Origin and Evolution of the Ore-Forming Fluids in the Liyuan Gold Deposit, Central North China Craton: Constraints from Fluid Inclusions and H-O-C Isotopic Compositions

1State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Resources, Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences, Wuhan 430074, China 2School of Earth Sciences, East China University of Technology, Nanchang 330013, China 3State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China


Introduction
Orogenic gold deposits are a distinctive class of mineral deposit that has been the source for at least one-third of world gold production [1][2][3]. This type of deposits is widely recognized in both old cratonic blocks and younger Phanerozoic mobile belts and mainly formed during transpressional to compressional events in collisional or postcollisional tectonic settings [1,2]. The orogenic gold deposits usually show the following characteristics: (1) Orebodies are structurally controlled by the faults or brittle to ductile zones, and host rocks are deformed and variably metamorphosed.
Orogenic gold deposits are extremely important gold exploration targets in China which comprise nearly 50% of the gold resources [8][9][10]. Most of the gold deposits in China concentrate in the North China Craton (NCC), and many of them have geological and geochemical signatures which suggest an orogenic gold mineralization style (e.g., [2,5,8,[11][12][13][14]). Previous studies on the gold deposits of NCC were mainly focused along the margin of the craton, such as the Jiaodong and Liaodong gold provinces (eastern margin), the Xiaoqinling gold province (southern margin), and the Jibei-Jidong and Chifeng-Chaoyang gold provinces (northern margin) (Figures 1(a) and 1(b)). However, few systematic investigations have examined the geology, geochemistry,    [28]). (b) Tectonic settings of the North China Craton, also showing the major gold districts in the North China Craton (after [29]). (c) Regional geological map of the Liyuan gold deposit, modified after Wang et al. [21]. "Red-circle": Au deposit, "orange-circle": Cu-Mo deposit, and "sky blue-circle": Pb-Zn deposit. Geofluids 3 ore-controlling structures, and ore genetic style of gold deposits within the interior NCC such as those in the northern Taihang Mountains (TM) region, central NCC, which limits our understanding of gold mineralization process in the whole NCC.
Most of the gold deposits in the northern TM region are distributed within the Precambrian metamorphic rocks and are strictly controlled by the brittle-ductile fractures, which share many similar features (i.e., mineralization style, mineral compositions of ores, hydrothermal alterations, and characteristics of ore-forming fluids) with those of orogenic gold deposits worldwide [15][16][17][18][19]. Niu et al. [16,17], Chen et al. [18], and Tian et al. [19] proposed that the deposits in the region belong to the metamorphic-hydrothermal or orogenic type gold deposits, whereas others (e.g., [20,21]) considered that the gold deposits were formed in magmatichydrothermal processes. The Shihu deposit is the largest gold deposit in the region with the gold resources of over 50 t (an average grade of 5.9 g/t), and some studies have been carried out on the Shihu gold deposit (e.g., [20][21][22][23][24]). It is suggested that the Shihu gold deposit was formed at ca.141 Ma (Quartz 40 Ar- 39 Ar age), and the ore-forming fluids in this deposit were characterized by low salinity, mesothermal, and CO 2 -H 2 O-NaCl system and were mainly derived from the lower crust with minor mantle component [20][21][22]. The Liyuan gold deposit is a newly discovered gold deposit in the northern TM region and is now still a small deposit due to the poor exploration work. Xu [25] has investigated the geology of this deposit and suggested that this deposit belongs to alteration rock type gold deposit controlled by fracture zone. Although temperature (140-350 ∘ C), salinity (1.8-12.6 wt.% NaCl equivalent), density (0.65-0.98 g/cm 3 ), and CO 2 -bearing fluid inclusions in the main stage ores were reported by Ju et al. [26], these authors did not study the early stage ore-barren quartz and related generation of fluid inclusions. Furthermore, detailed stable isotopes studies of the ore-forming fluids with an aim to constraint the gold deposition mechanism and ore genesis of the Liyuan deposit are still lacking.
Gold deposits generally form as a consequence of tectonic, geological, and geochemical conditions [27], in which the nature and evolution of ore-forming fluids are of fundamental importance [6]. Thus, an accurate assessment of characteristics and nature of the ore-forming fluids is a key to understand the origin of the deposit. In this paper, we carried out detailed field work, a combined petrographic, microthermometric, and laser Raman analysis of fluid inclusions, and H-O-C isotope analyses, to constrain the origin and evolution of the ore-forming fluids of the Liyuan gold deposit, in an attempt to provide new evidence to enhance our understanding of the ore genesis, and to promote and guide further research onto the mineralization potential and further gold exploration in the northern TM region, central NCC.

Regional Geology
The North China Craton (NCC), located between the Central Asian Orogenic Belt (CAOB) to the north and the Central China Orogen (CCO) to the south, is one of the oldest and largest cratons in the East Asia (Figure 1(a), [35,36]). The NCC consists of two Archean continental nuclei, the Western Block (WB), and the Eastern Block (EB), separated by the Trans-North China Orogen (TNCO) that formed at ca. 1.85 Ga during collision between the two blocks ( Figure 1(b), [37][38][39]).
The main geologic structures of the northern TM region are nearly NW-trending folds and NNE-trending faults. The folds which were formed primarily during Mesoarchean to Paleoproterozoic are coeval with the NNE-trending faults carrying the late Paleoproterozoic mafic-ultramafic dikes [48,49], while the regional NNE-trending faults, which cut across the Precambrian basement and Paleozoic strata, are mainly associated with the Mesozoic (Yanshanian) tectonic events [50,51]. Numerous Au, Ag, Cu, Mo, Pb, and Zn ore deposits, including the Liyuan gold deposit discussed in this paper, and the Lianbaling Pb-Zn, Mujicun Cu-Mo, Dawan Cu-Mo, and Shihu Au deposits, are distributed along the regional NNEtrending faults [52,53] (Figure 1(c)).
The Mesozoic magmatism in the northern TM region consists of volcanic rocks (Tiaojishan Formation) and large amounts of granitoids. The Tiaojiashan Formation with the age between 170 and 139 Ma is mainly composed of andesitic lava, tuff, and brecciated diorite porphyry [54][55][56]. Numerous Mesozoic granitoids such as Wang'anzhen, Mapeng, and Chiwawu batholiths (with recent obtained LA-ICP-MS and SHRIMP zircon U-Pb ages of 140-125 Ma) [57][58][59] are distributed along the NNE-trending faults in the region.

Geology of the Liyuan Gold Deposit
The Liyuan gold deposit (39 ∘ 05 00 to 39 ∘ 05 45 N, 114 ∘ 18 30 to 114 ∘ 19 30 E) is located in the central part of the northern TM region, 72 km south of the county of Lingqiu in northern Shanxi Province, China (Figure 1(c)). The gold orebodies are mainly hosted in the metamorphic rocks of the Archean Fuping Group gneisses and Archean coarsegrained K-feldspar granite (Figure 2, [26]  diabase dikes are cross cut by the auriferous quartz veins and have been strongly deformed and altered (Figure 3(a)). LA-ICP-MS zircon U-Pb analyses demonstrate that the emplace age of these diabase rocks is 1788 ± 13 Ma (our unpublished data). No granitic body has been found at the current mining levels at Liyuan, except for a few intermediate-felsic dikes, which trend NNE and NNW and are mainly composed of quartz porphyry and fine-grained granite that intruded to the Precambrian basement and yielded a LA-ICP-MS zircon U-Pb age of 134 ± 1 Ma ( Figure 2, our unpublished data).
Structurally, the ore district is characterized by the NWtrending anticline and some younger NNE-, NW-, and nearly W-E-trending faults. The F 3 fault is approximately 2,000 meters long and tens of centimeters to 15 meters wide, trending from 10 ∘ -30 ∘ and dipping 45 ∘ -65 ∘ to NW. Hydrothermal breccias, cataclastic rocks, and fault gouge are well developed along the faults, which may indicate multiple deformation events under different structural regimes [60]. Orebodies are mostly restricted to alteration zones developed along the NNE-trending F 3 fault and its subsidiary faults ( Figure 2). In addition, the NW-trending faults were formed primarily during the Precambrian and filled with diabase dikes, and many orebodies are cut by nearly W-E-trending postmineralization faults ( Figure 2).
Currently, three orebodies are mined in the Liyuan gold deposit. The mineralization features can be best represented by the largest number I orebody, which is controlled by the NNE-trending F 3 fault [25]. This orebody is more than 400 m long and 2-5 m wide (average 3.5 m), with strike 10 ∘ -30 ∘ and dips to northwest at an angle of 45 ∘ -65 ∘ . The gold grade of the number I orebody is up to 300.0 g/t Au with an average of 3.7 g/t, and the associated Ag is also of economic interests.
In the Liyuan gold deposit, the mineralization styles include veins, stockworks, disseminations, banded, and brecciated ores ( Figure 3). The Au-Ag series minerals (i.e., Agbearing native gold and Au-bearing native silver) are mainly found within the quartz-polymetallic sulfide veins/veinlets. Ag-bearing native gold typically occurs along the boundaries between pyrite and quartz (or other sulfides) (Figures 4(f) and 4(g)) or within the microfractures of pyrite (Figure 4(h)). Au-bearing native silver commonly occurs as inclusions within pyrite or quartz (Figure 4(i)). In addition, these veins/veinlets also comprise pyrite, galena, sphalerite, minor chalcopyrite, magnetite, and argentite. Most of the gold ores are associated with pyrite. Pyrite is the predominant sulfide mineral (over 80% of the total metallic minerals), shows euhedral to subhedral and granular crystals, and exhibits metasomatic dissolution textures ( Figure 4). The gangue minerals include over 85% quartz with minor amounts of sericite, muscovite, K-feldspar, chlorite, ankerite, calcite, and clay minerals ( Figure 4).
Hydrothermal alteration is well developed in the Liyuan deposit, and the main alteration types include silicic, Kfeldspar, sericite, chlorite, and carbonate alterations. Silicic alteration usually occurs as fine-grained quartz coexisting with sericite and muscovite in the alteration zone (Figures 4(a) and 4(b)) or presents as quartz-sulfide veinlets or stockworks (Figures 3(c)-3(g)). Pyrite shows a close association   ). Three hydrothermal stages have been identified in the Liyuan gold deposit on the basis of field evidence, mineralogical assemblages, petrographic observation, and cross-cutting relationships. The hydrothermal ore-forming process can be subdivided into an early quartz stage, a middle quartz-polymetallic sulfide stage, and a late quartz-carbonate stage ( Figure 5). The early stage is characterized by pyrite-sericitequartz alteration rocks (Figure 3(b)) or in places ore-barren quartz veins (Figures 3(a) and 3(c)). Minerals comprise coarse-grained milky white subhedral to anhedral quartz, sericite, muscovite (Figure 4(a)), and sparsely distributed minor coarse-to medium-grained, euhedral to subhedral pyrite. A small amount of Au-Ag series minerals is observed in this stage. Besides, the early stage quartz veins are commonly fractured and locally cut or cemented by middle stage and late stage veins (Figures 4(d) and 4(e)).
The middle stage, characterized by the quartzpolymetallic sulfide veins (Figures 4(f) and 4(g)), represents the most important gold-producing stage. Quartz in this stage is fine-grained and smoky gray and commonly shows no significant deformation, which indicates its formation in an extensional setting without compressional deformation. Sulfides, including pyrite, chalcopyrite, sphalerite, and galena, show coarse-to medium-grained euhedral crystals or subhedral aggregates and make up approximately

Fluid Inclusion Measurements.
Twenty-six doubly polished thin sections (about 200 m thick) for fluid inclusion study were selected from three mineralization stages in number I orebody of the Liyuan gold deposit. Fluid inclusions were carefully observed to identify their genetic and compositional types, vapour-liquid ratios, and spatial distribution. Eleven samples (three samples for early stage, five samples for middle stage, and three samples for late stage) with abundant and representative fluid inclusions were selected for microthermometric measurements and laser Raman spectroscopic analyses.
Fluid inclusion petrographic study was conducted using a Nikon Eclipse LV100POL microscope at the National Demonstration Center for Experimental Mineral Exploration Education, China University of Geosciences (Wuhan).
Microthermometric measurements were carried out using a Linkam THMS-600 heating-freezing stage (from −196 to 600 ∘ C) which is attached to a Leica DM2700P microscope connected to a television camera and screen at the China University of Geosciences (Wuhan). The stages were calibrated with synthetic fluid inclusions of pure H 2 O (ice melting temperature = 0 ∘ C; critical temperature = 374.1 ∘ C) and pure CO 2 (CO 2 melting temperature = −56.6 ∘ C). The heating/freezing rate is generally 0.2-5.0 ∘ C/min but was reduced to <0.2 ∘ C/min near the phase transformation. The uncertainties for the measurements are ±0.5, ±0.2 and ±2.0 ∘ C for runs in the range of −120 to −70 ∘ C, −70 to 100 ∘ C, and 100 to 600 ∘ C, respectively. Ice melting temperatures were observed at a heating rate of less than 0.1 ∘ C/min and homogenization temperatures at rate of ≤1 ∘ C/min. Five parameters of phase transformation were recorded, including final ice melting temperature ( m-ice ), melting temperature of clathrate ( m-cla ), melting temperature of solid CO 2 ( m-CO 2 ), homogenization temperature of CO 2 liquid and vapour ( h-CO 2 ), and total homogenization temperature ( h ), depending on the types of FIs. Salinities were calculated using the equations of Bodnar [61] for aqueous fluid inclusions and of Collins [62] for aqueous-carbonic fluid inclusions. Densities and pressures were estimated using the Flincor software [32] packages according to microthermometric data for CO 2 -H 2 O-NaCl system [33].
Fluid inclusion laser Raman spectroscopy analyses were performed at the State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences (Wuhan), using a Renishaw RM-1000 Raman microscope. The laser source was an argon laser with a surface power of 5 mW for exciting the radiation (514.5 nm). The area of charge-coupled device (CCD) detector is 20 m 2 , and the scanning range of spectra was set between 1200 and 3800 cm −1 , with an accumulation of 30 s for each scan and spectral resolution of ±2 cm −1 . Instruments settings were kept constant during all analyses.

Hydrogen, Oxygen, and Carbon Isotope Analyses.
Four quartz samples were selected for mineral oxygen and fluid inclusion hydrogen and carbon isotope analyses, and they represent the corresponding early (LY13-1-6), middle (LY13-2-1 and LY13-2-11), and late stages (15LYD-12). All the samples were crushed into 40-60 meshes and pure quartz separates were selected. The hydrogen, oxygen, and carbon isotopes were analyzed on a Finnigan MAT253 mass spectrometer in the Laboratory of Stable Isotope Geology, Chinese Academy of Geological Sciences, Beijing. Oxygen (O 2 ) was generated from the quartz by quantitatively reaction with BrF 5 [34] and converted to CO 2 on a platinum-coated carbon rod. Water extracted from the fluid inclusions within quartz separates was reacted with zinc at ca. 400 ∘ C and the released H 2 for mass spectrometric measurement. CO 2 was liberated from carbonic fluid inclusions in quartz separates by thermal decrepitation under vacuum and then measured for 13 C by mass spectrometry. The results were normalized to V-SMOW standards and the precisions were ±2‰ for D and ±0.2‰ for 18 O and 13 C. The oxygen isotope values of water in equilibrium with the minerals were calculated using the fractionation formula 1000 ln quartz-H 2 O = 3.38 × 10 6 −2 − 3.40 reported by Clayton et al. [34], where is the mean value of the homogenization temperature of fluid inclusions from the same quartz samples plus pressure-corrected temperature as discussed below.

Petrography and Types 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 [63,64]. Three types, including aqueous-carbonic fluid inclusions (C-type), pure carbonic fluid inclusions (PCtype), and aqueous fluid inclusions (W-type), were identified in quartz and calcite grains from the Liyuan gold deposit based on their phases at room temperature (25 ∘ C), phase transitions observed during heating and freezing, and compositions by using laser Raman spectroscopy. The classification of the FIs in this study is also consistent with previous study carried out by Ju et al. [26].

Aqueous-Carbonic Fluid Inclusions (C-Type
). The Ctype FIs are the most abundant type in the quartz grains of early and middle stages, which occupy 50% of the total primary FIs. They consist of two (liquid H 2 O + liquid CO 2 ) or three phases (liquid H 2 O + liquid CO 2 + vapour CO 2 ) at room temperature (25 ∘ C) (Figures 6(a)-6(d)), with the CO 2 volumetric proportion ranges of 10% to 90%. C-type FIs show ellipsoidal, irregular, or negative crystal shapes with sizes ranging from 4 to 30 m (mostly 5-15 m).

Pure Carbonic Fluid Inclusions (PC-Type).
The PC-type FIs, accounting for approximately 10% of the total primary FIs population, are mono-(vapour or liquid CO 2 ) or two-phase (liquid CO 2 + vapour CO 2 ) at room temperature (25 ∘ C) (Figures 6(e) and 6(f)) and commonly coexist with C-type FIs in the early and middle stages. They usually show oval and irregular shapes with the size varying between 4 and 12 m. Inclusions (W-Type). The W-type FIs are present in all mineralization stages and account for 40% of the total FIs population. They commonly appear as two-phase (liquid H 2 O + vapour H 2 O) FIs at room temperature (25 ∘ C), with bubbles accounting for 5-20% in volume (Figures 6(g) and 6(h)). The W-type FIs are negative crystal, ellipsoidal, and irregular in shape and 2 to 15 m in size. They commonly occur in an assemblage with C-and PC-type FIs in the middle mineralization stage (Figures 6(i)-6(k)). In addition, the primary W-type FIs occur as isolated or clusters, whereas some W-type FIs may distribute along microfractures ( Figure 6(l)), indicating a secondary origin.

Microthermometry.
Microthermometric data collected for the C-, W-, and PC-type primary fluid inclusions of the three mineralization stages are summarized in Table 1 and presented in Figures 7 and 8, which clearly show the association between mineral assemblages, physic-chemical conditions, and mineralization stages. The microthermometric data from Ju et al. [26] of middle and late stages are also listed in Table 1 for comparison.

Early Stage.
Fluid inclusions in the early stage quartz are dominantly C-type and W-type, with minor PC-type FIs. The melting temperatures ( m-CO 2 ) of solid CO 2 in the Ctype FIs range from −58.3 to −56.8 ∘ C (Figure 7(a)), which is very close to triple point of pure CO 2 (−56.6 ∘ C) and indicates that the carbonic phase in these FIs is nearly pure CO 2 . Clathrate melting temperatures ( m-cla ) show a range from 5.6 to 8.9 ∘ C, with corresponding salinities from 2.2 to 8.2 wt.% NaCl equivalent (Figure 8(b)). The carbonic phase ( h-CO 2 ) of C-type FIs was homogenized to liquid from 18.6 to 29.5 ∘ C (Figure 7(a)). The total homogenization temperatures ( h ) of C-type FIs range from 318 to 392 ∘ C (Figure 8(a)), generally homogenized to both liquid and vapour phases and with no systematic difference among homogenizing to liquid or vapour. The calculated CO 2 densities range from 0.62 to 0.79 g/cm 3 and bulk densities from 0.71 to 0.96 g/cm 3 (Table 1). For PC-type FIs, melting temperatures ( m-CO 2 ) of solid CO 2 show a range from −58.1 to −56.9 ∘ C (Figure 7(a)), suggesting that the components of these inclusions are nearly pure CO 2 . The homogenization temperatures ( h-CO 2 ) of CO 2 are 19.8 to 30.2 ∘ C (all homogenize to liquid phase) (Figure 7(b)), with corresponding densities of 0.59 to 0.78 g/cm 3 . The W-type inclusions yield final ice melting temperatures ( m-ice ) of −5.8 to −1.2 ∘ C and salinities of 2.1-8.9 wt.% NaCl equivalent (Figure 8(b)). In addition, these inclusions show a total homogenization to liquid at temperatures between 320 and 408 ∘ C (Figure 8(a)), with the calculated densities of 0.56 to 0.76 g/cm 3 (Table 1).

Middle Stage.
Middle stage quartz crystals mainly contain C-, W-, and PC-type fluid inclusions, with C-type FIs being most abundant. The solid CO 2 of the C-type FIs yielded the melting temperatures ( m-CO 2 ) of −60.3 to −56.9 ∘ C (Figure 7(c)), which are slightly lower than the triple point of pure CO 2 (−56.6 ∘ C), suggesting possible presence of minor amounts of other gases such as CH 4 , which were actually detected using Raman spectroscopy (Figure 9(d)). The CO 2 clathrate was melted at the temperatures of 3.1 to 9.6 ∘ C, with corresponding salinities range from 0.8 to 11.8 wt.% NaCl equivalent (Figure 8(d)). CO 2 generally homogenized to the liquid phase and h-CO 2 range from 18.7 to 30.8 ∘ C (Figure 7(d)). The densities of the CO 2 phase are 0.54 to 0.79 g/cm 3 , and the densities of the bulk inclusions are 0.70 to 0.95 g/cm 3 ( Table 1). The C-type FIs are totally homogenized to liquid or vapour at 229 to 327 ∘ C (Figure 8(c)). For PC-type FIs, final melting to liquid was observed during heating process, with m-CO 2 ranging from −59.3 to −56.9 ∘ C (Figure 7(c)). Homogenization (to liquid) temperatures of CO 2 ( h-CO 2 ) vary from 17.1 to 28.7 ∘ C (Figure 7(d)), corresponding to densities of 0.64 to 0.80 g/cm 3 (Table 1). W-type FIs in the middle stage quartz have final ice melting temperatures of −8.6 to −0.3 ∘ C and salinities of 0.5 to 12.4 wt.% NaCl equivalent (Figure 8(d)). They homogenized to liquid at 201 to 329 ∘ C (Figure 8(c)) with the densities of 0.75 to 0.95 g/cm 3 .

Late Stage.
Only W-type FIs are present in quartzcarbonate veins and veinlets in this stage. Their m-ice range from −3.9 to −0.2 ∘ C, corresponding to salinities of 0.4 to 6.3 wt.% NaCl equivalent (Figure 8(f)). They are totally homogenized to liquid at 134 to 227 ∘ C (Figure 8(e)), and their calculated densities are 0.85 to 0.96 g/cm 3 .

Raman Spectroscopy Analysis.
Representative fluid inclusions were measured with laser Raman spectroscopy to determine their gas compositions ( Figure 9). The results show that the vapour phase of PC-type and C-type FIs in the early stage was dominated by CO 2 (Figures 9(a) and 9(b)), and the liquid phase of C-type inclusions has large amount of water (Figure 9(b)). The PC-type and C-type FIs of the early stage show well-defined CO 2 peaks (Figures 9(a) and 9(b)). The PC-type FIs of the middle stage show well-defined CO 2 peaks and slightly CH 4 peaks (Figure 9(c)), and the C-type FIs from the middle stage contain a vapour phase of CO 2 with minor amounts of CH 4 and a liquid phase with a large amount of water (Figures 9(d) and 9(e)). Both vapour and liquid phases of the W-type FIs in the late stage are dominated by water; other gas phases were barely found (Figure 9(f)). The results are also consistent with previous gas analyses of crashed of groups of fluid inclusions [26] that the gas compositions of fluid inclusions are dominated by CO 2 , followed by trace amount of CH 4 , CO, H 2 , and N 2 . No CO, H 2 , or N 2 were detected by laser Raman in this study which might be due to their low content in the vapour phase of the fluid inclusions.

Fluid Immiscibility and Pressure Estimations.
In the Liyuan deposit, abundant primary W-type, C-type, and PC-type FIs coexist in a given cluster in both early and middle stage quartz grains (Figures 6(i)-6(k)). In spite of postentrapment modifications (such as subsequent necking during annealing of microfractures) which might cause coexistence of the W-type, C-type, and PC-type FIs in the same sample [65,66], fluid inclusions were studied in samples that are absence of abundant of secondary inclusions trails and exhibiting no evidence of postentrapment modification (i.e., stretching, leaking, and necking down; e.g., [67]). All the fluid inclusions selected for microthermometry and laser Raman spectroscopy are primary in origin, which generally occurred in undeformed quartz grains and with regular shapes. Fluid immiscibility is commonly associated with gold and sulfide deposition in a variety of hydrothermal deposits, including the Yanzhaiyu gold deposit in Xiaoqinling [68], the  Xincheng [69] and Taishang [70] gold deposits in Jiaiodong, and the Jinman Cu deposit in Yunnan, China [71]. Ramboz et al. [72] defined four criteria which can be used to recognize fluid immiscibility, including (1) the contemporaneous trapping of different types of fluid inclusions; (2) trapped fluid inclusions with varying scattered degree of filling; (3) two completely different modes of total homogenization of the liquid-rich (L + V → L) and vapour-rich (L + V → V) fluid inclusions within the same range of temperature; (4) the relative concentration of chemical components in the liquid and vapour phase which should be appropriate to equilibrium fractionation at total homogenization. Evidences for fluid immiscibility at the Liyuan gold deposit include the following: (1) There are three different types of fluid inclusions, including aqueous-carbonic (C-type), pure carbonic (PCtype), and aqueous fluid inclusions (W-type), in the Liyuan gold deposit. They coexist with each other in the early and middle stages.
(3) In the early and middle stages, W-type and C-type FIs have similar total homogeneous temperatures (Figures  [34]. The temperatures used in calculation are the mean values of the homogenization temperatures from the same quartz samples plus pressure corrected temperatures; that is, = h + (P correction); 13 C CO 2 values refer to CO 2 released from fluid inclusions. 8(a) and 8(c)). C-type FIs show two completely different modes of total homogenization of the liquid-rich (L + V → L) and vapour-rich (L + V → V) fluid inclusions within approximately the same temperature range (  Figure 7), indicating that they were trapped coevally and could be the same H 2 O-CO 2 -NaCl solution. PC-type FIs probably represent an unmixing CO 2 -rich endmember. Although a water phase is not visible, the PC-type inclusions may contain up to 15 vol.% water since the water prefers to adhere to the inclusion walls as a thin film [72][73][74].
(4) C-type FIs show slightly lower salinities (median values of 4.26 and 5.33 wt.% NaCl equivalent in the early and middle stages, resp.) than the W-type FIs (median values of 5.86 and 8.55 wt.% NaCl equivalent in the early and middle stages, resp.) (Figures 8(b) and 8(d)), which is consistent with phase separation [75].
Therefore, fluid immiscibility is a likely explanation for characteristics of fluid inclusions in the early and middle stages.
Trapping pressure-temperature estimation can be made using isochores determined from the fluid inclusions [76]. When immiscibility occurs, a homogenous fluid splits into a more dense and saline phase endmember and a volatilerich phase endmember [77,78]. Trapping pressure can be approximated from endmember inclusions trapped under immiscible conditions [74,[77][78][79]. Based on our study, immiscible entrapment is interpreted to have occurred at the early and middle stages at Liyuan. Therefore, we selected Ctype fluid inclusions in the early and middle stages with the low and high CO 2 contents to estimate trapping pressures. Isochores were calculated based on the H 2 O-CO 2 -NaCl system using the formula of Bowers and Helgeson [33] and Flincor program [10,33,80].
Given that the total homogenization temperatures of fluid inclusions can be only used to provide minimum formation temperature for the quartz veins/veinlets, the pressure estimates are always difficult [81]. Therefore, we use the same logic as used by Fan et al. [82] to revise the treatment of pressure and temperature. Measured total homogenization temperatures are set as , and using these values as minimum temperature, a preliminary minimum pressure 1 can be obtained from isochores. Using this value of 1, trapping temperatures could be used as + . This revised trapping Three purple arrows marked as A, B, and C represent the fluid evolution tendency from early to late stages. The arrows A and B mean that the FIs have wider salinities range from early to middle stage due to the fluid immiscibility. The arrow C represents that the temperature and salinities both decrease from middle to late stages. temperature could be used to calculate a more accurate pressure of 2.
In the case of Liyuan, since the total homogenization temperatures and bulk densities of C-type FIs have variable ranges, upper and lower quartiles are used to better reflect the distribution of the data (e.g., [83]). For the C-type FIs in the early stage, upper and lower quartiles of homogenization temperatures are 343 ∘ C and 367 ∘ C, respectively, and the associated C-type FIs with the low and high CO 2 contents have bulk densities of 0.89 g/cm 3 and 0.77 g/cm 3 (Figure 10). The preliminary pressure from Figure 11 is about 140 MPa at 343 ∘ C. The pressure correction at 140 MPa is about 120 ∘ C [63]. Thus, the trapping temperatures for the early stage Ctype fluid inclusions are from 463 to 487 ∘ C. The recalculated trapping pressure is 239 to 347 MPa ( Figure 11). In the middle stage, the upper and lower quartiles of total homogenization temperatures of C-type FIs are 246 to 278 ∘ C, respectively, and the associated C-type FIs with the low and high CO 2 contents have bulk densities of 0.92 g/cm 3 and 0.81 g/cm 3 . The preliminary minimum pressure from Figure 11 is about 70 MPa, and according to Roedder [63] the pressure correction at 70 MPa is about 70 ∘ C. Thus, the trapping temperatures for the C-type FIs from the middle stage are 316 to 348 ∘ C. The recalculated trapping pressure for the C-type FIs is 131 to 224 MPa ( Figure 11).
The trapping pressures for the C-type FIs of early stage (239-347 MPa) are higher than those of middle stage (131-224 MPa), indicating a pressure decrease during the oreforming process. The variations of the trapping pressures might reflect fluctuations between hydrostatic and lithostatic conditions during fluid immiscibility [84], and this situation is common in orogenic gold deposits (e.g., [85][86][87]). In the Liyuan deposit, minor CH 4 was found in the carbonic phase of the C-type and PC-type FIs in the middle stage. Additional volatile components such as CH 4 could expand the temperature and pressure range of fluid immiscibility and make the occurrence of fluid immiscibility much easier [88,89]. At Liyuan, we suggest that the fluid immiscibility was induced by pressure drop and temperature decrease, which could lower the CO 2 solubility in the H 2 O-CO 2 -NaCl system [33,90], and the presence of CH 4 in the middle stage could make the fluid immiscibility become easier.

Source of Ore-Forming Fluids.
The oxygen isotopic compositions of the early and middle stages ore-forming fluids of the Liyuan deposit are similar to those of magmatic water or metamorphic water, but the hydrogen isotopes plot slightly below the magmatic water or metamorphic water fields, and the late stage fluids show both lower oxygen and hydrogen isotopic compositions than the magmatic water or metamorphic water in the D vs 18 O diagram (Figure 12). Also shown are the isotopic fields for common metamorphic and primary magmatic water [30]. The field for orogenic gold deposit revised after Goldfarb et al. [31].
This feature suggests that the primary ore-forming fluids of the Liyuan gold deposit were genetically akin to either magmatic water or metamorphic water. The calculated 18 O w values of ore-forming fluids of the three hydrothermal stages show a decrease from 9.5‰ to 5.7‰ and to 2.2‰, and that from the late stage are significantly lighter than those of primary magmatic water (5 to 10‰, [91]), suggesting an increasing amount of meteoric water mixing with the magmatic or metamorphic fluids, especially in the late stage. The lower D values may also attribute to the existence of CH 4 in the ore-forming fluids. The ore-forming fluids at Liyuan have 13 C CO 2 values of −8.9 to −7.1‰, with an average of −7.9‰, which are similar to or slightly lower than those of average mantle value (-7 to -5‰, [92,93]), indicating that the carbon in the Liyuan deposit was likely derived from a deep magmatic source. In addition, the 13 C CO 2 values are slightly lower than those of average mantle and might be affected by an isotopic fractionation due to chemical reactions such as carbonate decomposition or contaminations of minor CH 4 gas related to metamorphism [94]. The characteristics of the ore-forming fluids, such as moderate to low temperature, low salinity, and high CO 2 content in the fluid inclusions, are typical of the majority of orogenic gold deposits worldwide (e.g., [1,2,4,95,96]). Although many of these deposits have been attributed to be formed by metamorphic fluids (e.g., [97,98]), models of magmatic fluid origin with admixing with local meteoric water have also been proposed in many orogenic gold deposits (e.g., [82,[99][100][101][102]). In the case of the Liyuan gold deposit, given that the late Mesozoic age of gold mineralization in the northern TM region is over 2 billion years younger than the age of metamorphic rocks, the regional metamorphic fluids associated with those Precambrian rocks are not a viable source, whereas the deep-seated magmatic water would have generated the ore-forming fluids, although currently we did not found any granite body at Liyuan but a concealed granite body at greater depths is possible according to recent geophysical survey (personal communications) although further work is still needed to test this hypothesis.

Fluid Evolution and Gold
Deposition. Fluid inclusion microthermometry and laser Raman spectroscopy indicate dominant C-type FIs and subordinate W-and PC-type FIs are developed in the early stage quartz. In this stage, ore-forming fluids belong to H 2 O-CO 2 -NaCl system, which featured by moderate temperatures (318 to 408 ∘ C), CO 2 -bearing, and low salinity (2.1 to 8.9 wt.% NaCl equivalent). In the middle stage, C-type FIs are dominant, and W-and PC-type FIs are also present. The ore-forming fluids change a little and belong to H 2 O-CO 2 -NaCl ± CH 4 system, which featured by moderate temperature (201 to 329 ∘ C), lower CO 2 with minor CH 4 , but higher salinity (0.5 to 12.4 wt.% NaCl equivalent), which is consistent with fluid immiscibility in this stage. During the phase separation, salt is preferentially fractionated into the aqueous phase, which leads to the result that the salinities of W-type FIs (0.5 to 12.4 wt.% NaCl equivalent, with a median value of 8.55 wt.% NaCl equivalent ) were slightly higher than those of C-type FIs (0.8 to 11.8 wt.% NaCl equivalent, with an median value of 5.33 wt. % NaCl equivalent) [75]. In the late stage, the ore-forming fluids finally evolved into a low temperature (136 to 229 ∘ C), low salinity (0.4 to 6.3 wt.% NaCl equivalent), and CO 2 -poor H 2 O-NaCl system which is similar to the fluids sourced from meteoric water and that is also supported by the hydrogen-oxygen isotope data ( Figure 12).
As shown above, the gold orebodies of the Liyuan deposit are mainly hosted within the Archaean gneisses, and plagioclase, hornblende, and ilmenite are very common in the gneisses, and the quartz, K-feldspar, titanite, and chlorite are the typical alteration minerals that are well developed in the altered gneisses [26]. The minor CH 4 found in the C-type and PC-type FIs in the middle stage could be the product of the following reactions [103]: Therefore, the presence of CH 4 in the middle stage was most likely related to the water-rock interaction between oreforming fluids and the wall rocks at Liyuan.
Gold is predominantly transported as gold chloride (AuCl 2− ) and gold bisulfide complexes (Au(HS) 2 − ) in hydrothermal solutions [104][105][106][107]. Commonly, the AuCl 2− complexes are predominant in near-neutral to weakly alkaline, high salinity, and H 2 S-poor ore fluids at high temperatures (>400 ∘ C, [108]), whereas for near-neutral to weakly acidic pH in sulfur-bearing solutions and at relatively low temperatures (<400 ∘ C), the Au(HS) 2 − complexes play a more important role to transport gold [109,110]. The K-feldsparsericite mineral assemblages, developed not only in the alteration zone but also in the quartz vein, indicate that the ore-forming fluids of the Liyuan gold deposit are nearneutral to weakly acidic (pH = 5.2-6.8, [69,111]). Taking into consideration that the ore-forming fluids were rich in CO 2 at 201-408 ∘ C, it is likely that the gold was transported by Au(HS) 2 − complexes in the ore-forming fluids, which is also consistent with the fact that gold is usually accompanied with sulfides (especially pyrite, Figures 4(h) and 4(i)). Thus, gold deposition at Liyuan probably is related to the breakdown of gold bisulfide complexes (Au(HS) 2 − ). Previous experimental and theoretical studies have shown that when temperature is below 400 ∘ C, a decrease in temperature at consistent fluid composition would enhance Au(HS) 2 − solubility [112]. Therefore, simple cooling of ore-forming fluids from early to middle stage is not an effective gold deposition mechanism for the Liyuan deposit. However, H 2 S loss and a decrease in HS − activity could decrease solubility effectively [88].
Fluid immiscibility and water-rock interaction are the two possible mechanisms that could result in the H 2 S loss of the ore-forming fluids [113][114][115][116]. In the case of Liyuan, primary fluid inclusions in quartz from both early and middle stages show evidence of fluid immiscibility. However, the early stage quartz veins/veinlets only contain minor pyrite and trace amount of gold. Thus, the fluid immiscibility might not play an important role of the gold deposition at Liyuan. Given that gold is associated with widespread pyrite-sericitequartz alteration in the Liyuan deposit ( Figure 3) and the close association of gold minerals and sulfides (especially pyrite, Figures 4(h) and 4(i)), the reaction between orebearing fluids and iron-bearing wall rocks to form pyrite is probably a more effective means of decreasing the H 2 S concentration and leads to gold deposition [113].
6.4. Comparison to Orogenic Gold Deposits. As mentioned above, many geological and geochemical features of the Liyuan gold deposit are consistent with typical orogenic gold deposits worldwide [5,86,117]. These features include the following: (1) The orebodies which are mainly hosted in the Archean Fuping Group metamorphic rocks and are structurally controlled by the NNE-trending faults.
(2) Multiple stages of mineralization of the disseminated and quartz vein and veinlet/stockworks ores.
(5) The ore-forming fluids which were low to moderate temperature, H 2 O-CO 2 -NaCl-(CH 4 ) system with low salinity, and the fluid which was rich in CO 2 .
(6) The fluids transformed from CO 2 -rich ore-forming fluids to CO 2 -poor meteoric water during fluid evolution.
These features supported the fact that the Liyuan deposit could be classified as an orogenic type gold deposit. Although hosted in the Archean basement rocks, the gold deposits in the TM region of central NCC are most likely Early Cretaceous in age. For example, quartz separates from the auriferous quartz veins hosted in the Precambrian metamorphic rocks in the Shihu and Yixingzhai gold deposits in the same region yielded 40 Ar-39 Ar ages of 141 ± 4 Ma [22] and 131 ± 3 Ma [118], respectively. These ages preclude a genetic relationship between gold mineralization and the Neoarchean metamorphism in the central NCC [40][41][42][43] but may suggest that the Liyuan deposit was formed during the Yanshanian magmatism although further direct age data are needed for this deposit [20,21,[119][120][121]. Moreover, the oreforming fluids of Liyuan belong to the H 2 O-CO 2 -NaCl ± CH 4 system with the characteristics of medium temperature (mostly < 400 ∘ C), enrichment of CO 2 , and medium-low salinities (<12.4 wt% NaCl equivalent). They are very different from typical magmatic fluids, which are characterized by high temperature and high salinity [122]. Thus, the Liyuan deposit could not be classified as "intrusion-related gold deposit." The eastern China became part of the Pacific margin tectonic during Jurassic to Cretaceous when the major tectonic transformation from N-S compression to NNE-SSW shearing was gradually completed. Maruyama et al. [123] considered this region as a continental margin orogenic belt, and its landward boundary was the NNE-trending N-S Gravity Lineament (NSGL, Figure 1(b)). The Taihang Mountains are part of the typical NSGL in the central NCC [124]; thus, although far from the eastern margin of NCC, the region could be affected by the subduction of the Paleo-Pacific plate, and large scale fluid circulation, granitic magmatism, and metallogenesis took place during the Jurassic to Cretaceous in the TM region.

Conclusions
(1) The Liyuan deposit is a structurally controlled gold deposit in the Taihang Mountains region of central NCC. The mineralization at Liyuan can be subdivided into three stages: an early quartz stage, a middle quartz-polymetallic stage, and a late quartz-carbonate stage.
(2) Three types of primary fluid inclusions are identified: the aqueous-carbonic (C-type), pure carbonic (PC-type), and aqueous fluid inclusions (W-type). The early and middle stage quartz crystals contain all three types of primary fluid inclusions, and the late stage quartz and calcite contain only W-type inclusions.
(3) The initial ore-forming fluids were either magmatic water, possibly derived from a concealed granite body, or metamorphic water, directly transported via deep faults from deep crust, and meteoric water played an increasing important role during the fluid evolution and in particular in the late stage.
(4) Pressure drop, temperature decrease, and the additional CH 4 derived from water-rock interaction caused large scale fluid immiscibility in the middle stage at pressures of 131-224 MPa. Fluid-rock interaction between ore-bearing fluids and iron-bearing wall rock is interpreted to be the main mechanisms of gold deposition at Liyuan. (5) The Liyuan gold deposit shares many similar features with those of orogenic gold deposits worldwide, and therefore we suggest that this deposit belongs to orogenic type gold deposit.

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