Scanning electron microanalysis indicates that the carnotite mineralization is part of a late-diagenetic mineral assemblage that critically includes Mg-clays (sepiolite and stevensite), amorphous magnesium silicate, and synsedimentary dolomite. This authigenic mineral assemblage is observed concentrated in fractures and pores in groundwater calcrete and silty salt marsh “palustrine” sediments. Drill-hole gamma ray and conductivity data from the Centipede-Millipede uranium deposit indicate that the locus of uranium mineralization occurs near the present-day water table where oxidizing fresh-to-brackish groundwater interacts with playa brine, forming a hypopycnal groundwater estuary beneath the clay pan and salt marsh. It is interpreted that effective U fixing occurs in areas where groundwater, near-saturated with respect to carnotite, is hydrologically focused upward and into the zone of evaporation. The appreciable precipitation deficit in the Northern Yilgarn is interpreted to produce an evaporation-driven positive feedback mechanism that results in the co-precipitation of Mg-clays, dolomite, and carnotite. The presence of vanadium-rich Mn-oxide phases in high-grade U ore zones indicates that Mn-redox cycling may serve an important role in increasing the local activity of V, and thus carnotite saturation. Mineralogical comparison of other channel and playa uranium deposits throughout Western Australia and Namibia have identified a similar mineral association and paragenetic trend, suggesting that contemporaneous evaporative precipitation of Mg-clays and dolomite are integral in achieving carnotite saturation and precipitation.

The structural controls on unconformity-related uranium deposits of the Athabasca basin, Saskatchewan, Canada, are a matter of debate regarding the role of inherited fault systems and their reactivation. This can be related to the lack of outcrops allowing for direct observations, the strong clay alteration halos wrapping deposits that often obliterate structures, and the poor core recovery related to drilling strongly altered and mineralized intervals, which limits observation of structures and reliable oriented measurements. Borehole imaging technology is an invaluable alternative for obtaining oriented data through challenging drilling intervals. The use of borehole Acoustic Televiewer (ATV) has been integrated in recent exploratory campaigns in the Athabasca basin by Orano Canada. Here, we present the inputs and benefits of the use of the ATV in the exploration of unconformity-related uranium deposits and the structural analysis of oriented data from seven inclined diamond drill holes completed in 2016 during the McClean project (Sue deposits). The main objectives were to precisely identify the structural controls of the basement-hosted mineralization, and to test the tool in a well-known site. This work shows the applicability and added value of using televiewer probes to provide reliable oriented data in zones where there is much less information available. The ATV data structural interpretation supports the concept of mineralization of dilational jogs opening during preexisting shear-related foliation under right-lateral reverse fault reactivation. The ATV provides robust oriented data, allowing for a better understanding of the meaning of flat-lying mineralized structures along the Sue trend.

Proterozoic continental sedimentary basins contain a unique record of the evolving Earth in their sedimentology and stratigraphy and in the large-scale, redox-sensitive mineral deposits they host. The Paleoproterozoic (Stratherian) Kombolgie Basin, located on the Arnhem Land Plateau, Northern Territory, is an exceptionally well preserved, early part of the larger McArthur Basin in northern Australia. This intracratonic basin is filled with 1 to 2 km-thick, relatively undeformed, nearly flat-lying, siliciclastic rocks of the Kombolgie Subgroup. Numerous drill cores and outcrop exposures from across the basin allow sedimentary fabrics, structures, and stratigraphic relationships to be studied in great detail, providing an extensive stratigraphic framework and record of basin development and evolution. Tectonic events controlled the internal stratigraphic architecture of the basin and led to the formation of three unconformity-bounded sequences that are punctuated by volcanic events. The first sequence records the onset of basin formation and is comprised of coarse-grained sandstone and polymict lithic conglomerate deposited in proximal braided rivers that transported sediment away from basin margins and intra-basin paleohighs associated with major uranium mineralization. Paleo-currents in the upper half of this lower sequence, as well as those of overlying sequences, are directed southward and indicate that the major intra-basin topographic highs no longer existed. The middle sequence has a similar pattern of coarse-grained fluvial facies, followed by distal fluvial facies, and finally interbedded marine and eolian facies. An interval marked by mud-rich, fine-grained sandstones and mud-cracked siltstones representing tidal deposition tops this sequence. The uppermost sequence is dominated by distal fluvial and marine facies that contain halite casts, gypsum nodules, stromatolites, phosphate, and “glauconite” (a blue-green mica group mineral), indicating a marine transgression. The repeating pattern of stratigraphic sequences initiated by regional tectonic events produced well-defined coarse-grained diagenetic aquifers capped by intensely cemented distal fluvial, shoreface, eolian, and even volcanic units, and led to a well-defined heterogenous hydrostratigraphy. Basinal brines migrated within this hydrostratigraphy and, combined with paleotopography, dolerite intrusion, faulting, and intense burial diagenesis, led to the economically important uranium deposits the Kombolgie Basin hosts. Proterozoic sedimentary basins host many of Earth's largest high-grade iron and uranium deposits that formed in response to the initial oxygenation of the hydrosphere and atmosphere following the Great Oxygenation Event. Unconformity-related uranium mineralization like that found in the Kombolgie Basin highlights the interconnected role that oxygenation of the Earth, sedimentology, stratigraphy, and diagenesis played in creating these deposits.

This thematic issue is in memory of Thomas Kurtis Kyser who died tragically in 2017. Kurt’s contribution to our understanding of the geochemistry of uranium deposits and the application of stable and radiogenic isotope studies to uranium ore systems is incalculable. His research on uranium deposits included the world-class unconformity-related uranium deposits from Canada, Australia, and Russia, and several other major types of uranium deposits worldwide such as Na-metasomatic deposit of Kurupung (Guyana) and Valhalla (Australia), volcanic-hosted deposits of Peru (Macusani), and vein-type uranium deposits from Beaverlodge district (Canada). He was the first to develop methods to use secondary dispersion of pathfinder elements to vector towards deeply buried uranium ore systems. These new geochemical and isotopic approaches are still successfully applied today. Kurt has been a pioneer in applying stable isotopic systems to characterize the paleofluid events associated with ore systems and to distinguish between barren and mineralized fluid pathways. Kurt was a prolific author with over 400 publications and over a hundred articles on uranium deposits. Kurt wrote three short courses for the Mineralogical Association of Canada dedicated to uranium deposits: Fluids and Basin Evolution (2000), Recent and not-so-Recent Developments in Uranium Deposits and Implications for Exploration (2008, in partnership with SGA), and Geology and Geochemistry of Uranium and Thorium Deposits (2015). However, his true legacy is the large number of undergraduate and graduate students, post-docs, and researchers that he taught, supervised, and mentored; most of whom are now working in industry, academia, and government institutions across the globe. He will be missed.

Research subject. The aim was to justify the prospects of uranium deposits in the Tyrminsk area of the Far East, which were briefly studied over 60 years ago. These deposits are highly similar to those located in the Streltsovsk mega uranium district. Materials and methods. The materials were geological data on uranium deposits in the promising Tyrminsk district and in the largest Streltsovsky district in the Far East. The research method was a comparison of the available information about the geological features of uranium deposits and ore areas as a whole. Results. The uranium ore regions under study are localized in Mesozoic volcanogenic calderas, within Paleozoic granites. Specific uranium deposits are formed at intersections of multidirectional faults, most frequently in the vicinity of volcanic caldera boundaries. The deposits formed within a contrasting geological section are represented by steep vein-like and stock-like bodies, as well as by gently sloping formation-like bodies under the screens of layers of dense sandstones and siltstones. In addition, deposits are found in favourable layers of conglomerates and effusives breccia. The length and thickness of ore bodies comprise 0.2–1 km and 1–80 m, respectively. Uranium ores belong to the fluorine-molybdenum-uranium type; near-ore rocks are represented by mudstones and hydrosludites. Some fields in the Tyrminsk district are opened by wells to a depth of 80– 160 m from the surface, while the fields in the Streltsovsk district are explored by wells and underground mine workings to a depth of 1.5 km from the surface. Conclusion. The extent of uranium deposits in the Tyrminsk district is predicted to reach a depth of 500–700 m, with their resources being estimated as “large” based on the high similarity of localities in the areas under study. As the most promising uranium area in the south Far East, the Tyrminsk district is recommended as a priority exploration site for uranium.

Magnetic and gravity data are used in the early stages of exploration for uranium deposits in the Athabasca Basin of Canada, just as for many other mineral exploration scenarios. Uranium mineralization in the Athabasca Basin is located where faults in the basement intersect the unconformity between the basement and the overlying sandstones. The gravity and magnetic data are dominated by signatures from the basement and an overburden of glacial sediments. The gravity and magnetic data are effective at mapping the basement geology. Any subtle gravity signal from the mineralization related to the formation of the uranium deposits is masked by the signal from the variable-thickness overburden. Three-dimensional joint inversion of gravity and magnetic data, first without and then with constraints, is evaluated as a means of better determining the structure of the three main lithologies (overburden, sandstones, and basement) in the Athabasca Basin. A significant amount of physical property information is available for the main rock units (and overburden), which makes the use of the compositional approach to joint inversion appropriate. For the joint inversion, the fuzzy c-means clustering method is used. Results from representative synthetic examples indicate that the joint inversions can construct the overburden and basement structures better than the independent inversions of gravity and magnetic data. Furthermore, constrained joint inversion allows delineation of all three major layers in the area. The same inversion strategies then are applied to real airborne gravity and magnetic data from the McArthur River area in the eastern Athabasca Basin. The results we obtain demonstrate the capabilities of joint inversion for real-life situations.

Genetic models for the formation of uranium deposits almost invariably invoke the reduction of U(VI) to U(IV) as the deposition mechanism. However, the questions of when and how this reduction occurred are often not clear in most uranium deposits. For example, mineralization in the giant Olympic Dam Cu-U-Au-Ag deposit (Australia) occurs mainly in hematite-rich breccias of the oxidizing hematite-quartz-barite assemblage. Similarly, unconformity-related uranium deposits in the Athabasca Basin (Canada) are accompanied by extensive and intensive hematite- and quartz-rich alteration halos, pointing to a possible linkage between uranium mineralization and silicification. However, this linkage is not understood due to the lack of knowledge about the speciation and mechanism of uranium incorporation in microcrystalline and macrocrystalline silicas. In this contribution, microcrystalline silicas (agate and opal-CT) containing 27–137 ppm U have been investigated by microbeam synchrotron X-ray fluorescence mapping, microbeam synchrotron X-ray diffraction analysis, synchrotron X-ray absorption spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy. These data unravel the binding mechanism of uranyl silicate complexes in microcrystalline silicas and suggest a new mechanism of uranium deposition involving uranyl co-precipitation without the putative reduction process, providing a potential missing link between uranium mineralization and silicification for the formation of uranium deposits. In addition, new data on the uptake and long-term retention of elevated uranyl contents in microcrystalline silicas extend previous findings from their amorphous counterparts on these materials as an effective sink for uranium attenuation to higher temperatures, important to the storage and disposal of heat-generating nuclear waste in deep geological repositories.

SUMMARY Uranium is an important energy material and a key exploration object in the field of radioactive geophysical exploration. Through field study, it was found that fractured uranium is one of the most abundant sources of uranium, therefore it is an important target of energy geophysical exploration. However, the fractured structure increases the difficulty of uranium exploration and analysis. Therefore, in order to identify the distribution and analyse the content of fractured uranium deposits, it is necessary to study the influence of fracture parameters on the occurrence state of uranium ore and its manifestation status in neutron logging response. Considering the fracture structure from different aspects, this study simulated the uranium ore occurrence state in the pore-fracture medium environment and obtained corresponding logging response results by setting two fracture parameters (number density and angle). The study generated several findings, namely: (1) high angle and high porosity fracture type uranium strata have the greatest influence on thermal neutrons; (2) under the condition of high porosity, the neutron energy spectrum displays a dispersion peak and moves towards the high energy region. It is noteworthy that in a high number density environment, medium-low angle uranium ore has the strongest thermal neutron absorption in a low energy range. (3) energy decay rate is the highest in the high angle fracture environment and (4) the higher the density of the fracture number, the more susceptible the counting is to the influence of the angle factor. These findings can be used to quantitatively analyse the effects of fracture parameters on uranium deposits. These results have important guiding significance to the evaluation of a fractured uranium reservoir.

To investigate the risks posed by trace and rare earth elements (REEs) in two tropical uranium ore fields, metal concentrations from 50 vegetable samples (corn and soybean) and their corresponding agricultural soils were evaluated in a U mining area and a U-rich coal mining area in Brazil. Samples from both areas had metal concentrations (REE: La to Lu, and trace elements: As, Pb, Cd, Ni, Cu, Cr, Mn, Zn, Ba, U, Sr) that were higher than the guidelines proposed by the Brazilian environmental agency. Soils from the U mining area (Pocos de Caldas) generally had higher contents of trace elements than the coal mining area (Figueira), with the exception of Ni and Cr, indicating a higher risk of pollution, which was confirmed by a pollution load index that was greater than unity. For both sites, concentrations of uranium in the soil and plants, its hazard quotients and the soil contamination factor were higher in agricultural fields closer to the mines, indicating that contamination and the consequent risks to human health were distance dependent. REE concentrations averaged 52.8 mg kg−1 in the topsoils and 0.76 mg kg−1 in the grains for Figueira, whereas higher values of 371 mg kg−1 (topsoils) and 0.9 mg kg−1 (grains) were found in Pocos de Caldas. Based upon corn and soybean consumption, the estimated intake dose of the REE was lower than the intake dose predicted to be problematic for human health for both sites, indicating limited risk related to the ingestion of REE.

Gravity signatures from features associated with the footprints of uranium deposits within the sandstones and basement of the Athabasca Basin are masked in the measured gravity by the contribution from glacial sediments (overburden), in particular by the variable thickness of the overburden. The 2D inversions of seismic refraction and gravity data are assessed as a means of reliably mapping overburden thickness, enabling the contribution to gravity from the overburden to be taken into account and density anomalies associated with deeper mineralization and alteration to be reconstructed through further inversion. Results show that independent inversion of seismic refraction data using the fuzzy c-means clustering method is able to determine the base of overburden well. Subsequent gravity inversion constrained by the overburden thickness reveals possible subtle density variations at depth, which could be associated with alteration in the sandstones associated with the uranium mineralization. Application of the seismic clustering inversion followed by constrained gravity inversion to both representative synthetic scenarios and real data from the Athabasca Basin, Canada, are considered. Drill-hole data show that the inversion results can predict the base of the overburden well, and there is an acceptable match between geologic information and possible alteration zones suggested by the inversions.

The Cigar Lake uranium deposit in the Athabasca Basin is the world’s second largest high-grade unconformity-related uranium deposit. Its distinct geological architecture includes heterogeneous basement lithologies, a local basement high and sub-vertical faults. The sub-vertical faults are classified further into two sub-types: faults that are restricted to the basement, termed ‘basement faults’, and faults that are distinctly related to post-Athabasca fault reactivation, in that they extend upward into the sandstone, termed ‘extended basement faults’. This study aims to evaluate the effects of the aforementioned geological factors on the fluid flow patterns under two different driving forces, buoyancy due to variation of fluid density or ‘thermal convection’ and deformation or a combination of them, and to determine the most probable fluid flow scenarios for the formation of the deposit. The numerical results show that fluid flow is strongly affected by the two types of faults. While the basement faults represent fluid paths with complex fluid flow patterns, depending on the driving forces, providing favourable physical conditions for different chemical processes, such as fluid-fluid and fluid-rock interactions, the extended basement faults enhance the permeability of an E-W corridor within the sandstone and significantly strengthen the overall upwelling flow above the basement faults, promoting sandstone-hosted mineralization. The numerical results also suggest that the main deposit likely formed via fluid convection during tectonically quiet periods, although faulting played a critical role in increasing permeability, in turn, enhancing thermal convection.

The Sanjiu uranium ore field, located in the central of Zhuguangshan granitic batholith, is a newly discovered granite-related uranium ore field in South China. The main sulfide in the ore field is pyrite, which is closely related to uranium mineralization. The textures, major and trace elements, S–He–Ar isotopes compositions of pyrites in ores of different grade were observed and/or analyzed by optical microscope, scanning electron microscope, electron microprobe, laser ablation inductively coupled plasma mass spectrometry, and noble gas mass spectrometer (Helix-SFT). It is observed that these U-related pyrites are generally euhedral–subhedral with dissolution textures, anhedral variety with colloform texture, veinlet and fine particles, and the color of the associated minerals is mostly dark hue, such as purple–black fluorite, dark-red hematite, and dark-green chlorite, etc. The analytical results show that the average compositions of major elements in pyrite are FeS1.944. Pyrites are characterized by S-deficiency, low content of Co and Th, and Co/Ni > 1, which indicate that these ores are of low-temperature hydrothermal origin. We found that the higher the grade of ore, the more deficient in S, the more obvious negative δ34S, and the higher REE content (close to U-rich granitic pluton) of pyrite. The S–He–Ar isotopic compositions of various varieties of pyrites indicate that the ore-forming fluids mainly come from crust-derived fluids and mixed with mantle-derived fluids.

The Patterson Lake corridor is a new uranium district located on the southwestern margin of the Athabasca Basin. Known resources extend almost one kilometer below the unconformity in graphite- and sulfide-bearing shear zones within highly altered metamorphic rocks. Despite different host rocks and greater depths below the unconformity, alteration assemblages (chlorite, illite, kaolinite, tourmaline and hematite), ore grades and textures are typical of unconformity-related deposits. This alteration includes at least three generations of Mg-rich tourmaline (magnesio-foitite). The boron isotopic composition of magnesio-foitite varies with generation: the earliest generation only observed in shallow samples from the Triple R deposit (Tur 1) contain the heaviest isotopic signature (δ11B ≈ +26 to +19 ‰), whereas subsequent generations (Tur 2, Tur 3) yield lighter and more homogeneous isotopic signatures (δ11B ≈ +17.5 to +19.9 ‰). These results are consistent with precipitation from low temperature, NaCl- and CaCl2-rich brine(s) derived from an isotopically heavy boron source (e.g. evaporated seawater) that interacted with tourmaline and silicates in the basement rocks and/or fluids derived from depth (with low δ11B values). The lower δ11B values in paragenetically later magnesio-foitite reflect greater contributions of basement-derived boron over time whereas minor compositional variations reflect local metal sources (e.g. Cr, V, Ti) and evolving fluid chemistry (decreasing Na and Ca, increasing Mg) over time. The δ11B and chemical variation in magnesio-foitite over time reinforce the strong interactions with basement rocks in these systems while supporting incursion of basinal brines well below the unconformity contact. Supplementary material:https://doi.org/10.6084/m9.figshare.c.5727555

International audience; The Zoovch Ovoo uranium roll-front-type deposit is hosted in the Sainshand Formation, a Late Cretaceous siliciclastic reservoir, which constitutes the upper part of the post-rift infilling of the Mesozoic East Gobi Basin in SE Mongolia. The Sainshand Formation consists of unconsolidated medium-grained sand, silt and clay intervals deposited in fluvial-lacustrine settings. The uranium deposit is confined within a 60-80 m thick siliciclastic sequence inside aquifer-driven systems. The overall system experienced shallow burial and was never subjected to temperatures higher than 40°C. This study proposes a comprehensive metallogenic model for this uranium deposit. Sedimentological and mineralogical observations from drill core samples to the microscopic scale (optical and Scanning Electron Microscopy) together with in situ geochemistry of late-formed phases (Laser Ablation-Inductively Coupled Plasma Mass Spectrometry, Electron Probe Microanalysis, Fourier Transform-Infrared Spectroscopy) were considered for the reconstruction of the main stages of U trapping. In the mineralized zone, the uranium ore is expressed as Ca-enriched uraninite (UO 2) and less commonly as Ca-enriched phospho-coffinite (U, P)SiO 4. Trapping mechanisms include i) complexation (i.e. uranyl-carboxyl complexes), ii) adsorption on organic or clay particles) and iii) reduction by pyrite and by bacterial activity to amorphous uraninite. In all cases, the organic matter plays either the role of trap for uranium or nutrient for bacteria that can trap uranium through their metabolism. The shallow burial diagenesis conditions do not allow direct reduction of U(VI) by organic carbon. The δ 34 S values of the iron disulfide are very diverse, fluctuating in extreme cases between-50 to +50‰, with an average δ 34 S value for framboidal pyrite at 2‰, and-20‰ for euhedral pyrite. The positive and negative values reflect close versus open fractionation systems, while bacterial sulphate reduction (BSR) is active during the whole diagenetic history of the deposit as an essential source of reduced sulfur. Therefore, using detrital organic matter as a carbon source, microorganisms play a significant role in uranium trapping, either as a direct reducing agent for uranium or pyrite formation, which will trap uranium redox driven epigenetic processes.

Mobile radiogenic lead isotopes (206Pb, 207Pb, 208Pb and 210Pb) represent products of radioactive decay of their parental uranium and thorium isotopes (238U, 235U, 232Th), and are considered potential geochemical pathfinders of buried sandstone-type uranium deposits. Soil samples collected along a geochemical traverse intersecting buried uranium roll front mineralization at the REB deposit in the Great Divide Basin, Wyoming, USA were studied. Mineralization of this deposit is hosted in weakly lithified arkosic sands, at a depth of 120–200 m, without a strong surficial expression of its presence at depth, which makes discovery of this deposit type difficult, slow and expensive. All soil samples have been analysed for ratios of the mobile long-lived Pb isotopes and their parental U and Th isotopes, determined from partial leach products obtained using a weak acid leaching technique. The samples were also analysed for trace elements, assayed both in the partial leach products and using conventional whole-soil sample assays. Ratios of the mobile radiogenic Pb isotopes to their parental U and Th isotopes (206Pb/238U, 207Pb/235U and 208Pb/232Th) determined in the partial leach products exhibit anomalous contents in the soil samples collected above the uranium rolls. The anomalous values are several times greater than background values, to lateral distances of 350–400 m outside of the roll fronts. Notably, conventional whole-soil assays have failed to detect the anomalies that were detected using mobile Pb isotopes. Supplementary material: Minor and trace elements assays of the geochemical samples are available at https://doi.org/10.6084/m9.figshare.c.5610980

The middle Jurassic Zhiluo Formation in the Dongsheng is comprised of a big set of green sandstone/mudstone with most of uranium orebodies occurring in close proximity to its footwall. By synthesizing field observations, region analysis, data collected from previous coal and uranium borehole, a regional north-south geological profile across the entire orefield is conducted. Experiments on sandstone/mudstone including rock mineral identification, clastic micromorphology and element geochemistry were carried out. Information from the geological profile indicates that green sandstone/mudstone is widely present in a stable horizon with clear boundaries to the country rock. Microscopic observations and geochemical data on sandstone/mudstone exhibit similar mineral composition with almost identical slightly flat, minor Eu enriched, Ce depleted chondrite-normalized REE patterns. Furthermore, the green clay membrane of the clasts has a complex composition containing chlorite/smectite, green smectite, chlorite, and green kaolinite, with elements including Fe, Mg, Si, and Al. These above results indicate that the green sandstone/mudstone underwent resemble sedimentary diagenetic processes as the country rock without transformation by large-scale regional fluid, while the existence of Fe2+-rich membrane is the main factor to the green sandstone/mudstone. Further concentration of the pre-enrichment uranium during diagenetic process led to the final formation for uranium deposits. The above studies are conducive to enrich the metallogenic mechanism of sandstone type uranium deposits and could provide certain reference for uranium exploration and deployment.

This paper considers examples of “hydrogenic” commercial uranium deposits termed as “sandstone-hosted” in the English literature and characterized by so-called “reduced-type alterations.” In addition to the case when they are absent at the deposit, a few characteristic alterations are identified in different ratios, such as rock bleaching, iron redistribution, argillization, carbonatization, bituminization, sulfidization, and other changes in the host rocks. The nature of these alterations varies in different areas, and their composition depends mainly on the groundwater composition and formation conditions in the studied region. Alterations related to waters of oil and gas basins, as well as to deep carbonic basement waters, are the most pronounced. The alterations are less significant at deposits related to “nitric spring” discharge.

The characteristic features of the manifestations of acid magmatism and metasomatic hydrothermal alteration processes in Southeastern Transbaikalia, an important mining region in Russia, are discussed. The zoned or beltlike distribution of the ores of various metals, uranium in particular, and other mineral deposits is due to the general evolutionary trends of the geodynamic regimes of the respective blocks of the territory and the subsequent magmatic differentiation trends. The highly differentiated and fluorine-rich granitoids of Southeastern Transbaikalia are located only in the zones of the consolidated crust, which are characterized by low gravity and negative quiet magnetic field values. In other zones, i.e., volcanic belts, volcanogenic troughs, and the regions dominated by the intrusive massifs of the Shakhtama complex, rare metal manifestations are absent. It was demonstrated that there is no correlation between the localization of large uranium deposits and leucocratic rare metal granite domains and that these deposits are concentrated in large volcanic calderas or volcanic troughs, controlled by long-lived fault systems. In addition to uranium, these volcanogenic structures control base metal, molybdenum, gold, fluorite, and some other mineral deposits. The results of a comparative analysis of the geochemical features of the acid volcanics that host the uranium mineralization and are classified as the last differentiation products of the contrasting latite volcanism, on the one hand, and the subvolcanic and hypabyssal derivatives of the rare metal granites, on the other, are presented. The analysis revealed the signatures of their fundamental difference from each other and that they belong to different branches of magmatic differentiation. A comparative analysis of the metasomatic aureoles of the altered wallrocks in the Late Mesozoic volcanics and the Shakhtama granitoid domains, on the one hand, and the leucocratic rare metal granite domain, on the other, is given. The data presented attest to the existence of a mixed geodynamic regime within the study area of the Central Asian mobile belt during the Jurassic and the earliest Cretaceous time: residual collisions with strong deformations in a compressional setting, accompanied by the development of late acid differentiation products of the magmatic latite series, and the incipient rifting associated with the intrusion of the leucogranites of the Kukul’bei complex in a more quiet extensional setting.

—Various hypotheses, including alternative ones, have been put forward in the literature about the sources of ore substance and ore-forming fluids in the deposits of the Streltsovka ore field, which hosts the world’s largest uranium reserves, in excess of 250 000 t U: (1) uranium transport by ascending flow of ore-forming fluids that have separated from a deep subcrustal or intracrustal uranium-bearing felsic magma chamber and (2) uranium mobilization from uranium-bearing rocks of the volcano-tectonic structure of the Streltsovka caldera via postvolcanic thermoconvective circulation of the ore-forming fluids. For both hypotheses, the authors previously developed computer models of the paleohydronamic formation conditions of the largest in the ore field Antei–Streltsovka deposit, which presumed different uranium transport mechanisms with forced and free thermal convection of fluids, respectively. Both models yielded calculation results consistent with data on the reserves and formation temperatures of the Antei–Streltsovka ores. However, since the computer models are a simplified image of ore-forming systems, such agreement between the modeling results and a natural prototype only indicate the possibility of the proposed hypotheses on the formation conditions of the deposits. To solve the problem of their reality, additional information is necessary. Therefore, the authors of this paper have attempted a comparative analysis of the proposed alternative interpretations of the formation of the Antei–Streltsovka deposit using a methodology elaborated within the mineral systems concept. The results of our analysis substantiate the idea of successive forms of uranium transport by magmatic melt and ore-forming fluid to the mineral system of the Streltsovka ore field. The deep magmatic source was a feeder chamber for uranium transport by magmatic melts to the upper horizons of the crust with the formation of uranium-bearing rocks of the subvolcanic chamber and volcanic eruptions of the Streltsovka caldera. After uranium transport via magmatic melts, its subsequent redistribution occurred in the paleohydrothermal system with free thermal convection of fluids in the residual temperature field of the Streltsovka subvolcanic chamber. In this case, in the thermoconvective fluid circulation loop, conjugate processes of uranium mobilization could have taken place: (1) from the consolidated subvolcanic chamber, (2) from granitoid rocks of the caldera’s basement, and (3) from igneous felsic rocks in the caldera’s volcano-sedimentary cover. The coparticipation of these three potentially highly productive uranium sources in the ore mineralization process explains the origin of the unique uranium ore reserves of the Streltsovka ore field deposits.

The Mesoproterozoic Pasha-Ladoga Basin is located at the margin of the Karelian Craton and Svecofennian Belt in northwest Russia. It is the only basin in Russia that is currently recognized to host unconformity-type uranium mineralization (e.g., Karku deposit). Mineralization is associated with the Priozersk Formation which comprises a basal sequence of immature arkosic sandstones and conglomerates interpreted as high-energy braided stream channel and channel bar facies within an alluvial fan depositional environment. Diagenetic and hydrothermal alteration of the Priozersk sandstones is similar to that observed in unconformity-related uranium deposits of the Athabasca Basin (Canada) and includes three main stages: (1) an early diagenetic assemblage comprising illite-smectite (K-rectorite), calcite, minor chlorite, and kaolinite precipitated at 100–150 °C; (2) a peak diagenetic-hydrothermal assemblage composed of kaolinite-dickite, Fe- and Fe-Mg chlorite, calcite, and sulfides formed at 150–250 °C; (3) late diagenetic-hydrothermal event involving fracture-controlled kaolinite and late hematite. Similar to the Athabasca Basin type uranium mineralization, the Karku deposit exhibits anomalous concentrations of LREE, Nb, Pb, and positive correlations between U and Cu, Zn, Ag, and Pb that is a geochemical signature of unconformity-type uranium mineralization. Based on oxygen and hydrogen stable isotope study, it was found that fluids in equilibrium with early and peak diagenetic clays were consistent with oxidized basinal brines and reduced basement fluids. In contrast to the Athabasca Basin, mineralizing fluids at the Karku area were more reduced and potassic. A new geological model for the Karku deposit is proposed based on petrographic observations and sandstones geochemistry. The proposed model envisages the permeable rocks of paleoregolith and the coarse-grained basal part of the Priozersk Formation as the main pathway for migration of uraniferous fluids responsible for the formation of Karku uranium deposit. From the perspective of the geological setting, the Karku deposit differs from deposits of the Athabasca Basin in that it occurs in a rapidly deposited rift-related basin with associated mafic magmatism, the host sandstone sequence is relatively thin (e.g., Priozersk Formation) and exhibits abrupt lateral facies variations, and the widespread presence of early diagenetic interstitial swelling clays. Based on these sedimentological characteristics, it is considered that the Karku deposit setting precludes optimum conditions to create high tonnage high-grade unconformity-type U deposits as found in the Athabasca basin.

The large ore clusters and uranium deposits detected in southeast Russia (eastern Transbaikal Region), eastern Mongolia, and northern China constitutes a large area, specialised in U, with a wide range of geological and mineral ore types. Amongst various types of uranium ore, hydrothermal deposits related to volcanic rocks, infiltration deposits of uranium sandstone and polygenous mineralisation in leucogranites represent main commercial interest. Large uranium clusters are located on the outer, side and peripheral parts of the Great Xing'an volcano-plutonic belt as well as in the framing taphrogenic depressions. The clusters formation timed to late Mesozoic (I-K) geodynamic processes. Deep geodynamics allowed the fluid flows of asthenosphere which were concentrating around the periphery of the slab stagnated in the transition mantle zone to penetrate into the above lithosphere and earth's crust with formation of pockets of magma, and ore-bearing volcano-tectonic depressions eventually. Therefore, the presence of igneous stocks, extrusions, and volcanic necks within the volcano-plutonic belts and depressions can be indicative of mineral potential of the area.

The data presented in the article consistently outlines the methodology for studying the orientation and morphogenetic characteristics of fracture systems of four scale levels including kilometers, meters, centimeters and millimeters. The Urtuisky granite massif, located in the South-Eastern Transbaikalia to the west of the Streltsovsky caldera, containing uranium deposits unique in their reserves was chosen as the object of the research. The massif is composed of Late Riphean granites and granite-gneisses, affected by dynamometamorphic and hydrothermal-metasomatic transformations in various degrees, and dissected by numerous faults with traces of fluid activity of various tectogenesis episodes. The interrelation between such geometrical parameters of fractures systems as specific density and specific length was established. It is shown that such geostructural data should be used for conceptual and numerical modeling of fluid filtration and radionuclides transport processes occurring in a three-dimensional fractured-pore space of crystalline rocks, as applied to the reconstruction and modeling of uranium ore formation and use of geological space for radioactive materials isolation.

The trend fractionation of the 238U and 235U isotopes and the extent of this process at the oxidative weathering of uranium minerals were evaluated by studying the variations in the U isotope composition of rocks, minerals, and fracture waters sampled in the quarry of the broadly known Tulukuevskoe uranium deposit in the Streltsovskoe ore field, eastern Transbaikalia. In the rock block in question, fine uranium minerals disseminated in the rocks were weathered under the effect of oxidizing fracture waters. Uranium isotope composition was measured in 22 water samples, eleven samples of the mineralized rocks, and eight uranium minerals. High-precision (±0.07‰, 2SD) measurements of the 238U/235U were carried out by MC-ICP-MS, using a 233U–236U double spike. The results involve the 238U/235U and 234U/238U ratios and the overall range of the δ238U variations determined in the rocks and waters (from –0.13 to –1.0‰ and from –0.22 to –0.59‰, respectively). Interaction between the waters and rocks induces U(IV) → U(VI) oxidation, U(VI) transfer into the aqueous phase, and 0.15–0.28‰ enrichment of U dissolved in the water in the 235U isotope. When the pitchblende is replaced by U(VI) minerals, the 238U and 235U isotopes also fractionate with ~0.3‰ enrichment of the younger U(VI) mineral phases in the light 235U isotope. The 238U/235U and 234U/238U ratios are proved to correlate, and hence, the fractionation of the 238U and 235U isotopes and the enrichment of the aqueous phase in the light 235U isotope proceed simultaneously with the well known shift in equilibrium the 238U–234U system with the accumulation of excess amounts of the 234U in the aqueous phase. Uranium leaching from uranium minerals, which is associated with the enrichment of the aqueous phase in excess amounts of the 234U isotope, can be viewed as a process that controls isotope fractionation in the 238U–235U system. This should be taken into account in describing the fractionation mechanism of the 238U and 235U isotopes at U(IV) → U(VI) oxidation. The fractionation of the 238U and 235U isotopes, which results in the isotopic "lightening" of U in the aqueous phase, largely controlled the complicated distribution pattern of the 238U/235U ratio in the quarry. In addition to isotope fractionation, this distribution was likely also affected by isotope exchange between uranium dissolved in the water and uranium in the finely dispersed minerals. The isotopically light uranium of the water could participate in forming U(VI) minerals at lower levels of the quarry.

The Southern High Plains (SHP) is a new and emerging U.S. uranium province. Here, uranyl vanadates form deposits in Pliocene to Pleistocene sandstone, dolomite, and limestone. Fifteen calcrete uranium occurrences are identified; two of these, the Buzzard Draw and Sulfur Springs Draw deposits, have combined in-place resources estimated at about 4 million pounds of U3O8. Ore minerals carnotite and finchite are hosted in dolomite at the Sulfur Springs Draw deposit, with accessory fluorite, celestine, smectite/illite, autunite, and strontium carbonate. Host carbonate at the Sulfur Springs Draw deposit is ∼190 ka and mineralization mobilized as recently as 3.8 ka. Ash collected near the deposit is 631 ka and erupted from the Yellowstone caldera complex. The Triassic Dockum Group that contains sandstone-hosted uranium deposits throughout the region and underlies the SHP is a potential source for uranium and vanadium. Regional uplift and dissection reintroduced oxygenated groundwater into the Dockum Group, mobilizing uranium. Additional uranium may have been contributed to groundwater by weathering of volcanic ash in Pliocene and Pleistocene host rocks. The locations of the uranium occurrences are mostly in modern drainage systems in the southeast portion of the SHP. Modelling of modern groundwater in the SHP carried out in a parallel study shows that a single fluid could form carnotite through evaporation, and that fluids of the requisite composition are more prevalent in the southern portion of the SHP. The southeastern portion of the SHP hosts more uranium occurrences due to a variety of factors including (1) upward transport of groundwater and connectivity between source and host rock, (2) higher uranium and vanadium content of groundwater, (3) higher rates of groundwater recharge in this region to drive the mineralizing system, and (4) shallower groundwater facilitating surface evaporation. Ongoing erosion of host rocks challenges preservation of deposits and may limit their size.

Large quantities of pyrite have been discovered near the ore-bearing segments of the sandstone-type uranium deposits in the North Ordos Basin. In this study, the electron probe microanalysis, scanning electron microscopy, and isotope geochemical analysis were used to systematically investigate the morphology, composition, and S isotopes of the pyrites in these deposits, and their genetic order relative to the other main altered minerals. Based on the regional tectonic evolutionary sequence of the basin, the relationships between the pyrites and uranium mineralization are discussed. Macroscopically, the pyrites were found to occur in lumpy, banded, disseminated, star-like or sheet-like form. Microscopically, they exist either as detrital grains or inside detrital grains, as idiomorphic grains, in colloidal or framboidal form, inside mica minerals, in banded or petaloid form, and in other forms related to biological cells or microorganisms. The δ34S‰ values in the pyrites are highly variable (−7.3‰ to −37.2‰), but principally in the proximity of −30‰, and display obvious biogenic origin. Taking into consideration the extremely low U contents in the pyrites and achavalites (iron selenide), the genetic order of the main altered minerals is determined as follows: detrital pyrites > clay mineral overgrowths > micritic calcites during the diagenetic stage; and achavalites > pyrites > coffinite/pitchblende > vanadium oxides > sparry calcites during the metallogenic stage. The broad presence of pyrites in the Jurassic Yan'an Formation during the basin’s evolution provided detrital pyrites and organic detritus for the overlying formations. The Zhiluo Formation, host of the uranium mineralization, is also home to some primary spherical colloidal or framboidal pyrites. The Cretaceous was the main epigenetic uranium metallogenetic period as well as the main pyrite development period of the area. The pyrites, which are typically colloidal, have cemented the earlier framboidal pyrites and other minerals and are closely associated with the redox reaction related to uranium mineralization. In the Paleogene, uranium mineralization mainly occurred by the redox migration of early shallow deposits to, and fresh pyrites forming in deeper levels.

The formation of sandstone-hosted uranium deposits has been linked to upward migration of hydrocarbons in many sedimentary basins. However, hydrocarbon-induced diagenetic alterations associated with uranium mineralization and preservation are rarely discussed. Here, we report the bleaching of red sandstone to green in the Middle Jurassic Zhiluo Formation from the uranium deposits in the northern Ordos Basin, China. The original gray sandstone underwent oxidation and was reddened by the downward flow of meteoric water from the northern margin of the basin during the Late Cretaceous–Cenozoic uplift. This process resulted in the leaching of preore-stage uranium concentration and oxidation of preore pyrite in the organic matter-rich sandstone. Uranyl and sulfate ions were carried downward through the host sandstone by oxygenated groundwater. Meanwhile, hydrocarbons migrated upward and reacted with the downward flowing oxidizing fluids, which induced uranium mineralization at the redox interface between the red and gray sandstone where iron was reduced to Fe2+ and incorporated into pyrite cement. The highly negative 34S values (−58.0‰ to −33.4‰ V-CDT) of the pyrite indicate that it originated from bacterial sulfate reduction during hydrocarbon biodegradation. Abundant organic acids and CO2 were generated from hydrocarbon oxidation, resulting in the dissolution of feldspars and the precipitation of kaolinite in the mineralized sandstone. With the formation of the Cenozoic Hetao graben to the north, the gravity-driven flow of meteoric water was limited, resulting in progressive halting of epigenetic uranium mineralization. Then, the upward hydrocarbons dominated the fluid system throughout the northern slope of the basin and bleached the previous red sandstone updip, which set up a strong reducing environment and protected the early formed uranium deposits at the paleo-redox front from oxidation and remobilization. Due to the oxygen and sulfur supply absence, the iron released from hematite during bleaching was immobilized and incorporated into Fe-rich chlorite that imparts the green color, rather than into pyrite or reprecipitation as hematite. This study provides insights for the processes of hydrocarbon-water-rock interactions associated with uranium mineralization and preservation, which can serve as exploration guides for sandstone-hosted uranium deposits.

Carbonaceous debris (CD) within uranium reservoir sandstones of the Zhiluo Formation has been studied in the Shuanglong uranium deposit of the southern Ordos Basin, northern China. Organic matter maturation (OMM) heterogeneity of uranium-bearing CD was analyzed by means of in-situ methods with optical microscope, laser Raman microprobe and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The results show that the contents of vitrinite, inertinite and minerals are 70.49%, 6.54%, and 22.97%, respectively, and the content of exinite is 0, indicating that vitrinite is the major maceral. Vitrinite reflectance (VR) of uranium-bearing CD ranges from 0.65 %Ro to 2.45 %Ro with the maximum difference of 1.80 %Ro and the mean value of 1.85 %Ro, and the distributions of VR frequencies are discrete, indicating that OMM of uranium-bearing CD might be heterogeneous. Uranium minerals in uranium-bearing CD are mostly coffinite and pitchblende (2–10 µm in diameter). For uranium-bearing vitrinite matrix in CD, VR, G peak shifts and uranium contents range from 1.14 %Ro to 1.84 %Ro, from 1586.5 cm−1 to 1600.1 cm−1 and from 0 to 6.0%, respectively, with the corresponding maximum difference of 0.71 %Ro, 13.6 cm−1 and 6.0%, respectively, indicating that uranium-bearing vitrinite matrix may be of heterogeneity. Shift differences of G peak (△G) and full width at half maxima of G peak (GFWHM) are unevenly distributed in uranium-bearing vitrinite matrix, and △G and GFWHM decrease with increased VR, which shows that OMM of uranium-bearing vitrinite matrix is heterogeneous, and its variation is associated with uranium enrichment. The uneven distributions of uranium minerals cause compositional contrast variations of vitrinite matrix. And compositional contrast is divided into three hierarchies by using backscattered electron images of SEM combined with the distributions of uranium contents, i.e., I (free uranium), II (uranium content less than 2%) and III (uranium content more than 2%). With uranium content increasing, VR shows an obvious rise while △G displays a significant decline. Moreover, when close to the uranium-rich area, VR exponentially increases, which implies that OMM increases with the increased difference in compositional contrast. And compared with VR in I hierarchy of compositional contrast, VR in II hierarchy and III hierarchy at least increases by 0.1 %Ro and 0.2 %Ro, respectively, which demonstrates that compositional contrast variation of organic matrix is relevant to OMM heterogeneity, and the increased difference in compositional contrast corresponds to higher OMM zone. Such findings are instructive in variation prospecting of OMM, and then OMM abnormality could act as a guide for uranium deposits prospecting.

The Meso-Cenozoic Yili Basin in northwestern China hosts important uranium and coal resources. The uranium deposits are mainly hosted by sandstones and are characterized by clay and carbonate alterations within and surrounding the orebodies. H-O-C isotopes of clay minerals and carbonate cement in the different alteration zones of the uranium deposits in the southern margin of the Yili Basin are summarized to examine the mechanisms of uranium mineralization. The alteration zones associated with the uranium mineralization include an intensely oxidized zone, a weakly oxidized zone, a transitional (mineralization) zone, and a reduced zone. The δD and δ18O values of the parent fluids calculated from clay minerals from the different alteration zones show a linear trend that intersects the global meteoric line at a point with a δ18OV-SMOW of ca. −5‰ and a δDV-SMOW of ca. −30‰, which coincide with the upper end of the δD – δ18O trend (along the meteoric line) of formation waters in the basin, suggesting that the uranium mineralizing fluid was of meteoric origin. The general trends of decreasing δD and δ13C and increasing δ18O from the intensely oxidized zone to the reduced zone, together with the fact that calculated δDV-SMOW values of the mineralizing fluids are lower than the original meteoric fluid, and that the δ13C values of the calcite cement overlap with those of organic matter in sedimentary rocks, suggest that hydrocarbons were involved in uranium mineralization. The hydrocarbons may have been important reducing agents causing precipitation of uranium minerals. The hydrogen ion released from oxidation of the hydrocarbons may have been buffered by the formation of clay minerals, which favors co-development of clay alteration and calcite cement. Deviation of H-O-C isotopes from the general variation trends as expected from the above redox mechanism likely resulted from the dynamic nature of the redox front, which changed its position in response to hydrodynamic balance between a downward flow of uranium-bearing, oxidizing fluid and an upward flow of reductants-bearing, reducing fluid from the lower part of the basin.

The molybdenum (Mo) isotope composition of uranium ore concentrate (UOC) has been proposed as a nuclear forensic signature due to the ubiquitous presence of Mo in the nuclear fuel cycle and to the possibility that different UOC production pathways lead to characteristic Mo isotope fractionation. Furthermore, Mo forms a volatile hexafluoride which leads to the retainment of Mo in the nuclear fuel cycle through U enrichment and reactor operation processes. As a result, it is possible that the nuclear fuel cycle generates a rich array of Mo isotope signatures. The first step in interpreting Mo isotope signatures generated during the nuclear fuel cycle is to characterize the Mo isotope composition of starting materials. In this study, a suite of UOC samples were analyzed for their Mo isotope composition in order to constrain the range of Mo isotope compositions entering the front end of the nuclear fuel cycle. Analytical protocols were developed to purify Mo from a U rich matrix prior to Mo isotope analysis by double spike MC-ICP-MS. A >3‰ variation in the 98Mo/95Mo ratio was observed in the suite of UOC samples with δ98Mo ranging from −1.15 to +1.96‰ relative to the NIST 3134 Mo standard reference material. The δ98Mo range observed in the UOC samples is very similar to the δ98Mo range observed in crustal igneous rocks and Mo-bearing minerals, suggesting that Mo isotope variability in UOC is driven by variability in the parent U ores. However, Mo isotope fractionation during UOC production is evident with one of the UOC samples being isotopically heavy relative to its parent U ore by 0.75‰, suggesting that different UOC production processes also contribute to the total Mo isotope variability observed. Overall, it is expected that the Mo isotope signatures generated during enrichment and reactor process will be clearly discernible from those generated during UOC production.

This study presents systematic chemical (U, Pb, Ca, Si, Fe) mapping coupled with in situ analyses of major, minor and trace elements, U/Pb, 207Pb/206Pb, and O isotopic compositions of natural uraninites (UO2) from two samples of the high-grade uranium ore from the Cigar Lake unconformity-related uranium deposit (Athabasca Basin, Saskatchewan, Canada). The studied uraninites are characterized by major chemical and isotopic heterogeneities expressed at small scale (µm to tens of µm), from almost pristine zones to strongly altered material. The 206Pb/238U and 207Pb/235U ratios of the different areas are widely spread and depict two similar and well-defined Discordia, providing an upper intercept age of crystallization at ca. 1300 Ma (1299 ± 4 and 1308 ± 14 Ma, respectively) and lower intercepts at 38 ± 13 and 72 ± 22 Ma, respectively. The freshest areas are characterized by sub-concordant 206Pb/238U and 207Pb/235U ratios, identical chemical compositions and similar very low δ18O values (−39.3 to −31.4‰). These data indicate that the two uraninites both crystallized at ca. 1300 Ma, from the same fluid and under identical physico-chemical conditions. Alteration is characterized by (i) the progressive incorporation of Ca, Si, and Fe, reaching several wt.%, which substitute to the radiogenic Pb and cause a progressive decrease in the Pb/U isotopic ratios. The radiogenic Pb is also substituted by water during the alteration, (ii) concomitant variations in trace element contents (As, Mn, V, LREEs, Sr, Th, B, Ba, Nb, for example) and (iii) heavier δ18O signatures (−22.5 to −8.91‰), typical of meteoric waters, in the altered zones. This combined approach demonstrates that fluid-driven post-crystallization exchanges affected each uraninite during recent fluid flow events (ca. 40 Ma and 70 Ma respectively). The relatively high dispersion of the Pb/U ratios in relation to the Discordia for both samples is considered as linked to a local (nm to µm-scale) differential mobility between lead and uranium within the uranium oxides. The chemical changes affecting elements previously considered as immobile in uraninite, such as REEs, indicate that these elements are not preserved during the post-crystallization alteration process studied here. Alteration processes may therefore have a major impact on the classical geochemical tracers, such as REE patterns or LREE/HREE ratios, currently used in nuclear forensic studies. The isotopic and chemical tracers currently used to track back the origin, age and history of natural uraninites should therefore be considered with a high degree of caution to avoid misleading and erroneous conclusions. Moreover, the comparison of calculated U-Th-Pb chemical, 207Pb/206Pb, and Pb/U isotopic ages shows that the use of age clustering for determining U-Th-Pb chemical ages and 206Pb/207Pb ages is not appropriate for constraining crystallization stage(s) of altered uraninites and for deciphering the different fluid events that potentially altered or recrystallized the uraninites over time. This study also indicates that estimation of the crystallization age of uraninite from substitution trends of Pb to Ca is not applicable to unconformity-related U deposits and results in overestimated ages because of an initial integration of calcium in the uraninite lattice at the time of crystallization.

A methodological proposal is presented for uranium prospection using multiple regression of aerogeophysical data (gamma-spectrometry and magnetometry) via a case study in a large and geologically complex area in central Brazil, where known uranium deposits occur. The model seeks to identify and minimize the background variability in uranium concentration for the different lithotypes present in the study area. The multiple regression model is obtained based on the thorium channel data, the eTh/K, eU/K ratios and the magnetic field analytical signal amplitude. Areas with high uranium concentration, in various lithotypes, are highlighted by the subtraction of the regression model obtained from the uranium values measured during the aerial survey. The study area covers part of the southeastern region of the Goias state, including the Catalao I Complex, which hosts a phosphate mine and includes well-known uranium anomalies as well as rare earth elements associated with the phosphate deposits. The proposed method reveals the presence of anomalous areas as a function of the independent variables, represented by the geophysical properties and the different lithotypes that constitute the geological environment. Besides areas with high uranium contents associated with the Catalao I Complex, new anomalous areas associated with other types of geological environments could be identified. The targets found by the application of the proposed method, were checked in the field via geochemical analysis of rock samples in addition to in situ measurements with a portable gamma-spectrometer. Field measurements showed that, in all cases analyzed, uranium contents presented values above the average concentration of this radioelement in the earth's crust. Geochemical analyzes results from the collected samples confirmed our results by revealing uranium levels ranging from 3.5 to 15.9 ppm. The presented method was successful in identifying specific sites of high uranium concentration via aerogeophysical data, previously concealed by the geological background.

The Patterson Lake corridor (PLC) in the southwestern margin of the Athabasca Basin hosts several high-grade uranium deposits. These deposits are located in the basement up to 900 m below the unconformity surface, raising questions about their affiliation with typical unconformity-related uranium (URU) deposits elsewhere in the basin. Based on cross-cutting relationships four pre- and three syn- to post-mineralization quartz generations were identified. Fluid inclusion analyses indicate that pre-mineralization fluids have salinities ranging from 0.2 to 27.2 wt% NaCl equiv. (avg. 9.0 wt%), whereas syn-mineralization fluids have salinities ranging from 8.8 to 33.8 wt% NaCl + CaCl2 (avg. 25.4 wt%), with NaCl- and CaCl2-rich varieties. The homogenization temperatures (Th) of fluid inclusions from pre-mineralization quartz range from 80 to 244°C (avg. 147°C), and from syn-mineralization quartz range from 64 to 248°C (avg. 128°C). Fluid boiling is indicated by the co-development of liquid-dominated and vapour-dominated fluid inclusions within individual fluid inclusion assemblages from the syn-mineralization quartz and is related to episodic fluid pressure drops caused by reactivation of basement faults. Our results indicate that composition and P–T conditions of the ore fluids in the PLC are comparable to those of typical URU deposits in the Athabasca Basin, indicating that the uranium deposits in the PLC formed under similar hydrothermal conditions. Episodic reactivation of basement faults was an important driving force to draw uraniferous fluids from the basin and reducing fluids from the basement to the mineralization sites, forming deep basement-hosted deposits. Supplementary material: Table 1, microthermometric results of type-2 and -5 fluid inclusion assembladges and isolated inclusions from the Patterson Lake corridor, and table 2, microthermometric results of type-6 fluid inclusions from the Patterson Lake corridor are available at https://doi.org/10.6084/m9.figshare.c.5510179 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

The Patterson Lake corridor is situated along the SW margin of the Athabasca Basin and contains several basement-hosted uranium deposits and prospects. Drill core investigations during this study have determined that granite, granodiorite, mafic and alkali intrusive basement rocks are entrained in a deep-seated NE-striking subvertical heterogeneous high-strain zone defined by anastomosing ductile to semi-brittle shears and brittle faults. The earliest phases of ductile deformation (D1/D2), linked with Taltson (1.94–1.92 Ga) orogenesis, involved interference between early fold sets (F1/F2) and development of an associated ductile transposition foliation (S1/S2). During subsequent Snowbird (c. 1.91–1.90 Ga) tectonism, this composite foliation was re-folded (D3) by NE-trending buckle-style folds (F3), including a regional fold centred on the Clearwater aeromagnetic high. In continuum with D3, a network of dextral-reverse chloritic-graphitic shears, with C–S geometry, formed initially (D4a) and progressed to more discrete, spaced semi-brittle structures (D4b; c. 1.900–1.819 Ga). Basin development (D5a; Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

The lower Proterozoic uranium deposits in the Franceville basin (Gabon) are mainly known to have hosted natural nuclear reactors. However, this basin also witnessed a former petroleum system which shows major occurrences of uranium ores associated with organic matter. The uranium mineralization is the result of several episodes of oil generation and migration, fluid mixing and hydrofracturing. During burial, at around 2.0 Ga, black shales were heated and expelled hydrocarbons which migrated in underlying sandstones. The first episode of oil migration was synchronous with a silicification event produced by a hot low salinity fluid, as shown by fluid inclusions trapped in quartz overgrowths. The second episode was contemporaneous with quartz dissolution and is characterized by the mixing of hydrocarbons and oxidizing U-bearing brine which resulted in the precipitation of UO2 inclusions in oil. Then, oil-to-gas conversion generated overpressured gas trapped in pores with uraniferous bitumen nodules. Changes in the stress regime probably favored radial hydrofracturing around bitumen nodules and subsequently the development of a pervasive micro-fracture network. There, UO2 precipitated together with Cu-Fe sulfides through thermochemical sulfate reduction favored by the presence of hydrocarbons and methane.

Rammelsbergite (NiAs2) is often present in ore deposits worldwide in association with other arsenides, sulphides and sulfarsenides. This work demonstrates the value of the application of the Re-Os isotopic system to the dating of rammelsbergite. Using the example of the Cigar Lake uranium deposit (Athabasca Basin, Saskatchewan, Canada), our results show that rammelsbergite concentrates significant and variable amounts of Re when crystallizing, up to 1.6 µg g−1, while incorporating minimal common Os. Such characteristics make this mineral an ideal target for Re-Os isotopic dating, by both isochron and model age methods. Remarkable agreement of the Re-Os age of the rammelsbergite with the U-Pb age of the uranium oxide grains found inside one of the massive rammelsbergite veins confirms the applicability of the method. The selected rammelsbergite samples from the Cigar Lake uranium deposit yield a 187Re-187Os isochron age of 1239 +33/−20 Ma. These results challenge a long-standing petrogenetic model which postulates contemporaneous U oxide and Ni-Co-arsenide deposition in unconformity-related uranium deposits. The new Re-Os age of rammelsbergite veins indicates that arsenide crystallization postdated the deposition of the primary uranium oxides at 1461–1341 Ma, implying that uranium oxides and nickel arsenides, as well as other sulphides and sulfarsenides, were not precipitated contemporaneously during a single hydrothermal event as previously proposed. The 187Re-187Os data suggest that crystallization of rammelsbergite and co-genetic sulphides, arsenides and sulfarsenides was linked to the ca. 1270 Ma Mackenzie magmatic event, marked by the emplacement of mafic dikes throughout the Canadian Shield, including in the area of the Athabasca Basin. This study therefore suggests a new hypothesis for the origin of nickel, cobalt, sulfur and arsenic in unconformity-related U deposits of the Athabasca Basin. More broadly, the successful application of the 187Re-187Os rammelsbergite chronometer in this geological context suggests that its application in other ore deposit environments worldwide is likely to provide similar insights. In this respect, the 187Re-187Os chronometer applied to rammelsbergite, and potentially to other arsenides and sulfarsenides, represents a valuable complement to molybdenite dating because it can be used in systems that lack molybdenite.

The ability to identify the sources of nuclear materials using various analytical methods has become a major target of nuclear forensics science. However, despite the emergence of novel tools developed in recent years, it has become obvious that accurate identification of sources requires multiple tools. Here, we have developed a new isotope tool based on Mo isotopes that could be used in nuclear forensics. Molybdenum is a trace element that is found in significant levels in uranium ores (0.5–4800 ppm), uranium minerals (0.02–6000 ppm) and uranium ore concentrates (UOCs, from 0.7 to 1400 ppm). The Mo isotope composition, reported as δ98Mo was analyzed in these materials and shown to have a significant variability from −2.62 to +0.30‰ in uranium ores and from −3.50 to +0.46‰ in UOCs. In uranium ores, the largest Mo isotope fractionation is found in deposits of sedimentary origin while the few ores with a magmatic origin show limited Mo isotope fractionation. Thus, the Mo isotope ratios may ultimately be used to distinguish high temperature from low temperature uranium ores. The δ98Mo values in UOCs also show a significant range that not only originate from variations in ore compositions but can also be attributed to ore processing such as leaching, solvent extraction, resin extraction and UOC precipitation. Laboratory experiments based on existing protocols of uranium ore separation demonstrate the existence of sizeable Mo isotope fractionation associated with these processes. Experiments with solvent extraction and precipitation of ammonium diuranate and peruranate show that the refined uranium fraction is enriched in light Mo isotopes in both cases. In the case of a uranium ore (SOMAIR, Niger) and associated UOC, the observed fractionation can possibly be assigned to the combined effect of solvent extraction and precipitation, although Mo isotope variability in ores and UOCs may make such a diagnostic difficult. Ultimately, these experiments can be used to reconstruct the Mo isotope composition of the ore, if there is sufficient knowledge about the ore processing flowsheet or to provide information about the type of uranium ore. Alternatively, it could be used to identify the process used for manufacturing a given UOC, if one can make an estimate about the composition of the ore, probably in conjunction with other tools. Our study demonstrates the potential of Mo isotopes as a useful tool in the field of nuclear forensics.

Paleoclimate significantly influenced continental sandstone-type uranium mineralization. To better understand continental climate change in the Late Cretaceous and its influence on uranium mineralization, we selected four boreholes from Late Cretaceous sedimentary successions from the southwestern Songliao Basin, in northeastern China. Twenty-one samples were collected for sporopollen analyses. Based on the variations in the relative abundances of the different sporopollen taxa, we observed the four following palynological assemblages (PAs) in ascending order: the Schizaeoisporites–Classopollis–Tricolporollenites assemblage, the Taxodiaceaepollenites–Exesipollenties–Cranwellia assemblage, the Schizaeoisporites–Classopollis–Lytharites assemblage, and the Biasccate–Taxodiaceaepollenites–Aquilapollenites assemblage. Based on the geological range of several important elements and correlations between relevant assemblages from other regions, we assigned a geological age of late Turonian to early Campanian (approximately 86.7–84.4 Ma) to the four PAs. In this study, we introduced a sporopollen–climate transforming methodology, which uses the percentage of drought, hygrophilous, and thermophilic taxa to indicate stratigraphic trends of humidity and temperature. On the one hand, the percentage of thermophilic taxa was characterized by a continuously decreasing trend, but remained at a relatively high abundance along the entire sedimentary sequence. This was indicative of a hot subtropical–tropical climate with declining temperatures. On the other hand, the percentage of drought and hygrophilous taxa exhibited a cyclical fluctuation between humid and semi-arid climates. A semi-arid climate existed in the PA I, but this then changed to humid conditions in the PA II. Subsequently, the climate evolved into semi-arid and semi-humid conditions as a result of strengthened drought during the PA III. Finally, in the PA IV, the climate became humid once again. In general, the Yaojia Formation’s synsedimentary paleoclimate (i.e., the PA II and PA III) mainly had semi-humid conditions. In the Qianjiadian area, the main prospect target layer is the Yaojia Formation. Compared with the typical sandstone-type uranium deposits throughout northern China, we suggest that the humid paleoclimate was essential for uranium mineralization. The humid climate favored the formation of reduced sandstones, which provided significant geochemical reducing barriers and uranium pre-concentrations essential for epigenetic uranium mineralization.

Color zoning of strata of Mesozoic continental basins in North China is closely correlated with the sedimentary environments of sandstone-type uranium deposits. Systematic research has been conducted on the colors of U-bearing rock series within the Ordos Basin by using data of several ten thousands of drill holes from coal and oil fields. Taking an ore cluster as the unit, well-tie sections of deposit areas were compiled to analyze the vertical and lateral spatial variations in different color zones of rocks. Taking the basin as the unit, two typical well-tie sections in the central-northern basin were compiled in order to summarize the rock color zoning laws of U-bearing rock series within ore clusters and the entire basin. Lithogeochemical characteristics of different color zones in deposit areas and ore barren areas were studied. Especially, variations in trace elements in different color zones and the paleo-sedimentary environments they reflect were discussed. Within the Ordos Basin, from the basin margin to the center and from the U ore clusters to the ore barren areas, colors of the Zhiluo Formation U-bearing rock series generally display vertical zoning (i.e., from up downwards, red–green–gray [grayish black]) and show no lateral zoning characteristics. U ore beds mostly occur in the lower part of the green zone and the upper part of the gray zone. This conclusion differs greatly from the understanding on rock color zoning related to classic interlayer-oxidation-zone metallogenic theory. Hence, a correct understanding on color zoning of U-bearing rock series within the Ordos Basin is of great significance not only to U mineralization exploration but also to research on sandstone-type U mineralization theory.

Carbonaceous debris (CD) within uranium-bearing strata has been studied in the Daying uranium deposit of the northern Ordos Basin, northern China. The influence of radiogenic heat from uranium on organic matter maturation was investigated through a series of tests including measurements of vitrinite reflectance (Ro), fission-track (FT) analysis in quartz grains, and the calculation of the radiogenic heat production rate of the samples. The results show that Ro in uranium-bearing strata generally increases as the burial depth increases, indicating that CD experienced normal burial coalification. However, Ro values of the samples rich in uranium are 0.062% Ro higher than those without uranium mineralization. Vitrinite reflectance bears a positive relationship with uranium content, and an inverse relationship with distance to the closest sandstone rich in uranium, indicating that uranium enrichment enhances organic matter maturation. The production of uranium decay makes FT observable in quartz grains, and the intensity of decay increases with proximity to the uranium ore body. The calculated radioactive heat production rate from the uranium ore body is 6.857 × 10−5 W/m3. During the long-term stable decay, as the uranium ore body theoretically results in an abnormal increase in temperature of 52°C without consideration of the loss of heat conduction, heat convection, and thermal radiation, this would yield a theoretical Ro increase of 0.209% Ro, reasonably greater than the observed. Therefore, the long-term stable radiogenic heat produced by uranium ore body can slightly enhance organic matter maturation, which is instructive in uranium prospecting.

The South Zhuguang uranium (U) ore field in South China, which was discovered in the 1950s, contains an estimated >20,000 t of recoverable U. The U ore is hosted by the Zhuguang granite massif, which is located in the South China Block. The main uranium-bearing mineral, pitchblende, is found mainly in veins along high-angle normal faults, associated with hematite, quartz, fluorite, calcite, and locally pyrite. The hydrothermal alteration is generally silicification, illitization, hematization, and chloritization that developed adjacent to the ore-bearing faults and outwards over hundreds of meters into the unaltered granites. The ore-forming fluids in the South Zhuguang U ore field can be classified into two stages. Stage I fluids were acidic and oxidizing, with fluoride and sulfate complexes of UO22+ being the dominant species in the fluid. Stage II fluids were alkaline and oxidizing, with carbonate and hydroxide complexes of UO22+ being the dominant species. δ18O values of syn-ore quartz range from +6.29‰ to +14.30‰, with calculated δ18OW-SMOW values from −1.85‰ to +5.35‰. δDW-SMOW values of the ore-forming fluids were −104.4‰ to −23.1‰. δ18O values of the altered granites increase gradually from ∼+4‰ close to ore, to ∼+12‰ in the nearly unaltered granites. The δ13C and δ18O values for syn-ore calcite range from −10.3‰ to −4.3‰ and −22.41‰ to −16.8‰ relative to Pee Dee Belemnite, respectively, with calculated δ18O and δ13C values for water and CO2 of −2.97‰ to +2.83‰ and −11.2‰ to −5.2‰, respectively. The δ34SV-CDT values of syn-ore pyrite range from −17.1‰ to −3.4‰. Initial 87Sr/86Sr ratios of syn- and post-ore fluorite vary from 0.719250 to 0.721327 and 0.714598 to 0.716299, respectively. The ore-forming fluids in the South Zhuguang U ore field were sourced primarily from Cretaceous–Paleogene redbed basins in the south, and initially derived from meteoric waters. Hydrothermal alteration associated with stage I fluids released considerable amounts of Ca2+ and F− into the ore-forming fluids, increased the fluid pH, decreased the stability of the dominant fluoride complexes, and lowered the saturation solubility of the uranyl ions. Reduction of the ore-forming fluid converted U6+ into U4+, which resulted in the deposition of pitchblende–fluorite–quartz veins of the stage I ore in fractures along with pyrite. During stage II, the U mineralization was controlled mainly by CO2 degassing, which resulted in decomposition of the carbonate complex UO2(CO3)2–, and deposition of U in pitchblende–calcite–quartz veins as the ore-forming fluid was reduced.

Recent discoveries of basement-hosted uranium deposits in the Patterson Lake corridor in the southwestern Athabasca Basin of Canada have brought vigorous exploration interest to the region. New lithostratigraphic constraints, geochronology and airborne geophysical surveys have dramatically improved the understanding of the host basement geology, warranting a re-examination of the remote predictive mapping and geophysical responses of the buried basement rocks. This study took a two-step approach to examine the regional basement geology and architecture. First, a mosaic of the long-wavelength response of potential field (gravity and magnetic) datasets was examined to divide the basement into regional domains based on bulk physical property variations. The interpretive geological model was then refined using textural and lineament analysis of new airborne gravity and magnetic datasets, geological drill hole logs and magnetic susceptibility measurements. The new basement map identifies and updates major features including a crustal-scale structure that separates the southern Tantato Domain from the newly defined eastern Taltson Domain. This structure may have played a role in localizing fluid flow in the Patterson Lake corridor, defining the spatial extents of structurally controlled buried felsic intrusions, and redefines the boundaries of the Taltson, Clearwater and Tantato Domains. In addition, the potential field enhancements delineated significant regional faults that controlled the geometry of Paleoproterozoic cover sequences and have implications for understanding the crustal architecture of the southern Rae Province. These new interpretations shed light on the tectonic history of the region to support on-going exploration activities and delineate regionally prospective areas in this understudied area of the Canadian Shield. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways