Genesis of the Bayan Obo Fe–REE–Nb deposit: Evidences from Pb–Pb age and microanalysis of the H8 Formation in Inner Mongolia, North China Craton

Abstract

The Bayan Obo Fe–REE–Nb deposit is a world-largest REE deposit in Inner Mongolia, North China Craton. It also contains large Fe and Nb reserves. The REE ore body is hosted in H8 dolomite of the Bayan Obo Group. Depositional time and genesis of H8 dolomite are still controversial in addressing the genesis of the REE deposit. In this study, Pb–Pb age and geochemical features of carbonate minerals in H8 dolomite have been studied, providing constraints to genesis of H8 dolomite and the REE deposit. Results of Pb–Pb dating from unmetamorphosed and non-mineralized domains of the H8 dolomite well constrain the depositional age as 1619±150Ma, which is earlier than both REE ore and carbonatite dykes in the Bayan Obo region. This indicates that REE mineralization has characteristics of epigenetic origin. Geochemical data of carbonate minerals in H8 dolomite have low REE contents, distinctly distinguished from those carbonatitic dykes. However, the fine-grained H8 dolomite whole rocks have high REE contents, similar to those carbonatitic dykes. Mineral analysis suggests a close relationship between REE mineralization and calcite carbonatite dykes and related-derived fluids, which could transport a large amounts of REE. Integrated with these new geochronological and geochemical data, we draw conclusion that the Bayan Obo Group was a Proterozoic depositional succession, REE mineralization is as result of sedimentary carbonate rocks once being metasomatised by fluids derived from regional REE-rich calcite carbonatitic magma at depth.