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Article

Ore Genesis of the Langcun Porphyry W-Mo Deposit, South China: Constraints from Fluid Inclusion and H-O-S Isotopics

by
Kai Cui
1,2,
Yunwei Qu
3,*,
Yuling Xie
3,
Kejun Yang
4 and
Zhaoqiang Huang
1
1
Institute of Mineral Recourses Research, China Metallurgical Geology Bureau, Beijing 101300, China
2
School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China
3
Civil and Resource Engineering School, University of Science and Technology Beijing, Beijing 100083, China
4
Jiangxi Copper (Hong Kong) Investment Co., Ltd., Beijing 100020, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(2), 109; https://doi.org/10.3390/min15020109
Submission received: 20 November 2024 / Revised: 21 January 2025 / Accepted: 21 January 2025 / Published: 23 January 2025
(This article belongs to the Special Issue Mineralogy of Porphyry Systems: Genesis, Exploration, and Applications)
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Abstract

:
The Langcun W-Mo deposit, located in the Zhejiang Province of South China, is a medium-sized porphyry deposit. The ore bodies mainly occur in aplite, granite porphyry, and the contact zone with hornfels of the Nanhua System. Four stages of mineralization are recognized in the Langcun deposit, including the quartz–K-feldspar stage (stage I), quartz–sericite–molybdenite stage (stage II), quartz–chlorite–pyrite stage (stage III), and calcite stage (stage IV). Stages I and II are the main ore-forming stages for wolframite and molybdenite. The petrographic and microthermometric results show that four types of fluid inclusions exist in the Langcun W-Mo deposit, including two-phase liquid-rich fluid inclusions (type LV), three-phase CO2-rich fluid inclusions (type LC), pure CO2 fluid inclusions (type C), vapor H2O inclusions (type V), and multi-phase inclusions with daughter minerals (type LDV). In stage I, the fluid inclusion assemblage is LDV + LV + V, and the LDV and LV fluid inclusions have similar homogenization temperatures (281–387 °C), indicating a boiling fluid inclusions association. In stage II, the fluid inclusion assemblage is LC + C, indicating immiscibility between CO2 and aqueous fluids. The homogenization temperatures of type LC are in the range of 228–342 °C, and the salinities are in the range of 2.77–5.14 wt.% NaCl equiv. The fluid inclusions in stages III and IV are type LV, with homogenization temperatures in the ranges of 224–275 °C and 200–225 °C, respectively, and salinities in the ranges of 1.74 to 4.96 wt.% NaCl equiv and 1.06 to 3.39 wt.% NaCl equiv, respectively. Hydrogen and oxygen isotopic results indicate that the ore-forming fluids mainly come from magmatic water in the early stage and may have received an input of meteoric water in the late stage, which results in the decrease in the temperature and salinity of ore-forming fluid. Early W-Mo precipitation was induced by CO2 escape because of decompression, and fluid mixing resulted in Mo precipitation in the later stage.

1. Introduction

The South China W-Sn metallogenic province is renowned for its substantial resources of tungsten (W) and tin (Sn), contributing more than 60% and 11% of the global reserves, respectively [1,2]. The majority of the W polymetallic deposits are intimately associated with Mesozoic granitoids [3,4]. The predominant deposit types in South China are skarn (e.g., the Shizhuyuan W-polymetallic deposit [5]), greisen and quartz-vein (e.g., the Hongshuizhai deposit [6]), and porphyry (e.g., the Yangchuling deposit [7]). Skarn and greisen–quartz-vein deposits have historically accounted for over 90% of identified tungsten resources. In recent years, the discovery of several giant porphyry W deposits, such as Dahutang [8], Dongyuan [9], and Xingluokeng [10], has highlighted the significance of porphyry systems. Despite these advancements, there is currently limited research on fluid inclusions in porphyry W deposits. Fluid inclusion studies are fundamental to understand the nature and evolutionary process of the ore-forming fluids in porphyry W deposits, and for the overall understanding of their genesis. In turn, this knowledge has important consequences for the research.
Langcun is a medium-size porphyry tungsten–molybdenum (W-Mo) deposit that was recently identified in South China, and it hosts significant resources with 18,500 tonnes of WO3. The mineralization of tungsten and molybdenum is predominantly as veins and stockworks within the aplite, granite porphyry, and surrounding rocks. Aplite is characterized by the presence of quartz–K-felspar druses, within which molybdenite occurs. Based on our detailed field observation, we conducted petrological, mineralogical, fluid inclusions, and stable isotopic analysis for the ore host granite porphyry, aplite, hydrothermal veins, and altered rocks. The findings reveal that the ore-forming fluids are CO2-rich NaCl-H2O fluids, marked by a relatively high temperature (~387 °C), high to medium salinities, and are predominantly derived from magmatic water. These research results have a direct and profound impact on guiding mineral exploration and improving exploration efficiency.

2. Geological Setting

2.1. Regional Geology

The Langcun W-Mo deposit is located in the southeastern part of the Yangtze Block, which is part of the South China Block (Figure 1). Tectonically, the South China Block was formed by the amalgamation of the Yangtze Block and Cathaysia Block during the Neoproterozoic era (ca. 850 Ma) (Figure 1) [11,12,13]. The region has undergone multi-phase tectonic–magmatic events. Notably, the most significant was associated with the Mesozoic subduction of the Paleo-Pacific plate beneath the South China Block, which resulted in a shift in the tectonic regime from the Tethys to the Paleo-Pacific [14,15,16]. Subsequently, alterations in the subduction angle of the Pacific plate shifted the tectonic setting from compression to extension, prompting lithospheric thinning and consequent extensive granitic magmatism and mineralization [17,18]. The transition is estimated to have begun around ca. 145 Ma [19] and lasted to approximately ca. 123 Ma. During this period, the South China Block entered a post-collisional phase, as evidenced by the emergence of extensive A-type granites and the diabase dike along the southern margin of the Yangtze Block [20,21,22,23].

2.2. Ore Deposit Geology

The Langcun W-Mo deposit, located in Anji County in Zhejiang Province in northwest South China, is characterized by diverse lithological units. The dominant units in the region include the Nanhua, Sinian, Cambrian, Silurian, Cretaceous, and Quaternary Systems (Figure 2) [25]. The Xiuning Formation, which is mainly silty hornfels and greywacke, is overlain by the manganese dolomite-bearing sandstone of the Nantuo Formation. The Doushantuo Formation is composed of siliceous rock and hornfels. The Dengying Formation with marbleized dolomite overlies the Doushantuo Formation. The Hetang Formation consists of interbedded shale and siliceous rock, while the Dachenling Formation comprises lime dolostone and thin layers of siliceous rock. The Yangliugang Formation is predominantly limestone. The Xiaxiang Formation consists of interbeds of siltstone and silty mudstone, which are an unconformity in relation to the clastic sedimentary and volcaniclastic rocks of the Laocun Formation. The area is crisscrossed by multiple fault systems, including NE-trending major thrusts, such as F1 and F3, and the NW-trending F2 fault (Figure 2). Furthermore, a series of secondary NE and NW trending faults have developed in the region, significantly influencing the occurrence and distribution of wolframite and molybdenite–quartz veins.
The Yanshanian magmatism was intense in the Langcun area, and is characterized by the presence of various intrusions, such as biotite monzonite porphyry, aplite, granite porphyry, and lamprophyre dykes. Aplite often occurs together with lamprophyre (Figure 3a), which can be observed cutting through biotite monzonite porphyry. These intrusions are often associated with quartz–K-feldspar–epidote druses and veins, indicating hydrothermal activity (Figure 3b,c). The zircon LA-ICP-MS U-Pb ages of these intrusions have yielded age values predominantly ranging from 144 to 132 Ma [26]. These data correlate well with the LA-ICP-MS U-Pb age of 127 Ma obtained from wolframite [26], suggesting a late-Mesozoic W mineralization event in the region.

2.3. Mineralization

The mineralization of wolframite and molybdenite mainly occurs within aplite and granite porphyry, with minor occurrences in hornfels (Figure 3d–f). Twenty wolframite vein ore bodies have been identified in the mine, with an average grade of 1.25% WO3 and an estimated resource of 18,500 t of WO3. Additionally, minor scheelite ore bodies in the Xiuning Formation have been observed, with an average grade of 0.153% WO3. Molybdenite’s mineralization, often associated with wolframite, is yet to be fully assessed for its resource potential. Mineralization appears as disseminated (Figure 3d,e), stockworks (Figure 3f), and veins (Figure 3e). Disseminated wolframite and molybdenite primarily occur in aplite and granite porphyry, while vein and stockwork types are the main types of W and Mo mineralization in the region, such as wolframite veins, quartz–wolframite veins, molybdenite veins, quartz–molybdenite ± pyrite veins, and quartz–sulfide–wolframite veins. These veins are composed of quartz, fluorite, wolframite, molybdenite, pyrite, magnetite, and minor amounts of chalcopyrite, pyrrhotite, galena, and sphalerite.
The host rock experienced extensive hydrothermal alterations in the Langcun deposit, including K-feldsparization, silicification, sericitization, chloritization, epidotization, and carbonation, among which silicification and sericitization are particularly associated with W and Mo mineralization. A field observation of vein crosscutting relationships, alteration patterns, and the mineral assemblages in the deposit suggests four continuous mineralization stages:
Stage I: Quartz–K-feldspar stage.
The main tungsten mineralization stage is characterized by quartz–K-feldspar veins and druses occurring within the aplite (Figure 3b,c and Figure 4a). The mineral assemblage includes quartz, K-feldspar, wolframite, and scheelite, and a small amount of molybdenite (Figure 5a,b).
Stage II: Quartz–sericite–molybdenite stage.
This stage includes the main molybdenite mineralization, in which the mineral assemblages are dominated by quartz, sericite, molybdenite, pyrite, and pyrrhotite (Figure 5c,d).
Stage III: Quartz–chlorite–pyrite stage.
Granitic wall rocks are pervasively altered by chlorite, quartz sulfide, and quartz–chlorite (pyrite) veins, which together with barren quartz veins crosscut older wolframite veins (Figure 4b), molybdenite veins (Figure 4c,d), and quartz–molybdenite veins (Figure 4e). The mineral assemblage of this stage includes quartz, chlorite, pyrite with minor molybdenite sphalerite, and galena (Figure 5e,f).
Stage IV: Calcite stage.
Extensive carbonation and calcite veins over country rocks characterize this stage—these carbonate veins crosscut quartz–chlorite veins (Figure 4f), with the mineral assemblage dominated by calcite with little quartz and chlorite.

3. Samples and Analytical Methods

Samples for fluid inclusion studies were carefully collected across all mineralization stages. These samples, mostly quartz and calcite veins, were prepared as double-polished thin sections (~300 μm thick) for further observation and tests.
The microthermometric study was conducted in the Fluid Inclusion Laboratory at the University of Science and Technology Beijing, Beijing, China (USTB), using a Linkam THMS600 heating–freezing stage with a temperature range from −196 °C to 600 °C. The accuracy of the temperature measurement was within ±0.2 °C for temperatures below 30 °C, ±0.1 °C for the interval between 30 and 100 °C, and approximately ±2 °C from 100 to 600 °C. The heating rate was initially set at 1–20 °C/min for the early stages of each run and was reduced to 0.3–1 °C/min as the samples approached the phase change point.
The compositions of individual fluid inclusions were identified using HORIBA Jobin Yvon HR800-type laser Raman spectroscopy with a laser wavelength of 633 nm in the Inclusion Analysis Laboratory at the Beijing Research Institute of Uranium Geology, Beijing, China (BRIUG). The scanning band was set to 1000–4000 cm−1 with a buildup time of 1 min for each scan. The spectral resolution was ±0.14 cm−1 with a beam size of 2 μm.
Scanning electron microscopy/X-Ray Energy Dispersive Spectrum (SEM/EDS) analysis of daughter minerals in the fluid inclusions was performed at the Fluid Inclusion Laboratory, USTB, using Phenom XL SEM equipped with an energy dispersive spectrometer (EDS) system and element identification software. The accelerating voltage was 15 kV, with a resolution of 10 nm and a maximum magnification of 100,000×. The samples used in the experiment were prepared using a natural opening method for fluid inclusions. The samples were crushed to yield particles 3 to 5 mm in size. Particles with relatively flat cross-sections were selected and mounted on a glass slide with their flat surfaces facing upward for observation.
Typical quartz samples from the first three stages were selected for H-O and S isotopic analyses at BRIUG. The samples were processed using the BrF5 method described by Clayton and Mayeda [27], and then the O-isotopic composition was determined using a MAT-253 mass spectrometer. All values are reported relative to the V-SMOW standard [28], with an uncertainty of ±0.2‰. H isotope analyses were conducted on the same samples using the Zn reduction method at 450 °C [29]. Measurements were performed by the MAT-252EM instrument, with analytical precision for δD of ±2‰. Thirteen pyrite and molybdenite samples were used for the sulfur isotope analysis. The method of SO2 extraction was described by Robinson and Kusakabe [30]; the S isotopic ratio (34S/32S) was measured on the Finnigan MAT-251 mass spectrometer with a precision of ±0.2‰.

4. Results

4.1. Fluid Inclusion Petrography

Through the phase composition of fluid inclusion at room temperature and the phase transition characteristics during heating and cooling [31], primary fluid inclusions were classified into four types: two-phase liquid-rich fluid inclusions (type LV), three-phase CO2-rich fluid inclusions (type LC), pure CO2 fluid inclusions (type C), and multi-phase inclusions with daughter minerals (type LDV).
Type-LV inclusion has a negative crystal, ellipsoidal, and irregular form, and consists of an aqueous liquid and vapor phase (Figure 6a) with a vapor volume percentage ranging from 12% to 45%.
Type-LC inclusion has liquid H2O, liquid CO2, and vapor CO2 (Figure 6b,c); only liquid H2O and vapor CO2 are partially seen at room temperature (Figure 6c) and CO2 liquid circles appear during cooling. Type-LC inclusions are mainly negative crystal and ellipsoidal in shape, 5–9 μm in size, and have gas phase filling in the range of 20%–80%, and such inclusions are developed in stages I and II.
Type-C inclusion contains little to no water with high CO2 phase volumetric proportions (VCO2 > 90%) (Figure 6e). It is generally 5–15 μm in size and has a sub-circular or elliptical shape, often in close association with LC-type inclusions. Type-C inclusion is mainly seen in stages I and II.
Type-LDV inclusions comprise an aqueous liquid, a vapor, and one or multiple daughter minerals (Figure 6e,f). Type-LDV inclusions are mostly isolated, with a negative crystalline or irregular shape, and 6–15 μm in size. Transparent daughter minerals are mainly halite, while opaque daughter minerals can be recognized as chalcopyrite (see Section 4.4). This type of inclusion occurs primarily in an early stage.

4.2. Microthermometry of Fluid Inclusion

The microthermometry results of fluid inclusions are summarized in Table 1 and Figure 7.
Stage I contains type-LV, -LC, -C, and -LDV fluid inclusions, which often have a close spatial association, showing the characteristics of non-homogeneous capture. Halite daughter minerals disappear later than vapor in type-LDV inclusions during the heating process, with melting temperatures of 362~392 °C and salinities of 43.5~46.6 wt.% NaCl equiv. Type-LV inclusions yielded ice-melting temperatures (Tmice) of −6.0~−1.1 °C and salinities of 1.74~9.21 wt.% NaCl equiv with homogenization temperatures (Thtotal) between 287 and 387 °C. Type-LC inclusion temperatures of CO2 solid melting (TmCO2) are between −58.0 and 57.0 °C; the melting temperatures of clathrate (Tmclath) range from 3.3 to 7.9 °C, with calculated salinities between 3.95 and 11.57 wt.% NaCl equiv (Collins, 1979). The temperatures of partial homogenization (ThCO2) for CO2 phases range from 20.8 to 30.5 °C with CO2 liquid disappearance. Thtotal for type LC range from 296 to 377 °C, with final homogenization generally occurring in the liquid phase. TmCO2 for type C is between −58.1 and −57.0 °C, and ThCO2 is between 27.7 and 31.1 °C.
The main types of fluid inclusions in stage II are types LC and C. TmCO2 is between −58.3 and −57.6 °C, Tmclath ranges from 7.5 to 8.6 °C, and the corresponding calculated salinities are 2.77~5.14 wt.% NaCl equiv. ThCO2 and Thtotal for type LC are 26.9~30.2 °C and 228~342 °C, respectively. TmCO2 for type C is between −58.4 and −57.6 °C, and ThCO2 is between 25.2 and 28.9 °C.
Stages III and IV are mainly type-LV inclusions. Stage III showed that Tmice ranged from −3.3 to −1.0 °C, with calculated salinities of 1.74 and 4.96 wt.% NaCl equiv, and Thtotal between 224 and 275 °C, which mainly occurs in the liquid phase. Type-LV inclusions in stage IV have Tmice values ranging from −2.4 to −0.6 °C, with calculated salinities ranging from 1.06 to 3.39 wt.% NaCl equiv, and Thtotal ranging from 200 to 225 °C, mainly in the liquid phase.

4.3. Laser Raman Spectroscopy Analysis

Representative FIs were measured using Laser Raman microspectroscopy to constrain their compositions. The results show that the vapor and liquid phases of the LV- and LDV-type inclusions are dominated by H2O (Figure 8a–d). The vapor phase of the LC- and C-type inclusions is dominated by CO2 (1284.82 cm−1 and 1388.53 cm−1, respectively, Figure 8e,g) with a small amount of N2 (2915.73 cm−1, Figure 8f) and CH4 (2915.73 cm−1, Figure 8h). The vapor and liquid phases of inclusion in stage III are all H2O (Figure 8i,j).

4.4. SEM Analysis of Daughter Minerals

The petrographic observation shows that the daughter minerals in type-LDV inclusions at the early stage are both transparent and opaque. SEM analysis results reveal that the daughter minerals include the following: (1) halite, which can occur alone or coexist with other daughter minerals (Figure 9a–c); (2) strontianite, as shown by peaks of C, O, and Sr in the SEM/EDS energy spectrum (Figure 9d), which is columnar in shape, and often coexists with halite in fluid inclusions; (3) ilmenite, occurring in fluid inclusions of rutile (Figure 9e), characterized by peaks of O, Fe, and Ti in the energy spectrum (Figure 9f); (4) sericite, which is flake-like in shape (Figure 9g) and contains O, Si, Al, and K with an Al/K atomic ratio of about 3:1 (Figure 9h), and is more common in the early stage and often co-occurs with other daughter minerals, such as calcite (Figure 9g,i), chalcopyrite (Figure 9j), and pyrite; (5) chalcopyrite, with a tetrahedral shape and peaks of S, Cu, and Fe in the energy spectrum (Figure 9k), which can coexist with sericite and rhodochrosite (Figure 9l); and (6) pyrite, characterized by its cubic shape (Figure 9m) and Fe and S in the X-ray map analysis.
All of the abovementioned daughter minerals were precipitated from the fluid after trapping. The evidence supporting this conclusion is multifaceted: (1) the daughter minerals exhibit regular crystal forms within the inclusions and have a growth relationship with the inclusions; (2) the presence of daughter minerals is exclusively observed within the primary inclusions of stage I, and the recurrence of identical daughter minerals is noted; and (3) minerals that correspond to the daughter minerals also develop within aplite and granite porphyry.

4.5. H-O Isotopic Compositions

The hydrogen and oxygen isotopic compositions of the analyzed quartz samples are presented in Table 2 and Figure 10. The δD values of the extracted water for quartz in stage I range from −70.4% to −89.0%. The δD values of quartz from stage II and III range from −68.5% to −76.9% and −80.5% to −85.1%, respectively. The measured δ18O value of quartz from stage I ranges from 9.4‰ to 12.8‰. Using the equations of Matsuhisa [32] and the corresponding fluid inclusion average homogenization temperature data, the δ18O value of aqueous fluid from stage I was calculated to range from 3.8‰ to 7.2‰. The δ18O value of quartz from stages II and III range from 9.0‰ to 9.6‰ and 6.5‰ to 11.3‰, respectively, and the calculated δ18O values of fluid range from 2.0‰ to 2.6‰ and −2.9‰ to 1.9‰, respectively.

4.6. Sulfide S Isotopic Compositions

The S isotope results are shown in Table 3. The δ34S values in pyrite and molybdenite range from 5.7‰ to 9.5‰ and 4.2‰ to 7.2‰, respectively, and are mostly concentrated in the range of 7‰ to 8‰.

5. Discussion

5.1. Source of the Ore-Forming Material

Because of the absence of sulfates, we assume that the sulfur isotopic composition of these sulfides can be considered as representative of the overall sulfur isotope signature of the Langcun deposit. The sulfur isotopes exhibit a narrow range from 4.2‰ to 9.5‰, clustering between 7.0‰ and 8.0‰ (Figure 11), and a variation not exceeding 5‰, slightly higher than the typical δ34S values of mantle sources [34], indicating that the sulfides in the mineralization stage were mixed with heavy sulfur isotopes.
Typically, the sulfur isotopic composition of primitive mantle is relatively homogeneous, with δ34S values ranging from 0 to 3‰. Magmatic sources generally have δ34S values near 0‰, rarely exceeding 8‰. In contrast, sedimentary rocks exhibit a broader isotopic variation, spanning from −40‰ to 50‰ [34]. Compared to the S natural reservoir, most of the δ34S values in the Langcun W-Mo deposit are close to the magmatic sulfur source. Moreover, the δ34S value at the early stage is slightly lower than that at the late stage in the Langcun deposit (Figure 11) and is higher than that of magmatic sulfur-source W deposits in South China, such as the Dongyuan W-Mo deposit [35] and Dahutang W deposit [36]. Higher δ34S values can be explained by the incorporation of some sedimentary sulfur through the interaction with sedimentary rocks [37].

5.2. Magmatic Fluid Evolution

Magmatic fluid exsolution is a pivotal process in magmatic–hydrothermal systems, which can transport a significant amount of metallic elements, thereby contributing to the formation of polymetallic deposits [38,39]. In the Langcun deposit, the mineralization of wolframite and molybdenite is intricately linked to granitic porphyry and aplite. The presence of quartz–K-feldspar druses and early quartz–K-feldspar veins within the aplite illustrate the magmatic fluid exsolution process. The aplite structure and the quartz–K-feldspar druses within aplite imply that decompression boiling (first boiling), triggered by the rapid ascent of magma, is likely the predominant mechanism of fluid exsolution.
The fluid inclusions study in stage I reveals a variety of inclusions with different vapor–liquid ratios and halite daughter minerals assemblage. Notably, liquid- and vapor-rich inclusions show different homogenization patterns during heating, with the former homogenizing to the liquid phase and the latter homogenizing to the vapor phase. On the other hand, the range of homogenization temperatures is similar, indicative of boiling fluid inclusions assemblage. This suggests that fluid boiling is a key process in the early stage of mineralization. Fluid boiling leads to phase separation, forming a high-salinity liquid-rich phase and a low-salinity CO2-rich phase fluid. The results of the fluid inclusion studies indicate that early magmatic exsolution fluid belongs to the NaCl-H2O-CO2 system with a high temperature (~387 °C) and high–medium salinity (1.74–46.60 wt.% NaCl equiv). H-O isotope analysis shows that the δD values and calculated δ18O values overlap near the primary magmatic fluids [33], implying that the ore-forming fluids are dominantly magmatic. In contrast, the isotopic data of fluids for stages III and IV show low δ18O values, and the plots in the δ18OH2O versus δDH2O diagram (Figure 10) indicate a significant influence of meteoric water, which is consistent with the fluid characterized by the low temperature and low salinity in late stages. Therefore, we suggest that the primary ore-forming fluids were derived from the magmatic exsolution and were predominantly of magmatic origin, whereas stages III and IV had the contribution of meteoric water.

5.3. Ore-Forming Mechanisms

The transport and precipitation of W in hydrothermal fluids involve complex mechanism. Wood and Samson [40] concluded through thermodynamic experiments that, under certain temperature and pressure conditions, tungsten migrates mainly as simple tungstic acid (WO42−, H2 WO4, HWO4−, and KHWO4) and basic tungstate ion pairs (KWO4−, NaWO4−, and NaHWO4) in the hydrothermal fluid of the NaCl-H2O-CO2 composition. The main mechanisms of tungsten precipitation include (1) the natural cooling of the fluid [41]; (2) a water–rock reaction [42,43]; (3) fluid immiscibility/boiling [44]; and (4) fluid mixing [45,46]. The role of CO2 in W mineralization is still a contentious issue. Ni et al. [41] suggested the minor role that CO2 plays in tungsten mineralization, given the absence of CO2-bearing inclusions in wolframite. However, several studies have documented variable CO2 contents in the ore-forming fluids of W and Mo deposits, such as the Dajishan W deposit, Xingluokeng W deposit [47], and Larong W-Mo deposit in Tibet [48]. The presence of CO2 may be a common feature of ore-forming fluids of W and Mo deposits [49]. Higgins [50] suggested that, under high-temperature conditions, tungsten in CO2-rich fluids may migrate as carbonates and bicarbonates. CO2 escape causes the pH fluctuation in the fluid and disrupts the tungsten complex equilibrium, which in turn causes the precipitation of W minerals [47,48]. Through the calculation of the reaction equilibrium model, Liu and Zhang [51] concluded that CO2 escape can reduce tungsten solubility in the fluid, promoting the precipitation of wolframite.
Microthermometry results show that the temperature from stages I to II does not decrease significantly, and there is no clear evidence of fluid mixing. However, wolframite predominantly precipitated during stage I, indicating that cooling and fluid mixing were not the primary drivers of precipitation for wolframite in the Langcun deposit. As described above, fluid boiling occurred in the early stage of mineralization, which likely reduced the stability of metal complexes in the fluid system and may have been a critical factor for wolframite mineralization. Although some molybdenite was present in the early stages, the main mineralization occurred later than that of wolframite. H-O isotopes suggest a minor addition of meteoric water to the fluid during stage II. In addition, the absence of CO2-bearing inclusions in stage III implies that CO2 escape occurred during or before that stage (Figure 12). A significant number of CO2-rich fluid inclusions developed during the early stages of W-Mo mineralization, especially in quartz associated with W-Mo mineralization, whereas such inclusions are rarely seen in the late stages. This suggests that CO2 escaping during the main mineralization stages could have triggered the precipitation of wolframite and molybdenite. Therefore, CO2 appears to be a favorable factor for W-Mo mineralization. In summary, fluid mixing and CO2 escape could be the primary mechanisms for Mo mineralization in the Langcun deposit.

6. Conclusions

1. The Langcun W-Mo deposit is a porphyry deposit that has evolved through four stages: quartz–K-feldspar of stage I, quartz–sericite–molybdenite of stage II, quartz–chlorite–pyrite of stage III, and calcite of stage IV. Among these, stages I and II are the main mineralization stages. The fluid inclusions encompass a variety of types: two-phase liquid-rich fluid inclusions (type LV), three-phase CO2-rich fluid inclusions (type LC), pure CO2 fluid inclusions (type C), and multi-phase inclusions with daughter minerals (type LDV).
2. The primary ore-forming fluids stem from magmatic exsolution, exhibiting high–moderate temperatures, high–moderate salinity, and are rich in CO2. The H-O isotope data indicate that the ore-forming fluids are predominately magmatic in origin, which, however, during the later stage of mineralization, were mixed with meteoric water, leading to the decrease in the temperature and salinity of ore-forming fluids.
3. Early W-Mo precipitation was triggered by fluid boiling. The escape of CO2, coupled with the mixing of magmatic fluid with meteoric water, facilitated the precipitation of molybdenite.

Author Contributions

Data curation, K.C.; funding acquisition, Y.X.; investigation, K.C., Y.Q. and K.Y.; project administration, Y.X.; resources, Y.X.; supervision, Y.X. and Z.H.; writing—original draft, K.C.; writing—review and editing, Y.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the 12th Five–Year Plan project of the National Science & Technology Pillar Program, grant number 2011BAB04B00, and the public welfare industry fund of the Ministry of Land and Resources, grant number 20101101.

Data Availability Statement

Data are contained within the article.

Acknowledgments

Many thanks to Qi Chen for his help with the fieldwork and Hao Cui for his help with the laser Raman spectroscopy analysis.

Conflicts of Interest

Kejun Yang is an employee of Jiangxi Copper (Hong Kong) Investment Co., Ltd. The paper reflects the views of the scientists and not the company.

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Figure 1. Distribution of the major W deposits in South China (modified from [24]).
Figure 1. Distribution of the major W deposits in South China (modified from [24]).
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Figure 2. Simplified geologic map of the Langcun deposit (modified from [25]).
Figure 2. Simplified geologic map of the Langcun deposit (modified from [25]).
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Figure 3. Photographs showing the ore geology and mineralization of the Langcun W-Mo deposit. (a) Aplite occurs together with lamprophyre; (b) quartz–K-feldspar druses occur in aplite; (c) quartz–K-feldspar–epidote veins occur in aplite; (d) wolframite occurs as disseminations in aplite; (e) molybdenite occurs as disseminations and veins in aplite; (f) quartz–molybdenite stockworks in aplite. Abbreviations: Ep—epidote; Kfs—K-feldspar; Mo—molybdenite; Py—pyrite; Qtz—quartz; Ser—sericite; Wf—wolframite.
Figure 3. Photographs showing the ore geology and mineralization of the Langcun W-Mo deposit. (a) Aplite occurs together with lamprophyre; (b) quartz–K-feldspar druses occur in aplite; (c) quartz–K-feldspar–epidote veins occur in aplite; (d) wolframite occurs as disseminations in aplite; (e) molybdenite occurs as disseminations and veins in aplite; (f) quartz–molybdenite stockworks in aplite. Abbreviations: Ep—epidote; Kfs—K-feldspar; Mo—molybdenite; Py—pyrite; Qtz—quartz; Ser—sericite; Wf—wolframite.
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Figure 4. Photographs showing various veins in the Langcun deposit. (a) Quartz–molybdenite vein in the aplite of stage I; (b) early wolframite vein cut by the quartz–chlorite–pyrite vein in stage III; (c) molybdenite vein cut by the quartz–epidote vein in stage III; (d) molybdenite vein cut by the quartz–pyrite vein in stage III; (e) quartz–molybdenite vein cut by the quartz vein in stage III; (f) quartz–chlorite vein cut by the calcite in stage IV. Abbreviations: Cal—calcite; Ep—epidote; Chl—chlorite; Kfs—K-feldspar; Py—pyrite; Qtz—quartz; Mo—molybdenite; Wf—wolframite.
Figure 4. Photographs showing various veins in the Langcun deposit. (a) Quartz–molybdenite vein in the aplite of stage I; (b) early wolframite vein cut by the quartz–chlorite–pyrite vein in stage III; (c) molybdenite vein cut by the quartz–epidote vein in stage III; (d) molybdenite vein cut by the quartz–pyrite vein in stage III; (e) quartz–molybdenite vein cut by the quartz vein in stage III; (f) quartz–chlorite vein cut by the calcite in stage IV. Abbreviations: Cal—calcite; Ep—epidote; Chl—chlorite; Kfs—K-feldspar; Py—pyrite; Qtz—quartz; Mo—molybdenite; Wf—wolframite.
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Figure 5. Photomicrographs of ores in the Langcun deposit. (a) Wolframite coexisting with pyrite in stage I; (b) molybdenite occurring in quartz–K-feldspar druses; (c) pyrite occurring in stage II; (d) molybdenite coexisting with magnetite and pyrrhotite in stage II; (e) molybdenite and magnetite occurring with chlorite in stage III; (f) pyrite, pyrrhotite, and chalcopyrite in stage III. Abbreviations: Chl—chlorite; Ccp—chalcopyrite; Kfs—K-feldspar; Po—pyrrhotite; Py—pyrite; Qtz—quartz; Mag—magnetite; Mo—molybdenite; Ser—sericite; Wf—wolframite.
Figure 5. Photomicrographs of ores in the Langcun deposit. (a) Wolframite coexisting with pyrite in stage I; (b) molybdenite occurring in quartz–K-feldspar druses; (c) pyrite occurring in stage II; (d) molybdenite coexisting with magnetite and pyrrhotite in stage II; (e) molybdenite and magnetite occurring with chlorite in stage III; (f) pyrite, pyrrhotite, and chalcopyrite in stage III. Abbreviations: Chl—chlorite; Ccp—chalcopyrite; Kfs—K-feldspar; Po—pyrrhotite; Py—pyrite; Qtz—quartz; Mag—magnetite; Mo—molybdenite; Ser—sericite; Wf—wolframite.
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Figure 6. Microphotographs showing different types of fluid inclusions at the Langcun W-Mo deposit. (a) Type-LV inclusion; (b) type-LC inclusion containing three phases (LH2O + LCO2 + VCO2); (c) type-LC inclusions with various filling percentages; (d) type-C inclusion; (e) type-LDV inclusion with a halite daughter mineral; (f) type-LDV inclusion with multiple daughter minerals. Abbreviations: Hl—halite.
Figure 6. Microphotographs showing different types of fluid inclusions at the Langcun W-Mo deposit. (a) Type-LV inclusion; (b) type-LC inclusion containing three phases (LH2O + LCO2 + VCO2); (c) type-LC inclusions with various filling percentages; (d) type-C inclusion; (e) type-LDV inclusion with a halite daughter mineral; (f) type-LDV inclusion with multiple daughter minerals. Abbreviations: Hl—halite.
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Figure 7. Salinity vs. homogenization temperatures of fluid inclusions in the Langcun W-Mo deposit.
Figure 7. Salinity vs. homogenization temperatures of fluid inclusions in the Langcun W-Mo deposit.
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Figure 8. Representative Raman spectra of fluid inclusions in quartz. (a,b) H2O spectra of the vapor phase and liquid phase in the type-LDV inclusion of stage I; (c,d) H2O spectra of the vapor phase and liquid phase in the type-LV inclusion of stage I; (e) CO2 spectrum of the vapor phase in the type-C inclusion of stage I; (f) H2O and N2 spectra of the vapor phase in the type-LC inclusion of stage I; (g) CO2 spectrum of the vapor phase in the type-C inclusion of stage II; (h) CO2 and CH4 spectra of the vapor phase in the type-LC inclusion of stage II; (i,j) H2O spectra of the vapor phase and liquid phase in the type-LV inclusion of stage III. “+” indicates the focus of Raman laser beam.
Figure 8. Representative Raman spectra of fluid inclusions in quartz. (a,b) H2O spectra of the vapor phase and liquid phase in the type-LDV inclusion of stage I; (c,d) H2O spectra of the vapor phase and liquid phase in the type-LV inclusion of stage I; (e) CO2 spectrum of the vapor phase in the type-C inclusion of stage I; (f) H2O and N2 spectra of the vapor phase in the type-LC inclusion of stage I; (g) CO2 spectrum of the vapor phase in the type-C inclusion of stage II; (h) CO2 and CH4 spectra of the vapor phase in the type-LC inclusion of stage II; (i,j) H2O spectra of the vapor phase and liquid phase in the type-LV inclusion of stage III. “+” indicates the focus of Raman laser beam.
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Figure 9. Backscattered electron images and X-ray spectrum of daughter minerals in inclusions. (a,b) Microphotograph and backscattered electron images of halite and strontianite in inclusions, respectively; (c,d) X-ray spectra of halite and strontianite, respectively; (e,f) backscattered electron image and X-ray spectrum of ilmenite in fluid inclusions of rutile; (g) backscattered electron images of sericite and calcite in inclusions; (h,i) X-ray spectra of sericite and calcite, respectively; (j) backscattered electron images of chalcopyrite, sericite, and rhodochrosite in inclusions; (k,l) X-ray spectra of chalcopyrite and rhodochrosite, respectively; (m) backscattered electron image of pyrite. Abbreviations: Cal—calcite; Ccp—chalcopyrite; Hl—halite; Ilm—ilmenite; Py—pyrite; Qtz—quartz; Rds—rhodochrosite; Rt—rutile; Ser—sericite; Str—strontianite.
Figure 9. Backscattered electron images and X-ray spectrum of daughter minerals in inclusions. (a,b) Microphotograph and backscattered electron images of halite and strontianite in inclusions, respectively; (c,d) X-ray spectra of halite and strontianite, respectively; (e,f) backscattered electron image and X-ray spectrum of ilmenite in fluid inclusions of rutile; (g) backscattered electron images of sericite and calcite in inclusions; (h,i) X-ray spectra of sericite and calcite, respectively; (j) backscattered electron images of chalcopyrite, sericite, and rhodochrosite in inclusions; (k,l) X-ray spectra of chalcopyrite and rhodochrosite, respectively; (m) backscattered electron image of pyrite. Abbreviations: Cal—calcite; Ccp—chalcopyrite; Hl—halite; Ilm—ilmenite; Py—pyrite; Qtz—quartz; Rds—rhodochrosite; Rt—rutile; Ser—sericite; Str—strontianite.
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Figure 10. δ18OH2O vs. δD diagram of the Langcun W-Mo deposit (base map after [33]).
Figure 10. δ18OH2O vs. δD diagram of the Langcun W-Mo deposit (base map after [33]).
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Figure 11. Frequency histogram and distribution of δ34S from the Langcun W-Mo deposit.
Figure 11. Frequency histogram and distribution of δ34S from the Langcun W-Mo deposit.
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Figure 12. Diagram of the fluid evolution process in the Langcun W-Mo deposit.
Figure 12. Diagram of the fluid evolution process in the Langcun W-Mo deposit.
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Table 1. Summary of fluid inclusion data from the Langcun deposit.
Table 1. Summary of fluid inclusion data from the Langcun deposit.
Host MineralMineralized StageInclusion TypeTm, CO2Tm, iceTm, clathTh, CO2Th, totalTm, haliteSalinity
°C°C°C°C°C°Cwt.% NaCl
QuartzILDV 281~312362~39243.5~46.6
LC−58.0~−57.0 3.3~7.920.8~30.5296~377 3.95~11.57
C−58.1~−57.0 27.7~31.1
LV −6.0~−1.1 287~387 1.74~9.21
QuartzIILC−58.3~−57.6 7.5~8.626.9~30.2228~342 2.77~5.14
C−58.4~−57.6 25.2~28.9
QuartzIIILV −3.3~−1.0 224~275 1.74~4.96
CalciteIVLV −2.4~−0.6 200~225 1.06~3.39
The pressure correction for LV inclusions has not been conducted.
Table 2. Oxygen and hydrogen isotopic data for quartz from the Langcun W-Mo deposit.
Table 2. Oxygen and hydrogen isotopic data for quartz from the Langcun W-Mo deposit.
Sample NumberMineralOre-Formation StageδDV-SMOWδ18 OV-SMOWδ18OH2O
(‰)(‰)(‰)
LC1064QuartzI−78.910.54.9
LC2018Quartz−79.69.43.8
LC1058Quartz−89.011.96.3
LC1036Quartz−81.412.77.1
LC2044Quartz−76.612.87.2
LC2035Quartz−70.412.16.5
LC2020QuartzII−68.59.62.6
LC2034Quartz−70.992.0
LC2021Quartz−76.992.0
LC1093QuartzIII−80.56.5−2.9
LC1013Quartz−85.16.5−2.9
LC1078Quartz−84.211.31.9
Table 3. S isotope of sulfide from the Langcun W-Mo deposit.
Table 3. S isotope of sulfide from the Langcun W-Mo deposit.
Sample NumberOre-Formation StageMineralδ34S V-CDT‰
LC1064IPyrite7.1
LC2018Pyrite6.7
LC2035Pyrite6.3
LC2044Pyrite7.1
LC2043Pyrite5.7
LC1047IIPyrite5.7
LC1070Molybdenite7.2
LC2021Molybdenite4.2
LC1093Pyrite8.3
LC1026IIIPyrite6.8
LC1046Pyrite9.5
LC1078Pyrite7.6
LC2033Pyrite9.3
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Cui, K.; Qu, Y.; Xie, Y.; Yang, K.; Huang, Z. Ore Genesis of the Langcun Porphyry W-Mo Deposit, South China: Constraints from Fluid Inclusion and H-O-S Isotopics. Minerals 2025, 15, 109. https://doi.org/10.3390/min15020109

AMA Style

Cui K, Qu Y, Xie Y, Yang K, Huang Z. Ore Genesis of the Langcun Porphyry W-Mo Deposit, South China: Constraints from Fluid Inclusion and H-O-S Isotopics. Minerals. 2025; 15(2):109. https://doi.org/10.3390/min15020109

Chicago/Turabian Style

Cui, Kai, Yunwei Qu, Yuling Xie, Kejun Yang, and Zhaoqiang Huang. 2025. "Ore Genesis of the Langcun Porphyry W-Mo Deposit, South China: Constraints from Fluid Inclusion and H-O-S Isotopics" Minerals 15, no. 2: 109. https://doi.org/10.3390/min15020109

APA Style

Cui, K., Qu, Y., Xie, Y., Yang, K., & Huang, Z. (2025). Ore Genesis of the Langcun Porphyry W-Mo Deposit, South China: Constraints from Fluid Inclusion and H-O-S Isotopics. Minerals, 15(2), 109. https://doi.org/10.3390/min15020109

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