Multi-stage REE mineralization in the Bayan Obo Fe-REE-Nb deposit: Constraints from biotite and apatite geochemistry

This new discovery of the silicate-calcite complex pluton for the Bayan Obo Formation in the giant Bayan Obo deposit, North China craton, with varied petrographic characteristics of biotite-, calcite-biotite-, calcite- and carbonatite units, from outer to inner, provides a new direction for us to explore the enrichment mechanism of REE mineralization in carbonatite and subsequent post-carbonatite hydrothermal fluid. As the sensibility to chemical and physical factors associated with magmatic crystallization fractionation and fluid activities, biotite and apatite from the Bayan Obo deposit were taken detailed petrographic, geochronologic and geochemical compositional study. Two groups of biotite and apatite have been divided, including primary magma-related ones with age of 1231 Ma, which is characterized by relatively high K/Rb ratios and temperature; and altered hydrothermal ones with ages of 280 to 240 Ma, which is characterized by relatively high XMg, AlIV, F, REE contents, high oxygen fugacity, and low temperature. Significantly discrepant geochemical characteristics of types of biotite and apatite indicate disparate REE mineralization in magmatic and hydrothermal processes. For the magmatic process, the crystallization of REE-poor minerals during the evolution of silicate-carbonatite complex results in the enrichment of REE and the differentiation between HREE and LREE in the residual carbonatite magma. For the hydrothermal process, the highest REE contents in altered hydrothermal biotites and apatites formed by REE/F-rich late hydrothermal fluids, which was supposed to source from the Hercynian magmatic activities. In addition, the high contents of F in late hydrothermal biotites and apatites are assumed to be important for the enrichment and precipitate of REE minerals.