Source of Ore-Forming Fluids and Ore Genesis of the Batailing Au Deposit, Central Jilin Province, Northeast China: Constraints from Fluid Inclusions and H-O-C-S-Pb Isotopes

Abstract

The Batailing Au deposit is a vein-type deposit in central Jilin Province, situated in the southern sector of the Lesser Xing’an–Zhangguangcai Range within the eastern Central Asian Orogenic Belt. NE-trending fault-controlled orebodies occur in the Upper Permian Yangjiagou Formation and quartz diorite–porphyrite. The mineralisation process was delineated into three stages: (I) quartz–arsenopyrite–pyrite, (II) quartz–polymetallic sulphides (main Au mineralisation stage), and (III) quartz–pyrite–carbonate. Fluid inclusions (FIs) in quartz were identified as four types: PC-type (pure CO₂), C₁-type (CO₂-bearing), C₂-type (CO₂-rich), and W-type (aqueous two-phase). Raman spectroscopy analysis revealed that the vapor components of the FIs predominantly comprised CO₂ with minor quantities of CH₄ in stages I–II. Stages I and II encompassed four types of FIs with homogenisation temperature ranging from 264 to 332 °C and 213 to 292 °C and salinity spanning from 4.7 to 11.2 wt% and 1.8 to 11.6 wt%, respectively. Stage III exclusively contained W-type FIs with homogenisation temperature ranging from 152 to 215 °C and salinity spanning from 1.4 to 6.4 wt%. H-O isotopic values (δD = −84 to −79.6‰, δ¹⁸O_H2O = 6.2 to 6.4‰ in stage I and δD = −96.4 to −90.4‰, δ¹⁸O_H2O = 2.8 to 4.4‰ in stage II) and microthermometric data indicated that the ore-forming fluids are initially from a magmatic source, with later meteoric water input. Low C isotopic data from CO₂ in FIs in quartz (−24.4 to −24.3‰ in stage I and −23.7 to −22.6‰ in stage II) indicated an organic carbon source. Ore precipitation is mainly attributable to fluid immiscibility. S-Pb isotopic data (δ³⁴S = −3.5 to −1.6‰; ²⁰⁶Pb/²⁰⁴Pb = 18.325–18.362, ²⁰⁷Pb/²⁰⁴Pb = 15.523–5.562, ²⁰⁸Pb/²⁰⁴Pb = 38.064–38.221) revealed that ore metals primarily originated from magma. Based on this research, the origin of the Batailing Au deposit is of the mesothermal magmatic–hydrothermal lode type.

1. Introduction

Jilin Province, an important Au-Cu polymetallic metallogenic area in Northeast (NE) China, is positioned at the junction of the eastern part of the Central Asian Orogenic Belt (CAOB) and the north-eastern segment of the North China Craton (NCC) (Figure 1a). This region has undergone complex tectonic evolution, from the subduction and closure of the Palaeo-Asian Ocean in the Palaeozoic to the superposition and transformation of the Palaeo-Pacific Ocean in the Mesozoic [1,2,3,4]. These complex tectonic events have made this region an important Au polymetallic metallogenic area [5,6,7]. In Jilin Province, the Au polymetallic deposits are mainly distributed in the southern part (Jiapigou gold belt), eastern part (Yanbian area), and central part of Jilin. Among them, the Jiapigou gold belt and Yanbian area have been studied relatively extensively, while there has been relatively little research on the central part of Jilin [8,9,10,11,12,13,14,15].

The Jiapigou gold belt contains many large to medium Au deposits, such as the Benqu, Bajiazi, Erdaogou, and Xiaobeigou. These deposits are of the magmatic–hydrothermal lode type and mainly formed in the Early–Middle Jurassic (170–178 Ma), in the tectonic setting of the Palaeo-Pacific Plate subduction [4,16,17,18,19,20].

The Yanbian area also contains many Au (Cu) deposits, such as the Xiaoxinancha, Duhuangling, and Ciweigou. These deposits were formed in the Early Cretaceous (101–118 Ma) in an extensional tectonic setting, attributed to the Palaeo-Pacific Plate subduction rollback [21,22,23]. The ore-forming fluids in these deposits were mainly NaCl-H₂O systems [23,24,25,26].

The central part of Jilin contains small to medium Au deposits such as the Cuyu, Guanma, Ailin, Lanjia, Batailing, and Jiangjiagou. Studies on some of these deposits show that they were formed in Early–Middle Jurassic (169–190 Ma) in the tectonic setting of the Palaeo-Pacific Plate subduction [10,14,15]. However, compared to the Au deposits in the southern and eastern parts of Jilin, most Au deposits in the central part of Jilin have not been subject to systematic research on the origins of fluids and materials, geochronology, fluid evolution, and ore genesis [9,11,12,13], which limits the development of metallogenic theory and the implementation of Au exploration in central Jilin.

The Batailing deposit is a small Au deposit situated in central Jilin in the south segment of the Lesser Xing’an–Zhangguangcai Range (LXZR), NE China. Since the discovery of the Batailing Au deposit, the total proven Au metal reserves have exceeded 2.5 t, averaging 3.78 g/t. The geochemical anomalies show that the Batailing Au deposit has more industrial value than the Lanjia Au deposit in Central Jilin [27]. At present, the drilling works are mainly concentrated in one of the three ore zones, but current data from a geological team show that Au mineralisation occurs in the two other ore zones, indicating that the Batailing Au deposit may have prospecting potential for a large resource. Previous research on the Batailing Au deposit only concentrated on ore deposit geology [9]. Nevertheless, the sources and evolution of fluids, origin of materials, mineralisation mechanisms, and ore genesis of the Batailing Au deposit are elusive.

In this study, we conducted comprehensive research on the geology, mineralisation, and fluid inclusions (FIs) of the Batailing Au deposit. We discuss findings from H-O-C-S-Pb isotope and FI analyses to elucidate the evolution and sources of fluids, the origin of metals, and mineralisation mechanisms, with the ultimate goal of characterizing the ore genesis of the Batailing Au deposit. This study can improve the metallogenic theory of Au deposits in the central part of Jilin and provide evidence for the comparison of those Au deposits with those in the eastern and southern parts of Jilin.

2. Regional Geology

Northeast China, specifically the eastern segment of the CAOB, adjacent to the NCC, was divided from south to north into the Jiamusi, Songnen, Xing’an, and Erguna blocks by the five regional faults (Figure 1a,b; [1,2,28,29]). Central Jilin lies on the south-eastern part of the Songnen Block in Northeast China (Figure 1b).

The strata in this region include Early–Late Palaeozoic, Mesozoic, and Cenozoic sediments (Figure 1c). The Early Palaeozoic strata consist of basic metamorphic and intermediate volcanic rocks, including metasandstones with marble. The Late Palaeozoic strata are composed of the Carboniferous Luquantun, Shizuizi, and Mopanshan Formations and the Permian Shoushangou, Fanjiatun, and Yangjiagou Formations. The Carboniferous and Permian strata consist of andesite, tuff and acidic tuff, sandstone, siltstone, marble, black mudstone, and carbonaceous slate [6,10,14,15,28]. The Carboniferous and Permian strata are the main Au ore hosts in this region [10,13,15]. The Mesozoic strata, including the Triassic Sihetun Formation and the Jurassic Yuxingtun and Nanloushan Formations, are composed of rhyolitic tuff, thin-layer black tuffaceous slate, and continental volcanic rock [6,30,31]. Cenozoic sediments are widely exposed in this region, as shown in Figure 1c.

The region’s structures include NE- and NW-trending faults and folds. The NE-trending faults control the distribution of deposits. The NW-trending faults are secondary faults (Figure 1c; [32]). These faults are characterised by ductile shearing and shallow brittle deformation [8]. The Jizhong synclinorium represents the main fold, and its axial direction is EW-trending [33].

Magmatic activities from multiple periods have been identified in the region, including the Palaeozoic, Late Triassic, and Early–Middle Jurassic periods. Palaeozoic rocks are exposed in the southern part of the region (Figure 1c). The tonalite reported in Zhangjiatun (443 ± 5 Ma; [34]) is one of the few Early Palaeozoic magmatic activities. Late Palaeozoic rocks are widely exposed and mainly include granodiorite (254 ± 1.5 Ma) and monzogranite (280 ± 1.2 Ma; 279 ± 0.9 Ma) [15]. Magmatic activity has been frequent since the Mesozoic, resulting in abundant intrusive rocks. Triassic intrusive rocks mainly include granodiorite and monzogranite, with minor amounts of diorite and gabbro, which are associated with the Palaeo-Asian Ocean tectonic domain (228–201 Ma; [35,36,37]). Early–Middle Jurassic rocks include granodiorite, diorite–porphyrite, and monzogranite (200–165 Ma; [11,15,38]), which were influenced by Palaeo-Pacific Ocean. Early–Middle Jurassic intermediate–acidic dioritic rocks are generally associated with Au mineralisation [11,13,15].

3. Deposit Geology

The Batailing Au deposit is located in the southern region of the LXZR in central Jilin, Northeast China (Figure 1c). The exposed strata in the mining area consist of the Lower Permian Fanjiatun and Upper Permian Yangjiagou Formations. The strata generally trend NE and dip at 40–60°. The Lower Permian Fanjiatun Formation, situated in the south-eastern sector of the Batailing Au deposit, consists of andesitic conglomerate, sandstone, andesite, andesitic tuff, and breccia tuff. Moreover, the Lower Permian Fanjiatun Formation has undergone low-level regional metamorphism, with visible silty metamorphic and metamorphic fine sandstone. The Upper Permian Yangjiagou Formation, situated in the north-western sector of the mining area, consists of black carbonaceous slate, phyllite, hornblende andesite, meta-andesite, and argillaceous sandy silty slate. The orebodies predominantly occur within the Yangjiagou Formation (Figure 2a–c).

Structurally, the Batailing Au deposit is characterised by the NE-trending Nuanquanzi anticline and many NE- and secondary NW-trending faults. The Nuanquanzi anticline is composed of the Yangjiagou and Fanjiatun Formations, and the orebodies are exposed on the NW side of the fold. The main NE-trending faults dip to the NW at 40–70° and are ore-controlled. Strongly silicified fractured altered rocks developed in NE-trending fault fracture zones. Secondary NW-trending faults disrupt the strata and have a destructive effect on the orebodies (Figure 2a).

The magmatic rocks in the Batailing Au deposit are mainly composed of quartz diorite–porphyrite and dikes of diorite–porphyrite. The quartz diorite–porphyrite, covering an area of approximately 2 km², is exposed as stock in the north-eastern part of the Batailing Au deposit and is partly covered by Quaternary sediments. The orebodies partly occur in the quartz diorite–porphyrite (Figure 2a). The NE-trending diorite–porphyrite dikes mainly occur in the Yangjiagou Formation and partly occur in the Fanjiatun Formation. The diorite–porphyrite dikes have opposite dip directions relative to the orebodies and cut through them, indicating that their formation occurred later than the mineralisation process (Figure 2b,c).

Seven vein-type orebodies have been identified; they can be divided into three ore zones (No. 1, No. 2, and No. 3) from west to east (Figure 2a). These orebodies mainly occur in the Yagjiagou Formation (No. 1 and No. 2) and are partly developed in the quartz diorite–porphyrite (No. 3). These NE-trending orebodies dip NW at 40–65°. The lengths and widths of the orebodies are 40–800 m and 0.5–2 m, respectively. The main orebody (2.5 t Au, average grade of 3.78 g/t; 143.7 t Ag, average grade of 194.38 g/t) in the No. 2 ore zone dips SE at 45–65° within this ore zone and measures approximately 800 m long and 0.3–4.7 m thick.

Hydrothermal alterations within the Batailing Au deposit are widely exhibited. The intensity and type of the alteration are related to the lithology of surrounding rock and the distance from the orebodies. The alterations gradually weaken outwards from the central orebodies and show certain zoning: strong silicification zones, silicification–sericitisation zones, sericitisation-chloritisation zones, and rare potassic alteration zones. Carbonatisation is widely distributed in the hanging wall of orebodies or inside and outside the orebody in the form of net veins. The ore minerals in the Batailing Au deposit are pyrite, arsenopyrite, chalcopyrite, galena, and sphalerite (Figure 3). The common gangue minerals are quartz, sericite, chlorite, and calcite.

Field studies, mineral paragenesis, and the relationships of crosscutting veins divide three mineralisation stages in the Batailing Au deposit (Figure 3 and Figure 4): quartz–arsenopyrite–pyrite (stage I), quartz–polymetallic sulphides (stage II), and quartz–pyrite–carbonate (stage III). Stage I is characterised by milky white quartz, arsenopyrite, and pyrite. Disseminated pyrite and pyrite–arsenopyrite veins are prevalent in this stage (Figure 3a), whereas euhedral–subhedral arsenopyrite and pyrite were later replaced by sphalerite and chalcopyrite (Figure 3g,h). Stage II is characterised by quartz, pyrite, chalcopyrite, galena, and sphalerite and represents the main stage of Au mineralisation. The quartzes are white–grey in colour, and the sulphides are distributed as the disseminated type in wallrocks or quartz veins cutting through the minerals of stage I (Figure 3b–e). In stage II, most chalcopyrite develops as anhedral crystals and fills in cracks of arsenopyrite and pyrite (Figure 3h–j); a few occur as emulsion drops in sphalerite (Figure 3k). Galena is observed as subhedral crystals with black triangular pores, while sphalerite occurs as anhedral crystals and coexists with solid-solution chalcopyrite (Figure 3i–k). Native gold (Au > 85%) mostly exists in mineral cracks as fissure gold, and electrum (Ag: 28%~34%; Au: 64%~69%) is mostly encased in other sulphides (galena and pyrite) [39]. Stage III is distinguished by calcite and quartz with a minor amount of anhedral pyrite (Figure 3l). A quartz–pyrite–carbonate vein has crosscut early-formed minerals and veins (Figure 3f).

4. Sampling and Analytical Methods

4.1. Fluid Inclusion Analysis

Twenty-five quartz samples were taken for FI studies from the outcrop and holes drilled in the Batailing Au deposit (Figure 2a–c). Quartz samples of stages I–III were prepared as doubly polished thin sections (approximately 150 μm thick) for FI petrographic observation using a Carl Zeiss Axiolab microscope (Jena, Germany). Microthermometric studies were realised using the Linkam THMS-600 (Redhill, UK) cooling–heating stage (−196 to 600 °C) at the Key Laboratory of Geological Fluids, Jilin University (Changchun, China). Measurement precision was corrected according to the triple point of CO₂ at −56.6 °C and the freezing point and critical point of pure water at 0.0 °C and 374.1 °C, respectively [40]. FIs were initially cooled to −190 °C and maintained for 5 min to ensure the freezing of the components. The FIs were then heated gradually until they were completely homogeneous. A low rate of 0.1 °C/min was used to observe the temperatures of ice melting (T_m,ice), solid CO₂ melting (T_m,CO2), CO₂ clathrate melting (T_m,cla), and CO₂ partial homogenisation (T_h,CO2). The total homogenisation temperature (T_h,total) was observed at a heating rate of approximately 1 °C/min. T_m,cla and T_m,ice were used to calculate salinities according to the equations proposed by Bodnar [41] and Collins [42]. The calculation method for FI densities was obtained from the formula proposed by Schmidt et al. [43]. The pressures were estimated using the measured data and plotted on a P–T diagram as proposed by Roedder and Bodnar [44].

Raman spectroscopy analysis was performed using a Renishaw RM-1000 instrument mounted on an Olympus BX40 microscope (Wotton-under-Edge, UK) in the above laboratory. The radiation was excited by an Ar ion laser (532 nm wavelength). Scans were performed every 30–60 s, covering a range of 1000–4000 cm⁻¹, with a 1–2 μm laser beam diameter.

4.2. H-O-C Isotope Analyses

Five quartz samples extracted from mineralised veins were employed for H-O-C isotope analyses. After obtaining crushed samples in the size range of 40–60 mesh, single mineral grains were selected using a microscope to improve purity (over 98%). The MAT 253 mass spectrometer used for conducting H-O-C isotope analyses was at the Analytical Laboratory, Beijing Research Institute of Uranium Geology, China National Nuclear Corporation (CNNC).

Oxygen isotopic analysis followed the method in [45], involving quantitative reaction of samples and BrF5 at a temperature of approximately 550 °C. This process resulted in the release of H₂O. Subsequently, hydrogen was generated by the reaction of zinc with the extracted H₂O at a temperature of 800 °C. δD and δ¹⁸O data obtained from the analysis were compared to Standard Mean Ocean Water (SMOW). The δ¹⁸O_H2O value was obtained using the method proposed by Clayton and O’Neil [46]. The analytical precision of δD was 2%, and that of δ¹⁸O was 0.2%.

Carbon isotopes were derived from the CO₂ components of the FIs. The analysis was conducted using a MAT-253 mass spectrometer (Thermo Fisher, Waltham, MA, USA) equipped with TC/EA accessory equipment and homemade heating and bursting devices, allowing online assessment. The samples were heated to break, and various carbon-containing gas components were frozen using liquid nitrogen and adsorption materials. Finally, the carbon isotope compositions (δ¹³C) were tested in comparison to the Pee Dee Belemnite. The analytical accuracy of δ¹³C was 0.2%.

4.3. S-Pb Isotope Analyses

Six samples comprising pyrite and galena from stages I–II were used to conduct S-Pb isotope analyses; their sampling positions are marked in Figure 2. S-Pb isotopes were analysed in the laboratory of the Beijing Research Institute of Uranium Geology, CNCC.

The sulphur isotopic analysis was conducted using the combustion method [47], which was performed using a Delta–V–Plus mass spectrometer (Thermo Fisher, Waltham, MA, USA). These samples were crushed to the size of 200 mesh and added to Cu₂O to produce SO₂ under certain conditions (0.02 Pa, 980 °C). A MAT 251 mass spectrometer (Thermo Fisher, Waltham, MA, USA) was used to determine the composition of sulphur isotopes. The δ³⁴S values are stated relative to CDT with analytical precisions of ±2‰.

For analysis, lead isotope samples weighing 10–50 mg were put into a mixed-acid liquid (HF + HNO₃ + HClO₄) and reacted with HBr-HCl to separate the lead. Lead isotopic analysis was conducted using an ISOPROBE-T thermal ionisation mass spectrometer (IsotopX Ltd., Middlewich, UK) with accuracy over 0.005% for ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁴Pb/²⁰⁶Pb values.

5. Results

5.1. The Types and Characteristics of Fluid Inclusion

According to the classification standard [48], the FIs appearing in all samples can be divided into primary inclusions (occurring in isolation or randomly distributed in groups in quartz) and secondary inclusions (distributed along transgranular microcracks and trails). Because secondary FIs generally represent fluid after mineralisation, only primary inclusions (occurring in groups or isolation) were examined in this research. In quartz grains of stages I–III, four types of FIs are classified according to the phases at ambient temperature, phase transition with temperature changes, and Raman spectroscopy component analysis (Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9), including CO₂-bearing (C₁-type), CO₂-rich (C₂-type), pure CO₂ (PC-type), and aqueous two-phase (W-type) FIs.

C₁-type FIs contain two phases (L_CO2 + L_H2O) or three phases (V_CO2 + L_CO2 + L_H2O) in stages I–II. These C₁-type FIs are 5–25 μm (mainly 5–20 μm) and are irregular and oval. The volume ratio of the CO₂ in C₁-type FIs is 20% to 40% (Figure 5a,c). Some C₁-type FIs coexist with W- and PC-type FIs.

C₂-type FIs contain two phases (L_CO2 + L_H2O) or three phases (V_CO2 + L_CO2 + L_H2O) during stages I–II. These C₂-type FIs are 5–20 μm and are oval and irregular. The volume ratio of the CO₂ in the C₂-type FIs is 70% to 90% (Figure 5b,d). C₂-type FIs generally coexist with W- and PC-type FIs in stages I–II (Figure 5i,j).

PC-type FIs have mainly carbonic components, consisting of CO₂ vapor (V_CO2) and liquid CO₂ (L_CO2) or only L_CO2. These PC-type FIs are 5–15 μm and oval and irregular (Figure 5e,f). In stages I–II, PC-type FIs coexist with C₂- and W-type FIs (Figure 5j,k).

W-type FIs occur in stages I–III, consisting of vapor H₂O (V_H2O) and liquid H₂O (L_H2O). The W-type FIs are 5–15 μm and oval, elongated, and irregular (Figure 5g,h). The V_H2O/V_H2O + L_H2O ratio of W-type FIs is 5% to 35%. W-type FIs coexist with C₂-and PC-type FIs in stages I–II (Figure 5i,k) and have low vapor–liquid ratios in stage III (Figure 5h,l).

5.2. Fluid Inclusion Microthermometry

Microthermometry was performed on C₁-, W-, C₂-, and PC-type FIs of quartz grains from stages I–III. A clear schematic diagram (Figure 5m) of the microthermometric results was drawn for the stage II (main mineralisation stage). After eliminating the influence of useless secondary FI results from the microthermometric data, the valid microthermometric data are integrated in

Table 1. Microthermometric data of FIs from different stages in the Batailing Au deposit.

Stage	Mineral	Type (Number)	Size (μm)	Tm,CO2 (°C)	Tm,ice (°C)	Tm,Cla (°C)	Th,CO2 (°C)	Th,total (°C)	Salinity (wt% NaCl Equiv.)	CO2 Density (g/cm3)	Bulk Density (g/cm3)
I	quartz	C1 (15)	5–25	−60.5 to −57.3		5.5 to 7.1	25.8 to 29.8	264 to 332	5.6 to 8.3	0.6 to 0.69	0.72 to 0.85
	quartz	C2 (24)	5–20	−59.7 to −57.2		6.1 to 7.6	25.3 to 30.1	278 to 328	4.7 to 7.3	0.59 to 0.7	0.73 to 0.80
	quartz	W (30)	5–15		−7.6 to −4.3			272 to 318	6.9 to 11.2		0.75 to 0.85
	quartz	PC (11)	5–15	−59.6 to −57			24.2 to 30.2			0.59 to 0.72
II	quartz	C1 (18)	5–20	−60.6 to −57.1		6.4 to 8.6	24.1 to 29.2	220 to 292	2.8 to 6.8	0.62 to 0.72	0.76 to 0.88
	quartz	C2 (37)	5–25	−60.2 to −56.9		6.8 to 9.1	25.6 to 30.3	231 to 286	1.8 to 6.1	0.58 to 0.69	0.77 to 0.83
	quartz	W (51)	5–15		−7.9 to −2.5			213 to 277	4.2 to 11.6		0.79 to 0.87
	quartz	PC (15)	5–10	−59.4 to −56.9			26.1 to 30.6			0.57 to 0.69
III	quartz	W (26)	5–10		−4.0 to −0.8			152 to 215	1.4 to 6.4		0.86 to 0.94

Note: T_m,CO2 (°C): solid-CO₂-melting temperatures; T_m,ice (°C): ice-melting temperatures; T_m,Cla (°C): clathrate-melting temperatures; T_h,CO2 (°C): partial homogenisation temperatures of the carbonic phase; T_h,total (°C): total homogenisation temperatures of fluid inclusions.

, shown as histograms in Figure 6 and Figure 7, and plotted in Figure 9. The fluid temperatures and pressure are estimated using the data of FIs from stages I–II and plotted in Figure 10.

5.2.1. Stage I

FIs in stage I contain C₁-, C₂-, W-, and PC-type FIs. For C₁-type FIs, the T_m,CO2 data are −60.5 to −57.3 °C, which are slightly lower than that of pure CO₂ (T_m,CO2 = −56.6 °C). The T_m,cla data are 5.5 to 7.1 °C with the salinities from 5.6 to 8.3 wt% NaCl equiv. CO₂ phase mostly homogenise into fluid, with T_h,CO2 data of the carbonic phase from 25.8 to 29.8 °C (Figure 6b). The T_h,total data with disappearance of CO₂ phase are 264 to 332 °C (Figure 7a). The bulk densities are 0.72 to 0.85 g/cm³. For C₂-type FIs, the T_m,CO2 are −59.7 to −57.2 °C. It indicates that there are minor CH₄ contents in C₂-type FIs, which is consistent with the Raman analysis (Figure 8a). The T_m,cla data are 6.1 to 7.6 °C with salinities of 4.7 to 7.3 wt% NaCl equiv. The T_h,CO2 data are 25.3 to 30.1 °C and mainly homogenise into the liquid CO₂ phase from 278 to 328 °C (Figure 7a). The bulk densities are 0.73 to 0.80 g/cm³. For PC-type FIs, the T_m,CO2 data are −59.6 to −57 °C, which again indicates that minor CH₄ components exist in the vapor phase (Figure 8b). The T_h,CO2 data are 24.2 to 30.2 °C (Figure 6b), and the densities are 0.59 to 0.72 g/cm³. For W-type FIs, the T_m,ice data are −7.6 to −4.3 °C with salinities ranging from 6.9 to 11.2 wt% NaCl equiv. The T_h,total data (homogenised into fluid) are 272 to 318 °C (Figure 7a), with densities ranging from 0.75 to 0.85 g/cm³.

5.2.2. Stage II

FIs in stage II consist of PC-, C₁-, C₂-, and W-type FIs. For C₁-type FIs, the T_m,CO2 data are −59.8 to −57.1 °C, indicating that a few other gas contents in the vapor phase, such as CH₄, may be present. The T_m,cla data are 6.4 to 8.6 °C, with salinities from 2.8 to 6.8 wt% NaCl equiv. The CO₂ phase in C₁-type FIs generally homogenises into fluid, and the T_h,CO2 data are 24.1 to 29.2 °C. The T_h,total (homogenise into liquid phase) data are 220 to 292 °C (Figure 7c). The bulk densities are 0.76 to 0.88 g/cm³. For C₂-type FIs, the T_m,CO2 values are −60.2 to −56.9 °C, indicating the existence of minor CH₄ contents, which is consistent with the Raman analysis (Figure 8c). The T_m,cla data are from 6.8 to 9.1 °C with salinities of 1.8 to 6.1 wt% NaCl equiv. The T_h,CO2 data are 25.6 to 30.3 °C, and the T_h,total (homogenised into CO₂ phase) data are 231 to 286 °C (Figure 7c). The bulk densities are 0.77 to 0.83 g/cm³. For PC-type FIs, the T_m,CO2 and T_h,CO2 data are −59.4 to −56.9 °C and 26.1 to 30.6 °C, respectively (Figure 6c,d), indicating minor amounts of other components in the vapor phase, such as CH₄ (Figure 8d). The densities are 0.57 to 0.69 g/cm³. For W-type FIs, the T_m,ice data are −7.9 to −2.5 °C with salinities from 4.2 to 11.6 wt% NaCl equiv. The T_h,total (homogenise into fluid) are 213 to 277 °C (Figure 7c), with densities from 0.79 to 0.92 g/cm³.

5.2.3. Stage III

FIs in stage III consist only of W-type FIs. The T_m,ice data are −4.0 to −0.8 °C, with salinities from 1.4 to 6.4 wt% NaCl equiv. The T_h,total (finally homogenised into fluid) data are 152 to 215 °C (Figure 7e), with densities of 0.86 to 0.94 g/cm³.

5.3. Raman Spectroscopy Analysis

In the Batailing Au deposit, the typical FIs from stages I–III were analysed through Raman spectroscopy to constrain the vapor-phase components (Figure 8). In stage I, the PC-type FIs showed significant CO₂ peaks (1278 and 1384 cm⁻¹) with weak CH₄ peaks (2914 cm⁻¹) (Figure 8b). The C₂-type FIs showed dominant CO₂ peaks (1281 and 1388 cm⁻¹) and a weak CH₄ peak (2915 cm⁻¹) (Figure 8a). In stage II, the C₂-type FIs showed spectral CO₂ peaks (1269 and 1384 cm⁻¹) (Figure 8c). The PC-type FIs showed spectral CO₂ peaks (1268 and 1382 cm⁻¹) and a weak CH₄ peak (2907 cm⁻¹) (Figure 8d). The W-type FIs showed a spectral H₂O peak with weak CO₂ peaks (1268 and 1383 cm⁻¹; Figure 8e). In stage III, the W-type FIs only showed a spectral H₂O peak (Figure 8f).

The peak areas of CO₂ and CH₄ can be calculated using the formula proposed by Burke [49]: X_a = [A_a/(σ_aζ_a)]/Σ[A_a/(σ_aζ_a)]. X_a, A_a, σ_a, and ζ_a are the molar fraction, the peak area, the Raman cross-section and the instrumental efficiency for species a. A_i, σ_i, and ζ_i represent the appropriate values for all species present in the inclusion. Σ is the sum. The ratio of peak area can be used to calculate the relative content (in percent) of different vapor phase [49]. According to the peak areas of CO₂ (2656~4800) and CH₄ (80~133) in PC- and C₂-type FIs calculated with the above formula, the contents of CH₄ in FIs are less than 5%. Therefore, the vapor-phase components of FIs are mainly CO₂, with a minor amount of CH₄.

5.4. H-O-C Isotopic Composition

The H-O isotope values of the quartzes from stages I–II in the Batailing Au deposit are shown in

Table 2. H-O-C isotopic data of fluid inclusions from quartz in Batailing Au deposit.

Sample Number	Mineral	Stage	δDV-SMOW (‰)	δ18OV-SMOW (‰)	Th (°C)	δ18OH2O (‰)	δ13CCO2 (‰)
BTL-1	quartz	I	−79.6	13.4	305	6.2	−24.3
BTL-2	quartz	I	−84	13.6	305	6.4	−24.4
BTL-3	quartz	II	−96.4	11.6	265	2.8	−23.5
BTL-4	quartz	II	−96.2	13.2	265	4.4	−23.7
BTL-5	quartz	II	−90.4	12.6	265	3.8	−22.6

Note: T_h (°C) is the calculated trapping temperature for stages I and II.

and plotted in Figure 11. The δD data in stages I–II are −84 to −79.6‰ and −96.4 to −90.4‰, respectively. The δ¹⁸O_V-SMOW data in stages I–II are 13.4 to 13.6‰ and 11.6 to 13.2‰, respectively. The δ¹⁸O_H2O of hydrothermal fluid in stages I–II can be calculated by following the formula proposed by Clayton and O’Neil [46] with T_h,total and δ¹⁸O_V-SMOW. The results of the calculated δ¹⁸O_H2O in stages I–II are 6.2 to 6.4‰ and 2.8 to 4.4‰, respectively.

The δ¹³C_CO2 values of FIs in quartz from stages I–II in Batailing Au deposit are shown in Table 2 and plotted in Figure 12. The δ¹³C_CO2 data of FIs are −24.4 to −24.3‰ in stage I and −23.7 to −22.6‰ in stage II.

5.5. S-Pb Isotopic Composition

The S-Pb isotopic data of the pyrite and sphalerite in the Batailing deposit are summarised in

Table 3. S-Pb isotopic data of sulphides in Batailing Au deposit.

Sample Number	Mineral	Stage	δ34S (CDT)	206Pb/204Pb	207Pb/204Pb	208Pb/204Pb
BTL-1	Pyrite	I	−2.1	18.362	15.562	38.165
BTL-2	Pyrite	I	−1.9	18.333	15.543	38.221
BTL-3	Pyrite	II	−1.6	18.325	15.523	38.064
BTL-4	Sphalerite	II	−2.4	18.334	15.522	38.082
BTL-5	Sphalerite	II	−2.3	18.246	15.548	38.072
BTL-6	Sphalerite	II	−3.5	18.234	15.533	38.023

and plotted in Figure 13 and Figure 14. The δ³⁴S data of three pyrite and three sphalerite samples in stages I–II have a narrow range of −2.1‰ to −1.6‰ (average = −1.9‰) and −3.5‰ to −2.3‰ (average = −2.7‰), respectively.

The lead isotopic values of three pyrites in stages I–II are ²⁰⁸Pb/²⁰⁴Pb = 18.325–18.362, ²⁰⁷Pb/²⁰⁴Pb = 15.523–15.562, and ²⁰⁶Pb/²⁰⁴Pb = 38.064–38.221, respectively. The lead isotopic data of three sphalerites in stage II are ²⁰⁶Pb/²⁰⁴Pb = 18.234–18.334, ²⁰⁷Pb/²⁰⁴Pb = 15.522–15.548, and ²⁰⁸Pb/²⁰⁴Pb = 38.023–38.082.

6. Discussion

6.1. Nature of Ore-Forming Fluids and Pressure Estimation

6.1.1. Nature of Ore-Forming Fluids

Microscopic observations show that there are C₁-, C₂-, PC-, and W-type FIs in stage I (Figure 5a,b,e,i). Microthermometric results show that FIs have similar homogenisation temperature ranging from 264 to 332 °C with salinities from 4.7 to 11.2 wt% NaCl equiv. The vapor-phase components of these FIs consist mainly of CO₂, H₂O, and a minor amount of CH₄ (Figure 5a,b), the calculated results in Section 5.3 can also prove this. These FIs characteristics indicate that there is a NaCl-H₂O-CO₂±CH₄ fluid system having moderate temperature and low salinity in stage I [15].

Similarly, C₁-, C₂-, PC-, and W-type FIs develop in stage II (Figure 5c,d,f,g,j,k,m). These FIs have homogenisation temperatures ranging from 213 to 292 °C with salinities from 1.8 to 11.6 wt% NaCl equiv. in stage II. The major vapor phase components of these FIs are CO₂ and H₂O with minor CH₄ (Figure 8c–e). These features indicate a moderate-temperature and low-salinity NaCl-H₂O-CO₂±CH₄ system in stage II [15].

The FIs are consistent with the following characteristics in stage I–II: (1) Microscopic observations show that the C₂-, W-, and PC-type FIs coexist in the same growth zone of a crystal, implying that the trapping of these FIs is simultaneous (Figure 5j,k; [59]). (2) The C₁- and C₂-type FIs have variable phase ratios (Figure 5a–d; [60]). The coexisting C₂- and W-type FIs homogenise at approximately the same temperature range with different homogenisation manners (Figure 5i,j). (3) The C₂-type FIs have relatively lower salinities than the W-type FIs [18,61,62]. (4) The T_h,CO2 of C₂-type FIs is similar to that of PC-type FIs (Figure 6; [5,18,59,63,64]). PC- and C₂-type FIs can represent a CO₂-rich endmember; the former likely contains H₂O on the FIs walls [59,61]. Furthermore, minor CO₂ components present in the W-type FIs (Figure 8e). These characteristics indicate that in stage I–II, ore-forming fluids have experienced fluid immiscibility, which is common in hydrothermal Au deposits, such as the Bajiazi, Benqu, Xiaobeigou, and Toudaoliuhe Au deposits in the Jiapigou gold belt [4,18,19,20,52,65]; the Qianchen Au deposit in the Jiaojia gold belt [66]; and the Fenghuangshan Au deposit in the Jidong area [53].

The W-type FIs occur in stage III (Figure 5h,l). The homogenisation temperatures and calculated salinities of W-type FIs are 152 to 215 °C and 1.4 to 6.4 wt% NaCl equiv., respectively, indicating the fluids of stage III evolve to a NaCl-H₂O system with low temperature and low salinity.

6.1.2. Pressure Estimation

The trapping pressure could be estimated using a P–T diagram (CO₂-H₂O) if PC- and W-type FIs coexisted at a microscopic mineral with similar homogenisation temperatures, namely, fluid immiscibility, indicating simultaneous trapping [67,68]. In stage I–II, the FIs exhibit the characteristics of fluid immiscibility and are trapped simultaneously (Figure 5; Table 1). Therefore, the P-T diagram can be used to infer the pressure of the fluid through the density data of W- and PC-type FIs. The estimated trapping temperature and pressure of FIs in stage I are 243–371 °C and 74–142 Mpa, respectively (Figure 10). The estimated trapping temperature and pressure of FIs in stage II are 179–324 °C and 46–116 Mpa, respectively (Figure 10). The estimated temperatures in stage I–II (243–371 °C; 179–324 °C) are similar to the complete homogenisation temperatures (stage I: 264–332 °C; stage II: 213–292 °C; Table 1), indicating the rationality of the estimation method [54]. Consequently, the estimated depths are 2.7–5.3 km and 1.7–4.3 km in stages I–II under the lithostatic condition, respectively.

6.2. Source and Evolution of Ore-Forming Fluids

6.2.1. Source of Ore-Forming Fluids

H-O isotopic data provides crucial insights into the source of ore-forming fluids [63]. The δD and the δ¹⁸O_H2O data of the FIs in stage I are −84 to −79.6‰ and 6.2 to 6.4‰. These H-O isotopic data are similar to the primary magmatic water (δD = −80 to −50‰, δ¹⁸O_H2O = 6 to 10‰; [51]) yet different from metamorphic water (δD = −65 to −20‰, δ¹⁸O_H2O = 5 to 25‰; [51]). The H-O isotopic data of stage I plot away from the area of most orogenic Au deposits (Figure 11; [69]). These data points align closely with the range of magmatic water, implying the ore-forming fluids are mainly magmatic origin in stage I. The δD and the calculated δ¹⁸O_H2O data of the FIs are −96.4 to −90.4‰ and 2.8 to 4.4‰ in stage II, which are relatively lesser than those in stage I and show a tendency to approach the meteoric water line (Figure 11). This decreasing trend observed of both δD and δ¹⁸O_H2O values from stages I and II indicates the mixing of meteoric and magmatic water have occurred in mineralisation process [4,15], which is consistent with the total trend of FIs temperatures and salinities (Figure 9). Therefore, we conclude that the initial ore-forming fluids in the Batailing Au deposit are mainly from a magma source. In stage II, the initial magmatic water was then mixed with a small amount of meteoric water. This situation of Batailing Au deposit also occurs in many mesothermal lode Au deposits in China (e.g., the Bajiazi, Xiaobeigou, Benqu, Toudaoliuhe, Cuyu, Heilongtan–Xiejiagou, and Fenghuanshan) [4,15,19,20,52,53,54,70].

Current studies on C isotopes have focused on testing carbonate minerals in the surrounding rock or calcite veins in the late stage of mineralisation. However, carbon isotope testing of the CO₂ components in Fis provide a more direct and accurate indication of the origin of the carbonaceous components in ore-forming fluids [54]. The major sources of carbon isotopes were generally divided into the following: (1) a marine carbonate source with an average ¹³C value of approximately 0‰ [71]; (2) a magmatic source with a range of ¹³C values from −9‰ to −3‰ [50]; and (3) an organic matter source with an average ¹³C value of −25‰ [72]. The results of the Raman spectroscopy and microthermometric-petrographic studies indicate that the vapor components in the Fis are mainly CO₂ with only minor CH₄ (Figure 6 and Figure 8). Therefore, we conclude that the ¹³C values of CO₂ can represent the total carbon content in the fluid.

The ¹³C values (−24.4‰ to −22.6‰) in stage I–II are significantly different from the magmatic carbon source and are consistent with the sedimentary organic carbon source (Figure 12). The Yangjiagou Formation contains carbonaceous slate with high organic carbon contents [30,73]; thus, the carbonaceous rocks in the Yangjiagou Formation may be the source of organic carbon. Many gold deposits (such as Tokuzbay Au deposit, −22.8‰ to −7.6‰; Zhuanghuhe Au-Sb deposit, −32.5‰ to −11.1‰; and Chertovo Koryto Au deposit, −17.9‰ to −16.6‰) have ore-forming fluids with low carbon isotopic characteristics, which are generally interpreted as derived from organic matter of sedimentary rocks [74,75,76]. However, the ¹³C values of these deposits vary widely and are interpreted as the addition of organic carbon to the mineralisation system through metamorphism or water-rock reactions [75,76].

The low ¹³C values (−24.4‰ to −24.3‰) in stage I indicate that sedimentary organic carbon was added before hydrothermal mineralisation. Organic carbon can be added by the water-rock reaction during the magmatic to hydrothermal transitional stage or assimilated in the magmatic stage [55]. In the former case, organic carbon can generally be added to the magmatic–hydrothermal system during the transitional stage by the water-rock reaction: 2C + 2H₂O → CH₄ + CO₂ [76,77,78,79]. However, the carbon phase in the Fis consists of abundant CO₂ contents and minor CH₄ contents in stage I (Figure 8a,b; Table 1), which is inconsistent with the results of the above reaction. Meanwhile, the absence of high ƒO₂ minerals such as magnetite and sulphate in stage I makes it impossible to explain the significantly higher CO₂ content than CH₄ content during the oxidation (high ƒO₂) condition through the reaction: CH₄ + 2O₂ → CO₂ + 2H₂O [80,81,82]. Therefore, we tend to the latter case, the signatures of organic carbon in the mineralisation system acquired by the assimilation of organic matter during the magmatic stage. Subsequently, the fluids exsolved from the magma exhibited characteristics of organic carbon.

Compared with the H-O-C isotopes of the adjacent Jiapigou gold belt, the H-O isotopic data in Batailing Au deposit are similar, but the ¹³C values of fluids (−24.4 to −22.6‰) in the Batailing Au deposit are much lower than the ¹³C values of calcites in the Jiapigou gold belt (−5 to −4.2‰; [56]). Organic matter occurred in the ore-hosting Yangjaigou Formation in the Batailing Au deposit, but there are no organic matters in the wall-rock or strata in the Jiapigou gold belt. Based on these geological and C-H-O isotopic data, we conclude that the ore-forming fluids in the Batailing Au deposit and Jiapigou gold belt are mainly magmatic sources (Figure 11). However, the mineralisation system of the Batailing Au deposit assimilates organic carbon in the Yangjiagou Formation during the magmatic stage before mineralisation, resulting in the carbon isotopic characteristics (organic carbon) of the Batailing Au deposit, which are different from the magmatic carbon of the Jiapigou gold belt [56]. Therefore, water and carbon in the fluid system have different sources in the Batailing Au deposit.

6.2.2. Evolution of Ore-Forming Fluids

Fis observed in stages I–II consist of C₁-, C₂-, PC-, and W-type with moderate temperatures and low salinities. As described in Section 6.1.1, the characteristics of these Fis demonstrates that fluid immiscibility occurs in the ore-forming fluid in stage I–II. During fluid upward migration, fluid immiscibility may be produced when the pressure decrease suddenly [50,83]. Fluid immiscibility occurs in stages I–II in the Batailing Au deposit, as in many other gold deposits [62,84], and is effective in promoting the precipitation of sulphides [85,86].

The migration modes of gold in hydrothermal solutions include gold sulphide complexes (such as Au(HS)₂⁻) and gold hydroxide/chloride complexes (such as AuOH (aq) and AuCl₂⁻). The former predominates in ore-forming fluids at moderate-low temperatures (<400 °C), whereas at higher temperatures (>400 °C), the latter plays an important role [87,88,89,90]. Therefore, through the temperatures of the fluids, gold-sulphide complexes are the main transport mode for gold in the Batailing deposit. Continuous fluid immiscibility can cause the volatiles to enter the gas phase, reducing the CO₂ and H₂S contents in the fluid [54]. This process can change the conditions (such as the pH, fO₂, and activity of HS⁻) in the fluids, reduce the stability of Au(HS)₂⁻ complexes, and accelerate the following reactions: (1) HS^– + H⁺ → H₂S; (2) HCO₃⁻ + H⁺ → H₂O + CO₂; and (3) Au(HS)₂⁻ → Au + 2HS^– [54], which promotes the precipitation of Au and sulphides.

In stage III PC-, C₁- and C₂-type FIs are not visible, and only W-type FIs occur, indicating the end of fluid immiscibility. FIs in stage III have much lower temperatures and salinities than those of stage I–II, indicating the addition of abundant meteoric water (Figure 7 and Figure 9). The abundant CO₂ components in the fluids are released by fluid immiscibility in stage I–II, resulting in no CO₂ components in the gas phase (Figure 8f) and no precipitation of Au in stage III. The magmatic fluid transforms to a low-temperature, low-salinity fluid system dominated by meteoric water. These characteristics and the existence of only minor pyrite may represent the end of the mineralisation process in stage III.

6.3. Source of Ore-Forming Materials

The sulphide mineral compositions and the inexistence of sulphate minerals in the Batailing deposit indicate that sulphur occurs mainly as S²⁻ and HS⁻ [89]. Therefore, the δ³⁴S data of sulphides can nearly represent the total sulphur isotopes and constrain the sulphur source [69]. δ³⁴S data of sulphides show a tendency of δ³⁴S_pyrite > δ³⁴S_sphalerite in stage I–II (Figure 13a; Table 3), which means the fundamental equilibrium of δ³⁴S values of sulphides in mineralisation process [91].

The δ³⁴S values of sulphides in the Batailing Au deposit show a narrow range (−3.5 to −1.6‰, mean = −2.3‰), which are analogous to the values of mantle sulphur (0 ± 3‰) or magmatic sulphur (0 ± 5‰) [91,92]. The δ³⁴S values of sulphides in Batailing Au deposit are close to those of gold deposits in the southern part of LXZR (such as Lanjia, Cuyu, Guanma in central Jilin; Figure 13b; [8,14,15]) but different from those of Au deposits in Jiapigou gold belt (such as Xiaobeigou, Bajiazi, Benqu, Sandaocha, Toudaoliuhe, Erdaogou in southern part of Jilin; Figure 13b; [18,19,20,51,52]). The surrounding rocks in Batailing and other Au deposits in central Jilin are mainly Permian-Carboniferous sedimentary rocks [8,13,14,15]. However, the surrounding rocks of Au deposits in Jiapigou gold belt are Neoproterozoic metamorphic rocks [18,20,52]. Previous studies have shown that the δ³⁴S values of sulphides in Jiapigou gold belt are higher than those of magmatic sulphur source due to the water–rock reaction between fluid and Neoproterozoic metamorphic surrounding rocks [5,15,18,51]. Therefore, the δ³⁴S values of Batailing Au deposits are similar to those of Au deposits in central Jilin but different from those of Au deposits in Jiapigou gold belt, which may be caused by the influence of surrounding rocks. In conclusion, the S isotopic signatures of sulphides support a mainly magmatic or mantle sulphur origin in the Batailing Au deposit.

The lead isotopic values of sulphides in the Batailing Au deposit exhibit a narrow range with minimal variations (²⁰⁶Pb/²⁰⁴Pb = 18.325–18.362, ²⁰⁷Pb/²⁰⁴Pb = 15.523–15.562, ²⁰⁸Pb/²⁰⁴Pb = 38.064–38.221), implying a relatively uniform or single lead source. In the Figure 14, the lead isotopic values of the sulphide samples are primarily distributed between the mantle and orogenic belt, indicating a crust-mantle mixed source of lead. The lead isotopes of sulphides are similar to those of Au deposits and quartz diorite–porphyrite in southern part of LXZR in central Jilin (Figure 14; [14,15]). It may indicate a genetic relationship between ore Pb and quartz diorite–porphyrite.

6.4. Ore Genesis

The FIs and H-O-C-S-Pb isotopic characteristics of the Batailing Au deposits align closely with those of the mesothermal magmatic–hydrothermal lode Au deposits [15,59,61,93,94], evidenced by geology, FIs studies, and isotope analyses as following. In Batailing Au deposit, Au orebodies controlled by NE-trending faults are mainly hosted in the Permian Yangjiagou Formation and partly hosted in quartz diorite–porphyrite. The main gold-related mineralisation types are vein and disseminated. The hydrothermal alterations and sulphide paragenetic assemblage in the Batailing Au deposit are consistent with those in magmatic–hydrothermal Au deposits [95]. The ore-forming fluids in Batailing Au deposit reveal a moderate-temperature and low-salinity NaCl-H₂O-CO₂±CH₄ system. H-O isotopic data indicated the mainly magmatic origin of the initial ore-forming fluids. C isotopic data of FIs show an organic source of C in the fluids, and the addition of organic carbon occurred before the mineralisation process. S-Pb isotopic values of the sulphides imply that the ore-forming materials are primarily magmatic source. Therefore, the Batailing Au deposit can be classified as a mesothermal magmatic–hydrothermal lode deposit in terms of its genetic type.

Previous researches have shown that Au mineralisation in central Jilin mainly occurred during two periods: (1) Early Jurassic during the initial stage of the Palaeo-Pacific Plate subduction [14,15]; and (2) Middle Jurassic, caused by lithospheric thinning during the Palaeo-Pacific Plate subduction [11,15]. Through the analysis of the H-O-C-S-Pb isotopes, we conclude that the ore-forming materials and fluids originate mainly from magma. Magmatic rocks in the Batailing Au deposit include quartz diorite–porphyrite and diorite–porphyrite dikes. However, diorite–porphyrite dikes cut through Au orebodies, indicating that the formation of diorite–porphyrite dikes occurred later than mineralisation (Figure 2b,c). Therefore, the diorite–porphyrite dikes are unrelated to mineralisation. The orebodies partly occur in the quartz diorite–porphyrite (Figure 2b), indicating that the quartz diorite–porphyrite is related to mineralisation in space. The (quartz) diorite–porphyrites in central Jilin formed during the Middle Jurassic (170.21 ± 0.73 Ma and 170.4 ± 1.8 Ma; [11,15]), which are temporally consistent with the regional Au mineralisation age (such as Cuyu; 169.3 ± 2.0 Ma; [15]). Therefore, we propose that the age of the quartz diorite–porphyrites (~170 Ma) can limit the maximum age of Au mineralisation in the Batailing Au deposit. The Middle-Jurassic quartz diorite–porphyrites in the Batailing deposit may form through the partial melting of the immature lower crust under the subduction of the Palaeo-Pacific Plate [11,15]. This subduction caused extensive tectonic and magmatic activity in central part of Jilin during Middle Jurassic. During the upward migration of the magma, the entire system obtained signatures of organic carbon from the Yangjiagou Formation via assimilation. Subsequently, during upward migration along the fracture, the pressure of fluids rich in metals and volatiles (CO₂, H₂O) suddenly decreases, resulting in fluid immiscibility. Continuous fluid immiscibility and the mixing of meteoric and magmatic water change the physico-chemical environment of fluids, then destroy the stability of Au–sulphur complex, finally cause the precipitation of Au and sulphides (Figure 15).

7. Conclusions

• The Batailing Au deposit in central Jilin Province is a mesothermal magmatic–hydrothermal lode Au deposit.
• The ore-forming fluids belong to a NaCl-H₂O-CO₂±CH₄ system with moderate temperatures and low salinities.
• The H-O isotopic data show a mainly magmatic source of ore-forming fluids. The C isotopic data and FIs’ characteristics show an organic carbon source of carbon in fluids. The organic carbon was added to the ore-forming system before mineralisation. The S-Pb isotopic data show a mainly magmatic source of ore-forming materials.
• The fluid immiscibility caused by sudden decrease in pressure may lead the precipitation of sulphides and Au in Batailing Au deposit.