*Article* **Geological Controls on Geochemical Anomaly of the Carbonaceous Mudstones in Xian'an Coalfield, Guangxi Province, China**

**Bo Li 1, Fuqiang Zhang 1,2, Jialong Liao 2, Baoqing Li 1,\*, Xinguo Zhuang 1, Xavier Querol 1,3, Natalia Moreno <sup>3</sup> and Yunfei Shangguan <sup>1</sup>**


**Abstract:** The anomalous enrichment of the rare earth elements and yttrium (REY), U, Mo, As, Se, and V in the coal-bearing intervals intercalated within the carbonate successions in South China has attracted much attention due to the highly promising recovery potential for these elements. This study investigates the mineralogical and geochemical characteristics of the late Permian coal-bearing intervals (layers A–F) intercalated in marine carbonate strata in the Xian'an Coalfield in Guangxi Province to elucidate the mode of occurrence and enrichment process of highly elevated elements. There are two mineralogical assemblages, including quartz-albite-kaolinite-carbonates assemblage in layers D–F and quartz-illite-kaolinite-carbonates assemblage in layers A–C. Compared to the upper continental crust composition (UCC), the REY, U, Mo, As, Se, and V are predominantly enriched in layers A and B, of which layer A displays the REY–V–Se–As assemblage while layer B shows the Mo–U–V assemblage. The elevated REY contents in layer B are primarily hosted by clay minerals, zircon, and monazite; Mo, U, and V show organic association; and As and Se primarily display Fe-sulfide association. Three geological factors are most likely responsible for geochemical anomaly: (1) the more intensive seawater invasion gives rise to higher sulfur, Co, Ni, As, and Se contents, as well as higher Sr/Ba ratio in layers A–C than in layers D–F; (2) both the input of alkaline pyroclastic materials and the solution/rock interaction jointly govern the anomalous enrichment of REY; and (3) the influx of syngenetic or early diagenetic hydrothermal fluids is the predominant source of U, Mo, V, Se, and As.

**Keywords:** geochemistry; rare earth elements and Y (REY); Yunkai Upland; Heshan Formation; mineral

#### **1. Introduction**

Coals formed in marine carbonate platforms are primarily distributed in South China, including Heshan [1–4], Fuisu [5], Xian'an [6], Yishan [7], Guiding [8], Yanshan (Yunnan Province) [9], and Chenxi (Hunan Province) coalfields [10], of which the first four are located within Guangxi Province, South China. These coals mostly contain highly enhanced concentrations of sulfur (especially organic sulfur) and, thus are classified as super-highorganic-sulfur (SHOS) coals [11,12].

The Late Permian is an important coal-forming period in Guangxi Province, South China. In recent years, the late Permian coal-bearing strata intercalated in marine carbonate strata in Guangxi Province, South China, have attracted much attention mainly due to the elevated concentrations of some critical elements, such as rare earth element and yttrium (REY), U, Mo, Se, V, and so on [3,5,6]. For example, the coals from the Heshan and Yishan

**Citation:** Li, B.; Zhang, F.; Liao, J.; Li, B.; Zhuang, X.; Querol, X.; Moreno, N.; Shangguan, Y. Geological Controls on Geochemical Anomaly of the Carbonaceous Mudstones in Xian'an Coalfield, Guangxi Province, China. *Energies* **2022**, *15*, 5196. https://doi.org/10.3390/en15145196

Academic Editor: Reza Rezaee

Received: 28 June 2022 Accepted: 16 July 2022 Published: 18 July 2022

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Coalfields are enriched in Mo, U, Se, and V [1–3,13] while those from the Fusui, Xian'an, and Yishan Coalfields are characterized by the elevated concentrations of REY, Zr (Hf), and Li [5,6,13].

The geological factors controlling the geochemical anomaly are still controversial. Dai et al. [3] attributed the elevated concentrations of Mo–U–Se–V assemblage in the coals from the Heshan and Yishan Coalfields in Guangxi Province to the joint influence of terrigenous detrital materials from Yunkai Upland and multistage low-temperature hydrothermal fluids. However, the marine invasion is also considered as being the predominant factor controlling the enrichment of Mo–U–Se–V assemblage in the coals from the Heshan Coalfield in Guangxi Province [1]. Additionally, Zeng et al. [13] attributed the enrichment of Mo, U, Se, and V in the coals from the Heshan Coalfield to the soil horizon at top of the middle Permian Maokou Formation. The anomalous enrichment of REY in the coals from the Yishan Coalfield is attributed to the influx of high-temperature hydrothermal fluids [13]. However, the joint influence of volcanic ash fall and water/rock interaction is regarded as a predominant factor influencing the enrichment of REY in the coals from the Xian'an Coalfield in Guangxi Province [6].

The Xian'an Coalfield contains relatively abundant coal resources, but the mineralogical and geochemical compositions of the coals are limitedly investigated [6] and report the enrichment of REY, Sc, U, Pb, and Mo in the lowermost coal seam of the Heshan Formation. However, whether the rare metals are enriched in other coal seams in the Xian'an Coalfield is still unclear. This study investigates the mineralogy and geochemistry of the late Permian coal-bearing intervals located in the middle and upper portions of the Heshan Formation in the Xian'an Coalfield, Guangxi Province, China, with an emphasis on the mode of occurrence and enrichment mechanism of some elements highly enriched in the coal-bearing intervals studied. It also provides an opportunity to determine whether the coal-bearing intervals in the Xian'an Coalfield can be considered as promising raw materials for certain rare metals.

#### **2. Geological Setting**

Late Permian is an important coal-forming period in South China, including Guizhou, Yunnan, Sichuan, and Guangxi Provinces [14]. The late Permian coalfields in Guangxi Province, South China, mainly include the Heshan, Fusui, Yishan, Xian'an, and Baiwang Coalfields (Figure 1A). The Shanglin exploration region studied is located within the Xian'an Coalfield in Guangxi Province.

The palaeogeographic environment of the late Permian in Guangxi Province is mainly represented by a series of isolated carbonate platforms surrounded by deep flume basins (Figure 1B). The late Permian strata include the Heshan and Dalong Formations, the former of which is approximately 140 m thick and consists mainly of carbonates intercalated with coal seam, mudstone, and carbonaceous mudstone while the latter of which is mainly composed of siliceous rocks and siltstones intercalated with tuffaceous sandstone and marlstone.

The late Permian coal-bearing stratum in the Xian'an Coalfield is primarily the Heshan Formation. There are mainly five layers of coal in the Heshan Formation, which are numbered from top to bottom as K1, K2, K3, K4, and K5 coals, respectively (Figure 2). The K3 and K4 coals are divided into two or three layers in this area. The lithology of the roof and floor of the coals is limestone or flint (Figure 2). Based on the lithological and coal-bearing characteristics, the Heshan Formation is subdivided into the upper and lower sections (Figure 2). The upper section of the Heshan Formation is the primary coal-bearing stratum and consists of bioclastic limestone and four coal seams while the lower section is a secondary coal-bearing stratum and composed of limestone, biolimestone, and gravel clastic limestone intercalated with one locally mined coal seam K5. The total thickness of the coal seam is 11.6 m, and the total recoverable thickness is 6.4 m, among which the K4 coal seam is the main mineable coal seam with an average of 1.80 m, and the K3 and K2 coal seams are locally workable or unworkable [1].

**Figure 1.** (**A**) Location of coalfields in Guangxi Province [15]; (**B**) location of the Xian'an Coalfield as well as the distribution of Late Permian sedimentary environments in southern China. CSS, China South Sea [16].

**Figure 2.** (**A**) The generalized stratigraphic column of the late Permian coal-bearing strata within the Xian'an Coalfield [6]; (**B**) sampling column of borehole SL.

#### **3. Methodology**

Sixteen samples of carbonaceous mudstone were collected from borehole SL located in the Wanfu exploration region of the Xian'an Coalfield in Guangxi Province, SW China (Figure 2B). The samples collected were then put in plastic bags to avoid contamination and oxidation.

The individual samples were ground to ≤0.2 mm and split into two representative portions. A portion of the sample (<0.2 mm) was directly used for proximate analysis based on the ASTM Standards D3173-11 (2011), D3174-11 (2011), and D3175-11 (2011) [17–19], while another portion was ground further to ≤0.076 mm (200 mesh) using an agate mortar and pestle for mineralogical and geochemical analysis. Mineralogical analyses of the

sample powders were performed by powder X-ray Diffraction (XRD) using a Bruker D8 A25 Advance (Bruker D8 A25 Advance, Leipzig, Germany) at Institute of Environmental Assessment and Water Research (Barcelona, Spain). The detailed XRD analysis procedure and semi-quantitative analysis were reported in the previous study [20]. The diffractograms were obtained at a 2θ interval of 5–90◦, with a step size of 0.01◦.

Approximately, a 0.1 g sample was weighed and digested based on the method proposed by Querol et al. [21] for geochemical analysis. Major elements (Al, Ti, Fe, Mg, Ca, Na, K, and P) and trace elements were performed by inductively coupled plasma atomicemission spectrometry (ICP-AES, Iris Advantage TJA Solutions, Thermo Fisher Scientific, Waltham, MA, USA) and inductively coupled plasma mass spectrometry (ICP-MS, X-Series II Thermo, Thermo Fisher Scientific, Waltham, MA, USA). Silicon content was measured by wavelength dispersive X-ray fluorescence spectrometry (XRF; ZSXPrimus II) following the methods for chemical analysis of silicate rocks (GB/T14506.28-2010).

A small portion of representative block samples was used to prepare the polished sections for the SEM-EDS analysis. The modes of occurrence of minerals were studied using field emission-scanning electron microscope (ZEISS Sigma300, Carl Zeiss AG, Jena, Germany), equipped with an energy-dispersive X-ray spectrometer (EDS) in the State Key Laboratory of Geological Processes and Mineral Resources (China).

#### **4. Results**

#### *4.1. Coal Chemistry*

The moisture content, ash yield, volatile matter yield, and total sulfur content of the samples from borehole SL are tabulated in Table 1. The moisture content and volatile matter yield of the samples range from 0.4% to 2.1% and 8.4% to 16.3%, respectively. The samples are characterized by high ash yield, which ranges from 54.4% to 86.9% with an average of 75.8% exceeding 50%, and thus are classified as carbonaceous mudstone rather than coal according to Chinese standard (>50% ash yield indicative of noncoal rock, GB/T 15224.1- 2018). However, these carbonaceous mudstones are used as high-ash coals due to relatively rare coal resources in Guangxi Province. Vertically, the ash yield is distinctly lower in layers A and C than in the other layers (Figure 3). The low-temperature ash yield (LTA) of the investigated samples is higher than the high-temperature ash yield (Table 1). The difference is partly due to dehydration of the clay minerals, oxidation of the pyrite, and/or CO2 release from the carbonate minerals during the high-temperature ashing process [3].

**Table 1.** Proximate analysis, total sulfur content, and low-temperature ash yield (LTA) of the samples from borehole SL.


Note: ad, air-dry basis; d, dry basis; daf, dry and ash-free basis.

**Figure 3.** Vertical distribution of ash yield, sulfur content, and the ratios of major element oxides throughout borehole SL profile.

The total sulfur contents of the samples from borehole SL vary between 2.0% and 8.6%, with an average of 4.2%, indicating a high sulfur content for them (<1.0, 1.0–3.0, and >3.0% indicative of low, medium, and high sulfur content, respectively) [12]. The sulfur content shows a distinct variation throughout the profile and displays higher sulfur contents in layers A–C than in layers D–F (Figure 3). Layer A (5.8% on average), B (4.7%), C (6.8%), and D (3.2%) contain a high sulfur content (>3.0%) while layers E (2.2%) and F (2.4%) are characterized by medium sulfur content.

#### *4.2. Mineralogy*

#### 4.2.1. Mineral Phases

The contents of the crystalline mineral phases of the samples taken from borehole SL are tabulated in Table 2. The minerals in the investigated samples consist mainly of quartz, and to a lesser extent, illite, albite, calcite, dolomite, kaolinite, and pyrite, along with trace amounts of bassanite and anatase (Figure 4). Paragonite is only present in sample SL-5. There are two types of mineral assemblage throughout the borehole SL profile. The first mineral assemblage (pyrite-quartz-albite-kaolinite-carbonates) is only present in layers D–F (Figure 5) while the second assemblage (pyrite-quartz-illite-kaolinite-carbonates) occurs in layers A–C (Figure 5).

**Table 2.** Mineralogical proportions and low-temperature ash yield (LTA) of the samples from the borehole SL (on whole-coal basis; unit in wt%).


**Figure 4.** X-ray diffractogram (XRD) patterns of minerals in selected samples to show mineral assemblage.

**Figure 5.** Vertical distribution of minerals throughout borehole SL profile.

Illite is only distributed in layer A (11.7% on average), layer B (12.6%), and layer C (14.0%) while kaolinite universally occurs in all layers (Figure 5). Similarly, pyrite is also abundant in layers A–C relative to layers D–F. Dolomite is more abundant in layers B, D, E, and F compared to layers A and C. The distribution of calcite, however, is different from that of dolomite. Notably, albite is a major mineral constituent in layers D–F but is absent in layers A–C (Figure 5).

#### 4.2.2. Mode of Occurrence of Minerals

Kaolinite mainly occurs in the following forms: kaolinite matrix (Figure 6A), pore/cavityfilling kaolinite (Figure 6B), vermiculate kaolinite (Figure 6C), and fracture-filling kaolinite (Figure 6D), of which the former three forms indicate the syngenetic to early diagenetic stage while the fourth suggests epigenetic stage.

**Figure 6.** Scanning electron microscope (SEM) back-scattered electron images of minerals (**A**–**H**): (**A**) euhedral pyrite embedded within kaolinite matrix (sample SL-9); (**B**) kaolinite and anatase (sample SL-9); (**C**) vermiculate kaolinite (sample SL-12); (**D**) fracture-filling kaolinite (sample SL-5); (**E**) massive pyrite (sample SL-5); (**F**) albite, kaolinite, pyrite, and finely-grained disseminated anatase particles (sample SL-5); (**G**) kaolinite, albite, and pyrite (sample SL-5); and (**H**) framboidal pyrite aggregates (sample SL-16).

Pyrite is a primary mineral phase in samples studied. Pyrite primarily occurs as single euhedral crystals (Figure 6A) or massive forms (Figure 6E–G) embedded in the kaolinite matrix, which indicates an approximately contemporaneous early diagenetic formation. In a few cases, framboidal pyrite aggregates are also observed in the samples studied (Figure 6H).

Albite is an important mineral constituent in layers D–F. Albite predominantly occurs as disseminated particles with irregular corroded borders (Figure 6F,G), indicating the alteration of albite. In most cases, albite is surrounded by flaky kaolinite, revealing the transformation of albite to kaolinite.

Anatase is also found in some samples and primarily occurs as dispersed particles embedded within a clay minerals matrix (Figure 6F), indicating a detrital origin. In a few cases, anatase is embedded within organic matter (Figure 6B), indicating an authigenic origin.

#### *4.3. Geochemistry*

The major and trace elements contents of the samples collected from borehole SL are listed in Table 3.

**Table 3.** Major-element oxides (wt%) and trace element concentrations (μg/g) of the samples from borehole SL-1 (on whole-coal basis).


<dl, below detection limit.

#### 4.3.1. Major Elements

The major elements in the investigated samples are predominantly composed of SiO2, and, to a lesser extent, Al2O3, with the remaining Fe2O3, CaO, MgO, Na2O, and K2O as minor or trace components. The SiO2/Al2O3 ratio (4.2–15.1, 6.6 on average) in the studied samples is evidently higher than the theoretical value of kaolinite (1.2) (Figure 3), illite (3.2), and albite (3.3), which would be explained by quartz-dominated mineral assemblage. The Na2O/Al2O3 value is obviously higher in layers D–F than in layers A–C (Figure 3), coinciding with the mineral assemblage where albite is an abundant constituent in layers D–F but is absent in layers A–C as mentioned above. The higher K2O/Al2O3 and Fe2O3/Al2O3 ratios in layers A–C than in layers D–F (Figure 3) would be ascribed to much more abundant occurrence of illite and pyrite, respectively, in layers A–C than layers D–F (Figure 5). Compared to layers A–C, layers D–F exhibit a distinctly higher CaO/Al2O3 ratio (Figure 3), which is due to the relatively abundant occurrence of carbonate minerals in layers D–F. This is also attested by the relatively significant correlation between MgO and dolomite (Figure 7A).

**Figure 7.** Relationship among selected elements and minerals: (**A**) plot of MgO vs. dolomite; (**B**) plot of K2O vs. illite; (**C**) plot of Fe2O3 vs. pyrite; and (**D**) plot of Fe2O3 vs. sulfur.

K2O significantly positively correlates with illite in layers A–C (Figure 7B), indicating an illite affinity of K2O. Fe2O3 and pyrite have a relatively significant linear correlation (Figure 7C), suggesting a sulfide affinity for Fe. Additionally, Fe and sulfur display a relatively significant correlation (Figure 7D); however, the slope (0.72) of the Fe–S regression equation is distinctly lower than the theoretical Fe/S ratio (0.87), appearing to indicate that other forms of sulfur (e.g., organic sulfur) contribute to the total sulfur.

#### 4.3.2. Trace Elements

To better elucidate the degree of enrichment or depletion of trace elements in coalbearing strata, the concentration coefficient (CC) is used in the present study; the CC is

a ratio of the studied samples versus referenced rocks, such as upper continental crust composition (UCC) [22] and world coal [23], with CC < 0.5, 0.5–2, 2–5, 5–10, and >10 indicative of depletion, similarity, slight enrichment, enrichment, and significant enrichment, respectively [8]. Compared to the average of trace elements in UCC [22], Se, Mo, and U are significantly enriched in the studied samples; As and B are enriched; V, Cr, Cs, Dy, Ho, Tm, and Yb are slightly enriched; and the remaining trace elements are depletion or similar to the values of UCC (Figure 8).

**Figure 8.** Concentration coefficient (CC) of trace elements in the studied samples, normalized to the respective average of upper continental crust composition (UCC; [22]).

Elements U, Mo, and Se in abundance show distinct variations throughout the vertical profile (Figure 9). The significantly enriched-U intervals are distributed in layer A (CC = 41.7), layer B (13.0), layer C (12.5), and layer D (CC = 21.2); enriched-U intervals are vertically located within layer E (CC = 5.1) and layer F (CC = 8.2). Mo and Se are significantly enriched among layers A–F, but they appear to be more elevated in layers A–D relative to layers E–F. As is significantly concentrated in layers A (CC = 11.9) and B (12.0), and other layers reach the enriched or slightly enriched level. Boron in abundance is enriched in layers B (CC = 6.8), C (CC = 6.8), and F (CC = 9.1) and slightly elevated in other horizons. Elements V, Cr, Dy, Ho, Tm, and Yb contents reach the enriched or slightly enriched level within layers A, B, and D.

**Figure 9.** Vertical distribution of selected trace elements, La/Ho and Y/Ho, throughout borehole SL profile.

4.3.3. Rare Earth Element and Yttrium (REY)

The rare earth elements and yttrium (REY) contents in the investigated samples range from 92 μg/g to 625 μg/g (625 μg/g; sample SL-12) with an average of 208 μg/g, which is similar to that of UCC (168 μg/g; [22]) but higher than that of world coal (68.5 μg/g; [23]).

A three-fold REY classification, namely LREY (La, Ce, Pr, Nd, and Sm), MREY (Eu, Gd, Tb, Dy, and Y), and HREY (Ho, Er, Tm, Yb, and Lu), is used in the present study. The UCC-normalized REY distribution pattern is used in the present study to elucidate the distribution and fractionation of REY. The REY distribution pattern can be represented by three types, namely LREE distribution type (L-type), MREY distribution type (M-type), and HREY distribution type (H-type) [24]. The UCC-normalized REY distribution of the investigated samples shows two patterns of REY distribution (Figure 10). The first pattern is represented by the HREY distribution type (H-type) in the samples containing REY content similar to that of UCC (Figure 10A). The second pattern is represented by the LREY distribution type (L-type) in the samples (e.g., SL-12, SL-13) with elevated concentrations of REY (Figure 10B). Moreover, all samples are universally characterized by weakly negative Eu anomaly (0.67–0.95, 0.77 on average) and slightly negative Y anomaly (0.63–0.95, 0.80 on average) (Figure 10). Except for samples SL-12 and SL-13 showing a pronounced positive Ce anomaly and no Ce anomaly, respectively, other samples display slightly negative Ce anomaly (0.82–0.95, 0.85 on average).

**Figure 10.** UCC-normalized REY distribution patterns in the studied samples (**A**,**B**). UCC data from Taylor and McLennan [22].

#### **5. Discussion**

#### *5.1. The Nature of Detrital Materials*

Some previous studies have indicated that the Yunkai Upland located to the east of the Heshan, Yishan, Fusui, and Xian'an Coalfields (Figure 1B) is the dominant sediment source region providing felsic detrital materials into the late Permian coal-bearing basins in Guangxi Province, China [5,6]. However, a few studies have demonstrated that some detrital materials within the late Permian coal-bearing strata in Guangxi Province are probably derived from the basaltic materials from the Kangdian Upland [25], the eroded materials from the felsic igneous rocks at the top of the Kangdian Upland [26,27]. The investigated samples are most likely derived from the eroded felsic detrital materials from the Yunkai Upland based on the following evidence:

(1) The Al2O3/TiO2 ratio has been extensively used to infer the parent rock composition of mudstones [28] and coals [20,29–31], with 3–8, 8–21, and 21–70 indicative of mafic, intermediate, and felsic igneous rocks, respectively [28]. The plot of Al2O3 versus TiO2 shows that the samples studied all fall within the category of felsic rocks (Figure 11A), indicating the input of felsic detrital materials. However, the samples from layers A–C display distinctly low Al2O3/TiO2 (20.6–41.7, 30.9) compared to those from layers D–F (37.7–67.7, 50.6 on average), appearing to indicate that the detrital materials in layers A–C are possibly derived from a mixture of much more felsic constituents and much less mafic materials.

**Figure 11.** (**A**) the plot of Al2O3 versus TiO2 contents for the investigated samples; (**B**) the plot of Zr/TiO2 versus Nb/Y contents for the investigated samples, the data of Qiandongbei [31], Heshan [3], and Fusui [5] are cited to compare.

(2) The Eu anomaly is also commonly used in monitoring parent rocks. In most cases, the coals with the input of mafic detrital materials display a positive Eu anomaly while the coals with the input of felsic detrital materials are characterized by a negative Eu anomaly [32]. The samples studied display negative Eu anomalies ranging from 0.67 to 0.95 with an average of 0.77, suggesting the input of felsic components. The Eu anomaly and Al2O3/TiO2 ratio in the samples studied appear to exclude the basaltic detrital materials from the Kangdian Upland as the predominant terrigenous sediment region.

(3) In the plot of Nb/Y versus Zr/TiO2 (Figure 11B), the samples studied fall within the field of rhyodacite-dacite, comparable to the late Permian Heshan, Fusui coals in Guangxi Province, China [3,5], indicating that the investigated samples have the same source region to the late Permian coals in Guangxi Province. Moreover, the studied samples differ from the felsic igneous rocks from the Kangdian Upland, which cluster within the field of alkaline rocks [31] (Figure 11B), appearing to exclude the possibility that the felsic constituents are sourced from the felsic rocks at the top of Kangdian Upland.

(4) The UCC-normalized REY distribution patterns in the investigated samples are very similar to that of Heshan coals with terrigenous materials from the Yunkai Upland [3,5] but distinctly differ from that of felsic igneous rocks from the Kangdian Upland [31], further confirming the input of felsic detrital materials from the Yunkai Upland.

(5) The samples studied are primarily composed of quartz, albite, clay minerals, and carbonate minerals. This mineral assemblage would correspond well to the Heshan and Fusui coals with a felsic detritus from the Yunkai Upland [3,5].

#### *5.2. Influence of Seawater Invasion*

The coal-bearing layers A–F studied are intercalated within the carbonate successions (Figure 2B), indicating a significant marine invasion during or shortly after coal-bearing deposition. The seawater invasion is also confirmed by geochemical indicators, such as sulfur content, boron content, and Sr/Ba ratio. The previous studies show that the mediumand high-sulfur coals are formed in marine-influenced environments, while the low-sulfur coals are deposited in freshwater-influenced environments [12,33]. The sulfur content in the investigated samples ranges from 2.1 to 8.6% with an average of 4.2% (high-S coal), indicating a significant seawater influence. The boron content in layers A–F range from 45 to 241 μg/g with an average of 100 μg/g, suggesting mildly brackish water-influenced coal-forming environments (B concentrations <50, 50–110, and >110 μg/g indicative of

freshwater, mildly brackish water, and brackish water, respectively [34]). The Sr/Ba ratio is considered a useful index of depositional environments with Sr/Ba > 1 and Sr/Ba < 1 indicative of marine-influenced environments and freshwater-influenced environments, respectively [33]. In the present study, the Sr/Ba ratio in layers A–F ranges from 2.6 to 10.6 with an average of 5.9 evidently exceeding 1.0, revealing seawater-dominatedinfluenced environments, which is consistent with the sedimentary setting of an isolated carbonate platform.

Layers A–C exhibit higher sulfur content (2.8–8.6%, 5.8% on average), Sr/Ba ratio (5.5–10.6, 8.3) than sulfur content (2.1–4.6%, 2.6%), and Sr/Ba ratio (2.4–5.9, 3.5) in layers D–F, indicating a more intensive marine injection during the deposition of layers A–C than layers D–F. The various degrees of marine injection result in different geochemical patterns. For example, layers A–C display higher contents of illite, pyrite, Co, Ni, As, and Se than in layers D–F. This is because Co, Ni, As, and Se are primarily associated with pyrite and the formation of pyrite is intimately associated with the degree of seawater injection [12]. Higher illite abundance in layers A–C is ascribed to the marine-influenced environments because illite is preferentially deposited in brackish-influenced alkali conditions [30,35]. Although the REY content (264 μg/g on average) in layers A–C is higher than that (152 μg/g on average) in layers D–F, seawater is not the primary geological control on REY distribution in layers A–F; otherwise, the pronounced positive Y and Gd anomalies would be expected in layers A–F [36–38], which is in sharp contrast to results shown in Figure 10. Additionally, the Y/Ho molar ratio in the samples ranges from 32.0 to 48.6 with an average of 41.0, which is essentially identical to that in UCC (51.0) but lower than that reported in seawater (90–110; [36]), further indicating a negligible influence of seawater control on the REY content in the investigated samples.

#### *5.3. Mode of Occurrence of Elements*

Zirconium and Hf are commonly hosted in zircon [31]. The Zr and Hf in the samples studied show a significant positive correlation (r = 0.95, Figure 12A), indicating that Zr and Hf are hosted in the same mineral carrier and do not evidently fractionate during the formation of coal-bearing strata. The slope of the Zr-Hf regression equation (34.6) is very comparable to that of zircon in granite (38.5, [39]), revealing zircon as the major carrier of Zr and Hf.

The REY contents in the samples from layers C–F display a negative correlation with LTA (r = −0.52, Figure 12B), positive correlation with Zr (r = 0.74, Figure 12C), and slight correlation with Al2O3 (r = 0.26) and P (r = 0.14), appearing to indicate that REY is jointly hosted by organic matter and heavy minerals, such as zircon. In most cases, the MREY and HREY are preferentially adsorbed to organic matter, thus leading to the H-type REY distribution patterns in layers C–F (Figure 10A). Additionally, the negative correlation between low-temperature ash yield and (MREY + HREY)/REY (r = −0.52; Figure 12D) indicates that the MREY and HREY comprise more proportions of total REY with increasing organic matter. By contrast, the REY content in the samples from layers A–B is positively correlated with LTA (r = 0.86, Figure 12B), Al2O3 (r = 0.99, Figure 12E), and Zr (r = 0.86, Figure 12C), suggesting that REY is predominantly hosted by inorganic matters (e.g., clay minerals, zircon) rather than organic matter. Although the REY in layers A–F shows a relatively significant correlation with Zr, zircon is not the major carrier for REY because the UCC-normalized REY distribution of the samples studied (Figure 10) is not consistent with the REY distribution of zircon (HREY enrichment type, positive Ce anomaly, negative Eu anomaly, and positive Y anomaly [40]). The REY-rich samples SL-12 and SL-13 from layer B display a higher La/Ho ratio compared to other benches (Figure 9), probably indicating precipitation of LREY-enriched mineral phases, such as monazite, which preferentially incorporate the LREE but do not fractionate Y and Ho (a rather constant Y/Ho ratio) as reported by Bau and Dulski [36] and Chesley et al. [41]. Additionally, the L-type REY distribution (Figure 10B) and relatively high P and Th content in the REY-rich samples SL-12 and SL-13 appear to indicate the occurrence of phosphate minerals, such as monazite.

**Figure 12.** Correlation among selected elements and LTA: (**A**) plot of Zr vs. Hf; (**B**) plot of REY vs. LTA; (**C**) plot of REY vs. Zr; (**D**) plot of (MREY + HREY)/REY vs. LTA; (**E**) plot of REY vs. Al2O3; (**F**) plot of U vs. Mo; (**G**) plot of U vs. V; (**H**) plot of Mo vs. V; (**I**) plot of Mo(U) vs. LTA; (**J**) plot of Mo vs. LTA; (**K**) plot of U vs. LTA; (**L**) plot of V vs. LTA; (**M**) plot of Se vs. As; (**N**) plot of As(Se) vs. Fe2O3; and (**O**) plot of As(Se) vs. Sulfur.

Mo, U, and V show a similar distribution throughout the borehole profile (Figure 9) and display a positive correlation between Mo and U (r = 0.91, Figure 12F), U and V (r = 0.82, Figure 12G), and Mo and V (r = 0.75, Figure 12H), indicating the same carrier for them or similar geological controls on their enrichment. Mo and U negatively correlate

with LTA (Figure 12I) in all samples; in layers A–B and D–F, Mo, U, and V significantly and negatively correlate with LTA (Figure 12J–L), appearing to confirm that Mo, U, and V are predominantly hosted by organic matter, which is consistent with the previous studies indicating that Mo, U, and V in the coals intercalated within carbonate succession commonly show an organic affinity [24,28].

The distribution of As is comparable to that of Se, and they display a relatively significant correlation (r = 0.66, Figure 12M), indicating the same carrier for As and Se. The As and Se positively correlate with Fe2O3 (Figure 12N) and sulfur (Figure 12O), suggesting a primary Fe-sulfide affinity for them. The lower correlation coefficient of As–S and Se–S than As–Fe2O3 and Se–Fe2O3 is ascribed to the additional existence of organic sulfur.

#### *5.4. Elevated Concentrations of Trace Elements*

#### 5.4.1. Rare Earth Elements and Yttrium (REY)

The REY content in layer B ranges from 278 to 619 μg/g with an average of 460 μg/g higher than the UCC (168 μg/g; [22]). The highly elevated concentration of REY in layer B is a joint result of the input of alkaline detrital materials and solution/rock interaction based on the following evidence:

(1) As mentioned above, the Yunkai Upland provides felsic detrital materials into the late Permian coal-bearing basin, which do not cause the geochemical anomaly of most lithophile elements in the samples except for those from layer B, appearing to indicate that the terrigenous felsic detrital materials from the Yunkai Upland contribute to normal geochemical background values and other geological factors possibly exert an important influence on REY enrichment in layer B.

(2) The REY-rich samples SL-12 and SL-13 in layer B have higher Nb/Y ratios compared to other bench samples (Figure 11B), appearing to indicate more input of alkaline detrital materials because alkaline rocks compared to sub-alkaline rocks have higher Nb/Y ratio [42]. Moreover, the elevated REY content in layer B is accompanied by elevated concentrations of Zr, Nb, and Ga, which are commonly found in the coals influenced by alkaline volcanic ash [13,24,31], also supporting the input of alkaline pyroclastic materials.

(3) The REY-rich samples SL-12 and SL-13 in layer B are characterized by L-type REY distribution (Figure 10B), which is commonly caused by the input of terrigenous clastic materials or pyroclastic materials [24]. As discussed above, terrigenous felsic detrital input is excluded as a predominant geological factor contributing to the REY enrichment, appearing to indicate that the pyroclastic material is responsible for the L-type REY distribution.

(4) The abundant occurrence of albite and vermicular kaolinite (Figure 6C,F,G), combined with the negative Eu anomaly, appears to indicate the influx of felsic pyroclastic materials because vermiculate kaolinite is commonly derived from the alteration of volcanic ash [43].

(5) The bench sample (sample SL-12) with the highest REY content shows a most notable feature of positive Ce anomaly compared to any of the other benches, revealing that the bench sample is subjected to the oxidation of oxygen-rich solutions, which oxidizes Ce3+ to Ce4+ that is preferentially adsorbed to clay minerals or deposits as cerianite [44]. In this case, the positive Ce anomaly is most likely caused by preferential adsorption of Ce4+ onto clay minerals rather than deposition of cerianite because Ce/La (3.2), Ce/Pr (13.2), and Ce/Nd (3.6) ratio in the bench sample SL-12 is markedly lower than Ce/La (222), Ce/Pr (257), and Ce/Nd (177) in cerianite [45]. The transformation from positive Ce anomaly, no Ce anomaly, to negative Ce anomaly in the upper, middle, and lower portions of layer B, respectively, are due to preferential adsorption of Ce4+ onto clay minerals in the upper portion of layer B and, consequently, Ce-poor solutions migrate downward into the lower portion. The lower Y/Ho ratio in the REY-rich benches (17.6) compared to other REY-poor benches (22.9) also confirms the solutions/rocks interaction during which Y and Ho are released, but Ho, relative to Y, is more readily adsorbed by clay minerals [36], causing a relatively low Y/Ho ratio in the REY-rich benches. During the interaction of solution/rock, the HREY are easily released and complexed relative to LREY; the released HREY ions

migrate downward and are ultimately absorbed by clay minerals with the increasing pH due to reaction between HREY-containing solutions with detrital materials, which explains the L-type REY distribution in the upper portion and H-type REY distribution in the lower portion of layer B (Figure 10).

(6) Albite is relatively high in layers D–F, but it is absent in layers A–C, indicating more intensive chemical weathering or solution/rock reaction in layers A–C, which results in the transformation of albite to clay minerals.

#### 5.4.2. Uranium, Mo, V, As, and Se

Uranium, Mo, and V are highly enriched in layer A and layer B and their contents are comparable to the U–Mo–V-rich coals intercalated within carbonate successions [3,5,8,46]. The highly elevated U content is commonly accompanied by Mo and V in the Heshan coals [1–3], Fusui coals [5], Yishan coals [13] (anomaly), and Shanglin coals [6], which were deposited in isolated carbonate platform environments. In the previous study, the highly enriched concentrations of U, Mo, and V are primarily governed by the infiltration of syngenetic or early diagenetic low-temperature hydrothermal fluids rather than the detrital materials from the sediment-source region and marine influence as confirmed by the following evidence:

(1) The felsic detrital materials can be excluded because the felsic rocks are usually depleted in compatible elements, such as V and Cr, which are enriched in the investigated samples (Figure 8).

(2) The previous studies demonstrate that seawater influence may exert an important effect on U and Mo enrichment [47]. In the present study, this is not the case because U (r = −0.67), Mo (r = −0.26), and V (r = −0.86) display a negative correlation with Sr/Ba ratio, which is a useful indicator of seawater invasion [3]. The negative correlation reveals an adverse influence of marine invasion on U, Mo, and V accumulation.

(3) The enhanced concentration of element assemblage of U–Mo–V–As–Se in the samples studied is essentially comparable to that in the coals subjected to the influence of hydrothermal fluids [3], revealing the infiltration of hydrothermal fluids. Moreover, the hydrothermal fluids infiltrating into the investigated samples are regarded as being low-temperature hydrothermal fluids because the high-U and low-U samples show similar negative Eu and Y anomaly, appearing to rule out the injection of high-temperature solutions; otherwise, the positive Eu and Y anomaly can be expected [44]. Furthermore, the lowtemperature hydrothermal fluids most likely migrate and permeate into the coal-bearing strata during peat accumulation (syngenetic stage) or shortly after peat accumulation (early diagenetic stage) because the U-rich layers are intercalated among the impermeable clays and limestones, which hamper the infiltration of hydrothermal fluids at the late diagenetic stage.

#### **6. Conclusions**

Based on the study of the mineralogy and geochemical characteristics of samples in the studied area, the following conclusions can be drawn:

(1) The minerals in layers A–C consist mainly of quartz, illite, kaolinite, and carbonate phases while those in layers D–F are predominantly composed by quartz, albite, kaolinite, and carbonate phases.

(2) The REY, U, Mo, As, Se, and V are predominantly enriched in layers A and B. The REY is hosted primarily by clay minerals, and U, Mo, and V are primarily associated with organic matter while As and Se show an Fe-sulfide affinity.

(3) The more extensive marine invasion results in the higher contents of pyrite, sulfur, As, Se, Co, and Ni in layers A–C than layers D–F; the input of alkaline pyroclastic materials and the interaction of O2-rich solutions and detrital materials jointly govern the REY enrichment and distribution pattern; the influx of low-temperature hydrothermal fluids at the syngenetic or early diagenetic stage is the predominant source of U, Mo, As, Se, and V in layers A–B. The elements U, Mo, V, Se, and REY are highly promising for recovery.

**Author Contributions:** B.L. (Bo Li), B.L. (Baoqing Li), X.Z., F.Z., X.Q., Y.S. and J.L. collected the samples; X.Q. and N.M. conducted the experiments; B.L. (Bo Li) wrote the original draft; B.L. (Baoqing Li) revised and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key Research and Development Program of China (No. 2021YFC2902005), National Science Foundation of China (No. 41972182), the National Science Foundation of Guangxi Province (No. 2018JJA150165), and Science Program of China National Administration of Coal Geology (no. ZMKJ-2021-ZX03).

**Acknowledgments:** The authors would like to give their sincere thanks to Guangxi Bureau of Coal Geology for assistance during sampling and Institute of Environmental Assessment and Water Research, CSIC, Spain, for assistance during the sample analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Article* **Mineralogy and Geochemistry of the Lower Cretaceous Coals in the Junde Mine, Hegang Coalfield, Northeastern China**

**Yingchun Wei 1,2, Wenbo He 2, Guohong Qin 3,\*, Anmin Wang <sup>2</sup> and Daiyong Cao <sup>1</sup>**


**Abstract:** Hegang coalfield is one of the areas with abundant coal resources in Heilongjiang Province. Characteristics of minerals and geochemistry of No. 26 coal (lower Cretaceous coals) from Junde mine, Hegang coalfield, Heilongjiang province, China, were reported. The results showed that No. 26 coal of Junde mine is slightly enriched in Cs, Pb, and Zr compared with world coals. The minerals in No. 26 coal of Junde mine primarily include clay minerals and quartz, followed by calcite, siderite, pyrite, monazite, and zircon. The diagrams of Al2O3–TiO2, Zr/Sc–Th/Sc, Al2O3/TiO2–Sr/Y, and Al2O3/TiO2–La/Yb indicate that the enriched elements in No. 26 coal were mainly sourced from the Late Paleozoic meta-igneous rocks in Jiamusi block. The volcanic ash contribution to No. 26 coal seems very low. Sulfate sulfur indicating oxidation/evaporation gradually decreases during No. 26 coal formation.

**Keywords:** trace elements; minerals; source; volcanic; sedimentary environment

#### **1. Introduction**

Coal is considered an important source of critical metals [1,2]. The critical metals in coal have become a research hotspot in recent years due to their potential economic significance [2,3] and the geological implications for coal basins [4,5]. Coals with the significant economic value of critical element production are usually called 'metalliferous coal' [2,6] or 'coal-hosted rare-metal deposit' [7]. More recently, further studies have paid close attention to the occurrence and recovery methods from these coals [8–19].

Heilongjiang Province, Northeastern (EN) China, has a vast territory and rich resources. In terms of coal resources, the Hegang coalfield is rich [20,21]. Although there has been much literature focusing on coal geochemical and mineralogical characteristics in China, these works are mainly concentrated in the coals in southwestern and northern China [13,22–29]. In Heilongjiang province, a few studies [30,31] have been carried out about coal-bearing sequences in the Hegang coalfield. Previous investigations have shown that the Hegang coalfield was affected by three periods of tectonic stress and accompanied by multiple periods of volcanic activity, which had an impact on coal quality in the coal basin [32]. There are still two problems to be solved about coal geochemistry in the Hegang coalfield: (1) did the volcanic activity influence the element enrichment in the coals? (2) The occurrence modes and the provenance of elements in the Palaeogene coal-bearing seams have been deeply studied recently in Jilin province [33–36], neighboring the Heilongjiang province. Is the provenance of trace elements in Hegang coals the same as in the coals from Jilin Province [33,35]? Further investigation of these issues has important theoretical and economic value for improving the comprehensive utilization efficiency of coal measures mineral resources.

**Citation:** Wei, Y.; He, W.; Qin, G.; Wang, A.; Cao, D. Mineralogy and Geochemistry of the Lower Cretaceous Coals in the Junde Mine, Hegang Coalfield, Northeastern China. *Energies* **2022**, *15*, 5078. https://doi.org/10.3390/ en15145078

Academic Editor: Dameng Liu

Received: 21 June 2022 Accepted: 11 July 2022 Published: 12 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

To address this issue, we study the minerals and elements characteristics of No. 26 coal in the Hegang coalfield. We also discussed the geological factors influencing element enrichment.

#### **2. Geological Setting**

Junde mine is situated in Hegang coalfield, Heilongjiang Province (Figure 1). Geologically, the Hegang coalfield is located on the Jiamusi block inside the Central Asian Orogenic Belt between the Chinese plate and the Siberia plate, and is generally characterized as a semi-covered monoclinic structure dipping eastward. The Hegang coalfield is an important part of the Mesozoic-Cenozoic basin group in the east of Heilongjiang. There are mainly fault structures in the Hegang coalfield, and there are relatively gentle folds in the local area, accompanied by multiple volcanic structures. Proterozoic, Mesozoic and Cenozoic strata developed in the study area [37,38].

The major coal-bearing formation in the Hegang coalfield is the Lower Cretaceous Chengzihe Formation. The Lower Cretaceous Chengzihe Formation consists of graywhite conglomerate, coarse and fine sandstone, gray-yellow medium sandstone, dark gray siltstone, mudstone, coal seams, and tuff. In Hegang coalfield, 36 coal seams are minable or

partially minable, which are divided into four coal bearing groups from top to bottom. The first coal seam group includes coal seams No. 1 and No. 2, which are unstable. The second coal seam group includes coal seams No. 3~22. The third coal seam group, including coal seams No. 23~26, is mainly developed in the south of the coalfield, and its thickness gradually thins or even pinches out to the north. The fourth coal seam group includes coal seams No. 27~32.

No. 26 coal, as the main mining seam in Junde mine from Hegang coalfield, is the main research object of this paper. The thickness of No. 26 coal is about 2.9 m, deposited in continental facies [37,38].

#### **3. Sampling and Analytical Techniques**

Ten samples were obtained in No. 26 coal in Junde mine, Hegang Coalfield, which include one roof sample, eight coal samples, and one floor sample (Figure 2). From top down, eight coal samples are identified as JD-26-01 to JD-26-08 (Figure 2B). The roof and floor are identified as JD-26-R and JD-26-F, respectively (Figure 2B). All samples are fresh, without pollution and oxidation.

**Figure 2.** (**A**) The strata histogram (modified after [38]) and (**B**) sampling profile from Junde mine, Hegang coalfield.

According to ASTM D3173-11, D3174-11 and D3175-11(2011) [39–41], the coal samples collected from Junde mine were tested for the proximate analysis. The macerals were obtained based on the ICCP System 1994 (ICCP, 1998, 2001) [42,43] and Pickel, Kus [44]. The total sulfur content was obtained by ZCL-3 automatic sulfur analyzer, and the contents of various sulfur forms in the samples were obtained according to ASTM D3177-02(2011) and ASTM D2492-02(2012) [45,46]. These analyses were made at the Société Générale de Surveillance S.A-China Stand Science and Technology Group Corporation (SGS-CSTC) Standards Technology Service Corporation.

Based on SY/T 5163-2010, the minerals were identified by X-ray diffractometer. The morphology of minerals was observed by the scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS). The working distance was 8.4–11.4 mm, and beam voltage is 15.0 kV during the SEM operation. These experiments were conducted at the Beijing Center for Physical and Chemical Analysis (BCPCA).

The cleaned samples used for the geochemical analysis were crushed and ground to less than 200-mesh size using a tungsten carbide ball mill. The Major Oxides contents were obtained using X-ray fluorescence spectrometry (XRF; PANalytical Axios). The trace elements were obtained by the inductively coupled plasma mass spectrometry (ICP-MS, ELEMENT XR). Before ICP-MS analysis, samples were digested by using an UltraClave Microwave High Pressure Reactor (Milestone, Milan, Italy). Multi-element standards (Inorganic Ventures: CCS-1, CCS-4, CCS-5, and CCS-6; GBW07103, GBW07104) were used for trace element concentrations calibration. More method details can be seen in [47]. XRF analyses and the ICP-MS analyses were performed at the China National Nuclear Corporation (CNNC) Beijing Research Institute of Uranium Geology.

In our study, concentration coefficients (CC) proposed by [48] is adopted. CC = ratio of the element contents in the coals/clays to average contents in worldwide coals/clays. On this basis, the concentrations of elements can be classified into six categories, i.e., unusually enriched (CC > 100), significantly enriched (10 < CC < 100), enriched (5 < CC < 10), slightly enriched (2 < CC < 5), normal (0.5 < CC < 2), and depleted (CC < 0.5).

#### **4. Results**

#### *4.1. Coal Characteristics and Coal Petrology*

As shown in Table 1, the ash yield and total sulfur of Junde coals is 8.6–17.6% (13.0% on average) and 0.54–0.72%, (0.62% on average). The contents above show that No. 26 coals are characterized by low ash and low sulfur based on GB15224.1-2010 and GB15224.2- 2010 [49,50]. In addition, sulfur in coal occurs mainly in organic and pyritic sulfur. Sulfate sulfur increases with depth.


**Table 1.** Proximate analysis and sulfur contents (%) of coals from Junde mine.

Mad—air-dry basis moisture; Ad—dry basis ash yield; Vdaf—dry and ash-free basis volatile matter.

The vitrinite (84.8 vol.% on average) is the most abundant macerals in No. 26 coal (Table 2). It is composed mainly of collotelinite (Figure 3A–C,E,I) and collodetrinite (Figure 3A,D,I), and to a lesser extent, by telinite (Figure 3H), with a few corpogelinite and virtrodetrinite. The collodetrinite is usually in matrix form, embedded in quartz (Figure 3B), clay minerals (Figure 3D,I), calcite (Figure 3E), or fracture-filling pyrite (Figure 3G).

*Energies* **2022**, *15*, 5078


**Table2.**CoalpetrologycharacteristicsofNo.26coals(Vol.%). Notes: T—telinite; CT—collotelinite; CD—collodetrinite; CG—corpogelinite; VD—Virtrodetrinite; T-V—total vitrinites; F—fusinite; SF—semifusinite; Mic—micrinite; ID—inertodetrinite; T-I—total inertinites; Sp—sporinite; Cut—cutinite; Res—resinite; Sub—suberinite; Bit—bituminite; T-L—total liptinites; -, no test or no result.

**Figure 3.** Coal petrology characteristics in No. 26 coal. (**A**) Collotelinite, collodetrinite, and pyrite in sample JD-26-01; (**B**) Collotelinite, clay, quartz, and pyrite in sample JD-26-01; (**C**) Collotelinite and clays in sample JD-26-04; (**D**) Collodetrinite, semifusinite, quartz, pyrite and clay minerals in sample JD-26-04; (**E**) Collotelinite and calcite in sample JD-26-05; (**F**) Fusinite in sample JD-26-07; (**G**) Pyrite in in sample JD-26-07; (**H**) Telinite and Fusinite in sample JD-26-07; (**I**) Collodetrinite, collotelinite and clays in sample JD-26-08. T—telinite; CT—collotelinite; CD—collodetrinite; F—fusinite; SF—semifusinite.

The inertinite (8.2 vol.% on average) is the second most abundant macerals in No. 26 coal in the Junde mine (Table 2). It primarily consists of semifusinite (Figure 3D), inertodetrinite, and fusinite (Figure 3F,H), with small amounts of micrinite. In the majority of cases, the cell walls of fusinite and semifusinite are swollen and not intact, indicating the existence of degradation [51,52] (Figure 3D,F,H).

The liptinite (2.2 vol.% on average) is mainly represented by sporinite (1.4 vol.% on average), with trace amounts of cutinite (0.3 vol.% on average), bituminite (0.3 vol.% on average), resinite (0.2 vol.% on average), and suberinite (0.1 vol.% on average).

#### *4.2. Geochemical Features*

#### 4.2.1. Major Oxides (MEO)

Table 3 shows the MEO percentages of studied coals and the average value of Chinese coals. The MEO in studied coals are primarily composed of SiO2 (average 7.63%), Al2O3 (average 2.95%) and Fe2O3 (average 1.62%). By comparison with the average values of Chinese coals [22], the percentages of MnO and K2O are a bit higher, while the content of other MEO is near or below (Figure 4). Value of SiO2/Al2O3 of No. 26 coals (average 2.65) exceeds that of Chinese coals [22] and the theoretical figure of SiO2/Al2O3 of kaolinite, probably because a high content of quartz existed in the studied coals (Figure 3B,D).


**Table 3.** The concentration of MEO in No. 26 coals, on a whole coal basis (%).

Notes: LOI, loss on ignition; a, concentration of MEO in Chinese coals [22].

**Figure 4.** Concentration coefficient (CC) of MEO in No. 26 coals.

#### 4.2.2. Trace Elements

Compared to the average figures for world hard coals [53], the trace elements Cs (CC, 3.58), Pb (CC, 2.05) and Zr (CC, 2.04) are slightly enriched in No. 26 coals. Cr, Cd, Ba, Sr, Cu and Bi are depleted (Table 4, Figure 5A).

By comparison with the average figures for world clays [54], the roof sample is slightly enriched in Er (CC, 2.92), Pb (CC,2.78), Bi (CC, 2.37), Yb (CC,2.32), Lu (CC, 2.23), Ho (CC, 2.11), and Dy (CC, 2.08). Other elements are normal or depleted (Figure 5B). In floor sample, only Pb (CC, 2.23) and Bi (CC, 2.19) are slightly enriched, and other elements are normal or depleted (Figure 5C).

In addition, from the vertical section of No. 26 coal seams, the contents of Cs, Pb, and Zr in coal near the host rocks (roof and floor) outclass those in the middle coal seams (Figure 6).

#### 4.2.3. Rare Earth Elements and Yttrium (REY)

REY geochemical classification adopted in our study is according to [55]. Previous studies have shown that because of interference of BaO or BaOH, the Eu content measured in coal by ICP-MS based on quadrupole should be carefully used to identify the positive Eu anomaly [56,57]. In our study, the Ba/Eu values (56 to 370, average of 185), and the relation degree of Ba and Eu in coal samples are low (Figure 7), indicating that Ba concentration has no interference with Eu content.

**Figure 5.** CC [48] of trace elements in (**A**) coals, (**B**) the roof, and (**C**) the floor in the Junde mine. Average concentration of trace elements of world hard coals is used to normalize the corresponding element concentration in studied coal samples [53]. Average concentration of trace element of world clays is used to normalize the corresponding element concentration in the studied roof and floor samples [54].

**Figure 6.** Variations of Hf, Cs, and Pb contents in the Junde mine.


 **Ce**

 122

 52

 14

 22

 72

 17

 12

 19

 34

 138

 30

 23

 75

 Hf

 -

 7.0

 0.98

 1.3

 1.3

 1.5

 1.5

 1.1

 1.8

 3.0

> 8 1.2 5.8 0.83 4.4 0.9 1.9 0.5ofworldcoals[53];b,averagesofworldclay[54];test

Average-coal

world coal a

world clay b

 3.3 3.5

10

 36 a, averages

 -, no

 or no result.

 12

 2

 0.47

 2.7 0.32

 12

 2.2

 0.28

 1.9 0.32

 1.7 0.34 0.99

 2.1 0.54 0.93

 0.16

 0.31

 1.0

 2.5 0.39

 1.4

 1.3

 14 0.38

 0.2 0.28

 1.0 0.17 0.55

 0.5

 0.63

 7.8 0.97

 16

 0.2

 3.7

 3.3

 14

 120

 1.3

 190

 5

 25

 0.92

 36

 1.2

 15

 0.95

 73

 2.0

*Energies* **2022**, *15*, 5078

**Figure 7.** (**A**) Correlation diagram of Eu and Ba contents in coals, roof, and floor samples. (**B**) Magnification of relationship between Eu and Ba in coal samples from Junde mine.

The contents of REY of No. 26 coal in the Junde mine (from 34.1 μg/g to 174.6 μg/g, average 80.6 μg/g; Table 5) slightly exceed those in the world's hard coal [53], but below those in Chinese coals [22] and the upper continental crust [58].


**Table 5.** REY geochemical parameters for the studied samples from the Junde mine.

Notes: Units for REYs: μg/g; REY = LREY + MREY + HREY; Calculation formula for LREY, MREY, HREY, Eu*N*/Eu*<sup>N</sup>* \*, Ce*N*/Ce*<sup>N</sup>* \*, δY are according to [56].

The REY contents in host rocks (roof and floor) are about four times that of coals (Table 5). Additionally, REY contents of the roof (340.9 μg/g) and the floor (327.8 μg/g) are well above those of average world clays [54] and the upper continental crust [58].

According to [55], the features of REY in No. 26 coals from Junde mine are LREY enrichment, with obviously negative Eu (Figure 8A), and, in the upper coal bench only, by HREY enrichment with obviously negative Eu, and positive Y anomalies. Correspondingly,

the feature of REY of the roof is HREY enrichment like the top coal sample, while the feature of REY of the floor is LREY enrichment, like other coal samples (Figure 8B).

**Figure 8.** REY characteristics for (**A**) coal samples, (**B**) roof and floor samples in Junde mine. REY contents were normalized by UCC.

#### *4.3. Mineralogy*

Based on X-ray diffraction experiment (XRD) results (Figure 9), clay minerals and quartz are the chief minerals in the Junde coals, and after them calcite, siderite, and pyrite. Some monazite and zircon were detected in No. 26 coal by SEM-EDS. The minerals in the floor and roof are primarily clay minerals, quartz, potassium feldspar, and plagioclase.

#### 4.3.1. Clay Minerals

Clay minerals in No. 26 coals primarily occur as massive lumps (Figure 10A,B) and cellfillings (Figure 3C), indicating authigenic and terrigenous origins. Kaolinite, illite/smectite formation (I/S), and illite have been detected in Junde coals.

**Figure 9.** XRD patterns of No. 26 coals of Junde mine. (**A**) Sample JD-26-02; (**B**) sample JD-26-04; (**C**) sample JD-26-05; (**D**) sample JD-26-08.

**Figure 10.** Minerals in samples from Junde mine (SEM, SE mode). (**A**) Kaolinite and quartz in sample JD-26-05; (**B**) Kaolinite and siderite in sample JD-26-06; (**C**) Siderite and quartz in sample JD-26-04; (**D**) Quartz in sample JD-26-05; (**E**) Pyrite in sample JD-26-04; (**F**) Pyrite in sample JD-26-05; (**G**) Calcite in sample JD-26-04; (**H**) Fracture-filling calcite in sample JD-26-05, Reflected light, oil immersion; (**I**) Siderite and kaolinite in sample JD-26-06.

#### 4.3.2. Quartz

In No. 26 coals, some quartz occurs as fine-grained particles (Figure 10C,D), indicating a syngenetic detrital origin. A lot of quartz in the studied coals has sharp edges Figures 3B,D and 10A) and indicates the influence of high temperature and volcanic activity [35,59,60].

#### 4.3.3. Pyrite

Some pyrite in No. 26 coal is in the form of discrete crystals (Figures 3B,D and 10E), suggesting an authigenic origin. In addition, fracture fillings pyrite has also been found (Figures 3A,G and 10F), suggestive of an epigenetic origin [61].

#### 4.3.4. Carbonate Minerals

Calcite and siderite are the major carbonate minerals. Calcite generally occurs as plates (Figure 10G) and crack fillings (Figure 10H), suggesting an authigenic origin and epigenetic origin, respectively. Additionally, the siderite occurs as fracture-fillings cutting through the kaolinite (Figure 10I), indicating an epigenetic origin.

#### 4.3.5. Other Minerals

Zircon (Figure 11) and monazite (Figure 12) were observed in No. 26 coal. Zircon occurs as corroded-crystal (Figure 11A), hexagon (Figure 11B) or incomplete quadrilateral bipyramid (Figure 11C), while monazite occurs mainly as nearly circular individual particles (Figure 12).

**Figure 11.** Zircon in samples from Junde mine. (**A**) Sample JD-26-03; (**B**) Sample JD-26-05; (**C**) Sample JD-26-06. (**D**) EDS spectrum of Spot 1 in Figure 11C.

**Figure 12.** Monazite in samples from Junde mine. (**A**) Sample JD-26-04; (**B**) Sample JD-26-06; (**C**) Sample JD-26-07; (**D**) EDS spectrum of Spot 1 in Figure 12C.

#### **5. Discussion**

#### *5.1. Sediment Source*

The value of Al2O3/TiO2 is commonly used to infer the sediment source (including coal deposits) [62,63]. The value of Al2O3/TiO2 in Junde coals varies from 26 to 45 (average 22.6) (Figure 13A), suggesting source rocks during No. 26 coal formation in Junde mine are primarily felsic rocks. This conclusion can also be evidenced by the diagrams of Zr/Sc—Th/Sc (Figure 13B), which have been proved a useful provenance indicator for coal deposits [64,65].

The No. 26 coals in the study area and the Fuqiang coals from the Hunchun Coalfield [33] have similar Cs-Pb-Zr enrichment characteristics. The provenance in Fuqiang coals is Mesozoic and Paleozoic igneous and metamorphic rocks [33]. To further explore the terrigenous supply of No. 26 coal, a comparison about Sr/Y, La/Yb, and Al2O3/TiO2 values of Junde coals with the local (the Jiamusi block) rocks of Neoproterozoic, Late Paleozoic, Mesozoic and Cretaceous ages [66–69] is shown in Figure 14. Figure 14 indicates that the source material of the Junde mine mainly comes from the Late Paleozoic igneous rocks from Jiamusi block. In addition, the REY patterns of No. 26 coal are akin to those of the Late Paleozoic igneous rocks with negative Eu anomalies (Figure 15). Sun et al. have also determined that detrital zircons of Mesozoic and Paleozoic ages predominate in sandstones of the Chengzihe Formation [70]. Those zircons are shaped similarly to the zircons described in this study (Figure 11A–C). Therefore, the terrigenous components in No. 26 coal from Junde mine were mainly derived from the Late Paleozoic meta-igneous rocks in local block.

**Figure 14.** Comparison of (**A**) Al2O3/TiO2 vs. Sr/Y and (**B**) Al2O3/TiO2 vs. La/Yb between the studied samples, and the local (the Jiamusi block) rocks of Neoproterozoic, Late Paleozoic, Mesozoic and Cretaceous ages [66–69].

**Figure 15.** Characteristics of REY of (**A**) Late Paleozoic rocks, (**B**) Early Cretaceous rocks, (**C**) the Neoproterozoic rocks, and (**D**) Mesozoic rocks in the Jiamusi block [66–69].

#### *5.2. Sedimentary Environment*

The value of Sr/Ba has been widely used to identify the depositional environment of sedimentary rocks and coals [71,72]. Generally, coals with Sr/Ba ratio greater than 1 and less than 1 was formed in the environment of seawater and freshwater intrusion, respectively. However, we should be cautious when using this indicator, because the content of Sr and Ba in terrigenous clastic minerals (especially feldspar) may cause misjudgment of the sedimentary environment [73,74]. In the present study, feldspar content is so low that it is not detected by XRD. Thus, Sr/Ba ratio is used to roughly distinguish sedimentary environment in this study. Sr/Ba value of No. 26 coal seam from Junde mine varies from 0.48 to 1.01, and the average Sr/Ba value of the coal seam is 0.81, suggestive of a primarily fresh-water affected process of No. 26 coal.

Because NE China is abundant with Cu porphyry ores [75] that may be eroded and consequently changing the ratio in terrigenous sediments, Sr/Cu ratio is not suitable to infer the sedimentary environment. The relationship of sulfur and the environment has been widely recognized [72]. In the Junde mine, sulfate sulfur increases with depth (Figure 16), which indicate oxidation/evaporation gradually decreases during No.26 coal formation.

**Figure 16.** Regularity of the variation of sulfate sulfur in the Junde samples.

#### *5.3. Influence of Volcanic Ash*

It is generally believed that the Eu anomaly in coal does not originate from the weathering process during the transportation of metals from provenance to coal forming peat, but is inherited from rocks in the source areas [56]. Eu anomalies characteristics of No. 26 coal from Junde mine (including roof and floor) resembles those of felsic volcanic rocks.

Previous studies suggested that the crystalline habit and morphology of terrigenous detrital zircons are quite different from those of igneous detrital zircons [76]. The characteristics of the former are tetragonal bipyramids with relatively short prisms, and the length width ratio (c/a values) is about two [77], while the feature of the latter is long and well-developed, with a c/a value of more than 2.5 [77]. In the present study, the ratio of c/a in the zircon found in No. 26 coal (Figure 11C) is > 2.5, indictive of a pyroclastic

origin. Another possibility that its angular and elongated shapes may evidence just short transportation by relatively quiet water streams cannot be ruled out.

A large number of sharp-edged quartz particles were observed in No. 26 coal (Figures 3B,D and 10A), indicating a volcanic origin rather than a terrigenous clastic origin [35,59,60].

Although zircon and some high-T quartzs have been found in the No. 26 coals, the vermicular kaolinite/chlorite particles that commonly indicate volcanic ash have not been found. Thus, the volcanic ash contribution to No. 26 coal seems very low, if occurred. This conclusion is consistent with [70].

#### **6. Conclusions**

The characteristics of minerals and elements of No. 26 coal from Junde mine, Hegang coalfield, northeastern China, were studied. The main conclusions are summarized as follows.


**Author Contributions:** Y.W.: Methodology, Data curation, Writing-original draft. W.H.: Data curation, Writing-original draft. G.Q.: Methodology, Data curation, Writing-original draft. A.W.: Supervision. D.C.: Conceptualization, Supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the National Key Research and Development Plan of China (2021YFC2902004), National Natural Science Foundation of China (41972174 and 42072197), and Science Foundation of Hebei Normal University (L2021B25).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Available from Yingchun Wei and corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

