Carbonatite dykes at bayan Obo, inner Mongolia, China

Calcite-rich dykes occur in the thrust fold belt near the Bayan Obo rare earth element (REE) deposit. They cut a thrust inlier of granitic migmatite within folded Bayan Obo Group sediments of Proterozoic age. Cathodoluminescence, X-ray fluorescence and microprobe studies show that the rock is a calcite carbonatite with Sr-Mn-bearing calcite, magnesio-riebeckite, apatite, pyrochlore, K-feldspar and biotite. One dyke was chosen for detailed analysis. Its margin is strongly REE-mineralized with much monazite developed adjacent to zoned apatite. Secondary alteration is marked by the introduction of Fe and Mn. The adjacent migmatite is fenitized to a magnesio-riebeckite-albite rock. The sedimentary dolomite of the Bayan Obo Group is composed mainly of Mn-Sr-RE-hearing ferroan dolomite and contains bands of opaque grains, apatite, monazite, fluorite and taeniolite. Many trace element and isotope similarities between the carbonatite dyke and the sedimentary dolomite are revealed, and the evidence supports the possibility that the dolomite is a dolomitized carbonatite tuff. The Bayan Obo REE mineralization also shows geochemical similarities with the mineralization seen in the carbonatites, and a possible genetic connection is presented.