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Article

Occurrence State and Genesis of Large Particle Marcasite in a Thick Coal Seam of the Zhundong Coalfield in Xinjiang

by
Xue Wu
1,2,
Ning Lü
1,2,3,
Shuo Feng
1,2,4,*,
Wenfeng Wang
1,2,4,*,
Jijun Tian
1,5,
Xin Li
1,2 and
Hayerhan Xadethan
6
1
Xinjiang Key Laboratory for Geodynamic Processes and Metallogenic Prognosis of the Central Asian Orogenic Belt, Xinjiang University, Urumqi 830047, China
2
School of Geology and Mining Engineering, Xinjiang University, Urumqi 830047, China
3
Xinjiang Natural Resources and Ecological Environment Research Center, Urumqi 830000, China
4
School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
5
School of Earth Resources, China University of Geosciences, Wuhan 430051, China
6
Zhundong Economic and Technological Development Zone Branch of Natural Resources Bureau of Changji Prefecture, Changji 831700, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(8), 816; https://doi.org/10.3390/min15080816
Submission received: 6 June 2025 / Revised: 19 July 2025 / Accepted: 28 July 2025 / Published: 31 July 2025
(This article belongs to the Special Issue Mineralogical and Geochemical Characteristics of Coal and Coal-Bearing Strata)
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Abstract

The Junggar Basin contains a large amount of coal resources and is an important coal production base in China. The coal seam in Zhundong coalfield has a large single-layer thickness and high content of inertinite, but large particle Fe-sulphide minerals are associated with coal seams in some mining areas. A series of economic and environmental problems caused by the combustion of large-grained Fe-sulphide minerals in coal have seriously affected the economic, clean and efficient utilization of coal. In this paper, the ultra-thick coal seam of the Xishanyao formation in the Yihua open-pit mine of the Zhundong coalfield is taken as the research object. Through the analysis of coal quality, X-ray fluorescence spectrometer test of major elements in coal, inductively coupled plasma mass spectrometry test of trace elements, SEM-Raman identification of Fe-sulphide minerals in coal and LA-MC-ICP-MS test of sulfur isotope of marcasite, the coal quality characteristics, main and trace element characteristics, macro and micro occurrence characteristics of Fe-sulphide minerals and sulfur isotope characteristics of marcasite in the ultra-thick coal seam of the Xishanyao formation are tested. On this basis, the occurrence state and genesis of large particle Fe-sulphide minerals in the ultra-thick coal seam of the Xishanyao formation are clarified. The main results and understandings are as follows: (1) the occurrence state of Fe-sulphide minerals in extremely thick coal seams is clarified. The Fe-sulphide minerals in the extremely thick coal seam are mainly marcasite, and concentrated in the YH-2, YH-3, YH-8, YH-9, YH-14, YH-15 and YH-16 horizons. Macroscopically, Fe-sulphide minerals mainly occur in three forms: thin film Fe-sulphide minerals, nodular Fe-sulphide minerals, and disseminated Fe-sulphide minerals. Microscopically, they mainly occur in four forms: flake, block, spearhead, and crack filling. (2) The difference in sulfur isotope of marcasite was discussed, and the formation period of marcasite was preliminarily divided. The overall variation range of the δ34S value of marcasite is wide, and the extreme values are quite different. The polyflake marcasite was formed in the early stage of diagenesis and the δ34S value was negative, while the fissure filling marcasite was formed in the late stage of diagenesis and the δ34S value was positive. (3) The coal quality characteristics of the thick coal seam were analyzed. The organic components in the thick coal seam are mainly inertinite, and the inorganic components are mainly clay minerals and marcasite. (4) The difference between the element content in the thick coal seam of the Zhundong coalfield and the average element content of Chinese coal was compared. The major element oxides in the thick coal seam are mainly CaO and MgO, followed by SiO2, Al2O3, Fe2O3 and Na2O. Li, Ga, Ba, U and Th are enriched in trace elements. (5) The coal-accumulating environment characteristics of the extremely thick coal seam are revealed. The whole thick coal seam is formed in an acidic oxidation environment, and the horizon with Fe-sulphide minerals is in an acidic reduction environment. The acidic reduction environment is conducive to the formation of marcasite and is not conducive to the formation of pyrite. (6) There are many matrix vitrinite, inertinite content, clay content, and terrigenous debris in the extremely thick coal seam. The good supply of peat swamp, suitable reduction environment and pH value, as well as groundwater leaching and infiltration, together cause the occurrence of large-grained Fe-sulphide minerals in the extremely thick coal seam of the Xishanyao formation in the Zhundong coalfield.

1. Introduction

Sulfur is one of the common hazardous elements in coal, and Fe-sulphide minerals are some of the most widely distributed sulfides in coal [1]. Sulfur in coal can be divided into inorganic sulfur and organic sulfur according to its occurrence form. Among them, inorganic sulfur mainly exists in the form of pyrite in coal, and there is also a small amount of sulfate sulfur [2]. The ankerite in coal is mainly formed by the precipitation of Ca2+ in the pore solution rich in Fe2+, Mn2+ and Mg2+ in the form of isomorphism [3]. Fe-sulphide minerals in coal primarily occur as pyrite and marcasite [4]. Both of them are homogeneous polymorphs, but pyrite has an equiaxed crystal structure, whereas marcasite has an orthorhombic crystal structure [5].
In the past, the research on pyrite in coal mainly focused on the electrical properties, magnetic characteristics, crystal structure and structural transformation between pyrite and marcasite [6,7,8]. Many scholars have studied Fe-sulphide minerals in coal from the aspects of geometric structure, crystal structure, crystal characteristics, microcrystalline type and structural transformation relationship [9,10,11]. It is generally believed that pyrite is a high-sulfur FeS2 formed in an weak acidic to weak alkaline conditions, while marcasite is a low-sulfur FeS2 formed in an acidic anoxic reduction environment, and the priority of forming marcasite in an acidic environment is greater than that of forming pyrite in a weak acidic to weak alkaline environment [12,13,14,15,16,17,18]. Many scholars have studied the macroscopic and microscopic morphological characteristics of pyrite. Liu et al. [19] divided the macroscopic and microscopic morphology of pyrite in the main coal seam of the Late Paleozoic in North China. Wang [20] divided the pyrite in the No. 8 coal seam of the Gujiao mining area in Shanxi into two categories: with biological fabric and without biological fabric. Based on optical microscope and scanning electron microscope analysis of the occurrence characteristics of inorganic sulfur in coal in the Zhijin area of Guizhou Province, Zhang et al. [21] found that the inorganic sulfur in coal mainly appears strawberry-like, and they divided its microscopic occurrence characteristics. According to the microscopic morphology and material composition of pyrite with different morphologies, Zhao et al. [22] divided membranous pyrite into epigenetic hydrothermal genesis minerals, and defined the nodular pyrite as syngenetic sedimentary minerals. Wang et al. [23] believed that the pyrite in coal can be divided into lenticular, nodular, film-like, disseminated, vein-like, shell-like and banded according to the macroscopic morphology. Under the microscope, the morphology can be divided into eight types of pyrite, namely raspberry spherical, self-shaped granular, caviar, block, uniform spherical, allomorphic, nodular and joint fissure filling. Previous studies have shown that marcasite usually appears in the form of isolated spheres or clusters of spheres, which tend to be distributed along a narrow area parallel to the coal seam [24]. Inside, the marcasite sphere exhibits a radial polycrystalline habit and is usually surrounded by thin spongy pyrite or, in some cases, well-crystallized pyrite. The elements associated with pyrite in coal are Sb, As, Cd, Co, Cu, Pb, Hg, Mo, Ni, Se, Zn, and 11 other species. The key factors affecting the formation of pyrite in coal are Fe2+, SO42−, organic matter, and bacterial activity [25]. Tang Yuegang et al. [26] studied the genesis of pyrite in late Permian high-sulfur coal in Sichuan. While distinguishing type I and epigenetic hydrothermal type II vein pyrite formed during diagenesis, they proposed two types of genetic evolution models: direct precipitation series (euhedral crystal, aggregate, polycrystal) and complex genesis (particles, berries, spheres, nodules, clumps). A petrographic study of Fe-sulphide minerals was carried out on the subject coals to deduce an Fe-sulphide minerals precipitation sequence for five principal stages: (a) early syngenetic stage; (b) late syngenetic stage; (c) syngenetic-diagenetic stage; (d) early epigenetic stage; and (e) late epigenetic stage [27]. Grady [28] identified four forms of marcasite in West Virginia coal: (1) a large number of massive grains of interlocking crystals; (2) spherical grains grown by radial crystal; (3) overgrowth of pyrite; (4) dendritic crystals. King and Renton [29] found that there are three basic forms of marcasite in West Virginia coal: irregular shape, fissure filling and spherical crystal aggregates. Boctor [30] and Parratt [31] described the relationship between marcasite and pyrite in Seelyville III and V coal samples, respectively. Lett and Fletcher [32] found similar structural relationships between strawberry-like and surrounding marcasite or chalcopyrite in the heavy medium separation of peat mires in British Columbia. Liu Bei et al. [8] found that the marcasite in the late Paleozoic coal in the Qinshui Basin is mainly in the form of plate, sheet and spearhead. Liu et al. [33] divided pyrite into syngenetic-penecontemporaneous pyrite, pyrite formed in the early diagenesis stage, pyrite formed in the late diagenesis stage and epigenetic stage according to the crystal form characteristics, particle size and S isotope combination characteristics of pyrite. Zhou et al. [34] added δ34S data on the basis of the generation division of Fe-sulphide mineral formation. According to the difference of δ34S in different formation generations of Fe-sulphide minerals, it is pointed out that early and late Fe-sulphide minerals have different δ34S characteristics. The δ34S of early Fe-sulphide minerals has more negative values, while that of late Fe-sulphide minerals is more positive. The research on the genesis of Fe-sulphide minerals in coal mainly focuses on the specific form of Fe-sulphide minerals in coal, the influence of different geochemical environments on it, and the contribution of different basin evolution histories to the formation of Fe-sulphide minerals [35]. The amount of sulfur content in coal can not only be used to define high-sulfur coal and low-sulfur coal, but also be used to reveal the sedimentary facies characteristics of the coal-forming period. Nielsen [36] systematically summarized the early research work on the isotopic composition characteristics of organic sulfur, elemental sulfur and Fe-sulphide mineral sulfur in coal around the world. The study of Goldhaber and Smith [37,38,39,40] showed that the variation range of sulfur isotope in Australian coal with sulfur content greater than 1% from Permian to Tertiary was wide (+2.9‰~+24‰), while the variation range of organic sulfur isotope in low sulfur coal with sulfur content less than 1% was narrow (+4.6‰~+7.3‰). The relatively uniform organic sulfur isotope composition in low sulfur coal may indicate the uniformity of fresh water sulfate isotope composition since the Permian. Hunt and Smith [41] conducted a detailed study on the sulfur isotope changes of low-sulfur coal (total sulfur ≤ 1%, pyrite sulfur ≤ 0.2%) in the Permian Basin of Australia. The results showed that the sulfur content and δ34S value increased towards the downstream of the river delta. This trend is parallel to the change of sedimentary environment from braided alluvium to fluvial-upper delta plain and lower delta plain to marginal marine conditions. The results showed that the availability of sulfate to plant growth increased, and 32S was preferentially assimilated during plant growth, and the impact of downstream oceans increased slightly. Compared with low-sulfur coal, the abnormal enrichment of some elements in high-sulfur coal provides evidence for the study of the source of sulfur in coal. The comprehensive research results of Kolker show that As is the most abundant secondary component in Fe-sulphide minerals in coal, and the content of trace elements such as Se and Ni is low [1]. Chou has shown that the contents of several major trace elements (including B, Fe, Mo, Hg, Tl, U) in high-sulfur coals are higher than those in low-sulfur and medium-sulfur coals, and B, Mo and U are likely to be derived from seawater that submerges swamps and terminates peat accumulations, like sulfur, while Fe, Tl and Hg are more likely to be terrestrial sources and combine with pyrite during peat and coal diagenesis in a sulfur-rich and reducing environment [42,43]. Gluskoter [44] and Love [45] discussed that Mo is very sensitive to the sedimentary environment during coal formation.
In recent years, numerous scholars have studied and analyzed pyrite in coal [6,7,46]. However, research on the crystal structure of large-particle Fe-sulphide minerals in ultra-thick, and inertinite-rich coal, geochemical environment during pyrite formation (ascribed from major and trace elements), occurrence state, and tracing of sulfur isotopes to the formation mechanism and sulfur source of Fe-sulphide minerals has not reached sufficient maturity; in these aspects, research on marcasite is even weaker. The study of the occurrence state and genesis of Fe-sulphide minerals in the Zhundong coalfield in Xinjiang will be beneficial to the separation, removal, and clean utilization of large-grained Fe-sulphide minerals in coal [46]. It will aid in the understanding of the occurrence state and genesis of minerals in coal [47,48,49,50,51], which will promote the realization of clean utilization of high-quality power and electric coal in Zhundong coalfield.

2. Geological Background

The Zhundong coalfield is located in the eastern Junggar uplift belt. The whole structure trends nearly east–west or NWW, and its overall structural pattern is complex, showing an asymmetric and open fold shape from east to northwest. It is composed of the Wutongwozi depression, Heishan uplift, Shiqiantan depression, Huangcaohu uplift, Shishugou depression, Shazhang fault fold belt, and Wucaiwan depression [39].
The study area is located in the Shazhang fault fold belt in the east of the Junggar Basin. It is adjacent to the southern foot of Kelameili Mountain in the north and the Shishugou depression in the east [52]. In this study, the boundary line of Huoshao Mountain was set as the western part of the sampling point. The area features only the Xidagou syncline, with no distribution of faults and magmatic rocks. Therefore, the tectonic setting is simple(Figure 1). The main coal-bearing strata in the study area are the Middle Jurassic Xishanyao formations. A set of extremely thick coal seams is developed in the group, and the thickness of the coal seam is consistent, with an average thickness of 68.4 m [12].

3. Samples and Research Methods

3.1. Sample Collection

According to the (”GB/T 482-2008” [54]) coal seam sampling method, fresh outcrop samples were selected from the bottom to the top in the Yihua open-pit coal mine of the Zhundong coalfield. Among them, seven coal samples were collected from the pyrite-bearing horizon, five coal samples from the adjacent horizon, and four coal samples from the horizon without pyrite (Figure 2). The samples were subjected to breaking, bricking, polishing, and Au coating for SEM-EDS, total sulfur and morphological sulfur analysis, coal ash composition analysis, identification of Fe-sulphide minerals occurrence state, and micro-area isotope study.

3.2. Research Method

3.2.1. Determination of Major Elements

The major element oxides (SiO2, Al2O3, Fe2O3, TiO2, CaO, Na2O, MgO, K2O, P2O5, MnO) in coal were determined using X-ray fluorescence spectrometry (XRF). Samples sieved through a 200 mesh were dried at 105–110 °C for 2 h, and a certain number of samples were weighed and tested with boric acid as the base. Scanning mode: continuous scanning; irradiation mode: lower irradiation mode; X-ray tube: Rh target, ceramic end window X-ray tube, light tube maximum power of 4.2 kW, with the maximum voltage and current at 70 kW and 140 mA; X-ray generator: maximum output power of 3.5 kW, and maximum rated voltage and current of 60 kV and 60 mA.

3.2.2. Determination of Microelements

In accordance with the national standard (“GB/T 14506.30-2010” [55]), 50 mg of the 200 mesh sample was weighed into a PTFE tank, after which 1 mL HF (1.16 g/mL) and 0.5 mL HNO3 (1.42 g/mL) were added, and the mixture was placed and sealed in a high-pressure reactor and heated at 185 °C for 24 h. After cooling, the inner tank was taken out and placed on an electric heating plate at 200 °C to evaporate the sample to near dryness, and 0.5 mL HNO3 (1.42 g/mL) was then added to evaporate to near dryness. Subsequently, 5 mL HNO3 (1:1) was added, and the sample was sealed and placed in an oven at 130 °C for 3 h. After cooling, the inner tank was taken out and quantitatively transferred to a plastic bottle. After dilution to 25 mL, the content of trace elements (Y-U rare earth, etc.) was determined by inductively coupled plasma mass spectrometry (ICP-MS).

3.2.3. Preparation of Pulverized Coal Light Sheet

A certain number of 18–40 mesh samples were taken in a Φ 2.5 cm PTFE mold, and epoxy resin and curing agent were mixed according to a fixed ratio of 3:1. The mixture was stirred until there were no bubbles, and the mixture of epoxy resin and curing agent was poured into the PTFE mold. After the sample was stirred evenly with the epoxy resin and curing agent mixture, it was labeled and placed in a vacuum device for curing. The sample was removed, ground and polished. The polishing solution was washed with water, and the water was dried for later use.

3.2.4. Observation of Light Microscopy

The identification and statistics of coal rock macerals were completed by the current author at Xinjiang University and the Coalbed Methane Test Center of Xinjiang Coalfield Geological Bureau. The preparation of coal rock light sheet refers to the (“GB/T 16773-2008” [56]) coal rock analysis sample preparation method. Using the optical system of Zeiss Axio Scope A1 (Zeiss, Oberkochen, Germany) optical microscope with magnification of 10 eyepieces and 5~40 times dry objective lens, the prepared coal rock light sheet was observed under oil immersion 50× objective lens (a minimum of 350 points on each sample [57]) and the content of each maceral component was counted. According to the national standard ‘ICCP 1994’ [58], about 500 component points were counted, and the maceral vitrinite, inertinite, liptinite group and minerals were quantified. Then the observation of the exinite group was accompanied by purple fluorescence, and the image acquisition was carried out through the Zeiss application software package (LAS).

3.2.5. Scanning Electron Microscopy-Raman Spectroscopy System

Pulverized coal was vacuumized under dry conditions, and coated with Au to increase its conductivity. Mineral morphology images of coal were obtained under a field emission scanning electron microscope (Gemini 450, Thermo Fisher Scientific, Hillsboro, OR, USA). At the same time, combined with Raman spectroscopy (RISE), the results of electron microscopy and light microscopy were superimposed to obtain superimposed results, and the Raman single spectrum of the suspected area was obtained. The Raman single spectrum was compared with the standard mineral Laman spectrum in the Ruff database to determine the specific occurrence state of Fe-sulphide minerals.

3.2.6. LA-MC-ICP-MS Isotope Analysis of Sulphide Minerals

The experimental instrument used in the in-situ sulfur isotope analysis of sulphide minerals in the study is an excimer laser ablation system with an accuracy of 193 nm. The system mainly includes a 193 nm ArF excimer laser generator (Cymer, LLC, San Diego, CA, USA), a sample chamber (double chamber) and a computer-controlled high-precision X-Y sample stage movement and positioning system. When testing the sulfur isotope of sulphide minerals, the deep ultraviolet beam generated by the excimer laser generator is first homogenized, so that the homogenized optical path is focused on the surface of sulphide minerals, and the energy density is 2.5 J/cm2. First, the gas was collected for 30 s, and then the 35 μm ablation spot beam was ablated at a frequency of 5 Hz for 40 s. The aerosol was sent out of the ablation cell using helium gas, mixed with argon gas, and then entered the MC-ICP-MS. The single integration time of MC-ICP-MS is 0.3 s, and there are about 133 sets of data within 40 s of erosion time. During the test, Wenshan pyrite was used as the test external standard, and the external standard was retested every three times. The data quality control was based on the National Geological Experiment and Test Center of the Chinese Academy of Geological Sciences, GBW07267 pyrite briquette and GBW07268 chalcopyrite briquette and NIST SRM 123 sphalerite particles of the National Institutes of Standards and Technology. The long-term external reproducibility was about ±0.6‰.

4. Results

4.1. Coal Property

The content of macerals on a whole-coal basis in coal of the Xishanyao formation of the Yihua open mine was more than 95%, with an average of 96.54%. The macerals were mainly inertinite and vitrinite, with a very small content of liptinite. Among them, the content of inertinite varied from 39.96% to 71.44%, with an average of 59.02%. In the inertinite group macerals, the content of vitrodetrinite was the largest, with an average of 27.99%, followed by the contents of semi-fusinite and fusinite, with averages of 22.67% and 7.10%, respectively. The contents of other inertinite components did not exceed 5%. The vitrinite content ranged from 21.37% to 58.35%, with an average of 37.52%. Among them, telocollinite had the highest content, with an average of 15.11%, followed by desmocollinite and vitrodetrinite, with averages of 11.65% and 9.07%, respectively; telinite and corpogelinite had small contents, with averages of 1.09% and 0.59%, respectively. The content of the liptinite was the lowest, with an average content of only 0.03%. Clay minerals accounted for the largest proportion of inorganic components, with an average content of 1.61%. Marcasite had the second largest content, ranging from 2.21% to 5.13%, with an average of 1.54%(Table 1), and it was mainly concentrated in individual horizons (Figure 3 and Figure 4).

4.2. Mineralogy

Due to differences in the peat-forming environment, the thick coal seam of the Yihua open-pit mine generally has low Fe-sulphide mineral content, with different occurrence characteristics [12]. Three morphological types of Fe-sulphide minerals in coal can be observed macroscopically: (1) Thin film Fe-sulphide minerals: these refer to the Fe-sulphide minerals attached to the surface of the endogenous fracture surface and the size characteristics are controlled by the fracture surface (Figure 5a). (2) Nodular Fe-sulphide minerals: they refer to nodular Fe-sulphide minerals formed by the dense accumulation of fine grains, and they mostly occur in polygonal and lenticular forms, distributed in the coal seam. In general, large particles can reach a diameter of 10 cm, and small particles can be as small as approximately 1 mm (Figure 5b). (3) Disseminated Fe-sulphide minerals: they refer to the Fe-sulphide minerals mainly composed of fine-medium self-shaped crystals and other-shaped crystals, evenly distributed in the coal seam in scattered or cloud-like aggregates, with a small particle size (Figure 5c).
Pyrite and marcasite are two main homogeneous polymorphs of sulphide minerals in coal, both of which are formed in a reducing environment [59]. However, pyrite belongs to the equiaxed crystal system, which is usually formed in alkaline (pH > 7) water, while marcasite belongs to the orthorhombic crystal system, which is formed in acidic (pH < 5) water, and will be converted into pyrite when the temperature is greater than 350 °C [60]. Therefore, the specific occurrence state of sulphide minerals cannot be accurately determined by a scanning electron microscope, X-ray diffraction, EDS energy spectrum analyzer or electron probe alone. Based on this, the ZEISS RISE system was used to obtain the micro-area composition, morphology and structure of the sample. Pyrite has a cubic symmetry structure. The distribution characteristics of Fe, S and other elements can be observed on the energy spectrum analysis diagram (Figure 6), but it is not confirmed whether the mineral type is marcasite or pyrite. The suspected area was transferred to Raman spectroscopy to determine the specific occurrence state of Fe-sulphide minerals [60], and the sample was then moved to the scanning electron microscope to determine the in-situ morphology of Fe-sulphide minerals (Figure 7). The occurrence of marcasite was observed in seven samples: YH-2, YH-3, YH-8, YH-9, YH-14, YH-15, and YH-16. No marcasite was detected in the remaining samples, and no pyrite was detected in all samples. The two main Raman peaks at 346 cm−1 and 382 cm−1 are assigned to Ag and Eg vibration modes, respectively. The secondary peak 432 cm−1 belongs to the Tg vibration mode [11]. By Gaussian fitting of the Raman spectrum at 2.87 mW, Raman peaks of 325 cm−1 and 382 cm−1 appeared, indicating the occurrence of marcasites (Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13). The XRD test also shows that the Fe-sulphide minerals in coal are mainly marcasite, as shown in Figure 14 and Figure 15.
The microscopic morphology of marcasite in the thick coal seam of the Zhundong coalfield was identified using SEM. The microscopic morphology of marcasite in the thick coal seam of the Zhundong coalfield is mainly patchy, massive, spearhead, and fissure filling. The morphology of flaky marcasite is similar to that of strawberry pyrite, but the difference is that flaky marcasite is not a spherical or ellipsoidal aggregate composed of multiple single particles, but a petal-like aggregate composed of multiple flakes, with a diameter of 5~30 μm (Figure 16a). The flaky marcasite only occurs in samples YH-2 and YH-3; the spearhead-like marcasite is wide in the middle and gradually thins to both ends, with a diameter of 30–50 μm (Figure 16b). The spearhead-like marcasite only occurs in the YH-8 sample. The surface of massive marcasite is usually flat, and the particle size is about 30 μm (Figure 16c). The massive marcasite occurs in YH-9 and YH-15 samples. Fissure-filled marcasite occurs in YH-8, YH-9, YH-14, YH-15 and YH-16 samples, and develops along fissures and joints. Its shape and size are restricted by the shape and size of the filling space. It is considered to be a product of epigenetic diagenesis (Figure 16d).

4.3. Geochemistry

4.3.1. Content Characteristics of Major Elements

The major element oxides in 16 coal samples of the Yihua coalmine are mainly CaO and MgO, accounting for 53.12% and 13.93% of the total content of major element oxides in coal, respectively. The content of SiO2, Al2O3, Fe2O3 and Na2O in coal is the second, accounting for 5%~10% of the major element oxides in coal. The content of major element oxides TiO2, K2O, P2O5 and MnO is very low, and their relative content is less than 0.5%. Specifically, the CaO content in coal ranges from 1.82% to 15.06% (average 3.89%); the MgO content ranges from 0.42% to 1.23% (average 1.02%). The content of SiO2 ranges from 0.21% to 0.96% (average 0.48%), Al2O3 ranges from 0.18% to 0.65% (average 0.70%), Fe2O3 ranges from 0.12% to 4.12% (average 0.75%) and Na2O ranges from 0.21% to 1.08% (average 0.41%), accounting for 6.57%, 9.56%, 10.24% and 5.60% of the total content of major element oxides in coal, respectively. The average contents of TiO2, K2O, P2O5 and MnO in coal were 0.13%, 0.19%, 0.09% and 0.02%, respectively. TiO2 accounted for 0.14% of the sum of the ten major element oxides, and K2O, P2O5 and MnO accounted for 0.27% of the sum of the ten major element oxides(Table 2). According to the results of the content of major elements, it is judged that the mineral composition in the extremely thick coal seam often contains more Fe-sulphide minerals in addition to clay minerals, quartz and calcite.

4.3.2. Content Characteristics of Trace Elements

Studying the content and occurrence form of trace elements in coal has important guiding significance for the study of coal-forming geological conditions, the evaluation of environmental effects in the process of coal utilization, and the development and utilization of rare metals. Table 3 lists the content changes and average values of trace elements in extremely thick coal seams. The results show that the main trace elements in the extremely thick coal seam of Yihua open-pit mine are Ba, Li and Sr. Compared with the average value of trace elements in Chinese coal, the content of trace elements in extremely thick coal seam is higher than that of Chinese coal, except that the content of Ni, Cr and Co is relatively low. The content of Ba ranges from 191.16 μg/g to 291.83 μg/g, with an average value of 232.76 μg/g, which is 1.47 times the average value of Chinese coal. The content of Sr ranges from 82.49 μg/g to 144.84 μg/g, with an average of 116.65 μg/g, which is 0.83 times the average value of Chinese coal. The content of Li ranges from 33.92 μg/g to 85.43 μg/g, with an average of 58.89 μg/g, which is 1.34 times the average value of Chinese coal. The content of V ranges from 12.46 μg/g to 85.64 μg/g, with an average of 40.76 μg/g, which is 1.17 times the average value of Chinese coal. The content of Ga varies from 17.09 to 24.76 μg/g, with an average of 20.03 μg/g, which is 3.01 times the average value of Chinese coal. The content of Ni ranges from 6.37 μg/g to 20.82 μg/g, with an average of 12.44 μg/g, which is 0.91 times the average value of Chinese coal. The content of Cr ranges from 3.31 to 19.68 μg/g, with an average value of 10.58 μg/g, which is 0.69 times the average value of Chinese coal. The content of U ranges from 0.75 to 45.30 μg/g, with an average value of 8.14 μg/g, which is 3.38 times that of Chinese coal. The content of TH ranged from 2.40 to 12.73 μg/g, with an average of 15.92 μg/g, which was 1.01 times that of Chinese coal. The content of Co ranges from 1.64 μg/g to 3.26 μg/g, with an average value of 2.35 μg/g, which is 0.33 times that of Chinese coal.

4.3.3. The Sulfur Isotope Characteristics

The sulfur isotope characteristics of Fe-sulphide minerals in coal can indicate not only the source of sulfur, but also the formation age of Fe-sulphide minerals [40]. The results of the in-situ sulfur isotope test of marcasite are as follows (Table 4): the overall δ34S of marcasite showed a wide range of variation, with the extreme value reaching up to 51.30‰, indicating a relatively long formation time of marcasite sulfur in the extremely thick coal seam. The δ34S values were not only positive, indicating sulfate origin, but also negative, indicating the biogenic origin; intermediate δ34S values indicate a combined sulfate and biogenic origin (Figure 17).

5. Discussion

5.1. Occurrence State of Marcasite in Thick Coal Seam

Macroscopically, disseminated marcasite mainly occurs in YH-2 and YH-3, and membranous and nodular marcasite are distributed in YH-8, YH-9, YH-14, YH-15, and YH-16. Microscopically, agglomerated flaky marcasite occurs only in YH-2 and YH-3, spearhead marcasite occurs only in YH-8, massive marcasite occurs in YH-9 and YH-15, and fissure-filled marcasite occurs in YH-8, YH-9, YH-14, YH-15, and YH-16 samples.

5.2. Environmental Indication of the Sulfur Isotope of Marcasite in the Extremely Thick Coal Seam

Overall, δ34S shows a wide variation range in marcasite, with the extreme value reaching up to 51.30‰. The δ34S values of marcasite in different samples vary widely and narrowly. The results show that the sulfur in the white iron ore in the thick coal seam includes both primary sulfur and secondary sulfur introduced after diagenesis. Both single biogenic and sulfate-genetic as well as composite sulfate and biogenic marcasite occur. Upon further investigation, the δ34S value of the same marcasite nodule was found to be smaller at the edges than in the combined interior, further confirming that the sulfur in these layers formed in multiple stages (Figure 18) [63].
Considering the macroscopic and microscopic occurrence state and isotope characteristics of marcasite in the super thick coal seam, it is concluded that during the peat accumulation, when the peat mire was in a reducing environment with sufficient supply of sulfate ions and organic matter, a large amount of sulfate ions—as the oxidant of sulfate reducing bacteria—oxidized organic matter anaerobically. The sulfate ions in the peat mire were reduced to H2S, which combined with divalent iron ions in water to form pyrite (Fe2S) precipitates (preserved in the coal seam). In this process, as 32S is preferentially utilized by sulfate-reducing bacteria, the sulfur isotope (δ34S) is significantly fractionated, and the fractionation range can reach −46‰. At the bottom and top of the extremely thick coal seam, the water body is an open system, and the sulfate is derived from terrigenous debris and groundwater. The relatively high sulfate concentration can promote the proliferation of sulfate-reducing bacteria, resulting in the formation of marcasite. The sulfur isotope value of the ore will continuously become lighter, reaching significantly negative values. However, when the concentration of sulfate ions in the environment is scarce, the limited sulfate ions continue to lose 32S under the long-term sulfate reduction, and the sulfur isotope fractionation in the bacteria-dominated sulfate reduction reaction is reduced. The δ34S value of the resulting marcasite will continue to become heavier and significantly positive. Therefore, the marcasite in YH-2 and YH-3 is ascertained to have formed in the early stage of diagenesis. Macroscopically, the marcasite is disseminated in the coal seam, and microscopically, it is distributed in the coal seam as a sheet. YH-8, YH-9, YH-14, YH-15, and YH-16 contain marcasite generated in the early stage of diagenesis and marcasite formed by the addition of secondary sulfur after diagenesis. Macroscopically, marcasite is distributed in the coal seam in the form of films and nodules, and microscopically, it is distributed in the coal seam in the form of fissure filling, spearheads, and blocks. Therefore, the formation period of disseminated marcasite and micro-flake marcasite is speculated to be earlier, and that of the fracture-filled marcasite to be during the epigenetic stage of diagenesis [64].

5.3. Genesis of Marcasite in Thick Coal Seam

The analysis shows high sulfur content in the matrix vitrinite, which would provide a certain amount of primary sulfur for the formation of marcasite. Compared with vitrinite, inertinite can provide sufficient space for the growth of marcasite, such that marcasite can grow into larger particles in the coal seam [65].
The entire thick coal seam features medium moisture, low–medium ash, medium sulfur, and medium–high volatile coal. The correlation between morphological sulfur and total sulfur was Sp, d > So, d > Ss, d. The raw coal Ad shows a strong positive correlation with St, d and Sp, d, indicating that the supply of terrigenous debris strongly affected the sulfur content of coal, especially sulfide sulfur content(Figure 19).
With reference to the average elemental contents of coal in China, the thick coal seam in the Zhundong coalfield is enriched in Si, Fe, Al, Na, and K among the major elements and Li, Ga, Ba, U, and Th among the trace elements. In addition, the main coal ash components are SiO2, Al2O3, and Fe2O3. The eight major elements in the extremely thick coal seam exhibit good positive correlation with the raw coal Ad, indicating that the major elements in the coal are closely related to the supply of terrigenous debris.
Trace elements such as Ni, U, Th, V, Cr and Co are very sensitive to the change of redox environment in water, and their enrichment in water is controlled by the change of paleo-oxygen phase. Therefore, these elements are usually regarded as redox indicators [66]. U has low solubility under reducing conditions, while under the conditions of oxidation, it is easy to be oxidized and migrated, resulting in the lack of U element in the sediment. Whether in a reducing environment or an oxidizing environment, the solubility of the Th element is not high, and it is easy to accumulate in the sediment. Therefore, δU and U/Th values can be used to determine the redox conditions of the sedimentary environment, where δU = 2U/(Th/3 + U) [67,68,69,70]. V, Ni, Cr and Co elements have similar characteristics of easy migration under oxidation conditions and easy precipitation under reduction conditions. Previous studies have proposed the determination of redox environments such as V/(V + Ni), V/Cr and Ni/Co [71,72,73,74].
The existing sample test data were analyzed, calculated, counted and evaluated. The results are shown in Table 5 and Figure 20. The δU values of extremely thick coal seams range from 0.80 to 1.92, with an average of 1.26. Among them, YH-2, YH-3, YH-8, YH-9, YH-14, YH-15 and YH-16 are located on the left side of the oblique line with the δU value of 1.0, indicating that these samples are from a reducing environment, while the remaining samples are located on the right side of the oblique line with the δU value of 1.0, indicating that these samples are in an oxidizing environment (Figure 20a). The V/Cr values ranged from 1.56 to 5.12, with an average of 3.03. YH-2, YH-3, YH-8, YH-9, YH-14, YH-15 and YH-16 were located on the left side of the V/Cr value equal to 4.25, indicating that these samples were formed in a reducing environment. YH-11 was in a weak oxidation-weak reduction environment, and the remaining samples were located on the right side of the oblique line with a V/Cr value equal to 2.0, indicating that these samples were formed in an oxidizing environment (Figure 20b). The values of Ni/Co ranged from 2.31 to 10.57, with an average of 5.75. Among them, YH-2, YH-3, YH-8, YH-9, YH-14, YH-15 and YH-16 were located on the left side of the oblique line with Ni/Co value equal to 7.00, indicating that these samples were derived from the reducing environment, while the remaining samples were located on the right side of the oblique line with Ni/Co value equal to 5.00, indicating that these samples were derived from the oxidizing environment (Figure 20c). The U/Th values ranged from 0.21 to 7.93, with an average of 1.70. Among them, YH-2, YH-3, YH-8, YH-9, YH-14, YH-15 and YH-16 were located on the left side of the oblique line with a U/Th value of 1.25, indicating that these samples were derived from a reducing environment, while the remaining samples were located on the right side of the oblique line with a U/ Th value of 0.75, indicating that these samples were derived from an oxidizing environment (Figure 20d).
The δU, V/Cr, Ni/Co, U/Th, and ash composition index consistently indicate the redox environment during the formation of the extremely thick coal seam. Specifically, YH-2, YH-3, YH-8, YH-9, YH-14, YH-15, and YH-16 were under a reducing environment, and YH-1, YH-4, YH-5, YH-6, YH-7, YH-10, YH-11, YH-12, and YH-13 were under an oxidizing environment. The triangular diagram of coal ash composition (Figure 21) indicates a weakly reducing environment during the formation of the entire thick coal seam that was relatively close to the terrestrial source area. The YH-2, YH-3, YH-8, YH-9, YH-14, YH-15, and YH-16 samples are relatively biased towards the Fe2O3 and SO3 ends, indicating that these samples formed under a strongly reducing environment. The calculation results of the pH value show that the whole thick coal seam formed in an acidic environment.
Sulfur produced by sulfur-containing plants and ancient wildfires, the supply of terrigenous debris during peatification, and the infiltration of freshwater sulfate in the coal seam roof jointly contributed to the sulfur and iron elements in the extremely thick coal seam. At the same time, the ancient wildfires led to the high inertinite content in the extremely thick coal seam, and the high inertinite content provides sufficient space for the growth of marcasite. The occurrence of large-grained Fe-sulphide minerals in the thick coal seam is indicative of good material basis, sufficient growth space, and a suitable water geochemical environment. After the formation of the coal seam, the infiltration of sulfate in the groundwater of the roof led to the occurrence of Fe-sulphide minerals not only in the syngenetic period, but also widely across the later stage of diagenesis.

6. Conclusions

Taking the ultra-thick coal seam of the Xishanyao formation in Yihua open-pit mine of the Zhundong coalfield as the research object, this study comprehensively analyzed the state and genesis of Fe-sulphide minerals in the ultra-thick coal seam of the Yihua open-pit mine. To this end, experiments such as micro-coal rock identification, industrial analysis, total sulfur and morphological sulfur analysis, ash composition analysis, determination of main and trace elements in coal, identification of occurrence form of Fe-sulphide minerals, and sulfur isotope test of marcasite were performed using basin evolution data, paleoenvironmental data, and other related data. The specific findings are as follows:
(1) Macroscopically, three morphological types of Fe-sulphide minerals could be observed in coal, namely, thin film Fe-sulphide minerals, nodular Fe-sulphide minerals, and disseminated Fe-sulphide minerals. Microscopically, the occurrence morphology mainly included flake, block, spearhead, and crack filling.
(2) Overall, the δ34S value of marcasite presented a wide variation range, and the extreme value reached up to 51.30‰. The δ34S values of marcasite in different samples vary widely and narrowly. The sulfur in marcasite in the thick coal seam has both primary sulfur and sulfur added after the formation of the coal seam, represented by biogenic marcasite and sulfate-genetic marcasite. Moreover, white iron ore composite of sulfate and biogenic origin was also identified.
(3) The content of organic components in the thick coal seam is more than 95%, and the inorganic components are mainly clay minerals and marcasite, with marcasite being concentrated in individual layers. The content of marcasite in coal is most strongly affected by the contents of matrix vitrinite and inertinite, exhibiting a positive correlation. The analysis shows high sulfur content of matrix vitrinite, which would have provided a certain amount of primary sulfur for the formation of marcasite. Compared with vitrinite, inertinite has a more developed pore structure, allowing the growth of marcasite into larger particles in the coal seam.
(4) The occurrence of sulfur and iron elements in the extremely thick coal seam is attributable to sulfur produced by sulfur-containing plants and ancient wildfires, the supply of terrigenous debris during peatification, and the infiltration of freshwater sulfate in the coal seam roof. At the same time, the ancient wildfires led to the high content of inertinite in the extremely thick coal seam, which provided sufficient space for the growth of marcasite. The good material basis, sufficient growth space, and suitable geochemical environment facilitated the occurrence of large-grained marcasite in the thick coal seam. After the formation of the coal seam, the infiltration of sulfate-containing groundwater through the roof of the coal seam not only enabled marcasite formation in the syngenetic period, but also its wide occurrences in later stages of diagenesis.

Author Contributions

Conceptualization, S.F.; Methodology, N.L., J.T. and X.L.; Software, W.W.; Validation, X.W. and H.X.; Formal analysis, N.L. and J.T.; Investigation, X.L. and H.X.; Resources, X.W.; Data curation, W.W.; Writing—original draft, S.F.; Writing—review & editing, X.W. and W.W.; Supervision, J.T.; Project administration, X.L.; Funding acquisition, S.F. and W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Science and Technology Program of Xinjiang Uygur Autonomous Region (2023D04056); Major Scientific and Technological Project in the Xinjiang Uygur Autonomous Region (2022A03014); the Science and Technology Program of Xinjiang Uygur Autonomous Region (2023B03013-1); the Natural Science Foundation of China (42162017); the Natural Science Foundation of China (42472236); the Third Xinjiang Scientific Expedition Program (2022xjkk1003).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 15 00816 g001
Figure 1. Structural feature map of the study area (modified after [53]). (a) Structural characteristics of the study area; (b) Structural characteristics of the eastern Junggar Basin.
Figure 1. Structural feature map of the study area (modified after [53]). (a) Structural characteristics of the study area; (b) Structural characteristics of the eastern Junggar Basin.
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Minerals 15 00816 g002
Figure 2. Lithologic column and sampling point of sampling strata in the Yihua open-pit mine of the Zhundong coalfield.
Figure 2. Lithologic column and sampling point of sampling strata in the Yihua open-pit mine of the Zhundong coalfield.
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Minerals 15 00816 g003
Figure 3. Typical maceral plate of the Yihua open-pit mine in the Zhundong coalfield. (a) YH-3 Corpogelinite; (b) YH-15 Fusinite; (c) YH-16 Fusinite; (d) YH-9 Funginite; (e) YH-15 Marcasite; (f) YH-6 Sporinite.
Figure 3. Typical maceral plate of the Yihua open-pit mine in the Zhundong coalfield. (a) YH-3 Corpogelinite; (b) YH-15 Fusinite; (c) YH-16 Fusinite; (d) YH-9 Funginite; (e) YH-15 Marcasite; (f) YH-6 Sporinite.
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Figure 4. Columnar distribution map of the proportion of the main components of coal rock from the Yihua open-pit mine of the Zhundong coalfield.
Figure 4. Columnar distribution map of the proportion of the main components of coal rock from the Yihua open-pit mine of the Zhundong coalfield.
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Minerals 15 00816 g005
Figure 5. Macroscopic occurrence characteristics of Fe-sulphide minerals. (a) YH-9 Medium-thin film Fe-sulphide minerals; (b) YH-15 Medium nodular Fe-sulphide minerals; (c) YH-2 Medium-disseminated Fe-sulphide minerals.
Figure 5. Macroscopic occurrence characteristics of Fe-sulphide minerals. (a) YH-9 Medium-thin film Fe-sulphide minerals; (b) YH-15 Medium nodular Fe-sulphide minerals; (c) YH-2 Medium-disseminated Fe-sulphide minerals.
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Minerals 15 00816 g006
Figure 6. YH-16 SEM and mapping overlay flow chart of the sample. (a) SEM images; (b) EDS spectrum mapping diagram. Element mapping diagrams of (c) S; (d) O; (e) Si; (f) Fe; and (g) C.
Figure 6. YH-16 SEM and mapping overlay flow chart of the sample. (a) SEM images; (b) EDS spectrum mapping diagram. Element mapping diagrams of (c) S; (d) O; (e) Si; (f) Fe; and (g) C.
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Minerals 15 00816 g007
Figure 7. YH-16 SEM-Laman Fe-sulphide minerals identification flow chart. (a) Fe-sulphide minerals marker image under SEM; (b) Marked position Raman spectrum; (c) Raman Ruff library comparison results.
Figure 7. YH-16 SEM-Laman Fe-sulphide minerals identification flow chart. (a) Fe-sulphide minerals marker image under SEM; (b) Marked position Raman spectrum; (c) Raman Ruff library comparison results.
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Minerals 15 00816 g008
Figure 8. SEM-Raman test analysis of the YH-2 sample.
Figure 8. SEM-Raman test analysis of the YH-2 sample.
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Minerals 15 00816 g009
Figure 9. SEM-Raman test analysis of the YH-3 sample.
Figure 9. SEM-Raman test analysis of the YH-3 sample.
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Figure 10. SEM-Raman test analysis of the YH-8 sample.
Figure 10. SEM-Raman test analysis of the YH-8 sample.
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Minerals 15 00816 g011
Figure 11. (a–c) SEM-Raman test analysis of the YH-9 sample.
Figure 11. (a–c) SEM-Raman test analysis of the YH-9 sample.
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Minerals 15 00816 g012
Figure 12. SEM-Raman test analysis of the YH-14 sample.
Figure 12. SEM-Raman test analysis of the YH-14 sample.
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Minerals 15 00816 g013
Figure 13. SEM-Raman test analysis of the YH-15 sample.
Figure 13. SEM-Raman test analysis of the YH-15 sample.
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Minerals 15 00816 g014
Figure 14. XRD pattern of YH-15. M: Marcacite; G: Gypsum; R: Rozenite.
Figure 14. XRD pattern of YH-15. M: Marcacite; G: Gypsum; R: Rozenite.
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Minerals 15 00816 g015
Figure 15. XRD pattern: YH-2, YH-3, YH-8, YH-9, YH-14, YH-16. M—Marcasite, C—Calcite, K—Kaolinite, Q—Quartz, G—Gypsum.
Figure 15. XRD pattern: YH-2, YH-3, YH-8, YH-9, YH-14, YH-16. M—Marcasite, C—Calcite, K—Kaolinite, Q—Quartz, G—Gypsum.
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Minerals 15 00816 g016
Figure 16. Microcosmic occurrence characteristics of Fe-sulphide minerals. (a) YH-2 Poly-flake Fe-sulphide minerals in coal; (b) YH-8 Spearhead Fe-sulphide minerals in coal; (c) YH-14 Plate-like Fe-sulphide minerals in coal; (d) YH-15 Disseminated Fe-sulphide minerals in coal.
Figure 16. Microcosmic occurrence characteristics of Fe-sulphide minerals. (a) YH-2 Poly-flake Fe-sulphide minerals in coal; (b) YH-8 Spearhead Fe-sulphide minerals in coal; (c) YH-14 Plate-like Fe-sulphide minerals in coal; (d) YH-15 Disseminated Fe-sulphide minerals in coal.
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Minerals 15 00816 g017
Figure 17. Sulfur isotopic composition of marcasite.
Figure 17. Sulfur isotopic composition of marcasite.
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Minerals 15 00816 g018
Figure 18. YH-8 LA-MC-ICP-MS δ34S test of marcasite.
Figure 18. YH-8 LA-MC-ICP-MS δ34S test of marcasite.
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Minerals 15 00816 g019
Figure 19. Composite genetic model of marcasite in the thick coal seam of the Zhundong coalfield.
Figure 19. Composite genetic model of marcasite in the thick coal seam of the Zhundong coalfield.
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Minerals 15 00816 g020
Figure 20. Oxidation-reduction index point diagram. (a) Ni/Co Point diagram; (b) V/CrPoint diagram; (c) U/Th Point diagram; (d) (2 × U)/(Th/3) + U) Point diagram.
Figure 20. Oxidation-reduction index point diagram. (a) Ni/Co Point diagram; (b) V/CrPoint diagram; (c) U/Th Point diagram; (d) (2 × U)/(Th/3) + U) Point diagram.
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Minerals 15 00816 g021
Figure 21. Triangular diagram of coal ash composition of the thick coal seam in the Yihua open-pit mine.
Figure 21. Triangular diagram of coal ash composition of the thick coal seam in the Yihua open-pit mine.
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Table 1. Zhundong coalfield Yihua Open-Pit Mine Xishanyao group coal maceral statistics table.
Table 1. Zhundong coalfield Yihua Open-Pit Mine Xishanyao group coal maceral statistics table.
SampleVitrinite (%)Inertinite (%)Liptinite (%)Clay (%)Marcasite (%)
TETELCODEVISUMFUSEMAMIFUNINSUM
YH-10.19 10.13 0.18 16.21 15.37 42.08 9.38 21.14 0.39 0.10 0.07 25.03 56.11 0.02 1.15 -
YH-21.12 13.79 0.49 3.93 8.02 27.35 11.69 23.94 0.03 0.07 - 31.23 66.96 -2.64 2.58
YH-30.28 13.94 1.12 7.53 5.85 28.72 10.47 20.52 0.37 0.32 0.16 34.03 65.87 0.02 2.20 2.21
YH-40.37 19.57 0.04 12.86 6.36 39.20 10.72 18.06 0.70 0.46 0.00 28.68 58.62 0.07 1.77 -
YH-54.55 17.09 1.71 7.81 9.09 40.25 9.76 16.48 0.73 0.46 0.16 30.91 58.50 0.01 1.20 -
YH-60.33 21.79 1.19 5.91 13.22 42.44 6.69 19.80 0.08 1.17 0.37 27.78 55.89 0.06 0.94 -
YH-71.14 19.76 0.95 10.41 7.26 39.52 5.25 22.15 2.08 0.55 0.76 26.14 56.93 0.04 1.57 -
YH-81.22 8.72 0.81 10.23 5.34 26.32 9.78 26.17 0.37 0.64 0.00 32.10 69.06 -1.83 2.76
YH-91.14 6.76 0.95 5.41 12.26 26.52 7.25 24.15 2.08 3.55 0.76 30.04 67.83 0.04 2.07 2.55
YH-100.55 21.09 0.09 13.91 7.36 43.00 6.18 25.82 0.63 0.03 0.84 22.27 55.77 0.01 1.08 -
YH-110.52 26.12 0.21 13.13 16.07 56.05 4.11 19.82 - - - 18.37 42.30 0.01 1.34 -
YH-123.70 24.87 0.35 10.43 13.07 52.42 7.56 23.06 0.70 0.46 - 14.08 45.86 0.04 0.78 -
YH-134.73 21.98 0.73 11.16 10.04 48.64 5.13 20.34 0.31 -0.17 23.49 49.44 0.01 1.65 -
YH-140.49 4.89 0.39 6.13 11.85 23.75 12.10 26.34 - - - 32.35 70.79 0.02 1.14 4.28
YH-150.26 1.99 0.16 3.07 6.45 11.93 10.39 32.18 0.11 - - 37.16 79.84 0.05 2.64 5.13
YH-161.14 1.90 0.03 11.84 8.82 23.73 16.19 24.70 0.45 - -29.16 70.50 0.01 2.07 3.56
TE—telinite, TEL—telocollinite, CO—corpogelinite, DE—desmocollinite, VI—vitrodetrinite, SUM—summation, FU—fusinite, SE—semifusinite, MA—macrinite, MI—micrinite, FUN—Funginite, IN—inerto-detrinite. ICCP System 1994 [58].
Table 2. Concentrations of major element oxides (wt%, on whole coal ash basis).
Table 2. Concentrations of major element oxides (wt%, on whole coal ash basis).
SampleSiO2Al2O3Fe2O3CaONa2OMgOTiO2K2OP2O5MnO
YH-10.560.280.212.050.461.120.0060.0050.0120.003
YH-20.450.360.922.520.431.230.0040.0060.0160.005
YH-30.360.251.682.350.421.040.0080.0080.1020.002
YH-40.850.650.212.360.411.150.0060.0070.0260.003
YH-50.420.320.192.120.381.060.0070.0060.0660.016
YH-60.310.220.122.450.211.020.0050.0050.01960.065
YH-70.520.280.424.320.351.210.0080.0580.0080.009
YH-80.366.621.652.400.460.420.0050.1560.0260.008
YH-90.540.320.722.380.471.180.0090.0070.0160.007
YH-100.480.250.212.320.151.160.0050.0080.0060.006
YH-110.210.180.3415.061.080.620.0060.0020.0090.008
YH-120.960.420.202.450.361.160.0110.0080.0380.009
YH-130.350.120.381.820.541.180.0120.0070.0180.106
YH-140.320.250.052.560.260.960.0150.0060.0080.007
YH-150.330.324.1212.60.250.820.0180.0030.0050.13
YH-160.620.410.622.620.351.060.0130.0060.0020.006
Ave0.480.700.753.890.411.020.010.020.020.02
China8.475.984.851.230.160.220.330.190.190.19
Ave, weighted average for Yihua coals. China coal data from [61].
Table 3. Concentration of trace elements in samples from the Yihua mine (μg/g).
Table 3. Concentration of trace elements in samples from the Yihua mine (μg/g).
SampleLiSrNiGaVCrCoBaUTh
YH-133.92106.829.6317.7629.1916.062.01191.161.073.34
YH-240.1995.4613.7720.9146.089.401.86204.054.262.51
YH-349.46108.3415.8718.8554.9410.731.64257.194.753.42
YH-455.16126.4610.1617.9612.467.312.47216.740.752.40
YH-557.46111.279.7219.0923.7315.263.14210.431.615.69
YH-665.46127.066.3717.7916.479.112.76247.882.3610.24
YH-780.47136.178.3821.0923.2413.212.04259.051.366.02
YH-885.43115.4412.2823.3260.6514.141.67248.4210.904.12
YH-973.09127.8115.6418.0685.6419.081.86291.838.345.27
YH-1083.46140.4910.4619.6730.116.852.41245.762.7112.73
YH-1170.76144.8412.8617.8218.928.712.86251.461.888.26
YH-1261.26116.769.1621.8614.107.783.26234.372.039.08
YH-1360.16122.3911.9517.0918.789.983.24217.922.057.18
YH-1448.72104.8216.1722.9458.6812.112.28208.4716.54.83
YH-1536.4399.7520.8224.7672.9616.981.97227.7224.33.87
YH-1640.7982.4915.7421.4384.2119.682.05211.7945.35.71
Ave58.89116.6512.4420.0640.6312.902.35232.778.145.92
China Coal43.91140.2013.726.6434.9715.357.07158.702.415.84
China coal data from [62].
Table 4. δ34S test results of marcasite.
Table 4. δ34S test results of marcasite.
Sample DescriptionTest ItemsUnitδ34SMean Value
YH-2Sulfur isotopic‰−25.4~−21.6−23.5
YH-3−34.6~−31.6−33.1
YH-8−7.2~+12.82.8
YH-9−3.5~+16.76.6
YH-14−6.3~+13.13.4
YH-15−19.5~+6.612.9
YH-16−21.6~+8.213.4
Table 5. Discrimination index of redox environment and statistical results of sample data.
Table 5. Discrimination index of redox environment and statistical results of sample data.
MicroelementReducing EnvironmentWeak Oxidation–Weak
Reduction Environment		Oxidizing MilieuSample
Min–MaxMean Value
δU>1.00.7~0.0<0.70.80~1.921.26
V/Cr>4.252.0~4.25<2.01.79~5.123.73
Ni/Co>7.05.0~7.0<5.02.31~10.575.75
U/Th>1.250.75~1.25<0.750.21~7.931.70
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Wu, X.; Lü, N.; Feng, S.; Wang, W.; Tian, J.; Li, X.; Xadethan, H. Occurrence State and Genesis of Large Particle Marcasite in a Thick Coal Seam of the Zhundong Coalfield in Xinjiang. Minerals 2025, 15, 816. https://doi.org/10.3390/min15080816

AMA Style

Wu X, Lü N, Feng S, Wang W, Tian J, Li X, Xadethan H. Occurrence State and Genesis of Large Particle Marcasite in a Thick Coal Seam of the Zhundong Coalfield in Xinjiang. Minerals. 2025; 15(8):816. https://doi.org/10.3390/min15080816

Chicago/Turabian Style

Wu, Xue, Ning Lü, Shuo Feng, Wenfeng Wang, Jijun Tian, Xin Li, and Hayerhan Xadethan. 2025. "Occurrence State and Genesis of Large Particle Marcasite in a Thick Coal Seam of the Zhundong Coalfield in Xinjiang" Minerals 15, no. 8: 816. https://doi.org/10.3390/min15080816

APA Style

Wu, X., Lü, N., Feng, S., Wang, W., Tian, J., Li, X., & Xadethan, H. (2025). Occurrence State and Genesis of Large Particle Marcasite in a Thick Coal Seam of the Zhundong Coalfield in Xinjiang. Minerals, 15(8), 816. https://doi.org/10.3390/min15080816

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