Sediment-Hosted Rare-Earth Elements Mineralization from the Dian-Qian District, Southwest China: Mineralogy and Mode of Occurrence

Abstract

The Xuanwei Formation’s claystones in the Dian-Qian District of Southwest China are rich in rare-earth elements (REEs), suggesting their potential as a source of medium and heavy rare earths. However, the REE content in these rocks is lower than other types of rare-earth deposits, and the interrelationship among clay minerals is intricate. There is no direct evidence indicating the mineralization of REEs, limiting their beneficiation and extraction. The objective of this study is the characterization of REE distribution in the Dian-Qian District. The sedimentary rocks in this district are mainly composed of kaolinite, boehmite, quartz, rutile, and pyrite. The results of continuous chemical extraction of REE-rich claystone and transmission electron microscope (TEM) observations have confirmed that REEs occurred as florencite in the rocks, and that the ion-absorption state makes only a negligible contribution to the REE content. A close relationship between florencite and kaolinite makes traditional mineral processing operations very difficult. Combined with the properties of kaolinite, roasting-acid leaching was the efficacious approach for rare-earth resources extracted from the rare earth-rich clay rocks of the Xuanwei Formation.

1. Introduction

Rare-earth elements (including lanthanides and yttrium, referred to as REEs hereafter) are critical for a wide range of high-tech applications due to their unique physicochemical properties [1]. The growing importance of REEs to technologies and economies as well as the potential risks brought by supply disruptions has led both the United States (US) and the European Union to label REEs as “critical materials” [2,3]. The strong demand for them has brought many new REE resources into people’s view in recent years, like deep sea mud [4], REEs associated with bauxite tailings [5], coal measure strata [6], and sediment-hosted REE resources [7].

Sediment-hosted REE deposits are a newly defined class of REE resources [7,8]. Although detailed research on deposits’ geological characters, geochemistry, and geochronology have been conducted for this type of deposit [7], the mode of occurrence of REE resources is still controversial. According to some researchers, the REEs in sedimentary rock occur as an ion-absorption state [9,10]; Xu (2018) [11] and Xu (2020) [12] utilized metallurgical experiments and mineralogical studies to propose that rare earths were present in kaolinite in the form of isomorphism. In recent studies, some researchers proposed that the REEs in claystones occur as REE minerals like florencite [13,14] and monazite [15,16,17,18], based on the evidence from X-ray diffraction (XRD) analysis and back-scattered electron (BSE) observation. The mode of occurrence of REEs is of great significance to mineral beneficiation and metallurgy [19]. The mode of occurrence and distribution of REEs are very important factors in understanding the development and exploitation of the deposit. This requires a thorough and detailed mineralogical analysis.

In our paper, we provide direct evidence of the nature of REEs in samples from the Xuanwei Formation of the Weining District, Southwest China, which is a typical sediment-hosted REE deposit (REEs; 1100–16,000 ppm, average 2700 ppm) [7]. These data can provide the mineralogical basis for the development and utilization of this type of REE deposit. Based on combined data from scanning electron microscope (SEM), the Advanced Mineral Identification and Characterization System (AMICS), electron microprobe analysis (EPMA), transmission electron microscope (TEM), and continuous extraction of the samples, we propose that REEs predominantly exist as independent minerals (florencite). The presence of rare-earth elements in an ion-adsorption or homogeneous state has a minimal impact on the overall rare-earth resources in the region.

2. Geological Background

The Weining District is situated at the intersection of the Tethys-Himalaya and the Pacific Coast, on the southwestern periphery of the Yangtze Continent, within the triangle zone delineated by the Ziyun-Yadu Deep Uplift (Figure 1). The area is distinguished by two prominent faults: Shizong-Guiyang-Jiujiang Deep Fault, which tends in a northwesterly direction, and the Xiaojiang Deep Fault, which extends in both northeasterly and southeasterly directions [7].

During the Late Paleozoic Era, extensive terrestrial–marine interactions were triggered by Cathaysia with the Yangtze plates amidst Caledonian Orogeny, which formed the Guizhou region. This tectonic activity facilitated both Emeishan large igneous rock formation and Maokou limestone deposition throughout the Middle and Upper Permian Period. Additionally, the erosional characteristics of Maokou limestone exhibit variability based on its altitude and regional positioning. Based on this, He (2003) [20] proposed that the Emeishan large igneous rock unit was categorized into inner, middle, and outer zones. As a result, significant basaltic volumes are produced within Kangdian Paleoland [11] following their emergence as continental segments. Subsequent weathering resulted in transportation and deposition of sediments onto ancient eastern lands, giving rise to diverse sedimentary formations. These contain Xuanwei Formation, Longtan Formation, Changxing Formation, Wujiaping Formation, and Dalong Formation, progressing from west to east [21].

The REE-bearing rock series of the Xuanwei Formation is primarily distributed from the Zhaotong-Xuanwei-Qujing in eastern Yunnan to the west of the Hezhang-Liupanshui region in western Guizhou. Sample locations are illustrated in Figure 1. The thickness of rare earth ore-bearing rock series varies from large to small due to the transition from continental to marine faces, with paleogeomorphology and volcanic basalt tuff interspersed. Additionally, the grade of rare earth decreases from high to low with decreasing thickness [7].

3. Sampling and Analytical Techniques

A total of 40 samples were collected from three stratigraphic sections of the Xuanwei Formation in the Weining, Maojiaping, and Chahe districts. These samples are characterized as off-white, blocky structure and are rich in kaolinite (Figure 2). Petrographic studies were carried out using scanning electron microscopy with backscattered electrons (BSE) acquired using a ZEISS Sigma 300 SEM at the Institute of Comprehensive Mineral Utilisation, Chinese Academy of Geological Sciences. The operating conditions were under an acceleration voltage of 20 kV and a beam current of 30 nA.

The chemical composition of minerals was determined using a Shimadzu EPMA-1720 electron microprobe at the Institute of Comprehensive Utilization of Minerals, Chinese Academy of Geological Sciences. The operating conditions were under an acceleration voltage of 20 kV and a beam current of 20 nA, with a beam diameter of 1–5 μm. The peak counting time was 10 s with a background counting time of 5 s. Natural minerals and synthetic materials were used as standards for Al₂O₃ (NaAlSi₃O₈), SiO₂ (SiO₂), MnO (MnSiO₂), FeO (Fe₃O₄), MgO (MgO), CaO (CaSO₄), TiO₂ (TiO₂), La₂O₃ (LaP₅O₁₄), Ce₂O₃ (CeP₅O₁₄), Nd₂O₃ (NdP₅O₁₄), Dy₂O₃ (DyP₅O₁₄), Ho₂O₃ (HoP₅O₁₄), Er₂O₃ (ErP₅O₁₄), Tm₂O₃ (TmP₅O₁₄), Yb₂O₃ (YbP₅O₁₄), Lu₂O₃ (LuP₅O₁₄), Eu₂O₃ (EuP₅O₁₄), Tb₂O₃ (Tb₃Ga₅O₁₂), Gd₂O₃ (GdP₅O₁₄), and Sm₂O₃ (SmP₅O₁₄).

Continuous extraction was used to study the nature of REE occurrence, which was carried out at the Institute of Comprehensive Utilization of Minerals, Chinese Academy of Geological Sciences. In this research, continuous extraction can be divided into five steps: (1) Stirring and leaching the water-soluble rare-earth mineral sample with 5 times the volume of deionized distilled water, followed by separation of the leached solution through centrifugation. This procedure was repeated 10 times under identical conditions, after which the filtrate was combined. Subsequently, after mixing and clarification, the rare-earth content was quantified using a preferred ICP-MS instrument. (2) The residue from leaching rare-earth ions was vigorously agitated and treated with 5 times the volume of 4% ammonium sulfate solution, followed by separation of the leached solution through centrifugation. The process was repeated 10 times under consistent conditions, after which the filtrate was combined. Subsequently, the combined solution underwent mixing, clarification, and analysis for rare-earth content using inductively coupled plasma mass spectrometry (ICP-MS). (3) The colloidal residue from leaching rare-earth elements was stirred and treated with 5 times a 0.5 mol/L hydroxylamine hydrochloride +2 mol/L hydrochloric acid solution, followed by separation of the leached solution through centrifugation. The process was repeated 10 times under consistent conditions, after which the filtrate was combined. After mixing and clarification, testing for rare-earth content was conducted using ICP-MS. (4) The rare-earth leaching residue was vigorously agitated and subjected to leaching with a 5-fold concentrated hydrochloric acid (36.5%) solution, followed by separation of the resulting leachate using centrifugation. The process was repeated 5 times under identical conditions, after which the filtrate was combined. The rare-earth content was then determined using the preferred ICP-MS instrument. (5) The residual rare-earth samples were melted with sodium peroxide and sodium hydroxide and subsequently acidified to obtain the samples. Upon adjusting the samples to a fixed volume, the rare-earth content was analyzed using the preferred ICP-MS instrument.

The AMICS analysis test in this paper was conducted at the Institute of Comprehensive Utilization of Minerals, Chinese Academy of Geological Sciences. The system used for the test included a ZEISS Sigma 300 high-resolution field emission scanning electron microscope (FESEM), a Bruker XFlash 6130 modern fast X-ray energy spectrometer (EDS), and a set of AMICS software consisting of four subroutines: AMICS Tool, Mineral STD Manager, Investigator, and AMICS Process. The experimental conditions were as follows: an accelerating voltage of 20 kV (since the accelerating voltage of the X-ray energy spectrum information in the AMICS mineral standard library was 20 kV, the scanning electron microscope’s accelerating voltage was set to 20 kV during the test), a working distance of 8.5 mm, a backscattered electron detector (HDBSD), an objective aperture of 60 μm, and high-vacuum mode.

Transmission electron microscopy (TEM) measurements were carried out at the General Research Institute of Mining & Metallurgy Technology Group using a JEOL JEM2100 transmission electron microscope equipped with a GATAN bottom-insert 832CCD Oxford INCA energy spectrometer. Samples for the TEM examination were prepared by placing a drop of suspension on a copper grid (300 mesh) coated with a carbon film, which was allowed to dry under ambient conditions.

4. Results

4.1. Mineralogical Composition of the Samples

The mineral composition in the samples was meticulously analyzed using SEM-EDS and AMICS (

Table 1. Mineralogy of the investigated samples.

Mineral	Content (wt.%)
Kaolinite (Ti + Fe)	86.51
Kaolinite (Fe)	2.94
Boehmite	6.40
Boehmite (Si + P)	0.92
Boehmite (Si + Ti)	0.67
Quartz	1.10
Other silicate minerals (Ti + Al)	0.24
Pyrite	0.17
Rutile/anatase	0.06
Unknow minerals + holes	0.99

, Figure 3). Kaolinite is the predominant mineral in the sample, comprising 89.45 wt.% of the total content. It can be further classified into titanium-rich, iron-rich kaolinite and iron-rich kaolinite based on trace-element disparities. Following kaolinite, boehmite constituted 7.99 wt.% of the total content as the second most abundant mineral in the sample. Some boehmite specimens containing trace amounts of P, Ti, Si, and other elements were detected in the samples. Additionally, traces of minerals such as pyrite, quartz, and rutile/anatase were detected.

4.2. Texture of the Samples

This study employed SEM to delineate the micromorphological characteristics of the specimen (Figure 4). An examination of the natural cross-section unveiled a close association of minerals such as kaolinite, boehmite, and quartz (Figure 4A–C). Kaolinite was predominantly observed in stratified and flaky conglomerates, with particle dimensions ranging from 0.1 to 20 microns (Figure 4A,C). These kaolinites show a typical lamellar structure under higher magnification of SEM (Figure 4E,F). Conversely, boehmite was primarily present in granular aggregates within kaolinite aggregates, occasionally exhibiting porous structures (Figure 4B). These voids often contained minerals like kaolinite (Figure 4B). Boehmite particles displayed a wide-ranging particle size distribution, ranging from 0.3 to 1000 microns (Figure 4B). Quartz, typically granular, was sporadically found within kaolinite conglomerates (Figure 4A,D), with particle sizes ranging from 5 to 50 microns.

4.3. EPMA Analysis of Clay Minerals

The samples are mainly composed of kaolinite, accounting for 90 wt.%, and the other minerals are present in small amounts. Therefore, electron probe analysis focused specifically on the kaolinite, and the findings are detailed in

Table 2. EPMA analysis of kaolinite (wt.%).

Sample	Al2O3	SiO2	MnO	FeO	MgO	CaO	TiO2	La2O3	Ce2O3	Nd2O3	Dy2O3
1	38.92	43.85	bdl	0.04	0.01	0.05	0.01	0.02	0.01	0.02	bdl
2	39.01	43.54	0.02	0.18	0.01	0.03	0.05	0.04	0.02	0.01	bdl
3	38.55	44.01	0.01	0.03	0.01	0.04	0.01	0.01	0.04	0.01	0.01
4	39.01	45.17	0.03	0.19	bdl	0.02	0.04	0.03	bdl	0.02	bdl
5	39.14	44.32	0.02	0.05	bdl	0.01	0.02	0.02	bdl	0.00	0.01
6	39.25	44.09	0.01	0.04	0.02	0.03	0.03	0.01	0.02	0.00	0.01
7	39.15	44.22	bdl	0.05	0.01	0.02	0.01	0.01	0.01	0.04	bdl
8	39.10	42.38	bdl	0.07	bdl	0.01	bdl	0.03	bdl	0.02	bdl
9	39.11	44.01	bdl	0.02	bdl	0.03	bdl	0.02	0.01	0.01	bdl
10	39.17	45.05	bdl	0.17	0.03	0.07	0.01	0.01	0.03	bdl	bdl
Average	39.04	44.06	0.01	0.08	0.01	0.03	0.02	0.02	0.01	0.01	0.02
Sample	Ho2O3	Er2O3	Tm2O3	Yb2O3	Lu2O3	Eu2O3	Tb2O3	Gd2O3	Sm2O3	Total	REO
1	0.02	0.03	0.02	0.02	0.03	0.07	0.03	bdl	0.01	83.16	0.28
2	bdl	0.08	bdl	0.02	bdl	0.04	bdl	bdl	bdl	83.05	0.21
3	0.01	0.06	0.01	bdl	0.01	bdl	0.02	bdl	bdl	82.84	0.18
4	0.03	0.01	0.01	bdl	0.01	bdl	0.01	bdl	bdl	84.58	0.12
5	bdl	bdl	0.02	0.01	bdl	bdl	bdl	bdl	bdl	83.62	0.06
6	0.01	0.02	bdl	0.01	bdl	0.02	bdl	bdl	bdl	83.57	0.10
7	bdl	0.01	0.01	0.02	0.01	0.01	bdl	bdl	bdl	83.58	0.12
8	0.01	bdl	0.02	bdl	0.02	0.03	0.01	bdl	0.01	81.71	0.15
9	0.03	0.01	0.01	bdl	bdl	0.01	0.02	bdl	bdl	83.29	0.12
10	0.04	0.01	0.02	0.01	0.05	0.02	0.06	bdl	bdl	84.75	0.25
Average	0.02	0.02	0.01	0.01	0.01	0.02	0.02	bdl	bdl	83.42	0.27

bdl: Below detection limit or under detection limit.

.

4.4. Continuous Extraction Results

The results of the sequential extraction process are presented in

Table 3. The results of continuous extraction.

REE Deportment	Ratio (wt.%)
Water-solution/ion-adsorbed	16.94
Carbonate-bound	0.76
Amorphous iron–manganese-bound	34.10
Crystalline iron–manganese-bound	3.48
Residue	44.65

. The analyzed sample contains 16.94 wt.% REEs in an ion-adsorbed state, with 0.76 wt.% bound to carbonate, 34.10 wt.% associated with amorphous iron–manganese compounds, and 3.48 wt.% within organic matter and sulfides. Meanwhile, 44.65 wt.% of REEs remain in the residue (Table 3).

4.5. TEM Analysis of Samples

In TEM high-angle annular dark-field (HAADF) images, the REE nanoparticles were clearly visible due to their higher gray contrast compared to the surrounding clays (Figure 5), which is attributed to their greater mass. TEM-EDS mapping of these particles revealed a chemical composition rich in P, Al, and Ce (Figure 6). Crystal plane calibration (Figure 5B) and selected area electron diffraction (SEAD, Figure 5C) analysis, conducted on the mineral’s high-resolution transmission electron microscope (HRTEM) image (Figure 5A), revealed that d_(1-1-3) = 0.433nm, d_(1-1-1) = 0.573 nm (Figure 5B), which is consistent with the lattice spacing characteristics of the florencite ((Ce,Sr,Ca) Al₃ (PO₄)₂ (OH,F)₅∙H₂O). Combined with its characteristic chemical composition, the mineral was identified as florencite. There is a close spatial relationship between these florencite and clay minerals (Figure 5A). Even at the nanometer level, the two minerals’ particles still have a close-contact relationship (Figure 5A). Different from previous studies, other types of rare-earth minerals were not found in the TEM observation in this study.

5. Discussion

5.1. Occurrence of REEs

The widespread presence of clay minerals and the strong correlation between rare-earth element content and clay minerals led some researchers to link the distribution of rare-earth elements in the Xuanwei Formation with ion adsorption of rare-earth elements deposits in South China [9,10]. This suggests that rare-earth elements might originate from being adsorbed onto the surface of clay minerals in ion form [18]. The continuous extraction method enables a systematic study into the various occurrence states of rare-earth elements in samples, making it an effective technique for the occurrence states of rare-earth elements in ion-adsorption rare-earth minerals analyzed [22,23,24,25]. The results of continuous extraction in this study indicated that approximately 17% of the rare earths found in the clay rocks of the Xuanwei Formation are in the ion-adsorbed state (Table 3), which was a lower proportion compared to ion-adsorbed rare earths typically found in ion-adsorbed rare-earth deposits (>40 wt.%). The limited occurrence of ion-adsorption states for rare-earth elements demonstrates that this is not their primary form in the samples being studied.

Based on the combined XRD and kaolinite EPMA study results of samples from the area, Xu et al. (2018) [13] proposed that rare-earth elements are present in the crystal lattice of clay minerals in an isomorphous manner. However, the EPMA analysis conducted on kaolinite in this study indicated a low rare-earth element content (Table 2), which is insufficient to support the presence of rare-earth reserves in the area. Furthermore, most EPMA analyses revealed rare-earth element contents near or below the detection limit, casting doubt on the credibility of the data (Table 2). The significant ionic radius gap between rare-earth elements and clay mineral elements makes it theoretically implausible for rare-earth elements to replace elements in clay minerals on a large scale, even if isomorphic interactions occur at a small scale. Therefore, it can be concluded that rare-earth elements existing in the crystal lattice of clay minerals in an isomorphic manner are not likely to be primary-occurrence forms of rare-earth elements in the study samples.

Recent studies have consistently shown that there are abundant unique minerals rich in rare earths in this area. Zhao et al. (2023) [26] used BSE and electron probe analysis to confirm the presence of cerium monazite in rare earth-rich clay rocks at relatively low levels. This discovery was further supported by our SEM and TEM observations, which revealed its limited prevalence. Previous research speculated on the existence of florencite, a rare earth-free mineral, in similar rock samples using XRD and EDS analyses, but direct evidence remains elusive [13,14]. Mapping results from TEM-EDS demonstrate a strong association between the rare-earth element Ce and elements such as P and Al (Figure 6). HRTEM observations suggest that these Ce-, P-, and Al-rich minerals are indeed florencite particles measuring <200 nm, which intricately intertwine with surrounding clay minerals (Figure 5). Their small size combined with intricate intergrowth patterns creates challenges for observation under optical microscopes or SEM.

5.2. Implications for Utilization of Rare-Earth Resources

The existence of rare-earth resources in the abundant clay formations hidden within the Xuanwei Formation has long been a subject of uncertainty due to uncertainties surrounding elemental occurrences and high levels of clay content [26]. This study reveals that predominant forms of these resources are identified as florencite, establishing a strong foundation for potential exploitation. While specialized extraction processes have demonstrated effectiveness for other rare-earth independent mineral deposits [27], conventional techniques encounter significant challenges when attempting to concentrate such fine-grained (<200 nm) materials closely associated with clays [17]. Although hydrometallurgy methods have exhibited promise in recovering valuable components from phosphates [28], it is important to note that because of the nanoscale nature of florencite and its intimate relationship with clays (Figure 5A), simple acid leaching may not fully capture all available reserves. Therefore, we recommend employing a roasting-acid leaching approach for extracting rare-earth elements from the host rocks. The process involves subjecting samples to temperatures between 500 and 600 °C, which serves to disrupt interlayer structures within clays [29], thereby exposing embedded rare-earth minerals and enhancing their susceptibility to acid-leaching solutions.

6. Conclusions

The rare-earth elements primarily manifest as independent minerals formed, namely fluorite, and present within the rare earth-rich clay rocks of the Xuanwei Formation. These fluorites typically possess a less than 200 nm particle size and are closely affiliated with clay minerals.

Mineralogical evidence indicates that one of the efficacious approaches for rare-earth resources extracted from the rare earth-rich clay rocks of the Xuanwei Formation is roasting-acid leaching.