Formation and preservation of the Bayan Obo Fe-REE-Nb deposit, Inner Mongolia: Insights from evidences of petrogenesis, geochemistry and apatite fission track dating

The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, has attracted considerable attention over the past decades. However, its genesis has been highly debated, especially whether the host dolomite is of sedimentary or igneous origin. In this study, we present a comprehensive study on field geology, petrographic, geochemical, and apatite fission track analyses on REE-mineralized dolomite (H8) and its wall-rock (H9), in an effort to provide additional insights into the formation and preservation of this giant ore deposit. According to field geology and petrology analyses, the ore-hosted dolomite of the H8 unit in/around the Bayan Obo deposit displays the characteristics of sedimentary carbonates and demonstrates complicated deformation and hydrothermal events that resulted in the complex REE mineralization. The parameter of w(As) in pyrite ranging from 100 to 1600 ppm suggesting that the pyrite of the Bayan Obo deposit was mainly formed in a hydrothermal environment, and the thermal event from pyrite at ~440 Ma may represent the mineralization event of the Bayan Obo deposit, or it is an important disturbance event after the mineralization of the deposit. The geochemical research depicts that the evolution process of carbonatite was originated from calcium carbonatites and magnesio-carbonatites to ferro-carbonatite, indicating that the different genesis models of the Bayan Obo deposit that ranging from sedimentary carbonate to volcano-sedimentary and igneous carbonatites are reasonable. The two apatite ages obtained from granite on the east side of the Bayan Obo deposit and the dolomite in the Bayan Obo deposit are ~173 Ma and ~54 Ma, respectively. Time–temperature (t–T) histories of rocks yield new information about the timing of deformation of the Bayan Obo deposit, implying that the east side of the deposit experienced a rapid cooling process between ~300 Ma to ~54 Ma (especially ~300–180 Ma), this reflects the granite underwent a rapid cooling process which coeval with the plate subduction during the closure of the Palaeo-Asian Ocean, while the tectonic movement has little effect on the broken of the H8 dolomite in the mining area. Although the slate of the H9 unit from the east of the Bayan Obo deposit experienced a strong fragmentation in the later tectonic movement, it still made a great contribution to the mineralization for better preservation of REE-rich fluids, leading to the formation of the giant Fe-REE-Nb deposit.