The Origin Of The Bayan Obo Ore Deposit, Inner Mongolia, China: The Iron And Magnesium Isotope Constraints

The Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world,which has attracted much attention for several decades. But its origin remainscontroversial. As Fe and Mg are the major elements of the H8ore-hosting dolomitemarble and Fe is also the ore-forming element, recent advances in Fe and Mg isotopesprovide new insights into the origin of the deposit. Here to constrain the origin of thegiant ore deposit, both the Fe and Mg isotope geochemistry are systematicallyinvestigated for the Bayan Obo ore deposit and related geological units, which isbased on the detailed field works and petrography and mineralography works. Inaddition, the metallogenic chronology of the deposit is restudied using Sm-Ndisotopes. Some significant advances are as follows:1) For REE-Nb-Mn enriched samples, methods of chromatographic separation ofFe and Mg are improved respectively for high-precision measurements of Fe and Mgisotope ratios using MC-ICP-MS.2) The comparision of Fe isotopes between Precambrian sedimentary ironformations and igneous rocks (including magmatic iron ores) is made. With thedeterminations of Fe isotopes of Mesoproterozoic sedimentary iron formations ofBayan Obo Group and mafic rocks in Bayan Obo mining area, combined with theprevious studies, it shows that the Precambiran sedimentary iron formations havevariable δ56Fe-IRMMvalues, with the majority>0, but the igneous rocks includingmagmatic iron ores have limited variation in δ56Fe-IRMMvalues which are clusteraround0.3) The comparision of Mg isotopes between sedimentary carbonates andmantle-derived igneous rocks is made. With the determinations of Mg isotopes ofMesoproterozoic sedimentary carbonates and carbonatites in Bayan Obo mining area,combined with the previous studies, it shows that sedimentary carbonates are enrichedin light Mg isotope with δ26Mg-DSM3values <-1‰, but the igneous rocks are enrichedin heavy Mg isotope with δ26Mg-DSM3values between-0.5and0‰.4) Fe isotopes are systematically investigated for Bayan Obo ore deposit basedon the detailed petrography and mineralography works. It shows that the whole rocksof Bayan Obo iron ores and “H8” ore-hosting dolomite marble have limited variationsof δ56Fe-IRMMvalues that cluster around0, with the former-0.03±0.16‰(2SD, n=14)and the later-0.07±0.24‰(2SD, n=19). The Fe isotope fractionation between magnetite and dolomite is limited, with Δ56Femagnetite-dolomite=0.22‰, indicating thatthey are formed in a high-temperature condition. These demonstrate that the BayanObo ore deposit is not sedimentary iron formations, but is of magmatic origin.5) Mg isotopes are detailedly investigated for the Bayan Obo H8ore-hostingdolomite marble. It has δ26Mg-DSM3values varying from-1.18‰to0.56‰, with anaverage of-0.42‰. Among these data, the majority fall into the field of igneous rocksand the minority fall into the field between igneous rocks and Mesoproterozoicsedimentary dolostones, but none fall into the field of Mesoproterozoic sedimentarydolostones. This demonstrates that the Bayan Obo ore deposit is neither a micritemound nor originally sedimentary dolostones. The H8ore-hosting dolomite marble isa dominantly carbonatite, although it cannot rule out some mixing of seawater orcrustal materials.6) A thorough review on geochronological data and re-assessments of Sm-Ndisotopic results in literatures has been performed to study the timing and episodes ofREE mineralization. It is clarified that the REE in the Bayan Obo ore deposit isenriched by a single episode of mineralization occurred in Mesoproterozoic andsourced from the mantle only. Some later thermal events occurred between ca.1.3Gaand ca.0.44Ga resulting in the REE remobilization within the ore body itself, but thecontribution from external source is minimal.7) Through combination of Fe, Mg isotope geochemistry, geochronology, and C,O, Sr, Nd, etc. isotope geochemistry, the model of Bayan Obo ore deposit is improved.The Bayan Obo ore deposit is a Mesoproterozoic carbonatite probably intruding orerupting in the seafloor, mixed with minor sedimentary carbonates or some seawater,and probably accompanying the hydrothermal activities. The later thermal eventsresult in the REE remobilization within the ore body itself, but the contribution fromexternal source is minimal.