The Genesis of the Ore Hosting “Dolomitic Marble” in the Bayun Obo Deposit, Inner Mongolia,China:Constrained by Isotopic Results

The Bayan Obo Fe-REE-Nb ore deposit, Inner Mongolia, China is a super large REE deposit. This deposit attracts attentions of many geologists and geochemists for its unique geological characters and geotectonic background. However, there are quite a number of controversies on various aspects of its genesis, such as when the ore deposit was formed, where the ore forming materials came from and what conditions and geotectonic environment the deposit was formed. Among them, the genesis of ore bearing “dolomitic marble” is a focus point of debating. Isotopic methods are important tools for study on ore deposits. Therefore, supplemental stable isotope investigation was undertaken in this study to reach more information on the forming conditions of “dolomitic marble”. The high contents of REE and Nb, the REE distribution pattern, the low initial 87Sr/ 86Sr ratio and low δ 30Si values of trace silica in “dolomitic marble” all indicate its magmatic origin. The high δ 18O values of carbonate and silicate minerals, and the relatively high δ 34S values indicate that this rock had experienced isotopic exchange with sea water, implying their submarine volcanic origin. The δ 13C values observed in carbonate and silicate minerals can also be explained by carbonatite assimilated by oceanic carbonate. Therefore, based on its isotopic characters, the “dolomitic marble” is more likely of volcanic carbonatite rock formed during middle Proterozoic era.