Sources of the Laoxiongdong carbonate-hosted Pb-Zn deposit in Southwest China: constraints from S-Pb-Zn isotopic compositions

Wenyuan Wang · Jianguo Gao · Keyong Wang · Yangxia Nong

Abstract The Sichuan-Yunnan-Guizhou (SYG) Pb-Zn mineral province,which has significant Pb-Zn repositories in China,is situated in the western Yangtze Block.Despite decades of research, the Pb-Zn source of deposits is still disputed between the basement rocks, sedimentary wall rocks,or the Emeishan flood basalts.The newly-discovered Laoxiongdong Pb-Zn deposit is hosted by the Late Ediacaran Dengying Formation in the SYG province. Moreover, the Laoxiongdong orebodies lie between regional deep faults and close to the Proterozoic basement and Emeishan basalts. Hence, this deposit represents a good case study on the ore-forming material source for the regional Pb-Zn mineralization. The Laoxiongdong Pb-Zn ores are massive, vein, or disseminated and have mainly sphalerite,galena, pyrite,quartz, and calcite.In this paper,we present new sulfide S-Pb-Zn isotope data of the deposit. The sulfide δ34SCDT values (+ 11.8 to + 16.5 ‰)suggest that the reduced sulfur was mainly sourced from evaporites in the Late Ediacaran-Cambrian sequences. Pb isotopic ratios (206Pb/204Pb = 18.004-18.107, 207Pb/204Pb= 15.652-15.667, and 208Pb/204Pb = 38.037-38.248) suggest that the lead metal was primarily originated from the basement rocks. The sphalerite δ66Zn values (+ 0.16 to+ 0.37 ‰)are also highly similar(within error)to those of basement rocks (+ 0.10 to + 0.34 ‰), suggesting a basement-rocks zinc source with minor contributions from the carbonate host rocks and Emeishan flood basalts. The narrow sphalerite Zn isotopic range(0.21 ‰)also indicates that the Zn isotopic fractionation between the sphalerite and initial fluid was limited during the sphalerite ore precipitation. Therefore, we propose that both the Late Ediacaran Dengying Formation rocks and Proterozoic basement rocks were important ore-forming material source for the Laoxiongdong deposit, whereas the Emeishan flood basalts represent only a minor ore-material source.

Keywords S-Pb-Zn isotopes · The Laoxiongdong Pb-Zn deposit · Sichuan-Yunnan-Guizhou Pb-Zn mineral province · Southwest China

1 Introduction

Carbonate-hosted Pb-Zn deposits represent a major part of the world’s Pb-Zn resource, of which the MVT (Mississippi Valley-Type) Pb-Zn deposits are the most important(Leach et al. 2005). The MVT deposits are hosted by carbonate rocks, and genetically related to basinal brines and basement rocks. In addition, these deposits are also featured with low mineralization temperatures(90-150 °C),medium-high ore-fluid salinities(10-30 wt%NaCleqv.), and commonly low ore grade (＜10 wt% Pb +Zn) (Leach et al. 2005). Although a great deal of research has been conducted on the MVT deposits (Leach et al.2005), the details of the metallogenic mechanism remain unclear because these deposits occur across various geological and tectonic environments (Wilkinson et al. 2009;Leach et al. 2010; Zhou et al. 2018a).

The SYG (Sichuan-Yunnan-Guizhou) Pb-Zn mineral province in southwestern (SW) China has about 400 carbonate-hosted lead-zinc deposits,hosting over 200 million tonnes(Mt)Pb-Zn ores at 10 wt%Zn and 5 wt%Pb(Zhou et al. 2015). Previous works suggested that these deposits have MVT affinities in terms of their host rock types,tectonic setting and the presence of a reduced-S source.However, these deposits also show features distinct from classic MVT deposits, including high ore grades(＞10 wt% Pb + Zn), intermediate mineralization temperatures (150-280 °C), and low ore-fluid salinities (usually ＜15 wt% NaCleqv.) (Luo et al. 2019).

Despite many studies have been conducted, the origins of metallogenic materials in the SYG province remain controversial (Chen 1986; Han et al. 2007, 2014; Zhou et al.2001,2013a,b,c,2015,2018a,b,c;Luo et al.2019;Tan et al.2019).For instance,Chen(1986)argued that the ore-forming materials were derived from the host rocks,while Zhou et al.(2001)considered that the materials were sourced from the Neoproterozoic igneous rocks. Other authors suggested a mixed source of the wall rocks, the basement rocks and Emeishan flood basalts (Zhou et al.2014a, 2015). The deposit type is also disputed to be:(i) MVT (Wu et al. 2013, Zhang et al. 2015; Kong et al.2017;Zhou et al.2018a);(ii)magmatic-hydrothermal-type related to the Emeishan mantle plume (Xie 1963); (iii) a unique huize-type(HZT)with deep fluid source(Han et al.2014,2019);(iv)a unique SYG-type that is between MVT and magmatic-hydrothermal deposits (Zhou et al. 2018c).

The newly-discovered Laoxiongdong Pb-Zn deposit is hosted in the Late Ediacaran Dengying Formation. Furthermore, the deposit is tectonically situated between two regional faults (i.e., Xiaojiang deep-seated fault and Puduhe fault)and spatially close to the basement rocks and Emeishan basalts(Figs. 1b and 2b).Therefore,the study of the Laoxiongdong deposit potentially holds significant information in disclosing the role played by the host rocks(Late Ediacaran Dengying Formation), basement rocks,Emeishan basalts, and faults in metallogenic processes of the SYG mineral province.

Sulfur (S) and Lead (Pb) isotopes are important tracers of the source of metallogenic elements (Zhou et al. 2015).Zinc isotopes has also been applied to trace the Zn metal origin of Pb-Zn deposits in recent years (2004;Kelley et al. 2009; Zhou et al. 2014a, b; He et al. 2016;Duan et al. 2016; Deng et al. 2017, 2019; Gao et al. 2017;Wang et al. 2017, 2018a, b; Zhu et al. 2018; Meng et al.2019; Xu et al.2019;Zhang et al.2019a,b;Li et al.2019).In this study, we present field geological information and new data on sulfide S-Pb isotopes and sphalerite Zn isotopes from the Laoxiongdong Pb-Zn deposit. The results were used to explore the ore metal and fluid source at Laoxiongdong, and can thus unravel the potential controls on Zn isotopic variation and the ore formation in the SYG mineral province.

2 Geological setting

2.1 Regional geology

In South China, the SYG mineral province is situated on the western Yangtze Block, which is bordered by the Sanjiang Orogenic Belt to the southwest, Cathaysia block to the south, Songpan-Ganzi Orogenic Belt to the northwest and Qinling-Dabie Orogenic Belt to the north(Fig. 1a; Hu and Zhou 2012; Kong et al. 2017;).

Stratigraphic sequences of the SYG province are composed of Archean crystalline basement,Mesoproterozoic to Neoproterozoic folded basement, and Neoproterozoic to Mesozoic sedimentary sequences. The folded basement is primarily made up of the Dongchuan (～ 1700 to ～1500 Ma) and Kunyang (～ 1200 to ～ 900 Ma) Groups that comprise mainly slate, greywacke, and other siliceous sedimentary rocks (Liu and lin 1999; Zhou et al. 2018a).Neoproterozoic to Mesozoic sequences comprise thickbedded marine sedimentary carbonate and clastic rocks(Kong et al. 2017; Fig. 1b). The deposits in this mineral province are mainly hosted in the Kunyang Group metamorphic basement rocks or Late Ediacaran to Permian sequences that are overlain by the Emeishan flood basalts(Zhou et al. 2002). Over 25% of the Pb-Zn deposits are hosted in the carbonates rock of Late Ediacaran Dengying Formation, such as the Jinshachang, Lehong, Maozu,Daliangzi, Yinchanggou, and Tianbaoshan Pb-Zn deposits(Figs. 1b and 3; Luo et al. 2019; Chen 2015). The lower part of Dengying Formation is made up of dark-grey thickbedded dolostone, while the upper part is made up of grey medium to thick-bedded dolostone and siliceous/phosphorous-bearing dolostones. The Late Ediacaran Dengying Formation covers other Ediacaran strata or the folded basement and is overlain by Early Cambrian sedimentary rocks(Fig. 3).The thickness of Dengying Formation varies significantly in the region,from over 900 m in the western SYG province to below 500 m in the central and eastern parts (Wang et al. 2018a, b). Furthermore, the Dengying Formation is less exposed in the central and eastern SYG province (Wang et al. 2018a, b). Therefore, the Late Ediacaran carbonate-hosted lead-zinc deposits are concentrated mainly in the western SYG province (Fig. 1b). The previous reports of gypsum(pseudomorphs)and bird’s eye texture dolostone in the Dengying Formation indicate the presence of sulfur-bearing evaporites in the Dengying Formation (Wang 1990). Besides, the previous literatures proved that S-bearing evaporites were a significant S origin for the sediment-hosted lead-zinc deposits in the SYG province (Luo et al. 2019).

Fig. 1 a Tectonic map of South China; b Geologic map of the SYG province (modified from Zhou et al. 2018a; Luo et al. 2019)

Regional faults are primarily NS-, NE- and NW-trending (Fig. 1b). NS-trending faults (e.g., the Xiaojiang,Lvzhijiang-Anninghe, Puduhe, and Huize-Zhaotong) are primarily distributed in the western SYG province(Fig. 1b).The faults(e.g.,the Shizong-Mile)in the central and southwestern SYG province mostly trend NE (Han et al. 2014), whilst the NW-trending faults (e.g., the Kangding-Yiliang-Ziyun) are mainly in the northwestern SYG province (Jin 2008). Many of these faults (e.g., the Xiaojiang deep fault and Puduhe fault) were active in the various orogenic cycles (e.g., Hercynian, Indosinian,Yanshanian and Himalayan), and controlled the regional distribution of the magmatism and Pb-Zn mineralization(Zhou et al. 2014a, b).

Fig. 2 a Simplified structural map of the Laoxiongdong ore district; b Geologic map and c Geologic profile a-b of the Laoxiongdong deposit

Fig. 3 Simplified stratigraphic columns of the major Pb-Zn deposits hosted by Ediacaran Dengying Formation carbonates in the SYG province(modified from Luo et al. 2019; Wu 2013)

Magmatic rocks in the region consist of the Neoproterozoic Longqiushan granite, Middle-Late Permian Emeishan flood basalts (263-258 Ma), and Triassic alkaline granites (239-204 Ma) (Song et al. 2005). The Emeishan basalts are especially thick-layered, extensively distributed and spatially related to the Pb-Zn deposits(Fig. 1b) (Zhou et al. 2002). Then, Indosinian Orogeny(257-205 Ma) occurred (Ren 1984; Carter et al. 2001),which was associated with the closure of the Paleo-Tethys Ocean, forming the principal thrust faults that controlled the occurrence of lead-zinc deposits(Reid et al.2007).The previous timing data showed that the lead-zinc deposits in this mineral province were mainly formed between 226 and 192 Ma (Li et al. 2007; Zhou et al. 2014b, 2015; Zhang et al. 2015; Liu et al. 2017) and coeval with the regional Indosinian orogenic event.

2.2 Deposit geology

In the Laoxiongdong ore district, the outcrops mainly contain Late Ediacaran, Early Cambrian, and Permian rocks(Fig. 2b).The Late Ediacaran Dengying Formation is made up of dolostone and phosphorous-bearing dolostone.Overlying the Dengying Formation rocks are shale and siltstone of the Early Cambrian Qiongzhusi Formation.These sedimentary rocks are unconformably overlain by the Permian Emeishan basalts (Fig. 3).

Faults are well-developed in the Laoxiongdong deposit,which includes near EW-trending Baotaichang-Jiulong fault,series of NS-trending faults,and NW-trending faults.The near EW-trending Baotaichang-Jiulong fault has dip angles between 50° and 70°NE and a length of 100 km.Series of NS-trending thrust faults consist of Puduhe regional fault, Maolu fault and F1fault with dip angles between 50°and 80°NW,which controlled the distribution of lead-zinc deposits in the research area (Fig. 2a). At Laoxiongdong,the minor NW-trending thrust fault(F2)has dip angles between 21°and 34°NE and controls the attitude of Pb-Zn orebodies(Fig. 2b,c).The F3fault has dip angles between 25° and 40°NW and is parallel to Baotaichang-Jiulong fault.

There are two main orebodies in the Dengying Formation,which are stratiform and vein-type(Fig. 2b).The No.V orebody is the main orebody in the deposit and is 700 m long, 2 m thick and 20 m wide, plus the orebody No. I is 300 m long, 2 m thick, and 15 m wide. The deposit contains a metal reserve of about 0.05 Mt Pb + Zn at 0.23 to 5.64 wt% Pb (average 2.01 wt%) and 2.01 to 31.60 % Zn(average 12.50 %).

Metallic minerals mainly include sphalerite, galena,pyrite, and minor chalcopyrite and tetrahedrite, whereas non-metallic minerals consist of quartz and calcite(Figs. 2 and 4). Ore structures primarily contain massive (Fig. 4a,b), vein (Fig. 4c, d), and disseminated (Fig. 4e-h), whilst ore textures mainly include granular (Fig. 5a-e), metasomatic (Fig. 5f), solid-solution (Fig. 5g), and enclosed(Fig. 5h).In the massive ores(Fig. 4a,b),galena is fine-to coarse-grained (0.1-1.5 mm) with anhedral to subhedral texture (Fig. 5b, c), which forms massive aggregates. In vein ores, sulfides occur as veined aggregation. Galena is fine-grained (0.05-0.5 mm) with anhedral texture and coexists with chalcopyrite(Fig. 5i).Chalcopyrite occurs in tiny veinlets (0.03 mm) and coexists with tetrahedrite(Fig. 5j).Pyrite is very fine-grained(0.005-0.1 mm)and is distributed in curved linear aggregates (Fig. 5e). Calcite appears as tiny veinlets(Fig. 4d).In the disseminated ores,sphalerite is dominantly fine- to coarse-grained(0.1-5 mm) with anhedral to subhedral texture (Fig. 5a, d,g-h), and coexists with galena and quartz (Fig. 4e, g).Galena is fine-grained (0.01-0.2 mm) with anhedral to subhedral texture (Fig. 5a). Pyrite is commonly replaced by chalcopyrite, forming metasomatic texture, which suggests that chalcopyrite is formed later than pyrite(Fig. 5f).Very fine-grained (0.01-0.05 mm) pyrite is enclosed in coarse-grained (1-3 mm) sphalerite, forming enclosed texture. It suggests that sphalerite is formed later than pyrite (Fig. 5h). Chalcopyrite forms solid-solution texture in sphalerite, which indicates that chalcopyrite coexists with sphalerite (Fig. 5g).

According to the mineral assemblage and crosscutting relationships, alteration/mineralization at Laoxiongdong occurred in three stages: (I): pyrite ± quartz alteration(Figs. 4f and 5e, h), (II): sphalerite + galena + pyrite ± chalcopyrite ± tetrahedrite ± quartz mineralization(Figs. 4b,d and 5g,i),and(III): carbonate (calcite + dolomite) alteration (Fig. 4d).The detailed mineral paragenesis is shown in Fig. 6. Orerelated wall-rock alterations are relatively simple and mainly include silicification (Fig. 5l), dolomitization, and calcitization, of which silicification (e.g., silicified dolostone and quartz cement) is closely Pb-Zn ore-related(Fig. 5). Carbonate (dolomite and calcite) alterations are also spatially ore-related and are important exploration indicators.

3 Samples and analytical methods

3.1 Samples

Samples were mainly collected from exploration trenches and mining tunnels of No. V (main) and I orebodies in the Laoxiongdong deposit (sample locations are shown in Fig. 2c and Tables 1, 2, 3). Stage II ore sulfide separates were obtained for the S and Pb isotope analysis,whilst five stage II ore sphalerite separates were obtained from different locations in the Dengying Formation (at Laoxiongdong) for the Zn isotope analysis (Fig. 2c). As shown in Fig. 2c,samples for the Zn isotope analysis were collected at depths from 2600 to 2670 m(the LXD-17 collected from level 2670 m, LXD-09 ～11 from level 2653 m, and LXD-14 from level 2600 m, respectively).

Fig. 4 Photos of the Laoxiongdong Pb-Zn ore structures and minerals.a and b Massive ores;c-d Veined ores;e-h Densely disseminated ores;(i) Crumby ores. Abbreviations: Sp-sphalerite; Gn-galena; Qtz-quartz; Cal-calcite

3.2 Sulfur isotope analysis

Sulfur isotope compositions were measured at the State Key Lab of Environmental Geochemistry, China. The Elemental Analyzer (EA) method was used with a continuous flow isotope ratio mass spectrometer. The samples were tightly wrapped in tin cans and put into the EA reactor,then burned and decomposed to produce SO2.The external standards are GBW 04415 and GBW 04414 Ag2S.The standard for sulfur is Canyon Diablo Troilite(V-CDT),and the analytical precision is ± 0.2 ‰ (2σ).

3.3 Pb isotope analysis

Analytical laboratory of Pb isotope analyses were conducted at the Beijing Institute of Uranium Geology with the GV Isoprobe-T thermal ionization mass spectrometer. The analytical procedures are as follows, HF and HClO4were used to dissolve the samples in crucibles and then an anion exchange resin was added to purify Pb. The analytical results for the NBS 981 standard yielded208Pb/204Pb = 36.611 ± 0.004 (2σ),207Pb/204Pb = 15.457± 0.002 (2σ), and206Pb/204Pb = 16.937 ± 0.002 (2σ).The uncertainties(2σ)are 0.005 %for208Pb/206Pb ratio of 1 μg lead.

3.4 Zinc isotope analysis

Zinc isotope analyses were conducted at the Key Laboratory of Isotope Geology of the Ministry of Land and Resources, China. The sphalerite sample was heated on an electric heating plate after adding 1 mL 15 mol/L HNO3until the sample was completely dissolved. The dissolved sample was steamed and dried with 2 mL 6N HCl, which was converted into HCl by repeated steaming. Ion exchange chromatography was used to separate zinc from other elements (Tang et al. 2006). Zinc isotope analysis was performed with a Nu Plasma HR-MC-ICP-MS. The test was carried out under the low-resolution mode. The quality fractionation of the instrument was corrected by the standard-sample cross method(Li et al.2008).The external reproducibility for δ66Zn measurement is based on international basalt standard BCR-2 (0.28 ± 0.07 ‰; n = 6;2σ), within the range of previous determinations (Zhou et al. 2014a). All reported results were obtained by the averaging values of N repetitions. In addition, the results were compared with the zinc reference material JMC3-0749L (Maréchal et al. 1999).

Fig. 5 Microphotographs of the Laoxiongdong Pb-Zn ore textures and minerals.a Sphalerite,pyrite and galena occur as subhedral or anhedral fine- to medium-granular texture.; b Anhedral, fine galena grains; c Subhedral, fine galena grains; d Spherical-granular sphalerite; e Tiny fine pyrite grains are distributed in curvy linear aggregation;f Pyrite is replaced by chalcopyrite;g Chalcopyrite occurs as within sphalerite,forming solid-solution texture and chalcopyrite coexists with sphalerite; h Pyrite is enclosed in sphalerite; i Galena and chalcopyrite are enclosed in quartz grains;j Chalcopyrite coexists with tetrahedrite; k Pressure shadow of granular sphalerite enclosed in dolostone;l Dolomite is generally replaced by anhedral granular quartz, forming silicified dolostone. Abbreviations: Sp-sphalerite; Gn-galena; Ccp-chalcopyrite; Tet-tetrahedrite;Qtz-quartz; Dol-dolomite

Fig. 6 Mineral paragenesis of the Laoxiongdong deposit

4 Results

4.1 Sulfur isotopic compositions

The sulfide δ34SCDTvalues are listed in Table 1. The sulfides of this deposit have δ34SCDTvalues of + 11.8 to+ 16.5 ‰ (average + 14.7 ‰, n = 9), of which those of galena and sphalerite are of + 11.8 to + 12.5 ‰ (average+ 12.2 ‰, n = 6) and + 15.0 to + 16.5 ‰ (average+ 16.0 ‰, n = 9), respectively. The δ34SCDTvalues of galena and sphalerite (Fig. 7a) show the order of δ34-Spyrite ＞ δ34Ssphalerite＞ δ34Sgalena, which indicates equilibrium sulfur isotopic fractionation, at least on a local scale.

4.2 Lead isotopic compositions

Pb isotopic data are presented in Table 2. The Galena separates have206Pb/204Pb,207Pb/204Pb and208Pb/204Pb ratios varying from 18.004 to 18.107,15.652 to 15.667 and 38.037 to 38.248, respectively.

4.3 Zinc isotopic compositions

Sphalerite δ66Zn values from the Laoxiongdong deposit are presented in Table 3 and vary from + 0.16 to + 0.37 ‰(arithmetic average: + 0.27 ‰; weighted average:+ 0.28 ± 0.09 ‰).

5 Discussion

5.1 Source of sulfur and formation mechanism

5.1.1 Sulfur isotope equilibrium

Coexisting minerals may coprecipitate from the same solution or form at different times from different solutions.Only if coprecipitated from the same solution, these minerals are likely to attend isotopic equilibrium(Ohmoto and Rye 1979). Coexisting sulfide minerals of this study are in agreement with this condition (Fig. 5d). Furthermore, the best-fit mineral pair for temperature calculation is the sphalerite-galena pair (Hoefs 2018). As shown in Table 1,the sphalerite δ34S values are higher than those of galena.In addition, the δ34Ssphalerite-δ34Sgalenavary from + 2.7 to+ 4.5 ‰ among the analyzed sulfide pairs, which furtherindicates that sulfur isotopic fractionation between sphalerite and galena is under equilibrium fractionation.Therefore, the temperature and δ34SH2Svalues for the ore fluids could be estimated. The formula 1000lnαSp-Gn-= 0.73*106/T2(1000lnαSp-Gn≈ ΔSp-Gn)(Ohmoto and Rye 1979) may be used to calculate the temperature of the coexisting sphalerite-galena pairs.As presented in Table 1,the calculated mineralization temperatures in the Laoxiongdong deposit vary from 132 to 243 °C (average 202 °C), which is distinct from those of classic MVT deposits.

Table 1 δ34S, Δ34SSp-Gn, and calculated temperatures of the coexisting sphalerite-galena pairs (Ohmoto and Rye 1979)from the Laoxiongdong deposit

Table 2 Pb isotopic compositions of Galena from the Laoxiongdong deposit

Table 3 Zinc isotopic compositions of the Laoxiongdong sphalerites and related rocks in the SYG Province

When sulfate and sulfide coexist, it is hard to calculate total sulfur isotopic compositions of the ore fluids since the δ34S values of minerals depend on both the temperature and the redox conditions (Ohmoto 1972; Liu et al. 2017).However,no sulfate is found in the Laoxiongdong deposit.Hence,the S isotope fractionation was primarily controlled by the temperature (Liu et al. 2017). Ohmoto and Rye(1979)summarized the available S isotope data to conclude S isotopic fractionation related to H2S (Fig. 8). As shown in Fig. 8,the sphalerite δ34S values are close to those of the H2S in the ore fluids under 132-243 °C. Besides, the 1000lnαsphalerite-H2Svalue becomes lower with decreasing temperatures. Hence, the lowest δ34S (+15.0 ‰) value of sphalerite can approximate the total δ34S value of the orefluids.Moreover,the total δ34S value for the Laoxiongdong deposit (+ 13.4 ‰) can also be obtained by the method proposed by Pinckney and Rafter(1972)(Fig. 9).Thus,the total δ34S values of the ore-forming fluids determined by the two distinct methods are largely similar (difference = 1.6 ‰). In summary, the compatible range of+ 13.4 to + 15.0 ‰ is adopted as the total δ34S (H2S)value for the Laoxiongdong deposit.

Fig. 7 a Sulfur isotope histogram for the Laoxiongdong deposit;b Sulfur isotopic compositions for seawater sulfates;c Comparison between the Laoxiongdong deposit and other Dengying Formation carbonate-hosted Pb-Zn deposits. Data of other deposits and seawater sulfates are from Claypool et al. (1980), Zheng and Wang (1991), Zhou et al. (2013a, 2015), Li et al. (2016); Zhu et al. (2018)

5.1.2 Source of sulfur

Several potential S origins were thought to have contributed to Pb-Zn ore deposits in the SYG, including Emeishan basalts related to mantle, basements, and evaporites (Guan and Li 1999; Zhu et al. 2016; Zhou et al.2013a). Sulfur-bearing minerals in the Laoxiongdong deposit primarily consist of galena, sphalerite and pyrite(Fig. 5), whereas sulfate minerals are absent. Therefore,the sulfide δ34S values can approximate those of the fluid system, i.e., δ34Ssulfide≈ δ34Sfluid(Ohmoto 1972; Seal 2006). The total δ34S (H2S) of the Laoxiongdong sulfides(+ 13.4 to + 15.0 ‰) is clearly higher than the mantlederived sulfur (0 ± 3 ‰; Hoefs 2018). Similarly, the sulfur isotopic compositions of the Laoxiongdong deposit are also distinct from those of the Tianbaoshan deposit(- 0.4 to + 5.1 ‰), of which the reduced S was mainly derived from the basement rocks(Fig. 6c;Zhu et al.2016;Xu et al.2019;Tan et al.2019).Hence,the basalts and the basements may not provide major ore-forming sulfur for the Laoxiongdong deposit. As above-mentioned, sulfurbearing evaporites are common in the Late Ediacaran Dengying Formation and Cambrian-to-Permian(especially Cambrian,Devonian,and Carboniferous) sequences (Zhou 2011), which likely served as a potential sulfur source for its hosting Pb-Zn deposits. The δ34SCDTvalues of evaporites in the Dengying Formation, Cambrian, Devonian,and Carboniferous strata range from + 20.0 to + 38.7 ‰(average 29.0 ‰;Zhang et al.2004), + 17.4 to+ 33.6 ‰(average 28.0 ‰;Shields et al.1999),+ 21.8 to+ 25.9 ‰(23.6 ‰; Ren et al. 2018) and + 10.4 to + 18.6 ‰(average 13.1 ‰; Zhang et al. 2016; Ren et al. 2018),respectively, resembling those of the coeval seawater(15-35 ‰) (Fig. 7b). Furthermore, the δ34Ssulfate-sulfidecould be as high as + 15‰ in the thermochemical sulfate reduction process (Ohmoto 1972). Therefore,evaporites in the Late Ediacaran to Permian strata all have the potential to provide reduced sulfur for the hydrothermal fluids of the Laoxiongdong deposit. However, the Ordovician to Permian sedimentary sequences are less exposed around the mining area (Figs. 1b and 2b). Hence, evaporites in Late Ediacaran Dengying Formation and Cambrian strata probably have more contributions to the source of sulfur.Thus, the total δ34S (H2S) signals of sulfides (+ 13.4 to+ 15.0 ‰) suggest that the reduced sulfur mainly originated from the evaporites in the Late Ediacaran-Cambrian sequences. Similar interpretations for the sulfur source are yielded from many other Dengying Formation carbonatehosted Pb-Zn deposits (e.g., Yinchanggou, Wusihe, Jingshachang, Daliangzi and Maozu) in the region (Fig. 7c;Zhou et al. 2013a, 2015; Li et al. 2016; Zhu et al. 2018).

Fig. 8 Equilibrium S isotope fractionation of sulfur compounds relative to H2S (solid lines and dashed lines show measured and calculated values, respectively) ( modified after Hoefs 2018)

Fig. 9 Plot of δ34S versus Δ34S for the coexisting sphalerite-galena pairs of the Laoxiongdong deposit (R2 represents the coefficient of determination)

5.1.3 Formation mechanism of reduced sulfur

Two major hypotheses for the formation offromare namely thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) (Machel et al 1995; Worden et al. 1995; Seal 2006). TSR and BSR can produce sulfate-sulfide δ34S fractionation of 0 to + 15 ‰(Ohmoto and Goldhaber 1997) and + 15 to + 60 ‰(Goldhaber and Kaplan 1975),respectively.Both processes are temperature dependent, with BSR (＜ 110 °C) occurring under lower temperatures than TSR (＞ 100-140 °C)(Jørgenson et al. 1992). For the Laoxiongdong deposit, S isotope equilibrium temperatures (132-243 °C) are higher than those of bacteria survival. Therefore, TSR likely produced the majority of S2-fromin the Laoxiongdong deposit.

5.2 The implication from Pb isotopes

Th and U contents in galena are extremely low, so the increase of radioactive Pb isotopes caused by Th and U decay is negligible after the galena formation (Zhou et al.2013a).However,Pb isotopic compositions of the potential Pb metal origins for the Pb-Zn deposits in the SYG province (i.e., basement rocks, sedimentary rocks, and the Emeishan basalts)need to be corrected for ages(Zhou et al.2013a). As above-mentioned, the ore-forming time is between 226 and 192 Ma in the region. So, the timing of 200 Ma is applied to calibrate the lead isotope compositions of the surrounding rocks. The206Pb/204Pb and207Pb/204Pb ratios from Laoxiongdong deposit plot along the upper crust Pb evolution curve, suggesting a major upper crustal source of Pb (Zartman and Doe 1981;Fig. 10). In addition, the Pb isotope data also plot into the field of basement rocks in the206Pb/204Pb-207Pb/204Pb diagram (Fig. 10), indicating that the basement rocks were also a source of Pb metal.

Fig. 10 Plot of 207Pb/204Pb vs. 206Pb/204Pb for galena from the Laoxiongdong deposit(modified from Huang et al.2004;Zhou et al.2013c). Curves for the lower crust (L), mantle (M), orogenic belt(O) and upper crust (U) are from Zartman and Doe (1981)

5.3 Causes of zinc isotopic fractionation and sourcecontrolled Zn isotopic variation

5.3.1 Causes of zinc isotopic fractionation

Before using the Zn isotopic compositions to trace the origin of the Zn element, the degree of the zinc isotope fractionation between sphalerite and initial fluid (Δ66-Znsphalerite-initialfluid) at the time of precipitation of sphalerite ought to be considered(Zhang et al.2019a,b,c).The zinc isotope fractionation in hydrothermal fluids may result from four processes including(i)temperature(Mason et al.2005; Pašava et al. 2014), (ii) mixing of multiple zinc sources (Wilkinson et al. 2005), (iii) changes in geochemical conditions (e.g., pH and Zn complex species)(Fujii et al. 2011; Pašava et al. 2014; Zhang et al.2019a,b,c),and(iv)kinetic fractionation(Wilkinson et al.2005; Zhou et al. 2014a, b; Deng et al. 2017, 2019; He et al. 2016; Gao et al. 2017; Wang et al. 2018a; Zhu et al.2018;).

The previous literature indicated that there was no relationship between δ66Zn and temperature at low to medium temperatures(＜ 300 °C)(Maréchal and Sheppard 2002). In the Laoxiongdong deposit, the S isotope equilibrium temperatures are primarily between 132 and 243 °C, which suggests that temperature plays an insignificant role in zinc isotope variations. In addition,such a narrow range of δ66Zn variation of this deposit suggests that the mixing of multiple zinc sources plays an unimportant part in the Zn isotope variations. Moreover,data compilation by Zhang et al. (2019a, b, c) concluded that pH and Zn complex species changes could account for the wide-range(＞ 0.6 ‰)and negative δ66Zn values(Fujii et al. 2011;et al. 2014), which is inconsistent with the narrow-range and positive δ66Zn values in the Laoxiongdong deposit. Furthermore, δ66Zn variations of the Laoxiongdong sphalerite also do not show any spatial patterns (Fig. 2c), which is different from Rayleigh fractionation, and the narrow δ66Zn range (0.21 ‰) is also inconsistent with those produced by kinetic fractionation(～ 0.8 ‰)(Zhang et al.2019a,b,c).Hence,none of these four processes were likely influential on the Zn isotopic variation of this deposit, and thus the value of Δ66Znspha-lerite-initial fluid is small.

Several other processes that may homogenize Zn isotopic compositions in hydrothermal fluids have been proposed, including (i) Slow sphalerite growth that allows isotopic re-equilibration in the fluids(Gagnevin et al.2012;Duan et al.2016);(ii)Homogeneity of hydrothermal fluids on small scales (He et al. 2016); (iii) The rapid and complete precipitation of Zn2+(Zhu et al. 2018).

Previous studies indicated that the input rate of fluid at Navan deposit (Ireland) was limited, as supported by the common occurrence of sphalerite grains with colloform or zoned textures (Gagnevin et al. 2012; Zhang et al.2019a,b,c),which are absent in the Laoxiongdong deposit(Fig. 5a).Therefore,slow sphalerite growth was unlikely a Valley-type cause for Zn isotopic homogenization in the Laoxiongdong deposit.

Fig. 11 Comparison diagram of δ66ZnJMC between the Laoxiongdong deposit,basement rocks, sedimentary rocks and Emeishan basalts and Dengying Formation carbonate-hosted Pb-Zn deposits (data source same as Table 3)

The sphalerite samples of homogeneous Zn isotopic compositions from the Tianbaoshan Pb-Zn deposit were carried out via using a microdrill sampling system which represents the small-scale system (He et al. 2016). Nevertheless, five sphalerite samples were gathered from different locations of the Laoxiongdong deposit (Fig. 2c),suggesting that the homogeneity of hydrothermal fluids on small scales is not the reason for homogeneous Zn isotopic compositions in this deposit.

Similar sphalerite colors(brown)(Fig. 4e-g)and lack of colloform and zoned textures could prove that Zn2+was rapidly and completely precipitated (Zhu et al. 2018).Therefore,the rapid and complete precipitation of Zn2+ion is the reason for the limited Zn isotope fractionation between sphalerite and initial hydrothermal solution.

5.3.2 Source-controlled Zn isotopic variation

The aforementioned discussion suggests that the Δ66-Znsphalerite-hydrothermalfluid value is low. Hence, the δ66-Znsphalerite(0.16 to + 0.37 ‰) value can similarly represent the δ66Znhydrothermalfluidvalue. The basement rocks, host rocks, other sedimentary rocks, and basalts are proved to be potential origins for the lead-zinc deposits in the region(above). Compared with the Emeishan basalts(+ 0.30 to + 0.44 ‰; average 0.35 ‰) (Zhou et al.2014a),the sphalerite of the Laoxiongdong deposit is richer in light Zn isotopes,even though the sphalerite Zn isotopic compositions (+ 0.16 to + 0.37 ‰) are partly overlapped with those of the Emeishan basalts (Fig. 11). Hence, the Emeishan basalts could not supply the Laoxiongdong deposit with the majority of the Zn metal, despite the source of zinc that may be partially supplied by the Emeishan basalts. Although the sphalerite Zn isotopic compositions(+ 0.16 to+ 0.37 ‰)are within the range of the Late Ediacaran carbonates (- 0.24 to + 0.41 ‰;Table 3) (Zhou et al. 2014a; He et al. 2016; Zhang et al.2019a), the former has a much narrower range (0.21 ‰)than the latter (0.65 ‰). Therefore, the carbonate host rocks are unlikely to be the main Zn metal source in the Laoxiongdong deposit, despite the Zn may also have been partially supplied by the host rocks. Devonian to Permian sedimentary rocks could not have been a major metal source either due to their lighter Zn isotopes (- 0.22 to+ 0.17 ‰) than the Laoxiongdong sphalerite (Zhou et al.2014a). Previous studies indicate that Zn isotopic compositions of basement rock assemblage (carbonates, graywackes, and phyllites) vary from + 0.10 to + 0.34 ‰(Table 3;Zhang et al.2019a),which are consistent(within error) with the sphalerite Zn isotopic compositions in the Laoxiongdong deposit (+ 0.16 to + 0.37 ‰). Thus, the zinc metal of the Laoxiongdong deposit was mainly derived from the basement rocks with a certain influence from the host rocks and basalts, which is similar to the Pb source in this deposit and the metal source interpretations for the Maozu deposit in the SYG province(Fig. 11;Zhang et al. 2019a).

5.4 Ore-forming process

The Laoxiongdong deposit is hosted by carbonate rocks and controlled by faults, indicating that this deposit ought to be of the epigenetic type.Mineral characteristics and S-Pb-Zn isotopic compositions indicate that the ore-material source at Laoxiongdong was mainly related to the carbonates and evaporites in the Dengying Formation and the basement rocks.In particular,the basement rocks may have provided the main metal source for the Laoxiongdong deposit, which is also supported by the wall rock element contents, H-O-Sr isotopic data and leaching experimental data from other deposits in the SYG province (Zhou et al.2013a, b, c; Tan et al. 2017; Luo et al. 2019). Recent studies indicated that the degree of Pb and Zn enrichment in the basement rocks is higher than that in carbonate sequences (Luo et al. 2019). Moreover, Kong et al. (2017)compiled Sr isotopic compositions of sulfides, gangue minerals, and the potential metal source rocks from other Pb-Zn deposits in the region, and found that metals of these Pb-Zn deposits were primarily derived from a mixed source of basement rocks and host rocks. In addition, Han et al. (2019) demonstrated that the ore fluids were mainly originated from deep sources based on H-O isotope evidence. Furthermore, Kong et al. (2017) summarized mass H-O isotopic data from the SYG Pb-Zn deposits and considered that basement-derived metamorphic fluids were an important ore fluid source. Leaching experimental results from previous research (Bao et al. 2017) also indicated that Pb and Zn in the basement clastic rocks were easily leached to form the ore deposits in the SYG province, e.g., the Huize Pb-Zn deposit (largest Pb-Zn deposits in the region; Fig. 1b). Hence, basement rocks played an important metallogenic role for the SYG Pb-Zn deposits. Despite the general ore-forming features (e.g.,host rocks, the origin of reduced sulfur and metals) are comparable to those of typical MVT deposits, the Laoxiongdong deposit is distinct in terms of the Pb + Zn ore grade and ore-forming temperatures (Table 1). Therefore,we deem that the Laoxiongdong deposit is different from a typical MVT deposit, and is best categorized as a carbonate-hosted, fault-controlled epigenetic Pb-Zn deposit with multiple ore-material origins.

As shown above, the lead-zinc deposits in the SYG province were mainly formed during the Indosinian orogenic event. Tectonic events associated with Indosinian Orogeny had also developed a series of thrust belts in the SYG province (Xu et al. 2019). Furthermore, such a favorable tectonic setting could have also provided the necessary fluid channel and precipitation sites for the ore fluids (Han et al. 2007). In summary, we propose a comprehensive mineralization mechanism:Indosinian Orogeny may have driven the deep-seated/basement-derived metalbearing(rich in Pb,Zn,and Ag)fluids(as evidenced by Pb and Zn isotopes) upward along regional deep and large faults (e.g., Puduhe and Xiaojiang faults; Fig. 1b). Subsequently, these fluids were controlled by local thrust faults,e.g., F2fault in Paleozoic sedimentary rocks. The thrust fault systems can also lead to the greater destruction of wall rocks,which could greatly promote water/rock interactions and Pb-Zn hydrolysis (Zhang et al. 2019c). Therefore,these fluids may have leached ore-forming materials from the Late Ediacaran sedimentary sequencesevaporites and carbonate rocks) and minor basalts (as evidenced by S-Zn isotopes). Such processes could have caused the formation of S2-fromby TSR and the rapid sulfide precipitation(as shown by S and Zn isotopes).

6 Conclusions

(1) Sulfur isotope evidence suggests that the sulfur was mainly originated from evaporites in the Late Ediacaran Dengying Formation and that TSR was pivotal in the formation of S2-. The sulfur isotope calculated mineralization temperatures vary from 132 to 243 °C (average 202 °C).

(2) Pb isotope evidence suggests that the Pb metal was mainly derived from the basement rocks.

(3) Zinc isotope evidence suggests that Zn isotopic fractionation between the sphalerite and initial fluids was limited during the sphalerite precipitation and that the Zn metal was mainly derived from the basement rocks with certain inputs from the carbonate host rocks and Emeishan flood basalts.

AcknowledgementsWe thank the editors and two anonymous referees for their constructive comments. Thanks are also given to Pro.Jia-Xi Zhou,Pro.Lai Chun Kit,Dr.Xin-kai Liu and Dr.Xue-li Ma for the fruitful discussions. The research is financially supported by the National Natural Science Foundation of China (No. 41272111).