The Metallogenic Chronology and Prospecting Indication of Tiechanghe Granite and Molybdenum Polymetal in Jiulong Area, West Sichuan, China

1. Introduction

Granite, broadly referred to as granitic rocks, is a crucial component of continental crustal rocks. It plays a significant role in studying the formation and evolution of the continental crust and holds substantial scientific significance in research on the genesis of endogenous metal deposits. Additionally, granite possesses strategic value for national economic development [1,2,3,4,5]. In the western Sichuan region, Indosinian to Yanshanian granites are distributed in areas such as Eastern Tibet Jiangda, Yidun Island Arc Belt, Kangding Songlinkou-Tagong, Maerkang, and Jiulong Fangmaping-Sanyanlong, and they have U-Pb zircon ages ranging from 225 to 205 Ma. These granites are primarily S-type, with minor occurrences of I-type granites [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. The Early Yanshanian (166–150 Ma) granites are only exposed in the Jiulong area in regions such as Tiechanghe, Xinhuoshan, Qiaopengzi-Landiao, and Huajiaoping granite [28,29,30,31,32,33,34,35,36,37,38,39]. Recent studies in the Danba area have reported magmatic and metallogenic ages of 153–177 Ma, which are considered to be metamorphic ages [40,41]. It is speculated that magmatic activity during the Yanshanian period may have occurred at the junction of the eastern margin of the Songpan-Ganzi Block and the western margin of the Yangtze Block, and such rocks are currently exposed only in the Jiulong area. These findings provide an opportunity to study the tectonic evolution of the Songpan-Ganzi Block during the Yanshanian. The Jiulong area in western Sichuan is rich in mineral resources related to Yanshanian granites, including rare metals (Li and Be), tungsten, and molybdenum. Representative deposits include the Daqiangou Li-Be deposit, the Daniuchang tungsten-molybdenum deposit, and the Ziershi copper-molybdenum deposit. Through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon dating of the Tiechanghe granite and Re-Os dating of molybdenite, in this study, we precisely determined the magmatic and metallogenic ages of the Ziershi Cu-Mo deposit. We explored the genesis of the deposit and summarized the metallogenic patterns, providing significant insights for guiding mineral exploration in the Jiulong area in western Sichuan.

2. Regional Geological Background

The Jiulong area in western Sichuan is located in the southern part of the Songpan-Ganzi orogenic belt and is bordered by the Ganzi-Litang fault zone to the west and the Longmenshan-Jinpingshan orogenic belt to the east (Figure 1a,b). The Xianshuihe fault is located to the north-northeast, and the Muli-Yanyuan arcuate structural belt is located to the south. The Triassic Xikang Group strata are extensively exposed in the study area [42,43], primarily including schist and slate. The Ordovician-Carboniferous strata are only exposed in the southern part of the study area, including schist, metagraywacke, and quartzite. The western part of the study area includes the Yulongxi fault zone, which is northeast-southwest trending and is connected to the Ganzi-Litang fault zone in the west and aligned with the long axis of the magmatic body. The central part is characterized by north-south trending faults, the eastern part by the Qingna fault zone, and the southern part by detachment faults within the dome structure (Figure 1b). Additionally, the area exhibits a complex fold structure with a continuous distribution of anticlines and synclines.

Controlled by tectonics, the Jiulong area experienced two periods of granite intrusion: the Indosinian period (220–205 Ma) and the Yanshanian period (165–160 Ma) (Figure 2). The Indosinian granites (220–205 Ma) include the Lanniba, Yangfanggou, Sanyanlong-Fangmaping, Galazi, and Dichishan granites [17,25,38,39,44,45]. The Yanshanian granites (165–150 Ma) include the Xinhuaoshan (also known as Wenjiaping), Tiechanghe (also known as Wulaxi), and Qiaopengzi-Landiao granites [24,26,28,29,30,31,32,33,34,35,36]. The early metamorphism in this region was primarily regional metamorphism (low greenschist facies), which was followed by contact metasomatism around the granites. Only the Jianglang and Taka domes exhibit high greenschist to low amphibolite facies metamorphism [34].

Moreover, this region contains copper-zinc, lithium-beryllium, lead-zinc, and tungsten-molybdenum deposits [34,35,46,47,48,49,50]. Except for the Liwu copper-zinc deposit, which has two mineralization ages (343 Ma and 153–151 Ma) [48,49], the ages of the other deposit are constrained within 166–150 Ma and 180–200 Ma [24,25,26,34,35]. In summary, the Jiulong area in western Sichuan is rich in Cu-Pb-Zn-W-Mo and rare metal deposits, and the mineralization ages are concentrated in the Indosinian and Yanshanian periods [24].

The Tiechanghe pluton (also known as the Wulaxi pluton) is located at the confluence of the Tiechanghe and Jiulonghe rivers. This pluton exhibits intrusive contact with the surrounding rocks and steep outward dipping contact interfaces. The surrounding rocks are Middle Triassic Zagashan Formation metamorphosed sandstone, as well as slate interbedded with marble, and the periphery includes a thermal contact metamorphic zone that varies from 50 to 100 m in size. Altered rocks include actinolite-diopside-albite rock, biotite-quartz hornfels, hornfelsed sandstone, mica schist, and skarn. Two sets of joints are developed within the pluton: one set trends 310–330°∠50–60°, and the other set trends 55–70°∠65–75°, with straight joint surfaces and no mineral filling observed. Pegmatite veins of varying scales are distributed within the pluton. The Tiechanghe pluton is composed of syenogranite and monzogranite. The central part consists of medium-grained monzogranite, transitioning outward to medium-grained syenogranite and fine-grained syenogranite.

3. Deposit Geology

Molybdenum occurrences have been discovered around the Tiechanghe pluton, including the Wulaxi Daniuchang Mo-W deposit and the Ziershi Cu-Mo deposit.

3.1. Daniuchang Mo-W Deposit

The stratigraphy of the deposit area belongs to the Zagashan Formation (Triassic Xikang Group), primarily composed of metamorphosed sandstone, slate, and interbedded marble. The structural setting of the deposit is simple, with only a few gentle small-scale folds observed. The area underwent regional metamorphism in its early stages, followed by thermal contact metamorphism due to the intrusion of the granite pluton, including hornfels, skarn, schist, and marble. These rocks are generally gray-green and exhibit granular metamorphic textures and massive or banded structures. The main minerals present are grossular, andradite, diopside, hedenbergite, and vesuvianite, minor minerals such as epidote, clinozoisite, tremolite, mica, calcite, tourmaline, fluorite, and metallic minerals, including hematite, sphalerite, and pyrrhotite.

Four ore bodies have been identified in this deposit, which are primarily hosted in skarn and quartz veins and exhibit stratiform, stratiform-like, and lenticular morphologies. Ore bodies I-III are located in the central part of the deposit and are the largest skarn belt in the area, with an exposed length of approximately 1000 m and a width of more than 10 m. The general orientation of the ore body is 120°∠8°, and it exhibits stratiform and lenticular occurrences within the layered skarn, which is consistent with the skarn’s orientation. The mineralization features scheelite disseminated within the skarn. The lithology is mainly composed of diopside skarn and diopside-clinozoisite skarn, and the surrounding rocks are schist, phyllite, and marble. The grade of the scheelite ore bodies ranges from 0.07% to 0.52% (average of 0.22%), and it has a total thickness of 6.37 m. Ore body IV features an upper quartz vein-type molybdenite ore body and a lower skarn-type molybdenite ore body. The exposed surface has a width of 2–3 m, and it exhibits stratiform, stratiform-like, and lenticular morphologies. The ore body generally trends east-west with an orientation of 131°∠10°, its thickness ranges from 2.45 to 7.28 m, and the molybdenite ore grades range from 0.14% to 0.27% (average of 0.17%). The alteration of the wall rocks includes skarnification and silicification, and the main mineral assemblages include diopside + tremolite + hornblende, clinozoisite + tremolite + secondary amphibole + calcite, and biotite + chlorite + epidote + vesuvianite. The main ore minerals are scheelite and molybdenite, and the gangue minerals include calcite, biotite, chlorite, and epidote. Seven molybdenite samples from the deposit yielded an Re-Os weighted average age of 166.8 ± 1.7 Ma (mean squared weighted deviation (MSWD) = 0.90), which is consistent within error margins with the magmatic age of the Tiechanghe granite (166.0 ± 0.9 Ma). This indicates that the magmatism and mineralization occurred during the early Yanshanian [35].

3.2. Ziershi Molybdenum Deposit

The Ziershi molybdenum deposit is a newly discovered deposit located approximately 2 km north of the Tiechanghe pluton in Wulaxi Township in the Jiulong region. The southern part of the mining area is dominated by the exposure of the Tiechanghe monzogranite. The ore-hosting wall rock belongs to the Lower Triassic Bozigou Formation, primarily including gray to dark-gray biotite-hornblende schist. The mining area covers approximately 6 km², is distributed in patches, and exhibits sporadic iron staining and hydroxyl-type anomalies. These anomalies are mainly caused by ferruginous and carbonate alterations of the strata, suggesting the influence of granite veins or concealed plutons and indicating that they are mineralization-related anomalies. The Ziershi copper-molybdenum deposit is mainly quartz vein type. Only one ore body has been identified in the mining area, which exhibits stratiform, stratiform-like, and lenticular occurrences. The ore body has an exposed length of about 300 m, a width of 1–3 m, and a trend of 310°. The surface of the quartz vein contains abundant malachite, spotty chalcopyrite, and cluster-like or flaky molybdenite (Figure 3), with strong limonitization. The general orientation of the ore body is 24°∠65°, which is consistent with that of the surrounding rock. The surrounding rock is grayish-white with a slightly greenish tint and exhibits granular metamorphic textures and massive to banded structures. The main mineral is molybdenite, and minor metallic minerals such as pyrrhotite, pyrite, and arsenopyrite are also present and are distributed in a spotted pattern. The gangue minerals include calcite, biotite, chlorite, and epidote. Geochemical analysis of five samples has revealed that it has tungsten (W) contents of 4.85–10.1 ppm, molybdenum (Mo) contents of 0.67%–1.59%, copper (Cu) contents of 0.39%–1.00%, lead (Pb) contents of 19.4–758 ppm, and zinc (Zn) contents of 123–196 ppm.

4. Sample Collection and Analytical Methods

4.1. Sample Collection and Description

The sampling locations and characteristics of the two molybdenite samples are shown in Figure 2. The main ore mineral in the deposit is molybdenite, minor associated minerals such as chalcocite, malachite, and chalcopyrite are also present, and the gangue main mineral is quartz. Molybdenite occurs in disseminated, flaky, and clustered forms on the surface of the quartz veins. It exhibits a flaky, bundled, and platy appearance, and there is local enrichment of molybdenite particles.

Sample T20200521-01 is a monzogranite (Figure 4a,b). It is grayish-white and has a fine-medium grained granitic texture and a massive structure. The rock is composed of plagioclase (25–40%), potassium feldspar (28–40%), quartz (25–32%), and biotite (3–7%), with minor titanite and hornblende. Sample T20200521-02 is a granitic pegmatite (Figure 4a), with grain sizes of 5–15 mm. Its primary mineral composition includes potassium feldspar (20%), plagioclase (40%), quartz (20%), biotite (5%), and muscovite (15%). Samples T20200521-03 and T20200521-04 are syenogranites (Figure 4b,d), which have a light gray color, fine-to-medium grained texture, and a massive structure. These rocks consist of plagioclase (10–12%), potassium feldspar (55%), quartz (25%), biotite (7%), and muscovite (1–3%). The plagioclase is primarily microcline, and under a microscope, albite and pericline twinning can be observed.

4.2. Analytical Methods

4.2.1. Zircon U-Pb Geochronology

Zircon grains were separated using standard heavy liquid and magnetic techniques, mounted in epoxy, and polished to expose the centers of the zircon grains. Cathodoluminescence (CL) images of the zircon grains were obtained, and the electron microprobe (JEOL JXA-8900RL) at Northwest University of China was used to examine the internal structures of the zircons. The U–Pb zircon dating was conducted via LA-MC-ICP-MS at the State Key Laboratory of the Continental Dynamics, Northwest University. The operating conditions for the laser ablation system and the multicoupled (MC)-ICP-MS instrument, as well as the data reduction, have been descried by Yuan et al. (2004) [51].

The laser ablation spot size was approximately 32 μm. The 207Pb/206Pb, 206Pb/238U, 237Pb/235U, and 208Pb/232Th ratios were calculated using GLITTER 4.0 (Macquarie University) and then were corrected using Harvard zircon 91500 as an external standard, which has a recommended 206 Pb/ 238U isotopic age of 1065.4 ± 0.6 Ma [52]. GJ-1 was also used as a standard sample, which has a recommended 206Pb/ 238U isotopic age of 603.2 ± 2.4 Ma. The details of the analytical techniques have been described by Yuan et al. (2004) [51]. The age calculations and plotting of the concordia diagrams were conducted using ISOPLOT (version 4.15) [53]. The uncertainties are quoted at the 2σ level.

4.2.2. Molybdenite Re-Os Isotope Dating

The samples were crushed, separated, and purified to obtain molybdenite with a purity >99%. Molybdenite Re-Os isotope analyses were carried out at the Re-Os Laboratory, National Research Center of Geoanalysis, Chinese Academy of Geological Sciences (CAGS, Beijing). The isotopic ratios of the molybdenite samples were analyzed using an inductively coupled plasma mass spectrometer (ICP-MS). Specifically, a JA X-series ICP-MS from TJA Inc was used. All of the molybdenite separated was prepared and analyzed using the procedures described by Du et al. (2004) [54].

5. Results

5.1. Zircon U-Pb Ages

In this study, we selected zircons from four samples (T20200521-01, T20200521-02, T20200521-03, and T20200521-04) of granite-pegmatite for U-Pb zircon isotopic measurement. The data are presented in Table 1, Figure 5 and Figure 6.

For sample T20200521-01, the zircons selected from the quartz monzonite are colorless and transparent, stubby to elongate prisms (120–300 μm long) with aspect ratios of 1:1–3:1 (Figure 5). In the CL images, most of the zircons exhibit well-defined oscillatory zoning, indicating a magmatic origin, and several zircons exhibit light cores, indicating that they are wall-rock xenocrysts or are inherited (Figure 5). Thirty spots were analyzed, and spots 3, 11, 13, and 30 have 206 Pb/ 238 U ages of 762 ± 7, 1022 ± 9, 571 ± 5, and 420 ± 4 Ma, respectively, which are the ages of the xenocrysts or zircons inherited from the surrounding strata (Table 1). The other 26 spots have concordant 206 Pb/ 238 U ages of 164–158 Ma, with a weighted mean age of 162.9 ± 0.7 Ma (MSWD = 0.31, n = 26), representing the peak crystallization age of the quartz monzonite. The age of 164 Ma is the starting crystallization time of magma, and the age of 158 Ma is the terminal crystallization time of magma. These zircons have variable Th contents of 69–574 ppm, U contents of 141–2199 ppm, and Th/U ratios of 0.05–1.09. Spot analyses of different zircon domains revealed that there is a significant difference in the rare earth element (REE) patterns (Figure 7). The spots on 26 zircons used in the age calculations exhibited HREE (158–1638 ppm) enrichment patterns and strongly negative Eu (Eu/Eu* = 0.03–0.32) and positive Ce (Ce/Ce* = 1.32–101) anomalies. The zircon Ti thermometer [55] yielded temperatures of 632 °C–784 °C (average 690 °C). The CL images and REE contents of the zircons confirm that the age of 162.9 ± 0.7 Ma represents the crystallization age of the quartz monzonite.

For sample T20200521-02, the zircons selected from the granite pegmatite are colorless and transparent, stubby to elongate prisms (140–340 μm long), with aspect ratios of 1:1–2:1 (Figure 5). In the CL images, most of the zircons exhibit well-defined oscillatory zoning, indicating that they have a magmatic origin, and several zircons exhibit light cores, are black, and have an irregular shape, indicating that they are wall-rock xenocrysts or are inherited and have experienced Pb loss (Figure 5). Thirty spots were analyzed, excluding six zircons with concordances <90% and one captured zircon (Table 1). Spots 8, 12, 21, and 22 have 206 Pb/ 238 U ages of 114 ± 1, 141 ± 1, 130 ± 1, and 132 ± 1 Ma, respectively, which represent the ages of tectonic thermal events (Table 1). The other 19 spots have concordant 206 Pb/238 U ages of 160–166 Ma, with a weighted mean age of 164.1 ± 0.9 Ma (MSWD = 1.3, n = 19), indicating that they represent the peak crystallization age of the granite pegmatite. The age of 166 Ma is the starting crystallization time of the magma, and the age of 160 Ma is the terminal crystallization time of the magma. These zircons have variable Th contents of 22–1154 ppm, U contents of 997–30496 ppm, and Th/U ratios of 0.01–0.06. The spot analyses on different zircon domains yielded significantly different REE patterns (Figure 7). The spots on 19 zircons included in the age calculations have HREE (300–11077 ppm) enrichment patterns and strongly negative Eu (Eu/Eu* = 0.06–0.76) and positive Ce (Ce/Ce* = 0.57–1.50) anomalies. The zircon Ti thermometer [55] yielded temperatures ranging from 578 °C to 838 °C (average of 681 °C). The CL images and REE contents of the zircons confirm that the age of 164.1 ± 0.9 Ma represents the crystallization age of the quartz monzonite. This age is consistent with the crystallization age of the monzogranite within the error range.

For sample T20200521-03, the zircons selected from the syenogranite are colorless, transparent, stubby to elongate prisms (140–300 μm long) with aspect ratios of 1:1–2:1 (Figure 5). In the CL images, most of the zircons exhibit well-defined oscillatory zoning, indicating that they have a magmatic origin, and several zircons have light cores, indicating that they are wall-rock xenocrysts or are inherited (Figure 5). Thirty spots were analyzed, and spots 14, 19, and 25 yielded 206 Pb/238 U ages of 191 ± 2, 177 ± 2, and 739 ± 7 Ma, respectively, and they represent the ages of the xenocrysts or inherited zircons from surrounding strata (Table 1). The other 26 spots have concordant 206 Pb/238 U ages of 166–160 Ma, with a weighted mean age of 163.4 ± 0.6 Ma (MSWD = 0.68, n = 26), and they represent the peak crystallization age of the syenogranite. The age of 166 Ma is the starting crystallization time of the magma, and the age of 160 Ma is the terminal crystallization time of the magma. These zircons have variable Th contents of 68–314 ppm, U contents of 175–1139 ppm, and Th/U ratios of 0.10–0.95. Spot analyses on different zircon domains revealed that they have significantly different REE patterns (Figure 7). The spots from 26 zircons involved in the age calculations have HREE (184–881 ppm) enrichment patterns and strong negative Eu (Eu/Eu* = 0.08–0.27) and positive Ce (Ce/Ce* = 1.49–34.87) anomalies. The zircon Ti thermometer [55] yielded temperatures ranging from 648 °C to 810 °C (average 714 °C). The CL images and REE contents of the zircons confirm that the age of 163.4 ± 0.6 Ma represents the crystallization age of the syenogranite.

For sample T20200521-04, the zircons selected from the syenogranite are colorless, transparent, stubby to elongate prisms (120–300 μm long), with aspect ratios of 1:1–3:1 (Figure 5). In the CL images, most of the zircons exhibit well-defined oscillatory zoning, indicating a magmatic origin, and several zircons have light cores, indicating that they are wall-rock xenocrysts or inherited zircons (Figure 5). Thirty spots were analyzed, and spot 3 had a 206 Pb/238 U age of 148 ± 2 Ma, which is the age of a tectonic-thermal event (Table 1). The other 29 spots yielded concordant 206 Pb/238 U ages of 161–170 Ma, with a weighted mean age of 163.2± 0.6 Ma (MSWD = 0.68, n = 29), representing the peak crystallization age of the syenogranite. The age of 170 Ma is the starting crystallization time of the magma, and the age of 161 Ma is the terminal crystallization time of the magma. These zircons have variable Th contents of 22–610 ppm, U contents of 129–2728 ppm, and Th/U ratios of 0.03–1.08. Spot analyses on different zircon domains yielded significantly different REE patterns (Figure 7). The spots from 29 zircons involved in the age calculations exhibited HREE (132–1408 ppm) enrichment patterns with strong negative Eu (Eu/Eu* = 0.04–0.29) and positive Ce (Ce/Ce* = 3.63–33.07) anomalies. The zircon Ti thermometer [55] yielded temperatures ranging from 614 °C to 768 °C (average 700 °C). The CL images and contents of the zircons confirm that the age of 163.2 ± 0.6 Ma represents the crystallization age of the syenogranite.

5.2. Re-Os Isotope Dating of Molybdenite

Re-Os data for two samples are presented in Table 2. The samples have moderate Re and Os concentrations (187Re concentration of 3.97–7.72 ppm, and 187Os concentration of 10.67–20.52 ppb) and model ages ranging from 161.1 ± 2.2 Ma to 159.4 ± 2.5 Ma, with an average of 160.3 ± 1.6 Ma (MSWD = 1.05).

6. Discussion

6.1. Implications of Zircon U-Pb and Molybdenite Re-Os Ages

Accurate dating of mineral deposits is fundamental for developing deposit models and interpreting the geodynamic background of the mineralization. It is crucial for understanding the formation processes of deposits, determining their genesis, exploring the coupling relationships between mineralization events and other geological events, and establishing mineralization and exploration models [56,57]. The reported crystallization ages of granitoids in the Songpan-Ganzi region predominantly range from 220 to 205 Ma, and most are I-type and S-type granitoids [13,14,15,16,17,20,21,22]. Only the Nianbaoyuze area has been reported to contain A-type granitoids [7]. A smaller group of granitoids have ages ranging from 165 to 150 Ma [31,32,33,34]. In the Jiulong area, the granitoids are primarily clustered into two age groups: 170–150 Ma and 220–200 Ma (Table 4), indicating that this region experienced two distinct magmatic events.

Table 3. Re-Os isotope data for molybdenite from the Ziershi W-Mo deposit in Jiulong region.

Sample	Weight (g)	Re (ppm)	2σ	Os (ppm)	2σ	187Re (ppb)	2σ	187Os (ppb)	2σ	Model age (Ma)	2σ
Z01	0.02	6.32	0.04	0.048	0.021	3.97	0.027	10.67	0.07	161.1	2.2
Z02	0.05	12.28	0.12	0.080	0.009	7.716	0.072	20.52	0.16	159.4	2.5

Table 3. Re-Os isotope data for molybdenite from the Ziershi W-Mo deposit in Jiulong region.

Sample	Weight (g)	Re (ppm)	2σ	Os (ppm)	2σ	187Re (ppb)	2σ	187Os (ppb)	2σ	Model age (Ma)	2σ
Z01	0.02	6.32	0.04	0.048	0.021	3.97	0.027	10.67	0.07	161.1	2.2
Z02	0.05	12.28	0.12	0.080	0.009	7.716	0.072	20.52	0.16	159.4	2.5

The 220–200 Ma ages of the granitoids indicate that their formation was related to the magmatic activity triggered by the westward subduction and collision of the Ganzi-Litang Ocean during the Indosinian. This is consistent with previous studies that have suggested that the peak age of the closure of the Ganzi-Litang Ocean and the associated magmatic activity is around 216 Ma [55,56]. The 170–150 Ma granitoids reflect the Early Yanshanian magmatic events or tectonic thermal events in the Jiulong area, and their formation was related to post-collision extensional processes. Notably, this period also marks the stabilization of the neighboring Jianglang Dome [30,34].

The LA-ICP-MS U-Pb zircon ages of the Tiechanghe granite studied in this paper range from 162.9 ± 0.7 Ma to 163.4 ± 0.6 Ma. These results are consistent with previously reported zircon U-Pb ages for this region and represent the crystallization age of the Tiechanghe granite. The weighted average Re-Os age of the molybdenite is 160.3 ± 1.6 Ma, which is also consistent with earlier reported Re-Os isotope ages for this region, indicating that this is the molybdenum mineralization age in the Jiulong area. Additionally, the LA-ICP-MS U-Pb zircon ages of the granitic pegmatites (114 ± 1, 141 ± 1, 130 ± 1, and 132 ± 1 Ma) suggest that these zircons may have been influenced by later magmatic or tectonic thermal events or possibly by the loss of Pb.

The Sm-Nd isochron age of the Xuebaoding tungsten deposit in the northern edge of the Songpan-Ganzi Block is 182.0 ± 9.2 Ma [58,59]. The Re-Os isotope age of the molybdenite in the Hede tungsten-tin deposit in the Kangding region is 199.0 ± 2.6 Ma [39], while the Re-Os isotopic age of the molybdenite in the Daniuchang molybdenum-tungsten deposit is 166.8 ± 1.7 Ma [35]. The weighted average Re-Os age of the molybdenite in the Ziershi deposit is 160.3 ± 1.6 Ma, which is consistent with the LA-ICP-MS U-Pb zircon ages of the Tiechanghe granite (162–166 Ma) within the error margins. These ages collectively confirm that the molybdenite in the Ziershi deposit formed during the Early Yanshanian and that their formation was associated with granitic magmatism. These ages are consistent with those of the Daniuchang molybdenum-tungsten deposit within error [31,60], but they differ from those of the Xuebaoding W-Sn-Be deposit and the Hedong tungsten-tin deposit. This discrepancy suggests that significant magmatic activity related to Mo mineralization occurred in the Jiulong area at ca.166 Ma.

The geological and mineralization events in the Jiulong Tiechanghe region occurred approximately 40 Ma after the crustal collision and thickening event in the Songpan-Ganzi Block during 220-205 Ma. This period represents the peak of the lithospheric extension along the southern margin of the Songpan-Ganzi Block and marks the beginning of the thermal uplift and extension of the lithospheric after the contractional thrusting and detachment (collision orogeny) associated with the Songpan-Ganzi orogenic belt [29,60]. Consequently, the Tiechanghe granite is a product of the transition from compressional to extensional tectonic environments in the Songpan-Ganzi Block from the Indosinian to the Early Yanshanian. It is also a crucial time marker of the regional transition from the Tethyan tectonic domain to the Pacific tectonic domain.

6.2. Ore Formation Models and Geological Exploration Directions

The Daniuchang tungsten-molybdenum deposit is spatially closely associated with the Tiechanghe granite, which primarily consists of syenite and monzonitic granite. Zhou et al. (2014) reported that the Tiechanghe granite exhibits the characteristics of an aluminum-rich A-type granite, including high SiO₂, Na₂O, and K₂O contents, high FeOₒ/MgO and Ga/Al ratios, and low TiO2, CaO, and MgO contents [30]. However, Liu Xiaojia and Xu Zhiqin (2021) argued that the Tiechanghe granite should be classified as a high-potassium calc-alkaline I-type granite [61]. This classification reflects the transition from collision orogeny to post-collisional extension, where decreased crustal stress led to partial melting of the ancient crust and contamination by newly formed crustal melts, resulting in Middle Jurassic magmatism. This perspective is consistent with the views of Dai et al. (2017) and Liu et al. (2022) [32,36].

The zircon Ti thermometer [63] can be used to infer the formation temperature of zircons, which represents the maximum temperature of the magma. Experimental petrology has revealed that A-type granites typically form at temperatures >800 °C. In contrast, the Ti thermometer calculations for the zircons analyzed in this study yielded temperatures of 690–714 °C. Therefore, based on the magma temperature, the Tiechanghe granite may not be an A-type granite but rather an I-type granite.

The Yanshanian granites, such as the Tiechanghe granite, which were formed in an extensional tectonic setting, provided the material and dynamic conditions necessary for the formation of tungsten-molybdenum-copper deposits due to multiple phases of granitoid magmatic activity. The Tiechanghe granite, characterized by high concentrations of mineralizing elements such as molybdenum (Mo), served as the primary mineralizing body in the region, and it supplied both the heat and material sources essential for the formation of the molybdenum deposits. During the Yanshanian, the intrusion of the Tiechanghe granite into the surrounding strata induced thermal contact metamorphism and metasomatism, resulting in the formation of skarns at the contact interfaces. The Triassic strata provided calcium, while the granite supplied tungsten and molybdenum, leading to the formation of scheelite and molybdenite in or near the skarn zone and thus creating the Daniuchang skarn-type W-Mo deposit. Additionally, in the schist areas located 2-3 km from the granite, quartz vein-type molybdenum deposits were more likely to form (Figure 8).

Previous studies have suggested that the mineralization depths of skarn-type and quartz vein-type tungsten-molybdenum deposits are typically 2–4 km [62,63,64]. Zircon and apatite fission track data for the Tiechanghe granite indicate that the granite body was exhumed to approximately 2170 m [34], which is within the depth range at which tungsten-molybdenum deposits form. This suggests that the tungsten-molybdenum deposits in this area may have experienced uplift and erosion. Field geological surveys have revealed the presence of large quantities of tungsten minerals in the colluvial deposits below the skarn-type scheelite bodies, providing evidence that erosion has affected these mineralized bodies [35].

Therefore, exploration of tungsten-molybdenum-copper deposits should be intensified around the Tiechanghe granite body. Additionally, the impact of the uplift and erosion in the Jiulong area should be considered as tungsten-molybdenum-copper deposits may have been significantly eroded and lost due to uplift and exposure [35]. This finding offers new insights for mineral exploration in the Jianglang area and its surrounding regions. In summary, the focus of molybdenum-polymetallic exploration in the Tiechanghe region should be within 2–3 km of the Yanshanian (ca. 160 Ma) granite bodies, and careful attention should be paid to the effects of tectonic uplift and erosion on mineral exposure and preservation. This understanding can be applied to the entire western Sichuan region.

7. Conclusion

(1) The formation age of the Zierzi molybdenum-polymetallic deposit was determined to be 160.3 ± 1.6 Ma. This age is consistent with the Tiechanghe granite LA-ICP-MS U-Pb zircon ages (162–164 Ma) within the error range, indicating that a molybdenum mineralization event associated with granitic activity occurred at ca. 160 Ma.

(2) The emplacement and mineralization ages of the Tiechanghe granite and the tungsten-molybdenum deposits correspond to the peak of lithospheric extension in the southern margin of the Songpan-Ganzi Block. They represent the transition from the Indosinian compressional to the Yanshanian extensional tectonic environment in the Songpan-Ganzi Block. This transition also reflects the occurrence of shallow emplacement and mineralization in response to the regional tectonic shift from the Tethyan to the Pacific tectonic domain in the Jiulong area.

(3) The focus of the exploration for molybdenum-polymetallic deposits related to the Yanshanian (circa 160 Ma) granites in the Jiulong area in western Sichuan should be concentrated within 2–3 km of the granite bodies. It is also crucial to consider the impact of structural uplift on the degree of erosion, which affects the exposure and preservation of mineral deposits.