A discussion on forming time of the Bayan Obo Group and ore-forming time of the Bayan Obo giant REE-Nb-Fe deposit, Inner Mongolia

Abstract：Mineralization of the Bayan Obo REE-Nb-Fe ore deposit presents very special geochemical characteristics, suggesting the multi-stage mineralization incredible. The forming age of dolomite as the major ore-bearing stratum must be no later than the mineralization of the Bayan Obo ore deposit. Geological age of Bayan Obo Group has been in dispute in long-term period, and changed many times, but abundant paleontological evidences prove that the age of the Bayan Obo Group is between Sinian and Ordovician. The dolomite is a hydrothermal sedimentary formation and the carbonate veins are formed from hydrothermal replacement of the metamorphic rock or sandstone, and consequently, the zircons from the carbonate veins must be originated from the zircons of metamorphic rock or sandstone. Even though these zircons are reformed by the hydrothermal fluid, it is reasonable that the calculated U-Pb ages of these zircons are older than the ore-forming age. Huge amounts of Sm-Nd isochron ages of the Bayan Obo deposit have been reported. Though they mainly concentrate in the range of 1.2-1.6 Ga, some of them are ~400 Ma, ~800 Ma, and ~1000 Ma, and most of them have large inaccuracy. Considering that the Bayan Obo deposit is formed in a single mineralization period, all the 98 Sm-Nd data from published papers about Bayan Obo are calculated by Isoplot program. 98 data can be synthesized in a straight line(only 2 data are far from the isochron line)with correlation coefficient R= 0.96325, and gield t=1125.8±32.5 Ma（λ=6.54×10-12a-1）, εNd=-3.02, which shows that the Sm-Nd isotope clock after the onset of 1125.8±32.5 Ma has never been disturbed by subsequent geological events. Therefore the ore-forming age of the Bayan Obo deposit should be no earlier than 1125.8±32.5 Ma. The molybdenite Re-Os model age and the pyrite Re-Os isochron age of 439 ± 8 Ma and 439 ± 86 Ma respectively, which is consistent with the age of paleomicroplant fossils. Assuming that Sm-Nd isotopic clock is very perseverant and difficult to be reset by general geological events, the ore-forming age of 439 Ma is admissible.