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Article

Paleoenvironmental Controls and Economic Potential of Li-REY Enrichment in the Upper Carboniferous Coal-Bearing “Si–Al–Fe” Strata, Northeastern Qinshui Basin

by
Ning Wang
1,2,*,
Jun Zhao
2,
Yingxia Xu
1,
Mangen Mu
1,
Shangqing Zhang
3,4,
Libo Jing
1,
Guoshu Huang
1,
Liang Liu
1 and
Pengfei Tian
1
1
Department of Earth Science and Engineering, Shanxi Institute of Technology, Yangquan 045000, China
2
Shanxi Key Laboratory of Bauxite Resources Exploration and Comprehensive Utilization, Jinzhong 030620, China
3
Shanxi Province Key Laboratory of Metallogeny and Assessment of Strategic Mineral Resources, Taiyuan 030006, China
4
College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(3), 269; https://doi.org/10.3390/min15030269
Submission received: 14 February 2025 / Revised: 28 February 2025 / Accepted: 4 March 2025 / Published: 5 March 2025
(This article belongs to the Special Issue Mineralogical and Geochemical Characteristics of Coal and Coal-Bearing Strata)
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Abstract

:
Critical metals in coal-bearing strata have recently emerged as a frontier hotspot in both coal geology and ore deposit research. In the Upper Carboniferous coal-bearing “Si–Al–Fe” strata (Benxi Formation) of the North China Craton (NCC), several critical metals, including Li, Ga, Sc, V, and rare earth elements and Y (REY or REE + Y), have been discovered, with notable mineralization anomalies observed across northern, central, and southern Shanxi Province. However, despite the widespread occurrence of outcrops of the “Si–Al–Fe” strata in the northeastern Qinshui Basin of eastern Shanxi, there has been no prior report on the critical metal content in this region. Traditionally, the “Si–Al–Fe” strata have been regarded as a primary source of clastic material for the surrounding coal seams of the Carboniferous–Permian Taiyuan and Shanxi Formations, which are known to display critical metal anomalies (e.g., Li and Ga). Given these observations, it is hypothesized that the “Si–Al–Fe” strata in the northeastern Qinshui Basin may also contain critical metal mineralization. To evaluate this hypothesis, new outcrop samples from the “Si–Al–Fe” strata of the Benxi Formation in the Yangquan area of the northeastern Qinshui Basin were collected. Detailed studies on critical metal enrichment were assessed using petrographic observations, mineralogy (XRD, X-ray diffractometer), and geochemistry (XRF, X-ray fluorescence spectrometer, and ICP-MS, inductively coupled plasma mass spectrometer). The results indicate that the siliceous, ferruginous, and aluminous rocks within the study strata exhibit varying degrees of critical metal mineralization, mainly consisting of Li and REY, with minor associated Nb, Zr, and Ga. The Al2O3/TiO2, Nb/Y vs. Zr/TiO2, and Nb/Yb vs. Al2O3/TiO2 diagrams suggest that these critical metal-enriched layers likely have a mixed origin, comprising both intermediate–felsic magmatic rocks and metamorphic rocks derived from the NCC, as well as alkaline volcaniclastics associated with the Tarim Large Igneous Province (TLIP). Furthermore, combined geochemical parameters, such as the CIA (chemical index of alteration), Sr/Cu vs. Ga/Rb, Th/U, and Ni/Co vs. V/(V + Ni), indicate that the “Si–Al–Fe” strata in the northeastern Qinshui Basin were deposited under warm-to-hot, humid climate conditions, likely in suboxic-to-anoxic environments. Additionally, an economic evaluation suggests that the “Si–Al–Fe” strata in the northeastern Qinshui Basin hold considerable potential as a resource for the industrial extraction of Li, REY, Nb, Zr, and Ga.

1. Introduction

In recent years, critical metals (including rare, dispersed, precious, and rare earth metals) have been widely utilized across industries such as semiconductors, renewable energy, electronic information, aerospace, and advanced weaponry due to their unique material properties like corrosion resistance, high-temperature stability, superconductivity, and superior thermal conductivity [1,2,3,4]. These metals are internationally recognized as strategic mineral resources [5]. With the ongoing development of the global economy and high-tech industries, the demand for critical metals is expected to continue growing, further intensifying the supply–demand imbalance. Consequently, identifying and exploiting novel sources of critical minerals has emerged as a global priority to address escalating resource demands [6,7,8].
Under distinct conditions, geological processes associated with coal-bearing strata may facilitate the accumulation of multiple technology-critical minerals (e.g., Ga/Al, Ge, REY, Li, Nb, Zr), leading to the formation of large-scale “coal-bearing critical metal deposits” or “coal-type critical metal deposits” [8,9,10]. For example, REY associated with the Appalachian coal basin have been regarded as unconventional deposits with promising development prospects [11]; other coal-bearing strata in the United States, including those in Kentucky, Texas, and Utah, have also discovered multiple occurrences of rare earth mineralization [12,13,14]. These coal-bearing critical metal deposits represent a cutting-edge research focus within the fields of coal geology and ore geology and are classified as a new type of mineral resource [6,10]. They can be considered an important supplementary source for augmenting traditional metal resource reserves, and have garnered significant attention from scholars (e.g., [12,15,16,17,18,19,20,21,22,23,24,25]).
Situated in eastern Shanxi Province, the Qinshui Basin hosts extensively developed Late Paleozoic coal-bearing sequences, presenting a unique potential for the extraction of critical metals associated with coal-hosted deposits. Currently, several anomalous critical metal occurrences within mineable coal seams in the basin have attracted the attention of scholars. For instance, elevated Li and REY concentrations in Coal Seam No. 15 of the Upper Carboniferous Taiyuan Formation within the Jincheng area [26,27,28]; Ge, U, Li, and REY enrichment across Coal Seam No. 3 of the Lower Permian Shanxi Formation, along with Coal Seams Nos. 9 and 15 of the Taiyuan Formation in the Changzhi area [29,30,31,32]; and anomalous Li, Ga, Zr, and REY occurrences in Coal Seams Nos. 8 and 9 of the Taiyuan Formation in the Yangquan area [33,34,35]. However, little has been reported on the enrichment and distribution of critical metals in non-coal lithologies at the base of these coal seams. Geochemical analyses of the sediment sources for the Carboniferous–Permian Taiyuan and Shanxi Formations in North China suggest that multiple coal-bearing basins, including the Qinshui Basin, may have received erosional input from the underlying Benxi Formation bauxite [26,27,36,37], indicating that some of the critical metals enriched in the coal seams could also originate from this process. Furthermore, it is worth noting that recent studies on the enrichment of critical metals in the Late Paleozoic coal-bearing basement (the “Si–Al–Fe” strata of the Benxi Formation) have been reported in southern and central-northern Shanxi province, North China. These studies consistently reveal abnormal enrichment of various critical metals in the “Si–Al–Fe” strata. For example, Zhang et al. [17] reported that a low-grade bauxite sample from Pinglu, Yuncheng, southern Shanxi, contains 4140 μg/g of Li, an abnormally high enrichment level compared to the average upper continental crustal (UCC, McLennan [38]) value of 20 μg/g (CC > 100; CC, concentration coefficient, namely, the ratio of trace element concentrations in analyzed samples relative to those in the UCC, Dai et al. [39]), along with associated Ga, V, and REY. Zhao et al. [40] identified Li-enriched bauxite layers in the Benxi Formation of Qinyuan, Changzhi, central and southern Shanxi, where the average Li content in clay rocks, bauxitic clays, bauxite ore, and iron-rich clays are 431 μg/g, 288 μg/g, 252 μg/g, and 202 μg/g, respectively. Similarly, the average contents of Li, B, and REY in the bauxite of Xingxian, Lvliang, northern Shanxi, reach 892 μg/g, 327 μg/g, and 584 μg/g, respectively, demonstrating significant industrial mining potential [16]. Therefore, we hypothesize that the “Si–Al–Fe” strata of the Benxi Formation, located beneath the lowermost coal seams of the Late Paleozoic in the Qinshui Basin, may also exhibit abnormal enrichment of various critical metals, acting as a material source member for such enrichments.
In this study, we selected an outcrop of the “Si–Al–Fe” strata of the Benxi Formation in the northeastern part of the Qinshui Basin (Yangquan area) for detailed petrographic, mineralogical, and geochemical analyses. The aim is to investigate the distribution, sources, and paleoenvironment of critical metal enrichment within the “Si–Al–Fe” strata at the base of the regional coal measures from a critical metal enrichment perspective. Additionally, the economic potential of these strata has been evaluated.

2. Geology Setting

The study area is located in the northeastern part of the Qinshui Basin, Shanxi Province, north China, and tectonically belongs to the Trans-North China Orogen (TNCO) within the North China Craton (NCC) (Figure 1 and Figure 2). The regional and local geological settings are described in detail below.

2.1. Regional Geology

The North China Craton (NCC), one of the oldest cratons on Earth, was formed during the Archean (~1850 Ma) through the amalgamation of multiple micro-continental blocks, including the Western Block, Eastern Block, and the TNCO along orogenic belts (Figure 2A) [41,42]. The NCC occupies a tectonic position flanked by the Qinling-Dabie and Central Asian Orogenic Belt. Its crystalline basement consists predominantly of Archean to Paleoproterozoic lithologies, including gneisses, granites, amphibolites, migmatites, and banded iron formations. These ancient rocks are unconformably capped by a succession of Mesoproterozoic to Neoproterozoic and Phanerozoic sedimentary [43,44,45,46,47].
Minerals 15 00269 g002
Figure 2. Geological map of the study area. (A) the North China Craton (NCC) (modified from [42]); (B) regional stratigraphy for the Middle Ordovician to Upper Carboniferous and stratigraphic sequences of the “Si–Al–Fe” strata within the Benxi Formation in the study area.
Figure 2. Geological map of the study area. (A) the North China Craton (NCC) (modified from [42]); (B) regional stratigraphy for the Middle Ordovician to Upper Carboniferous and stratigraphic sequences of the “Si–Al–Fe” strata within the Benxi Formation in the study area.
Minerals 15 00269 g002
The Phanerozoic strata in the region consist mainly of Cambrian to Middle Ordovician marine deposits, Carboniferous to Permian clastic rocks, and Mesozoic to Cenozoic terrigenous clastic rocks [43,48]. The evolutionary history of the Phanerozoic strata is as follows: During the Cambrian, much of the NCC was submerged beneath a shallow sea. By the Middle Ordovician, the Caledonian Orogeny caused regional uplift, resulting in widespread weathering and erosion. This uplift event led to a 150-million-year hiatus in sedimentation. Not until the Upper Carboniferous, under the influence of the Hercynian Orogeny, did the region experience another marine transgression, reestablishing a marine sedimentary environment. Over time, repeated episodes of transgression and regression led to the accumulation of several terrigenous clastic layers. Due to prolonged weathering, erosion, and interruptions in sedimentation, the Upper Ordovician, Silurian, Devonian, and Lower Carboniferous strata are absent in the study area (Figure 2B) [47,49].

2.2. Local Geology

The Qinshui Basin, located on the eastern margin of Shanxi, north China, preserved strata from the Cambrian, Ordovician, Carboniferous, Permian, Triassic, and Quaternary [50]. The principal coal-bearing sequences in the northeastern part of the basin include the Upper Carboniferous Benxi Formation (C2b), Upper Carboniferous Taiyuan Formation (C2t), and Lower Permian Shanxi Formations (P1s).
The Benxi Formation, which is unconformably overlain by the Ordovician Fengfeng Formation, primarily comprises bauxite-rich mudstone, interbedded with argillaceous siltstone and sporadic thin coal layers, and the uppermost section of the sequence is punctuated by localized limestone deposits (Figure 2B). The “Si–Al–Fe” strata examined in this study are situated at the foundation of the Benxi Formation and host numerous karst-type bauxite deposits (Figure 2B). The stratum is stratigraphically subdivided into three distinct layers from bottom to top, namely ferruginous, aluminous, and siliceous layers, respectively (Figure 2B). The lower ferruginous layer, which contains clays intermixed with iron-rich minerals such as hematite and goethite, is known as a “Shanxi-type” iron deposit in North China. The middle aluminous layer consists of bauxitic clay and clayey bauxite, while the upper siliceous layer is a clay-rich stratum [42].
The Taiyuan and Shanxi Formations are composed of a variety of sediments, including fine- to medium-grained sandstone, siltstone, mudstone, limestone, and coal layers. In the study area, the Taiyuan Formation was deposited in environments associated with tidal deltas, tidal flats, and lagoons, whereas the Shanxi Formation developed in a fluvial-lacustrine setting [51]. The primary mineable coal seams within these formations, listed from top to bottom, are the Nos. 3, 9, and 15 seams.

3. Sampling and Analytical Methods

In this study, nine representative bulk rock samples were collected from fresh outcrops in the lower horizons of the Upper Carboniferous Benxi Formation, located in the Yangquan area, northeastern Qinshui Basin (Figure 1 and Figure 2). The samples included five siliceous rocks, one ferruginous rock, and three aluminous rocks, and sampling locations are shown in Figure 1C. These samples were labeled YQ-1 to YQ-9 from top to bottom, respectively (Table 1). All samples were crushed and ground to <200 mesh before undergoing mineralogical and geochemical analysis.
Mineralogical identification was conducted using a D8-advance X-ray diffractometer (XRD, Bruker, Karlsruhe, Germany) with Jade 6.5 software. The XRD measurements were conducted on a Rigaku D/max-2500/PC diffractometer (Tokyo, Japan), set to 40 kV and 40 mA. Scans were recorded over a 2θ range of 2.5–70°, with a step size of 0.02° and a count time of 0.3 s per step. Quantification was performed using SiroquantTM software 4.0, developed by Taylor [52] based on the whole pattern analysis principles by Rietveld [53]. Details of this analytical method are further provided by Ward et al. [54] and Ruan and Ward [55].
The whole-rock abundances of major element oxides (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5) were determined using an Axios mAX X-ray fluorescence spectrometer (XRF, Malvern Panalytical, Almelo, the Netherlands). Samples were ashed at 815 °C for 1.5 h in a muffle furnace before analysis, and the loss on ignition (LOI) was calculated from the mass difference before and after ashing. Trace elements contents were determined using an ICAPQ inductively coupled plasma mass spectrometer (ICP-MS, Thermo Fisher, Waltham, USA). Prior to ICP-MS analysis, approximately 50 mg of each sample was digested in a mixture of purified nitric (HNO3) and hydrofluoric acids (HF), which were heated on an electric plate. The analysis followed the standard GB/T 14506.30-2010 [56].
The above mineralogical analysis was performed at the State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources, China University of Mining and Technology (Beijing), while the major and trace elemental analysis was performed at the Hebei Regional Geological and Mineral Survey Institute.

4. Results

4.1. Petrography

Based on the macroscopic petrological observations of fresh outcrops within the Upper Carboniferous Benxi Formation and hand samples collected, three rock types were identified in the present study: siliceous, ferruginous, and aluminous rocks (Figure 1C,D). The detailed lithological characteristics are described below: (1) Siliceous rocks (samples YQ-1 to YQ-5), exhibit a yellowish-white to yellow-brown color, with siltstone to sandstone texture, blocky structure, and siliceous cementation (Figure 3A); (2) ferruginous rock (sample YQ-6), appears yellow-brown, occurring as lenses or nodules at the base of the aluminous rock, displaying a lump-like structure (Figure 3B); (3) aluminous rocks (samples YQ-7 to YQ-9), range from gray-white to gray, with a brittle texture that feels slippery to the touch, clastic texture (locally oolitic texture), dense blocky structure, and localized yellow-brown ferruginous distribution on some surface areas (Figure 3C).

4.2. Mineralogy

The mineralogical phases and contents, as determined by XRD and quantified using SiroquantTM software 4.0, are summarized in Table 1. The XRD patterns for three representative rock powder samples, including siliceous rock YQ-3, ferruginous rock YQ-6, and aluminous rock YQ-9, are shown in Figure 3(A1)–(C1).
Results show that the minerals in siliceous rocks are dominated by kaolinite (33.7%–51.6%), quartz (24.7%–31.3%), and illite (15.0%–28.7%), along with varying proportions of anatase (0.2%–1.3%), rutile (0.2%–0.9%), goethite (0.15%–2.8%), and hematite (0.4%–1.8%). Sample YQ-5 also contains traces of marcasite, pyrite, and anorthite (Figure 3(A1), Table 1), possibly influenced by the underlying ferruginous horizon. Ferruginous rock (sample YQ-6) is primarily composed of illite (40.4%), quartz (27.5%), and kaolinite (22.5%), along with minor amounts of goethite (4.5%), hematite (2.0%), and anatase (1.0%), and traces of marcasite, pyrite, and anorthite (Figure 3(B1), Table 1). Aluminous rocks are predominantly composed of diaspore (10.4%–84.2%), kaolinite (11.2%–59.4%), and illite (26.0%–27.3%), with smaller amounts of anatase (2.0%–2.8%), florencite (0.6%–1.1%) (Figure 3(C1), Table 1). Additionally, samples YQ-7 and YQ-8 also contain traces of quartz (0.4% and 0.8%, respectively) and rutile (0.6% and 0.9%, respectively), while sample YQ-9 includes goethite (1%) (Table 1).

4.3. Elemental Geochemistry

The percentages of major element oxides (on an SO3-free ash basis, %) and the concentrations of trace elements (on a whole-rock basis, μg/g) in the studied samples are presented in Table 2. To assess the degree of element enrichment, the concentration coefficient (CC) proposed by Dai et al. [39] was applied for both major and trace elements. The CC values are categorized as follows: CC ≥ 100 (highly enriched), 10 ≤ CC < 100 (significantly enriched), 5 ≤ CC < 10 (moderately enriched), 2 ≤ CC < 5 (slightly enriched), 0.5 ≤ CC < 2 (normal), and CC < 0.5 (depleted). The reference standard values were based on the UCC averages reported by McLennan [38].

4.3.1. Major Elements

The siliceous rock samples (YQ-1 to YQ-5) dominantly consist of SiO2 (62.93%–73.37%) and Al2O3 (19.02%–25.87%), with varying proportions of TiO2 (0.99%–1.80%), Fe2O3 (0.37%–9.84%), and K2O (0.736%–3.085%), and with negligible amounts of MnO (0.003%–0.021%), MgO (0.187%–0.580%), CaO (0.128%–0.204%), Na2O (0.081%–0.170%), and P2O5 (0.038%–0.089%) (Table 2). The ferruginous sample (YQ-6) is mainly enriched in Fe2O3 (52.92%), SiO2 (29.62%), and Al2O3 (11.42%), and with less than 7% of TiO2, MnO, MgO, CaO, Na2O, K2O, and P2O5 combined (Table 2). Different from the two rock types mentioned above, the aluminous rock samples (YQ-7 to YQ-9) have a high content of Al2O3 (57.48%–93.44%), followed by SiO2 (1.61%–36.90%) and TiO2 (2.58%–4.05%), with erratic amounts of Fe2O3 (0.48%–2.90%) and K2O (0.088%–1.382%), and with less than 1% of MnO, MgO, CaO, Na2O, K2O, and P2O5 combined (Table 2). Meanwhile, the siliceous and ferruginous rocks show relatively low LOI values ranging from 6.60% to 8.62%, whereas aluminous rocks exhibit LOI values over 10%, ranging from 13.29% to 14.64% (Table 2).
In comparison with the average value for UCC [38], TiO2 is slightly to moderately enriched in both siliceous and aluminous rocks, with CC values ranging from 1.45 to 2.65 (average 2.19) and from 3.80 to 5.96 (mean 4.92), respectively (Figure 4A,C); Al2O3 is slightly to moderately enriched in aluminous rocks (3.79 ≤ CC ≤ 6.15, average 4.89) (Figure 4C); and Fe2O3 and MnO are highly enriched (CC = 10.58) and slightly enriched (CC = 3.54), respectively, in ferruginous rock (Figure 4B); whereas the proportions of remaining other elements in all studied samples are within the normal range (0.55 ≤ CC ≤ 1.54) or are deplete (0.01 ≤ CC ≤ 0.45) compared to the average values for UCC (Figure 4A–C).
Table 2. Concentration of major element oxides (on an SO3-free ash basis, %) and trace elements (on a whole-rock basis, μg/g) in the studied samples.
Table 2. Concentration of major element oxides (on an SO3-free ash basis, %) and trace elements (on a whole-rock basis, μg/g) in the studied samples.
SampleLOISiO2TiO2Al2O3Fe2O3MnOMgOCaONa2OK2OP2O5LiBeScVCrCoNiCuZnGaRbSrZrNbMoCd
YQ-17.7265.261.7721.379.840.0050.2650.1910.1071.1020.08828.51.0714.716299.61.9719.815.419.923.134.243.329829.31.400.76
YQ-27.8970.051.8025.470.870.0030.2470.1680.1151.2370.03964.32.3720.119183.22.8928.414.218.122.948.862.137936.70.510.55
YQ-37.8271.951.5524.630.370.0030.2050.1500.1130.9860.03852.01.9216.311982.71.7910.89.5910.420.134.253.627327.80.680.13
YQ-46.6073.370.9919.025.430.0210.1870.1280.0810.7360.04152.62.0318.020574.314.120.640.213.521.628.147.317613.80.480.13
YQ-57.4862.931.3425.875.730.0080.5800.2040.1703.0850.08976.62.9619.118875.713.420.351.517.828.210692.024532.00.710.07
YQ-68.6229.620.5611.4252.920.2830.4622.3180.1362.1030.17032.515.253.811851.126115776.697.617.575.782.41219.551.680.97
YQ-714.641.614.0593.440.480.0020.1060.0700.0000.0880.15122.97.4129.016140.20.815.1614.421.833.21.8855.592747.90.340.07
YQ-813.2919.723.4171.852.900.0020.2720.1570.0361.3820.2821456.3959.228381.81.5944.741.224.332.333.31598921570.980.13
YQ-913.4736.902.5857.480.970.0020.2510.1820.0581.2660.3073455.3522.95061152.0872.314.617.420.028.32105311040.540.10
UCC 66.000.6815.195.000.0802.2204.2003.9003.3700.16020.03.0013.610783.017.044.025.071.017.011235019012.01.500.10
SampleLaCePrNdSmEuGdTbDyYHoErTmYbLuInCsBaHfTaWTlPbBiThUA/S
YQ-121.9 42.8 4.66 16.1 2.92 0.64 2.71 0.50 2.96 15.1 0.57 1.63 0.30 2.02 0.31 0.11 3.08 77.0 8.36 1.32 2.28 0.25 16.3 0.57 22.5 3.56 0.33
YQ-220.8 41.6 4.53 16.2 3.22 0.77 3.19 0.56 3.20 15.8 0.60 1.71 0.29 1.92 0.30 0.14 4.77 118 10.3 1.65 3.18 0.32 10.6 0.55 21.8 4.39 0.36
YQ-317.3 31.9 3.23 11.3 2.51 0.74 2.65 0.47 2.53 9.87 0.46 1.34 0.23 1.52 0.24 0.08 3.96 68.5 7.57 1.20 2.25 0.24 11.4 0.53 10.5 3.21 0.34
YQ-418.1 38.3 4.63 18.5 5.58 1.63 10.6 1.97 12.0 52.5 1.88 4.38 0.58 3.33 0.46 0.09 3.24 60.6 5.16 0.85 1.66 0.25 8.32 0.24 13.1 2.53 0.26
YQ-594.3 227 39.1 134 11.6 1.62 10.1 1.35 8.51 41.2 1.67 5.25 0.87 6.21 0.93 0.10 7.47 246 7.04 1.49 2.50 0.63 10.2 0.63 30.6 6.73 0.41
YQ-655.8 128 24.2 102 18.1 4.29 22.1 3.76 26.1 123 4.70 12.7 1.94 12.9 1.83 0.45 4.39 206 3.29 0.66 1.20 1.74 7.41 0.25 9.33 3.94 0.39
YQ-738.0 56.7 5.80 15.3 2.24 0.40 2.40 0.43 2.61 10.1 0.52 1.76 0.31 2.15 0.32 0.08 0.25 9.74 27.7 2.56 1.39 0.06 41.70 1.91 49.9 67.3 58.20
YQ-8123 351 43.6 158 19.9 3.03 15.6 1.77 9.74 33.3 1.70 5.08 0.78 5.34 0.77 0.10 1.62 51.9 26.0 6.69 7.78 0.29 107 2.83 93.2 55.1 3.64
YQ-9143 523 52.4 198 24.2 3.79 19.8 1.98 9.67 30.4 1.53 4.20 0.62 4.08 0.59 0.09 1.55 52.3 15.9 4.51 4.28 0.29 67.9 1.70 84.9 31.1 1.56
UCC30.0 64.0 7.10 26.0 4.50 0.88 3.80 0.64 3.50 22.0 0.80 2.30 0.33 2.20 0.32 0.05 4.60 550 5.80 1.00 2.00 0.75 17.0 0.13 10.70 2.80
UCC, the average concentration of elements in the upper continental crust, data from McLennan [38]; A/S indicates Al2O3/SiO2.
Additionally, the values of Al2O3/SiO2 in the siliceous and ferruginous rocks range from 0.26 to 0.41, in accordance with the abundant quartz and relatively low contents of kaolinite (Table 1). The aluminous rock samples have an Al2O3/SiO2 ratio of 1.56 to 58.2 (Table 2), which is closely related to the high content of diaspore and far exceeds the cut-off grade for bauxite ore (Al2O3/SiO2 = 1.8–2.6, and Al2O3 ≥ 40%, DZ/T 0202-2020 [57]).

4.3.2. Trace Elements and Rare Earth Elements

Compared with the average values for UCC [38], trace elements, including Li (1.43 ≤ CC ≤ 3.83, average 2.74), Nb (1.15 ≤ CC ≤ 3.06, average 2.33), Cd (0.66 ≤ CC ≤ 7.76, average 3.34), In (1.62 ≤ CC ≤ 2.80, average 2.10), and Bi (1.89 ≤ CC ≤ 4.96, average 3.97) in the siliceous rocks (samples YQ-1 to YQ-5) are slightly enriched (Figure 5A). In contrast, the ferruginous rock (sample YQ-6) shows significant enrichment of Co (CC = 15.35), moderate enrichment of Be (CC = 5.07) and Cd (CC = 9.90), and slight enrichment of Sc (CC = 3.96), Ni (CC = 3.57), Cu (CC = 3.06), and Tl (CC = 2.32) (Figure 5B). For the aluminous rocks (samples YQ-7 to YQ-9), trace elements Bi (13.39 ≤ CC ≤ 22.28, average 16.90) and U (11.11 ≤ CC ≤ 24.04, average 18.27) are significantly enriched, while Li (1.15 ≤ CC ≤ 17.25, average 8.55), Nb (3.99 ≤ CC ≤ 13.08, average 8.58), and Th (4.66 ≤ CC ≤ 8.71, average 7.10) exhibit moderate enrichment. Other trace elements such as Be, Sc, V, Zr, Hf, Ta, W, and Pb show slight enrichment, with concentrations in the range of 2 ≤ CC < 5 (Figure 5C). The remaining trace elements in these investigated samples are generally at or below the average concentrations for the UCC (Figure 5A–C).
The distribution of REY concentrations among all studied samples is not uniform. The REY contents in siliceous rocks (samples YQ-1 to YQ-5) are relatively low, ranging from 86.29 to 583.71 μg/g. In contrast, the ferruginous rock (sample YQ-6) has a concentration of 541.42 μg/g, while the aluminous rocks (samples YQ-7 to YQ-9) exhibit higher concentrations, ranging from 139.04 to 1017.26 μg/g (Table 2). Furthermore, by comparing the total REY content in these rock types with the average values of the UCC, it is observed that the siliceous rocks fall within the normal range (CC = 1.28), whereas both the ferruginous and aluminous rocks show slight enrichment (CC = 3.22, and 3.82, respectively).
This study adopts the REY classification and enrichment types in coal-bearing sequences proposed by Seredin and Dai [9]. Specifically, LaN/LuN > 1, LaN/SmN < 1 and GdN/LuN > 1, and LaN/LuN < 1 are indicative of LREE-type (light rare earth elements: La, Ce, Pr, Nd, Sm), MREE-type (medium rare earth elements: Eu, Gd, Tb, Dy, Y) and HREE-type (heavy rare earth elements: Ho, Er, Tm, Yb, Lu), respectively, where N represents normalization of REY to UCC values. The REY anomaly parameters are presented in Table 3. All samples exhibit H-type REE distribution patterns, with the LaN/LuN ranging from 0.003 to 0.028 (Table 3). Additionally, several REY (i.e., Ce, Eu, Gd, and Y), which are influenced by specific geochemical environments, such as high-temperature, reducing, and oxidizing conditions (Dai et al. [58]), were examined. The results indicate that Ce, Gd, and Y exhibit significant negative anomalies across all samples in this study, with corresponding Ce/Ce*, Gd/Gd*, and Y/Y* values ranging from 0.115 to 0.187, from 0.217 to 0.377, and from 0.026 to 0.037, respectively, while Eu displays a positive anomaly (1.188 ≤ Eu/Eu* ≤ 2.070) (Table 3).

5. Discussion

5.1. Provenance Source Rock

The Al2O3/TiO2 ratio is widely used to discern the source of volcanic ash in coal-bearing sequences [19,59,60,61], the origin of inorganic matter in coal seams [28,62], and the provenance of clastic material in sedimentary rocks [63,64], owing to its relatively immobile geochemical behavior during both primary and secondary geological processes. Typical ranges for Al2O3/TiO2 ratio are 3–8, 8–21, and 21–70, which correspond to mafic, intermediate, and felsic parent rock types, respectively [64]. In this study, the Al2O3/TiO2 ratios of the siliceous rocks (samples YQ-1 to YQ-5) range from 12.06 to 19.31, while the ferruginous rock (sample YQ-6) has a ratio of 20.52, and the aluminous rocks (samples YQ-7 to YQ-9) range from 21.09 to 23.06 (Figure 6A), suggesting that the siliceous and ferruginous rocks likely originate from intermediate parent rocks, whereas the aluminous rock is probably derived from felsic sources. Furthermore, the studied samples predominantly fall within the intermediate–felsic region on the Nb/Y-Zr/TiO2 diagram proposed by Winchester and Floyd [65], including compositions such as trachyandesite and andesite (Figure 6B).
Previous studies have conducted detrital zircon U–Pb–Lu–Hf dating of the Benxi Formation “Si–Al–Fe” strata within the NCC (e.g., [42,49,66,67]). The results indicate that the detrital material in the southern NCC mainly originates from ~450 Ma igneous rocks of the North Qinling Orogenic Belt, while the central and northern regions derive mainly from ~320–200 Ma volcanic rocks of the Bainaimiao Arc Terrane. In addition, Precambrian ~950 Ma metamorphic rocks have also influenced the “Si–Al–Fe” strata across the entire NCC [49,66,68]. Recently, the detrital rutile U–Pb ages reported by Liu et al. [15], ranging from approximately 900 to 400 Ma, indicating that the sedimentation of the “Si–Al–Fe” strata in the northern and southern NCC regions mainly derived from the Bainaimiao Arc Terrane and the North Qinling Orogenic Belt, respectively. Based on these reports, the intermediate–felsic detrital materials in this studied horizon likely derive primarily from the volcanic rocks of the Bainaimiao Arc Terrane, as the Yangquan area is situated on the northern margin of the NCC (Figure 1B). However, Liu et al. [69] demonstrated through mercury isotope studies that the “Si–Al–Fe” strata within the NCC were also influenced by the input of volcanic detrital material from the northern NCC during the Upper Carboniferous. Furthermore, this study utilized the schematic diagram (Nb/Yb-Al2O3/TiO2) proposed by Zheng et al. [70] to distinguish the properties of volcanic clastic source rocks. As shown in Figure 7, areas with high-Ti basalt and alkaline intermediate–felsic rocks are linked to within-plate (plume-related) sources, whereas calc-alkaline intermediate–felsic rocks are associated with subduction/collision-related processes. Low-Ti mafic compositions are widespread. In this study, all aluminous rocks and most siliceous rocks fall within the alkaline intermediate–felsic area, while other rocks exhibit low-Ti basalt characteristics (remaining close to the intermediate–felsic area), indicating that the studied “Si–Al–Fe” strata within the NCC are primarily originated from within-plate sources, with some connection to subduction/collision-related provenance.
Based on extensive previously published geochemical data, Nechaev et al. [22] revealed that the widespread distribution of critical metals (Zr(Hf)-Nb(Ta)-REY-Ga-U) in the Carboniferous–Permian coal-bearing sequences of North China may have been influenced not only by the erosional input from surrounding source regions but also by alkaline volcanic ash from Tarim Large Igneous Provinces (TLIP)-related mantle plume activity. Similarly, Di et al. [71], while studying the source of critical metals (Li, Nb, Ta, Zr, Hf, Ga) in Late Carboniferous–Early Permian coal from the Antaibao surface mine in the Ningwu Coalfield, Shanxi Province, suggested that these elements are likely linked to the input of TLIP alkaline volcanic material. Notably, Di et al. [71] proposed a different hypothesis for the origin of Li, countering the view that Li in the Carboniferous–Permian coal measures of the southern Qinshui Basin formed from low-temperature hydrothermal fluids. In this study, the investigated “Si–Al–Fe” strata in the northeastern Qinshui Basin, which are located near the Ningwu Coalfield (as mentioned in Section 4.3), also show enrichment in Li, Nb, Zr, and Ga. Therefore, this suggests that the volcanic ash-derived clastic materials in the study area may also originate from the TLIP in the northwestern Qinshui Basin.
In summary, the “Si–Al–Fe” strata in the northeastern Qinshui Basin, enriched in critical metals (Li-REY), likely represent a mixed origin of terrigenous clastic material and volcanic detritus. This mixture comprises weathered and eroded products from intermediate–felsic magmatic rocks and metamorphic rocks along the northern margin of the NCC, as well as alkaline volcaniclastics from the TLIP.

5.2. Paleoenvironment

5.2.1. Paleoclimate

Paleoclimate plays a pivotal role in Late Paleozoic bauxite mineralization, significantly influencing both the intensity of weathering and the enrichment of critical metals [40,72]. The “SiO2—Al2O3—Fe2O3” ternary diagram for the study samples reveals that both siliceous and ferruginous rocks fall within the moderate to strong laterization range (Figure 8; [73]), indicating that these layers underwent intense weathering. In contrast, the aluminous rocks exhibit comparatively lower degrees of weathering, although kaolinization is evident in some samples (Figure 8). To further infer the paleoclimate during the enrichment of critical metals (Li-REY) within the “Si–Al–Fe” strata of the Benxi Formation, the chemical index of alteration (CIA) was employed. The CIA is a geochemical proxy highly sensitive to temperature and humidity [74]. It is calculated using the formula: CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100 [74], where CaO* specifically refers to the CaO from silicate minerals, and all oxides are measured in molecular proportions. Regarding CaO*, the method proposed by McLennan [75] was applied: CaOremian = CaO − P2O5 × 10/3. If CaOremian < Na2O, then CaO* = CaOremian; otherwise, CaO* = Na2O. According to Fedo et al. [76], a CIA of 50–60 indicates weak weathering, 60–80 indicates moderate weathering, and 80–100 indicates intense weathering. In this study, CIA values for all samples ranged from 80.7 to 100.1, indicating that significant chemical weathering occurred during the formation of the Li-REY-enriched “Si–Al–Fe” strata (Table 4).
Additionally, the geochemical parameter Sr/Cu has been widely used to infer the paleoclimate during bauxite deposition, as it reflects the temperature range of the climate [72]. Specifically, Sr/Cu > 5, 1.3 < Sr/Cu < 5, and Sr/Cu < 1.3 are associated with hot, warm, and cold climates, respectively, based on the precipitation behavior of Cu2+ before Sr2+ at higher temperatures [72]. The Sr/Cu values calculated in this study range from 1.08 to 14.38 (mean = 4.32) (Table 4), indicating a warm climate during the formation of the Li-REY-enriched “Si–Al–Fe” strata. Moreover, the Sr/Cu vs. Ga/Rb ratio is commonly used to assess the humidity of the paleoclimate [77,78]. As shown in Figure 9A, most of the studied samples are located within the warm, humid climate zone. Additionally, the relative stability differences in certain elements, expressed as the C ratio [C = Σ(V + Cr + Mn + Fe + Co + Ni)/Σ(Na + Mg + K + Ca + Sr + Ba)], provide another useful geochemical parameter for estimating paleoclimate humidity. Specifically, C > 0.6, 0.2 < C < 0.6, and C < 0.2 are indicative of humid, semi-arid, and arid climates, respectively [72,79]. Most of the C ratios in the study area exceed 0.6 (Table 4), suggesting that the paleoclimate was humid during deposition. Therefore, integrating these geochemical parameters, it is evident that the “Si–Al–Fe” strata, enriched in critical metals (Li-REY), formed under warm-to-hot, humid climate conditions. This aligns well with the observed intense weathering, as indicated by the high CIA values.

5.2.2. Paleoredox

Elements like Th, U, V, Cr, Co, and Ni are highly sensitive to depositional environments, and their ratios are commonly used to reflect the paleoredox conditions during the formation of sedimentary rocks [80,81,82]. Previous studies have established that Th/U ratios of <2, 2–7, and >7 correspond to anoxic, suboxic, and oxic environments, respectively [83]. Similarly, V/(V + Ni) values of >0.83, 0.46–0.83, and <0.46 correspond to anoxic, suboxic, and oxic conditions, respectively [72,81]. Furthermore, V/Cr > 4.25 and Ni/Co > 7.0 are indicative of anoxic conditions, while V/Cr < 2.0 and Ni/Co < 5.0 suggest oxic conditions [81]. In this study, the Th/U ratios ranged from 0.74 to 6.32 (average 3.53), suggesting suboxic-to-anoxic deposition (Table 4). The V/(V + Ni) ratios, ranging from 0.43 to 0.97 (average 0.85), further support the conclusion (Table 4). Additionally, the V/Cr ratios (1.44 to 4.40, average 2.75) and Ni/Co ratios (0.60 to 34.76, average 10.97) also reflect a predominantly suboxic environment (Table 4). The Ni/Co vs. V/(V + Ni) plot (Figure 9B) further reinforces this, showing that, with the exception of the ferruginous sample, the study layers primarily formed under suboxic-to-anoxic conditions.
In conclusion, the “Si–Al–Fe” strata, enriched in critical metals (Li-REY) in the northeastern Qinshui Basin were predominantly formed under warm-to-hot, humid climate conditions, with the depositional basin existing in a suboxic-to-anoxic environment during the period of formation.

5.3. Potential Economic Significance of Li-REY and Other Associated Critical Metals

As described in Section 4.3.2, Li is slightly enriched in the siliceous rocks at the upper portion of the studied “Si–Al–Fe” strata, while it exhibits moderate enrichment in the aluminous rocks at the base, with sample YQ-9 showing a particularly high level of enrichment (CC = 17.25). The average content of LiO2 is 195.10 μg/g (on a whole-rock basis), exceeding the minimum recoverable grade (80 μg/g) and the industrial index for comprehensive recycling (120 μg/g) for Li in coal-bearing sequences reported by Sun et al. [84]. Additionally, considering ash yield, the LiO2 content (854.32 μg/g) in sample YQ-9 exceeds the industrial development potential cut-off grade (800 μg/g) in coal-bearing sequences under high-temperature ash conditions (Table 5), as outlined by Zhao et al. [85]. Hence, the investigated “Si–Al–Fe” strata hold potential economic value for Li recovery.
The average content of REO (REY-oxides) in the studied samples is 515.88 μg/g, which meets the marginal grade for ion-adsorption type deposits (500 μg/g) (Table 5, DZ/T 0204-2020, 2020 [86]). Notably, samples YQ-8 and YQ-9 have REO concentrations of 1040.33 μg/g and 1373.33 μg/g, respectively, exceeding the recommended recovery grade of 1000 μg/g for REY in coal-bearing sequences, as proposed by Seredin and Dai [9]. Furthermore, according to the REYdef-Coutl scheme established by Seredin and Dai [9], the data points for the studied samples predominantly fall within the prospective region (Figure 10), indicating that the “Si–Al–Fe” strata hold potential as a source for REY recovery.
The concentration of (Nb, Ta)2O5 in the “Si–Al–Fe” strata is 85.67 μg/g on average, up to 268.36 μg/g. This exceeds the marginal grade (80 μg/g) for conventional weathering crust-type Nb(Ta) deposits but falls short of the minimum industrial grade (160 μg/g) (Table 5, DZ/T 0203-2020, 2020 [87]). The maximum concentration of (Zr, Hf)O2 in the “Si–Al–Fe” strata is recorded at 1510.32 μg/g, with an average of 670.87 μg/g, significantly exceeding the minimum marginal grade (400 μg/g) for coastal sand-type Zr(Hf) deposits (Table 4, DZ/T 0203-2020, 2020 [87]). Thus, both Nb(Ta) and Zr(Hf) in the studied “Si–Al–Fe” horizon have economic prospect value.
The average concentration of Ga in the “Si–Al–Fe” strata is 27.08 μg/g, which exceeds the associated industrial grade in bauxite (20 μg/g) (Table 4, Committee of National Reserves, 2010 [88]), suggesting the “Si–Al–Fe” strata as a potential source for Ga recovery.

6. Conclusions

Compared with the average values for the upper continental crust, the Upper Carboniferous coal-bearing “Si–Al–Fe” strata (Benxi Formation) in the northeastern Qinshui Basin, eastern Shanxi, exhibit varying degrees of enrichment in critical metals Li and REY, and with minor associated Nb, Zr, and Ga. Notably, aluminous rocks serve as the principal host for these critical metals, exhibiting peak concentrations of 345 μg/g Li (CC = 17.25), 1017 μg/g REY (CC = 3.82), 157 μg/g Nb (CC = 13.08), 927 μg/g Zr (CC = 4.88), and 33.2 μg/g Ga (CC = 1.95). These values represent the maximum metal concentrations documented across all investigated stratigraphic units in the study area.
The ratios of stable elements suggest that the provenance of the Li-REY-enriched “Si–Al–Fe” strata is a mixture of intermediate–felsic magmatic rocks and metamorphic rocks derived from the NCC, along with alkaline volcaniclastics associated with the TLIP. Furthermore, paleoenvironmental reconstructions based on multiple geochemical parameters reveal that these strata predominantly formed under warm-to-hot, humid climatic conditions, with deposition occurring in suboxic-to-anoxic environments.
Additionally, an economic significance evaluation highlights the potential of the “Si–Al–Fe” strata in the northeastern Qinshui Basin, eastern Shanxi, as a prospective source for the industrial utilization of critical metals such as Li, REY, Nb, Zr, and Ga.

Author Contributions

Conceptualization, N.W.; data curation, L.J. and G.H.; formal analysis, S.Z. and L.J.; funding acquisition, N.W. and S.Z.; investigation, M.M.; methodology, N.W., J.Z., Y.X. and M.M.; project administration, N.W.; resources, N.W.; software, N.W. and Y.X.; supervision, N.W.; validation, J.Z. and M.M.; visualization, L.L. and P.T.; writing—original draft, N.W.; writing—review and editing, J.Z. and S.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Fundamental Research Program of Shanxi Province (No. 202403021222346 and No. 202403021222498), and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2024L390).

Data Availability Statement

The data are contained within the article.

Acknowledgments

Thanks to all those who helped with the field sampling and experiments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the Qinshui Basin and the study area (Yangquan) in north China (modified from [26]). (A) China map; (B) Qinshui Basin; (C,D) outcrops of the Upper Carboniferous Benxi Formation in the study area.
Figure 1. Location of the Qinshui Basin and the study area (Yangquan) in north China (modified from [26]). (A) China map; (B) Qinshui Basin; (C,D) outcrops of the Upper Carboniferous Benxi Formation in the study area.
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Figure 3. Macroscopic petrological observation (A–C) and the corresponding XRD patterns for three representative rock samples (A1–C1). (A,A1) yellow-white siliceous rock (sample YQ-3); (B,B1) yellow-brown ferruginous rock (sample YQ-6); (C,C1) gray aluminous rock (sample YQ-9). Ilt, illite; Kln, kaolinite; Gth, goethite; Qz, quartz; Hem, hematite; Ant, anatase; Rt, rutile; An, anorthite; Py, pyrite; Mrc, marcasite; Dsp, diaspore; Flo, florencite.
Figure 3. Macroscopic petrological observation (A–C) and the corresponding XRD patterns for three representative rock samples (A1–C1). (A,A1) yellow-white siliceous rock (sample YQ-3); (B,B1) yellow-brown ferruginous rock (sample YQ-6); (C,C1) gray aluminous rock (sample YQ-9). Ilt, illite; Kln, kaolinite; Gth, goethite; Qz, quartz; Hem, hematite; Ant, anatase; Rt, rutile; An, anorthite; Py, pyrite; Mrc, marcasite; Dsp, diaspore; Flo, florencite.
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Figure 4. Concentration coefficient (CC) of major element oxides in the “Si–Al–Fe” strata from the northeastern Qinshui Basin. (A) siliceous rock; (B) ferruginous rock; (C) aluminous rock. The solid triangles, circles, and squares in (A,C) represent the maximum, average, and minimum values of CC in siliceous and aluminous rocks, respectively. CC is the ratio of trace element concentrations in analyzed samples relative to those in the UCC as reported by McLennan [38].
Figure 4. Concentration coefficient (CC) of major element oxides in the “Si–Al–Fe” strata from the northeastern Qinshui Basin. (A) siliceous rock; (B) ferruginous rock; (C) aluminous rock. The solid triangles, circles, and squares in (A,C) represent the maximum, average, and minimum values of CC in siliceous and aluminous rocks, respectively. CC is the ratio of trace element concentrations in analyzed samples relative to those in the UCC as reported by McLennan [38].
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Figure 5. Concentration coefficient (CC) of trace elements in the “Si–Al–Fe” strata from the northeastern Qinshui Basin. (A) siliceous rock; (B) ferruginous rock; (C) aluminous rock. The solid triangles, circles, and squares in (A,C) represent the maximum, average, and minimum values of CC in siliceous and aluminous rocks, respectively. CC is the ratio of trace element concentrations in analyzed samples relative to those in the UCC as reported by McLennan [38].
Figure 5. Concentration coefficient (CC) of trace elements in the “Si–Al–Fe” strata from the northeastern Qinshui Basin. (A) siliceous rock; (B) ferruginous rock; (C) aluminous rock. The solid triangles, circles, and squares in (A,C) represent the maximum, average, and minimum values of CC in siliceous and aluminous rocks, respectively. CC is the ratio of trace element concentrations in analyzed samples relative to those in the UCC as reported by McLennan [38].
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Figure 6. Discrimination diagrams for Al2O3 vs. TiO2 (A) and Zr×0.0001/TiO2 vs. Nb/Y (B) (modified from Winchester and Floyd [65]) of the studied samples from the “Si–Al–Fe” strata.
Figure 6. Discrimination diagrams for Al2O3 vs. TiO2 (A) and Zr×0.0001/TiO2 vs. Nb/Y (B) (modified from Winchester and Floyd [65]) of the studied samples from the “Si–Al–Fe” strata.
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Figure 7. The magma source discrimination diagram of Al2O3/TiO2 vs. Nb/Yb (modified from [70]) of the studied samples from the “Si–Al–Fe” strata.
Figure 7. The magma source discrimination diagram of Al2O3/TiO2 vs. Nb/Yb (modified from [70]) of the studied samples from the “Si–Al–Fe” strata.
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Figure 8. SiO2—Al2O3—Fe2O3 ternary diagram showing the laterization degree of the studied samples from the “Si–Al–Fe” strata (modified from [73]).
Figure 8. SiO2—Al2O3—Fe2O3 ternary diagram showing the laterization degree of the studied samples from the “Si–Al–Fe” strata (modified from [73]).
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Figure 9. Cross plots presenting paleoclimatic (A) (Sr/Cu vs. Ga/Rb) and paleoredox (B) (Ni/Co vs. V/(V + Ni)) conditions. The dashed lines indicate the ranges of the coordinate axes corresponding to the paleoclimatic and paleoredox conditions.
Figure 9. Cross plots presenting paleoclimatic (A) (Sr/Cu vs. Ga/Rb) and paleoredox (B) (Ni/Co vs. V/(V + Ni)) conditions. The dashed lines indicate the ranges of the coordinate axes corresponding to the paleoclimatic and paleoredox conditions.
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Figure 10. Evaluation of the prospect significance for REY in the studied samples from the “Si–Al–Fe” strata (modified from Seredin and Dai [9]).
Figure 10. Evaluation of the prospect significance for REY in the studied samples from the “Si–Al–Fe” strata (modified from Seredin and Dai [9]).
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Table 1. Mineralogical composition of studied samples by XRD + SiroquantTM analysis (on a whole-rock basis, wt%).
Table 1. Mineralogical composition of studied samples by XRD + SiroquantTM analysis (on a whole-rock basis, wt%).
SampleRock TypeKlnIltDspQzAntRtGthHemMrcPyAnFloClay
YQ-1Siliceous45.722.0 29.01.20.90.90.4 67.7
YQ-2Siliceous51.215.0 31.30.60.20.11.5 66.2
YQ-3Siliceous49.621.0 25.21.00.60.81.8 70.6
YQ-4Siliceous51.619.7 24.70.2 2.81.0 71.3
YQ-5Siliceous33.728.7 25.51.3 2.71.21.41.14.3 62.4
YQ-6Ferruginous22.540.4 27.51.0 4.52.00.90.90.2 62.9
YQ-7Aluminous11.2 84.20.42.80.6 0.711.2
YQ-8Aluminous32.727.335.60.82.10.9 0.660.0
YQ-9Aluminous59.426.010.4 2.0 1.0 1.185.4
Kln, kaolinite; Ilt, illite; Dsp, diaspore; Qz, quartz; Ant, anatase; Rt, rutile; Gth, goethite; Hem, hematite; Mrc, marcasite; Py, pyrite; An, anorthite; Flo, florencite; Clay, the total of kaolinite and illite; blank spaces, below the detection limit.
Table 3. The parameter values of REY anomaly in studied samples.
Table 3. The parameter values of REY anomaly in studied samples.
SampleLaN/LuNLaN/SmNGdN/LuNCe/Ce*Eu/Eu*Gd/Gd*Y/Y*TypeREYdelCoutl
YQ-10.0080.1690.0620.1791.6550.2170.035H32.08 0.8
YQ-20.0080.1450.0750.1801.7830.2280.035H33.34 0.9
YQ-30.0080.1550.0780.1872.0700.2270.028H30.42 0.8
YQ-40.0040.0730.1630.1671.1880.2220.037H52.16 2.0
YQ-50.0120.1830.0770.1261.4220.2920.033H32.88 0.8
YQ-60.0030.0690.0860.1151.5270.2370.035H50.21 1.8
YQ-70.0140.3820.0530.1761.2280.2250.026H22.01 0.5
YQ-80.0180.1390.1440.1711.8770.3360.026H27.30 0.6
YQ-90.0280.1330.2380.2132.0430.3770.026H24.38 0.5
N = the concentrations of elements in studied samples vs. the corresponding average values in the upper continental crust (UCC) (from [38]). Ce/Ce* = CeN/(0.5LaN + 0.5PrN), Eu/Eu* = EuN/(0.5SmN + 0.5GdN), Gd/Gd* = GdN/(SmN × 0.33 + TbN × 0.67), Y/Y* = YN/HoN (from [55]). REYdef = (Nd + Eu + Tb + Dy + Er + Y)/ΣREY (from [9]). Coutl = [(Nd + Eu + Tb + Dy + Er + Y)/ΣREY]/[(Ce + Ho + Tm + Yb + Lu)/ΣREY] (from [9]).
Table 4. Paleoenvironmental parameter values.
Table 4. Paleoenvironmental parameter values.
SampleCIASr/CuGa/RbCTh/UV/(V + Ni)V/CrNi/Co
YQ-193.4 2.810.683.026.320.891.6310.05
YQ-293.7 4.370.470.284.970.872.309.83
YQ-394.5 5.590.590.163.270.921.446.03
YQ-494.7 1.180.772.495.180.912.761.46
YQ-587.2 1.790.270.724.550.902.481.51
YQ-680.7 1.080.235.322.370.432.310.60
YQ-7100.1 3.8517.661.070.740.974.006.37
YQ-898.4 3.860.970.821.690.863.4628.11
YQ-998.2 14.380.710.342.730.874.4034.76
Table 5. The concentrations of selected critical metal oxides and Ga in the studied samples (μg/g, on an ash basis).
Table 5. The concentrations of selected critical metal oxides and Ga in the studied samples (μg/g, on an ash basis).
SampleLiO2REONb2O5Ta2O5ZrO2HfO2(Nb, Ta)2O5(Zr, Hf)O2Ga
YQ-166.18145.3145.411.75437.7510.6947.15448.4425.03
YQ-2149.59144.9956.982.19557.7713.1959.17570.9624.86
YQ-3120.88109.0543.131.59401.449.6944.72411.1321.80
YQ-4120.68216.2821.131.11255.446.5222.24261.9623.13
YQ-5177.42736.0149.461.97358.978.9851.43367.9530.48
YQ-676.21687.4814.950.88179.504.2515.83183.7519.15
YQ-757.48190.1180.253.661472.0438.2883.911510.3238.89
YQ-8358.331040.33258.949.421394.4635.38268.361429.8437.25
YQ-9854.321373.33171.876.36831.8021.68178.24853.4823.11
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Wang, N.; Zhao, J.; Xu, Y.; Mu, M.; Zhang, S.; Jing, L.; Huang, G.; Liu, L.; Tian, P. Paleoenvironmental Controls and Economic Potential of Li-REY Enrichment in the Upper Carboniferous Coal-Bearing “Si–Al–Fe” Strata, Northeastern Qinshui Basin. Minerals 2025, 15, 269. https://doi.org/10.3390/min15030269

AMA Style

Wang N, Zhao J, Xu Y, Mu M, Zhang S, Jing L, Huang G, Liu L, Tian P. Paleoenvironmental Controls and Economic Potential of Li-REY Enrichment in the Upper Carboniferous Coal-Bearing “Si–Al–Fe” Strata, Northeastern Qinshui Basin. Minerals. 2025; 15(3):269. https://doi.org/10.3390/min15030269

Chicago/Turabian Style

Wang, Ning, Jun Zhao, Yingxia Xu, Mangen Mu, Shangqing Zhang, Libo Jing, Guoshu Huang, Liang Liu, and Pengfei Tian. 2025. "Paleoenvironmental Controls and Economic Potential of Li-REY Enrichment in the Upper Carboniferous Coal-Bearing “Si–Al–Fe” Strata, Northeastern Qinshui Basin" Minerals 15, no. 3: 269. https://doi.org/10.3390/min15030269

APA Style

Wang, N., Zhao, J., Xu, Y., Mu, M., Zhang, S., Jing, L., Huang, G., Liu, L., & Tian, P. (2025). Paleoenvironmental Controls and Economic Potential of Li-REY Enrichment in the Upper Carboniferous Coal-Bearing “Si–Al–Fe” Strata, Northeastern Qinshui Basin. Minerals, 15(3), 269. https://doi.org/10.3390/min15030269

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