At the Zhaoxian gold deposit, exposed pre-Quaternary strata comprise mainly the Paleoproterozoic Lugezhuang Formation (Jingshan Group) granulite, marble and schist, and the Cenozoic Zhubidian Formation (Wutu Group) pebbly feldspar sandstone and clayey sandstone (Zhu, 2018). The Jiaojia fault (∼60 km long, max 1 km wide) at Zhaoxian is divided into three segments, i.e., (from north to south) the Gaojiazhuangzi-Xincheng, Xincheng-Jiaojia (Matang), and Sizhuang. The fault strikes 30°–50° and has a dip angle of 80° (Shu et al., 2022). The Jiaojia fault branches out into secondary structures locally and is featured by multistage movements (Song et al., 2010b; Hen et al., 2015; Cao et al., 2016; Sun et al., 2018). Magmatic rocks are widely distributed at Zhaoxian, and comprise (from old to young) the meta-gabbro in the Neoarchean Malianzhuang sequence (Figure 2), the Qixia gneiss, biotite tonalite, (weakly gneissic) early Yanshanian Linglong medium-grained monzogranite, the late Yanshanian Guojialing porphyritic granodioritez, and some younger dykes (Song et al., 2010b).

Alteration is well developed along the faults at Zhaoxian. In the hanging-wall, there is a cataclastic zone with sericitized wallrocks, which transitions into a partially pyrite-sericite-altered cataclastic zone. The footwall also contains a pyrite-sericite-altered cataclastic zone, which transitions into pyrite-sericite alteration in the granite wallrocks (Figure 3).

Major alteration styles in the wallrocks include mainly 1) silicification (Figure 4A), which is often associated with metal sulfides and gold mineralization; 2) pyrite-sericite alteration (Figure 4B), with the mineral assemblage of sericite + quartz ± pyrite in cataclastic rocks; 3) chloritization (Figure 4C), which is altered from magmatic hornblende and biotite, occurring locally as patches or veinlets; 4) K-feldspar alteration (Figure 4C), which occurred in an early hydrothermal stage and the altered rocks have a fleshy red color; 5) carbonate alteration (Figure 4D), which occurs as late calcite veining (cutting the ores) and locally massive aggregates.

The orebodies at Zhaoxian can be divided into four groups (I to IV) (Figure 5): I) The orebodies in the pyrite-sericite-altered cataclastic zone on the footwall (Figure 5), containing 20 orebodies and accounting for 71.04% of the estimated total resource. Among them, No. I-2 orebody is the main orebody, accounting for 42.44% of the total resource; II) The orebodies in the pyrite-sericite-altered granitic cataclastic zone below Group I (Figure 5), containing 13 orebodies and accounting for 19.93% of the estimated total resource. Among them, No. II-1 orebody is one of the largest, accounting for 10.48% of the total resource; III) The nine orebodies in the pyrite-sericite-altered granite belt below Group II (Figure 5), accounting for 3.95% of the estimated total resource; IV) The orebodies in the pyrite-sericite-altered granite cataclastic zone above the main fault surface (Figure 5), containing seven orebodies and accounting for 5.08% of the estimated total resource.

Based on the field crosscutting relationships and mineral assemblages, the alteration/mineralization at Zhaoxian can be divided into four stages, namely (from old to young):

1) Pyrite-quartz-K-feldspar (Figures 6A,B): Pyrite is coarse-grained subhedral, and occurs as patches and veins with cataclastic texture. The quartz is coarse-grained milky and with wavy extinction. The K-feldspar is coarse-grained anhedral granular, and is commonly altered to sericite in the later hydrothermal stages. At this stage, minor native gold mineralization is observed under the microscope. The gold particles are small and mostly included in pyrite.

The samples were collected from the ZK01 drill-hole at 2038–2,146 m depths (Figure 2). Because the Jiaojia fault has undergone multistage faulting and hydrothermal alteration and mineralization, we collected four ore samples at different depths to recover quartz grains from the different ore-stages. The samples consist of pyrite-sericite-altered cataclasitic granite (ZX5, ZX6) and pyrite-sericite-altered clastic rock (ZX11, ZX21). The samples were prepared into double-polished thin-sections (0.3 mm thick) for optical microscopic observations. Representative quartz-hosted fluid inclusions (FIs) from the early-ore I), main-ore (II, III), and late-ore IV) stages were selected for microthermometric and laser Raman spectroscopic analyses.

The FI microthermometric measurement was performed at the Fluid Inclusion Laboratory of the Analysis, Test and Research Center (Beijing Institute of Geology of Nuclear Industry), on a Linkam-THMS600 temperature control stage. The heating and freezing stages were calibrated by using the synthetic FI standard provided by Fluid Inc. (United States). The instrument measurement accuracy is ± 0.5°C down to −120°C, ±0.2°C at −70°C–100°C, and ±2°C at 100°C–500°C. The heating rate is 0.2°C–10°C/min, and that for the CO₂-bearing FIs decreased to 0.2°C/min near the melting point, and to 0.2°C–0.5°C/min near the freezing point and homogenization for the aqueous FIs (Hu et al., 2021). All the raw data were processed with the MacFlincor program (Brown and Hagemann, 1995). Laser Raman probe analysis (RAM) was performed at the same laboratory, using a LABHR-VIS LabRAM HR800 microlaser Raman spectrometer. The analysis used a YAG crystal frequency-doubled solid-state laser with 532 nm wavelength, 100–4,200 cm⁻¹ scanning range, and 2 cm⁻¹ spectral resolution. Detailed sample preparation and analytical procedures follow those by Wei et al. (2015) and Shu et al. (2022).

Syn-ore FIs at Zhaoxian can be divided into primary and secondary ones, with the former comprising four types: CO₂-H₂O three-phase I), H₂O aqueous II), CO₂ single-phase III) and gaseous CO₂ and H₂O with daughter minerals IV).

Type I FIs account for ∼30% of the total FIs, and can be divided into two subtypes (Ia and Ib): type Ia FIs contain a liquid water phase (L_H2O), a liquid CO₂ phase (L_CO2), and a gaseous CO₂ phase (V_CO2) (Figures 7E, F, I–J). The L_CO2+V_CO2 phase accounts for ca. 15–40 vol% of the FIs. V_CO2 has relatively large volume. L_CO2 usually forms a thin dark rim around V_CO2, and this FI type is mostly stripy. These FIs are generally 2 × 2 to 12 × 14 μm in size. In type Ib two-phase (L_H2O+V_CO2) FIs (Figures 7D, I–J), the volume percentage of V_CO2 is largely similar (ca. 15–20 vol%). Most of these FIs are oval or negative crystal shape, with sizes of 2 × 3 to 4 × 10 μm (mostly 3 × 4 μm).

Type II FIs can also be divided into two subtypes: type IIa gas-liquid (L_H2O+V_H2O) FIs (Figures 7C, L) have their gas phase accounting for ca. 15–20 vol% (and ∼60% of the total number of type II FIs). They have mostly oval or negative crystal shape, with sizes of 2 × 3 to 4 × 6 μm (mostly 2 × 3 μm). Type IIb (aqueous single-phase L_H2O) FIs (Figures 7C, K) are colorless, transparent, and without air bubbles. They are usually long or oval shaped and are typically 2 × 5 μm in size.

Type III FIs (Figures 7A, J) are composed of a pure CO₂ gas phase, accounting for ∼7% of the total number of FIs. These inclusions are usually dark black with size of 3 × 3 μm.

Type IV FIs (Figures 7G, K) are composed of the gas phase (V_H2O+V_CO2) and daughter minerals S), and account for ∼3% of the total number of FIs. Daughter minerals (size: 1–2 μm) consist mainly of halite, and occur in aqueous L_H2O at room temperature. Type IV FIs are generally 4 × 10 to 5 × 12 μm in size (Figures 7H).

The trapping pressure of our FI samples is 217–353 MPa (mainly 260–330 MPa). The metallogenic depth H) is estimated to be 7.4–12.0 km with the equation of: P = ρgH (ρ = 3 g/cm³), where P = metallogenic pressure (Sheperd et al., 1985).

Representative type Ia, Ib and IIa were analyzed, and the resulting laser Raman spectra show characteristic CO₂ peaks (1,386 cm⁻¹, 1,285 cm⁻¹, 1,282 cm⁻¹) (Figure 10) and a slightly weaker CH₄ peak (2,912 cm⁻¹) (Figure 10). Laser Raman spectra also show that the early ore stage type Ia FIs has pure CO₂ Besides, the gas phase basically does not contain CH₄.

The FI petrographic, microthermometric and laser Raman spectroscopic data show that the ore fluids are of medium-low salinity and belong to a CO₂-H₂O system, consistent with the published data from the shallow part of the Jiaojia gold deposit and most Jiaojia-type gold deposits in this area. The features are different from typical high-temperature and high-salinity magmatic ore-forming fluids, but resemble the ore-forming fluids of most orogenic-type gold deposits.

Our results show that the main ore stage fluids are hotter and are more CH₄-rich than those of the early ore stage. Some studies suggested that the early ore-forming fluids of Jiaojia-type gold deposits are CO₂-rich and CH₄-poor, while the main ore stage fluids contain more CH₄ (Fan et al., 2005; Wen et al., 2016). There are two possible sources of CH₄: 1) re-equilibrium by the infiltration of H₂ into the FIs (Hall and Bodnar, 1990; Ridley and Hagemann, 1999); 2) the H₂O-NaCl-CO₂ fluids produced a small amount of CH₄ with the decreasing temperature and oxygen fugacity from the early to main ore stage.

Because the fluid immiscibility would cause substantial CO₂ loss, the temperature drop would lead to a decrease in the SiO₂ solubility, forming the late-stage quartz-calcite veins (Shu et al., 2022). The low salinity in type II FIs suggests that the late-stage fluids are of low-salinity and CO₂-poor.

In the early ore stage, the FIs are dominantly CO₂-bearing three-phase, and thus the ore-forming fluid is CO₂-rich and of high temperature. Metal complexes are stable in this weakly acidic hydrothermal fluid, and thus it could dissolve and carry gold and other ore-forming elements (Yang, 1986; Liu and Shen, 1999; Phillips et al., 2004; Hou et al., 2007; Lu, 2011; Mao et al., 2013).

The homogenization temperature of type Ia FIs is higher than those of type Ib FIs, and the salinity and density of type Ib FIs are lower than those of type Ib FIs. This may be caused by fluid immiscibility (Huang, 2021). During fluid immiscibility, the homogeneous temperature and pressure of inclusions are similar and can represent the fluid trapping temperature and pressure. (Huang, 2021). Therefore, the homogenization temperature of type Ia FIs can represent the capture temperature of immiscible inclusions, i.e., the ore-forming temperature is 234°C–357°C. The formation of many CO₂-bearing three-phase inclusions in quartz indicates that the fluid is CO₂-rich or carbonic. The presence of CO₂ can increase the solubility of chlorine and water in magmatic fluids and facilitate metal migration (Rui et al., 2003; Chen et al., 2006).

According to previous studies, the ore-forming materials in northwestern Jiaodong gold province were mainly derived from the Jiaodong metamorphic rocks (Deng et al., 2004; Li et al., 2007; Guo et al., 2008; Liang et al., 2015), yet the average gold content of the latter (2.47 ppb) is lower than that of the crust average (4 ppb) (Li and Ni, 1990). The average Au content of the Neoarchean Jiaodong Group metagabbro in the Jiaojia fault hanging-wall at Zhaoxian is low (1.16 ppb) (Yang et al., 1991), and could unlikely represent a main gold source. The Au content of Linglong granite (19 ppb) and Guojialing granite (49 ppb) in the Jiaodong district area are higher (Lu et al., 1999; Xu et al., 2022), and both being interpreted to have a crust-mantle mixed magma source. An upper mantle source of Au has also been suggested for the Jiaodong gold deposits (Gui, 2014).

In the geochemical tectonic discrimination plots for granites (Figure 12), our granite samples all fall inside the volcanic arc granite field, reflecting that they were formed in an arc setting (Pearce et al., 1984).

Thus, the granites at Zhaoxian may have emplaced in an arc environment, with its formation related to mantle-derived magma underplating. The mantle source may have provided both the heat to partially melt the crust and mix with the crustal magma (Zhang and Guo, 2012). Our geochemical data on the Zhaoxian granite show that the granite is highly fractionated with dominantly continental crustal source. Therefore, we infer that the gold ore-forming materials were mainly derived from the upper mantle. During the upwelling of mantle materials, gold may have ascended with the magma and formed gold deposits.

Based on the close spatial relationship of the Jiaodong gold deposits with the Jiaodong Group metamorphic rocks, Linglong granite, and Guojialing granodiorite. Many works in the 1980s and 1990s proposed that the ore-forming fluids were magmatic-hydrothermal sourced with meteoric water input (Wang and Li, 1985; Zhang et al., 1994; Wang et al., 1995.; Zhang et al., 1995; Lin and Yin, 1998). In recent years, some works have invoked mantle-derived input for the ore fluids in the Jiaodong gold province, based on mineral isotope and FI studies (Sun and Shi, 1995; Mao et al., 2002; Zhang et al., 2002; Liu et al., 2003; Fan et al., 2005).

Published He and Ar isotope data of FIs in pyrite from the Denggezhuang and Jiaojia gold deposits indicated that the ore-forming fluids have high 3He/⁴He and 40Ar/³⁶AR values, resembling mantle-derived fluids (Zhang et al., 2002). By studying the carbonate minerals from the Rushan and Jiaojia gold deposits, Liu et al. (2003) concluded that the Sr-Nd isotopic compositions are similar to those of the mantle source rocks.

The northwest Jiaodong region has experienced a complex tectonic evolution. In the Late Jurassic, the regional tectonics was influenced by the Paleo-Pacific subduction beneath Eastern China. Since the Early Cretaceous, regional tectonics had evolved from intracontinental compressional orogeny and crustal thickening to intracontinental rifting and lithospheric thinning. The Paleo-Pacific subduction had also led to sinistral shearing of the transcrustal Tanlu (Yishu) fault and its associated NE-NNE-trending structures. The transtension may have provided open space for the ascent of deep magmatic fluid (and thus gold mineralization) and mafic dyke intrusion (Goldfarb and Groves, 2015; Guo, 2016; Dou et al., 2021). During the upwelling of mantle material, gold may have ascended with the magma, and deposited at the open-space formed by the transtensional faults.