Geochronological and Geochemical Constraints on the Origin of the Hutouya Polymetallic Skarn Deposit in the East Kunlun Orogenic Belt, NW China

Abstract

The Hutouya polymetallic skarn deposit lies in the Qimantagh area of the East Kunlun Orogenic Belt, NW China. Skarnization and mineralization at the deposit are closely associated with contemporary felsic intrusions. In this paper, zircon U-Pb ages and zircon Hf isotope as well as whole-rock geochemical and whole-rock Sr-Nd isotope data are reported for intrusive rocks and crystal tuff of the Elashan Formation in the Hutouya area. Moreover, Re-Os ages and S-Pb isotopes are also reported for the ore minerals in the Hutouya deposit. The Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U-Pb age of granodiorite and Re-Os isochron age of molybdenite suggest that mineralizations occurred at ca. 227 Ma and that the granodiorite and molybdenite are closely related petrogenetically. All the granitoids in the Hutouya deposit are high-K calc-alkaline and metaluminous to weakly peraluminous I-type granitoids. Among them, the ore-forming granitoids were derived by the mixing of crust-derived (either juvenile or ancient mature lower crust) and mantle-derived magmas, whereas the non-ore-related granite porphyry was generated by the partial melting of a single ancient mature lower crust. The magmas of all the granitoids underwent extensive fractionation–crystallization during the process of rising and emplacement. The sulfur of the analyzed samples from the northern and middle zone of Hutouya deposit (including No. II, III, IV, and VI ore belts) belongs to deep magmatic sulfur, while the sulfur of samples from the southern zone of Hutouya deposit (No. VII ore belt) includes not only deep magmatic sulfur but also a contribution of strata sulfur. All the ore mineral samples in the Hutouya deposit have similar Pb compositions that are consistently derived from a mixed source of upper crust and mantle. Tectonic discrimination diagrams indicate a post-collisional setting for all granitic rocks of the Hutouya skarn deposit, which is therefore considered a product of a the post-collision extensional system and is consistent with other porphyry-skarn deposits within the East Kunlun Orogenic Belt.

1. Introduction

The East Kunlun Orogenic Belt (EKOB; Figure 1a,b) is one of the most significant polymetallic belts in China [1], with many studies of its numerous porphyry- and skarn-type deposits having been undertaken in recent years (

Table 1. Magmatic and mineralization ages for porphyry-skarn polymetallic deposits of the East Kunlun area. LA–ICP–MS: laser ablation–inductively coupled plasma–mass spectrometry.

Deposit	Type	Analytical Methods	Age (Ma)	Reference
Yemaquan	Skarn (Fe, Cu, Pb, Zn)	Zircon LA–ICP–MS U–Pb	228 ± 3	[5]
		Zircon LA–ICP–MS U–Pb	225 ± 2
		Zircon LA–ICP–MS U–Pb	221 ± 2
		hydrothermal phlogopite 40Ar/39Ar step-heating dating	223 ± 3.3
Shuangqing	Skarn (Fe, Cu, Pb, Zn)	Zircon LA–ICP–MS U–Pb	225.8 ± 1.6	[1]
		molybdenite Re–Os	224.3 ± 1.6
Kendekeke	Skarn (Fe)	Zircon LA–ICP–MS U–Pb	230.5 ± 4.2	[11]
		Zircon LA–ICP–MS U–Pb	229.5 ± 0.5	[12]
Yazigou	Porphyry (Cu, Mo)	molybdenite Re–Os	224.7 ± 3.4	[7]
		Zircon SHRIMP U–Pb	224.0 ± 1.6	[8]
Wulanwuzhuer	Porphyry (Cu, Mo)	Zircon SHRIMP U–Pb	215.1 ± 4.5	[13]
Kaerqueka	Skarn (Cu, Pb, Zn)	molybdenite Re–Os	238.8 ± 1.3	[14]
		Zircon SHRIMP U–Pb	237 ± 2	[15]
Harrizha	Porphyry (Cu, Mo)	Zircon LA–ICP–MS U–Pb	234.5 ± 4.8	[16]
Qingshuihedong	Porphyry (Cu, Mo)	Zircon LA–ICP–MS U–Pb	224	[17]
Xiadeboli	Porphyry (Cu, Mo)	Zircon LA–ICP–MS U–Pb	244.2 ± 2.1	[18]
Jiadanggen	Porphyry (Cu, Mo)	Zircon LA–ICP–MS U–Pb	227.2 ± 1.9	[19]
Saishitang	Skarn (Cu)	molybdenite Re–Os	224.5 ± 1.8	[20]
		Zircon LA–ICP–MS U–Pb	222.7 ± 2.3	[21]
		Zircon LA–ICP–MS U–Pb	222.6 ± 2.4
		Zircon LA–ICP–MS U–Pb	223.3 ± 2.2
		Zircon LA–ICP–MS U–Pb	229.9 ± 2.6
Galinge	Skarn (Fe)	Zircon LA–ICP–MS U–Pb	228.3 ± 0.5	[2]
Lalingzaohuo	Skarn (Mo–Fe)	Zircon LA–ICP–MS U–Pb	228.49 ± 0.84	[22]
Changshan	Porphyry (Mo)	Zircon SHRIMP U–Pb	220 ± 1	[23]
Tawenchahan	Skarn (Fe-Cu-Zn)	Muscovite 39Ar–40Ar	230.7 ± 2.0	[3]
			229.9 ± 3.5

). In particular, the Qimantagh area (Figure 1b,c), located in the northern EKOB, is the top priority of researchers for skarn and porphyry-type Fe-Cu-Pb-Zn-Mo-W-Sn deposits, including the Galinge Fe skarn deposit [2], Tawenchahan skarn Fe-Cu-Zn deposit [3], Yemaquan skarn Fe-Cu-Pb-Zn deposit [4,5], Hutouya skarn Fe-Cu-Pb-Zn deposit [6], and Yazigou porphyry Cu-Mo deposit [7,8].

The Hutouya district includes seven major Fe-Cu-Pb-Zn skarn deposits accompanied by Mo-W-Sn-Ag mineralization, which are referred to as ore belts Nos. I–VII (Figure 2). The submitted Cu-Pb-Zn metal resources of ore belts Nos. VII, VI, and II are ca. 60 × 10⁴ t, 20 × 10⁴ t, and 5 × 10⁴ t, respectively, indicating they correspond to large-scale deposits and have important economic value [24]. To date, several issues need more reasonable research and interpretation as the prospecting work has deepened; thus, many scholars have become attracted to the discussion of the ore genesis in this deposit [6,24,25,26]. Although previous studies have investigated igneous rocks in this area, we notice that (1) most of them focused on the monzogranites in the Hutouya [24,26], but the ore-forming granodiorites of ore belts No. I and III have attracted less attention; (2) it still remains unknown whether all the granitic rocks in the Hutouya deposit belong to A-type granite [27]; (3) Sr-Nd-Hf isotopic data for granitoids are urgently needed to compare the magma sources between the ore-forming and non-ore-related granitoids; (4) the petrogenesis of a variety of rock types within the Hutouya deposit has not been systematically studied—especially the crystal tuff of the Elashan Formation—since many Pb-Zn-Ag deposits related to the volcanic strata were discovered [28]; (5) to date, the commonality and difference of ore-forming materials on S-Pb isotopes within different ore belts of the Hutouya deposit is unclear.

In this work, in order to address the above issues, we provide new geochronological and geochemical data for the Hutouya deposit, including molybdenite Re-Os dating, zircon LA–ICP–MS U-Pb dating, zircon Hf isotopic compositions, whole rock major and trace elements, and Sr-Nd isotopic compositions of ore-related granitoids, non-ore-related granitoids, and crystal tuff, as well as ore-mineral S-Pb isotopic compositions, which will place important constraints on the origin of the Hutouya polymetallic skarn deposit.

2. Regional Geology

The EKOB extends east–west (E–W) for >1000 km in the western part of the Central Orogenic Belt. The respective northern and southern boundaries of the EKOB are marked by the Qaidam Block and Bayan Har-Songpanganzi Terrane (Figure 1a,b), and this belt records the earliest history of assembly of the northern Tibetan Plateau [29,30]. The EKOB is divided into three parts by deep regional fractures: the North Kunlun, Central Kunlun, and South Kunlun belts by the boundaries between these units—from north to south, the North Kunlun fault (NKF), Middle Kunlun fault (MKF), and South Kunlun faults (SKF) [30,31,32] (Figure 1b). The Precambrian basement of the north Kunlun and Central Kunlun belts is the Paleoproterozoic Jinshuikou Group, which formed at ca. 1800–1600 Ma and underwent granulite facies metamorphism in the Paleozoic [33]. The Precambrian basement of the South Kunlun Belt is composed of the Proterozoic Wanbaogou Group, Kuhai Group, and the early Paleozoic Naij Tal Group [31].

Previous sufficient research works have identified two stages of the Phanerozoic orogeny and related magmatic events in the EKOB: the late Neoproterozoic–Devonian and Carboniferous–Triassic, which correspond to the evolution of the Proto-Tethys and Paleo-Tethys oceans, respectively [31,34,35,36,37]. The Hutouya deposit, one of the typical Cu-Fe-Pb-Zn polymetallic skarn deposits, is located in the northwestern EKOB (Figure 1b,c). As previously stated, fragments of the Paleo-Tethyan Oceanic crust have been identified by their 308 Ma MORB-type basaltic lavas and 333 Ma OIB-type gabbro [38,39,40]. During the Late Permian to Triassic, numerous igneous rocks formed in the subduction and continental collision settings during the Paleo-Tethyan Ocean evolution in the EKOB [41,42]. As previously stated, a large amount of Permian-early Triassic granitoids (ca. 263–240 Ma) with the geochemical characteristics of Andeans-type granites formed in an active continental margin setting within the EKOB [42,43]. In addition, the marine facies sedimentary formations in the forearc basin, including the Upper-Permian Gequ Formation, the Lower-Triassic Hongshuichuan Formation, and Mid-Triassic Naocangjiangou Formation, are believed to be formed in a tectonic setting of ocean subduction within the EKOB related to the evolution of Paleo-Tethys ocean during the Late-Permian to Mid-Triassic [44]. In summary, the Paleo-Tethyan oceanic crust in the EKOB was still in a subduction setting before the Middle Triassic. Subsequently, a continental collision event occurred in the late Middle Triassic (after ca. 240 Ma). Moreover, the Middle to Upper Triassic Babaoshan Formation comprises terrigenous clastic and volcanic rocks in angular unconformity with an underlying marine sequence, indicating that the EKOB entered a collision setting after the late Middle Triassic [20,45]. During the Late Triassic, the EKOB tectonic regime was transformed from a collision compressional to post-collision extensional system following the closure of the Paleo-Tethys Animaqen Ocean [46].

3. Deposit Geology and Sample Descriptions

The Hutouya polymetallic deposit is ≈345 km west of Golmud, Qinghai Province, NW China. The strata in the Hutouya mining area are characterized with a large time span, including the mid-Proterozoic Langyashan Formation limestone, Ordovician–Silurian Tanjianshan Formation marble, mafic volcanic rocks with clasolite, Lower Carboniferous Dagan’gou Formation limestone, Upper Carboniferous Di’aosu Formation dolomitic limestone, and marble and Upper Triassic Elashan Formation crystal tuff with rhyolitic tuff breccia. Fold and fault structures occur in the mining area. A near-E–W transpressive fault and fold system formed by north–south (N–S)-directed compression is the ore-controlling structure of the deposit (Figure 2). The Hutouya deposit is a large deposit, including seven main ore belts in the area. Ore belts No. I, II, III, and VI in the middle part of the Hutouya deposit, named the “inner contact skarn zones”, emerge as the contact area among the ore-forming intrusions, carbonate strata, and host Fe-Cu-Mo mineralization (Figure 2). In addition, ore belt No. VII, in the southern part of Hutouya deposit, named the “outer contact skarn zone”, emerges as the main Pb-Zn ore body bounded by carbonate strata and the fracture zones. By contrast, small granodiorite and granite porphyry dykes, without a contact area between granitoids and carbonate strata, appeared in the No. IV and V ore belts in the northern part of the Hutouya deposit, which is different from the No. VII ore belt (Figure 2). Based on the current field geology, a large polymetallic mineralization contact zone between the ore-forming intrusions and the carbonate strata probably exists in the deep part of the No. IV and V ore belts. The specific features of the main ore belts and sample locations are described below:

(1)

Ore belt No. I: the ore bodies occur within the granodiorite contact belt with the marble of the Lower Carboniferous Dagan’gou Formation. The No. I ore belt is 2 km in length and includes four ore bodies. The main ore body is ca. 560 m in length and ca. 6.6 m in thickness. The ore bodies generally dip south, and the dip angle is about 20–70°. The magnetite is the dominant opaque mineral in these ores;

(2)

Ore belt No. II: the ore bodies occur within the contact zone between the Upper Carboniferous Di’aosu Formation and the monzogranite. Ore belt No. II is 1.5 km in length and includes 25 ore bodies. The two main ore bodies are 500–600 m in length and 5–6m in thickness. The ore bodies generally dip south, and the dip angle is about 40~75°. The chalcopyrite is the dominant opaque mineral in these ores, with minor Fe-Sn-Zn mineralization;

(3)

Ore belt No. III: the mineralization produced in the contact zone between the granodiorite and the Upper Carboniferous Di’aoiosu Formation. Two iron ore bodies have been explored. The ore belt stretches 1.3 km. The alteration types are diopsidization, epidotiation, and choritization;

(4)

Ore belt No. VI: the mineralization occurs in the contact zone between the granites (including monzogranite and small granite porphyry veins) and the Upper Carboniferous Di’aosu Formation. The ore belt is 3 km in length and 10–50 m in width. Two main mineral bodies (Pb, Zn, and Cu) have been explored. The length of the two main ore bodies is ca. 830 m, and the thickness of the ore body is about 5~6 m;

(5)

Ore belt No. VI: the ore bodies occurs in the structure fracture of the Upper Carboniferous Di’aosu Formation. The main ore body is 490 m long, with an average thickness of 3.5 m and an average zinc grade of 2.52% and lead grade of 2.30%;

(6)

Ore belt No. VII: the ore bodies are located in the E–W trending tectono-fracturing belts in the mid-Proterozoic Langyashan Formation limestone. The No. VII mineralized zone contains four main ore bodies with strata-bound features.

Fresh samples of the ore-forming granitic rocks JR (37°03′52″ N, 91°39′32″ E) and DH10 (37°04′24″ N, 91°40′06″ E) and the non-ore-related HT (37°05′28″ N, 91°39′16″ E) were taken from the Hutouya deposit (Figure 2). The crystal tuff samples (37°05′47″ N, 91°38′17″ E) were also taken to examine the magmatism within the Hutouya deposit. The granodiorite sample (JR) was grayish white in surface outcrops. Microscopic examination revealed a granodiorite mineral composition (Figure 3a) composed mainly of plagioclase (≈45%; 0.6–1.0 mm), alkali feldspar (≈20%; 1.3–1.9 mm), quartz (≈20%; 0.6–1.0 mm), biotite (≈5%; 0.5–1.5mm) and amphibole (≈10%; 0.3–0.5 mm) with accessory minerals including zircon and apatite. The fresh face of the monzogranite sample (DH10) was light gray with a massive structure (Figure 3b) and a mineral composition of quartz (≈45%; 0.75–3.0 mm), plagioclase (≈35%; 1–2 mm), alkali feldspar (≈20%; 1.3–1.9 mm) and biotite (≈5%; 0.6–0.75 mm). The granite porphyry sample (HT) exhibited a porphyritic texture. The phenocrysts were mainly plagioclase (≈15%; 0.5–1.0 mm), K-feldspar (≈10%; 0.5–1.0 mm), quartz (≈30%), and biotite (≈2%). The groundmass was dominated by felsic minerals, biotite, and minor accessory minerals (Figure 3c). Most plagioclase was subhedral and short columnar in shape, with albite twinning and minor surface kaolinization and carbonate. K-feldspar was mostly subhedral short columnar, with Carlsbad twinning and surface kaolinization. The groundmass mainly comprised microcrystalline quartz and feldspar, and the main accessory minerals were apatite, zircon, and corundum. The Hutouya crustal tuff sample (DH09) was grayish green, with a crystalline–pyroclast tuffaceous texture. Crystal pyroclasts (≈30%) included predominantly quartz and pegmatolite (Figure 3d). Quartz pyroclasts were angular with visible embayments.

The main ore mineral samples of the Hutouya deposit were sphalerite, galena, magnetite, and chalcopyrite, followed by molybdenite, arsenopyrite, pyrophyllite, and small amounts of chalcocite, pyrrhotite, and pyrite. The ore mineral analytical samples in this study were collected from the No. IV ore belt and No. VI ore belt within the Hutouya deposit. The ore texture was predominantly metasomatic in structure and exsolution (Figure 3e–i). In particular, the molybdenite was flaky with bending deflection and showed a dissemination structure in the photomicograph (Figure 3e). The chalcopyrite was brass yellow. Microscopic examination revealed an allotriomorphic texture (Figure 3f–h) and milk-drop texture (Figure 3f). The pyrrhotite was cream yellow, tinged slightly pink, and anhedral in texture in the photomicrograph (Figure 3g). The galena was pure white by reflection. Microscopic examination revealed an allotriomorphic granular texture with black delthrium in the surface (Figure 3h). The sphalerite was shown to be dark gray by reflection and exhibited an allotriomorphic graular texture in the photomicrograph (Figure 3f,i). Gangue minerals mainly included calcite, garnet, diopside, fluorite, and quartz.

4. Analytical Methods

Zircons from the Hutouya granodiorite (sample JR-N) were dated at the State Key Laboratory of Continental Dynamics, Northwest University, Xi an, China. Zircons from the Hutouya granite porphyry (HT-N) and crystal tuff (DH09-N) were dated at the State Key Laboratory of Geological Processes and Mineral Resources (LGPMR), China University of Geosciences, Wuhan, China. Re-Os isotopic analyses were performed at the National Research Center of Geoanalysis, Chinese Academy of Geosciences, Beijing, China. Whole-rock major and trace-element analyses were conducted at the Analysis Center of Jilin University, Jilin Province, China. Hf isotopic compositions of zircons from granodiorite and crystal tuff were determined at the State Key Laboratory of Geological Processes and Mineral Resources (LGPMR), China University of Geosciences, Wuhan, China. Hf isotopic compositions of zircons from the granite porphyry were determined at the Beijing Yanduzhongshi Testing Technology, Beijing, China. Whole-rock Sr-Nd isotopic analyses of the Hutouya monzogranite and crystal tuff were undertaken at the Analytical Laboratory of the Beijing Research Institute of Nuclear Industry, Beijing. Sulfur and Lead isotopic analyses were undertaken at the Analysis and Test Research Center, Beijing Geological Research Institute of Nuclear Industry, Beijing. The analytical techniques used in the testing are described in Appendix A.

5. Results

5.1. Geochronology

5.1.1. Zircon U-Pb Dating

The results of zircon U-Pb dating are given in Supplementary Material Table S1, and their concordia diagrams are presented in Figure 4a–c. Zircons from the Hutouya granitic rocks and crystal tuff were generally light gray and columnar. All analyzed zircons were euhedral with lengths of 70–120 μm and aspect ratios of 1–3. Their oscillatory growth zoning indicated a magmatic origin [47]. The ²⁰⁶Pb/²³⁸U ages of zircons from sample JR-N ranged from 232 to 221 Ma, with a weighted-mean age of 227.1 ± 3.6 Ma (MSWD = 0.13; n = 16; Figure 4a). Zircons from the Hutouya granite porphyry (sample HT-N) yielded ²⁰⁶Pb/²³⁸U ages of 226–217 Ma and a weighted-mean age of 222.1 ± 1.0 Ma (MSWD = 0.66; n = 26) (Figure 4b). Zircons from the Hutouya crystal tuff (sample DH09-N) displayed prismatic forms, with ²⁰⁶Pb/²³⁸U ages of 233–215 Ma and a weighted-mean age of 222.7 ± 2.5 Ma (MSWD = 0.34; n = 18) (Figure 4c). These results together indicate that the crystallization of the Hutouya granitic rocks and the eruption of the crystal tuff both occurred during the Late Triassic.

5.1.2. Molybdenite Re-Os Dating

Re-Os analysis results for seven molybdenite samples are summarized in Supplementary Material Table S2. Re and ¹⁸⁷Os contents were 169.0–278.8 ppm and 403.8–669.4 ppm, respectively. Common Os (≈0.01 ppb) may have been derived from analytical artifacts and was negligible compared with ¹⁸⁷Os. Therefore, the molybdenite Re-Os ages were considered reliable [48]. The seven molybdenite samples from skarn ores yielded consistent model ages of 230–225 Ma (Supplementary Material Table S2), with an Re-Os isochron age of 227 ± 6 Ma (MSWD = 1.5) and an initial ¹⁸⁷Os content of 0 ± 14 ppm (Figure 4d). Furthermore, the Re content is commonly used to trace ore-forming materials [49], and the higher Re contents (169–278 ppm) recorded here may imply the involvement of mantle underplating [50].

5.2. Whole-Rock Major and Trace-Element Compositions

Whole-rock major and trace-element compositions of 23 representative samples of granitic rocks and crystal tuff are given in Supplementary Material Table S3 and are described separately for each rock type below.

5.2.1. Ore-Forming Granodiorite and Monzogranite

The six analyzed samples of the Hutouya granodiorite have SiO₂ and Al₂O₃ contents of 62.40–65.10 wt % and 16.38–16.83 wt %, respectively, and a high alkali content (Na₂O = 4.43–4.69 wt %; K₂O = 2.93–3.41 wt %; K₂O + Na₂O = 7.59–7.92 wt %) with K₂O/Na₂O ratios of 1.32–1.59. These samples are plot in the high-K calc-alkaline field in a SiO₂–K₂O diagram (Figure 5a). Their A/CNK and A/NK ratios (A = Al₂O₃; C = CaO; N = Na₂O; K = K₂O, molar values) are in the ranges of 1.00–1.02 and 1.47–1.55, respectively, plotting in the metaluminous to weakly peraluminous region (Figure 5b). The samples display subparallel, right-tilting, chondrite-normalized rare Earth element (REE) patterns (Figure 6a) and have high total REE contents (REE = 76.9–92.1 ppm). The fractionation of light REEs (LREEs) and heavy REEs (HREEs) is obvious ((La/Yb)_N = 5.66–7.92). The samples display weak positive Eu anomalies (Eu/Eu* = 1.09–1.44). In a primitive-mantle-normalized trace-element spider diagram (Figure 6b), the rocks are enriched in Rb, Ba, K, and other large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs; e.g., Nb, Ta, P, and Ti).

The monzogranite samples showed SiO₂, Na₂O and K₂O contents of 72.93–75.24 wt %, 2.85–3.16 wt % and 5.06–5.27 wt %, respectively, with Na₂O/K₂O ratios of 0.56–0.61. They are plotted in the high-K calc-alkaline field in a SiO₂–K₂O diagram (Figure 5a). Their A/CNK and A/NK ratios are in the ranges 0.86–0.95 and 1.17–1.19, respectively, indicating a metaluminous to weakly peraluminous nature (Figure 5b). The samples exhibited subparallel chondrite-normalized REE patterns, which are characterized by fractionation between LREEs and HREEs, with (La/Yb)_N ratios of 3.67–6.99 and strong negative Eu anomalies (Eu/Eu* = 0.54–0.64) (Figure 6a). The rocks were enriched in LILEs but depleted in HFSEs, with negative Ba, Sr, P, and Ti anomalies (Figure 6b).

5.2.2. Non-Ore-Related Granite Porphyry and Crystal Tuff

The granite porphyry samples had SiO₂, K₂O, MgO, TiO₂ and P₂O₅ contents of 63.83–73.26 wt %, 4.31–6.74 wt %, 0.51–1.24 wt %, 0.11–0.53 wt %, and 0.11–0.13 wt %, respectively. They had high total-alkali contents (K₂O + Na₂O = 7.03–8.53 wt %) with K₂O/Na₂O ratios of 0.20–0.63, plotting in the high-K calc-alkaline shoshonite field in a SiO₂–K₂O diagram (Figure 5a). The samples had A/CNK ratios of 0.73–1.16, indicating metaluminous to weakly peraluminous characteristics. Their REE contents ranged from 48.3 to 238.6 ppm (mean 164.6 ppm), including LREE contents of 38.01–215.6 ppm. The granite porphyry samples had similar chondrite-normalized REE patterns (Figure 6a), exhibiting negative Eu anomalies (Eu/Eu* = 0.34–0.74; Figure 6a) and LREE enrichment with a marked differentiation of LREEs.

The crystal tuff samples had SiO₂ contents of 73.85–74.75 wt % and appeared metaluminous to weakly peraluminous in an A/CNK–A/NK diagram (Figure 5b). All samples had total medium REE (ΣMREE) contents of 78.1–111.5 ppm (mean 101.4 ppm). In the chondrite-normalized REE plots (Figure 6a), they display a strong enrichment in LREEs and depletion in HREEs (mean (La/Yb)_N = 9.04). In the primitive-mantle-normalized trace-element spider diagram (Figure 6b), all samples display enrichment in LILEs (e.g., Rb, Th, U, and REEs), strong depletion in HFSEs (e.g., Nb, Ta, Ti, and P) and weak negative Sr anomalies. They had strong negative Eu anomalies (Eu/Eu* = 0.58–0.63), which is indicative of the possible crystal fractionation of plagioclase.

5.3. Sr-Nd Isotopic Compositions

The results of Sr-Nd isotopic analyses of the Hutouya monzogranite and crystal tuff are given in Supplementary Material Table S4 and plotted in Figure 7b. The monzogranite samples had ⁸⁷Sr/⁸⁶Sr ratios in the range of 0.731771–0.734463, with initial ⁸⁷Sr/⁸⁶Sr ratios (I_Sr) of 0.70635–0.70798. Their ¹⁴³Nd/¹⁴⁴Nd ratios were in the range 0.51228–0.51319 with εNd(t) values of −5.01 to −4.11 and T_DM2 model ages of 1406–1369 Ma. The crystal tuff samples had ⁸⁷Sr/⁸⁶Sr ratios in the range of 0.732635–0.739379, with initial ⁸⁷Sr/⁸⁶Sr ratios (I_Sr) of 0.70068–0.71639. Their ¹⁴³Nd/¹⁴⁴Nd ratios were in the range of 0.512252–0.512291 with εNd(t) values of −5.43 to −4.69 and T_DM2 model ages of 1442–1382 Ma.

5.4. Zircon Hf Isotopic Compositions

Hf isotopic data for zircons from the Hutouya granitic rocks and crystal tuff are given in Supplementary Material Table S5 and plotted in Figure 7a. All magmatic zircons from the granodiorite had initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282655–0.282755, positive εHf(t) values of 0.4–6.2, and T_DM2 ages of 1087–930 Ma, indicating that the primary magma was derived from Meso-Neoproterozoic juvenile components. Eighteen magmatic zircons from the crystal tuff were analyzed for Lu-Hf isotopes, yielding initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282524–0.282591, εHf(t) values of −4.0 to +0.3, and T_DM2 ages of 1345–1199 Ma, indicating magma sourced from Mesoproterozoic and reworked crustal materials. Fifteen magmatic zircons from the granite porphyry were also analyzed for Lu-Hf isotopes, yielding initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282524–0.282607, εHf(t) values of −5.3 to −0.9, and T_DM2 ages of 1312–1106 Ma. The predominantly negative εHf(t) values indicate that the magma was derived from reworked Mesoproterozoic crustal components.

5.5. Ore-Mineral S-Pb Isotopic Compositions

S isotopic data for 15 ore-mineral samples from the Hutouya No. IV and No. VI ore belts are presented in Supplementary Material Table S6. Together with the previous data, δ³⁴S_V-CDT values from the northern, middle, and southern Hutouya deposit were in the ranges +0.8‰ to +4.8‰ (mean 2.2‰; n = 21), +0.6‰ to +6.4‰ (mean 4.3‰; n = 24), and +1.6‰ to +9.8‰ (mean 7.6‰; n = 16), respectively.

Pb isotopic compositions for the 15 sulfide mineral samples together with previous published data are given in Supplementary Material Table S7. All of the 51 sulfides had ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.116–19.502, 15.560–15.813, and 38.186–39.021, respectively. The uniformity of these ratios indicates a homogeneous common Pb composition, with the deposit being derived from a relatively stable Pb source.

6. Discussion

6.1. Age of Mineralization

Accurate dating is crucial to the understanding of an ore deposit, and here, we discuss new dating results for the granitic rocks, crystal tuff, and molybdenite of the Hutouya deposit. Our zircon U-Pb dating of the crystal tuff indicates that its eruption occurred at 222.7 ± 2.5 Ma, and our zircon U-Pb dating of the granitic rocks indicates the emplacement and crystallization of granodiorite and granite porphyry at 227.1 ± 3.6 Ma and 222.1 ± 1.0 Ma, respectively. The molybdenite Re-Os isochron ages (227.5 ± 5.5 Ma) and zircon U-Pb dating of granodiorite are concordant within error at ca. 227.5 Ma. Thus, these ages indicate that magmatism and related skarn mineralization in the Hutouya deposit occurred during the Late Triassic. As stated previously, the closure temperature of the zircon U-Pb isotopic dating system and the Re-Os isotopic dating system is 800 °C and 500 ± 50 °C [56,57]. Therefore, the formation of the Hutouya deposit is related to the separation and exsolution of ore-forming fluids that occurred after the magma cooled rapidly from 800 to 500 ± 50 °C at ca. 227 Ma.

6.2. Genetic Types of the Igneous Rocks

The identification of the genetic type of felsic igneous rocks is critical to the understanding of their magma sources, magmatic evolution and tectonic settings [58]. Granitic rocks can be classified as either I, S, or A-types [59,60]. Our granodiorite, granite porphyry, and monzogranite samples had high K contents, low (Zr + Nb + Ce + Y) contents, and low (10,000 × Ga/Al) ratios. In ((Na₂O + K₂O)/CaO) − (10,000 × Ga/Al) (Figure 8a) and ((Na₂O + K₂O)/CaO) − (Zr + Nb + Ce + Y) (Figure 8b) diagrams, all samples were plotted in the I and S-type granite and non-fractionated granite fields, respectively.

A-type granites generally consist of alkaline minerals such as richterite, aegirine, and fayalite, unlike the granitic rocks of the Hutouya deposit, which are therefore not A-type granites. The mean A/CNK ratios of the Hutouya granitic rocks is 0.96, unlike S-type granites (A/CNK > 1.1) [61]. The solubility of P₂O₅ is high in strongly peraluminous melts but low in metaluminous or weakly peraluminous melts. Therefore, the P₂O₅ content increases in peraluminous S-type melts during crystal fractionation but decreases in I-type melts [62,63,64]. The Hutouya granitic rock samples generally have high SiO₂ contents and are high-K calc-alkaline and metaluminous to weakly peraluminous rocks. Additionally, their trends are not in accordance with S-type granites in the P₂O₅-SiO₂ diagram; on the contrary, they resemble the I-type granite trend (Figure 9h). The lack of Al-rich minerals such as cordierite and muscovite also supports their I-type rather than S-type origin [61]. Moreover, the low initial ⁸⁷Sr/⁸⁶Sr ratios of monzonitic granite ranging from 0.70635 to 0.70798 are clearly different from S-type granites (initial ⁸⁷Sr/⁸⁶Sr > 0.710) [65]. Therefore, we conclude that the Hutouya granitoids are I-type granites.

6.3. Magma Sources and Petrogenesis

As mentioned above, all the igneous rocks from the Hutouya deposit are high potassium calc-alkaline to shoshonite series, metaluminous to weakly peraluminous I-typed granites. Generally, the calc-alkaline I-type granites can be generated by the partial melting of mafic to intermediate igneous rocks [66] by mantle-derived basaltic magmas, which underwent extensive assimilation and fractional crystallization [67] or by the mixing of crust and mantle-derived magma [68].

6.3.1. Ore-Forming Granodiorite and Monzogranite

The granodiorite and monzogranite samples from the Hutouya deposit all showed high SiO₂ (62.40–75.24 wt %) and Al₂O₃ (12.15–16.83 wt %) contents; low MgO (0.45–1.26 wt %), Cr (2.03–10.48 ppm), Co (0.80–6.94 ppm), and Ni (3.19–8.21 ppm) contents (Supplementary Material Table S3); positive Rb and K anomalies; and negative Nb, Ta, P, and Ti anomalies (Figure 6b). Such characteristics are consistent with crust-derived rocks [69,70]. In addition, the granodiorite and monzogranite samples were unassociated with any contemporaneous gabbros and basalts in the Hutouya area. All these factors exclude the possibility that they could be generated directly by the partial melting of mantle peridotite. Moreover, experimental studies indicate that the Mg^# value is an effective way to evaluate the magma melts. In general, the dehydration melting of the basaltic lower crust generates low Mg^# (<40) melts, whereas the melts with mantle components involved can produce Mg^# (>40) [71,72]. According to the results, the granodiorite samples had a relatively low Mg^# value ranging from 27 to 29, which indicates that the melts were derived from mafic lower crust. Notably, the Mg^# value of the monzogranite samples ranged from 42 to 55, which likely suggests an involvement of mantle-derived components. Furthermore, the Nb/Ta ratios of the granodiorite and monzogranite samples ranged from 13.96 to 17.31 (mean value = 15.88) and from 6.52 to 11.08 (mean value = 9.66), respectively. Both of the granitoids are above the average Nb/Ta ratios of the lower crust (8.3; [73]) and below the average Nb/Ta ratios of mantle-derived rocks (17.4; [54]), further indicating that a certain number of mantle-derived components were involved in the generation of the granodiorite and monzogranite. Isotopically, zircon Hf isotopic analysis is an effective and widely used method to trace the source of magmas [74,75,76]. All the granodiorite samples had similar zircon Hf isotopic compositions with εHf(t) values of 0.0–6.2. In the εHf(t)–age diagram (Figure 7a), they were plot between the depleted mantle and chondrite lines. In addition, the T_DM2 ages ranged from 1123 to 930 Ma (Supplementary Material Table S5), implying that the granodiorite could be derived from reworked Meso-Neoproterozoic juvenile components. The monzongranite samples had a narrow range of high I_Sr values (0.70635–0.70798) and low εNd(t) values (−5.01 to −4.11) with the T_DM2 ages of 1406–1333 Ma, suggesting that the monzongranite may have been sourced from the partial melting of ancient lower crustal material. Notably, the monzongranite samples showed similar Sr-Nd compositions to other Triassic granitoids, plotting in the overlap field of the EKOB continental arc magmas and the EKOB-enriched lithospheric mantle (Figure 7b), which implies that the EKOB enriched lithospheric mantle might have been involved during the generation and/or evolution of the primary magma of the monzongranite.

All the granodiorite and monzongranite samples displayed low Sr (80.79–434.00 ppm), high Y (11.40–23.90 ppm), and high Yb (1.57–2.01 ppm) values, with low Sr/Y ratios (5.3–37.54), thus belonging to normal arc rocks rather than typical adakitic rocks [60]. The relatively high Yb concentrations (1.57–2.01 ppm) and relatively flat HREE characteristics (Figure 6a) indicate that partial melting occurred above the stable region of garnet [61]. Therefore, there was no garnet residue in the source. The granodiorite samples show a good linear relation in the Harker diagrams, which implies a strong fractionation of primary magma (Figure 9b–h). The relative negative correlations between SiO₂ and P₂O₅, CaO, Al₂O₃ imply the fractional crystallization of apatite and plagioclase. In the Rb-Sr and (Zr/Y)-(Hf/Sm) diagrams (Figure 10c,d), the trend of granodiorite samples indicates the crystallization of amphibolite. The crystallization of Fe-Ti oxides and plagioclase can be inferred from the relative negative correlations between SiO₂ and TiO₂, Fe₂O_3total and CaO, respectively (Figure 9b,c,f). The negative Eu anomalies (Eu/Eu* = 0.54–0.64, Figure 6a) and Sr negative anomalies of the monzongranite samples also indicate the fractionation crystallization of plagioclase (Figure 6b). The P negative anomalies of monzongranite samples suggest the fractionation crystallization of apatite (Figure 6b). In the Ba-Sr and (Zr/Y)-(Hf/Sm) diagrams, it can be seen that the magma of monzongranite experienced the fractionation–crystallization of K-feldspar and hornblende (Figure 10c,d).

In summary, the ore-forming granitoid intrusions were generated by the mixing of crust and mantle-derived magma. The mafic magma was generated by the partial melting of enriched lithospheric mantle and underplated on the bottom of lower crust. As the upwelling enriched lithospheric mantle-derived magma, which rose to the bottom of the lower crust, the partial melting of juvenile (i.e., the Hutouya granodiorite) and ancient mature lower crust (i.e., the Hutouya monzongranite) occurred, which then mixed with mantle-derived magmas. Then, the mixed magmas underwent extensive fractionation–crystallization during the process of rising and emplacement.

6.3.2. Non-Ore-Related Granite Porphyry and Crystal Tuff

All the granite porphyry and crystal tuff samples contained high concentrations of SiO₂ and Al₂O₃ and low concentrations of MgO, Cr, Co, and Ni (Supplementary Material Table S3). All the concentrations mentioned above are in accordance with crust-derived rocks [77]. The low Nb/Ta ratios of both the granite porphyry (6.39–8.25) and crystal tuff (6.33–8.17) indicate that the magmas of granite porphyry and crystal tuff probably originated solely from the anatexis of the lower crust. In addition, the low Mg^# values of the granite porphyry (21–43) further imply that the magma was derived from crustal material. In contrast, the relatively high Mg^# values of the crystal tuff (51–65) imply that the mantle-derived components were involved in the magma. The high concentrations of Y (12.25–33.33 and 10.90–17.60 of granite porphyry and crystal tuff samples, respectively) and relatively flat HREE pattern (Figure 6a) indicate the presence of residual amphibole [78].

The isotope geochemical data also contribute some proof of the magma source. The non-ore-related granite porphyry samples displayed the following features: their zircon εHf(t) values and T_DM2 ages ranged from −5.34 to −0.85 Ma and 1518 to 1312 Ma, respectively, indicating that the primary magma may have been derived from the partial melting of the Mesoproterozoic lower crust (Figure 7a); the nature of the magma source of crystal tuff was revealed through zircon Sr−Nd−Hf isotopic compositions; the εHf(t) values of −4.0 to +0.3 and T_DM2 ages of 1345–1106 Ma indicate that the primary magma may have originated from the partial melting of the Mesoproterozoic lower crust with a minute quantity of mantle components; and the fairly negative εNd(t) values (−5.43 to −4.69) and T_DM2 ages of 1442–1382 Ma also imply that the magma of the crystal tuff was derived from the partial melting of the Mesoproterozoic lower-crustal material.

In the Harker diagrams, the non-ore-related granite porphyry and crystal tuff samples show a slight fractionation of magmas with few relative negative correlations (Figure 9b–h). Plagioclase crystallization is indicated by their negative Eu anomalies (Eu/Eu* = 0.34–0.84 and 0.58–0.63 for the granite porphyry and crystal tuff samples, respectively; Figure 6a). Amphibolite crystallization is indicated in the (Hf/Sm) − (Zr/Y) diagram (Figure 10d). The crystallization and differentiation of K-feldspar is indicated in the Sr–Ba diagram (Figure 10a). The P and Ti negative anomalies of all the granite porphyry and crystal tuff samples suggest the fractionation–crystallization of apatite and the Fe-Ti oxides.

In summary, non-ore-related granite porphyry was generated by the partial melting of a single ancient mature lower crust, with its primary magma undergoing fractionation–crystallization during the process of rising and emplacement. In contrast, the crystal tuff of the Elashan Formation originated from the mixing of lower crust material and mantle-derived components and also underwent fractionation–crystallization in the process of magma rising. To date, many Pb-Ag polymetallic deposits (such as Hariza, Dongshan, Narimalahei, and Zamashan) closely hosted in the continental volcanic rocks of the Elashan Formation have been discovered in the eastern section of the EKOB [29]; therefore, we infer that the acid volcanic rocks of the Elashan Formation, with mixed magma sources, should probably also possess great prospecting potential in the western section of the EKOB.

6.4. Sources of Ore-Forming Materials

Sulfur obviously plays an important role in the mineralization and precipitation of metal sulfide deposits, and S isotopic data provide important information about the sources of ore-forming materials [79,80], as gained from sulfur isotopic characteristics of hydrothermal fluids [81]. The absence of sulfate minerals in all ore belts of the Hutouya deposit shows that the ore-forming hydrothermal fluid is a single component that did not experience complex strong fractionation. For this reason, the values of the δ³⁴S_sulfide from the sulfide mineral samples can approximately represent the values of δ³⁴S_fluid in the ore-forming hydrothermal solution (δ³⁴S_sulfide ≈ δ³⁴S_fluid; [82,83]). Overall, our data, together with the previous studies, show that the δ³⁴S values from the major ore minerals range from 0.6‰ to 9.8‰ with a mean value of 4.5 (Supplementary Material Table S6). The mean δ³⁴S value is higher than the mantle-derived sulfur (0‰; [84]), meteoritic sulfur (from −3‰ to +3‰; [85]), and the reduced sulfur in sediments (<0‰; [86]), while it is lower than the oceanic carbonate (Cambrian–Triassic δ³⁴S = 15–35‰; [87]) and approximately equal to deep magmatic sulfur (from −5‰ to +5‰; [81]), which indicates that the S predominately originated from magmatic sulfur. In addition, the δ³⁴S peaks intervals of sulfide mineral samples from the No. IV ore belt in the north zone (0.8–5.0‰; Supplementary Material Table S6; Figure 11b,c) are similar to ore belts No. II, III and VI in the middle zone, named the “inner contact skarn zones” (0.6–6.8‰; Supplementary Material Table S6; Figure 11b,c), which are more approximate to the deep magmatic sulfur (from −5‰ to +5‰; [81]), while they are significantly lower than the VII ore belt in the southern zone (6.5–9.8‰, except for one chalcopyrite sample of 1.6‰; Figure 11a–c). In consideration of all the ore bodies bounded by carbonate strata and fracture zones in the No. VII ore belt, named the “outer contact skarn zones”, located in the southern part of the Hutouya deposit, the higher δ³⁴S peak intervals are likely due to the influence of the sulfur from the strata. Therefore, we conclude that concealed ore-forming intrusions and “inner contact skarn zones” probably existed deep within the No. IV, V ore belts of the northern zone within the Hutouya deposit, which shows greater metallogenic potential in the deep zone of the northern part of the Hutouya deposit.

Lead isotope compositions can be used to indicate the evolution of the crust, the source of metallogenic minerals, and the genesis of deposits [88]. According to our data, together with previous studies, all the samples from the Hutouya deposit showed mean μ values of 9.55 (Supplementary Material Table S7), which is slightly lower than the mean value of the upper crust (9.58; [89]). The mean ω value (36.34; Supplementary Material Table S7) was between the mean value of the mantle (31.84; [90]) and upper crust (41.86; [90]). The mean Th/U ratio (3.68; Supplementary Material Table S7) was slightly lower than the mean Th/U ratio of the upper crust in China (3.76; [91]). All these data indicate that the Pb is likely derived from a mixed source of predominating crust-derived Pb and little mantle-derived Pb [92]. To further clarify the source of Pb in the Hutouya deposit, Pb isotopic data were also plotted in the Pb-isotope tectonic model diagram (Figure 12a,b) and tectonic environment discrimination diagram (Figure 12c,d). In a (²⁰⁷Pb/²⁰⁴Pb)-(²⁰⁶Pb/²⁰⁴Pb) diagram (Figure 11a), the samples were plot mainly between the orogenic and upper crust evolution line with a proximate linear correlation. In a (²⁰⁸Pb/²⁰⁴Pb)-(²⁰⁶Pb/²⁰⁴Pb) diagram (Figure 11b), the samples were plotted mostly on the orogenic evolution line. In tectonic environment discrimination diagrams (Figure 12c,d), the samples mainly fell in the orogen fields. Therefore, we conclude that Pb in the Hutouya deposit was sourced from the upper crust and the erosion zone of the orogenic belt [93]. In the ∆γ–∆β diagram, all the samples were mainly plotted in the field of “3a”, which indicates that the Pb was derived from mixed lead from upper crust and mantle subduction (Figure 13). Therefore, the Pb of all the mineral samples from the Hutouya deposit was derived from a mixed source: the majority from the crust and a small amount from the mantle.

In summary, the sulfur of all the analyzed samples from the northern and middle zone of the Hutouya deposit (including ore belts Nos. II, III, IV, and VI) was derived from deep magmatic sulfur, while the S of all the analyzed samples from the southern zone of the Hutouya deposit (ore belt No. VII) was derived from deep magmatic sulfur and the contribution of strata sulfur. All the ore mineral samples showed that Pb was consistently derived from a mixed source of upper crust and mantle in the Hutouya deposit.

6.5. Geodynamic Background

An understanding of the geodynamic background of the EKOB is important for elucidating regional metallogenic processes [95,96,97,98]. The EKOB records several different orogenic phases and corresponding orogeneses, in which the orogeny related to the consumption of Paleo-Tethys oceans is an important and complex episode [99,100].

According to data in our study, the Late Triassic granitic rocks and crystal tuff from the Hutouya deposit are calc-alkaline I-type granites. In Nb-Y, and Rb–(Yb + Ta) diagrams (Figure 14), most samples were plotted in the (volcanic arc granites (VAG) + syn-collision granites (Syn-COLG)) and Post-COLG fields, respectively, similar to other Late Triassic high-K calc-alkaline to shoshonite I-type granites in the EKOB, which were supposed to form in a post-collision extensional setting [46,101]. In addition, the presence of Late Triassic A-type granites and adakites suggests that lithosphere delamination occurred and that the EKOB as in post-collision extension [30,102,103,104,105].

In the EKOB, a series of large and medium-scale skarn–porphyry Fe-Cu (Mo-Pb-Zn) polymetallic deposits have been generated, and their related ore-forming intrusions were formed during the Late Triassic ([20,106]; Table 1). As previously stated, the Late Triassic post-collision extensional tectonic setting in the EKOB triggered the event of strong mantle-source magma underplating with the emergence of mafic–ultramafic rocks [107,108,109]. In addition to the mafic–ultramafic rocks, numerous intermediate–felsic rocks derived from a mixing source of mantle and crust also developed (ca. 237–204 Ma; [46]). Therefore, intermediate–felsic rocks were likely generated from the partial melting of a thickened crust heated by strong mantle-derived magma underplating in decompression caused by the post-collision extension during the Late Triassic. This provided abundant ore-forming material as well as a heat source for regional mineralization under the influence of the strong crust–mantle interaction and magmatism in EKOB [23,106], as with the ore-forming granitoid intrusions from the Hutouya deposit in this study. Our results for the magmatism and mineralization ages of the Hutouya polymetallic deposit confirm its association with the Paleo-Tethys evolution of the EKOB.

7. Conclusions

Our new geochronological and geochemical data for Late Triassic granitoids, crystal tuff, molybdenites, and other ore minerals of the Hutouya Cu-Fe-Pb-Zn polymetallic deposit allow us to draw the following conclusions:

(1)

Zircon LA–ICP–MS U-Pb and molybdenite Re-Os dating reveals that the emplacement of granitoids and mineralization of the Hutouya deposit occurred at ca. 227 Ma;

(2)

The ore-forming granitoids were generated by the mixing of crust-derived (either juvenile or ancient mature lower crust) and mantle-derived magma. The non-ore-related granite porphyry was generated by the partial melting of a single ancient mature lower crust. The granitic magmas underwent extensive fractional crystallization during the process of rising and emplacement;

(3)

The crystal tuff was derived from a mixed magma source of lower crust material and mantle-derived components, and the Elashan Formation probably has great prospecting potential in the western EKOB;

(4)

Sulfur in the northern and middle zone of the Hutouya deposit was derived from deep magmatic sulfur, while the S of the southern zone of Hutouya deposit was derived from both deep magmatic sulfur and strata sulfur. All the ore mineral samples show that the Pb was consistently derived from a mixed source of upper crust and mantle in the Hutouya deposit;

(5)

The new data for granitoids and crystal tuff in the the Hutouya area suggest that they were formed at 227–222 Ma in a post-collision extensional geodynamic setting, with the Paleo-Tethys Bayankala Ocean having closed before the Late Triassic.