1. Introduction
Orogenic gold deposits are referred to as the gold deposits formed at the convergent plate boundaries, and their space–time is related to accretional or collisional orogeny, controlled by ductile–brittle shear zones, and characterized by disseminated-/lode-style mineralization [1,2]. The West Qinling orogen represents one of the major gold metallogenic belts in China, with numerous large- and medium-sized gold deposits having been discovered since the 1980s. However, due to the presence of multi-phase tectonic superposition, complex fluid sources, and late-stage alterations in this region, an overlap or even mixing of metallogenic features among different types of gold deposits is frequently observed. This complexity increases the difficulty of accurately classifying metallogenic types, thereby affecting exploration strategies and predictive modeling. The Liba mega gold deposit, located at the northern edge of the West Qinling orogenic belt, contains quartz veins considered closely associated with mineralization. Its structural, lithological, and metamorphic characteristics are consistent with those typically observed in orogenic gold deposits. However, the ore is hosted in slate, and the gold occurs in the form of fine grains, which is consistent with Carlin-type gold deposits. Nevertheless, with respect to fluid properties and mineralization mechanisms, the Liba gold deposit exhibits certain atypical characteristics. Although the nature and origin of the ore-forming fluids have been previously investigated through analyses of quartz inclusions, homogenization temperatures, and H–O isotopic compositions, considerable debate remains. Key questions include whether the ore-forming fluids were of medium to low temperatures (254–356 °C) [3,4] or medium to high temperatures (300–420 °C) [5,6] and whether these fluids originated primarily from metamorphic sources [7,8] or constituted a mixture of meteoric water and magmatic water [3,6,9,10,11,12].
The disagreement in understanding mineralization temperatures and fluid sources may primarily lie in the fact that traditional methods of fluid inclusion studies—such as H–O isotopic and major element analyses—are limited by their inability to accurately resolve the distinct phases of mineralized quartz veinlets and their associated fluid events. As a result, these limitations often lead to inconsistent interpretations of the composition, temperature–pressure conditions, and sources of the mineralizing fluids [13]. In response to the inadequacy of conventional techniques in resolving multi-phase fluid overprinting and microchemical evolution, the combined use of scanning electron microscope–cathodoluminescence (SEM-CL) and laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) has proven particularly effective in the study of hydrothermal deposits. SEM-CL imaging can reveal structural features within quartz grains that represent successive episodes of precipitation, dissolution, fracturing, recrystallization, regrowth, and other processes. These features provide insight into the quartz growth environment and the geological events it has experienced—information that cannot be discerned using conventional petrographic techniques [14,15,16]. In recent years, the quartz SEM-CL technique has been successfully applied to the study of various hydrothermal deposits, such as in the Bilihe gold deposit [17], where SEM-CL was used to identify fan-shaped zoning of dendritic quartz during the magmatic phase and oscillatory ring zones in quartz during the hydrothermal phase. The data were integrated with LA-ICPMS-derived Al-Ti-Li patterns to propose a fluid evolution model from high-temperature magmatic fluids, through medium-temperature acidic hydrothermal fluids, to low-temperature antimony-rich fluids.
Based on this, the present study has employed SEM-CL and LA-ICPMS analyses to obtain detailed information on the quartz structure and trace element characteristics to characterize the fine-scale growth structures of quartz formed during the orogenic period, as well as the associated fluid evolution and mineral precipitation processes, and to further assess the extent to which magmatic–hydrothermal fluids contributed to the evolution of the deposit [18,19].
2. Regional and Ore Deposit Geology
The Liba gold deposit is located at the northern edge of the West Qinling orogenic belt. The West Qinling orogenic belt is the western extension of the Qinling orogenic belt, which lies at the junction between the Yangzi Craton and the North China Craton (Figure 1). The outcrops in the area mainly consist of the mottled slate of the Middle Devonian Liba Group, the metamorphosed quartz sandstone of the Middle Devonian Xihanshui Group, and the thick-bedded slate of the Middle Carboniferous Xiajialing Group, along with localized conglomerates and sandstones of the Middle and Lower Jurassic, as well as Cenozoic sediments. Magmatic activity in the region is intense and characterized by multiple phases of activity, with all discovered gold deposit sites distributed around the Indo-Chinese rock body. The regional tectonic trend is in the NWW direction, and the tectonic framework is characterized by a complex fold–fault system, with well-developed fractures that control the distribution of gold mineralization. Orogenic gold deposits (e.g., Liba, Baguamiao, and Ma’anqiao) are mainly distributed near the Shangdan Suture Zone and are controlled by shear zones that extend parallel to it.
The main outcrop of the Liba deposit is the Middle Devonian Liba Group (D2lb), as well as Palaeocene, Neocene, and Quaternary sediments (Figure 2). The Liba Group consists of flysch formations, which are divided into three layers, as follows: (1) slate with sandstone and siltstone (D2lb1); (2) sericite–chlorite slate with metamorphosed sandstone (D2lb2); and (3) slate, black sandstone, and siltstone (D2lb3), from bottom to top. Fracture structures are well-developed in the deposit area, with the main fault (F1) striking northwest and generally dipping south. This is a compression–torsion reverse fault, which runs diagonally through the northeast of the deposit, with no ore body formation observed in the F1 fracture zone. Several NWW and nearly E-W oriented fracture zones are present within the deposit, considered secondary fractures of F1 [21], all inclined southward, with dips ranging from 70° to 80°. These fracture zones strictly control the distribution of the ore body, exhibiting branching, composite, side-present, and cusp extinction patterns in their spatial distribution. Magmatic activity in the deposit area is intense, with Indosinian biotite monzogranite exposed to the south and west, representing the northern extension of the Zhongchuan rock body. Dykes are abundantly developed, often accompanying the ore body, and consist mainly of granite porphyry dykes and lamprophyre dykes, with an age of formation between 210 and 221.9 Ma [4,21,22], corresponding to the Late Triassic period. These dykes are closely associated with gold mineralization, providing a noteworthy source of both fluids and mineralizing materials [22,23].
The Liba gold mine contains a cumulative proven gold resource of more than 150 tons, with an average grade of 2.11 g/t [24], classifying it as a super-large gold deposit. The deposit mainly comprises the ore sections of Zhaogou, Wawugou–Wayaogou, Magou, Liba, Wanghe, Dugou, Madigou, Tanyaogou, Sanrengou, Daqing, and Jiangping (Figure 2), among which Magou and Zhaogou are currently mined via open-pit methods. A total of 133 ore bodies have been identified within the deposit, including 2 large ore bodies (the No. 6 ore body in Magou and the No. 26 ore body in Zhaogou), 8 medium-sized ore bodies, and 123 small-sized ore bodies. The ore bodies are usually vein-like, lenticular, podiform, or beaded and occur in single-phase or multi-phase filled composite veins in the form of sulfide-bearing altered rocks or quartz veins. Most exhibit poor continuity, and the gold grade varies significantly. The No. 6 ore body, located within the F3 tectonic zone, exhibits upward expansion, contraction, branching, and cusp extinction features (Figure 2 and Figure 3a). It strikes 90–110°, dips south at angles between 83° and 89°, and extends for approximately 946 m. The No. 26 ore body is situated in the F29 fracture alteration zone (Figure 2 and Figure 3b) and exhibits good continuity, with a strike of 85–90°, a northward dip of 60–90°, and a length of about 750 m.
Mineralization is closely associated with intense tectonic fragmentation and mylonitization. The alteration of surrounding rocks is characterized by assemblages of pyrite, chlorite, sericite, quartz, carbonates, and arsenopyrite. Ore types in the Liba deposit include vein-type quartz–pyrite, vein-type quartz–sericite–pyrite, and vein-type quartz–polymetallic sulphide ores. The internal textures of the ore are diverse, including sparsely mineralized, fine-veined, densely mineralized, banded, massive, and brecciated structures. Pyrite is the dominant metallic mineral, followed by arsenopyrite, chalcopyrite, sphalerite, and galena. The gangue minerals are primarily sericite and quartz, with minor amounts of feldspar, chlorite, carbonate, graphite, and organic carbon. Quartz within the veins can be divided into several stages, including early pyrite–milky quartz (Qz0), quartz–pyrite (Qz1), quartz–polymetallic sulphide (Qz2), and late-stage quartz–carbonate (Qz3).
3. Quartz Petrography
Multiple phases of hydrothermal vein bodies have been developed in the Liba deposit, recording complex processes of hydrothermal alteration and mineralization. Based on field investigations and hand specimen observations, the quartz in the Liba deposit has been classified into three phases and five stages according to crosscutting relationships, mineral assemblages, and alteration types. These include the following: pre-mineralization-phase quartz (Qz0); metallogenic-phase quartz, which comprises early-metallogenic-stage quartz (Qz1); main-metallogenic-stage quartz (Qz2, further subdivided into Qz2a and Qz2b); late-metallogenic-stage quartz (Qz3); and post-mineralization-phase quartz (Qz4) (Figure 4).
(1). Pre-mineralization quartz veins (Qz0) are typically represented by those found in the south wall of the Magou pit, specifically within the tectonic fracture zone outside the ore body. These veins, stretched by tectonic activity into a hook shape (Figure 4a), range from 4 to 7 m in length and are composed of coarse-grained, milky-white quartz. They are crosscut by fine sulphide-bearing veins, suggesting that the formation of Qz0 predates gold-related mineralization (Figure 4b). These quartz veins are dominated by quartz with traces of pyrite and pyrrhotite. Microscopic observation reveals that the quartz is coarse-grained and often displays pronounced undulatory extinction due to stress-induced deformation (Figure 4c).
(2). Early-metallogenic-stage quartz veins (Qz1) typically occur as a web-like arrangement of veins within the phyllitic slate, which is later crosscut by the main- and late-metallogenic-stage quartz veins (Figure 4d). Compared with the main metallogenic-stage quartz (Qz2), Qz1 quartz is fine-grained, with straight vein edges, and contains a more homogeneous, fine-grained texture (Figure 4e).
Main-metallogenic-stage quartz veins (Qz2) are commonly found in fine sandy slates containing trace amounts of very fine pyrite. They can be generally classified into two types: (i) Qz2a, sulphide-bearing fissure veins, which are mainly siliceous with fuzzy boundaries (Figure 4f) and (ii) Qz2b, quartz–carbonate reticulation veins (Figure 4g), which are marginally wider than Qz2a veins and are predominantly interspersed with them, as evidenced by Qz2b cutting through Qz2a, indicating that Qz2a quartz predates Qz2b.
Late-metallogenic-stage quartz veins (Qz3) invariably crosscut the earlier quartz veins Qz1, Qz2a, and Qz2b. These veins appear as comb structures. The quartz is primarily smoky grey, with more authomorphic, coarse-grained textures, and the quartz and carbonates form comb-like patterns. The vein margins contain small amounts of coarse-grained authomorphic pyrite (Figure 4h,i).
(3). Post-mineralization-stage quartz veins (Qz4) are large-grained and contain carbonates but no sulfides. The vein margins are straight (Figure 4j), and these veins were deposited late in the evolution of the hydrothermal system, cutting through all the pre-existing fine veins.
4. Samples and Analytical Methods
4.1. Sampling Selection
The samples in this study were collected from the surface outcrops and boreholes in the Liba deposit, yielding a total of 21 samples. Five samples were subjected to SEM-CL and an in situ trace element analysis, while 18 samples were analyzed for H-O isotopes, as shown in Table 1.
4.2. Quartz SEM-CL Imaging
Scanning electron microscope–cathodoluminescence (SEM-CL) images of quartz samples were obtained at the Nanjing Hongchuang Exploration Technology Service Co., Ltd. in Nanjing, China, using a TESCAN Mira3 LMH electron microprobe. Imaging was conducted under conditions of a 12 kV electric field voltage and a 3 nA current. Due to the small field of view, some images presented here are mosaics of multiple CL images digitally stitched together after acquisition.
4.3. LA-ICPMS
The LA-ICP-MS analysis was guided by the combination of CL images and transmitted light images to minimize the accidental ablation of mineral and fluid inclusions within the quartz. The Agilent 7900 ICP-MS (Santa Clara, CA, USA) with a Photon Machines Analyte HE 193-nm ArF excimer laser lblation system (Photon Machines Inc., Redmond, WA, USA) was used for the in situ LA-ICPMS analysis at the Nanjing Hongchuang Exploration Technology Service Co., Ltd. in Nanjing, China. Detailed operating conditions and data reduction methods followed those in [25,26]. The laser spot utilized was 60 μm in size. For calibrating trace elements, glasses derived from NIST610, NIST612, GSE-1G, and BCR-2G were employed as external standards [27]. The trace element analytical data were processed using ICPMSDataCal (version 12.2, China University of Geosciences, Beijing, China) [27,28].
4.4. Hydrogen and Oxygen Isotope
H-O isotopic testing was performed at the Nanjing Hongchuang Exploration Technology Service Co., Ltd. in Nanjing, China. The hydrogen isotope analysis was conducted using the high-temperature carbon reduction stable isotope method. The samples were entrained in a 5N helium carrier gas and passed through a pyrolysis furnace (Flash EA, Thermo Fisher, Waltham, MA, USA). The furnace is heated to a temperature of 1450 °C. The products of the reduction reaction (H2 and CO) are separated in a gas chromatographic column and analyzed directly in an isotope ratio mass spectrometer (253 Plus, Thermo Fisher, Waltham, MA, USA) [29]. The analysis of oxygen isotopes was conducted using the BrF5 method [30]. A 253-plus gas isotope ratio mass spectrometer was used as the testing instrument. The precision is 0.2‰ for the δ18O value and 1‰ for the δD value. The isotopic compositions of oxygen and hydrogen are expressed in ‰ relative to Vienna Standard Mean Ocean Water (V-SMOW).
5. Results
5.1. Quartz Generations and SEM-CL Imaging
The cathodoluminescence analysis of the quartz in the Qz0, Qz1, Qz2, Qz3, and Qz4 stages yielded the following results: Qz0 is characterized by a homogeneous dark grey color with prevalent microcracks and contains virtually no sulfides or gold (Figure 5a). Qz1 displays a dull grey color (Figure 5c,d), with small grains ranging from 70 to 100 µm in diameter and lacking internal growth bands. Qz2 is subdivided into Qz2a and Qz2b. Qz2a consists of semi-automorphic grains exhibiting a mottled grey luminescence, with grain diameters exceeding 100 µm. The presence of occasional unmodified, bright quartz grains is observed, accompanied by sericitization and biotitization alterations (Figure 5b). This stage also contains gold-bearing pyrite. Qz2b is composed of grains with greater automorphism and a darker grey luminescence. Sericite and carbonate alterations are observed at the vein margins (Figure 5e). Qz3 is defined by automorphic grains featuring bright nuclei and darker margins, with distinct bright and dark growth bands. This stage is commonly associated with sericitization and small amounts of authigenic pyrite and gold (Figure 5f,g). Qz4 exhibits euhedral characteristics, including well-defined grain boundaries and clean veins devoid of sulfides (Figure 5h).
5.2. Trace Elements
This analysis was conducted based on cathodoluminescence images of quartz grains from each stage (Qz0 to Qz4) to guide the selection of analytical points for the in situ trace element analysis. The results are summarized in Table S1. The test results indicate that the primary trace elements present in quartz include Li, Na, Al, K, and Ti, which exhibit considerable variation in concentration. In contrast, other elements such as Be, Rb, Ga, Sn, and Cs are generally close to or below the detection limit. The variation in quartz trace element concentrations across the different growth stages is illustrated in Figure 6. Specifically, trace element concentrations in Qz0 are generally low, reaching only background levels. Fluid-mobile elements, such as Al, Na, and K, are present in particularly low concentrations. In Qz1, the overall trace element concentrations are markedly elevated, with Na, Al, K, Ti, Ga, Rb, and Sr exceeding those of other phases by at least one order of magnitude. Within Qz2, Qz2a exhibits slightly lower concentrations compared to Qz1, particularly for Na, K, and Rb, and the data are more tightly clustered, indicating reduced variability. In contrast, Qz2b shows significant variability, with a notable enrichment of trace elements, including Al, K, Rb, and Sr, compared to Qz2a. The trace element concentrations in Qz3 are generally lower than those in Qz2, although Li, Na, and Al remain at relatively elevated levels. Qz4 displays substantially reduced trace element concentrations, resembling those observed in Qz0.
5.3. Hydrogen and Oxygen Isotopes of Quartz
The results of the hydrogen and oxygen isotope analyses of quartz samples from different mineralization stages (Qz0 to Qz3) are presented in Table 2. These samples exhibit distinct isotopic variations across the stages. The δ18O values of Qz0 range from 8.69‰ to 11.64‰, and the corresponding δD values range from −94.80‰ to −79.80‰. In Qz1, δ18O values increase to a range of 10.94‰ to 12.67‰, while δD values decrease to between −107.50‰ and −91.20‰. The δ18O values of Qz2 then decline to a range of 8.68‰ to 10.98‰, with δD values further decreasing to between −119.10‰ and −101.20‰. Finally, Qz3 exhibits δ18O values ranging from 8.25‰ to 10.10‰ and δD values from −116.50‰ to −101.20‰.
6. Discussion
6.1. The Characteristics and Genesis of the Liba Gold Deposit
The origin of the Liba gold mine has predominantly been classified as orogenic gold [4,9,23,31] or Carlin-like [6,32,33,34,35,36]. The deposit exhibits characteristics typical of orogenic gold systems in terms of its metallogenic tectonic setting (related to the Triassic orogeny of the Qinling orogenic belt), metallogenic element association (Au-As-Sb), primary ore-controlling structures (primarily influenced by tectonic incision), and fluid inclusion compositions (H2O-CO2-NaCl). However, the host rock assemblage is primarily composed of a turbidite sequence consisting of weakly metamorphosed fine clastic rocks. Gold occurs primarily as fine-grained inclusions within pyrite and arsenopyrite. This mineralization style differs significantly from the typical orogenic gold system. It is more analogous to the Carlin-type gold deposit in southwestern Guizhou, China. Consequently, many researchers have classified the Liba deposit as a “Carlin-like” gold deposit [6,37]. This study contends that the key geological characteristics of the Liba gold deposit are overall consistent with those of orogenic deposits. Although comparisons can be made to typical Carlin-type deposits in terms of host rocks and the occurrence of fine-grained gold, the Liba deposit is distinguished by the absence of a contemporaneous porphyry mineralization system in the region. This fundamental difference underscores a distinct genetic setting from that of Carlin-type gold deposits. Therefore, we consider it to be an orogenic deposit rather than a Carlin-type deposit. Recent studies have revealed that orogenic gold deposits encompass a wide spectrum of subtypes and variants, with mineralization characteristics exhibiting considerable regional variation. Some Chinese scholars have emphasized the role of magmatism and have described such deposits as medium- to low-temperature magmatic–hydrothermal gold systems. The metallogenic geological body of medium- to low-temperature magmatic–hydrothermal gold deposits is the intermediate–felsic intrusion. These deposits can be classified as proximal, medial–distal, or distal end-members based on their distance from the metallogenic geological body. They can also be classified as mesothermal or epithermal according to their ore-forming temperatures [38,39,40]. Accordingly, the core of the genetic debate surrounding the Liba gold deposit lies in the controversy between proximal, or medial–distal/distal genetic models, as well as high-temperature or medium- to low-temperature mineralization conditions.
6.1.1. Magmatic–Hydrothermal Attribution of the Liba Gold Deposit
This study is intended to clarify these genetic interpretations by examining the quartz texture and trace element geochemistry characteristics. Spatial distribution differences between ore deposits and metallogenic geological bodies have been roughly categorized into three types [40]: proximal, medial–distal, and distal. Proximal deposits are located adjacent to the contact interface between the metallogenic geological body and the surrounding rocks, with ore bodies situated within 500 m of the contact surface. Medial–distal deposits are found within the contact zone of the metallogenic geological body and surrounding rocks, ranging from 0.5 to 3 km, and are controlled by brittle–ductile shear zones, silica/calcium surfaces, unconformable surfaces, and folded tectonics, among others. Distal deposits are located at distances exceeding 3 km from the contact zone between the metallogenic geological body and the surrounding rocks. The Liba deposit is situated approximately 2 km northeast of the Zhongchuan rock body, and the ore body is structurally controlled by a shear zone, which is consistent with the characteristics of the medial–distal deposit.
In addition, trace element contents in quartz from different deposit types exhibit systematic variations, allowing deposit types to be identified through trace element compositions. Porphyry deposits, orogenic gold deposits, and epithermal deposits can be differentiated based on the Al and Ti contents within quartz from hydrothermal systems [16,41]. Porphyry deposits are characterized by quartz Ti contents ranging from 1 to 200 ppm, Al contents between 50 and 500 ppm, and Al/Ti ratios between 1 and 10. In contrast, epithermal deposits are marked by quartz Ti levels below 3 ppm, Al levels ranging from 20 to 4000 ppm, and Al/Ti ratios spanning from 100 to 10,000. Orogenic gold ore quartz exhibits intermediate values, with Ti contents ranging from 1 to 10 ppm, Al contents typically ranging from 100 to 1000 ppm, and Al/Ti ratios between 10 and 100.
The Ti content of quartz in the Liba deposit during the metallogenic phases was found to range from 2.03 to 93.34 ppm, with an average of 10.09 ppm. Al contents varied from 5.28 to 17,476 ppm, with an average of 1217.54 ppm, while Al/Ti ratios ranged from 0.99 to 372.50, averaging 102.17. These values are primarily distributed within the orogenic gold deposit area (Figure 7), though some data points of Qz2a and Qz2b are also observed within the porphyry-type deposit area. This distribution indicates the possible involvement of magmatic hydrothermal fluids in the mineralization process of the Liba deposit, further supporting the interpretation of a magmatic source background for the Liba gold deposit.
As shown in Figure 8, the Liba deposit’s δ18O–δD data provide evidence that the ore-forming fluids were primarily derived from magmatic sources. The δ18O–δD data from the Liba gold deposit can be broadly divided into two groups. The first group, comprising Qz0 and Qz1, exhibits similar isotopic signatures: δ18Owater values mostly range from 5‰ to 15‰, and δD values range from −80‰ to −110‰. These data plot mainly within the magmatic water range and are located near the lower part of metamorphic water, reflecting a predominantly magmatic water affinity with potential input from metamorphic fluids, suggesting that pre- and early ore-forming fluids were mainly magmatic hydrothermal fluids and may have been influenced to some extent by metamorphic fluids. The second group, consisting of Qz2 and Qz3, shows that δ18O values mostly range from 5‰ to 10‰, while δD values range from −100‰ to −120‰. These points plot directly below the magmatic water range, indicating that the ore-forming fluids were closely associated with magmatic water but were influenced by external factors (such as formation waters) during later mineralization stages, exhibiting more complex characteristics. A comparison of hydrogen and oxygen isotope compositions between the Liba deposit and other gold deposits in the West Qinling region (Zhaishang, Ma’anqiao, Baguamiao, and Yangshan; Figure 8, Table 3) shows general similarities. However, the Liba deposit differs somewhat from the late-stage isotopic signatures of the Ma’anqiao and Baguamiao deposits, where significant meteoric water involvement has been documented [42,43].
Although the δ18O–δD diagram provides important information for analyzing the source of mineralizing fluids, it also has an obvious issue of multiple interpretations. Therefore, this can thus serve only as a preliminary indicator of the fluid origin. To better understand the evolutionary characteristics of hydrothermal systems, in situ trace element indicators in quartz are of significant importance. Previous studies have demonstrated that Ge4+ can substitute for Si4+ within the quartz lattice [47,48]. The incorporation of Ge4+ is known to promote the formation of lattice defects during the crystallization of low-temperature quartz, thereby providing structural sites that facilitate the incorporation of Al3+ into the lattice [49]. After the statistical analysis of a large amount of quartz trace element data [50], it was concluded that the Ge/Al ratio can distinguish between magmatic quartz (Ge/Al ratio < 0.008) and hydrothermal quartz (Ge/Al ratio > 0.008). In the Liba gold deposit, this ratio effectively differentiates quartz types based on their stage of mineralization (Figure 9a). Specifically, Qz1 corresponds to magmatic quartz, Qz2a is classified as hydrothermal quartz, Qz2b exhibits characteristics of mixed magmatic–hydrothermal origin, and Qz3 also reflects magmatic quartz.
6.1.2. Medium- to Low-Temperature Characteristics of the Liba Gold Deposit
Previous studies have conducted fluid inclusion thermometry on the Liba deposit, revealing temperatures ranging from 192.6 °C to 363.9 °C [4]. Additionally, arsenopyrite geothermometry has been applied to estimate ore-forming temperatures, indicating ranges of 340–405 °C, 342–417 °C, and 331–381 °C for the early, main, and late mineralization stages, respectively [23]. These findings consistently indicate a general trend of a decreasing temperature throughout the mineralization process. However, these studies did not provide a detailed classification of the formation stages of quartz, the host mineral. This study will discuss the formation of quartz at different stages of the Liba deposit based on the trace element composition of quartz.
Previous studies have shown that the Al content in quartz exhibits distinct distribution patterns at different temperatures [51]. While the Al content cannot serve as a precise geothermometer, it provides valuable support for interpreting the thermal evolution of hydrothermal systems. High-temperature quartz typically contains Al concentrations ranging from 80 to 400 ppm, whereas low-temperature quartz (<350 °C) often exhibits a bimodal distribution of the Al content—namely, one group with low Al concentrations (4–50 ppm) and another with high Al concentrations (2000–4000 ppm) [51]. In the Liba gold deposit, the Al content of Qz2b can similarly be divided into two distinct ranges: 35–166 ppm (mean 97.29 ppm) and 502.8–1174 ppm (mean 940.98 ppm). For Qz3, the Al content ranges from 156 to 379.8 ppm (mean 263.77 ppm) and 610.3 to 2067 ppm (mean 980.71 ppm). These values exhibit the bimodal characteristics described in low-temperature quartz, indicating that both Qz2b and Qz3 correspond to quartz crystallized under low-temperature hydrothermal conditions.
The trace element characteristics of quartz indicate that quartz formation primarily occurred at low temperatures during the mineralization of the Liba gold deposit. It has been demonstrated that in high-temperature quartz (>400 °C), the SEM-CL intensity is strongly correlated with the substitution of Si4+ by Ti4+, whereas the synergistic relationship between Al3+ and monovalent cations such as Li+ is not significant. In contrast, in low-temperature quartz (<350 °C), the SEM-CL intensity is more strongly influenced by the synergistic association of Al3+ with monovalent cations such as Li+, K+, and Na+ [51,52]. In the Liba deposit, the Ti content in quartz (Qz1 to Qz3) during the mineralization stage remains relatively stable (mostly between 2 and 12 ppm) and does not exhibit a consistent trend. In comparison, variations in the concentrations of elements such as Al and K are consistent with changes in the SEM-CL intensity, indicating that they are the primary controlling factors and suggesting that the quartz in Liba corresponds to low-temperature quartz (<350 °C). This assessment aligns with previous interpretations that the mineralizing fluid temperature was within the medium- to low-temperature range [3,4], and it also provides independent evidence based on the correlation between the cathodoluminescence and quartz trace element composition. The trace element assemblage in quartz offers a more continuous and systematic record of the fluid evolution than the potentially unrepresentative or random results of single-point thermometry methods. This conclusion helps resolve previous discrepancies regarding the mineralization temperature of the Liba deposit and offers a more robust foundation for understanding its genetic processes.
6.2. Fluid Evolution and Metallogenic Mechanism
The ore-forming fluids of the Liba gold deposit record a complex evolutionary history characterized by multi-stage, multi-source superimposed metallogenic features. The evolutionary characteristics of the ore-forming fluids and the mineralization mechanism of the deposit can be accurately constrained through multi-parameter approaches, including in situ trace element analysis and H–O isotope measurements.
6.2.1. Substitution Mechanism of Trace Elements into Quartz
The atomic structure of the quartz lattice is composed of strong Si–O bonds and is characterized by the small size of the Si4+ ion and a tightly packed arrangement, which contributes to its simple chemical composition and relatively low capacity [16,53,54]. However, alternative mechanisms’ trace elements may still be incorporated into the quartz lattice through substitutional mechanisms, interstitial incorporation, or the presence of microinclusions (e.g., fluid or mineral inclusions) within crystal defects [47,48,55,56,57]. (1) Si4+ may be directly substituted by Ti4+ and Ge4+; (2) two neighboring Si atoms may be replaced via coupled substitution involving P5+ and trivalent cations such as Al3+, (Fe3+ or B3+); and (3) Al3+, Fe3+, and Ga3+ may be incorporated into the quartz lattice either through charge compensation by monovalent cations (Li+, Na+, K+, H+, Rb+, Cs+) or by occupying structural channel gaps aligned parallel to the c-axis of the quartz crystal.
Trace element data from the quartz of various paragenetic stages in the Liba gold deposit indicate that the Al content in quartz types Qz1, Qz2b, and Qz3—those closely associated with mineralization—is markedly higher than that in Qz0 and Qz2a, suggesting that Al3+ was likely incorporated into the quartz lattice during mineralization. Quartz from the main mineralization stage exhibits a strong positive correlation between Al3+ and the monovalent cations K+, Li+, and Rb+ (Figure 9b–d), implying that these cations were likely incorporated into the quartz lattice via charge-compensating mechanisms during the substitution of Si4+ by Al3+. This observation indicates that the hydrothermal system during the main mineralization stage was enriched in Al, K, and Li, consistent with the chemical characteristics of metamorphic fluids. Metamorphic minerals such as muscovite and chlorite release K+, Al3+, and Li+ during dehydration, thereby serving as the primary source of mineralizing fluids. The Liba gold deposit is situated within a regional shear zone, and hydrothermal fluids during the main metallogenic stage underwent an extensive interaction with the surrounding rocks during their migration along the tectonic zone, particularly through widespread silicification and Sericitic Alteration, which further facilitated the mobilization and enrichment of K+, Li+, and Rb+.
6.2.2. Advantages of Combined SEM-CL and LA-ICPMS Measurements
SEM-CL studies have further revealed a strong correlation between the trace element content in quartz and its luminescence intensity. Studies of quartz from hydrothermal deposits indicate that the SEM-CL intensity is often correlated with the content of trace elements, including Li, Na, Al, K, Ti, and Ge, within the lattice. The incorporation of these elements triggers lattice defects, thereby enhancing the SEM-CL luminescence intensity [16,58,59].
Quartz from the Qz0 stage exhibits low trace element concentrations (Figure 10a), with a weak SEM-CL intensity and evidence of recrystallization. In the Qz1 stage, quartz occurs as granular crystals (Figure 10b), suggesting precipitation from siliceous fluids under rapid crystallization within cleavages or pore spaces. This rapid crystallization likely resulted in crystal defects (e.g., dislocations, lattice distortions), which inhibit the cathodoluminescence (CL) emission. Quartz from the Qz2a stage shows significantly lower Al contents (average 33.30 ppm; Figure 10c), indicating reduced hydrothermal activity, relatively weak mineralization, and a correspondingly low CL intensity. In contrast, the Al content in quartz from the Qz2b and Qz3 stages is markedly higher, with average values of 519.13 ppm and 649.82 ppm, respectively (Figure 10c), suggesting a high degree of hydrothermal activity during mineralization. The Qz2b quartz also contains elevated Ge contents; however, it typically forms in hidden veins, which may suppress CL emissions, resulting in an overall weak CL intensity. In the Qz3 stage, both Al and K contents are elevated (averaging 649.82 ppm and 145.25 ppm, respectively), and quartz grains display oscillatory zoning (Figure 10d,e), characterized by a bright CL in the core and a darker CL at the rim. The Al concentration decreases from the core to the edge, indicating the compositional evolution of the hydrothermal fluids during crystal growth.
6.2.3. Evolution of Mineralizing Fluids
Based on the analysis of geological evidence, the mineralization of the Liba gold deposit experienced a complex, multi-stage mineralization process.
Qz1 Stage: δ18O-δD results indicate the activity of magmatic hydrothermal fluids, with relatively high contents of Na, Al, K, Ti, etc. The ore-forming temperature is within the medium-temperature range. Mineralization is characterized by the development of quartz stockworks, pyrite-bearing quartz veinlets, and widespread disseminated mineralization; however, no economic gold ore bodies had formed [23,24]. It is inferred that this stage was mainly characterized by the injection of magmatic residual fluids into the fault system through early fractures (which were later filled by granite porphyry dikes and lamprophyre veins). Although gold precipitation was not yet significant during this period, the hydrothermal system laid the foundation for subsequent fluid evolution and metal enrichment.
Qz2a Stage: Ore bodies commonly occur as vein or lenticular forms hosted within tectonic fracture zones. The tectonic activity intensified significantly, leading to the formation of siliceous veins. Gold precipitation occurred due to decreasing temperatures and the introduction of external fluids (such as formation water). It is inferred that the fracture zones not only provided effective migration pathways for meteoric fluids but also became important sites for gold deposition. Exploration data show that Au grades of this type of mineralization are around 2 g/t [24], making it one of the principal ore-forming stages in the entire gold mineralization process.
Qz2b Stage: δ18O-δD results indicate that the system evolved to be dominated by magmatic fluids. The tectonic activity remained strong during this stage, with frequent hydrothermal activity within the fracture zones, which led to extensive gold precipitation. Compared to the Qz2a stage, the contribution of metamorphic fluids decreased notably, and the hydrothermal fluids were mainly of medium- to shallow-sourced origins. Gold mineralization further intensified, resulting in the formation of stable, economic ore bodies.
Qz3 Stage: This represents the late phase of the residual magmatic hydrothermal activity, where hydrothermal temperatures and the metal-carrying capacity of the fluids declined significantly. Mineralization gradually waned, manifesting only as localized veinlet alterations or weak gold enrichment, with virtually no subsequent formation of economic ore bodies.
Cathodoluminescence (SEM-CL) imaging in this study reveals that Qz2b and Qz3 quartz samples display distinct oscillatory zoning patterns, characterized by alternating bright and dark non-periodic bands with sharply defined boundaries. These features indicate that the crystals experienced frequent and abrupt changes in their fluid composition during growth [60]. These observations suggest that the Liba hydrothermal system underwent multiple episodes of rapid pressure drops and fluid immiscibility between the main and late mineralization stages [61]. Each pressure drop likely led to the escape of dissolved CO2 and water vapor, resulting in phase separation. Subsequently, a rebound in pressure or the introduction of cooler external fluids may have triggered the re-homogenization of the fluid system and the subsequent precipitation of a new generation of quartz. The coupling between oscillatory zoning and inclusion characteristics essentially records a mineralization process driven by “periodic hydrodynamic perturbations”. Intermittent pressure fluctuations and the resulting periodic fluid immiscibility caused the destabilization and decomposition of gold–sulfur complexes, triggering large-scale gold precipitation. Consequently, the Qz2b and Qz3 stages record peak hydrodynamic instability within the hydrothermal system, corresponding to the principal episodes of gold and the associated metal deposition.
7. Conclusions
Through a comprehensive study of the ore-forming fluid at the Liba gold deposit, this paper explores the characteristics, genesis, and evolutionary mechanisms of the ore-forming fluid, revealing the complexity of the fluid system and its significant role in gold mineralization. The specific conclusions are as follows: (1). The Liba gold deposit is a medium- to low-temperature orogenic gold deposit. The gold enrichment process was primarily driven by a hydrothermal system, with variations in fluid composition during mineralization playing a key role in the gold concentration. (2). The ore-forming fluid exhibits significant temperature variation and cyclical disturbances during mineralization. SEM-CL imaging identifies five distinct quartz stages, each reflecting rapid changes in fluid properties and disturbances during the mineralization process. The main quartz stages during mineralization include the early-stage Qz1, the primary-metallogenic-stage Qz2 (divided into Qz2a and Qz2b), and the late-stage Qz3, followed by the post-mineralization-stage Qz4. (3). The ore-forming fluid of the Liba gold deposit underwent a multi-stage evolution from magmatic to metamorphic hydrothermal fluids. Based on the analysis of quartz trace elements (e.g., Al/Ti, Ge/Al ratios), fluid homogenization temperatures, and isotope data (δ18O = 8.25‰ to 12.67‰ and δD = −119.1‰ to −79.8‰), it is evident that the ore-forming fluids originated from a mixture of magmatic hydrothermal fluids and formation water, with a later-stage influence from the metamorphic water.
These three main conclusions provide a new perspective for a deeper understanding of the genesis and metallogenic mechanisms of the Liba gold deposit.
Conceptualization, Y.C.; investigation, Y.C., Y.W., D.L., J.L. and R.W.; resources, Y.W. and D.L.; data curation, Y.C.; writing—original draft preparation, Y.C.; writing—review and editing, Y.W., J.W. and J.G.; visualization, Y.C.; supervision, Y.W.; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.
The data presented in this study are available in this article.
We are grateful for the valuable support of Longnan Zijin Mining Co., Ltd. We would like to thank Changhao Li for insightful discussions. We are grateful to three anonymous reviewers for their constructive feedback that helped to improve an earlier draft of the manuscript.
Authors Yuwang Wang, Dedong Li, and Jian Geng are employees of the Beijing Institute of Geology for Mineral Resources Co., Ltd. Authors Jianxiang Luo and Rui Wang are employees of Longnan Zijin Mining Co., Ltd. This paper reflects the views of the scientists and not the companies. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.
Footnotes
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Figure 1 (a) Simplified map showing the location of the West Qinling Belt. (b) Tectonic framework of the Qinling orogenic belt (modified after [
Figure 2 Map showing geology of the Liba gold deposit and drill hole for sampling (modified after [
Figure 3 Geologic profile of the Magou line 16 (a) and Zhaogou line 119 (b) in Liba gold deposit (modified after [
Figure 4 Photographs and micrographs of typical quartz veins in the Liba gold deposit. (a) South wall of the Magou pit: hook-shaped Qz0 vein (field photo); (b) L3907-7: pyrite veins filling fractures within the Qz0 (hand specimen); (c) L3907-7: undulatory extinction in Qz0 with pyrite veins filling fractures (reflected light micrograph); (d) L3907-16: bedding-parallel arsenopyrite-Qz1 veinlets in phyllitic slate, crosscut by pyrite–carbonate-Qz3 vein (hand specimen); (e) L3907-16: bedding-parallel arsenopyrite–Qz1 veinlets in phyllitic slate, transected by pyrite–carbonate–Qz3 vein (plane-polarized light micrograph); (f) L3909-14: intergrown Qz2a veins and Qz2b siliceous stringers (hand specimen); (g) L3909-14: interlocking Qz2a veins and Qz2b siliceous stringers (cross-polarized light micrograph); (h) L3909-24: pyrite–carbonate–Qz3 vein in pyritic phyllite (hand specimen); (i) L3909-24: contact zone between pyritic phyllite and pyrite–carbonate–Qz3 vein (cross-polarized light micrograph); and (j) L3909-10: Qz4 in slate containing quartz–pyrite veins (hand specimen). Py—Pyrite; Apy—Arsenopyrite; and Cc—Calcite.
Figure 5 Representative images showing SEM-CL textures of quartz from the Liba gold deposit. (a) L3909-45, in the siliceous vein, Qz3 veinlets penetrate Qz0 along fractures; (b) L3908-1, pyrite–Qz3 veins transect sericitized Qz2a; (c) L3907-16, pyrite-bearing phyllite containing bedding-parallel Qz1 veins, overprinted by pyrite–carbonate–Qz2b veins; (d) L3907-16, enlarged view of Qz1 in (c); (e) L3907-16, enlarged view of Qz2b in (c); (f) L3909-24, euhedral Qz3 with distinct oscillatory zoning in pyrite–carbonate–Qz3 veins within pyritic phyllite; (g) L3909-10, Qz2a stringers and pyrite–Qz2b in silty slate, crosscut by carbonate–Qz3 veins; and (h) L3909-24, pyrite–carbonate–Qz4 assemblage in pyritic phyllite featuring subhedral grains with sulfide-free clean veins. Py—Pyrite; Cc—Calcite; Ser—Sericite; Circles—In situ microspot locations.
Figure 6 Box plots of trace elemental contents of quartz in different stages of the Liba gold deposit.
Figure 7 Plots of concentrations for Al vs. Ti. The colorful areas outlined for porphyry-type, orogenic Au, and epithermal deposits are after [
Figure 8 Hydrogen and oxygen isotopic compositions of quartz from the Liba gold deposit and the West Qingling gold deposits (modified after [
Figure 9 Binary diagrams of trace elements in quartz from Liba gold deposit: (a) Ge vs. Al. (b) K vs. Al. (c) Li vs. Al. (d) Rb vs. Al.
Figure 10 Comparison of SEM-CL texture and trace elements of quartz veins in the Liba gold deposit. (a) Points were derived from sample L3909-45; (b) points were derived from sample L3907-16; (c) points were derived from sample L3909-10; (d) points were derived from sample L3909-24; and (e) points were derived from sample L3909-24. The point numbers correspond to the points in
Statistics of sample collection in formation from the Liba gold deposit.
| Sample Number | Lithology | Sampling Location | Qz Type | Analytical |
|---|---|---|---|---|
| L3907-07 | Quartz vein | Zhaogou ZZK87-5, 315.3 m | Qz0 | H-O isotope |
| L3907-12 | Slate with quartz veins and pyrite | Zhaogou ZZK87-5, 122.5 m | Qz3 | H-O isotope |
| L3907-16 | Phyllitic slate | Magou MZK32-4, 163 m | Qz1, Qz2b | SEM-CL and LA-ICPMS |
| L3908-01 | Silty slate | Magou MZK32-4, 47 m | Qz2a, Qz3 | SEM-CL and LA-ICPMS |
| L3908-27 | Quartz vein | East side of platform 1833, Magou deposit pit | Qz1 | H-O isotope |
| L3908-29 | Quartz vein | North wall of platform 1833, Magou open pit | Qz0 | H-O isotope |
| L3908-30 | Slate with quartz veins and pyrite | South wall of platform 1833, Magou open pit | Qz0 | H-O isotope |
| L3908-32 | Slate with quartz veins and pyrite | West wall of platform 1833, Magou open pit | Qz0 | H-O isotope |
| L3909-03 | Slate with quartz veins and pyrite | Zhaogou ZG5210DD6, 76 m | Qz1 | H-O isotope |
| L3909-08 | Brecciated pyrite–quartz vein | Zhaogou ZG5210DD6, 134 m | Qz3 | H-O isotope |
| L3909-10 | Slate with quartz veins and pyrite | Zhaogou 99 line ZG5210DD6, 137.5 m | Qz2a, Qz2b, Qz3 | SEM-CL, LA-ICPMS |
| L3909-24 | Slate with quartz veins and pyrite | Zhaogou ZG5210DD6, 31.6 m | Qz2, Qz3, Qz4 | SEM-CL, LA-ICPMS, H-O isotope |
| L3909-38 | Quartz vein | Platform 1868, Zhaogou open pit, near granite porphyry vein | Qz2 | H-O isotope |
| L3909-45 | Siliceous vein | South of Platform 1868, Zhaogou open pit | Qz0, Qz3 | SEM-CL, LA-ICPMS, H-O isotope |
| L3909-50 | Slate with quartz veins and pyrite | West side of platform 1880, Zhaogou open pit | Qz3 | H-O isotope |
| L3911-25 | Slate with quartz veins and pyrite | Tanyaogou TZK52-2, 120.7 m | Qz2 | H-O isotope |
| L3911-27 | Slate with quartz veins and pyrite | Tanyaogou TZK52-2, 111.5 m | Qz2 | H-O isotope |
| L3911-53 | Slate with quartz veins and pyrite | Wanghe ZK51-2, 120.2 m | Qz2 | H-O isotope |
| L3911-55 | Slate with quartz veins | Wanghe TZK51-2, 137.5 m | Qz1 | H-O isotope |
| L3911-60 | Slate with quartz veins and pyrite | Madigou ZK215-2, 77.5 m | Qz2 | H-O isotope |
| L3911-61 | Slate with quartz veins and pyrite | Madigou ZK215-2, 81.6 m | Qz1 | H-O isotope |
Hydrogen and oxygen isotopic compositions of quartz from the Liba gold deposit.
| Samples | Types | δ18O (‰) | δD |
|---|---|---|---|
| L3907-07 | Qz0 | 11.64 | −90.80 |
| L3908-29 | Qz0 | 11.41 | −83.90 |
| L3908-30 | Qz0 | 10.00 | −89.10 |
| L3908-32 | Qz0 | 11.32 | −94.80 |
| L3911-25 | Qz0 | 10.17 | −82.00 |
| L3911-27 | Qz0 | 8.69 | −79.80 |
| L3911-55 | Qz1 | 12.67 | −96.20 |
| L3908-27 | Qz1 | 10.94 | −91.20 |
| L3909-03 | Qz1 | 11.32 | −96.90 |
| L3909-38 | Qz1 | 11.09 | −103.60 |
| L3911-61 | Qz1 | 11.82 | −107.50 |
| L3909-24 | Qz2 | 9.99 | −114.90 |
| L3911-53 | Qz2 | 10.98 | −119.10 |
| L3911-60 | Qz2 | 8.68 | −111.90 |
| L3909-08 | Qz3 | 8.42 | −101.20 |
| L3909-45 | Qz3 | 9.25 | −108.70 |
| L3909-50 | Qz3 | 10.10 | −116.50 |
| L3907-12 | Qz3 | 8.25 | −104.60 |
Hydrogen and oxygen isotopic compositions of quartz from the gold deposits in the West Qinling Orogen.
| Deposit | δD (‰) | δ18O (‰) | References |
|---|---|---|---|
| Yangshan | −86~−73 | 9.5~15.3 | [ |
| Zhaishang | −81~−69 | 8.71~11.01 | [ |
| Baguamiao | −78.6~−115.6 | 4.7~14.8 | [ |
| Ma’anqiao | −95~−64 | 1.4~10.6 | [ |
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; Li Dedong 2 ; Geng Jian 2 ; Luo Jianxiang 4 ; Wang, Rui 4 1 School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China, Beijing Institute of Geology for Mineral Resources Co., Ltd., Beijing 100012, [email protected] (J.G.)
2 Beijing Institute of Geology for Mineral Resources Co., Ltd., Beijing 100012, [email protected] (J.G.)
3 School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China
4 Longnan Zijin Mining Co., Ltd., Longnan 742500, China