| 其他摘要 | The Maoniuping REE deposit, hosted in a carbonatite-syenite complex, is the second largest primary light REE deposit in China, less voluminous than the Bayan Obo Fe-Nb-REE deposit. Carbonatites and syenites are spatially and temporally associated with rare earth mineralization. Based on the studies of main elements, trace elements, REE, PGE, C-O-Sr-Nd-Pb isotopes, the characteristics of the mantle sources and genetic relationship of carbonatites and syenites are discussed. According to geochemistry of REE, Sr-Nd-Pb isotopes and fluid inclusions of fluorites and the other igneous rocks, we further discuss the origin of REE ore-forming fluids, and propose the ore-forming model. The conclusions are following: ① It is overall and systemic to study geochemical characteristics of carbonatites and associated syenites, and reveal their source features and genetic relationship. Due to intracontinental subduction in Maoniuping, crustal materials with an EM2 signature, subducted and metasomatized EM1 type sub-continental lithosphere to form the primary mantle of carbonatites and syenites. This was followed by variable degrees and discrete parting melting of the resulting heterogeneous, metasomatized sub-continental lithosphere to produce carbonated silicate melts. The carbonatites and syenites were generated from liquid immiscibility of CO2-rich alkalic silicate melts. ② The PGE contents in carbonatites from Maoniuping REE deposit were analyzed. It shows that carbonatites have similar PGE distribution patterns to harzburgites in Dazhuka, Tibet; carbonatite melts can transport PGE; the PGE of carbonatites underwent multi-source evolution. Fluids derived by metasomatism and subduction may transport PGE into carbonatite magmas; besides the role of sulfides, volatile and alloys may modify PGE distribution patterns in carbonatite magma. But the results need more evidences. ③ The fluorites are the main gangue minerals of Maoniuping REE deposit. According to ratios of (La/Sm)n, the fluorites are divided LREE-rich, LREE-flat and LREE-loss models. The three REE type fluorites have identical initial Sr, Nd and Pb isotopic compositions. They were the products of the same source, but at different stages of evolution, and the LREE- rich fluorites were formed at a relatively early stage. ④ The Sr, Nd and Pb isotope ratios from fluorites are similar to those of Maoniuping carbonatites and syenites, different to those of granites. The early fluorites also show similar REE distribution patterns to carbonatites and syenites, different to granites, basalts and rhyolites. The fluid-melt inclusions were found in fluorites. These show that REE ore-forming fluids come from carbonatite and syenite melts. The Maoniuping REE deposit is the product of mantle-derived fluids. |
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