REEs upgrading by post-carbonatite fluids in the Huangshui’an Mo-REE deposit, eastern Qinling Orogen (central China)

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; εNd (t): –23.6 ∼ −14.3] are between the range of those reported for the Qinling carbonatites [(87Sr/86Sr)i: 0.70373–0.70624; εNd (t): −14.7 ∼ -3.0] and the early Cretaceous granites [(87Sr/86Sr)i: 0.70700–0.71090; εNd (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.