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10 November 2025

Isotopic and Elemental Constraints on Zircon, Garnet, and Uraninite from Nakexiuma: Implications for U–W Mineralization

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1
Qinghai Nuclear Industry Geological Bureau, Xining 810001, China
2
National Key Laboratory of Uranium Resources Exploration-Mining and Nuclear Remote Sensing, East China University of Technology, Nanchang 330013, China
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Authors to whom correspondence should be addressed.
Minerals2025, 15(11), 1182;https://doi.org/10.3390/min15111182 
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This article belongs to the Section Mineral Deposits
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Abstract

The Nakexiuma area in the East Kunlun Orogen Belt hosts two spatially distinct mineralization systems: uranium-molybdenum (U-Mo) in schist and granitoid, and tungsten-molybdenum (W-Mo) in skarn and granitoid. To clarify their genetic relationship, we conducted U-Pb dating and trace element analyses on zircon, garnet, and uraninite. Zircon from granitoids yields a crystallization age of 250 ± 2.3 Ma, followed by W-Mo mineralization at 245 ± 2.1 Ma (garnet) and U-Mo mineralization at 235 ± 9 Ma (uraninite), indicating a prolonged magmatic-hydrothermal history spanning approximately 15 million years. Trace element data reveal a shift in fluid chemistry over time: Skarn garnets show high W contents, suggesting oxidizing, high-temperature fluids; uraninite displays REE depletion and negative Eu anomalies, precipitated from oxidizing fluids encountering a reducing environment. We propose that the W, U, and Mo mineralization in Nakexiuma is the result of this long-lived magmatic-hydrothermal system. The spatial separation of these mineralization systems is attributed to a multi-stage process involving host rock lithology and fluid redox evolution. Early oxidizing fluids from granitoids metasomatized carbonates to form W-Mo mineralization skarn. Later, meteoric water influx increased oxygen fugacity, generating U-rich, highly oxidizing fluids that precipitated uraninite and molybdenite upon interaction with the reducing meta-mafic rocks. These results highlight the roles of lithology and fluid chemistry in controlling spatially separated mineralization within the same system. Furthermore, they expand the Early Mesozoic metallogenic spectrum of the East Kunlun Belt, providing a refined model for polymetallic ore formation in a post-collisional extensional setting.

1. Introduction

The East Kunlun Orogen Belt (EKOB), an orogenic belt marked by intense tectono-magmatic activity and complex geological architecture [,,], hosts a wide variety of metal deposits related to Cambrian to Early Paleozoic and Late Paleozoic to Early Mesozoic magmatism. During the Late Paleozoic to Early Mesozoic, the EKOB underwent multi-stage magmatic evolution and subsequent mineralization in a setting of northward Paleo-Tethys Ocean subduction followed by post-collisional extension [,]. This complex geodynamic history has endowed the region with abundant and diverse metallogenic systems, establishing the EKOB as one of the most mineral-rich belts in Qinghai Province [,,,,,,].
The Nakexiuma area, located in the eastern EKOB [], is primarily composed of Triassic granitoids and Ordovician Tanjianshan Group. Notably, the area hosts two spatially distinct mineralization systems: U-Mo mineralization distributed within schist and granitoid, and W-Mo mineralization strictly confined to the skarn belt at the contact zone between granite and marble. Despite their spatial separation, preliminary geochemical and geochronological evidence suggests that both mineralization types share a close genetic and temporal relationship with the Early Mesozoic granitic magmatic activity. This spatial separation raises a core scientific question: Within the same magmatic-hydrothermal system, what factors controlled the differentiation and spatial segregation of uranium and tungsten mineralization? Addressing this question is essential for developing a coherent metallogenic model in the Nakexiuma area.
To address this question, this study provides detailed U-Pb geochronological and trace elemental analyses of key ore-forming and magmatic minerals from the Nakexiuma area—zircon, garnet, and uraninite. By obtaining ages and elemental compositions for these minerals, we aim to determine the temporal relationship between granite emplacement and the two distinct mineralization events. This paper aims to establish a comprehensive metallogenic model that links the regional geodynamic setting, magmatism, fluid evolution, and mineralization. This research provides insights into the genesis of the Nakexiuma deposit and expands the Early Mesozoic metallogenic spectrum of the East Kunlun Orogenic Belt by documenting U-W-Mo mineralization linked to granitic activity.

2. Regional and Deposit Geology

The Kunlun Orogenic Belt is located in the northern Qinghai–Tibet Plateau, bounded by the Qinling Orogenic Belt to the east and the Pamir Plateau to the west []. It forms a major component of China’s central orogenic system and plays a critical role in the plateau’s tectonic evolution. The Altyn Tagh Fault marks the boundary between the East Kunlun Orogenic Belt (EKOB) and the West Kunlun Orogenic Belt []. Within the EKOB and its surrounding areas, four tectonic units are recognized: the Qimantage Belt, the Central EKOB, the Southern EKOB, and the Bayanhaer Terrane [].
Since the Phanerozoic, the East Kunlun Orogenic Belt (EKOB) experienced two major tectono-magmatic episodes: the Early Paleozoic and the Late Paleozoic–Early Mesozoic. These events produced widespread granitoid intrusions and associated metallogenesis. The Nakexiuma mining area is located in the eastern EKOB about 50 km east of Dulan County, Qinghai Province. The stratigraphy consists of Ordovician metavolcanic rocks of the Tanjianshan Group overlain by Quaternary alluvial–proluvial sand and gravel []. The Tanjianshan Group strikes northwest to west–northwest in a belt-like distribution exceeding 3 km2. Its lithologies include grayish-black biotite quartz schist, grayish-green to grayish-white muscovite quartz schist, grayish-green plagioclase–amphibole schist interbedded with grayish-white banded marble. These schists were derived from calc-alkaline intermediate to basic volcanic rocks interlayered with clastic and carbonate strata, later metamorphosed in an island-arc setting. The strata display conformable contacts, pronounced foliation, and evidence of ductile deformation. During the Late Triassic, intense magmatism led to the emplacement of granitic plutons and dikes. The intrusive rocks consist mainly of medium- to fine-grained granodiorite, medium- to coarse-grained biotite monzonitic granite, porphyritic biotite granite, and minor granite dikes. These bodies generally intrude the Tanjianshan Group with clear intrusive contacts. In carbonate-bearing areas, granitoid emplacement caused extensive skarn alteration of the surrounding strata. Faulting is well developed, dominated by northwest- and east–west-trending faults, with northeast-trending structures as secondary. Fault zones are marked by depressions, and host rocks in these zones are fractured, silicified, and limonitized.
Uranium–molybdenum (U-Mo) mineralization in the Nakexiuma area is distributed within an elongated, northwest-trending belt hosted by schists of the Tanjianshan Group and granitoid intrusions (Figure 1B). Seven ore bodies have been delineated, ranging from 100 to 1100 m in length and 0.36 to 2.91 m in thickness. Uranium grades reach up to 0.19%, and molybdenum grades up to 0.20%. The main host rocks include quartz schist, as well as tectonic breccia (Figure 2a,b). Mineralization typically occurs as veins and veinlets that fill fractures in the host rocks. The most widespread hydrothermal alteration is pervasive hematitization (Figure 2c,d). In contrast, tungsten-molybdenum (W-Mo) mineralization is spatially associated with the skarn belt developed at the contact between the Tanjianshan Group marble and granitic intrusions (Figure 1B). The skarn belt extends for 540–1000 m in length and 60–230 m in width. Within this zone, four W-Mo ore bodies have been identified, with dimensions ranging from 100 to 500 m in length and 1.65 to 7.72 m in thickness. Ore grades range from 0.09% to 1.56% WO3 and from 0.04% to 0.98% Mo. Garnet-rich skarn constitutes the principal host rock, displaying colors from grayish-white to reddish-brown (Figure 2e,f), and exhibiting granoblastic–prismatic textures with massive structures. Mineralization occurs mainly as disseminations and veinlets within the skarn. Scheelite and molybdenite are the dominant ore minerals, accompanied by minor chalcopyrite and malachite. The gangue mineral assemblage consists of garnet, quartz, calcite, diopside, and tremolite (Figure 2g–i). Surrounding wall rocks show extensive hydrothermal alteration, including epidotization, silicification, carbonatization, and chloritization. These alteration halos are closely associated with ore mineralization, indicating that fluid-rock interaction was critical for metal enrichment. Together, the U-Mo and W-Mo mineralization styles illustrate the complex metallogenic processes in the Nakexiuma area, reflecting both structurally controlled hydrothermal systems and skarn-related contact metasomatism.
Figure 1. (A) Map sketch of the Qinghai–Tibetan Plateau showing the dominant blocks in western China; (B) Simplified geological map of the East Kunlun orogenic belt; (C) Geological map of the Nakexiuma mining area. NEKT: northern East Kunlun orogenic terrane; SEKT: southern East Kunlun orogenic terrane.
Figure 2. Field and drill hole characteristics of typical samples from the Nakexiuma mining area. (a) Chloritized schist; (b) Granitoid-vein intruded into schist; (c,d) U-Mo ore samples displaying hematitization; (e) banded marble; (f) Skarnified Marble; (g) Radial tremolite in marble; (h) garnet-bearing marble; (i) W-Mo ore’s fluorescence under UV light. Gr: Garnet; Cal: Calcite; UV: Ultraviolet.

3. Analytical Methods

U-Pb dating of zircon was conducted by LA-ICP-MS at the Sincere Langfang Chengxin Geological Service Co., Ltd., China. Experiments were carried out on the PlasmaQuant MS030 elite ICP-MS instrument (Analytik Jena AG, Thuringia, Germany) in combination with an excimer 193nm laser ablation system (NewWave, Fremont, CA, USA). The spot size and frequency of the laser were set to 25 µm and 12 Hz, respectively, in this study. Helium was applied as a carrier gas. NIST SRM610 was used to optimize sensitivity, reduce background noise, and achieve stable analytical conditions. A dwell time of 15 ms was set for 204Pb, 206Pb, 207Pb, 208Pb, 232Th, and 238U. Zircon 91500 and Plesovice [] were used as external standards for U-Pb dating and a blind sample for monitoring the instrument condition, respectively. Data reduction was made using GLITTER 4.0 software and plotted using Isoplot/Ex_ver3.6 [].
U-Pb dating and trace element analysis of garnet were simultaneously conducted by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as described by []. Laser sampling was performed using a GeolasPro laser ablation system (Coherent Inc., Bremen, Germany) that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7900 ICP-MS instrument (Agilent Technologies, Tokyo, Japan) was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. A “wire” signal smoothing device is included in this laser ablation system []. The spot size and frequency of the laser were set to 44 µm and 5 Hz, respectively, in this study. Zircon standard 91500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. Garnet Mali and Willsboro were analyzed as unknown samples, with reference to isotopic composition []. Each analysis incorporated a background acquisition of approximately 20–30 s followed by 50 s of data acquisition from the sample. An Excel-based software, ICPMSDataCal, was used to perform offline selection and integration of background and analyzed signals, time-drift correction, and quantitative calibration for trace element analysis and U-Pb dating []. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3.6 [].
U-Pb isotope composition and trace element measurements of uraninite were performed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS; Agilent 7900) at Wuhan Sample Solution Analytical Technology Co., Ltd. The detailed methodology that was followed for LA-ICP-MS U-Pb isotope measurements is presented in []. The spot size and frequency of the laser were set to 16 µm and 1 Hz, respectively. The standard NIST610 was used as reference material to analyze trace elements in uraninite using U contents measured by EMPA as internal standards. The standard GBW04420 was used as reference material for analyses of U-Pb isotope composition. Then, data were processed using the ICPDATACAL 10.8 []. Age calculations and Tera-Wasserburg diagram plotting were carried out using Isoplot flowsheets [].

4. Result

4.1. Petrography

The analyzed uraninite sample was collected from schist in the Nakexiuma area. Uraninite and brannerite are the dominant uranium minerals, and molybdenite is the principal molybdenum mineral in the schist (Figure 3). Uraninite occurs either as isolated euhedral grains, several to tens of micrometers in size, enclosed within quartz, or as aggregates up to several hundred micrometers in size, common coexisting with brannerite and molybdenite (Figure 3).
Figure 3. (a,b) BSE images showing uraninite co-occurring with brannerite and molybdenite; (c,d) BSE images showing garnet zoning in skarn. Ur: Uranite; Bran: Brannerite; Qz: Quartz; Py: Pyrite; Gr: Garnet; Di: Diopside; Mol: molybdenite.
The garnet sample was collected from endoskarn, where the rock is mainly composed of garnet, epidote, plagioclase, and minor apatite and quartz. Garnet occurs as granular, subhedral to anhedral aggregates ranging from 0.7 to 1.9 mm in size (Figure 3c,d). Back-scattered electron (BSE) images distinguish two garnet types based on brightness: dark and light, the latter often containing irregular domains of dark garnet. Locally, fine-grained pyroxene is enclosed within garnet (Figure 3). In well-crystallized zones, tremolite aggregates are unevenly distributed, with most grains smaller than 1.0 mm × 2.0 mm. Epidote occurs as short prismatic, columnar–granular, and granular aggregates. Most grains are <0.5 mm, though larger aggregates are present. Epidote is colorless, exhibits high relief, and is commonly intergrown with andradite.

4.2. Geochronology

Thirty-five zircon grains from one granitic sample were selected from Nakexiuma granitoids for LA-ICP-MS U-Pb isotopic dating (Supplementary material Table S1; Figure 4a,b). Zircon grains are euhedral and prismatic, ranging from 150 to 300 μm in length. Zircon grains in the Nakexiuma granitoid rocks show clear oscillatory growth zoning, indicating magmatic zircon. Those thirty-five zircon grains have Th contents of 220 ppm to 2232 ppm, U contents of 428 ppm to 1676 ppm, with Th/U ratios ranging from 0.36 to 1.85, also indicating magmatic zircon. The 206Pb/238U ages range from 241 Ma to 256 Ma (Figure 4a), with a weighted mean age of 250 ± 2.3 Ma (MSWD = 0.11) (Figure 4b).
Figure 4. U-Pb isotopic dating diagrams of zircon (a,b), garnet (c), and uraninite (d,e), and time evolution from granitoid emplacement to W-Mo and U-Mo mineralization (f).
A total of thirty-nine analyses from garnet in skarn were analyzed, and the results are listed in Supplementary material Table S2 and plotted in Figure 4c. The analyzed garnet spots have U contents of 0.52 to 10.82 ppm, Pb contents of 0.05 to 55.72 ppm, and Th contents of 1.05 to 383 ppm. The Th/U ratios range from 0.87 to 12.25 with an average of 2.05. These analyses yielded a lower intercept 206Pb/238U age of 245 ± 2.1 Ma (MSWD = 1.2) and y-intercept age at 4960 ± 14 Ma on the Tera-Wasserburg diagram.
Due to their small grains, only six uraninite grains were selected for LA-ICP-MS U-Pb isotopic dating. Results are shown in Supplementary material Table S3 and plotted in Figure 4d and e. Using Stacey-Kramers Pb correction, those six analyses yield 206Pb/238U ages ranging from 226 Ma to 245 Ma, giving a concordia age of 235 ± 9.0 Ma (MSWD = 0.49). This age is consistent with the minimum age (232 Ma) of EMPA U-Th-Pb chemical ages of uraninite [].
The dating results demonstrate that the entire granite-related magmatic-hydrothermal system persisted for at least 15 Ma (Figure 4f), confirming a longevity of the magmatic-hydrothermal system associated with the emplacement of the Triassic granite.

4.3. Trace Elements

4.3.1. Uraninite

Uraninite trace elemental compositions are presented in Supplementary material Table S4 and Figure 5a. Six uraninite analyses in the Nakexiuma area have total REE contents ranging from 4000 ppm to 4394 ppm. In the chondrite-normalized REE pattern (Figure 5a), these analyses exhibit left-dipping REE patterns, characterized by LREE depletion and HREE enrichment with δEu ranging from 0.14 to 0.37. The (La/Sm)N ratios range from 0.01 to 0.03, (La/Yb)N ratios from 0.04 to 0.10, and (Gd/Yb)N ratios from 1.56 to 2.99.
Figure 5. Chondrite-normalization REE patterns for uraninite (a), Bright (b) and dark (c) zones of garnet from the Nakexiuma mining area. REE patterns of vein-related and intrusive-related uranium minerals are from [].

4.3.2. Garnet

Trace elemental compositions of garnet in the skarn are listed in Supplementary material Table S5 and plotted in Figure 5b,c and Figure 6. The trace elemental compositions of garnet can be divided into two groups, corresponding to dark and light zones in BSE images. Compared with analyses from light zones (Figure 6), those from dark zones exhibit high Al (10,708 ppm to 34,162 ppm, medium of 19,418 ppm), Y (11.6 ppm to 39.5 ppm, medium of 24.8 ppm), MREE and HREE concentrations, and low Ti (149 to 3939 ppm, medium of 624 ppm), Zr (22.3 ppm to 201 ppm, medium of 82.7 ppm), Nb (0.62 ppm to 11.7 ppm, medium of 2.21 ppm), W (0.83 to 110 ppm, medium of 23.2 ppm) and LREE concentrations. The analyses from the light zones have Al contents of 4077 ppm to 9067 ppm (medium of 6346 ppm), Y contents of 5.84 ppm to 13 ppm (medium of 10.1 ppm), Zr contents of 73.6 ppm to 234 ppm (medium of 161 ppm), Nb contents of 3.16 ppm to 24.8 ppm (medium of 12.9 ppm), and W contents of 25.4 ppm to 171 ppm (medium of 71.4 ppm). Sn contents are similar between light zones (127–269 ppm; median 180 ppm) and dark zones (106–383 ppm; median 195 ppm).
Figure 6. Al versus (a) Sn, (b) Ti, (c) Y, and (d) Nb diagrams showing the elemental differences between dark and bright zones of garnet from the Nakexiuma.
The garnet analyses from the light zones have total REE contents ranging from 41.6 ppm to 111 ppm (median of 58.5 ppm), and those from the dark zones have total REE contents varying from 53.7 ppm to 96.6 ppm (median of 79.0 ppm). In the chondrite-normalized REE patterns (Figure 5b,c), the analyses from the light zones show more depletion in MREE and HREE with more significant positive Eu anomalies, when compared to the analyses from the dark areas. The analyses from the light area have (La/Sm)N ratios ranging from 0.70 ppm to 2.58 (medium of 1.83), (La/Yb)N of 2.49 to 34.0 (medium of 5.46), and δEu ranging from 2.04 to 3.87 (medium of 3.55); and those from the dark zones have (La/Sm)N ratios ranging from 0.02 to 0.90 (medium of 0.19), (La/Yb)N ratios ranging from 0.10 to 7.30 (medium of 0.87), and δEu ranging from 0.89 to 2.67 (medium of 1.80).

5. Discussion

5.1. Genetic Type of Uraninite

Uraninite/pitchblende (UO2), the most common uranium oxide in uranium deposits, can form across both high-temperature and low-temperature environments. Uraninite that forms in high-temperature environments, including magmas and high-temperature hydrothermal fluids, is generally associated with brannerite, while pitchblende, formed in low-temperature environments. In the Nakexiuma area, uraninite exhibits a euhedral shape and coexists with brannerite, suggesting a high temperature origin [,,,,,]. In high-temperature settings, the similar ionic radii of U4+ with Th4+ and REE3+ allow significant incorporation of Th and REEs into the uraninite crystal lattice. This is commonly observed in magmatic uraninite from granitoids and pegmatites [,,] and in some hydrothermal uraninites [,]. In Nakexiuma, uraninite displays REE patterns comparable to intrusive-type uraninite, but its occurrence in schist suggests a high-temperature hydrothermal, rather than a magmatic origin. However, its low REE and Th contents appear inconsistent with such an origin. Previous studies have suggested that Th and REE contents of uraninite not only depend on the temperature, but also Th and REE contents of fluids from which uraninite crystallized [,]. This implies that even when crystallized from a high temperature environment, some uraninite grains may exhibit low REE and Th contents if their parent fluids were depleted in these elements.

5.2. U-W-Mo Mineralization-Related to Granitic Fluids

U-Pb isotopic geochronological results establish a clear temporal link between the granitoids and U-W-Mo mineralization in the Nakexiuma area: the granite formed at 250 ± 2.3 Ma, closely followed by W-Mo mineralization (garnet U-Pb isotopic age of 245 ± 2.1 Ma) and U-Mo mineralization (uraninite U-Pb isotopic age of 235 ± 9 Ma). This close temporal association indicates that the U-W-Mo mineralization in the Nakexiuma area is intimately related to Early Mesozoic granitic magmatic processes. The granitoids likely served as the primary heat source and/or source of ore-forming fluids and materials. The slight age difference between granite crystallization and U-W-Mo mineralization events (245 Ma and 235 Ma, respectively) suggests a long-lived magmatic-hydrothermal continuum: mineralization events occur not during, but shortly after, granite emplacement, as cooling intrusions released fluids that evolved and migrated into favorable host rocks [,]. This concept model is consistent with many ore systems, where crystallizing magmas provide heat and fluids for hydrothermal circulation, metal transport, and precipitation [,,].

5.3. Fluids of Skarn Formation

The general chemical formula for garnet is X3Y2[SiO4]3, where X represents divalent cations occupying eight-fold coordination (e.g., Ca2+, Mn2+, Mg2+, or Fe2+), and the Y site is typically occupied by trivalent cations (Fe3+, Al3+, or Cr3+) [,]. Rare earth elements (REEs) are incorporated into the mineral lattice through isomorphic substitution, primarily controlled by the crystal chemical structure. Generally, heavy rare earth elements (HREEs) have higher partition coefficients than light rare earth elements (LREEs) between magma/fluid and garnet. Therefore, garnet typically preferentially incorporates HREEs over LREEs, exhibiting a left-tilting, HREE-enriched chondrite-normalized REE pattern [,,,]. In Nakexiuma, however, garnet REE distributions deviate from typical HREE enrichment. Darker zones show variable patterns, including LREE enrichment with MREE–HREE depletion, dual enrichment in LREEs and HREEs, or preferential MREE enrichment. These patterns suggest the presence of other factors controlling the REE composition of skarn garnets in the Nakexiuma area.
Redox-sensitive elements (e.g., U and W) can occur in different valence states, and their behaviors are significantly affected by oxygen fugacity, making them an effective tool to estimate the redox state of hydrothermal fluids []. In oxidizing fluids, Fe and W exist as Fe3+ and W6+, respectively. The ionic radius of W6+ is similar to that of Fe3+, allowing W6+ to effectively substitute for Fe3+ into the garnet crystal structure. Therefore, garnets formed in oxidizing conditions are characterized by high W content. The Nakexiuma skarn garnets have a high W content (0.83–171 ppm, with a median of 55.8 ppm), which is significantly higher than the W content in garnets from domestic reducing skarn-type tungsten deposits, such as the Xintianling deposit (average W content of 6.5 ppm; []), the Xianghualing tungsten deposit (average W content of 11.4 ppm; []), and the Zhuxi tungsten deposit (6.64 ppm; []). Therefore, the high W garnet indicates that the Nakexiuma skarn formed under oxidizing conditions. However, the presence of high uranium content in the analyzed garnet suggests a more nuanced interpretation. While W can be mobile and incorporated into garnet under oxidizing conditions, the incorporation of U(IV) into garnet lattices generally requires less oxidizing conditions. If the fluids were sufficiently oxidizing to maintain tungsten mobility, they might also oxidize U(IV) to U(VI), which is typically more mobile and less likely to be incorporated into garnet [,]. The high U content in the Nakexiuma garnet, despite the high W content, implies that the oxygen fugacity of the fluids responsible for skarn formation was oxidizing enough to mobilize W, but not high enough to fully prevent the incorporation of U(IV) into the garnet lattice.
Furthermore, REE fractionation is strongly dependent on the hydrothermal fluid [,,]. Under mildly acidic conditions, fluids are typically LREE-enriched and HREE-depleted, often exhibiting positive Eu anomalies, whereas nearly neutral fluids tend to be HREE-enriched and LREE-depleted, displaying negative or no Eu anomalies []. The overall REE contents and the type of chondrite-normalized pattern of the fluids are also influenced by the presence of Cl-, which can transport Eu in EuCl42- complex, which often results in the formation of positive Eu anomalies [,,]. At Nakexiuma, both the dark and bright zones of garnet exhibit LREE enrichment and HREE depletion with positive Eu anomalies. This signature consistently indicates that the garnet crystallized from a mildly acidic and Cl-rich fluid.

5.4. Factors Controlled Spatial Separation of U and W Mineralization

In the Nakexiuma area, the granitoid emplacement (250 Ma) and the subsequent W-Mo (245 Ma) and U-Mo (235 Ma) mineralized events exhibit a continuous temporal succession, suggesting a genetic relationship among the three. These ages define a prolonged 15-million-year history of magmatic-hydrothermal activity spanning from Early to Late Triassic. The W, U and Mo mineralization is collectively the result of this multistage process. This extended and episodic magmatic-hydrothermal activity likely played a significant role by repeatedly supplying the necessary heat, fluids, and metals. However, a key distinction lies in their host rocks: U-Mo mineralization is primarily found in meta-mafic volcanic rocks, while W-Mo mineralization is hosted in skarn and granitoids. This raises the critical question: what controlled the separation between U and W mineralization in the Nakexiuma area.
Petrographic observations indicate that W-Mo mineralization within the skarn coexists with garnet and diopside but lacks hematite. The high W and U concentration in the garnet reveal that fluids indicate oxidizing fluids, though oxygen fugacity was below the stability field of U6+ []. Under such conditions, uranium, typically as U4+, has low solubility even in fluids with high mineralizer concentrations []. Conversely, uranium is highly soluble and mobile as U(VI) in oxidized fluids []. In the Nakexiuma, the later U-Mo mineralization is characterized by the coexistence of uraninite, molybdenite and hematite, with some pyrite grains. This mineral assemblage is common in hydrothermal uranium deposits, and is interpreted as a high oxygen fugacity fluid undergoing reduction during ore formation [,,]. This significant redox transition likely reflects meteoric water involvement [,,]. Further evidence supporting the presence of meteoric water is the high uranium grade (up to 0.19%). Although mineralizers, such as F and Cl, can increase the solubility of U4+ [], such high uranium concentrations strongly indicate that uranium exists in the ore-forming fluids in its hexavalent form []. The presence of hexavalent uranium is typically associated with high oxygen fugacity fluids, often derived from meteoric water []. Therefore, In the Nakexiuma area, uraninite precipitated from U6+-rich highly oxidizing ore-forming fluids upon encountering a reducing environment. Similarly, molybdenum, commonly precipitating as molybdenite, also favors deposition under reducing conditions, especially in the presence of sulfur. The necessary reduction is supplied by the local meta-mafic volcanic rocks. These rocks are rich in ferrous iron-bearing minerals (e.g., amphiboles, pyroxenes, or chlorite) and may also contain pre-existing sulfides. These phases function as effective reducing agents, promoting the simultaneous precipitation of both uranium and molybdenum.
Therefore, the spatial separation of W and U in Nakexiuma was primarily controlled by the combination of host rock differences and fluid redox evolution. During the Early Triassic, early oxidizing, W-rich, magmatic fluids exsolving from the emplaced granitoids in Nakexiuma, metasomatized carbonate rocks (Figure 7). This process led to the formation of skarn and associated W-Mo mineralization. Due to the low uranium content in these initial fluids, uranium mineralization is absent within the skarn. Subsequently, with the influx of meteoric water, the oxygen fugacity of the ore-forming fluids increased. These highly oxidizing fluids effectively leached uranium from the Nakexiuma granitoids, forming uranium-rich ore-forming fluids. These fluids then circulated along fractures within the meta-mafic rocks and reacted with the reducing materials present. As a result of this decrease in the oxygen fugacity of the ore-forming fluids, uraninite and molybdenite precipitated (Figure 7).
Figure 7. A schematic model of mineralization in the Nakexiuma mining area illustrating the intrusion of granitoids into schists, showing W–Mo mineralization associated with magmatic fluids and U–Mo mineralization controlled by a combination of magmatic and meteoric fluids.

5.5. Geodynamic Setting for U-Mo-W Mineralization at Nakexiuma

This study conducted U-Pb isotope dating on uraninite and garnet from the U-Mo-W mineralized area with the Nakexiuma area, determining the metallogenic ages of the deposit to be 235 ± 9 Ma (U-Pb isotope age of uraninite) and 245 ± 2.1 Ma (U-Pb isotope age of garnet). These two ages are closely associated with the magmatic crystallization age of nearby granites (250 ± 2.3 Ma). This confirms a genetic relationship between mineralization and Early Mesozoic magmatic-metallogenic processes. In the East Kunlun Orogenic Belt, metal mineralization elements associated with Early Mesozoic magmatism are primarily Cu, Fe, Pb, Zn, and Au [,,,], with W and U mineralization being less common. The Nakexiuma case refines this metallogenic spectrum, demonstrating that U-W-Mo can also be linked to Triassic magmatism in the EKOB.
The EKOB experienced a series of metallogenic events during the Early Mesozoic, including the Harizha Cu-Mo deposit in the eastern East Kunlun at approximately 239 Ma [], the Weibao Cu-Pb-Zn deposit at approximately 233 Ma [], the Galinge Fe-Cu polymetallic deposit in Qimantag at approximately 236 Ma [], the Kendekeke skarn-type polymetallic deposit in Qimantag at approximately 225 Ma [], the Haidewula volcanic-type uranium deposit at 234 Ma [], and the W-Mo mineralization identified in Nakexiuma of this study. These metallogenic events indicate that the Early-Middle Triassic was a significant metallogenic period for the EKOB, and also suggest that, at least from the Early-Middle Triassic, the East Kunlun Orogenic Belt had entered a post-collisional extensional environment []. Since the Late Paleozoic, the EKOB has been situated within the Paleo-Tethys Ocean tectonic domain, which began with the northward subduction of the Late Paleozoic Hercynian Animaqing Ocean and concluded with a post-orogenic extensional environment in the Late Triassic []. During the Early-Middle Triassic, the intense extensional activity triggered mantle magma upwelling. This process provided heat that promoted crustal partial melting, which, in turn, led to the formation of granitoids. The associated thermal and fluid activity drove the mineralization in the Nakexiuma area: U-Mo mineralization in metavolcanic rocks and W-Mo mineralization in skarns.

6. Conclusions

Based on our integrated analysis of zircon, garnet, and uraninite elemental and isotopic data, we conclude that U-Mo (235 ± 9 Ma) and W-Mo (245 ± 2.1 Ma) mineralization in the Nakexiuma area were broadly contemporaneous with Triassic granitoid emplacement (250 ± 2.3 Ma). Our findings reveal that the spatial separation of these two distinct mineralization systems was not simply controlled by fluid differentiation alone but involved a multi-stage process involving distinct host rocks and evolving fluid chemistry. W-Mo mineralization was formed by relatively oxidizing, high-temperature granite-derived fluids, producing tungsten-bearing skarn at the granite-marble contact. In contrast, the later U-Mo mineralization was associated with highly oxidizing fluids generated by the mixing of granite-derived with an influx of meteoric waters. Uraninite and molybdenite precipitated when these oxidizing fluids encountered reducing meta-mafic volcanic rocks. This highlights the crucial role of host rock lithology and fluid redox evolution in the formation of complex polymetallic ore systems. This study refines the known Early Mesozoic metallogenic spectrum of the East Kunlun Orogenic Belt, offering a model for spatially separated mineralization within a prolonged magmatic–hydrothermal system under post-collisional extension.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15111182/s1. Table S1: LA-ICP-MS U-Pb isotopic dating of zircon in granitoids from the Nakexiuma mining area; Table S2: LA-ICP-MS U-Pb isotopic dating of garnet in skarn from the Nakexiuma mining area; Table S3: LA-ICP-MS U-Pb isotopic dating of uraninite in the schist from the Nakexiuma mining area; Table S4: LA-ICP-MS trace elemental compositions (ppm) of uraninite from the Nakexiuma mining area; Table S5: LA-ICP-MS trace elemental compositions (ppm) of garnet in skarn from the Nakexiuma mining area.

Author Contributions

Conceptualization, Y.L., S.L., and J.D. (Jianhua Duan).; formal analysis Y.L., S.L., and J.D. (Jianhua Duan).; investigation, Y.L., S.L., J.D. (Jianhua Duan)., J.D. (Jiawen Dai)., K.W., H.S.; sampling, Y.L., S.L. and S.L.; experiment; Y.L., and K.W.; writing—original, Y.L.; writing—review, J.D. (Jianhua Duan)., and K.W.; supervision, J.D. (Jianhua Duan).; funding, J.D. (Jiawen Dai). All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Key R&D and transformation plan of Qinghai Science and Technology Department (No. 2025-SF-141).

Data Availability Statement

The original contributions presented in this study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflicts of interest.

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