Abstract
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.
Highlights
The Bayan Obo deposit is the largest rare earth element (REE) accumulation in the world, and an important iron and niobium resource in China (Xie et al, 2016)
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
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
Summary
The Bayan Obo deposit is the largest rare earth element (REE) accumulation in the world, and an important iron and niobium resource in China (Xie et al, 2016). 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. 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. 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
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