Geochemical and mineralogical evidence for a coal-hosted uranium deposit in the Yili Basin, Xinjiang, northwestern China

Ore Geology Reviews 70 (2015) 1–30

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Geochemical and mineralogical evidence for a coal-hosted uranium deposit in the Yili Basin, Xinjiang, northwestern China Shifeng Dai a,⁎, Jianye Yang b, Colin R. Ward c, James C. Hower d, Huidong Liu a, Trent M. Garrison d, David French c, Jennifer M.K. O'Keefe e a

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing), Beijing 100083, China Xi'an University of Science and Technology, Xi'an 710054, China School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia d University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, United States e Department of Earth and Space Sciences, Morehead State University, Morehead, KY 40351, United States b c

a r t i c l e

i n f o

Article history: Received 27 January 2015 Received in revised form 4 March 2015 Accepted 10 March 2015 Available online 20 March 2015 Keywords: Coal-hosted uranium deposit Jurassic coal Trace-element geochemistry Minerals in coal Hydrothermal solutions Yili coal basin

a b s t r a c t The petrological, geochemical, and mineralogical compositions of the coal-hosted Jurassic uranium ore deposit in the Yili Basin of Xinjiang province, northwestern China, were investigated using optical microscopy and field emission-scanning electron microscopy in conjunction with an energy-dispersive X-ray spectrometer, as well as X-ray powder diffraction, X-ray fluorescence, and inductively coupled plasma mass spectrometry. The Yili coal is of high volatile C/B bituminous rank (0.51–0.59% vitrinite reflectance) and has a medium sulfur content (1.32% on average). Fusinite and semifusinite generally dominate the maceral assemblage, which exhibits forms suggesting fire-driven formation of those macerals together with forms suggesting degradation of wood followed by burning. The Yili coals are characterized by high concentrations of U (up to 7207 μg/g), Se (up to 253 μg/g), Mo (1248 μg/g), and Re (up to 34 μg/g), as well as As (up to 234 μg/g) and Hg (up to 3858 ng/g). Relative to the upper continental crust, the rare earth elements (REEs) in the coals are characterized by heavy or/and medium REE enrichment. The minerals in the Yili coals are mainly quartz, kaolinite, illite and illite/smectite, as well as, to a lesser extent, K-feldspar, chlorite, pyrite, and trace amounts of calcite, dolomite, amphibole, millerite, chalcopyrite, cattierite, siegenite, ferroselite, krutaite, eskebornite, pitchblende, coffinite, silicorhabdophane, and zircon. The enrichment and modes of occurrence of the trace elements, and also of the minerals in the coal, are attributed to derivation from a sediment source region of felsic and intermediate petrological composition, and to two different later-stage solutions (a U–Se–Mo–Re rich infiltrational and a Hg–As-rich exfiltrational volcanogenic solution). The main elements with high enrichment factors, U, Se, As, and Hg, overall exhibit a mixed organic–inorganic affinity. The uranium minerals, pitchblende and coffinite, occur as cavity-fillings in structured inertinite macerals. Selenium, As, and Hg in high-pyrite samples mainly show a sulfide affinity. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Sandstone-hosted roll-type uranium deposits are economically important in many parts of the world (Dahlkamp, 1993; Min et al., 2001; Wu et al., 2009). In China, this type of deposit was first discovered in the Yili Basin, Xinjiang Province of northwestern China (Fig. 1A) during the early 1990s (Min et al., 2001; Wang et al., 2005, 2006a). The 511 deposit, which was subsequently explored and mined, is one of the most important U deposits in the Yili Basin (Min et al., 2001; Wang et al., 2006a). However, all the published literature on this deposit is related to the abundance, modes of occurrence, and geologic origin of uranium in the sandstone (Min et al., 2001, 2005a,b; Wang et al.,

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.oregeorev.2015.03.010 0169-1368/© 2015 Elsevier B.V. All rights reserved.

2005, 2006a,b; Zhang et al., 2006; Li et al., 1996, 2006; Li and Huang, 2001). Although this sandstone-hosted roll-type uranium deposit occurs in coal-bearing strata, no data are available on the uranium in the coal itself. In this study, three complete drill cores through the coal-bearing sequence were sampled and a geochemical and mineralogical study was conducted on the three main coal beds. The results may have implications in understanding the enrichment of U, Re, and Se in coal generally, and may help in better evaluating the deposit's economic significance. Under particular geological conditions, some rare elements (e.g., Ga, Ge, Zr, Nb, platinum group elements, Ag, Au, rare earth elements, and U) may be significantly enriched in coals, having concentrations equal to or even higher than those found in conventional types of ores (Seredin, 2004a; Seredin and Finkelman, 2008). Although coals with such high trace-element concentrations are not common and the deposits are generally small in volume, these metals may still represent

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S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

Fig. 1. Location of the Yili Basin (A) and a cross-section of the Yili Basin showing the location of the 511 Uranium deposit (B). B is from Feng and Jiang (2000) and Wang et al. (2005). γ4, Hercynian granite; C, Carboniferous; P, Permian; T1sh, Lower Triaasic Shangcangfanggou Group; T2–3xq, Middle-Upper Triassic Xiaoquangou Group; J1–2sh, Lower-Middle Jurassic Shuixigou Group; J3q, Upper Jurassic Qigu Formation; K, Cretaceous; E, Palaeogene N, Neogene; Q, Quaternary.

useful economic by-products of coal cleaning or combustion residues (Finkelman and Brown, 1991; Seredin and Shpirt, 1999; Seredin, 2004a; Dai et al., 2014a; Mastalerz and Drobniak, 2012). For example, uranium has been and Ge is currently being extracted from coal ash as a raw material for use by industry (Seredin and Finkelman, 2008; Hu et al., 2009; Seredin et al., 2013; Jaireth et al., 2014). Rare earth elements and yttrium (REY; or REE if Y is not included), Ga, Nb, Zr, Ag, Au, and platinum group elements (PGEs) in coal ash have the greatest potential

for such use (Seredin and Finkelman, 2008). However, uranium in coal has also attracted attention for the following reasons: (1) Coal has been and probably will again be one of the major sources of U for industrial utilization (Seredin and Finkelman, 2008). In post-WWII years, U-rich coal was one of the main sources of uranium for the nuclear industry in the Soviet Union and the United States (Kislyakov and Shchetochkin, 2000; Tang

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

and Huang, 2004; Seredin, 2012). Seredin (2004b) suggests that coal can be considered as a source of U for industrial extraction if the U concentration in the coal ash reaches 1000 μg/g. (2) Coal-hosted U deposits usually have high concentrations of other rare metals such as Re, Se, Mo, and V, which may also have potential economic significance (Dai et al., 2015b). (3) Uranium is a radioelement, and combustion residues derived from high-U coals may have adverse effects on the environment and human health (Swaine, 2000; Yang, 2007). (4) The abundance and mode of occurrence of U in coal may provide an indicator of the original peat-accumulation environments and of subsequent diagenetic and epigenetic processes (Bostrom et al., 1973; Bostrom, 1983; Wignall, 1994; Gayer et al., 1999; Dai et al., 2015b; Ketris and Yudovich, 2009).

The concentration of U in coal generally varies from 0.5 to 10 μg/g (Swaine, 1990), and is 2.9 and 1.9 μg/g on average for world low-rank and world hard coals, respectively (Ketris and Yudovich, 2009). The average U concentrations for common Chinese and US coals are 2.43 and 2.1 μg/g, respectively (Dai et al., 2012; Finkelman, 1993). However, the concentration in some U-rich coals varies from a few hundreds to a few thousands of μg/g (Seredin and Finkelman, 2008; Dai et al., 2015b). The largest coal-hosted uranium deposits occur in Middle Asia (Seredin and Finkelman, 2008) where the U resources are estimated to be as high as 37,000 t (Koldzhatsk) and 60,000 t (Nizhneillisk) (Kislyakov and Shchetochkin, 2000; Seredin and Finkelman, 2008). Smaller coalhosted U deposits have also been found in Russia, the United States, France, the former Czechoslovakia, and China (Yudovich and Ketris, 2001, 2006a,b; Seredin and Finkelman, 2008; Shao et al., 2003; Zeng et al., 2005; Dai et al., 2008, 2015b). Uranium in U-rich coals generally occurs as U(VI) in the organic matter, as U(IV) in oxides (e.g., pitchblende) and silicates (e.g., coffinite), and, in some cases, is incorporated in arsenates, vanadates, phosphates, and carbonates (Seredin and Finkelman, 2008). The Yili coals in the present study are characterized by high concentrations of U (up to 7207 μg/g); this paper is intended to evaluate the origin and modes of occurrence of these highly-elevated concentrations in the coal seams.

2. Geological setting of the Yili Basin The geological setting of the Yili Basin has been described in detail by Min et al. (2001, 2005a,b), Wang et al. (2005, 2006a,b), and Feng and Jiang (2000). The Yili Basin is a Mesozoic–Cenozoic continental basin, derived from a Paleozoic interarc taphrogenic trough. The sediment-source region mainly consists of Hercynian granite, Permo-Carboniferous intermediate and felsic igneous rocks, and pyroclastic rocks interbedded with carbonate layers. These units form the basement to the Yili Basin, which is infilled by Mesozoic conglomerate, sandstone and mudstone, and by Cenozoic clastic sediments (Fig. 1B). The coal-bearing strata are part of the Middle-Lower Jurassic Shuixigou Group, which consists of the Badaowan Formation (J1b), Sangonghe Formation (J1s), and Xishanyao Formation (J2x), and is mainly made up of conglomerate, medium- to coarse-grained sandstone, siltstone, mudstone, and lignite. The thicknesses of the Shuixigou Group and the Sangonghe Formation range from 223 to 437 m and from 56 to 119 m, respectively (Min et al., 2001). The 511 U deposit is located in the southern flank of the synclinal Yili Basin (Fig. 1B), where the sedimentary strata dip 3–8° northeast (Wang et al., 2005; Min et al., 2001). It is located within the lower Sangonghe Formation and includes lignite beds as well as the industrially exploited sandstone roll-type U deposit. The U deposit is hosted by medium- to coarse-grained sandstone, with the sandstone being composed of quartz (40–60%), feldspar (8–15%), carbonaceous debris (2–3%), and rock fragments (20–40%) (Min et al., 2001).

3

Three drill cores (ZK0407, ZK0161, and ZK0177), which had been drilled for exploration of the sandstone-hosted U accumulations, were sampled from locations within the area of the 511 uranium deposit. The sedimentary sequences in these three cores are presented in Fig. 2. Based on data from the 216 Team of the Xijiang Nuclear Bureau, the sandstone roof strata of the No. 10, 11, and 12 coals in drill hole ZK0407 contain high U concentrations, with averages of 261, 450, and 556 μg/g, respectively. 3. Samples and analytical procedures A total of 64 samples of the No. 10, 11, and 12 coals were taken from the three drill cores, including 40 coal bench samples, one full-seam sample, six roof samples, nine floor samples, and seven partings. All of the samples were immediately stored in plastic bags to minimize contamination and oxidation. From top to bottom, the roof strata (with suffix —R), coal benches, partings (with suffix — P), and floor samples (with suffix — F) are identified by the core number plus the coal bed number, with the coal seams numbered in decreasing order from top to bottom. Proximate analysis1 was conducted following ASTM Standards D3173-11, D3175-11, and D3174-11 (2011). Total sulfur was determined following ASTM Standard D3177-02 (2002). For ultimate analysis2, an elemental analyzer (Vario MACRO) was used to determine the percentages of C, H, and N in the coals. Maceral constituents were identified using white-light reflectance microscopy under oil immersion and more than 500 counts were measured for each polished pellet. The maceral classification and terminology applied in the current study are based on Taylor et al. (1998) and the ICCP System 1994 (ICCP, 1998, 2001). A field emission-scanning electron microscope (FE-SEM, FEI Quanta™ 650 FEG), in conjunction with an EDAX energy-dispersive Xray spectrometer (Genesis Apex 4), was used to study the morphology of the minerals, and also to determine the distribution of some elements. Samples were carbon-coated using a Quorum Q150T ES sputtering coater or were not coated for a low-vacuum SEM working condition, and were then mounted on standard aluminum SEM stubs using sticky conductive carbon tabs. The working distance of the FESEM-EDS was 10 mm, beam voltage 20.0 kV, aperture 6, and spot size 4.5–5.5. The images were captured via a retractable solid state backscattered electron detector. Samples were crushed and ground to pass 200 mesh (75 μm) for major and trace element analysis. The mineralogy was determined by X-ray powder diffraction (XRD) of the powdered samples, supplemented by SEM and optical microscope observation. Low-temperature ashing of the powdered coal samples was carried out using an EMITECH K1050X plasma asher prior to XRD analysis. XRD analysis of the low-temperature ashes, and also of the powdered but otherwise untreated non-coal samples (partings, roof and floor strata), was performed on a D/max-2500/PC powder diffractometer with Ni-filtered Cu-Kα radiation and a scintillation detector. Each XRD pattern was recorded over a 2θ interval of 2.6–70°, with a step size of 0.01°. X-ray diffractograms of the LTAs and non-coal samples were subjected to quantitative mineralogical analysis using Siroquant™, commercial interpretation software developed by Taylor (1991) based on the principles for diffractogram profiling set out by Rietveld (1969). Further details indicating the use of this technique for coal-related materials are given by Ward et al. (1999, 2001) and Ruan and Ward (2002). X-ray fluorescence (XRF) spectrometry (ARL ADVANT'XP+) was used to determine the major oxides, including SiO2, Al2O3, CaO, K2O, Na2O, Fe2O3, MnO, MgO, TiO2, and P2O5, in the coal ash (815 °C) and in the powdered non-coal samples. The samples for XRF analysis were prepared by borate fusion in an automated fusion furnace (CLAISSE TheBee-10). Each sample (1 g) was mixed and homogenized with lithium borate flux (10 g; CLAISSE, pure, 50% Li2B4O7 + 50% LiBO2). 1 2

Determination of the amounts of moisture, volatile matter, and ash yield in coal. Determination of chemical elements in coal including the percentages of C, H, N and S.

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Inductively coupled plasma mass spectrometry (X series II ICP-MS), in pulse counting mode (three points per peak), was used to determine trace elements in the coal samples, except for Hg and F. The ICP-MS analysis and microwave digestion procedures are outlined more fully by Dai et al. (2011). For boron determination, the addition of H3PO4 to the HNO3 and HF was used in the sample digestion process to reduce boron volatilization during acid-drying after sample digestion. A 2% ammonia solution was injected into the ICP-MS spray chamber to eliminate the memory effect of boron. Arsenic and Se were determined by ICP-MS using collision cell technology (CCT) in order to avoid disturbance of polyatomic ions (Li et al., 2014a). For ICP-MS analysis, samples were digested using an UltraClave Microwave High Pressure Reactor (Milestone). Multi-element standards (Inorganic Ventures: CCS-1, CCS-4, CCS-5, and CCS-6; NIST 2685b and Chinese standard reference GBW 07114) were used for calibration of trace element concentrations. Mercury was determined using a Milestone DMA-80 Hg analyzer. Fluorine was determined by pyrohydrolysis in conjunction with an ionselective electrode following ASTM method D 5987-96 (2002). The chemical composition of the high-temperature ash expected to be derived from the mineral assemblage indicated by the XRD and Siroquant analyses of each coal LTA or non-coal sample was calculated, using the methodology described by Ward et al. (1999). This process includes calculations to allow for the loss of hydroxyl water from the clay minerals and CO2 from the carbonates associated with hightemperature (815 °C) ashing and similar combustion processes. The major-element data obtained by XRF analysis for each sample were also recalculated (on an organic- and SO3-free basis) to provide normalized percentages of the major element oxides in the inorganic fraction. These represent the chemical composition of the (high-temperature) ash derived from each coal or non-coal sample. The inferred percentages of major element oxides in the coal LTAs, partings, roof and floor strata, as calculated from the XRD data, were then compared to the normalized percentages of the same oxides in the SO3-free ash as calculated from the normalized XRF analysis results. 4. Results 4.1. Coal chemistry and vitrinite reflectance Table 1 presents the thickness, proximate and ultimate analyses, and huminite reflectance for the benches of the No. 10, 11, and 12 coals collected from the three drill cores. Ash yield greatly varies not only within each coal seam (vertically and laterally) but also between the different coal seams. For example, the weighted average ash yields of the No. 12 coal from cores ZK0407, 0161, and 0177 are 41.96, 13.74, and 29.01%, respectively. Sulfur varies greatly through the vertical section of each seam (Table 1). For example, total sulfur varies from 0.07% to 4.61% (with an average of 0.75%) through the No. 11 coal of core ZK0407. The high-sulfur samples also have high percentages of pyrite, indicated by XRD, optical microscopy and SEM observations (described more fully below). Overall, the coals from the Yili Basin are medium-ash (26.88% on weighted average) and medium-sulfur coal (1.32% on weighted average) (coal with total sulfur b 3% and N 1% is mediumsulfur coal; Chou, 2012). 4.2. Petrology

Fig. 2. Sedimentary sequences of the three drill cores (ZK0407, ZK0161, and ZK0177) in the 511 uranium deposit.

Petrographic analyses for indicative coal samples are presented in Table 2. With the exception of sample 0177-10-3 (No. 10 coal) in core ZK0177, and samples 0161-12-2 (No. 12 coal) and 0161-11-5 (No. 11 coal) in core ZK0161, none of the coals have more than 50% vitrinite (mineral-free basis). In general, fusinite and semifusinite dominate the maceral assemblages. Owing to the paucity of vitrinite in some of the samples, which is as low as 6.3% (mineral-free basis) in sample 0177-10-1 (No. 10 coal from core ZK0177), vitrinite reflectance was not measured on all

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5

Table 1 Proximate and ultimate analysis (%) and random huminite reflectance (%) of the coals from the Yili Basin. Drill holes

Coal

Sample no.

Thickness (cm)

Mad

Ad

Vdaf

St,d

Cdaf

Ndaf

Hdaf

Rr

ZK0407

No. 12

0407-12-1 0407-12-2 0407-12-3 0407-12-4 WA 0407-11-1 0407-11-2 0407-11-3 0407-11-4 0407-11-5 0407-11-6 0407-11-7 0407-11-8 0407-11-9 WA 0407-10-1 0407-10-2 0407-10-3 0407-10-4 0407-10-5 0407-10-6 0407-10-7 0407-10-8 WA 0161-12-1 0161-12-2 0161-12-3 0161-12-4 0161-12-5 WA 0161-11-1 0161-11-2 0161-11-3 0161-11-4 0161-11-5 WA 0177-12-1 0177-12-2 WA 0177-10-1 0177-10-2 0177-10-3 WA All

10 10 10 10 40a 10 10 10 20 30 10 10 10 15 125a 40 10 10 10 8 10 10 10 108a 180 10 10 10 10 220a 10 20 10 20 20 80a 20 100 120a 10 20 60 90a

8.79 9.84 10.66 11.03 10.08 15.20 14.08 10.08 12.40 12.28 12.69 12.39 10.04 9.16 11.99 12.39 8.27 9.68 6.85 14.29 11.71 10.07 9.44 10.83 16.16 14.14 12.49 12.44 11.94 15.54 12.52 14.09 9.13 9.85 12.10 11.72 9.58 11.81 11.44 8.83 12.89 13.14 12.61 12.69

51.61 41.02 35.42 39.77 41.96 23.60 28.96 40.88 31.04 31.04 25.09 23.48 37.99 64.65 34.57 46.97 58.41 48.41 68.45 10.20 18.53 39.85 43.76 43.84 12.31 9.97 24.76 17.29 28.65 13.74 19.71 13.97 44.71 38.68 14.99 24.96 44.36 25.94 29.01 54.47 15.65 15.90 20.13 26.88

45.04 43.00 35.00 33.44 39.12 33.54 33.58 34.91 35.96 35.28 34.70 36.01 41.05 46.48 36.90 37.02 39.16 36.50 50.45 39.13 40.85 39.72 40.46 39.49 35.79 40.13 39.73 41.48 41.28 36.67 34.66 33.51 41.11 38.76 42.89 38.26 40.34 37.93 38.33 40.58 37.66 37.10 37.61 37.75

0.91 1.00 1.09 1.60 1.15 0.07 0.20 4.61 0.07 0.68 0.45 0.43 0.97 0.27 0.75 1.93 0.59 0.70 0.38 0.83 1.06 3.16 0.79 1.39 1.45 1.05 0.64 0.65 0.53 1.32 1.14 0.38 0.53 0.26 1.42 0.72 10.30 0.68 2.28 0.83 2.81 0.97 1.36 1.32

69.93 70.84 78.38 78.21 74.34 78.58 78.60 72.13 76.37 76.75 77.33 76.76 73.70 68.22 75.39 76.14 73.47 75.19 67.25 79.00 76.80 73.00 75.08 74.87 76.81 73.90 75.44 74.16 73.17 76.33 75.73 77.84 73.42 75.39 73.10 75.23 59.56 76.14 73.38 72.32 76.11 76.80 76.15 75.29

1.09 1.06 0.81 0.78 0.94 0.72 0.74 0.77 0.68 0.76 0.71 0.78 0.88 0.89 0.77 0.80 0.81 0.80 0.91 0.79 0.78 0.74 0.78 0.80 0.90 0.94 0.97 0.90 0.81 0.90 0.82 0.76 0.81 0.86 0.78 0.80 0.65 1.00 0.94 0.99 1.01 0.89 0.93 0.87

5.62 4.14 3.45 2.94 4.04 2.79 2.12 3.50 3.46 3.14 2.70 2.95 3.62 4.47 3.26 3.43 3.55 2.61 5.46 3.00 3.69 3.85 4.27 3.66 3.15 3.45 3.93 3.49 3.93 3.25 2.84 2.36 4.01 3.53 4.19 3.38 3.84 3.59 3.63 4.39 2.64 3.12 3.15 3.41

nd nd nd nd

No. 11

No. 10

ZK0161

No. 12

No. 11

ZK0177

No. 12

No. 10

0.51 nd nd nd nd nd nd nd nd 0.56 nd nd nd 0.49 nd nd nd 0.59 0.55 nd nd nd nd 0.53 nd nd 0.55 nd 0.52 nd 0.47 0.47

M, moisture; A, ash yield; V, volatile matter; St, total sulfur; C, carbon; H, hydrogen; N, nitrogen; ad, as-received basis; d, dry basis; daf, dry and ash-free basis; Rr, random reflectance of huminite; WA, weighted average by thickness of sample interval. nd, not detected. a Total thickness of the coal seam.

Table 2 Maceral contents determined under optical microscope for coals from the Yili Basin (vol.%; on mineral-free basis). Sample

T

CT

VD

CD

CG

G

T-V

F

SF

Mic

Mac

Sec

Fun

ID

T-I

Sp

Cut

Res

Alg

LD

Sub

Ex

T-L

0407-12-4 0407-11-1 0407-11-2 0407-11-3 0407-11-4 0407-11-5 0407-11-8 0407-11-9 0407-10-1 0407-10-5 0161-12-1 0161-12-2 0161-11-2 0161-11-5 0177-12-1 0177-12-2 0177-10-1 0177-10-2 0177-10-3

0.7 8.4 2.6 5.1 4.3 4.3 13.3 8.5 12.6 22.6 7.7 38.6 8.3 27.6 11.4 15.7 6.3 20.7 18.9

13.1 7.7 7.2 15.4 2.6 6.0 9.7 1.7 13.7 2.1 9.4 23.8 1.0 35.6 11.4 11.2 0 19.5 21.4

22.2 1.9 8.5 7.7 6.0 5.2 4.4 3.4 3.2 2.5 7.2 14.3 1.0 8.6 5.3 5.6 0 3.6 11.2

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0.6 0.7 1.3 0 1.7 1.8 0 2.1 0.4 1.7 1 0 8.0 1.8 4.1 0 0.6 1.9

0 0 0 0 0 0 0 0 0 1.2 1.7 5.2 0 0 0 5.6 0 0 0.5

35.9 18.7 19.0 29.5 12.9 17.2 29.2 13.6 31.6 28.8 27.6 82.9 10.4 79.9 29.8 42.1 6.3 44.4 53.9

50.3 65.8 73.9 39.7 69.8 44 48.7 64.4 28.4 39.9 45.3 5.2 81.3 2.9 12.3 49.2 70.8 46.7 37.9

5.9 7.7 5.2 28.2 13.8 34.5 13.3 18.6 35.8 19.8 24.9 4.8 7.3 2.3 53.5 5.6 22.9 7.7 5.8

0 0 0 0 0 0 0 0 1.1 0 0 0 0 0.6 1.8 0 0 0 0

0 5.2 1.3 0 0 0 0.9 0 0 1.2 0 0.5 0.5 1.1 1.8 0 0 0 0

2.6 0.6 0 0 0 0 0 0 0 0.8 0 0 0 0 0 0 0 0.6 0.5

0.7 0 0 0 0 0 0 0 0 0 0 0.5 0 4.6 0 0 0 0 0

3.9 0.6 0 0 2.6 1.7 6.2 1.7 0 0.4 0 0 0.5 0 0.9 0 0 0 1

63.4 80.0 80.4 67.9 86.2 80.2 69.0 84.7 65.3 62.1 70.2 11.0 89.6 11.5 70.2 54.8 93.8 55.0 45.1

0.7 0.6 0 2.6 0.9 0 0.9 1.7 2.1 4.9 2.2 2.9 0 1.1 0 0.5 0 0 0.5

0 0.6 0 0 0 2.6 0 0 1.1 2.9 0 0.5 0 5.7 0 0 0 0 0.5

0 0 0 0 0 0 0.9 0 0 0 0 1.4 0 1.7 0 1.5 0 0.6 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 1.2 0 0.5 0 0 0 0.5 0 0 0

0 0 0.7 0 0 0 0 0 0 0 0 1.0 0 0 0 0.5 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0.7 1.3 0.7 2.6 0.9 2.6 1.8 1.7 3.2 9.1 2.2 6.2 0 8.6 0 3.0 0 0.6 1.0

T, telinite. CT, collotelinite. VD, vitrodetrinite. CD, collodetrinite. CG,corpogelinite. G, gelinite. T-V, total vitrinite. F, fusinite. SF, semifusinite. Mic, micrinite. Mac, macrinite. Sec, secretinite. Fun, funginite. ID, inertodetrinite. T-I, total inertinite. Sp, sporinite. Cut, cutinite. Res, resinite. Alg, alginite. LD, liptodetrinite. Sub, suberinite. Ex, exsudatinite. T-L, total liptinite.

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of the coal samples. The values where measurements are available, 0.51–0.59% Rr (Table 1), indicate that the coal is in the high volatile C/ B bituminous rank range (ASTM Standard D388-12, 2012). Fusinite and semifusinite exhibit forms typically associated with firedriven formation of those macerals (Fig. 3A–E), as well as forms suggestive of wood degradation followed by burning (Fig. 3F–H). In particular, the forms illustrated in Fig. 3F–H resemble macerals which could, by

virtue of their brightness, superficially resemble the fusinite and semifusinite resulting from a fire-driven path directly from woody or herbaceous tissue. The possibility of multiple pathways for the development of inertinite macerals was discussed by Hower et al. (2013) and O'Keefe et al. (2013). The latter authors also emphasized the degradational origin of macrinite, with discussion of the role of the mire fauna in processing and recycling organic matter (see Hower et al., 2009, 2011a,b; O'Keefe

Fig. 3. Inertinite-group macerals in coal under the optical microscope (reflected white light, oil immersion). (A), Fusinite, sample 0407-11-5. (B), Semifusinite, sample 0161-12-3. (C), Broken fusinite, sample 0407-11-5. (D), Semifusinite with cell structure, sample 0407-10-1. (E), Broken fusinite with secretinite, sample 0407-10-1. (F), Degraded fusinite adjacent to undegraded fusinite, sample 0161-12-1. (G), Thickened cell walls in fusinite, sample 0407-10-5. (H), Thickened cell walls in fusinite, sample 0177-10-1. (I), Rounded to oval macrinitic coprolites associated with fusinite and semifusinite, sample 0407-10-1. (J), Degraded fusinite transitioning to unstructured inertinite identified as macrinite, sample 0407-11-5. (K), Coprolites in telinite, sample 0177-12-2. (L), Coprolites with gellinite, sample 0177-12-2. f, fusinite; sf, semifusinite; df, degraded fusinite; c, coprolite; t, telinite; sec, secretinite.

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

and Hower, 2011; O'Keefe et al., 2011). Such interactions are seen in macrinite (Fig. 3I, J) and in coprolites (Fig. 3K, L) in the present study. In particular, the oval macrinite in Fig. 3I closely resembles a form considered to represent macrinized/fusinized insect coprolites in the Cretaceous Wulantuga coal (Inner Mongolia) (Hower et al., 2013; forms previously described by Scott and Taylor, 1983, and Cohen et al., 1987). The vitrinite ranges from telinite (Fig. 4A), gelocollinite (Fig. 4B), and telinite/gelocollinite (Fig. 4C) to gelinite (Fig. 4D,E). Gelinite is rarely among the more abundant vitrinite macerals, but it does comprise over 5% (mineral-free basis) of samples 0161-12-2 and 0177-12-2, both from the No. 12 coal. The gelinite occurs as an amorphous to sub-micron granular maceral with remnants of cell walls (telinite), as in Fig. 4D,E. The granular matrix is coarser in some cases and could be considered to be micrinite (Fig. 5; block sample 0161-11-1 of the No. 11 coal). Liptinite macerals in the coals include sporinite (Fig. 6A, sporinite in a macrinite matrix), suberinite (Fig. 6B), thin-walled cutinite (Fig. 6C-D & E-F), and resinite associated with gymnospermous wood (Fig. 6G–H & I–J).

7

4.3. Geochemistry 4.3.1. Major element oxides When considered on a whole-coal basis (Table 3), the Yili coals contain higher proportions of SiO2, MgO, CaO, and K2O (except the No. 12 coal of core K0161), and lower proportions of Fe 2O 3, MnO, Na2O, and P2O5, than the average values for Chinese coals reported by Dai et al. (2012). The SiO 2 /Al 2O 3 ratios for the Yili coals are much higher than those of other Chinese coals (1.42) (Dai et al., 2012) and also than the theoretical SiO2 /Al2 O3 ratio of kaolinite (1.18), consistent with the relatively high proportions of quartz in the mineral matter (Table 4). When compared to the roof, floor and parting samples (see Supplementary Electronic File 1), all three seams are characterized by having higher CaO and lower K2O contents than the non-coal lithologies. Fe2O3 concentrations are variable, with locally high concentrations (e.g., No. 10 and 11 coals in hole ZK0407). MgO contents are also higher in the No. 10 and 11 coals from hole ZK0407.

Fig. 4. Vitrinite in the Yili coals under the optical microscope (reflected white light, oil immersion). (A) Telinite in juxtaposition with micrinite, sample 0161-12-2. (B) Corpogelinite in sample 0161-12-1. (C) Telinite (cell walls) and corpogelinite (cell interiors) in sample 0407-10-1. (D) Gelinite associated with resinite, cutinite, and suberinite in sample 0161-12-2. (E) Gelinite associated with cutinite and other liptinite macerals; fine coprolites are present in the gelinite/liptinite band above the larger, through-going band in the center of the image; sample 0161-11-1.

8

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

The positive relationship of Se, Mo and Re to U concentration in the Uenriched coal benches also supports this association (see Supplementary Electronic File 2). With the exception of As in the highest-U coal bench (0177-12-1), the concentrations of Hg, As, and Tl in the samples of Groups 1 and 2 are lower than or close to the averages for world low-rank coals. Mercury, As, and Tl are relatively enriched in one of the low-U coal benches (0177-10-2) (Fig. 7D,E), but they show no particular relationship to U concentration in the other samples (see the Supplementary Electronic File 2).

Fig. 5. Micrinite in sample 0161-11-1 under the optical microscope (reflected white light, oil immersion).

4.3.2. Trace elements 4.3.2.1. Grouping of coal bench samples based on U concentration. Based on comparison of the U concentrations of the coal benches in the present study with the average for world low-rank coals reported by Ketris and Yudovich (2009), three groups of coal bench samples can be identified in the Yili coals. These are described herein as Groups 1, 2, and 3, and have concentration coefficients for U (CC: concentrations of trace elements in the Yili coals vs. the world low-rank coals) that are N 50, between 8 and 20, and b 8, respectively (Fig. 7). This classification also corresponds to the natural grouping of the samples as shown by the histogram of U concentration for the overall sample suite (Fig. 8). Group 1 These coal benches are significantly enriched in U (CC N 50, with an average of 355, Fig. 7B; U concentration N 150 μg/g, Fig. 8). They are also enriched in Se, Mo, and Re (Fig. 7A,B), with an average CC of 19.8, 182, and 2377, respectively. In the bench sample 0177-12-1 with the highest-U concentration (7208 μg/g; Fig. 7A), elements Se (CC = 253), Mo (CC = 567), and Re (CC = 33,953) are significantly enriched; elements V, Co, As, Cd, and W are also enriched to a lesser extent, with CC N 10. Group 2 Uranium is enriched to a lesser extent in the coal bench samples of this group (8 b CC b 20, with an average of 14.4; U concentration between 30 and 60 μg/g, Fig. 8). Elements Mo (CC = 50.5) and Re (CC = 27.7) are significantly enriched in the samples of this group, and Ni (CC = 4.73) and W (CC = 10.7) are weakly enriched (Fig. 7C). Group 3 The concentration of U in the coals of this group (U concentration b25 μg/g; Fig. 8) is close to (0.5 b CC b 8) or much lower than (CC b 0.5) the average for world low-rank coals reported by Ketris and Yudovich (2009) (Fig. 7D). However, Mo (CC = 5.93), W (CC = 6.41) and Re (CC = 7.77) are slightly enriched (Fig. 7D). Sample 0177-10-2 in Group 3 has the highest Hg concentration (3858 ng/g; CC = 38.58), but the U concentration for this sample is only 3.07 μg/g (CC = 1.06). In addition to Re (CC = 10.33), elements As (CC = 25.48) and Tl (CC = 36.3), and to a lesser extent, Ni (CC = 19.39), Co (CC = 7.08), and Be (CC = 6.96) are also enriched in this sample.

Overall, the elements Mo, Re, Cd, and, to a lesser extent, Se, are also enriched in the U-enriched (U concentration N 150 μg/g) coal benches.

4.3.2.2. Variations in trace elements through vertical coal seam sections. The vertical and lateral distribution of minerals and trace elements within a coal seam is generally not uniform (Hower et al., 2002; Yudovich, 2003; Seredin et al., 2006; Dai et al., 2013, 2014b; Kelloway et al., 2014), reflecting gradual changes during peat formation and perhaps a wide range of post-depositional processes (e.g. Ward, 2002; Permana et al., 2013). One particular case is “Zilbermints Law” (Pavlov, 1966; Yudovich, 2003), which was first observed by Zilbermints et al. (1936) and indicates Ge-enrichment near the roof, floor, and partings of Donetsk Basin coals. Thus an evaluation of the distribution of minerals and trace elements in different benches within the seam may assist in developing a better understanding of element enrichment process that may occur during the syngenetic, diagenetic and epigenetic stages of the coal's geological history. Figs. 9–11 show the variations of selected trace element concentrations in the coal (U, Se, Re, Mo, W, As, and Hg), as well as the ash yield and total sulfur content, through the vertical sections of the coal beds. Particular points of significance include: (1) Elements U, Se, Re, and Mo are enriched in the coal benches that underlie the roof sandstones and parting sandstones (e.g., Figs. 9A, 10, 11B). (2) With the exception of 0161-11-P1 immediately underlying the U-enriched coal bench (Fig. 10A), the partings are not enriched in U, Re, and Se. (3) Uranium, Re, and Se in some cases are enriched (e.g., No. 11 coal of K0161 core, Fig. 10B) but in other cases are not enriched (e.g., No. 11 coal of K0407 core, Fig. 9B) in silty sandstone floors. (4) Arsenic, Hg, and total sulfur have similar variations through the coal seam sections. Most of the coal benches in which these three elements are enriched have low U contents. An exception, however, is bench 0177-12-1, which has the highest-U content and also elevated concentrations of both As-Hg-S and U-Se-Re-Mo assemblages.

4.3.2.3. Distribution pattern of rare earth elements and yttrium (REY). Rare earth elements and yttrium (REY, or REE if yttrium is not included) have been used for many years as geochemical indicators of the sedimentary environment and post-sedimentary history of coal deposits because of their coherent behavior during different geochemical processes and their predictable pattern of fractionation (Hower et al., 1999; Seredin and Dai, 2012; Bau et al., 2014; Dai et al., 2015b). A threefold classification of REY was used for this study: light (LREY: La, Ce, Pr, Nd, and Sm), medium (MREY: Eu, Gd, Tb, Dy, and Y), and heavy (HREY: Ho, Er, Tm, Yb, and Lu) (Seredin and Dai, 2012). Accordingly, in comparison with the upper continental crust (UCC; Taylor and McLennan, 1985), three enrichment types are identified (Seredin and Dai, 2012): L-type (light-REY; LaN/LuN N 1), M-type (mediumREY; LaN/SmN b 1, GdN/LuN N 1), and H-type (heavy REY; LaN/LuN b 1). The concentrations of REY in the Yili coals are slightly higher than the average values for world low-rank coals (59.7 μg/g) reported by Ketris and Yudovich (2009), with the exception of the highest-Hg sample (0177-10-2), which has lower concentrations of La to Nd (CC b 1; Fig. 7).

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

9

Fig. 6. Liptinite in the Yili coals (reflected light, oil immersion). (A) Sporinite in macrinite, sample 0177-10-2. (B) Compressed suberinite, sample 0161-12-2. (C) and (D) Cutinite under blue and white light, sample 0161-11-5. (E) and (F) Cutinite under blue and white light, sample 0161-11-5. (G) and (H) Resin ducts (fluorescing resinite in G) in gymnospermous wood, sample 0161-12-2. (I) and (J) Resinite under blue and white light, sample 0161-11-5. mac, macrinite; sp., sporinite; su, suberinite; c, cutinite; r, resinite.

With the exceptions of samples 0177-12-1 (highest U, L-type) and 0177-10-2, the REY enrichment patterns in the coal benches are dominated by H-type (LaN/LuN b 1) or in some cases H–M-type assemblages, all with weak negative/positive Eu and weak negative Ce anomalies (Fig. 12); an exception is sample 0407-10-3 which has a distinct Ce negative anomaly. Sample 0177-12-1 is characterized by an L-type assemblage. Relative to the UCC, the light, medium, and heavy REY in sample 0177-10-3 are weakly fractionated (Fig. 12F). The light, medium, and heavy REY in the pebbly sandstone roof samples are weakly fractionated, with weak positive Eu anomalies (Fig. 13A). The REY distribution patterns in the carbonaceous mudstones and silty sandstones of the roof strata are characterized by Htype patterns, without Eu anomalies (e.g., No. 10 coal of K0711 core; Fig. 13B). The sandstone parting (0407-11-P; Fig. 13F) has the same REY distribution pattern as those of the sandstone roof strata (e.g., 0161-11-R, 0177-12-R, 0407-10-R1, and 0407-10-R2; Fig. 13A). Heavy REEs are slightly enriched in the partings and seam floors (e.g., samples 0161-11-P1, 0161-11-F4; Fig. 13D). However, the partings (e.g., sample 0161-12P; Fig. 13F) have an L-type REY enrichment and negative Eu anomaly and are not enriched in U, Se, Re, and Mo.

Other floor strata, including samples 0407-10-F, 0407-11-F2, 0161-12F1 and 0161-12-F1, are characterized by slight enrichment of H-REY and weak Eu anomalies (Fig. 13C). The significance of the REY distribution patterns for coal benches and host rocks will be discussed below. 4.4. Mineralogy 4.4.1. Mineralogical compositions Table 4 and Supplementary Electronic File 3 respectively provide data on the mineralogical compositions of the coal-LTA and non-coal samples from the Yili deposit, based on Siroquant analysis of the XRD patterns. The results show that quartz and kaolinite are usually the most abundant minerals in the LTA or rock samples, along with illite and in some cases interstratified illite/smectite (I/S). Minor proportions of chlorite (probably Fe-rich) and K-feldspar (possibly microcline) also occur in most of the samples studied. Another feldspar phase, tentatively identified as albite (Na-feldspar) occurs with the K-feldspar in some of the samples as well. Although a significant degree of scatter is involved, Fig. 14A indicates that the proportion of quartz in the mineral matter increases with the

10

Table 3 Elemental concentrations in samples from the Yili Basin (elements in μg/g, oxides in %, Hg in ng/g; on whole-coal basis). LOI

SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

SiO2/Al2O3

Li

Be

B

F

Sc

V

Cr

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Rb

Sr

Y

Zr

0407-12-0 0407-12-1 0407-12-2 0407-12-3 0407-12-4 0407-11-1 0407-11-2 0407-11-3 0407-11-4 0407-11-5 0407-11-6 0407-11-7 0407-11-8 0407-11-9 0407-10-1 0407-10-2 0407-10-3 0407-10-4 0407-10-5 0407-10-6 0407-10-7 0407-10-8 0161-12-1 0161-12-1′ 0161-12-2 0161-12-3 0161-12-4 0161-12-5 0161-12-6 0161-11-1 0161-11-2 0161-11-3 0161-11-4 0161-11-5 0161-10-C 0177-12-1 0177-12-2 0177-10-1 0177-10-2 0177-10-3 Categary 1 Categary 2 Categary 3 World

91.10 48.39 58.98 64.58 60.23 76.40 71.04 59.12 68.96 68.96 74.91 76.52 62.01 35.35 53.03 41.59 51.59 31.55 89.8 81.47 60.15 56.24 87.69 68.19 90.03 75.24 82.71 71.35 56.89 80.29 86.03 55.29 61.32 85.01 75.15 55.64 74.06 45.53 84.35 84.1 74.66 71.88 71.63 na

2.08 36.24 26.93 20.21 23.00 11.13 14.73 20.31 16.28 18.66 14.31 13.59 22.93 43.29 28.31 38.42 30.28 44.93 2.15 8.31 21.13 25.78 3.62 18.71 3.19 14.01 8.63 18.56 30.63 9.52 6.24 30.72 25.33 7.67 12.24 20.74 14.22 33.77 4.76 6.02 13.07 15.50 15.77 8.47

0.030 0.450 0.355 0.213 0.203 0.125 0.111 0.145 0.260 0.389 0.287 0.304 0.385 0.896 0.413 0.484 0.411 0.739 0.050 0.155 0.307 0.403 0.105 0.23 0.059 0.244 0.123 0.234 0.44 0.201 0.114 0.532 0.465 0.115 0.11 0.168 0.196 0.558 0.104 0.082 0.201 0.23 0.24 0.33

1.33 10.77 9.71 5.28 6.07 3.89 4.28 5.36 7.34 7.04 5.34 5.40 9.58 16.09 10.80 13.06 11.46 17.13 1.25 3.39 9.94 10.12 1.50 7.60 2.58 5.90 4.14 6.18 7.29 4.89 2.58 9.69 8.63 3.65 3.01 6.35 5.89 13.9 1.52 2.0 5.03 5.67 5.40 5.98

0.02 1.23 1.26 2.55 2.66 0.63 1.30 7.62 1.19 0.75 0.44 0.34 1.39 1.01 2.32 1.70 1.43 1.57 0.76 1.21 3.75 1.62 1.15 0.55 0.45 0.49 0.42 0.52 0.64 1.05 0.18 0.78 0.69 1.10 1.88 10.05 0.80 2.04 2.79 1.33 1.64 1.51 1.42 4.85

0.005 0.007 0.010 0.033 0.023 0.017 0.018 0.025 bdl bdl bdl bdl bdl bdl 0.014 0.021 0.014 0.021 0.006 0.009 0.011 0.016 0.011 0.011 0.011 0.013 0.012 0.011 0.008 bdl bdl bdl bdl bdl 0.016 0.034 0.019 0.016 0.010 0.014 0.014 0.011 0.010 0.015

0.32 0.55 0.50 0.79 0.83 0.69 0.72 0.83 0.50 0.43 0.48 0.39 0.50 0.58 0.64 0.71 0.73 0.77 0.44 0.46 0.58 0.67 0.35 0.48 0.27 0.38 0.29 0.39 0.49 0.36 0.40 0.44 0.45 0.22 0.69 0.96 0.43 1.09 0.53 0.64 0.46 0.51 0.58 0.22

2.59 1.23 1.35 3.87 4.25 5.62 5.13 3.46 3.87 3.09 3.54 3.03 2.33 2.25 1.94 1.85 2.33 1.44 3.63 3.24 2.69 3.04 2.68 1.98 2.3 3.0 2.63 2.28 1.6 2.89 3.74 2.01 2.58 1.56 2.73 2.67 2.9 1.93 3.0 3.51 2.628 3.25 2.87 1.23

0.013 0.042 0.048 0.103 0.130 0.087 0.132 0.186 0.082 0.051 0.052 0.029 0.058 0.045 0.070 0.063 0.067 0.068 0.024 0.040 0.045 0.065 0.053 0.039 0.038 0.029 0.054 0.045 0.034 0.026 0.034 0.050 0.051 0.050 0.114 0.243 0.080 0.052 0.028 0.093 0.071 0.072 0.071 0.16

0.01 0.96 0.90 0.85 0.82 0.32 0.51 0.79 0.27 0.26 0.18 0.16 0.53 0.50 1.19 1.60 1.32 1.77 0.04 0.23 0.65 0.90 0.06 0.47 0.06 0.29 0.09 0.33 0.57 0.07 0.03 0.72 0.55 0.27 0.45 0.8 0.35 1.56 0.06 0.25 0.40 0.36 0.45 0.19

0.002 0.017 0.015 0.026 0.028 0.012 0.020 0.028 0.017 0.014 0.013 0.011 0.017 0.026 0.022 0.031 0.031 0.033 bdl 0.007 0.015 0.024 0.010 0.010 bdl 0.015 0.007 0.011 0.013 0.016 bdl 0.025 0.018 0.006 0.011 0.058 0.015 0.034 0.006 0.011 0.017 0.014 0.013 0.092

1.56 3.36 2.77 3.83 3.79 2.86 3.44 3.79 2.22 2.65 2.68 2.52 2.39 2.69 2.62 2.94 2.64 2.62 1.72 2.45 2.13 2.55 2.41 2.46 1.24 2.37 2.08 3.0 4.2 1.95 2.42 3.17 2.94 2.10 4.07 3.27 2.41 2.43 3.13 3.01 2.51 2.66 3.07 1.42

nd 23.1 19.0 7.75 9.30 9.33 7.65 8.17 7.41 7.39 7.10 6.88 12.7 25.2 12.7 18.2 16.7 26.1 2.56 4.24 11.3 11.5 3.57 9.83 2.95 9.39 6.52 9.91 14.2 10.3 5.97 12.0 11.5 5.31 4.90 9.50 8.60 27.2 3.44 4.64 7.85 7.77 8.53 10

nd 6.97 10.6 21.9 13.3 6.65 6.56 3.99 13.7 6.17 4.28 2.60 1.92 2.97 3.47 5.94 3.85 1.92 7.12 6.39 10.5 7.80 4.0 1.28 0.51 1.98 2.39 2.98 4.06 30.6 5.54 3.49 4.15 5.47 0.43 2.51 2.95 10.1 8.36 0.56 5.02 4.86 3.98 1.2

nd 44.7 44.7 13.8 15.9 30.0 25.7 30.2 16.5 27.9 26.8 34.9 37.2 37.8 56.6 31.5 38.1 43.2 47.2 43.7 29.6 31.0 61.0 nd 107 49.2 80.1 82.9 nd 20.9 49.1 60.6 50.4 105 118 50.3 57.6 33.8 58.2 80.1 54.0 39.6 67.1 56

nd 860 713 364 59.0 250 38 142 201 265 365 148 812 419 223 358 322 367 58.0 117 211 247 bdl nd 3.0 76 13 106 nd 17 0 192 159 19 nd 108 40 370 5 56 102 134 126 90

1.92 10.6 11.2 9.27 5.88 4.93 3.75 4.06 8.89 8.05 7.10 6.18 8.31 12.8 7.26 9.26 8.12 9.62 5.82 8.38 15.9 14.7 3.24 3.94 4.65 9.76 5.25 5.62 5.17 14.2 5.67 7.60 7.27 4.56 1.66 3.37 7.44 21.1 13.0 2.30 5.85 6.26 6.29 4.1

nd 89.6 144 118 36.2 52.6 37.2 31.4 82.0 36.4 28.3 25.7 44.8 81.5 87.4 192 90.3 103 29.3 56.2 112 100 18.5 30.8 32.3 68.9 31.5 43.7 44.2 161 25.1 41.8 38.9 24.5 15.3 745 47.9 208 60 15.5 85.2 39.3 43.5 22

nd 58.1 98.7 176 121 65.0 95.3 157 76.6 56.5 51.4 35.5 53.2 62.4 83.8 250 205 118 45.4 86.1 145 152 25.0 19.4 16.7 52.8 27.4 29.0 22.9 144 27.5 38.6 84.5 23.8 18.5 57.1 39.9 228 73.6 29.5 53.8 62.9 56.4 15

nd 6.0 9.54 146 43.5 3.74 4.05 8.37 5.13 4.51 4.88 4.58 5.52 2.77 10.4 21.0 16.6 6.30 9.33 8.85 5.32 6.98 7.01 5.13 6.62 5.68 6.98 6.54 5.88 4.76 6.95 3.85 3.97 9.64 5.91 184 7.01 8.25 29.7 10.3 16.4 9.36 11.7 4.2

nd 22.0 38.0 510 207 25.9 48.2 90.6 32.5 19.3 17.6 12.4 20.0 16.3 34.5 122 78.9 42.7 13.3 22.4 31.6 45.1 14.4 14.6 8.91 11.0 9.86 11.7 8.66 18.4 13.6 11.7 31.3 8.69 13.6 45.9 12.1 36.0 174 21.3 23.3 42.5 41.5 9

nd 53.9 55.5 36.9 29.6 20.5 11.8 17.6 14.2 14.8 13.9 14.7 17.5 21.9 24.8 38.4 42.9 39.8 6.1 12.1 40.6 33.1 13.7 17.1 21.4 41.6 15.1 18.3 25.4 10.9 10.9 16.5 16.1 12.3 18.4 21.7 24.3 68.3 19.1 15.0 21.0 20.7 19.8 15

nd 70.0 81.6 134 115 54.3 43.5 48.6 50.8 36.6 46.5 37.0 44.0 46.1 53.9 67.3 45.3 55.0 33.0 40.1 46.5 55.4 59.5 11.8 32.2 28.9 30.0 37.7 9.09 55.2 54.5 41.9 47.7 29.1 15.9 129 41.3 320 114 48.7 58.0 43.0 51.4 18

nd 14.9 23.2 8.60 7.33 7.83 6.62 6.49 9.30 9.52 7.28 8.29 14.8 23.7 10.4 15.4 13.1 16.1 5.8 7.83 9.9 11.5 4.70 6.62 4.13 8.63 5.38 7.70 12.2 11.7 8.21 18.9 12.0 8.08 2.72 8.75 7.29 12.6 4.74 1.83 7.64 7.50 7.54 5.5

0.18 2.22 4.27 0.50 0.49 0.82 0.63 1.38 3.68 0.68 0.44 3.24 2.76 1.18 4.67 3.93 1.29 0.98 2.24 1.92 0.75 0.71 5.79 1.46 2.05 0.39 3.14 3.36 2.47 3.72 1.52 9.32 1.47 2.62 0.37 16.4 2.39 0.69 3.57 0.31 4.94 1.18 1.46 2

0.87 9.1 8.57 35.1 20.2 2.09 9.27 125 8.18 2.83 2.17 2.60 15.7 2.30 12.5 12.0 4.14 2.87 0.89 1.12 5.19 2.75 22.2 0.87 2.71 8.66 2.85 1.69 1.74 6.01 1.41 0.58 1.05 1.49 2.84 234 29.0 3.14 194 1.40 30.8 17.0 13.8 7.6

0.38 1.27 0.84 2.29 1.74 1.13 2.04 1.10 76.8 1.14 0.94 0.85 0.98 0.86 5.83 23.9 2.85 0.53 1.28 1.05 1.82 1.25 0.78 0.38 0.56 0.82 0.53 0.63 0.83 2.23 0.96 0.95 0.90 0.79 0.74 253 8.16 1.07 0.84 0.43 19.8 0.97 0.89 1

nd 56.5 46.3 29.0 29.0 12.3 16.9 27.9 10.9 11.0 7.41 6.75 27.9 25.2 43.7 67.7 57.2 71.1 1.51 9.67 30.3 38.5 2.58 24.4 3.13 22.3 3.72 22.6 39.6 3.27 1.54 33.3 24.5 14.2 17.1 26.0 14.9 105 2.96 9.97 16.5 15.6 20.4 10

nd 66.8 69.4 151 155 226 210 129 183 148 161 139 116 134 90.1 117 82.3 65.9 181 176 177 160 105 81.1 96.0 139 113 110 80.1 133 176 110 125 86.1 140 69.9 112 146 186 187 106 138 148 120

4.9 29.2 28.6 80.5 47.4 47.4 44.7 29.6 39.2 29.6 26.9 24.3 23.9 42.4 25.2 39.7 27.8 22.1 43.5 44.5 47.0 45.1 16.6 10.1 12.5 25.5 24.8 29.5 27.7 120 38.1 40.4 40.8 24.4 4.38 20.5 28.8 46.7 42.8 6.67 25.7 28.6 26.0 8.6

5.76 91.1 131 81.6 68.7 54.1 40.5 60.5 77.4 108 74.5 84.5 102 225 86.6 109 87.5 124 15.9 45.1 80.5 95.7 38.3 40.3 21.4 71.7 34.3 61.1 73.3 149 40.5 150 138 32.7 26.8 54.0 61.3 134 71.6 20.7 60.5 67.4 64.4 35

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

Sample no.

Nb

Mo

Cd

In

Sn

Sb

Cs

Ba

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

Ta

W

Re

Hg

Tl

Pb

Bi

Th

U

0407-12-0 0407-12-1 0407-12-2 0407-12-3 0407-12-4 0407-11-1 0407-11-2 0407-11-3 0407-11-4 0407-11-5 0407-11-6 0407-11-7 0407-11-8 0407-11-9 0407-10-1 0407-10-2 0407-10-3 0407-10-4 0407-10-5 0407-10-6 0407-10-7 0407-10-8 0161-12-1 0161-12-1′ 0161-12-2 0161-12-3 0161-12-4 0161-12-5 0161-12-6 0161-11-1 0161-11-2 0161-11-3 0161-11-4 0161-11-5 0161-10-C 0177-12-1 0177-12-2 0177-10-1 0177-10-2 0177-10-3 Categary 1 Categary 2 Categary 3 Worlda

0.24 6.74 6.65 4.06 4.51 2.73 2.55 3.23 4.99 7.95 5.59 6.20 7.76 18.0 6.42 7.47 6.29 10.5 1.24 2.64 5.1 6.59 2.41 3.23 1.47 4.52 2.08 3.99 6.04 4.64 2.30 11.8 10.1 2.45 1.86 3.05 3.46 9.53 2.98 1.52 3.70 4.56 4.56 3.3

8.22 237 299 20.4 30.2 30.1 17.2 81.4 53.0 32.1 12.7 18.3 19.8 4.78 102 59.3 45.9 8.68 34.2 17.2 6.19 24.2 328 75.2 100 617 79.6 25.4 10.3 115 8.95 11.4 7.51 87.2 3.20 1248 679 1.26 3.75 3.19 401 111 13.1 2.2

nd 0.85 1.02 0.26 0.12 0.13 0.12 0.28 1. 90 0.16 0.11 0.13 0.09 0.11 0.45 3.06 0.65 0.25 0.06 0.06 0.20 0.23 0.61 0.31 0.31 1.10 0.18 0.18 0.15 0.35 0.09 0.06 0.21 0.21 0.08 9.14 1.36 0.86 0.37 0.03 1.33 0.29 0.15 0.24

nd 0.058 0.061 0.025 0.016 0.035 0.020 0.014 0.103 0.031 0.018 0.026 0.044 0.074 0.051 0.065 0.042 0.096 0.011 0.034 0.051 0.039 0.030 0.010 0.017 0.079 0.018 0.031 0.01 0.039 0.019 0.046 0.045 0.017 bdl 0.076 0.049 0.10 0.015 0.014 0.045 0.028 0.023 0.021

0.22 2.47 2.12 2.67 1.71 0.92 0.70 1.21 1.77 2.01 1.37 1.36 2.42 5.12 1.86 3.32 2.10 4.03 0.30 0.86 1.94 2.01 0.59 0.95 0.71 1.71 0.74 2.29 1.67 1.70 0.38 2.88 7.93 2.01 0.68 1.19 1.15 2.92 0.85 0.58 1.15 1.40 1.78 0.79

0.07 1.71 2.61 0.96 0.82 0.45 0.38 0.64 0.74 0.52 0.40 0.54 1.25 1.29 1.20 1.09 0.61 0.64 0.74 0.79 1.62 1.11 0.39 0.36 1.02 1.01 1.03 1.03 0.72 2.97 0.72 1.43 1.0 0.64 0.27 1.60 0.79 1.0 3.65 0.14 0.81 0.69 0.79 0.84

nd 10.1 6.37 1.25 1.48 0.64 0.70 1.20 0.65 0.71 0.48 0.60 3.66 2.50 4.39 8.10 7.47 11.5 0.12 1.12 3.71 4.07 0.15 2.67 0.24 2.90 0.45 4.07 7.48 0.20 0.13 3.71 2.81 1.23 0.84 1.46 1.20 11.3 0.22 0.64 1.68 1.17 2.06 0.98

nd 126 119 207 201 176 201 241 151 117 103 82.0 111 143 223 324 239 276 81.4 114 183 308 51.8 101 49.7 117 65.8 97.7 108 36.1 43.9 110 95.8 55.8 128 191 127 318 71.4 107 113 130 126 150

7.2 13.6 9.56 20.6 19.1 16.3 16.9 14.6 29.8 23.7 19.9 18.4 27.9 38.5 19.9 45.0 46.3 33.1 9.02 12.0 20.8 29.9 10.1 13.4 7.54 23.9 11.0 16.6 23.0 16.9 12.9 51.1 26.5 12.6 5.09 26.2 15.0 22.1 6.14 5.65 16.0 17.7 15.2 10

11.9 24.8 17.1 39.2 38.2 38.7 38.8 32.4 35.7 45.6 40.7 37.9 53.6 71.9 36.3 84.8 70.5 66.1 21.0 26.4 43.4 62.2 22.0 27.3 16.8 48.7 23.6 32.7 47.2 37.6 31.2 104 54.8 24.9 10.1 50.8 33.5 39.4 13.2 10.5 31.3 36.2 30.8 22

1.24 3.30 2.34 5.07 4.77 5.27 5.26 4.09 5.55 5.32 4.84 4.61 6.04 8.66 4.55 9.76 9.88 7.34 3.01 3.6 5.36 7.23 2.68 2.92 2.26 6.09 3.17 4.10 4.97 4.98 4.04 11.3 6.53 3.23 1.09 5.61 4.59 5.30 1.77 1.17 4.03 4.45 3.68 3.5

4.96 12.3 8.77 22.7 20.5 23.2 23.5 18.2 21.0 20.4 17.8 17.0 22.6 31.3 18.8 38.4 39.1 28.2 13.3 16.1 22.2 27.8 10.0 11.8 9.69 23.2 13.8 16.8 20.5 22.5 17.6 41.1 24.1 13.0 4.42 20.7 18.6 21.1 7.99 5.0 15.8 18.2 14.6 11

0.96 3.2 2.44 5.85 4.28 5.65 5.74 3.84 4.21 4.65 4.09 3.71 3.86 5.78 4.07 6.74 7.22 4.67 3.58 4.21 5.62 6.49 2.0 2.0 2.56 4.83 3.56 3.87 3.84 6.48 3.96 6.95 4.87 3.19 0.83 3.02 3.75 4.42 2.66 0.84 3.18 4.07 3.11 1.9

0.18 0.93 0.61 1.61 1.05 1.14 1.26 0.99 0.97 1.0 0.90 0.85 0.88 1.10 0.76 1.56 1.27 1.01 1.03 1.01 1.33 1.37 0.44 0.38 0.45 1.05 0.77 0.83 0.74 1.82 1.04 1.19 1.0 0.61 0.18 0.77 0.93 1.14 0.73 0.22 0.73 0.88 0.70 0.5

1.01 3.37 3.31 7.79 5.40 5.92 5.42 4.44 4.25 4.34 3.94 3.81 3.77 5.66 3.54 6.50 5.66 4.40 4.91 5.04 6.04 6.39 2.19 2.27 2.32 4.51 3.38 3.88 4.58 9.27 4.94 6.83 5.32 3.08 0.91 3.32 4.22 5.84 4.09 0.85 3.39 4.13 3.42 2.6

0.12 0.75 0.67 1.63 0.96 0.97 0.94 0.64 0.83 0.82 0.72 0.67 0.67 1.08 0.63 1.10 0.87 0.69 1.02 1.02 1.13 1.19 0.38 0.28 0.37 0.80 0.61 0.64 0.65 2.25 0.94 1.11 0.95 0.60 0.12 0.48 0.75 1.11 0.86 0.14 0.61 0.71 0.62 0.32

0.78 4.88 4.81 11.53 6.38 6.39 5.90 4.08 5.43 5.29 4.66 4.31 4.09 6.96 3.95 6.17 5.02 3.84 6.17 6.78 7.09 7.65 2.33 1.70 2.46 4.98 3.97 4.35 4.13 15.6 5.56 6.56 5.96 3.98 0.81 2.64 4.67 7.79 6.19 0.81 3.83 4.58 3.99 2

0.14 1.01 1.01 2.51 1.30 1.29 1.28 0.88 1.11 1.01 0.91 0.82 0.83 1.32 0.83 1.18 0.97 0.76 1.31 1.40 1.45 1.51 0.49 0.33 0.42 0.92 0.70 0.84 0.85 3.29 1.08 1.31 1.21 0.85 0.15 0.49 0.90 1.62 1.33 0.14 0.78 0.89 0.81 0.5

0.43 3.14 3.17 7.29 3.98 4.23 3.97 2.64 3.41 3.02 2.53 2.31 2.3 4.14 2.50 3.47 2.54 2.32 4.03 4.15 4.67 4.55 1.42 1.03 1.37 2.63 2.30 2.63 2.65 10.2 2.85 3.82 3.45 2.23 0.46 1.33 2.56 4.96 4.24 0.59 2.27 2.72 2.42 0.85

0.06 0.45 0.5 1.12 0.64 0.59 0.55 0.43 0.51 0.47 0.40 0.37 0.35 0.65 0.34 0.49 0.40 0.35 0.58 0.63 0.70 0.71 0.19 0.15 0.18 0.42 0.34 0.44 0.36 1.46 0.42 0.57 0.53 0.39 0.07 0.19 0.43 0.76 0.61 0.08 0.34 0.42 0.37 0.31

0.37 3.11 3.23 7.04 3.99 4.06 3.83 2.68 3.54 3.12 2.66 2.34 2.26 4.60 2.66 3.56 2.56 2.51 3.71 4.02 4.70 4.52 1.4 0.96 1.34 2.72 2.09 2.56 2.35 8.76 2.51 3.74 3.22 2.47 0.46 1.22 2.63 4.97 3.89 0.45 2.26 2.72 2.36 1

0.06 0.51 0.49 1.10 0.61 0.69 0.63 0.48 0.54 0.50 0.41 0.41 0.38 0.68 0.41 0.52 0.39 0.40 0.60 0.66 0.75 0.73 0.24 0.14 0.18 0.36 0.32 0.39 0.34 1.31 0.34 0.58 0.54 0.42 0.07 0.15 0.37 0.76 0.60 0.08 0.35 0.43 0.37 0.19

0.19 2.58 2.86 1.89 1.92 1.47 1.14 1.76 2.21 3.13 2.16 2.43 2.96 6.38 2.56 3.10 2.64 3.71 0.45 1.18 2.33 2.71 0.99 1.12 0.61 2.08 1.0 1.76 2.02 2.42 1.08 4.16 3.84 1.0 0.77 1.16 1.77 3.51 1.33 0.58 1.61 1.94 1.77 1.2

0.04 0.65 0.57 0.41 0.40 0.25 0.22 0.30 0.46 0.76 0.54 0.57 0.72 1.69 0.57 0.70 0.62 1.00 0.07 0.21 0.45 0.60 0.25 0.25 0.09 0.42 0.18 0.38 0.49 0.36 0.24 1.06 0.95 0.39 0.14 0.30 0.32 0.87 0.16 0.12 0.35 0.42 0.41 0.26

6.67 6.04 1.87 9.14 23.2 18.7 7.06 1.91 5.33 2.37 4.64 4.23 4.38 6.39 4.86 3.91 3.32 4.66 2.31 1.93 2.02 6.66 18.8 62.1 4.42 7.39 2.92 4.24 3.53 0.76 13.8 12.8 5.66 5.23 15.8 12.1 3.39 9.66 1.18 3.78 10.8 12.8 7.70 1.2

bdl 0.009 0.009 0.006 0.016 0.035 0.026 0.054 1.370 0.054 0.026 0.010 0.007 0.002 1.198 4.742 0.758 0.021 0.006 0.003 0.034 0.013 0.815 bdl 0.005 0.003 0.002 bdl bdl 0.033 0.002 0.006 0.002 0.003 bdl 34.0 0.179 0.010 0.010 0.014 2.377 0.028 0.008 0.001

nd 124 129 301 147 25 28 132 21 22 26 31 66 50 49 35 34 30 60 66 203 97 36 nd 34 44 29 14 nd 61 36 52 25 44 nd 116 37 127 3858 39 46 47.7 250 100

nd 0.40 0.30 2.32 1.03 0.12 0.22 1.66 0.71 0.88 0.36 0.41 1.18 0.41 2.0 1.72 1.31 0.95 0.66 0.69 0.68 0.66 0.8 0.16 0.05 0.29 0.10 0.21 0.24 0.25 0.04 0.22 0.15 0.25 0.09 4.53 0.34 0.89 24.7 0.14 0.98 0.61 1.66 0.68

nd 25.8 24.3 17.8 9.88 13.9 10 11.0 22.7 13.8 7.86 12.7 20.3 14.9 23.4 27.7 21.5 15.4 21.4 19.8 29.2 23.9 9.49 5.47 7.88 22.5 13.6 11.3 5.02 84.3 14.1 11.0 24.9 6.76 4.47 57.5 20.5 33.8 24.0 4.26 19.8 12.6 12.3 6.6

nd 0.63 0.58 0.18 0.11 0.3 0.14 0.13 0.29 0.29 0.2 0.19 0.29 0.41 0.30 0.54 0.55 0.73 0.08 0.18 0.38 0.36 0.18 0.15 0.21 0.41 0.17 0.28 0.27 0.33 0.11 0.28 0.25 0.14 0.03 0.09 0.29 0.57 0.13 0.06 0.26 0.24 0.18 0.84

1.95 10.5 8.82 4.76 3.43 6.43 3.72 3.48 10.1 13.3 10.3 9.3 10.8 20.5 7.27 11.9 11.0 12.2 2.37 5.65 15.4 12.2 3.97 7.93 4.93 14.9 5.07 5.66 8.24 10.7 5.65 12.3 11.9 3.75 2.74 2.73 9.25 23.7 5.21 1.35 6.63 8.61 6.70 3.3

9.52 161 307 17.6 31.8 43.0 21.9 51.8 334 31.5 14.8 11.8 14.6 17.0 331 1402 173 9.07 16.3 15.5 13.7 23.1 864 46.6 57.2 34.3 57.8 20.3 10.6 638 8.02 11.6 10.7 15.1 2.51 7207 761 5.87 3.07 1.76 1030 41.7 8.93 2.9

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

Sample no.

nd, not determined. bdl, below detection limit. LOI, loss on ignition. a Average concentrations of elements for the world low-rank coals (Ketris and Yudovich, 2009).

11

12

Table 4 Mineralogical compositions of coal LTA samples by XRD and Siroquant (wt.%). Coal

Sample no.

Ad

Quartz

Kaolinite

ZK0407

No.12

0407-12-0 0407-12-1 0407-12-2 0407-12-3 0407-12-4 0407-11-1 0407-11-2 0407-11-3 0407-11-4 0407-11-5 0407-11-6 0407-11-7 0407-11-8 0407-11-9 0407-10-1 0407-10-2 0407-10-3 0407-10-4 0407-10-5 0407-10-6 0407-10-7 0407-10-8 0161-12-1 0161-12-1′ 0161-12-2 0161-12-3 0161-12-4 0161-12-5 0161-12-6 0161-11-1 0161-11-2 0161-11-3 0161-11-4 0161-11-5 0161-10-C 0177-12-1 0177-12-2 0177-10-1 0177-10-2 0177-10-3

8.9 51.6 41.0 35.4 39.8 23.6 29.0 40.9 31.0 31.0 25.1 23.5 38.0 64.7 47.0 58.4 48.4 68.5 10.2 18.5 39.9 43.8 12.3 31.8 10.0 24.8 17.3 28.7 43.1 19.7 14.0 44.7 38.7 15.0 24.8 44.4 25.9 54.5 15.7 15.9

11.7 47.9 33.3 50.3 51.6 38.3 37.1 44.1 31.0 38.2 38.6 34.9 31.0 35.0 36.0 45.0 38.8 33.9 18.2 28.6 23.9 36.7 23.2 31.6 13.6 28.7 26.8 37.0 45.8 22.5 40.8 40.6 40.7 28.3 42.4 28.6 37.6 31.9 37.6 40.5

68.0 35.4 37.2 15.7 19.3 21.9 20.0 12.4 51.2 49.4 44.4 48.8 48.7 39.4 37.7 28.3 34.3 37.6 36.2 32.6 43.4 35.5 30.0 48.3 62.5 46.7 55.5 28.1 33.6 57.6 58.7 34.2 36.7 44.5 17.9 11.8 49.0 43.9 29.3 24.6

No. 11

No. 10

ZK0161

No. 12

No. 11

ZK0177

No. 12 No. 10

Illite 13.1 21.1 7.0 13.3 12.3 16.0 7.5 7.4 3.7 6.6 3.1 9.7 4.2 14.1 14.2 24.6 27.1 5.8 23.1 17.7 19.5 6.3 12.8 16.3 23.9 5.1 10.5 18.4 3.5 20.1 12.3 18.8 14.6 7.2 3.2 18.2 6.1 14.0

I/S

2.3

Chlorite 2.5 1.7 3.6 2.5 3.3 0.3 4.5

1.3

1.0

K-feldspar

1.5 8.2 3.3 11.6 8.7 1.7 8.9 5.2 3.2 5.0 1.1

Na-feldspar

3.0 2.3 5.0 6.8

Amphibole

0.6 4.4 3.5 2.0 1.5

21.1 2.2 3.8

1.7 3.2 0.5

4.3 3.4 1.0 1.0 6.1 0.8 3.1 3.7

3.6

0.6

2.2

22.2

Pyrite

0.4 14.9 0.4 0.9 0.3 0.2 2.8 0.3 1.5 0.8 0.6 1.1 6.3 9.7 1.4 8.7 0.4 7.6 0.7 2.0 0.4

Calcite

0.7 4.0 6.9 3.2

2.3

1.1

4.7

0.5

8.3

3.1

6.4 4.1

5.6 4.0 5.9

3.1 0.2 2.5

0.8

5.9

5.3 5.8

6.8 2.3 30.9 1.4 0.5 18.4 6.2

0.6

Bassanite

2.1

1.6

Gypsum

Anatase

20.4 1.1 4.6 7.8 1.3 3.5 1.8 7.4 1.0 1.2 1.1 7.9 5.7 4.1 0.8 0.7 0.4 22.5 8.6 2.1 0.4 28.5 4.1

10.6 1.9 1.4 11.9

4.5 0.5 0.4 2.7

Dolomite

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

Drill hole

3.4 4.0 1.7 5.7 4.9 0.5 0.3 8.6 0.6

0.8

1.3 0.5

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

Fig. 7. Concentration coefficients of trace elements in the Yili coals vs. world low-rank coals. Data for trace elements in world low-rank coals are from Ketris and Yudovich (2009).

Fig. 8. Frequency histogram of U concentrations for the Yili coal samples.

13

14

a

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

b

Fig. 9. Variations of ash yield, total sulfur, As, Se, Hg, Mo, Re, U, and W, through the coal beds of drillhole K0407. (A) No. 10 coal; (B) No. 11 coal.

a

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

b

15

Fig. 10. Variations of ash yield, total sulfur, As, Se, Hg, Mo, Re, U, and W, through the coal beds of drillhole K0161. (A) No. 11 coal; (B) No. 12 coal.

16

a

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

b

Fig. 11. Variations of ash yield, total sulfur, As, Se, Hg, Mo, Re, U, and W, through the coal beds of drillhole K0177. CS, carbonaceous sandstone. (A) No. 10 coal; (B) No. 12 coal.

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

17

Fig. 12. REY distribution patterns in the coal benches of the three drillholes. REY plots are normalized by Upper Continental Crust (UCC) (Taylor and McLennan, 1985).

ash yield (dry basis) of the samples. For example, quartz typically makes up b40% of the mineral matter in the coals with low (b20%) ash yield, but may make up to 80% of the mineral matter in some of the noncoal samples with very high ash yields. This may in part reflect greater relative input from detrital sources for the non-coal and higher-ash coal samples, which is also consistent with the modes of quartz occurrence as described below. By contrast, kaolinite is more abundant, as a fraction of the total clay minerals, in the lower-ash coal samples, and is increasingly admixed with other clay minerals, mainly illite and I/S, in the higher-ash samples (Fig. 14B). This may also suggest that the illite and I/S are mainly of detrital origin. The abundance of K-feldspar as a fraction of the mineral matter, on the other hand, does not seem to show any trend with ash yield, although most of the samples with b20% ash have low proportions (if any) of the K-feldspar component (Fig. 14C).

Traces (b1%) of pyrite are present in most of the samples; in some cases, however (e.g. samples 0177-12-1 and 0177-10-2), pyrite may make up to 20–30% of the total mineral assemblage (Table 4). Small proportions of calcite and/or dolomite also occur in some of the samples, although in one case (sample 0407-10-9) calcite makes up almost 20% of the crystalline mineral matter. Minor gypsum occurs in the roof samples of coal bed 11 in drill holes ZK0407 and ZK0161. Bassanite (CaSO4·1/2H2O) is present in the LTAs of most of the coal samples (Table 4). Fig. 14D indicates that bassanite is most abundant (up to 30%) in the LTA of the low-ash coal samples (samples with b20% ash), but makes up less than 1% of the mineral matter in samples with more than around 50% ash yield. Bassanite has not been identified at all in samples with more than around 70% ash yield. As discussed by Ward (2002) and other authors, bassanite in the LTA of coal samples may be formed during plasma-ashing by interaction of

18 S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

Fig. 13. REY distribution patterns in the roof, floor, and parting strata of coal beds in the Yili Basin. REY plots are normalized by Upper Continental Crust (UCC) (Taylor and McLennan, 1985).

S. Dai et al. / Ore Geology Reviews 70 (2015) 1–30

19

Fig. 14. Relations between ash yield and selected minerals in mineral matter of the samples investigated. (A) Quartz; (B) kaolinite in total clays; (C) K-feldspar; (D) bassanite.

organically-associated Ca with sulfur in the coal samples, especially with lower-rank materials. Alternatively, it may represent a dehydration product of gypsum, formed by crystallization of Ca2 + and SO24 − ions in the pore water of the coal sample (e.g., Ward, 2002). In either case, the bassanite would be expected to be most abundant in coals with a high proportion of organic matter and hence a low ash yield. The occurrence of gypsum in some high-ash (non-coal) samples, which have not been subjected to plasma-ashing, is also consistent ions in the pore waters of the with crystallization from Ca2+ and SO2− 4 samples concerned, but a combination of crystallization and artifactforming processes may apply to the LTAs of the lower-ash coal samples. Although gypsum and/or bassanite may also be formed by interaction of acids from pyrite oxidation with calcite in the samples (e.g., Rao and Gluskoter, 1973), the Yili samples typically have low pyrite and low calcite contents. Other pyrite oxidation products, such as jarosite, are also not present to support a process of this type. Other minerals with various percentages in the coal LTAs include smectite, amphibole, calcite, dolomite, and anatase in various samples. Some traces of other phases are also detected by SEM-EDS, but these have overall concentrations below the detection limit of the XRD technique. Such phases include millerite, chalcopyrite, cattierite, siegenite, ferroselite, krutaite, eskebornite, pitchblende, coffinite, U-bearing sulfates, silicorhabdophane, cheralite, Fe–Ti oxides, zircon, potash feldspar, calcite, and Fe-hydroxide. Overall, with the exception of more abundant bassanite and a slightly higher relative abundance of kaolinite, the mineral assemblages in the coal LTAs are similar to those in the partings, roof and floor strata (Table 4; Supplementary Electronic File 3). This may possibly indicate the same source of mineral input during sedimentation for both types of material, and also similar effects from any subsequent diagenetic or epigenetic activity. 4.4.2. Comparison between mineralogical and chemical compositions The percentages of major-element oxides observed by XRF have been recalculated to provide the normalized SO3-free oxide percentages for each sample (see Supplementary Electronic File 4). The majorelement oxide percentages inferred from the XRD data were calculated

as described by Ward et al. (1999) and are presented in Supplementary Electronic File 5. The relationship between the observed (directly analyzed) and inferred (from XRD) percentages of key oxides is given in Fig. 15. Data for low-ash (b18% ash) and higher-ash (N 18% ash) samples are plotted separately in each case. A diagonal line is provided on each plot to indicate where the data points would fall if the observed and inferred percentages were exactly equal for each sample in the series studied. With some exceptions in the case of the low-ash coals (Fig. 15; ash b 18%), discussed more fully below, most of the data points for SiO2, Al2O3 and Fe2O3 (Fig. 15A,B,C; ash N 18%) fall close to the diagonal equality line, suggesting that the mineral quantifications obtained from the XRD analysis for the higher-ash materials (at least) are consistent with the normalized chemical data. The plot for K2O is more scattered (Fig. 15D), partly because of the relatively large errors associated with XRD determinations at such low concentrations and of uncertainties in the chemistry of the illite and I/S, but still shows a broad correlation to the equality line. By contrast, the plots show considerably lower inferred proportions of CaO and MgO from the XRD data than are indicated by direct chemical analysis (Fig. 15D,E). The differences are especially great for the lowash samples (b 18% ash) in each case. Despite the abundance of poorlyordered bassanite noted in the low-ash coals as described above, which is included in the plot for CaO, the plots therefore suggest that the LTA of these samples may contain additional Ca and Mg in non-crystalline form. Such material, if present, would probably be made up of disordered Ca and Mg oxides released by plasma ashing for which insufficient S was available to allow sulfate (e.g. bassanite) formation. The data points for the low-ash coals (Fig. 15A,B; ash b 18%) in the plots for SiO2, and to a lesser extent for Al2O3, tend to fall above the equality line, providing a departure from the otherwise close relationship indicated by the data points for the higher-ash materials. This is consistent with the presence of non-crystalline Ca-rich material in the LTA of those samples, which would not have been included in the quantification for the crystalline components. Normalization of the crystalline components to 100% would therefore have resulted in higher proportions of Al- and Si-bearing phases in the Siroquant data, and

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Fig. 15. Comparison of observed normalized oxide percentages from chemical analysis (x-axis) to oxide percentages inferred from XRD data (y-axis). The diagonal line in each plot indicates equality.

higher inferred proportions of SiO2 and Al2O3 in comparison to the observed values (i.e. including the non-crystalline material). The plot for SiO2, representing the most abundant non-Ca oxide, would have been most affected by this process. Taken together, the XRD and chemical data indicate that the organic matter of the coals contains a significant proportion of Ca and Mg in non-crystalline form. This observation is further supported by the common occurrence of Ca in the SEM analyses of vitrinite. This may, for example, be represented by dissolved ions in the (relatively abundant) pore water, with 10–15% air-dried moisture typically occurring in the coal samples (Table 1), with the higher percentages typically occurring in the lower-ash materials. However, it may also represent Ca, Mg and possibly other ions attached in some way to the maceral components (Li et al., 2010; Mares et al., 2012). The relative significance of this non-mineral inorganic component is greatest in the low-ash coals (e.g. b18% ash), and decreases as the ash yield increases. While some of this material appears to form bassanite during plasma-ashing, there may be insufficient S in the coals to convert all of this Ca to crystalline sulfate material (or the Mg to phases such as hexahydrite). 4.4.3. Modes of occurrence of minerals and trace elements in coal The quartz and K-feldspar in the coal occur mainly as discrete grains in collodetrinite or in association with clay minerals (Fig. 16), suggesting that they are derived from detrital materials of terrigenous origin. The associated occurrence of sodic plagioclase and sparse amphibole

suggests derivation from a granitic source, which is consistent with the presence of Hercynian granite on the basin margins. Cell- and fracturing-filling quartz and feldspar of possible authigenic origin were not observed in the coal samples. Two modes of occurrence are observed for kaolinite in the Yili coals: coarse-grained kaolinite distributed along the bedding planes (Figs. 16A; 17A,B,C) and fine-grained kaolinite occurring as cellfillings (Fig. 17D,E,F). The first type of kaolinite is probably a detrital mineral of terrigenous origin and the second type is probably of authigenic origin. The illite, mixed-layer illite/smectite, Fe-rich chlorite and mica (with lath- and needle-shapes and long-axes parallel to the bedding planes; Fig. 16A,B,D) have modes of occurrence indicating that they are mainly detrital materials of terrigenous origin. In addition to the pyrite observed by XRD, SEM-EDS, and under the optical microscope (Fig. 18), other sulfide and selenide minerals identified by SEM-EDS in the Yili coals include millerite (NiS), chalcopyrite (FeCuS2), cattierite (CoS2), siegenite ((Ni,Co)3S4), ferroselite (FeSe2), krutaite (CuSe2), and eskebornite (CuFeSe2) (Fig. 19). Pyrite in the coal occurs as framboidal aggregates (Fig. 18A) or finegrained euhedral crystals (Fig. 18B) in collodetrinite. The framboids are sometimes associated with kaolinite layers (Fig. 18A), and compaction of the layering around the pyrite suggests that the framboids are of syngenetic origin. Cell-filling (Fig. 18C), massive, fracture-filling, and dendritic (Fig. 18D) forms have also been observed, with the latter two forms suggesting an epigenetic origin. The different forms of pyrite

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Fig. 16. SEM back-scattered electron images of quartz, clay minerals, K-feldspar, mica, pyrite and siderite in coal. (A) Quartz, illite and kaolinite (or mixed-layer I/S) of detrital materials of terrigenous origin, sample 0407-10-2. (B) quartz, illite, kaolinite of detrital materials of terrigenous origin, sample 0177-12-1. (C) quartz, K-feldspar, and siderite in sample 0177-12-2. (D) Quartz, kaolinite, K-feldspar, and mica of detrital materials of terrigenous origin; sample 0177-10-2.

(e.g., Fig. 18A,D), and also the occurrence of single pyrite particles with different textures in the internal and external parts (Fig. 18E), indicate multiple stages of pyrite formation. Fe-sulfate minerals, probably water-bearing Fe-oxysulfates that contain Si, are distributed within the organic matter (Fig. 18C). This material, being closely distributed with pyrite (Fig. 18C), is probably a pyrite oxidation product. Trace concentrations of other sulfide and selenide minerals occur either as cell-fillings (Fig. 19A,B) in collodetrinite (Fig. 19C,D) or as fracture-fillings (Fig. 19D). The modes of occurrence of these minerals suggest that they are mainly of epigenetic origin. Siegenite and cattierite are rarely observed in coal (Tang and Huang, 2004; Dawson et al., 2012; Dai et al., 2015a). Trace amounts of cattierite have been found in the Baralaba Coal Measures of Queensland's Bowen Basin (Dawson et al., 2012). Cattierite may directly be derived from hydrothermal solutions. However, Co is one of the most common elements that may substitute for Fe in the pyrite structure; if substitution is complete it may give rise to cattierite, which is isostructural with pyrite (Vaughan and Craig, 1978). The cattierite in the Yili coals contains minor amounts of Ni and Se, which substitute for Co and S, respectively. Ferroselite has been found in a few other coals (Goodarzi and Swaine, 1993). Krutaite and eskebornite have not previously been reported in coal, but the latter has been found in the Se-rich carbonaceous siliceous rocks and carbonaceous shales (known locally as “stone coal”) of the lower Permian Maokou Formation in Enshi, China (Belkin et al., 2003; Zhu et al., 2012). The krutaite in the Yili coals contains minor amounts of Co, Fe, and S.

Some of the minor minerals in the coals, such as ferroselite, krutaite, and cattierite, may be carriers of a proportion of Se in the Yili coals. The correlation coefficients of Se with Fe2O3 and S (Se–Fe2O3 = 0.70, Se– S = 0.80) are high, which seems to demonstrate the sulfide affinity of Se; however, the actual X–Y plots of Se against Fe2O3 and total S (see Supplementary Electronic File 6) show only a single high-Se sample with high Fe and S values (sample 0177-12-1). This clearly indicates a Se-bearing pyrite. However, the other two high-Se coals (sample 0407-10-2 and sample 0407-11-4) have relatively low Fe and S, and no particular correlation is apparent. U-bearing minerals identified by SEM-EDS but below the detection limit of the XRD technique include pitchblende, coffinite, and a Ubearing sulfate (Fig. 20A,B,C). These minerals are either distributed in the pores of the organic matter or occur along the edges of cavities/fractures/cleats. The low correlation coefficient for U-ash (r = 0.15) suggests that U has a mixed organic–inorganic affinity. The X–Y plot of U against ash yield (see Supplementary Electronic File 6) shows one high-U sample (sample 0177-12-1) with an ash yield of 44.36% and then a more or less random relationship between U and ash. Both the plot and the correlation coefficient therefore seem to indicate that there is no particular preference for U concentration in either the organic or mineral fraction. SEM/EDS analysis of some vitrinites nevertheless suggests that U also occurs in the organic matter. Uranium occurs in other U-rich coals mainly in the organic matter (Dai et al., 2008, 2015b; Seredin and Finkelman, 2008) or as coffinite (Van Der Flier and Fyfe, 1985; Seredin

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Fig. 17. SEM back-scattered electron images of kaolinite in the Yili coals. (A)–(C) Coarse kaolinite distributed in collodetrinite 0177-12-2. (D) Fine-grained cell-filling kaolinite in sample 0177-12-2. (E) and (F) fine-grained cell-filling kaolinite in sample 0177-12-1.

and Finkelman, 2008), pitchblende, torbernite, autunite, uranophanebeta, uranospinite, zeunerite, and renardite (Stoikov, 1976; Seredin and Finkelman, 2008); and brannerite (Dai et al., 2015b). Finkelman (1981) found that uranium in coal may occur in apatite, monazite, uraninite, zircon, calcite, rutile, and a lead-bismuth phase, as well as in the organic matter; he lists 14 other uranium minerals (many oxidation products) that have been found in mineralized coals. The correlation coefficients for Mo-ash (0.02) and Re-ash (0.16), as well as the X–Y plot of Mo or Re against ash yield (see Supplementary Electronic File 6), also

suggest that Mo and Re have no particular correlation with ash yield and therefore a mixed organic–inorganic affinity. The relationships of Hg and As to ash yield (see Supplementary Electronic File 6) show that the two elements have a mixed organic–inorganic affinity, with the exception of three high-pyrite samples (0177-10-2, 0177-12-1, and 0407-11-3) that have a sulfide affinity for As and a high-pyrite sample (0177-10-2) with a sulfide affinity for Hg. Sulfide and organic affinities for As and Hg have also been reported by previous studies (e.g., Belkin et al., 1997; Coleman and Bragg, 1990; Eskenazy,

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Fig. 18. Pyrite in sample 0177-10-2. (A) Framboidal and spherical forms of pyrite in collodetrinite, SEM back-scattered electron image, sample 0177-10-2. (B) Fine-grained crystals of pyrite in collodetrinite. (C) Cell-filling pyrite and water-bearing Fe(Si)-oxysulfate. (D) fracture-filling dendritic pyrite. (A)–(D) SEM back-scattered electron images. (E) Optical microscope, oil immersion, white light.

1995; Huggins and Huffman, 1996; Minkin et al., 1984; Riley et al., 2012; Ruppert et al., 1992; Ward, 2001; Yudovich and Ketris, 2005a,b; Zhao et al., 1998). The interstitial mode of occurrence of silicorhabdophane (Fig. 21), and that also of Fe-hydroxide (Fig. 21E,F), indicate that these phases are of authigenic origin. Authigenic silicorhabdophane has also been observed in other REY-rich coals (Seredin and Dai, 2012). The modes of occurrence of calcite and K-feldspar (Fig. 21E), not filling cavities or

fractures but occurring as individual particles in the kaolinite matrix, suggest a terrigenous material of detrital origin, consistent with the petrological compositions of the sediment-source region interbedded with carbonate layers. Syn-sedimentary calcite of detrital origin is seldom observed in coal because calcite can be easily-decomposed under acid conditions in the peat bog; however, syngenetic deposition of calcite (aragonite) is possible if a sediment source region mainly made up of carbonate rocks is located close to the peat mire (Bouška et al., 2000),

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Fig. 19. SEM backscattered electron images of sulfide or selenide in the Yili coals. (A) Siegenite and chalcopyrite in cavities of sample 0407-10-2. (B) Cell-filling cattierrrite in sample 017712-1. (C) Ferroselite and krutaite distributed in collodetrinite of sample 0177-12-1. (D) Ferroselite, eskebornite, and pyrite in sample 0177-12-1.

or if fragments of shells from organisms living in or around the peat swamp are preserved within the organic matter (Ward, 1991). 5. Discussion Sandstone-type U deposits account for ~ 18% of world U resources and are one of the main U mineralization types in the USA, Australia, Niger, South Africa, and central Asia (Hobday and Galloway, 1999; World Nuclear Association, 2009). The U in these deposits has generally accumulated within medium to coarse-grained sandstone beds in localized reduced environments, typically in curved zones known as rollfronts (Min et al., 2005a,b; Wu et al., 2009; Pirajno et al., 2011). Some of these sandstone-hosted U deposits, including the Yili U ore, are hosted in coal-bearing sequences (Min et al., 2005a,b; Wu et al., 2009). Min et al. (2005a,b), Wang et al. (2005, 2006a,b), and Feng and Jiang (2000) have studied and reported on the Yili sandstone-hosted roll-front type uranium deposits. The host sandstone for the U typically consists of quartz, feldspar, carbonaceous debris, and lithic fragments in a clay and silt matrix. The orebodies have a classic single or double Cshape (roll-front) at the interface between oxidized and reduced rocks. The ore minerals are uraninite, coffinite, pyrite, marcasite, galena, and urano-organic complexes (Min et al., 2005a,b). The plant fragments in the host sandstone in the ore zones are enriched in U, Se, Mo and Re. The mineralization has U–Pb ages of ca. 17 to 11 Ma (Min et al., 2001). Such sandstone-hosted roll-front U mineralization in the coal basin relates to the injection of Miocene low-temperature meteoric fluids into Jurassic clastic sediments (Min et al., 2001; Pirajno et al., 2011). Yue

and Wang (2011) showed that the dissolution and deposition of U in water are closely related to reactions with organic C and minerals such as sulfides, carbonates, and silicates along the relevant groundwater flow paths. However, U mineralization in coal itself has not been studied as fully as that in sandstone, even though coal may contain several hundreds to a few thousands μg/g of U (Seredin and Finkelman, 2008; Dai et al., 2015b). The elevated concentrations of U, Se, Mo, Re, Hg, and As, as well as the mineral assemblages in the Yili coals, are attributed to input from the sediment source region and two different solutions (U–Se–Re–Mo rich infiltrational and Hg–As-rich exfiltrational volcanogenic solutions).

5.1. Sediment source region The nature of the sediment-source region on the margin of a coal basin may be a major factor in determining the background concentrations of trace elements, not only in small-scale fault-controlled and larger-scale down-warped coal basins (Dai et al., 2012), but also in coal-hosted ore deposits (Dai et al., 2012; Zhuang et al., 2006; Qi et al., 2007). As discussed above, the modes of occurrence of the quartz, Kfeldspar, coarse-grained kaolinite, illite, chlorite, amphibole, sodic plagioclase, and calcite in the Yili coals suggest that they were derived from detrital materials of terrigenous origin. The angular nature of the detrital mineral grains (Fig. 16 A,B,C) and the poorly-sorted characteristics of the minerals (Fig. 16), as well as the presence of what appear to be epiclastic carbonate minerals (e.g., calcite, Fig. 21E), indicate that

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The Al2O3/TiO2 ratios for the partings and the roof and floor strata (Fig. 22A), as well as coal benches (Fig. 22B), indicate that the sediment source region for the coal-bearing strata in the Yili Basin was mainly of felsic and, in some cases, intermediate composition, consistent with the petrological compositions of the Hercynian granite, Permo-Carboniferous intermediate felsic igneous rocks, and pyroclastic rocks interbedded with carbonate layers in the surrounding region as described by Min et al. (2001, 2005a,b) and Wang et al. (2005, 2006a, b). The presence of K-feldspar (possibly microcline) in the coal and non-coal samples is also consistent with a felsic sediment source material. 5.2. Hydrothermal solutions Solutions that circulate in coal basins can be classified into descending (infiltrational) and ascending (exfiltrational) types (Seredin and Finkelman, 2008). Infiltrational solutions are mainly derived from meteoric waters migrating under gravity from peripheral to central parts of the basin. Exfiltrational solutions are dominantly derived from groundwater and are mainly driven by high gas pressures, penetrating into the coal basin along faults in the basement rocks (Seredin and Finkelman, 2008). Both infiltrational and exfiltrational solutions can occur at the syngenetic, diagenetic and epigenetic stages of a coal's history (Seredin and Finkelman, 2008). Solutions with a significant influence on the mineralogical and geochemical compositions of coals, partings, and host rocks (roof and floor strata) have been described in a number of different coal deposits around the world (Golab et al., 2006; Zhuang et al., 2006; Seredin and Finkelman, 2008; Dai et al., 2012, 2015b; Dawson et al., 2012; Sia and Abdullah, 2012; Permana et al., 2013). The enrichment of U in the Yili coals, as well as of Se, Re, and Mo, is of the epigenetic infiltrational type for the following reasons:

Fig. 20. Back-scattered electron images of pitchblende (A, sample 0177-12-1), coffinite (B, sample 0177-12-1), and U-bearing sulfate (C, sample 0177-12-2).

the sediment source region in the present case was located close to the coal basin. The Al2O3/TiO2 ratio is a useful provenance indicator of sedimentary rocks (Hayashi et al., 1997; He et al., 2010) and of the sediment-source region for coal deposits (Dai et al., 2013, 2015b), mainly because of the similar ratio of these elements in sedimentary materials to that in their parent rocks (Hayashi et al., 1997; Dai et al., 2013). Typical Al2O3/TiO2 ratios are 3–8, 8–21, and 21–70 for sediments derived from mafic, intermediate, and felsic igneous rocks, respectively (Hayashi et al., 1997).

(1) In addition to serving as the sediment source region for the Yili coals during peat-accumulation, the Permo-Carboniferous intermediate-felsic igneous rocks and Hercynian granite are both enriched in U (3.0–12.9 and 5.4–20.9 μg/g, respectively; Hou et al., 2010), and have provided epigenetic sources of U for this coal-hosted ore deposit, like for the sandstone-hosted U deposits (Min et al., 2001, 2005a,b; Wang et al., 2005, 2006a,b) overlying the U-rich coals. (2) The climate has been arid from the late stage of Early Cretaceous to the Quaternary (Wang et al., 2006a,b), which would easily cause oxidizing conditions and thus have favored leaching of U from the surrounding U-rich rocks to soluble U(VI) in solution. This has also been reported by a number of researchers (Min, 1995; Spirakis, 1996; Qin et al., 2009; Seredin and Finkelman, 2008). The waters in the fractures of the igneous rocks surrounding the basin are high in U (30–400 μg/L; Hou et al., 2010). (3) A high proportion of coarse sediment (e.g., pebbly and/or coarsegrained sandstone; Figs. 9–11) was available to serve as channels for migration of the U-bearing solutions. The portions of the coal beds immediately underlying the coarse sediments were accessible to oxygen-rich waters with high uranium contents during arid periods, allowing the U to be captured by the organic matter (Figs. 9–11). The highest U concentrations are found in the upper portions of the seams at or near contacts with the oxidized host rocks. The thickness of the coal benches with significant U enrichment (CC N 50 or concentration N150 ppm) is from 10 cm (e.g., ZK0161-11) to 180 cm (e.g., ZK0161-12). However, Seredin and Finkelman (2008) have reported that the thickness of near-contact U-bearing coal layers of epigenetic infiltration type is 0.1–0.5 m in most cases and rarely exceeds 1–2 m. Those portions of the seams adjacent to impermeable clay roofs, partings and floors are low in U (Figs. 9–11). The U ore bodies in infiltrational-type deposits with monoclinal dip for the coal

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Fig. 21. Back-scattered electron images of other trace minerals identified in the Yili coals. (A) and (B) Silicorhabdophane distributed in kaolinite in sample 0407-10-2. (C) Zircon in kaolinite matrix in sample 0177-12-2. (D) Siderite in sample 0177-12-2. (E) and (F) K-feldspar and poorly crystalline hydroxide in sample 0177-12-1, (F) is the enlargement of the rectangle area in (E).

beds are located only in the upper portions of those beds (Denson and Gill, 1965). (4) The occurrence of U-bearing minerals in the pores (e.g., cells, fractures, veinlets) of the inertinite macerals (Fig. 20) indicates that these minerals were deposited from solutions that permeated the micro-porous inertinite-rich coal (especially of the structured fusinite and semifusinite as described above) at some stage after coal deposition. Porous inertinite-rich coals may not only have provided channels for migration of the U-bearing

solutions but also acted as a reductant, leading to a change from U(VI) to U(IV) and then precipitation of U minerals (e.g., pitchblende and coffinite; Fig. 20) where favorable conditions exist, such as in the (inertinite-rich) coal near a porous sandstone roof horizon. The reducing conditions for U deposition from solutions are also evidenced by co-existing pyrite and trace amounts of other sulfide and selenide minerals in the coal. Additionally, adsorption and complexation of organic matter are the other two deposition mechanisms of U from U-rich solutions

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Fig. 22. Al2O3 vs. TiO2 in the Yili host rocks (A) and coal benches (B).

(Meunier et al., 1989; Wang, 1983), which is evidenced by the mixed organic–inorganic associations of U in the Yili coals. The coals' low rank (Ro = 0.51–0.59% Rr for Yili coals) is favorable for the two processes of U deposition (Wang, 1983). (5) The sediment source regions of the coals in the present study were mainly composed of intermediate to felsic igneous rocks, which would be expected to lead to an L-type REE enrichment. However, most of the coal benches are characterized by H-REY or H–M-REY enrichment types, further suggesting that the coal was subjected to hydrothermal solutions (Seredin and Dai, 2012). Coals with H-REY or H–M-REY enrichment types caused by hydrothermal solutions have been observed in other areas (Seredin and Finkelman, 2008). The weak fractionation of the light, medium, and heavy REY in the pebbly sandstone roof samples and in sample 0177-10-3 (Fig. 13) could also be attributed to the input of hydrothermal solutions; however, these samples should have had an L-REY categorization because of the intermediate-felsic sediment source rocks. Although the Yili coals are characterized by H-REY or H–M-REY enrichment types, no heavy- or medium-REY-bearing minerals were observed. To the contrary, a light-REY-being mineral, silicorhabdophane, has been identified in the Yili coal, which is generally considered as an authigenic mineral derived from hydrothermal solutions (Seredin and Finkelman, 2008; Seredin and Dai, 2012; Zhao et al., 2013).

The partings and seam floors (e.g., samples 0161-11-P1, 0161-11F4; Fig. 13D) are characterized by a slight H-REY enrichment, which was also due to input of hydrothermal solutions; otherwise, they would be expected to have an L-type REY enrichment because they have the same sediment source regions as the coals. However, not all partings in the present study have been subjected to the hydrothermal solutions (e.g., sample 0161-12P; Fig. 13F) and thus have an L-type REY enrichment, and a negative Eu anomaly, and are relatively low in U, Se, Re, and Mo, consistent with the nature of the sediment-source region. Note that not all of the Yili coals are enriched in U. Li et al. (2014b) showed that some of the Jurassic coals in the Yili Basin contain less than 2 μg/g U, suggesting that only the coals located close to the sandstone-type U deposit were able to be enriched in U. Based on the work of Danchev and Strelyanov (1979) and Seredin and Finkelman (2008), as well as the concentration variations of U, Se, and Mo through the coal seam section in the present study, the enrichment of U, Se, and Mo in the present study can be modeled as indicated in Fig. 23. The coal-hosted U deposit, produced by epigenetic infiltration, has a zoned distribution. The uppermost U(Se, Mo)-bearing zone (Zone 1), behind the fluid front and directly underlying the sandstone (Fig. 23), is composed of oxidized coals enriched in Fe hydroxide (Fig. 21E,F), and U in this zone generally occurs as U(VI) in the organic matter (Seredin and Finkelman, 2008). The U(Se, Mo)-bearing Zone 2

(Fig. 23; e.g., sample 0407-11-3), underlying Zone 1, is significantly mineralized and contains a high percentage of pyrite, as well as other epigenetic sulfides and selenides. Uranium in Zone 2 generally occurs as the tetravalent form in oxides and silicates (e.g., pitchblende, coffinite; sample 0177-12-2; Fig. 20) (see also Seredin and Finkelman, 2008). The unaltered coals (Zone 3), with low U–Se–Mo concentrations, underlie the front zone. The highest concentrations of U are found in the upper zones near the contacts with the oxidized host rocks and also within the front zone at some distance from the layers of oxidized rocks. Uranium, Se, and Mo are not evenly distributed in Zones 1 and 2 (in some cases, the two zones are overlapped), leading to inconsistent distribution of element concentration through the section (Figs. 9, 10, 11). Another previously-reported process of U enrichment in coal is a syngenetic or early diagenetic infiltration or exfiltration mechanism (Dai et al., 2015b; Seredin and Finkelman, 2008). In the syngenetic exfiltration type of deposit, U-enriched coal beds are usually interlayered between impermeable clays (Seredin and Finkelman, 2008; Dai et al., 2013) or limestones (Dai et al., 2015b). Apart from the Yili U-rich coals, which are of the epigenetic infiltration type, U-rich coals of the syngenetic exfiltration type are usually deficient in Hg, As, and W (Seredin and Finkelman, 2008; Dai et al., 2013, 2015b). For example, the Late Permian coal-hosted U deposits of Guiding, in Guizhou province of southern China, contain 9.24 μg/g As and 165 ng/g Hg, close to the averages for world coals (9.0 μg/g As and 100 ng/g Hg; Ketris and Yudovich, 2009). However, the Yili coals are not only significantly enriched in U, Se, Mo, and Re, but are also enriched in Hg and As, indicating additional

Fig. 23. The enrichment model of U, Se, and Mo in the Yili coal-hosted ore deposit. The mode plot was based on Danchev and Strelyanov (1979) and Seredin and Finkelman (2008), as well as the concentration variations of U, Se, and Mo through the coal seam section in the present study.

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input of hydrothermal solutions of an exfiltrational type. The locations of the Hg-As and U-Se-Re assemblages in the seam section are variable, suggesting that the sources of the Hg-As-rich and U-Se-Re-rich solutions are different. The enrichment of As and Hg in coal and in coal-hosted ore deposits is generally attributed to volcanogenic hydrothermal solutions, e.g., the significantly-enriched Hg and As in the coal-hosted Ge ore deposits in China (Lincang and Wulantuga coal-hosted Ge ore deposits; Hu et al., 2009; Dai et al., 2012; Zhuang et al., 2006) and the Russian Far East (Seredin et al., 2006). Arsenic and Hg are also associated with some of the pyrite in the Yili coals (see Supplementary Electronic File 6), indicating that the two elements are a result of separate volcanogenic hydrothermal solutions. 6. Conclusions The U-rich coal in the Yili Basin is a high volatile C/B bituminous coal (0.51–0.59% Rr), with medium-ash (average 26.88%, db), and mediumsulfur (St,d = 1.32% on average) characteristics. The enrichment and modes of occurrence of elements and minerals in the coal-hosted U ore deposit are attributed to detrital input from the sediment source region and to the influence of epigenetic solutions from two different sources. (1) The mineral matter in the U-rich coal mainly consists of quartz, kaolinite, illite, and, to a lesser extent, K-feldspar, sodic palagioclase, chlorite, and pyrite, as well as trace amounts of calcite, dolomite, amphibole, pitchblende, coffinite, silicorhabdophane, zircon, sulfides, and selenides. Quartz, coarse-grained kaolinite, illite, chlorite, K-feldspar, sodic plagioclase, amphibole, and at least some of the calcite were derived from detrital input from the sediment source region. Kaolinite occurring as cell infillings and pyrite are both of authigenic origin, with two generations of pyrite apparently being present. (2) In addition to having high U concentrations, the Yili coal-hosted U deposit is characterized by high concentrations of Se, Mo, Re, as well as As and Hg. The elevated U–Se–Mo–Re and As-Hg assemblages are, respectively, attributed to circulation of U–Se–Mo–Re rich infiltrational (meteoric) and Hg–As-rich exfiltrational (volcanogenic) solutions through the coal basin. The rare earth elements in the coals are characterized by heavy or/and medium REE enrichment, further indicating the role of hydrothermal solutions in generating the element and mineral assemblages in the Yili coal. (3) A U-enrichment model has been proposed for better understanding the modes of occurrence and distribution of U and its associated trace elements and minerals. U-bearing minerals (pitchblende and coffinite) occur as cavity-fillings in structured inertinite macerals. Porous inertinite-rich coal not only provided channels for migration of U-bearing solutions but also acted as a reductant, leading to a change from U(VI) to U(IV) and then the precipitation of U minerals.

Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.oregeorev.2015.03.010. Acknowledgments This research was supported by the National Key Basic Research Program of China (No. 2014CB238902), the National Natural Science Foundation of China (Nos. 41420104001 and 41172143), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13099). Many thanks are given to Shande Liu, Xibo Wang, and Lei Zhao for their help during sample preparation. We are very grateful to two anonymous reviewers for comments which greatly improved the quality of the paper.

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