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Article

40Ar-39Ar Chronometry Supports Multi-Stage Tectonic Thermal Events in the Bayan Obo Fe-Nb-REE Deposit

by
Xinke Gao
1,2,
Dongsheng Wang
1,2,*,
Hongying Li
1,*,
Yike Li
1,2,
Hongquan She
1,2,
Jianjun Yang
3,
Li Zhang
4,
Changhui Ke
1,2,
Jian Zhao
5,
Shouxian Ma
1,
Chenghao Ren
1,2 and
Futing Yin
1,2
1
MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
2
Bayan Obo Scientific Research Base of Baotou Steel Company, Institute of Mineral Resources, Chinese Academy of Geological, Baotou 014080, China
3
Inner Mongolia Key Laboratory of Magmatic Mineralization and Ore-Prospecting, Inner Mongolia Geological Survey Institute, Hohhot 010020, China
4
Bayan Obo Iron Mine, Baotou Iron and Steel Company, Baotou 014080, China
5
Barun Mining Co. of Steel Union Co., Ltd. of Baotou Steel (Group) Corp., Baotou 014080, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(7), 683; https://doi.org/10.3390/min15070683
Submission received: 20 May 2025 / Revised: 17 June 2025 / Accepted: 19 June 2025 / Published: 26 June 2025
(This article belongs to the Special Issue Mineralization and Metallogeny of Iron Deposits)
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Abstract

The Bayan Obo deposit, located on the northern margin of the North China Plate (NCP), is the world’s largest comprehensive Fe-REE-Nb deposit. After its formation, this deposit was affected by multiple tectonic thermal events, but the ages of these geological events are controversial. To determine the evolutionary history of the Bayan Obo deposit, we conducted a detailed study of the macroscopic and microscopic deformation characteristics of the ore district and selected representative minerals, such as riebeckite and biotite, which are widely present in the banded rocks of the deposit, for an 40Ar-39Ar isotopic analysis. The results show that a large number of deformation structures have developed in the carbonatite and surrounding rocks, including mineral bands, boudins, tight folds, and rotated porphyroclasts, suggesting that the region has undergone intense compression and shearing and that the deformation temperature can reach ~550 °C. 40Ar-39Ar plateau ages of 414.9 ± 1.4 Ma and 264.5 ± 2.5 Ma were obtained for the riebeckite and biotite, respectively. Using these results in conjunction with regional geological data and considering the closure temperature of the mineral isotope system, it was inferred that these two ages corresponded to two distinct reworking events experienced by the deposit during the Early Paleozoic and Late Paleozoic following its initial formation. These events corresponded to the collision between the Bainaomiao Arc and the NCP and the magmatic activity induced by a continental–continental collision during the closure of the Paleo-Asian Ocean (PAO), respectively.

1. Introduction

Within ore districts, various deformation structures facilitate the migration and precipitation of ore-forming fluids, thereby influencing the morphology and distribution of ore bodies [1,2,3,4]; they also serve as a driving force for material exchange, which is a crucial aspect of mineralization process studies [5]. Therefore, determining the structural styles from different periods within a deposit is a fundamental task. As part of this, the determination of the deformation age, as the decisive factor for the identification of different deformation stages, has become an important prerequisite for dividing geological structures into sections and reconstructing the development history of mineralization processes. 40Ar-39Ar isotope geochronology is a commonly used high-precision dating technique [6,7]. Tectonic thermal events cause the recrystallization of protolith minerals or the formation of new minerals such as amphibole and biotite. As tectonic activity ceases, a relatively closed isotopic system is formed within the minerals. Because of the different closure temperatures of different minerals, they can record the ages of geological events based on their respective closure temperatures, thereby providing temporal evidence for geological events and contributing to in-depth studies of geological evolutionary processes [8,9].
The Bayan Obo deposit is the largest rare-earth industrial base in the country [10]. Previous studies have extensively studied the deposit from basic geology [11,12], mineralogy [13,14,15,16], geophysics [17,18,19], geochemistry [20,21], chronology [22,23,24,25], and structural deformation [26,27,28,29] perspectives, accumulating a large amount of data and making considerable progress. Studies have shown that the Bayan Obo deposit is characterized by numerous minerals, diverse alteration types, and typical banded structures. However, the formation age of the banded structure in the ore district has not been well investigated. Some scholars have suggested that the banded structure formed during the Mesoproterozoic compression and tectonic replacement processes [15,18,30]. Others believe that the banded structure resulted from strong metamorphism and deformation during the Paleozoic [31,32]. When did the banded structures form in the ore district? Which geological processes controlled their formation? Was any other tectonic thermal event recorded within the ore district? The current data have not yielded definitive conclusions regarding these questions, thereby impacting the reconstruction of tectonic thermal events within the ore district and constraining our comprehension of the regional tectonic evolution process.
Addressing the issues outlined above, this study examined nearly east–west-oriented banded rocks within and adjacent to the Bayan Obo deposit. Through macroscopic and microscopic structural observations, the cause of the banded structure was determined, and representative minerals were selected for 40Ar-39Ar isotope dating to precisely define the age of the tectonic thermal events in the ore district. This study contributes significantly to the structural delineation of ore districts and enhances the regional chronological framework by supplying essential geological chronological data, which is crucial for advancing mineral exploration in the area.

2. Regional Geological Background

2.1. Regional Geology

The Bayan Obo Fe-REE-Nb deposit is located on the northern margin of the NCP and is separated from a series of subduction–accretion tectonic units of the Central Asian Orogenic Belt (CAOB) to the north by the Bayan Obo–Chifeng Fault (Figure 1a,b) [33]. The northern margin of the NCP entered a passive continental margin evolution stage during the Meso-Neoproterozoic [34], and several rifts developed, such as the Bayan Obo and Yanshan–Liaoning rifts [35,36]. The Bayan Obo deposit is located within the Bayan Obo rift, which extends approximately 700 km in an east–west direction and reaches a maximum width of 90 km in the north–south direction. The strata, from the bottom up, are the Archean–Paleoproterozoic metamorphic basement, Paleoproterozoic parametamorphic rocks, Mesoproterozoic parametamorphic rocks, Triassic quartz sandstone and shale, and Quaternary rocks [37,38]. The sedimentary thickness of the Mesoproterozoic stratum reaches 10,000 m. The main stratum is the Bayan Obo Group, which has lithologies that include clastic rocks, limestone, dolomite, carbonatite, and volcanic rocks. From the bottom to the top, the formations are the Dula Hala, Jianshan, Halahaogete, Bilute, Baiyingaolao, and Hujiertu Formations (Figure 1c and Figure 2) [39,40].
The main faults that have developed in the region are the Baiyin Juelake–Kuangou and Ulan Baolige faults. The former is closely adjacent to the ore district and controls the structural pattern within the ore district, whereas the Ulan Baolige Fault is part of the Bayan Obo–Chifeng fault in this region. The main folds are the Kuanggou Anticline and Bayan Syncline [31]. The core of the anticline is composed of the Palaeoproterozoic Baoyintu Group, while the limbs are formed from the Mesoproterozoic Bayan Obo Group. The syncline predominantly comprises the Bayan Obo Group, with its core consisting of the Jianshan Formation and the limbs consisting of the Dula Hala Formation. Regional magmatic activity occurred from the Proterozoic to the Mesozoic. The lithologies include biotite granite, granodiorite, and diorite, which are distributed in Saiwusu and Dahuer. The age of the magmatic rocks is Hercynian but is not related to mineralization [41,42]. In addition, gabbro-diorite and peridotite are distributed near Bilute. The rocks closely related to mineralization are carbonatites and alkaline rocks, which are mainly exposed within the Bayan Obo ore district, with only sporadic outcrops on the periphery [43,44]. In addition, a large number of veins have developed in the area, with lithologies such as granite, diorite, gabbro, and lamprophyre, most of which intruded along the faults.
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Figure 1. Tectonic location of CAOB (a) (according to [45]), division of structural units on northern margin of NCP (b) (according to [46,47,48], and geological map of Bayan Obo region (c) (modified from [49]). Stereogram is plot of rocks with banded structure (lower hemisphere, equal area projection; colored line, poles to foliation).
Figure 1. Tectonic location of CAOB (a) (according to [45]), division of structural units on northern margin of NCP (b) (according to [46,47,48], and geological map of Bayan Obo region (c) (modified from [49]). Stereogram is plot of rocks with banded structure (lower hemisphere, equal area projection; colored line, poles to foliation).
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2.2. Geology of Ore Deposits

The strata exposed near the ore district are mainly from the Mesoproterozoic Bayan Obo Group and Permian Suji Formation volcanic rock. The latter was only exposed in a small area in the northeastern corner of the study area. The Bayan Obo Group within the ore district includes the Dulahala, Jianshan, Halahuogete, and Bilute Formations. The lithologies of each group and their corresponding relationships with the 10 lithologic sections delineated by our predecessors are shown in Figure 2; the ore-bearing carbonatite mainly intruded into the Jianshan Formation. The ore body is divided into the west or body, main ore body, and east ore body from west to east and includes two ore sections: Gaoci and Dongjielegele. The ore body is predominantly hosted in carbonatite and is close to the contact zone between the carbonatite and slate on the south side. The ore body dips steeply to the south and is lenticular along the strike and dip with pinch-out and recurrence characteristics. Through electromagnetic analysis, He Lanfang [19] found that the “Y”-shaped ore-controlling structural characteristics of the Bayan Obo deposit align with the high-angle magmatic intrusion model within the context of a craton margin rift. This suggests that the carbonatite magma intruded southward at a high angle from the north within the ore district of the Bayan Obo Group. Furthermore, the substantial sedimentary deposits in the north served as a protective barrier, facilitating the preservation of the ore body.
The iron ore minerals in the ore district are primarily magnetite, hematite, and goethite, with small amounts of pyrite, siderite, specularite, and pyrrhotite. They have granular, banded, and mottled structures and locally exhibit a massive structure. The rare-earth minerals are mainly monazite, bastnaesite, parisite, synchysite, and huanghoite with disseminated or massive structures and locally show banded structures, which are products of deformation [50]. The niobium minerals are mainly pyrochlore and aeschynite. Additionally, a large amount of fluorite has developed in the ore district, which exhibits a disseminated and banded structure. The alteration types are mainly biotization, magnetization, albitization, riebeckitization, aegiritization, carbonatization, and K-feldspathization. The alteration intensity gradually weakens from the carbonatite to the surrounding rocks. The alteration intensity of the surrounding rock in the contact zone of the southern roof is significantly higher than that of the surrounding floor rock.
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Figure 2. Stratigraphic column of the Bayan Obo area (modified based on [36,43,49,51]).
Figure 2. Stratigraphic column of the Bayan Obo area (modified based on [36,43,49,51]).
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3. Deformation Characteristics

Banded structures are among the most frequently developed ore body structures. The bands include carbonatite and a series of minerals developed within them, including magnetite, pyrite, fluorite, sodium amphibole, barite, and rare-earth minerals (Figure 3a,b and Figure 4a–c). In the field, the thicknesses of bands composed of different minerals are laterally uneven with varying widths, and the extension lengths of different bands also differ. In most cases, we found that they were pinched out along the strike, representing lenses formed by compression perpendicular to the foliation. At the outcrop scale, banded structures also developed in the contact zones between carbonatite and the surrounding rocks. During the subsequent compressive shearing, these bands formed several folds. Owing to the differences in competence between the light and dark bands, the folds exhibited an inconsistent style (Figure 3a). In banded carbonatite ores, magnetite formed elongated strips intercalated with other minerals. Due to the relatively high competence of magnetite, its boundaries frequently became undulating rather than linear when subjected to compressive forces perpendicular to the foliation. As compression persisted, variations in the rock competence led to the contraction and fracturing of more rigid layers along zones of stress concentration, resulting in the formation of characteristic pudding structures, where adjacent weaker layers filled the spaces between the pudding-like fragments (Figure 4a).
Accompanied by intense compression, interlayer sliding occurred between the bands, causing foliations or bands to fold under interlayer shearing. At the outcrop scale, pyrite aggregates were sheared to form tight interlayer folds, and their axial planes were parallel to the main foliation (Figure 3b). In other ores, fluorite, dolomite, biotite, riebeckite, and albite underwent synchronous deformation and formed tight folds (Figure 4b,c). Compared with the limbs, their cores were significantly thickened, and locally rootless hook-shaped folds were formed. Similarly to the rock surrounding the deposit, the internal mineral bands in the biotitized siltstone also formed dense folds owing to tectonic activity. Penetrating cleavages were observed along the axial plane. The original bedding, as a deformation surface, was observed between the cleavages. Cleavages developed along the axial planes of the folds, representing the result of tectonic replacement (Figure 3c). The microscopic characteristics showed that the mineral aggregate bands were bent by compression to form buckle folds, and the minerals accumulated towards the core and underwent significant thickening (Figure 4b,c). Mirrored shearing patterns that had developed on the two limbs of the folds further confirmed the interlayer sliding. These characteristics indicate that the ores inside the carbonatite, rocks in the contact zone and slate of the surrounding rocks all underwent compression and interlayer shear action, forming numerous shear folds.
In addition, intense compression and shearing formed numerous lenses and rotated porphyroclasts within the ore district. In the banded ore, pyrite underwent shearing to form typical σ-type rotated porphyroclasts (Figure 3d). Within the slate, the competent layer formed large lenses during shearing, and the trailing shape indicated interlayer thrust shearing (Figure 3e). During carbonatite emplacement, numerous veins penetrated the surrounding rocks. Subsequently, during deformation, the veins were compressed and sheared, forming rotating porphyroclasts in the biotite-rich slate (Figure 3f). Upon microscopic examination, it was observed that magnetite, as a structurally competent mineral, developed a pressure shadow structure during the compression process, with the minerals within the shadow predominantly consisting of dolomite. The monoclinic symmetry of the dolomite suggests that the area underwent shearing (Figure 4d). Hematite porphyroclasts became rounded during the shearing process and formed tails composed of dolomite in the low-stress region, jointly forming a rotated porphyroclast system (Figure 4e). The quartz sandstone of the Dulahala Formation, similarly to the rock surrounding the Bayan Obo deposit, also underwent deformation to form a schistose quartzite. A microscopic analysis revealed that the quartz was organized in ribbons or bands and demonstrated subgrain rotational recrystallization, suggesting deformation under conditions of relatively medium–high temperatures (~550 °C) (Figure 4f).

4. Sample Collection and Testing Methods

The riebeckite sample (XK21) used for 40Ar-39Ar isotope dating in this study was collected from the northern side of the 21st exploration line in the west ore body of the Bayan Obo ore district. Under a microscope, we determined that the magnetite-bearing carbonatite ore had undergone obvious deformation. The main constituent minerals were dolomite, magnetite, riebeckite, and small quantities of biotite (Figure 5a). The dolomite was fine-grained with a particle size of approximately 0.05–0.1 mm and had a high-order white interference color. The riebeckite was in the form of long columns with a particle size of approximately 0.2–0.5 mm, arranged in parallel and directionally to form the main foliation. The characteristics of the riebeckite indicate that it was modified by strong deformation after formation. Biotite occurred in the form of small, scaly flakes with a particle size of approximately 0.05–0.1 mm, filling the space between the other mineral grains (Figure 5b).
A biotite sample (BY19) was collected from a slate on the south side of the No. 5 exploration line in the west ore body of the Bayan Obo ore district. The nearly vertical foliation of the slate had formed folds due to shearing. Under a microscope, we determined that the main minerals were quartz and biotite. Biotite is a scaly aggregate with a particle size of approximately 0.05–0.1 mm. The aggregates were arranged parallel and directionally to each other and had formed asymmetric folds under strong SN-directional compressive shear action (Figure 5c). In addition, the residual albite phenocrysts inside the slate had formed rotational porphyroclasts owing to shearing (Figure 5d), and fine-grained feldspar and quartz formed tails, indicating shear action along this direction. Albite alteration, potassic alteration, and carbonatization were locally observed in the slate.
The 40Ar-39Ar dating analysis of the riebeckite samples was completed at the Beijing Research Institute of Uranium Geology of the China National Nuclear Corporation. First, the selected samples were crushed and sieved through a 40–60 mesh and examined under a binocular microscope to ensure that the amphibole purity exceeded 99%. The samples were then wrapped in high-purity aluminum foil. A high-purity quartz tube was used for bottom sealing and was ultrasonically cleaned with deionized water and acetone successively. Subsequently, the samples were irradiated in the High-Flux Experimental Reactor at the China Institute of Nuclear Power Research and Design, and the irradiation lasted for 14 h. The correction factors obtained for the side reactions were (36Ar/37Ar)ca = 0.00025436, (39Ar/37Ar)ca = 0.00071651, and (40Ar/39Ar)k = 0.00293927. The stage heating of the samples and isotope mass spectrometry analysis were performed at the Beijing Research Institute of Uranium Geology of the China National Nuclear Corporation. A Thermo Fisher Helix SFT inert-gas isotope mass spectrometer was used for all the experiments. Following the calibration of the temperature control meter and the adjustment of the furnace temperature, a temperature range of 800–1500° was established. The argon isotope data collected at various temperatures using the step heating method were utilized to obtain pertinent age information, including the plateau, isochron, and inverse isochron ages of the samples, using ArArCALC software (v2.5.2) [52].
The 40Ar-39Ar dating analysis of the biotite samples was completed at the Institute of Geology, Chinese Academy of Geological Sciences. First, the minerals (purity of > 99%) were selected, cleaned using ultrasonic waves, and then dried. The cleaned samples were sealed in quartz vials and transferred to a nuclear reactor for neutron irradiation. Irradiation was carried out in the “swimming pool reactor” at the China Institute of Atomic Energy using the B4 channel, with a neutron flux density of approximately 2.65 × 1013 n cm−2S−1. The total irradiation time was 1440 min, and the integrated neutron flux was 2.29 × 1018 n cm−2. Simultaneously, the standard sample used as a monitor sample, a ZBH-25 biotite standard, which had a standard age of 132.7 ± 1.2 Ma and a K content of 7.6%, was irradiated with neutrons. The samples were heated stepwise in a graphite furnace, with each stage involving heating for 10 min and purification for 20 min. Mass spectrometry analysis was performed using a multicollector rare gas mass spectrometer (GV Helix MC), and 20 sets of data were collected for each peak. All the data were regressed to the time-zero value and subsequently subjected to corrections for mass discrimination, atmospheric argon, blank values, and interference element isotopes. The correction coefficients for the interference isotopes produced during neutron irradiation were determined by analyzing the irradiated K2SO4 and CaF2, and their values were (36Ar/37Aro)Ca = 0.0002398, (40Ar/39Ar)K = 0.004782, and (39Ar/37Aro)Ca = 0.000806. 37Ar was corrected for radioactive decay, with a decay constant of 40K and λ = 5.543 × 10−10 year−1. The plateau age and forward and reverse isochrons were calculated using the ArArCALC program [52], and the error in the plateau age was given as 2σ. Detailed experimental procedures are available in relevant articles [53].

5. Analytical Results

The analytical data obtained using the 40Ar-39Ar stepwise heating method for riebeckite and biotite are presented in Supplementary Tables S1 and S2, respectively, and the plateau and inverse isochron ages are shown in Figure 6. According to the 40Ar-39Ar stepwise heating analytical data and age spectra of riebeckite, the sample was heated in ten stages, and five temperature stages formed a relatively flat plateau. The apparent age values ranged from 366.7 to 437.9 Ma, and the plateau age was 414.9 ± 1.4 Ma (MSWD = 0.46), corresponding to 64.65% of the 39Ar release (Figure 6a). The riebeckite yielded an isochron age of 416.2 ± 2.4 Ma (Figure 6b). In the inverse isochron plot for the sample, the initial ratio of 40Ar/36Ar was 122 ± 95 Ma, and the isochron age was 416.1 ± 2.4 Ma (MSWD = 0.50) (Figure 6c), which was generally consistent with the plateau age within the error range, indicating that the measured data were reliable.
The 40Ar-39Ar stepwise heating analytical data and age spectra of the biotite showed that the sample was heated in 14 stages, and 11 temperature stages formed a flat age plateau. The apparent age values ranged from 263.1 to 269.2 Ma, and the plateau age was 264.5 ± 2.5 Ma (MSWD = 1.53), corresponding to 73.4% of the released 39Ar (Figure 6d). The biotite yielded an isochron age of 264.4 ± 2.6 Ma (Figure 6e). The inverse isochron age indicated by the temperature stages constituting the plateau age was 264.2 ± 2.5 Ma (MSWD = 1.22) (Figure 6f), and the initial 40Ar/36Ar ratio was (324.7 ± 32.6), which was very close to the Neil value (295.5 ± 5 Ma), indicating that there was no excess argon in the sample.

6. Discussion

6.1. The Multi-Stage Tectonic Thermal Events in Bayan Obo

The closure temperature of the argon isotope system of riebeckite is 500–550 °C [6], which is similar to the deformation temperature of rocks under medium and low temperatures (~550 °C) and can be used to better record the crystallization time of riebeckite. The closure temperature of the argon isotope system of biotite is 250–350 °C [6]. During the deformation process, the biotite system was reset, and the age after the latest regional tectonic thermal event, when the system cooled to approximately 350 °C, was recorded. The 36Ar/40Ar-39Ar/40Ar inverse isochron ages of the samples measured in this study were consistent with their respective plateau ages within the error range, indicating that the measured data were reliable. Moreover, the initial 40Ar/36Ar values of the samples were also consistent with the Neil value (295.5 ± 5) within the error range, further indicating that the samples had maintained a well-preserved, closed system since their formation. Simultaneously, owing to systematic regression and correction using the 40Ar/36Ar-39Ar/36Ar isochron method, the influence of argon losses or excess argon on the true age of the samples was very small. Therefore, the obtained argon–argon plateau ages better reflect the formation ages of riebeckite and biotite.

6.1.1. Early Paleozoic

Extensive data suggests that the carbonatite was emplaced approximately 1300 Ma, subsequently interacting with the surrounding rocks. This interaction resulted in significant hydrothermal alteration, metasomatism, and the mineralization of Fe-REE-Nb [22,54,55,56]. However, there is still considerable controversy regarding the development of deformation structures within ore districts. Chen Zhiyong [57] reported that a ductile shear deformation had developed within the Bayan Obo Group, including mylonitic foliation, a boudinage structure, and a folded layer structure, and its formation mechanism was the large-scale extensional detachment of the Changcheng System relative to the basement rocks, with an age of ~1400 Ma. He further pointed out that the ductile shear, uplift, and rifting in the Mesoproterozoic were the products of different stages of the same extensional tectonic process. Zhang Jien [30] pointed out that the original bedding was transformed into schistosity before the intrusion of carbonatite magma and provided a weak zone available for the ascent of carbonatite magma. However, the deformation preceding the emplacement of these rocks does not account for the lenticular configuration of the ore bodies in the present Bayan Obo ore district, nor does it elucidate the folding patterns of rare-earth elements and other minerals. This indicates that after the formation of the ore bodies, another stage of tectonic reformation should have occurred.
In recent years, during the testing of different minerals, ages of 1300–400 Ma have been widely reported within the ore district, with 1300 and 400 Ma being the main age peaks [32,36]. Based on this, some researchers have proposed a theory of long-term mineralization from 1300 to 400 [58]; however, the likelihood of such a long-term mineralization process is unconvincing. Based on research on Sm-Nd and Th-Pb isotopes, Li Xiaochun [22] believed that late hydrothermal activity had affected the isotopes in the Bayan Obo ore district, resulting in different degrees of resetting and a series of mixed ages determined during the testing process. Based on this, only the ages of approximately 1300 Ma and 400 Ma were of practical significance. Simultaneously, based on the monazite and bastnaesite at ~400 Ma, which showed signs of post-mineralization fluid activity, it was suggested that the geological event at ~400 Ma led to the transformation of the original system but was not sufficient to cause mineralization and did not support the previous theory of secondary mineralization [59]. Therefore, tectonic events that occurred at approximately 400 Ma have received increasing attention.
A large amount of age data has confirmed that the Bayan Obo ore district experienced significant tectonic thermal events during the Early Paleozoic. The Th-Pb age of monazite aggregates obtained by Wang Junwen [60] and Chao [61] was Caledonian; the Re-Os ages of molybdenite and pyrite are both 439 Ma [62]; multiple U-Th-Pb dating analyses of monazite and columbite-Mn along with cassiterite yielded peak ages of 425 Ma and 419 Ma [63]; the Ar-Ar ages of riebeckite in the ore district are 343–396 Ma [61] and 389.5 Ma [64]; and the age of the zircon edge in carbonatite is 455 Ma [55]. Based on the in situ geochemical data, Chen Wei [65] believed that coarse-grained dolomite of magmatic origin underwent a significant hydrothermal metasomatic event associated with the subduction of the Siberian Plate (SP) during the Early Paleozoic (~400 Ma). This process led to the recrystallization of dolomite, resulting in the formation of recrystallized fine-grained dolomite with non-uniform grain sizes. Zi Jianwei [24] obtained Caledonian age data for monazite in carbonatite and nephelinized surrounding rocks, reflecting the superimposed disturbed record of later tectonic thermal events. The field observations in this study showed that banded structures are widely distributed in the Bayan Obo ore district, and a large number of tight folds have developed with bands as the deformation surface, representing strong north–south compressions. In the banded ore, the plateau age given by the oriented riebeckite is 414.9 ± 1.4 Ma, which is consistent with the Early Paleozoic tectonic thermal event described in previous studies. Combined with the strong deformation characteristics of the ore district, this reflects the age of the superimposed and transformed tectonic deformation after the formation of the deposit.

6.1.2. Late Paleozoic

In the Late Paleozoic, a continent–continent collision triggered a series of magmatic activities within the region, forming a series of magmatic intrusions. Fan Hongrui [66] obtained zircon U-Pb dating results for different lithologies in the Bayan Obo area ranging from 281 to 263 Ma, with a peak value of 269 Ma. Lai Xiaodong [64] conducted Ar-Ar dating using biotite in the ore veins and obtained an age of 289.1 Ma. Li Xiaochun [67] conducted an analysis of undeformed apatite within the ore district, determining a Sm-Nd age of 270 Ma. A variety of Nb-bearing minerals have yielded Pb-Pb isochron ages of 257–277. Ma and Yang Lan [68] attributed the Nb mineralization to skarn formation during granite emplacement during the Late Paleozoic. Despite this finding, there remains an ongoing debate concerning the tectonic environment of the Bayan Obo area during the Late Paleozoic; for example, Zhao Lei [69] defined the formation environment of the 287–242 Ma diorite that developed along the northern margin of the NCP as an active continental margin and pointed out that the Paleo-Asian Ocean (PAO) was still in the subduction stage in the Late Paleozoic. Ling [70] reported that granitoids formed at 243.2–293.8 Ma outside the ore body, and all the granitoids (granite, quartz monzonite) were formed during the post-collisional tectonic regime at the convergent margin based on their geochemical characteristics. However, the formation of many Hercynian rocks on the northern margin of the NCP was undoubtedly related to the closure of the PAO. The plateau age of the biotite in the rocks surrounding the Bayan Obo deposit was 264.5 ± 2.5 Ma. Owing to the multiple stages of tectonic evolution, the effects of tectonic thermal events play a decisive role in the argon–argon geochronological data. Therefore, it is necessary to discuss this phenomenon in combination with the tectonic events and mineral isotopic systems. Owing to the low closure temperature of biotite, the deformation at medium–high temperatures (~550 °C) in the Early Paleozoic and the magmatic emplacement in the Late Paleozoic disrupted its Ar system, causing it to be reset and re-timed. Based on the above discussion, the age of 264.5 Ma in the ore district represents the time in the Late Paleozoic when the magmatic emplacement cooled to approximately 350 °C.

6.2. Tectonic Significances

Numerous studies on the chronology of carbonatites and ores have shown that after the emplacement and mineralization of carbonatite magma in the Bayan Obo ore district during the Middle Proterozoic, they experienced two stages of tectonic thermal events (Table 1). Statistics show that the two age peaks correspond to an Early Paleozoic arc–continent collision event and a Permian magmatic event [66], which were closely related to the evolution of the PAO on the northern side of the NCP. Zhou Jianbo [27] concluded that the Bayan Obo Group on the south side of Kuangou developed due to the marginal sedimentation of the Middle Proterozoic, was strongly transformed by mantle fluids during the Caledonian, and strongly metamorphosed regionally during the Hercynian, which was related to the subduction and accretion processes of the ancient plate. The findings from aeromagnetic and aeroradiometric surveys within the ore district indicate the presence of magnetic bodies of varying stages and intensities. This evidence further corroborates the hypothesis that the deposit experienced superimposition and transformation processes during multiple phases of tectonic thermal events [71]. Tian Pengfei [72] obtained a large number of ages of 1.0–0.2 Ga through the in situ monazite U-Pb dating of the ore and pointed out that due to multiple subduction and accretion events related to the PAO and the NCP, the Bayan Obo deposit was strongly superimposed and recorded these tectonic thermal events. Chen Biao [73] discussed previous isotopic chronology research results and suggested that Bayan Obo is a giant rare-earth deposit that was mineralized in a rift at about 1.3 Ga and superimposed with two thermal disturbance events in the Caledonian and Hercynian. The thermal disturbance events originated due to the influence of the subduction and collision orogeny of the PAO on the northern margin of the NCP. Based on existing achievements, it can be observed that the docking and collision between the NCP and SP caused intense compressive shortening, the intensity of which gradually weakened from the edge to the interior of the plate. The Bayan Obo area is located on the northern margin of the NCP. The banded characteristics of the ore structure within the ore district, along with the district’s 40Ar-39Ar ages (414.9 Ma and 264.5 Ma), recorded a geological process dominated by compression. This process reflects the closure of the PAO at the northern margin of the NCP, as observed on the scale of the ore district. Furthermore, it represents a distal response to an arc–continent collision during the Silurian and a continent–continent collision in the Permian.

6.3. Implications for the Orogenic Process

The PAO mainly developed during the Cambrian–Permian and underwent a relatively complete process of development and extinction [46,74,75,76,77,78]. The tectonic deformation sequence and evolutionary history of the PAO during its extinction stage can be categorized into two major stages: arc–continent collision and continent–continent collision.

6.3.1. The Age of Collisions Between the NCP and the Bainaimiao Arc

The PAO underwent a series of geological processes, such as subduction, closure, and collision, during the Paleozoic era, eventually forming the CAOB [46]. During this process, the southern part of the eastern section of the CAOB comprised from south to north the Bainaimiao Arc, Wenduermiao Ophiolitic Mélange, Solonker–Xar Moron Suture Zone (SXSZ), Erdaojing Ophiolitic Mélange, and Hegenshan Ophiolitic Mélange Belt. [46,47,79] (Figure 7a). The Early Paleozoic geological events on the northern margin of the NCP mainly involved collisions between the Bainaimiao Arc and the NCP (Table 1). The main evidence for this is as follows: (1) The existence of the Wude Mélange Zone has been reported in the western section of the Bayan Obo–Chifeng Fault between the NCP and the Bainaimiao Arc. Many lenticular serpentinite, gabbro, chert, and other tectonic blocks are confined by faults within the flysch matrix. This matrix contains Middle Ordovician fossils and is unconformably covered by the Upper Silurian Xibiehe Formation [80,81]. Simultaneously, Early Paleozoic (493–447 Ma) plutons related to subduction are also present near the mélange zone, indicating that the Wude Mélange Zone was formed in the Middle Silurian. (2) Sedimentology characteristics: The study of detrital zircons showed that there had been an ocean between the NCP and the Bainaimiao Arc during the Cambrian–Ordovician, followed by a subduction–accretion process. An arc–continent collision occurred in the Silurian (430–410 Ma), and the Bainaimiao Arc finally became attached to the northern part of the NCP [82]. (3) Tectonic aspects: Zhou Zhiguan [83] analyzed the Bainaomiao thrust–nappe structure and believed that it recorded tectonic activities from 450 to 410 Ma, corresponding to the collision between the Bainaomiao island arc belt and the NCP. Zhang Yuqing [29] believed that the Bayan Obo Group was transformed by Caledonian ocean–continent subduction orogeny and identified the fold structure formed by the tectonic deformation in this period in the Jianshan Formation. Hou [84] believed that the northern margin of the NCP experienced an oblique collision between the Bainaomiao Arc and NCP in the Silurian, corresponding to the formation of large-scale folds on a regional scale. This evidence confirms an arc–continent collision event in the Early Paleozoic on the northern margin of the NCP (Figure 7b). The results of this study show that this geological process led to the gradual parallelization of the riebeckite formed in the Mesoproterozoic in the Bayan Obo ore district to form a foliation under the influence of stress, as recorded in the 414.9 Ma 40Ar-39Ar data.

6.3.2. The Closure Age of the PAO

During the Permian–Triassic, the PAO subducted southward and eventually closed along the SXSZ [73], welding the SP and NCP together [46]. The interaction between the plates produced intense tectonic–magmatic activities in this area during the Late Paleozoic [46,85] (Table 1). The main manifestations were as follows: (1) Tectonic aspects: During the orogenic process, a plate collision manifested as crustal shortening caused by forward compression and strike-slips along the orogenic belt. Zhang Yuqing [29] identified a fold formed due to Late Paleozoic continent–continent collision orogeny in the Jianshan Formation of the Bayan Obo Group. Li Gang [86] believed that the Bainaimiao thrust–nappe structure underwent a south–north thrust–nappe from the Late Permian to the Early Triassic. This geological activity resulted in the displacement of the Bayan Obo Group from the northern margin of the NCP to the Bainaomiao island arc belt, located on the southern margin of the Xingmeng Orogenic Belt. This movement was a response to the continental margin dynamics following the closure of the PAO and the subsequent continent–continent collision. Zhou Zhiguang [83] also suggested that the Bainaimiao thrust–nappe structure recorded this Late Paleozoic tectonic event. Hou Liyu [84] suggested that the northern margin of the NCP underwent southward forward subduction and collided with the PAO during the Early Permian and that the corresponding structure involved the formation of superimposed folds on top of the early deformation structure. Wang Jun [28] reported a NE–SWW-trending reverse sinistral strike-slip ductile shear zone approximately 6.5 km northwest of the main ore deposit in Bayan Obo, which was related to the convergence and amalgamation of the Mongolian Ocean Plate and NCP in the Late Paleozoic. Studies have shown that during the period of 280–230 Ma, a dextral strike-slip occurred in the CAOB, which could be seen from Alxa, Beishan, and the northern margin of the NCP to the eastern part of the Greater Khingan Mountains, and its active age gradually decreased from west to east [87]. For example, the main shear zone in Tianshan was 280–250 Ma [88]; the South Alxa shear zone was 269–240 Ma [89]; the Langshan shear zone was 274–249 Ma [90]; the Guyang–Wuchuan shear zone was 253–247 Ma [91]; the Ondor Sum shear zone was 241 Ma [92]; the Xar Moron shear zone was 240 M [93]; the Chifeng shear zone was 220 M [94]; and the shear zone on the northern margin of the NCP was 255–241 Ma [95]. (2) Sedimentology aspects: Zhang Chen [96] pointed out that marine molasses with an angular unconformity from the Late Devonian–Early Carboniferous could be seen in the northern belt of Wenduermiao; thus, it was judged that the suture event involving the NCP and the SP most likely occurred in the Late Devonian–Early Carboniferous. (3) In terms of magmatic rocks, Wang Huichu [97] measured the SHRIMP zircon U-Pb age of dioritic rocks in Fengning and Chengde, northern Hebei Province, to be 288–280 Ma, indicating that the northern margin of the NCP was an Andean-type active continental margin in the Early Permian and that the PAO closed at the end of the Late Permian. The LA-ICP-MS zircon U-Pb age of the Xiyingzi granite in Guyang, Inner Mongolia, was 282 Ma. It was found in the Late Paleozoic magmatic accretion zone of the northern margin of the NCP, which was the response of the NCP to the subduction of the PAO (Figure 7c). Combining these findings with other evidence, Zeng Junjie [98] suggested that the PAO underwent a final closure during the Late Permian. This study calculated the 40Ar-39Ar age of biotite in the Bayan Obo area to be 264.5 Ma, reflecting the transformation of the early biotite Ar isotope system by the Late Paleozoic magmatic event, which was the response within the ore district to the final closure process of the PAO.
Table 1. List of ages for rocks in the Bayan Obo and its surrounding areas.
Table 1. List of ages for rocks in the Bayan Obo and its surrounding areas.
No.LocationLithologyMethodsInterpretationAge (Ma)References
1Bayan OboniobiteU-Pbintraplate extension268 ± 5Yu et al. (2024) [23]
2Bayan ObozirconTh-Pbextension1301 ± 12Zhang et al. (2017) [36]
3Bayan ObozirconTh-Pbextension1300Smith et al. (2015) [50]
4Bayan ObodolomiteTh-Pb, monazitesubduction455.6 ± 28.2Campbell et al. (2014) [55]
5Bayan ObodolomiteSHRIMP Th-Pbextension1325 ± 60Campbell et al. (2014) [55]
6Bayan ObodolomiteSm-Ndextension1286 ± 27 MaZhu et al. (2014) [59]
7Bayan ObodolomiteTh-Pb, monazitesubduction418 ± 11Wang et al. (1994) [60]
8Bayan ObodolomiteTh-Pb, monazitesubduction424 ± 5Wang et al. (1994) [60]
9Bayan ObodolomiteTh-Pb, monazitesubduction429 ± 11Wang et al. (1994) [60]
10Bayan ObodolomiteTh-Pb, monazitesubduction432 ± 8Wang et al. (1994) [60]
11Bayan ObodolomiteTh-Pb, monazitesubduction437 ± 9Wang et al. (1994) [60]
12Bayan ObodolomiteTh-Pb, monazitesubduction474 ± 4Wang et al. (1994) [60]
13Bayan ObodolomiteTh-Pb, monazitesubduction475 ± 8Wang et al. (1994) [60]
14Bayan ObodolomiteTh-Pb, monazitesubduction496 ± 13Wang et al. (1994) [60]
15Bayan ObodolomiteTh-Pb, monazitesubduction532 ± 11Wang et al. (1994) [60]
16Bayan Obobiotite rockAr-Arsubduction266.4 ± 3.9Lai et al. (2015) [64]
17Bayan OboslateCAMAECA U-Pb, zirconsubduction289.1 ± 1.8Lai et al. (2015) [64]
18Bayan ObonatroamphiboleAr-Arsubduction389.5 ± 3.0Lai et al. (2015) [64]
19Bayan OboslateCAMAECA U-Pb, zirconsubduction518.8 ± 7.5Lai et al. (2015) [64]
20Harihanatwo-mica granite LA-ICP-MS U-Pb, zirconmantle plume258 ± 2.5Hui et al. (2021) [88]
21Harihanamuscovite graniteAr-Armantle plume268 ± 2.72Hui et al. (2021) [88]
22HexipugraniteLA-ICP-MS U-Pb, zirconslippage fracture259Zhang et al. (2021) [89]
23HexipugraniteLA-ICP-MS U-Pb, zirconslippage fracture269 ± 1.2Zhang et al. (2021) [89]
24HexipumuscoviteAr-Arslippage fracture425 ± 2.9Zhang et al. (2021) [89]
25Fengningquartz dioriteSHRIMP U-Pb, zirconsubduction279.5 ± 5.6Wang et al. (2007) [97]
26GuyanggraniteLA-ICP-MS U-Pb, zirconsubduction282Zeng et al. (2008) [98]
27Bayan Obomonazite, fluoceriteSm-Ndextension1313 ± 41Zhang et al. (2001) [99]
28Bayan Oborare-earth minerals from late veinsSm-Ndsubduction422 ± 18Zhang et al. (1994) [100]
29Bayan ObocarbonatiteSm-Nd, whole rockextension1223 ± 65Zhang et al. (1994) [100]
30Bayan ObonatroamphiboleAr-Arextension1288 ± 12Ren et al. (1994) [101]
31Bayan Obomonazite, fluoceriteSm-Ndextension1312.5 ± 41.2Ren et al. (1994) [101]
32Bayan Obocarbonatite dikeSHRIMP U-Pb, zirconextension1374 ± 42Fan et al. (2006) [102]
33Bayan Obocarbonatite and fenitized country rocks Sm-Ndextension1308 ± 56Wang et al. (2018) [103]
34Bayan ObodolomiteSm-Nd, whole rockextension1305 ± 78Zhang et al. (2003) [104]
35Bayan Obomica-type ore magnesium biotiteRb-Srsubduction287 ± 16Yao et al. (2025) [105]
36Bayin Zhu Rihehornblende gabbroLA-ICP-MS U-Pb, zirconextension265 ± 2Luo et al. (2013) [106]
37Bayan OboplagioclasiteSHRIMP U-Pb, zirconintraplate extension285 ± 2Wang et al. (2010) [107]
38Bayan Obohydrothermal alkaline amphiboleAr-Arsubduction396 ± 4Zhao et al. (1991) [108]
39Bayan ObomonaziteTh-Pbsubduction423 ± 3Zhao et al. (1991) [108]
40Bayan Oboyicalcite, Huanghe oreTh-Pbsubduction438.2 ± 25.1Zhao et al. (1991) [108]
41Bayan Obohydrothermal alkaline amphiboleAr-Arsubduction719 ± 2Zhao et al. (1991) [108]
42Bayan Obohydrothermal alkaline amphiboleAr-Arsubduction720 ± 20Zhao et al. (1991) [108]
43Bayan Obohydrothermal alkaline amphiboleAr-Arsubduction818 ± 5Zhao et al. (1991) [108]
44Bayan ObodolomitePb-Pb, whole rockextension1283 ± 59Gao et al. (1995) [109]
45Bayan ObomonaziteLA-ICP-MS U-Pbintraplate extension249 ± 13Wang et al. (2018) [110]
46Bayan ObophlogopiteLA-ICP-MS U-Pb, zirconsubduction269.5 ± 3.1Wang et al. (2018) [110]
47Bayan ObodolomiteTh-Pb, monazitesubduction330Ling et al. (2013) [111]
48Bayan ObodolomiteTh-Pb, monazitesubduction420Ling et al. (2013) [111]
49Bayan ObodolomiteTh-Pb, monazitesubduction490Ling et al. (2013) [111]
50Bayan ObodolomiteTh-Pb, monazitesubduction545Ling et al. (2013) [111]
51Bayan ObodolomiteTh-Pb, monazitesubduction600Ling et al. (2013) [111]
52Bayan ObodolomiteSHRIMP Th-Pb, monazitesubduction745Ling et al. (2013) [111]
53Bayan Obocarbonatite dikeSHRIMP U-Pb, zirconextension1417 ± 19Fan et al. (2014) [112]
54Bayan Obocarbonatite dikeID-TIMS U-Pb, zirconextension1418 ± 29Fan et al. (2014) [112]
55Bayan ObobiotiteRb-Srsubduction240.2 ± 14.9Liang et al. (2024) [113]
56Bayan ObobiotiteRb-Srintraplate extension281.8 ± 92Liang et al. (2024) [113]
57Bayan ObodolomiteTh-Pb, monazitesubduction455 ± 28Campbell et al. (2000) [114]
58Bayan ObodolomiteSHRIMP Th-Pbextension1314 ± 56Campbell et al. (2000) [114]
59Bayan Obomonazite Sm-Nd, LA-ICP-MSextension1320 ± 210 MaYang et al. (2008) [115]
60Bayan Obobasement complexSm-Nd extension1354 ± 57 MaYang et al. (2010) [116]
61Bayan ObopyriteRe-Os subduction439 ± 86 MaLiu et al. (2005a) [117]
62Bayan Obobiotite rockAr-Arsubduction272.2 ± 1.9Qiu et al. (2009) [118]
63Bayan ObodolomiteSm-Nd, whole rockextension1341 ± 59Yang et al. (20011b) [119]
64Bayan Obocarbonatite dikeSm-Nd, whole rockextension1354 ± 57Yang et al. (2011b) [119]

7. Conclusions

In this study, the deformation characteristics and 40Ar-39Ar isotopes of banded ores in the Bayan Obo deposit in Inner Mongolia were investigated. The following conclusions were drawn regarding the formation and evolutionary history of these deposits:
(1)
The widely distributed banded structure within the ore district was the product of structural transformation under the influence of north–south compression, indicating strong ductile deformation in the region.
(2)
The 40Ar-39Ar chronology indicates that after the formation of the Bayan Obo deposit, it was affected by two geological events in the Early Paleozoic (414.9 Ma) and Late Paleozoic (264.5 Ma), which were recorded in the banded ores of riebeckite and biotite.
(3)
These two geological events correspond to an arc–continent collision event in the Early Paleozoic and a Permian magmatic event, respectively, which were closely related to the evolution of the PAO on the northern side of the NCP.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15070683/s1, Table S1: 40Ar–39Ar results of riebeckite from the Bayan Obo ore district; Table S2: 40Ar–39Ar results of biotite from the Bayan Obo ore district.

Author Contributions

Conceptualization, D.W., H.L. and H.S.; Methodology, H.L., Y.L., H.S. and C.K.; Investigation, X.G., D.W., Y.L., H.S., J.Y., L.Z., C.K., J.Z., C.R. and F.Y.; Data curation, S.M.; Writing—original draft, X.G.; Writing—review & editing, D.W. and S.M.; Funding acquisition, D.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Inner Mongolia Autonomous Region Department of Natural Resources Project (Grant No.150000235053210000203), the National Science and Technology Major Project (2024ZD1001001), the Science and Technology Achievements Transformation Projects (HE2333, HE2228, HE2332 and HE2503). The APC was funded by [150000235053210000203].

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Acknowledgments

We are very grateful to the Editor for their hard work and efficient editorial handling. We gratefully acknowledge the reviewers for their suggestions. The thorough constructive criticism and comments of the anonymous reviewers significantly improved this manuscript. We appreciate the assistance provided by Sun Wei during this study.

Conflicts of Interest

The authors declare no conflicts of interest. Among them, authors Li Zhang is employee of Bayan Obo Iron Mine, Baotou Iron and Steel Company and Jian Zhao is employee of Barun Mining Co. of Steel Union Co. Ltd. of Baotou Steel (Group) Corp. The paper reflects the views of the scientists and not the company.

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Figure 3. Deformation characteristics of rocks in the Bayan Obo ore district. (a) The folded calcite veins within the carbonatite; (b) the folds formed by the pyrite bands; (c) the dense crenulation cleavage that developed inside the slate; (d) rotated porphyroclasts formed by the shearing of pyrite; (e) lenses formed by the shearing of the strong layers in the slate; (f) the porphyroclasts of carbonatite formed by the shearing within the slate. The red arrow indicates shear sense.
Figure 3. Deformation characteristics of rocks in the Bayan Obo ore district. (a) The folded calcite veins within the carbonatite; (b) the folds formed by the pyrite bands; (c) the dense crenulation cleavage that developed inside the slate; (d) rotated porphyroclasts formed by the shearing of pyrite; (e) lenses formed by the shearing of the strong layers in the slate; (f) the porphyroclasts of carbonatite formed by the shearing within the slate. The red arrow indicates shear sense.
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Figure 4. Microstructural characteristics of rocks in the Bayan Obo ore district. (a) The magnetite bands compressed to form puddings; (b) the folds formed by dolomite, fluorite, biotite, and albite; (c) the folds formed by riebeckite and albite; (d) the pressure shadows of magnetite and the rotational porphyroclasts formed by dolomite aggregates in riebeckite carbonatite; (e) rotated porphyroclasts formed by hematite; (f) the subgrain rotation recrystallization of quartz that occurred in the surrounding rock of the ore district. The red arrow indicates shear sense. ab—albite; bt—biotite; dol—dolomite; flu—fluorite; hem—hematite; mag—magnetite; qtz—quartz; rbk—riebeckite.
Figure 4. Microstructural characteristics of rocks in the Bayan Obo ore district. (a) The magnetite bands compressed to form puddings; (b) the folds formed by dolomite, fluorite, biotite, and albite; (c) the folds formed by riebeckite and albite; (d) the pressure shadows of magnetite and the rotational porphyroclasts formed by dolomite aggregates in riebeckite carbonatite; (e) rotated porphyroclasts formed by hematite; (f) the subgrain rotation recrystallization of quartz that occurred in the surrounding rock of the ore district. The red arrow indicates shear sense. ab—albite; bt—biotite; dol—dolomite; flu—fluorite; hem—hematite; mag—magnetite; qtz—quartz; rbk—riebeckite.
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Figure 5. Microstructural characteristics of riebeckite and biotite from the Bayan Obo ore district. (a) The minerals arranged parallel and directionally to form the main foliation; (b) the main constituent minerals of sample XK21; (c) the asymmetric folds formed by biotite; (d) the rotational porphyroclasts formed by albite. ab—albite; bt—biotite; dol—dolomite; kfs—potassium feldspar; mag—magnetite; qtz—quartz; rbk—riebeckite.
Figure 5. Microstructural characteristics of riebeckite and biotite from the Bayan Obo ore district. (a) The minerals arranged parallel and directionally to form the main foliation; (b) the main constituent minerals of sample XK21; (c) the asymmetric folds formed by biotite; (d) the rotational porphyroclasts formed by albite. ab—albite; bt—biotite; dol—dolomite; kfs—potassium feldspar; mag—magnetite; qtz—quartz; rbk—riebeckite.
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Figure 6. The geochronology results for riebeckite and biotite from the Bayan Obo ore district. (a) 40Ar-39Ar age plateaus of riebeckite; (b) isochron diagram for riebeckite; (c) inverse isochron diagram for riebeckite; (d) 40Ar-39Ar age plateaus of biotite; (e) isochron diagram for biotite; (f) inverse isochron diagram for biotite.
Figure 6. The geochronology results for riebeckite and biotite from the Bayan Obo ore district. (a) 40Ar-39Ar age plateaus of riebeckite; (b) isochron diagram for riebeckite; (c) inverse isochron diagram for riebeckite; (d) 40Ar-39Ar age plateaus of biotite; (e) isochron diagram for biotite; (f) inverse isochron diagram for biotite.
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Figure 7. Tectonic model showing multi-stage tectonic thermal events on the northern margin of the NCP (see the text for further details).
Figure 7. Tectonic model showing multi-stage tectonic thermal events on the northern margin of the NCP (see the text for further details).
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MDPI and ACS Style

Gao, X.; Wang, D.; Li, H.; Li, Y.; She, H.; Yang, J.; Zhang, L.; Ke, C.; Zhao, J.; Ma, S.; et al. 40Ar-39Ar Chronometry Supports Multi-Stage Tectonic Thermal Events in the Bayan Obo Fe-Nb-REE Deposit. Minerals 2025, 15, 683. https://doi.org/10.3390/min15070683

AMA Style

Gao X, Wang D, Li H, Li Y, She H, Yang J, Zhang L, Ke C, Zhao J, Ma S, et al. 40Ar-39Ar Chronometry Supports Multi-Stage Tectonic Thermal Events in the Bayan Obo Fe-Nb-REE Deposit. Minerals. 2025; 15(7):683. https://doi.org/10.3390/min15070683

Chicago/Turabian Style

Gao, Xinke, Dongsheng Wang, Hongying Li, Yike Li, Hongquan She, Jianjun Yang, Li Zhang, Changhui Ke, Jian Zhao, Shouxian Ma, and et al. 2025. "40Ar-39Ar Chronometry Supports Multi-Stage Tectonic Thermal Events in the Bayan Obo Fe-Nb-REE Deposit" Minerals 15, no. 7: 683. https://doi.org/10.3390/min15070683

APA Style

Gao, X., Wang, D., Li, H., Li, Y., She, H., Yang, J., Zhang, L., Ke, C., Zhao, J., Ma, S., Ren, C., & Yin, F. (2025). 40Ar-39Ar Chronometry Supports Multi-Stage Tectonic Thermal Events in the Bayan Obo Fe-Nb-REE Deposit. Minerals, 15(7), 683. https://doi.org/10.3390/min15070683

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