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Article

The Mineralization Mechanism of the Axi Gold Deposit in West Tianshan, NW China: Insights from Fluid Inclusion and Multi-Isotope Analyses

by
Fang Xia
1,
Chuan Chen
1 and
Weidong Sun
2,*
1
Xinjiang Key Laboratory for Geodynamic Processes and Metallogenic Prognosis of the Central Asian Orogenic Belt, College of Geology and Mining Engineering, Xinjiang University, Urumqi 830047, China
2
Geological Bureau, Xinjiang Uyghur Autonomous Region, Urumqi 830000, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(5), 536; https://doi.org/10.3390/min15050536 (registering DOI)
Submission received: 2 April 2025 / Revised: 13 May 2025 / Accepted: 14 May 2025 / Published: 18 May 2025
(This article belongs to the Special Issue Mineralogy, Geochemistry and Fluid Inclusion Study of Gold Deposits Endowed in Critical Metals)
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Abstract

:
The Axi gold deposit, which is located in the Tulasu Basin of the West Tianshan orogenic belt in Northwest China, features vein-type ore bodies hosted in radial structural fractures formed due to volcanic activity. The deposit experienced three distinct mineralization stages: Stage I, characterized by the microcrystalline quartz–pyrite crust; Stage II, characterized by quartz–sulfide–native gold veins; and Stage III, characterized by quartz–carbonate veins. Fluid inclusion studies have identified four types of inclusions: pure vapor, vapor-rich, liquid-rich, and pure liquid. The number of vapor-rich inclusions decreases when moving from Stage I to Stage III, whereas the number of liquid-rich inclusions increases. The fluid temperature gradually decreases from 178–225 °C in Stage I to 151–193 °C in Stage II and further to 123–161 °C in Stage III, whereas the fluid salinity decreases slightly from 2.1%–5.1% wt.% NaCl eqv to 1.4%–4.6% wt.% NaCl eqv and finally to 0.5%–3.7% wt.% NaCl eqv. As suggested by the results of the oxygen, hydrogen, and carbon isotope analyses, the ore-forming fluids were primarily meteoric water. Sulfur isotopic compositions indicate a single deep mantle source. The lead isotopic compositions closely resemble those of Dahalajunshan Formation volcanic rocks, indicating that these rocks were the primary source of the ore-forming material. In addition, gold mineralization formed in a Devonian–Early Carboniferous volcanic arc environment. Element enrichment was mainly caused by the circulation of heated meteoric water through the volcanic strata, while fluid boiling and water–rock interactions were the main mechanisms driving element precipitation. The integrated model developed in this study underscores the intricate interplay between volcanic processes and meteoric fluids during the formation of the Axi gold deposit, offering a robust framework for an understanding of the formation processes and enhancing the predictive exploration models in analogous geological settings.

1. Introduction

The Axi gold deposit, located within the Tulasu Basin, West Tianshan, Northwest China, is a significant example of low-sulfidation epithermal gold mineralization [1]. This region has undergone complex Paleozoic collisional and accretionary orogenic processes, which have led to the formation of numerous Late Paleozoic porphyry Cu-Au deposits, epithermal Au deposits, and orogenic Au deposits [2]. The Tulasu Basin is renowned for its rich gold mineralization and has several large-scale epithermal gold deposits, including the Axi, Jingxi-Yierman, and Tawuerbieke deposits.
The Axi gold deposit is a focus of ongoing research because of its substantial gold reserves (exceeding 60 tons of gold) and unique geological characteristics. Previous studies on this deposit have focused primarily on the geological setting, mineralization age, fluid properties, wall–rock alteration, and ore-controlling factors of the deposit [3,4,5,6]. While fluid studies have highlighted the nature of hydrothermal fluid, the detailed processes linking fluid evolution to mineral precipitation remain poorly understood to date. Specifically, the roles of fluid boiling, water–rock interactions, and changes in the physicochemical conditions in driving metal deposition warrant a systematic analysis. Additionally, the mechanisms underlying the process of ore-forming material enrichment within the hydrothermal system are not well understood. It is, therefore, essential to trace the origins of critical elements and understand how these elements are concentrated in the ore-forming environment. Furthermore, the existing studies have largely been conducted in isolation, and a unified model that integrates the tectonic setting, material enrichment, and precipitation mechanisms has not been developed. Such a model is, however, essential in understanding the spatial and temporal distributions of mineralization and to guide exploration efforts.
In the above context, this study combined field investigations, mineralogical analyses, fluid inclusion studies, and H-O-C-S-Pb isotope analyses to achieve the following objectives: (1) characterize fluid evolution, (2) trace material sources, and (3) elucidate the precipitation mechanisms. Ultimately, a comprehensive ore-forming model that integrates the tectonic setting, material enrichment, and precipitation mechanisms was developed, providing a holistic understanding of the Axi gold deposit’s genesis. This model enhances the academic knowledge and offers practical insights for other explorations in similar geological settings.

2. Regional Geology

The Tulasu Basin is located within the northern segment of West Tianshan and constitutes one portion of the active continental margin of the Central Tianshan–Yili Plate [1]. The basin has the South Keguchqin Mountain Fault to the north and the Northern Yili Basin Fault to the south (Figure 1). The basin basement is composed of Proterozoic low-grade metamorphic rocks, primarily including limestone, argillaceous rocks, dolomite, and marble [2,3,4]. Overlying this basement are Ordovician to Carboniferous strata. The Ordovician strata include the Nailenggeledaban Formation, which is characterized by tuffaceous calcareous siltstone, and the Hudukedaban Formation, which includes limestone interbedded with clastic rocks. The Silurian Nilekehe Formation includes carbonate rocks interbedded with clastic rocks. The Devonian Tuhulasu Formation comprises conglomerate and quartz sandstone. The Carboniferous strata in this region include the Dahalajunshan, Aqialehe, Naogaoyi, and Aoyimanbulake Formations. The Dahalajunshan Formation primarily includes intermediate–acidic volcanic rocks and continental volcaniclastic rocks. The Aqialehe Formation comprises marine clastic rocks and bioclastic rocks. The Naogaoyi Formation is dominated by terrigenous clastic rocks interbedded with volcanic and volcaniclastic rocks. The Aoyimanbulake Formation primarily includes clastic and carbonate rocks, with intercalations of volcanic and volcaniclastic rocks.
Along the margins of the basin, magmatic intrusions are exposed, indicating extensive magmatic activity in the region. In the northern part of the basin, dioritic intrusions have developed in the Kexiaxi region, with the zircon U-Pb age range of 357–348 Ma. In the southeastern part of the basin, granitic intrusions are exposed, with the zircon U-Pb age range of 362–361 Ma [7]. Within the basin, granitic intrusions are exposed in the Tawuerbieke area, with the zircon U-Pb age range of 355–349 Ma [8]. The basin has typical well-developed fault structures, predominantly the NW-trending faults, which exist parallel to the northern and southern boundary faults (South Keguchqin Mountain Fault and Northern Yili Basin Fault). These faults control the structural framework of the basin and also play a significant role in the mineralization processes occurring in this region.

3. Ore Deposit Geology

The Axi gold deposit, discovered from the Tulasu Basin, is a tremendous gold deposit, with proven gold reserves exceeding 60 tons (Figure 2a). The mineralization is hosted within the volcanic rocks of the Dahalajunshan Formation, which have a high-precision zircon SHRIMP U-Pb age of 362 Ma [9]. The Dahalajunshan Formation can be divided into five lithological units from the bottom up, and their formation is associated with distinct volcanic facies [10,11,12,13]. The first unit comprises andesitic volcanic breccia, corresponding to the “explosive facies”. The second unit comprises andesite and basaltic andesite. The third unit comprises andesitic dacite and amygdaloidal andesite. The fourth unit comprises andesitic dacite and dacite. These second, third, and fourth units together form the “effusive facies”. The fifth unit is characterized by andesitic volcanic tuff, which is associated with the “pyroclastic facies”. The “volcanic neck facies” are not exposed at the surface. Under the influence of the volcanic structure, a series of ring-shaped, radial, and irregular faults have developed within the mining region. The arcuate fault F1 controls the occurrence of the main ore body—Ore Body No. 1 (Figure 2b). This fault generally forms a convex arc toward the southwest, and its southern segment strikes ~40° and dips to the northeast, while its northern segment strikes ~100° and dips to the east. The entire fault extends over 1300 m in length, with widths ranging from 20 to 60 m and depths of up to 500 m. Ore Body No. 1 contains gold resources exceeding 40 tons, and the mean gold grade is 5.57 g/t [11]. Ore Body No. 1 is more than 1000 m long, with a mean thickness of 11–15 m. It forms a thick, vein-like structure, exhibiting swell-and-narrow undulations along both the strike and dip directions.
The hydrothermal alteration exhibits a distinct zone distribution, progressing outward from the central quartz veins in the form of silicification, sericitization, and propylitization zones [4]. The silicification zone is primarily observed within the ore body and its hanging wall and is closely associated with extensional fracture zones (Figure 3a). This zone is characterized by veinlet and stockwork quartz cementing early-formed rock breccias. The sericitization zone is distributed outward from the silicification zone, and, in this zone, plagioclase is replaced by quartz and sericite, and disseminated pyrite is released. The propylitization zone, which is widely developed outward from the silicification zone (Figure 3b), is characterized by the replacement of plagioclase, amphibole, and pyroxene with minerals such as chlorite, epidote, and calcite. The alteration pattern in this zone reflects the progressive changes in temperature from the center of the hydrothermal system outward. The metallic minerals primarily include pyrite, arsenopyrite, marcasite, native gold, and electrum, accompanied by minor amounts of sphalerite and chalcopyrite. The gangue minerals include chalcedony, quartz, illite, sericite, calcite, adularia, and laumontite.
According to the mineral assemblages, structural features, and crosscutting relationships among the veins, this hydrothermal mineralization period is classified into three mineralization stages: Stage I is a microcrystalline quartz–pyrite stage characterized by light grayish–white microcrystalline quartz forming crustiform ores with minor fine-grained pyrite (Figure 3c,d); Stage II is a quartz–sulfide–native gold stage, which is the main gold mineralization stage, characterized by smoky gray quartz veins containing disseminated pyrite and arsenopyrite (Figure 3e–g); Stage III is the quartz–carbonate stage, featuring coarse-grained quartz–calcite forming irregular veins, representing late-stage mineralization (Figure 3h). Pyrite is the most critical gold-bearing mineral (Figure 3i–l).

4. Analytical Methods

4.1. Fluid Inclusion (FI)

Quartz samples at the three mineralization stages were used to conduct the FI study. FI petrography and microthermometry were carried out at the Geofluids Laboratory of Xinjiang University, Urumqi. Microthermometry was performed using a THMS600 heating–freezing stage (Linkam Scientific Instruments, Surrey, UK) coupled with a Zeiss microscope. Quartz samples associated with sulfides from different mineralization stages were selected and prepared into doubly polished thin sections of about 0.2 mm in thickness. In the temperature range of <0 °C to >200 °C, the accuracy of the temperature measurement was ±2 °C. The fluid inclusions’ salinity in the NaCl–H2O system was determined by evaluating their ice-melting temperatures [14].

4.2. H-O-C Isotopes

Quartz samples from three mineralization stages were selected for H-O-C isotope analysis at Beijing Zircon Chronology Navigation Technology Co., Ltd. (Beijing, China) A 253 Plus gas isotope ratio mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used. A hydrogen isotope analysis was conducted, for which samples were introduced into a high-temperature pyrolysis furnace (1420 °C) filled with glassy carbon granules. In this analysis, water obtained from FI can react with glassy carbon to produce H2 and CO, which are then carried by high-purity helium into a chromatographic column and finally into the mass spectrometer for measurement [15]. The analytical precision was over 1‰. Oxygen isotope analysis was conducted next using the traditional BrF5 method, in which BrF5 reacts with oxygen-bearing minerals under a vacuum at 580 °C to extract oxygen, generating O2 gas [16]. This O2 gas was collected using a 5 Å molecular sieve tube, and the external analytical precision was greater than ±0.2‰ for standard samples. For carbon isotope analysis, samples were heated at 550 °C for 5 min to release carbon-bearing gases, which were collected using a nickel filament trap [17]. When the temperature reached 960 °C and the CO2 background decreased below 10 mV, CO2 gas was introduced into the mass spectrometer for the carbon isotopic composition analysis, and the analytical precision was ±0.2‰.

4.3. S-Pb Isotopes

Pyrite samples from three mineralization stages were selected for S-Pb isotope analysis using a 253 Plus gas isotope ratio mass spectrometer (Thermo Fisher Scientific, USA) and a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, USA) at Beijing Zircon Chronology Navigation Technology Co., Ltd. For the sulfur isotope analysis, the samples were combusted at 960 °C, and the resulting gases were passed through the oxidation–reduction reactor containing Cu and WO3 wires under a helium flow [18]. This process ensured the complete oxidation of the gases and reduced the SO3 produced in trace amounts to SO2. The gases were then separated using a chromatographic column and introduced into the mass spectrometer for measurement, and the analytical precision was greater than 0.2‰ for standard samples. For the lead isotope analysis, samples were placed in Savillex digestion vessels and treated with 2 mL of HF and 1 mL of HNO3, followed by heating for 48 h [19]. After evaporation, the residues were redissolved in concentrated HNO3, evaporated again, and then dissolved in 1 mL of 3.5 M HNO3 for column separation. Lead was separated using a Sr-specific resin and collected using 5 mL of 8 M HCl. The purified Pb solution was evaporated, redissolved in concentrated HNO3, and finally diluted with 1 mL of 2% HNO3 for analysis. The isotopic ratios were corrected using 203Tl/205Tl = 2.3889, with analytical precision of over ±0.005‰.

5. Results

5.1. Fluid Inclusion and Microthermometry

Fluid inclusions (FIs) can be classified based on their distribution patterns [20,21,22,23]: isolated inclusions, randomly scattered clusters, or aggregated groups are typically primary in origin. In contrast, those arranged in short, discontinuous intracrystalline trails represent pseudo-secondary features, whereas long, continuous trails that transect crystal boundaries are indicative of secondary formation.
The primary FIs in the Axi gold deposit are relatively small, with a size range of 5–15 μm. These FIs exhibit various shapes, such as rounded, elongated, elliptical, and irregular forms, and are primarily distributed in clusters, linear arrays, or as isolated individuals (Figure 4). The FIs can be classified into four types according to their phase ratios: pure vapor, vapor-rich (vapor phase > 50%), liquid-rich (liquid phase > 50%), and pure liquid FIs. Upon heating, the vapor-rich FIs homogenize to the vapor phase, whereas the liquid-rich FIs homogenize to the liquid phase. The transparency increases from pure vapor to pure liquid FIs, transitioning from dark and opaque to nearly colorless.
The fluid inclusions’ salinity in the NaCl–H2O system was determined by evaluating their ice-melting temperatures [14]. Stage I contains liquid-rich, vapor-rich, and pure vapor FIs (Figure 4a,b). The ice-melting temperatures of liquid-rich FIs are in the range of −3.1 °C to −1.2 °C, the salinity range is 2.1%–5.1% wt.%, and the homogenization temperatures are 178–218 °C. The homogenization temperatures of vapor-rich FIs are 192–225 °C. Stage II contains liquid-rich, vapor-rich, and pure liquid FIs (Figure 4c,d). For liquid-rich FIs, the Tm-ice values are between −2.8 °C and −0.8 °C, the salinity is in the range of 1.4%–4.6% wt.%, and the homogenization temperatures are 151–193 °C. The homogenization temperatures of vapor-rich FIs are 172–190 °C. Stage III includes liquid-rich and pure liquid FIs (Figure 4e,f). Liquid-rich FIs display Tm-ice values between −2.2 °C and −0.3 °C, salinity in the range of 0.5%–3.7% wt.%, and homogenization temperatures ranging from 123 to 161 °C. The detailed data are presented in Table 1 and Figure 5.

5.2. H-O-C-S-Pb Isotopic Composition

The measured δ18Oquartz values of Stages I to Stage III are 9.26‰–11.34‰, 9.19‰–14.10‰, and 9.15‰–10.36‰, respectively. Using the fractionation equations established by Clayton et al. [24], the δ18Ofluid values were calculated, which ranged from −2.4‰ to −0.4‰ for Stage I, −4.6‰ to 0.3‰ for Stage II, and −7.3‰ to −6.1‰ for Stage III. The measured δDfluid values ranged from −117.2‰ to −110.2‰ for Stage I, −127.3‰ to −107.2‰ for Stage II, and −129.6‰ to −114.8‰ for Stage III. The δCfluid values for the main gold mineralization stage (Stage II) varied from −12.28‰ to −11.48‰. Additionally, the δ34S values of pyrite ranged from 0.50%–4.06‰, and its Pb isotope ratios are as follows: 206Pb/204Pb = 18.332–18.558, 207Pb/204Pb = 15.605–15.642, and 208Pb/204Pb = 38.293–38.544. The detailed data are presented in Table 2 and Table 3.

6. Discussion

6.1. Origin and Evolution of Ore-Forming Fluids

The Axi gold deposit typically features relatively low mineralization temperatures. In Stage I, the fluid temperature ranged from 178 to 225 °C, representing the initial phase of mineralization. The FIs in this stage exhibit two homogenization phases: a vapor phase and a liquid phase, indicating fluid boiling. In Stage II, the fluid temperatures range from 151 to 193 °C, reflecting prolonged ore-forming fluid activity. Similarly to Stage I, the FIs exhibit two homogenization phases, indicating that boiling continued into this stage. Intense boiling would increase the salinity concentration in the fluids, thereby trapping highly saline fluids [23]. The absence of high-salinity daughter minerals suggests that the boiling was not intense. Pure liquid inclusions were trapped from a homogeneous single-phase fluid [23], distinct from the characteristics of earlier ore-forming hydrothermal fluids. The abundance of pure liquid inclusions suggests the influx of low-temperature, low-salinity fluids into the system. In Stage III, the fluid temperatures are the lowest, ranging from 123 to 161 °C, indicating that the fluids were near the surface and operated in a relatively open system. The overall fluid salinity is low in this region, decreasing systematically from the early to late stages, likely due to increasing meteoric water input. This trend aligns with the changes in the FI types and temperature patterns.
This study shows that there are differences in the fluid characteristics at different stages, which suggests multiple sources. H-O isotope analyses were conducted to trace the fluid origins and evolution [25,26]. In the δDfluid vs. δ18Ofluid diagram (Figure 6a), the data points from Stage I to Stage III were plotted near the meteoric water line, suggesting meteoric water as the dominant ore-forming fluid, with minor magmatic contributions. When moving from Stage I to Stage III, the data points shifted closer to the meteoric water line, further supporting the dominance of meteoric water. These findings imply that volcanic influences gradually decreased. The heated meteoric water likely caused the leaching of ore-forming elements from the surrounding rocks, leading to the enrichment of these elements. As mineralization progressed, meteoric water continuously infiltrated the system through fractures, thereby reducing the temperature and salinity, promoting mineral crystallization, and forming vein-type ore bodies. Through field observations, we identified alteration features such as silicification, sericitization, and propylitization in the vein-hosted surrounding rocks. Additionally, fluid leaching caused shifts in the isotopic composition, resulting in H-O isotopic data points trending closer to the meteoric water line. The abovementioned fluid characteristics and H-O isotopic compositions are consistent with those observed in most low-sulfidation epithermal gold deposits, such as Harla in China, Buckskin National in New Zealand, and Pongkor in Indonesia [27,28,29]. Previous studies on fluid inclusion bulk compositions in the Axi gold deposit have shown the presence of carbonaceous components in the fluid [5]. This study, for the first time, measured the C isotopic composition of quartz from the main gold mineralization stage (Stage II). As visible in the δ13CPDB vs. δ18OSNOW diagram (Figure 6b), the data points were plotted near the field of igneous carbon origin, suggesting the Dahalajunshan Formation volcanic rocks as the carbon source. The shift in the δ18OSNOW values may have resulted from low-temperature alteration [25,26].

6.2. Sources of Ore-Forming Materials

Sulfides are widely distributed in various types of metal deposits, and, therefore, the sulfur isotope composition is a crucial indicator of the sources of ore-forming materials [32,33,34]. Ohmoto proposed that the sulfur isotopic composition of hydrothermal minerals is a function of the total sulfur isotopic composition, oxygen fugacity, temperature, pH, and ionic strength [35]. In the Axi gold deposit, pyrite is the dominant sulfide mineral and there is a lack of sulfate minerals. Sulfide minerals are present in many types of ore deposits, and their S isotopic compositions are useful to constrain fluid sources, as well as ore-forming processes [36]. Due to the absence of sulfates and oxides, the pyrite and chalcopyrite δ34SH2S values are therefore consistent with the fluid Σδ34S values [37]. The δ34S values of pyrite within the Axi deposit range from 0.50‰ to 4.06‰, which conform to the published in situ δ34S values ranging between −2.6‰ and 5.6‰ [6]. This indicates a single sulfur source for the ore-forming hydrothermal fluids (Figure 7a). On the basis of a comparative analysis of the sulfur isotopic compositions from various natural geological reservoirs, the Axi deposit has a sulfur isotopic composition similar to the mantle and igneous sulfur (Figure 7b). This finding further supports the hypothesis of a deep magmatic sulfur source. Minor variations in the sulfur isotope composition values among different samples may have resulted from changes in physicochemical conditions (such as oxygen fugacity, temperature, and sulfur fugacity). In the regional geological context, it was inferred that the sulfur source likely originated from the volcanic rocks within the Tulasu Basin.
In addition, the tectonic zone also plays an important role in the process of mineralization. The formation of tectonic fissures provides a channel for the transport of ore-bearing hydrothermal fluids, and the external atmospheric precipitation can also enter the mineralization system through the tectonic system, accelerating the dilution of the atmospheric water and the cooling effect. At the same time, the open tectonic space can provide a location for the precipitation of the deposit of the ore body.
The Pb isotopic composition of pyrite further elucidates the source of the ore-forming material [38]. As inferred from the Pb isotope tectonic discrimination diagram (Figure 8), the Axi pyrite samples are predominantly distributed near the orogenic evolution curve, indicating a mixed crust–mantle origin for the Pb source [38]. The Pb isotope pattern for pyrite overlaps with that of the Dahalajunshan Formation volcanic rocks. These findings confirm a genetic link between these rocks and suggest that the volcanic rocks provided the Pb source for mineralization. Additionally, the results of the 1:50,000 regional geological survey revealed that the average Au content in the Dahalajunshan Formation volcanic rocks within the Tulasu region is 110 × 10−9, representing enrichment of nearly 30 times the crustal Au Clarke value of 4 × 10−9 [39]. These findings indicate that these volcanic rocks can supply sufficient Au for mineralization.
In summary, the ore-forming materials in the Axi gold deposit have predominantly originated from Dahalajunshan Formation volcanic rocks, which provide the essential material foundation for element enrichment and mineralization.

6.3. Metallogenic Mechanism of the Axi Deposit

Previous studies have reported varying Re-Os ages for the auriferous Axi gold deposit, including 299 ± 35 Ma, 353 ± 6 Ma, and 332 ± 8 Ma [3,4,5]. The SIMS U-Pb ages of rutile coexisting with pyrite are 306.1 ± 16.9 Ma and 303.8 ± 14.6 Ma [6]. While the exact gold deposit mineralization age remained uncertain, the ore-forming event could be constrained to the period between 350 and 300 Ma, which corresponds to the island arc environment related to the North Tianshan Ocean’s subduction under the Kazakhstan–Yili Plate during the Carboniferous period [9,10,11].
During subduction, the crust partially melted and mixed with mantle components, generating intermediate-to-acidic magmas. These magmas then ascended to shallow crustal levels, forming shallow magma chambers and triggering volcanic activity. As the magma cooled, large amounts of hydrothermal fluids rich in volatiles and ore-forming materials were released. Subsequently, driven by the geothermal gradient, heated meteoric water infiltrated through the fault systems, forming a convective hydrothermal circulation system. During migration, the fluids leached metal elements from the Dahalajunshan Formation volcanic rocks, forming the initial ore-forming element-rich fluids. These fluids migrated upward along fractures or faults, eventually precipitating in the structural fissures associated with volcanic structures. During this process, the ore-forming fluids were accompanied by the dilution effect of meteoric fluids, and the mineralization temperature gradually decreased, which was also an important factor in the precipitation of minerals.
The occurrence of wall–rock alterations, primarily sericitization and carbonatization, indicates that the hydrothermal fluids were neutral-to-weakly alkaline and reducing in nature. This interpretation was further supported by the detection of abundant CH4 in fluid inclusion assemblages [5]. Under such conditions, sulfur predominantly exists as H2S or HS−, and gold migrates in the form of complexes such as Au(HS)2− [40,41]. The FI studies revealed the simultaneous trapping of vapor and liquid phases, indicating that fluid boiling occurred during the mineralization process. The reduced temperature and increased pH (due to the escape of acidic gases) under boiling destabilize metal complexes, leading to precipitation. Additionally, water–rock interactions, such as sericitization, consume H+, causing unstable changes in the fluid pH, which further destabilizes gold complexes and promotes Au precipitation [42]. This process can be described by the equation below:
Au(HS)2− + Fe2+ → Au + FeS2 + H+
Therefore, it was inferred that fluid boiling and water–rock interactions were the primary mechanisms for metal precipitation.

7. Conclusions

(1)
On the basis of FI studies and H-O isotope data, the ore-forming fluids are primarily derived from heated meteoric water, accompanied by fluid boiling.
(2)
Sulfur and lead isotope compositions indicate that the ore-forming materials have most likely originated from Dahalajunshan Formation volcanic rocks.
(3)
The Axi deposit is a representative epithermal gold deposit, in which water–rock interactions and fluid boiling are the primary mechanisms for metal precipitation.

Author Contributions

Methodology, C.C.; Investigation, F.X. and W.S.; Data curation, W.S.; Writing—original draft, F.X.; Writing—review & editing, W.S.; Project administration, C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Major Special Science and Technology Project of the Xinjiang Uygur Autonomous Region [2022A03010-2].

Data Availability Statement

The data used to support the findings of this study are included in the article.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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Minerals 15 00536 g001
Figure 1. (A) Location of the West Tianshan; (B) Location of Tulasu Basin in the West Tianshan; (C) Geological map depicting the Tulasu Basin. Modified after [1].
Figure 1. (A) Location of the West Tianshan; (B) Location of Tulasu Basin in the West Tianshan; (C) Geological map depicting the Tulasu Basin. Modified after [1].
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Figure 2. (a) Geological map depicting the Axi gold deposit; (b) geological cross-section of Ore Body No. 1. Modified after [1].
Figure 2. (a) Geological map depicting the Axi gold deposit; (b) geological cross-section of Ore Body No. 1. Modified after [1].
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Minerals 15 00536 g003
Figure 3. The field and microscopic images of the Axi gold deposit: (a) silicification; (b) propylitization; (c,d) Stage I microcrystalline quartz–pyrite crustiform; (e–g) stage II quartz–sulfide–native gold vein; (h) stage III quartz–carbonate vein; (i) bladed pyrite; (j) pyrite and arsenopyrite assemblage; (k) disseminated euhedral pyrite; (l) euhedral to subhedral pyrite. Abbreviations: Qz—quartz; Cal—calcite; Py—pyrite; Apy—arsenopyrite.
Figure 3. The field and microscopic images of the Axi gold deposit: (a) silicification; (b) propylitization; (c,d) Stage I microcrystalline quartz–pyrite crustiform; (e–g) stage II quartz–sulfide–native gold vein; (h) stage III quartz–carbonate vein; (i) bladed pyrite; (j) pyrite and arsenopyrite assemblage; (k) disseminated euhedral pyrite; (l) euhedral to subhedral pyrite. Abbreviations: Qz—quartz; Cal—calcite; Py—pyrite; Apy—arsenopyrite.
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Figure 4. Micropetrographic characteristics of the fluid inclusions. (a,b) Stage I: liquid-rich, vapor-rich, and pure vapor FIs; (c,d) Stage II: liquid-rich, vapor-rich, and pure liquid FIs; (e,f) Stage III: liquid-rich and pure liquid FIs. Abbreviations: L—liquid phase; V—vapor phase.
Figure 4. Micropetrographic characteristics of the fluid inclusions. (a,b) Stage I: liquid-rich, vapor-rich, and pure vapor FIs; (c,d) Stage II: liquid-rich, vapor-rich, and pure liquid FIs; (e,f) Stage III: liquid-rich and pure liquid FIs. Abbreviations: L—liquid phase; V—vapor phase.
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Figure 5. Histograms showing the homogenization temperatures (a–c) and salinities (d–f) of the fluid inclusions from Stage I (a,d), Stage II (b,e), and Stage III (c,f).
Figure 5. Histograms showing the homogenization temperatures (a–c) and salinities (d–f) of the fluid inclusions from Stage I (a,d), Stage II (b,e), and Stage III (c,f).
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Figure 6. (a) The δDfluid vs. δ18Ofluid plot and (b) δCPDB vs. δ18OSNOW plot for the fluid inclusions in the Axi gold deposit. Data for important geological categories were sourced from [30,31].
Figure 6. (a) The δDfluid vs. δ18Ofluid plot and (b) δCPDB vs. δ18OSNOW plot for the fluid inclusions in the Axi gold deposit. Data for important geological categories were sourced from [30,31].
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Figure 7. (a) Histogram and (b) comparison of pyrite δ34S values between the Axi gold deposit and other key geological reservoirs. Data for important geological categories were sourced from [29,31].
Figure 7. (a) Histogram and (b) comparison of pyrite δ34S values between the Axi gold deposit and other key geological reservoirs. Data for important geological categories were sourced from [29,31].
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Figure 8. (a) 206Pb/204Pb vs. 207Pb/204Pb and (b) 206Pb/204Pb vs. 208Pb/204Pb tectonic discrimination diagrams. Data for evolution curves were sourced from [32]. Abbreviations: UC—upper crust; LC—lower crust; O—orogenic; M—mantle.
Figure 8. (a) 206Pb/204Pb vs. 207Pb/204Pb and (b) 206Pb/204Pb vs. 208Pb/204Pb tectonic discrimination diagrams. Data for evolution curves were sourced from [32]. Abbreviations: UC—upper crust; LC—lower crust; O—orogenic; M—mantle.
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Table 1. Microthermometric data for the fluid inclusions in the Axi gold deposit.
Table 1. Microthermometric data for the fluid inclusions in the Axi gold deposit.
StageFI TypeTmelt-ice (°C)Salinity
(NaCl wt.%)		Th (°C)Peak
Th (°C)
ILiquid-rich–3.1 to –1.22.1–5.1178–218200
Vapor-rich 192–225
IILiquid-rich–2.8 to –0.81.4–4.6151–193170
Vapor-rich 172–190
IIILiquid-rich–2.2 to–0.30.5–3.7123–161140
Table 2. H-O-C isotope data for quartz from the Axi gold deposit.
Table 2. H-O-C isotope data for quartz from the Axi gold deposit.
Stageδ18Oquartz (‰)δDfluid (‰)Th (°C)δ18Ofluid (‰)δCfluid (‰)
I9.26−107.9200−0.4—
10.06−110.2−2.4
11.20−117.2−1.6
14.10−112.4−0.5
II10.45−123.01700.3−11.48
9.19−127.3−3.4−12.28
11.64−127.2−4.6−12.22
9.15−107.2−2.2−11.76
III9.56−114.8140−7.3—
10.14−120.8−6.9
10.36−129.6−6.3
9.26−129.0−6.1
Table 3. S-Pb isotope data for pyrite from the Axi gold deposit.
Table 3. S-Pb isotope data for pyrite from the Axi gold deposit.
Mineralδ34Spyrite (‰)206Pb/204Pb207Pb/204Pb208Pb/204Pb
Pyrite2.4318.40415.63238.409
Pyrite2.4918.55815.64038.544
Pyrite4.0618.33815.61738.284
Pyrite2.8918.35315.61738.325
Pyrite1.6318.37315.61738.356
Pyrite2.0518.33215.61738.293
Pyrite0.5018.45415.61738.482
Pyrite2.3418.47715.61738.504
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Xia, F.; Chen, C.; Sun, W. The Mineralization Mechanism of the Axi Gold Deposit in West Tianshan, NW China: Insights from Fluid Inclusion and Multi-Isotope Analyses. Minerals 2025, 15, 536. https://doi.org/10.3390/min15050536

AMA Style

Xia F, Chen C, Sun W. The Mineralization Mechanism of the Axi Gold Deposit in West Tianshan, NW China: Insights from Fluid Inclusion and Multi-Isotope Analyses. Minerals. 2025; 15(5):536. https://doi.org/10.3390/min15050536

Chicago/Turabian Style

Xia, Fang, Chuan Chen, and Weidong Sun. 2025. "The Mineralization Mechanism of the Axi Gold Deposit in West Tianshan, NW China: Insights from Fluid Inclusion and Multi-Isotope Analyses" Minerals 15, no. 5: 536. https://doi.org/10.3390/min15050536

APA Style

Xia, F., Chen, C., & Sun, W. (2025). The Mineralization Mechanism of the Axi Gold Deposit in West Tianshan, NW China: Insights from Fluid Inclusion and Multi-Isotope Analyses. Minerals, 15(5), 536. https://doi.org/10.3390/min15050536

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