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Article

The Differences in the Li Enrichment Mechanism between the No. 6 Li-Rich Coals and Parting in Haerwusu Mine, Ordos Basin: Evidenced Using In Situ Li Microscale Characteristics and Li Isotopes

by
Guohong Qin
1,†,
Jinhao Wei
2,†,
Yingchun Wei
2,3,*,
Daiyong Cao
2,3,*,
Xin Li
2 and
Yun Zhang
2
1
Hebei Key Laboratory of Environmental Change and Ecological Construction, School of Geographical Sciences, Hebei Normal University, Shijiazhuang 050024, China
2
College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China
3
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Minerals 2024, 14(8), 836; https://doi.org/10.3390/min14080836 (registering DOI)
Submission received: 15 June 2024 / Revised: 9 August 2024 / Accepted: 16 August 2024 / Published: 18 August 2024
(This article belongs to the Special Issue Mineralization Mechanism and Geochemical Characteristics of Coals and Associated Minerals)
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Abstract

:
As a potential strategic mineral resource, lithium (Li) in coal measures (including coal and parting) has attracted increasing attention from scholars globally. For a long time, Li in coal measures has been studied mainly on the macro-scale (whole rock); however, the microscopic characteristics of Li and Li isotope variations in coal measures are less well known. In this study, the No. 6 coal measures in the Haerwusu Mine were studied using ICP-MS, XRD, SEM-EDS, MC-ICP-MS, and LA-ICP-MS. The geochemical and mineralogical characteristics, the microscale distribution of Li in minerals, and the Li isotopes of Li-rich coal and parting in the No. 6 coal measure were investigated. The results show that the Li content in the No. 6 coal seam ranges from 3.8 to 190 μg/g (average 83 μg/g), which is lower than the parting (290 μg/g) and higher than the comprehensive evaluation index of Li in Chinese coal (80 μg/g). LA-ICP-MS imaging showed that Li in the coal is mainly contained within cryptocrystalline or amorphous lamellae aluminosilicate materials, and the Li content in lenticular aggregate kaolinite is low. The Li in parting is mainly found in illite/chlorite. The δ7Li of the coals was 3.86‰, which may be influenced by the input of the source rock. The δ7Li of the parting (7.86‰), which was higher than that of the coal, in addition to being inherited from the source rock, was also attributed to the preferential adsorption of 7Li by the secondary clay minerals entrapped in the parting from water during diagenetic compaction. Finally, by integrating the peat bog sediment source composition, sedimentary environment evolution, and Li isotope fractionation mechanism of No. 6 coal, a Li metallogenic model in the Li-rich coal measure was initially established. In theory, the research results should enrich the overall understanding of the Li mineralization mechanism in coal measures from the micro-scale in situ and provide a scientific basis for the comprehensive utilization of coal measure resources.

1. Introduction

Lithium (Li), as a new strategic metal, is widely used in the field of new energy [1,2,3]. However, with the rising demand for Li [4], the Li available in coal measures (including coal and parting) has gradually attracted global attention as a potential strategic resource [2,5,6,7,8,9,10,11,12,13,14,15]. In recent years, Li-rich coal has been discovered by Chinese scholars in the Jungar region north of Ordos, the Ningwu Coalfield in Shanxi, and the Hebi Coalfield in Henan. A systematic study focused on Li-rich coal showed that the lithium in coal mainly occurs in inorganic minerals, which is closely related to the provenance and sedimentary environment. Moreover, hydrothermal action has a certain influence. Li-rich coals have gradually become one of the hotspots of international coal geology research [16,17,18,19,20,21,22,23,24,25,26].
Most of the Li-rich coal is found in the Carboniferous–Permian strata in North China and the Permian strata in South China. The concentration of Li in coal is mainly controlled by provenance, sedimentary environments, regional geological structures, and low-temperature hydrothermal fluid [16,17,23]. With the improvement in the accuracy of Li isotope analysis, Li isotopes are gradually being applied to trace various geological processes [27,28,29,30,31,32,33,34,35,36,37,38]. It is known that the depositional processes involved in coal measures encompass a series of geological effects, such as weathering, basin deposition, sea-level change, and volcanism [39,40]. Furthermore, fluid activity usually results in different degrees of Li isotope fractionation [41,42,43,44,45,46,47].
With increasing metamorphism, Li and δ7Li values gradually decrease in sedimentary rocks. The sorting of Li isotopes depends mainly on the chemical composition of primary rocks and the contribution of exchange Li, which is usually relatively small during low-degree metamorphism and large when new minerals are formed.
Applying density functional theory combined with MC-ICP-MS to the compositional characteristics and enrichment mechanism of Li isotopes in coal measures has been verified by previous authors [38,48]. Sun [49,50] compared the Li isotopes of No. 8 coal from the Xinjing Mine in Yangquan and No. 11 coal from the East Open Pit Coal Mine in Ningwu, Shanxi Province, as well as the Li isotopes of the different forms present. It was found that the convergence of the δ7Li values in the coals may be the result of the redistributive effects of Li in organic and inorganic matter during the coalification stage. Moreover, the Li isotope ratio is not only affected by silicate minerals but also influenced by water–rock reactions during coalification.
Previous studies focusing on Li isotopes and Li enrichment mechanisms in coal measures were mostly conducted solely on coal samples, and few studies were conducted on partings, which are closely related to the sedimentation processes involved in coal seams [51]. In this study, the microscopic occurrence and isotopic characteristics of Li in coals and partings were compared to better explain the enrichment mechanisms of Li in coal measures as a whole.
Therefore, this study conducted sampling and testing analysis for the No. 6 coal of the Haerwusu Mine, thoroughly examining its geochemical and mineralogical characteristics. Subsequently, we selected samples of Li-rich coal and partings and explored the in situ geochemical characteristics of Li-bearing minerals to investigate the distribution patterns of Li. In addition, we explored the fractionation mechanisms of Li isotopes in coal and partings. Finally, based on these findings, we proposed a mechanism for the formation of Li-rich coal and partings, providing a theoretical basis for solving the origin and enrichment mechanisms involved in Li-rich coal in theory and prospecting and predicting coal Li deposits in practice.

2. Geological Setting

2.1. Stratigraphy

The Haerwusu Mine is located in the middle of the Jungar Coalfield in the Ordos Basin (Figure 1a,b). The main coal-forming era of the Haerwusu Mine occurred during the Late Paleozoic Carboniferous–Permian, and the paleogeographic environment is a paralic environment. The main outcrop strata in the mining area from the bottom to the top are the Benxi Formation (C2b), the Taiyuan Formation (C3t), the Shanxi Formation (P1s), the Xiashihezi Formation (P1x), the Shangshihezi Formation (P2s), the Upper Neogene Pliocene Series (N2), the Malan Formation (Q3m), and the Quaternary Holocene Series (Q4) [16,21,52].
The Shanxi Formation and the Taiyuan Formation are the most important coal-bearing strata in this area. There are five coal seams in the Taiyuan Formation: No. 6, No. 7, No. 8, No. 9, and No. 10. Of these, No. 6 coal has the greatest thickness and is located in the upper section of the Taiyuan Formation, which is a stable coal seam and can be mined in the whole area. The No. 7, No. 8, No. 9, and No. 10 coal seams are located in the lower section of the Taiyuan Formation. There are three coal seams in the Shanxi Formation, numbered No. 1, No. 3 and No. 5 from the top to the bottom.

2.2. Coal Seam Characteristics

The main research object of this study is the No. 6 coal seam in the Haerwusu Mine area (Figure 2). The No. 6 coal seam is a bituminous coal, with a thickness of 25.53–39.32 m and an average thickness of 33.70 m; the whole area can be mined as the thickness is vast, the structure is simple, and the partings are mostly concentrated in the middle and upper parts of the No. 6 coal seam [16,21].

3. Samples and Methods

3.1. Sample Collection

In this study, whole-seam sampling was carried out on the No. 6 coal seam of the Taiyuan Formation from the Haerwusu Mine, and a total of five samples were collected, including four coal samples (HEWS-1 to HEWS-4) and one parting sample (HEWS-P). All of the samples were collected in the open section, with a mixed sample collected every 6 m (no sampling when the parting was less than 0.1 m). Oxidized or contaminated surface layers were removed after sample collection to ensure the freshness of the samples.

3.2. Analytical Method

(1) Proximate analysis
In this study, in accordance with the international standard [53,54,55], the moisture, ash, and volatile matter of the coal and parting samples of the No. 6 coal seam of the Haerwusu Mine were directly determined by using the SDTGA5000 approximate analyzer, while the fixed carbon was calculated using the formula (FCad = 100 − (Mad +Aad +Vad)).
(2) Trace element analysis
After collection, the samples were first broken into small pieces with a hammer, and then an agate mortar and sieve were used to obtain the particle size of the samples to less than 170 µm. Agilent 7700e ICP-MS was used to determine the content of trace elements in the coal and rock samples. The specific pre-treatment process of each sample was as follows: (1) The samples with a particle size of less than 170 μm were placed in an oven at 110 °C to be dried for more than 10 h; (2) 50 mg of the sample was weighed and added to a Teflon bomb; (3) 1 mL of HF (purification: purified by distillation) and 1 mL HNO3 (purification) were added to the Teflon bomb; (4) a steel sleeve was placed on the outside of the Teflon bomb and heated to 200 °C in an oven for at least 1 day; (5) after the Teflon bomb cooled to room temperature, it was placed on a hotplate at 150 °C, and 1 mL of HNO3 (purification) was added again and evaporated to dryness; (6) 1 mL of MQ water, 1 mL of HNO3 (purification), and 1 mL of internal standard solution (the concentration was 1 μg/g) were added to the Teflon bomb, and the Teflon bomb was resealed and placed in an oven at 200 °C for >10 h; (7) the solution was transferred into a polyethylene bottle, and the solution was diluted to 100 g with dilute 2% HNO3 for subsequent test analysis. The data collection time was 100 ms, and the standard curve was established using the international standards BHVO-2, BCR-2, RGM-2, and JA-2. The test accuracy was set to 5% (RSD) and the accuracy was set to 10% (relative deviation (RD)).
(3) LTA-XRD analysis
A QuorumK1050X plasma ashing instrument was used to conduct low-temperature ashing (LTA) on the coal and waste from No. 6 coal. In order to preserve the original mineral structure, the temperature was set to 120 °C for 24 h, and the powder was turned every 2 h until the powder was finally gray and white.
Mineral particles with a particle size of less than 10 μm extracted via flotation were used to determine the total content of various clay minerals. Mineral particles with a particle size of less than 2 μm extracted by centrifugation were used to determine the content of different types of clay minerals.
The mineral contents of LTA-coals and the parting samples were determined by X-ray diffraction (XRD) analysis (D/max-2500/PC powder diffractometer, Rigaku, Tokyo, Japan). This equipment employed Ni-filtered Cu-Kα radiation and a scintillation detector. The XRD pattern was captured across a 2θ range spanning from 2.6° to 45°, with incremental steps of 0.02° for enhanced resolution. The semiquantitative mineralogical analysis was carried out using the Reference Internal Standard Method developed by Chung [56]. The mineral content results identified by XRD analysis are relative contents, rather than absolute contents.
(4) Li isotope analysis
(1) Sample digestion: The samples were digested using the method outlined by Lin et al. [57]: 50–100 mg of sample powder was weighed and placed in an in-house PTFE-lined bomb, HNO3-HF mixed acid (1:1, v/v) was added, and it was then dissolved at 190 °C for 48 h. After the sample cooled completely, it was taken out of the sealed bomb, and the solution was steamed; 1 mL of HNO3 was added, and the sample was steamed again. This step was repeated twice to remove the fluoride. Then, 1 mL of HNO3 and 2 mL of high-purity water were added, and the resulting mixture was dissolved at 190 °C for 12 h until the sample solution was clarified and completely dissolved. After the digestion solution dried, it was converted into HCl medium; finally, the sample was fixed in 0.28 mol/L HCl for column separation.
(2) Purification and separation: The separation of Li isotopes was achieved using Bio-Rad AG 50W-X8 cationic resin via ion exchange chromatography [57]. After the sample solution was removed to the pre-cleaned cation exchange column, the matrix elements were eluted with 24 mL of 0.28 mol/L HCl, and elution continued with 31 mL of 0.28 mol/L HCl to collect Li. Finally, before analysis, the collected Li components were dried and converted with nitric acid.
(3) Instrumentation: Li isotope analysis was performed on an MC-ICP-MS (Neptune Plus) provided by Thermo Fisher Scientific. The instrument is equipped with 9 Faraday cup receivers and 7 ion counters. MC-ICP-MS uses a Jet + X cone combination to improve instrument sensitivity. Depending on the Li content in the sample, a 50 µL/min low-flow PFA atomizer was selected to receive 7Li and 6Li simultaneously by optimizing the focusing and dispersion parameters. A 7Li+ sensitivity of ~2 V was obtained at a 50 μL min−1 injection rate in a 2% HNO3 medium with a sample concentration of 50 ppb. During sample injection, 5% NaCl solution was introduced into the ICP system every minute for 3 h to reduce the background effects relating to the instrument and memory in the case of lithium, thus improving the precision and accuracy of the measurements. After washing with 5% NaCl solution, the background signal of Li reduced from 30–110 mV to 1.5–2 mV. The isotope mass fractionation of the instrument was calibrated using the sample–standard interpolation method (SSB method). All Li isotope data are reported as thousandth differences with respect to the reference material, L-SVEC:
δ7Li (‰) = [(7Li/6Li) sample/(7Li/6Li) L-SVEC − 1] × 103
Each batch of samples included at least two international geological reference materials to monitor the instrument status and pre-treatment processes, ensuring the accuracy and precision of the results. Long-term measurement results from our laboratory confirmed that the isotopic composition of the geological RMs (e.g., BCR-2, RGM-2, GSP-2, and RMs; their recommended values are provided in the report) was consistent with previously reported values within the uncertainty range [57,58]. Overall, the long-term reproducibility of δ7Li in our laboratory is better than 0.40‰.
(5) Microscope and scanning electron microscope backscatter observations
Each sample taken from the Haerwusu No. 6 coal seam was observed under a microscope, and the samples were initially observed using a 5× eyepiece and a 10–20× objective lens to select the target minerals for LA-ICP-MS.
The samples were then subjected to backscattering observation and X-energy point analysis using a high-vacuum field emission scanning electron microscope (JSM-IT300HR) and subjected to EX-230 energy spectroscopy to obtain backscattering images of different minerals and their major elemental profiles.
(6) In situ imaging analysis via LA-ICP-MS
LA-ICP-MS was used to analyze the in situ micro-area using a laser denudation inductively coupled plasma mass spectrometer. The laser model is NEW193HE, and the ICP-MS model is Agilent 8900 ICP-MS Triple Quad. During laser denudation, He is used as the carrier gas, and Ar is used as the compensation gas to adjust the sensitivity, and the two gases are mixed through the T-joint and then input into the ICP [59]. The standard substance, NIST 610, was used as an external standard for the correction of the trace element content. Iolite 4.0 was adopted for offline analysis and to process the data (instrument sensitivity drift correction [60], sample and blank signal selection, and element content), and then XMap Tools was used for further map optimization [61,62].
Through the optical microscope, scanning electron microscope backscattering combined with X-ray diffraction results, and X-energy spectral point analysis, the clay minerals (kaolinite, illite/chlorite) in the HEWS-P sample and the kaolinite and cryptocrystalline or amorphous lamellae aluminosilicate materials found in the HEWS-4 sample of the Haerwusu Mine were selected as the main objects of this study, and the areas were divided and analyzed via LA-ICP-MS imaging analysis. The parameter selection for the areas of facet scanning analysis for different samples is shown in Table 1.

4. Results

4.1. Coal Chemistry

Proximate analysis was carried out on four coal samples as well as one parting sample from No. 6 coal of the Haerwusu Coal Mine (Table 2). The moisture was 2.79%–6.01%, with an average value of 4.31%. The ash yield was 14.14%–43.26%, with an average value of 29.71%. The volatile matter was 23.46%–32.73%, with an average value of 27.57%. The fixed carbon was 30.5%–47.13%, with an average value of 38.41%. According to the international ASTM standard [53,54,55], the No. 6 coal seam of the Haerwusu Mine contains high-ash-yield, medium–high volatile bituminous coal.

4.2. Trace Elements

The trace element concentrations in the coals and partings are shown in Table 3. The Li content in the No. 6 coal seam ranges from 3.84 to 190.33 μg/g (average: 83.40 μg/g), of which a maximum value of 190.33 μg/g is located at the bottom of the No. 6 coal seam (Table 3). The lithium content of the No. 6 coal seam is higher than the comprehensive evaluation index of Li in Chinese coal (80 μg/g) [63]. The Li content of the No. 6 coal seam is slightly enriched compared with common Chinese coals [12] and highly enriched (Figure 3) compared to the world hard coal [64]. The Li content of the parting is 290 μg/g, which is significantly higher than that of the No. 6 coal seam. Compared with the world average clays [65], the Li content of the parting is highly enriched (Figure 4).
Except for Li, when compared with common Chinese coal, only Be and Ga are slightly enriched in terms of other trace elements; the rest of the elements are normal or depleted. Compared with the world hard coals, Be, Ga, Y, Zr, Nb, Hf, Ta, Pb, and REE are slightly enriched, and the rest of the elements are normal or depleted. Compared with the world clays, Ga, Nb, Ta, Pb, and Th in the parting are slightly enriched, and the rest of the elements are normal or depleted.

4.3. Mineralogy

The mineral contents of the coal seam and parting samples taken from the No. 6 coal seam based on LTA-XRD are shown in Table 4 and Figure 5. Among them, the mineral composition of the coal is mainly composed of clay minerals (16%–89.6%, with an average of 52.7%), followed by quartz (with an average of 22.6%) and boehmite (with an average of 14.1%), and a small amount of carbonate minerals, such as calcite (with an average of 8.0%), siderite (4.1%, HEWS-2), and ankerite (6.4%, HEWS-2). The mineral composition of the parting is dominated by clay minerals (80.3%), followed by boehmite (19.4%), with quartz being the least abundant (0.3%).
In the coals, the semiquantitative X-ray diffraction results show that the clay minerals mainly include kaolinite (Figure 5a,c,d,e), and some occur as lenticular aggregates (Figure 6b and Figure 7b). Notably, massive lamellae materials were observed by SEM-EDS (Figure 6b and Figure 7b), and the chemical elements of these lamellae materials include Si, Al, Fe, and Mg. However, these materials were not observed by optical microscope (Figure 6a, Figure 7a), and not detected by XRD, indicating the mineral crystal grains of these materials are very small or even absent; therefore, these massive lamellae materials may be cryptocrystalline or amorphous aluminosilicate materials. The distribution of minerals other than clay minerals in the No. 6 coal seam is extremely heterogeneous (Figure 5); the minerals in the HEWS-1 sample are dominated by quartz (up to 84%) (Figure 5). The HEWS-2 sample has the highest content of boehmite, up to 42.2%, which is mostly fragmentary and closely related to vein calcite and ankerite (Figure 8b). The rest of the samples are all dominated by clay minerals and boehmite (Figure 8). Carbonate minerals, mainly calcite, are found in HEWS-2, HEWS-3, and HEWS-4; only HEWS-2 contains minor amounts of siderite and ankerite.
In the parting, the semiquantitative X-ray diffraction results show that the clay minerals mainly include kaolinite (Figure 5b), and some occur as cell-filling (Figure 9a,b). Notably, some clay minerals with high interference color levels (primary yellow to secondary green) were observed under an optical microscope (Figure 9a). SEM-EDS and LA-ICP-MS further suggest that the clay minerals are possibly illite/chlorite (Figure 10a–d and Figure 12) [11,21,66]. Because the contents of illite/chlorite were below the detection limit of XRD, they were not detected by XRD.

4.4. Li Isotopes

In this study, two samples taken from the coal sample of HEWS-4 (Li content: 190 μg/g) and the parting sample of HEWS-P (Li content: 290 μg/g) with the highest Li contents in the No. 6 coal measure were selected for Li isotope analysis (Table 5). The δ7Li of the parting sample (HEWS-P) was 7.86‰, and the δ7Li of the coal sample (HEWS-4) was 3.86‰. The δ7Li values of the coal samples were more similar to the δ7Li values found in previous research on Yinshan Oldland granite [67] (Table 5).

5. Discussion

5.1. In Situ Li Microscopic Characteristics

Previous studies have indicated that Li in coals primarily occurs in clay minerals [18,21,23,66,68,69,70]. In this study, LA-ICP-MS surface scanning analysis further revealed significant differences in terms of the Li distribution between lenticular aggregate kaolinite and cryptocrystalline or amorphous lamellae aluminosilicate materials in coals. Specifically, the results show that Li is mainly found in these cryptocrystalline or amorphous lamellae aluminosilicate materials and that the Li content found in lenticular aggregate kaolinite is low. Furthermore, the occurrence of Li in the coals is also different from that of the partings. Therefore, the microscopic characteristics of Li in coals and partings are discussed separately.

5.1.1. In Situ Micro-Geochemical Characteristics of Li in Coal

Using a microscope and SEM-EDS, it was found that the lenticular aggregate kaolinite is mostly distributed in the HEWS-4 sample (Figure 6). Notably, a large number of lamellae aluminosilicate materials can also be found distributed around the lenticular aggregate kaolinite (Figure 11b), meaning it is likely that these aluminosilicate materials are detrital and they have deformed around the kaolinite as a result of compaction during diagenesis [71]. Li is mainly enriched in these lamellae aluminosilicate materials, whereas the lenticular aggregate kaolinite is almost free of Li and Ga, indicating that the lenticular aggregate kaolinite formed before the enrichment of Li and Ga into the peat bog.
Comparative analyses of the two a-zone and b-zone regions in the imaging area of cryptocrystalline or amorphous lamellae aluminosilicate materials further revealed that the distribution characteristics of Li, Fe, and Mg are generally similar (Figure 11). Via SEM-EDS (Figure 11b), it can be found that the distribution of lamellae aluminosilicate materials in the a-zone is greater, while in the b-zone, there are some organic components and the content of Li and Mg is lower, suggesting that the differences in the distribution characteristics of Li, Mg, and Fe in the b-zone may be affected by the organic components.
The Li, Al, and Si contents in cryptocrystalline or amorphous lamellae aluminosilicate materials distributed around lenticular aggregate kaolinite were higher. This is because the dissolved Li originating from the provenance debris was transported directly into the peat bog by water. Then, Li was adsorbed in an ionic form onto the ferric hydroxide and parts of the organic matter during the depositional phase. This shows that the enrichment of Li in coal may have occurred during sedimentation and diagenesis, which is closely related to the formation of cryptocrystalline or amorphous lamellae aluminosilicate materials. After the lenticular aggregate kaolinite entered the peat bog and was deposited through transportation, the Li in the water was first combined with the ferric hydroxide formed by Fe3+, precipitated, and then fixed in place. During sedimentation and diagenesis, lamellae aluminosilicate materials began to form around the lenticular aggregate kaolinite, at which time the Li-bearing particulate matter and free Li+ precipitated in the water column were adsorbed by these materials. At the same time, the large number of organic components in the coal resulted in some of the Li also being adsorbed onto the organic components.

5.1.2. In Situ Micro-Geochemical Characteristics of Li in Parting

The clay minerals in the HEWS-P sample (Figure 10) were selected after microscope and SEM-EDS analyses, and the in situ micro-geochemical characteristics of the clay minerals in the a-zone and the minerals in the b-zone were observed (Figure 12).
LA-ICP-MS imaging analysis shows that Li is mainly concentrated in illite/chlorite in the a-zone on the left side. The minerals in the b-zone, on the right, are relatively enriched with Ga and Al, and the distribution characteristics of Ga and Al are relatively similar, and it was inferred via XRD that the mineral in the b-zone is boehmite (Table 4).
By comparing the overall distribution characteristics of Li, Fe, and Mg (Figure 12), it was found that there are distributions of Li and Fe outside of the mixed-layer clay minerals, and the distribution characteristics of Li and Fe are relatively similar, while Mg is only enriched inside the mineral. In the interior of the clay mineral, the distribution characteristics of Li and Fe are similar, but Mg is mostly concentrated in the central part of the mineral. This suggests that the occurrence of Li in clay minerals is either due to adsorption onto clay surfaces or through the substitution of Fe within the crystal lattice [72].

5.2. Li Isotopes

Lithium has two stable isotopes, 6Li and 7Li, with abundances of 7.52% and 92.48%, respectively [73]. The significant relative mass difference between these two isotopes, amounting to 16.7%, can lead to a pronounced fractionation of Li isotopes. In addition, Li is moderately incompatible during partial melting processes and strongly fluid-active during fluid-activity-related processes [74]; Li isotopes are significantly fractionated in nature [35,36,74,75,76,77,78,79,80,81].
The Li⁺ (0.057 nm) ion has a similar ionic radius to Mg2+ (0.059 nm) [82], and in addition to Mg2+, the ionic radius of the Fe2+ (0.061 nm) ion is also closer to that of the Li ion [83]. This similarity facilitates the occurrence of isomorphous substitution between Li⁺ and Mg²⁺ or Fe²⁺ during the weathering processes of silicate minerals. Furthermore, due to the adsorption of cations by some clay minerals, Li may occur due to both adsorption, such as in the case of kaolinite and also via ionic substitution, such as in the case of chlorite [37,82].
During geological processes related to fluid activity (e.g., weathering, hydrothermal activity, and seafloor alteration), 7Li is more abundant in low-coordinated fluid phases, while 6Li is more predominant in solid phases [84,85,86,87]. During silicate weathering, 6Li in the fluid preferentially binds to secondary minerals, leading to higher δ7Li values in the fluid (Figure 13). Therefore, the variation in δ7Li values in fluids is generally related to the intensity of silicate weathering [37,38,88]. Thermodynamic calculations and molecular dynamics simulations of the isotopic fractionation of Li+ for different binding sites in clay minerals revealed that the isotopic fractionation of Li located in interlayer exchange sites and aqueous solution in monolayer-structured clay minerals during the dissolution of the clay minerals is negligible (Δ7Limonolayer-aq =−2.0~0.0‰); however, multilayer-structured clay minerals in which Li is located in structural sites preferentially export 6Li to an aqueous solution (Δ7Liststructural-aq~−20.0‰) [88,89].
δ7Li values can also vary considerably depending on the weathering intensity and transportation distance. In the downstream (floodplain), where the weathering intensity is high and the annual precipitation is high, the Li residence time in the water body is long, and more secondary minerals can be formed, resulting in more 6Li being introduced into the secondary minerals, leading to higher δ7Li in the downstream water body. In contrast, in orogenic zones (mountainous areas) with lower weathering, the residence time of Li in water is short, and the secondary minerals are less developed; thus, the δ7Li value in the downstream river will decrease (Figure 13a) [38,88].
Since peat bogs as a whole are hydrostatic depositional systems, and the internal water has less exchange with the outside, the input of water (including rivers and other water bodies) and terrestrial debris into peat bogs may be an important factor affecting the Li isotopes of the interior. Based on the aforementioned established theories, the Li isotope fractionation of the coal sample (HEWS-4) and parting (HEWS-P) from the No. 6 coal seam was studied in this work.
Lenticular aggregate kaolinite in the coal seams may originate from feldspar, quartz, or clay minerals resulting from the in situ weathering of Yinshan Oldland granite, eventually entering the peat bog via water transportation. Due to the short transportation time, kaolinite can be better preserved during transportation. The large amount of lenticular aggregate kaolinite in HEWS-4 indicated that the Li content of the terrestrial debris input into the peat bog at this time was relatively low; the kaolinite does not contain Mg and Fe that can isomorphically exchange with Li. It is speculated that Li in the water at this time may mostly enter the peat bogs in the state of ionic adsorption. However, previous studies have shown that the Li isotope fractionation between the monolayer of clay minerals and the aqueous solution is negligible (Δ7Limonolayer-aq = −2.0~0.0‰). Therefore, the Li isotope (δ7Li = 3.86‰) of coal in the coal-forming process is mainly inherited from the provenance and is affected by the influence of water during weathering and transportation, showing a similar Li isotope value to provenance (δ7Li = 3.87‰).
The δ7Li value in the parting (HEWS-P, δ7Li = 7.86‰) is significantly higher than those in the coal and provenance. In addition to the terrestrial debris supply, the LA-ICP-MS imaging results show that the distributions of Li and Fe are generally consistent (Figure 12), indicating that the higher δ7Li value in the parting may be related to these clay minerals (illite/chlorite) containing Fe. The crystallization and deposition of secondary clay minerals during the transportation process caused the water to start losing 6Li [36,88], thus leading to higher δ7Li values in the water input into the peat bog, leading to the higher δ7Li value in the parting. In addition, after the sedimentation and diagenesis stages, the fluid in the coal seam diffused under the influence of compaction, and Li in the residual fluid migrated to the parting in the vicinity of the coal. Due to the continuous addition of secondary clay minerals in the processes of sedimentation and diagenesis, the δ7Li value in the parting increased further (Figure 14) when the residual fluid entered the parting under the action of compaction.

5.3. Lithium Enrichment Mechanism of the No. 6 Li-Rich Coals and Parting

By comparing the micro-geochemistry of Li and Li isotopes between the coal (HEWS-4) and parting (HEWS-P) samples, it was found that Li experienced different enrichment processes between the coals and partings in the Haerwusu Mine during the formation of the coal measures.
In the early stage of the No. 6 coal measure formation (corresponding to HEWS-4), the weathering degree of the source rocks was low, and the clastic material derived from the in situ weathering of granite in the Yinshan Oldland was carried and transported by water and then deposited in the peat bog (Figure 15a). In this stage, because the terrestrial debris was formed by the weathering of Yinshan Oldland granite, the transportation process was relatively short. This resulted in the better preservation of a large number of clay mineral agglomerates, while very few secondary clay minerals were formed. Therefore, the isotopic fractionation of Li in the provenance debris during water transportation is limited, enabling a much greater inheritance of Li isotopes from Yinshan Oldland granite to the coal seam. Namely, the dissolved Li originating from the provenance debris was transported directly into the peat bog by water. Then, Li was adsorbed in an ionic form onto the ferric hydroxide formed by Fe3+ and parts of the organic matter during the depositional phase (Figure 15b). Subsequently, during the deposition and diagenesis stages, the cryptocrystalline or amorphous lamellae aluminosilicate materials led to the adsorption of Li (Figure 15c), initiating the enrichment of Li in the coal. The residual fluids from the overlying coal seams migrated to the underlying coal layers under the influence of compaction, potentially resulting in the secondary enrichment of Li. This process contributed to the higher Li content observed in the bottom coal seam (HEWS-4) compared to the overlying coal seams (HEWS-2, HEWS-3).
In the middle stage of the No. 6 coal measure formation (corresponding to HEWS-2 and HEWS-3), fluid diffusion during sedimentation and diagenesis resulted in the occurrence of fracture-filling calcite and ankerite veins and other carbonate minerals in the coal seam. Concurrently, the presence of a large amount of detrital boehmite indicates that the provenance debris input into the peat bog gradually shifted from debris derived from the weathering of granite to that originating from the bauxite of the Benxi Formation.
In the later stages of the No. 6 coal measure formation (corresponding to HEWS-P), due to crustal uplift, the source rocks were exposed to the surface and subjected to continuous weathering and erosion, and water transported their debris into the peat bog (Figure 16a), resulting in increased Li being carried by the water body. At this time, due to intense weathering, a large amount of secondary clay minerals formed from the weathering of source rocks. During transportation, dissolved Li in the water combined with secondary clay minerals (Figure 16a,b), which led to the 6Li in the water fixing to the river under the aggregation and sedimentation of secondary clay minerals, and very few secondary clay minerals entered the peat bog. Consequently, this process resulted in an increased δ7Li content of the water that flowed into the peat bog. When the water entered the peat bog, Li in the water physically adsorbed with Fe3+ formed ferric hydroxide and was retained in situ (Figure 16c) [90,91]; then, it was adsorbed as ions on the surface of the secondary clay minerals or organic matter or in a form analogous to homogeneity in the lattice of secondary clay minerals during the stage of sedimentary diagenesis (Figure 16d). Under compaction, the fluid in the surrounding coal measure gradually diffused into the parting, and the secondary enrichment of Li potentially occurred (Figure 14). The presence of volcanic clastic quartz in HEWS-1 indicated that island arc volcanism, occurring during the closure of the Paleo-Asian Ocean north of the North China Craton [92,93], supplied volcanic ash clasts to the peat bog. The Li brought into the peat bog by the volcanic ash clasts may have been transported to the parting via leaching during compaction or metamorphism. Thus, this facilitated the further enrichment of Li in the parting, leading to a decreased Li content in HEWS-1.

6. Conclusions

  • By comparing the in situ micro-geochemistry of different elements in HEWS-P (parting) and HEWS-4 (coal), it was found that Li in the No. 6 coal seam is mainly found in the cryptocrystalline or amorphous lamellae aluminosilicate materials, and the lenticular aggregate kaolinite is almost free of Li, suggesting that the lenticular aggregate kaolinite formed before the Li element was enriched and that it might have been formed during transportation.
  • In the parting, the microscale distribution of Li is similar to that of Fe and Mg. Comparing the in situ micro-geochemical characteristics of Li, Mg, and Fe, it is assumed that the occurrence of Li in clay minerals is either due to adsorption onto clay surfaces or through the substitution of Fe/Mg within the crystal lattice.
  • The δ7Li value of the Li-rich coal (HEWS-1) in the No. 6 coal is 3.86‰, and the δ7Li value of the parting (HEWS-P) is 7.86‰. The δ7Li value in the parting is obviously higher than that in the bottom coal seam and Yinshan Oldland granite. When compared with the Li isotope of Yinshan Oldland granite, it was found that the Li isotope ratio in the bottom coal seam is similar to that in Yinshan Oldland granite.
  • The microscopic characteristics of Li in two different Li-rich samples taken from the No. 6 coal seam in the Haerwusu Mine were compared, and the provenance analysis, sedimentary environment evolution, and Li isotope fractionation mechanisms of the No. 6 coal seam were combined. It was found that the enrichment of Li in the coal mainly occurred during sedimentation and diagenesis and is related to the introduction of terrestrial debris and water into the peat bog. Late compaction might also lead to the secondary enrichment of Li in the coal seam and parting.

Author Contributions

Data curation, G.Q. and J.W.; formal analysis, J.W., Y.W. and G.Q.; methodology, J.W., X.L., Y.Z. and G.Q.; resources, J.W., G.Q. and Y.W.; supervision, D.C. and Y.W.; writing—original draft, J.W., Y.W. and G.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by National Natural Science Foundation of China (42372187 and 42202197), Science Research Project of Hebei Education Department (BJK2024073), the Special Project for Geological Development of Ningxia in 2023 (640000233000000011005), and the funding project of the Northeast Geological S&T Innovation Center of China Geological Survey (No. QCJJ2023-02).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data available on request due to restrictions.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geographical location and sampling location of the Haerwusu Mine: (a) Jungar Coalfield; (b) Haerwusu open pit mine (modified by [52]); (c) location of the sampling point of this study.
Figure 1. Geographical location and sampling location of the Haerwusu Mine: (a) Jungar Coalfield; (b) Haerwusu open pit mine (modified by [52]); (c) location of the sampling point of this study.
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Figure 2. Column chart of main coal-bearing strata in the Haerwusu Mine.
Figure 2. Column chart of main coal-bearing strata in the Haerwusu Mine.
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Figure 3. Concentration coefficient of trace elements in the No. 6 coal seam (a) compared with common Chinese coal; (b) compared with the world hard coal.
Figure 3. Concentration coefficient of trace elements in the No. 6 coal seam (a) compared with common Chinese coal; (b) compared with the world hard coal.
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Figure 4. Concentration coefficient of trace elements in the No. 6 coal seam (compared with world clays).
Figure 4. Concentration coefficient of trace elements in the No. 6 coal seam (compared with world clays).
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Figure 5. XRD pattern of the No. 6 coal seam: (a) HEWS-1; (b) HEWS-P; (c) HEWS-2, (d) HEWS-3; (e) HEWS-4.
Figure 5. XRD pattern of the No. 6 coal seam: (a) HEWS-1; (b) HEWS-P; (c) HEWS-2, (d) HEWS-3; (e) HEWS-4.
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Figure 6. Minerals in HEWS-4: (a) optical microscope; (b) backscatter image; (c,d) EDS spectra and element content (Kao: kaolinite; CK: lenticular aggregate kaolinite; BK: cryptocrystalline or amorphous lamellae aluminosilicate materials).
Figure 6. Minerals in HEWS-4: (a) optical microscope; (b) backscatter image; (c,d) EDS spectra and element content (Kao: kaolinite; CK: lenticular aggregate kaolinite; BK: cryptocrystalline or amorphous lamellae aluminosilicate materials).
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Figure 7. Kaolinite in HEWS-4: (a) optical microscope; (b) backscatter image; (c,d) EDS spectra and element content (Kao: kaolinite; CK: lenticular aggregate kaolinite; BK: cryptocrystalline or amorphous lamellae aluminosilicate materials).
Figure 7. Kaolinite in HEWS-4: (a) optical microscope; (b) backscatter image; (c,d) EDS spectra and element content (Kao: kaolinite; CK: lenticular aggregate kaolinite; BK: cryptocrystalline or amorphous lamellae aluminosilicate materials).
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Figure 8. Boehmite, calcite, and ankerite in HEWS: (a) optical microscope; (b) backscatter image; (c–e) EDS spectra and element content (Boe: boehmite; Cal: calcite; Ank: ankerite).
Figure 8. Boehmite, calcite, and ankerite in HEWS: (a) optical microscope; (b) backscatter image; (c–e) EDS spectra and element content (Boe: boehmite; Cal: calcite; Ank: ankerite).
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Figure 9. Kaolinite in HEWS-P: (a,b) backscatter image; (c) EDS spectra and element content (Kao: kaolinite).
Figure 9. Kaolinite in HEWS-P: (a,b) backscatter image; (c) EDS spectra and element content (Kao: kaolinite).
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Figure 10. Clay minerals in HEWS-P: (a) optical microscope; (b) backscatter image; (c,d) EDS spectra and element content (CM: clay mineral).
Figure 10. Clay minerals in HEWS-P: (a) optical microscope; (b) backscatter image; (c,d) EDS spectra and element content (CM: clay mineral).
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Figure 11. HEWS-4 LA-ICP-MS imaging results: (a) optical microscope; (b) backscatter image (yellow box line is the imaging area; the ordinate is μg/g). The a,b in the write circle represent a-zone and b-zone regions in the imaging area of cryptocrystalline or amorphous lamellae aluminosilicate materials from (b).
Figure 11. HEWS-4 LA-ICP-MS imaging results: (a) optical microscope; (b) backscatter image (yellow box line is the imaging area; the ordinate is μg/g). The a,b in the write circle represent a-zone and b-zone regions in the imaging area of cryptocrystalline or amorphous lamellae aluminosilicate materials from (b).
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Figure 12. HEWS-P LA-ICP-MS imaging results: (a) optical microscopy image; (b) backscatter image (yellow box line is the imaging area, the ordinate is μg/g). The a-zone and b-zone regions (in the write circle) represent illite/chlorite and boehmite, respectively.
Figure 12. HEWS-P LA-ICP-MS imaging results: (a) optical microscopy image; (b) backscatter image (yellow box line is the imaging area, the ordinate is μg/g). The a-zone and b-zone regions (in the write circle) represent illite/chlorite and boehmite, respectively.
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Figure 13. Li isotope behavior during weathering: (a) fractionation of Li entering the water under the action of weathering; (b) fractionation with the formation of secondary clay minerals in the case of transportation.
Figure 13. Li isotope behavior during weathering: (a) fractionation of Li entering the water under the action of weathering; (b) fractionation with the formation of secondary clay minerals in the case of transportation.
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Figure 14. The fractionation behavior of Li isotopes in coal measures during the compaction stage.
Figure 14. The fractionation behavior of Li isotopes in coal measures during the compaction stage.
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Figure 15. Li enrichment metallogenic model of the coal seam: (a) early formation of the No. 6 coal seam (HEWS-4); (b) Li exists in organic matter and ferric hydroxide colloid in an ionic adsorbed state; (c) formation of secondary clay minerals.
Figure 15. Li enrichment metallogenic model of the coal seam: (a) early formation of the No. 6 coal seam (HEWS-4); (b) Li exists in organic matter and ferric hydroxide colloid in an ionic adsorbed state; (c) formation of secondary clay minerals.
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Figure 16. Li enrichment metallogenic model of the parting: (a) parting formation (HEWS-P); (b) secondary clay minerals are formed during transportation; (c) Li exists in organic matter and ferric hydroxide colloid in an ionic adsorbed state; (d) formation of secondary clay minerals.
Figure 16. Li enrichment metallogenic model of the parting: (a) parting formation (HEWS-P); (b) secondary clay minerals are formed during transportation; (c) Li exists in organic matter and ferric hydroxide colloid in an ionic adsorbed state; (d) formation of secondary clay minerals.
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Table 1. Parameter selection of LA-ICP-MS mapping analysis for HEWS-P and HEWS-4.
Table 1. Parameter selection of LA-ICP-MS mapping analysis for HEWS-P and HEWS-4.
SampleArea/μmSpeed/μm/sLaser Size/μmFrequency/Hz
HEWS-P462 × 524510 × 1010
HEWS-4274 × 53846 × 1010
Table 2. Approximate analysis results of coal samples and parting samples of the No. 6 coal seam (wt%).
Table 2. Approximate analysis results of coal samples and parting samples of the No. 6 coal seam (wt%).
SampleMadAadVadFCad
HEWS-15.0131.7328.3934.88
HEWS-P1.8372.8416.648.70
HEWS-26.0114.1432.7347.13
HEWS-33.4229.7325.7041.16
HEWS-42.7943.2623.4630.50
Average4.3129.7127.5738.41
Table 3. Trace element content of the No. 6 coal seam (μg/g, coal-based).
Table 3. Trace element content of the No. 6 coal seam (μg/g, coal-based).
SampleLiBeBScVCrCoNiCuZnGaRbSr
HEWS-13.8 11 20 5.5 12 36 8.4 9.9 19 4.6 9.9 1.7 19
HEWS-P290 1.5 23 6.2 23 8.9 1.1 4.4 11 14.7 48 14 55
HEWS-231 1.1 243.7 9.3 4.3 1.3 1.5 8.8 2.9 13 1.8 175
HEWS-3108 2.1 16 8.6 24 5.9 1.7 3.3 11 6.1 24 2.2 146
HEWS-4190 2.5 23 8.4 21 8.0 1.9 3.9 11 7.9 28 2.9 360
Average83 4.2 216.5 17 13 3.3 4.6 13 5.4 19 2.2 175
Common Chinese Coal32 2.1 53 4.4 35 15 7.1 14 18 41 6.6 9.3 140
World Hard Coals12 1.6 52 3.9 25 16 5.1 13 16 23 5.8 14 110
World Clays54 3 110 15 120 110 19 49 36 89 16 133 240
SampleYZrNbSnCsBaHfTaTlPbThUREE
HEWS-121 56 2.9 1.08 0.10 72 1.6 0.18 0.06 13 2.8 1.9 108
HEWS-P19 247 41 5.30.77 42 9.2 3.02 0.13 33 285.4 95
HEWS-211 91 5.4 0.79 0.23 18 2.4 0.26 0.17 16 3.7 2.0147
HEWS-326 233 8.8 1.9 0.17 12 5.8 0.74 0.06 29 14 3.0 199
HEWS-425 281 22 4 0.29 29 8.2 1.40 0.07 3221 4.6 157
Average21 1669.7 1.9 0.20 33 4.5 0.65 0.09 23 10 2.9 153
Common Chinese Coal18 90 9.4 2.1 1.13 159 3.7 0.62 0.47 15 5.8 2.4 136
World Hard Coals8.4 36 3.7 1.1 1 150 1.2 0.28 0.63 7.8 3.3 2.4 68
World Clays31 190 11 3.5 13 460 5 1.4 1.3 14 14 4.3 226
Table 4. LTA-XRD analysis results of coals and parting in No. 6 coal.
Table 4. LTA-XRD analysis results of coals and parting in No. 6 coal.
SampleMineral Content (%)
QuartzCalciteBoehmiteSideriteAnkeriteClay Minerals
KaoItSCI/S
HEWS-184.0////16.0\\\\
HEWS-P0.3/19.4//80.3\\\\
HEWS-20.217.242.24.16.429.9\\\\
HEWS-31.713.010.1//75.2\\\\
HEWS-44.41.94.1//89.6\\\\
Average in coal22.68.0314.11.021.664.9\\\\
Kao: kaolinite; It: illite; S: smectites; C: chlorite; I/S: Aemon mixed layer; /: not detected.
Table 5. δ7Li in HEWS-P, HEWS-4, and Yinshan Oldland granite (-: no data; a: [68]).
Table 5. δ7Li in HEWS-P, HEWS-4, and Yinshan Oldland granite (-: no data; a: [68]).
SampleLi Content (μg/g)δ7Li (‰)2SDn
HEWS-P290.327.860.343
HEWS-4190.333.860.283
Granite-1 a10.13.87--
Granite-2 a12.63.21--
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Qin, G.; Wei, J.; Wei, Y.; Cao, D.; Li, X.; Zhang, Y. The Differences in the Li Enrichment Mechanism between the No. 6 Li-Rich Coals and Parting in Haerwusu Mine, Ordos Basin: Evidenced Using In Situ Li Microscale Characteristics and Li Isotopes. Minerals 2024, 14, 836. https://doi.org/10.3390/min14080836

AMA Style

Qin G, Wei J, Wei Y, Cao D, Li X, Zhang Y. The Differences in the Li Enrichment Mechanism between the No. 6 Li-Rich Coals and Parting in Haerwusu Mine, Ordos Basin: Evidenced Using In Situ Li Microscale Characteristics and Li Isotopes. Minerals. 2024; 14(8):836. https://doi.org/10.3390/min14080836

Chicago/Turabian Style

Qin, Guohong, Jinhao Wei, Yingchun Wei, Daiyong Cao, Xin Li, and Yun Zhang. 2024. "The Differences in the Li Enrichment Mechanism between the No. 6 Li-Rich Coals and Parting in Haerwusu Mine, Ordos Basin: Evidenced Using In Situ Li Microscale Characteristics and Li Isotopes" Minerals 14, no. 8: 836. https://doi.org/10.3390/min14080836

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