Abstract:

In order to place new constraints to the multiple metallogenic stages and genesis of the footwall dolomite, we carried out detailed petrographic studies on the ore related samples using optical microscope and scanning electron microscope in this study.The following genetic results have been achieved.Firstly, the mineral assemblages associated with the mineralized alteration show similar patterns among carbonatite (dyke), magnetite ore, altered rock, and mineralized dolomite.This pattern is characterized by the presence of magnetite, monazite, fluorine-carbonate rare earth minerals (e.g., bastnasite), calcite, fluorite, apatite, Ba-rich minerals (barite or witherite), biotite, Na-rich minerals (riebeckite and aegirine), and sulfide minerals.This suggests that these rocks were formed in association with a same fluid event that enriches in Fe, Ca, Ba, K, Na, Si, REE, and volatile components, such as CO₂, P₂O₅, F, and S.The textures of the rocks and ores also show distinct one-stage mineralization characterization, supporting that the Bayan Obo deposit is a product of one mineralization event.Secondly, the REE mineralization is not the product of the same fluid event as the footwall dolomite is.Instead, the REE mineralization is related to the fluid activity dominated by carbonate species, which belongs to the type of carbonatitic or magmatic-hydrothermal fluid.The footwall dolomite is REE-mineralized wall rock showing messages of sedimentary origin.The dolomite may represent hydrothermal sedimentary rocks or sedimentary carbonate rocks which have experienced hydrothermal modifications.Thirdly, we conclude that both the magnetite mineralization and the REE mineralization are the products of a same metallogenic event. However, the lack of magnetite in carbonatite raises the question of the relationship between the two sources of iron and REE.

Key words: magnetite, rare earth element, mineral constituent, fluid composition, Bayan Obo, InnerMongolia