Metallogenesis of the Hengjiangchong gold deposit in Jiangnan Orogen, South China

Journal Pre-proofs Metallogenesis of the Hengjiangchong gold deposit in Jiangnan Orogen, South China Cheng Wang, Yongjun Shao, Xiong Zhang, Chunkit Lai, Zhongfa Liu, Huan Li, Chao Ge, Qingquan Liu PII: DOI: Reference:

S0169-1368(19)30463-9 https://doi.org/10.1016/j.oregeorev.2020.103350 OREGEO 103350

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Ore Geology Reviews

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17 May 2019 19 December 2019 17 January 2020

Please cite this article as: C. Wang, Y. Shao, X. Zhang, C. Lai, Z. Liu, H. Li, C. Ge, Q. Liu, Metallogenesis of the Hengjiangchong gold deposit in Jiangnan Orogen, South China, Ore Geology Reviews (2020), doi: https://doi.org/ 10.1016/j.oregeorev.2020.103350

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1

Metallogenesis of the Hengjiangchong gold deposit in Jiangnan

2

Orogen, South China

3

Cheng Wanga, Yongjun Shaoa, Xiong Zhangb, Chunkit Laic, Zhongfa Liua, Huan

4

Lia, Chao Gea, Qingquan Liua

5

a

6

Environment Monitoring, Ministry of Education, School of Geosciences and Info-

7

Physics, Central South University, Changsha 410083, China

8

b 416 Geological Brigade, Hunan Bureau of Geology and Mineral Exploration (BGMR),

9

Zhuzhou 412007, China

10

Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological

c Faculty

of Science, University Brunei Darussalam, Brunei Darussalam

11 12 13

Abstract: The Hengjiangchong gold deposit is located in northeastern Hunan of the central

14

Jiangnan Orogen, South China. Distribution of auriferous sulfide–calcite–quartz vein-type

15

orebodies are controlled by NW-/WNW-trending ductile shear zones, and hosted in the Lengjiaxi

16

Group (Gp.) low-grade metamorphic sequences and the Hengjiangchong granite. Ore minerals

17

include mainly pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, and native gold,

18

whilst

19

Alteration/mineralization can be divided into three stages: quartz–calcite–pyrite–arsenopyrite 

the

major

alteration

styles

include

silicic,

sericite,

carbonate

and

chlorite.

Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, School of Geosciences and Info-Physics, Central South University, No. 932 Lushannan Street, Changsha 410083, China. E-mail address: [email protected] (Q. Liu).

20

mineralization (Stage 1), quartz–calcite–polymetallic sulfide mineralization (Stage 2), and quartz–

21

calcite ore-barren alteration (Stage 3). Two types of fluid inclusion (FI) are present in the auriferous

22

sulfide–calcite–quartz ore veins: CO2-bearing (C) and H2O-rich (W) type. Petrographic and

23

microthermometric analyses of the FIs yielded homogenization temperatures for Stage 1, 2, and 3

24

to be 254–377, 191–339, and 134–223 °C, respectively, with corresponding salinities of 2.22–10.37,

25

2.23–9.98, and 1.56–4.94 wt.% NaClequiv. Pressures of Stage 1 and 2 mineralization are estimated

26

to be 280–370 and 170–300 MPa, respectively. δ18O and δD values are determined to be 9.8–10.1‰

27

and −70.2 to −68.7‰ (Stage 1), 7.4–8.1‰ and −72.4 to −71.2‰ (Stage 2), and 2.7 to 2.9‰ and

28

−79.1 to −73.0‰ (Stage 3), respectively. These results indicate that the primary ore-forming fluids

29

were derived from a metamorphic source. For the auriferous sulfides, their δ34S values are of −15.4

30

to −7.5‰, whilst their

31

15.637−15.769, and 18.301−20.936, respectively. Both the stable and radiogenic isotopic data

32

indicate that ore-forming fluids and metals were derived from a deeper and higher metamorphic

33

grade source (e.g. underlying metamorphosed rocks). Fluid immiscibility and fluid–rock

34

interactions were likely critical for the gold ore precipitation. The Hengjiangchong deposit exhibits

35

many features of orogenic gold deposits, such as the structural control on orebody distribution,

36

alteration and mineralization styles, and FI microthermometric and H–O–S–Pb isotopic features.

37

Therefore, the Hengjiangchong is best classified as an orogenic gold deposit.

38

Keywords: Orogenic gold deposit; Fluid inclusions; Multi-isotopic (H-O-S-Pb) geochemistry;

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Hengjiangchong gold deposit; Jiangnan Orogen, South China

40

208Pb/204Pb, 207Pb/204Pb,

and

206Pb/204Pb

values are of 38.663−44.861,

41

1 Introduction

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Orogenic gold deposits constitute a major supply of gold worldwide (Groves, 2003; Goldfarb

43

and Groves, 2015). These deposits are typically associated with low-grade metamorphic host rocks

44

and convergent plate margins (including accretionary and collisional orogens), with wide variation

45

in mineralization ages and styles (Groves et al., 1998; Kerrich et al., 2000; Goldfarb et al., 2005;

46

Goldfarb and Groves, 2015). In China, orogenic gold deposits account for about half of its total gold

47

resource, and are distributed in six major gold provinces (Zhou et al., 2002; Deng et al., 2016): (1)

48

Jiaodong province along the southeastern margin of North China Craton; (2) Northern China

49

province along the northern margin of North China Craton; (3) Qinling and Xiaoqinling provinces

50

along the North China-Yangtze suture zone and southern margin of North China Craton,

51

respectively; (4) Central Asian Orogenic Belt in northern China; (5) Tibetan and Sanjiang Orogens

52

in SW China; and (6) Jiangnan Orogen in South China.

53

Different from many orogenic gold deposits that were formed in convergent plate margins

54

(accretionary and collisional orogens; Groves et al., 1998; Goldfarb and Groves, 2015), gold

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deposits in the Jiangnan Orogen are widely interpreted to have formed in an intracontinental

56

orogenic setting (Zhao et al., 2013; Ni et al., 2015; Zhu et al., 2015). Located between the Yangtze

57

and Cathaysia blocks, the Jiangnan Orogen was formed during the Grenvillian orogeny (ca. 1.1–0.9

58

Ga) (Fig. 1a) and related to Rodinia supercontinent assembly (Li et al., 1995, 2002; Guan et al.,

59

2013; Li et al., 2015; Li et al., 2018, 2019). The orogen contains over 250 Au–polymetallic deposits

60

with a reserve of ~970 t (tonnes) Au (Xu et al., 2017a; 2017b). To date, over 125 Au–polymetallic

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deposits have been discovered in northeastern (NE) Hunan Province (central Jiangnan Orogen). The

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Au deposits are mainly hosted in the Lengjiaxi Group (Deng et al., 2017), and their distribution

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controlled mainly by EW-trending thrust–ductile shear zones or NE- to NNE-trending strike-slip

64

faults (Fig. 1b), but their orebodies are located predominantly in NW- to WNW- or NE-striking

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inter-/intra-layer faults (Xu et al., 2017a; 2017b). Many gold mineralization in NE Hunan is spatially

66

associated with granitoids (Deng et al., 2017; Xu et al., 2017b and references therein), and the

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orebodies are largely hosted within Neoproterozoic and lower Paleozoic sequences (Luo, 1990; Hu

68

et al., 1995; Mao and Li, 1997; Dong et al., 2008; Han et al., 2010; Huang et al., 2012).

69

The various space-time relationships between Au mineralization and granitoids and the

70

complexity of metallogenic material sources have evoked different views on the Au metallogeny in

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NE Hunan. Some studies proposed that the Au ore-forming materials were sourced primarily from

72

the Neoproterozoic host rocks (Pirajno and Bagas, 2002; Zhao et al., 2013; Ni et al., 2015; Zhu and

73

Peng, 2015; Deng and Wang, 2016; Liu et al., 2019), whereas some other studies proposed that the

74

Au ore-forming materials in some deposits were magmatic-derived (Liu and Wu, 1993; Mao et al.,

75

2013; Cao et al., 2015; Deng et al., 2017; Xu et al., 2017b). Gold deposits such as the Huangjindong,

76

Wangu, and Nanjiao deposits were once considered to be telethermal (Xu et al., 1965), syn-

77

magmatic epi-/meso-thermal (Xu et al., 1965), or syn-diagenetic sedimentary (Meng and Xie, 1965).

78

More recent studies on these Au deposits in NE Hunan suggested them as orogenic (also referred to

79

as mesothermal or shear-zone-type), epithermal, or magmatic–hydrothermal types (Mao and Li,

80

1997; Dong et al., 2008; Zhao et al., 2013; Goldfarb et al., 2014). Thus, despite decades of research

81

and numerous studies, there is no consensus regarding the deposit type and metallogenic processes.

82

The Hengjiangchong Au deposit is one of the latest-discovered in NE Hunan, and has a total

83

resource of 1.9 t Au at 1.05–384.73 g/t (mean 3.12 g/t). The Hengjiangchong deposit represents a

84

good target to investigate the Au ore-forming material sources and the possible relationship between

85

Au mineralization, structures and granitoids nearby, because of the presence of NW-/NNW-trending

86

ductile shear zones and the Hengjiangchong granite in/around the deposit. In this paper, we present

87

new information on the deposit geology, fluid inclusion (FI) thermometry, and mineral H–O–S–Pb

88

isotopes of the Hengjiangchong deposit. We discuss the source and evolution of its ore-forming

89

fluids and its metal sources, as well as any implications on regional Au metallogeny in the NE

90

Hunan section of the Jiangnan Orogen.

91

2 Regional geology

92

Northeastern Hunan is located in the central Jiangnan Orogen (Xu et al., 2009) (Fig. 1).

93

Outcropping sequences in the region include Neoproterozoic Lengjiaxi Group (Gp.)

94

metasedimentary rocks and Mesozoic proluvial–alluvial–neritic–fluvial red clastic beds. The

95

Archean–Neoproterozoic Lianshan Gp. metamorphic rocks are also found around Huangjindong

96

(Fig. 1b). These metamorphic rocks comprise a series of amphibolite-facies biotite schist (volcanic–

97

sedimentary protoliths), and interlayers of plagioclase gneiss and plagioclase amphibolite.

98

Proterozoic Jiangxichong meta-volcanic/clastic rocks and magnetite quartzite are also present (ca.

99

2.0–1.8 Ga; Jia and Peng, 2005).

100

Regional faults in NE Hunan comprise three sets: NW-, EW-, and NE-/NNE-trending. After

101

the early Neoproterozoic Orogeny (Wuling Orogeny), N–S-directed compression has intensely

102

folded the Lengjiaxi Gp. sequences (Fig. 1b). The following middle Neoproterozoic Orogeny

103

(Xuefeng Orogeny) formed the second, NW-trending deformation phase (folding and faulting) (Xu

104

et al., 2017a; 2017b). EW-trending Caledonian structures include overturned/tight folds and

105

associated ductile compressive shear zones (Xu et al., 2017a; 2017b; Deng et al., 2017). According

106

to geophysical data, many of these E-trending folds may extend through the crust (Xu et al., 2009;

107

Wen et al., 2016; Deng et al., 2017). During the early Mesozoic, the Indochina-South China collision

108

may have shifted the regional stress direction into NW–SE (Chu et al., 2012; Wang et al., 2012),

109

forming a series of (thrust-)folds with ENE-/NNE-trending axes (Li et al., 2011; Xu et al., 2009),

110

while NS-directed compression may have continued but weakened. Under the influence of different

111

stress regimes, a series of NE-oriented tectonic belts (fold, ductile shear, and imbricated thrust-fold)

112

formed in NE Hunan (Xu et al., 2017b). During the late Mesozoic, the major regional stress direction

113

changed to WNW, and reactivated preexisting E- and NE-/NW-trending faults (as normal faults)

114

and formed a dominantly NNE-trending structural framework (Xu et al., 2017b).

115

Four magmatic episodes were identified in NE Hunan, i.e., during the Neoproterozoic, early

116

Paleozoic, early Mesozoic, and late Mesozoic (Zhou et al., 2006; Zhao and Cawood, 2012; Charvet,

117

2013; Shu et al., 2014, 2015). During the late Neoproterozoic, mantle plume-related or continental

118

arc magmatism may have rifted apart the South China Block, forming the South China Rift Basin

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and extensive magmatism. Magmatic peaks occurred at ca. 825 Ma, 800 Ma,

120

(Li et al., 2015). Representative rift-related granites in NE Hunan include the Changsanbei,

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Daweishan, Getengling, and Banbei plutons (Xu et al., 2017). During the early Paleozoic, intensive

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intracontinental orogeny may have closed the South China Rift Basin (Faure et al., 2009; Charvet

123

et al., 2010; Li et al., 2015), and led to crustal melting, syn-orogenic intrusion (e.g., the Banshanpu

124

and Hongxiaqiao granites), and high-grade metamorphism (Li et al., 2015). During the early

125

Mesozoic (ca. 250–230 Ma; Carter et al., 2008), the PaleoTethys closure had caused the South

126

China–Indochina and North China–South China collisions along the Ailaoshan-SongMa and

127

Qinling–Dabie suture zones, respectively. The post-orogenic extension soon after (ca. 230–210 Ma)

128

may have induced the widespread regional granitic intrusion, including in NE Hunan (Li et al., 2014;

780 Ma, and 750 Ma

129

2015). During the late Mesozoic (ca. 136–85 Ma), thickening of the South China crust due to the

130

NW-directed Paleo-Pacific subduction was followed by extension and then collapse, forming a

131

series of NE-oriented grabens, extensional domes, and the accompanied regional plutonism (e.g.,

132

Lianyunshan granite) and volcanism (Zhang et al., 2008, 2012; Yi et al., 2010; Li et al., 2012, 2013b,

133

2014).

134

3 Deposit geology

135

Outcropping sequences in the Hengjiangchong deposit (from base to top) include the

136

Huanghudong and Xiaomuping Formations (Fms) of the Neoproterozoic Lengjiaxi Gp. (Fig. 2). The

137

Huanghudong Fm. is distributed mainly in southwestern part of the deposit, and comprises mainly

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grey–greyish-green meta-sandstone/greywacke and slate. The Xiaomuping Formation is the major

139

ore host, and comprises dominantly meta-sandstone and slate with tuffaceous- and calcareous

140

turbiditic protoliths.

141

WNW-trending faults and ductile shear zones, and NE-/NNE-trending faults are well-

142

developed at Hengjiangchong (Fig. 2). Clearly-defined WNW-trending ductile shear zones (0.5–1.0

143

km long and 20–80 m wide) are developed along the sandy slate layers with quartz vein intrusion.

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Quartz veins in the shear zones show plastic deformation, with S–C fabrics being locally observed.

145

Gold orebody and granite distributions at Hengjiangchong are strictly controlled by the WNW-

146

trending ductile shear zones. The NW-trending faults are distributed at southwestern

147

Hengjiangchong, which dip toward 020–040° (dip angle: 30–70°) and extend 1–2 km along strike.

148

The NE- to NNE-trending faults, including the Daguanchong–Jinjiang fault (F1) and the

149

Zimuchong–Lishupo–Shamaojian fault (F2), dip toward 280°–320° (dip angle: 47–80°) and extend

150

500–1300 m along strike (3–15 m wide) (Fig. 2). The Hengjiangchong Au orebodies are cut by NE-

151

to NNE-trending post-mineralization faults (Fig. 2).

152

Granite is the dominant magmatic rock type exposed at Hengjiangchong, and intruded the ore-

153

hosting Lengjiaxi Gp. sequences. The granite is medium- to fine-grained and massive structure, and

154

has mainly K-feldspar, plagioclase, quartz, and biotite, along with accessory zircon and sphene.

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Silicic, chlorite, sericite, and (arseno)pyrite alteration halos are developed around the auriferous

156

veins.

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Three Au ore belts (No. I to III) are developed at Hengjiangchong (Fig. 2), among which

158

Orebody No. III is the largest and accounts for over 60% of the total Au resource of the deposit (Fig.

159

3). Gold mineralization occurs predominantly within the ductile shear zones and minor in the altered

160

granitic rocks (Fig. 4). Orebodies are aligned (sub)-parallel to the WNW-trending ductile shear

161

zones. Exposed orebodies are 200–800 m long and 1.2–12.1 m wide, and have average (for each

162

individual orebody) Au grades ranging 1.05–6.44 g/t. The Au ore belts strike 280–310° and dip to

163

the NE at 10–48°. Ore textures at Hengjiangchong include carbonate–quartz-vein (dominant) (Figs.

164

4a–e), disseminated (Fig. 4g), and breccia (Fig. 4h). Carbonate–quartz-vein-textured ores are found

165

mainly in the ductile-deformed slate and granite (Figs. 4f, i, l). Metallic minerals in the vein-type

166

ores include mainly native gold, electrum, pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, and

167

pyrrhotite (Figs. 4–5), whilst non-metallic minerals include mainly quartz, calcite, sericite, and

168

chlorite. Ores in altered rocks are mainly located in alteration halos along ore veins (Figs. 6b–c),

169

with disseminated metallic minerals such as pyrite, pyrrhotite, and arsenopyrite (Fig. 4g). Ores in

170

breccias are rare and contain mainly disseminated pyrite and arsenopyrite (Figs. 4h). The breccias

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contain clasts of mainly granite and metamorphic rocks. Quartz and calcite are the major minerals

172

in the matrix.

173

Ore minerals are dominated by native gold, electrum, pyrite, arsenopyrite, sphalerite, galena,

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pyrrhotite, and chalcocite, whilst major gangue minerals include quartz, calcite, sericite, and chlorite

175

(Figs. 5–6). Native gold and electrum occur principally within pyrite and arsenopyrite in fissures,

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interstitial, and inclusions (Figs. 5a–e).

177

Mineral textures of the Hengjiangchong ores include granular, metasomatic, inclusion, and

178

exsolution (Fig. 5). Wall-rock alteration styles include silicic, carbonate, sericite, and chlorite (Figs.

179

6a–i). Based on vein crosscutting relationships and mineral assemblages, alteration/mineralization

180

at Hengjiangchong can be divided into three stages (Figs. 6j–l and Fig. 7): pyrite–arsenopyrite–

181

calcite–quartz, with minor chalcopyrite (Stage 1), native gold–polymetallic sulfide–calcite–quartz

182

(Stage 2), and calcite–quartz (Stage 3).

183

4 Samples and analytical methods

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4.1 Sampling

185

Doubly-polished thin-sections were prepared for 13 quartz samples from the three

186

alteration/mineralization stages. Six representative samples from the three stages were also analyzed

187

for their O and H isotopes. All the samples were crushed and sieved to 40–80 mesh. Three pyrite,

188

three arsenopyrite, and three ore-bearing granite samples from Hengjiangchong were crushed to 200

189

mesh for the S and Pb isotope analyses.

190

4.2 Fluid inclusion analyses

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Fluid inclusion (FI) petrography was observed with an optical microscope on doubly-polished

192

sections (200–300 μm thick). Microthermometric analysis was conducted at the Key Laboratory of

193

Metallogenic Prediction and Geological Environment Monitoring of Non-ferrous Metals, Ministry

194

of Education, Central South University (Changsha, China). The analysis was conducted on a

195

THMSG600 heating–freezing platform (Linkam Scientific, UK), which has a temperature range of

196

−196 to 600 °C. The maximum testing errors are ±1 °C (in the 30–600 °C range) and ±0.1 °C (in

197

the −196 to 30 °C range). During the analyses, the rate of temperature change was controlled at 5

198

to 10 °C/min, and changed to 0.1−1 °C/min when the temperatures approached gas-liquid phase

199

transition.

200

Compositions of individual FIs were determined with a LABHRVIS HR800 Laser Raman

201

spectrometer at the Analysis and Testing Research Center of Nuclear Industry, Institute of Geology,

202

Beijing, China. Analytical conditions include 532 nm Ar+ laser wavelength, 20 s analysis time (per

203

FI), 100 to 4200 cm−1 spectral region, ±2 cm−1 spectral resolution, and 1 μm beam spot diameter.

204

4.3 Hydrogen-oxygen isotope analyses

205

Hydrogen-oxygen (H–O) isotopic analyses were performed at the same laboratory as the FI

206

analyses, using a Finnigan MAT 253 mass spectrometer. The quartz samples were collected from

207

various types of quartz veins, and the quartz was crushed and handpicked to reach >99% purity. For

208

the O isotope analysis, the oxygen was liberated from quartz by reaction with BrF5 and converted

209

to CO2 on a Pt-coated carbon rod (Clayton and Mayeda, 1963). Water in the FIs from the quartz

210

samples was released by heating the samples to > 500 °C in an induction furnace, and then reacting

211

the FIs with Zn powder at 410 °C to generate the hydrogen gas for the H isotope analysis (Friedman,

212

1953). The results are reported in per mil (‰) relative to Vienna Standard Mean Ocean Water (V-

213

SMOW), and the precisions were ±2‰ for δD and ±0.2‰ for δ18O. The ore-fluid δ18OH2O values

214

(determined from the quartz samples) were calculated with the equation 1000lnαquartz–H2O = 3.38 ×

215

106 T−2 − 3.40 (Clayton et al., 1972), where α = fractionation factor and T = mean FI homogenization

216

temperature of a particular alteration/mineralization stage.

217

4.4 Sulfur–lead isotope analyses

218

Sulfur–lead (S–Pb) isotope analyses were performed at the same laboratory as the H-O isotope

219

analyses. For the S isotope analysis, the samples (pyrite, galena, pyrrhotite) were collected from

220

quartz–sulfide veins, and then crushed and handpicked to >99% purity. Sulfur isotope analyses were

221

conducted with a MAT 253 gas isotope mass spectrometer. SO2 gas was emitted by combustion of

222

sulfide samples at 1000 °C with V2O5. Analyses of the sulfate minerals required H2S to be prepared

223

by reaction with KIBA solution at 350 °C, which was subsequently converted to Ag2S and oxidized

224

with V2O5 to SO2 at 1000 °C. The SO2 was then analyzed by mass spectrometry. The S isotope

225

ratios are reported relative to the CDT (Canyon Diablo Troilite) standard in δ34S notation. The

226

sulfide reference standards used are the GBW–04414 and GBW–04415 silver sulfide standards,

227

with δ34S = −0.07 ± 0.13‰ and 22.15 ± 0.14‰, respectively. Analytical precision was better than

228

±0.2‰. Lead isotope analyses were performed with an IsoProbe–T thermal ionization mass

229

spectrometer (TIMS). Lead was separated and purified with the conventional cation-exchange

230

technique (AG1-X8, 200–400 resin), with diluted HBr used as the eluent. The

231

207Pb/206Pb,

232

± 0.00033 (2σ), and 0.059042 ± 0.000037 (2σ), respectively.

233

5 Results

234

5.1 Fluid inclusions

235

208Pb/206Pb,

and 204Pb/206Pb ratios of the NBS981 Pb standard were 2.1681 ± 0.0008 (2σ), 0.91464

5.1.1 Fluid inclusion types and petrography

236

At the hand specimen scales, the auriferous veins are widespread over the whole length and

237

width of early barren quartz veins (Figs. 4d–f). In detail, the early barren quartz veins have been

238

reactivated as vein-parallel fractures, with the addition of some new vein-parallel or oblique

239

fractures. All these fractures may be filled with fine auriferous veins (Stage 1 and 2) to form banded

240

ores at Hengjiangchong (Figs. 4d–f). At the optical microscopic scales, the auriferous sulfide ore

241

veins (Stage 1 and 2) clearly widespread overprint the early barren quartz veins (Figs. 8a–c), which

242

was also reported in the Huangjingdong deposit nearby (Zhang et al., 2019). In places, a few late,

243

thin ore-barren carbonate–quartz veins (stage 3) crosscut both the early barren and auriferous (Stage

244

1 and 2) veins (Figs. 6k–l). Abundant FIs trapped in quartz from auriferous carbonate–quartz veins

245

of the Stage 1, 2 and 3 are observed. These isolated or randomly-clustered FIs in the quartz crystals

246

are interpreted as primary (Roedder, 1984; Figs. 8d–h). Based on their phase features under room

247

temperature, these primary FIs were divided into the CO2 (C)- and aqueous (W)-type (Figs. 8d–f).

248

C-type FIs are found in Stages 1 and 2, whilst W-type FIs are found in all the three stages. FI

249

assemblages (FIAs) typically comprise C- and W-type (dominant) FIs. C-type FIs have variable

250

CO2 ratios, and W-type FIs have variable liquid/vapor ratios (Figs. 8d–h).

251

C-type FIs contain three phases (liquid H2O, liquid CO2, and vapor CO2) (Fig. 8d) or two

252

phases (liquid H2O and vapor CO2) at room temperature, with carbonic phases (liquid CO2 + vapor

253

CO2) occupying 45–90% of the total volume (Fig. 8e). C-type FIs (diameter: 6–14 μm, mean 8 μm)

254

are mostly oval, elongated, or irregular ellipsoidal shape. C-type FIs are either isolated or occur in

255

aligned clusters, and homogenize to the vapor phase when heated. In addition, CO2-only FIs (PC

256

(pure CO2)-type) are observed in Stage 2, with liquid ± vapor CO2 at room temperature (Fig. 8g).

257

These PC-type FIs (diameter: 5–10 μm) have rounded isometric or negative crystal shape.

258

W-type FIs have two phases at room temperature: liquid H2O and vapor H2O (Fig. 8f). They

259

are mostly oval, elongated, or irregular ellipses. The volume percentages (vol.%) of the vapor phase

260

in W-type FIs are of 10–50 vol.% (mostly 20–40 vol.%). These FIs (diameter: 4–25 μm, mean 7

261

μm) are either isolated or clustered, and homogenize to the liquid phase when heated.

262

5.1.2 Microthermometric data

263

Salinities of C-type FIs were estimated using CO2-clathrate-melting temperatures by assuming

264

a simple NaCl–H2O–CO2 system (Collins, 1979), and their densities were calculated using the

265

FLINCOR program of Brown (1989) and the equations of Brown and Lamb (1989). Salinities of

266

W-type FIs were estimated using ice-melting temperatures based on the method of Bodnar (1993),

267

and their densities were calculated using the FLINCOR program of Brown (1989). Results of the

268

microthermometric analyses of the different types of FI are listed in Table 1. Figure 9 shows

269

histograms of FI homogenization temperatures and salinities for the two types of FI from Stage 1 to

270

3.

271

Stage 1 quartz crystals have W- and C-type FIs. The final ice-melting and homogenization

272

temperatures of W-type FIs are of −6.9 to −3.2 °C and 254–338 °C (mainly 275–355 °C),

273

respectively. These FIs homogenize to the liquid phase, and their densities range from 0.70 to 0.86

274

g/cm3. The calculated salinities ranging from 5.25 to 10.37 wt.% NaClequiv. For C-type FIs, their

275

melting temperatures of the carbonic phases (TmCO2 = −61.2 to −58.3 °C) are lower than the CO2

276

triple-phase point temperature (−56.6 °C), which indicates minor dissolved components in the

277

carbonic phase, such as CH4 and N2 (Roedder, 1984). Their Tmcla values range from 6.8 to 8.6 °C,

278

corresponding to salinities of 2.22–6.12 wt.% NaClequiv, and the carbonic phase homogenizes to

279

vapor or liquid at 22.5–30.8 °C (ThCO2). C-type FIs homogenize to vapor or liquid at total

280

homogenization temperatures (Thtot) of 314–377 °C, and their densities range from 0.78 to 0.87

281

g/cm3.

282

Stage 2 quartz grains have W-, C-, and PC-type FIs. The final ice-melting and homogenization

283

temperatures of W-type FIs are of −6.6 to −1.3 °C and 191–333 °C (mainly 195–300 °C),

284

respectively. These FIs homogenize to the liquid phase, and their densities range from 0.74 to 0.94

285

g/cm3. The calculated salinities range from 2.23 to 9.98 wt.% NaClequiv. For C-type FIs, melting

286

temperatures of the carbonic phases (TmCO2) range from −60.8 to −57.6 °C. Their Tmcla values range

287

from 5.9 to 8.7 °C, corresponding to salinities of 2.62–7.64 wt.% NaClequiv, and the carbonic phase

288

homogenizes to vapor or liquid at 14.4–30.2 °C (ThCO2). C-type FIs homogenize to the vapor or

289

liquid at Thtot = 231–339 °C, and their densities range from 0.76 to 0.97 g/cm3.

290

Quartz grains from Stage 3 carbonate–quartz veins have only W-type inclusions, with freezing

291

and homogenization temperatures of −3.0 to −0.9 °C and 134–223 °C, respectively. They

292

homogenize to the liquid phase, and their densities range from 0.85 to 0.96 g/cm3. The calculated

293

salinities range from 1.56 to 4.94 wt.% NaClequiv.

294

5.1.3 Laser Raman analysis

295

CO2 is the main volatile in the measured C-type FIs from Stages 1 and 2 (Figs. 10a–c), although

296

small quantities of CH4 and N2 are also found in Stage 2 C-type FIs (Figs. 10a–c). In contrast, H2O

297

is the main volatile in W-type FIs of Stage 3 (Fig. 10d).

298

5.2 Hydrogen–oxygen isotopic compositions

299

Hydrogen and oxygen isotopic compositions of quartz from the Hengjiangchong Au deposit

300

are given in Table 2. δ18O and δD values of the Stage 1 fluids are of 9.8−10.1‰ and −68.7 to

301

−70.2‰, respectively. For Stage 2 and 3 fluids, their δ18O values are of 7.4−8.1‰ and 2.7−2.9‰,

302

and their δD values of −72.4 to −71.2‰ and −79.1 to −73.0‰, respectively.

303

5.3 Sulfur–lead isotopic compositions

304

Results of the S isotope analyses of the Hengjiangchong gold deposit are listed in Table 3. δ34S

305

values of the pyrite, pyrrhotite and arsenopyrite samples are of −15.4 to −7.5‰ (mean −11.3‰; n

306

= 7). δ34SV-CDT values of the granite range from −11.7 to −10.8‰.

307

Lead isotope compositions of the Hengjiangchong gold deposit are listed in Table 4. The

308

sulfide

309

18.301−20.936, respectively. Granite samples yielded

310

15.753–15.857, and 206Pb/204Pb = 19.200–21.737.

311

6 Discussion

312

208Pb/204Pb, 207Pb/204Pb

and

206Pb/204Pb

ratios are of 38.663−44.861, 15.637−15.769, and 208Pb/204Pb

= 40.253–44.234,

207Pb/204Pb

=

6.1 Source of ore-forming materials

313

Quartz (and thus ore fluid) of the main ore-forming period (Stage 1 and 2) at Hengjiangchong

314

have δ18O values of 16.2–16.9‰ and 7.4–10.1‰, respectively (Table 2), similar to those of typical

315

lode gold deposits (δ18OQtz = 10–18‰; δ18OWater = 4–15‰; McCuaig and Kerrich, 1998; Chen et

316

al., 2012). δD values of the Stage 1 and 2 ore-forming fluids (−72.4 to −68.7‰; Table 2) are also

317

within the range of typical lode gold deposits (McCuaig and Kerrich, 1998; Ridley and Diamond,

318

2000; Chen et al., 2012). In the δD vs. δ18OH2O diagram (Fig. 11), the Stage 1 and 2 samples plot

319

within the fields of metamorphic water, Archean lode gold deposits and orogenic-type gold

320

deposits. This indicates that the ore-forming fluid may have derived from a metamorphosed terrane.

321

Furthermore, the moderate–low temperature, low salinity, and CO2-rich character of the FIs from

322

the Stage 1 and 2 also suggest a metamorphic origin. The gold deposits from NE Hunan have

323

similar O isotopic compositions to those of average metamorphic water and Archean vein-type

324

gold deposits (Fig. 11), again supporting a metamorphic fluid source.

325

The δ34S values of hydrothermal minerals depend not only on the δ34S values of the material

326

source, but also on the physicochemical conditions during the S-bearing fluid migration and

327

precipitation. Ohmoto (1972) proposed that the hydrothermal mineral δ34S value is a function of

328

the hydrothermal fluid δ34S value, oxygen fugacity (fO2), temperature, pH, and ionic strength, i.e.,

329

δ34Smineral = f(δ34S∑S, fo2, T, pH, I). Field geological and microscopic observations indicate that the

330

Hengjiangchong ore minerals comprise predominantly arsenopyrite and pyrite, with minor

331

pyrrhotite, chalcopyrite, galena, and sphalerite (Figs. 4–5). Sulfate minerals are lacking, suggesting

332

that the sulfide δ34S values approximate those of the ore-forming fluids. The Hengjiangchong

333

sulfides have δ34S = −15.4 to −7.5‰, different from those of the mantle (0 ± 3‰; Chaussidon and

334

Lorand, 1990; Hoefs, 1997; Ohmoto, 1972), but similar to those of Lengjiaxi Gp. sequences and

335

Hengjiangchong granite, and those of the Wangu, Huangjindong, Xiaojiashan, Zhengchong, and

336

Yanlinsi gold deposits, whose sulfur is interpreted to have sourced from metamorphic rocks (Fig.

337

12a; Xu et al., 2017b; Zhang et al., 2018; Liu et al., 2019). Although the δ34S values of the analyzed

338

samples marginally overlap with the Lengjiaxi Gp. metamorphic rocks and Hengjiangchong

339

granite, the median and interquartile ranges are distinctly different (Fig. 12b). This minor

340

overlapping can be best explained by the limited extraction of S from the Lengjiaxi Gp.

341

metamorphic rocks and Hengjiangchong granite by deep-sourced hydrothermal fluids.

342

Pyrite from the Hengjiangchong deposit have a single-stage model age of −508 Ma to 16 Ma,

343

which indicate the presence of excess radiogenic Pb in the fluid system, due either to U-Th decay

344

or to extract Pb from other sources (Johansson, 1983; Horner et al., 1997; Chen et al., 2012). For

345

a two Pb-source end-members mixing system, Pb isotopic data should fall on a line between the

346

two end-members (Peng et al., 2000). In the

207Pb/ 204Pb

vs.

206Pb /204Pb

diagram (Fig. 13a), the

347

Hengjiangchong ore sulfide samples plot close to the upper crustal evolution line and show a linear

348

trend, indicating a mixture of two sources. The Pb isotopic similarity between some of the most

349

Hengjiangchong samples and many other deposits in NE Hunan (e.g., the Huangjindong, Wangu,

350

Zhengchong and Xiaojiashan; Fig. 13) suggests a similar Pb source, which may have been deep

351

and high-grade metamorphosed (Xu et al., 2017b; Zhang et al., 2018). The shifting of Pb isotopic

352

compositions of some samples (auriferous sulfides in mineralized granite, i.e., sample ZK0409-2

353

and ZK408-3; Table 4) from Hengjiangchong, which is markedly different from many other

354

deposits in NE Hunan and similar to Lengjiaxi Gp. sequence and Hengjiangchong granite (Fig.

355

13), most likely indicate mixing between less and more radiogenic lead sources. Such shifting in

356

Fig. 13 cannot be explained by in situ growth of radiogenic Pb because these auriferous sulfides

357

are from the same hydrothermal event and should have similar radiogenic Pb contents after

358

trapping if they are from a single system (Chen et al., 2012). Since the amount of Pb that can be

359

transported in low-salinity ore-fluids in orogenic gold systems is very limited, the Pb present at the

360

deposit would have come mainly from local source by source rock alteration (Kerrich, 1983; Zhang

361

et al., 2018), which for the case of Hengjiangchong would have been the Lengjiaxi Group and

362

Hengjiangchong granite. Therefore, the reason for this Pb isotopic shift may be that the

363

Hengjiangchong ore fluids had extracted Pb from the Hengjiangchong granite and metamorphic

364

rocks.

365

For the main ore-stage (Stages 1 and 2) at Hengjiangchong, their H–O isotopic data indicate

366

that the ore fluids were derived from metamorphic water. In a metamorphic devolatilization model

367

for formation of gold deposits (Phillips and Powell, 2010; Tomkins, 2010; Zhong et al., 2015 and

368

references therein), dehydration of hydrous and carbonaceous greenschist-facies rocks would

369

produce significant amounts of gold-bearing fluids during orogenesis and metamorphism.

370

Experimental studies demonstrate that across the greenschist–amphibolite facies boundary, chlorite

371

is broken down, and the substantial volume of fluid released may have linked to gold ore formation

372

(Tomkins, 2010; Zhong et al., 2015; Pitcairn et al., 2015). However, the oldest rocks in NE Hunan

373

are highly deformed Mesoproterozoic greenschist-facies meta-sandstone, siltstone, and slates

374

(Hunan BGMR, 1988; Wang et al., 2005; Xu et al., 2017b; Zhang et al., 2018). Furthermore, chlorite

375

+ muscovite + albite + quartz mineral assemblages in the host rocks (Neoproterozoic Lengjiaxi Gp.

376

metamorphic strata) at Hengjingchong was observed in the optical microscope (Fig. 4l), indicating

377

that the host rocks have only undergone greenschist-facies metamorphism. The host rocks have not

378

across the greenschist–amphibolite facies boundary and cannot be an effective local dominant

379

metamorphic ore fluid or metal source. Thus, the formation of ore-forming sulfur-bearing fluids

380

related to the metamorphic dehydration of chlorite require a deeper and higher metamorphic grade

381

source (e.g. underlying metamorphosed rocks; Tomkins, 2010; Zhong et al., 2015; Pitcairn et al.,

382

2015). The mechanism of autogenous fluids to release Au, S and base metals from underlying deep-

383

sourced metamorphosed rocks remains unclear, with two hypotheses being proposed: (1) These

384

elements were released during the pyrite to pyrrhotite transformation by metamorphism (Tomkins,

385

2010; Pitcairn et al., 2015; Finch and Tomkins, 2017); and (2) Most Au and a small portion of S and

386

base metals can be extracted during chlorite dehydration from a source rock (Zhong et al., 2015).

387

6.2 Fluid immiscibility and FI trapping pressure

388

Trapping pressures can be estimated only when the exact trapping temperature is known or

389

when FIs are trapped during phase separation (Roedder and Bodnar, 1980; Brown and Hagemann,

390

1995). Microscopic observations reveal the coexistence of higher-salinity W-type FIs and lower-

391

salinity C-type FIs for both Stage 1 and 2. These two types of coexisting FI have different gas/liquid

392

ratios and modes of homogenization (V → L vs. L → V), despite have a narrow range of

393

homogenization temperatures (Figs. 8g–h, 9 and 14). This indicates that the FIs were trapped

394

simultaneously within the immiscible fluid (Figs. 14–15). According to Diamond (1994), if FIs are

395

formed in an immiscible two-phase field, their trapping pressures can be approximated from the

396

end-member FIs trapped nearest to the solvus. As the fluids exhibit immiscibility features, the

397

homogenization temperatures can be interpreted as representing the trapping temperatures (Li et al.,

398

2012). The average salinity of Stage 1 and 2 is 5.8 wt.% and 6.7 wt.% NaClequiv, respectively.

399

Therefore, a NaCl–H2O–CO2 phase diagram with a 6 wt.% NaClequiv salinity was used to estimate

400

the ore-forming pressure (Brown and Lamb, 1989). The trapping pressures and temperatures

401

estimated for Stage 1 are ca. 280–370 MPa and 314–329 °C (Fig. 16a), respectively, whilst for Stage

402

2 they are 170–300 MPa and 231–294 °C (Fig. 16b).

403

Based on the estimated pressures for the Hengjiangchong mineralization, the lithostatic

404

pressure is estimated using H = p/(ρg), where ρ refers to the average density (2.70 g/cm3) of continental

405

rock and g = 9.8 m/s2. This calculation yields the mineralization depths for Stage 1 and 2 to be around

406

10–13 km and 6–11 km, respectively, which are typical of mesozonal (6–12 km deep) orogenic Au

407

deposits worldwide (Groves et al., 1998; Kerrich et al., 2000).

408

6.3 Ore fluid evolution

409

Petrographic features and microthermometric data of the FIs from Hengjiangchong

410

demonstrate that several types of FI are present in the Stage 1 to 3 veins. From Stage 1 to 3, the

411

marked changes in the types of FI reflect evolution in the hydrothermal fluid temperature, salinity,

412

and pressure (Fig. 15), as described below:

413

The Stage 1 and 2 ore-fluid temperatures are calculated to be 254–377 °C and 191–339 °C,

414

respectively, whereas their salinities are of 2.22–10.37 wt.% and 2.23–9.98 wt.% NaClequiv,

415

corresponding to 280–370 MPa and 170–300 MPa. The presence of pyrite and arsenopyrite in the

416

orebodies indicates that H2S was present in the ore fluids. Furthermore, mineral assemblages in the

417

Stage 1 and 2 auriferous veins include carbonates, chlorite, quartz, and sericite. The absence of K-

418

feldspar implies that the fluid pH was close to neutral (Johnson et al., 1991; Mikucki and Ridley,

419

1993). Under 200–400 °C and near-neutral pH fluid conditions, Au would have migrated mainly in

420

the form of Au(HS)2− (Seward, 1973; Cole and Drummond, 1986; Hayashi and Ohmoto, 1991;

421

Stefánsson and Seward, 2004). The Hengjiangchong gold-quartz-carbonate vein is characterized by

422

vein-parallel banding structure (Figs. 4d–f and 8a–c), indicating multiphase hydrothermal activity

423

and mineral precipitation in the same calcite–quartz veins. This can be explained by episodic fluid

424

pressure (supralithostatic vs. hydrostatic) fluctuation accompanied by episodic opening and closing

425

of sub-horizontal fractures (Sibson et al., 1988; Goldfarb et al., 2005; Chi and Guha, 2011). Pressure

426

fluctuations, particularly for quartz vein-hosted orebodies, would lead to fluid immiscibility during

427

the transient pressure drop, and facilitate gold deposition (Goldfarb et al., 2005). Our FI analysis

428

suggest that large volumes of CO2-bearing fluids were present during Stage 1 and 2 mineralization.

429

According to Philips and Evans (2004), CO2 could have caused the fluid pH to stabilize and maintain

430

the Au solubility. The ore-fluid immiscibility may have caused phase separation and CO2 loss,

431

pushing Equation 1 to the left and Equation 2 to the right. This likely destabilized the Au complexes

432

and resulted in Au precipitation:

433

CO2 + H2O ⇌ H2CO3 ⇌ H+ + [HCO3]− ⇌ 2H+ + CO32−

434

Au(HS)2− + 0.5H2O ⇌ Au0 + 2HS− + H+ + 0.25O2

435

(1) (2)

The solubility of CO2 is lower than that of H2S (Reed and Spycher, 1985; Spycher and Reed, 1985),

436

which means that as the ore-fluid phase separation proceeded, the escaping volatiles changed from

437

CO2 to H2S and pushed reaction of Equation 3 to the right:

438

Au(HS)2− + H+ + 0.5H2(aq) ⇌ Au0 + 2H2S

(3)

439

This destabilized the Au bisulfide complexes and resulted in Au precipitation. The CO2 degassing

440

could have intensified the S2− activity, and the remaining S2− may have combined with As3− and

441

metal ions (e.g., Cu2+, Fe2+, Pb2+, Zn2+) to form quartz–calcite–pyrite–arsenopyrite veins and quartz–

442

calcite–polymetallic sulfide veins. Furthermore, interactions between the ore-fluids and host rocks

443

would have destabilized Au thiosulfate complexes, resulting in the precipitation of gold and

444

polymetallic sulfides at Hengjiangchong.

445

Our study revealed that W-type FIs are dominant in the Stage 3 fluids, whereas C-type FIs are

446

absent. Compared to Stage 1 and 2, the temperature, salinity, and pressure of Stage 3 fluids

447

continued to decrease (Fig. 15a), possibly resulted from further incursion of low-temperature/-

448

salinity meteoric water. The ore precipitation and dilution of the hydrothermal fluid by meteoric

449

water likely formed the late-stage barren quartz–carbonate veins. This conclusion is further

450

supported by our FI and H–O isotopic evidence (Figs. 11 and 15).

451

6.4 Genesis of the Hengjiangchong deposit

452

Gold mineralization in the NE Hunan section of the Jiangnan Orogen has produced the

453

Hengjiangchong, Yanlinsi, Xiaojiashan, Dayan, Wangu, Huangjindong, and Zhengchong deposits

454

(Ye et al., 1988; Wang et al., 2000; Huang et al., 2012; Deng et al., 2017; Xu et al., 2017b; Liu et

455

al., 2019), and the metallogenic models have been proposed to be intrusion-related and orogenic-

456

type (Mao and Li, 1997; Dong et al., 2008; Zhao et al., 2013; Goldfarb et al., 2014; Zhang et al.,

457

2018; Liu et al., 2019).

458

Some features of the Hengjiangchong deposit, such as the partial occurrence of orebodies in

459

granitic intrusions, medium-temperature auriferous vein formation, low-salinity aqueous–carbonic

460

fluids, Au–As–Pb–Zn–Cu metal assemblage, low sulfide content, reducing ore-mineral assemblage,

461

and S–Pb isotopic features, are consistent with the intrusion-related gold deposit type (Sillitoe and

462

Thompson, 1998; Thompson et al., 1999; Lang and Baker, 2001; Baker, 2002). However,

463

differences between the Hengjiangchong and the intrusion-related type occur in the lack of W-Sn

464

mineralization, biotite and K-feldspar alteration, and the presence of late Au-bearing magmatic

465

dikes/pods (pegmatite/aplite/granite) and/or magmatic silicate melt inclusions with Au–Bi-bearing

466

sulfide droplets (Rhys, 1995; Sillitoe and Thompson, 1998; Mustard et al., 2006; Zachariáš et al.,

467

2014). Furthermore, both the FI and H–O isotopic evidence indicate that the Hengjiangchong Au

468

mineralization was not related to magmatism, and is therefore unlikely an intrusion-related type

469

deposit.

470

The lithology, structure, and alteration/mineralization features at Hengjiangchong resemble

471

typical orogenic gold deposits (Groves et al., 1998; Goldfarb et al., 2005 and references therein):

472

(1) Auriferous carbonate–quartz ore veins are structurally controlled, by WNW-trending ductile

473

shear zones in the case of Hengjiangchong. (2) Gold orebodies are hosted in deformed metamorphic

474

rocks. At Hengjiangchong, they are hosted in the highly-deformed Lengjiaxi Gp. meta-graywacke

475

and slate. (3) The Hengjiangchong gold ores contain native gold, pyrite, and arsenopyrite, and minor

476

chalcopyrite, sphalerite, and galena, similar to many orogenic gold deposits. (4) Native gold occurs

477

as FIs and refractory Au in arsenopyrite and pyrite (Figs. 5a–e), a common phenomenon in many

478

orogenic gold deposits (Oberthür et al., 1994; Zoheir., 2008; Morey et al., 2008). (5) Ore-related

479

alteration is well developed and includes chlorite, sericite, silicic, and carbonate. (6) Ore-forming

480

fluids were derived from metamorphic source, and were CO2-rich H2O–NaCl–CO2 ± N2 ± CH4

481

solutions with low-medium salinities (1.56–10.37 wt.% NaClequiv) and temperatures (134–377 °C).

482

7 Conclusions

483

(1) Alteration/mineralization at Hengjiangchong comprises three stages: quartz–carbonate–pyrite–

484

arsenopyrite mineralization (Stage 1), quartz–carbonate–polymetallic sulfide (pyrite, pyrrhotite,

485

galena, sphalerite, chalcopyrite)–native gold mineralization (Stage 2), and quartz–carbonate

486

alteration (Stage 3).

487

(2) Two types of fluid inclusions (FIs) are found in the auriferous carbonate–quartz veins at

488

Hengjiangchong: CO2-bearing (C-type) and H2O-rich (W-type) FIs. The gold ore-forming fluids

489

were of medium-temperature, low-salinity and belong to the H2O–CO2–NaCl system. Fluid

490

inclusion petrographic and microthermometric data suggest the occurrence of fluid immiscibility,

491

which may have been key to gold ore deposition at Hengjiangchong. The trapping pressures

492

were estimated to be 280–370 MPa (Stage 1) and 170–300 MPa (Stage 2), with estimated

493

mineralization depths of 10–13 km and 6–11 km, respectively, resembling those of typical

494

orogenic gold mineralization.

495

(3) Isotopic geochemical features of the Hengjiangchong gold ores indicate that the ore-forming

496

fluids were likely derived from metamorphic dehydration (via broken-down of chlorite) of

497

regional (meta-) sedimentary sequences. The ore-forming materials (e.g., sulfur and lead) may

498

have derived from Lengjiaxi Gp. metamorphic sequences and Hengjiangchong granite through

499

fluid–rock interactions.

500

(4) The Hengjiangchong gold deposit shares many geological and geochemical similarities with

501

typical orogenic gold deposits, but displays major differences from typical intrusion-related gold

502 503

deposits. Thus, the Hengjiangchong gold deposit is best classified as orogenic-type.

Acknowledgments

504

We appreciate the Editor Prof. Jun Deng for handling the manuscript and the insightful review.

505

We sincerely thank the constructive suggestions from two anonymous reviewers, which greatly

506

improved this paper. This study was supported by the Innovation-driven Plan of Central South

507

University (2018zzts196), National Natural Science Foundation of China (41702078), Hunan

508

Geoscientific Research Project of the Hunan Land and Resources (2016-04), and the General

509

Financial Grant from the China Postdoctoral Science Foundation (2017M622596). We are grateful

510

to the staffs from #416 Brigade of Hunan BGMR for their field assistance.

511

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512

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fluid inclusion, and isotope constraints on ore genesis of the Neoproterozoic Jinshan orogenic

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gold deposit, South China. Geofluids 13, 506–527.

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during metamorphic devolatilization of a pelite. Geochimica et Cosmochimica Acta 171, 338–

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Zhou, T.H., Goldfarb, R.J., Phillips, G.N., 2002. Tectonics and distribution of gold deposits in China –an overview. Mineralium Deposita 37, 249–282. Zhou, X.M., Sun, T., Shen, W.Z., Shu, L.S., Yao, L., 2006. Petrogenesis of Mesozoic granitoids and volcanic rocks in South China: a response to tectonic evolution. Episodes 29, 26–33.

777

Zhu, Y., Peng, J., 2015. Infrared microthermometric and noble gas isotope study of fluid inclusions

778

in ore minerals at the Woxi orogenic Au-Sb-W deposit, western Hunan, South China. Ore Geol.

779

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780 781

Figure captions

782 783 784

Fig. 1. Location map (a) and simplified geologic map (b) of the central Jiangnan Orogen in NE

785

Hunan (modified from Deng et al., 2017 and Xu et al., 2017b).

786 787

Fig. 2. Simplified geologic map of the Hengjiangchong gold deposit (modified from Hunan BGMR,

788

2016).

789 790

Fig. 3. Generalized geologic cross-section along Prospecting Line No. 48 (across Orebody No. III

791

orebody) of the Hengjiangchong gold deposit (modified from Hunan BGMR, 2016; location shown

792

in Figure 2).

793 794

Fig. 4. Photographs and photomicrographs of the Hengjiangchong gold ores. (a) NW-trending

795

orebodies in Hengjiangchong granite. (b) Orebodies in Hengjiangchong granite. (c) Native gold–

796

pyrite–arsenopyrite–quartz–carbonate vein in granite. (d) Auriferous pyrite–arsenopyrite–quartz–

797

calcite vein in altered slate. (e) Auriferous pyrite–arsenopyrite–quartz–calcite vein in altered slate.

798

(f) Auriferous pyrite–quartz–calcite vein in mylonitized granite. (g) Disseminated pyrite and

799

pyrrhotite in granite. (h) Auriferous pyrite–quartz–calcite vein in altered/brecciated slate. (i)

800

Disseminated pyrite in mylonitized granite. K-feldspar, quartz, sericite, and pyrite are (sub)-parallel

801

lineated, and quartz has wavy extinction (cross-polarized light). (j) Disseminated pyrite occurring

802

in altered slate (cross-polarized light). (k) Disseminated pyrite occurring in altered sandy slate

803

(cross-polarized light). (l) Mylonitized quartz grains with wavy extinction (cross-polarized light).

804

Abbreviations: Cal: Calcite; Py: Pyrite; Po: Pyrrhotite; Asp: Arsenopyrite; Au: Native gold; Qtz:

805

Quartz; Ser: Sericite; Kfs: K-feldspar; Pl: Plagioclase; Chl: Chlorite; Ep: Epidote.

806 807

Fig. 5. Photomicrographs of the Hengjiangchong gold ores. (a) Native gold among euhedral–

808

subhedral pyrite and arsenopyrite grains in arsenopyrite–pyrite–carbonate–quartz vein (Stage 2)

809

(reflected light). (b) Native gold among pyrite and arsenopyrite grains in arsenopyrite–pyrite–

810

carbonate–quartz vein (Stage 2). Arsenopyrite replaced by galena and pyrrhotite (reflected light).

811

(c) Native gold among pyrite grains, which is replaced by chalcopyrite and contains euhedral

812

arsenopyrite (Stage 1) (reflected light). (d) Native gold at arsenopyrite grain boundaries, which is

813

replaced by galena and pyrrhotite (Stage 2) (reflected light). (e) Native gold, pyrite, and chalcopyrite

814

in fissures of arsenopyrite, which is replaced by pyrrhotite (Stage 2) (reflected light). (f) Euhedral

815

arsenopyrite replaced by sphalerite, pyrite, and chalcopyrite (Stage 2) (reflected light). (g)

816

Euhedral–subhedral pyrite and arsenopyrite (Stage 1) (reflected light). (h) Euhedral–anhedral pyrite

817

and pyrrhotite. Pyrrhotite has euhedral arsenopyrite, and is replaced by galena and chalcopyrite

818

(Stage 2) (reflected light). (i) Anhedral pyrite (Stage 1) (reflected light). Abbreviations: Ccp:

819

Chalcopyrite; Sp: Sphalerite, and as in Figure 4.

820

821

Fig. 6. Photographs and photomicrographs of host-rock alteration and cross-cutting relationships.

822

(a) Hand-specimen of quartz–calcite–pyrite–arsenopyrite vein. (b) Silicic and sericite alteration halo

823

along vein. (c) Silicic, carbonate and chlorite alteration halo along vein. (d) Silicic and carbonate

824

alteration. (e) Silicic, carbonate, sericite and chlorite alteration. (f) Silicic, carbonate and sericite

825

alteration (cross-polarized light). (g) Subparallel-lineated hydrothermal sericite (cross-polarized

826

light). (h) Recrystallized hydrothermal quartz (cross-polarized light). (i) Subparallel-lineated

827

hydrothermal sericite and pyrite (cross-polarized light). (j) Stage 1 pyrite–arsenopyrite–calcite–

828

quartz vein cut by Stage 2 polymetallic sulfide–native gold–calcite–quartz vein. (k) Stage 1 pyrite–

829

arsenopyrite–calcite–quartz vein cut by Stage 3 calcite–quartz vein. (l) Polymetallic sulfide–native

830

gold–calcite–quartz vein of Stage 2 cut by calcite–quartz vein of Stage 3. Abbreviations as in Figure

831

4.

832 833

Fig. 7. Alteration/mineralization paragenesis of Stage 1 to 3 for the Hengjiangchong Au deposit.

834 835

Fig. 8. Photomicrographs of various types of FIs from the Hengjiangchong gold deposit. (a) Pyrite–

836

carbonate–quartz vein (Stage 1) intruded early barren carbonate–quartz vein (plane-polarized light).

837

(b) Pyrite–carbonate–quartz vein (Stage 1) intruded early barren carbonate–quartz vein (cross-

838

polarized light). (c) Polymetallic sulfide carbonate–quartz vein (Stage 2) intruded early barren

839

carbonate–quartz vein (plane-polarized light). (d) C-type inclusion composed of vapor CO2, liquid

840

CO2, and liquid H2O. (e) C-type FIs composed of vapor CO2 and liquid H2O. (f) W-type FIs

841

composed of vapor H2O and liquid H2O. (g) PC-type FIs composed of vapor CO2 and liquid CO2.

842

(h) Stage 1 FI assemblage of C-type FIs with variable CO2 ratios and W-type FIs.

843 844

Fig. 9. Histograms of homogenization temperatures and salinities of FIs from the Hengjiangchong

845

gold deposit.

846 847

Fig. 10. Representative Raman spectra of vapor bubbles of FIs in quartz from the Hengjiangchong

848

gold deposit. (a–c) Vapor bubbles of Stage-1/-2 C-type FIs, with CO2 plus minor CH4 and N2. (d)

849

Vapor bubbles of Stage-3 W-type FIs, containing mostly H2O.

850

851

Fig. 11. Fluid δD vs. δ18OH2O diagram for the Hengjiangchong Au deposit. The magmatic and

852

metamorphic fluid δ18O fields were modified after Taylor (1974). The Archean lode gold deposits

853

field was modified after Chen et al. (2012) and references therein. The orogenic gold deposits field

854

was modified after Zhang et al. (2018). Data for the Huangjindong, Zhengchong, Yanlinsi, and

855

Wangu gold deposits are from Liu and Wu (1993), Mao and Li (1997), Deng et al. (2017), and Liu

856

et al. (2019).

857 858

Fig. 12. Sulfur isotopic compositions of sulfides and granite from the Hengjiangchong gold deposit,

859

and gold deposits and rocks in the region. Data sources: Hengjiangchong gold deposit (this study),

860

Lengjiaxi Group (Luo, 1990; Liu et al., 1999), regional gold deposits (Huangjindong, Yanlinsi,

861

Xiaojiashan, Zhengchong and Wangu) (Luo, 1988; Liu et al., 1999; Jiang et al., 2016; Xu et al.,

862

2017; Zhang et al., 2018; Liu et al., 2019).

863 864

Fig. 13. (a) 207Pb/204Pb vs. 206Pb/204Pb and (b) 208Pb/204Pb vs. 206Pb/204Pb diagrams for sulfides and

865

granite from the Hengjiangchong gold deposit and nearby gold deposits and rocks. Data for the

866

Huangjindong, Xiaojiashan, Zhengchong and Wangu gold deposits are from Luo (1989), Deng et

867

al. (2017), Xu et al. (2017), Zhang et al. (2018), Liu et al. (2019), and our unpublished data.

868

Evolution trend lines are from Zartman and Doe (1981). Abbreviations: UC, Upper crust; O, Orogen,

869

M, Mantle; LC, Lower crust.

870 871

Fig. 14. Fluid immiscibility in Stage 1 pyrite–arsenopyrite–calcite–quartz (a-b) and Stage 2 native

872

gold–polymetallic sulfide–calcite–quartz (c-e). Numbers next to the FIs denote their

873

homogenization temperatures, and the homogenization mode include to vapor (V) or liquid (L). The

874

FIs show heterogeneous homogenization, with some homogenized to vapor and others to liquid.

875 876

Fig. 15. (a) Salinity vs. homogenization temperature for different types of FIs in Stage 1 to 3. (b)

877

Zoom-in of Figure 15a for W- and C-type FIs, showing wide salinity variations that suggests

878

possible fluid immiscibility.

879 880

Fig. 16. Representative isochores for minimum and maximum bulk densities for C-type FIs and the

881

solvus for H2O–CO2 fluids containing 6 wt.% NaClequiv (after Bowers and Helgeson, 1989).

882 883

Table captions

884 885

Table 1. Microthermometric data for FIs from the Hengjiangchong Au deposit.

886 887

Table 2. δD and δ18O values for FIs in quartz and calculated values for equilibrium fluids from the

888

Hengjiangchong deposit.

889 890

Table 3.δ34S values of pyrite, arsenopyrite, pyrrhotite and granite samples from the Hengjiangchong

891

Au deposit.

892 893

Table 4. Lead isotope values for pyrite, arsenopyrite and granite samples from the Hengjiangchong

894

deposit.

895 896

Declaration of interests

897 898 899

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

900 901 902

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

903 904 905 906 907

908 909 910 911

Highlights:

912

Orebodies of Hengjiangchong Au deposit (Jiangnan Orogen) are controlled by NW-trending faults.

913

H–O–S–Pb isotopes indicate a metamorphic source for the ore fluid and components.

914

Fluid inclusions indicate that fluid immiscibility and fluid–rock interactions likely promoted Au

915

deposition.

916

The Hengjiangchong Au deposit is best classified as orogenic type, instead of intrusion-related.

917 918

919

920

921

922

923

924

925

926

927

928

929

930

931

932

933

Stage Stage 1

Stage 2 Stage 3

934 935 936

Type

Tm( CO2)

C

–61.2 to –58.3

W

Denstiny

% NaClequiv)

(g/cm3)

314–377(L,V)

2.22–6.12

0.78–0.87

254–338(L)

5.25–10.37

0.70–0.86

231–339(L,V)

2.62–7.64

0.76–0.97

Tm(clath)

Th(CO2)

Th(tot)

　

6.8–8.6

22.5–30.8

–6.9 to –3.2

　

Salinity(wt.

Tm(ice)

C

–60.8 to –57.6

　

PC

–60.5 to –57.6

　

　 5.9–8.7 　

　 14.4–30.2 17.1–26.3

　

　

W

　

–6.6 to –1.3

　

　

191–333(L)

2.23–9.98

0.74–0.94

W

　

–3.0 to –0.9

　

　

134–223(L)

1.56–4.94

0.85–0.96

Table 1 Microthermometric data for FIs from the Hengjiangchong Au deposit.

937 938 939 940

Table 2 δD and δ18O values for FIs in quartz and calculated values for equilibrium fluids from the Hengjiangchong deposit.

Sample No. ZK408–6 ZK608–3 ZK4801–1 ZK4801–4 ZK608–2 ZKk408–3

Stage 1 2 3

δ18OV–SMOW(‰)

δDV–SMOW(‰)

δ18OH2O(‰)

–70.2 –68.7 –71.2 –72.4 –79.1

16.8 16.5 16.9 16.2 16.3

10.1 9.8 8.1 7.4 2.9

–73.0

16.1

2.7

941 942 943 944

0.71–0.80

Table 3 δ34S values of pyrite, arsenopyrite, pyrrhotite and granite samples from the Hengjiangchong Au

945

deposit. Deposit

Hengjiangchong deposit

Sample No.

Samples

Mineral//Rock

δ34SV– CDT(‰)

ZK0409-2

Mineralized granite with polymetallic sulfide–calcite– quartz vein

Pyrite

-15.4

ZK4801-5

Polymetallic sulfide–calcite– quartz vein

Pyrite

-13.2

ZK408-2

Pyrite–arsenopyrite–calcite– quartz vein

Pyrite

-9.7

ZK408-5

Mineralized granite

Pyrrhotite

-11.3

ZK4801-5

Polymetallic sulfide–calcite– quartz vein

Arsenopyrite

-13.6

D018

Mineralized granite with quartz-sulfide vein

Arsenopyrite

-7.5

ZK408-3

Mineralized granite

Arsenopyrite

-8.5

ZK608-9

unaltered granite

Granite

–10.8

ZK608-11

unaltered granite

Granite

–11.7

ZK608-13

unaltered granite

Granite

–11.4

946 947 948 949

Table 4 Lead isotope values for pyrite, arsenopyrite and granite samples from the Hengjiangchong deposit. Deposit

Hengjiangchon g deposit

Sample No.

Samples

Mineral/Roc k

206Pb/204P

207Pb/204P

208Pb/204P

b

b

b

ZK0409 -2

Mineralized granite with polymetallic sulfide– calcite– quartz vein

Pyrite

19.469

15.689

42.207

ZK4801 -5

Polymetallic sulfide– calcite– quartz vein

Pyrite

18.345

15.637

38.712

ZK4082

Pyrite– arsenopyrite –calcite– quartz vein

Pyrite

18.758

15.681

39.716

ZK4801 -5

Polymetallic sulfide–

Arsenopyrite

18.301

15.637

38.663

calcite– quartz vein

950 951

D018

Mineralized granite with quartzsulfide vein

Arsenopyrite

18.442

15.647

39.027

ZK4083

Mineralized granite

Arsenopyrite

20.936

15.769

44.861

ZK60811

Unaltered granite

Granite

20.348

15.803

43.158

ZK6089

Unaltered granite

Granite

19.200

15.753

40.253

ZK60813

Unaltered granite

Granite

21.737

15.857

44.234