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[Display omitted] • The unique Mo-rich carbonatite was caused by country rock contamination. • The post-carbonatite fluids were remobilized and enriched REEs at ∼ 127 Ma. • The metasomatism of secondary uranium-fluorine rich oxidizing fluid is crucial to form the economic REE budget. The Huangshui'an carbonatite (∼210 Ma) in the Qinling orogenic belt (central China) is known for its unique enrichment of heavy rare earth elements (HREEs) and molybdenum (Mo). However, the origin of Mo in carbonatite veins and the contribution of post-carbonatite fluids to REE enrichment remain enigmatic. In this study, new data of U-Pb dating on zircon and bastnaesite, trace elements and Sr-Nd isotopic compositions of calcite and apatite are obtained to clarify both puzzles. Xenocrystic zircon U-Pb age dating for multiple carbonatite samples yields a concordia age of 2317.0 ± 7.2 Ma (MSWD = 1.2) and a 207Pb/206Pb weighted average age of 2305.2 ± 4.5 Ma (MSWD = 1.0), which are consistent with the age of the Taihua Group granitic gneisses and demonstrate that Mo enrichment in the carbonatite veins is caused by the contamination and assimilation of the wallrocks of the Mo-rich Taihua Group during the carbonatite magma ascending. Hand specimens and microtextural relationships show that magmatic calcite (∑REE: 180–1511 ppm) were extensively metasomatized by the late fluids, accompanied by abundant hydrothermal calcite, bastnaesite and allanite. Compared with magmatic calcite, hydrothermal calcite is significantly depleted in LREEs, Ba, Sr, Mn, Na and Mg, and shows elevated fractionation between LREE and HREE, which probably reflects the crystallization of cogenetic REE-bearing minerals and phases of barite-celestine series. Regardless of the different REE concentrations, similarities in elemental patterns and Y/Ho ratio of both calcites indicate that hydrothermal alteration remobilized REEs from magmatic calcite. The U-Pb dating of bastnaesite (127.5 ± 2.6 Ma) from the hydrothermal veins yields the same age as the early Cretaceous granitic intrusions and related molybdenum deposits (126–130 Ma) around the Huangshui'an deposit. The Sr-Nd isotopic values of the hydrothermal apatite [(87Sr/86Sr) i : 0.70607–0.70717; ε Nd (t): –23.6 ∼ −14.3] are between the range of those reported for the Qinling carbonatites [(87Sr/86Sr) i : 0.70373–0.70624; ε Nd (t): −14.7 ∼ -3.0] and the early Cretaceous granites [(87Sr/86Sr) i : 0.70700–0.71090; ε Nd (t): −28.7 ∼ -9.0]. These results suggest that the fluids involved in REE remobilization are exogenous and may be related to the late Mesozoic granitic intrusions in the Qinling orogenic belt, and that the metasomatism of the early carbonatite veins by the post-carbonatite hydrothermal fluids facilitates the re-enrichment of REEs with economic resources. [ABSTRACT FROM AUTHOR]

The rare earth elements (REEs) have unique and diverse properties that make them function as an "industrial vitamin" and thus, many countries consider them as strategically important resources. China, responsible for more than 60% of the world's REE production, is one of the REE-rich countries in the world. Most REE (especially light rare earth elements (LREE)) deposits are closely related to carbonatite in China. Such a type of deposit may also contain appreciable amounts of industrially critical metals, such as Nb, Th and Sc. According to the genesis, the carbonatite-related REE deposits can be divided into three types: primary magmatic type, hydrothermal type and carbonatite weathering-crust type. This paper provides an overview of the carbonatite-related endogenetic REE deposits, i.e., primary magmatic type and hydrothermal type. The carbonatite-related endogenetic REE deposits are mainly distributed in continental margin depression or rift belts, e.g., Bayan Obo REE-Nb-Fe deposit, and orogenic belts on the margin of craton such as the Miaoya Nb-REE deposit. The genesis of carbonatite-related endogenetic REE deposits is still debated. It is generally believed that the carbonatite magma is originated from the low-degree partial melting of the mantle. During the evolution process, the carbonatite rocks or dykes rich in REE were formed through the immiscibility of carbonate-silicate magma and fractional crystallization of carbonate minerals from carbonatite magma. The ore-forming elements are mainly sourced from primitive mantle, with possible contribution of crustal materials that carry a large amount of REE. In the magmatic-hydrothermal system, REEs migrate in the form of complexes, and precipitate corresponding to changes of temperature, pressure, pH and composition of the fluids. A simple magmatic evolution process cannot ensure massive enrichment of REE to economic values. Fractional crystallization of carbonate minerals and immiscibility of melts and hydrothermal fluids in the hydrothermal evolution stage play an important role in upgrading the REE mineralization. Future work of experimental petrology will be fundamental to understand the partitioning behaviors of REE in magmatic-hydrothermal system through simulation of the metallogenic geological environment. Applying "comparative metallogeny" methods to investigate both REE fertile and barren carbonatites will enhance the understanding of factors controlling the fertility. [ABSTRACT FROM AUTHOR]

[Display omitted] • Calcite from two mineralization stages displays distinct REE enrichment, the early calcite more enriches the HREE, and the late calcite depletes HREE and relatively enriches LREE. • Xenotime is an independent mineral mainly enriched HREE in the Jialu REE deposit, especially the Y element. • The REE mineralization that occurred at 239.2–240.9 Ma was first reported in the Jialu deposit, which recorded an important metallogenic event in the Triassic period. The Jialu carbonatites (∼240 Ma) in the Qinling orogenic belt are characterized by the enrichment of heavy rare earth elements compared with typical global carbonatites. The carbonatites are dominantly comprised of calcite, quartz, sulfate, K-feldspar, minor sulfides, and rare earth minerals such as monazite, bastnäsite, parisite, and xenotime, with two mineralization stages including the early quartz-K-feldspar-calcite stage (Stage I) and the late sulfide-rare earth mineral-calcite stage (Stage II). The rare earth element contents of Stage I calcite are higher than those of the Stage II, especially heavy rare earth elements. Calcite from the different mineralization stages exhibits variable chondrite-normalized REE patterns, with the HREE and REE abundances (134–1023 ppm) decreasing from the early to the late stage owing to the crystallization of xenotime. The δ13C (−5.39 ‰ to −6.68 ‰) and slightly higher δ18O (10.77 ‰ to 12.60 ‰) values for calcite from the Jialu carbonatites generally deviate from the values observed in the primary carbonatite field, which may be a result of Rayleigh-type fractionation. LA–ICP–MS U–Pb dating shows the lower-intercept ages of 243.8±5 and 237.6±1.3 Ma for the monazite and xenotime, respectively, with the weighted average 206Pb/238U ages of 240.9±6 and 239.2±1.3 Ma. These results indicate that the Jialu REE deposit was concurrent with Triassic carbonatite magmatism and REE mineralization observed on the southern margin of the North China Craton. This implies that this region may have great REE mineralization potential. [ABSTRACT FROM AUTHOR]