Genesis of W Mineralization in the Caledonian Granite Porphyry of the Chuankou W Deposit, South China: Insights from Fluid Inclusions and C–H–O–S Isotopes

Abstract

The Chuankou deposit is a super-large W deposit formed during the Indosinian collision event in South China, and its mineralization is suggested to be related to the Indosinian muscovite granite. However, two types of W mineralizations were discovered in the Caledonian granite porphyry in the Chuankou W deposit: disseminated scheelite and quartz-wolframite-scheelite vein mineralizations. The genesis of W mineralization in the Caledonian granite porphyry is not yet clear. This paper focuses on fluid microthermometry and stable isotopes (C, H, O, S) analysis of the quartz and scheelite in the ores in the Caledonian granite porphyry in the Chuankou W deposit. The aims are to determine the nature and evolution of the ore-forming fluids, the origin of the ore-forming materials involved in the two types of W mineralization in the Caledonian granite porphyry, and to provide a detailed discussion of the deposit’s genesis. Microthermometry results of fluid inclusions with scheelite and quartz from two stages show that the average homogenization temperature in the quartz-veins within the Caledonian granite porphyry is 248 °C, and the average salinity is 6.31 wt.% NaCl eq (n = 85), the average homogenization temperature in the quartz-veins within the slate is 219 °C, and the average salinity is 5.57 wt.% NaCl eq (n = 49). The ore-forming fluids experienced an evolution from high temperature and high salinity to low temperature and low salinity. Sulfur isotope compositions show that the δ³⁴S values of pyrite and arsenopyrite in the quartz-veins within the Caledonian granite porphyry are 2.06 to 3.28‰ and −0.38 to 0.21‰, respectively, and the δ³⁴S value of pyrite in the quartz-veins within the slate is −1.72 to 0.47‰. The δ³⁴S values of each stage are close to 0‰, indicating that the origin of sulfur mainly from magma. The H-O isotope compositions of the quartz indicate that the ore-forming fluid was primarily magmatic water. The low δ¹⁸O_H2O values (1.74 to 1.58‰) are influenced by fluid–rock interactions or the incorporation of atmospheric precipitation. The carbon isotopes (δ¹³C = −9.5 to 8.3‰) indicate a magmatic origin, but the C isotopes of quartz in the quartz-veins within the slate shift toward sedimentary rocks, reflecting the incorporation of rock components in the late mineralization period. These isotopic differences indicate that the fluid–rock interaction gradually strengthened during fluid evolution.

1. Introduction

Tungsten, a strategic metal, has extensive applications in defense, aerospace, electronics, and other fields [1]. According to statistics from the United States Geological Survey 2024, global W reserves and production will be approximately 4.6 million tons and 81,000 tons, respectively. China’s W reserves and production will be approximately 2.4 million tons and 67,000 tons, respectively, accounting for 52% and 81% of the global. With the continuous advancement of international industrialization, China’s industrialization process is also accelerating, leading to a continuous increase in demand for W and other mineral products. However, as a non-renewable metal resource, W faces an increasingly significant supply–demand imbalance, intensifying competition for resources and causing frequent price fluctuations. To ensure the sustainable development of economy, it is necessary to increase resource exploration efforts and ensure the stable progress of industrialization.

The Chuankou deposit, located in Hengnan County, Hunan Province, on the northern side of the Nanling metallogenic belt, is a super-large W deposit dating back to the Indosinian period [2,3]. W reserves are substantial, with proven WO₃ reserves exceeding 205,000 tons at an average grade of 0.42% [4]. The Chuankou ore field, encompassing the Chuankou, also includes more than a dozen large, medium, and small W deposits, including Sanjiaotan, Yaomuling, Huanglong, and Maowan. Previous studies have examined the geological characteristics, geochemistry, geochronology, and mineralization mechanisms of the Chuankou intrusion and ore deposits. However, compared to other large and super-large W deposits in South China [4], the characteristics and evolution of the ore-forming fluids in the Chuankou W deposit remain unclear, and controversy surrounding the deposit’s genesis has hampered prospecting and exploration.

Recently, during field geological surveys at the Yangling’ao deposit, our team discovered new mineralized granodiorite-porphyry dikes. W mineralization within granodiorite porphyry occurs in two forms: one is quartz veins containing scheelite and wolframite within the granodiorite porphyry, and the other is disseminated scheelite mineralization directly within the granodiorite porphyry. Preliminary studies indicate that the granodiorite porphyry formed during the Caledonian Period [5], but its origin and relationship to W mineralization remain unclear, hindering prospecting in the area.

This paper examines the newly discovered ore-bearing granodiorite porphyry in the Yanglin’ao W deposit within the Chuankou W ore field. Building on previous research, this paper focuses on fluid microthermometry and stable isotope (C, H, O, S) analysis of the quartz and scheelite in the newly discovered granodiorite porphyry quartz in the Yanglin’ao W deposit. The aims are to determine the nature and evolution of the ore-forming fluids, the origin of the ore-forming materials involved in the two types of W mineralization in the Caledonian granite porphyry, and to provide a detailed discussion of the deposit’s genesis.

2. Geological Background

2.1. Regional Geology

The Nanling Range is located in the northwest of the Cathaysia Block in South China (Figure 1a). Following the amalgamation of the Cathaysia Block with the Yangtze Craton along the Qinzhou–Hangzhou tectonic belt between 1100 and 830 million years ago, the Nanling Range underwent significant remodeling during tectonothermal events during the Caledonian, Indosinian, and Yanshanian periods [6,7]. These events led to the formation of a regional basement composed of Neoproterozoic metavolcanic and metasedimentary rocks, overlying a Devonian-Jurassic sedimentary sequence encompassing marine and terrestrial clastic and carbonate lithologies [8,9,10]. Multiple phases of magmatism formed numerous granites [11,12,13], which are believed to have played a key role in the accumulation of W and Sn, making the Nanling Range the largest tungsten reservoir on Earth [14].

The Nanling Range is a world-renowned W-Sn mineralization province and a significant source of rare metals such as niobium, tantalum, lithium, beryllium, and rubidium [8]. W-Sn and rare metal mineralization in the Nanling Range is primarily associated with Yanshanian granites, with smaller amounts associated with Indosinian and Caledonian granites [8,15,16]. The Caledonian tungsten mineralization is primarily concentrated in the Miaoershan–Yuechengling areas, primarily associated with S-type granites and minor I-type granites [17,18]. The Indosinian tungsten mineralization is primarily concentrated in the Hengyang Basin in central Hunan [19]. The Yanshanian tungsten mineralization mainly concentrated in the central-eastern part of the Nanling Range [20,21]. The Chuankou W orefield is one of the largest and most representative Indosinian orefields in the region. The Caledonian W deposits are differentiated and their mineralization prospects are not been estimated [14,15,22].

2.2. Deposit Geology

The Chuankou orefield is located on the northern side of the Nanling metallogenic belt, approximately 40 km east of Hengyang City, Hunan Province, at the junction of the Changde–Anren basement fault and the Chaling–Chenzhou fault [23]. Neoproterozoic Banxi Group (Pt₃bn) shallowly metamorphosed clastic rocks and argillaceous slates are exposed in the center of the orefield. Devonian mudstones and metamorphosed sandstones, Carboniferous clastic rocks, and minor Quaternary strata unconformably overlie the Banxi Group slates and are exposed at the orefield margin (Figure 2a) [24].

Multi-stage tectonic evidence is evident within the orefield, with the basic structural framework being a near north–south uplift [25]. Folds in the area primarily develop in two groups: EW and NS. The EW folds are the product of Caledonian tectonic movement. After later transformation, their axes shifted to NEE. These are relatively small, mostly concealed anticlines, and develop within the metamorphic rocks of the Banxi Group [26]. The NS folds formed later, developing contemporaneously with the Chuankou Uplift, and are larger in scale [26]. Faults within the ore field develop in two groups: north-northwest and northeast (Figure 2b). NNW-trending faults are the primary ore-guiding and ore-controlling structures in the area. They are often greater than 1 km in length, with widths ranging from tens to several dozen meters. They dip southwest with a wide range of inclinations, ranging from 45° to 78°. NE-trending faults are smaller in scale and have a highly variable dip, ranging from 70° to 80°. The F₂₄ fault is the most important among many faults. It is located in the contact zone between the Neoproterozoic Banxi Group slate and the Chuankou two-mica granite. It is a normal fault formed by compression and shear during the W mineralization process and plays a key role in controlling the migration and enrichment of ore-forming fluids (Figure 2c).

The Chuankou orefield has experienced intense magmatic activity, occurring from the Caledonian to the Yanshanian periods. Late Caledonian granite-diorite porphyry, occurring as dikes within Neoproterozoic slate (not seen at the surface), is a newly discovered intrusion and the subject of this study. Indosinian granite (210–240 Ma) is the most extensive intrusion in the orefield and the host rock for W mineralization. It includes biotite granite (Jiangjunmiao intrusion), two-mica granite, and muscovite granite, along with tourmaline granite, occurring in batholithic formations. Yanshanian granite (~135 Ma) is relatively rare, occurring as granite-porphyry dikes (Figure 2a). Due to the low level of exploration, the Caledonian granite porphyry has not been found in the previous exploration process, so its relationship with tungsten mineralization has not been studied in depth.

The ore bodies of the Yanglin’ao deposit are spatially distributed in a north-northeast direction (Figure 2c). According to the field occurrence characteristics of the ore bodies, they can be divided into two categories: (1) The ore-bearing host rock is Caledonian granodiorite porphyry (Figure 3a–c). (2) The ore-bearing host rock is Neoproterozoic Banxi Group slate and Devonian sandstone and mudstone [5]. And the first one can be further divided into two types: ① Disseminated scheelite mineralization: Scheelite is produced in the granodiorite porphyry in a disseminated manner. The scheelite particles are small and the grade is low (Figure 3d–f). The reserves of disseminated scheelite mineralization is 60,000 t. ② Quartz-wolframite-scheelite vein mineralizations: The quartz veins containing scheelite and wolframite are about 5~30 cm wide and are found in Caledonian granodiorite porphyry (Figure 3d–f). The reserves of vein type mineralization is 120,000 t. The vein generally strikes north-east—north-north-east (NE-NNE), extending over an area of approximately 1.3 km in length and 0.5 km in width. Individual orebodies range in thickness from 35 to 95 m, with an average dip of 36°. The ore grade (WO₃) ranges from 0.41 to 0.46 wt.%, and the orebodies vary significantly in size, with approximately 15 large orebodies and 40 small orebodies [4,27]. In terms of spatial distribution, this deposit differs significantly from the typical “five-story building + basement” mineralization model of Yanshanian quartz vein-type W deposits in the Nanling region [28]: Vertically, the upper orebodies are thicker and more stable, while the lower orebodies are thinner and more variable. Along strike, the orebodies are stable in the middle and gradually thin toward the ends, until they taper off. Both wolframite and scheelite in the Yanglin’ao deposit occur primarily in quartz veins, occurring within Neoproterozoic Banxi Group slates and Caledonian granodiorite porphyry (Figure 3a–c). Some scheelite occurs directly as disseminated within the Caledonian granodiorite porphyry veins (Figure 3d).

The ore-bearing quartz veins are often milky white with a strong oily luster. W-bearing minerals include scheelite and wolframite. Metallic sulfides are primarily euhedral pyrite and arsenopyrite, often interspersed with wolframite and scheelite formed in the early stages of dissolution. Small amounts of pyrrhotite, chalcopyrite, sphalerite, and galena are also observed. Gangue minerals primarily include quartz, muscovite, sericite, apatite, and fluorite. Muscovite coexists with both wolframite and scheelite, forming needle- and flake-like structures. Some muscovite can also be found within the scheelite grains. Apatite is long, columnar, and granular, approximately 50 to 400 μm in diameter. It is often coexisted with scheelite and, to a lesser extent, wolframite. Fluorite appears purple under natural light and is completely extinct under crossed polarizers. Mao et al. (2024) conducted a detailed description of the characteristics of the mineral [5].

The Yanglin’ao deposit exhibits a variety of wall rock alteration types, including greisenization, silicification, sericite, and carbonatization. Greisenization is primarily composed of muscovite or a quartz-muscovite combination, distributed along the sides of quartz veins in granite [29], and is a key prospecting indicator for this type of W deposit. Silicification is manifested by a strong enrichment of silica in the slates on both sides of the quartz veins, significantly increasing the rock hardness. This is caused by the replacement or filling of the wall rock with silica (SiO₂) from hydrothermal fluids, which increases the silica content of the original rock.

3. Sampling and Analytical Methods

3.1. Sampling

In this study, six granodiorite porphyry samples (22CK09, 22CK2-6, 23CK01, 23CK02, 23CK03, and 23CK04) were collected from the tunnel of the Yanglin’ao deposit for geochemical analysis. Sample 22CK09 was collected from the middle between line 02 and line 03 in the middle section of 5, sample 22CK2-06 was collected from 30 m south of line 468 and line 406, and the other samples were collected from the surface.

3.2. Fluid Inclusion and Raman Spectroscopy Analysis

Microthermometry of fluid inclusions was conducted at the Key Laboratory of Nonferrous Metal Ore Deposit Prediction and Environmental Monitoring (Ministry of Education), Central South University, Changsha, China. The cooling-heating stage used for temperature measurement was the Linkam THMS-600 model. The temperature range of the measurements was −196 to 600 °C, with an accuracy of ±1 °C. Prior to testing, the stage was calibrated using fluid inclusion standards encapsulating CO₂ and pure H₂O, with calibration temperatures at −56.6 °C, 0.0 °C, and 374.1 °C.

The composition of individual fluid inclusions was determined by laser Raman spectroscopy at the State Key Laboratory for the Prevention and Control of Heavy Metal Pollution Engineering, Central South University. The instrument used was the Renishaw confocal micro Raman spectrometer. The laser wavelength was 514.5 nm, with a power of 40 mW. The laser beam spot diameter was 2 μm, and the spectral resolution was 1 cm⁻¹.

3.3. C–H–O Isotope Analysis

Hydrogen, oxygen, and carbon isotope analyses were completed at Beijing Kehui Testing Technology Co., Ltd. (Beijing, China) The instruments used were the Thermo Fisher Scientific (Waltham, MA, USA) MAT-253 Plus mass spectrometer. Prior to isotope testing, the samples were crushed and cleaned, then carefully handpicked under a binocular microscope to select target minerals, and ground to below 200 mesh. Using the traditional BrF5 method, oxygen gas was generated, and δ¹⁸O was measured with the MAT-253 Plus (Finnigan, Germany). The water released from the fluid inclusions was instantly reduced to H₂ by reaction with a graphite carbon target, and then carried into the mass spectrometer using high-purity helium for δD measurement. The results were calibrated against Vienna Standard Mean Ocean Water (V-SMOW), with measurement precisions of 0.05‰ for δ¹⁸O and 1‰ for δD. The CO₂ gas produced from the reaction of the sample with phosphoric acid was separated from other impurity gases by a 70 °C fused silica capillary column and then entered a gas isotope ratio mass spectrometer for carbon isotope analysis. The results were calibrated against the international standard PDB (Pee Dee Belemnite), with a measurement precision of less than ±0.2%.

3.4. S Isotope Analysis

In situ sulfur isotope analysis of sulfides was conducted at the Mineral Geochemistry Laboratory of the State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences (Wuhan, China). The laser ablation system was manufactured by Resonetics, model Resonetics-S155. An ArF excimer laser generator produced 193 nm deep ultraviolet laser beams, which were homogenized and focused onto the surface of the sulfide minerals. The laser spot diameter was typically set at 33 μm, with an ablation frequency of 5 Hz, and ablation was performed for 40 s. High-purity helium was used as the carrier gas, mixed with argon and a small amount of N₂, then introduced into the mass spectrometer. The multi-collector inductively coupled plasma mass spectrometer was produced by Nu Instruments, model Nu Plasma II. The ³⁴S/³²S isotope ratio was directly measured on the standards and sample points, and the δ³⁴S values were calculated using an external standard calibration method (SSB method). The standard used was the laboratory internal standard, WS-1 pyrite.

4. Results

4.1. Fluid Inclusion Petrography

The metasomatic scheelite (Sch1) grains in the Yanglin’ao granite porphyry are small and relatively scarce, and no fluid inclusions suitable for thermometry have been identified. Quartz veins hosted in the granite porphyry and slate contain coarse-grained quartz and scheelite, both of which are characterized by abundant fluid inclusions. Therefore, this study selected fluid inclusions from the above two types scheelite, as well as those associated with coexisting quartz, to perform microthermometry in order to elucidate the evolution process of mineralizing fluids.

Quartz veins in the granite porphyry (Stage II), containing quartz (Qtz2) and scheelite (Sch2), host numerous fluid inclusions with larger sizes. The fluid inclusions range from 2 to 10 μm, with over 95% being two-phase gas–liquid inclusions (L + V), predominantly with gas–liquid ratios between 10% and 30%. A small number of inclusions are two-phase with liquid-dominant content or three-phase inclusions containing daughter minerals (Figure 4a–f). The fluid inclusions within the scheelite are slightly larger than those in quartz (2–15 μm) and contain more daughter minerals. Some inclusions show coexisting CO₂ in the vapor phase and aqueous liquid. The morphology is mainly negative crystal, elliptical, or irregularly shaped, distributed as isolated, linear, or clustered formations. Isolated inclusions are relatively large, while those distributed in linear or cluster forms tend to be smaller, possibly representing secondary fluid inclusions formed during later hydrothermal processes.

Fluid inclusions in quartz veins within slate (Stage III) contain scheelite and quartz with smaller sizes (2 to 8 μm) compared to Stage II, and the fluid inclusions in scheelite exhibit poorer clarity. Most inclusions are rounded, with few containing daughter minerals (Figure 4g–i). For this microthermometric analysis, only inclusions with relatively regular shape and dispersed or clustered distribution were chosen to avoid the influence of later hydrothermal modifications or secondary inclusions.

4.2. Microthermometry and Raman Spectroscopy

Microthermometry was conducted on regular large-particle fluid inclusions in scheelite and quartz from two stages. The experiments were carried out at the Key Laboratory of Mineralization Prediction and Environmental Monitoring (Ministry of Education) at Central South University, Changsha, China. Details of the methodology are provided in Attachment 1. A total of 134 valid datasets were obtained: during Stage II (quartz vein-type scheelite in the granite porphyry), 61 inclusions in scheelite and 24 in quartz were measured; during Stage III (scheelite in slate within the quartz vein), 15 inclusions in scheelite and 34 in quartz were measured. The calculation results for each stage are listed in

Table 1. Statistics of fluid inclusion microthermometry results of the Yanglin’ao tungsten deposit in Chuankou.

Stage	Mineral (Number)	Size (μm)	Tm, ice (°C)	Th, tot (°C)	Salinity (wt.% eqv)
II	Sch2 (61)	2~15	−7.1~−1.4	172~326 (avg. 250)	2.41~10.61 (avg. 6.26)
	Qtz2 (24)	2~10	−5.4~−1.5	198~313 (avg. 242)	2.57~8.41 (avg. 6.70)
III	Sch3 (15)	2~8	−5.8~−1.6	165~285 (avg. 223)	2.74~8.95 (avg. 5.56)
	Qtz3 (34)	2~8	−5.1~−1.9	163~289 (avg. 218)	3.23~9.60 (avg. 5.60)

. The distribution histograms of homogenization temperature and salinity are shown in Figure 5.

The fully homogeneous temperature and salinity of inclusions in Stage II are 248 °C and 6.31 wt.% NaCl equivalent (n = 85). Specifically, the average homogenization temperature and salinity for inclusions in scheelite are 250 °C and 6.26 wt.% NaCl equivalent (n = 61), while for quartz, they are 242 °C and 6.70 wt.% NaCl equivalent (n = 24). In Stage III, the fully homogeneous temperature and average salinity are 219 °C and 5.57 wt.% NaCl equivalent (n = 49). In scheelite, the average values are 223 °C and 5.56 wt.% NaCl equivalent (n = 15), and in quartz, 218 °C and 5.60 wt.% NaCl equivalent (n = 34). In both stages, the homogenization temperature in scheelite fluid inclusions is generally 5–10 °C higher than that in quartz inclusions, although the salinity values are similar. Overall, the homogenization temperatures and salinities in Stage II are slightly higher than those in Stage III.

Laser Raman spectroscopy was performed on regular-shaped, larger fluid inclusions in scheelite from both stages. The analysis was conducted at the Center for Heavy Metal Pollution Prevention and Control Engineering, Central South University. The detailed method is available in Attachment 1. The Raman spectral features (Figure 6) show that the vapor phase components within the fluid inclusions from the Yanglin’ao deposits are mainly H₂O, H₂S, and CO. Gases such as CH₄, CO₂, N₂, and H₂ were not detected, possibly due to their low concentration in the vapor phase.

Previous Raman spectral analyses of quartz veins in slate from the Kawaguchi Yanglin’ao deposit revealed that the gas–liquid two-phase inclusions mainly contained H₂O, with only small amounts of CO₂ and CH₄ detected [29,30]. This is consistent with the findings of this study.

In the quartz veins within the granite porphyry (Stage II), in addition to H₂O vapor, CO gas was also detected in the scheelite fluid inclusions. In the quartz veins of slate (Stage III), H₂S gas was observed in the fluid inclusions. These findings suggest that the mineralization environment may have been reducing.

4.3. C–H–O Isotopic Compositions

This study collected ore samples containing quartz from the Kawaguchi deposit for quartz C-H-O isotope analysis. The test results are shown in

Table 2. Hydrogen, oxygen, and carbon isotope characteristics of the Yanglin’ao tungsten deposit in Chuankou.

Sample	Mineral	δDV-SMOW‰	δ18OV-PDB‰	δOV-SMOW‰	δ18OH2O‰	δ13CV-PDB‰
22CK2-7	quartz	−53.5	−19.9	10.3	1.74	−9.5
22CK02	quartz	−53.1	−18.2	12.2	1.58	−8.3

. The analyses were completed at Beijing Kehui Testing Technology Co., Ltd. (Beijing, China). The δD values in quartz can represent the hydrogen isotope composition of the hydrothermal fluid during mineralization, while the δ¹⁸O values in quartz need to be combined with formation temperature. Using the quartz-water oxygen isotope equilibrium fractionation equation, the oxygen isotope composition of the hydrothermal fluid can be calculated. The formula is δ¹⁸O_Q − δ¹⁸O_H2O ≈ 3.38 × 10⁶/T² − 3.40 [31].

This equation is applicable for quartz formation temperatures between 200 °C and 500 °C, with T being the thermodynamic temperature in Kelvin. Using the calculated formation temperatures of 248 °C and 219 °C from the fluid inclusions, the oxygen isotope composition of the fluids was estimated. The results are presented in Table 2. The data indicate that the mineral-bearing quartz veins in the granite porphyry have δD, δ¹⁸O_V-SMOW, δ¹⁸O_V-PDB, δD_H2O, and δ¹³C_V-PDB values of −53.5‰, 10.3‰, −19.9‰, 1.25‰, and −9.5‰, respectively, while those in the slate-hosted quartz veins exhibit values of −53.1‰, 12.2‰, −18.2‰, 1.64‰, and −8.3‰, respectively. The slight differences in isotope compositions between the two stages may indicate the contribution of atmospheric precipitation or variations in the surrounding rock components to the mineralizing fluids.

Hydrogen and oxygen isotope results for the Kawaguchi Yanglin’ao W deposit are shown in Figure 7a. In both stages, the δ¹⁸O_H2O values in the mineralizing solutions (1.74‰ and 1.58‰) are lower than the δ¹⁸O_H2O values of magmatic water (5.5 to 10‰), indicating a shift toward the meteoric water line. However, considering previous studies and this research, it is believed that the main mineralizing fluid is magmatic in origin. The study suggests that fluid-rock interaction and atmospheric precipitation can influence the hydrogen and oxygen isotope compositions of the mineralization fluids [32,33,34], which also explains the relatively low δ¹⁸O_H2O values in the mineralizing fluids of the Kawaguchi Yanglin’ao W deposit.

Regarding carbon isotopes, the results are shown in Figure 7b. The quartz C-isotope values in the quartz veins within the granite porphyry mainly fall within the range typical of granite. In contrast, the C-isotope values in the slate-hosted quartz veins show a shift toward mixing with sedimentary rock signatures. This suggests that, while the primary source of carbon in the two stages is magmatic, the mineralizing fluids associated with the quartz vein-type mineralization in the slate have contributions from the surrounding strata.

4.4. S Isotopic Compositions

In situ sulfur isotope analysis was conducted on pyrite and pyrrhotite mineralizations coexisting with scheelite in quartz veins within the granite porphyry and slate of the Kawaguchi Yanglin’ao W deposit (

Table 3. In situ sulfur isotope composition of sulfides in the Yanglin’ao tungsten deposit in Chuankou.

Sample	Mineral	δ34S
22CK2-7-1-1	Apy1	−0.38
22CK2-7-1-2	Apy1	0.10
22CK2-7-1-3	Apy1	0.10
22CK2-7-1-4	Apy1	0.21
22CK2-7-2-1	Py1	2.06
22CK2-7-2-2	Py1	3.28
22CK2-7-2-3	Py1	2.96
22CK2-7-2-4	Py1	2.45

). The sulfur isotope analyses were performed at the Mineral Geochemistry Laboratory of the Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences (Wuhan, China).

The δ³⁴S values of pyrite and pyrrhotite from the quartz veins within the granite porphyry ranged from 2.06 to 3.28‰ and −0.38 to 0.21‰, respectively (Figure 8). In the quartz veins within the slate, the δ³⁴S values of pyrite ranged from −1.72 to 0.47‰. The δ³⁴S values across different stages show narrow ranges, mostly within the magmatic sulfur isotopic domain (−5 to +5‰) [35], indicating that the mineralizing sulfur primarily originated from the magmatic body.

5. Discussion

5.1. Nature and Evolution of the Ore-Forming Fluids

Fluid inclusions are important indicators of the physical and chemical conditions of mineralization. Parameters such as homogenization temperature and salinity reflect the temperature and fluid concentration during magmatic-hydrothermal evolution. Microthermometry of fluid inclusions in scheelite and quartz from two types of ores revealed that the average homogenization temperature of W mineralization in quartz veins within granodiorite porphyry was 248 °C and the average salinity was 6.31 wt.% NaCl eq. (n = 85). The average homogenization temperature of W mineralization in quartz veins within slate was 219 °C and the average salinity was 5.57 wt.% NaCl eq. (n = 49). This indicates that the temperature and salinity of the ore-forming fluids gradually decreased with their evolution (Figure 9 and Table 1).

5.2. Origin of the Ore-Forming Materials and Fluids

Carbon, hydrogen, and oxygen isotope tracing is an important geochemical tool that can effectively reveal the source of ore-forming fluids [31]. Isotopic studies of the Yanglin’ao W deposit in Chuankou suggest that the ore-forming fluids were primarily magmatic in origin, but underwent significant fluid-rock interaction and mixing. Hydrogen and oxygen isotope analysis (Figure 7a) reveals that the δ¹⁸O_H2O values of hydrothermal fluids from both mineralization stages (1.74‰ and 1.58‰) are lower than those of magmatic water (5.5 to 10‰), indicating a shift toward the atmospheric precipitation line. While this phenomenon could indicate the incorporation of atmospheric water, combined with carbon isotope and fluid inclusion data, a purely precipitation-driven mechanism can be ruled out. As shown in Figure 7b, the carbon isotope compositions of quartz in the ore-bearing quartz veins of the granodiorite porphyry fall into the granite, while the carbon isotope compositions of the slate shift toward sedimentary contamination. This suggests that the carbon in both stages was primarily derived from magma, but that the ore-forming fluids of the quartz vein-type W mineralization in the slate also contributed to the surrounding rock formations.

Previous studies have shown that factors such as magma degassing [32], fluid-rock interaction [33,34], the addition of high-altitude atmospheric precipitation [35], and fluid boiling can all influence the H and O isotopic compositions of ore-forming fluids, resulting in anomalously low δ¹⁸O_H2O values. Chuankou is located in the Hengyang Basin of South China, at a relatively low altitude, and it is believed that the low δ¹⁸O_H2O values are not solely due to atmospheric precipitation. Combining the carbon isotope signatures of the two mineralization stages, along with H₂S gaseous components detected in scheelite fluid inclusions in ore-bearing quartz veins within the slate (possibly a result of interaction between the ore-forming fluid and bioclastic host rock), we infer that the low δ¹⁸O_H2O values in the ore-forming fluids of the Yanglin’ao W deposit in Chuankou are primarily due to the combined effects of fluid-host rock interaction and fluid boiling.

The S values of quartz vein-type pyrite and arsenopyrite within the granodiorite porphyry range from 2.06 to 3.28‰ and −0.38 to 0.21‰, respectively, while the δ³⁴S value of quartz vein-type pyrite within the slate ranges from −1.72 to 0.47‰. The narrow range of δ³⁴S values across the stages, averaging close to 0‰ (Figure 8), suggests a magmatic source of sulfur [36].

5.3. Genesis of W Mineralization in the Caledonian Granite Porphyry

Disseminated and quartz vein-type mineralization within granodiorite porphyry is primarily derived from magmatic-hydrothermal systems, with some contributions from the host rock (granodiorite porphyry). The high Sr content (average 408 μg/g) and Sr/Mo ratio (average 530) in the scheelite (Sch1 and Sch2) are closely related to Sr released by plagioclase alteration in the Caledonian granodiorite porphyry [5], suggesting interaction between the ore-forming fluid and the host rock, which resulted in the addition of components from the host rock to the ore-forming fluid. Furthermore, the rare earth element (REE) distribution of the wolframite (Wol1) exhibits an up-arching distribution pattern with MREE enrichment and a tetrad effect, differing from typical quartz vein-type wolframite in the Nanling region and suggesting that its ore-forming fluids (materials) are distinct [5]. Early disseminated scheelite (Sch1) formed in a high-temperature, reducing fluid environment. Its high positive Eu anomaly (Eu_N/Eu*_N = 1.62) indicates that Eu²⁺ preferentially occupied the Ca site in scheelite under reducing conditions [37]. With fluid evolution, the mineralization temperature of quartz vein-type mineralization (Sch2 and Wol1) dropped to a moderately high temperature (average of 248 °C), and the salinity decreased slightly (average of 5.57 wt.% NaCl eq.). The Mo content of Sch2 decreased significantly (maximum 1.18 μg/g), and the Eu anomaly weakened (average 1.06), indicating an increase in fluid oxygen fugacity. Concurrently, the REE content increased (average of 607 μg/g in Sch2), believed to be the result of enhanced water-rock interaction between the fluid and the granodiorite porphyry.

Quartz vein-type mineralization in Neoproterozoic slate (Sch3 and Wol2) is primarily derived from magma, but it is also believed to have been influenced by the addition of wall rock materials and meteoric water [5]. Carbon isotopes reveal that the δ¹³C of quartz veins in the slate is shifted toward the sedimentary rocks, indicating that carbon from Pleizoic strata (such as the slate) was incorporated into the mineralizing fluids (Figure 7b). The Sr content (average of 296 μg/g) of scheelite (Sch3) is intermediate between that of Sch1 and Sch2, and the Sr/Mo ratio (average 35.4) is significantly lower, suggesting an enrichment of Mo in the fluids and a reduction in the Sr contribution of the rock mass. Mineralization in the slate formed in a medium-low temperature (average of 219 °C) and highly oxidizing environment. The Eu anomaly (average of 1.30) of scheelite (Sch3) is significantly lower than that of Sch1, and the Mo content (8.04 to 8.67 μg/g) is moderate, reflecting an increased Eu³⁺ fraction and enhanced Mo activity under oxidizing conditions. The fluid salinity (5.57 wt.% NaCl eq.) decreased in tandem with temperature, possibly due to mixing with meteoric water or fluid-rock interactions. The rare earth element content (average 174 μg/g in Sch3) is lower than that in Sch2, indicating that REEs were partially consumed during the early stages of mineralization. The precipitation of mineralization during this stage was primarily controlled by fluid-rock interactions. Oxidizing conditions resulted in Eu⁺ being predominantly present, while Mo⁶⁺ was more readily incorporated into scheelite. Further decreases in temperature and salinity weakened the stability of metal complexes, leading to rapid precipitation of REEs and W. Furthermore, the release of elements such as Fe and Mn from the host rock likely promoted the formation of wolframite (WoL2), whose coexistence with scheelite reflects fluctuations in local physicochemical conditions.

The spatial distribution of the two types of mineralization is closely related to the fluid evolution pathway. Early high-temperature reducing magmatic fluids formed disseminated mineralization within the granodiorite (Sch1). As the fluids migrated upward, fluid–rock interaction intensified, mixing with atmospheric precipitation and increasing oxygen fugacity, leading to the formation of quartz vein-type mineralization within the granodiorite porphyry (Sch2, Wol1). Later, the fluids migrated further up the faults into the slate, extracting some wall rock components through fluid-rock interaction, ultimately precipitating quartz vein-type mineralization within the slate under low-temperature oxidizing conditions (Sch3, Wol2). This process was accompanied by decreases in temperature, salinity, and oxygen fugacity, as well as the continuous consumption of REEs and Mo in the fluids by scheelite, resulting in low-REE, low-Mo scheelite. Figure 10 is a conceptual model for W mineralization in the Chuankou deposit.

6. Conclusions

(1)

Microthermometry results of fluid inclusions with scheelite and quartz from two stages show that the average homogenization temperature in the quartz-veins within the Caledonian granite porphyry is 248 °C, and the average salinity is 6.31 wt.% NaCl eq (N = 85), the average homogenization temperature in the quartz-veins within the slate is 219 °C, and the average salinity is 5.57 wt.% NaCl eq (N = 49). The ore-forming fluids experienced an evolution from high temperature and high salinity to low temperature and low salinity.

(2)

Sulfur isotope compositions show that the δ³⁴S values of pyrite and arsenopyrite in the quartz-veins within the Caledonian granite porphyry are 2.06 to 3.28‰ and −0.38 to 0.21‰, respectively, and the δ³⁴S value of pyrite in the quartz-veins within the slate is −1.72 to 0.47‰. The δ³⁴S values of each stage are close to 0‰, indicating that the origin of sulfur mainly from magma.

(3)

The H-O isotope composition of the quartz indicates that the ore-forming fluid was primarily magmatic water. The low δ¹⁸O_H2O values (1.74 to 1.58‰) are influenced by fluid-rock interactions or the incorporation of atmospheric precipitation. The carbon isotopes (δ¹³C =−9.5 to −8.3‰) indicate a magmatic origin, but the C isotopes of quartz in the quartz-veins within the slate shift toward sedimentary rocks, reflecting the incorporation of rock components in the late mineralization period. These isotopic differences indicate that the fluid–rock interaction gradually strengthened during fluid evolution.