The chemistry of niobium mineralisation at Bayan Obo, Inner Mongolia, China: constraints on the hydrothermal precipitation and alteration of Nb-minerals

As well as world class Fe and REE resources the Bayan Obo mineral deposits also hosts significant niobium resources (estimated as 2.2Mt Nb with an average grade of 0.13 weight % Nb). Niobium in this study is primarily hosted in aeschynite-(Ce) and –(Nd), but with subsidiary amounts of pyrochlore, fergusonite-(Ce), fersmite and columbite. Here we report on the paragenetic and textural setting of aeschynite, pyrochlore and fergusonite in the main ore bodies and in a carbonatite dyke. Niobium in a carbonatite sample is hosted in a phase tentatively (due to significant Ca, Mn and Ti contents) identified as fergusonite-(Ce). Aeschynite occurs overgrowing foliation in banded ores, in fractures and vugs in aegirine-rich rocks and in calcite veins. The composition in all settings is similar, but some examples in banded ores develop significant zonation in Y, Th and the REE, inferred to relate to buffering of halogen acid species to low levels by dissolution and fluoritisation of calcite, and the preferential precipitation of LREE from solution due to lower mineral solubility products compared to the HREE. Although lower in total concentration the ratios of REE in pyrochlore are similar to those of aeschynite and suggest the same metal source. The crystallisation of pyrochlore probably relates to growth in paragenetic settings where carbonates had already been eliminated and hence the buffering of F-species activities in the hydrothermal fluid was reduced. Both aeschynite and pyrochlore show evidence of alteration. Primary alteration of aeschynite resulted in leaching of A-site cations (Ca, REE, Th) and Nb, addition of Fe, and ultimately replacement by Ba-Ti phases (baotite and bafertisite). Secondary, metamictisation enhanced, possibly supergene alteration of pyrochlore resulted in hydration, leaching of A-site cations leading to the development of lattice vacancies and increases in Si. The presence of hydrothermal Nb resources at Bayan Obo suggests there may be potential for further Nb discoveries in the area, whilst the trends in element mobility during alteration have significant implications for the utility of A-B oxides as components of materials for immobilisation of radionuclides.