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Unconformity-related uranium (URU) deposits in the Athabasca Basin occur in local areas near the intersections of the basal unconformity of the basin and regional-scale, basement-rooted faults. This study examines the ore-controlling factors through analysis of fluid inclusions from syn-mineralization drusy quartz in mineralized and distal (> 500 m away from mineralized zones) barren areas of two major URU-hosting structures: the P2 fault in the eastern, and the Patterson Lake corridor (PLC) in the southwestern, parts of the basin. Microthermometric data indicate that fluids in mineralized zones have salinities ranging from 24.8 to 31.7 wt.% NaCl + CaCl2 and homogenization temperatures (Th) from 64 to 227 °C, which are comparable with those from distal barren areas with salinities ranging from 25.2 to 32.5 wt.% NaCl + CaCl2 and Th from 91 to 213 °C. Laser ablation-inductively coupled plasma-mass spectrometry analyses of individual fluid inclusions indicate that the fluids have elevated concentrations of U in both mineralized zones (0.50 to 109 mg/kg) and distal barren areas (0.32 to 73.8 mg/kg). Bulk fluid inclusion analyses also indicate elevated U concentrations for mineralized zones (0.39 to 1560 mg/kg) and distal barren areas (0.12 to 1.5 mg/kg), and U was detected in fluid inclusions from both mineralized and distal areas with synchrotron X-ray fluorescence mapping. The development of uraniferous fluids with similar geochemistry in distant areas with different basement lithologies supports the hypothesis that U for mineralization was mainly derived from the basin. The presence of uraniferous fluids with similar thermal and compositional characteristics in both mineralized zones and distal barren areas along the same structures with similar lithologies suggests that mixing of U-rich fluids with reductants-carrying fluids played a critical role in ore precipitation; lack or low flux of either or both fluids resulted in lack or poor development of mineralization. [ABSTRACT FROM AUTHOR]

The reactivation of inherited tectonic structures formed during the Paleoproterozoic Trans-Hudson Orogeny (THO) has played a significant role in generating high-grade unconformity-related uranium deposits in the eastern Athabasca Basin. The role of these tectonic structures is now investigated through a series of two-dimensional hydrothermal numerical models. Two modelling scenarios are considered: (1) models during the THO peak of metamorphism and (2) models with a permeable layer mimicking the presence of the Athabasca Basin, deposited unconformably over the THO basement. In the first scenario, general fluid patterns are strongly affected by the applied permeability configurations. Unidirectional high fluid flow zones (from 10-9 to 10-8 m·s-1) and high thermal gradients (up to 65°C·km-1) can be observed above and within the deep-seated tectonic structures. In the second scenario, well-established fluid convection cells or unidirectional fluid flow zones are observed within the basin layer, with upflow originating from the core of the deep-seated structures, regardless of the applied fluid pressure regime. These results highlight that these deep-seated structures can efficiently transport fluids and heat towards the upper parts of the crust and the basin. In the second scenario, the loci for preore alteration are then evaluated by computing a rock alteration index based on temperature and fluid velocity constraints. These alteration areas reside along and above the deep-seated structures and are potential regions for structural reactivation during mineralization. These results imply that the analysis of the inherited tectonic structures, combined with the alteration regions, can serve as markers for uranium exploration. [ABSTRACT FROM AUTHOR]

The unconformity-related uranium (URU) deposits in the Proterozoic Athabasca Basin (Canada) represent the richest, and one of the most important, uranium endowments in the world. Most of the URU deposits are associated with pre-existing graphitic basement faults that were reactivated after the formation of the basin. These graphite-rich structures have been widely used as a vector for exploration, but the nature of the association of the URU deposits with graphitic basement faults has been debated for over four decades. Proposed roles of graphite include: (1) as a direct reducing agent to reduce U6+ to U4+ and precipitate uraninite; (2) as a precursor of hydrocarbons (mainly CH4) produced in situ or nearby and then used as a reducing agent for uraninite precipitation; (3) as a precursor of hydrocarbons produced at depth that were remobilized to the site of mineralization and acted as a reducing agent for uraninite precipitation; and (4) as a lubricant facilitating faulting and fluid flow that led to uranium mineralization. This paper uses the Phoenix uranium deposit in the southeastern Athabasca Basin as a case study to address these uncertainties. Petrographic studies indicate that there is no direct contact between graphite and uraninite at microscopic scales, and the content of graphite in the graphitic metapelite along the ore-controlling WS Shear Zone does not show a systematic change with the distance from the unconformity surface. Raman spectroscopic studies of graphite suggest that the degree of structural disorder of graphite, expressed by various parameters related to the D bands and G band ratios, does not change systematically with the distance from the unconformity surface either. The minor irregularities in these parameters near the unconformity are better explained by paleo-weathering related to the unconformity and/or diagenetic processes than by hydrothermal activity related to uranium mineralization. Based on these observations and interpretations, the role of graphite as an in situ reducing agent, either directly or as a provider of hydrocarbons, is discounted. It is proposed that hydrocarbons derived from graphite at depth, tapped by episodic reactivation or seismicity of the basement faults that was facilitated by graphite as a lubricant, were responsible for URU mineralization. [ABSTRACT FROM AUTHOR]

The Patterson Lake corridor, located on the southwestern margin of the Athabasca Basin, contains several basement-hosted uranium deposits that formed via protracted, structurally controlled fluid–rock interactions. Using multiple generations of pyrite grains (pre-, syn- and post-mineralization) from the Triple R deposit, in situ iron isotopic analyses revealed large intra-sample and -grain variations (δ 56Fe values ranging from −2.21 to +1.67‰) whereas sulfur isotopes yielded minor variations (δ 34S values ranging from −4.44 to +5.3‰) relative to natural isotopic variations for both elements. The wide range in δ 56Fe values supports textural and chemical evidence that fluctuating oxidation states and chemistry in the fault zone fluids caused multiple generations of pyrite oxidation and precipitation. Sulfur isotope data from shallower mineralized zones show a slight enrichment in heavier isotopes, consistent with limited Rayleigh fractionation. However, when coupled with iron isotope data, the overall dataset supports a sulfur-rich, open system wherein heat from intrusions at depth and fault movements drove sulfur-bearing fluids upwards, causing precipitation of pre-mineralization pyrite and graphite. During fault reactivation, fluid pressure fluctuations between hydrostatic and sub-hydrostatic regimes drew oxidizing, uranium-bearing, basinal brines down into the basement to react with sulfides in the host rocks and deeply sourced, H2S-bearing reducing fluids. These redox reactions and fluid mixing resulted in precipitation of uraninite and syn-mineralization pyrite. These results further support the importance of structural control, repeated faulting and thermal anomalies in the basement for mineralization, necessitating re-examination of the current exploration model for unconformity-related uranium deposits. Supplementary material: Plot of δ 57Fe versus δ 56Fe values of all samples is available at https://doi.org/10.6084/m9.figshare.c.6026621 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways [ABSTRACT FROM AUTHOR]

The Zoujiashan uranium deposit in the Xiangshan ore field is the largest volcanic-related uranium deposit in China. Hematite- and fluorite-type ores are the predominant mineralization styles. Hematitization in the Xiangshan ore field is closely associated with uranium mineralization, mainly occurring as hematitized rocks enclosing fluorite-type vein ores developed in pre-ore illitized porphyritic lava. Detailed petrographic and mass balance calculation studies were conducted to evaluate the mechanisms for uranium precipitation and mass transfer during hematitization. Petrographic observations suggest that in the hematitized rocks, orthoclase is more altered than plagioclase, and quartz dissolution is common, whereas in the illitized rocks, pyrite commonly occurs within the altered biotite grains, and chlorite grains are locally found. Mass balance calculations indicate that Na2O and U were gained, K2O, CaO and SiO2 were lost, whereas Fe2O3-t remained more or less constant during hematitization. These observations suggest that the hydrothermal fluids were Na- and U-rich and Ca-K-poor, and the Fe2+ used for hematitization was locally derived, most likely from biotite, pyrite and chlorite in the host rocks. The Fe2+ is inferred to have played the role of reductant to precipitate uranium, and calculation indicates that oxidation of Fe2+ provided by host rocks is sufficient to form ores of economic significance. Consequently, the hematite-type ore is interpreted to be generated by the reaction between oxidized ore fluids and reduced components in host rocks. The development of calcite and pyrite in the fluorite ores suggests that perhaps mixing between the U-rich fluid and another fluid carrying reduced sulfur and carbon may have also contributed to uranium mineralization, in addition to temperature and pressure drop associated with the veining. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A number of large and high-grade unconformity-related uranium deposits formed in intracratonic Proterozoic basins, including the Athabasca Basin in Canada and the McArthur Basin in Australia. These deposits occur close to the unconformities between the Paleo- to Mesoproterozoic basinal sequences and their Archean to Paleoproterozoic metasedimentary basement, and are spatially associated with reactivated basement faults showing reverse offsets due to contractional deformation. Previous studies have suggested that fluid flow responsible for uranium mineralization in such basins may be related to thermal convection and tectonic deformation; however, the relative roles of these two driving forces, as well as their interrelationships, have not been explored in detail. In this study, two-dimensional numerical modeling of fluid flow in relation to coupled tectonic compression and heat transport was carried out to evaluate the relationships between deformation-driven flow and fluid convection, and their relative importance for uranium mineralization. The results indicate that tectonic compression at a relatively high strain rate (6.66 × 10−11 s−1) may instantaneously destroy pre-existing thermal convection developed in the sedimentary rocks in the basin, due to the rapid development of overpressure in the system. Convection may then reappear over time as deformation progresses and the overpressure wanes. By contrast, tectonic compression at a relatively low strain rate (6.66 × 10−13 s−1) does not affect pre-existing thermal convection, and deformation-driven fluid flow in the basement coexists with thermal convection in the basin. It is proposed that during an overall tectonically active period associated with a far-field tectonic event, compressional reactivation of basement faults creates permeability and causes fluid to flow toward dilatant zones accompanying individual seismic events, whereas during seismically quiet times, fluid flow driven by thermal convection dominates. The formation of the unconformity-related uranium deposits likely involves alternating relatively short-lived, deformation-driven fluid flow and relatively prolonged fluid convection events. [ABSTRACT FROM AUTHOR]

Thermal convection and fluid mixing are involved in the formation of many mineral deposits, but their relationships and roles in mineralization remain poorly understood because they are generally investigated separately in different studies. Here we present a case study combining thermal convection and fluid mixing to examine the critical factors that controlled the formation of high-grade, world-class unconformity-related uranium (URU) deposits in the Athabasca Basin (Canada). Reactive transport modeling of various scenarios with different degrees of involvement of thermal convection and fluid mixing shows that significant URU mineralization occurs at the intersection of a basement fault with the basin-basement unconformity only if thermal convection and basin-basement fluid mixing take place concurrently. If there is no thermal convection in the basin, only sparse U mineralization occurs along the unconformity. If insufficient amount of reducing fluid is provided from the basement along the fault, no significant U mineralization occurs either. Furthermore, no significant U mineralization occurs if the U concentration in the basinal fluid is low. We conclude that the exceptionally rich U endowment in the Athabasca Basin is the result of coupling of three critical factors: high-permeability sandstone favoring thermal convection in the basin, ample supply of reducing agents by basement fluids along reactivated basement faults, and abundant U-rich basinal fluid available in the basin sequences. Similar coupling of fluid processes may be essential for the formation of other types of mineral deposits. • Reactive transport modeling reproduces unconformity-related U mineralization. • Thermal convection and fluid mixing must occur simultaneously for U mineralization. • Permeable sandstone is critical for initiating and maintaining thermal convection. • Permeable basement faults linked with regional faults can provide ample reductant. • Availability of U-rich basinal fluids in sandstone is vital for U mineralization. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The formation of the Sue Trend deposits is related to tectonic compression. • Dilation is controlled by basement lithologies and degree of deformation. • Dilation is favoured by strong rheological contrast. Abstract Unconformity-related uranium deposits in the Athabasca Basin (Canada) are spatially associated with reactivated basement faults that cut the unconformity surface between Archean to Paleoproterozoic basement and Paleoproterozoic to Mesoproterozoic sedimentary rocks of the Athabasca Group. The Sue deposits (Sue A, B, C, D, and E) are located in the eastern Athabasca Basin along a 2-km-long north-northeast-trending structural corridor and have both sandstone-hosted (Sue A and Sue B; to the north) and basement-hosted (Sue C, Sue D, and Sue E; to the south) orebodies. All the deposits are structurally controlled by north-northeast-trending basement faults that demonstrate different degrees of reverse displacements of the unconformity surface. However, it is not clear why the mineralization is distributed in such a pattern along the corridor. In this study, both 2-dimensional (2D) and 3-dimensional (3D) numerical modeling of deformation were conducted to examine the relationship between the distribution of strain and uranium mineralization. The modeling shows that dilation (positive volumetric strain), thought to correlate with the development of extensional fracture systems and dilational jogs that represent potential mineralization sites, is primarily controlled by the rheological contrasts in basement rocks and the degree of deformation. The presence of faulted graphitic pelitic gneiss associated with the sandstone-hosted Sue A and Sue B deposits favours dilation localized within the sandstone at low degrees of deformation mainly due to the overlying competent hangingwall granitic gneiss. In contrast, the basement-hosted Sue C, Sue D and Sue E deposits are associated with dilation zones localized along the contact between the faulted graphitic pelitic gneiss and competent footwall silicified gneiss at higher degrees of deformation. The modeling results highlight the importance of both graphitic basement structures and strong rheological contrasts between basement rocks in uranium exploration in the Athabasca Basin. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Phoenix uranium deposit formed from basinal brines from the Athabasca Basin. • Mineralization took place at shallow burial. • Fluid boiling occurred during mineralization. • Faulting-boiling-ore precipitation are integrated processes related to mineralization. Abstract The Phoenix uranium deposit in the southeastern Athabasca Basin (Canada) is a typical unconformity contact-hosted uranium deposit, characterized by an association with reactivated basement faults, graphite-rich rocks in the underlying basement, and a pervasive clay alteration halo surrounding the mineralized zones. Petrographic results suggest that the mineralizing hydrothermal system was characterized by alternating desilicification and silicification events. Uraninite and clay-size minerals (tourmaline, kaolinite, illite and minor chlorite) mainly precipitated in the desilicification periods whereas hydrothermal quartz (mainly drusy quartz) formed during the silicification periods. Primary fluid inclusions in the hydrothermal quartz are inferred to represent the ore-forming fluids even though quartz did not co-precipitate with uraninite. The coexistence of multiple types of fluid inclusions (liquid-dominated biphase, vapor-dominated biphase, vapor-only and halite-bearing triphase) within individual fluid inclusion assemblages is interpreted to indicate fluid boiling and heterogeneous trapping. Bulk fluid inclusion volatile analysis by mass spectrometry indicates H 2 O as the dominant species, with less than 1 mol% non-aqueous volatiles that show a compositional trend typical of fluid boiling. Microthermometric and cryogenic Raman spectroscopic analyses indicate that the mineralizing fluids are of H 2 O-NaCl-CaCl 2 ± MgCl 2 composition, with salinities ranging mainly from 23.4 to 31.1 wt%. The liquid-dominated biphase inclusions (excluding those interpreted to have resulted from heterogeneous trapping) have homogenization temperatures from 90 to 157 °C. These data are generally consistent with the classical diagenetic-hydrothermal model in which basinal brines that extracted uranium from the basin or the basement were channeled along reactivated basement faults, and precipitated uraninite near the unconformity through reaction with reducing agents. However, the relatively low fluid inclusion homogenization temperatures documented in this paper, together with fluid boiling inferred from fluid inclusion assemblages, suggest that the deposit may have formed in a shallower environment (∼2.5 km) than assumed in the conventional diagenetic-hydrothermal model (>5 km). Abrupt fluid pressure drops during episodic faulting may have resulted in fluid boiling (flash vaporization) changing the fluid pH and promoting uraninite precipitation, with the alternating liquid-vapor conjugates of the boiling system resulting in alternating precipitation of quartz and uraninite respectively. [ABSTRACT FROM AUTHOR]

Previous studies on unconformity-related uranium deposits in the Athabasca Basin (Canada) suggest that egress flow and ingress flow can develop along single fault systems at different stages of compressional deformation. This research aims to examine whether or not both ingress and egress flow can develop at the same time within an area under a common compressional stress field, as suggested by the reverse displacement of the unconformity surface by the basement faults. The study considers the Phoenix and Gryphon uranium deposits in the Wheeler River area in the southeastern part of the Athabasca Basin. Two-dimensional numerical modeling of fluid flow, coupled with compressional deformation and thermal effects, was carried out to examine the fluid flow pattern. The results show that local variations in the basement geology under a common compressional stress field can result in both egress and ingress flow at the same time. The fault zone at Phoenix underwent a relatively low degree of deformation, as reflected by minor reverse displacement of the unconformity, and egress flow developed, whereas the fault zone at Gryphon experienced a relatively high degree of deformation, as demonstrated by significant reverse displacement of the unconformity, and ingress flow was dominant. The correlation between strain development and location of uranium mineralization, as exemplified by Gryphon and Phoenix uranium deposits, suggests that the localization of dilation predicted by numerical modeling may represent favourable sites for uranium mineralization in the Athabasca Basin. [ABSTRACT FROM AUTHOR]

The End deposit is one of several uranium deposits in the Kiggavik area near the Proterozoic Thelon Basin, which is geologically similar to the Athabasca Basin known for its unconformity-related uranium deposits. The mineralization occurs as uraninite and coffinite in quartz veins and wall rocks (psammopelitic gneisses) in the sub-Thelon basement and is associated with clay- and hematite-altered fault zones. Fluid inclusions were studied in quartz cementing unmineralized breccias formed before mineralization (Q2), quartz veins that were formed before mineralization but spatially associated with uranite (Q4), and calcite veins that were formed after mineralization. Four types of fluid inclusions were recognized, namely liquid-dominated biphase (liquid + vapor), vapor-dominated biphase (vapor + liquid), monophase (vapor-only), and triphase (liquid + vapor + halite) inclusions. The first three types were found in Q2, whereas all four types were found in Q4 and calcite. The coexistence of these different types of inclusions within individual fluid inclusion assemblages is interpreted to indicate fluid immiscibility and heterogeneous trapping. Based on microthermometry, the fluids associated with Q2 are characterized by low salinities (0.4 to 6.6 wt%) and moderate temperatures from 148 to 261 °C, and the fluids associated with calcite show high salinities (26.8 to 29.3 wt%) and relatively low temperatures from 146 to 205 °C, whereas the fluids associated with Q4 have a wide range of salinities from 0.7 to 38.8 wt% and temperatures from 80 to 332 °C. Microthermometric and cryogenic Raman spectroscopic studies indicate that the high-salinity fluids in Q4 and calcite belong to the HO-NaCl-CaCl ± MgCl system, with some dominated by NaCl and others by CaCl. The fluid inclusions in Q2 are interpreted to be unrelated to mineralization, whereas those in Q4 and calcite reflect the mineralizing fluids. The fluid inclusion data are consistent with a genetic link of mineralization with basinal brines derived from the Thelon Basin. However, unlike the conventional deep-burial (>5 km) diagenetic-hydrothermal model proposed for the unconformity-related uranium deposits, the uranium mineralization in the End deposit is inferred to have formed in a shallow environment (probably <2 km), based on fluid immiscibility and low fluid pressures obtained in this study. The U-Pb age of uraninite (1295 ± 12 Ma) is interpreted to reflect isotopic resetting after the primary mineralization. [ABSTRACT FROM AUTHOR]

The Beaverlodge district in northern Saskatchewan is known for “vein-type” uranium mineralization. Most of the uranium deposits are spatially related to major structures, and hosted by ca. 3.2–1.9 Ga granitic rocks (and albitite derived from them) and by ca. 2.33 Ga Murmac Bay Group amphibolite, all of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca. 1.82 Ga Martin Group, and by the flat-lying ca. 1.75–1.5 Ga Athabasca Group. The uranium mineralization is mainly hosted in fault rocks (breccias) and carbonate ± quartz ± albite veins, referred to as breccia-style and vein-style mineralization, respectively, with the latter being the focus of this study. Most of the mineralized veins occur in the basement rocks, although some crosscut the Martin Group. This study examines the field, petrographic, fluid inclusion and C-O isotope characteristics of mineralized and non-mineralized veins from 19 deposits/occurrences as well as from the Martin Group, with an aim to better understand the mineralizing environment and processes. The coexistence of liquid-dominated (L + V), vapour-dominated (V + L) and vapour-only (V) fluid inclusions within individual fluid inclusion assemblages (FIAs) in the veins suggests fluid immiscibility and heterogeneous trapping. The L + V inclusions with the lowest homogenization temperatures (T h ) within individual FIAs are interpreted to represent homogeneous trapping of the liquid phase, which yield T h values from 78° to 330 °C (mainly 100° to 250 °C), and salinities from 0.2 to 30.8 wt.% NaCl equivalent. Mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by H 2 O (average 97.2 mol%), with minor amounts of CO 2 , CH 4 , H 2 , O 2 , N 2 , Ar and He. Fluid pressures were estimated to be < 200 bars based on the inference of fluid immiscibility, fluid temperatures of 100° to 250 °C, and low concentrations of non-aqueous volatiles (< 3 mol%). The δ 18 O VPDB and δ 13 C VPDB of carbonate minerals associated with mineralization range from − 20.5 to − 8.9‰ and − 10.1 to − 0.9‰, respectively. The δ 18 O VSMOW values of the parent fluids calculated using the T h values range from − 9.6 to + 17.0‰, with the majority from 0 to + 5.0‰. O isotopes of paired equilibrium quartz and calcite, analyzed by secondary ion mass spectrometry (SIMS), yield temperatures from 161° to 248 °C, which are consistent with the fluid inclusion data. The new fluid inclusion and stable isotope data are inconsistent with a metamorphic or magmatic-hydrothermal model as proposed in some previous studies (for breccia-style and vein-style mineralization), but rather support a model in which the vein-type uranium mineralization took place at relatively low temperature (100° to 250 °C) and shallow (< 2 km) conditions, with fluid pressure fluctuating between hydrostatic and sub-hydrostatic regimes, possibly related to episodic faulting. The mineralizing fluids were mainly sourced from the Martin Lake Basin, and uraninite was precipitated as a result of mixing between this basin-derived fluid and fluids carrying reducing agents (Fe 2 + , CH 4 ) derived from the basement, although fluid-rock reactions and fluid immiscibility may have also played a role. [ABSTRACT FROM AUTHOR]

Uranium (U) and iron (Fe) are generally considered to have contrasting aqueous geochemical behavior, with U dissolution being favored in oxidizing fluids as oxidized U (U6+) and Fe in reducing fluids as reduced Fe (Fe2+) species. However, high concentrations of both U and Fe have been reported for the ore fluids of uranium deposits, suggesting that the two elements can be co-transported in the same fluid. In this study, geochemical modeling, based on recent thermodynamic and experimental data for U and Fe species and fluid inclusion compositions from two types of hydrothermal U deposits (volcanic-related and unconformity-related), was conducted to investigate the nature and conditions of U Fe co-transportation. The results of the modeling indicate that high concentrations of U (>10, up to >100 ppm) and Fe (>1000 ppm) can be dissolved in acidic aqueous fluids (pH < ∼3.3) of moderate to high chlorinity (∼ 55 to 250 g/L Cl), for a wide range of log f O 2 above and below the magnetite-hematite (MH) buffer (from ∼ MH-5 to MH + 30), at temperatures from 150 to 250 °C. There are four different combinations of dominant dissolved U and Fe species depending on the pH and redox conditions: U6+ and Fe3+ species (UO 2 Cl 2 0 and FeCl 3 0) at relatively high f O 2 , U4+ and Fe2+ species (UCl 4 0 and FeCl 4 2−) at relatively low f O 2 , and U6+ and Fe2+ species (UO 2 Cl 2 0 and FeCl 4 2−) or U4+ and Fe3+ species (UCl 4 0 and FeCl 3 0) at intermediate f O 2 conditions. Because the most important U mineral in hydrothermal uranium deposits is uraninite (UO 2), in which U is present as U4+, a necessary condition for U mineralization is that U is either transported as U4+ species or, if transported as U6+ species, precipitation of uraninite is induced by a reducing agent. The recognition that U6+ and Fe2+ species can be co-transported in the same fluid indicates that uraninite can be precipitated without the need to invoke a reducing agent external to the ore fluid. The driver of uranium mineralization in this case is an increase in pH due to fluid-rock interaction that leads to the alteration of the rocks by clay minerals and associated coupled sorption-reduction. These findings provide for a more extensive range of conditions for U transport and deposition than previously imagined and require that interpretation of the conditions of uranium mineralization consider both oxygen fugacity and pH. This study underscores the potential for uranium deposits to occur in geological settings where reducing agents are either absent or insufficiently abundant to account for the observed mineralization, and the possibility for the transport of U in environments that might otherwise be considered unfavorable. [ABSTRACT FROM AUTHOR]

• Thermal convection can be revealed by integrated diagenetic and numerical studies. • Coupled quartz cementation-dissolution pattern is indicative of thermal convection. • Large-scale thermal convection did occur in the Proterozoic Athabasca Basin. • Fluid convection was vital for unconformity-related uranium mineralization. Thermal convection has been shown to be operating in many magmatic-hydrothermal systems, but whether thermal convection driven by thermal gradients occurs in sedimentary basins devoid of magmatic activity remains disputed. Here we report a case study of the Proterozoic Athabasca Basin in which the thermal convection hypothesis is strongly supported by petrographic observations and reactive transport modeling. The basin is characterized by thick (up to 1500 m) sandstone successions separated by relatively thin (<300 m), mud-rich intervals. The top of the sandstone succession is characterized by extensive quartz overgrowths and point contacts of detrital grains, whereas the basal part contains little cement and shows extensive dissolution features and sutured contacts. This spatial distribution pattern of diagenetic features cannot be explained by gradual burial of the basin, and is instead interpreted as a result of simultaneous quartz precipitation at the top and dissolution at the bottom due to fluid convection. Numerical modeling of reactive transport of SiO 2 indicates that this quartz cementation-dissolution distribution pattern can be produced only if the sandstones are sufficiently permeable and thick so that the Rayleigh number exceeds the critical value for convection. Thus, the development of such a coupled quartz cementation-dissolution distribution pattern can be used as an indicator of thermal convection. This recognition is of importance for explaining the large amounts of fluids required to form mineral deposits in sedimentary basins, as exemplified by the world-class unconformity-related uranium deposits in the Athabasca Basin. [ABSTRACT FROM AUTHOR]

• Most of the types of uranium deposits classified by IAEA (2018a) can find examples in China. • The most important ones in China are sandstone-, granite-, volcanic- and carbonate types. • Fourteen papers in the Special Issue represent the different types of uranium deposits in China. • Examples of the intrusive-type and IOCG-type are also presented. • The papers in the Special Issue highlight the diversity of uranium deposits in China. As one of the top ten countries with important uranium resources, China hosts most of the fifteen types of uranium deposits identified by International Atomic Energy Agency (IAEA), among which the economically most important ones are sandstone-type, granite-type, volcanic-type, and carbonaceous-siliceous-pelitic rock- or carbonate-type. This Special Issue provides an up-to-date perspective of the geological characteristics, metallogenic environments, ore-forming mechanisms and exploration methods by the Chinese uranium geoscience community, of the more important types of uranium deposits in China. Among the fourteen papers, six are on sandstone-type, two on volcanic-related, two on granite-related, two on intrusive-type, one on carbonaceous-siliceous-pelitic rock-type or carbonate-type, and one on IOCG or polymetallic iron oxide breccia complex-type. These papers provide useful references for comparison between the Chinese uranium deposits and their counterparts in other parts of the world, and will contribute to further advancement of the global uranium geoscience and resource exploration. [ABSTRACT FROM AUTHOR]

• Beaverlodge (Canada) and South China granite-related uranium deposits share many similarities. • Uranium mineralization in both areas are associated with red bed basins and mafic magmatism in an extensional setting. • Red bed basins served as reservoirs of oxidizing fluids, and elevated geothermal gradients facilitated fluid circulation. • Coupling of shallow (red bed basins) and deep (mantle-derived magmatism) processes is critical for uranium mineralization. Many uranium deposits are related to granitic rocks, but the mineralization ages are much younger, thus excluding a direct magmatic-hydrothermal link between the mineralization and the granites. Two such examples are the Proterozoic "vein-type" uranium deposits in the Beaverlodge district in Canada and the Mesozoic granite-related uranium deposits in South China. Both areas have been extensively studied, but the critical factors that control the mineralization remain unclear. The uranium mineralization in the Beaverlodge district occurs in quartz – carbonate ± albite veins and breccias developed within and near major deformation zones, and are mainly hosted by ca. 2.33 – 1.90 Ga granitic rocks and ca. 2.33 Ga Murmac Bay Group amphibolite. These rocks are unconformably overlain by the Martin Lake Basin, which was formed during a period of regional extension in the later stage of the Trans-Hudson orogeny and is filled with red beds. A ca. 1820 Ma mafic magmatic event is manifested as volcanic rocks occurring within the Martin Lake Basin and as dikes crosscutting the basement rocks and lower Martin Group strata. Uraninite U-Pb and Pb-Pb ages range from ca. 2290 Ma to <300 Ma, with a peak overlapping the mafic magmatism. The granite-related uranium mineralization in South China occurs mainly as quartz – carbonate ± fluorite veins and as disseminations in the host rocks adjacent to fracture zones. The deposits are hosted by, or occur adjacent to, granitic rocks, and are spatially close to Cretaceous – Tertiary red bed basins that were formed in a Basin-and-Range like tectonic setting related to roll back of the Pacific plate. These red bed basins contain intervals of bimodal volcanic rocks and are crosscut by coeval mafic dikes. Uraninite U-Pb ages dominantly range from 100 to 50 Ma, which is much younger than the host granites (>145 Ma) and broadly contemporaneous with development of the red bed basins and mafic magmatism. Comparison of the two study areas reveals striking similarities in geologic attributes related to uranium mineralization, specifically, the development of large volumes of granitic rocks that are relatively enriched in uranium, and the development of red bed basins with accompanying coeval mafic magmatism in extensional tectonic settings. It is proposed that oxidizing basinal fluids from the red bed basins circulated into the underlying fertile granitic rocks along high-permeability structural zones, acquired uranium in the path through fluid-rock interaction, and precipitated the uranium where they encountered reducing agents. Elevated geothermal gradients associated with the mantle-derived magmatism greatly enhanced the mineralization by facilitating the uranium extraction and transport processes. Thus, it is the coupling of shallow (red bed basin and oxidizing basinal fluid development) and deep-seated (mantle-derived magmatism and related thermal activity) processes, together with the pre-enrichment of uranium in basement rocks (particularly granitic rocks) and pre-existing deformation zones, that controlled the formation of the granite-related uranium deposits in both Beaverlodge and South China, and perhaps elsewhere in the world with similar geologic setting. [ABSTRACT FROM AUTHOR]

• Clay minerals and carbonate cementation are associated with U mineralization. • δD and δ18O values of clay minerals suggesting meteoric origin of the ore fluid. • Hydrocarbons may have played an important role in U mineralization. • H-O-C isotopes suggest a transient interface between oxidizing/reducing fluids. 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 δ18O V-SMOW of ca. −5‰ and a δD V-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 δD V-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. [ABSTRACT FROM AUTHOR]

• Baimadong U deposit is hosted by carbonate rocks and is associated with black and red alterations. • Black alteration is characterized by exogenous graphite and bitumen. • Red alteration is marked by precipitation of hematite and goethite. • Black alteration provides reducing agents for, and red alteration results from, U mineralization. The Baimadong uranium deposit is hosted in carbonate rocks that were subjected to extensive black and red alterations, silicification and argillization, and is assigned to the carbonaceous-siliceous-pelitic rock-type uranium deposits in the Chinese literature. However, the nature of the mineralization, especially the black and red alterations and their relationship with uranium mineralization, remains poorly understood. Petrographic and Raman spectroscopic studies indicate that the black material are organic matter mainly composed of graphite and bitumen, whereas the red-altered rocks are characterized by abundant iron oxides. Crosscutting relationships suggest that the black alteration occurred before the red alteration. Comparison of whole-rock geochemistry between the different types of rocks suggests that total organic carbon (TOC), U and total organic sulfur (TOS) were gained through the black alteration, and FeOT was gained through the red alteration, whereas total inorganic carbon (TIC), CaO and MgO were lost in both types of alterations. The δ13C org (PDB) values and biomarker characteristics of the black-altered rocks suggest that the organic matter was derived from the black shale of the Niutitang Formation underneath the deposit. U enrichment is associated with both the black and red alterations, and shows positive correlations with Al 2 O 3 , K 2 O, TiO 2 , SiO 2 , TOC and TOS. It is proposed that the uranium mineralization at Baimadong resulted from superposition of two fluid events controlled by a common structural system. First, petroleum generated in the black shales of the Niutitang Formation and hydrothermal fluids migrated along cross-formational faults to the carbonate rocks of the Qingxudong and Shilengshui formations, causing the black alteration and preliminary uranium enrichment. Second, an oxidizing, U-bearing fluid flowed through the same structure and precipitated uraninite through reaction with the black-altered rocks, accompanied by precipitation of hematite and goethite as marked by the red alteration. The combination of black and red alterations with major faults may be used as a mineralization indicator in the exploration of this type of U deposits. [ABSTRACT FROM AUTHOR]