Hydrothermal processes at the Axi epithermal Au deposit, western Tianshan: Insights from geochemical effects of alteration, mineralization and trace elements in pyrite

Accepted Manuscript Hydrothermal processes at the Axi epithermal Au deposit, western Tianshan: Insights from geochemical effects of alteration, mineralization and trace elements in pyrite Zhankun Liu, Xiancheng Mao, Hao Deng, Bin Li, Shugen Zhang, Jianqing Lai, Richard C. Bayless, Min Pan, Longjiao Li, Qinghua Shang PII: DOI: Reference:

S0169-1368(17)30892-2 https://doi.org/10.1016/j.oregeorev.2018.09.009 OREGEO 2688

To appear in:

Ore Geology Reviews

Received Date: Revised Date: Accepted Date:

17 November 2017 21 August 2018 10 September 2018

Please cite this article as: Z. Liu, X. Mao, H. Deng, B. Li, S. Zhang, J. Lai, R.C. Bayless, M. Pan, L. Li, Q. Shang, Hydrothermal processes at the Axi epithermal Au deposit, western Tianshan: Insights from geochemical effects of alteration, mineralization and trace elements in pyrite, Ore Geology Reviews (2018), doi: https://doi.org/10.1016/ j.oregeorev.2018.09.009

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Hydrothermal processes at the Axi epithermal Au deposit, western Tianshan: Insights from geochemical effects of alteration, mineralization and trace elements in pyrite Zhankun Liua,b, Xiancheng Maoa,b, , Hao Denga,b,*, Bin Lia,b, Shugen Zhanga,b, Jianqing Laia,b, Richard C. Baylessc, Min Pana,b, Longjiao Lia,b, Qinghua Shanga,b a

Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring ，

Central South University, Ministry of Education, Changsha 410083, China

b

School of Geosciences and Info-Physics, Central South University, Changsha 410083, China

c

Department of Natural Sciences, Shawnee State University, Portsmouth, OH 45662, USA

Abstract As the largest low-sulfidation (LS) epithermal deposit in the Tulasu basin, western Tianshan, NW China, the Axi gold deposit has a poorly understood geologic history. It is not clear whether hydrothermal mineralization occurred during a single event or multiple events. In this paper, new data consist of whole rock geochemistry and masschange calculations for hydrothermal alteration. LA-ICP-MS trace element analysis of pyrite was conducted in order to develop a comprehensive metallogenic model. The pyrite-sericite-quartz alteration at the Axi deposit is characterized by replacement of plagioclase and mafic phenocrysts by sericite, quartz, pyrite (py1), smectite, and illite, indicating slightly acidic and reducing conditions during the early ore stage. Medium

Corresponding author. E-mail address: [email protected] (X. Mao); [email protected] (H. Deng). 1

subhedral pyrite crystals (py1) with porous texture are typically sited adjacent to ironrich minerals (e.g., hornblende) and are characterized by Co-Ni-Mn enrichment with high ratios of Co/Ni (avg. 1.114) and Cu/As (avg. 0.655), implying preferential sulfidation reactions in magmatic hydrothermal systems. The quartz ± calcite ± pyrite ± adularia assemblages in silicified rocks and the strong mobility of LREE both reflect a slightly acidic chloride-rich solution phase during the later ore stage. Coarse anhedral pyrites with cataclastic texture (py2) and medium euhedral pyrites (py3) have a mixed hydrothermal origin and were deposited with quartz during Au-pyritequartz and Au-polysulfide-quartz stages, respectively. Py2 and py3 both have significantly lower Cu/As ratios (avg. 0.103 and avg. 0.088, respectively) than py1, indicating the input of meteoric fluid during the later ore stage. The three generations of pyrite, especially py3, have significant lattice-bound gold contents (py1: 0.01– 38.86 ppm, median 1.51 ppm; py2: 0.03–234.37 ppm, median 1.21 ppm; py3: 3.31– 231.56 ppm, median 42.72 ppm). Gold precipitation is associated with the destabilization of Au–bearing chlorine (Au(Cl)2–) and bisulfide complexes (Au(HS)2–) via sulfidation and fluid boiling. All of these characteristics indicate the Axi epithermal deposit is the result of two stages of mineralization. Keywords: Alteration-mineralization effects; Pyrite trace elements; Mass-change; Hydrothermal process; Axi epithermal Au deposit; NW China

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1. Introduction The Tulasu basin is an important mineral district in the western Tianshan, NW China (Fig. 1). The basin hosts numerous low-sulfidation (LS) (e.g., Axi, Tawuerbieke, and Tabei), and high-sulfidation (HS) epithermal deposits (e.g., JingxiYelmend), as well as porphyry-skarn deposits (e.g., Abiyindi and Kexiaxi) (Fig. 2a). All these deposits are hosted by andesites and granitic and dioritic porphyries, which have similar trace element and Sr–Nd–Pb–Hf isotopic composition, with a broadly contemporaneous age of ca. 370 to 347 Ma (Sha et al., 2005; Tang et al., 2013; Xue et al., 2013; Zhao et al., 2014a, 2014b; Peng et al., 2016; Gu et al., 2016). The S–Pb–C– O isotopic composition of ore minerals indicates that mineralization is intimately related to Carboniferous tectonic-magmatic and hydrothermal events (Zhao et al., 2014a; Gu et al., 2016). These characteristics provide compelling evidence for a causal link among these epithermal and porphyry deposits, which likely resulted from the Tulasu polymetallic mineralization system (Zhao et al., 2014a; Peng et al., 2016; Gu et al., 2016). The Axi gold deposit is the largest (over 50 t) LS epithermal deposit in the Chinese Western Tianshan. Gold mineralization is typically identified as disseminated-style and vein-style (Li and Xue, 1994; Feng, 2005; Dong et al., 2018). The crosscutting relationships (An and Zhu, 2017; Liu et al., 2017; Zhang et al., 2017) clearly indicate that the low-grade disseminated ore was followed by the overprinting

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of high-grade vein-style ore. In previous studies, however, there is little information about whether a single mineralization event or multiple events were involved in the formation of the Axi deposit. Thus, a detailed study of the hydrothermal process at the Axi deposit is necessary to better understand ore genesis. The Axi deposit exhibits clear alteration-mineralization zonation, which transitions outwards from silicification, quartz-carbonate alteration, to pyrite-sericitequartz alteration, and propylitic alteration (Zhai et al., 2009; Zhao et al., 2014a; An and Zhu, 2017; Liu et al., 2017). The hydrothermal alteration and mineralization provide a convincing geological record of hydrothermal events at the Axi district. In addition, pyrite, as the most abundant host mineral for gold, is ubiquitous in all the alteration-mineralization zones in the Axi deposit, which is also regarded as a sensitive record of the ore-forming process (Large et al., 2007, 2009; Zhang et al., 2014; Franchini et al., 2015). However, to date, few studies in this area have focused on hydrothermal alteration and pyrite as means for understanding the evolution of mineralizing events. This study focuses on alteration-mineralization assemblages and zonation as well as geochemical behavior during water-rock interactions at the Axi deposit in order to determine its hydrothermal evolutionary history. Laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analysis of hydrothermal pyrite is conducted to further constrain the hydrothermal environment during different ore-forming stages. Our results, in combination with previous studies, yielded a 4

refined model of the hydrothermal processes at the Axi gold deposit, which has important implications for understanding the ore genesis of epithermal deposits. 2. Geological setting 2.1. Regional geology The Tulasu basin is located between the south Keguqin Mountain and north Yili Basin thrust fault zones in the Borohoro area (Fig. 2a). The two fault zones strike NNE and dip 50° to 70°, extending for more than 10 km. Their structural directions are parallel to the boundary of sutures between the Yili and Kazakhstan domains (Zhang et al., 2002). The secondary structures in the Tulasu basin are commonly NW– and N–trending steep strike-slip faults that control the occurrence of Paleozoic strata (Fig. 2). Ring and radial fractures developed around the volcanic edifice within the Jingxi, Abiyindi, and Axi districts (Zhang et al., 2002; Chen et al., 2012). The basement of the Tulasu basin, which formed during the Mesoproterozoic to Paleozoic, consists of two suites of rocks. The lower part consists of metamorphosed shallow marine carbonates, siltstones and sedimentary clastic rocks of the Mesoproterozoic Jixian and the Neoproterozoic Qingbaikou Group. The upper part consists of the Middle Ordovician Nailenggeledaban Formation, which comprises basalt and limestone intercalated with arkose, and the Lower Silurian Nileke Formation, mainly consisting of intermediate to felsic volcaniclastic rocks, as well as carbonate and clastic rocks. Unconformably overlying these basement rocks are the

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subaerial intermediate to intermediate-felsic volcanic and volcaniclastic rocks of the Paleozoic Dahalajunshan Formation (C1d), which comprise tuff, basalt, andesite, dacite, rhyolite, as well as volcaniclastic rocks (e.g., volcanic breccia, volcanic agglomerate, and ignimbrite), with a wide range of ages between ca. 417 and 347 Ma (An et al., 2013; Tang et al., 2013; Zhao et al., 2014a, 2014b; Peng et al., 2016). The C1d is further subdivided into five lithologic members, from bottom to top: gray conglomerate (C1d1), acid tuff (C1d2), lower andesite (C1d3), volcanoclastic (C1d4), and upper andesite members (C1d5). It is generally associated with the late Paleozoic subduction of the South Tianshan Ocean (Zhu et al., 2009) or North Tianshan Ocean (An et al., 2013; Tang et al., 2010, 2013; Zhao et al., 2014b). These volcanic rocks host numerous large (> 10 t) epithermal deposits, including the Axi and JingxiYelmand deposits (Fig. 2). Several granitic intrusions, which are comprised of syenite porphyry, granitic porphyry, and diorite, intruded into the basement or volcanic strata and are exposed in the Tulasu basin (Fig. 2a). They show similar geochemical and isotopic characteristics to the C1d volcanic rocks, and are broadly contemporaneous with the nearby ca. 357 to 348 Ma volcanic rocks (Tang et al., 2013; Xue et al., 2013; Zhao et al., 2014a). The granitic intrusions that outcrop in the southeastern, northeastern, and western parts of the Tulasu basin lack economic mineralization, while the Abiyindi, Kexiaxi, and Tabei intrusions host some porphyry-skarn Au-Cu-Mo deposits and an epithermal Pb-Zn deposit. Some younger dioritic aplites are observed in the Tawuerbieke area (315.2 ± 6

3.5 Ma; Peng et al., 2016). 2.2. Ore deposit geology The strata overlying the Axi district consist of the uppermost member of the Dahalajunshan (C1d5) and Aqialehe (C1a) Formations (Fig. 2b). The former comprises tuff, hornblende andesite, andesite, and dacite, with zircon SHRIMP ages of ca. 367 to 356 Ma (Zhai et al., 2006; An et al., 2013). These volcanic rocks are commonly mauve, light gray-brown, or gray-green, with textures variable from massive or flow structures to porphyritic. The phenocrysts, dominated by plagioclase, are euhedral– subhedral and platy in shape (0.5 ~ 5 mm in size). Porphyroclasts mainly consist of euhedral and subhedral plagioclase together with hornblende and quartz. The Lower Carboniferous Aqialehe Formation (< 288 Ma; Dong et al., 2018), which is only exposed in the northern segment (Fig. 2b), consists mainly of conglomerate, calcareous sandstone, and mudstone. Calderas with ring and radial faults are well developed around the paleovolcanic zone (Zhang et al., 2002; Sha et al., 2005). The ore-controlling fault, F2, strikes to NW 340°, NE 25°, and NW 325° and dips 60°-70°, 50°-80°, and 55°-85° in the northern section (north of line No.44), middle section (between line No.44 and No.27), and southern section (south of line No.27), respectively (Fig. 2b, 3). Orebodies generally occur in the hanging wall of F2 and the footwall of the pyritesericite-quartz zone (Fig. 3, 4), extending to ~1 km along the strike (Fig. 2b), with varying thickness (11-15 m), dip (46°-78°), and grade (2 to 16 g/t and mean 5.6 g/t). 7

The ores can be subdivided into two types: silicified vein type ore (Fig. 5a-e) and disseminated ore (Fig. 5a, 5f). The silicified vein type is the main economic ore, accounting for more than 80% of gold reserves. It typically has a relatively higher gold grade (>5 g/t) than disseminated ore (ca. 2 g/t). Silicified vein type ore is closely associated with quartz ± chalcedony ± sulfide vein and overprinting silicification (Fig. 5b-d), and mainly consists of quartz (or quartz-chalcedony, quartz-sulfide) veins, silicified wall rocks, and breccias (Fig. 5a-d). Most of the ores are smoky or dark gray (Fig. 5a, 5c), due to the fine-grained sulfide and metasomatic debris. The Au-bearing minerals in this type of ore are identified as native gold, electrum, and sulfide (Fig. 5g-h) that are commonly located in the quartz-sulfide aggregate (Fig. 5b, 5e), and strongly altered breccia (Fig. 5d). Gold grade correlates with sulfide content. Gold disseminations in altered rocks are associated with extensive pyrite-sericite-quartz alteration, showing breccia structure and pyrite disseminated texture (Fig. 5a, 5f). Neither visible gold nor electrum are observed in the disseminated ore. The Axi deposit displays a successive alteration-mineralization zoning pattern centrally from the core outward that mainly consists of several zones of silicification, quartz-carbonate, pyrite-sericite-quartz, and propylitic alteration (Fig. 4). The alteration-mineralization zones are parallel to F2 (Fig. 3, 4), with the widths variable from several meters to a few hundred meters. Propylitic alteration, as the earliest alteration (Chen et al., 2003), forms the outer zone with a wide halo (> 500 m) from the present-day surface to depth. Propylitic alteration is characterized by pale-green or 8

green rock and chlorite-epidote-calcite assemblages (Fig. 6a-b). The alteration partially to completely transformed mafic minerals (e.g., pyroxene and amphibole) and plagioclase into chlorite, epidote, sericite, carbonate, quartz, and pyrite (Fig. 6cd). Pyrite-sericite-quartz alteration forms in the outer zone and partially overprints the propylitic alteration. This alteration is characterized by gray-white and tan coloration (Fig. 5a, 5f) and partial to complete replacement of plagioclase and mafic phenocrysts by sericite, quartz, pyrite, and epidote (Fig. 6e-g). Sericite partially or totally replaces plagioclase (Fig. 6e) or appears as veinlets filling in fractures of the former alteration minerals (Fig. 6g). Most quartz grains are fine-grained, with irregular shapes (Fig. 6ef). The silicification and quartz-carbonate alteration zone, which hosts the main orebodies, is well developed in the pattern center and has a variable thickness (15–40 m). These zones have gradational or sharp contact with the surrounding rocks and are characterized by quartz, chalcedony and/or quartz-carbonate vein, veinlet, stockwork, and breccia (Fig. 5a, 6h-i). Hydrothermal breccias are typically cemented by quartz ± chalcedony stockworks and veinlets, which have irregular and angular shapes, and can be jointed (Fig. 5b-c). The clasts of these breccias also occur in quartz veins, which have been totally silicified (Fig. 6j). The silicification is successive with near quartz veins (Fig. 5c, 6h) and varies from gray to smoky-gray in color (Fig. 6h-i). Alteration minerals are mostly comprised of quartz, chalcedony, and sericite, with a 9

small and variable component of pyrite, arsenopyrite, and marcasite. Multiple episodes of quartz deposition occurred in the Axi deposit. The early quartz typically associates with pyrite-sericite-quartz altered rock (Fig. 6g). It is commonly crosscut by late veins or coeval calcite veins in propylitic alteration (An and Zhu, 2017). The second episode of silicification is characterized by black chalcedony veins and veinlets (Fig. 5b, 6j), white or light gray quartz-chalcedony veins (Fig. 7a), and minor pyrite veins (Fig. 7c). The chalcedony veins and breccias are cut or cemented by coeval quartz-chalcedony veins (Fig. 6j-k). Most quartz grains are fine- to medium-grained in veins, but this is not the identifying characteristics. The late silicification typically occurs as gray and smoky-gray quartz-chalcedony stockworks, veins (Fig. 5a, 5c) or tan quartz-polysulfide veins (Fig. 5e). Vug infill (Fig. 6l), comb, and crustiform textures and drusy cavities are common in this type of silicification. Quartz-carbonate veins commonly crosscut (Fig. 6m), cement (Fig. 5c-d, 6n, 7d) and replace (Fig. 6o) all other stages, indicating later formation. The veins are mainly composed of carbonate (e.g., calcite, dolomite, and siderite), quartz, and barite, with minor amounts of fine-grained pyrite. Comb texture (Fig. 6p) and platy calcite (Zhai et al., 2009) are commonly observed in the quartz-carbonate veins. Pyrite is the predominant sulfide mineral in the Axi epithermal system. Based on hand specimen and microscopic-observations of samples, four main types of pyrites have been recognized. Pyrite 1 (py1) is commonly distributed as disseminations in 10

phyllic-altered rock or breccia (Fig. 5f, 7a-b). It spatially associates with altered hornblende (Fig. 6e-f). Py1 rarely occurs as pyrite clusters. The textures of py1 are variable: mainly medium- to coarse-grained, euhedral to subhedral crystals, typically with porous texture, filled by quartz (Fig. 7e-f). Some grains of py1 have complex core-mantle-rim textures, revealed by SEM-BSE imaging (Zhang et al., 2017), or develop oscillatory zonation (Fig. 7g). Py1 grains are overgrown or replaced by pyrite 2 (Fig. 7g-h), pyrite 3, and another sulfide (e.g., arsenopyrite; Fig. 7f), and are considered to be the oldest pyrite type. Pyrite 2 (py2) occurs as disseminations, veins or veinlets (Fig. 6k, 7a-b), and massive accumulations (Fig. 7c) in quartz veins and silicified rocks. Py2 is commonly observed in the contact areas between quartz veins and altered breccias (Fig. 6k, 7b, 7i). Py2 is characterized by coarse- to medium-grained, subhedral to anhedral grains, with cataclastic textures (Fig. 7j). They are typically intergrown, filled, or sealed by late marcasite (Fig. 7k), arsenopyrite, native gold (Fig. 7k; Wei, 2012) and electrum (An and Zhu, 2017). Obvious zonation or textures, related to the crystallization path, rarely occur in py2. Pyrite 3 (py3) is mainly distributed as random disseminations or aggregates (Fig. 7d) in quartz-sulfide veins, breccia, and silicified rocks. Py3 is distinguished from py1 by its relatively small size, clean surface, and pyrite-marcasite-arsenopyrite assemblage. Py3 grains are dominantly medium- to fine-grained, euhedral to subhedral crystals ranging from 20 to 80 μm in size (Fig. 7l-n), occasionally up to 150 11

μm. Py3 overgrows py2, showing zonation (Fig. 7l-m) or developing clear crystallization features (Wei et al., 2011; Zhang et al., 2017). Py3 grains are locally crosscut or overgrown by marcasite and arsenopyrite (Fig. 7n). Py3 commonly coexists with other sulfide minerals (arsenopyrite, chalcopyrite, sphalerite, and galena), comprising polymetallic sulfide veins/veinlets. Pyrite 4 (py4) generally occurs as veinlets or isolated grains in carbonate-quartz veins. It is characterized by fine-grained (less than 40 μm) and cubic crystals (Li and Xue, 1994; Wei et al., 2011). Py4 characteristics that vary most from other pyrite generations are the small size, low content of sulfides, and unique gangue minerals (i.e., calcite-quartz veins). Based on mineral assemblages, textural characteristics, crosscutting and overprinting relationships, four paragenetic sequences have been identified (Fig. 8): (1) pyrite-sericite-quartz (Stage I), characterized by the assemblage of quartz + py1 + sericite; (2) gold-pyrite-quartz (Stage II), identified in black chalcedony veinlets, white and gray quartz veins, stockworks, and breccias, and pyrite ± marcasite ± arsenopyrite ± electrum assemblage; (3) gold-polysulfide-quartz (Stage III), dominated by gray and smoky-gray quartz veins, stockworks and veinlets, as well as mahogany quartz + polysulfide (e.g., pyrite, marcasite, arsenopyrite, sphalerite, galena, and chalcopyrite) ± chalcedony veins; (4) quartz-carbonate (Stage IV), marked by white and light gray carbonate + quartz ± barite veins or veinlets.

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3. Samples and analytical methods 3.1. Whole rock geochemistry A total of 21 samples from the Axi district, which consisted of unaltered or minimally altered andesites (n = 4), altered rocks ( = 9) and vein type ores (n = 8), were analyzed for geochemical composition. The four andesite samples were taken from near the quartz-sericite-pyrite halo. The altered rocks are related to pyritesericite-quartz alteration (Stage I, n = 4), silicification (Stage II, n = 3) and quartzcarbonate alteration (Stage IV, n = 2) (Fig. 8). Vein type ores included two gray quartz veins from Stage II, three smoky-gray quartz veins from Stage III, and three white quartz-carbonate veins from Stage IV (Fig. 8). The detailed locations of these samples are listed in Supplementary Table 1. Analysis of whole rock major and trace elements was performed at ALS Chemex (Guangzhou) Co Ltd. Samples, which were first culled out of breccias, xenoliths, and veins, were crushed to less than 200 mesh by using a steel mortar. Major elements were determined by using X-ray fluorescence spectrometry, with the analytical precision better than 1 %. Trace elements were detected by using the lithium borate dissolution method and ICP-MS, with the analytical precision better than 5 %. 3.2. Trace element analyses by LA-ICP-MS A total of 73 spot analyses (22 on py1; 27 on py2; 24 on py3) were completed on pyrites. Py4 was not analyzed in this study, due to its small grain size. Spot analyses

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were conducted on both rim and core of three types of pyrite (6 on py1; 7 on py2; 3 on py3). Note that py2 does not show the rim-core texture, and we define that rim and core of py2 as the edge and central parts of subhedral crystals. The detailed localities of pyrite-bearing rocks are listed in Supplementary Table 2. Analytical instrumentation employed in this study at the Institute of Geochemistry, Chinese Academy of Sciences, Guiyang consists of a New Wave UP-213 nm Laser Ablation System coupled with an Agilent 7700x Quadrupole ICP-MS. Each analysis was performed by ablating 40-um diameter spots at 8 Hz with a laser beam energy of 6-7 J/cm2. The analysis time for each sample was 80 seconds, consisting of ~15 seconds of background measurement with laser off, and a ~45 seconds analysis with laser on. Helium was applied as a carrier gas for the ablated materials. Every set of eight analyses was followed by one analysis of GSE-1G as quality control of the timedependent signal drift and mass discrimination. Data were processed using ICPMSDataCal (Liu et al., 2008). The following elements were monitored: 49Ti, 51V, 55

Mn, 59Co, 60Ni, 65Cu, 66Zn, 75As, 77Se, 95Mo, 107Ag, 111Cd, 121Sb, 125Te, 197Au, 205Tl,

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Pb, 209Bi.

4. Results 4.1. Whole rock geochemistry Analytical results for the whole rock geochemistry of the Axi gold deposit are presented in Supplementary Table 1.

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4.1.1. Geochemical composition and REE pattern Quartz veins in Stages II and III share similar major elements, which are mainly SiO2 (84.43 ~ 97.70 wt%), with minor CaO (0.06 ~ 3.58 wt%), MgO (0.03 ~ 1.98 wt%), Total iron (TFe2O3) (0.47 ~ 3.25 wt%), and K2O (0.07 ~ 0.22 wt%). The SiO2 content of the samples in Stage IV ranges widely from 21.09 to 61.22 wt%, with high CaO (10.49 ~ 24.20 wt%), MgO (4.63 ~ 11.90 wt%) and TFe2O3 (4.83 ~ 8.37 wt%). There are good negative correlations between SiO2 and MgO, CaO, MnO, TFe2O3, and LOI with hydrothermal processes (Fig. 9). In addition, Al2O3 and K2O show a significant positive correlation (Fig. 9), implying that Al and K are jointly located in sericite. Two quartz veins in Stage II are characterized by the lowest abundance of Ag, As, Co, Ni, Li, Pb, Mo, and Sb. Quartz veins in Stage III contain higher concentrations of Ag (avg. 7.49 ppm) and Li (avg. 70.63 ppm). Total rare earth elements concentrations (∑REE) of andesite samples vary from 73.83 to 101.30 ppm, with (La/Yb)N values from 6.37 to 7.72, showing enrichment of LREE, depletion of HREE (Fig. 10a) and no Eu anomaly (δEu = 0.95 ~ 1.02). Regardless of slightly negative Eu anomalies (avg. δEu = 0.87), the samples which underwent the pyrite-sericite-quartz alteration and silicification share similar REE patterns with protoliths. The pyrite-sericite-quartz alteration is more enriched in REE (∑REE = 74.79 ~ 92.07 ppm) in comparison with the silicification (∑REE = 46.06 ~ 87.38 ppm). The rocks with quartz-carbonate alteration have an overall parallel REE 15

pattern, but with low ∑REE (26.28 ~ 30.79 ppm) and a relatively weak LREE enrichment (Fig. 10a). Quartz vein samples show a low ∑REE (avg. 8.52 ppm) and present a systematic increase from Stage II to IV (1.55 ~ 3.84 ppm → 2.05 ~ 9.94 ppm → 10.37 ~ 24.88 ppm; Supplementary Table 1). The REE distribution patterns of these samples show slight LREE enrichment, low degrees of REE (avg. (La/Yb)N = 6.56), and positive Eu anomalies (avg. δEu = 1.36; Fig. 10b; Supplementary Table 1). 4.1.2. Mass change In this study, pyrite-sericite-quartz alteration, silicification, and quartz-carbonate alteration were selected to calculate mass change by the isocon method (Grant, 1986, 2005). The method is derived from the mass balance equation of Gresens, which can be written as CiA = (MO/MA)(CiO+∆Ci), where MO and MA are the masses of protolith and corresponding altered rock, respectively, and CiA and CiO are the concentrations of element i in rocks. This equation can be formulated as ∆Ci/CiO = (MA/MO)(CiA/CiO) −1, where ∆Ci is the change in concentration of element i, and ∆Ci/CiO represents the relative intensity of hydrothermal alteration. The immobile element during hydrothermal processes is an integrant in the isocon, given ∆Cimmobile = 0. The equation for immobile elements is CimmobileA = (MO/MA)CimmobileO. The immobile elements in pyrite-sericite-quartz alteration at the Axi deposit include TiO2, Ge, Zr, Hf, and In. In addition, P2O5, Hf, Zr, and Nb remain immobile during silicification, and Al2O3, In, Hf, Nb are immobile in quartz-carbonate alteration. We 16

choose TiO2, P2O5, and Al2O3 as immobile elements in the isocon equation (Fig. 11). The calculated mass change values are listed in Table 1, and gains and losses are shown in Fig. 12. Pyrite-sericite-quartz altered samples are enriched in SiO2, MnO, K2O, and P2O5, and are depleted in CaO and Na2O (Fig. 12a). In silicification, there is a significant enrichment in SiO2 and TFe2O3 with the obvious reduction of MnO, MgO, CaO, and Na2O. The quartz-carbonate altered rock is strongly enriched in MgO, CaO, MnO and TFe2O3 and significantly depleted in P2O5 and TiO2 (∆Ci/CiO>25%; Table 1). Most trace elements, particularly Ag, As, Mo, Sb, Tl, and W, are enriched in pyrite-sericite-quartz alteration, while only Ba, Cs, and Sr are depleted. Silicified samples show significant mass gains in Pb, Ag, Sb, Ni, Li, Cr, Co, and As and losses of Ba, V, and U (Fig. 12b). Quartz-carbonate alteration and silicification have a similar pattern of trace element migration, whereas As and Zn are strongly depleted. Pyrite-sericite-quartz alteration generally enriches REE, particularly La and Ce (∆Ci/CiO>10%; Table 1). The geochemical behavior of REE in silicification is analogous to pyrite-sericite-quartz alteration but differs in the enrichment of Eu and the mass change (Fig. 12c). During the process of quartz-carbonate alteration, REE shows different features, such as the depletion of LREE and the enrichment of HREE. Europium, Gd, Tb, and Dy have a weaker mobility than other REE in three types of hydrothermal alteration.

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4.2. Pyrite geochemistry Trace element concentrations in the three types of pyrite are listed in Supplementary Table 2. Selected trace element compositions are shown with boxplots in Fig. 13. In general, concentrations of most trace elements (e.g., Au, As, Co, Bi, and Mo) in the three types of pyrite grains vary considerably (Fig. 13). Gold content in py1 and py2 are similar, with medians of 1.513 ppm (n = 20) and 1.212 ppm (n = 24), respectively. There are several outliers of 38.86 ppm and 21.57 ppm in py1, and 234.4 ppm and 22.28 ppm in py2. Gold content in py3 is distinctly higher than py1 and py2, varying from 3.101 to 231.6 ppm, with a median of 42.72 ppm (n = 24). Gold has a clear positive correlation with Se in the three generations of pyrite (R = 0.620; Fig. 14a), but no covariation with other analyzed elements. Silver contents in the three types of pyrite show a similar variation with Au, such that py1 and py2 contain lower medians of 16.00 ppm and 12.38 ppm, and py3 grains host a significantly higher content (57.43 ppm). Silver and gold generally show a minor positive correlation in py3, but there is no correlation in neither py1 nor py2 (Fig. 14b). The ratios of Au/Ag in most of the pyrite crystals are lower than 10 (Fig. 14b), with an upward trend (0.23, 0.91 and 1.60). There is a positive correlation between Ag with Sb (Fig. 14c). Arsenic is the most abundant trace element in all pyrite crystals in the Axi deposit (Supplementary Table 2). The As contents in py1, py2, and py3 range from 14.48 to 23467 ppm, 2.66 to 30946 ppm, and 5029 to 55735 ppm, respectively, and 18

py3 contains a narrower range of As compared with py1 and py2 (Fig. 13). Although there is no significant correlation between As and Au (Fig. 14d), gold and Au/As ratios show a good correlation (R = 0.612) with the strongest positive correlation (R = 0.791) in py3 (Fig. 14f). Plots of Au/Ag vs. Au/As allow discrimination of the three types of pyrite (Fig. 14g). Cobalt, Ni, Cu, Pb, Ti, and Sb are enriched in pyrite grains (Supplementary Table 2). Py1, the early pyrite, contains higher concentrations of Co, Mn, V, and Bi than py2 and py3, and concentrations of Ni, Pb, Zn, and Cu that are similar to py3 (Fig. 13). Cobalt correlates closely with Ni and Bi in the analyzed pyrite grains (R = 0.981 and 0.967; Fig. 14h). The Co/Ni ratios of py1, py2, and py3 range from 0.057 to 5.460 (avg. 1.114), 0.043 to 1.716 (avg. 0.584), and 0.001 to 0.638 (avg. 0.142), respectively. This trend contrasts with Au/Ag and Au/As. There is no correlation between Cu and As, but a notably high ratio of Cu/As is observed in py1 (Fig. 14i). Comparing py1 and py3, only py2 crystals are only enriched in Tl, and Ti (Fig. 13). Our results show that the rims of analyzed pyrites are enriched in Au and As, with a high Au/As ratio. Detailed analysis reveals that py1 rims have a higher concentration of Au, Ag, As, Sb and Bi, with a higher Au/Ag ratio (avg. 0.12) than cores (n = 6; Fig. 15a). Py2 has an Au-As-rich rim (Fig. 15b). In this type of pyrite, silver shows a strong enrichment in cores, causing a low Au/Ag ratio. The rims of py3 grains are enriched in Au, Ag, As, Co, and Mn. Trace element concentrations within the core and rim of py3 grains share a similar pattern (Fig. 15c). 19

5. Discussion 5.1. Genetic relationship between pyrite and hydrothermal alteration Pyrite is the most typical and abundant mineral at every stage in the Axi deposit and it has a close association with the hydrothermal alteration. Since pyrite in the deposit often contains ample gold whereas only minor native gold grains occur as inclusions in pyrite (Fig. 5f), the existence of abundant invisible gold is implied. In this study, the contents of Au and Se show a significant correlation in pyrites, but without the parallel spectra in LA-ICP-MS profiles (Fig. 16). Notwithstanding the occurrence of some spiky Au profiles (Fig. 16e-f), gold distribution in pyrite crystals exhibits a general flat signal that is similar to As, especially in the pyrites with high Au contents (Fig. 16a, 16c, 16e). This is consistent with recent EPMA studies (Zhang et al., 2017). In addition, the Au-bearing pyrites from the Axi deposit reveal the relatively low Au/As ratios that plot below the gold saturation line (Fig. 14d; Reich et al., 2005). These key observations suggest that gold is mainly present in solid solutions in the lattices of three types of pyrite (e.g., Large et al., 2007; Zhang et al., 2014; Franchini et al., 2015; Gregory et al., 2015). This also indicates that pyrite coprecipitated with Au. Thus, discussion of relationships between pyrite and hydrothermal alteration help us to not only investigate pyrite genesis but also to further understand the mineralization conditions and mechanisms of gold precipitation.

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In pyrite-sericite-quartz alteration, feldspars are commonly destabilized by low pH hydrothermal fluids that are rich in Si, K+, H2S, and HS- (Dai et al., 2008; Parsapoor et al., 2009). Some H+, K+, and Si may be transported from the peripheral propylitic zone (Liu et al., 2017). The altered rock is characterized by the noticeable addition of SiO2 and K2O, and removal of Na2O, CaO, Ba, Cs, and Sr (Fig. 12), which results from feldspar dissolution as well as sericite and quartz generation (Fig. 6e-f). Although TFe2O3 is weakly depleted in this alteration, we observe that py1 is typically located adjacent to iron-rich minerals (e.g., hornblende; Fig. 6e-f), implying the original preferential sulfidation of hornblende. Some py1 is associated with sericite, chlorite, and quartz (Fig. 6e, 6g), via this reaction: 3Al(Mg, Fe)5AlSi3O10(OH)8 (chlorite)+ 2K+ + 18H2S + 7O2 + 10H+ → 2KAl3Si3O10(OH)2 (sericite) + 9FeS2 (pyrite) + SiO2 (quartz) + 33H2 + 6Mg2+. Additionally, the trace elements (Co, Ni, Cu, Pb, Mn, and Zn) are enriched in py1 (Fig. 13), consistent with the whole-rock elemental gains during pyrite-sericite-quartz alteration (Fig. 12b). These characteristics suggest that py1 directly precipitated during hydrothermal fluid/rock interaction. During silicification, feldspar, sericite, carbonate, chlorite, and epidote are totally or partly replaced by quartz and pyrite in a high fluid/rock ratio environment. Silicification strongly increases Ag, As, Co, Cr, Ni, Pb, and Sb (Fig. 12). Generally, these element gains during silicification are likely partly related to the formation of pyrite, which can incorporate some contents of the chalcophile and siderophile 21

elements from hydrothermal fluid during crystallization (e.g., Large et al., 2009; Reich et al., 2013; Franchini et al., 2015). Py2 and py3 show a close association with quartz (Fig. 6k, 7), indicating the generation through silicification. The Ag/Pb ratios of py2 and py3 range from 0.017 to 4.217 (avg. 0.945) and 0.355 to 1.130 (avg. 0.954), close to the values of mass change (0.857; Table 1). Additionally, the Co/Ni ratio associated with the mass change in silicification is 0.568 (Table 1), similar to py2 (avg. 0.584), indicating a hydrothermal origin for py2. Hydrothermal pyrite is typically characterized by Co/Ni >1, whereas the Co/Ni ratios of py2 and py3 are mostly below 1, especially in py3 (< 0.4). The likely conclusion then is that py2 and py3 are syn-sedimentary pyrites (Bralia et al., 1979; Bajwah et al., 1987; Koglin et al., 2010) or diagenetic pyrites associated with deposition of organic matter (Large et al., 2007, 2009; Berner et al., 2013; Gregory et al., 2015). However, if the early pyrite was remobilized by fluid (Koglin et al., 2010; Pasava et al., 2013), or the hydrothermal fluid mixed metamorphic and/or epizonal fluid (Clark et al., 2004; Zhang et al., 2014), hydrothermal pyrite could form at low Co/Ni ratios (< 1). Previous studies of silicifying hydrothermal fluids in the Axi deposit show mixing between magmatic and external water (Jia et al., 1999; Feng, 2005; Zhang et al., 2007; Zhai et al., 2009; An and Zhu, 2017). Considering the clear links between pyrite and quartz veins and their common coarse and euhedral textures, we suggest that both py2 and py3 were deposited together with quartz and are related to a mixed hydrothermal origin. 22

5.2. Ore-forming fluid characteristics 5.2.1. Fluid composition and source inferred by pyrite Gold and many other trace elements, such as Ag, As, Co, Ni, Cu, Pb, Se, and Te, can partition from hydrothermal fluids into pyrite in solid solution and/or as micro- to nano-sized mineral inclusions during growth and/or recrystallization (Reich et al., 2005, 2013; Large et al., 2009; Deditius et al., 2009; Gregory et al., 2015; Tardani et al., 2017). The trace element concentrations and distributions in pyrite depend on both the geochemical properties of elements and the nature of hydrothermal fluid (e.g., composition, temperature, and phase). Thus, the diverse Au-bearing pyrites in the Axi deposit have tremendous potential for providing useful information on the evolving composition and physical-chemistry of ore-forming fluids (e.g., Zhang et al., 2014; Franchini et al., 2015). Three types of pyrite contain variable concentrations of siderophile and chalcophile elements (e.g., As, Cu, Co, Ni, Pb, and Sb) from py1 to py3 as well as from the core to rim of one pyrite crystal (Fig. 15), suggesting that the hydrothermal fluids vary in composition. The early pyrites occurring in pyrite-sericite-quartz altered rock generally have high concentrations of Co, Ni, Cu, Pb, Zn, and Mn (Fig. 13), with high Co/Ni ratios. In addition, those elements are notably enriched in the cores of py1 (Fig. 15a). High Co and Ni concentrations in pyrite with a higher Co/Ni ratio are typically interpreted to be produced in high-temperature magmatic-hydrothermal systems (Reich et al., 2016). The reported temperatures of mineralization at the Axi 23

deposit suggest that Stage I was likely above 285°C; subsequently Stages II and III were at temperatures of ~285°C to 120°C, and Stage IV was below 120°C (Li and Xue, 1994; Jia et al., 1999; Feng, 2005; Zhai et al., 2009; An and Zhu, 2017). These temperature conditions are appropriate for the substitution of Co and Ni during pyritesericite-quartz alteration (Huston et al. 1995). Previous studies have shown that Mn can be enriched in hydrothermal fluids by extensive interactions with carbonate rocks. The fluid inclusions from the Axi deposit typically contain some organic gas (e.g., C2H4, C2H6, C4H6, and C6H6) (Jia et al., 1999; Feng, 2005; Sha et al., 2005; Zhang et al., 2007). δ13C values of calcite vary between 1.7‰ and +4.9‰ (Zhang et al., 2007; Zhai et al., 2009), approximately coincident with the δ13C of marine carbonate (–2‰ ~ +6‰; Veizer et al., 1999), reflecting derivation of the carbon from basement limestone. The δ34S values of py1 are +4.03‰ to +4.30‰ (Li and Xue, 1994), indicating a magmatic source. In summary, py1 was likely formed in magmatichydrothermal systems, and hydrothermal fluids contained some magmatic waters derived from deep intrusion. Py2 and py3 generally occur in quartz veins and silicified rocks associated with SiO2–bearing fluids. The SiO2 content of quartz veins shows a descending trend from Stages II to IV, whereas MgO, CaO, MnO, and TFe2O3 are the opposite (Fig. 9), implying that the major element composition of ore-forming fluids transitioned from SiO2-rich to MgO-, CaO-, MnO-, and TFe2O3-rich. Our data clearly show that py2 and py3 both have lower Cu/As ratios (0.103 and 24

0.088) than py1 (0.655), but are nearly the same as the ratios in quartz veins (avg. 0.014) and in bulk pyrite geochemistry (avg. 0.022; An and Zhu, 2017). The difference likely represents chemical changes in hydrothermal ore fluids (Deditius et al., 2009, 2014; Reich et al., 2013; Tardani et al., 2017). Copper is slightly depleted, and arsenic strongly enriched during silicification (Fig. 12b), which seems to suggest that water/rock reactions result in the decrease in the Cu/As ratios of py2 and py3. The Cu abundances in several generations of pyrite are, however, relatively constant (Fig. 13). In addition, our observation and analysis show that some pyrites were replaced by arsenopyrite (Fig. 7n), and these pyrites generally occur in the As-rich growth zones, with relatively low ratios of Cu/As. Therefore, the high-As fluid was more likely to occur in association with the low Cu/As ratios of py2 and py3, and their As-rich zones, rather than being due to the Cu losses during water/rock reaction. Experimental data, fluid inclusion and geochemical analysis of pyrite have determined that Cu has a higher affinity for the high-density saline brine, while As preferentially partitions into low density and low salinity aqueous vapor (Pokrovski et al., 2002, 2005; Seo et al., 2009; Tardani et al., 2017). The H–O isotope data suggest that there was an input of meteoric fluid during Stages II and III in the Axi epithermal deposit (Sha et al., 2005; Zhai et al., 2009; An and Zhu, 2017). It is plausible that the diluted meteoric water decreased the density, salinity, and temperature of ore fluid, thereby promoting the partitioning of As into the ore system (e.g., Tardani et al., 2017). Thus, the As-rich pyrite may be related to a low density and low salinity fluid 25

environment that resulted from a large input of meteoric fluid. 5.2.2. Physicochemical conditions of hydrothermal fluids The sericite-pyrite-quartz altered rock at the Axi deposit contains some quartz, kaolinite, illite, and smectite, which indicates that the infiltrating fluid was slightly acidic (Simmons et al., 2005; Dai et al., 2008). Due to the loss of H+ during alteration, the pH might have increased, resulting in the formation of calcite (Fig. 6f). The presence of pyrite in sericite-pyrite-quartz alteration is originally associated with reduced sulfur, indicating that local conditions were reducing during Stage I. The quartz-pyrite ± sericite ± adularia ± marcasite silicification assemblages (Stages II and III) represent formation from slightly acidic pH solutions (Simmons et al., 2005; Cooke et al., 2011). During Stage IV, the occurrence of calcite and dolomite reflects decreasing acidity of fluids during quartz-carbonate alteration (Dai et al., 2008). Europium has two oxidation states, Eu3+ and Eu2+, and the oxidation state of Eu changes from Eu3+ to Eu2+ in a reducing-high temperature (> 250 °C) environment (Sverjensky, 1984). The geochemical behavior of Eu can be used to indicate the relative oxygen fugacity of hydrothermal fluids. Our data show that quartz and carbonate veins have positive Eu anomalies (avg. δEu = 1.36; Fig. 10b; Supplementary Table 1), reflecting the relative gain of Eu2+. In addition, fluid inclusion studies show that the ore-forming fluid typically contains H2O, CO2, and CH4, with small amounts of CO, H2S and organic matter (Jia et al., 1999; Feng, 2005; 26

Zhang et al., 2007). Moreover, pyrrhotite was formed in quartz veins during Stages II and III (Li and Xue, 1994; Jia et al., 1999; An and Zhu, 2017). These observations indicate that Stages II to IV all had relatively reducing conditions. 5.2.3. Effects of complexes in hydrothermal fluids on REE mobility Complexes are important for the transportation and precipitation of metal elements in magmatic-hydrothermal ore deposits (Heinrich et al., 2004; WilliamsJones and Heinrich, 2005; Seo et al., 2009). REE are supposed to be carried in aqueous solutions as complexes (Taylor and Fryer, 1980; Michard, 1989; Lottermoser, 1990; Haas et al., 1995; Williams-Jones et al., 2012) and the occurrence as complexes affects their geochemical behavior (e.g., Bau, 1991; Allen and Seyfried, 2005; Parsapoor et al., 2009; Craddock et al., 2010). REE in pyrite-sericite-quartz alteration (Stage I) exhibit no significant variation (Table. 2; Fig. 12c), resulting in the parallel pattern of altered rock and protoliths (Fig. 10a). As mentioned earlier, this alteration was related to high temperature and reducing environment in the early hydrothermal stage. It was detrimental to REE enrichment since REE3+ rarely combines with sulfide-complexes (HS－ or S2－) and even then the complex is unstable at high temperature. The dissolution of plagioclases may release REE into fluids, nevertheless low fluid/rock ratios restrict the subsequent migration. The chloride anion (Cl－) is regarded as a good complexing agent for LREE and Eu (Taylor and Fryer, 1980; Sverjensky, 1984; Michard, 1989; Lottermoser,

27

1990; Allen and Seyfried, 2005; Craddock et al., 2010). Complexation with Cl－ may result in the loss of Eu (Fig. 12c) and weak negative Eu anomalies (Fig. 10a). Although most major elements were strongly depleted by silicification, the REE distribution patterns of the silicified rock were weakly affected (Fig. 10a). There is a clear positive correlation (R2 > 0.89) between REE and P2O5, TiO2, Al2O3, and K2O in the Stages II to IV (Fig. 17), suggesting that the REE-bearing minerals may include apatite, titanite, epidote, and sericite (Gieré and Sorensen, 2004; Vuorinen and Hålenius, 2005; Horie et al., 2008; Williams-Jones et al., 2012). Ore-forming fluids were supplemented by much meteoric water during Stages II and III. It increased the fluid/rock ratio and introduced ligands that could transport REE (e.g., Cl－, F－, SO42－, and CO32－), resulting in REE mobilization and enrichment. Jia et al. (1999) reported that the Cl－ concentration in fluid inclusions trapped in quartz veins can reach up to 8.0 ppm (avg. 2.3 ppm). LREE exhibits relatively strong mobility in silicification (Fig. 12c), which may be attributed to the dissolution of REE-bearing minerals to form REE chloride species in acid chloride-rich hydrothermal fluids (Bau, 1991; Parsapoor et al., 2009; Craddock et al., 2010). Although fluoride is perhaps responsible for LREE behavior (Oreskes and Einaudi, 1990; Ayers and Watson, 1993), chloride species dominate and are able to transport REE under acidic conditions (Bau, 1991; Benaouda et al., 2017). Additionally, F-rich aqueous liquids also enrich HSFE, with high Hf/Sm, Nb/La, and Th/La ratios (>1). However, in our samples, the ratios of mass change and geochemistry of altered rock and quartz veins are below 1 28

(Supplementary Table 1). This suggests that Cl－ was the most important factor for LREE enrichment. Quartz fluid inclusion data reveal that the Axi epithermal deposit hosts some H2O-CO2 fluid inclusions (Jia et al., 1999; Feng, 2005; Zhang et al., 2007; Wei, 2012), so the enrichment of HREE may be attributed to CO32－(Haas et al., 1995). During Stage IV, LREE, P2O5, TiO2, Na2O, and K2O display strong compositional drifts (Fig. 12), whereas Al2O3 remains constant. The losses of LREE (Fig. 12c) reflect the consumption of LREE–rich accessory minerals (e.g., sericite, apatite, and titanite), due to the leaching and/or dissolution of susceptible ions by suitable complexing agents in fluids. However, HREE clearly undergo a different process, since carbonate and barite crystallization deplete the complexing agents SO42－ and CO32－(Haas et al., 1995), and result in the decomposition of HREE complexes. 5.3. Gold deposition mechanism Gold deposition from hydrothermal fluids in epithermal deposits is induced by the decoupling of Au-bearing complexes via water/rock interaction, and fluid boiling or mixing with different fluids (Sillitoe and Hedenquist, 2003; Simmons et al., 2005; Li et al., 2015; Zhong et al., 2017a, 2017b). As discussed above, py1 was formed in a magmatic-hydrothermal system, and the concentrations of sulfur and chlorine are relatively high in the early pyrite-forming fluids. The high temperature and the reducing environment in Stage I is also inferred from mineral assemblages. These fluid conditions are conducive to gold transportation as Au–bearing chlorine- and/or 29

sulfur- complexes, such as AuCl2－and Au(HS)2－ (Gammons and Williams-Jones, 1995; Sverjensky et al., 1997; Williams-Jones et al., 2009). The precipitation of gold during pyrite-sericite-quartz alteration accompanies the base-metal minerals (predominantly sulfides) and sericitic assemblages (Fig. 6f). An important gold mineralization process in the disseminated ore of the Axi deposit involved hydrothermal fluid/rock interaction, especially sulfidation, during which Fe consumed the reduced sulfur, thereby resulting in the destabilization of Au–bearing agents causing gold to deposit (Phillips and Powell, 2010). It is noted that the ore and py1 generated during the early stage typically contain relatively low gold grade. There are two possible reasons: (1) the limitation of water/rock interaction, because the lowpermeability of wall rocks (Fig. 5f) constrains the fluid infiltration (e.g., Ronacher et al., 2004), and (2) the constrained breakdown of Au–bearing complexes because of the weak changes in physicochemical conditions, such as fO2 and fS2 (e.g., Li et al., 2015) or the limited iron supplied by country rocks (i.e., dacitic andesite) (Feng et al., 2005; Sha et al., 2005; and references therein). As discussed above, the geochemical compositions of pyrite indicate that the oreforming fluids in Stages II and III are composed mostly of low-salinity meteoric water under slightly acidic and reduced conditions. In this environment, gold is most likely transported as the bisulfide complex (Au(HS)2－) rather than as chloride complexes (Benning and Seward, 1996; Williams-Jones et al., 2009). The formation of this complex is expressed as: Au + H2S + HS－ = Au(HS)2－+ 0.5H2. The ore-forming 30

fluids contain some carbonate ions, which may play an important role in gold transportation and enrichment. Due to the presence of CO2 within the fluid, the pH of fluid would be buffered through CO2 + H2O ↔ H2CO3, H2CO3 ↔ H(CO)3－ + H+ and H(CO)3－ ↔ CO32－ + H+. This buffering effect enhances the stability of Au(HS)2 －

and elevates gold solubility in fluids (Phillips and Evans, 2004; Brugger et al.,

2008; Chi et al., 2009). The mineralized quartz veins and breccia systems at the Axi deposit, which constitute most of the Au reserves, are controlled by hydraulic fracturing and/or seismic movement along fault zones. Fracture zones typically surrounded by pyritesericite-quartz wall rocks, serve as efficient channels for gold transport into shallow epithermal domains, due to the limited loss of auriferous complexes in the relatively nonreactive pathways in deeper levels (Heinrich et al., 2004; Williams-Jones and Heinrich, 2005; İmer et al., 2016). The formation of structural fracture zones can result in the hydrothermal eruption and fluid boiling, accompanied by an intense liquid-vapor separation involving a very rapid and total loss of volatiles to the vapor phase, thereby resulting in major gold deposition events. The occurrence of boiling events is also supported by fluid inclusion characteristics (Feng, 2005; Zhai et al., 2009), the presence of adularia (Jia et al., 1999; Zhai et al., 2009) and platy calcite (Zhai et al., 2009). It follows that the silicic alteration and/or breccia zones at the Axi deposit are proximal to fluid conduits that allow gold precipitation throughout the deposit. This mechanism can explain both the formation of native gold and the high 31

gold grade in quartz veins (e.g., Canet et al., 2011), suggesting that boiling is a more important gold depositional mechanism than sulfidation in the Axi deposit. 5.4. Evolution model of hydrothermal processes and its implications Our studies of hydrothermal processes at the Axi LS epithermal deposits indicate a two-stage mineralization history: the early ore stage (Stage I) intimately associated with pyrite-sericite-quartz alteration in magmatic hydrothermal systems, while the later ore stage (Stages II and III) is related to silicification and large input of meteoric fluid. During the early ore stage (Stage I), magmatic fluid at depth migrated upward along foundational faults, shallow faults or cracks, (Fig. 18a), and extracted minor amounts of CO2 and organic gas from the basement strata. Meanwhile, the infiltration and diffusion of meteoric water occurred along fracture zones, leading to propylitic alteration in surrounding rock, which increased the content of K+, H+, and Si in the fluid. Gold was transported as Au–bearing chloride- and sulfur- complexes in hydrothermal fluids (An and Zhu, 2017; and references therein). Due to the destabilization of Au–bearing complexes during sulfidation, gold and py1 deposition occurred in pyrite-sericite-quartz alteration around flow paths (Fig. 18a). During the later ore stage (Stages II and III), ore-forming fluids were composed of minor magmatic water and much meteoric water. There was a deep, wide, and persistent hydrothermal fluid circulation (Fig. 18b), accompanied by acid weakened, temperature and sulfur fugacity decreased, and oxygen fugacity increased. The components of the ore-forming fluid were rich in Si, K+, Cl－, and CO32－. It is notable 32

that ore-forming metals, such as Au, As, Ag, Cu, Pb, Zn, and Sb, while depleted in Stage II, are remarkably enriched in Stage III. In addition, the primary pyrites are typically replaced by the late sulfides (Fig. 7n) and the rims of all generations of pyrite are enriched in Au and As (Fig. 15), with high Au/As ratios. Moreover, the Au vs. Au/As and Au/Ag vs. Au/As plots discriminate different types of pyrite (Fig. 14fg). These geochemical characteristics indicate that metamorphic devolatilization in Stage III mobilized some gold into the circulating ore-forming fluids from multiple sources (Fig. 18b) (e.g., Large et al., 2007; Zhang et al., 2014). Due to liquid-vapor separation resulting from hydraulic fracturing and/or seismic activity, the hydrothermal fluids concentrated in and around flow channels in which the major gold precipitated. During the post-mineralization stage (Stage IV), the circulating hydrothermal fluids were composed mostly of meteoric water (Fig. 18c), enriched in Ca, Mg, CO32－ and SO42－, and the pH changed from weakly acid to near neutral. A reductant infiltrated into the hydrothermal system because of the deep convective circulation. Possibly associated with the elevated setting and frequent tectonic movements in the Axi district (Wei, 2012), the previously generated altered rocks and orebodies continue to be broken and leached. Generally, LS epithermal deposits rarely co-occur in the same region with porphyry deposits. But several co-occurring porphyry and LS epithermal deposits have been observed, such as the Zhilingtou, Jinjiyan, Yueyang Au-Ag (± Cu) deposits 33

in South China (Zhong et al., 2017a, 2017b) and the Ladolam Au deposit in Lihir Island (Müller et al., 2002). The Tulasu basin hosts numerous porphyry and epithermal deposits, which likely formed during two major episodes as discussed above. Recently, Zhong et al. (2017b) proposed a two-stage mineralization model for the Yueyang deposit in the Zijinshan district, including early porphyry-type Cu mineralization related to magmatic water and late LS epithermal Ag-Au-Cu mineralization related to meteoric water. Only one sericite-quartz-pyrite ± chlorite alteration event was recognized in the Yueyang deposit, which is closely associated with porphyry Cu mineralization. Notably, the early gold mineralization in the Axi deposit and porphyry Au ± Cu mineralization in the Abiyindi deposit both have the close genetic association with the magmatic water and show disseminated-style mineralization related to the sericite-quartz-pyrite alteration. The potential porphyry mineralization within the Axi district must be further explored. 6. Conclusions (1) There are four different types of hydrothermal pyrite in the paragenetic sequence of mineralization at the Axi deposit. Py1 was mostly formed in sulfidation reactions during pyrite-sericite-quartz alteration. Py2 and py3 formation occurred in a mixed hydrothermal environment and were deposited together with quartz during silicification. (2) Two types of ore, i.e., disseminated type and silicified vein type have been

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distinguished in the Axi gold deposit. Gold was mainly deposited in solid solution in pyrite grains. Sulfidation was the mineralization mechanism of disseminated ore while boiling by hydraulic fracturing and/or seismic movement resulted in more gold deposition in silicified vein type ores. (3) The Axi LS epithermal deposit had a two-stage mineralization history during which gold mineralization occurred under acidic/near-neutral and reducing conditions. The ore-forming fluids in the early ore stage had a relatively high proportion of magmatic water, which contained a high concentration of Cu, Co, Ni, Mn, Cl－, HS－, K+, and H+. The fluids in the later ore stage and post-ore stage were mostly composed of meteoric water enriched in As, Si, Cl－, HS－, CO32－, and Ca, Mg, CO32－, SO42－, respectively. Acknowledgments We thank the Editor-in-Chief Franco Pirajno and Associate Editor Alexander Yakubchuk for their editorial assistance and supportive comments. We are very grateful to three anonymous reviewers for their critical and constructive comments to improve our paper. We thank Prof. Jianfeng Gao, Dr. Jianping Liu, and Dr. Renyu Zeng for their assistance in this study. We appreciate Prof. Jeffrey M. Dick and Syed Muzyan Shahzad for their linguistic assistance. Thanks are given to Jianmin Han, Hongxi Fan, and Xudong Han for their support during fieldwork. This study is supported by projects from National Key R&D Program of China (No.

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Table names Supplementary Table 1 Bulk rock and trace element data for the samples from the Axi deposit (major wt%, trace

ppm).

Supplementary Table 2 LA-ICP–MS analyses of different types of pyrite from the Axi deposit.

Table 1 The mass change of selected elements during hydrothermal alteration.

Figure names Fig. 1. (a) Schematic map showing the position of the Central Asian Orogenic Belt. (b) Geological and tectonic

map of the Chinese Western Tianshan Belt (modified from Zhang et al., 2012).

Fig. 2. (a) Geological map of the Tulasu basin showing the locations of porphyry and epithermal deposits

(modified from Zhai et al., 2009; ages are taken from An et al., 2013; Tang et al., 2013; Xue et al., 2013; Zhao et

al., 2014a, 2014b; Peng et al., 2016; Dong et al., 2018). (b) Geological map of the Axi district (ages are taken from

Zhai et al., 2006; An et al., 2013; Dong et al., 2018).

Fig. 3. 3D models of orebodies, F2 and pyrite-sericite-quartz zone.

Fig. 4. The distribution of orebody and alteration at 1370 m level (a) and section No. 24 (b).

Fig. 5. Images of different types of ore from the Axi gold deposit. (a) Silicified vein type ore and disseminated ore; 45

(b) Andesite breccias, which are cemented by quartz-chalcedony-sulfide veins, can be jointed, implying the

hydraulic fracturing; (c) and (d) Gray quartz or silicified breccias cemented by white quartz-carbonate veins; (e)

chalcedony-quartz-sulfide vein; (f) Pyrites are disseminated in pyrite-sericite-quartz altered andesite; (g) Native

gold occurring as an inclusion in pyrite crystal; (h) Electrum intergrown with pyrite. Abbreviations: Au-native

gold; Apy-arsenopyrite; Cab-carbonate; Cln-chalcedony; El-electrum; Py-pyrite; Qtz-quartz.

Fig. 6. Images showing characteristics of hydrothermal alteration-mineralization at the Axi epithermal deposit. (a)

and (b) Propylitic alteration gives the rock a green color. The altered hornblendes are infilled locally by calcite and

quartz in Fig. 6a. (c) Augites were locally replaced by chlorite. (d) Plagioclases were locally or totally replaced by

epidote and calcite. (e) Mineral assemblage associated with pyrite-sericite-quartz alteration comprises sericite,

quartz and disseminated pyrite. The plagioclase was totally replaced by sericite and quartz and reserved intergrown

apatites. (f) The hornblendes have been slightly replaced by illite-smectite and pyrite. (g) Sericite forms veinlet

filling in fractures of chlorite/epidote. (h) and (i) Silicified and pyrite-sericite-quartz altered rock showing a

gradational relationship and quartz veinlet crosscutting altered rock; (j) Chalcedony and quartz vein cutting and

replacing rock breccia. (k) Early formed chalcedony cut by quartz-chalcedony-pyrite vein. (l) Quartz with vug

infill texture. (m) Carbonate veinlet cutting the early formed silicified rocks. (n) Calcite containing the quartz-

chalcedony breccia (o) Calcite-quartz vein. (p) Calcite vein with comb texture. Abbreviations: Ap-apatite; Aug-

augite; Cal-calcite; Chl-chlorite; Cln-chalcedony; Ep-epidote; Hbl-hornblende; Ill-Illite; Qtz-quartz; Pl-

plagioclase; Py-pyrite; Ser-sericite, Sm-smectite.

Fig. 7. Images showing textures of different types of pyrite at the Axi epithermal deposit. (a), (b) and (c) Hand 46

specimen photographs showing disseminated pyrite 1 (py1) in altered andesite breccia and pyrite 2 (py2) occurring

as veins. (d) Assemblages of pyrite 3 (py3), marcasite and arsenopyrite in quartz breccia. (e) and (f) Disseminated

medium- to coarse-grained euhedral to subhedral py1. (g) Py1 with oscillatory zoning. (h) Py2 overgrown on py1.

(i) Clusters of anhedral to subhedral py2 in quartz vein. (j) Coarse-grained py2 with cataclastic texture in quartz

vein. (k) Free gold intergrown with py2 (from Wei, 2012). (l) and (m) Euhedral py3 overgrown with fractured py2

showing zonation. (n) Py3 locally or totally replaced by arsenopyrite. Abbreviations: Au-native gold; Apy-

arsenopyrite; Cab-carbonate; Cln-chalcedony; Mar-marcasite; Py-pyrite; Qtz-quartz.

Fig. 8. Paragenetic sequence of mineral, alteration, and mineralization at the Axi deposit.

Fig. 9. Binary plots of selected major elements for quartz vein ores. Pearson correlation coefficients are displayed

with each binary plot.

Fig. 10. Chondrite-normalized patterns of REE for andesites, altered rocks (a), and veins (b). Chondrite

normalization values are from Sun and McDonough (1989).

Fig. 11. Isocon diagrams of pyrite-sericite-quartz alteration (a), silicification (b) and quartz-carbonate alteration (c)

at the Axi deposit. Elements above the isocon line are enriched in the altered rock, and elements below the isocon

line are depleted. The protolith for pyrite-sericite-quartz alteration is andesite and the protolith for silicification and

quartz-carbonate alteration is pyrite-sericite-quartz altered andesite.

47

Fig. 12. Histograms showing gains and losses of selected elements during hydrothermal alteration.

Fig. 13. Comparative box plots of trace element concentration in three types of pyrites. Copper, Co, Ni, Pb, Mn, V

and Bi are enriched in py1; Tl and Ti are enriched in py2; Au, Ag, As, Se, Zn, and Sb are enriched in py3. Median

metal content in py1–3, respectively, is as follows: Cu (129.3, 93.25, 126.0), Co (60.45, 18.13, 10.37), Ni (102. 6,

36.51, 105.4), Pb (171.3, 20.74, 151.4), Mn (10.48, 2.87, 2.50), V (0.70, 0.41, 0.40), Bi (2.09, 0.72, 0.25), Tl (3.23,

13.27, 7.58), Ti (28.47, 38.11,31.1), Au (1.51, 1.21, 42.72), Ag (16.00, 12.38, 57.43), As (5670, 8695, 15380), Se

(1.68, 1.888, 3.75), Zn (3.74, 2.06, 4.82), and Sb (0.53, 0.32, 2.74).

Fig. 14. Binary plots of selected trace elements (in ppm) for three types of pyrites. The blue dashed curve in Fig.

14d represents the solubility limit of Au as a function of As, as determined by Reich et al. (2005).

Fig. 15. Spidergram showing the average concentrations of trace elements at core and rim of three pyrite types.

The far outliers of Co and Ni in the rim of py2 (G045-3, Co 24454 ppm, Ni 22945 ppm) were rejected when

calculating the average concentrations.

Fig. 16. Representative time-resolved depth profiles of pyrite LA-ICP-MS analyses. (a) and (b) are from Py1; (c)

and (d) are from Py2; (e) and (f) are from Py3.

Fig. 17. Plot of ∑REE versus P2O5, TiO2, Al2O3, and K2O for veins and altered rocks related to silicification and

quartz-carbonate alteration. 48

Fig. 18. Schematic model showing hydrothermal processes of the Axi epithermal deposit. (a) Pyrite-sericite-quartz

alteration developed along the inchoate F2, as fluid conduits, during the early ore stage. (b) F2 occurred right strike-

slip movements during the later ore stage, resulting in the dilatation zones, which are conducive to fluid flow and

confinement. The mineralized quartz vein and breccia system are located in the center of the pyrite-sericite-quartz

zone. (c) The orebodies were broken by the fault activity and overprinted by quartz-carbonate alteration during the

post-ore stage.

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53

54

55

56

57

58

59

60

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63

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Table 1 The mass change of selected elements during hydrothermal alteration.

Pyrite-sericite-quartz

Silicification

Quartz-carbonate

Element

SiO2

∆Ci

∆Ci/∆CiO

∆Ci

∆Ci/∆CiO

∆Ci

∆Ci/∆CiO

1781.57

29.84

4550

69.43

5704

87.04

67

TiO2

0.00

0.00

-6.25

-8.31

-21.79

-28.96

Al2O3

96.51

6.33

277.11

20.21

0.00

0.00

MgO

-26.36

-12.84

-50.71

-33.53

1067.17

705.57

MnO

2.42

35.79

-3.81

-49.12

49.05

632.91

TFe2O3

-41.57

-7.57

136.06

31.70

412.73

96.15

CaO

-130.94

-34.12

-77.23

-36.13

1992.55

932.19

Na2O

-57.08

-86.16

-1.84

-23.69

-4.41

-56.89

K2O

217.57

57.75

-45.13

-8.98

-111.58

-22.21

P2O5

2.65

14.94

0.00

0.00

-6.11

-35.44

Ag

215.42

3746

378.31

202.30

247.35

132.27

As

236110

4734

95905

47.05

-118761

-58.26

Ba

-3011.63

-17.21

-6828

-55.74

-6681.36

-54.54

Co

415.99

28.35

1182

74.24

568.13

35.68

Cr

3045.26

35.51

10677

108.68

7215.05

73.44

Cs

-720.61

-31.30

8.99

0.67

721.78

53.97

Cu

1563.31

42.80

-546.00

-12.38

-1492.03

-33.83

Li

584.88

26.23

2864.00

120.34

6273.67

263.60

Mo

588.08

1104.38

-29.68

-5.47

454.54

83.82

Ni

19.14

63.96

31.07

49.44

130.81

63.53

Pb

351.66

55.82

1959.57

236.09

963.10

116.04

68

Rb

8647.09

50.20

-2007.93

-9.18

32.04

0.15

Sb

6322.36

277.42

10406

143.09

31206

429.11

Sr

-6971.96

-42.83

-925.64

-11.76

6029.33

76.61

Th

91.34

19.27

155.81

32.60

-18.03

-3.77

Tl

393.80

194.95

-88.76

-17.62

-34.87

-6.92

U

117.24

83.74

-45.00

-20.69

5.25

2.41

V

6735.30

69.62

-5595.00

-40.32

1271.71

9.17

W

1027.59

178.71

-206.64

-15.25

-475.15

-35.07

Zn

1576.41

25.43

-118.57

-1.80

-4124.80

-62.73

La

297.09

19.23

542.07

34.80

-310.12

-19.91

Ce

396.59

11.45

1062.28

32.54

-781.39

-23.93

Pr

33.18

8.10

105.54

28.18

-115.00

-30.71

Eu

-6.12

-5.54

6.38

7.23

1.96

2.22

Nd

101.59

6.07

358.07

23.87

-486.51

-32.43

Sm

11.14

3.22

57.05

18.91

-56.73

-18.80

Gd

8.37

2.52

32.36

11.24

-24.05

-8.35

Tb

0.17

0.35

4.33

10.31

1.44

3.42

Dy

-1.64

-0.56

16.41

6.61

21.27

8.57

Ho

1.50

2.64

3.49

7.08

7.55

15.33

Er

4.67

3.01

14.83

10.98

22.04

16.32

69

Tm

1.15

5.13

3.16

15.82

4.50

22.51

Yb

10.53

7.43

20.59

15.99

48.33

37.54

Lu

0.13

0.60

4.17

22.55

6.00

32.44

Y

135.14

8.82

78.43

5.56

449.93

31.91

Note: The units for ∆Ci of major elements and trace elements are % and ppm, respectively. The ∆Ci/∆CiO unit is %.

70

Graphical abstract\

71

Highlights

(1) Four types of pyrite precipitate from hydrothermal fluid. (2) Ore-forming fluids are hybrid magmatic-meteoric waters. (3) Solid-solution gold is incorporated in three generations of pyrite. (4) The Axi epithermal gold deposit has a two-stage mineralization history.

72