A tectono-genetic model for porphyry–skarn–stratabound Cu–Au–Mo–Fe and magnetite–apatite deposits along the Middle–Lower Yangtze River Valley, Eastern China
Highlights
► Two episodes of magmatism and mineralization in the YRB. ► Porphyry-skarn Cu-Au-Mo system dated at 148–135 Ma is related to the tear up of the Izanagi plate, which oblique subducted beneath Eurasian continent. ► Magnetite-apatite ore system dated at 135–123 Ma occurred within the pull apart volcanic basins of regional striking-slip fault zones.
Introduction
The Middle–Lower Yangtze (or Changjiang) River Valley (hereinafter referred to as YRB) is one of the most important metallogenic belts of igneous rock-related Cu–Fe–Au–Mo ore deposits in East China. It extends from Wuhan (Hubei Province) in the west to Zhenjiang (Jiangsu Province) in the east. It comprises several ore clusters along the Yangtze River valley, such as Edong (southeastern Hubei province), Jiurui (Jiujiang–Ruichang), Anqing–Guichi, Tongling, Luzong, Ningwu and Ningzhen (Fig. 1). There are two major types of ore deposits: 1) porphyry–skarn–stratabound Cu–Au–Mo–Fe; and 2) magnetite–apatite (or magnetite porphyry) (Mao et al., 2006, Pan and Dong, 1999, Zhai et al., 1996). The former are related to high-K calc-alkaline granitoids, comprising diorite, quartz diorite, and granodiorite. The latter belong to the shoshonite series consisting of pyroxene diorite porphyry, diorite porphyry, syenitic granite porphyry and their corresponding eruptive rocks. The high-K calc-alkaline granitoids were classed as I-type (Pei and Hong, 1995) or magnetite-series granitoids, (Ishihara, 1977), respectively. Nevertheless, these have recently been recognized to be of adakitic affinity (Wang et al., 2004a, Wang et al., 2004b, Wang et al., 2006a, Wang et al., 2007, Xu et al., 2002, Zhang et al., 2001a). In addition to the above-mentioned mineral systems, there are also a few, albeit uneconomic, Cu–Au hydrothermal veins associated with the A-type granitoids, consisting of syenite, quartz syenite, alkaline granite and their corresponding eruptive rocks (Fan et al., 2008, Tang et al., 1998, Zhang et al., 1988).
The ore deposits in the YRB have been extensively studied in the past few decades. Chang et al., 1991, Zhai et al., 1992a, Zhai et al., 1992b, Tang et al., 1998 and Pan and Dong (1999) provided detailed descriptions of the regional geology and characteristics of the ore deposits. The magnetite–apatite deposits in both Ningwu and Luzong Cretaceous basins are considered to be typical continental subvolcanic hydrothermal systems or porphyry-type iron deposits (Ningwu Research Group, 1978, Zhang, 1986), similar to the Kiruna type (Yu and Mao, 2002a, Yu et al., this issue). The genesis of the porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits has been debated for a long time. These ore systems are thought to be typical skarns, since most orebodies are hosted in both calc- and magnesian-silicate rocks formed by metasomatism of carbonate rocks and hornfels (Guo, 1957, Guo, 1963, Guo, 1957, Hu et al., 1979, Lai et al., 2007, Li et al., 2008, Liu et al., 1988, Mao et al., 2006, Mao et al., 2009, Pan and Dong, 1999, Tang et al., 1998, Wu et al., 2003, Xie et al., 2007, Zhao et al., 1999, Zhao et al., 1999, Zhou et al., 2007a, Zhou et al., 2008a). However, because of the layer or layer-likely shape of the orebodies, their laminated structures and the presence of framboidal pyrite in some localities, other workers (Gu, 1984, Gu and Xu, 1986, Gu et al., 2003, Gu et al., 2007, Ji et al., 1990, Yan and Yuan, 1977, Yang et al., 2004, Yue et al., 1993, Xu and Zhou, 2001) considered these ore systems to be Devonian volcanogenic massive sulfide (VMS)-type deposits, later overprinted by Mesozoic granite-related mineralization.
The South China Block, comprising the Cathaysian block and the Yangtze craton (Fig. 1, map inset), is well-endowed with orebodies derived from granite-related W–Sn–Mo–Bi–Sb mineralization of Mesozoic age. However, the east–west trending YRB on the northern margin of the Yangtze craton is dominated by Mesozoic granite-related Cu–Fe–Au–Mo mineralization. The contrasting tectonic–metallogenic settings for these mineral systems remain an unsolved problem. Works from the 1980s implied that the YRB is genetically linked to the subduction of the Pacific plate in the Jurassic and Cretaceous (e.g. Deng et al., 1992, Wu et al., 1982). Chang et al., 1991, Zhai et al., 1992a and Tang et al. (1998). Deng et al. (1999) suggested that the YRB is located over an elongated uplift zone with several intra-continental fault blocks and that mineralization is associated with the magmatism derived from both mantle and lower crust. Utilizing precise molybdenite Re/Os dating and sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating of the host rocks, along with geological observations, Mao et al. (2006) proposed a two-stage tectonic–metallogenic model. It consists of porphyry–skarn–stratabound Cu–Fe–Au–Mo and magnetite–apatite deposits. They are related to the subduction of the paleo-Pacific plate and subsequent lithospheric extension. Zhang et al., 2001a, Zhang et al., 2001b, Xu et al., 2002 and Wang et al., 2004a, Wang et al., 2004b, Wang et al., 2006a, Wang et al., 2007 recognized the porphyry–skarn Cu–Au–Mo–Fe mineralization-related granitoids in the YRB as adakitic and proposed that they are derived by partial melting of thickened or delaminated lower crust. Based on other work, Hou et al. (2007) suggested a thick delaminated lower crust model to explain the magmatism (150–100 Ma) and associated mineralization, whereas the 140–125 Ma magmatism and associated mineralization, was attributed by Ling et al. (2009) to ridge subduction in the Cretaceous.
In this paper we attempt to establish models of ore genesis and related tectonic settings for the porphyry–skarn–stratabound and magnetite–apatite ore systems in the YRB. The work is based on detailed field investigation and the synthesis of published geological, geochemical and geophysical data. Particular emphasis has been placed on the wealth of newly available data from precise geochronological and geochemical investigations of both ore deposits and host rocks.
Section snippets
Geological setting
The YRB is located on the northern margin of the Yangtze craton, south of the North China craton and Qinling–Dabie orogenic belt. Several large strike-slip fault systems characterize the border zone between the Yangtze and North China cratons (Fig. 1). The stratigraphic sequence in the YRB consists of three units: 1) pre-Sinian (Sinian = Late Proterozoic) metamorphic basement, 2) Sinian to Early Triassic submarine sedimentary cover and 3) Middle Triassic to Cretaceous terrigenous clastic and
Petrology and petrochemistry of the Mesozoic igneous rocks
There are three types of granitoids along the YRB (Zhou et al., 2008b). 1) High-K calc-alkaline intermediate-felsic (or high-K calc-alkaline) granitoids, comprising gabbro, diorite, quartz diorite, and granodiorite, which belong to I-type (Pei and Hong, 1995, Xie et al., 2008a, Zhou et al., 2007a) or magnetite-series granitoids (Ishihara, 1977). Zhang et al., 2001a, Zhang et al., 2004, Xu et al., 2002, Wang et al., 2001, Wang et al., 2004a, Wang et al., 2004b, Wang et al., 2006a, Wang et al.,
Principal characteristics of the ore deposits
As mentioned above, there are three types of mineral deposits, i.e. porphyry–skarn–stratabound Cu–Au–Mo–Fe (including skarn Fe–Cu deposits), magnetite–apatite (or magnetite porphyry) (Chang et al., 1991, Mao et al., 2006, Pan and Dong, 1999, Zhai et al., 1996) and hydrothermal vein Au (Fan et al., 2008, Tang et al., 1998). The first two are economically important, whereas presently the latter are not economically viable. In the uplift areas the porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits
Two igneous and ore-forming events
The Mesozoic igneous activity and ore-forming processes in the YRB display a break at ca. 135 Ma. This has been observed and proved by Mao et al., 2006, Zhao et al., 2006, Xie et al., 2007, Zhou et al., 2008a, Li et al., 2010. The 148–135 Ma porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits are genetically associated with 156–137 Ma high-K calc-alkaline granitoids, whereas the 134.9–122.9 Ma magnetite–apatite deposits are genetically related to 135–123 Ma igneous rocks of shoshonite affinity (Fig. 2).
Conclusions
Following detailed field investigations and a thorough review of the published data, we recognize two episodes of magmatism and mineralization in the YRB: 1) 156–137 Ma high-K calc-alkaline granitoids, associated with 148–135 Ma porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits and 2) 135–123 Ma shoshonite rocks, associated with 134.9–122.9 Ma magnetite–apatite deposits. A-type granitoids and phonolites have a small range of ages (126.5 to 124.8 Ma). They are temporally, spatially and genetically
Acknowledgments
This work was jointly supported by Projects 2007CB411405 and 2007CB411407 of the State Key Fundamental Program, Geological Survey Project (1212010634001) and the National Natural Science Foundation of China (No. 40434011). The authors wish to thank Prof Taofa Zhou, his team and local geologists from the various mines visited for providing invaluable assistance and constructive discussions during our field investigations. We are grateful to Profs Li Jianwei and Lin Jinwen and an anonymous
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