Samples were collected from the gold-bearing quartz vein-type ores located in the NWW-trending brittle-ductile shear zone of the Saibagou gold deposit. These ores have plagiogranites or granodiorites as the surrounding rocks and exhibit an average gold grade of 5 g/t. A total of 25 pieces of quartz vein-type gold ores were collected from the Saibagou gold deposit, which was approximately 145 kg. During the collection of ore samples from mining roadways, most of the surrounding rocks attached to the gold-bearing quartz vein-type ores were removed in advance, the total weight of 145 kg. Among these samples, two samples were utilized to prepare polished thin sections for observational purposes, and the remaining 23 samples were used for sorting out zircons and pyrite. All the samples were processed and analyzed at the Wuhan Sample Solution Analytical Technology Co., Ltd. in Hubei Province, China. A total of 1650 various zircon grains and several pyrites were obtained. Tests and analyses were conducted on these samples in this study, including the cathodoluminescence (CL) imaging, U-Pb isotopic dating, and trace element analysis of zircons, as well as the backscattering (BSE) imaging, S isotope analysis, and trace element analysis of pyrite. Zircon CL images of all zircon grains were obtained and pyrite BSE imaging was conducted for some pyrite grains. All obtained microphotographs were carefully observed and tested to contribute to the broader understanding of the Saibagou gold deposit’s geological characteristics.

The zircon CL images were obtained using a high-vacuum scanning electron microscope (JSM-IT300) equipped with a Delmic sparc CL probe. This instrument operated at a voltage of 0.5–30 kV, a filament emission current of 72 μA, an accelerating voltage of generally 20–30 kV in spectral analysis and tests, and a working distance of 9.5–10.5 mm. Zircon U-Pb isotopic dating and the determination of the trace element concentrations of zircons were simultaneously completed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Wuhan Sample Solution Analytical Technology Co. Ltd. (Zong et al., 2017). A GeolasPro laser ablation system was used for laser sampling. During the laser ablation, helium was used as the carrier gas, argon was used as the make-up gas to adjust the sensitivity (Hu et al., 2015), and the laser beam spots and frequency were set at 32 µm and 5 Hz, respectively. Moreover, zircon reference material 91,500 and glass reference material NIST 610 were used as external standards for the fractionation corrections of zircon isotopes and trace elements, respectively. Each piece of the time-resolved analytical data included a blank signal of about 20–30 s and sample signals of 50 s. The data were processed using the ICPMSDataCal (Liu et al., 2008; Liu et al., 2010) and IsoplotR software. LA-ICP-MS was used to conduct the in situ microanalysis of major and trace elements in pyrite, with the GeolasPro laser ablation system used for laser sampling under the same laser ablation conditions stated above. The glass reference materials NIST 610 and NIST 612 served as external standards for calibration, while no internal standard was employed for calibration. To verify the reliability of the calibration method, USGS sulfide reference material MASS-1 was used to monitor standard samples. Each piece of time-resolved analytical data also included a blank signal of about 20–30 s and sample signals of 50 s and was processed using the ICPMSDataCal software.

The in situ microanalysis of the sulfur isotope ratio of sulfides in pyrite was conducted using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) (Fu et al., 2016), with a Geolas HD system used as the laser ablation system and a Neptune Plus multicollector ICP-MS employed for MC-ICP-MS. The laser ablation system utilized helium as the carrier gas, with analysis conducted in single-point mode. Large, large laser beam spots of 44 µm were used, alongside a low frequency of 2 Hz. Moreover, the system was designed to cover about 100 laser pulses for analysis. To guarantee stable signals at low frequencies, a signal smoothing device was employed, and the laser energy density was kept constant at 5.0 J/cm². A Neptune Plus mass spectrometer was adopted, which was equipped with nine Faraday cups and a 1011 Ω resistance amplifier. Among them, three Faraday cups (L3, C, and H3) were employed to simultaneously receive ³²S, ³³S, and ³⁴S signals. To increase the signal intensity, a high-performance combination of the Jet sample cone and the X-skimmer cone was used. Nitrogen (4 mL/min) was introduced into the plasma to reduce the interference from polyatomic ions. Moreover, the system operated in the medium resolution mode (about 5000), and the mass fractionation of sulfur isotopes was corrected using the SSB method. To prevent any potential matrix effects, the pyrite samples were calibrated using the pyrite reference material PPP-1.

The zircon dating results and trace element analysis results of the selected gold ore samples are shown in Tables 1 and 2, respectively. A total of 50 survey points were selected for zircon U-Pb dating, with 35 representative survey points being determined after some points with a low degree of concordance were screened out. The zircons were divided into three groups according to their morphologies, CL images, ages, and trace element distribution (Figure 5). The first group of zircons was mainly short prismatic. They had a length of generally less than 150 μm and a width of less than 50 μm, with length-to-width ratios of 1:2–1:3. They exhibited intact crystal morphologies. In the CL images, they were gray or hoary and had irregular luminous areas. The zircons of the first group were rich in inclusions, and some of them were black and not luminous. They had Th concentrations of 200 ppm–2884 ppm, a U concentration of 157 ppm–3114 ppm, and Th/U ratios of 0.24–1.69 (average: 0.72), suggesting significant characteristics of hydrothermal origin (Rubatto and Hermann, 2003; Hoskin, 2005; Zhong et al., 2018). They had ²⁰⁶Pb/²³⁸U ages of 417–429 Ma and 455–485 Ma, with weighted averages of 423.91 ± 4.5 Ma (MSWD=1.1) and 470.18 ± 4.92 Ma (MSWD=3.7), respectively. The second and third groups of zircons were captured zircons. They were mostly short prismatic or granular. They had varying grain sizes, with lengths generally less than 100 μm (nearly 200 μm for some zircons), widths less than 50 μm (nearly 100 μm for some zircons), and length-to-width ratios of 1:2–1:1. They exhibited a high idiomorphic degree. The second group of zircons had a Th concentrations of 11.6 ppm–359 ppm, U concentrations of 35.6 ppm–620 ppm, and Th/U ratios of 0.30–0.80 (average: 0.45), implying significant characteristics of magmatic origin (Rubatto and Hermann, 2003; Vavra et al., 1999; Belousova et al., 2006; Zhou et al., 2022). They had two sets of ²⁰⁶Pb/²³⁸U ages, namely, 455–497 Ma and 519–557 Ma, with a weighted average of 476.90 ± 3.92 Ma (MSWD = 2.6))for the former set. The weighted average of the latter set was not calculated due to the small number and the large value range of the survey points. The weighted average of the former set was consistent with the age of tonalite in the study area obtained in previous studies (Wu et al., 2022). The zircons with ²⁰⁶Pb/²³⁸U ages of 455–497 Ma were inferred to be zircon minerals trapped when ore-forming fluids upwelled and formed gold-bearing quartz vein-type ores. The third group of zircons had Th concentrations of 9.73 ppm–913 ppm, a U concentrations of 48.4 ppm–751 ppm, and a Th/U ratios of 0.15–1.30 (average: 0.56). They had scattered ²⁰⁶Pb/²³⁸U ages of 610–2360 Ma, which could be roughly divided into three age ranges, namely, 600–1000 Ma, 1500–2000 Ma, and greater than 2500 Ma (Figure 6). As revealed by the CL images, the third group of zircons exhibited significant core-mantle structures. The cores of the zircons were more luminous than the mantles and did not bear zoning textures. The mantles developed slightly oscillatory zoning or no zoning. Some zircons developed narrow altered rims, indicating that they might be magmatic zircons formed in the early stage and altered due to late metamorphism. According to the analysis, the age range of 600–1000 Ma is close to that (750–1000 Ma) of high to ultra-high-pressure metamorphic rocks in the northern Qaidam Basin (Yang et al., 2006; Zhang et al., 2006). The zircons older than 1500 Ma might be derived from the ancient crystalline basement of the tectonic belt at the northern margin of the Qaidam Basin (Chen et al., 2007).

The three metallogenic stages of the Saibagou gold deposit are dominated by stages II and III, during which large quantities of gold elements precipitated and formed gold ores. Therefore, trace element analysis was conducted for the pyrite grains of the two stages. The trace element composition of pyrite in the gold ore samples is shown in Table 3. According to the test results, the pyrite in the Saibagou gold deposit has generally low concentrations of trace elements. Only a few elements, namely, Ag, As, Bi, Co., and Ni, had concentrations tested above their detection limits, while most trace elements showed concentrations below or near their detection limits (Figure 7). The pyrite in the gold ore samples was relatively enriched in trace elements Co., Ni, and As. Specifically, Py2 and Py3 had Co. concentrations of <63.36 ppm and <12.63 ppm, respectively, Ni concentrations of <9.38 ppm and <7.81 ppm, respectively, and As concentrations of <61.70 ppm and <827.57 ppm, respectively. For the pyrite of stages II and III, its Au concentrations were mostly close to the detection limit. An exception was observed at survey point S-307, where the Au concentrations exceeded that of other points and the detection limit. This higher concentrations, which might be attributed to the presence of small native gold inclusions in the pyrite grains, agrees well with the ablation curve of LA-ICP-MS analysis.

As shown in the test results of sulfur isotopes (Table 4), the δ³⁴S values of pyrite of metallogenic stages I, II, and III varied slightly, in the ranges of 1.99 ‰–2.44‰ (average: 2.22‰; extreme differences: 0.45‰), 1.99 ‰–3.28‰ (average: 2.64‰; extreme differences: 1.29‰), and 2.38 ‰–3.14‰ (average: 2.76‰; extreme differences: 0.76‰), respectively. It is noteworthy that the average δ³⁴S value of pyrite in the Saibagou gold deposit varied with the metallogenic stages in theorder of δ³⁴S_Ⅰ < δ³⁴S_Ⅱ < δ³⁴S_Ⅲ. The main reason for this phenomenon is that the quantity of sulfur isotopes of pyrite increased with a decrease in temperature, as shown in the equilibrium fractionation curve of sulfur isotope exchange (Ohmoto, 1972). As the gold mineralization process continued, the ore-forming temperature decreased, leading to increased δ³⁴S values.

All the hydrothermal zircons in the gold ore samples were characterized by relatively intact crystal forms, irregularly luminous areas, rich inclusions, and certain oscillatory and weak zoning textures. They are gray or hoary, with some beingblack and not luminous. These characteristics are significantly different from those of magmatic zircons, indicating that hydrothermal zircons were formed in saturated fluids or after transformation by fluid alterations during the formation of the gold-bearing quartz veins. This inference is consistent with the characteristics of hydrothermal zircons proposed by some researchers (Rubatto and Hermann, 2003; Hoskin, 2005; Zhong et al., 2018). Compared with the magmatic zircons in the gold ore samples, the hydrothermal zircons of the Saibagou gold deposit are significantly rich in LREEs (the first, second, and third groups of zircons had average LREE concentrations of 86.2 ppm, 7.44 ppm, and 11.99 ppm, respectively), especially rich in elements La and Ce (the first, second, and third groups of zircons had average La concentration of 6.53 ppm, 0.05 ppm, and 0.07 ppm, respectively and average Ce concentration of 44.49 ppm, 4.44 ppm, and 6.50 ppm, respectively), have high U and Th concentrations (the first, second, and third groups of zircons had an average U concentrations of 755.97 ppm, 177 ppm, and 286.49 ppm, respectively and average Th concentrations of 643.81 ppm, 94 ppm, and 78.82 ppm, respectively) and are rich in common lead (Pb; the first, second, and third groups of zircons had an average Pb concentrations of 1.94 ppm, 0.38 ppm, and 0.74 ppm, respectively). The hydrothermal zircons in the gold ore samples exhibit relatively flat distribution patterns of REEs, with weak or no Ce anomalies (Figure 8A). These characteristics are consistent with the results of the studies on the characteristics of hydrothermal zircons (Figure 8B) (Hoskin and Ireland, 2000; Rubatto, 2002; Geisler et al., 2003; Hoskin and Schaltegger, 2003). However, they differ significantly from the REE distribution patterns of the magmatic zircons in the gold ore samples (Figures 9A,B).

The gold-bearing quartz vein-type ores of the Saibagou gold deposit have relatively high gold grades (average: 5 g/t). The formation conditions of these gold-bearing quartz veins, with temperatures of 180 C‒350 C and a pressure of 46 MPa (Liu et al., 2005), are similar to those of hydrothermal zircons in the Sudetic green schists in Poland (T: 207 C‒300°C; P: 100 MPa) and the hydrothermal zircons in the gold-bearing quartz veins of the Shuangkoushan area at the northern margin of the Qaidam Basin (T: 280 C; P: 67.37 MPa) (Rubatto, 2002). As shown in the diagram of chondrite-normalized REE patterns, the hydrothermal zircons (group 1) in gold-bearing quartz veins exhibit consistent REE distribution with the hydrothermal zircons trapped in the Boggy Plain pluton in Australia, the ore-bearing quartz veins of the Shuangkoushan gold deposit at the northern margin of the Qaidam Basin in China, and gold ore bodies in the Baolun gold deposit in China (Figure 8B) (Hoskin, 2005; Zhang X. W. et al., 2009; Yu et al., 2020). This result indicates that the dating results of the hydrothermal zircons in the gold-bearing quartz vein-type ores can accurately represent the actual metallogenic age of the Saibagou gold deposit.

The dating results show that the hydrothermal zircons in the gold-bearing quartz veins have ages of 423.91 ± 4.5 Ma and 470.18 ± 4.92 Ma, indicating at least two distinct metallogenic periods of the Saibagou gold deposit. The metallogenic period is dominated by 423.91 ± 4.5 Ma (Figure 9C), which is consistent with the age of the regional NWW-trending ductile-brittle shear zone in the Saibagou area (426 ± 2 Ma) (Feng et al., 2002) but is slightly later than the ages of granodiorites (946 ± 26 Ma) and the Late Ordovician plagiogranites—the surrounding rocks of the ore bodies (Zhang et al., 2005). This result indicates that the formation of the gold-bearing quartz veins in the Saibagou gold deposit is closely related to the ductile-brittle shear zone in this area (Liu et al., 2005). Furthermore, the gold-bearing quartz vein-type ores are the major type of ores in the Saibagou gold deposit, and the gold ore bodies occur in the NWW-trending ductile-brittle shear zone. The ore body attitudes are roughly consistent with the characteristics of the ore deposi (e.g., fault strikes). Additionally, the dominant metallogenic period is slightly later than the ultrahigh-pressure metamorphic stage (428–435 Ma) of the northern margin of the Qaidam Basin (Chen et al., 2018a; Chen et al., 2018b). This result is consistent with the metallogenic regularity proposed by Goldfarb, which suggests that orogenic gold deposits were formed during or after the orogenic peak period (Goldfarb et al., 2001). Therefore, it can be inferred that the hydrothermal zircon age of 423.91 ± 4.5 Ma represents the primary metallogenic epoch of the Saibagou gold deposit.

The dating results also indicate a zircon age of 470.18 ± 4.92 Ma, which represents another metallogenic age of the gold deposits in the study area (Figure 9D). It is believed that the hydrothermal zircon dating results of the Saibagou gold deposit are reliable based on the morphologies, CL images, ages, and trace element distribution of zircons. Interestingly, the metallogenic age of 470.18 ± 4.92 Ma falls within the formation age (400–490 Ma) of the metavolcanic rock series in the Tanjianshan Group at the northern margin of the Qaidam Basin (Zhang et al., 2005; Cao et al., 2019). Moreover, this age belongs to the early stage of the Caledonian orogenic period and is close to the age (480 Ma) of the ophiolites in the Tanjianshan Group in the Saibagou area (Cao et al., 2019). This metallogenic age is consistent with the established understanding of moderate-to low-temperature magmatic-hydrothermal deposits (Ye et al., 2014). From these results, it can be inferred that during the subduction of the paleo-oceanic crust (South Qilian Ocean) toward the northern Qilian block, the crust was uplifted and thickened, and accordingly, fluids rose and metasomatized the upper mantle, leading to the partial melting of the lower crust. As a result, the metavolcanic rocks of the Tanjianshan Group with high background values of gold were formed. Concurrently, some ore-bearing hydrothermal fluids rose along regional deep faults, forming gold ore bodies in the local fractures derived near the deep faults.However, the formed gold ore bodies have a smaller scale and lower gold grades than those formed at 423.91 ± 4.5 Ma. Therefore, it can be concluded that the age 470.18 ± 4.92 Ma represents the secondary metallogenic period of the Saibagou gold deposit.

As the most common gold-bearing sulfide in gold deposits, pyrite is an important mineral rich in multiple trace elementssuch as Co, Ni, Cu, Zn, As, and Au (Reich et al., 2005; Large et al., 2009; Berner et al., 2013). The differences in the degrees of enrichment of these trace elements in pyrite are frequently used to indicate the physical and chemical environment in which ore-forming materials were formed and to constrain the properties of ore-forming fluids (Liu et al., 2018; Li H. et al., 2022). In the Saibagou gold deposit, the Au concentrations in pyrite grains is generally near or below the detection limit, signifying an extremely low lattice gold concentration. Therefore, it can be inferred that the majority of the gold in the deposit occurs as native gold. However, an exceptional case is survey point S-307, where the pyrite displayed an unusually high Au concentrations, which considerably exceeded the values of other survey points and the detection limit. In addition, several extremely significant Au peaks appeared on the time-resolved LA-ICP-MS ablation curve of S-307 (Figure 10). Some researchers found that native gold is paragenetic with sulfide in the Saibagou gold deposit under the microscope (Liu et al., 2005). This finding suggests that the anomalous increase in the Au concentration of S-307 is likely due to the presence of small gold-bearing mineral inclusions in the pyrite rather than lattice gold. As shown in the time-resolved LA-ICP-MS ablation curves of pyrite, the Au signal exhibited very similar distribution characteristics to the Ag and Te signals, and the anomalous Au peaks tended to be accompanied by the anomalous peaks of Ag and Te. Given the mineral formation sequence in the Saibagou gold deposit, the small gold-bearing mineral inclusions in the pyrite could possibly be fine-grained petzite.

As shown by the time-resolved LA-ICP-MS ablation curves of pyrite, the Co, Ni, and As signals showed relatively stable intensities and similar distributions to the Fe signal. It can be inferred that Co, Ni, and As mainly infiltrated into pyrite grains through isomorphism. During metallogenic stages II and III, the pyrite had a stable As concentration in each epoch, and ferric sulfarsenide, such as arsenopyrite, appeared. Similar findings have also been reported by otherresearchers (Liu et al., 2005). These phenomena further indicate that the ore-forming fluids of the Saibagou gold deposit are As-bearing fluids. As revealed by the time-resolved LA-ICP-MS ablation curves of pyrite, the signals of Mo and Bi frequently varied with the Pb signal, and anomalous Pb peaks were frequently accompanied by the Mo and Bi peaks. In addition, galena trapped in pyrite grains was discovered under the microscope, indicating that Mo and Bi might be present as the micro-inclusions of Mo-Bi sulfides in the galena.

Pyrite in the quartz vein-type gold ores developed relatively slowly, resulting in relatively low trace element concentrations inside. Nevertheless, pyrite directly crystallized and precipitated in the ore-forming fluids, being free from the interference by other impurity elements. As a result, pyrite can effectively reflect the properties of the ore-forming fluids (Zhen et al., 2020). The pyrite in gold-bearing quartz vein-type ores in the Saibagou gold deposit has Co concentrations of 0.82 ppm–115.10 ppm, which are less than the maximum Co concentrations (1000 ppm) observed in the pyrite of mesothermal deposits (Roman et al., 2019). Furthermore, the pyrite has Ni concentrations of 0.60 ppm–19.29 ppm. Therefore, the Co and Ni concentrations in the pyrite are both less than 200 ppm and exactly fall within the ranges of those in acidic magmatic fluids (Tao et al., 2020). Many mesothermal-epithermal minerals, such as pyrite, chalcopyrite, galena, and sphalerite, have also been discovered in the Saibagou gold deposit. The fluid inclusions within the veins of the Saibagou gold deposit have homogeneous temperatures of mostly 160 C‒350°C, indicating that the ore-forming fluids might be derived from acidic magmas and are moderate-to low-temperature As-bearing fluids.

Pyrite plays a significant role in the Saibagou gold deposit as one of the primary gold-bearing sulfides. Its characteristics, such as morphology, size, and elemental composition, provide valuable insights into the formation of gold in ores. In some specific geological environments, pyrite can be used to determine the properties of ore-forming fluids and the source of deposit materials (Ohmoto, 1986; Lu et al., 2022). As indicated by previous studies, the Saibagou gold deposit is a typical hydrothermal deposit (Liu et al., 2005). The sulfur isotopic composition in this deposit is closely related to the mineral crystallization temperature, isotopic composition, pH, and oxygen fugacity of hydrothermal fluids. The sulfides in the Saibagou gold deposit primarily include pyrite, chalcopyrite, galena, pyrrhotite, and sphalerite, with the absence of sulfate minerals. The alterations in the surrounding rocks are dominated by moderate-to low-temperature alterations, such as silicification and the alterations of pyrite, chlorite, and sericite. The fluid inclusions in the veins of the gold deposit have homogeneous temperatures of mostly 160 C‒350 C (Liu et al., 2005). Accordingly, it can be inferred that the ore-forming hydrothermal fluids of the Saibagou gold deposit might have originated from a moderate-to low-temperature environment with a low oxygen fugacity. The S element in the ore-forming fluids of the Saibagou gold deposit mainly occurs as HS⁻ and S^2- ions, and the pyrite precipitating from fluids has similar δ³⁴S values to the ore-forming fluids (Ohmoto, 1972; Zhen et al., 2020). Therefore, it is feasible to use the sulfur isotopes of pyrite to indicate the characteristics of sulfur isotopes in the ore-forming fluids and determine the sources of ore-forming materials in the Saibagou gold deposit.

There are three main sources of sulfur on earth, namely, mantle-derived (magmatic) sulfur, seawater sulfur, and sedimentary sulfur (Ohmoto, 1986; Chaussidon and Lorand, 1990; Chen et al., 2015). As indicated by the geochemical characteristics of pyrite in the Saibagou gold deposit, pyrite formed at different metallogenic stages shows relatively concentrated δ³⁴S values of 1.99 ‰–3.28‰, with an average of 2.54‰ and an extreme difference of 1.29‰. The highly homogeneous δ³⁴S values indicate significant characteristics of mantle-derived sulfur (Ohmoto, 1986). In the Saibagou area and its adjacent regions, gold ores exhibit δ³⁴S values of 0.50 ‰–3.92‰, with an extreme difference of 3.42‰. The quartz veins bearing no gold ores (formed after the metallogenic period) and the surrounding rocks have δ³⁴S values of 3.7 ‰–4.00‰ (Zhang et al., 2001; Tang et al., 2021b). Gold deposits in the western part of the tectonic belt at the northern margin of the Qaidam Basin, such as Hongliugou, Tanjianshan, Yuka, and Qinglonggou (Guo and Shuwang, 1998; Fan, 2017; Zhang, 2017), have δ³⁴S values of 0.50 ‰–11.00‰ (extreme difference: 10.5‰), which are significantly higher than those in the eastern part of the tectonic belt. Although the δ³⁴S values of the gold deposits in the tectonic belt at the northern margin of the Qaidam Basin all fall within the range of the δ³⁴S values of orogenic gold deposits (Liu et al., 2021), the gold deposits in the eastern and western parts of the tectonic belt have greatly different sources of ore-forming materials. The relatively high δ³⁴S values of the gold deposits in the western part of the northern Qaidam Basin tectonic belt indicate that the ore-forming fluids underwent substantial water-rock interactions with the surrounding rocks near the pathway as they ascended, thus significantly increasing the δ³⁴S values of the fluids a phenomenon commonly observed in orogenic gold deposits (Hodkiewicz et al., 2009; Liu et al., 2021; Sugiono et al., 2021). The relatively low δ³⁴S values of gold deposits in the study area indicate that the chlorite alteration in shear zones served as a barrier to the migration of ore-forming minerals in hydrothermal fluids, hindering the water-rock interactions between the ore-forming fluids and the surrounding rocks in the pathway (Wang et al., 2022) (Figure 11). As shown in this study, the gold-bearing quartz vein-type ores in the Saibagou gold deposit have δ³⁴S values of 1.99 ‰–3.28‰, with an average of 2.54‰ and an extreme differences is 1.29‰. These highly homogeneous δ³⁴S values are concentrated within the δ³⁴S range of mantle-derived sulfur (0‰±3‰), suggesting significant characteristics of mantle-derived sulfur. This result indicates that the surrounding rocks of ore bodies had a minor influence on the ore-forming fluids during the metallogenic process. Therefore, it can be inferred that the ore-forming materials of the Saibagou gold deposit mainly include mantle-derived materials in the deep earth.