Re–Os geochronology of Cu and W–Mo deposits in the Balkhash metallogenic belt, Kazakhstan and its geological significance

Geoscience Frontiers (2010) 1, 115e124

available at www.sciencedirect.com

China University of Geosciences (Beijing)

Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf

ORIGINAL ARTICLE

ReeOs geochronology of Cu and WeMo deposits in the Balkhash metallogenic belt, Kazakhstan and its geological significance Xuanhua Chen a,b,*, Wenjun Qu c, Shuqin Han b, Seitmuratova Eleonora d, Nong Yang a,b, Zhengle Chen a,b, Fagang Zeng c, Andao Du c, Zhihong Wang b a

Key Laboratory of Neotectonic Movement & Geohazard, Ministry of Land and Resources, Beijing 100081, PR China Institute of Geomechanics, Chinese Academy of Geological Sciences, 11 Minzudaxue Nan Road, Beijing 100081, PR China c National Research Center for Geoanalysis, Beijing 100037, PR China d Laboratory of Geological Formations, K. Satpaev Institute of Geological Sciences, Almaty 050010, Kazakhstan b

Received 23 April 2010; accepted 20 June 2010 Available online 8 October 2010

KEYWORDS ReeOs geochronology; Metallogenic age; Porphyry CueMo deposit; Greisen WeMo deposits; Balkhash metallogenic belt; Kazakhstan

Abstract The Central Asian metallogenic domain (CAMD) is a multi-core metallogenic system controlled by boundary strike-slip fault systems. The Balkhash metallogenic belt in Kazakhstan, in which occur many large and super-large porphyritic CueMo deposits and some quartz vein- and greisen-type WeMo deposits, is a well-known porphyritic CueMo metallogenic belt in the CAMD. In this paper 11 molybdenite samples from the western segment of the Balkhash metallogenic belt are selected for ReeOs compositional analyses and ReeOs isotopic dating. Molybdenites from the Borly porphyry Cu deposit and the three quartz vein-greisen WeMo depositsdEast Kounrad, Akshatau and Zhanetdall have relatively high Re contents (2712e2772 mg/g for Borly and 2.267e31.50 mg/g for the other three W eM o d e p o s i t s ) , a n d l ow e r c o m m o n O s c o n t e n t s ( 0 . 6 7 0 e2 . 6 9 6 n g / g f o r B o r l y a n d 0.0051e0.056 ng/g for the other three). The molybdenites from the Borly porphyry CueMo deposit

* Corresponding author. Institute of Geomechanics, Chinese Academy of Geological Sciences, 11 Minzudaxue Nan Road, Beijing 100081, PR China. Tel.: þ86 10 68994510, þ15801123149 (mobile). E-mail address: [email protected] (X. Chen). 1674-9871 ª 2010, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. Peer-review under responsibility of China University of Geosciences (Beijing). doi:10.1016/j.gsf.2010.08.006

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X. Chen et al. and the East Kounrad, Zhanet, and Akshatau quartz vein- and greisen-type WeMo deposits give average model ReeOs ages of 315.9 Ma, 298.0 Ma, 295.0 Ma, and 289.3 Ma respectively. Meanwhile, molybdenites from the East Kounrad, Zhanet, and Akshatau WeMo deposits give a ReeOs isochron age of 297.9 Ma, with an MSWD value of 0.97. ReeOs dating of the molybdenites indicates that CueWeMo metallogenesis in the western Balkhash metallogenic belt occurred during Late Carboniferous to Early Permian (315.9e289.3 Ma), while the porphyry CueMo deposits formed at w316 Ma, and the quartz vein-greisen WeMo deposits formed at w298 Ma. The ReeOs model and isochron ages thus suggest that Late Carboniferous porphyry granitoid and pegmatite magmatism took place during the late Hercynian movement. Compared to the Junggar-East Tianshan porphyry Cu metallogenic belt in northwestern China, the formation of the CueMo metallogenesis in the Balkhash metallogenic belt occurred between that of the Tuwu-Yandong in East Tianshan and the Baogutu porphyry Cu deposits in West Junggar. Collectively, the large-scale Late Carboniferous porphyry CueMo metallogenesis in the Central Asian metallogenic domain is related to Hercynian tectono-magmatic activities. ª 2010, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction The Balkhash metallogenic belt lies at the inner margin of the U-shaped KazakhstaneTianshan orocline and the core of the BalkhasheJunggar orogenic belt of the Central Asian metallogenic domain (CAMD; also called the Paleo-Asian metallogenic tectonic system; Chen et al., 2009; Fig. 1). It is the main porphyry copper mineralization area in Kazakhstan, and one of the ten most important porphyry Cu metallogenic belts in the world (He and Zhu, 2006; Zhu et al., 2007). The Kounrad Cu deposit in the belt is one of the world’s ten largest porphyry Cu deposits, the formation of which is mainly related to tectono-magmatism in the later stage of the relict ocean basin’s extinction during the Devonian-Carboniferous. Other noteworthy deposits in the region include the super-large porphyry Aktogay CueMo deposit and the large skarn Sayak Cu-polymetallic deposit (Li et al., 2007). Furthermore, there are also porphyry Borly CueMo deposit and the East Kounrad, Akshatau, and Zhanet quartz vein-greisen WeMo deposits in the belt. Molybdenite can provide information critical to understanding sulfide mineral deposition in ore systems; this information is not recorded by other isotopic systems in alteration minerals (Selby and Creaser, 2001). ReeOs isotopic dating of molybdenites is considered to be a high-accuracy dating method usable for direct determination of metallogenic age (Mao et al., 2003, 2006; Du et al., 2007; Nie et al., 2007). Based on the ReeOs dating of molybdenites, this paper defines ages of the Late Paleozoic porphyry CueMo metallogenesis and quartz vein-greisen WeMo mineralizations in the Balkhash metallogenic belt. In addition, it also correlates them with the ages of East and West Junggar and the East Tianshan porphyry Cu metallogenic belts in China.

2. Geology of the Balkhash metallogenic belt in the Central Asian metallogenic domain 2.1. The Central Asian metallogenic domain The Central Asian metallogenic domain begins in the west in the Ural Mountains on the Euro-Asia boundary, and extends eastwards through Kazakhstan, Uzbekistan, parts of Kyrgyzstan, Xinjiang, Qinghai, northern Gansu, western Inner Mongolia, and western Mongolia to the southern Siberia region. In the latter area, it also includes the eastern part of Lake Baikal, consisting of the SayaneErgunaeXinkai orogen, the TianshaneXing’an Hercynian

orogen, the Ural-South Tianshan Hercynian orogen, the Tarim para-platform, and partial Yanshanian orogens on the eastern margin of Asia (Chen et al., 2009; Fig. 1). Multi-stage and multi-type zones of volcanic rocks, granites, basic rocks, ultrabasic rocks, ophiolites, and metamorphic rocks are distributed in this metallogenic domain. Geologically, this domain has experienced the formation of continental basement, pericontinental accretion of the Paleo-Asian Ocean, and continental margin activities in the western Pacific, as well as the elevation and subsidence of intracontinental blocks resulting in the presence of various environments favorable to the formation of ore (Chen et al., 2009). The Central Asian metallogenic domain is world famous for its metallic and non-metallic deposits, the ore-forming processes of which are extremely complicated and diversified (He and Zhu, 2006; Zhu et al., 2007). The mineralizations of porphyry CueMo and quartz vein-greisen WeMo deposits occupy a particularly important place in these processes. The main body of the Central Asian metallogenic domain is the BalkhasheJunggareSouth Mongolian porphyry Cu(Mo) metallogenic belt (Li et al., 2007), in which there are many porphyry deposits: Tsagaan Suvarga and Ouyu Tolgoi in Mongolia; the Kounrad, Aktogay, Koksai, and Borly in Kazakhstan; the Olmaliq in Uzbekistan; the Baogutu in West Junggar; and the Tuwu-Yandong in the East Tianshan Mountains, etc. (Fig. 1). Most of these deposit formed during the CarboniferousePermian, although some are of Early Paleozoic age.

2.2. The Balkhash metallogenic belt The Balkhash metallogenic belt is one of the cores of the Central Asian metallogenic domain (Zhu et al., 2007), and is also a part of the (circum-)Balkhash-(circum-)Junggar-South Mongolian porphyry Cu(Mo) metallogenic belt (Li et al., 2007). The formation of its Late Paleozoic porphyry Cu deposits is related to Devonian and CarboniferousePermian volcano-magmatic arcs in the region (Li et al., 2008). This metallogenic belt consists of north and south zones of mineralization. The former begins at the Mointy block in the west, and extends eastwards via Aktogay to Baotugu, western Junggar, Xinjiang, China. It is about 1000 km long from east to west and contains nearly one hundred deposits of various sizes that have already been discovered, amongst which are both the Kounrad (with Cu reserves > 8 Mt) and Aktogay (Cu Z 5.88 Mt), two world famous super-large porphyry CueMo deposits. The Borly porphyry Cu deposit and the quartz veingreisen WeMo deposits such as Akshatau, East Kounrad, and

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Figure 1 Sketch map of the multi-core metallogenic system and distribution of porphyry CueMo deposits in the Central Asian metallogenic domain. The fault system is modified from Ren et al. (1999). The distribution of porphyry CueMo deposits is modified from Li et al. (2007). 1-sinistral strike-slip fault; 2-dextral strike-slip fault; 3-thrust fault; 4-fault; 5-the four cores of the multi-core metallogenic system of the Central Asian metallogenic domain: (1) Balkhash core, (2) Junggar core, (3) Mongolia core, and (4) Altay core; 6-porphyry CueMo deposits in superlarge to large sizes and in medium to small sizes.

Zhanet (Fig. 2), all studied in this paper, are found in the western area of the Balkhash metallogenic belt.

3. Geological features of the deposits 3.1. The Borly porphyry Cu deposit The Borly porphyry Cu deposit is situated in the Prioziorny district, Dzhezkazgan Province, 60 km north of Balkhash City, and 45 km away from the Kounrad Cu deposit, at N 47 110 400 , E 74 420 4100 , and at an elevation of w493 m a.s.l. It lies mainly in the southern part of the Tokrausky synclinorium, and its Cu reserves amount to some 600 kt Cu @ 0.34%, and a total Cu:Mo ratio Z 33:1. The rocks in the deposit are dominated by aplitic rhyoliteedacite tuff. These include (1) a suite of lithic-crystal tuff, lava, and subvolcanic rocks of the Lower Carboniferous Karkaralinskaya Formation, and (2) a suite of dacite, sometimes trachdacite or andesiteedacite ignimbrite, microlitic tuff, tufflava, lava, and subvolcanic rocks of the Upper-Middle Carboniferous Keregetass Formation. The center of the Borly deposit is the Borlinksy apophysis of the Kyzylzhalsky intrusion. The Borlinskaya apophysis contains three petrofaciesdthe first being quartz diorite, the second (and the main one) being biotite amphibole granodiorite, and the third, light-colored granite-porphyry. These petrofacies are cut by a granodiorite pluton with a complex dendritic texture. The intrusion of that rock body was accompanied by intensive cryptoexplosive brecciation, hydrothermal alteration, and formation of quartz-sulfide stockworks. The youngest magmatic assemblage is the alkaline graniteporphyry dikes and subvolcanic rocks in the Early Permian Zhaksitagalinsky complex (Abdulin et al., 1998).

3.2. The Akshatau WeMo deposit The Akshatau large-scale quartz vein-greisen WeMo deposit occurs in the Paleozoic orogen in Central Kazakhstan, 150 km from Balkhash City, at N 47 580 5200 , E 74 30 2200 , at an elevation of

approximately 740 m. It is a disseminated quartz vein-greisen BeeWeMo deposit (Yefimov et al., 1990), having resources as follows: 2.741  106 t (first-grade reserves) of 0.50% WO3; 6.55  107 t of 0.1%e0.3% WO3; 1.75  107 t of 0.04%e0.07% Mo; and 1.60  107 t of 0.03%e0.07% Be. As part of the Permian Akshatau metallogenic province, the deposit is located in the rear part of a volcano-intrusive zone into a flysch belt in the Late Paleozoic continental margin. It is controlled by linear and circular faulted structures, and by the intersection by structural belts of different trends (Burmistrov et al., 1990). It occurs within Permian leucogranites of the Akshatau multi-stage complex that intrude Silurian sandstones. Greisenquartz vein type WeMo deposits are closely related to the tops of ore-forming intrusives in both endocontacts and exocontacts. The greisen bodies consist of root, intermediate, and front zones. Most lie within the intermediate zone, occurring inside granite cupolas or at their wings and ridges of different sizes. Enriched deposits are most easily found at the tops of granites in mono-dome structures (Daukeev et al., 2004). The ore-forming process has mainly undergone 2 stages and 4 phases: the first stage is the pneumatolyticehydrothermal stage, comprising the molybdeniteequartz phase (440e340  C) and a complex rare-metal phase (480e250  C); the second stage is the real hydrothermal stage, containing the galenaesphaleriteequartz phase (310e150  C) and the calciteefluoriteequartz phase (180e60  C) (Yefimov et al., 1990).

3.3. The East Kounrad WeMo deposit The East Kounrad WeMo deposit lies about 11 km east of the Kounrad porphyry Cu deposit, at N 47 10 800 , E 75 80 600 , and an elevation of about 432 m. It is an underground-mined postmagmatic pegmatite-quartz vein type WeMo deposit, which played a role during the Second World War, but is now abandoned. It has reserves of 200e250 kt of Mo, with a grade of 0.056%. It is of the quartz vein-greisen type with the major ore minerals being wolframite and molybdenite (Fig. 3). The WeMo mineralizations

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Figure 2 Regional geological sketch map of the western Balkhash metallogenic belt 1-Quaternary System; 2-Permian System; 3-CarboniferousePermian System (undivided); 4-Carboniferous System; 5-Devonian System; 6-Silurian System; 7-Precambrian System; 8-Triassic granitoids; 9-Permian granitoids; 10-Carboniferous granitoids; 11-Devonian granitoids; 12-Ordovician granitoids; 13-Precambrian granitoids; 14the Balkhash Lake area; 15-thrust fault; 16-dextral strike-slip fault; 17-fault; 18-locations of deposits studied in this paper.

ReeOs geochronology of Cu and WeMo deposits in the Balkhash metallogenic belt, Kazakhstan

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age of the ore-forming granites in the deposit is post-Middle Carboniferous.

4. ReeOs isotopic compositions of the molybdenites 4.1. Sample processing and analytical methods

Figure 3

A quartz vein from the East Kounrad WeMo deposit.

occur mainly in quartz veins and quartz veinlets, and also at the tops of cupolas and in greisens surrounding quartz veins (Burmistrov et al., 1990). The main mineral assemblages of the deposit are: scheelite, wolframite, molybdenite, apatite, phenakite, beryl, biotite, muscovite, bismite, bismuthinite, calcite, chalcopyrite, ferromolybdite, fluorite, prosopite, helvite, microcline, pyrite, phlogopite, powellite, quartz, rhodochrosite, salite, and topaz.

3.4. The Zhanet Mo deposit The Zhanet Mo deposit is a medium-sized quartz vein-greisen Mo deposit, located at N 47 310 1700 , E 74 180 5500 , and an elevation of approximately 618 m. It was first explored in 1948, and mined for some time, but at present mining has been temporarily suspended. The ores contain molybdenite, wolframite, topaz, fluorite, and beryl. The main ore mineral is molybdenite (Fig. 4), which also has high contents of rare-earth and rare elements. Molybdenites occur mainly in Mo-bearing granite-porphyries and in late-stage quartz veins and fissures assuming disseminated and veined shapes. In the late-stage quartz veins the molybdenites are associated with fluorite. Wall-rock alterations include potassic-alteration (e.g. K-feldspathization, biotitization), pyritization, greisenization, and epidotization. Pegmatite veins formed in the late stage. The

The 11 molybdenite samples used for ReeOs isotope dating were collected from the Borly porphyry Cu deposit, the Akshatau WeMo deposit, the East Kounrad WeMo deposit, and the Zhanet Mo deposit in the western section of Kazakhstan’s Balkhash metallogenic belt. After grinding and sorting, all the molybdenite samples reached the purity (>98%) and homogeneity requireddwith a grain size of 200 meshesdfor the purpose of reducing the effect of decoupling (Stein et al., 2001, 2003; Du et al., 2007). ReeOs isotope dating of the molybdenite samples was performed at the ReeOs Isotope Chronology Laboratory, National Research Center for Geoanalysis. The chemical separation and processing processes of Re and Os as well as the mass spectrometry technology have been described by Du et al. (1994, 2001, 2004) and Qu and Du (2003, 2004). The isotope ratio was determined using the TJA X-series ICP-MS at the National Research Center for Geoanalysis. For Re, the mass numbers 185 and 187 were chosen and 190 was used to monitor Os. For Os, 186, 187, 188, 189, 190 and 192 were chosen, and 185 was used to monitor Re. The common Os was obtained from the measured ratio of 192 Os/190Os following the table of atomic weight (Wieser, 2006) and the table of isotope abundances (Bohlkea et al., 2005). The uncertainties of Re and Os contents consisted of the weighing errors of the samples and diluents, the nominal errors of the diluents, the correction errors of fractionation in mass spectrometry, and the measurement errors of isotope ratios of the samples. The confidence level was 95%. The uncertainties of model ages also included the uncertainty of the decay constant (1.02%), and the confidence level was also 95%. The model age t (Ma) of molybdenites was calculated according to the formula given by Du et al. (1994) and Qu and Du (2003), where 1.666  1011 yr1 (Simoliar et al., 1996) was taken for the decay constant of 187Re. Ludwig’s (2003) method was used to process the Re and Os isotope data relating to molybdenites, and obtain the isochron ages of ReeOs.

4.2. Analytical results The analytical results are listed in Table 1. The table also gives the measured results of the standard material GBW04436 (JDC) and its certified values (Du et al., 2004). Table 2 provides the blank background of the whole procedure of the analyses.

5. Results and discussions 5.1. Metallogenic ages and series of CueWeMo deposits in the Balkhash metallogenic belt

Figure 4

A quartz vein from the Zhanet Mo deposit.

The model ages of molybdenites obtained in the experiment are 316.1  7.0 Ma and 315.6  5.9 Ma, averaging 315.9 Ma for the Borly porphyry Cu deposit; 297.8  5.3 Ma, 297.9  4.5 Ma, 297.9  4.4 Ma, and 298.4  4.1 Ma, averaging 298.0 Ma for the East Kounrad WeMo deposit; 295.2  5.3 Ma, 295.8  4.3 Ma, and 294.1  4.3 Ma, averaging 295.0 Ma for the Zhanet Mo

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Table 1 Nos.

ReeOs isotopic data for molybdenites from CueWeMo deposits in the Balkhash metallogenic belt, Kazakhstan and the standard material. Original sample Nos.

The Borly porphyry Cu deposit 090414e17 xh080912-9(1) 090414e18 xh080912-9(5)

Sample weight (g)

0.00118 0.00109

Re (mg/g)

Common Os (ng/g)

187

187

Re (mg/g)

Os (ng/g)

Model age (Ma)

Measured

Uncertainty

Measured

Uncertainty

Measured

Uncertainty

Measured

Uncertainty

Measured

Uncertainty

2712 2772

50 40

2.696 0.670

3.464 3.004

1705 1742

32 25

9001 9184

80 79

316.1 315.6

7.0 5.9

The East Kounrad greisen WeMo deposit 090330e10 xh080910-11(3) 0.05070 090420e5 xh080910-11(1) 0.04976 090420e6 xh080910-11(2) 0.05036 090420e7 xh080910-11(3) 0.05378

26.19 31.50 27.30 25.92

0.34 0.27 0.26 0.21

0.0051 0.0301 0.0428 0.0262

0.0227 0.0796 0.0444 0.0277

The Zhanet greisen Mo deposit 090330e11 xh080915-5(1) 090420e10 xh080915-5(1) 090420e11 xh080915-5(4)

0.03580 0.05002 0.05272

9.664 9.099 15.25

0.128 0.076 0.14

0.0212 0.0223 0.0103

0.0002 0.0286 0.0763

The Akshatau greisen WeMo deposit 090420e8 xh080914-9(2) 0.05606 090420e9 xh080914-9(3) 0.05118

6.942 2.267

0.055 0.021

0.0488 0.0560

0.0251 0.0720

16.46 19.80 17.16 16.29

0.22 0.17 0.16 0.13

81.87 98.50 85.37 81.19

0.71 0.90 0.68 0.65

297.8 297.9 297.9 298.4

5.3 4.5 4.4 4.1

6.074 5.719 9.58

0.081 0.048 0.09

29.94 28.26 47.07

0.27 0.25 0.40

295.2 295.8 294.1

5.3 4.3 4.3

4.363 1.425

0.034 0.013

21.21 6.840

0.17 0.061

291.1 287.5

4.0 4.4

Results of measurement of the standard material in the experiments GBW04436 (JDC ) 090403e19 JDC 0.10038 16.99 0.31 090420e19 JDC 0.10128 17.15 0.13

24.98 25.16

0.48 0.21

140.2 139.9

3.9 1.9

Certified values* of the measured values of the standard material GBW04436 (JDC) (Du et al., 2004) 17.39 0.32

25.46

0.60

139.6

3.8

Note: The measurements were done at the Re-Os Isotope Chronology Laboratory, National Research Center for Geoanalysis.

X. Chen et al.

ReeOs geochronology of Cu and WeMo deposits in the Balkhash metallogenic belt, Kazakhstan Table 2

121

Blank background of the whole procedure of analyses.

Serial Nos.

090414e20 090420e20

187

Original sample Nos.

Re (ng)

Common Os (ng)

Measured value

Uncertainty

Measured value

Uncertainty

Measured value

Uncertainty

BK BK

0.0117 0.0269

0.0017 0.0018

0.0003 0.0001

0.0000 0.0000

0.0005 0.0000

0.0000 0.0000

deposit; and 291.1  4.0 Ma and 287.5  4.4 Ma, averaging 289.3 Ma for the Akshatau WeMo deposit. Among them, the ReeOs isochron age for the deposits such as East Kounrad, Akshatau and Zhanet is (297.9 þ 0.99/3.4) Ma, and the MSWD value is 0.97 (Fig. 5). All of the above ages are greater than the metallogenic ages of the Akshatau and East Kounrad WeMo deposits previously reported (which are w285 Ma and 285e283 Ma; He and Zhu, 2006). The ReeOs isochron dating of molybdenites is considered to be a high-accuracy dating method usable for direct determination of the metallogenic age (Mao et al., 2003, 2006; Du et al., 2007; Nie et al., 2007). The formation ages of the East Kounrad WeMo deposit, Akshatau WeMo deposit, and Zhanet Mo deposit are of the same period, their molybdenites come from the same material source, and the ReeOs isotope system can be regarded as under a closed status, so they, therefore, agree with the determination criterion of isochron ages of isotopes (Nie et al., 2007). The ReeOs isochron ages given in the present paper represent the ages of the quartz vein-greisen WeMo mineralization in the western Balkhash metallogenic belt. The analytical results of ReeOs isochron dating of molybdenites indicate that the mineralization related to CueWeMo in the western Balkhash metallogenic belt took place during the interval 315.9e289.3 Ma. The formation of CueWeMo deposits can be divided into two stages: the first stage is that of porphyry CueMo deposits, occurring at w316 Ma; the second, of quartz veingreisen WeMo deposits, occurring at w298 Ma. Thus, CueWeMo mineralization in the western Balkhash metallogenic belt has resulted in a metallogenic series with porphyry CueMo deposits in the early stage and quartz vein-greisen WeMo deposits in the late stage.

Os (ng)

5.2. Characters of ReeOs isotope systems of CueWeMo deposits in the Balkhash metallogenic belt ReeOs data is available for a number of ore deposits (Lambert et al., 1999), among them mantle-sourced materials (xenoliths, komatiites, and basalts), sulfides-bearing sediments, a series of magma-sulfides deposits including the Ni deposit associated with komatiites (Kambalda), the CueNi deposit associated with basalts/ gabbros (Sudbury, Noril’sk-Talnakh, and Duluth), and the PGErich deposit associated with basalts/gabbros in basic and ultrabasic layered intrusives (e.g. JeM Reef, Stillwater; Fig. 6). In contrast, the molybdenites from the Borly porphyry Cu deposit and the three quartz vein-greisen WeMo depositsdEast Kounrad, Akshatau and Zhanetdhave relatively higher Re contents (2712e2772 mg/g for Borly and 2.267e31.50 mg/g for the other three WeMo deposits), and lower common Os contents (0.670e2.696 ng/g for Borly and 0.0051e0.056 ng/g for the other three). Therefore, they have very high Re/Os ratios (1.006  106e4.137  106 for Borly and 4.048  104e5.135  106 for the other three). Just as magmatogenic sulfide-bearing rocks (and ores) have similar but slightly lower Re/Os ratios when compared with their parent silicate magmas (Lambert et al., 1999), the magma source rocks of the porphyry CueMo deposits and quartz vein-greisen WeMo deposits in the Balkhash metallogenic belt have higher Re/Os ratios. Hence, the mantle melts of this region had very high Re/Os ratios. We can see in the diagram of Re/Os ratios vs. common Os contents that the molybdenites from the porphyry CueMo deposits and from the quartz vein-greisen WeMo deposits fall in different zones, which indicates that the two mineralizations had different paths (Fig. 6). The former (mineralization of porphyry CueMo deposits) has suffered contamination of more crustal materials while the latter is more directly related to the mantle process. Nevertheless, they may have come from the same mantle source region.

5.3. Tectono-magmatism and metallogenic ages in the BalkhasheJunggar metallogenic belt

Figure 5 ReeOs isochron diagram of the molybdenites from quartz vein-greisen WeMo deposits in the western Balkhash porphyry CueMo metallogenic belt. The zero intercept on the 187Os axis is expected because molybdenite contains no initial or common 187Os (Morgan et al., 1968; Markey et al., 1998).

The metallogenic series of CueWeMo deposits in the western Balkhash metallogenic belt may reflect the evolutionary process of magmatism in the lithosphere of a mobile continental margin controlled by polycycle geodynamics and an orogenic history of multi-stage subduction, crustal accretion, and lateral growth in the core part of Central Asia during the Paleozoic, especially the Middle and Late Paleozoic (Heinhorst et al., 2000; Heubeck, 2001; Xiao et al., 2008, 2009). It developed for multiple reasonsdFrom the tectonic setting of a back-arc ocean (volcano-massive sulfide CueAu deposits), to the cal-alkaline magmatism controlled by subduction (porphyry Cu deposits), and to the crustal differentiation of low-grade partial melting and internal separation of magma in a broad sense (porphyry Mo deposits and quartz vein-greisen WeMo deposits), and finally to the setting of a nonorogenic

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Figure 6 Re/Os vs. Os diagram of molybdenites from the western Balkhash porphyry-type CueMo metallogenic belt Re/Os concentration ratio calculated using common Os. (A) CueNieCo magmatic metallogenic system, after Lambert et al. (1999). (B) Porphyry CueMo metallogenic system. RGs, RHs and RPs are from the Hercynian Uralide orogen. Among them, the RGs and RHs are molybdenites from the quartzemolybdenite stockwork in the Malyshevo leucogranite stock and molybdenites from the hornfels surrounding the stock respectively, which are hosted in the Shameika porphyry Mo deposit, and their RPs represent the pegmatite-hosted molybdenite from the Lipovy Log rare-metal deposit (Mao et al., 2003).

Permian continental rift valley (the peralkaline riebeckite granite REEeZreNb-enriched system; Heinhorst et al., 2000). Some areas of China, e.g. East and West Junggar and the East Tianshan Mountains of Xinjiang and Inner Mongolia, also developed banded porphyry CueMo and quartz vein-greisen WeMo (Sn) metallogenic belts, which are of similar age and type to the Balkhash metallogenic belt. For example, the Paleozoic Baogutu porphyry CueMo deposit in the southern Darabut island-arc of West Junggar occurs in rock body V of Baogutu, from which LA ICP-MS dating has yielded a zircon UePb age of quartz diorite porphyry of 309.9  1.9 Ma (Tang et al., 2009). Sulfide minerals of Cu, Fe, Mo, and Zn were formed at an early stage (Shen et al., 2009), in which the ReeOs age of molybdenite is 310 Ma (Song et al., 2007). The Beierkuduke quartz vein-greisen Sn deposit in East Junggar also occurs in Late Carboniferous granites, with a metallogenic age of 296.3  2.6 Ma (Tang et al., 2006; KeAr age of muscovite). In the Sareshike Sn deposit of East Junggar, the ReeOs age of molybdenite is 307  11 Ma (Tang et al., 2007). The Tuwu-Yandong porphyry Cu deposit in the East Tianshan Mountains occurs in a plagiogranite porphyry that is Carboniferous (334  3 Ma to 333  4 Ma; Chen et al., 2005), and the ReeOs isochron age of molybdenite in the copper ore body is 323  2 Ma (Rui et al., 2002). The Chihu porphyry MoeCu deposit in the East Tianshan Mountains also occurs in a plagiogranite porphyry intrusion, and zircon SHRIMP UePb dating reveals its crystallization age to be 322  10 Ma (Wu et al., 2006). It can thus be seen that the metallogenic age of the porphyry CueMo deposit in the western Balkhash metallogenic belt (315.9 Ma, the Borly Cu deposit) is between that in the East Tianshan Mountains (323  2 Ma, the Tuwu-Yandong porphyry Cu deposit, Rui et al., 2002; 322  10 Ma, the Chihu porphyry MoeCu deposit, Wu et al., 2006) and that in West Junggar (310 Ma, the Baogutu porphyry CueMo deposit, Song et al., 2007). On the other hand, the metallogenic age of quartz vein-greisen WeMo deposits in the western Balkhash metallogenic belt (297.9 Ma) is younger

than or identical with that of quartz vein-greisen Sn deposits in East Junggar (307  11 Ma, the Sareshike Sn deposit, Tang et al., 2007; 296.3  2.6 Ma, the Beierkuduke Sn deposit, Tang et al., 2006). In summary, the mineralizations of large-scale porphyry CueMo deposits in the western BalkhasheJunggar metallogenic belt of the Central Asian metallogenic domain were concentrated in the interval of 323e310 Ma of the Late Carboniferous, whereas younger quartz vein-greisen WeMo(Sn) deposits occurred between 297.9 Ma and 296.3 Ma, and were products of late Hercynian tectono-magmatism. The age of BalkhasheJunggar porphyry CueMo mineralization in the Central Asian metallogenic domain agrees well with that of post-collisional plutonism in East and West Junggar in the Late Paleozoic (Han et al., 2006). It is obviously younger than the metallogenic age of the Elegen porphyry Mo(Cu) deposit in Beishan Mountain, Inner Mongolia, which occurred in a Late Paleozoic island-arc environment (332.0  9.0 Ma, ReeOs isochron age of molybdenites; Nie et al., 2005). On the other hand, the age of the quartz vein-greisen WeMo(Sn) mineralization in the BalkhasheJunggar porphyry CueMo metallogenic belt coincides with that of post-collisional plutonism in the Late Paleozoic in East and West Junggar (Han et al., 2006). It is slightly older than of the age of (1) CueNi sulfide deposits in East Junggar that formed under a post-collisional extension environment (e.g. the ReeOs isochrone ages of the No. 1 and No. 2 intrusive bodies of the Karatungk deposit, and CueNi sulfide ore of the Huangshandong deposit in the East Tianshan Mountains are 282.5  4.8, 290.2  6.9 and 284  14 Ma respectively; Zhang et al., 2008), and (2) the Kanggur gold deposit in the East Tianshan Mountains. The latter is controlled by a ductile shear zone (RbeSr and SmeNd isochrons give the main stage as 290e282 Ma and a late stage as 275e254 Ma; Zhang et al., 2003). Porphyry Mo deposits and pegmatite-type rare-metal deposits were also later formed in the Hercynian Uralide orogen. For example, the quartzemolybdenite stocks in the Malyshevo leucogranite stocks

ReeOs geochronology of Cu and WeMo deposits in the Balkhash metallogenic belt, Kazakhstan and the molybdenites in the hornfels surrounding the stocks hosted in the Shameika porphyry Mo deposit in the Uralide orogen yielded ReeOs isochron ages of 273  5 Ma and 282  6 Ma, respectively, while the molybdenites from the pegmatite-hosted Lipovy Log TaeNbeMo deposit gave an isochron age of 262.0  7.3 Ma (Mao et al., 2003).

6. Conclusions The Balkhash metallogenic belt is an important porphyry CueMo metallogenic belt in the multi-core metallogenic system of the Central Asian metallogenic domain. This paper, based on (1) ReeOs isotope analyses of 11 molybdenite samples from the BalkhasheAkshatau region in the western Balkhash metallogenic belt, and (2) a preliminary correlation between the EasteWest Junggar and the East Tianshan metallogenic belts, has reached the following conclusions: (1) The model ReeOs ages (mean values) of the Borly porphyry Cu (Mo) deposit, and the East Kounrad, Zhanet, and Akshatau quartz vein-greisen WeMo deposits are 315.9 Ma, 298.0 Ma, 295.0 Ma, and 289.3 Ma, respectively, amongst which the latter three deposits have a ReeOs isochron age of (297.9 þ 0.99/3.4) Ma, and an MSWD value of 0.97; (2) ReeOs ages of molybdenites reveal that the CueWeMo mineralization of the Balkhash metallogenic belt occurred during 315.9 Ma to 289.3 Ma. The formation of CueWeMo deposits can be divided into two stages: the first, of porphyry CueMo deposits occurring at w316 Ma; and latter, quartz vein-greisen WeMo deposits at w298 Ma; (3) It is inferred on the basis of model and isochron ages of molybdenites that the formation of metallogenic granite-porphyry and pegmatite is roughly synchronous with that of the corresponding deposits in the regiondall being of Late Carboniferous age, the products of late Hercynian tectono-magmatism; (4) A correlation of the Balkhash deposit with EasteWest Junggar and the East Tianshan porphyry Cu ore belts in China shows that large-scale metallogenesis of porphyry CueMo in the Central Asian metallogenic domain was concentrated between 323 and 310 Ma, whereas that of quartz vein-greisen WeMo(Sn) deposits occurred between 297.9 Ma and 296.3 Ma, the products of late Hercynian tectono-magmatism.

Acknowledgments We wish to express here our heartfelt gratitude to the National No. 305 Project Office of Xinjiang Uygur Autonomous Region of China and the Satpaev Institute of Geological Sciences of Kazakhstan for their support and assistance in the field investigation and sampling for this study. Financial support supplied by the key project in the National Science & Technology Pillar Program in the Eleventh Five-year Plan Period (No. 2007BAB25B02) is gratefully acknowledged. We thank the reviewer for his criticism, which has greatly benefitted the manuscript.

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