The Xiangshan uranium ore field is structurally situated within a large-scale volcanic collapse basin. This volcanic-intrusive basin, located in the Cretaceous extensional tectonic zone between the Yangtze and Cathaysia blocks (Li et al., 2014), occupies a critical metallogenic position in southeastern China. Specifically, it is positioned at the southwestern margin of the Gan-Hang tectonic belt and the northeastern periphery of the Qin-Hang metallogenic belt (Figure 1), where it developed through intense magmatic-tectonic interactions during the Late Mesozoic continental extension.

The basement of the volcanic basin consists mainly of low greenschist facies to low amphibolite facies Paleo-Mesoproterozoic metamorphic rocks (amphibolite and schist) and the less-metamorphosed Neoproterozoic (Sinian system) rocks, including phyllite, slate, and metasandstone (Yang et al., 2010). The volcanic rocks from the Xiangshan include rhyolitic crystal tuffs, welded tuffs, rhyodacites, porphyroclastic rhyolite, associated subvolcanic rocks such as monzogranite-porphyry and granite-porphyry, and late dykes such as quartz monzonitic porphyry and lamprophyre. Previous studies showed that the volcanic cover was formed by two cycles (Figure 2). The first volcanic cycle represents a fissure eruption and is composed mainly of rhyodacite that yielded a zircon U-Pb age of 135 Ma (Li et al., 2014; Yang et al., 2010) and rhyolitic welded tuff. The second volcanic cycle was a central vent eruption and was composed mainly of extrusive facies of felsic lava that formed the main part of the Xiangshan volcanic-intrusive complex, yeilding a zircon U-Pb age of 132 Ma (Li et al., 2014).

So far, almost 30 uranium deposits have been discovered, mainly distributed in the western and northern Xiangshan. Uranium mineralization occurs in fractures attached to NE-trending faults, which cut the volcanic-intrusive complex without dependence on wall rock. Polymetallic mineralization has been discovered at depths of 500–1,500 m below uranium ore bodies, such as in the Niutoushan area (boreholes ZK26-9, ZK26-11, ZK26-101), the Heyuanbei area (CUSD3), and the Niuonaoshang area (CUSD1). The mineralization occurs in both overlying volcanic rocks and basement metamorphic rocks, showing no selectivity toward host rocks. Spatially, the two mineralization types (uranium and polymetallic) exhibit a vertical zoning characterized by “uranium above and polymetallic below” (Figure 3).

The CUSD1 has been carried out in the Xiangshan volcanic basin, 2 km southeast of the Zoujiashan uranium deposit, with a final depth of 2818 m for the main hole. Three uranium mineralizations and five polymetallic mineralizations, with hydrothermal alteration developing, have been recovered by CUSD1.

Uranium mineralization occurs in cataclastic rhyolite at shallow depths of 200–900 m in borehole CUSD1, with strongly developed hydrothermal alteration such as albitization, hematitization (Figure 4a), fluoritization, carbonatization (Figure 4b), and pyritization. The ore exhibits vein-type and stockwork-type mineralization. The main ore minerals are pitchblende (Figure 4c), brannerite, conffinite, thorite, pyrite, minor molybdenite, chalcopyrite, galena, sphalerite, and native bismuth. The main gangue minerals are calcite, fluorite, quartz, apatite, minor rutile and monazite.

Polymetallic mineralization occurs within the deep-seated basement metamorphic rocks at vertical depth of 1,400–2,800 m in borehole CUSD1, primarily characterized by hydrothermal alterations such as silicification (Figure 4d), carbonatization, sericitization, and chloritization. The main ore minerals are galena, sphalerite (Figure 4f), arsenopyrite (Figure 4e), pyrite, pyrrhotite, minor chalcopyrite, and argentite. The main gangue minerals are quartz, calcite, siderite, minor sericite, chlorite, and cassiterite.

All samples used in this study were collected from CUSD1, including monomineralic samples for multi-isotope analysis and fresh rock samples. Mineral separates were extracted from crushed and washed sample fragments (40–60 mesh) and were handpicked under a binocular microscope to achieve a purity of >99%. Each grain was carefully examined in order to avoid inclusions.

H-O-S-Pb-Sr isotope analyses were performed in the Analytical Laboratory of the Beijing Research Institute of Uranium Geology.

The H-O isotope analysis of calcite and quartz single minerals was conducted using a MAT253 isotope mass spectrometer. The δ¹⁸O measurement was obtained by generating O₂ using the BrF₅ method, which was then converted to CO₂ for analysis on the mass spectrometer to obtain the hydrogen and oxygen isotope composition. Through standard calculations, the ΔD_H2O of the inclusions was obtained, with an analytical precision of ±2%.

The sulfur isotope compositions of sulfide samples were analyzed on a MAT251E gas mass spectrometer by using Cu₂O to oxidize the sulfides. The analytical procedure usually yielded an in-run precision of 0.2‰. The calibration were performed with regular analyses of internal δ³⁴S standard samples.

The lead isotope compositions of sulfide samples were analyzed on the GV IsoProbe-T Thermal Ionization Mass Spectrometer. The analytical procedure involved dissolution of samples using HF and HClO₄ in crucibles, followed by basic anion exchange resin to purify Pb. Analytical results for the standard NBS 981 are ²⁰⁸Pb/²⁰⁴Pb = 36.611 ± 0.004 (2σ), ²⁰⁷Pb/²⁰⁴Pb = 15.457 ± 0.002 (2σ), and ²⁰⁶Pb/²⁰⁴Pb = 16.937 ± 0.002 (2σ), in agreement with the reference value (Belshaw et al., 1998).

The strontium isotope composition was determined on a multicollector IsoProbe-T Thermal Ionization Mass Spectrometer. Details of the chemical separation and mass spectrometric procedures are described by Wang et al. (2014b). ⁸⁷Sr/⁸⁶Sr is normalized to ⁸⁸Sr/⁸⁶Sr = 8.375219 to correct for instrumental fraction. During the period of this study, measurement for the American Standard Reference Material NBS 987 Sr standard gave ⁸⁷Sr/⁸⁶Sr = 0.710250 ± 0.000007 (2σ). The total procedural blanks for Rb and Sr were 0.2 ng and 0.2 ng, respectively.

The analytical results of hydrogen and oxygen isotopes for uranium-polymetallic mineralization are presented in Table 1. The δ¹⁸O_H2O values were obtained based on the modal temperature measurements of fluid inclusions and the oxygen isotope equilibrium fractionation formulas for the SiO₂-H₂O and CaCO₃-H₂O systems. During the uranium mineralization period, the δ¹⁸O values of four calcite samples ranged from 8.6‰ to 12.6‰, with an average of 10.63‰. The calculated δ¹⁸O_H2O values ranged from 0.95‰ to 4.95‰, averaging 2.98‰, while the δ¹⁸D values ranged from −87.3‰ to −78.1‰. During the polymetallic mineralization period, the δ¹⁸O values of ten quartz samples ranged from 10.4‰ to 14.7‰, with an average of 13.7‰. The calculated δ¹⁸O_H2O values ranged from 3.51‰ to 7.81‰, averaging 6.80‰, while the δ¹⁸D values ranged from −91.3‰ to −71.4‰.

S isotope compositions of pyrite, pyrrhotite, sphalerite, and arsenopyrite separated from the polymetallic mineralization of CUSD1, together with previously published data (Nie et al., 2015), are listed in Table 2 and showed in histogram (Figure 5). Fourty-five δ³⁴S values of sulfides range from 2 to 6.9‰, with an average value of 4.16‰. The sulfur isotope compositions of pyrite, pyrrhotite, sphalerite, and arsenopyrite range from 2.2‰ to 6.9‰ (n = 21), from 2 to 5.8‰ (n = 12), from 3.3‰ to 3.9‰ (n = 4), and from 3.1 to 4.8 (n = 6), respectively. The obtained single sulfur isotope values of galena and chalcopyrite from polymetallic mineralization are 2.5‰ and 4.6‰. The S isotope compositions of pyrite in uranium mineralization differ significantly from those of galena and chalcopyrite. The S isotope values of 24 pyrites show strong δ³⁴S enrichment, with a wide distribution range and values of δ³⁴S between 6.2‰ and 19.55‰. The average δ³⁴S value of pyrite in uranium ore is 13.15‰.

Pb isotope compositions of polymetallic mineralization, uranium ore, porphyroclastic rhyolite, rhyodacite, and metamorphic basement are listed in Table 3, respectively. We estimate the Pb isotope compositions of metamorphic basement with U, Th, and Pb values, denoting an age of 130 Ma, which is the sulfide Rb-Sr dating age (Yang et al., 2015; Guo et al., 2018), to exclude the influence of radioactive lead. We use the Pb isotope compositions of volcanic rock, porphyroclastic rhyolite, and rhyodacite without estimating because of the same age between polymetallic mineralization and the formation of volcanic rock.

²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios for sulfide minerals of polymetallic mineralization ranging from 17.599 to 18.262, 15.558 to 15.725, and 37.934 to 38.875; for pyrite from uranium ore ranging from 18.221 to 19.372, 15.591 to 15.717, and 38.504 to 38.910; for volcanic rock ranging from 18.471 to 18.792, 15.618 to 15.652, and 38.634 to 39.182. The estimated ratios of metamorphic basement range from 17.968 to 18.623, 15.553 to 15.633, and 38.304 to 38.753.

Sulfide samples have relatively homogeneous and characteristically low radiogenic lead; those from uranium ore is more radiogenic than sulfide from polymetallic mineralization.

The Sr isotope compositions of uranium mineralization, polymetallic mineralization, and major geological units in the Xiangshan ore field were obtained (Table 4). The analyzed results indicate that the Sr isotope composition of cataclastic rhyolite ranges from 0.710511 to 0.713861, with an average of 0.712225. The Sr isotope composition of rhyodacite ranges from 0.710773 to 0.711841, averaging at 0.711147. The Sr isotope composition of the basement metamorphic rocks ranges from 0.718085 to 0.724013, averaging at 0.720212, which is significantly higher than that of the volcanic rocks. The Sr isotope composition of individual fluorite minerals in uranium mineralization ranges from 0.71447 to 0.72072, averaging at 0.71752 (Jiang et al., 2006). The Sr isotope composition of individual pyrite minerals in polymetallic mineralization ranges from 0.719656 to 0.720133, averaging at 0.719833, which is higher than that of individual fluorite minerals in uranium mineralization and similar to the whole-rock Sr isotope composition of the basement metamorphic rocks.