The fluid inclusions in this study can be classified into primary, secondary, and pseudo-secondary. Primary fluid inclusions (size: 5–25 μm) are usually found in mineral grains or along the crystal growth direction. Secondary fluid inclusions (size: 1–6 μm) are distributed along the mineral crystal fractures. Pseudo-secondary fluid inclusions (size: <5 μm) are mainly present in mineral crystals in a banded distribution or overlap with primary fluid inclusions. Our analysis is focused on primary fluid inclusions. According to the classification criteria of fluid inclusions at room temperature and the phase changes during the heating-freezing process (Roedder, 1984; Lu et al, 2004), we divide the primary fluid inclusions in Stage I to III quartz into four types: L-type, LV-type, C-type, and CO₂-type. The characteristics of each fluid inclusion type are described as follows:

L-type: At room temperature, they occur as pure liquid-phase and are colorless transparent (Figure 7A). This inclusion type is common in quartz but rare in calcite, and is usually elongated.

LV-type: they consist of both liquid and vapor phases and are widely distributed in both quartz and calcite (Figures 7B–F, H, K–M). According to the volume percentage of the vapor and liquid phases in the inclusion, they can be further divided into the liquid-rich (LV1-type) and vapor-rich (LV2-type) subtypes. At room temperature, LV1-type inclusions are composed of gas and saline phases, with the latter accounting for >50% of the total volume of the inclusion. The inclusion size varies greatly (diameter: 10–20 μm), whereas the gas-liquid ratio variation is relatively narrow. These inclusions are often elliptical, elongated, quadrangular, or irregular (Figures 7D, F). During the heating, the gas bubble volume gradually decreased and finally homogenized with the liquid phase. In a few cases, the gas bubble volume increased with increasing temperature, and finally homogenized with the gas phase. Some larger inclusions show “necking” phenomenon (Figure 7D). LV2-type inclusions are more common than LV1-type inclusions and they commonly coexist. They are composed of gas and saline phase at room temperature, with the former occupying >50% of the total volume. The inclusion size varies greatly (diameter: 15–20 μm, up to ∼30 μm). They are often negative crystal or elliptical in shape (Figures 7A–F, H). During the heating, the gas bubble gradually increased and finally homogenized with the gas phase (Figures 7L, M).

C-type: They are well-developed in stage I and II (but not stage III) quartz. The H₂O liquid phase is mostly colorless and transparent, while the CO₂ phase is mostly gray-black. C-type inclusions account for ∼25% of the total number of fluid inclusions. These inclusions can be of two-phase (C1-type) or three-phase (C2-type) at room temperature. C1-type contains liquid water and gaseous CO₂, and liquid CO₂ appears at ca. 5–20°C (Figures 7C, I). C1-type can be further classified into H₂O-rich (C1a-type) and CO₂-rich (C1b-type), according to its proportion of CO₂: C1a-type (size: 5–18 μm) is mostly quadrangular or elliptical, while C1b-type (size: 8–15 μm) homogenized mostly into liquid (minor into gas) when heated, with negative crystal or irregular shapes. C2-type (size: 8–24 μm) contains gaseous CO₂, liquid CO₂, and liquid H₂O (Figures 7E, F, H, K). These inclusions are mostly elliptical, elongated or irregular, and homogenized into gas when heated.

CO₂-type: At room temperature, these inclusions comprise pure liquid. They are grayish-black and negatively crystal-shaped (Figure 7G), and are mostly hosted in quartz and largely absent in calcite.

In this study, laser Raman spectroscopic analysis was performed on all L-, LV-, C-, and CO₂-type fluid inclusions from quartz, with the results shown in Figure 8. For gas-liquid two-phase fluid inclusions, the characteristic peaks appearing before 1,000 cm^-1 in Raman shifts are generally affected by the host mineral quartz or the outer wall of the fluid inclusion, and thus they are not further discussed here. The results show that the liquid phase component is mainly CO₂ (characteristic peaks at 1,282–1,387 cm^-1) and the gas phase component is mainly CH₄ (characteristic peaks at 2915 cm^-1).

In this study, microthermometric measurements were conducted on seven well-preserved fluid inclusion (LV- and C-type) samples, yielding a total of 168 data points. The fluid inclusions analyzed were selected based on micro-petrographic observations (Figure 9).

A: C2-type fluid inclusion at −100°C; B: Solid-state CO₂ is still visible at −56.9°C; C: Solid CO₂ melts at −56.7°C, which is the initial melting temperature; D: At 8.4°C, the daughter mineral is not completely dissolved, and the gaseous-liquid CO₂ boundary becomes blurred; E: At 8.5°C, the daughter mineral disappears, and the gaseous-liquid CO₂ boundary becomes clear, and the gaseous CO₂ becomes a complete bubble, Consistent with room temperature state; F: At 23.2°C, the C2-type inclusion homogenized partially to gaseous phase; G: At 110°C, the gaseous CO₂ expands gradually; H: At 173.2°C, C2-type inclusion homogenized completely to gaseous phase.

From Table 1, it is shown that the homogenization temperatures of LV-type fluid inclusions vary greatly from 185.2 to 380.6°C. Stage I LV-type fluid inclusions have homogenization temperatures of 285.2–380.6°C (avg. 349.8°C) and ice melting temperatures of −5.7 to −0.1°C (avg. −3.1°C), whilst stage II LV-type inclusions have homogenization temperatures of 232.1–351.3°C (avg. 276.1°C) and ice melting temperatures of −13.8 to −0.1°C (avg. −3.1°C). Stage III LV-type inclusions have homogenization temperatures of 185.2–237.3°C (avg. 213.2°C) and ice melting temperatures of −5.6 to −0.3°C (avg. −2.5°C). Accordingly, mineralization temperatures at Zhangjiapingzi are mainly 320 and 380°C in stage I, 240–300°C in stage II, and 220 and 240°C in stage III (Figure 10).

The vast majority of C-type fluid inclusions would burst or leak when the temperature exceeds 190°C during the heating process. Moreover, the larger the inclusion volume, the easier it is to burst. This is why only a small number of C-type inclusions could yield complete homogenization temperature in this study (Table 2).

From Table 2, it is shown that the initial melting temperatures of stage I C-type fluid inclusions are −57.8 to −55.8°C (mean −56.6°C). The daughter minerals disappeared at 6.1–9.8°C (mean 8.7°C). Partial homogenization temperatures are 18.4–29.8°C (mean 25.4°C), whilst total homogenization temperatures are 196.7–317.6°C (mean 265.3°C). In stage II, the initial melting temperatures of C-type inclusions are −57.5 to −56.1°C (mean −56.6°C). The daughter minerals disappeared at 6.7–9.7°C (mean 9.1°C). Partial homogenization temperatures are 20.1–23.2°C (mean 21.6°C), whilst the total homogenization temperatures are 119.8–253.2°C (mean 180.0°C). From Figure 10, it is shown that the mineralization temperatures at Zhangjiapingzi are concentrated around 240–300°C in stage I, and 140–160°C and 220–260°C in stage II.

The salinity and density for LV-type inclusions were calculated using the HokieFlincs2012Excel spreadsheet (Matthew et al, 2012), while those of C-type inclusions were calculated using the HokieFlincs2018Excel spreadsheet (Matthew, 2018).

In stage I, the calculated salinity of LV-type and C-type fluid inclusions is of 0.18%–8.81% (avg. 5.02%) and 0.42%–7.29% (avg. 2.57%), respectively. The calculated fluid density of LV-type and C-type inclusions is of 0.54–0.78 g/cm³ (avg. 0.67 g/cm³) and 0.79–1.01 g/cm³ (avg. 0.90 g/cm³), respectively. In stage II, LV-type and C-type inclusions have salinity of 0.18%–17.61% (avg. 4.94%) and 0.63%–6.28% (avg. 1.84%), respectively, and density of 0.66–0.91 (avg. 0.80 g/cm³) and, 0.87–0.98 g/cm³ (avg. 0.94 g/cm³). In stage III, LV-type inclusions have salinity of 0.53%–8.68% (avg. 4.12%), and density of 0.84–0.93 g/cm³ (avg. 0.88 g/cm³) (Table 3).

Petrographic observations indicate that LV-type and C-type inclusions coexist in quartz veins. LV1-type inclusions homogenized to the liquid phase, while some LV2-type inclusions homogenized to the gas phase. There are also some high-salinity LV1-type and C-type inclusions, indicating fluid boiling during mineralization. Figures 10C, D shows that the fluid salinity decreases from the early to late stage, whilst Figure 11 shows that the fluid salinity and density are correlated positively. Throughout the mineralization, the density increases and the salinity decreases gradually. The overall fluid salinity and density are below 10% and 0.79–1.01 g/cm³, indicating that the Zhangjiapingzi gold ore fluids are of low salinity and medium-low density.

Fluid inclusion pressure estimation has always been challenging, and the estimated mineralization pressure usually refers to the trapping pressure of fluid inclusions. The homogenization pressure of fluid inclusions can be obtained based on their homogenization temperature, but the trapping pressure needs to be corrected. In nature, the most common ore-forming fluid system is the NaCl-H₂O system, which exists in almost any type of deposits. The Zhangjiapingzi gold ore-forming fluids belong to the H₂O-NaCl-CO₂ system, and thus different formulas are needed to calculate the pressure.

Matthew (2018) proposed a new pressure calculation method for the NaCl-H₂O fluid system, which is adopted here for LV-type inclusions. Meanwhile, pressure calculation of C-type inclusions used the formula for the H₂O-NaCl-CO₂ system (Pudack et al, 2009; Matthew et al, 2012). The calculation results are shown in Table 4.

In stage I, the estimated pressure of LV-type and C-type fluid inclusions is of 6.75–22.48 MPa (avg. 16.27 MPa) and 8.28%–42.46% (avg. 21.92%), respectively. In stage II, the estimated pressure of LV-type and C-type inclusions is of 2.82–16.30 MPa (avg. 6.25 MPa) and 10.23–29.08 MPa (avg. 22.14 MPa), respectively. In stage III, the estimated pressure of LV-type inclusions is 1.08–3.12 MPa (avg. 2.04 MPa). Estimated pressure of the Zhangjiapingzi gold mineralization ranges from 1.08 to 42.46 MPa. As shown in Figure 12, the pressure of quartz fluid inclusions decreases from the early to the late stage, except for some outliers.

Primary fluid inclusions preserve the features and origin of ore-forming fluids, which are crucial for the understanding of the ore genesis (Hou et al, 2006a; Hu et al, 2014). Based on microthermometry and petrographic observations, the fluid inclusions from Zhangjiapingzi gold deposit are dominated by CO₂-H₂O-NaCl inclusions. Our microthermometric analysis indicates that the main ore stage fluid inclusions range from 220 to 300°C, with low salinities (<10%) and density (0.79–1.01 g/cm³). Laser Raman analysis reveals that the inclusions are CO₂-rich, suggesting that the ore-forming fluid was of medium-temperature, low-salinity, medium-low-density, and CO₂- and CH₄-rich. Previous studies have shown that most of the H-O isotope data fall within the metamorphic water field (Figure 13), and the C-O data imply that the carbon of the ore-forming fluid was originated from the mantle (Figure 14). This indicates that the Zhangjiapingzi gold deposit has similar carbon (δC¹³ = −7.40 to −3.44‰), oxygen (δO¹⁸ _H2O = 4.4‰–6.7‰) and sulfur (δ³⁴S = 3‰–7‰) isotope compositions (Table 5) to those of typical mantle-derived metamorphic-hydrothermal fluids (Luo et al, 2013; Xin et al, 2016; Xu et al, 2019; Li, 2001; Li et al, 2007; Li et al, 2005).

Generally, water-rock interaction, fluid mixing and CO₂ degassing are common mechanisms for hydrothermal ore deposition (Zheng, 1990; Zhang, 1997). Previous studies have demonstrated (Zheng et al, 1993) that C and O isotopic compositions are negatively correlated when the fluids are affected by CO₂ degassing or fluid-wallrock interaction, and such negative correlation is found in our data. However, a simple fluid-wallrock interaction cannot explain the wide δ¹³C_V-PDB range (−7.40 to −3.44) observed in the Zhangjiapingzi samples (Table 5). This implies that water-rock interaction alone is insufficient to account for metal precipitation. Pressure drop (e.g., boiling) is a key process for CO₂ degassing. We found that coexisting fluid inclusions with different types and gas-liquid ratios have similar total homogenization temperatures (within the same field of view).

Figure 15 shows that although the homogenization temperature variation is minor, their salinities vary widely. Some main-ore stage fluid inclusions have high salinities (max 17.61%), while some have low salinities (min 0.18%), implying fluid boiling. Table 4 shows that from the early to late mineralization stages, LV-type inclusions indicate a gradual decrease of ore-forming pressure (from 22.48 to 1.08 Mpa). During CO₂ degassing of the ore-forming fluids, metallic minerals precipitated as the fluid degassed CO₂ and its pH increased. The H-O isotope data plot within the metamorphic water field (Figure 13), with minor contribution from the mantle-derived magmatic water. This suggests that the ore-forming fluids may have involved a mixture of metamorphic and magmatic fluids, although fluid mixing alone cannot account for all the ore precipitation.

Based on previous studies (Lan, 2013; Luo et al, 2013; Yang et al, 2015; Bao, 2016; Xin et al, 2016; Zhou et al, 2016; Wang et al, 2019) (Table 6), a comparison of Zhangjiapingzi gold deposit with epithermal and orogenic types reveals that Zhangjiapingzi gold deposit matches with orogenic features in terms of regional tectonic setting, orebody features, ore types, wallrock alteration, and the ore-forming fluid features and sources (Groves et al, 1998; Kerrich et al, 2000; Chen, 2006; Chen et al, 2007; Cline, 2018; Deng et al, 2020; Wang et al, 2022). The H isotope composition (δD) of Zhangjiapingzi gold deposit ranges from −78.06 to −23.35‰ (Table 5), which is consistent with those of typical orogenic gold deposits (δD = −80 to −20‰) (Ohmoto, 1997), further supporting that the Zhangjiapingzi is orogenic type.

The Zhangjiapingzi gold deposit is situated in the Danba-Mianning gold metallogenic belt, which borders the southwest margin of Yangtze block and Songpan-Ganzi orogenic belt. This area underwent the Yanshanian and Himalayan orogenic events, leading to the development and modification of shear (and alteration) zones that controlled the gold transport and concentration (Bao et al, 2016). The regional orogeny may have also provided the ore-forming materials, fluids and structures for gold mineralization. Within the Zhangjiapingzi mining area, the F1 fault (Figure 2) and its associated structures and alteration zones are the main ore-hosting sites, as these structures created favorable conditions for gold deposition. Based on our petrographic and Raman spectroscopic analyses of fluid inclusions, the ore-forming fluid was likely of medium-temperature (220–300°C), low-salinity (<10%), medium-low-density (0.79–1.01 g/cm³), and CO₂- and CH₄-rich, resembling orogenic-type ore-forming fluid and suggesting an orogenic origin for this deposit.