Metallogeny of the Dagangou Au-Ag-Cu-Sb Deposit in the Eastern Kunlun Orogen, NW China: Constraints from Ore-Forming Fluid Geochemistry and S-H-O Isotopes

A study of ore-forming ﬂ uid geochemistry and S-H-O isotopes has been conducted to reveal the metallogeny of the Dagangou Au-Ag-Cu-Sb deposit, NW China. Three mineralization stages are identi ﬁ ed within Dagangou, including ankerite-quartz-pyrite (Ank-Qz-Py), quartz-Au-sul ﬁ de (Qz-Au-Sul), and quartz-calcite (Qz-Cal) stages. Four types of primary ﬂ uid inclusions (FIs) and various ﬂ uid inclusion assemblages (FIAs) are identi ﬁ ed by petrographic observations, including pure CO 2 , CO 2 -bearing, vapour-liquid and halite-bearing FIs, most of which are distributed randomly or along growth zones. Raman spectroscopy suggests that the ﬂ uid vapour is mainly H 2 O and CO 2 with minor N 2 , CH 4 , and C 2 H 6 . The homogenization temperatures of primary FIs from the Ank-Qz-Py to Qz-Au-Sul to Qz-Cal stages are 208.7 – 313.4 ° C, 184.3 – 251.7 ° C, and 167.3 – 224.2 ° C, with salinities of 3.4 – 13.4 wt.%, 5.4 – 39.5wt.% and 4.9 – 11.4wt.% NaCl equiv. . Sul ﬁ de S isotopes show a predominant source of sedimentary sulfur with minor magmatic sulfur, as evidenced by δ 34 S values of -1.4 – 5.1 ‰ . The FI H-O isotopes reveal that the primary ﬂ uid was derived from magmatic waters mixed with minor sedimentary water ( δ D = − 90 : 0 to -86.8 ‰ , δ 18 O H2O = 6 : 7 – 7.4 ‰ ) and then was signi ﬁ cantly diluted by meteoric water in the Qz-Cal stage ( δ D = − 97 : 2 to -95.7 ‰ , δ 18 O H2O = 3 : 9 – 4.3 ‰ ). Thus, the ore-forming ﬂ uid geochemistry and S-H-O isotopes indicate that ﬂ uid boiling occurred in the Qz-Au-Sul stage due to a pressure reduction from 85 MPa to 70MPa, which led to the phase separation of ﬂ uid, loss of CO 2 , increase in residual ﬂ uid salinity, and formation of immiscible FIAs and halite-bearing FIs in the Qz-Au-Sul stage. This indicates a system moving away from a primary equilibrium, resulting in the deposition of metallic elements. The Dagangou Au-Ag-Cu-Sb deposit, therefore, is an orogenic gold-polymetallic deposit with a mesozonal depth of 5.9 – 7.5km.

Studies show that the gold-polymetallic deposits in the EKO are closely related to tectono-hydrothermal activity in metallogenesis, and they are also very important deposit types in the Eastern Kunlun Metallogenic Belt (EKMB), producing most of the gold resources [14,15,17,18]. Many studies have been performed on these gold-polymetallic deposits, mainly involving geological characteristics, oreforming fluid characteristics and evolution, ore-forming material sources, metallogenic ages, ore genesis, and metallogeny [4-6, 15, 17-22]. First, researchers have found that the ore-forming fluids of these gold-polymetallic deposits generally have the characteristics of medium to high temperatures and medium to low salinities, with a wide range of variations. The temperature of some deposits in the early mineralization stage can reach 350°C or even higher, and the metallogenic temperature of most deposits is between 240°C and 340°C. However, due to the mixing of meteoric water, the temperature in the late mineralization stage often dropped to less than 200°C or even less than 150°C, such as Dachang, Xizangdagou, Kaihuabei, Dongdatan, Guoluolongwa, and Annage deposits [1][2][3][4][5][6][15][16][17][18][19][20][21][22][23][24][25]. Meanwhile, fluid inclusions (FIs) with high salinity appear in the ore-forming fluids of some ore deposits, such as Guoluolongwa and Annage deposits, which are up to 22% or higher [20,25,26]. Second, most researchers suggest that the origin of the primary oreforming fluids of these gold-polymetallic deposits is closely related to magmatic water with δD values of -59.6 to -122‰ and δ 18 O H2O of 0.6-10.7‰, which generally belongs to the NaCl-H 2 O-CO 2 system with minor CH 4 and N 2 , but it will be mixed to varying degrees by metamorphic water meteoric water and/or other fluids with evolution [4,[18][19][20][21][22][23][24][25]. Third, a large number of dating results (e.g., zircon fission track or U-Pb dating, hydrothermal mica Ar-Ar dating, and FI Rb-Sr dating) indicate that these gold-polymetallic deposits were mainly formed in the late Indosinian period   [21,[27][28][29][30][31], which responded to the strong tectono-magmatism during the geological evolution of the EKO.
The recently discovered Dagangou Au-Ag-Cu-Sb deposit is located between the Middle and South Kunlun Faults [32]. It is part of the Eastern Kunlun Gold-Polymetallic Metallogenic Belt and belongs to a series of Au deposits that exemplify the highly prospective nature of this region. In 2012-2013, the Xi'an Institute of Geological and Mineral Exploration prescreened this area defined by stream sediment anomalies at a 1: 50,000 scale and identified 3 mineralized alteration zones, 6 ore bodies, and 11 mineralized bodies. Then, the Dagangou Au-Ag-Cu-Sb deposit was prospected as a commercial project by the Qaidam Integrated Institute of Geology and Mineral Resource Exploration, Qinghai Province. Presently, only minimal basic geological research has been undertaken in this area [32], leaving many key issues, including the origin and evolution of the oreforming fluid, the source of ore-forming materials, the processes involved in the formation, and the genesis of the Dagangou deposit, unclear. Here, we combine new FI data with the results of field investigations to determine the nature, composition, and evolution of the ore-forming fluids and discuss the ore genesis and metallogenic processes of the Dagangou Au-Ag-Cu-Sb deposit. We provide new information on the prospects of the Dagangou deposit and the wider Eastern Kunlun Metallogenic Belt for gold-polymetallic mineralization.

Regional Geological Setting
The Eastern Kunlun Orogen is located in the western segment of the Central Orogenic Belt in China (Figure 1(a)) and is bordered by the Qaidam Massif to the north and the Bayan Har-Songpanganzi Terrane to the south [33]. The EKO trends E-W and is approximately 1500 km long and 50-200 km wide [34]. Its northern and southern parts differ significantly in terms of basement and geological evolution and are separated by the Middle Kunlun Fault. The granulite facies metamorphic rocks of the Paleoproterozoic Jinshuikou Group constitute the crystalline basement of the northern part, whereas the southern part consists of greenschist facies metamorphic rocks of the Meso-Neoproterozoic Wanbaogou Group [35]. The Wanbaogou Group is mainly composed of basalt, carbonate rock, and clastic rock from bottom to top, in which the basalt has a thickness of >4000 m and a pillow structure at the bottom, and is significantly characterized by the features of Oceanic Island Basalt [36]. Meanwhile, carbonate rock formations are carbonate deposits that occur far away from the mainland [37]. Therefore, Sun et al. [34,35] suggested that the Wanbaogou Group basalt belongs to the Oceanic Basalt Plateau (OBP) based on its distribution characteristics, geochemical characteristics, and geological significance, such as the Ontong Java OBP in the southwestern Pacific, which is currently the largest igneous province on Earth. They further believed that the Wanbaogou basalt has the characteristics of OBP within the oceanic plate, which is related to mantle plume activity in the ocean located far away from the Qaidam Massif in the late Meso-Proterozoic. The Wanbaogou OBP played a significant role in the tectonic evolution and Ni-Cu mineralization of the Eastern Kunlun area in the late Caledonian-early Hercynian. Sun et al. [35] considered the EKO to be a continental marginal Orogen resulting from the Wanbaogou OBP, which converged northward to the southern margin of the Qaidam Massif in the late Silurian and records multiple orogenic stages in the Phanerozoic Eon.

Geofluids
The EKO can be divided tectonically into several subparallel E-W-striking belts by the North Kunlun Fault, Middle Kunlun Fault, South Kunlun Fault, and Anyemaqen Fault from north to south. These belts include the Caledonian back-arc rift belt of the Northeastern Kunlun (CNEK), the basement uplift and granite belt of Middle Eastern Kunlun (BMEK), the composite collage belt of Southeastern Kunlun (CSEK), and the Anyemaqen Suture Belt (ASB) (Figure 1(b)) [35].
Precambrian metamorphic rocks and Phanerozoic sedimentary rocks are widespread in the EKO. Paleoproterozoic Jinshuikou Group metamorphic rocks, Middle Permian Ma'erzheng Formation limestones, sandstones and slates, Lower Triassic Bayan Har Group flysch sediments, and Lower Triassic Naocangjiangou Formation foreland basin sedimentary rocks are the main ore-hosting rocks of the hydrothermal deposits in the EKO [2]. The EKO also records intense magmatism that formed numerous suites of basicacid magmatic rocks during various magmatic events, especially within the BMEK [34,35]. The CSEK is located between the Middle and South Kunlun Faults and represents an area of folded basement OBP and microcontinental debris basement material that records two main tectonic events: (a) the formation of the Meso-Neoproterozoic Wanbaogou OBP and Caledonian orogenic belt and (b) a Paleo-Tethyan active continental margin between the Late Palaeozoic and Early Mesozoic [35]. The widely exposed Meso-Neoproterozoic Wanbaogou and Palaeozoic Nachitai Groups record greenschist-facies metamorphism and deformation, and the presence of Late Triassic Babaoshan volcanic rocks and syntectonic emplacement of Indosinian granitoids indicate that the EKO underwent intracontinental tectonism and magmatism [35].

Deposit Geology
The Dagangou Au-Ag-Cu-Sb deposit is located in the middle of the CSEK, and its ore bodies are hosted by the Middle Permian Ma'erzheng Formation and Early-Middle Triassic Naocangjiangou Formation [39]. The strata are dominated by Ma'erzheng Formation limestones, sandstones, and slates; Naocangjiangou Formation clastic rocks and slates; and Quaternary sediments (Figure 2(a)). Structures in this area mainly include folds and faults, the latter generally trend nearly E-W, and the main mineralization-related structure is the Dagangou ductile shear zone [39]. However, this area contains few magmatic rocks, but small granitic porphyry Indosinian dikes with unclear mineralization relationships are found near the northern part of the Dagangou area.   [38]); (b) geological map of the Eastern Kunlun Orogen (modified after Xu et al. [33]). 3 Geofluids foliation, S-C fabrics, and rotational augens, of rocks in the shear zones is remarkable. The ductile shear zones are approximately 7-11 km long from east to the west and 50-600 m wide, and they are characterized by expansion, contraction, branching, and compounding. Sericitization, silicification, carbonization, and other hydrothermal alterations have occurred in these shear zones and are suitable for metallic element deposition and enrichment. Based on the ore-forming element and mineralization combinations, three main mineralized zones of Au-Ag-Cu-Sb have been identified in these ductile shear zones (Figure 2(a)) [10]: a Cu-Ag mineralized zone, Sb-Au mineralized zone, and Au mineralized zone. According to the ore cut-off grade and production grade, respectively, of Au (1 g/t; 2 g/t), Ag (50 g/t; 100 g/t), Cu (0.3%; 0.5%), and Sb (0.7%; 1.5%), six ore bodies and ten mineralized bodies have been identified in the mineralized zones ( Table 1). The ore grade of the ore body in Dagangou is higher than the production grade, while the ore grade of the mineralized body is between the production grade and cut-off grade. Moreover, the trend of ore bodies is basically consistent with shear zones and is obviously controlled by shear zones. The ore bodies have been investigated mainly by trenching, and the reserve of the Dagangou deposit is difficult to estimate presently due to the low exploration degree [39].   Geofluids (i) Cu-Ag mineralized zone (SB I): SB I is hosted in the Ma'erzheng Formation feldspar lithic sandstone and mainly in copper and silver mineralization. It is approximately 5 km long and 5-20 m wide, trends nearly E-W, and is basically consistent with the stratum trending northward at 40°-50°. The rocks are characterized by sericitization, carbonization, and silicification. Three Cu ore bodies, two Cu-Ag ore bodies, and four Cu-(Au)-mineralized bodies have been identified in SB I (Table 1).

Quaternary sediments
(a) K1 Cu ore body: K1 is approximately 25 m long and 0.5 m wide, dips to the north at 50°-60°, and has the same shape as the shear zone. The ore is mainly composed of hydrothermal chalcocite and quartz veins with a Cu grade of 2.25%. The ore minerals are mainly chalcocite, followed by chalcopyrite and malachite, which occur in the fissures of quartz veins, but the wall rock has weak mineralization.
The rest of the Cu-(Au)-(Ag) ore/mineralized bodies in SB I have lenticular, banded, and vein-like geological features. They have the same shape as the shear zone, showing the characteristics of structural ore control. Among them, the K4 Cu, K5 Cu, and K3-2 Cu-Ag ore bodies contain average Cu grades of 0.57%, 3.72% and 2.12%, respectively, with an average Ag grade of 141 g/t for K3-2 (Table 1).
(ii) Au-Sb mineralized zone (SB II): SB II is hosted in the Lower-Middle Triassic Naocangjiangou Formation schist, calcareous slate, and limestone and mainly in gold and antimony mineralization. It is approximately 3.5 km long and 3-6 m wide, runs nearly E-W, and dips to the south at 45°-55°. The rocks in SB II are characterized by strong hydrothermal alteration, including sericitization, calcitization, and silicification. One Au ore body, one Sb ore body, and five Au-(Sb)-mineralized bodies have been identified in this zone (Table 1).
(a) K8 Au ore body: K8 is approximately 50 m long and 1.45 m wide, and its Au grade is 2.48 g/t with a certain amount of Sb. It is lenticular in the ductile shear zone, in which the rocks are fragmented and foliated. This ore body is close to the K9 Sb ore body, and they have the same occurrence of dipping south at approximately 80° (Figure 2(b)). In K8, hydrothermal alterations, including sericitization, carbonization, and silicification, are intense, and pyrite is the common sulfide and main gold-bearing mineral.
(b) K9 Sb ore body: the K9 Sb ore body is close to K8, and they both dip south at approximately 80°( Figure 2(b)). K9 is 120 m long and 2.80 m wide and controlled by an E-W-directed fault. The ores are mainly composed of stibnite (30-60%) and gangue minerals (40-70%); stibnite is the most important sulfide. The altered schist-and calcareous slate-type ores comprise the main ore, containing average Sb and Au grades of 3.23% and 0.48 g/t, respectively. Many ore samples were collected from the K8 and K9 ore bodies to perform FI microthermometry and H-O-S isotope analysis ( Figure 2(b)).
(iii) Au-mineralized zone (SB III): SB III is located in the southern Dagangou area and is hosted in the Naocangjiangou Formation schist, calcareous slate, and limestone. It is approximately 3 km long and 20 m wide, runs nearly E-W, and dips to the south at 40°-50°. The rocks in SB III are heavily altered, including by sericitization, carbonization, and silicification. The KH13 Au-mineralized body identified in this zone is approximately 50 m long and 1.5 m wide and is lenticular and veined in the EW ductile shear zone. It is hosted in the Naocangjiangou Formation, trends E-W, dips to the south at 50°, and is controlled by the fault in this zone. In KH13, the hydrothermal alterations, including sericitization, carbonization, and silicification, are remarkable. Pyrite is the common sulfide that contains 0.57 g/t Au.
3.2. Ore Characteristics. The metallic minerals are mainly chalcocite, pyrite, stibnite, and chalcopyrite ( Figure 3), followed by argentite, galena, sphalerite, malachite, native gold, and azurite, while the gangue minerals mainly include quartz, feldspar, calcite, ankerite, sericite, calcite, and chlorite. The Dagangou deposit has two main types of ore: primary sulfide ore and oxidized ore. The primary sulfide Cu-(Ag) ore is mainly composed of chalcocite, pyrite, and chalcopyrite, which account for 10-50%, while gangue minerals account for 50-90%. Chalcocite is the most important Cu mineral, with minor chalcopyrite, azurite, and malachite, and chalcocite contains a small amount of Ag. Chalcocite is generally present as granular aggregates between 0.5 and 1.0 mm. Chalcopyrite is a granular aggregate with a grain size of 0.10-0.15 mm, is distributed in quartz gaps in the form of stripes and microveins, and is metasomatized by chalcocite ( Figure 3(b)). Some chalcocites fill in between ankerite and quartz locally (Figure 3(a)), indicating that ankerite crystallized earlier than chalcocite. The copper sulfides (e.g., chalco-pyrite and chalcocite) are readily oxidized to azurite and malachite, forming oxidized Cu ore. Cu-(Ag) ores show mainly automorphic, hypautomorphic, and xenomorphic granular textures; intersertal and metasomatic textures; and massive, vein, and disseminated structures.
The primary Sb-Au sulfide ore is mainly composed of stibnite, pyrite, and chalcopyrite, which account for 30-60%, while gangue minerals account for 40-70% (e.g., quartz, feldspar ankerite, sericite, and calcite). The Sb ore is composed of stibnite with an abundance of approximately 30-60% and gangue minerals with abundances of 40-70%, while the Au ore is composed of sulfide with an abundance of approximately 1-3% and gangue minerals with abundances of 97-99%. Stibnite is greyish white with highly grey blue heterogeneity (Figure 3(f)) and hypautomorphic with lath, strip, and granular shapes and ranges from 0.5 to 2.0 mm, and most grains are distributed unevenly in lumps and veins. Native gold, which is distributed in gangue minerals and pyrite, is the main Au-bearing mineral in the Au ore. It is golden in colour and low in hardness and is classified as microgold with a particle size of 0.001-0.020 mm [10]. The ore and gangue minerals mainly show automorphic, hypautomorphic, and xenomorphic granular textures and intersertal and metasomatic textures between them. The Sb-Au ores show massive, vein, crumby, and disseminated structures ( Figure 3).

Alteration and Mineralization Stages.
Hydrothermal alteration is widespread in the Dagangou deposit, with the most intensive alteration occurring in and around the mineralized Au-Ag-Cu-Sb veins. The key alteration assemblages mainly include ankeritization (Figure 3(a)), silicification (Figures 3(a), 3(c), and 3(f)), chloritization, sericitization, calcitization, and kaolinitization. The alteration of the surrounding rock in each alteration zone is centred on the ore body with a zoning of outward weakening, and the distribution direction is basically the same as that of the ore-bearing faults. The alterations from the centre to the wall rock are generally ankeritization, silicification, sericitization, kaolinitization, chloritization, and calcitization ( Figure 4).
The Dagangou deposit consists of Cu-(Ag), Sb, and Au ore bodies, among which Au and Sb mineralization is closely related and often occurs in the same ore body. The mineralization in the Dagangou area shows zoned characteristics of Sb-Au in the middle of the deposit, Cu-Ag in the northern part of the deposit, and Au in the southern part of the deposit. Moreover, these mineralizations have similar altered features: (i) they are obviously controlled by the same hydrothermal field, but differences in the surrounding rocks lead to slight differences in these minerals; (ii) the ore bodies are controlled by nearly E-W-trending faults; and (iii) the alteration of surrounding rock in each alteration zone is centred on ore bodies with outwardly weakening zoning, and the distribution direction is basically the same as that of the orebearing faults.
Among the alterations discussed above, silicification is the most widespread and predominant; silicification constitutes the main part of the quartz vein-type ore body and coexists with minor early precipitated ankerite and late 6 Geofluids 7 Geofluids silicification, which generally occurs on the periphery of the alteration zone ( Figure 4) and overprints all the previous alteration types that coexist with minor pyrite.
In conclusion, crosscutting relationships, ore fabrics, and the paragenesis of minerals in Dagangou enable the identification of hydrothermal and supergene periods, with the former being divided into three distinct mineralization stages as follows ( Figure 5). The three mineralized zones and the ore or mineralization bodies therein have similar alterations and zoning, and their alterations can also generally be divided into the following three stages.
(ii) Quartz-gold-polymetallic sulfide stage (Qz-Au-Sul stage): this stage was characterized by precipitation of large amounts of sulfides, such as chalcocite, stibnite, pyrite, chalcopyrite, and minor sphalerite, representing the main stage of mineralization. Some hypautomorphic-xenomorphic pyrites may be Aubearing minerals containing notable gold [10]. Mineralization is present in massive sulfide, vein, lumpy, and fine-grained disseminated forms and likely represents the main stage of Au mineralization associated with silicification. Chalcopyrite is generally filled in between the quartz grains, and chalcocite is evident along chalcopyrite fissures or the interior (Figure 3(b)). Stibnite-quartz veins in this stage commonly cut the quartz veins in the Ank-Qz-Py stage (Figure 3(c)).
(iii) Quartz-calcite stage (Qz-Cal stage): this stage is mainly represented by quartz and calcite with minor pyrite, chalcocite, gold, and stibnite. Veins of finegrained ore-barren quartz that formed during this stage are interspersed with stibnite in the Qz-Au-Sul stage (Figure 3(f)).
In the supergene period, azurite, malachite, and limonite are formed by the oxidation of primary sulfides, such as chalcocite, chalcopyrite, and pyrite ( Figure 5). Remarkably, some xenomorphic malachites fill in the interspace between ankerite and quartz locally (Figure 3(a)). However, secondary minerals (e.g., azurite, malachite, and limonite) are not a major part of mineral resources in the Dagangou deposit.

Fluid Inclusion Petrography
Hydrothermal quartz samples from each mineralization stage ( Figure 5) were collected from the SB I-and SB IImineralized zones (Figure 2  Groups of individual FIs in a particular growth zone of the host mineral or in the same pseudosecondary trails are considered a fluid inclusion assemblage (FIA) [41,42]. Where growth zones and pseudosecondary trails are absent, FIs that are distributed randomly or in a cluster with similar phase ratios and microthermometric measurements are also treated as an FIA in this study (Figures 6(a), 6(c), and 6(f)) [43]. Therefore, a total of 15 groups of FIAs were recognized in this study, and they usually contain various types of FIs. FIs are abundant in quartz within the Dagangou deposit, but the types of FIs contained in each mineralization stage are quite different. The FIs in Ank-Qz-Py stage quartz are mainly type I, II, and III FIs, which are further classified into     Figure 7.

Microthermometry.
FIs in quartz were studied to determine the temperature, salinity, density, pressure, and depth of trapping of the fluids. Prior to analysis, approximately eight double-sided polished inclusion wafers (0.05-0.2 mm thick) were prepared. A microthermometric study of FIs was carried out using a Linkam THMS 600 programmable heating-freezing stage [44] at the Geological Fluid Laboratory of Jilin University. Most measurements were made at a heating rate of 0.2 to 0.4°C/min. Carbonic phase melting (T mðCO2Þ ) and clathrate melting (T mðclaÞ ) were determined by temperature cycling [45]; the heating rate near T mðCO2Þ and T mðclaÞ was 0.1 to 0.2°C/min during the measurements. The measurement accuracy was ensured by calibration against the triple point of pure CO 2 (-56.6°C), the freezing point of water (0.0°C), and the critical point of water (374.1°C) using synthetic fluid inclusions supplied by FLUID INC. The reproducibility of measurements was ±0.2°C below +30°C and ±2°C at temperatures of final homogenization when the chips were centred in the specimen holder [46]. T h values and halite consumption temperatures (T mðSÞ ) were determined for halitebearing triphase inclusions.
The microthermometric data for all the FIs are summarized in Table 2 and Figures 7 and 8. Calculations of salinity, the mole fraction of components (X H2O , X CO2 , and X NaCl ), the densities of carbonic and bulk fluids, the salinities from clathrate melting temperatures, and the bulk molar volumes of the FIs were made using FLUIDS software [47].

Stable Isotope Analysis. S, H, and O isotope analyses were undertaken at the Analytical Laboratory of the Beijing
Research Institute of Uranium Geology. The S isotope analyses were performed using a MAT-253 stable isotope ratio mass spectrometer. δ 34 S values are analysed in thirteen sulfides (e.g., stibnite, pyrite, and chalcopyrite) from the Qz-Au-Sul stage. The sampled location, mineralization stage, and analysed results of the S isotopic samples are listed in Table 3. Sulfur isotopic compositions of sulfide minerals were measured using the conventional combustion method, and the testing procedure is described by Liu et al. [48]. The results are reported as δ 34 S relative to the Vienna-Canyon Diablo Troilite (VCDT) standard, and the analytical precision was ±0.2‰.
A total of eight samples were used for H-O isotope analysis. First, the 40-60 mesh quartz inclusion samples are weighed at 5-10 mg, baked in a 105°C constant temperature oven for more than 4 hours, and then wrapped in a clean and dry tin cup for use. Second, the air inside the elemental analyser Flash EA was flushed with high purity helium to reduce the H 2 background. The sample can be tested when the temperature rises to 1400°C and the background drops below 50 mV. The sample burst in a ceramic tube containing vitreous carbon, releasing H 2 O, H 2 , and other H-containing gases. H 2 O and other possible organic matter can be reduced with glass carbon at high temperature to reduce the Hcontaining gas to H 2 . Finally, H 2 was driven by high purity helium flow into a MAT-253 gas isotope mass spectrometer for analysis. The results are reported as δD V-SMOW values with the Standard Mean Ocean Water (SMOW) as the standard, and the analysis accuracy was better than ±1‰. The reference standards of hydrogen isotopes are the national standard materials of Peking University and Lanzhou standard water, whose δD V-SMOW values are -64.8‰ and -84.55‰, respectively.
First, the samples were placed in a vacuum of 10 -3 Pa and reacted with pure bromine pentafloride at a constant temperature of 500-680°C for 14 hours to release O 2 and impurity components. Second, pure O 2 reacts with graphite to produce CO 2 at 700°C and a platinum catalyst after separating SiF4, BrF3, and other impurity components by the freezing method. Finally, CO 2 was collected by the freezing method, and the O isotope composition was analysed by MAT-253 gas isotope mass spectrometry. The results are reported as δ 18 O V-SMOW values with SMOW as the standard, and the analysis accuracy was better than ±0.2‰. The reference standards of oxygen isotopes are the quartz standards of GBW-04409 and GBW-04410, whose δ 18 O values are 11:11 ± 0:06‰ and −1:75 ± 0:08‰, respectively. Oxygen isotopic compositions of hydrothermal waters in equilibrium with quartz were calculated using an extrapolation of the fractionation formula of 1000 ln α quartz−water = 3:38 × 10 6 · T -2 -3:40 from [49]. The sampled location, mineralization stage, and analysed results of the H and O isotope samples are listed in Table 4.     (Figures 7(a)-7(d)). The vapour phases of the Qz-Au-Sul    (Figures 7(e)-7(h)), but the CO 2 Raman intensity is weaker than that in the Ank-Qz-Py stage. In comparison, the vapour phase of the Qz-Cal stage FIs is mainly H 2 O (Figure 7(i)). Therefore, the vapour components of ore-forming fluids that formed the Dagangou deposit consist of CO 2 -H 2 O-CH 4 -N 2 -C 2 H 6 . Therefore, ore-forming fluid in the Dagangou deposit approximately belongs to the NaCl-H 2 O-CO 2 system, although it contains minor other vapours, such as CH 4 , N 2 , and C 2 H 6 , and other chlorides, including KCl, CaCl 2 , and likely MgCl 2 and Fe chlorides. This FI system is used to calculate the salinity and density in this paper.

Homogenization Temperature, Salinity, and Density.
Microthermometric data were measured in some representative primary FIs. The microthermometric data and other related calculations for all FIs are summarized in Table 2 and Figures 8 and 9. Calculations of salinity, the mole fractions of components (X H2O , X CO2 , and X NaCl ), the densities of carbonic and bulk fluids, the salinities from clathrate melting temperatures, and the bulk molar volumes of FIs were made using FLUIDS software [47]. The salinity and density of the type IV FIs were calculated according to [50].  The melting temperature of CO 2 clathrates (T mðclaÞ ) in type II FIs is 5.5-8.3°C while heating up after cooling down, yielding salinities of 3.4-8.3 wt.% NaCl equiv. , with values concentrated in the range of 4-6 wt.% NaCl equiv. (Figure 8). CO 2 clathrates have CO 2 homogenization-to-vapour temperatures (T hðCO2Þ ) of 26.1-29.2°C, yielding CO 2 densities (ρ CO2 ) of 0.62-0.70 g/cm 3 . The corresponding bulk inclusion densities are 0.70-0.89 g/cm 3 , and they homogenize to liquid at 226.4-308.4°C with values concentrated in the range of 240-270°C ( Figure 9).
(ii) Fluid inclusions in the Qz-Au-Sul stage: the microthermometric data of the four types of FIs in the Qz-Au-Sul stage are listed in The melting of CO 2 clathrate (T mðclaÞ ) in the presence of CO 2 liquid for type II FIs occurs between 4.6 and 7.2°C, yielding salinities of 5.4-9.7 wt.% NaCl equiv. , with values concentrated in the range of 6-7 wt.% NaCl equiv. (Figure 8 16 Geofluids The type III FIs in the Qz-Au-Sul stage show final homogenization to liquid temperatures (T h ) of 184.3-251.7°C, with values concentrated in the range of 210-230°C (Figure 9), and ice melting temperature (T mðiceÞ ) values are in the range of -11.6 to -6.3°C. Their T mðiceÞ values correspond to salinities from 9.5 to 15.6 wt.% NaCl equiv. , with values concentrated in the range of 12-14 wt.% NaCl equiv. (Figure 8), and their densities are approximately 0.90-1.00 g/cm 3 .
The vapour-liquid phase of type IV FIs has first homogenization temperature (T h ) to liquid at 214.8-235.2°C during heating, and as the temperature continued to rise, halite   17 Geofluids melted at 286.3-321.5°C (T mðSÞ ) and eventually homogenized to the liquid phase. The melting temperature of halite (T mðSÞ ) yields salinities of 37.0-39.5 wt.% NaCl equiv. and densities of 1.07-1.08 g/cm 3 .
(iii) Fluid inclusions in the Qz-Cal stage: FIs in the Qz-Cal stage are mainly type III FIs with minor liquid monophase FIs, and the microthermometric data are shown in Table 2. Type III FIs are particularly common and have final T h to liquid of 167.3-224.2°C, with values concentrated in the range of 180-200°C (Figure 9). The T mðiceÞ of type III FIs is -7.8 to -3.0°C, which yields salinities of 4.9-11.4 wt.% NaCl equiv. , with values concentrated in the range of 6-8 wt.% NaCl equiv. (Figure 8), and their densities range from 0.89 to 0.97 g/cm 3 .
The calculated temperature of Ank-Qz-Py and Qz-Au-Sul stages fluid are medium of the trapping temperature of 260-310°C and 230-280°C in Figure 10, and the Qz-Cal stage temperature is the highest of 224°C.

Physicochemical Conditions of Fluid Inclusion Trapping.
Studies show that fluid immiscibility is a common phenomenon in the evolution of natural fluids [52][53][54], and it is believed that the immiscibility of NaCl-H 2 O-CO 2 system fluids plays a significant role in the mineralization of gold deposits [55][56][57][58]. The fluid immiscibility of the NaCl-H 2 O-CO 2 system has been studied extensively by many researchers [59][60][61][62]. Among them, Frantz et al. [61] believed that immiscibility in the NaCl-H 2 O-CO 2 system often occurred between 1-3 kbar and 500-700°C and mainly depended on the pressure and temperature. In addition, the ratio of CO 2 /H 2 O is also very important for immiscibility. Lode gold deposits usually contain aqueous FIs, H 2 O-CO 2 FIs, CO 2 FIs, and mineral-bearing FIs, the first three types of which dominate the FI type [57]. Ramboz et al. [52] suggested that the heterogeneous trapping of immiscible fluids would result in FI populations with variable phase ratios that homogenized in two different ways.
Clusters of type I, II, and III FIs are distributed randomly or along growth zones in the Dagangou hydrothermal quartz. They have varied vapour phases (e.g., CO 2 , CO 2 -HO 2 , and H 2 O) and continuous variations in vapour/liquid ratios, type II FIs homogenize to the liquid or vapour phase, and some type IV FIs are also present in the Qz-Au-Sul stage. This indicates that these FIs were formed in heterogeneous trapping during fluid immiscibility of an originally uniform primary CO 2 -H 2 O-NaCl fluid [44,47,63]. Type II FIs have higher homogenization temperatures, lower salinities, and lower densities than type III FIs but also have variable homogenization temperatures. Moreover, the microthermometric FI results are approximate for the FIA groups within a mineralization stage, and no more than 10% of FIs with different sizes and shapes have homogenization temperature variations greater than 15°C [41], all of which suggest that they are likely heterogeneous and captured synchronously or almost synchronously. This phenomenon has also been described in similar Au deposits elsewhere, such as Sigma and Star Lake Au deposits in Canada, and it is considered to be the result of several continuous fluid immiscibility events caused by fluctuations in pressure [64,65].
Pressure-temperature conditions can be estimated from FIs by constructing isochores from microthermometric and fluid composition data [44,46,66]. Isochores were constructed for the FIs of Ank-Qz-Py and Qz-Au-Sul stages ( Figure 11) based on the H 2 O-NaCl-CO 2 system, which is dominated by saline solution and CO 2 , with less CH 4 , N 2 , and C 2 H 6 in the vapour phase. As such, we obtained corrected temperatures (trapping temperatures) for the Ank-Qz-Py and Qz-Au-Sul stages of 260-310°C and 230-280°C, respectively ( Figure 11). In trapped fluid inclusions in the process of fluid immiscibility, many researchers have suggested that the homogenization temperature and pressure of the trapping end-member components are similar and basically represent the trapping temperature and pressure [44,63,67]. The endmember composition inclusions trapped during these immiscibility events have similar homogenization temperatures and pressures and are close to the trapping temperatures and pressures for these FIs [44,63,[67][68][69]. The homogenization temperature of type II FIs trapped in the Qz-Au-Sul stage may actually reflect their captured temperature as a result of oreforming fluid immiscibility. Since the Qz-Au-Sul stage is the peak of sulfide precipitation and mineralization of Dagangou, the homogeneous temperature (230-280°C) in this stage approximately represents the formation temperature of this deposit ( Figure 11).
The corrected pressures (trapping pressures) in the Ank-Qz-Py and Qz-Au-Sul stages are estimated to be 70-100 MPa 18 Geofluids (average 85 Ma) and 55-85 MPa (average 70 Ma), respectively ( Figure 11). The pressure in the Qz-Au-Sul stage is lower than that in the Ank-Qz-Py stage, and we consider the trapping pressure in the Qz-Au-Sul stage  to be the mineralization pressure since most of the sulfide precipitation and mineralization took place in this stage. In turn, this yields a formation depth of 5.9-7.5 km for the Dagangou deposit, as defined by Sun et al. [70]. This result indicates that the Dagangou Au-Ag-Cu-Sb deposit formed at a mesozonal depth.  (Figure 10), which range from -6.3‰ to 11.1‰ and include pyrite, arsenopyrite, chalcopyrite, stibnite, galena, and sphalerite [2,4,17,18,24]. These δ 34 Hoefs (1973) [51], and the data of gold-polymetallic deposits in EKO are collected from [2,4,17,18,24]. Geofluids is closely related to magmatic water, which generally belongs to the NaCl-H 2 O-CO 2 system with minor CH 4 and N 2 [2,[4][5][6][17][18][19][20][21][22]. Therefore, considering that the fluid was derived from depth (>5 km) and has a high salinity and high CO 2 content, the primary ore-forming fluids of the Dagangou deposit likely originated from magmatic waters that mixed a few sedimentary waters rather than meteoric waters. The δD values in the Qz-Cal stage (-97.2 to -95.7‰) decrease abruptly compared with those of the Ank-Qz-Py and Qz-Au-Sul stages (-90.6 to -86.8‰), which suggests that an increasing amount of meteoric water was mixed, resulting in a reduction in the δD value as the mineralization depth became shallower in the Qz-Cal stage. Previous studies have shown that the δD and δ 18 O H2O values of hydrothermal quartz within gold-polymetallic deposits of the EKO show a clear evolution in the δD vs. δ 18 O H2O diagram ( Figure 13). It has been suggested that the primary ore-forming fluids mainly involved magmatic fluids, whereas the Qz-Cal stage involved mixing with meteoric water [4][5][6][17][18][19][20][21][22]. Most researchers suggest that mixing large amounts of meteoric water in Qz-Cal stage fluid is a common phenomenon for gold-polymetallic deposits in the EKO [4][5][6][17][18][19][20][21][22]. Therefore, we suggest that the fluids in the Ank-Qz-Py and Qz-Au-Sul stages were most likely derived from magmatic water mixed with sedimentary water (e.g., Permian-Triassic sedimentary wall rock), whereas the fluid in the Qz-Cal stage was greatly diluted by meteoric water.

Sources and Evolution of
The evolution of ore-forming fluid in the Dagangou deposit can be divided into two stages. First, a significant phenomenon is that the CO 2 content decreased abruptly and type IV FIs appeared in the Qz-Au-Sul stage compared with the Ank-Qz-Py stage. The fluids in the Qz-Au-Sul stage underwent a significant geological process that resulted in reduced CO 2 and elevated salinity. Lu et al. [71] suggested that an S (wt.% NaCl)-T h (°C) diagram plays a role in demonstrating the evolution of ore-forming fluids. The ore-forming fluids within the Dagangou deposit record an evolution from the Ank-Qz-Py stage to the Qz-Au-Sul stage, which is similar to trend 3 in Figure 14, where the salinity increases and the homogenization temperature rapidly decreases with the emergence of type IV FIs. The evolution of trend 3 is usually interpreted as fluid boiling [44], the Qz-Au-Sul stage fluid within the Dagangou deposit as well. This resulted in fluid phase separation and release of CO 2 [71] and formed a great amount of FIs with highly varied CO 2 and liquid phase contents [72]. The boiling event also increased the fluid salinity and caused physicochemical changes that led to sulfide formation and accelerated deposition of Au [72,73]. In conclusion, the fluids in the Ank-Qz-Py stage that had moderate temperatures (trapping temperatures of 260-310°C) and low-moderate salinities and were CO 2 -rich that evolved into Qz-Au-Sul stage CO 2 -bearing fluids with moderate temperatures (trapping temperatures of 230-280°C) and high salinities due to fluid boiling.
Fluid boiling within the Dagangou deposit is also supported by H-O isotopic variations. Wagner et al. [74] and Zhu [75] suggested that lighter isotopes are concentrated in vapour phases, which indicates that the residual fluid has higher δ 18 O values after boiling. In comparison, fluid mixing has the opposite effect on the oxygen isotope composition of the hydrothermal system, with δ 18 O values generally decreasing after mixing, although these values may increase during mixing at temperatures of approximately 250°C [74]. The δ 18 O values of the Dagangou ore-forming fluid vary from 6.7‰ to 7.4‰ during the Ank-Qz-Py stage, to 8.1-8.6‰ during the Qz-Au-Sul stage, and to 3.9-4.3‰ during the Qz-Cal stage. This indicates that the primary fluid underwent boiling, which elevated the fluid δ 18 20 Geofluids fluid cooling. However, boiling is still the main mechanism for fluid evolution and metallic element deposition in the Dagangou deposit. Therefore, fluid boiling occurred in the Qz-Au-Sul stage due to a pressure reduction from 85 MPa (average in the Ank-Qz-Py stage) to 70 MPa (average in the Qz-Au-Sul stage), which was the most important mechanism of metallic element precipitation and enrichment in the Dagangou deposit. This led to the phase separation of fluid, a loss of CO 2 due to escape, an increase in the salinity of the residual fluid, and the formation of immiscible inclusion assemblages and halite-bearing fluid inclusions in the Qz-Au-Sul stage, all of which indicate a system moving away from the original equilibrium, which resulted in the deposition of Cu, Sb, and Au. Second, the evolution of Qz-Au-Sul to Qz-Cal stage fluids is similar to trend 1 in Figure 14, which shows a rapid decrease in salinity and a less rapid decrease in homogenization temperature. This reflects a change from moderatetemperature, high-salinity, and CO 2 -bearing fluids in the Qz-Au-Sul stage to low-temperature, low-salinity, and CO 2 -free fluids in the Qz-Cal stage (Figures 7 and 8). It is suggested that the Qz-Au-Sul stage fluids mixed with lowtemperature and low-salinity meteoric water and formed liquid-rich inclusions (including vapour-liquid biphase and liquid monophase inclusions) and a minor number of CO 2bearing triphase inclusions [71,76]. [77] suggested that ore-forming fluid carrying Au may contain H 2 S and CO 2 , primarily because H 2 S can complex with Au to form Au-S complexes and CO 2 plays an important role in transporting Au. CO 2 is a weak acid that can adjust the pH value  [49]). Isotopic compositions of magmatic and metamorphic waters after Taylor (1974) and meteoric water line after Craig (1961). The data of gold-polymetallic deposit in EKO are collected from [2,[4][5][6][17][18][19][20][21][22]. or Au(HS) 0 [78][79][80]. Cooling, boiling, attenuation, changes in pH, and sulfide deposition can all reduce the H 2 S content to precipitate Au. The solubility of Au in a H 2 S-HS to -SO 4 2system reaches a maximum value in the reduced sulfur field at temperatures >400°C [81]. Oxidation of this fluid can cause the dissociation of Au-S complexes to precipitate Au. The reduction in CH 4 vapour during the Ank-Qz-Py stage of mineralization can also cause Au deposition, as HScan easily react with H + to form H 2 S, thereby reducing the stability of Au(HS) 2 and resulting in the deposition of Au from the sulfide solution. The reaction between HSand H + also consumes the latter, leading to the loss of CO 2 and increasing the pH of the ore-forming fluid as a result of the following reaction:

Au HS
ð Þ -2 + 0: This reaction produces H + , and then stibnite precipitates as follows: In conclusion, it is suggested that the primary H 2 O − C O 2 − NaCl fluid underwent a significant boiling event, which caused immiscibility and the loss of CO 2 [53,71,73], leading to Au and stibnite precipitation in the massive and vein-type ores.
7.4. Ore Genesis. Generally, the ore-forming fluids associated with hydrothermal vein-type gold-polymetallic deposits have low salinities (generally <10%), contain high concentrations (5-30 mol% or higher) of CO 2 + CH 4 , and contain immiscible H 2 O and CO 2 , although some of the fluids can have salinities of 10-20% or higher [82]. Similar to the Dagangou Au-Ag-Cu-Sb deposit, the salinity of the fluid can be as high as 40 wt.% NaCl equiv . Mineral-bearing FIs (such as the type IV FIs in the Dagangou deposit) have been described from some hydrothermal gold-polymetallic deposits, including the Wiluna deposit in Australia [83] and the Lengshuibeigou Pb-Zn deposit in Henan Province, China [84]. These highsalinity or mineral-bearing FIs were trapped during the main or late stage but were generated by CO 2 -H 2 O fluid boiling during the early or main stage .
The primary H 2 O-NaCl-CO 2 fluid in the Ank-Qz-Py stage was mainly derived from magmatic water, which mixed with minor crustal water and contained significant amounts of CO 2 with minor CH 4 , N 2 , and C 2 H 6 . It is characterized by a moderate-high temperature and a low-moderate salinity and density, while the salinity in the Qz-Au-Sul stage can reach 40 wt.% NaCl equiv. due to fluid boiling. Then, fluid boiling and immiscibility took place in the Qz-Au-Sul stage, which led to phase separation, a rapid loss of CO 2 , an increase in the salinity of the residual fluid, and the formation of halite-bearing inclusions in the Qz-Au-Sul stage. This indicates that a system moving away from equilibrium would have resulted in the deposition of Cu, Sb, and Au. Finally, the fluids in the Qz-Cal stage were mixed with meteoric water, which significantly reduced the fluid temperature and salinity. The evolution characteristics of the ore-forming fluids in the Dagangou deposit were consistent with the fluid system and evolution regularity of orogenic deposits [72,82], especially the hydrothermal vein-type gold-polymetallic deposits in the EKO, such as Dachang, Wulonggou, Xintuo,

Geofluids
Asiha, Guoluolongwa, and Annage. They are all identified as orogenic gold deposits with the same or similar geological setting, metallogenic age, and fluid composition and evolution [1][2][3][4][5][6][7][15][16][17][18][19][20][21][22][23][24][25][26]. Finally, the Au-Ag-Cu-Sb ore bodies within the Dagangou deposit are hosted by a nearly E-W-trending fault zone within Permian-Triassic strata with no selectivity to the surrounding rock. All of them are structurally controlled by a series of nearly E-W-trending faults in the ductile shear zone, which shows an epigenetic deposit. The ore is produced in the form of sulfide-rich altered rocks or a small amount of quartz veins, both hosted in the fracture zone within the shear zone. The shear zone determines the shape, occurrence, scale, and placement of the ore bodies, and sulfides are mainly hosted in the fragmented rocks in the shear zone, producing veins of different scales, such as microveins, veinlets, or veins. Meanwhile, the wall rock around the shear zone is also altered to different degrees due to the hydrothermal fluid but weak mineralization. The ore-forming element assemblage (Au-Ag-Cu-Sb) is close to the Au-Sb of the epigenetic gold deposit, and the alteration mineral assemblage shows that the ore-bearing hydrothermal fluid is rich in CO 2 , S, and K. The alteration of the surrounding rock is linear and mainly spreads along the fault zone, with transverse zoning of tens of metres, but vertical zoning is not obvious. The Dagangou Au-Ag-Cu-Sb deposit, therefore, most likely represents an orogenic goldpolymetallic deposit with a mesozonal depth of 5.9-7.5 km.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.