Genesis of the Banxi Sb deposit, South China: Constraints from wall-rock geochemistry, fluid inclusion microthermometry, Rb–Sr geochronology, and H–O–S isotopes

Ore Geology Reviews 115 (2019) 103162

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Genesis of the Banxi Sb deposit, South China: Constraints from wall-rock geochemistry, fluid inclusion microthermometry, Rb–Sr geochronology, and H–O–S isotopes

T

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Huan Lia, Hua Konga, , Zhe-Kai Zhoub, Thomas Tindellb, Yu-Qiang Tanga, Qian-Hong Wua, Xiao-Shuang Xia a

Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, School of Geosciences and Info-Physics, Central South University, Changsha 410083, China Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Low-temperature mineralization Banxi group Xuefeng Mountain Xiangzhong Basin Jiangnan Orogen

The Banxi Sb deposit is located in the world-class central-western Hunan Sb belt in South China, with quartz-vein type Sb-only mineralization hosted by Neoproterozoic epimetamorphic clastic sedimentary rocks. In this contribution, microthermometric, Rb–Sr geochronological and H–O isotopic analyses on fluid inclusions trapped in quartz, S isotopic studies on stibnite, and major-trace elemental measurements on fluid-altered and unaltered wall-rocks are presented to constrain the genesis of the deposit. The fluid inclusions are characterized by low homogenization temperature (170–260 °C) and low salinity 3–7 wt% NaCl equiv., with a low fluid density of 0.72–0.93 g/cm3 and a pH of 5.59. The low δDw (fluid δ2H, –140‰ to –107‰) whereas positive δ18OV-SMOW (15.9–17.2‰) and δ18Ow (fluid δ18O, 5.9–8.4‰) values in quartz indicate that the mineralizing fluid was generated from the low-grade metamorphic basement rocks and received a certain input of meteoric water. The Rb–Sr isotopic compositions of quartz samples yielded an isochron age of 196 ± 4 Ma (1σ, MSWD = 0.70) for quartz-stibnite ores. A highly radiogenic 87Sr/86Sr value (0.72663–0.74002, initial 87Sr/86Sr = 0.72640) indicates that ore-forming materials are derived mainly from basement rocks, and a direct material contribution from local granitic rocks can be ruled out. The δ34SV-CDT values of stibnite range narrowly from 4.81‰ to 6.72‰ and are in the δ34SV-CDT range of the metamorphic rocks, indicating that the sulfur was sourced from the basement rocks by sulfate reduction. Compositional differences between fluid-altered (decolorized) and unaltered wall-rocks are obvious: the migrated SiO2, SO3, LREE, Hf, Nb, Zr, Ta, Th, U, Mo, Sn, W into altered wallrocks suggests a deep basement metamorphic rocks-derived Si-rich alkaline brine for the fluid-rock interaction; while the leached Fe2O3, MgO, MnO, TiO2, Ba, Cs, Rb, As, Co, V, Ni, Sb, Sc, Cu, Pb and Zn from the altered rocks imply a material contribution of Banxi Group wall-rocks during the Sb mineralization process. Consequently, we conclude that the quartz-stibnite stage of the Banxi Sb deposit was formed during the Late Triassic orogeny in the Xuefeng Mountain region. Heated by granitic magmatism, Sb and S were leached from the basement rocks, forming an initial Si-rich and alkaline ore-bearing fluid. Precipitation of stibnite from the fluid resulted from interacting with wall-rocks and mixing of meteoric water, coupled by releasing of stress, decreasing of temperature and lowering of the pH at higher crustal levels.

1. Introduction A large number of Sb deposits occur in the central-western Hunan of South China, best exemplified by the giant Xikuangshan Sb deposit and surrounding Woxi, Fuzhuxi, Zhazixi, Longshan and Banxi Sb deposits (Fig. 1a). When grouped with hundreds of other medium-small scale Sb deposits, central-western Hunan represents the largest Sb metallogenic

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belt in the world, with a total Sb metal reserve of ~3 Mt (Wu et al., 1993). Tectonically, these deposits were mainly developed in the Xuefeng Mountain (western Hunan) and Xiangzhong Basin (central Hunan), within or around the Jiangnan Orogen (Fig. 1b). In addition, this Sb belt is also an important component of the South China lowtemperature metallogenic domain, containing key information on the origin of the low-temperature province of South China (Hu et al., 2016;

Corresponding author. E-mail address: [email protected] (H. Kong).

https://doi.org/10.1016/j.oregeorev.2019.103162 Received 23 June 2019; Received in revised form 23 September 2019; Accepted 4 October 2019 Available online 08 October 2019 0169-1368/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Simplified geological map of central-western Hunan Sb belt (after Lin, 2014); (b) Tectonic map of South China Block, showing the locations of the centralwestern Hunan Sb belt and Jiangnan Orogen.

trace elemental measurements on fluid-altered and unaltered wallrocks. This study aims to reveal the genesis of the Banxi Sb deposit and shed new light on the origin of the world-class low-temperature mineralization in the Hunan Sb belt of South China.

Xie et al., 2017). Though a large number of studies have been carried out for these Sb deposits (e.g., Zhu and Peng, 2015; Xu et al., 2017; Chen et al., 2018), their genesis has been controversial (Peng and Frei, 2004; Fan et al., 2004; Yang et al., 2006a, b; Peng et al., 2006, 2008; Gu et al., 2007, 2012; Liang et al., 2014), and the timing of the Sb mineralization is also hotly debated (Peng et al., 2003a, b; Wang et al., 2012a; Fu et al., 2016; Li et al., 2018a). The Banxi Sb deposit is situated at the SE margin of the Jiangnan Orogen, a transitional region from the Xuefeng Mountain to the Xiangzhong Basin (Fig. 1a). It has a long mining history (> 100 years), with a total Sb metal reserve of ~100,000 tons (Li et al., 2018a). The quartz-vein type ore bodies are hosted in the Neoproterozoic epimetamorphic rocks, with stibnite as the only economic product. As one of the most representative Sb deposits in central-western Hunan, this deposit has not received as much attention as it should, and only a few basic geological studies on ore body structures, and ore-controlling factors have been carried out by previous studies (e.g., Zeng et al., 1998; Wu and Cheng, 2004). The fluid sources and metal precipitation processes are undefined, and no geochronological studies have been carried out for the quartz-stibnite ores for the Banxi Sb deposit. In order to understand the metal source and metallogenic epoch of the giant Sb mineralization in the central-western Hunan region, we took the Banxi deposit as an example and performed microthermometric, Rb–Sr geochronological and H–O isotopic analyses on fluid inclusions trapped in quartz, S isotopic studies on stibnite, and major-

2. Geology of South China and the Banxi deposit The South China Block is comprised of the Cathaysia Block to the southeast and the Yangtze Block to the northwest, separated by the Jiangnan Orogen (Xu et al., 2018; Fig. 1b). The Jiangnan Orogen is a NE-trending tectonic belt which represents the Neoproterozoic orogenic zone between the Yangtze and Cathaysia Blocks (Zhao and Guo, 2012; Zhang et al., 2012; Li et al., 2019a). This orogenic belt contains the Xuefeng Mountain in its central part (western Hunan Province, Fig. 1b), at which the Precambrian strata and the Silurian-Sinian units crop out extensively (Fig. 1a). The Precambrian strata, involving the Neoproterozoic Lengjiaxi and Banxi groups, are regarded as the basement rocks in the region. After the Neoproterozoic orogeny, the South China Block experienced two major regional tectonic events which are known as the Early Palaeozoic and Triassic Orogenies (Xu et al., 2009; Chu et al., 2015; Li et al., 2017), typified by coeval strong tectonic deformation and intense granitic magmatism in the Xuefeng Mountain region (Qiu et al., 1998; Ding et al., 2007; Liu et al., 2012; Wang et al., 2012b, 2013; Fu et al., 2015; Zhang et al., 2015; Fig. 1a). Outcrops at the Banxi deposit belong to the Neoproterozoic 2

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Fig. 3. Cross section of Line 08 at the Banxi Sb deposit. Fig. 2. Simplified geological map of the Banxi Sb deposit.

The quartz-stibnite ores at the Banxi deposit have a parallel banded structure (Fig. 4a) or a breccia-like occurrence (Fig. 4b) and are mainly composed by quartz, stibnite and rare pyrite and arsenopyrite (Fig. 4c). Microscopically, the subhedral–anhedral stibnite occurs in the cracks in quartz (Fig. 4d), whereas the euhedral arsenopyrite and pyrite are sporadically distributed in quartz (Fig. 4e, f). Wall-rocks in the Banxi Sb deposit can be divided into two categories based on the extent of hydrothermal alteration: unaltered and altered. The fresh, unaltered wall-rocks retain their original color and show a clear stratification (Fig. 5a), without obvious decolorization (Fig. 5b) and existence of sulfide minerals (Fig. 5c). In contrast, the altered wall-rocks show opposite characteristics. Wall-rock alterations at the Banxi deposit are characterized by decolourization, a form of bleaching of the country rocks (Fig. 5d, e). They vary by the distance to the quartz-stibnite ore bodies, successively occurring sericitization (Fig. 5f), arsenopyritization (Fig. 5g), pyritization, silicification (Fig. 5h), carbonation and chloritization (Fig. 5i). These medium–low temperature hydrothermal alterations often superimpose over one another in locations where Sb mineralization is strongly developed. Arsenopyritization and silicification are positively related to the Sb mineralization intensity, whereas carbonation and chloritization are always developed far away from the ore bodies or distributed in lowgrade ore sections. After examining the mineralization-alteration assemblages, four hydrothermal stages can be differentiated: quartz stage, quartz-stibnite stage, stibnite stage and carbonate stage (Fig. 6). Among them, the quartz-stibnite stage and stibnite stage are the two major metallogenic stages at the Banxi deposit.

Wuqiangxi Formation of the Banxi Group, a set of epimetamorphic littoral facies-neritic facies clastic rocks. The Wuqiangxi Formation can be divided into three lithologic sections from bottom to top (Fig. 2). The first and second sections are distributed at the core and the wings of the Jiangjiachong anticline, respectively; and the third section is mostly located at (or just beyond) the wings and their outsides of the Jiangjiachong anticline. The first and the second sections are the Sb mineralization-bearing strata, and tuff and tuffaceous slate are the host rocks for the Sb ore bodies. In addition, a small quartz porphyry dike is exposed at the northern part of the Banxi region (Xiaogang area), with quartz and a small amount of K-feldspar (sanidine) as its phanerocrysts (Zhao et al., 2005). Ore body veins of the Banxi deposit are controlled by a regional deep fault (F1, also referred to as the Dafuping Fault) and a series of secondary NE-trending compressional torsional faults (Fig. 2). The tension fault F1 is part of the regional Taojiang–Chengbu fault (Fig. 1a). It trends NE and dips to NW, with a steep dip angle of > 70° (Fig. 3). The NE-trending, en-echelon-arranged secondary faults are mainly developed on the eastern side of the F1; some of them intersecting F1 and occur along axises of folds. These secondary faults provided favorable conduits for Sb-bearing fluids, forming quartz-stibnite lodes which are divided into three ore body veins referred to as V1, V2 and V3 respectively (Fig. 2). The actively being mined No. 2 ore vein (V2) is the largest and most representative ore body at the Banxi deposit (Fig. 3). It extends along the axis of the Jiangjiachong anticline, has an elongated “S“ shape in plane view, and a full-length of over 2000 m, occurring as a subparallel vein which is 300–500 m away from the No. 1 vein (V1). This ore vein is generally NW-inclined with a dip direction of 315–330° and a steep dip angle of 46–89°, having a Sb grade of 0.02–64.5% and a thickness ranging from 0.3 m to 1 m.

3. Samples and analytical methods Quartz-stibnite ores were sampled from the No. 2 ore vein (V2) in 3

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Fig. 4. Field occurrences (a–c) and photomicrographs (d–f) of quartz-stibnite ores at the Banxi Sb deposit. See descriptions in the text for details. Qz: quartz; Sti: stibnite; Ars: arsenopyrite; Py: pyrite.

Fig. 5. Field occurrences, hand specimens and photomicrographs of unaltered and altered wall-rocks at the Banxi Sb deposit. (a) fresh, unaltered tuff with clear stratification; (b) unaltered tuff retains original sedimentary color; (c) unaltered tuff without hydrothermal sulfide minerals; (d) decolorization in tuffaceous slates; (e) decolourization in tuffs; (f) sericitization in decolorized rocks; (g) arsenopyritization in tuffs; (h) silicification in decolorized rocks; (i) carbonation and chloritization in tuffs. Ser: sericite; Ars: arsenopyrite; Cal: calcite; Chl: chlorite. 4

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Fig. 6. Simplified paragenetic sequence of ore and gangue minerals at the Banxi Sb deposit.

from quartz using a quantitative reaction with BrF5 in heated nickel vessels (ca. 700 °C) and converted to CO2 on a platinum-coated carbon rod. For hydrogen isotopic analysis, degassing of labile volatiles and secondary fluid inclusions in quartz separates was conducted by heating under vacuum to 120 °C for 3 h. The released water was trapped, purified and converted to hydrogen by passing over heated zinc, and then the hydrogen isotopic compositions were analyzed with the mass spectrometer. The detailed analytical methods were similarly described in Li et al. (2014), Ni et al. (2017) and Zeng et al. (2017). The isotopic data are reported in per mil relative to the V-SMOW for oxygen and hydrogen, and the analytical precision is about ± 0.2‰ for δ18O and ± 2‰ for δD. Sulfur isotopes were analyzed at the Wuhan Institute of Geology and Mineral Resources (China) using a MAT-251 EM mass spectrometer, with analytical procedures similar to Li et al. (2016a). Powdered pure stibnite samples (200-mesh) were combusted under vacuum with CuO in a 1000 °C oven. Liberated SO2 was frozen in a liquid nitrogen trap after cryogenic separation from other gases. All values are reported as per mil (‰) relative to Canyon Diablo Troilite (CDT), and the analytical precision is better than ± 0.2‰. Altered and unaltered wall-rocks (mainly tuffs) were sampled proximal and distal to the No. 2 ore vein (V2). Altered samples were from the decolorization zones, with strong silicification and sericitization; unaltered samples were recognized by their original color, with no obvious fluid modification. These samples are taken from underground mining tunnels at different exploration levels (Level Nos. 6, 10 and 14) and drilling holes. Samples were polished to thin-sections and observed by a petrographic microscope to establish their altered or unaltered affiliations. The freshest parts of the samples were crushed by a hammer, and powdered to 200-mesh by a vibration agate mill for major and trace element analysis. Major-element analyses for wall-rock samples were performed at the Central South University, using X-ray fluorescence (XRF) spectroscopy. Powdered samples were fused with lithium metaborate-lithium tetraborate flux, included lithium nitrate as an oxidizing agent, and then poured into a platinum mold to generate a fused disk for XRF analysis. Loss-on-ignition (LOI) was measured for each sample at 1000 °C, and elemental concentrations were calculated from the results of the XRF and LOI analyses. Errors of 1–5% for most major elements were estimated by using the analysis of standards. Whole-rock trace element analyses for wall-rock samples were carried out at the Guangzhou ALS Laboratory, China. Rare earth elements (REEs), Ba, Cs, Hf, Nb, Rb, Sr, Ta, Th, U, Y and Zr were analyzed using Agilent 7700x inductively coupled plasma-mass spectroscopy (ICP-MS). Powdered samples (> 200-mesh) were added to lithium tetraborate flux, mixed well and fused in a furnace at 1025 °C. The resulting melts were then cooled and dissolved in a mixture of nitric and hydrofluoric acids prior to analysis. Other metallic elements (Ag, As, Be, Bi, Cd, Co, Cr, Cu, Ga, Ge, In, Mo, Ni, Pb, Re, Sb, Sc, Se, Sn, Te, Tl, V, W, and Zn)

underground mining tunnels at different exploration levels (from No. 8 Level to No. 18 Level). Microthermometric sections were made from typical ores, and remaining quartz and stibnite grains were obtained by crushing using a hammer, initial separation using a heavy liquid, and hand-picked using a binocular microscope. Microthermometric sections were then observed by microscope, and fluid inclusion assemblages were assessed. Quartz grains (40–60 mesh) from the ores with abundant primary fluid inclusions were chosen for Rb–Sr and H–O isotopic analysis, and powdered stibnite samples (200-mesh) prepared for sulfur isotopic analysis. The Rb–Sr isotopic analysis of quartz (fluid inclusions) was performed at Wuhan Institute of Geology and Mineral Resources (China). Quartz grains were washed with super pure HCl, HNO3 and distilled water sequentially. After drying at room temperature, thermal explosion (120–180 °C) and ultrasonic washing methods were used to clean the secondary inclusions in quartz. Mixed diluents (85Rb＋84Sr) were added with the samples, and hydrofluoric acid and perchloric acid were used to dissolve quartz grains. Ion exchange method was used to separate and purify Rb and Sr. 87Rb/86Sr and 87Sr/86Sr ratios were measured with a MAT261 mass spectrometer. Reference material GBW04411, NBS 607 and NBS987 were used to monitor the analytical process and instrument condition, ensuring the reliability of the measurement. The analytical results of NBS987 (87Sr/86Sr = 0.71023 ± 0.00004), NBS607 (Rb = 523.2, Sr = 64.7, 87 Sr/86Sr = 1.20037 ± 0.00003) and GBW04411 (Rb = 248.8, Sr = 158.9, 87Sr/86Sr = 0.75994 ± 0.00002) were consistent with their certificate values, with a relative error of < 0.5%. The full preparation process is conducted in an ultra-clean chamber, with a blank background of Rb and Sr at 0.9 × 10–11 g and 0.5 × 10–11 g, respectively. More detailed analytical procedures can be found in Hu et al. (2017), Zhang et al. (2017) and Li et al. (2018b). The Rb–Sr isochron ages (2σ) were obtained using Isoplot software (Version 3.75) (Ludwig, 2012), with values of λRb = 1.42 × 10–11 a–1. Microthermometry analysis of fluid inclusions in quartz was carried out at China University of Geosciences (Wuhan) and Central South University. Microthermometric data were measured using a LINKAM THMSG 600 heating-freezing system (temperature ranges: from −190 to 600 °C). The accuracy is about ± 0.1 °C on freezing, ± 2 °C below 350 °C and ± 4 above 350 °C on heating. Heating rates of 0.1 °C/min were chosen when phase transitions were approached. Fluid salinity of two-phase liquid–vapor inclusions was calculated based on the analysis of final ice melting temperature (Tm) and the equation of Bodnar (1993). Fluid density was calculated using the computer program FLINCOR (Brown, 1989). The hydrogen and oxygen isotopic compositions of fluid inclusions in quartz were determined with a Finnigan-MAT253 mass spectrometer at the Stable Isotope Laboratory, Chinese Academy of Geological Sciences. Hydrogen and oxygen isotopic analysis of fluid inclusions were conducted on the same quartz samples. Oxygen was extracted 5

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Table 1 Rb–Sr isotopic compositions of fluid inclusions in quartz at the Banxi Sb deposit. Sample No.

Sample location

Rb (ppm)

Sr (ppm)

87

BX30-1 BX30-6 BX1-6 BX4-1 BX7-5 BX7-7

Level Level Level Level Level Level

1.115 1.436 0.3758 0.02206 0.3267 3.642

2.123 2.505 1.726 0.5354 2.297 2.136

1.518 1.657 0.629 0.119 0.4109 4.93

19, 19, 14, 14, 11, 08,

Line Line Line Line Line Line

07 03 17 36 12 18

Rb/86Sr

87

Sr/86Sr (1σ)

0.73072 0.73122 0.72937 0.72663 0.72743 0.74002

± ± ± ± ± ±

0.00004 0.00005 0.00006 0.00006 0.00006 0.00001

density ranges from 0.72 g/cm3 to 0.93 g/cm3 (average 0.87 g/cm3). These data suggest a low temperature, salinity and density for the fluids of the Banxi Sb deposit. In addition, 10 temperature measurements of liquid CO2 homogenization and solid CO2 melting temperature on Type II inclusions from sample 21D1S1 were carried out and yielded results ranging from 6 °C to 20 °C (average 13 °C) (Fig. 9c) and from –61 °C to –58.8 °C (average –59.8 °C) (Fig. 9d), respectively, with the calculated fluid density of 0.79–0.88 g/cm3 (average 0.83 g/cm3).

were analyzed using Varian VISTA inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Powered samples (> 200-mesh) were digested with perchloric, nitric and hydrochloric acids. The residues were topped up with dilute hydrochloric acid and the resulting solution was analyzed by ICP−AES. Results were corrected for spectral interelement interferences. Analytical error for both ICP-MS and ICP-AES were relatively < 10% for most elements, based on the standard analysis. 4. Results

4.3. H–O isotopes

4.1. Rb–Sr geochronology

The H–O isotopic compositions of fluid inclusions in quartz are shown in Table 3. Four quartz samples have low δDV-SMOW (δDw = δ2H in fluid) values ranging from –140‰ to –107‰. They possess a relatively low variability of δ18OV-SMOW and δ18Ow (δ18O in fluid) values, ranging from 15.9‰ to 17.2‰ and from 5.9‰ to 8.4‰, respectively.

The Rb–Sr isotope analytical results are shown in Table 1. Six quartz samples show variable Rb (0.02206–3.642 ppm) and Sr (0.5354–2.505 ppm) compositions, with 87Rb/86Sr and 87Sr/86Sr ratios ranging from 0.119 to 4.93 and from 0.72663 to 0.74002, respectively. The 87Rb/86Sr and 87Sr/86Sr values of quartz samples yielded an Rb–Sr isochron age of 196 ± 4 Ma (1σ, MSWD = 0.70), with an initial 87 Sr/86Sr value of 0.72640 ± 0.00011 (1σ) (Fig. 7).

4.4. S isotope The S isotopic compositions of stibnite are presented in Table 4. Five stibnite samples are characterized by low positive δ34SV-CDT values, with a narrow range of 4.81‰ to 6.72‰ (average 5.59‰).

4.2. Fluid inclusions Two types of fluid inclusions in quartz were identified by microscope at room temperature: abundant, liquid-rich two-phase inclusions (Type I, Fig. 8a–d) as well as subordinate CO2-rich single phase (liquid) inclusions (Type II, Fig. 8e–f). These fluid inclusions mostly possess ellipsoidal and amygdaloidal shapes, and vary in sizes from 2 to 15 μm (mostly 4 to 8 μm). The volatile-liquid ratios of these fluid inclusions are between 5% and 30%. In total, 236 temperature and 149 salinity measurements for Type I inclusions were conducted (Table 2). The homogenization temperatures and salinities mainly fall into the ranges of 170–260 °C (Fig. 9a) and 3–7 wt% NaCl equiv. (Fig. 9b), respectively, and the calculated fluid

4.5. Wall-rock major and trace elements Whole-rock major- and trace-element compositions of the altered and unaltered wall-rocks are listed in Table 5. These rocks are characterized by variable contents of SiO2 (59.86–70.88 wt%), Al2O3 (12.41–17.09 wt%), total Fe2O3 (2.47–9.89 wt%), CaO (0.46–2.00 wt %), Na2O (1.34–3.46 wt%) and K2O (2.14–4.84 wt%). They are enriched in Rb, Ba, Th, U whereas relatively depleted in Ta, Nb, Sr and Ti (Fig. 10a), with moderately fractionated REE patterns (LREE/ HREE = 7.65–11.44) and weak Eu anomalies (Eu/Eu* = 0.43–0.81) (Fig. 10b). Compared to the unaltered wall-rocks, the fluid-altered wallrocks have higher average compositions of SiO2 (70.72 wt%), SO3 (0.15 wt%), LREE (208.62 ppm), Hf (6.0 ppm), Nb (12.0 ppm), Zr (194 ppm), Ta (0.7 ppm), Th (10.47 ppm), U (2.23 ppm), Mo (1.65 ppm), Sn (1.9 ppm), W (3.0 ppm) whereas lower contents of total Fe2O3 (3.19 wt%), Al2O3 (13.61 wt%), MgO (1.08 wt%), MnO (0.07 wt %), TiO2 (0.42 wt%), Ba (816 ppm), Cs (6.61 ppm), Rb (127.8 ppm), As (2.1 ppm), Co (6.6 ppm), Cu (9.0 ppm), Ni (9.0 ppm), Pb (13.9 ppm), Sb (22.2 ppm), Sc (9.0 ppm), V (26 ppm) and Zn (60 ppm) (Table 5). 5. Discussion 5.1. Ore-forming epoch of central-western Sb belt The ore-forming epoch of Sb deposits in the central-western Sb belt have been discussed for a long time, and variable opinions ranging from Silurian to Cretaceous have been reported using multiple dating methods (Hu et al., 2016). Three major age ranges for Sb mineralization have been determined by previous studies: Ordovician-Devonian (435–380 Ma), Late Triassic (230–200 Ma) and Late Jurassic–Early Cretaceous (160–130 Ma) (Hu et al., 2017). The former two ranges

Fig. 7. Rb–Sr isochron age of fluid inclusions in quartz at the Banxi Sb deposit. 6

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Fig. 8. Photomicrographs of (a–d) liquid-rich two phase inclusions (Type I) and (e–f) CO2-rich single phase inclusions (Type II) at the Banxi Sb deposit. V: vapor; L: liquid.

Antimony mineralization may be associated with magmatic activities (Pochon et al., 2016). The Late Triassic Sb–Au mineralization event in the Xuefeng Mountain area is considered to be temporally and spatially related to the coeval orogenic and magmatic activity (Xu et al., 2017). This is based on the geochronological work on the Chanziping and Daping deposits and neighboring Baimashan granitic intrusion, where the gold-bearing quartz veins have been dated at 205.6 ± 9.4 Ma and 204.8 ± 6.3 Ma (quartz fluid inclusion Rb–Sr ages), respectively, and the biotite granite was dated at 205–222 Ma (zircon SHRIMP U–Pb ages) (Chen et al., 2007; Li et al., 2008; Fu et al., 2015). More recent studies on the Darongxi deposit also revealed a Triassic age of 223.3 ± 3.9 Ma (molybdenite Re–Os isochron age) for the ore-bearing quartz vein, which is consistent with the intrusive age of the neighboring Dashenshan granite (224.3 ± 1.0 Ma, zircon LAICP-MS U–Pb age) (Zhang et al., 2012, 2014). In this study, the quartz fluid inclusion age of 196 ± 4 Ma is also consistent with the emplacement time (~200 Ma, whole-rock K–Ar age) of the Xiaogang quartz porphyry within a reasonable error range (Zhao et al., 2005). These

were favored by the Sb (Au) deposits in the Xuefeng Mountain area, whereas the last period was mainly determined for the Sb deposits in the Xiangzhong Basin region. For example, quartz Ar–Ar and scheelite Sm–Nd dating of the Woxi Sb–Au deposit yielded a result of 420–402 Ma (Peng et al., 2003a); scheelite Sm–Nd dating of the Zhazixi Sb–Au deposit yielded an isochron age of 227.3 ± 6.2 Ma (Wang et al., 2012a); syn-sulfide calcite Sm–Nd dating of the Xikuangsha Sb deposit gave a result of 156–124 Ma (Peng et al., 2003b); and stibnite/arsenopyrite Rb–Sr and Sm–Nd dating of the stibnite ore stage at the Banxi deposit yielded an age of ~130 Ma (Li et al., 2018a). In this study, the Rb–Sr dating of stibnite-quartz ores at the Banxi Sb deposit show an isochron age of 196 ± 4 Ma, which corresponds to the second major mineralization epoch (Late Triassic) of the central-western Hunan Sb belt. Combined with the previous dating work for the Banxi deposit (Li et al., 2018a), it can be inferred that the Banxi Sb deposit may have formed through several mineralization stages, crossing the two major Sb mineralization stages (230–200 Ma and 160–130 Ma) in South China.

Table 2 Characteristics of liquid-rich fluid inclusions (Type I) in quartz at the Banxi Sb deposit. Sample No.

BX2-8 BX4-3 BX2-2 BX7-6 BX4-6 0812D8S2 17D8S2 12D19S1 801-6S1 0817D3S5 802-1S1 21D1S1

Sample location

Level Level Level Level Level Level Level Level Level Level Level Level

16, 14, 17, 11, 15, 14, 17, 08, 14, 18, 17, 15,

Line Line Line Line Line Line Line Line Line Line Line Line

28 32 14 16 28 07 32 12 09 20 12 11

Vapor/liquidratio (%)

15–30 10–20 10–20 10–25 5–15 10–20 10–20 10–20 10–20 10–20 10–20 10–20

Size (μm)

3–10 3–5 3–8 4–8 3–6 2–4 2–5 2–8 3–8 2–5 2–4 2–6

7

Homogenization temperature (oC)

Salinity (wt% NaCl equiv.)

Range

Average

Range

Average

165–317 173–291 178–259 181–253 164–248 221–313 185–340 207–253 203–293 183–294 181–225 193–233

250 221 220 219 193 266 259 226 250 237 216 220

2.2–9.8

5.7

3.0–6.7

4.8

5.3–7.4 5.9–7.7 4.7–5.9 3.4–5.9 4.6–6.4 4.7–6.5 4.6–5.9

6.2 6.5 5.3 4.6 5.2 6.0 5.5

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Fig. 9. Frequency histograms of (a–b) homogenization temperature for Type I liquid-rich two phase inclusions, (c) liquid CO2 homogenization temperature for Type II CO2-rich single phase inclusions, and (d) solid CO2 melting temperature for Type II CO2-rich single phase inclusions.

combined factors strongly reinforce the close link between Triassic magmatism and Sb–Au mineralization in the Xuefeng Mountain region.

Table 4 S isotopic compositions of quartz at the Banxi Sb deposit.

5.2. Nature of ore-forming fluids The positive correlation between homogenization temperatures and salinities of Type I fluid inclusions (Fig. 11) indicate that fluid mixing may have occurred to form the quartz-stibnite ores (Li et al., 2013; Zhu and Peng, 2015). Previous research documented that fluid inclusions captured during the fluid mixing process normally show positive correlations between temperature and salinity, while those captured during fluid boiling or phase separation processes show the opposite characteristics (Shepherd et al., 1985; Daniel, 1997). In addition, Type I and II fluid inclusions both occur in sample 21D1S1. These paragenetically linked fluid inclusions in a single quartz grain may also suggest the mixing of two types of fluid (Hagemann and Lüders, 2003). The paragenetic Type I and Type II fluid inclusions can be regarded to represent the H2O end-member and CO2 end-member of a hydrothermal system, respectively. Plotting the average fluid density data on a P-T diagram, combined with a H2O–CO2 system, an approximate

Sample No.

Sample location

Mineral

δ34SCDT (‰)

801-6S1 804-3S3 0812D8S2 0821D1S1 0817D3S5

Level Level Level Level Level

stibnite stibnite stibnite stibnite stibnite

5.23 4.81 5.64 6.72 5.53

14, 14, 14, 15, 18,

Line Line Line Line Line

09 09–11 07 11 20

mineralizing temperature of 280 °C and pressure of 1.48 kbar can be estimated. For the breccia-containing Sb ore bodies (Fig. 4b) formed at a moderate distance below the surface, the original mineralization depth can be calculated and converted from the value of lithostatic pressure (Hagemann and Lüders, 2003). Given a lithostatic pressure gradient of 3.3 km/kbar, a mineralization depth of 4.9 km can be estimated for the Banxi Sb deposit. pH values of ore-forming fluid can be determined using the equations proposed in Liu et al (2011) and parameters mentioned in Ryzhenko and Bryzgalin (1985). Based on the data of average NaCl

Table 3 H–O isotopic compositions of quartz at the Banxi Sb deposit. Sample No.

Sample location

δDV-SMOW (‰)

δ18OV-SMOW (‰)

δ18Ow (‰)

Th (℃)

12D19S1 21D1S1 17D8S2 802-1S1

Level Level Level Level

–140 –124 –121 –107

17.2 16.5 15.9 16.1

8.4 6.7 6.5 5.9

253 233 240 225

08, 15, 17, 17,

Line Line Line Line

12 11 32 12

Note: 1) subscript “w” is for “water” (fluid); 2) T is the peak homogenization temperature value that was used to calculate δ18Ow values; 3) δ18Ow calculations use the fractionation equation: 1000lnαquartz−water = 3.38 × 106/T2–3.40 (Clayton et al., 1972). 8

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Table 5 Major and trace element composition of altered and unaltered wall-rocks at the Banxi Sb deposit. Altered wall-rocks

Unaltered wall-rocks

Sample No.

806-4S1

807-3S2

811-1S2

Average

806-4S2

807-3S1

811-1S1

Average

SiO2 (wt. %) Al2O3 TFe2O3 MgO CaO Na2O K2O P2O5 MnO TiO2 SO3 LOI Total Ba (ppm) Cs Hf Nb Rb Sr Ta Th U Y Zr La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ΣREE LREE HREE LREE/HREE Eu/Eu* Ce/Ce* Ag (ppm) As Be Bi Cd Co Cr Cu Ga Ge In Mo Ni Pb Re Sb Sc Se Sn Te Tl V W Zn

70.88 14.54 2.47 0.94 2.00 1.35 4.18 0.51 0.08 0.34 0.18 2.41 99.89 815 7.48 6.4 14.2 141.0 137.0 0.9 11.95 4.24 35.3 189 52.1 105.0 12.00 42.9 8.66 1.15 7.13 1.14 6.26 1.31 3.71 0.66 4.41 0.79 247.22 221.81 25.41 8.73 0.43 0.99 0.01 1.0 2.23 0.20 < 0.02 3.4 8 5.9 17.90 0.17 0.063 0.31 7.2 1.6 < 0.002 35.5 8.6 1 2.0 < 0.05 0.56 18 6.1 53

69.75 13.88 4.41 1.73 0.54 1.34 4.84 0.05 0.05 0.55 0.05 1.93 99.11 1235 7.88 5.4 9.5 180.5 67.3 0.5 6.72 0.67 27.9 202 38.4 79.4 9.01 32.2 6.19 1.16 4.91 0.78 5.04 1.08 3.03 0.47 2.99 0.46 185.12 166.36 18.76 8.87 0.62 1.01 0.13 0.4 3.03 0.14 0.03 11.9 21 10.4 18.15 0.22 0.061 3.01 12.6 33.4 < 0.002 19.9 11.3 <1 1.5 0.08 0.71 27 1.8 109

71.54 12.41 2.68 0.57 0.46 3.46 2.14 0.04 0.07 0.36 0.23 4.66 98.61 398 4.47 6.1 12.3 61.9 119.0 0.8 12.75 2.22 31.9 191 59.7 111.5 12.40 44.8 8.13 1.16 5.89 0.88 5.09 1.11 3.30 0.55 3.41 0.55 258.47 237.69 20.78 11.44 0.49 0.95 0.05 4.9 1.56 0.21 < 0.02 4.6 15 10.7 16.60 0.24 0.053 1.63 7.1 6.8 < 0.002 11.2 7.1 <1 2.2 < 0.05 0.26 32 1.0 18

70.72 13.61 3.19 1.08 1.00 2.05 3.72 0.20 0.07 0.42 0.15 3.00 99.20 816 6.61 6.0 12.0 127.8 107.8 0.7 10.47 2.38 31.7 194 50.1 98.6 11.14 40.0 7.66 1.16 5.98 0.93 5.46 1.17 3.35 0.56 3.60 0.60 230.27 208.62 21.65 9.68 0.51 0.98 0.06 2.1 2.27 0.18

65.57 14.65 4.70 1.20 1.68 2.27 3.51 0.37 0.13 0.55 0.10 4.25 98.98 790 7.33 4.5 10.0 125.0 114.0 0.6 9.53 2.13 31.4 157 38.9 74.3 8.74 32.7 6.63 1.10 5.65 0.91 5.44 1.10 3.23 0.47 2.94 0.47 182.58 162.37 20.21 8.03 0.54 0.95 0.10 1.9 2.09 0.25 0.03 10.1 20 44.2 18.00 0.17 0.068 0.54 19.6 37.3 < 0.002 53.9 12.0 1 1.6 0.05 0.52 53 1.3 113

59.86 14.72 9.89 1.50 1.07 2.28 4.73 0.23 0.05 1.18 0.05 3.83 99.40 1515 7.60 4.8 6.4 181.5 104.5 0.4 4.54 1.07 34.5 181 39.4 77.6 9.27 37.1 7.79 1.94 6.51 1.07 5.97 1.30 3.52 0.54 3.13 0.58 195.72 173.10 22.62 7.65 0.81 0.96 0.02 3.5 3.23 0.27 < 0.02 11.1 12 1.3 20.50 0.21 0.095 0.15 7.9 9.3 < 0.002 41.7 23.5 <1 1.3 0.06 0.65 132 3.6 74

66.03 17.09 4.21 1.11 1.08 2.74 3.54 0.34 0.10 0.49 0.10 2.78 99.61 757 9.86 4.8 9.5 120.5 98.1 0.6 7.91 1.13 25.1 158 35.5 69.7 8.54 32.0 5.93 1.14 5.17 0.80 4.78 0.96 2.84 0.37 2.83 0.41 170.97 152.81 18.16 8.41 0.62 0.95 0.01 1.3 1.91 0.04 < 0.02 8.7 17 8.1 21.80 0.24 0.064 0.17 14.4 2.9 < 0.002 9.9 13.3 <1 1.7 < 0.05 0.50 51 1.2 65

63.82 15.49 6.27 1.27 1.27 2.43 3.93 0.31 0.09 0.74 0.08 3.62 99.33 1021 8.26 4.7 8.6 142.3 105.5 0.5 7.33 1.44 30.3 165 37.9 73.9 8.85 33.9 6.78 1.39 5.78 0.93 5.40 1.12 3.20 0.46 2.97 0.49 183.09 162.76 20.33 8.03 0.65 0.95 0.04 2.2 2.41 0.19

6.6 15 9.0 17.55 0.21 0.059 1.65 9.0 13.9 < 0.002 22.2 9.0 <1 1.9 0.51 26 3.0 60

9

10.0 16 17.9 20.10 0.21 0.076 0.29 14.0 16.5 < 0.002 35.2 16.3 <1 1.5 0.56 79 2.0 84

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Fig. 10. (a) Primitive mantle-normalized diagram and (b) chondrite-normalized REE diagram of the wall-rocks at the Banxi Sb deposit. Normalized values for primitive mantle and chondrite are from Sun and McDonough (1989), and Taylor and McLennan (1985), respectively.

Fig. 12. The δ18Ow vs. δDw diagram of stibnite ores from typical Sb deposits in the central-western Hunan Sb belt. Data sources: Banxi: this study; Xikuangshang: Xie et al. (1996), Yang et al. (1998), Yin and Dai (1999), Peng et al. (2002), Ma et al. (2003), and Lin (2014); Zhazixi: He et al. (1996); Longshan: Yang (1986), Liang (1991); Fuzhuxi: Yao and Zhu (1993); Woxi: Yang (1986), Liang and Zhang (1986). δ18Ow = δ18O in water (fluid); δDw = δ2H in water (fluid). Fig. 11. Salinity vs. homogenization temperature of Type I liquid-rich two phase inclusions in quartz at the Banxi Sb deposit.

–140‰ to –50‰ and from –8‰ to 12‰, respectively. The wide ranging but low δDw values may indicate the variable proportions of three end-member fluids: magmatic hypothermal brine, metamorphic flow and D-depleted meteoric water (Yang et al., 2017). The strongly depleted δDw values (< –80‰), which are especially reflected in the Banxi deposit and partially in the Xikuangshan deposit (Fig. 12), can be ascribed to boiling of hydrothermal fluids (Imai et al., 1998; Fifarek and Rye, 2005) or interactions between water and organic matter (Kesler et al., 1997; Polya et al., 2000). The existence of breccia-bearing ores (Fig. 4b) at the Banxi deposit suggests the boiling of hydrothermal fluids. On the other hand, the low CO2 melting temperature (–61 °C to –58.8 °C) relative to pure CO2 melting temperature (−56.6 °C) also indicates the existence of other reductive components such as CH4 and N2 in the Banxi fluid inclusions (Martín-Izard et al., 2009; Li et al., 2019b). This inference is also consistent with previous studies on compositions of fluid inclusions from the central-western Sb belt, which suggest that the gas components are dominantly H2O, followed by CO2 and minor CH4 and H2 (Li, 1996; Guo, 2014; Lin, 2014; Li et al., 2016b). More abundant H2O content relative to CO2 in the fluid inclusions is a typical characteristic of a fluid originating from lowgrade metamorphic rocks (Zhu and Peng, 2015). Though the basement epimetamorphic rocks may be short of organic matter to decrease the δDw value of fluids (Goldfarb et al., 2004), the H concentration in the

molarity (converted from the fluid salinity) and estimated mineralizing temperature and pressure discussed above, a pH value of 5.59 can be discerned at deposition. This value is consistent with those of other Sb deposits in the central-western Hunan Sb belt, for example, pH = 5.6–7.3 at Xikuangshan (Lin, 2014), pH = 5.5–5.9 at Dong’an (Guo, 2014), and pH = 4.6–6.2 at Longshan (Liang, 1991). All of these indicate that the mineralizing fluid is neutral to weakly acidic during stibnite precipitation. However, the original (earlier stage) fluid could be weakly alkaline. This inference is based on the widely developed early stage sericite alteration (Fig. 6), an indicator of alkaline fluid-rock interaction (Meriaud and Jebrak, 2017). Thus, a transition in the nature of the fluid may exist at the Banxi deposit, i.e., evolving from early alkaline state, transitioning later to neutral to weakly acidic. Stibnite solubility in a NaCl-H2O solution is positively related to pH and temperature (Hu, 1995). Moreover, Sb-bearing fluid is easy to migrate in an alkaline environment, and tends to precipitate in acidic to neutral conditions (Obolensky et al., 2007). The H–O isotopic characteristics of major Sb deposits in the centralwestern Hunan Sb belt are shown in Fig. 12. It can be concluded that the δDW and δ18OW values of these Sb deposits dominantly range from 10

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basement rocks is low enough to remarkably affect the hydrogen isotopic ratios of the fluid (Deen et al., 1994; Taylor, 1997; Zhou et al., 2017), compared with the amount of hydrogen involved in hydrothermal fluids. On the other hand, the input of meteoric water would also significantly reduce the δDw value (Wang et al., 2017; Zeng et al., 2017). Consequently, it can be indicated that the hydrothermal fluid at the Banxi Sb deposit could be derived from the basement epimetamorphic rocks and mixed with meteoric water during stibnite precipitation. Oxygen isotope analyses of quartz may provide key information about hydrothermal fluid conditions, ore-material sources and ore-formation processes (Lubben et al., 2012). The positive δ18OW values (5.9‰ to 8.4‰) cannot be solely due to magmatic fluids, and they probably point to the 18O-enriched fluid source reservoir of meta-sedimentary rocks (Suchecki and land, 1983; McCuaig and Kerrich, 1998). It is notable that the δ18OW and δDw values of the Banxi Sb deposit are negatively correlated (Fig. 12). The basement rocks are mainly lowgrade metamorphosed tuff, siltstone, slate and sandstone, thus fluid circulation in the basement rocks would increase the δ18OW value of the fluid but decrease the δDw value at the same time (Yeh, 1980; Moldovanyi et al., 1993; Zhou et al., 2017). In conclusion, the observed ranges in δDw and δ18OW values of the Banxi Sb deposit suggest that the mineralizing fluid was generated from the basement epimetamorphic rocks and received a certain input of meteoric water trapped at high crustal levels.

Fig. 13. The δ34SV-CDT ranges of stibnite ores from typical Sb deposits in South China Sb belt. Data sources: Banxi: this study; other deposits: Shen et al. (2013) and references therein.

5.3. Sources of strontium and sulfur belt (mostly –10‰ to 10‰, Fig. 13), indicating a close genetic relationship between the basement rocks and the Sb mineralization. As discussed above, the original ore-forming fluid of the Banxi deposit was a weakly alkaline solution. Previous research pointed out that sulfate in the sedimentary rocks can be easily reduced when the permeable fluid is alkaline (Zhang et al., 2010; Esteban-Arispe et al., 2016). Thus, the S that constitutes the stibnite at the Banxi Sb deposit may be sourced from the basement rocks by sulfate reduction. This further confirms the inference that the basement metamorphosed clastic rocks have largely contributed to the Sb mineralization.

The 87Sr/86Sr ratios of fluid in quartz at the Banxi deposit are highly radiogenic (0.72663–0.74002) and higher than those of the Banxi Group low-grade metamorphic clastic rocks (0.71306–0.72874, Peng et al., 2002), implying a deeper metamorphic basement source beneath the Banxi Group. The most recent study (Li et al., 2019e) on ore zircon geochronology, geochemistry and Hf isotopes from the Banxi deposit also suggests a deeper fluid source from the Lengjiaxi Group and the crystalline basement beneath. The highly radiogenic 87Sr/86Sr ratios have also been reported for the scheelites at the Zhazixi Sb–W deposit (0.7304–0.7329) (Peng et al., 2008) and the Woxi Sb–Au deposit (0.7476–0.7504) (Peng and Frei, 2004). In the central-western Sb belt, fluids that leached Neoproterozoic shales, clastic rocks and argillaceous rocks always have high 87Sr/86Sr ratios (Peng et al., 2006, 2008). 87Srenriched ore-forming fluids are considered to be derived from basement clastic rocks in many cases (e.g., Subías et al., 1998; Gromek et al., 2012; Saintilan et al., 2016). The initial 87Sr/86Sr values of the fluid (0.72640) are also highly radiogenic (> 0.710), compared to those of the Xiaogang quartz porphyry dikes (0.6653–0.7149, mean = 0.6817 (n = 6); Zhao et al., 2005). This implies a dominant contribution of basement metamorphosed clastic rocks for the Sb mineralization, and a direct material contribution from the local magmatic rocks seems impossible. The δ34SV-CDT values of stibnite from quartz-stibnite ores narrowly ranges from 4.81‰ to 6.72‰. The relatively tight clustering δ34SV-CDT values indicate that the sulfur source was isotopically uniform, and the fluid redox state was below the SO2/H2S boundary (Yang et al., 2017; Zeng et al., 2017; Li et al., 2019c). Ore minerals from the quartz-stibnite ores chiefly consist of stibnite and minor pyrite and arsenopyrite, with no sulfate minerals occurring. Thus, H2S was the dominant sulfur species in the hydrothermal system (Tran et al., 2016; Li et al., 2019b; Huang et al., 2019a,b). This is suggestive that the sulfur isotopes of stibnite can approximately represent the total S isotopic compositions of the hydrothermal fluid (i.e., δ34SΣS ≈ δ34Sstibnite). The basement rocks in the central-western Sb belt (i.e., Banxi Group and underlying Lengjiaxi Group) have variable bulk δ34SV-CDT values ranging from –7.0‰ to 19.2‰ and from 15.7‰ to 23.5‰, respectively (Yang, 1986; Tang, 2017). These δ34SV-CDT ranges roughly correspond to those of stibnite from the majority deposits in the central-western Sb

5.4. Fluid-rock interaction in wall-rocks A simple and effective means of quantitatively estimating changes in element concentrations during hydrothermal alteration can be referenced to Grant (2005). In the method of “isocon analysis”, element migrations may be calculated graphically by plotting an altered composition against an original composition. Elements that have remained immobile in the alteration process define the isocon, which is a straight line through the origin. During hydrothermal processes, Al2O3 and TiO2 are regarded immobile, thus are normally used as inert components (MacLean and Kranidiotis, 1987; Niri, 2001). In this study, Al2O3 composition in the altered and unaltered wall-rocks is used to define the isocon. The isocon analytical results of the hydrothermal alteration process at the Banxi Sb deposit are shown in Fig. 14a, b. It can be concluded that SiO2, SO3, K2O, Hf, Nb, Zr, Ta, Th, U, Mo, Sn and W have been migrated into the altered rocks whereas total Fe2O3, MnO, TiO2, As, Co, Cu, Ni, Sb, Sc, V, In and Zn have been leached out from the altered rocks during the fluid-rock interaction. This is also consistent with the compositional variations in elements of the unaltered and altered wall-rocks (Table 5). The enrichment of SiO2 and K2O in the altered wall-rocks may suggest an original Si-rich alkaline fluid, macroscopically manifested as silicification and sericitization closed to the Sb ore bodies. In contrast, the migrated CaO and MgO in the decolorized zones may have contributed to the carbonation and chloritization which are developed far from the ore bodies. The slightly increased high field-strength elements (Hf, Nb, Zr, Ta, Th and U) may indicate an interaction of the basement metamorphic rocks-originating deep brine (Katayama et al., 2003; He 11

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Fig. 14. Isocon analysis of the altered and unaltered wall-rocks for (a) normal major and trace elements, and (b) mineralization-related metallic elements at the Banxi Sb deposit. Elements above the isocon lines are enriched during hydrothermal alteration process. CA and CO are the average values (major element: wt. %; trace element: ppm) of the altered and original rocks (Table 5). The detailed calculation process can be found in Grant (2005).

2. The homogenization temperature and salinity analysis of fluid inclusions in quartz-stibnite ores suggests a low temperature, low salinity, low density and weakly acid fluid for the stibnite precipitation. The low δDw (–140‰ to –107‰) and positive δ18Ow (5.9–8.4‰) values indicate that the mineralizing fluid was generated from the basement epimetamorphic rocks and received a certain input of meteoric water trapped at high crustal levels. 3. The δ34SV-CDT values (4.81‰ to 6.72‰) of stibnite from quartzstibnite ores represent the assumed isotope composition of the total sulfur in the hydrothermal fluid. This range also falls into that of the metamorphosed clastic rocks, implying that the sulfur was sourced from the basement rocks by sulfate reduction. 4. The enrichment of SiO2, SO3, LREE, Hf, Nb, Zr, Ta, Th, U, Mo, Sn and W in the fluid-altered wall rocks are consistent with a Si-rich alkaline brine derived from deep basement rocks. In contrast, the higher compositions of Fe2O3, MgO, MnO, TiO2, Ba, Cs, Rb, As, Co, Cu, Ni, Pb, Sb, Sc, V and Zn in the unaltered wall-rocks implies a potential provenance role of the Banxi Group during the Sb mineralization process. 5. The intensive granitic magmatism heated the basement rocks, leaching out Sb and related materials to form a silicon-rich and weakly alkaline ore-bearing fluid. Formation of quartz-stibnite ores resulted from interacting with wall-rocks, mixing of meteoric water, releasing of stress, decreasing of temperature and lowering of the pH at higher crustal levels.

et al., 2010). However, the addition of SO3, Mo, Sn and W may require a contribution from the deep magmatic intrusion (Jiang et al., 2018; Li et al., 2019d; Jiang et al., 2019; Girei et al., 2019; Liu et al., 2019), which may have provided heat to generate the circulated fluid in the basement. Previous studies on the typical Sb deposits from the centralwestern Hunan Sb belt also suggest the existence of deep magmatic intrusion that may have aided the fluid circulation (e.g., Li et al., 2018a; Zheng et al., 2018). On the other hand, the leached Fe2O3, As and Sb from the altered wall-rocks provided a certain number of elements to form the arsenopyrite in the neighbouring arsenopyritization zones and stibnite in the ore bodies. The low-temperature mineralization-related elements such as Mn, Co, Ni, Sc, V, Pb and Zn have also been leach out with Sb from the altered wall-rocks, suggesting a potential provenance role of Banxi Group wall-rocks in the mineralization process. 5.5. Mechanism of mineralization in the Banxi Sb deposit Based on the microthermometric, Rb–Sr, H–O and S isotopic analyses on the quartz-stibnite ores and whole-rock geochemical measurements on the wall-rocks, the mechanism of Sb mineralization in the Banxi area can be summarized as follows. During the Triassic orogeny, intense granitic magmatism occurred at the Xuefeng Mountain region. Fluid was generated from the deep metamorphosed rocks in the heated basement by dehydration, leaching out Sb, S and other mineralization-related elements and evolving into a medium–low temperature, low salinity, low density, silicon-rich and alkaline ore-bearing fluid. This fluid migrated to the upper levels of the crust through regional deep faults (i.e., Taojiang–Chengbu fault in the region and fault F1 at the Banxi deposit), interacted with the Banxi Group low-grade metamorphic clastic rocks and mixed with the circulating meteoric water near the surface. Thereafter, the mixed fluid underwent boiling due to stress release, decreasing of temperature and lowering of the pH, causing the precipitation of stibnite and formation of quartz-stibnite ores at the secondary faults of the Banxi deposit.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements 6. Conclusions

This work was financed by the Special Survey for Mineral Resources Evaluation from the China Geological Survey (Grant Nos. 1212011085372 & 1212011121116). We thank Dr. Chris Harris (University of Cape Town) for his comments on the earlier version of the manuscript. We are also grateful to two anonymous reviewers for their insightful and constructive comments. Guest Editor Prof. Xiaoyong Yang and Editor-in-Chief Prof. Franco Pirajno are thanked for their editorial handling of the manuscript.

1. The Rb–Sr isochron age of 196 ± 4 Ma represents the formation time of the quartz-stibnite ores at the Banxi deposit, which was related to the Late Triassic orogeny in the Xuefeng Mountain region. The highly radiogenic 87Sr/86Sr value indicates that ore-forming materials were mainly derived from deep basement rocks rather than granitic rocks. 12

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Appendix A. Supplementary data

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