Deeply Derived Magma Controlling the Polymetallic Mineralization at Shuikoushan, South China: Constraints from Mineral U–Pb Dating and Whole-Rock Geochemistry

Abstract

Various magmatic–hydrothermal activities have resulted in different styles of polymetallic mineralization in South China. Shuikoushan is a large Fe-Cu-Pb-Zn-Au-Ag orefield situated in fold-and-thrust belts within the South China Block. Two types of granodiorite have been identified in recent drilling work. The early-stage, coarse-grained granodiorite has developed magnetite-bearing skarns in the deep level. The late-stage, fine-grained granodiorite is associated with garnet-hematite–magnetite–pyrite–sphalerite–chalcopyrite-bearing skarns in its contact zone. Away from the garnet-bearing skarn are calcite–quartz–pyrite–sphalerite–galena veinlets in faulted breccia. Fieldwork has identified iron mineralization in both skarns, whereas copper mineralization was only discovered in the garnet-bearing skarns. Lead, zinc, gold, and silver mineralization were observed in the garnet-bearing skarns and faulted breccia. Zircon U–Pb analyses suggested the emplacement of two granodiorite at 167.8 ± 0.8 Ma (MSWD = 1.1, N = 31) and 163.6 ± 0.7 Ma (MSWD = 1.3, N = 32). Apatite and garnet U–Pb dating further indicated the magnetite-bearing skarns of 166.2 ± 1.9 Ma (MSWD = 4.5, N = 27), the hematite–magnetite–sulfide-bearing skarns of 158.6 ± 2.8 Ma (MSWD = 1.3, N = 34), and the calcite–quartz–sulfide veinlets of 159.5 ± 5.2 Ma (MSWD = 1.7, N = 24). The time–space relationship between the two intrusions and hydrothermal activities suggests that the fine-grained granodiorite is responsible for polymetallic mineralization. Whole-rock geochemistry analyses demonstrated the enrichment of LILEs and the depletion of Nb and Ta in two granodiorites, with a slight enrichment in LREEs and flat HREE patterns. These granodiorite bodies therefore belong to high-K calc-alkaline magma generated via the crust’s partial melting. The fine-grained granodiorite generally has a lower HREE and higher Dy/Yb, Sr/Y ratios than coarse-grained granodiorite, corresponding to the source of magma in garnet stable lower crust. The residual garnet keeps ferric iron in melts, leaving the fine-granodiorite more oxidized for copper and gold concentration. Through these analyses and our drilling work, a continuous skarn–hydrothermal–epithermal system has been identified for Cu-Pb-Zn-Au-Ag targeting in Shuikoushan.

1. Introduction

The fertilization of Cu, Mo, Pb, Zn, W, Sn, Au, Ag, and rare earth elements (REEs) in magmatic–hydrothermal systems is crucial for ore deposits within the continental crust [1,2,3,4,5]. Pb, Zn, W, and Sn are believed to be enriched in the continental crust, especially Neoproterozoic sediments [3,6,7,8,9]. On the other hand, Cu, Au, Mo and other chalcophile elements take low concentrations in the upper crust [10,11,12]. The deeply derived magma plays a fundamental role in fluxing chalcophile elements into the crust from the metasomatized lithosphere and depositing them alone or in combination with other metals [13,14,15,16]. Identifying the exact metal-rich cargoes among multistage magmatism is thus important for mineral exploration in igneous provinces.

The South China Block (SCB) is one of the most important metallogenic provinces in China, characterized by many polymetallic deposits related to Mesozoic magmatism [17,18,19,20]. Triassic peraluminous granites are related to W-Sn-Nb-Ta mineralization within the Cathaysian Block and were formed in a post-collisional process of 230–210 Ma [19,21]. The uprise of the asthenospheric mantle in the Jurassic Era has led to the formation of both I-type and metaluminous granitic magma, which further deposited porphyry Cu-Au, skarn–hydrothermal Pb-Zn-Au-Ag, and polymetallic W-Sn within the SCB (170–150 Ma) [22,23]. The subduction-related continental extension tectonics further promoted the formation of epithermal Cu-Au, granite-related Sn-(W), and hydrothermal U mineralization in the eastern SCB (120–80 Ma) [21,24,25]. Among these tectonic–magmatic–hydrothermal terranes, South Hunan has attracted much attention for its preservation of both W-Sn and its polymetallic Cu-Pb-Zn-(Au-Ag) mineralization during the intracontinental extension process [26,27,28]. The convergence of the E-W striking Nanling Range and NE-striking Qingzhou–Hangzhou Metallogenic Belt in South Hunan complicated the relationship between multiple magmatism and polymetallic mineralization [29].

The Shuikoushan ore field from South Hunan has preserved large-sized Fe-Cu-Pb-Zn-Au-Ag deposits within the SCB, featuring skarn-type Fe-Cu-Pb-Zn mineralization around the intrusion, vein-type Pb-Zn-Au-Ag mineralization in the breccia, and epithermal Au-Ag deposits in the shallow depth of intrusive rocks. Previous studies have confirmed the long-term magmatism of 172~149 Ma and the multistage metal deposition of 159~140 Ma in Shuikoushan, but the relationship between specific magmatic and hydrothermal events remains unclear [30,31,32,33,34,35]. Meanwhile, the vein-type Pb-Zn-Au-Ag mineralization was believed to be the product of regional magmatism, but its magma source and fluid migration direction were controversial [32,35,36]. Skarn-type Fe-Cu mineralization was discovered in the western part of the Shuikoushan ore field (e.g., Yagongtang, Laoyachao). However, there are no more clues about its parent magma and its distribution beneath the fold-and-fault belts. The situation has become even worse in recent years with regard to mining Pb-Zn ores in the shallow depth. Because polymetallic Cu-Pb-Zn deposits are rare in the Nanling Range and their magmatism afffinity is different from those associated with W-Sn mineralization [28,37,38], a closer look at magmatic evolution and hydrothermal activities would throw light on regional Cu-Pb-Zn exploration.

In this study, we present in situ U–Pb dating of zircon from different granodiorite types, together with hydrothermal garnet and apatite U–Pb analyses from skarns and hydrothermal veins. These geochronological attempts help to explain the magmatic–hydrothermal processes at Shuikoushan. We also report whole-rock major and trace element analyses of these two granodiorites, with the intention of revealing their genesis and potential for copper mineralization. Through these analyses and our drilling work, a continuous skarn–hydrothermal–epithermal system has been identified for Cu-Pb-Zn-Au-Ag targeting in Shuikoushan.

2. Geological Setting

Among the major blocks of East Asia, the South China Block is characterized by its complex collision–extension–magmatism histories and multiple W-Sn-Pb-Zn-Cu-Au-REE mineralization events [16,39,40]. The architecture of the SCB is the result of the amalgamation between the Yangtze Block (YZB) and the Cathaysia Block (CAB) before 860 Ma, when the Precambrian metamorphic rocks and sediments finally welded along the folded Jiannan Orogen [41,42]. During the breakup of the Rodinia supercontinent in the Neoproterozoic Era, some rift-related magmatic rocks were also emplaced along the pre-existing suture. The SCB then behaved as a stable continent until the closure of the ancient Tethys Ocean on its northern margin and the collision between the SCB and the North China Craton during the late Mesozoic Era [43,44,45]. The westward subduction of the Paleo-Pacific Ocean further led to collision–extension tectonics, extensive magmatism, and multi-stage magmatic–hydrothermal mineralization within the SCB [46,47].

Central Hunan lies in the central part of the SCB, where the world-famous Qingzhou–Hangzhou Metallogenic Belt encounters the Nanling Metallogenic Belt (Figure 1a) [48]. This area also features the junction zone between the YZB and CAB, with Proterozoic meta-sandstones, slates, and volcanic rocks on its northern part and Neoproterozoic–Silurian marine and glacial sediments on its southeastern part (Figure 1b) [49,50]. To the southern part of this area are shallow marine carbonates, slates, and littoral sandstones ranging from Devonian to Triassic. The central northern part of this area, also named Hengyang Basin, is dominated by Jurassic and Triassic continental sediments of sandstone and shale. Multiple orogeneses have resulted in N-striking anticlinoria and E-, NNE-, and NNW-striking faults in Central Hunan, further controlling the distribution of intrusions and mineral deposits [51]. Phanerozoic magmatic activities in South Hunan can be divided into three periods. Early Paleozoic intrusive rocks are primarily composed of biotite granite and monzogranite in the southeast Chenzhou area [52,53]. Biotite granite and granodiorite of the Triassic era are mainly distributed in the central area [54,55,56], along with abundant muscovite and tourmaline. Mesozoic intrusions are the most abundant in South Hunan, including two-mica granite, granite porphyry, biotite granite, quartz porphyry, and granodiorite, which occur along N-striking folds and NNE-striking faults [57,58]. The emplacement of felsic magma and deeply seated faults have facilitated magmatic–hydrothermal, skarn and epithermal polymetallic mineralization in South Hunan, including the W-Sn-Pb-Zn deposits in Shizhuyuan, Furong, Yaogangxian, Xianghualing, Huangshaping, and Baoshan, as well as Pb-Zn-W-(Au-Ag-Cu) deposits in Shuikoushan, Chuankou, and Shuangjiangkou [59].

3. Deposit Geology

The Shuikoushan ore field is located about 35 km south of Hengyang City and preserves about 0.44 Mt Pb (3.68%), 0.49 Mt Zn (4.06%), 12126 t Cu (0.15%), 1.45 Mt S (12.93%), 36.62 t Au (3.26 g/t), and 1297.78 t Ag (115.50 g/t) by 2021 [60]. This ore field mainly consists of Layachao Fe-Cu-Pb-Zn-Au-Ag, Yagongtang Fe-Cu-Pb-Zn, and Kangjiawan Pb-Zn-Au-Ag deposits (Figure 2). Previous studies have identified skarn Cu-Pb-Zn, vein-type hydrothermal Pb-Zn-Au-Ag, and epithermal Au-Ag mineralization within Shuikoushan [30,32,61]. Apart from these skarn–hydrothermal systems, some epithermal Au-Ag deposits (Longwangshan, Xianrenyan) and sedimentary Cu deposits (Baifang) have been discovered near Shuikoushan.

The stratum outcropped in the Shuikoushan district is more than 3 km thick, with Devonian to late-Triassic shallow marine carbonate rocks, sandstones, and shales in the south and late-Triassic to Cretaceous continental molasses in the north [49]. Skarn and hydrothermal activities were mainly developed within the Permian marlstone–shale of the Dangchong Formation and the siltstone shale of the Douling Formation, with minor mineralization in the lower limestone of the Qixia Formation and the upper sandstone of the Gaojiatian Formation (Figure 2). The Devonian–Triassic strata in Shuikoushan suffered multiple deformations in the Mesozoic Era. From the west to the east, there are now overturned Yagongtang anticline, Yangjialing syncline, Xianrenyan anticline, Miaoqian syncline, and Kangjiawan anticline in the ore field, which extend in an N-striking direction and dip to the west [34]. NE-, NW-, and EW-striking faults then cut these folds and thrust the siliceous rocks of the Dangchong Formation onto the sandstones of the Gaojiatan Formation. Magmatic rocks mainly intrude the Neopaleozoic strata in Shuikoushan and are bounded by NE-striking faults (Figure 2). The intrusions in Yagongtang and Laoyachao are mainly granodiorite (1.8 km², 172~152 Ma) [30,33,35], with minor granite porphyry. The shallow depth dacite porphyry and rhyolite porphyry are distributed in the east Laomengshan and Xinmengshan area, with their emplacement ages being 157~149 Ma and 153 Ma [38,62].

Hydrothermal activities in this area are associated with granodiorite, which intrude into the Laoyachao anticline and are bounded by F1, F2, and F3 faults (Figure 2 and Figure 3). These granodiorite bodies are composed of plagioclase, K-feldspar, quartz, biotite, with minor quantities of hornblende, chlorite, and magnetite (Figure 4). The granodiorite at the ground surface has been eroded and only the porphyritic textures can be observed [35]. Where the intrusion is deep and close to the fault plane, our fieldwork discovered coarse-grained granodiorite (Figure 3 and Figure 4a–c). Typical minerals include plagioclase (45~55%), biotite (10~20%), K-feldspar (15~30%), quartz (10~15%), and minor magnetite (<1%, Figure 4f). This type of coarse-grained granodiorite generally develops magnetite-bearing skarn in the contact zone and minor chloritic alteration within the intrusion. The alteration transformed biotite into chlorite and magnetite (Figure 4f). Another type of fine-grained granodiorite is distributed in a shallow space or intrudes the coarse-grained granodiorite at depth (Figure 3b and Figure 4a–e). The mineral assemblage of fine-grained granodiorite is similar to coarse-grained granodiorite, with higher contents of plagioclase (55~70%) and magnetite (2~3%) and lower contents of K-feldspar (10~15%), quartz (5~15%), and biotite (5~10%, Figure 4g). The fine-grained granodiorite is also altered during lateral hydrothermal activities, as evidenced by a potassic zone and quartz–calcite–sulfide veins that were observed in the drilling hole (Figure 4d,e and Figure 5a). Compared to coarse-grained granodiorite, there is no obvious chloritic alteration within fine-grained granodiorite.

The Shuikoushan orefield is characterized by the extreme development of skarn between granodiorite and carbonate rocks (Figure 3a). The coarse-grained granodiorite from the deeper space intrudes the marlstone of the Dangchong Formation and helps in the development of magnetite-type skarn. This type of skarn (Type I, magnetite-bearing) is characterized by abundant magnetite intergrown with chlorite, diopside, epidote, apatite, and quartz, with minor or no sulfides observed under a microscope (Figure 5b,g,h). The distribution of magnetite-type skarn is narrow and was fractured during deformation (Figure 5e). The fine-grained granodiorite, however, is associated with extensive garnet-bearing skarn between the intrusion and limestone of the Qixia Formation in the shallower space (Figure 3 and Figure 5c,d). This type of skarn (Type II, garnet-bearing) features abundant garnet, quartz, calcite, hematite, and magnetite, with some chalcopyrite and sphalerite grains (Figure 5i–k). Garnet from this skarn is zoned and belongs to andradite, with hematite and minor sulfides as mineral inclusions.

Away from the garnet-bearing skarns, we found extensive silicification and carbonatization within the breccia zone of the Qixia and Dangchong Formations (Figure 3 and Figure 5). The components of breccia is rather complicated and includes limestone, shale of sedimentary origin, granodiorite of magmatic origin, and magnetite–hematite-bearing skarns of hydrothermal origin (Figure 5d–f). Calcite–quartz–sulfide stockworks have filled the fractures of the breccia and have developed abundant sphalerite and galena in the altered rocks (Figure 5e,f). Typical hydrothermal minerals are calcite, quartz, pyrite, sphalerite, galena, and chalcopyrite, with minor native gold grains (Figure 5l,m). These stockworks also cut the preformed skarns and felsic intrusions (Figure 5f).

Here, we classify the hydrothermal minerals at Laoyachao into two periods (Figure 6). The first period is characterized by magnetite-bearing skarn related to coarse-grained granodiorite. The second period can be further divided into a garnet-bearing skarn stage and a calcite–quartz–sulfide stage. It is important to note that the magnetite-bearing stage and garnet-bearing stage belong to the products of different magmatism and that no genetic relationship exists between them.

Orebodies at Laoyachao can be further categorized as skarn-type Fe-Cu-Au-(Zn-Pb) ores in the depth, skarn-type Pb-Zn(Fe-Cu) ores surrounding the fine-grained granodiorite in the shallow space, and breccia-type Au-Pb-Zn ores along the NNE-striking F1 fault (Figure 3). Fe-Cu-Au-(Zn-Pb) orebodies are of stringer or tubular shape and extend for 150~200 m. This type of ore contains mainly hematite, magnetite, garnet, chalcopyrite, and pyrite, with minor sphalerite, galena, and bornite contents observed under a microscope (Figure 5i,j). Recent drilling work has discovered the extension of Fe-Cu-Au-(Zn-Pb) orebodies into the deeper region where the Cu grade can reach 0.79% [60]. Pb-Zn-(Fe-Cu) ores generally take tubular or lenticular shapes and extend for 10~400 m, with Pb, Zn, Cu, and Au grades of 0.34~6.24%, 0.51~5.74%, 0.03~0.24%, and 1~5 g/t [60]. The typical sulfides found in these ores are pyrite, sphalerite, galena, chalcopyrite, and native gold (Figure 5k). Other minerals such as hematite and magnetite are also observed in Pb-Zn-(Fe-Cu) ores. Au-(Pb-Zn) ores in the Laoyachao district occur in the hanging wall breccia of the F1 fault and exhibit a lenticular shape (Figure 3). This type of orebody can extend up to 600 m wide and 700 m deep, with an average thickness of 8.51 m and an Au grade of 4.3 g/t. Native gold in Au-(Pb-Zn) ores is intergrown with calcite, quartz, and pyrite, with minor amounts of sphalerite, galena, tetrahedrite, and bismuthinite (Figure 5l,m).

4. Sampling and Analytical Methods

Altogether, nine samples of the granodiorite that had undergone the least significant alterations were collected from drill holes ZK10001 and ZK10102 for chemistry analysis and zircon U–Pb dating (Figure 3b). To ascertain the relationship between magmatism and hydrothermal activities, both hydrothermal apatite and garnet were collected from skarns and hydrothermal breccia for chronological analysis. There are abundant apatite grains that coexist with magnetite in magnetite-bearing skarns (Figure 5g), which is suitable for in-situ U–Pb dating. As for the garnet-bearing skarns, their andradite is zoned and intergrown with hematite and sulfides (Figure 5i). In-situ U–Pb dating on andradite would constrain the timing of the skarn. We also collected one sample of the quartz–calcite–sulfide veinlet from the breccia for hydrothermal apatite U–Pb dating. Mineral separation and whole-rock mesh were prepared at Xi’an Leijie Geological Technology Ltd. Epoxy resin fabrication, backscattered imaging (BSE), and cathodoluminescence imaging (CL) were conducted at the Analytical Laboratory for Beijing Research Institute of Uranium Geology (ALBRIUG), China.

4.1. Zircon U–Pb Chronology

Samples selected for zircon isotopic analysis were collected from drill hole ZK10001 at 445 m (LSZ-1, coarse-grained granodiorite) and 110 m (LSZ-2, fine-grained granodiorite). Zircon grains were separated using heavy-liquid and magnetic methods and cast into an epoxy mount. To avoid possible mineral inclusions and fractures, both optical observation and cathodoluminescence imaging were conducted before analysis. The LA-ICP-MS zircon U–Pb dating and trace element analysis were applied at ALBRIUG using a GeoLasPro 193 nm excimer laser and a ThermoFisher Scientific Element XR ICP-MS instrument (Bremen, Germany). Zircon 91500 and NIST 610 were analyzed as external standards for isotopic and elemental calibration. The laser spot for each analysis was 32 μm, with a laser frequency of 6 Hz and an energy density of 7 J/cm². Zircon-standard Plešovice was tested as a monitoring standard. Off-line signal selection, integration, and calibration were processed using ICPMSDataCal [63,64]. Common lead correction followed the procedure proposed by Andersen (2002) [65]. U–Pb geochronological calculation and diagrams were then performed using Isoplot/Ex_ver 3.75 [66].

4.2. Apatite U–Pb Chronology

Apatite from magnetite-bearing breccia near the coarse-grained granodiorite (B13-1K, 425 m) and the quartz–calcite–sulfide veinlet (B11-1K, 380 m) was selected for U–Pb dating (Figure 3). Apatite grains were prepared following the same procedure as zircon and we carried out BSE imaging to identify the inner textures. In situ apatite U–Pb analysis was carried out at ALBRIUG, using a CAMECA IMS 1280HR Secondary Ion Mass Spectrometry (SIMS, Gennevilliers Cedex, France). The ${\mathrm{O}}_2^{-}$ primary beam was accelerated at −10 kV under the Kohler illumination mode, with an intensity of 10~12 nA and a primary beam mass filter (PDMF) of 200 μm. The ellipsoidal spot was therefore approximately 20 × 30 μm. After coating with high-purity gold, the sample charging effects were minimized by optimizing the energy in a 60 eV energy window at the start of each analysis. ⁴⁰Ca₂³¹P¹⁶O₃⁺ (mass 159) was acquired as a reference peak for centering the secondary ion beam, energy, and mass adjustments [67]. We also tested ³¹P₄¹⁶O₅⁺ (mass 203.87) for ²⁰⁴Pb adjustment. Apatite MAD was analyzed for U, Th, and Pb isotope calibration in this study, with Durango and McClure as mass monitoring standards [68]. Each analysis included the signals of ⁴⁰Ca₂³¹P¹⁶O₃⁺, ³¹P4¹⁶O₅⁺, ²⁰⁴Pb⁺, ²⁰⁶Pb⁺, ²⁰⁷Pb⁺, ²⁰⁸Pb⁺, ²³²Th⁺, ²³⁸U⁺, ²³²Th¹⁶O⁺, ²³⁸U¹⁶O⁺, ²³²Th¹⁶O₂⁺, and ²³⁸U¹⁶O₂⁺ in order to produce one set of data. After ten cycles of analysis for each spot, the results were further calibrated following the method proposed by Li et al. (2012) [67] and illustrated using Isoplot/Ex_ver 3.75.

4.3. Garnet U–Pb Chronology

Garnet from hematite-, chalcopyrite-, and sphalerite-bearing skarns was selected for analysis (Figure 5c,i). These samples were cut into thin sections and observed under the microscope to avoid mineral inclusions. In situ garnet U–Pb dating was applied at Yanduzhongshi Geological Analysis Laboratories Ltd. (Beijing, China), using the AnalytiKjena PlasmaQuant MS quadrupole ICP-MS (Jena, Germany) with a 193 nm MWR193 Ar-F excimer laser. Detail analysis procedure is referred to [69]. The laser spot was set to 53 μm, with a laser frequency of 7 Hz and an energy density of 3 J/cm². Each analysis included a 15-s blank gas measurement followed by a further 40 s of analysis time. Grandite MALI (202.0 ± 1.2 Ma) [70] was analyzed as the primary standard for isotope calibration. Two other garnets, including Willsboro andradite (1022 ± 16 Ma) [70] and in-house standard Zsls-01 (155 ± 2 Ma), were employed as quality monitors. The signals were calibrated offline using ICPMSDataCal software (ver. 10.8) and the ZSKits program [64,71]. ²⁰⁷Pb-based common lead correction and a Tera–Wasserburg diagram were further applied for U–Pb calculation [72].

4.4. Whole Rock Geochemistry

Samples of the least-altered granodiorite were selected for major and trace element analysis at the Northwest Mineral Geological Analytic Centre of Nonferrous Metals, Xi’an. After being subjected to grinding in an agate mill to ~200 meshes, these samples were analyzed using Rigaku ZSX Primus Ⅱ (Tokyo, Japan) for major oxides. Its precision is better than 1% for all major elements. As for trace element analysis, the power was dissolved using HNO₃+HF. Rh was added as an internal standard and the sample solutions were converted into 1% HNO₃ medium. Trace element abundance was then measured using Agilent 7700 ICP-MS (Santa Clara, CA, USA), with precision better than 5% and accuracy better than 5% for most elements.

5. Results

5.1. Zircon U–Pb Dating

Altogether, 31 zircon grains from coarse-grained granodiorite (LSZ1) and 32 zircon grains from fine-grained granodiorite (LSZ2) were selected for LA–ICP–MS U–Pb dating. These zircon grains were of euhedral shape, being 50–300 μm in length and 50–200 μm in width. They were colorless and transparent under the microscope and exhibited strong oscillatory zoning under CL. U–Pb isotopes, calculated ages, and trace element concentrations are listed in Supplementary Table S1 and illustrated in Figure 7.

The zircon grains from LZS1 had variable Th contents of 143–548 ppm and U contents of 240–794 ppm, with Th/U ratios between 0.47 and 1.06, which typical for grains of magmatic origin. These zircon grains yielded a concordia age of 167.8 ± 0.8 Ma (2δ, MSWD = 1.1, N = 31). Although some zircon grains possessed Ti contents lower than the detection limit, twenty-two grains yielded average Ti content of 3.22 ppm, which suggests an average crystallization temperature of 676 °C [73].

Zircon grains from LZS2 generally take similar Th (121–868 ppm) and U (222–857 ppm) contents compared to LZS1, with an average Th/U ratio of 0.61. Thirty-two zircon grains produced a concordant U–Pb age of 163.6 ± 0.7 Ma (2δ, MSWD = 1.3, N = 32), which is slightly younger than coarse–sized granodiorite. The zircon from fine-grained granodiorite took a higher Ti content (average 4.34 ppm), with an average crystallization temperature of 708 °C.

5.2. Apatite U–Pb Dating

The apatite from magnetite-bearing zones (B13-1K) was intergrown with magnetite, chlorite, and quartz, with magnetite and pyrite as its inclusions (Figure 5c and Figure 8d). Compared to B13-1K, the apatite from quartz–calcite–sulfide veinlets (B11-1K) was rich in quartz and sphalerite inclusions (Figure 8d). The analyzed apatite U–Pb isotopic data are listed in Supplementary Table S2, and illustrated as U–Pb Tera-Wasserburg diagrams in Figure 8a,b.

The apatites from the magnetite-bearing zones were 80–200 μm in length and homogeneous under BSE imaging (Figure 8d). Twenty-seven apatite grains were analyzed and the U contents were between 16.9 and 387 ppm (average 184 ppm). Because apatite generally takes initial lead during its formation, a substantial, common Pb correction is required before age illustration. Here, the ²⁰⁷Pb correction method proposed by Chen et al. (2014) [17] was applied. Based on the upper initial ²⁰⁷Pb/²⁰⁶Pb ratio of 0.670 ± 0.098 on the Tera–Wasserburg diagram, the f206 (molar ratio of common lead) was between 1% and 30%. These apatite grains therefore yielded the lower intercept age of 166.2 ± 1.9 Ma (2δ, MSWD = 4.5, N = 27), corresponding to a ²⁰⁷Pb corrected mean ²⁰⁶Pb/²³⁸U age of 165.0 ± 1.3 Ma (2δ, MSWD = 6.1, N = 23, four outliers excepted).

The apatites from quartz–calcite–sulfide veinlet (B11-1K) were of an anhedral shape and exhibited metasomatic textures under BSE imaging. Uranium contents of these apatites were significantly lower than those in B13-1K, with the average U content of 11.5 ppm. Their isotopes were also variable, with f206 ranging between 9% and 97% and an initial ²⁰⁷Pb/²⁰⁶Pb ratio of 0.851 ± 0.032. Twenty-four spots yielded the lower incept age of 159.5 ± 5.2 Ma (2δ, MSWD = 1.7, N = 24), with the weighted mean ²⁰⁶Pb/²³⁸U age of 161.5 ± 2.2 Ma (2δ, MSWD = 3.1, N = 28, six outliers excepted).

5.3. Garnet U–Pb Dating

Garnet from the skarn zone near fine-grained granodiorite (BCK46904-12) generally coexisted with hematite and calcite (Figure 5i and Figure 8d). Minor chalcopyrite and sphalerite were also discovered with the garnet grains. The results of U–Pb analysis are summarized in Supplementary Table S3 and presented in Figure 8c. Thirty-four analyses of the garnet yielded U contents of 2.10–10.1 ppm (average 4.46 ppm) and Pb contents lower than 0.5 ppm. These analyses demonstrated a lower intercept age of 158.6 ± 2.8 Ma (2δ, MSWD = 1.3, N = 34), similar to the ²⁰⁷Pb-corrected ²⁰⁶Pb/²³⁸U age of 158.5 ± 1.6 Ma (2δ, MSWD = 0.74, N = 34).

5.4. Whole-Rock Major and Trace Element Abundance

The whole-rock major and trace element analyses of coarse-grained granodiorite (LSZ1) and fine-grained granodiorite (LSZ2) are illustrated in Figure 9 and Figure 10 and summarized in Supplementary Table S4. To evaluate the possible influence of hydrothermal alteration, the alteration index (AI) versus the chlorite–carbonate–pyrite index (CCPI) diagram and isocon diagram are illustrated (Figure S1) [74,75]. The results indicate the rocks with minor or no alteration for both intrusions, according to Large et al. (2001) [75]. Most elements from two granodiorites have constant concentrations except for Cu, Cd, Zn, Mo, In, Pb, Bi, Cs, Sb, Tl, S and LOI, which show no obvious trend for chloritic alteration. Our samples were therefore only affected by sulfide veins and related elements were excluded for further discussion.

Four samples of coarse-grained granodiorite exhibited low SiO₂ (60.20–63.14 wt.%), Na₂O (2.05–2.94 wt.%), and K₂O (2.93–4.05 wt.%), and exhibited high MgO (1.78–2.16 wt.%), ^TFe₂O₃ (5.12–7.39 wt.%), and TiO₂ (0.62–0.67 wt.%) contents, with A/CNK ranging between 1.02 and 1.15. Compared to the coarse-sized granodiorite, five samples of fine-grained granodiorite took higher SiO₂ (63.41–66.03 wt.%), Na₂O (2.91–3.98 wt.%), and K₂O (3.24–3.79 wt.%) contents and lower MgO (0.84–1.15 wt.%), ^TFe₂O₃ (4.13–5.53 wt.%), and TiO₂ (0.44–0.50 wt.%) contents, corresponding to the decreasing Mg^# values from coarse-grained granodiorite (61.1–51.7) to fine-grained granodiorite (47.7–42.4). Their Na₂O and K₂O contents were similar to diorite and granodiorite, respectively (Figure 9a). The abundance of MgO and FeO in different intrusions is consistent with the enrichment of biotite in coarse-grained granodiorite and hematite in fine-grained granodiorite. When presented in the A/CNK versus the A/NK diagram, those two types of intrusion range from metaluminous to weakly peraluminous (Figure 9c). The K₂O versus SiO₂ diagram also suggests that these two intrusions are the product of high-K calc–alkaline magma (Figure 9b).

These two types of felsic intrusions share many differences among rare earth elements (REE). The primitive mantle-normalized multi-element diagram indicates that they are both enriched in Rb, Th, U, K, and Pb and depleted in Nb, Ta, and Ti, with no obvious Eu anomalies (Eu/Eu* = 1.08–1.20, Figure 10a). However, the coarse-grained granodiorite (LZS1) generally had a higher content of Dy~Lu and Y, with a flat HREE pattern and (La/Yb)_CN of 12.04–15.47. The fine-grained granodiorite was depleted in HREE, but its Lu content was slightly higher than Yb and Tm.

6. Discussion

6.1. Timing of Magmatism and Polymetallic Mineralization

It has long since been proposed that the skarn–hydrothermal Pb-Zn-Au-Ag mineralization at Shuikoushan was related to Jurassic granitoids, but the emplacement of magmatism was reported to be 172~149 Ma [30,31,33,34,35,62,77,78,79,80,81,82,83]. Different chronological attempts at determining mineralization age also indicated hydrothermal activities between 161 and 140 Ma [30,31,32,35]. Because there are different types of felsic intrusions and hydrothermal systems at Shuikoushan (Table S5), it is reasonable to conclude that different types of intrusions result in different stages of hydrothermal activities.

Our research has identified two types of granodiorite at Laoyachao, consisting of early coarse-grained granodiorite of 168 Ma intruded by fine-grained granodiorite of 164 Ma. The timing of the coarse-grained granodiorite is coeval with that of the granodiorite reported by Wang et al. (2001) [33] and slightly older than the previously reported Pb-Zn-related intrusions at Shuikoushan (158 to 163 Ma) [30,35,79]. The hydrothermal apatite from magnetite-bearing skarn also suggests the occurrence of an early hydrothermal event of 166 Ma, in which magnetite was intergrown with chlorite and Cu-Pb-Zn mineralization is relatively weak (Figure 3a and Figure 5).

However, our analyses of the fine-grained granodiorite-related hydrothermal system have indicated garnet-bearing skarn and vein-type hydrothermal activity near 159 Ma. Because garnet-bearing skarns were mainly developed in the contact zone between fine-grained granodiorite and limestone, there is no doubt that the skarn-type Cu-Pb-Zn mineralization is the product of ~164 Ma magmatism. The development of vein-type Au-(Pb-Zn) mineralization is restricted to the silicified breccia near the fine-grained granodiorite and NNE-striking fault. This type of hydrothermal vein also cut the early magnetite-bearing skarn (Figure 5b–f), suggesting late-stage hydrothermal activity near the felsic intrusion and regional fault. It is concluded that regional magmatism peaked at 170 Ma, 163 Ma, and 155 Ma at Shuikoushan (Figure 11). The ~159 Ma vein-type Au-(Pb-Zn) mineralization at Laoyachao is most likely the product of ~164 Ma fine-grained granodiorite and older than the ~155 Ma granodiorite reported. The Au-(Pb-Zn) mineralization at Laoyachao also occurred earlier than the vein-type hydrothermal Pb-Zn-Au-Ag mineralization at Kangjiawan [32,35].We therefore established a temporal link between magmatism and mineralization at Shuikoushan. The early stage of coarse-grained granodiorite (172~168 Ma) was emplaced at the depth of Laoyachao, with a magnetite-bearing skarn but no obvious Cu-Pb-Zn mineralization. The middle stage of fine-grained granodiorite (164~157 Ma) resulted in skarn-type Fe-Cu-Pb-Zn mineralization and vein-type Au-(Pb-Zn) mineralization at Laoyachao and Yagangtang. The late-stage granodiorite and granite porphyry (156~152 Ma) may have resulted in skarn and hydrothermal Pb-Zn-Au mineralization at Laoyachao, Yagongtang, and Kangjiawan. The development of the post-magma hydrothermal system should also be examined in future work.

6.2. Magma Generation in Extensional Setting

The polymetallic W-Sn-Nb-Ta-Cu-Mo-Pb-Zn-REE mineralization in the Nanling Range is attractive for its close relationship with multistage granites [46,85]. Our geochemical analyses on granodiorite have suggested the continuous generation of felsic magma at Shuikoushan (Figure 9). These granodiorites all belong to sub-alkaline and high-K calc-alkaline series, with the coarse-grained granodiorite being rich in Mg, Fe, Ti and the fine-grained granodiorite being rich in Si, Na, and K. The A/NK-A/CNK diagram suggests a uniform reservoir, ranging from metaluminous to weakly perluminous (Figure 9c), with an A/CNK lower than that of the S-type magmatism in Chuankou [54]. The enrichment of LILEs (large ion lithophile elements, e.g., K and Rb) and the depletion of HFSEs (high-field-strength elements, e.g., Nb and Ta) and Ti in granodiorite are similar to those of arc-related magmatism [86]. Central South China underwent intracontinental orogenesis in the Triassic Era and developed multiple fold-and-thrust belts and S-type granites in the Nanling Range [87]. These Indosinian granitoids are late-collisional and formed in a locally extensional environment, which is typical for Chuankou W deposits in the east of Hengyan Basin [54]. The Jurassic Era witnessed the low-angle subduction of the Paleo-Pacific Plate, which resulted in NNE-striking faults, basin-range tectonics, and associated alkaline–sub-alkaline magmatism within the SCB [51,88]. The development of the Hengyan Basin and the widespread intrusions in Shuikoushan are features of the intracontinental lithospheric extension setting for the Late Mesozoic period (175–80 Ma) [87,88]. The subduction of the oceanic block released abundant fluids into the superjacent lithosphere, which has been proven by the granodiorite Nb/U ratios of 3.99–5.55 at Shuikoushan. This Nb/U value is lower than the typical continental crust value of 6.2 [89], suggesting the modification of the magma reservoir by slab-derived fluids.

We further investigated the genesis of Shuikoushan granodiorite in an intracontinental extension setting. The rare earth element diagram in Figure 10b suggests a flat MREE and HREE pattern in two types of granodiorite, with a slight enrichment of LREE and insignificant Eu anomalies. The absence of Eu anomalies suggests that the behavior of feldspar is not critical for magma differentiation at Shuikoushan. Whole-rock geochemistry further indicates no obvious Zr/Hf vibration between different samples (Figure 12a). Because the crystallization of magma significantly lowers the Zr/Hf ratio in the residual melt when Zr enters other minerals [90], it is unlikely that the magma crystallization–differentiation process controlled the multistage magmatism at Shuikoushan. Nevertheless, a positive correlation exists between the La/Sm and Th/Nd ratios (Figure 12b), which corresponds to the partial melting of basement rocks at different degrees.

Compared to the coarse-grained granodiorite, the fine-grained granodiorite is more depleted in HREEs, which indicates that the source of fine-grained granodiorite is in the garnet and/or hornblende stable region. This is confirmed by the decrease in Dy/Yb along SiO₂ and Nb/Ta (Figure 13a,b), where residual garnet has captured HREEs along melting [91,92]. The Nb/Ta ratios are also believed to be controlled by the crustal thickness and to increase at a deeper region where rutile solubility is significantly reduced [92]. Elemental Nb-Y and Rb-(Y+Nb) diagrams thus suggest a post-orogenic setting for Shuikoushan magmatism (Figure 13c,d), with the source of fine-grained granodiorite taking higher pressure than coarse-grained granodiorite. The presence of two granodiorite at Shuikoushan may therefore be primarily an outcome of magma partial melting at different depths.

6.3. Magma Oxidation and Metallogenic Potential

It was believed that the long-term and continuous crust partial melting and mineral–magma separation have led to different styles of mineralization in South China [17,38,95]. Skarn–hydrothermal W-Sn-Ta-Li-Be deposits are mainly related to the S-type granites formed by upper-crustal melting in an compression–extension transformation process [54,84]. Skarn–hydrothermal Cu-Fe deposits are associated with I-type granites derived from the lower crust, where the lithosphere thinning was triggered by asthenospheric upwelling [96]. Our studies on granodiorite from Shuikoushan polymetallic deposits also demonstrate a source of magma at different depths. The higher Sr/Y ratios in fine-grained granodiorite suggest the transition from arc-like magma to adakitic rocks (Figure 9d), which is typical for materials related to porphyry copper deposits worldwide [97,98]. The fine-grained granodiorite takes higher temperatures (~708 °C) and fO₂ of ΔFMQ +0.08–+5.80. The coarse-grained granodiorite is of slightly lower temperature (~676 °C) and fO₂ of ΔFMQ +0.90–+2.93 (Figure 13e,f). The zircon Ce⁴⁺/Ce³⁺ calculations also indicate heightened and more variable Ce anomalies for fine-grained granodiorite from Shuikoushan, with features mostly similar to those possessed by porphyry Cu deposits (>120) [99].

The oxidation state of the parent magma impacts the solubility of copper and gold in the magmatic system. Because copper and gold are more enriched in the continental lower crust and mantle, the oxidized magma generally has a higher capacity for transporting metals and sulfur from the deeper lithosphere to a shallower depth [100,101]. The residual garnet in the lower crust leaves ferric iron in melting magma, which progressively oxidizes sulfide into sulfate and releases copper and gold into a solution [92,102]. As a result, the fine-grained granodiorite from Shuikoushan tends to take more copper and gold when it migrates from the deeper space as compared to the coarse-grained granodiorite. This interpretation is consistent with the hydrothermal alteration in the field. The skarn zone related to coarse-grained granodiorite is rich in magnetite but develops less copper mineralization (Figure 3 and Figure 5). Garnet-bearing skarns and calcite–quartz–sulfide veinlets are associated with fine-grained granodiorite, with abundant hematite and chalcopyrite along the metasomatism (Figure 4 and Figure 5). The ~164 Ma magmatism in Shuikoushan therefore has more potential for copper mineralization.

6.4. The Development of Skarn and Hydrothermal System

Although there have been debates on the mineralization styles of Cu-Pb-Zn-Au-Ag deposits at Shuikoushan, recent studies have confirmed the skarn, vein-type hydrothermal, and epithermal systems around the Laoyachao, Yagongtang, Kangjiawan, and Longwangshan ore districts [32,35,103]. Our analyses on hydrothermal minerals confirmed three different hydrothermal activities related to two stages of granodiorite. The development of the skarn was focused on the contact zone between the granodiorite and Late Paleozoic carbonate rocks, with the ~166 Ma skarn formation related to magnetite deposition and the ~159 Ma skarn formation associated with Fe-Cu-Pb-Zn-Au mineralization. Along the emplacement and the cooling of felsic intrusions, the hydrothermal fluids were derived from magma and migrated upwards along the NNE-striking fault. The precipitation of chalcopyrite, pyrite, galena, sphalerite, and native gold in this zone was due to the interaction between the high-temperature magmatic–hydrothermal components and the wall rocks. Unlike those related to the skarn system, fluids from the vein-type hydrothermal system had low temperatures but possessed high salinities [35,104] that were favorable for the transportation of lead, zinc, gold, and silver over a long distance [105,106]. Where the hydrothermal fluids were mixed with meteoric water or decompressed in the fractures, abundant metals were trapped in the distal zones of the mineralization system. Where granite porphyry developed, or deeply seated faults reached the ground surface (e.g., Xianrenyan, Kangjiawan), epithermal Au-Ag deposits dominate the mineralization styles at the Shuikoushan Orefield [107].

The continuous evolution of the skarn-vein-type hydrothermal–epithermal mineralization system is vital for polymetallic exploration around Shuikoushan. The ~159 Ma skarn formation event has also been reported in Yagongtang and Kangjiawan [32]. Previous fieldwork has reorganized the vein-type Pb-Zn-Au ores and epithermal Au-Ag ores in the shallow depth of the Kangjiawan anticline, where the hydrothermal minerals include quartz, calcite, chlorite, sericite, adularia, fluorite, and dikite [60,107]. There is likely a skarn-type Fe-Cu mineralization system beneath the overturned anticline. To verify this hypothesis, we applied the wide-field electromagnetic method (WFEM) and performed drilling work in the south of Kangjiawan (Figure 14) [107,108]. The WFEM results suggested a series of NNE-striking faults from Yagongongtang to Kangjiawan (Figure 14a), where the Paleozoic sediments were thrusted and covered by Mesozoic sandstones. Because these faults extended into the deeper space and cut across the pre-existing folds, it is unlikely that the hydrothermal fluids migrated eastwards from the Laoyachao intrusion without being blocked by faults. Nevertheless, we found a typical magnetite–hematite–chalcopyrite-bearing skarn at a 500 m depth southeast of the Kangjiawan anticline (Figure 14b) [60]. These discoveries, combined with our analyses in Laoyachao, indicate that the in situ magmatic–hydrothermal system was responsible for the Pb-Zn-Au-Ag-(Cu-Fe) mineralization in Kangjiawan. This mineralization style is similar to those in Laoyachao and is controlled by both deeply seated faults and an overturned anticline.

6.5. Implications for Mineral Exploration

Jurassic intra-continental extension and asthenosphere upwelling are important for polymetallic Fe-Cu-Pb-Zn-Au-Ag mineralization within the SCB. The thermal-driven partial melting of basement rocks has formed mafic–ultramafic intrusions, A-type granites, I-type granites, bimodal basalt–rhyolite volcanic rocks, and S-type granites at different depths (Figure 15a) [18,46,47]. The addition of mantle materials and slab-derived fluids further fertilized the magmatism system and led to the polymetallic mineralization along the Qingzhou–Hangzhou Metallogenic Belt [18,29]. Our analyses confirmed the development of Cu-Pb-Zn mineralization at Shuikoushan, where the partial melting of the lower crust in the garnet stable region resulted in ~164 Ma oxidized magma. The extensional tectonics and deeply seated faults facilitated the upwards migration of magma and the emplacement of granodiorite in fold-and-thrust belts (Figure 15b). Continuous skarn-vein-type hydrothermal–epithermal mineralization systems were then developed in contact zones, faults, and folds. This type of mineralization has been reported in Yinkeng, Qibaoshan, Tongshanling, and Baoshan [109,110,111], where the partial melting of the lower crust within the Qinzhou-Hangzhou Metallogenic Belt has facilitated the formation of skarn-hydrothermal Fe-Cu-Pb-Zn-Au-Ag mineralization within fold-and-fault belts. Future explorations should therefore focus on the deeper space of intrusion and the overturned anticline.

7. Conclusions

Altogether, two stages of magmatism and three types of magmatic–hydrothermal activities have been identified at Shuikoushan, and the early-stage, coarse-grained granodiorite was emplaced near 168 Ma, resulting in the development of the magnetite-bearing skarn of 166 Ma in the deeper space. The late-stage, fine-grained granodiorite formed around 164 Ma, developing the ~159 Ma skarn-type Cu-Pb-Zn-Au-Cu ores in the contact zone and vein-type Au-(Pb-Zn) ores in the breccia. The source magma of each granodiorite was generated via the partial melting of the lower crust at different levels, resulting in high-K calc-alkaline and weakly peraluminous magmatism at Shuikoushan. The residual garnet led to the deeply derived fine-grained granodiorite being more oxidative, which further developed skarn Fe-Cu-Pb-Zn and vein-type Pb-Zn-Au-Ag mineralization systems at Shuikoushan. Further exploration should focus on the continuous skarn-vein-type hydrothermal–epithermal mineralization systems within fold-and-fault belts.