Mineralogy and Critical Metal Distribution in Upper Carboniferous Aluminum-Bearing Strata from the Yangquan Mining Area, Northeastern Qinshui Basin: Insights from TIMA

Abstract

Critical metals associated with aluminum-bearing strata have garnered increasing attention due to their considerable economic potential. Recent investigations have identified notable enrichment of Li, Ga, Zr, Nb, REEs (rare earth elements), etc., within the Upper Carboniferous Benxi Formation in the Yangquan mining area, the Northeastern Qinshui Basin, Northern China. However, their mineralogical characteristics and micro-scale modes of occurrence remain insufficiently constrained. In this study, we employed the TESCAN Integrated Mineral Analyzer (TIMA) in combination with X-ray diffraction (XRD) and clay-separation experiments to provide direct mineralogical evidence for the occurrence of Ti, Li, Ga, Zr, and REEs in claystone and aluminous claystone from the Benxi Formation, Yangquan mining area, Northeastern Qinshui Basin. Our results indicate that both lithologies are primarily composed of kaolinite and diaspore, with minor amounts of anatase and cookeite; illite is additionally present in the claystone. Titanium predominantly occurs as anatase in both lithologies, though a portion in aluminous claystone may be incorporated into kaolinite and other Ti-bearing minerals such as rutile and leucoxene. Lithium is primarily hosted by cookeite in both rock types. Mineral assemblage variations further suggest that kaolinite may have partially transformed into Li-rich chlorite (i.e., cookeite) during the transformation from aluminous claystone to claystone. Gallium is chiefly associated with diaspore and kaolinite, with a stronger correlation with diaspore in the aluminous claystone. Zircon is the sole carrier of Zr in both lithologies. Importantly, La and Ce show a consistent spatial association with O–Al–Si–Ti–P mixed aggregates in TIMA maps, particularly in aluminous claystone. Based on these spatial patterns, textural relationships, and comparisons with previous studies, phosphate minerals are inferred to be the dominant REE hosts, although minor contributions from other phases cannot be completely excluded. These findings highlight a previously underexplored mode of critical-metal enrichment in Northern Chinese bauxite-bearing strata and provide a mineralogical basis for future extraction and utilization.

1. Introduction

In recent years, critical metals such as lithium (Li), gallium (Ga), zirconium (Zr), niobium (Nb), and rare earth elements (REEs) have garnered growing attention due to their strategic importance in the advancement of emerging technologies, including new energy vehicles, advanced electronics, and national defense systems [1,2]. Owing to their low crustal abundance, high economic value, and indispensable roles in high-tech applications, these elements have been designated as strategic or critical resources in many countries [3,4,5]. However, the conventional supply of these metals is becoming increasingly constrained, prompting global efforts to explore and exploit alternative resources, including recovery from unconventional geological settings such as aluminum- and coal-bearing sequences [6,7,8].

Aluminum-bearing strata, particularly those formed during the Carboniferous–Permian periods, are widely distributed across both Southern and Northern China and have been increasingly recognized as promising alternative sources for strategic critical metals [9]. These strata are typically dominated by clay-rich lithologies such as aluminous claystone, ferruginous claystone, argillaceous claystone, and mudstone, which have a strong capacity to adsorb or incorporate trace metals during sedimentation and early diagenesis. In Southern China, a series of studies have demonstrated significant enrichment of critical metals within these sequences. For example, Permian bauxite deposits in the Central Yunnan, Southwestern China, exhibit elevated contents of REEs (up to 286 ppm) and Li (ranging from 171 ppm to 2083 ppm, with an average of 957 ppm), and they are also enriched with B, Be, Ga, Cr, V, Sc, Zr, Hf, Nb, Ta, W, Th, and U [10]. In the Pingguo area of Guangxi, Late Permian claystones are characterized by high Li contents (up to 1.05 wt.% Li₂O, averaging 0.45 wt.% Li₂O), primarily hosted in cookeite [11]. Similarly, in Western Guizhou, Late Permian claystones in the aluminum-bearing strata show enrichment in Nb, Zr, REEs, and Ga, with average contents of 298 ppm, 2231 ppm, 1669 ppm, and 67 ppm, respectively [12]. Internationally, comparable bauxitic systems have also been identified as important critical metal repositories. In the Mediterranean region, the Greek bauxite ores of the Parnassos–Ghiona area host REE contents of up to 2782 ppm (average 527 ppm), largely associated with diasporic phases, and they include monazite, xenotime, and cerianite [13]. In the Eastern Alborz Mountains of Northern Iran, the Gano bauxite deposit has been investigated for its Ga, Nb, and V recovery potential, with REE contents ranging from 23 ppm to 767 ppm [14]. In Northwestern Iran, Permian bauxite deposits such as Kanirash, Shahindezh, Qopi, and Kanigorgeh have been reported to have high contents of elements such as Ti, Ga, V, and LREE [15,16,17,18]. Additionally, extensive studies have already been conducted on REE-enriched bauxite deposits in Iran that share a similar genetic setting. For example, work in the Zagros Fold–Thrust Belt (REY: ~264–1637 ppm) and the Sanandaj–Sirjan Zone (REY: ~256–1322 ppm) has meticulously identified the host minerals and geochemical behavior of REEs [19,20,21]. Furthermore, in Central and Southern Italy, Cretaceous bauxite deposits have been reported as potential sources of critical metals such as Co, Ni, and LREEs (light rare earth elements, including La, Ce, Pr, Nd, and Sm [22]) [6]. Multivariate R-mode statistical analysis indicates that the distribution of these metals is primarily controlled by Al-, Fe-, and Ti-oxhydroxides, and to a lesser extent, by detrital phases [6]. These examples underscore the geochemical affinity of critical metals for aluminum-rich sedimentary environments and highlight their resource potential beyond conventional deposits.

Despite these advances, relatively few studies have been conducted on similar aluminum-rich strata in Northern China, particularly within the Upper Carboniferous Benxi Formation (e.g., [23,24,25]). Preliminary geochemical data suggest that the Benxi Formation in the Yangquan mining area, located in the Northeastern Qinshui Basin, has considerable potential for critical-metal enrichment [26,27]. Recent analyses have reported elevated contents of TiO₂, Li, Ga, Zr, and REEs in the “Si-Al-Fe” lithological interval, with maximum values reaching 3.09 wt.% for TiO₂ (on an SO₃-free ash basis), 345 ppm for Li, 33 ppm for Ga, 927 ppm for Zr, and ΣREEs + Y exceeding 1017 ppm, especially in aluminous claystones [27]. However, the absence of systematic mineralogical and microtextural investigations has hindered a comprehensive understanding of the modes of occurrence and host phases of these metals.

In this study, the TESCAN Integrated Mineral Analyzer (TIMA) combined with clay separation experiments were employed to characterize the mineral assemblages and to investigate the distribution and modes of occurrence of Ti, Li, Ga, Zr, and REEs in two representative rock samples from the Upper Carboniferous Benxi Formation in the Yangquan area, Northeastern Qinshui Basin, Northern China, as previously reported by Wang et al. [26]. The objective is to provide a mineralogical basis and theoretical support for the future extraction and utilization of these strategic metals.

2. Geology Setting

The Yangquan mining area is situated in the central part of the North China Craton (NCC), extending north–south along the eastern margin of the Qinshui Basin (Figure 1A). The region hosts extensively developed Upper Carboniferous Benxi Formation, characterized by aluminum-bearing strata with unique potential for the exploration and development of critical metals.

The NCC was assembled during the Archean (ca. 1800–1900 Ma) through the amalgamation of its eastern and western blocks and the intervening Central Orogenic Belt [28,29]. Geographically, it is bounded by the Central Asian Orogenic Belt to the north and the Qinling–Dabie Orogenic Belt to the south, with the Yangtze Craton bordering it to the south (Figure 1A). The basement of the NCC mainly comprises Archean to Paleoproterozoic gneisses, granites, amphibolites, migmatites, and banded iron formations, which are unconformably overlain by Mesoproterozoic–Neoproterozoic and Phanerozoic sequences [30,31]. During the late Middle Ordovician, the NCC experienced regional uplift in response to the Caledonian orogeny, followed by prolonged weathering and erosion lasting approximately 150 million years. By the Upper Carboniferous, an extensive weathering-derived residual sedimentary layer had developed across the craton, forming the Benxi Formation [32,33].

Figure 1. Regional geological setting (modified from [29]). (A) Location of the Qinshui Basin and the Yangquan mining area within the North China Craton (NCC). (B) Stratigraphic column of the Benxi Formation and adjacent units, along with sampling locations from this study.

Figure 1. Regional geological setting (modified from [29]). (A) Location of the Qinshui Basin and the Yangquan mining area within the North China Craton (NCC). (B) Stratigraphic column of the Benxi Formation and adjacent units, along with sampling locations from this study.

In the Yangquan mining area, the Benxi Formation is 20–50 m thick. It conformably underlies the Upper Carboniferous Taiyuan Formation and unconformably overlies the Middle Ordovician Fengfeng Formation, forming a typical vertical profile of an aluminum-bearing sequence (Figure 1B), consistent with the field outcrop profile documented by Wang et al. [26]. Based on lithological characteristics, the Benxi Formation in the study area can be subdivided into three members, from bottom to top: (1) the lower member consists predominantly of yellow-brown pyrite-bearing claystone, locally hosting Shanxi-type iron ore bodies composed mainly of hematite and limonite, which occur in nest-like, concretionary, or lenticular forms; (2) the middle member is dominated by grayish-white bauxite, featuring oolitic, clastic, or massive textures; and (3) the upper member comprises light-yellow claystone, gray hard refractory clay, and aluminous claystone, interbedded with thin coal seams and locally with silty mudstone. The samples analyzed in this study were collected from the middle and upper members of the Benxi Formation (Figure 1B).

3. Samples and Analytical Methods

3.1. Sample Collection

Two rock samples (a claystone and an aluminous claystone) were meticulous selected from borehole No. 7602, located within the Upper Carboniferous aluminum-bearing strata of the Yangquan mining area, Northeastern Qinshui Basin, Northern China, as previously reported by Wang et al. [26]. The sampling locations are indicated in Figure 1A, and the two samples are re-labeled as 7602-a and 7602-b in Figure 1B.

3.2. Analytical Methods

3.2.1. XRD

X-ray diffraction (XRD) analyses were conducted using a D/max-2500 PC diffractometer (Rigaku, Tokyo, Japan) equipped with a Cu-Kα radiation source and a Ni filter. The instrument was operated at 40 kV and 100 mA, with a step size of 0.02°, a scanning speed of 2°/min, and a scanning range from 2.5° to 70° 2θ. The obtained XRD patterns were first subjected to qualitative mineral identification using MDI Jade 6.0 software.

3.2.2. TIMA

Quantitative mineralogical analyses were carried out using the TESCAN Integrated Mineral Analyzer (TIMA3 X GHM, Brno, Czech Republic), which integrates a TESCAN MIRA 3 scanning electron microscope (SEM) with nine detectors, including four high-throughput silicon drift energy-dispersive X-ray spectrometers (EDAX Element 30). This system enables rapid determination of mineral species, modal abundances, mineral associations, and elemental occurrence modes. Data acquisition was conducted in dot-mapping mode. For the study samples, each EDS point was collected with 1500 X-ray counts, a BSE pixel spacing of 3 μm, and an EDS dot spacing of 9 μm. All measurements were performed under high-vacuum conditions, with an accelerating voltage of 25 kV, a beam current of 9 nA, and a working distance of 15 mm. Beam current and BSE signal intensity were automatically calibrated using a platinum Faraday cup, while EDS signals were calibrated against a manganese (Mn) standard. The TIMA software (TIMA3 X GHM) can automatically compare the measured BSE and EDS data of each different phase with the database and then distinguish their mineral phases and compute mineral abundances.

To ensure the reliability of the automated mineralogical analyses, several quality assurance and control (QA/QC) procedures were implemented during TIMA measurements. Prior to each analytical session, the EDS detectors were calibrated against a manganese (Mn) standard, while the beam current and BSE signal intensity were automatically adjusted using a platinum Faraday cup to guarantee instrumental stability. Mineral identification was achieved by comparing the measured BSE and EDS data with an extensive internal mineral database, and ambiguous or rare mineral phases were further verified through manual inspection of representative spectra. The modal mineral abundances obtained from TIMA were also cross-checked against bulk XRD results to validate the consistency of the mineral assemblages. These QA/QC measures collectively ensure that the TIMA-derived mineralogical data are both quantitative and reproducible within the analytical resolution of the method.

3.2.3. Clay Separation Experiment

Our clay separation experiment was conducted following a standard six-step procedure. First, powdered samples were dispersed in ultrapure water, using 3 min of ultrasonication, followed by 4 h of natural settling. The resulting suspensions were pipetted into two centrifuge tubes (~10 mL each) and then centrifuged at 3000 rpm for 15 min to obtain concentrated clay fractions. One portion was used to prepare air-dried (AD) slides by smearing the clay onto heat-resistant glass and allowing it to dry naturally. The AD slides were then treated with ethylene glycol vapor at 60 °C for 12 h to produce EG slides, and subsequently heated at 400 °C and 550 °C for 2 h in a muffle furnace to obtain H-400 °C and H-550 °C slides, respectively. To identify the presence of cookeite, distinguishable from Fe-Mg-rich chlorite by its resistance to acid dissolution, the second clay concentrate was treated with 5 mL of 6 mol/L HCl and heated in an 80 °C water bath for 15 min. The suspension was then centrifuged, washed 3 to 5 times with ultrapure water until chloride-free, and the final residue was smeared onto a slide and air-dried (HCl slide). The detailed experimental procedures are described in Wang et al. [34].

Each oriented clay specimen prepared through the above procedures was analyzed by XRD. The measurements were carried out using a Bruker D8 Advance powder diffractometer equipped with Ni-filtered Cu-Kα radiation and a scintillation detector. Diffraction patterns were recorded over a 2θ range of 4° to 30°, with a step size of 0.02°.

4. Results

As reported by Wang et al. [26], samples 7602-a and 7602-b exhibit significantly elevated contents of critical metals compared to average upper continental crust (UCC, [35]) values, including TiO₂ (2.16 wt.%, 2.95 wt.%), Li (224 ppm, 1700 ppm), Ga (30.2 ppm, 49.3 ppm), Zr (474 ppm, 1050 ppm), and REEs (356.42 ppm, 659.01 ppm). This study therefore focuses on identifying the host minerals responsible for the enrichment of Ti, Li, Ga, Zr, and REEs and examining their distribution and modes of occurrence in the samples.

4.1. Mineralogical Phases Identified by XRD and TIMA

XRD patterns (Figure 2) show that the claystone (sample 7602-a) is dominated by kaolinite and diaspore, with minor illite, anatase, and chlorite. The aluminous claystone (sample 7602-b) is also dominated by kaolinite and diaspore, with minor anatase. However, a key difference lies in the more prominent chlorite peak and the absence of illite in sample 7602-b (Figure 2).

TIMA-automated mineralogical results are broadly consistent with the XRD findings (Figure 3). Quantitative data indicate that both samples are primarily composed of kaolinite and diaspore, accounting for 54.58 wt.% and 28.51 wt.% in sample 7602-a, and 79.71 wt.% and 15.19 wt.% in sample 7602-b, respectively. In addition, several minor mineral phases were identified in both samples, including cookeite, schorl, limonite, montmorillonite, quartz, and a O-Al-Si-Ti-P mixed phase (representing mixtures of aluminosilicates, Ti-bearing minerals, and phosphates) (Figure 3A,C). Notably, cookeite occurs in both samples, with a relatively high abundance of 12.02 wt.% in sample 7602-a, but only 0.02 wt.% in sample 7602-b. The O-Al-Si-Ti-P mixed phase constitutes 1.54 wt.% and 4.06 wt.% of the mineral assemblages in the claystone and aluminous claystone, respectively. Other identified minerals occur in trace amounts (<1 wt.%) (Figure 3B,D).

In addition, TIMA whole-scan images (Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9) further illustrate the modes of occurrence of several mineral phases:

(1) Kaolinite and diaspore: In both lithologies, kaolinite forms a widespread matrix (Figure 4A and Figure 5A). In the claystone, diaspore also occurs as a matrix mineral intergrown with kaolinite (Figure 4B and Figure 5B), whereas in the aluminous claystone, it typically forms detrital or oolitic grains interbedded with kaolinite. Locally, kaolinite is enclosed within diaspore grains (as indicated by irregular circular outlines in Figure 5A–C), suggesting formation via reaction between diaspore and silica-rich fluids.

(2) Cookeite: In the claystone, cookeite is extensively disseminated in the matrix, embedded within kaolinite- and diaspore-rich domains (Figure 6). Although less abundant in the aluminous claystone, cookeite can still be observed in localized areas, where it closely coexists with kaolinite and diaspore, primarily associated with kaolinite. In particular, cookeite is distributed within the kaolinite phase that occurs inside oolitic diaspore grains (Figure 6C,D, indicated by irregular circular outlines).

(3) Anatase and zircon: Both occur as fine-grained particles sparsely disseminated in the kaolinite–diaspore matrix of both lithologies (Figure 7A,B and Figure 8A,B).

(4) O-Al-Si-Ti-P mixed phase: This phase is irregularly distributed within the matrix of both lithologies. It is more abundant in the aluminous claystone, where it predominantly surrounds the detrital/oolitic diaspore grains, with only minor occurrences within the diaspore grains themselves (Figure 9A,B).

4.2. XRD of Oriented Specimens

Clay fraction separations were conducted for both samples. XRD patterns under various treatments are shown in Figure 10, from top to bottom, including AD (air-dried) slides, EG (ethylene glycol) slides, HCl (dilute hydrochloric acid) slides, H-400 °C (heated at 400 °C) slides, and H-550 °C (heated at 550 °C) slides.

In the claystone, chlorite in the AD slide displays two broad but discernible characteristic diffraction peaks at d = 14.52 Å and d = 3.51 Å. These peaks remain largely unchanged after treatment with ethylene glycol. Upon heating to 400 °C, the 001 peak shifts from d = 14.38 Å to d = 14.02 Å. At 550 °C, and all diffraction peaks disappear, indicating structural collapse. After HCl treatment, peak intensities remain nearly unchanged, though the 001 peak shifts slightly from d = 14.52 Å to 14.07 Å (Figure 10A). In contrast, the chlorite in the aluminous claystone exhibits three relatively distinct characteristic peaks in the AD slide at d = 14.20 Å, d = 4.72 Å, and d = 3.52 Å. These peaks persist unchanged in the EG slide. At 400 °C, all three peaks remain but with slightly reduced intensity. At 550 °C, only two peaks remain (d = 13.29 Å, 3.51 Å). After HCl treatment, all three peaks are preserved, with slight shifts to d = 13.98 Å, 4.71 Å, and 3.51 Å, respectively (Figure 10B).

Illite in the claystone shows little change after AD, EG, HCl, and 400 °C treatments, but most peaks disappear after heating to 550 °C (Figure 10A). In the aluminous claystone, illite maintains an ~10 Å peak under all treatments (Figure 10B). Kaolinite shows similar behavior in both samples: its two characteristic peaks at d = ~7.12 Å and d = ~3.56 Å are stable after AD, EG, HCl, and 400 °C treatments but disappear after heating to 550 °C, indicating complete structural collapse (Figure 10A,B).

5. Discussion

5.1. Modes of Occurrence of Ti

In recent years, investigations into aluminum- and coal-bearing sequences have identified multiple modes of Ti occurrence, most commonly as discrete Ti-bearing mineral phases [37]. For instance, SEM-EDS and EPMA analyses of basal claystones (Nb-Zr-REE-Ga polymetallic horizons) within the Upper Permian Xuanwei and Longtan formations of Eastern Yunnan and Western Guizhou indicate that Ti predominantly occurs in anatase, rutile, and ilmenite, with minor titanite [12,38,39]. In contrast, Ti in Late Paleozoic coals from the Daqingshan coalfield, Inner Mongolia, is more commonly associated with clay minerals, predominantly hosted in kaolinite and, to a lesser extent, illite [40]. Additionally, in some low-grade metamorphic coal-bearing strata, Ti may also occur within organic matter [41].

In the present study, TIMA whole-scan imaging reveals that Ti in the claystone is mainly hosted in anatase (Figure 7A,C). In the aluminous claystone, Ti is not only present in anatase but also likely associated with kaolinite. As illustrated in Figure 7D, certain Ti-enriched zones (indicated by irregular outlines) spatially correspond to the distribution of kaolinite enclosed within diaspore grains, consistent with the mineralogical features observed in Figure 5A,B. Furthermore, unpublished heavy mineral separation data suggest the presence of additional rutile and leucoxene (weathered ilmenite) in minor amounts, within the aluminous claystone, indicating a more complex assemblage of Ti-bearing phases.

5.2. Modes of Occurrence of Li

In aluminum- and coal-bearing sequences, Li typically occurs in two principal forms. (1) The first form is as discrete Li-rich minerals (e.g., lepidolite, elbaite, and hectorite) [41,42,43]. Among these, hectorite is characteristic of clay-type Li deposits, such as those found in Clayton Valley and McDermitt Caldera (USA), Sonora (Mexico), and Jadar (Serbia) [44,45,46]. (2) The second form as isomorphous substitutions or visa surface adsorption in clay minerals, with host phases varying regionally [9]. For example, Li in the Lower Permian Daoshitou Formation (Central Yunnan) and the Lower Carboniferous Jiujialu Formation (Guizhou) is chiefly associated with smectite-group minerals [47,48]; in the Upper Permian Heshan Formation bauxite of Central Guangxi, Li is mainly hosted in diaspore [49]; and in Southwestern Guangxi, Li is dominantly enriched in cookeite [11]. In the Upper Carboniferous Taiyuan Formation coals of the Northern Qinshui Basin, Li is primarily associated with kaolinite [50], while in the southern basin, cookeite is the principal host [51,52,53].

In this study, TIMA analyses revealed that cookeite is widely disseminated within the kaolinite matrix in the claystone (Figure 4A,B and Figure 6A). Similarly, in the aluminous claystone, cookeite is enclosed within kaolinite domains that are themselves embedded in diaspore grains (Figure 6B–D). These observations collectively suggest that the high Li contents in both lithologies are primarily contributed by cookeite. In addition, clay fraction separation further confirms the identity of the Li-bearing chlorite as cookeite (Figure 10). This is supported by the results of thermal- and acid-stability tests: Fe-Mg chlorites such as chamosite and berthierine lose their diagnostic XRD peaks after HCl treatment, whereas cookeite retains them [54]. In our samples, HCl-treated slides consistently preserve the cookeite diffraction peaks, supporting cookeite’s identification (Figure 10).

Interestingly, the lower aluminous claystone contains significantly more Li (1700 ppm) than the overlying claystone (224 ppm), yet TIMA data indicate that cookeite is more abundant in the upper unit (12.02 wt.%) than in the lower one (0.02 wt.%). This apparent contradiction may result from misidentification during TIMA scanning: due to the intimate intergrowth of kaolinite and cookeite in the upper claystone, regions rich in kaolinite may have been mistakenly assigned as cookeite, particularly given their 20.56% spatial overlap. This suggests a possible mineralogical transformation from kaolinite to cookeite during diagenesis, consistent with the stratigraphic evolution from aluminous claystone to claystone. A similar transformation mechanism was proposed by Ling et al. [11], who documented interlayered associations of cookeite with kaolinite and detrital illite in Late Permian Li-rich claystones from the Pingguo mining area, Guangxi. They inferred that cookeite likely formed from Li-enriched kaolinite during diagenesis. While our findings support a similar mechanism, the specific transformation process warrants further investigation.

5.3. Modes of Occurrence of Ga and Zr

Gallium exhibits similar geochemical behavior to Al and is commonly hosted in Al-bearing minerals (including kaolinite, diaspore, boehmite, goyazite, etc.) within aluminum- and coal-bearing sequences through isomorphous substitution [25,37]. For example, in the basal aluminous clay layer of the Late Permian Xuanwei Formation (Eastern Yunnan), Ga is primarily hosted in kaolinite [38,55]. In Inner Mongolia, Ga has been reported in association with kaolinite and diaspore (Adaohai Mine, Daqingshan Coalfield), boehmite (Heidaigou Mine, Junggar Coalfield), and goyazite (Guanbanwusu Mine, Junggar Coalfield) [56,57,58]. In this study, TIMA whole-scan images revealed that Ga is evenly distributed within kaolinite- and diaspore-rich matrices in both lithologies (Figure 4A,B and Figure 5A,B), with spatial distribution patterns closely matching those of Al (Figure 4C and Figure 5C). In particular, Ga shows a stronger affinity with diaspore in the aluminous claystone (Figure 5D), suggesting that diaspore plays a more prominent role as a Ga host in this lithology. These observations are broadly consistent with Ga occurrence modes reported from other aluminum-bearing strata.

Zirconium in aluminum- and coal-bearing sequences is typically hosted in primary zircon grains [37,59]. However, previous studies have identified additional occurrence modes, including (1) as adsorbed or structurally bound species in clay minerals such as I/S (illite–smectite mixed layers) and kaolinite. For instance, Zhao et al. [60] reported a positive correlation between Zr and I/S in the mudstone layers of the Upper Permian Xuanwei Formation, Eastern Yunnan, and similarly, Li et al. [61] observed a positive correlation between Zr and both Al₂O₃ and SiO₂ in coal from the Weibei Coalfield, Southern Shaanxi. (2) Another occurrence mode is in association with Ti-bearing minerals such as anatase, rutile, and ilmenite, as demonstrated by Zhao et al. [62], who documented a positive correlation between Zr and TiO₂ in the CP2 coal seam of the Daqingshan Coalfield, Inner Mongolia. (3) A third occurrence mode is within phosphate minerals. For instance, Wang et al. [38] reported a positive correlation between Zr and P₂O₅ in mudstone of the Upper Permian Xuanwei Formation, Eastern Yunnan.

In contrast, our TIMA results demonstrate that Zr in both the claystone and aluminous claystone occurs exclusively within discrete zircon grains, with clear spatial correspondence (Figure 8). No evidence was found for Zr associated with clay minerals, Ti-bearing phases, or phosphates, suggesting that zircon is the sole carrier of Zr in the studied samples.

5.4. Modes of Occurrence of REEs

REEs in aluminum- and coal-bearing sequences are typically hosted in two principal forms: organically bound and inorganically bound (i.e., mineral-bound) forms [41,63]. The inorganic association primarily includes (1) detrital terrigenous or volcanogenic minerals derived from source areas during syndepositional stages, such as zircon, monazite, apatite, and xenotime; and (2) authigenic phosphate or sulfate minerals containing REEs that formed during syn- or post-depositional stages, such as aluminum phosphate–sulfate (APS) minerals, hydrated phosphates, carbonates, or fluorocarbonates (e.g., florencite, rhabdophane, and parisite) [22,37].

Consistent with the findings of Wang et al. [26], both the claystone and aluminous claystone in this study show significant enrichment with LREEs. Accordingly, La and Ce were selected as representative elements for spatial mapping using TIMA analysis. The results reveal that La and Ce are sparsely distributed in the claystone (Figure 9C,E), but they are more extensively and systematically concentrated in the aluminous claystone, especially around detrital or oolitic diaspore grains (Figure 9D,F). Notably, the spatial patterns of La and Ce in aluminous claystone closely correspond to those of O-Al-Si-Ti-P mixed phases (Figure 9B). This spatial correspondence suggests that REEs (especially LREEs) in both lithologies are primarily hosted within these mixed mineral aggregates. Based on the mineral characteristics, phosphate minerals within these aggregates, such as APS minerals and florencite, are inferred to be the major REE carriers.

Although the REE signals do not form large, concentrated grains, their repeated spatial overlap with P-bearing microdomains and their occurrence as discrete inclusions within kaolinite–diaspore matrices support an interpretation of authigenic phosphate precipitation during diagenesis. Importantly, we find no systematic association of La and Ce with Fe-rich zones or with Al-only domains, observations that argue against clay minerals or Fe-oxides/hydroxides as their primary hosts. While more detailed microanalytical approaches (e.g., SEM-EDS point analyses or pixel-level statistics) could provide further confirmation, the convergence of TIMA spatial patterns, petrographic context, and comparisons with similar REE-enriched bauxite systems in China and Iran provides a reasonable basis to conclude that phosphate minerals are the dominant REE carriers in the Benxi Formation.

6. Conclusions

This study employed the TIMA to characterize the mineralogical composition, spatial distribution, and modes of occurrence of critical metals (Ti, Li, Ga, Zr, and REEs) in representative claystone and aluminous claystone samples from the Upper Carboniferous Benxi Formation in the Yangquan mining area, Northeastern Qinshui Basin, Shanxi Province, Northern China. Both lithologies are dominated by kaolinite and diaspore, with minor anatase and cookeite; illite occurs only in the claystone, whereas cookeite is more prominent in the aluminous claystone, as confirmed by XRD analysis.

Titanium primarily occurs as anatase in both rock types, with additional Ti in the aluminous claystone likely incorporated into kaolinite and minor Ti-bearing phases such as rutile and leucoxene. Lithium occurs almost exclusively in cookeite in both lithologies, and mineral assemblage evidence suggests a partial diagenetic transformation of kaolinite to cookeite during the transition from aluminous claystone to claystone. Gallium is chiefly associated with diaspore and kaolinite, with a stronger affinity for diaspore in the aluminous claystone. Zircon is the sole carrier of Zr in both lithologies, indicating a predominantly detrital origin. REE mapping (La and Ce) shows close spatial correspondence with O-Al-Si-Ti-P mixed phases, suggesting phosphate minerals as the principal REE hosts.