The Occurrences and Geochemical Characteristics of Thorium in Iron Ore in the Bayan Obo Deposit, Northern China

Acta Geochim (2020) 39(1):139–154 https://doi.org/10.1007/s11631-019-00331-3

ORIGINAL ARTICLE

The occurrences and geochemical characteristics of thorium in iron ore in the Bayan Obo deposit, Northern China

1 1,2,3 3 1 Xiaozhi Hou • Zhanfeng Yang • Zhenjiang Wang • Wencai Wang

Received: 3 December 2018 / Revised: 23 February 2019 / Accepted: 8 March 2019 / Published online: 13 March 2019 Ó Science Press and Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract The Bayan Obo deposit in northern China is an ThO2 and TFe, and REO and TFe in the six types of iron ultra-large Fe–REE–Nb deposit. The occurrences, and ore samples showed that ThO2 did not always account for geochemical characteristics of thorium in iron ores from the highest distribution rate in rare earth minerals, and the the Bayan Obo Main Ore Body were examined using main occurrence minerals of ThO2 were closely related to chemical analysis, field emission scanning electron iron ore types. microscopy, energy dispersive spectrometer, and automatic mineral analysis software. Results identified that 91.69% of Keywords Thorium Á Occurrence state Á Distribution law Á ThO2 in the combined samples was mainly distributed in Geochemical characteristics Á Iron ore Á Bayan Obo deposit rare earth minerals (bastnaesite, huanghoite, monazite; 56.43% abundance in the samples), iron minerals (magnetite, hematite, pyrite; 20.97%), niobium minerals 1 Introduction (aeschynite; 14.29%), and gangue minerals (aegirine, riebeckite, mica, dolomite, apatite, fluorite; 4.22%). An The Bayan Obo deposit, an importantly associated thorium unidentified portion (4.09%) of ThO2 may occur in other deposit, is internationally recognized as an ultra-large Fe– niobium minerals (niobite, ilmenorutile, pyrochlore). Only REE–Nb deposit (Drew et al. 1990; Chao et al. 1997; a few independent minerals of thorium occur in the iron ore Kynicky et al. 2012). Recent studies on this deposit have samples. Thorium mainly occurs in rare earth minerals in examined its formation processes and age (Yang et al. the form of isomorphic substitution. Analyses of the geo- 2014, 2017; Lai et al. 2015, 2016; Smith et al. 2015; Zhang chemical characteristics of the major elements indicate that et al. 2001, 2002; Fan et al. 2010, 2015, 2016; Zhu et al. thorium mineralization in the Main Ore Body was related 2015; Liu et al. 2004), its metallogenic tectonic environ- to alkali metasomatism, which provided source material ment (Yuan et al. 1992; Zhang et al. 2012), and its geo- and favorable porosity for hydrothermal mineralization. chemical characteristics (Lai et al. 2012a, b; Lai and Yang Trace elements such as Sc, Nb, Zr, and Ta have higher 2013; Yang et al. 2000, 2009). However, studies on tho- correlation coefficients with thorium, which resulted from rium geochemistry are generally lacking, especially on being related to the relevant minerals formed during tho- distribution law and occurrence of thorium in iron ores of rium mineralization. In addition, correlation analysis of the Bayan Obo deposit. About 0.221 Mt of thorium is present in the Bayan Obo deposit, accounting for 77% of China’s entire thorium & Xiaozhi Hou reserves (Xu et al. 2005). Although China is short of ura- [email protected] nium resources, it has an abundance of thorium. Therefore, 1 Mining Research Institute, Inner Mongolia University of the exploitation and utilization of thorium resources not Science and Technology, Baotou 014010, China only compensate for the uranium shortage, but also pro- 2 Rare Earth Research and the Comprehensive Utilization State mote the development of other advanced industries that Key Laboratory of Bayan Obo, Baotou 014000, China need thorium. However, although iron and some rare earth 3 Baotou Rare Earth Research Institute, Baotou 014000, China resources in the Bayan Obo deposit are exploited and 123 140 Acta Geochim (2020) 39(1):139–154 utilized, the utilization rate of thorium resources is zero (Su 3 Samples and experiments et al. 2005). Based on the industrial value of thorium (Kazimi 2003), it is necessary to investigate the occur- 3.1 Sample collection rences and distribution law of thorium in the Bayan Obo deposit in order to fully identify thorium resources. Tho- The mining level of the Bayan Obo Main Ore Body was rium occurrences in the ores are important for the suit- selected for this study. Sampling was undertaken in 2–14 ability of using the current technology to separate and exploration lines in the W–E trending, and six mining recycle thorium (Liu et al. 2016; Luo et al. 2010). There- levels (1528–1458) in the vertical direction. Reference was fore, iron ore from the Bayan Obo Main Ore Body was made to the original geology or production exploration selected as the research subject in this study. Mineral network degree (50–200 m 9 50–100 m), and sampling composition and contents of the iron ore, as well as the was undertaken using a block-picking method. The diam- distribution law, occurrence state and geochemical char- eter of the ore block was controlled at about 50–100 mm acteristics of thorium, were examined using chemical (Fig. 2a), and the coordinates of sample points were analysis, field emission scanning electron microscopy recorded using a GPS. Ore samples weighing about 5 kg (FESEM), energy spectrum analysis and automatic mineral were collected from each sampling point. A total of 101 analysis software. Findings from our study provide a the- samples were collected, and the samples all had typical oretical foundation for the exploitation of thorium resour- representation. TFe (total iron), REO (rare earth oxides) ces and the study of thorium metallogeny in the Bayan Obo and ThO2 grades of each ore sample were tested after deposit. crushing. Thirty-five samples met the requirements for REE–Fe ore (TFe C 20%, REO C 1%; Table 1). The iron ore samples included six major types of iron ore in the 2 Geological features of the deposit Main Ore Body of the Bayan Obo deposit: fluorite-type REE–Fe ore (Fig. 2b), aegirine-type REE–Fe ore (Fig. 2c), The Bayan Obo deposit is located 150 km north of Baotou, riebeckite-type REE–Fe ore, dolomite-type REE–Fe ore Inner Mongolia (Xiao and Kusky 2009; Bai et al. 1996). (Fig. 2d), mica-type REE–Fe ore and massive-type REE– The deposit is distributed in the southern limb of the Fe ore. Kuangou anticline, between dolomite and slate in both limbs of the southern syncline. A narrow ore belt (about 3.2 Experimental analysis 18 km long and 2–3 km wide) was formed in an east–west direction. Large and small iron ore bodies exist across an Although iron ore currently extracted from the Bayan Obo area of about 48 km2, among which the largest two ore deposit (Table 1) is the main raw material for ore dressing deposits are the Main Ore Body and the East Ore Body and smelting, a single iron ore type sample is not repre- located in the central part of the ore-bearing belt (Fig. 1). sentative of the whole deposit. Iron ore combined samples Sixteen iron ore bodies of different sizes are present in the were therefore prepared and tested to accurately identify western part of the ore-bearing belt, known as the West ore the average distribution rate of thorium in each mineral. body. The 35 typical REE–Fe ore powder samples from the Main The Main Ore Body is mainly composed of various Ore Body were combined, mixed and divided in equal types of iron ores containing niobium and rare earth ele- quantities and split into three parts of 50, 50, and 2000 g. ments. The ore types are diverse and the mineral compo- These samples were mainly used to study the distribution sition and structures complex. In the ore body, the fluorite- law and occurrence state of thorium. Multi-element type Nb–REE–Fe ore is distributed in the lower part, and chemical analysis was undertaken on one portion of the the aegirine-type Nb–REE–Fe ore and riebeckite-type Nb– 50 g sample. Na2O, K2O, MgO, and CaO were measured REE–Fe ore are distributed in the upper part near the using atomic absorption spectrometry (AAS) (SHIMADZU siliceous slate. The massive-type Nb–REE–Fe ore is dis- AA-6300CF). BaO, TiO2,Sc2O3, MnO2,Nb2O5,Al2O3, tributed in the middle section. The surrounding rock in the ThO2 and REO were measured using inductively coupled hanging side of the iron ore body is either mainly black plasma atomic emission spectrometry (ICP-AES) (Thermo siliceous slate or biotitization slate with interlayers of iCAP-6300). S was measured using a high frequency biotite. Biotitization of the slate becomes weakened with infrared carbon and sulfur analyzer (LECO CS-400). SiO2 increasing distance from the ore body. Due to sodium and P2O5 were measured by spectrophotometry, and F was metasomatism, an aegirine-type Nb–REE ore belt also determined by EDTA complexometric titration. FeO and developed, and the surrounding rock in the heading side of TFe were measured by potassium dichromate oxidation the iron ore body is dolomite-type Nb–REE ore. reduction titration.

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Fig. 1 Geological map of the Bayan Obo area, northern China (modiﬁed after Fan et al. 2016)

The other 50 g sample fraction was used for particle graphic processing technology (automatic mineral analysis analysis using mesh sizes of ? 200 mesh ([ 0.074 mm), software). The experimental conditions were: acceleration - 200 to ? 500 mesh (0.030–0.074 mm) and - 500 mesh voltage was 20 kV, the resolution was 0.8 nm, and the (\ 0.030 mm). These mesh sizes were prepared into probe current was 40–100 nA. mosaic samples and their surfaces were spray-coated with The 2000 g sample was screened into particle fractions platinum. The phase composition and the micro-area of - 80 to ? 120 meshes. Magnetite and hematite were composition of the samples were then characterized via separated using the weak and strong magnetic function of FESEM (ZEISS Sigma-500) and energy dispersive spec- the magnetic tube (RK/CXG-U50). Mineral separation of trometer (EDS; BRUKER XFlash 6 | 60) using a the remaining samples was conducted using magnetic backscattered electron imaging analysis technique, com- separators (S.G. FRANTZ LB-1) according to different bined with micro-area energy spectrum analysis. Minerals specific magnetization coefficients. Mixing powder for were quickly and accurately classified and quantified using each of the six types of iron ores was prepared to study its the automatic mineral recognition function and advanced geochemical characteristics. Major elements were

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Fig. 2 Main iron ores recovered from the Bayan Obo Main Ore Body. a Iron ore block. b Fluorite-type REE–Fe ore. c Aegirine-type REE–Fe ore. d Dolomite-type REE–Fe ore measured using ICP-AES. Trace and rare earth elements 4.1.2 Mineral composition analysis were measured by inductively coupled plasma mass spectrometry (ICP-MS) (NEXion300Q). All analyses were Mineral content results (Table 3) record iron minerals in undertaken in the Baotou rare earth research institute. the iron ore combined samples to be mainly magnetite, hematite, and pyrite. The rare earth minerals are mainly bastnaesite, huanghoite, and monazite, and the niobium 4 Results and discussion minerals are mainly aeschynite. The gangue minerals mainly consist of carbonates, silicates, phosphates, and 4.1 Sample analysis results ﬂuorides. The sum of iron minerals, rare earth minerals, and niobium minerals account for more than 55% of the 4.1.1 Chemical analysis total minerals. Although a few particles of independent minerals of thorium (thorite and ferrothorite) are recorded, Chemical analysis results of the iron ore combined samples their content is less than the detection limit. Therefore, from the Bayan Obo Main Ore Body (Table 2) contain thorite, ferrothorite, and other minerals with contents lower

32.38% TFe, 6.36% REO and 0.038% ThO2. ThO2 content than the detection limit are recorded as zero and they are in this ore is signiﬁcantly higher than the Clark value for not listed in Table 3. Th in the earth’s crust (9.6 9 10-4%) (Taylor 1964); the enrichment ratio of ThO2 in the Main Ore Body is nearly 4.2 Distribution law of thorium 35 times greater. Pure minerals in the samples were collected from the mineral composition analyses (Table 3) using a microscope (ZEISS Stemi 2000-C). A total of 16-types of single

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Table 1 Chemical analysis Number Iron ore type TFe (wt%) REO (wt%) ThO2 (wt%) results of iron ore samples from the Bayan Obo Main Ore Body 1 Fluorite-type REE–Fe ore 31.95 13.41 0.046 2 Fluorite-type REE–Fe ore 31.33 13.20 0.020 3 Fluorite-type REE–Fe ore 39.85 3.34 0.024 4 Fluorite-type REE–Fe ore 36.75 7.11 0.014 5 Fluorite-type REE–Fe ore 25.89 10.28 0.034 6 Fluorite-type REE–Fe ore 30.65 8.49 0.037 7 Fluorite-type REE–Fe ore 28.81 10.70 0.068 8 Fluorite-type REE–Fe ore 39.15 3.60 0.044 9 Fluorite-type REE–Fe ore 21.76 5.41 0.013 10 Fluorite-type REE–Fe ore 31.36 6.46 0.029 11 Fluorite-type REE–Fe ore 21.83 11.35 0.052 12 Fluorite-type REE–Fe ore 27.03 6.92 0.082 13 Fluorite-type REE–Fe ore 29.26 3.93 0.077 14 Fluorite-type REE–Fe ore 20.95 5.98 0.050 15 Fluorite-type REE–Fe ore 28.83 8.88 0.030 16 Fluorite-type REE–Fe ore 25.17 10.42 0.045 17 Fluorite-type REE–Fe ore 28.31 8.10 0.043 18 Fluorite-type REE–Fe ore 26.60 7.09 0.029 19 Fluorite-type REE–Fe ore 21.49 10.49 0.026 20 Fluorite-type REE–Fe ore 21.83 11.68 0.018 21 Aegirine-type REE–Fe ore 29.97 8.36 0.0086 22 Aegirine-type REE–Fe ore 30.40 8.28 0.019 23 Aegirine-type REE–Fe ore 28.95 8.68 0.019 24 Riebeckite-type REE–Fe ore 27.15 8.92 0.012 25 Riebeckite-type REE–Fe ore 39.37 2.10 0.087 26 Riebeckite-type REE–Fe ore 37.76 1.10 0.044 27 Riebeckite-type REE–Fe ore 42.36 2.71 0.064 28 Dolomite-type REE–Fe ore 36.57 4.48 0.0086 29 Dolomite-type REE–Fe ore 22.58 9.88 0.013 30 Mica-type REE–Fe ore 36.56 2.53 0.044 31 Mica-type REE–Fe ore 38.96 3.62 0.085 32 Mica-type REE–Fe ore 22.88 1.12 0.079 33 Massive-type REE–Fe ore 49.11 4.24 0.076 34 Massive-type REE–Fe ore 58.73 2.66 0.039 35 Massive-type REE–Fe ore 46.51 2.07 0.074

Table 2 Chemical analysis Elements Na2OK2O MgO CaO BaO SiO2 TiO2 FeO F results of iron ore combined samples from the Bayan Obo Contents (wt%) 0.74 0.47 2.23 16.94 2.48 8.49 0.66 11.23 8.33 Main Ore Body Elements Sc2O3 MnO2 Nb2O5 P2O5 Al2O3 S ThO2 TFe REO

Contents (wt%) 0.01 1.28 0.12 2.2 1.0 1.06 0.038 32.38 6.36

minerals were separated. Each mineral was more than 1 g thorium (thorite), rare earth minerals, niobium minerals, and purity was greater than 99%. The content of ThO2 in iron minerals, silicate minerals, carbonate minerals, phos- each mineral was chemically analyzed (Table 4). phate minerals, and halide minerals. The average content of

Minerals containing thorium mainly include eight cate- ThO2 in thorite is 55.2700%. The average content of ThO2 gories (14 types of minerals): independent minerals of in magnetite and hematite were basically the same, but it

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Table 3 Mineral composition and content of the iron ore combined samples from the Bayan Obo Main Ore Body Minerals name Magnetite Hematite Pyrite Pyrrhotine Siderite Ilmenite Rhodochrosite Psilomelane Galena Sphalerite

Content (wt%) 33.04 9.57 0.75 0.11 0.13 0.66 0.25 0.10 0.12 0.06 Minerals name Bastnaesite Parisite Huanghoite Monazite Allanite Aeschynite Niobite Ilmenorutile Pyrochlore Quartz Content (wt%) 7.52 0.53 0.26 2.23 0.10 0.21 0.07 0.06 0.06 1.24 Minerals name Feldspar Amphibole Pyroxene Mica Calcite Dolomite Fluorite Apatite Barite Other Content (wt%) 0.84 3.29 3.87 3.15 2.35 6.98 13.99 3.91 4.36 1.18 caGohm(00 39(1):139–154 (2020) Geochim Acta Acta Geochim (2020) 39(1):139–154 145

Table 4 Average ThO2 content of partial minerals in iron ore combined samples of the Bayan Obo Main Ore Body

Minerals categories Minerals name Chemical formula Average ThO2 content (wt%)

Thorium minerals Independent Thorite ThSiO4 55.2700* minerals

Minerals containing thorium Iron minerals Magnetite Fe3O4 0.0180

Hematite Fe2O3 0.0210

Pyrite FeS2 0.0015

Rare earth minerals Bastnaesite (Ce,La)(CO3)F 0.1889

Huanghoite (Ce,La)Ba(CO3)2F 0.2700

Monazite (Ce,La)PO4 0.2931

Niobium minerals Aeschynite (Ce,Nd)(Ti,Nb)2O5 2.5850 3? Silicate minerals Pyroxene (aegirine) NaFe [Si2O6] 0.0027

Amphibole Na2Fe3Fe2[Si4O11]2(OH)2 0.0043 (riebeckite) 2? 3? Mica K(Mg,Fe )3(Al,Fe )Si3O10(OH,F)2 0.0270

Carbonate minerals Dolomite CaMg(CO3)O2 0.0021

Phosphate minerals Apatite Ca5(PO4)3F 0.0046

Halide minerals Fluorite CaF2 0.0013 Minerals containing no Sulﬁde minerals Galena PbS – thorium Sulfate minerals Barite BaSO4 –

Oxide minerals Quartz SiO2 –

*ThO2 results of thorite were calculated from the average values of the Th energy spectra of 10 thorite samples according to the ThO2 molecular formula—indicates that the chemical analysis result was below the detection limit

was notably higher than that in pyrite. The average content monazite is 10.01%, and the total ThO2 distribution rate is of ThO2 in bastnaesite, huanghoite, and monazite is 0.1889, 56.43%, indicating that the rare earth minerals are the most 0.2700, and 0.2931%, respectively. The average content of important minerals containing thorium. The average con-

ThO2 in monazite is significantly higher than that in bast- tent of ThO2 in monazite is higher than that in bastnaesite. naesite and huanghoite. The average content of ThO2 in However, because the mineral content of bastnaesite is aeschynite is 2.5850%, this mineral having the second significantly higher than that of monazite, the distribution highest ThO2 content after thorite. The average content of rate of ThO2 in bastnaesite is about 2.17 times greater than ThO2 in mica is obviously higher than that in aegirine and that in monazite. Although the average content of ThO2 in riebeckite. The average content of ThO2 in dolomite, aeschynite is high, total ThO2 distribution rate is 14.29% apatite, and fluorite is low, with a value less than 50 ppm. due to the low mineral content of aeschynite. The

The chemical measurement value of ThO2 in the iron unidentiﬁed 4.09% of ThO2 might be more likely to occur ore samples is 0.038% (Table 1). Based on the detection in other niobium minerals (niobite {(Fe,Mn)(Nb,Ti,Ta)2- value of ThO2 in minerals containing thorium and their O6}, ilmenorutile {(Ti,Nb,Fe)O2} and pyrochlore mineral amount, the equilibrium of ThO2 in iron ore {(Ca,Na,Ce)2(Nb,Ti,Ta)2O6(F,OH)}). The total mineral samples was calculated (Table 5). The sum of the mineral content of aegirine, riebeckite, mica, dolomite, apatite and, content of the 16 target minerals is 94.49%, and the total ﬂuorite is 35.19%, and the total distribution rate of ThO2 is distribution quantity of ThO2 is 0.03644%. 95.91% of 4.22%. Because ThO2 is not present in galena, barite, and ThO2 is found to be distributed in 13 minerals, except for quartz, the distribution rate of ThO2 is ‘‘0’’. In summary, it thorite. Because thorite has a low mineral content, its dis- can be concluded that the distribution rate of ThO2 in rare tribution rate is recorded as ‘‘0’’. The total mineral content earth minerals is the highest, followed by iron minerals and of magnetite, hematite, and pyrite is 43.36%, and the total niobium minerals.

ThO2 distribution rate was 20.97%. Due to the high min- From the aspect of existing mineral processing tech- eral content of magnetite, the distribution rate of ThO2 in nology, and according to existing technical conditions, rare magnetite is about 3 times greater than that in hematite. earth minerals and niobium minerals containing high tho- The total mineral content of bastnaesite, huanghoite and rium could be adopted as the main objects for thorium

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Table 5 ThO2 equilibrium calculation results of iron ore combined samples in the Bayan Obo Main Ore Body

Containing thorium ThO2 content Mineral content Distribution quantity Distribution rate Total distribution rate of similar minerals (wt%) (wt%) (wt%) (wt%) minerals (wt%)

Thorite 55.2700 – – – – Magnetite 0.0180 33.04 0.00595 15.65 20.97 Hematite 0.0210 9.57 0.00201 5.29 Pyrite 0.0015 0.75 0.00001 0.03 Bastnaesite 0.1889 7.52 0.01421 37.38 56.43 Huanghoite 0.2700 0.26 0.00070 1.85 Monazite 0.2931 2.23 0.00654 17.20 Aeschynite 2.5850 0.21 0.00543 14.29 14.29 Pyroxene (aegirine) 0.0027 3.87 0.00010 0.27 2.88 Amphibole 0.0043 3.29 0.00014 0.37 (riebeckite) Mica 0.0270 3.15 0.00085 2.24 Dolomite 0.0021 6.98 0.00015 0.39 0.39 Apatite 0.0046 3.91 0.00018 0.47 0.47 Fluorite 0.0013 13.99 0.00018 0.48 0.48 Galena – 0.12 – – – Barite – 4.36 – – – Quartz – 1.24 – – – Total 94.49 0.03644 95.91 95.91

recovery. Due to the low content of ThO2 in the minerals, minerals might be concentration of Si, pressure of CO2, such as the magnetite, hematite, mica, dolomite, apatite, and other components in the ore-forming fluid. In the CO2- and fluorite, or the low content of the mineral, it is hard to rich carbonate medium environment, thorium readily recycle and extract these thorium resources using current formed thorianite. Conditions poor in CO2 and rich in Si 4- technology. result in Th being readily combined with [SiO4] to form thorite. If the concentration of Fe3? is high, Fe3? would be 4.3 Occurrence state of thorium involved in a crystal lattice of thorite to form ferrothorite (Institute of Geochemistry 1988). 4.3.1 Independent minerals 4.3.2 Isomorphism Mineral measurement and chemical analysis of the combined samples indicate that there are two main types of The second occurrence state of thorium exists in the form thorium occurrences in iron ore from the Bayan Obo Main of isomorphism. In this study, the occurrence of thorium in Ore Body. The first type exists in the form of independent bastnaesite and monazite is mainly discussed. Most bast- minerals, mainly thorite (Fig. 3a, b) and ferrothorite naesite is yellow or brown-yellow in color, presenting in

{(Th,Ca,RE,Fe)[(Si,P)(O,OH)4]ÁnH2O} with less amount the form of small particles, and it is imbedded in various of thorianite. Thorite output is mainly in the form of par- types of ore. The particle size of bastnaesite is usually less ticles with round or other shapes, with a particle size than 0.1 mm and it is commonly presented as aggregates of ranging from 0.005 to 0.05 mm. The majority of the par- fine particles. Monazite is mostly light-yellow or brown- ticles are disseminated and imbedded in magnetite or other yellow in color, the majority of which occurs as aggregates minerals in the form of fine particles. The main paragenetic of fine particles. Large particles are presented in a plate or minerals are aegirine, biotite, and riebeckite, followed by irregular shape, and particle sizes range from 0.005 to fluorite, bastnaesite, monazite, pyrochlore, etc. Mineral 3 mm. Fluorite, aegirine, amphibole, quartz, barite, and paragenesis analysis results indicate that the main con- magnetite are the common paragenetic minerals of the rare trolling factors for the formation of thorium independent earth minerals. Because the ion radius and the negative

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Fig. 3 a Back scattering images of Thorite. b Energy spectrum analysis images of Thorite charge of thorium are similar to cerium group rare earth thorium is relatively rich in these two minerals. Figure 4d elements, some thorium occurrs in rare earth minerals in shows that the brightness of monazite is obviously greater the form of isomorphism. The electrovalence compensation than that of bastnaesite, reﬂecting a higher content of ThO2 3? for Th to replace Ce Pin rare earth minerals is carried out in monazite than that in bastnaesite. through Ca or Ba as: 2 Ce3? / Th4? ? (Ca, Ba)2? (Hou et al. 2018). 4.4 Geochemical characteristics of thorium In order to better illustrate the occurrence state of thorium, the surface of bastnaesite, monazite, and other min- 4.4.1 The relationship between thorium enrichment erals were scanned using SEM. Results from this analysis and major elements are shown in Fig. 4. Surface scanning analysis of Ce and P elements The analytical results of major elements, rare earth ele- (Fig. 4b, c) in the minerals (Fig. 4a) reﬂect the distribution ments, and trace elements are listed in Tables 6, 7 and 8, of the main elements in bastnaesite and monazite. Surface respectively. Analysis results of major elements, thorium, scanning analysis of Th (Fig. 4d) indicates that that the and uranium (Table 6) indicate that the ratio of Th:U in brightness degree of minerals differs, suggesting that different types of iron ore are relatively diverse, generally minerals generally contained Th. Obvious bright spots in between 10 and 320. Thorium-rich and depleted uranium bastnaesite and monazite were recorded, illustrating that are the geochemical characteristics of the Main Ore Body.

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Fig. 4 Surface scanning images of bastnaesite and monazite

Table 6 Contents of major Elements REE–Fe ore type (wt%) elements, Th and U (ppm) elements of different Fluorite-type Aegirine-type Riebeckite-type Mica-type Dolomite-type Massive-type types of iron ore from the Bayan Obo Main Ore Body Na2O 0.46 2.95 1.99 0.80 0.14 0.28

K2O 0.14 0.066 0.38 0.75 0.24 0.15 MgO 1.61 1.02 2.52 2.10 2.08 1.08 CaO 23.63 13.20 10.73 13.85 12.86 12.38

SiO2 6.67 14.62 12.00 11.55 1.63 3.18

TiO2 0.31 0.72 1.15 1.95 0.15 0.38 FeO 8.68 8.37 14.86 18.84 8.56 22.03

MnO2 0.41 0.32 0.67 2.10 0.58 0.79

Al2O3 0.66 0.13 0.34 0.95 0.21 0.22

P2O5 2.31 3.64 1.60 1.79 6.82 1.17 TFe 28.40 29.19 36.85 38.77 30.66 51.19 Th 377.9 562.4 764.5 641.5 44.8 483.3 U 1.7 2.1 2.5 2.0 3.4 2.2

The content of Na2O in most samples is higher than that of et al. 2015; Institute of Geochemistry 1988; Zhang 1995; K2O, which is related to the hydrothermal metasomatism Du 2002) indicates that the alteration of ore bodies and process in this area. A large volume of research data (Yang surrounding rock in the Bayan Obo deposit are mainly

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Table 7 Contents of rare earth (ppm) elements of different types of ore from the Bayan Obo Main Ore Body Elements REE–Fe ore type Fluorite-type Aegirine-type Riebeckite-type Mica-type Dolomite-type Massive-type

La 17,737 22,768 9209 2558 18,760 4263 Ce 34,430 37,441 17,744 7977 32,558 12,616 Pr 3311 3477 2069 910.9 2980 1656 Nd 10,457 10,457 7971 4200 8829 6857 Sm 733.1 810.7 569.2 517.5 655.5 413.9 Eu 120.9 155.5 120.9 38.7 146.8 53.55 Gd 329.6 407.7 260.27 364.37 242.9 85.01 Tb 8.50 3.40 5.10 3.40 10.20 5.95 Dy 24.40 8.71 20.04 8.71 34.85 17.43 Ho 5.65 4.28 1.92 2.17 3.31 1.05 Er 17.16 14.68 7.4 8.45 9.78 2.68 Tm 1.22 0.73 0.31 0.33 0.62 0.27 Yb 3.58 3.21 1.35 1.61 2.78 1.75 Lu 0.45 0.40 0.19 0.27 0.35 0.16 P REE 67,180 75,552 37,980 16,591 64,234 25,974 LREE 66,789 75,109 37,683 16,202 63,929 25,859 HREE 390.6 443.1 296.6 389.3 304.8 114.3 LREE/HREE 171.01 169.50 127.06 41.62 209.75 226.24

Table 8 Contents of trace Elements REE–Fe ore type (ppm) elements of different types of ore from the Bayan Fluorite-type Aegirine-type Riebeckite-type Mica-type Dolomite-type Massive-type Obo Main Ore Body Ba 28,900 36,400 14,200 17,900 39,600 1000 Sc 71.72 71.72 169.5 117.3 39.77 84.76 Nb 699.2 1678 1887 1048 153.8 559.4 Y 267.7 228.4 181.1 362.2 220.5 53.5 V 110 320 190 310 220 92 Cr 51 55 60 590 69 36 Co 12 7 17 29 11 5 Ni 49 23 31 110 52 98 Rb 8 10 9 23 7 6 Sr 800 1600 790 460 700 490 Zr 400 330 620 740 400 350 Hf 15 14 19 30 17 14 Pb 400 49 200 130 78 350 Ta 1.6 1.8 2.1 2.3 1.7 1.4 caused by the metasomatism of fluorine, sodium, and mainly formed mica-type Nb–REE–Fe ore. The contents of potassium. The metasomatism of fluorine mainly reacts thorium in aegirine-type Nb–REE–Fe ore, riebeckite-type with calcareous rock to form fluorite-type Nb–REE–Fe ore. Nb–REE–Fe ore, and mica-type Nb–REE–Fe ore, which The metasomatism of sodium is mainly represented by were formed by strong sodium–potassium metasomatism in aegirinization, riebeckitization, and albitization, predomi- the Main Ore Body, are obviously higher than that of nantly forming aegirine-type Nb–REE–Fe ore and dolomite-type Nb–REE–Fe ore with weak fluorine–sodium riebeckite-type Nb–REE–Fe ore. The metasomatism of metasomatism. potassium is mainly represented by biotitization and 123 150 Acta Geochim (2020) 39(1):139–154

Correlation coefficients between Na2O, SiO2, TiO2, and between Th and P2O5 is due to a large amount of monazite Th are 0.60, 0.78 and 0.72, respectively, showing a sig- and apatite produced in the process of mineralization. nificant positive correlation. The correlation coefficient between P2O5 and Th is - 0.81, showing a significant 4.4.2 The relationship between thorium enrichment negative correlation. Correlation between Th and other and trace elements elements is not obvious (Table 9). The correlation coefficients of Na2O and K2O are 0.60 The REE characteristic values of theP samples (Table 7) and 0.38, respectively (Table 9), indicating that the role of indicate that the fluctuation range of REE in different potassium in the mineralization process of the Main Ore types of iron ore is relatively large. The average value of Body is not significant. The metasomatism of sodium was LREE/HREE is 139.2, which is higher than the average very uneven in the Main Ore Body; the precipitants of value of LREE/HREE in the crust (Li 1976), indicating that sodium are mainly iron and silicon, and their metasoma- the fractionation of light and heavy rare earth elements is tism is mainly developed along rocks containing silicon significant. The rare earth elements are mainly composed and iron. Due to the regular distribution of the original of the cerium group light rare earth elements, and the rocks, the intensity of metasomatism varies in different content of Ce accounts for more than 50% of the total rare areas (Institute of Geochemistry 1988). Alkaline metaso- earth elements. The enrichment degree of heavy rare earth matism can bring alkaline ions such as Na?,Ca2?,OH-, elements is low. - 2- Cl and CO3 from the ore-forming hydrothermal solu- The REE distribution patterns of different types of ore in tion into the rock. An alkaline hydrothermal solution can the Main Ore Body have the characteristics of being extract thorium elements from the surrounding rock, but inclined to the right and having a large slope (Fig. 5a). also can increase the porosity of rock by hydrothermal Rare earth elements have similar activity and have tracer metasomatism, providing favorable conditions for the effects in the metallogenic process, which can indicate the precipitation and enrichment of thorium (Meng and Fan metallogenic environment and physical and chemical

2013). Under the action of volatiles (F, S, P, CO2, etc.), conditions (Hanson 1980). In the ore-forming hydrother- with the precipitation of alkali metals, alkalinity gradually mal solution, thorium and rare earth elements have similar decreases, and the complex in the solution decomposes geochemical characteristics, so they were simultaneously accompanied by mineralization of thorium (Institute of activated and transported over long distances in the form of

Geochemistry 1988). Liu and Cao (1987) noted that Th a carbonate complex (Huang et al. 2010). When CO2 in the migration was controlled by the proportion of volatiles. ore-forming fluid escapes, the carbonate complex disinte- Therefore, the mineralization of thorium was the result of grates at the same time. The solubility of the LREE car- various components, various conditions restricting each bonate anion complex is higher than that of the HREE other and comprehensive action. Alkali metasomatism carbonate anion complex, resulting in the content of LREE occupies a relatively important position among them. in the hydrothermal solution being higher than that of There is a significant positive correlation between Th HREE. Under the same conditions, LREE carbonate anion and SiO2 (Table 9). It was speculated that at least a part of complex is more prone to precipitate than HREE carbonate U and Th in the Main Ore Body may be transported in the anion complex (Huang et al. 2010; Cantrell and Bryne form of a silicate complex, and the silicate complex of U 1987). During the precipitation process, due to a large and Th are stable only in an alkaline environment. When amount of thorium in the hydrothermal solution, some the pH value decreases, the silicate complex of U and Th thorium can occur in rare earth minerals in the form of are decomposed. After U was precipitated, it was dispersed isomorphism. in the minerals such as fergusonite, pyrochlore, and Cao et al. (1994) and Tu (1998) combined isotopes, xenotime, and there was no excess U to form independent geochemistry and other methods to demonstrate that the minerals of uranium. Due to the solution containing a large ore-forming elements of REE and Nb in the Bayan Obo amount of Th, some Th entered the rare earth minerals, and deposit are from the mantle. The ore-forming fluid was there was excess Th to form independent minerals of tho- relatively rich in sodium, potassium and other alkali met- rium. As a result, the Th value is larger (Institute of Geo- als, siliceous and rare earth elements, niobium and other chemistry 1988). The significant negative correlation incompatible elements, and rich in a deep volatile system (H–A–C–O–N–S) (Du 1996, 2000). It can be seen from the

Table 9 Correlation Elements Na2OK2O MgO CaO SiO2 TiO2 FeO MnO2 Al2O3 P2O5 TFe coefﬁcients of Th and major elements of samples Th 0.60 0.38 0.11 - 0.26 0.78 0.72 0.47 0.33 0.26 - 0.81 0.30

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100000 thus forming thorium mineralization (body) (Wu et al. Fluorite type 2005). Aegirine type The correlation coefﬁcient between trace elements 10000 Riebeckite type Mica type (Table 10) indicates that Th has a signiﬁcant positive Dolomite type correlation with Sc, Nb, Zr, and Ta. Among minerals such Massive type 1000 as zircon, titanite, allanite, monazite, pyrochlore, and ilmenorutile associated with thorium minerals, and in addition to containing higher levels of Sc, Nb, Zr, Ta, etc., 100 it also contains more Th. Some minerals have similar

SampleChondrite / geochemical properties and some were closely associated 10 with each other, leading to a closer correlation between Th and trace elements in the MainP Ore Body. Th was nega- (a) tively correlated with Ba, U, REE and LREE, while 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu other elements had a reduced correlated with Th. Correlation analysis of elements identiﬁed from chem- 100000 (b) Fluorite type ical analysis of the iron ore samples (see Table 1) indicate 10000 Aegirine type that correlations exist between ThO and REO and ThO Riebeckite type 2 2 Mica type and TFe (Fig. 6a, b). 1000 Dolomite type Massive type Although the distribution rate of ThO2 in rare earth 100 minerals was the highest, ThO2 and REO were negatively correlated from the perspective of different iron ore types

10 (Fig. 6a). This indicates that not all ThO2 in the six different types of iron ore had the highest distribution rate in 1 the rare earth minerals. However, ThO2 and TFe were

Sample / Primitive Mantal / Primitive Sample 0.1 positively correlated (Fig. 6b), indirectly indicating that the distribution rate of ThO2 in iron minerals might be higher 0.01 than that of rare earth minerals in certain types of iron ores. According to the main distribution positions, the scattered Rb Ba U Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y TmYb points in Fig. 6a, b were divided into three regions. Results indicate that the scattered points in the riebeckite-type Fig. 5 a Chondrite-normalized REE abundance diagram for samples from Bayan Obo Main Ore Body. Chondrite values are from Masuda REE–Fe ore, mica-type REE–Fe ore and massive REE–Fe et al. (1973). b Primitive mantle-normalized trace element abundance ore were mainly distributed in area I. The contents of ThO2 pattern for samples from Bayan Obo Main Ore Body. Primitive and TFe in this area are relatively high, and the content of mantle values are from Sun and McDonough (1989) REO is relatively low, indicating that the distribution rate of ThO in iron minerals would occupy a high proportion. primitive mantle-normalized trace element abundance 2 SEM analysis also identified that small amounts of thorite pattern of trace elements that Th element was a strong and ferrothorite are mixed in the magnetite of the three positive anomaly (Fig. 5b), and it was obviously enriched types of iron ores, therefore ThO and TFe has a positive in Ba, Th, Nb, and REE, and depleted in U, K, Ta, Sr, P, Zr 2 correlation in these types of iron ore. The scattered distri- and Hf. This finding indicates that Th, Nb, and REE have bution of fluorite-type REE–Fe ore is relatively concen- the same matter source and the same mineralization pro- trated and mainly distributed in area II. The scattered spots cess. In summary, we believe that the ore-forming fluid in of aegirine-type REE–Fe ore and dolomite-type REE–Fe this area originates from the mantle. Mantle fluids from ore are mainly distributed in area III. In areas II and III, a deep areas have a super ability of dissolving and extracting. positive correlation between ThO and REO exists, but During the upward migration of the mantle fluid, it can not 2 ThO is negatively correlated with TFe, indicating that the only extract the relatively enriched and incompatible Th, 2 distribution rate of ThO in rare earth minerals is higher in REE and other metallogenic elements in the crust, it can 2 these three types of iron ore. also react with basement and surrounding rocks to extract Th, REE etc. The content of metallogenic elements such as Th and REE in the final hydrothermal fluid is much higher 5 Conclusions than that of the mantle fluid itself and surrounding rock. With the change of physical and chemical conditions, (1) The distribution of 95.91% of ThO in minerals was precipitation is unloaded in favorable metallogenic space, 2 identified. Among them, 56.43 and 14.29% of ThO2 123 152 Acta Geochim (2020) 39(1):139–154

Table 10 Correlation Elements Ba Sc Nb Y V Cr Co Ni Rb coefﬁcients of Th and trace elements of samples Th - 0.55 0.87 0.85 0.06 0.21 0.30 0.39 0.03 0.42 P Elements Sr Zr Hf Pb Ta U REE LREE HREE

Th 0.10 0.54 0.34 0.05 0.51 - 0.57 - 0.48 - 0.49 0.09

Fig. 6 Correlation between ThO2 and REO and ThO2 and TFe

were distributed in rare earth minerals (bastnaesite, ThO2 did not always account for the highest huanghoite and monazite) and niobium minerals distribution rate in rare earth minerals, and the main

(aeschynite), respectively. The remaining 25.19% of occurrence minerals of ThO2 were closely related to ThO2 was distributed in iron minerals and some iron ore types. gangue minerals. (2) Thorium independent minerals are sparse in iron ore Acknowledgements This study was supported by the National Basic combined samples from the Bayan Obo Main Ore Research Program of China (973 Program) (2012CBA01200) and Body. Thorium occurs in in rare earth minerals, Northern Rare Earth Science and Technology Project (BFXT-2015- niobium minerals, and other minerals mainly in the D-0002) and (2016H1928). form of isomorphism. (3) Thorium mineralization of the Main Ore Body was References mainly related to alkali metasomatism, which pro- vides material conditions and porosity for later Bai G, Yuan ZX, Wu CY, Zhang ZQ, Zheng LX (1996) Demonstra- hydrothermal mineralization. Ba, Th, Nb, and REE tion on the geological features and genesis of the Bayan Obo ore were obviously enriched in the Main Ore Body, deposit. Geological Publishing House, Beijing, pp 24–46 indicating that they were consistently enriched in the Cantrell KJ, Bryne RH (1987) Rare-earth element complexation by carbonate and oxalate ions. Geochim Cosmochim Acta process of mineralization, and at the same time, a 51:597–605 large area of alkaline alteration was developed in the Cao RL, Zhu SH, Wang JW (1994) Source materials for the Bayan ore body, both of which indicated that the ore- Obo Fe–REE–Nb ore deposit and problems of the genetic theory. forming ﬂuid have the characteristics of mantle Sci China (Ser B) 24:1298–1307 (in Chinese with English abstract) source. The correlation coefﬁcients of trace elements Chao ECT, Back JM, Minkin JA, Tatsumoto M, Wang JW, Conrad Sc, Nb, Zr, Ta and thorium were higher, which was JE, Mckee EH, Hou ZL, Meng QR, Huang SG (1997) The related to the associated minerals in the formation of sedimentary carbonate-hosted Giant Bayan Obo REE–Fe–Nb ore thorium mineralization. deposit of Inner Mongolia, China: a cornerstone example for giant polymetallic ore deposit of hydrothermal origin. USGS (4) Correlation analysis of ThO2 and TFe, and REO and Bull 2143:1 TFe in the six types of iron ore samples showed that

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