The Sm-Nd and Rb-Sr isotopic systems of the dolomites in the Bayan Obo oredeposit, Inner Mongolia, China.

The Bayan Obo Fe-Nb-REE ore deposit, Inner Mongonia, China is the largest R EE deposit in the world. Many research works on this deposit have been made and vast amount of scientific data have been accumulated. However, its gen esis, especially the ore-forming age and REE sources, have been under dispu te for a long time. This paper reports the Sm-Nd and Rb-Sr isotopic analyti cal results of the dolomites for one of the major REE ore types and their m inerals in the Bayan Obo deposit. The Sm-Nd isotopic analytical results of the 15 dolomite whole rock samples from the Main ore body and the East ore body in the deposit present an isochron of t = 1273 +/- 100(2 sigma )Ma, co rresponding to I-Nd = 0.510909 +/- 36(2 sigma), MSWD 1. 01, but their Rb-Sr isotopic analytical data, with the ranges of 0 similar to 2.092 x 10(-2) f or Rb-87/Sr-86 ratios and of 0.70341 similar to 0.70541 for Sr-87/Sr-86 rat ios, are quite scattered. The Sm-Nd isotopic analytical results for the min eral samples of the dolomite give an analogous isochron age of t = 1250 +/- 210 (2 sigma) Ma, with I-Nd = 0.510914 +/- 77 (2 sigma), MSWD0. 56, to the whole rock samples. The obtained Sm-Nd ages of the dolomite whole rock and their mineral samples are in agreement with the reported age data of the o ther samples from this deposit. For example, the Sm-Nd age of the whole ore samples from the Main ore body and East ore body of this deposit is 1286 /- 91 Ma, the ones of the potassium-rich slates and dark slates in the over lying H9 formation of the main ore body are 1096 +/- 56Ma and 1208 +/- 123M a respectively, the one of the monazite and bastnaesite mineral samples, so rted from the Main ore body, is 1313 +/- 41Ma, and their corresponding epsi lon (Nd)(t) are all negative values in range of - 0.06 similar to - 4.4. Be sides, the Ar-40/Ar-39 age of 1260Ma, 1209 +/- 12Ma and the Sm-Nd age of 12 23 +/- 65Ma for the alkali amphibole and the whole rock samples of carbonat ite dykes, being similar geochemical feature to the deposit in the Dulahala area of the north of the Main ore body have been measured. This data state clearly indicates that the Bayan Obo deposit is possibly products of magma tazation from the slightly riched mantle in large ion lithophile trace elem ents in the late Mesoproterozoic era.