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[Display omitted] • Fluorocarbonates and monazites have recorded geochemical evolution of REE-rich magma and fluids. • Fluorocarbonates and monazites has potential as indicators to REE mineralization processes. • Mesoproterozoic REE minerals inherit geochemical features from carbonatite melt. • Mesoproterozoic and Paleozoic hydrothermal REE minerals reveal contamination of crustal materials. Fluorocarbonates and monazite are common economic REE minerals in carbonatite-related REE deposits, such as the largest known REE deposit at Bayan Obo, China. The geochemical evolution of REE minerals is a sensitive and effective indicator of delicate REE mineralization processes. In this study, the fluorocarbonates and monazites were classified into three generations, corresponding to three REE mineralizing stages. The first generation (Gen-1) of bastnäsite and monazite are generally characterized by elevated La/Ce, La/Nd and La/Lu ratios, with right-declining chondrite normalized REE distribution patterns. They have low Th and radiogenic Sr content, with forming age of about 1.3 Ga and εNd(t) around 0, indicating a primary origin and orthomagmatic crystallization from carbonatite melts. The second generation (Gen-2) of REE minerals, formed at 1.25–1.2 Ga, normally constitute banded aggregates with associated hydrothermal gangue minerals (fluorite, aegirine, sodium amphibole etc.). The Gen-2 REE minerals are featured by moderate-low La/Ce, La/Nd, La/Lu ratios but slightly elevated 87Sr/86Sr 0 ratios, indicating a Mesoproterozoic hydrothermal origin related to carbonatitic fluids. Gen-2 bastnäsite and monazite precipitated earlier in low-fluid/rock-ratio environment with right-declining LREE distribution patterns, while the subsequent Gen-2 parisite and huanghoite display convex LREE patterns and higher OH/F ratios, indicative of relative La-depletion and increased ambient fluid/rock ratios during progressive hydrothermal REE mineralization. The third generation (Gen-3) of REE minerals are generally megacrysts in the vein-type ores formed in the early Paleozoic REE mineralization stage (470–420 Ma). The Gen-3 REE minerals are typified by significantly variable REE, Sr and Nd isotopic components, indicating extensive hydrothermal metasomatism and intense contamination of crustal material. Induced by activities of early Paleozoic subduction-related fluids, Gen-3 REE minerals were formed through local REE re-enrichment within the ore-hosting dolomite, rather than by introduction of REE from external sources. In this case study, a comprehensive model of the delicate REE mineralization processes of the giant Bayan Obo REE-Nb-Fe deposit has been established. This study highlights the great potential of fluorocarbonates and monazites widely distributed in carbonatite-related REE deposits as universal geochemical indicators of late-stage evolution of carbonatite and delicate magmatic-hydrothermal REE mineralization processes. [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 Bayan Obo deposit has notable differences with skarn/BIF in mineralogy and magnetite geochemistry. • Trace element contents in Bayan Obo magnetite controlled by coexisting mineral, f O 2 and T. • Bayan Obo iron deposit was a hydrothermal deposit related to igneous carbonatite. The giant Bayan Obo REE deposit bears a large amount of iron resources as well. However, the genesis of iron mineralization is still highly controversial with limited research. In this study, new data of trace elements of magnetite from Bayan Obo ores, Eastern contact zone skarn and Heinaobao BIF are obtained by Laser ablation (LA) ICP-MS analyses, revealing the genesis of Bayan Obo iron resources through comparing study. The Bayan Obo deposit contains disseminated, banded and massive ores, and in which magnetite is associated with REE minerals, fluorite, aegirine, riebeckite, and biotite. Euhedral magnetite in the skarn of Eastern contact zone is usually associated with pyrite, biotite, fluorite, and bastnaesite. There are two types of magnetite in the Heinaobao BIF: (1) euhedral to subhedral magnetite associated with quartz and a small amount of amphibole; (2) anhedral magnetite associated with quartz, garnet, plagioclase, amphibole and biotite. Magnetite in the Bayan Obo is enriched in Ni, but depleted in Mg, Al, Ti, Ge, Mn, Ga and Zn, showing low and variable concentrations, which is similar to the hydrothermal magnetite. The contents of some trace elements (e.g. Ti, Mn, Sc, Zn, Nb and REE) of magnetite from the skarn ores are generally higher, indicating a relatively high formation temperature. Magnetite in the Heinaobao BIF is generally characterized by low and uniform trace element contents, except for the abnormal values of Ti, Al, Cr and Mn, suggesting the involvement of terrigenous clastic materials. Compared with Bayan Obo magnetite, the skarn magnetite has the highest Sn/Ga, Co/Ni, Nb/Ta, La/Yb, Ti/V and Ti/Al ratios, while magnetite in the BIF has the highest Al/Co and lowest Nb/Ta ratios. The significant difference in mineralogical and trace element characteristics among the three types of magnetite indicates that the iron resources in Bayan Obo deposit are unlikely to be a skarn Fe deposit nor a BIF deposit. However, the Bayan Obo magnetite shows similar geochemical characteristics to hydrothermal magnetite related to carbonatite, both of which are depleted in high field strength elements, such as Zr, Hf and Ta, and show strong positive anomalies of Mn and Zn and negative anomalies of Co and Ga. In addition, in the diagrams of Ti vs. Nb + Ta and Ti vs. Zr + Hf, the Bayan Obo magnetite falls into the field of hydrothermal magnetite associated with carbonatite. In conclusion, it is recommended that the Bayan Obo iron deposit is a typical hydrothermal deposit related to carbonatite. In this contribution, the skarn magnetite formed at high temperature and the BIF magnetite was contaminated by terrigenous materials deviating from the expected region in discriminant diagrams. It is therefore proposed that the validity of these diagrams depends on a full understanding of mineralogical characteristics of samples and magnetite precipitation environment, and that multiple discrimination diagrams should be used in combination. [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]

[Display omitted] • Hydrothermal zircon is identified in the Bayan Obo REE deposit for the first time. • Dating of hydrothermal zircon yielded an age of ~1.3Ga for the Bayan Obo deposit. • Substantial LREE depletion and Th/U fractionation are two discriminative features for hydrothermal zircon. The Bayan Obo deposit in China hosts the largest REE resource in the world. However, the genesis of this deposit is still highly debated, largely due to a lack of consensus on the mineralization age. The Banyan Obo deposit has experienced intensive post-ore metamorphic and metasomatic overprints, so it is crucial to choose robust chronometers to determine the primary mineralization age. In this study, zircon is identified in the REE ore of the Bayan Obo deposit. The zircon grains are in contact with or contain inclusions of hydrothermal minerals (e.g., magnetite, biotite, monazite, allanite, apatite and ilmenorutile). They have variable Th contents (14 to 1139 ppm) and are poor in U (<0.8 ppm), consistent with the Th-rich and U-poor feature of the Bayan Obo REE ore. Moreover, they have substantially low contents of light REE (La to Nd; LREE), which may be due to zircon coprecipitation with LREE-rich monazite and allanite. The combined textural and compositional features imply a hydrothermal origin of the zircon grains. The hydrothermal zircon grains have a weighted mean 208Pb-232Th age of 1297 ± 13 Ma, which places a direct constraint on the timing of REE mineralization at Bayan Obo. Notably, substantial LREE depletion and Th/U fractionation have not been documented before for hydrothermal zircon from other environments, and thus they could be taken as discriminative features for hydrothermal zircon from carbonatite-related REE deposits. Thorium-lead dating was also conducted on monazite from the zircon-bearing sample, yielding 208Pb-232Th ages (993 to 319 Ma) variably younger than that of zircon. The younger monazite Th-Pb ages could have resulted from disturbance of the Th-Pb isotopic system during post-ore modification process. Similarly, monazite has younger Th-U-Pb dates than zircon in many other geologically-complicated carbonatites or REE deposits. Such a phenomenon implies that the zircon Th-U-Pb chronometer is robust even if the host-rocks have experienced intensive modification, whereas monazite Th-U-Pb chronometer is more susceptible to modification. Therefore, younger Th-U-Pb dates of monazite should be interpreted with caution as they may record the timing of post-ore modification event or represent partial resetting, and hence geologically meaningless results. [ABSTRACT FROM AUTHOR]