Fan, and 4 co-authors, reported in 2016 that the Bayan Obo ore deposit is the largest rare-earth element (REE) resource, and the second largest niobium(Nb) resource in the world. Due to the complicated element/mineral compositions and involving several geological events, the REE enrichment mechanism and genesis of this giant deposit still remains intensely debated. The deposit is hosted in the massive dolomite, and nearly one hundred carbonatite dykes occur in the vicinity of the deposit. The carbonatite dykes can be divided into three types from early to late: 1) dolomite, 2) co-existing dolomite-calcite and 3) calcite type, corresponding to different evolutionary stages of carbonatite magmatism based on the REE and trace element data. The latter always has higher REE content.
The origin of the ore-hosting dolomite at Bayan Obo has been addressed in various models, ranging from a normal sedimentary carbonate rocks to volcano-sedimentary sequence, and a large carbonatitic intrusion. Geochemical evidence shows that the coarse-grained dolomite represents a Mesoproterozoic carbonatite pluton and the fine-grained dolomite resulted from the extensive REE mineralization and modification of the coarse-grained variety. The ore bodies, distributed along an E to W striking belt, occur as large lenses and underwent more intense fluoridation and fenitization.
The first-episode mineralization is characterized by disseminated mineralization in the dolomite. The second or main-episode is banded and/or massive mineralization, cut by the third episode consisting of aegirine-rich veins. Various dating methods gave different mineralization ages at Bayan Obo, resulting in long and hot debates. Compilation of available data suggests that the mineralization is rather variable with two peaks at ~1400 and 440 Ma. The early mineralization peak closes in time to the intrusion of the carbonatite dykes. A significant thermal event at ca. 440 Ma resulted in the formation of late-stage veins with coarse crystals of REE minerals. Fluids involving in the REE-Nb-Fe mineralization at Bayan Obo might be a REE-F-CO2-NaCl-H2O system.
The presence of REE-carbonates as an abundant solid in the ores shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo. The Bayan Obo deposit is a product of mantle-derived carbonatitic magmatism at ca. 1400 Ma, which was likely related to the breakup of Columbia. Some remobilization of REE occurred due to subduction of the Palaeo-Asian oceanic plate during the Silurian, forming weak vein-like mineralization.
The authors concluded that the giant Bayan Obo deposit suffered repeated tectonic-magmatic re-working at the north margin of the NCC. Recent researches have provided some evidence that the REE mineralization at Bayan Obo was a product of mantle-derived carbonatitic magmatism. But where do the REE come from? How were the REE transported and deposited from ore fluids?
The Bayan Obo mine is also a large Nb deposit, the scale and mineralizing style of Nb are still unclear. Where can we find another giant LREE deposit like Bayan Obo in China or in the World? Understanding of the Bayan Obo deposit would be enhanced by recent advances in-situ micro-analytical studies of the geochemical and isotopic signature of the mineralization and more-detailed geological mapping.
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