The geology and genesis of the Bayan Obo Fe-REE-Nb deposit: a review

The Bayan Obo Fe-REE-Nb deposit is currently the world's largest REE resource. It has estimated reserves of up to 1500Mt of iron oxides (35 wt. % Fe), 48-1 OOMt REE (6 wt. % REE,03) and IMt Nb (0.13 wt. % Nb). The deposits are hosted in the Proterozoic Bayan Obo group sediments, mostly in dolomite marble, although the deposits themselves are principally Caledonian in age (555-420Ma). Fe occurs as magnetite and hematite, whilst the REE occur principally as monazite and bastnasite, although over 16 individual REE-minerals and 17 REE-bearing niobium minerals are also present. The deposits are accompanied by an alteration assemblage of apatite, aegirine, aegirine-augite, fluorite, alkali amphibole, phlogopite and barite. Albite and K-feldspar occur in the overlying slates and schists.

The deposits were formed by multistage hydrothermal replacement of marble during Caledonian subduction. The source of metals and fluids is uncertain, although carbonatites, alkaline igneous rocks, A-type granites and subduction-derived fluids have all been suggested as possibilities. Late stage, low salinity fluids were responsible for extensive modification of the deposit, and an overprint of sulphide and barite alteration. The deposits show many similarities in processes to others of the Fe-oxide class, but there are important differences including the absence of significant base metal sulphide mineralisation, no enrichment in U, and the absence of evidence for the involvement of hypersaiine brines in ore genesis.