Introduction

The eastern Tianshan region has recently emerged as an important metallogenic ore province in China. More than ten larger porphyry copper–molybdenum, orogenic gold, magmatic copper–nickel, and epithermal gold deposits have been discovered in the eastern Tianshan orogen since the late-1980s. The radiometric ages of these deposits (Table 1) were determined by several researchers (Li et al. 1998; Chen et al. 1999; Rui et al. 2002a; Mao et al. 2003; Qin et al. 2003; Zhang et al. 2003a, 2003b). Isotopic data available show that most of these deposits were formed in the late Paleozoic (e.g. Tuwu–Yandong porphyry copper deposit; Kanggur gold deposit; Huangshang copper–nickel deposit; Shiyingtan gold vein I and II, Fig. 1). Therefore, the main metallogenic epoch in the Tianshan region is considered of Variscan age (Pirajno et al. 1997; Li et al. 1998; Mao et al. 2003; Qin et al. 2003).

Table 1 Age data on the Kanggurtag Au–Cu–Mo metallogenic belt in eastern Tianshan
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Fig.  1
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Simplified geological map of the eastern Tianshan, NW China. Numbers in triangles (1–6) show location of deposits: (1) Baishan Mo–Re deposit; (2) Huangshan Cu–Ni deposit; (3) Tuwu–Yandong Cu (Mo) deposit; (4) Kanggur Au deposit; (5) Shiyingtan Au deposit; (6) Jinwozi Au deposit. The faults in Fig. 1 are: 1 Kalamaili fault; 2 Kanggurtag fault; 3 Kushui fault; 4 Weiya fault; 5 Kumishi fault.

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However, Mesozoic ages of intrusions and mineralization in the eastern Tianshan have been repeatedly reported in recent years (Chen et al. 1999; Zhang et al. 2003b). This paper, based on the geochronology of the Baishan Mo–Re deposit, gives further evidence of a Mesozoic mineralization event.

Mapping by the Xinjiang Bureau of Geology and Mineral Resources (XBGMR) shows that some Mesozoic granites (245–180 Ma, XBGMR 2001, unpublished data; Deng et al. 2003) are emplaced adjacent to the Baishan Mo–Re deposit. However, so far there is no direct information on the age of ore formation. In this study, molybdenite and pyrite samples from the Baishan deposit were chosen for Re–Os isotope measurements in order to constrain the timing of mineralization. The source of the ore-forming fluids and the tectonic setting are also discussed.

Geological setting

The east Tianshan mountains, a part of the central Asian Paleozoic collisional orogenic belt, have been studied by many researchers (Windley et al. 1990; Allen et al. 1992, 1993; He et al. 1994; Biske and Shilov 1998; Qin et al. 2003; Xiao et al. 2004). The main structures of the orogen are characterized by a series of approximately east-west-trending faults, including the regional-scale Kalamaili fault (Late Paleozoic suture), Kanggurtag fault, Kushui (or Yamansu) fault, and Weiya fault (Fig. 1). Of these faults, the Kanggurtag fault, expressed by mylonite, tectono-clastic rocks, tectonic lenses, and breccia, is an important structural zone along which intense magmatic activity and associated mineralization took place (Ma et al. 1997; Zhang et al. 2004). The Dananhu–Tousuquan Paleozoic island arc consisting mainly of Devonian to Carboniferous (locally also Ordovician-Silurian, Qin et al. 2003) volcanic-intrusive rocks and hosting most of the porphyry copper deposits (e.g. Tuwu and Yandong) occurs to the north of the Kanggurtag fault (Fig.1). The Kanggurtag–Huangshan Late Paleozoic arc-related basin, consisting of Carboniferous sedimentary rocks, some Cu–Ni deposits associated with mafic complexes (e.g. Huangshan and Huangshandong), and Cu–Mo (Re) deposits related to granite porphyry (e.g. Baishan Mo–Re), is located between the Kanggurtag and Kushui faults. The Carboniferous Aqishan-Yamansu volcanic arc, and associated Au deposits (e.g. Shiyingtan Au, Kanggur Au; Fig. 1), are located between the Kushui and Weiya faults. The middle Tianshan block consisting of Precambrian and early Paleozoic metamorphic rocks is located south of the Weiya fault.

Intrusive activity in the eastern Tianshan mainly includes Variscan (350–270 Ma) granite, plagiogranite porphyry, tonalite, quartz porphyry, dacite porphyry, and mafic-ultramafic bodies. They are related to most of the gold–copper mineralizations. However, there are also some Mesozoic granites reported recently in the eastern Tianshan, such as the Weiya granite, which has a SHRIMP zircon age of 233–246 Ma (Gu et al. 2004, unpublished data) and Ar–Ar age of 223–246 Ma (Li et al. 2002), the Xiaobaishitou granite (located 20 km northeast of Weiya) with a SHRIMP zircon age of 249±3 Ma, and the Baishitou granite (40 km northeast of Weiya) with a Rb–Sr isochron age of 209±9.6 Ma (Gu et al. 2003).

The development of some metal ore deposits in the eastern Tianshan is closely related to subduction and closure of the ancient Tianshan ocean intervening between the Tarim and Siberian plates (He et al. 1994; Qin et al. 2003; Zhang et al. 2004). From Late Devonian to Early Carboniferous, the northern margin of the Tarim plate was a passive continental margin, whereas the ancient Tianshan ocean was subducted to the south along the Kalamaili fault, resulting in the development of the Dananhu–Tousuquan magmatic arc and associated porphyry-type Cu–Mo deposits. In Late Carboniferous, the ancient Tianshan ocean was closed and continent–continent collision occurred, leading to the formation of the east Tianshan orogen and orogenic Au deposits. The collisional event was followed by an extensional event in the Permian, large ultramafic–mafic complexes were emplaced, and a number of large-scale magmatic copper–nickel ore deposits were formed along the Kanggurtag fault. In Early Mesozoic, the tectonic evolution of the east Tianshan is characterized by post-orogenic extension with associated intrusion of granites and volcanic hydrothermal activity (i.e. Jinwozi Au, Shiyingtan III Au and Baishan Mo–Re etc.).

Local geology and sampling

The Baishan porphyry Mo–Re deposit, located ~140 km southeast (42°31′N, 95°55′E) of Hami city, is situated 2 km south of the Kanggurtag fault. The host rocks are composed of sedimentary rocks of the Lower Carboniferous Gandun Formation (Fig. 2). The sedimentary rocks are divided into four sequences (Zhou et al. 1996). The first sequence consists of siltstone interlayered with carbonaceous shale. The second sequence is composed of tuffaceous graywacke interlayered with tuff and basalt. The third sequence contains graywacke, tuff, and shale. The fourth sequence consists of siltstone, carbonaceous shale, and sandstone. The stratigraphic units trend WNW and dip ENE with an angle of 65–80°. The sedimentary rocks are thermometamorphically overprinted by deeper-seated intrusive rocks (Deng et al. 2003).

Fig.  2
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Geological map of the Baishan Mo–Re deposit (modified from Deng et al. 2003).

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Some granitic porphyries occur as stocks and dikes in the south of the mineralized area (Fig. 2). The porphyry dikes exhibit minor pyrite and molybdenite mineralization. An unmineralized biotite granite is emplaced 1.5 km southwest of the mining area. The granite body cuts the granitic porphyry dikes, which implies the intrusion of the granite as a late event. A zircon SHRIMP U–Pb age of 181±3 Ma recently obtained (Li et al. 2004, unpublished data) further indicates that the granite was intruded after Mo–Re mineralization. In the ore district (Fig. 2), minor mineralized quartz veins are widely developed. A stratabound-fracture zone is recognized, which hosts the main mineralization trending EW, and dipping north with an angle of 60–70°. A group of NE-trending faults are post-ore structures.

More than 16 mineralized bodies have been identified and 7 of these are rich orebodies (No. 4, 5, 1, 8, 9, 10 and 15; Fig. 2). Some of the larger orebodies vary between ~2 and 124 m in thickness, and 100–2,000 m in length (e.g. No. 4 and 5). They trend 85° and dip north with an angle of 65–75°. In dipping direction, the explored orebodies extend over 600 m in depth. These orebodies exhibit stratiform and lenticular shape. The average grade of the ore is 0.06% Mo (range from 0.03 to 0.14%) and 1.4 g/t Re (range from 0.7 to 1.9 g/t; Zhou et al. 1996).

In accordance with the mineral association and their occurrence, the ores can be divided into three types, i.e., molybdenite–pyrite–quartz vein, molybdenite–polymetallic sulfide–quartz vein and molybdenite-bearing altered rock. The main ore minerals include molybdenite, chalcopyrite, and pyrite, with minor pyrrhotite, magnetite, galena, sphalerite, and marcasite. The gangue minerals mainly include quartz, sericite and microcline, secondary biotite, calcite, and chlorite. The size of molybdenite ranges from 0.2 to 0.6 mm (individually up to 2 mm). Ores are characterized by subhedral-euhedral textures, veinlet-disseminated and brecciated structures.

Hydrothermal alteration is strongly developed around the orebodies, and includes phyllic and potassic alteration, biotitization, chloritization, and carbonatization. Phyllic alteration, occupying the central part of the alteration zones, consists dominantly of sericite and quartz. This zone hosts most of the molybdenite. Potassic alteration is the most conspicuous feature of the altered zones in the deposit. It is characterized by the formation of microcline throughout the orebody. Chloritization is weakly developed and related to polymetallic sulfides, whereas carbonatization is related to a late-stage vein composed of quartz and calcite. According to mineral assemblages and crosscutting relationships of the ore veins, four hydrothermal stages and one supergene stage can be identified (Table 2). The first mineralization stage (I) is characterized by quartz veins, with magnetite and ilmenite. The second stage (II) is an assemblage consisting of quartz, pyrite, and a little molybdenite. The third stage (III, main mineralization stage) consists of polymetallic sulfides including molybdenite, chalcopyrite, and pyrite, with minor galena and sphalerite. The fourth stage (IV) is marked by barren calcite-quartz veins. The supergene assemblage, only seen at the surface of the ore district, consists of copper, iron, and molybdenum oxides.

Table 2 ...
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According to drillcore samples (ZK15-1, 600 m in depth), an altered and mineralized granitic porphyry dike was found at a depth of ~580 m (Deng et al. 2003). We further infer that a large granite porphyry body was emplaced at the depth. Based on studies of ore geology and mineral assemblages, it is suggested that the Baishan Mo–Re deposit is of quartz vein-porphyry type.

Two groups of samples were collected from drill hole ZK15-1 in the Baishan mine area (Fig. 2). One of them (BSH-302–309), molybdenite, representing the main mineralization stage (III), was collected from 220–280 m at the depth of the central part of orebody 5. The molybdenite occurs on veinlets dissemination. The second sample (BSH-F21–F27) consists of pyrite of mineralization stage II and was collected from 30–100 m depth, at the top of orebody 5; pyrite occurs or veinlets in the orebody.

Chemical procedure and analytical technique

Re–Os isotopic analyses were performed in the National Research Center of Geoanalysis, Chinese Academy of Geosciences. The details of the chemical procedure have been described by Du et al. (1995, 2001), Shirey and Walker (1995), Stein et al. (1998), and Markey et al. (1998). They are briefly described here.

The Carius tube (a thick-walled borosilicate glass ampoule) digestion technique was used. The weighed sample was loaded in a Carius tube through a long thin-neck funnel. The mixed 190Os and 185Re spike solution and 2 ml of 10 N HCl and 6 ml of 16 N HNO3 were added while the bottom part of the tube was frozen at −80 to −50°C in an ethanol-liquid nitrogen slush; the top was sealed using an oxygen–propane torch. The tube was then placed in a stainless-steel jacket and heated for 10 h at 230°C. Upon cooling, the bottom part of the tube was kept frozen, the neck of the tube was broken, and the contents of the tube were poured into a distillation flask and the residue was washed out with 40 ml of water.

Separation of osmium by distillation and separation of rhenium by extraction was performed based on the analytical method from Du et al. (1995 and 2001). A TJA PQ-EXCELL ICP-MS was used for the determination of the Re and Os isotope ratio.

Average blanks for the total Carius tube procedure were ca. 10 pg Re and ca. 1 pg Os. The analytical reliability was tested by repeated analyses of molybdenite standard HLP-5 from a carbonatite vein-type molybdenum-lead deposit in the Jinduicheng–Huanglongpu area of Shaanxi Province, China. Fifteen samples were analyzed over a period of 5 months. The uncertainty in each individual age determination was about 0.35% including the uncertainty of the decay constant of 187 Re, uncertainty in isotope ratio measurement, and spike calibrations. The average Re–Os age for HLP-5 is 221.3±0.3 Ma (95% confidence limit, Stein et al. 1997). Median age and mean absolute deviation were 221.34±0.12 Ma. The average Re concentration was 283.71±1.54 μg/g. The average Os concentration was 657.95±4.74 ng/g.

Results

The concentrations of Re and Os and the osmium isotopic compositions of molybdenite and pyrite from the Baishan Mo–Re deposit are shown in Tables 3 and 4. The total Re and Os concentrations of molybdenite range from 74 to 250 μg/g and 0.18 to 0.60 μg/g, respectively, whereas those of pyrite vary from 93 to 331 ng/g and 0.1 to 0.8 ng/g, respectively.

Table 3 Re–Os isotopic data for molybdenite from the Baishan Mo–Re deposit, eastern Tianshan
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Table 4 Re–Os isotopic data for pyrite from the Baishan Mo–Re deposit, eastern Tianshan
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As molybdenite has extraordinarily high Re/Os ratios, the Re–Os chronometer is used for dating. Raith and Stein (2000) suggested that molybdenite does not contain any initial or common Os, and all measured Os is monoisotopic (187Os), the product of decay of 187Re. But, analysis actually indicates that molybdenite from some deposits contains minor common Os or initial Os (Mao et al. 2002; Hou et al. 2004). Our analyses indicate that the concentrations of common Os in molybdenite from the Baishan deposit is <0.05 ng/g. In contrast to the 187Os value of ~170 to 600 ng/g (Table 3), the common Os values can be ignored (close to zero). Therefore, the Re–Os dating of molybdenite is based on a simplified age equation, that is, 187Os=187Re (eλt−1).

A regression analysis was applied to eight analytical data of molybdenite, which yields an isochron with an age of 224.8±4.5 Ma (2σ), initial 187Os of 0.0038±0.0066, and mean square weighted deviation (MSWD) of 1.2 (Fig. 3). Model ages for individual analyses range from 222.0±3.5 to 229.9±3.7 Ma (Table 3).

Fig. 3
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Re–Os isochron plot for molybdenite samples from the Baishan Mo–Re deposit, eastern Tianshan.

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Stein et al. (2000) suggested that low-level, highly radiogenic sulfide minerals, recognized by 187Re/188Os ratios of ≥5,000, must be plotted in 187Re−187Os space to obtain meaningful precise ages. Barra et al. (2003) found that reliable geochronological and tracer information can be obtained using the 187Re/188Os versus 187Os/188Os isochron plot. For the Baishan deposit, 187Re/188Os ratios of pyrite are greater than 5,000 (Table 4). Analyses of seven pyrites yield one isochronal age of 225±12 Ma (2σ) with an initial 187 Os/188Os of 0.3±0.07 on 187 Re/188Os versus 187 Os/188Os plot (MSWD=6.3, Fig. 4a), whereas another isochronal age is 233±14 Ma (2σ) with an initial 187Os of−0.008 on the 187Os versus 187Re correlation diagram (MSWD=3.9, Fig. 4b). Consequently, the two ages of pyrite are close to error. These isochrones are calculated based on the 187Re decay constant of 1.666×10−11/year (Smoliar et al. 1996) and Isoplot software (Ludwig 1999).

Fig. 4
figure 4

(a) 187Re/188Os versus 187Os/188Os isochron plot for pyrite samples from the Baishan Mo–Re deposit, eastern Tianshan. (b)187Os versus 187Re correlation diagram for pyrite samples from the Baishan Mo–Re deposit, eastern Tianshan.

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Discussions

Content of rhenium and its significance

The ore from Baishan has a high concentration of Re between 0.7 and 1.9 ppm, which reflects unusually high Re concentrations in molybdenite (74 to 250 ppm, this study; 800–900 ppm, Zhou et al. 1996), higher than in most other deposits in China and infrequent in the world (Table 5). Geological exploration in recent years indicates that the deposit may reach the scale of a large Re (Mo) deposit (Nie et al. 2001; Deng et al. 2003).

Table 5 Concentrations of rhenium in molybdenite from some molybdenite-bearing deposits
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By gathering information on Re content in molybdenite for molybdenite-bearing deposits published in recent years, we feel that the Re contents vary greatly (Table 5). Mao et al. (1999) suggested that the Re content of molybdenite could reflect the source of the deposits. Re content would decrease from mantle-related deposits, to I-type and S-type granite-related deposits. Stein et al. (2001) also suggested that deposits with a mantle component in their source have significantly higher Re contents than those deposits that are crustally derived. We suggest that the controlling factors for Re content in molybdenite include the mineral assemblage, temperature, and source of the ore-forming material. In general, high-temperature mineral assemblages (i.e. quartz-molybdenite-chalcopyrite) have high Re content (e.g. Baishan, Gangdese, and Bagdad deposits etc.; Table 5).

Initial 187Os/188Os and source of ore-forming metals

The Re–Os isotope system has been recognized as a possible geochemical tool not only for directly dating mineralization but also for tracing the source of metals (Ruiz and Mathur 1999). The 187Os/188Os ratios in the crust (0.2–10), compared to mantle values (0.11–0.15, Meisel et al. 2001), can be used to readily discern different sources (Barra et al. 2003).

The initial 187Os/188Os ratio from the pyrite isochron is 0.3±0.07 (Fig. 4a), and is slightly more radiogenic than the chondritic 187Os/188Os mean ratio of 0.13 at 225 Ma. This indicates that crustal components were only a little involved in the source of osmium for pyrite. Osmium and molybdenum should have similar geochemical behavior in this system, as they are both chalcophile elements. If osmium behaves like molybdenum, then we can infer the source of Mo from understanding the source of osmium (Barra et al. 2003). According to Nie et al. (2003), there is a narrow range of positive δ34S values (+0.5 to +3.6‰) for five molybdenite samples from the Baishan deposit. The δ18O values of the ore fluids vary from +4.3 to +5.4‰ and the δD values from −78.8 to −56.6‰ (Zhou et al. 1996). These data further support the above hypothesis that the ore-forming fluids were derived mainly from a mantle source and crustal components were less involved in the ore-forming processes.

Age of mineralization and its geological significance

The Re–Os geochronometer, applied to molybdenite, has been demonstrated to be remarkably robust, even in situations of overprint by where metamorphism and deformation (Stein et al. 1998). If molybdenite does not contain any initial or common Os, all measured Os is monoisotopic (187Os) as the product of decay of 187Re, and the isochron age then represents the depositional age of molybdenite (Suzuki et al. 1996; Brenan et al. 2000; Barra et al., 2003). For the Baishan Mo–Re ore deposit, the analysis of eight molybdenite samples yields an isochron age of 224.8±4.5 Ma (2σ) with an initial 187Os of 0.0038±0.0066. It is shown that the initial187Os values from the molybdenite samples are close to zero and the Re–Os isochron ages reflect the time of sulfide deposition. The Mo–Re mineralization of the Baishan deposit took place after regional low-grade metamorphism and folding, and was not influenced by later geological events.

Analyses of seven pyrite samples yield one isochron age of 225±12 Ma (2σ) on the 187Re/188Os versus 187 Os/188Os plot and another isochronal age of 233±14 Ma (2σ) on the 187Os versus 187Re correlation diagram. As the pyrite samples contain minor initial Os, the molybdenite age is more reliable. However, both ages of molybdenite and pyrite are consistent within their errors, which implies that the time of ore formation for the Baishan deposit is between 225 and 233 Ma, and the Mo–Re mineralization is derived from magmatic-hydrothermal activity.

Mineralization in the eastern Tianshan, reported by some researchers (Li et al. 1998; Mao et al. 2003; Qin et al. 2003), is mainly of Late Paleozoic age (330 to 260 Ma). Younger mineralization ages from the Indosinian epoch (Mesozoic) have rarely been reported in the literature. However, recent studies indicate that the ages of the Jinwuozi gold deposit are 228–230 Ma (Chen et al. 1999), the Au-bearing quartz vein III of the Shiyingtan gold deposit is 244±9 Ma (Zhang et al. 2003b), and the Xiaobaishitou W–Mo deposit (20 km northeast of Weiya) is 248 Ma (Li et al. 2004, unpublished data).

Similarly, some ages of 245–211 Ma of metal deposits in the western Tianshan were reported (Ye and Ye 1999; Liu et al. 2002). These ages are close to the Re–Os ages (225–233 Ma) of molybdenite and pyrite from the Baishan Mo–Re deposit in the eastern Tianshan, and indicate that the Indosinian period is also an important mineralization epoch in the Tianshan region.

Conclusions

The Baishan deposit is controlled by a strata-bound fracture zone. The Mo–Re mineralization is ascribed to the quartz vein-porphyry type. The Re–Os isochron ages from the sulfides of the main mineralization are between 225 and 233 Ma. Based on the regional tectonic evolution, it is indicated that the deposit formed in an intracontinental extensional setting in the Early Mesozoic .

The initial187Os/188Os ratio of 0.3±0.07 and δ34S values of +0.5 to +3.6‰ (Nie et al. 2003) from the sulfides indicate that the ore-forming materials are derived mainly from a mantle source.