1 Introduction

The South China antimony (Sb) metallogenic belt is the most concentrated area of antimony deposits in the world. This belt contains many mid-to-large scale antimony deposits, which can be divided from west to east into two basins: (1) Xikuangshan, Banxi, Woxi, etc. in the Xiangzhong basin; and (2) Qinglong and Banpo, Banian, etc. in the Youjiang basin (Fig. 1a). Neoproterozoic epimetamorphic rocks of southeast Guizhou Province (the exposed area of basement of the Southern China fold belt), located in the uplift between the Youjiang and Xiangzhong basins, are distributed throughout the Bameng, Dacao, Peize, and Paidiao Sb deposits (Fig. 1a). These deposits spread along a NE trend, and they are all hosted by metamorphic rocks of the pre-Sinian Xiajiang Group, referred to as the Leishan-Rongjiang Sb ore field (LAOF) (Wang et al. 1994; Fig. 1b). Over the past two decades, many studies have been carried out on the South China Sb metallogenic belt to determine the metallogenic age, the evolution of metallogenic fluid, and the sources of metallogenic material (Hu et al. 2002, 2007; Peng et al. 2002, 2003a; Su et al. 2008, 2009, 2012; Huang et al. 2011; Hu and Zhou 2012; Chen et al. 2015; Zhu and Peng 2015). However, few studies of ore deposit geology, mineralogy, and element geochemistry have been carried out on the LAOF (Zhang 1985, 1994; Chen et al. 1991; Wang et al. 1994; Zhu et al. 1999), and most held different opinions and were published in Chinese. In addition, there has been no related detailed study carried out on the nature of the ore-forming fluids or on the metallogenic mechanism of the LAOF, an omission which limits understanding of the genetic relationships between different Sb deposits in the South China Sb metallogenic belt.

Fig. 1
figure 1

Location map showing the South China antimony ore belt in SW China (a), and tectonic schematic map of the Leishan-Rongjiang antimony ore field (b) (a after Hu and Zhou 2012 and b after Wang et al. 1994)

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Infrared microthermometry has been widely applied in geology in recent years, especially in the study of metallic minerals (e.g., stibnite, pyrite, enargite, wolframite) (e.g., Campbell and Panter 1990; Mancano and Campbell 1995; Kouzmanov et al. 2002). These minerals can be directly observed and analyzed to determine the geophysical and geochemical conditions of the ore-forming fluid (Buchholz et al. 2007; Ge et al. 2011; Ni et al. 2014, 2015; Sun et al. 2014, 2015; Zhu and Peng 2015). Here we present a systematic mineralogic study of the fluid inclusions (FIs) in stibnite and coexisting quartz and the sulfur isotopic geochemistry of stibnite of the Peize and Bameng Sb deposits. As these two deposits are representative deposits in the LAOF, our analysis provides a springboard to discuss the nature of the ore-forming fluids and the metallogenic process in the region.

2 Regional and ore-deposit geology

2.1 Geologic setting

Since the Wuling orogeny, crustal rigidity in southeastern Guizhou Province has increased. At the end of the late Proterozoic, the study area completed the process of cratonization (Wang et al. 1994). Multi-period tectonic movement in this area formed NNE-oriented large-scale circular and linear structures. The circular structures correspond to the large complex folds (Leigongshan anticline) controlling these deposits (Fig. 1b); the linear structures correspond to deep faults (i.e., the Songtao-Rongjiang deep fault) and to dykes. For example, a large number of lamprophyre and alkaline ultrabasic dykes are exposed in the Leigongshan area along the east side of the Songpan-Rongjiang faults, which are a part of the Hubei-Hunan-Guizhou lithospheric faults. However, the relationship between these rock bodies and the antimony veins is still not clear.

2.2 Geology of the Bameng and Peize deposits

The LAOF contains several Sb deposits, namely, the Peize, Gandang, Kaitun, and Dacao in the north; and the Bameng, Huoshaozhai, Paishao, and Gaopai in the south (Fig. 1b). The Bameng Sb deposit is the largest and accounts for 70% of the LAOF Sb reserves (Zhu et al. 1999). The exposed strata in the ore field mainly include the pre-Sinian Xiajiang Group, which has an angular unconformable contact with the underlying Sibao Group. From bottom to top, the Xiajiang Group can be divided into the Jialu, Wuye, Fanzhao, Qingshuijiang, Pinglüe, and Longli Formations. The Pinglüe and Fanzhao Formations are the ore-hosting strata of the Bameng and Peize Sb deposits, respectively. Igneous rocks are scattered within the ore field as small plutons and dykes. According to isotopic age data (No. 104 geological team of the Bureau of Geology and Mineral Resources of Guizhou Province), the emplacement age of these igneous rocks has a wide range and is different from the mineralization age of antimony. The deposits in the LAOF are mainly controlled by the Gula anticline (Fig. 1b), but the ore bodies are significantly controlled by faults (Fig. 2) and mostly occur as lenticular and vein-like shapes 60–650 m in length and 10–30 m in width, with dip angles of 40°–45° (Chen et al. 1991). The ore bodies that have widespread silicification always occur as veins in the fracture zone of steep faults and in the interlayer fracture zone (Fig. 2).

Fig. 2
figure 2

(modified from Wang et al. 1994)

Geological map of the Bameng Sb deposit in the Leishan-Rongjiang antimony ore field

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The mineral assemblages of different antimony deposits in the LAOF are generally similar. Ore minerals are dominated by stibnite (Fig. 4a–h) and pyrite (Fig. 4a, d) with minor sphalerite, chalcopyrite, and arsenopyrite (Wang et al. 1994); the gangue minerals include quartz (Fig. 4a–f), calcite (Fig. 4e), and minor amounts of ankerite (Fig. 4b), sericite (Fig. 4a), and pyrophyllite (Fig. 4h). Detailed mineralogical features have been documented (Zhang 1985, 1994; Wang et al. 1994; Zhu et al. 1999). The deposits of the LAOF can be divided into three types: stibnite-quartz, stibnite-quartz-calcite, and stibnite-quartz-pyrite, showing that the quartz is closely related to Sb mineralization. Common ore textures include brecciated (Fig. 3a), massive (Fig. 3b), fine-grained (Fig. 3c), disseminated (Fig. 3d), and especially replacement structures, showing characteristic hydrothermal alteration. At the LAOF, Sb mineralization is spatially associated with silicification and dolomitization. In addition, pyritization, sericitization, and silicification occur in wall rocks from the alteration center to the edge (Fig. 4a).

Fig. 3
figure 3

The main structures and tectonics of antimony ores of the Leishan-Rongjiang antimony ore field. a Breccia ore. b Drusy ore at the main-ore stage (Stb-II). c Bedded orebody in fault zone and altered minerals including ankerite, kaolinite, and limonite. d Disseminated ore of greenschist rocks at late-ore stage (Stb-I)

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Fig. 4
figure 4

Photomicrographs of metallic and gangue minerals from the Leishan-Rongjiang antimony ore field. a Alteration zone changes from sulfide, sericite, quartz, and silicification belts from center to edge. b Stibnite-II vein cuts across the quartz-ankerite-stibnite ore. c Vein-like stibnite cuts across greenschist. d Subhedral pyrite filled in the quartz-stibnite ore. e Stibnite-II formed in breccia ore. f Stibnite-II and coexisting quartz. g, h Fine-grained and anhedral stibnite-I and coarse-grained and subhedral stibnite-II. Stb stibnite, Qtz quartz, Py pyrite, Ser sericite, Si-Z silicification zone, Ank ankerite, Cal calcite, Prl pyrophyllite

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Events recognized at the Bameng and Peize deposits include pre-ore, main-ore, and late-ore stages, of which the former two are associated with the Sb mineralization. The pre-ore stage is characterized by finely-disseminated stibnite in the silicified wall rocks (Stb-I) (Figs. 3d, 4g, h). At the main-ore stage, the minerals included quartz and stibnite (Stb-II) with euhedral to subhedral crystals and grain sizes between 0.8 and 10 mm (Fig. 3a–c). Ankerite, pyrophyllite, and sericite were the hydrothermal alteration products in this stage (Figs. 3a–c, 4a–b, h). At the late-stage, minor pyrite appeared in the fine veins (Fig. 4d).

3 Analytical methods

Samples were collected from the Bameng and Peize Sb deposits, and examined and analyzed using an electron probe micro analyzer (EPMA) to identify the chemical compositions and paragenesis of the minerals. All of the FI studies were carried out in the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. Microthermometric measurements were obtained using a Linkam THMSG600 heating-freezing stage attached to an Olympus BX51 infrared microscope connected to a ROLERA-XR infrared digital camera. The estimated temperature measurement accuracy was ±0.1 °C. The rate was ≤15 °C/min, and from 0.1 to 1 °C/min near the phase transition point. To minimize the effects of infrared light heat on the homogenization and freezing point temperatures of FIs, we set the infrared light intensity as low as possible during the course of the experiment (Ge et al. 2011). Additionally, in the freezing experiment of the medium and low-salinity inclusions, we first carried out uniform heating, followed by freezing, to prevent the leakage of inclusion due to freezing expansion (Su et al. 2015).

Stibnite was crushed to 200 mesh using an agate mortar for sulfur isotope analysis, which was performed at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. The samples were reacted with heated CuO to produce sulfur dioxide, and their δ34S values were measured by a Thermo Finnigan MAT-253 mass spectrometer. The results are expressed as δ34S relative to the Vienna Canyon Diablo Troilite (V-CDT). The analytical precision was ±0.1‰.

Mineral compositions were determined using an EPMA-1600 operated with an accelerating voltage of 25 kV, a beam current of 10 nA, and an ablating diameter of 10 μm at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences.

4 Results

4.1 Mineral chemistry

We carried out the EPMA analysis for stibnite and pyrite at the main-ore and late-ore stages, respectively. The test results are in Table 1. Sulfur content in the stibnite samples averaged 26.93% and Sb content averaged 71.38%. Arsenic was the main accompanying element, with an average content of 2.47%, which is similar to that of the stibnite (2.12%–2.99%) in the Woxi Au–Sb–W deposit in the Xuefengshan Range (Zhu and Peng 2015). In addition, As can displace S, causing the loss of S in the stibnite. Moreover, some of the data indicate that the sulfophilic elements, such as Cu and Zn, are associated with stibnite, indicating that these elements were probably enriched in the ore-forming fluid during the main stage of Sb mineralization.

Table 1 Representative electron probe micro analyzer data of stibnite and pyrite in the Leishan-Rongiang antimony ore field (wt%)
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The late-ore stage samples were characterized by minor pyrite. Iron content in the pyrite averaged 45.69%; S content averaged 53.78% (Table 1). In addition, pyrite generally contained trace Sb (averaging 0.14%) as well as As (averaging 0.58%), albeit in a concentration much lower than that in stibnite. Gold content of the pyrite averaged 0.03%, suggesting that Au mineralization probably occurred in the same stage as pyrite precipitation and that the precipitation of Au may have been caused by the decrease in As content in the ore-forming fluid. The Au-bearing pyrite has little economic value due to low contents in the deposit.

4.2 Fluid inclusion petrography

The stibnite-quartz type ore was chosen for analysis of the main-ore stage. Quartz displayed a close paragenetic relationship with stibnite (Fig. 4e–f). Using the standard for identifying primary, secondary, and pseudosecondary inclusions by Roedder (1984) and Lu (2014), this paper mainly focuses on primary FIs in the LAOF.

Type-I FIs consisted of a vapor bubble and a liquid phase (Fig. 5a–c). These FIs were negative crystals, usually having long columnar and irregular short columnar shapes. The vapor phase occupied between 5 and 30% of the inclusion volume and homogenized into a liquid phase. The sizes of the inclusions ranged from 12 to 30 μm (long axis). Type-I FIs were abundant in stibnite samples and distributed along the {010} cleavage plane of the stibnite crystal.

Fig. 5
figure 5

Photomicrographs of representative fluid inclusion types at room temperature in stibnite and coexisting quartz. a–c Type-I mass fluid inclusions in stibnite; d–f Type-IIa individual fluid inclusions in stibnite; g–i Type-IIb fluid inclusions in quartz

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Type-II FIs (two-phase) were observed randomly as irregular shapes that developed in both stibnite and quartz. These FIs in stibnite and quartz were subdivided into Type-IIa and Type-IIb, respectively.

Type-IIa FIs appeared as isolated inclusions in stibnite as oval and irregular polygons with sizes of 10–40 μm. Vapor accounted for approximately 25% of the total volume of the inclusions (Fig. 5d, e).

Type-IIb FIs were scattered, mostly as oval and irregular shapes, with sizes of 6–13 μm. Vapor accounted for approximately 10–40% of the total volume of the inclusions and homogenized into a liquid phase (Fig. 5g–i).

Type-III included aqueous daughter minerals, sparsely distributed with Type-IIb in quartz, with sizes of 10–12 μm (Fig. 5h).

Clusters of primary FIs forming at the same time show considerably variable liquid/vapor ratios and generally exhibit roughly similar homogenization temperatures, suggesting that they form during the boiling of fluid (Roedder 1984; Lu et al. 2004). Similarly, this type of FIs occurred in stibnite samples (Fig. 5d, f), indicating that fluid boiling may have been a key factor responsible for the precipitation of stibnite during the main-ore stage.

4.3 Results of microthermometry

The phase transformation process of FIs in stibnite and quartz during temperature measurement is shown in Fig. 6.

Fig. 6
figure 6

The homogenization temperature (a), and the final ice-melting temperature (b) behavior of fluid inclusions in stibnite

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The Type-I FIs had melt temperature of ice (Tm-ice) values ranging from −0.9 to −3.2 °C and salinity varying from 1.57 wt% to 5.26 wt% NaCl equiv. (n = 43, mean = 3.61 wt% NaCl equiv.), with a peak value of 3.0 wt%–4.5 wt% NaCl equiv (Fig. 7b). Homogenization temperature values ranged from 142 to 210 °C (n = 49, mean = 171 °C) with a peak value between 160 and 180 °C (Fig. 7a). Bulk densities (Table 2) averaged 0.921 g/cm3.

Fig. 7
figure 7

Histograms of salinities and homogenization temperatures of fluid inclusions in stibnite (a, b), and coexisting quartz (c, d), collected from the Leishan-Rongjiang antimony ore field

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Table 2 Microthermometric data for the Leishan-Rongiang antimony ore field
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The Type-IIa FIs had Tm-ice in the range of −1.2 to −2.3 °C, corresponding to salinities of 2.07–4.65 wt% NaCl equiv. (n = 13, mean = 3.55 wt% NaCl equiv.), with peak values varying from 3.0 to 4.0 wt% NaCl equiv (Fig. 7b). Their homogenization temperature values mostly ranged from 155 to 184 °C (n = 13, mean = 161.97 °C) with a peak value between 150 and 180 °C (Fig. 7a). Bulk densities (Table 2) averaged 0.934 g/cm3.

For the type-IIb FIs, Tm-ice ranged from −1.1 to −3.6 °C, and salinities mostly varied from 1.91 to 5.86 wt% NaCl equiv. (n = 26, mean = 3.81 wt% NaCl equiv.) with two peak values (Fig. 7d). The main peaks ranged from 3.1 to 4.5 wt% NaCl equiv. whereas the sub-peak values were from 2.0 to 2.5 wt% NaCl equiv., consistent with the peak value of the Type-IIa inclusions (Fig. 7b). Type-IIb homogenization temperature values ranged from 138 to 252 °C (n = 26, mean = 172.8 °C), with two peak values ranging from 150 to 160 °C and from 170 to 190 °C (Fig. 7c). Bulk densities (Table 2) averaged 0.922 g/cm3.

4.4 Sulfur isotopes

Eight stibnite samples were collected for sulfur isotope analysis (Table 3). Their δ34S values averaged −6.30‰, with a range of 4.45‰. The δ34S values of coexisting fluids (δ34S∑S) are related to temperature, pH, and fO2 during sulfide precipitation (Ohmoto and Rye 1979). The LAOF samples had an assemblage of stibnite and pyrite, but lacked sulfates, indicating relatively low fO2 conditions (Ohmoto 1972) and fluids dominated by H2S. Previous studies have shown that temperature has a subtle effect on the δ34S value of H2S when H2S is the dominant species of S in the ore-forming fluid. Therefore, the δ34S values of H2S likely closely reflect the bulk fluid values (Ohmoto and Rye 1979; Saravanan and Mishra 2009). We calculated δ34S H2S values of H2S based on the fractionation factors [1000 lnα(Sb2S3–H2S) = −0.75 × 106/T2] of Ohmoto and Goldhaber (1997) and the average temperature of the stibnite fluid inclusions (171 °C) (Table 3).

Table 3 Sulfur isotopic compositions of stibnite collected from the Leishan-Rongjiang antimony ore field
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5 Discussion

5.1 Sources of sulfur

Calculated δ34S∑S values of the LAOF ore-forming fluid (Table 3) are close to the δ34S∑S values of the Woxi (−5.85‰ to −0.90‰) and Wangjiacun (−4.16‰ to −2.1‰) antimony deposits in the Xiangzhong basin (Bao 1989; Bao et al. 1999; Gu et al. 2004; Xiao 2014), indicating that these deposits probably have similar sulfur sources. Previous studies suggest that hydrothermal sulfur has three potential sources (Ohmoto 1972; Zheng 1991; Seal 2006): (1) sulfur originating from seawater and evaporates, with δ34S∑S ≈ +20‰; (2) sulfur derived from the mantle and deep crust, with δ34S∑S ≈ 0‰; and (3) biogenic sulfur in sedimentary rocks, with δ34S∑S values ranging from −20‰ to −10‰ (Machel 1989; Dixon and Davidson 1996). Clearly, our δ34S values of LAOF stibnite are distinctly different from sulfur originating from seawater, and fall in between the sulfur isotope compositions of the mantle source and of biogenic sulfur in sedimentary rocks. However, while our δ34S∑S values of the ore-forming fluid (−4.41‰ to +0.04‰) are mostly negative, they are generally close to the sulfur isotopic composition of the mantle source. Additionally, previous studies have shown that negative δ34S values in hydrothermal deposits may also result from gas phases (such as H2S) escaping during fluid boiling (Hagemann et al. 1994; Hodkiewicz et al. 2009). We conclude that sulfur in the LAOF was dominated by a mantle source, with possible involvement of biogenic sulfur.

5.2 The nature of ore-forming fluids and mineral precipitation

Type-I and type-II FIs in stibnite from the LAOF had similar salinities and homogenization temperatures. This suggests that these inclusions were captured by stibnite from the same ore-forming fluid. However, the salinities (1.57 wt%–5.26 wt% NaCl equiv.) and homogenization temperatures (150–180 °C) of the stibnite showed smaller ranges than those of the coexisting quartz (1.91 wt%–5.86 wt% NaCl equiv. and 138.2–251.5 °C, respectively). It is noteworthy that homogenization temperatures and salinities of FIs in stibnite were consistent with the main peak values of FIs in quartz (170–190 °C and 3.1 wt%–4.5 wt% NaCl equiv., respectively), which suggests that these FIs with relatively high temperature and high salinity were captured by stibnite and quartz at the main-ore stage. This could indicate that the stibnite and coexisting quartz formed under the same geophysical and geochemical conditions. Generally, fluid mixing and boiling are the two most important ore-forming processes for hydrothermal ore deposits (Wilkinson 2001), and the formation of most antimony and gold deposits is related to fluid boiling (e.g., Guillemette and Williams-Jones 1993; Bailly et al. 2000; Zhu and Peng 2015). In our study, evidence of boiling was locally observed in stibnite at the main-ore stage (Fig. 5d, f); a negative correlation between homogenization temperatures and salinities for FIs in stibnite (Fig. 8) further supports boiling as a key mechanism responsible for the precipitation of stibnite at the main-ore stage (Mernagh 2001; Chen 2012; Lawrence et al. 2013). EPMA data show that the As content of pyrite at the late-ore stage sharply decreased. However, the Au content increased compared with stibnite at the main-ore stage, which implies minor Au precipitation during the collapse of the hydrothermal system.

Fig. 8
figure 8

Diagram of salinity versus homogenization temperatures for the different types of fluid inclusions collected from the Leishan-Rongjiang antimony ore field

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Analysis of FIs in stibnite and quartz suggest that the ore-forming fluid was a low-temperature (150–210 °C), low-salinity (1.5 wt%–6.0 wt%. NaCl equiv.) NaCl–H2O hydrothermal system. The δ34S data suggest that sulfur was mainly derived from a mantle source. As mentioned previously, the magmatism in this area is not directly related to the Sb mineralization (Wang et al. 1994; Zhang 1994). However, previous studies have shown that the average content of Sb is approximately 15.77 ppm in pre-Sinian Xiajiang Group epimetamorphic rocks (Zhang 1994; Zhang et al. 2012). Moreover, boiling is sometimes caused by an abrupt drop in pressure induced by the fault-valve mechanism in hydrothermal systems (Wilkinson 2001) and structural evidence for fluid-pressure fluctuations (Fig. 3) is common in the LAOF. Similarly, the diagram of the homogenization temperature, density, and pressure relationships (D–T–P) of the NaCl–H2O system in stibnite and quartz (Fig. 9) indicates that the pressures of stibnite and quartz FIs were generally between 250 and 800 atmospheres, belonging to the range of low-pressure fluid (Lu et al. 2004). Therefore, it can be concluded that low-temperature, dilute hydrothermal fluids with mixed origins migrated along the regional fault and interacted with the wall rock, extracting the ore-forming materials. Then, through boiling, the ore-forming fluids were injected into the fault fracture zones.

Fig. 9
figure 9

D-T-P diagram of NaCl–H2O system for fluid inclusions collected from the Leishan-Rongjiang antimony ore field (after Roedder 1984)

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5.3 Implications for the South China Sb metallogenic belt

The South China Sb metallogenic belt is composed of the Youjiang basin antimony ore field, the LAOF, and the Xiangzhong basin from west to east (Fig. 1a). However, the LAOF’s location in the uplift between the other two ore fields has significantly restricted understanding of Sb mineralization in South China.

In this paper, which compared the LAOF with typical deposits in the Youjiang and Xiangzhong basins, we found that the Youjiang basin Sb deposit samples (e.g., Qinglong) were characterized by higher temperature and higher salinity and were conspicuously different from the LAOF and Xiangzhong basin deposits (Table 4). However, the LAOF and Xiangzhong Sb deposits exhibited some similar characteristics, including the following: (1) the Sb ore bodies were all hosted in Neoproterozoic low-grade metamorphic clastic rocks; (2) they were formed in a similar metallogenic epoch and geological setting; and (3) the ore-forming fluid was characterized by low homogenization temperature and low salinity, and boiling was the dominant factor affecting mineral precipitation. Thus, we suggest that the Neoproterozoic low-grade metamorphic area in eastern Guizhou Province is a potential target for finding new large antimony deposits.

Table 4 Characteristics of hydrothermal mineralization of the representative deposits of the South China antimony ore belt
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6 Conclusions

  1. 1.

    Ore-forming fluids in the LAOF were characterized by low temperature (150–210 °C), low salinity (1.5 wt%–6.0 wt% NaCl equiv.), and low density (0.872 and 0.961 g/cm3).

  2. 2.

    Fluid boiling may have been a key factor responsible for the precipitation of stibnite, and it may have been caused by an abrupt drop in pressure when the ore-forming fluids were injected in the fault fracture zones of the LAOF.

  3. 3.

    The δ34S values of stibnite ranged from −8.21‰ to −3.76‰ with an average −6.30‰. The δ34S∑S values of the ore-forming fluid varied from −4.41‰ to +0.04‰ (average: −2.49‰), indicating that the sulfur in the LAOF was dominated by a mantle source, but involvement of biogenic sulfur remains a possibility.