A PRECISE TH-232-PB-208 CHRONOLOGY OF FINE-GRAINED MONAZITE - AGE OF THE BAYAN OBO REE-FE-NB ORE DEPOSIT, CHINA

We have obtained precise Th-Pb internal isochron ages on monazite and bastnaesite for the world's largest known rare earth elements (REE)-Fe -Nb ore deposit, the Bayan Obo of Inner Mongolia, China. The monazite samples, collected from the carbonate-hosted ore zone, contain extreme ly small amounts of uranium (less than 10 ppm) but up to 0.7% ThO2. Pr evious estimates of the age of mineralization ranged from 1.8 to 0.255 Ga. Magnetic fractions of monazite and bastnaesite samples (<60-mu m size) showed large ranges in Th-232/Pb-204 values (900-400,000) and pr ovided precise Th-Pb internal isochron ages for paragenetic monazite m ineralization ranging from 555 to 398 Ma within a few percent error (0 .8% for two samples). These results are the first indication that REE mineralization within the giant Bayan Obo ore deposit occurred over a long period of time. The initial lead isotopic compositions (low Pb-20 6/Pb-204 and high Pb-208/Pb-204) and large negative epsilon(Nd) values for Bayan Obo ore minerals indicate that the main source(s) for the o res was the lower crust which was depleted in uranium, but enriched in thorium and light rare earth elements for a long period of time. Zirc on from a quartz monzonite, located 50 km south of the ore complex and thought to be related to Caledonian subduction, gave an age of 451 Ma , within the range of monazite ages. Textural relations together with the mineral ages favor an epigenetic rather than a syngenetic origin f or the orebodies. REE mineralization started around 555 Ma (disseminat ed monazite in the West, the Main, and south of the East Orebody), but the main mineralization (banded ores) was related to the Caledonian s ubduction event ca. 474-400 Ma.