Exhumation of the Koktokay rare-metal pegmatite group in the China Altai: insights from low-temperature thermochronology of the No. 3 pegmatite and Aler S-type granitic batholith

Wang, Haoyu; Ren, Guangming; Tang, Yong; Zhang, Hui; Lv, Zhenghang; Huang, Zhilong

doi:10.3389/feart.2025.1573046

ORIGINAL RESEARCH article

Front. Earth Sci., 10 September 2025

Sec. Structural Geology and Tectonics

Volume 13 - 2025 | https://doi.org/10.3389/feart.2025.1573046

This article is part of the Research TopicStructural Processes, Petrogenesis, Mineralization, and Geochronology in the Earth’s CrustView all 9 articles

Exhumation of the Koktokay rare-metal pegmatite group in the China Altai: insights from low-temperature thermochronology of the No. 3 pegmatite and Aler S-type granitic batholith

Haoyu Wang^1,2^†

Guangming Ren¹^†

Yong Tang²*

Hui Zhang²

Zhenghang Lv²

Zhilong Huang²

¹Chengdu Center of China Geological Survey (Geoscience Innovation Center of Southwest China), Chengdu, Sichuan, China
²State Key Laboratory for Critical Mineral Research and Exploration, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China

The China Koktokay Pegmatite Group is an important metallogenic region where major rare-metal ores are mined. Here, we present new low-temperature thermochronological data to contribute to the understanding of the Koktokay Pegmatite Group’s exhumation and preservation history. Apatite U–Pb ages of ∼176 Ma, zircon fission-track ages of ∼150 Ma, zircon (U–Th)/He ages of ∼82–52 Ma, apatite fission-track ages of ∼69–49 Ma, and apatite (U–Th)/He ages of ∼90–52 Ma were obtained from three samples of the No. 3 pegmatite and the contemporaneous Aler granitic batholith in the Koktokay area. Our thermochronological data and inverse thermal history modeling reveal a moderate-to-rapid basement cooling phase during the Cretaceous (∼150–65 Ma), with an average cooling rate of ∼1.53 °C–1.06 °C/Ma. It is envisaged that this phase eventually uplifted and exhumed the pegmatite group, with erosion in the Cenozoic era being limited. Combined with previously published geochronological and thermochronological data, a multi-stage cooling history for the pegmatite group can be established. Following its magmatic–hydrothermal formation in the Late-Triassic (∼220–200 Ma), two phases of accelerated regional cooling (i.e., in the late Triassic to Early Jurassic, ∼200–180 Ma; and the mid-Jurassic to Late Jurassic, ∼176–150 Ma) can be recognized. The intense cooling in the Cretaceous is associated with the final exhumation of the pegmatite group to the surface, and some Li–Ru–Cs-mineralized pegmatites formed at the distal end of the Koktokay Pegmatite Group may have been exhumed and denuded. Furthermore, we propose a relatively intense denudation of the Koktokay Pegmatite Group, which is unfavorable for the preservation of rare-metal pegmatite bodies.

1 Introduction

The preservation of ore deposits is an essential component of the mineralization system. The formation of an economic mineral deposit requires both a genetic process and subsequent exhumation events that enable commercial mining (Zhai et al., 2000; Kesler and Wilkinson, 2006). Therefore, understanding the erosion and uplift of deposits, mining areas, and larger regions not only contributes to the understanding of deposit preservation but also provides valuable information for regional deposit prospecting. Low-temperature thermochronological tools can determine the timing and rates at which a geo-body approaches the upper crust and surface and subsequently cools as a result of exhumation. Hence, these tools place constraints on the thermal history of a geo-body as it passes through a shallow crustal isothermal structure, undergoing cooling from ∼450 to ∼45 °C, and are used to reveal the complex dynamic mechanisms involved in the preservation of the geo-body (Reiners and Brandon, 2006; Enkelmann and Garver, 2016). Low-temperature thermochronology is majorly used to reconstruct the post-mineralization thermal history of deposits (Marton et al., 2010). It is useful for evaluating the preservation potential of deposits. Furthermore, when combined with medium- to high-temperature geo-/thermochronometry tools, such as apatite U–Pb, muscovite ⁴⁰Ar–³⁹Ar, and zircon U–Pb, a comprehensive history of cooling/preservation since the formation of the ores can be reconstructed (Evans et al., 2013; Zhang L. et al., 2017; Zhang J. et al., 2017).

Pegmatites have long received considerable attention due to their colorful gemstones, fine mineral specimens, and industrial minerals, such as feldspar and quartz. Additionally, pegmatites are significant reservoirs of rare metal elements (Linnen et al., 2014; Glover et al., 2012; Linnen et al., 2012; Simmons et al., 2012; London, 2008). London (2008) proposed a definition for pegmatite that provides a comprehensive overview of its characteristics. According to the definition, granitic pegmatite is an essentially igneous rock commonly granitic in composition. Fundamentally, pegmatite is distinguished from other igneous rocks by its extremely coarse, variable grain size or by the abundance of crystals with skeletal, graphic, or other strongly directional growth habits. Furthermore, pegmatites typically occur as a cluster, and within a pegmatite field, the distribution of rare-metal-mineralized pegmatites follows a systematic pattern reflecting their geochemical and textural evolution: microcline-rich pegmatites are found abundantly in the center, while Ta-, Li-, Rb-, and Cs-bearing minerals increase in abundance toward the outermost parts (Jahns and Burnham, 1969; Černý, 1991; Dill et al., 2012; London, 2018; Zhang and Li, 2024). For instance, within a pegmatite group, Be-mineralized pegmatites typically form near the center, while Li–Rb–Cs-mineralized pegmatites are found at the periphery (London, 2018), such as the Oxford pegmatite Field in Maine (Webber et al., 2019; Simmons et al., 2020). Therefore, determining the preservation status and erosion thickness is crucial for evaluating the potential for exploration of hidden ore bodies in pegmatite groups. However, there are only a few case studies on the preservation history of pegmatites, such as the Jiajika pegmatite in China (Liu et al., 2023).

The Altai Orogen, located in the western part of the Central Asian Orogenic Belt (CAOB) (Figure 1a), stretches approximately 2,500 km from Russia and Kazakhstan through China to Mongolia. The Altai Orogen is a significant producer of rare metals globally, with abundant rare-metal-mineralized pegmatites. It is referred to as the Kalba–Narym–Koktokay rare-metal (Li–Be–Nb–Ta–Cs) pegmatite belt (Annikova et al., 2016a; Annikova et al., 2016b; Murzintsev et al., 2019; Khromykh et al., 2020). The Koktokay Pegmatite Group, located in the central part of the Chinese Altai, is the characteristic rare-metal-producing area with massive contents of mineralized pegmatites. In the Koktokay Pegmatite Group, the dominant mineral is beryllium (Be). The most famous, No. 3 pegmatite, which contains 10 internal zones and vast resources of Be–Nb–Ta–Li, consists of large amounts of beryllium but moderate amounts of lithium (Zou and Li, 2006). Previous studies have primarily focused on the chronology, geodynamic processes, internal evolution, and mineralization of the Koktokay Pegmatite Group (Wang et al., 2007; Tang et al., 2018; Zhao and Yan, 2023; Shen et al., 2022; Zhang and Li, 2024). However, little attention has been paid to their long-term post-mineralization evolution (i.e., their exhumation and preservation). Exhumation and preservation of the ore-bearing bodies in the area remain poorly understood, which is important for mineral exploration.

Figure 1

Map detailing geological features of the Chinese Altai region, including faults, strata, and rock formations. It highlights diverse formations like Dongxileke (Ordovician-Devonian) and Baihaba (Devonian), with various color codes depicting sedimentary covers, mafic intrusions, and granitoids. Insets show geographic context with countries and domains, and arrows indicate fault directions. Scale and compass are included for orientation.

Figure 1. (a) Location of the Koktokay region; (b) geological map of the China Altai on the basis of regional 1: 200,000 geological maps (Li et al., 2019; BGMRX, 1993); and the low-temperature data are collected from Wu et al. (2023) and references therein.

It is clear that understanding the exhumation history of the Koktokay Pegmatite Group has valuable implications for gaining deeper insights into the preservation and exposure of the pegmatite group. We focus on the thermal history of the Koktokay Pegmatite Group. Moreover, the contemporaneous Aler granitic batholith in the Koktokay area is used as a reference for evaluating the preservation history. Pegmatite and granite samples are analyzed using medium- and low-temperature thermochronological techniques, including apatite U–Pb, zircon fission track (ZFT), zircon (U–Th)/He (ZHe), apatite fission track (AFT), and apatite (U–Th)/He (AHe). Their integrated thermal history is then derived through inverse modeling. In addition, based on compiled thermochronological data of this area, we further discuss the preservation history of the Koktokay pegmatite to shed more light on mineral exploration and provide a case study on pegmatite preservation research.

1.1 Geological setting

The Altai region occupies a key position within the CAOB (Figure 1a). It originated in the Late Paleozoic and is the result of the conglomeration of various geological entities, such as island arcs, accretionary wedges, seamounts, and microcontinents, during the Cambrian–Carboniferous period of the ancient Asian oceanic system (Buslov et al., 2001; Buslov et al., 2004; Windley et al., 2007; Glorie et al., 2011). Based on isotopic studies and the formation of a series of rift basins such as Ashele, Chongkuer, Kelan, and Maizi basins, a foreland basin developed in the southern part of the China Altai, characterized by extensional tectonic settings that persisted at least until the Late Carboniferous (Yuan et al., 2007). During this period, a limited amount of syn-orogenic pegmatite was formed, predominantly exposed in the Qiongkuer domain of the Altai, with small quantities and scale of rare-metal mineralization (Lv et al., 2018). The Carboniferous suturing between the Altai and the Junggar terranes is represented by the Irtysh Shear Zone (Figure 1b; Laurent-Charvet et al., 2002; Briggs et al., 2007). Despite the prolonged tectonic activity during the Mesozoic–Cenozoic, the west Junggar Mountains have retained the records of upper-crustal exhumation caused by Permian shortening along the Irtysh Shear Zone (Gillespie et al., 2020). The granite intrusion around the Irtysh Shear Zone recorded the intensity of deformation in different episodes. Devonian granitoids preserve deformational fabrics (Li et al., 2017; Li et al., 2022). However, Permian aged granites are commonly undeformed (Sun et al., 2008; Tong L. X. et al., 2014). The Tarim Craton collided with the Siberian Craton during the Triassic period (Xiao et al., 2009; Xiao et al., 2015; Lv et al., 2012; Zheng et al., 2015). Monazite U–Pb ages and biotite Ar–Ar cooling ages recorded in the Altai indicate exhumation of up to 20 km in relation to thrust faulting in the Late Permian to Early Jurassic (Briggs et al., 2009; Li et al., 2015). Late in this period, numerous post-orogenic rare-metal pegmatites were formed in the Altai (Lv et al., 2012; Lv et al., 2018; Che et al., 2015; Zhou et al., 2015a; Ma et al., 2015).

The Koktokay pegmatite district is located in the middle of the China Altai (Figure 1B), where more than 3,000 pegmatites have been formed (Tang et al., 2018). These pegmatites belong to the LCT family (Černý, 1991; Černý and Ercit, 2005) and the Pegmatite family (Lv et al., 2018) based on the chemical composition–mineral assemblage–structural geology (CMS; Dill, 2016) classification with the mineralogical range of barren to Be-, Ta-, and Nb-enriched types. However, rare-metal-mineralized pegmatites account for less than 2% of the total pegmatite bodies within individual pegmatite groups (Tang et al., 2018). Within the Koktokay Pegmatite Group, the No. 3 pegmatite is the most strongly mineralized rare-metal pegmatite, containing 10 internal zones (Zou and Li, 2006).

The famous Koktokay No. 3 pegmatite is 1.5 km south of the Koktokay Town (Figure 2). Igneous rocks are widely exposed in the area. The igneous comprises a biotite batholith intruded by two-mica and muscovite granite dike (Figure 2). Zircon U–Pb dating yields ages pf 396–415 Ma for the biotite batholith (Wang et al., 2007), 390–400 Ma for the two-mica granite, and 390–396 Ma for the muscovite granite dikes (Zhou et al., 2015a). The metagabbro pluton is the host rock of the No. 3 pegmatite (Figure 2), and it was dated at 408 ± 6 by zircon U–Pb (Wang et al., 2006; Cai et al., 2012). The formation age of the No. 3 pegmatite is Late Triassic to Early Jurassic (Zircon U–Pb, Rb–Sr, and coltan U–Pb, 220–190 Ma; Wang et al., 2007; Zhou et al., 2015b; Che et al., 2015; Shen et al., 2022), and it is determined from a geochronological perspective that it has no genetic connection with the surrounding igneous rocks. Detailed geochronology studies identified that the geodynamics setting for the formation of No. 3 pegmatite is the extension stage after the collision between the Tarim Craton and Siberian Craton.

Figure 2

Geological map showing the Koktokay town area, highlighting various geological formations. Key features include faults, rivers, quaternary deposits, pegmatites, ultramafic-mafic rocks, and different types of granite and group formations. Locations such as No. 3 pegmatite and points A102 and A103 are marked. A legend explains the color coding for each geological feature.

Figure 2. Geological map of the Koktokay area showing the distribution of the No. 3 pegmatite and the Aler granitic batholith (after Zou and Li, 2006).

The Aler Granite is a batholith with the area exposed exceeding 1,400 km², located approximately 15 km north of the No. 3 pegmatite (Figure 2). This batholith extends along the trend of the Altai orogen and intrudes into the Paleozoic metasedimentary rocks (Kuwei group) and Devonian plutons. The lithology of the Aler Granite in the northern part predominantly consists of medium-fine-grained biotite granite, while in the southern part, it is predominantly porphyritic biotite granite with larger phenocrysts of quartz and feldspar (Liu et al., 2014). The primary mineral composition is quartz, K-feldspar, and biotite, while the accessory minerals include magnetite, garnet, zircon, apatite, and xenotime. According to lithology and occurrence location, the Aler Granite can be classified into central zone, transitional zone, and marginal zone (Zhang et al., 2015). Some dating methods with relatively low closure temperature had been used in the Aler Granite and revealed a younger age (e.g., biotite K-Ar 138–150 Ma and biotite ⁴⁰Ar/³⁹Ar 131.39 ± 4.25 Ma; Zhang et al., 1994; Chai et al., 2010; Han, 2008), leading to the recognition of the Aler Granite as a product of the Indosinian period. Subsequent extensive precise zircon U–Pb dating has determined its age to be Late Triassic (Liu and Han, 2019; Zhang et al., 2015; Liu et al., 2014). Nevertheless, the reported Cretaceous origin of the Aler Granite holds significance as it is correlated with the time when the Chinese Altai cooled to upper-crystal temperatures (Yuan et al., 2006; Pullen et al., 2020). It provides significant reference information for studying the preservation of the Aler Granite. Several Mesozoic granites developed in the Altai Orogenic Belt (Vladimirov et al., 2005; Annikova et al., 2006; Han, 2008), but only a few in the Chinese Altai [Jiangjunshan granite, 150 Ma, Chen and Jahn (2002); Shangkelan, 202 Ma, Wang et al. (2010); and Aler Granite, 210–219 Ma]. There is no evidence indicating that the Mesozoic Chinese Altai is not conducive to the formation of granites, so the preservation of Mesozoic granites in this area is questionable. Obviously, spherical weathering with large height differences in the Aler Granite (Liu and Han, 2019) also implies multiple stages of exhumation and denudation in this area.

2 Methods

2.1 Sample preparation

The sample locations are remarked in Figure 2. The formation of the Koktokay pegmatites is a complex process, including the initial magmatic stage, magmatic–hydrothermal transition stage, and hydrothermal stage (Zhao and Yan, 2023 and references therein). According to the occurrence of the Koktokay Pegmatite and the simulation experiment of London (2014), the graphic zone (zone Ⅰ) in the external of the Koktokay No. 3 pegmatite can represent the initial pegmatitic magma. Most zircons in pegmatites are damaged by radiation due to high U and Th contents and lose their original crystal structure (Lv et al., 2018). This process turns the zircon crystals into materials filled with small pores. The destroyed crystal structure of zircon grains is not conducive to the preservation of products from multiple isotope systems (e.g., Pb_radiogenic and He), and the fission track is even invisible. Therefore, AFT, AHe, and U–Pb ages from the No. 3 pegmatite were utilized to constrain the multistage thermal history, tracking cooling from higher to lower temperatures (Table 1). Apatite was obtained by collecting typical bulk samples from zone Ⅰ (Figure 3d), followed by crushing and flotation. Then, they were divided into two groups, one for fission-track analysis (Figure 3e) and the other mounted in epoxy resin for LA-ICPMS analysis (Figure 3f).

Table 1

Table 1. Table of the main mid-low-temperature thermochronology tools and their closure-temperature and corresponding closure depths (after Huntington and Klepeis, 2018).

Figure 3

Panel a shows Aler granite with a pen for scale. Panels b and c display zircon and apatite from Aler granite under a microscope. Panel d depicts a pegmatite outcrop with a blue hammer for scale. Panels e and f show apatite crystals from pegmatite under a microscope.

Figure 3. Sampled porphyritic biotite granite (a), with obtained zircon (b) and apatite (c) grains; sampled graphic pegmatite (d), and obtained apatite (e,f).

Aler granite samples were collected from porphyritic biotite granite in the southern part of the batholith. The zircon U–Pb age and whole-rock geochemical composition are reported in Liu et al. (2012). Zircon and apatite are invisible in the porphyritic biotite granite (Figure 3a), which are obtained by crushing typical bulk rocks, flotation, and magnetic separation. Complete zircon (Figure 3b) and apatite (Figure 3c) of grains were selected for fission-track and (U–Th)/He analyses.

2.2 Apatite and zircon fission-track thermochronology

The fission-track analysis experimental works were performed at the Beijing Zekang’en Technology Co., Ltd. The obtained apatite and zircon concentrates were separated using conventional magnetic and heavy-liquid techniques. The zircon grains were mounted in glass slides, heated, and covered by FEP Teflon sheets. Their external prismatic surfaces were ground and polished. The etching duration was approximately 20–35 h with NaOH/KOH (1:1) used as the eutectic etchant at 210 °C (Garver, 2003; Yuan et al., 2007). Different from zircon, apatite was mounted in epoxy resin on glass slides, ground, and polished to an optical finish to expose the internal grain surface. Spontaneous tracks were revealed using 5.5 N HNO₃ for 20 s at 21 °C. Thin low-uranium muscovite used as external detectors were packed together with sample grain mounts, CN2 (apatite), and CN5 (zircon) uranium dosimeter glass (Bellemans, 1995) irradiated in the well-thermalized hot-neutron nuclear reactor (Yuan et al., 2006). After irradiation, the muscovite external detectors were detached and etched in 40% HF for 20 min at 25 °C to reveal the induced fission tracks (Yuan et al., 2003). Track densities for both natural and induced fission track populations were measured using a dry objective at ×100 magnification. Fission-track ages were measured using the IUGS-recommended zeta calibration approach. The zeta values used in this study have been determined from repeated measurements of standard apatite (Hurford and Green, 1983; Hurford, 1990). The weighted mean zeta value obtained is 391 ± 17.8 a/cm² for apatite and 88.2 ± 2.9 a/cm² for zircon.

2.3 Apatite and zircon (U–Th)/He thermochronology

Apatite and zircon (U–Th)/He analyses were conducted at the Sate Key Laboratory of Ore Deposit Geochemistry (SKLODG), Institute of Geochemistry, Chinese Academy of Sciences (IGCAS), Guiyang, China. Inclusion-free apatite and zircon grains for (U–Th)/He analysis were handpicked from concentrated separates, based on their size and euhedral crystal shape. The representative apatite and zircon grains are shown in Figure 3. The crystal size of each grain was measured microscopically for applying the alpha-ejection age corrections (Farley et al., 1996). The apatite grains were loaded into platinum capsules, and the zircon grains were loaded into niobium capsules and outgassed under vacuum at 900 °C for 5 min and 1,300 °C for 15 min. The protocols of He analysis followed the established laboratory routine extraction. The ⁴He abundances were determined as an isotope ratio using a pure ³He spike that has been calibrated against an independent ⁴He standard (House et al., 2000; Wu et al., 2016). The outgassed apatite grains were dissolved with platinum capsules in 7 mol/L HNO₃ and analyzed for ²³⁸U, ²³⁵U, ²³²Th, and ¹⁴⁷Sm by ICP–MS. The degassed zircon grains were transferred to Parr bombs, where they were spiked with ²³⁵U and ²³⁰Th and digested at 240 °C for 40 h in HF. A second bombing in HCl for 24 h at 200 °C ensured the dissolution of fluoride salts. Zircon solutions were then dried down, dissolved in HNO₃, and diluted in H₂O to 5% acidity for the analysis of ²³⁸U, ²³⁵U, and ²³²Th by solution ICP–MS (Goeadow et al., 2015).

2.4 Apatite U–Pb thermochronology

Apatite U–Pb isotopic and trace element analyses were performed at the SKLODG, IGCAS, using a GeoLas Pro 193-nm excimer ArF LA system coupled with an Agilent 7500a quadrupole-based Q-ICP-MS. During Laser Ablation (LA), a National Institute of Standards and Technology (NIST) 610 SEM reference material glass was used to optimize the instrumental parameters. Using OD 306 as an external standard, the QH apatite standard [∼150 Ma, Chew et al. (2016)] was used to verify the analytical accuracy and return the weighted mean ²⁰⁶Pb/²³⁸U age of 156 ± 4.7 Ma. NIST 612 was used for apatite trace-element calculation using Ca as the internal standard. In routine analysis, every 15 samples were followed by two runs of SEM 610, NIST 612, and OD306 and one run of Durango and QH, with a repetition of 10 Hz, a laser energy density of 3 J/cm², and a spot size of 60 μm. Every analysis consists of 30-s background acquisition, 60-s sample data acquisition, and approximately 60-s blank for flashing. The fractionation correction and U–Pb ages were calculated using GLITTER 4.0 (GEMOC, Macquarie University). The Concordia and weighted mean U–Pb age were calculated using the ISOPLPT/EX 4.15 software package.

2.5 Thermal history modeling

The cooling histories of the pegmatite and granite (KT3, A102, and A103) were derived using the HeFTy program (v 2.0.0; Ketcham et al., 2007). The dominant input data are apatite fission-track spontaneous and induced track densities, length–frequency distributions, and kinetic parameters D_par. Monte Carlo simulation was used in modeling operation. The model was constrained at the start by the ages and closure temperatures of apatite U–Pb and ZFT in the Koktokay No. 3 pegmatite and the Aler Granite, respectively. We used the multi-kinetic fission-track annealing model of Ketcham et al. (2007), and the initial track length in apatite was set as 16.3 μm (Ketcham, 2005). Available AHe and ZHe data were integrated using the radiation damage accumulation and annealing, with helium diffusion models (Flowers et al., 2009; Gautheron et al., 2013). The present-day surface temperature is constrained at 20 °C ± 10 °C.

3 Results

3.1 Fission-track analysis

3.1.1 AFT data

Apatite fission-track dating was performed on samples KT3, A102, and A103. The detailed results are shown in Table 2. For every sample, the central age is calculated based on 35 grains in KT3 and 42 grains in A102 and A103. The samples A102 and A103 passed the χ² test [P(χ²)-values are 12% and 11.3%, respectively], indicating a relatively concentrated age of single particles. Their central ages were determined to be 60.69 ± 8.08 Ma (Figure 4b) and 48.13 ± 5.73 Ma (Figure 4c), respectively. Although the sample KT3 did not pass the χ² test, the dispersion value is 11.8% ± 5.2%, indicating a relatively discrete age of single particles. It may be caused by the high content of U in pegmatitic apatite (Glorie et al., 2023; McDannell, 2020). The pooled age was used to replace the central age in KT3, and its pooled age is 68.78 ± 7.22 Ma (Figure 4a). Mean track lengths of the three samples vary between 11.29 and 13.08 μm (Table 2). They are relatively short confined track lengths, suggesting more intensive thermal annealing. The average etch diameter (D_par) values of the samples range from 1.426 to 1.714 μm, which were shorter than those of the Durango apatite (∼1.75 μm), indicating relatively weaker resistance to fission-track annealing (Donelick et al., 2005).

Table 2

Table 2. Apatite and zircon fission-track results.

Figure 4

Five radial plots labeled a to e display data about central and pooled ages in millions of years (Ma) with a color gradient indicating Dpar values. Details include:a. Central age: 68.78 ± 7.22 Ma, Pooled age: 69 ± 4 Ma.b. Central age: 60.69 ± 8.08 Ma, Pooled age: 60 ± 4 Ma.c. Central age: 48.13 ± 5.73 Ma, Pooled age: 49 ± 3 Ma.d. Central age: 150.1 ± 12.9 Ma, Pooled age: 150.73 ± 11.56 Ma.e. Central age: 154.1 ± 14.8 Ma, Pooled age: 145.89 ± 8.52 Ma.Each plot shows a mathematical standard deviation value (MSWD) and a p-value.

Figure 4. Radial plots of apatite FT (a–c) and zircon FT (d,e) ages using the Radialplotter (Vermeesch, 2009).

3.1.2 ZFT data

The zircons from the Aler Granite were analyzed using ZFT (samples A102 and A103). Both of their ZFT ages passed the χ² test (Table 2). Their central ages are 150.73 ± 11.56 Ma (Figure 4d) in A102 and 145.89 ± 8.52 Ma (Figure 4e) in A103. ZFT analysis predominantly focused on density measurement owing to the lack of unified standards, while the neglected track length analysis would be a significant parameter for evaluating the thermal histories. All the tracks in zircon have the same initial track length (∼10 μm; Tagami et al., 1998). Thus, we selected the classical standard of Garver (2003) and obtained the mean track lengths of 9.97 and 9.88 μm in A102 and A103, respectively (Table 2). The track length in zircon is nearly initial, indicating that the obtained tracks did not suffer annealing and that the two concentrated ages of ZFT have not been reset.

3.2 (U–Th)/He analysis

Apatite helium ages were measured on pegmatite and granite (samples KT3 and A102), and zircon helium ages were measured on granite (samples A102 and A103); all results are shown in Table 3. The obtained datasets of pegmatite and granite samples show a low degree of single-grain helium age dispersion. Five apatite grains from the pegmatite sample KT3 yielded a weighted mean AHe age of 83.9 ± 4.5 Ma, and three grains from the sample A102 yielded an age of 56.5 ± 3.2 Ma. Meanwhile, five zircon grains of the sample A102 and five zircon grains of A103 produced mean ZHe ages of 65.1 ± 3.3 Ma and 64.8 ± 3.3 Ma, respectively. Comparison between (U–Th)/He and fission-track ages within a single sample shows that dating methods involving a higher isotope closure temperature generally display older corresponding ages. Moreover, all of them are significantly younger than the Koktokay Pegmatite and Aler Granite formation (∼220 Ma; Wang et al., 2007; Liu et al., 2014).

Table 3

Table 3. Apatite and zircon (U–Th)/He results.

3.3 Apatite U–Pb data

The apatite grains of KT3 selected for U–Pb dating are shown in Figure 5a. The analytical spots yielded a Tera-Wasserburg U–Pb lower intercept age of 176 ± 11 Ma (n = 14, MSWD = 0.41, Figure 5b).

Figure 5

Three-panel image consisting of: (a) a black and white electron microscope image of apatite grains, labeled with SEM settings; (b) a concordia diagram with a red ellipse indicating lower intercept age of \(176 \pm 11\) million years and MSWD of 0.41; (c) a plot of Apatite/Chondrite ratios against elements La to Lu, showing trends with various lines.

Figure 5. Back-scattered electron (BSE) images of the KT3 apatite grains for U–Pb dating (a); Tera–Wasserburg diagram for apatite U–Pb dating (b); and the chondrite-normalized REE pattern (c).

The trace element content of apatite is shown in Table 4. The apatite Rare Earth Elements (REE) content ranges from 300 to 600 ppm, which is consistent with previous investigations (Zhang, 2001; Cao et al., 2013) and is different from apatite of other zones in No. 3 pegmatite. In addition, the chondrite-normalized REE patterns of apatite have obvious negative Eu anomalies, with Eu/Eu* ratios of 0.16–0.04. According to Masuda et al. (1994) and Irber (1999), the provided quantitative method is used to describe the REE distribution characteristic (Table 4; Figure 5c). The t₁, t₃, and t₄ values of all apatite are >1.1, and t₃>t₄>t₁, which indicates that the “quadruple effect” of apatite is most developed in the third segment (Gd-Ho). All values of t_3,4 and t_1,3,4 > 1.5, implying that the apatite analyzed have a significant M-type tetrad effect. This is consistent with the REE distribution features of minerals in the external zones in No. 3 pegmatite (Cao et al., 2013; Zhang, 2001). Therefore, the analyzed apatite crystallized during the pegmatitic magma stage.

Table 4

Table 4. The REE contents of apatite.

3.4 Thermal history modeling results

All three samples (KT3, A102, and A103) in this study obtained acceptable [GOF (good of fit) > 0.3] or good (GOF >0.7) thermal modeling results. Inverse thermal history models are detailed in Figures 6–8. All the three samples show observed similar cooling paths (i.e., the best-fit path). They entered the ZHe PRZ in the Early Cretaceous, followed by prolonged slow cooling since the Middle Cretaceous. All three samples display a complex three-stage cooling, and a change in the cooling rate (to a slower rate) can be observed in the Cretaceous (∼120 Ma). The modeling shows an accelerated cooling between ∼170 and 120 Ma, with a cooling rate of >3 °C/Ma, and subsequent moderate cooling between ∼120 and 60 Ma, with a cooling rate of ∼1.5 °C/Ma, followed by a slow cooling rate of <0.5 °C/Ma until the present. Cooling rates of <∼0.5 °C/Ma, ∼0.5 °C–2.0 °C/Ma, and >∼2.0 °C/Ma are defined as slow, moderate, and rapid, respectively, in an intracontinental setting (based on empirical values, Wu et al., 2023; He et al., 2022).

Figure 6

Six graphical plots illustrating temperature in degrees Celsius against age in million years (Ma) for apatite samples. Plot (a) shows a green cluster with an age of 68.78 ± 7.22 Ma. Plots (b) to (f) display purple clusters labeled AHe with ages ranging from 75.9 ± 4.14 Ma to 86.9 ± 4.69 Ma. Goodness of fit (GOF) values vary, indicating data accuracy.

Figure 6. Results of HeFTy thermal history modeling of the sample KT3 from the No. 3 pegmatite. Green and purple colors show accept-fit solutions (goodness of fit >5%) and good-fit solutions (goodness of fit >50%), respectively; the rectangular boxes represent the time–temperature constraints; the best-fit path is plotted with a black curve. (a) AFT: Apatite fission track; (b–f) AHe: Apatite (U-Th)/He; GOF: good of fit; Old: The age (Ma) of the oldest fission track that has not yet been fully annealed.

Figure 7

Nine-panel chart showing temperature (degrees Celsius) versus age (million years ago) for thermochronological data. Panels labeled a to i include three thermal histories: AFT, AHe, and ZHe, represented by different colored lines. Each plot includes a measure of goodness of fit (GOF) and age calculations with respective uncertainties. The temperature ranges from 0 to 400 degrees Celsius, and the age ranges from 200 to 0 million years ago. Pink and green colors indicate different data subsets.

Figure 7. Results of HeFTy thermal history modeling of the sample A102 from the Aler granitic batholith. Green and purple colors show accept-fit solutions (goodness of fit >5%) and good-fit solutions (goodness of fit >50%), respectively; the rectangular boxes represent the time–temperature constraints; the best-fit path is plotted with a black curve. (a) AFT: Apatite fission track; (b–d) AHe: Apatite (U-Th)/He; (e–i) ZHe: zircon (U-Th)/He; GOF: good of fit; Old: The age (Ma) of the oldest fission track that has not yet been fully annealed.

Figure 8

Graphical representation of temperature versus age (in millions of years, Ma) across six panels labeled a to f. Panel a shows numerous green plots with a fit line, and AFT data indicating 48.13 ± 5.73 Ma. Panels b to f display ZHe data in pink and green with individual GOF values of 1.0. Specific ZHe ages for each panel range from 52.06 ± 2.98 Ma to 82.96 ± 4.59 Ma. Temperature ranges from -20 to 400 degrees Celsius, and age from 0 to 200 Ma.

Figure 8. Results of HeFTy thermal history modeling of the sample A103 from the Aler granitic batholith. Green and purple colors show accept-fit solutions (goodness of fit >5%) and good-fit solutions (goodness of fit >50%), respectively; the rectangular boxes represent the time–temperature constraints; the best-fit path is plotted with a black curve. (a) AFT: Apatite fission track; (b–f) ZHe: zircon (U–Th)/He; GOF: good of fit; Old: The age (Ma) of the oldest fission track that has not yet been fully annealed.

4 Discussion

4.1 Cooling history of the Koktokay district

The previous studies on the uplift of high-elevation low-relief surfaces in the China Altai proposed a rapid cooling in the Cretaceous (Yuan et al., 2006; Jolivet et al., 2007; Vassallo et al., 2007; Pullen et al., 2020). Our results from the Koktokay No. 3 pegmatite and Aler Granite show a history of relative intact cooling/exhumation, roughly consistent with those of the whole China Altai, which shed light on the preservation of rare-metal pegmatite groups. Given the exhumation scenarios of the Koktokay, we further provide constraints on the preservation of the rare-metal pegmatite ore deposit and discuss the potential of pegmatite-type rare-metal mineral exploration in the Koktokay region.

4.1.1 Middle Jurassic to Cretaceous (∼170–120 Ma) rapid cooling of Koktokay

As described above, a significant cooling phase in the Mesozoic in Koktokay district is recorded by the thermal history modeling of our pegmatite and granite samples. This phase of the cooling prevails within the whole Altai, and numerous studies have even documented consistent low-temperature cooling ages from eastern Kazakhstan to Lake Baikal in south-eastern Siberia in the Mesozoic (Glorie et al., 2023; Glorie and De Grave, 2016; McDannell et al., 2018; Glorie et al., 2012; Vassallo et al., 2007). Previous low-temperature thermochronology studies in the Altai shows that different Early Jurassic and Cretaceous AFT ages were obtained from the Mongolia Altai (McDannell et al., 2018; Jolivet et al., 2007; Vassallo et al., 2007). Siberia Altai-Sayan also experienced a rapid cooling phase in the Jurassic period (Glorie et al., 2012). The apatite fission-track thermochronology revealed a cooling phase in the Late Jurassic–Cretaceous (De Grave et al., 2013; Guenthner et al., 2013). All these results investigated from the Altai and adjacent regions imply that this area experienced accelerated cooling during the Mesozoic, while the exhumation process differs spatiotemporally (Wu et al., 2023).

In the Early Mesozoic, there were active intracontinental fault movements in the China Altai. The intracontinental A-type subduction and obduction nappe were associated with strong magmatic activities. A biotite ⁴⁰Ar/³⁹Ar plateau age of 186 Ma was obtained for the Kangbutiebao granite, and a whole-rock Rb–Sr isochrone age of 172.8 Ma was determined for the Aweitan two-mica granite (Hu et al., 2000). These ages reflect tectonic–magmatic mineralization in 170–120 Ma, corresponding to the Cretaceous or late Yanshan period, consistent with the Aler Granite ages of 150.1 ± 12.9 Ma (Figure 4d) and 154.1 ± 14.8 Ma (Figure 4e) from ZFT ages and the Koktokay No. 3 pegmatite of 176 ± 11 Ma from apatite U–Pb age (Figure 5b). Moreover, the muscovite granite and altered biotite granite in Koktokay district also reported ⁴⁰Ar/³⁹Ar plateau ages of 181 Ma and 177 Ma, respectively (Shen et al., 2022). The Early Mesozoic ages of these geobodies are all derived from dating methods using moderate closure temperatures (<350 °C). It indicates that Koktokay district, even the middle Altai, cooled to a moderate temperature range until the Early Mesozoic. Actually, during the formation of the Koktokay Pegmatite Group and the Aler Granite (∼220–210 Ma), the region experienced the end stage of Permian–Triassic high-temperature retrograde metamorphism (Tong et al., 2013; Tong Y. et al., 2014) and the onset of extensive hydrothermal events (Glorie et al., 2023; Zhou et al., 2015a). This makes it exceptionally difficult to constrain the preservation of pegmatite from magma consolidation to the Late Cretaceous period. The rapid cooling rate of >3.0 °C/Ma between approximately 170 and 120 Ma (Figure 6) would be caused at least by the combined effect of the following reasons: 1) in response to the large-scale cooling event of the Mesozoic Altai, which was inferred to be a distant effect from the North China–Eurasia collision (Buslov, 2011; De Grave and Buslov, 2007) or closure of the Mongolo–Okhotsk Ocean (Zorin, 1999); 2) a relatively high temperature (>350 °C) had been maintained by the extensive hydrothermal events in Koktokay district (stated at ∼170 Ma; Glorie et al., 2023; Zhou et al., 2015b). This event had contributed to the mineralization of hydrothermal deposits such as Au–Cu–Pb–Zn (Liu et al., 2014; Zheng et al., 2017). Meanwhile, it ceased at the latest at ∼150 Ma recorded by granite apatite U–Pb (Zhou et al., 2015b). Subsequently, the temperature rapidly cooled down; 3) the crystallization and cooling processes of pegmatitic melt are controversial. One speculation is that the internal temperature of pegmatite remained above the liquidus line until the Jurassic. The thermal relaxation or cooling of the rock mass itself may also contribute to the Early Jurassic apatite U–Pb age of the pegmatite sample. Both reasons 1) and 2) are not conductive to the preservation of rare-metal pegmatite, especially reason 2), where extensive hydrothermal activity is destructive to the active element ore such as Li–Rb–Cs ore. Therefore, the stage ranging from Middle Jurassic to Early Cretaceous is not conducive to the preservation of Koktokay rare-metal pegmatites and may even damage the deposit to a certain extent.

4.1.2 Middle to Late Cretaceous (∼120–66 Ma) moderate rate of exhumation

As shown in Figure 9, a cooling/exhumation episode since the Middle to Late Cretaceous, which is also a widespread cooling event in the China Altai, is recorded in our samples. The cooling rates we obtained are more consistent with those reported in previous studies, rather than the extremely rapid cooling rates of the Early Cretaceous. It indicates complete consolidation of the pegmatite without regional hydrothermal activity. Notably, by summarizing the low-temperature thermochronological data on the whole Altai in the Mesozoic (Glorie et al., 2023), it was found that, at higher elevations (>1,500 m), such as the middle-northern China Altai and Siberia Altai, AFT data showed concentrated ages with narrow fission-track distributions (De Grave et al., 2008; De Grave et al., 2009; Jolivet et al., 2007; Yuan et al., 2006), suggesting rapid cooling rates. Meanwhile, a range of apparent AFT ages and AHe age of pegmatite (>75 Ma) can be observed in moderate topographic relief of Koktokay, reflective of slow cooling at low elevations (<1,500 m). As these rapid Late Cretaceous cooling ages are preserved at high altitude, it indicates that a younger, secondary exhumation event was limited to preserve the Late Cretaceous AFT ages.

Figure 9

Graph depicting temperature in degrees Celsius versus time in millions of years (Ma). It features three lines, labeled A102, KT3, and A103, showing different temperature trends. Time periods are highlighted at the bottom with different colors. Labels AHe, AFT, ZHe, and ZFT indicate different thermal events.

Figure 9. Interpreted thermal history of the Koktokay Pegmatite Group generated in the HeFTy modeling. It is derived from the weighted average path of each sample.

Furthermore, for Koktokay district located in the east of the Kalaxianger Fault, contrasting AFT data are evident on either side of the Kalaxianger Fault (Figure 1). The Kalaxianger dextral transpressional fault is the largest NNW–SSE fault in the China Altai. The average strike of this NE dipping fault is 165° with a dip angle of 65°–85° (Fu et al., 2010). K–Ar (∼276 Ma) and ⁴⁰Ar–³⁹Ar (∼282 Ma) ages from an associated pseudotachylite indicate that the earliest movement of this fault can be traced back to at least the Permian (Briggs et al., 2009). The samples taken to the east of the Kalaxianger Fault recorded rapid Late Cretaceous exhumation, while samples to the west did not. Away from the WNW–ESE-striking faults associated with the Irtysh Shear Zone, Late Jurassic–Early Cretaceous thermal processes underwent gradual cooling. The differential factor of AFT ages and cooling with respect to the Kalaxianger Fault is observed in the Late Mesozoic as samples from the more uplifted west of the fault preserved shorter apatite mean fission track lengths (<13 μm; Glorie et al., 2023), suggesting preservation of a single substantive cooling event in the Late Cretaceous, as opposed to a mixed uplift history including later differential uplift during the Cenozoic (Fu et al., 2010; Yuan et al., 2006). Therefore, it is considered that the Kalaxianger Fault was active during the Late Cretaceous (Glorie et al., 2023). Due to the differential uplift, topography preserved to the west of the Kalaxianger Fault is lower than that to the east, which, to some extent, caused erosion and exhumation of shallow pegmatite in the Koktokay district. Late Cretaceous exhumation is coeval with a series of far-field tectonic events, although the precise cause of reactivation within the Altai remains unclear. This exhumation coincided with the collapse of the Mongol–Okhotsk Orogen to the east and slab rollback in the Tethys Ocean to the south, both occurring approximately 100–80 Ma (Yin et al., 2019; Glorie et al., 2019; Ma et al., 2013; Metelkin et al., 2012; Dilek and Furnes, 2009; Jolivet et al., 2009). These events are potential drivers of stress propagation into the continental interior, leading to reactivation and exhumation. However, the current data do not clarify the extent to which these factors influenced reactivation within the Altai.

4.1.3 Cenozoic slow exhumation process

Following the Mesozoic, there was a slow and slight exhumation in the studied area (Figure 9). Many scholars consider that much of the modern topography was formed in response to deformation caused by the India–Asia collision (Molnar and Tapponnier, 1975; Sobel et al., 2006). The Altai Mountains experienced enhanced stress caused by the India–Asia collision, which reactivated preexisting basement structures and led to extensive uplift (Yuan et al., 2006; Jolivet et al., 2007; Vassallo et al., 2007). Moreover, the Cenozoic deformation and stable isotope evidence also support surface uplift since the early Oligocene (Caves et al., 2017; Caves et al., 2014; De Grave and Buslov, 2007). However, on the one hand, the low elevation (<1,500 m) of the China Altai orogenic belt is relative to other ranges of whole Altai. The majority of pre-Mesozoic basement rocks in Altai, and even across Central Asia, show evidence of Mesozoic low-temperature thermochronology, whereas Cenozoic rapid cooling and exhumation are observed only in very localized regions with high elevation (Wu et al., 2023; He et al., 2022; Gillespie et al., 2021; Jepson et al., 2018; Wang et al., 2007). On the other hand, crustal deformation alone is not a sufficient and necessary condition for driving exhumation, and the topographic and denudational evolution of orogenic belts are also strongly coupled with climate (Jepson et al., 2021; Pullen et al., 2020). Aridity has been connected to mountain building in CAOB since the Late Cretaceous (Jepson et al., 2021), which was also marked by the sedimentary records in the adjacent Junggar basin. Since the Late Cretaceous, the Junggar basin has been affected by global climate cooling and uplift of the Tibetan plateau, and the degree of dryness has been continuously intensified (Wang et al., 2019). On the contrary, during the Late Triassic to Early Jurassic, humidity is thought to have reached a maximum in the Junggar basin and adjust area, as evidenced by the retrogradation of deltaic and fluvial facies and the expansion of deep-water facies (Yang et al., 2015). The recorded humid climate coincided with the large-scale Mesozoic exhumation in the Koktokay area, while the arid climate limited denudation during the Cenozoic. The earliest Paleogene ages of (U–Th)/He in this study also contribute to minimal denudation in the China Altai since the Late Cretaceous (Caves et al., 2014; Caves et al., 2017), which is conductive to the preservation of the Koktokay Pegmatite Group.

4.2 Estimation of Koktokay rare-metal pegmatite group exhumation

The uplift and exhumation history of deposit is significant evidence to estimate the potential for further deep exploration. Thermal history modeling has been used to estimate the exhumation rates and erosion thickness of the Koktokay Pegmatite Group. Although the Koktokay Pegmatite apatite U–Pb age limits the rapid cooling since the Late Jurassic (∼176 Ma; Figure 5a), the ⁴⁰Ar/³⁹Ar plateau ages of muscovite granite and altered biotite granite in the Koktokay area support this onset of cooling (181 Ma and 177 Ma, respectively; Shen et al., 2022). The recorded ages of the above dating tools, with closure temperatures of ∼400 °C, probably fall within a delicate evolutionary range of the fluid stage before complete crystallization of pegmatite (Thomas and Davidson, 2015; London, 2018). In addition, the Early Jurassic extensive hydrothermal event in Koktokay (Glorie et al., 2023; Zhou et al., 2015b) interfered with the calculation of exhumation in this stage. Therefore, we here calculate exhumation rates since ∼150 Ma with constraints by ZFT ages (Figures 4d,e).

The constant geothermal gradient is assumed to be 30 °C ± 2 °C/km. We utilized various low-temperature geochronological tools to reconstruct the thermal history of the Koktokay Pegmatite Group area, and the predominant exhumation stage in the Cretaceous has been identified. Based on the obtained thermal history paths (Figure 9), we estimated the maximum and minimum temperature changes of each stage, divided by the assumed geothermal gradient, to derive the exhumation thickness. Subsequently, we divided the exhumation thickness by the duration of rapid uplift to estimate the rate. The results indicate a temperature change of ∼110 °C ± 20 °C, an exhumation thickness of 3.6 ± 0.65 km, and an exhumation rate of 0.046 ± 0.008 km/Ma during the Cretaceous. After this time, there is a final significant cooling stage between the Paleocene and the Eocene, with a temperature change of 25 °C ± 5 °C, an exhumation thickness of 0.83 ± 0.16 km, and an exhumation rate of 0.027 ± 0.005 km/Ma. Therefore, the total thickness of exhumation in the Koktokay Pegmatite Group area since ∼150 Ma is approximately 4.43 ± 0.81 km, which is shallower than the crystallization pressure constrained by fluid inclusions in the Koktokay Pegmatite (6.8–11.4 km, Zhou et al., 2015b). The reason for this difference is the lack of calculation of the exhumation thickness during the Late Triassic to Early Cretaceous in the Koktokay Pegmatite Group.

Generally, the calculated exhumed thickness of the Koktokay Pegmatite Group is not conducive to the preservation of rare-metal pegmatites. Furthermore, the Jurassic extensive hydrothermal events and potential exhumation from 220 Ma to 150 Ma also contribute to the damage of the pegmatite group. According to the mineralization model of pegmatites, the rare-metal-mineralized pegmatites usually formed at the distal end in the pegmatite group, especially Li–Rb–Cs mineralized pegmatite. Meanwhile, the Be-mineralized pegmatite formed near the end within the pegmatite group (London, 2018; Dill et al., 2012). The preservation history of the Koktokay Pegmatite coincided with the exploration situation, that is, the scale of Li–Rb–Cs-type pegmatite in Koktokay is limited, while that of Be-type pegmatite is huge. In addition, thermochronology study suggest a bad prospect for Li–Rb–Cs-type pegmatite exploration in the Late Triassic Koktokay Pegmatite Group.

5 Conclusion

In this contribution, the samples of the No. 3 pegmatite and the Aler granitic batholith are collected and a series of low-temperature thermochronology analyses and thermal history modelling were applied to explore the denudation and exhumation history of the Koktokay rare-metal pegmatite group and further to provide guidance for subsequent exploitation in this region.

It clearly shows a consistent younger trend of our low-temperature thermochronology apparent ages from the high to low closure temperature of different thermochronology tools, with an apatite U–Pb age of ∼176 Ma, a ZFT age of ∼150 Ma, a ZHe age of ∼70 Ma, an AFT age of ∼60 Ma, and an AHe age of 50 Ma. The thermal history modeling results present that the Koktokay Pegmatite Group went through two predominant phases of rapid cooling, occurring in the Middle Jurassic to the Early Cretaceous and the Middle-to-Late Cretaceous. The earlier phase is coupled with the cooling event in the whole Altai and the extensive hydrothermal event in the Koktokay area. The disturbances caused by multiple thermal events make it impossible to calculate exhumation thickness during this stage, but they can be determined to be harmful to the preservation of pegmatite. The later phase would be related to the activity of the Kalaxianger Fault and a series of far-field tectonic events (such as the collapse of the Mongol–Okhotsk Orogen to the east and slab rollback in the Tethys Ocean to the south). A total of ∼4.43 ± 0.81 km has been denuded since ∼150 Ma, with the Middle-to-Late Cretaceous period accounting for the majority of denudation. Overall, the denudation of the Koktokay Pegmatite Group is relatively intense, which is unfavorable for the preservation of rare-metal pegmatite bodies.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material; further inquiries can be directed to the corresponding author.

Author contributions

WH: Conceptualization, Formal Analysis, Investigation, Methodology, Resources, Software, Validation, Writing – original draft, Writing – review and editing. RG: Funding acquisition, Project administration, Writing – review and editing. TY: Data curation, Funding acquisition, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing. ZH: Funding acquisition, Supervision, Writing – review and editing. LZ: Writing – review and editing, Investigation. HZ: Conceptualization, Writing – review and editing.

Funding

The author(s) declare that financial support was received for the research and/or publication of this article. This work was financially supported by the China Geological Survey Project (Grant No. DD20240033) and the National Science Foundation of China (Grant Nos. 91962222 and 41773053).

Conflict of interest

The 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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