Abstract

The Bayan Obo deposit in China is endowed with the largest rare earth element (REE) resource in the world. The mechanism resulting in this REE enrichment has been the focus of many studies. Carbonatite is known globally as the most favorable carrier of REE ores. In the Bayan Obo deposit, REE ores are hosted in dolomites (including coarse-grained and fine-grained varieties), and many carbonatite dikes (ferroan, magnesian, and calcic) have been identified. All of the dolomites and carbonatite dikes appear to be broadly coeval and possess similar geochemical characteristics. The Sm-Nd isochron age of apatite (1317 ± 140 Ma) from coarse-grained dolomite is consistent with the Th-Pb age of monazite (1321 ± 14 Ma) from a calciocarbonatite dike. The εNd(t) values and initial 87Sr/86Sr ratios at 1.3 Ga of apatite from coarse-grained dolomite show a tight cluster between −2.5 and +1.0 and between 0.70266 and 0.70293, respectively. The δ18OVSMOW values (relative to Vienna standard mean ocean water) of apatite also vary narrowly from 5.0‰ to 6.2‰. These results are consistent with primary mantle-derived carbonatite and prove a magmatic origin for the ore-hosting dolomite. Furthermore, the rim and core texture of dolomite and calcite in the magnesian and calcic carbonatite dikes shows that carbonatite at Bayan Obo has an evolutionary sequence from ferroan through magnesian to calcic in nature. There is a clear negative correlation between the iron content and REE concentration in different stages of carbonatite. Intense magmatic differentiation of carbonatite is likely the critical factor for the giant REE accumulation.

INTRODUCTION

The Bayan Obo deposit is the largest rare earth element (REE) accumulation in the world, and also an important iron and niobium resource in China (Xie et al., 2016). However, its genesis is still highly debated, largely due to pervasive post-ore modifications (Smith et al., 2015). In this deposit, the REE ores are mainly hosted in a suite of dolomite, which is divided into fine-grained and coarse-grained varieties. Moreover, various types of carbonatite dikes occur in the deposit; these dikes follow an intrusive sequence of dolomite (ferroan) type, dolomite-calcite (magnesian) type, and calcite (calcic) type (Yang et al., 2011). Due to the broad distribution of carbonatite dikes, it has long been envisaged that the REE mineralization has a genetic association with carbonatite. Moreover, the REE concentrations vary in different types of carbonatites. The calcic carbonatite dikes have much higher REE concentrations (REE2O3 up to 20%; Yang et al., 2011) as compared to the other two types. Therefore, the elemental differentiation process between various types of carbonatites is likely a critical factor for understanding the genesis of massive REE enrichment. However, the mechanism of carbonatitic magmatic evolution at Bayan Obo remains ambiguous. Whether or not the ore-hosing carbonate rocks are even magmatic in origin is a point of considerable debate.

In this paper, the ages of coarse-grained ore-hosting dolomite and a calciocarbonatite dike were determined through apatite Sm-Nd and monazite Th-Pb geochronometers, respectively. The results show that these carbonate rocks were all formed at ca. 1.32 Ga. In situ Sr-Nd-O isotopic compositions that are clearly indicative of a magmatic origin were obtained for apatite from coarse-grained dolomite. Furthermore, in situ REE concentrations obtained for carbonate minerals from ore-hosting dolomites and carbonatite dikes indicate that an intense magmatic differentiation, from ferroan to magnesian and further to calcic, is the main reason for the giant REE accumulation.

GEOLOGICAL SETTING AND SAMPLES

The Bayan Obo REE deposit is located in the northern margin of the North China craton (Fig. 1). The deposit occurs within a suite of low-grade metamorphic rocks of the Mesoproterozoic Bayan Obo Group. The Bayan Obo Group is divided into 18 units (H1–H18). The lower sequence (H1–H9 units) has gentle fold structures in this region and unconformably overlies the Neoarchean to Paleoproterozoic metamorphic basement rocks (Fig. 1). All of the REE-Nb-Fe orebodies (East, Main, and West) are hosted in a suite of dolomite that was previously classified as the H8 unit. Based on the grain size of major rock-forming minerals, the ore-hosting dolomite can be divided into coarse-grained and fine-grained dolomite. Coarse-grained dolomite occurs locally to the north of the Main and East orebodies and to the south of the West orebodies, commonly with layered and interbedded features (Fig. DR1A in the GSA Data Repository1). It mainly contains coarse-grained (300–1000 μm) dolomite with subordinate apatite, magnetite, and pyrochlore (Fig. DR1C). Fine-grained dolomite is much more widespread than coarse-grained dolomite, and contains obviously smaller dolomite crystals (50–150 μm) with subordinate magnetite, hematite, apatite, and monazite (Fig. DR1E).

A large number of carbonatite dikes can also be observed within the Bayan Obo region. The dikes intruded into the Bayan Obo Group and basement rocks (Fig. 1), causing intense fenitization of surrounding wall rocks (Fig DR2A). Based upon mineralogy, the dikes can be divided into dolomite, dolomite-calcite, and calcite varieties, geochemically corresponding to ferroan, magnesian, and calcic carbonatite (Le Maitre, 2002). Field crosscutting relationships show that emplacement of the calciocarbonatite dikes postdated that of the ferrocarbonatite dikes (Yang et al., 2011). Furthermore, our observations indicate that the dolomite in the magnesiocarbonatite dikes and calcite in the calciocarbonatite dikes exhibit obvious dark core and bright rim textures in cathodoluminescence images (Fig. DR2B). This reflects a reduction of iron content during the growth of carbonate minerals (Table DR3 in the Data Repository).

RESULTS

In situ Sm-Nd isotope measurements for apatite from coarse-grained dolomite yield a Sm-Nd isochron age of 1317 ± 140 Ma (Fig. 2C). This age is coincident with the Th-Pb date of zircon in the fine-grained dolomite (1301 ± 12 Ma; Zhang et al., 2017). The 208Pb/232Th ages of monazite collected from a calciocarbonatite dike vary over a wide range from 411 ± 6 Ma to 1321 ± 14 Ma (Fig. 2D; Fig. DR3B). As proposed by Song et al. (2018), the Bayan Obo deposit was intensely overprinted by externally derived fluids after the 1.3 Ga carbonatitic magmatism, and the Th-Pb isotopic composition was modified over an extended period of time. Therefore, only the oldest age of 1321 ± 14 Ma is interpreted here to represent the intrusion time of the calciocarbonatite dike, and is also consistent with the Sm-Nd isochron age of apatite from coarse-grained dolomite. Moreover, the εNd(t) values (t = 1.3 Ga) of apatite in the coarse-gained dolomite show a tight cluster between −2.5 and +1.0, the initial 87Sr/86Sr ratios of apatite at 1.3 Ga show a narrow range between 0.70266 and 0.70293 (Fig. 2B), and the δ18OVSMOW values (relative to Vienna standard mean ocean water) of apatite also vary little from 5.0‰ to 6.2‰ (Fig. 2A).

In situ major and trace element analyses show that the dolomites from coarse-grained dolomite are characterized by light REE (LREE) enrichment [(La/Yb)N = 13–36] and high REE concentration (∑REE = 55–99 ppm; Table DR3; Fig. 3A). In comparison, dolomites from ferrocarbonatite dikes have higher iron contents and exhibit LREE depletion [(La/Yb)N = 0.07–0.13] and low REE concentrations (∑REE = 22–27 ppm; Table DR3; Fig. 3B). Regarding zoned dolomite and calcite crystals from magnesian and calcic carbonatite dikes, their cores and rims have different compositions. The rim of dolomite with lower iron content from magnesiocarbonatite dikes contains a higher REE concentration (∑REE = 357–441 ppm) and is more enriched in LREEs [(La/Yb)N = 36–55] than the core (∑REE = 206–310 ppm, (La/Yb)N = 24–29; Table DR3). Similar REE variations also appear in calcite from calciocarbonatite dikes (Fig. 3C). Overall, there is a negative correlation between the iron content and REE concentration in carbonate minerals from coarse-grained dolomite and carbonatite dikes. A similar negative correlation characterizes whole-rock samples of the carbonatite dikes from ferroan through magnesian to calcic carbonatite (Fig. 3D).

DISCUSSION

A Magmatic Origin for Coarse-Grained Ore-Hosting Dolomite

Coarse-grained dolomite is commonly layered and shows interbedded features with wall rocks of the H4 unit; these features were once considered as important evidence for a sedimentary origin (Chao et al., 1997). However, our field observations indicate that the so-called layering is in fact structural foliation, rather than sedimentary strata. We also found euhedral pyrochlore and ovoid carbonates occurring as cogenetic inclusions trapped in apatite (Figs. DR1D and DR2C). Such observations are very similar to those noted for typical carbonatite (Chakhmouradian et al., 2017). Furthermore, disseminated riebeckite crystals locally occur in the coarse-grained dolomite near the contact with wall rocks (Fig. DR1B); riebeckite is commonly accepted as a metasomatism-induced phase related to carbonatite (Cooper et al., 2016). It is notable that apatite from coarse-grained dolomite has Sr-Nd isotopic compositions that differ from those of Mesoproterozoic sedimentary carbonate rocks in the region [εNd(t) = −5.3 to −6.1, 87Sr/86Sri = 0.730–0.731; Table DR1]. The δ18OVSMOW values of apatite also narrowly range from 5.0‰ to 6.2‰, which is coincident with mantle-derived carbonatite (5.3‰–8.4‰; Taylor et al., 1967; Deines, 1989). Collectively, these data prove that the coarse-grained dolomite has a magmatic origin, rather than being sedimentary carbonate rocks. Geochemically, it is very similar to magnesiocarbonatite dikes, and thus can be classified as magnesiocarbonatite.

Formation of the Giant REE Accumulation during Magmatic Evolution

A complete evolutionary sequence of carbonatitic magma, from ferroan through magnesian to calcic composition, developed in the Bayan Obo region. Pirajno et al. (2014) suggested that ferrocarbonatite can be generated from mantle plume-related magmatism. Wyllie and Lee (1998) proposed that carbonatitic magma can differentiate from magnesian to calcic. During the magmatic evolution, fractional crystallization can lead to the enrichment of incompatible elements in the residual magma (Yang and Le Bas, 2004). Indeed, the REE concentration increased during differentiation of the carbonatitic magma in the Bayan Obo deposit (Fig. DR4). The ferrocarbonatite dikes have the lowest REE concentration, showing slight enrichment in LREEs. In comparison, the magnesiocarbonatite dikes have a higher REE concentration with significant enrichment in LREEs. The calciocarbonatite dikes have an extremely high REE concentration and are most enriched in LREEs (Fig. 3A). The overall REE concentrations are, therefore, enriched with the decrease of iron contents from ferroan through magnesian to calcic carbonatite dikes (Fig. 3D).

The gradual increase in REEs and decrease in iron are due to separation of iron during different stages of carbonatite evolution. Microscopic observations show ∼5 vol% of cogenetic spherical hematite and carbonate inclusions trapped in dolomite phenocrysts from fine-grained dolomite (Fig. 3G), which resulted from immiscibility of iron and carbonatitic magma. Yang et al. (2011) proposed that the fine-grained dolomite has whole-rock major element and Sr-Nd isotope compositions comparable to those of ferrocarbonatite dikes. Nevertheless, the major element content of dolomite from fine-grained dolomite is consistent with dolomite from magnesiocarbonatite dikes (Wang et al., 2019). Therefore, we suggest that the fine-grained dolomite was derived from the ferrocarbonatite and, because of the separation of hematite from the iron-rich melts of primary magma, that the ferroan carbonatite evolved to a magnesian variety (Fig. 4B). The iron isotopes of hematite from fine-grained dolomite and iron ore, in fact, exhibit magmatic signatures (Sun et al., 2013). Furthermore, magnetite is closely accompanied by monazite in fine-grained dolomite (Fig. 3F). This mineral assemblage, in the form of veins and aggregates, is paragenetically later than deposition of spherical hematite and dolomite phenocrysts (Fig. 3F; Figs. DR1E–DR1F). Magnetite fractional crystallization has previously been proposed to be the product of breakdown of ankerite during decarbonization (Lentz, 2014). Considering the low REE concentrations in magnetite (Huang et al., 2015), we thus deduce that crystallization of magnetite triggered the precipitation of REE-bearing minerals during carbonatite differentiation. The negative correlation between iron and REEs from ferroan to magnesian carbonatite supports this hypothesis (Fig. 3D). Moreover, the early magnetite separation is also supported by the core and rim textures of dolomite from magnesiocarbonatite, where magnetite is commonly associated with the rim with a low iron content and high REE concentration (Fig. 3E). In general, the immiscibility of hematite and crystallization of magnetite in the ferrocarbonatite magma chamber would have removed a considerable amount of siderophile elements, leading to the evolution of carbonatite from ferroan to magnesian and gradual accumulation of incompatible elements (e.g., Th, Nb, and LREEs).

The highest REE concentration and (La/Yb)N ratio are observed in the calciocarbonatite dikes, implying that further enrichment of REEs occurred while magnesiocarbonatite evolved to calciocarbonatite. The calciocarbonatite dikes at Bayan Obo are always accompanied by intense fenitization (Fig. DR2A). Mian and Le Bas (1986) and Kresten (1988) suggested that fenitization between carbonatite and wall rocks induces removal of Mg and Fe and metasomatic gain in Si. The calciocarbonatite contains much lower Mg and Fe contents than magnesiocarbonatite, which is consistent with fenitization between a deep-seated magnesiocarbonatite pluton and wall rocks in the Bayan Obo rift (Fig. 4B). Fenite, composed of riebeckite and aegirine, not only consumes the Fe and Mg from the carbonatitic magma, but also has a low LREE concentration (Liu et al., 2018), which would lead to further accumulation of LREEs within residual calciocarbonatite. Moreover, as demonstrated by experiments and mineralogical observations (Wyllie et al., 1996), calcite from calciocarbonatite in the Bayan Obo region has lower degrees of differentiation between LREEs and heavy REEs [(La/Yb)N = 7.0–7.8; Table DR3; Fig.e 3B], which may result in extreme concentration of the LREEs in bastnaesite and parisite during the REE mineralization stage.

There are >500 carbonatite occurrences in the world (Woolley and Kjarsgaard, 2008), but only 5% of those are ferrocarbonatite. Significantly, carbonatite in the Bayan Obo region is a rare example where an intense differentiation process, shifting from ferroan to calcic magmas, took place. This is the real reason for the giant REE accumulation at Bayan Obo.

CONCLUSIONS

The REE mineralization in the Bayan Obo deposit is genetically related to mantle-derived carbonatite. The carbonatitic magmas show an evolutionary trend from ferroan through magnesian to calcic composition. Such an evolutionary process is responsible for progressive REE concentration. Immiscibility of iron-rich melts in the form of hematite and crystallization of magnetite in the magma chamber removed a considerable amount of iron. This led to the differentiation of carbonatite magma from ferroan to magnesian and resulted in the primary enrichment of REEs. Fenitization consequently consumed large amounts of Fe and Mg of magnesiocarbonatite, further leading to the immense accumulation of LREEs in residual calciocarbonatite. An intense magmatic evolution process was responsible for the giant REE accumulation of the Bayan Obo ore system.

ACKNOWLEDGEMENTS

We are grateful to Yue-heng Yang, Ting-guang Lan, Yan-wen Tang, and Zhi-hui Dai for their kind supports of the analytical experiments, and to Richard Goldfarb for insightful suggestions. This work was financially supported by the National Key R&D Program of China (grant 2017YFC0602302), the National Natural Science Foundation of China (grants 41372099, 41930430), and the Zhongke Developing Science and Technology Co., Ltd. (projects 2017H1973 and ZK2018H003). We are particularly grateful to David Lentz, Rex Taylor, and an anonymous reviewer for their constructive comments, and to editor Dennis Brown for handling our paper.

1GSA Data Repository item 2019404, analytical methods, Figures DR1–DR4, and Tables DR1–DR3, is available online at http://www.geosociety.org/datarepository/2019/, or on request from editing@geosociety.org.
Gold Open Access: This paper is published under the terms of the CC-BY license.