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

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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 (mag- netite, 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. 123 Acta Geochim (2020) 39(1):139–154 141 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).
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