Gold Mineralization at the Syenite-Hosted Anwangshan Gold Deposit, Western Qinling Orogen, Central China

Abstract

The Anwangshan gold deposit is located in the northwestern part of the Fengtai Basin, Western Qinling Orogen (WQO). The gold ore is hosted within quartz syenite and its contact zone. The U–Pb weighted mean age of the quartz syenite is 231 ± 1.8 Ma. It is characterized by high potassium (K₂O = 10.13%, K₂O/Na₂O > 1) and high magnesium (Mg^# = 55.31 to 72.78) content, enriched in large ion lithophile elements (Th, U, and Ba) and light rare earth elements (LREE), with a typical “TNT” (Ti, Nb, and Ta) deficiency. The geochemical features and Hf isotope compositions (ε_Hf(t) = −6.68 to +2.25) suggest that the quartz syenite would form from partial melting of an enriched lithospheric mantle under an extensional setting. Three generations of gold mineralization have been identified, including the quartz–sericite–pyrite (Py1) stage I, the quartz–pyrite (Py2)–polymetallic sulfide–early calcite stage II, and the epidote–late calcite stage III. In situ sulfur isotope analysis of pyrite shows that Py1 (δ³⁴S = −1.1 to +3.8‰) possesses mantle sulfur characteristics. However, Py2 has totally different δ³⁴S (+5.1 to +6.7‰), which lies between the typical orogenic gold deposits in the WQO (δ³⁴S = +8 to +12‰) and mantle sulfur. This suggests a mixed source of metamorphosed sediments and magmatic sulfur during stage II gold mineralization. The fluid inclusions in auriferous quartz have three different types, including the liquid-rich phase type, pure (gas or liquid)-phase type, and daughter-minerals-bearing phase type. Multiple-stage fluid inclusions indicate that the ore fluids are medium-temperature (concentrated at 220 to 270 °C), medium-salinity (7.85 to 13.80% NaCleq) CO₂–H₂O–NaCl systems. The salinity is quite different from typical orogenic gold deposits in WQO and worldwide, and this is more likely to be a mixture of magmatic and metamorphic fluids as well. In summary, the quartz syenite should have not only a spatio-temporal but also a genetical relationship with the Anwangshan gold deposit. It could provide most of the gold and ore fluids at the first stage, with metamorphic fluids and/or gold joining in during the later stages.

1. Introduction

The Qinling orogenic belt is famous for numerous Triassic magmatic activities and sediment-hosted gold deposits. The Triassic magmatism mainly developed in the early Indosinian period (245–235 Ma) and the late Indosinian period (227–205 Ma), during which there was an intermission of ca. 10 Myr [1,2,3,4,5,6,7]. It is widely recognized that late Indosinian magmatic activity was formed in a post-collision setting [5,6,7,8,9,10,11]. However, the tectonic setting of early Indosinian magmatic activity is still under debate, including the active continental margin model and the early stage of a post-collision setting model [2,4,9,12]. The major conflict over the two models concerns the closure time of the Mianlve Ocean. The active continental margin model is founded on the geochemical characteristics of early Indosinian magmatism, which resemble those of island arc magmatic rocks [12]. The post-collision setting model is based on geological observation. The conglomerate at the bottom of the lower part of the molasse formation in the Upper Permian Gequ Formation in the East Kunlun area is disconformably overlaid on the ophiolite suite of the Mianlve suture zone, indicating that the Mianlve Ocean had already closed and a continent–continent collision had occurred in the Late Permian [13]. Therefore, understanding the genesis of ca. 230 Ma magmatic activity will be critical to solving the controversy.

Veins, characterized as magmatic intrusions with vein-like or wall-like structures, are widely distributed around orogenic belts [14,15,16]. Sedimentary hosted gold deposits are generally structurally controlled in the Western Qinling Orogen (WQO, western part of the Qinling Orogen, divided by the Baoji–Chengdu railway), with abundant magmatic veins sharing the same pass with gold mineralization or hosting in nearby unmineralized faults. The genetic relationship between magmatic veins and gold mineralization is a hot topic in the region [17,18,19,20,21,22]. Some scholars believe that magmatic veins are merely spatially related to gold mineralization without genetic connection, since the magmatic veins are earlier or later than the gold mineralization [23]. Other scholars argue that magmatic veins are closely genetically related to gold mineralization, not only providing a heat source but also supplying mineralizing substances [20,21,24,25]. It is vital to clarify the genetical relationship between syenite veins and gold mineralization, especially in terms of its theoretical significance and implications for regional mineral exploration.

The Anwangshan gold deposit is situated in the Fengtai polymetallic metallogenic belt, WQO [26]. Field investigations show that multiple quartz syenite veins have been introduced into the pyroxenite stock, and gold mineralization is highly spatially related to the quartz syenite veins. Zircon U–Pb dating of quartz syenite renders an age of 231 ± 1.8 Ma, which is between the early and late Indosinian magmatic activity. In order to understand the genesis of the ca. 230 Ma quartz syenite and its genetical relationship with gold mineralization, this study conducts zircon U–Pb geochronology, whole-rock geochemistry, fluid inclusion, and pyrite in situ sulfur isotope analysis on representative quartz syenite and ore samples. Through a comparison study of the nearby Jiuzigou diopsidite and syenite complex, the petrogenesis of quartz syenite has been put forward. The fluid inclusion and pyrite sulfur isotope study build a connection between gold mineralization and the quartz syenite veins. This finally establishes the foundation for exploring the genetic relationship between magmatic veins and gold mineralization in the WQO.

2. Regional Geology

The Qinling Orogen is a unique continental composite orogenic belt spanning the central part of China (Figure 1A). Since the break-up of the Rodinia supercontinent in the Neoproterozoic, the Qinling Orogen has undergone multiple tectonic evolutionary stages, including the formation of the Qin-Qi-Kun Ocean, oceanic crustal subduction orogeny, continental collision orogeny, intraplate extension, and intracontinental superimposed orogeny [1,27,28,29]. The magmatic intrusions are widely spread and of long duration in the Qinling Orogen, among which the Indosinian period is the most common. The Indosinian magmatic activity in the western segment of the orogeny is mainly dated between 245 and 235 Ma, while the eastern segment is concentrated between 227 and 205 Ma [30,31,32].

The research area is situated at the northwest part of the Fengtai basin in the WQO, which is famous for its numerous Pb–Zn and gold deposits [33,34]. The outcropped strata covered a wide time span from the Early Proterozoic to the Carboniferous (Figure 1B) [35]. This includes the Paleoproterozoic Qinling Group (Pt₁Q), which is in fault contact with the Lower Paleozoic Danfeng Group (Pz₁D) to the south and the Ordovician Longwanggou Formation (Ol) to the north, consisting mainly of gneiss, amphibolite, granulite, and marble [36]. The northern part of the Lower Paleozoic Luohansi Group (Pz₁L) is bordered by the Luohansi–Wayaoshang ductile–brittle fault and the Dacaotan Formation, and is a set of shallowly metamorphosed and strongly deformed volcanic–sedimentary rock series. The Pz₁D is a structural mélange belt which has undergone multiple stages of metamorphism and deformation, and mainly comprises arc-related volcanic–sedimentary rocks in low amphibolite facies [37]. The Ordovician Caotangou Group is characterized by a series of sub-greenschist facies terrigenous volcaniclastic rocks and can be divided into the Zhangjiazhuang Formation (Ozh) and the Longwanggou Formation (Ol) from the bottom to the top. The Upper Devonian Dacaotan Formation (D₃d) comprises greenschist facies clastic rocks with well-preserved original sedimentary structures. The Carboniferous Caoliangyi Formation (Cc) is in fault contact with the Ozh in the south and is intruded by the Baoji batholith in the north. It is mainly composed of quartz sandstone, quartz conglomerate, etc. (Figure 1B).

Multiple stages and types of fault are well-developed in the research area [38]. Under the compression of north–south stress in the late Caledonian, nearly east–west- to northwest-oriented faults were formed, represented by the Miaoping–Kangjialiang fault and the Majiayao–Baijiadian ductile–brittle fault which are distributed in the central part of the region (Figure 1B). The east–west Luohansi–Wayaoshang ductile–brittle fault was formed during the Indosinian period. The northeast Anwangshan normal fault and the northeast Yujiayao–Weijiawan strike-slip fault were formed in the Himalayan period, cutting through earlier northwest-oriented structures. All those different faults together shaped the basic structural framework of the area. Intensive magmatic activities were developed in the area as well, represented by the northern Baoji batholith which is a multiphase magmatic intrusion formed at 217 Ma, 212 Ma, and 196 Ma [39]. The Tangzang pluton in the southern part was formed at 428 Ma by fractional crystallization of mafic melt from enriched mantle with crustal contamination [40]. The Jiuzigou stock exposed in the east is a composite intrusion with diopsidite (235 Ma), biotite diopsidite (232 Ma), and diopside-bearing syenite (230 Ma) from early to late [41].

3. The Anwangshan Gold Deposit

The Anwangshan gold deposit is newly discovered in the WQO [42], and it is still undergoing continuous exploration. The Pt₁Q, Ozh, Ol, Cc, and Quaternary (Q) sediments have been found in the mining area (Figure 2). The Ol can be further divided into Ol^s (composed mainly of tuffaceous siltstone and sericitized) and Ol^f (composed mainly of slightly metamorphosed quartz sandstone). The gold ore bodies and magmatic intrusions are generally hosted within the Ol^s.

The two northwest-trending Wuxingtai–Nianziba and Miaoping–Kangjialiang faults are parallel to each other, and crosscut by the late northeast-trending Yujiayao–Weijiawan fault (Figure 2). Under the influence of the two northwest-trending faults, three strong mylonitic zones with a width of 20–50 m are distributed between them. Silicification, sericitation, potassization, and pyritization are developed within the mylonitic zone.

The magmatic rocks are highly related to the gold mineralization in the Anwangshan deposit, including the pyroxenite and the quartz syenite veins. Minor granite porphyry veins display no spatial relation with gold mineralization and will not be discussed here. The pyroxenite is mainly exposed in the area sandwiched between the Jiangjiagou and Hujiagou river, which is 100–200 m in width and approximately 1 km in length. The quartz syenite veins intruded into the pyroxenite and Ol^s tuffaceous siltstone (Figure 3A,B). The quartz syenite veins vary from 0.5 to 30 m in width and from 10 to 1000 m in length, showing obvious structural control, with a general northwest trending.

Pyroxenite, displaying a grayish-green color (Figure 3C), is mainly made up of pyroxene (75%–80%), plagioclase (10%–15%), and apatite (5%–10%) (Figure 3D). The size of different mineral grains ranges from 0.2 to 2 millimeters, with medium- to fine-grained euhedral textures. The quartz syenite veins display flesh-red color, weathering to grayish-white to brownish-yellow (Figure 3E). Quartz syenite is mainly made up of K-feldspar (80%–90%), quartz (5%–10%), plagioclase (5%–10%), and minor biotite (Figure 3F). The mineral grain size ranges from 0.1 to 3 millimeters. The plagioclase is generally undergoing sericitization. Accessory minerals, including magnetite, apatite, and pyrite, can be observed in quartz syenite. Field investigation and microscopic observation both indicate that the combination of pyroxenite and quartz syenite in the Anwangshan gold deposit have similar texture and mineralogical features to the 3 km-southeast Jiuzigou complex [41].

Four gold ore bodies (No.1, 2, 3, and 4) have been identified within the mining area, with gold grades peaking at 62.3 g/t. They share a strong spatial relationship with quartz syenite veins, either enclosed within the veins or situated within contact zones between the veins and the Ol^s (Figure 2 and Figure 3G). Importantly, ore grades within the veins generally exceed those in the Ol sediments. The ores can be divided into altered quartz–syenite type and altered sandstone type. Silicification, sericitization, carbonatization, epidotization, pyritization, and chloritization are widely developed in the mining area.

Based on the field evidence and observation under the microscope, the paragenesis of the Anwangshan gold deposit have been divided into three stages. The quartz–sericite-pyrite (Py1) stage I is characterized by hydrothermal veins comprising coarse-grained quartz, sericite, and sparsely distributed euhedral Py1 (Figure 4A–C). The quartz–polymetallic sulfide–early calcite stage II is characterized by anhedral pyrite (Py2) occurring in association with sphalerite (Figure 4D,E). Minor chalcopyrite presents as solid solutions within sphalerite. Additionally, early calcite veins crosscutting stage I quartz veins are widely distributed (Figure 4D,F). The epidote–late calcite stage III is characterized by epidote predominantly developing along fissures. The epidote veins cut through early calcite veins (Figure 4G,H) and are subsequently cut by even later calcite veins (Figure 4I), with minimal to no association with gold mineralization.

4. Sampling and Analytical Methods

The quartz syenite samples for zircon U–Pb dating, Hf isotope, and whole-rock major and trace element analysis in this study were all collected from the TC-14. Samples for fluid inclusion micro-thermometry and in situ sulfur isotope analysis were collected from the No.2 ore bodies.

Zircon from quartz syenite was separated using conventional heavy liquid and magnetic techniques in the laboratory of Langfang Yantuo Geological Service Co., Ltd., Hebei province (Langfang, China). Handpicked zircon grains were mounted in epoxy blocks, polished to obtain an even surface, and cleaned in an acid bath before laser ablation–inductively coupled plasma mass spectrometry (LA–ICP-MS) analysis. Cathodoluminescence (CL) imaging, U–Pb dating, and Hf isotope analysis of zircon were conducted at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) (Wuhan, China), China University of Geosciences, Wuhan. LA–ICP-MS U–Pbdating was conducted using an Agilent 7500a ICP-MS (Palo Alto, CA, USA) instrument equipped with a GeoLas2005 193 nm ArF excimer laser, with a beam spot diameter of 32 μm and a frequency of 8 Hz. The background acquisition time for each zircon analysis point was 20 s, and the target signal acquisition time was 40 s. Isotope ratio fractionation correction was performed using external standard 91500. Detailed operating conditions and data reduction follow those in Liu et al. [44]. Zircon Hf isotope LA–MC-ICP-MS (Resolution S155-Nu Plasma II) analysis (Charlestown, SC, USA) was conducted near the U–Pb dating sites, with a laser beam diameter of 44 μm and an ablation rate of 8 Hz. The carrier gas is high-purity helium mixed with argon and a small amount of nitrogen before entering the mass spectrometer. For detailed instrument parameters and analytical methods, refer to Hu et al. (2012) [45].

Whole-rock major and trace element analysis were completed at ALS Chemex (Guangzhou) Co., Ltd (Guangzhou, China). Major elements were analyzed using X-ray fluorescence spectroscopy (XRF), method code ME-XRF26d, with the PANalytical Axios Max (Malvern, UK) instrument manufactured in the Netherlands. Trace elements were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES), method code ME-ICP61, with the Agilent VISTA instrument manufactured in the United States.

Fluid inclusion microthermometry was conducted at GPMR using a LINKAM TMS93 stage (Waltham, MA, USA), capable of temperature measurements ranging from 194 to 600 °C, with a temperature reproducibility error of ±1 °C and an ice point temperature error of ±0.1 °C. Phase transition temperatures of fluid inclusions were carefully observed and recorded, and salinity was calculated using appropriate formulas.

LA–MC-ICP-MS in situ sulfur isotope analysis was conducted at GPMR, employing a Resolution S-155 laser ablation system and Nu Plasma II multi-collector inductively coupled plasma mass spectrometer. The laser spot diameter was set at 33 μm, with an ablation rate of 5 Hz and an energy density of 2 J/cm². High-purity helium gas was used as the carrier gas, mixed with argon and a small amount of nitrogen before entering the mass spectrometer, with a single test duration of about 120 s. The test employed a sample-standard cross-matching method, with the laboratory internal standard pyrite sample WS-1 (δ³⁴S_V-CDT = 0.3 ± 0.1‰, Zhu et al. [46]) used for analysis, with an analytical precision of ±0.5‰.

5. Results

5.1. Zircon U–Pb Ages and Hf Isotope Composition

Age data for zircon samples from the quartz syenite are shown in

Table A1. LA–ICP-MS zircon U–Pb data of quartz syenite from the Anwangshan gold deposit.

Spot No.	Content/ppm				207Pb/206Pb		207Pb/235U		206Pb/238U		207Pb/206Pb		207Pb/235U		206Pb/238U
	U	Th	Pb	Th/U	Ratio	1σ	Ratio	1σ	Ratio	1σ	Age	1σ	Age	1σ	Age	1σ
No.1	181	259	17.3	1.43	0.05473	0.00199	0.27235	0.00819	0.03667	0.00053	466	81	245	7	232	3
No.2	627	609	46.6	0.97	0.05172	0.00136	0.26284	0.00629	0.03695	0.00046	272	59	237	5	234	3
No.3	748	820	60.4	1.09	0.05313	0.00115	0.26769	0.00581	0.03656	0.00037	345	50	241	5	231	3
No.4	1274	958	85.1	0.75	0.05029	0.00131	0.25228	0.00666	0.03633	0.00043	209	94	228	5	230	3
No.6	1497	1071	93	0.71	0.05158	0.00124	0.26096	0.00668	0.03657	0.00031	265	56	235	5	232	2
No.7	919	880	66.8	0.95	0.05537	0.00102	0.28097	0.00564	0.03673	0.0005	428	45	251	4	233	3
No.8	1074	1137	82.8	1.05	0.05289	0.00099	0.26991	0.00505	0.03695	0.00036	324	43	243	4	234	2
No.10	574	587	43.9	1.02	0.05205	0.00126	0.25865	0.00664	0.03603	0.00046	287	56	234	5	228	3
No.11	1233	894	75.7	0.72	0.05092	0.00086	0.25275	0.00436	0.03593	0.00031	235	39	229	4	228	2
No.13	460	620	41.8	1.32	0.05491	0.0013	0.27372	0.00659	0.03618	0.00043	409	54	246	5	229	3
No.14	1249	992	88.1	0.73	0.05727	0.00319	0.28469	0.01538	0.03599	0.00087	501	122	254	12	228	5
No.15	169	420	23.4	2.48	0.05424	0.00151	0.2686	0.00786	0.0361	0.00062	389	63	242	6	229	4
No.16	241	396	24.8	1.64	0.05481	0.00651	0.26891	0.03092	0.03574	0.00085	406	264	242	25	226	5
No.17	1744	1014	102	0.58	0.05088	0.00118	0.2537	0.00678	0.03608	0.00058	235	49	230	5	229	4
No.18	374	968	49.5	2.58	0.0516	0.00091	0.26408	0.0052	0.03698	0.000382	333	38	238	4	234	2
No.20	906	746	62.9	0.828	0.05523	0.00109	0.27166	0.00517	0.03568	0.0003	420	44	244	4	226	2
No.21	662	634	43.5	0.95	0.05251	0.00122	0.26702	0.00636	0.03686	0.00037	309	562	240	5	233	2
No.22	213	394	23.6	1.84	0.05156	0.00139	0.2555	0.00674	0.03598	0.00031	265	58	231	5	228	2
No.23	638	601	49	0.94	0.05602	0.00137	0.28401	0.00755	0.03662	0.00038	454	54	254	6	232	2
No.24	398	396	30.9	0.99	0.05478	0.00214	0.27394	0.01035	0.03629	0.0004	467	82	246	8	230	2

. Zircon grains are grayish and euhedral. Their size generally ranges from 50 to 110 μm, with aspect ratios ranging from 2:1 to 4:1. Most of the zircon grains in the CL image exhibit typical oscillatory zoning (Figure 5). The analyzed zircons have high Th and U, with Th/U ratios from 0.58 to 2.58 (averaging 1.18), indicating their magmatic affinition [47]. A total of 24 zircon grains were analyzed. Four of them with less than 90-percent concordance have been excluded, with the remaining data showing ²⁰⁶Pb/²³⁸U ages ranging from 226 to 234 Ma, and their weighted-mean ²⁰⁶Pb/²³⁸U age is 231 ± 1.8 Ma (1σ; MSWD = 1.15) (Figure 5), representing the crystallization age of the quartz syenite. During this period (ca. 234–224 Ma), the WQO underwent a short quiet period of magmatic activity [48].

The Hf isotope composition of zircon grains in the quartz syenite are presented in

Table A2. Hf isotope composition of zircons in the quartz syenite.

Analysis No.	Age	$\frac{{}^{176}\mathbf{Yb}}{{}^{177}\mathbf{Hf}}$	$\frac{{}^{176}\mathbf{Lu}}{{}^{177}\mathbf{Hf}}$	$\frac{{}^{176}\mathbf{Hf}}{{}^{177}\mathbf{Hf}}$	2α	εHf	εHf (t)	tDM (Hf)	tDM (C)	fLu/Hf
LH-4-1	232 ± 3	0.025914	0.0011569	0.282589	0.000022	−6.47	−1.44	942	1359	−0.97
LH-4-2	234 ± 3	0.034333	0.0015881	0.282555	0.000025	−7.67	−2.72	1002	1440	−0.95
LH-4-3	231 ± 3	0.046215	0.0019609	0.282566	0.000029	−7.28	−2.4	996	1419	−0.94
LH-4-4	230 ± 3	0.050392	0.0020349	0.282564	0.000037	−7.37	−2.47	1002	1426	−0.94
LH-4-5	232 ± 2	0.033383	0.0016093	0.282556	0.000026	−7.65	−2.68	1002	1440	−0.95
LH-4-6	233 ± 3	0.046072	0.0020993	0.282594	0.000022	−6.31	−1.44	960	1359	−0.94
LH-4-7	234 ± 2	0.028519	0.0013355	0.282535	0.000019	−8.37	−3.39	1023	1482	−0.96
LH-4-8	228 ± 3	0.015010	0.0006804	0.282554	0.000024	−7.72	−2.61	980	1435	−0.98
LH-4-9	228 ± 2	0.019341	0.0007752	0.282484	0.000036	−10.2	−5.12	1080	1592	−0.98
LH-4-10	229 ± 3	0.004002	0.0001692	0.282568	0.000021	−7.23	−2.04	948	1399	−0.99
LH-4-11	228 ± 5	0.011150	0.0004350	0.282571	0.000024	−7.13	−1.97	950	1394	−0.99
LH-6-1	229 ± 4	0.050771	0.0020629	0.282705	0.000028	−2.36	2.52	797	1107	−0.94
LH-6-2	226 ± 5	0.004477	0.0001959	0.282437	0.000028	−11.84	−6.68	1128	1692	−0.99
LH-6-3	229 ± 4	0.004669	0.0001967	0.282464	0.000034	−10.9	−5.72	1091	1631	−0.99
LH-6-4	234 ± 2	0.007766	0.0003070	0.282531	0.000037	−8.53	−3.35	1002	1482	−0.99
LH-6-5	226 ± 2	0.012339	0.0004776	0.282575	0.000032	−6.96	−1.83	945	1384	−0.99
LH-6-6	233 ± 2	0.038755	0.0014199	0.282629	0.000040	−5.05	−0.06	892	1272	−0.96
LH-6-7	228 ± 2	0.050945	0.0018332	0.282586	0.000023	−6.58	−1.65	964	1373	−0.94
LH-6-8	232 ± 2	0.021080	0.0008728	0.282610	0.000030	−5.74	−0.66	906	1310	−0.97
LH-6-9	230 ± 2	0.011014	0.0004390	0.282515	0.000029	−9.11	−3.95	1028	1520	−0.99
LH-6-10	234 ± 2	0.053334	0.0019466	0.282612	0.000025	−5.65	−0.73	929	1315	−0.94
LH-6-11	228 ± 3	0.027761	0.0010363	0.282632	0.000034	−4.95	0.08	879	1262	−0.97
LH-6-12	228 ± 2	0.008001	0.0003248	0.282605	0.000063	−5.92	−0.77	900	1316	−0.99

. Two zircon grains display ¹⁷⁶Lu/¹⁷⁷Hf ratios at 0.002099 and 0.002063, and the remaining 21 points have ¹⁷⁶Lu/¹⁷⁷Hf ratios less than 0.002, indicating minimal radiological Hf accumulation after zircon crystallization. Therefore, the measured ¹⁷⁶Hf/¹⁷⁷Hf values can represent the Hf isotopic composition of the system at the time of zircon formation [49]. The measured ¹⁷⁶Hf/¹⁷⁷Hf ratios in the quartz syenite zircon grains range from 0.2824 to 0.2827, resulting in calculated ε_Hf(t) values of −6.68 to +2.25. The single-stage Hf model ages range from 797 to 1128 Ma. The two-stage Hf model ages range from 1107 to 1692 Ma, predominantly centered around 1310 to 1482 Ma.

5.2. Rock Geochemistry

The major and trace elements of the quartz syenite have been presented in

Table A3. Major (wt%) and trace elements (ppm) content of quartz syenite.

Sample No.	L7-2	L7-7	L8-7	YFG-11	YFG-15	YFG-16
SiO2	63.64	64.42	60.19	56.68	59.25	60.94
TiO2	0.31	0.28	0.39	0.69	0.59	0.61
Al2O3	17.65	17.48	16.58	14.7	14.88	15.29
MgO	0.44	0.52	1.33	2.97	2.64	2.82
CaO	0.72	0.29	2.39	4.76	3.76	2.69
Na2O	4.63	3.95	3.71	3.92	3.32	4.54
K2O	8.09	8.75	7.08	3.55	5.42	3.84
Na2O+K2O	12.72	12.7	10.79	7.47	8.74	8.38
K2O/Na2O	1.75	2.22	1.91	0.91	1.63	0.85
Mg#	55.31	72.78	68.52	61.27	64.46	65.19
A/CNK	1	1.06	0.91	0.77	0.82	0.92
A/NK	1.08	1.09	1.2	1.43	1.31	1.31
δ	7.84	7.53	6.77	4.08	4.7	3.91
Cs	1.17	1.45	1.3	1.09	2.03	0.92
Rb	131	175.5	131	95.7	133.5	87.3
Ba	7590	6280	3160	3570	4290	3350
Th	21.7	38.6	23.8	48.4	44.7	43.7
U	1.18	3.54	2.61	5.41	5.75	5.78
V	44	43	88	143	108	111
Cr	10	10	40	40	40	30
Ga	17.5	18	16.5	20.1	18.8	19.2
Sr	2350	842	1320	994	973	755
Y	24.8	24.5	19.5	36.8	29.5	28.4
Zr	64	96	180	321	336	339
Nb	17.4	18.3	18.5	32	28.4	27.4
La	168	190.5	241	184	144	146
Ce	294	318	327	333	257	261
Pr	29.3	29.7	26.8	33.4	25.8	26
Nd	95.8	93.7	75.9	115.5	88.2	86.9
Sm	16.5	14.75	9.83	18.6	14.65	14.35
Eu	3.41	3.13	2.06	4	3.01	2.98
Gd	10.4	9.23	6.43	12.6	9.55	9.31
Tb	1.37	1.17	0.84	1.59	1.24	1.15
Dy	6.12	5.44	3.74	7.52	5.7	5.9
Ho	0.94	0.89	0.69	1.36	1.03	1.03
Er	2.11	2.11	1.72	3.31	2.72	2.68
Tm	0.26	0.29	0.24	0.47	0.37	0.4
Yb	1.56	1.73	1.48	3	2.33	2.47
Lu	0.22	0.26	0.22	0.45	0.35	0.39
Hf	2.1	2.9	4.1	7.4	7.6	7.8
Ta	1.5	1.4	0.9	1.5	1.3	1.4
W	2	8	2	3	2	2
Sn	3	3	3	4	4	4
ΣREE	629.99	670.9	697.95	718.8	555.95	560.56
LREE	607.01	649.78	682.59	688.5	532.66	537.23
HREE	22.98	21.12	15.36	30.3	23.29	23.33
LREE/HREE	26.41	30.77	44.44	22.72	22.87	23.03
(La/Yb)N	77.25	78.99	116.8	43.99	44.33	42.4
δEu	0.74	0.76	0.74	0.75	0.73	0.74

. The quartz syenite exhibited low contents of SiO₂ (56.68–64.42%), TiO₂ (0.28–0.69%), and Na₂O (3.32–4.63%). In contrast, the contents of Al₂O₃ (14.88–17.65%), Mg^# (55.31–72.78), K₂O (3.55–8.75%), total-alkali (Na₂O + K₂O = 7.47–12.72%) and potassium–sodium ratio (K₂O/Na₂O = 0.85–2.22) were generally high in the quartz syenite. In the total-alkali-silica (TAS) diagram, the samples are plotted in the syenite and syenite diorite fields (Figure 6A). All samples plot in the shoshonite field in the K₂O–SiO₂ diagram (Figure 6B). Furthermore, the Al₂O₃/ (Na₂O + K₂O) (A/NK) and Al₂O₃/ (CaO + Na₂O + K₂O) (A/CNK) ratios are from 1.08 to 1.43 and from 0.77 to 1.06, respectively, indicating metaluminous to weakly peraluminous characteristics (Figure 6C). On the basis of the Zr + Nb + Ce + Y versus Na₂O + K₂O/CaO diagram, all samples are collectively plotted in the field of the A-type granitoids (Figure 6D).

The quartz syenite generally has relatively high total rare earth element (ΣREE) concentrations, ranging from 555.95 ppm to 718.8 ppm (average at 639.03 ppm). The LREE/HREE ratio ranges from 22.72 to 44.44, and the (La/Yb) _N varies from 42.4 to 116.8 (Table A3), indicating significant fractionation between light and heavy rare earth elements. The chondrite-normalized rare earth element distribution curves display strong rightward slopes (Figure 7A).

The δEu values range from 0.73 to 0.76, indicating weakly negative anomalies. This suggests the magma source from a plagioclase-stable reservoir or undergoing plagioclase fractionation. δCe values range from 0.82 to 0.96, with no significant anomalies. The trace element distribution patterns of all quartz syenite samples are similar to each other. The large ion lithophile elements (LILE), such as Ba, Th, K, Sr, and LREE, are enriched, and the high-field-strength elements (HFSE), including Nb, Ta, Ti, are depleted (Figure 7B).

5.3. Fluid Geochemistry

Fluid inclusions in the Anwangshan deposit are characterized by being numerous, small-sized, and morphologically complex. Three types of fluid inclusions are observed in auriferous quartz formed during stages I and II: the liquid-rich phase (Figure 8A,B), pure (gas or liquid) phase (Figure 8C), and daughter-minerals-bearing phase (Figure 8D). The liquid-rich inclusions are elliptical or negatively crystalline, with sizes ranging from 3 to 7 μm and gas-phase percentages ranging from 5% to 30%. The pure phase inclusions are commonly 2 to 9 μm in diameter, and are mostly elliptical in shape. The daughter-minerals-bearing inclusions exhibit elliptical, elongated, or irregular shapes, with sizes ranging from 3 to 15 μm.

Primary fluid inclusions from the two stages were selected for microthermometric analysis. Results are presented in

Table A4. Physical and chemical characteristics of fluid inclusions in the Anwangshan gold deposit.

Stage	Mineral	Size (μm)	Gas–Liquid Ratio (%)	Homogenization Temperature (°C)	Freezing Point (°C)
I	Quartz	2 × 4	10	289	−9.83
I	Quartz	3 × 4	10	315	−7.72
I	Quartz	3 × 3	10	280
I	Quartz	3 × 4	15	256
I	Quartz	4 × 4	10	244
I	Quartz	3 × 4	10	278
I	Quartz	4 × 3	15	262
I	Quartz	2 × 4	10	234
I	Quartz	2 × 3	10	243
I	Quartz	2 × 4	10	303
I	Quartz	2 × 3	10	287
I	Quartz	6 × 4	30	206	−5
I	Quartz	3 × 3	10	254
I	Quartz	2 × 4	10	212
I	Quartz	4 × 7	10	273
I	Quartz	2 × 3	20	264
I	Quartz	2 × 4	10	211
I	Quartz	6 × 12	15	235
I	Quartz	4 × 10	30	258
I	Quartz	2 × 3	20	234
I	Quartz	3 × 4	25	255
I	Quartz	2 × 4	15	260
I	Quartz	4 × 6	10	227
I	Quartz	2 × 4	20	321	−5.92
II	Quartz	3 × 4	10	276
II	Quartz	3 × 6	15	210	−7.52
II	Quartz	4 × 4	10	215	−6.98
II	Quartz	4 × 5	10	230
II	Quartz	4 × 4	10	252
II	Quartz	4 × 4	10	230	−6.03
II	Quartz	3 × 4	10	244
II	Quartz	3 × 4	10	201
II	Quartz	4 × 4	10	229	−8.13
II	Quartz	3 × 4	10	235
II	Quartz	3 × 5	10	215
II	Quartz	3 × 2	20	229	−6.43
II	Quartz	2 × 3	20	231
II	Quartz	2 × 4	10	211
II	Quartz	2 × 3	10	252	−6.83
II	Quartz	6 × 16	20	243
II	Quartz	2 × 3	20	186
II	Quartz	2 × 2	20	190
II	Quartz	3 × 2	10	240	−5.9
II	Quartz	2 × 4	15	155
II	Quartz	2 × 6	20	237
II	Quartz	3 × 5	25	221
II	Quartz	2 × 5	15	230
II	Quartz	1 × 2	30	179
II	Quartz	2 × 3	30	193
II	Quartz	2 × 3	20	235

and Figure 9A. Stage I homogenization temperatures range from 206 to 321 °C, with a concentration between 250 and 260 °C. Stage II homogenization temperatures range from 155 to 276 °C, with a concentration between 220 and 230 °C. There is a trend of decreasing fluid temperature from stage I to stage II. Salinity was calculated based on the ice point data using the formula proposed by Potter et al. [56]: w(NaCleq)/% = 0.00 + 1.76985Tm − 4.2384 × 10⁻²Tm² + 5.2778 × 10⁻⁴Tm³, where Tm is the absolute value of the ice point temperature. A total of 11 ice point temperature data were collected. The calculated salinity values range from 7.85 to 13.80%NaCleq (Figure 9B), predominantly concentrated between 9 and 12%NaCleq, which is of moderate salinity.

5.4. In Situ Sulfur Isotope Composition of Pyrite

This study completed in situ sulfur isotope composition analysis of pyrite at 18 points across four ore samples (

Table A5. Sulfur isotope composition of pyrite in the Anwangshan gold deposit.

Sample Number	Pyrite Type	δ34S (‰)
Aws-55-1	Py2	6.2
Aws-55-2	Py2	6.4
Aws-55-3	Py2	6.5
Aws-55-4	Py2	6.7
Aws-55-5	Py2	5.1
Aws-55-6	Py2	5.8
Aws-55-7	Py2	5.8
Aws-8-1	Py1	3.3
Aws-8-2	Py2	5.6
Aws-4-1	Py1	0.7
Aws-4-2	Py1	−1.1
Aws-4-3	Py1	3.8
Aws-21-1	Py1	0.5
Aws-21-2	Py1	2.6
Aws-21-3	Py1	2.7
Aws-21-4	Py1	2.9
Aws-21-5	Py1	2.1
Aws-21-5	Py1	3.0

). For stage I pyrite (Py1), 10 analysis points showed a relatively concentrated distribution between −1.1 and +3.8‰. For stage II pyrite (Py2), all eight analysis points exhibit values higher than those of Py1, ranging from +5.1 to +6.7‰, indicating the addition of more ³⁴S during the stage II gold mineralization.

6. Discussion

6.1. Genesis of Ore-Bearing Quartz Syenite and Its Tectonic Implication

The syenite–ultramafic complex is generally interpreted as partial melting of mantle-derived rocks in an extensional tectonic setting [57,58,59,60,61], or as a result of lower crust melting triggered by mantle-derived magma diapir [62,63,64,65]. The quartz syenite in the Anwangshan gold deposit exhibits characteristics of high potassium (average K₂O of 10.13%, K₂O/Na₂O > 1) and high magnesium values (Mg^# 64.59). It is difficult for a single crustal-source rock to generate rocks with high potassium and Mg^# greater than 40 through partial melting [66,67]. Liu [68] suggests that crustal materials in the northern Qinling terrane are moderately enriched in LREE, with obvious negative Eu anomalies, and have high HREE contents. These differ from the quartz syenite, which shows high enrichment in LREE, weak negative Eu anomalies, and low HREE contents (Figure 7A). Additionally, no mafic micro-granular enclave (MME) was observed in the field within the syenite veins, excluding the possibility that the syenite veins in Anwangshan were formed by direct partial melting of crustal rocks or the mixing of crust and mantle magma caused by mantle-derived magma diapir [69].

The ε_Hf(t) values of quartz syenite range from −6.68 to +2.25, slightly lower than those of the diopside-bearing syenite in Jiuzigou, and much lower than the depleted mantle values during the same period (Figure 10). When the ε_Hf(t) of zircon is negative, it is generally believed that magma originated from the crust or enriched mantle, or resulted from crustal mantle mixing. The average Sr content of quartz syenite is 1205 ppm, which is much higher than the Sr contents of depleted mantle and lower crust (20 ppm and 290 ppm, respectively; Wang et al. [70]), but similar to the enriched mantle (1100 ppm; Chen and Zhai [71]). Moreover, the higher K₂O + Na₂O content (7.47%–12.72%) and (La/Yb) _N ratio (42.4–116.8) of quartz syenite is comparable to the composition of alkaline rocks formed by the low-level partial melting of lithospheric mantle ((La/Yb) _N = 23.56–130.64, K₂O + Na₂O = 9.39%–14.28%, Eby [72], Pin et al. [73]; Laporte et al. [74]), suggesting that the quartz syenite may be derived from the low-level partial melting of the enriched lithospheric mantle [75,76]. Therefore, the crustal source and crustal mantle mixing could have been ruled out again. The relatively positive ε_Hf(t) values of the quartz syenite and diopside-bearing syenite could not exclude the involvement of asthenosphere mantle material or newborn crustal material [41]. Previous research of the nearby Jiuzigou complex, including diopsidite (235.2 Ma), biotite diopsidite (232.8 Ma), and diopside-bearing syenite (230.7 Ma), indicate that they resulted from partial melting of an enriched lithospheric mantle in an extensional background [41]. The phenomenon in the field and under the microscope aligns with the observation of quartz syenite veins intruded into pyroxenite in the Anwangshan deposit. The emplacement age of quartz syenite at 231 ± 1.8 Ma is also consistent with the diopside-bearing syenite in the Jiuzigou complex. It is not easy to deny that the two simultaneous syenite–ultramafic complexes should generate from the same source region.

Moreover, the lg [CaO/(Na₂O + K₂O)] − SiO₂ diagram indicates that the 231 Ma quartz syenite should form in an extensional background (Figure 11A). The enrichment of Th, Rb, Ba, and LREE and the negative anomalies of Ta, Nb, and Ti (“TNT” deficiency) also indicate that the source mantle has already been metamorphized by the melt generated by subducting plates [55,77]. The w (Rb) − w (Nb + Y) diagrams show that the quartz syenite has a strong post-collisional affiliation (Figure 11B). If the early Indosinian magmatic activity (245–235 Ma) is still under an active continental margin environment [12,13], there will not be enough time to finish the subsequent continental collision and transform the stress environment from compression to extension. Therefore, the post-collision setting model is preferred to describe the tectonic setting of the early Indosinian magmatic activity. In summary, the 231 Ma quartz syenite is formed by a low-level partial melting of the enriched lithospheric mantle under the early stage of a post-collision setting [2,4]. This is consistent with the characteristics of the Qinling orogenic belt transitioning from compression to extension at around 230 Ma [41,78].

Figure 11. Discriminant diagram of the Anwangshan quartz syenite. lg [CaO/(Na₂O + K₂O)] − SiO₂ discriminant (A), according to Brown et al. [79]. (B) Rb − (Nb + Y) diagram (Pearce [80]). ORG: ocean ridge granite, post-COLG: post-collision granites, syn-COLG: syn-collision granites, VAG: volcanic arc granites, WPG: within-plate granites.

Figure 11. Discriminant diagram of the Anwangshan quartz syenite. lg [CaO/(Na₂O + K₂O)] − SiO₂ discriminant (A), according to Brown et al. [79]. (B) Rb − (Nb + Y) diagram (Pearce [80]). ORG: ocean ridge granite, post-COLG: post-collision granites, syn-COLG: syn-collision granites, VAG: volcanic arc granites, WPG: within-plate granites.

6.2. Relationship between Quartz Syenite and Gold Mineralization

The Anwangshan gold deposit exhibits a close spatial–temporal relationship with quartz syenite. The occurrence of ore bodies is identical to that of the quartz syenite vein. The intensity of gold mineralization and alteration exhibits zoning characteristics, with the quartz syenite vein serving as the center of gold mineralization (Figure 3E). The silicification, sericitization, pyritization, and the gold grade on both sides of the vein gradually weaken, indicating that quartz syenite has a direct controlling effect on gold mineralization.

Alkaline rocks usually have low background of gold (average gold content at about 3.4 ppb; Boyle [81]), and they generally cannot provide enough gold for mineralization. However, Mutschler [82] proposed that when the gold content in alkaline rocks exceeds 10 ppb, they can serve as part of the sources for gold deposits. Both the Hougou gold deposit and the Dongping gold deposit in the Zhangxuan area, north China, have close tempo-spatial and genetic links with syenites [83], and the gold contents in the syenites of the two are 22.2 ppb and 10.1 ppb, respectively [84]. The quartz syenite veins in the Anwangshan gold deposit have an average gold content of 12.8 ppb [41]. The sulfur isotope composition of Py1 (−1.1‰ to +3.8‰) shows clear mantle sulfur affiliation, which is significantly different from orogenic gold deposits related to regional metamorphism in the WQO (Figure 12). Thus, the quartz syenite could provide part of the gold source during the first mineralization stage. During stage II, the sulfur isotope composition of Py2 ranges between Py1 and the majority of orogenic gold deposits (deep gray) in the WQO (Figure 12). This indicates that metamorphic gold may join in during the late gold mineralization stage in the Anwangshan gold deposit.

The point that syenite could provide ore-forming fluids during gold mineralization process has also been reported. Ludovic and Michel [86] suggest that metamorphic fluids and magmatic fluids from ore-bearing syenite jointly provided ore-forming fluids for late-stage gold mineralization in the Beattie gold deposit, Canada. The study in the Kirkland Lake–Larder Lake gold mineralization belt in Ontario, Canada, also suggested that regional gold mineralization fluids originated from a deep alkaline magma–hydrothermal system [87]. Previous studies on gold deposits closely related to magmatic dykes in the WQO have also shown that intermediate to mafic dykes formed by partial melting of enriched mantle provided ore-forming fluids for gold mineralization [20,88,89]. The fluid inclusion studies of the Anwangshan gold deposit show that stage I displays mid-temperature ranges (206 to 321 °C), with a trend of decline from early to later stages. The appearance of daughter-mineral-bearing inclusions and the medium salinity (7.85 to 13.80% NaCleq) are quite different from those of typical orogenic gold deposits which are dominated by CO₂-rich vapor–liquid two-phase fluid inclusions with 3 to 7% NaCl eq in salinity [90,91]. This indicates the involvement of high-salinity fluid, most likely magmatic fluids in the Anwangshan deposit.

In conclusion, the in situ sulfur isotope and fluid inclusion study both support strong magmatic affiliation during stage I gold mineralization. Combined with the geological observation, the quartz syenite is an unavoidable source to provide gold and ore fluid in the Anwangshan gold deposit. Mao [19] and Wang et al. [65] suggest that the Triassic large-scale lead–zinc–gold mineralization in the WQO is a product of tectonic–magmatic-fluid activity. The age of the quartz syenite (231.0 ± 1.8 Ma) is younger than the nearby Tangzang pluton (428 Ma, Wu et al. [39]) and the regional magmatic activity (ca. 240 Ma; Hejiazhuang pluton, Liujiagou pluton, granite porphyry dykes in the Pangjiahe and Matigou gold deposit; Ma, 2018 and references therein [81]). However, it is consistent with the mineralization age of the nearby orogenic gold deposits, including the Pangjiahe (231.2 Ma) and the Matigou (234 Ma) gold deposit [34,92]. Furthermore, the Qinling Orogen’s tectonic setting changed from compression to extension at around 230 Ma, which would have provided a favorable environment for auriferous fluid migration and precipitation in the shear zones. Therefore, the ore-bearing quartz syenite should be the product of the same tectonic–magmatic–fluid activity at around 230 Ma [19,93]. Unlike typical orogenic gold deposits formed by Indosinian metamorphism in the WQO, the Anwangshan gold deposit has the same mineralization age as the Pangjiahe and Matigou gold deposits (ca. 230 Ma), but a totally different metal and fluid source in stage I. The mixing of magmatic fluids with metamorphic fluids led to stage II sulfur isotope compositions and ore-forming fluid temperatures similar to those of orogenic gold deposits (Figure 9A and Figure 12).

7. Conclusions

The quartz syenite in the Anwangshan gold deposit was formed at 231 ± 1.8 Ma, during an intermittent period of two-stage Indosinian magmatic activity in the WQO. The quartz syenite exhibits characteristics of high potassium, high magnesium, enrichment of LILE, LREE, and depletion of HFSE, with a typical “TNT” (Ti, Nb, and Ta) deficiency. Combined with the Hf isotope composition, it should be the product of partial melting of enriched lithospheric mantle in the post-collision extension background of the WQO. The geological characteristics, in situ sulfur isotope composition, and fluid inclusion studies all indicate that the early stage I gold mineralization in the Anwangshan gold deposit have strong magmatic affiliation, with metamorphic substance involvement in the late stage II gold mineralization.