Introduction

China is the world’s largest producer of tungsten both in terms of reserves and production. According to USGS (2006), the tungsten production of China in 2004 accounted for 90.9% of the total world production for that year, and China hosts 62.1% of the world’s total tungsten reserves. More than 90% of the Chinese tungsten resources are distributed in the Nanling region, South China (Hsu 1943; RGNTD 1985; Lu 1986), and the Yaogangxian deposit is the largest of these.

Previous studies have significantly advanced our understanding of the tungsten ore formation in the Nanling region (e.g., RGNTD 1985; Lu 1986), but the exact timing of tungsten mineralization has been poorly constrained. Early isotopic dating, which focused mainly on granites related to the mineralization, yielded very scattered dates ranging from 195 to 65 Ma (RGNTD 1985). These have led to great ambiguity in our understanding of the timing of regional ore formation. Thus, direct dating of the deposits is required.

Molybdenite Re–Os (Suzuki et al. 1993; McCandless and Ruiz 1993; Huang et al. 1994; Stein et al. 1997, 1998; Mao et al. 1999, 2003; Selby et al. 2002; Kohút and Stein 2005; Zhang et al. 2005) and mica Ar–Ar dating (Snee et al. 1988; Cheilletz et al. 1993; Marsh et al. 1997; Reynolds et al. 1998; Garnier et al. 2002; Mello et al. 2006) have been used elsewhere to date in hydrothermal ore deposits. In this paper, we report first Re–Os ages of molybdenite and Ar–Ar ages of hydrothermal phlogopite and muscovite from the Yaogangxian tungsten deposit. The Re–Os ages and 40Ar/39Ar ages are indistinguishable and coincide well with the field geological relationships and other published geochronological results for W–Sn mineralization in the central Nanling region. Our work provides the first precise constraints on the absolute timing of hydrothermal tungsten mineralization in this deposit and further documents a regional tungsten–tin mineralization event in the central Nanling region, South China.

Geological background

South China comprises the Yangtze Block to the west and the Cathaysian Block to the east. The two blocks amalgamated along a Neoproterozoic collision belt at about 1000 Ma. The Cathaysian Block consists of a Proterozoic basement overlain by a cover sequence of Sinian to Triassic sedimentary strata (Chen and Jahn 1998). The Yaogangxian tungsten deposit is located in the Nanling district, which lies in the northwestern part of the Cathaysian Block (Fig. 1).

Fig. 1
figure 1

Sketch map of tungsten and tin deposits in the central Nanling region, South China. Modified after Huang et al. (2003), age data for Qitianling granite from Fu et al. (2004) and Li et al. (2005), Qianlishan granite from Li et al. (2004), Huangshaping granite from Yao et al. (2005), Jiufeng granite from RGNTD (1985), and others from HBGMR (1988)

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Late Paleozoic sedimentary strata, especially Devonian and Carboniferous limestone, are widespread in the central Nanling region. Mesozoic granitoid intrusions are well developed throughout the region, and Jurassic and Cretaceous granites are abundant in the central part (Institute of Geochemistry 1979; Mo et al. 1980). The Jurassic plutons consist predominantly of biotite granite with lesser amounts of two-mica granite, and they were previously considered to be typical S-type granites (Mo et al. 1980; RGNTD 1985), but some have recently been identified as A-type (Zhao et al. 2000; Zheng and Jia 2001; Fu et al. 2005; Bo et al. 2005). Spatially, these granitoid rocks are controlled by Mesozoic NE-striking faults.

The central Nanling region, covering an area of 6,600 km2, contains numerous tungsten and tin deposits (Huang et al. 2003). These deposits are typically enriched in Mo, Bi, Pb, Zn, Cu, and Ag, and in some cases, Pb–Zn are the dominant metals. Several giant ore deposits in this region include the Shizhuyuan W–Sn–Mo–Bi–F deposit (Lu et al. 2003), the Furong Sn deposit (Huang et al. 2001, 2003; Li et al. 2006; Peng et al. 2006, submitted for publication), the Yaogangxian W deposit, the Xianghualing Sn–W–Pb–Zn deposit (Xiong et al. 2002; Yuan et al. 2006, submitted for publication), and the Huangshaping Pb–Zn–W–Sn–Ag deposit (Fig. 1). The Furong Sn deposit was discovered in the late 1990s and is expected to become a world-class tin producer (Huang et al. 2001). Despite extensive mining since the 1930s, the central Nanling region is still rich in W and Sn, with metal reserves of 1.7 million tons (metric) tungsten and 1.2 million tons tin (Che et al. 2005).

There are three main types of tungsten mineralization in this region, i.e., greisen-, skarn-, and quartz vein-type. All three types are commonly found together in most deposits but they can also occur individually. In addition to these three types, tin mineralization also occurs as stratiform cassiterite + sulfide type, altered granite type, and stockwork or network type in some deposits. Spatially, all these deposits are associated with the widespread Mesozoic granitic intrusions. Tungsten and tin mineralization usually occurs along the contact zones between the granitic intrusions and the sedimentary strata and is hosted in both Devonian to Permian sedimentary rocks and in granitic rocks. The field relations suggest that W–Sn deposits in this region are essentially coeval with the host Mesozoic granites (RGNTD 1985).

Yaogangxian tungsten deposit

The Yaogangxian tungsten deposit is mined since 1914 and has an average ore grade of about 1.27% WO3 (Zhu 1999). The total production of the Yaogangxian mine until 2000 was 65,000 tons of refined tungsten concentrates (WO3>65%). Being the largest W producer in China, the Yaogangxian mine currently has an annual production of refined tungsten concentrates of about 2,300 metric tons (Yaogangxian Mine, unpublished data). The proven reserve of wolframite ores is estimated to be 2,360,000 tons.

In the Yaogangxian mining district, the stratigraphic sequence consists of Cambrian metasandstone and slate, unconformably overlain by Devonian and Carboniferous sandstone and limestone, and Jurassic sandstone (Fig. 2) (Chen 1981). A two-mica granite, with an exposed surface area of 1.2 km2, intruded Cambrian and Devonian strata at 173–175 Ma (muscovite, biotite, K–Ar method) (Institute of Geochemistry 1972; HBGMR 1988). Some small dikes including diabase, aplite, and granite porphyry also occur in the mining district. They postdate the emplacement of the two-mica granite (Chen 1981).

Fig. 2
figure 2

Geological map of the Yaogangxian tungsten deposit, South China. Modified after Chen (1981)

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The Yaogangxian deposit consists of more than 200 ore veins. These veins are usually NNW-, NW-, and NWW-striking (Fig. 2). Most ore veins occur along the northern contact zone between the granite and the sedimentary strata and commonly crosscut both lithologies. Individual veins are up to 1,200 m long, 1.5 m wide, and typically extend for 100 to 1,000 m downdip (Chen 1981). Generally, the veins increase in thickness with depth, but the number of veins decreases.

Ore minerals in this deposit are mainly wolframite and molybdenite, with minor amounts of arsenopyrite, cassiterite, chalcopyrite, pyrite, bournonite, and bismuthinite. The gangue minerals in the ores are predominately quartz, with minor amounts of mica, feldspar, fluorite, and calcite. The veins display mineral zoning, with quartz, sulfides, and occasionally wolframite in the innermost part, whereas the selvages of the veins typically contain wolframite, molybdenite, and mica. These minerals are usually oriented perpendicular to, or oblique to, the vein walls.

Sampling and analytical methods

Molybdenite occurs as fine- to coarse-grained euhedral platelets along the margins of the veins (Fig. 3a,b) where it is typically intergrown with other minerals such as wolframite, mica, fluorite, and pyrite. Molybdenite separates were collected from underground exposures of quartz–molybdenite veins hosted in the Cambrian metasandstones in the Yangmeiling mining adit (Fig. 2).

Fig. 3
figure 3

Photographs of ore veins from the Yaogangxian tungsten deposit, South China. a Quartz–molybdenite ore vein. b Quartz–molybdenite ore vein. c Quartz–feldspar–phlogopite–sulfide vein. d Quartz–muscovite vein

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The phlogopite sample was collected from a quartz–feldspar–mica vein hosted in Devonian sandstone in the Dayanmen mining adit (Fig. 2). The vein has a sharp contact with the wall rocks, and phlogopite forms aggregates of euhedral crystals oriented perpendicular to the vein walls (Fig. 3c). The central part of the vein consists of feldspar, fluorite, and chalcopyrite, with minor quartz and fine-grained wolframite. The muscovite sample was collected in the Yangmeiling mining adit, and it is from a mica–quartz vein in Cambrian slate that contains disseminated fine-grained aggregates of sulfide. The mica forms euhedral aggregates with a diameter of about 1 to 3 mm (Fig. 3d).

Molybdenite for Re–Os dating was magnetically separated and then handpicked under a binocular microscope. The selected separates were crushed in an agate mortar to about 200 mesh. The analytical procedures followed those of Du et al. (2004). Molybdenite separates were digested using the Carius tube method (Shirey and Walker 1995) and equilibrated with 185Re and 190Os spikes in HNO3-HCl by sealing in a thick-walled glass ampoule. The tube was then placed in a stainless steel jacket and heated for 10 h at 230°C. Osmium was recovered by distilling directly from the Carius tube and purified by microdistillation. The Re was recovered by anion exchange. Re and Os concentrations and isotopic compositions were determined using a TJA PQ-Excell inductively coupled plasma mass spectrometer (ICP-MS) in the National Research Center of Geoanalysis, Beijing. The molybdenite standard GBW04436 (JDC) used in this study gave a mean value of 139.0±3.3 Ma compared with the certified value of 139.6±3.8 Ma (Du et al. 2004). Blanks during this study were 26–88 pg for Re and 1.6–1.7 pg for Os. The Re–Os isochron age was calculated using the least-squares method of York (1969), as implemented in the ISOPLOT 3.00 program (Ludwig 2004). The decay constant used in the age calculation is λ 187Re=1.666×10−11 year−1 (Smoliar et al. 1996).

Phlogopite and muscovite grains (about 60 mesh) for Ar–Ar dating were carefully handpicked under a binocular microscope from the crushed materials and washed in an ultrasonic bath using ultrapure water. The mineral concentrates were analyzed by X-ray diffraction to ensure their purity. Samples were irradiated with fast neutrons for 62 h at the Chinese Academy of Nuclear-Energy Sciences and subsequently cooled for about 100 days. 40Ar/39Ar stepwise heating analyses were performed at the Guilin Institute of Geology and Mineral Resources using an MM-1200 gas-source mass spectrometer. Samples were analyzed in 11 temperature steps from 300°C to total fusion at 1,300°C. The biotite standard JBH (132.5 Ma) was used to monitor the neutron flux in the study. 40Ar/39Ar plateau ages were calculated using ISOPLOT 3.00 (Ludwig 2004).

Analytical results

Re–Os age of molybdenite

The eight molybdenite samples have a relatively narrow range of Re–Os mineral ages varying from 152.0±3.5 to 161.1±4.5 Ma, with an average of 156.0 Ma (Table 1), showing excellent reproducibility. The samples yield a well-constrained 187Re–187Os isochron, which corresponds to an isochron age of 154.9±2.6 Ma (MSWD=2.4) and an intercept of 0.0006±0.0034 (Fig. 4). A zero intercept confirms that all the 187Os in the molybdenite are radiogenic and that the molybdenite contains no measurable common Os. This indicates that the model age for the molybdenite samples is reliable (Hintenberger et al. 1954; Luck and Allègre 1982). Noticeably, no correlation between Re and Os contents and molybdenite type (2H or 3R) was found in this study (Table 1).

Table 1 Re and Os isotopic data for molybdenite from the Yaogangxian tungsten deposit, South China
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Fig. 4
figure 4

187Re–187Os isochron diagram for eight molybdenite samples from the Yaogangxian tungsten deposit, South China

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Ar–Ar age determination of phlogopite and muscovite

Ar–Ar analytical results for phlogopite and muscovite are summarized in Tables 2 and 3, respectively, and illustrated in Fig. 5. In this study, an age plateau is defined as a sequence of five or more consecutive steps corresponding to at least 60% of the total 39Ar released that yield apparent ages reproducible at the 95% confidence level (2σ). The two samples yielded well-defined plateau ages at 153.0± 1.1 and 155.1±1.1 Ma (Fig. 5), and both dates overlap the Re–Os date within uncertainties. The plateaus comprise ten continuous steps accounting for 99% of the total 39Ar released. The characteristics of the spectra suggest that argon loss and excess argon did not occur in the samples and that the hydrothermal minerals host their radiogenic and nucleogenic gas fractions in tight crystallographic reservoirs which remained closed during the geological history of the sample.

Table 2 40Ar/39Ar analytical data for phlogopite from sample YGX-20 of the Yaogangxian tungsten deposit, South China
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Table 3 40Ar/39Ar analytical data for muscovite from sample YGX-52 of the Yaogangxian tungsten deposit, South China
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Fig. 5
figure 5

40Ar/39Ar age spectrum for phlogopite (a) and muscovite (b) from the Yaogangxian deposit, South China

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Discussion

Age of the Yaogangxian deposit

In the Yaogangxian mining district, homogenization temperatures (T h) for quartz from quartz–wolframite ore veins range from 252 to 323°C (RGNTD 1985) or from 220 to 340°C (Ni 1994), and the decrepitation temperatures (T d) for wolframite from the same veins range from 150 to 305°C (Ni 1994). These temperatures are obviously lower than the closure temperatures of the Re–Os molybdenite and Ar–Ar mica chronometers. The closure temperature for the Re–Os isotope system in molybdenite is estimated at about 500°C (Suzuki et al. 1996), and the closure temperature for the K–Ar isotope system in phlogopite is 400–510°C according to Giletti and Tullis (1977) or about 450°C according to Dodson (1979). The K–Ar closure temperature for muscovite is less certain, somewhere in the range of 350–640°C (Hames and Bowing 1994). Therefore, we can assume that both the Re–Os system for molybdenite and the K–Ar system for phlogopite and muscovite remained closed after mineral precipitation, and thus, the Re–Os and Ar–Ar dates reported in this study are taken as the age of ore formation in the Yaogangxian tungsten deposit.

Therefore, the absolute timing of tungsten mineralization in Yaogangxian is about 154 Ma, obviously younger than K–Ar dates for magmatic biotite and muscovite in the granite (173∼178 Ma, Institute of Geochemistry 1972; HBGMR 1988), which coincides with the field fact that the ore veins cut the granite and that granitic breccias occur in molybdenite-bearing quartz veins (Lin et al. 1986).

Timing of the regional W–Sn mineralization

Recent studies reveal that tungsten and tin mineralization in the central Nanling region took place at 150–161 Ma. The Re–Os date of molybdenite from the Shizhuyuan deposit is 151.0±3.5 Ma (Li et al. 1996), whereas the Sm–Nd isochron age of skarn minerals from the same deposit is 160.8±2.4 Ma (Liu et al. 1997) or 149±2 Ma (Li et al. 2004). This deposit also has a 40Ar/39Ar mica date of 153.4± 0.2 Ma (Mao et al. 2004a) or 148.2±1.1 Ma (Peng et al., unpublished data). Two hydrothermal muscovite samples from the Furong tin mining district have plateau ages of 159.9±0.5 and 154.8±0.6 Ma, and three hydrothermal phlogopite samples yielded 40Ar/39Ar plateau ages of 150.8± 0.8, 154.7±1.1, and 157.3±0.6 Ma (Peng et al. 2006, submitted for publication). Hydrothermal hornblende from the same deposit gave a 40Ar/39Ar plateau age of 156.9± 1.1 Ma. Analyses for muscovite from the Furong ore field have yielded similar 40Ar/39Ar plateau ages of 156.1±0.4 and 160.1±0.9 Ma (Mao et al. 2004b). In the Xingtianling scheelite deposit, the 40Ar/39Ar plateau age of a zinnwaldite sample from skarn is 157.1± 0.3 Ma (Mao et al. 2004a). Three hydrothermal muscovite samples from the Xianghualing mining district have 40Ar/39Ar plateau ages of 154.4±1.1, 158.7±1.2, and 161.3±1.1 Ma (Yuan et al. 2006, submitted for publication). Thus, the age of tungsten mineralization in Yaogangxian coincides closely with that of tungsten–tin mineralization elsewhere in the central Nanling region. This large-scale mineralization event at about 154 Ma was probably associated with Mesozoic lithospheric extension in South China (Hu et al. 2004; Mao et al. 2004a; Li et al. 2006). Recent studies have identified several granite belts in South China with low T DM values and relatively high ɛ Nd(t) values (−4.0 to −8.0), which are considered to result from crust–mantle interaction during lithospheric extension (Gilder et al. 1996; Chen and Jahn 1998; Hong et al. 1998; Zhu et al. 2003). Lithospheric extension in this region is supported by the presence of numerous Mesozoic basic dikes and some A-type granites in the central Nanling region (Zhao et al. 2000; Zheng and Jia 2001; Fu et al. 2005; Bo et al. 2005).

Conclusions

The Re–Os mineral ages and isochron age of molybdenite coincide well with 40Ar/39Ar plateau ages of hydrothermal phlogopite and muscovite from the Yaogangxian tungsten deposit. The results constrain the absolute timing of mineralization in this deposit at about 154 Ma, further documenting a Late Jurassic regional tungsten–tin mineralization event in the central Nanling region.