Geology | 2019
The Bayan Obo (China) giant REE accumulation conundrum elucidated by intense magmatic differentiation of carbonatite
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 δOVSMOW 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 finegrained 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 orehosting 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 *E-mails: [email protected]; fanhr@ mail.iggcas.ac.cn 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 [email protected]. Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G46674.1/4848238/g46674.pdf by guest on 03 November 2019 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America 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 SmNd 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 δOVSMOW 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 Figure 1. Geological map of Bayan Obo (China) rare earth element (REE) deposit with sampling locations (blue triangles) (modified after Yang et al., 2011). Inset shows location marked by red star.