Fe, Mg, Sr-Nd-Pb, C-O isotopes and geochemical constraints on the genesis of Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, Northern China

Abstract

Bayan Obo is one of the largest iron ore deposits in China and holds the world’s largest resources of rare earth elements (REE). The enrichment mechanisms of ore-forming elements and the metallogenic model of the deposit have been the focus of extensive research. Various carbonatites in the deposit are complex in composition and contain a notable amount of REE-bearing minerals, making them among the most favorable hosts for REE-rich ores globally. The carbonatites at Bayan Obo follow an evolutionary sequence from iron-rich to magnesium-rich and finally to calcium-rich types, with iron-rare-earth ores primarily hosted in dolomitic and siderite carbonatites. In this study, we reveal the genesis of the Bayan Obo deposit by examining siderite carbonatites using Fe-Mg, Sr-Nd-Pb and C-O isotopes, as well as whole-rock major and trace geochemical analyses. The results show that the Fe isotopic variation in the siderite carbonatites is narrow, with δ⁵⁶Fe ranging from −0.33 ‰ to −0.08 ‰. The δ²⁶Mg values range from −1.31 ‰ to −0.09 ‰, indicating a mantle source. The ε_Nd(t) values and initial ⁸⁷Sr/⁸⁶Sr ratios of the siderite carbonatites are concentrated between −0.72 to 1.65 and 0.702869 to 0.704619, respectively. The Pb isotopic compositions exhibit comparable ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios (15.992 to 16.482 and 15.271 to 15.412, respectively), but variable ²⁰⁸Pb/²⁰⁴Pb ratios (36.701 to 51.566). The δ¹³C_V-PDB values range from −3.66 ‰ to −0.13 ‰, and δ¹⁸O_V-SMOW values range from 10.89 ‰ to 13.66 ‰. Major and trace elements composition show comparable geochemical properties to the dolomitic carbonatites but differ from sedimentary carbonate rocks. These isotopic characteristics are consistent with those of carbonatites originating from primitive mantle sources, confirming the magmatic origin of the siderite carbonatites. The Fe isotope fractionation between magnetite and siderite is small (with an average of 0.17 ‰), suggesting a high-temperature (>1000 °C) formation environment. Based on field observations and analytical results, combined with the regional geological context, we propose a metallogenic model for the Bayan Obo deposit. In this model, ore-forming materials (e.g., Fe, REE) are sourced from an enriched lithospheric mantle, undergoing partial melting to form initial carbonatite magma. As the magma ascends, it undergoes differentiation and evolution. The differentiated carbonatite magma carries ore-forming materials along fractures to the shallow crust. During the decline phase of the magmatic-hydrothermal events, the ore-forming materials finally precipitate within pre-existing faults or fractures in the Bayan Obo Group, forming the major ore bodies in the deposit.