Uranium-bearing Sandstones Research Articles

Pore structure characterization and permeability prediction of uranium-bearing sandstone based on digital core

The permeability of the ore-bearing layer is an important indicator affecting the in-situ leaching (ISL) of uranium-bearing sandstone, which is related to various factors such as pore shape, distribution, and size. In order to study the effect of pore structure on seepage in low-permeability uranium-bearing sandstone, CT scanning tests were conducted to create a 3D digital core based on scanning images and to calculate the fractal dimension using the box counting dimension method, which integrated fractal theory to define the core samples' pore structure. The permeability prediction was realized based on the porosity-permeability model and the fractal theory model. Results indicated that this type of sandstone is obviously characterized by pore connectivity, large differences in distribution, and strong microscopic inhomogeneity. The pores are dominated by micro- and nano-pores, as well as small pores, accounting for 90 %; macropores are few in number, but the diameters of their single pores are large. The distribution of pore structure in this type of sandstone exhibits a good fractal characteristic; the three-dimensional fractal dimensionality is 2.044–2.310. The porosity-permeability model was established, and permeability prediction was realized by combining the fractal theory to provide theoretical support for determining the values of well field parameters in ISL.

Progress on enhancing seepage-leaching mass-transfer research for in-situ leaching mining of low-permeability uranium-bearing sandstone: a review

Progress on enhancing seepage-leaching mass-transfer research for in-situ leaching mining of low-permeability uranium-bearing sandstone: a review

Seepage characteristics of the leaching solution during in situ leaching of uranium

Investigating the seepage characteristics of the leaching solution in the ore-bearing layer during the in situ leaching process can be useful for designing the process parameters for the uranium mining well. We prepared leaching solutions of four different viscosities and conducted experiments using a self-developed multifunctional uranium ore seepage test device. The effects of different viscosities of leaching solutions on the seepage characteristics of uranium-bearing sandstones were examined using seepage mechanics, physicochemical seepage theory, and dissolution erosion mechanism. Results indicated that while the seepage characteristics of various viscosities of leaching solutions were the same in rock samples with similar internal pore architectures, there were regular differences between the saturated and the unsaturated stages. In addition, the time required for the specimen to reach saturation varied with the viscosity of the leaching solution. The higher the viscosity of the solution, the slower the seepage flow from the unsaturated stage to the saturated stage. Furthermore, during the saturation stage, the seepage pressure of a leaching solution with a high viscosity was greater than that of a leaching solution with a low viscosity. However, the permeability coefficient of the high viscosity leaching solution was less than that of a low viscosity leaching solution.

Characteristics of fluid inclusions and fluid coupling mineralization of the Pengyang uranium deposit, Ordos Basin

The Pengyang uranium deposit, located in the Tianhuan Depression in the southwestern Ordos Basin, is a large-scale sandstone-type uranium deposit in aeolian sediments. The uranium-bearing rock series in the deposit is mainly sandstone and conglomerate from the Lower Cretaceous Luohe Formation, which is dominated by desert facies. Based on our investigation of the geological characteristics of this deposit, a systematic study of the type, characteristics of the ore-forming fluid and ore genesis was conducted. Through the petrographic study of inclusions, three stages of inclusions have been identified in the quartz overgrowth, carbonate cements, and secondary microfractures in the ore-bearing sandstone of the Luohe Formation, and the third-stage inclusions in the secondary microfractures or late calcite cements represent the ore-forming fluids. There are three types of inclusions captured from ore-forming fluids: liquid-dominated aqueous inclusions, liquid-dominated hydrocarbon inclusions, and natural gas inclusions. Their gaseous components are mainly CH4 and H2O, with a small amount of N2. The homogenization temperatures of liquid-dominated aqueous inclusions range from 58 to 147 ℃, and the salinity ranges from 0.35 to 6.88 wt%. The homogenization temperature of liquid-dominated hydrocarbon inclusions mainly exhibits two peaks (at 60–140 ℃ and 160–190 ℃). Our studies of carbon-hydrogen–oxygen isotopes indicate that the formation of uranium-bearing sandstone is closely related to the participation of organic matter such as oil and gas. Based on the comprehensive researches of fluid inclusions and isotope analysis, it is concluded that the ore-forming fluid of the Pengyang uranium deposit is a mixture of meteoric water and hydrocarbon fluid. The deep hydrocarbon fluid traveled upward along the structural fractures, acting as a reductive agent and participating in the oxidation–reduction reaction of the Pengyang uranium deposit. Although hydrocarbon fluid can bring some uranium from deep area, the uranium from the provenance area of the basin may be the main source of mineralization in the Pengyang uranium deposit. The Pengyang uranium deposit is a sandstone-type uranium deposit related to the reduction of oil and gas. Uranium mineralization is mainly caused by the coupling between the uranium-bearing oxygen-rich fluid infiltrated from the surface and the deep hydrocarbon fluid.

Lithology Identification of Uranium-Bearing Sand Bodies Using Logging Data Based on a BP Neural Network

Lithology identification is an essential fact for delineating uranium-bearing sandstone bodies. A new method is provided to delineate sandstone bodies by a lithological automatic classification model using machine learning techniques, which could also improve the efficiency of borehole core logging. In this contribution, the BP neural network model for automatic lithology identification was established using an optimized gradient descent algorithm based on the neural network training of 4578 sets of well logging data (including lithology, density, resistivity, natural gamma, well-diameter, natural potential, etc.) from 8 boreholes of the Tarangaole uranium deposit in Inner Mongolia. The softmax activation function and the cross-entropy loss function are used for lithology classification and weight adjustment. The lithology identification prediction was carried out for 599 samples, with a prediction accuracy of 88.31%. The prediction results suggest that the model is efficient and effective, and that it could be directly applied for automatic lithology identification in sandstone bodies for uranium exploration.

Characteristics of Altered Ilmenite in Uranium-Bearing Sandstone and Its Relationship with Uranium Minerals in the Northeastern Ordos Basin

Characteristics of Altered Ilmenite in Uranium-Bearing Sandstone and Its Relationship with Uranium Minerals in the Northeastern Ordos Basin

Ore-controlling structures of sandstone-hosted uranium deposit in the southwestern Ordos Basin: Revealed from seismic and gravity data

We present the seismic and 3-D gravity study to define the ore-controlling structures of sandstone-hosted uranium deposits in the southwestern Ordos Basin, the second largest sedimentary basin in China. The instantaneous amplitude and frequency are extracted from seismic reflection data, and the seismic impedance is obtained by joint inversion of well-log and post-stack seismic data. Results of the seismic attributes and inversion show that the uranium-bearing deposits exhibit low instantaneous frequencies, low instantaneous amplitudes and medium–low seismic impedances. The scratch analysis method is employed on the residual gravity anomaly generated from the gravity bouguer anomaly by using the potential field separation. The edge coefficient image of the residual gravity anomaly shows that the fault structures are composed of NE-trending faults in the west and EW-trending faults in the east of the study area. The 3-D gravity inversion constrained by the seismic data was performed to reveal that uranium-bearing deposits are characterized by low gravity anomaly, which is related to the larger porosity of the uranium-bearing sandstones. Geophysical results show that the uranium-bearing deposits are controlled by flower structures, EW-trending normal faults and NW-trending thrust faults. Flower structures are produced by the tectonic inversion as a result of the uplift and NE-directed growth of the Tibetan Plateau. Ore-controlling faults cut crossing the floor of the ore-bearing strata as channels, along which the ore-forming fluids move upward and are enriched in the sandstone strata with high permeability and porosity in Luohe Formation. In addition, the roof and floor of the ore-bearing strata in Luohe Formation are characterized by high impedance and gravity anomaly, which has a sealing effect on uranium deposits. Then we predicted the thickness of the sandstones in Luohe Formation, as the sandstone is a prerequisite for mineralization.

Fractal kinetic characteristics of uranium leaching from low permeability uranium-bearing sandstone

The pore structure of uranium-bearing sandstone is one of the critical factors that affect the uranium leaching performance. In this article, uranium-bearing sandstone from the Yili Basin, Xinjiang, China, was taken as the research object. The fractal characteristics of the pore structure of the uranium-bearing sandstone were studied using mercury intrusion experiments and fractal theory, and the fractal dimension of the uranium-bearing sandstone was calculated. In addition, the effect of the fractal characteristics of the pore structure of the uranium-bearing sandstone on the uranium leaching kinetics was studied. Then, the kinetics was analyzed using a shrinking nuclear model, and it was determined that the rate of uranium leaching is mainly controlled by the diffusion reaction, and the dissolution rate constant (K) is linearly related to the pore specific surface fractal dimension (DS) and the pore volume fractal dimension (DV). Eventually, fractal kinetic models for predicting the in-situ leaching kinetics were established using the unreacted shrinking core model, and the linear relationship between the fractal dimension of the sample's pore structure and the dissolution rate during the leaching was fitted.

Alteration, uranium occurrence state, and enrichment mechanism of the Cretaceous Luohe Formation, southwestern Ordos Basin, western China

To ascertain the characteristics of mineral assemblages associated with uranium mineralization and enrichment mechanism of deep sandstone-type uranium deposits, drill-core logging, petrography and mineralogy, scanning electron microscopy (SEM), electronic probe micro analysis (EPMA), whole-rock analysis, X-ray diffraction (XRD) were used to identify the types, redox state of uranium minerals, and typical uranium-related mineral assemblages in the sandstone of the Cretaceous Luohe Formation, southwestern Ordos Basin. The results showed that the clastic grains in the gray and red uranium-bearing sandstone have undergone relatively strong alteration, while alteration of the clastic grains in the gray and red calcareous sandstone is relatively weak. The alteration minerals mainly include pyrite, anatase, kaolinite, chlorite, calcite, apatite, and feldspar. Uranium contents is highly consistent with S, P2O5, CaO, and FeO concentrations in the vertical direction and has a weak correlation with the total organic carbon. Uranium ores are mainly pitchblende and titanium-bearing uranium minerals. The Luohe Formation experienced transformation from a weak alkaline oxidation environment during the diagenetic stage to a weak acid reduction conditions in the early mineralization stage and then to a weak alkaline reduction environment in the middle–late mineralization stages. Typical alteration minerals associated with uranium ores include two types, which are formed in acid reduction and alkaline reduction redox stages, respectively. The alteration minerals related to acidic fluid process are pyrite, anatase, kaolinite, and apatite; those related to alkaline fluid mechanism consist of calcite and chlorite. Deep-seated uranium mineralization was formed by both acidic and alkaline reducing fluids, being dominated by acidic fluid conditions. In-depth investigation of the alteration and uranium ore minerals are of great scientific value to infer the fluid type of mineralization stage and reveal the enrichment mechanism of uranium deposits.

Geochemical Characteristics, Palaeoenvironment and Provenance of Uranium-Bearing Sandstone in the Sifangtai Formation, Northern Songliao Basin, Northeast China

During the Cretaceous period of the northern Songliao Basin (northeast of China), a 100 m thick layer of fluvial-phase sandstone (Sifangtai Formation) with uranium potential was widely deposited, but its geochemical characteristics, paleoenvironment, and provenance remain unknown. This research proposes a new set of relevant geochemical data for sandstones to investigate their paleoenvironment, provenance and tectonic setting. The results revealed that: (1) The sandstone of the Sifangtai Formation was dominated by feldspar lithic sandstone. Geochemical signatures demonstrate that these sandstones have a high silicon content (SiO2 = 68.30~83.60 wt%) and total alkali content, but are poor in magnesium and calcium. They are also enriched in Rb, Th, U, K and LREE, and depleted HFSE (e.g., Nb, Ta), with crustal magmatic source. (2) The paleoclimate discriminant indicated that the rocks of the Sifangtai Formation might that the climate of Sifangtai Formation is semi-arid, and the chemical weathering of the source rocks is weak under the semi-arid climate environment. (3) The combination of element Sr/Ba, 100 MgO/Al2O3 and the combination of v/v + Ni, V/Cr, Ni/Co, and Sr/Cu indicated that the paleo-water medium was deposited in an oxygen-rich freshwater environment when the Sifangtai Formation was deposited. (4) The discriminate diagrams showed that almost all the sandstones of the Sifangtai Formation fell in the range of the active continental margin, indicating that the source area of the sandstones of Sifangtai Formations is an active continental margin tectonic environment, and the source is a felsic rock developed in the Xiaoxing’an Ridge and Zhangguangcailing area.

Multi-scale visualization of uranium-rich domains dispersed in U-Zr mineralization of sandstone-type (Břevniště, Czech Republic)

Multi-level visualization of U-Zr mineralization in sandstones is discussed, aiming at the identification of mineral phases, imaging of their distribution using 2D and 3D projections, and quantification of the presence of U-rich domains. For this purpose, uranium-bearing sediments with anomalous concentrations of Zr, Nb ± Fe, Sr were studied (Břevniště deposit, Czech Republic). The applied multi-instrumental approach included selection of the most suitable areas in core samples (highest U and Zr concentrations) using X-ray fluorescence analysis (XRF). The presence of numerous detrital and authigenic accessories, hardly discernible using optical methods, was confirmed with the use of automated mineralogical analysis (TIMA). Hydrozircon and metacolloidal mixed phases were shown to be the carriers of uranium. Their close relationship to sulfides, Fe oxyhydroxides and clay minerals was documented using phase distribution maps. X-ray micro-computed tomography (μCT) was used for the visualization of internal structures of uranium-bearing sandstones. Based on a series of thresholding of μCT data, segmentation of selected samples was performed with subsequent visualization of selected mineral phases. The combination of TIMA and μCT analysis allowed for an effective discrimination of U-rich areas and quantification of their volume. This approach provides new possibilities in imaging elementary complex disseminated uranium mineralizations.

Full-scale pore size distribution features of uranium-bearing sandstone in the northwest of Xinjiang, China.

As an important nuclear fuel, uranium in sandstone uranium deposits is mainly extracted by in situ leaching. The porosity of sandstone is one of the important indexes determining in situ leaching efficiency. Moreover, the microscopic pore size distribution (PSD) of the uranium-bearing layer has an important effect on porosity. It is necessary to feature the pore structure by various techniques because of the different pore types and sizes in the uranium layer. In this paper, combined with nitrogen gas adsorption, nuclear magnetic resonance techniques and scanning electron microscopy, the full-scale PSD features of uranium-bearing sandstone in the northwest of Xinjiang are effectively characterized. The results show that pores structure of uranium-bearing sandstone include dissolution pores (d ≤ 50 nm), intergranular pores (50 nm < d ≤ 200 µm) and microfractures. Intergranular pores of 60 nm and 1 µm are the significant contributors to pore volume. The effects of the pore volume of two pore types (dissolution pores and intergranular pores) on the porosity of uranium-bearing sandstone are analysed. The results show that intergranular pores have the greater influence on the porosity and are positively correlated to the porosity. Dissolution pores have little effect on the porosity, but it is one of the key factors for improving uranium recovery. Moreover, the greater the difference of PSD between sandstones, the stronger the interlayer heterogeneity of uranium-bearing sandstone. This kind of interlayer heterogeneity leads to the change of permeability in the horizontal direction of strata. It provides a basis for a reasonable setting of well type and well spacing parameters.

3D characterization of porosity and minerals of low-permeability uranium-bearing sandstone based on multi-resolution image fusion

In the process of in situ leaching of uranium, the microstructure controls and influences the flow distribution, percolation characteristics, and reaction mechanism of lixivium in the pores of reservoir rocks and directly affects the leaching of useful components. In this study, the pore throat, pore size distribution, and mineral composition of low-permeability uranium-bearing sandstone were quantitatively analyzed by high pressure mercury injection, nuclear magnetic resonance, X-ray diffraction, and wavelength-dispersive X-ray fluorescence. The distribution characteristics of pores and minerals in the samples were qualitatively analyzed using energy-dispersive scanning electron microscopy and multi-resolution CT images. Image registration with the landmarks algorithm provided by FEI Avizo was used to accurately match the CT images with different resolutions. The multi-scale and multi-mineral digital core model of low-permeability uranium-bearing sandstone is reconstructed through pore segmentation and mineral segmentation of fusion core scanning images. The results show that the pore structure of low-permeability uranium-bearing sandstone is complex and has multi-scale and multi-crossing characteristics. The intergranular pores determine the main seepage channel in the pore space, and the secondary pores have poor connectivity with other pores. Pyrite and coffinite are isolated from the connected pores and surrounded by a large number of clay minerals and ankerite cements, which increases the difficulty of uranium leaching. Clays and a large amount of ankerite cement are filled in the primary and secondary pores and pore throats of the low-permeability uranium-bearing sandstone, which significantly reduces the porosity of the movable fluid and results in low overall permeability of the cores. The multi-scale and multi-mineral digital core proposed in this study provides a basis for characterizing macroscopic and microscopic pore-throat structures and mineral distributions of low-permeability uranium-bearing sandstone and can better understand the seepage characteristics.

Radiation damage in quartz from uranium-bearing sandstones

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Research on diagenesis of the sandstone-type uranium deposits in Dongsheng area, Ordos Basin

Synthetic methods of thin section petrography, scanning electron microscope, electron microprobe, energy spectrum analysis, cathodoluminescence, isotopic analysis and temperature measuring for fluid inclusions were used in analyzing sandstone samples collected from the Zhiluo Formation in order to fully understand the diagenesis evolution and the mineralizing response as well as the genesis of the uranium-bearing sandstone in Dongsheng area. The result shows that (1) the sandstone include lithic silicarenite, feldspathic litharenite and litharenite; (2) the authigenic minerals include clay minerals, carbonate minerals, siliceous and ferric minerals; (3) the physical property of sandstone is obviously controlled by diagenesis; and (4) the sandstone with favorable physical property is propitious to migration and storage of ore-forming fluid, and finally, forming the ore deposit. The sandstone of the Zhiluo Formation had undergone the early diagenetic stage (periods A and B) and the epidiagenetic stage. The evolution of diagenetic environment is in the order of acidic oxidation, alkalescent deoxidization, acidity to transitional environment of oxidation-deoxidization and acidity-alkalescence. The uranium exists in forms of pre-enrichment uranyl ion, active uranyl ion, dispersive adsorptive uranium and uranium mineral, respectively. In addition, the authors also hold that the formation of the sandstone-type uranium is not only related to the oxidation-deoxidization environment, but also closely related to the acidic-alkaline transitional environment, which are propitious to uranium mineralization in sandstone.

Genesis of stratiform UMo deposits in the Karoo Basin of South Africa

Evaluation of existing theories on the formation of uranium orebodies in the Karoo shows that no single model can adequately explain their distribution, size and geochemistry, so that a combination of various ore-forming processes should be considered instead. Extra-basinal leaching of granitic and volcanic source rocks and the expulsion of fresh, uraniferous pore waters from tuffs interbedded with the argillaceous overbank and lacustrine sediments during early diagenesis probably account for most of the uranium, although the breakdown of feldspars, glass shards and heavy minerals within the sandstones probably contributed to the widespread occurrence of uranium deposits in the Karoo. Mineralization in some of the main uranium-bearing sandstones in the beaufort Group seems to correlate in time with major paroxysms of the Cape orogeny. Tectonism and uplift in these provenance areas probably coincided with an increase in volcanic activity as well as the orographic rainfall, which would have enhanced chemical weathering and erosion of the source rocks and amplified the volume of ash and detritus supplied to the basin. Low-temperature ore fluids migrated down the paleoslope along permeable conduits in channel sands, possibly driven in part by seismic pumping associated with the early stages of the Cape orogeny. Locally, the uranium was precipitated by H 2S accumulating along the permeable pathways and in the deeper portions of channels where plant material was protected from oxidation by stagnant groundwaters. Some mineralization could have taken place at the interface between slightly oxidized ore fluids and reduced groundwaters. Calcite gangue introduced by the mineralizing fluids drastically reduced the permeability of the host sandstones and effectively sealed the mineralization in place in the Beaufort Group, but in the Molteno and Elliot Formations, coarser grain sizes and a lack of carbonate allowed the redistribution of uranium by circulating groundwaters. This process apparently continued until at least after the intrusion of dolerite dykes and sills during the Jurassic.

Radiation-damage rims in quartz from uranium-bearing sandstones

Radiation-damage rims in quartz from uranium-bearing sandstones

Radiation-Damage Rims in Quartz from Uranium-Bearing Sandstones

ABSTRACT Using cathodoluminescence (CL) mode of a scanning electron microscope (SEM), radiation damage in quartz is made visible in uranium-vanadium-bearing sandstones located in the Salt Wash Member of the Morrison Formation (Jurassic) from the Colorado Plateau (U.S.A.). Detrital quartz grains surrounded by uranium minerals display radiation-damage rims. Three concentric rims with thicknesses of 37 µm, 24 µm and 16 µm have been observed. The rims are similar to halos surrounding uranium- and thorium-rich accessory minerals observed by Owen (1988). They are due to alpha particles, which create damage when penetrating a medium. The width of the rims agrees well with the Bragg-Kleeman rule and corresponds to the most energetic alpha particles in the 238U decay series Sandstones entirely cemented by vanadium clays also exhibit radioactive rims, which demonstrates that uranium has been removed during diagenesis. These results have important implications for the uranium ore genesis in the Salt Wash sandstones, because they imply an ancient episode of uranium leaching prior to recent weathering. Therefore, cathodoluminescence in sandstones may be useful in recognizing ancient zones or episodes of accumulation and leaching of alpha-emitting nuclides from the uranium series.

Geochemical hosts of solubilized radionuclides in uranium mill tailings

The solubilization and subsequent resorption of radionuclides by ore components or by reaction products during the milling of uranium ores may have both economic and environmental consequences. Particle-size redistribution of radium during milling has been demonstrated by previous investigators; however, the identification of sorbing components in the tailings has received little experimental attention. In this study, uranium-bearing sandstone ore was milled, on a laboratory scale, with sulfuric acid. At regular intervals, filtrate from this suspension was placed in contact with mixtures of quartz sand and various potential sorbents which occur as gangue in uranium ores; the potential sorbents included clay minerals, iron and aluminum oxides, feldspar, fluorspar, barite, jarosite, coal, and volcanic glass. After equilibration, the quartz sand-sorbent mixtures were separated from the filtrate and radioassayed by gamma-spectrometry to determine the quantities of 238U, 230Th, 226Ra, and 210Pb sorbed, and the radon emanation coefficients. Sorption of 238U was low in all cases, with maximal sorptions of 1–2% by the bentonite- and coal-bearing samples. 230Th sorption also was generally less than 1%; maximal sorption here was observed in the fluorspar-bearing sample and appears to be associated with the formation of gypsum during milling. 226Ra and 210 Pb generally showed higher sorption than the other nuclides - more than 60% of the 26Ra solubilized from the ore was sorbed on the barite-bearing sample. The mechanism (s) for this sorption by a wide variety of substrates is not yet understood. Radon emanation coefficients of the samples ranged from about 5 to 30%, with the coal-bearing samples clearly demonstrating an emanating power higher than any of the other materials.

In situ calibration of a high-resolution gamma-ray borehole sonde for assaying uranium-bearing sandstone deposits

A method is presented for assaying radioactive sandstone deposits in situ by using a high-resolution borehole gamma-ray spectrometer. Gamma-ray photopeaks from the same spectrum acquired to analyze a sample are used to characterize gamma-ray attenuation properties, from which a calibration function is determined. Assay results are independent of differences between properties of field samples and those of laboratory or test-hole standards generally used to calibrate a borehole sonde. This assaying technique is also independent of the state of radioactive disequilibrium that usually exists in nature among members of the natural-decay chains.