A newly Found Biotite Syenogranite in the A newly Found Biotite Syenogranite in the Huangshaping Polymetallic Deposit, South China: Insights into Cu Mineralization

The Huangshaping W-Sn-Mo-Cu-Pb-Zn polymetallic deposit is located in the central Nanling Range, severing as the largest lead-zinc production base in the Hunan Province, South China (Xu et al., 2007). The polymetallic mineralization has been proven to be related to three types of granites, including quartz porphyry, granophyre and granite porphyry. In pace with the continuous exploration in recent years, increasing Cu polymetallic ore bodies have been discovered, showing good prospecting prospect at depth (Lei et al., 2010). However, the origin of the Cu mineralization is still controversial (Li et al., 2014a, b; Wang et al., 2011; Zeng, 2001; Zhong, 1996). Previous studies pointed out that the Cu mineralization is related to the quartz porphyry, evidenced by the sporadic Cu mineralization hosted in the quartz porphyry bodies (Li et al., 2014a, b). In contrast, Pb-Zn mineralization is considered to be generated by granophyre, and granite porphyry is closely associated with the W-Sn-Mo-Fe mineralization (Ding et al., 2016a; Li et al., 2014a, b; Yuan et al., 2014; Zhu et al., 2012; Wang et al., 2011; Zhong, 1996; Tong, 1986). These inferences were further illustrated by the whole-rock geochemical studies on the granites, which showed that concentrations of some ore-forming elements such as Pb, Zn are in the same level among three types of granites, while Cu is considerably enriched in quartz porphyry and W-Sn are mostly concentrated in granite porphyry (Li et al., 2014b; Qi et al., 2012; Quan et al., 2012; Liu et al., 2009; Yao et al., 2005). Though the corresponding relations between the granite and mineralization types were determined based on factual basis, the magmatic evolutionary processes and their bearings on different types of mineralization are increasingly questioned by the recent studies (Ding et al., 2016a, b; Li et al., 2014a, b). The latest- prospected Cu ores are found much deeper than the intrusive depth of the quartz porphyry, thus the source of Cu at the Huangshaping Deposit is still unclear.

Recently, a new type of granite, biotite syenogranite, has been revealed to be spatially associated with the deep Cu ore bodies by the systematic drilling work. In this contribution, analysis on petrology, whole-rock major-trace element geochemistry, zircon U-Pb chronology and Hf isotope of the syenogranite has been conducted in comparison with those of quartz porphyry, granophyre and granite porphyry from previous studies. We are aiming to reveal the genesis of this new type granitoid and provide some insights into the origin of the Cu-polymetallic mineralization in the Huangshaping Deposit.

The Nanling Range is located in the central part of the South China Block, geographically including southern Jiangxi, southern Hunan and northern Guangdong and eastern Guangxi (Fig. 1a). It is the largest W-Sn polymetallic ore province in China and even in the world (Mao et al., 2007). In the southern Hunan Province (central part of Nanling region), abundant Jurassic granitic plutons (e.g., Qianlishan and Qitianling) intruded along the Chenzhou-Linwu deep fault, forming a regional magmatic-tectonic belt (Fig. 1b). The granite-associated W-Sn polymetallic deposits are represented by Xianghualing, Shizhuyuan, Huangshaping, Xintianling and Yaogangxian in the region (Tong et al., 2000).

Figure 1.  (a) Sketch geological map of South China (after http://bzdt.nasg.gov.cn/, No. GS(2016)1597), showing the distribution of Early Jurassic magmatic rocks; (b) regional geological map of southern Hunan; and (c) sketch geological map of the Huangshaping Deposit (modified from Li et al., 2014a, b).

In the Huangshaping area, the outcropping strata are relatively simple, including Upper Devonian Shetianqiao Formation and Xikuangshan Formation, and Lower Carboniferous Ceshui Formation, Menggong'ao Formation, Shidengzi Formation and Zimenqiao Formation (Fig. 1c; Wang et al., 2011; Yao et al., 2007). Lithology of these strata mainly contains limestone, dolomitic limestone, calcareous sandstone, bioclastic limestone and sand-shale. The most advantageous ore- bearing wall-rock is Shidengzi Formation limestone; and Ceshui Formation sandstone is the secondary ore-bearing stratum which also served as a significant shelter layer for the mineralization at the Huangshaping Deposit (Lei et al., 2010).

The tectonic framework of the Huangshaping Deposit consists of a series of nearly SN-striking reversed faults (F₁, F₂, F₃) and overturned anticlines, coupled with three groups of EW-striking faults (F₀, F₆ and F₉). As the foremost ore-control structures, these faults and folds in the two directions not only provided a well-shaped trap space for the mineralization but also dominated the emplacement of various granitic plutons (Ma et al., 2007).

Extensive and frequent magmatism resulted in the intrusions of various granitic plutons at the Huangshaping area. Among them, the quartz porphyry and dacite porphyry are exposed to the surface (Fig. 1c), while granite porphyry and granophyre are completely concealed for hundreds of meters beneath the surface (Fig. 2a). These four types of granitoids all belong to Jurassic hypabyss intrusions with a variable intrusive time span ranging of 180-150 Ma (Ding et al., 2016b; Li et al., 2014a; Yuan et al., 2014; Ai, 2013; Quan et al., 2012; Wang et al., 2011; Lei et al., 2010; Yao et al., 2005). With a dumbbell shape, the quartz porphyry is situated in the middle part of the Huangshaping area (Zhong, 1996). The concealed tongue- shaped granophyre intruded in footwall of the fault F₁ (Li et al., 2014a, b). Occurring in the southeast portion of the Huangshaping area, the concealed granite porphyry is located between the faults F₁ and F₂ and has the largest intrusive volume (Li et al., 2014a, b; Yuan et al., 2014; Zhong, 1996). The newly found syenogranite was revealed beneath the granophyre in the northern portion of the deposit, probably with a smallest intrusive scale (Fig. 2b).

Figure 2.  (a) Geological map of 56 m level and (b) cross-section of No. 13 exploration line in the Huangshaping Deposit (after Wang et al., 2011). The newly found syenogranite is located in the northern portion of the deposit, beneath the granophyre body without absolute boundaries (noted by dotted lines in Fig. 2b).

In the Huangshaping Deposit, hundreds of hydrothermal filling replacement-type Pb-Zn and dozens of skarn-type W-Mo-Bi-Sn-Fe ore bodies were discovered during past exploration activities (Chen et al., 2013). The former is mainly distributed along the contact zone between limestone and granophyre, and the latter occurs in massive skarns around the concealed granite porphyry. These ore bodies are in different sizes, ranging from 800 to 1 000 m in length, 60 to 400 m in width and 1 to 310 m in thickness. Ore bodies are complicated in morphology, showing vein, crescent, lentoid, irregular, stratoid and columnar shapes. Ore minerals are mostly composed of sphalerite, chalcopyrite, galena and pyrite for the Pb-Zn ores, and magnetite, scheelite, molybdenite and bismuthinite for the W-Mo-Bi-Sn-Fe ores (Chen et al., 2013). Four types of Cu ore bodies have been differentiated (Wang et al., 2011): (1) skarn-type distributed around the granite porphyry, (2) hydrothermal filling-replacement-type located close to the granophyre and partially hosted in the syenogranite, (3) stratoid-type existed in the Ceshui Formation, and (4) fine disseminated porphyry-type hosted in the quartz porphyry. Recent exploration revealed that the second type Cu ore bodies with high grade, bulk mass and deep burial depth possess the greatest prospecting potential for Cu resources. Additionally, strong wall-rock alterations such as silicification, K-feldspathization, propylitization, marbleization, muscovitization and biotitization were closely related to the Cu mineralization.

The syenogranite is flesh red-colored with euhedral- subhedral granular texture and massive structure (Fig. 3a). The main rock-forming minerals are fine-grained (0.5-2 mm) and mainly composed of quartz (40%), K-feldspar (30%), plagioclase (15%) and biotite (5%) (Fig. 3b). Among them, plagioclase possesses the best automorphic degree, followed by K-feldspar and quartz. In addition, a small amount of K-feldspar occurs as Carlsbad twin whereas plagioclase is commonly found in polysynthetic twin. The biotite is characterized by relatively large granularity and good automorphic extent. The accessory minerals are mainly composed of zircon and chalcopyrite. Moreover, the syenogranite is locally interspersed by thin chalcopyrite- quartz veins (Fig. 3c), in which sphalerite and pyrite were partly replaced by chalcopyrite (Fig. 3d).

Figure 3.  Hand specimens of the syenogranite (a), photomicrographs of the syenogranite (b), quartz-chalcopyrite veins in the syenogranite (c), typical Cu-ores related to the syenogranite (d). Py. Pyrite; Cp. chalcopyrite; Sp. sphalerite; Qtz. quartz; Pl. plagioclase; Kf. K-feldspar; Bi. biotite.

Five syenogranite samples were collected at 160-240 m depth of No. ZK1301 drill hole which was arranged at the No. 13 exploration line, No. 56 m level at the Huangshaping Deposit. Thus, the freshness of these samples is fully guaranteed. After sampling, each sample was cut and polished into thin sections and then was crushed by a hammer. Subsequently, a vibration agate mill was used to grind the crushed samples into 200-mesh size.

The measurement of major elements was performed using X-ray fluorescence spectrometry (XRF) at Kyushu University (Japan). Standard sample JA-3 was used to evaluate accuracy, and the results show that the experimental error was less than 5%. Trace elements encompassing rare earth elements (REE) were determined by inductively coupled plasma mass spectrometry (ICP-MS) at the ALS Laboratory in Canada. Ascertained by the analysis of standard sample JG-2, the experimental error of most trace elements was less than 10%.

Zircon crystals were separated from a mineralized syenogranite sample (HSP-2) by using standard techniques: hand- picked to purify zircon crystals under binocular microscope→ mounted in epoxy resin→polished to expose the grain center. Cathodoluminescence (CL) imaging was implemented by scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) system and a CL3+ detector under an operating condition at 15 kV and 20 nA at the Beijing Geo- Analysis Co. Ltd. Zircon U-Pb dating and trace element analyses were conducted by LA-ICP-MS at Beijing Geo-Analysis Co. Ltd., using a GeoLas 2005 excimer laser ablation system that coupled to an Agilent 7500a ICP-MS. For the instrument, laser energy and frequency were 70 mJ and 8 Hz, respectively; spot size and ablated depth were 32 and 20-40 μm, respectively. During the experimental process, helium acted as a carrier gas whereas argon was used as the make-up and the mixture gas in carrier gas. In order to improve precision and reduce detection limit for the analysis, nitrogen was added into central gas stream (Ar+He) of the Ar plasma (Liu et al., 2010; Hu et al., 2008). Each analysis contained a background acquisition of approximately 20-30 s (gas blank) followed by 50 s of data acquisition from the sample. Zircon 91500 is the external standard for U-Pb dating which was analyzed twice every 5 samples. As the other standard sample, zircon standard GJ-1 was analyzed as an unknown. The weighted mean ²⁰⁶Pb/²³⁸U age for GJ-1 is 597.51± 5.26 Ma (2σ, n=10), which corresponds to the recommended values within uncertainty (599.81±1.7 Ma, 2σ) (Jackson et al., 2004). Time-dependent drifts of U-Th-Pb isotopic ratios were calibrated using a linear interpolation (with time) for every five analyses under variations in the 91500 standard. Preferred U-Th-Pb isotopic ratios used for 91500 were referenced from Wiedenbeck et al. (2004). Additionally, NIST SRM 610 was analyzed every 10 samples to make a correlation of time- dependent drift of sensitivity and mass discrimination in trace element results. Trace element compositions of zircons were calibrated by internal standardization (²⁹Si, assumed 32.7 wt.% SiO₂ for zircon unknowns). Detailed experimental methods and conditions for LA-ICP-MS trace element analyses were introduced in Liu et al. (2008) and Chen et al. (2011). Off-line background and analytical signals, time-drift correction and quantitative calibration of zircon U-Pb dating and trace element compositions were selected and integrated by ICPMSDataCal 10.1 (Liu et al., 2010). The calibration of common Pb correction based on the method was proposed by Andersen (2002). U-Pb concordia diagrams and weighted mean ages were calculated by Isoplot 4.5 (Ludwig, 2003).

In-situ analysis of zircon Lu-Hf isotopes was tested by laser ablation-multi collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) at Beijing Createch Testing Technology Co. Ltd. In the course of the experiment, helium was applied as the carrier gas of denudation material, and the diameter of denudation was 40 μm. The international standard sample GJ-1 of zircon was used as reference material during the determination, and most of the analytical spots are the same as those of U-Pb dating. Related instrument operation conditions and detailed analysis process are described in Wu et al. (2006) and Hou et al. (2007). In the analysis process, the ¹⁷⁶Hf/¹⁷⁷Hf ratios obtained for zircon standard GJ-1 are 0.282 007±0.000 007 (2σ, n=36), which is in good agreement with the reported values (Morel et al., 2008) within the error range.

The analytical results of major element compositions of the syenogranite are presented in Table 1. The loss on ignition (LOI) for each sample is less than 2 wt.%, indicating high reliability of the data. The syenogranite samples have consistent chemical compositions: SiO₂=75.58 wt.%-76.02 wt.% (average=75.76 wt.%), Al₂O₃=12.23 wt.%-12.46 wt.% (average=12.36 wt.%), K₂O=5.24 wt.%-5.78 wt.% (average=5.44 wt.%), Na₂O=2.73 wt.%-3.36 wt.% (average=2.97 wt.%), CaO=0.81 wt.%-1.35 wt.% (average=1.07 wt.%), MgO=0.18 wt.%-0.27 wt.% (average=0.22 wt.%) and FeO_t=0.37 wt.%-1.15 wt.% (average= 0.72 wt.%). Compared to the other granite types at the Huangshaping Deposit, the syenogranite is characterized by high silicon, aluminum and alkalinity (K₂O+Na₂O=8.08 wt.%-8.66 wt.%) (Table 1). The average content of SiO₂ in syenogranite is higher than that in other granite types by 1 wt.%-2 wt.%. The average content of K₂O in syenogranite is only slightly higher than that in granite porphyry and is lower than that in quartz porphyry and granophyre by ~1 wt.%. The average content of Na₂O is gradually increased by the order of quartz porphyry, granophyre, granite porphyry and syenogranite. In addition, the average contents of FeO_t (0.72 wt.%) and MgO (0.22 wt.%) in syenogranite samples are both the minimum quantities among the four types of granites, reflecting a relatively less abundant of mafic minerals in syenogranite. Based on the SiO₂ vs. (K₂O+Na₂O) diagram (Fig. 4a) and microscopic studies (Fig. 3b), the new type granite can be formally classified as fine-grained biotite syenogranite. Furthermore, the syenogranite can be divided into high K calc-alkaline shoshonite granite due to its low Rittmann indexes (2-3) whereas high K₂O compositions (Fig. 4b). In addition, the syenogranite can be classified as metaluminous to peraluminous granite, similar to other types of granites from the Huangshaping Deposit (Fig. 4c).

Figure 4.  Classification diagrams for granites in the Huangshaping Deposit. (a) SiO₂ vs. (K₂O+Na₂O) diagram (after Le Bas et al., 1986 and Le Maitre, 1989); (b) SiO₂ vs. K₂O diagram (after Wheller et al., 1987); (c) A/KNC vs. A/NK diagram, where A/KNC=Al₂O₃ (molar)/(K₂O+Na₂O+CaO) (molar) and A/NK=Al₂O₃ (molar)/(Na₂O+K₂O) (after Frost et al., 2001). The fields of quartz porphyry, granophyre and granite porphyry are from Li et al. (2014a).

Trace element compositions (including rare earth elements) of the syenogranite samples are listed in Table 2. Compared to the other granite types in the Huangshaping Deposit, the syenogranite has similar characteristics to quartz porphyry and granophyre in terms of most trace element compositions, such as Cs, Rb, Ta, Zr, Ce, Sm, Gd, Tb, Ho, Dy, Tm and Lu. In contrast, compositions of Ba (average=9.92 ppm), Sr (17.2 ppm), V (< 5 ppm) and Eu (average=0.05 ppm) in the syenogranite are close to those of granite porphyry. Moreover, elements such as Ga, Nb, Rb, Ta, U, Y, Sm, Gd, Tb, Er, Tm, Yb and Lu in syenogranite are slightly higher than those of quartz porphyry and granophyre but are comparatively lower than those of granite porphyry, with reverse trends for Ba, Sr, Ce and Eu. For the major ore-forming elements in syenogranite, the average value of Cu (134 ppm) is much higher than that in granophyre (73 ppm) and granite porphyry (1 ppm), and is almost the same as that in quartz porphyry (132 ppm). The average contents of Pb (27 ppm) and Zn (12 ppm) in syenogranite are extremely less than those in quartz porphyry, granophyre and granite porphyry (Table 2). In addition, the average concentration of Mo among the four types of granites shows little difference whereas the As content displays large variations: 120 ppm in granite porphyry, 31 ppm in quartz porphyry, 29 ppm in granophyre and 23 ppm in syenogranite.

On the primitive mantle-normalized multi-elements diagram (Fig. 5a), the four types of Huangshaping granites exhibit strongly negative Ba, Sr, P and Ti anomalies and positive Rb, Th and U anomalies. Overall, more pronounced anomalies are shown for the granite porphyry and syenogranite. The abnormity degrees of Ba, U, Sr and P in the syenogranite are almost the same as those of the granite porphyry, whereas Ti and Rb show significant disparities between the other two types of granites. Additionally, several elements such as Ta, Y, Yb and Lu also act with relatively independent states between granite porphyry and other granites.

Figure 5.  (a) Primitive mantle-normalized trace element, and (b) chondrite- normalized REE patterns for the granites in the Huangshaping Deposit. The average values of quartz porphyry, granophyre and granite porphyry are from Li et al. (2014a). Normalized values for primitive mantle and chondrite are both from Sun and McDonough (1989).

The REE distribution curves of all the granite types are generally similar to each other but also show some distinctions (Fig. 5b). Overall, the granite porphyry shows the most significant negative Eu anomaly, followed by the syenogranite, granophyre and quartz porphyry. The difference of average ΣREE between the syenogranite (140.6 ppm) and the quartz porphyry (139.5 ppm) can be ignored whereas that in the granophyre (147.3 ppm) and the granite porphyry (172.1 ppm) is relatively high. The average LREE/HREE ratios among these granites display an increasing trend from granite porphyry (1.55) to syenogranite (2.99), granophyre (4.07) and then to quartz porphyry (4.55). This difference is manifested by a LREE-enriched pattern for the granite porphyry whereas slightly fractionated REE patterns for the other three types of granites. Furthermore, lanthanide tetrad effects are shown on the REE patterns and are most obvious in the granite porphyry, followed by the syenogranite.

In brief, the syenogranite is characterized by enrichment of LREE, relatively flat HREE patterns and pronounced lanthanide tetrad effect, with a deep ''V'' shape REE pattern. Additionally, characteristics of trace elements in the syenogranite imply a transitional trend between granite porphyry and the other two kinds of granites (quartz porphyry and granophyre), indicating its own uniqueness.

On CL images (Fig. 6), most of the zircon grains separated from the syenogranite possess euhedral-subhedral short angular column characteristics and range from 70 to 150 μm in size, with length/width ratios ranging from 1 : 1 to 2 : 1. Overall, most zircons show oscillatory zoning characteristics with relatively smooth surfaces, suggesting a typical magmatic origin. However, several zircons (represented by Nos. 2, 4, 6, 9, 10, 13, 16, and 20) are characterized by inferior euhedral degree and encircled dark rims, probably indicating a hydrothermal origin. Furthermore, Nos. 7, 12, 14, 17 and 18 zircons show characteristics of moderate roundness and irregular internal texture, implying a detrital origin.

Figure 6.  CL-images of analyzed zircons from syenogranite sample HSP-2, showing U-Pb dating (yellow circles) and Hf measuring (green circles) spots.

Zircon trace element compositions are shown in Table 3. In most cases, the distinct dark and bright colors in zircons on the CL images can be probably ascribed to different trace element compositions and U/Th ratios. Zircons with oscillatory zoning characteristics (Group 1) have Hf concentrations ranging from 7 672 ppm to 13 276 ppm, Y from 305 ppm to 14 699 ppm, Nb from 0.34 ppm to 336 ppm, Ta from 0.23 ppm to 34.9 ppm, Ti from 1.81 ppm to 35.6 ppm, Pb from 10.1 ppm to 350 ppm, Th from 67.4 ppm to 2 955 ppm and U from 105 ppm to 7 243 ppm. In addition, Group 1 zircons have high U/Th ratios (averaged at 0.8), suggesting a typical magmatic origin. In contrast, dark zircons with irregular shape on CL images (Group 2) possess obviously higher average contents of Hf (14 146 ppm), Y (29 488 ppm), Nb (1 737 ppm), Ta (115 ppm), Ti (130 ppm), Pb (265 ppm), Th (5 448 ppm) and U (10 923 ppm). Moreover, the U/Th ratios (averaged at 0.5) are comparatively lower than those in Group 1 zircons, further indicating their hydrothermal origin.

Two types of REE patterns can be differentiated on the chondrite-normalized diagram (Fig. 7a). Group 1 zircons are characterized by low contents of LREE and significant positive Ce anomalies, showing the characteristics of magmatic zircon. In contrast, Group 2 zircons (Nos. 2, 4, 6, 9, 10, and 20, Fig. 6) possess higher LREE compositions, unobvious Ce anomalies and more pronounced negative Eu anomalies, representing hydrothermal origin. This inference can be further proved by using zircon discrimination diagrams (Figs. 7b, 7c), on which Group 1 zircons mainly plotted in the magmatic field whereas Group 2 zircons plotted in the hydrothermal zircon domain. Thus, we can infer that Group 2 zircons have undergone hydrothermal alteration during postmagmatic stage.

Figure 7.  Discriminant diagram of zircon types for the syenogranite in the Huangshaping Deposit. (a) Chondrite-normalized average REE patterns for the syenogranite; (b) Sm_N/La_N vs. Ce/Ce* diagram; (c) La vs. Sm_N/La_N diagram. Normalized values for chondrite are from Sun and McDonough (1989); (b) and (c) after Fu et al. (2009), Hoskin (2005), and Li et al. (2014a).

Zircon LA-ICP-MS U-Pb dating results are presented in Table 4 (excluded No. 6 zircon data with a low analytical concordance < 90%). The ²⁰⁶Pb/²³⁸U age data of magmatic zircons and hydrothermal zircons were plotted separately, the obtained concordant ages can represent magma crystallization age and hydrothermal event age (or even mineralization age), respectively (Li et al., 2017a; Li, 2009; Zhang et al., 2009; Hu et al., 2004; Geisler et al., 2003). After removing old inherited zircons (498.1-2 030.3 Ma, Fig. 8a), the ²⁰⁶Pb/²³⁸U ages of remaining 14 zircons range from 138.2 to 158.2 Ma. Seven magmatic zircons (Group 1) yielded a concordant U-Pb age of 156.5±1.8 Ma (MSWD=0.46, Fig. 8b), representing the crystalizing age of the syenogranite. Another five hydrothermal zircons (Group 2) yielded a concordant U-Pb age of 154.4±2.9 Ma (MSWD=0.25, Fig. 8c). This hydrothermal zircon U-Pb age is consistent with the previous molybdenite Re-Os dating results (ranging from 153.8±4.8 to 159.4±3.3 Ma) at the Huangshaping Deposit (Qi et al., 2012; Lei et al., 2010; Ma et al., 2007; Yao et al., 2007). In addition, the two zircons with ages of 138 Ma (Nos. 16 and 19) have unclear zoning characteristics in CL images and high concordances in U-Pb dating, probably related to a later hydrothermal event.

Figure 8.  U-Pb concordant ages of (a) all zircons from syenogranite, (b) Group 1 magmatic zircons, and (c) Group 2 hydrothermal zircons.

Zircon Hf isotope analytical results are displayed in Table 5. Since the analytical result of No. 2 spot is abnormal to others, it has been excluded for further discussion. The remaining5 data are divided into two categories: Nos. 1, 3 and 4 are magmatic zircons; Nos. 5 and 6 are inherited zircons. The ¹⁷⁶Hf/¹⁷⁷Hf ratios and ε_Hf(t) values of the magmatic zircons ranging from 0.282 603 to 0.282 390 and from -10.2 to 2.87, respectively, with single-stage Hf model ages (T_DM1) of 741- 1 206 Ma and two-stage Hf model ages (T_DM2) of 1 025-1 850 Ma. On the other hand, the ¹⁷⁶Hf/¹⁷⁷Hf ratios of the inherited zircons range from 0.281 404 to 0.281 713 which are significant lower than those of magmatic zircons. In addition, the ε_Hf(t) values are -6.78 and -3.77 for the inherited zircons, with the T_DM1 and T_DM2 ages ranging from 2 188 to 2 550 Ma and from 2 644 to 2 904 Ma, respectively.

Coupled with the magmatic and hydrothermal zircon dating in this study, the previous zircon U-Pb dating results on different types of granites and molybdenite Re-Os dating on W-Sn polymetallic ores at the Huangshaping Deposit are summarized in Table 6. The variable results on the granites may be ascribed to multiple episodes of magmatism and prolonged magmatic evolution in the deposit area, or could be caused by the dating of samples from distinct magmatic facies from variable sampling locations (Ai, 2013). Thus, it can be concluded that the different ages of crystallization and mineralization are within reasonable error range, further indicating the close genetic relationship between the magmatism and ore formation (Li et al., 2017b; Yuan et al., 2014; Ai, 2013; Yao et al., 2005).

The dating results of the magmatic and hydrothermal event associated with the syenogranite in this study are consistent with the magmatic and ore-forming ages of neighboring world-class W-Sn polymetallic deposits, such as the Shizhuyuan W-Sn- Mo-Bi Deposit, the Yaogangxian W Deposit, the Baoshan Pb-Zn Deposit, the Furong Sn Deposit, the Xianghualing Sn Deposit, the Xintianling W Deposit and the Jinchuantang Sn-Bi Deposit (Table 7). They correspond to the Mesozoic flame up granitic magmatism and polymetallic mineralization in South China (Mao et al., 2013, 2007; Hua et al., 2005).

Since the close relationship between granitic plutons and ore bodies in the Huangshaping Deposit (Yuan et al., 2014; Lei et al., 2010; Zhong, 1996; Tong, 1986), it is very important to determine the origin of different types of the granites. Although the analytical quantity for Hf isotopes in this study is small, the data are still meaningful in indicating the origin of syenogranite, especially when comparing the data with previous Hf analysis on the various types of granite in the Huangshaping Deposit.

Previous Hf isotopic data on the different types of granites at the Huangshaping Deposit are listed in Table 8. The values of ε_Hf(t) among the granites are generally variable and negative (-19.7 to -0.1) and the two-stage model ages (T_DM2) vary from 1 263 to 2 459 Ma, implying that the magmas were mainly derived from Neoarchean-Mesoproterozoic crust but received a certain amount of mantle contribution (Yuan et al., 2014; Ai, 2013; Quan et al., 2012). It is generally accepted that the crust-mantle interaction and mantle contribution in the Nanling region probably have also played an important role in the formation of the world-class polymetallic deposits (Wang et al., 2011; Zhu et al., 2009, 2008; Zhao et al., 2001; Zhou and Li, 2000).

In this study, the ε_Hf(t) values of spots No. 1 and No. 4 are -2.71 and 2.87, respectively, indicating the magma of syenogranite was generated by crust-mantle interaction. The positive ε_Hf(t) value further suggests the participation of mantle material into the syenogranite magma, which is substantially different from other granite types in the Huangshaping Deposit (Table 8). On the other hand, the relatively lower ¹⁷⁶Hf/¹⁷⁷Hf ratios and negative ε_Hf(t) values of spots No. 5 and No. 6 (inherited zircons) imply partial melting of a crustal source; and their old T_DM1 (2 188-2 550 Ma) suggest that the source region of the syenogranite magma could probably the Paleoproterozoic or even older basement rocks. Thus, it can be concluded that the syenogranite was derived from the partial melting of Proterozoic basement rocks, with significant input of mantle materials.

Different types of granites produced by frequent magmatism at the Huangshaping Deposit resulted from multi-stage magmas (Yuan et al., 2014; Wang et al., 2011; Liu et al., 2009; Tong et al., 2000; Zhong, 1996). The chondrite-normalized REE patterns and the primitive mantle-normalized multi-element patterns of the syenogranite are quite similar to those of quartz porphyry and granophyre, indicating a similar magma evolutionary process among the three granites. In contrast, distinguished patterns are shown for the granite porphyry, suggesting the highest evolved degree for its magma (Li et al., 2014a, b). The potential location of the syenogranite is just beneath the granophyre (Fig. 2b), also indicating the closest relationship between the granophyre and the syenogranite in space. Geochemically, the trace elements such as Ba, Sr and Nb appear only in silicate minerals thus can apparently indicate the evolution of granitic magma. The average compositions of Ba (9.92 ppm) and Sr (17.2 ppm) in syenogranite are close to those in granite porphyry (7.24 ppm and 13.54 ppm, respectively) whereas less than those in quartz porphyry (61.42 ppm and 47.07 ppm, respectively) and those in granophyre of (46.91 ppm and 44.47 ppm) (Table 2). In contrast, the average content of Rb in the syenogranite (463.8 ppm) is close to that of quartz porphyry (510.56 ppm) and granophyre (425 ppm) but is quite different from that of granite porphyry (973.78 ppm). The average Rb/Sr ratios are 10.85 for quartz porphyry, 9.56 for granophyre, 71.89 for granite porphyry and 33.36 for syenogranite, demonstrating that the granite porphyry experienced the strongest fractional crystallization, followed by syenogranite, quartz porphyry and then granophyre. All of these evidences fully proved that the evolutionary sequence is as follows: quartz porphyry→granophyre→syenogranite→granite porphyry. Therefore, we can infer that the syenogranite is a transitional magmatic phase between the granophyre and the granite porphyry.

The Huangshaping granites have been recognized as S-type granites or transitional type between S- and Ⅰ-type for a long time (Liu et al., 2009; Yao et al., 2005; Zhong, 1996; Tong, 1986). However recently, Quan et al. (2012) divided these ore-forming related granites into the strong peraluminous A-type range and Li et al. (2014b) proposed that the quartz porphyry and granophyre can be classified as A₁-type alkaline granites whereas the granite porphyry is considered to be A₂-type aluminous granite. Similarly in this study, the syenogranite can be also classified as A-type granite (Figs. 9a-9d). Moreover, the syenogranite samples fall into the A₁ field (similar to the quartz porphyry and granophyre) whereas the granite porphyry plots in the A₂ field (Figs. 9e-9f). Moreover, the Huangshaping granite samples mainly fall into the "within-plate granite" field on the tectonic discrimination diagrams (Figs. 10a, 10b), overlapping with each other on the patterns except for the granite porphyry. Previous studies believed that A₁-type alkaline or peralkaline granites were mostly formed in within-plate anorogenic tectonic environment (Zhang et al., 2017; Wu et al., 2002; Patiño Douce, 1997; Hong et al., 1996). As a transitional type between the granophyre and granite porphyry, the magma of syenogranite may be generated in an intra-continental rift setting during the large-scale continental extension process.

Figure 9.  Geochemical classification for the A-type granites in the Huangshaping Deposit. (a) 10 000Ga/Al vs. Na₂O+K₂O diagram; (b) 10 000Ga/Al vs. Nb diagram; (c) 10 000Ga/Al vs. Ce diagram; (d) 10 000Ga/Al vs. Zr diagram; (e) Y/Nb vs. Ce/Nb diagram; (f) Nb-Y-Ce diagram; (a)-(d) after Whalen et al. (1987); (e) and (f) after Eby (1992).

Figure 10.  Discriminate diagrams of tectonic settings for the Huangshaping granites (after Pearce, 1996). (a) Y vs. Nb diagram and (b) Y+Nb vs. Rb diagram. WPG. Within plate granites; VAG. volcanic arc granites; ORG. ocean ridge granites; syn-COLG and post-COLG. syn- and post-collision granites.

The relationship between variable magmatic rocks and distinct mineralization types in the Huangshaping Deposit has been discussed since 1980s (He et al., 2010; Zhong, 1996; Tong, 1986). Some studies suggested that the granite porphyry provided the most ore-forming materials and energy for the mineralization (Zhu et al., 2012; Tong, 1986). In other opinions, the primary ore-forming materials were derived from the granophyre and supplemented by the granite porphyry (Zhong, 1996). Recently, it is believed that the Cu, Pb, Zn mineralization is intimately associated with the quartz porphyry whereas the W-Sn polymetallic mineralization is closely related to the granite porphyry (Liu et al., 2009). Afterwards, Lei et al. (2010) pointed out that the generation of W-Sn-Mo-Bi skarn-type ores are both associated with quartz porphyry and granite porphyry. Li et al. (2014b) reported that the quartz porphyry is weakly mineralized by the porphyry type Cu; the granophyre is related to the vein-type Pb-Zn mineralization; and the granite porphyry is closely associated with the skarn type W-Sn-Mo-Fe mineralization. However, a distinct perspective has been proposed in a most recent study (Ding et al., 2016a): significant amount of base metals (Pb and Zn) and S were extracted from basement material beneath southern Hunan whereas granitoids merely provided heat and fluid during the ore-forming process, except for the W-Sn mineralization which is still considered to be associated with granite porphyry.

The Jurassic granites were formed in the extensional environment and have experienced a stronger differentiation and evolution process (Li et al., 2018; Zhu et al., 2009; Bai et al., 2007; Wang et al., 2007; Zhou et al., 2006; Hua et al., 2005). The A₁-type granites in the Huangshaping Deposit, which include quartz porphyry, granophyre and the newly found syenogranite, are more or less associated with the Cu mineralization. The rapid extension of the crust may result in the deposition of deeply sourced Cu in a low-pressure environment. Partial melting of the lower crust which experienced intense mantle-crust interaction could produce A₁-type alkaline magmas as well as leach a large amount of Cu from the mantle, forming the Cu-dominated ore bodies in the earlier period of the Huangshaping Deposit. It is commonly approved that the early Jurassic syenite magmas in South China were closely associated with asthenospheric mantle under an intra-continental extension setting (Bai et al., 2015; Liu et al., 2013). Granitic magmas that underwent crust-mantle interaction are likely to result in Cu mineralization. This is consistent with our Hf data of the syenogranite, and we can infer that the participation of mantle material is a favorable factor for Cu mineralization.

On the other hand, the syenogranites also possess some similar geochemical characteristics to the granite porphyry. They contain more incompatible elements, volatile components, higher Rb/Sr ratios, lower Eu/Eu* values and higher alkalinity compared to quartz porphyry and granophyre in the Huangsha- ping Deposit. For the W-Sn polymetallic mineralization, the ore-forming ability and specialization of related granites depend on the contents of mineralizing elements, volatiles and heat producing elements (such as U, Th and other radioactive elements), oxidative statuses and Ph values (Bai et al., 2007). In contrast to other granites, the concentrations of U and Th in syenogranite samples are similar to those in granite porphyry whereas higher than those in quartz porphyry and granophyre. The average Rb/Sr and Eu/Eu* ratios also show a downward trend from granite porphyry and syenogranite to quartz porphyry and granophyre (Table 2). Therefore, the syenogranite possesses a strong W-Sn mineralization ability theoretically. The Cu-mineralized syenogranite is geochemically a transitional phase between the granophyre and the granite porphyry, indicating that the syenogranite may have not only contributed to the deep Cu ore formation but also played a role of "vanguard" for the later W-Sn polymetallic mineralization in the Huangshaping Deposit. Many scholars pointed out that mantle-derived material have played an important role during the emplacement of Jurassic granites (Zhou et al., 2016), and the crust-mantle interaction may have played an important role in the formation of world class W-Sn polymetallic deposits in the Nanling region (Chen et al., 2016; Zhu et al., 2009; Jiang et al., 2008; Li et al., 2006; Zhao et al., 2001).

(1) The newly found fine-grained biotite syenogranite in the Huangshaping Deposit is a transitional phase between the granophyre and the granite porphyry, belonging to A₁-type alkaline granite and forming under an intraplate extensional setting.

(2) The U-Pb dating of magmatic and hydrothermal zircons in the syenogranite yielded the ²⁰⁶Pb/²³⁸U concordant ages of 156.5±1.8 and 154.4±2.9 Ma, representing the crystallization and related Cu mineralization ages, respectively. The syenogranite magma may be derived from Paleoproterozoic basement (lower crust) and experienced substantial mantle-crust interaction.

(3) The syenogranite may have not only contributed to the deep Cu mineralization but also induced the later W-Sn polymetallic ore formation in the Huangshaping Deposit.

This work was co-financed by the National Natural Science Foundation of China (Nos. 41502067, 41672074) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG150612). The authors thank researcher Li Liu for his technical assistance in LA-ICP-MS analysis. The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0974-7.