Shao-qing Huang, Shu-zheng Ning, Jian-qiang Zhang, Li Zhang, Kang Liu

Exploration and Research Institute, China National Administration of Coal Geology, Beijing 100039, China

Keywords:Rare earth elements (REE)Distribution pattern type Concentration factor Alatanheli Group Baiyanhua Group Economic value Erlian Basin Inner Mongolia China

ABSTRACT The rare earth elements (REE) content of the coal in the Erlian Basin was determined by inductively coupled plasma mass spectrometry (ICP-MS), and it turns out that the REE content from different geological age shows a significant difference: The REE content of the coal in the Jurassic Alatanheli Group is from 152.05×10−6 to 1416.21×10−6, with an average value of 397.31×10−6, and the relative concentration factor shows enriched; the REE content of the coal in Early Cretaceous Baiyanhua Group is from 20.65×10−6 to 102.53×10−6, the mean value is 49.06×10−6, and the relative concentration factor shows normally. The REE distribution patterns samples in Jurassic and Cretaceous shows the difference:The REE pattern in Jurassic coal mainly manifests as H-type distribution, with the Y, Lu positive anomaly,it is speculated that the fluid carried REE ions into the coal-bearing basin, and the heavy REE gather in the coal due to the different chemical properties of each REE. The REE occurrence mode is presumed to be mainly organic. Flat type is the REE main distribution pattern in Cretaceous coal. The REE patterns in clastic rocks of the roof, parting and floor of coal seam are similar to the REE patterns in the coal and the most possible reason is that the REE main source is from the clastic rock. It showed that the coal of the Early Jurassic, especially of Amugulen coalfield has resource value.

1. Introduction

The research on rare earth elements (REE) has two significant meanings. Firstly, the different REE combinations can reflect the different geochemical environments, and it could be an effective tracer to indicate the origin and environmental information (Zhao ZG, 2002; Ren DY et al.,2006). Secondly, although most REE has low content, Ketris MP and Yudovich YE estimated the REE average value is 68.5×10−6(Ketris MP and Yudovich YE, 2009), which are not valuable for industrial purposes. While in some situations,REE could be gathered and enriched. Especially in the coal ash after burning, the REE grade could be reached to the grade in some current mining rare earth ore deposits, which has higher industrial value. The anomalously enriched REE has been found such as Cenozoic coal in Pavlov of Russia,Cretaceous coal in Mongolia, Jurassic coal in Tajikistan,Carboniferous coal in Kentucky in the USA, Late Permian and Late Triassic coal at the southwestern area in China and some REE contents in the coal have been exceeded 2000×10−6(Seredin VV and Dai SF, 2012). Among them, the oxide extraction has been achieved in Pavlov in Russia(Seredin VV and Dai SF, 2012), and it has been moved to the pilot stage at the development demonstration project in the USA (Honaker R, 2018). Chinese scholars (Chen RB et al.,1985; Zhao ZG et al., 1998; Zhuang XG et al., 1998; Huang WH et al., 1999; Dai SF et al., 2008, 2016; Wu D et al., 2013)studied the REE genesis. However, those researches almost focused on the Carboniferous‒Permian coal in north China,Late Permian and Late Triassic coal in south China, while researches are still insufficient in other areas. There are large reserves and high yields of coal in the Erlian Basin, which is one of the most important energy bases in north China. The rare metal resources have been found in coal and coal-bearing series. Some scholars (Niu L et al., 1995; Du G et al., 2004;Huang WH et al., 2008; Huang SQ et al., 2018; Zhao Y et al.,2018) studied the Ge, U of the coal in the Erlian Basin, but the research on REE is bare. This study discusses the REE geochemical characteristics in the Erlian Basin, China (Fig. 1).

2. Geological background

Erlian Basin is composed of several small continental rifted-basins (Fig.1), which have dissociation reactions and generation relations (Li ST, 1997). Those basins are located in the Hercynian fold base and every lake basin has its independent deposition system, depocenter and marginal face belt. The evolution process of the Erlian Basin including the stage of lifting and weathering in Triassic, rifting to the basin at Early Jurassic, contracting and inversion at Middle‒Late Jurassic, rifting and forming into the basin at Early Cretaceous, then the structural configuration of Erlian Basin formed at present style (Meng QR, 2003). The mainly coalbearing stratum is the Bayanhua Group in the Early Cretaceous and Alatanheli Group in the Early‒Middle Jurassic. The Alatanheli Group is mainly bearing coal construction clastic rocks, which suffered serious erosion by contracting and inversion at the Middle‒Late Jurassic (Yang FJ, 2003). This formation mainly remains in the northeastern area of the Erlian Basin, and the sedimentary characteristics of the Alatanheli Group are mainly composed of the northeastern small rifted-basins. Lithological characteristics are fluvial and lacustrine clastic rocks, which occur sporadically. The Baiyanhua Group mainly distributes inside of the northeastern small fractures and is controlled by the boundary faults, which are composed of gray san-mudstone with coal seam (Li XP et al., 2015).

Depending on the analysis results and exploration materials, the coal type of Jurassic is mainly the long flame coal in the Erlian Basin. The ash content is between 5% and 15%. The sulfur content is generally less than 1%, and the volatilization yield is generally 35%‒40%. For Cretaceous coal, it is mainly composed of lignite. The ash content is generally between 10% and 30%. The volatile content is generally more than 40%, and the sulfur content is less than 1%.

3. Sampling and analyzing

There are 46 samples including coal, roof, gangue and floor samples were collected in the Erlian Basin. There are 29 samples collected from the Bayanhua Group of the Early Cretaceous, and 17 samples are collected from the Alatanheli Group of Early‒Middle Jurassic (Fig. 1). The Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) has been utilized to analyze the REE contents. The analysis method is depending on the DZ/T 0223-2001 , which proceeded in the 20℃, 35% relative humidity. The analysis results were achieved at the Key Laboratory of Coal Series Minerals of Jiangsu Geological and Mineral Research Institute.

Fig. 1. Geological background of the Erlian Basin and sampling points distribution.

4. REE characteristics

4.1. REE abundance

According to the different coal-forming ages, the data shows in Table 1 and Table 2. The REE average contents of the Jurassic and Cretaceous coal are shown in Table 3. It is shown that the REE contents of the coal have obvious differences between Jurassic and Cretaceous coal in the Erlian Basin. The REE ranges from 152.05×10−6to 1416.21×10−6with an average value of 397.31×10−6in the Jurassic coal,which far exceeds the average value of the world’s coal(Ketris MP and Yudovich YE, 2009), and even 2.3 times more than REE of upper continental crust (UCC). The REE ranges from 20.65×10−6to 102.53×10−6in the Cretaceous coal with an average of 49.06×10−6, which is less than the average value of the world’s coal, and it’s only a third of the UCC.

The REE are divided into three categories according to light rare earth (LREE: La, Ce, Pr, Nd, Sm), middle rare earth(MREE: Eu, Gd, Tb, Dy, Y), heavy rare earth (HREE: Ho, Er,Tm, Yb, Lu). According to the enrichment coefficient (CC) of trace elements in coal proposed by Dai SF et al. (2015)(CC=average REE content in coal sample/REE average value in world’s coal; 10

(i) Pr, Gd and Tm in the Cretaceous coal are slightly depleted, and the other elements are normal (Fig. 2a).

(ii) The REE content in Jurassic coal is higher than it’s in Cretaceous coal. According to the CC, the LREE is slightly enriched (Fig. 2b); the MREE is enriched with CC=9.75 and close to HREE, in which the element yttrium (Y) CC=12.7 is significantly enriched; HREE is enriched. Among them, the REE enrichment in Amugulen is particularly obvious: The CC of MREE in coal samples reaches 16.7, the CC of yttrium (Y)reaches 22, and the CC of HREE reaches 11.1. All ofthe them are significant at a high level.

4.2. REE distribution characteristics

The REE curve average content was drawn from the east to the west in Cretaceous coal field mines (Figs. 1, 3a). It can be seen that the REE content is low in Cretaceous coal.Except for the REE content in the coal of Ulagai River(102.53×10−6) coalfield are close to the world’s coal average value, the other coalfields’ REE content in the coal is much less than the average value of world’s coal and ranges from 20×10−6to 60×10−6; but the REE content in the gangue of Cretaceous coal is much higher than the world’s coal and UCC value, such as Baiyinhua 2# Mine (Fig. 3c). The REE content in gangue samples such as BYH2-1-R, BYH2-1-3D,BYH2-2-R, are much higher than REE in the coal, and the REE values of the gangue range from 172×10−6to 386×10−6.

According to the distribution of sampling locations, a curve of the REE average content in the Jurassic is made from north to south (Figs. 1, 3b). It is shown that the REE in Jurassic coal has a higher concentration, which is higher than the average value of the world’s coal, and the REE content in Amugulen coalfield has changed greatly compared to the other mines. The REE average value in Amugulen coal has reached 632.70×10−6, and the REE content in other coalfields are about 200×10−6. The Amuguleng Coal Mine sample shows (Fig. 3d) that the REE content in the roof mudstone sample (ALM1-R) has little difference from the REE content in the surrounding rock of the Cretaceous coal seam in the Erlian Basin, but the REE is significantly enriched in the coal seam of Amuguleng Coal Mine. This is quite different from Early Cretaceous coal-seam.

Table 1. REE content of the Early‒Middle Jurassic coal in the Erlian Basin (10−6).

Table 2. REE content of the Early Cretaceous coal in the Erlian Basin (10−6).

Table 3. REE average value of the Early Cretaceous and Early‒Middle Jurassic coal in the Erlian Basin, UCC, and the world’s coal(10−6).

Fig. 2. REE concentration coefficient of the coal in the Erlian Basin. a‒Cretaceous coal; b‒Jurassic coal.

4.3. Characteristics of REE types

To draw the distribution model curve, the REE average value in the UCC proposed by Taylor SR and Mclennan SM(1985) was used to standardize the REE contents in Jurassic,Cretaceous and world’s coals in the Erlian Basin (Fig. 4a).

The distribution pattern of the world’s coal is flat and the Coal/UCC ratio is less than 1.0, which indicating that the REE average content of the world’s coal is consistent with the UCC REE combination but less than the UCC value. The REE distribution pattern of Cretaceous and world’s coal are similar and tend to coincide, indicating that the combination type and REE element content value of Cretaceous coal and world’s coal are the same. The distribution pattern curve in Jurassic coal shows a leftward deviation, and the REE content of the coal is about 2‒5 times to UCC. The value shows that(LREE/HREE)=12.41 in Jurassic and (LREE/HREE)=14.60 in Cretaceous, and the differentiation degree in Jurassic coal is less than that of Cretaceous. Eu shows a significantly depleted, andδEu=0.70. Y shows an obvious positive anomaly andδY=1.41.

The REE distribution pattern enriched the Amugulen coal samples of Jurassic coal in the Erlian Basin shows certain differences among the samples (Fig. 4b; Table 4). Sample ALM1-2, ALM1-3, ALM2-6 with higher REE contents showing more obvious positive Y anomaly, andδY value are 1.82, 1.86, and 1.57 respectively. While sample ALM1-1,ALM2-1, and ALM2-3 with relatively lower REE contents are 1.47, 1.13, and 1.23, respectively (Fig. 5a; Table 4);δLu is similar toδY, also shows a strong positive correlation with total REE content (∑REE) (Fig. 5b; Table 4); heavy REE content (∑HREE) and ∑REE also show an obvious positive correlation (Fig. 5c; Table 4);δEu shows a strong negative correlation with ∑REE. It means that the greater the REE contents in coal, the more obvious negative anomaly of the Eu element (Fig. 5d; Table 4). ICP-MS method was utilized to test the occurrence of Eu anomalies, meanwhile, the interference of Ba on Eu content, which has been discussed by Dai SF et al. (2016). Eu always shows a strong positive anomaly by Ba interference because the Ba content is more than Eu. However, Eu is dominated by negative anomalies in this research, so that the content of Eu is not interfered with by Ba during the analysis process.

The REE distribution patterns are relatively consistent in the Erlian Basin during the Cretaceous period, which is generally flat (Fig. 4c; Table 5). The REE further study in the coal and surrounding rocks in the Baiyinhua coalfield (Fig. 4d),shows that the REE distribution pattern is similar to the surrounding rocks and gangue, with a flat pattern.

5. Discussion

5.1. REE source and occurrence

Fig. 3. REE distribution of the coal in the Erlian Basin. a‒REE distribution in the Early Cretaceous coal; b‒REE distribution in the Jurassic coal; c‒REE vertical distribution of the coal in the Baiyanhua No.2 Coalfield; d‒REE vertical distribution of the coal in Amugulen.

Fig. 4. REE distribution patterns of coal-seam, roof, and floor in the Erlian Basin, China. a‒distribution patterns of the REE average value of the world’s coal, Jurassic coal, and Early Cretaceous coal in Erlian Basin; b‒REE distribution patterns of the coal in Amugulen; c‒REE distribution patterns of the Early Cretaceous coal; d‒REE distribution patterns of the coal, roof, and floor, dirt band in the Baiyinhua coalfield.

Table 4. REE geochemical parameters of the Jurassic coal in the Erlian Basin, China.

Fig. 5. REE correlation parameter of the Amugulen coal in the Erlian Basin, China.

In general, the REE content is highly positively correlated with main components of coal ash, such as Si, Al, Fe, etc.,and has a significant positive correlation with typical terrigenous ash content such as Ti and Pb (Eskenazy GM,1987a). The REE type standardized distribution pattern in the coal and gangue is similar, and also similar to the UCC distribution. It is speculated that the REE in the coal is mainly derived from terrigenous debris (Huang WH et al., 1999;Zhao ZG, 2002; Eskenazy GM, 1987a). It is found that the roof-floor and gangue distribution pattern is the same in the Cretaceous coal of the Erlian Basin (Fig. 4d), which is consistent with the results of the above studies. It is suggested that terrigenous debris might be the REE main source in the coal. However, there is a significant difference in the REE distribution pattern between the high REE coal (such as samples ALM1-2, ALM1-3, ALM2-6) and the roof mudstone(ALM1-R) from Amugulen coalfield (Fig. 4b). The Amugulen’s roof-floor mudstone and the Cretaceous coal seam surrounding rock has a similar distribution pattern. The high REE coal sample shows a clear leftward pattern,enriched in HREE.δY,δLu, and the ∑REE show a strong positive correlation. It might be due to the certain differences in the atomic structure and radius of each REE, which leads to its fractionation in nature. Follows the sequence of La→Ce→Pr→Nd→Sm→Eu→Gd→Tb→Dy→Ho→Y→Er→Tm→Yb→Lu, the atomic radius of REE gets smaller, the alkalinity of the REE gradually decreases. Similarly, the pH value of hydroxide dissolved in the deposit also decreases in this order,leads to the difference in migration ability and precipitation order. For the trivalent ion, the ions in smaller radii are easier to be adsorbed than those in larger ions. The adsorption capacity of REE ions increases to the ion radius decrease. On the other hand, the ability of REE to form complexes is increasing follows the sequence of La→Ce→Pr→Nd→Sm→Eu→Gd→Tb→Dy→Ho→Er→Tm→Yb→Y→Lu, and the ability of HREE to form complexes is greater than that of LREE so that the migration capacity of HREE is greater than that of LREE in nature (Chen DQ et al., 1990). Therefore, the possible sources of REE in HREE coal are the pH value of terrigenous materials decreases when entering the peat swamp, and the absorption capacity of REE increases from La to Lu, which leads to the accumulation of HREE in peat.Meanwhile, due to the stronger migration ability of HREE,the groundwater or river water flowing into the mud is relatively rich in HREE (Eskenazy GM, 1987b).

Generally, Eu anomalies are not caused by source rock weathering or transportation. Eu anomalies in sedimentary rock are generally inherited from source rocks or affected by high-temperature hydrothermal fluids.δEu is distributed between 0.73 and 1.25 in the Cretaceous coal, and the average value is 0.97. It’s similar to most coals in China and has no obvious abnormalities (Dai SF et al., 2016). The averageδEu is 0.70 in Jurassic coal and it distributes between 1.08 and 0.34 with a wide range, which reflects the REE multi-source.

Previous studies have been found that the REE occurrence state is mainly related to the minerals in coal. As the ash content increases, the REE abundance gradually increases.Extrinsic debris is the main carrier of REE, and the debris mainly is clay minerals. (Shao JB et al., 1997; Huang WH et al., 1999; Zhao ZG, 2002). For low ash and high REE coal, it is difficult to form such a high accumulation level of REE only by terrigenous materials in this coal. In low ash and high REE coal, organic matter plays an important role in the REE accumulation (Zhao ZG, 2002). Seredin VV (1996) found that the REE content between 0.03% and 0.10% in the Far Eastern District of Russian, Kuznets and some other coals, the organic matters in high REE coals play a leading role which differs from other coals of inorganic substances such as clay minerals and so on. The adsorption of clay minerals and the finegrained minerals have secondary significance.

Due to the difference in the REE chemical properties,fractionation may occur when the REE enter the peat swamp with an ionic state, which is mainly manifested by the difference in the distribution pattern of the REE in coal and surrounding rocks. The HREE in coal is relatively enriched.The above-mentioned fractionation may not occur when REE has been carried into the peat swamp by fragments. Therefore,the REE samples in the Cretaceous coal in this study are presumed to be mainly brought into the coal by clastic rocks,which mainly exist in inorganic minerals. While for Jurassic high REE coal, the REE enter the coal basin mainly in a dissolved state quite possibly, because of the differences between REE in the coal and its gangue in REE distribution patterns and content, and the more convincing evidence, the authors will conduct further studies in the future. For the REE occurrence in high REE coal, the authors prefer to reckon thatthe organic matter is a dominant factor because the REE content gangue of the coal lower than in coal, which is rich in organic matter.

Table 5. REE geochemical parameters of the Early Cretaceous coal in the Erlian Basin, China.

5.2. REE resources in coal

The REE ore deposits mainly include carbonate type and weathered crust ion adsorption type. Generally, the carbonate type of REE ore deposits is mainly composed of LREE, the mining grade of REE oxides is 1.5%‒2.0%. Ion adsorption type of REE ore deposits has a complete REE variety, but the global reserves are relatively small, and the REE mining grade oxides is between 0.06% and 0.15%. The current REE supply situation is that the reserves of middle and heavy REE are insufficient, and it is expected to be used up in the next 10‒15 years (Chegwidden J and Kingsnorth DJ, 2011). In terms of the REE price, the HREE price is generally much higher than that of LREE. According to an estimation by Seredin VV and Dai SF (2012), the REE content oxide in many samples of coal ash more than 0.1%, which means that it has industrial utilization value.

Take the Early Jurassic Amugulen coalfield as an example, the REE average value in the coal is about 600×10−6and the ash yield of the coal is about 7%. Therefore, it is estimated that the REE content of the coal ash is about 8000×10−6(0.8%) and the REE content oxides is about 0.9%,far above the industrial-grade (Seredin VV and Dai SF, 2012)and it is far beyond the conventional ionized REE deposit.These coal resources are mainly used for coal-fired power plants, and the fly ash pile has formed a large artificial REE deposit. Compared with conventional deposits, it saves exploration and mining costs and has a high value.Meanwhile, the development and protection of such deposits should still be strengthened, this will greatly strengthen China’s REE reserve. However, because the REE occurrence state in fly ash (Brown P et al., 2011; Neupane G and Donahoe RJ, 2012; Zhang Y et al., 2017; Shao P, 2019) is different from that in conventional deposits (Chi R et al, 2012;Han YS et al, 2019), the current extraction process has the problems of high energy consumption, environmental pollution and so on. It leads to the REE cannot be used in industrial production on a large scale. The economical and environmentally friendly extraction technology of REE has not been getting breakthrough, so it needs more further research and investment in scientific research these researches and investment will help to utilize the REE which concentrated in coal to be the real industrially available and economic value resources.

6. Conclusions

(i) The REE average content of coal samples from the Alatanheli Group of the Early Jurassic in the Erlian Basin is 397.31×10‒6. Among them, the REE average value in the Amugulen coalfield reaches 632.70×10-6, which is much higher than the world’s average value. According to the CC,the elemental abundance of LREE is slightly enriched, MREE and HREE are enriched, and Y is highly enriched. The average content of the Early Cretaceous Baiyanhua Group is 49.06×10−6which is less than the average value of REE content in the world’s coal, and the REE abundance is generally normal.

(ii) The REE average content of the UCC was used to standardize the REE content of the collected samples. The distribution pattern shows that the MREE and HREE are enriched in the Early Jurassic coal samples from Amugulen in the Erlian Basin. The abnormal enrichment of Y, Lu, and other elements is particularly obvious. It is speculated that due to the REE enter the peat swamp in the solution state, and the chemical properties of REE are different, which resulting in fractionation, and the occurrence is suggested to be mainly combined with organic matter. The distribution pattern of REE in Baiyanhua Group coal is mainly flat in the Early Cretaceous and is consistent with the pattern of coal roof,floor, and gangue in clastic rocks, both of which are flat types.It is inferred that the Cretaceous REE are mainly from clastic rocks in the Erlian Basin.

(iii) Depending on the current situation of the REE supply-demand relationship and the results of sampling and testing, the REE in some coals of Anlatanheli Group is of high quality to developed and utilized. A new type of artificial REE deposit has formed by fly ash accumulation in a coalfired power plant. While due to the difference between REE occurrence state and conventional REE deposit, the further economic and environmentally friendly extraction technology needs to be further studied..

CRediT authorship contribution statement

Shao-qing Huang conceived of the presented idea. Shaoqing Huang, Jian-qiang Zhang, Li Zhang, and Kang Liu wrote the manuscript in consultation. Shu-zheng Ning supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgment

This study was supported by the project of the China Geological Survey (DD20160187) and the Science and Technology Special Project of the China National Administration of Coal Geology (ZMKJ-2019-J02).