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[Display omitted] • There are multiple stages of carbonatites in Bayan Obo, mainly formed in ∼1350 Ma. • ∼420 Ma and ∼270 Ma remelted carbonatites are identified. • Neither of the Paleozoic remelting events introduced exogenous ore-forming metals. Currently, it has been confirmed that the ore-hosting dolomite of the giant Bayan Obo deposit is the Mesoproterozoic igneous carbonatite, but the roles, that the Paleozoic tectonothermal events played, are still poorly understood. In this study, we present a comprehensive investigation on the geochemistry of various carbonates to understand the generation of the Bayan Obo carbonatite. Hydrothermal monazite Th-Pb dating (peaking at ∼ 420 Ma) identified an atypical coarse-grained dolomite, which occurring as veins, and cutting through typical fine dolomitic facies. Typical coarse dolomitic facies and atypical coarse-grained dolomite are significantly different in terms of MgO, FeO, MnO, Y, Al, Si, V, Ti, Ni, Sc, and ∑REE contents and the C-O-Sr isotopic compositions, and atypical coarse-grained dolomite has lower Ba and Sr contents, lower Co/Ni ratio, with strong Eu positive anomaly and more scattered element concentration, suggesting that it has a different origin from typical coarse dolomitic facies, crystallizing from a relatively low-temperature and oxidized environment. The pervasiveness of magma flow structure and magnetite mass in the contact zone between the granite plutons and the fine dolomitic facies indicates that, at ∼ 270 Ma, the heat flux from granitic magma triggered melting of the fine dolomitic facies, forming remelted calcitic carbonatite. In addition, Th-Pb ages of monazite ranging from 1350 Ma to 240 Ma and the REE-poor properties of both remelted carbonatites suggest that both Paleozoic thermal events only caused REE activation and redistribution, without introducing additionally exogenous ore-forming metals. Accordingly, we propose that the dolomite mass in Bayan Obo are Mesoproterozoic mantle-derived carbonatites (including ore-hosting dolomite and carbonatite dike), which underwent remelting at ∼ 440 Ma and ∼ 270 Ma, respectively. [ABSTRACT FROM AUTHOR]

Fenitization, a unique type of alkali metasomatism related to carbonatite or alkaline igneous rocks, is widely distributed around the Mesoproterozoic carbonatite dykes in the Bayan Obo giant REE-Nb-Fe deposit. Meanwhile, represented by alkali silicates (sodic amphibole or aegirine) enriched REE-(Fe) ores, there is extensive alkali alteration in ores at Bayan Obo, whose similarity to fenitization has been proposed. Fenitization associated with carbonatite is an appropriate indicator of REE mineralizing process. Alkali silicates (sodic amphibole, aegirine and phlogopite) from the East and Main open pits, and fenite around the Wu dyke at Bayan Obo were collected for EMPA and laser ablation ICPMS analyses. Aegirine and phlogopite from the open pits and fenite around the Wu dyke contain similar major components. Sodic amphiboles from those two localities contain comparable trace element composition but with different Cao, MgO and F contents. Alkali silicates from fenites around the Wu dyke show systematic enrichment of Zr, Hf and Ti over alkali silicates from the open pits. Considering different wallrocks and other factors influencing the composition, combined with previous results of geochronology, mineralogy and fluid inclusion, the alkali alteration in the open pits is essentially fenitization in ore-hosting dolomite. A two-stage fenitization process is supported by the zonal texture of sodic amphiboles from Wu carbonatite dyke. The early stage fenitization was Mg, Na, K, F rich and Fe, La, Ce depleted, distributed in alkali silicates from the banded REE-Fe ores and in the core of sodic amphiboles from the Wu dyke. The late stage was Fe, LREE rich and Mg, Na, K, F depleted, distributed in the massive REE-(Fe) ores and the rim of sodic amphiboles from the Wu dyke. The initial alkali fluids derived from carbonatite commonly exhibits LREE-rich patterns. Therefore, the LREE-depletion and low La/Nd ratio of the early stage silicates indicates intense LREE-bearing mineral precipitation and the REE mineralization gradually weakened in the late stage when alkali fluids kept the initial REE patterns unchanged. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mineralizing process of carbonatite system can be divided into six stages. • The associated behaviors of REE and Nb are different in migration and deposition. • The sequence of REE minerals is phosphate, fluorcarbonate, Ba-fluorocarbonate. • The main body of the deposit was formed in the autometasomatism of carbonatite. The Bayan Obo REE-Nb-Fe deposit with giant rare earth reserves, still has no unified understanding of the REE and Nb-bearing mineral evolution and the division of mineralization stages, due to the complicated metallogenic process and the huge controversy on the important issues such as the origin of ore-hosting dolomite. In this work, the whole process of carbonatite magma solidification and mineralization is divided into six stages: coarse dolomitic facies, fine dolomitic facies, disseminated mineralization, banded mineralization, massive mineralization and vein mineralization. The mineralogical composition and paragenesis of various types of rocks and ores have been systematically studied using the automatic mineral analysis technology-Advanced Mineral Identification and Characterization System (AMICS), supplemented by optical microscopy and cathodoluminescence. In order to establish the internal relationship between different stages, trace elements of six kinds of rocks and ores were analyzed. The mineralogical studies show that the rare earth minerals appear in the order of transition from the phosphate minerals in the brine-melt stage (fine dolomitic facies), through the fluorcarbonate minerals in the hydrothermal stage (disseminated, banded and massive mineralization), to the Ba-fluorcarbonate minerals in the Paleozoic vein mineralization, and all of these are generally associated with apatite, baryte, fluorite, aegirine and quartz. The distribution of niobium minerals in all kinds of ores is in the form of disseminated grains, and there is no obvious paragenetic relationship with other minerals, which indicate that niobium mineralization may only be related to the content of niobium in hydrothermal solution. The geochemical data show that all types of rocks and ores have positive anomalies of Ba and Nb, negative anomalies of U, Ta, Sr, Zr and Y. From coarse dolomitic facies to massive mineralization, the content of REE, Nb and Th increases, the content of Sr decreases, and the ratio of (La/Yb) N and (La/Nd) N increases, all of which change continuously. Vein mineralization has similar (La/Nd) N and (La + Ce + Pr) N /REE ratios compared with coarse dolomitic facies and has a wide range of Sr, Th, Nb and REE compositions, and elemental ratio and content of vein mineralization are within the distribution range of ore-hosting dolomite and different types of mineralization. Based on the above, it is considered that the main mineralization body at Bayan Obo was formed in the process of auto-metasomatism of Mesoproterozoic igneous carbonatite, while vein mineralization was the product of homogenization and re-precipitation of early ore-forming material by the Paleozoic fluids, without the addition of exogenous metallogenic materials. [ABSTRACT FROM AUTHOR]