Formation Of Uraninite Research Articles

Constraints of the geochemical characteristics of apatite on uranium mineralization in a uraninite-rich quartz vein in the Haita area of the Kangdian region, China

Uraninite is commonly smaller than 1 mm and widespread in felsic or alkaline igneous rocks as accessory minerals. However, abundant coarse-grained euhedral uraninites, up to 1 cm in size, are intergrown with sphene, apatite, zircon, and some other accessory minerals in a quartz vein from the Haita area of the Kangdian region, China. Although previous studies have examined uraninite in detail, the genesis of the uraninite-rich quartz vein remains controversial. In this study, we investigate the relationships among euhedral single-particle apatite (ApA), aggregated apatite (ApB), and uraninite in a uraninite-rich quartz vein. TESCAN integrated mineral analysis, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry were used to investigate the nature of the ore-forming fluids of U mineralization. The two types of apatites have similar major element compositions and rare earth element patterns, whereas trace elements, such as Mn, U, and Cu, exhibit different characteristics, indicating a two-stage evolution of the ore-forming fluids. In addition, both types of apatite are fluorapatite, indicating high F concentrations in the ore-forming fluid, which favors U mobility. Changes in the concentrations of multivalent elements (such as Mn, Eu, and Ce) in the two types of apatite indicate that a slow increase in the oxygen fugacity promotes megacrystalline uraninite formation. A significant increase in the U concentrations from ∼46.56 ppm in the ApA to ∼7,169.12 ppm in the ApB likely represent an enrichment of U in the fluid. This study provides a new geochemical basis for understanding the mechanism of megacrystalline uraninite formation.

Immobilization of U(VI) by stabilized iron sulfide nanoparticles: Water chemistry effects, mechanisms, and long-term stability

• CMC-stabilized FeS (CMC-FeS) nanoparticles can rapidly remove U(VI) from water. • CMC-FeS worked effectively under normal groundwater conditions (pH, DOM, co-solutes) • Reduction accounts for 90% of U(VI) removal and S 2− and S 2 2− are primary e-donors. • High concentrations (≥5 mM) of bicarbonate or DOM may inhibit the U(VI) removal. • Immobilized U kept stable in anoxic condition, while 26% remobilized under air in 180 days. Carboxymethyl cellulose stabilized iron sulfide (CMC-FeS) nanoparticles have been shown promising for reductive immobilization of U(VI) in water and soil. This work aimed to fill some critical knowledge gaps on the effects of the stabilizer and water chemistry, reaction mechanisms, and long-term stability of stabilized uranium. The optimal CMC-to-FeS molar ratio was determined to be 0.0010. CMC-FeS performed effectively over pH 6.0–9.0, with the best removal being at pH 7.0 and 8.0. The retarded first-order model adequately interpreted the kinetic data, representing a mechanistically sounder model for heterogeneous reactants of decaying reactivity. The presence of Ca 2+ (1 mM) or bicarbonate (1 mM) lowered the initial rate constant by a factor of 1.6 and 9.5, respectively, while 1 mM of Na + showed negligible effect. Humic acid at 1.0 mg/L (as total organic carbon) doubled the removal rate, but inhibited the removal at elevated concentrations (≥5.0 mg/L). Fourier transform infrared spectroscopy, X-ray diffractometer, X-ray photoelectron spectroscopy, and extraction studies indicated that reductive conversion of UO 2 2+ to UO 2 (s) was the primary reaction mechanism, accounting for ~90% of U removal at pH 7.0. S 2− and S 2 2− were the primary electron sources, whereas sorbed and structural Fe(II) acted as supplementary electron donors. The immobilized U remained stable under anoxic conditions after 180 days of aging, while ~26% immobilized U was remobilized when exposed to air for 180 days. The long-term stability is attributed to the protective reduction potential of CMC-FeS, the formation of uraninite and associated structural resistance to oxidation, and the high affinity of FeS oxidation products toward U(VI).

Geochronology of Uraninite Revisited

Identification of uraninite provenance for the purpose of nuclear forensics requires a multifaceted approach. Various geochemical signatures, such as chondrite normalized rare earth element patterns, help identify and limit the potential sources of uraninite based on the geological setting of the uranium ore mineralization. The inclusion of accurate age determinations to discriminate geochemical signatures for natural uranium ores may help to potentially restrict geographical areas for provenance consideration. Determining a robust age for uraninite formation is somewhat difficult, due to well known, inherent difficulties associated with open system behavior that involve either uranium and/or lead loss or gain. However, open system behavior should not perturb their Pb isotopic compositions to the same degree as Pb isotopes should not fractionate during alteration processes. Here, a suite of pristine and altered samples of uraninite was examined for their Pb isotope compositions, and these yielded geologically meaningful secondary Pb–Pb isochron ages. The degree of alteration within individual uraninite samples, which is extremely variable, does not appear to affect the calculated ages. The approach adopted here yields insightful age information, and hence, is of great value for source attribution in forensic analyses of raw nuclear materials.

Development of U–Pb dating of uraninite using a secondary ion mass spectrometer: Selection of reference material and establishment of calibration method

AbstractA method for U–Pb isotopic dating using secondary ion mass spectrometer (SIMS) was developed for uraninite. Correlation between 251(UO)+/235U+ and 206Pb+/235U+ obtained by a sensitive high‐resolution ion microprobe (SHRIMP) was adopted for a calibration from secondary ion ratios (Pb+/U+) to the atomic abundance ratios (Pb/U). In this study, a uraninite sample (206Pb/238U = 0.1647) collected from Faraday mine, Bancroft, Canada, is used as a reference material for the U–Pb calibration. The established method was applied to three uraninite samples collected from the Chardon, Ecarpière, and Mistamisk mines. The calibrated 206Pb*/238U ratios of the three uraninites show correlation with Pb/U elemental ratios obtained using an electron probe microanalyzer (EPMA) (correlation coefficients: 0.98, 0.99, and 0.97, respectively), which indicates the reliability of the SHRIMP calibration method used in this study. The analysis of Ecarpière uraninite provides concordant U–Pb data, and a weighted average of the 206Pb*/238U age is 287 Ma ±8 Ma (95 % conf.) which is consistent with the previous chronological results by SIMS. Mistamisk uraninite provides discordant U–Pb data with the upper and lower intercept ages of 1 729 and 421 Ma, which correspond to uraninite formation in association with the Hudsonian orogeny and the remobilization of uranium as pitchblende, respectively. The U–Pb age of Chardon uraninite (315 Ma) is consistent with the igneous activity of Mortagne granite, but is older than the previously reported age (264 Ma). Marcasite in the Chardon uraninite altered to goethite under the oxidizing condition, which indicates that U–Pb system in the uraninite crystallized at 315 Ma was disturbed under the oxidizing condition. The established calibration method using Faraday uraninite is useful for U–Pb isotopic dating on the scale of a few micrometers to tens of micrometers, which make it possible to obtain the accurate age of uraninite.

Geochemistry and chemical dating of uraninite in the Samarkiya area, central Rajasthan, northwestern India – Implication for geochemical and temporal evolution of uranium mineralization

In the Samarkiya area, located at the central part of the Aravalli-Delhi Fold Belt (ADFB), uranium mineralization is hosted both by the basement Mangalwar Complex and the overlying supracrustal rocks of the Pur-Banera belt. The present study aims to appraise the geochemical and temporal evolution of uranium mineralization from the basement and the adjoining supracrustals in the Samarkiya area integrating textural features, geochemistry, and in situ U-Th-PbTotal dating of uraninite.Uraninite occurs as inclusions in the major rock forming minerals, viz. plagioclase, quartz, biotite, and chlorite. Based on the shape, location in the host mineral (well inside/at the grain boundary/along or connected to micro-cracks etc.) and association with other secondary uranium minerals, uraninites are classified into different groups, which are compositionally distinct, barring some exceptions. Integrating texture, geochemistry and in situ electron probe dating we propose that in addition to an old event at ∼1.88Ga in the basement rocks, there are two major events of uraninite formation at ∼1.24–1.20Ga and ∼1.01–0.96Ga in both the basement and supracrustal rocks. Although none of the pristine, unaltered uraninites that formed during the above mentioned events contain significant intrinsic minor or rare earth elements, the basement uraninites are consistently much enriched in thorium compared to those from the supracrustal. Based on the compositions, we propose that the basement uraninites formed from a high temperature magmatic/metamorphic fluid, whereas those in the supracrustal rocks crystallized from a low temperature, presumably oxidized fluid. Back-scattered electron images, X-ray elemental mapping of selected elements and EPMA spot analysis of large uraninite grains (both from the basement and the supracrustals) collectively demonstrate that subsequent to the major mineralizing event at ∼1.24–1.20Ga, the mineralized rocks were subjected to fluid-mediated alteration, which resulted in ∑REE+Y- and Si (Ca)-enrichment of existing ∼1.24–1.20Ga uraninites in the basement and supracrustal rocks, respectively. We cannot constrain the exact timing of this alteration event. However, as this event altered the ∼1.24–1.20Ga uraninites and as spot ages of the altered grains yield ages largely between ∼1.24 and 0.96Ga, it is reasonable to place this event between the second and third stages of uranium mineralization/mobilization at ∼1.20Ga and ∼1.01Ga, respectively. The last event that took place at ∼1.01–0.96Ga most likely represent an episode of recrystallization/alteration of existing uraninite leading to complete Pb-loss and resetting of the isotopic clock. However, we do not entirely reject the possibility of neo-mineralization.The discrete events deciphered from uraninite in the Samarkiya area can also be broadly linked to some major magmatic-metamorphic events, identified from other independent studies, in the ADFB. For example, the earliest ∼1.88Ga event displayed by basement uraninite is most likely related to a pervasive magmatic-metamorphic event (∼1.86–1.82Ga) that affected the basement, whereas the last/latest event ∼1.01–0.96Ga can be linked to a pervasive metamorphic event that affected perhaps the entire ADFB. This last episode can also be linked to the tectono-thermal event related to the Rodinian amalgamation. The ∼1.24–1.20Ga event appears to be somewhat enigmatic in the context of well-known geological events in the area. However, based on some very recently published data, we interpret this to be a post-peak metamorphic (∼1.37–1.35Ga) hydrothermal event or even a new metamorphic event, hitherto unknown.

Mononuclear U(IV) complexes and ningyoite as major uranium species in lake sediments

Natural attenuation of uranium in subsurface environments is usually assigned to immobilisation processes due to microbially mediated reduction of U(VI). Recent laboratory studies have established that the end products of such a process include both low solubility biogenic uraninite and more labile non-crystalline U(IV) species. Indeed, biogenic uraninite formation may be inhibited in the presence of organic or inorganic phosphoryl ligands, leading to the formation of non-crystalline U(IV)-phosphate complexes or nanoscale U(IV)-phosphate solids. Such species have been observed in shallow contaminated alluvial aquifers and can thus be suspected to form in other important environments, among which lacustrine sediments have a global environmental significance since they may represent major uranium accumulation reservoirs in riverine watersheds. Here, on the basis of microscopic, spectroscopic and chemical extraction analyses, we report the occurrence of mononuclear U(IV)-phosphate/silicate complexes, accompanied by nano-crystalline ningyoite-like U(IV)-phosphate minerals, as major scavengers for uranium in lacustrine sediments downstream from a former uranium mine in France. This observation reveals that uranium trapping mechanisms during early diagenesis of lacustrine sediments can virtually exclude uraninite formation, which has important implications for better modelling uranium cycling in natural and contaminated freshwaters. Moreover, our results raise issues concerning the long term fate of mononuclear U(IV) complexes and U(IV) phosphate nano-minerals, especially with respect to re-oxidation events. © 2016 European Association of Geochemistry.

Contrasting distributions of groundwater arsenic and uranium in the western Hetao basin, Inner Mongolia: Implication for origins and fate controls

Although As concentrations have been investigated in shallow groundwater from the Hetao basin, China, less is known about U and As distributions in deep groundwater, which would help to better understand their origins and fate controls. Two hundred and ninety-nine groundwater samples, 122 sediment samples, and 14 rock samples were taken from the northwest portion of the Hetao basin, and analyzed for geochemical parameters. Results showed contrasting distributions of groundwater U and As, with high U and low As concentrations in the alluvial fans along the basin margins, and low U and high As concentrations downgradient in the flat plain. The probable sources of both As and U in groundwater were ultimately traced to the bedrocks in the local mountains (the Langshan Mountains). Chemical weathering of U-bearing rocks (schist, phyllite, and carbonate veins) released and mobilized U as UO2(CO3)22− and UO2(CO3)34− species in the alluvial fans under oxic conditions and suboxic conditions where reductions of Mn and NO3− were favorable (OSO), resulting in high groundwater U concentrations. Conversely, the recent weathering of As-bearing rocks (schist, phyllite, and sulfides) led to the formation of As-bearing Fe(III) (hydr)oxides in sediments, resulting in low groundwater As concentrations. Arsenic mobilization and U immobilization occurred in suboxic conditions where reduction of Fe(III) oxides was favorable and reducing conditions (SOR). Reduction of As-bearing Fe(III) (hydr)oxides, which were formed during palaeo-weathering and transported and deposited as Quaternary aquifer sediments, was believed to release As into groundwater. Reduction of U(VI) to U(IV) would lead to the formation of uraninite, and therefore remove U from groundwater. We conclude that the contrasting distributions of groundwater As and U present a challenge to ensuring safe drinking water in analogous areas, especially with high background values of U and As.

The Ranger uranium deposit, northern Australia: Timing constraints, regional and ore-related alteration, and genetic implications for unconformity-related mineralisation

The Ranger uranium deposit, northern Australia: Timing constraints, regional and ore-related alteration, and genetic implications for unconformity-related mineralisation

Predominance Diagrams of Dissolved Uranium Species and log<i>f</i>O<sub>2</sub>(g)-pH Diagrams of U-PO<sub>4</sub><sup>3-</sup>-Nacl-H<sub>2</sub>O System at 25°C, PCO<sub>2</sub>=10<sup>-3.5</sup> Mpa

To explore the effect of logfO2(g), pH, uranium concentration, phosphate concentration and NaCl concentration on the predominance diagrams of dissolved uranium species and formation of solid phases containing uranium in the U-PO43--NaCl-H2O system at 25 °CandPCO2=10-3.5MPa, the thermodynamic model of this system was constructed. Based on the results of calculation, the logfO2(g)-pH diagrams were drawn. It can be found that: 1) the formation of uraninite needed enough reductive condition (about logfO2(g) < -50 ), while the formation of Na-Autunite needed the strict pH range (5<pH<8) in oxidative condition. 2)The addition of phosphate concentration could promote the formation of Na-Autunite, and inhibit the formation of U4O9(C). 3)The independent increasing of uranium concentration can lead to more kinds of uranium species.4) The variance of NaCl concentration had little impact on the formation of solid phases containing uranium.

Uranium(VI) Interactions with Mackinawite in the Presence and Absence of Bicarbonate and Oxygen

Mackinawite, Fe(II)S, samples loaded with uranium (10(-5), 10(-4), and 10(-3) mol U/g FeS) at pH 5, 7, and 9, were characterized using X-ray absorption spectroscopy and X-ray diffraction to determine the effects of pH, bicarbonate, and oxidation on uptake. Under anoxic conditions, a 5 g/L suspension of mackinawite lowered 5 × 10(-5) M uranium(VI) to below 30 ppb (1.26 × 10(-7) M) U. Between 82 and 88% of the uranium removed from solution by mackinawite was U(IV) and was nearly completely reduced to U(IV) when 0.012 M bicarbonate was added. Near-neighbor coordination consisting of uranium-oxygen and uranium-uranium distances indicates the formation of uraninite in the presence and absence of bicarbonate, suggesting reductive precipitation as the dominant removal mechanism. Following equilibration in air, mackinawite was oxidized to mainly goethite and sulfur and about 76% of U(IV) was reoxidized to U(VI) with coordination of uranium to axial and equatorial oxygen, similar to uranyl. Additionally, uranium-iron distances, typical of coprecipitation of uranium with iron oxides, and uranium-sulfur distances indicating bidentate coordination of U(VI) to sulfate were evident. The affinity of mackinawite and its oxidation products for U(VI) provides impetus for further study of mackinawite as a potential reactive medium for remediation of uranium-contaminated water.

Bioreduction of Hydrogen Uranyl Phosphate: Mechanisms and U(IV) Products

The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, redox, and precipitation reactions. Previous work demonstrated the formation of nanometer-sized hydrogen uranyl phosphate (abbreviated as HUP) crystals on the cell walls of Bacillus subtilis, a non-U(VI)-reducing, Gram-positive bacterium. The current study examined the reduction of this biogenic, cell-associated HUP mineral by three dissimilatory metal-reducing bacteria, Anaeromyxobacter dehalogenans strain K, Geobacter sulfurreducens strain PCA, and Shewanella putrefaciens strain CN-32, and compared it to the bioreduction of abiotically formed and freely suspended HUP of larger particle size. Uranium speciation in the solid phase was followed over a 10- to 20-day reaction period by X-ray absorption fine structure spectroscopy (XANES and EXAFS) and showed varying extents of U(VI) reduction to U(IV). The reduction extent of the same mass of HUP to U(IV) was consistently greater with the biogenic than with the abiotic material under the same experimental conditions. A greater extent of HUP reduction was observed in the presence of bicarbonate in solution, whereas a decreased extent of HUP reduction was observed with the addition of dissolved phosphate. These results indicate that the extent of U(VI) reduction is controlled by dissolution of the HUP phase, suggesting that the metal-reducing bacteria transfer electrons to the dissolved or bacterially adsorbed U(VI) species formed after HUP dissolution, rather than to solid-phase U(VI) in the HUP mineral. Interestingly, the bioreduced U(IV) atoms were not immediately coordinated to other U(IV) atoms (as in uraninite, UO2) but were similar in structure to the phosphate-complexed U(IV) species found in ningyoite [CaU(PO4)2·H2O]. This indicates a strong control by phosphate on the speciation of bioreduced U(IV), expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.

Uranium(VI) Reduction by Iron(II) Monosulfide Mackinawite

Reaction of aqueous uranium(VI) with iron(II) monosulfide mackinawite in an O(2) and CO(2) free model system was studied by batch uptake measurements, equilibrium modeling, and L(III) edge U X-ray absorption spectroscopy (XAS). Batch uptake measurements showed that U(VI) removal was almost complete over the wide pH range between 5 and 11 at the initial U(VI) concentration of 5 × 10(-5) M. Extraction by a carbonate/bicarbonate solution indicated that most of the U(VI) removed from solution was reduced to nonextractable U(IV). Equilibrium modeling using Visual MINTEQ suggested that U was in equilibrium with uraninite under the experimental conditions. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the U(IV) phase associated with mackinawite was uraninite. Oxidation experiments with dissolved O(2) were performed by injecting air into the sealed reaction bottles containing mackinawite samples reacted with U(VI). Dissolved U measurement and XAS confirmed that the uraninite formed from the U(VI) reduction by mackinawite did not oxidize or dissolve under the experimental conditions. This study shows that redox reactions between U(VI) and mackinawite may occur to a significant extent, implying an important role of the ferrous sulfide mineral in the redox cycling of U under sulfate reducing conditions. This study also shows that the presence of mackinawite protects uraninite from oxidation by dissolved O(2). The findings of this study suggest that uraninite formation by abiotic reduction by the iron sulfide mineral under low temperature conditions is an important process in the redistribution and sequestration of U in the subsurface environments at U contaminated sites.

Non-uraninite Products of Microbial U(VI) Reduction

A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO(4))(2)], U(2)O(PO(4))(2), and U(2)(PO(4))(P(3)O(10)) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.

Geological conditions of safe long-term storage and disposal of depleted uranium hexafluoride

The production of enriched uranium used in nuclear weapons and fuel for atomic power plants is accompanied by the formation of depleted uranium (DU), the amount of which annually increases by 35–40 kt. To date, more than 1.6 Mt DU has accumulated in the world. The main DU mass is stored as environ-mentally hazardous uranium hexafluoride (UF6), which is highly volatile and soluble in water with the formation of hydrofluoric acid. To ensure safe UF6 storage, it is necessary to convert this compound in chemically stable phases. The industrial reprocessing of UF6 into U3O8 and HF implemented in France is highly expensive. We substantiate the expediency of long-term storage of depleted uranium hexafluoride in underground repositories localized in limestone. On the basis of geochemical data and thermodynamic calculations, we show that interaction in the steel container-UF6-limestone-groundwater system gives rise to the development of a slightly alkaline reductive medium favorable for chemical reaction with formation of uraninite (UO2) and fluorite (CaF2). The proposed engineering solution not only ensures safe DU storage but also makes it possible to produce uraninite, which can be utilized, if necessary, in fast-neutron reactors. In the course of further investigations aimed at safe maintenance of DU, it is necessary to study the kinetics of conversion of UF6 into stable phases, involving laboratory and field experiments.

THE URANINITE-PYRITE ASSOCIATION, A SENSITIVE INDICATOR OF CHANGES IN PALEOFLUID COMPOSITION: AN EXAMPLE FROM THE OHRE (EGER) GRABEN, BOHEMIAN MASSIF, CZECH REPUBLIC

Trace-element variations in uraninite were employed to decipher its chemical age and to evaluate the As-rich nature of fluids during its formation at Jenikov, Teplice area, in the central part of the Cenozoic Ohře Graben, Bohemian Massif, Czech Republic. Cretaceous sandstones of the graben fill show two distinctive stages of young mineralization (less than 18 Ma), most probably related to topography-driven flow of epithermal fluid along the northwestern boundary-fault of the graben (the Krusne hory Fault). The older stage is dominated by pervasive precipitation of silica accompanied by pyrite, sphalerite, galena, uraninite and kaolinite, whereas the younger stage is represented by barite with pyrite inclusions and fluorite. The uraninite formed together with microcrystalline quartz (Qtz–4) at the end of the first stage. Uraninite of subgroup A shows high As/Pb values (14.6–69.9), higher Ca (1.02–1.99 wt.%), lower Ti (0.27–0.44 wt.%), and other trace elements compatible with the structure of uraninite. Uraninite of subgroup B shows low As/Pb values (2.5–8.4), lower Ca (0.39–0.90 wt.%), higher Ti (0.43–8.94 wt.%), and the presence of elements incompatible with the structure (Ti, Fe). A significant correlation between As and Pb was found in both subgroups. The calculated apparent radiometric age of the uraninite samples shows a broad dispersal, suggesting the presence of common lead, which was corrected using the correlation between the contents of As and Pb tot (Pb common + Pb radiogenic ). This procedure allowed us to assign an age of uraninite formation of about 0.5 Ma (± 0.6 Ma), which fits the geological data, and to demonstrate its formation during a single episode of mineralization. An approximately five-fold increase in As content of the fluid phase during uraninite and pyrite formation can be inferred from the correlation between the intragroup As/Pb values and the observed intragrain As max /As min values in the associated arsenian pyrite. If amounts of common lead are introduced into uraninite in conjunction with another conservative element, a correlation between the conservative element and total lead can be used to decipher the age of the mineralization and to track element variations in the parent fluid phase.

Two-Fluids (Oxic and Reduced) Model for the Formation of Uraninite in Early Proterozoic Quartz-Pebble Conglomerates of the Elliot Lake District, Ontario, Canada

Uraninite in quartz-pebble conglomerate beds of ~2.8 to ~ 2.2 Ga has been considered to be detrital in origin by many previous researchers because of the presence of rounded grains of uraninite and pyrite. The survival of these minerals during fluvial transport and deposition has been used as an important line of evidence for an anoxic atmosphere prior to 2.2 Ga, because both minerals are unstable under an oxygen-rich environment. An alternative model, diagenetic/hydrothermal model, has been suggested for uraninite and pyrite in the q u a r t s p e b b l e c o n g l o m e r a t e beds in the Wi twa te r s rand dis t r ic t , South Af r ica (e.g. Barnicoat et al., 1997). If uraninite and pyrite are not of detrital in origin, the occurrence of this type of deposits may not be used as evidence for an anoxic atmosphere. On the other hand, if these minerals were formed by diagenetic or hydrothermal processes, a series of redox reactions must have occurred to form uranium-bearing minerals, such as the reactions 6+ between U -beanng, oxic groundwater and reductants (e.g. organics and sulphides) to precipitate UO2 (uraninite). Such formational processes would have been favoured under an oxic atmosphere. The objective of this research is, therefore, to better understand the origin(s) of uraninite in quartz conglomerate beds of >2.2 Ga, and to constraint the atmospheric pO2 level. This objective has been pursued mostly from a detailed study, using ore microscopy and electron microprobe analyser, of the textures and paragenesis of U-Th-bearing minerals in ores from the Stanleigh mine, Elliot Lake district, Ontario, Canada. The ores are hosted in quartzpebble conglomerate beds of the Matinenda Formation which is ~2.4 Ga in age. Paragenesis and textures of U-Th.bearing minerals

Experimental evidence of uraninite formation from diagenesis of uranium-rich organic matter

This study focuses on the experimental diagenetic evolution of an uranium-rich lignite from the Coutras uranium deposit (Gironde, France) located in Eocene sediments. Three samples containing 6 to 15.5 wt% uranium were analysed and compared to two barren samples located outside the ore deposit in a stratigraphically equivalent unit. Uranium in the mineralized samples is present as uranyl-carboxylic complexes. Dry thermal maturation was simulated in a cold-seal autoclave at 150, 200, 250, 300, and 350°C at 100 MPa for 24 h. With increasing temperature, uranyl-carboxylic complexes decompose while uraninite forms. The chemical composition of the experimentally matured barren samples, when plotted in a Van Krevelen diagram (H/C vs. O/C), parallels the natural trend of land-derived type-Ill organic matter evolution, while mineralized samples are characterized by a higher loss of hydrogen above 250°C. Reduction to form uraninite is directly related to the dehydrogenation of lignite. The resulting uraninite has a similar unit cell edge to that of natural uraninites observed in sedimentary environments. Uraninite not only forms in the pore space but also replaces the lignite constituents. The results demonstrate that uraninite may form during thermal evolution of uranium-rich organic matter during diagenesis.

Chronology of magmatism, skarn formation, and uranium mineralization, Mary Kathleen, Queensland, Australia

Uranium mineralization is hosted in a metasedimentary sequence intruded by felsic and mafic bodies in which the zircon U-Pb systems are 1,730 to 1,740 m.y. old. Felsic dikes characterized by exceptionally high uranium contents crystallized at essentially the same time (1,737 + or - 15 m.y.). Primary Rb-Sr systematics in these igneous rocks are profoundly disturbed. U-Pb discordia establish age of uranium mineralization as 1,550 + or - 15 m.y. Derivation of uranium protore in the host rocks prior to regional metamorphism and 1,550-m.y. uraninite formation was probably by ca 1,740-m.y. hydrothermal activity.--Modified journal abstract.