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Abundance and geological implication of rare earth elements and yttrium in coals from the Suhaitu Mine, Wuda Coalfield, northern China Jian Kang, Lei Zhao*, Xibo Wang, Weijiao Song, Peipei Wang, Ruixue Wang, Tianjiao Li, Jihua Sun, Shaohui Jia and Qin Zhu State Key Laboratory of Coal Resources and Safe Mining; College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China *Author for corresponding. E-mail: [email protected] (Received 23 March 2014; accepted 28 July 2014)

Abstract Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentrations of rare earth elements and yttrium (REY) in coal and associated rock samples from the Suhaitu Mine, Wuda Coalfield, Northern China. The concentration of REY in the No. 15 coal of the Suhaitu Mine is lower than that in normal Chinese coal, but is slightly higher than that in normal world hard coal. The roof and some partings of the No. 15 coal have been leached by ground water. The geochemical patterns of REY indicate that the LREY-HREY have been slightly fractionated, which may be related to the homogenization of rare earth elements and yttrium in seawater. The REY distribution patterns for most of coal benches of the No. 15 coal are of H-REY type or H-M-REY type. This may be attributed to the influence of seawater and stronger organic affinity of the HREY than that of the LREY. The correlations of REY concentration and the ash yield indicate that REY in the No. 15 coal are associated with clay minerals and REY-bearing organic compounds. Keywords: Coal, Rare earth elements, Yttrium, Geochemistry, Wuda Coalfield

1. INTRODUCTION Lanthanides and yttrium (REY) is a unique group of elements with similar geochemical properties (McLennan, 1989; Seredin and Dai, 2012). From a genetic point of view, REY abundance can be used to identify various coal-forming environments and their geochemical distribution patterns have been used to provide significant information about the nature of sediment source rocks (Eskenazy, 1987a; 1987b; Finkelman, 1995; Ketris and Yudovich, 2009; Dai et al., 2008a; Zhao et al., 2012). Though there are a variety of classifications of REY, from geochemical points of view, two classifications of them are commonly used. One is a geochemical classification, which classify REY into light (LREE—La, Ce, Pr, Nd, Sm, and Eu) and

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heavy (HREY—Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y) groups (Han and Ma, 2003). The other classification divides REY into 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) groups. The latter, however, is more convenient for the description of REY distribution either in coals or conventional REY ores (Seredin and Dai, 2012). In this paper, based on the three-fold geochemical classification of REY, some important aspects related to geochemistry of REY in the No. 15 coal of the Suhaitu Mine, Wuda Coalfield are reported, including the concentration and distribution patterns of REY, as well as the modes of REY occurrence. 2. GEOLOGICAL SETTING The Wuda Coalfield is located in Inner Mongolia, northern China, covering an area of 35 km2 (Fig. 1). It is one of the major coking coal mining areas in Northern China. The coalfield includes the Huangbaici, Wuhushan, Suhaitu Mines (Fig. 1). The sedimentary sequences in the Wuda Coalfield include the Upper Carboniferous Benxi Formation, Taiyuan Formation, the Lower Permian Shanxi and Xiashihezi Formation, and the Upper Permian Shangshihezi Formation. The Taiyuan Formation is the major coal-bearing formation which consists of sandstone, limestone, mudstone and mineable coal beds including the No. 9, No. 10, No. 12, No. 13, and No. 15 coals. The Taiyuan Formation has a thickness between 70 and 140 m, decreasing from the center towards both the south and north (Dai et al., 2002; 2003). The coal seams of Taiyuan Formation in the Wuda Coalfield were formed in delta sedimentary system (Peng and Zhang, 1995). The No. 15 coal was formed in a tidal plain setting (Fig. 2) and influenced strongly by seawater during coal-forming period (Dai et al., 2002; 2003). The sediment-source region for the Wuda Coalfield is the Alxa plate, situated to the west of the Wuda Coalfield (Peng and Zhang, 1995). The Alxa plate has double layers structure, namely crystalline basement and sedimentary cover. The basement construction called Alxa group is mainly composed of

Figure 1. Location of the Wuda Coalfield and coal mines in the coalfield.

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Figure 2. Paleogeography of Late Paleozoic in North China (modified after Dai, 2002). precambrian metamorphic rocks, with the metamorphic grade reached amphibolite facies as a whole, and partially achieved greenschist facies; Sedimentary cover is mainly composed of clastic rocks, volcanic rocks and carbonate rocks (Peng and Zhang, 1995). 3. SAMPLING AND ANALYTICAL METHODS A total of 12 bench samples were taken from the face of the mined coal at the Suhaitu Mine, Wuda Coalfield. These samples include 8 coal bench samples, 3 partings and 1 roof sample. Each coal bench sample was cut over an area 10-cm wide, 10-cm deep, and 30-cm thick. All collected samples were immediately stored in plastic bags to minimize contamination and oxidation. Samples were crushed and ground to pass 200 mesh for geochemical analysis. Inductively coupled plasma mass spectrometry (ICP-MS, Xseries II), in a pulse counting mode (three points per peak), was used to determine rare earth elements and yttrium in the coal and rock samples. 50-mg sample were weighed into PFA vessels. For ICP-MS analysis, samples were digested using an UltraClave Microwave High Pressure Reactor (Milestone). The basic load for the digestion tank was composed of 330-ml distilled H2O, 30-ml 30% H2O2, and 2-ml 98% H2SO4. Initial nitrogen pressure was set at 50 bars and the highest temperature was set at 240℃ that lasted for 75 minutes (Dai et al., 2011a). The reagent for coal samples digestion was a mixture of 2-ml 40% HF and 5-ml 65% HNO3, while the reagent for parting and roof samples digestion was a mixture of 5-ml 40% HF and 2-ml 65% HNO3. After digestion, the digests were transferred to PFA (polytetrafluoro ethylene) volumetric flasks (resistant to HF corrosion). Two drops of 72% HClO4 were added to PFA flasks. The resultant digests then were subjected to drying on electric hot plate at 180℃. 5-mL 50% (V/V)

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HNO3 was added in the PFA flasks, which were then covered by the lids and were kept at 180℃ on the electric hot plate for four hours. After that, the samples were cooled to room temperature and then were diluted with ultra-pure water to 100 ml for ICP-MS analysis. A standard (Inorganic Ventures, CCS-1) were used for calibration of rare earth elements and yttrium. 4. RESULTS AND DISCUSSION 4.1. Coal characteristics The moisture, ash yield, and volatile matter were analysed according to Chinese Standard GB/T 212-2008 (2008). Moisture of Suhaitu coals varies narrowly from 0.94% to 1.49%, with an average of 1.09%, while ash yield varies widely from 6.33% to 45.05%, with an average of 22.12%. The weighted average volatile matter (dry and ash free basis) and the vitrinite random reflectance are 29.49% and 1.20%, respectively, indicating a medium volatile bituminous coal according to the ASTM classification (ASTM D388-12, 2012). The No. 15 coal is classified as medium-ash coals (coals with ash yield 20.01–30.00% are medium-ash coals) according to Chinese Standards GB/T 15224.1-2010 (2010). The total S was analysed according to Chinese Standard GB/T 214-2007 (2007). Total S in the No. 15 coal varies from 0.65% to 4.17%, with an average of 1.66%. The maximum and minimum concentrations of total S occur in the coal bench samples SHT15-3 and SHT15-6, respectively. The No. 15 coal is a medium-sulfur coal (coals with total sulfur contents between 1.01% and 2.00% are medium-sulfur coal) according to Chinese Standards GB/T 15224.2-2010. 4.2. REY concentrations The concentration of REY (from La to Lu plus Y) in the No. 15 coal of the Suhaitu Mine varies from 43.47 μg/g to 150.16 μg/g and averages 76.87 μg/g, lower than that in normal Chinese coal (136 μg/g; Dai et al., 2012a) and slightly higher than that in average world hard coals (68.6 μg/g; Ketris and Yudovich, 2009). The maximum REY concentration (150.16 μg/g) in the No. 15 coal occurs in the SHT15-6 coal bench sample which also has a relatively high ash yield (24.38%), and the minimum value (43.47 μg/g) in the SHT15-4 coal sample with relatively low ash yield (6.33%) (Fig. 3). It has been reported that REY in coal is often positively correlated with ash yield and generally related to minerals, primarily clay minerals (Chou, 1997; Dai et al., 2008a; Finkelman, 1995; Dai, 2011a), while the proportions of clay minerals in

Figure 3. REY contents in the section of the No. 15 coal of the Suhaitu Mine, Wuda Coalfield.

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inorganic matter mainly depends on characteristic of parent rock and supplies of terrigenous detrital material at the peat accumulation stage. The concentration of REY in coal depends not only on the composition and quantity of terrigenous detrital matters but also the sedimentary environment. The concentration of REY is low in seawater due to the very short retention time in seawater and the low REY content in marine biological detritus (Table 2), only a few ng per liter of water (Liu and Cao, 1993; Wang et al., 1989), much lower than REY in terrigenous detritus. This indicates that coal-forming peat swamp influenced by seawater may have a small amount of rare earth elements. On the contrast, REY concentration of coal mainly influenced by terrestrial sources is generally high (e.g. Dai, 2002). The concentration of REY in the No. 15 coal that formed in a tidal plain setting of delta sedimentary system (Dai et al., 2002; 2003) is generally low, due to seawater impact and the lack of terrigenous supplies in peat accumulation period. 4.3. REY parameters The value of Eu/Eu* in the No. 15 coal in the Suhaitu Mine varies from 0.63 to 0.93, with an average of 0.78. The partings and roof sample in the No. 15 coal also have negative Eu anomalies (Fig. 4). Generally, a negative Eu anomaly of coal is inherited from the parent rocks, because Eu3+ can not be reduced to Eu2+ to separate with other rare earth elements in supergene reductive environment; Eu can separate with other rare earth elements only in the magmatic differentiation extreme reducing conditions (such as: in mid-ocean ridge hydrothermal fluids) (Tang and Huang, 2004). As mentioned above, the sediment-source region for the Wuda coal basin is the Alxa plate. Magmatic activity was extremely strong in late Paleozoic in Alxa plate

Figure 4. Upper Continental Crust-normalized REY distribution pattern in the No. 15 coal.

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(Peng and Zhang, 1995). Thus, acidic intrusive mass was formed widely (Peng and Zhang, 1995). The average value of Eu/Eu* of the No. 15 coal is 0.78, which is higher than that of acid rock (0.45) (Zhao, 2002). Negative Eu anomalies may have been inherited from the source region, and the impact of seawater reduced the extent of negative anomalies (Zhao, 2002). Seawater has an adjusting role to Eu anomalies. It not only reduces the degree of negative Eu anomalies, but also makes the REY distribution curve flattening (Birk and White, 1991). The No. 15 coal formed in a tidal plain setting of delta sedimentary system (Dai et al., 2002; 2003), having a Eu/Eu* value of 0.78, do not have a strong Eu negative anomaly. This may be closely associated with weakened terrigenous control and enhanced seawater influence. The value of Ce/Ce* in the No. 15 coal varies from 0.94 to 1.18, with an average value of 1.02 (Table 1). One important feature of REY in seawater is the obvious loss of Cerium (Liu and Cao, 1993; Zhao, 2002). Cerium has a strong negative anomaly and its content is less than La in seawater, which is significant different from river water and other geologic bodies on the earth (Liu and Cao, 1993; Elderfield and Greaves, 1982). The loss of Ce may be due to the oxidation of Ce3+ to Ce4+ and the precipitation in the form of CeO2, but other rare earth elements retain still trivalent state (Liu and Cao, 1993). The value of Ce/Ce* in seawater is 0.1904 (Table 2), having a strong negative Ce anomaly, which is much lower than that of the No. 15 coal (1.02). The loss of Cerium is heavy in the open sea, while the concentration of Ce is basically normal in marginal sea, shallow sea area, and the sea enclosed by land (Wang et al., 1989; Elderfield and Greaves, 1982). Therefore, the concentration of Ce in No. 15 coal formed in a tidal plain setting of delta sedimentary system (Dai et al., 2002; 2003) is basically normal, with a slightly positive anomaly due to relatively shallow seawater depth and narrow sea area. The value of Y/Y* in the No. 15 coal varies from 0.64 to 0.95, with an average value of 0.86, having a slightly negative Y anomaly. The partings and roof sample in the No. 15 coal also have negative Y anomalies (Fig. 4). The Yb/La ratio in the No. 15 coal ranges narrowly from 0.05 to 0.17. The Yb/La ratio in SHT15-2P parting is lower than that in the adjacent coal seams, while the Yb/La ratio in SHT15-8P parting is higher than that in underlying coal bench (Fig. 5). The contents of La and Yb are higher in the middle portions than that in the lower portions and the upper portions through the No. 15 coal seam section (Fig. 5). The concentrations of La and Yb are relatively low in SHT15-2P parting, while the contents of La and Yb are 35.13 μg/g and 3.15 μg/g in SHT15-7P parting, reaching their maximum respectively through the seam section. @Body:The concentrations of La, Yb, and REY show similar variation tendency from top to bottom through the No. 15 coal seam section (Fig. 5). The value of (La/Yb)N in the No. 15 coal is 0.74, which is slightly higher than that in seawater, the value of which is 0.3, indicating low fractionation between LREY and HREY. The fractionation between LREY and HREY in seawater and coral is not obvious (Liu and Cao, 1993). The low fractionation of LREY and HREY commonly occur in low-ash and middle-high-sulfur coals in comparison with high-ash and lowsulfur coals (Dai, 2002). The LREY and HREY have been slightly fractionated in the No. 15 coal, which is associated with the homogenization of seawater to rare earth elements and yttrium.

Sample SHT15-R SHT15-1 SHT15-2P SHT15-3 SHT15-4 La 5.88 11.72 6.13 12.66 7.11 Ce 21.01 26.32 19.08 24.52 13.79 Pr 0.95 3.13 1.38 2.76 1.56 Nd 3.48 13.22 5.56 11.58 6.22 Sm 0.53 2.97 1.01 2.46 1.27 Eu 0.11 0.67 0.19 0.53 0.24 Gd 0.67 3.51 1.02 2.89 1.51 Tb 0.10 0.61 0.10 0.46 0.22 Dy 0.80 4.04 0.53 3.20 1.49 Y 4.70 19.68 2.52 17.16 7.98 Ho 0.18 0.73 0.09 0.64 0.27 Er 0.67 2.23 0.27 2.06 0.84 Tm 0.11 0.28 0.03 0.28 0.11 Yb 0.91 2.00 0.28 2.06 0.76 Lu 0.14 0.27 0.04 0.28 0.10 ΣREE 35.54 71.70 35.70 66.38 35.49 ΣREY 40.24 91.38 38.22 83.54 43.47 LaN/LuN 0.43 0.43 1.44 0.45 0.70 LaN/SmN 1.67 0.59 0.91 0.77 0.84 GdN/LuN 0.39 1.02 1.89 0.81 1.17 δCe 1.99 0.99 1.50 0.95 0.94 δEu 0.82 0.93 0.85 0.90 0.77 δY 0.95 0.86 0.87 0.91 0.95

SHT15-5 SHT15-6 SHT15-7P SHT15-8P SHT15-9 SHT15-10 SHT15-11 15.55 29.26 35.13 30.99 16.27 8.75 10.44 29.91 65.53 75.33 96.35 30.56 18.96 26.30 3.20 5.66 7.69 6.32 3.32 2.01 2.46 12.31 20.49 27.84 22.37 12.26 8.15 9.82 2.43 3.69 5.17 3.71 2.22 1.74 1.99 0.46 0.63 0.87 0.55 0.32 0.34 0.33 2.82 4.07 6.06 4.13 2.39 2.08 2.02 0.43 0.50 0.89 0.45 0.27 0.33 0.27 2.89 2.93 5.67 2.39 1.44 2.19 1.46 15.68 13.43 28.46 11.28 4.99 10.50 5.04 0.56 0.52 1.06 0.41 0.22 0.41 0.24 1.68 1.55 3.29 1.25 0.63 1.21 0.71 0.21 0.21 0.46 0.17 0.08 0.16 0.09 1.47 1.48 3.15 1.17 0.56 1.13 0.68 0.19 0.20 0.43 0.16 0.07 0.15 0.09 74.10 136.73 173.05 170.43 70.62 47.60 56.91 89.79 150.16 201.51 181.71 75.61 58.10 61.95 0.82 1.49 0.82 1.92 2.37 0.59 1.18 0.96 1.19 1.02 1.25 1.10 0.75 0.79 1.18 1.64 1.12 2.02 2.76 1.10 1.81 0.96 1.16 1.04 1.57 0.95 1.03 1.18 0.79 0.74 0.71 0.64 0.63 0.80 0.74 0.94 0.82 0.88 0.85 0.66 0.84 0.64

Table 1. Concentrations and geochemical parameters of REY of the No. 15 coal (elements in μg/g).

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0.43 0.17 57.36 28.50 5.51 1.06 2.25

1.61 0.05 33.15 4.36 0.71 1.75 3.11

0.45 0.16 53.98 24.24 5.32 1.05 1.94

0.69 0.11 29.95 11.43 2.08 1.22 1.94

0.78 0.09 63.40 22.28 4.11 1.22 1.92

1.45 0.05 124.64 21.56 3.96 1.56 2.24

0.82 0.09 151.17 41.95 8.39 1.48 2.14

1.94 0.04 159.74 18.80 3.17 2.43 3.11

2.12 0.03 64.64 9.41 1.56 1.51 1.88

0.57 0.13 39.60 15.43 3.07 1.30 2.17

Sample La Seawater 3.4 Coral 0.34

Ce 1.2 0.29

Pr 0.64 0.129

Nd 2.8 0.65

Sm 0.45 0.21

Eu 0.13 0.037

Gd 0.7 0.171

Tb 0.14 0.02

Dy 0.91 0.14

Ho 0.22 0.037

Er 0.87 0.121

Tm 0.17 0.022

Yb Lu ΣRE δCe 0.82 0.15 12.6 0.1904 0.14 0.024 2.331 0.2885

Table 2. Concentrations and geochemical parameters of REY in seawater and coral (elements in μg/g) (from Wang and Zhao, 1989).

(La/Yb)N 0.48 Yb/La 0.15 LREE 31.86 MREE 6.38 HREE 2.00 δCe/δEu 2.43 Ce/La 3.57

δEu 0.7154 0.6659

1.13 0.06 51.02 9.12 1.80 1.59 2.52

880 Abundance and geological implication of rare earth elements and yttrium in coals from the Suhaitu Mine, Wuda Coalfield, northern China

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Figure 5. Vertical variations of ash yield, REY, and geochemical parameters in the No. 15 coal.

4.4. REY distribution patterns In comparison with upper continental crust (UCC; Taylor and McLennan, 1985), three enrichment types of REY in coal are classified, L-type (light-REY type enrichment; LaN/LuN >1), M-type (medium-REY type; LaN/SmN 1), and H-type (heavy-REY type; LaN/LuN 500℃) volcanogenic fluids (Rybin et al., 2003). Seredin (2001) considered that Htype REY distributions pattern of coal is mainly due to seawater. Elderfield and Greaves (1982) also indicated that the enrichment degree of HREY in shallow sea is stronger than that in deep sea. The No. 15 coal in Suhaitu Mine was formed in delta sediment system and more strongly affected by seawater (Dai et al., 2002; 2003). Thus, H-type distribution patterns of some Suhaitu coals may be attributed to influence of seawater. HREY have stronger ability to form soluble complexes and can be more easily to dissolve and migrate in seawater in comparison with LREY, due to shorter ionic radius of HREY (Eskenazy, 1987a; 1987b). Fine clay minerals and organic matter give priority to adsorb and precipitate HREY, which leads to relative enrichment of HREY (Seredin and Shpirt, 1999; Liang et al., 2013). Eskenazy (1999) considered a stronger ability of HREY to form complexes with organic matter relative to LREY, which increases the stability of the HREY. The distribution patterns of most coal benches of the No. 15 coal seam of the Suhaitu Mine are H-type, which may be because of stronger organic affinity of HREY and the influence of seawater. The coal benches (SHT15-1, SHT15-4, SHT15-5, SHT15-10, and SHT15-11) and parting (SHT15-2P) are enriched in MREY. Three subtypes of M-type distribution can be identified: 1) with a strong pronounced Eu-minimum; 2) with a strong pronounced

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Eu-maximum; and 3) swell-like without Eu-anomalies (Seredin and Dai, 2012). The coal benches and partings enriched in MREY in the No. 15 coal are subtype 1, while subtype 2 or subtype 3 was not present. The M-type of REY plot normalized to UCC is rather typical for acid natural water (Johanneson and Zhou, 1997; Dai, 2012b). The M-type REY distribution patterns are caused mainly by two reasons: one is the acid natural water circulating in coal basins including acid hydrothermal solutions (McLennan, 1989; Michard, 1986; Seredin, 2011). The other reason is stronger sorption of organic matter for MREY in comparison with LREY and HREY (Seredin and Shpirt, 1999; Seredin and Finkelman, 2008). Thus, the coal benches and partings with M-type REY distribution patterns in the No. 15 coal in the Suhaitu Mine may be attributed to acid natural water and stronger adsorption of organic matter for MREY. The REY distribution pattern curves of coal beaches samples (SHT15-1, SHT15-3, SHT15-4, SHT15-5, and SHT15-10) are similar in shape, but slightly different in concentrations due to different coal-forming sedimentary micro-environments (Fig. 4). The fractionation among their LREY is relatively low, the ratio of LaN/SmN ranging from 0.59 to 0.96; Their MREY (Eu, Gd, Tb, Dy, and Y) have been highly fractionated, characterized by relatively high distribution pattern curves slope (Fig. 4), the ratio of GdN/LuN ranging from 0.81 to1.18; The distribution pattern curves of their HREY (Ho, Er, Tm, Yb, and Lu) are gentle inclined (Fig. 4), with relatively low fractionation. The distribution pattern curves of the No. 15 coal roof sample (SHT15-R) and partings (SHT15-2P) without obvious fractionation among LERY, MREY, and HREY are characterized by two gentle straights (Fig. 4). The HREY fractionation of the SHT15-7P and SHT15-8P partings are relatively low, but the fractionation of LREY or MREY is relatively high (Fig. 4). 4.5. Leaching process of REY The roof sample and parting SHT15-2P in the No. 15 coal of the Suhaitu Mine have lower REY contents than those of the underlying coal benches. The content of REY of roof sample is 40.24 μg/g, lower than that in the underlying coal bench (91.38 μg/g). The content of REY in the coal bench SHT15-3 is 83.54 μg/g, higher than that in the overlying parting (38.22 μg/g). The fractionation of HREY and LREY in the parting SHT15-2P and roof sample is much higher than that in their underlying coal benches. The ratio of (La/Yb)N in the roof sample SHT15-R and parting SHT15-2P is 0.48 and 1.61, respectively, higher than that in their underlying coal benches which are 0.43 and 0.45. In general, trivalent REE are able to migrate along with the groundwater rather than being easily adsorbed by clay minerals under the acidic conditions of supergene leaching (Liu and Cao, 1993). Eskenazy (1999) noted that 20%–95% of the rare earth elements in clay minerals are easy to be leached. Low REY contents in the partings and host rocks are attributed to leaching by groundwater. When REY in roof and parting samples is leached by groundwater, HREY are more easily to form soluble bicarbonate, and thus are preferential entering the leachate compared with LREY (Balashov, 1976; Duddy, 1980). REY in the leachate can be absorbed by the organic matter, leading to low REY content in the partings but high REY content in the underlying coal benches. The LREY and HREY in overlying

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parting have been more highly fractionated than that in underlying coal benches due to higher HREY content in leachate and more HREY adsorbed by the organic matter in underlying coal benches. Generally, rare earth elements and yttrium are positive trivalent, while their trivalent valence changes rarely occur in nature (Zhao, 2002; Elderfield and Greaves, 1982). However, Ce3+ can be oxidized to Ce4+ in supergene zone of high oxidation potential and separate with other trivalent rare earth elements and yttrium, resulting in a loss of Ce in supergene zone (Liu and Cao, 1993). Cerium is the only rare earth element that can be oxidized to Ce4+ and could be precipitated in-situ in groundwater leaching process, leading to the generation of Ce-poor leachates from the partings or host rocks (Dai, 2012b). This may cause the partings to be lower in total REY content, but higher in Ce content and Ce/Ce* value than that in the underlying coal benches. The Ce/Ce* value of roof sample SHT15-R and the parting SHT15-2P is 1.99 and 1.5, respectively, higher than that in their underlying coal benches (Ce/Ce* value of 0.99 and 0.95), indicating that the roof sample and the parting SHT15-2P may have been subjected to significant leaching by groundwater. When REY-enriched leachate infiltrated the underlying coal benches, three possible processes may have happened in the coal: 1) Authigenic REY-rich minerals, such as cell-filling goyazite, were precipitated from the leachate; 2) REY in the leachate were incorporated in aluminum hydrate minerals, e.g., boehmite; 3) REY in the leachate were absorbed by the organic matter (Dai et al., 2008b). The parting SHT15-8P has higher REY content and lower (La/Yb)N value than the underlying coal bench, sharply contrasting to the roof sample and the parting SHT152P, indicating that the parting SHT15-8P probably have not been subjected to groundwater leaching. 4.6. Modes of REY occurrence The correlation coefficient between the concentration of individual REY and SiO2 in the No. 15 coal of the Suhaitu Mine varies from 0.2 (Y-SiO2) to 0.75 (Sm-SiO2) and the correlation coefficient between concentrations of REY and SiO2 is 0.66 The correlation coefficient between the concentration of individual REY and Al2O3 varies from 0.05 (Y-Al2O3) to 0.77 (Nd-Al2O3) and the correlation coefficient between total REY and Al2O3 is 0.71 On the whole, HREYs in the No. 15 coal have lower correlation coefficients with SiO2 and Al2O3 in comparison with LREYs. The correlation coefficient between the concentration of individual REY and ash yieldvaries from 0.45 (Y-Ad) to 0.79 (Sm-Ad) and the correlation coefficient between total REY and ash yield is 0.6, indicating that REY in the No. 15 coal have both organic and inorganic affinities. The different modes of REY occurrence may reflect the different sources of REY in the coals (Dai et al., 2011b). Rare earth elements and yttrium (REY) in coal may be present in two forms, namely, a, clastic minerals (mainly monazite and to a lesser extent xenotime) or as isomorphic admixtures in minerals of terrigenous origin (e.g., zircon, apatite); b, fine grained minerals of authigenic origin, such as water-bearing REY phosphates (rabdophane group and cherchite) and carbonates (Seredin and Dai, 2012). However, REY may also be partly associated with the organic matter in coal

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Figure 6. Correlation coefficients between ash yield and individual REY of the No. 15 coal. (Eskenazy, 1987a, b). REY was adsorbed by organic matter mainly in peat accumulation stage (Seredin, 2001). Due to acidic environment in peat swamp, organic matter has a strong adsorption capacity in the gelation process, leading to the formation o REY-bearing organic compounds (Eskenazy, 1987b). Seredin and Shpirt (1999) suggested that REY-bearing organic compounds in coal are normally rare. Finkelman (1993) noted that REY-bearing organic compounds are less than 10% in the United States lignite. However, Eskenazy (1987a) considered that organic REY compounds have a great influence on the geochemistry of Bulgaria low ash coal. The REY of Ge-bearing coal in Lincang, SW China, has a clearly negative correlation with ash yield, indicating that a portion of REY in Ge-bearing coal occurs in organic complexes (Chen et al., 1996). The positive but low correlation coefficients between REY and ash yield in the No. 15 coal of the Suhaitu Mine indicate that REY may be associated with clay minerals, and REY-bearing organic compounds may also occur. Overall, the HREY in the No. 15 coal of the Suhaitu Mine have lower positive correlation coefficients with ash yields than MREY and LREY, indicating that HREY have stronger organic affinities than MREY and LREY. The correlation coefficient between individual LREY (La, Ce, Pr, Nd, and Sm) and ash yields increases with increasing atomic number, indicating LREY have decreasing organic affinity with increasing atomic number (Fig. 6). However, individual MREY have increasing organic affinity with increasing atomic number (Fig. 6). The individual HREY (Ho, Er, Tm, Yb, and Lu) has similar correlation coefficients with ash yield, indicating that individual HREY have a similar organic affinity. 5. CONCLUSIONS The average concentration of REY in the Suhaitu No. 15 coal that formed in a tidal plain setting is 76.87 μg/g, lower than that in normal Chinese coal and slightly higher than that in average world hard coals. This may be due to the impact of seawater and the lack of terrigenous supplies during peat swamp accumulation period. The REY distribution patterns of most coal benches in the No. 15 coal are characterized by

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H-type or H-M-type, probably because of relatively stronger organic affinity of HREY and seawater influence. The roof sample and parting SHT15-2P in the Suhaitu No. 15 coal have lower REY contents than those in the underlying coal benches, while the fractionation of HREY and LREY, Ce/Ce* value, and the ratio of (La/Yb)N in the parting SHT15-2P and roof sample are much higher than those in their underlying coal benches, indicating that the roof sample and the parting SHT15-2P have been subjected to leaching by groundwater. Clay minerals and REY-bearing organic compounds are the main modes of REY occurrence for the Suhaitu No. 15 coal. ACKNOWLEDGEMENTS This research was supported by the National Key Basic Research Program of China (No. 2014CB238902) and National Natural Science Foundation of China (No. 41302128). REFERENCES ASTM Standard D388-12, 2012. Standard Classification of Coals by Rank. ASTM International, West Conshohocken, PA. Balashov Y.A., 1976. Geochemistry of Rare Earth Elements. Nauka, Moscow, pp. 267 (in Russian). Birk D. and White J.C., 1991. Rare earth elements in bituminous coals and underclays of the Sydney Basin, Nova Scotia: Element sites, distribution, Mineralogy. International Journal of Coal Geology 19(1), 219–251. Chen Z.C., Yu S.Y., Fu Q.C., Wang J.Y. and Gan Q.H., 1996. A model study on the formation of compound of humic acid and REE under weathering conditions in the weathering crust REE deposits. Acta Scientiarum Naturaliu Universitatis Sunyatseni 35(5), 102–107 (in Chinese with English abstract). Chou C.L., 1997. Abundances of sulfur, chlorine, and trace elements in Illinois Basin coals, USA. Proceedings of the 14th Annual International Pittsburgh Coal Conference & Workshop, Taiyuan, China, Sept. 23–27, Section 1, pp. 76–87. Dai S.F., 2002. Geological-geochemical behaviors and enrichment models of associated elements in coal: Doctoral Thesis, China University of Mining and Technology, Beijing, China, pp. 143 (in Chinese with English abstract). Dai S.F., Hou X.Q., Ren D.Y. and Tang Y.G., 2003. Surface analysis of pyrite in the No. 9 coal seam, Wuda Coalfield, Inner Mongolia, China, using high-resolution time-of-flight secondary ion mass-spectrometry. International Journal of Coal Geology 55(2), 139–150. Dai S.F., Li D., Chou C.L., Zhao L., Zhang Y., Ren D.Y., Ma Y.W. and Sun Y.Y., 2008b. Mineralogy and geochemistry of boehmite-rich coals: new insights from the Haerwusu Surface Mine, Jungar Coalfield, Inner Mongolia, China. International Journal of Coal Geology 74(3–4), 185–202. Dai S.F., Ren D.Y., Chou C.L., Finkelman R.B., Seredin V.V. and Zhou Y.P., 2012a. Geochemistry of trace elements in Chinese coals: a review of abundances,

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