Abstract: The results of lead isotope analysis indicate that the materials of the Bayan Obo Group originated at 1314 Ma B. P. and that the mineralization occurred at 6650 Ma B.P. The Jianshan Formation at the base of the group has Rb-Sr isochron ages of 749.6±62.11 Ma, 753.99±42.28 Ma and 685.91± 24.12 Ma. The Bilute Formation above it has a K-Ar age of 607.9 Ma. The two Formations may be assigned ot the Sinian. The Hujiltu Formation and the strataabove it have a Rb-Sr isochron age of 353±30 Ma and a K-Ar age of about200 Ma, so should be assigned to the pateozoic. The Pb-Sr isochron ages of the Silurian strata overlying the Bayan Obo Group are 327±36Ma, 314.29±28Ma and 260±11 Ma respectively.