SOURCE OF ORE-FORMING SUBSTANCES AND THEORETICAL PROBLEMS OF METALLOGENY RELEVANT TO THE BAYAN-OBO FE-REE ORE-DEPOSITS IN INNER-MONGOLIA, CHINA

The source of rare earth elements (REE) ore-forming substances is identified to be extremely distinct from that of iron ores. The Bayan Obo Fe-REE ore deposits were generated by a composite process of both crustal and mantle source mineralization. The original iron bodies are of a sedimentary deposit from supergenesis, while the REE ores have been formed by mantle fluid metasomatism superimposed upon the pre-existing iron bodies. It is believed that the REE ore deposit would be controlled by intracontinental hot spot. The Hg dolomite in mine regions belong to normal sedimentary carbonate rock, its C and O isotopic composition rules out the possibility comparable with magmatic carbonatite. The Sm-Nd isochrons of separated REE minerals have shown two REE peak mineralization periods: early-middle Proterozoic (1 700 Ma +/- 480 Ma) and Caledonia (424 - 402 Ma)