Abstract:

The giant Bayan Obo REE-Nb-Fe deposit is the world's largest REE deposit, which is hosted mainly by Middle Proterozoic Na-rich rocks, dolomitic rock and K-rich slate. Na-rich rocks occured together with ore-bearing dolomitite and K-rich slate are distributed widely at Bayan Obo, especially in the south area of the Main orebody and East orebody，also occur in the ore-bearing country rocks and the  footwall. Na-rich rocks include albitite, arfvedsonite-bearing albitite dyke, arfvedsonite dyke, and arfvedsonite-bearing carbonatite dyke and arfvedsonitized dolomitite. These all occur together in one geologic unit but differ slightly in their compositions. Whole rock analyses of Na-rich rocks have revealed Na-rich compositions. Na₂O contents of most of the analysed samples are much higher than 6 %. The Na₂O+K₂O content is more than 8 %, with Na₂O in excess of K₂O. The Rittmann index (δ) of the albitite is between 2.96 and 8.56，with Al/(K+Na) more than 1 and Al/(K+Na+Ca) less than 1, indicating that these are alkali metaluminous rocks. The Na-rich rocks are characterized by enrichment of crustal-derived incompatible trace elements, and depletion of mantle-derived elements. Rare earth elements, especially light REE in Na-rich rocks are extremely abundant. There is no distinct Eu anomaly. The total amounts of REE in Na-rich rocks are lower than that of mineralized dolomitic rocks. These characteristics are similar to those of the dolomitic volcanic rocks. Based on their occurrence, structure and chemical compositions, the albite rocks are inferred to have been Na-rich volcanic rocks, whereas the arfvedsonite-rich dykes are magmatic hydrothermal dykes and the dykes of magmatic differentiation.Sodic alterations at Bayan Obo mainly occur in the forms of fenitization, arfvedsonization and albitization. In addition, the reaction of Na-rich fluid with dolomitite has produced an arfvedsonitized dolomitite.The most intensive alterations at Bayan Obo occur in the Main and East orebodies, where REE have also been enriched. Compared to the compositions of unaltered and altered dolomitites in terms of bulk rock, trace elements and REE, alteration has caused REE differentiation and a weak negative Eu anomaly. The arfvedsonitized dolomitite is high in SiO₂, TiO₂, K₂O and Na₂O, and depleted in P₂O₅  compared to the protolith. It is indicated that REE had already been enriched in the original dolomitite prior to sodic alteration (fenitization). Hydrothermal process caused modification and redistribution of REE but did not add any significant new REE to the deposit.

Key words: sodic rock, albitite, arfvedsonite, arfvedsonitized dolomitite, Bayan Obo