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Bayan Obo |
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Inner Mongolia, China |
| Main commodities:
REE Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Bayan Obo iron and REE deposit is located ~200 km NW of Hohot in Inner Mongolia, China, and is the world's largest known and most significant Rare Earth Element source (#Location: 41° 47' 58"N, 109° 58' 30"E).
It is a large REE-iron deposit containing more than 1.5 Gt @ 35% Fe ore and 48 Mt @ 6% Rare Earth Oxides.
The deposit lies within a sequence of late Palaeoproterozoic to early Mesoproterozoic sedimentary rocks deposited within an intracratonic rift basin within the larger Columbia supercontinent. This basin was formed in response to extension during the global breakup of Columbia, starting at ~1.8 Ga and culminating at
~ 1.2 Ga (Zhao et al., 2003). However, this section of the supercontinent remained intact and became part of the interior of the succeeding Rodinia supercontinent. Intrusion of a series of carbonatite dykes attended the extension related to the basin development. During the breakup of Rodinia at ~830 Ma, the deposit was located near the northern margin of the fragment that became the North China Craton. During the Palaeozoic, volcanic arcs were accreted to the northern margin of the North China Craton. By the late Palaeozoic, the craton and the attached terranes on its northern margin approached the Central Asian Orogenic belt to the north, with attendent subduction below the craton from the north, accompanied by widespread Permian magmatism. Collision and accretion occurred in the early Triassic along the Solonker suture, which is ~100 km north of Bayan Obo. This collision was followed by a period of thrusting and imbrication during the Jurassic as compression continued resulting in a thrust and fault complex.
Palaeoproterozoic basement in the deposit area comprises, east-west-trending ~2350 Ma migmatite, granulite, gneisses and schists of the Wu Tai group. The Mesoproterozoic Bayan Obo group, which unconformably overlies the basement, is composed of coarse- to medium-grained clastic and carbonate rocks. Cambrian-Ordovician sediments are locally distributed in the areas south and east to the Bayan Obo area. Carboniferous and Permian continental coal-bearing
clastic sedimentary sequences are variable in thickness and distributed further to the south on the basement of the craton. In the fold belt to the north, several Precambrian terrains (mainly composed of quartzite, amphibolite, gneisses and marbles) are surrounded by thick Palaeozoic marine volcanic-sedimentary packages, which are dominated by basaltic and dacitic lavas, tuffs, pyroclastic and sedimentary rocks.
The Bayan Obo group is considered to be stratigraphically equivalent to the Chartai group, which was deposited on the North China Craton between 1650 to 1350 Ma; Ren et al., 1987; Qiao et al., 1991).
The Bayan Obo ore bodies are hosted within the late Palaeo- to early Mesoproterozoic Bayan Obo group, which is divided into 18 horizons with the lower 9 present in the Bayan Obo area (Drew et al., 1990; Bai et al., 1996). The Bayan Obo Group sequence begins with the H1 basal conglomerate, followed by the H2-H7 horizons, a sequence of quartzites, carbonaceous slates, sandstones, limestones and siltstones. The ore bodies are hosted in the H8 dolomite marble, and to a much lesser extent in the H9 slates and biotites schists.
The H8 dolomitic marble occurs as a spindle-shaped concordant stratabound body, and extends 18 km from east to west, with a width of tens to a 1000 metres. In the orefield the H8 marble has a fine- to medium-grained crystalline texture, massive and banded structure, and consists mainly of dolomite and calcite, together with abundant feldspar, quartz, Na-tremolite, magnesio-arfvedsonite, phlogopite, apatite, fluorite and baryte. It hosts the Main, East and West orebodies, and is pervasively REE- and Nb-mineralised itself. Away from the Bayan Obo orefield, the H8 carbonate rocks appear to be normal limestone. The H9 unit is dominated by feldspar rock that has light-coloured and dark-coloured varieties, and forms the hanging wall of the iron orebodies.
Petrographic studies by Chen et al. (2020) showed that the H8 dolomitic marble can be subdivided into:
• Coarse-grained marble which predominantly comprises 85 vol.% dolomite with a grain size of >200 µm, with mosaic-textured twinned dolomite that has triple junctions. Euhedral to subhedral granular monazite is the most common accessory mineral, the bulk of which are disseminated along dolomite grain boundaries or fracture forming veins, although some are included within dolomite crystals. Apatite was the most abundant accessory mineral in one of the samples studied, and contains monazite inclusions. Magnetite is also common in coarse grained marble. Scarce pyrite, siderite, fluorite, bastnäsite and quartz occur as late-stage hydrothermal minerals, and where present usually occur as fillings along fractures.
• Fine-grained marble is primarily composed of <200 µm dolomite and fluorite. The principal REE-bearing mineral is bastnäsite, either disseminated along dolomite grain boundaries or forming veinlets or aggregates with fluorite. Monazite is rare, with only a few anhedral crystals found intergrown with fluorite. Other accessory minerals include barite, magnetite and phlogopite.
• Heterogeneous grain-size marble which occurs as both both coarse- and fine-grained dolomite occurring together, with coarse dolomite typically embedded within a matrix of fine-grained dolomite. Fluorite and bastnäsite are commonly found between coarse- and fine-grained dolomite, with bastnäsite being the dominant REE mineral. Other accessory minerals include fluorite, apatite, monazite, quartz and barite.
The earliest magmatic activity in the immediate region of Bayan Obo was the intrusion of a group of carbonatite dykes described by Le Bas et al. (1992), Zhang et al. (1994) and Tao et al. (1998). At least 28 carbonatite dykes have been recognised, ranging from 0.8 to 2.6 m wide. The majority cut the Archaen Wu Tai gneiss to the north of the ore bodies, but others have been reported from within the Bayan Obo group sedimentary rocks. They have all been affected by subsequent deformation and in some cases the dyke margins are intricately folded (Le Bas et al., 1992). The majority of the dykes are composed of calcite carbonatite (Le Bas et al., 1992; Zhang et al., 1994) although dolomite and calcite-dolomite carbonatites have also been recognised (Tao et al., 1998). Other mineral constituents typically include apatite, magnetite, quartz, amphibole, K feldspar and biotite. Secondary monazite mineralisation occurs in some dykes (Le Bas et al., 1992). These carbonatites have been dated at 1426±40 Ma (Sm–Nd mineral age) and of 1350±149 Ma (monazite age; Nakai et al., 1989).
The dykes are surrounded by fenite haloes, composed of aegirine, sodic amphibole, albite and K feldspar. Several gabbros and alkalic gabbros outcrop both to north and to south of the ore bodies in the Bayan Obo area, occurring as stocks and dykes with dimensions ranging from tens of square meters to 2000 m long by 50 to 60 m wide. These basic bodies intrude into the Bayan Obo group and are intruded by, or occur as xenoliths enclosed by, Permian granite. One of these gabbroic intrusion has been dated at 316 Ma using Rb-Sr isochron techniques (Hu et al., 1988; Bai et al., 1996).
A large volume of Permian granitoid rocks outcrops in the Bayan Obo area, mainly to the south of the ore bodies. These include diorite, granodiorite, biotite granite, and leucogranite (Drew et al., 1990) and post-date the main mineralisation. Caledonian granite magmatism is not exposed in the vicinity of the ore bodies, but Chao et al. (1997) drew attention to the Hejao plutonic rocks which are exposed ~50 km south of Bayan Obo, where they comprise granodiorite, monzonite and quartz-biotite diorite, and have been dated to 455±3.4 Ma. They are inferred to be either A-type or S-type granitoids.
The earliest stages of REE mineralisation at Bayan Obo occurs as stringers of monazite, associated with ferroan dolomite, ankerite and magnetite, occurring in fractures and along grain boundaries in relatively unaltered dolomite marble. In the two main ore bodies the disseminated mineralisation has been overprinted by later stages forming the main banded ores. The banding appears to be a metasomatic effect that has been enhanced by deformation during the Permian tectonism (Drew et al., 1990). Massive, unbanded pods are locally preserved. In the banded ores, monazite, bastnäsite and apatite occur associated with the development of several stages of magnetite and hematite. Major monazite mineralisation preceded the development of bastnäsite, although there are several generations of both. This mineralisation was followed by the widespread replacement of the host dolomite by aegirine and aegirine-augite, followed by fluoritisation (and further REE and Fe mineralisation) of much of the remaining carbonate. Fluoritisation is accompanied by the development of phlogopite and alkali-amphibole in the banded ores and further magnetite and hematite mineralisation.
Variations in the degree of overprinting of the different alteration assemblages produced banded ores of varying mineralogy, defining a broad scale zonation of the two deposits. The banded ores are cut by veins containing assemblages of aegirine-apatite, aegirine-calcite, aegirine-barite and fluorite-apatite-alkali amphibole all with associated REE-fluorocarbonate mineralisation. Ca-REE fluorocarbonates including parisite and unnamed polysomatic minerals of varying Ca:REE ratio occur associated with fluorite alteration. The final stage of REE mineralisation involves barite in veins and vugs, which is associated with parisite, huanghoite (BaREE(CO3)2F) and other Ba-REE fluorocarbonates.
The sequence of Fe-oxide mineralisation is also complex. According to Chao et al. (1997) the earliest phase was the deposition of euhedral magnetite in partly altered marble, which was then replaced by hematite to form martite. This was followed by the deposition of granular hematite, banded magnetite and finally massive magnetite. Further development of hematite may be related to the late stage alteration of the ores.
Nb mineralisation is concentrated in the west ore bodies, where it occurs as disseminated minerals within the H8 dolomite marble (Chao et al., 1997). Nb-rutile is associated with granular hematite, columbite is associated with magnetite in disseminated ores, and aeschynite and pyrochlore occur as coarse grains in veins associated with huanghoite and alkali-amphibole which cut the main and east ore bodies. Pyrochlore also occurs in skarns associated with the Permian granites. Other Nb minerals occur throughout the dolomite marble including fergusonite and fersmite. Sulphide mineralisation is a minor feature of the ore deposits and is dominated by pyrite with subsidiary pyrrhotite, sphalerite, galena and chalcopyrite. It is mostly associated with barite veins or alteration, and hence represents a manifestation of the late stage modification of the deposit. Gold has been reported at Bayan Obo, but only in quartz veins hosted in shear fractures cutting the quartzo-feldspathic metasediments of the Bayan Obo group which are probably unrelated to the main Fe-oxide deposits (Chen et al., 1990).
The deposits have been interpretted to have formed by multistage hydrothermal replacement of marble (e.g., Smith and Wu, 2000). However, a detailed isotopic and geochemical study by Chen et al. (2020) indicated that the whole rock trace element and major element composition of the H8 dolomite are similar to those of mantle-derived carbonatites and clearly distinct compared to that of sedimentary limestone and carbonate phases elsewhere within the Bayan Obo district. They concluded that Sm-Nd isotope results for the entire complex are homogeneous and indicate derivation from the same mantle source, and were not perturbed by subsequent metasomatic activity (Zhu et al., 2015; Song et al., 2018). The coarse-grained marble is interpreted to represents the crystallisation product of a 1.3 to 1.2 Ga Mesoproterozoic carbonatite melt with little open-system alteration (Chen et al., 2020). This carbonatite magma is possibly related to the 1.33 to 1.30 Ga Yanliao large igneous province (Zhang et al., 2017). However, the Mesoproterozoic carbonatite may have been thermally affected by widespread Early Palaeozoic (~440 Ma) metasomatic events, as indicated by the variable Th-Pb ages recorded by monazite within the coarse-grained marble. This metasomatism is interpreted to have imparted the most intense alteration on the fine-grained and heterogeneous-grained marble. Dolomite grains within these two fractions are recrystallised and characterised by radiogenic 208Pb/204Pb and enriched Sr isotope signatures.
Various characteristics have been taken to imply Bayan Obo belongs to the Iron-Oxide Copper-Gold family of deposits, although there are important differences including the absence of significant base metal sulphide mineralisation, no enrichment in U, and the absence of evidence for the involvement of hypersaline brines in ore genesis.
It is the subject of a paper in the monograph: Porter T M (Ed.) "Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective" volume 1, published by PGC Publishing, Adelaide, Australia. The description above is drawn from that paper.
The full Abstract and Reference list from the paper can be displayed by selecting the option offered below.
For detail consult this paper or other reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2012.
Record last updated: 13/7/2015
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Bayan Obo
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Chen, W., Liu, H.-Y., Lu, J., Jiang, S.-Y., Simonetti, A., Xu, C. and Zhang, W., 2020 - The formation of the ore-bearing dolomite marble from the giant Bayan Obo REE-Nb-Fe deposit, Inner Mongolia: insights from micron-scale geochemical data: in Mineralium Deposita v.55, pp. 131-146.
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Deng, M., Xu, C., Song, W., Tang, H., Liu, Y., Zhang, Q., Zhou, Y., Feng, M. and Wei, C., 2017 - REE mineralization in the Bayan Obo deposit, China: Evidence from mineral paragenesis: in Ore Geology Reviews v.91, pp. 100-109.
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Drew L J, Meng Qingrun, Sun Weijun 1990 - The Bayan Obo iron-rare-earth-niobium deposits, Inner Mongolia, China: in Lithos v26 pp 43-65
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Fan, H.-R., Hu, F.-F., Yang, K.-F., Piranjo, F., Liu, X. and Wang, K.-Y., 2014 - Integrated U-Pb and Sm-Nd geochronology for a REE-rich carbonatite dyke at the giant Bayan Obo REE deposit, Northern China: in Ore Geology Reviews v.63, pp. 510-519.
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Fan, H.-R., Xie, Y.-H., Wang K.-Y. and Wilde, S.A., 2004 - Methane-rich fluid inclusions in skarn near the giant REE-Nb-Fe deposit at Bayan Obo, Northern China: in Ore Geology Reviews v25 pp 259-283
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Fan, H.-R., Yang, K.-F., Hu, F.-F., Liu, S. and Wang, K.-Y., 2016 - The giant Bayan Obo REE-Nb-Fe deposit, China: Controversy and ore genesis: in Geoscience Frontiers v.7, pp. 335-344.
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Hu, L., Li, Y.-K., Wu, Z.-J., Bai, Y. and Wang, A.-J., 2019 - Two metasomatic events recorded in apatite from the ore-hosting dolomite marble and implications for genesis of the giant Bayan Obo REE deposit, Inner Mongolia, Northern China: in J. of Asian Earth Sciences v.172, pp, 56-65.
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Huang, X.-W., Zhou, M.-F., Qiu, Y.-Z. and Qi, L., 2015 - In-situ LA-ICP-MS trace elemental analyses of magnetite: The Bayan Obo Fe-REE-Nb deposit, North China: in Ore Geology Reviews v. 65, pp. 884-899.
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Lai, X., Yang, X., Santosh, M., Liu, Y. and Ling, M., 2015 - New data of the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia:Implications for ore genesis: in Precambrian Research v.263, pp. 108-122.
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Lai, X.-D. and Yang, X.-Y., 2013 - Geochemical characteristics of the Bayan Obo giant REE-Nb-Fe deposit: Constraints on its genesis: in J. of South American Earth Sciences v.41, pp. 99-112.
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Lai, X.-D., Yang, X.-Y. and Sun, W.D., 2012 - Geochemical constraints on genesis of dolomite marble in the Bayan Obo REE-Nb-Fe deposit, Inner Mongolia: Implications for REE mineralization: in J. of Asian Earth Sciences v.57, pp. 90-102.
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Lai, X.-D., Yang, X.-Y., Liu, Y. and Yan, Z.-Q., 2016 - Genesis of the Bayan Obo Fe-REE-Nb deposit: Evidences from Pb-Pb age and microanalysis of the H8 Formation in Inner Mongolia, North China Craton: in J. of Asian Earth Sciences v.120, pp. 87-99.
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Li, X.-C., Fan, H.-R., Su, J.-H., Groves, D.I., Yang, K.-F. and Zhao, X.-F., 2024 - Giant Rare Earth Element Accumulation Related to Voluminous, Highly Evolved Carbonatite: A Microanalytical Study of Carbonate Minerals From the Bayan Obo Deposit, China: in Econ. Geol. v.119, pp. 373-393. doi: 10.5382/econgeo.5060.
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Ling, M.-X., Zhang, H., Li, H., Liu, Y,-L., Liu, J., Li, L.-Q., Li, C.-Y., Yang, X.-Y., and Sun, W., 2014 - The Permian-Triassic granitoids in Bayan Obo, North China Craton: A geochemical and geochronological study: in Lithos v.190-191, pp. 430-439.
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Liu, S., Fan, H.-R., Yang, K.-F., Hu, F.-F., Wang, K.-Y., Chen, F.-K., Yang, Y.-H., Yang, Z.-F. and Wang, Q.-W., 2018 - Mesoproterozoic and Paleozoic hydrothermal metasomatism in the giant Bayan Obo REE-Nb-Fe deposit: Constrains from trace elements and Sr-Nd isotope of fluorite and preliminary thermodynamic calculation: in Precambrian Research v.311, pp. 228-246.
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Liu, Y,-L., Fan, H.-R., Yang, K.-F., Hu, F.-F., Rusk, B., Liu, X., Li, X.-C., Yang, Z.-F., Wang, Q.W. and Wang, K.-Y., 2018 - Fenitization in the giant Bayan Obo REE-Nb-Fe deposit: Implication for REE mineralization: in Ore Geology Reviews v.94, pp. 290-309.
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Liu, Y,-L., Ling, M.-X., Williams, I.S., Yang, X.-Y., Wang, C.Y. and Sun, W., 2018 - The formation of the giant Bayan Obo REE-Nb-Fe deposit, North China, Mesoproterozoic carbonatite and overprinted Paleozoic dolomitization: in Ore Geology Reviews v.92, pp. 73-83.
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Lu, J., Chen, W., Ying, Y., Jiang, S. and Zhao, K., 2021 - Apatite texture and trace element chemistry of carbonatite-related REE deposits in China: Implications for petrogenesis: in Lithos v.398-399, 20p. doi.org/10.1016/j.lithos.2021.106276
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Nie, F.-J., Jiang, S.-H., Su, X.-X. and Wang, X.-L., 2002 - Geological features and origin of gold deposits occurring in the Baotou-Bayan Obo district, south-central Inner Mongolia, Peoples Republic of China: in Ore Geology Reviews v21 pp 139-169
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Pirajno, F., 2014 - Intracontinental anorogenic alkaline magmatism and carbonatites, associated mineral systems and the mantle plume connection: in Gondwana Research v.27, pp. 1181-1216.
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Ren, Y., Yang, X., Wang, S. and Ozturk, H., 2019 - Mineralogical and geochemical study of apatite and dolomite from the Bayan Obo giant Fe-REE-Nb deposit in Inner Mongolia: New evidences for genesis: in Ore Geology Reviews v.109, pp. 381-406.
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Smith M and Chengyu W, 2000 - The Geology and Genesis of the Bayan Obo Fe-REE-Nb Deposit: A Review: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective PGC Publishing, Adelaide v.1 pp. 271-281
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Smith M P, 2007 - Metasomatic silicate chemistry at the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China: Contrasting chemistry and evolution of fenitising and mineralising fluids: in Lithos v93 pp 126-148
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Smith M P, Henderson P 1999 - Fluid inclusion constraints on the genesis of the Bayan Obo Fe-REE-Nb deposit: in Stanley, et. al., (Eds), Mineral Deposits: Processes to Processing Balkema, Rotterdam pp 103-106
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Smith M P, Henderson P 2000 - Preliminary fluid inclusion constraints on fluid evolution in the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China: in Econ. Geol. v95 pp 1371-1388
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Smith, M.P., Campbell, L.S. and Kynicky, J., 2015 - A review of the genesis of the world class Bayan Obo Fe-REE-Nb deposits, Inner Mongolia, China: Multistage processes and outstanding questions: in Ore Geology Reviews v.64, pp. 459-476.
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Smith, M.P., Moore, K., Kavecsanszki, D., Finch, A.A., Kynicky, J. and Wall, F., 2016 - From mantle to critical zone: A review of large and giant sized deposits of the rare earth elements: in Geoscience Frontiers v.7, pp. 315-334.
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Sun, J., Zhu, X., Chen, Y. and Fang, N., 2013 - Iron isotopic constraints on the genesis of Bayan Obo ore deposit,Inner Mongolia, China: in Precambrian Research v.235, pp. 88-106.
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Wang, K., Fang, A., Zhang, J., Yu, L., Dong, C., Zan, J.-F., Hao, M. and Hu, F., 2019 - Genetic relationship between fenitized ores and hosting dolomite carbonatite of the Bayan Obo REE deposit, Inner Mongolia, China: in J. of Asian Earth Sciences v.174, pp. 189-204.
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Wang, Z.-Y., Fan, H.-R., Zhou, L., Yang, K.-F. and She, H.-D., 2020 - Carbonatite-Related REE Deposits: An Overview: in Minerals (MDPI) v.10, 26p. doi:10.3390/min10110965.
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Wei, C.W., Deng, M., Xu, C., Chakhmouradian, A.R., Smith, M.P., Kynicky, J., Song, W.L., Chen, W. and Fu, B., 2022 - Mineralization of the Bayan Obo Rare Earth Element Deposit by Recrystallization and Decarbonation: in Econ. Geol. v.117, pp. 1327-1338.
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Xu, C., Wang., L., Song, W. and Wu, M., 2010 - Carbonatites in China: A review for genesis and mineralization: in Geoscience Frontiers v.1, pp. 105-114.
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Yang, K.-F., Fan., H.-R., Santosh, M., Hu, F.-F. and Wang, K.-Y., 2011 - Mesoproterozoic mafic and carbonatitic dykes from the northern margin of the North China Craton: Implications for the final breakup of Columbia supercontinent: in Tectonophysics v.498, pp. 1-10.
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Yang, K.-F., Fan., H.-R., Santosh, M., Hu, F.-F. and Wang, K.-Y., 2011 - Mesoproterozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, North China: Constraints for the mechanism of super accumulation of rare earth elements: in Ore Geology Reviews v.40, pp. 122-131.
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Yang, X.-M. and Le Bas, M.J., 2004 - Chemical compositions of carbonate minerals from Bayan Obo, Inner Mongolia, China: implications for petrogenesis: in Lithos v72 pp 97-116
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Yang, X.-Y., Lai, X.-D., Pirajno, F., Liu, Y., Mingxing, L. and Sun, W.-D., 2017 - Genesis of the Bayan Obo Fe-REE-Nb formation in Inner Mongolia, North China Craton: A perspective review: in Precambrian Research v.288, pp. 39-71.
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Zhang, J., Fan, H., Xiao, W., Xu, X., Wakabayashi, J., Zhang, L., Zhao, Y., Wang, Q., Zhao, Y. and Wang, K., 2024 - Configuration of Carbonatite Constrained in Preintrusion Transpositional Foliation in the Bayan Obo Giant Rare Earth Element Deposit, China: in Econ. Geol. v.119, pp. 853-869. doi: 10.5382/econgeo.5076.
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Zhang, S.-H., Zhao, Y. and Liu, Y., 2017 - A precise zircon Th-Pb age of carbonatite sills from the worlds largest Bayan Obo deposit: Implications for timing and genesis of REE-Nb mineralization: in Precambrian Research v.291, pp. 202-219.
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Zhang, S.-H., Zhao, Y., Li, Q.-L., Hu, Z.-C. and Zhen, Z.-Y., 2017 - First identification of baddeleyite related/linked to contact metamorphism from carbonatites in the worlds largest REE deposit, Bayan Obo in North China Craton: in Lithos v.284-285, pp. 654-665
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Zhu, X,-K., Sun, J. and Pan, C., 2015 - Sm-Nd isotopic constraints on rare-earth mineralization in the Bayan Obo ore deposit, Inner Mongolia, China: in Ore Geology Reviews v.64, pp. 543-553.
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