The occurrences and geochemical characteristics of thorium in iron ore in the Bayan Obo deposit, Northern China

Abstract

The Bayan Obo deposit in northern China is an ultra-large Fe–REE–Nb deposit. The occurrences, and geochemical characteristics of thorium in iron ores from the Bayan Obo Main Ore Body were examined using chemical analysis, field emission scanning electron microscopy, energy dispersive spectrometer, and automatic mineral analysis software. Results identified that 91.69% of ThO2 in the combined samples was mainly distributed in rare earth minerals (bastnaesite, huanghoite, monazite; 56.43% abundance in the samples), iron minerals (magnetite, hematite, pyrite; 20.97%), niobium minerals (aeschynite; 14.29%), and gangue minerals (aegirine, riebeckite, mica, dolomite, apatite, fluorite; 4.22%). An unidentified portion (4.09%) of ThO2 may occur in other niobium minerals (niobite, ilmenorutile, pyrochlore). Only a few independent minerals of thorium occur in the iron ore samples. Thorium mainly occurs in rare earth minerals in the form of isomorphic substitution. Analyses of the geochemical characteristics of the major elements indicate that thorium mineralization in the Main Ore Body was related to alkali metasomatism, which provided source material and favorable porosity for hydrothermal mineralization. Trace elements such as Sc, Nb, Zr, and Ta have higher correlation coefficients with thorium, which resulted from being related to the relevant minerals formed during thorium mineralization. In addition, correlation analysis of ThO2 and TFe, and REO and TFe in the six types of iron ore samples showed that ThO2 did not always account for the highest distribution rate in rare earth minerals, and the main occurrence minerals of ThO2 were closely related to iron ore types.