Geochemical Characteristics and Influencing Factors of Germanium in Early Permian Coal from the Southern Margin of the North China Craton Basin: Constraints from Coal Petrology, Mineralogy, and Germanium Isotopes

Abstract

The geochemical behavior of germanium (Ge) in Early Permian coal from the southern margin of the North China Craton remains unclear, and the enrichment mechanism of Ge under low-content conditions urgently needs to be elucidated. This study employed a variety of analytical techniques to conduct coal petrology, mineralogy, and geochemistry analyses on 16 Early Permian coal samples from the Huainan–Huaibei Coalfield on the southern margin of the North China Craton. The results show that the Ge content in coal ranges from 0.9 to 19.1 μg/g (average 6.33 μg/g, n = 16), generally at a low level. Ge content exhibits a significant positive correlation with vitrinite content, gelification index (GI), vegetation index (VI), and pyrite content, while showing a negative correlation with inertinite content, indicating that a reducing, water-saturated peat swamp environment favors Ge enrichment through organic matter complexation-adsorption and pyrite as a carrier. The δ⁷⁴Ge values range from 1.85‰ to 3.52‰ and show a negative correlation with Ge content, confirming that organic matter preferentially incorporates the heavy isotope δ⁷⁴Ge during Ge fixation. Both the Huainan–Huaibei Coalfield and the eastern United States coalfields are located in tropical-subtropical low-latitude humid climate zones, but due to the lack of intense synsedimentary hydrothermal activity and limited swamp development, Ge enrichment in the former is significantly lower than that in the Lincang Coalfield in Yunnan and the Wulantuga Coalfield in Inner Mongolia. The findings of this study provide a theoretical basis for understanding the geochemical behavior of Ge in coal from the Huainan–Huaibei Coalfield.

1. Introduction

Coal is not only one of the most crucial fossil energy sources globally but also an important carrier of various strategic and critical mineral resources, including lithium, gallium, germanium, rare earth elements, and precious metals. , With the increasing depletion of traditional independent ore deposits and the growing difficulty in their extraction, the resource utilization of associated critical elements in coal and its combustion by-products has garnered increasing attention worldwide. , Among these critical metal elements, Ge occupies an irreplaceable position in modern high-tech industries due to its unique optical, electrical, and semiconductor properties. Germanium is one of the core foundational materials for manufacturing high-end equipment such as infrared optical devices, fiber-optic communication systems, high-performance solar cells, and integrated circuit chips. , In recent years, with the construction of 5G network infrastructure, the upgrading of global military infrared thermal imaging systems, and the development of space solar cell technologies, the global demand for Ge has continued to rise. On a global scale, countries and regions such as the United States, the European Union, and China have classified Ge as a strategic critical mineral or key raw material and have actively taken measures to ensure the security and stability of its supply chain. ,

However, germanium has an extremely low abundance in the Earth’s crust, approximately only 1.5 μg/g, and mainly exists in a dispersed state in sphalerite, coal, and some hydrothermal sulfide deposits, rarely forming independent industrial minerals. , The global distribution of Ge resources is highly uneven, with proven reserves highly concentrated in a few countries, including China, the United States, and Russia. , Coal is currently recognized as one of the most promising carriers of Ge globally. World-class super-large coal-hosted germanium deposits, represented by Lincang in Yunnan, China, Wulantuga in Inner Mongolia, China, and Pavlovka in Russia, have been discovered worldwide. Additionally, notable background levels of Ge have been commonly detected in many other coalfields. , The Ge content in these high-Ge coal seams can generally reach tens to thousands of μg/g, with enrichment levels significantly higher than those in the upper crust. In some mining areas, it even reaches industrial recovery grades, making coal-hosted Ge deposits an important source of Ge for global industrial use. , Numerous studies indicate that the enrichment of Ge in coal is not the product of a single geological process but is controlled by the complex coupling of multiple factors, primarily including: (1) adsorption and complexation by organic matter–Germanium mainly exists in an organically bound state in coal, with oxygen-containing functional groups such as phenolic hydroxyl and carboxyl groups in peat during coal formation considered the primary active sites for fixing germanium. , Germanium selectively enriches by forming stable germanium–oxygen complexes with these functional groups; (2) hydrothermal activity and ore-forming material sourcesin the Wulantuga mining area, germanium enrichment is closely related to the involvement of hydrothermal fluids during the syn-sedimentary or post-sedimentary stages. Similar hydrothermal processes are also observed in the Lincang mining area, where Ge-rich fluids interact with organic matter, leading to the precipitation of Ge in coal seams; (3) sedimentary environment and paleoclimate during coal formationwarm and humid climatic conditions ²⁵and weakly reducing to reducing peat swamp environments with water coverage facilitate the extensive accumulation of organic matter and the effective preservation of Ge. Furthermore, with the maturity and development of MC-ICP-MS analytical techniques, Ge stable isotopes (primarily δ⁷⁴Ge) have become an important geochemical tool for tracing the material sources of Ge in coal and revealing its enrichment and fractionation mechanisms. , Qi et al. found through MC-ICP-MS analysis of Ge-rich lignite in the Lincang mining area that the δ⁷⁴Ge values exhibited a wide range from −2.59‰ to +4.72‰, with a significant negative correlation between δ⁷⁴Ge values and Ge content. This can be explained by the Rayleigh fractionation model in an open system, indicating that the isotopic fractionation of Ge in coal is primarily controlled by the preferential enrichment of light isotopes. These key insights provide important isotopic constraints on the material sources and ore-forming processes of continental coal-hosted Ge deposits.

The Huainan–Huaibei Coalfield, located on the southeastern margin of the North China Craton, is one of China’s most significant Carboniferous-Permian coal-bearing regions and an important component of the coal-accumulating basins on the North China Plate. , Geochemical surveys of Ge have been conducted to varying degrees in the Late Paleozoic coal-bearing strata of this area. However, overall, the Ge content in the Huainan–Huaibei Coalfield is relatively low (with most samples below 10 μg/g), , far lower than that in world-class high-Ge ore districts such as Wulantuga in Inner Mongolia and Lincang in Yunnan. Nevertheless, it is precisely this characteristic of “low background and weak enrichment” that makes the Huainan–Huaibei Coalfield an ideal natural laboratory for studying the distribution patterns, enrichment mechanisms, and controlling factors of Ge in different coal facies under background conditions. Compared to high-Ge ore districts that have been systematically studied, the low-level enrichment of Ge in the Huainan–Huaibei Coalfield better reflects the normal processes of Ge geochemical cycling under regional geological settings, which is of great significance for understanding the macroscopic distribution patterns and occurrence states of Ge in the Carboniferous-Permian coal measures of the North China Craton. , However, systematic research on the occurrence characteristics, main controlling factors, and the spatiotemporal coupling relationship between Ge and the coal-forming environment in this area is still relatively lagging. There is a lack of systematic integration and in-depth exploration of multidimensional information ranging from coal quality parameters, mineral composition, and microlithotype components to isotope geochemistry. Based on the above understanding, this study conducted systematic industrial and elemental analyses, thin-section identification, quantitative analysis of macerals, X-ray diffraction mineralogical characterization, and Ge content testing on 16 Carboniferous-Permian coal samples from the Huainan–Huaibei Coalfield. Additionally, δ⁷⁴Ge isotope analysis was performed on eight representative samples. This study aims to systematically ascertain the content levels and distribution characteristics of Ge in Carboniferous-Permian coal from the Huainan–Huaibei Coalfield. It seeks to reveal the enrichment patterns of germanium, as well as its correlations with coal quality parameters, mineral composition, and maceral constituents. Furthermore, by integrating germanium isotope compositions, the study constrains its material sources and enrichment processes, and explores the controlling effects of coal-forming climatic backgrounds and regional tectonics on germanium enrichment. The ultimate goal is to deepen the understanding of the geochemical behavior of germanium in the Carboniferous-Permian coal measures of the North China Craton and provide a fundamental scientific basis for the potential evaluation of coal-associated germanium resources in the region.

2. Geological Background

The North China Craton Basin is situated between the Yinshan-Yanshan Orogenic Belt and the Qinling-Dabie-Sulu Orogenic Belt. Its northern boundary extends along the southern margin of the Central Asian Orogenic Belt, its western boundary lies west of the Helan Mountains-Liupanshan line, and its eastern boundary reaches the Bohai Bay and the coastline, encompassing vast areas such as North China, eastern Liaoning, and the Bohai Bay region. As shown in Figure A, within this basin, a sedimentary environment dominated by late Paleozoic Carboniferous-Permian marine-continental transitional facies has formed China’s most coal-rich interactive strata of carbonate rocks and clastic rocks, with a wide range of significant coalfields represented by the Qinshui Coalfield, Huainan Coalfield, Huaibei Coalfield, among others. The main coal-bearing strata in these coalfields are the late Paleozoic Carboniferous-Permian offshore coal-bearing rock series, which represent the primary coal-accumulating period in the North China Craton Basin. The North China Craton Basin has undergone superimposed transformations from multiple tectonic movements, including the Indosinian, Yanshanian, and Himalayan movements, resulting in a complex and diverse geological structural framework.

1.

Location overview and comprehensive stratigraphic column of the study area. (A) Distribution map of major basins in North China. (B) Distribution map of sampling sites. Note: In the figure A, TC stands for Tarim Craton, QB for Qaidam Block,and QM for Qiangtang Block; In the figure B, ZZ stands for Zhu Zhuang, ZXZ for Zhu XianZhuang, WG for Wu Gou, YL for Yang Liu, LL for Lu Ling, XH for Xin Hu, XT for Xu Tong, BJ for Ban Ji, GQ for Gu Qiao, PJ for Pan Ji, KZD for Kou Zi Dong, and ZJ for Zhang Ji.

The Huainai-Huaibei Coalfields are located in the Xuhuai Block on the southeastern margin of the North China Plate, bordered by the southwestern Shandong Uplift to the north, the Fufeng Thrust Nappe Belt to the south, the Henan Depression to the west, and the Tanlu Fault Zone to the east. In the north-south direction, they are further divided into the Huaibei Coalfield and the Huainan Coalfield by the Bengbu Uplift (Figure B). The sedimentary environments in the Huainai-Huaibei region are mostly coastal plains or inland lake-swamp environments. Such environments are conducive to plant proliferation and accumulation, and can effectively preserve plant remains, leading to the formation of coal resources through prolonged geological processes.

3. Sampling and Testing

3.1. Sample Collection

The samples for this study were collected from different mining areas in the Huainan and Huaibei Coalfields, with 7 samples from the Huainan Coalfield and 9 from the Huaibei Coalfield. All these mines are operational, providing a solid foundation for the work. Taking into full consideration the safety of sampling and the diversity of coal mines, 14 different coal mine areas were selected for this study. All samples were collected from underground working faces, following a strictly standardized sampling procedure. Large coal lumps were randomly hand-picked from the coal mine working faces to ensure the representativeness and reliability of the samples. It should be noted that although the Huainan and Huaibei Coalfields are located within the same coal-bearing strata, they employ different coal seam numbering systems. Therefore, the coal seam numbers referred to in this paper directly follow the original numbering systems of each coalfield (the Huainan samples use the coal seam numbers of the Huainan Coalfield, and the Huaibei samples use those of the Huaibei Coalfield), without any unified consolidation. The basic information about the samples is shown in Table .

1. Basic Information of Coal Samples.

sample	coalfield	coal mine	coal seam	sampling depth (m)
C-1	Huainan	KZD	no. 9 coal seam	800
C-2	Huaibei	ZXZ	no. 10 coal seam	956
C-3	Huaibei	JG	no. 7 coal seam	366
C-4	Huaibei	XH	no. 8-2 coal seam	890
C-5	Huaibei	LL	no. 10 coal seam	684
C-6	Huaibei	TT	no. 3 coal seam	310
C-7	Huainan	BJ	no. 8 coal seam	700
C-8	Huaibei	WG	no. 10 coal seam	460
C-9	Huaibei	XT	no. 7 coal seam	550
C-10	Huaibei	YL	no. 5 coal seam	264
C-11	Huainan	BJ	no. 8 coal seam	659
C-12	Huaibei	ZZ	no. 4 coal seam	269
C-13	Huainan	PJ	no. 4 coal seam	506
C-14	Huainan	PJ	no. 3 coal seam	588
C-15	Huainan	ZJ	no. 13-2 coal seam	642
C-16	Huainan	GQ	no. 13 coal seam	725

3.2. Test Analysis

3.2.1. Proximate Analysis and Ultimate Analysis

The moisture, ash, and volatile matter contents of each sample were determined in accordance with ASTM standards D3173-23(2023), D3174(2019), and D3175-20(2020). For specific experimental analysis procedures, please refer to our previous study. Ultimate analysis was conducted at the Anhui Provincial Coalfield Geology Testing Center using a Vario EL III elemental analyzer (manufactured by Elementar Analysensysteme GmbH, Hanau, Germany). The temperatures of the combustion tube and reduction tube were set at 1150 °C and 850 °C, respectively. Helium was used as the carrier gas with a flow rate of 230 milliliters per minute. The instrument was calibrated using sulfanilamide, and the sample weight was controlled at 20 ± 2 milligrams to determine the contents of C _d, H _d, S _d, and N _d. The content of O _d was derived using the subtraction method, as shown in formula

$$O_{\mathrm{d}}(\%)=100\%-(C_{\mathrm{d}}+H_{\mathrm{d}}+N_{\mathrm{d}}+S_{\mathrm{d}}+A_{\mathrm{d}})$$ 1

The experimental data are shown in Table .

2. Proximate and Ultimate Analysis of Coal Samples .

sample	proximate analysis (wt %)				ultimate analysis (%)
	M ad	A d	V daf	FCad	C d	H d	N d	O d	S d
C-1	3.04	19.76	25.32	53.64	63.22	4.42	0.93	11.14	0.53
C-2	1.02	18.13	28.34	53.06	67.52	4.12	1.17	8.86	0.2
C-3	2.11	12.23	26.59	64.07	74.82	4.41	1.27	6.9	0.37
C-4	0.54	18.89	26.64	56.38	66.02	4.46	0.92	9.31	0.4
C-5	1.33	5.56	34.88	58.8	76.2	4.48	1.07	12.42	0.27
C-6	1.06	19.58	24.46	60.89	60.84	4.76	0.16	14.57	0.09
C-7	2.16	11.56	30.03	57.75	63.62	4.81	1.31	18.55	0.15
C-8	2.1	18.41	33.15	46.82	68.04	4.37	1.13	7.65	0.4
C-9	1.83	11.03	29.81	60.96	76.47	4.73	0.77	6.77	0.23
C-10	0.88	8.67	36.26	58.21	73.28	4.79	1.4	11.58	0.28
C-11	1.43	15.87	30.8	58.27	70.8	4.74	1.1	7.29	0.2
C-12	1.12	25.53	13.72	61.83	57.75	4.5	1.14	10.83	0.31
C-13	1.04	13.38	35.35	56.7	66.45	4.67	1.26	13.37	0.87
C-14	0.8	7.27	36.84	60.08	77.59	4.52	1.37	8.95	0.3
C-15	1.12	10.5	44.05	52.28	75.86	5.22	1.04	7.25	0.13
C-16	1.82	15.32	29.44	58.34	66.63	4.09	1.14	12.34	0.48

^a“ad” stands for air-dried basis; “d” stands for dry basis; “daf” stands for dry ash-free basis.

3.2.2. Thin Section Identification

Thin section identification is conducted using a ZEISS AxioSkop 40 polarizing microscope. Micrographs of the thin sections are taken to record information such as the observed mineral types and rock structural characteristics. The captured micrographs are then transferred to a computer, where image processing software (such as Adobe Photoshop) is used to crop, adjust contrast and brightness, etc., to enhance the quality of the photographs. Accurately weigh 0.5–1.0 g of fine powder sample using an electronic balance and pour it into a pellet press mold. Gradually apply pressure up to 20-30 MPa and maintain it for 1–2 min. After pressing, remove the thin section. If there are burrs, polish it with sandpaper, and then place it in a drying oven at 60–80 °C for 1–2 h of drying.

3.2.3. Maceral Measurement

Quantitative analysis of the macerals in coal samples is carried out according to the standard methods 74 and 75 of the International Committee for Coal and Organic Petrology (ICCP). For specific experimental analysis procedures, please refer to our previous study. To further explore the relationship between the depositional environment of the coal samples and their elemental contents, we introduce two coal facies indices used to determine the depositional environment. Among them, the gelification index (GI) is a dimensionless indicator that reflects the water coverage and reducing conditions of the coal-forming peat bog, as shown in formula

$$\mathrm{G}\mathrm{I}=\mathrm{V}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}/\mathrm{I}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}$$ 2

The vegetation preservation index (VI) is a dimensionless indicator that represents the degree to which plant remains are preserved through gelatinization. The formula is shown in ()­

$$\mathrm{V}\mathrm{I}=(\mathrm{V}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{L}\mathrm{i}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e})/\mathrm{I}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}$$ 3

The GI/VI ratio reveals the extent of the contribution of exinite to the VI value. When the GI/VI ratio is close to 1, it indicates a low exinite content, with the coal-forming plant tissues primarily consisting of woody fibers and undergoing intense gelification. Conversely, if the GI/VI ratio is significantly less than 1, it suggests that exinite makes a substantial contribution to the VI value, indicating that herbaceous plants or lipid-rich plants account for a certain proportion in the coal-forming vegetation.

Meanwhile, to ensure the reliability of experimental data, the experimental process for each sample was repeated 4×, and the data for each sample is the average of the four measurements.

3.2.4. XRD Analysis

In this experiment, a PANalytical X’Pert PRO X-ray powder diffractometer was used to conduct phase analysis on unknown samples in reflection mode. Prior to the experiment, the samples were processed as follows: The samples to be tested were thoroughly ground into a uniform powder of approximately 40 μm. The powder was then filled into the groove of a flat sample holder, pressed tightly with a glass slide, and the surface was scraped flat to ensure that the testing surface was even, dense, and flush with the surface of the sample holder, thereby guaranteeing the consistency of diffraction geometric conditions and the reliability of diffraction data. Subsequently, the X-ray generator and the circulating water cooling system were activated to preheat the X-ray tube to a stable operating state. The instrument parameters were set as follows: The X-ray source was a Cu target (Kα radiation), with a tube voltage of 40 kV and a tube current of 40 mA. The wavelengths were λKα₁ = 1.540598 Å and λKα₂ = 1.544426 Å, and the intensity ratio of Kα₂/Kα₁ was 0.50. A reflection-transmission rotating stage was used for the sample holder, and the sample plane was kept horizontal during the scanning process to eliminate the influence of preferred orientation on diffraction intensity. The goniometer employed a θ/θ mode, with a scanning range of 2θ = 5–45°, a step size of 0.0167°, a scanning rate of 0.0678°/s, and a counting time of 13.770 s per point. The entire measurement process was programmed and executed using the PANalytical Data Collector software, and the data was saved after testing.

3.2.5. ICP–MS Analysis

The Ge element content in the samples was determined using an ICP–MS instrument. The ICP–MS instrument used was an X Series 2 model (Thermo Fisher Scientific). The sample treatment procedure was as follows: First, the 200-mesh samples were dried in a 105 °C oven for 12 h. Then, precisely 50 mg of the powdered sample was weighed and placed into a Teflon digestion vessel. Subsequently, 1 mL of high-purity HNO₃ and 1 mL of high-purity HF were added sequentially and slowly. The Teflon digestion vessel was placed into a steel jacket, tightly sealed, and heated in a 190 °C oven for more than 24 h. After the digestion vessel cooled down, it was opened and placed on a 140 °C hot plate to evaporate to dryness. Then, 1 mL of HNO₃ was added and evaporated to dryness again. After that, 1 mL of high-purity HNO₃, 1 mL of MQ water, and 1 mL of internal standard In (with a concentration of 1 μg/g) were added. The Teflon digestion vessel was then placed back into the steel jacket, tightly sealed, and heated in a 190 °C oven for more than 12 h. Finally, the solution was transferred into a polyethylene bottle and diluted to 100 g with 2% HNO₃ for ICP–MS testing. During the testing process, standard samples OU-6, BCR-1, and GBPG-1 were used to monitor the accuracy of the data results, with the relative standard deviation (RSD) of the data results being better than 5%. The Ge element content values for each sample were thus obtained. Replicate measurements (n = 4) were carried out for the treatment process of each sample, and the Ge element content of the sample was the arithmetic mean of the four measurements.

3.2.6. Germanium Isotope Analysis

3.2.6.1. Sample Dissolution

Accurately weigh 100–150 mg of the sample into a Teflon digestion vessel. The sample is first reacted with 10 mL of concentrated nitric acid at 120 °C for over 48 h, followed by drying on a hot plate at 120 °C until dry. If any residue remains, an additional 15 mL of concentrated nitric acid should be added. For the final digestion of the dry residue obtained after nitric acid dissolution, 1 mL of concentrated hydrofluoric acid and 5 mL of ultrapure water are used. The solution is heated on a hot plate in a sealed Teflon digestion vessel at 120 °C for over 24 h, with periodic stirring. The solution and insoluble precipitate are diluted with water to obtain an approximately 1 mol/L hydrofluoric compound solution.

3.2.6.2. Chemical Purification

The 1 mol/L hydrofluoric compound solution containing Ge is loaded onto an anion exchange chromatography column packed with 1.8 mL of AG1-X8 resin (Bio-Rad, Hercules, CA, USA, 100–200 mesh), following the sample treatment method described by Rouxel et al. After Ge adsorption onto the column, 10 mL of 1 mol/L HF and 2 mL of H₂O are passed through the column to elute the remaining matrix. Subsequently, Ge is eluted using 12 mL of 3 mol/L HNO₃, and the solution is dried to dryness at a temperature below 80 °C. After drying on a hot plate, the solution is redissolved in 0.28 mol/L HNO₃ and continuously heated at 80 °C for over 1 h. The final solution is ready for isotope analysis.

3.2.6.3. MC-ICP-MS Analysis

Germanium isotope values are measured using a MC-ICP-MS instrument in conjunction with continuous-flow hydride generation technology, with the instrument operating conditions and data acquisition parameters identical to those used by Rouxel et al. in their Ge isotope studies. The original sample solution is diluted according to its germanium concentration for Ge isotope measurements. The mass separation process of the instrument is corrected using the standard sample bracketing method, which involves measuring a standard sample (NIST SRM3120a Ge standard solution) before and after each analysis.

4. Results

4.1. Thin-Section Identification and Analysis of Coal

In this study, thin-section identification and analysis were conducted on 16 coal samples from the Huainan–Huaibei Coalfield, with selected samples displayed (Figure ). Through systematic observation and testing using a polarizing microscope, the distribution patterns and existing states of major minerals such as quartz, clay minerals, and pyrite were precisely identified, revealing the symbiotic relationship between organic matter and inorganic minerals.

2.

Thin-section identification images of some samples. (A) Sample C-1; (B) sample C-2; (C) sample C-6; (D) sample C-8; (E) sample C-10; (F) sample C-12; (G) sample C-13; (H) sample C-14; (I) sample C-15.

Figure A shows that the rock is primarily composed of argillaceous material and organic matter, presenting as black and reddish-brown clumps. There are small amounts of detrital components, mainly quartz and a small amount of feldspar, mostly scattered in angular shapes with particle sizes mainly ranging from 0.02 mm to 0.06 mm, and the largest reaching 0.12 mm. A small amount of pyrite is present as fine granules mixed within the rock; Figure B illustrates that the rock is mainly composed of organic matter and a small amount of argillaceous material, appearing as black clumps that are difficult to distinguish from each other. There is a small amount of calcite, filling the rock fractures in the form of bright granules. A small amount of pyrite is present as fine granules and aggregate clumps mixed within the rock. The rock has well-developed fractures that crisscross each other, with widths ranging from 0.01 mm to 0.2 mm. Most fractures are unfilled, while a small number are filled with calcite; Figure C shows that the rock is primarily composed of argillaceous material, presenting as yellowish-brown and mainly consisting of clay mineral aggregates of micritic mud. There are some detrital components, mainly quartz and a small amount of feldspar, mostly scattered in angular shapes with particle sizes mainly ranging from 0.03 mm to 0.06 mm, and the largest reaching 0.12 mm. A small amount of organic matter is present in clumps and stripes within the argillaceous material; Figure D indicates that the rock is mainly composed of organic matter and a small amount of argillaceous material, appearing as black clumps. There is a small amount of calcite, filling the rock fractures as fine granules. A small amount of pyrite is distributed as fine granules and aggregate clumps. The rock has several fractures with widths ranging from 0.01 mm to 0.05 mm, and a small number of fractures are filled with calcite; Figure E shows that the rock is mainly composed of organic matter and a small amount of argillaceous material, appearing as black clumps that are mixed together. Some organic matter has residual plant cell cavity structures, presenting as a meshwork filled with a small amount of siliceous material and clay minerals. A small amount of pyrite is distributed as fine granules and aggregate clumps; Figure F illustrates that the rock is mainly composed of organic matter and some argillaceous material, appearing as black clumps with some argillaceous material aggregated locally into yellowish-brown clumps. A small amount of pyrite is present as fine granules mixed within the rock. The rock has several developed fractures with widths ranging from 0.01 mm to 0.2 mm, and no fillings are observed; Figure G shows that the rock is mainly composed of organic matter and a small amount of argillaceous material, which are mixed together, presenting as black and reddish-brown clumps. Some organic matter has residual plant cell cavity structures, presenting as a meshwork filled with a small amount of authigenic clay minerals, mainly kaolinite; Figure H indicates that the rock is mainly composed of organic matter and a small amount of argillaceous material, presenting as black and reddish-brown clumps. Some organic matter has residual plant cell cavity structures, presenting as a meshwork filled with a small amount of authigenic clay minerals and siliceous material. A small amount of pyrite is distributed as fine granules and aggregate clumps; Figure I shows that the rock is mainly composed of organic matter and a small amount of argillaceous material, presenting as black and reddish-brown clumps. Some organic matter has residual plant cell cavity structures, presenting as a meshwork filled with a small amount of calcite and siliceous components. The rock has well-developed pores, mostly irregular in shape, with pore sizes ranging from 0.02 mm to 0.5 mm, and a small number are filled with clay minerals and siliceous material.

From the micrographs of the coal samples, it can be observed that the rocks in the coal samples of this experiment are generally composed of organic matter intermingled with a small amount of argillaceous material, presenting black and reddish-brown clumps. Some organic matter exhibits a residual plant cell-cavity reticular structure. Fine granular or aggregate clump-like pyrite is commonly seen, and irregular pores (with pore diameters ranging from 0.02 to 0.7 mm) are often developed. These microscopic images are of great significance for understanding the distribution and occurrence states of the main mineral components in the coal samples. This is crucial for in-depth exploration of the occurrence state of Ge in coal and its geological significance.

4.2. Maceral Analysis of Coal

According to the test results of maceral composition for each coal sample in Table , the vitrinite content in the coal samples ranges from 48.83% to 88.72%, the inertinite content ranges from 7.84% to 37.89%, and the exinite content ranges from 2.06% to 15.19%. Among them, samples C-6, C-11, and C-12 have notably high vitrinite contents (>80%), while samples such as C-15, C-10, and C-16 have relatively larger proportions of exinite and inertinite. The Gelification Index (GI) reflects the degree of water coverage and reductivity in the coal-forming peat swamp. The GI values of the coal samples range from 1.34 to 11.32, with most concentrated between 1.5 and 6.0, indicating an overall relatively reductive coal-forming environment, but with significant differences among different samples. The vitrinite index (VI) represents the degree to which plant remains are preserved through gelification, with data ranging from 1.64 to 11.76, generally following the same trend as the GI. The GI/VI ratios are all less than 1 (ranging from 0.77 to 0.98), indicating that the contribution of exinite to the VI values makes VI generally slightly higher than GI. Samples with ratios close to 1 (such as C-6 and C-12) have lower exinite contents, reflecting the characteristics of their coal-forming period, where plant tissues were mainly composed of lignocellulose and gelification was intense. In summary, the micropetrographic composition of the coal samples in this experiment is dominated by vitrinite, followed by inertinite, with relatively low exinite content. The coal-forming environment generally leans towards reductive and humid peat swamp conditions.

3. Maceral Composition Parameters of Samples.

sample	vitrinite (%)	inertinite (%)	liptinite (%)	GI	VI	GI/VI
C-1	59.43	26.39	14.18	2.25	2.79	0.81
C-2	62.43	29.67	7.90	2.10	2.37	0.89
C-3	58.94	33.26	7.80	1.77	2.01	0.88
C-4	60.04	31.09	8.87	1.93	2.22	0.87
C-5	57.44	34.63	7.93	1.66	1.89	0.88
C-6	83.40	14.54	2.06	5.74	5.88	0.98
C-7	70.58	16.06	13.36	4.39	5.23	0.84
C-8	62.50	32.57	4.93	1.92	2.07	0.93
C-9	58.09	37.89	4.02	1.53	1.64	0.94
C-10	50.48	35.73	13.79	1.41	1.80	0.79
C-11	80.35	15.29	4.36	5.26	5.54	0.95
C-12	88.72	7.84	3.44	11.32	11.76	0.96
C-13	72.46	18.72	8.82	3.87	4.34	0.89
C-14	78.59	9.84	11.57	7.99	9.16	0.87
C-15	48.83	36.51	14.66	1.34	1.74	0.77
C-16	55.67	29.14	15.19	1.91	2.43	0.79

4.3. Main Mineral Types in Coal

X-ray diffraction is one of the core characterization techniques for studying the mineral composition in coal. This method offers prominent advantages in coal structure characterization, such as non-destructive analysis, rapidity, accuracy, simplicity, as well as the capability for both qualitative and quantitative analysis. In this experimental process, the internal standard method was introduced in the quantitative analysis strategy to quantitatively determine the mass fractions of various mineral phases in coal by referencing the standard curves of diffraction peaks. For clay minerals, which exhibit severe diffraction peak overlap and are prone to preferred orientation, the Rietveld full-pattern fitting refinement method was further employed. This method performs a global fit of the entire diffraction pattern based on a crystal structure model, thereby yielding more reliable results for mineral mass fractions compared to the single-diffraction-peak method and significantly enhancing the quantitative accuracy for clay minerals and minerals with low concentrations.

The XRD spectra of the coal samples are shown in Figure . To further explore the types of mineral compositions in the coal samples, Table presents the XRD mineral analysis results of the coal samples. All samples exhibit a strong P peak at approximately 2θ ≈ 25°, representing microcrystalline carbon (graphite-like structure) in coal, which reflects the main organic structural characteristics of coal. Meanwhile, the mineral diffraction peaks of different samples highly correspond to the mineral composition data in the table: C-1 and C-14 show distinct quartz peaks at approximately 2θ ≈ 12°, corresponding to their quartz contents of 8.6–7.5% respectively in the table, while the absence of this peak in the other samples aligns with their zero quartz content; C-9 exhibits carbonate peaks near 2θ ≈ 20° and sulfide peaks near 2θ ≈ 35°, corresponding to its 1.5% calcite and 1.6% siderite content. However, the pyrite content of C-9 in the table is 0, which is consistent with the absence of typical pyrite diffraction peaks in this sample. Additionally, the clay mineral contents of C-9, C-10, C-12, C-13, C-14, and C-1 are 96.9%, 94.3%, 94.0%, 95.5%, 90.2%, and 88.0% respectively, representing the highest proportion of mineral components among all samples. This corresponds to the broadened background and weak peaks in the spectra, excluding the characteristic mineral peaks. Moreover, the pyrite contents of C-10, C-12, C-13, C-14, and C-1 (5.7%, 6.0%, 4.5%, 2.3%, and 3.4% respectively) also match the intensities of the weak sulfide diffraction peaks at corresponding positions in the spectra. Overall, this reflects the differences in XRD characteristic peaks among different coal samples due to variations in mineral composition, while maintaining a consistent main carbon structure. Among them, clay minerals are the absolutely dominant minerals in all samples, quartz is only present in C-1 and C-14, carbonate minerals are only present in C-9, and pyrite exists in varying amounts in most samples except C-9.

3.

XRD spectra of some coal samples.

4. XRD Mineral Analysis Results of Coal Samples.

sample	component content (%)
	quartz	calcite	siderite	pyrite	clay mineral
C-1	8.6	0.0	0.0	3.4	88.0
C-9	0.0	1.5	1.6	0.0	96.9
C-10	0.0	0.0	0.0	5.7	94.3
C-12	0.0	0.0	0.0	6.0	94.0
C-13	0.0	0.0	0.0	4.5	95.5
C-14	7.5	0.0	0.0	2.3	90.2

4.4. Germanium Content in Coal

The Ge content results of various coal samples are presented in Table . According to Table , the Ge content in the 16 coal samples from the Huaibei and Huainan coalfields ranges from 0.9 to 19.1 μg/g, with an average of 6.33 μg/g, indicating an overall low background level. Additionally, there is a significant coefficient of variation among the samples (with a standard deviation 1σ ranging from 0.46 to 2.66), reflecting the non-uniform distribution of Ge in coal within this region. To delve into the distribution of Ge element content across global regions, this study incorporated Ge content data from various scholars regarding the Wulantuga Coal Mine in China (with sample data from the Wulantuga Coal Mine sourced from Yao et al. (n = 9), Wang et al. (n = 13), and Wei et al. (n = 6)), the Lincang Coal Mine in China (with sample data from the Lincang Coal Mine sourced from Qi et al. (n = 37) and Wei et al. (n = 7)), the Pavlovka Coalfield in the Russian Far East (with sample data from the Pavlovka Coalfield sourced from Qi et al. (n = 37)), and Ge content data from the United States (with sample data from the Indiana Coalfield sourced from Seredin et al. (n = 7) and sample data from the Kentucky Coalfield sourced from Hower et al. (n = 37)). The Ge content in these coalfields exhibits significant differences, with specific content ranges shown in Figure . The study found that the Ge content in coal from the United States ranges from 0.21 to 290 μg/g, predominantly concentrated in the lower range (<40 μg/g), sharing a similar low Ge content background with the Huainan and Huaibei Coalfields. In the Pavlovka Coalfield in the Russian Far East, the Ge content in coal ranges from 5.9 to 566.7 μg/g, with a median of approximately 136 μg/g, and some samples exceeding industrial grade. The Ge content in coal from the Wulantuga area ranges from 23 to 1036 μg/g, with an average of approximately 276.71 μg/g, indicating a medium-to-high enrichment level. In contrast, the Ge content in coal from the Lincang area is exceptionally high, ranging from 22 to 2585 μg/g, with an average of approximately 993.8 μg/g, representing a typical coal mine with extraordinary Ge enrichment.

5. Ge Content in Various Coal Samples .

sample	average content of Ge (μg/g)	max (μg/g)	min (μg/g)	standard deviation (1σ)
C-1	5.4 (n = 4)	6.7	2.39	2.02
C-2	3.4 (n = 4)	6.67	1.06	2.36
C-3	3.6 (n = 4)	5.22	1.28	1.69
C-4	4.7 (n = 4)	6.94	2.2	2.01
C-5	1.2 (n = 4)	3.03	0.6	1.11
C-6	19.1 (n = 4)	20.54	18.14	1.16
C-7	13.3 (n = 4)	14.04	12.41	0.67
C-8	4.2 (n = 4)	5.46	2.79	1.19
C-9	1.8 (n = 4)	3.31	0.03	1.41
C-10	3.0 (n = 4)	5.16	1.86	1.53
C-11	8.4 (n = 4)	9.28	7.15	0.9
C-12	16.9 (n = 4)	17.37	16.3	0.46
C-13	7.5 (n = 4)	9.84	4.32	2.66
C-14	5.3 (n = 4)	6.15	3.55	1.21
C-15	0.9 (n = 4)	2.5	0.4	1.02
C-16	2.6 (n = 4)	3.65	2.16	0.71

^a1σ represents one standard deviation of the data.

4.

Ge element contents in coalfields across various regions. (Note: Data for the Wulantuga region are sourced from refs , and ; data for the Lincang region are from refs and ; data for the Pavlovka coalfield in the Russian Far East are from ref ; data for the Indiana coalfield in the United States are from ref ; data for the Kentucky coalfield in the United States are from ref ).

4.5. Ge Isotope Geochemical Characteristics of Coal

In this study, eight coal samples were selected for ⁷⁴Ge isotope analysis, with the test results presented in Table . The δ⁷⁴Ge values (‰) represent the per mil deviation of the Ge isotope ratio of the samples relative to the international standard NIST SRM 3120a, used to quantitatively characterize the degree of isotope fractionation. The study found that the δ⁷⁴Ge values of the coal samples ranged from 1.85‰ to 3.52‰, with relatively low 2σ values (0.03‰–0.08‰), indicating that the analytical results are precise and reliable. This range of δ⁷⁴Ge variation (1.84‰) is significantly smaller than the fractionation range of up to 7.31‰ observed in coal from the super-large Ge deposit in Lincang, Yunnan. This suggests that the geochemical processes (such as adsorption, complexation, or redox reactions) experienced by the Huainan–Huaibei coal samples were relatively limited in intensity, or that the source region was relatively homogeneous.

6. Ge Isotope Contents in Eight Coal Samples .

sample	δ74Ge (‰)	2σ
C-1	3.06	0.05
C-9	3.26	0.08
C-10	2.88	0.05
C-11	1.85	0.08
C-12	1.93	0.05
C-13	2.94	0.03
C-14	2.32	0.05
C-16	3.52	0.03

^a2σ represents twice the standard deviation of the data.

5. Discussion

5.1. Relationship between the Germanium Content in Coal and the Coal Analysis Results

There is also a certain relationship between the Ge content in coal and coal analysis. This study investigated the correlations between the Ge content and other elemental contents, as well as industrial analysis results, in 16 coal samples from the Huainan and Huaibei coalfields. The study found that the Ge content exhibits a negative correlation trend with C _d, a positive correlation trend with O _d, a weak correlation with N _d, and no obvious statistical pattern with H _d and S _d (as shown in Figure A). This indicates that the enrichment of Ge is more closely associated with coal that has a lower degree of coalification and a higher content of active functional groups in the organic matter. This correlation may be attributed to the fact that Ge primarily exists in organic matter rather than inorganic minerals. Coal with a high carbon content has undergone more extensive coalification, during which a significant number of oxygen-containing active functional groups (such as carboxyl and phenolic hydroxyl groups) are lost through decarboxylation and dehydration reactions. These functional groups are key sites for the adsorption and complexation of Ge. Meanwhile, samples with higher Ge content correspond to lower C _d and higher O _d, indicating that these coals are at a lower coalification stage and retain more oxygen-containing functional groups capable of forming stable complexes with Ge, thereby providing favorable chemical conditions for the organic adsorption and fixation of Ge. Additionally, the study by Jiu et al. showed an opposite trend and a weak correlation with Ge content, while Zhuang et al. also found no obvious correlation between Ge content and elemental analysis results, reflecting significant differences in the controlling factors for Ge enrichment in coal from different study areas.

5.

Correlation between the Ge content in coal and the content analyzed in coal quality assessments. (A) Ultimate analysis; (B) proximate analysis.

Meanwhile, the study found that the Ge content exhibits a negative correlation trend with V _daf , a trend also observed by Shang et al. No obvious statistical patterns were identified with M _ad (moisture on an as-received basis) and FC_ad, as shown in Figure B. During the coal-forming period of the Taiyuan Formation in the Huaibei–Huainan Coalfield, the coastal reducing environment was influenced by seawater, significantly affecting the marsh peat conditions. Ge in coal primarily exists in organic-bound and adsorbed states, with its enrichment mechanism mainly related to the adsorption and complexation by organic matter. Volatile matter on a dry ash-free basis primarily represents the pyrolyzable portion of organic matter in coal, and its content is closely related to the coal’s metamorphic grade: the higher the metamorphic grade of coal, the lower the volatile matter yield. During the coal-forming process, as the coalification and metamorphic grades increase, organic matter gradually loses volatile matter. Ge, as a volatile element, escapes and migrates from highly metamorphosed coal, resulting in relatively lower Ge content in low-volatile coal and higher Ge enrichment in high-volatile coal. Notably, in this study, the Ge content shows a significant positive correlation with A _d, which differs from the negative correlation trend found by Yao et al. This discrepancy may indicate that, in addition to organically bound Ge, a certain proportion of inorganic-bound Ge (such as that associated with clay minerals) exists in some samples from the Huaibei–Huainan Coalfield, or that the input of terrigenous detritus influenced by seawater has had a superimposed effect on the Ge content.

5.2. Relationship between the Content of Ge Element in Coal and Minerals

There is a close relationship between the content of Ge in coal and its mineral composition. In recent years, numerous scholars have conducted systematic geochemical and mineralogical analyses on different coal mining areas, revealing the varying affinities of Ge with various minerals. Ding et al. studied the Wulantuga Ge-rich coal through density fractionation experiments and found that Ge was significantly enriched in low-density clean coal, indicating its predominant existence in an organic form. In contrast, Co, As, Sb, and Hg were found in pyrite, suggesting that some of these elements might be associated with fine-grained minerals embedded in the organic matter. Additionally, Xu et al. investigated high-Ge coal in Lincang, Yunnan, using correlation analysis algorithms. They discovered that when the pyrite content in coal exceeded approximately 16.8 wt %, the occurrence states of As and Hg shifted from organic–inorganic mixed affinity to primarily pyrite-hosted, quantitatively revealing the influence of hydrothermal fluids on element occurrence patterns. The aforementioned studies indicate that the enrichment of Ge in coal is closely related to organic matter and is also influenced by inorganic minerals such as clay minerals, pyrite, and calcite.

This study systematically analyzed the correlation between the Ge element and mineral composition in coal samples from the Huainan–Huaibei Coalfield using thin-section identification and X-ray diffraction experiments. Meanwhile, sample burial depth and δ⁷⁴Ge were also incorporated into the correlation analysis to explore their intrinsic relationships, as detailed in Figure . According to the data in Table , it was found that sample C-12, with the highest Ge content (Ge = 16.9), had a clay mineral content of 94.0% and a pyrite content of 6.0%. Sample C-13, with the second-highest Ge content (Ge = 7.5), had a clay mineral content as high as 95.5% and a pyrite content of 4.5%. Sample C-9, with a lower Ge content (Ge = 2.37), had the highest clay mineral content (96.9%) but only a 1.6% pyrite content, indicating that the content of a single mineral alone cannot fully explain the enrichment pattern of Ge. The correlation between the Ge element content and mineral content in Figure shows a significant positive correlation between Ge and pyrite (r = 0.63), a marked negative correlation with calcite and siderite (r = −0.44), a weak negative correlation with quartz (r = −0.19), and virtually no correlation with clay minerals (r = −0.019). Although clay minerals are the predominant inorganic component in coal (ranging from 88.0% to 96.9%), higher clay mineral content does not necessarily correspond to higher Ge content in all samples, possibly reflecting the complex selectivity or saturation effect of clay minerals on Ge adsorption. In contrast, the significant positive correlation between pyrite and Ge suggests that pyrite formed under reducing conditions in the coal-forming environment of the Huainan–Huaibei Coalfield may be one of the important carrier minerals for Ge enrichment. This mineral–element correlation reflects the unique metallogenic geological environment of the Carboniferous-Permian coal measures in the Huainan–Huaibei Coalfield. The negative correlation between sample burial depth and pyrite reflects the weakening of reducing conditions in deep diagenetic environments. The negative correlation between δ⁷⁴Ge and pyrite indicates that pyrite formed under reducing conditions preferentially enriches light Ge isotopes, resulting in an inverse coupling between mineral phases and isotopic composition. Against the geological backdrop of the Huainan–Huaibei Coalfield, the strongly reducing environment of peat bogs during the coal-forming period was conducive to the formation of pyrite. In the coal sample data of this study, the pyrite content is generally low (ranging from 0% to 6.0%), while the clay mineral content is extremely high (ranging from 88.0% to 96.9%). This implies that, even though the correlation coefficient between Ge and pyrite is 0.63, the absolute contribution of pyrite to Ge enrichment may be limited, and a significant amount of Ge may still exist in an organic form.

6.

Correlation between sample burial depth, Ge content, δ⁷⁴Ge, and the content of various minerals.

5.3. Relationship between Germanium Content in Coal and Macerals

The content of macerals in coal also has a certain impact on its Ge content. The enrichment of Ge in coal is primarily controlled by the adsorption and complexation of Ge by organic matter. Wei et al. conducted systematic research on two super-large coal-hosted Ge deposits in Wulantuga, Inner Mongolia, and Lincang, Yunnan, using electron probe microanalysis and density gradient centrifugation techniques. They found that in the high-Ge coal from Wulantuga, Ge, As, and W were more enriched in the huminite group than in the inertinite group, and were also more enriched in dense macerals. Further density gradient centrifugation experiments on finely ground coal demonstrated that huminite was more prone to enrich Ge than inertinite, and that denser huminite contained more Ge binding sites than less dense huminite, indicating that Ge exists in coal organic matter through weak organic bonding. The aforementioned studies indicate that the enrichment of Ge in coal is closely related to organic macerals, with vitrinite (huminite) in particular exhibiting strong affinity for Ge in most Ge-rich coals.

This study systematically investigated the correlation between the Ge content and maceral parameters in 16 coal samples from the Huainan–Huaibei Coalfield, as shown in Figure . The study found a significant positive linear correlation between Ge content and vitrinite content (r = 0.839), and a significant negative correlation with inertinite content (r = 0.801). The correlation with exinite content was relatively weak (r = 0.441), which is similar to the research findings of Zhang et al. In terms of the gelification index (GI) and vegetation preservation index (VI), the Ge content showed a positive correlation with GI (r = 0.75) and also exhibited a positive correlation trend with VI (r = 0.727). Moreover, the GI/VI ratio was generally close to or slightly higher than 0.95 in high-Ge samples. This indicates that a strongly reducing ecosystem environment and gelification are conducive to Ge enrichment, a viewpoint further confirmed by Wang et al. in their study of the sedimentary environment of Ge-rich coal in the Wulantuga Coal Mine of the Shengli Coalfield. This correlation suggests that Ge-rich coal seams formed in reducing swamp environments characterized by long-term stable water coverage, weak hydrodynamic conditions, and strong suppression of oxidation. Such environments provided the necessary geological conditions for the continuous supply, complexation-adsorption by organic matter, and effective preservation of Ge. In the Huainan–Huaibei Coalfield, Ge primarily exists in vitrinite, and the degree of Ge enrichment increases with the increase in vitrinite content.

7.

Correlation between Ge content in coal and maceral parameters. (A) Vitrinite; (B) inertinite; (C) exinite; (D) GI; (E) VI; (F) GI/VI.

5.4. Relationship between Germanium Content and Its Isotopes in Coal

In recent years, with the development of non-traditional stable isotope geochemistry, the correlation between Ge content and its isotopic composition (δ⁷⁴Ge) in coal has become an important means of tracing the sources and enrichment mechanisms of Ge materials. In this study, a linear fitting analysis of Ge content and δ⁷⁴Ge values was conducted on coal samples from the Huainan and Huaibei Coalfields (as shown in Figure ). The fitting results reveal a negative correlation between Ge content and δ⁷⁴Ge values in coal from these coalfields (n = 8, r = 0.794, p = 0.011), meaning that as Ge content increases, δ⁷⁴Ge values gradually decrease. This trend is consistent with the negative correlation observed in the Lincang Coal Mine. It indicates that during the process of Ge fixation by coal-forming organic matter, the heavy isotope ⁷⁴Ge is preferentially taken up. This negative correlation trend suggests that the enrichment process of germanium may be accompanied by kinetic isotope fractionation caused by organic complexation or sulfide precipitation. In this process, the light isotopes are preferentially incorporated into the solid phase or complexes, leading to the gradual enrichment of heavy isotopes in the residual solution, which is then subsequently re-adsorbed by the organic matter. The negative correlation between the Ge content and δ⁷⁴Ge in coal from the Huainan–Huaibei Coalfield may represent a geochemical evolutionary pathway dominated by open-system Rayleigh fractionation with a negative correlation under conditions of low background germanium element content. This implies that in the coal-forming swamp environment, the supply and adsorption processes of germanium continue continuously, and the degree of isotope fractionation is limited by the saturation level of adsorption sites on the organic matter and the fluid exchange rate. Furthermore, the selective adsorption differences of Ge by different coal macerals (such as vitrinite and inertinite) and the mixed contributions from trace pyrite may, to a certain extent, interfere with the linear relationship between δ⁷⁴Ge and Ge content. The positive deviation of δ⁷⁴Ge may be attributed to equilibrium fractionation resulting from the reductive absorption of Ge­(OH)₄ and the coordination with functional groups of organic matter in the peat swamp environment.

8.

Correlation between Ge content and Ge isotopes in coal.

5.5. Relationship between Germanium Content in Coal and Paleoclimate

There is a close relationship between the Ge content in coal and the paleoclimatic conditions during coal formation. The paleoclimate influences the migration, enrichment, and preservation of Ge in coal by controlling the hydrological conditions, redox environment, and vegetation types of peat swamps. In recent years, many scholars have conducted in-depth research on the relationship between Ge enrichment in coal from different coal-forming periods and paleoclimate. Zeng et al. found that gallium-Ge deposits in China are mainly concentrated in coal-bearing basins from the Carboniferous-Permian, Triassic-Jurassic, Early Cretaceous, and Neogene periods on the North China and Yangtze platforms, and that the formation of coal-type Ge deposits is closely related to magmatism and hydrothermal activity. Ning et al. discovered that, constrained by sedimentary background and tectonic conditions during coal formation, lithium and gallium enrichment zones in coal are mainly distributed in two coal-forming periods: the Carboniferous-Permian in North China and the Permian in South China, while Ge in coal is primarily distributed in the Early Cretaceous in Northeast China and the Neogene in western Yunnan. Cao et al., after re-examining the latitudinal distribution of climate-sensitive lithological indicators such as global coal, evaporites, and glacial deposits since the Devonian period, found that there was a significant poleward migration of coal distribution in the early Permian, and that the distribution of evaporites exhibited a distinct bimodal pattern, indicating significant latitudinal zonation in the global paleoclimate during the Early Permian.

This study systematically compared the differences in Ge enrichment in coal under different latitudinal paleoclimatic backgrounds by combining a paleogeographic reconstruction map of the Early Permian (∼296.8 Ma) (see Figure ) with Ge content data from typical coal mining areas domestically and abroad. The study found that both the Lincang coal mine in Yunnan, located in a low-latitude tropical humid climate region (with a Ge content as high as 993.8 μg/g), and the Wulantuga coal mine in Inner Mongolia, situated in a mid-latitude tropical humid climate region (with a Ge content of 276.71 μg/g), exhibited significant Ge enrichment characteristics. The former is located on the eastern margin of the Tethys Ocean near the equator, where abundant organic matter accumulated under humid and hot climatic conditions, and hydrothermal fluids leached Ge from the basement granite and enriched it in the peat; the latter is located in the mid-latitude region of the Northern Hemisphere, where favorable ore-forming conditions also exist under humid conditions. In contrast, the Ge content in coal from the Pavlovka coalfield in the Russian Far East (196.1 μg/g) and the Indiana and Kentucky coalfields in the United States (19.55 μg/g) decreased sequentially. The former may be due to its relatively high latitude and weaker hydrothermal activity, while the latter may be influenced by the expansion of arid climate in the North American continent during the Early Permian, leading to the shrinkage of peat swamps. The Ge content in coal from the Huainan–Huaibei coalfield on the southeastern margin of the North China Platform is 6.33 μg/g, which is much lower than that in the Russian Far East, Lincang in Yunnan, and Wulantuga in Inner Mongolia, and is close to the average Ge content level in the United States. From the perspective of Early Permian paleogeography, both the Huainan–Huaibei coalfield and the United States are located in tropical-subtropical low-latitude regions, which benefited from the humid climate that dominated the global tropical regions from the Late Carboniferous to the Early Permian, providing a basis for the development of large-scale coal-forming swamps. Geographically, coal mines in both regions were mainly formed in extensive coastal plains or delta environments. The collision and amalgamation of the South China and North China plates during the Early Permian formed large coastal plain swamps in the Huainan–Huaibei region; similarly, coal accumulation in the early Pennsylvanian in eastern North America also occurred in the extensive and long-term subsiding coastal plains of the Appalachian Basin. This similar paleogeographic environment provided optimal conditions for the development of large-scale peat swamps. This similarity is mainly attributed to the fact that Ge enrichment in coal in both regions is primarily controlled by organic matter. The lush coal-forming plants that grew under warm and humid climates constituted a rich source of organic matter in peat swamps and efficiently absorbed Ge elements from the source area. The common “organic-dominated” enrichment pathway ultimately laid the intrinsic foundation for the similar Ge content in coal in both regions. Meanwhile, this paleoclimatic background limited the development of peat swamps, resulting in low organic matter input and unfavorable to early Ge enrichment; the region lacks intense syn-sedimentary hydrothermal activity like that in Lincang and Wulantuga, resulting in insufficient material sources and enrichment drivers for Ge.

9.

Map of the ancient continent in the early Permian at 296.8 Ma. (A) Locations of various coal mines; (B) paleoclimatic environment of the ancient continent. (Modified from Scotese, with coal mine and climate data sourced from Boucot et al.). Note: Adapted with permission from [Scotese, C.R. Atlas of Permo-Carboniferous Paleogeographic Maps (Mollweide Projection), Maps 53–64, Volumes 4, The Late Paleozoic, PALEOMAP Atlas for ArcGIS, PALEOMAP Project, Evanston, IL]. Copyright [2014] [PALEOMAP Project].

6. Conclusions

The primary conclusions drawn from this study are as follows:

• (1)The germanium (Ge) content in coal from the Huainan–Huaibei Coalfield ranges from 0.9 to 19.1 μg/g, with an average of 6.33 μg/g. The significant coefficient of variation among samples reflects the marked heterogeneity in Ge distribution.
• (2)Low coalification degree, high oxygen-containing functional groups, and high vitrinite content facilitate the adsorption and complexation of germanium; a strongly reducing peat swamp environment with deep water coverage (high gelification index (GI) and vegetation index (VI)) significantly promotes Ge preservation.
• (3)A significant positive correlation exists between Ge content and pyrite content (r = 0.63), while the correlation with clay minerals is weak, indicating that in strongly reducing peat swamp environments, a small portion of Ge may be hosted by pyrite. The δ⁷⁴Ge values range from 1.85‰ to 3.52‰, showing a negative correlation with Ge content, reflecting that coal-forming organic matter preferentially incorporates the heavy isotope δ⁷⁴Ge during Ge fixation.
• (4)Although both the Huainan–Huaibei Coalfield and the eastern United States coalfields are located in low-latitude humid zones, Ge enrichment in the former is significantly lower than that in the Lincang Coalfield and the Wulantuga Coalfield due to the lack of synsedimentary hydrothermal activity and limited swamp scale. This finding reveals the geochemical behavior of Ge in coal under different climatic backgrounds.

The authors declare no competing financial interest.