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Article

Geochemistry, Mineralogy, and Geochronology of the NYF Pegmatites, Jiaolesayi, Northern Qaidam Basin, China

by
Long Zhang
1,2,
Xianzhi Pei
1,*,
Yongbao Gao
2,
Zuochen Li
1,
Ming Liu
2,
Yongkang Jing
2,
Yuanwei Wang
2,
Kang Chen
2,3,
Nan Deng
2,
Yi Zhang
2 and
Junwei Wu
2
1
School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
2
Innovation Center for Gold Ore Exploration, Xi’an Center of Mineral Resources Survey, China Geological Survey, Xi’an 710100, China
3
State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(8), 805; https://doi.org/10.3390/min14080805
Submission received: 9 July 2024 / Revised: 2 August 2024 / Accepted: 8 August 2024 / Published: 9 August 2024
(This article belongs to the Special Issue Rare Metal and Related Deposits: Geology, Geochemistry and Mineralization)
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Abstract

:
A significant amount of pegmatite has been discovered on the northwest margin of the Qaidam basin. Among this, the Jiaolesayi pegmatite, located in the northwestern margin of the Quanji Massif (Oulongbuluke micro-continent), shows rare element mineralization potential. Detailed field investigations, along with mineralogical, geochemical, and zircon U-Pb geochronological studies, were carried out on the pegmatite. The results show that the Jiaolesayi pegmatite is syenite, without obvious compositional zoning in the outcrop. It exhibits a peraluminous, high-K calc-alkaline nature with strong depletions in Eu, Sr, Ba, Ti, and P, and high contents of Nb, Ta, Y, Ti, U, Th, and heavy rare earth elements (HREEs), which are primarily concentrated in allanite-(Ce), euxenite-(Y), limonite, thorite, and zircon. The geochemical and mineralogical features of the syenite pegmatite indicate it belongs to the euxenite-type in the rare element class (REE) of the NYF family, with the characteristic accessory mineral being euxenite-(Y). Its 10,000 Ga/Al ratios (2.46 to 2.96), Zr + Nb + Ce + Y contents (998 to 6202 ppm), Y/Nb ratios (0.62 to 0.75), and Yb/Ta ratios (0.80 to 1.49) show an affinity with A1-type granite. Zircons from the syenite sample yielded a weighted mean 206Pb/238U age of 413.6 ± 1.4 Ma, while the elevated U and Th concentrations in the zircons and Th/U ratios (0.04 to 0.16) suggest the possible influence of hydrothermal processes in the late-stage fractional crystallization. In the context of the regional tectonic evolution, the syenite pegmatite may have formed from a basic alkaline magma derived from an OIB-like melt with minor crustal contamination, under the post-collisional extension setting.

1. Introduction

Competition among countries for strategic resources such as lithium (Li), niobium (Nb), tantalum (Ta), and rare earth elements (REEs) is intensifying due to their critical role in emerging technologies and industries [1]. About 1%–2% of all pegmatites contain these rare elements, making these pegmatite deposits have significant economic value [2]. Pegmatites are primarily classified into two chemical families based on their distinctive rare element enrichment: the LCT (Lithium–Cesium–Tantalum) family and the NYF (Niobium–Yttrium–Fluorine) family [3,4]. Recent studies [5,6,7] have also proposed a mixed family (LCT + NYF) for pegmatites exhibiting characteristics of both.
Despite advancements in classification, LCT pegmatites dominate in both quantity and subtype diversity among rare element pegmatites. They also represent the most significant type of rare earth deposits and have become a primary focus of research [8]. LCT pegmatites are enriched in Li, Cs, and Ta, exhibit peraluminous features, and show compositional affinity with S-type granites [9]. They are believed to form during the late-stage crystallization differentiation of granitic melts, primarily in late orogenic and post-orogenic settings [4]. Globally renowned deposits such as Bikita [10] in Zimbabwe; Tanco [11] in Canada; Greenbushes [12] in Australia; and Dahongliutan [13] in China all belong to the LCT family. In contrast, NYF pegmatite signatures are relatively obscure. Pegmatites that fit this type enrich Nb, Ta, Y, Zr, U, Th, HREEs, and F. They often contain exotic minerals such as gadolinite, euxenite-(Y), polycrase-(Y), aeschynite-(Y), samarskite, thorite, allanite-(Ce), or fergusonite [14,15,16,17,18], and rarely exhibit regional zoning patterns [8,19,20]. NYF pegmatites typically show subaluminous or peraluminous characteristics, are closely associated with A-type granites, and form under non-orogenic settings [4]. NYF pegmatites also hold significant value for rare element resources, for instance, the Strange Lake deposit in Canada hosts over one million tons of high-grade REEs, ZrO2, Nb2O5, and BeO [21].
Since 2020, researchers have discovered over two hundred pegmatites with rare element mineralization potential in the western Quanji Massif, the northwest margin of the Qaidam basin (Figure 1C–E). These pegmatites were identified in the Niubiziliang, Dachaigou, and Jiaolesayi areas using remote sensing interpretation and identification technology [13,22]. Many pegmatites exhibit Nb, Ta, Rb, and HREE mineralization. The largest continuous vein verified in the field extends approximately 7 km. Previous researchers [23] reported a small Nb-Ta deposit in the Jiaolesayi area, but detailed studies were not conducted. Recently, the authors found several rare element-bearing pegmatites while examining the Nb-Ta deposit. Besides known Nb-Ta enrichment, the newly discovered pegmatites also significantly enrich Zr, Y, U, Th, and HREEs, showing the characteristics of NYF pegmatite. Therefore, through petrological, mineralogical, and geochemical analyses, combined with electron probe microanalysis (EPMA) and zircon U-Pb dating, this study aims to determine the geological features, mineral assemblages, possible petrogenesis, and geochronology of newly discovered rare element-bearing pegmatites in Jiaolesayi.

2. Geological Background

2.1. Regional Geology

The Jiaolesayi area is on the north margin of the Quanji Massif (Figure 1B), close to the Altun Tagh Fault which marks the northern margin of the Qinghai–Tibet Plateau [24]. The Quanji Massif, also known as the Oulongbuluke micro-continent, is one of several micro-massifs situated between the North China Craton (NCC) and Tarim Craton (TC) [25,26,27]. It extends WNW-ESE for nearly 500 km and measures about 50–100 km wide. Bounded by the Qilian orogenic belt to the northeast and the early Paleozoic north Qaidam high- to ultrahigh-pressure (HP-UHP) belt to the southwest [28,29] (Figure 1A), the Quanji Massif features a Paleoproterozoic medium to high-grade metamorphosed crystalline basement overlain by unmetamorphosed Neoproterozoic to Mesozoic strata [25,28,30,31,32]. This typical cratonic double-layered structure along with similarities to the Tarim Craton in terms of stratigraphic sequences, magmatic evolution, and metamorphic histories [33], suggests that this narrow ancient cratonic fragment was once part of the TC [26,34].
The metamorphic basement of the Quanji Massif is mainly composed of the Delingha complex and the Dakendaban Group paragneisses. The Delingha complex is primarily a suite of metamorphosed granitoids with ca. 2.41 Ga [35] amphibolite enclaves, grouped into Quanjishan, Delingha, Mohe, and Hudesheng granitoid plutons [36,37]. The Dakendaban Group comprises amphibolite-facies volcano-sedimentary rocks for its lower subgroup and metasedimentary rocks for its upper subgroup. The primary rock types include graphite–marble, sillimanite–garnet–biotite–quartz schist, graphite–biotite–quartz schist, amphibolite–plagioclase schist, garnet–biotite–plagioclase gneiss, and some garnet–amphibole pyroxene granulite lenses [38]. These aluminum-rich metamorphic rocks are identified as khondalite-series and have suffered multiple regional and dynamic metamorphic events [39,40]. The unconformably overlaying sedimentary cover, the Quanji Group, mainly consists of quartz sandstone with minor tuff beds [29]. Its maximum deposition age is determined at ca.1.73 Ga [41]. The long-term left-lateral strike-slip activity [42] of the Altun Tagh Fault has induced significant structural deformations in the adjacent Quanji Massif, resulting in intense deformation of the rocks. Combined with multiple metamorphic events, the original geological features within the rocks have been completely altered. The orientation of structural fabric within the Dakendaban Group is notably shifted from predominantly N-S to NEE-SWW as it approaches the Altun Tagh Fault.
The Silurian diorite is the only known massive intrusion in Jiaolesayi, as Mesozoic–Cenozoic sediments covered almost half of the area (Figure 2). Additionally, there are several small granites, granodiorite, alkali feldspar granite, granitic pegmatites, and quartz veins, including the previously reported Nb-Ta-bearing pegmatite, intruded in the Dakendaban Group and the Silurian diorite. Most of their ages remain uncertain due to the limited investigation, while the early studies suggested these veins formed during the Yanshanian Movement [23], without detailed geochronology data. However, this hypothesis has yet to be verified.

2.2. Geological Characteristics of Jiaolesayi Pegmatite and Sample Description

Jiaolesayi pegmatites are generally exposed as lenses intruded into the garnet–biotite–plagioclase gneiss and marble of the Dakendaban Group, with clear boundaries and no signs of migmatization or anatexis. Four primary outcrops are located within a 2 km range from west to east, with widths ranging from 10 to 80 m (Figure 2). The previously reported Nb-Ta deposit is at the western end (Figure 3A,B), while the newly found pegmatite is at the eastern end. Pervasive joints and brittle fractures are well-developed in the pegmatite due to the activities of the Altun Tagh Fault (Figure 3C). Since the newly discovered pegmatite is close to the regional NEE-NSS trending faults, its northern margin exhibits brecciation and has developed some quartz veins that indicate hydrothermal activities caused by later tectonic activities (Figure 3E). Magnetites are widespread in the pegmatite, with a maximum grain size exceeding 3 cm (Figure 3D), and exhibit strong magnetic properties. No obvious mineral zoning features were observed in the pegmatite outcrops, distinguishing them from the significant zoning patterns of LCT-type pegmatite.
To investigate the rare element mineralization of the pegmatite, eight fresh pegmatite samples (D3301-1 to D3301-8) were collected from the newly discovered outcrop (Figure 2). All samples underwent bulk rock major and trace element analyses. D3301-1 was selected for zircon U-Pb dating. D3301-1 and D3301-4 were chosen for petrographic observation and EPMA analyses. These samples are generally flesh-red or brick-red in color, displaying a massive structure (Figure 3C) and typical graphic texture (Figure 3D). They mainly consist of fine- to coarse-grained, subhedral-to-euhedral alkali feldspar, orthoclase, quartz, mica, biotite, and plagioclase. The maximum grain size of orthoclase is about 8.3 cm, and quartz exceeds 10 cm. In the photomicrographs, quartz, alkali feldspar, and orthoclase exhibit typical graphic structures (Figure 3G,H), and sericite indicates the sericitization of plagioclase (Figure 3I). Accessory minerals including magnetite, allanite, zircon, limonite, pyrite, and thorite are observed under a reflecting microscope.

3. Analytical Methods

3.1. Major and Trace Element Analyses

Major element compositions were determined using a Zetium X-ray fluorescence spectrometer (XRF) manufactured by Malvern Panalytical Ltd., Almelo, The Netherlands, at the Yanduzhongshi Geological Analysis Laboratories Ltd., Beijing, China. The analytical errors for major elements were better than 1%. Trace elements of these samples were analyzed using an Aurora M90 inductively coupled mass spectrometer (ICP-MS) manufactured by Analytik Jena Ltd., Jena, Germany, with an analytical uncertainty better than 5%. The obtained trace element values in the GSR-2 standard are all consistent with their recommended values.

3.2. LA-ICP-MS Zircon U-Pb Dating

Zircon U-Pb dating and element analysis were conducted using a Coherent GeoLasPro 193 nm laser ablation system, Saxonburg, PA, USA, coupled to an Agilent 7700× ICP-MS at the Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, Xi’an Center of Geological Survey (CGS), China. Helium was the carrier gas, and argon was used as the make-up gas to adjust sensitivity. The laser beam is focused at 36 µm. Each analysis included a background acquisition of approximately 10 s (gas blank), 40 s of sample signal acquisition, and 10 s for rinsing. U-Pb dating employed 91,500 and GJ-1 zircon standards, with common lead correction based on measured 204Pb [43]. Trace element compositions of zircons were calibrated using artificial synthetic silicate glass NIST610 of the American National Standard Substance Bureau as an external standard, with Si as the internal standard. Data analysis and element concentration calculations were performed using Glitter (ver. 4.4) [44]. The recommended element content values for NIST glass are sourced from the GeoReM database (http://georem.mpch-mainz.gwdg.de accessed on 28 March 2022). Detailed analytical procedures were described by Li [45].

3.3. EPMA Analysis

Mineral compositions were determined using a JEOL JXA-8230 Electron Probe Micro Analyzer (EPMA), Tokyo, Japan, equipped with four wavelength-dispersive spectrometers at the Yanduzhongshi Geological Analysis Laboratories Ltd., Beijing, China. Before the analyses, the samples were coated with a ca. 20 nm thin conductive carbon film. The analyses include an accelerating voltage of 15 kV, a beam current of 20 nA, and a 5 µm spot size. Natural minerals and synthetic oxides were used as standards. Data were corrected online using a modified ZAF (atomic number, absorption, fluorescence) correction procedure. Detailed EPMA methods are based on Yang [46].

4. Results

4.1. Major and Trace Elements

Whole-rock major and trace element compositions of the Jiaolesayi pegmatite samples are listed in Supplementary Table S1.
The Jiaolesayi pegmatite samples (D3301-1 to D3301-8) have medium SiO2 (60.61~63.02 wt.%) and Al2O3 (20.53~21.14 wt.%) contents, both in a narrow range, and high total alkali abundances (Na2O + K2O = 9.82~11.26 wt.%), with low MgO (0.07~0.18 wt.%), FeOT (1.04~2.68 wt.%), MnO (0.04~0.05 wt.%), TiO2 (0.09~0.35 wt.%), and P2O5 (<0.01~0.10 wt.%) contents. All samples plot in the syenite field of the TAS diagram (Figure 4A) and show weakly peraluminous characteristics in the A/NK vs. A/CNK diagram (A/CNK values of 1.00~1.05) (Figure 4B). They are located in the high-K calc-alkaline field in the K2O vs. SiO2 diagram (Figure 4C).
The chondrite-normalized rare earth element (REE) patterns (Figure 4D) of the syenite pegmatite samples exhibit a medium left-sloping REE distribution, showing strong enrichment of HREEs, with (La/Yb)N ratios ranging from 0.14 to 0.37, and pronounced negative Eu anomalies (Eu/Eu* = 0.09–0.15). The ΣREE (total REE concentrations) of all samples range from 162 to 1286 ppm. On the primitive mantle-normalized trace element spider diagrams (Figure 4E), all samples exhibit similar wave-like patterns, characterized by pronounced negative anomalies of Ba, Sr, P, and Ti, and strong enrichment in large ion lithophile elements (LILEs, e.g., Cs and Rb) and high field strength elements (HFSEs, e.g., Nb, Ta, Zr, Hf, U, and Th).

4.2. Zircon U-Pb Geochronology

Zircons from the syenite pegmatite sample D3301-1 are subhedral to euhedral, transparent, and dark brown (Figure 5A). Most of them are incomplete crystal fragments. The complete ones have lengths ranging from 100 to 300 μm and widths from 50 to 150 μm, with a length/width ratio between 3:1 and 1.5:1. Core mantling structures and weak oscillatory zoning in the rims are visible. Their cathodoluminescence (CL) images are opaque and spongy (Figure 5A), which is common when zircons contain elevated rare elements [51]. The uranium (U) concentrations of the zircons range from 5795 to 12,393 ppm, thorium (Th) contents vary from 268 to 1169 ppm, and the Th/U ratios range from 0.04 to 0.16, indicating the possible influence of metamorphism or hydrothermal processes [52]. Therefore, the obtained data have been carefully checked, and 24 out of 30 concordant data points were chosen, resulting in a weighted mean 206Pb/238U age of 413.6 ± 1.4 Ma with a mean square weighted deviation (MSWD) of 1.2 (Figure 5B). Zircon data are listed in Supplementary Table S2.

4.3. EPMA Analysis

Under reflected light microscopy, some anhedral to euhedral, transparent dark brown, columnar, or needle-like minerals were observed in samples D3301-1 and D3301-4. They are embedded in the blocky K-feldspar and quartz with a grain size of 100 to 500 μm (Figure 6A–C,G). EPMA later identified these minerals as euxenite-(Y). Their chemical compositions mainly consist of Nb2O5 (28.88 to 32.64 wt.%), TiO2 (21.97 to 24.11 wt.%), Y2O3 (15.65 to 21.20 wt.%), UO2 (5.52 to 11.88 wt.%), ΣREE (7.16 to 8.70 wt.%), ThO2 (3.06 to 4.22 wt.%), and Ta2O5 (1.05 to 2.57 wt.%). Other typical minerals in NYF pegmatites, including zircon, thorite, limonite, and allanite-(Ce), were also observed and identified (Figure 6C–G). The thorite is translucent yellow-brown, subhedral, with a grain size over 200 μm and ThO2 content up to 71.96 wt.%. The allanite-(Ce) is significantly enriched in REE (ΣREE content 20.54 to 24.95 wt.%) with Ce2O3 up to 12.00 wt.%. The limonite may be the oxidized product of pyrite from the late mineralization period. These HREE-bearing minerals could explain the significant enrichment of HREE in the chondrite-normalized REE patterns.
In the BSE images (Figure 6K,L), euhedral euxenite-(Y) grains appear almost homogenous without any obvious zoning. Some cracks and fractures are well-developed, extending into the host minerals (Figure 6H,I). High concentrations of UO2 and ThO2 in the euxenite-(Y) could produce radiogenic helium, which may form nano-bubbles within the crystal. These bubbles can contain pressures up to 500 MPa, causing fractures in the euxenite-(Y) and the surrounding minerals [53]. Detailed EPMA data are listed in Supplementary Table S3.

5. Discussion

5.1. Pegmatite Classification and Petrogenetic Type

Pegmatites are categorized based on their distinctive characteristics, such as the host rock’s metamorphic environment, mineralogy, element composition, and internal structure [14]. Since the subdivision of pegmatite classes was first proposed in 1979 [54], the classification of granitic pegmatites has become increasingly complex and detailed. The acknowledged LCT and NYF families have now been extended into the class–subclass-type–subtype hierarchy to describe their specific features and diversities [14].
In the rare element class of the NYF family, euxenite-type pegmatites typically contain aeschynite-group minerals (AGMs) and euxenite-group minerals (EGMs). These Y, REE, Nb, Ta, and Ti-bearing minerals share a common formula of AB2O6 and exhibit similar mineralogical characteristics, making them indistinguishable in hand specimens or under the microscope. Therefore, canonical discrimination analysis is used to distinguish between the AGMs and EGMs [55,56].
In the syenite pegmatite samples, those columnar, or needle-like minerals, with chemical compositions acquired by the EPMA (N = 9), were plotted in the EGM field (Figure 7A) and identified as euxenite-(Y) in the triangular discriminant graph (Figure 7B). According to the classification of Černý and Ercit [4], the geochemical and mineralogical features of syenite pegmatite indicate its affinity to the euxenite-type in the rare element class (REE) of the NYF family, with the characteristic accessory mineral being euxenite-(Y).
Most NYF pegmatites are associated with A-type granite, sharing similar features with their granite origin. While the definition and petrogenesis of A-type granite, first proposed in 1979 [59], remain debated [60], there is consensus on certain characteristics based on previous studies. These characteristics include high Fe*, K2O, Ga/Al ratios, enrichment in fluorine, high field strength elements (HFSEs), REEs, and low concentrations of Al2O3, Sr, Ba, Ti, P, with pronounced negative Eu anomalies [61,62,63,64].
However, whether syenite belongs to the A-type granite remains controversial due to its relatively low SO2 content. The Jiaolesayi syenite pegmatite’s SO2 content ranges from 60.61 to 63.2 wt.%, within the SO2 content range (60.4 to 79.8%) of the initial definition of the A-type granite [57]. Moreover, the syenite pegmatites are significantly enriched in HSFEs and REEs, exhibiting low tetrad effect in REE patterns (Figure 4D), and displaying some non-CHARAC (CHArge- and RAdius-Controlled) trace element ratios such as Zr/Hf (11.83 to 14.60), Nb/Ta (11.24 to 19.17), K/Rb (42.51 to 57.50), and La/Nb (0.01 to 0.04) [65]. Their 10000 Ga/Al ratios range from 2.46 to 2.96, and Zr + Nb + Ce + Y contents range from 998 to 6202 ppm, typical of A-type granite [57,62,64,66] (Figure 7C,E). Combined with their peraluminous, high-K calc-alkaline nature (Figure 4C), and the high content of Nb-Ta, low Y/Nb (0.62 to 0.75) and Yb/Ta (0.80 to 1.49) ratios, the Jiaolesayi pegmatites can be classified as A1-type granite in the ternary plot of Nb-Y-Ce after Eby [56] (Figure 7D). In some instances, Ga enrichments could be observed in the highly fractionated S- and I-type granites [67]. However, the Jiaolesayi syenite pegmatites have a shallow content of P2O5 (0.01~0.10 wt.%), which is significantly different from the fractionated S-type granite [68,69]. Their enrichment of HFSEs distinguishes them from the HFSE-depleted I-type granite [57]. Therefore, we consider the Jiaolesayi syenite pegmatites indicative of A1-type affinity [60].

5.2. Petrogenesis and Tectonic Setting

Granitic pegmatites are primarily formed by fractional crystallization of granitic magmas or by anatexis during high-grade metamorphism [70]. Although the Jiaolesayi pegmatites occur within the amphibolite- to granulite-facies metamorphosed Dakendaban Group, no migmatization or anatexis veins are observed along the contact boundaries, distinguishing them from pegmatites of anatexis origin [71]. Additionally, the syenite pegmatites have very low MgO (<0.18%) and P2O5 (<0.10%) contents, along with strong negative anomalies in Eu, Sr, P, and Ti, indicating intensive crystal fractionation [66,72]. Therefore, the Jiaolesayi syenite pegmatites are likely formed by fractional crystallization of their parental magma.
Generally, syenite/trachytic rocks are considered to form through three possible processes: fractional crystallization of the mantle-derived magma [73,74], anatexis of the thickened lower crust [75], and mixing of mafic and felsic magmas [76]. To reveal the geochemical signatures of the parental magma, incompatible element ratios such as Nb/Yb, Th/Yb, Y/Nb, and Yb/Ta are used as geochemical indicators due to their resistance to fractionation during partial melting and fractional crystallization [77].
As shown in Figure 8, the syenite pegmatite samples fall into the MORB-OIB array in the Nb/Yb vs. Th/Yb plot and the OIB-A1-type granite field, exhibiting OIB-like geochemical properties. This suggests a possible mantle-derived parental magma. The enrichment of incompatible elements (e.g., Nb, Ta) in the syenite pegmatite, with high Nb/La ratios (27 to 67), also supports a mantle-deep source [72,78]. Furthermore, melts derived from lower crust typically have high Sr/Y ratios (>40) [79], while the Jiaolesayi syenite pegmatites show low Sr/Y ratios (0.07 to 0.65), suggesting insignificant crustal material involvement. The elevated Nb and Ta contents also indicate minor crustal contamination, as Nb and Ta would significantly decrease during the crustal contamination process [80].
Combined with the restricted SiO2 content, high alkali, and weakly peraluminous nature, the syenite pegmatite is likely formed from a basic alkaline magma derived from OIB-like melt, with minor crustal contamination. However, constraints on the magma source provided by incompatible elements are limited. It is necessary to combine these with isotopic characteristics of elements such as Pb, Nd, and Hf to more accurately reveal the magma source and the processes of magma evolution.
Previous studies suggest that the North Qaidam UHP continental subduction belt formed at ca. 440 to 420 Ma, during the transition from subduction to continental collision between the Qaidam and Central Qilian Block [81,82]. The Quanji Massif, located on the northern margin of the Qaidam Block, experienced post-collisional extension at ca. 415 Ma. This is indicated by the formation of the Subei pluton, which formed through low-pressure processes accompanied by a significant influx of mantle-derived OIB-like magma during the crustal thinning [83]. Therefore, under the same tectonic regime, the parental magma of the Jiaolesayi syenite pegmatite could have been derived from the same source, explaining the OIB-like geochemical signature in the syenite pegmatite, and experienced the protracted fractional crystallization with minor crustal contamination.

5.3. Implications for REE Mineralization in the Northwest Margin of the Qaidam Basin

NYF-type pegmatites have significant metallogenic potential to form rare earth/rare element deposits. The Jiaolesayi syenite pegmatite is the first reported NYF-type pegmatite in the western Quanji Massif in the northern margin of the Qaidam basin. The characteristic accessory minerals in the pegmatite, such as euxenite-(Y) and thorite, can be used to extract Nb, Ta, U, and other rare earth elements, providing great economic value. During 410–420 Ma, the Quanji Massif was under an extensional tectonic setting, providing a favorable geological environment for the formation of NYF-type pegmatites. The discovery of the Jiaolesayi syenite pegmatite has indicative significance for the exploration of rare earth/rare element mineralization in the northern margin of the Qaidam basin.

6. Conclusions

The Jiaolesayi syenite pegmatite contains high contents of Nb, Ta, Y, Ti, U, Th, and HREE, forming typical rare element-bearing minerals such as allanite-(Ce), euxenite-(Y), limonite, thorite, and zircon. The geochemical and mineralogical features of the syenite indicate it belongs to the euxenite-type in the rare element class (REE) of the NYF family, with the characteristic accessory mineral being euxenite-(Y) and showing an affinity with A1-type granite. The syenite pegmatite may have formed from a basic alkaline magma derived from an OIB-like melt, with minor crustal contamination, under the post-collisional extension setting at ca. 413.6 ± 1.4 Ma.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14080805/s1, Table S1: Bulk rock major and trace element geochemical data for Jiaolesayi pegmatite; Table S2: The LA-ICP-MS analytical results of U-Pb isotopes for zircons of Jiaolesayi pegmatite; Table S3: EPMA compositions of minerals from the pegmatite samples in Jiaolesayi.

Author Contributions

Conceptualization, L.Z. and X.P.; methodology, Y.G. and M.L.; software, Y.W.; validation, N.D., Y.Z. and Y.J.; formal analysis, K.C.; investigation, L.Z., Y.J., N.D., Y.Z. and J.W.; data curation, Y.J.; writing—original draft preparation, L.Z.; writing—review and editing, Z.L.; visualization, L.Z.; funding acquisition, L.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Natural Science Basis Research Program of Shaanxi (Grant No. 2023-JC-QN-0342), the China Geology Survey project (Grant No. DD20211551 and DD20243309), and the Youth Innovation Team of Shaanxi Universities (Grant No. 2022-36).

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgments

The authors thank Zonghui Li and Huanhuan Wu of the Xi’an Center of Mineral Resources Survey, CGS, for their guidance, and appreciate all the editors and reviewers for their constructive suggestions.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Location of the study area (based on Google Earth image). (B) Geological map of the northwest margin of Quanji Massif. Figure 2 is shown as red rectangle. (C–E) Remote sensing images of pegmatites in Niubiziliang, Dachaigou, and Jiaolesayi (unpublished images from Xi’an Center of China Geological Survey, 2020).
Figure 1. (A) Location of the study area (based on Google Earth image). (B) Geological map of the northwest margin of Quanji Massif. Figure 2 is shown as red rectangle. (C–E) Remote sensing images of pegmatites in Niubiziliang, Dachaigou, and Jiaolesayi (unpublished images from Xi’an Center of China Geological Survey, 2020).
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Figure 2. Simplified geological map of Jiaolesayi.
Figure 2. Simplified geological map of Jiaolesayi.
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Figure 3. Photographs and photomicrographs of pegmatite outcrops and rock samples from Jiaolesayi. (A) Previous reported Nb-Ta deposit, represented by the blue star in Figure 2. (B) Newly discovered pegmatite, represented by the green star in Figure 2, a person in the yellow circle as a scale. (C) Conjugated joints and brittle fractures developed in the flesh-red pegmatite. (D) Graphic structure and magnetite aggregates in the pegmatite. (E) Quartz veins intruded into the brecciated pegmatite. (F) Perthitic texture of perthite under a polarizing microscope. (G,H) Photomicrographs of graphic structure under a polarizing microscope. (I) Pegmatite with sericitization under a polarizing microscope. Abbreviations: Afs—alkali feldspar; Mc—mica; Or—orthoclase; Pl—plagioclase; Pth—perthite; Qz—quartz; Ser—sericite.
Figure 3. Photographs and photomicrographs of pegmatite outcrops and rock samples from Jiaolesayi. (A) Previous reported Nb-Ta deposit, represented by the blue star in Figure 2. (B) Newly discovered pegmatite, represented by the green star in Figure 2, a person in the yellow circle as a scale. (C) Conjugated joints and brittle fractures developed in the flesh-red pegmatite. (D) Graphic structure and magnetite aggregates in the pegmatite. (E) Quartz veins intruded into the brecciated pegmatite. (F) Perthitic texture of perthite under a polarizing microscope. (G,H) Photomicrographs of graphic structure under a polarizing microscope. (I) Pegmatite with sericitization under a polarizing microscope. Abbreviations: Afs—alkali feldspar; Mc—mica; Or—orthoclase; Pl—plagioclase; Pth—perthite; Qz—quartz; Ser—sericite.
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Figure 4. Classification diagrams for the Jiaolesayi pegmatite. (A) Total alkalis (Na2O + K2O) vs. SiO2 (TAS) diagram after Middlemost [47]; (B) (A/NK) vs. (A/CNK) diagram, solid lines are after Peccerillo [48]; (C) K2O vs. SiO2 diagram after Rickwood [49], and (D) Chondrite-normalized REE patterns. (E) Primitive mantle-normalized trace element spider diagrams for the Jiaolesayi pegmatite samples. Normalizing data for the chondrite and primitive mantle are from Sun and McDonough [50]. Symbols: green diamond—syenite pegmatite samples.
Figure 4. Classification diagrams for the Jiaolesayi pegmatite. (A) Total alkalis (Na2O + K2O) vs. SiO2 (TAS) diagram after Middlemost [47]; (B) (A/NK) vs. (A/CNK) diagram, solid lines are after Peccerillo [48]; (C) K2O vs. SiO2 diagram after Rickwood [49], and (D) Chondrite-normalized REE patterns. (E) Primitive mantle-normalized trace element spider diagrams for the Jiaolesayi pegmatite samples. Normalizing data for the chondrite and primitive mantle are from Sun and McDonough [50]. Symbols: green diamond—syenite pegmatite samples.
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Figure 5. (A) Photomicrographs (cross-polarized light) and cathodoluminescence images of all tested zircons from Jiaolesayi pegmatite. (B) U-Pb Concordia diagram of sample D3301-1.
Figure 5. (A) Photomicrographs (cross-polarized light) and cathodoluminescence images of all tested zircons from Jiaolesayi pegmatite. (B) U-Pb Concordia diagram of sample D3301-1.
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Figure 6. Photomicrographs under a reflecting microscope. (A) Typical subhedral euxenite-(Y) grain. (B) Short columnar euxenite-(Y) aggregates. (C) Thorite and anhedral euxenite-(Y). (D) Zircon, euxenite-(Y), and later-formed limonite in between. (E) A partial enlargement of Figure (D). (F) Image (E) in plane-polarized light. (G) An allanite-(Ce) grain. (H,I) Euhedral euxenite-(Y) grains and the EPMA test locations. (J) Limonite pseudomorph with residual pyrite in the core. (K,L) BSE images of euhedral euxenite-(Y) grains. Abbreviations: Aln-(Ce)—allanite-(Ce); Eux—euxenite-(Y); Lm—limonite; Py—pyrite; Thr—thorite; Zr—zircon.
Figure 6. Photomicrographs under a reflecting microscope. (A) Typical subhedral euxenite-(Y) grain. (B) Short columnar euxenite-(Y) aggregates. (C) Thorite and anhedral euxenite-(Y). (D) Zircon, euxenite-(Y), and later-formed limonite in between. (E) A partial enlargement of Figure (D). (F) Image (E) in plane-polarized light. (G) An allanite-(Ce) grain. (H,I) Euhedral euxenite-(Y) grains and the EPMA test locations. (J) Limonite pseudomorph with residual pyrite in the core. (K,L) BSE images of euhedral euxenite-(Y) grains. Abbreviations: Aln-(Ce)—allanite-(Ce); Eux—euxenite-(Y); Lm—limonite; Py—pyrite; Thr—thorite; Zr—zircon.
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Figure 7. (A) Canonical discrimination analysis of AGM and EGM and (B) triangular discriminant graph after Škoda [55]; (C) FeO*/MgO vs. Zr + Nb + Ce + Y; (D) ternary plot of Nb-Y-Ce after Eby [57] and (E) Zr vs. 10,000 Ga/Al, A-type granite discrimination diagrams after Whalen [58]. Symbols: pink squares—AGM, grey squares—EGM from Škoda [55], orange crosses—EPMA data in this study (Supplementary Table S3).
Figure 7. (A) Canonical discrimination analysis of AGM and EGM and (B) triangular discriminant graph after Škoda [55]; (C) FeO*/MgO vs. Zr + Nb + Ce + Y; (D) ternary plot of Nb-Y-Ce after Eby [57] and (E) Zr vs. 10,000 Ga/Al, A-type granite discrimination diagrams after Whalen [58]. Symbols: pink squares—AGM, grey squares—EGM from Škoda [55], orange crosses—EPMA data in this study (Supplementary Table S3).
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Figure 8. (A) Plots of Nb/Yb vs. Th/Yb, after Wang [78], and (B) plots of Y/Nb vs. Yb/Ta, after Eby [56]. Black squares represent the three end-members in the MORB-OIB array. Abbreviations: OIB, oceanic island basalt; IAB, island arc basalt; N-MORB, normal middle oceanic ridge basalt; E-MORB, enriched middle oceanic ridge basalt; A1, A1 type granite; A2, A2 type granite.
Figure 8. (A) Plots of Nb/Yb vs. Th/Yb, after Wang [78], and (B) plots of Y/Nb vs. Yb/Ta, after Eby [56]. Black squares represent the three end-members in the MORB-OIB array. Abbreviations: OIB, oceanic island basalt; IAB, island arc basalt; N-MORB, normal middle oceanic ridge basalt; E-MORB, enriched middle oceanic ridge basalt; A1, A1 type granite; A2, A2 type granite.
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Zhang, L.; Pei, X.; Gao, Y.; Li, Z.; Liu, M.; Jing, Y.; Wang, Y.; Chen, K.; Deng, N.; Zhang, Y.; et al. Geochemistry, Mineralogy, and Geochronology of the NYF Pegmatites, Jiaolesayi, Northern Qaidam Basin, China. Minerals 2024, 14, 805. https://doi.org/10.3390/min14080805

AMA Style

Zhang L, Pei X, Gao Y, Li Z, Liu M, Jing Y, Wang Y, Chen K, Deng N, Zhang Y, et al. Geochemistry, Mineralogy, and Geochronology of the NYF Pegmatites, Jiaolesayi, Northern Qaidam Basin, China. Minerals. 2024; 14(8):805. https://doi.org/10.3390/min14080805

Chicago/Turabian Style

Zhang, Long, Xianzhi Pei, Yongbao Gao, Zuochen Li, Ming Liu, Yongkang Jing, Yuanwei Wang, Kang Chen, Nan Deng, Yi Zhang, and et al. 2024. "Geochemistry, Mineralogy, and Geochronology of the NYF Pegmatites, Jiaolesayi, Northern Qaidam Basin, China" Minerals 14, no. 8: 805. https://doi.org/10.3390/min14080805

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