SIMS Dating of Granite-Hosted Uranium Deposits in the Xiazhuang Ore Field and Its Geological Significance

Abstract

Using pitchblende uranium ore GBW04420 as the standard material and through the secondary ion mass spectrometry (SIMS) technical method, the in situ U-Pb isotopic chronology characteristics of the main granite-type uranium deposits in the Xiazhuang ore field in the Nanling area of southern China were studied. Firstly, the suitability of GBW04420 as the in situ U-Pb isotopic dating standard material for uranium minerals was verified. On this basis, the in situ U-Pb isotopic ages of the three main granite-type uranium deposits in the Xiazhuang ore field, namely the Xianshi, Zhaixia, and Xiwang deposits, were obtained by SIMS dating. The results show that the overall mineralization period of the Xiazhuang ore field is mainly in Late Cretaceous and the Eocene-Oligocene. The mineralization ages indicate that the uranium deposits are of post-magmatic, medium-low temperature hydrothermal origin rather than the magmatic uranium deposit type. The hydrothermal fluids originate from the combined effect of the crust-mantle hydrothermal fluid and atmospheric precipitation; the uranium source originates from the extraction of the Indosinian-Early Yanshanian diagenetic granite by atmospheric water and partly from the mantle source of the basic dike.

1. Introduction

Micro-scale in situ geochronology comprises two principal methodological frameworks: isotopic microanalysis and chemical microanalysis. This approach demonstrates significant advantages over conventional whole-rock U-Pb dating, including enhanced cost-effectiveness, rapid analytical throughput, and superior spatial resolution (at the micrometer scale). These technical merits collectively enhance the likelihood of obtaining age determinations with increased geological relevance.

The electron probe microanalysis (EPMA) chemical micro-scale in situ dating technique has progressively expanded its application to magmatic and metamorphic petrological investigations in recent decades, originally pioneered by Suzuki et al. (1991) [1]. In China, electron microprobe analysis of known monazite samples has been used to determine the age of these rocks by Zhou Jian, Chen Qiang, and Chen Nengsong [2,3,4]. This technique has been employed in research conducted by Forster H J, Kample U., Hurtado, and others. In addition, domestic researchers, including Zhang Zhaoming, Guo Guolin, and Ge Xiangkun, have explored the application of this method in the analysis of crystalline uranium ore and asphalt uranium ore. However, it has been determined that this method is only suitable for measuring younger samples [5,6,7,8,9]. Since the three Pb isotope ratios cannot be used to assess age concordance, its interpretation must be constrained by additional regional geological evidence to be geologically meaningful.

In recent decades, laser ablation-(multi-collector) inductively coupled plasma mass spectrometry (LA-(MC)-ICP-MS) and the ion probe method have become the focus of intense research activity. This is due to the technique’s higher sensitivity, in situ analysis, rapidity, economy, and spatial resolution [10,11]. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method is distinguished by its high accuracy, rapidity, and high spatial resolution (approximately 20–40 μm), which can achieve 4% repeatability accuracy [12] and yield more satisfactory results; concurrently, the method can also provide trace element and hafnium isotope testing, which provides a substantial amount of information for the understanding of mineral or ore deposit genesis. Zircon has become the mineral of choice for geochronology owing to its favorable properties, including the near absence of common lead during formation and higher closure temperatures. These characteristics make zircon dating a precise dating method in modern mineral deposit chronology [13,14,15,16,17].

SIMS, an instrument with the highest spatial resolution and sensitivity, enables scientists to perform mass spectrometry of mineral particles on the micrometer scale. The instrument has been shown to determine high-precision (1%–2%) zircon U-Pb ages at a spatial resolution of more than 5 μm [18,19]. This allows for the elemental and isotopic analyses of finer particles that cannot be analyzed by other methods. However, it is more expensive to use. Conversely, LA-(MC)-ICP-MS, while economically viable, is constrained in its application to larger particles with a deeper stripping depth (20 μm). Uranium minerals, particularly those in sandstone-hosted uranium deposits, typically exhibit fine grain sizes, ranging from 5 to 20 μm. Although uranium minerals in granite-related hydrothermal deposits are slightly coarser, most grains still fall within the 10–20 μm size range. Due to these small grain dimensions, secondary ion mass spectrometry (SIMS) proves to be a more practical method for uranium mineral geochronology compared to techniques requiring larger sample volumes.

To date, limited scholarly efforts have been dedicated to micro-scale in situ U-Pb isotopic dating of uranium minerals. For instance, in studies in Canada and Australia, zircon standards were utilized as the external standard for U-Pb isotope correction in the analyses [20,21]. The results yielded the age of the intersection point of the U-Pb unconformity but did not provide a harmonic age. In China, geochronological investigations have been conducted on uranium deposits across various geological settings. For granite-hosted uranium deposits in northern Guangdong Province, isotopic dating was performed by Zou Dongfeng et al. and Luo Jincheng et al. Additionally, fs-LA-ICP-MS U-Pb dating has been applied to study sandstone-hosted uranium deposits in the northern Ordos Basin [22,23,24,25].

In micro-area in situ dating applications, variations in material composition between the analyzed target and reference standard (termed the matrix effect) can significantly impact age determination accuracy. This phenomenon has been recognized as one of the primary constraints on precise isotopic ratio analysis in micro-scale in situ studies [26,27,28,29]. Recent advancements have seen researchers employ uranium mineral standards for age determination studies of granite-hosted uranium deposits. Representative examples include the dating of uraninite from Namibian alaskites [30], where the certified pitchblende reference material GBW4420 was utilized. This standard has been thoroughly characterized by the China Nuclear Geology Bureau, with comprehensive geological documentation and research validation [31]. The application of GBW4420 as a primary standard in conjunction with femtosecond laser ablation inductively coupled plasma mass spectrometry (fs-LA-ICP-MS) has facilitated successful geochronological investigations of sandstone-type uranium deposits, including those in the northern Ordos Basin and the Qianjiadian uranium deposit in the Songliao Basin [32,33]. The temporal consistency observed between the obtained ages and regional geological events demonstrates methodological reliability and reinforces the credibility of these dating results.

This investigation employed fs-LA-ICP-MS to conduct a comprehensive homogeneity assessment of the GBW4420 reference material. Analytical results demonstrate satisfactory age distribution uniformity across the standard, confirming its suitability for micro-scale in situ U-Pb isotopic dating of uranium minerals (as detailed below). The methodological validation was achieved through systematic characterization of intra-grain isotopic consistency and inter-fragment reproducibility. Furthermore, synergistic applications combining fs-LA-ICP-MS with complementary techniques such as secondary ion mass spectrometry (SIMS) represent a promising frontier in micro-analytical geochronology.

The Xiazhuang field is an exemplary granitic uranium mine of hydrothermal origin in South China [34,35]. The primary exposed magmatic rocks are Indo-Chinese granites, in addition to late Yanshan and Cenozoic acidic and medium-basic magmatic rocks [36,37,38,39]. The representative deposits, Xianshi and Zhaixia uranium deposits, are hosted within medium-grained porphyritic black mica granites with an age of ~210 Ma [40]. It is hypothesized that the uranium mineralization is governed by the convergence of the north-north-easterly silicification and the east-west pyroxene zone. The age of uranium mineralization is not concurrent with that of the medium-basalt veins and is devoid of specificity. Consequently, the age of uranium mineralization for the Xianshi deposit was ascertained to be two phases of 123.3 Ma breccia uranium mineralization and 84.1 Ma vein uranium mineralization using the electron microprobe method [41,42]. For the Zhaixia Uranium deposit, the U-Pb age of asphalt uranium ore was determined to be 73.1 Ma through in situ testing of zircon with LA-ICP-MS [22]. The U-Pb age of gabbro constrained the primary metallogenic period of the Zhaixia Uranium deposit to 81–96.4 Ma [34], with the late 61 Ma period exhibiting signs of hydrothermal alteration [43]. Some ages have also been obtained by the isochron method, and it is believed that two phases of mineralization, 81 Ma and 125 Ma, existed in the Xianshi deposit [44]. The age of the Zhaixia uranium deposit (335) is 59.5–71 Ma [45], and the age of the Hope deposit (330) is 64–66.8 Ma (Cretaceous-Palaeoproterozoic).

In summary, the age of principal uranium deposits in the Xiazhuang ore field remains subject to ongoing academic debate. This study focuses on the Xianshi, Zhaixia, and Hope uranium deposits, employing GBW4420 bituminous uranium ore as specimens. The study utilized the secondary ion microprobe (SIMS) technique to perform in situ micro-area U-Pb isotope dating of ore and bituminous uranium ore minerals. The results of the fs-LA-ICP-MS rare earth element (REE) analysis of the uranium ore minerals, as well as the regional geological background, are then combined to analyze the reasonableness of the age and the ore deposit. This work contributes to the development of the Xiazhuang mine field. This work has certain theoretical and practical significance for analyzing the genesis of uranium deposits in the field and promoting the development of the discipline of uranium ore fine dating.

2. Basis for the Applicability of Reference Material Gbw04420 to Micro-Area In Situ U-Pb Isotope Dating in Sandstone-Hosted Uranium Deposits

2.1. GBW04420: A Certified Uranium-Lead Isotopic Age Reference Material Jointly Developed by Six Domestic Institutions

GBW04420 is a certified pitchblende uranium-lead isotopic age reference material developed under the coordination of the China National Nuclear Corporation (CNNC) by six domestic institutions. These institutions include the Beijing Research Institute of Uranium Geology (lead organization), Mineral Resources of the Nuclear Industry, the Institute of Geology of the Chinese Academy of Geological Sciences, the Yichang Institute of Geology and Mineral Resources, the China National Center for Reference Materials, and the Institute of Geology of the Chinese Academy of Sciences.

According to the scientific report Pitchblende Uranium-Lead Isotopic Age Reference Material by Zhao Puyun, this reference material originated from pitchblende veins within the No. 201 granite-hosted uranium deposit in South China. This deposit is a representative example of granite-type uranium mineralization, supported by extensive geological data and research. The purity of the sample reached 99.8% [31].

Homogeneity Testing: Homogeneity testing was conducted in compliance with the National Metrological Technical Specifications for Primary Reference Materials (JJG1006-86). Key parameters tested included ²⁰⁷Pb/²⁰⁶Pb (radiogenic), U content, and total Pb content, as these determine the calculated surface age. Statistical analysis of variance (ANOVA) confirmed that the F-values for all parameters were below the critical F-value at the 5% significance level, verifying the material’s homogeneity.

Collaborative Certification Analysis: Eighteen randomly selected samples were analyzed in triplicate by participating laboratories using isotope dilution mass spectrometry (ID-TIMS). The results demonstrated that the U and total Pb contents, as well as the ²⁰⁷Pb/²⁰⁶Pb (radiogenic) ratio of GBW04420, met precision requirements. Statistical tests confirmed a normal distribution of data, and retesting after one year revealed no significant temporal variation in properties, confirming stability [31].

The certified values (95% confidence interval) for the three key parameters are U content: 69.48% ± 0.34%; total Pb content: 6869 ± 17 ppm; ²⁰⁷Pb/²⁰⁶Pb (radiogenic): 0.04909 ± 0.00004. Using these values, the calculated model age of GBW04420 is 69.8 ± 0.6 Ma [T = (1/λ²³⁸) ln(1 + ²⁰⁶Pb/²³⁸U)]. This result aligns with the K-Ar age of illite from the deposit’s alteration zone, 70.3 ± 0.5 Ma, within error margins [31].

2.2. Validation of GBW04420 as a Reliable Uranium Mineral Dating Standard

Zong Keqing Backscattered electron imaging revealed a uniform microstructure in GBW04420, except for minor dark fillings in uranium mineral fractures. Using GBW04420 as an external standard, the U-Pb ages of uraninite from two Rössing-type alaskite uranium deposits in Namibia matched ID-TIMS results, validating GBW04420′s reliability for in situ microanalysis [30].

In a study published in Acta Petrologica Sinica, Luo Jincheng reported that SIMS analysis with LAMNH as an external standard yielded an average age of 73.3 ± 4.7 Ma for GBW04420, consistent with the TIMS ²⁰⁶Pb/²³⁸U age of 70.3 ± 0.5 Ma, within uncertainties. This further supports the credibility of its certified model age, 69.8 ± 0.6 Ma [23].

2.3. Current Study: Validation of GBW04420 for Microscale U-Pb Dating

2.3.1. Methodology and Result

Samples were prepared as electron probe thin sections with a thickness of 100–200 μm for in situ microanalysis. While absolute age determination was secondary (as prior studies established this), the focus was on verifying age homogeneity across mineral grains. Zircon 91,500 served as the external standard, and femtosecond laser ablation multi-collector ICP-MS (fs-LA-MC-ICPMS) at Northwest University’s State Key Laboratory of Continental Dynamics was employed to minimize matrix effects. Analytical conditions were 3 Hz laser pulse, 3 W average power, and 10 μm beam spot. Data processing used IsoplotR [46].

A total of 25 effective data points of uranium mineral (pitchblende) particles were obtained through the test (

Table 1. In situ micro-area U-Pb age dating results and homogeneity analysis (variance) of uranium mineral particles in certified reference material GBW04420.

Sample Number	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	208Pb/232Th	1σ	207Pb/235U age	Age Error (Ma)	206Pb/238Uage	Age Error (Ma)	Variance
GBW04420-001	0.04674	0.00047	0.07411	0.00075	0.0115	0.00012	93.28062	20.42509	72.59	±1.38	73.71	±1.49	3.50
GBW04420-003	0.04714	0.00047	0.07214	0.00073	0.0111	0.00011	29.24225	6.15272	70.73	±1.35	71.16	±1.37	0.08
GBW04420-007	0.04791	0.00048	0.06661	0.00068	0.01008	0.0001	6.64627	0.35004	65.48	±1.26	64.65	±1.25	9.22
GBW04420-008	0.04711	0.00047	0.08032	0.00082	0.01235	0.00013	13.649	1.31843	78.45	±1.51	79.12	±1.62	11.09
GBW04420-009	0.04782	0.00048	0.07189	0.00074	0.01089	0.00011	18.8446	2.1794	70.49	±1.37	69.82	±1.37	1.97
GBW04420-010	0.04729	0.00047	0.07738	0.0008	0.01185	0.00012	27.51763	2.61259	75.68	±1.47	75.94	±1.49	6.63
GBW04420-011	0.04644	0.00046	0.073	0.00075	0.01139	0.00012	16.77671	2.7532	71.54	±1.39	73	±1.49	2.50
GBW04420-012	0.04773	0.00047	0.06354	0.00066	0.00964	0.0001	24.81961	1.48822	62.55	±1.23	61.84	±1.25	13.17
GBW04420-013	0.04807	0.00048	0.07447	0.00078	0.01122	0.00012	20.00116	2.02329	72.93	±1.44	71.92	±1.49	0.98
GBW04420-014	0.04679	0.00046	0.06937	0.00072	0.01074	0.00011	30.43513	2.66617	68.1	±1.33	68.86	±1.37	3.31
GBW04420-015	0.04638	0.00046	0.07758	0.0008	0.01211	0.00013	65.30703	16.40085	75.87	±1.47	77.59	±1.62	8.94
GBW04420-016	0.04694	0.00046	0.0745	0.00077	0.01149	0.00012	128.10147	24.73144	72.96	±1.42	73.64	±1.49	3.40
GBW04420-017	0.04771	0.00047	0.06226	0.00065	0.00945	0.0001	161.02843	60.03397	61.33	±1.21	60.63	±1.25	14.87
GBW04420-018	0.04695	0.00046	0.07474	0.00078	0.01153	0.00012	82.06559	12.92677	73.19	±1.44	73.9	±1.49	3.76
GBW04420-022	0.0469	0.00046	0.07372	0.00077	0.01138	0.00012	5.88539	0.29386	72.22	±1.42	72.94	±1.49	2.42
GBW04420-023	0.04561	0.00044	0.06831	0.00072	0.01085	0.00012	94.0015	29.81958	67.09	±1.34	69.56	±1.49	2.33
GBW04420-026	0.04589	0.00044	0.07669	0.00081	0.01211	0.00013	86.75533	25.92044	75.03	±1.49	77.59	±1.62	8.94
GBW04420-025	0.04581	0.00045	0.06631	0.00071	0.01049	0.00012	34.29167	7.6253	65.19	±1.32	67.27	±1.50	5.55
GBW04420-030	0.04446	0.00043	0.06372	0.00068	0.01039	0.00012	26.44255	3.57038	62.72	±1.27	66.63	±1.50	6.44
GBW04420-002	0.0472	0.00048	0.06277	0.00064	0.00965	0.0001	8.04932	0.56565	61.82	±1.19	61.9	±1.25	13.09
GBW04420-019	0.04368	0.00044	0.07172	0.00076	0.01189	0.00013	19.65723	4.83027	70.33	±1.41	76.19	±1.62	6.98
GBW04420-021	0.04493	0.00044	0.07168	0.00076	0.01155	0.00012	86.79568	37.20795	70.29	±1.41	74.02	±1.49	3.93
GBW04420-024	0.04596	0.00044	0.07718	0.00081	0.01216	0.00013	7.46836	1.00378	75.49	±1.49	77.91	±1.62	9.39
GBW04420-027	0.04584	0.00044	0.07676	0.00081	0.01213	0.00013	8.63204	0.49288	75.09	±1.49	77.72	±1.62	9.13
GBW04420-028	0.04709	0.00046	0.06389	0.00068	0.00983	0.00011	25.35171	2.03068	62.88	±1.27	63.05	±1.37	11.47
Mean Deviation													6.52

). Among them, 19 points have good harmony, and the harmonious age obtained is 69.44 ± 0.14 Ma (Figure 1).

2.3.2. Analysis of Age Uniformity and Reasonableness

An analysis of 25 uranium mineral grain spot age data points (²⁰⁶Pb/²³⁸U, Table 1) reveals that the variance ranges from a maximum of 14.87% to a minimum of 0.08%, with an average of 6.52%. Only four data points (16%) exhibit variances exceeding 11%, while 12 data points (43%) show deviations below 6%. The overall distribution follows a normal pattern (Figure 2). These results indicate good uniformity in the ages of uranium mineral grains for this reference material, supporting the suitability of GBW04420 as a reference standard for in situ microanalysis of U-Pb isotopes in uranium minerals.

The average ²⁰⁶Pb/²³⁸U age of 69.347 ± 0.28 Ma aligns within error margins with the ID-TIMS age of the same sample (70.3 ± 0.5 Ma), demonstrating that femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICPMS) achieves acceptable analytical precision. This consistency suggests effective control of matrix effects when using zircon (91,500) as an external standard. However, studies report significantly larger errors (up to 17%) when using nanosecond laser ablation (ns-LA-MC-ICPMS) [30], indicating pronounced matrix effects. Therefore, the fs-LA-MC-ICPMS method is recommended for higher precision, regardless of the availability of matrix-matched reference materials.

3. Geological Setting

The Xiazhuang uranium ore field is situated within the peripheral zone of the Cathaysian Old Land, occupying a fault depression controlled by the NNE-trending Huangpo quartz fault and the NNE-trending Mashishan fault. Granite-hosted uranium mineralization in this mining district primarily occurs in Meso-Cenozoic magmatic uplift zones, with exposed magmatic rocks dominated by the Guidong composite pluton, a geological entity formed through superimposed Late Yanshanian and Cenozoic acidic magmatism and intermediate-mafic magmatic activities upon the Indosinian granitic basement. Representative deposits include the Xiwan deposit (No. 330), Zhaixia uranium deposit (No. 335), and Xianshi uranium deposit (No. 339). Notably, the Xiwan deposit, being the earliest discovered in the ore field, along with other deposits, is hosted within medium-grained porphyritic biotite granite plutons dated at approximately 210 Ma (Figure 3).

The mining district is underlain by Precambrian strata exhibiting relatively enriched uranium concentrations. Within this region, all stratigraphic units from Cenozoic to Quaternary are exposed, with the notable exception of the general absence of Middle Triassic strata. Mesoproterozoic sedimentation comprises metamorphic series and argillaceous formations, while Late Proterozoic-Early Paleozoic sequences consist of low-grade metamorphic sandstones and slates. Carboniferous-Devonian systems are characterized by terrigenous clastic deposits intercalated with carbonate sequences, whereas Meso-Cenozoic units predominantly feature continental sedimentary rocks and volcanic-sedimentary assemblages. Jurassic strata of the Mesozoic era, distributed predominantly in the northern sector, are principally composed of intermediate-acidic volcanic rocks and pyroclastic formations. Cretaceous strata, developed predominantly in the southeastern sector of the study area, overlie the Cambrian basement with angular unconformity. All exposed intrusive bodies maintain intrusive contact relationships with their host rocks.

The Zhuguang-Xiazhuang metallogenic cluster represents China’s most significant concentration area for granite-hosted uranium mineralization, hosting multiple granite-type uranium ore fields containing dozens of uranium deposits and numerous mineralized occurrences. This district exhibits a spatially clustered distribution pattern of uranium mineralization, constituting the most representative granite-hosted uranium resource base in South China. The Xiazhuang ore field, situated in the eastern sector of the Guidong composite granite pluton, occupies the central-eastern segment of the Nanling E-W trending tectonic-magmatic belt. Tectonically constrained by the intersection of the Dadongshan-Guidong E-W trending magmatic belt with the Huangpi Fault, Mashishan Fault, and Youshan-Xiazhuang Neocathaysian fault system, the ore field is enveloped by Sinian-Paleozoic country rocks. Since the Late Yanshanian period, the Xiazhuang district has experienced extensional rifting and differential uplift-subsidence movements, with regional-scale deep faults penetrating the upper mantle and extensive development of secondary fractures and subsidiary fissures. These structural features collectively establish the area as a favorable zone for mantle-derived fluid mineralization (Figure 3). These fractures serve as pathways formed by granite and subsequent hydrothermal activity. The interaction of hydrothermal fluids with the intrusive rocks facilitated the leaching of uranium, providing a critical uranium source for mineralization. This interpretation is supported by fs-LA-ICP-MS rare earth element (REE) data from uranium minerals in Section 6.3.

The three uranium deposits are spatially distributed along the western extension of the eastern Guidong composite granite pluton, which covers approximately 1000 km² with 186 km² surface exposure. The pluton predominantly consists of medium-grained porphyritic two-mica granite, intruded by multi-phase acidic magmatic intrusions and intermediate-mafic magmatic intrusions. Within these deposits, lamprophyres and diorite porphyries occupy NNE-trending trans-tensional fractures, while diabase porphyries infill SN-oriented fault zones. Previous geochronological investigations have demonstrated that these intermediate-mafic dikes primarily formed during the Cretaceous period.

The three deposits exhibit distinct geological characteristics. The Xiwan uranium deposit (No. 330), a significant mineralization body along the northeastern margin of the ore field, is spatially distributed within the Wengyuan-Quannan adjacent area of the Guangdong-Jiangxi border tectonic zone. Mineralization occurs in silicified and altered granite within a silicified fault zone, typically forming either fine veinlets concentrated along fracture swarms or large silicified veins. This pitchblende-microcrystalline quartz mineralization type originated from Late Yanshanian acidic hydrothermal fluids enriched in uranium and silica. Ore bodies predominantly display vein-like and lenticular morphologies, hosted primarily in porphyritic-textured granite (emplacement ages 193–128 Ma) and lamprophyre (emplacement ages 110–100 Ma). This deposit type is classified as silicified cataclastic zone mineralization.

Previous studies [43,48] have demonstrated that the Xianshi and Zhaixia uranium deposits exhibit close spatiotemporal associations with intermediate-mafic rocks, forming intersection-type deposits through the convergence of NNE-trending structural zones and diabase dike swarms. The Xianshi deposit extends approximately 4 km in the EW dimension and 0.2 km in NS width, where orebody distribution is structurally controlled by the intersection of NWW-trending Luxi-Xianrenzhang diabase dikes and NNE-oriented Damaofeng-Shijiaowei silicified zones. Zhaixia mineralization is principally constrained by convergence nodes between NE- and NNE-trending silicified zones and NW-oriented hornblende diabase dikes, with subordinate ore occurrences localized at structural intersections of multiple silicified zones.

This study systematically analyzed ore mineral assemblages from three deposits using electron probe microanalysis (EPMA). The experimental protocol commenced with in situ characterization of uranium mineral spatial distribution patterns and paragenetic relationships through color backscattered electron (BSE) imaging. Subsequent energy-dispersive X-ray spectroscopy (EDS) enabled precise determination of target mineral elemental compositions. Secondary electron imaging (SEI) further elucidated surface morphological characteristics and elemental distribution patterns of uranium minerals. Uranium-bearing phases demonstrating optimal surface flatness and sufficient grain size were selected for subsequent geochronological analyses based on these comprehensive evaluations (Figure 4).

Petrographic observations reveal that uranium mineralization in the Xiwan deposit predominantly exhibits veinlet-disseminated, vein-type, and dispersed spherulitic distribution patterns, lacking coarse-grained massive mineral aggregates. The predominance of fine-grained particulates poses significant challenges for conventional geochronological approaches, necessitating the application of secondary ion mass spectrometry (SIMS) to obtain reliable age data due to its superior spatial resolution capabilities. The mineralization demonstrates a stable paragenetic assemblage dominated by quartz-fluorite-hematite-pyrite associations, with subordinate occurrences of sulfides including sphalerite. This diagnostic mineral assemblage indicates hydrothermal conditions within the mesothermal to hypothermal range (150–250 °C), consistent with low-temperature hydrothermal genetic classification. Previous investigations have proposed that initial ore-forming fluids originated as magmatic water derived from granitic magma differentiation, with CO₂ in mineralizing fluids primarily contributed by lamprophyric magmatism, and uranium predominantly sourced from granitic protoliths. These constraints collectively demonstrate dual genetic relationships between uranium mineralization and both granitic and lamprophyric magmatic systems, with granites serving as primary metal sources and lamprophyres acting as critical volatile contributors during mineralization.

Mineralogical investigations of the Xianshi and Zhaixia deposits reveal that pitchblende constitutes the primary uranium-bearing phase. The metallic mineral assemblage is characterized by a pitchblende-pyrite paragenetic system, accompanied by subordinate sulfides including galena, chalcopyrite, and chalcocite. Gangue mineral associations are dominated by calcite, with fluorite and microcrystalline quartz as secondary constituents. Pitchblende predominantly occurs in vein-type and massive aggregates, providing favorable textural characteristics for micro-scale geochronological analysis. Paragenetic sequence analysis identifies three distinct mineralization types: (1) pitchblende-pyrite-red microcrystalline quartz association; (2) pitchblende-fluorite assemblage; (3) pitchblende-calcite mineralization. The mineralized zones exhibit intense alteration processes, prominently featuring albitization, K-feldspathization, silicification, chloritization, hematitization, and epidotization. Among these, hematitization and chloritization demonstrate the strongest spatial correlation with uranium mineralization. Previous research data indicate a mineralization temperature range of 250–150 °C, classifying these deposits as typical mesothermal to hypothermal hydrothermal systems.

Chronostratigraphic investigations of the Xiazhuang ore field, based on previous isochron age determinations, reveal two principal mineralization epochs: an Early Cretaceous phase (122–138 Ma), characterized by post-magmatic hydrothermal uranium mineralization, and a Late Cretaceous-Paleogene phase (54–96 Ma), representing magmatically reactivated hydrothermal reworking mineralization. However, the limited isochron age data available undermine the overall persuasiveness of the interpretation. The metallogenic evolution exhibits distinct multistage characteristics. Early-stage uranium mineralization primarily occurs within granitic plutons, forming pitchblende-albite-chlorite paragenetic assemblages. Late-stage mineralization is distinguished by abundant calcite development, generating stable calcite-pitchblende or pyrite-pitchblende associations concentrated within contact zones between granitic rocks and mafic dikes, constituting typical intersection-type deposits.

4. Test Methods and Procedures

4.1. Technical Methods and Instrument Performance

The present study collected high-grade ore samples from the No. 8 pit of the Hope deposit, 510 middle section of the Xianshi deposit, and 410 middle section of the Zhaixia deposit (

Table 2. The sampling position of Xiazhuang ores.

Sampling Location		Sample
Xianshi mineral deposit	High-grade ore, 510 level	510-1
	High-grade ore, 511 level	510-2
	High-grade ore, 512 level	510-3
	High-grade ore, 513 level	510-4
	High-grade ore, 514 level	510-5
	High-grade ore, 515 level	510-6
	High-grade ore, 516 level	510-7
	High-grade ore, 517 level	510-8
Xiwang mineral deposit	Pit No. 8	XW-1
Zhaixia mineral deposit	Section 410 Middle	ZX-13

). Thickened electron probe sections (0.15–0.20 mm thick) were prepared at the Xi’an Geological Survey Center. Large-grained pitchblende minerals were identified and mapped using an electron probe microanalyzer (EMX-SM7 model). Standard pitchblende reference material GBW04420 from the 201 uranium deposit in the Xiaozhuang ore field (same mining area) was selected as a calibration standard. Isotopic age analyses were conducted using the SHRIMP-RG ion microprobe at the Research School of Earth Sciences (RSES), Australian National University (ANU). Age results were complemented by in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of rare earth elements in pitchblende to investigate uranium mineralization processes and genetic mechanisms.

The collected samples were processed into thickened probe sections for electron probe analysis and optical microscopic observation. Uranium minerals with well-developed crystal forms, minimal alteration, and no fractures were selected. The uranium ore samples were then mounted on targets, ensuring flat and regular surfaces free from impurities. Gold-coated sample targets were stored in the sample chamber overnight prior to analysis.

The SHRIMP RG (reverse geometry) secondary ion mass spectrometer comprises four core components: a sample chamber, magnetic sector system, electrostatic analyzer, and mass spectrometry detector. During analysis, an O²⁻ primary ion beam (4–6 nA intensity) was accelerated to bombard the sample surface, forming elliptical beam spots through an entrance aperture, with sputtering depths controlled at ~1 μm. The initial sample acceleration voltage of +7.95 kV was precisely adjusted to a +8.0 kV reception voltage using an electrostatic analyzer to optimize ion transmission efficiency. The mass spectrometry detection system sequentially measured ion beams, including ²⁰⁴Pb⁺, ²⁰⁶Pb⁺, ²⁰⁷Pb⁺, ²⁰⁸Pb⁺, ²³⁵U⁺, and ²³⁸U⁺. To effectively mitigate hydride isobaric interferences, systematic application of a 50 V compensation potential was implemented to optimize matrix effect suppression. Controlled reduction of inlet/outlet aperture dimensions achieved peak flattening. Analytical uncertainties primarily stem from mass fractionation effects (IMF), with statistical analysis of extensive experimental datasets confirming this phenomenon as attributable to chemical compositional discrepancies between samples and reference materials. Therefore, certified reference material GBW04420 with similar composition to the samples was used for calibration. During the analytical phase, standard-sample bracketing was employed for calibration. This involved comparing SIMS-measured values of the reference material against its certified TIMS isotopic values to derive correction factors. These factors were then applied to implement mass bias correction for U-Pb isotopic age calculations using SIMS data. All results were processed with ISOPLOT software.

4.2. Preparation Before Sample Testing

Probe Slice Processing: Based on preliminary backscattered electron images and electron probe microanalysis (EPMA) results, Based on prior EPMA work, larger-sized uranium mineral grains were selected under the microscope as targets for SIMS U-Pb isotope analysis. Selected areas were excised from the thin sections and polished into sample mounts. During mounting, double-sided adhesive tape was affixed to glass to prepare standard cylindrical mounts (25 mm diameter). To ensure analytical precision, reference standards were placed at the center of the mount, while target mineral samples were tightly arranged around the standards. A blank margin of no less than 3 mm was reserved at the edge of the mount to meet the technical requirements of the SHRIMP (Sensitive High-Resolution Ion Microprobe) instrument’s fixation device.

Epoxy Resin Embedding: A mixture of 0.6 g EPIREZ curing agent and 5 g EPIREZ compound was thoroughly stirred to prepare epoxy resin, which was then poured into a 25 mm polytetrafluoroethylene (PTFE) hollow cylinder. The selected mineral samples were positioned at the center of the mold. To eliminate bubbles generated during resin curing, the mold was placed in a vacuum drying oven for 5 min, followed by suspension on a 60 °C constant-temperature hotplate for over 12 h to complete curing. The cured resin column was dried, then cut and polished into a 7 mm-thick disc.

Cleaning and Coating: Prior to testing, surface treatment was performed. A 1 μm diamond abrasive polishing solution was used to remove surface carbon layers, followed by sequential ultrasonic cleaning with ultrapure water and alcohol to thoroughly eliminate residual carbon. After drying, a gold film was sputtered onto the surface. Two sample mounts, Z6734 and Z6833, were prepared.

Mount Mapping: To precisely locate SHRIMP analysis positions, systematic SEM imaging and coordinate mapping were conducted for both mounts. Uranium mineral grains with intact crystal forms and clean surfaces were selected as test points, with their spatial coordinates recorded. To ensure data independence and representativeness, a minimum spacing of 30 μm between test points was strictly maintained.

5. Test Results

5.1. Age Results

5.1.1. Xianshi Ore Deposit

The test results of the Xianshi ore deposit are shown in

Table 3. SIMS U-Pb isotopic data and ²⁰⁶Pb*/²³⁸U ages of uraninite from the Xianshi deposit.

Sample_ Grain.Spot	206Pb*/238U (204Pb Corrected)	±%	207Pb*/235U (204Pb Corrected)	±%	Error Correlation	206Pb*/238U Age (Ma) (207Pb Corrected)	± 1 s
510-4-2_U_0.1	0.00730	7.90	0.0470	32.60	0.240	46.70	4
510-4-2_U_0.2	0.00760	8.20	0.0530	35.20	0.230	48.50	4
510-4-2_U_1.1	0.00660	7.10	0.0460	7.60	0.940	42.60	3
510-4-2_U_2.1	0.00690	7.00	0.0480	7.10	0.990	44.60	3
510-4-2_U_3.1	0.00770	7.00	0.0510	7.20	0.980	49.20	3
510-4-2_U_4.1	0.00740	7.10	0.0510	7.30	0.980	47.40	3
510-4-2_U_6.1	0.00720	7.10	0.0490	7.90	0.890	46.30	3
510-4-2_U_5.1	0.00720	7.00	0.0490	7.50	0.940	46.30	3
510-4-2_U_8.1	0.00720	7.00	0.0490	7.20	0.980	46.20	3
510-4-2_U_7.1	0.00700	7.10	0.0490	7.20	0.980	45.10	3
510-7-2_U_12.1	0.01140	7.10	0.0740	8.10	0.870	72.90	5
510-4-2_U_9.1	0.00720	7.00	0.0490	7.20	0.970	46.00	3
510-7-2_U_18.1	0.01220	7.10	0.0850	7.90	0.900	78.40	6
510-4-2_U_10.1	0.00720	7.00	0.0500	7.30	0.970	46.00	3
510-7-2_U_15.1	0.01040	7.10	0.0760	7.90	0.890	66.90	5
510-7-2_U_11.1	0.01040	7.10	0.0740	7.90	0.890	66.60	5
510-7-2_U_14.1	0.01160	7.10	0.0790	8.50	0.830	74.60	5
510-7-2_U_16.1	0.01180	7.00	0.0780	8.10	0.870	75.60	5
510-7-2_U_19.1	0.01210	7.10	0.0820	8.00	0.890	77.70	5
510-7-2_U_20.1	0.01070	7.10	0.0790	8.70	0.810	68.70	5
510-7-2_U_21.1	0.01270	7.10	0.0920	7.70	0.920	81.10	6
510-7-2_U_22.1	0.01230	7.10	0.0840	7.60	0.930	79.00	6
510-7-2_U_25.1	0.01130	7.10	0.0820	7.80	0.910	72.70	5
510-7-2_U_23.1	0.01220	7.10	0.0820	7.70	0.910	78.10	5
510-7-2_U_24.1	0.01130	7.10	0.0760	7.90	0.900	72.20	5

Note: Errors are reported at 1σ. The 1σ mean error in the calibration of reference material GBW04420 is <1.025%, where Pb* denotes radiogenic lead.

. Twenty-five valid data points were obtained from uranium mineral grain analyses. Based on SIMS U-Pb isotopic and ²⁰⁶Pb/²³⁸U data of uranium minerals, the calculated mineralization ages predominantly range from 42.6 ± 3 Ma to 81.1 ± 6 Ma. Combined with the ²⁰⁶Pb/²³⁸U weighted mean ages of uranium minerals, the ages can be further categorized into two groups: 46.1 ± 1.9 Ma (MSWD = 0.28) and 73.1 ± 2.9 Ma (MSWD = 0.69), corresponding to samples from different ore sections (Figure 5). Within these two mineralization age ranges, 12 and 13 data points fall into the two groups, respectively. The low MSWD values indicate reliable results. The mineralization occurred in two stages: the Late Cretaceous and the Eocene.

5.1.2. Zhaixia Ore Deposit

The SIMS U-Pb isotopic geochronology analysis of uranium minerals from the Zhaixia deposit (

Table 4. SIMS U-Pb isotopic data and ²⁰⁶Pb*/²³⁸U ages of uraninite from the Zhaixia deposit.

Sample_ Grain.Spot	206Pb*/238U (204Pb Corrected)	±%	207Pb*/235U (204Pb Corrected)	±%	Error Correlation	206Pb*/238U Age (Ma) (207Pb Corrected)	±1 s
ZX-13-54_U_1.1	0.0116	7.10	0.0850	9.20	0.770	74.50	5
ZX-13-54_U_2.1	0.0113	7.10	0.0840	10.00	0.700	72.50	5
ZX-13-54_U_5.1	0.0111	7.10	0.0860	8.00	0.880	71.30	5
ZX-13-54_U_3.1	0.0111	7.10	0.0900	9.20	0.760	71.20	5
ZX-13-54_U_13.1	0.0110	7.10	0.0770	11.10	0.640	70.80	5
ZX-13-54_U_4.1	0.0121	7.10	0.0860	9.70	0.730	77.60	5
ZX-13-54_U_10.1	0.0121	7.10	0.0870	8.40	0.840	77.80	5
ZX-13-54_U_6.1	0.0122	7.10	0.0890	8.80	0.800	78.10	5
ZX-13-54_U_9.1	0.0113	7.10	0.0870	8.10	0.870	72.50	5
ZX-13-54_U_15.1	0.0113	7.10	0.0810	9.30	0.760	72.60	5
ZX-13-54_U_11.1	0.0118	7.10	0.0880	8.50	0.830	75.40	5
ZX-13-54_U_12.1	0.0115	7.10	0.0820	8.00	0.880	73.80	5
ZX-13-9-3_U_11.1	0.0115	7.10	0.0850	8.30	0.850	73.80	5
ZX-13-9-3_U_2.1	0.0109	7.10	0.0780	8.10	0.870	70.00	5
ZX-13-9-3_U_1.1	0.0115	7.10	0.0850	7.80	0.910	73.60	5
ZX-13-9-3_U_6.1	0.0123	7.10	0.0950	8.40	0.860	78.80	6
ZX-13-9-3_U_3.1	0.0113	7.10	0.0880	11.40	0.620	72.30	5
ZX-13-9-3_U_4.1	0.0102	7.10	0.0690	9.00	0.780	65.60	5
ZX-13-9-3_U_5.1	0.0100	7.10	0.0740	8.50	0.830	64.30	5
ZX-13-9-3_U_10.1	0.0098	7.10	0.0710	8.50	0.830	62.90	4
ZX-13-9-3_U_8.1	0.0097	7.10	0.0680	8.80	0.800	62.20	4
ZX-13-9-3_U_13.1	0.0099	7.10	0.0690	8.60	0.820	63.80	4
ZX-13-9-3_U_7.1	0.0110	7.10	0.0770	7.90	0.890	70.80	5
ZX-13-9-3_U_15.1	0.0105	7.10	0.0690	8.70	0.810	67.50	5
ZX-13-9-3_U_12.1	0.0108	7.10	0.0740	8.10	0.870	68.90	5

Note: Errors are reported at 1σ. The 1σ mean error in the calibration of reference material GBW04420 is <1.025%, where Pb* denotes radiogenic lead.

) reveals that 25 valid measurements yield ²⁰⁶Pb/²³⁸U mineralization ages ranging from 62.2 ± 4 Ma to 78.8 ± 6 Ma. These data points cluster around a weighted average ²⁰⁶Pb/²³⁸U age of 70.7 ± 2 Ma (MSWD = 0.97), demonstrating excellent age concordance (Figure 6). Statistical results indicate that all analytical data fall within this confidence interval, confirming the high reliability of the measurements. Similar to the Xianshi deposit, the mineralization period corresponds to the Late Cretaceous epoch.

5.1.3. Xiwang Ore Deposit

The SIMS U-Pb isotopic geochronology analysis results of uranium minerals from the Hope deposit (

Table 5. SIMS U-Pb isotopic data and ²⁰⁶Pb*/²³⁸U ages of uraninite from the Xiwang deposit.

Sample_ Grain.Spot	206Pb*/238U (204Pb Corrected)	±%	207Pb*/235U (204Pb Corrected)	±%	Error Correlation	206Pb*/238U Age (Ma) (207Pb Corrected)	±1 s
XW-1-1_U_5.1	0.00420	7.10	0.028	7.20	0.980	27.200	2
XW-1-1_U_16B.1	0.00540	7.20	0.037	7.30	0.980	34.400	2
XW-1-1_U_16B.2	0.00510	7.60	0.034	7.80	0.970	32.700	2
XW-1-1_U_9.2	0.00500	8.00	0.034	8.10	0.980	32.200	3
XW-1-3_U_21.1	0.00430	7.00	0.032	7.10	0.990	27.500	2
XW-1-1_U_7.2	0.00530	7.00	0.036	7.20	0.980	34.000	2
XW-1-3_U_23.2	0.00530	7.10	0.037	7.30	0.980	34.200	2
XW-1-3_U_25.2	0.00530	7.10	0.038	7.30	0.980	34.400	2
XW-1-1_U_8.2	0.00650	7.20	0.044	7.30	0.990	41.500	3
XW-1-3_U_26.2	0.00480	7.00	0.034	7.10	0.990	31.100	2
XW-1-3_U_30B.1	0.00440	7.00	0.032	7.10	0.990	28.200	2
XW-1-1_U_1.1	0.00580	7.40	0.041	7.60	0.970	37.100	3
XW-1-1_U_1.2	0.00710	7.10	0.049	7.20	0.990	45.800	3
XW-1-1_U_2.1	0.00630	7.50	0.043	7.60	0.990	40.300	3
XW-1-1_U_2.2	0.00590	7.10	0.042	7.10	1.000	37.800	3
XW-1-1_U_3.1	0.00690	7.00	0.047	7.10	1.000	44.400	3
XW-1-1_U_4.1	0.00650	7.00	0.043	7.10	0.990	41.800	3
XW-1-3_U_24B.1	0.00760	7.00	0.055	7.10	0.990	49.000	3
XW-1-3_U_1.3	0.00660	7.10	0.045	7.20	0.990	42.700	3
XW-1-3_U_4.2	0.00660	7.00	0.044	7.10	0.990	42.500	3
XW-1-3_U_17.2	0.00750	7.00	0.050	7.10	1.000	48.000	3
XW-1-3_U_18.2	0.00650	7.00	0.044	7.10	0.990	42.000	3
XW-1-3_U_3B.1	0.00640	9.00	0.044	9.10	0.990	41.000	4
XW-1-3_U_29B.1	0.00690	8.40	0.049	9.50	0.890	44.600	4

Note: Errors are reported at 1σ. The 1σ mean error during calibration with reference material GBW04420 is <1.025%. Pb* denotes radiogenic lead.

) indicate that 24 valid data points were selected from 25 tested spots. After ²⁰⁴Pb correction, the ²⁰⁶Pb/²³⁸U apparent ages range from 27.2 ± 2 Ma to 49.0 ± 3 Ma. Statistical processing reveals that these age data can be divided into two distinct age groups with significant differences: The first group comprises 10 data points, yielding a weighted mean age of 31.0 ± 2.2 Ma (MSWD = 1.9), while the second group contains 14 data points, with a weighted mean age of 42.3 ± 2.0 Ma (MSWD = 1.2). The mineralization age corresponds to the Eocene-Oligocene epoch, indicating a relatively young metallogenic timing (Figure 7).

5.2. Micro-Area In Situ Uranium Mineral Rare Earth Element Analysis

The in situ micro-scale rare earth element (REE) analysis of pitchblende from the Xianshi and Zhaixia deposits reveals highly similar geochemical characteristics. Both show REE distribution patterns with weak fractionation between light and heavy REEs (LREE/HREE), though generally exhibiting relative LREE enrichment, as evidenced by their gently sloping normalized distribution curves. Notably, despite generally high total REE contents (ΣREE), both display significant negative δEu anomalies. The consistent REE characteristics of pitchblende formed during different mineralization stages in both deposits indicate continuity in genetic environments and material sources. Particularly significant is the striking similarity between these pitchblende REE patterns and those of crust-mantle hybrid granites, strongly suggesting that uranium and other ore-forming elements in the mineralizing fluids likely originated from deep crust–mantle interaction processes, involving both ancient crustal material contributions and mantle-derived components. The REE concentrations in these deposits are significantly higher than surrounding rock background values, with statistical data showing marked content differences. Importantly, the strong positive correlation between uranium and REEs indicates that uranium mineralization processes dominated the abnormal REE enrichment in these deposits.

Based on high-precision geochronological dating of pitchblende from three typical deposits (Xianshi, Zhaixia, and Xiwang) in the study area, this research innovatively conducted in situ micro-scale REE geochemical analysis (detailed data in

Table 6. In situ REE data of uraninite in the Xianshi deposit.

Sample	ppm															ΣREE	LREE	HREE	LaN/YbN	δEu	δCe
	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y
510-1-2-3-1	1262	4606	695	3011	742	289	497	143	821	146	341	44	243	30	229	12,870	10,605	2265	3.5	1.38	1.17
510-1-2-3-2	1221	5140	886	3584	924	346	564	155	897	164	359	50	264	33	216	14,587	12,100	2487	3.11	1.36	1.14
510-4-2-1-1	1162	4626	838	3647	999	385	701	181	1046	177	445	60	334	40	327	14,640	11,656	2984	2.34	1.34	1.08
510-4-2-2-3	1056	3682	681	3261	1014	364	880	182	1078	193	460	61	370	42	324	13,324	10,058	3266	1.92	1.15	1.01
510-7-2-2-1	2253	6091	861	4642	1266	575	1886	307	1862	395	913	112	659	81	2061	21,903	15,688	6215	2.31	1.14	1.05
510-7-2-2-1	2366	6929	997	5192	1492	661	2098	343	2033	415	1006	123	707	82	2982	24,443	17,636	6807	2.25	1.14	1.09
510-7-2-2-2	2752	5811	776	3862	998	469	1607	237	1361	295	701	84	429	57	1086	19,438	14,668	4770	4.33	1.13	0.94
510-7-2-2-3	2755	5402	718	3753	1080	510	1664	267	1673	325	822	103	578	73	1316	19,725	14,218	5507	3.21	1.16	0.91

,

Table 7. In situ REE data of uraninite in the Zhaixia deposit.

Sample	ppm															ΣREE	LREE	HREE	LaN/ YbN	δEu	δCe
	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y
ZX-13-5-3-1-5	1262	4606	695	3011	742	289	497	143	821	146	341	44	243	30	2836	12,870	10,605	2265	3.5	1.38	1.17
ZX-13-5-3-1-6	1221	5140	886	3584	924	346	564	155	897	164	359	50	264	33	2741	14,587	12,100	2487	3.11	1.36	1.14
ZX-13-9-1-2	1234	4021	598	2692	750	306	540	137	832	155	383	50	297	34	2493	12,030	9602	2428	2.8	1.4	1.12
ZX-13-5-1-2-1	1611	5524	928	3865	1279	385	1008	216	1349	223	592	88	594	63	3633	17,724	13,592	4133	1.83	1	1.07
ZX-13-5-1-2-2	1514	4987	833	3626	1175	349	933	204	1199	219	540	82	552	61	3522	16,275	12,484	3791	1.85	0.99	1.05
ZX-13-5-1-2-3	1484	4342	705	3079	997	308	864	181	1123	205	524	74	507	56	3410	14,449	10,915	3534	1.97	0.99	1.02
ZX-13-5-1-2-4	1300	3966	640	2809	865	274	776	163	1035	182	476	67	453	54	3144	13,061	9855	3207	1.93	1	1.04
ZX-13-5-1-2-5	1443	4516	713	3306	1035	309	854	179	1123	191	509	73	474	57	3305	14,782	11,322	3459	2.05	0.98	1.06
ZX-13-5-1-2-6	1463	4545	738	3271	1057	328	863	187	1226	210	510	77	513	61	3305	15,048	11,401	3647	1.92	1.02	1.05
ZX-13-5-1-2-9	1549	5202	860	3912	1198	374	947	205	1268	226	554	85	567	59	3488	17,006	13,095	3911	1.84	1.04	1.07

and

Table 8. In situ REE analytical data of uraninite from the Xiwang uranium deposit.

Number	Sample	Lithology	ppm															ΣREE	HREE	LREE	LREE/ HREE	LaN/ YbN	δEu	δCe
			Ce	Sm	Er	Dy	Lu	Tm	La	Pr	Nd	Eu	Gd	Tb	Ho	Yb	Y
1	xw-1-5-1-1	Volcanic rock	6490	448	261	484	35	33	2537	669	2394	30	169	81	96	197	2348	13,926	1358	12,568	9.25	8.67	0.28	1.18
2	xw-1-5-1-2		5990	341	205	384	30	30	2279	592	2011	22	133	70	77	168	1970	12,331	1097	11,234	10.24	9.13	0.27	1.22
3	xw-1-5-2-1		2912	103	60	109	9	8	1471	233	704	7	45	21	23	44	694	5748	317	5431	17.12	22.72	0.28	1.08
4	xw-1-5-2-2		3905	170	98	179	13	13	1638	333	1030	11	66	32	35	79	1014	7602	516	7086	13.73	13.95	0.27	1.21
11	XW-1-4-3-3		3176	350	193	342	26	27	1669	367	1522	23	388	71	70	189	2358	8413	1306	7107	5.44	5.96	0.19	0.94
12	XW-1-4-3-4		3277	443	254	465	38	34	1598	438	1859	35	473	77	90	232	2946	9315	1664	7651	4.60	4.65	0.23	0.93
7	XW-1-3-2-6		2012	80	35	64	6	6	1160	191	577	5	50	13	13	35	397	4247	223	4025	18.08	22.11	0.24	0.94
8	XW-1-3-2-7		2156	73	27	57	4	4	1268	190	508	4	41	11	12	28	412	4385	184	4200	22.77	30.72	0.22	0.94
5	xw-1-5-3-1		2499	64	37	65	5	4	1378	170	475	4	24	11	13	26	433	4774	185	4589	24.81	36.35	0.25	1.06
6	xw-1-5-3-2		2508	55	31	54	4	4	1309	170	452	4	21	11	12	24	388	4657	160	4498	28.17	36.35	0.28	1.11
9	XW-1-3-2-8		2547	80	34	67	5	5	1456	211	597	5	42	13	13	28	537	5101	205	4896	23.88	35.68	0.24	0.98
10	XW-1-3-2-9		3082	95	48	93	7	7	1545	269	735	6	36	17	19	36	623	5997	265	5732	21.66	29.30	0.27	1.06

; distribution patterns in Figure 8, Figure 9 and Figure 10). The REE characteristics of the study area show the following features.

All three deposits exhibit ΣREE values significantly higher than the average surrounding rock content, with particularly prominent REE enrichment in high-uranium samples. This indicates uranium mineralization as the primary geological process driving REE enrichment in the ores.

Xianshi and Zhaixia deposits demonstrate highly consistent REE distribution patterns: gently-sloping curves with weak LREE/HREE fractionation and distinct negative δEu anomalies. The identical REE characteristics of pitchblende from different mineralization stages in both deposits reflect continuous genetic environments and material sources. The remarkable similarity between these pitchblende patterns and those of crust–mantle hybrid granites strongly suggests that ore-forming elements (including uranium) in the mineralizing fluids originated from deep crust–mantle interaction processes, involving both ancient crustal materials and mantle-derived components.

The Xiwang deposit displays unique REE characteristics: its distribution pattern shows significant LREE/HREE fractionation and extreme δEu depletion, indicating mantle-derived metallogenic materials.

6. Discussion

Since the discovery of the Xiwang uranium deposit in northern Guangdong, numerous studies have been conducted on its uranium sources, tectonic controls, multi-stage mineralization, and hydrothermal activities, leading to various insights into its genesis and metallogenic processes. However, due to limitations in analytical techniques, uncertainties remain regarding the precise chronology of uranium mineralization. This study employs state-of-the-art microanalytical techniques to investigate the mineralization age spectrum, providing new evidence for the coupling between hydrothermal activity and tectonic events in the Xiazhuang ore field, as well as constraints on uranium sources. The results lay a crucial foundation for a deeper understanding of the genetic mechanisms of granite-hosted uranium deposits in the Xiazhuang area.

6.1. In Situ Microscale Geochronology of Uranium Mineralization in the Xiazhuang Ore Field

Using GBW04420 as the reference material, we conducted in situ SIMS U-Pb isotopic dating on uranium minerals from the Xiwang, Xianshi, and Zhaixia deposits. The results, along with previous in situ dating data, are summarized in

Table 9. Summary of micro-scale in situ metallogenic ages for uranium deposits in the Xiazhuang ore field.

Ore Field	Metallogenic Age (Ma)	Data Sources	Test Method
Xianshi	Late Cretaceous (K2): 73.1 ± 2.9 Eocene (E2): 46.1 ± 1.9	This study	SIMS (SHRIMP RG)
	Early Cretaceous (K1): 113 ± 2; 104 ± 2	Luo Jincheng, HOu Ruizhong, 2015 [49]	SIMS (CAMECA ims)
	Late Cretaceous (K2): 96.4 ± 1.4	He Debao, 2016 [50]	LA-ICP-MS
	Latest Late Cretaceous (K3): 79 ± 11	Zheng Xin, 2019 [34]	SIMS (CAMECA ims)
Zhaixia	Late Cretaceous (K2): 70.7 ± 2	This study	SIMS (SHRIMP RG)
	Late Cretaceous (K2): 92	He Debao, 2016 [50]	LA-ICP-MS
	Late Cretaceous (K2): 93.5 ± 1.2; 73.1 ± 1.4	Zou Dongfeng, 2011 [22]	LA-ICP-MS
Xiwang	Eocene (E2): 42.3 ± 2.0; Oligocene (E3): 31.0 ± 2.2	This study	SIMS (SHRIMP RG)
	Late Cretaceous (K2): 81.8 ± 1.1	He Debao, 2016 [50]	LA-ICP-MS
	Early Cretaceous (K1): 107 ± 16	Zheng Xin, 2019 [34]	SIMS (CAMECA ims)

. The three deposits share broadly similar age characteristics, with formation concentrated in the Early Cretaceous, Late Cretaceous, and Eocene-Oligocene, respectively. This indicates a unified metallogenic background and environment across the Xiazhuang ore field.

Although the deposit types vary—Xiwang is a silicified fracture zone-type deposit, while Xianshi and Zhaixia are intersection-type uranium deposits—their metallogenic age characteristics exhibit similarities, further supporting a unified geodynamic setting. Locally, the Xiwang deposit exhibits slightly younger ages, including Oligocene mineralization, which may be related to its specific geological characteristics. Electron probe microanalysis (EPMA) reveals that uranium minerals in the Xiwang deposit occur as fine veinlets with small grain sizes and cross-cutting relationships with pitchblende, suggesting late-stage secondary uranium mineralization. All three deposits record Late Cretaceous mineralization, highlighting this period as a critical and dominant metallogenic stage.

The mineralization ages obtained in this study are younger than the Late Cretaceous and differ from the emplacement ages of the Early Yanshanian granites in the region. Therefore, the uranium deposits in this study are not magmatic-type but rather post-magmatic hydrothermal deposits.

6.2. Coupling Characteristics Between Mineralization and Tectonic Dynamics

6.2.1. Tectonic Evolution Background

The regional tectonic evolution underwent a transition from compression to extension and subsequently to transpression (

Table 10. Coupling relationships between mineralization and tectonic-magmatic-hydrothermal events in the Xiazhuang ore field.

Age			Tectonic Stage	Tectonic Activity	Geological Event	Magmation	Basic Dike Emplacement	This Age (Ma)
Kz	Q		60 Ma~Now, Himalayan Stage
	N	N2		Fault block differential rise and fall	Frequent hydrothermal activity, small fracture activity
		N1
	E	E3			Intermittent lifting motion			31.0 ± 2.28, E3
		E2			Strong differential ascending and descending motion, NW–SE stretching		(55~45) Ma	46.1 ± 1.9, 42.3 ± 2.0, E2
		E1
Mz	K		(205~60) Ma, Yanshanian Stage	NW–SE spreading, late Yanshan stage	Elongation rift period, U-rich mantle fluid rising; Intermediate-base magma emplacement along faults; Lots of high angle normal faults and formation of slip fault	multi-stage secondary basic dike emplacement, deep silicon-rich uranium-rich fluid rising	(75~70) Ma, (95~85) Ma, (110~100) Ma, (125~115) Ma	73.1 ± 2.9, 70.7 ± 2, K2
	J			Early Yanshan stage, spreading from south to north	Post-orogeny, Formation of near E–W extension basin and NW–W diabase dikes	Yanshanian Stage granite	(140~135) Ma
	T		(257~205) Ma, Indochinese stage	Lithospheric compression-collision	Intracontinental orogeny, Intracontinental subduction and orogeny; Deep lithosphere extension	Indosinian granite

). Existing research confirms that the main uranium mineralization stages in South China exhibit clear spatiotemporal consistency with regional extensional phases. The specific tectonic evolution process is outlined as follows.

The Sibao Orogeny formed the earliest crystalline basement (Proterozoic basement), which exerted significant control over both the formation and subsequent development of the ore field. During the Indosinian Orogeny, the Huangpo Fault (northern sector) and Mashishan Fault (southeastern sector) were developed, collectively constituting the fundamental structural framework of the ore field.

The Yanshan and Himalayan tectonic movements had their most significant influence during the Jurassic-Cretaceous period. The Early Yanshanian stage was in a state of compressional uplift, forming acidic to intermediate-acidic magmatic eruptions and intrusions in this area. Among these, the Guidong composite granite batholith, affected by multi-phase Indosinian-Yanshanian movements, mainly formed through three rock-forming stages: first, the Late Indosinian to Early Yanshanian biotite granite formed during T_2-3-J₁; second, the Middle Yanshanian muscovite granite formed in the Late Jurassic; and finally, the Late Yanshanian granite syenite formed in the Early Cretaceous. During this stage, the geodynamic regime underwent significant transformation, gradually transitioning from the earlier compressional environment to an extensional setting, with the principal compressive stress direction adjusted to NE-SW. This tectonic characteristic continued to evolve until the Paleocene. Under this tectonic background, intermediate-acidic magmas and basic magmas intruded along NNE-trending faults, forming two typical types of dikes: acidic and basic.

During the Early Cretaceous, uranium-bearing hydrothermal fluids migrated upward along fault structural channel systems, forming an important early uranium mineralization event in the Xiazhuang ore field. By the end of the Late Cretaceous, a low-temperature hydrothermal mineralization system developed in association with the final remelting event triggered by regional tectonic-thermal activity.

The Cenozoic tectonic evolution of the Xiazhuang ore field underwent three main stages: The first stage was characterized by differential uplift and subsidence; the second stage featured intermittent uplift and subsidence; while the third stage was dominated by fault activity. These tectonic activities vertically formed a graben-style structural framework composed of three fault sets: NNE trending, NEE trending, and near-EW trending [36].

Although the tectonic activity during this period was relatively weak in intensity, it still produced significant geological effects: First, it formed late-stage calcite veins and white quartz veins; Second, it reactivated uranium and caused small-scale mineralization. The Eocene-Oligocene metallogenic ages obtained in this study were formed under this tectonic background.

This tectonic evolutionary process can reasonably explain the unique mineralization characteristics of the Xiwang deposit (Eocene-Oligocene mineralization). Electron microprobe analysis shows that the uranium mineral grains are fine-grained. Backscattered electron images indicate that uranium minerals mainly occur as veinlets. These characteristics are distinctly different from the mineralization styles of other deposits in the ore field.

6.2.2. Metallogenesis-Regional Tectonic Event Coupling Mechanism: Regional Magmatic-Hydrothermal Activity

Since the Mesozoic Era, South China has undergone significant crust–mantle interaction, triggering large-scale lithospheric thinning and magmatism, which provided an essential geological framework for the polymetallic mineralization in the granitic belt of the region. Within the study area, Indosinian acidic granitic bodies, represented by the Guidong composite pluton, are extensively developed.

During the Middle Yanshanian Movement (150–100 Ma), the regional tectonic environment transitioned into a shallow-level thermal dome extensional regime. Against this backdrop, deep-seated magmas intruded upward in a pulsating, small-scale manner along structural conduits, forming a series of small intrusions in the northern part of the ore field. Concurrently, deep-derived mafic magmas ascended to form diabase dikes. Subsequently, silica- and uranium-rich hydrothermal fluids rose along fault zones, with their activity periods closely coinciding with multiple uranium mineralization events in the ore field.

This tectonic-magmatic evolutionary process persisted until the late Yanshanian Movement (55–45 Ma), ultimately concluding with the emplacement of diabase porphyrite and diabase dikes. The complete evolutionary sequence thoroughly documents the dynamic processes of crust–mantle interaction and metallogenic response in South China since the Mesozoic.

Research indicates that hydrothermal uranium deposits exhibit multi-source characteristics for their ore-forming fluids, typically forming under oxidized conditions at medium-high pressure (50–200 MPa) and medium-low temperature (150–300 °C) environments. The fluid system consists of weakly acidic to weakly alkaline (pH 5–8) aqueous solutions containing CO₂. In northern Guangdong, uranium mineralization initiated during the Early Cretaceous (approximately 145–100 Ma), coinciding with the emplacement period of the earliest diabase dikes in the region.

Regional hydrothermal activity primarily manifests as (1) activation and migration of uranium in granites by hydrothermal fluids; (2) spatiotemporal coupling between hydrothermal alteration and uranium mineralization; (3) critical control of multi-stage hydrothermal superposition on uranium enrichment.

Uranium exhibits distinct late-stage enrichment characteristics during magmatic evolution, forming initial uranium pre-enrichment bodies. In magmatic-hydrothermal systems, uranium primarily exists in the U4+ form. Through leaching by meteoric water and interaction with mantle-derived fluids, it oxidizes to U6+ and forms stable complexes (e.g., [UO₂(CO₃)₃]⁴⁻), thereby achieving uranium activation and migration [43,46].

During the Late Yanshanian to Eocene-Miocene period, the Xiazhuang ore field experienced extensional rifting and differential uplift-subsidence movements, creating favorable conditions for mantle-fluid mineralization. Deep-seated faults control the occurrence of granite-hosted uranium deposits, while the hydrothermal circulation systems that had developed within the fault depression zones provided conducive pathways for mineral transport. Consequently, the deep faults formed during the Late Yanshanian movement, along with the numerous fractures and fissures developed during the Cenozoic Era, created an advantageous environment for the ascent of mantle-derived fluids and the leaching effects of meteoric water, ultimately leading to mineralization [51].

Based on comprehensive analysis of the aforementioned characteristics of tectonic evolution, magmatic activities, and hydrothermal activities, the following conclusions can be drawn: magmatic activities were mainly concentrated during the Indosinian-Early Yanshanian period, while hydrothermal activities occurred during the Late Yanshanian period and E₂-N₁. According to the results of in situ micro-area dating, post-magmatic hydrothermal activities formed granite-hosted uranium deposits, with additional participation of meteoric hydrothermal fluids during late stages. In summary, the formation of rocks and the uranium mineralization process did not occur simultaneously, indicating that the uranium deposits in this region are not magmatic-type uranium deposits but rather hydrothermal-type uranium deposits formed after magmatism.

Therefore, the fundamental cause of granite-hosted uranium deposit formation lies in the presence of large-scale post-magmatic hydrothermal activity. However, a significant temporal gap exists between granitic rock formation and subsequent mineralization. The hydrothermal systems themselves originated from regional large-scale tectonic events. Such causal relationships have been confirmed by multidisciplinary studies from various scholars [23,36,43,48,51]. Consequently, the coupling between mineralization events and tectonic events is not coincidental. Conversely, the accuracy of experimentally determined mineralization ages can be partially validated through correlations with regional tectonic events.

6.3. Evidence from Uranium Sources and REE Characteristics

The question of whether uranium was derived from the mantle or from granites themselves has seen significant debate in recent years. The mantle-derived uranium hypothesis has gained support, with studies suggesting that mantle fluids can contain uranium concentrations of (6.0–51.0) × 10⁻⁶, forming uranium-rich mantle-derived fluids [52,53,54,55].

During in situ micro-scale dating of uraninite from the three deposits, rare earth element (REE) analyses were also conducted. The results are detailed in Table 6, Table 7 and Table 8 and Figure 8, Figure 9 and Figure 10. Key findings include the following.

The REE (Rare Earth Element) distribution curve of the Xiwang deposit shows a severe depletion of δEu, accompanied by a slight fractionation between light and heavy rare earth elements, with a slight enrichment of light rare earth elements. This characteristic clearly indicates that the ore-forming materials of this deposit are derived from the mantle.

The total amount of rare earth elements ΣREE is relatively large, which is much higher than that of the surrounding rocks. This demonstrates that uranium mineralization is the main factor contributing to the enrichment of rare earth elements in the ores.

The REE distribution curves of the Xianshi and Zhaixia deposits are extremely similar. There is a mild depletion of δEu, and the total content of rare earth elements ΣREE is relatively high. The characteristics of these REE distribution curves are quite similar to those of granites with a mixed crust-mantle origin. Therefore, it can be inferred that the sources of ore-forming materials for these two deposits include the participation of crustal materials, and at the same time, there is also the mixing of mantle-derived materials [35].

In view of this, taking into account the understanding that the ore-forming geological conditions and tectonic backgrounds of the three deposits are similar, we can draw the conclusion that the ore-forming materials of the Xianshi deposit and the Zhaixia deposit are derived from the mixture of crustal materials and mantle materials. In contrast, the ore-forming materials of the Xiwang deposit mainly come from the mantle. In addition, based on the previous research on the temperature measurement characteristics of fluid inclusions, it was found that there are two unified homogeneous temperature peaks in these three deposits. This phenomenon indicates that the reduction and precipitation of uranium elements are more likely to be the result of the mixing of fluids from different sources, namely fluids derived from the crust and fluids derived from the mantle.

7. Conclusions

It is reasonable to use GBW04420 as the reference material for in situ U-Pb isotopic dating of uranium minerals in the micro-region. During the work of analyzing and testing the uniformity of the ages of uranium mineral grains in BW04420 by the fs-LA-MC-ICP MS method, a total of 25 uranium mineral grain ²⁰⁶Pb/²³⁸U age data were analyzed. The result shows that the average deviation of each datum is only 6.52%, and the data as a whole present a normal distribution, indicating that the ages of each uranium mineral grain are uniform; this shows that GBW04420 is suitable as a dating standard sample.

Using GBW04420 as the reference material, through the SIMS method, the in situ micro-region U-Pb isotopic series of ore-forming chronologies of the Xiwang, Xianshi, and Zhaixia deposits were obtained, and a set of advanced dating methods was summarized. From this, the age of the Xiwang deposit was measured to be Eocene-Oligocene; the Zhaixia deposit was of Late Cretaceous age; the Xianshi deposit had two ages, namely the Late Cretaceous and the Eocene. It is proved that the overall ore-forming epochs of the Xiazhuang ore field are mainly the two periods of Late Cretaceous and Eocene-Oligocene. Different from the formation ages of the Indosinian and Early Yanshanian diagenetic granites in this area, it is the same as the intrusion age of the basic dikes in the Late Yanshanian period. This indicates that the Xiazhuang ore field is a hydrothermal uranium deposit formed after the magmatic period, rather than a magmatic uranium deposit.

The research results of in situ SIMS U-Pb isotopic dating of uranium minerals in the micro-region and the study of rare earth elements strongly support the view that the mineralization is coupled with regional tectonic dynamics, magmatic activities, and hydrothermal activities. By comparing the research results with the tectonic-magmatic activity background, it is found that the mineralization period does not coincide with the magmatic diagenetic period but is consistent with the post-magmatic hydrothermal activity period. This indicates that the uranium deposits in the Xiazhuang ore field belong to the post-magmatic medium-low temperature hydrothermal origin type, and the hydrothermal fluids are derived from the combined action of crust-mantle hydrothermal fluids and meteoric water. From another perspective, these results and understandings also verify the rationality of the data obtained from the in situ SIMS U-Pb isotopic dating of uranium minerals in the micro-region.