Short communicationDiscovery and geological significance of REE deposit in southern Songxian County, East Qinling, China
Graphical abstract
Introduction
The development of emerging industries has prompted the wide application of rare earth elements (REEs) in high-tech fields, making them an important strategic resource (Wang et al., 2013, Zhang et al., 2016, Liu et al., 2017, Moura et al., 2019, Taylor et al., 2019). China owns abundant rare earth resources and is the largest supplier of REEs in the world (Liu and Hou, 2017). Bayan Obo and Maoniuping are the world-class REE deposits (Xie et al., 2016), and these two REE Deposits are closely related to alkaline rock. Alkaline rocks are the most important sources of REEs, hosting the majority of known REE deposits in the world (Chakhmouradian and Zaitsev, 2012).
Alkaline rocks include an expansive category of igneous rocks. Using a broad definition, alkaline rocks are deficient in SiO2 relative to Na2O, K2O, and CaO (Winter, 2001). Alkaline rocks are classified as alkaline or peralkaline based on their chemical composition. Alkaline rocks primarily originate from the upper mantle and are formed in the extensional environment (Liu et al., 2020). Most of the world’s REEs are currently supplied from ore deposits related to alkaline rocks (Xie et al., 2016). REE mineralization related to alkaline rocks often involves multiple stages and a complex combination of processes, such as, magma crystal fractionation and/or hydrothermal activities coupled with chloride-rich fluid immiscibility (Morgenstern et al., 2019). REEs cannot easily enter into the crystal structure of common rock-forming minerals and become gradually enriched in magma during the crystallization process. REE minerals are closely related to the crystallization history of magma, especially the late stage of magma (Sheard et al., 2012). Furthermore, REE mineralization is usually related to hydrothermal alteration, especially to high fractionation of late magmatic fluids (Vrublevskii et al., 2019, Decree et al., 2015).
East Qinling Orogenic Belt of West Henan Province, located in the eastern part of the Qinling Orogenic Belt, is an attractive polymetal minerogenic belt (Mo, Au, Ag, Pb and Zn) to researchers, as well as its production of alkaline rocks (Zhang et al., 2003, Mao et al., 2005, Mao et al., 2008, Du et al., 2007, Li et al., 2008, Wu et al., 2014, Jin et al., 2019, Tang et al., 2019a). Alkaline rocks of East Qinling Orogenic Belt are mainly distributed in Lushi-Luanchuan and southern Songxian County areas. The alkaline rocks in Lushi-Luanchuan area are small stocks, and their emplacement age belongs to Neoproterozoic Era. The alkaline rocks in southern Songxian County are concentrated in the Zhifang-Huangzhuang area, including 32 alkaline rock masses, of which the representative ones are Mogou, Longtou, Langaogou, Jiaogou and Wusanggou rock bodies. The majority of rock types in southern Songxian County are syenite and syenite porphyry, and the stocks intruded in Mesoproterozoic Xiong’er Group at Triassic period.
To the best of our knowledge, despite the large amount of alkaline rocks produced in East Qinling Orogenic Belt, REE mineralization has not been reported in this area until now. In this work, one REE ore occurance is firstly discovered in an alkaline complex from southern Songxian County in East Qinling. The REE content of ore vein reaches the industrial cut-off grade according to standards of China, The Manual of Industrial Requirements for Mineral Resources (Editorial board of the Manual of Industrial Requirements for Mineral Resources, 2012). This study provides a preliminary report about the geological and geochemical characteristics of the REE mineral occurrence. Geological and geochemical characteristics of rock mass and ore body, zircon geochronology of syenite porphyry, mineralogical research of the ores, and REE content of ores and minerals were investigated in detail. This research aims to achieve breakthroughs in REE deposit exploration of the local area.
Section snippets
Regional geology
The study area is at the north of the Shangdan Suture Zone and located in the Xiaoshan-Lushan Structural Province in the Huaxiong Block, which belongs to the southern margin of the North China Craton (Fig. 1) (Cui et al., 2010, He et al., 2010, Li et al., 2013). This area mainly consists of hypometamorphic rocks from Archean Taihua Group, volcanic rocks from Mesoproterozoic Xiong’er Group, and Mesozoic/Cenozoic clastic rocks (Zhao et al., 2004, Xu et al., 2009, Wang et al., 2010). The major
Sampling
In this study, one geochronological sample was collected from the central facies of the quartz syenite stock. Five syenite samples were selected from the central facies of the rock mass. Five ore samples were picked from the inner ore veins at the outcrops. Two single-mineral samples (LGK-1 and LGK-2) were picked from ore samples LG-1 and LG-2, respectively.
Zircon U-Pb analysis
Zircons were separated from the sample using standard heavy liquid separation techniques (Wang et al., 2018), hand-picked under a binocular
Zircon U-Pb ages
Zircon cathodoluminescence images (Fig. 6) and polarizing microscopy suggest that most of the zircon grains are colorless to pale yellowish-brown, with a major axial length of 200 to 400 μm and a minor axial length of 180 to 300 μm, and a length-to-width ratio of approximately 1:1 to 2:1. Cathodoluminescence images also suggest that most zircons show clear magmatic oscillatory zoning (Kosler et al., 2004, McCombs et al., 2004).
U-Th-Pb isotope analyses were performed on the zircons from the
Emplacement ages of syenite porphyry
Our study has shown that the emplacement age of the syenite porphyry is 236.3 ± 2.6 Ma, which is in early Middle Triassic, indicating that these rocks are products of magmatism during the Indosinian period. Previous studies (Lu et al., 2013, Liang and Lu, 2018) suggested that the alkaline rock body in Mogou have a similar emplacement age of 245.5 ± 8.0 Ma, indicating that they were all formed during the Indosinian period. The results of Hf isotope analyses conducted on the zircons of the
Conclusions
The major conclusions of the study are summarized as follows:
1) The REE2O3 content of the REE deposit reaches the cut-off grade. The dark mineral of the REE ores discovered in southern Songxian County is aegirine. The aegirine is characterized with high REE content and LREE enrichment. This REE deposit belongs to the cerium-family REE deposits associated with alkaline magmatism. The formation of this REE deposit is closely associated with alkaline intrusive rocks, which deserves further
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (41773024, 21507145) and the Natural Science Foundation of Henan Province (172102210331). Discussions with Xucheng Pang significantly improved this paper. The laboratory assistance of Dangyu Song are greatly appreciated.
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