The Zhaoxian gold deposit is located within the central part of the Jiaojia fault zone, where it is exposed to Paleoproterozoic granulite, marble, and schist of the Jingshan Group, as well as pelitic arkose and clay sandstone (Wutu Group) of the Cenozoic Zhubidian Formation (Zhu et al., 2018; Figure 2).

The Jiaojia fault zone is a large fault zone within the Jiaodong gold province, comprising more than 10 parallel northwest-trending faults with an approximate width of 6 km. The fault zone has a slightly undulating shape and a width ranging from 500 m to 1,000 m. The strikes of the faults are nearly northwest, and their dips average around 40°. Both ends of the fault zone are covered by Quaternary strata along its strike. The Jiaojia fault, located within the Jiaojia fault zone, plays a significant role as the main controlling fault governing mineral deposition. The fault has a considerable width ranging from 160 m to 500 m and has a slightly undulating shape. The fault exhibits varying trends between 0° and 30° in a west-northwest direction with a gentle dip angle. The northern segment of the fault has an average dip angle of 19°, ranging from 10° to 26°, while the southern segment exhibits an average dip angle of 27°, ranging from 12° to 40°. The central region of the Jiaojia fault exhibits a continuous and stable main fracture plane, characterized by fault gouge with thicknesses ranging from 1 cm to 30 cm. The tectonic rocks in the Jiaojia fault zone can be classified into three lithologic zones based on their degree of fragmentation. The internal zone consists of mylonites and cataclastic rocks with an approximate average thickness of 20 m. The middle zone primarily comprises granitic cataclastic and metabrro cataclastic rocks, with an average thickness of around 50 m. The outer zone encompasses sericite granite along with sericite metabrro belts, which have an average thickness of approximately 200 m. While most boundaries between these lithologic zones demonstrate a gradual transition, certain boundaries exhibit more distinct characteristics. Within the mining area, various magmatic rocks can be found including metamorphic gabbros (Figure 2), Qixia gneisses, biotite schists; early Yanshanian Linglong medium-grained monzonitic granites displaying weak gneissosity; late Yanshanian Guojialing porphyritic granodiorites; in addition to several younger dikes.

According to estimates, the potential gold resource in the Zhaoxian gold deposit exceeds 100 tonnes (Survey report on the gold deposit in the Zhaoxian area, Laizhou County, Shandong Province, Shandong Institute of Geological Survey, 2017). The Zhaoxian gold deposit likely consists of fissure-altered ore, containing a total of 27 orebodies located within faults and clastic zones (Figure 3). These ore bodies range in elevation from 2,144 m to 1,260 m below sea level and are aligned along fault lines trending between 236° and 345° with a dip angle ranging from 11° to 27°. The thicknesses of the orebodies range from 0.95 m to 13.83 m, exhibiting gold grades varying between 0.10 g/t and 18.40 g/t.

We conducted comprehensive mineralogical investigations, including both macroscopic and microscopic observations. By examining the mineral characteristics under an optical microscope, we can distinguish different mineral phases. In cases where identifying certain minerals becomes difficult, we can use a scanning electron microscope-backscattered electron (SEM-BSE). For example, native gold appears as a brighter and more vibrant golden yellow color compared to chalcopyrite; when the mineral grains are indistinguishable at small scales, SEM analysis helps in identifying the minerals: native gold appears as a brilliant white shade in SEM images while chalcopyrite manifests as a dark gray color. The metallogenic process can be categorized into four stages based on the crosscut relationship and mineral coexistence association. Figures 4A–D display the representative ores in each stage, while Figures 4E–G display the crosscuts between these stages.

Stage 1 (early stage, barren coarse-grained quartz vein with fine-grained pyrite, Figure 4A) primarily consists of milky quartz veins ranging from 1cm to 4 cm in width. The quartz grains show both ductile and brittle deformation, and there is a small amount of fine-grained pyrite (Py1, Figures 5A, B, 7A, 8A), with K-feldspar also present (Figure 4A). Only a little fine-grained pyrite found in this stage, however, other sulfide and native gold did not found.

Stage 2 (gold stage, composed of smoky fine-grained quartz, coarse-grained pyrite, allotriomorphic chalcopyrite, and native gold, Figures 4B, E) is characterized by the abundant presence of pyrite (Py2) among the ore minerals. The pyrite crystals in this stage have a relatively high degree of crystallinity and generally possess coarse grains and of idiomorphic-hypidiomorphic aggregates with oscillating zones (Figures 5C, D, 6B, 7B), which is an identifying mark of this stage. Native gold grains can be observed within the pyrite grains or within fissures in quartz or pyrite grains (Figures 5C, 6B, C, 7B). The fine-grained smoky appearance of the quartz veins signifies their significance as primary conduits for gold mineralization (Abdelkarim et al., 2022; Khedr et al., 2013; 2023; Figure 4B). Chalcopyrite is also common in this stage, which filled in the fractures of quartz and pyrite (Figures 5D, 7B). Stage 2 veins often crosscut Stage 1 veins, which shows that Stage 2 veins formed latter than preexisting Stage 1 veins (Figure 4E). The gold mineralization in this stage exhibits the most prominent manifestation compared to all other stages.

Stage 3 (polymetallic stage, composed of fine-grained smoky quartz, coarse-grained pyrite, sphalerite, galena, chalcopyrite, and native gold, Figures 4C, F) predominantly exhibits a fine and idiomorphic-hypidiomorphic morphology, with pyrite serving as the primary sulfide mineral (Figures 5E, F, 6D, 7C) that frequently undergoes replacement by chalcopyrite, galena, and sphalerite (Figures 5F, 6E, 7C), sulfides mainly filled in the fissures of quartz (Figure 5F–H, 6E). In this stage, the proportion of sulfide is larger than that of previous stages. Stage 3 veins often crosscut Stage 2 veins, which shows that Stage 3 veins formed latter than preexisting Stage 2 veins (Figure 4F).

Stage 4 (late stage, composed of transparent quartz, calcite with a little pyrite, Figures 4D, G) contains an abundance of off-white quartz alongside transparent quartz-calcite veins containing some hypidiomorphic pyrite (Figures 5I, 6F, 7D). Stage 4 veins often crosscut Stage 3 veins, which shows that Stage 4 veins formed latter than preexisting Stage 3 veins (Figure 4G).

The major mineral paragenetic sequence is illustrated in Figure 8.

The wall rock alteration within the Zhaoxian gold deposit is widespread and predominantly characterized by potassic alteration, beresitization, chloritization zone, and carbonatization alteration. The alteration phenomenon exhibits evident symmetry (Figure 3), with the ore body as the focal point and a gradual decrease in alteration intensity. Most of the ore bodies belong to altered-rock type. Each prospecting profile often encompasses multiple zones of alteration, including beresitization cataclastic zone, beresitization granitic cataclastic zone, beresitization granite zone, and phyllic granite zone (reference to Survey report on the gold deposit in the Zhaoxian area, Laizhou County, Shandong Province, Shandong Institute of Geological Survey, 2017). Potassic alteration is a high-temperature alteration, often found in the wall rock alongside high-temperature veins (Figure 9A). Beresitization refers to a moderate to high-temperature wall-rock alteration characterized by the presence of pyrite, quartz, sericite, etc. (Figure 9B). It primarily occurs in medium-acid igneous rocks and metamorphic rocks under the influence of hydrothermal fluids at moderate to low temperatures. Beresitization may have a strong relationship with gold mineralization (Figure 9B). During this process, feldspar undergoes decomposition into sericite and quartz, while dark minerals are replaced by pyrite (Khedr et al., 2013; 2023; Abdelkarim et al., 2022). Chloritization (Figure 9C) appears to have a lower temperature than that of beresitization, and it can be found near the polymetallic stage. In this process, dark minerals like biotite and amphibolite are altered by chlorite. Carbonatization (Figure 9D) occurs at low temperatures and can be found near the late stage. Comparing with the wall-rock alteration works in previous researches (Li et al., 2022; Xu et al., 2022), we found that the alteration type in the Zhaoxian gold deposit is similar at different depths.

In order to conduct a detailed analysis of the minerals, representative samples from the Zhaoxian gold deposit were selected for mineralogy and petrology analyses. The Phenom XL scanning electron microscope (SEM) was utilized for precise mineral identification at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing, China. Equipped with an energy-dispersive X-ray spectrometer (EDS), the SEM captured backscattered electron (BSE) images using an accelerating voltage of 15 kV and beam current of 3 nA or 4 nA through its user interface. Each BSE image was acquired over a duration of 1 min, while semi-quantitative point analyses were performed for 30 s.

The quartz veins thin sections, with a thickness of approximately 0.2mm, were meticulously polished for the purpose of studying fluid inclusion and used for mineralogical and fluid inclusion analyses. The studies on inclusion were conducted using microthermometry and Raman spectroscopy techniques. All samples from four different stages of quartz veins contained suitable fluid inclusion assemblages for analysis, including primary, secondary, and pseudo-secondary fluid inclusions with identified fluid phases according to (Lu et al., 2004). Microthermometric measurements were conducted at IGGCAS using a THMS 600 programmable heating/freezing stage equipped with a Carl Zeiss Axioskop 40 and Nikon transmission/reflection optical microscope+camera system that allowed direct observation of changes in the phase states during heating or freezing and measured the final melting temperature of solid CO₂ phase (T_{m, CO₂}), the final melting temperature of CO₂-H₂O clathrate (T_{m, Cl}), the homogenization temperature of CO₂ phases in the inclusion (T_{h, CO₂}) of CO₂-rich inclusion; the final melting temperature of ice (T_m-ice) of biphase aqueous inclusion; and the homogenization temperature (T_h-tot) of CO₂-rich inclusion and biphase aqueous inclusion. The accuracy of the freezing/heating measurements was estimated to be ± 0.1°C below 30°C, ± 1°C between 31°C and 300°C, and ± 2°C above 300°C. Salinities of H₂O-NaCl fluids were calculated based on the measured T_m-ice using the method described in (Bodnar, 1993), while salinities of H₂O-CO₂ fluids were calculated based on the measured T_{m, Cl} using the method described in (Collins, 1979). The T_h-tot provided minimum temperatures associated with different stages of ore-forming fluid evolution. Salinity is quantified as the weight percentage of NaCl equivalent (wt %NaCl eqv.) (Fan et al., 2006).

The in-situ analysis of pyrite was conducted using LA-ICP-MS at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The operating conditions and data reduction for laser ablation systems and ICP-MS instruments were performed as described by (Zong et al., 2017). Laser sampling was carried out using the GeolasPro laser ablation system, which comprised a MicroLas optical system and a COMPexPro 102 ArF excimer laser (maximum energy of 200 mJ and wavelength of 193 nm). Ion signal intensities were acquired using the Agilent 7,700e ICP-MS instrument. Helium served as the carrier gas while argon acted as the supplementary gas; both gases were mixed prior to entering ICP. A “wire” signal smoothing device (Hu et al., 2015) was also incorporated into the laser ablation system. The spot frequency and size of the laser were set at 5 Hz and 32 μm, respectively. The elemental compositions of pyrite was calibrated using various reference materials (NIST 610 and NIST 612) without the use of an internal standard (Liu et al., 2008). The sulfide reference material MASS-1 (USGS) was utilized as the unknown sample to validate the calibration method’s accuracy. Each analysis included a 20s–30s background acquisition followed by 50s of data acquisition from the sample. The off-line selection and integration of background and analyzed signals, time-drift correction, and quantitative calibration for trace element analysis were performed using an Excel-based software called ICPMSDataCal (Liu et al., 2008).

The in-situ S isotope analysis of pyrite was conducted using the Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Bremen, Germany) equipped with a Geolas HD excimer ArF laser ablation system (Coherent, Gottingen, Germany) at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Helium was employed as the carrier gas for the laser ablation system. To mitigate potential sulfur isotope ratio downhole fractionation effects during analysis (Fu et al., 2016), we opted for a large beam spot size (44 μm), low frequency (2 Hz) laser conditions, and performed approximately 100 laser pulses per single analysis. Additionally, a signal smoothing device (Hu et al., 2021) was utilized to ensure stable signals under slow frequency conditions. The fixed laser energy density was set at 5.0 J/cm². Equipped with 9 Faraday cups and 10¹¹Ω resistance amplifiers, the Neptune Plus employed L3, C, and H3 Faraday cups for simultaneous static reception of ³²S, ³³S, and ³⁴S signals. A high-performance Jet + X cone combination was used to enhance signal strength. Nitrogen gas (4 mL/min) was introduced into the plasma to minimize polyatomic ion interference. Medium-resolution modes (∼5,000) were adopted for data acquisition purposes. Pyrite samples were calibrated against pyrite standard PPP-1 to avoid the matrix effect, and reference values of δ³⁴S_v-CDT were obtained from previous work (Fu et al., 2016). All analyzed data underwent processing using professional isotope data processing software “ISO-Compass” (Zhang et al., 2020).

In the Zhaoxian gold deposit, pyrite was identified as the predominant and significant sulfide and gold-bearing mineral throughout all stages of ore formation (Figures 6, 7). Native gold was also frequently observed in the Zhaoxian gold deposit, exhibiting diameters ranging from 2 μm to 30 μm. It displayed a distinct bright golden yellow color and commonly occurred as inclusions or within fissures under reflected light or BSE images (Figures 6B, C, 7B). Other metallic minerals such as chalcopyrite, galena, and sphalerite were also present (Figures 6E, 7C). Gangue minerals including quartz, sericite, chlorite, and K-feldspar were predominantly observed but on a smaller scale compared to sulfides and quartz.

The SEM-BSE image of Py1 primarily exhibits idiomorphic to hypidiomorphic grains without an oscillating zone (Figure 7A). Py2 exhibits idiomorphic to hypidiomorphic grains with mineral inclusions such as chalcopyrite and natural gold present. Oscillating zones are visible in Py2 indicating a relatively volatile formation environment (Figure 7B). In contrast, oscillating zones and mineral inclusions were not observed in Py3, while sulfides (e.g., chalcopyrite and galena) often filled the fissures of Py3 suggesting stable physical and chemical conditions during this stage (Figure 7C). Finally, Py4 appears as hemidiomorphic grains with minimal cracks or mineral inclusion without any oscillating zones (Figure 7D).

The acidic environment in the ore-forming fluid can enhance the stability of metal complexes, which facilitate the transportation of gold and other elements involved in ore formation through various types of complexes (Yang, 1986; Phillips and Evans, 2004; Hou et al., 2007; Lu, 2011; Mao et al., 2013). The properties of ore-forming fluids can be directly elucidated by measuring the homogenization temperature and salinity of fluid inclusion. In our research of quartz veins in the Zhaoxian gold deposit, we have identified and classified four types of fluid inclusion (Figure 10): CO₂-rich inclusion (Type I), biphase aqueous inclusion (Type II), monophase gaseous inclusion (Type III), and monophase aqueous inclusion (Type IV).

Type I inclusion: CO₂-rich inclusion, primarily consists of NaCl, H₂O, CO₂, and CH₄, with a diameter ranging from 3μm to 17 μm (Figures 10A–C). These inclusions exhibit an elliptical or irregular shape and display in many styles: three phases (clathrate, luquid CO₂, and gas CO₂), biphase (clathrate, liquid CO₂), monophase (liquid or gas CO₂).

Type II inclusion is predominantly composed of NaCl solution (liquid) phase and bubble (gaseous) phase, with a diameter ranging from 2 μm to 12 μm (Figures 10B, D–F, H), accounting for the largest proportion. These inclusions display elliptical, linear, irregular, and negative crystalline features. They are commonly found in conjunction with other inclusions and occur across all metallogenic stages. Based on their vapor/liquid ratios (VLRs), these inclusions can be further classified into liquid-rich (Type II-L) and vapor-rich (Type II-V). Type II-L inclusions exhibit VLRs ranging from 10% to 50% at room temperature, whereas Type II-V inclusions exhibit VLRs ranging from 50% to 90% at room temperature.

Type III inclusion consists primarily of gaseous carbon dioxide with traces of methane or water vapor (Figures 10E, G, I). They have a diameter range of 4 μm–10 μm. Type III inclusion exhibits irregular shapes. These inclusions are present in all stages of the ore formation process.

Type IV inclusion is primarily aqueous inclusion, with a diameter range of 4 μm–10 μm (Figures 10A, F, G). They exhibit an irregular shape. Type IV inclusion can be observed across all stages of ore formation.

The compositions of the volatile component of fluid inclusions were determined by micro-laser Raman spectroscopy analysis on representative samples from each stage (Figure 11). Type I inclusions exhibit distinct peaks of CO₂ and CH₄ in either the vapor or clathrate phase (Figures 11A, B). Type II inclusions show a prominent CO₂ peak (Figures 11C–E), H₂O peak (Figure 11C), and CH₄ peak (Figure 11E). Type III inclusions at each stage display noticeable CO₂ and CH₄ peaks (Figure 11F). Laser Raman spectrum analysis has confirmed that the ore-forming fluid consists of an H₂O-NaCl-CO₂-CH₄ system (Figure 11).

Microthermometry analysis of fluid inclusion in quartz from different stages in Zhaoxian gold deposit has been carried out (Table 1; Figure 12).

In Stage 1, the milky quartz vein primarily contains two types of inclusion: types I and II. Unfortunately, the T_{m, CO₂} for Type I inclusion are unavailable due to their small volume. The final melting temperature of T_{m, cl} ranges from 4.7°C to 7.5°C, and the T_{h, CO₂} ranges from 25°C to 29°C. The majority of these inclusions completely homogenized into the liquid phase, while a minority of them homogenized into the gas phase. The T_h-tot ranges from 300°C to 336°C, and the salinity ranges from 4.9 wt% NaCl eqv to 9.5 wt% NaCl eqv. As for Type II inclusion, after undergoing complete freezing and subsequent warming, the T_m-ice ranging from −5.8°C to −2.5°C, while the T_h-tot ranges from 272°C to 311°C. Additionally, the calculated salinity varies between 4.2 wt% NaCl eqv. and 9.0 wt% NaCl eqv. Most of Type II have undergone complete homogenization into the liquid phase, while only a minority have transitioned into the gas phase.

In Stage 2, a large number of primary fluid inclusions were discovered in smoky quartz veins. The predominant inclusion species in this stage is Type II, with Type I also being commonly observed, while Types 3 and 4 appear to be present in smaller quantities. For Type I inclusion, the T_{m, CO₂} ranges from −58°C to −57°C, the T_{m, Cl} ranges from 5.4°C to 7.8°C, and the T_{h, CO₂} ranges from 26°C to 28°C. In this stage, the quantity of Type I inclusion that homogenized into the gas phase surpasses that observed in Stage 1. The T_h-tot of the Type I inclusion ranges from 255°C to 273°C, the salinity of which ranges from 4.3 wt% NaCl eqv. to 8.5 wt% NaCl eqv. For Type II inclusion, the T_m-ice of the Type II inclusion ranges from −7.1°C to −2.3°C, the T_h-tot ranges from 247°C to 308°C and the calculated salinity ranges from 3.9 wt% NaCl eqv. to 10.6 wt% NaCl eqv. The quantity of inclusion homogenized into the gas phase at this stage is also higher compared to Stage 1.

In Stage 3, the dominant fluid inclusion in smoky gray quartz veins mainly consists of Type II, with a small amount of types III and IV. The T_m-ice of the Type II inclusion ranges from −4.8°C to −2.9°C, the T_h-tot ranges from 243°C to 283°C, and the calculated salinity ranges from 4.8 wt% NaCl eqv. to 7.6 wt% NaCl eqv.

In Stage 4, the dominant fluid inclusion in smoky gray quartz veins mainly consists of types II and IV. The T_m-ice of the Type II inclusion ranges from −3.7°C to −1.2°C. The T_h-tot, and calculated salinity of Type II inclusion in Stage 4 is significantly lower compared to earlier stages, ranging from 210°C to 242°C and 2.1 wt% NaCl eqv to 6.0 wt% NaCl eqv, respectively.

Regarding the distribution of trace elements in pyrite, Table 2; Figures 13, 14 present the analytical results for each stage.

The size of Py1 is smaller than the beam spot applied for LA-ICP-MS analysis.

The Py2 sample exhibits significant variations in Au and As content, with the highest Au content observed among all stages. In Py2, each stage demonstrates the highest content of Ga, Sn, Sb, W, and Tl; whereas Co, Ni, Cu, Zn, Ag, and In exhibit the lowest content. Distinct zones of Au-As-Cu enrichment and depletion can be identified within Py2, indicating the presence of oscillating regions, other elements generally exist in solid solution with their contents showing a horizontal trend corresponding to ablation depth (Figures 14A, B).

The Py3 sample exhibits elevated levels of Ag and base metal elements, including Cu, Zn, Cd, In, Pb, and Bi. However, the contents of Co and Ni are comparatively higher in Py3 compared to Py2. Additionally, polymetallic sulfide inclusions become prevalent in Py3 samples; this phenomenon suggests the precipitation of polymetallic sulfides from the ore-forming fluid (Figures 14C, D).

Except for Co and Ni which have equal content to that in Py2, the content of Py4 is significantly lower than that observed in both Py2 and Py3. Other elements generally exist in solid solution with their contents showing a horizontal trend corresponding to ablation depth, and their contents are generally the lowest among all stages (Figures 14E, F).

Table 3 and Figure 15 present the sulfur isotope composition in pyrite of Zhaoxian gold deposit. The δ³⁴S values of Py2 range from +10.51‰ to +11.01‰, with an average value of +10.75‰; for Py3, the δ³⁴S values range from +9.56‰ to +10.10‰, with an average value of +9.80‰; and for Py4, the δ³⁴S values range from +12.02‰ to +12.44‰, with an average value of +12.18‰.

In the discussion, we examine the characteristics and evolution of deep ore-forming fluids. Previous studies have investigated the ore-forming fluids of gold deposits in the Jiaodong gold province and have found that they are characterized by medium temperatures (200°C–400°C) and low salinity (0 wt% NaCl eqv.–10 wt% NaCl eqv.) (Fan et al., 2003; Wen et al., 2016; Liu et al., 2019; Hu et al., 2021; Wu et al., 2022). The fluid inclusion composition primarily consists of H₂O-NaCl-CO₂ ± CH₄.

Researchers have also studied the fluid system of gold deposits in the Sanshandao gold belt and found similarities with the Jiaodong gold district. Specifically, the ore-forming fluids at shallow depths (100 m–600 m below sea level) show similarities between the Jiaodong gold district and the Sanshandao gold belt. In this study, we focus on the ore-forming fluids at depths ranging from 1,731 m to 1,372 m below sea level in the Zhaoxian gold deposit.

It is worth noting that CH₄ has been generally detected in this study. Previous work suggests that CH₄ often appears in the main ore stage at a depth ranging from 2,146 m to 2,038 m below sea level (Li et al., 2023). In this study, it can be found that CH₄ is common in the Type I fluid inclusion of Stage 2. The possible sources of CH₄ in hydrothermal fluids previous study summarized are fluid-rock interaction (Berndt et al., 1996; Potter et al., 1998; 2004; Cao et al., 2014), the mantle (Abrajano et al., 1988; Kendrick et al., 2011), or biological processes (Des-Marais et al., 1988; Ueno et al., 2006). The fluid inclusion in the samples collected from gold deposits below 3,000 m in the Sanshandao gold belt has been extensively investigated; however, no presence of CH₄ was detected in these samples (Wen et al., 2016). This implies that the mantle source and biological source are unlikely to be responsible for the CH₄ in this district, as there is no sedimentary stratum present. We propose that a fluid-rock interaction involving hornblende, ilmenite, and plagioclase in the wall rock with the ore-forming fluid may have been the initial source of CH₄ in both Sanshandao and Jiaojia gold belt gold deposits. Therefore, the most likely source of CH₄ can be attributed to a fluid-rock interaction wherein hornblende, ilmenite, and plagioclase in the host rock underwent reactions with the ore-forming fluid, resulting in the formation of sphene and liberation of hydrogen gas. The hydrogen further catalyzed the reduction of CO₂ to CH₄ (Wen et al., 2016; Hu et al., 2021). Consequently, the alteration of the wall rock adjacent to the orebodies at depths ranging from 1,731 m to 1,372 m below sea level may exhibit a higher degree of intensity compared to that observed at depths ranging from 2,146 m to 2,038 m below sea level.

The ore-forming fluid system in the Zhaoxian gold deposit is characterized as a medium-temperature (210°C–336°C) and moderate-low salinity (3.9 wt% NaCl eqv.–10.6 wt% NaCl eqv.) system. We have compiled and analyzed temperature and salinity data (Figure 12) from various stages of fluid inclusion. It was observed that the ore-forming temperature and salinity generally decreased from earlier to later stages. In Stage 1 veins, the ore-forming fluids may be primary magmatic water released from magma. The fluid inclusions in Stage 1 primarily consist of Type I and Type II inclusions, with Type I accounting for the largest percentage among all stages (25%), while Type II accounts for 75%. In Stage 2, the salinity data can be divided into two clusters, while the temperature remains relatively constant. Furthermore, the simultaneous presence of liquid-rich Type II inclusion and Type III inclusion can be observed in various microscopic regions within quartz sections of Stage 2. In Stage 2, the fluid inclusions consist of Type I inclusion (10%), Type II inclusion (65%), Type III inclusion (25%), with Type IV being insignificantly present. These phenomena potentially indicate the occurrence of fluid boiling. However, the predominant sight within quartz during Stages 3 and 4 is an abundance of liquid-rich type II inclusion. Homogeneous temperatures and salinities exhibited by these stages’ fluid inclusions suggest a stable mineralizing process. In Stage 3, fluids include type I (5%), type II (80%), type III (5%), and type IV (10%). In Stage 4, fluids include Type II (80%) and Type IV (20%). It is hypothesized that decreases in temperature and pressure are the primary factors influencing the precipitation of each element. Gas phase analysis through laser Raman spectrum reveals composition mainly consisting of CO₂, CH₄, and H₂O. The homogenization temperature appears similar, suggesting a distinct composition of the H₂O-CO₂ fluid phase during gold and pyrite precipitation, implying fluid boiling. Cooling can induce instability in metal complexes within the ore-forming fluid, leading to gold precipitation. In stages 3 and 4, a gradual decrease in both the homogeneous temperature and salinity of fluid inclusion is observed, with relatively minor variations in salinity. Additionally, the presence of CH₄ in fluid inclusion suggests a contribution of fluids from fluid-rock interaction.

In the Zhaoxian gold deposit, the metallogenic process follows a similar evolution trend to most gold deposits in the Jiaodong gold district, but there are differences in ore-forming temperature and salinity range. As the fluid moves upward, there is a decrease in pressure, which can result in phase separation (Hu et al., 2021). This phase separation is significant in the partitioning of elements during fluid chemistry evolution (Ulrich et al., 2002; Klemm et al., 2007; 2008; Landtwing et al., 2010) and can lead to fluid boiling. There is also a slight change in salinity (Figure 12), with salinity decreasing with stage, depth, and temperature, possibly due to fluid-rock interaction and decompressing effect. Overall, it is believed that the Jiaodong gold province has a unified ore-forming fluid system. The Sanshandao gold belt and Jiaojia gold belt have similar ore-forming fluid properties, dominated by H₂O-NaCl-CO₂ ± CH₄, with medium temperature and medium-low salinity characteristics. Metal precipitation in these systems is caused by decompression, cooling, phase separation, fluid mixing, and fluid-rock interaction. Fluid boiling and fluid-rock interactions are likely important mechanisms of mineralization.

Recent studies have shown that Au, Ag, As, and heavy metals can enter the pyrite lattice through solid solutions or mineral inclusion at the micron to nanometer scale (Reich et al., 2005; Deditius et al., 2008; 2009; 2011; Barker et al., 2009; Large et al., 2009). The examination of pyrite not only enhances our understanding of the characteristics and distribution of metals, but also provides insights into the underlying processes governing mineralization events such as hydrothermal alteration, metamorphism, and deformation (Reich et al., 2005; 2016; Cook et al., 2009; Thomas et al., 2011; Deditius et al., 2014; Peterson and Mavrogenes, 2014; Steadman et al., 2015; Meffre et al., 2016).

In Stage 2 (Py2), the gold content is observed to be the highest compared to other stages, as determined by in-situ analysis of trace elements in pyrite. The variations in gold and arsenic content in Py2 indicate a clear positive correlation between these two elements, suggesting their formation as a distinct element group during this stage. Oscillating zones enriched in gold have been identified, indicating rapid decompression and the presence of volatile substances that trigger sudden changes in the chemical environment of the fluid (Large et al., 2009; Muntean et al., 2011; Steadman et al., 2015; Gregory et al., 2016; Tardani et al., 2017). Morphologically, the porous structure of pyrite particles (Simon et al., 1999) and abundance of mineral micro-inclusions (such as chalcopyrite; Figure 5D) suggest fluid boiling (Román et al., 2019). High-temperature elements such as Ga, Sn, Sb, W, and Tl exhibit higher concentrations compared to other stages while Co, Ni, Cu, Zn, Ag, and In show lower concentrations (Figures 13, 17). However, there exists a negative relationship between (Ag, Cu, Pb, Sb) and Au whereas positive relationships are observed among these four elements (Figure 16). By cross-referencing the primary data, we have identified the presence of these elements in a solid solution, which has been documented in several research studies (e.g., Large et al., 2007; Large et al., 2009; Román et al., 2019). Oscillating zones and sulfide inclusions can be readily identified in the SEM-BSE image of Py2 grains (Figure 7B), the Au-As-Cu enrich and deplete zones within Py2 (Figures 14A, B) is also an evidence, thus implying that fluid boiling mechanisms are involved in ore formation. We noticed that Au exist in native gold grains (Figures 5C, 6B, 6C, 7B), while in the Au vs. As diagram (Figure 16A), all data points fall below the Au saturation line indicating potential undersaturation of gold within ore-forming fluids (Reich et al., 2005), considering the enrichment of native gold in this stage, this suggesting that the ore-forming fluid in this stage saturated at the beginning, the precipitation of native gold led to the unsaturation of Au in later fluids and Py2.

In Stage 3 (Py3), the concentrations of Ag, Cu, Zn, In, Pb, and Bi exhibit the highest contents among all stages (Figures 13, 17). Furthermore, the contents of Co and Ni are higher in Py3 compared to Py2. A relatively weaker positive correlation can be observed between Au, Sb, and Bi contents (Figures 16G, H). Additionally, a significant positive correlation is observed between the element groups (Ag, Pb, Sb, and Bi) and (Cu, Zn, and In), indicating extensive involvement of polymetallic sulfides (Figure 17). Furthermore, the coexistence of chalcopyrite, galena, and sphalerite in the fissures in Py3 and quartz further supports this opinion. Significant Pb-(Zn) inclusions were detected in Py3 (Figures 14C, D), along with the corresponding BSE image (Figure 7C) showed that chalcopyrite and galena are common in pyrite grains and fissures, indicating the precipitation of multiple polymetallic sulfides during this stage.

In Stage 4 (Py4), it appears that the chemical properties of the ore-forming fluid have reached a state of equilibrium. The formation of Py4 occurred through a tranquil process, with other sulfides being scarce in Stage 4. The pyrite content in stage 4 (Py4) is significantly lower compared to stages 2 and 3 (Py2 and Py3), except for Co and Ni, which exhibit similar levels as Py2 (Figures 13, 17). A relatively weak positive correlation can be observed between Zn, Pb, Sb, and Bi content in Py4. This suggests that stage 4 represents the final phase of ore formation, with most elements having undergone nearly complete precipitation from the ore-forming fluid. The occurrence of trace elements in Py4 often found in pyrite solid solution, few sulfide inclusions was found (Figures 14E, F).

The substitution of Fe²⁺ in pyrite with Co²⁺ and Ni²⁺ is favored by an increase in fS₂ and/or a decrease in temperature (Maslennikov et al., 2009). As gold and polymetallic sulfide gradually precipitates from the ore-forming fluid at each stage, without any supplementary source, the fS₂ should be decreasing. Therefore, an increase in Co and Ni contents (Figure 13) would indicate a decrease in temperature. This is coincide with the result of fluid inclusion microthermometry analysis (Figure 12).

The δ³⁴S values of pyrite from each stage in the Zhaoxian gold deposit exhibit similarity, ranging from +10.51‰ to +11.01‰ for Py2, +9.56‰ to +10.10‰ for Py3, and +12.02‰ to +12.44‰ for Py4, with mean values of +10.75‰, +9.80‰, and +12.18‰ respectively (Figure 15). The δ³⁴S values of the Zhaoxian gold deposit are comparable to those of Jiaojia gold deposit (shallow part, 8‰–13‰) and Sanshandao-Jiaojia gold belt (+1.9‰ to +13.0‰), while a little higher than those of Linglong granitoid (+6.1‰ to +10.1‰) and the Guojialing granitoid (+2.7‰ to +10.0‰), indicating a medium-to-low temperature system without significant isotopic fractionation.

Therefore, there may exist certain disparities in the characteristics of the ore-forming fluids during each stage, albeit originating from a common source. Fluid-rock interactions could potentially contribute to the elevation of δ³⁴S values. As the ore-forming fluids ascended, modifications in temperature, pressure, and oxygen or sulfide fugacity would likely occur. Findings from fluid inclusion studies propose that Au precipitation might primarily occur through fluid boiling, while multiple sulfides could precipitate via fluid-rock interactions. These processes are predominantly chemical in nature and relatively unaffected by external materials. Furthermore, the δ³⁴S values of Zhaoxian gold deposit overlap with those observed in the Jiaojia gold district at depths around −300 m (Mills et al., 2015), ranging from +8‰ to +13‰. This implies that the shallower portion of the ore-forming fluids shares a similar origin with its deeper counterpart.

In conclusion, it can be inferred that the primary ore-forming fluid in the Zhaoxian gold deposit is most likely to be magmatic-hydrothermal fluids. Moreover, considering the occurrence of orebodies within surrounding rocks, it is plausible that the ore-forming fluid has undergone extensive reactions with the host rock, potentially resulting in some sulfur inheritance from these surrounding rocks.

Stage 1 is characterized by a deep-source magmatic fluid, exhibiting high temperature and salinity, along with a predominant presence of Type I fluid inclusion (25%, the highest proportion among all stages) (Figure 18). The occurrence of idiomorphic-hypautomorphic pyrite grains indicates a thermally stable fluid environment.

Stage 2 is the most crucial stage for gold mineralization in Zhaoxian gold deposit, providing compelling evidence of fluid boiling through the coexistence of liquid-rich Type II and Type III fluid inclusions, significant variations in salinity while maintaining a relatively constant temperature, oscillating zones, and the concomitant variations and fluctuations in the concentrations of Au and As (Figure 18, Zhang et al., 2023). Fluid boiling plays a crucial role in the formation of magmatic-hydrothermal gold deposits (Brown, 1986; Seward, 1989). During this process, Au ( HS ) 2 ‐ may be the most significant form of gold transformation under low-temperature and neutral conditions (Stefansson and Seward, 2004). The release of H₂S not only leads to a sharp decrease in the concentration of dissolved gold but also controls the evolution of sulfide and oxide complexes. These processes facilitate gas escape and result in an increased concentration of metal elements. Furthermore, medium-low salinity fluids rich in CO₂ and H₂O may be released during the evolution of mantle-derived magma (Ridley and Diamond, 2000). CO₂ functions as a regulator of gold solubility while maintaining stability within a specific range for Au-S complexes (Liu and Shen, 1999). This facilitates the generation of vapor in gaseous form, ultimately leading to a reduction in pressure and an elevation in fluid pH upon its evaporation (Lowenstern et al., 1991). Additionally, CH₄ was detected in the gas phase of the inclusion during Stage 2, while wall-rock alteration, including potassic alteration, sericitization, chloritization, and carbonatization, can be observed in the ores (Figure 4B). This suggests that fluid-rock interaction also plays a significant role in gold precipitation by influencing the pH of ore-forming fluids through CO₂ mineral reactions (Khedr et al., 2013; 2023; Abdelkarim et al., 2022). As the ore-forming fluid ascends, it inevitably interacts with surrounding rocks leading to changes. During this stage, gold primarily exists as native gold and solid-solution within the pyrite lattice. The presence of elevated levels of Ga, Sn, Sb, W, and Tl in the ore indicates high temperatures during ore formation as indicated by fluid inclusion analysis results. Concentrations of Ag, Cu, Pb, and Sb often exhibit a positive correlation, the representative LA-ICP-MS time-resolved depth profiles suggesting their existence may be solid forms within pyrite (the signals of these elements almost remain horizontal, Figures 14A, B). These micro-inclusions further indicate a process of fluid boiling.

Stage 3 is the primary polymetallic stage in the Zhaoxian gold deposit. The dominant type of fluid inclusion is Type II, which is rich in water. The temperature, salinity, and trace element content in pyrite (Py3) appear to be stable, and the salinity is relatively low. Additionally, CH₄ has been detected in the gas phase of the fluid inclusion in Stage 3, and alterations in the surrounding rock can be observed in the ores. This suggests that fluid-rock interaction plays a significant role in the precipitation of polymetallic minerals. An oscillating zone can hardly be found in Py3, chalcopyrite, sphalerite, and galena filled in the fissures of Py3 and quartz, this can also be proved in the representative LA-ICP-MS time-resolved depth profiles (Figures 14C, D). In this aspect, Py3 is the precipitant of polymetallic sulfides. Therefore, the ore-forming fluids in this stage seem to be stable, the most important inducer of the metallogenic event is fluid-rock interaction (Figure 18). In this stage, multiple sulfides precipitate from the ore-forming fluids, with Cu, Zn, Ag, Cd, In, Pb and Bi showing the highest concentrations among all stages. The precipitation temperatures of these elements are generally lower than that of gold, indicating a decrease in temperature, which is consistent with the data from fluid inclusion analysis.

Stage 4 is primarily characterized by late barren quartz veins, (Figure 18) which generally have the lowest homogeneous temperature and salinity of fluid inclusion among all stages. Similarly, the trace element content (except Co and Ni) in pyrite (Py4) is also the lowest among all stages, they exist in pyrite solid solution (Figures 14E, F).