Origin and Evolution of Ore-Forming Fluid and Metallogenic Mechanism of the Baoshan Cu-Pb-Zn Deposit, South China: Constraints of Fluid Inclusion and C-H-O Isotopes

Abstract

The Southern Hunan area is located in the superposition of the Qin-Hang Cu-Pb-Zn polymetallic ore belt and the Nanling W-Sn-Mo polymetallic ore belt, which is an important window to study the mineralization of W-Sn-Mo and Cu-Pb-Zn polymetallic deposits. The Baoshan deposit is a large Cu-Pb-Zn polymetallic deposit in Southern Hunan Province with obvious zones of Cu mineralization and Pb-Zn mineralization: the central part of the Baoshan deposit demonstrates contact metasomatic (skarn) Cu mineralization, while the western, northern and eastern parts demonstrate hydrothermal vein Pb-Zn mineralization. However, the origin and evolution of the ore-forming fluid and mechanism of Cu and Pb-Zn mineral precipitation are still unclear. The metallogenic process of the Baoshan Cu-Pb-Zn deposit can be divided into four stages: (1) the early skarn stage (S₁); (2) the late skarn stage (S₂); (3) the Cu-Fe sulfide stage (S₃); and (4) the Pb-Zn sulfide stage (S₄). The results of microtemperature measurements and a Raman spectrometric analysis of fluid inclusions show that the ore-forming fluid was the H₂O-NaCl (-CO₂ ± N₂ ± C₂H₆) system in the skarn stages (S₁ + S₂) and changed into the H₂O-NaCl-CO₂ (±N₂ ± C₂H₆) system in the sulfide stages (S₃ + S₄). The temperature (S₁: 436.6~548.2 °C; S₂: 344.1~435.1 °C; S₃: 134.1~413.1 °C; S₄: 183.9~261.0 °C) and salinity (S₁: 17.4~51.2 wt.%NaClequiv; S₂: 13.6~41.7 wt.%NaClequiv; S₃: 1.2~32.3 wt.%NaClequiv; S₄: 1.8~9.6 wt.%NaClequiv) showed a downward trend from the early to late stages. From the skarn stages (S₁ + S₂) to the sulfide stages (S₃ + S₄), the ore-forming pressure results from the static rock pressure and the hydrostatic pressure, and the ore-forming depth is estimated to be about three to six km. The C-H-O isotopic compositions of hydrothermal minerals such as quartz and calcite indicate that the ore-forming fluid is predominately magmatic fluid, but a significant amount of meteoric water is added in the Pb-Zn sulfide stage (S₄). The formation of the mineralization zonation of the Baoshan deposit is the result of many factors (e.g., stratigraphy, structure and metal precipitation mechanism): the Cu mineralization is controlled by the contact zone, and the Pb-Zn mineralization is controlled by the fault. In addition, the precipitation of Cu is mainly controlled by fluid boiling, while the precipitation of Pb and Zn is mainly controlled by the mixing of magmatic fluid and meteoric water.

1. Introduction

The formation of skarn deposits is usually restricted by medium-acid intrusive rocks, wall rocks and structural conditions, and the contact zone between intrusive rocks and wall rocks often shows obvious mineralization alteration zoning [1,2,3,4]. The Southern Hunan area is located in the superposition of the Qin-Hang Cu-Pb-Zn polymetallic ore belt and the Nanling W-Sn-Mo polymetallic ore belt (Figure 1), which is an important window to dissect the mineralization of W-Sn-Mo and Cu-Pb-Zn deposits [5,6,7]. Skarn-type Cu mineralization and hydrothermal vein-type Pb-Zn mineralization are the most important mineralization types in Cu-Pb-Zn polymetallic deposits in this region [5,6,8,9]. Hydrogen and oxygen (H-O) isotopes show that the ore-forming fluid of Cu-Pb-Zn deposits is predominately magmatic fluid, with the addition of meteoric water at a later stage [10,11]. The sulfur in Cu-Pb-Zn polymetallic deposits mainly comes from magma [9,12,13,14], and there may be a small amount of formation sulfur [9,12]. Pb is mainly derived from the upper crust [9,11,13]. However, studies on the precipitation mechanism of metal elements in Cu-Pb-Zn polymetallic deposits in the region are relatively weak, and a unified understanding has not been obtained [10,15,16].

The Baoshan Cu-Pb-Zn deposit is a large polymetallic deposit with obvious Cu and Pb-Zn mineralization zoning in Southern Hunan Province (Cu reserves: 0.9 Mt) [6]. According to its mineralization alteration characteristics, the Baoshan deposit can be divided into Cu (Mo) mineralization in the central part and Pb-Zn (Ag) mineralization in the west, north and east. Among them, the eastern section is located below Guiyang County and has been closed due to safety concerns, so this study focuses on the central, western and northern sections of the Baoshan deposit. The central part of the Baoshan deposit is skarn-type Cu mineralization, with skarn, silicitization, sericitization and marbling alteration, and the western, northern and eastern parts of the deposit transition to hydrothermal vein-type Pb-Zn mineralization, dominated by marbling alteration [6,7,8,9,11,12,13]. Previous studies mainly focused on the geological characteristics, structure and mineralization zonation of the Baoshan deposit [6,7,8,9,10,11,12,13,14,15,19,20]. With the establishment of its ore-forming relationship with granodioritic porphyry, this pluton has been widely considered by scholars to be the ore-forming granite of the Baoshan deposit [21,22,23,24,25,26,27,28,29]. Previous studies have suggested that the ore-forming fluid was magmatic fluid in the early stage and meteoric water in the late stage [10]. However, some scholars have suggested that the addition of meteoric water is not obvious [15]. Through the study of trace elements in the Baoshan deposit’s pyrite, it is suggested that the ore-forming materials and fluids of the deposit mainly come from its granodiorite porphyry, wall rocks and deep basement [30,31]. In addition, previous studies of fluid inclusion have suggested that the ore-forming fluids were mainly the H₂O-NaCl (±CO₂) system, while the temperature and salinity show an overall downward trend [10,15,22]. However, there are disputes over whether there is fluid boiling in the ore-forming process, and there are also differences in opinion on the mechanism of metal precipitation. Xuan et al. [10] suggested that the metallogenic process is associated with an obvious addition of meteoric water and mineral precipitation is mainly controlled by fluid mixing. However, Qi [32] suggested that the precipitation of minerals was related to the decrease in temperature and the solubility of various metal elements. It is necessary to carry out more detailed research on the ore-forming fluid and its evolution. In this study, we report new fluid inclusions and C-H-O isotope data from different mineralization stages in the Baoshan Cu-Pb-Zn polymetallic deposit.

2. Geology Background

2.1. Regional Geology

The Southern Hunan area is located at the intersection of the Yangtze Block and the Cathaysia Block in South China and is also the superposition of the Qin-Hang Cu-Pb-Zn polymetallic ore belt and the Nanling W-Sn-Mo polymetallic ore belt (Figure 1). It has experienced many complex tectonic movements, among which the Jinningian, Caledonian, Indosinian and Yanshanian tectonic activities were the most intense [33,34,35]. Many microplates in the range of the Yangtze Block and the Cathaysia Block coalesced during the JinningIstage (1.0~0.9 Ga), forming the Ancient Yangtze Block and the Ancient Cathaysia Block, which combined along the Jiangshan-Shaoxing line during the Jinning II stage (~800 Ma) to form the Ancient South China Block [33,34,35,36]. Subsequently, under the dynamic background of the cracking of the Rodinia supercontinent, South China entered the tensile rift stage, which lasted until the early Paleozoic [37]. In the late Early Paleozoic, the Yangtze Block and the Cathaysia Block once again coalesced, resulting in the loss of Silurian strata, and the Devonian strata overlayed the Ordovician strata in an angled unconformity, forming a series of near-EW inverted folds and thrust fault structures [34,38]. During the Indosinian, under the remote influence of the collision and orogeny of the Tethys Indosinian block, the North China Block and the South China Block, a series of NNE thrust nappe structures formed in the area, which constituted the main tectonic framework of the area [39,40,41,42]. The Yanshanian tectonic activities are complex, which mainly manifested due to the remote influence of the Paleo–Pacific subduction and the north–south land mass extrusion, resulting in the unconformity contact between the Jurassic layers and the development of a series of northeast thrust nappe structures [43,44,45,46,47]. During the mid-Late Jurassic regional extension, strong magmatic activity in South China occurred and formed a large number of granites accompanied by extensive W-Sn-(Mo)-Cu-Pb-Zn polymetallic mineralization [45,48,49,50].

In addition to the missing Ordovician, Silurian and Jurassic, the Sinian to Quaternary are all exposed in the Southern Hunan area (Figure 2). Among them, the Sinian to Cambrian are mainly metamorphic sandstone and carbonaceous slate, with some unstable marl. The Devonian to Triassic are the most widely distributed and are also an important ore-bearing horizon in this area, which contains a lot of rich metal mineral resources. The Triasis to Tertiary are mainly distributed in the southern and northwestern parts of the region, with a small amount in the southeastern parts. The Quaternary is mainly a set of loose deposits deposited by rivers and beaches, mainly composed of a semi-consolidated gravel layer and sandy clay, with a complex composition. The faulted structures were mainly formed in the Indosinian and Yanshanian stages, were most developed in the northeast and the south, and had a large scale and the largest number. The secondary faults are NW and NE, and a small number of E-W faults can be seen. The fold structure in this area is developed, and most folds are compound folds. These folds mainly strike northeastward and are closely related to the subduction of the Paleo–Pacific plate under the South China continent. Magmatic activity was frequent in the area, and strong magmatic activity developed from the Caledonian to Yanshanian (Figure 2). Previous studies have shown that the Caledonian magmatic activity in the area may have little relationship with regional mineralization, while the Indosinian magmatic activity caused the initial enrichment of W, Sn, Fe and other elements. The Yanshanian magmatic activity is the most intense and widely distributed, and almost all the discovered non-ferrous metal, rare and precious metal deposits are closely related to the Yanshanian magmatic activity.

2.2. Deposit Geology

The exposed stratigraphy mainly included the Upper Devonian Xikuangshan Formation (D₃x); the Lower Carboniferous Menggongao (C₁m), Shidengzi (C₁sh), Ceshui (C₁c), Zimenqiao Formations (C₁z); and the Middle–Upper Carboniferous Hutian Group (C₂₊₃h). The Lower Carboniferous Shidengzi limestone, Ceshui quartz siltstone, carbonaceous shale and Zimenqiao dolomite are the main ore-bearing wall rocks in the Baoshan mining area (Figure 4). The structure mainly consists of a series of inverted folds and compression–torsion thrust faults. The central Baoling inverted anticline, the Western Niuxin inverted anticline and the Northern Caishenmiao inverted anticline are closely related to mineralization. According to the different strikes, the faults in the mining area can be roughly divided into two groups (Figure 3): NE-trending faults are closely related to mineralization and are important ore-guiding and ore-controlling structures (such as F₀, F₁, F₂₁ and F₂₅); and the NW fault is located in the middle of the mining area and diagonally spans the entire mining area from the NW to the SE, cutting the NE fault. There is strong magmatic activity in the mining area, which is mainly Yanshanian middle-acid epigenetic intrusion, including granodiorite porphyry, porphyritic granodiorite and a small amount of lamprophyre (Figure 3). The granodiorite porphyry is enclosed in the core of the Baoling reversed anticline, and the upper part is shielded by quartz-siltstone and carbonaceous argillaceous shale. Porphyritic granodiorite is mostly found on the surface of the mining area or in the western and northern ore sections and is often in fault contact with the wall rocks. No obvious mineralization alterations have been observed in the contact area between the intrusive rock and the wall rocks, and its age was previously obtained as 180.5 ± 1.6 Ma [40]. Lamprophyre can only be found at line 151 in the middle of −70~−110 m, with a dendritic production, and its rock formation age is 156 ± 2 Ma [52], which is obviously later than the formation time of garnet skarn and molybdenite mineralization (162~160 Ma [51,53]).

The orebodies are produced in veins and have a lamellar or lenticular shape. The contact boundary with the skarn is clear (Figure 4). The central Cu orebody is mainly stratiform, vein, lentil or lens-like, with a continuous distribution, a large scale and a high grade of orebodies, which are closely related to skarn and are obviously controlled by the Baoling inverted anticline. The western and northern Pb-Zn sections are dominated by hydrothermal and metasomatic vein-filling, stratified and lenticular-shaped Pb-Zn mineralization. The orebodies are mainly located in the fracture zone (F₂₁ and F₂₅) and the interlayer fracture zone of the fault footwall, and a small amount is located in the southern flank of the Baoling inverted anticline. The metal minerals mainly include chalcopyrite, molybdenite, pyrite, sphalerite and galena, followed by tetrahedrite, scheelite, magnetite, arsenopyrite, natural gold and a small amount of sulfide minerals. Non-metallic minerals are mainly garnet, diopside, epidote, tremolite, actinolite, chlorite, quartz, fluorite and carbonate minerals. The alteration types of wall rocks in the central Cu section mainly include skarn, silicitization, sericite and fluorination. The related mineral composition is more complex, including garnet, diopside, epidote, chlorite, actinolite, quartz, fluorite and sericite. Skarn is not developed in the western and northern Pb-Zn sections, and the alteration types of the wall rocks are mainly carbonation and a small amount of fluoridation. The Pb-Zn sections are usually distributed in the fracture zone, a large number of carbonate veins are developed in the wall rocks, and a small amount of fluorite alteration can be seen locally (Figure 5c,d).

Figure 4. Geological cross-section of line 169 through the Baoshan Cu-Pb-Zn polymetallic deposit (modified from Ref. [17]).

Figure 4. Geological cross-section of line 169 through the Baoshan Cu-Pb-Zn polymetallic deposit (modified from Ref. [17]).

Combined with the interpenetration relationship of veins and the sequence of mineral association and replacement, the Baoshan Cu-Pb-Zn polymetallic deposit can be divided into four ore-forming stages (Figure 6). (1) In the early skarn stage (S₁), early anhydrous skarn minerals such as garnet and diopside are mainly formed, and radial and clustered wollastonite are rare (Figure 5a). (2) The late skarn stage (S₂) is manifested by the degeneration of early skarn minerals, mainly forming epidote, actinolite, chlorite and other minerals, and a disseminated scheelite formation is locally visible (Figure 5b,c). At this stage, quartz veins began to form (Figure 5d). Small amounts of pyrite and magnetite were precipitated in the late stage, and the scheelite particles formed in the early stage were cut and replaced (Figure 5e). (3) In the Cu-Fe sulfide stage (S₃), which is the main ore-forming stage of the central Cu section, a large amount of chalcopyrite, pyrite and molybdenite precipitation is characteristic (Figure 5f,g). Moreover, a small amount of sphalerite can be observed, which is closely related to chalcopyrite (Figure 5h,i). (4) The Pb-Zn sulfide stage (S₄) is another main metallogenic stage of the western and northern Pb-Zn sections. At this stage, the pyrite–sphalerite–galenite–quartz (±fluorite) veins can be seen in the early stage, and the pyrite–sphalerite–galenite–calcite (±fluorite) veins dominate in the late stage. In sulfide ores, pyrite, sphalerite and galena are deposited and replaced in turn (Figure 5j). Galena is the main silver-carrying mineral in the mining area, which is replaced by late sulfide minerals (Figure 5k). At the end of mineralization, a large amount of pyrite precipitated and produced metasomatic galena as a fine granular aggregate (Figure 5l).

3. Sampling and Analytical Methods

3.1. Fluid Inclusion Analysis

In this study, 23 skarn and ore samples from different ore-forming stages of the Baoshan deposit were selected for fluid inclusion analysis according to the interpenetration relationship of their veins, mineral metasomatism and mineral association characteristics. Firstly, the samples were ground into double-sided polished thermometry sheets (with a thickness of 0.06~0.08 mm), and the petrography of the fluid inclusions of minerals in each mineralization stage was carefully observed using an optical microscope.

The laser Raman spectroscopic composition analysis of fluid inclusions was performed at the Chinese National Engineering Research Center for Control and Treatment of Heavy Metal Pollution (Central South University) with RENISHAW microscopic laser Raman spectrometer. The laser wavelength is 514.5 nm, the laser power is 40 mw, the laser beam spot diameter is 2 μm and the spectral resolution is 1 cm⁻¹. The microthermometry study of fluid inclusion was carried out using LinkamTHMS600 heating–freezing stage (−196~600 °C) connected to a Zeiss microscope at the Laboratory of Fluid Inclusions Temperature Measurement at the Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University). When the test temperature is below 30 °C, the error is ±0.1 °C, and when the test temperature is between ~30 and 600 °C, the homogenization temperature error is ±1.0 °C. Prior to testing, standard samples of synthetic CO₂ three-phase and pure H₂O fluid inclusions were used for calibration. The heating and cooling rate of the hot and cold station is limited to 10–15 °C/min and is reduced to less than 1 °C/min when approaching the phase transition point. The resulting temperature measurements were used to calculate the salinity and density of fluid inclusions in different composition systems using Microsoft Excel (2016) spreadsheet [54,55].

3.2. Carbon, Hydrogen and Oxygen Isotope Analysis

In order to further reveal the ore-forming fluid source in the Baoshan Cu-Pb-Zn deposit, quartz in the late skarn stage, quartz and calcite in the Cu-Fe sulfide stage and calcite in the Pb-Zn sulfide stage were selected for C-H-O isotope analysis. C-H-O isotopes were tested by Beijing Kehui Testing Technology Co., Ltd., Beijing, China. The instrument used was Thermo Fisher MAT-253Plus mass spectrometer (Massachusetts, MA, USA). Before isotope testing, the samples to be tested were crushed and cleaned, and the target mineral was carefully selected using binoculars and ground to less than 200 mesh. The traditional BrF₅ method was used to generate oxygen, and 253 Plus spectrometer was used to determine δ¹⁸O [56]. Fluid inclusion in the release of H₂O and glassy carbon during the instantaneous reaction to H₂ was tested using a high-purity helium mass spectrometer to determine the delta δD with load. The test results were compared with the Vienna Standard Mean Ocean Water (V-SMOW), and the test accuracy was 0.05‰ and 1‰, respectively [57]. After the reaction of the sample with phosphoric acid, the CO₂ gas was separated from other impure gases by a 70 °C molten silicon capillary column, and the gas stable-isotope spectrometer was used to determine carbon isotope. Compared with international standard material PDB (Pee Dee Belemnite), the test accuracy is less than ±0.2‰.

4. Results

4.1. Characteristics of Fluid Inclusions

Fluid inclusions are widely distributed in the Baoshan deposit and can be observed in garnet, pyroxene, epidote, fluorite, quartz, sphalerite and calcite (Figure 7). The laser Raman spectral analysis shows that the fluid inclusions in each stage of the Baoshan deposit are mainly composed of H₂O, with a small amount of CO₂, N₂ and C₂H₆ (Figure 8). A large number of opaque daughter minerals can be seen in the fluid inclusions of garnet and pyroxene in the skarn stage, which are mainly composed of metal minerals such as magnetite (Figure 8a,b). However, CO₂-containing inclusions are mainly found in sphalerite in the Pb-Zn sulfide stage, and a small amount exists in quartz, fluorite and calcite (Figure 8c,d).

Most of the fluid inclusions are distributed in groups or produced as isolated inclusions with negative crystal, sub-round and round shapes and with a few of an irregular shape, showing the characteristics of primary inclusions (Figure 7). The secondary inclusions with leakage, necking phenomena or a linear or planar distribution along cracks across the mineral crystal face were not analyzed in this study. According to the phase characteristics of fluid inclusions at room temperature (20 °C), fluid inclusions in the Baoshan deposit can be divided into four types (Figure 7): (1) polyphase inclusions containing daughter minerals (type I, 4~50 μm), (2) aqueous solution inclusions (type II, 4~25 μm), (3) CO₂-containing inclusions (type III, 8~17 μm) and (4) pure CO₂-containing inclusions (type IV, 4~12 μm). The petrographic characteristics of each type of fluid inclusion and the phase changes during the heating process are described as follows:

(1) Polyphase inclusions contain daughter minerals (Type I). This type of inclusion is distributed in all ore-forming stages (Figure 7a–d). Type I inclusions are mainly composed of brine, bubbles, daughter crystals and opaque daughter minerals (e.g., magnetite). In this study, only the three-phase type I fluid inclusions containing transparent daughter crystals were measured for microthermometry, and the gas-phase volume accounted for about 15~35 vol.% (Figure 7b–d). They were usually negative crystalline inclusions, with a sub-circular to circular distribution, and were rarely irregular. During the heating process, the transparent daughter minerals disappear first, and then the whole inclusions homogenize into the liquid phase.

(2) Aqueous solution inclusions (Type II). This type of fluid inclusion is also widely distributed in various ore-forming stages (Figure 7b–d,h,i). They are composed of brine and bubble phases, are negative crystalline and are subcircular to circular or irregular. In addition, they have a wide range of gas-phase volumes. When the gas-phase volume ratio of the inclusion is less than 45 vol.%, most of them are homogeneous to the liquid phase during heating, and when the gas-phase volume ratio of the inclusions is greater than 65 vol.%, most of them are homogeneous to the gas phase.

(3) CO₂-containing three-phase inclusions (Type III). This type of inclusion is mainly found in the Pb-Zn sulfide stage (Figure 7e–g,i). At room temperature, they are mainly composed of aqueous solution and two-phase CO₂. They are mostly negative crystalline or subcircular, and the volume proportion of the CO₂ phase in the inclusions ranges from 35 to 65 vol.%. During the heating process, they are mainly homogenized to the liquid phase, and a small amount can homogenize to the gas phase. In addition, a small amount of gas-phase CO₂, liquid-phase CO₂ and three-phase aqueous solution inclusions can be seen (Figure 7e–g), and the volume proportion of the CO₂ phase in the inclusions can reach 85 vol.%. During the heating process, the gas-phase CO₂ and liquid-phase CO₂ first reach partial homogenization and then disappear completely.

(4) Pure CO₂ inclusion (Type IV). Similar to type III inclusion, this type of inclusion is mainly found in sphalerite at the Pb-Zn sulfide stage (Figure 7e). At room temperature, it presents the two-phase characteristics of CO₂, mostly in the form of secondary round and round output. In the heating process (<31 °C), most of them are homogeneous to the liquid phase, and a small amount are homogeneous to the gas phase.

4.2. Microthermometry of Fluid Inclusions

The microtemperature measurement results of the fluid inclusions in the Baoshan deposit are shown in

Table 1. Microtemperature results of fluid inclusions in the Baoshan Cu-Pb-Zn deposit.

Ore-Forming Stages	Type (Number)	V/T	Size	Tm, ice	Tm, hal	Tm, CO2	Tm, clath	Th, CO2	Th, tot	Salinity	Density
		(C/T)	(μm)	(°C)	(°C)	(°C)	(°C)	(°C)	(°C)	(wt.% eqv.)	(g/cm3)
Early skarn stage (S1)	Type I (7)	15~30	10~50		273.7~437.3				469.5~548.2	36.1~51.2	0.90~1.03
	Type II (7)	15~40	8~25	−20.8~−13.6					436.6~522.1	17.4~22.9	0.6~0.79
Late skarn stage (S2)	Type I (2)	15~30	4~18		323.9; 343.5				401.2; 427.6	40.0; 41.7	0.98; 1.02
	Type II (4)	15~75	4~20	−18.3~−9.7					410.4~435.1	13.6~21.2	0.7~0.78
Cu-Fe sulfide stage (S3)	Type I (5)	15~35	6~23		98.5~208.4				344.1~399.8	27.9~32.3	0.9~0.98
	Type II (23)	10~100	6~17	−20.9~−1.5					287.8~413.1	2.6~23	0.57~0.89
Pb-Zn sulfide stage (S4)	Type I (5)	15~35	7~26		43.3~132.8				255.9~309.7	26.7~29	0.98~1.03
	Type II (52)	0~85	4~12	−18.4~−0.7					134.1~334.7	1.2~21.3	0.78~0.98
	Type III (8)	35~85	8~17			−59.7~−56 (to L)	4.6~9.1 (to L)	20.2~29.8 (to L)	183.9~261	1.8~9.6	0.67~0.93
						−56.7 (to V)	8.7; 7.9 (to V)	27.7; 30.8 (to V)	239.4; 228.8	2.6; 4.1	0.54; 0.55
	Type IV (4)	100	4~12			−57.4 (to L)	28.6~30.4 (to L)				0.55~0.64
							29.3 (to V)				0.33

Note: V/T(C/T): Percentage of gas phase (CO₂ phase); T_{m, ice}: Ice dissolution temperature; T_{m, hal}: Daughter mineral dissolution temperature; T_{m, CO2}: CO₂ dissolution temperature; T_{m, clath}: CO₂—trapped melt temperature; T_{h, CO2}: CO₂—partial homogeneous temperature; T_{h, tot}: perfectly uniform temperature.

and Figure 9a, and the temperature measurement results of the fluid inclusions at each ore-forming stage are as follows:

In the early skarn stage, the main fluid inclusions are type I and type II, and the host minerals are garnet and diopside. As mentioned earlier, type I inclusions containing opaque daughter minerals were not measured in this study. At the same time, while the temperature of most of the inclusions in this stage increased to close to 600 °C, it fails to reach uniformity, which is beyond the measurement range. The dissolution temperature of the daughter crystals of the remaining type I inclusions was 274~437 °C. The homogenization temperature ranged from 470 °C to 548 °C, with an average of 496 °C, and the salinity ranges from 36.1 to 51.2 wt.%NaClequiv, with an average of 44.2 wt.%NaClequiv. The freezing point dissolution temperature of type II inclusions is −20.8~−13.6 °C. The homogenization temperature was 437~522 °C, with an average of 464 °C, and the salinity ranges from 17.4 to 22.9 wt.%NaClequiv, with an average of 20.4 wt.%NaClequiv. In this stage, the gas-phase volume of type I and type II inclusions is relatively small (V/T < 40 vol.%), and most of them are uniform to the liquid phase during heating.

In the late skarn stage, the fluid inclusions in epidotes were selected as temperature measurement objects and are considered as type I and type II inclusions. Among them, two data of type I inclusions in this stage were measured, and the dissolution temperatures of the sub-minerals were 324 °C and 344 °C, respectively. The homogenization temperatures were 401 °C and 428 °C, respectively. The salinity values were 40.0 and 41.7 wt.%NaClequiv, respectively. The freezing point dissolution temperature of type II inclusions is −18.3~−9.7 °C, with an average of −14.6 °C. The homogenization temperature was 410~435 °C, with an average of 421 °C, and the salinity ranges from 13.6 to 21.2 wt.%NaClequiv, with an average of 18.1 wt.%NaClequiv. In this stage, the type II inclusions are mainly liquid-rich (V/T < 40 vol.%), and they are homogeneous to the liquid phase. The rare gas-rich inclusions (V/T > 65 vol.%) homogenized to the gas phase after heating. They do not exist in the same field of view. The difference in homogenization temperature is large (>15 °C), and no obvious signs of fluid boiling are found.

The Cu-Fe sulfide stage is the main ore-forming stage of the Baoshan deposit, and the fluid inclusions in quartz are selected as test objects in this study. The main inclusion types are type I and type II. Among them, the dissolution temperature of the daughter crystals of type I inclusions is 99~208 °C, and the salinity of the corresponding inclusions ranged from 27.9 to 32.3 wt.%NaClequiv, with an average of 29.5 wt.%NaClequiv. The homogenization temperature of inclusions ranges from 344 °C to 400 °C, with an average of 368 °C. The gas-phase ratio of type II inclusions varies greatly (10–100 vol.%), the freezing temperature of inclusions is −20.9~−1.5 °C and the salinity of the corresponding inclusions ranged from 2.6 to 23 wt.%NaClequiv, with an average of 14.1 wt.%NaClequiv. The homogenization temperature ranges from 288 °C to 413 °C, with an average of 364 °C. At the same time, two sets of boiling inclusion group data were measured in this study. The homogenization temperature ranges between 400~413 and 389~397 °C (range < 15 °C); the salinity is between 6.9~30.2 and 14.4~28.6 wt.%NaClequiv, which has the characteristics of non-uniform trapping; and the fluid density is 0.57~0.91 g/cm³ based on temperature and salinity. Compared with the previous stage, the ore-forming temperature and salinity in this stage have changed greatly and show an obvious decreasing trend.

The Pb-Zn sulfide stage is another main ore-forming stage in the Baoshan deposit, and the fluid inclusions used for temperature measurement mainly exist in sphalerite, fluorite and calcite. At this stage, type I~type IV inclusions can be found. Among them, the dissolution temperature of the daughter crystals of type I inclusions is 43~133 °C, the homogenization temperature is 256~310 °C (average: 282 °C) and the salinity is 26.7~29 wt.%NaClequiv. It is suggested that the ore-bearing hydrothermal fluid in the early stage still had a high temperature and level of salinity. The type II inclusions were widely distributed in this stage, and their freezing point temperature, homogenization temperature and salinity showed a wide range of −18.4~−0.7 °C, 134~335 °C and 1.2~21.3 wt.%NaClequiv, respectively. In this study, we also found that there are CO₂ inclusions (types III and IV) at this stage. Among them, the solid-phase dissolution temperature of the type III inclusion is between −59.7 and 56 °C (<56.6 °C), which is consistent with the characteristics of other organic components, including C₂H₆ in the fluid inclusion (Figure 8a). The dissolution temperature of the trapped compound was 4.6~9.1 °C, and the corresponding salinity is 1.8~9.6 wt.%NaClequiv. The partial homogenization temperature of CO₂ is 20.2~29.8 °C, and the complete homogenization temperature is 184~261 °C (homogenized to the liquid phase). The solid-phase dissolution temperature of the two type III inclusions is −56.7 °C, and the dissolution temperature (8.7 and 7.9 °C), the corresponding salinity (2.6 and 4.1 wt.%NaClequiv), the partial CO₂ homogenization temperature (27.7 and 30.8 °C) and the complete homogenization temperature (239 and 229 °C) of the trapped complex are obtained, respectively. Only one solid-phase dissolution temperature (−57.4 °C) is detected for type IV inclusions. According to the homogenization temperature and state of the CO₂ (28.6~30.4 °C, homogenized to the liquid phase; 29.3 °C, homogenized to the gas phase), the density is estimated to be 0.33~0.64 g/cm³.

4.3. C-H-O Isotopic Composition

The results of C-H-O isotopes obtained from the Baoshan Cu-Pb-Zn deposit are shown in Table 2. The δD, δ¹⁸O_V-SMOW and δ¹⁸O_H2O values of the late skarn stage are −56.4~−54.9‰, 18.0~19.6‰ and 7.5~9.2‰, respectively. The δD, δ¹⁸O_V-SMOW, δ¹⁸O_H2O and δ¹³C_V-PDB values in the Cu-Fe sulfide stage range from −84.4 to −70.5‰, 15.6 to 17.7‰ and 5.4 to 6.0‰ and 1.0‰, respectively. The δD, δ¹⁸O_V-SMOW, δ¹⁸O_H2O and δ¹³C_V-PDB values of the Pb-Zn sulfide stages are −101.7~−83.7‰, 15.4~16.2‰, 1.2~2.0‰ and −6.5~6.3‰, respectively. The values of δD, δ¹⁸O_V-SMOW, δ¹⁸O_H2O and δ¹³C_V-PDB generally show a downward trend from the late skarn stage to the sulfide stage.

Table 2. Hydrogen, oxygen and carbon isotope composition of the Baoshan Cu-Pb-Zn deposit.

Number	Mineral	δDV-SMOW‰	δ18OV-PDB‰	δOV-SMOW‰	δ18OH2O‰	δ13CV-PDB‰	Stages
475-5	Quartz	−56.4	−12.55	17.97	7.52		Late skarn stage
475-4	Quartz	−54.9	−10.96	19.61	9.16		Late skarn stage
723-15	Quartz	−70.5	−12.79	17.72	6.02		Cu-Fe sulfide stage
821-3	Calcite	−84.4	−14.84	15.61	5.39	1	Cu-Fe sulfide stage
819-5	Calcite	−83.7	−15.09	15.35	1.20	−6.28	Pb-Zn sulfide stage
730-3	Calcite	−101.7	−14.35	16.12	1.96	−6.45	Pb-Zn sulfide stage

The standard conversion formula for V-SMOW to PDB is δ¹⁸O_V-PDB‰ = 0.97002 × δO_V-SMOW‰ − 21.98 [58]; δ¹⁸O_H2O‰ was calculated according to the equations 1000lnα_quartz-water = 3.38 × 10⁶/T² − 3.40 [59] and 1000lnα_{calcite-water} = 2.78 × 10⁶/T² − 3.39 [60]. Temperatures are calculated as the average temperature of fluid inclusions, which are 419 °C, 365 °C and 235 °C for the late skarn stage, the Cu-Fe sulfide stage and the Pb-Zn sulfide stage, respectively.

Table 2. Hydrogen, oxygen and carbon isotope composition of the Baoshan Cu-Pb-Zn deposit.

Number	Mineral	δDV-SMOW‰	δ18OV-PDB‰	δOV-SMOW‰	δ18OH2O‰	δ13CV-PDB‰	Stages
475-5	Quartz	−56.4	−12.55	17.97	7.52		Late skarn stage
475-4	Quartz	−54.9	−10.96	19.61	9.16		Late skarn stage
723-15	Quartz	−70.5	−12.79	17.72	6.02		Cu-Fe sulfide stage
821-3	Calcite	−84.4	−14.84	15.61	5.39	1	Cu-Fe sulfide stage
819-5	Calcite	−83.7	−15.09	15.35	1.20	−6.28	Pb-Zn sulfide stage
730-3	Calcite	−101.7	−14.35	16.12	1.96	−6.45	Pb-Zn sulfide stage

The standard conversion formula for V-SMOW to PDB is δ¹⁸O_V-PDB‰ = 0.97002 × δO_V-SMOW‰ − 21.98 [58]; δ¹⁸O_H2O‰ was calculated according to the equations 1000lnα_quartz-water = 3.38 × 10⁶/T² − 3.40 [59] and 1000lnα_{calcite-water} = 2.78 × 10⁶/T² − 3.39 [60]. Temperatures are calculated as the average temperature of fluid inclusions, which are 419 °C, 365 °C and 235 °C for the late skarn stage, the Cu-Fe sulfide stage and the Pb-Zn sulfide stage, respectively.

5. Discussion

5.1. Origin of Ore-Forming Fluid

The H-O isotope compositions are shown in Figure 9c. All the samples in the late skarn stage fall into the range of magmatic fluid. Combined with the study of mineral geochemical characteristics [6,7], it is indicated that the initial ore-forming fluid of the Baoshan deposit comes from magmatic–hydrothermal fluid. Compared with the skarn stage, the δD and δ¹⁸O values of the mineral-bearing hydrothermal fluids in the Cu-Fe sulfide stage and the Pb-Zn sulfide stage are significantly reduced (Figure 9c; δD = −101.7~−70.5‰, δ¹⁸OH₂O = 1.20~6.02‰). Previous studies have shown that fluid evaporation, biochemistry, fluid–rock interaction, magmatic degassing and meteoric water are important factors affecting the H-O isotopic composition of ore-forming fluids [61]. No fluid evaporation or biochemical effects have been observed in the Baoshan deposit [6,15]. Meanwhile, no obvious positive or negative characteristics have been observed in the in-situ S isotope of the Baoshan pyrite, indicating that evaporation and biochemical effects are unlikely to be the cause of the decrease in δD and δ¹⁸OH₂O values in hydrothermal minerals [6]. In addition, although magmatic degassing can lead to a significant decrease in δD value, it has little effect on the δ¹⁸OH₂O value [62,63]. The fluid–rock reaction between the hydrothermal fluid and the formation containing organic carbon can reduce the δD value in the fluid, but the strong fluid–rock reaction will lead to the increase in the δ¹⁸OH₂O value [64], which is contrary to the trend of the decrease in the δ¹⁸OH₂O value in the ore-forming fluid of the Baoshan deposit. Therefore, it is inferred that the significant decrease in δD and δ¹⁸OH₂O values in the ore-forming fluids of the Baoshan deposit is closely related to the addition of meteoric water. During the evolution of ore-forming hydrothermal fluids in typical skarn deposits, the proportion of circulating precipitation in late hydrothermal fluids gradually increases, and even circulating heated precipitation can be used as the main source of late ore-forming fluids [61]. Compared with the Cu-Fe sulfide stage, the fall point of the Pb-Zn sulfide stage leaves the magmatic fluid range and rapidly approaches the meteoric water line, suggesting that the proportion of meteoric water in the Pb-Zn sulfide stage increases significantly.

The C-O isotope compositions are shown in Figure 9d. In the magmatic–hydrothermal system, the carbon of the ore-forming fluid is mainly divided into magmatic origin, oxidation and hydrolysis of reducing carbon, dissolution and decarbonization of sedimentary carbonate. In this study, only one δ¹³C_V-PDB value (1.0‰) of the Cu-Fe sulfide stage is obtained, and its settling point is close to the stratum range of the Shidengzi Formation (limestone) and the Zimenqiao Formation (dolomite) (Figure 9d). Combined with the previous C-O isotope data, the fall point of hydrothermal calcite is mostly between the wall rocks and granite, but there is a significant difference from the wall rocks, suggesting that the carbon in the ore-forming fluid is of magmatic origin as well as the contribution of the wall rocks. In this study, the δ¹³C value of the Cu-Fe sulfide stage samples is far away from the granite, indicating that there may be a strong fluid–rock reaction at this stage. Compared with the Cu-Fe sulfide stage, the δ¹³C value of the Pb-Zn sulfide stage decreased significantly (−6.64~−6.28‰), which may be related to the addition of reduced organic carbon in the Ceshui Formation. During this process, the δ¹⁸O_V-SMOW value did not increase with the addition of reduced organic carbon, suggesting that the fluid–rock reaction may be gradually weakened in the Pb-Zn sulfide stage. Therefore, it is speculated that the addition of organic carbon may come from the leaching of meteoric water to the Ceshui Formation.

5.2. Metallogenic Pressure and Depth

Previous studies have shown that obtaining the capture temperature of mineralized hydrothermal fluid inclusions is a prerequisite for calculating the fluid pressure. For boiling inclusions or immiscible inclusions, the fluid capture pressure can be estimated based on the homogeneous temperature [61,65]. In addition, in deposits associated with intrusive rocks, the crystallization pressure of minerals in magmatic rocks is often used to replace the trapping pressure of ore-forming fluids at the static rock pressure stage [66,67].

The homogenization temperature of fluid inclusion in the skarn stage of the Baoshan deposit is greater than >400 °C, suggesting that the formation may be in a ductile deformation state and the system pressure is mainly static rock pressure [66,67]. The results of the whole-Al manometer for the Baoshan granodiorite porphyry biotite show that the crystallization pressure of biotite ranges from ~105 to 125 MPa [68], which corresponds to the metallogenic depth of the Baoshan deposit ranging from ~3.89 to 4.63 km (the rock density is 2.7 g/cm³).

The homogenization temperature of most fluid inclusions in the Cu-Fe sulfide stage is lower than 400 °C, indicating that the rock has basically completed the transformation from plastic deformation to brittle deformation (from a static rock pressure to hydrostatic pressure) in this stage [66,69]. Based on the temperature measurement results of boiling inclusions in quartz at the Cu-Fe sulfide stage (Figure 9a), it is estimated that the vapor pressure (the true trapping pressure of fluid inclusions) at the boiling time of the fluid is between 20.4 and 30.4 MPa, corresponding to the hydrostatic pressure depth range of 2~3 km (the water density is 2.7 g/cm³), which is significantly lower than the depth range estimated by the biotite crystallization depth in the skarn stage. It shows that a single hydrostatic pressure system is not enough to describe the pressure environment of the ore-forming fluid at this stage, suggesting that the ore-forming fluid may be in the hydrostatic pressure semi- to open-system environment. In the Cu-Fe sulfide stage, the “scarp-like” downward trend of the ore-forming pressure may be related to the hydraulic fracturing of wall rock strata. This is consistent with the macroscopic geological phenomena of the widespread hydrothermal breccia and fluid inclusion structures in the Baoshan deposit [6]. The skarn and Cu ore bodies can be found in the Baoshan deposit (−100~−270 m) to migrate intermittently through the NW wing of the Baoling reversed anticline fold to the F₂₁ fault. In this process, ore-forming fluid is connected with F₂₁ and other fault systems through fractured rock and further forms a semi-connected state with the outside world.

Figure 9. (a) Homogenization temperature vs. salinity of fluid inclusions; (b) T-P trajectory of ore-forming fluid (according to Ref. [70]); (c) Diagram of ore-forming fluid δD-δ¹⁸OH₂O in the Baoshan Cu-Pb-Zn polymetallic deposit (based on previous data [10]); (d) δ¹³C-δ¹⁸OSMOW diagram of ore-forming fluid of the Baoshan Cu-Pb-Zn polymetallic deposit (based on previous data from Ref. [13]).

Figure 9. (a) Homogenization temperature vs. salinity of fluid inclusions; (b) T-P trajectory of ore-forming fluid (according to Ref. [70]); (c) Diagram of ore-forming fluid δD-δ¹⁸OH₂O in the Baoshan Cu-Pb-Zn polymetallic deposit (based on previous data [10]); (d) δ¹³C-δ¹⁸OSMOW diagram of ore-forming fluid of the Baoshan Cu-Pb-Zn polymetallic deposit (based on previous data from Ref. [13]).

In the Pb-Zn sulfide stage, aqueous solution inclusions (type II) are closely associated with CO₂-containing inclusions (type III) and pure CO₂-containing inclusions (type IV) (Figure 7e,f). The temperature measurement results show that the H₂O-rich inclusions and CO₂-rich inclusions have roughly the same homogenization temperatures (218~237 °C and 225~239 °C), indicating that the trapped inclusions are immiscible inclusions. In this study, two end-element components captured from sphalerite were used to estimate the ore-forming pressure by the isotropic crossing method. The end-element density of H₂O is estimated to be between 0.86 and 0.89 g/cm³ by selecting the inclusion of type II aqueous solution with a salinity lower than 5%. At the same time, the CO₂ end-element density was calculated to be 0.33~0.64 g/cm³ by selecting pure CO₂-containing inclusions (type IV). Combined with the isovolumetry diagram of the two end-elements, it is estimated that the trapping pressure of the fluid inclusion ranges from 23 to 70 MPa (Figure 9b), and the corresponding hydrostatic pressure depth ranges from about two to seven km (the water density is 2.7 g/cm³). Compared with the Cu-Fe sulfide stage, the trapping pressure of the ore-forming fluid in this stage showed an upward trend, which is speculated to be related to the rapid decrease in the porosity and system connectivity of the wall rock due to the general precipitation of minerals in the Cu-Fe sulfide stage. This indicated that the mineralization depth range of the sulfide stage coincides with the estimated mineralization depth range of the skarn stage, and it is inferred that the mineralization depth of the Baoshan Cu-Pb-Zn deposit may be concentrated in the range of about three to six km.

5.3. Properties and Evolution of Ore-Forming Fluids

The laser Raman spectroscopy and microthermometry studies show that the ore-forming fluids have different characteristics in different ore-forming stages of the Baoshan deposit. From the early skarn stage to the Cu-Fe sulfide stage, the primary fluid inclusions are mainly polyphase fluid inclusions (type I) and aqueous solution inclusions (type II), and the ore-forming fluid is the H₂O-NaCl (-CO₂ ± N₂ ± C₂H₆) system. At the Pb-Zn sulfide stage, the primary fluid inclusions can be seen as polyphase fluid inclusions containing crystallites (type I), aqueous inclusions (type II), CO₂-containing inclusions (type III) and pure-CO₂-containing inclusions (type IV), and the ore-forming fluid changes into the H₂O-NaCl-CO₂ (±N₂ ± C₂H₆) system.

In the early skarn stage, the measured fluid inclusions show that the ore-forming fluid has a high temperature (437~548 °C) and high level of salinity (17.4~51.2 wt.%NaClequiv.), which is consistent with the characteristics of typical magmatic–hydrothermal fluids. In the late skarn stage, the temperature (401~435 °C) and level of salinity (13.6~41.7 wt.%NaClequiv.) of the ore-forming fluid showed a downward trend compared with the previous stage (Figure 9a). Therefore, the ore-forming fluid had extensive fluid–rock interactions with the carbonate rock, and small amounts of scheelite and magnetite are formed. The above-mentioned process leads to the evolution of ore-forming fluid in the direction of decreasing oxygen fugacity and increasing pH [65,71]. The metallogenic pressure of the skarn stage is 105~125 MPa, and the corresponding metallogenic depth is 3.89~4.63 km. In the Cu-Fe sulfide stage, the homogenization temperature and level of salinity of the fluid inclusions are 288~413 °C and 2.6~32.3 wt.%NaClequiv, respectively. The extensive evolution of boiling inclusions at this stage indicates that the low salinity of fluid inclusions is mainly caused by fluid phase separation (boiling). Studies have shown that the boiling process leads to the escape of acidic gases such as CO₂ and H₂S in the hydrothermal fluid, which will lead to the decrease in oxygen fugacity and the increase in the pH of the ore-forming fluid [71]. The boiling inclusion group indicates that the ore-forming pressure is 20.4~30.4 MPa, corresponding to the ore-forming depth of 2~3 km. It is much lower than the depth of the skarn stage, suggesting that the ore-forming fluid in this stage was connected with the outside world through the fault system, and the estimation of the ore-forming depth may be inaccurate. The temperature (134~335 °C) and level of salinity (1.2~29 wt.%NaClequiv.) of the ore-forming fluid decreased significantly in the Pb-Zn sulfide stage (Figure 9a). With the precipitation of sphalerite and galena, the temperature measurement of the late calcite fluid inclusions showed that the ore-bearing hydrothermal fluid gradually evolved into low-temperature and low-salinity fluid. The phase separation (fluid immiscibility) of CO₂ and H₂O ends occurred in the ore-forming fluid at this stage, which led to the continuous evolution of the ore-forming fluid in the direction of decreasing oxygen fugacity and increasing pH. At the same time, the co-precipitation of Pb, Zn, Ag and other metals also indicates that the ore-forming fluid is under the conditions of low levels of oxygen fugacity and high pH levels. The combined phase diagram of the two immiscible end-elements calculated that the ore-forming pressure was 23–70 MPa, corresponding to the ore-forming depth of 2–7 km. It is consistent with the range of the ore-forming depth of the skarn stage, indicating that the porosity and connectivity of the wall rocks rapidly decreased with the progress of mineralization and mineral precipitation.

The ore-forming fluids in the Baoshan deposit changed from the early H₂O-NaCl (-CO₂ ± N₂ ± C₂H₆) system to the late H₂O-NaCl-CO₂ (±N₂ ± C₂H₆) system. With the progress of mineralization, the temperature, salinity and oxygen fugacity of the ore-forming fluid showed a decreasing trend, while the pH showed an opposite trend. The metallogenic pressure showed a “cliff-like” downward trend from the skarn stage to the sulfide stage, which may be related to hydraulic fracturing. With the precipitation of a large number of minerals filling the hydraulic fracturing channel, the metallogenic pressure showed a slight upward trend in the Pb-Zn sulfide stage.

5.4. Metal Migration and Precipitation Mechanism

The main mineralization mechanisms leading to metal precipitation in the ore-forming fluids of porphyry and skarn deposits are as follows: (1) a decrease in metallogenic temperature; (2) phase separation (fluid immiscibility or fluid boiling); (3) fluid–rock reactions; and (4) fluid mixing [61]. Previous studies have shown that with the decrease in temperature, the solubility of Cu, Fe, Pb, Zn and other metals shows an obvious decreasing trend [72,73], and 350–425 °C and 275–375 °C are the favorable temperature ranges for Cu, Pb and Zn precipitation, respectively [61,74]. The homogenization temperature of fluid inclusions in the Cu-Fe sulfide stage (288~413 °C) and the Pb-Zn sulfide stage (134–335 °C) in the Baoshan deposit is basically consistent with the favorable metallogenic temperature range (Figure 9a), indicating that the decrease in temperature is one of the important factors controlling the precipitation of metal sulfide in the Baoshan deposit. However, the central part of the Baoshan deposit is dominated by Cu (Mo) mineralization, while the western, northern and eastern parts are dominated by Pb-Zn (Ag) mineralization (Figure 10). The selective precipitation of different metals in different spaces is difficult to explain simply by the change in temperature, which indicates that the decrease in temperature may not be the only factor of metal precipitation in the Baoshan deposit. Compared with the Pb-Zn sulfide stage, the widespread boiling inclusion groups and colloidal pyrite in the Cu-Fe sulfide stage suggest that the ore-forming fluids in this stage experienced intense boiling [6]. The relationship between cutting and metasomatism between minerals shows that chalcopyrite precipitation was induced immediately after the formation of colloidal pyrite (i.e., after fluid boiling), suggesting that fluid boiling may be an important factor leading to Cu precipitation in ore-bearing hydrothermal solutions of the Baoshan deposit.

Studies have shown that it is difficult for a single fluid to transport enough metal and sulfur (non-sulfate forms) and eventually precipitate them to form metal sulfide deposits [61]. In the hydrothermal system, Cu, Mo, Au, Ag, S and other elements tend to be enriched in the low-density gas phase, while Pb, Zn, Mn and other elements tend to be enriched in the brine phase with a high level of salinity (35–70 wt.%) [66,75,76,77]. This indicates that Cu, Mo and other elements are more likely to form complexes with HS⁻ in the hydrothermal system [77], while Pb, Zn and other elements are more likely to form complexes with Cl⁻ [78,79]. It can be predicted that the escape of H₂S and other gases during the boiling process will have a significant impact on the former [61,80], which is consistent with the geological characteristics of chalcopyrite precipitation after the stage involving the boiling of Cu-Fe sulfide. It can be inferred that the precipitation of chalcopyrite and molybdenite in the Baoshan deposit is controlled by fluid boiling, but the boiling effect does not cause significant changes in the Cl⁻ concentration in the fluid [78]. Previous studies have pointed out that fluid mixing between ore-bearing hydrothermal fluid and meteoric water is an important mechanism leading to Pb and Zn precipitation in skarn deposits [78]. The addition of meteoric water causes the temperature and Cl⁻ concentration of ore-forming fluid to decrease significantly, which will further reduce the bearing capacity of the hydrothermal fluid to Pb, Zn and other metals and will eventually lead to the precipitation of sphalerite and galena. The H-O isotope composition of the Baoshan deposit (Figure 9c) shows that the amount of meteoric water added in the Pb-Zn sulfide stage increases significantly, suggesting that the addition of meteoric water is an important mechanism leading to the precipitation of Pb and Zn in the Baoshan deposit. In addition, the increase in pH levels of the ore-forming fluid is also one of the important factors causing the precipitation of Pb, Zn and Ag [81]. During the boiling process of the fluid, acidic gases such as CO₂ and H₂S escape from the hydrothermal solution. At the same time, the fluid–rock interaction between the ore-forming fluid and carbonate formation will further consume the H⁺ of the fluid, and the above two processes will increase the pH value of the ore-forming fluid [61,71], which may be an important factor causing a small amount of sphalerite and chalcopyrite to precipitate together in the Cu-Fe sulfide stage. The immiscibility between the CO₂-rich phase and the H₂O-rich phase in the Pb-Zn sulfide stage of sphalerite further intensifies the escape of acidic gases, such as CO₂, H₂S, SO₂, HCl and HF [81]. With the mixing of meteoric water, the decrease in mineralization temperature and the continuous increase in pH level provide favorable conditions for the precipitation of Pb, Zn, Ag and other metal elements (Figure 10).

To sum up the above discussion, the Baoshan Cu-Pb-Zn polymetallic deposit forms Cu mineralization in the centre and Pb-Zn mineralization in the edge, which, on the one hand, is a manifestation of the migration and evolution of ore-forming fluids, and, on the other hand, is the result of the selective precipitation of different metal elements to the change in physical and chemical conditions (Figure 10).

6. Conclusions

(1) The ore-forming fluid was the H₂O-NaCl (-CO₂ ± N₂ ± C₂H₆) system in the skarn stage and changed into the H₂O-NaCl-CO₂ (±N₂ ± C₂H₆) system in the Pb-Zn sulfide stage, and the temperature and level of salinity of the ore-forming fluid showed a downward trend from the early to the late stage.

(2) From the skarn stage to the sulfide stage, the ore-forming pressure system experienced a change from the static rock pressure to the hydrostatic pressure, and the ore-forming depth is estimated to be about three to six km, which is obviously greater than the depth of the W-Sn deposits in the area (which are generally less than 3 km).

(3) The C-H-O isotope of hydrothermal minerals such as quartz and calcite shows that the ore-forming fluid of the Baoshan deposit is mainly magmatic fluid, but a significant amount of meteoric water is added in the Pb-Zn sulfide stage.

(4) The precipitation of Cu is mainly controlled by fluid boiling, while the precipitation of Pb and Zn is mainly controlled by the mixing of magmatic fluid and meteoric water.