The Xiaotazigou gold deposit is situated on the northern margin of the North China Craton (NCC), at the junction between the Inner Mongolia Uplift and the Yanliao Depression Zone, bounded by the Chifeng–Kaiyuan and Chengde–Beipiao faults (Figure 1a). From the Paleozoic to the early Mesozoic orogeny, the NCC experienced multiple large-scale collisional and accretionary events. During the late Paleozoic, the NCC collided and accreted with the Siberian Craton, and subsequently, under a north–south compressional regime, interacted with the Yangtze Craton, resulting in deformation of the sedimentary cover, including widespread intraplate folding and faulting (Tang, 1990; Ames et al., 1993; Li, 1994). In the Mesozoic, within a compressional orogenic setting, lithospheric thinning facilitated the ascent of mantle-derived magmas into the deep crust, triggering widespread granitoid intrusion, volcanic activity, and large-scale metallogenic events (Jia et al., 2011). This complex tectono-magmatic evolution created favorable conditions for the formation of hundreds of gold deposits of varying scales along the northern NCC, accompanied by numerous Fe, Cu, and Mo deposits.

Regionally, Archean metamorphic rocks of the Jianping Group, widely exposed in the Chifeng-Chaoyang area, (Jia et al., 2011), represent an ancient crustal sequence and serve as the primary host rocks for gold mineralization (Figure 1b). These metamorphic rocks are dominated by gneisses, diorite–amphibolites, and banded magnetite–quartzites. Intrusive rocks are primarily granitoids, and faults are well-developed, with major NE- and N–S-trending structures. In the northern part of the region, the Early Yanshanian Jianggoushan pluton exhibits a close spatiotemporal and genetic relationship with regional gold mineralization. This pluton is predominantly composed of monzogranite and quartz monzonite. Numerous gold deposits, including Dongwujiazi, Beidi, and the Xiaotazigou deposit investigated in this study, are distributed within 5–8 km south of this pluton. The Beidashan pluton, which occurs as a stock south of the Jianggoushan pluton, represents a southward extension of the Jianggoushan intrusive system into the Xiaotazigou area. (Li et al., 2010). Industrially significant gold veins in the Xiaotazigou deposit are all hosted within a 0.5–1.5 km zone south of this pluton, indicating a strong genetic relationship between the plutonic intrusion and gold mineralization in the region.

The exposed stratigraphy of the Xiaotazigou mining area is dominated by metamorphic rocks of the Archean Jianping Group Xiaotazigou Formation, including biotite–hornblende monzogranitic gneiss, biotite monzogranitic gneiss, hornblende monzogranitic gneiss, diorite–amphibolite interlayered with banded magnetite–quartzite, and lens-shaped diorite porphyry bodies. This formation constitutes the primary host rock for gold mineralization in the area (Figure 2a). Fault structures are well-developed throughout the mining district. Near east–west trending faults, NE-to ENE-trending faults, and NNW-trending faults are the main ore-controlling structures, with the NNW-trending faults interpreted as post-mineralization fractures. In the northern part of the district, the Beidashan monzogranite pluton is exposed, with most gold veins occurring along its southern margin. Polymetallic sulfide veins are locally observed in this rock mass (Figure 2b). Minor intrusions of porphyritic rocks, including syenite porphyry, diorite (or diorite porphyry), and quartz porphyry, are also present. Based on cross-cutting relationships, the quartz porphyry and diorite veins are pre-mineralization intrusions, whereas syenite porphyry represents post-mineralization intrusion.

Currently, over 20 gold-bearing veins have been identified in the mining area, among which veins No. 1, 4, 5, and 6 are of economic significance. The majority of veins exhibit a NE-trending orientation, dipping northwest at angles between 64° and 86°. The largest vein, No. 1, has been traced over 1,200 m by trenching and deep drilling. Ore types are predominantly sulfide-bearing quartz veins (Figures 2c,d), with pyrite as the principal metallic mineral, accompanied by chalcopyrite, galena, and sphalerite. Ore textures include brecciated, inclusion, and replacement–dissolution structures, while ore fabrics are mainly veinlet, massive, and disseminated (Figures 2e,f). Gold occurs primarily as fracture-hosted mineralization. Alteration of the surrounding rocks is well-developed and includes silicification, sericitization, chloritization, and, carbonatization.

Rare earth and trace element analysis of whole rock were conducted on Elan DRC-e ICP–MS at the Harbin Natural Resources Comprehensive Survey Center, China Geological Survey. The detailed sample-digesting procedure was as follows: (1) Sample powder (200 mesh) were placed in an oven at 105 °C for drying of 12 h; (2) 50 mg sample powder was accurately weighed and placed in a Teflon bomb; (3) 1 mL HNO₃ and 1 mL HF were slowly added into the Teflon bomb; (4) Teflon bomb was putted in a stainless steel pressure jacket and heated to 190 °C in an oven for >24 h; (5) After cooling, the Teflon bomb was opened and placed on a hotplate at 140 °C and evaporated to incipient dryness, and then 1 mL HNO₃ was added and evaporated to dryness again; (6) 1 mL of HNO₃, 1 mL of MQ water and 1 mL internal standard solution of 1 ppm In were added, and the Teflon bomb was resealed and placed in the oven at 190 °C for >12 h; (7) The final solution was transferred to a polyethylene bottle and diluted to 100 g by the addition of 2% HNO₃.

Sulfur isotope analyses were conducted at the Tianjin Center, China Geological Survey. Pyrite samples were first purified to >99% and ground to 200 mesh. For analysis, approximately 15 mg of sample powder was weighed into a tin capsule with ∼10 mg of tungstic oxide (WO₃) as an accelerator. The mixture was flash-combusted at 1800 °C in an Elementar Vario Micro Cube elemental analyzer coupled to a Delta V Plus mass spectrometer. The released SO₂ gas was purified by a He carrier stream and separated via a trap-and-purge method before isotopic measurement. The results were normalized against the international standard CDT (Cañon Diablo Troilite), with an analytical precision better than ±0.2‰ (2σ).

The rare earth element (REE) compositions of ore, monzogranite, and gneiss from the Xiaotazigou mining area are summarized in Table 1. All data were normalized to C1 chondrite values (Boynton, 1984), and the resulting REE distribution patterns are shown in Figure 3.

Oresfrom Vein No. 1 exhibit relatively uniform total REE contents (∑REE = 24.48–28.05 ppm, average 26.75 ppm), with LREEs ranging from 19.54 to 23.11 ppm and HREEs from 4.27 to 5.84 ppm. The LREE/HREE ratios (3.80–5.42) and La_N/Yb_N values (13.99–18.94) indicate pronounced LREE enrichment and right-inclined REE patterns (Figure 3a). LREE fractionation is significant (LaN/Sm_N = 4.23–5.52), whereas HREE fractionation is weak (Gd_N/Yb_N = 1.89–2.22). Europium anomalies (δEu = 0.65–0.72) are strongly negative, while Ce anomalies (δCe = 0.69–0.81) are minor.

Monzogranitesdisplay higher ∑REE (62.66–87.42 ppm, average 78.25 ppm) with LREEs of 53.05–73.14 ppm and HREEs of 9.61–15.87 ppm. The LREE/HREE ratios (4.13–5.52) and La_N/Yb_N values (15.75–26.20) indicate strong LREE enrichment. They exhibit pronounced negative Eu anomalies (δEu = 0.68–0.80) but minor Ce anomalies (δCe = 0.67–0.82) (Figure 3b).

Gneissesshow the highest ∑REE (72.96–150.18 ppm, average 106.63 ppm), with LREEs of 55.16–128.48 ppm and HREEs of 17.80–21.70 ppm, LREE/HREE ratios of 3.10–5.92, and La_N/Yb_N values of 7.99–19.51. Negative Eu anomalies are evident (δEu = 0.64–0.74), whereas Ce anomalies remain minor (δCe = 0.87–0.95) (Figure 3b).

The trace element compositions are summarized in Table 2. The data were normalized to primitive mantle values, and the resulting spider diagrams are shown in Figure 4. The geochemical behavior of the ore closely resembles that of the associated monzogranite and gneiss, being enriched in large-ion lithophile elements (LILEs) such as Cs, Rb, and Ba, as well as in La, while showing relative depletion in high-field-strength elements (HFSEs) such as Nb, Ta, Ti, and P. In the ores, the trace element ratios Hf/Sm, Th/La, Nb/La, Y/Ho, Co/Ni, Nb/Ta, Zr/Hf, and Th/U range from 0.58 to 0.91, 0.21 to 0.27, 0.32 to 0.39, 13.67 to 28.90, 1.64 to 4.06, 16.00 to 20.36, 20.46 to 61.15, and 5.07 to 6.82, respectively.

The sulfur isotope compositions of pyrite from Vein No. 1 of the Xiaotazigou gold deposit are presented in Table 3. The δ³⁴S values of pyrite vary within a narrow range of 2.67‰–3.70‰, with an average of 3.14‰.

Geochemically, the monzogranitesexhibit light REE enrichment and heavy REE depletion, with pronounced negative anomalies in Nb, Ta, Ti, and P (Figure 4b). These characteristics indicate fractional crystallization of minerals such as apatite and ilmenite. Furthermore, the monzogranites possess relatively high Sr contents (262–445 ppm) but low Y (9.88–11.8 ppm) and Yb (0.97–1.25 ppm) concentrations, suggesting that the magmatic source was likely relatively mafic igneous rocks and/or involved contributions from mantle-derived components. The rocks show weak negative Eu anomalies, enrichment in Ba, and insignificant Th anomalies (Figure 4b), consistent with features of I-type granites. On the (Y + Nb)–Rb tectonic discrimination diagram (Figure 5a), all monzogranitesplot within the field of volcanic arc granites, further indicating that their formation was likely related to orogenic processes (Miao et al., 2003).

The gneisses are characterized by low concentrations of Nb, Ta, and Sr Their chondrite-normalized REE patterns are strongly right-sloping, indicating significant fractionation between LREEs and HREEs. Pronounced negative Eu anomalies are observed, suggesting plagioclase retention in the source region, which is consistent with formation under low-pressure conditions given the pressure-sensitive stability of plagioclase. The rocks exhibit variable Sr/Y ratios (7.58–45.17), depletion in HREEs and Y, and plot within or near the adakite field on the Sr/Y–Y diagram (Figure 5b). These features imply that their protoliths likely belong to the Archean TTG (tonalite–trondhjemite–granodiorite) suite. Geochemically, the gneisses are enriched in LILEs (e.g., Ba, Rb) but depleted in HFSEs (e.g., Nb, Ta, P, Ti), reflecting an arc magmatic affinity. This interpretation is supported by their placement in the volcanic arc granite field on the (Y + Nb)–Rb diagram (Figure 5a), suggesting a genetic link to orogenic events.

The study of the geochemical characteristics of ore-forming fluids and the sources of ore-forming materials represents a key focus in modern ore deposit research. Rare earth elements, due to their geochemical stability, can effectively trace fluid–rock interactions during mineralization (Henderson, 1984; Li et al., 2019; 2020) and help constrain ore-forming processes (Schade et al., 1989; Zhao and Jiang, 2007). Moreover, in hydrothermal deposits, the ionic radii of key ions such as Cu²⁺, Zn²⁺, and Fe²⁺ are much larger than those of REE³⁺, making the latter less likely to enter mineral lattices, thus preserving original REE signatures in the ore (Bi et al., 2004; Mills and Elderfield, 1995). Previous studies have shown that REEs and HFSEs are often concentrated in fluid inclusions within sulfide minerals in hydrothermal systems (Bi et al., 2004; Mao et al., 2006). Consequently, the composition of REEs and HFSEs in pyrite can reflect the characteristics of the ore-forming fluids (Bi et al., 2004; Mao et al., 2006). In the Xiaotazigou gold deposit, the total REE content in pyrite is relatively low (24.47–28.05 ppm), with moderate fractionation between LREEs and HREEs (LREE/HREE = 3.80–5.51; La_N/Yb_N = 13.99–18.94; Table 1), and the normalized REE patterns are strongly right-inclined (Figure 3a), indicating LREE enrichment during mineralization.

Experimental and theoretical studies suggest that Cl and F exhibit differential complexing behavior with REEs: Cl preferentially complexes with LREEs, while F favors HREEs (Flynn and Burnham, 1978; Alderton et al., 1980; Hass et al., 1995). HFSEs also show contrasting behavior in Cl-versus F-rich fluids. In Cl-rich fluids, LREEs are enriched and ratios such as Hf/Sm, Nb/La, and Th/La are generally <1, whereas in F-rich fluids, HFSEs are enriched, and these ratios exceed 1 (Oreskes and Einaudi, 1990; Ayers and Watson, 1993; Keppler, 1996). This feature is consistent with the fact that the fluid phase components of fluid inclusions in many typical gold deposits within the same mineralization belt generally contain Cl-, such as the Anjiayingzi gold deposit (Sun, 2013), Honghuagou gold deposit (Tang, 2018), Jinchanggouliang gold deposit (Hou, 2011), etc. Moreover, on the spider web diagram of trace elements (Figure 4), it is shown that the significant deficiency of high-field-strength elements (Nb, Ta, Ti, etc.) in pyrite also rules out the possibility that the ore-forming fluid is of the F system (Li et al., 2022). In Xiaotazigou pyrite, the ratios Nb/La (0.32–0.39), Th/La (0.21–0.27), and Hf/Sm (0.58–0.91) are all <1, consistent with LREE enrichment and indicating that the ore-forming fluids were predominantly Cl-rich. Fluid inclusion studies (Li et al., 2010) suggest temperatures of 174 °C–348 °C and salinities of 2.06–11.72 wt% NaCl equivalent, with the presence of CO₂, indicating that the mineralizing fluid system was a Cl-rich, medium-to low-temperature NaCl–H₂O–CO₂ hydrothermal system.

The redox state of the mineralizing fluid can be inferred from Eu and Ce anomalies in REEs. In reducing conditions, Eu²⁺ is preferentially separated from trivalent REEs, while Ce remains as Ce³⁺ and does not show significant fractionation (Ma et al., 2013). In contrast, oxidizing environments typically produce negative Ce anomalies and minimal Eu anomalies (An and Zhu, 2014; Chen et al., 2013). In the Xiaotazigou deposit, pyrite exhibits pronounced negative Eu anomalies (δEu = 0.65–0.72) with no significant Ce anomalies (Figure 3a), and field observations confirm the absence of sulfate minerals indicative of strongly oxidizing conditions (e.g., barite, gypsum). These observations indicate that the ore-forming fluids were reducing. Impurity elements in pyrite, particularly Co and Ni, and the Co/Ni ratio, are valuable indicators of ore genesis (Brill, 1989; Gregory et al., 2015). Sedimentary pyrite typically exhibits Co/Ni < 1 (average 0.63) (Price, 1972), hydrothermal pyrite ranges from 1.17 to 5 (average 1.7), and volcanogenic pyrite generally has Co/Ni > 1, concentrated between 5 and 10 (average 8.7) (Campbell and Ethier, 1984; Bajwah et al., 1987; Bralia et al., 1979). In Xiaotazigou, Co/Ni ratios in pyrite range from 1.64 to 4.06 (Table 2), consistent with a magmatic–hydrothermal origin (Meng et al., 2018). The pyrite compositions also fall within the porphyry-type field in the Ni–Co diagram (Figure 6), further supporting a magmatic–hydrothermal source for the fluids.

Elemental ratios such as Y/Ho, Zr/Hf, and Nb/Ta, which have similar ionic radii and charges, are generally stable in a closed hydrothermal system but can vary significantly due to fluid mixing or wall-rock interaction (Bau and Dulski, 1995; Yaxley et al., 1998; Douville et al., 1999). In Xiaotazigou pyrite, Y/Ho (13.67–28.90) and Zr/Hf (20.46–61.15) show wide variations, while Nb/Ta (16.00–20.36) is relatively stable, suggesting that the ore-forming fluids underwent fluid–rock interaction or mixing during evolution. Hydrogen and oxygen isotope testing and analysis show that the δD value of quartz minerals in the ore samples of the Xiaotazigou gold deposit ranges from −82.61‰ to −91.79‰, and the δ¹⁸O value ranges from 5.55‰ to 7.56‰. The δD values of the Dongwujiazi gold deposit in the neighboring area range from −89.58‰ to −92.66‰, and the δ¹⁸O values range from −1.40‰ to −0.23‰. It is believed that the ore-forming fluids of these two gold deposits both originate from magmatic water and are mixed with atmospheric precipitation (Zhang et al., 2007; Park et al., 2008), indicating that the initial mineralizing fluid was magmatic in origin with some incorporation of meteoric water.

Overall, the ore-forming fluids of the Xiaotazigou gold deposit were Cl-rich, medium-to low-temperature NaCl–H₂O–CO₂ hydrothermal fluids, reducing in nature, and derived primarily from magmatic sources with minor meteoric water input. This phenomenon can be explained by differences in mineralization depth and distance from intrusive bodies between the Xiaotazigou gold deposit and the neighboring Dongwujiazi gold deposit. The Xiaotazigou deposit is located approximately 1 km from the Beidashan monzogranite intrusion, which has a surface exposure of less than 1 km², and its orebodies are hosted in the metamorphic rocks of the Xiaotazigou Formation. In contrast, the Dongwujiazi deposit lies about 6 km from a quartz monzonite intrusion to the north, with its orebodies also occurring within the Xiaotazigou Formation metamorphic rocks. The mineralization depth of the Xiaotazigou deposit ranges from 2.8 to 4.72 km (Park et al., 2008), indicating a relatively short migration distance for the ore-forming fluids. It is thus inferred that the ore-forming fluids at Xiaotazigou were predominantly derived from magmatic water associated with the monzogranite, with a minor component of meteoric water. In comparison, the Dongwujiazi deposit formed at a shallower depth of 1.39–1.84 km (Park et al., 2008) and is situated farther from the intrusion horizontally. The extended lateral migration and ascent of magmatic fluids in this setting resulted in the incorporation of a significantly higher proportion of meteoric water.

Sulfur isotope analysis of the Xiaotazigou gold ore reveals a narrow range of δ³⁴S values (2.67‰–3.70‰; mean = 3.14‰) with a small variation (1.07‰). These values are close to the typical mantle sulfur range (δ³⁴S = −3‰–3‰; Chaussidon and Lorand, 1990), suggesting a predominantly deep mantle origin for the sulfur. As shown in Figure 7, the δ³⁴S values of the Xiaotazigou deposit fall within a relatively tight range, similar to those reported from other regional gold deposits such as the Jinchanggouliang (−7.21‰–1.27‰), Erdaogou (−2.2‰–3.4‰), and Dongwujiazi (1.91‰–3.14‰) deposits. The consistently mantle-like sulfur isotope signatures among these deposits indicate a shared deep magmatic source for the sulfur. Furthermore, tellurium (Te) is generally a dispersed element in the crust, and its local enrichment is widely interpreted as indicative of mantle-derived inputs (Du, 1996; Mao and Li, 2004). It is transported via mantle-derived volatile exhalations despite its low solubility in water (Du, 1996). Electron microprobe analysis has identified the presence of Te in electrum from the No. 1 vein of the Xiaotazigou deposit (Li et al., 2010), providing further evidence for the involvement of mantle-derived components in the mineralization process. Meanwhile, the similarity of REE and trace element patterns between ore, granitic intrusions, and host gneiss indicates elemental inheritance from both the intrusive rocks and country rocks (Figures 3, 4). The ore-forming fluids likely leached REEs and other trace elements during fluid–rock interaction, producing geochemical signatures that overlap those of the host rocks. Such dual inheritance is a common feature of orogenic gold systems where syn-to post-collisional granitic magmatism interacts with ancient metamorphic basement. As illustrated in the δEu–δCe, ΣREE–LREE/HREE, Nb–Ta, and Zr–Hf diagrams (Figure 8), the oreplot in similar ranges or exhibit trends consistent with those of monzogranite and gneiss. This further supports the interpretation that the ore-forming materials of the Xiaotazigou gold deposit were likely derived from the monzogranite and gneiss.

From the Late Indosinian to Middle Jurassic, northern China, including the study area, experienced intracontinental collisional orogeny, with the southward subduction of the Mongolia–Okhotsk Ocean at ∼160 Ma providing the primary tectonic driver (Mao et al., 2005; Zhao et al., 2004). During the compressional stage, crustal thickening concentrated energy and facilitated the formation of mineralized magmatic–hydrothermal systems. Subsequent extensional stages triggered lithospheric thinning and ascent of mineralized granitic magmas along faults, where fluids were enriched and precipitated in favorable structural sites, forming numerous gold deposits in the region, including Dongwujiazi, Beidi, Huojiazi, and Xiaotazigou.

Field observations indicate that Xiaotazigou gold mineralization is structurally controlled by the Chifeng–Kaiyuan and Chengde–Beipiao faults, which provided conduits for deep-seated ore-forming fluids. Ore bodies are confined to compressional–shear fractures, with zoned hydrothermal alteration of the wall rocks progressing outward from silicification to sericitization, chloritization, and carbonate alteration. Mineral assemblages are relatively simple, dominated by pyrite with subordinate chalcopyrite, sphalerite, and galena, and quartz as the primary gangue mineral with minor calcite. These geological features closely resemble those of typical orogenic gold deposits worldwide (Table 4).

Fluid inclusion studies indicate that homogenization temperature and salinity are negatively correlated (Figure 9), consistent with fluid evolution patterns observed in classic orogenic gold systems (Chen et al., 2007). Proximity to the Dongwujiazi gold deposit (∼7 km to the northeast) with a similar tectonic setting suggests that Xiaotazigou shares the same orogenic gold-forming mechanism, confirming its classification as an orogenic.