By Sergey M. Rodionov1, Alexander A. Obolenskiy2
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Langshan-Bayan Obo Metallogenic Belt ofSedimentary Exhalative Pb-Zn (SEDEX) andPolygenetic REE-Fe-Nb Deposits(Belt LB) (Northwestern and North-Central China)This Mesoproterozoic metallogenic belt occurs in the central part of Inner Mongolia, along the Yinshan Mountains. The belt is 600 km long and 50 km wide, strikes northeast in the western part, and changes to east-west strike in the eastern part. The belt is hosted in the Early Mesoproterozoic Zhangbei-Bayan Obo-Langshan rift-related metasedimentary and metavolcanic rocks deposit onthe Sino-Korean Craton. The sedimentary exhalative Pb-Zn (SEDEX) and Pb-Zn-Cu deposits in the belt are large to superlarge, and the Bayan Obo Fe-Nb-REE deposit is world class. The stratigraphic horizons hosting SEDEX deposits are in the Mesoproterozoic Zhartaishan and Agulugou Formations though the horizon varies for different SEDEX deposits (Xu Guizhong and others, 1998). The Bayan Obo Fe-Nb-REE deposit is hosted in the 8th of 9 members in the Mesoproterozoic Bayan Obo Group. The significant deposits in are at Bayan Obo and Hugeqi. Bayan Obo Polygenic REE-Fe-Nb Deposit DepositThis deposit (Lin Chuanxian and others, 1994; Lin, Chuanxian and others, 1994; Tu Guangzhi, 1996; Qiao, Xiufu and others 1997) occurs in am east-west trending Mesoproterozoic rift zone along the northern margin of Sino-Korean Craton. The mining district containing the deposit contains several ore bodies that occur in a zone that is about 18 km long along an east-west trend and 5 km wide. Host strata are quartzite, slate, limestone, and dolomite that is main host rock. The bodies are stratiform and lenticular, with masses, bands, layers, and veins, and disseminations. Based on mineralogy, nine types of ores are identified about sixty Nb, REE, Ti, Zr, Nb, and Fe minerals including 19 new minerals such as Huanghoite and others. Besides clear features of hot water sedimentation, the deposit also exhibits Mg, Fe, Na and F metasomatism. Sm-Nd monazite isochron age for bastnaesite and riebeckite is 1200 to 1300 Ma, whereas Th-Pb and Sm-Nd age of Ba-REE-F carbonates and aeschynite is 474 to 402 Ma. Recent years Qiao Xiufu and others (1997) suggest that some host strata are early Paleozoic. The deposit is superlarge with reserves of 40.1 million tonnes with average grade of 3-5.4% REE; Reserves of more than 1 million tonnes Nb2O5 have an average grade of 0.1-0.14% Nb2O5. Huogeqi Sedimentary Exhalative Pb-Zn (SEDEX) DepositThis stratiform deposit (Ge, Chaohua and others, 1994) occurs in the Langshan Mountains and consists of stratiform bodies hosted in phyllite, schist, and quartzite of the Proterzoic Langshan group that has a Rb-Sr isotopic age of 1100 Ma. Ore minerals are mainly chalcopyrite, pyrite, pyrrhotite, magnetite, galena, and sphalerite, with small amounts of arsenopyrite and hematite. Wall rocks are altered to silica, diopsie-grunerite, biotite, sericite, and chlorite. The deposit is large with reserves of 0.973 tonnes Pb, 0.782 tonnes Zn, 0.711 million tonnes Cu. Average grades of Pb, Zn, and Cu are 1.44%, 1.46%, 1.35%, respectively. Origin and Tectonic Controls forLangshan-Bayan Obo Metallogenic BeltThe Bayan Obo deposit interpreted as a SEDEX deposit related to a carbonatite magma and associated hydrothermal activity. The belt hosted in a Mesoproterozoic overlap sedimentary assemblage deposit formed in the Zhangbei-Bayan Obo-Langshan rift along the passive continental margin of the Sino-Korean Craton. The Early Mesoproterozoic overlap assemblage hosting the belt in the Yinshan Archean terrane consists of: (1) metasedimentary schist, biotite gneiss, quartzite, marble, (2) metaconglomerate, quartzite, stromatolite-bearing crystalline limestone, phyllite, slate, mica schist, actinolite schist, and minor metamorphosed intermediate and siliceous volcanic rock of the Zhartai Group with an age of 1500 to 1600 Ma; and (3) Phyllite, slate, quartzite, meta-sandstone, and dolomite of the Bayan Obo Group with an age 1350 to 1650 Ma. Some authors interpret the assemblage as the Mesoproterozoic Langshan-Zhartaishan basin that formed along the northwestern margin of North China Plate (Xu Guizhong and others, 1998). The world class Bayan Obo Fe-Nb-REE deposit is a non-conventional super-large of deposit (Tu Guangzhi, 1998) is unique in the world. The origin is still debated (Chao ECT and others, 1992, Tu Guangzhi, 1998). Tu Guangzhi (1998) suggested that Bayan Obo deposit is a SEDEX deposit related to the carbonatite magma and associated hydrothermal activity. Various studies on the Bayan Obo deposit focus on the syngenetic nature of igneous carbonatite and the epigenetic replacement of the sedimentary dolomite. These two types processes are not strictly exclusive and both may be part of a SEDEX deposit model. REFERENCES: Chao Ect and others, 1992.Shi Lindao and others, 1994; Tu Guangzhi, 1998; Xu Guizhong and others, 1998; Xu Guizhong and others, 1998. Wuenduermiao Metallogenic Belt ofVolcanogenic-Sedimentary Fe Deposits(Belt WD) (North-Central China)This Mesoproterozoic through early Neoproterozoic metallogenic belt occurs in the eastern Inner Mongolia and is hosted in the Mesoproterozoic through Middle Ordovician Wenduermiao accretionary wedge terrane. The belt extends east-west, is more than 80 km long and 20 km wide. The significant deposit is at Wundurmiao. Wuenduermiao Volcanogenic-Sedimentary Fe DepositThis deposit (Chen Qi and others, 1994) consists of several stratiform deposits of banded magnetite hematite quartzite, and hematite jasper silexite. Fe oxide minerals are very fine-grained. The host rocks metamorphosed mafic lava, spilite tuff and argillite that are strongly folded. A Sm-Nb isotopic age for host rocks in the deposit is 1500 to 850 Ma. The host rocks are part of an ophiolite suite composed of chert, pillow lava, and ultramafic rock. The host rocks are metamorphosed into quartzite, sericite schist, Fe ore, greenschist, and glaucophane schist. The belt is 50 km long and 20 km wide and contains more than ten moderate and small Fe deposits and occurrences. The deposit is medium size with reserves of 120 million tonnes total FE with an average grade of 36.04%Fe (20-57.8% Fe). Origin and Tectonic Controls for the Wenduermiao Metallogenic BeltThe belt is interpreted as forming during Mesoproterozoic volcanism and sedimentation with subsequent metamorphism and deformation occurring during accretion of the Wenduermiao terrane The Wenduermiao terrane hosting the metallogenic belt consists of an ophiolite complex and was interpreted as early Paleozoic. Recent studies suggest the Wenduermiao Group Mesoproterozoic. REFERENCES: Chen Qi and others, 1994. |