Geofluids
Volume 2019 (2019), Article ID 3726465, 23 pages
https://doi.org/10.1155/2019/3726465

Research Article

Geology, Fluid Inclusion, and H–O–S–Pb Isotope Constraints on the Mineralization of the Xiejiagou Gold Deposit in the Jiaodong Peninsula

Peng Chai, Zeng-qian Hou, Hong-rui Zhang, and Lei-lei Dong

Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China

Correspondence should be addressed to Peng Chai; cx001chaipeng@163.com

Received 15 November 2018; Revised 29 December 2018; Accepted 8 January 2019; Published 3 April 2019

Academic Editor: Paolo Fulignati

Copyright © 2019 Peng Chai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The Xiejiagou deposit is a representative medium-sized gold deposit in Jiaodong the Peninsula, which contains gold reserves of 37.5 t. The orebodies are hosted in the Linglong biotite granite with a zircon LA-ICP-MS U–Pb age of (, ) and are characterized by disseminated- or stockwork-style ores. Mineralization and alteration are structurally controlled by the NE-striking fault. Three stages of mineralization were identified with the early stage being represented by (K-feldspar) sericite quartz pyrite, the middle stage by quartz gold polymetallic sulfide, and the late stage by quartz carbonate. Ore minerals and gold mainly occurred in the middle stage. Three types of primary fluid inclusions were distinguished in the Xiejiagou deposit, including carbonic-aqueous, pure carbonic, and aqueous inclusions. The primary fluid inclusions of the three stages were mainly homogenized at temperatures of 262–386°C, 192–347°C, and 137–231°C, with salinities of 2.22–8.82, 1.02–11.60, and 1.22–7.72 wt% NaCl equivalent, respectively. These data indicate that the initial ore-forming fluids were a medium temperature, CO₂-rich, and low-salinity H₂O–CO₂–NaCl homogeneous system, and the ore-forming system evolved from a CO₂-rich mesothermal fluid into a CO₂-poor fluid. Considering the fluid inclusion characteristics, H–O–S–Pb isotopes, and regional geological events, the ore-forming fluid reservoir was likely metamorphic in origin. Trapping pressures of the first two hydrothermal stages estimated from the carbonic aqueous inclusion assemblages were ~224–302 MPa and ~191–258 MPa, respectively. This suggests that the gold mineralization of the Xiejiagou gold deposit occurred at a lithostatic depth of ~7.2–9.7 km. Au(HS)₂⁻ was the most probable gold-transporting complex at the Xiejiagou deposit. Precipitation of gold was caused by a CO₂ effervescence of initial auriferous fluids.

1. Introduction

The Jiaodong Peninsula (Figure 1) is an important gold metallogenic belt in China hosting about 4000 t of gold reserves and also considered as one of the most famous gold mineralization belts in the world [1–11]. The majority of gold deposits in the region are temporally and spatially related to Mesozoic NE-trending faults, and the deposits have been classified as Jiaojia-type and Linglong-type mineralization, where the former refers to a combination of disseminated and stockwork/veinlet gold mineralization and the latter to auriferous quartz veins [1, 12–15]. The Jiaodong gold belt is dominated by Jiaojia-type gold deposits, which make up more than 80% of its gold resources [16, 17], and extensive research has been under taken on them ([14, 15, 18–22] and the references therein). Gold mineralization in the region is generally considered to have occurred during the Early Cretaceous, with a peak age at [23], although there are still controversies regarding the ore-forming age [20].

Figure 1: Geology of the Jiaodong gold province showing locations of large (>20 t Au), medium (5–20 t Au), and small (<5 t Au) gold deposits (modified after Goldfarb and Santosh, [21]).

Gold deposits are generally related to tectonic, geological, and geochemical processes, in which the nature of the hydrothermal fluids is of fundamental importance [24–26]. Several issues related to the nature and sources of the ore-forming fluids remain debated, and a number of hypotheses have been proposed. Initially, Zhang [27] speculated that the ore-forming fluids were meteoric in origin. More recently, Fan et al. [16, 17] suggested that the ore-forming fluids originated from magmatic water, and Zeng et al. [28] suggested that the gold mineralization resulted from magmatic-hydrothermal processes. In addition, because some of the orebodies within these deposits are accompanied by intermediate-mafic dikes, therefore, the ore-forming fluids have been inferred to be derived from the mantle [28], although a combination of crustal and mantle components has also been proposed [16, 17]. Alternatively, based upon ore geological characteristics and mineralization style, the Jiaodong gold mineralization has been treated as an “orogenic type” and it has been proposed that the ore-forming fluids were derived from metamorphic water [21]. Although some geologists classified these as orogenic gold deposits, the nearly 1.7-billion-year gap between gold mineralization and the latest regional metamorphism, together with the tectonic settings, has raised doubt on the orogenic gold deposit model [5, 9, 18, 23]. Therefore, it is difficult to apply the conventional classification schemes for common ore deposit types (such as epithermal gold, orogenic gold, porphyry gold,…) to define the formation of the Jiaodong gold mineralization.

The Xiejiagou deposit is a representative medium-sized Jiaojia-type gold deposit, which contains gold reserves of >35 t. Given that this deposit is an important producer of gold, some geochemical studies have been undertaken with the aim of characterizing the nature of the Au mineralization (e.g., [29–34]). Nevertheless, the nature and evolution of the fluid associated with gold mineralization have not been well documented through petrographic, isotopic, and fluid inclusion studies, which limit our understanding of gold mineralization of the Xiejiagou deposit. Based on previous geological information on the Xiejiagou deposit, in particular, petrographic, microthermometric, and Raman spectroscopic analyses for inclusions contained within different stage veins were conducted to investigate the ore-forming P-T conditions and examine the mechanisms of gold transport and deposition. In addition, based on the geologic characteristics and isotopic compositions (H, O, S, and Pb) of the ores from the deposit, we try to trace the origin of ore-forming fluids. We hope that this study can further address the implications for the better understanding of the gold mineralization in the Jiaodong Peninsula.

2. Regional Geology

The Jiaodong Peninsula, located in east Shandong Province, occupies the southeastern margin of the North China Block (NCB) is and bounded by the Tanlu fault in the west (Figure 1). This region consists of two tectonic units, the Jiaobei terrane in the west and Sulu terrane in the east, bordered by the Wulian-Qingdao-Yantai fault (WQYF) (Figure 1). The Jiaobei terrane comprises the Jiaobei uplift in the north and the Jiaolai basin in the south (Figure 1). The Jiaobei uplift is the primary host of gold on the Jiaodong Peninsula with gold reserves of more than 3600 t ([19], 90% of the proven gold resources). The main lithological units in the Jiaobei uplift include the metamorphosed Precambrian basement, as well as widespread Mesozoic intrusive rocks [35], Figure 1. The Precambrian basement includes the Neoarchean Jiaodong, Paleoproterozoic Jingshan, and Fenzishan groups [36], Figure 1. The Jiaodong group is characterized by widely distributed late Archean tonalitic, trondhjemitic, and granodioritic gneisses, as well as supracrustal amphibolites and gneisses that yield U–Pb zircon ages of mainly between 2.9 and 2.5 Ga [37]. The Paleoproterozoic Jingshan and Fenzishan groups contain widespread schist, marble, and amphibolite, plus minor amounts of mafic granulite. These groups yield metamorphic zircon ages of about 1.8 Ga and underwent amphibolite- to granulite-facies metamorphism and deformation [38]. Mesozoic granitoids that intruded into the Jiaobei uplift have been traditionally divided into the Late Jurassic Linglong granite and the Early Cretaceous Guojialing granite, with both of them being significant hosts for gold mineralization ([39–41], Figure 1). Mesozoic volcanic rocks were widespread in the Jiaolai basin (Figure 1). The volcanic sequences mainly include trachybasalt, trachyte, and rhyolite, forming at about 110–95 Ma [42]. The Sulu terrane is formed by the subduction of the Yangtze Block beneath the NCB in the Triassic [43] and is characterized by the presence of high and ultra-high-pressure (HP to UHP) metamorphic rocks as well as Late Triassic, Late Jurassic, and Early Cretaceous granitoids (Figure 1). Subsequently, these rocks were unconformably overlain by Cenozoic basalt and Quaternary sedimentary rocks (Figure 1).

Most of these gold deposits in the Jiaobei uplift are controlled by the NE- to NNE-trending fault systems ([39], Figure 1): from west to east, the Sanshandao fault (SSDF), Jiaojia fault (JJF), Zhaoping fault (ZPF), Muping-Jimo fault (MJF), and Qixia fault (QXF). These faults are subsidiary faults to the continental-scale Tanlu fault zone [44]. Among them, the ZPF is one of the most important tectonic structures controlling second-order ore-hosting faults in the Jiaobei uplift [1, 44, 45].

3. Ore Deposit Geology

The Xiejiagou gold deposit is located about 5 km southwest of Zhaoyuan city in the northwestern part of the Jiaobei uplift and near the middle part of the ZPF (Figure 1). Structurally, the mineralization and hydrothermal alteration are controlled by a series of NE-trending secondary faults (ductile shear structures and brittle faults) located in the hanging wall of the ZPF fault zone (Figure 2). The principal lithologic unit in the Xiejiagou deposit is a pluton of biotite granite, the so-called Linglong granite. Numerous intermediate-mafic dikes are widespread in the Xiejiagou gold deposit. Among them, the lamprophyre dikes (pre-ore, Figure 2(b)) are cross-cut by orebodies or faults and display intense hydrothermal alteration with a whole rock K–Ar age of 123.6 Ma [29], whereas the orebodies are cut by the NW-trending diorite porphyrite, gabbro diorite, and dolerite dikes (post-ore, Figures 2(a) and 2(b)) with whole rock K–Ar ages of 98.6 to 115.2 Ma [29], 97.2 to 98.8 Ma [29], and 100.5 to 107.6 Ma [29], respectively. These geochronological data indicate that the age of Xiejiagou gold mineralization occurred at ~123.6–115.2 Ma, which is identical to , the generally considered main mineralizing period of the major gold deposits within the Jiaodong Peninsula [46]. The formation of the Jiaodong gold mineralization including Xiejiagou deposit was closely related with subduction of the Paleo-Pacific Plate beneath the Eurasian continent accompanied by lithospheric delamination, asthenospheric upwelling, and intense craton destruction [4–9].

(a)

(b)

Figure 2: (a) Geological sketch map of the Xiejiagou gold deposit; (b) geological section of the Xiejiagou gold deposit (modified after Ding [33]).

The Xiejiagou deposit currently has proven reserves of about 37.5 t of Au [33]. Most of the orebodies are lenticular or irregular lodes and are controlled by the NE-trending faults, which cut the granitoids (Figure 2). The ore-controlling faults are 200–1000 m long and 2–15 m in width with a strike of 20 to 35° and a dip of 60° to 85° (Figure 2). Four major orebodies have been identified in the ore district, and no. 3 is the largest and most representative (Figure 2), containing roughly 70% of the total proven gold reserves of the deposit [32]. The no. 3 orebody is 600 m in length with a gentle dip of 76° to 85° and contains an ore grade of 3.0 to 6.8 g/t Au [31]. We have focused on the no. 3 orebody, as it is the most productive for Au. The mineralization style of the Xiejiagou deposit appears associated with pyrite sericite silica-altered granites or fine pyrite veinlets, belonging to the disseminated- and stockwork-style gold mineralization. Extensive hydrothermal alteration also occurs in ore-controlling faults and is characterized by K-feldspar, quartz, sericite, pyrite, chlorite, and carbonate (Figures 3(a)–3(f)). Major ore minerals in the Xiejiagou deposit include pyrite, galena, sphalerite, and chalcopyrite, along with minor pyrrhotite (Figures 3(g)–3(l)). The gangue minerals include 80% quartz with amounts of K-feldspar, sericite, chlorite, calcite, and clay minerals (Figure 3). Au occurs mostly as native gold, followed by electrum. Native gold grains occur mainly as inclusions in pyrite crystals (Figure 3(k)) and less commonly in tiny fissures of pyrite. Electrum occurs mainly as inclusions in sphalerite or galena crystals (Figure 3(l)).

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

Figure 3: Photographs showing the alteration assemblage and ore minerals of the Xiejiagou gold deposit. (a) Potassic feldspathization and sericitization; (b) silicification, sericitization, and pyritization; (c) silicification and sericitization; (d) silicification with rare carbonate alteration; (e) pyritization; (f) silicification, sericitization, and chloritization; (g) euhedral cube pyrite in the early stage; (h) middle-stage pyrite occurs as subhedral aggregates; (i) middle-stage subhedral galena coexists with sphalerite and chalcopyrite; (j) middle-stage subhedral chalcopyrite coexists with pyrrhotite; (k) native gold occurs as inclusions in pyrite crystals; (l) electrum occurs as inclusion in sphalerite crystals. Kfs: K-feldspar; Ser: sericite; Chl: chlorite; Py: pyrite; Sp: sphalerite; Gn: galena; Ccp: chalcopyrite; Po: pyrrhotite; Au: native gold; El: electrum; Si: silicification; Carb: carbonate alteration.

4. Hydrothermal Quartz Vein Sequences and Mineral Assemblages

Three hydrothermal stages and a supergene stage have been identified in the Xiejiagou deposit on the basis of mineralogical assemblage, textures, and cross-cutting relationships observed in hand specimens and thin sections (Figures 3 and 4). The hydrothermal ore-forming processes occurred during an early (K-feldspar) sericite quartz pyrite stage, a middle quartz gold polymetallic sulfide stage, and a late quartz carbonate stage (Figure 5). The gold mineralization occurred in the middle stage.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 4: Photographs showing hand specimens and cross-cutting relationships of different veinlets of the Xiejiagou gold deposit. (a) Early-stage white quartz in K-feldspar pyrite sericite silica-altered rocks; (b) early-stage white quartz sericite K-feldspar assemblages cemented by middle-stage smoky gold quartz pyrite assemblages; (c) early-stage white quartz pyrite assemblages infilled by middle-stage smoky gold quartz pyrite assemblages; (d) middle-stage smoky gold polymetallic sulfide quartz assemblages; (e) middle-stage smoky gold polymetallic sulfide quartz assemblages cross-cut by late quartz carbonate pyrite veinlet; (f) pyrite sericite silica-altered rocks cross-cut by late quartz carbonate veinlet.

Figure 5: Paragenetic sequence of the main minerals in the Xiejiagou gold deposit.

The early hydrothermal stage is characterized by (K-feldspar) sericite pyrite silica alteration granites (Figure 4(a)) or in ore-barren quartz veinlet places (Figure 4(b)). Minerals comprise milky quartz, sericite, K-feldspar, and sparsely distributed euhedral cube pyrite (Figures 3(a), 3(b), 3(g), 4(a), and 4(b)). The main mineral assemblage in this stage is milky subhedral/anhedral quartz (Qz1), sericite, K-feldspar, and sparsely distributed euhedral cube pyrite, with variables of albite and muscovite (Figures 3(a), 3(b), 3(g), 4(a), 4(b), and 5). Not any gold was found in our observation. The early-stage ores were locally cut or cemented by middle- or late-stage veinlets (Figures 4(c) and 4(f)).

The middle stage is the main gold-producing stage and is characterized by the quartz gold polymetallic sulfide veinlets (Figures 4(c) and 4(d)). During this stage, large amounts of sulfide minerals were precipitated including pyrite, galena, sphalerite, chalcopyrite, and minor pyrrhotite (Figures 3(h)–3(j)). Quartz in the middle stage (Qz2) is commonly dark-gray (smoky) in color coexisting with sulfide minerals and minor calcite (Figures 4(c)–4(e)). Gold mainly occurs as native gold and that coexists with other sulfide minerals (Figures 4(e) and 4(f)).

The late stage is characterized by quartz carbonate veinlets. This stage mainly contains calcite, quartz, and siderite, with trace amounts of pyrite (Figures 4(e) and 4(f)). No gold has been identified under reflected light in this stage. The paragenetic sequence of the Xiejiagou gold deposit is summarized in Figure 5.

5. Sampling and Analytical Methodology

5.1. Zircon U–Pb Dating

Zircons were separated by an unaltered Linglong biotite granite sample (C156-13) from the south of the Xiejiagou gold deposit (Figure 2(a)). The main minerals of the medium-grained biotite granite sample (C156-13) are plagioclase (30%), K-feldspar (30%), quartz (35%), and biotite (5%) (Figure 6(a)), with accessory titanite, zircon and apatite. Zircon sample preparation, cathodoluminescence (CL), and back-scattered electron imaging were completed at the Nanjing Hongchuang Exploration Technology Service Company Limited. U–Pb dating was performed on a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for constraining the age of the wall-rocks of the deposit.

Figure 6: (a) The Linglong biotite granite (C156-13) under polarized light; (b) representative zircon CL images of the biotite granite; (c) U–Pb concordia diagrams of the biotite granite; (d) the ²⁰⁶Pb/²³⁸U-weighed mean age of the biotite granite.

LA-ICP-MS U–Pb isotope analyses were conducted using an Agilent 7500 mass spectrometer connected to a 193 nm ArF excimer laser ablation system at the Wuhan Sample Solution Analytical Technology Company Limited. The synthetic silicate glass standard reference material NIST SMR610 was used to calibrate the instrument, and the international reference standard zircon 91500 was used as the external standard for age calibration. During the analyses, high-purity He gas was used to transfer the ablated materials. The spot size was 32 mm. For detailed procedures and methods, as well as the instrument settings, see Yuan et al. [47]. The ICPMSDataCal [48, 49] and Isoplot 3.0 software [50] were used for data reduction. Common Pb corrections were undertaken using the approach of Andersen [51]. Fifteen zircon analyses were performed via LA-ICP-MS. Uncertainties on individual LA-ICP-MS analyses are quoted at the 1σ level, with results given in Table 1.

Table 1: Results of LA-ICP-MS zircon U-Pb analysis for the biotite granite (C156-13) in the Xiejiagou gold deposit.

Sample no.
(ppm)
U (ppm)
/U
Isotopic ratios
Ages (Ma)

²⁰⁷Pb/²⁰⁶Pb
²⁰⁷Pb/²³⁵U
²⁰⁶Pb/²³⁸U
²⁰⁷Pb/²⁰⁶Pb
²⁰⁷Pb/²³⁵U
²⁰⁶Pb/²³⁸U

Ratio
1σ
Ratio
1σ
Ratio
1σ
Age
1σ
Age
1σ
Age
1σ

D01
439
4744
0.09
0.0473
0.0015
0.1655
0.0052
0.0254
0.0003
62
50
156
4
162
2

D02
161
219
0.73
0.0518
0.0053
0.1741
0.0148
0.0256
0.0008
278
137
163
13
163
5

D03
176
404
0.44
0.0503
0.0043
0.1720
0.0115
0.0257
0.0005
207
114
161
10
163
3

D04
319
2686
0.12
0.0481
0.0018
0.1684
0.0060
0.0254
0.0003
106
63
158
5
162
2

D05
145
1734
0.08
0.0489
0.0028
0.1662
0.0096
0.0246
0.0004
143
104
156
8
157
2

D06
235
622
0.38
0.0504
0.0032
0.1663
0.0087
0.0245
0.0004
212
89
156
8
156
3

D07
139
811
0.17
0.0491
0.0034
0.1658
0.0114
0.0245
0.0004
152
128
156
10
156
2

D08
159
245
0.65
0.0518
0.0097
0.1727
0.0304
0.0256
0.0009
278
307
162
26
163
5

D09
321
2678
0.12
0.0483
0.0017
0.1668
0.0055
0.0251
0.0002
112
59
157
5
160
1

D10
971
2493
0.39
0.0482
0.0021
0.1685
0.0072
0.0254
0.0003
107
77
158
6
162
2

D11
166
218
0.76
0.0518
0.0066
0.1743
0.0181
0.0255
0.0008
276
177
163
16
163
5

D12
321
2701
0.12
0.0497
0.0021
0.1762
0.0071
0.0258
0.0003
179
73
165
6
164
2

D13
125
1445
0.09
0.0494
0.0037
0.1728
0.0127
0.0254
0.0005
168
133
162
11
162
3

D14
511
598
0.85
0.0482
0.0035
0.1679
0.0117
0.0254
0.0005
110
118
158
10
161
3

D15
157
852
0.18
0.0473
0.0032
0.1662
0.0111
0.0255
0.0004
62
116
156
10
162
3

5.2. Fluid Inclusion Studies

Samples for the fluid inclusion study were collected from the main orebody (lode no. 3) of the Xiejiagou gold deposit to determine the nature of the fluids associated with mineralization. (Figure 4). Doubly polished thin sections (about 0.20 mm thick) were made from 45 quartz samples associated with different stages. Fluid inclusion petrography included careful observation of the shapes, characteristics of spatial distribution, genetic and composition types, and vapor/liquid ratios. Twenty-nine typical samples with abundant and representative fluid inclusions were selected for microthermometric measurements and laser Raman spectroscopy analyses. Various inclusions in quartz were selected for microthermometry based on the distribution, size, types, and textural relationship. The microthermometric study was carried out at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, and Institute of Geology, Chinese Academy of Geological Science, using a Linkam THMS-600 heating-freezing stage with a temperature range of −195°C to +600°C. The equipment was calibrated with synthetic samples of fluid inclusions: pure water inclusions (0°C), pure CO₂ inclusions (−56.6°C), and potassium bichromate (398°C). The estimated precision of the measurements was ±0.2°C for temperatures lower than 31°C, ±1°C for the interval of 31–300°C, and ±2°C for temperatures higher than 300°C. The salinities of NaCl–H₂O inclusions were calculated using the final melting temperatures of ice [52]. The salinities of CO₂-bearing fluid inclusions were calculated using the melting temperatures of clathrate [53]. Densities and pressure were calculated using FLINCOR software according to the microthermometry data [54, 55]. Representative fluid inclusion volatiles were analyzed using a Renishaw System 2000 Raman Microspectrometer at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, with a blazing power of 20 MW and blazing wave length of 514.5 nm according to the method of Burke [56].

5.3. Oxygen Isotope and Hydrogen Isotope Analyses

Six quartz samples that were intergrown with ores during the different stages of mineralization were collected for detailed hydrogen and oxygen isotope analyses. All samples were handpicked and checked under a binocular microscope to achieve >95% purity. The isotopic compositions of oxygen and hydrogen were analyzed using a MAT-253 stable isotope ratio mass spectrometer in the analytical laboratory of the Beijing Research Institute of Uranium Geology. Oxygen was extracted for analysis using the BrF₅ technique [57], and hydrogen from H₂O was released from fluid inclusions [58]. The isotopic ratios are reported in standard δ notation (‰) relative to standard mean ocean water (SMOW) for oxygen and hydrogen. The analytical precision was better than ±0.2‰ for δ¹⁸O and ±2‰ for δD. The oxygen isotope ratios of water in equilibrium with quartz were calculated by the equation [59]. is the average homogenization temperature of fluid inclusions from different stages.

5.4. Sulfur Isotope Analysis

Sulfur isotope analyses were carried out from 50 to 100 mg of sulfide minerals including pyrite, sphalerite, and galena. The sulfide grains were mixed with cuprous oxide and crushed into 200 mesh powder. SO₂ was produced through the reaction of sulfide and cuprous oxide at 980°C under a vacuum pressure of . The SO₂ was then measured by MAT-252 mass spectrometer for sulfur isotope. The sulfur isotope analysis was carried out at the analytical laboratory of the Beijing Research Institute of Uranium Geology. All the analytical uncertainties were better than ±0.2‰.

5.5. Lead Isotope Analysis

For lead isotope ratios, approximately 10 to 50 mg of five sulfide samples was first leached in acetone to remove surface contamination and then washed by distilled water and dried at 60°C in the oven. Washed sulfides were dissolved in dilute mix solution of nitric acid and hydrofluoric acid. Following ion exchange chemistry, the lead in the solution was loaded onto rhenium filaments using a phosphoric acid silica gel emitter. The lead isotopic compositions were measured on MAT-261 thermal ionization mass spectrometer with the standard sample NBS 981. The lead isotope analysis was carried out at the analytical laboratory of the Beijing Research Institute of Uranium Geology. The analytical precision of Pb isotope is better than ±0.09‰.

6. Results

6.1. Zircon U–Pb Age of Biotite Granite

Zircons from the biotite granite (C156-13) are subhedral to euhedral, with a crystal length of 100 to 300 μm and aspect ratios of 2 : 1 to 4 : 1. Most of the grains display well-developed oscillatory zoning (Figure 6(b)), indicating a typical magmatic origin [60, 61]. The zircons contain variable moderate concentrations of U (1445 – 2493 ppm) and Th (125 – 971 ppm). The Th/U ratios range from 0.09 to 0.85. They have concordant ²⁰⁶Pb/²³⁸U ages of to , yielding a concordia age of (, ; Figure 6(c)) and a weighted mean ²⁰⁶Pb/²³⁸U age of (, ; Figure 6(d)). The weighted mean ²⁰⁶Pb/²³⁸U age with a low mean square of weighted deviation (MSWD) of 1.2 is interpreted as the crystallization age of the biotite granite.

6.2. Type and Occurrence of Fluid Inclusions

Fluid inclusions (FIs) occurring as isolated inclusions, randomly distributed groups, or in clusters were considered to be primary or pseudosecondary in origin, whereas those aligned along microfractures in transgranular trails were interpreted as secondary [62, 63]. Abundant primary, secondary, and pseudosecondary fluid inclusions are observed (Figure 7(a)).

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 7: Photomicrographs of typical fluid inclusions in the Xiejiagou gold deposit. (a) Primary, pseudosecondary, and secondary fluid inclusions in the same sample; (b) three-phase type Ιa fluid inclusion; (c) two-phase type Ιa fluid inclusion; (d) three-phase type Ιb fluid inclusion; (e) type ΙΙ fluid inclusion; (f) type ΙΙΙa fluid inclusion; (g) type ΙΙΙb fluid inclusion.VCO2: vapor CO₂; LCO2: liquid CO₂; VH2O: vapor H₂O; LH2O: liquid H₂O.

Three compositional types of fluid inclusions (Figure 7) were identified based on their phases at room temperature, their observed phase transitions during heating and cooling runs, and laser Raman spectroscopy results. The FI types, in decreasing order of abundance, are aqueous carbonic (Figures 7(b)–7(d); CO₂–H₂O–NaCl; type Ι), pure carbonic (Figure 7(e); CO₂; type II), and aqueous (Figures 7(f) and 7(g); H₂O–NaCl; type IΙΙ).

Type Ι inclusions are as follows: CO₂-bearing aqueous solution inclusions comprise 60% of the total inclusions and are mainly three-phase CO₂-bearing fluid inclusions () at room temperature (Figure 7(b)). Occasionally, two phases can be observed (; ) (Figure 7(c)). Besides, two subtypes of aqueous carbonic FIs were identified (type Ιa and type Ιb). The aqueous carbonic type Ιa and type Ιb inclusions contain two or three phases, but the volumes of the carbonic phases are 10–50% and 50–80%, respectively (Figures 7(b)–7(d)). These inclusions have ellipsoidal or irregular shapes with a long axis ranging from 5 to 20 μm (mainly 10–15 μm). Type Ιa inclusions commonly coexist with type ΙΙ inclusions in the early stage. Types Ιa and Ιb inclusions commonly coexist with type ΙΙ and type ΙΙΙ inclusions in the middle stage.

Type ΙΙ inclusions are as follows: these types of inclusions consist of pure CO₂ (Figure 7(e)). The pure CO₂ inclusions are less than 5% of the total population and mainly occur in the early and middle stages. They are composed of single or two phases (LCO2, VCO2, or ). These types of inclusions are oval to negative crystal morphologies with a long axis of about 5–15 μm, with most measuring between 5 and 10 μm. Type ΙΙ FIs are distributed in clusters with type Ι in quartz grains from the early stage and type Ι and type ΙΙΙ FIs in quartz grains from the middle stage.

Type ΙΙΙ inclusions are as follows: NaCl-H₂O-type inclusions consist of H₂O liquid and H₂O vapor at room temperature and comprise 35% of total inclusions. Three subtypes of aqueous FIs were identified (type ΙΙΙa, type ΙΙΙb, and type ΙΙΙc). The aqueous type ΙΙΙa and type ΙΙΙb inclusions contain two phases, but the volumes of the vapor phase are 10–50% and 50–80%, respectively (Figures 7(f) and 7(g)). Besides, a small number of one-phase aqueous inclusions (type ΙΙΙc) contain liquid water at room temperature; these inclusions are confined to the late stage and occur commonly in clusters with the two-phase type ΙΙΙa inclusions (Figure 8(g)). The type ΙΙΙ inclusions are oval, nearly round, or irregular. The long axis of this type of inclusion ranges from 5 to 20 μm, with most measuring between 8 and 15 μm. Type ΙΙΙ FIs commonly coexist with type Ι and type ΙΙ inclusions in the middle stage.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 8: Photomicrographs of fluid inclusion assemblages from different hydrothermal stages in the Xiejiagou gold deposit. (a) Primary type Ιa FIs in early-stage quartz; (b) type Ιa FIs coexist with type ΙΙ FIs in early-stage quartz; (c) type Ι FIs coexist with type ΙΙ and type ΙΙΙa FIs in middle-stage quartz; (d) type Ιb FIs coexist with type ΙΙ FIs in middle-stage quartz; (e) type Ιa FIs coexist with type ΙΙΙa FIs in middle-stage quartz; (f) primary type ΙΙΙa and ΙΙΙb FIs in middle-stage quartz; (g) type ΙΙΙa and ΙΙΙc FIs in late-stage quartz.

6.3. Fluid Inclusion Microthermometry

Microthermometric data obtained from primary, pseudosecondary, and secondary type Ι, ΙΙ, and ΙΙΙ FIs of the three mineralization stages are summarized in Table 2 and presented in Figure 9. Microthermometric measurements were not possible for some FIs due to their small sizes (i.e., for type Ι and type ΙΙ, for type Ι, and for type ΙΙΙ).

Table 2: Microthermometric data for fluid inclusions of different stages from the Xiejiagou gold deposit.

Stage
Type
Number

Salinity
CO₂ density
Bulk density

°C
°C
°C
°C
°C
wt% NaCl equiv.
g/cm³
g/cm³

Early
Ia
67
-59.4 to -56.6
5.1 – 8.9
21.3 – 30.6

262 – 386
2.22 – 8.82
0.562 – 0.760
0.773 – 0.912

II
6
-57.9 to -56.7

16.7 – 28.4

0.651 – 0.813

Sec Ia
4
-58.1 to -56.7
5.8 – 7.3
21.6 – 30.5

245 – 301
5.14 – 7.70
0.569 – 0.756
0.857 – 0.920

Sec Ib
1
-56.9
8.7
25.3

272
2.58
0.707
0.798

Sec II
2
-57.4 to -57.1

20.4 – 25.6

0.702 – 0.770

Sec IIIa
6

-1.3 to -3.6
165 – 233
2.23 – 5.85

0.845 – 0.939

Middle
Ia
53
-59.1 to -56.6
4.6 – 8.8
20.4 – 30.7

235 – 347
2.39 – 9.59
0.581 – 0.773
0.839 – 0.908

Ib
7
-57.8 to -56.6
6.0 – 9.5
25.0 – 30.2

251 – 302
1.02 – 7.48
0.586 – 0.734
0.779 – 0.846

II
9
-57.6 to -56.8

13.1 – 25.6

0.690 – 0.830

IIIa
61

-1.9 to -7.9
192 – 313
3.21 – 11.60

0.740 – 0.896

IIIb
6

-1.5 to -1.6
242 – 266
2.56 – 2.73

0.800 – 0.828

Sec IIIa
10

-1.0 to -3.3
144 – 187
1.73 – 5.40

0.915 – 0.941

Late
IIIa
69

-0.7 to -4.9
137 – 231
1.22 – 7.72

0.850 – 0.950

Sec: secondary FIs; : melting temperature of solid CO₂; : temperature of CO₂ clathrate dissociation; : homogenization temperature of CO₂; : temperature of final ice melting; : homogenization temperature; wt% NaCl equiv.: weight percent NaCl equivalent.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 9: Histograms of total homogenization temperatures () and salinities of fluid inclusions in different stages of Xiejiagou gold deposit. (a) Homogenization temperatures for primary and secondary FIs in early-stage quartz; (b) salinity for primary and secondary FIs in early-stage quartz; (c) homogenization temperatures for primary and secondary FIs in middle-stage quartz; (d) salinity for primary and secondary FIs in FIs of middle-stage quartz; (e) homogenization temperatures for type IIIa FIs of late-stage quartz; (f) salinity for type IIIa FIs of late-stage quartz.

6.4. Laser Raman Microprobe Analysis

To constrain the fluid inclusion compositions, representative samples of the three different hydrothermal stages were examined using laser Raman microspectroscopy (Figure 10). The type Ι and ΙΙ FIs from the early and middle hydrothermal stages contain a vapor phase of CO₂ with minor amounts of CH₄ (Figures 10(a)–10(d)). The main volatile component of the type ΙΙΙ FIs for the last two stages is H₂O with minor amounts of CO₂ and H₂S (Figures 10(e) and 10(f)).

(a)

(b)

(c)

(d)

(e)

(f)

Figure 10: Representative laser Raman spectra of fluid inclusions. (a) Spectrum for the gas phase of type Ia inclusion in the early stage with few contents of CH₄; (b) spectrum for the gas phase of type II inclusion in the early stage; (c) spectrum for the gas phase of type Ib inclusion in the middle stage; (d) spectrum for the gas phase of type II inclusion in the middle stage; (e) spectrum for the gas phase of type IIIa inclusion in the middle stage with few contents of H₂S and CO₂; (f) spectrum for the gas phase of type IIIa inclusion in the late stage.

6.5. Oxygen and Hydrogen Isotopes

The oxygen and hydrogen isotopic results of Xiejiagou gold deposit are given in Table 3, including the data from previous studies [30]. The measured δ¹⁸O values for quartz crystals from the gold mineralization at Xiejiagou have intervals between 3.1‰ and 13.2‰. The δ¹⁸Owater was calculated according to the oxygen isotopic compositions for quartz and the corresponding homogenization temperatures obtained from our fluid inclusion study (Table 3); average temperatures for early stage, middle stage, and late stage are estimated to be 327, 268, and 183°C, respectively. The calculated δ¹⁸Owater values of the fluid range from −9.7‰ to 5.7‰, and the δD values range from −101.8‰ to −83.1‰.

Table 3: The δ¹⁸Om, δ¹⁸Ow, and δD ratios (‰) for quartz from the Xiejiagou gold deposit.

No.
Sample no.
Mineral assemblages
δ¹⁸Om
δDW
Stage
(°C)
δ¹⁸Ow
Data sources

1
C156-07

11.2
-92.0
Early
327
5.2
This study

2
C156-12

12.3
-99.7
Middle
268
4.2

3
C156-28

13.2
-95.4
Middle
268
5.1

4
C156-36

12.8
-99.4
Middle
268
4.7

5
C156-201

10.0
-94.6
Middle
268
1.9

6
C156-29

3.1
-101.8
Late
183
-9.7

7
X-07

120
-83.8
Early

5.7
Sun, 2006 [30]

8
X-73

9.7
-83.1
Early

3.5

9
05021

12.3
-91.7
Middle

5.4

6.6. Sulfur Isotopes

The sulfur isotopic results of Xiejiagou gold deposit are given in Table 4, including the data from previous studies [34]. The sulfur isotope compositions of hydrothermal sulfide have a narrow range of δ³⁴S between 4.7‰ and 7.8‰; the δ³⁴S values of pyrite samples are between 5.9‰ and 7.8‰; the δ³⁴S value of one sphalerite sample is 6.4‰; and the δ³⁴S value of one galena sample is 4.7‰.

Table 4: Sulfur isotope compositions for sulfides from the Xiejiagou gold deposit.

No.
Sample no.
Mineral assemblages
Stage
Mineral
δ³⁴SCDT (‰)
δ³⁴SH2S (‰)
Data sources

1
C156-13

Early
Py
7.5
6.4
This study

2
C156-24

Middle
Py
6.3
4.9

3
C156-38

Middle
Py
6.5
5.1

4
C156-38

Middle
Sp
6.4
6.1

5
C156-38

Middle
Gn
4.7
6.9

6
JT02

Early
Py
7.1
6.0
Ding et al., 2017 [34]

7
JT09

Middle
Py
6.2
4.8

8
JT19

Middle
Py
6.5
5.1

9
JT25

Middle
Py
5.9
4.5

10
JT41

Middle
Py
6.7
5.3

11
JT43

Middle
Py
6.5
5.1

12
JT65

Middle
Py
6.6
5.2

13
JT68

Middle
Py
6.8
5.4

14
JT85

Middle
Py
7.6
6.2

15
JT86

Early
Py
5.9
4.8

16
JT90

Early
Py
6.7
5.6

17
JT94

Early
Py
7.8
6.7

18
JT95

Early
Py
6.7
5.6

6.7. Lead Isotope

Lead isotope data were obtained for sulfides in ore samples from Xiejiagou deposit (Table 5). The ²⁰⁶Pb/²⁰⁴Pb (17.251 to 17.315), ²⁰⁷Pb/²⁰⁴Pb (15.486 to 15.519), and ²⁰⁸Pb/²⁰⁴Pb (37.904 to 38.029) ratios are relatively constant.

Table 5: Lead isotope compositions for sulfides from the Xiejiagou gold deposit.

No.
Sample no.
Mineral assemblages
Mineral
²⁰⁶Pb/²⁰⁴Pb
2σ
²⁰⁷Pb/²⁰⁴Pb
2σ
²⁰⁸Pb/²⁰⁴Pb
2σ

1
C156-13

Py
17.271
0.002
15.498
0.002
37.953
0.004

2
C156-24

Py
17.315
0.002
15.519
0.002
38.008
0.005

3
C156-38

Py
17.285
0.004
15.486
0.004
37.904
0.009

4
C156-38

Sp
17.280
0.002
15.509
0.002
37.993
0.004

5
C156-38

Gn
17.251
0.005
15.517
0.005
38.029
0.005

7. Discussion

7.1. Evolution of Ore-Forming Fluids

In this study, fluid inclusion microthermometry and laser Raman spectroscopy indicate that dominant type Ιa and rare type ΙΙ FIs are developed in the early-stage quartz crystals. The homogenization temperatures of the early-stage FIs are from 262 to 386°C with salinities of 2.22–8.82 wt% NaCl equivalent. These factors indicate that the initial ore-forming fluids were characterized by medium-high temperatures, low salinities, and enrichment in CO₂ with minor CH₄ (Table 1, Figures 9 and 11). Therefore, the initial ore fluids belonged to a homogeneous fluid system. Quartz crystals formed during the middle stage of mineralization contain type Ι, ΙΙ, and ΙΙΙ FIs that yield moderate homogenization temperatures (192°C–347°C) and variable salinities (1.02–11.60 wt% NaCl equivalent) (Table 1, Figures 9 and 11). These features indicate that the ore-forming fluids evolved into a system at moderate temperatures and variable salinities. The ore-forming fluid then became an H₂O–NaCl system at the end of mineralization as only type ΙΙΙ FIs existed in the late stage. In addition, FIs trapped during the late stage of mineralization have low temperatures (137°C–231°C) and salinities (1.22–7.72 wt% NaCl equivalent) similar to that of meteoric water [7], which indicated that the ore-forming fluids experienced cooling and diluting (Figure 11).

Figure 11: Plots of homogenization temperature () vs salinity of different types of FIs from the Xiejiagou deposit.

In another way, the average δ¹⁸O value of ore-forming fluids slightly decreases from 4.8‰ to 4.2‰ and to −9.7‰ in stages 1–3, as well as the δD values for each stage (median, −86.3‰ → −96.2‰ → −101.8‰). Thus, the O–H isotopic compositions obviously have a shift towards meteoric water from the early to late hydrothermal stage (Figure 12), indicating that significant amount of meteoric water infiltrated into the fluid system during the late mineralization stage.

Figure 12: δ¹⁸O–δD plots of the ore fluids at the Xiejiagou gold deposit. The base map is cited from Taylor [74]. δ¹⁸OH2O and δD values of the Mesozoic meteoric water in Jiaodong after Tan et al. [7]. δ¹⁸OH2O and δD values of the Magmatic water of Guojialing granite after Guo et al. [76]. δ¹⁸OH2O and δD values of the orogenic gold deposits revised after Goldfarb et al. [82].

7.2. Source of Metallic and Hydrothermal Components of the Ore-Forming Fluids

Sulfur isotopic composition of sulfide minerals can be used to trace the source of sulfur in ore fluids [64, 65]. The δ³⁴S values of the sulfides in Xiejiagou deposit display a narrow range (4.7‰–7.8‰, with an average of 6.6‰, Figure 13), suggesting an extremely homogeneous source; this is similar to the majority of gold deposits in Jiaodong [2, 3, 66]. The average δ³⁴S values of those sulfide minerals from Xiejiagou deposit show a trend (Figure 13, Table 4), suggesting equilibrium fractionation of sulfur isotopes among the sulfide minerals. Based on the average homogenization temperatures of the corresponding fluid inclusions from the first two stages (327°C and 268°C), the δ³⁴SH2S values of the mineralizing fluid are 4.5 to 6.9‰ using the equations of Ohmoto and Rye [67]. These δ³⁴SH2S values are obviously higher than that of chondrite () indicating that the sulfur is unlikely to have originated from the mantle. In addition, the average δ³⁴SH2S value is within the typical range of the orogenic gold deposits worldwide (Figure 13, 0 to 10‰) and also within the ranges of whole rocks and pyrite from the Archean Jiaodong group, the Mesozoic Linglong and Guojialing granites, and the Early Cretaceous mafic-intermediate dikes. Even though the average δ³⁴SH2S value of the Xiejiagou deposit is close to that of the Late Jurassic Linglong granite (host rock, pre-ore), it is still difficult to establish whether the ore-forming fluids had a magmatic source or other sources.

Figure 13: δ³⁴S values of sulfides from the Xiejiagou deposit and related lithologies. δ³⁴S values of different lithologies at the Jiaodong deposit are quoted from Yang et al. [19] and the references therein. The field for orogenic gold deposits revised after Goldfarb et al. [82].

Lead isotope compositions (particularly of sulfide minerals) are an excellent proxy for constraining the sources of lead in ore-forming fluids [68, 69]. Sulfide minerals in the Xiejiagou deposit show Pb isotope compositions of , , and . In a similar manner to S isotope data, all of the Pb isotope data overlap with the compositions of whole rocks from Mesozoic granites and dikes, as well as Precambrian metamorphic rocks (Figure 14), and are consistent with the lead isotope compositions of most gold deposits in Jiaodong [19]. These values overlap the field of Late Jurassic Linglong granite, which is the main host rock of the Xiejiagou deposit. Thus, the lead isotopic compositions of sulfides can be interpreted to suggest that the host rocks maybe one of the lead sources for sulfides but can not be used to trace the source of gold. In another way, on plumbotectonic diagrams (Figure 14, Table 5), the Pb isotope compositions of sulfide minerals fall within a range that spans within the orogene and lower crust lead isotope evolution curves [70], indicating that the lead in ore-forming fluids may be originated from the lower crust reservoir in an orogenic regime. The Jiaodong gold mineralization had a genetic relationship with continental extension that was induced by the upwelling of hot felsic magma during the Mesozoic [8, 71, 72]. Therefore, the lead in the ore-forming fluids, and included lead in the Mesozoic granites and dikes, may be originated from the Mesozoic orogenic lead reservoir.

(a)

(b)

Figure 14: Lead isotopic compositions of sulfides from the Xiejiagou deposit and related lithologies. The base map from Zartman and Doe [70]. Lead isotopic compositions of different lithologies at the Jiaodong deposit are quoted from Yang et al. [19] and the references therein.

The oxygen and hydrogen isotopic compositions of hydrothermal quartz crystals are useful tracers for determining the source of ore-forming fluids. Unlike hydrothermal minerals that form during the late stage of mineralization, those that form during the early and middle stages preserve O and H isotope ratios that should not have been significantly influenced by meteoric water and thus have the potential to accurately reflect the nature of the original ore-forming fluids [26, 73]. The O–H compositions of ore-forming fluids are shown in Figure 12. The calculated δ¹⁸OH2O (1.9‰ to 5.7‰) and δD (–99.4‰ to –83.1‰) values for fluid of the first two stages are slightly lower than the magmatic water composition ([74], −50‰ to −85‰ for δD; 5.5‰ to 9.0‰ for δ¹⁸OH2O). As such, it is difficult to establish whether the ore-forming fluids had a magmatic source. Mesozoic granites are widespread on the Jiaodong Peninsula (Figure 1), and many researchers have therefore suggested that magmatic water is likely to be the dominant source of the fluids that formed the Jiaodong gold deposits [16, 17, 28]. However, FIs of the Xiejiagou deposit are characteristic of low-salinity mesothermal CO₂-rich fluids, which are widely accepted to be of metamorphic origin and differ markedly from typical high-temperature, high-salinity exsolved magmatic fluids [73, 75]. Furthermore, because the mineralization of the Xiejiagou deposit occurred at about 123.6–115.2 Ma, it is reasonable to narrow the field of magmatic water compositions to include only those of Early Cretaceous Guojialing granite (132–126 Ma; [39]) in the Jiaodong Peninsula. None of the analyzed compositions fall within this narrower field ([76]; magmatic water of Guojialing granite: −50‰ to −85‰ for δD; 5.5‰ to 9.0‰ for δ¹⁸OH2O), indicating that magmatic water may not to be the source of the fluids that formed the Xiejiagou deposit. On the other hand, it is worth noting that the ore-forming fluids of reduced intrusion-related gold deposit are also characterized by CO₂-rich fluids with variable temperature and salinity [77, 78]. This raises the possibility that the Xiejiagou deposit is a reduced intrusion-related gold deposit. Unfortunately, as with previous researchers, we were unable to find any syn-ore-reduced igneous rocks in the Jiaodong Peninsula and we did not observe any of the exsolved textures (e.g., unidirectional solidification textures and quartz eyes) that are typical of primary magmatic water [77]. There is also no evidence of sheet-formed gold-bearing veins ([78]; representative mineralization style of reduced intrusion-related gold deposit) in the Xiejiagou deposit or, for that matter, in any other gold deposits within the Jiaodong Peninsula. In combination, these factors indicate that Xiejiagou is not a reduced intrusion-related deposit.

Representatively, because the nature of ore-forming fluids of the gold deposits in the Jiaodong Peninsula was mesothermal, has low salinity, and enriched in CO₂, it has been suggested that the Jiaodong gold deposits including Xiejiagou might be orogenic gold deposit [21] and that the ore-forming fluids were of metamorphic origin. In addition, the secondary FIs within quartz crystals of the early and middle stages (Table 1; Figure 9) probably lead to inaccurate δD values for the hydrothermal fluids in each stage [22, 79–81], making the calculated O–H values slightly less than those of the typical range of the majority of orogenic gold deposit (Figure 12; [82]). Therefore, the true δD values for the hydrothermal fluids are higher than the δD values of FIs and may come to lie in the metamorphic water field.

In summary, we speculate that the most probable explanation is that the fluids that formed the Xiejiagou deposit were dominated by a metamorphic source. It is commonly believed that orogenic gold deposits, which are predominantly hosted by greenschist facies or low-amphibolite facies metamorphic terranes, have gold-forming ages that are either synchronous with, or slightly younger than, terrane metamorphism and deformational events [82]. However, the Jiaodong gold deposits including Xiejiagou are of the granite-hosted “gold-only” type [1], with only a limited number of metamorphic rocks within the vicinity of the ore deposit (Figure 2). In addition, the ages of mineralization at the Jiaodong Peninsula postdate peak at high amphibolite to granulite facies metamorphism in the area by at least 1.7 Ga, indicating that the source of ore-forming fluids can not come from the Neoarchean and Paleoproterozoic basement rocks [22]. Therefore, the original ore fluids may in fact have formed via metamorphic dehydration, decarbonization, and desulfidation of the subducting Paleo-Pacific slab within an accretionary orogeny [14, 15, 21, 22].

7.3. Trapping Pressure of FIs and Metallogenic Depth

The assemblage of the fluid inclusions from the different hydrothermal stages can estimate the pressure conditions of the fluid inclusions trapped during ore forming [25, 26]. Given that the type Ι FIs are common in the early and middle hydrothermal stages, these FIs exhibit values of approximately -56.6°C; as such, the pressure can be estimated based on the CO₂–H₂O–NaCl system with a range of isochores calculated using FLINCOR software [54] and the formula of Burke [56].

Based on the homogenization temperatures and salinity of the type Ι FIs in the first two stages, a representative isochore calculated for the H₂O–CO₂–NaCl system containing 6 wt% NaCl equivalent and 15 mol% CO₂ ([55], Figure 15) was constructed to estimate the trapping pressures of type Ι FIs from the early and middle stages. Given that the homogenization temperatures of FIs from the first two stages have wide ranges and can not provide the accurate temperature and pressure of fluid entrapment, upper and lower quartiles are more suitable than the average values to reflect the data distribution [80, 83]. Therefore, intersections of isochores of FIs with the upper and lower quartiles of homogenization temperatures and bulk densities are used to constrain the trapping pressure range of the fluids.

(a)

(b)

Figure 15: Representative isochores for minimum and maximum CO₂ densities for H₂O–NaCl–CO₂ inclusions (type I) and the solvus for H₂O–NaCl–CO₂ fluids containing 6 eq. wt% NaCl and 15 mol% CO₂ (after Bowers and Helgeson, [55]). (a) The estimated trapping pressure-temperature diagram of the early stage; (b) the estimated trapping pressure-temperature diagram of the middle stage. Isochores of the type I FIs are calculated using the FLINCOR software.

The trapping pressure for FIs from quartz veinlets of the early hydrothermal stage were calculated to be 224–302 MPa using homogenization temperatures ranging from 305 to 359°C and CO₂ densities ranging from 0.574 to 0.751 g/cm³ (Figure 15(a)). The pressure of the gold polymetallic quartz veinlets from the middle hydrothermal stage was estimated ranging from 191 to 258 MPa by homogenization temperatures of 246 to 289°C and CO₂ densities ranging from 0.589 to 0.767 g/cm³ (Figure 15(b)). Applying an ancient ground pressure gradient of 0.0265 GPa/km (mean density of upper crust), the calculated trapping pressure for the gold polymetallic quartz veinlets corresponds to a depth of 7.2 to 9.7 km within a lithostatic fluid system. The thermochronologic study indicates that the erosion depth of the Jiaobei uplift has been calculated as about 3.0 to 7.0 km between the Early Cretaceous and the present [84]. Consequently, it is inferred that the Xiejiagou gold deposit has undergone very less total denudation since its formation. Accordingly, it can be predicted that large gold reserves could still exist at deep levels within the Xiejiagou deposit.

7.4. Mechanisms of Gold Transport and Deposition

Gold is commonly dissolved and transported in the form of gold bisulfide complexes (Au(HS)⁰ and Au(HS)₂⁻) in hydrothermal solutions at temperatures of <400°C [85]. The aqueous speciation of gold(I) sulfide complexes is sensitive to physical and chemical conditions of solution [86]. For example, when the temperature is below 400°C, Au(HS)⁰ dominates under acidic conditions, whereas Au(HS)2– is more likely to form under weakly acidic to neutral conditions [86–88]. The alteration mineral assemblages of the Xiejiagou deposit consist of quartz sericite K-feldspar chlorite association (Figures 3 and 4), indicating that the pH of the ore solution was near-neutral to weakly acidic [89, 90]. Thus, gold was most probably transported as the Au(HS)₂⁻ complex at the Xiejiagou deposit. It is also supported by the observation that gold is generally accompanied by pyrite and other sulfides in the Xiejiagou deposit (Figures 3(k) and 3(l)), as is typical in orogenic gold deposits worldwide, in which gold(I) sulfide complexes (especially Au(HS)₂–) were considered the most likely species for transporting gold [82]. Besides, the abundance of type Ι and ΙΙ FIs in quartz from the early hydrothermal stage demonstrates that the ore-forming solution was enriched in CO₂. CO₂ can buffer or neutralize the fluid within the pH range in which the gold bisulfide complexes are stable and increase its solubility in the ore-forming fluids [91], which provides favorable conditions for gold bisulfide migration. Therefore, the initial auriferous hydrothermal fluids need changes of physical and chemical conditions to reduce solubility of Au(HS)₂⁻ so that gold can be precipitated.

In the pressure-temperature isochore diagram (Figure 15(b)), the P–T window with the yellow-shaded area of the middle stage is mainly below the solvus (6 wt% NaCl equivalent with 15 mol% CO₂), indicating that the initial homogeneous fluids entered the two-phase field and experienced phase separation process in this stage [55]. Besides, the existence of the types Ι, ΙΙ and ΙΙΙ FIs within a single thin-section of the middle stage with different compositions and densities is inferred to be caused by the CO₂ effervescence from an initial homogeneous fluid system [92, 93]. In addition, the homogenization temperatures of type ΙΙΙ FIs are slightly lower than those of coexisting type Ι FIs within middle-stage samples, also because the homogenization temperatures of aqueous inclusions can thus be significantly lowered by CO₂ effervescence resulting from pressure fluctuations [94]. Such CO₂ effervescence in auriferous hydrothermal systems could have led to the exsolution of the volatiles such as H₂S (Figure 10(e)). The exsolution of carbon dioxide of the original ore fluid led to an increase in pH values and a decrease in temperature. This process, in conjunction with the exsolution of H₂S from the ore-forming fluid, which may have made gold bisulfide complexes unstable and reduced their solubility, resulted in the precipitation of abundant gold.

8. Conclusions

(1)Three types of primary fluid inclusion were identified in the Xiejiagou gold deposit: type Ι (aqueous carbonic), type ΙΙ (pure carbonic), and type ΙΙΙ (aqueous). Type Ι and minor type ΙΙ inclusions exist in quartz from the early stage of mineralization, whereas all three types of inclusions are observed in quartz from the middle period of mineralization. In contrast, quartz from the late stage of mineralization only contains type ΙΙΙ inclusions(2)The initial ore-forming fluids of the Xiejiagou deposit belonged to a medium-high temperature (262–386°C), CO₂-rich, low salinity (2.22–8.82 wt% NaCl), and homogeneous fluid system. During mineralization, the fluid finally evolved into a medium-low temperature NaCl–H₂O system as a result of an influx of meteoric water. Considering the fluid inclusion characteristics, H–O–S–Pb isotopes, and regional geological events, the ore-forming fluid reservoir of the Xiejiagou deposit was likely metamorphic in origin(3)Trapping pressures of the first two hydrothermal stages estimated from the type Ι inclusion assemblages were ~224–302 MPa and ~191–258 MPa, respectively. The gold mineralization of the Xiejiagou gold deposit occurred at a lithostatic depth of ~7.2–9.7 km(4)Au(HS)₂⁻ was the most probable gold-transporting complex at the Xiejiagou deposit. CO₂ effervescence in auriferous hydrothermal systems resulted in the precipitation of gold

Data Availability

The manuscript is a data self-contained article, whose results were obtained from the laboratory analysis, and the entire data is presented within the article.

Conflicts of Interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the submitted paper.

Acknowledgments

This research work was jointly supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA20070304), the China Geological Survey (DD20160024 and grant no. 121201102000150011), the National Natural Science Foundation of China (grant nos. 41320104004 and 41602084), and the China Postdoctoral Science Foundation funded project (grant no. 2016M590119). We especially thank four reviewers for their kind and critically constructive comments and suggestions, which greatly improved the quality of our manuscript. We are grateful to the Institute of Mineral Resources, Chinese Academy of Geological Science, for laser Raman spectroscopic analyses. We are also grateful to the Beijing Research Institute of Uranium Geology for H–O–S–Pb isotopic analyses.

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