Genesis of the Dongtangzi Zn-Pb Deposit of the Fengxian-Taibai Ore Cluster in West Qinling, China: Constraints from Rb-Sr and Sm-Nd Geochronology, and In-Situ S-pb Isotopes

1. Introduction

The Fengxian–Taibai (abbr. “Fengtai”) ore cluster in the Shaanxi Province, located in the west Qinling orogenic belt, China, is an important part of the giant Qinling Pb-Zn-Au metallogenic belt. Several large and medium-sized Zn-Pb deposits are present, including, from west to east, the Fengya, Yindongliang, Qiandongshan-Dongtangzi, Bafangshan-Erlihe, and Yinmusi deposits, along with many small-sized deposits and occurrences. Besides, there are two large Au deposits of the Baguamiao and Shuangwang, along with many small-sized Au deposits, and several small Cu deposits. The total reserve is over 5 Mt Pb+Zn and more than 200 t gold in the ore cluster (Wang R.T. et al., 2007; Li, 2008). The large Dongtangzi Zn-Pb deposit, located in the southwest of the Fengtai ore cluster (Figure 1c), contains over 1.5 Mt Pb+Zn, with an average grade of 7.7 wt% Zn and 1.7 wt% Pb (Wu et al., 2016). The Dongtangzi Zn-Pb deposit was discovered in the late 1950s, which was formerly known as the Qiandongshan deposit. With the deepening of the exploration work, around the year 2002, the newly discovered concealed west part of the main ore body is called the Dongtangzi mine section. After the year 2004, the two mine sections were merged into one company, collectively called the Dongtangzi Zn-Pb deposit. The previous research work was mainly done on the Qiandongshan mine section, while less work has been done on the Dongtangzi mine section. Since the 1980s, extensive investigations into the Dongtangzi Zn-Pb deposit have been done into its geological characteristics and ore-controlling structures (Zhang, 1986; Wang J.L. et al., 1996; Wu W.T. et al., 2015; Wu X.D. et al., 2016; Wang Y.T. et al., 2020a), stable isotope and trace elements geochemistry (Fang, 1999a, 1999b; Huang et al., 2003; Ren et al., 2014; Zhang et al., 2020), and ore genesis (Li, 1986; Yang X.K. et al., 1991; Li et al., 1992; Qi and Li, 1993; Wang J.L. et al., 1996; Zhang et al., 2018; Zhang et al., 2020; Wang et al., 2021; Shi et al., 2022). Despite decades of research, the origin of the ore remains contentious.

Prevailing perspectives (e.g., Zhang and Wang., 1988; Qi and Li, 1993; Wang J.L. et al., 1996; Wang D.S. et al., 2009; Wang R.T. et al., 2011) believed that it is a SEDEX deposit developed in Devonian or a sedimentary-reformed deposit. Conversely, proponents of the epigenetic hydrothermal ore deposit theory (Wang Y.T. et al., 2009, 2011, 2013, 2020a, 2021; Hu et al., 2015, 2020; Zhang et al., 2018; Shi et al., 2022) contend that it is a product of Triassic tectonic-hydrothermal processes. Due to the lack of a precise geochronology data and comprehensive isotopic geochemistry studies, especially the lack of in-situ technique analysis, the metallogenic processes of the Dongtangzi Zn-Pb deposit remains poorly understood. Therefore, this study presents a comprehensive recognition of its deposit geology, along with Rb-Sr and Sm-Nd geochronology and in-situ S-Pb isotopic studies, aiming to enhance the comprehension of the ore genesis. The new results in this study, provide valuable information about the timing of Zn-Pb mineralization and the source of ore-forming materials, and allow us to refine the origin of the Dongtangzi Zn-Pb deposit, which will be an essential factor for understanding the Zn-Pb metallogeny throughout the entire Fengtai ore cluster and the West Qinling Zn-Pb belt.

2. Regional Geology

The Qinling orogen was formed from the convergence and collision of the North China block and the Yangtze block (Figure 1a), which is a multi-stage composite orogenic belt composed of two suture zones and three tectonic units (Zhang et al., 2004). The northern suture zone is the Early Silurian Shangdan suture (Figure 1b), and the southern is the Late Triassic Mianlue suture (Figure 1b). The three tectonic units are, from north to south, the south margin of the North China block and the North Qinling belt, the South Qinling belt, and the north margin of the Yangtze block (Zhang et al., 2004). The Fengtai ore cluster is located in the middle-west of the South Qinling belt (Figure 1c), which is bordered by the NWW-trending faults of the Xiangzihe－Huangbaiyuan fault (F₁, part of the Shangdan suture) in the north and the Jiudianliang－Jiangkou fault (F₅) in the south (Figure 1c). There are three NWW-trending faults inside the ore cluster called Xiushiyan－Guanyinxia (F₂), Wangjialeng－Erlangba (F₃) and Daohuigou－Zheliyuan (F₄) faults (Figure 1c). The basic tectonic framework of the ore cluster is a compound fold with well-developed secondary folds and faults. The tectonic style is a large-scale transpressional strike-slip duplex system resulted from regional sinistral strike-slip faulting (Wang Y.T. et al., 2009, 2018).

The stratigraphic sequence of the Fengtai ore cluster is comprised of the Middle-Upper Devonian epimetamorphic littoral-neritic facies clastic and carbonate rocks (Figure 1c). This vertical succession comprises, from bottom to top, fine clastic rocks and metasandstone of the Middle Devonian Macaogou Formation (D₂m), followed by carbonate rocks of the Middle Devonian Gudaoling Formation (D₂g), epimetamorphic clastic-carbonate rocks of the Upper Devonian Xinghongpu Formation (D₃x), and fine clasolite of the Upper Devonian Jiuliping Formation (D₃j). Around the periphery of the ore cluster, the high grade metamorphic pararock and volcanics of the Proterozoic Qinling Group (Pt₁q) and Danfeng Group (Pt₁d) appear to the north, the carbonate and clastic rocks of the Lower Carboniferous (C₁) and breccia with siltstone of the Lower Cretaceous (K₁) to the northwest (Figure 1c), the marine carbonate rocks with continental clastic rocks of the Carboniferous (C), and limestone of the Lower Triassic Liufengguan Group (T₁l) to the southwest (Figure 1c).

Intense Triassic magmatism was well developed in the ore cluster. The Xiba intermediate-felsic pluton, trending NWW, is positioned to the southeast of the cluster (Figure 1c), comprising medium-fine grained granodiorite, monzogranite, quartz diorite, and tonalite, with transitional contact relations. The small Huahongshuping granodioritic intrusive stock is located in the north (Figure 1c). In addition, numerous dikes of diorite, granite porphyry and lamprophyre commonly develop over the ore cluster (mainly concentrated adjacent to the Pb-Zn and Au deposits), filling the NE- or NWW-trending faults, with a thickness of tens of centimeters to several meters.

3. Ore deposit geology

3.1. Structures

The “M”-shaped Qiandongshan-Dongtangzi brachy-anticline is the principle ore-hosting structure, with the anticlinal axis plane strike range from 275° to 285° (Figure 2). The hinge of the anticline pitches toward west, with the plunge angles ranging from 27° to 37° (Tang, 2013). There are EW-, NW- and NE-trending faults in the mine area. The EW-trending faults are largest in scale, developed mostly in the interface between Gudaoling formation and Xinghongpu formation, belonging to reverse faults (Figure 2). The NW- and NE-trending faults are small in scale and large in number, belonging to the "X" type conjugate shear faults formed under regional shearing events, cutting through the main ore body with small fault displacement.

3.2. Rocks

The strata within the mine area contains the upper section of the Gudaoling Formation of middle Devonian (D₂g₂) and the the Xinghongpu Formation of middle Devonian (D₃x) (Figure 2). The EW-striking upper section of the Gudaoling Formation, consisting of microcrystalline biolithite and carbonaceous microcrystalline limestone, silicified intensively in the top by meters to tens of meters in thickness, hosts the main orebody. The first lithologic section of the Xinghongpu Formation consists of ferrodolomitic phyllite with sericite phyllite and thin layer of carbonaceous limestone at the bottom, ferrodolomitic phyllite with thin layer of limestone and silty sericite phyllite in the middle, and carbonaceous calcareous sericite phyllite with thin layer of carbonaceous biolithite at the top. The second lithologic section of the Xinghongpu Formation consists mainly of chlorite sericite phyllite, with less ferrodolomitic sericite phyllite at the bottom (Figure 3).

No large scale of igneous rocks have been found in the mine area. However, the intrusive dikes are well developed. There are several diorite porphyry dikes and granite porphyry dikes, filling the NE-trending and NWW-trending faults, respectively. The former dikes crosscut the main orebody, while the latter dikes with zircon U-Pb ages of 226 ~ 221 Ma (LA-ICP-MS; Chen S.C. et al., 2020) are roughly parallel to the main orebody (Figure 3, Figure 4a). The main minerals of the granite porphyry dikes are plagioclase (30%), potash feldspar (35%), quartz (30%), biotite (3%), pyrite, zircon, sphene, etc (Figure 4b, c). The existance of cataclastic quartz suggests that the dike has underwent tectonic deformation after its emplacement (Figure 4b, c).

3.3. Orebodies

Controlled by the Qiandongshan-Dongtangzi anticline, the main orebody occurs at the contact zone between the Devonian Gudaoling Formation (D₂g) and the Xinghongpu Formation (D₃x) (Figure 2, 3), which is composed of the No.1 orebody (north subsidiary anticline) and the No.2 ore body (south subsidiary anticline), accounting for more than 90% of the total Zn-Pb reserves. The No.1 ore body extending 2.35 times its depth (Zeng L.G.; 2009). The other ore bodies occur within the fault fracture zones and irregular tensile fractures in limestone, or within interlayer fractures in phyllite, with relatively small scale and unstable occurrence (Figure 2b, 3). The eastern part of the No.1 ore body occurs in the northern wing of the anticline and the saddle of the northern subsidiary anticline, and there is a trend of pinching out westward (Figure 2). The No. 2 ore body, located on the southern wing of the anticline, is larger in scale in the west part within the Dongtangzi mine section (Figure 2). The orebodies in the north wing of anticline are thiner and the wall rocks are strong foliated, while the orebodies in the south wing of anticline are thicker, with wall rocks not obviously foliated. In addition, within the carbonaceous limestone near the footwall of the No. 2 orebody, a cryptoexplosion breccia pipe was discovered, with different sizes of angular-sub angular breccias, composed of granite porphyry, quartz vein, altered rock, and massive sulfide ore, cemented with argillaceous and carbonaceous (Figure 5a).

The ore minerals are mainly comprised of sphalerite, galena, pyrite, chalcopyrite, and arsenopyrite. The gangue minerals include quartz, ankerite, dolomite, calcite, siderite, sericite, chlorite, and apatite. The ore structures are mainly veinlets (Figure 4e), banded, brecciated (Figure 4d, f), blocky (Figure 4g), banded, and disseminated. Extensively vein filling and metasomatic mineralization can be observed in all the orebodies. The ore textures include granular (Figure 4h, i), metasomatic (Figure 4h, i, j), fragmented (Figure 4j), and wrinkling pressure shadow texture. The wall-rock alteration is widely and intensively developed throughout the floor and roof rocks, including silicification, dolomitization, calcitization, pyritization, sericitization, graphitization, etc. In addition, skarnization was developed in the ore beside the granite porphyry dikes (Figure 5b, c, d).

3.4. Mineralogy assemblage and paragenetic sequence

Based on systematic observation of mineral assemblages, ore structure and texture, hydrothermal alteration, and crosscutting relationships of multistage veins in the Dongtangzi Zn-Pb deposit, the paragenetic association can be divided into two periods, namely the sedimentary period and the hydrothermal mineralization period, respectively. The Hydrothermal mineralization period can be further divided into three stages (Figure 6): Stage I, a polymetallic sulfide – ankerite – quartz stage; Stage II, a quartz – pyrite-rich sulfide – ankerite stage; and stage III, a quartz – carbonate – sulfide stage.

3.4.1. Sedimentary period

There is no obvious Zn-Pb mineralization in the sedimentary period, and the syn-sedimentary pyrite (Py₀) are observed, which are mostly metasomatized by sulfides and quartz during the hydrothermal period (Figure 8a, b, c). No significant syn-sedimentary characteristics like clastic texture or laminated texture have been found in Pb-Zn ores, as the primary sedimentary textures were mostly destroyed by the structural deformation and metamorphism, syngenetic minerals are mostly replaced, deformed, and cemented by hydrothermal minerals during the hydrothermal mineralization period.

Figure 7. Occurrence features of the different mineralizaion stages in the Dongtangzi Zn-Pb deposit. a. The Stage I massive and brecciated ores intersected by the Stage II quartz – pyrite - ankerite veins; b. The granite porphyry dike is crosscut by the Stage II quartz – pyrite - sphalerite vein; c. The Stage II pyrite-rich ore is crosscut by Stage III quartz – calcite veins; d. The Stage II pyrite - ankerite vein is crosscut by the Stage III calcite veins; e. The Stage III coarse-grained calcite - pyrite vein; f. The Stage III quartz - pyrite - sphalerite veins are arranged in echelon, crosscutting the Stage I ores; g. The Stage I quartz - sphalerite - galena - pyrite - ankerite parallel veins crosscutting the carbonaceous alterated rocks. Note: Sp - sphalerite, Q - quartz, Gn - galena, Ank - ankerite, Sil - siliceous rock.

Figure 7. Occurrence features of the different mineralizaion stages in the Dongtangzi Zn-Pb deposit. a. The Stage I massive and brecciated ores intersected by the Stage II quartz – pyrite - ankerite veins; b. The granite porphyry dike is crosscut by the Stage II quartz – pyrite - sphalerite vein; c. The Stage II pyrite-rich ore is crosscut by Stage III quartz – calcite veins; d. The Stage II pyrite - ankerite vein is crosscut by the Stage III calcite veins; e. The Stage III coarse-grained calcite - pyrite vein; f. The Stage III quartz - pyrite - sphalerite veins are arranged in echelon, crosscutting the Stage I ores; g. The Stage I quartz - sphalerite - galena - pyrite - ankerite parallel veins crosscutting the carbonaceous alterated rocks. Note: Sp - sphalerite, Q - quartz, Gn - galena, Ank - ankerite, Sil - siliceous rock.

Figure 8. Microfabric characteristics of the ore in the Dongtangzi Zn-Pb deposit. a, b. Metasomatic residual skeleton texture of the pyrite (Py₀) replaced by the sphalerite (Sp₁), under reflected light; c. Droplet chalcopyrite, irregular galena and metasomatic residual Py₀ are developed in the massive sphalerite (Sp₁), under reflected light. d. Densely disseminated pyrite (Py₂) + arsenopyrite (Asp₂) developed in the ferrodolomite siliceous rock, under reflected light; e, f. The ankerite in the siliceous rock is replaced by quartz, forming skeletal crystal texture, BSE image; g. Ankerite in the siliceous rocks are metasomatized by quartz and sphalerite; h. Microfine quartz, sericite and ankerite together in the siliceous rocks, with sphalerite filling in cracks; i. Scanning electron microscopy (SEM) images show that the surface of the quartz developed triangular pores, with tightly cemented grains, flat edges, and crystal textures. j. Cathode luminescence (CL) image of the ankerite siliceous rock. The residual of metasomatized calcite is orange colored, while the ankerite is black, and the quartz is deep bluish purple; k, l. the silicate rocks are metasomatized by quartz and sphalerite (k - cathode luminescence, l - reflected light).

Figure 8. Microfabric characteristics of the ore in the Dongtangzi Zn-Pb deposit. a, b. Metasomatic residual skeleton texture of the pyrite (Py₀) replaced by the sphalerite (Sp₁), under reflected light; c. Droplet chalcopyrite, irregular galena and metasomatic residual Py₀ are developed in the massive sphalerite (Sp₁), under reflected light. d. Densely disseminated pyrite (Py₂) + arsenopyrite (Asp₂) developed in the ferrodolomite siliceous rock, under reflected light; e, f. The ankerite in the siliceous rock is replaced by quartz, forming skeletal crystal texture, BSE image; g. Ankerite in the siliceous rocks are metasomatized by quartz and sphalerite; h. Microfine quartz, sericite and ankerite together in the siliceous rocks, with sphalerite filling in cracks; i. Scanning electron microscopy (SEM) images show that the surface of the quartz developed triangular pores, with tightly cemented grains, flat edges, and crystal textures. j. Cathode luminescence (CL) image of the ankerite siliceous rock. The residual of metasomatized calcite is orange colored, while the ankerite is black, and the quartz is deep bluish purple; k, l. the silicate rocks are metasomatized by quartz and sphalerite (k - cathode luminescence, l - reflected light).

3.4.2. Hydrothermal mineralization period

Stage I: A polymetallic sulfide – ankerite - quartz stage

The stage I is rich in sphalerite and galena, with pyrite locally enriched, accompanied with less chalcopyrite. The sphalerite is fine-grained, mainly dark brown in color. The galena and pyrite are fine-grained - aphanitic, cementing the breccia of the carbonaceous altered rock (Figure 7a). The stage I ore developed silicification, dolomitization, chloritization, and locally sericitization, replacing the banded marl along bedding, or filling the tension fractures within the carbonaceous limestone (Figure 7g). Carbonates are extensively metasomatized by quartz and sulfides (Figure 8e, f, g), and some quartz and sulfides retain the skeleton crystal structure of carbonates (Figure 8e, f). The stage I is the main mineralization stage, which is generally affected by later tectonic deformation, developing fragmentation and locally recrystallization (Figure 4d, 7a, 7f).

No obvious sedimentary mineralization characteristics has been found in the ore of the Dongtangzi Zn-Pb deposit. For example, (1) the sheet-like sericite among quartz grains is irregularly (Figure 8h), without obvious orientation, and does not have typical sedimentary texture of directional arrangement of argillaceous minerals; (2) Quartz grains are crystalline and tightly cemented between grains (Figure 8i), with no obvious grinding round texture, showing no syn-sedimentary characteristics, indicating that the quartz in the altered rocks could be brought by the hydrothermal fluid running through the fracture systems of different scales; (3) Most of the quartz in the altered rocks are obviously formed by metasomatism, as the calcite (orange region in Figure 8j) is replaced by ankerite (black region in Figure 8j) and quartz (blue-purple region in Figure 8j), while the ankerite (Figure 8k, l) is replaced by quartz (blue-purple region in Figure 8k) and sphalerite (Figure 8k, l), indicating that the altered rocks were formed by remarkably silicification and replacement of carbonate rocks by siliceous hydrothermal fluid, rather than by sedimentary exhalative processes suggested by some researchers (Lu and Wei, 1992; Qi et al., 1993; Fang W.X.; 1997, 1999b).

Stage II: A quartz - pyrite-rich sulfide – ankerite stage

The stage II quartz - pyrite - ankerite - veins are accompanied by polymetallic sulfides such as sphalerite, galena, chalcopyrite and arsenopyrite, which are generally coarse-grained. The Zn-Pb mineralization scale of this stage is smaller than the stage I, but is more enriched in pyrite, locally intersecting and replacing the stage I ore and wall rock (Figure 7a), meanwhile crosscutting the granite porphyry dikes (Figure 7b), with scattered pyritization within the granite porphyry dike on both besides of the stage II quartz vein. According to the interspersion relationship, the NWW-trending granite porphyry dike locally crosscuts the Stage I orebody, and is cut through by the quartz-sulfide vein of the Stage II, indicating that it was emplaced at a time between stage I and stage II. The sphalerite is medium-fine grained, mainly light brown in color. No apparent replacement was observed on the pyrite (Py₂) and arsenopyrite (Asp₂) in the altered rocks (Figure 8d). Dolomitization, pyritization, and silicification are well developed in the wall rocks during the stage II (Figure 7c, d).

Stage III: A quartz – carbonate – sulfide stage

The Stage III quartz – carbonate – sulfide veins are widely developed in the whole deposit, forming NE-trending joint veins or en echelon veins (Figure 7f). It mainly consists of coarse-grained quartz and calcite (Figure 7e), with coarse– medium-grained ankerite, pyrite, galena, and sphalerite. The extension range of the veins is small, crosscutting the wall rocks and the ore veins of the first two stages (Figure 7c, d). The wall rock alteration is weak, including silicification, pyritization, carbonation.

4. Samples and analytical methods

All samples utilized in this work were collected from underground tunnels at diverse depths within the Dongtangzi Zn-Pb deposit. After making the polished thin sections, comprehensive microscopic observation was carried out to preliminarily characterize the morphology, textures and paragenesis of ore-related minerals. Subsequently, representative samples were chosen for further analyses (Table 1). A subset of these samples underwent crushing, with sulfide minerals, including sphalerite, galena, and pyrite, meticulously hand-picked under a binocular microscope. The purity of individual mineral separations exceeded 99%, and all mineral isolations underwent thorough cleaning in an ultrasonic bath. Concurrently, other portions of the samples were polished into thin sections, facilitating high-precision in-situ LA-MC-ICP-MS sulfur and lead isotope analyses.

4.1. In-situ S isotope analysis

Representative sphalerite, pyrite, and galena crystals of hydrothermal stage I to stage III, pyrite of sedimentary period, and pyrite of granite porphyry dike were selected for in-situ LA-MC-ICP-MS sulfur isotope analysis by using a Resonetics-S155 excimer ArF laser ablation system with Nu Plasma II multicollector ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. Detailed test methods follow Fu et al. (2016) and Zhu et al. (2016, 2017). The energy fluence of the laser is approximately 3 J/cm². For single spot analysis, the diameter is 33 μm with a laser repletion rate of 8 Hz. The true sulfur isotope ratio was calculated by correction for instrumental mass bias by linear interpolation between the biases calculated from two neighboring standard analyses. Isotope data are reported in delta notation (‰) in comparison with Vienna Cañon Diablo Troilite (V-CDT):

δ³⁴S_V-CDT = [((³⁴S/ ³²S)_sample/ (³⁴S/³²S)_V-CDT) -1] × 10³

Where (³⁴S/³²S) sample is the measured ³⁴S/³²S ratio in the sample and (³⁴S/³²S)_V-CDT is defined as 0.044163 (Ding et al., 2001). The precision of ³⁴S/³²S analysis is less than 0.00003 (1δ). An in-house pyrite standard named WS-1 was used to calibrate the mass bias for S isotopes (Zhu et al., 2016). This consists of a natural pyrite crystal from the Wenshan polymetallic skarn deposit, Yunnan Province, China. The ³⁴S_V-CDT value (0.3 ± 0.1%) for WS-1 natural pyrite was determined using the CF-IRMS method on a MAT 253 isotope ratio mass spectrometer (Thermo Finnigan, Bremen, Germany) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Standards were measured before, and after every four spot analyses. We also use the pyrite standard to get data of galena and sphalerite, and these data should be considered as roughly results due to no proper standard.

4.2. In-situ Pb isotope analysis

26 analysis spots, including pyrite, galena and sphalerite of hydrothermal stages I~III and pyrite of the granite porphyry dike, were selected for in-situ LA-MC-ICP-MS lead isotope analysis. The examination was conducted utilizing a 193 nm laser ablation system (RESOlution M-50, ASI) interfaced with a Nu Plasma II multicollector ICP-MS (Wrexham, UK) at the State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an. Homogeneous nano-particulate pressed sulfide powder tablets (PSPTs) served as reference materials to ensure measurement accuracy. Instrument parameters were optimized using the NIST610 standard (Pb=426 μg/g) to achieve maximal analytical sensitivity, signal stability, and optimal peak shape and alignment. Analytical signals were deduced using the Time Resolved Analysis (TRA) mode, with an integration time of 0.2 s. Each measurement spot encompassed a 30 s background measurement, followed by 50 s of ablation for signal collection, and an additional 120 s for wash time to mitigate memory effects. Galena laser ablation operated at a frequency of 2 Hz with a spot size of 9 μm, while for bismuthinite, the frequency was 6 Hz with a spot size of 30 μm, respectively. The ²⁰²Hg, ²⁰³Tl, ²⁰⁴Hg+²⁰⁴Pb, ²⁰⁵Tl, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb ion beams were collected by corresponding Faraday cups. The ²⁰⁴Hg/²⁰²Hg natural abundance ratio (0.229883) was used to calculate and determine the interference from the ²⁰⁴Hg species on the ²⁰⁴Pb intensity obtained. Analytical procedures, reference material PSPTs, and data processing has been previously described in detail by Chen et al. (2014), Yuan et al. (2015), and Bao et al. (2017).

4.3. Rb–Sr isotope analyses

The sulfide samples of the Stage I were rinsed several times in distilled water, dried, and crushed to 40–60-mesh size. The individual minerals of sphalerite and pyrite were handpicked under a binocular microscope, with purity levels of >98% being used for analysis. Sulfide grains were crushed to <200 mesh using an agate ball mill, then washed in an ultrasonic bath and dried. For Rb−Sr isotope analyses, powder weighing 0.2–0.3 g of each sample were dissolved in Teflon beakers with a mix of HF and HNO₃ acids. Rb−Sr have been separated for isotopic analysis by adopting resin of AG50W×8 and different eluent reagents. Firstly, REEs are separated from Rb−Sr by using normal method of cation−exchange chromatography with eluent of HCl. Then Rb and Sr were separated using cation ion exchange column.

Sulfide samples underwent Rb–Sr isotopic analyses using a VG 354 mass spectrometer equipped with five collectors at the Center of Modern Analysis, Nanjing University. The chemical separation and mass spectrometric procedures are outlined by Wang et al. (2006). ⁸⁷Sr/⁸⁶Sr is normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194, to correct for instrumental fractionation. During the period of this study, measurements for the American Standard Reference Material NBS 987 Sr standard gave ⁸⁷Sr/⁸⁶Sr = 0.710236 ± 0.000007 (2σ).

4.4. Sm-Nd isotope analyses

The carbonate minerals of ankerite, dolomite and calcite in the Dongtangzi Zn-Pb deposit were systematically collected from hydrothermal mineralization period sphalerite–galena–pyrite–quartz–ankerite–dolomite–calcite veins within the main orebody, with 6 samples of the Stage I, 2 of the Stage II, and 1 of the Stage III, respectively (Table 5). These carbonate minerals were separated from the ore samples after crushing and were powdered in an agate mill before Sm–Nd isotopic analysis.

Powders of carbonate mineral specimens underwent dissolution utilizing purified 0.5 M dilute acetic acid at ambient temperature over a four-hour duration. After centrifugation, acid-insoluble residues underwent digestion employing a mixture of HNO₃, HF, and HClO₄, rendering them amenable for Nd isotopic analyses. The Nd solutions were subjected to separation through conventional ion exchange techniques. The isotopic compositions of the refined Nd solutions were quantified utilizing a VG 354 mass spectrometer equipped with five collectors at the Center of Modern Analysis, Nanjing University. A comprehensive account of the experimental methodology can be found in the reports by Wang Y.X. et al. (2007) and Li et al. (2007).

5. Results

5.1. In-situ S isotopic compositions

A total of 50 spots were performed for in-situ sulfur isotope study covering the pyrite of sedimentary period (n=4), the sulfides of the hydrothermal period stage I (n=23), stage II (n=4), stage III (n=7), and the pyrite in the granite porphyry dike (n=4). The results are listed in Table 2 and shown in Figure 11b.

The measured δ³⁴S data of the pyrite of the sedimentary period and the granite porphyry dike vary from 15.8 to 21.4‰ (averaging 18.5‰), and from 2.1 to 4.3‰ (averaging 3.2‰), respectively. While those of the sulfides from the three hydrothermal mineralization stages vary from 1.1 to 9.4‰ (averaging 7.1‰), from 6.4 to 7.4‰ (averaging 6.9‰), from 3.8 to 10.2‰ (averaging 7.3‰), respectively, showing a relatively restricted range, indicating that the source of sulfur has no significant difference during the hydrothermal mineralization period.

5.2. In-situ Pb isotopic compositions

All the sulfides analyzed have similar lead isotope values and the radioactive lead content are low (Table 3). The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of the Stage I are 18.074-18.107, 15.641-15.663 and 38.217-38.764, respectively. And those of the Stage II are 18.073-18.082, 15.652-15.659 and 38.301-38.324, respectively. For the Stage III, the ratios are 18.078-18.265, 15.645-15.654 and 38.293-38.310, respectively. While for the granite porphyry dike, the ratios are 18.091-18.101, 15.585-15.610 and 38.169-38.241, respectively.

5.3. Rb-Sr isochron age

The analytical data are listed in Table 4. The Rb contents of the samples are low, ranging from 0.235 × 10⁻⁶ to 1.207 × 10⁻⁶, and the Sr contents are also low, ranging from 0.3105 × 10⁻⁶ to 5.218 × 10⁻⁶. The value of ⁸⁷Rb/⁸⁶Sr range from 0.528 to 6.971. The value of ⁸⁷Sr/⁸⁶Sr has a wide range from 0.71204 to 0.73148. Regression and age calculations of isochrons were performed using Isoplot/Ex Version 3.00 software (Ludwig, 2008), and with λ = 1.42 × 10⁻¹¹a⁻¹, using 1% errors for ⁸⁷Rb/⁸⁶Sr ratios and 0.05% errors for ⁸⁷Sr/⁸⁶Sr ratios at a confidence level of 95%. The analytical data yielded an isochron age of 211.6 ± 2.6 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71046 ± 0.00012 and MSWD value of 0.31 (Figure 9a).

Considering the potential for physical deformation of the ore, there exists a plausible scenario where the determined ages might be influenced by tectonic overprint, necessitating further investigation of this aspect. The assessment of 1/Rb versus ⁸⁷Rb/⁸⁶Sr diagrams provides a way to discern whether the initial (⁸⁷Sr/⁸⁶Sr)_i values remained constant throughout the sulfide growth process (Li et al., 2002), thereby enabling an evaluation of data integrity. In the present investigation, the absence of covariation between 1/Rb values and ⁸⁷Rb/⁸⁶Sr (Figure 9b) signifies that the isochron age data are meaningful, offering a reliable age of the ore formation.

5.4. Sm-Nd isochron age

All ¹⁴³Nd/¹⁴⁴Nd ratios determined for the samples of the hydrothermal mineralization period ankerite, dolomite and calcite analyzed during this study were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, with results and analytical uncertainties (2σ) given in Table 5. The ¹⁴⁷Sm/¹⁴⁴Nd values for carbonate minerals from stage I range from 0.0731 to 0.7195, with corresponding ¹⁴³Nd/¹⁴⁴Nd values range from 0.511851 and 0.512739.

Table 5. The Sm-Nd isotopic data of the stage I carbonates from the Dongtangzi Zn-Pb deposit.

Sample No.	Mineral	Stage	Sm(×10-6)	Nd(×10-6)	147Sm/144Nd	143Nd/144Nd
D22	Dolomite	I	0.1605	3.527	0.1351	0.511943±9
D28-4	Ankerite	I	0.6537	2.678	0.7195	0.512739±7
D29-1	Dolomite	I	0.4058	4.634	0.2586	0.512081±8
D45-31	Dolomite	I	0.1325	5.336	0.0731	0.511851±8
D45-32	Dolomite	I	0.3536	5.781	0.1803	0.511982±7
D48	Dolomite	I	0.2953	4.013	0.2175	0.512024±9
D55	Dolomite	II	0.4831	3.907	0.3656	0.512229±14
D58	Calcite	II	0.5029	2.694	0.5498	0.512501±8
D58	Dolomite	II	0.1847	5.325	0.1023	0.511875±9

Table 5. The Sm-Nd isotopic data of the stage I carbonates from the Dongtangzi Zn-Pb deposit.

Sample No.	Mineral	Stage	Sm(×10-6)	Nd(×10-6)	147Sm/144Nd	143Nd/144Nd
D22	Dolomite	I	0.1605	3.527	0.1351	0.511943±9
D28-4	Ankerite	I	0.6537	2.678	0.7195	0.512739±7
D29-1	Dolomite	I	0.4058	4.634	0.2586	0.512081±8
D45-31	Dolomite	I	0.1325	5.336	0.0731	0.511851±8
D45-32	Dolomite	I	0.3536	5.781	0.1803	0.511982±7
D48	Dolomite	I	0.2953	4.013	0.2175	0.512024±9
D55	Dolomite	II	0.4831	3.907	0.3656	0.512229±14
D58	Calcite	II	0.5029	2.694	0.5498	0.512501±8
D58	Dolomite	II	0.1847	5.325	0.1023	0.511875±9

Regression and age calculations of isochrons were performed using Isoplot/Ex Version 3.00 software (Ludwig, 2008). The ratio error of ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd is 1% and 0.03%, respectively. The carbonate minerals have yielded an isochron age of 211 ± 4 Ma (Figure 10a), with the value of mean square of weighted deviate (MSWD) 7.3 and the initial ¹⁴³Nd/¹⁴⁴Nd ratio 0.511731 ± 0.000009 (Figure 10a). The nonlinear relationship between the plots on the 1/Nd vs. ¹⁴³Nd/¹⁴⁴Nd diagram (Figure 10b) excludes the possibility of a mixing line. Thus, the Sm-Nd isochron age could represent the crystallization ages of the stage I carbonate minerals.

6. Discussion

6.1. Sources of sulfur and metals

6.1.1. Source of sulfur

The sulfur isotopic compositions of sulfides extracted from mineral deposits traditionally serve as discerning indicators of sulfur sources, offering insights into ore formation conditions and genesis. Alterations in δ³⁴S values are commonly associated with shifts in source, temperature, pH, and oxygen fugacity (fO₂) (Ohmoto, 1972; Ohmoto and Rye, 1979; Wilkin and Barnes, 1996). The array of δ³⁴S values within a specific region furnishes valuable information regarding the dynamics of sulfide precipitation, sulfate reduction processes, the extent of isotopic fractionation, and discerns whether the fluid system was open or closed contingent on limited or unlimited sulfate supply (Jørgensen, 1979; Ohmoto and Rye, 1979; Ohmoto, 1992).

Many views about the sulfur source of Zn-Pb deposits in the Fengtai ore cluster have been proposed, such as, (1) a mixed source of reduced sulfur in seawater sulfate and sulfur in deep-sea hydrothermal fluids, by decomposition of metal sulfide complexes in hydrothermal fluids or the combination of metal chloride complexes in hydrothermal fluids with basinal H₂S (Wei and Lu, 1990); (2) a mixture of ³²S concentrated sulfur formed by biological reduction with sulfur extracted from the sulfate and sulfide in the sedimentary formation (Wang J.L.; 1996); (3) the seawater sulfate mixed with the sulfur leached from the basement rock (Wang J.F.; 1991; Ren et al., 2013). However, previous works has not separately studied different sulfides from different mineralization stages, with no in-situ analysis, which cannot avoid the mixing of sulfides from different stages and sources.

Using the most advanced in-situ LA-ICP-MS analysis method, this work obtained distinct sulfur isotopic compositions of sulfides from different mineralization stages. Pyrites in sedimentary period have the highest positive δ³⁴S values (15.8‰~21.4‰), which are identical to Middle Devonian seawater sulfate (Claypool et al.,1980). The sulfides from the hydrothermal mineralization period have relatively uniform δ³⁴S values (1.1‰~10.2‰), peak values of +6 ~ +10‰ (Figure 11), obviously lower than that of the sedimentary period pyrite (15.8‰~21.4‰), partly overlapping that of the granite porphyry dike pyrite (2.1‰~4.3‰) (Table 2, Figure 11), suggesting that the sulfur of the hydrothermal mineralization period have a mixed source of magmatic sulfur and seawater sulfate, while the source of the sedimentary period sulfur is dominated by seawater sulfate. Wilkinson and Hitzman (2014) have advanced the hypothesis that magmatic heat, originating from the sub-crustal emplacement of mid-crustal sills, may serve as a catalyst for the regional fluid flow associated with Irish-type deposits. Similarly, Slack et al. (2015) have demonstrated that the ore-forming fluid of the black shale-hosted Red Dog Zn-Pb-Ag deposit emanated from hydrothermal fluids leached from mafic and ultramafic rocks at depth. Therefore, the contemporaneous magmatic activities at depth could have functioned as a primary or partial source of sulfur (S) for carbonate-hosted Pb-Zn deposits, including the Dongtangzi Zn-Pb deposit.

Compared with the pyrite from the hydrothermal mineralization period (averaging 7.2‰), the pyrite from the sedimentary period is remarkably rich in heavy sulfur (averaging 18.5‰), showing a seawater sulfate source, while the values of the pyrite of the granite porphyry dike (averaging 3.2‰) are closer to mantle-derived sulfur. The veined Stage I sphalerite in the siliceous rock range from 7.3 to 8.7‰ (sample D216.3), while the syngenetic disseminated pyrites from the siliceous rock are the most rich in heavy sulfur, ranging from 15.8 to 21.4‰ (sample D216.3), indicating that they have different sulfur sources and obviously do not reach isotopic equilibrium, even hosted in the same thin section. In general, if the δ³⁴S_pyrite > δ³⁴S_sphalerite > δ³⁴S_galena, then the fractionation of S isotopes among minerals reached equilibrium (Zheng et al., 2000). The δ³⁴S values of the sulfides from the hydrothermal mineralization period show δ³⁴S_sphalerite > δ³⁴S_pyrite > δ³⁴S_galena, displaying that the sphalerite and galena have reached equilibrium, but the pyrite and sphalerite have not, indicating that sulfur of the pyrite may be derived from a mixture of several sources.

Wang X. et al. (1996) obtained a range of 165-320°C for the homogenization temperature of the fluid inclusions in quartz and sphalerite from the main mineralization stage of the Dongtangzi Zn-Pb deposit. Usually, the TSR reaction at around 200°C will cause about 10‰ sulfur isotope fractionation between sulfates and reduced sulfurs (Kiyosu and Krouse, 1990; Machel et al., 1995). As the temperature increases, the degree of sulfur isotope fractionation caused by TSR (Thermochemical Sulfate Reduction) reaction gradually decreases, and there is generally no obvious sulfur isotope fractionation between sulfide minerals and reduced sulfur during mineral precipitation (Fry et al., 1986). Holser and Kaplan (1966) first proposed that the sulfur isotope values of seawater reached a minimum in the middle Devonian and then rapidly reached a maximum in the late Devonian (about 17‰ in the middle Devonian, about 30‰ in the late Devonian). The rapid increase in global seawater sulfur isotope values during the Devonian and early Triassic periods was possibly due to the rapid precipitation of pure sulfide minerals (Claypool et al., 1980).

If the sulfur in the Pb-Zn sulfides of the Dongtangzi deposit mainly comes from Devonian seawater sulfate, then the sulfur isotope composition range of sulfides formed by the TSR reaction of seawater sulfate at around 200°C would be the light blue area in Figure 11a, which is obviously higher than the sulfur isotope composition range of the measured sulfides in this study (Figure 11a, b). The single TSR process of Devonian seawater sulfate could not provide such low sulfur isotope composition, hence there must have involved a lot of light sulfur during mineralization processes. Possible sources include the following hypotheses: 1) Magmatic sulfur, as some of the Pb-Zn sulfides has consistent sulfur isotope compositions with pyrite in the granite porphyry dike (Figure 11) obtained in this study; 2) Sulfides enriched in light sulfur isotope through biogeochemical reduction reaction (BSR) of seawater sulfate during sedimentation, which is supported by the light sulfur isotope enriched pyrite (δ³⁴S<0‰）in the wall rock of the Dongtangzi deposit (Figure 11a); 3) The seawater sulfate of the Triassic sedimentary basin distributed to the south of the Fengtai ore cluster is a potential sulfur source, as the Triassic seawater sulfate are relatively enriched in light sulfur isotope, that could potentially generate sulfides by TSR (Figure 11a, light blue area) with similar δ³⁴S range compared to the Pb-Zn sulfides of the Dongtangzi deposit (Figure 11a). During Middle Triassic, the west Qinling orogenic belt have experienced a rapid transformation from Yangtze-type stable shallow carbonate sedimentation to Tethys-type deep-sea thick turbidite sedimentation. The Tethys-type rift basin opened in middle Triassic, and rapidly closed in Middle Triassic to early Late Triassic, developing complexly folded deformation in the rigid massif, sandwiched between the two ancient blocks (Li Y.J.; 2003). This rapid basin closure during Triassic may have created a suitable environment for the formation of evaporites, and the subsequent folding deformation provided the possibility of dissolution, infiltration, and long-distance transportation of the evaporites. For example, to the southeast of the Fengtai ore cluster, abundant evaporites are developed in the sedimentary rocks of the Middle Triassic Jialingjiang Formation in the northern margin of the Yangtze Block, adjacent to south Qinling belt, such as the Wadaozi, Huodigou and Zuoxigou gypsum deposits (Zheng et al., 1988; Yang et al., 2018).

In short, the sulfur associated with Zn-Pb mineralization of the Dongtangzi deposit has a mixed source of magmatic sulfur and seawater sulfate of evaporites from regional sedimentary formation.

6.1.2. Source of lead

Given the markedly diminished concentrations of thorium (Th) and uranium (U) in sulfide minerals, particularly in galena, the accrued influence of U and Th on radiogenic lead (Pb) isotopes within sulfides is deemed negligible, as attested by previous studies (Carr et al., 1995; Muchez et al., 2005; Pass et al., 2014). Consequently, the lead isotopic ratios derived from galena, sphalerite, and pyrite in the Dongtangzi deposit could represent the isotopic composition of the associated hydrothermal fluids.

Earlier studies suggested that metals of the Dongtangzi Zn-Pb deposit were originated from host Devonian to Carboniferous carbonate rocks (e.g. Wang, 1994; Zheng, 1994; Wang X. et al., 1996), and/or from magmatic activity in deep (Zhang et al., 2020). Li et al. (1999) suggested that Sinian to Carboniferous sedimentary rocks were important sources of metals. Wang et al. (1991) reported that the lead isotopic composition of the major deposits in Fengtai ore cluster (eg. The Qiandongshan, the Yindongliang, the Bafangshan, the Yinmusi, etc.) exhibited a relatively uniform and stable nature, with variations typically smaller than 1%. These findings indicated the presence of stable and normal lead with a high μ-value, suggesting a mixed source of lead originating from the crust within the orogenic belt. Qi et al. (1993) further noted that the sulfur and lead in the lead-zinc deposits of the Fengtai ore cluster originated from two distinct source regions, that is, the sulfur was derived from sea water while the lead is originated from the basement rock. Wang X. et al. (1996) proposed that the primary Pb-Zn source of the Pb-Zn deposits in the Fengtai ore cluster was the high-uranium crust rather than the mantle. This ore displayed characteristics of mixed lead, while the source region exhibited typical features associated with orogenic belts. Wang J.L. et al. (1996) further postulated that the initially enriched lead in ancient strata within the source region underwent leaching by subterranean hot water and was subsequently transported along growth faults to the seafloor, where mineralization occurred. The anomalous lead observed in the ore was attributed to the incorporation of crustal lead during later modification processes (Wang J.L. et al., 1996). However, prior investigations failed to differentiate sulfides from different mineralization stages and were not conducted in situ, resulting in the unavoidable mixing of lead from various stages and sources within the same sample.

In this study, the in-situ Pb isotope compositions of sulfides from different mineralization stages in the Dongtangzi deposit demonstrate a consistent and stable lead isotope composition across different hydrothermal mineralization stages. The narrow range of bulk isotopic data suggests a single source or else a well-mixed source of Pb (Table 3, Figure 12) (Zhou et al., 2018; Wei et al., 2020). However, the lead isotope composition of ore sulfides shows slight variations compared to the pyrite found in granite porphyry dikes, indicating some inconsistency in their source regions. That is, the ore sulfides are closer to the upper crust while the pyrite of the granite porphyry dikes are closer to orogenic belt (Figure 12a). This suggests that the primary source of lead-zinc elements may be from metamorphic basement rather than from magmatic sources. The lead isotope composition diagram illustrates that ore sulfides of the Dongtangzi deposit and Triassic granitic rocks such as the Xiba intrusion in the West Qinling orogen (Figure 12) have similar lead isotope compositions, indicating that they may share the same source area. Meanwhile, the lead isotope compositions of ore sulfides partially overlap with those of the Neoproterozoic Yaolinghe Group and Bikou Group metamorphic basement (Figure 12) beneath the Fengtai ore cluster. Such Pb isotopic signatures rule out contributions of Pb from the Paleozoic Mianlue oceanic crust (Figure 12).

Trends for the upper crust (U), orogenic belt (O), mantle (M), and lower crust (L) are taken from Zartman and Doe (1981). Data for Triassic granites from the west Qinling orogenic belt from Zhang et al. (2006), Zhang et al. (2007a, b), Qin et al. (2007, 2008a, 200b, 2010); Neoproterozoic Yaolinghe and Bikou meta-volcanic rocks from Xia et al (2007, 2008); and Paleozoic Mianlue oceanic crust from Xu et al. (2002b).

These findings suggest that the metallic materials involved in ore formation originate from a mixture of Triassic magmatic processes and metamorphic basement. The remarkably homogeneous lead isotope composition of the ore sulfide (Table 3, Figure 12) mirrors that of global MVT or SEDEX-type lead-zinc deposits (Broadbent et al., 1998; Leach et al., 2005), indicating the thorough mixing of ore-forming fluids within the source region.

6.2. Timing of the ore formation

The isotope dating of ore minerals is the best method to determine the age of a hydrothermal deposit. Obtaining the age of associated gangue minerals can further determine the metallogenic age. Since different mineral phases have different chemical potential, the paragenetic minerals with different chemical properties precipitated from the same ore forming fluid may have different Rb/Sr values and Sm/Nd values. Consequently, for the dating of hydrothermal deposit, reliance on hydrothermal mineral assemblages proves more advantageous than exclusive reliance on a single mineral species (Liu et al., 1998; Liu X.L. et al., 2014). Sulfide Rb–Sr isochron dating method has been widely used for dating the sediment-hosted Pb-Zn deposits (Nakai et al., 1990, 1993; Zhang R.B. et al., 2008; Zhang C.Q.; et al., 2008; Hu et al., 2012, 2015), as it could directly determine the age of a hydrothermal deposit. The calcium-bearing minerals such as calcite and fluorite in hydrothermal deposits are ideal objects for Sm-Nd isotopic dating (Halliday et al., 1990; Nie et al., 1999; Jiang et al., 2000). The temporal evolution of ore genesis within a hydrothermal deposit may span several million years, contrasting with a narrower timeframe of hundreds of thousands of years observed for a cohort of paragenetic hydrothermal minerals. Consequently, in the context of Rb–Sr isochron dating, it is reasonable to treat these paragenetic minerals as having formed synchronously (Liu et al., 1998). A similar situation also exists for the Sm-Nd isotopic system.

In this study, the handling of samples involved pulverizing sphalerite, galena, and pyrite single mineral particles to sizes below 200 mesh, followed by ultrasonic cleaning, effectively mitigating the interference from fluid inclusions within the minerals. Following the discriminant diagram proposed by Pettke et al. (1996), the constancy of the initial ⁸⁷Sr/⁸⁶Sr values during sulfide mineral growth can be determined by utilizing 1/Rb-⁸⁷Rb/⁸⁶Sr relation diagram, thus assessing the validity of the data. The 1/Rb-⁸⁷Rb/⁸⁶Sr relation diagram (Figure 9b) demonstrate that the data points do not exhibit linear correlation. The Rb and Sr contents vary across distinct sphalerite, galena, and pyrite single mineral samples, while ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr values remain relatively stable. This observation indicates that the initial ⁸⁷Sr/⁸⁶Sr values of sphalerite, galena, and pyrite remain essentially unchanged during their growth, lending geological significance to the isochron ages derived, effectively representing the primary mineralization age. The fundamental premise of Sm-Nd isochron dating for hydrothermal minerals is homogeneity, contemporaneity, and closure of the isotopic system. To ensure the basic conditions of contemporaneity and homogeneity for all samples, in this study, the samples were carbonates associated with the ore sulfides of the main ore-forming stage (the mineralization Stage I and II). Additionally, there is no linear relationship between 1/Nd and (¹⁴³Nd/¹⁴⁴Nd) (Figure 11b), indicating that the isochron formed by the 9 data points (Figure 11a) has practical significance and could represent the primary ore-forming age.

The isochron ages of sulfides Rb-Sr (211.6 ± 2.6 Ma, Figure 10a) and carbonates Sm-Nd (211 ± 4 Ma, Figure 11a) are closely consistent within the error range, indicating that the main mineralization stage of the Dongtangzi Pb-Zn deposit occurred at about 211 Ma, that is, the Late Triassic.

The ages of some Zn-Pb and Au deposits in the Fengtai ore cluter have been reported, such as the Bafangshan-Erlihe Zn-Pb-Cu deposit has yielded a pyrite Re–Os isochron age of 226 ± 17 Ma (Zhang et al, 2011) and a sphalerite Rb–Sr isochron age of 220.7 ± 7.3 Ma (Hu et al., 2012). A quartz ⁴⁰Ar–³⁹Ar isochron age of 222.1 ± 3.5 Ma (Feng et al., 2003), a muscovite ⁴⁰Ar–³⁹Ar age of 209.5 ± 1.4 Ma, and Sm-Nd isochron ages of 209 ~ 208 Ma (Wang Y.T. et al., 2020b) have been obtained for the large Baguamiao gold deposit. A carbonate minerals Sm-Nd isochron age of 203.2 ± 1.6 Ma (Liu et al., 2014) and a sphalerite Rb-Sr isochron age of 210.2 ± 2.4 Ma (Wang Y.T. et al., 2018) for the Chaima gold deposit have been obtained. Wang et al. (2014) obtained a ⁴⁰Ar–³⁹Ar plateau age of 211.9 ± 1.5 Ma for sericite from the Simaoling gold deposit. In addition, to the west of the Fengtai ore cluster, the giant Changba-Lijiagou Pb-Zn deposit has a sulfide Rb-Sr isochron age of 222.3 ± 2.2 Ma (Hu et al, 2015). The large Liba gold deposit has yielded a mica ⁴⁰Ar–³⁹Ar age of 216.4 ± 1.5 Ma (Zeng et al., 2012).

Similar ages have been obtained from the igneous rocks of the Fengtai ore cluster, such as, the NWW-trending granite porphyry dike of the Dongtangzi deposit have yielded a zircon U-Pb age of 221.3 ± 1.4 Ma (Chen S.C. et al., 2020), the NE-trending diorite dikes crosscutting the main orebody of the Bafangshan-Erlihe deposit and the granite porphyry dike within the mine area have yielded zircon U-Pb ages of 221 ± 3 Ma (Zhang et al., 2011) and 217.9 ± 4.5 Ma (Wang et al., 2011a), respectively. The monzonitic granite and the granodiorite of the Xiba pluton (Figure 1c) yielded zircon U-Pb age of 219 ± 1 Ma, and 218 ± 1 Ma, respectively (Zhang et al., 2009). The ages of the Taibai pluton and the Baoji pluton to the north of the Fengtai ore cluster are 216 Ma (Zhang et al., 2006) and 216 to 210 Ma (Liu et al., 2011a), respectively. The main ages of the west Qinling magmatic belt are within a similar age range of approximately 220 Ma to 205 Ma (Sun et al., 2000; Chen, 2010; Qin et al., 2010). It is summarized that the ages of polymetallic deposits in west Qinling are ranging from 231 to 197 Ma, concentrated around 220 ~ 200 Ma; While the ages of the magmatic rocks are ranging from 248 to 195 Ma, concentrated around 230 ~ 200 Ma (Wang Y.T. et al., 2021).

The ages described above indicate that the Zn - Pb metallogenic events, magmatic activities and regional tectonic processes of west Qinling orogenic belt are consistent in time (Wang et al., 2009b; Mao et al., 2012) and were closely related to the post-collisional regional extensional regime between the Qinling micro-block and the Yangtze block along the Mianlue suture zone during Late Triassic -Middle Jurassic (Mao, 2001; Mao et al., 2002, 2012), belonging to the east Paleo-Tethys tectonic domain (Wang et al., 2014).

6.3. Ore genesis

As mentioned above, the M-shaped anticline structure hosting the Dongtangzi Zn-Pb deposit could be regarded as the first-order ore-controlling factor. Structural deformation, hydrothermal fracture-filling, hydrothermal replacement mineralization, and intensely wall-rock alteration occurred after sedimentation and diagenesis. Additionally, the foreland basin environment that formed ore-hosting Devonian carbonate and clastic rocks (Dong et al, 2011, Dong and Santosh, 2016) is totally different from that of the SEDEX Pb-Zn deposits in the world, which formed in intra- and/or epicratonic rift and passive margin environments (Large et al., 2002; Leach et al., 2004, 2005). Therefore, Zn-Pb enrichment in this area should be dominated by epigenetic hydrothermal mineralization rather than syngenetic exhalative sedimentary processes.

Combining with the results of the Rb-Sr and Sm-Nd chronology and in-situ S-Pb isotopic studied in this work, the genesis of the Dongtangzi Zn-Pb deposit can be attributed to the multi-stage epigenetic hydrothermal fluid processes, driven by late Triassic regional tectono-magmatic activities. This work holds promising implications for understanding analogous Zn-Pb deposits of the west Qinling orogenic belt, given their similar geological settings and metallogenic characteristics.

7. Conclusions

1. The sulfur and Zn-Pb materials are believed to have originated from mixed sources of magmatic fluid and regional metamorphic basement.

2. The Dongtangzi Zn-Pb deposit is an epigenetic hydrothermal deposit, which formed in regional extensional regime during late Triassic, through epigenetic hydrothermal fluid processes.