Geochemistry and Zircon LA–ICP–MS U–Pb Geochronology of the Shuangwang Au Deposit, Shaanxi Province: Implications for Tectonic Evolution and Metallogenic Age

Abstract

The Shuangwang Au deposit (with a gold resource of approximately 70 t Au), is located in the Fenxian-Taibai fore-arc basin in the West Qinling Orogen of central China. Igneous intrusions in the region include the Xiba granitic pluton and granite porphyry and lamprophyre dykes. The Xiba pluton is composed of granodiorite and monzonite granite. The granodiorite is typical I-type granite, and it yields a crystallization age of 221.1 ± 1.2 Ma and a two-stage Hf model age of 1432–1634 Ma. The monzonite granite shows a transitional characteristic between I-type and A-type granite, and it yields a crystallization age of 214.8 ± 1.2 Ma and a two-stage Hf model age of 1443–1549 Ma. The granitoid was derived mainly from a crust–mantle mixed source. The ages indicate that the granodiorite and monzonite granite formed during two different stages. The REE distribution patterns of the Xiba granitoid exhibit significant fractionation between LREE and HREE, showing right-dipping curves, with an enrichment of LREE and a deficit of HREE. The granodiorite displays a light negative Eu anomaly, while the monzonite granite displays an obvious negative Eu anomaly. The granite porphyry dikes are distributed in the No. I breccia and Jiupinggou granite porphyry, and they yield crystallization ages of 219.9 ± 1.5 Ma and 213.1 ± 0.89 Ma, respectively, and two-stage Hf model ages of 1382–1501 Ma and 1373–1522 Ma, respectively. The lamprophyre dikes in the deposit yield a crystallization age of 214.4 ± 2.7 Ma. After the collision event between the Yangtze and the North China Plates along the Qinling orogenic belt, at approximately 220 Ma in the Late Triassic, the detachment of the slab produced the upwelling of the asthenosphere material. Under conditions of mantle heat and tectonic stress, widespread partial melting of the subducted continental crust and the upper lithosphere mantle occurred, forming granitoids with various degrees of adakite characteristics.

1. Introduction

The Qinling Mountains region is the natural geographical boundary between the north and the south of China. It is also a complex continental collision orogenic belt with a complex crustal composition and structure, which has undergone long-term evolution of different tectonic geological systems [1]. The Qinling orogenic belt is located between the North China Craton and the Yangtze Craton. It is connected to the Dabie orogenic belt in the east and the Qilian orogenic belt and the Kunlun orogenic belt in the west. It was formed by the convergence and collision of the North China Craton and the Yangtze Craton along the Mesozoic Shangdan suture zone and the Early Mesozoic Mianlue suture zone. In the western Qinling orogenic belt (WQO), a large number of early Mesozoic granites intruded into the pre-Triassic strata on the north side of the Mianlue suture zone [2,3,4,5,6,7,8,9].

The western Qinling, as the core area of the Qinling orogenic belt (Figure 1b), contains a large number of Yanshanian–Indosinian intermediate–acid rock bodies, as well as Au, Pb, Zn, U, Mo, and other types of deposits [10,11,12,13]. These deposits mainly formed in the late Triassic period, including some large-scale Au deposits and Pb-Zn deposits, such as the Baguamiao large-scale Au deposit (210–208 Ma [8,9]), Yangshan large Au deposit (220–211 Ma [7,8]), Liba large Au deposit (217–216 Ma [14]), and Bafangshan-Erlihe large Pb–Zn deposit (226–221 Ma [15,16,17,18,19]). These Au deposits and Pb–Zn deposits are usually closely adjacent to the Late Triassic granitic intrusions, and they are even associated with granitic dikes.

However, regardless of whether these deposits in the WQO are related to granitic magmatism [18,21,22], the Mesozoic magmatic mechanism, dynamic background, and relationship between magmatic activity and mineralization in the western Qinling have attracted much attention from geologists.

The Xiba rock mass is the largest magmatic rock mass exposed in the Shuangwang Au deposit in Shaanxi Province (Figure 2). It is distributed in the Xiba–Taibaihe area at the junction of Taibai County and Liuba County in Shaanxi Province. It is sandwiched between the Shangdan fault and the Shanyang–Fengzhen fault. It is bounded by the Wulong rock mass near the Niuweihe Zaojiao Bay. Previous studies on the Xiba pluton and its enclaves have been carried out from the aspects of petrology, geochemistry, chronology, and isotope composition, and the ages of the plutons are quite different [1,3,23,24]. At the same time, several granite porphyry dikes and lamprophyre veins are present in the Shuangwang Au deposit, and the relationship between these intrusive dikes and mineralization is not clear. Most previous studies focused on the Xiba rock mass and lacked a chronological study of the intrusive dikes, which restricted the understanding of the genesis of the Shuangwang Au deposit to a certain extent [3,9,18,21,23,24,25,26,27]. Therefore, in this paper, zircon U–Pb dating and Lu–Hf isotope tests were carried out on samples of granodiorite, monzonitic granite, and granite porphyry dikes and lamprophyre dikes from the Xiba rock mass and the mining area. Combined with the characteristics of major and trace elements, the crystallization ages of the granite porphyry dikes and lamprophyre dikes in the Xiba rock mass and the mining area were studied, and the multi-stage magmatic events related to the intrusion of the rock mass and the corresponding tectonic evolution stages were discussed.

2. Geological Background

The Qinling orogenic belt is located in the middle part of the central orogenic belt of the Chinese mainland. It is a composite orogenic belt formed by the convergence of the North China plate and the Yangtze plate. Long multi-cycle, multi-stage, and multi-body orogenic processes have led to the formation of various metal deposits in the orogenic belt, making the region one of the important metallogenic belts in China [20,28,29,30,31,32,33,34,35,36,37]. The area experienced the oceanic subduction of the Shangdan Ocean and the Mianlue Ocean in the Paleozoic and Mesozoic, respectively. After the closure of the Shangdan Ocean and the Mianlue Ocean, the plates in the adjacent area collided with each other along the suture zone. Most researchers believe that the Qinling orogenic belt can be divided into four regional blocks: the southern margin of the North China plate, the North Qinling tectonic belt, the South Qinling tectonic belt (SQB), and the northern margin of the South China plate bounded by the Luonan–Luanchuan fault, the Shangdan suture zone, and the Mianlue suture zone.

The SQB is mainly composed of Early Paleozoic limestone and shale, Late Paleozoic clastic rocks with limestone, Mesozoic sandstone, and Early Mesozoic granites [38]. There is no Precambrian basement exposed in the western section of the SQB [30]. The crustal growth history and radiogenic Pb isotopic composition of the Mesozoic granitoids in the western SQB are similar to those of the Mesozoic granitoids in the Yangtze Block, indicating that the basement of the western SQB is genetically related to that in the northern margin of the Yangtze Block. Generally, the easternmost part of the Triassic strata in the Songpan–Ganzi residual oceanic basin is regarded as the boundary between the WQO and the East Qinling orogenic belt, roughly along the Qinling orogenic belt [9] to the narrow downward part (the so-called “wasp waist”). Early Mesozoic granitic intrusions are widely distributed in the western Qinling Mountains, including Xiahe, Darzang, Dewulu, Meiwu, Mishuling, Huangzhuguan, Dangchuan, Baoji, Guangtoushan, Wulong, and Dongjiangkou rock groups [6,7,8,9,32]. The outcrop area of these granitic rocks is about 500 km² [36]. Geochronological data indicate that they formed at ca. 248–195 Ma. Minor volcanic rocks of the same age are present in the WQO. In addition, pre-Mesozoic granite and late Mesozoic granite are also exposed in the WQO.

The Shuangwang Au deposit is located in the southwest of Taibai County, Baoji City, Shaanxi Province. It is a large, easy-to-mine, and easy-to-separate low-grade breccia-type Au deposit (Figure 2). It is distributed between Wangjialing of Hongya River and Wangjiazhuang of Taibai River in Taibai County in the WNW-ESE direction [9,21,28]. The mining area is located in the north wing of the Xiba–Songping composite anticline. The tectonic position is located in the strong collision junction of the Yangtze plate, the Qinling microplate, and the North China plate. The regional deep faults, the Shangxian–Danfeng fault, and the Fengzhen–Shanyang fault, are distributed on both sides of the mining area. The study area belongs to the Middle Qinling foreland basin system [38].

The exposed strata in the mining area from old to young are the Lower Devonian Wangjialing Formation (D1w), the Middle Devonian Gudaoling Formation (D2g), the Upper Devonian Xinghongpu Formation (D3x), and the Jiuliping Formation (D3j) (Figure 2). The Xinghongpu Formation is located in the middle of the mining area and it is the main ore-bearing stratum of Au-bearing breccia. Its lithology is mainly metamorphic siltstone, silty sericite slate, layered siltstone, and layered sodium rock series. The structure in the mining area is generally NW–SE, and it is composed of a series of linear folds and faults. The main fold structure in the mining area is the Xiba composite anticline, and the secondary folds developed in the north wing of the composite anticline are the Yindonggou anticline, Yuanbazi syncline, and historical anticline. The fault structure of the mining area is mainly NW-trending, and there are two main faults, namely, the Wangjialeng fault in the south and the Xiushiya fault in the north, followed by small-scale nearly N–S-trending faults formed after mineralization [3,28,31].

The magmatic rocks in the Shuangwang Au deposit mainly include an exposed granitic rock mass, and lamprophyre dikes and quartz albite veins intrude the Au-bearing breccia body of the main ore belt. The Xiba rock mass can be divided into two intrusion periods according to its contact relationship and lithological composition: (1) the early intrusion exposed area accounts for about 80% of the rock mass, and the lithological composition is dominated by medium-fine-grained granodiorite and quartz monzodiorite; (2) the late intrusion (20%) is mainly composed of medium-coarse-grained monzonitic granite, which is exposed mainly in the west of the Hongya River and Xigou area. The contact boundary of the intrusions in the early and late stages is straight, and the lithology changes sharply. The samples used for petrological and geochemical analyses were collected from the Xiba pluton, the granite porphyry of the No. I Au-bearing breccia body, and the magmatic dikes that intrude into the main ore belt. Most of the rock samples were very fresh, except for the samples of granite porphyry from the No. I breccia body, which had a certain degree of alteration [3,25,26].

3. Materials and Methods

3.1. Sampling and Descriptions

The granodiorite sample of the Xiba rock mass was collected from the Bianjiagou–Shiziling area of the Xiba rock mass (SH–116), with a medium-fine-grained structure and a block structure (Figure 3a and Figure 4b). The sample’s content of plagioclase was 50%–60%, which was semi-automorphic–automorphic plate-like, with the development of lamellar double crystals, and some particles showed an obvious ring structure. Its content of K-feldspar was 15%–20%, showing double crystals. Its quartz content was about 20%, with a mainly semi-automorphic–allotriomorphic filling with the larger sized plagioclase and hornblende, as well as wavy extinction development. Its content of amphibole was about 10%, and the sizes of semi-automorphic particles were different. Under a single polarizer, it was brown-yellow–brown-green, with obvious polychromaticity, simple twin crystal development, and a certain degree of alteration, mainly chloritization. Its biotite content was 5%, leaf-like and dark brown–dark green under a single polarizer. A group of parallel poles was completely cleaved, mostly symbiotic with hornblende, with a certain degree of chloritization. It also contained a small amount of accessory minerals: titanite, zircon, magnetite, and apatite.

The monzonitic granite sample of the Xiba rock mass was collected from the Qingganggou (SH–117) of the Xiba rock mass with a medium-coarse-grained structure and block structure (Figure 3c). Its content of plagioclase was 30%–35%, mostly columnar and plate-like, and the development of polymorph double crystals, card sodium composite double crystals, and a ring structure. Its content of potassium feldspar was 30%–35%, the plate-like particles were coarse, the micro-oblique stripe feldspar was developed, and a lattice bicrystal was clearly visible. Its quartz content was 25%, and it was filled with other mineral particles. The dark minerals were mainly hornblende and biotite, with a certain degree of chloritization and epidotization. It contained a small amount of accessory minerals: sphene, zircon, magnetite, apatite, etc.

The lamprophyre sample (14–221) was collected from the 257th round of drilling in hole zk172–6 in Jiupinggou, with a hole depth of 454 m. The samples were gray-black, fine-grained, and dense (Figure 3d). The main minerals were plagioclase (40%), hornblende (35%), and biotite (10%), followed by quartz (5%), pyrite (3%), chlorite, and epidote (2%), and the accessory minerals were magnetite, apatite, and zircon. The hornblende was a self-shaped crystal output, with a typical lamprophyre structure and a block structure. The dark minerals had a good self-shaped degree and serious alteration, forming chlorite, epidote, and carbonate minerals. The matrix was mostly composed of light-colored mineral plagioclase and quartz, and its self-shaped degree was general. The plagioclase showed clear twin crystal lines, and an electron probe showed that it was mainly albite.

Granite porphyry occurs as dikes, with widths of 10–30 m and lengths of 100–1000 m. It is controlled by the regional tectonic line and is distributed along the WNW direction. Sample T–2 was collected from the No. I Au-bearing breccia body with a porphyritic structure (Figure 3e). Its phenocrysts were mainly plagioclase, potassium feldspar (microcline), and quartz. Sample T–5 was collected from the Jiupinggou magmatic rock mass (Figure 3f). Compared with T–2, its phenocryst particles were relatively large and mainly feldspar (potassium feldspar and striped feldspar), its quartz phenocrysts were relatively few, and the dark minerals (mainly biotite) in its matrix were relatively more.

3.2. Analytical Methods

3.2.1. Major and Trace Element Analyses

The major, trace, and rare earth elements of the samples were tested at the Analysis and Testing Research Center of the Beijing Institute of Geology, Nuclear Industry. The major elements were analyzed using a Philips PW244 X-ray fluorescence spectrometer (XRF, Amsterdam, The Netherlands), and the analytical accuracy was better than 1%. The analysis of trace elements and rare earth elements was performed using a Finnigan MAT Element I inductively coupled plasma mass spectrometer (ICP–MS, Bremen, Germany), with an RSD (10 min) <1.0% and an RSD (4 h) <5%. The analytical test methods are described in detail in [39].

3.2.2. LA–ICP–MS Zircon U–Pb Dating

LA–ICP–MS zircon U–Pb dating was carried out at the Key Laboratory of Orogenic Belt and Crust Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University. A COMPex 102 ArF excimer laser (working substance ArF, wavelength 193 nm) from Lambda Physik in Germany was used for laser ablation. The COMPex 102 ArF excimer laser (Coherent, Germany) was connected with an Agilent 7500ce ICP–MS (Agilent, Santa Clara, CA, USA) and GeoLas 200 M optical system from MicroLas (Göttingen, Germany). Shield Touch and a cs lens were used. The carrier gas of the erosion material was helium. The instrument was optimized with the synthetic silicate glass standard reference material NIST610 developed by the National Institutes of Standards and Technology, and a zircon in situ U–Pb analysis was performed using the Plesovice standard zircon external correction method [40]. The laser beam spot diameter was 32 μm, the laser ablation depth was 30–40 μm, the frequency was 5 Hz, and the energy density was 5 J/cm². The data were collected via a 20 s gas blank and 65 s laser ablation. Isotope ratio data processing and U–Pb age calculation were carried out using the Glitter program (version 4.0), and ordinary Pb correction was carried out according to the Anderson [41] method. The error of a single data point was 1, and the weighted average had 95% confidence.

4. Results

4.1. Major and Trace Elements

In the TAS rock classification diagram (Figure 5), the samples of the Xiba rock mass fall in the granodiorite and monzonitic granite area fields. The contents of SiO₂, Al₂O₃, CaO, and total alkali (Na₂O + K₂O) and the K₂O/Na₂O ratio of the granodiorite were 64.94%–66.05%, 14.41%–16.13%, 3.72%–4.18%, 6.95%–7.58%, and 0.70%–1.01% (with an average of 0.82%), respectively. The content of main oxides in the monzonitic granite changed little. The content of SiO₂ was relatively high, at 67.29%–67.54%. The content of Al₂O₃ was basically uniform, at 14.97%–15.34%. The content of CaO was 2.68%–2.9%, and the total alkali content (Na₂O + K₂O) was 7.69%–8.52%. The K₂O/Na₂O ratio of the monzonitic granite was 1.22–1.42, with an average of 1.32. Its contents of SiO₂, Al₂O₃, CaO, and MgO were 52.79%–53.88%, 15.54%–15.61%, 5.84%–6.28%, and 4.71%–4.46%, respectively. Its total alkali content (Na₂O + K₂O) was 4.90%–5.28%. The K₂O/Na₂O ratio was 0.47–0.69, with an average value of 0.58. The SiO₂ content of the granite porphyry ranged from 73.11 to 74.19%. The TiO₂, Al₂O₃, and CaO contents were in the ranges of 0.07%–0.09%, 13.96%–14.63%, and 0.55%–0.72%, respectively. The Na₂O and K₂O contents were 3.91%–4.01% and 4.86%–5.1%, respectively, with K₂O/Na₂O ratios ranging from 1.24 to 1.27. In the A/NK–A/CNK diagram (Figure 4), the samples fall in the quasi-aluminum field, showing the characteristics of I-type granitic rocks. In the discrimination diagram of the K₂O–SiO₂ magmatic rock series (Figure 4), the granodiorite and lamprophyre show high-K calc–alkaline characteristics, while the monzonitic granite was of the shoshonite series.

The trace element analysis of the magmatic rocks in the Shuangwang Au deposit (Figure 6b) showed an enrichment of Rb, Ba, K, Ta, and Sr elements; a relative loss of Nb, P, and Ti; and a weak loss of Th and Zr. The Sr content of the granodiorite was high (521.50–775.00 × 10⁻⁶), the Y content was low (10.73–13.99 × 10⁻⁶), and the Yb content was also low (1.07–1.21). The rocks showed the characteristics of adakitic rocks, with high Sr and low Y and Yb. The granodiorite had a high content of Co and Ni, 13.0–15.61 × 10⁻⁶ (with an average of 14.76 × 10⁻⁶) and 28.3–43.74 × 10⁻⁶ (with an average of 35.57 × 10⁻⁶), respectively, indicating the formation of the rock in the process of the addition of mantle material; however, the Sr content of the monzonitic granite was 391.50–413.50 × 10⁻⁶, which was significantly lower than that of the monzonitic granite, and the contents of Co and Ni were also lower, at 9.96–10.50 × 10⁻⁶ and 26.20–28.20 × 10⁻⁶, respectively, indicating that the contamination of mantle material was weak. In general, the granites of the Xiba pluton are depleted in high-field-strength elements, such as Nb, Ce, Zr, and Ti, and they are enriched in large-ion lithophile elements, such as Ba and Sr, indicating that the source rocks may be dominated by crust-derived components and mixed with mantle materials [42]. However, the similarity in the content and composition of rare earth elements and other trace elements between the granodiorite and monzogranite indicates that they have the same or similar magmatic source rocks.

Figure 4. A/NK–A/CNK diagram (after [43]) and K₂O–SiO₂ diagram (after [44]) of granitoids from the Shuangwang Au deposit.

Figure 4. A/NK–A/CNK diagram (after [43]) and K₂O–SiO₂ diagram (after [44]) of granitoids from the Shuangwang Au deposit.

Figure 5. (Na₂O + K₂O)–SiO₂ diagram of granitoids from the Shuangwang Au deposit (after [45]).

Figure 5. (Na₂O + K₂O)–SiO₂ diagram of granitoids from the Shuangwang Au deposit (after [45]).

Figure 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) of the Xiba granitoids. Chondrite and primitive mantle normalizing values are from [46].

Figure 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) of the Xiba granitoids. Chondrite and primitive mantle normalizing values are from [46].

The ∑REE of the Xiba granodiorite (Figure 6a) was 99.31–141.07 × 10⁻⁶, with an average of 112.83 × 10⁻⁶; the LREE/HREE was 10.19–14.62, with an average of 12.16; the(La/Yb)_N was 13.16–21.13, with an average of 16.11; the δEu was 0.77–0.87, with an average of 0.84; and the δCe was 0.93–1.01, with an average of 0.96. The total amount of rare earth elements (∑REE) of the monzonitic granite was 114.18–194.76 × 10⁻⁶, with an average of 166.37 × 10⁻⁶; the LREE/HREE was 10.94–17.01, with an average of 14.62; the (La/Yb)_N was 11.47–22.83, with an average of 18.36; the δEu was 0.60–0.66, with an average of 0.63; and the δCe was 0.97–0.99, with an average of 0.98.

The total content of rare earth elements (ΣREE) of the granite porphyry was in the ranges of 122.52–184.13 × 10⁻⁶, with a (La/Yb)_N ratio of 30.83–63.56 and a δEu of 0.54–0.58 (

Table 1. Major element and trace element analyses of granitoids and lamprophyre from the Shuangwang Au deposit.

Lithology	Granodiorite				Monzonitic Granite			Granite Porphyry		Lamprophyre
Sample	SH-116	QGG-4 *	XB-17 *	XB-24 *	SH-117	QGG-3 *	QGG-5 *	T-2	T-5	14-221
Major element (wt%)
SiO2	64.94	64.62	64.04	66.05	67.47	67.54	67.29	74.19	73.11	52.79
TiO2	0.49	0.61	0.59	0.49	0.40	0.52	0.56	0.07	0.09	0.70
Al2O3	16.13	15.87	15.05	14.41	14.97	15.34	15.06	13.96	14.63	15.54
Fe2O3	1.23	1.18	1.84	1.59	1.04	1.08	1.31	0.43	0.21	1.83
FeO	3.15	2.97	2.34	2.09	2.52	1.81	1.95	1.00	1.25	4.93
MnO	0.07	0.07	0.07	0.07	0.06	0.04	0.05	0.05	0.04	0.10
MgO	2.17	2.80	3.04	2.68	1.95	1.77	1.99	0.11	0.17	4.71
CaO	3.81	3.91	4.18	3.72	2.85	2.68	2.90	0.72	0.55	5.84
Na2O	3.90	3.55	4.52	4.23	3.18	3.67	3.63	3.91	4.01	3.59
K2O	3.05	3.59	3.18	3.35	4.51	4.85	4.43	4.86	5.10	1.69
P2O5	0.14	0.15	0.17	0.13	0.11	0.11	0.12	0.03	0.04	0.17
LOI	0.82	0.63	0.75	0.93	0.93	0.53	0.59	0.58	0.69	8.02
TOT	99.90	99.95	99.77	99.74	99.99	99.94	99.88	99.91	99.9	99.91
K2O/Na2O	0.78	1.01	0.70	0.79	1.42	1.32	1.22	1.24	1.27	0.47
K2O + Na2O	6.95	7.14	7.70	7.58	7.69	8.52	8.06	8.77	9.11	5.28
Σ	2.20	2.36	2.82	2.49	2.42	2.96	2.67	2.47	2.76	2.85
A/CNK	0.97	0.94	0.81	0.83	0.98	0.95	0.94	1.07	1.11	0.85
A/NK	1.66	1.63	1.38	1.36	1.48	1.36	1.40	1.19	1.21	2.01
Trace element (ppm)
La	26.50	35.64	21.10	19.91	24.30	49.02	48.41	31.10	44.70	19.90
Ce	47.90	63.08	40.67	41.89	48.80	88.76	90.75	51.10	83.80	37.10
Pr	5.43	6.44	4.84	4.90	5.86	8.64	9.04	6.32	8.79	4.57
Nd	198.90	22.29	19.84	20.16	21.50	28.09	29.60	23.20	30.70	18.30
Sm	3.26	3.63	3.61	3.52	3.46	4.29	4.62	3.94	4.98	3.32
Eu	0.76	0.96	0.95	0.92	0.70	0.82	0.83	0.63	0.87	0.82
Gd	2.65	2.99	3.02	2.79	2.90	3.30	3.59	2.91	3.89	2.85
Tb	0.40	0.46	0.44	0.39	0.46	0.51	0.57	0.44	0.57	0.48
Dy	2.20	2.31	2.31	2.00	2.39	2.72	2.94	1.63	2.89	2.54
Ho	0.43	0.44	0.42	0.37	0.49	0.52	0.57	0.21	0.44	0.52
Er	1.10	1.25	1.22	1.05	1.34	1.49	1.63	0.55	1.17	1.36
Tm	0.17	0.19	0.19	0.17	0.22	0.23	0.26	0.08	0.15	0.21
Yb	1.13	1.21	1.15	1.07	1.52	1.54	1.67	0.35	1.04	1.34
Lu	0.18	0.18	0.18	0.17	0.23	0.25	0.28	0.06	0.14	0.20
Y	11.90	13.99	12.20	10.73	14.30	16.79	18.74	6.99	11.40	13.90
Rb	129.00	138.30	114.20	100.7	239.60	222.60	221	174.00	173.00	55.20
Ba	956.00	1114.70	1054.20	1245.20	985.30	897.30	815	380.00	737.00	803.00
Th	9.74	12.7	8.92	8.55	22.43	21.97	18.60	17.00	20.70	4.50
U	2.79	1.76	2.02	2.72	4.97	4.50	3.52	5.03	3.93	3.86
Ta	0.89	0.75	0.99	0.93	1.70	1.61	1.78	1.86	2.27	0.53
Nb	11.00	10.20	11.59	10.55	13.50	13.90	17.20	25.00	27.30	7.48
Sr	626.00	521.50	610.00	775.00	413.50	391.50	402.00	121.00	199.00	551.00
Zr	161.00	207.30	132.90	113.00	270.70	262.30	194.00	89.60	121.00	122.00
Hf	4.49	7.24	6.15	5.10	8.40	8.49	6.10	3.53	5.45	3.44
Co	13.00	15.61	15.00	15.42	10.50	10.10	9.96	0.11	0.23	23.70
Ni	28.30	38.80	31.42	43.74	28.20	26.70	26.2	0.62	1.08	61.20
ΣREE	111.01	141.07	99.94	99.31	114.18	190.18	194.76	122.52	184.13	93.51
LaN/YbN	16.82	21.13	13.16	13.35	11.47	22.83	20.79	63.56	30.83	10.65
δEu	0.77	0.86	0.86	0.87	0.66	0.64	0.60	0.54	0.58	0.80
δCe	0.93	0.94	0.95	1.01	0.97	0.97	0.99	0.84	0.97	0.92

* The data are from [23].

). The total amount of rare earth elements in the lamprophyre was 93.51–110.46 × 10⁻⁶, with an average of 101.99 × 10⁻⁶; the LREE/HREE value was 8.84–9.00, with an average of 8.92; the (La/Yb)_N value was 10.65–11.24, with an average of 10.95; the δEu value was 0.80–0.81, with an average of 0.80; and the δCe value was 0.90–0.92, with an average of 0.91. The rare earth element characteristics of the granodiorite and monzonitic granite in the Xiba rock mass were roughly the same, but there were also differences. They all showed a right-leaning chondrite distribution curve with a strong differentiation of light and heavy rare earth elements. The ratio of LREE/HREE was large, showing the characteristics of light rare earth enrichment and heavy rare earth loss. The (La/Yb)_N ratio was larger, indicating that the light and heavy rare earth fractionation was strong. The difference was that the total amount of rare earth elements in the granodiorite was relatively low, and there was a weak negative Eu anomaly. The monzonitic granite had an obvious negative Eu anomaly, and its Ce anomaly was not obvious.

4.2. Zircon U–Pb Ages

The zircon Th/U values of six magmatic rock samples from the Shuangwang Au deposit were all greater than 0.3, except for the individual measuring points in the granite porphyry (T-2) of the No.1 breccia body (T-2), which may be affected by some zircon metamorphic cores. The zircon particles were large, the crystal shape was single, and the composition was single, with obvious oscillation zones, indicating that they were all magmatic crystallization zircons. The results of the zircon U–Pb isotopic dating of the magmatic rocks in the Shuangwang Au deposit are shown in

Table 2. Zircon LA–ICP–MS U–Pb isotopic compositions of the magmatic rocks in the Shuangwang Au deposit.

Spot	WB/10−6			Th/U	207Pb/206Pb		207Pb/235U		206Pb/238U		206Pb/238U Age/Ma
	Pb*	Th	U		Result	±1σ	Result	±1σ	Result	±1σ	Result	±1σ
SH-116
SH-116-01	12.4	195	291	0.17	0.0518	0.0012	0.2468	0.0052	0.0345	0.0003	219	2
SH-116-02	7.8	96	188	0.51	0.0508	0.0015	0.2440	0.0067	0.0349	0.0004	221	2
SH-116-03	5.8	75	136	0.55	0.0502	0.0018	0.2455	0.0084	0.0355	0.0004	225	2
SH-116-04	15.3	193	362	0.53	0.0533	0.0011	0.2580	0.0048	0.0351	0.0003	223	2
SH-116-05	16.5	243	380	0.64	0.0521	0.0010	0.2540	0.0046	0.0354	0.0003	224	2
SH-116-06	12.0	163	284	0.57	0.0519	0.0012	0.2517	0.0053	0.0352	0.0003	223	2
SH-116-07	10.3	135	248	0.54	0.0502	0.0012	0.2417	0.0055	0.0350	0.0003	221	2
SH-116-08	11.0	121	263	0.46	0.0527	0.0012	0.2536	0.0056	0.0349	0.0003	221	2
SH-116-09	8.8	116	208	0.56	0.0532	0.0014	0.257	0.0065	0.0350	0.0003	222	2
SH-116-10	12.1	234	248	0.95	0.0529	0.0016	0.2552	0.0074	0.0350	0.0004	222	2
SH-117
SH-117-01	5.8	86	139	0.62	0.0520	0.0020	0.2445	0.0086	0.0341	0.0006	216	4
SH-117-02	5.6	91	135	0.68	0.0491	0.0019	0.2308	0.0084	0.0341	0.0006	216	4
SH-117-03	5.2	83	123	0.67	0.0513	0.0020	0.2412	0.0089	0.0341	0.0006	216	4
SH-117-04	5.3	78	129	0.60	0.0546	0.0021	0.2521	0.0091	0.0335	0.0006	213	4
SH-117-05	4.7	65	117	0.56	0.0523	0.0021	0.2434	0.0091	0.0338	0.0006	214	4
SH-117-06	11.2	84	301	0.28	0.0517	0.0015	0.2407	0.0058	0.0338	0.0005	214	3
SH-117-07	5.0	78	122	0.64	0.0518	0.0021	0.2395	0.0088	0.0335	0.0006	213	4
SH-117-08	4.5	75	111	0.68	0.0487	0.0021	0.2240	0.0092	0.0334	0.0006	212	4
SH-117-09	5.8	90	138	0.65	0.0513	0.0020	0.2398	0.0084	0.0339	0.0006	215	4
SH-117-10	4.6	82	110	0.74	0.0516	0.0022	0.2406	0.0095	0.0338	0.0006	214	4
SH-117-11	7.3	137	164	0.84	0.0504	0.0018	0.2376	0.0076	0.0342	0.0006	217	3
SH-117-12	6.2	106	142	0.74	0.0530	0.0019	0.2511	0.0084	0.0344	0.0006	218	4
14-221
14-221-01	49.4	390	1226	0.32	0.0503	0.0015	0.2401	0.0068	0.0346	0.0004	219	3
14-221-02	10.5	165	242	0.68	0.0551	0.0035	0.2526	0.0156	0.0332	0.0006	211	3
14-221-03	6.3	111	140	0.79	0.0569	0.0052	0.2678	0.0239	0.0341	0.0007	216	4
14-221-04	6.4	110	138	0.80	0.0540	0.0055	0.2550	0.0257	0.0342	0.0008	217	5
14-221-05	9.7	142	227	0.63	0.0505	0.0038	0.2358	0.0173	0.0338	0.0006	214	4
14-221-06	4.0	56	93	0.60	0.0495	0.0072	0.2336	0.0334	0.0342	0.0009	217	6
14-221-07	4.8	58	110	0.52	0.0475	0.0058	0.2248	0.0273	0.0343	0.0008	218	5
14-221-08	5.6	115	122	0.95	0.0595	0.0057	0.2698	0.0254	0.0329	0.0007	208	5
14-221-09	8.4	175	172	1.02	0.0550	0.0043	0.2583	0.0197	0.0340	0.0006	216	4
14-221-10	7.0	148	153	0.97	0.0515	0.0047	0.2347	0.0211	0.0330	0.0006	210	4
14-221-11	7.3	141	167	0.84	0.0503	0.0043	0.2294	0.0192	0.0330	0.0006	209	4
14-221-12	65.9	718	1648	0.44	0.0517	0.0015	0.2379	0.0067	0.0333	0.0004	211	2
14-221-13	4.0	64	84	0.76	0.0464	0.0092	0.2226	0.0436	0.0348	0.0011	221	7
T-2
T-2-01	83.6	1012	2192	0.46	0.0514	0.0011	0.2441	0.0048	0.0345	0.0004	218	2
T-2-02	59.6	670	1627	0.41	0.0502	0.0011	0.2384	0.0052	0.0344	0.0004	218	2
T-2-03	71.5	575	1945	0.30	0.0527	0.0011	0.2509	0.0052	0.0345	0.0004	219	2
T-2-04	59.1	626	1633	0.38	0.0523	0.0012	0.2476	0.0054	0.0343	0.0004	217	2
T-2-05	129.7	907	3615	0.25	0.0510	0.0010	0.2406	0.0045	0.0342	0.0004	217	2
T-2-06	55.7	544	1550	0.35	0.0523	0.0012	0.2466	0.0055	0.0342	0.0004	217	2
T-2-07	321.4	1828	8747	0.21	0.0513	0.0009	0.2494	0.0044	0.0353	0.0004	223	2
T-2-08	93.5	836	2483	0.34	0.0515	0.0011	0.2499	0.0051	0.0352	0.0004	223	2
T-2-09	129.7	941	3519	0.27	0.0513	0.0010	0.2482	0.0048	0.0351	0.0004	222	2
T-2-10	257.2	2666	6618	0.40	0.0517	0.0010	0.2512	0.0047	0.0352	0.0004	223	2
T-2-11	660.7	7414	17294	0.43	0.0508	0.0010	0.2423	0.0044	0.0346	0.0004	219	2
T-2-12	161.2	1081	4357	0.25	0.0496	0.0010	0.2417	0.0047	0.0353	0.0004	224	2
T-2-13	110.0	1211	2921	0.41	0.0528	0.0011	0.2529	0.0051	0.0347	0.0004	220	2
T-2-14	138.3	1232	3645	0.34	0.0522	0.0013	0.2479	0.0058	0.0344	0.0004	218	2
T-2-15	62.3	811	1546	0.52	0.0518	0.0014	0.2489	0.0063	0.0349	0.0004	221	3
T-5
T-5-01	16.4	240	420	0.57	0.0489	0.0018	0.2260	0.0083	0.0335	0.0004	213	3
T-5-02	30.5	401	806	0.50	0.0501	0.0013	0.2302	0.0055	0.0333	0.0004	211	2
T-5-03	35.4	344	976	0.35	0.0500	0.0012	0.2309	0.0051	0.0335	0.0004	212	2
T-5-04	87.1	645	2444	0.26	0.0513	0.0010	0.2363	0.0043	0.0334	0.0004	212	2
T-5-05	12.4	156	322	0.48	0.0511	0.0021	0.2381	0.0096	0.0338	0.0004	214	3
T-5-06	20.0	268	517	0.52	0.0510	0.0015	0.2380	0.0066	0.0339	0.0004	215	2
T-5-07	13.6	234	338	0.69	0.0522	0.0019	0.2405	0.0086	0.0334	0.0004	212	3
T-5-08	119.9	874	3228	0.27	0.0504	0.0016	0.2375	0.0071	0.0342	0.0004	217	2
T-5-09	56.8	487	1574	0.31	0.0521	0.0011	0.2394	0.0048	0.0334	0.0004	211	2
T-5-10	32.0	472	796	0.59	0.0505	0.0013	0.2372	0.0058	0.0341	0.0004	216	2
T-5-11	37.9	426	1000	0.43	0.0494	0.0012	0.2301	0.0053	0.0338	0.0004	214	2
T-5-12	13.9	313	319	0.98	0.0516	0.0020	0.2377	0.0089	0.0334	0.0004	212	3
T-5-13	18.5	313	458	0.68	0.0500	0.0016	0.2308	0.0070	0.0335	0.0004	213	3
T-5-14	29.1	369	752	0.49	0.0502	0.0013	0.2345	0.0059	0.0339	0.0004	215	2
T-5-15	82.3	909	2212	0.41	0.0510	0.0011	0.2343	0.0048	0.0333	0.0004	211	2
T-5-16	16.3	234	414	0.57	0.0512	0.0017	0.2362	0.0076	0.0335	0.0004	212	3
T-5-17	12.9	286	295	0.97	0.0504	0.0021	0.2322	0.0096	0.0334	0.0005	212	3
T-5-18	16.5	368	378	0.97	0.0493	0.0017	0.2275	0.0078	0.0335	0.0004	213	3
T-5-19	37.0	397	994	0.40	0.0511	0.0013	0.2359	0.0057	0.0335	0.0004	212	2
T-5-20	17.0	167	444	0.38	0.0499	0.0016	0.2383	0.0076	0.0347	0.0004	220	3
T-5-21	25.4	395	631	0.63	0.0514	0.0015	0.2381	0.0067	0.0337	0.0004	213	3
T-4
T-4-01	66.0	538	1831	0.29	0.0491	0.0012	0.2307	0.0053	0.0341	0.0004	216	2
T-4-02	69.5	544	1928	0.29	0.0492	0.0012	0.2323	0.0053	0.0342	0.0004	217	2
T-4-03	89.2	1135	2481	0.46	0.0516	0.0012	0.2399	0.0052	0.0337	0.0004	214	2
T-4-04	16.6	239	442	0.54	0.0529	0.0020	0.2422	0.0087	0.0332	0.0005	211	3
T-4-05	15.7	279	400	0.69	0.0520	0.0021	0.2356	0.0094	0.0328	0.0005	208	3
T-4-06	17.7	299	450	0.66	0.0510	0.0019	0.2328	0.0085	0.0331	0.0005	210	3
T-4-07	15.8	285	394	0.72	0.0513	0.0020	0.2340	0.0090	0.0330	0.0005	209	3
T-4-08	60.3	697	1664	0.42	0.0505	0.0012	0.2302	0.0055	0.0331	0.0004	210	2
T-4-09	6.4	110	158	0.69	0.0497	0.0034	0.2332	0.0156	0.0340	0.0006	216	4
T-4-10	18.5	312	464	0.67	0.0511	0.0019	0.2381	0.0087	0.0338	0.0005	214	3
T-4-11	18.9	309	470	0.65	0.0502	0.0019	0.2371	0.0085	0.0342	0.0005	217	3
T-4-12	12.4	260	301	0.86	0.0513	0.0023	0.2342	0.0103	0.0331	0.0005	210	3
T-4-13	14.2	219	371	0.58	0.0522	0.0021	0.2411	0.0096	0.0335	0.0005	212	3
T-4-14	28.2	736	623	1.18	0.0527	0.0017	0.2459	0.0078	0.0338	0.0005	214	3
T-4-15	19.5	338	505	0.66	0.0510	0.0018	0.2314	0.0081	0.0329	0.0005	209	3
T-4-16	10.0	216	229	0.94	0.0494	0.0027	0.2300	0.0121	0.0338	0.0005	214	3
T-4-17	45.1	387	1252	0.30	0.0505	0.0014	0.2362	0.0062	0.0339	0.0004	215	3
T-4-18	9.0	197	211	0.93	0.0509	0.0029	0.2333	0.0128	0.0332	0.0005	211	3
T-4-19	25.7	403	672	0.59	0.0527	0.0016	0.2382	0.0071	0.0328	0.0004	208	3
T-4-20	16.1	254	403	0.63	0.0520	0.0020	0.2418	0.0089	0.0337	0.0005	214	3

Pb* refers to the amount of Pb in the measurement point, but does not include the amount of ²⁰⁴Pb.

.

The zircon of SH-116 (granodiorite) was colorless and a transparent euhedral crystal, with particle sizes of 0.10–0.30 mm and a length–width ratio of 2:1–6:1. Cathodoluminescence (CL) images show that the zircons had a complete crystal shape, and most of the zircons showed a core–mantle structure with obvious oscillatory zoning (Figure 7a). They were magmatic zircons. The U–Pb isotope analysis data and apparent ages of the representative zircon particles are listed in the table. The U and Th contents of the zircons in this sample varied greatly, and the U content was 136–380 × 10⁻⁶ (with an average of 261 × 10⁻⁶). The Th content was 75–243 × 10⁻⁶ (with an average of 157 × 10⁻⁶). The concordant age was 222.5 ± 2.1 Ma (MSWD = 0.74), and the weighted average age of ²⁰⁶Pb/²³⁸U was 221.1 ± 1.2 Ma (MSWD = 0.75), which represents the crystallization age of the sample (Figure 8a).

The zircon of SH-117 (monzonitic granite) was a euhedral crystal, with particle sizes of 0.15–0.3 mm and a length–width ratio of 2:1–3:1. The cathodoluminescence (CL) image shows that the zircons had obvious oscillation zones, which were magmatic zircons (Figure 7c). The contents of zircon U and Th in this sample varied in a small range: the content of U was concentrated in the range of 110–164 × 10⁻⁶ (with an average of 144 × 10⁻⁶), except at measuring point 06, where it was 301 × 10⁻⁶. The Th content was 65–137 × 10⁻⁶ (with an average of 88 × 10⁻⁶). The ratio of Th/U was concentrated in the range of 0.56–0.84, except at point 06, where it was 0.28, and the change was stable. The concordant age was 214.9 ± 2.4 Ma (MSWD = 0.32), and the weighted average age of ²⁰⁶Pb/²³⁸U was 214.8 ± 1.2 Ma (MSWD = 0.90), which represents the crystallization age of the sample (Figure 8b).

The zircons of 14-221 (lamprophyre) were mostly euhedral crystals with particle sizes of 0.10–0.20 mm and a length–width ratio of 2:1–1:1 (Figure 7b). The contents of U and Th in the zircons of this sample varied greatly. The U contents of measuring points 01 and 12 were 1226 × 10⁻⁶ and 1648 × 10⁻⁶, respectively, and the rest were 84~242 × 10⁻⁶ (with an average of 150 × 10⁻⁶). The content of Th was 75–243 × 10⁻⁶ (with an average of 184 × 10⁻⁶). The Th/U ratios of all measuring points were between 0.32 and 1.02. The concordant age was 217.4 ± 5.5 Ma (MSWD = 0.46), and the weighted average age of ²⁰⁶Pb/²³⁸U was 214.4 ± 2.7 Ma (MSWD = 0.9), which represents the crystallization age of the sample (Figure 8e).

The T-2 (granite porphyry) zircon had a good self-shaped degree, mostly long columnar, with particle sizes of 0.10–0.25 mm and a length–width ratio of 2:1–5:1 (Figure 7e). The content of U and Th in the zircon was very high, and the content of U was 1546–6618 × 10⁻⁶ (with an average of 4246 × 10⁻⁶). The content of Th was 575–2666 × 10⁻⁶ (with an average of 1490 × 10⁻⁶). The Th/U ratio of all the measuring points ranged from 0.21 to 0.52. The concordant age was 222.9 ± 2.8 Ma (MSWD = 0.46), and the weighted average age of ²⁰⁶Pb/²³⁸U was 219.9 ± 1.5 Ma (MSWD = 4.6), which represents the crystallization age of the sample (Figure 8c).

The T-5 (granite porphyry) zircons were mostly long columnar, with particle sizes of 0.05–0.30 mm and a length–width ratio of 1:1–6:1 (Figure 7f). The zircon U content was 295–3228 × 10⁻⁶ (with an average of 920 × 10⁻⁶); the Th content was 156–909 × 10⁻⁶ (with an average of 395 × 10⁻⁶). The Th/U ratio of all measuring points ranged from 0.26 to 0.97. The concordant age was 213.8 ± 1.8 Ma (MSWD = 0.37), and the weighted average age of ²⁰⁶Pb/²³⁸U was 213.1 ± 0.89 Ma (MSWD = 0.89), which represents the crystallization age of the sample (Figure 8d).

The zircons of the quartz albite (T-4) were mostly colorless euhedral crystals with grain sizes of 0.10–0.30 mm and a length–width ratio of 1:1–2:1, and they had the characteristics of magmatic zircons (Figure 7d). The measured zircon U content was 229–2481 × 10⁻⁶ (with an average of 743 × 10⁻⁶), the Th content was 158–1135 × 10⁻⁶ (with an average of 380 × 10⁻⁶), and the Th/U ratio was 0.28–1.18. The concordant age of all 20 measuring points was 214.5 ± 1.2 Ma (MSWD = 0.35), and the weighted average age of ²⁰⁶Pb/²³⁸U was 213.0 ± 1.5 Ma (MSWD = 1.30), which can represent the intrusion age of the quartz albite in the mining area (Figure 8f).

4.3. In Situ Zircon Hf Isotopic Composition

The results of the in situ Lu–Hf isotope analysis of the zircon samples are shown in

Table 3. Zircon Hf isotopic compositions in the Shuangwang Au deposit.

Spot	Age (Ma)	176Yb/177Hf	±1σ	176Lu/177Hf	±1σ	176Hf/177Hf	±1σ	εHf (0)	εHf (t)	±1σ	tDM1 (Ma)	tDM2 (Ma)	fLu/Hf
SH-116-01	222	0.020210	0.001019	0.000766	0.000038	0.282481	0.000012	−10.29	−5.59	0.688145	1083	1432	−0.977
SH-116-02	222	0.015475	0.000224	0.000595	0.000010	0.282418	0.000019	−12.54	−7.77	0.842290	1167	1554	−0.982
SH-116-03	222	0.011182	0.000079	0.000442	0.000004	0.282375	0.000015	−14.03	−9.16	0.751234	1220	1634	−0.987
SH-116-04	222	0.013660	0.000136	0.000531	0.000007	0.282458	0.000015	−11.10	−6.28	0.748375	1108	1473	−0.984
SH-116-05	222	0.013371	0.000293	0.000521	0.000010	0.282461	0.000013	−11.01	−6.17	0.701969	1105	1468	−0.984
SH-116-06	222	0.012165	0.000128	0.000475	0.000005	0.282445	0.000014	−11.56	−6.74	0.724417	1125	1499	−0.986
SH-116-07	222	0.018814	0.000896	0.000702	0.000031	0.282470	0.000016	−10.69	−5.94	0.772209	1097	1453	−0.979
SH-116-08	222	0.019336	0.000455	0.000739	0.000014	0.282445	0.000015	−11.58	−6.84	0.738637	1133	1503	−0.978
SH-116-09	222	0.014830	0.000107	0.000579	0.000003	0.282439	0.000014	−11.78	−6.99	0.734266	1137	1512	−0.983
SH-116-10	222	0.013987	0.000173	0.000517	0.000008	0.282471	0.000014	−10.63	−5.84	0.719404	1090	1448	−0.984
SH-117-01	214	0.011240	0.000251	0.000432	0.000007	0.282447	0.000015	−11.48	−6.80	0.737860	1121	1497	−0.987
SH-117-02	214	0.025048	0.001363	0.000900	0.000047	0.282473	0.000017	−10.59	−5.97	0.797670	1099	1451	−0.973
SH-117-03	214	0.008714	0.000107	0.000330	0.000003	0.282441	0.000014	−11.71	−7.01	0.728640	1126	1508	−0.990
SH-117-04	214	0.007409	0.000145	0.000287	0.000005	0.282442	0.000015	−11.66	−7.03	0.736388	1123	1507	−0.991
SH-117-05	214	0.013595	0.000326	0.000510	0.000012	0.282470	0.000015	−10.67	−6.05	0.744949	1091	1454	−0.985
SH-117-06	214	0.015483	0.000186	0.000582	0.000007	0.282457	0.000013	−11.16	−6.54	0.710832	1112	1481	−0.982
SH-117-07	213	0.013257	0.000171	0.000498	0.000006	0.282422	0.000014	−12.39	−7.79	0.727372	1158	1549	−0.985
SH-117-08	214	0.007813	0.000124	0.000298	0.000004	0.282462	0.000014	−10.96	−6.35	0.726451	1096	1469	−0.991
SH-117-09	214	0.008880	0.000547	0.000352	0.000018	0.282432	0.000012	−12.03	−7.36	0.686482	1139	1527	−0.989
SH-117-10	214	0.014274	0.000230	0.000532	0.000007	0.282446	0.000015	−11.53	−6.90	0.745378	1125	1501	−0.984
SH-117-11	214	0.014351	0.000229	0.000535	0.000009	0.282452	0.000015	−11.32	−6.63	0.738132	1117	1488	−0.984
SH-117-12	214	0.011409	0.000040	0.000434	0.000001	0.282475	0.000014	−10.52	−5.79	0.722327	1083	1443	−0.987
T-2-01	220	0.030422	0.000165	0.000925	0.000003	0.282508	0.000015	−9.35	−4.69	0.743928	1051	1382	−0.972
T-2-02	220	0.040693	0.000095	0.001186	0.000006	0.282498	0.000014	−9.69	−5.08	0.717633	1072	1403	−0.964
T-2-03	220	0.018169	0.000477	0.000570	0.000014	0.282486	0.000013	−10.12	−5.39	0.699498	1071	1421	−0.983
T-2-04	220	0.026919	0.000192	0.000989	0.000008	0.282498	0.000015	−9.70	−5.08	0.736920	1067	1402	−0.970
T-2-05	220	0.035117	0.001235	0.001087	0.000037	0.282447	0.000013	−11.48	−6.87	0.713007	1140	1501	−0.967
T-2-06	220	0.034276	0.000790	0.001208	0.000026	0.282492	0.000013	−9.91	−5.32	0.692911	1081	1416	−0.964
T-2-07	220	0.019280	0.000166	0.000608	0.000004	0.282480	0.000014	−10.32	−5.51	0.712387	1080	1431	−0.982
T-2-08	220	0.028940	0.000272	0.000916	0.000003	0.282458	0.000013	−11.10	−6.34	0.689573	1120	1477	−0.972
T-2-09	220	0.017574	0.000527	0.000575	0.000016	0.282473	0.000013	−10.58	−5.79	0.707270	1089	1446	−0.983
T-2-10	220	0.028508	0.000551	0.000897	0.000015	0.282466	0.000013	−10.84	−6.08	0.696975	1109	1462	−0.973
T-2-11	220	0.027844	0.000276	0.000861	0.000008	0.282466	0.000013	−10.81	−6.12	0.702715	1106	1462	−0.974
T-2-12	220	0.041257	0.000915	0.001507	0.000032	0.282460	0.000018	−11.02	−6.33	0.821186	1134	1477	−0.955
T-5-01	214	0.008939	0.000257	0.000385	0.000006	0.282439	0.000014	−11.78	−7.16	0.721786	1131	1514	−0.988
T-5-02	214	0.038920	0.000626	0.001447	0.000022	0.282514	0.000015	−9.11	−4.68	0.748377	1056	1375	−0.956
T-5-03	214	0.021567	0.000348	0.000712	0.000011	0.282479	0.000017	−10.36	−5.80	0.794200	1084	1438	−0.979
T-5-04	214	0.041896	0.001049	0.001634	0.000045	0.282507	0.000021	−9.38	−4.95	0.907341	1072	1391	−0.951
T-5-05	214	0.043085	0.000423	0.001628	0.000017	0.282511	0.000015	−9.23	−4.76	0.759529	1066	1382	−0.951
T-5-06	214	0.013501	0.000164	0.000546	0.000006	0.282435	0.000017	−11.92	−7.28	0.802656	1141	1522	−0.984
T-5-07	214	0.054383	0.000491	0.002094	0.000019	0.282524	0.000019	−8.76	−4.40	0.847535	1060	1361	−0.937
T-5-08	214	0.048077	0.000535	0.001881	0.000022	0.282502	0.000019	−9.54	−5.04	0.859499	1086	1400	−0.943
T-5-09	214	0.034449	0.000629	0.001237	0.000021	0.282496	0.000013	−9.75	−5.29	0.703224	1076	1409	−0.963
T-5-10	214	0.019780	0.000453	0.000613	0.000013	0.282511	0.000016	−9.21	−4.56	0.774128	1037	1373	−0.982

. The ratio of ¹⁷⁶Lu/¹⁷⁷Hf of only one measuring point (T-5-07) of the five samples for the Lu–Hf isotope analysis was 0.002094, and the ¹⁷⁶Lu/¹⁷⁷Hf values of the remaining measuring points were less than 0.002, indicating that there was a lack of radioactive Hf enrichment after zircon formation. The f_Lu/Hf values of the zircons in the samples were −0.937–−0.991, which were significantly lower than the f_Lu/Hf values of −0.72 in the aluminosilicate crust and −0.34 in the basic crust.

The analytical results of 10 zircons in the granodiorite sample SH-116 of the Xiba rock mass showed that the ¹⁷⁶Lu/¹⁷⁷Hf isotope ratio was generally low, less than 0.002, and the ¹⁷⁶Hf/¹⁷⁷Hf isotope ratio was relatively stable. The distribution range was 0.282375–0.282481, and the ε_Hf (t) values were all negative, distributed in the range of −9.16–−5.59. The corresponding single-stage Hf model age was 1083–1220 Ma (with an average of 1127 Ma). The two-stage Hf model age was 1432–1634 Ma (with an average of 1498 Ma).

The analytical results of 12 zircons in the monzogranite sample SH-117 of the Xiba rock mass were similar to those of SH-116. The ¹⁷⁶Lu/¹⁷⁷Hf isotope ratio was generally low, and the ¹⁷⁶Hf/¹⁷⁷Hf isotope ratio was distributed in the range of 0.282422–0.282475; the ε_Hf (t) value was also negative, distributed in the range of −7.79–−5.79. The corresponding single-stage Hf model age was 1083–1158 Ma (with an average of 1116 Ma). The two-stage Hf model age was 1443–1549 Ma (with an average of 1489 Ma).

The analytical results of 12 zircons in the granite porphyry sample T-2 showed that the ¹⁷⁶Hf/¹⁷⁷Hf isotope ratio was distributed in the range of 0.282458–0.282508, and the ε_Hf(t) value was also negative, distributed in the range of −6.87–−4.69. The corresponding single-stage Hf model age was 1051–1140 Ma (with an average of 1093 Ma). The two-stage Hf model age was 1382–1501 Ma (with an average of 1440 Ma).

The ¹⁷⁶Hf/¹⁷⁷Hf isotope ratios in T-5 ranged from 0.282435 to 0.282524, and the ε_Hf(t) values were also negative, ranging from −7.16 to −4.40. The corresponding single-stage Hf model age was 1037–1141 Ma (with an average of 1081 Ma). The two-stage Hf model age was 1373–1522 Ma (with an average of 1417 Ma).

In the five in situ zircon Lu–Hf isotope samples, the ¹⁷⁶Lu/¹⁷⁷Hf ratios were all less than 0.002, indicating that the zircons had low radioactive Hf element accumulation after formation. In addition, the zircons had stable physical and chemical conditions and a high closure temperature [35], which are rarely affected by magmatism and tectonic thermal events. Therefore, the 176Lu/177Hf ratio basically represents the initial Hf isotope composition [47].

5. Discussion

5.1. Type of Granitoids

The most common scheme for the classification of granites denotes them as I-type, S-type, A-type, and M-type based on the nature of the source area [48]. However, when these four types of granites undergo a high degree of fractional crystallization, their mineral composition and chemical composition tend to be similar to those of low eutectic granite, making it difficult, or even impossible, to distinguish the above three granites [48]. Chappell and White [49] proposed that P₂O₅, Th, Ba, Rb, and other elements can be used as reliable markers to distinguish the above four types of granites.

Based on the study of I-type and S-type granites in the Lachlan fold belt in Australia and the early Yanshanian granites in South China, Chappell [49] and Li et al. [50] concluded that there is a significant negative correlation between the P₂O₅–SiO₂ of I-type/differentiated I-type granites. This is because apatite, as the main accessory mineral of granite, has a very low solubility in quasi-aluminous/weakly peraluminous (A/CNK < 1.1) magma, and its solubility decreases with the decrease in temperature and the increase in SiO₂ during magmatic differentiation evolution. However, under the condition of strong peraluminous (A/CNK < 1.3) magma, the solubility of apatite increases linearly with an increase in A/CNK [50]. There is a significant negative correlation between SiO₂ and P₂O₅ in I-type granites, while the P₂O₅ in S-type granites tends to increase or remain mostlyunchanged with an increase in SiO₂ [50]. However, Y-rich minerals do not crystallize in the early stage of quasi-aluminous I-type magma evolution, resulting in a high Y content in differentiated I-type granites and a positive correlation with the Rb content [50].

The granodiorite (SH-116) and monzogranite (SH-117) of the Xiba rock mass have a high SiO₂ content, both of which belong to intermediate–acid rocks and cannot directly originate from the mantle peridotite source area. Experimental petrology shows that the partial melting of basic rocks (basaltic components) in the crust forms quasi-aluminous granites with a basic chemical composition [51], while the partial melting of clastic sedimentary rocks in the crust forms acidic peraluminous granites. Only the partial melting of sandy sedimentary rocks can producestrongly peraluminous granites [52]. The geochemical and petrological characteristics of the Xiba rock mass indicate that it is a quasi-aluminous intermediate–acid rock, and so its source area cannot be entirely composed of sedimentary rock; otherwise, the rock formed will inevitably show strong peraluminous granites. Judging from the genetic type of granitic rocks, A/CNK = 1.11 is the most widely used boundary between I-type and S-type granites (A/CNK > 1.11 is S-type granite, and A/CNK < 1.11 is I-type granite) [49].

The trend of Th and Y with the evolution of magmatic crystallization differentiation is an effective criterion for distinguishing I-type and S-type granites. Because Th and Y preferentially enter Th- and Y-rich minerals in the early stage of peraluminous magmatic evolution, the Th and Y of S-type granites decrease with an increase in Rb, while those of I-type granites show the opposite trend [49,50]. In the samples of the Xiba rock mass, there is a positive correlation between Rb and Th and Y (Figure 9a,b), thus demonstrating obvious non-S-type granite characteristics, excluding it from being S-type granite. In the Zr–SiO₂ discriminant diagram (Figure 9c), the granodiorite samples fall into the I-type granite field, while the monzogranite samples are located at the junction of I-type and A-type granites. In the Na₂O–K₂O diagram (Figure 9d) of the granite genesis series [53], the granodiorite samples fall into the I-type granite field, while the three samples of the monzonitic granite fall into the A-type granitic rock field.

Traditionally, A-type granites are anhydrous, alkaline, and non-orogenic [54], but the current definition has undergone major changes. For example, A-type granites are not always water-deficient, many A-type granites are peraluminous, and most A-type granites were formed in post-orogenic rather than non-orogenic environments [48]. Most of the current studies agree that A-type granites come from a source area of igneous rocks [55,56]. Therefore, some researchers believe that A-type granite is simply a type of I-type granite, making the A-type classification redundant [57]. However, because of its special mineral composition and its significance in the reactive tectonic setting, most researchers retain this type. The main geochemical characteristics of A-type granites are high contents of SiO₂, K₂O + Na₂O, Zr, Nb, REE, Y, and Ga; low contents of CaO, Eu, Sr, and Ba; high FeO/MgO, Ga/Al, and (K₂O + Na₂O)/CaO; and generally 1000 × Ga/Al > 2.6, Zr > 250 × 10⁻⁶, and Zr + Nb + Ce + Y > 350 × 10⁻⁶ [58]. As mentioned above, the Xiba monzogranite has a higher SiO₂ content and total alkali content than the Xiba granodiorite. K₂O + Na₂O is between 7.69% and 8.52%, with a higher ∑REE content (114.18–194.76) × 10⁻⁶, a lower Sr content, more obvious Ba and Eu negative anomalies, and a higher Zr content (194.00–270.70) × 10⁻⁶. Compared with the typical characteristics of I-type granitic rocks in the granodiorite of the Xiba rock mass, the monzonitic granite of the Xiba rock mass also has the characteristics of A-type granitic rocks. However, compared with standard A-type granitic rocks, the content of Eu is higher, and the differentiation of light and heavy rare earth elements is larger. The chondrite normalized curve of rare earth elements in the general standard A-type granitic rocks shows a flat type with a small differentiation of light and heavy rare earth elements, while the rare earth element distribution curve of the Xiba monzonitic granite shows a right-leaning type with an obvious differentiation of light and heavy rare earth elements. Therefore, it may be a transitional rock type between I-type and A-type granites, having the characteristics of both I-type and A-type granitic rocks. Thus, it represents a special rock produced in a post-collisional tectonic setting [23].

5.2. Source Characteristics

Due to the high Hf content and low ¹⁷⁶Lu/¹⁷⁷Hf ratio of zircons, the content of ¹⁷⁶Hf formed by ¹⁷⁶Lu decay is very low. In addition, zircons have a high Hf isotope closure temperature, and so the measured zircon Hf isotope composition can represent the Hf isotope composition of zircon crystallization [48]. Combined with the U–Pb geochronology of zircons, the Lu–Hf isotope system has become an effective means fortracing magma sources and crustal evolution [48].

In the samples measured in this study, the ¹⁷⁶Lu/¹⁷⁷Hf ratio of zircon was less than 0.002 (Table 3), indicating that the zircon had a low radioactive Hf accumulation after formation. In addition, zircon has stable physical and chemical conditions and a high sealing temperature, which are rarely affected by magmatism and tectonic thermal events. Therefore, the ¹⁷⁶Lu/¹⁷⁷Hf ratio basically represents the initial Hf isotopic composition [59]. Zircon ε_Hf(t) values represent the composition characteristics of the magma source area. In the zircon U–Pb age–ε_Hf(t) diagram (Figure 10a), the ε_Hf(t) values of the granodiorite and monzonitic granite in the Xiba pluton were close, and the Hf isotope characteristics were similar, showing obvious homology. The granite porphyry of the No. I breccia body had obvious homology with the Jiupinggou granite porphyry. If the ε_Hf(t) of the granitic rocks is concentrated and mostly distributed on the chondrite, that is, if ε_Hf(t) > 0, then it is generally believed that the granitic rocks are derived from the partial melting of a depleted mantle or the newly generated young crustal material from a depleted mantle [48,59,60,61]. If ε_Hf(t) < 0, then it indicates that the rock has ancient crust or mantle-derived magma mixed with ancient crustal material [48,62]. Based on the change in the zircon ε_Hf(t) value of the Xiba rock mass, the ε_Hf(t) value of the magmatic zircon was negative, distributed in the range of −9.16–−5.59, indicating that the rock was formed by the partial melting of crustal material. The two-stage Hf model ages calculated using the crust as the standard were concentrated in the range of 1400–1500 Ma. Therefore, it is speculated that the original magma that formed the granitic rock and granite porphyry of the Xiba rock mass experienced a long period of crustal retention after differentiation from the depleted mantle. In the ε_Hf(t)–age diagram (Figure 10b), the two-stage Hf model age calculated with the crust as the standard has a time gap of 1100–1300 Ma from the formation age of the rock mass, indicating that the parent magma of the granite was the product of the melting of the crustal material retained in the crust for a long time after the differentiation of the depleted mantle in the Mesoproterozoic period.

Figure 9. (a) Y–Rb diagram, (b) Th–Rb diagram, (c) Zr–SiO₂ diagram, and (d) Na₂O–K₂O diagram of the Xiba granite intrusion from the Shuangwang Au deposit (after [49]). The data for the Xiba granites of West Qinling are from [63]. Granodiorite includes sample SH-116, QGG-4, XB-17 and XB-24; Monzonitic granite includes sample SH-117, QGG-3 and QGG-5.

Figure 9. (a) Y–Rb diagram, (b) Th–Rb diagram, (c) Zr–SiO₂ diagram, and (d) Na₂O–K₂O diagram of the Xiba granite intrusion from the Shuangwang Au deposit (after [49]). The data for the Xiba granites of West Qinling are from [63]. Granodiorite includes sample SH-116, QGG-4, XB-17 and XB-24; Monzonitic granite includes sample SH-117, QGG-3 and QGG-5.

Figure 10. (a) ε_Hf(t) vs. U–Pb age diagram. (b) ε_Hf(t) vs. U–Pb age diagram (after [49]).

Figure 10. (a) ε_Hf(t) vs. U–Pb age diagram. (b) ε_Hf(t) vs. U–Pb age diagram (after [49]).

The Xiba rock mass and the granite porphyry in the mining area were formed at 220–210 Ma [9,64]. In addition, the Hf isotope composition of the samples measured in this paper was similar to that of the contemporary Wulong rock mass in the South Qinling Mountains, suggesting that there is a similarity in the magma source area, and the source material was mainly the Mesoproterozoic crustal basement material [48,62].

5.3. Formation Age of Ore Body

Accurately determining the metallogenic age of Au deposits is very significant for studying their genesis and establishing their metallogenic models, as well as for studying and establishing those of other similar deposits.

However, for the Shuangwang Au deposit, the metallogenic age has not been accurately determined due to the lack of an effective isotopic dating of characteristic minerals. The ⁴⁰Ar/³⁹Ar dating of pyrite in its main metallogenic stage by Shi et al. [28] yielded a metallogenic age of 183.09 ± 20.64 Ma, which can only be used as a reference.

Previous studies on the formation ages of Au deposits and Pb-Zn deposits adjacent to the Shuangwang Au deposit in the same stratum or with a similar tectonic environment can be used as references and for comparisons [65,66,67,68,69,70,71,72]. The quartz vein ⁴⁰Ar/³⁹Ar plateau age of the Baguamiao Au deposit in Fengxian County, Shaanxi Province, is 232.58 ± 1.59 Ma, and its isochron age is 222.14 ± 3.45 Ma [27]. The Ma’anqiao Au ore body occurs in the brittle–ductile shear zone on the southern margin of the Shangdan fault zone. The Xianggou rock mass on the south side of the mining area is the main magmatic rock mass in the mining area. The rock mass is only tens of meters to hundreds of meters away from Au mineralization. Zhu et al. [13] carried out a laser probe LA–ICP–MS single-grain zircon U–Pb precise dating of the Xianggou monzonitic granite porphyry, and the obtained age was 242.0 ± 0.8 Ma. The Re–Os age of pyrite is 226 ± 17 Ma, and the Rb–Sr isochron age of sphalerite is 220.7 ± 7.3 Ma [17].

When there is a lack of suitable characteristic minerals or specific isotope dating methods, an effective indirect method that can be used to determine the metallogenic age of Au deposits is structural interpenetration or the relationship between rocks and mineralization [2]. This indirect method to determine the metallogenic age uses the spatial cutting relationship between magmatic rocks, dikes, and ore bodies to limit the time of mineralization. The lamprophyre dikes in the main ore belt of the Shuangwang Au deposit cut through the breccia body and do not present obvious alterations, and their emplacement time is obviously after the formation of breccia. The zircon U–Pb age of the lamprophyre is 214.4 ± 2.7 Ma, which can be used to limit the upper limit of the formation time of the breccia and ore body in the Shuangwang Au deposit. This indicates that the mineralization age of the Shuangwang Au deposit is not later than the magmatic activity in the late Indosinian period.

5.4. Tectonic Implications

The geochemical data of rocks are often used to determine the tectonic background of the rock formation. Maniar and Piccoli [43] proposed to divide the tectonic environment of granite formation into orogenic granite and non-orogenic granite by using their element characteristics and mineralogical characteristics. Orogenic granites are divided into island arc granites (IAGs), continental arc granites (CAGs), continental collision granites (CCGs), and post-orogenic granites (POGs) [73]. Non-orogenic granites can be divided into rift granites (RRGs), continental orogenic uplift granites (CEUGs), and oceanic plagioclase granites (OPs). The granodiorite and monzogranite samples from the Xiba pluton were all cast in the IAG + CAG + CCG field (Figure 11a). In addition, the inactive elements in the alteration process were used to distinguish the tectonic environment. In the Nb–Y tectonic diagram (Figure 11b), all sample points are located in the volcanic arc granite and the syn-collision granite fields. In the Rb–Y + Nb tectonic diagram (Figure 11c), the granodiorite is located in the volcanic arc granite field, and the monzonitic granite falls into the critical fields of syn-collision granite and the volcanic arc granite, indicating that the Xiba rock mass formed in a post-collision tectonic environment after the main collision orogenic metamorphic peak of the Yangtze plate and the North China plate. The current research data also show that the collision of the Qinling orogenic belt along the southern Qinling Mianlue belt–Dabie Mountain occurred mainly in the early Mesozoic, forming the Qinling orogenic belt, and this finally completed the comprehensive collision between the Yangtze plate and the North China plate [8,10,12,21].

The amphibolite lamprophyres in the Shuangwang Au deposit show the enrichment of large-ion lithophile elements and the depletion of high-field-strength elements, especially the depletion of Ta–Nb, which makes them show the composition characteristics of mantle-derived rocks in a subduction zone [9,26]; the high Zr/Hf ratio may reflect the metasomatism and enrichment of carbonate fluids [71,72,74]. In summary, the lamprophyres in the mining area may be the product of the partial melting of an enriched mantle formed by the contamination and metasomatism of a subducted continental crust in the mantle source area.

The ultrahigh-pressure metamorphic age of the Dabie Mountains is 232–221 Ma, its peak age is 226 ± 2 Ma, and the time of the South Qinling metamorphic deformation and the closure of the Mianlue ocean basin is 242–221 Ma [2,4,10,26,64]. The crystallization age of the granodiorite in the Xiba rock mass is 222 Ma, and the crystallization age of the monzonitic granite is 214 Ma, which are slightly later than the main collision period of the Qinling orogenic belt. Therefore, it is speculated that the Xiba rock mass is a large complex rock mass, and two different stages of the same period were invaded to form granodiorite and monzonitic granite. The stage I granodiorite in the late Indosinian period formed in the syn-collision tectonic volcanic arc environment of the Yangtze plate and the North China plate after the closure of the oceanic basin of the Mianlue tectonic belt in the South Qinling Mountains. The late Indosinian stage II monzogranite formed in a syn-collisional extensional environment. After the collision between the Yangtze plate and the North China plate in the Qinling area, the Qinling orogenic belt changed from compression to extension. Due to the partial melting of the subducted oceanic crust and the gravity of the eclogite-facies oceanic crust, slab break-off finally occurred, which induced the upwelling of the asthenosphere mantle material. At the same time, the subducted oceanic crust induced tectonic exhumation due to buoyancy. Under the conditions of mantle heat and tectonic decompression, the subducted oceanic crust and the overlying lithospheric mantle partially melted, forming Late Triassic granites with different degrees of adakitic texture [10,21,26,35].

Figure 11. (a) SiO₂–TFeO/(TFeO + MgO) diagram (after [43]), (b) Y–Nb diagram, and (c) (Y + Nb)–Rb diagram (after [75]) of the Xiba granite intrusion from the Shuangwang Au deposit. Granodiorite includes sample SH-116, QGG-4, XB-17 and XB-24; Monzonitic granite includes sample SH-117, QGG-3 and QGG-5.

Figure 11. (a) SiO₂–TFeO/(TFeO + MgO) diagram (after [43]), (b) Y–Nb diagram, and (c) (Y + Nb)–Rb diagram (after [75]) of the Xiba granite intrusion from the Shuangwang Au deposit. Granodiorite includes sample SH-116, QGG-4, XB-17 and XB-24; Monzonitic granite includes sample SH-117, QGG-3 and QGG-5.

6. Conclusions

• The Xiba rock mass belongs to the high-K calc–alkaline series of quasi-aluminous granite. The granodiorite therein shows characteristics of I-type granitic rocks, and the monzonitic granite therein shows characteristics of I-type and A-type transition.
• The rare earth distribution patterns of the granites in the Xiba rock mass are basically uniform, characterized by the enrichment of light rare earth elements, a loss of heavy rare earth elements, and a strong fractionation of light and heavy rare earth elements. These granites are depleted in high-field-strength elements, such as Nb, Ce, Zr, and Ti, but enriched in large-ion lithophile elements, such as Ba and Sr.
• The intrusions or dikes in the Shuangwang Au deposit formed in the Late Triassic of the Indosinian period, but the crystallization ages are different, and so they can be considered products of two different stages in the same period.
• The two-stage Hf model age indicates that the magmatic parent magma in the mining area was the product of the melting of crustal material that had been retained for a long time in the crust after the differentiation of the depleted mantle in the Mesoproterozoic.
• The formation time of the magmatic rocks in the mining area is slightly later than the main collision period of the Qinling orogenic belt. Therefore, it is speculated that the Xiba rock mass is a large complex rock mass, and two different stages of the same period were invaded to form granodiorite and monzonitic granite. After the collision of the Yangtze plate and the North China plate in the Qinling area, the partial melting of the subducted oceanic crust and the overlying lithospheric mantle occurred, forming late Triassic granites with different degrees of adakitic texture.