Provenance and Paleo-Environment of the Late Carboniferous Bauxite Formations in Southern Shanxi

Abstract

The Carboniferous Benxi Formation in southern Shanxi of North China has significant bauxite resource potential; however, the source of its metallogenic material and its sedimentary environment remain unclear. The microscopy, X-ray diffraction, X-ray fluorescence spectroscopy, and inductively coupled plasma mass spectrometry methods were applied in this study to examine the mineralogical, petrographic, and geochemical characteristics. Geochemical proxies of La/Y, Sr/Ba, Al₂O₃/TiO₂, Zr/Sc, Th/Sc, La/Sc, and Th/Co were analyzed to investigate the paleo-depositional environment and provenance of the aluminum-bearing strata. The findings indicate that diaspores are the primary ore minerals in bauxite, while kaolinite and rutile are the predominant gangue minerals. Both the bauxite and claystone/aluminous rocks exhibit high enrichment in Li, Bi, and U, with relative enrichment in In, Sb, Th, Nb, and Ta. Li is notably concentrated in the claystone/aluminous rocks, reaching up to 1994.00 μg/g, primarily occurring in cookeite and boehmite, while U is highly concentrated in the bauxite. The aluminum-bearing strata were primarily formed under alkaline-reducing conditions, with changes in acidity and alkalinity of the environment during the sedimentary diagenetic process. Marine transgressions significantly impacted the sedimentary environment of the aluminum-bearing strata, and the paleoclimate was characterized as hot and humid. The principal factors contributing to enrichment of aluminum in the sedimentary basin were the in situ weathering of aluminum-rich source rocks and the transport of clastic materials from high-aluminum source rocks. The source rocks were closely associated with intermediate-acidic magmatic rocks and potentially related to the weathering of Ordovician carbonates.

1. Introduction

Based on the lithology of the underlying bedrock and metallogenic characteristics of bauxite, bauxite deposits are categorized into lateritic, karst, and sedimentary types [1]. In China, sedimentary bauxite is the most prevalent, comprising over 80% of the country’s total bauxite reserves. These deposits are extensively found in Shanxi, Henan, Guizhou, Guangxi, and other regions of China. Bauxite has wide-ranging industrial applications and is classified as a strategic critical mineral by the United States, European Union, Japan, and China [2].

During the Late Carboniferous, the bauxite in the North China Craton (NCC) was relatively uniform, primarily concentrated at the base of the Benxi Formation. They conform to the coal-bearing strata of the Taiyuan Formation from the Late Carboniferous and parallel unconformity with the underlying Ordovician Majiagou or Fengfeng Formation [3]. Recently, numerous studies have been conducted on North China bauxite in terms of mineralogy, geochemistry, metallogenic materials source, ore deposit genesis, and metallogenic age [4,5,6,7,8,9,10,11,12]. The researchs show that differences exist in the sedimentary sources of bauxite in the northern, central, and southern regions of the NCC. The substantial clastic material present in the southern to central parts of the NCC is derived from the uplifted North Qinling orogenic belt [6,13], while in the northern part of the NCC, the materials originate from the uplift zone caused by the southern branch of the Central Asian Orogenic Belt (Bainaimiao continental arc) [12,14]. The detrital zircon age spectra in the bauxite deposits of the central part of the NCC demonstrate characteristics of both southern and northern sources [10]. However, the absence of any evidence pertaining to fluvial deposits has been observed in the bauxite beds thus far, suggesting that the convergence mechanism of the southern and northern sources in the central and southern parts of the NCC is still unclear, and the source of materials remaining a matter of debate.

Shanxi Province is a major bauxite-producing region in China, with the large bauxite resource reserves in the country. These resources are distributed across the province and can be categorized into seven primary bauxite concentration areas from south to north and east to west: Pinglu, Wuxiang, Qinyuan, Linxian-Jiaokou, Yangquan, Yuanping-Ningwu, and Baode-Xingxian [15,16].

According to the mineralization environment, bauxite can be classified into three types around the Pinglu mine area: marine, terrestrial, and marine-terrestrial transitional facies. Some geologists think it may be occurred in a weakly oxidizing environment of marine or marine-terrestrial transitional facies [17], coastal lake facies [15], marine sedimentary [18], or surface alkaline-reducing karst depression environments with microorganisms widely participating in the mineralization process [19]. And there are mainly three theories regarding the material sources of karst-type bauxite deposits in Shanxi Province. Liu Changling [20], Lu Jingwen et al. [21], and Zhao Yunfa et al. [22] suggested that the bauxite originated from ancient landmass aluminosilicate rocks. Wen Tongxiang [23] and Meng Jianyin et al. [24] hold that the bauxite is a mixed source from both the basement carbonate rocks and ancient landmass aluminosilicate rocks. However, Sun Xuefei et al. [19] thinks that the main body of the iron-rich clay rocks at the bottom of large karst-type bauxite deposits was formed by in situ weathering of the underlying carbonate rocks, while the upper bauxite and clay rocks are of allochthonous origin.

In the geological history, the area of Pinglu County has experienced the ancient rock metamorphism and tectonic movement, forming a unique geological structural characteristics and rich mineral resources. Although some scholars have conducted research on the sedimentary bauxite in the Pingguo mining area, the number of samples is too small, and the testing data is limited. The Jinyu and Jinshi profiles, investigated in this study, are located in the Pinglu bauxite concentration area. The bauxite-bearing strata in the Carboniferous Benxi Formation mainly consist of clastic rocks rich in iron, aluminum, and coal. However, there is ambiguity in the sedimentological markers that indicate the depositional environment and facies, and the “iron-aluminum-coal” depositional system has few primary sedimentary structures. Recent research has involved U–Pb dating and Hf isotope analysis of detrital zircons, in addition to trace element analysis of detrital rutile in the Shanxi bauxite [25,26]. These studies indicate that the igneous rocks from the North Qinling orogenic belt, arc granites along the northern margin of the NCC, and regional metamorphic rocks could be important sources of the metallogenic materials for bauxite [6,7,13]. This indicates the complex and diverse origins of metallogenic materials for the Late Carboniferous bauxite in North China.

Shanxi Province is a large coal province in China, and the lithium-rich aluminum-bearing rock system is symbiotic/associated with coal-bearing strata (such as Benxi Formation, Taiyuan Formation and Shanxi Formation). The provenance and sedimentary environment of aluminum-bearing rock strata in south Shanxi Province are controversial. Therefore, this paper takes the aluminum-bearing rock strata of Benxi Formation in Pinglu area of southern Shanxi Province as the research object and carries out a series of research work (Figure 1). This study aims to ascertain the mineralogical, petrographic, and geochemical characteristics of aluminum-bearing strata, explore their metallogenic sources and paleo-depositional environments, and offer theoretical insights for further clarifying the metallogenic model. This will aid in the development and utilization of bauxite and underlying coal-based aluminum resources in the region.

2. Geological Background

As one of the oldest cratons in the world, the NCC is bordered on the south by the Qinling Orogenic belt, on the north by the Central Asian Orogenic Belt, and on the east by the Pacific Plate [1,2]. The craton’s basement predominantly comprises Archean to Paleoproterozoic metamorphic rocks, which are covered by Mesoproterozoic to Neoproterozoic unmetamorphosed volcanic–sedimentary rocks, as well as Phanerozoic Cambrian–Ordovician marine carbonates, Carboniferous–Permian iron–aluminum strata and coal-bearing strata, and Mesozoic Triassic–Cretaceous clastic strata [10,27]. The region features two major ancient weathering crust deposit-type bauxite metallogenic areas: the Shanxi fault-uplift aluminum district and the southern margin of the North China Block aluminum district (Figure 2a).

The Pinglu bauxite mining area is situated in Pinglu County, Yuncheng City, southern Shanxi Province, China. In the geological history, the area of Pinglu County has experienced the ancient rock metamorphism and tectonic movement, forming a unique geological structural characteristics and rich mineral resources. It lies on the southern edge of the NCC, at the intersection of the western Henan uplift and Zhongtiao Mountain fault arch. The regional stratigraphy, from oldest to youngest, includes Lower Proterozoic Zhongtiao Group metamorphic rocks, Mesoproterozoic Ruyang Group carbonate clastic rocks, Cambrian–Ordovician carbonates, Carboniferous–Permian clastic coal-bearing strata, and Cenozoic Paleogene, Neogene, and Quaternary formations (Figure 2b).

The study area features intricate tectonics, with principal structural lines extending northeast and predominantly featuring high-angle normal faults. The Pinglu bauxite reserves are primarily found in the Benxi Formation of the Late Paleozoic Carboniferous system, with the underlying bedrock consisting of Ordovician Majiagou Formation carbonates. The bauxite-bearing strata, from bottom to top, primarily include ferruginous claystone, bauxite, and claystone, interspersed with occasional coal seams or thin mudstone beds. The overlying strata primarily consist of mudstone and sandstone.

The morphology of the ore bodies within the region varies significantly owing to changes in the deposition basement topography. In areas with expansive, relatively flat dissolution basin or depression, the ore bodies typically exhibit a lamellar or quasi-lamellar with good continuity. In contrast, where there are notable height differences in the basement paleotopography, the ore bodies tend to be lenticular with poor continuity. In cases where the basement paleotopography resembles a dissolution funnel, the ore bodies take on a funnel-like shape.

3. Sample Collection and Test

The Pinglu bauxite rock strata can be vertically divided into three parts, namely, from bottom to top, iron-aluminum rock, bauxite, and clay rock (locally transformed into coal seams). In order to further study its sedimentary paleoenvironment and provenance characteristics, two representative profiles were selected. A total of 20 samples (Figure 3c) were selected from the typical profiles (Jinyu and Jinshi profile) of Pinglu mine area. The samples comprised claystone, bauxite, and coal and sandstone. From top to bottom, eleven samples were obtained from the Jinyu profile (Y1–Y11, Figure 3a), and nine from the Jinshi profile (S1–S9, Figure 3b).

Microscopic optical, X-ray diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS) analyses were conducted to determine the mineralogical and geochemical characteristics of the samples accurately. Microscopic optical observations were conducted at the Geological Experimental Center of Taiyuan University of Technology using a Leica 2700P microscope. Whole-rock XRD was performed at the Analytical Testing Center of Taiyuan University of Technology, using a PANalytical Aeris instrument Netherlands, Cu-Kɑ radiation, a graphite monochromator, and continuous scanning applying the following settings: 40 kV voltage, 80 mA beam current, 6.5°/min scanning speed, and a scanning range of 5°–70° under an ambient temperature of 18 °C and 30% humidity. The software Jade 6.5 was used to match the main diffraction peaks (d-value and intensity) with standard cards to determine the main mineral phase composition.

Elemental geochemical analysis was performed at the Analytical Testing Research Center of the Beijing Research Institute of Uranium Geology. Major element analysis was carried out at 24 °C and 24% humidity, adhering to the standards GB/T 14506.14-2010 [28] (Suitable for the determination of ferrous oxide content in silicate rocks, and also applicable for the determination of ferrous oxide content in soil and fluvial sediments.) “Methods for Chemical Analysis of Silicate Rocks Part 14: Determination of Ferrous Oxide Content”, GB/T14506.34-2019 [29] (Suitable for the gravimetric determination of loss on ignition in soil and sediment samples.) “Methods for Chemical Analysis of Silicate Rocks Part 34: Determination of Loss on Ignition by Gravimetric Method”, and YS/T 575.23-2021 [30] (Suitable for the determination of Al₂O₃ SiO₂, Fe₂O₃, TiO₂, K₂O, MgO, CaO, MnO and P₂O₅ content in bauxite.) “Chemical Analysis Methods of Bauxite Part 23: Determination of Element Content by X-ray Fluorescence Spectroscopy”. The XRF fusion method was used, employing the wavelength-dispersive X-ray fluorescence spectrometers AB104L and AXIOS-MAX.

Trace and rare earth element (REE) analyses were performed at 24.5 °C and 45% humidity, following the standard GB/T14506.30-2010 [31] (Suitable for silicate rock, like Li, Be, Sc and other 41 elements determination, also suitable for the determination of the above elements in soil, sediment samples.) “Methods for Chemical Analysis of Silicate Rocks Part 30: Determination of 44 Elements,” using a mixed acid digestion ICP-MS method and an ELEMENT XR plasma mass spectrometer.

4. Result

4.1. Petrological Characteristics

Based on observations of the profile strata, rock specimens, and thin sections, the aluminum-bearing strata are composed of five rock types: gray to gray-brown fine-grained sandstone with iron staining (Figure 4a,f); yellowish muddy to silty iron-bearing silty mudstone (Figure 4d); thin black coal beds that stain the hands (Figure 4g); yellowish-gray to gray to gray-black claystones (Figure 4b,e,h); and gray high-aluminum rocks and bauxite (Figure 4c,i).

Sandstone forms the overlying top layer of the ore-bearing strata. In the Jinyu profile, mudstone acts as a transitional layer between claystone and sandstone. The coal bed, identified only in the Jinshi profile, appears in interbedded structures with claystone and fine-grained sandstone. In the Jinyu profile, the bauxite is immediately overlain and underlain by claystones, with the bottom layer being an iron-bearing claystone. In the Jinshi profile, the immediate top and bottom layers of the ore-bearing strata are high-aluminum rock, with the top layer comprising coal and claystone. The ore in the bauxite in this area are mainly massive and earthy structures, but there are also layered, cryptocrystalline and euhedral-subhedral structures. Stomata, calcite, and iron weathering are generally developed.

4.2. Mineral Characteristics

Pinglu bauxite is a typical sedimentary bauxite with a highly complex mineral composition and extremely fine mineral particles. XRD analysis and microscopic observation of bauxite samples (Y8, S8), iron-bearing claystone samples (Y10), aluminous rock samples (S7, S9), and claystone samples (Y7, Y9, S5) revealed the following observations:

In the Jinyu profile, the claystones (Y7, Y9) are predominantly composed of kaolinite, boehmite, and illite, with some samples containing trace amounts of diaspores. The bauxite sample (Y8) contains diaspores, kaolinite, and rutile. The iron-bearing claystones (Y10, Y11) include kaolinite, illite, diaspore, and trace amounts of hematite.

In the Jinshi profile, the claystone (S5) is primarily made up of kaolinite, followed by mica. The high-aluminum rocks (S7 and S9) are chiefly composed of kaolinite, followed by diaspore, with some samples containing minor amounts of cookeite, quartz, and mica. The bauxite sample (S8) consists of diaspores, kaolinite, and small quantities of rutile.

4.2.1. Aluminum-Bearing Minerals

The bauxite, along with its immediate top and bottom beds (high-aluminum rock and claystone), contains a significant number of diaspores. The ore structure is mainly microcrystalline (Figure 5b,h), though some areas exhibit cryptocrystalline (Figure 5a,e,i), clastic (Figure 5c), and oolitic structures (Figure 5e). Diaspore is primarily found in a cryptocrystalline form and is the key aluminum mineral in the bauxite ore of the study area (Figure 6). Small particles of diaspores are often uniformly distributed in clastic forms, displaying oolitic, clumpy, and irregular shapes (Figure 5c–e). Diaspores are prevalent in both the oolitic and matrix parts of the bauxite ore, and some occur in euhedral to subhedral forms. Field profile surveys and XRD analysis did not identify significant metamorphic minerals in any of the layers, suggesting that diaspores often coexisted with rutile, indicating that diaspores likely crystallized during the metallogenic process rather than being transported.

4.2.2. Clay Minerals

Kaolinite is the most prevalent clay mineral in the ore-bearing strata (Figure 6), occurring in all layers and at the highest concentrations. Under a polarized microscope, kaolinite appears pale yellow, with a first-order gray-white interference color, positive low relief, oblique extinction, and is typically seen in fine-grained scales or vermicular shapes (Figure 5b). The primary forms of kaolinite in the ore-bearing strata include the following: 1. Coexisting with diaspores, forming a cryptocrystalline to microcrystalline matrix of clastic and oolitic bauxite ores. 2. Present as part of clasts or ooids or appearing as single clasts. 3. In the late diagenetic stage, coexisting with diaspores or forming microveins independently (usually microscopic veins). 4. Filling cracks, dissolution pores, and pyrite crystal interstices. 5. Moreover, the XRD diffraction spectra identified the presence of cookeite (Figure 6b), boehmite (Figure 6e), and illite (Figure 6i).

4.2.3. Other Minerals

In the aluminum-bearing strata of Pinglu, hematite is the predominant iron-bearing mineral (Figure 6h), typically found in the lower iron-bearing claystones. Under a polarized light microscope, hematite appears black and opaque, irregularly distributed around clay minerals (Figure 5f). Small amounts of quartz and mica were also observed (Figure 6d,i). Quartz is colorless and transparent with a clean surface, lacking cleavage, and exhibits first-order gray to first-order yellow-orange interference colors. Mica, on the other hand, appears flaky with perfect cleavage and bright interference colors above the second order (Figure 5g).

Zircon, though not abundant in the bauxite, is commonly present, typically colorless, light red-brown, or pale yellow, and is characterized by large grain sizes and ring structures (Figure 5d), suggesting long-distance transport. Rutile is also present (Figure 6c,f), but consists of fine mineral particles that are not easily discernible under a microscope.

4.3. Geochemical Characteristics of Elements

4.3.1. Major Elements

Based on the test results for the major elements (

Table 1. Geochemical data of the Pinglu aluminiferous rock series (wt %).

Sample	SiO2	Al2O3	TFe2O3	TiO2	CaO	MgO	K2O	MnO	P2O5	Na2O	CIA	Al2O3/TiO2
Y1	88.20	5.79	3.30	0.63	0.18	0.16	1.01	0.01	0.03	0.02	84.65	9.18
Y2	63.00	18.10	5.03	1.46	0.50	0.60	2.57	0.01	0.06	0.12	86.56	12.40
Y3	61.00	17.90	7.56	1.39	0.41	0.56	2.41	-	0.05	0.06	87.62	12.88
Y4	39.60	27.50	9.68	1.63	0.21	0.68	3.15	0.06	0.13	0.08	89.26	16.87
Y5	39.60	34.20	0.50	0.94	0.21	0.11	0.53	-	0.06	-	98.47	36.23
Y6	32.10	30.10	0.25	0.37	0.34	0.12	0.67	-	0.16	0.04	97.57	81.57
Y7	38.60	35.40	4.58	1.02	0.73	0.23	0.20	0.01	0.13	0.01	99.38	34.71
Y8	20.00	60.80	2.21	2.60	0.11	0.45	0.32	0.01	0.17	0.13	99.09	23.38
Y9	44.20	36.00	2.20	1.65	0.21	0.70	2.57	-	0.23	0.07	93.00	21.82
Y10	43.80	30.90	9.06	1.45	0.22	0.47	1.07	0.02	0.10	0.05	96.35	21.31
Y11	43.30	32.70	5.99	1.59	0.26	0.95	3.68	-	0.09	0.08	89.49	20.57
Max.	88.20	60.80	9.68	2.60	0.73	0.95	3.68	0.06	0.23	0.13	-	-
Min.	20.00	5.79	0.25	0.37	0.11	0.11	0.20	0.01	0.03	0.02	-	-
Av.	46.67	29.94	4.58	1.34	0.31	0.46	1.65	0.02	0.11	0.06	-	-
CC	0.71	1.97	1.02	2.68	0.07	0.21	0.49	0.22	0.55	0.02	-	-
S1	75.40	10.90	5.55	1.16	0.39	0.32	1.17	0.01	0.07	0.09	89.04	9.40
S2	1.90	1.00	0.48	0.04	6.26	0.28	0.04	-	0.01	0.02	92.76	25.00
S3	79.60	11.50	1.81	1.20	0.24	0.30	1.12	0.01	0.04	0.04	90.58	9.58
S4	58.40	21.60	4.97	1.78	0.47	0.64	2.21	0.01	0.07	0.06	90.28	12.13
S5	43.30	31.70	4.13	2.06	0.65	0.80	2.92	0.01	0.11	0.10	91.02	15.39
S6	3.78	1.52	1.38	0.13	6.02	0.36	0.04	-	0.02	0.06	90.8	11.69
S7	38.20	45.00	0.86	1.00	0.09	0.17	0.08	-	0.03	-	99.81	45.00
S8	21.30	61.10	1.89	1.25	0.10	0.15	0.12	0.03	0.06	-	99.8	48.88
S9	31.70	50.00	1.72	2.32	0.14	0.63	0.77	-	0.13	0.07	98.22	21.55
Max.	79.60	61.10	5.55	2.32	6.26	0.80	2.92	0.03	0.13	0.10	-	-
Min.	1.90	1.00	0.48	0.04	0.09	0.15	0.04	0.01	0.01	0.02	-	-
Av.	39.29	26.04	2.53	1.22	1.6	0.41	0.94	0.01	0.06	0.05	-	-
CC	0.60	1.72	0.56	2.43	0.38	0.18	0.28	0.17	0.30	0.01	-	-
Crust	65.89	15.17	4.49	0.50	4.19	2.20	3.39	0.07	0.20	3.89	-	-

Note: Chemical weathering index CIA = [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)] * 100, where CaO* is only the content of CaO in silicate minerals, and CaO* = Na₂O when CaO* > Na₂O, CaO* = CaO when CaO* < Na₂O; ‘-’ is lower than the limitation.

), the bauxite (Y8) in the Jinyu profile primarily comprises Al₂O₃ with a mass fraction of 60.80 wt.%, followed by SiO₂ (20.00 wt.%), TiO₂ (2.60 wt.%), TFe₂O₃ (2.21 wt.%), and minor amounts of MgO, K₂O, and CaO, resulting in a chemical index of alteration (CIA) of 99.09. The clay rocks above the bauxite (Y5, Y6, Y7, and Y9) are chiefly composed of SiO₂ (38.63 wt.%) and Al₂O₃ (33.93 wt.%), with secondary amounts of TFe₂O₃ (1.89 wt.%) and minor amounts of TiO₂, CaO, MgO, K₂O, and a CIA ranging from 92.98 to 99.39. The iron-rich clay rocks below the bauxite (Y10, Y11) mainly contain SiO₂ (43.55 wt.%), followed by Al₂O₃ (31.80 wt.%), TFe₂O₃ (7.53 wt.%), TiO₂ (1.52 wt.%), and minor amounts of K₂O, MgO, and CaO, with a CIA ranging from 89.48 to 96.37. The relatively low CIA value of Y11 is attributed to its high K₂O content (3.68 wt.%), which is likely associated with mica (Figure 6).

In the Jinshi profile, bauxite (S8) is predominantly composed of Al₂O₃ with a mass fraction of 61.10 wt.%, followed by SiO₂ (21.30 wt.%), TFe₂O₃ (1.89 wt.%), TiO₂ (1.25 wt.%), and small quantities of MgO, K₂O, and CaO, resulting in a CIA of 99.80. The clay rock (S5) is chiefly composed of SiO₂ (43.30 wt.%) and Al₂O₃ (31.70 wt.%), with secondary amounts of TFe₂O₃ (4.13 wt.%), K₂O (2.92 wt.%), and TiO₂ (2.06 wt.%), and minor amounts of MgO and CaO, and a CIA of 91.02. The aluminous rocks (S7 and S9) are primarily made up of Al₂O₃ (47.50 wt.%), followed by SiO₂ (34.95 wt.%), TiO₂ (1.66 wt.%), TFe₂O₃ (1.29 wt.%), and minor amounts of MgO, CaO, and K₂O, with a CIA range of 98.04–99.61.

Based on the test results for the major elements (Table 1), in the Jinyu profile, the content of SiO₂ is the highest in the uppermost sandstone (Y1) and the lowest in the bauxite layer (Y8). The content of Al₂O₃ is the opposite of SiO₂. In addition, the TiO₂ and CAI content of the bauxite layer are also the highest. The content of TFe₃O₂ is the highest in iron-bearing clay rocks. The content of CaO, MgO, K₂O, and Al₂O₃/TiO₂ is higher in clay rocks. The content of other major elements was low, mostly less than 0.20 wt.%. In the Jinshi profile, the content of SiO₂, Al₂O₃, TFe₃O₂, and TiO₂ in the coal seam (S2) is the lowest. But the content of CaO and K₂O in the coal seam (S2) is the highest. The content of SiO₂ is the highest in the sandstone (S3). Al₂O₃ content is highest in bauxite. In addition, the CAI and TiO₂ content of the aluminiferous rock series is higher, 89.04 to 99.81. The content of MgO and K₂O is higher in clay rocks. The content of other macroelements was low, mostly less than 0.20 wt.%.

In a lateral comparison, the Al₂O₃ and SiO₂ contents in the bauxite of both profiles were comparable. However, the Jinyu profile showed higher levels of TFe₂O₃, TiO₂, MgO, K₂O, and CaO than those in the Jinshi profile, indicating that the bauxite in the Jinyu profile is of relatively higher quality. Moreover, in the Jinshi profile, the Al₂O₃ content in the top and bottom layers of the bauxite exceeded 45.00%, while the TFe₂O₃ content was below 2.00%, suggesting good metallogenic potential.

In a vertical comparison, the trends of Al₂O₃, TiO₂, TFe₂O₃, MgO, and K₂O were similar (Figure 7). The CIA values for the stable overlying sandstone layers in both profiles were below 90.58 (S3). In the Jinshi profile, the bauxite was directly overlain by coal seams, with CIA values exceeding 99.09, while the CIA values of the bottom layers were notably lower than those of the bauxite and exhibited a decreasing trend. This signifies that the overall weathering degree of the profile is relatively low, and suggests that the bauxite metallogenic materials may have undergone significant weathering prior to the deposition of the overlying layers, presumably occurring prior to or during the deposition stage of the metallogenic materials in the basin.

4.3.2. Trace Elements

The formation of Shanxi bauxite took place within the metallogenic environment of the NCC, where the attributes of rocks involved are comparable to those of the upper crust [7,9,32]. Consequently, this study employed upper crust trace element data to standardize the geochemical data of profile rock samples to analyze the geochemical characteristics of trace elements.

The test data revealed that in the Jinyu profile, the average Li content was highest in clay rocks (464–732 µg/g, averaging 580 µg/g), followed by bauxitic ore (314 µg/g), iron–bearing clays (104–362 µg/g, averaging 210 µg/g), and sandstones (31.7–51.3 µg/g, averaging 46.93 µg/g) (

Table 2. Geochemical data of trace elements (×10⁻⁶) in samples from the Jinyu section of Pinglu.

Sample	Y-1	Y-2	Y-3	Y-4	Y-5	Y-6	Y-7	Y-8	Y-9	Y-10	Y-11	C	CC1	Av.	CC2
Li	31.70	57.80	51.30	104.00	490.00	634.00	732.00	314.00	464.00	362.00	164.00	20.00	15.70	474.33	23.72
Be	0.39	1.45	1.51	2.12	1.83	2.61	2.77	3.92	7.74	5.25	5.60	3.00	1.31	4.30	1.43
Sc	6.05	28.20	25.00	31.90	23.70	47.80	22.50	37.20	38.00	23.90	28.20	13.60	2.74	30.68	2.26
V	60.30	178.00	154.00	210.00	46.30	128.00	94.50	145.00	156.00	146.00	122.00	107.00	1.36	115.47	1.08
Cr	1100	362.00	307.00	243.00	55.40	60.10	40.90	641.00	177.00	142.00	152.00	83.00	7.72	104.57	1.26
Co	2.23	2.37	2.47	10.50	0.45	0.50	1.45	2.61	1.93	7.62	2.65	17.00	0.15	2.43	0.14
Ni	16.80	15.40	12.40	33.50	5.03	7.42	15.60	38.10	87.80	49.80	80.20	44.00	0.87	40.98	0.93
Cu	10.70	24.50	35.50	38.00	18.50	45.10	14.20	20.20	17.30	44.70	47.70	25.00	0.81	31.25	1.25
Zn	24.80	37.50	36.60	48.80	21.60	12.80	28.00	49.80	55.80	40.80	51.00	71.00	0.70	35.00	0.49
Ga	6.33	25.20	22.60	35.00	26.10	22.00	30.40	76.40	50.70	45.20	48.70	17.00	4.49	37.18	2.19
Rb	41.50	105.00	89.70	131.00	17.40	15.40	7.04	8.53	94.50	40.50	155.00	112.00	0.08	54.97	0.49
Sr	62.40	100.00	82.30	418.00	111.00	548.00	126.00	433.00	440.00	143.00	208.00	350.00	1.24	262.67	0.75
Mo	1.08	1.07	0.66	1.36	0.92	0.42	1.33	1.71	0.53	0.28	0.41	1.50	1.14	0.65	0.43
Cd	0.04	0.05	0.05	0.08	0.09	0.04	0.06	0.08	0.07	0.10	0.06	98.00	-	0.07	-
In	0.02	0.08	0.07	0.09	0.10	0.75	0.14	0.19	0.11	0.07	0.17	0.05	3.80	0.22	4.47
Sb	1.76	0.78	0.64	0.84	0.72	1.03	2.99	1.48	0.45	0.61	0.56	0.20	7.40	1.06	5.30
Cs	2.17	11.30	11.00	17.50	1.45	1.31	2.00	0.82	8.13	3.43	13.00	4.60	0.18	4.89	1.06
Ba	89.00	433.00	323.00	460.00	21.00	179.00	27.40	62.90	338.00	125.00	350.00	550.00	0.11	173.40	0.32
W	1.34	3.15	2.46	4.16	4.20	1.34	4.32	5.13	3.94	4.45	4.03	2.00	2.57	3.71	1.86
Tl	0.37	0.75	0.56	0.75	0.31	0.28	0.19	0.11	0.51	0.43	0.96	0.75	0.15	0.45	0.60
Pb	8.28	52.30	21.70	36.30	17.10	49.50	33.20	67.50	84.90	48.20	36.20	17.00	3.97	44.85	2.64
Bi	0.14	0.58	0.58	1.01	2.70	1.00	1.37	2.02	0.94	1.21	1.45	0.13	15.91	1.45	11.38
Th	4.62	19.80	14.30	23.20	54.40	33.30	39.20	81.80	51.20	42.70	49.70	10.70	7.64	45.08	4.21
U	1.42	5.37	4.14	5.45	13.70	12.30	13.00	26.80	9.95	13.00	9.78	2.80	9.57	11.96	4.27
Nb	8.71	24.10	19.90	28.20	27.40	10.40	30.10	61.60	35.20	35.30	37.40	12.00	5.13	29.30	2.44
Ta	0.68	1.69	1.60	1.99	0.94	0.81	1.43	5.36	3.34	2.94	3.02	1.00	5.36	2.08	2.08
Zr	109	285.00	241.00	292.00	507.00	211.00	273.00	596.00	281.00	304.00	340.00	190.00	3.14	319.33	1.68
Hf	3.28	8.55	7.57	9.03	16.20	7.37	9.27	18.70	9.92	10.90	10.10	5.80	3.22	10.63	1.83
La	18.40	50.10	46.90	66.60	69.70	96.30	55.10	163.00	152.00	134.00	121.00	30.00	5.43	104.68	3.49
Sr/Ba	0.70	0.23	0.25	0.91	5.29	3.06	4.60	6.88	1.30	1.14	0.59	-	-	-	-
Sr/Cu	5.83	4.08	2.32	11.00	6.00	12.15	8.87	21.44	25.43	3.20	4.36	-	-	-	-
Zr/Sc	18.02	10.11	9.64	9.15	21.39	4.41	12.13	16.02	7.39	12.72	12.06	-	-	-	-
Th/Sc	0.76	0.70	0.57	0.73	2.30	0.70	1.74	2.20	1.35	1.79	1.76	-	-	-	-
Th/Co	2.07	8.35	5.79	2.21	120.89	66.60	27.03	31.34	26.53	5.60	18.75	-	-	-	-
La/Sc	3.04	1.78	1.88	2.09	2.94	2.01	2.45	4.38	4.00	5.61	4.29	-	-	-	-

Note: C denotes the average value of the upper crust, CC represents the enrichment coefficient, CC1 signifies the bauxite samples, and Av. and CC2 are for the clay samples Y5, Y6, Y7, Y9, Y10, and Y11.

). The enrichment factors for Li and Bi in the bauxite are greater than 10 (Figure 8), indicating substantial enrichment (Figure 8). Enrichment factors for U, Th, Cr, Sb, Nb, and Ta ranged from 5 to 10, suggesting moderate enrichment. Sc, Ga, In, W, Pb, Zr, and Hf all had enrichment factors above 2, indicating slight enrichment. In the clay rocks, Li and Bi remain highly enriched, Sb is moderately enriched, and In, U, Th, Pb, Nb, Ga, Sc, and Ta are slightly enriched.

In the Jinshi profile, the average Li content was highest in aluminum rocks (967–1994 µg/g, averaging 1480.5 µg/g), followed by bauxitic ore (915 µg/g), clay rock (137 µg/g), sandstones (33.9–76.5 µg/g, averaging 53.47 µg/g) and coals (22.6–43.8 µg/g, averaging 33.2 µg/g) (

Table 3. Geochemical data of trace elements (×10⁻⁶) in samples from the Jinshi section of Pinglu.

Sample	S-1	S-2	S-3	S-4	S-5	S-6	S-7	S-8	S-9	C	CC1	Av.	CC2
Li	39.90	22.60	44.00	76.50	137.00	43.80	1994.00	915.00	967.00	20.00	45.75	1032.67	51.63
Be	0.84	1.96	0.87	1.73	2.37	2.22	1.37	2.18	4.75	3.00	0.73	2.83	0.94
Sc	11.70	2.29	15.40	28.40	32.50	15.50	24.50	19.10	34.10	13.60	1.40	30.37	2.23
V	142.00	11.80	116.00	245.00	326.00	45.80	115.00	292.00	155.00	107.00	2.73	198.67	1.86
Cr	458.00	12.50	494.00	289.00	248.00	27.80	106.00	417.00	273.00	83.00	5.02	209.00	2.52
Co	2.53	0.95	1.85	2.36	2.69	0.43	0.54	1.16	3.43	17.00	0.07	2.22	0.13
Ni	11.10	4.83	13.20	15.30	16.10	9.34	15.60	16.90	81.70	44.00	0.38	37.80	0.86
Cu	13.90	2.52	12.30	23.80	23.40	20.40	9.38	15.90	22.90	25.00	0.64	18.56	0.74
Zn	37.50	7.10	37.30	43.40	48.80	9.68	26.20	37.90	49.70	71.00	0.53	41.57	0.59
Ga	14.60	12.50	12.50	35.00	48.80	24.00	62.50	77.30	55.80	17.00	4.55	55.70	3.28
Rb	44.70	1.05	40.00	88.30	126.00	1.21	4.45	4.57	30.50	112.00	0.04	53.65	0.48
Sr	157.00	214.00	96.70	159.00	296.00	268.00	60.50	158.00	260.00	350.00	0.45	205.50	0.59
Mo	2.35	0.98	1.00	1.49	0.67	0.79	0.61	1.35	1.25	1.50	0.90	0.84	0.56
Cd	0.05	0.03	0.04	0.07	0.06	0.04	0.06	0.06	0.06	98.00	-	0.06	-
In	0.04	-	0.05	0.07	0.07	0.03	0.29	0.17	0.21	0.05	3.40	0.19	3.80
Sb	0.72	0.16	0.51	0.88	1.17	0.47	0.46	0.76	1.21	0.20	3.80	0.95	4.73
Cs	6.84	0.21	4.70	17.40	26.90	0.21	1.33	0.70	2.71	4.60	0.15	10.31	2.24
Ba	239.00	232.00	215.00	415.00	514.00	204.00	10.70	16.30	97.00	550.00	0.03	207.23	0.38
W	2.48	0.41	2.42	4.02	2.45	0.37	2.79	5.75	4.22	2.00	2.88	3.15	1.58
Tl	1.24	1.78	0.36	1.14	1.83	1.42	0.06	0.04	0.19	0.75	0.05	0.69	0.92
Pb	19.30	0.83	26.20	25.20	40.70	32.80	18.50	11.10	57.60	17.00	0.65	38.93	2.29
Bi	0.39	0.02	0.38	1.07	1.59	0.30	1.97	1.33	1.70	0.13	10.47	1.75	13.81
Th	13.80	1.36	14.20	26.20	28.80	4.58	36.50	39.40	64.50	10.70	3.68	43.27	4.04
U	3.90	20.10	3.85	6.38	9.76	28.90	21.30	34.00	25.40	2.80	12.14	18.82	6.72
Nb	15.20	0.84	17.30	32.20	30.60	3.13	31.40	33.10	56.90	12.00	2.76	39.63	3.30
Ta	1.33	0.07	1.29	2.37	1.16	0.23	2.54	2.52	4.62	1.00	2.52	2.77	2.77
Zr	259.00	16.10	311.00	360.00	413.00	65.30	353.00	379.00	535.00	190.00	1.99	433.67	2.28
Hf	8.90	0.55	8.81	11.00	12.10	1.62	13.30	11.30	16.50	5.80	1.95	13.97	2.41
La	29.20	1.65	33.10	59.10	96.80	11.20	10.60	20.20	321.00	30.00	0.67	142.80	4.76
Sr/Ba	0.66	0.92	0.45	0.38	0.58	1.31	5.65	9.69	2.68	-	-	2.48	-
Sr/Cu	11.29	84.92	7.86	6.68	12.65	13.14	6.45	9.94	11.35	-	-	18.25	-
Zr/Sc	22.14	7.03	20.19	12.68	12.71	4.21	14.41	19.84	15.69	-	-	14.32	-
Th/Sc	1.18	0.59	0.92	0.92	0.89	0.30	1.49	2.06	1.89	-	-	1.14	-
Th/Co	5.45	1.43	7.68	11.10	10.71	10.65	67.59	33.97	18.80	-	-	18.60	-
La/Sc	2.50	0.72	2.15	2.08	2.98	0.72	0.43	1.06	9.41	-	-	2.45	-

Note: C represents the average value of the upper crust, CC denotes the enrichment coefficient, CC1 is for the bauxite samples, and Av. and CC2 stand for the clay samples S5, S7, and S9.

). Li, Bi, and U exhibit significant enrichment within the bauxite, while Cr displays moderate enrichment, and V, Ga, In, Sb, W, Th, Nb, and Ta demonstrate slight enrichment (Table 3). In the clay/aluminous rocks, Li and Bi continue to be highly enriched, U is moderately enriched, and Sc, Ga, In, Sb, Th, Pb, Nb, Ta, Zr, and Hf are lightly enriched, with the highest Li content reaching 1994.00 μg/g (S7) and an average content of 1032.67 μg/g.

In comparison, the bauxite and clay rocks of both profiles exhibit significantly high enrichment levels of Li, Bi, and U, with relatively high enrichment of In, Sb, Th, Nb, and Ta. The Li content is notably higher in the clay/aluminous rocks, whereas the U content is more concentrated in the bauxite. The Jinyu profile shows greater enrichment in Bi, Sb, Th, Nb, and Ta within the bauxite, while the clay rocks exhibit higher enrichment in In. The Jinshi profile’s clay rocks are more enriched in In, Bi, Sb, Th, Nb, and Ta. As noted in Section 4.2.2, cookeite was found in the S7 sample (Figure 6b), and boehmite was identified in the Y7 sample (Figure 6e). The Li concentrations in S7 and Y7 are markedly higher than those in other samples (Table 3). S7 and Y7 are located in the upper part of the ore bed, while the underlying substrates S9 and Y9, despite having the same lithology, show significantly lower Li content. This suggests that cookeite and boehmite are likely the primary carriers of high Li content in these profile samples, and that the enrichment of lithium may also be controlled by modulating the depositional and diagenetic environments.

4.3.3. Rare Earth Elements

The total amount of REEs (∑REY) in the Jinyu profile varied significantly (

Table 4. Geochemical data of rare earth elements (×10⁻⁶) in samples from the Jinyu and Jinshi sections of Pinglu.

Sample	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho
Y1	18.40	32.20	3.88	14.10	2.38	0.44	1.88	0.33	1.78	8.67	0.34
Y2	50.10	87.50	10.90	36.50	6.22	1.22	4.71	1.03	5.64	24.70	1.05
Y3	46.90	83.70	9.99	35.80	5.43	1.04	5.01	1.00	5.03	24.40	1.07
Y4	66.60	116.00	16.00	53.00	10.00	2.09	8.24	1.43	7.22	34.50	1.43
Y5	69.70	112.00	12.90	35.80	5.23	0.97	5.47	1.43	9.17	43.50	2.15
Y6	96.30	154.00	17.20	44.30	4.53	0.81	5.93	1.00	7.76	43.70	1.79
Y7	55.10	94.30	12.90	47.80	8.58	1.51	7.24	1.48	9.13	39.00	1.89
Y8	163.00	229.00	25.70	77.60	12.50	2.36	11.70	1.75	9.78	42.80	1.79
Y9	152.00	340.00	75.50	537.00	97.90	15.60	44.30	7.60	30.90	80.90	3.97
Y10	134.00	152.00	27.00	101.00	18.60	3.69	15.20	2.34	12.00	48.50	2.05
Y11	121.00	191.00	23.60	86.90	13.90	2.00	10.60	1.85	10.30	51.60	2.05
Av.	88.46	144.70	21.42	97.25	16.84	2.88	10.93	1.93	9.88	40.21	1.78
S1	29.20	57.90	7.43	29.10	5.39	1.12	3.75	0.66	3.63	18.90	0.78
S2	1.65	3.74	0.55	2.62	0.83	0.20	0.86	0.28	2.03	14.70	0.56
S3	33.10	62.70	8.34	30.30	4.28	0.80	3.88	0.70	3.91	18.90	0.69
S4	59.10	114.00	14.40	54.90	10.70	2.05	7.24	1.36	6.58	33.20	1.38
S5	96.80	165.00	23.30	78.50	12.70	2.45	9.32	1.47	7.73	36.00	1.55
S6	11.20	17.50	1.99	8.23	1.73	0.41	1.56	0.34	2.21	12.40	0.50
S7	10.60	23.10	2.56	8.56	2.66	0.83	2.32	0.61	3.69	17.00	0.72
S8	20.20	23.50	3.24	10.70	2.20	0.48	2.13	0.54	3.53	20.30	0.79
S9	321.00	360.00	50.20	140.00	12.90	2.67	16.80	2.04	10.70	44.70	1.87
Av.	64.76	91.94	12.45	40.32	5.93	1.22	5.32	0.89	4.89	24.01	0.98
Sample	Er	Tm	Yb	Lu	∑REE	∑REY	Ce/Ce*	Eu/Eu*	La/Y	(La/Yb)N
Y1	1.05	0.18	1.17	0.17	78.30	86.97	0.87	0.98	2.12	10.60
Y2	3.08	0.50	3.68	0.58	212.71	237.41	0.86	1.06	2.03	9.19
Y3	2.77	0.48	3.43	0.48	202.13	226.53	0.89	0.94	1.92	9.23
Y4	4.32	0.66	4.77	0.70	292.46	326.96	0.83	1.08	1.93	9.42
Y5	6.57	1.11	7.33	1.02	270.85	314.35	0.84	0.84	1.60	6.42
Y6	5.97	0.96	5.79	0.83	347.17	390.87	0.84	0.71	2.20	11.20
Y7	4.91	0.75	4.63	0.60	250.82	289.82	0.82	0.90	1.41	8.03
Y8	5.82	0.97	6.07	0.94	548.98	591.78	0.76	0.92	3.81	18.10
Y9	12.80	1.55	9.54	1.40	1330.06	1410.96	0.76	1.06	1.88	10.80
Y10	6.45	1.00	6.24	0.99	482.56	531.06	0.58	1.03	2.76	14.50
Y11	6.23	1.02	6.77	1.03	478.25	529.85	0.81	0.77	2.34	12.10
Av.	5.45	0.83	5.40	0.79	408.57	448.78	0.81	0.94	2.18	10.87
S1	2.39	0.42	2.68	0.39	144.84	163.74	0.92	1.17	1.54	7.35
S2	1.62	0.29	1.77	0.28	17.28	31.98	0.94	1.11	0.11	0.63
S3	2.24	0.40	2.50	0.41	154.25	173.15	0.89	0.92	1.75	8.94
S4	4.17	0.75	4.84	0.68	282.15	315.35	0.91	1.09	1.78	8.24
S5	4.92	0.78	5.29	0.84	410.65	446.65	0.81	1.06	2.69	12.40
S6	1.20	0.22	1.44	0.23	48.77	61.17	0.83	1.18	0.90	5.25
S7	2.17	0.39	2.36	0.35	60.92	77.92	1.04	1.57	0.62	3.03
S8	2.20	0.38	2.51	0.40	72.78	93.08	0.63	1.04	1.00	5.43
S9	6.20	0.95	6.07	0.98	932.39	977.09	0.62	0.83	7.18	35.70
Av.	3.01	0.51	3.27	0.51	236.00	260.01	0.84	1.11	1.95	9.66

Note: Eu/Eu* = (2Eu/Eu_ch)/(Sm/Sm_ch + Gd/Gd_ch), Ce/Ce* = (2Ce/Ce_ch)/(La/La_ch + Pr/Pr_ch). Eu_ch—The content of Eu in chondrite.

). The highest concentration, 1410.96 µg/g, was found in the clay rock of the bauxite bottom layer (Y9), while the lowest, only 86.97 µg/g, was in the sandstone top layer (Y1). The bauxite and its underlying layers show higher REE concentrations, whereas the overlying layers, even if they are clay rocks, have markedly lower levels. In the Jinshi profile, only the clay rock S5 and the bauxite bottom layer S9 exhibited higher REE concentrations, ranging from 446.65 to 977.09 µg/g, while the other layers had relatively low concentrations. Eu mostly demonstrated negative anomalies in the Jinyu profile and positive anomalies in the Jinshi profile, with both profiles displaying negative Ce anomalies.

The distribution curves reveal varying degrees of REE enrichment in the Jinyu profile (Figure 9a), notably in the clay rock of the bauxite bottom layer Y9. Overall, the curve is relatively flat, showing slight enrichment of heavy REEs in some clay rocks. Conversely, the Jinshi profile exhibits REE depletion (Figure 9b), particularly in the bauxite and the overlying aluminous rock S7, where there is a significant depletion of light RREs. However, in the bauxite bottom layer S9 and the adjacent clay rock, there is a modest enrichment of light REEs.

5. Discussion

5.1. Metallogenic Environment

REEs exhibit good stability in sedimentary environments and throughout diagenesis, making them valuable for identifying sedimentary source rocks [33]. The occurrence of Ce anomalies is frequently observed in aluminium-bearing rock strata, similarly to the Wuchuan–Zheng’an–Daozhen bauxite deposit, the Zagrad karstic bauxite deposit, Montenegro, and the Apulian karst bauxite deposit, southern Italy. Ce anomalies serve as key indicators of redox conditions [34]. Under oxidizing conditions, Ce³⁺ is converted to Ce⁴⁺, resulting in a positive Ce anomaly, whereas under reducing conditions, soluble Ce³⁺ predominates, leading to a negative Ce anomaly [35,36,37]. All samples from the Jinyu profile displayed negative Ce anomalies. In the Jinshi profile, all samples except for clay rock S7, which had a slightly positive Ce anomaly (1.04), exhibited negative Ce anomalies. These variations in Ce anomalies suggest that the primary diagenetic environment of the aluminous strata was reducing, with occasional weakly oxidizing conditions.

The La/Y ratios can indicate the acidity or alkalinity of the bauxite depositional environment [36,37]. When La/Y > 1, the environment is alkaline, while it is acidic when La/Y < 1 [34,36,38,39]. In the Jinyu profile, La/Y values were all greater than 1; however, a significant difference was observed between bauxite Y8 and clay rocks Y7 and Y9 (Table 4). In the Jinshi profile, the La/Y ratios varied significantly, ranging from 0.11 (S2) to 7.18 (S9), with La/Y values less than 1 for the bauxite top S7 and the coal seam S6. This typically suggests that the diagenetic environment of the bauxite and its substrate was alkaline, whereas the top environment gradually became weakly acidic during diagenesis. XRD diffraction patterns reveal the presence of cookeite in S7, rutile in S8, quartz and mica in S9, and kaolinite and boehmite across all samples (Figure 5). Rutile, primarily derived from the weathering of titanite and associated with REEs, particularly the Y group [40], suggests that the changes in the depositional diagenetic environment’s acidity and alkalinity of the bauxite substrate are likely influenced by the source rock composition. This influence could be due to mixed materials from the in situ weathering of intermediate-acidic rocks and carbonates, indicating a genetic relationship between bauxite and acidic rocks, as well as carbonates.

Sr and Ba are both prevalent in the crust and share similar chemical properties in some respects, but they behave quite differently in various sedimentary environments. Sr has a greater mobility in water than Ba. As water salinity increases, Ba precipitates first as BaSO₄, followed by the gradual accumulation of Sr, eventually forming SrSO₄. Consequently, the Sr/Ba ratio can indicate whether sediments are influenced by marine or terrestrial environments. Typically, when Sr/Ba > 1, it suggests saline deposits; when 0.6 < Sr/Ba < 1, it indicates brackish deposits; and when Sr/Ba < 0.6, freshwater deposits [41,42]. The data in Table 2 signify that in the Jinyu profile, clay and bauxite rocks have Sr/Ba > 1, with the Y1 and mudstone Y4 bed having 0.6 < Sr/Ba < 1, and Y2 and Y3 having Sr/Ba < 0.6. Likewise, in the Jinshi profile, the clay and bauxite rocks exhibit Sr/Ba > 1, with the two coal layers and S1 layer having 0.6 < Sr/Ba < 1, and S3 and S4 having Sr/Ba < 0.6. These geochemical characteristics imply that during the diagenesis of the aluminum-bearing strata, the water body was predominantly influenced by a marine environment, mainly saline, with transitional brackish and terrestrial freshwater environments within the strata. The identification of coal seams and mudstones further corroborates the transitional environmental changes during the metallogenic period, with the observed decrease in Sr/Ba in the top sandstone suggesting a shift to a freshwater–brackish swamp environment conducive to coal formation during diagenesis.

In weathering processes, the Sr/Cu ratio serves as an effective indicator of climatic conditions. A ratio of 1.3 < Sr/Cu < 5 suggests a warm climate, while Sr/Cu > 5 indicates a hot climate [43]. In the Pinglu area, only the top sandstone samples Y2 and Y3 and the bottom samples Y10 and Y11 of the Jinyu profile have Sr/Cu values of 1.3 < Sr/Cu < 5, whereas the remaining are greater than 5, indicating a hot climate at that time.

The CIA can also be employed to infer paleoclimate. When CIA > 80, it reflects intense chemical weathering in hot and humid climates; when 60 < CIA < 80, it signifies moderate weathering in warm and humid climates; and when 50 < CIA < 60, it indicates weak weathering in cold and dry climates [44]. In the study area, CIA values range from 84.6 to 99.8, signifying that the aluminum-bearing strata underwent intense weathering during sedimentary diagenesis. This suggests a hot and humid tropical rainforest climate [45], consistent with the North China Craton’s position at 17°–18°N during the Late Carboniferous to Early Permian, belonging to a tropical–subtropical climate [46].

In summary, the layers enriched with bauxite were formed in a reducing environment predominantly shaped by marine conditions, with a sedimentary environment that exhibited a tendency toward weak alkalinity, but primarily acidic in nature. Through the analysis of the regional metallogenic environment, the lithofacies paleogeography and the size of the ore, Wang Zhuquan [47] put forward the bauxite in the area also conforms to the environment of weak alkaline reduction. The water environment is primarily saline, accompanied by a hot and humid climate and intense chemical weathering. The depositional diagenetic stage underwent significant environmental changes, evolving into a freshwater-brackish environment influenced by marine–terrestrial transitional environments. During the humid and hot climate stage, weathering is further intensified, and the desilication and deferritization of aluminous rocks become more pronounced. At the same time, abundant rainfall and the flourishing of vegetation create extensive swamp environments in this area, providing favorable conditions and material sources for the formation of the extensive coal series found in the upper part of the bauxite deposits in this region, favorable to Late Carboniferous coal formation.

5.2. Source of Metallogenic Materials

The metallogenic materials of weathering sedimentary type bauxite originate from multiple exogenous sources [7,13,34,48,49]. Any igneous, metamorphic, or sedimentary rock exposed to a warm and humid environment can contribute metallogenic materials essential for bauxite formation [50].

The Al₂O₃/TiO₂ ratio remained relatively stable during sedimentation and can be utilized for provenance analysis. The Al₂O₃/TiO₂ values in basic rocks are 3–8, and those of intermediate rocks are 8–21, and those of acidic rocks 21–70, and occasionally above 70. The Al₂O₃/TiO₂ ratios of the Pinglu bauxite (Table 1) indicate that, except for S5, the Al₂O₃/TiO₂ ratios of other bauxite and clay rocks are all greater than 21, with coal seam S2 reaching 25. The other rock samples, particularly the upper sandstone, had Al₂O₃/TiO₂ ratios between 8 and 21. Furthermore, the Al₂O₃–TiO₂ diagram (Figure 10a) reveals that nearly all points fall within the provenance areas of intermediate and acidic rocks, especially the bauxite and their top and bottom beds. This suggests that the source of metallogenic material for the Pinglu bauxite is closely related to acidic igneous rocks, whereas the source for the overlying rock layers transitioned gradually to intermediate rocks.

Studies indicate that Th, Sc, and Zr are minimally impacted by later diagenesis and are primarily related to the rock composition of their source areas, with Zr being particularly stable. The element Zr is predominantly found in highly stable zircons and exhibits enrichment in sediments through recycling processes. The ratio of Zr/Sc can serve as an indicator for the degree of recycling. Felsic rocks tend to be rich in La and Th compared to basic rocks, whereas Sc and Co are more abundant in mafic rocks [50]. Consequently, relatively stable trace elements such as La, Th, Zr, Sc, Co, and their ratios—like Zr/Sc, Th/Sc, La/Sc, and Th/Co—can provide insights into potential source areas and lithology, thereby offering source information [51,52]. In the Zr/Sc–Th/Sc and La/Sc–Th/Co diagrams (Figure 10b,c), the sample points cluster around intermediate-acidic igneous rocks, further suggesting a close relationship between the metallogenic materials of the bauxite and clay rocks and intermediate-acidic igneous rocks. The lack of a significant increase in Zr in Figure 9b implies that the diagenetic materials of the Pinglu bauxite series may not have undergone sedimentary recycling.

Compatible and incompatible elements serve as essential parameters for the differentiation of felsic and mafic rocks. Acidic rocks typically exhibit Th/Sc values exceeding 1, while intermediate rocks range from 0.6 to 1, and basic rocks less than 0.6 [53]. Table 2 and Table 3 show that the Th/Sc values for the aluminum-bearing strata samples in Pinglu are all greater than 1.0, suggesting that the clastic source rocks are predominantly felsic igneous rocks. For the upper layer samples (except for coal seam S6), the Th/Sc values are all greater than 0.6, indicating that the source rocks of the overlying beds of the aluminum-bearing strata are likely to be intermediate rocks.

Previous research on U−Pb dating and Hf isotopes of detrital zircons from the Late Carboniferous bauxite on the southern margin of the North China Craton points out that the primary metallogenic materials of the bauxite originated from the Neoproterozoic Kuanping Group and the Mesoproterozoic Qinling Group of the North Qinling orogenic belt [13]. It is well-established that the North Qinling tectonic belt underwent granitoid magmatism between 500 and 410 Ma prior to the Early Devonian. Nevertheless, the final collision and subsequent amalgamation of the North China and Yangtze blocks along the Mianlue suture, resulting in the formation of the Qinling orogenic belt, occurred in the Early Mesozoic [54,55]. The complete amalgamation between the North China Block and the Qinling Block occurred between the Early and Middle Triassic [56], during a time when the two blocks were separated by an ocean.

Other scholars investigating the provenance of Carboniferous bauxite on the southern margin of North China contend that the bottom iron-rich clay rocks are primarily products of in situ weathering and karstification of carbonate rocks, whereas the bauxite and top clay rocks are allochthonous deposits [19]. As previously discussed, the lithologies of the metallogenic source rocks for the iron-rich clay rocks and bauxite are generally consistent, although the source of the top beds indicates slight deviations. This study posits that the bauxite and underlying iron-rich clay rocks may be linked to the weathering of Ordovician carbonates, further influenced by high-aluminum, high-iron materials resulting from the weathering of terrestrial source rocks; conversely, the overlying clay rocks may be products transported from weathered source rocks.

5.3. Iron-Aluminum-Coal Structure

The physicochemical environment plays a pivotal role in determining the mineral composition and rock-forming elements of aluminum-bearing strata. In nature, aluminum is primarily found in its trivalent form, making its geochemical behavior largely independent by redox conditions [57]. The solubility of aluminum silicate minerals is closely related to acidity and alkalinity: at pH < 4, these minerals dissolve into Al³⁺, while at pH > 10, they dissolve into AlO₂. However, since the natural pH typically ranges from 5 to 9, primary aluminum minerals are minimally affected. During the weathering of source rocks, aluminum is transformed into various mineral components without being carried away by aqueous solutions [58]. Instead, it may be transported as terrestrial clastic material, suggesting that the accumulation of aluminum in sedimentary basins is primarily due to the in situ weathering of aluminum-rich source rocks and the clastic transport of high-aluminum materials.

During weathering, the mobility of Fe and Si is significantly influenced by acidity and alkalinity levels. When pH levels exceed 8, Si dissolves rapidly, whereas Fe remains largely insoluble, resulting in the removal of Si from primary aluminosilicate minerals while most Fe is retained. When 5 < pH < 8, the dissolution rate of Si is in the range of 10–20 times greater than that of Fe, leading to significant Si removal and minimal Fe dissolution from primary aluminosilicate minerals, with most Fe and stable Al remaining. At pH < 5, both Si and Fe solubility increase, but Fe dissolves faster than Si, resulting in substantial Fe removal and limited SiO₂ retention in primary aluminosilicate minerals [58]. This phenomenon signifies that alkaline and neutral conditions are more conducive for de-siliconizing while retaining Fe. The La/Y ratios further confirmed that the diagenetic environment of bauxite and its substrate tend towards alkalinity. The XRD spectra demonstrating the mineral composition of illite−kaolinite in Y9, Y10, and Y11 suggest that the iron-rich clay rocks of the bauxite substrate were originally deposited in a neutral-alkaline environment. This environment promotes de-siliconization and iron retention during the in situ weathering of source rocks, which, combined with the intermediate-acidic igneous characteristics of the source rocks, likely elucidates the high iron content in the bauxite substrate.

During the late diagenetic stage of bauxite formation, the depositional environment shifted to a marine–terrestrial interaction or freshwater swamp environment. Subsequently, the overlying beds transitioned to sand–mudstone or coal seams, signaling the end of bauxite metallogenesis and the onset of coal formation, thereby forming typical iron–aluminum–coal structural layers [59].

6. Conclusions

The predominant minerals found in the iron-bearing claystone or aluminum rock are hematite, illite, kaolinite, and muscovite, the major elements are SiO₂, Al₂O₃, and Fe₂O₃. The principal minerals in bauxite are diaspore, rutile, and kaolinite, with elements primarily comprising SiO₂, Al₂O₃, and TiO₂. The primary minerals in the upper claystone are kaolinite, mica, and boehmite, with the major elements being SiO₂ and Al₂O₃. The Li, Bi, and U are concentrated in the bauxite and claystone/aluminous rocks, while In, Sb, Th, Nb, and Ta are also relatively enriched. Li content is higher in claystone/ aluminous rocks, with cookeite and boehmite being the main carrier minerals.

The bauxite formed in a saline, reducing environment largely influenced by marine conditions. The depositional environment exhibited a tendency toward weak alkalinity; however, it remained predominantly acidic, characterized by a hot and humid climate and intense chemical weathering.

The source rocks for the bauxite and its top and bottom beds are intricately linked to intermediate-acidic igneous rocks. The principal factors contributing to aluminum accumulation in the sedimentary basin are the in situ weathering of aluminum-rich source rocks and clastic transport of high-aluminum source rocks. The elevated iron content observed in the iron-bearing claystone is attributed to its original depositional environment being neutral to alkaline and the intermediate-acidic igneous nature of the parent rock. Furthermore, the formation of a freshwater to brackish environment, influenced by a marine–terrestrial transitions in the late stage, facilitated the occurrence of coal formation during the Late Carboniferous.