Genesis of the Bayan Obo Fe-REE deposit: evidence from Mg isotope and geochemistry of siderite carbonatites, Inner Mongolia, China

The Bayan Obo Fe-REE deposit, a world-renowned giant polymetallic ore concentration area, is located on the northern edge of the North China Craton, which borders on the Central Asian orogenic belt. Many studies have been conducted since the primary orebody was discovered, but its genesis has always been a fiercely debated issue. In recent years, the rapid development of Mg isotope research has shown promise in tracing mantle-derived igneous rocks and carbonatites, providing a new approach to studying the formation of the Bayan Obo deposit. In this study, we conducted Mg isotope composition and whole-rock geochemical analysis on the siderite carbonatites closely associated with the formation of the Bayan Obo deposit to reveal its genesis. Results show that the siderite carbonatites have high levels of Sr (796.3–6,343.1 ppm), Ba (84.8–11593.0 ppm), and Mn (8,319.8–48680.8 ppm), as well as rare earth element abundance (2,696.8–20763.6 ppm), distinguishing them from sedimentary carbonate rocks. The δ 26 Mg variation range for the siderite carbonatites is −1.31 to −0.09‰, with a mean value of −0.37‰, similar to characteristics of dolomite carbonatites in the mining area. Integrating the results of this study with previous research, we propose that the Bayan Obo deposit formed through the intrusion of carbonatitic magma and subsequent metasomatic processes. The genesis of the deposit was primarily governed by mantle-derived carbonatitic magmatism, with progressive magma differentiation and evolution likely facilitating the enrichment of Fe, REE, Sr, Ba, and Y.