关键词: 碱性正长岩, 铌矿床, 金红石, 成矿作用, 庙垭

Abstract: The Miaoya alkaline complex is mainly composed of syenite and carbonatite enriched in Nb and REE, with a verified
Nb₂O₅ reserve of 0.93 Mt @ 0.12%. It is the second largest carbonatite-hosted Nb-REE deposit in China, second only to Bayan Obo. Previous studies have focused on the genesis of carbonatite and the process of REE enrichment, but the occurrence and enrichment mechanism of Nb in syenite are still unclear. Based on field geological surveys, detailed petrography, mineral chemistry, and U-Pb dating of the Nb-bearing minerals in syenite were obtained using TIMA, EMPA and LA-ICP-MS analyses. The results show that the main Nb-bearing mineral in the Miaoya syenite is rutile, which can be divided into magmatic and hydrothermal generations based on texture and chemical composition. Magmatic rutile is rare, while hydrothermal rutile is common in various syenites, mostly disseminated along the edges or fissures of primary rutile or closely associated with biotite and ilmenite. Magmatic rutile shows low contents of Nb2O5 (1.43%~2.56%), FeO (0.74%~1.01%), and other trace elements (e.g., Ta, Cr, V, W, Mo, Sb); while hydrothermal rutile has variable enrichments of Nb2O5 (3.48%~20.68%), FeO (1.18%~6.92%), and other trace elements. In-situ U-Pb dating of rutile illustrates that the formation age of magmatic rutile is 446±21 Ma, while the formation age of hydrothermal rutile is 240±19 Ma, indicating that Nb mineralization in syenite experienced initial enrichment during the early Paleozoic magmatic stage and secondary enrichment during the Triassic hydrothermal metasomatic stage. In summary, we believe that Nb enrichment and mineralization in the Miaoyao complex are controlled by both magmatic and hydrothermal processes. During the early Paleozoic magmatic stage, niobium within syenite is mainly hosted by rutile, biotite and Ti-bearing minerals (e.g., ilmenite); in the Triassic hydrothermal metasomatic stage, hydrothermal fluids can decompose biotite to form secondary Nb-rich rutile and columbite, or metasomatize primary rutile and ilmenite to generate hydrothermal Nb-rich rutile. 

Key words: alkaline syenite, Nb deposits, rutile, mineralization, Miaoya