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Article

The Yanshanian Uranium Mineralization Age and Its Geological Significance in the Dashigou Carbonatite-Type Mo-REE-U Deposit, East Qinling Orogen, China

by
Guangwen Huang
1,
Dehai Wu
2,*,
Furong Li
3,
Chunrong Pan
3,*,
Pengfei Fan
3 and
Zhuang Min
3
1
School of National Safety and Emergency Management, Qinghai Normal University, Xining 810008, China
2
Key Laboratory of Ionic Rare Earth Resources and Environment, Ministry of Natural Resources, Ganzhou 341000, China
3
State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(9), 928; https://doi.org/10.3390/min14090928
Submission received: 15 July 2024 / Revised: 23 August 2024 / Accepted: 5 September 2024 / Published: 11 September 2024
(This article belongs to the Special Issue Uranium: Geochemistry and Mineralogy)
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Abstract

:
The Dashigou deposit is one of the most representative carbonatite-type Mo-REE deposits in the East Qinling metallogenic belt of China, with a molybdenum resource of more than 180 kt and a rare earth resource of 37.8 kt. Recent exploration has revealed a considerable scale of uranium mineralization within this deposit. Therefore, this study conducted detailed mineralogical and EPMA U-Th-Pb chemical dating on the uranium mineralization in the Dashigou deposit. The results indicate that the U-ore body in the Dashigou deposit mainly consists in carbonatite veins, and principally as anhedral, mesh-like uraninite. The mineral assemblage is characterized by uraninite + rutile + bastnasite + parisite or brannerite. The uraninite displays geochemical compositions of high Y and Ce and low Si, Ti, and Mg. The EPMA U-Th-Pb chemical dating is 144 ± 3.1 Ma, representing the Yanshanian uranium mineralization age in the region. The newly discovered uranium mineralization age indicates that the deposit experienced a uranium remobilization event during the Cretaceous and was formed in an intracontinental orogenic and extensional environment post-collision orogeny.

Graphical Abstract

1. Introduction

Igneous carbonatites have been found in continental rifting environments, and their genesis is closely associated with deep-mantle magmatism beneath the continental crust. In addition to REE enrichment, carbonatite affects the mineralization of Nb, Mo, U, and Pb, thereby reflecting mantle components in continental rifts or orogenic belts [1,2], such as the Mountain Pass in the USA, the Kola Peninsula in Russia, and the large-scale carbonatite-type rare earth polymetallic Bayan Obo and Maoniuping deposits in China [2,3,4,5,6,7]. Carbonatite-type uranium deposits, an important type of uranium deposit, are primarily associated with carbonatites. These deposits are structurally controlled by fractures and are characterized by low grades (100–500 ppm), large reserves, and the coexistence (accessory) of rare earth elements, such as the Araxá uranium deposit in Brazil, with grades ranging from 100 to 500 ppm and a U3O8 resource amounting to 139,700 tons [8]. The Qinling orogenic belt, a remarkable tectonic unit in central China, is not only a crucial metallogenic region for strategic mineral resources such as gold and molybdenum [9,10,11,12,13] but also an important uranium metallogenic belt [5,6,14,15,16,17]. Known carbonatite-associated uranium deposits in China are mainly distributed in the East Qinling region, such as the superlarge Huayangchuan uranium–niobium–rare earth polymetallic deposit. These uranium deposits are characterized by multiple large-scale, low-grade minerals and significant potential, with uranium minerals primarily comprising pyrochlore and uraninite [15,16,18,19].
The Dashigou Mo deposit, belonging to the Huanglongpu Mo ore field, is situated adjacent to the Huayangchuan deposit to the north and is the first molybdenum deposit discovered in carbonatite veins in China, also hosting significant rare earth elements (REE), rhenium, and other polymetallic mineralizations [5,20,21,22,23]. Additionally, the deposit contains a notable scale of uranium mineralization [6,24]. We carried out radiometric spectrometer measurements on some mineralized carbonatite veins in the Dashigou deposit. The results showed that the uranium content varied from 100 to 5400 ppm, with uranium concentrations in some ore samples being several times higher than the industrial uranium grade of 500 ppm.
For a long time, most scholars have conducted in-depth studies on the genesis of the carbonatites [21,22], the molybdenum metallogenic age [25,26,27], the age of REE mineralization [5,22,23], and the metallogenic mechanisms and geodynamic background of this deposit [5,28]. However, there is limited research on the characteristics of the uranium mineralization in this deposit, with existing studies primarily focusing on the types of ores, distribution characteristics, and the state of uranium occurrence [6,24]. Although we conducted LA-ICP-MS U-Pb dating analysis on uraninite in this deposit in earlier studies, yielding a uranium metallogenic age of approximately ~223 Ma [6], it remains uncertain whether this age fully represents the uranium metallogenic era in the region. Therefore, further detailed research is needed on the uranium metallogenic age in the area. The electron probe microanalysis (EPMA) U-Th-Pb chemical dating technique offers advantages such as simplicity, rapidity, micro-area in situ, and non-destructive testing. Coupled with backscattered electron (BSE) imaging, it can effectively avoid cracks, pits, inclusions, and inaccurate surface locations [6,29]. In recent years, numerous scholars both domestically and internationally have conducted extensive EPMA in situ U-Th-Pb chemical dating studies on uraninite in uranium deposits, obtaining a series of reliable chronological data [15,29,30,31,32,33]. Therefore, this study aimed to select uraninite from the Dashigou Mo deposit as the research object. Based on indoor rock and mineral identification, we conducted U-Th-Pb chemical dating of uraninite using scanning electron microscopy (SEM) and EPMA in situ micro-area analysis techniques. The study will preliminarily explore the uranium metallogenic age and its tectonic setting, providing an important basis for enriching the uranium mineralization theory in the region.

2. Regional Geology

The Qinling orogenic belt is located in central China (Figure 1a), bordered to the north by the Sanmenxia–Baofeng fault and to the south by the Longmenshan–Dabashan fault, generally extending in an east–west direction. The belt comprises four structural units: the southern margin of the North China Craton, the North Qinling Belt, the South Qinling Belt, and the northern margin of the South China Craton (Figure 1b), which are segmented from north to south by the Luanchuan fault, the Shangdan fault, and the Mianlue fault, respectively [5,13,34,35,36]. The Qinling orogenic belt has undergone complex orogenic events, including the collision between the North Qinling and South Qinling units during the Carboniferous, and the final collision between the North China Craton and the Yangtze Craton during the Triassic [12,37,38,39,40].
The East Qinling Mo-U-REE ore-concentrated area is located within the tectonic unit of the southern margin of the North China Block, between the Sanbao fault and the Shangdan fault (Figure 1b) [6,11,21,22]. The region is predominantly characterized by the exposure of cratonic crystalline basement and sedimentary cover. The crystalline basement is mainly composed of the Archean–Paleoproterozoic Taihua Group TTG metamorphic rock series, while the sedimentary cover consists of the Mesoproterozoic calc-alkaline to alkaline volcanic rocks from the Xionger Group and the Meso- to Neoproterozoic shallow marine metamorphic clastic rocks-carbonatite from the Guandaokou Group and Luanchuan Group [5,6,27]. The area has extensive fault structures, primarily featuring the NWW-trending Machaoying fault, Luanchuan fault, and Shangdan fault as major deep faults. The East Qinling region exhibits intense magmatic activity, predominantly during the Indosinian and Yanshanian periods. During the Indosinian epoch, mainly alkaline rocks formed as stocks and veins, such as syenite dikes in the southern Huashan rock mass and Huanglongpu alkaline carbonatites. Yanshanian magmatic activity, on the other hand, is represented by granite batholiths and porphyry bodies, as typically shown in the Huashan and Laojunshan area [5,6,41,42].

3. Deposit Geology

The Huanglongpu Mo deposits consist of separate mineralized bodies at Wengongling, Yuantou, Dashigou, Shijiawan I and II, and Taoyuan (Figure 1c), which are all situated in Luonan County, China. With the exception of Wenggongling and Shijiawan I, which are hosted by granite porphyry, the rest of the deposits are associated with carbonatite dykes [5,21,23,43,44]. The Dashigou Mo (REE) deposit is located at the southern margin of the North China Platform, belonging to the northern part of the Huanglongpu ore field [6]. The stratigraphic units exposed in the mining area, from oldest to youngest, are the Mesoproterozoic Huanglongpu Formation, Gaoshanhe Formation, and Quaternary deposits. The Huanglongpu Formation is represented by metamorphosed amygdaloidal spilite, porphyrite, and tuffaceous slate, while the Gaoshanhe Formation mainly consists of biotite schist, plagiogneiss, tuffaceous slate, and sericite slate [6]. These two strata exhibit a parallel (or slightly angular) unconformity contact in the Songjiagou–Wengongling area, while an evident angular unconformity contact is observed in the Songjiagou–Shijiawan Xiliang area [6,45]. The primary ore-controlling structures in the region are two sets of NW- and NE–NEE-trending faults, mainly distributed in the northeastern limb of the Bancaliang–Maying Hill anticline. Magmatic rocks are abundant and occur as Proterozoic gneissic granite, Mesozoic granite, and several dike rocks (including mineralized carbonatite dikes) within the studied area, with the dike rocks predominantly consisting of diabase and carbonatite dikes (Figure 1c).
The Dashigou deposit has large Mo-mineralized bodies with reserves of 8.9 × 104 t and a grade of 0.075%~0.114% [18,46], and has a molybdenite Re-Os age of 221.5 ± 0.3 Ma [25,26], with associated rhenium and rare earth resources of 92.32 tons and 378,000 tons, respectively [5]. Currently, the Dashigou deposit has revealed six mineralized carbonatite veins, predominantly trending NE, with a few trending NW. The uranium-mineralized carbonatite veins within the area are typically stratiform, vein-like, and lens-like, with individual uranium-mineralized carbonatite veins being 0.1–1.0 m thick and 20–100 m long. Both thick and thin veins are present in various scales (Figure 2), with some NE- and NW-trending carbonatite veins intersecting in a grid-like or mesh-like pattern [6,28]. The main host rock in the area is quartz–calcite carbonatite, composed primarily of calcite (50%–80%), quartz (10%–20%), K-feldspar (5%–25%), sulfides (molybdenite), and uranium minerals. The ore exhibits a disseminated and breccia structure (Figure 2e,f). Exposed ore minerals include molybdenite, brannerite, pyrochlore (mendeleevite), galena, pyrite, monazite, bastnäsite, synchysite, and xenotime (Figure 3). Among them, molybdenite, brannerite, and monazite coexist closely and are products of the same period (221 Ma) [25,26]. Bastnäsite and xenotime belong to the Yanshanian period (145–142 Ma) [5]. Gangue minerals consist of calcite, K-feldspar, and quartz. Surrounding rock alterations include varying degrees of carbonatization, biotitization, pyritization, and anhydritization [28]. Additionally, our research team discovered high levels of radioactive anomalies in some uranium-bearing carbonatite veins in the area, with radiometric measurements showing uranium content of 100–5400 ppm, with local uranium content being several times higher than the industrial grade of uranium ore (0.05%) [6], indicating significant uranium mineralization potential.

4. Samples and Analytical Methods

4.1. Sample Preparation

All samples for this analysis were collected from ore-bearing carbonatite veins from the open pit of the Dashigou molybdenum (rare earth) deposit (Figure 2a–d). The sampling coordinates were E110°01′33″, N34°21′59″. Two samples of uranium-mineralized carbonatite were collected, showing field gamma-ray values ranging from 140 to 385 ppm, exhibiting an overall light flesh–red color, and occasionally containing gray–white quartz nodules. They display sub-idiomorphic to xenomorphic granular structures, metasomatic residual structures, angular and nodular textures (Figure 2e,f). Ore minerals are molybdenite, uranium minerals, and pyrite, while gangue minerals consist of calcite, quartz, and a small amount of K-feldspar.

4.2. Analytical Methods

SEM and EPMA analyses of uranium-mineralized samples were conducted at the State Key Laboratory of Nuclear Resources and Environment at East China University of Science and Technology. Prior to testing, the probe samples underwent polishing to remove surface oxide layers. Following Huang et al. (2022) [47] method, the samples were carbon-coated (approximately 20 nm thick) to enhance conductivity before analysis [6,47]. The microscope used was an Nova Nano-SEM 450 from FEI Company (Hillsboro, OR, USA), with resolutions of 1.0 nm (15 kV) and 1.4 nm (1 kV). The EPMA instrument employed was a JEOL JXA-8230 from Shimadzu Company (Kyoto, Japan), operating at 15 kV accelerating voltage, probe current of 20 nA, with an electron beam diameter of 1 μm. All test data underwent correction using the ZAF procedure, referencing standard samples as detailed in Huang et al. (2023b) [6]. The detection limit for elements in this analysis was 200 ppm, with a major element error of 1.5% and a trace element error of 5.0%. Additionally, EPMA U-Th-Pb chemical dating relied on radioactive decay, measuring Th, U, and Pb content in uraninite. After data processing, apparent ages of uraninite at measurement points were calculated using the EPMA U-Th-Pb age calculation method proposed by Bowles (1990) [48].

5. Results

5.1. Mineralogy of Uranium Minerals

The uranium minerals exposed in the uranium-mineralized carbonate veins of the Dashigou deposit primarily consist of uraninite and brannerite. They exhibit distinct differences in mineral morphology, crystal shape, and mineral associations. Uraninite often appears as irregularly shaped, grid-like, or flocculent particles (Figure 4), with particle sizes ranging from 8 to 20 μm. Some grains are commonly found surrounding monazite, concurrently associated with fluorcarbonates such as bastnäsite, galena, and parisite (Figure 4a–c), and cracks are commonly developed within these crystals. Furthermore, a minority of uraninite displays grid-like or flocculent morphologies due to hydrothermal alteration (Figure 4b–d), filled along the grid with small grains of galena or rare earth minerals such as brannerite and parisite. In addition, localized amounts of uraninite exhibit irregular granular, star-shaped, or linear occurrences within brannerite crystals (Figure 4e,f). These grains are generally smaller and are coexistent with bastnäsite, and galena. Brannerite appears as dark gray, with hypidiomorphic and idiomorphic granular and brecciform distributions (Figure 3e,f), ranging in size from 20 to 300 μm. Irregular microcracks (veins) are commonly developed within these crystals, filled with irregular granular uraninite, bastnäsite, and galena along the vein (Figure 4e,f). Partial brannerite grains exhibit distinct hydrothermal alteration phenomena, with alteration products mainly consisting of Ti-U-Fe oxides. In summary, the uranium minerals exhibit a characteristic mineral assemblage of uraninite + rutile + bastnäsite + parisite or uraninite + galena + bastnäsite + brannerite.

5.2. Chemical Composition of Uraninite

In this study, EPMA was employed for in situ chemical composition determination of uraninite in the uranium-mineralized samples, with a total of 21 test points conducted, and the results are detailed in Table 1. The chemical composition of uraninite exposed in the Dashigou deposit mainly comprises UO2, accompanied by a slight amount of Y2O3, PbO, Ce2O3, ThO2, etc. The total sum of chemical components in uraninite ranges from 94.74% to 100.29%, with a mean of 97.16%. Specifically, the UO2 content remains relatively stable overall, varying from 85.61% to 91.75%, with a mean of 88.45%; Y2O3 content ranges from 1.50% to 4.40%, with a mean of 2.14%; PbO content varies from 1.67% to 1.86%, with a mean of 1.72%; ThO2 content ranges from 0.76% to 2.13%, with a mean of 1.75%; Ce2O3 content ranges from 1.32% to 3.78%, with a mean of 2.23%; CaO content ranges from 0.15% to 0.43%, with a mean of 0.31%; FeO content ranges from 0.00% to 0.93%, with a mean of 0.12%. Additionally, the total content range of w (CaO + SiO2 + FeO) in the tested uraninite samples in this study is between 0.22% and 1.37% (a mean of 0.47%). Some uraninite samples have P2O5, TiO2, SiO2, and MgO content below the detection limit of the electron probe microanalyzer.

5.3. U-Th-Pb Chemical Age of Uraninite

Regarding EPMA U-Th-Pb micro-area chemical dating of uranium minerals, the following conditions must be met: firstly, the initial Pb content should be extremely low and negligible; secondly, the uranium mineral should not have undergone subsequent hydrothermal alteration after its formation, ensuring that the U-Th-Pb isotope system remains closed; moreover, it is assumed that Pb only originates from the decay of U and Th [6,48,49,50]. For the Dashigou deposit, we selected uraninite grains with smooth surfaces and no alteration or cracks for in situ single-point U-Th-Pb chemical dating using EPMA. Following the EPMA U-Th-Pb age calculation method proposed by Bowles (1990) [48] and using the age calculation program developed by Guo et al. (2012) [51] based on UO2, ThO2, and PbO content, this study calculated the apparent ages for 21 uraninite measurement points, ranging from 155 to 138 Ma (Table 1). Additionally, utilizing the Isoplot R software [52,53], the weighted mean U-Th-Pb chemical age and the weighted mean square of weighted deviations (MSWD) were calculated as 144 ± 3.1 Ma and MSWD = 0.39, respectively (Figure 5).
Table 1. EPMA analyses results (wt. %) and U-Th-Pb chemical ages (Ma) of uraninite in the Dashigou deposit.
Table 1. EPMA analyses results (wt. %) and U-Th-Pb chemical ages (Ma) of uraninite in the Dashigou deposit.
Sample NoY2O3Nb2O5P2O5FeOCaOTiO2SiO2La2O3Ce2O3PbOThO2UO2Yb2O3MgOTotalAge (Ma)
06BT-11.770.060.000.070.190.000.000.061.971.681.9489.010.040.0196.80140
06BT-21.840.000.000.000.220.000.000.011.621.682.0089.430.170.0296.98139
06BT-32.190.070.030.020.320.000.000.092.051.681.5787.770.000.0095.78142
06BT-42.390.120.000.000.430.000.000.001.531.711.7288.660.060.0096.61143
06BT-51.990.000.020.020.290.000.010.001.491.812.0188.060.060.0095.77152
06BT-61.620.000.000.240.240.370.010.423.411.681.6686.400.190.0196.25144
06BT-71.500.090.000.080.330.040.070.153.331.671.5587.100.180.0396.12142
06BT-81.860.120.010.000.360.080.510.163.781.721.7585.860.270.0196.51148
06BT-91.660.000.010.030.320.000.070.363.191.681.5285.610.280.0194.74145
06BT-102.390.020.000.110.340.230.000.223.411.711.8186.600.000.0096.84146
06BT-112.070.030.010.030.330.080.010.273.131.701.6686.400.040.0095.77146
10-Ur-11.850.000.000.000.250.000.030.001.721.702.0990.000.220.0397.88140
10-Ur-21.690.020.000.070.270.000.060.001.551.682.0990.000.020.0297.46138
10-Ur-31.720.110.020.110.390.000.030.002.201.722.1387.950.000.0296.39145
10-Ur-41.980.060.000.000.390.000.030.001.961.671.7388.430.390.0096.63140
10-Ur-51.870.020.000.060.340.000.040.121.651.811.6589.040.060.0096.65150
10-Ur-61.940.060.010.000.260.000.000.071.671.741.8091.750.300.0199.60140
10-Ur-71.920.000.000.150.380.000.000.002.311.721.7390.700.000.0298.93140
10-Ur-84.400.000.010.360.280.000.100.001.321.750.7689.770.260.0199.01145
10-Ur-93.270.000.010.190.150.580.000.022.241.861.9988.720.160.0399.21155
10-Ur-103.020.080.050.930.390.170.050.141.371.821.6490.170.450.02100.29149
Explanatory note: Age is the U-Th-Pb chemical age of uraninite. Calculation method is based on Bowles (1990) [48] and Guo et al. (2012) [51].

6. Discussion

6.1. Composition Characteristics and Indicative Significance of Uraninite

Although the theoretical chemical formula of uraninite is UO2, in reality, uranium is often replaced by certain impurity elements (such as Ca, Th, REE, etc.) from magma/fluids during its formation process, and this substitution may be related to the element’s ionic radius and temperature [53,54,55]. These impurity elements in uraninite are commonly used to indicate the nature and formation conditions of uraninite [6,55,56,57,58,59]. Uraninite tested from the Dashigou deposit contains small amounts of Y2O3, Ce2O3, PbO, ThO2 ( ), as shown in the Hacker diagram (Figure 6). Ce2O3 and CaO exhibit a significant negative correlation with UO2, indicating that the impurity elements in uraninite mainly enter the lattice through Ce3+ and Ca2+ replacing U4+, while SiO2, FeO, MgO, etc., are present in very low amounts (≤0.5%), suggesting minimal substitution of U4+ into the lattice.
Previous studies have indicated that the ThO2 content in uraninite is an important indicator for determining its formation temperature [55,58,60]. Due to the similar ionic radius and properties of U and Th, Th4+ can partially replace U4+ in the lattice during the crystallization process of uraninite. At low temperatures, Th4+ has low solubility, whereas at high temperatures, the solubility of Th4+ increases [61,62,63]. Therefore, uraninite formed under high-temperature magmatic conditions typically exhibits higher ThO2 than that formed under low-temperature hydrothermal conditions [5,58,60,64], and the content of rare earth elements in uraninite also increases with rising crystallization temperature [55,65]. The ThO2 content in the tested uraninite samples from the Dashigou deposit is all >0.1% (mean value is 1.75%), preliminarily indicating a high-temperature magmatic origin of the uraninite. Additionally, brannerite of the Dashigou deposit is mostly hypidiomorphic and idiomorphic granular (Figure 3e,f), closely associated with uraninite, confirming its high-temperature magma genesis [6]. Therefore, this further corroborates that the uraninite exposed in the Dashigou deposit is a product of high-temperature magma genesis.

6.2. Determination of the Validity of U-Th-Pb Chemical Ages of Uraninite

When utilizing EPMA for U-Th-Pb chemical dating of uraninite, two conditions must first be met. Firstly, the uraninite should be in a closed system after formation, where U, Th, and Pb have not been lost. Secondly, the initial Pb content in uraninite should be zero or negligible [15,29,51,57,66]. In testing minerals, the initial Pb content is zero, and certain impurity elements (such as Si, Ca, etc.) can leach into the uraninite lattice, leading to Pb loss, during the late-stage alteration process of uraninite [29,58,59]. Therefore, the quality fractions of Si, Ca, and Fe in uraninite are typically used to assess whether the U-Th-Pb system within the mineral remains closed. When the quality fractions of Si, Ca, and Fe are low in uraninite, it indicates that the mineral has not undergone late-stage alteration, confirming a closed system where radiogenic Pb loss has not occurred [15,29,67]. In the current study, data from EPMA analysis on uraninite (Table 1) show overall low w(SiO2 + CaO + FeO) (ranging from 0.22% to 1.37%, with a mean of 0.47%). There is no significant correlation observed between SiO2 + CaO + FeO (wt.%) and UO2 + PbO (wt.%), PbO (wt.%), and the age of uraninite (Figure 7), indicating no significant Pb loss in uraninite. Therefore, the U-Th-Pb chemical ages obtained in this study are reliable.
Previous studies have shown that uraninite generally contains negligible initial Pb, and even if present, its content is extremely low, typically not exceeding 0.36% [29,32,33]. In contrast, the content of U, Th, and radiogenic Pb is generally higher [29,32,33,58,59]. Thus, relative to radiogenic Pb, the influence of initial Pb in uraninite on its chemical dating is minimal. In summary, this indicates that the micro-area in situ electron probe U-Th-Pb chemical dating method used in this study is reliable. The weighted average age of 144 ± 3.1 Ma (MSWD = 0.39) obtained from 21 measurement points of uraninite can represent the Yanshanian uranium mineralization age in the study area.

6.3. Yanshanian Uranium Mineralization Age and Its Geological Significance

The Dashigou deposit is the most representative carbonate-type molybdenum deposit in the Huanglongpu ore field [5,25,26,27,28,43,44]. With continuous exploration efforts, U-REE mineralization on a certain scale has been discovered in the deposit [5,6,24,43,44]. Previous studies on the deposit’s metallogeny have yielded a wealth of precise metallogenic ages (Table 2), including molybdenum mineralization ages constrained to 225–221 Ma [25,26,27,46], REE mineralization ages ranging from 219.4 to 213.6 Ma [5,22], and a uranium mineralization age of 223 Ma [6]. These ages imply that the Mo-REE-U mineralization associated with carbonatites formed contemporaneously with magmatic activity, suggesting a massive Mo-REE-U mineralization period in the late Triassic in the eastern Qinling area. However, the reported uranium mineralization age of 223 Ma [6] only covers the previous findings and does not reveal whether there was uranium mineralization during the Yanshanian period in the region.
Previous studies have shown that around 140 Ma, another important rock-forming and ore-forming age occurred in the eastern Qinling orogenic belt [10,69,70], characterized by widespread Yanshanian molybdenum mineralization events, such as the Jinduicheng and Shijiawan granite porphyry molybdenum deposits [68,71,72,73]. These deposits are spatially adjacent to the Dashigou deposit, with the Shijiawan molybdenum deposit belonging to the Huanglongpu molybdenum field (Figure 1c). Existing geochronological data demonstrate that the Jinduicheng molybdenum deposit has rock- and ore-forming ages concentrated around 143.7–139 Ma [25,26,72,73], while the rock- and ore-forming ages of the Shijiawan molybdenum deposit are concentrated in 144–138 Ma [25,68]. Additionally, the Huayangchuan carbonate-type U-Nb-REE deposit, about 8 km north of the Dashigou deposit, also experienced Yanshanian uranium mineralization events, with a uranium mineralization age of 137.1 Ma [16,29]. It is noteworthy that previous studies by our team have identified Yanshanian rare earth mineralization events in the ore-bearing carbonate veins of the Dashigou deposit, with a rare earth mineralization age of 145.9–142.6 Ma [5]. The abovementioned rock- and ore-forming ages and the newly discovered Yanshanian uranium mineralization age of 144 ± 3.1 Ma in this study are generally consistent within the error range, and they are all located in the eastern Qinling metallogenic belt. Therefore, the newly obtained uranium mineralization age of 144 ± 3.1 Ma in this study may to some extent respond to regional geological events during the Yanshanian period. In summary, in addition to the primary uranium mineralization period around ~223 Ma [6], the Dashigou deposit also experienced Yanshanian uranium mineralization around 144 ± 3.1 Ma. The discovery of this age further enriches the understanding of carbonate-type uranium mineralization in the region and indicates the presence of two uranium mineralization events at the Dashigou deposit (Figure 8).
As mentioned earlier, uraninite at the Dashigou deposit often exhibits irregular, subangular, grid-like, or flecked distributions, coexisting with bastnäsite and parisite (Figure 4). Our previous studies demonstrated that bastnäsite and parisite from the Dashigou deposit may have resulted from hydrothermal activation and modification of early-stage rare earth minerals such as monazite [5]. Additionally, a few uraninite grains show grid-like or flecked morphologies due to hydrothermal alteration (Figure 4b–d), suggesting that the uranium mineralization in this study area responded to extensive Yanshanian magmatic activities to some extent. On account of crustal remelting (or deep melting), the mixing of melts and upwelling mantle materials formed rejuvenated magmas, which intruded and overlapped along the cross of faults in the area. In the late stage of magma evolution, differentiated hydrothermal fluids provided ample heat and fluid sources for the Yanshanian uranium mineralization developed in the carbonate veins of the study area [3,5,74]. This process facilitated the activation and migration of uranium during the early uranium mineralization (~223 Ma) [6]. As the concentration of [UO2]2+ in the late-stage magma fluids reached saturation, U precipitated and crystallized along the margins or fractures of local Ti-U-Fe oxides, brannerite, and calcite under the action of reducing medium in fluid, resulting in the formation of late uraninite (144 Ma). Thus, the newly discovered Yanshanian uranium mineralization age (144 Ma) indirectly represents the occurrence of the late uranium activation–deposition event.

6.4. Structural Background of Yanshanian Uranium Mineralization

The Qinling orogenic belt is a composite orogenic belt that may have undergone multiple stages of tectonic processes including rift, collision, and accretion from the Archaean to the Cenozoic [9,75,76]. Following the closure of the branch of the Paleo-Tethyan Ocean, the Qinling orogenic belt recorded a significant Triassic collision orogeny between the NCC and the SCC [39,77,78]. The final collision between the NCC and the SCC occurred during the Triassic period [12,79,80]. The Indosinian period marked the peak of intense compression and collision between the two continental blocks, followed by a period of extension after the collision [81,82].
Previous studies hold that the late-stage extensional geodynamic events in eastern China occurred around 140 Ma [5,29,69,70,72]. Regionally, with the continuous weakening of tectonic compression, a transition towards extensional tectonic regimes began. Since the Jurassic, the eastern Qinling region has experienced intracontinental orogenesis followed by post-orogenic extensional processes [10,29,76,83]. Spatially, the ore-bearing carbonate veins exposed in the Dashigou deposit are primarily controlled by near-EW- and NE–NEE-trending faults (Figure 1c) [5,6,25,28]. Temporally, the eastern Qinling region extensively developed Yanshanian magmatic activities, resulting in a series of Yanshanian granitic bodies (146–132 Ma) [72,73,84,85,86] and porphyry molybdenum deposits (144–138 Ma) [25,26,68,72]. The aforementioned magmatic and mineralization events occurred during the Jurassic–Cretaceous transition from compression to extension, corresponding to the tectonic setting of intracontinental orogenesis and post-orogenic extension between the North China Block and the Yangtze Block [5,71,72,76,79]. Additionally, similar Yanshanian REE mineralization events (127.5 ± 2.6 Ma) have been identified in the nearby Huangshuian carbonate-type Mo-REE deposit, confirming them as products of intracontinental orogenesis and extension following collisional orogeny [3]. The newly discovered Yanshanian uranium mineralization age in this study closely matches the aforementioned rock- and ore-forming ages, and it is spatially connected to them. Therefore, we preliminarily propose that the Yanshanian uranium mineralization in the Dashigou deposit may have formed in an intracontinental orogenic and extensional environment following collisional orogeny.

7. Conclusions

(1)
The Yanshanian uranium minerals in the carbonates of the Dashigou deposit are primarily represented by uraninite, which often exhibits irregular, subangular, grid-like, or flecked distributions, and overall presents a mineral assemblage characterized by uraninite + rutile + bastnäsite + parisite or brannerite.
(2)
The EPMA U-Th-Pb chemical age of uraninite is 144 ± 3.1 Ma, which represents the Yanshanian uranium mineralization age in the deposit. The newly discovered uranium mineralization age indicates that the deposit experienced a uranium remobilization event during the Cretaceous, possibly representing reactivation of early uranium mineral assemblages.
(3)
The Yanshanian uranium mineralization in the Dashigou deposit formed in an intracontinental orogenic and extensional environment following collisional orogeny.

Author Contributions

Data curation, G.H. and Z.M.; writing—original draft, G.H.; writing—review and editing, G.H. and D.W.; supervision, F.L. and P.F.; project administration, G.H. and C.P. All authors have read and agreed to the published version of the manuscript.

Funding

Qinghai Normal University Doctoral Initiation Fund Project (grants 21001060127 and 23401023); Jiangxi College of Applied Technology Doctoral Initiation Fund Project (Teaching and Research Team for Rare Earth and Uranium Exploration and Development (grant JXYY-G2022004); Open Fund Project of the Key Laboratory of Ionic Rare Earth Resources and Environment, Ministry of Natural Resources (grant 2023IRERE105); National Natural Science Foundation of China (grant 42002091).

Data Availability Statement

Data are contained within the article.

Acknowledgments

Special thanks go to the anonymous reviewers for their helpful suggestions and comments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Tectonic map of China showing the location of the Qinling Orogen. (b) Geological sketch of the Qinling Orogenic Belt. (c) Simplified geological map of the Huanglongpu deposit [5]. Abbreviations: S-NCC: Southern North China Craton, NQL: North Qinling unit, SQL: South Qinling unit, N-SCC: Northern South China Craton.
Figure 1. (a) Tectonic map of China showing the location of the Qinling Orogen. (b) Geological sketch of the Qinling Orogenic Belt. (c) Simplified geological map of the Huanglongpu deposit [5]. Abbreviations: S-NCC: Southern North China Craton, NQL: North Qinling unit, SQL: South Qinling unit, N-SCC: Northern South China Craton.
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Figure 2. Field photos of ore-bearing carbonate veins and representative macroscopic features of ores in the Dashigou deposit. (a–d) Ore-bearing carbonate veins intruding the Huanglongpu Formation of the Xiong’er Group, exhibiting irregular grid-like, coarse vein-like, and fine vein-like structures. (e,f) Representative macroscopic features of ores, uranium minerals occurring as massive, irregular granular, disseminated distributions, with ore bodies showing massive and disseminated textures. Qtz: quartz; Mol + U: molybdenite + uraninite; Cal: calcite; Py: pyrite; Kfs: K-feldspar; Mol: molybdenite; Gn: galena; Bnr: brannerite; Urn: uraninite.
Figure 2. Field photos of ore-bearing carbonate veins and representative macroscopic features of ores in the Dashigou deposit. (a–d) Ore-bearing carbonate veins intruding the Huanglongpu Formation of the Xiong’er Group, exhibiting irregular grid-like, coarse vein-like, and fine vein-like structures. (e,f) Representative macroscopic features of ores, uranium minerals occurring as massive, irregular granular, disseminated distributions, with ore bodies showing massive and disseminated textures. Qtz: quartz; Mol + U: molybdenite + uraninite; Cal: calcite; Py: pyrite; Kfs: K-feldspar; Mol: molybdenite; Gn: galena; Bnr: brannerite; Urn: uraninite.
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Figure 3. Microphotographs of representative ore-bearing carbonatites from the Dashigou deposit. (a) Ore-bearing carbonatites exhibiting granular structure. (b) Monazite, fluorapatite, bastnäsite, and uraninite irregularly fill interstices between calcite grains or along quartz margins. (c) Gangue minerals K-feldspar and calcite displaying irregular granular distribution. (d) Monazite occurring as subhedral grains within calcite crystals, with pyrochlore irregular grains of epidote along the edges. (e,f) Coexisting minerals molybdenite, monazite, and uraninite showing evident paragenetic relationships, featuring alteration remnants. Qtz: quartz; Mol: molybdenite; Urn: uraninite; Cal: calcite; Par: parisite; Xtm: xenotime; Kfs: K-feldspar; Mnz: monazite; Pcl: pyrochlore; Gn: galena; Bnr: brannerite; Py: pyrite.
Figure 3. Microphotographs of representative ore-bearing carbonatites from the Dashigou deposit. (a) Ore-bearing carbonatites exhibiting granular structure. (b) Monazite, fluorapatite, bastnäsite, and uraninite irregularly fill interstices between calcite grains or along quartz margins. (c) Gangue minerals K-feldspar and calcite displaying irregular granular distribution. (d) Monazite occurring as subhedral grains within calcite crystals, with pyrochlore irregular grains of epidote along the edges. (e,f) Coexisting minerals molybdenite, monazite, and uraninite showing evident paragenetic relationships, featuring alteration remnants. Qtz: quartz; Mol: molybdenite; Urn: uraninite; Cal: calcite; Par: parisite; Xtm: xenotime; Kfs: K-feldspar; Mnz: monazite; Pcl: pyrochlore; Gn: galena; Bnr: brannerite; Py: pyrite.
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Figure 4. BSE images of uranium minerals in the Dashigou deposit. (a) Uraninite exhibits irregular granular distribution. (b) Enlarged portion of image a, showing the coexistence of uraninite with bastnäsite and galena. (c,d) Uraninite with crack structures, irregular granular and flocculent distribution due to hydrothermal alteration. Image d is an enlarged portion of image a. (e,f) Brannerite with irregular granular distribution of bastnäsite, uraninite, and galena. Urn: uraninite; Bnr: brannerite; Cal: calcite; Par: parisite; Gn: galena; Rt: rutile; Bst: bastnäsite.
Figure 4. BSE images of uranium minerals in the Dashigou deposit. (a) Uraninite exhibits irregular granular distribution. (b) Enlarged portion of image a, showing the coexistence of uraninite with bastnäsite and galena. (c,d) Uraninite with crack structures, irregular granular and flocculent distribution due to hydrothermal alteration. Image d is an enlarged portion of image a. (e,f) Brannerite with irregular granular distribution of bastnäsite, uraninite, and galena. Urn: uraninite; Bnr: brannerite; Cal: calcite; Par: parisite; Gn: galena; Rt: rutile; Bst: bastnäsite.
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Figure 5. EPMA U-Th-Pb chemical age histogram (a) and weighted average age diagram (b) of the uraninite from the Dashigou deposit.
Figure 5. EPMA U-Th-Pb chemical age histogram (a) and weighted average age diagram (b) of the uraninite from the Dashigou deposit.
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Figure 6. Hacker diagrams of the uraninite in the Dashigou deposit.
Figure 6. Hacker diagrams of the uraninite in the Dashigou deposit.
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Figure 7. SiO2 + CaO + FeO (wt.%) versus UO2 + PbO (wt.%) (a), PbO (wt.%) (b) and age (Ma) (c) diagrams of uraninite in the Dashigou deposit.
Figure 7. SiO2 + CaO + FeO (wt.%) versus UO2 + PbO (wt.%) (a), PbO (wt.%) (b) and age (Ma) (c) diagrams of uraninite in the Dashigou deposit.
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Figure 8. Summary of mineralization age in the Dashigou deposit. Age data and references are listed in Table 2.
Figure 8. Summary of mineralization age in the Dashigou deposit. Age data and references are listed in Table 2.
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Table 2. Summary of mineralization age in the Dashigou deposit.
Table 2. Summary of mineralization age in the Dashigou deposit.
Test MineralsMethodAge (Ma)Reference
MonaziteLA-ICP-MS U–Pb219.4 ± 6.0[5]
BastnäsiteLA-ICP-MS U–Pb145.9 ± 5.3
XenotimeLA-ICP-MS U–Pb142.6 ± 1.8
UraniniteLA-ICP-MS U–Pb223 ± 1.0[6]
UraniniteEPMA U–Th–Pb144 ± 3.1This study
MolybdeniteRe–Os221[25]
MolybdeniteRe–Os221.5 ± 0.3[26]
MonaziteSIMS Th–Pb213.6 ± 4.0[22]
MolybdeniteICP-MS Re–Os225 ± 7.6[46]
MolybdeniteICP-MS Re–Os145.4 ± 2.1[68]
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Huang, G.; Wu, D.; Li, F.; Pan, C.; Fan, P.; Min, Z. The Yanshanian Uranium Mineralization Age and Its Geological Significance in the Dashigou Carbonatite-Type Mo-REE-U Deposit, East Qinling Orogen, China. Minerals 2024, 14, 928. https://doi.org/10.3390/min14090928

AMA Style

Huang G, Wu D, Li F, Pan C, Fan P, Min Z. The Yanshanian Uranium Mineralization Age and Its Geological Significance in the Dashigou Carbonatite-Type Mo-REE-U Deposit, East Qinling Orogen, China. Minerals. 2024; 14(9):928. https://doi.org/10.3390/min14090928

Chicago/Turabian Style

Huang, Guangwen, Dehai Wu, Furong Li, Chunrong Pan, Pengfei Fan, and Zhuang Min. 2024. "The Yanshanian Uranium Mineralization Age and Its Geological Significance in the Dashigou Carbonatite-Type Mo-REE-U Deposit, East Qinling Orogen, China" Minerals 14, no. 9: 928. https://doi.org/10.3390/min14090928

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