Abstract:

The enrichment mechanism of rare earth element (REE) in the Bayan Obo deposit is of global interest, and it is of great importance to define the REE mineralization process for high-quality development and utilization of REE resource. According to scanning electron microscope (SEM) observation of samples from the main orebody, two main REE minerals, bastnaesite and monazite, occur commonly as veinlets formed by filling localized micro fissures of host minerals. Of note, small and globular-shaped bastnaesite and monazite are wrapped by dolomite or occur on the grain boundaries of dolomite outside the veinlets of REE-bearing minerals; occasionally they also appear within the veinlets. The above observations indicate that REE is initially hosted by dolomitic carbonate magma, then migrates to the shallow crust, and eventually assumes its occurrence state as solidification of carbonate magma occurs. The globular droplet-like crystals also might be attributed to strong liquid immiscibility in carbonate magma during solidification. Furthermore, Nb-Cr-rich monazite can be observed in the REE-bearing mineral veinlets, also seen is Si-rich monazite from the early stage wrapped around by Si-poor monazite, suggesting that the enrichment of bastnaesite and monazite is closely associated with magma at depth. In addition, most of bastnaesite and monazite are characterized by subhedral-anhedral crystals apart from a few globular ones in the veinlets. Dolomite commonly undergoes dissolution and is wrapped by bastnaesite, monazite and fluorite. Especially, abundant fluorite and apatite are enriched together with bastnaesite and monazite veinlets, and globular-shaped monazite wrapped by apatite can be observed. These features indicate that bastnaesite, monazite and dolomite crystalized from early-stage magmatism are strongly modified by late-stage F-P-rich hydrothermal fluids—that is, the formation of bastnaesite and monazite veinlets is also closely linked to hydrothermal fluids. The above mineralogical phenomenon shows that REE enrichment in the Bayan Obo deposit underwent metallogeny at least twice: REEs are firstly carried by carbonate magma and migrate to the shallow crust where magmatic immiscibility leads to the initial enrichment; then F-P-rich hydrothermal fluids cause re-mobilization and cyclic migration of REEs, resulting in further enrichment.

Key words: Mineralogical phenomenon, Rare earth element enrichment, Magmatism, Hydrothermal process, Bayan Obo