The concentrations of major element oxides and trace elements of the Nos. 3 and 5 coals from the Toushi open-pit mine are shown in Table 1 . The major element oxides of the No. 3 coal are dominantly SiO₂ and Al₂O₃, ranging from 11.9% to 28.7% (avg. 21.5%, on a whole coal basis) and 8.1% to 16.0% (avg. 12.9%), respectively ( Table 1 ). The major element oxides of the No. 5 coal are also primarily SiO₂ (2.2-26.3%, avg. 14.8%) and Al₂O₃ (2.0-14.6%, avg. 8.6%). Compared to the average values of common Chinese coals reported by Dai et al. (2012b), the Nos. 3 and No. 5 coals both have relatively higher concentrations of SiO₂, Al₂O₃, K₂O, MgO and TiO₂, and the No. 5 coal also has relatively higher CaO content. The roof, parting, and floor samples of the Nos. 3 and 5 coals are dominantly composed of SiO₂ (avg. 53.9% and 56.0%, respectively) and Al₂O₃ (avg. 22.1% and 18.6%, respectively), followed by minor proportion of K₂O, Fe₂O₃, TiO₂, MgO, CaO, Na₂O, P₂O₅, and MnO (on a whole rock basis; Table 1 ).

The SiO₂/Al₂O₃ ratios of the Nos. 3 and 5 coals respectively range from 1.48 to 1.80 (avg. 1.65), and 1.10 to 2.08 (avg. 1.58), slightly higher than those of average Chinese coals (1.42; Dai et al., 2012b) and the theoretical value of kaolinite (1.18), tobelite (1.35), and similar to that of illite (1.57). This indicates that the major host of Si and Al is kaolinite, illite, and tobelite, which are the primary minerals in coal, whereas quartz is an additional host of Si in some coal samples. The elevated CaO in some coal samples of the No. 5 coal seam reflects the high proportion of carbonate minerals in the studied samples (e.g., calcite, Table A2 ).

Concentration coefficient (CC, ratio of the average elemental concentration in investigated coals vs. that in common world hard coals), proposed by Dai et al. (2015), is widely used to evaluate the enrichment degree of trace elements in coal (Dai et al., 2015). Compared with the common world hard coals (Ketris and Yudovich, 2009), the Nos. 3 and 5 coals in the Toushi mine are significantly enriched in Li ( Figure 5 ), with CC up to 17.8 and 13.4, and average Li concentration as high as 214 μg/g and 160 μg/g (on a whole-coal basis), respectively. In addition, the No. 3 coal is also enriched in V (CC = 5.6), Cr (CC = 5.6), Sm (CC = 5.4), and Th (CC = 5.1), and the No. 5 coal is enriched in Pb (CC = 5.5), Sm (CC = 5.2), and Co (CC =5.0). Manganese is depleted in the Nos. 3 and 5 coals (CC < 0.5; Figure 5 ), and Zn, Rb, Sr, Ba and Lu in the Nos. 3 and 5 coals are relatively close to the average values of world hard coals (0.5 < CC < 2; Figure 5 ). The remaining elements are slightly enriched in the Nos. 3 and 5 coals in the Toushi mine (2<CC< 5).

The concentration of total rare earth elements and Y (ΣREY) in the Nos. 3 and 5 coals ranges from 112 μg/g to 459 μg/g, and 41 μg/g to 620 μg/g, with an average of 276 μg/g and 241 μg/g, respectively, which is 3–4 times higher than the average (68.6 μg/g) for common worldwide coals (Ketris and Yudovich, 2009). With respect to non-coal parting, roof, and floor samples, the REY concentration in roof and floor samples from the No. 3 coal are 276 μg/g and 269 μg/g, respectively, similar to that in No. 3 coal. The REY concentration in roof sample (105 μg/g) from the No. 5 coal is relatively lower than that in No. 5 coal, and those in the parting and floor samples are slightly higher than the average in No. 5 coal ( Table 2 ).

Normalized to REY concentration in the upper continental crust (UCC, Taylor and McLennan, 1985), REY is correspondingly classified to light REY (L-type, LREY; La_N/Lu_N > 1), medium REY (M-type, MREY; La_N/Sm_N < 1 and Gd_N/Lu_N > 1), or heavy REY (H-type, HREY; La_N/Lu_N < 1) enrichment pattern (Seredin and Dai, 2012). According to this classification, the REY enrichment patterns for both Nos. 3 and 5 coal samples are predominantly of L-M type with the exception of TS-5-5 (L type, Table 2 ). Unlike in the Nos. 3 and 5 coals, the REY enrichment patterns in the non-coal roof and floor samples are primarily of M-H-type and H-type, but the parting of the No. 5 coal (TS-5-7) is characterized by L-type enrichment pattern ( Table 2 ).

As pointed out by Dai et al. (2016) and Yan et al. (2018), Eu concentrations can be interfered by high concentrations of Ba (Ba/Eu ratio >1000) during analysis using a quadrupole-based ICP-MS, for example, interference by ¹³⁵Ba¹⁶O and ¹³⁴Ba¹⁶OH to ¹⁵¹Eu, and ¹³⁷Ba¹⁶O and ¹³⁶Ba¹⁶OH to ¹⁵³Eu. Nonetheless, the Ba/Eu ratio in the samples investigated in this study are less than 1000, indicating that there is no interference of Ba concentration to Eu concentration ( Figure 6d ), consequently, the Eu concentrations determined are correct. The average δEu values of the Nos. 3 and 5 coal are 0.54 and 0.50, respectively ( Table 2 ), both showing a remarkable negative anomaly. The roof and floor samples from No. 3 coal seam show weak positive Eu anomaly, and the roof samples from No. 5 coal seam show no obvious Eu anomaly. With respect to Gd anomaly, the roof, floor, and parting samples from Nos. 3 and 5 coal seams present no obvious or weak negative Gd anomalies, while the Nos. 3 and 5 coal show remarkable Gd anomalies ( Table 2 ). With respect to Ce anomaly, the coal and non-coal rock samples all present weak to obvious negative Ce anomalies.

Elements, such as Al, Ti, Zr, Nb, and REY, are relatively immobile during the weathering, migration, hydrothermal alteration, deposition, diagenesis and/or volcanic process (Hayashi et al., 1997), which makes them effective indicators of the initial provenance (terrigenous clast or volcanic ash) of inorganic matter in coals, tonsteins and other sedimentary rocks (Dai et al., 2014b, 2015, 2017a; Zhao et al., 2019a; Liu et al., 2020; Zhang et al., 2022; Di et al., 2023; among others). For instance, the Al₂O_3/TiO₂ ratios of 3-8, 8-21, and 21-70, represent igneous rocks of mafic, intermediate, and felsic compositions, respectively (Hayashi et al., 1997). As illustrated in Figure 9a , the Al₂O₃/TiO₂ ratios of the coal and non-coal rock samples from the Nos. 3 and 5 coal seams are predominantly higher than 21, indicating that the Nos. 3 and 5 coals most probably have the same sediment source region characterized by felsic-intermediate compositions. In addition, the diagram of Zr/TiO₂ vs. Nb/Y has also been used to discriminate the sediment source of coal and coal-bearing sequences (Liu et al., 2020; Spears, 2012; Zhao et al., 2019a, b), according to which, the sediment provenance of the Toushi coals is of also felsic-intermediate composition ( Figure 9b ).

The Eu anomalies in coals are commonly inherited from their original inorganic source material (Eskenazy, 1987a, b) and/or attributed to high temperatures coupled with highly reduced environments (Dai et al., 2014a; Dai et al., 2016; Dai et al., 2018a). Specifically speaking, negative Eu anomaly in coals is generally derived from intermediate-felsic source rocks, while positive Eu anomaly is probably ascribed to the influence of mafic igneous rocks or high temperature fluids (Dai et al., 2016). The Nos. 3 and 5 coal are both characterized by remarkable negative Eu anomalies, representing a felsic-intermediate sediment provenance as well.

According to He (2008), the sea regression and deposition of terrigenous debris started since the early stage of the Ceshui Formation in the Lianshao and other coalfields in the central Hunan Region. The Ceshui Formation coal-bearing strata adjacent to the oldland in NE Hunan are characterized by coarse lithology with multi-layer conglomerate and a large total thickness. Furthermore, the lithology and thickness of the coal measures both become thinner, and the composition maturity, structure maturity and cement structure maturity of clastic minerals in coarser sandstones all become higher from the northeast to the southwest. This indicates that the main sediment source area is located in the northeast while the sea regressed towards the southwest direction. It is concluded that the source rock of the Ceshui Formation coal-bearing sequence was probably the Caledonian granite according to the analyses of isotopic dating and geochemical index such as Co/Ni ratio (He, 2008). Based on the contour maps of the sand-mud ratio, stratigraphic thickness, mudstone thickness percentage and sandstone thickness percentage of the Ceshui Formation coal-bearing sequence in the Lianshao Coalfield, it is further indicated that the terrigenous detrital supply of the Ceshui Formation in the Lianshao Coalfield is mainly sourced from the granite in the Xuefeng Oldland in the northeast of the study area ( Figure 1 , Yang, 2012).

The enrichment of lithium is enhanced by lower degrees of sub-arc mantle melting and higher extents of intracrustal differentiation (Chen et al., 2020). Arc magmas that traverse thick crust begin with higher initial Li contents due to lower extents of decompression melting and then evolve to higher Li contents through more extensive intracrustal differentiation. Magmatism and differentiation in continental arcs (e.g., the Andes) usually gives rise to higher lithium content than their island arc counterparts (Chen et al., 2020). Weathering of these enriched source rocks mobilizes and transports such Li into the hydrologic system, ultimately developing Li enrichment in landlocked extensional basins in thickened orogenic settings (Chen et al., 2020). He (2008) reported that the provenance area of Lianshao coalfield has suffered from intensive chemical weathering during the Permo-Carboniferous coal-forming period, and the enrichment of lithium in the Ceshui Formation coal measures was largely ascribed to the weathering products of Li-enriched granite source rocks.

The chemical index of alteration (CIA=Al₂O₃/(Al₂O₃+CaO*+Na₂O+K₂O)×100%, CaO*=CaO-(10/3×P₂O₅)) has been widely used to quantitatively reflect the intensity of chemical weathering, with CIA values of 80-100, 70–80 and 60–70 represent intense weathering under hot and humid tropical climatic conditions, moderate weathering under warm and humid climatic conditions, and weak weathering under cold and dry climatic conditions, respectively (Nesbitt and Young, 1982). However, due to Ca, Na and K are sensitive to leaching during syngenetic and diagenetic process (Dai et al., 2020), the CIA value may not accurately reflect the degree of chemical weathering, in which cases the use of chemical index of weathering (CIW=Al₂O₃/(Al₂O₃+CaO*+Na₂O)×100%) can circumvent this problem. If the variation trend of CIW is consistent with that of CIA, it can indicate the accuracy of the CIA (Harnois, 1988). The CIA values of the non-coal rocks in the Toushi mine ranges from 85.3 to 92.9 (avg. 90.9), is consistent with the variation trends of CIW (88.4 to 99.0, avg. 97.3), indicating the reliability of CIA for judging the chemical weathering intensity. Therefore, the CIA and CIW values of the no-coal rocks in the studied area further confirm the intense chemical weathering of granite source rocks during the coal-forming period. The study area is located in fold orogenic belt with complex tectonic movement, concomitant with the generation of more felsic magmas and development of high orogenic uplands (Yang, 2012; Chen et al., 2020). Weathering and leaching of the Li-enriched granitic source rocks from the uplands by surface runoff or groundwater flow transports soluble Li into the peat swamp of confined basins, where it is further concentrated and enriched (Rudnick et al., 2004; Tomascak, 2004; Henchiri et al., 2014; Chen et al., 2020).

Infiltration of seawater and its interaction with Li-bearing terrestrial waters has contributed to Li enrichment in coals from several coalfields (Dai et al., 2018b; Xie et al., 2021). The Sr/Ba ratio is regarded as an empirical indicator of paleosalinity, and is widely utilized as a indicator of fresh water or seawater influence in sedimentary environments (Dai et al., 2020; Tribovillard et al., 2006; Zhang et al., 2018). Sr/Ba ratio of 1.0 is used as the boundary for marine/freshwater deposition, with Sr/Ba>1 for seawater deposition, Sr/Ba<1 for freshwater deposition, and 0.6<Sr/Ba<1 for brackish water deposition (Chen et al., 2015b; Dai et al., 2013b, 2013a; Spiro et al., 2019; Yan et al., 2019). In the current research, the Sr/Ba ratios of the Toushi coals fluctuatingly range from 0.5 to 6.2 (avg. 1.8), indicating that the coal seams were intermittently influenced by seawater. The marine influence on the Toushi coals is further indicated by the relatively high content of illite in coals, as infiltration of seawater would favor the transformation of kaolinite into illite and/or montmorillonite (Wei et al., 2020; Zhao et al., 2019b; Zheng et al., 2017). This is consistent with previous research that the Ceshui Formation coals in the Lianshao Coalfield was predominantly formed in continental and marine transitional sedimentary environment, e.g., bay lagoon and tidal flat, where the peat swamp was mildly marine influenced (Wang et al., 1995; Yang, 2012). It is worth noting that coals formed in seawater-influenced sedimentary environment were usually characterized by high−sulfur content, as seawater sulfate is more available in seawater-influenced environment (Ward et al., 2007; Chou, 2012). However, although formed in marine-influenced environment, the Toushi coals are characterized by low sulfur content (avg. 0.72% and 0.77% in Nos. 3 and 5 coal), which is ascribed to the seawater desalination. The Lianshao coalfield was formed in restricted epicontinental-marine environment, where the seawater was desalinated by the heavy rainfall due to the warm and humid climate during the coal formation period (Wang et al., 1995; Yang et al., 1996). In addition, the Ce anomalies of the Toushi coals range from 0.64 to 0.97 (avg. 0.85), and the Th/U ratios from 2.3 to 6.0 (avg. 3.8), both reflecting a relatively anoxic condition in the marine-influenced environment (Wright et al., 1987; Chen et al., 2015a).

As the Sr/Ba ratio is regarded as an empirical of paleosalinity, which is supposed to increase with salinity (Meng et al., 2012; Tao et al., 2017), the vertical variation of Li concentration along with the fluctuation of ash yields and Sr/Ba ratios reflects that Li is highly enriched in coal sections that was formed during slow sea regressive process ( Figure 7 ). This is consistent with previous research that the infiltration of seawater and its interaction with Li-bearing terrestrial materials is favorable for Li enrichment in coals that were formed in the marine-influenced sedimentary environment, especially under the sea regressive background (Dai et al., 2018b; Nechaev et al., 2020; Xie et al., 2021).

Magmatic activities frequently occurred in the Lianshao coalfield, among which the Caledonian, Indosinian, and Yanshanian are three predominant large-scale ones. These magmatic activities are characterized by multicyclic granite intrusion widely distributed in the northeast, east and southern margin of the coalfield (Yang, 2012).

Igneous intrusion is not uncommon in coal beds and coal-bearing sequences. The high-temperature hydrothermal solutions accompanied with the granite intrusion resulted in regional magmatic metamorphism, and subsequently give rise to the high coal rank not only of the Lianshao coals present in this study, but also of coals from other areas (e.g., Dai and Ren, 2007; Finkelman et al., 1998; Sanders et al., 2023; Stewart et al., 2005; Wang et al., 2024a, b; Wang et al., 2023, 2024c). Hydrothermal influences are generally indirectly reflected by several geochemical indicators. e.g., positive Gd anomaly, and Eu anomaly (Ward et al., 1996; Dai et al., 2017a, 2016; Shand et al., 2005). In the current research, almost all the UCC-normalized REY patterns of the studied coal samples show positive Gd anomalies ( Table 2 , Figure 10 ), was probably caused by circulation of hydrothermal solutions or acidic waters in the coal-bearing basins (Dai et al., 2016; Di et al., 2023; Johannesson and Zhou, 1997). Furthermore, the influence of hydrothermal solution is also evidenced by the presence of tobelite in the Toushi coals, which may reflect the decomposition of organic nitrogen in the coal during rank advance or hydrothermal alteration, and associated release of aqueous ammonium ions (NH₄ ⁺) into the pore fluids (Daniels and Altaner, 1993; Ward and Christie, 1994; Nieto, 2002; Dai et al., 2012c). The tobelite in coals has been proved to be formed by reaction between kaolinite already present in the coal and NH₄ ⁺ derived from decomposition of the organic matter during hydrothermal alteration at a relatively high temperature (Boudou et al., 2008; Dai et al., 2012c, 2017). Furthermore, the occurrence of fracture-filling chlorite and nanoscale cookeite in the Toushi coals ( Figures 3B , 8 ) is also indicative of hydrothermal activity, which was commonly ascribed to the reaction of kaolinite with Li-bearing solutions or were directly precipitated by Li-bearing fluids, serving as the main carrier of Li in coals (Dai et al., 2012a; Permana et al., 2013; Zhao et al., 2018; Zhou et al., 2021). As a consequence, the elevated Li concentration in the Toushi coals is, at least in part, related to the injection of hydrothermal solutions (submarine hydrothermal fluids or acidic waters).

It is worth noting that apart from Li, V and Cr are also enriched in the Toushi coals, which is indicative of hydrothermal influence as well. Previous research has found that the injection of V-Cr-bearing magmatic hydrothermal solution during coal formation process contributed to the Cr and V enrichment in coals (Brownfield et al., 1995; Wu et al., 2005; Yang, 2012). Dai et al. (2013, 2017b) ascribed the abnormal enrichment of V-Cr-Ni-U element assemblages in organic-rich rocks, e.g., coal and black shale, to submarine hydrothermal activities. In the current study, the terrigenous provenance of the Toushi coals is primarily of intermediate to felsic composition, in which the contents of V, and Cr are much lower than those in basic and ultrabasic rocks (Brownfield et al., 1995). Therefore, the enrichment of V and Cr in the marine-influenced Toushi coals is not derived from the intermediate-felsic terrigenous provenance, but attributed to the submarine hydrothermal activities instead.