Mineral chemistry of the Xiasai Ag–Pb–Zn deposit in the central Yidun Terrane, SW China: Insight into Ni–Ag–Bi mineralization and formation conditions

Ore Geology Reviews 114 (2019) 103136

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Mineral chemistry of the Xiasai Ag–Pb–Zn deposit in the central Yidun Terrane, SW China: Insight into Ni–Ag–Bi mineralization and formation conditions

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Yan-Jun Lia,b,c, , Jun-Hao Weia,b, Thomas Ulrichc, Meng-Ting Chena, Hong-Mei Lia,d, Ming-Wei Niua, Ben Liua a

School of Earth Resources, China University of Geosciences, Wuhan 430074, China National Demonstration Center for Experimental Mineral Exploration Education, China University of Geosciences, Wuhan 430074, China Department of Geoscience, Aarhus University, Aarhus 8000, Denmark d Hubei Geological Survey, Wuhan 430034, China b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrothermal Ag–Pb–Zn deposit Nickel-mineral Silver-mineral Native bismuth Formation conditions Yidun Terrane

Sulfides and sulfosalts are major ore minerals for most of Ag–Pb–Zn deposits and can be used to study enrichment and distribution of ore-forming elements and formation conditions. The Xiasai Ag–Pb–Zn deposit (Zn 0.141 Mt @ 4.30%, Pb 0.132 Mt @ 4.03%, Ag 1028 t @ 337.8 g/t, Cu 10058 t @ 0.56%, and Sn 20000 t @ 0.75%) is a hydrothermal vein-type deposit in the central Yindun Terrane, SW China. It consists of proximal quartz-cassiterite veins and distal Ag–Pb–Zn sulfide veins to a Cretaceous monzogranite. Three successive mineralization sub-stages are recognized for Ag–Pb–Zn veins, i.e., arsenopyrite + coarse-grained pyrite (Py1) + pyrrhotite, sphalerite + chalcopyrite + fine-grained pyrite (Py2), and silver-minerals + native bismuth + galena. Nickel-minerals, such as breithauptite and ullmannite were identified as inclusions hosted within pyrrhotite using Energy Dispersive X-ray Spectroscopy (EDS) and Electron probe microscopy (EPM) analysis and mapping. Trace amounts of invisible silver have been detected in chalcopyrite and galena, whereas a significant amount of silver is incorporated in pyrargyrite, freibergite, miargyrite, and electrum as intimate intergrowths with galena or inclusions within galena. A high content of invisible bismuth is detected within galena (0.21–16.95 wt%), as well as numerous grains of native bismuth occur as inclusions in galena. Based on the Bi concentrations determined and the modal composition a large-sized bismuth deposit would be estimated with about 23,800 t of Bi resource. The presence of nickel-minerals and native bismuth, together with Zn/Cd ratios (58–151) of sphalerite, indicate that the ore-forming materials are derived from the Xiasai monzogranite. Based on mineral assemblages and chemical features for sulfides and sulfosalts, the formation of Ag–Pb–Zn ores was triggered by a decrease in temperature, fs2, fO2, and sulfidation. The early episode of arsenopyrite, Py1, pyrrhotite, and nickel-minerals formed at temperature of 320–470°C, lower fs2 and decreasing sulfidation conditions from low alkaline fluids. In contrast, the late episode of silver-minerals, native bismuth, and galena deposited at 160–270 °C with a lower fs2 state and neutral or acidic pH. However, a slight increase of fs2, fO2, and sulfidation (intermediate sulfidation) occurred during precipitation of Zn–Cu.

1. Introduction Hydrothermal vein-type Ag–Pb–Zn deposits are characterized by high grades of ore-forming elements (e.g., Baumgartner et al., 2008; Lawley et al., 2010; Wang et al., 2014; Mango et al., 2014; Liu et al., 2016b; Mehrabi et al., 2016; Zhai et al., 2019; Li et al., 2019), some even up to 2000–3000 g/t Ag (Baumgartner et al., 2008; Mango et al., 2014; Zhai et al., 2019). Many hydrothermal vein-type deposits have

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been documented, possessing large- to giant-scale resources of silver and base metals, such as the Guanajuato epithermal Ag–Au–Cu–Pb–Zn deposit in Mexico (34,000 t Ag and 170 t Au, Mango et al., 2014), the Weilasituo-Bairendaba deposit (6200 t Ag, 2.6 Mt Pb + Zn, 1.5 Mt Cu, > 1.0 Mt Sn and > 15 Kt W, Liu et al., 2016b), and Shuangjianzishan Pb–Zn–Ag deposit (0.026 Mt Ag, 1.1 Mt Pb, 3.3 Mt Zn, Liu et al., 2016a) in Inner Mongolia, China, to name a few. Ag–Pb–Zn veins have been regarded as the distal part of the magmatic-hydrothermal systems

Corresponding author at: School of Earth Resources, China University of Geosciences, Wuhan 430074, China. E-mail address: [email protected] (Y.-J. Li).

https://doi.org/10.1016/j.oregeorev.2019.103136 Received 30 April 2019; Received in revised form 5 September 2019; Accepted 19 September 2019 Available online 20 September 2019 0169-1368/ © 2019 Published by Elsevier B.V.

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veins and skarn Sn–Ag mineralization. The former is exemplified by the large-scale Shaxi, Jiaogenma, and Xiasai deposits, and the latter is represented by the Lianlong and Cuomolong deposits. A genetic link was proposed between the mineralized systems and the granitic magmatism in the area (Qu et al., 2001, 2002; Liu, 2003; He et al., 2004; Ying et al., 2006; Hou et al., 2007; Zou et al., 2008; Li et al., 2019). The Xiasai Ag–Pb–Zn deposit is the largest deposit in the metallogenic belt. It contains metals of Zn 0.141 Mt @ 4.30%, Pb 0.132 Mt @ 4.03%, Ag 1028 t @ 337.8 g/t, Cu 10,058 t @ 0.56% and Sn 20,000 t @ 0.75% (Li et al., 2019). It is characterized by enrichment of silver, zinc and lead with highest grades up to 4368.50 g/t, 31.89%, and 58.86% for Ag, Zn and Pb, respectively (Xiasai Mining Co. Ltd. Sichuan Province 2008, unpublished data). The description of the mineralization, fluid inclusions, and partial S–Pb isotopes on sulfides from Ag–Pb–Zn ore bodies were reported by Liu (2003), Ying et al. (2006), and Zou et al. (2008). In a previous study we documented systematic studies regarding geochronology for mineralization and magmatism, fluid inclusions and H–O–S–Pb isotopes for ores (Li et al., 2019). The ore bodies were recognized as a typical hydrothermal vein-type Ag–Pb–Zn deposit genetically associated with the Xiasai monzogranite (Li et al., 2019). Although an earlier study of ore minerals has been reported by Huang and Hu (2000), conditions excepting formation temperature are rarely studied for the genesis of Ag–Pb–Zn veins at Xiasai so far. In addition, previously not documented nickel-minerals such as breithauptite and ullmannite as inclusions in pyrrhotite and native bismuth in galena were identified at the Xiasai deposit during our mineral investigation. We here present inductively-coupled plasma mass spectrometry (ICPMS) analyses of major sulfides, Energy Dispersive X-ray Spectroscopy (EDS) and Electron probe microscopy (EPM) analysis and mapping for sulfides, nickel-minerals, silver-minerals and native bismuth, in order to further constrain the Ni–Ag–Bi mineralization and formation conditions for Ag–Pb–Zn-rich ores. 2. Geological setting The YDT lies between the Qiangtang and the Songpan–Garze terranes and is separated by two oceanic suture zones, corresponding to former Paleo-Tethys oceanic subduction zones, i.e., the Jinshajiang suture to the west, and the Garze–Litang suture to the north and east (Fig. 1; Wang et al., 2013; Peng et al., 2014). It can be divided into the western YDT (also known as the Zhongza Massif) and the eastern YDT (Fig. 1) by the strike-slip Xiangcheng–Geza Fault. The Xiasai–Lianlong metallogenic belt is 10–20 km in width and over 100 km in length, extending along the western margin of the eastern YDT (Hou et al., 2007). This area is dominantly composed by the Triassic Yidun Group, which includes sandstone, limestone, with intercalations of thin-bedded volcanic rocks. The Yidun Group is divided into four lithological formations from the base to the top, i.e., the Lieyi, Qugasi, Tumugou, and Lanashan (or Labaya) formations (Wang et al., 2013). Lithologies of these four formations are described in detail by Wang et al. (2013). Although no basement rocks are exposed, Wu et al. (2016) argued that there is a Paleoproterozoic basement in the YDT based on U–Pb ages on detrital and inherited zircons at 2.5–2.4 Ga and 1.9–1.8 Ga. The basement has affinities of the west Yangtze Craton (Wu et al., 2016). Volcanic and igneous rocks are widespread in the central to northern YDT. The volcanic sequence consists of basalts, andesites, dacites, and rhyolites with U–Pb ages on zircons of 231 ± 1–228 ± 2 Ma (Wang et al., 2013). Igneous rocks include Middle-Late Triassic and Cretaceous granites. The Triassic granites have U–Pb ages on zircons of 225 ± 1–215 ± 3 Ma (Reid et al., 2007; Weislogel, 2008; He et al., 2013; Peng et al., 2014), and are characterized by abundant hornblende, enrichment in LILE and LREE, and depletion in HFSE (e.g., Reid et al., 2007; Deng et al., 2014). The Cretaceous granites correspond to a NW-trending alignment of intrusions, such as the Rongyicuo (RYC), Lianlong, Ruorolong (K-feldspar Ar–Ar ages at 75.2 ± 0.3 Ma to 60.2 ± 3 Ma and whole-rock Rb–Sr

Fig. 1. (a) Simplified geological map of the YDT (modified from Hou et al., 2007, and Wang et al., 2013), showing the distribution of the main polymetallic ore deposits and granitic intrusions; (b) Simplified geological map showing the distribution of Sn and Ag–Pb–Zn deposits and spatial relationship with the Cretaceous granites in the central YDT (modified from He et al., 2004). Abbreviations: GLS – Garze–Litang suture, JS – Jingshajiang suture, and XGF – Xiangcheng–Geza fault.

with proximal porphyry and/or skarn Cu–Mo systems (e.g., Mao et al., 2009; Sillitoe, 2010; Lawley et al., 2010; Wang et al., 2014; Liu et al., 2016a, 2016b; Li et al., 2017; Zhai et al., 2018, 2019), even vein- or greisen-type W–Sn mineralization (Audétat et al., 2000; Zhao et al., 2018). However, the critical parameters which lead to the formation of Ag–Pb–Zn veins remain contentious. Some researchers proposed that precipitation of sulfides from hydrothermal fluids was governed by temperature decrease (Audétat et al., 2000; Müller et al., 2001; Mango et al., 2014), whereas others argued that it was caused by fluid mixing and dilution (Baumgartner et al., 2008; Ke et al., 2017; Xie et al., 2017; Zhou et al., 2018a, 2018b), or decrease in pH, fO2 and fS2 (Fontboté et al., 2017; Zhai et al., 2019). Formation conditions for hydrothermal vein-type Ag–Pb–Zn deposits need to be further studied. The Xiasai–Lianlong metallogenic belt is located in the central Yidun Terrane (YDT), and is composed of four large-sized, two mediumsized, and numerous smaller-sized Ag–Sn polymetallic deposits (Fig. 1). It is one of the most significant economic metallogenic belts for Ag, Pb–Zn, and Sn in China (Zhang et al., 2014, 2015a, 2015b; Chen et al., 2015). The occurrences are dominated by hydrothermal Ag–Pb–Zn 2

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the formation consists of grey slates, sandstones, intermediate to felsic volcanic rocks, and tuffs, with minor interlayers of mafic volcanic rocks. They can be divided into five units. Unit 1 consists dominantly of thick medium-grained sandstones with thin-bedded limestones. Unit 2 is dominated by alternating, thin-bedded medium-grained sandstones and limestones. Unit 3 corresponds to thick-bedded tuffaceous siltstones with tuffaceous slates and siliceous rocks. Unit 4 consists of siltstones and chlorite slates, with minor tuffs. Unit 5 contains siltstones, mediumbedded siltstones, and gray-black calcareous slates. The main intrusion is the XSM batholith, which extends along NE–SW trend over an area of about 1.5 km2 in the Xiasai district. Unaltered samples are generally light yellowish-red with porphyritic texture. The major phenocryst minerals are quartz (~10%), plagioclase (~10%), and K-feldspar (~15%). Anhedral quartz ranges from 1 to 12 mm. Sub-euhedral plagioclase and K-feldspar phenocrysts are up to 10 mm in length, while feldspars in the groundmass are < 4 mm in size. The groundmass dominantly consists of quartz (~20%), plagioclase (~20%), K-feldspar (~20%), and biotite (~5%). Interstitial biotite is observed between feldspars and quartz and ranges from 1 to 3 mm. LA–ICP–MS zircon U–Pb dating of the Xiasai monzogranite gave ages between 102 ± 1 and 101 ± 1 Ma (Li et al., 2019). It has a weakly peraluminous A-type granite composition, and is probably derived from mixing of melts of partial melting of crustal basement and mantle in an extensional tectonic setting (Li et al., 2019). The structures in the Xiasai deposit are dominated by NNW-trending

ages at 95 ± 3 Ma to 89 ± 5 Ma, Qu et al., 2002; Ying et al., 2006), Genie (U–Pb age of 105 ± 2 Ma, Wang et al., 2008), Chuershan and Haizi granites (U–Pb age of 104 ± 1 Ma and 94 ± 2 Ma, respectively, Reid et al., 2007), as well as the Xiasai monzogranite (U–Pb age of 102 ± 1~101 ± 1 Ma, Li et al., 2019). These granites show A-type granite affinities (Hou et al., 2001; Qu et al., 2002; Li et al., 2019). The tectonic structures in the YDT are dominated by NNW-trending faults, with subordinate NW-striking faults (Fig. 1b). The formers are more than 100 km long with strike directions of ~340° and consist of numerous parallel strike-slip faults. They probably formed during the early Cretaceous by multiple activation stages of regional faults that controlled the intrusion of the A-type granites. In addition, subsidiary NNW-trending faults are mineralized with Sn–Ag–Pb–Zn mineralization. NW-striking faults are represented by parallel sinistral strike-slip faults with strike directions of 300–310° and are steeply dipping to the south at angles of 50–65°. They cut the NNW-trending faults and mineralized veins, indicating that they formed after the main mineralizing episode. 3. Geology of the Xiasai Ag–Pb–Zn deposit The Xiasai Ag–Pb–Zn deposit is located in the northern part of the Xiasai monzogranite (XSM) and about 2 km away from the northern margin of the RYC granite (Fig. 1b). The host-rocks belong to the upper part of the Tumugou Formation (Fig. 2). This sedimentary sequence of

Fig. 2. Geological map of the Xiasai Ag–Pb–Zn deposit. 3

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divided into three zones vertically, i.e., a Pb–Ag-rich zone, a Zn–Cu-rich zone, and a Cu-bearing zone. Galena, silver-minerals, and native bismuth occur at the surface. Arsenopyrite, pyrrhotite, and minor sphalerite and chalcopyrite occur in the lowest zone. The intermediate zone is dominated by sphalerite and chalcopyrite, with minor galena (Fig. 3). According to this mineral zonation, the primary ores can be classified into three types, i.e., Cu–Zn-rich veins, Cu–Pb–Zn–Ag-rich veins, and Pb–Ag-rich veins (Fig. 4a–c). Hydrothermal alteration is characterized by an early skarn mineral assemblage composed by garnet, actinolite, and epidote, which is followed by a late quartz-sericite ± chlorite assemblage (Li et al., 2019). Skarn minerals are associated to the cassiterite formation, whereas the quartz and sericite are the most important alteration minerals associated to the Cu–Pb–Zn–Ag mineralization. Chlorite is present locally and is mostly found within chalcopyrite-sphalerite veins. A horizontal and vertical alteration zonation is developed. Three alteration zones are recognized from south to north (Fig. 2), i.e., epidote + actinolite + garnet zone, quartz + sericite + chlorite zone, and weak quartz + sericite zone. The intermediate zone is represented by quartz + sericite + chlorite. In addition, the same alteration zones are developed upward corresponding to the mineralized zones described previously (Fig. 3).

faults, with subordinate NW-striking faults, as observed at the district scale (Fig. 2). The NNW-trending faults are the primary ore-bearing structures and dip steeply to the south with angles of 50–70°. They are characterized by an early sinistral strike-slip motion overprinted by a late dextral strike-slip reactivation (Li et al., 2019). Faults F1 and F2 are the two longest faults and represent the main ore-bearing structures for S1 and S2 ore bodies. The F1 is more than 3 km long and about 10 cm to 12 m wide, and dips to the south with angles of 55–66°, and strike directions of 330–360°. The F2 is more than 3 km long and about 10 cm to 7 m wide, and dips to the south with angles of 50–68°. The NW-striking faults cut the mineralized veins and ore bodies. 4. Characteristics of vein-type ores Two types of mineralization, i.e., Sn-bearing veins and Ag–Pb–Znbearing veins occur in the Xiasai deposit. The Sn mineralization is hosted by cassiterite-quartz veins in a proximal location within 800 m to the XSM. Seven Sn ore bodies are identified, which are 100–400 m in length and 0.40–7.70 m in width, with grades of 0.44–4.20% Sn (Xiasai Mining Co. Ltd. Sichuan Province 2008, unpublished data). Ore-related alteration minerals such as garnet, actinolite, and epidote, are present locally. The Ag–Pb–Zn mineralization is hosted in sulfide veins that are located distal and to the north of the XSM with distance of ~500–3500 m (Fig. 2). Nine Ag–Pb–Zn ore bodies have been recognized and they commonly occur as lodes, lenses, and disconnected pods, dipping SW and pitching to NW at angles of 20–25° along NNWtrending faults. The contact between the mineralized veins and the host-rocks is sharp (Fig. 3). Individual ore veins are 200–3000 m in length and 0.2–14 m in width. The S1, S2, and S5 are the three biggest ore bodies The S1 ore body is ~2300 m long and 0.2–14 m wide, and continues to depths of 260 m below surface (Fig. 2). It has average grades of 2.86% Pb, 3.17% Zn, and 251.40 g/t Ag. The S2 ore body is more than 2500 m in length and 0.16–7.0 m in width, and continues to depths of approximately 230 m below surface (Fig. 2). It has average grades of 4.71% Pb, 5.45% Zn, and 395.48 g/t Ag. The S5 ore body is divided into two segments, i.e., the southern and northern segments. The southern segment is ~350 m long and 2 m wide with average grades of 2.62% Pb, and 173.49 g/t Ag. The northern segment is ~800 m in length and 1.85 m in width with average grades of 191.34 g/ t Ag, 3.11% Pb, and 4.62% Zn. The Ag–Pb–Zn mineralized veins exhibit horizontal and vertical zonation in terms of mineral assemblages. It can be divided into three zones from south to north, i.e., a pyrrhotite-arsenopyrite-rich zone, a Zn–Cu-rich zone, and a Pb–Ag-rich zone (Fig. 2). The first zone is dominated by arsenopyrite, and pyrrhotite, with minor cassiterite, chalcopyrite and sphalerite. The second zone consists of sphalerite and chalcopyrite, with minor galena. The third zone contains galena, silverminerals, and native bismuth. Similarly, the mineralization can also be

5. Mineral paragenesis and textures The ore minerals were studied in more than 20 polished sections, collected from Ag–Pb–Zn veins at different levels. According to the detailed microscopic observations and mineral chemical analyses, the mineral paragenesis of the Xiasai ore, from early to late episodes, comprises cassiterite, coarse-grained pyrite (Py1), arsenopyrite, pyrrhotite, marcasite, sphalerite, chalcopyrite, fine-grained pyrite (Py2), tetrahedrite, stannite, pyrargyrite, freibergite, miargyrite, native bismuth and galena, with minor breithauptite, ullmannite, and Küstelite. Quartz, chlorite, and other alteration minerals occur as gangue minerals. Based on mineral assemblages, textural observations, and crosscutting relationships, three hydrothermal stages are recognized at Xiasai (Fig. 5). Stage I corresponds to quartz-cassiterite veins, with minor alteration mineral assemblage of garnet, actinolite, and epidote locally. Stage II comprises of sulfide veins for the Cu–Pb–Zn–Ag mineralization, and stage III is represented by quartz and calcite veinlets with scarce galena and fine-grained pyrite. In addition, stage II can be further divided into three sub-stages, i.e., sub-stage II-1, sub-stage II-2, and sub-stage II-3. Sub-stage II-1 is represented by mineral assemblages of Py1, arsenopyrite, and pyrrhotite with minor marcasite in sulfide veins. Py1 and arsenopyrite are present as euhedral to anhedral coarse grains and are replaced by pyrrhotite, marcasite and sphalerite (Fig. 4d, e, h, i). Pyrrhotite is common in ore veins and is replaced by chalcopyrite and galena (Fig. 4f, g, i). Some

Fig. 3. Geological cross-section P7 of the Xiasai Ag–Pb–Zn deposit. 4

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(caption on next page)

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Fig. 4. Hand specimens and reflected-light photomicrographs, showing type and mineral paragenesis of the Ag–Pb–Zn vein ores. (a) Cu–Zn-rich vein ore; (b) Cu–Pb–Zn–Ag-rich vein ore; (c) Pb–Ag-rich vein ore; (d) Coarse-grained pyrite (Py1) replaced by pyrrhotite; (e) Py1 replaced by marcasite; (f) Breithauptite as inclusions in pyrrhotite, and both replaced by galena; (g) Pyrrhotite replaced by chalcopyrite in cusp-style, and both replaced by pyrargyrite; (h) Py1 and marcasite replaced by sphalerite in cusp-style; (i) Arsenopyrite replaced by pyrrhotite and sphalerite, and both replaced by galena; (j) Replacement relics of pyrrhotite in chalcopyrite, and galena partially replacing previous pyrrhotite, chalcopyrite, and sphalerite; (k) Sphalerite stars hosted within chalcopyrite; (l) Tetrahedrite and stannite associated with sphalerite with replacement textures by chalcopyrite crosscutting veinlets; (m) Sphalerite with chalcopyrite ‘disease’ replaced by stannite as surrounding rim; (n) Fine-grained pyrite (Py2) associated with sphalerite, and sphalerite replaced by stannite in veinlets; (o) Freibergite associated with galena and as infill veinlets in sphalerite; (p) Freibergite as inclusions within galena; (q) Replacement relics of pyrrhotite in galena, and pyrargyrite as irregular gains associated with galena; (r) Native bismuth as irregular grain associated with galena and as inclusions included in galena. Mineral abbreviations: Py = pyrite, Py1 = Coarsegrained pyrite, Py2 = fine-grained pyrite, Apy = arsenopyrite, Po = pyrrhotite, Mrc = marcasite, Bhp = breithauptite, Sp = sphalerite, Ccp = chalcopyrite, Gn = galena, Ttr = tetrahedrite, Stn = stannite, Pyr = pyrargyrite, Fbg = freibergite, Bi = native bismuth.

(Fig. 4j), and as small inclusions (so-called chalcopyrite disease) (Fig. 4l, m, r). The texture of ‘chalcopyrite disease’ was also explained as a replacement texture (Barton and Bethke, 1987). Both sphalerite and chalcopyrite partially replace arsenopyrite, pyrrhotite, and marcasite. Tetrahedrite is associated with sphalerite and chalcopyrite with replacement textures by chalcopyrite crosscutting veinlets (Fig. 4l). Stannite always replaces sphalerite as surrounding rim or in veins (Fig. 4m–n). Py2 coexists with sphalerite in sulfide veins (Fig. 4n). Substage II-3 is characterized by an assemblage of galena, silver-minerals,

nickel-minerals such as breithauptite and ullmannite occur as inclusions in pyrrhotite (Fig. 4f). Sub-stage II-2 consists of (quartz-) chalcopyrite-sphalerite veins, with minor Py2, tetrahedrite and stannite, associated with intense sericitization and chloritization. Sphalerite occurs either as massive sulfides with chalcopyrite and galena (Fig. 4j, l), or as ‘stars’ in chalcopyrite (Fig. 4k). Sphalerite stars could be the result of local supersaturation in sphalerite at the growing fronts of chalcopyrite with a high precipitation rate (Marignac, 1989). Chalcopyrite mainly occurs in association with sphalerite with replacement textures

Mineralogy

Stage II Stage I

Sub-stage II-1

Sub-stage II-2

Sub-stage II-3

Stage III

Alternation and gangue minerals Ga r net Actinolite Epidote Zoisite Axinite Quartz Chlorite Sericite Calcite Ore minerals Cassiterite Pyrite

Py1

Py2

Arsenopyrite Pyrrhotite Breithauptite Ullmannite Marcasite Sphalerite Chalcopyrite Tetrahedrite Stannite Native bismuth Galena Jamesonite Küstelite Pyrargyrite Miargyrite Freibergite Argentite major

minor

Fig. 5. Paragenetic sequence of the Xiasai Ag–Pb–Zn deposit. 6

accessory

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%) and some host Au content of 0,01–0.14 wt%. For trace elements, arsenopyrite contains Sb ranging from 1018 to 4818 ppm, Zn from 209 to1420 ppm, and Pb from 3151 to 17,855 ppm, with low concentrations of other elements (Table S1). The major elements of pyrrhotite grains are dominated by S (37.70–40.87 wt%) and Fe (57.42–61.39 wt%), with low contents of Co (0.08–0.12 wt%) and Bi (0.09–0.21 wt%) (Table 2). The calculated contours of FeS (mol.%) in pyrrhotite range from 0.91 to 0.96. However, Ni content is below the detection limit of EPM analysis. The Cu (914–9929 ppm), Pb (154–2593 ppm), and Zn (134–2734 ppm) are the three most abundant trace elements for pyrrhotite. The Co concentration ranges from 68.4 to 181 ppm and Ni ranges from 32.7 to 108 ppm (Table S1). Furthermore, nickel-minerals were observed under the microscope as inclusions in pyrrhotite with 10–40 μm in size (Fig. 4f). They were identified as breithauptite and ullmannite by EDS (Fig. 6a, b) and EPM analyses (Table S2). Breithauptite yields an average crystallochemical formula of (Ni1.04Fe0.13)1.17SbS0.05 (four analyses), and contains high Ni (30.80–31.81 wt%) and Sb (60.31–63.66 wt%) contents, but low Fe (2.78–4.56 wt%) and S (0.16–1.60 wt%) contents. Whereas, another analysis for ullmannite ((Ni1.10Fe0.74)1.84SbS0.74) shows higher Fe (16.47 wt%) and S (11.94 wt%) contents, lower Ni (25.88 wt%) and Sb (52.78 wt%) contents. The Py1 exhibits S content ranging from 51.55 to 54.61 wt% and Fe from 47.18 to 47.32 wt%, with low contents of Zn (0.02–0.15 wt%), Co (0.04–0.08 wt%), and Bi (0.16–0.23 wt%) (Table S2). Arsenic content is below the detection limit. By contrast, Py2 contains higher S contents (53.98–54.80 wt%) and lower Fe contents (43.50–43.82 wt%) with lower than 0.07 wt% As (Fig. 7a). The massive sphalerite grains contain S contents of 31.47–34.30 wt % and Zn of 50.99–57.57 wt%, and are characterized by high Fe contents (8.75–15.20 wt%) with Fe/Zn ratios of 0.18–0.30 (Table 3). The Fe contents are similar to those of three sphalerite samples (10.74–13.61 wt%) detected by Huang and Hu (2000). The Cd contents range from 0.53 to 0.88 wt% with Zn/Cd ratios of 70–103. Low contents of Cu (< 0.81 wt%) and Bi (0.05–0.26 wt% with an average of 0.16 wt%) were also detected by EPM analyses. The most important trace elements in sphalerite include Cd, Sb, Bi, and In (Table S1). The Cd concentration varies between 3313 and 4301 ppm with Zn/Cd ratios of 80–93. The Sb concentration ranges from 1283 to 4786 ppm, Bi from 62.9 to 205 ppm, and In from 215 to 799 ppm. Other trace elements detected by ICP–MS such as Co (4.49–31 ppm), Ni (1.61–4.46 ppm) and Ga (4.82–10.6 ppm) concentrations in sphalerite are relatively low. However, in comparison to the massive sphalerite, the sphalerite stars show higher Zn (55.59–58.99 wt%) and Cu (0.16–3.27 wt%), but lower S (31.17–33.79 wt%) and Fe (7.88–9.00 wt%) contents with lower Fe/ Zn ratios of 0.13–0.15 (Fig. 7b–d; Table 3). Chalcopyrite grains have high contents of S (34.17–35.56 wt%), Fe (29.00–31.09 wt%), and Cu (33.85–34.73 wt%) (Table S2). Although sphalerite stars are common in chalcopyrite, the Zn content in chalcopyrite is less than 0.23 wt%. Ag content in chalcopyrite ranges from 0.01 to 0.37 wt% and Bi content is less than 0.22 wt%. Relatively low contents of Ge (< 0.03 wt%) and Cd (< 0.02 wt%) were detected in some grains. In addition, the Zn concentration varies between 9447 and 15,139 ppm and Pb varies between 487 and 5478 ppm (Table S1). Cd, In, and Bi are moderately enriched, with concentrations of up to 215 ppm, 347 ppm, and 597 ppm, respectively. The concentrations of Co and Ni are relatively low, with values of up to 29.2 ppm and 55.2 ppm, respectively. Based on results of EDS and eleven EPM analyses, stannite is characterized by high S (28.90–31.18 wt%), Fe (12.45–13.42 wt%), Cu (27.45–30.30 wt%), and Sn (23.14–27.54 wt%) contents (Fig. 6h; Table S2). Zn content ranges from 0.69 to 5.55 wt%. Besides, low contents for Ag (0.05–0.48 wt%) and Bi (0.05–0.17 wt%) were detected in stannite.

and native bismuth, with weak silicification and sericitization. Galena partially replaces previous arsenopyrite, chalcopyrite, and sphalerite (Fig. 4f, I, j, l). In some cases, replacement relics of other minerals were observed in galena (Fig. 4p–r). Silver-minerals including pyrargyrite, freibergite, miargyrite, and Küstelite, are present as irregular grains intergrown with galena or as inclusions in galena (Fig. 4o–q). Native bismuth occurs as disseminated micro-inclusions in and/or as fine irregular grains on the margin of galena (Fig. 4r). 6. Analytical methods Fourteen sulfide samples, including arsenopyrite, pyrrhotite, sphalerite, chalcopyrite, and galena, were collected for ICP–MS trace elements analysis from different levels though the S1 and S2 ore bodies. The samples were broken into 40–80 mesh fragments and sulfide grains were hand-picked under a binocular microscope to obtain a 99% pure separate. Analyses were carried out at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology, China, using an ELEMENT 2/XR ICP–MS. Approximately 50 mg of sulfide samples were crushed to 200 mesh and dissolved in Teflon containers with a HF + HNO3 + HClO4 mixture at 200 °C. After drying, the products were redissolved by HClO4 and 3 ml 1:1 HNO3 mixture in screw-top beakers for several hours. The solution was diluted by 1% HNO3 in 50 ml containers and then measured by ICP–MS. A set of 26 elements was chosen for analysis of sulfides (Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Nb, Mo, Cd, In, Sb, Ba, Ta, W, Tl, Pb, Bi, Th, and U) in this study. The analytical precision is better than 5%. Twelve polished thin sections were prepared for Energy Dispersive X-ray Spectroscopy (EDS) and Electron probe microscopy (EPM) analysis. The EDS analysis was performed at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences in Wuhan, using an Oxford SDD Inca X-Max 50 EDS complied with a FEI Quanta 450 FEG-SEM. An accelerating voltage of 10 kV and beam current of 20 nA was used. Following careful petrographic inspections, the major element compositions of different minerals from ores, such as arsenopyrite, pyrrhotite, sphalerite, chalcopyrite, as well as silver-minerals, were obtained on polished thin sections using a JEOL JXA-8100 electron microprobe at the State Key Laboratory for Mineral Deposits Research (MDR), Nanjing University. The operating conditions were set at an acceleration voltage of 15 kV, a beam current of 20 nA for all elements, and a nominal beam diameter of 1 μm. Fifteen elements, including S, As, Fe, Cu, Pb, Zn, Ag, Ge, Cd, Co, Ni, Sb, Au, Sn, and Bi, were chosen for analysis. The detection limits for all elements are 0.02–0.05 wt%. All data were corrected with standard ZAF correction procedures. Natural minerals (arsenopyrite, pyrite, sphalerite, chalcopyrite, galena, stibnite, cassiterite, native gold, native silver, native nickel, native cobalt, and native bismuth) were used as standards. The EPM mapping was performed using a JEOL JXA-8600 electron microprobe at Department of Geoscience, Aarhus University, Denmark. The operating conditions were 15 kV acceleration voltage, 10nA beam current and 1 μm spot size. 7. Mineral chemistry 7.1. Silver-free minerals The silver-free minerals in the Xiasai deposit are dominated by arsenopyrite, pyrrhotite, pyrite, sphalerite, chalcopyrite, with trace amounts of stannite, marcasite, and tetrahedrite. The geochemistry of the main ore minerals is described by their paragenetic relations in the mineralization cycle. Arsenopyrite contains Fe contents of 32.95–35.79 wt%, S contents of 18.60–21.75 wt%, and As contents of 44.71–47.13 wt% (Table 1), with atomic weight of As (At%) ranging from 31.8 to 34.8%. Co (0.01–0.11 wt%), Sb (0.03–0.37 wt%), and Bi (0.03–0.17 wt%) contents were detected in arsenopyrite. Some analyses have low Ag (< 0.03 wt 7

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Table 1 EPMA compositions (wt.%) of arsenopyrite from the Xiasai Ag–Pb–Zn deposit. Sample

Ag

Ge

S

As

Fe

Zn

Cd

Cu

Co

Ni

Pb

Sb

Au

Sn

Bi

total

Calculated formula

Mol. Prop. As

K68-1-11 K68-1-13 K22-13 k18-3-01 k18-3-04 k18-3-05 4335-1-3 K34-02

– – – 0.01 – – 0.01 0.03

– – – – – – – –

20.01 18.60 19.44 21.75 20.69 21.25 19.96 19.57

45.85 47.13 46.56 44.71 45.32 45.08 46.88 46.98

34.13 33.39 34.22 33.65 33.28 32.95 35.79 34.10

– – 0.12 – – 0.02 – 0.02

– – 0.02 – – 0.00 – 0.01

0.01 – 0.08 – – – – 0.02

0.11 0.02 0.07 0.05 0.01 0.06 0.04 0.07

– – – – – – – –

0.02 0.05 – – – – 0.02 0.01

0.05 0.08 0.03 0.26 0.37 0.14 0.12 0.22

0.02 0.05 – – 0.14 0.07 0.07 –

– – – – – – – 0.01

0.05 0.07 0.05 0.17 0.15 0.03 0.10 –

100.26 99.38 100.58 100.58 99.96 99.61 102.97 101.00

Fe0.99As0.99S Fe1.03As1.08S Fe1.00As0.88S Fe0.89As0.88S Fe0.92As0.93S Fe0.89As0.91S Fe1.03As1.01S Fe1.00As1.03S

33.1 34.8 33.8 31.8 32.8 32.4 33.1 33.9

– below the detection limits. Table 2 EPMA compositions (wt.%) of pyrrhotite from the Xiasai Ag–Pb–Zn deposit. Samples

Ag

Ge

S

As

Fe

Zn

Cd

Cu

Co

Ni

Pb

Sb

Au

Sn

Bi

Total

Calculated formula

Mol. Prop. FeS

K1-08 K1-09 K68-1-12 K22-03 K22-11 k8-2-12 k8-2-17 k10-1–3-03 k10-1-3-07 k10-1-3-14 k10-1-2-05 k10-1-1-05

0.01 – 0.03 – – 0.06 – – – – – 0.01

– – 0.01 – – – – – – 0.01 – –

40.87 39.96 39.51 40.25 37.70 39.60 39.94 37.98 40.03 38.21 38.53 38.68

– – – – – 0.03 – – – 0.03 – –

60.67 60.55 60.63 60.90 61.13 57.42 59.97 61.39 57.71 60.25 60.19 60.51

– 0.05 – – 0.19 0.02 0.53 – – – – –

0.01 – 0.02 – – 0.00 0.02 – – – – 0.02

– – 0.01 – 0.24 0.01 0.03 – – – 0.12 –

0.10 0.12 0.10 0.11 0.08 0.08 0.11 0.09 0.11 0.09 0.10 0.09

– – – – – – – – – – – –

0.11 – – – 0.07 0.02 – – – – 0.03 0.02

0.02 – – – – 0.03 – – 0.02 0.03 – –

– – – 0.07 0.04 0.04 – – 0.12 – – –

– – – – – – – – – – 0.02 0.01

0.17 0.21 0.09 0.18 0.18 0.18 0.19 0.11 0.16 0.14 0.14 0.17

101.96 100.92 100.43 101.55 99.66 97.52 100.78 99.60 98.15 98.76 99.13 99.55

Fe1.09S1.27 Fe1.08S1.25 Fe1.09S1.23 Fe1.09S1.25 Fe1.09S1.18 Fe1.03S1.23 Fe1.07Zn0.01S1.25 Fe1.10S1.18 Fe1.03S1.25 Fe1.08S1.19 Fe1.08S1.20 Fe1.08S1.21

0.92 0.93 0.94 0.93 0.96 0.91 0.93 0.96 0.91 0.95 0.95 0.95

– below the detection limits.

grains also contain Cd contents ranging from below detection limit to 0.13 wt%. One analysis (K1-25) exhibits Au content of 0.17 wt%. Miargyrite (AgSbS2) is identified in the Xiasai deposit as veinlets in sphalerite and galena (Fig. 6d). One grain in this study hosts Ag content of 46.14 wt%, S of 20.35 wt%, Sb of 22.29 wt%, with Fe of 8.28 wt% and Cu of 3.43 wt% (Table S1). Three other analyses from Huang and Hu (2000) display lower Ag (36.40–38.24 wt%), Fe (0.10–0.24 wt%), and Cu (below detection limit to 0.22 wt%) contents, with higher S (21.13–22.18 wt%) and Sb (39.63–40.58 wt%) contents. Freibergite ((Cu,Ag)10(Fe,Zn,Hg)2(Sb,As)4S13) has intimate intergrowths with galena in general, and occurs as inclusions in galena (Fig. 4p) or irregular grains between sphalerite and galena (Fig. 4o). It is characterized by much higher contents of Cu (13.23–36.25 wt%) than the other sliver-bearing minerals (e.g., pyrargyrite and miargyrite) (Table 4). Fifteen analyses also display variable Ag contents of 4.23–33.22 wt%, S contents of 21.30–29.16 wt%, and Sb contents of 1.84–27.25 wt%. Ag has a clear negative correlation with Cu (correlation coefficient of R2 = 0.99, Fig. 9a) excluding one analysis (K22-02). Freibergite contains intermediate Fe content (3.28–11.91 wt%), low Zn (< 1.73 wt%), Pb (< 1.70 wt%), and Bi (< 0.17 wt%). The mole ratios of Ag/(Ag + Cu) for freibergite vary from 0.43 to 0.55 and Zn/ (Zn + Fe) from 0.05 to 0.20, which both are identical to those of samples (Ag/(Ag + Cu) = 0.42–0.59 and Zn/(Zn + Fe) = 0.09–0.21) from Huang and Hu (2000). Küstelite (AgAu) is very rare in the Xiasai deposit and occurs as fine grains surrounded by pyrrhotite (Fig. 6e) or miargyrite (Huang and Hu, 2000) and are 2–20 μm in size. One analysis in this study shows the highest content of Ag (73.22 wt%), high Au content (21.03 wt%), low S (0.35 wt%), Fe (0.92 wt%) and Sb (2.35 wt%) contents. However, another sample from Huang and Hu (2000) contains higher Au (55.85 wt %) and Cu (1.69 wt%) contents and lower Ag (42.03 wt%) and Fe (0.23 wt%) contents.

7.2. Silver and silver–bearing minerals In the Xiasai Ag–Pb–Zn deposit, several silver and silver-bearing minerals, including galena, pyrargyrite, freibergite, miargyrite, as well as electrum, were detected by EDS and EPM analyses. The major element composition of these minerals, together with previous data published by Huang and Hu (2000), are present in Table S2. Galena is characterized by low S (10.64–14.10 wt%) contents but high Pb (71.58–88.56 wt%) contents with calculated formula of Pb0.93–1.16S (Table S2). Fe content is mostly less than 0.53 wt%, only one analysis (K1-30) yielded up to 3.05 wt%. Similarly, Cu and Zn contents are less than 1.54 wt% and 1.85 wt%, respectively. Sb content varies between below the detection limit and 0.48 wt%. Low contents of Ge (0.03–0.12 wt%) and Cd (0.13–0.20 wt%) were also detected by EPM analyses. The Ag content for most galena grains is lower than 0.36 wt%. However, some grains contain Ag contents up to 2.77 wt%. Ag/Pb ratios for these grains vary from 0.00008 to 0.0350 with a peak value of 0.0005. Galena also contains high contents of Bi, ranging from 0.21 to 4.53 wt%, with a peak Bi/Pb ratio of 0.018. The mole proportion (mol.%) of Ag, Bi, and Ag + Bi in galena exhibit a linear correlation with those of Pb (Fig. 8). The Cu (372–942 ppm), Zn (1131–10,484 ppm), and Sb (1029–4432 ppm) are the most important trace elements in galena (Table S1). The concentrations of Cd and Bi are moderate and range from 65.5 to 172 ppm and 23.3 to 815 ppm, respectively. In addition, the concentrations of Co (0.11–0.53 ppm) and Ni (0.60–1.14 ppm) detected by ICP–MS are extremely low. Pyrargyrite (Ag3SbS3) is one of the most significant silver-minerals in the Xiasai Ag–Pb–Zn deposit. It generally occurs as intimate intergrowths with galena (Fig. 4q), and in places replaces pyrrhotite or chalcopyrite (Fig. 4g). It is mainly comprised of Ag, Sb and S (Fig. 6c). The Ag contents vary between 55.30 and 67.54 wt% with an average value of 59.46 wt%. The contents of S range from 13.05 to 19.00 wt% (excluding K1-02 with 7.15 wt% S) and Sb ranges from 19.67 to 30.77 wt% (Table S1). In addition, low abundances of As (< 0.37 wt%), Fe (0.01–0.25 wt%), Zn (< 0.63 wt%), Cu (< 0.44 wt%), Pb (< 0.27 wt %), and Bi (< 0.28 wt%), were detected in pyrargyrite grains. Some

7.3. Native bismuth Native bismuth is common in the Xiasai Ag–Pb–Zn deposit (Fig. 6h). 8

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Fig. 6. BSE images and EDS spectra of (a–b) nickel- and (c–f) silver-minerals, (g) stannite and (h) native bismuth.

9

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Fig. 6. (continued)

It always occurs as fine irregular grains in and/or on the margin of galena (Fig. 4r) with 1–5 μm in size, few grains are up to approximate 100 μm. The composition is ranging from 97.44 to 99.60 wt% Bi (Table S2). Eleven analyses exhibit Sb content of 0.07–1.01 wt%. In addition, other elements such as Ag (< 0.06 wt%), S (< 0.44 wt%), Fe (< 1.54 wt%), Zn (< 0.29 wt%), as well as Cu (< 0.13 wt%), are relatively low in native bismuth. One analysis (k8-2–13) is a mixture with high Pb content (17.35 wt%) and low Bi content (72.05 wt%).

8. Discussion 8.1. Mineralization of elements Ni, Ag, and Bi 8.1.1. Ni concentration Nickel as a trace element is dominantly hosted by pyrrhotite and chalcopyrite with Ni concentration of 32.7–108 ppm and 19.1–55.2 ppm, respectively as determined by ICP–MS analysis. The nickel concentrations in sphalerite and galena are extremely low (0.60–4.46 ppm). No nickel-mineral was recognized in chalcopyrite by 20

55

a

Sp star 16

Fe(wt.%)

S(wt.%)

54

53

52 Py1 51

b

Sp

12

8

Py2 4

50 42

44

46 Fe(wt.%)

48

50

50

35

52

56 54 Zn(wt.%)

58

60

3. 5

c

d

3. 0

34

Cu(wt.%)

S(wt.%)

2. 5 33

32

2. 0 1. 5 1. 0

31 0. 5 30 50

52

54

56 Zn(wt.%)

58

0. 0

60

50

52

54 56 Zn(wt.%)

58

Fig. 7. (a) S vs. Fe diagram for Py1 and Py2, and (b–d) S, Fe, and Cu vs. Zn diagrams for sphalerite and sphalerite stars. 10

60

K1-10 K1-19 K1-22 K1-23 K68-1-01 K68-1-05 K22-12 k8-2-14 k8-2-18 k10-1-2-04 k10-1-2-06 k10-1-2-07 4645-1-2-03 4645-1-2-04 4645-1-2-05 4335/2/4 4645-1-2-07 K1-12 K22-06 K22-07 k10-1-3-04 k10-1-3-05 K1-13

Sphalerite

11

0.00 – – 0.02 – 0.01 – – – – – – – – – – – 0.01 – – 0.02 – 0.04

Ag

S 31.47 32.84 32.92 33.81 34.30 34.28 32.61 33.79 34.24 33.97 31.49 31.51 33.79 32.45 33.88 32.38 32.43 33.79 31.17 31.43 31.34 31.22 31.43

Ge

– – – 0.00 – – – 0.03 – – – – – – – – – – – – – – –

– – – – – – – – – – 0.05 0.02 0.06 0.02 – 0.02 – – 0.02 0.02 – – 0.02

As 10.67 11.65 10.44 8.75 12.61 12.56 11.38 14.86 15.20 12.34 12.68 11.20 11.67 10.64 12.31 13.94 10.64 7.98 8.41 8.37 8.25 7.88 9.00

Fe 55.58 54.41 52.70 57.57 53.93 53.12 55.84 51.40 50.99 53.32 53.45 52.53 54.03 57.72 52.77 52.60 56.49 57.39 57.32 55.59 57.12 58.99 58.39

Zn 0.54 0.58 0.55 0.71 0.54 0.63 0.88 0.60 0.59 0.61 0.58 0.53 0.65 0.63 0.68 0.75 0.67 0.71 0.98 0.87 0.38 0.40

Cd 0.81 0.19 0.17 0.22 – 0.18 0.06 0.11 0.11 0.11 0.03 0.10 – 0.16 0.25 0.63 0.17 1.04 2.20 3.27 2.23 1.54 0.16

Cu

– below the detection limits. * T was calculated using formula of Fe/Znsphalerite = 0.0013(T) − 0.2953 (Keith et al., 2014).

Sphalerite star

Samples

Mineral

Table 3 EPMA compositions (wt.%) of sphalerite and sphalerite star from the Xiasai Ag–Pb–Zn deposit.

0.02 0.04 0.00 0.03 0.01 – 0.01 – 0.01 0.03 0.03 0.02 0.04 0.02 0.01 0.00 0.05 0.04 – – 0.01 0.03 0.10

Co – – – – – – – – – – – – – – – – – – – – – – –

Ni 0.08 – 12.51 0.02 0.02 – – – – 0.14 – – – – – – – – – – – – –

Pb 0.01 0.01 – – – 0.00 – – 0.03 – 0.02 – 0.00 – – – – 0.04 – – 0.00 0.02 –

Sb 0.01 0.03 – – – – – – 0.03 0.03 – 0.02 0.03 – – – – 0.10 – – – 0.06 –

Au – – – – – – – – – – – – – – 0.04 0.23 0.02 – – – 0.10 – –

Sn 0.17 0.16 0.26 0.14 0.12 0.12 0.11 0.13 0.20 0.26 0.12 0.17 0.21 0.05 0.20 0.19 0.15 0.09 0.13 0.18 0.16 0.17 0.13

Bi 99.35 99.89 109.55 101.28 101.51 100.90 100.87 100.93 101.40 100.82 98.44 96.08 100.47 101.68 100.14 100.74 100.63 101.18 100.23 99.72 99.61 100.30 99.25

total

(Zn0.87Fe0.19Cu0.01)1.07S (Zn0.82Fe0.20Cd0.01)1.03S (Zn0.78Fe0.18Pb0.06)1.02S (Zn0.84Fe0.15Cd0.01)1.00S Zn0.77Fe0.21S (Zn0.77Fe0.21Cd0.01)0.99S (Zn0.84Fe0.20Cd0.01)1.05S (Zn0.75Fe0.25Cd0.01)1.01S (Zn0.73Fe0.25)0.98S (Zn0.77Fe0.21Cd0.01)0.99S (Zn0.83Fe0.23Cd0.01)1.07S (Zn0.82Fe0.20)1.02S (Zn0.79Fe0.20Cd0.01)1.00S (Zn0.87Fe0.19Cd0.01)1.07S (Zn0.76Fe0.21Cd0.01)0.98S (Zn0.80Fe0.25Cu0.01Cd0.01)1.07S (Zn0.86Fe0.19Cd0.01)1.06S (Zn0.84Fe0.14Cu0.02Cd0.01)1.01S (Zn0.90Fe0.16Cu0.04Cd0.01)1.11S (Zn0.87Fe0.15Cu0.05Cd0.01)1.08S (Zn0.89Fe0.15Cu0.04)1.05S (Zn0.93Fe0.15Cu0.02)1.10S (Zn0.91Fe0.16)1.07S

Calculated formula

0.19 0.21 0.20 0.15 0.23 0.24 0.20 0.29 0.30 0.23 0.24 0.21 0.22 0.18 0.23 0.26 0.19 0.14 0.15 0.15 0.14 0.13 0.15

Fe/Zn

103 95 96 81 100 84 63 85 87 87 92 99 84 92 78 70 84 81 58 64 151 149

Zn/Cd

375 392 379 344 407 409 384 449 457 405 410 391 393 369 407 431 372 334 340 343 338 330 346

T*

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0.14

0 . 03

0.10

Sb(mol.%)

Fe/Cu/Zn(mol.%)

0 . 12

b

a

Fe Cu Zn

0.08 0.06

0 . 02

0 . 01

0.04 0.02 0 . 00 0.6

0.8

1.0

0 . 006

1.0

1.2

d

0.05

0 . 004

0 . 003

0 . 002

0.04 0.03 0.02

0 . 001

0.01

0 . 000

0.6

0.8

1.0

0.06

0.00 0.6 0.14

1.2

e

Ag+Bi(mol.%)

0.04 0.03 0.02

1.0

1.2

f

0.10 0.08 0.06 0.04

0.01 0.00 0.6

0.8

0.12

0.05

Bi(mol.%)

0.8

0.06

Ag(mol.%)

Ge/Cd(mol.%)

c

Ge Cd

0 . 005

0 . 00 0.6 0.07

1.2

0.02

0.8

1.0

0.00

1.2

Pb(mol.%)

0.6

0.8

1.0

1.2

Pb(mol.%)

Fig. 8. Fe, Cu, Zn, Sb, Ge, Cd, Ag, Bi, and Ag + Bi vs. Pb diagrams for galena.

8.1.2. Ag element Silver is one of the major mineralization elements in the Xiasai Ag–Pb–Zn deposit with total metal resource of 1028 t (Li et al., 2019). The results of the EPM analyses demonstrate that most of the major sulfides including arsenopyrite, pyrrhotite, pyrite, and sphalerite grains do not show Ag contents, except of some analyses containing Ag content lower than 0.06 wt%. Nine measurements for chalcopyrite all host Ag contents ranging from 0.01 to 0.37 wt%, with a peak Ag/Cu ratio of 0.005. EPM mapping further suggest that no Ag inclusions are hosted in chalcopyrite (Fig. 11b). The Ag contents within chalcopyrite is not correlated with the Cu and Fe contents. However, when one analysis (K1-11) is precluded, a rough linear correlation is observed between Ag + Bi (mol.%) and Cu (mol.%) (not shown). In terms of the potential of invisible Ag hosted in chalcopyrite, about 50.29 t is estimated based on Cu resources in the Xiasai deposit. Most galena grains host low contents of Ag (< 1.22 wt%), just two analyses show higher Ag contents of 2.07–2.77 wt%. The peak value for Ag/Pb is 0.0005, indicating a low silver content hosted within

EDS and EPM analysis (Fig. 10d). Likewise, no nickel-mineral was hosted by arsenopyrite, in spite of a sample K81-2 containing Ni concentration of 62.7 ppm (Table S1; Fig. 10e). No or extremely low (< 0.02 wt%) abundances of Ni were detected for sphalerite and galena by EPM analyses (Table S2). However, small nickel-mineral inclusions in pyrrhotite or as irregular grains between pyrrhotite and galena (Fig. 4f) were observed. These nickel-minerals (light gray) can be distinguished from pyrrhotite (dark gray) and galena (white) in BSE images (Fig. 6a-b and Fig. 10b–c). Two types of nickel-minerals, i.e., breithauptite and ullmannite, were identified in the Xiasai Ag–Pb–Zn deposit. They are both characterized by high Ni (25.88–31.81 wt%) and Sb (52.78–63.66 wt%) contents. One analysis of ullmannite hosts higher Fe (16.47 wt%) and S (11.94 wt%) contents. The nickel-minerals can be clearly seen in maps acquired by EPM mapping (Fig. 9b–c), but the Ni concentration in pyrrhotite is too low to be revealed in the maps (Fig. 10a).

12

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a

0.55 0.47 0.43 0.89 0.06 0.28 0.29 0.34 0.42 0.45 0.45 0.45 0.54 0.59 0.59 (Ag5.71Cu4.75)10.46(Fe1.93Zn0.50Pb0.01Cd0.01)2.45(Sb3.89As0.03)3.92S13 (Ag0.69Cu6.14)6.83(Zn0.17Pb0.12Cd0.01Bi0.01)0.31Sb0.22S13 (Ag4.90Cu5.58)10.48(Fe2.29Zn0.12Pb0.02Cd0.02Bi0.01)2.46(Sb3.98As0.01)4.01S13 (Ag4.37Cu5.79)10.18(Fe2.08Zn0.26Pb0.02Cd0.01)2.37(Sb3.78As0.01)3.79S13 (Ag0.66Cu9.55)10.21(Fe1.64Zn0.33)1.97(Sb3.71As0.05)3.76S13 (Ag2.86Cu7.52)10.38(Fe1.84Zn0.25)2.09(Sb4.00As0.07)4.07S13 (Ag3.03Cu7.57)10.60(Fe1.55Zn0.25)2.10(Sb4.06As0.02)4.08S13 (Ag3.41Cu6.75)10.16Fe2.27Sb4.11S13 (Ag3.90Cu5.45)9.35(Fe1.65Zn0.16)1.81(Sb3.81As0.21)4.02S13 (Ag4.52Cu5.55)10.07(Fe1.81Zn0.20)2.01Sb4.10S13 (Ag4.36Cu5.30)9.66(Fe1.81Zn0.48)2.29Sb3.66S13 (Ag4.41Cu5.36)9.77Fe1.71Sb1.06S13 (Ag5.35Cu4.64)9.99Fe1.81Sb4.17S13 (Ag5.64Cu3.88)9.52Fe1.75(Sb3.93As0.08)4.01S13 (Ag5.78Cu4.02)9.80(Fe1.10Zn0.28)1.38(Sb3.98As0.02)4.00S13 99.41 100.90 99.87 100.87 99.46 100.97 100.58 99.96 99.55 99.20 99.50 100.57 101.12 99.59 99.19 0.05 0.12 0.05 0.11 n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d

y = -0.7369x + 0.4619 2 R =0.99

0.20 0.15

K22-02

0.05 0.00 0.10

0.20

0.30 0.40 Cu(mol.%)

0.50

0.60

0.70

b

Ag(mol.%)

0.60 0.50 0.40 0.30 0.20

Freibergite Pyrargyrite Miargyrite

0.10 0.00 0.15

0.20

0.25 Sb(mol.%)

0.30

0.35

Fig. 9. Ag vs. Cu and Ag vs. Sb diagrams for silver minerals.

colloform galena. Based on the amount of galena the corresponding Ag concentration, approximate 68.16 t of invisible silver is possibly hosted in colloform galena. In addition, the Ag and Bi (both in mol.%) in colloform galena, show negative correlations with Pb (Fig. 8). Particularly a linear correlation was displayed between the Ag + Bi and Pb (both in mol.%) (Fig. 8f). These further reflect a coupled substitution of Ag++Bi3+↔2Pb2+ (Chutas et al., 2008) for the colloform galena. It is important to note that the distribution of high Ag and Bi coincides with Cu, Fe, and Sb in EPM images (Fig. 11c–e). Besides invisible silver hosted within colloform galena, several types of silver minerals, including pyrargyrite, freibergite, miargyrite, and Küstelite, were also identified from the sub-stage II-3 (Figs. 4, 6, and 11c–e). Pyrargyrite has Ag contents ranging from 55.30 to 67.54 wt% and miargyrite from 36.40 to 46.14 wt%. Two analyses for Küstelite contain Ag contents ranging from 42.03 to 73.22 wt% with Au contents of 21.03–55.85 wt%. In freibergite, 17.16–33.22 wt% Ag were detected by EPM analyses for most grains. Ag has a negative correlation with Cu (except analysis K22-02) in the diagram of Ag vs. Cu (mol.%) (Fig. 9a). Whereas, no correlation is observed between Ag and Sb for freibergite (Fig. 9b). These diagrams reflect a possible coupled substitution of 2Ag+↔Cu2+ in freibergite (Miller and Craig, 1983). The low Ag freibergite (4.23–5.22 wt%) formed due to coupled substitution of silver by copper. Furthermore, it can be interpreted that about 909 t of silver resource occurs as silver-minerals, which have an evaluated proportion of 88.7% for resource in the Xiasai Ag–Pb–Zn deposit.

0.02 – 0.02 – n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d 24.02 25.33 24.79 22.29 26.97 27.14 27.00 27.16 26.13 26.49 24.56 27.25 27.04 25.70 25.81 0.13 0.26 0.20 – 0.14 – – – – – – – – – – – – – – n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d – – – 0.02 n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d

– below the detection limits. n.d not detect. * Samples were reported by Huang and Hu (2000).

15.32 18.55 19.81 3.43 36.25 26.58 26.28 23.30 19.53 18.69 18.59 18.80 15.70 13.23 13.59 0.05 0.1 0.03 0.02 n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d 1.64 0.41 0.91 0.19 1.30 0.93 0.88 – 0.60 0.70 1.73 – – – 0.99 5.48 6.70 6.25 8.28 5.46 5.70 5.66 6.90 5.21 5.37 5.58 5.26 5.38 5.25 3.28 0.11 0.04 0.05 0.00 0.21 0.29 0.10 – 0.88 – – – – 0.34 0.10 21.30 21.73 22.39 20.35 24.90 23.17 22.78 22.62 23.50 22.08 23.03 23.00 22.21 22.38 22.20 – – – 0.04 n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d 31.28 27.65 25.38 46.14 4.23 17.16 17.88 19.98 23.70 25.87 26.01 26.26 30.79 32.69 33.22 K68-1-02 K22-04 K22-09 k8-2-11 YM5b* III3-1* III3-2* III7-1* G2* G31A* YM419* G1* G2-1* YM4b-6* YM1*

0.25

0.10

– – – – n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d

Total Sn Au Sb Pb Ni Co Cu Cd Zn Fe S Ag

Ge

As

Ag(mol.%)

0.20 0.05 0.11 0.02 0.17 0.12 0.12 0.00 0.09 0.10 0.21 0.00 0.00 0.00 0.20

0.30

Samples

Table 4 EPMA compositions (wt.%) of freibergite from the Xiasai Ag–Pb–Zn deposit.

0.35

Bi

Calculated formula

Ag/(Ag + Cu)

Zn/(Zn + Fe)

Y.-J. Li, et al.

8.1.3. Bi element The average bismuth concentrations of major sulfides detected by ICP–MS range from 16.95 ppm to 329.10 ppm. From arsenopyrite and pyrrhotite in sub-stage II-1, through sphalerite and chalcopyrite in substage II-2, to galena in sub-stage II-3, the Bi concentrations gradually increase (Table S1). The EPM results for major sulfides reveal low Bi 13

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Fig. 10. EPM mapping images for nickel in (a) pyrrhotite, (b–c) nickel-minerals, (d) chalcopyrite, and (e) arsenopyrite.

8.2. Implication for source of ore-forming materials

contents for arsenopyrite (< 0.17 wt%), pyrrhotite (0.09–0.21 wt%), Py1 (0.16–0.23 wt%), Py2 (0.19–0.22 wt%), and sphalerite (0.05–0.26 wt%). Chalcopyrite grains also host low Bi contents (0.08–0.22 wt%) with Bi concentrations of 21–507 ppm detected by ICP–MS. The highest contents of Bi were detected for colloform galena, ranging from 0.21 to 4.53 wt% (Table S2). Large amounts of native bismuth grains were observed as fine irregular grains in and/or on the margin of galena by microscopy and EPM mapping (Figs. 4r and 11e). They are dominantly comprised of bismuth (Fig. 6h) with proportions of 97.44–99.60 wt% (Table S2). These scenarios manifest two types of bismuth, i.e., invisible bismuth and inclusions of native bismuth. However, the Bi resources in the Xiasai deposit have not been estimated so far. Only considering bismuth associated with galena and a given proportion of 90% for native bismuth, a large-sized bismuth deposit would be estimated with about 23,800 t of Bi resources.

Nickel-minerals in (Ag–Cu–) Pb–Zn deposits are very rare, and some previous studies reported the presence of nickel-minerals such as breithauptite, nickeline, and ullmannite in Sedex-type (Vishwakarma, 1996; Cook et al., 1998), VMS + metamorphic-type (Zhong et al., 2012), stratabound, volcanic-associated marble- and skarn-hosted (SVALS) type (Jansson et al., 2017), skarn-type (Radosavljević et al., 2015), as well as hydrothermal vein-type (Staude et al., 2007; Staude et al., 2012; Marques De Sá et al., 2014) (Ag–Cu–) Pb–Zn deposits. The vein-type nickel-bearing deposits are exemplified by the Wenzel Ag deposit in southern Germany (Staude et al., 2007), the Wittichen Co–Ag–Bi–U district in SW Germany (Staude et al., 2012), and the Palhal Pb–Zn–Cu–Ag deposit in Portugal (Marques De Sá et al., 2014). However, the origin of nickel within (Ag–Cu–) Pb–Zn deposits remains 14

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Fig. 11. EPM mapping images for silver and bismuth in (a) sphalerite, (b) chalcopyrite, (c) pyrrhotite, (d) pyrargyrite associated with galena, and (e) freibergite and native bismuth within galena.

Ganze (zircon U–Pb age at 292 ± 4 Ma, Yan et al., 2005) and Jinshajiang (346–341 Ma, Jian et al., 2009) ophiolitic rocks, and mafic volcanic rocks (zircon U–Pb ages of ~231–230 Ma, Wang et al., 2013) from the Tumugou Formation. In addition, the calculated δ34S value of the hydrothermal fluids is −8.5‰ (Li et al., 2019), which is similar to those of one whole-rock sample from the Lianlong granite (Qu et al., 2001). Ores also have similar Pb isotopic compositions to K-feldspar, and share the same linear relationship (Li et al., 2019). These further preclude the possibility that the Ni content in the Xiasai deposit was derived from ophiolitic rocks or mafic volcanic rocks. A plausible source for nickel may be the Xiasai monzogranite. The Zn/Cd ratio of sphalerite has been proposed as indicators of the classification of diverse genetic deposits (Jonasson and Sangster, 1978; Zaw and Large, 1996), and Gottesman and Kampe (2007) further

contentious. Staude et al. (2007) proposed that the ascending hydrothermal Ni-bearing fluid for the Wenzel Ag deposit was driven by seismic pumping. Radosavljević et al. (2015) argued that the nickel in skarn (Ag–) Pb–Zn deposits in the Serbo-Macedonian Metallogenic Province was most probably from the ophiolitic rocks. However, a granitic source for the Wittichen district in SW Germany (Staude et al., 2012), and an original constituent of mafic rocks for the Zinkgruvan stratiform Zn–Pb–Ag deposit in Sweden (Jansson et al., 2017), were proposed. In the Xiasai deposit, breithauptite and ullmannite occur as inclusions in pyrrhotite from substage II-1, unliking the occurrence of nickel-minerals associated with Ag–Sb mineralization at late stage of the Wenzel Ag deposit (Staude et al., 2007). Rb–Sr isochron dating of sphalerite samples from the Xiasai Ag–Pb–Zn deposit yielded an age of 99 ± 3 Ma (Li et al., 2019), which is much younger than the Songpan15

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reflects a hydrothermal system providing fluids with intermediate sulfidation (Fig. 11c; Einaudi et al. 2003). A geo-thermometer was proposed as a formula of Fe/Znsphalerite = 0.0013(T)–0.2953, based on the Fe/Zn ratio of sphalerite (Keith et al., 2014). Fe/Zn ratios ranging from 0.18 to 0.30 for massive sphalerite yield fluid temperatures of 344–457 °C (average = 398 °C). Likewise, temperatures of 330–346 °C (average = 338 °C) were estimated for sphalerite stars based on Fe/Zn ratios of 0.13–0.15. In addition, Fe contents of stannite range from 12.45 to 14.03 wt% and Cu contents from 27.45 to 30.30 wt%. For two stannite-sphalerite pairs, temperatures between 285 and 345 °C with fs2 of –8.3 to –7.8 were estimated using the method of Shimizu and Shikazono (1985). These calculated temperatures together with results of fluid inclusions hosted in associated quartz, indicated that these Zn-, Cu- and Sn-sulfides formed at intermediate-high temperature. Fe content of sphalerite was proposed to be a function of temperature and sulfur fugacity of the fluids (e.g., Scott and Barnes, 1971; Lusk and Calder, 2004; Keith et al., 2014; Demir et al., 2013). The incorporation of Fe into the crystal lattice of sphalerite is related to Fe–Zn substitution caused by a temperature increase and a decrease in fs2 and fO2 (or sulfidation state) of the hydrothermal fluid (Keith et al., 2014; Lepetit et al., 2003). The very low iron content of the sphalerite (< 0.89 wt%) at the Istala deposit was interpreted as indicating low temperature and high fs2 conditions (Lusk and Calder, 2004). Keith et al. (2014) also proposed that sphalerite from sediment-hosted vents with higher Fe/Zn ratios and S contents indicated lower fs2 and logfO2 than those occurring at sediment-starved vent sites. Therefore, the presence of high-Fe sphalerite indicates low fs2 and fO2 conditions for precipitation of sphalerite and chalcopyrite at Xiasai. The fs2 and fO2 could increase slightly for the ore-forming fluids system after precipitation of sphalerite, due to the lower Fe/Zn ratios and higher S contents for sphalerite stars hosted within chalcopyrite. The sub-stage II-3 is characterized by the mineral assemblage of silver-minerals, native bismuth and galena. Fluid inclusions hosted in quartz have Th of 158–242 °C with salinities of 3.4–5.7 wt% NaCl (Li et al., 2019). In low temperature environments, the dissolution of silver in galena is difficult (Sack and Goodell, 2002). The low content of invisible silver in galena is suggested to be due to increasing antimony content of hydrothermal fluids. A high amount of Sb causes the incorporation of silver in As–S–Sb sulfosalts at lower temperatures, i.e., 300 °C, prior to the crystallization of galena (Miller and Craig, 1983; Demir et al., 2013). The detection of low silver content of galena (< 2.77 wt%, Table S2) and the presence of silver-minerals associated with galena (Fig. 4o–q) may indicate lower temperature for silver mineralization. According to the thermodynamic database of Sack (2005), temperatures of 160–270 °C (Fig. 12b) were suggested for the formation of freibergite based on the variable Zn/(Zn + Fe) (0.05–0.21) and Ag/ (Ag + Cu) (0.42–0.59) ratios (Table 4). Meanwhile, native bismuth as disseminated micro-inclusions within galena (Fig. 4o) indicates moderately-low temperatures (< 271 °C, Liu and Bassett, 1986) and sulfurpoor affinity. The fluids were progressively evolved to a lower fs2 state (Fig. 12c), corresponding to precipitation of silver sulfosalts, such as pyrargyrite and freibergite. These conditions for silver mineralization are similar to those of the Bianjiadayuan hydrothermal vein-type Ag–Pb–Zn deposit in Inner Mongolia, NE China (Zhai et al., 2019). Silver occurs dominantly as Ag+ (Williams-Jones and Migdisov, 2014), and is transported by bisulfide and chlorite-complexes in hydrothermal fluids (Audétat et al., 2000; Müller et al., 2001; Stefansson and Seward, 2003; Harlaux et al., 2017). Silver bisulfide-complexes (AgHS) predominate at near-neutral conditions at lower temperatures (< 200 °C), whereas chloride-complexes (AgCl2−) are dominant under mildly acidic to acidic conditions (e.g., Seward, 1976; Gammons and Barnes, 1989). A plausible interpretation for transport in silver bisulfide-complexes at Xiasai likes the Bianjiadayuan deposit (Zhai et al., 2019), which was supported by conditions inferred above. As oreforming fluids evolved to lower temperature due to fluids mixing and reacting between fluids and host rocks (Li et al., 2019), the

suggested that a Zn/Cd ratio lower than 250 is characteristically for felsic sources. Considering these suggestions, the low Zn/Cd ratios of sphalerite grains (70–103) and stars in chalcopyrite (58–151) (Table 3) may indicate a hydrothermal fluids source linked to the felsic monzogranite in the Xiasai deposit. In addition, bismuth is a common element in granitic rocks (Heinrichs et al., 1980) and is enriched in granitederived fluids (Shin et al., 2004). As inferred above, the high contents of invisible bismuth in galena and the presence of inclusions of native bismuth in and/or on the margin of galena, can further indicate a granite-derived source. This hypothesis can be supported by our previous study on the geochronology of the mineralization and magmatism, and multiple isotopic compositions (Li et al., 2019). We proposed that the Xiasai deposit is a typical hydrothermal vein-type deposit and is associated to a coeval A-type granite at Albian–Cenomanian (Li et al., 2019). The A-type granite had variable εHf(t) values (–2.7 to 0.6) and Mesoto Neo-Proterozoic two-stage Hf model ages (TDM2 = 925–1095 Ma), and was probably derived from the mixing of melts generated by the partial melting of the Paleoproterozoic crustal basement and the early Cretaceous mantle (Li et al., 2019). 8.3. Conditions of Ag–Pb–Zn mineralization Precipitation of sulfides and sulfosalts from hydrothermal vein-type Ag–Pb–Zn deposits is governed by diverse parameters, such as temperature decrease (Audétat et al., 2000; Müller et al., 2001; Mango et al., 2014), decrease in pH, fO2 and fS2 (Fontboté et al., 2017; Zhai et al., 2019). Our previous microthermometric results for fluid inclusions reveal that temperatures for cassiterite and Ag–Pb–Zn mineralization at Xiasai show a wide range between 480 and ~160 °C caused by mixing of magmatic fluids with hot spring water and organic matter over time (Li et al., 2019). Other physicochemical conditions, such as pH, fS2, and sulfidation state, were further estimated for the Ag–Pb–Zn mineralization based on mineral assemblage and chemistry in this study. The sub-stage II-1 consists of the assemblage Py1, arsenopyrite, pyrrhotite and marcasite. The atomic weight (At%) of As for arsenopyrite ranges from 31.8 to 34.8%, and FeS (mol.%) in pyrrhotite ranges from 0.91 to 0.96. These data indicate that temperatures range from 320 to 470 °C with fs2 of –4.1 to –10.9 on the diagram of logfs2 vs. temperature for arsenopyrite (Fig. 12a, Tomkins and Grundy, 2009). The assemblage arsenopyrite and pyrite is stable up to 491 °C (Clark, 1960), which is coincident with the results inferred above. Precipitation of iron-sulfides begins with the deposition of Py1, which could be under high-sulfidation conditions (Tomkins et al., 2006). In contrast, according to Einaudi et al. (2003) and Baumgartner et al. (2008), the high-temperature hydrothermal pyrrhotite-marcasite assemblage is composed of minerals of low-sulfidation state, deposited from low alkaline fluids. It is here proposed that the change of conditions led to the hydrothermal paragenetic sequence of Py1 → arsenopyrite → pyrrhotite → marcasite. The presence of arsenopyrite in the Xiasai deposit, as the principal As-mineral, suggests an alkaline state for the oreforming fluids because arsenopyrite is stable in an alkaline environment under strong reducing conditions (Vink, 1996). However, replacement of arsenopyrite by pyrrhotite (Fig. 4i) has been observed, which manifests an As-S melt. For many ore deposits, arsenopyrite and pyrite react to form pyrrhotite and an As–S melt at high temperature (Radosavljević-Mihajlović et al., 2017). The melt will be enriched in elements such as Ag, As, Au, Bi, Hg, Sb, and Sn (Frost et al., 2002; Radosavljević-Mihajlović et al., 2017). When the temperature decreased to 285–386 °C (average = 346 °C) based on homogenization temperatures of fluid inclusions hosted in quartz (Li et al., 2019) at sub-stage II-2, the precipitation of Zn-, Cusulfides and Sn-sulfide (stannite) begun with intensive silicification and sericitization. Massive sphalerite is characterized by high-Fe contents up to 15.20 wt% and sphalerite stars up to 9.00 wt% (Table 3). The assemblage of pyrite + chalcopyrite + high-Fe sphalerite probably 16

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0.75 92 0.

94

Ag/(Ag+Cu)

95

96

0.45

250°

97

0.4 0.6 Zn/(Zn+Fe)

300

Lo Apy Po S

ite on te +b pyri e t o ri py halc c

=1 N

400

500

600

700

800

3

Temperature (°C)

IN

R TE

NE

e llit ite

As e e+ rit s

D

rit y TE py enop IA s ar

em

s

py

II-2

te ni tib Sb

800

n atio

W

RY

ve

n H c od i g e GH HI

G HI

ond

LO

0. .0

0

ite tin ma drite fa rae tet

470 320

400

99

VE

-18

30

-12

-6

-14

Fe

S-rich Apy

300

Sc

-10 31

S

Py Ap As( s y S )

Xiasai Ag-Pb-Zn deposit

Arsenopyrite stability field

-10.9

200

T(°C)

rit

e

II-1

ti en arg Ag

te

ite g an ph e+A ste rgyrit a r py py

32

Ap

AsS=As-S (l) Realgar

100

-2

logfs2

-8

yA Lo S( S s)

Py S Po

33

c

py ars VE rr en pyr ho o R r Y tit py ho tit e+ rit LO e le e W ol lin gi te

2

0.8

te

98

0.2

0.

0

rr h Fe oti

0.25

34

As 2S 3=As-S (l) Orpiment

C

200° C 170° C

35

Py Ap As y -S S

logfs2 (atm)

400° C 300°C

0.35

0.

-4.1

0.55

0.

l) -S( S-poor As Po y S Apy 36 Ap

-6

-10

This study Huang and Hu (2000)

0.65

S ( l) Ass)+S As(

-4

b

0.

l) S( ) v S(

0.

0.9

5

Contours of FeS ( mol.%) in Po

+Po

0. 93

Contours of At% As in Apy

35

-2

a

II-3

2 1000/T( K )

1

Fig. 12. (a) LogfS2–temperature grid showing the stability field of arsenopyrite (light yellow shaded area). The phase diagram was from Tomkins and Grundy (2009). (b) Molar Ag/(Ag + Cu) and Zn/(Zn + Fe) ratios of primary freibergite in the Xiasai deposit. The isotherms are calculated on the basis of Sack (2005). The data for freibergite are from Huang and Hu (2000) and this study. (c) LogfS2 vs. temperature diagram showing the relative sulfidation state and the evolutionary path of hydrothermal fluids in the Xiasai deposit. Temperatures were estimated from FIs (Li et al., 2019) and sulfides studies, and logfS2 values from equilibrium mineral assemblages. Sulfidation state determinations and sulfidation reactions are from Zhai et al. (2019). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

was triggered by a decrease in temperature, fs2, fO2, and sulfidation of the hydrothermal fluids with a small increase of fs2, fO2, and sulfidation during precipitation of Zn–Cu.

hydrothermal system transformed to lower fs2 and fO2 state with nearneutral pH, which led to precipitation of silver (Zhai et al., 2019). Meanwhile, native bismuth deposited related to the pH decreasing to neutral or acidic state (Staude et al., 2012). Therefore, we conclude that the formation of Ag–Pb–Zn at Xiasai was triggered by decreasing temperature, fs2, fO2, and sulfidation of the hydrothermal fluids. However, a slight increase of fs2, fO2, and sulfidation was stated during precipitation of Zn–Cu.

Acknowledgements We thank two anonymous reviewers for their critical reviews and constructive comments. The authors also thank Editor-in-Chief Prof. F. Pirajno and Associate Editor Dr. P.C. Lightfoot for their editorial help and useful suggestions. This study was co-supported by the Natural Science Foundation of China (grant 41672083), the Fundamental Research Founds for National Universities, China University of Geosciences (Wuhan) (grant CUGCJ1817), and the Chinese Scholarship Council (support for Yan–Jun Li during his stay at Aarhus University, Denmark). Prof. Wen–lan Zhang at the MDR, Nanjing University, is thanked for her help during EPM analysis.

9. Conclusions (1) Nickel-minerals such as breithauptite and ullmannite occur as inclusions in pyrrhotite in the Xiasai hydrothermal vein-type Ag–Pb–Zn deposit. (2) Trace amounts of invisible silver have been detected in chalcopyrite and galena, whereas a significant amount of silver is incorporated in pyrargyrite, freibergite, miargyrite, and Küstelite as intimate intergrowths with galena or inclusions within galena. These silver minerals have an evaluated proportion of 88.7% for silver resource contribution. (3) Besides invisible bismuth hosted within galena, large amounts of native bismuth grains occur as inclusions in galena with an estimated Bi resource of about 23,800 t. (4) The presence of nickel-minerals and native bismuth, together with Zn/Cd ratios of sphalerite, indicate the ore-forming materials derived from the Xiasai monzogranite. (5) Mineral chemical studies indicate the formation of Ag–Pb–Zn ores

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.oregeorev.2019.103136. References Audétat, A., Gunther, D., Heinrich, C.A., 2000. Causes for Large-Scale Metal Zonation around Mineralized Plutons: Fluid Inclusion LA–ICP–MS Evidence from the Mole Granite, Australia. Econ. Geol. 95, 1563–1581. Barton, P.B., Bethke, J.P.M., 1987. Chalcopyrite disease in sphalerite: Pathology and

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