Genesis of the Supergiant Shuangjianzishan Ag–Pb–Zn Deposit in the Southern Great Xing’an Range, NE China: Constraints from Geochronology, Isotope Geochemistry, and Fluid Inclusion

Abstract

The supergiant Shuangjianzishan (SJS) Ag–Pb–Zn deposit, located in the southern Great Xing’an Range (SGXR), is the largest Ag deposit in China. The SJS deposit can be divided into two ore blocks: the Shuangjianzishan ore block and the Xinglongshan ore block. Given the importance of the Xinglongshan ore block in the SJS deposit, our work is focused on the Xinglongshan ore block. The vein orebodies in the Xionglongshan ore block mainly occur in the NW-, NNW-, and NNE-trending fault zones, and its mineralization is mainly related to a deep concealed syenogranite. Here, we present new geochronology, isotope geochemistry, and fluid inclusion data for the Xinglongshan ore block and provide additional insights into the metallogenic mechanism of the deposit. The dating results show that the syenogranite related to the mineralization formed at approximately 137 Ma, which is coherent with some previous age determinations in sulfides from the ore deposit. The mineralization of the Xinglongshan ore block can be divided into four stages: sphalerite–arsenopyrite–pyrite–chalcopyrite–quartz stage (stage I), sphalerite–galena–pyrite–silver-bearing mineral–quartz stage (stage II), sphalerite–galena–silver-bearing mineral–quartz–calcite stage (stage III), and weakly mineralized quartz–calcite stage (stage IV). Four types of fluid inclusions (FIs) have been identified within quartz and calcite veins: liquid-rich, gas-rich, pure-liquid, and pure-gas FIs. The homogenization temperatures in the four stages exhibit a gradual decrease, with stage I ranging from 253 to 302 °C, stage II from 203 to 268 °C, stage III from 184 to 222 °C, and stage IV from 153 to 198 °C, respectively. The salinity for stages I, II, III, and IV falls within the ranges of 3.4–6.6 wt% NaCl eqv., 2.6–7.2 wt% NaCl eqv., 2.9–7.0 wt% NaCl eqv., and 1.2–4.8 wt% NaCl eqv., respectively, indicative of a low-salinity ore-forming fluid. The δ¹⁸O_water and δD values of the ore-forming fluid span from −13.9‰ to 7.4‰ and −145‰ to −65‰, with δ¹³C_V-PDB values between −11.0‰ and −7.9‰. These values suggest that the ore-forming fluid predominantly originated from a mixture of magmatic and meteoric water. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios of sulfides range from 18.278 to 18.361, 15.530 to 15.634, and 38.107 to 38.448, respectively. These ratios imply that the ore-forming material was primarily derived from the Early Cretaceous granitic magma, which resulted from the mixing of depleted mantle- and crustal-derived magmas. The fluid mixing was the dominant mechanism for mineral precipitation. The Xinglongshan ore block belongs to a magmatic-hydrothermal vein-type deposit related to the Early Cretaceous syenogranite, and the Shuangjianzishan ore block belongs to an intermediate sulfidation epithermal deposit related to coeval subvolcanic rocks. The Ag–Pb–Zn mineralization at Shuangjianzishan is genetically related to the Early Cretaceous volcanic–intrusive complex.

1. Introduction

Silver (Ag), lead (Pb), and zinc (Zn) are widely developed in different genetic types of deposits [1,2,3]. The Ag–Pb–Zn deposits in the world can be divided into the following genetic types: (1) volcanic-hosted massive sulfide (VHMS or VMS) deposit [4,5,6]; (2) sedimentary exhalative (SEDEX) deposit [7,8,9]; (3) carbonate-hosted Mississippi Valley type (MVT) deposit [10]; (4) skarn-type deposit [11,12,13]; (5) magmatic-hydrothermal vein-type deposit [14,15]; and (6) epithermal deposit [16,17,18]. In addition, a few researchers have identified porphyry-type Ag–Pb–Zn deposits [19]. The vein-type Ag–Pb–Zn deposits occupy an important position in the global supply of Ag and base metals of Pb–Zn [18] and have become the hotspot of deposit research in the world. The southern Great Xing’an Range (SGXR), located in the eastern section of the Central Asian Orogenic Belt, spanning approximately 600 km in the northeast direction with a width of around 200 km [20,21,22,23], represents the preeminent Ag–Pb–Zn–Sn metallogenic belt in northern China. To date, forty-two Pb–Zn–Ag deposits have been discovered in the SGXR (Figure 1a,b), boasting proven reserves totaling 8,130,000 tons of Pb + Zn and 57,000 tons of Ag [21,24,25,26]. This area is distinguished by three types of Pb–Zn–Ag deposits: skarn type, subvolcanic-related vein type, and intrusion-related vein type (as depicted in Figure 1b). Some of these deposits also exhibit the presence of Cu and Sn, such as the Hua’aobaote deposit [26]. The ore-forming element assemblages of these deposits are mainly Ag–Pb–Zn, Ag–Pb–Zn–Cu, and Ag–Pb–Zn–Sn–Cu [21,27]. Recently, many studies have shown that the mineralization age of these deposits and the formation age of magmatic rocks related to mineralization are concentrated in the Late Jurassic to Early Cretaceous, suggesting that the mineralization is closely related to the late Yanshanian magmatic activity in the SGXR [22,28,29,30,31,32]. Among these Ag–Pb–Zn deposits in the SGXR, the Shuangjianzishan (SJS) deposit is the largest one, and it contains proven reserves of 15,214 tons Ag, 392,200 tons Pb, and 1,523,000 tons Zn, with average grades of 139.3 g/t Ag, 1.03% Pb, and 1.46% Zn, respectively, making this deposit the largest Ag deposit in China [18,33]. According to the classification criteria for the size of Zn–Pb–Ag deposits proposed by Large et al. [8,34], the SJS deposit belongs to a supergiant deposit.

Figure 1. (a) Simplified geotectonic division of NE China (after [35]); (b) geological map of the southern Great Xing’an Range, highlighting the key Pb–Zn–Ag polymetallic deposit locations (modified after [27,36]). Notes: MOS = Mongol–Okhotsk suture zone; EGM = Ergun massif; SLB = Songliao basin; JMM = Jiamusi massif; SZM = Songnen–Zhangguangcai Range massif; NCC = North China craton; XMOB = Xar Moron orogenic belt; XAM = Xing’an massif. Detailed names of Pb–Zn–Ag–polymetallic deposits marked with numbers: 1 = Baiyinchagandongshan; 2 = Maodeng–Xiaogushan; 3 = Aobaoshan; 4 = Harchulutu; 5 = Bayanwula; 6 = Weilasituo; 7 = Bairendaba; 8 = Anle; 9 = Shidi; 10 = Dadi; 11 = Yonglong; 12 = Huangtuliang; 13 = Hadatu; 14 = Nasitai; 15 = Shalonggou; 16 = Xishijiangshan; 17 = Erbadi; 18 = Bianjiadayuan; 19 = Dajing; 20 = Hongguangmuchang; 21 = Daihuanggou; 22 = Chaowula; 23 = Wulanbaiqi; 24 = Baiyinnuoer; 25 = Nailinba; 26 = Shuangjianzishan; 27 = Aguihundelun; 28 = Baiyinwula; 29 = Biliutaibei; 30 = Taipingdi; 31 = Erdaoyingzi; 32 = Dongshan; 33 = Haobugao; 34 = Bujinhei; 35 = Shabulengshan; 36 = Huaaobaote; 37 = Zhamuqin; 38 = Fuxingtun; 39 = Meng’entaolegai; 40 = Maohuduger; 41 = Shuiquan; 42 = Aobaotu; 43 = Panjiaduan.

Figure 1. (a) Simplified geotectonic division of NE China (after [35]); (b) geological map of the southern Great Xing’an Range, highlighting the key Pb–Zn–Ag polymetallic deposit locations (modified after [27,36]). Notes: MOS = Mongol–Okhotsk suture zone; EGM = Ergun massif; SLB = Songliao basin; JMM = Jiamusi massif; SZM = Songnen–Zhangguangcai Range massif; NCC = North China craton; XMOB = Xar Moron orogenic belt; XAM = Xing’an massif. Detailed names of Pb–Zn–Ag–polymetallic deposits marked with numbers: 1 = Baiyinchagandongshan; 2 = Maodeng–Xiaogushan; 3 = Aobaoshan; 4 = Harchulutu; 5 = Bayanwula; 6 = Weilasituo; 7 = Bairendaba; 8 = Anle; 9 = Shidi; 10 = Dadi; 11 = Yonglong; 12 = Huangtuliang; 13 = Hadatu; 14 = Nasitai; 15 = Shalonggou; 16 = Xishijiangshan; 17 = Erbadi; 18 = Bianjiadayuan; 19 = Dajing; 20 = Hongguangmuchang; 21 = Daihuanggou; 22 = Chaowula; 23 = Wulanbaiqi; 24 = Baiyinnuoer; 25 = Nailinba; 26 = Shuangjianzishan; 27 = Aguihundelun; 28 = Baiyinwula; 29 = Biliutaibei; 30 = Taipingdi; 31 = Erdaoyingzi; 32 = Dongshan; 33 = Haobugao; 34 = Bujinhei; 35 = Shabulengshan; 36 = Huaaobaote; 37 = Zhamuqin; 38 = Fuxingtun; 39 = Meng’entaolegai; 40 = Maohuduger; 41 = Shuiquan; 42 = Aobaotu; 43 = Panjiaduan.

Previous researchers have extensively investigated the SJS deposit, covering a range of aspects such as the geological characteristics of the deposit [33], geochronology and petrogeochemistry of ore-forming rocks [18,37,38], mineralogical assemblages [39], and the origins of ore-forming fluids and materials [18,40]. Nonetheless, the mineralization age and the ore genesis of the SJS deposit still remain controversial. The mineralization age of the SJS deposit, as investigated by previous researchers, falls within a wide range of 160–130 Ma [18,24,27,33,37,39,40]. Furthermore, there is an ongoing debate regarding the types of mineralization in the SJS deposit, which primarily revolves around two main types: the epithermal type [18,37] and the magmatic-hydrothermal vein type [27,33]. Further study is needed on the origin and evolution of ore-forming fluids, source of ore-forming materials, mechanisms of metal precipitation, and genetic types of the SJS deposit. In this study, we present new data concerning the zircon U–Pb age, fluid inclusions, and H–O–C–Pb isotopes associated with the SJS deposit. These various data are used to decipher the source, feature, and evolution of the ore-forming fluid and to explore the metallogenic mechanism. As a result, we are able to constrain the genesis of the SJS Ag–Pb–Zn deposit.

2. Regional Geology

The SGXR is located in the central section of the Songnen–Zhangguangcai Range block (SZM, Figure 1a), and four boundaries of the east, west, north, and south are the Nenjiang–Balihan fault, East Ujimqin Banner–Xilinhot belt, Hegenshan fault, and Xar Moron fault, respectively. The SGXR underwent sequential tectonic control from the Paleo-Asian Ocean in the Paleozoic, followed by the Mongol–Okhotsk Ocean and Paleo-Pacific tectonic domains in the Mesozoic [27,41,42]. These tectonic transitions are documented as intricate geodynamic processes marked by multiple tectonic events. The geological formations visible in the SGXR comprise the Mesoproterozoic metamorphic rock series, Paleozoic marine volcanic–clastic rocks, Paleozoic continental clastic rocks, Mesozoic continental volcanic–clastic rocks, as well as Cenozoic basalt, and Quaternary loose sediment (Figure 1b). Prominent geological structures within the SGXR encompass the Ganzhuermiao anticline, Linxi syncline, Hegenshan fault, and Xar Moron fault, which originated during the Late Paleozoic [43,44], and the Nenjiang–Balihan fault formed during the Mesozoic [45,46], which constitute the basic tectonic framework of the SGXR. This region underwent widespread Variscan, Indosinian, and Yashanian volcanic and intrusive activity, and different types of granitoids emplaced at different stages are widely distributed. Most importantly, the Mesozoic I- and A-type granitoids comprise > 20% of the surface area in the SGXR [14], which was triggered by the subduction of the Paleo-Pacific plate, making the eastern Asian Mesozoic continental margin the most important metallogenic area [18]. Numerous studies showed that the Jurassic subduction of the Paleo-Pacific plate led to the extension of the back-arc area and the lithospheric delamination and that the subsequent rollback of the subduction plate triggered the large-scale emplacement of the Early Cretaceous volcanic–intrusive complex [47,48].

The SGXR hosts a number of intrusion-related vein types, subvolcanic-related vein types, and skarn-type polymetallic (Ag–Pb–Zn–Cu–Sn) deposits [18,22,27]. Regional mineralization is spatially and temporally closely related to the Jurassic to Cretaceous volcanic–intrusive complex [40]. These deposits are predominantly distributed along an approximately 600-km-long and 200-km-wide zone of NE-trending and are mainly hosted by the Permian strata and Mesozoic granites. With the discoveries of a large number of new Ag–Pb–Zn deposits recently [28,49], the Ag–Pb–Zn–Sn–Cu metallogenic belt of the SGXR has become more important.

3. Ore District and Deposit Geology

3.1. Ore District Geology

The supergiant SJS Ag–Pb–Zn deposit (44°29′30″–44°31′30″ N, 119°03′30″–119°10′00″ E) is located in the north of Baarin Left Banner, Chifeng City, Inner Mongolia Autonomous Region, which is tectonically located in the central part of the Songnen–Zhangguangcai Range massif (Figure 1a). The exposed lithological units in the SJS ore district can be seen in Figure 2.

Figure 2. Simplified regional geological map of the Shuangjianzishan area (modified after [25,33]).

Figure 2. Simplified regional geological map of the Shuangjianzishan area (modified after [25,33]).

The Permian strata, including the Shoushangou, Dashizhai, Zhesi, and Linxi formations, are mainly composed of marine clastic rocks and intermediate–felsic volcanic rocks, with a small amount of continental calstic rocks. The ore bodies of the SJS deposit are mainly hosted in the Dashizhai Formation, which is predominantly characterized by marine thick sandstone layers, along with andesite, silty slate, altered andesite, basalt, and andesitic tuff, with a total thickness exceeding 900 m. The Mesozoic strata, including the Xinmin, Manketouebo, Manitu, and Baiyin’gaolao formations, are mainly composed of continental intermediate–felsic volcanic rocks and pyroclastic rocks with a small amount of calstic rocks. The Yanshanian granites are exposed 2 km northwest of the SJS deposit and lie concealed in the central part of the ore district. The concealed granite bodies revealed by deep drilling include biotite granite, syenogranite, and granite. In addition, granite porphyry, diorite porphyry, and rhyolite porphyry occur as NW- or NE-trending veins, dipping SW at ~60° and typically measuring up to 300 m long and 8 m wide, some of which are crosscut by Ag–Pb–Zn mineralized veins. The main structures in the ore district include the NE-trending Yaoerya–Pangjiawan anticline, NW-trending fracture zone, and NNE- and NW-trending faults.

3.2. Ore Deposit Geology

The SJS Ag–Pb–Zn deposit was discovered in 2013 and was divided into two ore blocks, namely the western ore block (Shuangjianzishan ore block) and the eastern ore block (Xinglongshan ore block) [18] (Figure 2). The Xinglongshan ore block accounts for the vast majority of the total reserves of the SJS deposit [18,33]. A total of 276 industrial orebodies have been delineated in the block, including 39 large-sized, 32 medium-sized, and 205 small-sized orebodies [18,33]. However, only 4 industrial orebodies, including 1 medium-sized and 3 small-sized orebodies, have been delineated in the Shuangjianzishan ore block [18,33].

3.2.1. Xinglongshan Ore Block

The exposed strata in the Xinglongshan ore block include the Lower–Middle Permian Dashizhai Formation, Middle Jurassic Xinmin Formation, and Upper Jurassic Manketouebo Formation (Figure 3a). Except for a small amount of rhyolite porphyry veins exposed on the surface, other granitic intrusions, including granite, biotite granite, and syenogranite, are all concealed below 500 m of the ore block surface (Figure 3a,c). The fault structures, including NE-, NW-, and NEE-trending faults, are well-developed (Figure 3a). According to the direction of orebodies, the orebodies in the Xinglongshan ore block are mainly divided into (1) NW-trending Ag–Pb–Zn (Cu) ore vein group, (2) NNW-trending Ag–Pb–Zn orebodies, and (3) NNE-trending Ag–Pb–Zn orebodies. In addition, small amounts of near E–W-trending Au-bearing Ag–Pb–Zn orebodies are also developed in this ore block.

(1)

The NW-trending Ag–Pb–Zn (Cu) ore vein group: The orebodies in the ore vein group distributed in the central and eastern parts of the ore block are controlled by a NW-trending thick and large fractured zone. The ore-hosting rocks are slate and silty slate of the Lower–Middle Permian Dashizhai Formation (Figure 3b). The overall strike of the ore-vein group is 300°–310°, inclined towards the southwest, with dip angles of 50°–65° and a length > 2000 m and a width > 1200 m. The ore-vein group is mainly composed of Ag–Pb–Zn orebodies and Ag–Zn orebodies, with a small amount of Zn orebodies and Ag–Cu–Pb–Zn orebodies (Figure 3b). The individual orebodies occur as veins, usually having a length of 100–800 m and a thickness of 1–10 m, with a maximum thickness exceeding 100 m (Figure 3b). The ore structure is mainly of fine-vein, stockwork, disseminated, and dense disseminated (Figure 4a–d). The major wall-rock alteration includes silicification and chloritization, with minor sericitization (Figure 4a–d). These altered minerals mainly occur in the form of fine veins and stockworks within a thick NW-trending alteration zone, with minor veins and crumbs of quartz. The grade of the Ag–Pb–Zn orebodies is relatively low, with average grades of 98 g/t Ag, 1.6% Zn, and 0.6% Pb, respectively [23]. The Ag–Cu–Pb–Zn orebodies usually occur at the top of the ore-vein group, with a controlled length greater than 300 m and a thickness of 0.6–2.8 m. The Ag–Cu–Pb–Zn orebodies have average grades of 263 g/t Ag, 0.7% Cu, 2.2% Zn, and 0.9% Pb, respectively [23].

(2)

The NNW-trending Ag–Pb–Zn orebodies: These orebodies, mainly distributed in the central and eastern parts of the ore block, are controlled by NNW-trending faults. The overall trend of these orebodies is approximately 310°, with a thickness of 3–15 m and a length of > 400 m. The ore structure is mainly of massive and vein (Figure 4e,f). The wall-rock alteration is characterized by silicification and carbonation, with minor chloritization, and quartz and calcite mainly occur as fine veins within the orebodies and within a range of no more than two meters nearby (Figure 4e,f). The NW-trending Ag–Pb–Zn orebodies have average grades of 400 g/t Ag (some up to 10,000 g/t), 2.8% Zn, and 4.3% Pb, respectively [23].

(3)

The NNE-trending Ag–Pb–Zn orebodies: These orebodies, distributed in the eastern part of the ore block, are composed of five parallel orebodies. They are controlled by NNE-trending faults, which dip to NWW at > 65°. The ore structure is mainly of massive and vein (Figure 4g,h). The types and distribution characteristics of wall-rock alteration are similar to those of the NNW-trending orebodies. These orebodies have a thickness of 2–6 m and a control length > 600 m, with average grades of 400 g/t Ag, 2.5% Zn, and 3.2% Pb, respectively [23].

The near E–W-trending Au-bearing Ag–Pb–Zn orebodies occur as veins and are interspersed within the aforementioned NW-, NNW-, and NNE-trending Ag–Pb–Zn orebodies. The ore structure is mainly of fine veins and disseminated (Figure 4i). The wall-rock alteration is characterized by silicification and carbonation, which develop in the form of clumps and veins within the orebodies and nearby wall-rocks (Figure 4i).

3.2.2. Shuangjianzishan Ore Block

The exposed strata in the Shuangjianzishan ore block include the Lower–Middle Permian Dashizhai Formation and Upper Jurassic Manketouebo Formation (Figure 2). The ore block is located within a Yanshanian volcanic edifice, and no intrusives are exposed. The orebodies are controlled by two sets of NW- and NE-trending faults, which belong to the derived structures of the volcanic edifice. The NW-trending faults dip to NE at approximately 50°, and NE-trending faults dip to SE at 57°–61°. The NW-trending #501 orebody is the largest in the Shuangjianzishan ore block. Its ore-hosting rocks consist of andesitic tuff, breccia tuff, altered andesite, and argillaceous slate. The orebody is 332 m long, with an inclined depth of 240 m and a thickness of 4.88 m. Its average grades are 99 g/t Ag, 1.0% Pb, and 0.8% Zn, respectively.

Figure 3. (a) Geologic map of the Xinglongshan ore block (modified after [39]); (b) simplified geological section along the 00-exploration line, showing the occurrence of the NW-trending Ag–Pb–Zn (Cu) ore vein group (modified after [39]); (c) log of drill no. ZK12-37, showing the distribution of granites (modified after [18]).

Figure 3. (a) Geologic map of the Xinglongshan ore block (modified after [39]); (b) simplified geological section along the 00-exploration line, showing the occurrence of the NW-trending Ag–Pb–Zn (Cu) ore vein group (modified after [39]); (c) log of drill no. ZK12-37, showing the distribution of granites (modified after [18]).

Figure 4. Representative photos of orebodies in the Xinglongshan ore block. (a) fine-vein Ag–Pb–Zn orebody of the NW-trending ore vein group; (b) stockwork Pb–Zn orebody of the NW-trending ore vein group orebody; (c) disseminated Pb–Zn ore of the NW-trending ore vein group, with silification; (d) dense disseminated Cu–polymetallic ore of the NW-trending ore vein group, with silification; (e) high-grade Ag–Pb–Zn orebody of the NNW-trending orebodies, with silification; (f) Ag–Pb–Zn orebody of the NNW-trending orebodies, with silification and carbonation; (g) high-grade Ag–Pb–Zn orebody of the NNE-trending orebodies; (h) high-grade Ag–Pb–Zn orebody of the NNE-trending orebodies, with silification and carbonation; (i) Au-bearing Ag–Pb–Zn orebody with silification and carbonation, occurring in the near E–W-trending faults. Abbreviations: Cal = calcite; Ccp = chalcopyrite; Gn = galena; Kut = kustelite; Qtz = quartz; Sp = sphalerite.

Figure 4. Representative photos of orebodies in the Xinglongshan ore block. (a) fine-vein Ag–Pb–Zn orebody of the NW-trending ore vein group; (b) stockwork Pb–Zn orebody of the NW-trending ore vein group orebody; (c) disseminated Pb–Zn ore of the NW-trending ore vein group, with silification; (d) dense disseminated Cu–polymetallic ore of the NW-trending ore vein group, with silification; (e) high-grade Ag–Pb–Zn orebody of the NNW-trending orebodies, with silification; (f) Ag–Pb–Zn orebody of the NNW-trending orebodies, with silification and carbonation; (g) high-grade Ag–Pb–Zn orebody of the NNE-trending orebodies; (h) high-grade Ag–Pb–Zn orebody of the NNE-trending orebodies, with silification and carbonation; (i) Au-bearing Ag–Pb–Zn orebody with silification and carbonation, occurring in the near E–W-trending faults. Abbreviations: Cal = calcite; Ccp = chalcopyrite; Gn = galena; Kut = kustelite; Qtz = quartz; Sp = sphalerite.

3.2.3. Ore Mineralogy and Textures of the Xinglongshan Ore Block

The ore minerals in the NW-trending ore vein group are mainly sphalerite, arsenopyrite, and pyrite, followed by galena, chalcopyrite, and pyrrhotite, with minor canfieldite and cassiterite (Figure 5a–d). The major gangue minerals are quartz, followed by chlorite, with minor sericite (Figure 5m,p). The ore texture includes anhedral granular, subhedral granular, euhedral granular, metasomatic dissolution, poikilitic, and exsolution textures (Figure 5a–d). Sphalerite is chiefly of anhedral granular texture (Figure 5a–c) and occasionally involves cassiterite particles, developing a poikilitic texture (Figure 5d); arsenopyrite is mainly of subhedral–euhedral granular texture (Figure 5c), with minor anhedral granular texture (Figure 5a); pyrite is mainly of anhedral granular texture, followed by subhedral–euhedral granular texture (Figure 5b); galena, pyrrhotite, canfieldite, and cassiterite are chiefly of anhedral granular texture (Figure 5a,c,d); chalcopyrite occurs as emulsion droplet within sphalerite, developing an exsolution texture (Figure 5a,b), and chalcopyrite is generally replaced by galena and canfieldite, developing a metasomatic dissolution texture (Figure 5c).

The ore minerals in the NNW-trending orebodies are mainly sphalerite and galena, followed by freibergite, polybasite, pyrargyrite, and aguilarite, with minor canfieldite, chalcopyrite, and argentite (Figure 5e–h). The major gangue minerals are mainly quartz and chlorite (Figure 5n,p). The ore texture includes anhedral granular, metasomatic dissolution, and exsolution textures (Figure 5e–h). Chalcopyrite occurs as an emulsion droplet within sphalerite, developing an exsolution texture (Figure 5e,f); galena is often replaced by freibergite and polybasite (Figure 5e), chalcopyrite and pyrargyrite (Figure 5f), and canfieldite and aguilarite (Figure 5g), developing a metasomatic dissolution texture.

The ore minerals in the NNE-trending orebodies are mainly sphalerite and galena, followed by pyrite, freibergite, polybasite, pyrargyrite, canfieldite, and argentite (Figure 5i–k). The gangue minerals are mainly quartz and calcite (Figure 5o,p), with minor chlorite. The ore texture includes anhedral granular, metasomatic dissolution, poikilitic, and exsolution textures (Figure 5i–k). Canfieldite replaces galena along the edge of galena, developing a metasomatic dissolution texture (Figure 5i); chalcopyrite occurs as an emulsion droplet within sphalerite, developing an exsolution texture (Figure 5j); sphalerite involves cassiterite and galena, developing a poikilitic texture, and canfieldite replaces galena and sphalerite, developing a metasomatic dissolution texture (Figure 5k).

The ore minerals in the near E–W-trending Au-bearing Ag–Pb–Zn orebodies are mainly sphalerite, galena, pyrite, pyrargyrite, freibergite, argentite, and polybasite, with minor native silver and kustelite (Figure 5l). The gangue minerals are mainly quartz and calcite (Figure 5p). The ore texture includes anhedral granular and metasomatic dissolution textures (Figure 5l).

Figure 5. Representative photos of ores and major wall-rock alteration types in the Xinglongshan ore block. (a–d) Mineral assemblages of the NW-trending ore vein group, including sphalerite, arsenopyrite, pyrite, chalcopyrite, galena, pyrrhotite, canfieldite, and cassiterite; (e–h) mineral assemblages of the NNW-trending orebodies, including sphalerite, galena, freibergite, polybasite, pyrargyrite, aguilarite, canfieldite, chalcopyrite, and argentite; (i–k) mineral assemblages of the NNE-trending orebodies, including sphalerite, galena, pyrite, freibergite, polybasite, pyrargyrite, canfieldite, and argentite; (l) kustelite occurring within quartz and/or freibergite in the near E–W-trending Au-bearing Ag–Pb–Zn orebodies; (m) sericitization and silicification in stage I, superimposed by later carbonation; (n) silicification and chloritization developing in stages II and III, superimposed by later carbonation; (o) silicification and carbonation developing in stage III; (p) vein bodies cutting relationship, displaying quartz vein of stage I, quartz–pyrite vein of stage II, and quartz–calcite–pyrite vein of stage III. (a–c,e,f,i,j,l): plainlight under reflected light; (d,g,h,k): back-scatter electron (BSE) images; (m) and (o): cross-polarized light under transmitted light; (n) and (p): plainlight under transmitted light. Abbreviations: Agl = aguilarite; Apy = arsenopyrite; Arg = argentite; Caf = canfieldite; Cal = calcite; Ccp = chalcopyrite; Chl = chlorite; Cst = cassiterite; Fre = freibergite; Gn = galena; Kut = kustelite; Po = pyrrhotite; Pol = polybasite; Py = pyrite; Pyr = pyrargyrite; Qtz = quartz; Ser = sericite; Sp = sphalerite.

Figure 5. Representative photos of ores and major wall-rock alteration types in the Xinglongshan ore block. (a–d) Mineral assemblages of the NW-trending ore vein group, including sphalerite, arsenopyrite, pyrite, chalcopyrite, galena, pyrrhotite, canfieldite, and cassiterite; (e–h) mineral assemblages of the NNW-trending orebodies, including sphalerite, galena, freibergite, polybasite, pyrargyrite, aguilarite, canfieldite, chalcopyrite, and argentite; (i–k) mineral assemblages of the NNE-trending orebodies, including sphalerite, galena, pyrite, freibergite, polybasite, pyrargyrite, canfieldite, and argentite; (l) kustelite occurring within quartz and/or freibergite in the near E–W-trending Au-bearing Ag–Pb–Zn orebodies; (m) sericitization and silicification in stage I, superimposed by later carbonation; (n) silicification and chloritization developing in stages II and III, superimposed by later carbonation; (o) silicification and carbonation developing in stage III; (p) vein bodies cutting relationship, displaying quartz vein of stage I, quartz–pyrite vein of stage II, and quartz–calcite–pyrite vein of stage III. (a–c,e,f,i,j,l): plainlight under reflected light; (d,g,h,k): back-scatter electron (BSE) images; (m) and (o): cross-polarized light under transmitted light; (n) and (p): plainlight under transmitted light. Abbreviations: Agl = aguilarite; Apy = arsenopyrite; Arg = argentite; Caf = canfieldite; Cal = calcite; Ccp = chalcopyrite; Chl = chlorite; Cst = cassiterite; Fre = freibergite; Gn = galena; Kut = kustelite; Po = pyrrhotite; Pol = polybasite; Py = pyrite; Pyr = pyrargyrite; Qtz = quartz; Ser = sericite; Sp = sphalerite.

3.2.4. Mineralization Stages

Based on mineral assemblages, ore fabrics, and crosscutting relationships between the veins, the ore-forming process in the Xinglongshan ore block can be divided into four stages (Figure 6): (1) sphalerite–arsenopyrite–pyrite–chalcopyrite–quartz stage (stage I); (2) sphalerite–galena–pyrite–silver-bearing minerals–quartz stage (stage II); (3) sphalerite–galena–silver-bearing minerals–quartz–calcite stage (stage III); and (4) weakly mineralized quartz–calcite stage (stage IV). The ore minerals of stage I are mainly iron sphalerite, arsenopyrite, and pyrite, followed by chalcopyrite, galena, pyrrhotite, and aguilarite, with minor canfieldite, freibergite, polybasite, and cassiterite. The ore minerals of stage II are mainly sphalerite and galena, followed by pyrite, aguilarite, canfieldite, freibergite, polybasite, and pyrargyrite, with minor pyrrhotite, arsenopyrite, and chalcopyrite. The ore minerals of stage III are mainly sphalerite and galena, followed by pyrite, canfieldite, freibergite, polybasite, pyrargyrite, and argentite. In addition, kustelite and native silver can be observed within Au-bearing Ag–Pb–Zn ores of stage III. The ore minerals of stage IV are dominated by pyrite, with minor sphalerite, galena, argentite, and native silver.

Figure 6. The paragenetic sequence of minerals in the Xinglongshan ore block.

Figure 6. The paragenetic sequence of minerals in the Xinglongshan ore block.

4. Sampling and Analytical Methods

4.1. Sampling

In this research, we carefully handpicked 37 samples sourced from diverse mining levels and drill holes in the Xinglongshan ore block. These samples were chosen to undergo a range of analytical processes, including U-Pb dating, fluid inclusion analysis, and H–O–C–Pb isotope investigations.

These samples, mainly medium-coarse-grained syenogranite specimens, characterized by their predominant composition of K-feldspar (50%), quartz (35%), plagioclase (10%), and biotite (5%), were carefully selected for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating, as indicated in

Table 1. List of analytical samples for the Xinglongshan ore block.

Sample	Orebody No./Lithology	Position	Stage	Mineral	Analysis
SJ49-1	Syenogranite	Drill No. ZK12-37 at 780 m deep		Zrn	U–Pb dating
SJ49-10	Syenogranite	Drill No. ZK12-37 at 1071 m deep		Zrn	U–Pb dating
SJ14	No. 2 Pb–Zn–Cu orebody	625-m level	I	Qtz	FIs study
SJ6	Disseminated Pb–Zn–Ag orebody	625-m level	II	Qtz	FIs study
SJ31	No. 2 Pb–Zn–Ag orebody	Drill No. ZK1501 at 1207 m deep	II	Qtz	FIs study
SJ11	No. 1-3 Pb–Zn–Ag orebody	625-m level	III	Qtz	FIs study
SJ12	No. 2 Pb–Zn–Ag orebody	625-m level	III	Qtz	FIs study
SJ13-1	Pb–Zn–Ag–Au orebody	625-m level	III	Cal	FIs study
SJ8	No. 2 Pb–Zn–Ag mineralized orebody	625-m level	IV	Qtz	FIs study
SJ45	No. 2-16 Pb–Zn–Ag mineralized body	665-m level	IV	Cal	FIs study
2-3m-3	No. 2-3 Pb–Zn–Cu orebody	625-m level	I	Qtz	H–O isotopes
8-16m-1	No. 8-16 Pb–Zn–Cu orebody	665-m level	I	Qtz	H–O isotopes
1m-3	No. 1 Pb–Zn–Cu orebody	625-m level	I	Qtz	H–O isotopes
8-16m-3	No. 8-16 Pb–Zn–Cu orebody	665-m level	I	Qtz	H–O isotopes
8-16m-4	No. 8-16 Pb–Zn–Cu orebody	665-m level	I	Qtz	H–O isotopes
2-3m-7	No. 2-3 Pb–Zn–Cu orebody	625-m level	I	Qtz	H–O isotopes
2-3m-6	No. 2-3 Pb–Zn–Ag orebody	625-m level	II	Qtz	H–O isotopes
1-5-1m-2	No. 1-5-1 Pb–Zn–Ag–Au orebody	625-m level	II	Qtz	H–O isotopes
5#KD-2-1	No. 2 Pb–Zn–Ag orebody	625-m level	II	Qtz	H–O isotopes
5#KD-2-2	No. 2 Pb–Zn–Ag orebody	625-m level	II	Qtz	H–O isotopes
ZD4-1-1m-1	No. 1-1 Pb–Zn–Ag orebody	665-m level	III	Cal	H–O–C isotopes
ZD4-1-1m-2	No. 1-1 Pb–Zn–Ag orebody	665-m level	III	Cal	H–O–C isotopes
ZD4-1-1m-3	No. 1-1 Pb–Zn–Ag orebody	665-m level	III	Cal	H–O–C isotopes
ZD4-1-1m-4	No. 1-1 Pb–Zn–Ag orebody	665-m level	III	Qtz	H–O isotopes
ZD4-1-1m-4	No. 1-1 Pb–Zn–Ag orebody	665-m level	III	Cal	H–O–C isotopes
5#KD-1	No. 1-1 Pb–Zn–Ag mineralized body	665-m level	IV	Qtz	H–O isotopes
8-12m-1	No. 8-12 Pb–Zn–Ag mineralized body	665-m level	IV	Cal	H–O–C isotopes
8-12m-2	No. 8-12 Pb–Zn–Ag mineralized body	665-m level	IV	Qtz	H–O isotopes
8-12m-2	No. 8-12 Pb–Zn–Ag mineralized body	665-m level	IV	Cal	H–O–C isotopes
8-12m-3	No. 8-12 Pb–Zn–Ag mineralized body	665-m level	IV	Cal	H–O–C isotopes
2-3m-1	No. 2-3 Pb–Zn–Cu orebody	625-m level	I	Gn, Sp, Py	Pb isotope
2m-3	No. 2-3 Pb–Zn–Cu orebody	625-m level	I	Gn, Sp, Py	Pb isotope
1m-1	No. 1 Pb–Zn–Cu orebody	625-m level	I	Gn, Sp, Py	Pb isotope
ZD5-1-1m-1	No. 1-1 Pb–Zn–Ag orebody	625-m level	II	Gn, Sp	Pb isotope
ZD4-1-1m-1	No. 1-1 Pb–Zn–Ag orebody	665-m level	III	Gn, Sp, Py	Pb isotope
8-16m-2	No. 8-16 Pb–Zn–Ag mineralized body	665-m level	IV	Gn, Sp, Py	Pb isotope
8-12m-1	No. 8-12 Pb–Zn–Ag mineralized body	665-m level	IV	Gn, Sp, Py	Pb isotope

Abbreviations: Cal = calcite; Gn = Galena; Py = pyrite; Qtz = quartz; Sp = sphalerite; Zrn = zircon.

(refer to Figure 3c and Figure 7). For fluid inclusion (FI) analysis, we collected six quartz samples and two calcite samples, each sourced from four distinct mineralization stages, as outlined in Table 1. In addition, thirteen quartz samples obtained from these mineralization stages underwent H–O isotope analysis, while seven calcite samples, specifically from stages III and IV, were subjected to H–O–C isotope analysis. Detailed information about these samples can be found in Table 1. Furthermore, our investigation extended to the analysis of twenty sulfide samples derived from seven different samples across the four mineralization stages. This set of samples encompassed seven galena, seven sphalerite, and six pyrite specimens. A comprehensive overview of these samples can be found in Table 1, alluding to the extensive and thorough nature of our analytical approach.

Figure 7. Field (a) and microscopic (b) photos of concealed syenogranite in the Xinglongshan ore block.

Figure 7. Field (a) and microscopic (b) photos of concealed syenogranite in the Xinglongshan ore block.

4.2. Syenogranite Zircon U–Pb Dating

Zircon grains were separated through traditional heavy liquid and magnetic separation techniques. Following this, they underwent meticulous purification through handpicking under a binocular microscope, a process carried out at the Langfang Chengxin Geological Service Co., Ltd. in Hebei Province, China. The handpicked zircon grains were mounted in epoxy and polished to expose the cores of the grains for subsequent cathodoluminescence (CL) imaging and zircon U–Pb analyses. The CL images were obtained at the Zircon Navigation Technology Co., Ltd., Beijing, China. The LA-ICP-MS zircon U–Pb dating was undertaken at the Yandu Zhongshi Testing Technology Co., Ltd., Beijing, China, using a Finnigan Neptune MC-ICP-MS attached to a New Wave UP 213 laser ablation system with an in-house sample cell. The detailed analytical procedures were similar to those described by Griffin et al. [50] and Hou et al. [51]. U–Pb fractionation was corrected using zircon standard GEMOC GJ-1 (²⁰⁷Pb/²⁰⁶Pb age of 608.5 ± 1.5 Ma, [52]), and accuracy was controlled using zircon standard Mud Tank (intercept age of 732 ± 5 Ma, [53]). U–Pb ages were calculated from raw signal data using ISOPLOT 3.0 [54]. Because ²⁰⁴Pb could not be measured due to a low signal and interference from ²⁰⁴Hg in the gas supply, a common lead correction was carried out using the EXCEL program ComPbCorr#3.15G [55]. Errors on individual analyses by LA-ICP-MS were quoted at the 1σ level, while errors on pooled ages were quoted at the 95% (2σ) confidence level.

4.3. Fluid Inclusion Microthermometry and Laser Raman Spectroscopy

Microthermometric measurements of fluid inclusions (FIs) were conducted at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, located in Beijing, China. This process utilized a LINKAM THMSG 600 programmable heating-freezing stage paired with a high-quality German Zeiss microscope. The homogenization temperature spanned from −190 °C to 600 °C, with estimated accuracies of ±0.1 °C between −100 °C and 25 °C, ±1 °C from 25 °C to 400 °C, and ±2 °C above 400 °C, respectively. The heating rate during testing typically ranged from 0.2 °C to 5 °C per minute, with a reduction to 0.2 °C per minute near phase transitions. To determine the salinity of the gas–liquid two-phase aqueous solution inclusions, ice-melting temperatures were employed [56], and the density of the fluid inclusions was calculated using Flincor software (version 1.4) [57]. Additionally, the volatile compositions of individual fluid inclusions were identified using a Renishaw RM-2000 Raman probe located at the Institute of Mineral Resources, Chinese Academy of Geological Sciences in Beijing, China. The excitation wavelength utilized was 514.53 nm from an argon laser, with a measurement spectrum time of 20 s. The counting rate remained consistent at one per centimeter, maintaining precision throughout. The laser beam was consistently set at a 1 μm size, offering a spectral resolution within the range of 1–2 cm⁻¹. Spectral data were acquired over the extensive wavenumber span of 1000–4000 cm⁻¹, ensuring comprehensive coverage.

4.4. H–O–C–Pb Isotope Analyses

4.4.1. H–O–C Isotope Analyses

All the samples were smashed at the Langfang Chengxin Geological Service Co., Ltd., Hebei Province, China. The mineral separates were extracted and handpicked under a binocular microscope to achieve a purity of 99%. Isotope analyses for H–O–C were conducted utilizing a MAT 251EM mass spectrometer located at the Beijing Research Institute of Uranium Geology in China. The analytical precision was outstanding, with results surpassing the following margins: ±2‰ for δD, ±0.2‰ for δ¹⁸O, and ±0.1‰ for δ¹³C. In the case of quartz, O isotope analysis was performed using the BrF₅ method to extract CO₂ [58]. The calcite samples reacted with phosphoric acid at 25 °C to produce CO₂ for the C–O isotope analyses of calcite. The H isotope of water in fluid inclusions of quartz and calcite was measured for vapors released from FIs in quartz and calcite grains with the thermal crack method. Hydrogen, produced by the reaction of the released water and zinc at 400 °C [59], was used for H isotope analysis. The δ¹⁸O of water for quartz was calculated from the O isotopes of quartz by using the fractionation equation 10001n a_{quartz–water} = (3.38 × 10⁶) T⁻² − 3.40, and the δ¹⁸O of water for calcite was calculated from the O isotopes of calcite by using the fractionation equation 10001n a_{calcite–water} = (4.01 × 10⁶) T⁻² − (4.66 × 10⁶) T⁻¹ + 1.71 [60], where the T (in Kelvin) represents the average fluid inclusion homogeneous temperature associated with the mineralization stage.

4.4.2. Pb Isotope Analyses

The procedure for isolating individual sulfides followed the methodology outlined in Section 4.4.1. Subsequently, the Pb isotope composition of the sulfides was determined at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology, China, using a GV IsoProbe-T multi-collector thermal ionization mass spectrometer. Remarkably, the measurement precision for both ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios exceeded 0.005‰ (2σ). The isotopic ratios were reported relative to the Pb standard reference NBS-981 values, as follows: ²⁰⁶Pb/²⁰⁴Pb = 16.934 ± 0.007, ²⁰⁷Pb/²⁰⁴Pb = 15.486 ± 0.012, and ²⁰⁸Pb/²⁰⁴Pb = 36.673 ± 0.033, respectively [61].

5. Results

5.1. Syenogranite Zircon U–Pb Dating

Fifteen zircon grains and twenty zircon grains from the syenogranite samples SJ49-1 and SJ49-10, respectively, were selected for LA-ICP-MS U–Pb dating. Zircon U–Pb compositions were analyzed based on the texture shown on CL images (Figure 8a,b). The analytical data are presented in

Table 2. LA-ICP-MS zircon U–Pb dating data for the syenogranite in the Shuangjianzishan area.

Spot No.	Isotopic Ratios						Age (Ma)
	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	238U/206Pb	1σ	206Pb/238U	1σ
Sample SJ49-1
SJ49-1-5	0.05207	0.00419	0.15345	0.00467	0.02148	0.00034	145	11	137	3
SJ49-1-6	0.05151	0.00219	0.15133	0.00427	0.02143	0.00029	143	6	137	2
SJ49-1-7	0.05404	0.00366	0.15977	0.00201	0.02136	0.00036	151	11	136	2
SJ49-1-8	0.05460	0.00363	0.16211	0.00490	0.02168	0.00035	153	9	138	3
SJ49-1-9	0.04866	0.00210	0.14663	0.00447	0.02204	0.00039	139	6	141	2
SJ49-1-10	0.05433	0.00359	0.16148	0.00411	0.02166	0.00032	152	10	138	3
SJ49-1-11	0.05142	0.00149	0.15133	0.00442	0.02139	0.00030	143	4	136	2
SJ49-1-12	0.05311	0.00273	0.15878	0.00418	0.02162	0.00034	150	7	138	2
SJ49-1-13	0.05426	0.00331	0.16184	0.00580	0.02173	0.00033	152	9	139	5
SJ49-1-14	0.05297	0.00267	0.15271	0.00588	0.02122	0.00037	144	6	135	2
SJ49-1-15	0.05326	0.00183	0.16031	0.00529	0.02201	0.00032	151	5	140	2
Sample SJ49-10
SJ49-10-1	0.05360	0.00234	0.15744	0.00366	0.02150	0.00036	354	98	149	6
SJ49-10-2	0.05298	0.00273	0.15447	0.00351	0.02152	0.00036	328	117	146	7
SJ49-10-3	0.05103	0.00178	0.15186	0.00530	0.02159	0.00033	243	77	144	5
SJ49-10-4	0.05306	0.00211	0.16012	0.00674	0.02171	0.00027	332	91	151	6
SJ49-10-5	0.05211	0.00203	0.15586	0.00647	0.02165	0.00035	300	89	147	6
SJ49-10-6	0.05381	0.00304	0.16331	0.00512	0.02198	0.00037	365	128	154	8
SJ49-10-7	0.05146	0.00127	0.15120	0.00409	0.02123	0.00030	261	53	143	4
SJ49-10-8	0.05294	0.00370	0.15308	0.00681	0.02095	0.00047	328	155	145	9
SJ49-10-9	0.05087	0.00207	0.15079	0.00669	0.02135	0.00032	235	94	143	6
SJ49-10-10	0.05095	0.00659	0.15175	0.00241	0.02123	0.00048	239	283	144	21
SJ49-10-11	0.05309	0.00244	0.16196	0.00318	0.02201	0.00039	332	101	152	7
SJ49-10-12	0.05295	0.00226	0.15167	0.00605	0.02077	0.00046	328	94	143	5
SJ49-10-13	0.05161	0.00190	0.14800	0.00525	0.02093	0.00029	333	81	140	5
SJ49-10-14	0.05390	0.00150	0.16319	0.00505	0.02185	0.00034	369	63	154	4
SJ49-10-15	0.05386	0.00215	0.15455	0.00616	0.02084	0.00027	365	91	146	5
SJ49-10-16	0.05468	0.00306	0.16463	0.00531	0.02207	0.00042	398	94	155	7
SJ49-10-17	0.05391	0.00209	0.16044	0.00622	0.02162	0.00038	369	89	151	5
SJ49-10-18	0.05461	0.00307	0.15926	0.00573	0.02129	0.00036	395	126	150	8
SJ49-10-19	0.05024	0.00245	0.14607	0.00689	0.02130	0.00033	206	118	138	6
SJ49-10-20	0.05368	0.00154	0.16087	0.00483	0.02167	0.00028	367	60	152	4

and are plotted on Concordia diagrams (Figure 8c,d).

Zircon grains retrieved from the syenogranite exhibit euhedral–subhedral shapes and demonstrate distinctive oscillatory zoning in CL images (Figure 8a,b), which, combined with their relatively high Th/U ratios (0.25–0.58), indicates their magmatic origin. Fifteen analyses from sample SJ49-1 form a tight cluster on a concordial diagram and yield a weighted mean ²⁰⁶Pb/²³⁸U age of 137 ± 1.4 Ma (MSWD = 1.5; Figure 8c), and twenty analyses from sample SJ49-10 form a tight cluster on a concordial diagram and yield a weighted mean ²⁰⁶Pb/²³⁸U age of 137 ± 1.2 Ma (MSWD = 1.6; Figure 8d), suggesting that the syenogranite formed during the Early Cretaceous.

Figure 8. Representative cathodoluminescence (CL) images of zircon grains for samples sj49-1 (a) and sj49-10 (b) from the Shuangjianzishan syenogranite, showing U–Pb analytical spots and corresponding ²⁰⁶Pb/²³⁸U ages, and zircon U–Pb concordia diagrams of samples sj49-1 (c) and sj49-10 (d).

Figure 8. Representative cathodoluminescence (CL) images of zircon grains for samples sj49-1 (a) and sj49-10 (b) from the Shuangjianzishan syenogranite, showing U–Pb analytical spots and corresponding ²⁰⁶Pb/²³⁸U ages, and zircon U–Pb concordia diagrams of samples sj49-1 (c) and sj49-10 (d).

5.2. Fluid Inclusion Data

5.2.1. Petrography

A fluid inclusion assemblage (FIA) is defined as a collection of fluid inclusions that were contemporaneously trapped, enabling us to obtain more reliable and precise data through fluid inclusion microthermometry using the FIA approach [62]. In this research, the criteria put forth by Goldstein and Reynolds [62] were applied to designate fluid inclusions that occur in close proximity or isolated positions as having a primary origin, thereby classifying them as FIA. Here, we focus on the primary inclusions (Figure 9), of which there are four types in the Xinglongshan ore block, using criteria such as phase ratios at room temperature (21 °C), phase transitions observed during heating and cooling, and findings from laser Raman spectroscopy.

(1)

Liquid-rich inclusions (WL-type): Within the examined quartz and calcite samples from all stages, these fluid inclusions are prevalent, constituting approximately 85% of the total number of inclusions. They exhibit elliptical, elongated columnar, and irregular shapes, with long axes ranging from 5 to 35 μm. Notably, the bubbles within them occupy 5%–45% of the total volume at room temperature (Figure 9a–f). When subjected to heating, these fluid inclusions underwent homogenization, transitioning into a liquid phase.

(2)

Gas-rich inclusions (WG-type): Exclusively found in quartz from stage I, these inclusions make up approximately 5% of the total number of fluid inclusions, with long axes ranging from 45 to 60 μm. WG-type inclusions are typically oval or circular in shape, with bubbles occupying 55%–70% of their total volume (Figure 9b). Upon heating, these inclusions homogenized into a vapor phase.

(3)

Pure gas inclusions (G-type): Predominantly present in quartz from stage I, these inclusions measure 5–10 μm in size and display irregular or round shapes (Figure 9a). They account for 3% of the total number of fluid inclusions and remain in a gaseous phase at room temperature, undergoing no phase change when heated.

(4)

Pure liquid inclusions (L-type): Primarily hosted within quartz and calcite from stages III and IV, these inclusions exhibit irregular and negative crystal shapes and have a size of 5–10 μm (Figure 9f). They constitute approximately 7% of the total number of fluid inclusions and remain in a liquid phase at room temperature. Upon heating, the L-type inclusions do not undergo any phase change.

Figure 9. Illustrative microphotographs of fluid inclusions within quartz and calcite crystals sourced from the Xinglongshan ore block. (a) G- and WL-type FIs in quartz of stage I; (b) WG- and WL-type FIs in quartz of stage I; (c) WL-type FIs in quartz of stage II; (d) WL-type FIs in quartz of stage II; (e) WL-type FIs in quartz of stage III; (f) WL- and L-type FIs in calcite of stage IV. L_H2O = liquid phase H₂O; V_H2O = vapor phase H₂O.

Figure 9. Illustrative microphotographs of fluid inclusions within quartz and calcite crystals sourced from the Xinglongshan ore block. (a) G- and WL-type FIs in quartz of stage I; (b) WG- and WL-type FIs in quartz of stage I; (c) WL-type FIs in quartz of stage II; (d) WL-type FIs in quartz of stage II; (e) WL-type FIs in quartz of stage III; (f) WL- and L-type FIs in calcite of stage IV. L_H2O = liquid phase H₂O; V_H2O = vapor phase H₂O.

5.2.2. Microthermometry

The microthermometric results, along with the parameters of the FIA, are presented in

Table 3. Microthermometric data and relevant parameters of the fluid inclusion assemblage (FIA) in quartz and calcite from the Xinglongshan ore block.

Type	Host Mineral	FIA No.	No.	Size (μm)	V (vol.%)	Tm (ice) (°C)	Th (°C)		Salinity (wt% NaCl Eqv.)	Density (g/cm3)
						Range	Range	Mean	Range
Stage I: sphalerite–arsenopyrite–pyrite–chalcopyrite–quartz stage (sample SJ14)
WL	Quartz	1	6	5–30	5–40	−3.2 to −1.9	278–302	288	3.4–5.3	0.74–0.79
WL	Quartz	2	5	10–30	10–35	−3.2 to −2.2	253–274	263	3.7–5.3	0.80–0.83
WG	Quartz	3	3	45–60	55–70	−4.1 to −2.4	279–289	284	4.0–6.6	0.79–0.80
Stage II: sphalerite–galena–pyrite–silver-bearing minerals–quartz stage (samples SJ6 and SJ31)
WL	Quartz	4	11	5–30	10–25	−4.5 to −2.5	224–248	238	3.9–7.2	0.84–0.88
WL	Quartz	5	16	10–35	5–45	−4.2 to −1.6	229–268	238	3.2–6.7	0.83–0.87
WL	Quartz	6	6	10–30	5–20	−2.3 to −1.8	203–221	220	2.6–3.9	0.80–0.88
Stage III: sphalerite–galena–silver-bearing minerals–quartz–calcite stage (samples SJ11, SJ12, and SJ13-1)
WL	Quartz	7	20	10–35	5–20	−3.5 to −1.8	200–222	212	3.1–5.7	0.86–0.91
WL	Quartz	8	12	10–30	10–20	−2.8 to −1.9	200–216	208	3.2–4.6	0.87–0.90
WL	Calcite	9	12	10–30	10–20	−4.4 to −1.7	184–199	192	2.9–7.0	0.89–0.92
Stage IV: weakly mineralized quartz–calcite stage (samples SJ8 and SJ45)
WL	Quartz	10	4	15–20	5–20	−2.2 to −1.5	185–198	191	2.6–3.7	0.89–0.91
WL	Calcite	11	9	10–35	10–20	−2.9 to −0.7	153–187	176	1.2–4.8	0.90–0.93

Abbreviations: T_{m (ice)} = final melting temperature of ice; T_h = total homogenization temperature; V = volume fraction of gas phase in the total volume of inclusion.

and depicted in Figure 10.

(1)

FIs in quartz of stage I (sample SJ14): Three FIAs have been recognized. The homogenization temperature of two WL-type FIAs varies from 253 °C to 302 °C (Figure 10a), and the final ice melting temperature is −3.2 °C to −1.9 °C, corresponding to salinities of 3.4–5.3 wt% NaCl eqv. (Figure 10b), and the density of the fluid is 0.74–0.83 g/cm³. The homogenization temperature of one WG-type FIA varies from 279 °C to 289 °C, and the final ice melting temperature is −4.1 °C to −2.4 °C, corresponding to salinities of 4.0–6.6 wt% NaCl eqv., and the density of the fluid is 0.79–0.80 g/cm³.

(2)

FIs in quartz of stage II (samples SJ6 and SJ31): Three FIAs have been recognized. The homogenization temperature of these FIAs varies from 203 °C to 268 °C (Figure 10c), and the final ice melting temperature ranges from −4.5 °C to −1.6 °C, corresponding to salinities of 2.6–7.2 wt% NaCl eqv. (Figure 10d), and the fluid density is 0.80–0.88 g/cm³.

(3)

FIs in quartz and calcite of stage III (samples SJ11, SJ12, and SJ13-1): Three FIAs have been recognized. The homogenization temperature of two WL-type FIAs in quartz varies from 200 °C to 222 °C (Figure 10e), and the final ice melting temperature ranges from −3.5 °C to −1.8 °C, corresponding to salinities of 3.1–5.7 wt% NaCl eqv. (Figure 10f), and the fluid density is 0.86–0.91 g/cm³. The homogenization temperature of one WL-type FIA in calcite varies from 184 °C to 199 °C, and the final ice melting temperature ranges from −4.4 °C to −1.7 °C, corresponding to salinities of 2.9–7.0 wt% NaCl eqv., and the fluid density is 0.89–0.92 g/cm³.

(4)

FIs in quartz and calcite of stage IV (samples SJ8 and SJ45): Two FIAs have been recognized. The homogenization temperature of one WL-type FIA in quartz varies from 185 °C to 198 °C (Figure 10g), and the final ice melting temperature ranges from −2.2 °C to −1.5 °C, corresponding to salinities of 2.6–3.7 wt% NaCl eqv. (Figure 10h), and the fluid density is 0.89–0.91 g/cm³. The homogenization temperature of one WL-type FIA in calcite varies from 153 °C to 187 °C, and the final ice melting temperature ranges from −2.9 °C to −0.7 °C, corresponding to salinities of 1.2–4.8 wt% NaCl eqv., and the fluid density is 0.90–0.93 g/cm³.

Figure 10. Histograms illustrating the homogenization temperatures and salinity of fluid inclusions within quartz and calcite from the Xinglongshan ore block. (a) Homogenization temperature of stage I; (b) salinity of stage I; (c) homogenization temperature of stage II; (d) salinity of stage II; (e) homogenization temperature of stage III; (f) salinity of stage III; (g) homogenization temperature of stage IV; (h) salinity of stage IV.

Figure 10. Histograms illustrating the homogenization temperatures and salinity of fluid inclusions within quartz and calcite from the Xinglongshan ore block. (a) Homogenization temperature of stage I; (b) salinity of stage I; (c) homogenization temperature of stage II; (d) salinity of stage II; (e) homogenization temperature of stage III; (f) salinity of stage III; (g) homogenization temperature of stage IV; (h) salinity of stage IV.

5.2.3. Laser Raman Spectra

The results of comprehensive laser Raman spectroscopic examinations on individual fluid inclusions within quartz and calcite from various stages are visually represented in Figure 11. In stage I, the gas-phase composition of FIs primarily consists of H₂O, although an inconspicuous CH₄ peak is also displayed. For stages II and III, the gas-phase composition is predominantly H₂O. In stage IV, the gas-phase of FIs is mainly composed of H₂O with a minor presence of C₆H₆. Overall, the ore-forming fluid is characterized as belonging to an H₂O–NaCl ± C₆H₆ system. Raman spectroscopy analysis shows that the ore-forming fluid of stages I, II, and III belongs to an H₂O–NaCl system and that the fluid of stage IV belongs to an H₂O–NaCl–(C₆H₆) system. The presence of a small amount of C₆H₆ in stage IV indicates that the late meteoric water carries organic matter components from the strata into the ore-forming fluid.

Figure 11. Laser Raman spectra for fluid inclusions in quartz and calcite from the Xinglongshan ore block. (a) Gas-phase composition of WL-type inclusions in quartz of stage I; (b) gas-phase composition of WL-type inclusions in quartz of stage II; (c) gas-phase composition of WL-type inclusions in quartz of stage III; (d) gas-phase composition of WL-type inclusions in calcite of stage IV.

Figure 11. Laser Raman spectra for fluid inclusions in quartz and calcite from the Xinglongshan ore block. (a) Gas-phase composition of WL-type inclusions in quartz of stage I; (b) gas-phase composition of WL-type inclusions in quartz of stage II; (c) gas-phase composition of WL-type inclusions in quartz of stage III; (d) gas-phase composition of WL-type inclusions in calcite of stage IV.

5.3. Isotope Data

5.3.1. Hydrogen and Oxygen Isotopes

Isotope data for hydrogen and oxygen are presented in

Table 4. Oxygen isotopic composition (‰) of quartz and calcite, hydrogen and oxygen isotopic compositions (‰) of fluid inclusions, and carbon isotopic composition (‰) of calcite from the Xinglongshan ore block.

Sample No.	Mineral	Stage	Th (°C)	δ18Oquartz (V-SMOW)	δ18Owater (V-SMOW)	δDV-SMOW	δ13CV-PDB
2-3m-3	Quartz	I	278	4.1	−3.6	−65
8-16m-1	Quartz	I	278	5.8	−1.9	−75
1m-3	Quartz	I	278	5.8	−1.9	−126
8-16m-3	Quartz	I	278	5.1	−2.6	−106
8-16m-4	Quartz	I	278	5.1	−2.6	−120
2-3m-7	Quartz	I	278	15.1	7.4	−117
2-3m-6	Quartz	II	234	4.1	−5.6	−97
5#KD-2-1	Quartz	II	234	4.4	−5.3	−118
5#KD-2-2	Quartz	II	234	4.5	−5.2	−103
1-5-1m-2	Quartz	II	234	6.1	−3.6	−94
ZD4-1-1m-1	Calcite	III	206	2.5	−7.0	−133	−11.0
ZD4-1-1m-2	Calcite	III	206	1.9	−7.6	−138	−10.7
ZD4-1-1m-3	Calcite	III	206	1.0	−8.5	−145	−10.5
ZD4-1-1m-4	Quartz	III	206	7.4	−3.9	−123
ZD4-1-1m-4	Calcite	III	206	1.3	−8.2	−121	−10.9
5#KD-1	Quartz	IV	180	7.9	−5.2	−106
8-12m-1	Calcite	IV	180	4.5	−6.5	−123	−9.9
8-12m-2	Quartz	IV	180	−0.8	−13.9	−116
8-12m-2	Calcite	IV	180	3.6	−7.4	−112	−7.9
8-12m-3	Calcite	IV	180	2.5	−8.5	−134	−9.4

and depicted in Figure 12. Within stage I, six quartz samples exhibit δD values ranging from −126‰ to −65‰, δ¹⁸O _quartz values spanning 4.1‰ to 15.1‰, and δ¹⁸O_water values ranging from −3.6‰ to 7.4‰. For stage II, four quartz samples display δD values between −118‰ and −94‰, δ¹⁸O_quartz values in the range of 4.1‰ to 4.6‰, and δ¹⁸O_water values from −5.6‰ to −3.6‰. In stage III, one quartz and four calcite samples yield δD values between −145‰ and −121‰, δ¹⁸O_mineral values within the range of 1.0‰ to 7.4‰, and δ¹⁸O_water values spanning −8.5‰ to −3.9‰. Similarly, within stage IV, two quartz and three calcite samples exhibit δD values ranging from −134‰ to −106‰, δ¹⁸O_quartz values spanning from −0.8‰ to 7.9‰, and δ¹⁸O_water values between −13.9‰ and −5.2‰. Notably, theδ¹⁸O_mineral values of quartz samples (ranging from −0.8‰ to 15.1‰, mostly >4‰) in stages I to IV are generally higher than those of calcite samples (ranging from 1.0‰ to 4.5‰, mostly <3‰) from stages III to IV.

Figure 12. δ¹⁸O_water versus δD diagram of the Xinglongshan ore block (base map from [63]).

Figure 12. δ¹⁸O_water versus δD diagram of the Xinglongshan ore block (base map from [63]).

5.3.2. Carbon Isotope

Carbon isotope data are given in Table 4. Four calcite samples from stage III have δ¹³C values of −11.0‰ to −10.5‰, and three calcite samples from stage IV have δ¹³C values of −9.9‰ to −7.9‰.

5.3.3. Lead Isotope

The lead isotopic data for 21 sulfide samples are listed in

Table 5. Lead isotopic composition of sulfide samples from the Xinglongshan ore block.

Sample No.	Mineral	Stage	206Pb/204Pb	207Pb/204Pb	208Pb/204Pb	t (Ma)	μ	ω	κ	Δα	Δβ	Δγ
2-3m-1	I	Galena	18.308	15.569	38.229	137	9.31	35.5	3.69	65.2	15.9	26.1
2-3m-1	I	Sphalerite	18.285	15.536	38.130	137	9.29	35.1	3.65	63.9	13.7	23.5
2-3m-1	I	Pyrite	18.303	15.564	38.205	137	9.31	35.4	3.67	64.9	15.6	25.5
2m-3	I	Galena	18.361	15.634	38.448	137	9.37	36.3	3.75	68.3	20.1	32.0
2m-3	I	Sphalerite	18.290	15.545	38.179	137	9.29	35.3	3.67	64.2	14.3	24.8
2m-3	I	Pyrite	18.308	15.566	38.240	137	9.31	35.5	3.69	65.2	15.7	26.4
1m-1	I	Galena	18.355	15.626	38.419	137	9.36	36.2	3.75	67.9	19.6	31.2
1m-1	I	Sphalerite	18.323	15.585	38.261	137	9.33	35.6	3.69	66.1	16.9	27.0
1m-1	I	Pyrite	18.302	15.561	38.211	137	9.31	35.4	3.68	64.8	15.4	25.6
ZD5-1-1m-1	II	Galena	18.358	15.630	38.438	137	9.36	36.3	3.75	68.1	19.9	31.7
ZD5-1-1m-1	II	Sphalerite	18.311	15.567	38.234	137	9.31	35.5	3.69	65.4	15.8	26.2
ZD4-1-1m-1	III	Galena	18.278	15.530	38.107	137	9.28	35.0	3.65	63.5	13.3	22.8
ZD4-1-1m-1	III	Sphalerite	18.310	15.570	38.234	137	9.31	35.5	3.69	65.3	16.0	26.2
ZD4-1-1m-1	III	Pyrite	18.299	15.555	38.199	137	9.30	35.3	3.68	64.7	15.0	25.3
8-16m-2	IV	Galena	18.311	15.569	38.239	137	9.31	35.5	3.69	65.4	15.9	26.4
8-16m-2	IV	Sphalerite	18.293	15.571	38.234	137	9.30	35.5	3.69	64.3	16.0	26.2
8-16m-2	IV	Pyrite	18.332	15.597	38.319	137	9.34	35.8	3.71	66.6	17.7	28.5
8-12m-1	IV	Galena	18.346	15.616	38.389	137	9.35	36.1	3.74	67.4	19.0	30.4
8-12m-1	IV	Sphalerite	18.329	15.598	38.331	137	9.33	35.9	3.72	66.4	17.8	28.9
8-12m-1	IV	Pyrite	18.307	15.564	38.229	137	9.31	35.5	3.69	65.1	15.6	26.1

Abbreviations: μ = ²³⁸U/²⁰⁴Pb; ω = ²³²Th/²⁰⁴Pb; κ = Th/U; Δα = [(²⁰⁶Pb/²⁰⁴Pb)_d(t)/(²⁰⁶Pb/²⁰⁴Pb)_m(t) − 1] × 1000; Δβ = [(²⁰⁷Pb/²⁰⁴Pb)_d(t)/(²⁰⁷Pb/²⁰⁴Pb)_m(t) − 1] × 1000; Δγ = [(²⁰⁸Pb/²⁰⁴Pb)_d(t)/(²⁰⁸Pb/²⁰⁴Pb)_m(t) − 1] × 1000; d is the Pb of ore mineral; and m is mantle Pb calculated by Chen et al. [64].

. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios for 3 galena, 3 sphalerite, and 3 pyrite samples from stage I are 18.285–18.361, 15.536–15.634, and 38.130–38.448, respectively. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios for 1 galena and 1 sphalerite samples from stage II are 18.311–18.358, 15.567–15.630, and 38.234–38.438, respectively. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios for 1 galena, 1 sphalerite, and 1 pyrite samples from stage III are 18.278–18.310, 15.530–15.570, and 38.107–38.234, respectively. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios for 2 galena, 2 sphalerite, and 2 pyrite samples from stage IV are 18.293–18.346, 15.564–15.616, and 38.229–38.389, respectively.

6. Discussion

6.1. Timing of the Syenogranite and Mineralization

Different attempts to date mineralization have been made in the last years, either by dating igneous rocks considered to be genetically related to mineralization or by dating hydrothermal alteration and ore minerals themselves. Cui [65] proposed that the age of Ag–Pb–Zn mineralization is 249 ± 2 Ma based on a zircon U–Pb age from one of the diorite dikes. Based on a zircon U–Pb date for granite porphyry that was believed to be related to the mineralization, Ouyang et al. [66] proposed an age of 159.3 ± 2.3 Ma. Wang [67] reported a U–Pb age of 148 ± 1 Ma for hydrothermal zircons. Wang et al. [37] indirectly dated the mineralization by determining the age of sericite using the Ar–Ar method and obtained an age of 147 ± 2 Ma. Zhai et al. [18] reported Re–Os ages of 135 ± 3.4 Ma for molybdenite and 135 ± 0.6 Ma for pyrite. Wu et al. [39] obtained an age of 133 ± 4 Ma from a sphalerite Rb–Sr isochron. Wang [67] used the Re–Os method on arsenopyrite and pyrite to obtain an age of 159 ± 6 Ma for the Ag–Pb–Zn mineralization. However, the wide range of the dating results shows that the age of the mineralization is still controversial.

The discovery of the concealed syenogranite by geophysical methods and drilling and evidence provided by the geophysical and geochemical models [68] suggest that the syenogranite intrusive is closely related to the mineralization and provide new insights into the timing of the mineralization processes. It can be seen from field observations that the syenogranite intrusion has undergone strong potassium and silicide alteration and contains many fine ore veins, as well as a large amount of potassium feldspar–quartz veinlets/stockworks and magmatic-hydrothermal breccias. Strong sericitization and silicification are seen in the contact between the intrusion and wall-rocks, and there are molybdenite–quartz ± albite fine veins and hydrothermal breccia zones in the local area [18]. The contact relationship between the syenogranite and mineralization suggests that the crystallization age of the syenogranite can be considered to be a pre-ore mineralization age close to the mineralization age. As described earlier, the concealed syenogranite yields a zircon-weighted mean ²⁰⁶Pb/²³⁸U age of 137 Ma, which is basically consistent with the peak ages of the Early Cretaceous granites in the SGXR obtained from previous studies and suggests that the dioritic dike and the granite porphyry dated in earlier studies [65,66] are unlikely linked to the mineralization. Furthermore, this age is earlier than the average Re–Os model ages of molybdenite (135 ± 3.4 Ma) and pyrite (135 ± 0.6 Ma) reported by Zhai et al. [18] and the Rb–Sr isochron age of sphalerite (133 ± 4 Ma) reported by Wu et al. [68], thus, discarding ages determined from sericite [69], hydrothermal zircon, pyrite, and arsenopyrite [67]. Therefore, we infer that the mineralization age of the SJS deposit is not earlier than 137 Ma, and taking into account the above sphalerite, pyrite, and molybdenite dating results [18,37], an average 135 Ma age for the SJS deposit is likely to be a good estimation. This age is comparable to that of numerous other magmatic-hydrothermal ore deposits located in the SGXR, such as the Baiyinnuoer skarn Pb–Zn deposit (135 ± 1 Ma, [22]), the Bairendaba Ag–Pb–Zn deposit (135 ± 11 Ma, [70]), and the Weilasituo Sn–Li–Zn–Pb–Cu–Mo deposit (135 ± 7 Ma, [71]). Considering the U–Pb age of the syenogranite obtained by us in this study and previous dating results together, we conclude that the Early Cretaceous was an important mineralization period in the SJS ore district and that the vein-type Pb–Zn–Ag mineralization was closely related to granitic magma activity.

6.2. Source and Evolution of Ore-Forming Fluids

The δ¹⁸O_water and δD values of the ore-forming fluids in the Xinglongshan ore block vary from −3.6‰ to 7.4‰ and −126‰ to −65‰ for stage I, −5.6‰ to −3.6‰ and −118‰ to −94‰ for stage II, −8.5‰ to −3.9‰ and −145‰ to −121‰ for stage III, and −13.9‰ to −5.2‰ and −134‰ to −106‰ for stage IV, respectively (Table 4). As shown in Figure 12, all samples, with a single exception, are plotted between the magmatic water box and the meteoric water evolution line. This feature indicates that the ore-forming fluid has a mixture source between magmatic and meteoric water and that the initial fluid may have derived from magmatic water. The δ¹⁸O_water and δD values of the Xinglongshan ore block are consistent with those of high-latitude meteoric water [71,72]. In addition, there is a gradually decreasing trend in H–O isotope composition from the early to late stage, indicating a continuous influx of meteoric water with the evolution of ore-forming fluids. This is also supported by C isotopic compositions. The δ¹³C values of stages III and IV vary from −11.0‰ to −7.9‰ (mean −10.0‰). In general, there are three major carbon reservoirs on Earth: marine carbonate with a δ¹³C mean of 0‰ [73]; carbonatite with δ¹³C values varying from −7‰ to −3‰, averaging −5‰ [73,74]; and organic matter having a δ¹³C mean of −25‰ [75]. The carbon isotope composition of the ore block is significantly different from that of marine carbonate and organic matter but slightly lower than that of carbonatite or granite. Given the geological context, we suggest that the carbon mainly came from granitic magma rather than carbonatites. In the δ¹⁸O_calcite–δ¹³Cv-_PDB diagram (Figure 13), all samples are projected on the left side of the C–O isotope composition region of igneous rocks, which is similar to the C–O isotope compositions reported by Zhang et al. [40] and Wang et al. [37] (Figure 13). The δ¹³C value of atmospheric CO₂ ranges from −11‰ to −7‰ proposed by Hoefs [76]. In addition, compared to magmatic water, meteoric water has lower δ¹³C values. Therefore, the involvement of meteoric water will cause the C–O isotope composition of the ore-forming fluid to migrate toward the lower left. Therefore, the C–O isotope composition of the ore-forming fluid in the Xinglongshan ore block shows a mixing of magmatic and meteoric water, indicating that the initial ore-forming fluid came from magma and mixed with meteoric water with the evolution of ore-forming fluid.

Figure 13. δ¹⁸O versus δ¹³C diagram of calcite from stages III and IV of the Xinglongshan ore block (base map from [37,40]).

Figure 13. δ¹⁸O versus δ¹³C diagram of calcite from stages III and IV of the Xinglongshan ore block (base map from [37,40]).

The laser Raman spectroscopy results indicate that the ore-forming fluid of the Xinglongsahn ore block overall belongs to an H₂O–NaCl–(C₆H₆) system. The FIs in stage I, mainly consisting of WL-type with minor WG-type, are characterized by medium–high temperature (mean 278 °C) and low salinity (mean 4.7 wt% NaCl eqv.). The FIs in stages II and III, predominantly composed of WL-type, are featured by medium–low temperature (mean 234 °C for stage II and mean 206 °C for stage III) and low salinity (mean 4.5 wt% NaCl eqv. for stage II and mean 4.0 wt% NaCl eqv. for stage III). The FIs in stage IV, mainly consisting of WL-type, have low temperature (mean 180 °C) and low salinity (mean 2.8 wt% NaCl eqv.). It should be noticed that the coexisting WL- and WG-type FIs were developed in stage I, while only WL-type FIs occurred in stages II, III, and IV. The homogenization temperature and salinity of the ore-forming fluid in the main mineralization stage (stages II and III) of the Xinglongshan ore block are consistent with the previous study [40], in which the homogenization temperature and salinity range from 201 °C to 280 °C and 0.5 wt% NaCl eqv. to 8.4 wt% NaCl eqv., respectively, indicating that this ore block mainly formed in a medium- to low-temperature and low salinity environment. The evolution of the system from early to late stage indicates that the temperature gradually decreases and that the salinity slightly changes but still shows a downward trend (Figure 10 and Figure 14).

In summary, the initial fluid of the Xinglongshan ore block was derived from magma, the ore-forming fluid is characterized by a mixture of magmatic and meteoric water, and the involvement of meteoric water gradually increased with the evolution of ore-forming fluids. Additionally, the homogenization temperature of the ore-forming fluid gradually decreases from stage I to stage IV, while the salinity is nearly constant, except for a significant decrease in stage IV (Figure 14). Overall, the ore-forming fluid is featured by medium–low temperature and low salinity and roughly belongs to an H₂O–NaCl ± C₆H₆ system.

Figure 14. Diagram of homogenization temperature versus salinity of FIs in quartz and calcite from the Xinglongshan ore block. NaCl saturation curve from [56].

Figure 14. Diagram of homogenization temperature versus salinity of FIs in quartz and calcite from the Xinglongshan ore block. NaCl saturation curve from [56].

6.3. Sources of Ore-Forming Materials

The Pb isotopic composition is rarely fractionated in the processes of elemental migration and precipitation. Thus, it can be used to trace the source of metallogenic materials [77,78,79,80,81]. The μ, ω, and κ values of the Pb isotope for crust and mantle are 9.60 and 8.92, 36.84 and 31.84, and 5.85 and 3.45, respectively [82]. These values of the Xinglongshan ore block are 9.28–9.37, 35.0–36.3, and 3.65–3.75, with mean values of 9.32, 35.6, and 3.70, respectively (Table 5), which are between those of crust and mantle. This feature implies that Pb was derived from a mixed source of the crust and mantle. In Figure 15a, all samples are plotted between the upper crust and mantle evolution curves, forming a steep straight line crossing the evolution line of the orogenic belt; in Figure 15b, all samples are projected between the lower crust and orogenic belt evolution lines and near the orogenic belt evolution line. The Pb isotopic composition of the Xinglongshan ore block indicates that Pb was derived from an orogenic belt or a mixed source of mantle and crust.

Zhu et al. [83] proposed that combining Δβ and Δγ values can distinguish the tectonic backgrounds from which the ore minerals originated. In Figure 16, these data points of the Xinglongshan ore block are projected into the domain of magmatism, where the lead isotope composition has the characteristics of mixed lead sources from the upper crust and mantle [83]. Given that the SGXR was in an extensional background during the Early Cretaceous [14,27], we suggest that the ore-related magma originated from the mixing of depleted mantle- and crustal-derived magmas. Moreover, this primitive magma underwent the MASH (melting-assimilation-storage-homogenization) process in the lower crust, as proposed by Richards [84]. Wang et al. [28] reported the δ³⁴S values of 28 sulfide samples from the Xinglongshan ore block, ranging from −4.70‰ to 1.40‰. Zhai et al. [18] obtained 77 in situ sulfur isotope data for sulfides and Ag-bearing sulfosalts from the ore block, and their δ³⁴S values vary from −4.67‰ to 2.44‰, with a mean value of −2.11‰. The sulfur isotope composition of the Xinglongshan ore block is consistent with that of the magmatic and meteorite, indicative of a magmatic sulfur source.

In summary, the lead isotope composition obtained in this study and the sulfur isotope composition obtained by predecessors suggest that the ore-forming material of the Xinglongshan ore block mainly came from granitic magma.

Figure 15. Diagrams of ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb (a) and ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁸Pb/²⁰⁴Pb (b) for sulfides from the Xinglongshan ore block (base map from [82]).

Figure 15. Diagrams of ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb (a) and ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁸Pb/²⁰⁴Pb (b) for sulfides from the Xinglongshan ore block (base map from [82]).

Figure 16. Δβ versus Δγ genetic classification diagram for lead isotopes of ore minerals from the Xinglongshan ore block (base map after [83]). Names of the number: 1 = mantle-derived lead; 2 = upper crust lead; 3 = subduction zone lead originated from the mixing of the upper crust and mantle lead (3a = lead related to magmatism; 3b = lead related to sedimentation); 4 = chemically deposited lead; 5 = submarine hydrothermal lead; 6 = medium–high grade metamorphism lead; 7 = lower crust lead of high-grade metamorphism; 8 = orogenic belt lead; 9 = upper crust lead in ancient shale; 10 = retrograde metamorphism lead.

Figure 16. Δβ versus Δγ genetic classification diagram for lead isotopes of ore minerals from the Xinglongshan ore block (base map after [83]). Names of the number: 1 = mantle-derived lead; 2 = upper crust lead; 3 = subduction zone lead originated from the mixing of the upper crust and mantle lead (3a = lead related to magmatism; 3b = lead related to sedimentation); 4 = chemically deposited lead; 5 = submarine hydrothermal lead; 6 = medium–high grade metamorphism lead; 7 = lower crust lead of high-grade metamorphism; 8 = orogenic belt lead; 9 = upper crust lead in ancient shale; 10 = retrograde metamorphism lead.

6.4. Mechanism of Mineral Deposition

The sulfides of the Xinglongshan ore block are dominated by sphalerite, galena, pyrite, chalcopyrite, and pyrrhotite, and no sulfate minerals were detected, indicating that H₂S dominated the hydrothermal system with a low oxygen fugacity during the mineralization. The mean homogenization temperatures of FIs are 278 °C for stage I, 234 °C for stage II, 206 °C for stage III, and 180 °C for stage IV, respectively, indicating that the ore block formed in a medium–low temperature hydrothermal environment. The ore-forming fluid is characterized by low salinity, with salinities of 4.7 wt% NaCl eqv. for stage I, 4.5 wt% NaCl eqv. for stage II, 4.0 wt% NaCl eqv. for stage III, and 2.8 wt% NaCl eqv. for stage IV, respectively. Silver mainly exists in the form of Ag⁺ in hydrothermal fluids and preferentially bonds with HS⁻ [85], and in S-bearing systems, Ag (HS)₂⁻ is the main species up to T = 300–400 °C and P = 50 MPa [86]. Pb (HS)₂⁰ is the main Pb complex in a reduced hydrothermal fluid system under the conditions of low salinity (3.4 wt% NaCl eqv.), a temperature between 150 °C and 500 °C, and pressure greater than 100 MPa [66]. In a low salinity and reduced hydrothermal fluids system, Cu⁺ is the dominant form of Cu, forming strong complexes with HS⁻ [87]. As for zinc, considering the low salinity of the ore-forming fluid, we speculate that it mainly exists in the form of Pb (HS) ₂⁰, although some researchers proposed that ZnCl⁺ is the main Zn species in low salinity (3.4 wt% NaCl eqv.) hydrothermal fluid under the conditions of temperature ranging from 200 °C to 400 °C and pressure greater than 100 MPa [87,88]. In summary, the ore-forming elements, such as Ag, Pb, Zn, and Cu in the Xinglongshan ore block migrate in the form of hydrosulfide complexes.

The following mechanisms may cause the decomposition of complex, inducing mineral precipitation from solution [89,90,91,92]: (1) temperature decrease, (2) fluid immiscibility or boiling, (3) fluid mixing/dilution, and (4) fluid/rock interaction. The H–O isotopic compositions indicate that the mixture of magmatic and meteoric water took place during the mineralization of the Xinglongshan ore block (Figure 12); thus, fluid mixing is one of the main mechanisms for mineral precipitation. The homogenization temperatures of FIs from stage I to stage IV gradually decrease (Figure 13), implying that fluid cooling is also one of the mineral precipitation mechanisms. The quartz veins from stage I consist of WL- and WG-type FIs. These FIs coexist in the same quartz matrix (Figure 6b) and have similar homogenization temperatures and different salinities (Figure 10a,b). The WL-type FIs were homogenized to liquid, and the WG-type ones were homogenized to vapor when heated. The FIs of stage I exhibit typical features of fluid immiscibility. The Xinglongshan ore block belongs to a medium- to low-temperature hydrothermal deposit, and its δ¹³C values range from −11.0‰ to −7.9‰, which is slightly lower than that of carbonatite (mean −5.0‰), but much higher than that of organic matter (mean −25‰). Therefore, we infer that the fluid–rock interaction was not the main mechanism of mineral precipitation in the ore block. Given the supergiant size of the SJS deposit, we do not believe that simple cooling is an efficient mechanism of ore precipitation because it does not sharply change the stability conditions of the complexes. In brief, fluid mixing was the main mechanism for mineral precipitation in the Xinglongshan ore block, despite the presence of a small amount of fluid immiscibility in stage I.

6.5. Ore Deposit Type and Metallogenic Model

The genetic type of the SJS deposit has always been controversial, with two main viewpoints: epithermal type [18,28,40] and magmatic-hydrothermal vein type [27,33,68]. The main difference between the two views is whether the igneous rocks related to mineralization are granites, subvolcanic rocks, or porphyry intrusives. Zhai et al. [18] considered that the SJS mineralization is related to a granite porphyry, where shallow Ag–polymetallic vein ores and deep disseminated Mo ores form a porphyry Mo and epithermal Ag–polymetallic mineralization system. Jiang et al. [93] argued that the mineralization of the deposit is related to a highly-fractionated syenogranite and belongs to a magmatic-hydrothermal vein-type deposit. Zhao et al. [94] also deemed that the deposit belongs to a magmatic-hydrothermal vein-type deposit, but they thought that the mineralization is related to a highly-fractionated quartz syenite porphyry. The ore-forming fluid of the Xinglongshan ore block is characterized by medium–low temperature and low salinity, with a mineralogical assemblage including sphalerite, galena, arsenopyrite, pyrite, pyrrhotite, chalcopyrite, and silver-bearing sulfosalt minerals. These are typical characteristics of intermediate sulfidation epithermal (IS) deposits proposed by Chinchilla et al. [95] and Wang et al. [17]. Our study using field and microscopic investigation showed that the concealed granitic intrusive rocks in the Xinglongshan ore block have obvious granitic and porphyritic-like textures but do not show porphyritic structures. From bottom to top, the granitic complex consists of coarse-grained granite, medium to coarse-grained syenite granite, and fine-grained biotite granite (Figure 3c and Figure 7). These indicate that the mineralization of the ore block is genetically related to granitic plutons and that the ore-forming depth is relatively deep. The main types of Ag–Sn–polymetallic deposits in the SGXR are classified into skarn, epithermal, and magmatic-hydrothermal vein types [14,27,30]. Of them, deposits of magmatic-hydrothermal vein and skarn types are genetically related to granitic intrusions, while epithermal deposits are related to subvolcanic rocks [27]. Except for a small amount of porphyry Cu–Mo deposits, such as the Banlashan Cu–Mo, Aolunhua Mo–Cu, and Laojiagou Mo–polymetallic deposits, occurring on the eastern slope of the SGXR, no significant porphyry deposits occur on the main ridge and western slope of the SGXR, which, by contrast, is a huge Ag–Sn–polymetallic metallogenic belt characterized by magmatic-hydrothermal veins and epithermal deposits [27]. It is worth noting that in the same mining area, magmatic-hydrothermal vein-type and epithermal deposits often occur simultaneously, such as Baiyinchagandongshan, where the Baiyinchagandongshan magmatic-hydrothermal vein-type Ag–Sn–polymetallic and the Budunwula epithermal Ag–Pb–Zn deposits are developed [14]. In fact, the magmatic-hydrothermal vein-, porphyry-, and epithermal-type deposits in a region usually belong to the same magmatic-hydrothermal system [96], and the determination of different types of deposits mainly depends on their respective formation depths. Sillitoe [96] referred to vein ores that occur within noncarbonate rocks on the lateral to porphyry intrusives as subepithermal deposits. Strictly speaking, these vein deposits are not considered epithermal since they form in a deeper environment (lateral to a porphyry intrusive), although in common with IS epithermal veins, they also form from IS-type fluids and, therefore, can share similar mineralogy and alteration patterns [16]. Our study shows that the mineralization of the Xinglongshan ore block is closely related to the syenogranite and that it belongs to a magmatic-hydrothermal vein-type deposit; the mineralization of the Shuangjianzishan ore block is closely related to the Early Cretaceous subvolcanic rocks and that it belongs to an IS epithermal deposit. We therefore conclude that the Ag–Pb–Zn mineralization at Shuangjianzishan is genetically related to granites and subvolcanic rocks and that it belongs to a magmatic-hydrothermal vein–epithermal metallogenic system rather than a porphyry–epithermal metallogenic system.

Based on the above views, we establish a metallogenic model of volcanic–intrusive complex system to explain the Ag–Pb–Zn mineralization at Shuangjianzishan (Figure 17). Details are as follows:

During the Early Cretaceous, the rollback of the subducted Paleo-Pacific plate resulted in an extensional environment in the SGXR, triggering asthenospheric mantle upwelling [14,27,30]. Mantle-derived basaltic magma underplated beneath the lower crust, leading to partial melting of the crust. Crustal-derived magma mixed with mantle-derived magma to form crust–mantle mixed magma, and the mixed magma rose to the shallow part to form magma chambers [27,30]. The magma underwent fractional crystallization in the shallow magma chamber, forming metal-bearing granitic magma. The metal-bearing magma further rose, forming hypabyssal granitic intrusions. Ore-forming fluid exsolved from the ore-bearing granite mixed with meteoric water, leading to the precipitation of ore-forming materials within the preexisting faults, forming magmatic-hydrothermal vein-type Ag–polymetallic orebodies in the Xinglongshan ore block (Figure 17a). The metal-bearing granitic magma further rose, forming volcanic rocks on the surface and subvolcanic cupolas in volcanic edifices. Ore-forming fluid separated from the subvolcanic cupola mixed with meteoric water, resulting in mineral precipitation to form IS epithermal Ag-dominated Ag–Pb–Zn orebodies in the Shuangjianzishan ore block (Figure 17a). The Early Cretaceous volcanic rocks and intrusive rocks, sharing a common magma chamber, constitute coeval volcanic–intrusive complexes, forming a magmatic-hydrothermal vein-type and epithermal Ag–Pb–Zn–Cu–Sn metallogenic system related to granites and subvolcanic rocks at Shuangjianzishan (Figure 17b).

Figure 17. Metallogenic model of the SJS deposit. (a) magmatic-hydrothermal vein-type Ag–polymetallic orebodies in the Xinglongshan ore block. (b) magmatic-hydrothermal vein-type and epithermal Ag–Pb–Zn–Cu–Sn metallogenic system.

Figure 17. Metallogenic model of the SJS deposit. (a) magmatic-hydrothermal vein-type Ag–polymetallic orebodies in the Xinglongshan ore block. (b) magmatic-hydrothermal vein-type and epithermal Ag–Pb–Zn–Cu–Sn metallogenic system.

7. Conclusions

(1)

The concealed syenogranite genetically related to the mineralization of the Xinglongshan ore block formed at approximately 137 Ma.

(2)

The ore-forming fluid of the Xinglongshan ore block originated from a mixture of magmatic and meteoric water. With the evolution of ore-forming fluid, the amount of meteoric water increased gradually. The ore-forming fluid is characterized by medium–low temperature and low salinity and has an affinity of H₂O–NaCl ± CH₄ ± C₆H₆ in composition.

(3)

The ore-forming material dominantly came from the Early Cretaceous granitic magma. Fluid mixing was the main mechanism for mineral precipitation.

(4)

The Xinglongshan ore block belongs to a magmatic-hydrothermal vein-type deposit related to the Early Cretaceous syenogranite, and the Shuangjianzishan ore block belongs to an intermediate sulfidation epithermal deposit related to subvolcanic rocks. The Ag–Pb–Zn mineralization at Shuangjianzishan was genetically closely related to the Early Cretaceous volcanic–intrusive complex.