Bayan Obo

Inner Mongolia, China

Main commodities: REE Fe
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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, although 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.

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 are interpretted to have formed by multistage hydrothermal replacement of marble. Various characteristics have been taken to imply it 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 summary 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

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
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
Feng-Jun Nie, Si-Hong Jiang, Xin-Xu Su and Xin-Liang Wang  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
Hong-Rui Fan, Yi-Han Xie, Kai-Yi Wang 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
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.
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
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
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
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
Xue-Ming Yang, 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

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Smith M, Wu Chengyu, 2000 - The Geology and Genesis of the Bayan Obo Fe-REE-Nb Deposit: A Review,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 271-281
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