Mineralogical study on the distribution regularity of niobium in various types of ores in the giant Bayan Obo Fe-REE-Nb deposit

Abstract

The Bayan Obo Fe-REE-Nb deposit is the largest rare earth deposit in the world, which also contains huge amounts of niobium resources, hosting 83% of industrial reserves in China. The previous researches focused on the REE mineralogy, mineralization age and REE source, whereas only a few studies paid attention to the genesis of Nb mineralization. The REE and Nb might present systematic mineralization difference as the degree of REE and Nb mineralization is distinct in different types of ores in host dolomite. Many complicated factors such as low grade, fine-grained dissemination and complex mineral composition are the main obstacles to hinder the effective utilization of Nb. In this study, a variety of niobium-bearing minerals have been identified, including aeschynite, fergusonite, columbite, pyrochlore, fersmite, baotite, and ilmenorutile. The distribution regularity of niobium in various types of ores in the giant Bayan Obo Fe-REE-Nb deposit is asserted. In massive Nb-REE-Fe ore and dolomite-type Nb-REE-(Fe) ore, niobium-bearing minerals are represented by columbite, while niobium mineralization in aegirine-type Nb-REE-Fe ore is characterized by pyrochlore and aeschynite. The major niobium-bearing mineral in the riebeckite -type Nb-REE-(Fe) ore and fluorite-type (banded) Nb-REE-Fe ore is aeschynite, and major niobium-bearing mineral in biotite-type Nb-REE-Fe ore and diopside-type niobium ore is fergusonite. The μ-XRF results and SEM observations display that columbite contains dolomite, magnetite and monazite inclusions and show obvious genetic links with apatite. Columbite, apatite and magnetite in massive Fe-REE-Nb ore and dolomite-type Nb-REE-(Fe) ore occur at same mineral generation, both having magmatic origins. Pyrochlore from aegirine-type Fe-REE-Nb ore also showsclose genetic relationship with apatite, which is formed by fractional crystallization during the magmatic stage of carbonatite. Aeschynite in fluorite-type (banded) Fe-REE-Nb ore, riebeckite-type Nb-REE-(Fe) ore, and fergusonite in biotite-type Fe-REE-Nb ore and diopside-type niobium ore are large in grain size and have hydrothermal origins. Moreover, intensive Nb mineralization in a series of ores related to fluoritization and alkali-metasomatism demonstrates the critical role of hydrothermal process in niobium enrichment in the Bayan Obo deposit. Fluid-rock interaction leads to the generation of alteration minerals, such as fluorite, biotite and aegirine, and contributes to a slash of K, Na and/or F in hydrothermal fluids, facilitating precipitation of niobium-bearing minerals. By using scanning electron microscope (SEM), X-ray energy dispersive analysis (EDS), Micro-XRF spectral analysis (μ-XRF) and TESCAN Intergrated Mineral Analysis (TIMA), detailed mineralogical study on niobium occurrence and typical niobium mineral compositions in various types of Fe-REE-Nb ores was conducted to provide an important reference for mineral processing and niobium prospecting and give a deeper understanding of the niobium anomalous enrichment mechanism of the Bayan Obo Fe-REE-Nb deposit.