Ore genesis of Axi post-collisional epithermal gold deposit, western Tianshan, NW China: Constraints from U–Pb dating, Hf isotopes, and pyrite in situ sulfur isotopes

Journal Pre-proofs Ore genesis of Axi post-collisional epithermal gold deposit, western Tianshan, NW China: constraints from U–Pb dating, Hf isotopes, and pyrite in situ sulfur isotopes Jiahao Zheng, Ping Shen, Changhao Li PII: DOI: Reference:

S0169-1368(19)30784-X https://doi.org/10.1016/j.oregeorev.2019.103290 OREGEO 103290

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Ore Geology Reviews

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Please cite this article as: J. Zheng, P. Shen, C. Li, Ore genesis of Axi post-collisional epithermal gold deposit, western Tianshan, NW China: constraints from U–Pb dating, Hf isotopes, and pyrite in situ sulfur isotopes, Ore Geology Reviews (2019), doi: https://doi.org/10.1016/j.oregeorev.2019.103290

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1

Ore genesis of Axi post-collisional epithermal gold deposit,

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western Tianshan, NW China: constraints from U–Pb dating,

3

Hf isotopes, and pyrite in situ sulfur isotopes

4 5

Jiahao Zheng1,2*, Ping Shen1*, Changhao Li1

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7

1

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Beijing 100029, China

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2

Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences,

Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen 518055,

10

China

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*Corresponding authors: Jiahao Zheng (e-mail: [email protected]); Ping Shen (e-mail:

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[email protected])

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Abstract

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The Axi gold deposit has long been regarded as a typical Paleozoic low

16

sulfidation epithermal deposit, located in the western Tianshan, in the Central Asian

17

Orogenic Belt (CAOB). SIMS zircon U–Pb dating of the ore-bearing andesite and

18

dacite yielded concordia ages of 350.8 ± 2.7 Ma and 351.3 ± 3.1 Ma, respectively. In

19

situ SIMS hydrothermal rutile U-Pb dating yielded two 1

207Pb

corrected ages as 306.1

20

± 16.9 Ma and 303.8 ± 14.6 Ma, respectively. These ages are considerably younger

21

than the zircon SIMS U-Pb ages of ore-bearing volcanic rocks, which precludes a

22

genetic link between the epithermal gold mineralization and the subduction-related

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Late Devonian-Early Carboniferous magmatism in the western Tianshan. Instead, the

24

gold mineralization is interpreted to be genetically related to a buried ~300 Ma pluton

25

that formed in a post-collisional environment. Combined with previously published

26

zircon Hf isotopic compositions of ~300Ma felsic intrusive rocks from western

27

Tianshan, the results show that ~300Ma felsic rocks have higher zircon εHf(t) values

28

than those from ~351Ma felsic rocks in the Axi deposit. These elevated εHf(t) values

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suggest that mantle material inputs of ~300 Ma Axi epithermal deposit in a

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post-collisional environment. Pyrite, the most common sulfide in the Axi ores,

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displays heterogeneous textures and has a large variation of As contents and sulfur

32

isotopes, suggesting complex ore-forming processes in the Axi deposit. This study

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highlights the importance of in situ isotopes of hydrothermal minerals to decode the

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ore-forming histories of epithermal systems.

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Keywords: Mineralogy; chronology; Axi gold deposit; Western Tianshan

37

38

39 40

1. Introduction

Epithermal precious metal deposits often associated with volcanoes in continental and island arcs above subduction zones, and they formed near the surface 2

41

with typical temperatures less than 300 °C (Hedenquist and Henley, 1985; Simmons

42

et al., 2005). Most epithermal deposits that survived formed during Cretaceous and

43

younger due to their shallow settings and relatively rapid erosion (Saunders et al.,

44

2014; Sholeh et al., 2016). However, little is known regarding to the ore formation

45

processes of the ancient epithermal deposits.

46

The Chinese Tianshan orogenic belt hosts many Paleozoic gold deposits and

47

occurrences, several of which contain ore reserves more than 50 tons, and is

48

considered to be one of the most important gold ore belts in China (Yang et al., 2009;

49

Deng and Wang, 2016; Zhu et al., 2016; Zheng et al., 2017, 2018). Recent researches

50

have documented the geological characteristics, nature of the ore fluids, ore-forming

51

ages, stable and radioactive isotopes, as well as geodynamic settings of these gold

52

deposits (e.g., Chiaradia et al., 2006; Yang et al., 2006; Liu et al., 2007; Zhu, 2011;

53

Chen et al., 2012; An et al., 2013; Zheng et al., 2016). Located in the western

54

Tianshan, the Axi deposit is one of the largest epithermal gold deposits (>50 t at 3g/t)

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in the Tianshan orogenic belt. Recent researches have elaborated on the chronology of

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Axi deposit, and have proven it was a Paleozoic epithermal deposit (Zhai et al., 2006;

57

An and Zhu, 2018). However, due to a lack of suitable dating methods, the formation

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age and the corresponding tectonic environment of Axi deposit remain controversial.

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In addition, detailed mineralogy and hydrothermal evolutions of the Axi deposit is not

60

clear.

3

61

Pyrites are common major mineral phases in gold deposits and are known for

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their close association with gold, and their texture, chemical and isotopic variations

63

makes them ideal indicators of hydrothermal evolution and ore genesis for gold

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deposits (Large et al., 2007; Ulrich et al., 2011; Cook et al., 2013; Hou et al., 2016).

65

Most recently, Tanner et al. (2016) have documented complex intracrystalline δ34S

66

values of pyrites in intermediate-sulfidation and high-sulfidation epithermal deposits.

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However, δ34S values of pyrites in low-sulfidation epithermal deposits remain poorly

68

understood. In addition, hydrothermal rutile has shown the potential to record the

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ore-formation age of gold deposit (Pi et al., 2017). In this contribution, we present

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geological observation, zircon U-Pb dating for ore-hosted volcanic rocks, in-situ rutile

71

U-Pb dating, in situ chemical and isotopic analyses of pyrite to determine the rock -

72

and ore -forming ages, as well as hydrothermal evolution and ore genesis of the

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Paleozoic Axi epithermal gold deposit.

74

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2. Regional geology

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The western Tianshan, situated in the southern part of the Central Asian

77

Orogenic Belt (CAOB; Fig. 1a), is herein defined as all parts of the mountain range

78

located west of the Urumuqi-Korla Road, and bounded by the southern margin of the

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Junggar Basin and the northern margin of Tarim Basin (Fig. 1b). It was formed by the

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amalgamations of the Tarim, Yili, and Junggar blocks (Gao et al., 1998; Zhu et al.,

81

2009). It can be further divided into North Tianshan Accretionary Complex (NTAC), 4

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the Yili –Central Tianshan, and the South Tianshan Orogenic Belt (STOB) from north

83

to south.

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The NTAC is mainly composed of Devonian to Early Carboniferous volcanic

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and sedimentary rocks, and ophiolitic slices (Feng and Zhu, 2018). It was formed by

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southward subduction of North Tianshan ocean beneath Yili–Central Tianshan along

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the North Tianshan suture zone. The Yili –Central Tianshan contains a Precambrian

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basement and overlying Paleozoic volcanic-sedimentary strata. Voluminous granitoid

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plutons intruded into the Ordovician–Early Carboniferous volcanic-sedimentary strata

90

(Feng and Zhu, 2019). The STOB mainly consists of Lower Cambrian–Carboniferous

91

sedimentary rocks and interlayered volcanic rocks, high/ultrahigh pressure

92

metamorphic rocks, ophiolitic components, and Permian granitoids (Gao et al., 2009).

93

Situated at west part of the north margin of the Yili–Central Tianshan, the Tulasu

94

volcanic-sedimentary Basin is bounded by the NWW-trending South Keguqin Fault in

95

the north and the North Yili Fault in the south, and other NWW- and E-W trending

96

faults occur as secondary structures (Fig. 2). The basement of the Tulasu Basin

97

consists mainly of the limestone and calcareous rocks of the Neoproterozoic

98

Kusongmuqieke Formation and Ordovician Hudukedaban Formation, as well as

99

calcareous muddy siltstone of the Ordovician Nailengeledaban Formation. Late

100

Devonian to Early Carboniferous volcanic-sedimentary rocks, consisting mainly of

101

tuff, rhyolite, dacite, and andesite, unconformably overlie the basement. Axi and

102

Jingxi-Yelmand gold deposits are hosted in these volcanic-sedimentary rocks. Some

103

~350 Ma and ~300Ma granitoid intrusions crop out in the Tulasu Basin (Tang et al., 5

104

2013; An and Zhu, 2018).

105 106

3. Deposit geology and mineralization

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Located in the central part of the Tulasu Basin, the Axi deposit contains gold

108

reserve of about 50t at an average grade of 3g/t, and is one of the largest epithermal

109

gold deposits in Xinjiang. The stratigraphic sequence in the Axi area is dominantly

110

composed of the Late Devonian to Early Carboniferous lavas (andesite and dacite) as

111

well as volcaniclastic rocks (tuff and volcanic breccia). These rocks are exposed

112

concentric in the ore district, indicating the existence of a volcano. The major

113

structures in the Axi area are NNE- and NW- striking faults (Fig. 3a). No intrusions

114

are exposed at the surface near the deposit. No exposed intrusions were found in the

115

ore district.

116

The gold orebodies of Axi deposit are hosted predominantly in the andesite,

117

andesitic breccia, and dacite (Fig.3a). Major orebodies have lengths of 100–1000 m

118

and widths of less than 1 m to tens of meters, with a depth of ~300 m (Fig. 3).

119

Ore-hosted rocks in the Axi deposit record various degrees of hydrothermal alteration

120

styles, which can be divided into sulfide-quartz vein, sericitic alteration, to propylitic

121

alteration zones distributed outward from the center of the main orebodies (Fig. 3b).

122

Sulfide-quartz vein is the main orebody, and average gold grade in sulfide-quartz vein

123

is at 8.27 g/t.

124

Sericitic altered volcanic rocks consist mainly of sericite, chlorite, and

125

disseminated pyrite (Fig.4a) as well as minor fine grains of arsenopyrite. Feldspar and 6

126

mafic minerals in the volcanic rocks have undergone various degrees of alteration

127

(Fig. 5a). Quartz-sulfide veins crosscut or enclosed the altered volcanic rocks

128

(Fig.4b-e, 5b). Quartz-sulfide ores consist predominantly of gray quartz and pyrite,

129

with small contents of arsenopyrite. Post-ore carbonate quartz vein crosscut the

130

quartz-sulfide vein (Fig. 4f). Some altered volcanic rocks contain quartz phenocrysts

131

with inclusions (Fig. 5c), and quartz grains in the quartz-sulfide ores vary from <10

132

um to more than 100um (Fig. 5d). Ore minerals are composed of pyrite and minor

133

arsenopyrite, with a small amount of sphalerite, galena, chalcopyrite and rare electrum

134

(An and Zhu, 2018). Gangue minerals include quartz, sericite, chlorite, calcite,

135

ankerite, rutile, apatite, and adularia (Zhai et al., 2009).

136

Three main paragenetic stages of hydrothermal evolution have been recognized

137

based on petrographic observations (Fig. 6), which show a sericite-pyrite assemblage

138

(Stage I), a quartz-pyrite assemblage (main gold ore stage, stage II), and a

139

carbonate-quartz assemblage (stage III).

140 141

4. Analytical techniques

142

4.1. Zircon U–Pb dating

143

Zircon grains from the ore-hosted volcanic rocks (sample AX16-62 and

144

AX16-63; Fig.4b and c) were separated using a conventional magnetic and density

145

technique and hand-picked under a binocular microscope. The selected zircon grains

146

were mounted in epoxy resin. Prior to analyses, all the selected zircon grains were

147

examined with reflected and transmitted light photomicrographs combined with 7

148

cathodeluminescence (CL) images (Fig.8a and b) to reveal their internal structures.

149

Zircons with few inclusions or fissures were chosen for U–Pb dating during this study.

150

Zircon U–Pb analyses were performed using the Cameca IMS 1280 ion microprobe at

151

the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) in

152

Beijing. The ellipsoidal spot for zircon U-Pb dating is about 30× 20μm in size.

153

Detailed operating and data processing procedures are similar to those described by Li

154

et al. (2009).

155

U-Th-Pb ratios and absolute abundances were determined relative to the standard

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zircon 91500 (Wiedenbeck et al. 1995). Measured compositions were corrected for

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common Pb using non-radiogenic 204 Pb. Uncertainties on individual analyses in data

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tables are reported at a 1σ level; mean ages for pooled U/Pb (and Pb/Pb) analyses are

159

quoted with 95% confidence interval. Data reduction was carried out using the Isoplot

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3.00 program (Ludwig 2003).

161 162 163

4.2. In situ rutile Raman spectroscopy and U-Pb dating Because TiO2 has three mineral polymorphs as rutile, anatase, and brookite, and

164

they cannot be distinguished by their geochemical composition. The most reliable

165

method to identify mineral polymorphs is laser Raman spectroscopy (Meinhold,

166

2010). Thus, before U-Pb isotope analysis, Raman spectroscopy was conducted on the

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rutile sample at 100 - 4000 cm -1 using a LabRam HR800 laser Raman

168

microspectrometer at IGGCAS. The exciting radiation was provided by an argon ion

169

laser with a wavelength of 532 nm and a source power of 44 mW. 8

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Rutile crystal coexist with pyrite (Fig.9a and b) was drill from a thin section and

171

then mounted in a transparent epoxy together with an in-house rutile standard JDX

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(~6 ppm U, 207Pb/206Pb age=521 Ma, 206Pb/238U age=500–520 Ma, Li et al., 2011).

173

The in-situ measurements of U - Pb isotopes of rutile were performed using a

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CAMECA IMS-1280 ion microprobe at IGGCAS. The instrumental conditions and

175

measurement procedures were similar to those described by Li et al. (2011). The

176

ellipsoidal spot was about 30 × 20μ m in size. Each measurement comprises 10 cycles

177

during a total analytical duration of ∼ 15 minutes, including 2 minutes rastering prior

178

to the actual analysis to reduce the contribution of surface contaminant Pb.

179 180

4.3. Electron microprobe (EMP) analysis

181

Major and minor element compositions of the selected sulfides were determined

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using a JEOL JXA-8100 electron probe under the operation conditions of 15 kV, 10

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nA with a beam size of 1 μm in diameter, count time 10 s (peak) and 5 s (upper and

184

lower background), at IGGCAS. The ZAF correction method was used to correct the

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atomic number (Z), absorption (A) and fluorescence (F) effects for all analyzed

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minerals.

187 188 189

4.4. In-situ S isotope analysis In-situ S isotope analysis and trace element mapping of representative pyrite grains

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from thin sections were conducted using a CAMECA Nano SIMS 50L instrument at

191

IGGCAS. A primary Cs+ ion beam of 1–2 pA and 100 nm in diameter was used for 9

192

analysis. The FC-EM-EM method was used for the in-situ S isotope analysis (Zhang

193

et al., 2014), and 32S was counted with Faraday cup (FC) to avoid the

194

quasi-simultaneous arrival (QSA) effect and 34S and other elements were counted with

195

electronic multipliers (EMs). The certified international pyrite standards (CAR-123)

196

and working reference pyrite samples (PY-1117 and CS01) were used during in-situ S

197

isotope analyses. The total count time for each S isotope analyses was 150 s, with 300

198

cycles of 0.5 s, and the analysis spot was 2 µm in diameter. The in situ S isotope

199

analyses generally have analytical errors <0.4‰.

200 201 202

4.5. Zircon Hf isotopes Zircons from the ore-hosted volcanic rocks were analysed for Lu-Hf isotopic

203

compositions. The Hf isotope analysis was carried out using a Newwave UP213

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laser-ablation microprobe, attached to a Neptune multi-collector ICP-MS at the

205

Peking University, China. Instrumental conditions and data acquisition methods were

206

comprehensively described by Wu et al. (2006) and Hou et al. (2007). The analytical

207

spot size was 44 μm in diameter. The 176Lu/175Lu=0.02658 and 176Yb/173Yb=0.796218

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ratios were used to correct the isobaric interferences of 176Lu and 176Yb on176Hf (Chu

209

et al., 2002). The mass bias behavior of Lu was assumed to follow that of Yb, mass

210

bias correction protocol details were reported by Wu et al. (2006) and Hou et al.

211

(2007). Zircon sample GJ1 was used as the reference standard, with a weighted

212

mean176Hf/177Hf ratio of 0.282001±11 (2σ, n=11) during our routine analyses. It is not

10

213

distinguishable from a weighted mean 176Hf/177Hf ratio of 0.282015 ± 19 (2σ) from

214

in–situ analysis by Elhlou et al. (2006).

215 216

5. Results

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5.1. Zircon U-Pb-Hf isotopes

218

The CL images of representative zircon grains from ore-hosted volcanic rocks

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(Sample AX16-62 and AX16-63) are shown in Figure 8A and B. SIMS zircon U-Pb

220

results are given in Table 1. Zircon grains from sample AX16-62 have a size range

221

of 60-130μm with a length/width ratio of 1:1-2:1(Fig. 8a). Most zircon grains exhibit

222

oscillatory zoning and high Th/U ratios of 0.47–0.84, consistent with magmatic

223

origins.

224

Ma (MSWD=0.097) (Fig. 8c). This age is interpreted as the crystallization age of the

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ore-hosted andesite. Zircon grains in the sample AX16-63 have a size range

226

of 90-150μm with

227

analyses gave a concordia age of 351.3 ± 3.1 Ma (MSWD=0.077) (Fig. 8d), which is

228

interpreted as the crystallization age of the ore-hosted dacite.

All fifteen spot analyses yielded a concordia age of 350.8 ± 2.7

a

length/width

ratio

of 1:1-1.5:1(Fig.

8b).

Twelve

spot

229

Zircon Hf isotopic results are listed in Supplemental Table 1 and presented in

230

Figure 13. The andesite and dacite have similar positive εHf(t) values ranging from 2.3

231

to 7.9.

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5.2. In situ rutile Raman spectrum and U–Pb ages

11

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Two TiO2 minerals in the Axi ores are characterized by the peaks at

235

wavenumbers 142, 240, 442, and 612 cm-1, and 141, 238, 443, and 611 cm-1,

236

respectively (Fig.9 c). These spectrums are similar to those from rutile, but

237

inconsistent with those from brookite and anatase (Fig.9d; Meinhold, 2010),

238

suggesting the TiO2 mineral in the Axi ores is rutile. Two analyzed rutile grains have

239

high U contents as 25.51 and 20.05 ppm, and Th/U ratios as 1.59 and 1.61,

240

respectively. They yield two 207Pb corrected ages as 306.1 ± 16.9 Ma and 303.8 ±

241

14.6 Ma, respectively.

242 243 244

5.3. Textures and geochemistry of pyrite Two major types of pyrite occur in the Axi ores. The altered volcanic rocks

245

mainly consist of disseminated pyrite, and quartz-sulfide veins are composed

246

predominantly of massive pyrite (Fig.7a and b). Combined with their optical and BSE

247

characteristics, four subtypes of pyrite were recognized. Py1 and Py2 occur as

248

disseminated grains in the altered volcanic rocks (Fig.7 c and d). Py1 occurs as

249

subhedral-euhedral cores, which is characterized by relatively dark BSE images, and

250

often contains silicate inclusions. Py2 commonly overgrows Py1, and it is

251

characterized by relatively bright BSE images with rare or no silicate inclusions. Py3

252

and Py4 occur either as elongated anhedral to subhedral clusters in the quartz-sulfide

253

veins (Fig.7 e and f). Py3 is relatively small in scale, and it occurs as anhedral cores

254

with bright BSE images. Py4 is ubiquitous in quartz-sulfide veins, in which it either

255

encloses Py3 or occurs independently. 12

256

Arsenic concentrations have systematic variations among pyrite of different

257

textural subtypes (Fig.10a). Py1 has relatively low As concentrations, mostly ranging

258

from 0.08 to 0.68 wt.%. Py2 and Py3 contain similar high As concentrations, ranging

259

from 1.22 to 2.58wt.%, and 0.82 to 2.92 wt.%, respectively. The concentrations of As

260

in Py4 are mostly below the detection limit of EPMA but with values reaching up to

261

0.40 wt.%. All pyrite grains contain similar measurable concentrations of Pb and Bi,

262

ranging from 0.14 to 0.67wt.%, and 0.05 to 0.32 wt.%, respectively. Gold

263

concentrations of different types of pyrite are mostly below detection limit but with

264

values reaching up to 0.29 wt.% (Fig. 10b). The concentrations of Cu in all types of

265

pyrite are mostly below the detection limit of EPMA.

266

A total of 40 sulfur isotope measurements on different types of pyrite were

267

obtained, and the representative analysis spots and results are show in the Fig.12 and

268

Table 3, respectively. In situ analytical results for Py1, Py2, Py3 and Py4 show

269

obvious systematic variation in sulfur isotopes (Fig.12e). Py1 and Py2 both have

270

positive δ34S values, ranging from 0.5 to 3.1‰, and 4.4 to 5.5‰, respectively. Values

271

of δ34S from Py3 are all negative (-2.6 to -0.5‰), whereas those from Py4 are all

272

positive (0.1–2.7‰).

273 274

6. Discussion

275

6.1. Age constraints on magmatism and hydrothermal activity

276

Previous attempts to date the ore formation age of Axi gold deposit have yielded

277

various isotopic ages, however, the precise ore formation age of Axi gold deposit has 13

278

not been determined due to the scarcity of suitable dating minerals. Whole-rock Rb-Sr

279

and zircon U-Pb ages of volcanic rocks in the Axi deposit are 345.9 ± 9 Ma and 363.2

280

± 5.7 Ma, respectively (Li et al., 1998; Zhai et al., 2006). In addition, Rb-Sr isotopic

281

ages of fluid inclusions in quartz broadly range from 340 ± 8 Ma to 301 ± 29 Ma (Li

282

et al., 1998). Thus, Axi was thought to be a Late Devonian- Early Carboniferous

283

epithermal deposit and genetically related to the ore-bearing volcanic rocks (Zhai et

284

al., 2009). However, most recently, An and Zhu (2018) obtained a much younger

285

pyrite Re-Os isochron age of 299 ± 35 Ma, which is in agreement with K-Ar ages of

286

intensively sericitized andesitic rocks that vary from 293 to 286 Ma.

287

Our zircon SIMS U-Pb age of 350.8 ± 2.7 Ma and 351.3 ± 3.1 Ma for the

288

andesite and dacite confirm that ore-bearing volcanic rocks formed at ~351Ma. In situ

289

U-Pb dating of hydrothermal rutile intergrown with pyrite, however, has yielded two

290

ages of 306.1 ± 16.9 Ma and 303.8 ± 14.6 Ma, which are broadly consistent with the

291

Re-Os isochron age (299 ± 35 Ma) of pyrite and K-Ar ages (293 - 286 Ma) of

292

sericitized andesitic rocks (An and Zhu 2018), and considerably younger than the

293

zircon SIMS U-Pb age of ~351Ma for the ore-bearing volcanic rocks. Accordingly,

294

we propose that the gold mineralization of the Axi deposit is genetically unrelated to

295

the ~351Ma host volcanic rocks. Instead, it formed by hydrothermal fluids associated

296

with a buried ~300 Ma pluton. This ore-forming event is consistent with the ~300Ma

297

porphyry Cu mineralization in the western Tianshan (Zhang et al., 2006, 2012).

298 299

Most epithermal deposits that survived formed during Cretaceous and younger because of their shallow ore-forming environment and relatively rapid erosion 14

300

(Simmons et al., 2005). Previous studies have shown that the ores in these young

301

low-sulfidation epithermal deposits are generally have similar ages with the host

302

volcanic rocks (Leavitt et al., 2004; Saunders et al., 2014). Ore-forming ages slightly

303

younger than the host volcanic rocks have also been reported, which may due a

304

second magmatic event (Sholeh et al., 2016). Our study reveals that ore-forming age

305

of Paleozoic low-sulfidation epithermal deposit can be much younger, by as much as

306

~50Ma, than the volcanic host rocks. Thus, the ages of host volcanic rocks may not

307

represent the ore-forming ages in the low-sulfidation epithermal deposits, especially

308

in the old ones.

309

The low-sulfidation epithermal deposits are typically hosted by calc-alkaline

310

volcanic rocks in predominantly regional extensional environments (Sillitoe and

311

Hedenquist, 2003). The Axi low-sulfidation epithermal ores formed during ~300Ma,

312

corresponding to a post-collision extension environment in the Western Tianshan

313

(Feng and Zhu, 2019). The ~300Ma granitoids have higher zircon εHf(t) values than

314

those from ~351Ma felsic rocks in the Axi deposit (Fig.13), indicating obvious mantle

315

components involved in the epithermal ore-forming systems, which is supported by

316

the Re-Os isotopes of pyrite (An and Zhu, 2018).

317 318

6.2. Application of hydrothermal rutile U-Pb geochronometer in gold deposits

319

Due to a lack of suitable dating minerals, determining the ore-forming ages of

320

many gold deposits has long been problematic. Muscovite 40Ar-39Ar, pyrite Rb-Sr,

321

arsenopyrite Re-Os are commonly used dating method in gold deposits (Goldfarb et 15

322

al., 1991; Yang and Zhou, 2001; Mao et al., 2004; Morelli et al., 2007). However,

323

muscovite with low closure temperatures may yield mixed ages or record the final,

324

waning stages induced by multiple hydrothermal events (Chiaradia et al., 2013), and

325

low Rb, Sr, Re, and Os contents in sulfides make them difficult to produce available

326

isochron ages. Rutile as an accessory mineral can occur in many gold deposits (Wong

327

et al., 1991; Dostal et al., 2009; Meinhold, 2010), and its high U concentrations as

328

well as high closure temperatures (Shi et al., 2012; Chiaradia et al., 2013) make it an

329

ideal mineral for determining the metallogenic age of gold deposits.

330

Traditional rutile U-Pb dating method is to pick rutile grains and mounted them

331

in a transparent epoxy. This method is suitable for gold deposits with relatively

332

abundant and large rutile grains (e.g., Pi et al., 2017). In the case of Axi gold deposit,

333

the rutile grains in the ores are relatively small and rare (Fig.9a), which is unsuitable

334

for the traditional rutile dating method. Thus, we present the first application of in situ

335

SIMS U-Pb dating on hydrothermal rutile that drilled form a thin section in the Axi

336

gold deposit. Although only two rutile grains were analyzed, the results (306.1 ± 16.9

337

Ma and 303.8 ± 14.6 Ma) is in good agreement with the pyrite Re-Os isochron age of

338

299 ± 35 Ma obtained by An and Zhu (2018). In addtion, our results are more precise

339

than the pyrite Re-Os isochron age, and we suggest that in situ SIMS U-Pb dating on

340

hydrothermal rutile that drilled form a thin section has the potential to be a powerful

341

chronometer for gold deposits, particularly in those where rutile grains are rare.

342 343

6.3. Mineralization events recorded in pyrites 16

344

The concentrations of Cu in all types of pyrite are negligible, which is different

345

from those reported in high-sulfidation epithermal deposits (Tanner et al., 2016). The

346

texture and microanalytical data in the Axi deposit show that the disseminated pyrite

347

(Py1 and Py2) in the altered volcanic rocks and massive pyrite (Py3 and Py4) in the

348

quartz sulfide viens both have As-poor (Py1 and Py4) and As-rich (Py2 and Py3)

349

pyrites (Fig.10a and 12). Arsenic -poor Py1 and Py4 have similar positive δ34S values

350

ranging from 0.5 to 3.1‰ and 0.1-2.7‰, respectively. By contrast, As-rich Py2 and

351

Py3 exhibit distinct δ34S values ranging from 4.4 to 5.5‰ and -2.6 to -0.5‰,

352

respectively (Fig.12). The range of in situ δ34S values in Axi pyrite from this study

353

(-2.6 to 5.5‰) exceeds the eleven bulk analyses of pyrites (-4.0 to 3.1‰; Zhai et al.,

354

2009). However, the range of δ34S from this study is well within the range of bulk

355

δ34S measured from other low-sulfidation epithermal deposits (Yilmaz et al., 2007;

356

Vidal et al., 2016). Moreover, a simple mixture of intracrystalline in situ δ34S values

357

in Axi pyrites (Fig.12b and d) would have yield a much lower bulk δ34S values. Thus,

358

we consider the narrow range of bulk analyses of pyrites may due to a mixture of

359

different types of pyrite.

360

The As-rich Py2 and Py3 show a systematic decoupling of As contents with δ34S

361

values, which may reflect changes in fluid chemistry during pyrite precipitation,

362

triggered by changes in temperature, pressure, oxidation state, and pH, similar to

363

those in other gold deposits (Baker et al., 2009; Peterson and Mavrogenes, 2014; Hou

364

et al., 2016). In the Axi deposit, a decrease of temperature and sulfur fugacity induced

17

365

by a mixture of meteoric and magmatic water (An and Zhu, 2018) may yielded such

366

fluid chemistry variations recorded in pyrites.

367

Based on the results of this study, it is reasonable to infer that if intracrystalline

368

in situ δ34S values were analyzed for the other epithermal deposits, it is likely that

369

their δ34S range would be expanded and reveal more complex physicochemical

370

variations in ore-formation processes.

371 372

7. Conclusions and implications

373

In situ SIMS U-Pb dating of rutile intergrown with pyrite shows that the Axi

374

epithermal deposit formed at ~300Ma, whereas SIMS U-Pb dating of zircon in the

375

ore-bearing andesite and dacite confirms that they formed much earlier at ~351Ma.

376

This precludes a genetic link between epithermal gold mineralization and subduction

377

related felsic magmatism in the Axi deposit. Instead, the gold mineralization is

378

interpreted to be genetically related to a buried ~300 Ma pluton that formed in a

379

post-collisional environment. Our study shows that in situ SIMS U-Pb dating on

380

hydrothermal rutile has the potential to be a powerful chronometer for gold deposits,

381

particularly in those where rutile grains are rare. In situ δ34S values and trace elements

382

in pyrite can record fluid chemistry variations in epithermal systems, which may be

383

caused by changes in physicochemical conditions.

384 385

Acknowledgments

18

386

This research was jointly supported by National Key R&D Program of China

387

(2018YFC0604004 and 2018YFC0603801), NSFC (No.41903042 and No.41530206)

388

and

389

No.2017M610984). Editor-in Chief Prof. Franco Piranjo, Handling Editor Alla

390

Dolgopolova and two anonymous reviewers are thanked for their constructive and

391

valuable comments.

the

China

Postdoctoral

Science

Foundation

(No.

2016LH0003

and

392 393

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597

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601

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602

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603 604

Figure Captions

605

Fig.1. (a) Geological map showing the location of western Tianshan in Central Asian Orogenic

606

Belt. (b) Simplified geological map of the western Tianshan (modified from Gao et al., 2009;

607

Wang et al., 2018; Feng and Zhu, 2019). The tectonic subunits include the North Tianshan

608

Accretionary Complex (NTAC), the Yili–Central Tianshan Block, and the South Tianshan

609

Orogenic Belt (STOB).

610 611

Fig. 2. Simplified geological map of the Tulasu Basin and major gold deposits in the western

612

Tianshan (after An and Zhu, 2018).

613 614

Fig. 3. (a) Geological map of the Axi gold deposit. (modified from Zhai et al., 2009). (b)

615

Geological cross-section showing the orebodies and alteration zones in wall rock (modified from

616

An and Zhu, 2018).

617 618

Fig. 4. Photographs of various ores in the Axi gold deposit.

619

(a) Disseminated pyrite grains in volcanic rocks. (b) Quartz vein crosscut andesite. (c) Quartz

620

sulfide vein crosscut dacite. (d) Volcanic rocks enclosed by massive sulfides. (e) Volcanic

621

breccias in smoky gray quartz sulfide. (f) Smoky gray quartz vein crosscut by carbonate quartz 29

622

vein.

623 624

Fig. 5. Photomicrographs of various ores in the Axi gold deposit.

625

(a) Feldspar has undergone sericitic alteration. (b) Quartz-sulfide veins crosscut the altered

626

volcanic rocks. (c) Altered volcanic rocks contain quartz phenocrysts with fluid inclusions. (d)

627

Quartz grains in the quartz-sulfide ores vary from <10 um to more than 100um.

628 629

Fig. 6. Paragenetic sequence of minerals in the Axi gold deposit.

630 631

Fig. 7. Photomicrographs showing different types of pyrite in the Axi gold deposit.

632

(a) Pyrite grains in the altered volcanic rocks, reflected light. (b) Pyrite grains in the quartz-sulfide

633

veins, reflected light. (c-d) Light color Py1 enclosed by dark color Py2 in the altered volcanic

634

rocks, BSE. (e) Light color Py3 enclosed by dark color Py4 in the quartz-sulfide veins, BSE. (f)

635

Py4 crosscut by galena in quartz-sulfide veins, BSE.

636 206Pb/238U

637

Fig. 8. (a-b) Cathodoluminescence images, analyzed spots, and

ages of zircon in the

638

ore-bearing andesite and dacite in the Axi deposit. (c-d) 207Pb/235U– 206Pb/238U concordia diagrams

639

of zircon from the mineralized volcanic rocks in the Axi deposit.

640 641

Fig. 9 (a) Rutile coexists with pyrite in the Axi ore, most rutile grains are relatively small in size.

642

(b) Two analyzed rutile spots and their

643

among the brookite, anatase, and rutile (Meinhold, 2010) and TiO2 minerals from the Axi gold

206Pb/238U

30

ages. (c-d) Comparison of Raman spectra

644

deposit.

645 646

Fig. 10. (a) EMPA results of As versus S and (b) Au versus As from the Axi gold deposit.

647 648

Fig. 11. BSE (a) and EDS (b-d) mapping results of pyrites in a quartz-sulfide ore from the Axi

649

gold deposit.

650 651

Fig. 12. (a-d) Nano SIMS mapping results and (e) in situ δ34S values of Py1-Py4 in the Axi gold

652

deposit.

653

Compilation of zircon εHf(t) versus U–Pb ages for the ore-hosted volcanic rocks in the

654

Fig. 13.

655

Axi deposit and ~300Ma granotoids related to the porphyry Cu deposit (Zhang et al., 2012) in the

656

Western Tianshan.

31