2013, Sun Jian, PR-Iron isotopic constraints on the genesis of Bayan Obo ore deposit, Inner...

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Precambrian Research 235 (2013) 88– 106

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Precambrian Research

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Iron isotopic constraints on the genesis of Bayan Obo ore deposit,Inner Mongolia, China

Jian Suna,1, Xiangkun Zhua,∗, Yuelong Chenb,1, Nan Fanga,b

a Laboratory of Isotope Geology, MLR, State Key Laboratory of Continental Dynamics, Institute of Geology, CAGS, Beijing 100037, Chinab School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China

a r t i c l e i n f o

Article history:Received 24 November 2012Received in revised form 11 June 2013Accepted 15 June 2013Available online xxx

Keywords:Bayan OboIron isotopesIron oreREECarbonatiteMagmatic origin

a b s t r a c t

The giant polymetallic Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world. Despitethe fact that a great number of works have been done on the deposit, its origin remains contro-versial. Various genetic models have been proposed, including sedimentary origin, magmatic origin,hydrothermal origin or origin with multiple processes. Here the Fe isotope compositions of differ-ent types of rocks from the deposit and related geological formations, such as carbonatites, maficdykes, and Mesoproterozoic sedimentary iron formation and carbonates, are systematically investi-gated. For the ore deposit, the average ı56Fe values are −0.03 ± 0.16‰ (2SD, n = 14), 0.01‰ ± 0.24‰(2SD, n = 6), −0.07 ± 0.24‰ (2SD, n = 19) for bulk samples of fine-grained iron ores, gangue rocks andore-hosting dolomite marble, respectively; and 0.01 ± 0.14 (2SD, n = 14), 0.08 ± 0.18 (2SD, n = 3), −0.21for the magnetite, hematite and dolomite, the main Fe oxide and carbonate minerals in the deposit.The narrow range of the near-zero ı56Fe values of fine-grained iron ores and Fe oxide minerals are con-sistent with those of magmatic products such as igneous rocks and magmatic iron ores, but differentfrom those of sedimentary or hydrothermal products like Precambrian sedimentary iron formationsand hydrothermal iron ores reported previously. The slightly negative Fe isotope values of the ore-hosting dolomite marble are consistent with those of the carbonatite dykes in Bayan Obo area and thetypical carbonatites worldwide, but different from those of Mesoproterozoic sedimentary carbonates.The small Fe isotope fractionations between magnetite and dolomite (�56Femagnetite-dolomite ≈ 0.23‰),and between hematite and magnetite (�56Fehematite-magnetite ≈ 0.07‰), indicate that the ore depositexperienced a very high temperature. Overall, the Fe isotope compositions are inconsistent witheither sedimentary or hydrothermal origin, but support a magmatic origin for the Bayan Obodeposit.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Bayan Obo REE-Nb-Fe ore deposit in Inner Mongolia is thelargest light REE deposit in the world. It is located in the north-ern margin of the North China Craton (Wang et al., 1992), andhosted in Mesoproterozoic Bayan Obo Group (Li, 1959). The BayanObo Group consists of an over 10 km thick succession of low-grademetamorphosed sandstones, siltstones, limestones and dolomites.It is divided into 6 Formations and 18 Members (numbered as H1to H18 from the bottom to top), and the ore deposit is hosted in“H8” dolomite marble (Fig. 1).

∗ Corresponding author at: Institute of Geology, CAGS, China.Tel.: +86 10 68999798.

E-mail addresses: [email protected] (J. Sun), [email protected],[email protected] (X. Zhu), [email protected] (Y. Chen).

1 Address: China University of Geosciences, Beijing, China.

A great number of studies have been carried out over the last80 years on the ore deposit since its discovery (e.g. Li, 1959; Meng,1982; Wei and Shangguan, 1983; Liu, 1985, 1986; Zhang and Tao,1986; Institute of Geochemistry, Academia Sinica, 1988; Drewet al., 1990; Le Bas et al., 1992, 1997, 2007; Meng and Drew, 1992;Yuan et al., 1992; Wei et al., 1994; Bai et al., 1996; Chao et al., 1997;Smith et al., 2000; Zhang et al., 2003, 2009; Fan et al., 2004, 2010;Liu et al., 2004, 2005; Smith, 2007; Yang et al., 2009, 2011a, 2011b;Sun et al., 2012a, 2012b; Zhu and Sun, 2012). These previousstudies have shown that the mineral constituent, origin, andevolution of Bayan Obo ore deposit are very complex. For example,more than 170 minerals have been identified in this ore deposit(Zhang and Tao, 1986), and geochronological data ranging fromProterozoic to Phanerozoic have been reported regarding the age ofthe deposit (Chao et al., 1997; Zhang et al., 2003). As to the genesisof the deposit, main viewpoints include: (1) Sedimentary origin.It is proposed that the ore-hosting dolomite marble is of normalsedimentary origin (Meng, 1982; Wei and Shangguan, 1983; Meng

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J. Sun et al. / Precambrian Research 235 (2013) 88– 106 89

Fig. 1. Sketch Geological map of Bayan Obo ore deposit (modified after Bai et al., 1996). (1) overthrust; (2) inferred fault; (3) geological boundaries; (4) inferred geologicalboundaries; (5) iron bodies; (6) carbonatite dykes; (7) ore-hosted dolomite; (8) fluorite type iron ores; (9) massive type iron ores; (10) aegirine type iron ores; (11) riebeckitetype iron ores; (12) dolomite type iron ores; (13) biotite type iron ores; (14) sample location; (15) location of drill hole. (H1–H15) members of Bayan Obo Group, see Table 1;(C) carboniferous; (Q) quaternary; (H) relicts of Bayan Obo Group; (DT) dolomite marble; (ST) mineralized slate; (�) meta-ultrabasic rocks; (�) migmatitic gneisses; (�)granites; (��) migmatitic granites; (�) basic-intermediate rocks.

and Drew, 1992; Wei et al., 1994), or is formed by hydrothermalsedimentation (Chen and Shao, 1987; Yang and Drew, 1994;Gao et al., 1999), and that the ore materials are enriched duringthe sedimentary process. (2) Magmatic origin (Zhou, 1980; Liu,1986; Chao et al., 1992; Le Bas et al., 1992, 1997, 2007; Bai et al.,1996; Yuan et al., 1992; Hao et al., 2002; Yang et al., 2011b; Xiaoet al., 2012). It is believed that the ore-hosting dolomite marble iscarbonatite, rather than sedimentary carbonates. (3) Hydrother-mal metasomatic origin (Chao et al., 1992, 1997; Yang et al.,2009). It is suggested that the host rock is sedimentary carbonateformed in early Proterozoic, while the ore materials are enrichedby late-stage hydrothermal metasomatism. (4) Multi-origins(Institute of Geochemistry, Academia Sinica, 1988; Cao et al., 1994;Kynicky et al., 2012). It is proposed that the ore deposit cannot beinterpreted by a simple model. Cao et al. (1994) suggested that theiron ore and host rock are of sedimentary origin, while REE andNb are derived from mantle by metasomatism of mantle-sourcedfluids. Kynicky et al. (2012) argued that the ore deposit is formedby intrusion of carbonatites into sedimentary marbles, followedby multiple periods of magmatic (carbonatite)-hydrothermalfluid infiltration, metamorphism and deformation. It is also pro-posed that the ore deposit is of multi-source, multistage andmulti genetic origin (Institute of Geochemistry, Academia Sinica,1988).

Recently, great advances have been made in using Fe isotopesto constrain some important geological processes due to the revo-lutionary development in mass spectrometry (Beard and Johnson,1999; Belshaw et al., 2000; Zhu et al., 2000, 2001, 2002; Dauphasand Rouxel, 2006; Johnson et al., 2008b; Wang and Zhu, 2012).As Fe is the main ore-forming element in Bayan Obo ore deposit,Fe isotope technique is probably among the most direct means toaddress the issue of the origin of the deposit. Previous studies haveshown that igneous rocks, mantle xenoliths, and magmatic ironore deposit have relatively homogeneous Fe isotope compositions,and their average ı56Fe values are close to that of internationalreference material IRMM-014 (Beard and Johnson, 1999; Beardet al., 2003a; Dauphas and Rouxel, 2006; Schoenberg and vonBlanckenburg, 2006; Weyer and Ionov, 2007; Teng et al., 2008; Zhaoet al., 2010, 2012; Wang et al., 2012; Wang and Zhu, 2012). Fe oxidesof sedimentary iron formation, such as banded iron formation (BIF),usually show heavy Fe isotope enrichment relative to IRMM-014,and large variations in Fe isotope compositions (Johnson et al., 2003,2008a,b; Dauphas et al., 2004, 2007b; Frost et al., 2007; Hyslopet al., 2008; Li et al., 2008a, 2012; Yan et al., 2010; Li and Zhu,2012; Planavsky et al., 2012). On the other hand, hydrothermalore deposits usually show light Fe isotope enrichment with a largevariation in Fe isotope compositions (Horn et al., 2006; Markl et al.,2006; Wang et al., 2011).


90 J. Sun et al. / Precambrian Research 235 (2013) 88– 106

In this study, we systematically investigate various types ofrocks in Bayan Obo ore deposit for their Fe isotope composi-tions, complemented with related geological formations includingigneous rocks and sedimentary iron formations and rocks for com-parison, to constrain the genesis of this giant polymetallic oredeposit.

2. Field geology and sample characterization

2.1. Geological setting

Bayan Obo ore deposit is located in the northern margin of theNorth China Craton (Fig. 1). Crystalline basement in the region isconsisted of the Archean Se’ertengshan Group, which is uncon-formably covered by Proterozoic Bayan Obo Groups (Bai et al.,1996; Bureau of Geology and Mineral Resources of Nei MongolAutonomous Region, 1996). These strata are shaped by gentle foldstructures including Kuangou anticline and Bayan syncline, and areseparated by a nearly E-W trending Kangou fault. The intrudedigneous rocks in the area include Proterozoic mafic and carbon-atite dykes (Le Bas et al., 1992, 2007; Tao et al., 1998; Yang et al.,2011a, 2011b), and Phanerozoic granite (Institute of Geochemistry,Academia Sinica, 1988; Zhang et al., 2003). Basic-ultrabasic andalkaline volcanic rocks are also reported in the region (Bai et al.,1996; Wang et al., 2002; Xiao et al., 2012).

The ore deposit is hosted in the Bayan Obo Group. Its stratigra-phy is shown in Table 1. The Bayan Obo sediments were depositedin the Mesoproterozoic Langshan-Bayan Obo rift zone in the north-ern of North China Craton, and the ore deposit are considered to beformed in the late stage of the rift development (Wang et al., 1992;Zhao et al., 2003).

Bayan Obo deposit is located in the core of Bayan syncline southof Kuangou. The ore-hosting rock is dolomite marble, which is suf-fered intensive REE and Nb mineralization. The dolomite marble istraditionally considered as a component of Bayan Obo Group andtermed as H8, but the actual origin of which is till under debate assummarized in last section. The H8 dolomite marble extends 18 kmfrom east to west, with a width of ten to one thousand meters andoccurs as a spindle-shaped stratiform body, widening in the middleand thinning toward the ends (Yuan et al., 1992). The lower part ofH8 dolomite marble is contacted with quartzite and slate, and onthe uppermost is covered by H9 K-rich slate. All layers being sub-parallel while both quartzite and slate contacted with H8 dolomitemarble being suffered fenitization. Numerous carbonatite dykesoccur in the vicinity of the ore deposit, with fenitization of the wallrocks. Their geochemical features are very similar to those of theore-hosting dolomite marble (Le Bas et al., 1997, 2007; Zhang et al.,2003), indicating that they may have genetic relationship. The ore-hosting dolomite marble is also generally explained as sedimentaryorigin. It is supposed that the typical H8 unit on north of Kuan-gou, which consists of sedimentary limestone and dolostone, is theoriginal sediments of ore-hosting dolomite marble. But these sed-iments show well developed stratification with weak deformationor metamorphism.

The dolomite marble contains many iron ore bodies, scatteringfrom west to east and occurring as large lens or beds as individualones. They may be divided into three portions. The biggest onesare the Main ore body and the Eastern ore body, which are in theeast part of Bayan Obo ore deposit area. In the west is the Westernore body, which is composed of many smaller sub-ore bodies. TheMain and the Eastern ore bodies are situated closely to the bound-ary between ore-hosting H8 dolostone marble and H9 K-rich slate(Fig. 1c). These two ore bodies suffered significant fluorization andfenitization. The ore bodies are zoned and have iron-rich cores:the stratigraphically lower portions (north of the core) have a high

fluorite content with hematite and magnetite as the main Fe min-erals, and the upper portions (south of the core) have a high sodicamphibole content with magnetite as the main Fe mineral. The orebodies are covered by biotite schist with Fe-sulfide and biotitizedK-rich slate. The Western ore body occurs mainly in the massivedolomite marble with less fluorization and fenitizaiton, and themain Fe mineral is magnetite.

2.2. Sample characterization

2.2.1. Bayan Obo ore depositThe petrographic classification of the rocks from the ore deposit

is not easy, as the mineral constituents are complicated. The mainrock-forming minerals include carbonates, Fe oxides, silicates, flu-oride, etc. According to the mineral phases and Fe content, therocks are roughly divided into three types: iron ores (with Fe con-tent > 15%), dolomite marble (with dolomite content > 50% and Fecontent < 15%), and gangue rocks (with dolomite content < 50% andFe content < 15%).

Dolomite marble is the ore-hosting rock of Bayan Obo oredeposit, which extents widely and constitutes the main part ofthe ore deposit. It commonly appears massive in outcrops, with-out clear sedimentary bedding structure (Fig. 2a and b). Themain mineral is dolomite or ankerite, which is usually granularor sheared (Fig. 3f and g). The minerals are mostly fine-grainedwith 0.05–0.1 mm in diameter, and some are coarse-grained with1–2 mm in diameter. Other common minerals include calcite, mag-netite, hematite, pyrite, fluorite, riebeckite, aegirine, phlogopite,apatite, barite, monazite, bastnaesite, etc. According to the mineralconstitutes, the dolomite marble can be further divided into twotypes: relatively pure dolomite marble (Fig. 3f) and Fe-mineralizeddolomite marble (Fig. 3g). The relatively pure dolomite marble iscomposed almost entirely of dolomite. It is mostly coarse-grainedwithout or with weak mineralization, and some are fine-grainedwith REE mineralization. Fe-mineralized dolomite marble is alwaysfine-grained and contains Fe-bearing minerals including mag-netite, hematite, pyrite, riebeckite, phlogopite, and other mineralsincluding fluorite, barite, monazite, bastnaesite, etc.

The major-element compositions of dolomite marble are shownin Table 2. The contents of MgO and CaO cluster around ca.10–17 wt% and ca. 25–30 wt% respectively, with CaO/MgO valuebetween ca. 1.5 and ca. 2. The FeO contents vary from ca.1 wt% to ca. 7 wt%, with an average of ca. 5 wt%. The contentsof (FeO + Fe2O3 + MnO) are varying from ca. 5 wt% to ca. 20 wt%,and MgO/(FeO + Fe2O3 + MnO) is between ca. 1 and ca. 3, show-ing the dolomite marble is Fe-enriched. The SiO2 content varieswidely, from ca. 0.1 wt% to ca. 10 wt%, and the Al2O3 content isnormally low, less than 0.2 wt%. The MnO, SrO, RE2O3, and BaOabundances are usually higher than normal carbonate rocks. Thesefeatures are consistent with those of magnesio-carbonatites orferro-carbonatites (Woolley and Kempe, 1989).

The iron ores, together with gangue rocks, constitutes the ironore bodies. The main Fe minerals are magnetite and hematite, asso-ciated with gangue minerals fluorite, aegirine, riebeckite, biotite,dolomite, apatite, barite, pyrite, monazite, bastnaesite, etc. Theseminerals usually form banded structure, alike banded iron forma-tion (Fig. 2c–e). The gangue rocks are generally the parts with littleFe oxide minerals, and the iron ores the parts that enriched in Feoxide minerals. Most major elements of gangue rocks and iron oresvary widely according to the variation of mineral constitutes of thesamples. The (FeO + Fe2O3) contents vary from ca. 10 wt% to ca.80 wt%. The SrO, RE2O3 and BaO abundances are similar to thoseof ore-hosting dolomite marble.

On the basis of textural and structural information, the ironores can be further divided into three types: disseminated (Fig. 3c),banded (Figs. 2c–e and 3b), and massive type (Figs. 2d and 3a);



Table 1Stratigraphy of Bayan Obo Group (after Bai et al., 1996; Institute of Geochemistry, Academia Sinica, 1988).

Group Formation Member Depth (m) Sedimentary formation Volcano formation

Upper Bayan Obo Hujiertu formation H18 882 LimestoneH17 406 SandstoneH16 410 Slate with limestone bedsH15 >280 Epidosite with quartzite Basic-intermediate volcanicsH14 >45 Gray limestone

Baiyinbaolage formation H13 473 Sandstone and slateH12 1584 Silty sandstoneH11 109 Gray quartzite

Lower Bayan Obo Bilute formation H10 >2340 Carbonaceous and siliceous slate withsandstone beds

Serpentine melange

H9 >161 Black slateHalahuoqite formation H8 435 Limestone and dolomite with quartzite beds Alkaline and basic volcanics,

carbonatiteH7 337 Sandstone with quartzite and limestone bedsH6 126 Arkosic sandstone and quartzite

Jianshan formation H5 >135 Thin-bedded carbonaceous slateH4 >236 Dark quartziteH3 >258 Dark carbonaceous slate and iron-rich slate

Dulahala formation H2 369 Tan to white quartziteH1 >225 Coarse sandstone and basal conglomerate

whereas on the basis of mineral assemblages, they can be dividedinto five types: fluorite type, aegirine type, riebeckite type, biotitetype and dolomite type.

A notable feature of the iron ores is that they are mostly fine-grained (Fig. 3a–c), with the Fe-oxide minerals normally <0.1 mmin diameter. The fine-grained Fe-oxide minerals have sufferedreplacement or recrystallization to varying degrees (Fig. 3a, d, ande), as it is common to observe that the boundary of some Fe oxideminerals is extremely irregular and that veins with larger-grainedand euhedral–subhedral minerals occur inside the fine-grainediron ores.

The recrystallized iron ores include the medium- to coarse-grained subhedral iron ores (Fig. 3d) and coarse-grained euhedraliron ores (Fig. 3e). The medium- to coarse-grained subhedral ironores are generally riebeckite type or dolomite type iron ores,with magnetite or hematite medium- to coarse-grained (normally0.2–1 mm) and subhedral. The coarse-grained euhedral iron oresare generally fluorite type iron ores, consisting of coarse-grained(normally 0.5–1 mm) and euhedral Fe minerals hematite, mag-netite and martite (pseudomorphs of hematite after euhedralmagnetite), and the gangue minerals fluorite, calcite, monazite, etc.

Moreover, the iron ores are also cut by aegirine-rich vein andpyrite vein (Fig. 2f), both of which are formed in ca. 440 Ma (Zhanget al., 2003; Liu et al., 2004; Hu et al., 2009). The aegirine-richvein contains aegirine, fluorite, huanghoite, albite, calcite, quartz,biotite, etc., and the pyrite vein consists mainly of pyrite, calcite,aegirine, etc. All of these late-stage minerals are coarse grained andeuhedral.

Various types of iron ores, dolomite marble, and some ganguerocks were collected from different parts of the ore bodies fromwest to east for analysis. The locations are shown in Fig. 1.

2.2.2. Related igneous rocksAs mentioned above, numerous carbonatite dykes incise the

sediments of Bayan Obo Group and the basement rock adjacent tothe Bayan Obo ore deposit (Fig. 2g), and a possible genetic connec-tion between the “H8” ore-hosting dolomite marble and the dykeshas been proposed (Le Bas et al., 1992, 1997; Yang et al., 2011a).The minerals of carbonatite dykes mainly include dolomite, calcite,barite, monazite, bastnaesite, apatite, magnetite, etc. (Fig. 3h); butthey may vary for dyke to dyke. The major-element compositionof the carbonatites is similar to those of the ore-hosting dolomitemarble, as shown in Table 2. 6 samples from Bayan Obo carbon-atite dykes, together with 9 samples of typical carbonatites from

elsewhere in China, were collected and analyzed for comparison.These typical carbonatites were collected from the Laiwu-Zibo inwestern Shandong Province, from Maoniuping-Dalucao in westernSichuan Province, and from the eastern Himalayan syntaxis in Tibet.Details of these carbonatites are reported by Ying et al. (2004); Houet al. (2006); Liu et al. (2006) respectively.

Besides carbonatite dykes, many mafic dykes occur in Bayan Oboarea. The geochemical similarity between the mafic and carbon-atite dykes suggests common source characteristics, and they arerecognized as the signature of mantle magmatism associated withthe final fragmentation of the Columbia supercontinent (Yang et al.,2011b). 2 samples (BM10-50, BM10-55) are collected from the Mainore body, and the sample BH09-1 were collected from Heinaobao,ca. 25 km southeast of Bayan Obo ore deposit. All of these samplesare diabase.

2.2.3. Related sedimentary iron formation and rocksTypical sedimentary iron formation and rocks occur in Bayan

Obo Group on north of Bayan Obo deposit. The iron formation isobserved in H3 Member, Jianshan Formation, about 0.5 km northof the Bayan Obo deposit (Fig. 1). Lithologically, it is iron-rich slate,interbedded with quartzite. The minerals in iron-rich slate mainlyinclude hematite, quartz, and clay, all are fine grained and formed asthin layers or lens. 5 samples (BN10-13, BN10-14, BN10-15, BN10-16, and BN09-61B) were collected and analyzed.

The sedimentary carbonates on north of Bayan Obo deposit aresuggested as the original sediments of the ore-hosting dolomitemarble. They are dolostone or limestone consisting of very fine-grained calcite, dolomite, quartz, clay, etc. The SiO2 and Al2O3contents are relatively high while MnO and P2O5 contents are rel-atively low (Table 2). 3 samples (BN09-62, BN09-64 and BN09-68),together with 4 samples of carbonates from a classic Mesoprotero-zoic section in Pingquan, Hebei province, China were collected andanalyzed. These carbonates are similar in age (Mesoproterozoic),and are all deposited in rift environment in North China Carton.

3. Analytical methods

Analyses of major elements were undertaken at the NationalResearch Center for Geoanalysis, China, using XRF (PW4400) bythe melting film method, calibrated against international standardsof appropriate compositions. REE, Ba, and Sr elements were mea-sured using ICP-MS (X-series). FeO content in the samples wasmeasured using the solution Potassium Permanganate Titration

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Table 2Major elements of Bayan Obo ore deposit, carbonatite dykes, and sedimentary carbonates.

Sample ID SiO2% TiO2% Al2O3% Fe2O3% FeO % MnO % MgO % CaO % Na2O % K2O % P2O5% CO2% H2O % LOI % F % S % RE2O3% BaO % SrO % CaO/MgO FeO + Fe2O3

+ MnOMgO/(FeO+ Fe2O3

+ MnO)

The ore-hosting dolomite marbleB2083 0.51 0.06 0.08 0.78 3.70 0.54 15.53 31.03 0.16 0.02 4.86 38.97 0.31 0.02 0.94 0.03 0.49 2.00 5.02 3.09BM10-1 0.5 0.04 0.06 0.44 5.32 1.22 15.26 26.66 0.12 0.08 1.11 41.37 0.67 0.079 4.09 0.11 0.26 1.75 6.98 2.19WK-41 0.87 0.09 0.03 0.19 6.7 1.12 14.8 25.7 0.12 0.04 0.11 41.9 0.62 0.036 4.16 0.04 0.29 1.74 8.01 1.85BE09-16b 7.7 0 0.15 2.83 5.73 2.86 10.9 22.3 0.97 0.34 0.44 31.04 0.89 1.03 3.74 4.38 0.19 2.05 11.42 0.95B2084a 0.25 0.01 0.01 0.98 7.24 2.41 16.72 27.29 0.09 0.02 0.78 42.5 0.66 0.03 1.79 0.02 0.27 1.63 10.63 1.57B2082a 2.03 0.01 0.39 3.9 5.35 1.07 16.58 26.92 0.13 0.39 0.03 41.64 0.9 0.12 0.08 0.10 0.43 1.62 10.32 1.61B9131a 2.7 0.21 0.01 1.72 4.54 0.83 17.73 27.45 0.07 0.06 0.6 39.46 1.2 2.26 2.04 0.50 0.20 1.55 7.09 2.50B9157a 1.58 0.41 0.22 7.72 1.28 0.83 14.58 30.35 0.06 0.03 1.16 34.82 1.42 0.021 1.42 4.37 1.45 2.72 2.08 9.83 1.48B9438a 3.26 0.41 0.03 4.73 1.62 0.92 15.21 30.6 0.01 0.09 0.79 33.03 1.06 7.76 0.6 3.75 0.58 0.24 2.01 7.27 2.09B9450a 11.72 0.07 0.9 17.58 0.01 2.18 7.26 30.58 1.06 1.06 0.57 23.27 1.48 1.87 0.68 0.28 0.09 4.21 19.77 0.37

Gangue rocksBE09-35 22.16 0.17 3.53 0.36 11.21 0.85 5.73 10.45 1.78 2.84 7.07 3.62 3.02 3.64 15.36 0.23 0.12 1.82 12.42 0.46B9451a 3.25 0.06 1.01 11.42 0.23 1.02 1.55 53.49 0.17 0.69 1.02 0.89 0.84 42.22 1.20 0.15 0.11 34.51 12.67 0.12B9453a 27.93 0.14 0.64 13.56 4.15 1.4 7.75 21.62 3.81 0.68 0.46 11.62 1.9 3.84 0.49 0.06 0.20 2.79 19.11 0.41

Iron oresBM09-6 1.67 0.4 0.33 17.1 10.6 0.21 0.42 31.45 0.31 0.17 3.64 4.32 9.06 0.064 7.00 0.09 0.07 74.88 27.91 0.02BM09-41 0.78 0.05 0.25 83.39 1.02 0.01 0.23 2.94 0.38 0.04 1.11 1.32 0.88 0.072 2.69 0.19 0.01 12.78 84.42 0.00BE09-7 11.76 0.46 0.36 5.89 16.15 3.55 6.52 5.44 1.69 0.76 3.54 10.23 1.91 1.3 13.82 0.03 0.13 0.83 25.59 0.25BE09-30b 0.22 0.18 0.06 16.51 9.74 2.38 5.08 29.75 0.18 0.02 0.61 20.88 9.36 0.37 1.51 1.66 0.12 5.86 28.63 0.18BM09-55 6.52 0.32 0.17 13.12 5.57 2.19 4.99 20.53 0.86 0.39 11.78 9.48 5.55 2.83 9.21 0.03 0.24 4.11 20.88 0.24B2076a 12.75 0.04 0.68 40.41 14.82 6.86 5.28 6.78 1.05 0.95 7.32 1.5 1.53 0.44 0.04 1.28 62.09 0.09B2086a 7.34 0.23 0.1 68.11 11.2 3.45 1.39 2.86 0.06 0.01 3.21 1.96 0.46 0.17 0.11 2.06 82.76 0.02B9440a 20.66 0.3 0.38 38.48 11.1 0.08 0.25 10.03 4.58 0.01 1.93 1.43 0.58 5.56 6.45 1.30 0.05 40.12 49.66 0.01

Carbonatite dykesBN09-13b 1.61 0.02 0.14 0.26 1.33 0.36 8.82 15.61 0.26 0.07 7.6 25.68 0.12 1.03 21.94 4.84 0.35 1.77 1.95 4.52BN09-35b 6.59 0.12 0.98 5.95 2.29 1.46 8.45 30.56 0.13 0.73 3.99 33.37 0.24 0.014 1.99 0.22 0.34 3.62 9.70 0.87BN09-48 0.42 0.01 0.05 0.06 1.49 0.37 17.23 33.36 0.11 0.02 3.61 41.18 0.18 0.02 0.19 1.94 1.92 8.97

Sedimentary carbonatesBN09-68b 26.07 0.07 1.33 0.08 0.88 0.07 14.94 21.64 0.02 0.44 0.04 33.75 – – 0.00 0.02 0.01 1.45 1.03 14.50

a Data from Zhang et al. (2003).b The major-element data are from Sun et al. (2012a).



Fig. 2. Field photographs of Bayan Obo ore deposit: (a) ore-hosting dolomite marble; (b) dolomite marble with disseminated irregular fine-grained magnetite clusters; (c)massive-banded type iron ore, with bands of magnetite, aegirine, fluorite, and rare earth minerals; (d) aegirine type iron ore, consisting of bands of fine-grained magnetiteand aegirine. Notice that the magnetite disseminated in the aegirine band is medium- to coarse-grained. The diameter of the coin is 25 mm; (e) fluorite type banded ironore, with bands of fine-grained hematite, fluorite, and rare earth minerals. The length of the pen is ca. 15 cm; (f) late-stage pyrite veins cutting the fluorite type iron ores;(g) a carbonatite dyke intruding to H2 quartzite, with intensive fenitization. Abbreviations: Aeg = aegirine; Dol = dolomite; Fl = fluorite; Hem = hematite; Mag = magnetite;Py = pyrite; RE = rare earth minerals.

method, under protection of CO2 gas, after dissolving with HF-H2SO4. TFe content was also measured using a Nu Plasma HRMC-ICP-MS by solution method at Laboratory of Isotope Geology,Institute of Geology, Chinese Academy of Geological Sciences.

The Fe isotope analysis was undertaken at the Laboratory of Iso-tope Geology, Institute of Geology, Chinese Academy of Geological

Sciences. The protocol for sample dissolution, chemical separation,and isotope analysis of Fe has been described elsewhere (Zhu et al.,2002, 2008; Zhao et al., 2010; Wang et al., 2011), and is only brieflyreviewed here. For Fe isotope analyses, whole rock power or min-eral separates of samples was digested with HCl, HNO3, HF, HClO4,or their mixture. After complete dissolution, the final solution was



Fig. 3. Photomicrographs of rocks from Bayan Obo ore deposit: (a) BSE image of massive iron ore, consisting of fine-grained magnetite and gangue minerals aegirine, monazite,etc. The magnetite suffers weak recrystallization; (b) photomicrograph of banded iron ore with plane-polarized light, consisting of fine-grained magnetite, fluorite, carbonate,barite, monazite, etc.; (c) photomicrograph of disseminated iron ore with plane-polarized light, consisting of fine-grained magnetite, riebeckite, fluorite, etc.; (d) medium- tocoarse-grained iron ore with cross-polarized light, consisting of medium- to coarse-grained subhedral magnetite, riebeckite, dolomite, phlogopite, etc.; (e) photomicrograph ofcoarse-grained iron ore with reflected light, consisting of euhedral coarse-grained magnetite or martite, fluorite, monazite, phlogopite, etc.; (f) photomicrograph of relativelypure dolomite marble with plane-polarized light, consisting mainly of fine-grained dolomite and monazite; (g) photomicrograph of mineralized dolomite marble with plane-polarized light, metasomated by fine-grained magnetite and rare earth minerals; (h) photomicrograph of carbonatite with cross-polarized light, consisting of medium- tocoarse-grained carbonate, monazite, barite, etc. Abbreviations: Aeg = aegirine; Brt = barite; Carb = carbonate; Dol = dolomite; Fl = fluorite; Hem = hematite; Mag = magnetite;Mar = martite; Mnz = monazite; Phl = phlogopite; Py = pyrite; Rbk = riebeckite; RE = rare earth minerals.



treated with concentrated HCl repeatedly to convert the cations toa chloride-form. It was finally dissolved in 1 mL of 6 M HCl + 0.001%H2O2 for chemical purification of Fe.

The procedure for chromatographic separation of Fe using AGMP-1 anion exchange resin (100–200 mesh) is based on Tang andZhu (2006) and Tang et al. (2006). Samples were loaded and firstwashed with 6 M HCl + 0.001% H2O2 to remove ions other than Feand Zn, then 2 M HCl + 0.001% H2O2 was used to strip Fe. AlthoughREE and Nb are extremely enriched in Bayan Obo ore materials, ithas been found that REE can be separated completely from Fe, andthat nearly 90% of Nb can be separated from Fe using the establishedmethod, and that for most Bayan Obo ore samples, the influence ofNb that left in the final Fe solution to Fe isotope ratio measurementis minimal (Sun et al., 2010, 2013). During the column chemistry, atleast one basaltic standard sample (GBW-07105, Table 3) and oneduplicated sample were processed with each batch of unknownsamples.

Fe isotope ratios were determined in high mass resolution modeon a Nu Plasma HR MC-ICP-MS using standard-sample bracketing(SSB) approach. Samples were introduced into the mass spectrome-ter in 0.1 mol L−1 HNO3 using a DSN 100 desolvating nebulizer withFe concentrations at ca. 5 �g mL−1, where sample and standardsolutions were matched to give total beam intensities with differ-ences less than 10%. Runs of sample and standard were separatedby washes using 1 M and 0.1 M HNO3 for 3 and 2 min, respectively.Data were acquired in blocks of 10 ratios with 10-s integrationtimes, and background measurements were taken prior to eachdata block.

The Fe isotope results are expressed as deviations of an Fe iso-tope ratio of a sample from that of the reference material IRMM-14:

ı56FeIRMM-014(‰) =[

(56Fe/54Fe)sample

(56Fe/54Fe)IRMM-014− 1

]× 103

ı57FeIRMM-014(‰) =[

(57Fe/54Fe)sample

(57Fe/54Fe)IRMM-014− 1

]× 103

The long-term external reproducibility at high-resolution modeis better than 0.04‰ amu−1 at 2SD level estimated from repeatedmeasurements of in-house CAGS Fe solutions and national basalticstandard reference material CAGSR-1 (GBW-07105) against IRMM-014 (Zhu et al., 2008). To assess the accuracy of our analyses, the Feisotope results of some reference standards analyzed in our lab-oratory are compared with previously reported data in Table 3.The average Fe isotope values obtained for CAGSR-1 during thisstudy are ı56Fe = 0.14 ± 0.07% and ı57Fe = 0.23 ± 0.10% (2SD, n = 11),identical to those obtained by Craddock and Dauphas (2011). TheFe isotope compositions of BHVO-2, BCR-2 and BIR-1a interna-tional basaltic standards analyzed in our laboratory by Zhao et al.(2012) and Tang et al. (2012) are identical to the values reported byCraddock and Dauphas (2011) and references therein, suggestingthat inter-laboratory biases for Fe isotopes are negligible.

4. Results

Samples from Bayan Obo ore deposit, and related geological for-mations including igneous rocks and sedimentary iron formationand rocks, were measured for Fe isotope compositions as well as Fecontent. The results are presented in Table 4.

4.1. Bayan Obo ore deposit

The Fe isotope compositions of both bulk samples and mineralseparates of various types of rocks in Bayan Obo deposit are listedin Table 4 and illustrated in Fig. 4. Overall, most samples cluster

Relatively

Fig. 4. Fe isotope compositions of various types of rocks from Bayan Obo ore deposit.

around 0‰ in ı56Fe values, and there is no correlation between theı56Fe values and Fe content (Fig. 5).

Bulk samples of fine-grained iron ores show ı56Fe values varyfrom −0.17‰ to 0.13‰, with average ı56Fe = −0.03 ± 0.16‰ (2SD,n = 14). The ı56Fe values of magnetite and hematite vary from−0.11‰ to 0.12‰ and −0.02‰ to 0.15‰, respectively, with an aver-age ı56Fe of 0.01 ± 0.14 (2SD, n = 14) and 0.08 ± 0.18 (2SD, n = 3),respectively. The Fe isotope compositions are similar to each otherfor different types of fine-grained iron ores (Table 4) and for thosefrom different ore bodies (Fig. 6). For both bulk samples and Fe-oxide mineral separates of fine-grained iron ores, the ı56Fe valuesare rather homogeneous, similar to published mantle xenoliths Feisotope values (Schoenberg and von Blanckenburg, 2006; Weyerand Ionov, 2007; Zhao et al., 2010, 2012), and they give a normaldistribution (Fig. 7).

Bulk sample ı56Fe values of the ore-hosting dolomite mar-bles cover a range from −0.26‰ to 0.21‰, with averageı56Fe = −0.07 ± 0.24‰ (2SD, n = 19), whereas the relatively puredolomite marble exhibits variation from −0.24‰ to −0.06‰, withan average of −0.13‰ ± 0.14‰ (2SD, n = 19). Mineral separates ofdolomite from the ore-hosting dolomite marble yields ı56Fe val-ues from −0.69‰ to 0‰, with an average of −0.21; whereas themagnetites have slightly higher ı56Fe values between −0.14‰ and0.19‰, with an average of 0.02‰.

Fig. 5. The relationship between ı56Fe values and Fe content (wt%) for various typesof rocks from Bayan Obo ore deposit.



Table 3Fe isotope compositions of standard references.

Standard Description n ı56Fe 2SD ı57Fe 2SD Reference

CAGSR-1 (GBW-07105) Basalt, Zhangjiakou, Hebei, China (IGGE) 11 0.14 0.07 0.23 0.10 This study3 0.154 0.022 0.232 0.028 Craddock and Dauphas (2011)

BHVO-2 Basalt, Kilauea, Hawaii, USA 16 0.112 0.043 0.167 0.066 Our laboratory, Zhao et al. (2012)12 0.114 0.011 0.174 0.016 Craddock and Dauphas (2011)

BCR-2 Basalt, Columbia River, Oregon, USA (USGS) 16 0.071 0.064 0.116 0.084 Our laboratory, Zhao et al. (2012)8 0.091 0.011 0.126 0.017 Craddock and Dauphas (2011)

0.054 0.077 0.159 0.212 Dauphas et al. (2004)BIR-1a Basalt, Reykavik Dolerite, Iceland (USGS) 3 0.044 0.026 0.07 0.03 Our laboratory, Tang et al. (2012)

5 0.053 0.015 0.087 0.023 Craddock and Dauphas (2011)

Fig. 6. Comparison of Fe isotope compositions for fine-grained iron ores (bulk sam-ples and Fe oxide minerals) from different ore bodies in Bayan Obo deposit.

Fig. 7. The normal distribution of ı56Fe values for fine-grained iron ores (bulk sam-ples and Fe oxides).

Coarse-grained euhedral iron ores

Medium- to coarse-grained anhedral iron ores

Fig. 8. Fe isotope compositions of pyrite veins, coarse-grained iron ores and fine-grained iron ores.

Bulk samples of gangue rocks show small variation from −0.13‰to 0.20‰ in ı56Fe values, with an average of 0.01‰ ± 0.24‰ (2SD,n = 6).

In contrast to the fine-grained iron ores, the coarse-grained ironores and late-stage pyrite veins show a larger variation in ı56Fevalues (Fig. 8). The medium- to coarse-grained subhedral iron oresvary from −0.25‰ to 0.18‰ in ı56Fe values. The coarse-grainedeuhedral iron ores vary from −0.20‰ to 0.62‰ in ı56Fe values, withaverage ı56Fe = 0.41. The pyrite veins show an even larger variationin ı56Fe values from −0.67‰ to 0.44‰, with average ı56Fe = −0.17.

4.2. Related igneous rocks

Carbonatite dykes in Bayan Obo area and typical carbonatitesfrom elsewhere in China have similar Fe isotope compositions(Table 4). The former have ı56Fe values ranging from −0.26‰ to−0.03‰, with an average ı56Fe of −0.14 ± 0.18‰ (2SD, n = 6), andthe latter give a range from −0.47‰ to 0.08‰ with an average ı56Feof −0.12 ± 0.18‰ (2SD, n = 8). Their ı56Fe values are both consis-tent with published carbonatite Fe isotope values (Dauphas et al.,2007b; Johnson et al., 2010). And these carbonatites agrees wellwith the ore-hosting dolomite marble in Fe isotope compositions.

For mafic dykes in Bayan Obo area, ı56Fe values of 3 diabasesamples were 0.11‰, 0.12‰, and 0.16‰ respectively, which areconsistent with those of basalts reported previously (Beard et al.,2003a; Weyer and Ionov, 2007; Zhao et al., 2012) and similar tothose of the fine grained iron ores.



Table 4Fe isotope compositions of Bayan Obo ore deposit and related geological formations.

Sample Description Whole rock Dolomite Magnetite Hematite Fe/%

ı56Fe ı57Fe ı56Fe ı57Fe ı56Fe ı57Fe ı56Fe ı57Fe

Fine-grained iron oresB9440 Fluorite-aegirine type iron ore,

Main ore deposit−0.05 −0.04 35.6a

B2075 Massive magnetite iron ore,Western ore deposit

0.07 0.15 0.08 0.20 68.3


−0.04 0.04 0.10 0.19 50.0


−0.01 −0.07 0.01 −0.02 56.4a

B2087 Disseminated magnetite ironore, Western ore deposit

−0.16 −0.20 31.7

B9141 Banded Fluorite type iron ore,Eastern ore deposit

0.12 0.25

BM10-21 Banded fluorite type hematiteiron ore, Main ore deposit

0.03 −0.01 50.3

BM09-6 Banded fluorite type magnetiteiron ore, Main ore deposit

−0.09 −0.09 20.2

BM09-14 Massive aegirine typemagnetite iron ore, Main oredeposit

0.03 0.05 53.1

BM09-41 Massive type hematite ironore, Main ore deposit

−0.17 −0.19 59.2

BM09-47 Massive type hematite ironore, Main ore deposit

0.13 0.21 45.7

BM09-43 Massive hematite-magnetiteiron ore, Main ore deposit

0.02 0.05 0.15 0.24

BM09-49 Massive hematite-magnetiteiron ore, Main ore deposit

−0.11 −0.15 −0.02 0.01

BM10-46 Disseminated-Massiveriebeckite-fluorite typemagnetite iron ore, Main oredeposit

0.04 0.03

BM10-47 Disseminated-Massive biotitetype magnetite iron ore, Mainore deposit

−0.06 −0.06

BE10-135 Disseminated riebeckite typemagnetite iron ore, Eastern oredeposit

−0.03 −0.02

WK1-13 Massive biotite type magnetiteiron ore, Western ore deposit

−0.02 −0.04

BM09-56 Disseminated dolomite typemagnetite iron ore, Main oredeposit

−0.07 −0.09 22.1

WK-20 Massive dolomite typemagnetite iron ore, Westernore deposit

0.12 0.22

BE09-7 Banded riebeckite-dolomitetype magnetite iron ore,Eastern ore deposit

−0.02 −0.02 −0.01 −0.02 16.7

BM09-19 Massive fluorite-aegirine typemagnetite iron ore, Main oredeposit

0.01 0.05 45.6

BS10-5 Disseminated dolomite typemagnetite iron ore, Dojiegele

−0.05 −0.07 41.7

Average (ı56Fe) −0.03 ± 0.16 (2SD) 0.01 ± 0.14 (2SD) 0.08 ± 0.18 (2SD)

Ore-hosting dolomite marbleBN09-44 Relative pure dolomite marble,

coarse-grained, North of Mainore deposit

−0.06 −0.10 2.8

BM10-1 Relative pure dolomite marble,fine-grained and REEmineralized, North of Main oredeposit

−0.15 −0.24 4.5

BM09-63 Relative pure dolomite marble,fine-grained and REEmineralized, North of Main oredeposit

−0.24 −0.32 −0.31 −0.44 3.3

BM10-65 Relative pure dolomite marble,without obvious REEmineralization, South of Mainore deposit

−0.13 −0.12 1.1

B2083 Relative pure dolomite marble,REE mineralized, Western oredeposit

−0.07 −0.09 3.4



Table 4 (Continued)



B2084 Relative pure dolomite marble,REE mineralized, Western oredeposit

−0.20 −0.35 5.5

WK-41 Relative pure dolomite marble,fine-grained and REEmineralized, Western oredeposit

−0.07 −0.15 5.4

WK-15 Relative pure dolomite marble,coarse-grained, Western oredeposit

−0.08 −0.14 5.7

WK-36 Relative pure dolomite marble,fine-grained and REEmineralized, Western oredeposit

−0.07 −0.11 4.2

BE09-26 Weakly Pyrite-fluorite-REEmineralized dolomite marble,fine-grained, North of Easternore deposit

−0.01 0.05 −0.12 −0.18 0.08 0.19 6.2

BM09-64 Weakly Fe-REE-fluorite-apatitemineralized dolomite marble,North of Main ore deposit

0.01 −0.01 −0.23 −0.30 5.6

BE09-32 Weakly Fe-REE mineralizeddolomite marble, Boluotou

0.09 0.21 4.8

B9131 Weakly Fe-REE mineralizeddolomite marble, North ofMain ore deposit

−0.03 −0.02 4.7a

B2082 Weakly Fe mineralizeddolomite marble, Western oredeposit

−0.13 −0.20 6.9a

B9157 Fe-REE-barite mineralizeddolomite marble, Boluotou

0.02 0.04 5.3a

B9438 Fe-mineralized dolomitemarble, fluorite-REEmineralized, Northeast of Mainore deposit

−0.22 −0.29 4.6a

BE09-16 Fe-riebeckite-REE-baritemineralized dolomite marble,Near the Eastern ore deposit

−0.26 −0.41 −0.69 −1.04 −0.04 −0.07 6.5

BE09-22 Fe-REE-fluorite-riebeckitemineralized dolomite marble,Near the Eastern ore deposit

0.05 0.07 10.2

BE09-23 REE-fluorite-riebeckitemineralized dolomite marble,Near the Eastern ore deposit

0.21 0.30 7.9

B9450 Fe mineralized dolomitemarble, Western ore deposit

−0.09 −0.13 12.3a

3082531 Fe-REE-fluorite mineralizeddolomite marble, Near theMain ore deposit

−0.08 −0.18 8.7

BW10-14 Fe-REE mineralized dolomitemarble, fine-grained, Westernore deposit

−0.22 −0.34 0.19 0.29 5.9

BW10-86 Riebeckite mineralizeddolomite marble,coarse-grained, Western oredeposit

0.00 0.03 7.5

BW10-94 Fe mineralized dolomitemarble, coarse-grained,Western ore deposit

−0.21 −0.34 −0.14 −0.16 4.2

Average (ı56Fe) −0.07 ± 0.24 (2SD) −0.21 0.02

Gangue rocksBE09-49 Banded fluorite REE-Nb ore,

Boluotou0.05 0.09 0.5

BM10-28 Banded fluorite-hematiteREE-Nb ore, Main ore deposit

0.05 0.04 9.2

BM09-55 Disseminated riebeckite-fluorite-dolomite-magnetiteREE-Nb ore, Main ore deposit

−0.13 −0.08 13.5

B9451 Disseminated fluorite REE-Nbore, Western ore deposit

0.20 0.30 8.2a

B9453 Disseminatedriebeckite-fluorite REE-Nb ore,Western ore deposit

0.00 0.02 12.7a

BE09-35 Riebeckite-dolomite REE-Nbore, Boluotou

−0.09 −0.16 9.0



Table 4 (Continued)



Average 0.01 ± 0.24 (2SD)

Recrystallized iron ores and pyrite veinsMedium-coarse grained iron oresBE09-30 Medium-coarse grained,

subhedral dolomite-fluoritetype hematite iron ore, Easternore deposit

0.18 0.27 19.2

BE10-114 Medium-coarse grained,subhedral dolomite type ironore, Eastern ore deposit

−0.25 −0.40 35.4

BE09-3 Medium-coarse grained,subhedral riebeckite-dolomitetype magnetite iron ore,Eastern ore deposit

−0.11 −0.19 35.2

B2076 Medium-coarse grained,subhedral riebeckite-fluoritetype magnetite iron ore,Western ore deposit

−0.19 −0.24 −0.14 −0.14 39.8a

BW10-31 Medium-coarse grained,subhedral riebeckite typemagnetite iron ore, Westernore deposit

0.05 0.03

WK1-11 Medium-coarse grained,subhedral biotite typemagnetite iron ore, Westernore deposit

0.07 0.09

Coarse grained iron oresBM09-22 Coarse-grained euhedral

magnetite iron ore, Main oredeposit

−0.20 −0.28 32.7

BM09-42 Coarse-grained euhedralfluorite type magnetite ironore, Main ore deposit

0.33 0.49 42.0

BM10-30 Coarse-grained euhedralfluorite typemagnetite-hematite iron ore,Main ore deposit

0.55 0.79 0.62 0.86

BM10-36 Coarse-grained euhedralfluorite type martite iron ore,Main ore deposit

0.52 0.73

BE10-123 Coarse-grained euhedralfluorite type martite iron ore,Eastern ore deposit

0.60 0.90

Pyrite VeinsBM09-2 Coarse-grained

pyrite-calcite-aegirine vein,Main ore deposit

0.11 0.16

BM09-3 Coarse-grainedpyrite-calcite-aegirine vein,Main ore deposit

0.44 0.65


−0.57 −0.85


−0.67 −1.01

Related igneous rocksCarbonatite Dykes in Bayan OboBN09-13 Carbonatite, Bayan Obo area −0.25 −0.43 1.2BN09-23 Carbonatite, Bayan Obo area −0.20 −0.30 6.3BN09-35 Carbonatite, Bayan Obo area −0.06 −0.10 6.0BN09-41 Carbonatite, Bayan Obo area −0.03 −0.03 3.1BN09-45 Carbonatite, Bayan Obo area −0.16 −0.21 2.9BN09-48 Carbonatite, Bayan Obo area −0.18 −0.25 1.2Average −0.15 ± 0.17 (2SD)Typical carbonatite samples in ChinaYMQ-1 Calcio-carbonatite from

Laiwu-Zibo, Shandongprovince, China

−0.18 −0.26 1.7b

02DSM-1 Magnesio-carbonatite fromLaiwu-Zibo, Shandongprovince, China

0.06 0.15 4.8b



Table 4 (Continued)



02LT-1 Ferro-carbonatite fromLaiwu-Zibo, Shandongprovince, China

0.04 0.09 8.1b

09xy-1 Ferro-carbonatite fromLaiwu-Zibo, Shandongprovince, China

0.08 0.09 6.3b

HJZ-4-1 Calcio-carbonatite fromLaiwu-Zibo, Shandongprovince, China

−0.12 −0.14 3.2b

MNP-133 Calcio-carbonatite fromMaoniuping, Sichuan province,China

−0.17 −0.17 0.2

MNP-139 Calcio-carbonatite fromMaoniuping, Sichuan province,China

−0.28 −0.41 0.2

DL-138 Calcio-carbonatite fromDalucao, Sichuan province,China

−0.47 −0.62 0.6

99051104 Calcio-carbonatite, Tibet, China −0.08 −0.12 0.2Average −0.12 ± 0.18 (2SD)Mafic Dyke in Bayan Obo areaBM10-50 Diabase 0.11 0.17 6.0BM10-55 Diabase 0.12 0.17 5.3BH09-1 Diabase 0.16 0.18 9.9

Related sedimentary iron formations or rocksIron-rich Slate in Bayan Obo areaBN10-13 Iron-rich slate −0.49 −0.73 38.7BN10-14 Iron-rich slate −0.39 −0.58 34.3BN10-15 Iron-rich slate 0.29 0.41 18.5BN10-16 Iron-rich slate 0.48 0.71 3.7BN09-61B Iron-rich slate −0.03 −0.04 13.1Mesoproterozoic sedimentary carbonatesBN09-62 Dolostone, Bayan Obo area 0.09 0.15 0.7BN09-64 Dolostone, Bayan Obo area 0.25 0.39 0.7BN09-68 Dolostone, Bayan Obo area 0.20 0.32 0.6SD-1058-15 Dolostone, Hebei province,

China0.32 0.50 0.2

WZZ-1058-26 Dolostone, Hebei province,China

0.29 0.46 0.1


0.36 0.58 0.2


0.11 0.18 0.8

Values of ı56Fe and ı57Fe for each sample are the averages of two or three separate analyses. The data quality was also controlled by repeated measurements of in-houseCAGS Fe solution and national basaltic standard reference material CAGSR-1 (GBW-07105) versus IRMM-14 Fe-isotope reference material during the run. The long-termrepeatability in this study is better than 0.04‰ amu−1 at the 2-standard-deviation level. The internal precision for individual analysis is generally better than 0.03‰ amu−1

at the 2� level.a Data from Zhang et al. (2003).b Data from Ying et al. (2004).

4.3. Related sedimentary iron formations and rocks

The typical sedimentary iron formation of H3 iron-rich slate dis-play a wide range from −0.49‰ to 0.48‰ in ı56Fe values, with anaverage ı56Fe of −0.03. Their ı56Fe values are positively correlatedwith Fe contents (Fig. 9), which is consistent with that observed byLi (2007) in Archean Anshan-benxi BIF, NE China.

Mesoproterozoic carbonate rocks from Bayan Obo area andfrom Pingquan, Hebei province range from 0.04‰ to 0.36‰ inı56Fe values, with average ı56Fe = 0.22, which show heavy isotopeenrichment relative to H8 dolomite marble and carbonatites.

5. Discussion

5.1. Inconsistency with a sedimentary origin for iron ore

One of the hypotheses about the genesis of the ore deposit isthat the Fe ore is originally of sedimentary origin (Meng, 1982; Weiand Shangguan, 1983; Institute of Geochemistry, Academia Sinica,

1988; Meng and Drew, 1992; Wei et al., 1994). The Fe isotope resultsobtained in this study are inconsistent with this hypothesis.

A piece of evidence used to argue for sedimentary origin is thatthe commonly displayed banded structure of the Fe ore is simi-lar to Precambrian banded iron formation (BIF) (Qiu et al., 1981;Zhao, 2010). BIFs form predominantly by chemical sedimentation,where the main Fe mineral magnetite and/or hematite precipitatefrom Fe enriched seawater by oxidation. It has been demonstratedby experiments that Fe isotopes fractionate significantly betweenFe(II) and Fe(III) species during redox process and that the Fe(III)species enriches heavy Fe isotopes relative to Fe(II) species (Bullenet al., 2001; Balci et al., 2006). The Fe isotope fractionation duringthis process may be further explained on the basis of Rayleigh frac-tionation model (Zhu et al., 2002; Dauphas et al., 2007a; Li et al.,2008a; Yan et al., 2010). The Fe isotope compositions of iron for-mations depend on the degree of Fe precipitation in the watercolumn, which is a function of the seawater redox state. As theocean redox state changes during Earth’s history, the Fe isotopecompositions of iron formations modify accordingly (Rouxel et al.,2005).



Fig. 9. The relationship between ı56Fe values and Fe content (wt%) for H3 iron-richslate.

During Archean to Paleoproterozoic (ca.3.8 Ga to ca.1.9 Ga), theoxygen level in atmosphere and ocean is very low. Fe oxides of BIFsare formed by partial oxidation of Fe(II) species in the water column.As a consequence, the Fe isotope compositions of BIF show twoprominent features: generally heavy isotope enriched and highlyvariable (Dauphas et al., 2004, 2007a, 2007b; Johnson et al., 2008b;Li et al., 2008a, 2012; Planavsky et al., 2012).

The Fe isotope compositions of Bayan Obo Fe ore are homo-geneous with average ı56Fe values near 0‰, which are in sharpcontrast to those of BIFs (Fig. 10).

The situation might be different in Phanerozoic, when the oceanand atmospheric oxygen was elevated. Fe oxides are formed bynearly completely oxidation of Fe2+ aqueous species, and thus theiron formations would be close to the ocean Fe2+ aqueous speciesin ı56Fe values. A crucial question arises then is what the Fe isotope

Carbonatites

Fig. 10. Fe isotope compositions of iron ores, ore-hosting dolomite marble in BayanObo ore deposit and their related geological formations. a–j, literature data; (a) Marklet al. (2006); (b) Wang et al. (2011); (c) Dauphas et al. (2004, 2007a, 2007b); (d)Planavsky et al. (2012); (e) Li et al. (2008a); (f) Johnson et al. (2008a), Planavskyet al. (2012); (g) Frost et al. (2007), Hyslop et al. (2008); (h) Li and Zhu (2012); (i)Wang et al. (2012); (j) Beard et al. (2003a,b), Weyer and Ionov (2007), Zhao et al.(2012); (k) Schoenberg and von Blanckenburg (2006), Weyer and Ionov (2007), Zhaoet al. (2010, 2012).

compositions of the iron formations in Mesoproterozoic are, whenthe Bayan Obo ore deposit formed.

A typical Mesoproterozoic iron formation in North China Cratonis Xuanlong type iron ore deposit, which is famous for the stro-matolitic and oolitic structures. The deposit mainly distributes inthe Xuanhua-Chicheng area, Hebei province, China, and occurs inChuanlinggou Formation (ca 1.6 Ga) of Changcheng System. It con-sists mainly of hematite layer (iron ore), accompanied with thinlayer of siderite layer. The Fe isotope compositions of hematite partin Xuanlong iron ore deposit are similar to those of BIFs, varyingfrom 0.25‰ to 0.74‰, with average ı56Fe values of 0.45‰ (Li andZhu, 2012). Another Mesoproterozoic iron formation is H3 iron-rich slate in Bayan Obo Group on north of Bayan Obo deposit,where its Fe isotope compositions are also widely variable, varyingbetween −0.49‰ and 0.48‰ in ı56Fe values, with averageı56Fe of−0.03 (Table 4). Its Fe isotope compositions are in good line with Fecontents (Fig. 9), consistent with the Rayleigh fractionation model.In summary, Mesoproterozoic iron formations have variable Fe iso-tope compositions or are enriched in heavy Fe isotopes relative toIRMM-014, indicating relatively low oxygen level in the ocean oratmosphere.

On contrast, the Fe isotope compositions of Bayan Obo Fe oreare rather homogeneous and cluster around 0‰ in ı56Fe values(Figs. 4 and 10), and no linear correlation between ı56Fe values andFe content is observed (Fig. 5). The possibility of rehomogenizationof the Fe isotopes in the fine grained Fe ores during metamor-phism or deformation is small, as the Bayan Obo area only sufferedlower greenschist facies metamorphism (Wang et al., 1992). It hasbeen reported that even during granulite facies metamorphism, Feisotope compositions would not be significantly modified at bulksample scale, although Fe isotope exchange and equilibrium maytake place at mineral scale during greenschist-lower amphibolitefacies (Dauphas et al., 2004; Frost et al., 2007; Li et al., 2008b). Fur-thermore, the Fe isotope compositions of samples from H3 iron-richslate collected near the Bayan Obo ore deposit show large variationand have not been homogenized. And all types of the Bayan Obofine-grained iron ores, including both banded fine-grained iron oresand the undeformed disseminated iron ores are very similar in Feisotope compositions, ruling out the possibility that Fe isotope com-positions of the Bayan Obo ores have been modified significantlyduring deformation. Consequently, we argue that the feature of Feisotope compositions of the fine-grained Fe ores is largely origi-nal and no significant homogenization have occurred during eithermetamorphism or deformation.

Overall, the Fe compositions the Bayan Obo are incompatiblewith a sedimentary origin.

5.2. Fe isotope fractionation between magnetite and dolomiteand its implications for ore genesis

The degree of fractionation between phases is a function oftemperature. Take Fe oxides and carbonate minerals as examples.Under equilibrium conditions, �56Femagnetite-siderite is ca. 1.1‰ at200 ◦C and it decreases to ca. 0.35‰ at 600 ◦C according to the-oretical predictions (Polyakov and Mineev, 2000; Polyakov et al.,2007). For the sedimentary iron formations, �56Femagnetite-carbonateis usually relatively high as the carbonates and Fe oxides are formedunder low temperature conditions, which cause a large Fe isotopefractionation between them. Although magnetite may not equil-ibrate with other phases, part of the variations can be inheritedfrom fractionation during deposition and low-grade metamor-phism (Johnson et al., 2003; Dauphas et al., 2007b). For example,the �56Fehematite-siderite in Xuanlong iron ore deposit is ca. 1.11‰ (Liand Zhu, 2012), and the �56Femagnetite-ankerite of BIF from TransvaalCraton, South Africa is ≥0.8‰ (Johnson et al., 2003). For those suf-fered high-grade metamorphism, which is expected to promote



inter-mineral equilibration, the Fe isotope fractionation betweenmagnetite and carbonates phases may be lower. It is calculatedthat the �56Fehematite-siderite in the early Archean supracrustalrocks from SW Greenland (amphibolite to granulite facies) is ca.0.38‰ (Dauphas et al., 2007b). On contrast, for igneous rocks,�56Femagnetite-carbonate is very small as the Fe oxides and carbonatesare formed under high temperature conditions. In carbonatite,the average ı56Fe value of carbonate phases (calcite & dolomite)is −0.23‰ and the Fe oxide phase (magnetite) −0.03‰, with�56Femagnetite-carbonate ≈ 0.20‰ (Dauphas et al., 2007b; Johnsonet al., 2010), much lower than that of sedimentary iron formations.

In Bayan Obo ore deposit, the ı56Fe values of magnetites inboth ion ores and ore-hosting dolomite marble are similar, withan average of 0.02‰, and the dolomite mineral is −0.21‰ in aver-age ı56Fe values, resulting in �56Femagnetite-dolomite = 0.23‰. Basedon the magnetite–ankerite (Ca1.1Mg0.5Fe0.3Mn0.1(CO3)2) Fe iso-topic equilibrium fractionation factor (Polyakov and Mineev, 2000;Polyakov et al., 2007), the calculated temperature is ca. 900 ◦C(Fig. 11a). The Fe isotope fractionation between hematite and mag-netite is also limited, with �56Fehematite-magnetite = 0.07‰ (Table 4)and the calculated temperature ca. 700 ◦C (Fig. 11b). Although thecalculated temperatures may not be precise, the limited fraction-ations between magnetite and dolomite, and between hematiteand magnetite, indicate that the ore deposit has experienced high-temperature processes such as magmatism or high-temperaturehydrothermal metasomatism.

5.3. Constraints on hydrothermal processing

Another hypothesis about the genesis of Bayan Obo deposit isthat it is of hydrothermal origin (Chao et al., 1992, 1997; Yanget al., 2009). According to this hypothesis, the ore-hosting dolomitemarble is originally of sedimentary origin but are metasomated byeither epigenetic hydrothermal fluids (Chao et al., 1992, 1997) orhigh temperature fluids that are derived from mantle or carbonatitemagma (Yang et al., 2009). These hypothesis are mainly based onthe studies of the REE mineralization of the ore-hosting dolomitemarble. Here the Fe isotopes of Fe ore can provide directly insightsinto its genesis.

Both in low-temperature and high-temperature hydrothermalscenario, Fe is derived from leaching of preexisting protoliths orfrom exsolving of magma. It has been demonstrated that the leach-ing or exsolving of the fluid from the stock and the evolution of thefluid both fractionated Fe isotope significantly. The leached fluidfrom the stock is preferentially enriched in light Fe isotopes, andleaves the residue enriched in heavy Fe isotopes (Sharma et al.,2001; Rouxel et al., 2003; Horn et al., 2006; Markl et al., 2006; Wanget al., 2011).

A typical study on low-temperature hydrothermal iron oredeposit is done by Markl et al. (2006) in the Schwarzwald region,southwest Germany. The iron ores and Fe oxide mineral separatesshow variable Fe isotope compositions, covering a range in ı56Febetween −1.5‰ and 0.9‰, with the alteration products (secondaryhematite) between −1.5 and −0.5‰ (Fig. 10). Large Fe isotope vari-ations are also observed in the basaltic section with positive ı56Fevalues up to 1.37‰ for highly altered basalts and negative val-ues down to −1.66‰ for the associated alteration products andhydrothermal deposits in Jurassic Pacific oceanic crust seaward ofthe Mariana Trench (Rouxel et al., 2003). If the Bayan Obo fine-grained Fe ore was of epigenetic hydrothermal origin, it would havewidely variable and overall negative ı56Fe values. In contrast, thevariation in Fe isotope composition of the fine-grained Fe ores isvery small (Table 4, Fig. 10) and the ı56Fe values cluster around 0‰for the Bayan Obo fine-grained Fe ore. This, together with the factthat the Fe isotope fractionation between the magnetite and the

dolomite is minimal, totally rules out any possibility that the Fe oreis of low-temperature hydrothermal origin.

A few studies have been carried out on Fe isotope fraction-ation in high-temperature hydrothermal systems. For example, Itis found that the MORB hydrothermal fluids (>350 ◦C), where theFe is leached from the basalt (ı56Fe ≈ 0.1‰), have negative ı56Fevalues (ca. −0.7‰ to ca. −0.1‰) (Sharma et al., 2001; Beard et al.,2003b; Rouxel et al., 2004, 2008; Severmann et al., 2004; Bennettet al., 2009). Even in the process of mantle metasomatism, Fe iso-topes may fractionate remarkably. And relative large Fe isotopicvariations in ı56Fe (ca. −0.7‰ to ca. 0.2‰) have been observed inmetasomatized mantle xenoliths (Williams et al., 2005; Weyer andIonov, 2007; Zhao et al., 2010, 2012).

Wang et al. (2011) carried out a detailed investigation on the Feisotope fractionation during fluid exsolved from magma and dur-ing the following fluid evolution in the case of a skarn-type Cu–Fedeposit in the Middle-Lower Yangtze valley, China. They found thatı56Fe values of endoskarn and the earliest precipitated Fe mineralphase magnetite from the exsolving fluid are ca. 0.8‰ and ca. 0.2‰lower, respectively, relative to the diorite stock, indicating that theore-forming fluids exsolved from the magma is lighter than that ofthe magma. They also observed the systematic variations in ı56Fevalues both spatially and temporally, showing that Fe isotopes frac-tionate during fluid evolution. The pyrite in all varies from −0.83‰to 0.46‰ in ı56Fe values and the pyrite formed in the same stagehave a variation of ca. 1‰ in ı56Fe values. This is because Fe iso-tope fractionates during the mineral precipitation in the fluids. Thepyrite and siderite, for example, incorporate lighter Fe isotopes rel-ative to the Fe(II)aq (Wiesli et al., 2004; Butler et al., 2005), and themagnetite and hematite incorporate relatively heavier Fe isotopes(Johnson et al., 2002; Anbar et al., 2005). Graham et al. (2004) alsoobserved wide variation of Fe isotope compositions in Grasbergporphyry and skarn sulfides, pyrite and chalcopyrite (the ı56Fe val-ues range from ca. −0.6 to ca. 1.2‰ for pyrite and ca. −2.1 to ca.−0.5‰ for chalcopyrite in skarns), indicating significant Fe isotopefractionation during hydrothermal metasomatic processes.

The Fe isotope compositions of various types of rocks in the oredeposit show minimal variation and cluster around 0‰ in ı56Fevalues (Figs. 4 and 10), with the fine-grained Fe ores a little heavierthan the dolomite marble and the carbonatite dykes. The bulk sam-ples and Fe oxide minerals of the fine-grained Fe ores are ratherhomogeneous in ı56Fe values, ranging from ca. −0.15 to ca. 0.15‰.They show a normal distribution of Fe isotope compositions (Fig. 7),and no spatial Fe isotope variation has been observed—their Fe iso-topes are not related with the Fe contents (Fig. 5) or the samplelocation (Fig. 6). Therefore, it is very unlikely for Bayan Obo ironore to be formed by high temperature hydrothermal processes as awhole to the best of current understanding of Fe isotope system.

It should be emphasized that this interpretation does not actu-ally mean that hydrothermal activities exert no influence on theBayan Obo mineralization. Instead, there is plenty of evidence thathydrothermal activities played a very important part in the forma-tion of the deposit, as illustrated detailedly by Chao et al. (1997) andmany others as well as in this study. For example, pyrite in veinsand coarse massive Fe ore incise or replace the primary Fe ore, witheuhedral and coarse minerals (Fig. 2f), and the dolomite marble ismetasomatized by fluorite, riebeckite, pyrite, etc. (Fig. 3g). Thereis a tendency that the Fe isotope compositions are more variablefor mineral grains which are coarser or more euhedral (Fig. 8). Thefine-grained Fe ores have variation in ı56Fe values from −0.17‰ to0.13‰; the medium- to coarse-grained anhedral Fe ores have ı56Fevalues between −0.25‰ and 0.18‰; and the ı56Fe values rangefrom −0.20‰ to 0.62‰ for coarse massive Fe ores and from −0.67‰to 0.44‰ for pyrite in veins. This suggests that the Fe isotope com-positions of the primary Fe ore were modified as hydrothermalactivities went on.



a b

Magnetite-Ankerite Hematite-Magnetite

Δ F

e5

6 Δ F

e5

6

Fig. 11. �56Femagnetite-dolomite and �56Fehematite-magnetite values for Bayan Obo deposit minerals and theoretical equilibrium fractionation factors as a function of temperature.The gray solid lines show fractionation factors for (a) ankerite (Ca1.1Mg0.5Fe0.3Mn0.1(CO3)2) (Polyakov and Mineev, 2000) relative to magnetite (Polyakov et al., 2007), and(b) magnetite relative to hematite (Polyakov et al., 2007). The red square represents the average �56Femagnetite-dolomite (a) and �56Fehematite-magnetite (b) for Bayan Obo depositminerals. Error bars represents 2SE uncertainties. The calculated temperature is ca. 900 ◦C from (a) and ca. 700 ◦C from (b). The data are from Table 4.

Overall, the Bayan Obo deposit is modified to some extent byhydrothermal activities from the Fe isotope point of view, but itsorigin is not hydrothermal as a whole.

5.4. Evidences for magmatic origin

The occurrence of carbonatite dykes near the Bayan Obo depositmanifests the existence of magmatic carbonatites. The field rela-tionship, mineral constituents, and geochemistry of ore-hostingdolomite marble and their comparison with those of the carbon-atite dykes support a magmatic origin of Bayan Obo ore deposit(Zhou, 1980; Liu, 1986; Le Bas et al., 1992, 1997, 2007; Yuan et al.,1992; Bai et al., 1996; Wang et al., 2010; Yang et al., 2011b). The Feisotope results obtained in this study lends further support for thispoint of views.

It has been demonstrated that the igneous rocks have relativelyhomogeneous Fe isotope compositions (Beard and Johnson, 1999;Beard et al., 2003a; Dauphas and Rouxel, 2006; Schoenberg andvon Blanckenburg, 2006; Weyer and Ionov, 2007; Zhao et al., 2010,2012; Wang et al., 2012; Wang and Zhu, 2012). The mantle xeno-liths have ı56Fe values that cluster around 0‰. The silicate igneousrocks formed at different times, locations, and tectonic settings areclose to 0.1‰ in ı56Fe values. As for carbonatite, a special kind ofigneous rocks with carbonates as the main minerals, the whole-rock samples yield slightly negative ı56Fe values (ı56Fe ≈ 0.1‰ to−0.5‰, with average ı56Fe = −0.16‰). The average ı56Fe value ofcarbonate phases (calcite & dolomite) is −0.23‰ and the Fe oxidephase (magnetite) −0.03‰, with �56Femagnetite-carbonate ≈ 0.20‰(Dauphas et al., 2007b; Johnson et al., 2010). The differencesbetween the carbonate phases and Fe oxide or silicate phases inFe isotope compositions are consistent with the fact that carbon-ate minerals slightly enrich lighter Fe isotopes relative to silicateminerals or Fe-oxide minerals in high-temperature conditions asdemonstrated both experimentally and theoretically (Polyakov andMineev, 2000; Polyakov et al., 2007).

Few investigations have been done on the Fe isotope compo-sitions of magmatic iron ore deposits, which should be similar tothose of igneous rocks in principle. Recently, Wang et al. (2012) hasstudied the iron isotope compositions of Panzhihua V–Ti–Fe oredeposit, a typical magmatic iron ore deposit in Sichuan province,southwestern China, and the results show that the bulk samplesof both iron ores and ore-host rocks have small variation in ı56Fe

values (from −0.03 to 0.15‰), which are in good agreement withthose of basalts.

In this study, the results show that mafic dykes in Byan Oboarea range from 0.13‰ to 0.16‰ in ı56Fe values, consistent withthose of basalts that reported by previous studies. The carbonatitesfrom Shandong and Sichuan province vary from −0.47‰ to 0.08‰,with average ı56Fe = −0.12‰, and the carbonatite dykes in BayanObo ore deposit area show more homogeneous Fe isotope compo-sitions ranging from −0.26 to −0.03‰ in ı56Fe values, with averageı56Fe = −0.14‰, which are also consistent with those reported pre-viously.

For Bayan Obo ore deposit, both whole rocks and Fe oxide miner-als of Fe ore have rather homogeneous Fe isotope compositions thatrange from ca. −0.17‰ to ca. 0.15‰ in ı56Fe values, with averageı56Fe value of ca. 0‰, well agreed with those of mantle xeno-liths, silicate igneous rocks or magmatic V–Ti–Fe ores in Panzhihua(Fig. 10). Additionally, the Fe isotope compositions of Bayan OboFe ore show a normal distribution (Fig. 7) and they have no rela-tionship with Fe content (Fig. 5). These are also the characters ofigneous rocks (Beard and Johnson, 1999; Heimann et al., 2008; Tenget al., 2008).

As for the carbonate phases, the dolomite from the ore-hostingdolomite marble and the whole rock of relatively pure dolomitemarble ranges from ca. −0.3‰ to ca. 0‰ in ı56Fe values, and theFe-related mineralized dolomite marble show a larger Fe isotopevariation ranging from ca. −0.25 to ca. 0.20‰ in ı56Fe values. Theyare similar to those of the carbonatite dykes in Bayan Obo area andthe carbonatites worldwide, but distinct from those of Mesopro-terozoic sedimentary carbonates from Bayan Obo area and Hubeiprovince, China (Fig. 10).

Magnetite, hematite, and dolomite are the dominant Fe mineralsof Bayan Obo ore deposit, the Fe isotopes of which are most unlikelymodified on a large scale during metamorphism or deformation.The results show that the Fe isotope fractionation between mag-netite and dolomite, and between hematite and magnetite in BayanObo deposit is minimal (the �56Femagnetite-dolomite is 0.23‰, and the�56Fehematite-magnetite is 0.07‰). Based on the Fe isotopic equilib-rium fractionation predicted by theoretical calculation (Polyakovand Mineev, 2000; Polyakov et al., 2007), the calculated tempera-ture is around 700–900 ◦C.

In summary, Fe isotopes of Bayan Obo Fe ores and ore-hosting dolomite marble, and the Fe isotope fractionation between



magnetite and dolomite and between hematite and magnetite areall well consistent with those of magmatic system, but are distinctwith those of sedimentary or hydrothermal systems as describedabove, demonstrating that the deposit is of magmatic origin.

This interpretation is not inconsistent with observations ofhydrothermal metasomatic textures in the deposit as illustrated byChao et al. (1997) and others, showing that the dolomite is replacedby monazite, fluorite, etc. These replacement textures can be inter-preted as the result of post magmatic hydrothermal fluids thatcommonly occurs following the intrusion of igneous rocks or car-bonatites (Luo et al., 2007; Hou et al., 2009; Smith and Henderson,2000; Xie et al., 2009; Xu et al., 2010). But the ore deposit as a wholeis of magmatic origin.

6. Conclusion

The Fe isotope compositions of Bayan Obo ore deposit and therelated geological formations are investigated for the first time inthis study. Bulk samples and Fe-oxide mineral separates of thefine-grained iron ores in Bayan Obo ore deposit have relativelyhomogeneous Fe isotope compositions, ranging from −0.17‰ to0.15‰ in ı56Fe values. The ore-hosting dolomite marble has slightlynegative ı56Fe values varying from −0.26‰ to 0.1‰ (with oneexception). The Fe isotope fractionation between magnetite anddolomite, and between hematite and magnetite in Bayan Obodeposit is small, indicating that they formed in very high tempera-ture conditions. The Fe isotope systematics for Bayan Obo depositis consistent with those of magmatic products, but different fromthose of sedimentary or hydrothermal products reported previ-ously. It is thus concluded that the Bayan Obo ore deposit is ofmagmatic origin.

This conclusion is not contradictory with observations ofhydrothermal metasomatic textures in the deposit, which can beinterpreted to be formed by post magmatic hydrothermal fluids.

Acknowledgements

This study was financially supported by the Natural ScienceFoundation of China (Grant No. 40973037) and by the MLR PublicBenefit Research Foundation of China (Grant No. 200911043-14).We thank Prof. Yuxu Zhang, and chief engineer Gui Wen, seniorgeologist Jianyong Liu and other staff at the Bayan Obo Mine andBaotou Rare Earths and Steel Company, and thank Feifei Zhang,Zhaofu Gao, and Shixia Wang for their assistance during the fieldwork. Drs Yueling Guo, Drs Zhihong Li, Drs Yue Wang, Dr NickBelshaw, Dr Biao Jin, Prof. Suohan Tang and Dr Bin Yan are thankedfor assistance with the isotopic analyses. We thank ZhongqingZhang, Xinmiao Zhao, Jifeng Ying, Shihong Tian, and Yan Liu forproviding samples.

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