how might pyrite play a role in creating an ore deposit?

Abstract

The Zaozigou gold deposit lies in the West Qinling orogenic belt, Gansu Province, China. It is one of the largest gold deposits, and the orebodies are hosted in fine-grained slates intercalated with limestone of the Middle-Triassic Gulangdi Formation and varied dykes. The gold orebodies are strictly controlled by the NE-, NW-, and SN-trending tensional and shearing faults with high dipping angle. The mineralogy and geochemistry of pyrite and arsenopyrite are measured by electron microprobe. Pyrite has up to 0.12 wt.% Au, and arsenopyrite contains up to 0.17 wt.% Au. The antithetic correlation between S and As indicates the substitution of As for S in pyrite, and arsenic occurs in anionic As¹⁻ state in the pyrite structure under the reduced conditions. Pyrite has relatively high Co (~364–2248 ppm) but relatively low Ni (~109–497 ppm) contents, with Co/Ni ratios ranging from ~1.63 to 10.50, indicating that the deposit originated from a volcanogenic fluid and remobilized by hydrothermal fluid. Au in arsenopyrite occurs as cationic Au in solid solution, whereas Au in pyrite is in solid solution and metal nanoparticles (Au⁰). The texture characteristics and trace element geochemistry among cores, transition zones, and rims of pyrites demonstrate that there are at least four pulses of fluid participating in the generation of pyrite in the deposit. The calculated formation temperatures of the Zaozigou deposit vary from 148°C to 304°C, with an average temperature of 213°C based on Au contents in pyrite. The Pb isotopic compositions of pyrite samples suggest that the metallogenic materials of the Zaozigou deposit were derived from the mantle and upper crust. All the characteristics above lead us to draw the conclusion that the Zaozigou gold deposit is classified as an epithermal deposit.

1 Introduction

The Zaozigou gold deposit in Hezuo City, Gansu Province, was discovered in 1996 during a regional geochemical reconnaissance survey. It has a reserve of 116 t Au with an average grade > 3.50 g t⁻¹ (Liang et al., 2013). The deposit occurs in the south of West Qinling orogenic belt. This belt belonged to a famous golden triangle area named "Shan-Gan-Chuan" in the northwestern part of China, the world's second largest Carlin and Carlin-like gold deposit province (Fig. 1; Chen, 2010; Yu & Guo, 2010; Chen & Santosh, 2015). The West Qinling orogenic belt is divided into northern, central, and southern metallogenic belts. Most of the deposits in the northern metallogenic belt (e.g. Liba, Baguamiao) belong to orogenic gold deposit, and those in the central and southern belts (e.g. Dashui, La'erma, Dongbeizhai) are chiefly ascribed to Carlin-like type (Goldfarb et al., 2014).

Simplified regional geological map of Western Qinling, China. (a) Simplified tectonic map of China (modified from Zhao *et al.,* 2001); (b) Simplified regional geological map of Western Qinling, showing the location of major Au deposits (modified from Dong *et al.,* 2011, Yang *et al.,* 2016). NCB, North China Block; YB, Yangtze Block; SGT, Songpan-Ganze Terrane.

Geological and geochemical studies have been conducted on the Zaozigou gold deposit in recent years. Several reports on the geology of the deposit have been published (e.g. Liu, 2011; Cao et al., 2012; Chen et al., 2018; Sui et al., 2018). It has been identified as a product of the Triassic (Liu et al., 2012; Sui et al., 2018). However, there is a debate concerning the genesis classification of the Zaozigou gold deposit regarding whether it is Carlin-like type (Cao et al., 2012), porphyry type (Liu et al., 2012), or epithermal type (Jiang & Wang, 2010; Liu, 2011; Chen et al., 2018). The Au is mainly contained in pyrite and arsenopyrite in the Zaozigou deposit. Previous research has shown that pyrite and arsenopyrite can provide important information about the elemental and isotopic evolution of the fluids from which they precipitate (Morey et al., 2008; Zhao et al., 2011; Su et al., 2012; Zhang et al., 2014; Morishita et al., 2018). Gold–arsenic relations in pyrite can be used to distinguish the different types of ore deposits and provide significant constraints on the approximate temperature of Au incorporation into arsenian pyrite (Deditius et al., 2014). Lead isotope of pyrite plays a pivotal role in identifying Pb sources and ore genesis (Kamona et al., 1999; Wang & Zhang, 2001; Liu et al., 2015; Jin et al., 2017).

In this paper, we report the result of mineralogy and geochemistry studies of pyrite and arsenopyrite from the Zaozigou gold deposit. The purpose of this research was to demonstrate the gold occurrence in pyrite and arsenopyrite and the evolution of hydrothermal fluids. Combined with petrological observations, the electron microprobe data enable us to discuss the ore genesis and genetic type of the Zaozigou deposit, thereby providing an insight into the gold mineralization in the West Qinling orogenic belt, China.

2 Geological background

The Qinling Orogenic Belt in central China is >1500 km in length and ~200–250 km in width and separates the North China Block (NCB) and South China Block (SCB) (Fig. 1) (Zhang et al., 1995; Meng & Zhang, 1999, 2000; Dong et al., 2014). It can be divided into the North Qinling Belt, the South Qinling Belt, and the northern SCB from north to south (Meng & Zhang, 1999, 2000). The three zones are separated by the Shangdan and Mianlue suture zones (Dong et al., 2011, 2013). The South Qinling Belt can be further separated into western (West Qinling) and eastern (East Qinling) segments, with the boundary of the Baoji-Chengdu railway between them (Zhang et al., 1995, 1996, 2007; Zheng et al., 2010; Ping et al., 2013). The Shangdan suture is considered to preserve the Paleozoic collision between the North Qinling Belt and South Qinling Belt, whereas the Mianlue suture preserves the Late Triassic collision between the South Qinling Belt and northern SCB and the final amalgamation of the NCB and SCB (Meng & Zhang, 1999, 2000). The closure of the Mianlue Ocean and collision of the SCB is ca. 228 Ma (Dong et al., 2011).

The West Qinling orogenic belt experienced an extended and complicated tectonic evolution from Grenvillian orogenesis, Late Paleozoic slab subduction, Triassic continent collision, and the Cretaceous reactivation, each of which is related to the formation of widespread granitoids. Gold resources in the orogen consist of >500 t Au with more than 10 of >20 t gold deposits summarized by Mao et al. (2002). They were ascribed to orogenic gold deposits and Carlin-like gold deposits (Goldfarb et al., 2014).

The gold mineralization in the West Qinling orogenic belt probably occurred mainly between Late Triassic and Middle Jurassic (ca. 220–170 Ma). The gold deposits in the West Qinling orogenic belt can be subdivided into the northern, central, and southern belts. The northern gold belt between the Shangdan suture and Fengzhen fault is the most productive gold belt of West Qinling. It consists chiefly of Devonian sedimentary rocks, which unconformably overlie the Lower Paleozoic rocks. It contains most of the gold deposits, such as the Baguamiao, Liba, and Zhaishang deposits (Ma et al., 2004; Liu et al., 2015). The central belt lies between the Fengzhen fault and the Mianlue suture. It is predominantly composed of Cambrian to Devonian carbonaceous slate, cherts, and carbonate rocks and siliceous mudstone. There are some dykes but few granitoids in this belt. This gold belt hosts many gold deposits such as the Dashui, La'erma, and Zaozigou deposits (Fig. 1). The southern belt lies between the Mianlue suture and the Aba-Heishui fault. It is mainly composed of Triassic slate and feldspathic greywacke, which host many deposits including the Yangshan, Jianchaling, and Dongbeizhai gold deposits (Deng & Wang, 2016). Many of these deposits in the northern metallogenic belt (e.g. Liba, Baguamiao) belong to orogenic gold deposit, and those in the central and southern belt (e.g. Dashui, La'erma, Dongbeizhai) are chiefly ascribed to Carlin-like type (Goldfarb et al., 2014).

3 Ore deposit geology

The Zaozigou gold deposit in Gansu province is located in the West Qinling orogenic belt (Dong et al., 2011). The orebodies are hosted in fine-grained slates intercalated with limestone of the Middle-Triassic Gulangdi Formation and varied dykes (Fig. 2). The gold orebodies are strictly controlled by the NE-, NW-, and SN-trending tensional and shearing faults with high dipping angle. The mineralized orebodies in NE-trending belt are >1000 m long and >300 m wide, with the dip angle changing from 50° to 70°. Those trending NW are >300 m long and >50 m wide, with the dip angle ranging from 300° to 320°, and those trending SN are >1000 m in length and >100 m in width, and the dip angle varies from 350° to 10°(Cao et al., 2012). The deposit consists of 104 orebodies, but about 80% of gold reserves are contained in 10 major orebodies. The largest orebody contains 23.8 t gold at an average grade of 4.6 g t⁻¹. It is 1160 m long and 1–17 m thick, with a minimum vertical extent of 1030 m (Chen et al., 2015; Sui et al., 2018).

Geological map of the Zaozigou gold deposit, West Qinling, China (modified after Cao *et al.,* 2012).

The granodioritic dykes that intrude the middle Triassic Gulangdi Formation are widespread in the area and are comprised of diorite porphyrite, granodiorite porphyry, and plagioclase granite porphyry from the margin to the center (Fig. 2). They are composed of phenocrysts of K-feldspar, plagioclase, quartz, and minor biotite and small amounts of pyrite, zircon, and apatite. The dykes display fine- to medium-grained porphyritic and hypidiomorphic textures and are intensively altered, with sericitization, silicification, and pyritization, and most of the dykes are spatially associated with gold mineralization. In dyke-hosted ores, pyrite-arsenopyrite aggregates generally replace biotite and hornblende. Such gold mineralization is commonly accompanied by gold-bearing sulfide-quartz-ankerite veinlets, typically 0.5–5 mm in thickness (Sui et al., 2018).

The main stage of gold mineralization at the Zaozigou deposit consists of disseminated fine-grained pyrite and arsenopyrite in hydrothermally altered clastic rocks of the Gulangdi Formation and, less importantly, in porphyritic quartz-diorite to granodiorite dykes (Fig. 3). Both sediment- and dyke-hosted ores have gold grades ranging from 0.1 to 25 g t⁻¹ but up to 224 g t⁻¹ (Chen et al., 2015). Wall rock alteration types observed at the Zaozigou mining area include silicification, sericitization, carbonatization, pyritization, arsenopyritization, and limonitization. Sericitization is always associated with disseminated pyrite and arsenopyrite in high-grade gold ores. Sericite, quartz, and ankerite typically replace feldspars both in sedimentary rocks and igneous dykes. Chloritization is best developed in sedimentary rocks peripheral to sericitized zones. The stronger the mineralization alteration is, the more enriched the gold mineralization is. Wall rock alteration has a certain alteration zoning, which is characterized by strong silicification and pyritization near the orebody.

Representative field and microscopic photographs of the Zaozigou gold deposit. (a) A sharp contact between quartz-stibnite ore and slate; (b) the orebody occurred in the contact zone; (c) the cataclastic quartz vein cemented by stibnite; (d) the cataclastic quartz vein cemented by hematite; (e) transmitted light of quartz diorite porphyrite; (f) transmitted light of granite porphyry; (g–h) backscattered electron image; (g) native gold in arsenopyrite; (h) pyrite grains showing well-developed fractures and erratic grain boundaries; (i) quartz veinlet fills with pyrite and arsenopyrite at the late stage. Asp, arsenopyrite; Ank, ankerite; Bt, biotite; Ilt, illite; Pl, plagioclase; Py, pyrite; Qz, quartz; Ser, sericite.

The ore minerals are pyrite, arsenopyrite, galena, tetrahedrite, and stibnite. The non-metallic minerals are mainly quartz, muscovite, sericite, chlorite, illite, and ankerite. The SHRIMP U–Pb ages of zircons from the two diorite porphyrites related to mineralization are 215.5 ± 2.1 and 216.2 ± 2.4 Ma (Liu et al., 2012). The zircon LA-ICP-MS U–Pb dating from granodiorite and diorite porphyry is from 233 to 249 Ma (Sui et al., 2018). Ore-related sericites have ⁴⁰Ar/³⁹Ar plateau ages of 245–242 Ma (Sui et al., 2018). The results show that they are the products of multiple stages of emplacement, and geochronological results constrain magmatism and gold mineralization at the Triassic (Sui et al., 2018).

Based on the mineral associations, ore textures, and tectonic structures, the paragenetic sequence of the Zaozigou gold deposit can be divided into sedimentary, magmatic–hydrothermal mineralization, and supergene epochs (Fig. 4). The metallogenic processes of the hydrothermal epoch are subdivided into four stages: pyrite-quartz stage (I), pyrite-arsenopyrite-quartz stage (II), stibnite-quartz-calcite stage (III), and quartz-calcite stage (IV). Stages I, II, and III represent the main metallogenic stages, and the richest ores in this deposit occur within the superposition of these three stages. Irregular native gold grains mainly occur as inclusions in the pyrite, arsenopyrite, and stibnite (Fig. 3g), and also occur in the silicate minerals.

Paragenetic sequence for major minerals of the Zaozigou gold deposit.

Pyrite is the main gold-bearing mineral in this deposit (Figs 3,4) and occurs throughout the various stages of mineralization. It is dominant in the main ore-forming stages I and II and is sparse in the early stage III.

4 Samples and analytical methods

The samples from stages I and II used in this study were collected from tunnels and outcrops in the Zaozigou gold deposit. Pyrite and arsenopyrite were from diorite porphyrite, granodiorite porphyry, granite porphyry, and marble. Pyrites, up to 500 μm in diameter, are generally euhedral but have porous cores or rims (Figs 3, 5). Pyrites from stage I have well-developed fractures and erratic grain boundaries, and pyrites from stage II fill in the quartz veinlet with arsenopyrite. Arsenopyrites are 30–120 μm with a maximum size of 200 μm, occurring in the inner part or along the edge of pyrite. The pyrites used for lead isotopes are from stages I and II.

Spot Electron microprobe analyses (EMPA) of selected pyrites from the Zaozigou gold deposit. Spot position of the EMPA analyses are shown in yellow circles. Ap, apatite; Asp, arsenopyrite; Ank, ankerite; Gn, galena; Ms, muscovite; Py, pyrite; Qz, quartz; Ttr, tetrahedrite.

4.1 Mineral compositions analysis

Backscattered electron imaging and mineral compositional analyses of pyrite and arsenopyrite were determined by wavelength dispersive X-ray analysis using a JEOL JXA-8230 electron microprobe in Xi'an Institute of Geology and Mineral Resources, China. The analytical conditions for major elements were 20 kV, 10-nA beam current, 1-μm beam size, and peak counting time of 20 s. The detection limit for Au, Ag, Cu, Ni, Zn, and Sb under such conditions is ~100 ppm; for Pb, Fe, Co, and W is ~200 ppm; and S is ~50 ppm. Sulfide mineral standards GB/T15246-2002 were used for calibration. Analytical reproducibility was within 1% relative.

4.2 Lead isotope analysis

Pb isotope analyses of pyrite were performed at the Beijing Research Institute of Uranium Geology on an ISOPROBE-T thermal ionization mass spectrometer. The mass fractionation corrections were based on the Tl isotope standard (He et al., 2005). The measurement values for NBS 981 standard are as follows: ²⁰⁸Pb/²⁰⁶Pb = 2.1672 ± 0.0003, ²⁰⁷Pb/²⁰⁶Pb = 0.91476 ± 0.00023, ²⁰⁶Pb/²⁰⁴Pb = 16.9445 ± 0.0096, ²⁰⁸Pb/²⁰⁴Pb = 36.7211 ± 0.0217, and ²⁰⁷Pb/²⁰⁴Pb = 15.4980 ± 0.0077 (±2σ).

5 Results

5.1 Chemical compositions of pyrite and arsenopyrite

The contents of Au, As, and other trace elements in pyrite and arsenopyrite from the Zaozigou gold deposit are summarized in Table 1.

Table 1. EMPA analyses of selected pyrite and arsenopyrite from the Zaozigou gold deposit

Sample no.	Point	Rock type	Mineral	Spot position	Types of pyrites	As	Fe	Cu	W	Zn	S	Pb	Ag	Sb	Au	Co	Ni	Total
ZH-9-1	1	Granodiorite porphyry	Pyrite			1.03	45.74	—	—	—	51.82	0.14	—	—	0.03	0.04	—	98.80
ZH-9-1	2	Granodiorite porphyry	Pyrite			0.72	45.75	0.04	—	—	52.20	0.13	—	0.01	—	0.05	—	98.89
ZH-9-1	3	Granodiorite porphyry	Pyrite			0.15	45.84	0.06	0.13	0.048	51.96	0.18	0.01	—	—	0.04	—	98.43
ZH-9-2	4	Granodiorite porphyry	Pyrite			0.44	45.16	—	0.07	—	52.59	0.19	—	—	—	0.08	—	98.53
ZH-9-2	5	Granodiorite porphyry	Pyrite			0.45	46.64	0.05	—	0.042	52.10	0.07	0.017	0.02	0.01	0.11	—	99.51
ZH-9-3	6	Granodiorite porphyry	Pyrite			0.92	45.59	0.06	—	0.018	52.21	0.19	—	—	—	0.10	—	99.09
ZH-9-3	7	Granodiorite porphyry	Pyrite			1.84	46.18	—	—	—	50.95	0.16	—	—	—	0.08	0.05	99.26
ZH-9-4	8	Granodiorite porphyry	Pyrite			0.53	46.13	—	—	—	52.15	0.17	—	—	0.11	0.09	—	99.18
ZH-9-4	9	Granodiorite porphyry	Pyrite			1.03	45.95	—	—	0.041	52.14	0.16	—	—	—	0.10	—	99.42
ZH-9-4	10	Granodiorite porphyry	Pyrite			0.20	46.22	0.03	—	0.052	52.70	0.07	—	0.01	—	0.07	0.04	99.40
ZH-9-5	11	Granodiorite porphyry	Pyrite	Core	Type 1	0.20	46.18	0.02	0.03	0.015	53.13	0.08	0.01	0.02	—	0.11	—	99.78
ZH-9-5	12	Granodiorite porphyry	Pyrite	Transition zone	Type 1	0.98	45.87	0.04	—	0.008	52.79	0.08	—	0.03	0.03	0.04	—	99.87
ZH-10-3	13	Granodiorite porphyry	Pyrite			0.05	46.11	—	0.04	0.040	52.34	0.11	—	—	0.04	0.08	—	98.81
ZH-10-3	14	Granodiorite porphyry	Pyrite			1.15	45.96	0.02	0.07	—	52.49	0.09	0.02	—	0.03	0.06	—	99.88
ZH-10-4	15	Granodiorite porphyry	Pyrite	Core	Type 2	1.11	46.15	0.02	—	—	52.04	0.17	0.03	—	0.04	0.09	0.01	99.65
ZH-10-4	16	Granodiorite porphyry	Pyrite	Transition zone	Type 2	0.30	46.12	—	0.07	0.007	52.80	0.17	—	0.02	—	0.09	0.01	99.59
ZH-10-4	17	Granodiorite porphyry	Pyrite	Rim	Type 2	0.74	46.24	—	—	—	52.74	0.16	—	0.01	0.09	0.09	—	100.06
ZH-10-2	18	Granodiorite porphyry	Pyrite	Core	Type 1	0.44	45.98	0.01	0.10	0.048	52.38	0.16	0.03	0.01	0.04	0.13	—	99.32
ZH-10-2	19	Granodiorite porphyry	Pyrite	Transition zone	Type 1	1.79	45.27	0.03	0.06	0.022	51.89	0.16	—	0.01	—	0.07	—	99.29
ZH-10-2	20	Granodiorite porphyry	Pyrite	Rim	Type 1	0.17	46.38	—	0.11	0.015	53.44	0.21	—	—	0.01	0.08	0.01	100.42
ZH-10-1	21	Granodiorite porphyry	Pyrite			0.23	46.18	—	—	—	52.34	0.14	—	—	0.04	0.07	0.01	99.00
ZH-10-1	22	Granodiorite porphyry	Pyrite			0.18	45.76	0.03	—	0.008	52.55	0.08	0.02	0.04	0.03	0.08	—	98.77
ZH-10-1	23	Granodiorite porphyry	Pyrite			0.11	46.07	—	0.12	0.012	53.36	—	0.01	0.01	—	0.07	—	99.77
ZH-11	24	Granodiorite porphyry	Pyrite			1.48	46.05	0.04	0.12	—	52.79	0.09	—	0.01	0.03	0.07	—	100.66
ZH-11	25	Granodiorite porphyry	Pyrite			0.61	45.14	0.03	0.08	—	51.57	0.19	0.01	0.03	—	0.09	1.35	99.09
ZH-11	26	Granodiorite porphyry	Pyrite			1.35	45.94	—	0.05	0.012	52.77	0.13	—	—	—	0.07	—	100.32
ZH-12-2	27	Diorite porphyrite	Pyrite			1.78	45.98	0.01	0.08	0.058	52.48	0.13	—	0.03	—	0.14	0.03	100.72
ZH-12-2	28	Diorite porphyrite	Pyrite			1.92	45.42	—	0.06	0.036	51.64	0.26	0.01	—	—	0.06	—	99.41
ZH-12-2	29	Diorite porphyrite	Pyrite			2.23	46.09	0.04	—	0.01	51.16	0.25	—	0.07	0.02	0.06	—	99.93
ZH-12-1	30	Diorite porphyrite	Pyrite	Transition zone	Type 2	0.17	46.10	0.03	—	—	52.53	0.16	0.02	0.01	—	0.07	—	99.11
ZH-12-1	31	Diorite porphyrite	Pyrite	Rim	Type 2	0.16	46.30	—	—	0.04	52.53	0.16	—	0.03	—	0.11	—	99.33
ZH-32-1	33	Granite porphyry	Pyrite			0.17	46.10	0.03	—	—	52.53	0.16	0.02	0.01	—	0.07	—	99.11
ZH-32-1	34	Granite porphyry	Pyrite			0.16	46.30	—	—	0.04	52.53	0.16	—	0.03	—	0.11	—	99.33
ZH-32-2	35	Granite porphyry	Pyrite	Rim	Type 1	2.44	45.32	0.01	—	—	50.83	0.05	—	0.02	0.04	0.08	0.03	98.82
ZH-32-2	36	Granite porphyry	Pyrite	Core	Type 1	2.26	45.32	0.03	—	—	51.00	0.11	0.05	0.01	—	0.06	—	98.86
ZH-32-4	39	Granite porphyry	Pyrite	Core	Type 1	1.44	45.67	0.03	0.10	0.02	51.72	0.05	0.01	0.02	0.07	0.08	—	99.21
ZH-32-4	40	Granite porphyry	Pyrite	Transition zone	Type 1	4.47	44.78	0.02	0.03	0.01	49.31	0.02	—	—	—	0.08	—	98.73
ZH-32-4	41	Granite porphyry	Pyrite	Rim	Type 1	1.79	45.60	0.03	—	—	51.41	0.11	0.01	—	—	0.08	0.01	99.04
ZH-32-5	42	Granite porphyry	Pyrite			4.09	44.96	—	—	—	49.75	0.10	0.02	0.01	—	0.08	—	99.01
ZH-32-5	43	Granite porphyry	Pyrite			0.34	46.33	—	0.13	—	52.93	0.13	0.05	—	0.03	0.10	—	100.03
ZH-32-6	44	Granite porphyry	Pyrite			3.02	44.78	0.03	0.02	0.09	50.41	—	0.01	0.03	—	0.10	—	98.48
ZH-32-6	45	Granite porphyry	Pyrite			2.63	45.14	0.05	—	—	50.39	0.10	0.04	—	0.07	0.11	0.05	98.58
ZH-32-7	46	Granite porphyry	Pyrite			2.37	45.01	—	—	—	50.63	0.13	0.03	—	—	0.06	—	98.23
ZH-32-7	47	Granite porphyry	Pyrite			3.04	44.65	0.01	—	—	50.35	0.16	0.01	—	0.02	0.08	—	98.31
ZH-32-8	48	Granite porphyry	Pyrite			0.81	46.04	—	0.09	—	52.25	0.14	—	0.01	—	0.07	—	99.41
ZH-51-4	67	Granite porphyry	Pyrite			0.15	46.64	—	—	0.02	53.13	0.07	—	0.05	0.02	0.07	0.02	100.17
ZH-51-4	68	Granite porphyry	Pyrite			0.04	45.76	0.04	0.18	0.04	52.92	0.14	—	—	—	0.07	—	99.19
ZH-51-5	69	Granite porphyry	Pyrite	Rim	Type 1	0.10	45.99	—	0.13	0.05	52.82	0.14	—	—	—	0.06	0.04	99.33
ZH-51-5	70	Granite porphyry	Pyrite	Core	Type 1	1.53	45.76	0.04	—	—	51.79	0.21	0.02	0.03	0.07	0.07	—	99.51
ZH-51-6	71	Granite porphyry	Pyrite			0.02	46.39	—	0.07	—	53.28	0.20	—	—	——	0.15	0.01	100.11
ZH-51-6	72	Granite porphyry	Pyrite			0.02	46.81	—	—	0.01	53.70	0.12	—	—	—	0.08	0.03	100.76
ZH-14-1	73	Granodiorite porphyry	Pyrite			0.88	45.98	—	0.03	0.01	52.37	0.16	—	—	—	0.05	0.02	99.48
ZH-14-2	74	Granodiorite porphyry	Pyrite			3.50	45.93	0.08	—	—	50.77	0.14	—	0.11	0.06	0.07	0.01	100.67
ZH-14-2	75	Granodiorite porphyry	Pyrite			0.94	46.41	0.02	—	—	51.85	0.06	—	—	—	0.06	0.01	99.36
ZH-14-2	76	Granodiorite porphyry	Pyrite			0.89	46.43	—	—	—	51.38	0.14	—	0.04	0.05	0.10	0.02	99.03
ZH-14-3	77	Granodiorite porphyry	Pyrite			0.02	46.22	0.06	0.04	—	52.87	0.06	0.02	—	0.08	0.22	0.03	99.63
ZH-14-3	78	Granodiorite porphyry	Pyrite			0.11	46.35	0.02	—	—	53.35	0.06	—	—	—	0.11	—	100.01
ZH-14-3	79	Granodiorite porphyry	Pyrite			0.97	46.13	0.02	—	—	53.01	0.14	—	0.02	—	0.08	—	100.36
ZH-14-4	80	Granodiorite porphyry	Pyrite			3.29	44.88	—	0.03	—	49.76	0.10	—	0.03	—	0.11	—	98.20
ZH-14-4	81	Granodiorite porphyry	Pyrite			0.77	45.73	0.04	—	0.01	51.71	0.16	—	0.07	—	0.09	—	98.57
ZH-14-5	82	Granodiorite porphyry	Pyrite	Rim	Type 1	0.47	46.29	0.05	—	0.03	52.93	—	0.01	0.04	—	0.08	—	99.89
ZH-14-5	83	Granodiorite porphyry	Pyrite	Core	Type 1	0.78	46.28	0.03	0.08	0.03	52.68	0.12	—	—	0.04	0.07	—	100.11
ZH-14-5	84	Granodiorite porphyry	Pyrite			1.56	45.89	0.05	—	—	51.70	0.16	—	0.01	—	0.10	—	99.47
ZH-12-3	32	Diorite porphyrite	Arsenopyrite			44.65	34.42	—	—	—	20.74	0.03	—	0.01	0.06	0.09	—	100.00
ZH-32-3	37	Granite porphyry	Arsenopyrite			43.12	35.08	0.02	—	—	20.60	0.15	—	0.02	0.000	0.04	0.02	99.05
ZH-32-3	38	Granite porphyry	Arsenopyrite			43.86	34.69	—	0.09	0.04	20.12	0.13	0.04	0.04	0.07	0.04	0.01	99.11
ZH-32-9	49	Granite porphyry	Arsenopyrite			42.43	35.70	—	0.08	—	22.17	0.04	0.02	0.01	—	0.07	—	100.51
ZH-32-9	50	Granite porphyry	Arsenopyrite			38.92	35.84	0.04	0.10	—	23.67	0.02	—	0.07	—	0.05	—	98.72
ZH-50-1	51	Granite porphyry	Arsenopyrite			43.42	34.80	—	—	0.05	22.02	0.05	0.02	—	0.03	0.01	—	100.39
ZH-50-1	52	Granite porphyry	Arsenopyrite			39.16	36.05	—	0.05	0.04	23.39	0.09	—	—	0.14	0.05	0.01	98.98
ZH-50-2	53	Granite porphyry	Arsenopyrite			39.71	35.62	—	—	0.02	23.06	0.03	—	0.04	0.05	0.04	—	98.56
ZH-50-2	54	Granite porphyry	Arsenopyrite			40.80	35.67	0.04	0.07	0.05	22.64	0.08	—	0.02	0.01	0.03	—	99.41
ZH-50-3	55	Granite porphyry	Arsenopyrite			41.31	35.35	—	—	—	22.25	0.07	—	—	—	0.05	—	99.04
ZH-50-4	56	Granite porphyry	Arsenopyrite			41.20	35.08	—	—	0.02	22.40	0.15	—	0.04	—	0.04	—	98.93
ZH-50-4	57	Granite porphyry	Arsenopyrite			43.52	35.29	0.01	—	0.02	21.66	0.11	—	0.03	0.18	0.10	—	100.92
ZH-58	58	Marble	Arsenopyrite			43.98	34.57	0.04	—	—	21.50	0.11	0.02	—	0.06	0.06	—	100.32
ZH-58	59	Marble	Arsenopyrite			44.40	35.07	—	—	—	20.83	0.06	—	—	0.04	0.06	—	100.45
ZH-51-1	60	Granite porphyry	Arsenopyrite			42.22	35.29	—	0.07	0.05	21.33	—	—	0.05	—	0.06	0.01	99.09
ZH-51-1	61	Granite porphyry	Arsenopyrite			42.84	34.73	0.04	0.05	0.08	21.14	—	0.01	0.01	—	0.08	—	98.99
ZH-51-2	62	Granite porphyry	Arsenopyrite			41.17	35.33	—	—	0.01	21.76	0.11	—	0.01	—	0.07	—	98.45
ZH-51-2	63	Granite porphyry	Arsenopyrite			42.98	34.87	0.04	—	0.03	20.71	—	0.01	0.02	0.07	0.06	—	98.79
ZH-51-3	64	Granite porphyry	Arsenopyrite			43.84	34.09	0.03	—	—	22.61	0.06	—	0.09	0.02	0.06	0.05	100.85
ZH-51-3	65	Granite porphyry	Arsenopyrite			43.49	34.64	0.04	—	—	22.28	0.11	—	0.04	0.04	0.06	—	100.69
ZH-51-3	66	Granite porphyry	Arsenopyrite			42.29	34.26	—	0.05	—	21.52	0.09	0.01	—	0.04	0.07	—	98.33

—, Values below the limit of detection, EMPA, electron microprobe analysis.

Arsenopyrite has As concentrations between 27.3 and 32.0 at % (33 at.% is the stoichiometric value), with corresponding variations in S contents from 34.1 to 38.8 at % S. Iron concentrations, however, do not vary greatly (32.1–34.0 at %). The Fe/S atom ratios in arsenopyrite vary from 0.86 to 0.96, and Fe/(As+S) atom ratios = 0.48–0.52, indicating that arsenopyrite in the Zaozigou gold deposit has relative low As contents. The antithetic correlation between As and S, As, and Fe and positive correlation of Au and Co in arsenopyrite has been observed in the Zaozigou deposit. There are no obvious correlations between Au and S, Fe, Sb, and Pb plus Cu (Fig. 6).

Plots of As and Au versus S (a, b); As and Au versus Fe (c, d); and Sb (e), Pb (f), Co (g) and Cu (h) versus Au (in wt.%) of pyrite and arsenopyrite from the Zaozigou gold deposit.

Sulfur contents in pyrite vary from 49.95 to 53.30 wt.%, and Fe contents vary from 45.36 to 46.89 wt.%, and Fe/S mole ratios = 0.49–0.52, indicating that pyrite composition is similar to the theoretical ratio. Note that high concentrations of Au in individual concentrates are accompanied by a significant increase of the concentrations of Ag, Pb, and As, and to a lesser extent Fe and Sb, which provides evidence for the important role of mineral associations with As-rich pyrite, fahlore (and other sulfosalts), and arsenopyrite as concentrators of gold and by the presence of Au–Ag minerals in them.

The pyrite has lower Au (130–1149 ppm Au) content than arsenopyrite (121–1734 ppm Au). Besides Au, pyrite and arsenopyrite also contain Co, varying from 0.04 to 0.22 wt.% and 0.03 to 0.10 wt.%, respectively (Table 1). There is a negative correlation of As with S in pyrite and arsenopyrite (Fig. 6a). The relationships indicate the replacement of As for S in the pyrite and arsenopyrite structure (Fleet & Mumin, 1997; Reich et al., 2005). There is an antithetic correlation between Fe and As, as well as Au and S in arsenopyrite. On the contrary, no obvious correlation between these element pairs showed in pyrite (Fig. 6b, c), indicating that As and Au are incorporated into the arsenopyrite structure, resulting in the decrease of Fe and S contents in arsenopyrite. In contrast to pyrite, arsenopyrite have relatively high contents of Sb (to 0.09 wt.%) and Zn (to 0.08 wt.%) with low contents of Cu, W, Pb, Co, and Ni (Fig. 6).

Au concentrations in pyrite and arsenopyrite have no correlation with the As contents. It is common to see the negative relationship between Au and Fe in arsenopyrite from various deposits (e.g. Pals et al., 2003), but there are no clear correlations between them in the Zaozigou deposit.

Eight grains were analyzed from the core, transition zone, and rim. The Au contents for the core, transition zone, and rim are 380–706 ppm, 250 ppm, and 425–889 ppm, respectively. The As contents for the core, transition zone, and rim are 0.20–2.29 wt.%, 0.17–4.53 wt.%, and 0.10–2.47 wt.%, respectively. Combined with the analyses on both the core and the transition zone, and between the transition zone and the rim, the trace element distribution pattern supports evidence that there are two kinds of pyrite formed in different environments. Some pyrites contain high As with low Fe and Pb contents in the transition zone compared to the corresponding core and the rim, whereas others contain elevated Co, Fe, and Pb contents in the transition zone (Fig. 7).

Au, As, Co, Sb, Pb, and Fe concentration variations of cores, transition zones, and rims from two types of pyrites. TZ1, transition zone of type 1 pyrite; TZ2, transition zone of type 2 pyrite; Rim1, rim of type 1 pyrite; Rim2, rim of type 2 pyrite.

5.2 Lead isotopic compositions

The Pb isotopic compositions of pyrites are listed in Table 2. Six pyrite samples from the granodiorite porphyry in the Zaozigou gold deposit exhibit limited Pb isotopic compositional variety with ²⁰⁶Pb/²⁰⁴Pb values of 18.190–18.727 (average 18.531), ²⁰⁷Pb/²⁰⁴Pb values of 15.555–15.682 (average 15.634), and ²⁰⁸Pb/²⁰⁴Pb values of 38.250–39.103 (average 38.645). These values are similar to the data obtained from stibnite in the gold-bearing quartz vein from the Zaozigou gold deposit (Chen et al., 2018) and the range of Triassic granitic intrusions in the West Qinling orogenic belt (Zhang et al., 2007).

Table 2. Lead isotopic composition of pyrites from the Zaozigou gold deposit

Sample no.	Rock type	²⁰⁶Pb/²⁰⁴Pb	2SE	²⁰⁷Pb/²⁰⁴Pb	2SE	²⁰⁸Pb/²⁰⁴Pb	2SE
ZD-2	Granodiorite porphyry	18.590	0.002	15.635	0.002	38.569	0.004
ZH-9	Granodiorite porphyry	18.644	0.002	15.639	0.002	38.536	0.004
ZH-11	Granodiorite porphyry	18.635	0.003	15.656	0.003	38.573	0.007
ZH-28	Granodiorite porphyry	18.190	0.003	15.555	0.002	38.250	0.005
ZH-29	Granodiorite porphyry	18.398	0.002	15.682	0.002	39.103	0.009
ZH-57	Granodiorite porphyry	18.727	0.002	15.635	0.002	38.825	0.005

6 Discussion

6.1 Trace element occurrence in pyrite and arsenopyrite

Pyrites and arsenopyrite display some similarities and diversities in chemical compositions. Chalcophile and siderophile elements, including Co, Ni, and As, are primarily distributed in pyrite and arsenopyrite. The negative correlations of Ni + Co versus Fe contents and As versus S contents in arsenopyrite (Fig. 5; Table 1) indicate that Ni and Co go into the crystal lattice through isomorphous substitution of Fe in arsenopyrite, and As enters the crystal lattice by means of replacing S.

The ore-forming elements are dominated by Au, Ag, Cu, and Pb, with minor Zn in the Zaozigou gold deposit. Cu and Pb mainly occur in pyrite as visible or invisible chalcopyrite, tetrahedrite, or galena inclusions (Fig. 5). The relationships between Ag, Pb, and Sb suggest that most Ag presents as a solid solution or inclusion of Sb compounds in galena.

6.2 Pyrite genesis

The element variation indicates that there are two types of pyrites. The cores of type 1 pyrites hosted in diorite porphyrite and granite porphyry have the highest contents of Au and Co (Fig. 7), suggesting that the initial igneous rocks and/or magmatic fluids are rich in these metallogenic elements, whereas the transition zones and rims of the type 1 pyrite are depleted in these metals. The transition zones of the type 1pyrites have higher contents of As and Sb with lower Fe, Zn, W, Co, and Pb contents than the cores of type 1 pyrite, which is different from the transition zones of the type 2 pyrite. However, the rims of the type 1pyrite have high contents of Pb, with the chemical compositions and associations resembling the transition zone of the type 2 pyrite; this suggests that the rims of the type 1 pyrite originated from a fluid system, different from the origination of the cores and the transition zones of type 1 pyrite but same as the origination of the transition zone of type 2 pyrite, indicating that there are multiple pulses of hydrothermal fluid associated with the ore-forming process, and this pulse of hydrothermal fluid is depleted in these ore metals. The rims of the type 2 pyrite have the highest Au contents, indicating that there are multiple pulses of hydrothermal fluid associated with the ore-forming process, and this pulse of hydrothermal fluid is enriched in Au.

The low Au contents may indicate sulfidation from a gold-poor fluid or that the sulfidation reaction has less influence on Au solubility under the environments of this type of pyrite formed (Sung et al., 2009). The higher Au contents could be largely controlled by the nature of the ore-forming fluid. Based on the texture characteristics and chemical compositions discussed above, there are at least four pulses of fluid participating in the generation of pyrite in the Zaozigou deposit. The first pulse of hydrothermal fluid is enriched in Au, which is the most important Au deposition age. The second pulse of hydrothermal fluid is enriched in As and depleted in Fe; the third pulse of hydrothermal fluid is enriched in Pb but depleted in Au; and the fourth pulse of hydrothermal fluid is enriched in Au, which is another important Au deposition event. The reduction, pH change, or the decrease in temperature and the H₂S activity may result in Au and As enrichment in pyrite, and arsenopyrite probably (Arehart et al., 1993).

The Co and Ni contents can used to infer the origin of pyrite (Cook et al., 2009). Pyrite determined by electron microprobe analyses (EMPA) has relatively high Co contents (~364–2248 ppm) but low Ni contents (~109–497 ppm, except one spot (13,574 ppm)), and Co/Ni ratios range from ~1.63 to 10.50 except one spot (0.07), with an average of 4.2. In the plot of Co versus Ni contents for pyrite and arsenopyrite, all the samples plot away from the fields of the sedimentary and magmatic deposits (Large et al., 2014), and the data fall in the volcanogenic field with low Ni contents, and seven spots plot in the hydrothermal field (Fig. 8), indicating they originated from a volcanogenic fluid and were remobilized by hydrothermal fluid.

Plot of Co versus Ni for pyrite and arsenopyrite from the Zaozigou gold deposit. Reference fields for different types of deposits or geological environments are defined according to Co and Ni values from Bralia *et al.* (1979) and Bajwah *et al.* (1987).

6.3 Gold source and occurrence

Arsenic is one of the most substantial and vital elements entered into pyrite (Deditius et al., 2008; Morey et al., 2008). There are three forms of arsenic in pyrite, As¹⁻, As⁰, and As³⁺. The As⁰ occurs as amorphous Fe–As–S nanoparticles (Deditius et al., 2009), whereas the As¹⁻ and As³⁺ are structurally bound and are incorporated into pyrite that forms in reduced and oxidizing conditions, respectively (Simon et al., 1999; Deditius et al., 2008). In the Fe–S–As ternary diagram (Fig. 9), all of the pyrite from the Zaozigou deposit falls into a zone parallel to the S–As axis, suggesting replacement of As for S in pyrite (Fleet & Mumin, 1997; Reich et al., 2005), and arsenic is present in the anionic As^1- state in the pyrite structure (Simon et al., 1999) from the Zaozigou deposit under the reduced conditions.

Composition of pyrite from ore deposits on As-Fe-S ternary. Four different trends show substitution of (i) As for S (As¹⁻-pyrite; red arrow); (ii) As²⁺ for Fe (As²⁺-pyrite; gray arrow); (iii) As³⁺ for Fe (As³⁺-pyrite; green arrow); and (iv) divalent metals Me²⁺ for Fe (yellow arrow). Modified from Deditius *et al.* (2008, 2014).

Gold generally occurs as structurally bound Au¹⁺ or gold-bearing nanoparticles in pyrite and arsenopyrite. Solid solubility of Au in pyrite and arsenopyrite has been studied in Carlin-type and epithermal Au deposits according to secondary ion mass spectometry and EMPA analyses of pyrite (Reich et al., 2005). When the formation temperatures vary from ~150°C to 250°C, solid solubility of Au in As-pyrite can be calculated using the empirical formula C_Au = 0.02 xC_As + 4 × 10⁻⁵ (C_Au and C_As represent the mole percentage of Au and As concentration, respectively). In the plot of Au versus As (Fig. 10), the arsenopyrite data from the Zaozigou deposit fall below the solubility limit, indicating that gold in arsenopyrite occurs as cationic Au in solid solution. Inexistence of native gold in arsenopyrite may be related to the cognition that arsenopyrite has large capacity for Au incorporation compared to pyrite (Liang et al., 2014). The data of pyrite fall below and above the solubility limit, suggesting that the Au in pyrite is not only in solid solution but also forms metal nanoparticles (Au⁰) (Simon et al., 1999; Chouinard et al., 2005; Reich et al., 2005; Morishita et al., 2018). Excess charge due to cationic As accommodation in pyrite suggests the existence of vacancies (Deditius et al., 2008). The distribution of Au–As pyrite analyses below the solubility limit shows that the fluids are gold-undersaturated when these pyrites are formed, whereas the gold-arsenic clusters of points above the solubility limit confirm the cooling of the temperature and/or the gold supersaturation for ore-forming fluids in the system(Reich et al., 2005).

Plot of Au versus As concentrations (in mol%) for pyrite and arsenopyrite from the Zaozigou gold deposit. Dashed lines indicate the Au: As ratios. Solubility limit for gold is based on Reich *et al.* (2005). Reference fields for different types of deposits are defined from Deditius *et al.* (2014).

Sulfur and arsenic from metallogenic hydrothermal fluid reacted with iron-bearing minerals such as magnetite, biotite, and chlorite in the host rocks to produce pyrite and arsenopyrite. The introduction of Fe²⁺ caused ankeritization along dolomite crystal boundaries during gold mineralization. Deposition of quartz, ankerite, and sericite was accompanied by pyrite, arsenopyrite, and gold mineralization (Saunders & Tuach, 1991).

6.4 Source of ore-forming materials

In Figure 11, the Pb isotope compositions of most of pyrites from the samples plot within the zone between the orogenic and the upper crust Pb evolution lines. One of those samples plot within the zone between the orogenic and the mantle Pb evolution lines in the ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb diagram (Fig. 11a, c). This reflects mixing of mantle- and the upper crust-derived materials. The Δγ and Δβ values can be used to trace the lead origins. In the Δγ–Δβ diagram (Fig. 11d), most of the pyrites fall into the field of mixed mantle and upper crust Pb related to magmatism, which indicates that the metallogenic materials possibly originated from granitoids in Zaozigou deposit. Only three of the pyrites fall in the field of the upper crust. Combined with the field observations (Fig. 3) and the Δγ and Δβ values, it thus can be deduced that the metallogenic materials of the Zaozigou deposit were derived from the mantle and upper crust.

Lead isotope diagrams for the Zaozigou gold deposit. (a–c) Based on Zartman and Doe (1981). (d) Based on Zhu (1993).

6.5 The genesis of the Zaozigou gold deposit

Gold–arsenic relations may be an effective approach to calculate the approximate temperature as Au incorporates into pyrite and to explore their potential application in constraining the chemical limits of Au and As elements for different types of ore deposits (Deditius et al., 2014). Au and As contents have a negative correlation with temperature in different types of ore deposits. According to the method provided in the Deditius et al. (2014), the calculated formation temperatures range from 148°C to 304°C, with an average temperature of 213°C based on Au contents in the Zaozigou gold deposit, which are consistent with the homogenization temperature of 120.8–360°C obtained from the fluid inclusion in quartz veins (Cao et al., 2012; Chen et al., 2014, 2018). The estimated formation temperature of pyrites has a negative correlation with the Au/As ratios (Fig. 12). The cooling of hydrothermal fluid is helpful to the formation of metal nanoparticles of native gold, and the temperature increase may lead to Au incorporation into pyrite by dissolution of metal nanoparticles (Deditius et al., 2014).

Variation of Au/As ratio (log scale) as a function of temperature (°C) in the Zaozigou gold deposit.

Pyrites from various types of hydrothermal ore deposits are characterized by diverse ranges of Au–As contents (Deditius et al., 2014). It is obviously shown that the concentrations of Au and As in pyrites from epithermal and Carlin gold deposits are significantly higher than those from porphyry Cu and orogenic Au deposits (Fig. 10). In the plot of Au versus As (Fig. 10), the range of the mole percentage of Au and As concentrations of pyrite from the Zaozigou gold deposit overlapped with the range of the epithermal deposits and the Carlin-type hydrothermal deposit but was different from the orogenic deposits and porphyry deposits (Deditius et al., 2014), indicating that the Zaozigou deposit is ascribed to the epithermal deposit or the Carlin-type hydrothermal deposit.

The Zaozigou deposit lies in the central gold belt of West Qinling. The Dashui gold deposit is the representative Carlin-type gold deposit in this belt (Peng et al., 2018). The Dashui deposit is hosted in Middle Triassic shallow marine carbonate rocks and younger igneous dykes. However, the Zaozigou deposit is hosted in fine-grained slates intercalated with limestone of the Middle-Triassic Gulangdi Formation and varied acidic–basic dykes. Wall rock alteration has a certain alteration zoning, which is characterized by strong silicification and pyritization near the orebody. The tensional and shearing faults controlled the gold orebodies strictly. The Au and As contents in the Zaozigou deposit suggest the gold supersaturation for ore-forming fluids in the system. Generally, hydrothermal fluids of epithermal deposits are saturated with gold, but those of Carlin-type deposits are gold-undersaturated (Reich et al., 2005; Zhu et al., 2011). In addition, Au occurs in solid solution, metal nanoparticles (Au⁰), and irregular native gold grains in the Zaozigou gold deposit. The formation temperatures of the Zaozigou gold deposit are relatively low, ranging from 148°C to 304°C. Furthermore, the pyrite Pb isotopic compositions of the granodiorite porphyry indicate that the metallogenic materials of the Zaozigou deposit primarily originated from the mantle and upper crust. The fluid inclusion compositions and C-H-O isotopes showed that the ore-forming fluid in the Zaozigou deposit were primarily magmatic mixed with meteoric water formed under low temperature, low salinity, and low density (Chen et al., 2018). Therefore, the Zaozigou gold deposit is better defined as an epithermal deposit (Chen et al., 2018). The Gangcha gold deposit in the West Qinling orogenic belt also belongs to the epithermal deposit (Kong et al., 2018).

The metallogenesis of the Zaozigou deposit was most likely associated with a Late Triassic continental arc setting of the Qinling orogenic belt (Chen, 2010; Li et al., 2014). The subduction of the Mianlue Oceanic plate triggered large-scale magmatism at the lower curst, and the late-stage magmatic fluid carried ore-forming materials and moved upward; finally, the ore-forming materials precipitated and settled in the fractures at shallow crust.

7 Conclusions

Several important conclusions can be made from this study. They are summarized below.

The relationship between As and S indicates the replacement of As for S in pyrite, and arsenic is present in anionic state in pyrite under the reduced conditions.
The pyrite Co/Ni ratios indicate that the Zaozigou gold deposit originated from a volcanogenic fluid and was remobilized by hydrothermal fluid.
Au in arsenopyrite occurs as cationic Au in solid solution, whereas Au in pyrite is in the form of solid solution and metal nanoparticles (Au⁰) in the Zaozigou deposit.
The Zaozigou deposit has multiple pulses of fluid participating in the generation of pyrite.
The pyrite Pb isotopic compositions indicate that the metallogenic materials of the Zaozigou deposit were derived from the mantle and upper crust.

Acknowledgments

This study was finally supported by the Natural Science Foundation of China (41472070 and 41872073); the Science and Technology Planning Project of Gausu Province, China (18JR3RA266); the Second Tibetan Plateau Scientific Expedition (STEP) program and the Fundamental Research Funds for the Central Universities of China (lzujbky-2017-77). We thank Xiangsheng Tian for assistance in sampling.

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