Introduction

Identifying the source region of metals contained in hydrothermal ore deposits hosted in sedimentary rocks is a challenge as conventional isotope signatures may not necessarily constrain metal sourcing. Over the past decade, several studies used unconventional isotopes of ore metals (e.g., Hg, Cu, Fe, Zn, Mo, Pb, Cd, Ge, Re, and Os) as a direct tracer of metal sources (Mathur et al. 2002, 2010; Dauphas et al. 2004; Morelli et al. 2007; Smith et al. 2008; Kelley et al. 2009; Darling et al. 2012; Meng et al. 2015; Yin et al. 2016; Zhu et al. 2017; Debret et al. 2018). However, the application of metal isotopes to hydrothermal ore deposits hosted in sedimentary rocks remains scarce, and therefore, case studies are needed to validate metal isotopes as a viable method for constraining the sources of metals.

Mercury is abundant in sulfide minerals in hydrothermal deposits (Schwartz 1997; Rytuba 2003; Yin et al. 2016). Mercury isotope geochemistry has recently been employed as a tool for understanding the sources of Hg and perhaps other chalcophile metals in hydrothermal ore deposits. Mercury isotopes undergo mass-dependent fractionation (MDF). Since Hg has seven natural stable isotopes (196Hg, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg, and 196Hg), with a relative mass difference of 4%, Hg-MDF is readily occurring during most chemical (e.g., photoreduction, ligand exchange, sorption, precipitation, and abiotic methylation), physical (e.g., diffusion, evaporation, and volatilization), and biological processes (e.g., microbial methylation, demethylation) (Bergquist and Blum 2009; Yin et al. 2010). In some cases, the isotope fractionation of Hg is mass-independent, resulting in so-called mass-independent fractionation (MIF). Unlike Hg-MDF, Hg-MIF mainly occurs during photochemical reactions (Blum et al. 2014). A large variation of Hg-MDF and Hg-MIF signals, in terms of δ202Hg and △199Hg, respectively, have been reported in natural samples. The MIF signature of Hg observed in natural samples is thought to be related to Hg photochemical reactions (Blum et al. 2014; Yin et al. 2016). In hydrothermal processes, the release of Hg from source rocks seems to cause little Hg-MDF (< ± 0.5‰ in δ202Hg; Smith et al. 2008), but the boiling of hydrothermal fluids and redox reactions can result in significant Hg-MDF (up to 4‰ in δ202Hg; Smith et al. 2005). Hydrothermal processes do not cause significant Hg-MIF (Smith et al. 2005, 2008; Sherman et al. 2009; Tang et al. 2017; Yin et al. 2019). The effects of Hg-MIF can therefore act as a direct source tracer of Hg in hydrothermal deposits since syngenetic Hg in magmatic/mantle materials is characterized by the absence of MIF (Δ199Hg ~ 0), but epigenetic Hg in sedimentary rocks show a large variation of Δ199Hg (Yin et al. 2016 and references therein). Some hydrothermal sulfide deposits show small but significant Hg-MIF signals, which have been interpreted by inheritance of epigenetic Hg via sedimentation and hydrothermal leaching (Sonke et al. 2010; Yin et al. 2016; Xu et al. 2018).

In addition, sulfur isotopes can provide a direct constraint on the sources of sulfur in hydrothermal deposits. Although bulk analysis methods for sulfur isotopes can produce high-precision analyses, recent studies have shown that laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analyses of sulfur isotopes offer several advantages over bulk analysis method such as higher spatial resolution across individual mineral grains (10–100 μm) and provide useful geochemical information such as features of mineral growth at the sub-grain scale (Craddock et al. 2008; Bühn et al. 2012; Yuan et al. 2018). Therefore, in situ sulfur analysis has been employed as a tool to understand the sources of sulfur in hydrothermal fluids (Mason et al. 2006; Hou et al. 2016).

The Xikuangshan Sb deposit in southern China is the world’s largest source of Sb. This deposit has a total proven Sb metal reserve of about 2.5 Mt, with an average grade of about 4% Sb (Hu et al. 2017). Although the deposit has been the focus of several previous studies, the genesis of the ore deposit remains controversial owing to a lack of conclusive constraints on the sources of metals and sulfur. Two opposing viewpoints have been proposed to explain the ore genesis of this deposit: (i) intrusion-related and (ii) strata-bound models (Hu et al. 2017 and references therein). The former argued that ore materials were mainly derived from deep-seated magmas without significant contribution of Devonian host rocks (Liu et al. 1985), whereas the latter proposed that metals in this deposit were dominantly derived from sedimentary and metamorphic rocks without any genetic links to magmas (Chen et al. 1983; Ma et al. 2003).

In this study, we report Hg isotope data from Xikuangshan in conjunction with in situ sulfur isotope data to provide new insights into the sources of Sb, Hg, and S. The study highlights the potential of combined Hg-S isotope measurements to constrain the sources of ore materials for sedimentary-hosted Sb deposits.

Regional and ore deposit geology

The South China Block is made up of the Yangtze Block to the northwest and the Cathaysia Block to the southeast which were welded together along the Jiangshao suture zone (Zhao et al. 2011; Hu and Zhou 2012; Yao et al. 2016; Fig. 1). The Xikuangshan Sb deposit is located in central Hunan province, in the eastern part of the Yangtze Block. Outcropping strata in central Hunan can be defined by two major lithotectonic units: (i) basement low-grade (greenschist facies) metasedimentary rocks of Middle to Late Proterozoic and Early Paleozoic age, and (ii) overlying sedimentary rocks of Late Paleozoic to Cretaceous age (BGMRHN 1988). The former is composed lithologically of thin-layer metasandstones, tuffaceous slates, and megafossiliferous metasedimentary rocks, which are mainly distributed along the margins of the Xiangzhong Basin and some interior sub-uplifts. The latter consists of mainly thick-bedded, massive carbonates,with minor amounts of shales, siltstones, and iron formations (Fig. 1; Liu et al. 1985; Shi et al. 1993; Ma et al. 2002). Early Devonian (410–400 Ma) and Late Triassic (230–200 Ma) graitic rocks outcrop in the periphery of the district (Fu et al. 2015; Chen et al. 2016; Xie et al. 2019).

Fig. 1
figure 1

Location of the Xikuangshan Sb deposit in southern China (modified from Hu et al. 2017). CB Cathaysia Block, IB Indochina Block, JOB Jiangnan Orogen Belt, JSS Jiang-Shao Suture, NCC North China Craton, QDOB Qinling-Dabie Orogen Belt, SMS Song-MA Suture, YB Yangtze Block, XLF Xikuangshan-Lianyuan Fault, CTF Chengbu-Taojiang Fault

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The Xikuangshan Sb deposit (111° 27′ 30″ E, 27° 40′ 30″ N) is located at the intersection of the NE-striking Chengbu-Taojiang regional fault and NW-striking Xikuangshan-Lianyuan basement fault (Fig. 1). Middle-Upper Devonian and Lower Carboniferous strata are the only rocks exposed in the deposit area, which are dominated by carbonates and locally interbedded with siltstone, argillite, and shale (Peng et al. 2003; Fan et al. 2004). The Sb mineralization is mainly hosted in the central portion of the Devonian Shetianqiao Formation and, to a lesser extent, in the lower part of the Devonian Shetianqiao Formation. The central part of the Devonian Shetianqiao Formation is composed of sandstone, carbonate, and mudstone layers from lower to upper and has a total thickness of > 350 m. The carbonate layer hosts the bulk of mineralization and is dominantly limestone interlayered with thin sandy mudstone. The mudstone layers above the carbonate layer are thought to have acted as a barrier to rising ore fluids, resulting in the precipitation and accumulation of Sb in the limestone (Jin et al. 2001; Yang et al. 2006a). These strata have been intensely deformed into a kilometer-scale, NNE-trending major fold structure referred as the Xikuangshan complex anticline (Fig. 2a; Peng et al. 2003; Yang et al. 2006b). The fault F75, which is a part of the regional Chengbu-Taojiang fault, cuts the northwest limb of the Xikuangshan complex anticline. Almost all orebodies occur in the footwall of this fault (Fig. 2a). A NNE-trending lamprophyre dike is the only outcropping intrusive rock in the deposit area (Hu et al. 1996; Peng et al. 2003; Fan et al. 2004). Although the precise age remains unclear, the dike is believed to have formed prior to the Sb mineralization (Liu and Jian 1983; Liu et al. 1985; Xie et al. 2001).

Fig. 2
figure 2

Geological setting of the Xikuangshan Sb deposit in southern China. a Geological map showing the locations of the four ore blocks Laokuangshan, Tongjiayuan, Feishuiyan, and Wuhua (modified after from Peng et al. 2003). b A-A’ section showing the occurrence of orebodies and their relationship with silicic alteration (modified after Tao et al. 2002)

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The Sb mineralization in the Xikuangshan deposit consists of the four ore blocks, referred to as Laokuangshan, Tongjiayuan, Wuhua, and Feishuiyan (Hu et al. 1996; Peng et al. 2003; Fig. 2a). Orebodies in the Xikuangshan deposit are chiefly stratiform, generally extending from 30 to 600 m along the strike and 1300–1800 m down in dip with a thickness of 1–5 m (locally up to 20 m), and are strictly controlled by interlayer fault zones in silicified carbonates (Fig. 2b). More than 80% of the Sb metal reserves come from stratiform orebodies with irregular hydrothermal veins (Fig. 3). The ores are monotonous in composition with stibnite as the only economic metallic mineral, and no mercury minerals were found; gangue minerals are dominated by quartz and calcite with minor barite, fluorite, talc, and gypsum (Peng et al. 2003; Hu and Peng 2018). The detailed mineral paragenesis for the Xikuangshan deposit is divided into early-stage and late-stage mineralization, and the features of each stage have been documented by Lin (2014) and Hu and Peng (2018). Wall rock alteration related to Sb mineralization comprises dominantly silicification and carbonatization, as well as local weak fluorite alteration (Kuang 2000; Peng et al. 2003). Silicification is pervasive in the deposit and Sb orebodies never appear to exceed the limit of silicic alteration (Fig. 2b; Hu et al. 1996; Yang et al. 2006b). Available fluid inclusion data suggest that the ore-forming fluids at Xikuangshan had epithermal temperatures of 150 to 250 °C and salinities < 5 wt% NaCl equiv. (Lin 2014; Hu and Peng 2018). The data from hydrothermal calcite Sm-Nd dating and (U-Th)/He dating of zircon in altered wall rocks showed that the mineralization is Mesozoic with an age of ca. 120–150 Ma (Peng et al. 2003; Fu et al. 2019).

Fig. 3
figure 3

Photographs of ore types. a Stratiform ores. b, c Vein ores. d Irregular ores. e Lentiform ores. f Breccia ores

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Samples and analytical methods

Fifteen stibnite ore samples were collected from underground workings at different mine levels ranging from level 25# at an elevation of − 142 m to level 2# at an elevation of + 320 m in the Feishuiyan and Laokuangshan blocks. Three major types of ores were collected (Fig. 4), namely (i) quartz-stibnite; (ii) calcite-quartz-stibnite; and (iii) calcite-stibnite. In addition, seven wall rock samples, including four barren rocks (XKS-2@1, XKS-3@1, XKS-4@2, and XKS-7@2) and three altered rocks (XKS-2@2, XKS-7@6, and XKS-9@2), were collected. As no metamorphic basement rocks were exposed in the Xikuangshan district, four metamorphic rocks were collected from the nearby Xuefengshan uplift (Fig. 1), which is dominated by the metamorphic slate of the Banxi Group.

Fig. 4
figure 4

Scanning electron microscopy images of ore types encountered at the Xikuangshan Sb deposit. a Quartz-stibnite ores. b, c Quartz-calcite-stibnite ores with minor pyrite. d Calcite-stibnite ore. Cal calcite, Py pyrite, Qtz quartz, Stib stibnite

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Mercury isotope analysis

Approximately 100 g of each ore sample was crushed. Stibnite grains were handpicked under a binocular microscope, and handpicked stibnites in each sample were combined as a sample, which was manually crushed into 150 mesh in an agate mortar prior to chemical analysis. About 1 kg of each rock sample was crushed and sieved to 150 mesh. Approximately 0.1 g of ore samples and 0.4 g of rock samples were digested (95 °C, 6 h) with 5 mL aqua regia (HCl:HNO3 = 3:1, v/v). Certified reference material (NIST SRM 2711a, Montana soil) was digested using the same process. After storing overnight, the Hg concentrations in the digests were measured using cold vapor atomic absorption spectrometry (CVAAS), following the procedures detailed in Li et al. (2005). Recoveries of Hg for NIST SRM 2711a were between 90 and 110% (n = 3), and the relative variability of sample triplicates were < 8%.

The Hg isotope compositions of all samples were determined using Neptune Plus MC-ICP-MS (Thermo Electron Corp, Bremen, Germany) at the State Key Lab of Ore Deposit Geochemistry, Institute of Geochemistry, following the method by Yin et al. (2016). Mercury concentrations in sample digests were monitored by MC-ICP-MS using 202Hg signals, and the Hg concentrations estimated by 202Hg signals were within 10% of that measured by CVAAS. The 202Hg intensities were about 1.3 V per ng/mL Hg and 202Hg signals for blanks were about 1.0 × 10−3 V. Standard-sample bracketing (SSB) was used during the sample analysis. MDF is reported as xxxHg, which means the per mil deviations from an international Hg standard, NIST SRM 3133 (Blum and Bergquist 2007):

$$ {\updelta}^{\mathrm{xxx}}\mathrm{Hg}=\left\{\left[{\left({}^{\mathrm{xxx}}\mathrm{Hg}/{}^{198}\mathrm{Hg}\right)}_{\mathrm{sample}}/{\left({}^{\mathrm{xxx}}\mathrm{Hg}/{}^{198}\mathrm{Hg}\right)}_{\mathrm{NIST}3133}\right]-1\right\}\times 1000 $$

where xxx is the mass of each Hg isotope ranging from 196 to 204. MIF of Hg isotopes is reported using ΔxxxHg, which means the deviation of the measured isotope ratio from the theoretical ratio predicted by MDF and can be calculated using the following equations (Blum and Bergquist 2007):

$$ {\Delta}^{199}\mathrm{Hg}={\updelta}^{199}\mathrm{Hg}-\left({\updelta}^{202}\mathrm{Hg}\times 0.2520\right) $$
$$ {\Delta}^{200}\mathrm{Hg}={\updelta}^{200}\mathrm{Hg}-\left({\updelta}^{202}\mathrm{Hg}\times 0.5024\right) $$
$$ {\Delta}^{201}\mathrm{Hg}={\updelta}^{201}\mathrm{Hg}-\left({\updelta}^{202}\mathrm{Hg}\times 0.7520\right) $$

The diluted solutions were prepared to have an acid concentration of 10–20%, and Hg concentrations and acid matrices of the NIST SRM 3133 were matched to the bracketed samples. UM-Almadén secondary standard solutions were diluted to 1 ng/mL Hg in 10% acid and measured in the same way as the samples. The overall mean and uncertainty for UM-Almadén (δ202Hg = − 0.55 ± 0.15‰; Δ201Hg = − 0.04 ± 0.06‰; Δ199Hg = − 0.01 ± 0.04‰, 2σ, n = 7) and NIST SRM 2711 (δ202Hg: − 0.14 ± 0.09‰; Δ199Hg: − 0.21 ± 0.04‰; Δ201Hg: − 0.20 ± 0.05 ‰; 2σ, n = 3) are comparable with previous studies (Blum and Bergquist 2007; Sherman et al. 2009; Donovan et al. 2013; Yin et al. 2016).

In situ sulfur isotope analysis

In situ sulfur isotopic compositions of stibnites in ore samples and sedimentary pyrites in host rocks were analyzed by LA-MC-ICP-MS at the State Key Laboratory of Continental Dynamics, Northwest University, China. The laser ablation system (RESOlution M-50), consisted of an excimer laser (193 nm), a two-volume laser ablation cell (Laurin Technic S155, 155 mm × 105 mm), a squid smoothing device, and a computer-controlled high-precision X-Y stage, was linked to MC-ICP-MS (Nu Plasma 1700) for in situ S isotope analysis. The Nu 1700 MC-ICP-MS system equipped with sixteen Faraday cups and three ion counters was used to determine the sulfur isotopic compositions of the PSPTs. The cup configurations for sulfur were as follows: an H5 cup for 34S, an Ax cup for 33S, and an L4 cup for 32S. Helium was used as carrier gas (0.28 L/min) for the laser ablation process, and it entered the cell body at its bottom to fill the cell. Helium from both bottom and top through the funnel cell entrained the sample aerosol and argon (0.98 L/min) was admixed downstream, in front of the squid signal smoothing device, into the MC-ICP-MS. Details on the method are given by Chen et al. (2017). Instrumental mass bias was corrected using the SSB approach with repeated measurements of the standard reference (IAEA-S-1, Ag2S), before and after each sample. To monitor the accuracy of data during analysis, the in-house standard Cpy-1/GC (δ34S = − 0.7 ± 0.3‰) was analyzed every 8 unknown spots. Analytical uncertainty of δ34S is < 0.1‰.

Results

Mercury concentrations and isotopic compositions

The total Hg (HgT) concentrations and Hg isotopic compositions of the samples are summarized in Table 1. The Proterozoic basement metamorphic rocks and barren host rocks have low HgT concentrations of 0.02 to 0.06 ppm (mean = 0.04 ppm) and 0.44 to 1.25 ppm (mean = 0.77 ppm), respectively. In contrast, stibnite samples and altered host rocks showed high HgT concentrations, ranging from 11.25 to 97.43 ppm (mean = 37.51 ppm) and 3.85 to 8.09 ppm (mean = 6.21 ppm), respectively. The 15 stibnite samples show δ202Hg ranging from 0.04 to 1.15‰ (mean = 0.41 ± 0.68‰, 2σ) and Δ199Hg ranging from − 0.17 to − 0.03‰ (mean = − 0.11 ± 0.08‰, 2σ). The three altered rocks have δ202Hg values of 0.07 to 0.52‰ (mean = 0.33 ± 0.30‰, 2σ) and Δ199Hg of − 0.14 to − 0.02‰ (mean = − 0.04 ± 0.12‰, 2σ), which are similar to that of ore samples. The four barren rock samples show relatively lower δ202Hg values (− 2.22 to − 0.04‰; mean = − 0.71 ± 2.04‰; 2σ) but similar Δ199Hg (− 0.11 to 0.16‰; mean = − 0.01 ± 0.24‰, 2σ). The four metamorphic basement rocks have δ202Hg values of − 0.36 to 0.60‰ (mean = 0.11 ± 1.04‰, 2σ) and Δ199Hg of − 0.03 to 0.07‰ (mean = 0.00 ± 0.08‰, 2σ). The overall variations of Δ199Hg for all samples (− 0.17 to − 0.02‰) are much higher than the analytical uncertainty for the UM-Almadén (2σ = 0.04‰). Most ore samples (11 of 15) with reliable MIF have Δ199Hg/Δ201Hg ratios of 1.12 ± 0.38 (σ, n = 11), which approximate to the photo-reductive Δ199Hg/Δ201Hg ratios of 1 to 1.3 (e.g., Bergquist and Blum 2007; Zheng and Hintelmann 2009). However, no discernable correlation trends were observed between HgT and either δ202Hg or Δ199Hg.

Table 1 Mercury concentration and isotopic compositions in stibnite ores and potential source rocks for the Xikuangshan Sb deposit in central Hunan province
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Sulfur isotopic compositions

A total of 50 spots were analyzed, including 43 stibnite grains in ore samples and 7 pyrite grains in host rocks, and the results are shown in Table 2 and Fig. 5. The 43 stibnite grains show relatively narrow δ34S ranges of + 6.8 to + 10.2‰, with an average of + 9.2 ± 0.6‰ (n = 43, 2σ). No systematic trends in δ34S values are observed among all samples. In contrast, sedimentary pyrite grains from host rocks yield much higher δ34S values of − 26.0 to + 21.9‰ (+ 19.1 ± 6.0‰, 2σ, n = 7), suggesting they have a distinct origin from stibnite.

Table 2 In situ sulfur isotopic compositions of sulfide minerals in the Xikuangshan Sb deposit in central Hunan province
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Fig. 5
figure 5

Histogram of sulfur isotopic values of sulfide minerals from the Xikuangshan Sb deposit and potential source rocks. Data sources: mantle-derived sulfides: Chaussidon et al. (1989); pyrite in lamprophyre dikes: Yang (1986); sulfides in basement metamorphic rocks: Jiang (1990), Ma et al. (2003), and Gu et al. (2012)

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Discussion and conclusions

Hg isotope constraints on the sources of ore metals

In the Xikuangshan deposit, no Hg minerals were identified. It is believed that stibnite is the major carrier of Hg, not only as stibnite is the dominant sulfide mineral and the most Hg-enriched mineral (mean of HgT = 37.5 ppm) in this deposit but also due to that Hg can substitute with Sb in stibnite during its formation (Rytuba 2003). In fact, Sb and Hg are both chalcophile elements with similar chemical properties, suggesting that Hg generally has the same or similar sources as Sb (Arehart 1996; Hu et al. 2017; Nevolko et al. 2019).

The δ202Hg values (0.04 to 1.15‰) and Δ199Hg (− 0.17 to − 0.03 ‰) of stibnite, which present the isotopic signature of the majority of Hg in Xikuangshan, are within the range of reported in Hg and Pb-Zn deposits (Smith et al. 2005, 2008; Yin et al. 2016; Tang et al. 2017; Xu et al. 2018). Hg-MIF signals in natural samples were induced by Hg(II) photoreduction at Earth’s surface (Blum et al. 2014; Yin et al. 2016). As hydrothermal processes do not induce Hg-MIF (Smith et al. 2005, 2008; Tang et al. 2017), Hg-MIF can be directly used as a source tracer of Hg in hydrothermal deposits. As shown in Fig. 6, significant Hg-MIF signals, with △199Hg ranging from − 0.3 to 0.3‰, have been reported in sedimentary and metamorphic rocks, whereas magmatic rocks showed insignificant Hg-MIF (△199Hg = 0 ± 0.1‰). Most of our stibnite samples showed significant Hg-MIF signals with △199Hg of < − 0.1‰, which is distinct from that of magmatic rocks, suggesting either sedimentary rocks or metamorphic rocks contribute Hg in the Xikuangshan deposit. However, according to δ202Hg values, our samples differ from sedimentary rocks but overlap with metamorphic rocks. As limited Hg-MDF (δ202Hg < ± 0.5‰) occurs during the leaching of Hg from source rocks (Smith et al. 2008), the large difference of δ202Hg between stibnite samples (0.41 ± 0.68‰, 2σ, n = 15) and sedimentary rocks (− 3 to 1‰, with an average of − 0.73 ± 0.88‰, 2σ, n = 69) suggests that Hg was not originated from sedimentary rocks. We, therefore, hypothesized that Hg, and Sb because of its similar chemical properties, were mainly derived from the Proterozoic basement metamorphic rocks. This is supported by the following additional evidence: (i) relatively “fresh” sedimentary rocks generally have very low Sb contents (average 0.9 ppm), whereas the Sb contents range up to 46 ppm in “altered” host rocks, indicating an external source of Sb (Ma et al. 2003; Hu et al. 2017); (ii) the Proterozoic basement metamorphic rocks have the highest Sb abundance (7.8 to 27.2 ppm; Lu et al. 2001; Ma et al. 2002) in the studied region; and (iii) leaching experiments demonstrated that the extractable Sb from basement rocks range between 20 and 90% at 200 °C (Ma et al. 2002), suggesting that they are likely significant metal contributors for this giant Sb deposit.

Fig. 6
figure 6

Plot of δ202Hg vs. △199Hg values in stibnite from Xikuangshan Sb deposit, displaying a comparison of Hg isotopes between Xikuangshan deposit and lithological reservoirs. The range of sedimentary rocks was defined by the data from Smith et al. (2008), Thibodeau et al. (2016), Grasby et al. (2017), Gong et al. (2017), Yin et al. (2017), and Wang et al. (2018); the range of magmatic rocks was defined by the data from Smith et al. (2008) and metamorphic rocks are defined by the data from Smith et al. (2008) and Xu et al. (2018)

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Although the leaching of Hg from source rocks does not induce a significant change of δ202Hg (< ± 0.5‰; Smith et al. 2008), the stibnite samples show a variation of 1.11‰ in δ202Hg, suggesting MDF may have occurred within the ore bodies. The boiling of hydrothermal fluids and redox reactions can result in significant Hg-MDF (up to 4‰ in δ202Hg; Smith et al. 2005). Our results show a positive correlation between δ202Hg and sampling depth (Fig. 7). A similar increase of δ202Hg with depth has also been observed in epithermal systems in northern Nevada, USA, which has been explained by the evaporation of isotopically light Hg(0) during boiling of hydrothermal fluids (Smith et al. 2005). Boiling results in the loss of H2S, CO2, and Hg0 from hydrothermal fluids, which cause the precipitation of sulfide minerals (Spycher and Reed 1989; Simmons and Christenson 1994; Christenson and Mroczek 2003). Laboratory experiments also confirmed that the volatilization of Hg0 from aqueous phase cause Hg-MDF of > 1‰ in δ202Hg, with the vaporized Hg0 preferentially enriched in lighter Hg isotopes (Zheng et al. 2007). At Xikuangshan, recent fluid inclusion data provide some convincing evidence for the boiling process. For example, fluid inclusions with variable vapor/liquid proportions were observed within hydrothermal gangue minerals (Lin 2014). Additionally, dramatic fluctuations of pressures were recorded (Hu and Peng 2018) which could promote the occurrence of boiling during ore formation. Therefore, boiling is suggested as a major process responsible for the variation of δ202Hg observed in this study.

Fig. 7
figure 7

Covariation of the sampling elevations vs. δ202Hg in stibnite from the Xikuangshan Sb deposit

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Sources of sulfur in ore fluids

Stibnite is the principal sulfide in the ores of the Xikuangshan deposit, and in comparison with sulfide, the amount of sulfate minerals (barite and gypsum) is negligible; therefore, the δ34S values of stibnite can represent that of the total sulfur in ore fluids of this deposit (Seal 2006). Distinct from previous results using traditional methods (δ34S values ranging from − 3.3 to + 16.8‰; Zou 1988; Ma et al. 2003; Fan et al. 2004; Yang et al. 2006a), our new results by in situ analysis suggest that the δ34S values of stibnite ores, regardless of mineral paragenesis, are concentrated in a narrow range (+ 6.8 to + 10.2‰; Fig. 5). The clustering of the sulfur isotope values indicates that the sulfur in ore fluids of different mineralization stages had a common origin or was derived from the same source region. As documented by Hu et al. (1996) and Peng et al. (2003), the NNE-striking 10-km-long lamprophyre dike is the only intrusion identified in the deposit area (Fig. 2a). Pyrite in this dike exhibits lighter δ34S values (− 6.3 to − 3.9‰; Yang 1986), implying that the dominant sulfur in ore fluids was unlikely derived from the lamprophyric dike or the upper mantle. Additionally, a large variation of δ34S values (− 26.0 to + 21.9‰) within pyrite grains within Devonian host rocks was observed (Table 2). If the ore fluids whose sulfur component was derived dominantly from the host rocks, the δ34S values of ore fluids should be identical to that of Devonian initial seawater sulfate (δ34S of + 12.2 to + 17.2‰; Ohmoto and Rye 1979). These δ34S values, however, are much heavier than those of ore fluids obtained in this study (+ 6.8 to + 10.2‰; mean = + 9.2 ± 0.6‰), suggesting no significant contribution of sulfur from the Devonian host rocks.

Alternatively, our new in situ sulfur isotope data is comparable with the mean δ34S values of sulfides in the Proterozoic basement rocks (δ34S = + 5.6 to + 11.5‰; Jiang et al. 1990; Ma et al. 2003; Gu et al. 2012; Fig. 5). Therefore, we propose that the Proterozoic basement metamorphic rocks are likely the predominant source of sulfur during the formation of stibnites in the Xikuangshan deposit.

Implications for ore genesis

Available C-H-O isotopic data suggested that ore fluids for the Xikuangshan Sb deposit were dominated by deep-circulated meteoric water (Ma et al. 2003). Numerical modeling also suggested that the ore fluids mainly originated from meteoric water (Yang et al. 2006a). In addition, previous work noted that the Devonian host strata had low Sb content (0.7–2.3 ppm; Peng et al. 2001) making them an unlikely source of Sb. Using evidence from previous work in conjunction with Hg-S isotopes data obtained in this study, we propose that the Sb, Hg, and S transported by the ore fluids were predominantly derived from the underlying Proterozoic basement metamorphic rocks. A model can be invoked that assumes deep circulation of meteoric water, mobilization of Sb, Hg, and S from basement metamorphic rocks underlying the Xiangzhong basin, ascending of the hydrothermal fluids along deep faults, and subsequently deposition of Sb ore minerals within favorable structural traps resulting in the formation of the world-class Xikuangshan Sb deposit.