HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS

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HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS ( handbook-onphysics-and-chemistry-rare-earths )

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194 Handbook on the Physics and Chemistry of Rare Earths Bayan Obo China Eastern Western Main 2 km Tertiary Diorite Permian granite Middle Proterozic sedimentary rocks Middle Proterozoic dolomite Archean gneiss Orebody Carbonatite dike Thrust fault FIG. 31 Geological map of the Bayan Obo area. After Smith, M.P., Wu, C., 2000. The geology and genesis of the Bayan Obo Fe-REE-Nb deposit: a review. In: Porter, T.M. (Ed.), Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, vol. 1, PGC Publishing, Adelaide, pp. 271–281. The Maoniuping carbonatite in Sichuan province of China forms a dike swarm present in quartz syenite intruding in the Mesozoic Mianxi alkaline granite. The carbonatite dikes are up to 400 m long and 90–200 m thick with 24–40 Ma radiometric ages (Xie et al., 2009). Fenitization is well developed around the carbonatite. The carbonatite dikes comprise mainly cal- cite (50–90%), aegirine, barite, fluorite, and bastnäsite-(Ce), with other REE minerals such as chevkinite-(Ce), parisite-(Ce), cerianite, titanite, britholite- (Ce), aeschynite-(Ce), and monazite-(Ce) (Yang and Wooley, 2006). Esti- mated REE reverses of the deposit are 1.2 Mt REO with an average grade of 2.89 wt.% (Xie et al., 2009). Rare earth mineralization at Maoniuping occurs as veins hosted in quartz syenite and carbonatite. Four main mineralization stages are recognized: (1) carbonatite stage, (2) pegmatite stage, (3) barite-bastnäsite stage, and (4) calcite stage. Xie et al. (2009) interpreted the mineralization of the deposit based on melt and fluid inclusion study as that ore-forming fluids resulted from the unmixing of carbonatite melt and carbonatitic fluid. Initial ore- forming fluids were high-temperature (600–850°C), high-pressure (>350 MPa), and high-density supercritical magmatic fluids, characterized by SO4-rich and multicomponent composition (eg, K, Na, Ca, Ba, Sr, and REEs). These fluids evolved to aqueous-rich fluid at low temperature though a CO2-rich fluid at high to medium temperature, which was responsible for REE precipitation. The main mechanism for REE precipitation was phase separation of CO2 and aqueous fluids resulting from a decrease of temperature and pressure.

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