Journal of Earth Science  2019, Vol. 30 Issue (1): 142-157   PDF    
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Relations of Uranium Enrichment and Carbonaceous Debris within the Daying Uranium Deposit, Northern Ordos Basin
Fan Zhang1,2, Yangquan Jiao1,2, Liqun Wu1,2, Hui Rong1,2, Longhui Wang3    
1. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China;
2. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;
3. No. 208 Geological Brigade, Inner Mongolia Nuclear Industry, Baotou 014000, China
ABSTRACT: Carbonaceous debris (CD) is widely distributed in the sandstone of the Daying Uranium Deposit, northern Ordos Basin, and coexists with uranium minerals, which provides a favorable case for studying their relationship. Vitrinite reflectance (VR), macerals, moisture, volatile matter, ash, total sulfur (St) and uranium concentration of CD within the sandstone were studied. The results show that VR ranges from 0.372%Ro to 0.510% Ro with an average value of 0.438%Ro, indicating that CD is in the stage of lignite. The contents of vitrinite (V), inertinite (I) and minerals range from 83.18%-99.48%, 0-7.70%, and 0.34%-15.72%, respectively, with the corresponding average value of 95.51%, 1.34%, and 3.15%, respectively which indicates that V is the major maceral. Moisture on air dried basis (Mad), volatile matter yield on dry, ash-free basis (Vdaf), ash yield on dried basis (Ad) and St mostly range from 7.95%-16.09%, 44.70%-66.54%, 4.84%-26.24% and 0.24%-1.12%, respectively, while their average values are 12.43%, 53.41%, 16.57% and 0.77%, respectively. It suggests that CD is of medium-high moisture, super-high volatile matter, low-medium ash and low sulfur. Uranium concentration ranges from 29 ppm to 92 ppm with an average value of 50 ppm, and uranium concentration increases with the decreased distance to CD. On the whole, Mad and Vdaf decrease with increasing burial depth, which indicates that CD experienced the burial metamorphism. However, Mad and Vdaf obviously decrease in uranium-rich areas whereas Ad and St noticeably increase. Comprehensive studies suggest that there is a certain relationship between uranium enrichment and CD. CD in the stage of lignite helps the adsorption of uranium. On one hand, radioactivity uranium enrichment makes organic matter maturation increase with a decrease in moisture and volatile matter. On the other hand, an increase in organic matter maturation, caused by radioactivity uranium enrichment, results in an increase in uranium minerals, which is instructive in the study of regional uranium mineralization and metallogenic regularity.
KEY WORDS: carbonaceous debris    coal property parameters    Daying Uranium Deposit    Ordos Basin    

0 INTRODUCTION

The close coexistence between organic matter (OM; e.g., coal, bitumen, and petroleum) and uranium in sedimentary basins is reported (Riegler et al., 2016; Dai et al., 2015a, b ; Havelcová et al., 2014; Cuney, 2010; Deng et al., 2006; Gauthier-Lafaye and Weber, 1993). It has been documented that in basins OM plays a key role of concentration, complexation and reduction in uranium mineralization (Bone et al., 2017; Janot et al., 2016; Dai et al., 2015b; Ortaboy and Atun, 2014; Bordelet et al., 2013; Douglas et al., 2011; Giordano, 2000; Spirakis, 1996). Günther et al. (2011) determined stability constants for the uranium (Ⅵ)-OM bond using UO22+ compounds with OH, CO32– and HA species. Riegler et al. (2016) suggested that the solid properties of carbonaceous material provided a favorable environment for uranium oxide and metallic sulfide from the perspective of nanoscale. Recently, more attention is paid to biologically induced uranium precipitation (Li et al., 2017; Xu et al., 2015; Greenwood et al., 2013; Haferburg and Kothe, 2007; Reith, et al., 2007; Bazylinski and Frankel, 2003). For example, microorganisms (e.g., sulfate reducing bacteria (SRB)) can accumulate and immobilize uranium biologically (e.g., Li et al., 2017; Xu et al., 2015; Newsome et al., 2014; Yang et al., 2012; Min et al., 2005, 2001; Lovley et al., 1993). However, uranium also causes radioactive alterations of uraniferous OM with a higher reflectance and structural changes (e.g., localised cleavage of OM-OH bonds and damage to OM-functional groups; Cumberland et al., 2016; Sýkorová et al., 2016; Smieja-Król et al., 2009; Drennan and Robb, 2006; Schlepp et al., 2001; Landais, 1996; Nagy et al., 1993; Leventhal et al., 1986; Sassen, 1984; Gentry et al., 1976). Xu et al. (2015) found that coffinite- produced radiation resulted in the carbonization of the surrounding OM and the lattice defects of the coffinite itself within black shale-hosted polymetallic sulfide ore layer. Havelcová et al. (2014) observed that there was a decrease in H/C, O/C ratio with the increased uranium concentrations within coals in the Sokolov Basin.

It has been widely reported that the structure of OM partly breaks down, and organic matter maturation increases for uranium-bearing coal, carbonaceous debris (CD), and bitumen by the methods of optical microscopy, electron microscopy, organic elemental analysis, pyrolysis GC-MS, and micro- spectroscopic techniques (Cumberland et al., 2016; Sýkorová et al., 2016; Havelcová et al., 2014; Smieja-Król et al., 2009; Drennan and Robb, 2006; Landais et al., 1990; Leventhal et al., 1986; Liu et al., 1980; Gentry et al., 1976). However, CD is rarely studied by the methods of coal property analyses. CD within the sandstone is abundant in the Daying Uranium Deposit, and bears a spatial relationship with uranium, which provides a favorable chance for investigating relations of uranium enrichment and CD. In view of this, coal property of CD within uranium-bearing strata in the Daying Uranium Deposit, northern Ordos Basin, was investigated in an effort to obtain the information about connections between uranium enrichment and CD, which is instructive for further ore exploration.

1 GEOLOGICAL SETTING

The Ordos Basin, northern China is situated between Qilian-Qinling fold belt and Inner Mongolia-Daxing'anling fold belt with an area of about 250 000 km2 (Fig. 1a), and developed an intracontinental depression basin during the Mesozoic (Du et al., 2016, 2013; Lü et al., 2010; Xue et al., 2010; Ren et al., 1994). The Daying Uranium Deposit, a super-large sandstone-type one, is located in the Yimeng uplift, northern Ordos Basin (Fig. 1a, Xie et al., 2016). The folds in the approximate east-west trend and local small normal faults develop in the study area (Fig. 1b), and the strata that drill holes penetrate from bottom to top are mainly Jurassic Yan'an, Zhiluo, Anding formations and Cretaceous (Jiao et al., 2016; Fig. 1c).

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Figure 1. (a) Regional tectonic setting of the Ordos Basin (the location of the Ordos Basin in China is shown in the insert); (b) distributions of bottom buried depth of the Zhiluo Formation and locations of sampling wells in the Daying Uranium Deposit; (c) stratigraphic column of the Daying Uranium Deposit, showing the distributions of the Yan'an, Zhiluo, Anding formations and cretaceous strata (after Jiao et al., 2016). SQ. Third-order sequence; SB. sequence boundary; LST. lowstand systems tract; EST. expanding systems tract; HST. highstand systems tract; M. mudstone; St. siltstone; FS. fine sandstone; MS. medium sandstone; CS. coarse sandstone; Cong. conglomerate.

The Zhiluo Formation is the main uranium-bearing strata with the thickness of about 270 m, and divided into three members, i.e., lower member of Zhiluo Formation (J2z1), middle member of Zhiluo Formation (J2z2) and upper member of Zhiluo Formation (J2z3) (Jiao et al., 2005a). J2z1 is primarily composed of gray sandstone and mudstone, a few scattered coals as well as abundant retented sediments, i.e., CD and mud-gravel, corresponding to lowstand system tract (LST). J2z2 is mostly made up of gray-green, purple-red mudstone, corresponding to lacustrine-expanding system tract (EST). J2z3 is mainly formed of coarse sandstone, corresponding to highstand system tract (HST). Moreover, J2z1 is the major uranium reservoir, and subdivided into lower sub-member in lower member of Zhiluo Formation (J2z1-1) and upper sub-member in lower member of Zhiluo Formation (J2z1-2). J2z1-1 is braided river and braided river delta sedimentary systems, while J2z1-2 is meandering river and meandering river delta sedimentary systems (Jiao et al., 2005a, b ; Fig. 1c), and fine-grained sediments (e.g., coal and mudstone) are underlain at the bottom of J2z1-2 (Fig. 1c).

In the study area, sand bodies of J2z1 comprise paleo- oxidation sand bodies (i.e., paleo-oxidation zone) and reduction sand bodies (i.e., reduction zone, Fig. 2). The paleo-oxidation sand bodies are mostly composed of green, gray-green sandstones and scattered red sandstones with few CD and pyrites (Li et al., 2007), while the reduction sand bodies are made of gray, light gray sandstones with abundant CD and pyrites (Figs. 3a, 3b and 3c). Uranium mineralization occurs in gray sandstones close to the paleo-oxidation zone, i.e., gray uranium-bearing sandstones with abundant CD and pyrites (Xue et al., 2010, 2009), and distributes in lenticular or tabular shape (Zhu et al., 2003; Fig. 2). Uranium-bearing sandstones are mostly characterized by pyritization, montmorillonitization, sericitization and late calcitization (Xie, 2016; Figs. 3a and 3c). Uranium enrichment occurs in the forms of uranium minerals and scatted uranium absorbed in organic matter and pyrites. In general, coffinite is the main uranium mineral (Cun et al., 2016; Xie, 2016).

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Figure 2. Regional lithologic geochemical profile in the Daying Uranium Deposit and locations of sampling points (the location of the sampling section is seen Fig. 1b). CD. Carbonaceous debris; IUE. industrial uranium enrichment; BUE. boundary uranium enrichment; AUE. abnormal uranium enrichment; NUE. no uranium enrichment.

Closely associated with uranium minerals, CD is widely distributed in sandstone in the forms of lumps and bands (Fig. 3). The lumpy or banded CD is the most common sedimentary phenomenon, with certain brittleness and vitreousluster, ranging from 1 to 10 mm in thickness, which would be classified into humic coal (Figs. 3a, 3b and 3c).

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Figure 3. Petrologic photographs of CD within sandstone. (a) CD in massive forms and pyrites within gray calcareous uranium-rich coarse sandstone, ZKD95-16, 714.9 m; (b) CD in gray medium sandstone in vein and massive forms, ZKBW2015-4, 484.30 m; (c) CD within gray calcareous uranium-rich medium sandstone distributed in vein forms, ZKT63-0, 634.7 m; (d) CD and uranium mineral in gray uranium-rich medium sandstone, showing a closely spatial relationship between uranium and CD, D192-31, 623 m; (e) the same view as (d) showing distributions of the fission track (FT) by the means of microradiography. Py. Pyrite; U. uranium.
2 SAMPLING AND METHODS

Quantitative gamma measurement (γ) could reflect uranium concentration to some extent. In the study area, the grades of uranium enrichment are divided into 4 groups by γ, i.e., industrial uranium enrichment (IUE, more than 90 nc/kg), boundary uranium enrichment (BUE, 30 to 90 nc/kg/h), abnormal uranium enrichment (AUE, 15 to 30 nc/kg/h), and no uranium enrichment (NUE, less than 15 nc/kg/h). Aqueous uranium in migration process could unevenly alter CD (Xue et al., 2009). Therefore, covering different zones (i.e., paleo- oxidation zone, uranium-rich zone and reduction zone), 34 samples characterized by different uranium concentration, were systematically collected from CD within sandstone in uranium-bearing strata from boreholes T111-7, T143-16, D111-16, D80-39, D112-39, D160-39 and D192-55 for coal property analyses. Among 34 samples, 2 samples were from CD within gray-green sandstones in NUE, 13 samples were from CD within gray uranium-rich sandstone (5 samples (i.e., D112-39-1, D112-39-2, D80-39-2, D80-39-3, D160-39-3) in IUE; 5 samples (i.e., D1116-1-2, T143-16-4, D80-39-4, D80-39-6, D80-39-8) in BUE; 3 samples (i.e., D111-16-1, D111-16-3, D192-55-1) in AUE), and the others are from CD within gray sandstone in NUE. Specific sampling locations, lithology, uranium enrichment information and γ are shown in Fig. 2, Table 1.

Table 1 Data of depth, lithology, uranium enrichment information and γ of CD within sandstone inthe Daying Uranium Deposit, northern Ordos Basin

Vitrinite reflectance (VR), coal petrography and coal property analyses were conducted at the China University of Geosciences (CUG) Laboratory of Coal and Coalbed Methane. CD was selected from sandstone by using tweezers, and crushed to less than 0.2 mm, between 0.2 and 1 mm, respectively. Then, CD with diameters less than 0.2 mm is mixed with those with diameters 0.2 to 1 mm by the proportion of 1 : 4 or 1 : 5. Finally, a random-chosen pile of the above air-dried CD was mixed with epoxy resin by 5 : 1 and solidified to form a cylindrical sample. The samples were polished and analyzed by optical microscope with an immersion lens to make VR and coal petrography analyses. The measurements of maximum reflectance of vitrinite were made on non-oriented telocollinite grains by a ZEISS microscope equipped with a HD photometer, an instrument with a monochromatic light (light λ=546 nm), at a temperature of 23 ℃ with an oil immersion lens. During a complete 360° stage rotation, the maximum reflectance value was recorded. Before measurements, the microscope was calibrated by using sapphire (R=0.590 %Ro), yttrium aluminum garnet (R=0.904 %Ro) and gadolinium-gallium-garnet (R=1.719 %Ro).

Besides, the samples with diameters less than 0.2 mm were conducted to make coal property analyses. Another random-chosen pile of less than 0.2 mm, air-dried CD was weighed in 1±0.1 g with an accuracy of 0.000 2 g for moisture, volatile matter, and ash yield, respectively, and also weighed in 50±2 mg for total sulphur (St) analyses. For moisture, samples were conducted in the stoving chest at 105–110 ℃ for an hour in the absence of air, then cooled to room temperature in the drying basin for weighing. For volatile matter, samples were conducted in the muffle furnace JXL-620 at 900±10 ℃ for 7 minutes in the absence of air, then cooled for 5–6 minutes in air and then to room temperature in the drying basin for weighing. For ash yield, samples were measured by means of slow-ashing method. Samples were conducted in the muffle furnace JXL-620 at less than 100 ℃ with about 15 mm gap, followed by 500 ℃ for half an hour, finally followed by 815±10 ℃ for an hour in the absence of air; after the heating process, cooled for 5–6 minutes in air and then to room temperature in the drying basin for weighing. For St, samples were conducted in the automation measuring sulfur instrument WDL-100C which was designed by coulometric titration. However, two waste samples of CD were introduced to balance electrolyte before testing. The detailed experimental procedures of moisture, volatile matter, ash yield and St were referred to standard procedures GB212-2008. The data of VR, maceral and coal property analyses are shown in Tables 2, 3, respectively.

Table 2 Vitrinite reflectance (VR) analysis of CD within sandstone inthe Daying Uranium Deposit, northern Ordos Basin
Table 3 Data of maceraland coal property analyses of CD within sandstone in the Daying Uranium Deposit, northern Ordos Basin

Additionally, uranium concentration was directly measured in Shenshangou outcrop. The area of gray calcareous CD-bearing coarse sandstone is 1.72 m×1.89 m, in which CD was mostly distributed in lumps forms with diameters ranging from 0.5 to 4 cm (Fig. 4a). The sandstone was conducted by methods of grid (20 cm) and divided into 94 testing points, which were directly tested by environment gamma radiation spectrometer FD-3013 for 30 seconds so as to obtain uranium concentration (Figs. 4b and 4c). The data of uranium concentrations are shown in Table 4.

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Figure 4. Distributions of uranium concentration (ppm) in the gray calcareous CD-bearing coarse sandstone in Shenshangou outcrop. (a) CD within gray calcareous CD-bearing coarse sandstone; (b) gray calcareous CD-bearing coarse sandstone with an area of 1.72 m×1.89 m, the white box is the same view as (a); (c) a sketch of (b) showing distributions of testing points; (d) distributions of uranium concentration (ppm) in the gray calcareous CD-bearing coarse sandstone.
Table 4 Distributions of uranium concentrations (ppm) in gray calcareous CD-bearing coarse sandstone, Shenshangou outcrop
3 RESULTS 3.1 VR and Characteristics of Coal Petrology

VR is an effective indicator to evaluate organic matter maturation. VR ranges from 0.372–0.510 %Ro with an average value of 0.438 %Ro, suggesting that the CD within sandstone is in the stage of lignite (Table 2).

The contents of V, I and minerals range from 83.18% to 99.48%, 0 to 7.70%, and 0.34% to 15.72%, respectively, with a corresponding average value of 95.51%, 1.34% and 3.15%, respectively, and the content of exinite (E) is 0, indicating that V is the major maceral (Table 2, Fig. 5). However, the contents of V and I range from 92.91% to 100% and 0 to 7.79%, respectively, in the condition of ignoring minerals. According to the classification of microlitho types, CD in the study area can be classified into vitrite (V > 95%) and vitrinertite-V (V+I > 95%, V > I) (Coalfield Staff Room of Wuhan Geological Institute, 1982; Fig. 6), among which four samples (i.e., D111-16-1, D111-16-2, D111-16-3, D80-39-6) are vitrinertite-V CD, while the others are vitrite CD.

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Figure 5. Photomicrographs of CD within sandstone. (a) Telocollinite within CD in gray medium-coarse sandstone in NU, D112-39, 662.32 m; (b) telocollinite within CD in gray medium sandstone in AUC, D111-16, 616 m; (c), (d) semifusinite within CD in gray medium sandstone in AUC, D111-16, 618 m. T. Telocollinite; Sf. semifusinite.
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Figure 6. Scatter diagrams of maceral in mineral-free basis. V. vitrinite; I. inertinite; E. exinite; A. vitrinertite-V; B. vitrite.
3.2 Characteristics of Coal Property

Moisture on air dried basis (Mad), volatile matter yield on dry, ash-free basis (Vdaf), ash yield on dried basis (Ad) and St mostly range from 7.95% to 16.09%, 44.70% to 66.54%, 4.84% to 26.24% and 0.24% to 1.12%, respectively, with a corresponding average value of 12.43%, 53.41%, 18.57% and 0.77%, respectively (Table 3). CD in the study area is of medium-high moisture, super-high volatile matter, low-medium ash and low sulfur, referred to Chinese Standard GB/T 15224.1-2004 (2004).

It shows that Mad and Vdaf generally decrease with an increase of burial depth, while Ad and St do not change significantly, indicating that CD underwent the burial metamorphism. However, Mad and Vdaf decrease significantly in uranium-rich areas whereas Ad and St noticeably increase (Fig. 7). It indicates that uranium enrichment might have an influence on the coal property of CD.

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Figure 7. Coal property parameter profiles for well ZKD112-39 (the location of the well is shown in Fig. 1b). Mad. Moisture on air dried basis; Vdaf. volatile matter yield on dry, ash-free basis; Ad. ash yield on dried basis; St. total sulfur; IUM. industrial uranium mineralization; BUM. boundary uranium mineralization; IUE. industrial uranium enrichment; NUE. no uranium enrichment.

On the basis of these, uranium-rich samples are emphatically analyzed. Vitrinertite-V samples are all rich in uranium, so coal property parameters of vitrite CD within paleo- interlayer oxidation zones are analyzed (Fig. 8). Mad and Vdaf are the lowest in the uranium-rich zone while the highest in the reduction zone (Figs. 8b, c). On the contrary, Ad and St are the highest in the uranium-rich zone while the lowest in the reduction zone (Figs. 8d and 8e). Aqueous uranium in migration process could unevenly alter CD, but CD in reduction zone is hardly alternated. Therefore, the CD in reduction zone is set as the standard, and coal properties of CD in uranium-rich zone and paleo-oxidation zone are compared with those in reduction zone. It shows that the difference values of Mad, Vdaf and Ad between paleo-oxidation zone and reduction zone are lower than those between uranium-rich zone and reduction zone (Figs. 8b, 8c and 8d), indicating the uranium can unevenly alter CD in migration process, and the variations of coal property parameters of CD in uranium-rich zone are more intense than those in paleo-oxidation zone. An increase in St is associated with an increase in pyrite in the uranium-rich zone (Figs. 3a and 8e).

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Figure 8. (a) The regional uranium enrichment mode of sandstone-type uranium deposits, northern Ordos Basin (after Jiao et al., 2016); (b), (c), (d) and (e) histograms showing variations of Mad, Vdaf, Ad and St in paleo-interlayer oxidation zones, respectively.
3.3 Uranium Concentration in CD-bearing Sandstone

Uranium concentration ranges from 29 ppm to 92 ppm with an average value of 50 ppm in the gray calcareous CD- bearing coarse sandstone (Table 4). Moreover, higher uranium concentration is mostly distributed in CD-rich area, and uranium concentration increases while it is close to CD (Fig. 4d). It indicates that CD is beneficial to the adsorption of uranium.

Comprehensive studies show that moisture and volatile matter decrease while ash yield and St increase in uranium-rich area, and the variations of coal property parameters of CD in uranium-rich zone are more intense than those in paleo- oxidation zone. Moreover, CD helps the adsorption of uranium. It indicates that there are certain genetic relationships between uranium enrichment and coal property parameters of CD.

4 DISCUSSION

Sandstone-type uranium deposit is the yield of uranium (Ⅵ)-bearing groundwater's migration to the edge of interlayer oxidation zone in sandstone to precipitate and form uranium ore body by adsorption or reduction (Harshman, 1972), which indicates it is epigenetic. For coal-hosted uranium deposit, it is classified into an epigenetic type and a syngenetic or early diagenetic type, among which uranium is mostly hosted in the upper unit of coal close to oxidation sand bodies for a epigenetic type while is evenly hosted in coal for a syngenetic or early diagenetic type (e.g., Dai et al., 2015a, b ; Seredin and Finkelman, 2008). However, there are many differences between coal-hosted and sandstone-type uranium deposits. For example, compared with coal-hosted uranium deposit, uranium is hosted in sandstone for sandstone-type one, and distributions and shapes of uranium ore body are controlled by reductant of uranium-bearing strata (e.g., CD, pyrite and dark mud-gravel; Jiao et al., 2006; Harshman, 1972). Therefore, CD within sandstone plays a key role in absorption and reduction of uranium for sandstone-type uranium deposit. Besides, uranium enrichment can alter CD during interaction process.

4.1 Influences of Uranium Enrichment on Coalification of CD

Mad and Vdaf could reflect the degree of coalification to some extent. Mad and Vdaf decrease with the increased degree of coalification (Coalfield Staff Room of Wuhan Geological Institute, 1982). As mentioned above, Mad and Vdaf obviously decrease in uranium-rich areas, less than that of the normal burial metamorphism, which indicates that the degree of coalification might increase in uranium-rich areas. VR measurements indicate that because of the influences of radioactive uranium enrichment, the degree of coalification increases, and particularly for CD in IUE, VR is obviously higher (0.102 %Ro) than that of free uranium enrichment (Table 2). Besides, Mad and Vdaf decrease with the increased quantitative gamma (Figs. 9a and 9c) and the decreased distance to the uranium-rich sandstone (Figs. 9b and 9d). Moreover, the difference values of Mad and Vdaf between IUE and NUE are higher (2.28% and 13.43%), respectively, than those between BUE and NUE for vitrite CD (Figs. 10a, 10c). Similar phenomena could be found in vitrinertite-V CD. Mad and Vdaf in BUE are lower than those in AUE (Figs. 10b and 10d). Comprehensive analyses indicate that the degree of coalification increases with the increased uranium concentration and the decreased distance to uranium- rich area. Similar phenomenon has been reported (e.g., Sýkorová et al., 2016; Smieja-Król et al., 2009; Sassen, 1984).

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Figure 9. (a), (c) Relationships between quantitative gamma and coal property parameters (i.e., Mad and Vdaf); (b), (d) relationships between distances to the closest uranium-rich sandstone and coal property parameters (i.e., Mad and Vdaf) (data seen in Table 3). γ. Quantitative gamma; d. distances to the closest uranium-rich sandstone.
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Figure 10. Histograms showing Mad and Vdaf of vitrite CD and vitrinertite-V CD in different uranium concentration. (a), (b) Moisture; (c), (d) volatile matter.

Uranium, a radioactive element, can produce decay accompanying with alpha, beta, gamma rays and certain energy. Radioactive uranium enrichment can cause the polymerization and condensation of molecular structure with a decrease in hydrogen atom and functional groups, thus increasing the degree of coalification, accompanying with a decrease in Mad and Vdaf (e.g., Jaraula et al., 2015; Court et al., 2006; Landais, 1996).

4.2 Influences of Coalification on Occurrences of Uranium within CD

Relations of uranium concentration and ash yield could reflect the occurrences of uranium in OM. An obvious positive relationship between them indicates that uranium occurs in mineral forms, conversely, uranium occurs in organic forms (Wang, 2016). In the study area, an unobvious correlation between uranium concentration and ash yield (r=0.32, Fig. 11) indicate that occurrences of uranium are complex and diverse, and uranium in minerals and organic forms coexist. It has been found that the occurrences of uranium are mainly coffinite and scattered adsorption uranium in CD in the Daying Uranium Deposit (Cun et al., 2016). On one hand, uranium mainly occurs in organic forms in OM at the stage of low organic maturation (Liu et al., 2016). In the study area, CD is in the stage of lignite (i.e., VR ranges from 0.372 %Ro to 0.510 %Ro with an average value of 0.438 %Ro) and plays an important role in the adsorption of uranium. As mentioned above, the distributions of uranium concentrations are restricted by CD in gray calcareous CD-bearing coarse sandstone, and uranium concentration increases with the increased CD (Table 4, Fig. 4d). It suggests that CD is beneficial to the adsorption of uranium, and uranium mainly occurs in organic forms. On the other hand, uranium also occurs in mineral forms according to an unobvious correlation between uranium concentration and ash yield (γ=0.32, Fig. 11). CD in NUE is set as the standard. The difference values of Ad between IUE and NUE are obviously 34.49% higher than those between BUE and NUE (Fig. 12a). Similar phenomena could be found in vitrinertite-V CD. Ad in BUE is higher than in AUE (Fig. 12b). Comprehensive analyses show that Ad increases with the increased uranium concentration, which indicates that uranium in mineral forms may increase with the increased uranium concentration in uranium- rich sandstone. Although, calcite cementation and pyrite are widespread in uranium-rich sandstone (Xie, 2016, Fig. 3a), which could cause an increase in ash yield of CD. Calcite cementation and pyrite are not associated with uranium concen-tration in uranium-rich sandstone. Therefore, uranium minerals increase with the increased uranium concentration in uranium- rich sandstone. Similar phenomena have been reported (e.g., Gluskoter et al., 1977).

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Figure 11. Relationships between quantitative gamma and ash yield.
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Figure 12. Histograms showing ash yield of vitrite CD and vitrinertite-V CD in different uranium concentrations.

According to the above analyses and related papers (e.g., Jaraula et al., 2015; Havelcová et al., 2014; Landais, 1996), the tracing paths of relations of uranium enrichment and CD are summarized (Fig. 13). CD in the stage of lignite, contains a large sum of humic acid with abundant functional groups, e.g., carboxyl (-COOH), hydroxyl (-OH), which participate in the adsorption and complexation of uranium (e.g., 2R-COOH+ UO22+=RCOO-(UO2)-OOCR+2H+; Landais, 1996). Furthermore, the structure of CD in the stage lignite is of a larger porosity and a higher specific surface area, which is beneficial to the physical adsorption of uranium (Bordelet et al., 2013; Li et al., 2009; Xiang et al., 2000), thus uranium in organic forms is the main occurrence in CD. On one hand, radioactive uranium enrichment causes polymerization and condensation of molecular structure with a decrease in hydrogen atom, thus increasing the degree of coalification, accompanying with a decrease in Mad and Vdaf. Moreover, the degree of coalification increases with the increased uranium concentration in uranium- rich sandstone and the decreased distance to uranium-rich area. On the other hand, an increase in the degree of coalification causes a decrease in functional groups and an increase in reducibility of CD (e.g., an increase in CH4 and H2S; Coalfield Staff Room of Wuhan Geological Institute, 1982), thus reducing hexavalent uranium (Ⅵ) to tetravalent uranium (IV), which can result in an increase in uranium minerals. Besides, uranium minerals and pyrites coexist (Fig. 3a), which indicates that pyrites play an important role in uranium enrichment. Some gas of hydrogen sulfide (H2S) which CD produces by sulphate reducing-bacterial reduces hexavalent uranium (Ⅵ) to tetravalent uranium to form uranium minerals (Min et al., 2001), and the other reacts with Fe2+ to form pyrite resulting in the coexistence with uranium minerals and an increase in St of CD (Fig. 13).

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Figure 13. Schematic diagram showing evo-paths of uranium enrichment and CD involved.
5 CONCLUSIONS

(1) CD in the study area is in the stage of lignite, and mostly composed of V. CD is classified into vitrite and vitrinertite-V. CD is of medium-high moisture, super-high volatile matter, low-medium ash and low sulfur.

(2) Mad and Vdaf generally decrease with the increased burial depth, while Ad and St do not change obviously. Compared with CD in reduction zone, Mad and Vdaf of CD in uranium-rich area significantly decrease whereas Ad and St increase.

(3) CD plays an important role in the adsorption of uranium, and uranium concentration increases with the increased CD. On one hand, uranium enrichment causes an increase in the degree of coalification of CD with a decrease in Mad and Vdaf. Moreover, Mad and Vdaf decrease with an increase of uranium concentration and the decreased distance to approaching to uranium-rich area. On the other hand, uranium concentration bears an unobvious positive relationship with ash yield. Furthermore, uranium minerals increase with the increased degree of coalification.

ACKNOWLEDGMENTS

This study was supported by the 973 Project (No. 2015CB453003), Open Fund of Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education (No. TPR-2015-09), Geological Survey Foundation of Ministry of Finance of the People's Republic of China (No. 12120115013701), Natural Science Foundation of China (No. 41502105), and Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. G1323511660). We are appreciative of the collaboration with and enthusiastic support from Aisheng Miao, Yunbiao Peng and many other coworkers from No. 208 Geological Brigade of China Nuclear Corporation in this study. Besides, we thank Sidong Pan from Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education for providing guidance during testing samples. We thank Journal of Earth Science editors and reviewers for their thorough and critical reviews of an early version of this manuscript and suggestions to improve the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0952-0.


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