Constraints of S – Pb – Sr Isotope Compositions and Rb – Sr Isotopic Age on the Origin of the Laoyingqing Noncarbonate-Hosted Pb – Zn Deposit in the Kunyang Group, SW China

The Laoyingqing Pb – Zn deposit is located on the southwestern margin of the Yangtze block and on the east side of the Xiaojiang deep fault in the Sichuan – Yunnan – Guizhou Pb – Zn metallogenic triangle area (SYGT). This deposit was ﬁ rst discovered in the silty and carbonaceous slate of the Middle Proterozoic Kunyang Group that is structurally controlled by thrust faults and anticlines. This study is aimed at investigating whether the Laoyingqing deposit has the same ore-forming age and type as other Pb – Zn deposits related to the Pb – Zn metallogenic system and prospecting prediction of the deep and peripheral areas of the deposits in the SYGT. Based on the sphalerite Rb – Sr age dating and S – Sr – Pb isotopic composition analysis of the Laoyingqing Pb – Zn deposit, the following results were obtained. First, the Rb – Sr isochron age of sphalerite is 209 : 8 ± 5 : 2 million years (Ma), consistent with the ages of most Pb – Zn deposits in the SYGT (approximately 200Ma), thereby potentially indicating that these Pb – Zn deposits may have been formed synchronously during the late Indosinian orogeny. Second, the Pb isotopic compositions of sul ﬁ des show a linear trend on the average crustal Pb evolution curve in 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb plot. In addition, Pb isotopic ratios were consistent with the age-corrected Pb isotopic ratios of basement rocks, consequently suggesting that the source of mixed crustal Pb is mainly derived from basement rocks. Combined with the initial 87 Sr/ 86 Sr ratios of sphalerite between the ( 87 Sr/ 86 Sr) 200Ma value of the basement rocks and that of the Upper Sinian – Permian carbonates, it can be concluded that the ore-forming metals were mainly derived from basement rocks. Third, sulfur isotopic composition of sphalerite from the Laoyingqing deposits shows δ 34 S CDT values that range mainly from -2.62 ‰ to 1.42 ‰ , which is evidently lower than the δ 34 S CDT values of sul ﬁ des (8 – 20 ‰ ) from other Pb – Zn deposits in the SYGT. This can be interpreted as a result of mixing with reduced S that was mainly derived from the thermochemically reduced S in the overlying strata and a small amount of reduced S produced by the pyrolysis of S-containing organic matter. We conclude that the Laoyingqing deposit and most of the Pb – Zn deposits in the SYGT are Mississippi Valley-type deposits, thereby providing new ideas for investigating the deep and peripheral areas of Pb – Zn deposits.


Introduction
The contiguous Sichuan-Yunnan-Guizhou Pb-Zn metallogenic triangle area (SYGT) (Figure 1(a)), located in the southwestern margin of the Yangtze Block in China, is an important part of the South China low-temperature metallogenic domain [1][2][3][4]. There are more than 440 carbonatehosted Pb-Zn deposits distributed in the SYGT and the metal resources exceed 26 million tons, which makes SYGT the largest Pb-Zn mining area and the area with the highest metallogenic potential in China [5]. These Pb-Zn deposits have high grades of Pb+Zn (generally >15%) and are rich in various elements, such as Cd, Ga, Ge, and Ag [6][7][8]. The deposit concentration district in Northeast Yunnan is confined by the NS-trending Xiaojiang Fault zone, NE-trending Mile-Shizong Fault zone, and NW-trending Ziyun-Yadu Fault zone. The area's strong multistage structure superposition, variable geological environment with complex metallogenic dynamic mechanisms, and superior metallogenic conditions formed the unique Ge-rich Pb-Zn polymetallic deposit concentration district and created a massive world-class deposit (Huize Pb-Zn deposit, with a Pb-Zn reserve of 7 million tons) of a rare super-high grade (Pb+Zn: 25-35%) (Figure 1(b)). The area is typical in the SYGT [9], and it has become a hot spot for ore deposit studies.
The Kunyang or Huili Groups in the basement are generally considered as sources for ore-forming elements in the Pb-Zn deposits of the SYGT [24,32,33]. Currently, two Pb-Zn deposits hosted in the Mesoproterozoic strata have been found in the SYGT. One is the Xiaoshifang Pb-Zn deposit in Huili, whose ore body is hosted in the calcareous slate of the Huili Group from the Mesoproterozoic; this is a   [7,31]). Panel B: regional geological map of the SYGT, SW China, showing the spatial relationships among the fault belts, Emeishan basalts, and major Pb-Zn deposits (modified from [6]).  4 Geofluids volcanic-hosted massive sulfide (VHMS) type Pb-Zn deposit formed in the Early Cambrian [34,35]. The other example is the Laoyingqing Pb-Zn deposit located in the deposit concentration district in Northeast Yunnan. The ore body is hosted in the fracture zone of the silty and carbonaceous slate of the Kunyang Group from the Mesoproterozoic. The Laoyingqing Pb-Zn deposit is unique within the SYGT and evidently different from the Xiaoshifang Pb-Zn deposit. Determining whether the Laoyingqing deposit has the same ore-forming age and type as those of other Pb-Zn deposits is directly related to understanding the Pb-Zn metallogenic system of the region and can benefit the prospecting prediction of the deep and peripheral areas of the deposits in the SYGT. However, there are few previous studies on the oreforming age, material source, and genesis of the Laoyingqing Pb-Zn deposit. Isotopic geochemistry is a powerful tool for studying hydrothermal deposits. Direct dating of hydrothermal deposits is key for the correct evaluation of their relationships with geotectonic events. However, due to the lack of suitable minerals for traditional radioisotope dating, this process is more difficult [36][37][38]. In recent decades, the sphalerite Rb-Sr age dating method has been greatly improved and has become an ideal tool for the direct dating of minerals in Pb-Zn deposits [37,[39][40][41][42][43]. However, the reliability of the sphalerite Rb-Sr dating method has been questioned due to the possibility of contamination with carbonate, clay, or volcanic ash inclusions resulting in erroneous 87 Sr/ 86 Sr ratios ( [44,45]). It has been experimentally demonstrated that the Rb-Sr isotopic compositions of residual solid samples after crushing and cleaning can be used to date a deposit [46]. The Rb-Sr isochron ages obtained by this method in the MVT deposits of the Polaris and upper Mississippi Valley areas are the same as their paleomagnetic ages; therefore, these ages were considered to represent the mineralisation ages [46]. At present, in terms of constraining the geodynamic settings of carbonate-hosted Pb-Zn deposits, the sphalerite Rb-Sr geochronology has been widely used (e.g., [47][48][49][50][51]). For instance, it has been recently reported that high-spatial in-situ Rb-Sr dating of fine-grained mineral slickenfibres within faults can constrain the time of fault activity and resolve the complex histories of fault reactivation [58]. The stable isotopes of S, Pb, and Sr are powerful tools that can be used to constrain the source of oreforming metals and fluids, as well as fluid migration pathways (e.g., [57,[59][60][61][62]). However, there are few studies on these isotopic systems in the Laoyingqing Pb-Zn deposit.

Geofluids
In this study, we performed a comparative analysis of the Laoyingqing Pb-Zn deposit and other relevant Pb-Zn deposits in the SYGT, in terms of ore deposit geology, oreforming ages, and ore-forming material sources, so as to determine the genesis of the Laoyingqing Pb-Zn deposit and the relationship with other deposits in the SYGT. The comparison is based on sphalerite Rb-Sr isochron age dating and systematics of S-Pb-Sr stable isotopes which helped reveal the metallogenic dynamic background and the differences in the source of ore-forming materials of different deposits in the SYGT. The results provide new insights into the genesis of the Laoyingqing Pb-Zn deposit, a reference for the study of Pb-Zn metallogenic systems in the SYGT, and provide new ideas for investigating the deep and peripheral areas of Pb-Zn deposits in the SYGT.

Regional Geology
The SYGT is located in the southwestern margin of the Yangtze block (Figure 1(a)), which is a triangular area bounded by the SN-trending Anninghe Fault, the NW-trending Yadu-Ziyun Fault, and the NE-trending Mile-Shizong Fault. There are many SN faults matched with NE and NW secondary faults and folds in the area [63]. Five thrust-fold belts controlled by NE structures have developed in the deposit concentration district in Northeast Yunnan, namely, Xundian-Xuanwei, Kuangshanchang-Jinniuchang, Huize-Yiliang, Qiaojia-Daguan, and Qiaojia-Jinshachang structural belts. These belts are distributed in a "multi-character" pattern, which is typical of the wide-scale patterns in Northeast Yunnan [10,64,65] (Figure 1(b)).
The regional strata are mainly composed of basement and sedimentary covers. The basement is mainly composed of the Archaeozoic-Palaeoproterozoic crystalline basement and the Mesoproterozoic folded basement [6]. Exposures of this region include the folded basement of the Dongchuan and Kunyang Groups (Huili Group) which are a set of shallow marine flysch clastic rocks intercalated with volcanic rocks mainly composed of graywackes, slates, and other carbonaceous to siliceous sedimentary rocks [66,67]. Thick marine sedimentary rocks were deposited from Neoproterozoic to the early Mesozoic. Among them, Sinian rocks mainly consist of thick dolomite; Cambrian deposits are characterised by shale and sandstone; Ordovician to Silurian strata are mainly composed of carbonate, argillaceous siltstone, and shale; Devonian sediments mainly comprise sandstone, siltstone, and carbonate; Carboniferous to Permian strata are mainly composed of carbonate and basalt; Triassic sediments mainly comprise sandstone, mudstone, and partly carbonate rock. During the Middle-Late Triassic, a strong oblique thrust-slip was induced in the foreland basin due to the closure of the Paleo-Tethys Ocean and orogenic events in the region (Indosinian orogeny, 250-230 Ma; [68,69]), which resulted in the formation of a series of thrust-fold structures (zones) [70][71][72].
The SYGT Pb-Zn deposits are mainly controlled by thrust faults. The ore bodies occur in the carbonate rocks from Neoproterozoic to Permian and occur in stratiform lenticular and veined forms (Table 1). Metallic minerals that occur in the deposits are mainly sphalerite, galena, and pyrite, whereas the nonmetallic minerals are mainly calcite, dolomite, and quartz. The wall rock alterations of the SYGT deposit are mainly dolomitisation, calcitisation, and silicification. The structural system in the region is very complex, which controls the distribution of the Pb-Zn deposits. The NS-trending faults in the west of the SYGT controlled the distribution of the Daliangzi, Tianbaoshan, Maozu, and Chipu deposits. The Tianqiao, Shaojiwan, and Shanshulin deposits in the east are distributed along the NW-trending Ziyun-Yadu Fault. The larger-sized deposits are usually distributed near the regional deep faults and the intersection of 5 Geofluids faults. The Laoyingqing Pb-Zn deposit is located at the intersection of the SN-trending Xiaojiang fault and the NEtrending Dongchuan-Huize-Zhenxiong Fault (Figure 1(b)).

Geology of the Laoyingqing Deposit
The Laoyingqing Pb-Zn deposit is located towards the east of the Xiaojiang Fault and in the core of the Wuxing anticline located in the western portion of the NNE-trending Dongchuan-Wuxing-Luozehe structural belt. The strata cropping out in the Laoyingqing ore-field include the Huangcaoling (Pt 2 h) and Heishantou Formations (Pt 2 hs) of the Mesoproterozoic Kunyang Group, as well as the Sinian Dengying Formation (Zz 2 dn) and Quaternary deposits (Q). The faults and folds in the ore-field mainly include NE-trending and NWtrending secondary faults and the SN-trending Wuxing anticline, which is a symmetrical anticline with a vertical axis and a strong secondary fold in its core. No magmatic rocks were exposed in the ore-field and surrounding areas. Under lowtemperature dynamic metamorphism, the Huangcaoling (Pt 2 h) and Heishantou (Pt 2 hs) Formations of the Mesoproterozoic Kunyang Group in the ore-field formed a shallow  6 Geofluids metamorphic rock series such as slate and phyllite with metamorphic quartz sandstone and marble. The wall rock alterations that are mainly distributed in the fault fracture zone and the wall rocks near the ore body are strong and mainly demonstrate signs of silicification and carbonatisation ( Figure 2(a)). The V 1 and V 2 ore bodies in the Laoyingqing and Laolongtian sections, respectively, discovered in the Laoyingqing Pb-Zn deposit occur in the silty and carbonaceous slate of the Huangcaoling Formation (Pt 2 h) and are, respectively, controlled by an NW-trending F 1 reverse fault and NEtrending F 2 reverse fault. The occurrence of the ore body is basically consistent with the fault. The F 1 fault has an inclination of 55°-65°, a dip angle of 70°-83°, and a fault fracture zone width of 1.6 m-16 m. The F 2 fault has an inclination of 292°-300°, a dip angle of 76°-78°, and a fault fracture zone width of 1.5 m-7.5 m. Both fracture zones are composed of zinc-bearing structural breccia and slate (fractured in F 1 and fragmented in F 2 ). The tunnels are used to control the blind ore bodies due to the steep occurrence of the two ore bodies (Figure 2(b)). The V 1 ore body has a length of 288 m, thickness of 3.56 m, and depth of 130 m. It contains 22764 tons of Zn metals and has a grade of 4.92 wt.% Zn. The V 2 ore body has a length of 250 m, a thickness of 3.10 m, a depth of 56 m, contains 10228 tons of Zn metals, and has a grade of 5.26 wt.% Zn (Figure 2(c)). The ore bodies have irregular vein shapes and are locally expanded and continuous. The ore metal minerals are mainly sphalerite, followed by a small amount of galena and pyrite; the ore gangue minerals are mainly quartz and calcite, followed by mica, dolomite, and chlorite. The ore contains inclusion structures and filling metasomatic structures; these structures are mainly breccia structures, followed by fine-grained star point structures, disseminated structures, fine vein-network structures, and massive structures (Figures 3(a) and 3(b)).
The ores of the Laoyingqing deposit have experienced syn-sedimentary, hydrothermal, and supergene processes. There are two main mineral assemblages formed during the hydrothermal period ( Figure 4). In the sphalerite-pyritequartz assemblage formed in the early stage of mineralisation, the dark brown fine-grained sphalerite occurs in a lumpy and patchy form, the fine-grained pyrite is patchy in

Sample Collection.
In this study, eight representative primary metal sulfide ore samples from different middle sections of the V 1 and V 2 ore bodies were collected-six in V 1 and two in V 2 . In each sample, sphalerite, galena, pyrite, quartz, calcite, and dolomite occurrence were observed, with seven samples of sphalerite, one sample of pyrite, and eight samples of quartz. Five sphalerites samples from V 1 were used for Rb-Sr radiogenic isotope analyses, and the seven sphalerites samples and one pyrite sample were used to analyse S and Pb stable isotopes. Seven ore samples were prepared by grinding them into double-sided polished fluid inclusion sheets (approximately 200 μm in thickness).

Analytical Methods.
Microthermometry was performed at the fluid laboratory of the Southwest Institute of Geological Survey Centre for Non-ferrous Metal Resources. The instruments used included the Linkam THMS600 heating and freezing stage from the United Kingdom and a standard microscope equipped with an image analysis system. Prior to the experiment, the heating and freezing stages were calibrated with international synthetic fluid inclusion standard samples. When the temperature was lower than 0°C and higher than 200°C, the instrument error was ±0.1 and ±2°C, respectively.
The ore samples from the Laoyingqing Pb-Zn deposit were crushed to 40-60 mesh, and the minerals with high purity, as determined by microscope, were selected. These minerals were then ground to less than 200 mesh and sent to the laboratory for isotopic analyses.
For the Rb-Sr isotope analyses, the sample was washed with ultrapure water three times in an ultrasonic bath to remove the salts coming from broken fluid inclusions. An appropriate amount of mixed diluent of known concentration was added to the sample and dissolved with ultrapure 6 mol/L HCl and HNO 3 in a sealed Teflon cup. Then, the sample was heated in an oven at 200°C and evaporated until dry. Next, ultrapure 6 mol/L HNO 3 was used to redissolve the sample. These samples were then transferred to a cation exchange resin column to separate and purify Rb and Sr. For a detailed description of the experimental process, please refer to the description of Wang et al. [73,74]. The isotopic ratios were measured on a Triton (00682t) Multicollector Thermal Ionization Mass Spectrometer (TIMS) at the Wuhan Geological Survey center, China Geological Survey. The instrument fractionation of the Sr isotope was corrected by analysing the international standard NBS-987. The measured standard value of 87 Sr/ 86 Sr was 0:710233 ± 6, which was consistent with the recommended value (0:71023 ± 5; [47]). The errors (2σ) of the 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios were 2% and 0.02%, respectively. The Rb-Sr isochron age was calculated using the Isoplot/Ex version 3.22 software [75].
The S isotope analyses were performed at the Beijing Kehui Testing Technology Co. Ltd. using a 253 plus 10 kV Isotope Ratio Mass Spectrometer, a FlashSmart Elemental Analyzer, and a ConFlo IV multipurpose interface of Thermo Fisher Scientific of the United States. The sample was fully burned, and all the gases generated were fully oxidised through the redox reaction tube with the layered filling of WO 3 and Cu wires. Meanwhile, a small amount of generated SO 3 was reduced to SO 2 when passed through Cu wires. Subsequently, the SO 2 was separated from other impurity gases by a chromatographic column and supplied to the mass spectrometer for analysis. Three standard substances, IAEA-S3, GBW04414, and GBW04415, were used in the analysis, and the analysis accuracy of the standard samples was better than 0.2‰.
The Pb isotope analyses were conducted using the GV Isoprobe-T TIMS at Beijing Kehui Testing Technology Co. Ltd. The sample was first decomposed with HF+HNO 3 mixed acid, then evaporated to dryness after digestion. Concentrated nitric acid was added and evaporated to dryness 8 Geofluids again. Then, the F ions were removed from the sample for conversion to chloride. The chloride sample was dissolved with 1 mL of 2 mol/L hydrochloric acid, and Pb was separated by the resin exchange method. After evaporation to dryness, the Pb isotopes were measured by thermal surface ionization mass spectrometry. The laboratory background level of Pb during the whole process was 1 × 10 −10 g.   Figure 5(a)), which corresponded to an isochron age of 209:8 ± 5:2 Ma with an initial 87 Sr/ 86 Sr ratio of 0:71834 ± 0:00012 and a mean squared weighted deviation (MSWD) of 0.85.

Sulfur Isotope
Compositions. S isotope compositions of sphalerite and pyrite are shown in Table 4. The S isotope compositions of sphalerite were homogeneous, except for an outlier (δ 34 S =11.93‰) measured from pyrite ( Figure 6). The δ 34 S values ranged from -2.62‰ to 1.42‰ with an average of -1.11‰ at the Laoyingqing Pb-Zn deposit, thereby indicating that S may have come from a single source. The δ 34 S values of sphalerite in the Laoyingqing Pb-Zn deposit were significantly lower than those of most Pb-Zn deposits in the SYGT (Figure 7).   9 Geofluids [86]. For the medium and low-temperature Pb-Zn deposits, the difference between the homogenisation temperature of the secondary inclusions and the primary inclusions in the minerals was small, complicating the separation of inclusions [15]. During the experiment, both crushing sphalerite to a less than 200 mesh and ultrasonic cleaning greatly diminished the interference of primary and secondary fluid inclusions; thus, the data obtained from the solid residue is more likely to represent the mineralisation age [44,87]. In this study, dense massive ores from the same ore body and same middle section were selected for analyses. The sphalerite grains had a high purity, with no gangue mineral interpenetration and only a few fractures, which satisfied the precondition of Rb-Sr isotopic dating to the greatest extent. The 87 Sr/ 86 Sr and 1/Sr ratios of the sphalerite were not linearly correlated (Figure 5(b)), so the isochron (Figure 5(a)) was not a pseudoisochron of two-component mixing. Therefore, the isochron age accurately reflected the timing of the Pb-Zn mineralisation [40,49,51,88]. The Rb-Sr dating of sphalerite in the Laoyingqing Pb-Zn deposit showed that the four samples were all located on the isochron ( Figure 5(a)), indicating that the Sr isotopes were homogeneous and the sealing conditions were adequate in the  10 Geofluids process of ore mineral formation; thus, the fitted isochron age had a high accuracy. Nakai et al. [40] interpreted that the previous failure in Rb-Sr dating of sphalerite occurred because either the ore-forming system was not closed or the fluid inclusions in minerals were not cleaned out prior analyses. It is possible that for similar reasons, e.g., interference of fluid inclusions, the sample LYQ-7 in this study did not yield results that fall on the isochron. Geologically, the Laoyingqing Pb-Zn deposit is located in the secondary fault in the core of the Wuxing anticline and is spatially controlled by the fault-fold structure composed of the Xiaojiang Fault and Wuxing anticline. This implies that the Pb-Zn mineralisation should have occurred earlier than the formation of the Wuxing anticline. However, there is no data to constrain the timing of the formation of the Wuxing anticline. The youngest strata involved in the anticline are the sediments of the Triassic Guanling Formation, which were deposited at 240-230 Ma. Therefore, the formation may provide an upper age limit for the Laoyingqing deposit. Thus, the formation of the Pb-Zn deposits may occur after the deposition of the Guanling Formation. The sphalerite Rb-Sr isochron age (209:8 ± 5:2 Ma) for the Laoyingqing deposit is in accordance with this conclusion. In comparison, geological inferences, structural deformation screening, structural paleo-stress analyses, and isotopic dating [5] infer that the main age of the formation of the thrust-fold structure and ore formation period of the deposits in the SYGT was the late Indosinian (200-230 Ma), and some ore-forming ages of the deposits could be extended to the early Yanshan. The Rb-Sr isochron age of sphalerite in the Laoyingqing Pb-Zn deposit was also consistent with this conclusion.

Lead
The Laoyingqing Rb-Sr age is approximately the same as that of most other SYGT Pb-Zn deposits within a reasonable degree of uncertainty (Table 1 and Figure 7; [7,12,14,55,80,89]). Although many ages are derived by Rb-Sr isotopic dating, this coincidence not only provides more credibility for the Laoyingqing Rb-Sr date but also indicates that the Rb-Sr geochronology is a suitable dating method for most SYGT Pb-Zn deposits.
The presently available ages of the SYGT Pb-Zn deposits show that most formed between 230 and 190 Ma (Figure 8). The age of the Laoyingqing deposit also falls within in this period, indicating that deposits in the area may have been formed synchronously under the background of a regional hydrothermal event triggered by the orogenic collision event in the late Indosinian ( [68,69,90]).

Possible Sources of Sulfur.
The composition of S isotopes has always been one of the important methods used to understand the sources of ore-forming materials. An S-source analysis of a deposit must be based on the total S isotope composition in the hydrothermal fluid during sulfide precipitation [92]. Primary ores from the Laoyingqing Pb-Zn deposit were composed primarily of sphalerite, with a small amount of pyrite and galena. The lack of barite and other sulfate minerals in the ores indicated a low oxygen fugacity. Therefore, the δ 34 S value of sulfide was mostly representative of the total S isotope composition of the hydrothermal fluid, which can be used to directly trace the S source [93].
The Xiaojiang Fault is a regional deep fault, which provides a channel for the flow of fluid in the rocks. The ore mineral assemblage of the Laoyingqing Pb-Zn deposit is simple, and sulfate minerals are not developed in the wall rocks and ore. However, previous studies have shown that sulfate layers developed in the overlying Sinian and Cambrian marine carbonate rocks [6,52]. The δ 34 S values of marine sulfate in the Sinian Dengying period of the Yangtze Plate range from 20.2‰ to 38.7‰ [94], and the δ 34 S values of marine sulfates in phosphorites of the Lower Cambrian Meishucun Formation (Yuhucun Formation) range from 17.4‰ to 33.6‰ [95]. Marine sulfate can form reduced S through bacterial reduction of sulfur (BSR) and TSR [96], providing a sufficient S source for metal deposits.
Generally, BSR mainly occurs at relatively low temperatures (less than 120°C, generally 50-70°C) [97,98]. The S isotope fractionation caused by BSR is generally 4.0-46.0‰, and the δ 34 S value of H 2 S is generally negative [97]. However, TSR often occurs at relatively high temperatures (above 150°C), and TSR can cause 10-25‰ fractionation between seawater sulfate rock and reduced S at 150°C. With an increase in temperature, Δδ 34 S decreases as follows: at 100°C, Δδ 34 S = 20‰; at 150°C, Δδ 34 S = 15‰; and at 200°C, Δδ 34 S = 10‰ [98][99][100]. The homogenisation temperature of primary fluid inclusions in sphalerite and quartz grains in the Laoyingqing Pb-Zn deposit ranges from 130 to 275°C, with an average of 206.3°C (Table 2). Therefore, at this temperature, sulfate developed in the overlying marine strata may produce reduced S through TSR, and provide the main S source for the deposit through the fractures.
In the process of mineral precipitation, there is usually no obvious S isotope fractionation between reduced S and metal sulfides. For metal deposits with reduced S provided by TSR, the δ 34 S value of ore minerals should be approximately 15-20‰ lower than that of seawater sulfate in the formation [13]. The δ 34 S value of pyrite in the sulfides of the deposit is 11.93‰, approximately 8-26‰ lower than that of marine sulfate in the overlying strata, which is close to the expected S isotope fractionation caused by TSR. However, the δ 34 S value of sphalerite grains ranges from -2.62‰ to 1.42‰, with an average of -1.11‰, which is 23‰ to 36‰ lower than that of marine sulfate in the overlying strata. The S source may be a contribution of deep mantle-derived S (-3.0‰-+3.0‰) or 11 Geofluids organic matter pyrogenic S (Figure 7). However, the main magmatic rocks around the ore district and in the SYGT are the Late Permian Emeishan basalts (256 Ma) [101] and the basic diabase vein (156-166 Ma) [16], with the formation age of the basalt being older than the ore-forming age (209:8 ± 5:2 Ma). The formation age of the diabase vein is younger than the ore-forming age, and no magmatism that coincides with the ore-forming age has been observed. Any other instances of concealed Indosinian magma of unknown depth that can provide mantle-derived S to participate in mineralisation are unknown. However, some studies report that there is a genetic link between the Emeishan basalts and the Pb-Zn deposits, characterised in either providing oreforming materials or energy transfers [30,[102][103][104]. Therefore, it cannot be ruled out that the ore-forming fluid activated the Emeishan basalts to provide a small amount of S.
At temperatures higher than 50°C, S-containing organic matter can be thermally decomposed to produce H 2 S [59].  Figure 7: Comparison of sulfur isotopic compositions between the Laoyingqing Pb-Zn deposit and the major deposits in the SYGT (modified from [26]). S isotope compositions of the Huize deposit are taken from Liu and Lin [6], Li et al. [76], Han et al. [52], and Zhang [15]; those of the Maoping deposit are taken from Liu and Lin [6] and Wang et al. [11]; those of the Maozu deposit are taken from Liu and Lin [6], Zhang [15], and Zhou et al. [12]; those of the Lehong deposit are taken from Zhang et al. [14]; those of the Daliangzi deposit are taken from Liu and Lin [6], Zhang [15], Wu [13], and Yuan et al. [77]; those of the Tianbaoshan deposit are taken from Liu and Lin [6], Zhang [15], Zhou et al. [17], Zhu et al. [78], and He et al. [79]; those of the Chipu deposit are taken from Zhang [15] and Wu [13]; those of the Shanshulin deposit are taken from Zhou et al. [54]; those of the Shaojiwan deposit are taken from Zhou et al. [55]; and those of the Tianqiao deposit are taken from Zhou et al. [8].

Geofluids
The carbonaceous slate of the Kunyang Group contains a large amount of dispersed carbonised S-containing organic matter, and the δ 34 S value of organic S is generally negative [18]. The thermal energy provided by the ore-forming hydrothermal fluid causes the pyrolysis of S-containing organic matter in the wall rock to produce reduced S, which is mixed with the reduced S formed by the sulfate in the overlying strata through TSR. This mechanism is different from other Pb-Zn deposits in the SYGT. Therefore, the reduced S in the sulfide of the Laoyingqing Pb-Zn deposit may have mainly been derived from the mixture of the reduced S produced by the marine sulfate in the overlying strata through TSR and the reduced S produced by the pyrolysis of S-containing organic matter in the carbonaceous slate of the Kunyang group, with a possible addition of a small amount of mantle-derived S at a later stage.  (Figure 9(b)), which is usually interpreted as a mixed source of Pb [105][106][107]. The Pb isotopic compositions of all samples plot on the upper crust evolution curve in a nearly EW-trending linear array in the 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb space (Figure 9(a)), thereby indi-cating that the Pb ore had a predominantly upper crust source, with a possible minor Pb contribution from an orogenic belt.
Most of the Pb-Zn deposits in the SYGT were hosted in carbonate rocks, and large areas of Emeishan basalt distribution and basement rocks (Kunyang and Huili Groups) were exposed in the region. Therefore, most scholars believe that the ore-forming materials were provided by basement rocks, carbonate rocks, and Emeishan basalts; however, their proposed contribution compositions are different as follows: (1) mainly provided by sedimentary rocks, that is carbonate rocks [27]; (2) mainly originating from Precambrian basement rocks [24]; or (3) Emeishan basalt providing the main source of thermal power as well as part of the ore-forming material [102].
The Pb isotopic compositions of sulfides in the Laoyingqing Pb-Zn deposit were different from those of Cambrian sedimentary rocks, basalt, Sinian dolomite, and Devonian to Permian carbonate rocks in this region. However, they were mostly consistent with the basement rock (Kunyang Group) ( Table 6). The Pb isotopic compositions of sulfides in the Laoyingqing Pb-Zn deposit showed high consistency with the age-corrected Pb isotopic compositions of the basement rocks, yet minimal consistency with the Devonian to Permian carbonate rocks and the Sinian Dengying Formation of dolomites (Figure 9(a)). This indicated that a mixture of multiple Pb sources may have occurred, that is the Pb ore was largely derived from the basement rocks and the Pb isotopes were homogenised in the ore-forming process. This interpretation supports the argument that the basement rocks provided  [91]). Sources of the data are taken from Yin et al. [53], Lin et al. [89], Wu [13], Zhou et al. [12], Zhang et al. [14], Zhang et al. [7], Zhou et al. [80], and this study.
13 Geofluids ore-forming materials, which is differed from the conclusion that the Pb ore of the majority of other SYGT Pb-Zn deposits was predominantly derived from carbonate rocks.
The age difference between the Emeishan basalt magmatic activity (approximately 260 Ma) and the regional Pb-Zn mineralisation (approximately 200 Ma) may be more than 50 Ma [112,113], thereby indicating that there is no direct genetic link between the two events. However, the ore-forming fluids likely activated part of the ore-forming metals in Emeishan basalt, especially the ore-forming element Zn, during the processes of atmospheric precipitation or interlayer water infiltration [8]. 206   Pb isotope compositions of the Huize deposit are taken from Zhou [109], Han et al. [65], and Li [110]; those of the Maoping deposit are taken from Liu and Lin [6] and Wang et al. [11]; those of the Maozu deposit are taken from Liu and Lin [6] and Zhou et al. [12]; those of the Chipu deposit are taken from Wu [13]; those of the Daliangzi deposit are taken from Zhu et al. [111]; those of the Tianbaoshan deposit are taken from Zhou et al. [17]; those of the Tianqiao deposit are taken from Zhou et al. [8]; those of the Shaojiwan deposit are taken from Zhou et al. [55]; those of the Shanshulin deposit are taken from Zhou et al. [54]. The age-corrected isotopic ranges (200 Ma) for the Sinian (Z) Dengying Formation dolostone, the Devonian to Permian (D-P) carbonates, the Proterozoic basement rocks, and the Emeishan basalts are delineated by the data from Hu [33], Liu and Lin [6], Zhou et al. [24], Han et al. [52], and Yan et al. [84].
14 Geofluids 6.3.2. Constraints from the Sr Isotopes. 87 Sr/ 86 Sr isotopes are often used to trace the source of ore-forming materials, magmatic fluids, and crust-mantle mixing of deep source fluids [106]. However, when a Sr isotopic composition is used to trace the source of ore-forming fluids/materials, the Sr isoto-pic composition of potential source areas must be corrected by the ore-forming age. In recent years, considerable progress has been made in the chronology of the Pb-Zn deposits in the SYGT. The results of isotope dating indicate that the deposits in the region were mainly formed in the Late
The initial 87 Sr/ 86 Sr value of sphalerite in the Laoyingqing Pb-Zn deposit obtained in this study is 0.71834, which is close, albeit slightly lower than the mean continental crust 87 Sr/ 86 Sr value of 0.719, and higher than the initial mantle Sr value of 0.707 [114]. Therefore, the ore-forming material of this deposit was likely predominantly derived from the continental crust, which is consistent with the Pb isotopic tracer results in this study.
Previous studies have suggested that the source of oreforming fluids in the SYGT may have flowed through the basement rocks [65,102], implying they are likely the result of a mixture of basement rocks, sedimentary rocks in different strata, and Emeishan basalt [8,80,115]. According to Figure 10, the ( 87 Sr/ 86 Sr) 200 Ma values of the main deposits in the SYGT were significantly higher than the mantle and Emeishan basalt, which seemed to exclude the possibility that the ore-forming materials were completely provided by the mantle and Emeishan basalt. According to several previous geological and geochemical studies on ore deposits, the possibility of the Emeishan mantle plume contributing a large amount of ore-forming materials and fluids has mostly been excluded [8,36,120]. The ( 87 Sr/ 86 Sr) 200 Ma value of the main deposits in the SYGT was higher than the Upper Sinian-Permian sedimentary rocks and lower than the basement rocks, thereby suggesting that the ore-forming materials were not entirely derived from the hosted sedimentary rocks or basement rocks. The Laoyingqing, Huize, and Daliangzi deposits all show high ( 87 Sr/ 86 Sr) 200Ma values, indicating that Sr was predominantly derived from the basement rocks; in comparison, the ( 87 Sr/ 86 Sr) 200Ma values of the Maoping, Shanshulin, Shajiwan, and Tianqiao deposits are more similar to the range (0.7073-0.7111) of the Upper Sinian to Permian carbonate rocks, thereby indicating that Sr was predominantly derived from marine carbonate rocks and less from the basement rocks. At the same time, the initial Sr isotopic values of sphalerite residues in the Laoyingqing, Maoping, Shanshulin, Shaojiwan, and Tianqiao deposits show minimal change, thereby indicating that the Sr isotopes in the ore-forming fluids of each deposit were relatively homogeneous and had the same source. This dynamic may have been detrimental in yielding accurate and reliable Rb-Sr ages of sphalerite.
Therefore, we conclude that the Laoyingqing Pb-Zn deposit has the same characteristics as the major deposits in the SYGT. The main source of metal-rich ore-forming fluids may be predominantly derived from the mixture of the fluid flowing through the basement rock and the fluid flowing through the sedimentary rock of the caprock; however, most of the metal in the ore-forming fluid of the Laoyingqing Pb-Zn deposit is provided by the basement rock, consistent with the Pb isotopic tracer results in this study.

Possible Ore Genesis and Mineralisation Mechanism
6.4.1. Ore Genesis. The MVT Pb-Zn deposit is a shallow epigenetic deposit that is separated from the dense basin brine at approximately 75-200°C [56]. These deposits occur in the platform carbonate rocks; they are not related to the magmatic activity [56] and are associated with the extensional basin setting [57,121]. Many previous studies have suggested that the SYGT carbonate-hosted Pb-Zn deposits belong to the MVT considering their similarities in host rocks, tectonic setting, and ore-forming fluids [2,7,24,85,122]. By comparison (Table 1), the Laoyingqing Pb-Zn deposit may have formed synchronously with most deposits under the background of a regional hydrothermal event triggered by an orogenic collision event in the late Indosinian (~200 Ma) in the SYGT. This deposit demonstrates a series of characteristics that are prevalent in most deposits, such as a clear compressional tectonic setting, ore bodies controlled by a thrust-fold structure, an epigenetic structure, simple ore mineral compositions, vein and lenticular ore bodies, silicification, carbonatisation of wall rocks, and a minimal relationship with magma (Emeishan mantle plume) activity. Moreover, the homogenisation temperature of primary fluid inclusions in sphalerite and quartz grains in the Laoyingqing Pb-Zn deposit ranges from 130 to 275°C, the salinity ranges from 7.17 to 20.67 wt% NaCleqv (Table 2), and the ore-forming fluid is the gas-rich fluid with medium-high temperature and medium-low salinity, which is consistent with most of the SYGT Pb-Zn deposits (fluid inclusion homogenisation temperature ranges from 115 to 280°C, and salinity ranges from 0.9 to 20 wt% NaCleqv; Table 1). Therefore, the origin of the Laoyingqing Pb-Zn deposit is obviously different from the VHMS type and Sedimentary Exhalative type (SEDEX) Pb-Zn deposits, and most likely belongs to the MVT deposits like most deposits in the SYGT.

6.4.2.
Ore-Forming Mechanism. The ore-forming mechanism of the deposit should be mainly described in terms of its metal sources, compositional characteristics of fluid components, ability to carry ore-forming metal ions, migration mode, and metal precipitation mechanisms. Therefore, the ore-forming mechanism of the Laoyingqing Pb-Zn deposit can be described as follows. The late Indosinian (~200 Ma) collision orogenic event and the closure of the Tethys Ocean induced a strong oblique strike-slip in the foreland basin on the southwestern margin of the Yangtze Block, forming a large number of thrust nappe fold structures (i.e., Xiaojiang Fault and the Wuxing anticline) [5]. At the same time, large-scale basin brine migrated along regional faults and fissures in the basement and overlying strata, extracted metal elements (such as Pb and Zn), and finally migrated to the basement rocks (Kunyang Group). Simultaneously, extensive hydrothermal flow and circulation reduced S in the sulfate in the overlying strata into thiosulfuric acid and hydrogen sulfuric acid, migrated with the infiltrating fluid, and finally 16 Geofluids mixed with the reduced S produced by the pyrolysis of Scontaining organic matter in the wall rock (Kunyang group). The thermochemically reduced S of sulfate is the main source of reduced S. When metal-containing fluids and reduced Scontaining fluids mixed at favourable fracture sites in the core of the Wuxing anticline, accompanied by changes in ore-forming conditions, metal sulfides precipitated to form ore bodies of industrial value. Leach et al. [123] calculated the paleomagnetic ages of six major MVT Pb-Zn deposits in North America, and the results showed that the mineralisation process can last for 25 Ma. The S, Pb, and Sr isotope tracing results in this paper cannot entirely exclude the possibility that the Emeishan basalt provided ore-forming metals and reduced sulfur. Therefore, extensive hydrothermal flow may have activated the Emeishan basalt and extracted part of the ore-forming metals or reduced sulfur into the fluid. The ore-forming materials migrated with the infiltration of the fluids and participated in the formation of the ore in the late stage of the ore-forming process.

Conclusions
The sphalerite Rb-Sr dating results show that the Laoyingqing Pb-Zn deposit and most of the Pb-Zn deposits in the SYGT were formed at approximately 200 Ma. These deposits are related to the strong regional hydrothermal event triggered by the orogenic collision event in the late Indosinian, and the hydrothermal event had no direct relationship with the Emeishan mantle plume. The low δ 34 S value of sphalerites is the result of mixing between reduced S produced by the marine sulfate in the overlying strata through the TSR and reduced S produced by the pyrolysis of S-containing organic matter in the carbonaceous slate of the Kunyang group. Thermochemical reduction of sulfate is the main mechanism for reducing S. However, the addition of a small amount of mantle-derived S in the later period cannot be excluded. The Pb and Sr isotopic compositions indicate that the ore-forming metal was predominantly derived from wall rocks (Kunyang Group) and the contribution of dolomite of the Sinian Dengying Formation and Devonian-Permian carbonate rocks was minimal. It cannot be entirely excluded that the Emeishan basalt provided a small amount of metal elements. This deposit and most of the Pb-Zn deposits in the SYGT belong to the MVT deposits.

Data Availability
The underlying data are already in the manuscript.

Conflicts of Interest
The authors declare that they have no conflicts of interest.