Re–Os dating of the Kalatongke Cu–Ni deposit, Altay Shan, NW China, and resulting geodynamic implications
Introduction
Development of NTIMS and HR-ICP-MS analytical techniques has allowed the Re–Os isotopic system to serve as a tool for the study of ore-forming ages of magmatic sulfides in many of the world's largest Cu–Ni–PGE deposits (Table 1; Foster et al., 1996, Shirey and Walker, 1998, Lambert et al., 1998, Lambert et al., 2000, Mao and Du, 2002, Mao et al., 2002). The method has been applied to the Noril'sk Camp (Russia; Walker et al., 1994), the Stillwater Complex (Montana, USA; Lambert et al., 1989, Martin, 1989), the Pechenga Complex (Kola Peninsula, NW Russia; Walker et al., 1994, Walker et al., 1997), the Sudbury Complex (Ontario, Canada; Hart and Kinloch, 1989), Voisey's Bay (Labrador, Canada; Lambert et al., 1999) and Sally Malay (Western Australia; Sproule et al., 1999). Several deposits in China (Table 1) have also been investigated using the Re–Os isotopic system, including Baotan (Guangxi; Mao and Du, 2002), Jianchaling (Shanxi; Wang et al., 2005), Jinchuan (Gansu; Zhang et al., 2004) and Huangshandong (Xinjiang; Mao et al., 2002).
The Kalatongke area, Xinjiang, NW China has received renewed exploration interest for biotite–hornblende–olivine–norite-associated Cu–Ni sulfide deposits (Yan et al., 2003). The Kalatongke mafic–ultramafic intrusion hosts a concentration of massive Cu–Ni-bearing sulfide bodies in the postulated feeder conduit. Indicated and inferred resources in the Kalatongke ore deposit included 1,326,600 t Cu, 777,400 t Ni, more than 4000 t Co, 2.5 t Pt, and 3.4 t Pd (Yan et al., 2003, Gong et al., 2005).
In the present study, we use Re–Os dating of Cu–Ni-bearing sulfide ores from the Kalatongke intrusion to further constrain the timing of mineralization. In addition, the source of the metals is identified and an attempt is made to understand the geodynamic environment that controlled ore formation. An understanding of these mineralizing processes and geodynamic environment has important implications for Cu–Ni exploration programs in Northern Xinjiang.
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Geological setting
The Altay Shan, a part of the central Asian Paleozoic collisional orogenic collage, has been studied by many researchers (Travin et al., 2001, Chen and Jahn, 2002, Laurent-Charvet et al., 2002, Windley et al., 2002, Wang et al., 2002, Laurent-Charvet et al., 2003, Goldfarb et al., 2003, Xiao et al., 2004). The Middle Paleozoic evolution of the Altay was characterized by both north- and south-directed plate subduction, with terrane accretion on both margins of the Paleozoic Asian Ocean (Goldfarb
Local stratigraphy
The Kalatongke Cu–Ni sulfide deposit is situated 15 km south of the Irtysh Fault. The Middle Devonian Yundukala Formation, the Lower Carboniferous Nanmingshui Formation, Tertiary and Quaternary systems outcrop in the deposit area (Fig. 2). The Kalatongke magmatic Cu–Ni deposit is hosted in the Lower Carboniferous Nanmingshui Formation of the Perkin-Ertai terrane (Windley et al., 2002, Goldfarb et al., 2003, Yan et al., 2003).
The Middle Devonian Yundukala Formation is located in the southwestern
Characteristics of the Kalatongke orebodies
There are nine orebodies in the Kalatongke deposit mine. Among these the No. 1 orebody is the largest; it is 695 m in length along strike, with a width between 90 and 280 m, and a depth of 400 m. Cu–Ni sulfide ores are hosted in the biotite–hornblende–olivine–norite and biotite–hornblende–norite units at depths ranging from 550 to 1000 m below surface (Fig. 3, Fig. 4). The orebody is oxidized from the surface to depths of more than 40 m. The orientation of the orebody is almost consistent with
Sampling and analytical methods
We collected 6 samples from the Kalatongke deposit for Re–Os dating (Fig. 3), all from fresh open-pit mining faces. Two groups were collected at the 300 m depth in the eastern part of orebody No. 1; sampling locations are marked on Fig. 3. To make the samples more representative, we chose samples of different ore types, including disseminated (samples KLTK-9, -5 and -6) and massive Cu–Ni ores (samples KLTK-1, -5 and -8). Variable chalcopyrite–pyrrhotite–pentlandite ratios in the samples provide
Major and trace element geochemistry
Representative whole-rock major and trace element analyses are given in Table 2. Rocks of the Kalatongke intrusion fall into a relatively narrow compositional range (44 to 59 wt.% SiO2, 5 to 17 wt.% Al2O3, 5 to 17 wt.% Fe2O3T and 2 to 8 wt.% CaO; Table 2). The samples have variable REE contents, reflecting different abundances and compositions of intercumulus liquids, but they are uniformly enriched in LREE relative to HREE, with (La/Yb)N and (La/Sm)N ratios of 6.1 to 9.3 and 1.3 to 2.0,
Initial 187Os/188Os and source of the ore-forming metals
The Re–Os isotope system has been recognized as a geochemical tool, not only for directly dating mineralization, but also as a tracer for metal source area. It can be a highly sensitive monitor of the extent of crustal involvement during ore genesis processes (Foster et al., 1996, Ruiz and Mathur, 1999, Mao et al., 2002, Zhang et al., 2005). 187Os/188Os ratios are higher for the crust (0.2 to 10), compared to mantle values (0.11 to 0.15; Meisel et al., 2001), and can thus be used to readily
Conclusions
The Kalatongke deposit is a magmatic Cu–Ni sulfide ore deposit in the Chinese Altay Shan. We have obtained an Re–Os isochron age of 305 ± 15 Ma for the deposit. The initial187Os/188Os ratio of 0.352 ± 0.044, and γOs values ranging from 171.60 to 193.49 for the Kalatongke sulfide ores reflect only a minor crustal contribution into the metal-rich magmatic system. Geochemical data suggest the metal-bearing intrusive complexes are mainly products of an MORB-type mantle, although with some crustal
Acknowledgments
This paper benefited greatly from the many achievements of the Xinjiang Bureau of Geology and Mineral resources and the National 305 Project. We are grateful to R.J. Goldfarb for constructive discussions and review on an early version of the manuscript. We are also indebted to Jingwen Mao, Bin Cui, Kezhang Qin, Zhaochong Zhang, Yitian Wang, Jinyi Li, Tianlin Ma, and Liangchang Zhang for discussions; many ideas in this paper having been initiated and rectified during these discussions. In
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