Compared with the arithmetic mean value of Chinese coal and US coal (Table 3) [6Dai, S.F.; Ren, D.Y.; Chou, C.L.; Finkelman, R.B.; Seredin, V.V.; Zhou, Y.P. Geochemist of trace elements in Chinese coals: A review of abundances, genetic types, impacts on human health, and industrial utilization. Int. J. Coal Geol., 2012, 94(1), 3-21.
[http://dx.doi.org/10.1016/j.coal.2011.02.003] , 21Finkelman, R.B. Trace and Minor Elements in Coal. In: Organic Geochemistry; Plenum: New York, 1993; pp. 593-607.], the content of CaO and MgO is higher in the coal of Gashunwusan Mine. Content of P₂O₅ is relatively enriched in some coal seams. Content of Na₂O and Fe₂O₃ are also enriched in the bottom layers. The other elements belong to a relatively depleted type.

Table 3
The content of common elements and trace elements in Xishanyao Formation of Gashunwusan Mine (μg/g).

Compared with clark value [22Li, T. Element abundances of Chinese continental crust and its sedimentary layer and upper continental crust. Geochemica, 1994, 23(2), 140-145. [in Chinese with English abstract].], Chinese coal [6Dai, S.F.; Ren, D.Y.; Chou, C.L.; Finkelman, R.B.; Seredin, V.V.; Zhou, Y.P. Geochemist of trace elements in Chinese coals: A review of abundances, genetic types, impacts on human health, and industrial utilization. Int. J. Coal Geol., 2012, 94(1), 3-21.
[http://dx.doi.org/10.1016/j.coal.2011.02.003] ], and US coal [21Finkelman, R.B. Trace and Minor Elements in Coal. In: Organic Geochemistry; Plenum: New York, 1993; pp. 593-607.], the contents of Co, Ga, Sr, and Ba in Xishanyao Formation of Gashunwusan Mine is higher. Influenced by sedimentary microenvironment, the trace elements are not evenly distributed, even in the same coal seam. Fig. (3) shows that most trace elements are concentrated at the top, floor and parting samples. The distribution of Li, Be, V, Zn, Tl, and Pb in B4, as well as vertical variation of Be, V, and Pb in B9 belong to the above situation. The causes of these differences may be influenced by the property of the parent rock, the chemical conditions of the coal-forming swamp, and the species of coal-forming plants, groundwater, etc [23Sun, Y.Z.; Zhao, C.L.; Li, Y.H.; Wang, J.X. Anomalous concentrations of rare metal elements, rare-scattered (dispersed) elements and rare earth elements in the coal from iqe coalfield, qinghai province, china. Acta. Geol. Sin., 2015, 89(1), 229-241.
[http://dx.doi.org/10.1111/1755-6724.12407] ]. At the beginning of accumulation of peat swamp, minerals and trace elements in the bottom of the swamp were affected by complex physical chemistry, biochemistry, and water, which mixed together in the swamp. As a consequence, the concentration of minerals and trace elements liters was high, which should be one of the reasons for high content of trace elements near the floor coal seam. When the top took shape, trace elements came into the swamp with provenance incessantly. With the changes of the geochemical environment, the swamp became unfavorable for the growth of vegetation. In this case, minerals and trace elements subsided and enriched. Obviously, contents of trace elements in coal were influenced by sediments at the top or on the floor.

Fig. (3) The content distribution of 20 selected trace elements in the vertical section.

Rare earth elements refer to lanthanide elements. Yttrium has similar properties with lanthanide. Thus, they are collectively called rare earth elements. Rare earth elements are composed of light rare earth elements and heavy earth elements. The former refers to total content of La-Eu, which is expressed in LREE. The latter refers to total content of La-Lu and Y, which is expressed in HREE Table 4.

Table 4
The geochemical parameters of rare earth elements (μg/g).

There are many geochemical parameters of rare earth elements, but ∑REE, LREE, HREE, LREE/HREE, (La/Yb)_N, δEu and δCe are commonly used to reflect the characteristics of rare earth elements [24Zhao, C.L.; Duan, D.J.; Li, Y.H.; Zhang, J.Y. Rare earth elements in No.2 coal of Huangling mine, Huanglong coalfield, China. Energy Explor. Exploit., 2012, 30(5), 803-818.
[http://dx.doi.org/10.1260/0144-5987.30.5.803] ]. ∑REE reflects the total content of rare earth elements, LREE reflects the content of light rare earth elements, HREE reflects the content of heavy rare earth elements and LREE/HREE reflects the degree of enrichment of light rare earth element and heavy rare earth elements. The ratio is small, which means relatively depletion of light rare earth elements. δEu and δCe reflect the extent of the Eu and Ce anomaly reaction. Usually, 1 is the dividing line: the ratio greater than 1 indicates positive anomaly, otherwise, the negative anomaly.

The average value of ∑REE in each coal seam of Gashunwusan Mine ranges from 6.57 to 34.45 μg/g, which is almost a tenth to a third of the world’s coal (68.5 μg/g) [25Ketris, M.P.; Yudovich, Y.E. Estimations of Clarkes for carbonaceous biolithes: world average for trace element contents in black shales and coals. Int. J. Coal Geol., 2009, 78, 135-148.
[http://dx.doi.org/10.1016/j.coal.2009.01.002] ] and far lower than the Chinese average value (137.9μg/g) [6Dai, S.F.; Ren, D.Y.; Chou, C.L.; Finkelman, R.B.; Seredin, V.V.; Zhou, Y.P. Geochemist of trace elements in Chinese coals: A review of abundances, genetic types, impacts on human health, and industrial utilization. Int. J. Coal Geol., 2012, 94(1), 3-21.
[http://dx.doi.org/10.1016/j.coal.2011.02.003] ] while basically remaining within Chinese coal range.

LREE concentration of all samples ranges from 4.95 μg/g to 29.71 μg/g, HREE concentration varies from 1.62 μg/g to 4.91 μg/g. The average value of LREE/HREE in each coal seam ranges from 3.06 to 4.91 μg/g, which indicates LREE obviously concentrated.

HREE shows a relatively depleted type. δEu reflects the degree of anomaly (Table 4), which is less than 1 in coal seams B1, B3, B8, and B9. These samples show negative anomaly, while in B2 and B4 coal seams, there is a slightly positive anomaly. The research suggests that anomaly of Eu is influenced by continental matrix and redox properties of aqueous media during the time that coal seam is deposited in swamps. The stronger the reductibility is, the more obvious the negative anomaly is, which suggest a weak redox coal-forming environment. δCe in each coal seam varies from 0.88 to 1.03 μg/g (Table 4), which shows slight depletion or no anomaly.

After standardization by average of upper crust [26Taylor, S.R.; McLennan, S.M. The Continental Crust: Its Composition and Evolution; Blackwell: Oxford, 1985. ], the distribution patterns of rare earth elements in samples is shown below Fig. (4), from which we can see that distribution patterns are basically in accordance with each other. All of them slant rightwards. From La to Lu, standardized values of rare earth elements increase gradually, with a notable rise at the point of Eu. The left of the distribution curve is high, and the right is low; it looks like a “Λ” as a whole. The La-Sm curve is steep, and its slop is higher. However, the La-Sm curve is flat and has a lower slop. Eu presents with a “hill” in the curve. The fractionation degrees of light and heavy rare earth elements are different; the former is higher.

Fig. (4) Distribution patterns of rare earth elements in Gashunwusan Mine.

Geochemical parameters of common elements show medium conditions of peat accumulation period. However, it must be used in the same conditions of metamorphic grade and in similar ash content [30Wu, C.D.; Xu, Y.L.; Guo, Z.J.; Quan, S.J.; Li, Y.L.; Feng, X. Geochemistry characteristics and palaeogeographic significance of Jurassic coal seam in Yanqi Basin of Xinjiang. Prog. Nat. Sci., 2006, 16(2), 199-206. [in Chinese with English abstract].]. In this research, samples were taken from the same mine and the same formation, with the same metamorphic grade and similar ash content scope to satisfy above conditions.

Phosphate mainly forms iron phosphate in fresh water environment and calcium phosphate in saline water environment. That is why CaO/(Fe₂O₃+CaO) is used to estimate salinity of sedimentary water medium, with high value indicating high salinity. Table 5 reflects the salinity of sedimentary water medium as it increases from B1 to B9.

Table 5
The parameters of constant elements in geochemical characteristics.

Content of Co is used to confirm types of swamp, the values of Sr/Ba, V/Ni and V/Zn are used to confirm salinity of sedimentary water medium, and the value of Ni/Co is used to confirm types of redox in this study [31Liu, H.J.; Zhang, Y.J.; Wang, H.W. Palaeogeography Study of Coal Bearing Formation Lithofacies in Jungar Coal Fields; Geological Publishing House: Beijing, 1991, pp. 1-128. (in Chinese with English abstract), 32Tu, G.C. Geochemistry; Shanghai Scientific and Technical Publishers: Shanghai, 1982, pp. 190-243. (in Chinese)]. Basin swamp from intermediate swamp (B1) rose to raised swamp (B2) and then lowered to intermediate swamp (B3). Afterwards, from raised swamp (B4), it lowered to low-level swamp (B8), and then rose to intermediate swamp (B9). The changing process was indicated by the content of Co. In general, the swamp that stayed at high level received less provenance clastic. Xishanyao Formation in Gashunwusan Mine basically stayed in intermediate and raised swamp, which may be why content of trace elements is less. The values of Sr/Ba, V/Ni, and V/Zn reflected that salinity of sedimentary water medium is large. Jurassic coal seams were deposited in an inland lake, the reasons of which may be changes of climate, leading to water salinization in the basin swamp [33Wang, J.X.; Deng, X.L.; Kalkreuth, W. The distribution of trace elements in various peat swamps of the No.11 coal seam from the Antaibao Mine, Ningwu coalfield, China. Energy Explor. Exploit., 2011, 29(4), 517-524.
[http://dx.doi.org/10.1260/0144-5987.29.4.517] ]. The value of Ni/Co is smaller in enriched oxygen sediment than in low oxygen sediment [34Jones, B.; Manning, D.A. Comparison of geochemical indicies used for the interpretation of palaeoredox conditions in ancient mudstones. Chem. Geol., 1994, 111(1-4), 111-129.
[http://dx.doi.org/10.1016/0009-2541(94)90085-X] ], making it a possible indicator of oxygen enrichment degree. The value in this study shows that coal-forming swamp was in relatively weak redox environment.

δEu reflects the extent of Eu abnormal valence reaction. Generally, rare earth elements mainly have charge of plus three; seldom could they change valences. However, in acid medium or under strong reduction condition, Eu³⁺ is reduced to Eu²⁺, which makes it separate from other trivalency rare earth elements. Thus, negative anomaly of δEu indicates an acid or reduction environment. δEu in coal seam of Xishanyao Formation Gashunwusan Mine ranges from 0.69 to 2.31, which in B3 (0.86), B8 (0.80), and B9 (0.92) shows a slightly negative anomaly. At the top, floor and parting, samples range from 0.67 to 0.99, indicating mild to moderate negative anomaly. Study suggests that negative anomaly of Eu is influenced by terrigenous parent rock and redox of water medium when coal seam is deposited. Negative anomaly of Eu increases when reduction condition gets stronger, which indicates the redox of coal forming environment was weak.