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Article

Evaluated Utilization of Middle–Heavy REE Resources in Bayan Obo Deposit: Insight from Geochemical Composition and Process Mineralogy

by
Hailong Jin
1,2,
Qing Sun
1,2,*,
Biao Chen
1,2,
Wei Wei
1,2,
Yanjiang Liu
1,2 and
Qiang Li
1,2
1
Baotou Rare Earth Research Institute, Baotou 014000, China
2
National Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization, Baotou 014000, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(3), 212; https://doi.org/10.3390/min15030212
Submission received: 12 December 2024 / Revised: 8 February 2025 / Accepted: 12 February 2025 / Published: 22 February 2025
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
The Bayan Obo is the largest carbonatite-type rare earth deposit in the world. It not only has a large amount of light rare earth element (LREE) resources but also hosts approximately several million tons of medium and heavy rare earth element (M+HREE) resources. However, the M+HREE resources have not received enough attention, which hinders their further utilization. In this study, we conduct a systematic investigation of the distribution and process mineralogy properties of M+HREE in different types of ores in the Bayan Obo deposit. The high-value area (>0.1%) of M+HREE elements is found concentrated in the central and deeper parts of the Main and East orebodies. The content of M+HREE varies among different types of ores, with the Aegirine type (1005 ppm) and Fluorite type (1204 ppm) showing a higher average M+HREE concentration. The minerals rich in M+HREE include bastnäsite, monazite, Ca-fluorocarbonate, Ba-fluorocarbonate, allanite, aeschynite, and fergusonite, each with concentrations exceeding 4000 ppm. Aeschynite and fergusonite, in particular, exhibit high M+HREE concentrations and are enriched in fluorite-type and aegirine-type ores. Analysis of the mixed raw ores from the production line at the concentrating plant reveals an M+HREE concentration of approximately 0.2% and a concentration of the seven target minerals at around 12%. However, the particle size distribution and monomer dissociation degree are limited to below 22.3 µm and 40%, respectively. Based on these integrated analyses, we propose that the fluorite-type and aegirine-type ores within the Main and East open-pits are potential M+HREE targets. Furthermore, the recycling and utilization of M+HREE resources in the Bayan Obo deposit require a well-structured process flow and the selection of advanced processing equipment in the future.

Graphical Abstract

1. Introduction

Rare earth elements (REEs), encompassing lanthanide elements (i.e., La to Lu) and yttrium (Y), have attracted global attention owing to their growing relevance in “low-carbon” energy technologies and national security [1,2]. REEs are categorized into light rare earth elements (LREE) ranging from La to Nd, medium rare earth elements (MREE) ranging from Sm to Gd, and heavy rare earth elements (HREE) ranging from Tb to Lu and Y [3,4]. Earth’s crust contains relatively fewer medium and heavy rare earth elements (M+HREE) compared to LREE [5], which merely occupies about 1% of total REE reserves [6,7].
Carbonatite is widely famous for its high abundance of rare earth elements, and is the predominant host rock for numerous REE deposits, such as the Mountain Pass deposit in America [8], the Mount Weld deposit in Australia [9], the Amba Dongar deposit in India [10], and the Maoniuping deposit in China [11]. The LREE is the main REE resource in these carbonatite-related deposits and is targeted for exploration and utilization [12,13]. Nevertheless, many pioneer works have identified some M+HREE resources within these deposits, including Chilwa Island, Kangankunde, Songwe, and Tundulu in Malawi [12,14,15,16,17,18,19,20]; Salpeterkop in South Africa [21]; Lofdal in Namibia [22,23]; Huanglongpu and Huayangchuan in China [24,25,26,27,28]; Pivot Creek in New Zealand [29]; and Bear Lodge in America [30,31]. The M+HREE resources in carbonatite-related deposits can alleviate the M+HREE supply worldwide, and address environmental issues in M+HREE extraction from ion-adsorption deposits [32,33]. However, the evaluation of M+HREE resources in carbonatite-related deposits appears to be limited.
The Bayan Obo deposit in China is the largest carbonatite-related REE deposit in the world and accompanies significant Fe and Nb resources. The carbonatite in the mining area extends about 48 km2, and all the carbonatite contains REE mineralization. For a long time, the Bayan Obo deposit has been considered the primary source of LREE resources, although some authors have suggested that there may be local reserves of M+HREE resources [34,35]. However, the specific composition and distribution of M+HREE, as well as key parameters of process mineralogy, remain unclear, which hinders their effective utilization. In this study, we systematically collected representative samples of different types of ores from the Main and Eastern orebodies in the Bayan Obo deposit and conducted geochemical and mineralogical analyses to obtain the spatial distribution and mineral composition of M+HREE resources, respectively. Additionally, the key parameters of the processing mineralogy of target minerals were also investigated. According to these analyses, the potential M+HREE resources in the Bayan Obo deposit were evaluated. This evaluation provided theoretical support for the efficient development and comprehensive utilization of Bayan Obo’s M+HREE resources.

2. Geology Background

The Bayan Obo Fe-REE-Nb deposit is located on the northern margin of the North China Craton. The North China Craton is one of the oldest cratons in the world and has undergone complex tectonic evolution since the Proterozoic [36,37,38], including a multiphase rifting event in the late Palaeoproterozoic-Neoproterozoic [39], the Palaeoproterozoic subduction-related multiphase magmatism [40,41], and metamorphism and deformation [42,43]. The stratigraphy exposed in the mine area includes the Paleoproterozoic Sertenshan Group and the Middle Proterozoic Bayan Obo Group. The Bayan Obo Group is deposited through two sedimentary cycles (regressive and transgressive) and is generally divided from bottom to top into 6 formations and eighteen units, the Dulahala (H1–H3), Jianshan H4–H5), Halahuoqite (H6–H8), Bilute (H9–H10), Baiyinbaolage (H11–H12) and Hujiertu Formation (H13–H18) [44]. The lower 4 formations and nine units (H1–H9) are the dominant strata that have close spatial relationships with the ore deposit [45], which are mainly composed of quartzite, sandstone, slate, and dolomite.
The Bayan Obo REE-Nb-Fe deposit holds the world’s biggest LREE deposit, ranks as the second-largest Nb deposit, and is a significant Fe deposit. It contains 57.4 million tonnes of LREE with an average grade of 6% of REE2O3, 2.16 million tonnes of Nb with an average grade of 0.13% of Nb2O5, and 1500 million tonnes of Fe with an average grade of 35% of iron oxide [46,47,48,49]. The deposit resides within a west-east oriented dolomite unit, spanning 18 km from the west to the east and occupying roughly 48 km2 (Figure 1). The deposit consists of the East, Main, and Weste orebodies. Compared to the West orebody, the Main and East orebodies suffered strongly alkaline hydrothermal metasomatism, which is characterized by alkaline mineral composition anywhere (e.g., aegirine and sodium-amphibole). Therefore, the mineralization intensity in the West orebody is relatively weak [50,51]. Monazite, bastnasite, parisite, cebaite, and huanghoite, among others, are the primary rare earth minerals in the deposit. Niobium-containing minerals include columbite, ilmenorutile, aeschynite, pyrochlore, baotite, and fergusonite-(Y), and magnetite and hematite are iron-bearing minerals [52,53]. Previous studies have identified over 100 carbonatite dikes in the Bayan Obo mining area [54,55]. The carbonatite dikes are categorized into three types: ferrocarbonatite, magnesiocarbonatite, and calciocarbonatite, and the REE concentrations vary remarkably. The REE levels rise when transitioning from ferrocarbonatite to magnesiocarbonatite and calciocarbonatite is the most enriched in REE [55,56,57].

3. Sampling and Analytical Methods

3.1. Sampling

Collected samples in this study contain outcropped hand-specimens in the Bayan Obo open-pits and mixed raw ore from the Bayan Obo concentrating plant. The sampling of hand-specimens primarily focuses on various types of Fe-REE-Nb ores. A total of 74 dolomite-type ore samples, 24 fluorite-type ore samples, 27 aegirine-type ore samples, 15 riebeckite-type ore samples, 6 massive iron ore samples, and 6 mica-type ore samples were sampled from the Main and East open-pits. Overall, a collection of 152 typical samples was made, comprising 86 from the Main open pit and 66 from the Eastern open pit, with a portion of these lacking a definitive sampling site record. In contrast, mixed raw ore comprises variable types of ores, as current beneficiation flows have not specified raw ore with clear ore types. Therefore, 100 kg mixed samples were collected for subsequent study on process mineralogy.

3.2. Integrated Mineral Analyzer

Sample observation and analysis were completed using ZEISS Sigma 500 high-resolution field emission scanning electron microscope (SEM), coupled with a Bruker XFlash 6160 modern fast X-ray spectrometer (EDS) at the National Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization. The mineral composition was identified and characterised using the advanced mineral identification and characterisation system (AMICS). Prior to the SEM analysis, a thin section was coated with gold.

3.3. Whole-Rock Geochemical Composition

Clean Ore blocks (>50 g), free from weathering rinds or alteration, were collected in the open-pits for geochemical analysis. The whole rock geochemical composition analysis of different types of ores in the Bayan Obo deposit was finished at the National Key Laboratory of Bayan Obo Rare Earth Resource Research and Comprehensive Utilization (as indicated in Supplementary Table S1). First, ore blocks were prepared for analysis by grinding to <200 mesh using an agate pulverizer. Then, the contents of TFe (Total) and F were determined by chemical titration, while SiO2 and P were measured using spectrophotometry. The contents of BaO, MgO, CaO, MnO2, TiO2, SrO, and Al2O3 were determined by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, 5110 VDV, Agilent, Santa Clara, CA, USA). The contents of S were measured using a Carbon and Sulfur Analyzer (EMIA-220V, Horiba, Kyoto, Japan), while the contents of Na2O and K2O were determined by an atomic absorption spectrometer (AA-6300CF, Shimadsu, Kyoto, Japan). Nb, Th, Sc, and rare earth elements (REE) were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, NexION5000G, PerkinElmer, Waltham, MA, USA). For the specific measurement conditions and methods of the above elements in Bayan Obo ore, please refer to DZ/T 0452.1-2023 [59].

3.4. Multivariate Statistics Analysis

In order to explore the distribution of medium and heavy rare earth elements in ore bodies, ores, and minerals, multivariate statistical analysis methods are adopted. In this study, Surfer.8 software was used to establish a spatial distribution model illustrating the content of REE and M+HREE within these mines. In order to investigate the possible factors controlling the variations of M+HREE composition of ore types, the whole-rock geochemical analysis was investigated by partial least squares-discriminant analysis (PLS-DA). PLS-DA can also identify discriminant elements separating different sample classes based on compositions [60,61].

3.5. Process Mineralogy Analysis

Process mineralogy plays a very important role in the entire industrial chain of mineral resources from exploration and evaluation to development and utilization [62]. The systematic process of mineralogy research will provide strong technical support for the study of the occurrence rules of key metals and effective development and utilization [63]. The research in process mineralogy of ore contains two aspects: chemical and physical properties. The chemical properties of ore involve the chemical composition, the distribution of useful, beneficial, and harmful components, and the equilibrium distribution of the main target components. The physical properties of ore focus on mineral composition, ore structure, mineral distribution characteristics, process granularity, and so on. These mineralogical indicators are essential for evaluating the ore quality, establishing mining technical conditions, determining the optimal extraction processes, and assessing the industrial value of ore deposits [64].
The 100 kg samples underwent coarse crushing, fine crushing, mixing, and shrinking processes, ultimately yielding multi-element analysis samples. An additional 1 kg of fine crushing products will be taken for grinding fineness and particle size screening tests. The samples from each particle size classification were the targets for inlaying to measure grinding fineness and particle size distribution parameters.

4. Results

4.1. Mineral Composition and Distribution

The Bayan Obo deposit has variable alteration styles according to the concentration of sodium (Na), potassium (K), and fluoride (F) elements. Fluoride and sodium metasomatism are notably high in the Main and East ores, which caused strong Nb and REE mineralization [65]. Based on the different compositions of altered minerals, the ores within the Bayan Obo deposit were classified into six types, containing dolomite-, fluorite-, aegirine-, riebeckite-, mica-, and massive magnetite-types. The distribution of ore types is obviously controlled by the fenitization between the dolomites and the wall rocks. In the upper layer of the orebody adjacent to the Bilute Formation, the ores are mainly mica- and riebeckite-types. Conversely, the lower layer dolomites are enriched in fluorite, exhibiting a pronounced zonation in their distribution (Figure 2) [65]. The detailed mineral compositions and contents of different types of ores are presented in Table 1.

4.1.1. Dolomite-Type

Dolomite-type Fe-REE-Nb ore is the most widely distributed ore type in the Bayan Obo deposit, and the ore has a disseminated or finely vein-disseminated structure. Dolomite (~47.26 wt.%), magnetite (~41.39 wt.%), monazite (~4.85 wt.%), and bastnäsite (~0.7 wt.%) are the primary minerals that are accompanied by a minor presence of mica in some regions (Figure 3A–C). Dolomite is categorizable into coarse and fine particles; magnetite spreads across mineral particles of dolomite with an inequigranular texture, and ferrodolomite emerges as Fe substitutes Mg in the dolomite (Figure 3A–C).

4.1.2. Aegirine-Type

Aegirine-type Fe-REE-Nb ores are distributed in the hanging wall of the Main and East orebody in a near east-west direction and are in close contact with the Riebeckite-type and mica-type ores (Figure 2). The main minerals in this type of ore are aegirine (~45.64 wt.%), magnetite (~30.3 wt.%), monazite (~0.27 wt.%), bastnäsite (~5.14 wt.%), fluorite (~13.48%), apatite (~2.03 wt.%), etc., displaying disseminated and disseminated-banded structures (Figure 3D–F). The alternating distribution of magnetite, aegirine, bastnäsite, and other minerals can be observed by the naked eye. Additionally, all minerals exhibit obvious directionality under the microscope (Figure 3D–F).

4.1.3. Fluorite-Type

Fluorite-type Fe-REE-Nb ore is the crucial ore type in the Bayan Obo deposit, which is mainly distributed in the Main orebody or the part near the footwall of the East orebody, although a few are exposed in the West orebody. Separate fluorite-type ore from dolomite-type ore is by its banded or vein-banded architecture (Figure 3G–I). Primary minerals include fluorite (~36.97 wt.%), magnetite (~44.20 wt.%), bastnäsite (~1.63 wt.%), Ca-fluorocarbonate (~2.7 wt.%), barite (~5.03 wt.%) and apatite (~1.32 wt.%) (Table 1). Occasionally, a magnetite residue phase emerges, etched by fluorite and other minerals (Figure 3G–I).

4.1.4. Riebeckite-Type

Riebeckite-type Fe-REE-Nb ore is widely distributed in the East and West orebodies, whereas its presence in the Main orebody is infrequent. Within the West ore, the distribution of riebeckite-type ore occurs close to the hanging wall, known as biotitite in H9 slate (Bilute Formation). Within the Eastern ore deposits, such ores are concentrated in the layers interbedded by aegirine-type and dolomite-type ore (Figure 2). Primarily REE minerals are monazite (~12.01 wt.%), which are disseminated and distributed in the shape of fine veins. The primary Nb minerals are columbite (~0.26 wt.%) and pyrochlore (~0.1 wt.%) which are characterized by their honeycomb-like and dispersed structure (Figure 3J–L).

4.1.5. Mica-Type

Mica-type Fe-REE-Nb ore is the most common ore type of the West orebody, only sporadically distributed in the Main and East orebodies. Within the Main orebody, such ore is exclusively found in the transitional contact zones between the hanging wall and H9 slate (Figure 2). The mineral composition of this type of ore includes mica (~38.59 wt.%), ilmenite (~9.45 wt.%), magnetite (~8.43 wt.%), dolomite (~14.01 wt.%), along with a few rare earth element minerals (~0.52 wt.%). This particular type of ore displays a typical schistose formation, with minerals aligned in a distinct direction and elongated iron minerals (Figure 3M–O).

4.1.6. Massive-Type

Massive-type Fe-REE-Nb ore is mainly distributed in the Main orebody and the middle part of the East orebody (Figure 2). In the Bayan Obo deposit, this type of ore is primarily utilized for extracting iron resources. The magnetite (~78.61 wt.%) in the massive ore shows two different structures: coarse and fine grain, accompanied by late-stage hydrothermal aegirine-aeschynite-calcite, fluorite-bastnäsite, fluorite-calcite-bastnäsite veins (Figure 3P–R). Within the West orebody, such type ore is present as lenticular formations incorporated into the dolomite-type ore.

4.2. Geochemical Composition

The Bayan Obo rare earth deposit not only hosts abundant LREE reserves but also has high concentrations of M+HREE resources. Figure 4A reveals that the distribution pattern of enriched zones for REE aligns with the overall trend of the orebody. The Main orebody exhibits a northwest enrichment zone, whereas the Eastern orebody shows a distinct northeast enrichment zone. Due to the strong correlation among REE elements [66], greater REE content (>3 wt.%) shows increased M+HREE levels (>0.1 wt.%) (Figure 4A). Furthermore, the Main orebody (average 0.09 wt.%) contains more M+HREE than the East orebody (average 0.07 wt.%), suggesting a distinct gradient from the peripheral to the middle and from the shallower to the deeper parts of the ore body (Figure 4A,B). Additionally, the deeper segments show more distinct patterns of enrichment for M+HREE elements (>0.1 wt.%; Figure 4B), indicating a higher likelihood of exploration in the orebody’s deeper areas.
There was a broad variation in the chemical composition of various types in the Bayan Obo deposit (Figure 5). In addition to the valuable elements, such as Fe, REE, and Nb, dolomite-type ore mainly contains MgO and CaO, with an average content of about 13.14 wt.% and 26.76 wt.%, respectively. Fluorite-type ore mainly contains CaO and F, the average content of which is about 32.80 wt.% and 20.02 wt.%, respectively. Aegirine-type ore mainly contains Na2O and SiO2, with average concentrations of about 5.85 wt.% and 27.20 wt.%, respectively. Riebeckite-type ore is mainly composed of SiO2 and Na2O, with an average content of about 18.80 wt.% and 2.66 wt.%, respectively. Mica-type ore contains mainly K2O and SiO2, with average concentrations of about 2.79 wt.% and 17.88 wt.%, respectively. Total iron content in massive ore averages more than 40 wt.%. In the Bayan Obo deposit, rare earth elements are predominantly found in aegirine-type, dolomite-type, riebeckite-type, and fluorite-type ores, with aegirine-type and fluorite-type ores showing a notable concentration of M+HREE. On average, aegirine-type and fluorite-type ores contain M+HREE at concentrations of 1005 and 1204 ppm, peaking at 3247 and 2426 ppm, respectively.

4.3. Statistics Analysis

As shown in Figure 6, PLS-DA’s statistics categorize the 19 elements into three primary classifications (the first: CaO, MgO, MnO, and SrO; the second: LREE, M+HREE, Sc, P, and F; the third: K2O, Na2O, SiO2, Al2O3, Fe, TiO2, Nb, Th, S, and BaO), potentially signifying varied geological phenomena in the mining region. There is a notable correlation between M+HREE and elements such as LREE, Sc, P, and BaO, as well as a close association with aegirine-type and fluorite-type ores (Figure 6A,C). Dolomite-type ores predominantly appear on the positive aspect of t1, owing to their positive associations with CaO, MgO, MnO, and SrO. The remaining three types of ores, distinguished by the relative enrichment of valuable elements like Fe, Nb, and Th, predominantly occupy the second quadrant in the t1-t2 (Figure 6B,C).
The composition of 15 rare earth elements (including yttrium) in 22 primary minerals of the Bayan Obo deposit was systematically analyzed (Supplementary Table S2). They consist of 5 rare earth minerals (bastnäsite, Ca-fluorocarbonate, Ba-fluorocarbonate, monazite and allanite), 6 niobium minerals (aeschynite, fergusonite, ilmenorutile, columbite, pyrochlore, and baotite), 3 iron minerals (magnetite, hematite, and pyrite), and 8 gangue minerals (calcite, dolomite, aegirine, riebeckite, fluorite, mica, apatite, and barite). The distribution of medium–heavy rare earth is highly uneven among different minerals (Figure 7A–C). Rare earth minerals contain medium–heavy rare earth at a concentration of 4781~17,357 ppm, while niobium minerals contain it at a concentration of 154~543,324 ppm. Iron minerals typically contain a relatively low concentration of medium–heavy rare earth elements, ranging from 2 to 7 parts per million (ppm), while gangue minerals can have a much higher concentration, ranging from 6 to 1442 ppm. Only a select few minerals have a medium–heavy rare earth element content exceeding 4000 ppm, including bastnäsite, Ca-fluorocarbonate, Ba-fluorocarbonate, monazite, allanite, aeschynite, and fergusonite. In addition, other minerals’ M+HREE accounted for a substantial proportion of the total rare earth content, including ilmenorutile, pyrochlore, columbite, apatite, and fluorite (Figure 7D). Among the 22 minerals, baotite and barite show obvious Eu and Gd anomalies (Figure 7B,C). However, it is likely that these anomalies are analytical artifacts due to the interference by BaO and BaOH during the ICP-MS measurement [67]. Thus, we excluded the data of Eu and Gd from the calculation on the M+HREE estimation for the two minerals (Figure 7D, Table S2).

4.4. Process Mineralogy

4.4.1. Component of Ore Sample

The result of multi-element analysis on the mixed raw ore is listed in Table 2, where detection revealed a REE comprising 6.21 wt.%, with M+HREE making up approximately 0.196 wt.%, representing 3.15% of the REE. This ore is rich in beneficial elements containing Fe, Nb, Th, and Sc that have 25.51 wt.%, 0.09 wt.%, 0.01 wt.%, and 0.01 wt.% concentrations, respectively. The primary impurities are Si, Ca, P, F, and S, cumulatively constituting 28.81 wt.%. It is crucial to be vigilant during the ore processing phase because impurities significantly impact the grade and quality of iron, rare earth elements, and niobium concentrates.
Detailed mineral compositions and contents of the raw ore sample are presented in Table 3, encompassing over thirty distinct minerals such as iron minerals, rare earth minerals, niobium minerals, and gangue minerals, among others. The seven target minerals have M+HREE concentrations of 6.64 wt.%, 4.18 wt.%, 0.53 wt.%, 0.45 wt.%, 0.12 wt.%, 0.06 wt.%, and 0.02 wt.%, respectively. Additional minerals include magnetite (32.67 wt.%), fluorite (16.75 wt.%), and niobium-bearing minerals like Ilmenorutile, Columbite, Pyrochlore, and Baotite.

4.4.2. Distribution of Target Minerals

The monomer dissociation degree and particle size distribution of these seven target minerals were statistically analyzed by AMICS, and the results are presented in Table 4, Supplementary Table S3, and Supplementary Table S4. When the grinding fineness reaches below 74 μm and accounts for a 90% share, the monomer dissociation degrees of bastnäsite, monazite, Ca-fluorocarbonate, Ba-fluorocarbonate, allanite, aeschynite, and fergusonite reach 37.67%, 36.60%, 20.46%, 33.04%, 31.11%, 34.48%, and 0.07%, respectively (Figure 8A). Despite the close interconnection of the target minerals, approximately half of them remain unbound or only partially released due to their symbiotic relationship with other minerals (Table 4). When the particle size of target minerals is below 22.3 μm, the cumulative distribution rate of these seven minerals has reached 64.89%, 68.96%, 83.89%, 66.80%, 66.95%, 78.83%, and 100% (Figure 8B). This suggests that the target minerals may not only have fine particle dimensions but also exhibit complex mineral symbiosis.

5. Discussion

5.1. Resources Potential of M+HREE

Igneous carbonates are globally significant ore-forming rocks for rare earth deposits [68,69]. However, only about 10% of carbonates worldwide are related to rare earth mineral deposits [5]. Fan et al. (2022) analyzed the three-dimensional distribution pattern of carbonatite masses in the Bayan Obo deposit using geology, geochemistry, and geophysics [70]. They identified the ore-bearing carbonatite mass as a rare earth orebody. The researchers estimated the potential rare earth resources within the Bayan Obo deposit to be approximately 333 million tons within a depth range of 0 to 1000 m. This estimation was based on the volume, range, and minimum density of the carbonatite rock (rare earth orebody) with an average rare earth content of 2%, which was a conservative average derived from historical data.
The composition of light, medium, and heavy rare earth elements in different types of ores in the Bayan Obo deposit varied widely (Figure 5). According to statistical analysis in this study, the proportions of M+HREE from highest to lowest are as follows: Mica-type ore (5.254 wt.%) > Riebeckite-type ore (3.735 wt.%) > Fluorite-type ore (3.169 wt.%) > Massive iron ore (3.138 wt.%) > Aegirine-type ore (3.07 wt.%) > Dolomite-type ore (2.30 wt.%) (Figure 9). Dolomite-type ore is the most widely distributed ore type in the Bayan Obo mining area (Figure 2). Therefore, estimated M+HREE resources can reach about several million tons according to M+HREE average concentration in the dolomite-type ore. This also led to the proportion of M+HREE in the mixed raw ore being only 3.15%.
After exploring the Bayan Obo deposit, both the Main and East ores are planned for deep mining. In recent years, Baotou Iron and Steel (Group) Co., Ltd. has conducted over 1000 m of drilling for the deep orebodies in the Main and East ores. The drilling revealed that the ore deposits extend deep and are of significant thickness, and furthermore, these ore bodies are still accessible [71,72]. The borehole chart for WK20-02 (1775.4 m), the deepest exploration project in the East orebody, reveals the presence of rare earth minerals such as monazite and bastnäsite at the depth of the ore body. Data analyses indicate that the REO grade remains significantly elevated under 1000 m, occasionally exceeding 10% [51,73,74]. Meanwhile, REE-mineralization shows no signs of diminishing with increasing depth, and the potential for M+HREE resources at deeper levels is immense (Figure 4).

5.2. Influence Factors for M+HREE Enrichment

The transportation, enrichment, and precipitation of rare earth elements in carbonatites have long been a topic of interest [69,75,76]. Several theories have been suggested to elucidate the enrichment of heavy rare earth elements in carbonatites. These processes include the following: (1) the precipitation of light rare earth (LREE)-rich minerals, such as bastnesite or monazite, from a carbonatite melt/fluid, resulting in the depletion of LREEs in the residual melt phase and the relative enrichment of HREEs [24,31,69]; (2) redistribution of REE during late-stage hydrothermal alteration, owing to the preferential stability of REE-chloride complexes [15,18]; and (3) the melting of eclogitic garnets from the mantle source region during the rising or stalled rising of a carbonatite melt [26]. The whole-rock analyses of the Bayan Obo deposit show that the different type-ores underwent different M+HREE enrichment processes (Figure 5), with obvious heterogeneity and significant differences in the mineral contents of different ores (Table 1). The highest concentration of M+HREE was scored in aegirine-type ores and fluorite-type ores (Figure 6C), indicating that the enrichment of M+HREE may be linked to sodicisation and fluoritization.
The iron, rare earth, and niobium resources in the Bayan Obo deposits have experienced a long enrichment process in different degrees. At about 1.3 Ga, a strong carbonatite magmatism occurred in the Bayan Obo area, resulting in the formation of ore-bearing dolomites and widely developed carbonate-walls in the area [53]. The H2O-CO2-NaCl (F-REE) carbonatite melts underwent three stages: magma, magmatic-hydrothermal, and hydrothermal. In the late hydrothermal stage, the H2O-CO2-NaCl-F hydrothermal fluids from the magma differentiation of carbonatite magma upwelled, leading to intense alkaline alteration and fluoritization [44,54,77,78], followed by multiple phases of large-scale hydrothermal metasomatism and alteration until the Late Paleozoic [79,80]. The strong alteration evolution promoted the migration and precipitation of REE and Fe in the mine area, resulting in the formation of major rare-earth minerals such as monazite and bastnäsite [81], iron minerals such as magnetite, gangue minerals [82] such as fluorite, aegirine, riebeckite, and mica, and some fine-veined sulfides [78], which eventually formed different types of ore intergrowth (Figure 2). The transport-precipitation of REEs in hydrothermal fluids is mainly controlled by the high content of F− [83,84,85,86]. Meanwhile, during hydrothermal fluid evolution, the addition of Na and K increased the solubility of HREE in the fluids, resulting in a greater enrichment of HREE in the late-stage fluids, and facilitating the differentiation of light and heavy rare earth elements [69]. This suggests that the enrichment of medium and heavy rare earth elements may have occurred at a later stage of hydrothermal fluid evolution.

5.3. Targeted M+HREE-Rich Minerals

The Bayan Obo deposit has undergone a complex ore-forming process, resulting in a diverse range of minerals, unique structures, and varied mineral assemblages. To date, 71 elements and over 180 varieties of minerals have been identified. There are 29 rare-earth minerals and their variants, some of which are both rare-earth minerals and niobium minerals [65]. With the continuous progress of new detection techniques and theories, new minerals have been discovered, such as nioboixiolite-(□) [87], niobobaotite [88], bayanoboite-(Y) [89]. Bayanoboite-(Y) is a recently discovered fluorocarbonate heavy rare earth mineral found in deep drill cores within the Bayan Obo deposit. This mineral exhibits a unique chemical composition and crystal structure, containing elements such as Y, Dy, Ga, Er, Lu, and other heavy rare earth elements. This discovery offers valuable insights into the presence of heavy rare earths within the Bayan Obo deposit.
In this study, medium and heavy rare earth elements were mainly found in bastnäsite, monazite, Ba-fluorocarbonate, Ca-fluorocarbonate, allanite, aeschynite, and fergusonite (Figure 7, Supplementary Table S2). Bastnäsite and monazite are the most widely distributed and high REE abundant in the Bayan Obo mining area, occurring in a variety of types of ores with disseminated or banded structures (Table 1). Ba-fluorocarbonates are characteristic minerals at Bayan Obo. The alteration sub-types of Ba-fluorocarbonates with different Ba/REE ratios have formed in the mining area due to the enrichment of Ba, REE, CO32−, and F−. These minerals occur in aegirine-type and dolomite-type ores, which are relatively enriched in Na element [65,90]. Ca-fluorocarbonates also have some sub-type minerals according to hydrothermal alteration and mainly occur in fluorite-type ores and late-stage thin veins of massive iron ores [65,90]. Allanite is mainly found in skarns in the contact zone between Hercynian granites and ore-bearing dolomites and is rarely distributed within the ore body [65]. In this study, it was found to be present in very small quantities in fluorite-type ores (Table 1). Aeschynite is one of the most widely distributed and relatively coarse-grained niobium minerals in the mine area. It is found in all types of ores, mainly in sodium-metasomatic ores or sodium-bearing mineral veins, and is often found in close symbiosis with minerals such as aegirine and riebeckite [65]. Fergusonite is unevenly distributed in the mining area, and was initially discovered in dolomite-type Fe-REE-Nb ores in the Western orebody [91], and is also sporadically found in other types of ores (Table 1). Through previous studies and observations, it has been identified that seven minerals are the main focus for extracting and utilizing M+HREE resources. Among these minerals, bastnäsite, and monazite remain the primary sources of M+HREE resources. Aeschynite and fergusonite are not only primary sources for the extraction of medium and heavy rare earths but also crucial targets for the recovery of niobium resources.

5.4. M-HREE Utilizability

Although the Bayan Obo deposit has made a significant contribution to global LREE production, only a limited amount of M+HREE resources are utilized in this deposit. When considering the volume of REE reserves, the importance of M+HREE resources is immeasurable. Nonetheless, the distribution and occurrence of M+HREE in the process of exploitation and utilization of Bayan Obo resources remain largely unclear. Process mineralogy research is the basis of resource development and utilization. At present, research results on the process mineralogy of iron, rare earth, and niobium in the Bayan Obo deposit are relatively abundant. These results mainly include various types of ores [92,93], iron processing tailings [94], rare earth concentrates [95], and rare earth tailings [96]. However, there are few studies on the medium and heavy rare earths. The present study found that approximately 0.2 wt.% of mixed raw ore contains M+HREE (Table 2), while the proportion of mineral content of the seven target minerals is around 12% (Table 3). As the fineness of the grinding process dips below 74 μm, the degree of monomer dissociation in the targeted minerals remains below 40%. If the particle size of the target minerals falls under 22.3 μm, the aggregate distribution rates of these minerals have surpassed 60% (Figure 8). This indicates that the dissociation of the target mineral monomer and the grinding fineness in the mixed ore has not yet reached the ideal state. Therefore, efficient separation of fine minerals and fine grinding of the ore is crucial for the extraction of medium-heavy rare earth resources.
Meanwhile, the distribution and trend of M+HREE in the current beneficiation process at the Bayan Obo concentrating plant were investigated [66]. In the process of iron processing, 96.40% of the M+HREE entered the beneficiation tailings, and the grade of M+HREE increased from 0.24 wt.% to 0.31 wt.%. The rare earth concentrate obtained through additional sorting of rare earth from iron tailings revealed a 54.1% recovery rate of M+HREE, with the grade increasing to 1.2%. Approximately 42.4% of M+HREE was transferred to REE tailings, and subsequently, CaF2 and Nb were separated from the REE tailings. The mineral composition of REE tailings mainly includes Nb-rich minerals, medium and heavy rare earth minerals, fluorite, and silicate minerals, which are important research objects for the comprehensive recovery and utilization of Nb and M+HREE resources. While the Bayan Obo deposit holds immense promise for M+HREE resources, the complex symbiotic conditions among beneficial minerals (Table 4), along with a long production process, make the recovery and utilization of middle and heavy rare earths more challenging.

6. Conclusions

(1) The Main orebody contains a higher concentration of M+HREE compared to the East orebody and is significantly enriched in the deep orebody. The Bayan Obo deposit is estimated to have approximately several million tons of available M+HREE resources.
(2) Aegirine-type and fluorite-type ores are relatively enriched in M+HREE. The enrichment of M+HREE may be related to the extensive hydrothermal alteration, which was mainly influenced by the late stage of hydrothermal fluid evolution.
(3) The M+HREE target minerals include bastnäsite, monazite, Ca-fluorocarbonate, Ba-fluorocarbonate, allanite, aeschynite, and fergusonite, all of which exhibited content levels above 4000 ppm. Among them, bastnäsite and monazite are still the main contributors of M+HREE resources.
(4) The mixed raw ore selected from the Bayan Obo concentrator has a high content of medium and heavy rare earth elements, with numerous target minerals that are closely symbiotic. This composition is conducive to the comprehensive recovery of primary resources. The current beneficiation process has enriched a large amount of Nb and M+HREE elements in the dilute tailings, creating a secondary resource of high value.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15030212/s1, Table S1 Multi-element analysis of different ore types in the Bayan Obo deposit. Table S2: Rare earth composition of different minerals in the Bayan Obo deposit [45,50,54,77,97,98,99,100,101,102,103,104,105]. Table S3: The monomer dissociation degree of seven M+HREE target minerals in the mixed raw ore. Table S4: Particle size distribution of seven M+HREE target minerals in the mixed raw ore.

Author Contributions

Conceptualization, Q.S.; methodology, software, validation, formal analysis, investigation, H.J. and B.C.; resources, Q.L.; data curation, W.W. and Y.L.; writing—original draft preparation, H.J.; writing—review and editing, Q.S.; funding acquisition and supervision, Q.S. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the National Key Research and Development Program of China (2023YFC2908700, 2020YFC1909101, and 2022YFC2905300), Inner Mongolia Autonomous Region Natural Science Fund Program (2021MS04004, 2023LHMS05050), Opening Project of the National Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization (2021Z2349, 2022Z2406), and the scientific research projects were supported by Baotou Steel (Group) Co., Ltd. (HE2228).

Data Availability Statement

Original data for this research are included in the article and the Supplementary Materials; other data used are from cited publications.

Conflicts of Interest

The authors declare no conflict of interest.

Correction Statement

This article has been republished with a minor correction to resolve spelling and grammatical errors. This change does not affect the scientific content of the article.

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Figure 1. Geological sketch map of the Bayan Obo deposit [58]. (The scale of carbonatite dikes is drawn in greater proportion than the reality).
Figure 1. Geological sketch map of the Bayan Obo deposit [58]. (The scale of carbonatite dikes is drawn in greater proportion than the reality).
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Figure 2. Distributions of different types of ores of the Main orebody and Eastern orebody [65].
Figure 2. Distributions of different types of ores of the Main orebody and Eastern orebody [65].
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Figure 3. Typical ore types and structures of the Bayan Obo deposit. (A–C) Dolomite-type Fe-REE-Nb ore with disseminated structure, bastnäsite and monazite with a honeycomb distribution, different particle sizes, and closely associated with magnetite and barite within Dolomite-type ore; (D–F) Aaegirine-type Fe-REE-Nb ore with banded structure, alternating distribution of magnetite, aegirine, and bastnäsite, bastnäsite with a particle size between 10 and 100 μm; (G–I) Fluorite-type Fe-REE-Nb ore with banded structure, Ca-fluorocarbonate in association with bastnäsite and fluorite, distributing around magnetite; (J–L) Riebeckite-type Fe-REE-Nb ore with banded structure, fine veins or disseminated monazite interspersed in the ore; (M–O) Mica-type Fe-REE-Nb ore with schistose structure, a small amount of rare earth minerals scattedly distributed within Mica-type ore; (P–R) Massive Fe-REE-Nb ore, bastnäsite, Ca-fluorocarbonate, Ba-fluorocarbonate, and monazite filling between magnetite particles. Abbreviation: Aeg = aegirine, Dol = dolomite, Fl = fluorite, Mt = magnetite, Rbk = riebeckite.
Figure 3. Typical ore types and structures of the Bayan Obo deposit. (A–C) Dolomite-type Fe-REE-Nb ore with disseminated structure, bastnäsite and monazite with a honeycomb distribution, different particle sizes, and closely associated with magnetite and barite within Dolomite-type ore; (D–F) Aaegirine-type Fe-REE-Nb ore with banded structure, alternating distribution of magnetite, aegirine, and bastnäsite, bastnäsite with a particle size between 10 and 100 μm; (G–I) Fluorite-type Fe-REE-Nb ore with banded structure, Ca-fluorocarbonate in association with bastnäsite and fluorite, distributing around magnetite; (J–L) Riebeckite-type Fe-REE-Nb ore with banded structure, fine veins or disseminated monazite interspersed in the ore; (M–O) Mica-type Fe-REE-Nb ore with schistose structure, a small amount of rare earth minerals scattedly distributed within Mica-type ore; (P–R) Massive Fe-REE-Nb ore, bastnäsite, Ca-fluorocarbonate, Ba-fluorocarbonate, and monazite filling between magnetite particles. Abbreviation: Aeg = aegirine, Dol = dolomite, Fl = fluorite, Mt = magnetite, Rbk = riebeckite.
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Figure 4. Horizontal (A) and depth (B) distribution map of REE and M+HREE in the Bayan Obo deposit.
Figure 4. Horizontal (A) and depth (B) distribution map of REE and M+HREE in the Bayan Obo deposit.
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Figure 5. Box and whisker plots for multielement analysis of different types of ore in the Bayan Obo deposit.
Figure 5. Box and whisker plots for multielement analysis of different types of ore in the Bayan Obo deposit.
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Figure 6. PLS-DA results of the chemical composition of different ore types in Bayan Obo deposit. (A) Plot of qw*1 vs qw*2 (first and second loadings) showing correlation among elements and type of ore. (B) Plot of t1 vs t2 (first and second scores) showing the distribution of individual analyses in the latent variable space defined by qw*1–qw*2 in (A). (C–H) Score contribution plots of elements for different ore types in Bayan Obo deposit.
Figure 6. PLS-DA results of the chemical composition of different ore types in Bayan Obo deposit. (A) Plot of qw*1 vs qw*2 (first and second loadings) showing correlation among elements and type of ore. (B) Plot of t1 vs t2 (first and second scores) showing the distribution of individual analyses in the latent variable space defined by qw*1–qw*2 in (A). (C–H) Score contribution plots of elements for different ore types in Bayan Obo deposit.
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Figure 7. The REE content (A–C) and proportion of M+HREE/REE (D) of each mineral in the Bayan Obo deposit. (the data can be found in Supplementary Table S2).
Figure 7. The REE content (A–C) and proportion of M+HREE/REE (D) of each mineral in the Bayan Obo deposit. (the data can be found in Supplementary Table S2).
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Figure 8. The monomer dissociation degree (A) and particle size distribution (B) of M+HREE target minerals.
Figure 8. The monomer dissociation degree (A) and particle size distribution (B) of M+HREE target minerals.
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Figure 9. M+HREE/REE ratio (%) of different ore types in Bayan Obo deposit.
Figure 9. M+HREE/REE ratio (%) of different ore types in Bayan Obo deposit.
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Table 1. Minerals and contents (wt.%) of different types of ores in the Bayan Obo deposit.
Table 1. Minerals and contents (wt.%) of different types of ores in the Bayan Obo deposit.
MineralDolomite-Type Nb-REE-(Fe) OreFluorite-Type Nb-REE-(Fe) OreAegirine-Type Nb-REE-(Fe) OresRiebeckite-Type Nb-REE-(Fe) OreMica-Type Nb-REE-(Fe) OreMassive Nb-REE-Fe Ore
Magnetite41.3944.230.327.612.4378.61
Hematite0.050.01--0.030.05
Ilmenite-1.96-0.029.450.12
Pyrite-0.52-0.011.050.01
Pyrrhotite-0.03-8.690.01-
Bastnäsite4.851.635.140.220.110.63
Ca-fluorocarbonate0.062.70.190.01-0.92
Ba-fluorocarbonate0.160.040.01--0.06
Monazite0.70.060.2712.010.520.43
Allanite-0.01----
Aeschynite0.050.090.20.070.010.07
Pyrochlore--0.120.1-0.19
Columbite0.03--0.26--
Ilmenorutile---0.03--
Fergusonite0.020.010.010.01--
Aegirine0.010.5245.640.06-1.9
Fluorite-36.9713.480.91-5.13
Riebeckite--0.1545.3220.662.29
Dolomite480.420.330.0214.110.58
Calcite-2.820.13--4.28
Mica0.10.430.051.8238.590.03
Apatite-1.322.030.02-3.04
Barite3.475.030.680.011.4-
Other1.111.231.272.811.631.66
Table 2. Multi-element analysis results (wt.%) of the mixed raw ore of the production line at Bayan Obo concentrating plant.
Table 2. Multi-element analysis results (wt.%) of the mixed raw ore of the production line at Bayan Obo concentrating plant.
ComponentSiCaPFSThScNbFe
Content6.6310.631.38.891.360.0140.00590.0925.51
ComponentLaCePrNdSmEuGdTbDy
Content1.683.120.340.870.0940.0210.0340.00330.0111
ComponentHoErTmYbLuYM+HREEM+HREE/REE
Content0.00150.00270.00020.00170.00020.0260.1963.15
Table 3. Minerals and contents (wt.%) of the mixed raw ore of the production line at Bayan Obo concentrating plant.
Table 3. Minerals and contents (wt.%) of the mixed raw ore of the production line at Bayan Obo concentrating plant.
MineralMagnetiteHematitePyritePyrrhotiteSideriteIlmeniteBastnaesiteMonazite
Content32.67 2.82 1.86 0.16 0.51 0.40 6.64 4.18
MineralCa-fluorocarbonateBa-fluorocarbonateAllaniteAeschyniteFergusoniteIlmenorutile ColumbitePyrochlore
Content0.53 0.45 0.12 0.06 0.02 0.10 0.08 0.03
MineralBaotiteRiebeckiteAegirineMicaDolomiteCalciteFluoriteApatite
Content0.01 5.34 5.71 3.01 6.38 1.66 16.75 3.63
MineralBariteQuartzFeldsparOphioliteBafertisiteRhodochrositeGalenaSphalerite
Content3.51 1.77 1.06 0.09 0.12 0.26 0.04 0.06
Table 4. Symbiotic association of target minerals in the mixed raw ore (%).
Table 4. Symbiotic association of target minerals in the mixed raw ore (%).
MineralLiberatedAssociation
BastnaesiteMonaziteCa-FluorocarbonateBa-FluorocarbonateAllaniteAeschyniteFergusoniteOthers
Bastnaesite37.67 6.00 2.48 1.00 0.09 0.05 0.03 52.69
Monazite36.60 7.65 0.56 0.33 0.23 0.06 0.03 54.54
Ca-fluorocarbonate20.46 23.40 3.42 2.50 0.06 0.20 0.02 49.94
Ba-fluorocarbonate33.04 10.21 2.46 2.85 0.05 0.10 0.02 51.28
Allanite31.11 6.35 3.07 0.46 0.26 0.06 0.00 58.69
Aeschynite34.48 7.51 3.78 1.19 0.62 0.04 1.15 51.23
Fergusonite0.07 19.54 11.68 0.84 0.47 0.04 1.30 66.07
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Jin, H.; Sun, Q.; Chen, B.; Wei, W.; Liu, Y.; Li, Q. Evaluated Utilization of Middle–Heavy REE Resources in Bayan Obo Deposit: Insight from Geochemical Composition and Process Mineralogy. Minerals 2025, 15, 212. https://doi.org/10.3390/min15030212

AMA Style

Jin H, Sun Q, Chen B, Wei W, Liu Y, Li Q. Evaluated Utilization of Middle–Heavy REE Resources in Bayan Obo Deposit: Insight from Geochemical Composition and Process Mineralogy. Minerals. 2025; 15(3):212. https://doi.org/10.3390/min15030212

Chicago/Turabian Style

Jin, Hailong, Qing Sun, Biao Chen, Wei Wei, Yanjiang Liu, and Qiang Li. 2025. "Evaluated Utilization of Middle–Heavy REE Resources in Bayan Obo Deposit: Insight from Geochemical Composition and Process Mineralogy" Minerals 15, no. 3: 212. https://doi.org/10.3390/min15030212

APA Style

Jin, H., Sun, Q., Chen, B., Wei, W., Liu, Y., & Li, Q. (2025). Evaluated Utilization of Middle–Heavy REE Resources in Bayan Obo Deposit: Insight from Geochemical Composition and Process Mineralogy. Minerals, 15(3), 212. https://doi.org/10.3390/min15030212

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