Genesis of the Gongjuelong Sn Polymetallic Deposit in the Yidun Terrane, China: Constraints from the In Situ Geochemistry of Garnet, Cassiterite, and Quartz

Abstract

Numerous skarn-type Sn and hydrothermal vein-type Pb–Zn–Ag deposits occur in the northern Yidun Terrane, China. The Gongjuelong skarn Sn polymetallic deposit, adjacent to the Haizishan granite, is situated in the central region of Yidun Terrane. The genesis of the Gongjuelong Sn deposit and its relationship with the adjacent Pb–Zn–Ag deposits remains controversial. The ore-forming process can be divided into three stages: the prograde stage (I), marked by the formation of garnet and pyroxene; the retrograde stage (II), which includes the epidote–actinolite sub-stage (II-1) and the quartz-cassiterite sub-stage (II-2); and the sulfide stage (III), consisting of the chalcopyrite–pyrrhotite sub-stage (III-1) and the arsenopyrite–sphalerite sub-stage (III-2). Two types of garnet (Grt-I and Grt-II) have been identified in stage I and both belong to the grossular–andradite solid solution. Grt-II (Gro_52-73And_25-45Spe+Pyr+Alm_2-3) contains slightly more Fe than Grt-I (Gro_64-76And_20-28Spe+Pyr+Alm_2-10). Grt-I is enriched in heavy rare-earth elements (HREEs) and depleted in light rare-earth elements (LREEs), whereas Grt-II is enriched in LREEs and depleted in HREEs. Grt-I has higher U contents and lower Th/U ratios than those of Grt II, indicating a lower oxygen fugacity for the earlier skarn alteration. In contrast to Grt-I, Grt-II shows a more significant negative Eu anomaly along with lower LREEs/HREEs. Therefore, Grt-I and Grt-II likely formed under mildly acidic and near-neutral conditions, respectively. The W (350–3015 ppm) and Fe (235–3740 ppm) contents and Zr/Hf ratios (18.7–49.4) of cassiterite from Gongjuelong are similar to those of cassiterite from the granite-related Sn deposits, as well as the Xiasai hydrothermal vein-type Pb–Zn–Ag deposit in the northern Yidun Terrane. The Ti/Ge ratio (0.06–1.13) and P contents (13.9–173 ppm) of quartz are also similar to those from the Xiasai Pb–Zn–Ag deposit, both of which resemble those of skarn-type deposits and Sn-associated quartz. Furthermore, the Ti/Zr ratio (average 33.2) of cassiterite at Gongjuelong are much higher than that of cassiterite at Xiasai (average 3.7), indicating that the Pb–Zn–Ag veins could represent the distal product of the “parent” granite. On the basis of combined evidence from geology, geochemistry, and published geochronology data, we propose that the proximal skarn-type Sn deposits and distal hydrothermal vein-type Pb–Zn–Ag±Sn deposits in the northern Yidun Terrane constitute an integrated ore system, which is genetically related to the late Cretaceous highly fractionated granites. This proposed hypothesis highlights the potential prospecting of Sn mineralization beneath the hydrothermal Pb–Zn–Ag veins, as well as the hydrothermal Pb–Zn–Ag veins controlled by faults/fractures within the strata around the Sn deposits and highly fractionated granites.

1. Introduction

Polymetallic vein systems associated with magmatic centers exhibit distinct element zoning patterns [1]. In the central area of the systems, metals such as Sn and Cu are typically enriched, with the minerals predominantly consisting of oxides (e.g., cassiterite) and sulfides, which precipitate under high-temperature, low-oxidation conditions. As the mineralizing fluids cool, the wall rocks adjacent to the veins gradually become enriched in Pb, Zn, and Ag through the precipitation of disseminated Pb–Zn sulfides [2,3]. Element zoning within the veins is more complex, resulting from the interplay between magmatic fluid evolution and subsequent mineralization processes. The spatial distribution relationship between Sn and Pb, Zn, and Ag suggests that these elements may originate from the same mineralizing system, with fluid evolution and magmatic processes controlling their distribution patterns [4,5,6]. The presence of Sn, along with Pb–Zn–Ag–bearing minerals, not only enhances the economic potential of the deposit but also supports the possibility of polymetallic mining. Understanding the relationship between Sn and Pb–Zn–Ag mineralization is essential for effective mineral exploration and economic evaluation.

Over the past 20 years, a number of skarn Sn and hydrothermal vein-type Ag–Pb–Zn±Sn deposits have been identified in the central Yidun Terrane, located in southwestern China [7,8,9,10,11,12,13,14,15,16,17] (Figure 1a,b). These mineral deposits form the Xiasai–Lianlong metallogenic belt, which has contributed over 4.6 million tons of Pb + Zn, 17,600 tons of Ag, and 0.27 million tons of Sn, making it one of the most important metallogenic regions in China for Ag, Pb–Zn, and Sn [17,18,19,20,21]. The skarn deposits of Sn–Ag (–Pb–Zn) include the Cuomolong, Lianlong and Gongjuelong deposit, whereas the hydrothermal (Sn–) Ag–Pb–Zn deposits are best represented by the Shaxi, Jiaogenma, and Xiasai sites. A genetic relationship between the mineralized systems and nearby granitic magmatic activity has been suggested through isotopic analyses. These analyses include Biotite ⁴⁰Ar-³⁹Ar dating of Haizishan and Queershan, zircon U-Pb dating of Rongyicuo, Xiasai, and Haizishan, as well as whole-rock Rb–Sr dating of Lianlong, Rongyicuo, and Haizishan. Geochronology suggests that ore-associated granites formed between 105 and 89 Ma [8,14,22,23]. These late Cretaceous granites are classified as A-type granites, which are characterized by high contents of SiO₂, Nb, Ta, Ga, and Y, and lower concentrations of Al₂O₃, CaO, MgO, Sr, and Eu [8,24].

Previous studies on the Sn–Pb–Zn–Ag polymetallic deposits in the central Yidun Terrane have primarily focused on the genesis of individual deposits [4]. However, whether the skarn Sn deposits and hydrothermal vein-type (Sn-) Pb–Zn–Ag deposits in this region formed as a result of the same metallogenic system remains a subject of debate. Li et al. [4,5] conducted geochronological studies using multiple methods: (1) LA-ICP-MS U-Pb dating of cassiterite from quartz-cassiterite veins yielded an age of 99.2 ± 0.8 Ma, (2) Rb–Sr dating of sphalerite from quartz–sulfide veins gave an age of 99 ± 3 Ma, and (3) zircon U-Pb dating of the mineralizing host rocks produced an age of 101 ± 1 Ma. Their studies confirmed a genetic link between Sn mineralization and Pb–Zn–Ag mineralization at the Xiasai deposit. However, research on their link to skarn Sn deposits remains insufficient.

The Gongjuelong Sn deposit is situated within the central Yidun Terrane. This study employs EMPA major element analysis on garnet and cassiterite from Gongjuelong, along with LA-ICP-MS trace element analysis on indicator minerals, including garnet, cassiterite, and quartz. The physicochemical conditions of mineralization remain unclear, and the genetic relationship between Sn mineralization and Pb–Zn–Ag mineralization has not yet been well defined. To address these questions, we will use geochemical data from garnet to discuss the metallogenic physicochemical conditions of the Gongjuelong Sn deposit. Additionally, we will analyze the geochemical data of cassiterite and quartz, combined with existing data from the Pb–Zn–Ag deposit in the central Yidun Terrane, to investigate the genetic relationship between Sn mineralization and Pb–Zn–Ag mineralization. This study aims to provide a theoretical foundation for future mineral exploration in the region.

2. Geological Setting

The Yidun Terrane is located in the eastern Tibetan Plateau, between the Qiangtang and Songpan–Garze terranes (Figure 1a). They are separated by two major oceanic sutures: the Ganze–Litang Suture and the Jinsha River Suture, which extend several thousand kilometers in the NNW direction [29,30,31,32]. The Xiangcheng–Geza Fault divides the Yidun Terrane into the Western Yidun Terrane (Zhongza Block) and the Eastern Yidun Terrane (Figure 1b). The Gongjuelong mining area lies on the western flank of the Yidun-Moyaba synclinorium, exhibiting diverse stratigraphy and lithology, with Permian, Triassic, and Quaternary sequences. The Permian strata (P) mainly consist of carbonate and clastic rocks, with minor volcanic intercalations, forming a belt along the Shamake-Namaqu Fault. The Triassic strata are the most widely exposed, including the Tumugou, Qugasi, Dang’en, Lieyi, and Labaya Formations, which are primarily composed of metamorphic sandstone, slate, and limestone.

The Yidun Terrane forms a part of the Yangtze–Qiangtang tectonic domain. Its southeastern boundary adjoins the Yangtze Craton, whereas its western boundary is linked to the Qiangtang–Changdu Block. The regional tectonic framework includes the Yajiang Back-Arc Basin Fold Belt, the Ganza–Litang Ophiolitic Mélange Zone, the Yidun Arc Fold Belt, the Zhongza Block, and the Jinsha River Ophiolitic Mélange Zone (Figure 1b). The Yidun Terrane has undergone tectonic processes from the Indosinian to the Yanshanian and Himalayan periods, encompassing seafloor spreading, subduction, arc-continent collision, and intracontinental convergence [8,25,33]. The region is dominated by three primary structural orientations: NNW-SN, NW, and EW-NE.

Permian basalts and metamorphosed volcanic rocks are limited in exposure, primarily along the southern margins of the Zhongza Block and Yidun Terrane. The volcanic rocks of the Yidun Terrane display a north–south differentiation: the northern region is dominated by calc-alkaline andesites, rhyolites, and basalts [31], whereas the southern region features calc-alkaline andesites, trachytes, dacites, and minor alkaline basalts [26].

The Yidun Terrane underwent multiple magmatic events from the Triassic to Late Cretaceous, producing hundreds of intermediate to felsic plutonic intrusions. The distribution of these intrusions indicates the formation of several large-scale plutons during 230–204 Ma, whereas smaller, more dispersed intrusions and porphyries formed during 105–80 Ma [15,27,28,34,35]. The southern Yidun Terrane hosts Late Triassic oxidized and less-differentiated granitic porphyries and Late Cretaceous acidic porphyries. In the northern central part of the Yidun Terrane, the Changduoke–Genie Granite Belt extends for approximately 160 km and is composed of eight major granitic plutons, along with smaller stocks and dikes, including those at Hagala (Haizishan), Rongyicuo, Lianlong, and Ruoluolong. These NNW-trending plutons intrude Permian to Upper Triassic strata, creating alteration zones several hundred to over a thousand meters wide. The plutons are generally homogeneous, primarily composed of biotite monzogranite.

3. Ore Deposit Geology

The Gongjuelong Sn deposit is located in the eastern part of the Batang Deda Township, approximately at 99°27′10″ E and 30°14′10″ N, with an elevation of 4590 m. From west to east, the stratigraphy in the mining area includes the Upper Triassic Qugasi Formation (T₃q), comprising the third subunit (T₃q³), composed of sandstone, slate, and quartzose conglomerates, and the fourth subunit (T₃q⁴), consisting of crystalline limestone, bioclastic limestone, minor sandstone, and slate. The Tumugou Formation (T₃t¹) consists of quartzose conglomerates, pebbly sandstones, siltstones, and rhythmic sequences of slate (Figure 2a). These rocks show varying degrees of hornfelsic and marble alteration. The area is characterized by significant folding and faulting, with four major fault sets (NNW-, NW-, EW-, and NE-trending) intersecting, causing widespread deformation. NNW-trending faults dominate the structural framework, hosting NEE-trending ore-bearing secondary faults that intersect with EW-trending faults, reflecting compressional stress followed by extension. NW-trending faults, arranged en echelon, extend 2–5 km and show evidence of at least two periods of movement, playing a key role in ore control [36]. Two sets of joints (NW- and NE-trending) are also present: ore veins and dikes are located along the NW-trending joints, whereas NE-trending joints crosscut the ore veins. The fault activity sequence is NNW, followed by NW, EW, and NE, with the NNW- and NW-trending faults showing multiple phases of movement.

Magmatic rocks in the area belong to the Haizishan complex granite intrusion, which was formed during the first stage of the Late Yanshanian period. These rocks primarily consist of fine-grained biotite monzogranite, with common occurrences of granite stocks, apophyses, and dikes trending NNW and NW. The main ore-related pluton is a medium- to coarse-grained biotite monzogranite stock, measuring approximately 280 m in length and 50–70 m in width. It intrudes conformably into the fourth member of the Qugasi Formation, with a NW strike and NE dip.

Within an area of approximately 1 km², 11 ore bodies have been identified (Figure 2b,c). These deposits are mainly hosted in skarns, occurring along the contact zones between granite and marble or hornfels, as well as within secondary faults and interlayered slip zones. The ore deposits align in a NW direction along the eastern slope of Gongjuelong. The No. III orebody is situated along the contact zone between the granite stock and marble. Due to the NE dip of the contact planes of the stock, the SW section has marble as the footwall and granite as the hanging wall, whereas this relationship is reversed in the NE section. No. III orebody has five surface outcrops, with the longest being 163 m and a thickness ranging from 0.9 to 3.2 m, exhibiting vein-like or banded morphologies and local bead- or spindle-shaped structures. Other outcrops extend 1065 m, with a thickness of 0.24 m, showing vein-like and lenticular forms, all dipping NE at angles between 35° and 77°. Four additional ore bodies (I, II, IV, V) occur within minor faults or interlayered slip zones, showing vein-like and lenticular morphologies, with lengths of 132 m and thicknesses of 0.24 m. Some ore zones are concordant with the host rock structures, whereas others exhibit oblique relationships. The majority of the ore exhibits disseminated textures, with minor massive and vein-like ores. Alteration associated with mineralization is predominantly skarnization, along with greisenization, tourmalinization, fluorite alteration, silicification, and carbonate alteration.

The ore textures are mainly disseminated and massive, with minor vein and network-like structures. Ore minerals include cassiterite, chalcopyrite, pyrrhotite, and arsenopyrite, with minor sphalerite and pyrite. Gangue minerals consist primarily of quartz, calcite, garnet, diopside, epidote, and actinolite, with secondary and accessory minerals including hedenbergite, sillimanite, chlorite, apatite, and fluorite. Based on macroscopic observations and microscopic analysis, the mineralization of the Gongjuelong deposit can be divided into three stages: the prograde stage (I), represented by the garnet-pyroxene stage; the retrograde stage (II), which includes the epidote–actinolite sub-stage (II-1) and the quartz–cassiterite sub-stage (II-2); and the sulfide stage (III), consisting of the chalcopyrite–pyrrhotite sub-stage (III-1) and the arsenopyrite–sphalerite sub-stage (III-2).

The mineralization stages are described as follows (Figure 3 and Figure 4): The skarn formation begins with the garnet–pyroxene stage (I), characterized by high-temperature, anhydrous silicate minerals. Hand specimens exhibit a distinct reddish hue, dominated by massive garnet and locally associated with diopside, while columnar apatite is occasionally enclosed by early garnet (Grt-I). As the system cools, the epidote–actinolite stage (II-1) initiates, marked by the formation of hydrous minerals such as coarse epidote (up to 15 cm) and radiating actinolite, accompanied by minor chlorite and sillimanite. The ore exhibits a dark green color, with quartz and calcite filling the interstitial spaces, while actinolite is partially altered to chlorite. The quartz–cassiterite stage (II-2), where cassiterite becomes predominant, occurring in disseminated textures with quartz and calcite. Cassiterite is brown, short-prismatic, and exhibits distinct zoning, occasionally intergrown with actinolite and sillimanite. Under the microscope, cassiterite displays well-developed euhedral forms, with some grains showing surface dissolution features and minor fractures. The chalcopyrite–pyrrhotite stage (III-1) follows, characterized by the precipitation of chalcopyrite and pyrrhotite in disseminated forms, closely associated with quartz and calcite. These sulfides replace skarn minerals, including epidote and actinolite, with minor arsenopyrite and sphalerite also present. Finally, the arsenopyrite–sphalerite stage (III-2) is marked by quartz–calcite–sulfide veins cross-cutting earlier disseminated ores. These veins contain large quartz and calcite crystals, with arsenopyrite and sphalerite as the main sulfides, accompanied by minor chalcopyrite, pyrrhotite, and fluorite. This sequence of stages reflects the evolution from high-temperature skarn formation to low-temperature hydrothermal alteration and sulfide mineralization.

4. Sampling and Analytical Methods

In this study, major element analysis of garnet and cassiterite from the Gongjuelong deposit was conducted using electron probe microanalysis (EPMA). Additionally, trace element analysis was performed on two types of garnet from the garnet–skarn stage, cassiterite from the quartz–cassiterite stage, and quartz from the sulfide stage of the Gongjuelong deposit using LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry).

4.1. Electron Probe Micro-Analysis (EPMA)

The composition of garnet was analyzed by Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd., Nanjing, China. The electron probe microanalyzer used was a JEOL JXA-iSP100 (JEOL, Tokyo, Japan). The instrument was operated with an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam diameter of 5 µm. For the elements Ti and Y, the peak counting time was set to 10 s, whereas for Si, Ca, K, P, Na, Mg, Al, Cr, Fe, and Mn, it was 5 s. The background counting time was set to half the peak time for both high- and low-background positions. All data were corrected for matrix effects using the ZAF correction method. The following standards were employed: 53-19 Chrome Diopside (Si), 53-23 Titanium Oxide (Ti), 53-19 Chrome Diopside (Ca), 53-40 Yttrium Al Garnet (Y), 53-13 Orthoclase (K), 53-20 Ap-5nA-20 um (P), 53-06 Albite (Na), 53-19 Chrome Diopside (Mg), 53-28 Almandine (Al), 53-24 Chromium Oxide (Cr), 53-26 Rhodonite (Mn), and 53-28 Almandine (Fe); the standard deviation (sigma) is typically 0.1–0.5 wt% for major elements (>1 wt%) and 0.01–0.1 wt% for trace elements (<1 wt%).

The composition of cassiterite was also analyzed by Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd., Nanjing, China. The electron probe microanalyzer used was a JEOL JXA-iSP100 (JEOL, Tokyo, Japan). The accelerating voltage, beam current, and beam diameter were identical, set at 15 kV, 20 nA, and 5 µm, respectively. For Sn, the peak counting time was 5 s, whereas for Ti, Nb, Zr, Si, Al, W, Ta, Fe, and Mn, it was 10 s. Similar to the garnet analysis, the background counting time was half the peak time for both high- and low-background positions. The ZAF correction method was applied to all data to account for matrix effects. The following standards were used: 53-23 Titanium Oxide (Ti), 53-43 Cassiterite (Sn), 44-21 Niobium (Nb), 53-41 Cubic Zirconia (Zr), 53-19 Chrome Diopside (Si), 53-06 Albite (Al), 44-34 Tungsten (W), 44-33 Tantalum (Ta), 53-33 Hematite (Fe), and 53-26 Rhodonite (Mn).

4.2. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)

Trace element analysis of cassiterite was performed at Guangzhou Tuoyan Analytical Technology Co., Ltd., Guangzhou, China. The laser ablation system used was a GeoLasPro 193 excimer laser solid sample system (Coherent, Santa Clara, CA, USA), coupled with an X Series 2 quadrupole inductively coupled plasma mass spectrometer (ICP-MS) from Thermo Fisher (Waltham, MA, USA). Helium (He) was employed as the carrier gas, whereas argon (Ar) served as the make-up gas. Analysis was carried out in single-point ablation mode with a laser spot size of 60 μm, frequency set to 6 Hz, and energy density of 7 J/cm². A 20 s background signal acquisition period was followed by 50 s for sample signal collection. NIST610 standard glass [37] was used as the external standard, and the theoretical Sn concentration in cassiterite was applied as the internal standard for trace element calibration. NIST612 was utilized as the monitoring standard to ensure data accuracy. Post-analysis data processing was conducted using software ICPMSDataCal 10.7; the standard deviation is generally 0.1–1 wt% for major elements and 0.01–0.1 wt% for trace elements.

For the trace element analysis of quartz, the study was performed at the Ore Deposit Geochemistry Microanalysis Laboratory, which is part of the State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences, Beijing. The analysis was conducted using a GeoLasPro 193 nm excimer solid-state sampling system (Coherent, Santa Clara, CA, USA) coupled with an X Series 2 quadrupole ICP-MS (Thermo Fisher Scientific, Waltham, MA, USA). Helium (He) was used as the carrier gas, and argon (Ar) served as the compensating gas. The analysis was performed in single-spot ablation mode with a laser spot size of 60 μm, a frequency of 6 Hz, and an energy density of 12 J/cm². A 20 s period was used for background signal acquisition, followed by 50 s for the sample signal collection. NIST610 standard glass was employed as the external standard, and the theoretical silicon (Si) content in quartz was used as the internal standard for trace element calibration. A natural quartz standard, Q7, was utilized for monitoring purposes [38]. Data processing was conducted using software ICPMSDataCal 10.7 [39].

The trace element analysis of garnets was carried out at the Ore Deposit Geochemistry Microanalysis Laboratory at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing. The GeoLasPro-193 nm system from Coherent was used for laser sampling, and ion signal intensities were measured using Thermo Fisher’s X-Series 2 ICP-MS. Helium was used as the carrier gas, with argon mixed as the make-up gas. The analysis was performed in single-spot ablation mode with a 32 μm spot size and a frequency of 6 Hz. Further details regarding the operating conditions and procedures are provided by Peng et al. [40].

5. Results

5.1. Major Elements of Garnet and Cassiterite

5.1.1. Garnet

The electron probe microanalysis (EPMA) results for garnet samples are presented in Table S1. The composition of garnet in Gongjuelong is presented in Figure 5 and Figure 6.

Grt-I: The SiO₂ content ranges from 35.9% to 35.3%, with an average of 35.7%. The CaO content is 33.6–35.4%, averaging 35%. Al₂O₃ is relatively high, ranging from 13.8% to 16.1%, with an average of 14.9%. FeO content ranges from 5.7% to 7.6%, with an average of 6.6%. TiO₂, MgO, and MnO are lower (0.5%–3.1%, 0.4%–2.2%, and 0.2%–0.3%, respectively), and P₂O₅, Na₂O, and K₂O are all ≤0.01%. Based on the major elements, the garnet is primarily composed of grossular (Gro), with 64%–76%, followed by andradite (And) at 21%–28%, with small amounts of pyrope (Pyr) (2%–10%) and spessartine (Spe) (0.5%–0.8%). Grt-I belongs to the grossular (Gro)–andradite (And) series (Gro _64-76, And _20-28, Spe + Pyr + Alm _2-10, average = And₇₀, Gro₂₄, Spe + Pyr + Alm₆) (Figure 5 and Figure 6, Table S1).

Grt-II: SiO₂ ranges from 36.7% to 37.9%, with an average of 37.3%. CaO ranges from 33.6% to 34.3%, with an average of 33.9%. The FeO content is higher, ranging from 10.4% to 15.7%, with an average of 15.5%, whereas Al₂O₃ content decreases (10.4%–15.4%, average 13.5%). The content of MnO is 0.5%–0.9%, and TiO₂ content is lower than Grt-I, ranging from 0.04% to 0.20%, with an average of 0.12%. Other elements (MgO, Na₂O, P₂O₅, K₂O) are all < 0.01%. Grt-II is mainly grossular (Gro) with some andradite (And), belonging to the Gro-And series (Gro_52-73, And_25-45, Spe + Pyr + Alm_1.5-2.4). Spe and Pyr range from 1.2% to 2.1% and 0.1% to 0.4%, respectively. The major element variations indicate an increase in FeO and a decrease in Al₂O₃ from Grt-I to Grt-II. Both Grt-I and Grt-II belong to the grossular–andradite solid solution series, but Grt-II contains a higher proportion of andradite and a lower proportion of grossular compared to Grt-I. The contents of other components, such as spessartine, pyrope, and almandine, show a significant decrease.

5.1.2. Cassiterite

The major element composition of cassiterite from the main mineralization stage of the Gongjuelong deposit was analyzed using electron probe microanalysis, and the results are presented in Table S2. The two samples (GJL-3-1 and GJL-3-3) reveal that the most abundant trace elements include Fe, Ti, Nb, Ta, and W, whereas the secondary trace elements are Zr, Al, Mn, and Si. Electron probe analysis detected small amounts of TiO₂ (<0.72 wt%), WO₃ (<0.35 wt%), FeO (<0.65 wt%), Nb₂O₅ (<0.07 wt%), and Ta₂O₅ (<0.07 wt%) in cassiterite from the Gongjuelong deposit. Additionally, trace amounts of ZrO₂ (<0.04 wt%), Al₂O₃ (<0.04 wt%), MnO (<0.02 wt%), and SiO₂ (<0.04 wt%) were also detected. In the binary diagrams, it is observed that the SnO₂ content decreases gradually from the core to the rim of the cassiterite, whereas the contents of TiO₂ and FeO increase. The contents of Ti, Fe, W, Nb, and Sn exhibit a negative correlation (Figure 7).

5.2. In Situ Trace Elements of Garnet, Cassiterite, and Quartz

5.2.1. Garnet

The LA-ICP-MS rare-earth element (REE) data for garnet (Table S3) reveal relatively high total REE contents, with ΣREE (excluding Y) ranging from 42.6 ppm to 203 ppm. For Grt-I, the total REE content is significantly higher, with ΣREE spanning from 165 ppm to 3973 ppm, averaging 1022 ppm. The content of LREEs ranges from 114 ppm to 3757 ppm, comprising 93% of the total rare-earth element (REE) content, whereas the HREEs range from 27.2 ppm to 216 ppm. The LREE/HREE ratio ranges from 2.3 to 32.8, with a mean of 11.6. The REE distribution pattern shows a pronounced enrichment in light REEs and a marked rightward inclination (Figure 8). A significant negative Eu anomaly (δEu = 0.1 to 0.7, average = 0.28) and a slight negative Ce anomaly (δCe = 0.8 to 1.0, average = 0.9) are observed. The (La/Yb)_N ratios range from 1.8 to 97.3, indicating notable fractionation between light and heavy REEs, as well as between LREEs and HREEs.

For Grt-II, the REE content is comparatively lower, with ΣREE values ranging from 31.2 ppm to 402 ppm, with an average of 62.9 ppm. The LREE content varies from 6.4 ppm to 332 ppm, whereas the HREE content ranges from 15.2 ppm to 70 ppm. The LREE/HREE ratio ranges from 0.2 to 4.7, with an average of 0.8. The REE distribution pattern shows enrichment in HREEs, with a moderate leftward slope. A moderate negative Eu anomaly (δEu = 0.04 to 0.2, average = 0.10) and a weak positive Ce anomaly (δCe = 0.7 to 1.9, average = 1.3) are noted. The (La/Yb)_N ratios range from 0.01 to 5.4, indicating significant fractionation between LREEs and HREEs.

5.2.2. Cassiterite

The trace element content of cassiterite from the Gongjuelong deposit is summarized in Table S4. The results indicate that the trace element concentrations in cassiterite vary significantly (Figure 9). The most abundant trace elements are Fe, Ti, and W, with concentrations ranging from 235 to 3740 ppm, 350 to 3015 ppm, and 1.9 to 6364 ppm, respectively. All cassiterite grains analyzed contain detectable amounts of Nb and Zr, with Nb concentrations ranging from 1.6 to 694 ppm and Zr concentrations varying from 1 to 190 ppm. Additionally, other trace elements present in cassiterite include Mn (<23.7 ppm), Hf (0.1–5.8 ppm), Ta (<6.5 ppm), Sc (<14.8 ppm), Ga (0.2–1.7 ppm), Ge (<2.1 ppm), and U (0.7–138 ppm). The concentrations of REEs, Rb, and Sr in cassiterite are generally below detection limits. A positive correlation is observed between the contents of W, Mn, Fe, Hf, Zr, Nb, and Ta.

5.2.3. Quartz

The trace element content of quartz from the Gongjuelong deposit are provided in Table S5. and illustrated in Figure 10. The most abundant trace elements in quartz are Li, Al, Ti, Ge, and P. Among these, Li, Al, and Ti are primarily used for interpretation. The trace element concentrations of quartz in stage III of the Gongjuelong deposit are as follows: Li content ranges from 0.1 to 23.4 ppm, with an average of 10.0 ppm. Al concentrations vary between 35.8 and 185 ppm, with an average of 86.5 ppm. Ti ranges from 0.9 to 2.8 ppm, with an average of 1.0 ppm. Ge concentrations range from 1.6 to 5.5 ppm, with an average of 4.0 ppm. P content varies from 14.0 to 173 ppm, with an average of 64.3 ppm.

6. Discussion

6.1. Substitution Mechanisms

6.1.1. Garnet

The REEs in garnet are valuable for tracing fluid sources and fluid–rock interactions, owing to their similar chemical properties, which allow for their fractionation under specific geological conditions [43]. The general formula of garnet is X₃Y₂Z₃O₁₂, where X denotes divalent cations, Y represents trivalent cations, and Z predominantly contains Si⁴⁺ in the silicon tetrahedral structure. The primary mechanisms of trace element incorporation into garnet crystals are surface adsorption, substitution, and isomorphic replacement [44]. U, REE³⁺, and Y³⁺ commonly substitute for Ca²⁺ in the X site of the garnet lattice [45,46,47]. Based on the ionic radii and their effects on garnet’s crystal structure, the likely substitution site for REEs is the octahedral X site (e.g., Ca²⁺) [48]. Given that Eu²⁺ has an ionic radius of 1.12 Å, which is similar to that of Ca²⁺ (0.99 Å), and both ions share the same charge, Eu²⁺ is more likely to substitute for Ca²⁺ at the X site [45]. However, the substitution behavior of REE³⁺ and Eu²⁺ in garnet differs, which requires coupled substitution or the formation of vacancies to maintain charge balance [45,49]. Previous studies have proposed several mechanisms to maintain charge balance [45,48,50,51], which are summarized as follows:

[X²⁺]VIII-2[X⁺]VIII+[REE³⁺]VIII

(1)

[X²⁺]VIII-[REE³⁺]VIII+[Si⁴⁺]IV-[Z³⁺]IV

(2)

[X²⁺]VIII-[REE³⁺]VIII+[Y³⁺]IV-[Y²⁺]IV

(3)

[X²⁺]VIII-3[]VIII+[REE³⁺]VIII

(4)

In these equations, X⁺ represents Na⁺, X²⁺ mainly refers to Ca²⁺, Z³⁺ represents Al³⁺ or Fe³⁺, Y³⁺ is predominantly replaced by Al³⁺, Y²⁺ corresponds to Mg²⁺ or Fe²⁺, and [ ] denotes vacancies (Ca). When the Na⁺ content is high in garnet, Na⁺ can substitute into the lattice to maintain charge balance, thereby promoting the incorporation of REEs through the first mechanism [50,52]. However, the content of Na in garnet from Gongjuelong is predominantly below detection limits, indicating that the first mechanism is not the primary mechanism for REE incorporation. If REEs are incorporated into garnet through the second mechanism, charge compensation occurs by substituting trivalent cations into the tetrahedral sites [45,53]. In the element correlation diagrams for garnet samples from the Gongjuelong deposit, no clear correlation is observed between ΣREE contents and Al or Fe (Figure 11b,d), indicating that the second substitution mechanism is unlikely to be the dominant mechanism for REE incorporation. Furthermore, the low content of Mg in both garnet types (Table S1) further indicates that Mg is insufficient to facilitate this substitution. Consequently, the third substitution mechanism is unlikely to be the dominant process in this region. A certain degree of linear positive correlation with ΣREE is exhibited only by Eu. Therefore, the fourth substitution mechanism is likely the primary mechanism for REE incorporation into garnet. However, previous studies have demonstrated the substitution of REEs in garnet is not solely governed by crystal chemistry [54,55]; it is also significantly influenced by the physicochemical features of the fluid (e.g., pH, f_O2), the water/rock ratio, and the dynamics of mineral growth.

6.1.2. Cassiterite

The substitution mechanisms of elements in cassiterite primarily involve: (1) Fe³⁺ + (Nb, Ta)⁵⁺ → 2Sn⁴⁺; (2) 2(Nb, Ta)⁵⁺ + (Fe, Mn)²⁺ → 3Sn⁴⁺; (3) Ti⁴⁺, Zr⁴⁺, U⁴⁺ → Sn⁴⁺; (4) V⁵⁺ + Sc³⁺ → 2Sn⁴⁺ [55,56,57,58]. In magmatic cassiterite, such as that found in rare-element-rich granites and pegmatites, the dominant substitution mechanism is 2(Nb, Ta)⁵⁺ + (Fe, Mn)²⁺ → 3Sn⁴⁺ [59].

The Nb and Ta contents of cassiterite in the Gongjuelong Sn deposit are similar to those in the Xiasai deposit, whereas the Fe and Mn contents are significantly higher (Figure 12b). These values are comparable to those observed in cassiterite from tin-bearing granites and skarn ores in the Gejiu deposit [60]. The lack of clear correlation between Fe + Mn and Nb + Ta (Figure 12f) further indicates that none of these substitution mechanisms are the dominant processes in the Gongjuelong deposit. In cassiterite, the simplest charge balance mechanism is achieved by substituting two Sn⁴⁺ ions with a trivalent cation (e.g., Fe³⁺, Al³⁺, Sc³⁺, Ga³⁺) and a pentavalent cation (e.g., V⁵⁺, Nb⁵⁺, Ta⁵⁺). Should this mechanism be dominant, a 1:1 charge balance between trivalent and pentavalent cations would be expected. However, the contents of trivalent cations (predominantly Fe³⁺) are much greater than pentavalent cations (Table S4), and this saturation is also observed in the large tin deposits of Gejiu, Dachang, and Xiasai [5,60,61]. This suggests the need for an additional mechanism to accommodate the surplus trivalent cations. Therefore, we propose that the substitution mechanism Fe³⁺ + OH⁻ → Sn⁴⁺ + O²⁻ is the primary process [58,59].

Moreover, cassiterite from the Gongjuelong deposit displays relatively high concentrations of W and Ti, which show a significant negative correlation with Sn (Figure 7d). These elements (W⁴⁺, Ti⁴⁺) can directly substitute for Sn⁴⁺ in the lattice.

6.1.3. Quartz

To interpret the incorporation and coupled substitution of major cations into the quartz crystal lattice, trace element concentrations were converted to atoms per formula unit (Table S5, Figure 13a–c). The content of Al³⁺ (190 apfu) is significantly higher than that of other trivalent cations, such as B³⁺ (73.5 apfu) and Sb³⁺ (0.5 apfu), in quartz from the Gongjuelong deposit. This is attributable to the fact that Al is commonly found in the Earth’s crust, and its ionic radius is similar to that of Si⁴⁺ (r = 0.34 Å) and Al³⁺ (r = 0.47 Å). Consequently, the substitution of Al³⁺ for Si⁴⁺ in the quartz lattice, resulting in the formation of the [AlO₄/M⁺]⁰ structural center, is the primary mechanism for trace element incorporation in quartz [63]. The M+ ions can include H⁺, Li⁺, Na⁺, K⁺, Cu⁺, and Ag⁺ [64]. The trivalent cations are mainly dominated by Al³⁺, followed by Sb³⁺ and B³⁺. For monovalent cations, Li⁺ is the most abundant, with Na⁺ and K⁺ following. The pentavalent cations are predominantly represented by P⁵⁺. Overall, the incorporation of cations into the quartz crystal lattice is mainly governed by the coupled substitution processes: (Al³⁺, Sb³⁺, B³⁺) + (Li⁺, Na⁺, K⁺, H⁺) ↔ Si⁴⁺ and (Al³⁺, Sb³⁺, B³⁺) + (P⁵⁺) ↔ 2Si⁴⁺ [64,65,66,67].

Previous studies have shown that the substitution mechanism for trace elements entering quartz may involve Sb³⁺ + Li⁺ ↔ Si⁴⁺, according to the Li⁺/Al³⁺ ratio in quartz higher than 1:1. By contrast, the Li⁺/Al³⁺ ratio is approximately 1:1, and the coupled substitution mechanism may be Al³⁺ + Li⁺ ↔ Si⁴⁺. Conversely, when the Li⁺/Al³⁺ ratio is less than 1:1, it suggests competition between Na⁺, K⁺, or P⁵⁺ with Li⁺, which couples with Al³⁺ to replace Si⁴⁺ in the quartz lattice [63,68]. In the Gongjuelong deposit, the Li⁺/Al³⁺ ratio (≈1:2) and (M⁺+M⁵⁺)/M³⁺ ratio (≈1:1) in quartz suggest two potential cation substitution mechanisms: (1) coupled substitution via Al³⁺ + (Li⁺/Na⁺/K⁺) ↔ Si⁴⁺ and (2) charge-balanced substitution through Al³⁺ + P⁵⁺ ↔ 2Si⁴⁺.

6.2. Geochemistry Significance of the Garnets

Variations in the physicochemical conditions during the formation of garnet in skarn deposits affect the chemical composition of the garnet. Thus, the chemical compositions of garnet can serve as a proxy for the physicochemical conditions during its formation [53]. Previous studies have demonstrated that andradite typically forms in oxidizing environments, whereas grossular tends to form in reducing environments [68,69,70].

Grt-I in the Gongjuelong Sn deposit is primarily composed of grossular and minor amounts of andradite and trace amounts of manganese–aluminum garnet and magnesium–aluminum garnet (Gro_64-76And_20-28Spe+Pyr+Alm_2.39-10.33). It is similar to late-stage garnets from the skarn-type copper deposits in the Hongniu area of western Yunnan [71]. This indicates that Grt-I is enriched in Al and formed in a low-oxygen fugacity, moderately acidic environment. In terms of Grt-II, grossular remains dominant, and the contents of andradite increase. Grt-II is part of the solid solution series between grossular and andradite (Gro_52-73And_25-45Spe+Pyr+Alm_1-2), with the andradite component being slightly lower than that of grossular, suggesting that Grt-II formed in a weakly reducing, near-neutral environment. From Grt-I to Grt-II, a marked increase in FeO content is observed, whereas the Al₂O₃ content decreases slightly (Figure 5; Table S1). In Grt-II, the grossular component decreases, whereas the andradite component gradually increases. This suggests that the oxygen fugacity of garnet in the Gongjuelong deposit increased from Grt-I to Grt-II, with the ore-forming environment evolving from relatively reducing to relatively oxidizing conditions, reflecting the progressive changes in the ore-forming environment. However, the end-member composition remains dominated by grossular, indicating that both types of garnet in the Gongjuelong deposit formed under reducing conditions. Notable differences in the REE characteristics are observed between these two types (Figure 8). The REE distribution pattern of Grt-I exhibits a steep rightward skew, whereas Grt-II shows a more gradual leftward skew. The left-skewed REE distribution pattern in Grt-II is similar to that in garnet from the Shizhuyuan [72] and Jinchuantang [73] deposits in southern Hunan, China, whereas the right-skewed pattern in Grt-I is comparable to that in garnet from the Huangshaping deposit [74] in the same region. In addition to the differences in REE distribution patterns, the Eu anomalies in the two types of garnet also differ significantly with Grt-II showing a more pronounced negative Eu anomaly (Figure 8). Moreover, the LREE/HREE ratios differ between the two garnet types. Grt-I exhibits a higher LREE/HREE ratio (2.3 to 32.8, average 11.6). In contrast, Grt-II shows a lower ratio (0.2 to 4.7, average 0.8).

It has been proposed that pH is a key factor in REE fractionation [75]. In mildly acidic environments, the REE distribution pattern of garnet is more significantly influenced by Cl⁻, which enhances the stability of soluble Eu²⁺ relative to REE³⁺, leading to a pronounced positive Eu anomaly, LREE enrichment, and HREE depletion. On the other hand, near-neutral conditions lead to fluids that are enriched in HREEs and depleted in LREEs, typically showing negative or no Eu anomalies [75,76]. At Gongjuelong, Grt-I exhibits a clear negative Eu anomaly and a steep rightward skew in the REE distribution pattern, whereas Grt-II shows a more significant negative Eu anomaly than Grt-I, along with a gradual leftward skew in the REE distribution pattern. This suggests that Grt-I and Grt-II likely formed under mildly acidic and near-neutral conditions, respectively. The U content in garnet can act as a proxy for the redox conditions of hydrothermal fluid systems [41,59,72]. Due to differences in ionic radii, U⁴⁺ is more likely to substitute into garnet than U⁶⁺. As a result, the lower U concentration and higher Th/U ratio in Grt-II suggest a more oxidizing environment (higher f_O2) compared to Grt-I (Figure 11e). Both Grt-I and Grt-II exhibit a strong linear correlation between Y and Ho (Figure 14a). In the Y/Ho-La/Ho diagram (Figure 14b), both Grt-I and Grt-II align along the same horizontal line, suggesting that they are products of the same ore-forming fluid. Wang [77] conducted micro-thermometry and Laser Raman Spectroscopy on fluid inclusions in garnet from the Gongjuelong deposit. The results indicate that the early ore-forming fluids were characterized by high homogenization temperatures (T_h = 435.2–500 °C) and high salinities (36.9–42.5 wt%), representing a NaCl-H₂O-CH₄ system. The presence of CH₄ indicates that the early ore-forming fluids were relatively reducing, consistent with the discussion of garnet.

6.3. Genetic Model and Implications for Exploration

To address the genetic relationship between Sn mineralization and Pb–Zn–Ag mineralization in the Yidun Terrane, this study systematically compares the geology, major/trace elements of cassiterite and quartz, isotopes (e.g., S, Pb, and H-O), fluid inclusions, and mineralization ages of the two deposit types using the data from this study and references (

Table 1. Comparison of metallogenic factors between the Gongjuelong Sn deposit and the Xiasai Ag–Pb–Zn deposit.

Type	Location	Host Rock	Mineralization Age	Mineral Assemblage	Sulfur Isotopes (‰)	Lead Isotopes	Hydrogen and Oxygen Isotopes of Quartz (‰)	Fluid Inclusions		Reference
								Th (°C)	Salinities (wt%)
Gongjuelong skarn Sn deposit	Situated within the contact zones between granite and Triassic carbonate rocks.	Skarns, marble, or hornfels	Cassiterite U-Pb 97 ± 2 Ma	Ore minerals include cassiterite, chalcopyrite, pyrrhotite, and arsenopyrite, with minor sphalerite and pyrite.	δ34S = (−6.74)–(−3.56)	206Pb/204Pb = 18.182–18.826	δDV-SMOW = (−117)–(−98.4) δ18O = 1.1–1.8(unpublished)			[77]
						207Pb/204Pb = 15.575–15.704		Quartz: 172.6–232.1	5.2–13.6
						208Pb/204Pb = 38.444–39.083
Xiasai hydrothermal vein type Ag–Pb–Zn deposit	Occurs within the metamorphosed sandstone and shale that are located distal to the causative granite intrusions.	Sandstones, carbonate rock, and siliceous slate	sphalerite Rb–Sr 99 ± 3 Ma	Cassiterite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, Ag-bearing minerals, and native bismuth.	δ34S = (−10.1)–(−1.2)	206Pb/204Pb = 18.711–18.748	Quartz from sub-stage II-2 δDV-SMOW= −104, δ18O = 8.4	Quartz from stage I: 423–481	14.9–19.0	[4]
						207Pb/204Pb = 15.711–15.752	Quartz from sub-stage II-3 δDV-SMOW = (−105)–(−138), δ18O = (−3.3)–(−0.9)	Quartz from stage II-1: 285–386	3.5–8.0
						208Pb/204Pb = 39.104–39.225		Quartz from stage II-3: 158–242	3.4–5.7

).

The Sn deposits and hydrothermal vein-type Pb–Zn–Ag±Sn deposits in the central Yidun Terrane are commonly located in the proximal and distal locations to the Late Cretaceous reduced and highly differentiated granite, respectively [78,79]. Most of the Sn deposits are situated within the contact zones between granite and Triassic carbonate rocks. Notable examples include the large-scale Cuomolong and Jiaogenma Sn deposit in the north of Rongyicuo pluton, as well as the small-scale Gongjuelong Sn deposit in the west of Haizishan pluton. In contrast, the hydrothermal vein-type Pb–Zn–Ag±Sn deposits generally occur within the metamorphosed sandstone and shale that are located distal to the causative granite intrusions. From the perspective of wall rock alteration, Sn±Ag deposits typically exhibit high-temperature alteration, including skarnization, greisenization, and tourmalinization. The primary ore minerals in these deposits include cassiterite, chalcopyrite, pyrite, and to a lesser extent, galena and sphalerite. Alternatively, Pb–Zn–Ag±Sn deposits are enriched in F, Si, and carbonates, with localized skarnization and greisenization. The primary ore minerals in these deposits include sphalerite and galena, with minor cassiterite and other minerals.

Based on the comparison of ion coordination and ionic radius with Sn⁴⁺, several trace elements, such as Fe, Ga, V, Cr, Sc, Sb, W, U, Zr, Hf, Ti, Nb, and Ta, are considered to be compatible with cassiterite [56,60]. The fluid source, depositional circumstances, and kind of mineralization can all be inferred from the composition of cassiterite [60,61,62,80,81]. Cassiterite of the Gongjuelong Sn deposit is characterized by relatively high concentrations of W (1.9 to 6364 ppm) and Fe (235 to 3740 ppm), which are closely consistent with those of cassiterite from granite-related Sn deposits and Xiasai Pb–Zn–Ag deposit (Figure 12a). Cassiterite of the Gongjuelong deposit shows high contents of Zr and Hf, with Zr/Hf ratios ranging from 18.7 to 49.4. Cathodoluminescence (CL) imaging of cassiterite reveals alternately light and dark zoned bands, and no zircon inclusions are detected (Figure 7c). This suggests that hydrothermal alteration, rather than fluid exchange or zircon fractionation, is the main factor influencing the Zr/Hf ratio variations. Additionally, the presence of fluorite in the Gongjuelong deposit indicates that the mineralizing fluids are fluorine-rich. Fluorine-enhanced fluids are known to increase the mobility of Zr and preferentially extract Zr over Hf. The Zr/Hf ratios in Gongjuelong cassiterite are comparable to those in cassiterite from skarn ores (17.8–61.5, mean = 34.0) within the Gejiu deposit [60], aligning with characteristics typical of granite-related tin mineralization. Compared to the cassiterite of the Xiasai deposit, the cassiterite of the Gongjuelong deposit exhibits similar Ti concentrations (350 to 3015 ppm) and significantly lower Zr concentrations [5] (Figure 12c). The Ti/Zr ratios in Gongjuelong cassiterite vary by an order of magnitude, likely due to local element fractionation during crystal growth. However, the absence of Ti-rich or Zr-rich minerals in Gongjuelong suggests that crystallization has a minimal role in Ti/Zr fractionation. In granite-associated tin deposits, a declining Ti/Zr ratio in cassiterite typically reflects the gradual loss of Ti relative to Zr as the mineralizing fluids evolve and migrate away from the granite intrusion. This trend is consistent with the higher solubility of Zr compared to Ti in hydrothermal fluids [82]. Studies in the Gejiu polymetallic tin deposit have demonstrated that Ti/Zr ratios in cassiterite from various ore types (e.g., tin-bearing granite, greisenized ores, skarn ores, sub-oxidized ores, oxidized ores, and vein ores) steadily decrease with increasing distance from the host granite. A similar pattern is observed in the Bolivian tin metallogenic belt in Central South America [83]. The Fe/W and Zr/Hf ratios in cassiterite indicate that both belong to a granite-related magmatic–hydrothermal genesis. Moreover, the Ti/Zr ratio in cassiterite from Gongjuelong, located closer to the granite (average = 36.8), is markedly higher than that of the more distant Xiasai deposit (average = 3.7).

Under magmatic–hydrothermal ore-forming systems, hydrothermal quartz formed under low-temperature conditions (300 °C or below) contains high contents of Al and Li, whereas quartz formed under high-temperature conditions (approaching 400 °C) generally exhibits lower contents of Al and Li [84]. The Ge/Ti ratio serves as an effective indicator of magmatic fractionation due to the contrasting geochemical behaviors of Ge and Ti during magma evolution. Germanium (Ge) is a moderately incompatible element that tends to enrich in the melt as fractionation progresses, while titanium (Ti) is a compatible element primarily hosted in minerals such as ilmenite and rutile, causing its concentration to decrease with fractionation. As a result, the Ge/Ti ratio increases significantly with advancing magmatic fractionation, where higher Ge/Ti values indicate magmas with a higher degree of evolution [85]. Wang [42] collected trace element data from quartz in 1220 samples of various deposit types, including quartz from 196 pegmatites, 66 Carlin-type deposits, 52 porphyry deposits, 74 epithermal deposits, 203 skarn deposits, 565 granites, and 64 orogenic deposits. Based on machine learning, a novel discrimination diagram using the Ti/Ge ratio vs. P in quartz was proposed to classify deposit types. Therefore, the chemical composition of quartz can provide valuable insights into the hydrothermal environmental conditions and deposit types. The average concentrations of Li and Al of quartz in the Gongjuelong deposit are 10 ppm and 86.5 ppm, respectively. The quartz from the Xiasai deposit is enriched in both Li and Al, with concentrations approximately one order of magnitude higher than those in the Gongjuelong deposit. The variations in trace element compositions of quartz can effectively distinguish the genetic types of hydrothermal deposits with different styles of mineralization, which is due to the significant influence of temperature, pH, and pressure on the distribution of elements like Li, Al, P, Ti, and Ge in quartz [42,68,86,87]. The P (13.9 to 173.1 ppm; average 64.3 ppm), Al, and Li contents and Ti/Ge ratios (0.1 to 1.1; average 0.4) of stage III quartz from Gongjuelong are similar to those of sulfide-bearing quartz from the Xiasai Pb–Zn–Ag deposit (Figure 15a,b). Interestingly, both the quartz from Gongjuelong and Xiasai are consistent with those from the skarn-type deposits and reduced Sn-related Pb–Zn veins in trace elements (Figure 15). On the other hand, the Al and Ti contents of quartz from the Gongjuelong Sn deposit are closely consistent with those of skarn deposits and significantly distinct from those of epithermal and porphyry-type deposits (Figure 15c). In contrast to the Gongjuelong Sn deposit, the Xiasai Pb–Zn–Ag deposit has similar Ti content but higher Al content. Both of the date ranges of the two deposits are located between the fields of epithermal- and porphyry-type deposits, overlapping with the range of skarn deposits. However, the quartz from the Xiasai deposit shows higher Al content, positioning it closer to the range of epithermal-type deposits. The trace element contents in quartz and cassiterite from the Gongjuelong deposit exhibit certain similarities with those from the Xiasai deposit, both indicating a magmatic-related origin.

Recently, during our investigation of the Shaxi Pb–Zn–Ag deposit in the central Yidun Terrane, we identified high-temperature, halite-bearing tri-phase (solid–liquid–vapor) aqueous inclusions in quartz associated with sulfides. The vapor phase of these inclusions contains volatile components, including CH₄, N₂, and H₂S, further supporting a genetic link between Pb–Zn mineralization and magmatic–hydrothermal fluids in the region. On the other hand, the δ³⁴S values of sulfides from the Gongjuelong Sn deposit range from −6.74‰ to −3.56‰ [77], overlapping with those from the Xiasai Pb–Zn–Ag deposit (−10.1‰ to −1.2‰) [4]. Previous δ³⁴S measurements for the Lianlong pluton in the region yielded a value of −8.1‰ [7], similar to those from both the Gongjuelong Sn deposit and the Xiasai Pb–Zn–Ag deposit. This suggests that sulfur in both types of deposits possibly originates from the granitic magmas, with additional sulfur from wall-rock contamination. The Pb isotopic compositions of sulfides from the Gongjuelong deposit (²⁰⁶Pb/²⁰⁴Pb = 18.182–18.826, ²⁰⁷Pb/²⁰⁴Pb = 15.575–15.704, ²⁰⁸Pb/²⁰⁴Pb = 38.444–39.083) are similar to those from the Xiasai deposit (²⁰⁶Pb/²⁰⁴Pb = 18.702–18.731, ²⁰⁷Pb/²⁰⁴Pb = 15.680–15.718, ²⁰⁸Pb/²⁰⁴Pb = 39.000–39.128) [4,77]. These values are consistent with those found in orogenic belts and the upper crust, suggesting a shared crustal source for the lead in both deposits. The H-O isotopic compositions of quartz from the Gongjuelong deposit (δD_V-SMOW = −117 to −98.4‰, δ¹⁸O = 1.1 to 1.8‰) and the Xiasai deposit (substage II-2: δD_V-SMOW = −104, δ18O = 8.4; substage II-3: δD_V-SMOW = −105 to −138, δ¹⁸O = −3.3 to −3.9) [4] both show characteristics of mixing between magmatic hydrothermal fluids and meteoric water. In conclusion, the ore-forming materials in both deposits primarily come from the granitic rocks, with sulfur and lead contamination from surrounding wall-rocks, while the ore-forming fluids are a combination of magmatic hydrothermal fluids and meteoric fluids. The above evidence indicates that the Pb–Zn mineralization seems to be influenced not only by a single magmatic hydrothermal process. The strata or surrounding rocks can also provide ore-forming materials. Therefore, during the infiltration of meteoric fluids or the intrusion of magmatic hydrothermal fluids, there is also a certain possibility that the Pb–Zn mineralization occurs due to the water–rock reaction with the surrounding wall-rocks. However, the magmatic hydrothermal process still plays an important role in the mineralization of Pb–Zn.

In the central part of the Yidun Terrane, the Cretaceous granitoids include the Rongyicuo, Ruolulong, Lianlong, Xiasai, and Haizishan plutons. Radiometric dating of whole-rock Rb–Sr isochrons has yielded ages ranging from 81 to 93 Ma for the Lianlong, Rongyicuo, and Haizishan plutons [8,14,34,88]. More precise zircon LA-ICP-MS U-Pb dating indicates that these granites of the central Yidun Terrane were emplaced between 94.4 and 108 Ma [4,78,89,90]. These findings confirm that the granitoids in this region primarily formed during the late Early Cretaceous to early Late Cretaceous periods. Previous studies on diagenetic and metallogenic ages have established that magmatic activity and metallogenesis in the central Yidun Terrane predominantly occurred during the Late Cretaceous. These processes were associated with an extensional environment that followed tectonic collision [77,91]. Two types of mineralization are identified in the central Yidun Terrane: skarn-type Sn and hydrothermal vein-type Pb–Zn–Ag. Both mineralization types are closely associated with the late Cretaceous A-type granites [4,5,6,7,8,17]. Furthermore, Li et al. [4] obtained an Rb–Sr isochron age of 99 ± 3 Ma for sphalerite in the quartz–chalcopyrite and sphalerite veins, which provides a close approximation of the Pb–Zn–Ag mineralization age. Regarding Sn mineralization, LA-ICP-MS U-Pb dating of cassiterite from the Xiasai quartz–cassiterite veins yielded a U-Pb age of 99.2 ± 0.8 Ma [5]. The cassiterite from the quartz–cassiterite vein of the Gongjuelong Sn deposit revealed a Tera–Wasserburg U-Pb lower-intercept age of 96.88 ± 1.36 Ma (unpublished data), which is closely consistent with the published ages of both the Pb–Zn–Ag mineralization (99 ± 3 Ma) and late Cretaceous granites (94.4–108 Ma).

These combined findings from the geology, geochronology, and geochemistry of hydrothermal minerals convince us to propose that the Sn mineralization is closely linked with the Pb–Zn–Ag mineralization in terms of time, space, and genesis. This magmatic–hydrothermal ore system consisting of proximal Sn and distal Pb–Zn–Ag mineralization was the product in response to the fluid exsolution of reduced and highly fractionated granites during the late Cretaceous. The ore metal zones are common in the magmatic–hydrothermal mineralization system, including the porphyry Cu-Mo-Au, greisen W-Sn, and skarn Fe-Cu/W-Sn ore systems [92,93,94,95]. Porphyry-type Cu-Mo-Au and skarn-type Fe-Cu mineralization are generally associated with oxidized, lowly differentiated and water-rich magmatic rocks, whereas skarn W-Sn mineralization is linked to reduced, highly differentiated and water-poor magmatic rocks [13,22,95]. Fe, Cu, and Au mineralization mainly come from partial melting of the mantle and magmatic crust, whereas W and Sn are primarily sourced from partial melting of the sedimentary crust [96]. Pb–Zn–Ag mineralization generally occurs in the peripheral strata around these systems. For instance, the Weilasituo Zn-Cu-Ag deposit in the southern Great Xing’an Range recently revealed W-Sn mineralization at depth [97,98]. Similarly, a series of hydrothermal Pb–Zn-Sb-Ag deposits (e.g., Zhaxikang, Keyue, and Suoyue) formed around the Cuonadong leucogranites, during which skarn-type W-Sn-Be and pegmatite-type Be-Nb-Ta mineralization were progressively discovered [99,100].

During exploration, areas exhibiting skarn alteration between the contact zones highly fractionated granite and carbonates should be prioritized for potential Sn mineralization, with Pb–Zn–Ag mineralization more likely to occur in the surrounding shallower strata. Recently, 16 rare metal veins (e.g., Be, Nb, Ta) were identified in the northern Haizishan granite [101]. Therefore, the region has potential not only for Sn–Pb–Zn–Ag mineralization but also for rare metal exploration.

7. Conclusions

• The two types of garnet in the Gongjuelong Sn deposit, Grt-I and Grt-II, are part of the solid solution series of grossular and andradite and reflect an initially reducing ore-forming environment. The ore-forming environment shifted from a moderately acidic reducing to a weakly reducing neutral environment during the transition from Grt-I to Grt-II.
• The trace element contents of garnet, cassiterite, and quartz from the Gongjuelong Sn deposit suggest that the Sn mineralization is favored by the reducing conditions developed by or around the magma intrusions. The quartz in this deposit formed at relatively high temperatures. The ore-forming fluid evolved from early-stage, high-temperature, reducing, moderately acidic fluids associated with garnet indicates that the Gongjuelong deposit is a proximal skarn deposit.
• The magmatic–hydrothermal system in the central Yidun Terrane is centered on the Late Cretaceous reduced, highly differentiated granites. The system includes proximal skarn Sn deposits and distal hydrothermal vein-type Pb–Zn–Ag±Sn deposits. Further exploration should focus on identifying Pb–Zn–Ag mineralization and rare metal mineralization in distal, structurally controlled areas. The locations of Pb–Zn mineralization could serve as pathfinders for hindered Sn deposits at depth.