39Ar dating of the Xihuashan tungsten deposit, central Nanling district, South China

39Ar dating of the Xihuashan tungsten deposit, central Nanling district, South China

Lithos 150 (2012) 111–118 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Molybdenite Re...

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Lithos 150 (2012) 111–118

Contents lists available at SciVerse ScienceDirect

Lithos journal homepage: www.elsevier.com/locate/lithos

Molybdenite Re–Os and muscovite 40Ar/ 39Ar dating of the Xihuashan tungsten deposit, central Nanling district, South China Rui-Zhong Hu ⁎, Wen-Feng Wei, Xian-Wu Bi, Jian-Tang Peng, You-Qiang Qi, Li-Yan Wu, You-Wei Chen State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China

a r t i c l e

i n f o

Article history: Received 1 September 2011 Accepted 17 May 2012 Available online 30 May 2012 Keywords: Molybdenite Re–Os dating Mica Ar–Ar dating Tungsten mineralization Nanling China

a b s t r a c t The Xihuashan tungsten deposit in the central Nanling region, South China, is an important vein-type ore deposit hosted in Cambrian strata and Mesozoic granitic intrusions. Wolframite and molybdenite are the principal ore minerals. The gangue minerals are mainly quartz and muscovite. Wolframite and molybdenite are products of the first stage hydrothermal activity, whereas muscovite formed dominantly at the second stage. Molybdenite Re–Os and muscovite 40Ar/ 39Ar dating have been carried out to investigate the age of mineralization. Re–Os isotopic dating for molybdenite associated with wolframite yield a precise, wellconstrained isochron age of 157.8 ± 0.9 Ma (MSWD = 1.5). Ar–Ar isotopic analyses of muscovite yield a plateau age of 152.8 ± 1.6 Ma, in agreement with an inverse isochron age of 152.8 ± 1.6 Ma, which is ~5 mys younger than the Re–Os age. The molybdenite Re–Os age is interpreted as the age of tungsten mineralization. This age coincides well with the zircon U–Pb age of the host granitic intrusion reported previously. The ~5 mys difference between molybdenite Re–Os and muscovite 40Ar/39Ar ages probably represents the duration of hydrothermal activity. The results show that the Xihuashan tungsten deposit is one of many important tungsten–tin deposits formed during 150 to 160 Ma associated with large-scale lithospheric extension in South China. © 2012 Elsevier B.V. All rights reserved.

1. Introduction China ranks first in the world in terms of tungsten resources and reserves (USGS, 2010). More than 90% of the Chinese tungsten resources occur in the Nanling region, South China (Hsu, 1943; Lu, 1986) as tungsten polymetallic deposits. The origin of these polymetallic deposits has been investigated by many researchers, and various magmatic-hydrothermal genetic models have been proposed (Chang et al., 2007; Lu, 1986; Wu et al., 2011b). Geochronological data have played a critical role in improving our understanding of the deposits. Recently, some accurate and precise ages for W–Sn mineralization and associated granites in the region have been published, but mostly in the Chinese literature. These data indicate that the ages of mineralization range from 160 to 150 Ma (Feng et al., 2011; Fu et al., 2004, 2008; Guo et al., 2011; Liu et al., 2008a; Liu et al., 2008b; Peng et al., 2006, 2007, 2008; Wang et al., 2010, 2011; Zeng et al., 2009; Zhang et al., 2009). However, due to the lack of highprecision dating, the age of the Xihuashan deposit, as well as other W deposits in the Nanling region, has not been universally agreed upon. In addition, the duration of the tungsten ore-forming hydrothermal solution is unknown. Being the largest quartz vein-type tungsten deposit in the Nanling region and the first tungsten deposit discovered in China, the

⁎ Corresponding author. Tel.: + 86 851 5891962; fax: + 86 851 5891664. E-mail address: [email protected] (R.-Z. Hu). 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.05.015

Xihuashan deposit is well-known to many researchers and exploration geologists. It is a classical example of tungsten mineralization associated with granitic intrusions. Comprehensive geochemical, fluid inclusion and isotope data are available for the deposit (Giuliani et al., 1988; Lu, 1986; Mu et al., 1981; Shen et al., 1994; Wei et al., 2011b; Wu et al., 1987; Zhang et al., 1981). However, the temporal and genetic relationship between tungsten mineralization and associated granite pluton is still controversial. This is mainly due to indiscriminate use of a wide range of reported ages (from 184 to 137 Ma) that include less reliable Rb–Sr, Sm–Nd, and K–Ar isotope dates (Li et al., 1992; Wu et al., 1987). For example, Hua (2005) and Tan et al. (2007) believed that there is a large age gap between tungsten mineralization and the emplacement of the host pluton based on the Sm–Nd and Rb–Sr isochron ages of hydrothermal quartz, fluorite and wolframite which vary from 137.4 ± 3.0 to 139.8 ± 4.5 Ma, and the K–Ar and Rb–Sr isochron ages of granitic rocks varying from 157 to 150 Ma (Li et al., 1992). These mineralization ages are obviously younger than the Re–Os ages of W–Sn mineralization in the Nanling region which vary between 160 and 150 Ma based on available data (Feng et al., 2011; Fu et al., 2004, 2008; Guo et al., 2011; Liu et al., 2008a; Liu et al., 2008b; Peng et al., 2006, 2007, 2008; Wang et al., 2010, 2011; Zeng et al., 2009; Zhang et al., 2009). Obviously, more reliable ages for both tungsten mineralization and granite crystallization are urgently required to resolve the debate. Molybdenite Re–Os and muscovite 40Ar/ 39Ar isotope dating has been successfully used to determine the ages of W–Sn mineralization (e.g., Feng et al., 2011; Guo et al., 2011; Peng et al., 2006). Molybdenite

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2. Geological background

sedimentary strata (Chen and Jahn, 1998; Yu et al., 2005; Zhao, 1999). The Cathaysia block can be further divided into the West Cathaysia block and the East Cathaysia block. The boundary between these two blocks is marked by a major fault zone with occurrence of Mesozoic S-type granites to the northwest and I-type granites to the southeast (Chen et al., 2008; Xu et al., 2007; Zheng et al., 2004). Temporally, the Triassic granitic rocks are widely dispersed in western Cathaysia, whereas the Jurassic and Cretaceous rocks are mainly distributed in the central part and a narrow zone close to the southeast coastal area of the Cathaysia, respectively (Chen et al., 2008; Hsieh et al., 2008).

2.1. Regional geology

2.2. Geology of the Nanling region

South China is composed of the Yangtze Block to the northwest and the Cathaysian Block to the southeast, separated by the JiangshanShaoxing Fault (Fig. 1). The two blocks came together during the Neoproterozoic (Yan et al., 2003; Zhao et al., 2011; Zhou et al., 2002). To the north, the late Paleozoic and early Mesozoic Qingling–Dabie orogenic belt lies between the Yangtze Block and the North China Block (Li et al., 2000). To the west, the Yangtze Block is bounded by the Tibetan Plateau (Fig. 1). The Yangtze Block is composed of a crystalline basement overlain by the Neoproterozoic (Sinian) to Cenozoic cover sedimentary sequences (Zhou et al., 2006). The Cathaysian Block consists of the Precambrian basement and the Sinian to Mesozoic

The studied Xihuashan tungsten deposit is located in the central Nanling region, the western part of the Cathaysian Block. The basement in this region is made up of weakly metamorphosed Precambrian folded strata, which are unconformably overlain by the folded Paleozoic and Lower Mesozoic strata of shallow marine origin (Yan et al., 2003). Abundant Mesozoic granitic intrusions are well exposed in the region (Fig. 1). Metallic mineralization in this region was mainly related to the widespread Jurassic to Cretaceous granitic magmatism (Hsieh et al., 2008). These intrusions, with peraluminous composition and relatively high initial 87Sr/86Sr ratios of 0.710–0.735, are widely regarded as typical S-type granites (Lu, 1986; Mao et al., 2008; Mo et al.,

is regarded as the most robust mineral for Re–Os dating because it contains abundant Re and negligible initial or common 187Os (Barra et al., 2005; Selby and Creaser, 2004; Stein et al., 1998). Os in molybdenite is almost entirely radiogenic 187Os derived from the decay of 187Re. This paper reports both muscovite 40Ar/ 39Ar age and molybdenite Re–Os age for the Xihushan tungsten deposit. These data are used to evaluate the genetic relationship between tungsten mineralization and associated granite pluton.

Fig. 1. Distribution of important tungsten deposits in the central Nanling region, South China. Modified after Mao et al. (2008) and Yan et al. (2003). Sources of age data: Shizhuyuan (Li et al., 1996); Yaogangxian (Peng et al., 2006); Shigushan and Shirenzhang (Fu et al., 2008); Maoping (Zeng et al., 2009); Piaotang (Zhang et al., 2009); Hongling (Wang et al., 2010); Taoxikeng (Guo et al., 2011); Baxiannao, Hongshuizhai, Maoping, Niuling, and Yaolanzhai (Feng et al., 2011).

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1980; Shen et al., 1994; Xiao et al., 2009; Zeng et al., 2001). Various types of tungsten mineralization, i.e., greisen-, skarn-, altered granite-, and quartz vein-type are present. Among them the quartz vein type is the most important in the region. These tungsten deposits are spatially and temporally associated with S-type granites. Recently, SHRIMP or LA-ICPMS zircon U–Pb ages for the associated granite plutons in the region reveal that the plutons were emplaced between 160 and 150 Ma (Feng et al., 2011; Fu et al., 2004; Guo et al., 2011; Liu et al., 2008a; Mao et al., 2007; Yang et al., 2009, 2012). 2.3. Geology of the ore deposit In the Xihuashan mining district, Cambrian strata of meta-sandstone and slate are intruded by a Mesozoic granite pluton (Fig. 2). This pluton

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is a polyphase intrusion composed of coarse-grained, medium-grained and medium- to fine-grained porphyritic biotite granites (Wang et al., 2003; Yang et al., 2012), with SiO2 ranging from 74.6 to 79.2 wt.%, Al2O3 from 9.2 to 13.0 wt.%, K2O + Na2O from 7.5 to 10.2 wt.%, K2O/ Na2O > 1 and A/CNK> 1.1 (Xiao et al., 2009; Yang et al., 2012). The granites are strongly peraluminous and belong to the calc-alkaline series and are generally an S-type granite (Shen et al., 1994; Wu et al., 1987; Xiao et al., 2009; Zeng et al., 2001). These granites have bulk-rock and quartz δ18O values ranging from 11.4 to 12.5‰, and 12.1 to 13.4‰, respectively, which also suggests that they are of S-type affinity (Shen et al., 1994; Wu et al., 1987). The zircon U–Pb (LA-ICP-MS) age of the pluton was recently determined at 155.7 ± 2.2 Ma (Wang et al., 2011). The Xihuashan deposit consists of more than 700 individual ore veins. These veins can be divided into two groups based on their

Fig. 2. Geological map of the Xihuashan tungsten deposit (a) and cross section with the sampling locations (b). Panel (a) is modified from Wu et al. (1987).

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orientation and localities. In the northern part of the mining area, they dip steeply (75–85°) with an EW strike. However, they dip more steeply (80–85°) and strike NEE in the south. Individual veins are up to 1075 m long, 3.6 m wide, and typically extend for 60 to 200 m downward (Wu et al., 1987). These veins usually show an echelon structure and occur as clusters (Giuliani et al., 1988). Ore minerals in these veins are mainly wolframite and molybdenite with minor arsenopyrite, cassiterite, chalcopyrite, pyrite and bismuthinite. Gangue minerals are predominately quartz with minor mica, feldspar, fluorite, beryl and calcite. Ore textures and mineral assemblages as well as vein crosscutting relationships indicate the following general paragenetic sequence: an early oxide-sulfide or ore-forming stage followed by a silicate stage and finally by a carbonate stage. Wolframite, molybdenite and quartz formed dominantly during the oxide-sulfide stage (Fig. 3a–c), whereas muscovite veins, which cut the first stage veins (Fig. 3d), formed during the silicate stage. Hydrothermal alteration mainly includes greisenization, silicification and sericitization (Wu et al., 1987). 3. Sampling and analytical methods 3.1. Molybdenite Re–Os dating method The hexagonal and tabular molybdenites of the ore-forming stage are selected for dating (Fig. 3c). Seven molybdenite separates were carefully handpicked under a binocular microscope from samples of the silicate-oxide stage at different exploration levels in the Xihuashan Mine (Fig. 2b). Re–Os isotope analyses were performed at the Re–Os Laboratory, National Research Center of Geoanalysis, Chinese Academy of Geological Sciences in Beijing. The analytical procedures are similar

to those described in Du et al. (2004), Li et al. (2010) and Shirey and Walker (1995). Re and Os concentrations and isotopic compositions were determined using the inductively coupled plasma mass spectrometer (TJA X-series ICP-MS). Repeated analyses of the molybdenite standard GBW04435 (HLP) give a mean age of 221.2 ± 3.3 Ma, which is in good agreement with the certified value of 221.4 ± 5.6 Ma (Du et al., 2004). Blanks used were 25.2–41.4 pg for Re and 0.1–0.3 pg for Os. The Re–Os isochron age was calculated using the least-squares method of York (1968) that is included in the ISOPLOT 2.49 program (Ludwig, 2001). The decay constant for 187Re used in the calculation is 1.666 × 10 − 11 year− 1. The absolute uncertainty is 1.02% (Smoliar et al., 1996).

3.2. Muscovite Ar–Ar dating method Muscovite for dating, selected from a muscovite vein (Fig. 3d) formed during the silicate stage at a 378 m level in the Xihuashan mine (Fig. 2b), occurs as euhedral aggregates with a diameter of about 1 to 4 mm, is relatively fresh and shows no pleochroism. Muscovite separates were carefully handpicked under a binocular microscope, with purity over 99%. The sample separates, together with the monitoring standard samples were irradiated within a quartz vial in a nuclear reactor at the Chinese Institute of Atomic Energy, Beijing. Step-heating 40Ar/ 39Ar analyses were performed at the Ar–Ar laboratory in the Institute of Geology and Geophysics, Chinese Academy of Sciences. The decay constant for 40K used in the calculation is 5.543 × 10 − 10 year − 1 (Steiger and Jäger, 1977). More details about the analytical procedures are described by Mao et al. (2006) and Wang et al. (2006).

Fig. 3. Photographs of hand specimens from the Xihuashan tungsten deposit, South China. (a) Molybdenite associated with wolframite and quartz. (b) Wolframite associated with disseminated molybdenite and quartz. (c) Quartz-molybdenite ore vein. (d) The muscovite vein cuts the first stage vein (the analyzed muscovite 378‐41 was collected from this muscovite vein).

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4. Analytical results 4.1. Re–Os ages of molybdenite The Re–Os isotopic compositions of 7 molybdenite separates from the Xihuashan tungsten deposit are listed in Table 1 and illustrated in Fig. 4. Incorporating the analytical error and the uncertainty of the decay constant, the results yield model ages ranging from 150.7 ± 2.3 to 158.4 ± 2.4 Ma, with a well-constrained 187Re– 187Os isochron age of 157.8 ± 0.9 Ma (MSWD = 1.5) (Fig. 4a), and a weighted average age of 156.3 ± 2.6 Ma (Fig. 4b). A zero intercept shows that there is no detectable common Os in molybdenite and that all the 187 Os in molybdenite are radiogenic, suggesting that the molybdenite ages are reliable (Luck and Allègre, 1982). A relatively wide range of the Re–Os model ages (150–158 Ma) in the Xihuashan deposit was probably caused by Re/Os decoupling due to the grain effect of coarsegrained molybdenite (Li et al., 2009). In contrast, the Re–Os isochron age, consistent with the weighted average age within the uncertainty, is well-constrained and therefore reliable. 4.2. Ar–Ar age of muscovite The Ar–Ar isotopic data of muscovite are given in Table 2 and illustrated in Fig. 5. The results yield a well-defined plateau age of 152.8 ± 1.6 Ma (Fig. 5a), consistent with the inverse isochron age of 152.8 ± 1.6 Ma (Fig. 5b). The initial 40Ar/36Ar value of 295.1 ± 8.5 is very close to the atmospheric value (295.5 ± 5), which suggests that this age is reasonable. 5. Discussion 5.1. Comparison of molybdenite Re–Os with muscovite

40

Ar/ 39Ar ages

Re–Os dating of molybdenite and Ar–Ar dating of muscovite are widely used to date hydrothermal ore deposits (e.g. Peng et al., 2006; Wu et al., 2011a; Xie et al., 2007). In comparison with Ar–Ar ages, the Re–Os chronometer of molybdenite is considered to be more robust because the Re–Os isotope system is less sensitive to hydrothermal and metamorphic overprinting (Selby and Creaser, 2001a,b; Stein et al., 1998, 2001). The closure temperature of the Re–Os isotope system for molybdenite is estimated to be around 500 °C (Suzuki et al., 1993). As a result, the Re–Os system of molybdenite is not easily affected by overprinting of hydrothermal fluids with temperatures b 500 °C (Raith and Stein, 2000; Selby and Creaser, 2001a,b; Stein et al., 1998). In the Xihuashan ore deposit, homogenization temperatures of fluid inclusions in wolframite and quartz from ore veins range from 239 to 380 °C and from 177 to 329 °C, respectively (Wei et al., 2011a). These temperatures are obviously lower than the closure temperatures of the Re–Os isotope system of molybdenite. Therefore, Re–Os ages of molybdenite truly record the age of molybdenite crystallization (Raith and Stein, 2000; Selby and Creaser, 2001a,b; Watanabe and Stein,

Table 1 Re and Os isotopic data for molybdenite from the Xihuashan tungsten deposit, South China. Sample

Weight(g)

Re(± 2σ) (ppb)

187 Re(± 2σ) (ppb)

187 Os(± 2σ) ( ppb)

Age (± 2σ) (Ma)

378-19 378-20 378-34 431-1 431-6 431-7 270-28

0.154 0.151 0.151 0.153 0.201 0.151 0.500

1520 ± 15 1544 ± 12 1009 ± 9 956.1 ± 9.2 446.9 ± 3.8 743.6 ± 7.3 52.45 ± 0.47

955.2 ± 9.6 970.4 ± 7.7 634.3 ± 5.4 600.9 ± 5.8 280.9 ± 2.4 467.3 ± 4.6 32.97 ± 0.30

2.513 ± 0.021 2.551 ± 0.022 1.673 ± 0.013 1.587 ± 0.014 0.7242 ± 0.0063 1.228 ± 0.012 0.0828 ± 0.0008

157.7 ± 2.4 157.6 ± 2.2 158.1 ± 2.2 158.4 ± 2.4 154.6 ± 2.3 157.5 ± 2.5 150.7 ± 2.3

Uncertainty for the calculated ages is 1.02% at the 95% confidence level.

Fig. 4. Re–Os isochron diagram (a) and weighted average age diagram (b) for molybdenite separates from the Xihuashan tungsten deposit, South China.

2000), and thus represent the age of tungsten mineralization because molybdenite is associated with the main ore mineral wolframite. The closure temperature of the muscovite Ar–Ar system is about 350 °C (Hames and Bowring, 1994). As described above, the analyzed muscovite is a product of the second hydrothermal stage, i.e. the silicate stage. According to the homogenization temperatures of the first stage wolframite and quartz, the formation of muscovite should be under the condition of much lower than 350 °C. Secondly, no obvious evidence of overprinting exists in the argon age-spectrum plateaus. Thus, the 40Ar/ 39Ar age of muscovite was neither related to the closure temperature nor affected by late thermal disturbance. Therefore, the 40Ar/ 39Ar age of muscovite should represent the age of muscovite crystallization. Both the 40Ar/39Ar plateaus and inverse isochron ages of the second stage muscovite are 152.8 ± 1.6 Ma, ~5 mys younger than the first stage molybdenite Re–Os isochron age of 157.8± 0.9 Ma. The ~5 mys difference between molybdenite Re–Os and muscovite 40 Ar/39Ar ages probably represents the duration of hydrothermal activity from the first to the second stages. 5.2. Origin of the deposit Based on the above analysis, the molybdenite Re–Os isotope age of 157.9 ±0.9 Ma is regarded as an accurate age of tungsten mineralization in the Xihuashan deposit, which coincides well with the molybdenite Re–Os age (157.0±2.5 Ma) determined by Wang et al. (2011) for the same deposit. All these Re–Os ages fall in the range of 160 to 150 Ma for many W–Sn deposits in the Nanling region reported previously (Feng et al., 2011; Guo et al., 2011; Peng et al., 2006; Wang et al., 2010; Zeng et al., 2009). Li et al. (1992) reported a fluorite Sm–Nd isochron age of 137.4 ±3.0 Ma, a wolframite Sm–Nd isochron age of 139.2 ±2.8 Ma and a quartz Rb–Sr isochron age of 139.8 ±4.5 Ma for the Xihuashan deposit, which are much younger than the whole-rock Rb–Sr isochron age of 155.0± 2.0 Ma (Le Bel et al., 1984; Li et al., 1986), and zircon U–Pb age of 156.0 ±4.0 Ma for the associated granite pluton (Liu et al., 2002). As a result, Hua (2005) and Tan et al. (2007) concluded that the Xihuashan tungsten deposit was much younger than the associated granite pluton. The new Re–Os age for the deposit reported here, together with zircon U–Pb ages for the host granite

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Table 2 40 Ar/39Ar analytical data for muscovite from the Xihuashan tungsten deposit, South China. H.S.

T (°C)

(40Ar/39Ar)m

(36Ar/39Ar)m

378-41, sample weight = 0.006 g, J = 0.002513 ± 0.000013, H.S. Heating step 1 650 243.25114 0.71056 2 700 314.64702 0.93450 3 750 115.66144 0.27371 4 790 42.44444 0.02540 5 820 35.71194 0.00307 6 850 35.24875 0.00108 7 880 35.21207 0.00082 8 910 35.2395 0.00077 9 940 35.39219 0.00072 10 970 35.45958 0.00135 11 1000 36.02559 0.00165 12 1400 34.95951 0.00169 13 1080 35.55717 0.00151 14 1120 36.28866 0.00230 15 1200 33.72874 0.00260 16 1280 33.31437 0.00486 17 1360 40.99011 0.02015

(37Ar/39Ar)m

40

0.06933 0.03135 0.01193 0.00108 0.00082 0.00039 0.00000 0.00002 0.00067 0.00086 0.00098 0.00000 0.00300 0.01186 0.00541 0.00785 0.02894

33.287879 38.504319 34.780783 34.938176 34.804594 34.929287 34.967951 35.010692 35.178363 35.059222 35.537012 34.458612 35.112079 35.61112 32.961593 31.877449 35.038174

indicates that there is no age gap between granite crystallization and ore formation. Clearly, tungsten mineralization and granite magmatism in the region are not only spatially associated but also coeval. The δ34S values of the vein sulfides are very homogeneous and range from −1.6 to +0.1‰, with an average of −0.8‰, which indicates that sulfur in hydrothermal fluids was derived from associated granitic magma (Wei et al., 2011a). Oxygen and hydrogen isotopic data also suggest that the hydrothermal fluids of the Xihuashan deposit are magmatic in origin, although there is some meteoric water in the later fluids (Liu et al., 2002; Wei et al., 2011a). Consequently, the Xihuashan deposit was spatially, temporally and genetically related to the host granitic pluton.

Ar*/39Ark

39 Ark (%)

40 Ar* (%)

Apparent age (± 2σ) (Ma)

0.15 0.31 0.87 3.65 6.88 11.88 20.49 20.48 16.32 6.36 3.48 2.95 2.39 1.39 1.35 0.78 0.26

13.68 12.24 30.07 82.32 97.46 99.09 99.31 99.35 99.4 98.87 98.64 98.57 98.75 98.13 97.73 95.69 85.48

145.28 ± 37.65 167.02 ± 46.00 151.52 ± 14.21 152.18 ± 2.27 151.62 ± 1.81 152.15 ± 1.57 152.31 ± 1.52 152.49 ± 1.53 153.19 ± 1.56 152.69 ± 1.69 154.68 ± 1.57 150.18 ± 2.43 152.91 ± 2.29 154.99 ± 2.12 143.91 ± 3.13 139.35 ± 2.46 152.6 ± 18.65

5.3. Geodynamic setting of metallogeny Based on field relations and existing geochronological data, Mao et al. (2008) and Peng et al. (2008) pointed out that the most important granite-associated W–Sn polymetallic mineralization in South China took place during the Late Jurassic (160–150 Ma) and was coincident with pervasive granitoid magmatism. This study reveals that the Xihuashan deposit belongs to this important W–Sn polymetallic mineralization event in South China. In South China, there are alkali basalts (177 to 178 Ma), bimodal volcanic rocks (158 to 179 Ma) and A-type granites (176 to 178 Ma), indicative of a post-orogenic extensional setting in the Jurassic (Chen et al., 2002; Fan and Chen, 2000). Li et al. (2007a) proposed that Jurassic granites in this region are I- and fractionated I-type granites, rather than S-type granites thought previously. These granites are temporally and spatially associated with A-type felsic and mafic volcanic and intrusive rocks as well as alkaline rocks, suggesting an extensional setting for South China in the late Jurassic (Li et al., 2007a). In other words, South China is mainly under a lithospheric extension-thinning setting since the Jurassic (180 Ma) (Hua et al., 2005). Several granite belts in South China with low tDM (depleted mantle model age) values and relatively high εNd values have been identified, and these granite belts are considered evidence for lithospheric extension and intense crust–mantle interaction (Chen and Jahn, 1998; Gilder et al., 1996; Hong et al., 1998). Large scale crustal–mantle interactions led to extensive W–Sn polymetallic mineralization in South China at 160–150 Ma (Hu et al., 2007, 2010; Mao et al., 2008; Peng et al., 2006). He and Ar isotopic compositions of fluid inclusions in sulfides from the typical W–Sn polymetallic deposits in the Nanling region (Hu et al., 2012; Li et al., 2007b; Wu et al., 2011b) including the Xihuanshan deposit (Wei et al., unpublished data) indicate that the ore-forming fluids contained both crustal and mantle volatiles. Clearly, a crust–mantle interaction was involved in the formation of the granite-related W–Sn polymetallic deposits in the Nanling region, possibly due to magmatism and hydrothermal activity associated with lithospheric thinning and crustal extension in South China. The Xihuashan tungsten deposit thus can be regarded as the evidence for such crust–mantle interaction. 6. Conclusions

Fig. 5. Plateau and inverse isochron Ar–Ar isotopic ages of muscovite from the Xihuashan deposit, South China.

Wolframite and molybdenite are products of the first stage or oreforming hydrothermal activity, whereas muscovite formed dominantly at the second stage. The precise muscovite 40Ar/39Ar and molybdenite

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Re–Os dating reported here suggests that the duration of hydrothermal activity from the first to second stages is about 5 mys in the Xihuashan tungsten deposit. The molybdenite Re–Os isochron age of 157.8± 0.9 Ma represents the age of tungsten mineralization of the deposit, which is in excellent agreement with zircon U–Pb age of the host granite reported previously by Liu et al. (2002) and Wang et al. (2011), indicating a genetic link between tungsten mineralization and granite magmatism. The new age reported here confirms that the Xihuashan tungsten deposit is part of a large belt of tungsten–tin polymetallic ore deposits formed during the Late Jurassic (160–150 Ma) in the Nanling region, SE China. Acknowledgments The field work was supported by the Xihuashan Mine Company. We are indebted to Prof. Mei-Fu Zhou and Dr. Chusi Li who read the early versions of this manuscript. The authors are grateful to three referees for their thoughtful reviews. 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