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Article

Aeschynite Group Minerals Are a Potential Recovery Target for Niobium Resources at the Giant Bayan Obo Nb–REE–Fe Deposit in China

by
Bo Yang
1,2,3,
Li Yang
3,
Yong-Gang Zhao
4,5,*,
Guo-Ying Yan
3,
Jian-Yong Liu
3,
Wen-Xiang Meng
3,
Jun-Fang Yu
3,
Lei Chen
5,6,
Xiao-Chun Li
1 and
Xian-Hua Li
1,2,5,*
1
State Key Laboratory of Lithospheric and Environmental Coevolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
3
Mining Research Institute of Baotou Steel (Group) Corp., Baotou 014030, China
4
Baogang Group, Baotou 014030, China
5
State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization, Baotou 014000, China
6
School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(10), 1029; https://doi.org/10.3390/min14101029
Submission received: 22 August 2024 / Revised: 4 October 2024 / Accepted: 8 October 2024 / Published: 14 October 2024
(This article belongs to the Special Issue Mineral Resources in North China Craton)
Browse Figures
Versions Notes

Abstract

:
With the development of the steel industry, China’s demand for niobium is increasing. However, domestic niobium resources are not yet stably supplied and are heavily dependent on imports from abroad (nearly 100%). It is urgent to develop domestic niobium resources. The Bayan Obo deposit is the largest rare earth element deposit in the world and contains a huge amount of niobium resources. However, the niobium resource has not been exploited due to the fine-grained size and heterogeneous and scattered occurrences of Nb minerals. To promote the utilization of niobium resources in the Bayan Obo deposit, we focused on the mineralogical and geochemical characterization of six types of ores and mineral processing samples from the Bayan Obo deposit, using optical microscopes, EPMA, TIMA, and LA–ICP–MS. Our results show that: (1) the niobium mineral compositions are complex, with the main Nb minerals including aeschynite group minerals, columbite–(Fe), fluorcalciopyrochlore, Nb–bearing rutile, baotite, fergusonite–(Y), fersmite, and a small amount of samarskite–(Y). Aeschynite group minerals, columbite–(Fe), and fluorcalciopyrochlore are the main niobium-carrying minerals and should be the primary focus of industrial recycling and utilization. Based on mineralogical and geochemical investigation, the size of the aeschynite group minerals is large enough for mineral processing. Aeschynite group minerals are thus a significant potential recovery target for niobium, as well as for medium–heavy REE resources. The Nb–rich aegirine-type ores with aeschynite group mineral megacrysts are suggested to be the most significant niobium resource for mineral processing and prospecting. Combined with geological features, mining, and mineral processing, niobium beneficiation efforts of aeschynite group minerals are crucial for making breakthroughs in the utilization of niobium resources at the Bayan Obo.

1. Introduction

The excellent characteristics of niobium (Nb), such as high-temperature resistance, corrosion resistance, and superconductivity, make it widely used in high-strength corrosion-resistant low-alloy steels, aerospace materials, superconducting materials, advanced electronics, medicine, the nuclear industry, and many other areas [1,2,3,4]. Brazil as a single country is responsible for over 90% of global niobium production [5,6]. The growing demand for niobium will lead to a combination of supply risk and economic importance indices [4]. Niobium in China is heavily dependent on imports (with an extent of more than 95% [7]), thus posing significant supply risks. China stands as the largest consumer of niobium products globally [2,3,5,8], with a demand for niobium that has been increasing annually (Figure 1). This demand is fueling the exploration and extraction of niobium resources. The Bayan Obo deposit is a very large REE–Nb deposit [9,10,11,12,13], with prospective reserves of Nb2O5 as high as 1 million tons [14]. As a complex and refractory ore associated with multi-metals, the Bayan Obo ore contains various complex minerals and low content of uncertain target niobium minerals. The content of Nb2O5 in the Bayan Obo deposit ranges from 0.08% to 0.25%, and the niobium minerals are complex within the particles, mostly less than 20 μm in size [15,16]. At present, these ores can only be used for iron and rare earth minerals, while the niobium resources have not been effectively utilized [17].
This study reports a detailed investigation of the occurrence and compositional characteristics of niobium minerals in different types of ores from the Main Pit and East Pit, as well as the process mineralogical characteristics of niobium minerals in the raw ore. It clarifies the occurrence of niobium in different types of ores and the raw ore from the Bayan Obo deposit. This study identifies aeschynite group minerals, columbite–(Fe), and fluorcalciopyrochlore as the main minerals for niobium resources. We propose that aeschynite group minerals are the priority recovery target mineral to be utilized for niobium resources in the Bayan Obo deposit.

2. Geological Background

The giant Bayan Obo REE–Fe–Nb deposit is located on the northern margin of the North China Craton, adjacent to the Central Asian Orogenic Belt in the north. The Bayan Obo deposit contains >100 million metric tons of REE2O3 [18,19], as well as significant resources of Nb (1 million tons with an average grade of 0.13% Nb2O5) and Fe (1500 million metric tons with an average grade of 35%) [14].
The Bayan Obo deposit is hosted in the Paleoproterozoic to Mesoproterozoic metasedimentary rocks of the Bayan Obo Group (Figure 2). The predominant strata within the mining area are the Middle to Upper Mesoproterozoic Lower Bayan Obo Group, encompassing the Dulahala Formation, Jianshan Formation, Halahuogete Formation, and Bilute Formation. Notably, the ore-hosting dolomite serves as the principal host rock for mineralization [20,21,22], with the rock body exhibiting pervasive REE and Nb mineralization across its entirety [23]. The geological history of the mining area is marked by three significant events: carbonatite magmatism activity in the Mesoproterozoic, subduction of the Paleo-Asian Ocean in the Early Paleozoic, and magmatic activity in the Late Paleozoic [24,25], which correspond to the three stages of niobium mineralization [(1312 ± 47 Ma (n = 99), 438 ± 7 Ma (n = 93), and 268 ± 5 Ma (n = 19)] reported by Yu et al. (2024) [13].
Mining is currently active in three open pits, namely the Main, East, and West Pits (Figure 2). The Main and East Pits serve as the primary ore supply areas for the comprehensive utilization of Fe, REE, and Nb in the Bayan Obo deposit. In terms of resource distribution, the Nb, REE, and Fe are not uniform in different ores, with Nb–REE ores being the most widely distributed. Structurally, the ores exhibit massive, banded, and disseminated textures. Regarding the alteration characteristics, some ores have undergone alterations such as aegirine alteration, riebeckite alteration, and diopside alteration. Based on the aforementioned geological features of the ores and in conjunction with production needs, the ores in the mining area are divided into several types of ores, which are dolomite-type Nb–REE–(Fe) ores, fluorite-type-banded Nb–REE–Fe ores, massive Nb–REE–Fe ores, aegirine-type Nb–REE–(Fe) ores, riebeckite-type Nb–REE–Fe ores, biotite-type Nb–REE–Fe ores, and diopside-type Nb–REE ores [26].
Dolomite-type Nb–REE–(Fe) ores are the most extensively distributed in the mining area. These ores are predominantly dolomite Nb–REE–Fe ores in the West Pit. However, they are primarily dolomite Nb–REE ores in the East and Main Pits. The ores exhibit a grayish-white color (Figure 3a), characterized by a disseminated structure. The main minerals are dolomite and magnetite, with magnetite often occurring as subhedral granular interstitial fillings among the dolomite grains. Other minerals mainly include fluorite, barite, apatite, monazite, bastnäsite, and aegirine.
Fluorite-type-banded Nb–REE–Fe ores are distributed in the Main Pit and the eastern part of the East Pit near the lower section, with a small amount exposed near the upper section, and are rarely seen in the West Pit. They exhibit a grayish-purple color (Figure 3b), predominantly characterized by a banded texture. The bands are primarily composed of iron minerals, fluorite, and rare earth minerals, with varying widths and limited extensions. Other minerals mainly include dolomite, apatite, and barite.
Massive Nb–REE–Fe ores are primarily distributed in the central part of the iron ore bodies in the Main and East Pits, as well as in some local areas of the West Pit. They exhibit a grayish-black color (Figure 3c) with a massive texture and are predominantly composed of fine-grained and coarse-grained magnetite, featuring a granular structure. Other associated minerals include fluorite, monazite, bastnäsite, aegirine, riebeckite, phlogopite, and hematite.
Biotite-type Nb–REE–Fe ores are the main type of ore in the West Pit, predominantly distributed near the upper section of the West Mine, with sporadic occurrences also found in the upper sections of the Main and East Pits. They exhibit a grayish-black color (Figure 3d) and have a schistose structure. The principal minerals are biotite, magnetite, phlogopite, riebeckite, and monazite. Other minerals include fluorite, pyrite, pyrrhotite, and bafertisite.
Riebeckite-type Nb–REE–Fe ores are primarily distributed in the West Mine and the areas close to the upper section of the East Pit, with a small amount also found near the upper section of the Main Pit. They exhibit a blueish-gray color (Figure 3e) and have a disseminated structure. The primary minerals are riebeckite, magnetite, and dolomite. Other minerals include pyrite, monazite, bastnäsite, and fluorite.
Aegirine-type Nb–REE–(Fe) ores are predominantly distributed in the areas near the upper section of the Main Pit and in both the upper and lower sections of the East Pit. They exhibit a greenish-gray color (Figure 3f) and possess either a disseminated structure or a disseminated banded texture. The main minerals are aegirine, magnetite, and REE minerals. Other minerals include aeschynite group minerals, apatite, fluorite, barite, and bastnäsite.
Diopside-type Nb–REE ores are less common and only occur along the contact zone between the granite and dolomite in the eastern part of the mining area. The principal minerals are diopside and phlogopite. Other minerals include apatite, feldspar, titanite, and dolomite [26].

3. Samples and Analytical Methods

3.1. Sample Description

This study primarily focuses on two types of samples: (1) Different types of ores from the Main and East Pits (collected from the east of platform 1388 and the north of platform 1402, respectively), including aegirine-type Nb–REE–(Fe) ores, fluorite-type-banded Nb–REE–Fe ores, riebeckite-type Nb–REE–Fe ores, massive Nb–REE–Fe ores, biotite-type Nb–REE–Fe ores, and dolomite-type Nb–REE–(Fe) ores. (2) Raw ores from the beneficiation process, with the samples in powder form (85% grain size < 74 μm), collected from the Bayan Obo comprehensive utilization processing production line (sampled continuously over 31 days). Currently, the raw ores are used for Fe beneficiation, followed by REE beneficiation, with the tailings used as material for niobium beneficiation [27]. The samples were dried and mixed evenly before being used for further study.

3.2. Petrographic Observation

Microscopic analysis was conducted at the Mining Research Institute of Baotou Steel (Group) Corp., Baotou, China, utilizing the Zeiss Scope A1 instrument (Oberkochen, Germany). The minerals were described and identified for their optical characteristics under transmitted light, crossed polarized light, and reflected light.
The particle size sieving of the raw ores was conducted at the Mining Research Institute of Baotou Steel (Group) Corp., employing a wet sieving method with the AS200 CONTROL instrument (RETSCH, Haan, Germany).
TIMA testing was obtained on carbon-coated thin sections (mounts) using the MIRA3 scanning electron microscope equipped at Guangzhou Tuoyan Analytical Technology Co., Ltd., Guangzhou, China, with an acceleration voltage of 25 kV and a probe current of 8.24 nA. The working distance was set to 15 mm. Pixel spacing was set to 3 μm, and spot spacing was set to 9 μm. The current and backscattered-electron (BSE) signal intensity was calibrated on a platinum Faraday cup using the automated procedure. Energy-dispersive spectroscopy (EDS) performance was checked using the manganese standard. The samples were scanned using the TIMA liberation analysis module.

3.3. Major and Trace Element Compositions of Niobium Minerals

The XRF analysis was conducted at the Mining Research Institute of Baotou Steel (Group) Corp. using the Shimadzu XRF-1800 instrument (Shimadzu, Tokyo, Japan). The determination of Nb2O5 was performed using the fusion sample preparation method followed by XRF spectrometry. After crushing and grinding the samples, a mixture of Li2B4O7, LiBO2, and LiBr was used as a flux. The samples were pre-oxidized using a high-frequency furnace with staged temperature increase, fused at 1050 °C, and upon cooling, formed a glassy frit, which was then analyzed using an XRF spectrometer.
The EPMA analysis was conducted at the Mining Research Institute of Baotou Steel (Group) Corp. using the Jeol XM-ISP100 instrument (JEOL Ltd., Akishima city, Tokyo, Japan). The operating conditions were set with an accelerating voltage of 15 kV and a beam current of 10 nA, with a beam spot diameter ranging from 1 to 10 µm. Specific elemental standards used were as follows: K–KNbO3; Ca–apatite; Ti–rutile; Na–jadeite; Si–quartz; Mg–forsterite; Al–kyanite; Cr–Cr2O3; Fe–magnetite; Mn–pyrophanite, F–phlogopite; Cl–halite; P–apatite; Sr–celestite; Ba–barite; Sn–cassiterite; Nb–LiNbO3; Ni–NiO; Co–CoO; U–Uraninite; Th–ThO2; Pb–PbVGe glass; Ta–LiTaO3; Sc–metallic scandium. For the rare earth elements: La–H2LaO5P; Ce–H2CeO5P; Pr–H2PrO5P; Nd–H2NdO5P; Sm–H2SmO5P; Eu–EuF3; Gd–H2GdO5P; Tb–Tb3Ga5O12; Dy–H2DyO5P; Ho–H2HoO5P; Er–H2ErO5P; Tm–H2TmO5P; Yb–H2YbO5P; Lu–LSO; Y–H2YO5P.
Trace element concentrations of aeschynite group minerals, columbite–(Fe), and fluorcalciopyrochlore were determined by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) employing an Agilent 7500a Q-ICP-MS instrument (Agilent Technologies, Santa Clara, CA, USA) coupled to a 193 nm ArF excimer laser system (Geolas HD, Lambda Physik, Göttingen, Germany) or an Analyte G2 193 nm ArF excimer laser ablation system at the State Key Laboratory of Lithospheric and Environmental Coevolution, Institute of Geology and Geophysics, Chinese Academy of Sciences. The approach is similar to those outlined in Wu et al. (2018) with isotopes measured using a peak-hopping mode with a laser beam diameter of ca. 44 μm and 5 Hz repetition rate for aeschynite group minerals and columbite–(Fe), while ca. 24 μm and 5 Hz for fluorcalciopyrochlore [28]. The laser energy density is ~4.0 J/cm2f. Helium was employed as ablation gas to improve the transporting efficiency of ablated aerosols. ARM-1 reference glass [29,30] was used as the calibration material, and NIST SRM 610 and BCR-2G were analyzed for data quality control. SRM 610 was selected as the external standard material since there is no available international standard material for niobium oxides. Considering potential problems with absolute concentration, these analytical results were only used to calculate REE patterns. The resulting data were reduced based on the GLITTER program [31]. For most trace elements (>0.10 μg/g), the accuracy is better than ±10% with analytical precision (1 RSD).

4. Results

4.1. Petrography and Mineralogy of Nb Minerals in the Ores

Over 200 samples from six types of ores hosting Nb mineralization were examined in detail. The petrological and mineralogical characteristics of the Bayan Obo carbonatite and ore were described in detail by several previous studies [10,22,32,33,34]. Eight types of Nb minerals are recognized in this study, including aeschynite group minerals, columbite–(Fe), fluorcalciopyrochlore, Nb–bearing rutile, baotite, fergusonite–(Y), fersmite, and samarskite–(Y).

4.1.1. Aeschynite Group Minerals

Aeschynite group minerals are one of the most common Nb minerals at Bayan Obo. Aeschynite group minerals are mainly observed from aegirine-type Nb–REE–(Fe) ore and widely distributed in dolomite-type Nb–REE–(Fe) ore, fluorite-type-banded Nb–REE–Fe ore, massive Nb–REE–Fe ore, biotite-type Nb–REE–Fe ore, and riebeckite-type Nb–REE–Fe ore.
Aeschynite group minerals are predominantly a dark brown to black color in hand specimens and have an adamantine luster (Figure 4a). Aeschynite group minerals exhibit a light brown to reddish-brown color under transmitted light (Figure 4b–h), they have positive relief (Figure 4b,c) and weak pleochroism, and they commonly present one set of cleavage (Figure 4b). In the aegirine-type Nb–REE–(Fe) ores, aeschynite group minerals typically occur in euhedral to subhedral forms (Figure 4b,c), with a grain size ranging from 100 to 400 μm and a few reaching centimeter-scale (Figure 4a). The euhedral aeschynite group mineral mega-crystals in the aegirine-type Nb–REE–(Fe) ores are generally associated with apatite and aegirine (Figure 4b,c). In the riebeckite-type Nb–REE–Fe ores, aeschynite group minerals are characteristically found in shapes that are either subhedral or euhedral (Figure 4d). The grains of these minerals are typically measured between 100 and 200 μm (Figure 4d). Some grains are anhedral granular and less than 50 μm in size. Aeschynite grains in this type of ore are always associated with riebeckite and dolomite aggregates (Figure 4d). In the dolomite-type Nb–REE–(Fe) ore, Nb minerals are generally encountered in forms that are subhedral to anhedral (Figure 4e), and the grains vary in size from 20 to 100 μm (Figure 4e). Aeschynite grains in this type of ore are always disseminated in dolomite (Figure 4e). In the massive Nb–REE–Fe ores, aeschynite grains are commonly found in irregular shapes (Figure 4f), with grain sizes varying from 20 to 100 μm (Figure 4f). Aeschynite grains in this type of ore are always interstitial to dolomite or magnetite aggregates (Figure 4f). In the banded ores, aeschynite always appears in subhedral to anhedral forms (Figure 4g), with grain sizes ranging from 30 μm to 50 μm (Figure 4g). Aeschynite grains in this type of ore are always crystallized along the fluorite vein. In the biotite-type Nb–REE–Fe ores, aeschynite usually takes on shapes from anhedral to subhedral (Figure 4h). Aeschynite grains are typically 10 to 50 μm in size, with some grains extending up to 100 μm (Figure 4h). Biotite and dolomite matrices are the relatively earlier forms of the minerals, which are replaced by the fine-grained aggregates of aeschynites occasionally (Figure 4h).

4.1.2. Columbite–(Fe)

Columbite–(Fe) is also a common Nb mineral at Bayan Obo. Columbite–(Fe) is mainly observed in biotite-type Nb–REE–Fe ore and also widely distributed in dolomite-type Nb–REE–(Fe) ore, fluorite-type-banded Nb–REE–Fe ore, massive Nb–REE–Fe ore, riebeckite-type Nb–REE–Fe ore, and aegirine-type Nb–REE–(Fe) ore.
Columbite–(Fe) is mostly opaque under transmitted light, with the translucent parts showing a dark red color (Figure 5a–c). Columbite–(Fe) exhibits pleochroism when the stage is rotated. Under reflected light, it exhibits significant heterogeneity. In the biotite-type Nb–REE–Fe ores, columbite–(Fe) predominantly exhibits anhedral forms, with grain sizes typically between 20 and 50 μm (Figure 5a,b). The columbite–(Fe) grains in the biotite-type Nb–REE–Fe ores are generally associated with dolomite aggregates or replaced biotite (Figure 5a,b).
In the context of other ore types, columbite–(Fe) maintains its anhedral fine-grained appearance in each ore type. In the dolomite-type Nb–REE–(Fe) ores, columbite–(Fe) typically occurs in anhedral fine-grain forms (Figure 5c), with grains ranging from 10 to 20 μm (Figure 5c). The columbite–(Fe) grains in the dolomite-type Nb–REE–(Fe) ores are generally associated with dolomite and magnetite (Figure 5c). In the aegirine-type Nb–REE–(Fe) ores and riebeckite-type Nb–REE–Fe ores, the grain size is typically within the range of 10 to 50 μm. Within the fluorite-type-banded Nb–REE–Fe ores, the grains are approximately 10 to 30 μm in size. In the massive Nb–REE–Fe ores, the size of grains generally ranges between 10 μm and 20 μm.

4.1.3. Fluorcalciopyrochlore

Fluorcalciopyrochlore is one of the most common Nb minerals at Bayan Obo. Fluorcalciopyrochlore is mainly observed in aegirine-type Nb–REE–(Fe) ore and dolomite-type Nb–REE–(Fe) ore. It is also widely distributed in fluorite-type-banded Nb–REE–Fe ore, massive Nb–REE–Fe ore, biotite-type Nb–REE–Fe ore, and riebeckite-type Nb–REE–Fe ore.
Fluorcalciopyrochlore exhibits a dark yellow to tangerine color under transmitted light and shows extremely positive relief. Fluorcalciopyrochlore is homogeneous under crossed polarized light. It also has a relatively high reflectivity under reflected light (Figure 5d). In aegirine-type Nb–REE–(Fe) ores, fluorcalciopyrochlore is commonly found in subhedral to euhedral forms (Figure 5d), and the grains of fluorcalciopyrochlore range in size from 50 to 200 μm (Figure 5d). The fluorcalciopyrochlore grains in the aegirine-type Nb–REE–(Fe) ores are always interstitial to aegirine aggregates (Figure 5d). In dolomite-type Nb–REE–(Fe) ores, fluorcalciopyrochlore generally appears in subhedral to euhedral forms (Figure 5e) and has grain sizes ranging from 20 to 50 μm (Figure 5e). The fluorcalciopyrochlore grains in the dolomite-type Nb–REE–(Fe) ores are generally disseminated in dolomite (Figure 5e).
In other types of ore, fluorcalciopyrochlore exhibits its subhedral to euhedral appearance. Specifically, in the riebeckite-type and massive-type Nb–REE–Fe ores, the grain size typically ranges from 20 to 50 μm. In fluorite-type-banded Nb–REE–Fe ores, the grains are approximately 30 to 50 μm in size, whereas in biotite-type Nb–REE–Fe ores, the grain sizes generally vary from 10 to 50 μm.

4.1.4. Nb–Bearing Rutile

Nb–bearing rutile is predominantly observed in fluorite-type-banded Nb–REE–Fe ore and also distributed across other types of ore, including aegirine-type Nb–REE–(Fe) ore, dolomite-type Nb–REE–(Fe) ore, massive Nb–REE–Fe ore, biotite-type Nb–REE–Fe ore, and riebeckite-type Nb–REE–Fe ore.
Nb–bearing rutile exhibits a brownish-yellow to brownish color under transmitted light (Figure 5f,g), with the color deepening as the Nb content increases. Nb–bearing rutile has a positive relief and exhibits weak pleochroism, showing higher-level interference color under crossed polarized light. In fluorite-type-banded Nb–REE–Fe ores, Nb–bearing rutile typically occurs in anhedral forms or as fine-grained aggregates (Figure 5d), with grain sizes ranging from 10 to 50 μm (Figure 5f). The Nb–bearing rutile grains in the fluorite-type-banded Nb–REE–Fe ores are generally disseminated in fluorite (Figure 5f).
In other ore types, Nb–bearing rutile retains its fine-grained appearance. In dolomite-type Nb–REE–(Fe) ores, aegirine-type Nb–REE–(Fe) ores, riebeckite-type Nb–REE–Fe ores, and biotite Nb–REE–Fe ores, the grain size typically ranges from 10 to 30 μm. In massive-type Nb–REE–Fe ores, the grains are approximately 10 to 50 μm. Nb–bearing rutile grains larger than 50 μm are occasionally found in aegirine-type Nb–REE–(Fe) ores and are always associated with aegirine (Figure 5g).

4.1.5. Baotite, Fergusonite–(Y), Fersmite, and Samarskite–(Y)

Baotite, fergusonite–(Y), fersmite, and samarskite–(Y) are characterized by their low abundance in each ore type, with fine particle sizes ranging from 10 to 30 μm. A few grains exceeding 100 μm in size are occasionally observed.
Baotite exhibits significant pleochroism under transmitted light, with colors ranging from yellow-brown to dark brown (Figure 5h). Fergusonite–(Y) exhibits a red to dark red color under transmitted light, with some small grains being opaque. Fersmite exhibits a yellowish to brownish-yellow color and shows pleochroism. Samarskite–(Y) exhibits a brownish yellow to brownish color under transmitted light. These Nb minerals are always scattered in minerals such as dolomite (Figure 5h), fluorite, magnetite, and aegirine.

4.2. Chemical Compositions of Ores

All the types of ores in the Bayan Obo deposit generally contain niobium. However, the distribution of niobium mineralization is heterogeneous among the samples. The contents of Nb2O5 in the aegirine-type Nb–REE–(Fe) ore, fluorite-type-banded Nb–REE–Fe ore, and riebeckite-type Nb–REE–Fe ore are relatively high (Supplementary Table S1). The average grade of Nb2O5 in the six types of ores is 0.14 wt.%, which is much higher than the Nb abundance in the crust. The content of Nb2O5 is the highest in the aegirine-type Nb–REE–(Fe) ore (0.18 wt.%) and the lowest in the dolomite-type Nb–REE–(Fe) ore (0.09 wt.%) (Figure 6; Table S1). In the fluorite-type-banded Nb–REE–Fe ores, riebeckite-type Nb–REE–Fe ores, biotite-type Nb–REE–Fe ores, and massive Nb–REE–Fe ores, the Nb2O5 content ranges from 0.05 to 0.24 wt.%, 0.11% to 0.27 wt.%, 0.05% to 0.20 wt.%, and 0.06% to 0.20 wt.%, respectively.

4.3. Major Elements of Aeschynite Group Minerals

The major element compositions of each Nb mineral are presented in Supplementary Table S2.
Aeschynite (AB2O6) group minerals in Bayan Obo contain Nb and Ti cations in the B site and LREE (mainly Ce and Nd) cations in the A site. Following the International Mineralogical Association’s classification system for aeschynite group minerals [35], aeschynite with Ti or Nb was named “aeschynite” and “nioboaeschynite”, respectively, with suffixes of “–Ce”, “–Nd”, or “–Y” when its corresponding concentration exceeds the rest of REEs. The chemical structures of aeschynite group minerals have been calculated for six oxygen atoms per formula listed in Table S2. Four types of aeschynite have been identified in this study (Figure 7): aeschynite–(Nd), nioboaeschynite–(Nd), aeschynite–(Ce), and nioboaeschynite–(Ce).
Four types of aeschynite contain variable contents of Nb2O5 (27.54~40.10 wt.%), TiO2 (19.33~25.33 wt.%), Ce2O3 (8.06~17.53 wt.%), and Nd2O3 (13.33~17.52 wt.%). The chemical formula of aeschynite group minerals is
  • (Nd0.39Ce0.29Th0.10Ca0.06Pr0.05Sm0.04Fe0.04La0.02Y0.01Gd0.01Ba0.01)Σ1.02(Ti1.10Nb0.90)Σ2.00O6.00 for aeschynite–(Nd),
  • (Nd0.33Ce0.20Th0.07Ca0.10Pr0.03Sm0.05Fe0.07La0.01Y0.07Gd0.02Dy0.01Er0.01Lu0.01)Σ0.98(Nb1.09Ti0.93)Σ2.02O6.00 for nioboaeschynite–(Nd),
  • (Ce0.37Nd0.32Th0.01Ca0.12Pr0.04Sm0.02Fe0.03La0.04Y0.03Gd0.01Lu0.01)Σ1.00(Ti1.07Nb0.96)Σ2.03O6.00 for aeschynite–(Ce), and
  • (Ce0.36Nd0.32Th0.01Ca0.14Pr0.04Sm0.04Fe0.03La0.04Y0.03Gd0.01)Σ1.02(Nb1.07Ti0.92)Σ1.99(O5.96F0.04)Σ6.00 for nioboaeschynite–(Ce), respectively (Table S2).

4.4. REE and Trace Elements of Nb Minerals

In-situ REE and trace element analyses were conducted on the aeschynite group minerals and fluorcalciopyrochlore. The results are presented in Supplementary Table S3. The chondrite normalized REE patterns were calculated and plotted based on these in-situ REE concentrations (Figure 8), including light rare earth elements (LREE, La, Ce, Pr, and Nd) and middle and heavy rare earth elements (M–HREE, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y).
Aeschynite group minerals in the aegirine-type ores were identified as aeschynite–(Ce), nioboaeschynite–(Ce), and nioboaeschynite–(Nd). Aeschynite–(Nd) was identified in the dolomite-type ores. All the aeschynite group minerals in this study presented nearly flat REE patterns (Figure 8). Aeschynite–(Nd) contains variable contents of LREE (266,169~280,185 ppm) and M–HREE (30,900~40,506 ppm). Nioboaeschynite–(Nd) contains variable contents of LREE (209,957~222,014 ppm) and M–HREE (83,715~94,649 ppm). Aeschynite–(Ce) contains variable contents of LREE (292,983~303,922 ppm) and M–HREE (36,435~43,542 ppm). Nioboaeschynite–(Ce) contains variable contents of LREE (269,716~297,901 ppm) and M–HREE (55,253~62,358 ppm). Four types of aeschynite contain variable contents of LREE (209,957~303,922 ppm) and M–HREE (30,900~94,649 ppm). Fluorcalciopyrochlore contains variable contents of LREE (19,901~23,133 ppm) and M–HREE (1867~2295 ppm).

4.5. Occurrence and Distributions of Nb Minerals in the Raw Ores

The raw ores, which are the materials for the beneficiation process, were in powdered form. A total of 148.5 g of raw ores were separated using a series of sieves into five size fractions: greater than 74 μm, 45 to 74 μm, 32 to 45 μm, 25 to 32 μm, and smaller than 25 μm. Each fraction was dried and weighed to calculate the weight distributions of each size fraction (weight of each size fraction/total weight × 100). The Nb2O5 distribution rate was calculated for each size fraction (the weight distributions × the grade of Nb2O5). These results are shown in Table 1.
The main minerals were identified using the TIMA analysis, including magnetite, aegirine, riebeckite, dolomite, calcite, bastnäsite, monazite, apatite, barite, and biotite. The Nb mineral composition of the raw ores is shown in Figure 9. The particle size of niobium minerals in the raw ores shows varying sizes. The majority of aeschynite group mineral grains are in the 50 to 100 μm range, with some grains reaching sizes of up to 200 μm. Additionally, there are many aeschynite group mineral grains smaller than 25 μm in size. The majority of pyrochlore group mineral grains are smaller than 25 μm, while only a few grains reach sizes up to 50 μm. The majority of Nb–bearing rutile grains are smaller than 25 μm, although some grains can occur as larger particles that are often intergrown with other minerals. In contrast, other Nb minerals are smaller than 25 μm.
The Nb2O5 distribution rate of each mineral phase was calculated by Nb2O5 average content of mineral × mineral content/the average grade of Nb2O5. The Nb2O5 grade of the total raw ores is 0.13% (Table 1). According to the niobium mineral composition, the total distribution rate of Nb2O5 in the various niobium minerals is 90.32% (Figure 9a), with an additional 9.68% of Nb2O5 distributed among other minerals (Figure 9a). The highest Nb2O5 distribution rate of the mineral phase is aeschynite group minerals (30.66%), followed by columbite–(Fe) (26.89%), pyrochlore group minerals (mainly fluorcalciopyrochlore) (20.53%), Nb–bearing rutile (5.06%), baotite (4.10%), and fergusonite–(Y) along with samarskite–(Y) (3.08%).

5. Discussion

5.1. Aeschynite Group Minerals Are the Major Source of Niobium at Bayan Obo

The main niobium ore minerals in the three carbonatite-hosted deposits worldwide (Araxá, St Honoré, and Catalão I) that are responsible for most of the world’s niobium production is pyrochlore group minerals [38,39,40,41]. Based on petrographic observation, the predominant Nb minerals at Bayan Obo include aeschynite group minerals, columbite–(Fe), fluorcalciopyrochlore, and Nb–bearing rutile, accompanied by minor amounts of baotite, fergusonite–(Y), and samarskite–(Y) (Table 2). These minerals are widely distributed in various types of ores without showing obvious concentrated distribution. Compared to other Nb minerals, the aeschynite group minerals have a relatively high volume proportion [11,15]. Aeschynite group minerals are the most significant Nb mineral in various types of ore, including aegirine-type Nb–REE–(Fe) ore, riebeckite-type Nb–REE iron ore, dolomite-type Nb–REE–(Fe) ore, and massive Nb–REE–Fe ore (Table 3). Aeschynites in the dolomite-type and fluorite-type-banded ores are generally fine-grained and interstitial to magnetite grains (Figure 4e–g). Aeschynite group minerals in the riebeckite-type and biotite-type ores replace massive sodium–amphibole as well as biotite (Figure 4d,h). However, aeschynite group minerals in aegirine-type ores are euhedral in shape (Figure 4a–c), with grain sizes reaching 400 μm~4 mm. Aeschynite group minerals are the largest particle-sized minerals among the Nb minerals in the six types of ore. The mineral species of Nb minerals in the raw ores show that the minerals mainly include aeschynite group minerals, columbite–(Fe), pyrochlore group minerals (mainly fluorcalciopyrochlore), and Nb–bearing rutile, accounting for more than 60% (Figure 9b). The distribution rate of Nb2O5 in different Nb minerals is displayed in Figure 9a, showing that the aeschynite group minerals, columbite–(Fe), and pyrochlore group minerals contain more than 70% niobium. A previous study concluded that Nb minerals with a high average content of Nb2O5 in the Bayan Obo deposit are columbite–(Fe) (Nb2O5 76.057 wt.%) and pyrochlore group minerals (Nb2O5 62.933 wt.%) [11]. However, the Nb2O5 distribution rate in the aeschynite group minerals is much higher than pyrochlore group minerals and columbite–(Fe) (Figure 9), which means the aeschynite group minerals are the most important niobium resource at Bayan Obo.

5.2. Aeschynite Group Minerals Are Also the Major Source of M-HREE at Bayan Obo

Monazite and bastnasite are the main REE minerals in the deposit that lack Nb. REE and Nb have long been regarded as cogenetic resources, originated from carbonatite magma, transported through the same batches of melts or hydrothermal fluids, and coprecipitated during the same metasomatic process [32,48]. Petrographic observations show that niobium mineralization is genetically related to REE and Fe mineralization (Figure 4c). Some Nb minerals are enriched in REEs, such as aeschynite group minerals and fergusonite–(Y) [11]. Compositionally, the aeschynite group minerals include Ce– and Nd–rich varieties, containing individual amounts of Ce [14.62~17.53 wt.% Ce2O3 in aeschynite–(Ce) and nioboaeschynite–(Ce); 8.06~13.77 wt.% Ce2O3 in aeschynite–(Nd) and nioboaeschynite–(Nd)] and similar amounts of Nd (13.33~17.52 wt.% Nd2O3; Table S2; Figure 7). Chondrite-normalized REE patterns of four types of aeschynite group minerals are generally similar (Figure 8). The REE patterns of the four types of aeschynite group minerals present LREE– and M–HREE-rich concentrations. The concentrations of M–HREE range from 30,900 ppm to 94,649 ppm (Table S3), which is much higher than fluorcalciopyrochlore (2142 ppm), monazite (8471 ppm), and bastnasite (9246 ppm), respectively. Therefore, aeschynite group minerals with high contents of Nb and M–HREE are potentially the most important M–HREE minerals among Nb minerals.

5.3. Aegirine-Type Nb–REE–(Fe) Ore Has Important Insights for Mineral Processing and Prospecting

To promote the utilization of niobium resources, we focused on the composition and distribution of niobium minerals of different types of Nb–REE–Fe ore in the Bayan Obo deposit. The mineralogical and geochemical results demonstrate that the aegirine-type Nb–REE–(Fe) ores can serve as an important reserve of niobium. The niobium takes the form of euhedral aeschynite group mineral megacrysts, which occur in the niobium-mineralized aegirine–magnetite rocks. Aegirine alteration ores exhibit distinct spatial distribution patterns [26] and are mainly found in the Main Orebody in this study. Two belts are found in East Orebody [32]. The Nb–aegirine-rich alteration zones are the most significant niobium resource for mineral processing and prospecting at Bayan Obo. Aeschynite group minerals can be taken as the main recovery target.

6. Conclusions

(1) The main niobium minerals in the Bayan Obo deposit are aeschynite group minerals, columbite–(Fe), fluorcalciopyrochlore, Nb–bearing rutile, along with minor amounts of fergusonite–(Y), samarskite–(Y), and baotite. Among them, aeschynite group minerals, columbite–(Fe), and fluorcalciopyrochlore are the predominant niobium resources.
(2) Aeschynite group minerals with high contents of Nb and M–HREE, especially the nioboaeschynite–(Nd) and nioboaeschynite–(Ce), are a major source of niobium and M–HREE.
(3) The Nb–rich aegirine-type ores with aeschynite group mineral mega-crystals are proposed to have great potential for mineral processing and prospecting at Bayan Obo.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14101029/s1, Table S1. The results of Nb2O5 content for different types of ores in the Bayan Obo deposit (wt.%). Table S2. Electron microprobe results of the main niobium minerals in the Bayan Obo deposit. Table S3. The REE compositions for aeschynite group minerals and fluorcalciopyrochlore from the Bayan Obo deposit by LA-ICP-MS.

Author Contributions

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

Funding

This work was supported by the National Natural Science Foundation of China [92262303] and Baotou Iron and Steel (Group) Co., Ltd. Joint Funds of State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization [GZ-2023-1-LH-001/002].

Data Availability Statement

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

Acknowledgments

We thank Shitou Wu from the Institute of Geology and Geophysics, Chinese Academy of Sciences and Xinhao Guo from Guangzhou Tuoyan Testing Technology Co., Ltd. for their help with LA-ICP-MS and TIMA analyses. Wenli Liu from Baoshan Mining Company, Bin Guo and Xingyu Yuan from Baiyun Iron Mine for their help in the field are gratefully appreciated. Jinlong Wang, Maosen Shen, Hongjing Li, and Liming Wang from the Mining Research Institute of Baotou Steel (Group) Corp. provided valuable suggestions during the data processing. The manuscript benefited from detailed and constructive reviews by two anonymous reviewers, which are gratefully appreciated. We thank the editor for editorial handling.

Conflicts of Interest

Bo Yang, Li Yang, Guo-Ying Yan, Jian-Yong Liu, Wen-Xiang Meng and Jun-Fang Yu were employed by the Mining Research Institute of Baotou Steel (Group) Corp., Yong-Gang Zhao was employed by the Baogang Group. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Trend chart of niobium demand in China in recent years (data from https://www.huaon.com/channel/trend/804683.html, accessed on 1 February 2022).
Figure 1. Trend chart of niobium demand in China in recent years (data from https://www.huaon.com/channel/trend/804683.html, accessed on 1 February 2022).
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Figure 2. Simplified regional geological map of the Bayan Obo deposit (modified after Fan et al., 2016 [23]).
Figure 2. Simplified regional geological map of the Bayan Obo deposit (modified after Fan et al., 2016 [23]).
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Figure 3. Photographs of the main ore types in the mining area at the Bayan Obo deposit: (a) dolomite-type Nb–REE–(Fe) ores; (b) fluorite-type-banded Nb–REE–Fe ores; (c) massive Nb–REE–Fe ores; (d) biotite-type Nb–REE–Fe ores; (e) riebeckite-type Nb–REE–Fe ores; (f) aegirine-type Nb–REE–(Fe) ores. Mineral abbreviations: Aeg = Aegirine; Bt = Biotite; Dol = Dolomite; Flr = Fluorite; Mag = Magnetite; Rbk = Riebeckite.
Figure 3. Photographs of the main ore types in the mining area at the Bayan Obo deposit: (a) dolomite-type Nb–REE–(Fe) ores; (b) fluorite-type-banded Nb–REE–Fe ores; (c) massive Nb–REE–Fe ores; (d) biotite-type Nb–REE–Fe ores; (e) riebeckite-type Nb–REE–Fe ores; (f) aegirine-type Nb–REE–(Fe) ores. Mineral abbreviations: Aeg = Aegirine; Bt = Biotite; Dol = Dolomite; Flr = Fluorite; Mag = Magnetite; Rbk = Riebeckite.
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Figure 4. The occurrence of aeschynite group minerals in different ore types is shown as follows: (a) centimeter-sized aeschynite group minerals in aegirine-type ore; (b) millimeter-sized aeschynite group minerals in aegirine-type ore (transmitted light); (c) aeschynite group minerals in aegirine-type ore (transmitted light); (d) subhedral aeschynite group minerals in riebeckite-type ore (transmitted light); (e) anhedral aeschynite group minerals disseminated in dolomite-type ore (transmitted light); (f) anhedral aeschynite group minerals distributed among interstitial magnetite grains in massive ore (BSE image); (g) anhedral aeschynite in banded ore (transmitted light); (h) subhedral to euhedral aeschynite group minerals in biotite-type ore (transmitted light). Mineral abbreviations: Aeg = Aegirine; Aes = Aeschynite; Ap = Apatite; Bsn = Bastnäsite; Bt = Biotite; Dol = Dolomite; Flr = Fluorite; Mag = Magnetite; Rbk = Riebeckite.
Figure 4. The occurrence of aeschynite group minerals in different ore types is shown as follows: (a) centimeter-sized aeschynite group minerals in aegirine-type ore; (b) millimeter-sized aeschynite group minerals in aegirine-type ore (transmitted light); (c) aeschynite group minerals in aegirine-type ore (transmitted light); (d) subhedral aeschynite group minerals in riebeckite-type ore (transmitted light); (e) anhedral aeschynite group minerals disseminated in dolomite-type ore (transmitted light); (f) anhedral aeschynite group minerals distributed among interstitial magnetite grains in massive ore (BSE image); (g) anhedral aeschynite in banded ore (transmitted light); (h) subhedral to euhedral aeschynite group minerals in biotite-type ore (transmitted light). Mineral abbreviations: Aeg = Aegirine; Aes = Aeschynite; Ap = Apatite; Bsn = Bastnäsite; Bt = Biotite; Dol = Dolomite; Flr = Fluorite; Mag = Magnetite; Rbk = Riebeckite.
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Minerals 14 01029 g005
Figure 5. Occurrences of other Nb minerals in different types of ores are as follows: (a) micro-fine-grained columbite–(Fe) in biotite-type ore (transmitted light); (b) disseminated columbite–(Fe) in biotite-type ore (transmitted light); (c) fine-grained columbite–(Fe) in dolomite-type ore (transmitted light); (d) anhedral to subhedral fluorcalciopyrochlore in aegirine-type ore (reflected light); (e) relatively euhedral fluorcalciopyrochlore in dolomite-type ore (transmitted light); (f) Nb–bearing rutile in fluorite-type-banded ore (transmitted light); (g) anhedral Nb–bearing rutile in aegirine-type ore; (h) large grains of baotite in dolomite-type ores (transmitted light). Mineral abbreviations: Aeg = Aegirine; Bao = Baotite; Bt = Biotite; Clb–Fe = Columbite–(Fe); Dol = Dolomite; Flr = Fluorite; Mag = Magnetite; Pcl = Fluorcalciopyrochlore; Rt = Nb–bearing rutile.
Figure 5. Occurrences of other Nb minerals in different types of ores are as follows: (a) micro-fine-grained columbite–(Fe) in biotite-type ore (transmitted light); (b) disseminated columbite–(Fe) in biotite-type ore (transmitted light); (c) fine-grained columbite–(Fe) in dolomite-type ore (transmitted light); (d) anhedral to subhedral fluorcalciopyrochlore in aegirine-type ore (reflected light); (e) relatively euhedral fluorcalciopyrochlore in dolomite-type ore (transmitted light); (f) Nb–bearing rutile in fluorite-type-banded ore (transmitted light); (g) anhedral Nb–bearing rutile in aegirine-type ore; (h) large grains of baotite in dolomite-type ores (transmitted light). Mineral abbreviations: Aeg = Aegirine; Bao = Baotite; Bt = Biotite; Clb–Fe = Columbite–(Fe); Dol = Dolomite; Flr = Fluorite; Mag = Magnetite; Pcl = Fluorcalciopyrochlore; Rt = Nb–bearing rutile.
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Figure 6. The content of Nb2O5 (wt.%) in different types of ores.
Figure 6. The content of Nb2O5 (wt.%) in different types of ores.
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Figure 7. Nb/(Nb + Ti) vs. Ce/(Ce + Nd) plot of aeschynite group minerals from the Bayan Obo deposit in China. Four mineral species can be identified: open circles, aeschynite–(Ce); filled circles, nioboaeschynite–(Ce); open squares, aeschynite–(Nd); filled squares, nioboaeschynite–(Nd).
Figure 7. Nb/(Nb + Ti) vs. Ce/(Ce + Nd) plot of aeschynite group minerals from the Bayan Obo deposit in China. Four mineral species can be identified: open circles, aeschynite–(Ce); filled circles, nioboaeschynite–(Ce); open squares, aeschynite–(Nd); filled squares, nioboaeschynite–(Nd).
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Figure 8. REE distribution pattern in major Nb minerals, bastnäsite, and monazite [36] from the Bayan Obo deposit. REE used in chondrite normalization is from Taylor and McLennan (1985) [37].
Figure 8. REE distribution pattern in major Nb minerals, bastnäsite, and monazite [36] from the Bayan Obo deposit. REE used in chondrite normalization is from Taylor and McLennan (1985) [37].
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Figure 9. The Nb2O5 distribution rate and mineral species of Nb minerals in the raw ores. (a) the Nb2O5 distribution rate (%) of each mineral phase; (b) weight percentage (%) of each niobium mineral relative to the total niobium minerals in the raw ores.
Figure 9. The Nb2O5 distribution rate and mineral species of Nb minerals in the raw ores. (a) the Nb2O5 distribution rate (%) of each mineral phase; (b) weight percentage (%) of each niobium mineral relative to the total niobium minerals in the raw ores.
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Table 1. Particle size and Nb2O5 distribution from the raw ores after sieving.
Table 1. Particle size and Nb2O5 distribution from the raw ores after sieving.
Particle Size (μm)Weight (g)Weight Distribution (%)Nb2O5 Grade
(%)		Nb2O5 Distribution Rate
(%)
>7421.014.140.1211.96
45~7430.920.810.1219.21
32~454.53.030.082.56
25~3212.88.620.154.64
<2579.353.400.1361.62
Total148.51000.13100
Table 2. The Nb minerals in different niobium deposits worldwide.
Table 2. The Nb minerals in different niobium deposits worldwide.
DepositMain Niobium and Niobium–Bearing MineralsReferences
Araxápyrochlore group minerals[42,43]
Catalão Ipyrochlore group minerals[38]
Saint-Honorépyrochlore group minerals, columbite–(Fe), phosphorite, magnetite, phlogopite[39,44]
Bayan Oboaeschynite–(Nd), nioboaeschynite–(Nd), aeschynite–(Ce), nioboaeschynite–(Ce), pyrochlore group minerals, Nb–bearing rutile, columbite–(Fe), samarskite–(Y), columbite–(Mn), fersmite, fergusonite–(Ce), fergusonite–(Ce)–β, fergusonite–(Nd), fergusonite–(Nd)–β, fergusonite–(Y), baotite, niobobaotite, nioboixiolite–(□), chevkinite–(Ce), titanite, bafertisite[13,23,45,46,47]
Table 3. Characteristics of Nb minerals in different types of ores at the Bayan Obo deposit.
Table 3. Characteristics of Nb minerals in different types of ores at the Bayan Obo deposit.
OreAeschynite
Group Minerals		Columbite–(Fe)Pyrochlore
Group Minerals		Nb–Bearing
Rutile		Baotite, Fergusonite–(Y), Samarskite–(Y), Fersmite, etc.
Volume ContentGrain Size (μm)Volume Content Grain Size (μm)Volume ContentGrain Size (μm)Volume Content Grain Size (μm)Volume Content Grain Size (μm)
Aegirine-type Nb–REE–(Fe) ore~50%100~400~10%10~50~20%50~200~10%10~30~10%10~30
Riebeckite-type Nb–REE–Fe ore~40%100~200~15%10~50~10%20~100~25%10~30~10%10~30
Dolomite-type Nb–REE–(Fe) ore~30%20~100~15%10~20~20%20~100~25%10~30~10%10~30
Massive Nb–REE–Fe ore~30%20~100~15%10~20~15%20~100~25%10~50~15%10~30
Fluorite-type-banded Nb–REE–Fe ore~15%30~50~5%10~30~10%30~50~60%10~50~10%10~30
Biotite-type Nb–REE–Fe ore~25%10~50~30%20~50~15%10~50~20%10~30~10%10~30
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Yang, B.; Yang, L.; Zhao, Y.-G.; Yan, G.-Y.; Liu, J.-Y.; Meng, W.-X.; Yu, J.-F.; Chen, L.; Li, X.-C.; Li, X.-H. Aeschynite Group Minerals Are a Potential Recovery Target for Niobium Resources at the Giant Bayan Obo Nb–REE–Fe Deposit in China. Minerals 2024, 14, 1029. https://doi.org/10.3390/min14101029

AMA Style

Yang B, Yang L, Zhao Y-G, Yan G-Y, Liu J-Y, Meng W-X, Yu J-F, Chen L, Li X-C, Li X-H. Aeschynite Group Minerals Are a Potential Recovery Target for Niobium Resources at the Giant Bayan Obo Nb–REE–Fe Deposit in China. Minerals. 2024; 14(10):1029. https://doi.org/10.3390/min14101029

Chicago/Turabian Style

Yang, Bo, Li Yang, Yong-Gang Zhao, Guo-Ying Yan, Jian-Yong Liu, Wen-Xiang Meng, Jun-Fang Yu, Lei Chen, Xiao-Chun Li, and Xian-Hua Li. 2024. "Aeschynite Group Minerals Are a Potential Recovery Target for Niobium Resources at the Giant Bayan Obo Nb–REE–Fe Deposit in China" Minerals 14, no. 10: 1029. https://doi.org/10.3390/min14101029

APA Style

Yang, B., Yang, L., Zhao, Y.-G., Yan, G.-Y., Liu, J.-Y., Meng, W.-X., Yu, J.-F., Chen, L., Li, X.-C., & Li, X.-H. (2024). Aeschynite Group Minerals Are a Potential Recovery Target for Niobium Resources at the Giant Bayan Obo Nb–REE–Fe Deposit in China. Minerals, 14(10), 1029. https://doi.org/10.3390/min14101029

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