Abstract:Bayan Obo Ore Deposit is a giant Fe-REE-Nb ore deposit, the largest light REE deposit in the world. However, its genesis has been a subject of longstanding debate, the main hypotheses including sedimentary origin and magmatic origin. Recently, the rapid development of high-precision measurement of Fe isotope ratios enables it to be used as a new proxy to trace the source of Fe for Bayan Obo ore deposit. To obtain direct Fe isotope baseline for Bayan Obo ore deposit of possible origins, Fe isotopes of geological formation related to the Bayan Obo ore deposit in Bayan Obo area were analyzed, including iron slate of Jianshan Formation (Bayan Obo Group), sedimentary iron ore deposit in north of Kuangou, micrite mound of Sailinhudong Group, and diabase dykes. The results show that iron slate of Jianshan Formation (Bayan Obo Group) yields δ56Fe values between -0.49‰ to 0.48‰, with the average -0.03‰±0.84‰, 2SD, n=5; that sedimentary iron ore deposit in north of Kuangou yields δ56Fe values between -0.68‰ to 0.23‰, with the average -0.10‰±0.78‰, 2SD, n=5; that micrite mound of Sailinhudong Group yields δ56Fe values between -0.64‰ to 0.12‰, with the average -0.28‰±0.57‰, 2SD, n=6; and that diabase dykes yields δ56Fe values between 0.11‰ to 0.16‰. Compared to ore hosting dolomitic marble, the Fe isotope composition of micrite mound of Sailinhudong Group is systematically lighter, and is more variable, indicating the different origins between them. The iron slate of Jianshan Formation and sedimentary iron ore deposit in north of Kuangou display a similar Fe isotope composition to that of Banded Iron Formation (BIF) all over the world. All of them show heavy Fe isotope enriched and large variation in Fe isotopes. Their Fe isotope composition is different from that of Bayan Obo ore deposit, which yields δ56Fe values of ca. 0‰. The Fe isotope composition of Bayan Obo ore deposit is identical with that of diabase dykes in Bayan Obo, magmatic rocks, and magmatic iron ore deposit, supporting the magmatic origin of Bayan Obo ore deposit.
