Mineralogical characteristics of the pyrochlore group minerals in the Bayan Obo deposit and insights into the utilization of niobium resources

The Bayan Obo deposit is a world-famous Fe-REE-Nb syngenetic deposit, in which the most common niobium minerals include pyrochlore, columbite and aeschynite. But the basic mineralogical study of its niobium minerals has been relatively weak during the century-long exploration and development process. In this paper, detailed petrographic observations and geochemical analyses of pyrochlore group niobium minerals in different ore types of the Bayan Obo deposit were carried out using polarized reflectance microscopy, field emission scanning electron microscopy(FE-SEM)and electron probe microanalyzer(EPMA). The results show that the pyrochlore group minerals in the Bayan Obo ore body are mainly divided into three types: fluorcalciopyrochlore, fluornatropyrochlore and hydroxycalciopyrochlore, which show six different types of production distribution. The morphology of pyrochlore group minerals is octahedral granular, platelike or xenomorphic-granular, with a particle size of 0.03~0.2mm, mainly coexisting with dolomite, aegirine, fluorite, monazite, magnetite and sulfide minerals, which are more often altered to account for the in-situ replacement with other niobium minerals such as columbite, fergusonite and aeschynite. The main chemical constituents of the pyrochlore group minerals are Nb, Ca, Na, REE, Ti, which are also replaced to varying degrees by hydrothermal fluids rich in Sr, Th, U and Si. Based on the detailed petrographic characteristics and chemical composition analysis, it is presumed that the pyrochlore minerals in the Bayan Obo deposit are the products of different stages of carbonate magma evolution. There are pyrochlore minerals with different structures and chemical compositions that were precipitated and crystallized in the magmatic, magmatic-hydrothermal and fenitization hydrothermal phases, reflecting that the carbonate evolution process is enriching the REE and Th elements in the fluids, and depleting the Nb elements. Due to the activation and modification of the Paleozoic hydrothermal fluids, some of the megacrystalline calcium fluorcalciopyrochlore is vein-like in the late crystallization. In addition, the coarse-grained dolomitic niobium rare-earth ores in the mine have simple mineral compositions, with Nb2O5 contents ranging from 0.1% to 0.2%, while the niobium minerals are mostly coarse-grained authigenic pyrochlore group minerals and columbite group minerals, which may become the preferred raw materials for efficient niobium beneficiation nowadays.