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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 Proterozoic Athabasca Basin hosts a large number of high-grade, large-tonnage unconformity-related uranium (U) deposits, many of which are also enriched in rare earth elements (REE). The basin also contains hydrothermal REE mineralization unassociated with U. Previous studies postulated that U and REE were derived from either the basin or the basement; however, the exact source of the metals remains ambiguous. This study provides evidence of U- and REE-rich fluids throughout the Athabasca Basin through laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) mapping of barren sandstone distal to mineralized areas. The results indicate that elevated U and REE concentrations mainly occur in the matrix; there are strong positive correlations between U and REE, Th, P and Sr, and moderate positive correlations between U and Zr, Ba, Fe, Al, K and Ca, but the few spots with the highest U are unrelated to these elements. Quantitative evaluation of the element correlations, together with scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, suggests that most of the elevated U and REE are hosted in aluminum phosphate sulfate (APS) minerals rather than apatite and monazite. As the APS minerals are of diagenetic-hydrothermal origin, the results testify to the presence of U- and REE-rich fluids within the Athabasca Basin. The elevated Th/U ratio (~10) and REE pattern (strong heavy rare earth element (HREE) depletion) are consistent with a model in which large amounts of U and REE (especially HREE) were leached from the sandstone within the Athabasca Basin and contributed to U and REE mineralization near the unconformity between the sedimentary rocks in the basin and underlying basement rocks. This study demonstrates that LA-ICP-MS mapping can be effectively used to evaluate microscale distribution of elements and their mobility in sedimentary rocks to address mineralization related problems. [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 Patterson Lake corridor is a new uranium district located on the southwestern margin of the Athabasca Basin. Known resources extend almost 1 km 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, which is only observed in shallow samples from the Triple R deposit (Tur 1), contain the heaviest isotopic signature (δ 11B ≈ 19–26‰), whereas subsequent generations (Tur 2 and Tur 3) yield lighter and more homogeneous isotopic signatures (δ 11B ≈ 17.5–19.9‰). 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 δ 11B values). The lower δ 11B 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 δ 11B 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: Complete analytical dataset including reference materials are available at https://doi.org/10.6084/m9.figshare.c.5727555 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]

Elevated concentrations of uranium have been found in many bauxite deposits, but the status of uranium in the ores and the mechanisms of enrichment have not been well understood. In this paper, we report a new case of uranium enrichment in a paleo-karstic bauxite deposit at Yunfeng, southwestern China, present electron probe micro-analyzer (EPMA) and Raman spectroscopic evidence for the presence of separate U-minerals, and propose a model in which uranium was enriched through successive processes from chemical weathering through early to burial diagenesis. The Yunfeng bauxite ores, developed in Lower Carboniferous mudrocks overlying Middle to Upper Cambrian carbonate rocks, contain 18.0 to 62.4 ppm (average 35.1 ppm) U, which is much high than the abundances in average crustal rocks (1–3 ppm). Micron-sized uraninite occurs as rims of Ti-oxides, fillings of micro-fractures in kaolinite, and disseminated grains in association with sulfides in the matrix of diaspore and kaolinite. Trace and rare earth elements analysis indicates that the bauxite ores could not be produced by in-situ weathering of the carbonates underlying the orebodies. Instead, the precursor rocks of the bauxite ores are inferred to be black shales originally enriched in uranium that were exposed tens of kilometers away. Detrital grains of the black shales were transported and accumulated in depressions above karstified carbonates in the Yunfeng area, where they were subjected to intense weathering to form bauxite ores. The weathering of the shale clasts liberated much of the uranium due to oxidation of uranium from U 4+ to U 6+ . However, the majority of the U 6+ was reduced to U 4+ again due to availability of reducing agents such as organic matter and sulfides originally in the shales, forming nanocrystals of UO 2+x in close association with Ti-oxides disseminated in Al-hydroxides and clay minerals. These nanocrystals of UO 2+x were converted to uraninite during diagenesis, together with the convention of gibbsite and boehmite to diaspore and the formation of anhedral to euhedral pyrite and various Cu-, Pb-, Ni- and Co-rich sulfides. The recognition of uraninite in the bauxite ores is important for uranium exploration and for evaluation of the economic value of uranium as a byproduct of bauxite deposits. [ABSTRACT FROM AUTHOR]

A number of Carboniferous paleo-karstic bauxite deposits occur in the Xiuwen-Qingzhen area, central Guizhou Province, which constitute an important part of the central Guizhou-Southern Chongqing bauxite belt in southwestern China. These deposits consist of lenticular and stratiform orebodies hosted in the Lower Carboniferous Jiujialu Formation, which lies unconformably above carbonate rocks (mainly dolomite) of the Middle and Upper Cambrian Loushanguan Group, Shilengshui Formation and Lower Cambrian Qingxudong Formation. Although it is generally agreed that the bauxite resulted from weathering of Cambrian rocks below the unconformity, it remains unclear whether the carbonates alone can provide sufficient material for the formation of the deposits. In this study, we examined the mineralogy and major, trace and rare earth elements geochemistry of bauxite and bauxitic clays from the Yunfeng deposit in the Xiuwen-Qingzhen area and compared them to previously published data of a paleo-weathering profile consisting of Loushanguan dolomite underlying the sub-Carboniferous unconformity and the “Black Rock Sequence” of the Lower Cambrian Niutitang Formation, in order to evaluate the sources of the bauxite. The bauxite ores are composed of diaspore as the main ore mineral, along with small amounts of pyrite, arsenopyrite, galena, stibnite, xenotime and uranium-bearing minerals, and are characterized by high contents of uranium and phosphorus. Chondrite-normalized REE patterns and various bivariant diagrams commonly used to study the provenance of bauxite including Eu/Eu ∗ vs TiO 2 /Al 2 O 3 , Eu/Eu ∗ vs Sm/Nd, TiO 2 vs Al 2 O 3 , TiO 2 /Th, Th vs Ta, Th vs Nb and Nb vs Ta suggest that the Loushanguan dolomite and the Niutitang “Black Rock Sequence” are both potential source rocks. However, a comparison in U and Th contents between the paleo-weathering profile of the Loushanguan dolomite and the bauxite samples suggests that the dolomite alone cannot provide the materials for the bauxite ores. Instead, the “Black Rock Sequence” may have contributed most of the ore-forming materials, which are mainly transported from source regions where the Niutitang Formation was exposed, weathered and eroded. Therefore, the Yunfeng bauxite deposit is of mixed autochthonous and allochthonous origin. [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]

The Proterozoic Athabasca basin and underlying basement host numerous unconformity-related uranium deposits that were formed from extensive fluid circulation near the basement-cover interface. Although it is generally agreed that the mineralizing fluids were basinal brines, it is still unclear what driving forces were responsible for the circulation of the basinal fluids. Because different fluid flow driving forces are associated with different thermal profiles, knowing the basin-scale distribution of paleo-fluid temperatures can help constrain the fluid flow mechanism. This study uses fluid inclusions entrapped in quartz overgrowths and authigenic illite in sandstones from three drill cores (WC-79-1, BL-08-01, and DV10-001) in the central part of the Athabasca basin as thermal indicators of paleo-fluids in the basin. A total of 342 fluid inclusions in quartz overgrowths were studied for microthermometry. The homogenization temperatures (T h ) range from 50° to 235 °C, recording the minimum temperatures in various diagenetic stages. Temperatures estimated from illite geothermometry (121 points) range from 212° to 298 °C, which are systematically higher than (partly overlapping) the T h values, suggesting that illite was precipitated in hotter fluids following the formation of quartz overgrowths. Neither the fluid inclusion T h values nor the illite temperatures show systematic increase with depth in individual drill cores. This, together with the high illite temperatures that cannot be explained by burial at a normal geothermal gradient (35 °C/km), is interpreted to indicate that basin-scale fluid convection took place during the diagenetic history of the basin. Prolonged fluid convection is inferred to be responsible for delivering uranium (extracted from the basin or the upper part of the basement) to the unconformity, where uranium mineralization took place due to redox reactions associated with fluid-rock interaction or structurally controlled fluid mixing. [ABSTRACT FROM AUTHOR]

The Proterozoic Athabasca basin, located in northern Saskatchewan and Alberta, Canada, is known for its richness in uranium resources. The lithologies in the basin are dominated by sandstones, with minor amounts of mudstones and conglomerates. It has been the subject of debate whether the uranium in the deposits, located near the sub-Athabasca unconformity, was derived from the sedimentary rocks in the basin or from the igneous and metamorphic rocks in the basement. A diagenetic and geochemical study of sedimentary rocks from unmineralized areas in the basin may provide information about the geochemistry of the background basinal fluids, thus helping to evaluate whether or not the sedimentary rocks in the basin may have contributed uranium to the mineralizing systems. In this paper, we present the study results of sandstone samples from drill core DV10-001, which was drilled in the central part of the Athabasca basin, and includes petrographic studies of 80 polished thin-sections, point-counting data of 30 polished thin-sections, density measurements of 80 samples, and major and trace element analyses of 135 samples. The strata in the basin are generally characterized by red beds due to the presence of iron oxides and/or hydroxides (IOHs), indicating overall oxidizing conditions during sedimentation and diagenesis, but a significant portion of them have been bleached to a variety of relatively light colours. Petrographic studies indicate that precipitation of IOH and kaolinite started in early diagenesis, and may have lasted throughout much of the diagenetic history, except during bleaching events, when IOHs were dissolved. Some bleaching events may have occurred in early diagenesis, but the majority of bleaching events likely took place after a significant burial of the rocks, as indicated by the sharp contacts between the red beds and bleached parts that appear to have been controlled by fractures. Quartz overgrowths were formed after the early phase of IOH, and may have been interrupted by quartz dissolution events. Illite was formed relatively late in diagenesis, and both kaolinite and illite, as well as quartz overgrowths, are relatively enriched in the bleached rocks. Bleaching is interpreted to have been caused by introduction of relatively reducing fluids, which lowered the oxygen fugacity and increased the pH of the basinal fluids. These changes could have caused precipitation of uraninite if the basinal fluids initially contained tens of ppm of uranium, as demonstrated by some previous studies assuming that uranium was extracted from the sedimentary rocks and carried by the basinal fluids. No systematic gain or loss of uranium during bleaching has been found in our study, suggesting low concentrations of uranium in the basinal fluids at the time of bleaching. However, this interpretation does not exclude the possibility that high concentrations of uranium may have been developed in basinal fluids in early diagenesis, especially during the reddening processes. Although this study can neither confirm nor discard the hypothesis that uranium found in the unconformity-related uranium deposits was extracted from the sedimentary rocks in the basin, it puts a constraint on the timing of such uranium extraction, limiting it to early diagenesis, which is consistent with the first mineralization ages. [ABSTRACT FROM AUTHOR]

Uranium deposits in sedimentary basins can be formed at various depths, from near surface to the basement. While many factors may have played a role in controlling the location of mineralization, examination of various examples in the world, coupled with numerical modeling of fluid flow, indicates that the hydrodynamic regime of a basin may have exerted a major control on the localization of uranium deposits. If a basin is strongly overpressured, due to rapid sedimentation, abundance of low-permeability sediments or generation of hydrocarbons, fluid flow is dominantly upward and uranium mineralization is likely limited at shallow depths. If a basin is moderately overpressured, upward moving fluids carrying reducing agents may meet downward moving, oxidizing, uranium-bearing fluids in the middle of the basin, forming uranium deposits at moderate depths. If a basin is weakly or not overpressured, either due to slow sedimentation or dominance of high-permeability lithologies, minor topographic disturbance or density variation may drive oxidizing fluids to the bottom of the basin, leaching uranium either from the basin or the basement, forming unconformity-type uranium deposits. It is therefore important to analyze the hydrodynamic regime of a basin in order to predict the most likely type and location of uranium deposits in the 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]

• Elevated U concentrations (up to 266 ppm) were found in the Lala IOCG ores. • Uraninite, uranothorite, polycrase and coffinite were recognized. • Uranium was initially enriched with Cu, Mo, Th and REE at ca. 1086 to 1053 Ma. • U and Mo were remobilized and re-enriched together at ca. 888 to 828 Ma. Elevated uranium concentrations have been reported in many IOCG deposits, but detailed information about the occurrences of uranium minerals and their relationships with other minerals remains scarce, which hinders understanding of the mineralization mechanisms and the nature of the association of these metals. This paper reports a new case of uranium enrichment in an IOCG system, the Lala Cu-Fe deposit in the Kangdian region, SW China. Whole-rock analyses of ores and barren rocks yielded highly variable U concentrations ranging from 0.76 to 266 ppm. Uranium shows a strong positive correlation with Mo, broadly positive correlations with Cu and ΣREE, and a vague, negative correlation with Th. Four uranium minerals, uraninite [UO 2 , uranothorite [(Th,U)SiO 4 , polycrase [(Y,Ca,Ce,U,Th)(Ti,Nb,Ta) 2 O 6 , and coffinite [U(SiO 4) 1-x (OH) 4x , were identified. The uraninite is divided into two phases: phase 1 (Ur1) characterized by corroded or irregular crystal shapes, and phase 2 (Ur2) showing well-developed, subhedral to euhedral crystal shapes. Ur1 is interpreted to be formed in the primary Cu – Fe – Mo – REE – U mineralization, together with the bulk of Cu and other metal sulfides, as well as other U and REE minerals, including uranothorite, polycrase, bastnäsite and xenotime, and accompanied by K-alteration (K-feldspar and biotite). Ur2 is interpreted to be formed in a secondary hydrothermal event characterized by introduction of fluorite and carbonates, in which U and Mo, and to lesser extent Cu and REE, were remobilized, while Th remained relatively stable. The primary mineralization is inferred to have taken place in a period from ca. 1086 to 1053 Ma based on previous Re-Os geochronology, and the secondary mineralization (remobilization) likely took place from ca. 888 to 828 Ma, as suggested by uraninite chemical ages obtained in this study and other published geochronological data. These ages correspond to tectonomagmatic events taking place in generally extensional environments, in the western margin of the Yangtze Block during the Meso- to Neoproterozoic time. The Lala Cu-Fe deposit represents a new case of IOCG deposits with U (and REE) enrichment, the economic potential of which warrants further examination. [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]

[Display omitted] • Uranium enrichment mainly occurs at the transition zone between Neogene and Quaternary sandstones. • The C-O isotopes of carbonates associated with U suggest involvement of hydrocarbons. • Disseminated uranyl silicates (coffinite) are major constituents of ore. There are several sandstone-type uraniferous basins around Tibetan Plateau. The Qaidam Basin is the largest sedimentary basin in the internal Tibetan Plateau, hosting important oil and coal resources, and has good potential for sandstone-hosted uranium resources. In this study, we examine the petrographic and mineralogical characteristics of a uranium mineralization interval at the Neogene-Quaternary transition (NQT) in the western Qaidam Basin using optical microscopy and electron probe microanalyzer (EPMA), including back scattered electron (BSE) imaging and energy-dispersive spectroscopy (EDS). The sedimentary lithofacies features and C-O stable isotopes of carbonate cement from two boreholes were analyzed, especially for the uraniferous interval. Whole-rock analyses of ores and barren rocks yielded highly variable U concentrations ranging from 1.5 to 2548 ppm. The main uranium minerals are coffinite and uraninite. The co-occurrence of uranium minerals with calcite indicates that uranium mineralization and calcite cementation were coeval. Coffinite mainly occurs at the interface between calcite cement and other minerals. The carbonates have δ13C V-PDB ranging from −29.67 to +3.93‰, suggesting involvement of organic carbon during uranium mineralization. This is also supported by the general trend of decreasing δ13C V-PDB and increasing U content from the conglomerate/coarse sandstone to the mudstone samples, and the overlap of δ13C V-PDB values between the calcite cement and organic matter in sedimentary rocks. The organic carbon may have been derived from hydrocarbons sourced from the deeper part of the basin and/or biogenic gases at shallow depths. The unconformity at the NQT may represent a favorable interval where oxidizing fluids carrying uranium met with reducing fluids carrying hydrocarbons and resulted in uranium mineralization. Thus, the NQT may serve as a guide for uranium exploration in this 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]