Uraninite Crystals Research Articles

Uranium Mineralogical and Chemical Features of the Na-Metasomatic Type Uranium Deposit in the Longshoushan Metallogenic Belt, Northwestern China

The Longshoushan Metallogenic Belt (northwestern China) is known for its word-class Jinchuan Ni-Cu sulfide (Pt) deposit and is also an important uranium metallogenic belt. The Jiling uranium deposit in this belt is a typical Na-metasomatic uranium deposit, which rarely occurs in China. Mineralization in the Jiling uranium deposit is hosted in granitoids that have suffered a Na-metasomatic alteration. There are three kinds of uranium minerals, including uraninite, pitchblende, and coffinite in the Jiling uranium deposit. Pitchblende is the predominant uranium mineral. Integrating the mineralogy and geochemistry of uranium minerals, and in situ electron microprobe analyzer (EMPA) U-Th-Pb chemical dating, we aimed to unravel the age and nature of the mineralization, to decipher the characteristics of the hydrothermal alteration and the U mineralization process. Based on the microtextural features and compositional variations, primary uraninite was altered to uraninite A and B, and fresh pitchblende was altered to pitchblende A and B. The best-preserved uraninite crystals displayed a euhedral-shape with high Pb and low SiO2, CaO, FeO, and Al2O3 contents, and was interpreted as primary uraninite. The EMPA U-Th-Pb chemical ages revealed that uraninite may have formed at 435.9 ± 3.3 Ma. High ThO2 + ΣREE2O3 + Y2O3 contents illustrated that the best preserved uraninite crystallized at a high temperature. Altered pitchblende A showed a relatively brighter gray color in backscattered electron (BSE) images and with a lower SiO2 content than B. Three analysis spots of the fresh pitchblende showed low contents of ΣSiO2 + CaO, indicating no obvious alteration. EMPA U-Th-Pb chemical dating gave a mean chemical age of 361 Ma. The low Th + ΣREE2O3 contents indicated that this pitchblende formed at a relatively low temperature. According to the different characteristics of occurrence and chemical composition, the coffinite in the Jiling uranium deposit can be divided into coffinite A and B, respectively. The compositional variation of the fresh and altered uraninite and pitchblende indicated that both uraninite and pitchblende underwent at least two discrete hydrothermal fluid alterations. The U mineralization was divided into two stages; uraninite was formed at a high temperature and possibly from a magmatic-hydrothermal fluid during ore stage I. Then, pitchblende was formed at a low temperature, during ore stage II. According to the petrographic observations and their chemical compositions, coffinite A and B resulted from the alterations of uraninite and pitchblende, respectively.

Release, Migration, Sorption, and (Re)Precipitation of U during Peraluminous Granite Alteration under Oxidizing Conditions in Central Portugal

In this work, in order to study the release, migration, sorption, and (re)precipitation of uranium (U) during alteration under oxidizing conditions, we carried out a systematic study using scanning electron microscopy, X-ray maps, and electron microprobe analyses on uranium minerals—such as uraninite, coffinite, saleeite, meta-saleeite, and thorite—and U-bearing minerals—such as xenotime, monazite, apatite, and zircon—from unaltered and altered Variscan peraluminous granites and related hydrothermal brecciated uranium–quartz veins. The paragenetic sequence of the granite and the mineralized quartz veins from Vale the Abrutiga is presented. Uraninite is magmatic in origin and occurs mainly in unaltered granite; it is rare in altered granite, and was not found in the mineralized quartz veins. Uraninite from the altered granite was fractured and hydrated; it had radioactive damage halos filled with late pyrite, U–S-bearing phases, and Fe oxyhydroxides; its analytical totals were also lower than in the uraninite from the unaltered granite. The alteration zones and crystal rims were poorer in U (86.7 wt.% UO2) than in the cores and unaltered zones (90.2 wt.% UO2), and some uraninite crystals were replaced by coffinite, which resulted from uraninite alteration. The U contents in the coffinite crystals ranged between 65.0 wt.% UO2 in the rims to 84.0 wt.% UO2 in the cores of the crystals. Thorite was found in all of the granite samples, and its composition was variable from 0.5 wt.% UO2 to 10.4 wt. % UO2. Some thorite seemed to be primary, whereas the other thorite was related to the granite alteration, replaced apatite and monazite, was associated with xenotime, and filled the fractures of several minerals. In the altered granite, thorite had low UO2 contents (0.46 wt. %) in the fractured crystal zones. Monazite from the altered granite had a pervasive porosity; some crystals were formed by the alteration of apatite, and were frequently replaced by thorite. Monazite and xenotime from the altered granite and hydrothermal veins had lower U contents than these minerals from the unaltered granite. In the altered granite, xenotime crystals were zoned, and their cores were richer in U than the rims. Apatite from the altered granite was fractured, showed dissolution, and had lower U and P contents than the apatite from the unaltered granite. In the quartz veins, apatite crystals were replaced by uranium phosphates and had high U contents (~1.1 wt. % UO2). In the quartz veins, zircon rims had an extraordinary U enrichment (up to 18 wt. % UO2). The most altered rims of chlorite and anatase from the quartz veins were partially replaced by U-bearing Fe oxyhydroxides containing up to 5.7 wt. % UO2. Meteoric water warmed by deep circulation through granite faults, shear zones, and quartz veins became enriched in U, P, and Mg due to the solubilization of mainly uraninite, coffinite, thorite, monazite, apatite, and chlorite. Uranium from these solutions was later adsorbed on Fe oxyhydroxides and the weathered surfaces of anatase, zircon, and apatite, or precipitated as saleeite and meta-saleeite on the surface of Fe minerals and the apatite-weathered surface due to local saturation.

Chemical and textural interpretation of late-stage coffinite and brannerite from the Olympic Dam IOCG-Ag-U deposit

AbstractThe Olympic Dam iron-oxide copper-gold-silver-uranium deposit, South Australia, contains three dominant U minerals: uraninite; coffinite; and brannerite. Microanalytical and petrographic observations provide evidence for an interpretation in which brannerite and coffinite essentially represent the products of U mineralizing events after initial deposit formation at 1.6 Ga. Marked compositional and textural differences between the various types of brannerite and coffinite highlight the role of multiple stages of U dissolution and reprecipitation.On the basis of petrography (size, habit, textures and mineral associations) and compositional variation, brannerites are divided into four distinct groups (brannerite-A, -B, -C and -D), and coffinite into three groups (coffinite-A, -B and -C). Brannerite-A ranges in composition from what is effectively uraniferous rutile to stoichiometric brannerite, and has elevated (Mg +Mn + Na + K) and (Fe + Al) compared to other brannerite types. It displays the most diverse range of morphologies, including complex irregular-shaped aggregates, replacement bands, and discrete elongate seams. The internal structure of brannerite-A consists of randomly-oriented hair-like needles and blades. Brannerite-B (>5 μm in size) is generally prismatic and typically associated with baryte and REY minerals (REE+Y= REY). Brannerite-C and -D are both associated with Cu-(Fe)-sulfides and are typically composed of irregular masses and blebs (10–50 μm in size) with a more uniform or massive internal structure. Brannerite-D is distinct from -C and always contains inclusions of galena. Brannerite-B to -D all contain elevated ΣREY, with brannerite-B and -C having elevated As, and brannerite-D having elevated Nb.All coffinite is typically globular (each globule is 2–10 μm in size) to collomorphic in appearance. Coffinite-A ranges from discrete globules to collomorphic bands completely encompassing quartz. Coffinite-B is always found with uraninite, and includes collomorph coffinite enveloped by massive uraninite, as well as aureoles of coffinite on the margins of uraninite crystals. Coffinite-C is associated with brannerite and REY minerals. The majority of coffinite is heterogeneous.Brannerite and coffinite have probably precipitated as part of a late-stage hydrothermal U-event, which might have involved the dissolution and/or reprecipitation of earlier precipitated uraninite, or could represent the products of a later U mineralizing event. Evidence which supports formation of late-stage coffinite and brannerite includes: (1) low-Pb contents of both minerals; (2) coffinite is commonly found on the edges of uraninite, implying later deposition; and (3) coffinite is often found on the edge of brannerite aggregates, suggestive of brannerite precipitation occurred before coffinite. Moreover, there are many features (e.g. banding, scalloped edges, alteration rinds, variable compositions etc.) indicative of hydrothermal alteration processes.

Mobilization and agglomeration of uraninite nanoparticles: A nano-mineralogical study of samples from the Matoush Uranium ore deposit

The occurrence of uraninite nanoparticles in the alteration zones of uranium ore deposits suggests potential mobilization of U(IV) under reducing conditions, which is important for understanding the mobility of uranium in contaminated sites and potential repositories for nuclear waste. This study investigates the occurrence of uraninite nanoparticles in the outer alteration zone of the Matoush uranium ore deposit, Quebec, Canada. Selected samples with finely disseminated uraninite from the outer alteration zone of the deposit are examined by X-ray fluorescence spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy on specimens prepared using the focused ion beam milling technique. Uraninite nanoparticles occur as single particles, in clusters, and in larger aggregates in close association with the Cr-rich phases chromceladonite (Cr-rich mica), ideally KMgCr3+(Si4O10)(OH)2, eskolaite, ideally Cr23+O3, bracewellite, Cr3+OOH, and an amorphous Cr-rich oxide matrix as well as with fluorapatite and galena. Nanoparticles on the surface and in the outer rim of single uraninite crystals indicate the growth of larger uraninite crystals via crystallization through particle attachment and Oswald ripening. The flow texture of the uraninite nanoparticles in the amorphous Cr-rich oxide matrix, their aggregation on the surface of nanocrystals of bracewellite, the absence of products of a redox reaction involving U(VI) and Fe(II), and the occurrence of amorphous Fe-depleted alteration layers between uraninite and eskolaite, and uraninite and Cr-rich mica indicate that the uraninite nanoparticles have been mobilized under reducing conditions (leaching of Fe2+ from the alteration layer) at low T (amorphous character of the alteration layer) after the main mineralization event from the center of the mineralization to the outer parts of the Matoush dike complex. These results indicate that fluids can mobilize U(IV) under reducing conditions in the form of uraninite nanoparticles albeit over limited distances. The potential mobilization of these nanoparticles may also explain the occurrence of proximal mineralized zones in U-ore deposits that lack common products resulting from the reduction of U(VI) by Fe2+ (e.g., hematite and other Fe3+-phases).

Replacement of Uraninite By BorniteViaCoupled Dissolution-Reprecipitation: Evidence From Texture and Microstructure

Abstract The occurrence and nature of rhythmically intergrown crystals of uraninite and bornite from the Olympic Dam iron oxide-copper-gold (IOCG)-U-Ag deposit, South Australia, is reported. Distinct zones within primary, euhedral uraninite crystals have been replaced by bornite and minor fluorite leaving a skeleton of uraninite, infilled with these minerals. The partially replaced uraninite crystals are always closely associated with locally abundant bornite and fluorite. The textural features of the intergrowth are consistent with partial replacement of uraninite by bornite via a coupled dissolution-reprecipitation reaction driven by a F-rich and Cu-Fe-sulfide-bearing hydrothermal fluid rather than a form of oscillatory growth or exsolution from a U–Cu–Fe–S solid solution. The crystallographic relationship between the parent uraninite and the daughter bornite and fluorite were explored by electron backscattered diffraction, as all three minerals share common crystal structural features, despite their chemical diversity. Generally speaking, the crystallographic orientation of the uraninite parent is initially inherited by the replacing bornite, but later the orientation of the bornite changes.

Nanoscale relationships between uranium and carbonaceous material in alteration halos around unconformity-related uranium deposits of the Kiggavik camp, Paleoproterozoic Thelon Basin, Nunavut, Canada

Concentrations of 7% U and 1% Cu were identified in massive, brecciated, and amorphous carbonaceous materials (CM) characterized by strongly negative values of carbon stable isotopes (δ13C=−39.1‰ relative to PDB). The anomalies are restricted to clay alteration halos developed in Neoarchean Woodburn Lake group metagreywacke that is the predominant host of unconformity-related uranium (U) deposits in the Kiggavik exploration camp. Petrographic and microstructural analyses by SEM, X-ray Diffraction, HRTEM and RAMAN spectroscopy identified carbon veils, best described as graphene-like carbon, upon which nano-scale uraninite crystals are distributed. CMs are common in U systems such as the classic Cretaceous roll-front deposits and the world-class Paleoproterozoic unconformity-related deposits. However, the unusual spatial and textural association of U minerals and CM described herein raises questions on mechanisms that may have been responsible for the precipitation of the CM followed by crystallization of U oxides on its surfaces. Based on the characteristics presented herein, the CMs at Kiggavik are interpreted as hydrothermal in origin. Furthermore, the nanoscale organization and properties of these graphene-like layers that host U oxide crystallites clearly localized U oxide nucleation and growth.

Uraniferous bitumen nodules in the Talvivaara Ni–Zn–Cu–Co deposit (Finland): influence of metamorphism on uranium mineralization in black shales

In the central part of the Fennoscandian Shield, the Talvivaara Ni–Zn–Cu–Co deposit, hosted by Palaeoproterozoic metamorphosed black schists, contains low uranium concentrations ranging from 10 to 30 ppm. The Talvivaara black schists were deposited 2.0–1.9 Ga ago and underwent subsequent metamorphism during the 1.9–1.79 Ga Svecofennian orogeny. Anhedral uraninite crystals rimmed by bitumen constitute the main host of uranium. U–Pb secondary ion mass spectrometry dating indicates that uraninite crystals were formed between 1,878 ± 17 and 1,871 ± 43 Ma, during peak metamorphism. Rare earth element patterns and high Th content (average 6.38 wt%) in disseminated uraninite crystals indicate that U was concentrated during high temperature metamorphism (>400 °C). The formation of bitumen rims around uraninite may be explained by two distinct scenarios: (a) a transport of U coincident with the migration of hydrocarbons or (b) post-metamorphic formation of bitumen rims, through radiolytic polymerization of gaseous hydrocarbons at the contact with uraninite.

Application of remote sensing in exploration for uranium mineralization in Gabal El Sela area, South Eastern Desert, Egypt

This research aims at integrating remote sensing data and field studies to prospect for radioactive materials at Gabal El Sela area, South Eastern Desert of Egypt. This area is a part of the Arabo-Nubian shield, which is constituted by Late Precambrian basement rocks and developed during the Pan African tectono-thermal event. Geological interpretation of Landsat ETM+ image and field studies revealed that the study area is mainly covered by ophiolitic ultramafic rocks, intermediate metavolocanics, granodiorites, biotite granites and garnetiferous muscovite granites. These rocks are injected by pegmatitic and quartz veins and cut by acidic and basic dykes. The processed Landsat ETM+ data enable also to identify uraniferous alteration zones hosted in the granitic rocks (using band ratio 5/7, 4/5, 3/1 in RGB, density slicing and supervised classification techniques). The measured reflectance values (DNs) of six spectral bands of Landsat ETM+ for all exposed rock types in the study area show that the alteration zones have the highest reflectance values. The study recorded the presence of uranium mineralizations in alteration zones associating with the granitic rocks occurring along ENE–WSW trending shear zones. Three uranium-bearing alteration zones hosted in granitic rocks were detected. These zones are characterized by ferrugination, silicification, grizenization and kaolinitization. The most promising alteration zones (Site 1) occurs at the northeastern part of Gabal El Sela and is characterized by very high radioactive anomalies (up to 5000ppm eU). Microprobe analyses of samples collected from these alteration zones reveal the presence of uraninite crystals. It seems that the high fractures density of the granitic rocks acted as good channels for the ascending hydrothermal fluids and the percolating meteoric water, that leached the uranium minerals disseminated all over the rocks and redeposited them along the fractures of the shear zones. This study demonstrates the utility of orbital remote sensing data for finding unknown mineralized zones in the Eastern Desert of Egypt and similar arid regions.

Whiteite-(CaMnMn), CaMnMn2Al2[PO4]4(OH)2·8H2O, a new mineral from the Hagendorf-Süd granitic pegmatite, Germany

AbstractWhiteite-(CaMnMn), CaMnMn2Al2[PO4]4(OH)2·8H2O, is a new hydrous phosphate of Ca, Mn and Al, which is closely related to both jahnsite-(CaMnMn) and the minerals of the whiteite group. It is monoclinic, P2/a, with a = 15.02(2), b = 6.95(1), c =10.13(3) Å, β = 111.6(1)°, V = 983.3(6) Å3, Z = 2 (from powder diffraction data) or a = 15.020(5), b = 6.959(2), c = 10.237(3) Å, β = 111.740(4)°, V = 984.3(5) Å3, Z = 2 (from single-crystal diffraction data). The mineral was found in the Hagendorf Süd granitic pegmatite (Germany) as small (up to 0.5 mm in size) crystals elongated on a and tabular on {010}. The crystals are either simply or polysynthetically twinned on {001}. They crystallize on the walls of voids within altered zwieselite crystals or form coronas (up to 1 mm in diameter) around cubic crystals of uraninite. The mineral is transparent, colourless to pale yellow (depending on Al–Fe3+ substitution), with a vitreous lustre and a white streak. The cleavage is perfect on {001}, the fracture is stepped and the Mohs hardness is 3½. In transmitted light, the mineral is colourless; dispersion was not observed. Whiteite-(CaMnMn) is biaxial (+), α = 1.589(2), β = 1.592(2), γ = 1.601(2) (589 nm), 2Vmeas = 60(10)°, 2Vcalc = 60.3°. The optical orientation is X = b, Z^a = 5°. The calculated and measured densities are Dcalc = 2.768 and Dmeas = 2.70(3) g cm–3, respectively. The mean chemical composition determined by electron microprobe is Na2O 0.53, MgO 0.88, Al2O3 11.66, P2O5 34.58, CaO 4.29, MnO 17.32, FeO 8.32, ZnO 2.60 wt.%, with H2O 19.50 wt.% (determined by the Penfield method), giving a total of 99.68 wt.%. The empirical formula calculated on the basis of four phosphorus atoms per formula unit, with ferric iron calculated to maintain charge balance, is (Ca0.63Zn0.26Na0.14)Σ1.03(Mn0.60Fe0.402+)Σ1.00(Mn1.40Fe0.372+Mg0.18Fe0.063+)Σ2.01(Al1.88Fe0.123+)Σ2.00[PO4]4(OH)2·7.89H2O. The simplified formula is CaMnMn2Al2[PO4]4(OH)2·8H2O. The mineral is easily soluble in 10% HCl at room temperature. The strongest X-ray powder-diffraction lines [listed as d in Å (I) (hkl)] are as follows: 9.443(65)(001), 5.596(25)(011), 4.929(80)(210), 4.719(47)(002), 3.494(46)(400), 2.7958(100)(022). The crystal structure of whiteite-(CaMnMn) was refined for a single crystal twinned on (001) to R1 = 0.068 on the basis of 5702 unique observed reflections. It is similar to the structures of other members of the whiteite group. The mineral is named for the chemical composition, in accordance with whiteite-group nomenclature.

U[sbnd]Mo mineralization potential in Pan-African granites, southwestern Cameroon: Economic geology of the Ekomédion prospect

Uranium and molybdenum mineralization at the Ekomédion prospect of southwestern Cameroon occurs within pegmatite pods in Pan-African two-mica granite. The two-mica granite and the associated pegmatite experienced brittle deformation. Mineralization occurs as veinlets following early fractures and as irregular bodies intimately associated with pegmatite pods. Syn-tectonic alteration comprises albitization, silicification, ferruginization, seriticitization and chloritization. Mica in the ore zone is less magnesian and F-rich relative to mica in the host two-mica granite. Uraninite crystals are anhedral to subhedral and molybdenite is commonly encapsulated by phengitic muscovite. The two-mica granite is a strongly fractionated high-K, low P ferroan granite. The pegmatite pods hosting UMo mineralization are suggested to have been derived from the fractionation of magma parental to the two-mica granite as observed from Rare Earth Element (REE) patterns and the trend shown by Th/U ratios of the host granite and the pegmatite. This implies that uranium and molybdenum fractionated and concentrated preferentially into the late stage pegmatitic fluid that formed the pods and subsequently into the hydrothermal fluid phase. This therefore points to the two-mica granite as well as the metamorphic wall rocks as potential sources for UMo mineralization in southwestern Cameroon.

Crystal chemistry and radiation-induced amorphization of P-coffinite from the natural fission reactor at Bangombe, Gabon

Phosphorous-rich coffinite, U(Si, P)04Ti20, from the natural nuclear reactor at Bangombe, Gabon (depth 12.25 m), has been examined as an important primary mineral and alteration product of uraninite under reducing conditions. Based on electron microprobe analyses (EMPA) and textural relationships, two distinct types of coffinite have been identified: (1) P-coffinite-(i) [with P2C)5 and (REE+Y)20, as high as 9.84 and 8.66 wt%, respectively] replaces uraninite and has a chemical formula of (FJ0.84±.0.05Y, REE 0.12±0.02Ca0.10±0.02Th 0.003±0.002)Σ1.07±o.o6(Si 0.41±0.07P0.34±0.03S 0.08±0.05) Σ0.84±0.05O4 and; (2) coffmite-(ii), lacks uraninite inclusions [with P20 5 and (Y+REE)203 up to 1.45 and 1.79 wt%, respectively] and has a chemical formula of (U0.78±0.02Ca 0.05±0.003YREE0.03±0.01 Th0.002 ±0.001) Σ0.87±0.02(Si 1.02±0.02P0.06±0.01) Σ 1.08±0.01O4- The EMPA elemental maps reveal a homogeneous distribution of P, Si, Nd, and FT in P-coffinite-(i). Charge-balance calculations indicate that S substitution for Si in the coffinite structure results in the enhancement of the incorporation of P and REEs. High EMPA totals (95-100 wt%), suggest that water is not an essential component of the coffmite structure. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveal the decomposition of single crystals of uraninite into 50 nm grains during the alteration process to P-coffmite-(i). High-resolution transmission electron microscopy (F1RTEM) and selected area electron diffraction (SAED) patterns confirm that P-cofiinite-(i) has the coffinite structure. However, P-coffmite-(i) has a large amorphous fraction probably due to radiation damage caused by a-decay events. The calculated cumulative dose for both types of coffinite varies from 1.6-1.9 x 10 18 (a-decay events/mg), which is equivalent to 134.2-161.5 displacement per atom (dpa) that have accumulated most probably during the past 800 million years.

Biomineralization of Uranium: A Simulated Experiment and Its Significance

Abstract A simulated experimental reduction of UVI and the synthesis of uraninite by a sulfate‐reducing bacteria, Desulfovibrio desulfuricans DSM 642, are first reported. The simulated physicochemical experimental conditions were: 35°C, pH=7.0–7.4, corresponding to the environments of formation of the sandstone‐hosted interlayer oxidation‐zone type uranium deposits in Xinjiang, NW China. Uraninite was formed on the surface of the host bacteria after a one‐week's incubation. Therefore, sulfate‐reducing bacteria, which existed extensively in Jurassic sandstone‐producing environments, might have participated in the biomineralization of this uranium deposit. There is an important difference in the order‐disorder of the crystalline structure between the uraninite produced by Desulfovibrio desulfuricans and naturally occurring uraninite. Long time and slow precipitation and growth of uraninite in the geological environment might have resulted in larger uraninite crystals, with uraninite nanocrystals arranged in order, whereas the experimentally produced uraninite is composed of unordered uraninite nanocrystals which, in contrast, result from the short time span of formation and rapid precipitation and growth of uraninite. The discovery has important implications for understanding genetic significance in mineralogy, and also indicates that in‐situ bioremediation of U‐contaminated environments and use of biotechnology in the treatment of radioactive liquid waste is being contemplated.

Enrichment of Uranium inside Wood: a Natural Analog from a Sandstone-hosted Roll-type Uranium Ore Deposit

AbstractFurniture contaminated with uranium will be disposed of, together with U-bearing waste, in ageological repository such as the WIPP site in New Mexico. It is important to understand the effect of the wooden furniture on the migration of uranium in order to predict long-term behavior of uranium in a geological repository environment. In this paper, we present natural uraninite-bearing carbonized wood pieces from a sandstone-hosted roll-type uranium ore deposit in NW China. Results from SEM and TEM observations show that there are nanometer sized, and micron-sized, uraninite crystals that have accumulated on cell walls of the carbonized wood. Some uranitite crystrals display oval and round shapes that may be the result of microbial-induced reduction of uranium from groundwater. The wood carbonized fragments are the most uranium-rich “phase.” In some areas, aggregates of pyrite crystals occur withthe uraninite. It is proposed that organic components from the decay of the wood cells provide nutrientsfor the anaerobic bacteria to grow. The wood pieces with the bacteria inside may serve as scavengers of uranium because of the local reducing chemical environment.

Mesostructural and microstructural evidences for a two stage tectono-metallogenetic model for the uranium deposit at Mika, Northeastern Nigeria: A research note

The Mika uranium deposit is of the Central Massif vein-type. The deposit consists of a series of subparallel veins, similar to Lilljuthatten deposit (Sweden), with variable ore grade. Two main structural trends occur within the vicinity of Mika namely: (1) vertical N160E-N180E or N-S normal fault trend in which the brecciated granite has been permeated and cemented by silica ribbons; (2) N130E trend associated with reddish brown hematitized siliceous breccias filling open fractures. The N130E trend is younger, and it is a local, passive shear trend developed at the bends of the N-S faults. On a microscopic scale, samples from the N-S trend are rich in crude cubic-tetragonal prismatic crystals of uraninite and/or pitchblende ores occurring in association with a low-temperature greenschist mineral assemblage of chlorite, epidote, and mica. Conversely, samples from the N130E trend show widespread kaolinitisation, goethite formation and layering reminiscent of groundwater activity at shallow depths near the earth’s surface. The main uranium mineral in this zone is yellowish autunite. The model proposed here envisages that the mineralization was related to two separate processes. A first mineralization episode developed simultaneous with the greenschist mineral assemblage, and a second episode was related to remobilization and subsequent mineral concentration in the passive N130E open fractures by groundwater to form autunite. Such a model has far reaching implications for exploration. Although the secondary minerals along the N130E fractures are more obvious in the field, the potentially economic primary mineral concentrations may be concealed at greater depth in the N-S structures.

The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16

In an underground nuclear waste repository, the chemical behavior of some stored fission products and actinides depends on the redox conditions during their long-term evolution. In this respect, radiolysis is an important phenomenon which can significantly modify the local redox conditions. The Oklo natural fission zones are good examples where the effect of radiolysis can be deduced from a mineralogical and geochemical study. Zones 10 and 16 were studied because they are located at depth of 270 m in an area devoid of any recent water circulation and not subject to the effect of the lateritic alteration occurring elsewhere in this area. In zone 10, there is a marked evolution of the UPbFeS mineralogy from the center to the periphery of the reactor zone. In the center, uraninite shows silicification and coffinitisation with the formation of galena and native lead; the PbO content of uraninite can be as much as 20 wt%. In the periphery of the reactor zone, some radiogenic lead is present as minium (Pb 30 4) and in Pb-bearing calcite. In the surrounding sandstones, hematite is widespread. In zone 16, the mineral paragenesis is generally comparable with that of zone 10 but with some differences. Galena is the only Pb-bearing mineral associated with uraninite crystals. The PbO content of uraninite is always <7 wt%. In the periphery of the alteration zone, barite partly replaces quartz. In the reactor zone, hematite is sometimes replaced by pyrite. In an area where the fission zone 10 is in contact with sandstones devoid of organic matter, H 2OH 2O 2 and H 20H 2 ± CH 4 inclusions were observed in healed microcracks in the detrital quartz grains. Based on microthermometric measurements, the salinity of the aqueous solution ranges from 0.2 to 18 wt% eq. NaCl. Raman analysis of the gas phase indicates that the hydrogen to oxygen ratio differs from an inclusion to the other. The presence of H 2− and O 2-bearing fluid inclusions confirms the existence of water radiolysis in the reactor zone. In zones rich in organic matter, oxygen was preferentially consumed in reactions with organic matter and its radiolytic products. In the absence of organic matter and its radiolytic products, oxidizing conditions prevailed and hematite and minium have formed. Consequently, presence of highly oxidizing conditions has important consequences on uranium and fission products mobility. Water radiolysis is also evidenced during the subsequent 1.97 By history of the reactor. During the fission reactions, redox conditions deduced from Pb-bearing minerals indicate that the sulfur fugacity was very low in zone 10 as shown by the formation of minium and higher in zone 16 with the formation of barite and pyrite. In this paper we discuss the geochemical consequences of such contrasting redox zones and the resulting redox heterogeneity created by water radiolysis.

Characterization of a La-Ce-Sr-Ca aluminous hydroxy phosphate in nuclear zone 13 in the Oklo uranium deposit (Gabon)

A La-Ce-Sr-Ca aluminous hydroxy phosphate of the goyazite-florencite-crandallite series was observed in the nuclear reactor zone 13 in the Oklo uranium deposit (Gabon). It has a trigonal elementary cell with a = 6.98 ± 0.03 A and c = 16.35 ± 0.04 A (space group P3m). It occurs in a U-rich sample depleted in 235U (235U/238U = 0.0047). Its chemical composition, measured by ion microprobe analyses, shows that it contains fission products (Zr, LREE) whereas the isotopic composition of Ba and Sr is natural. Ion microprobe imaging indicates that the nearby uraninite crystals are depleted in fissiogenic LREE. This study shows that the migration of some fission products, probably related to a dolerite dike intrusion, is limited by the formation of aluminous hydroxy phosphates.

Organic free radicals and micropores in solid graphitic carbonaceous matter at the Oklo natural fission reactors, Gabon

The presence, concentration, and distribution of organic free radicals as well as their association with specific surface areas and microporosities help characterize the evolution and behavior of the Oklo carbonaceous matter. Such information is necessary in order to evaluate uranium mineralization, liquid bitumen solidification, and radionuclide containment at Oklo. In the Oklo ore deposits and natural fission reactors carbonaceous matter is often referred to as solid graphitic bitumen. The carbonaceous parts of the natural reactors may contain as much as 65.9% organic C by weight in heterogeneous distribution within the clay-rich matrix. The solid carbonaceous matter immobilized small uraninite crystals and some fission products enclosed in this uraninite and thereby facilitated radionuclide containment in the reactors. Hence, the Oklo natural fission reactors are currently the subjects of detailed studies because they may be useful analogues to support performance assessment of radionuclide containment at anthropogenic radioactive waste repository sites. Seven carbonaceous matter rich samples from the 1968 ± 50 Ma old natural fission reactors and the associated Oklo uranium ore deposit were studied by electron spin resonance (ESR) spectroscopy and by measurements of specific surface areas (BET method). Humic acid, fulvic acid, and fully crystalline graphite standards were also examined by ESR spectroscopy for comparison with the Oklo solid graphitic bitumens. With one exception, the ancient Oklo bitumens have higher organic free radical concentrations than the modern humic and fulvic acid samples. The presence of carbon free radicals in the graphite standard could not be determined due to the conductivity of this material. Oklo solid bitumen samples were subjected to various pressures of O 2, a paramagnetic gas. O 2-organic free radical interactions, as revealed by ESR spectral line broadening, indicate that the organic free radicals of the Oklo solid bitumens are, with one exception, accessible to O 2. This indicates that the organic free radicals are located at or very near the surfaces of these organic solids, including the internal surfaces of pores. The presence of pores was deduced from the BET analyses. The presence of porosity and related permeability has important implications for fluid flow through the uraniferous graphitic bitumen affecting radionuclide containment at Oklo. However, the very small average pore sizes (18.5–38.1Å) together with the non-wettability by aqueous solutions of the uraniferous graphitic bitumen indicate that the pores did not serve as effective conduits for the transport of radionuclides by aqueous solutions through and out of the solid bitumen matrix. The ESR data obtained from the O 2 experiments is comparable to the BET N 2 adsorption data, indicating that the ESR O 2 experiment reflects physical adsorption, and not chemisorption, of O 2 on the Oklo bitumen surfaces.

Weathering Conditions and Behavior of Fission Products in Oklo Reactors

Two billion years ago, natural fission reactions occured in the uranium ore deposit of Oklo (Gabon), mainly due to the high concentrations of fissile 235U. At that time, high uranium contents (40-60 % U) allowed reaching criticality when oxidised uranium bearing fluids met reduced fluids accompanying the migration of hydrocarbons. Natural fission reactors consist of a core made of uraninite crystals embedded in a clay matrix (mainly chlorites). The thickness of the clay layer range between few centimetres to 2 meters. Fission products are mostly located in the uraninite grains but newly crystallised minerals such as phosphate minerals, clays and metallic aggregates retain some proportions of migrated fissiogenic elements [1].

Petrographic relationships between mineral phases and bitumen in the Oklo Proterozoic natural fission reactors, Gabon

AbstractSeveral of the Proterozoic natural fission reactors at Oklo, Gabon contain abundant organic matter (bitumen), with which much of the reactor uranium is associated. An understanding of the petrography of the bitumen is important in assessing its role in element retention following fissiogenic reactions. The bitumen is replacive and includes vestiges of detrital grains and clays of the host sandstones. It also contains several generations of mineral phases related to the migration of uranium and daughter lead, and other elements mobilized by hydrothermal activity in the reactor zones. The minerals include (primary) uraninite precipitated after reduction of uranium by organic matter, silicates concentrated in radiation halos around the uraninite, and native lead, galena and further uraninite which migrated from the primary uraninite. The silicates are illitic clays in the immediate vicinity of the reactors, and chloritic more distant from the reactors. Authigenic clays in radiation halos are compositionally distinct from earlier host rock clays. Local migration of uranium and lead is evident as dispersion of the elements through the radiation halos; cross-cutting veinlets of uraninite, native lead and galena, including veinlets extending directly from the primary uraninite crystals; and concentrations of the elements at the margins of bitumen masses. The limit of observed uranium dispersion is within clay immediately beyond the bitumen margins.

Organic matter and containment of uranium and fissiogenic isotopes at the Oklo natural reactors

SOME of the Precambrian natural fission reactors at Oklo in Gabon contain abundant organic matter1,2, part of which was liquefied at the time of criticality and subsequently converted to a graphitic solid3,4. The liquid organic matter helps to reduce U(VI) to U(IV) from aqueous solutions, resulting in the precipitation of uraninite5. It is known that in the prevailing reactor environments, precipitated uraninite grains incorporated fission products. We report here observations which show that these uraninite crystals were held immobile within the resolidified, graphitic bitumen. Unlike water-soluble (humic) organic matter, the graphitic bituminous organics at Oklo thus enhanced radionu-clide containment. Uraninite encased in solid graphitic matter in the organic-rich reactor zones lost virtually no fissiogenic lan-thanide isotopes. The first major episode of uranium and lead migration was caused by the intrusion of a swarm of adjacent dolerite dykes about 1,100 Myr after the reactors went critical. Our results from Oklo imply that the use of organic, hydrophobic solids such as graphitic bitumen as a means of immobilizing radionuclides in pretreated nuclear waste warrants further investigation.