BACKGROUND The demand for rare metals and rare earth elements has been steadily rising as they play an important role in the high-tech industry. In response, there is an urgent need to study the exploration, development, and utilization of them. The formation of deposits containing rare metals and rare earth elements is intricately linked with igneous rocks, and it has been found that numerous large-scale rare metal and rare earth mines, both domestically and internationally, are associated with alkaline rock complex and carbonatite. In Dagele, East Kunlun, a groundbreaking discovery of the first occurrence of rare and rare-earth mineralized carbonatite-alkaline rock complex predominantly containing niobium was made. This finding represents a significant advancement in the understanding of mineralization in East Kunlun. Currently, the existing research has been primarily focused on surface rock assemblages and mineral characterization investigations. Hornblendite, being the predominant lithology in this complex, exhibits varying degrees of niobium and rare earth mineralization. However, the occurrence state of niobium and rare earth elements in hornblendite remains unclear. Studying the occurrence characteristics of niobium and rare earth elements is crucial for identifying the types of minerals present in the ores, summarizing distribution in their patterns, and exploring the enrichment mechanisms. Comprehensive knowledge of the mineralization laws within the alkaline complex and breakthroughs in mineral exploration should subsequently follow. Due to the small particles and complex dissemination characteristics of niobium minerals and rare earth minerals, precise identification and mineralogical and occurrence analysis under polarized light microscope pose significant challenges^18-19. Fortunately, electron probe microanalyzers are well-suited for identifying minerals containing key metallic elements like niobium and rare earth minerals. They also enable analysis of the mineral forms in which these elements are present. Recent reports have highlighted their successful applications in related studies.

OBJECTIVES In order to find out the existing forms of niobium and rare earth elements in hornblende rocks and the host minerals of niobium and rare earth elements.

METHODS In this study, the hornblendite was analyzed by electron probe on the basis of petrographic observations under a microscope. The primary focus is on investigating the characteristics of niobium minerals and rare earth minerals, such as their species, dissemination relationships, and chemical composition. Additionally, the aim is to accurately analyze the occurrence state of niobium and rare earth elements. The polished thin section of the electron probe was polished and prepared at the Shougang Geological Exploration Laboratory, and subsequently examined and identified using a polarized light microscope (Leica DM4500p) at the Rock and Mineral Identification Center of the Qinghai Geological and Mineral Research Institute. A JEOL JXA-iHP200F electron probe was adopted, and its analysis and test were conducted at the Electron Probe Laboratory of the Institute of Mineral Resources, the Chinese Academy of Geological Sciences. To facilitate the analysis, a conductive carbon film was sprayed onto the surface of the electron probe sheet in a high vacuum environment. Subsequently, JED-2300 X-ray energy spectrum analysis and quantitative electron probe spectrum analysis were performed using an electron probe analyzer. For the quantitative analysis of oxides, the acceleration voltage was 15kV, acceleration current 20nA, and beam spot diameter 3μm. For the quantitative analysis of rare metals and rare earth minerals, the acceleration voltage was 15kV, acceleration current 20nA, and beam spot diameter less than 3μm. During the analysis, the elemental peak measurements were conducted for a duration of 10s, followed by 5s of pre-background measurement and 5s of post-background measurement time. To ensure accurate results, all the collected data were processed using the ZAF matrix correction method. The detection limits for different elements range from 50×10⁻⁶ to 300×10⁻⁶.

RESULTS (1) The analysis results with the polarized light microscope suggest that hornblendite primarily consists of hornblende, pyroxene, phlogopite/biotite, apatite, and other minerals, with a minor presence of aeschynite. The electron probe study indicates that ① In hornblendite, niobium elements are primarily present in niobium aeschynite and niobium-bearing ilmenite, while rare earth elements are predominantly found in allanite and niobium aeschynite. Notably, these elements exhibit significant enrichment in light rare earth elements; ② Niobium aeschynite typically contains an average of 42.98% to 51.96% Nb₂O₅, 4.63% La₂O₃, and 12.16% Ce₂O₃. The mineral grains range in diameter from 15 to 90μm and are located within hornblende crystals or between hornblende and phlogopite crystals. In certain areas, they exhibit intergrowth with hornblende and are closely associated with allanite and apatite; ③ The average content of Nb₂O₅ in niobium-bearing ilmenite is 2.01%; ④ allanite inclusions exhibit an average Ce₂O₃ content of 10.73% and an average La₂O₃ content of 9.89%. These mineral particles have diameters ranging from 10 to 40μm and are primarily found in apatite marginal pores and fissures. They demonstrate a close association with apatite, displaying characteristic mutual intergrowth.　　(2) The analysis of chemical samples of rocks shows that the highest grade of Nb₂O₅ reaches 0.1% in hornblendite in the middle of complex (close to carbonatite and peridotite), and about 0.02% in hornblende at the edge of the complex. The analyzed sample (21DGb11) of apatite-bearing phlogopite hornblendite is situated in the central region of the complex. This specific sample exhibits a closer spatial relationship with the overall mineralization of carbonatite, peridotite, and pyroxene within the area. Notably, valuable ore minerals such as niobium aeschynite, allanite, and niobium-bearing ilmenite have been identified within this sample.　　(3) The presence of niobium minerals and rare earth minerals in hornblendite in Dagele is likely attributed to late-stage hydrothermal processes. Furthermore, the hornblendite that is in closer proximity to the whole-rock mineralized carbonatite and peridotite exhibits a higher degree of influence from late-stage hydrothermal processes, resulting in more significant mineralization.

CONCLUSIONS The formation of niobium minerals and rare earth minerals in rocks is primarily attributed to late-stage hydrothermal processes. Moreover, the hornblendite that is in closer proximity to the whole-rock mineralized carbonatite and peridotite exhibits a higher susceptibility to late-stage hydrothermal processes, resulting in enhanced mineralization. In the East Kunlun orogenic belt, rare and rare-earth mineralized alkaline complex predominantly enriched with niobium elements has been discovered for the first time. These findings highlight the exceptional concentration of niobium in this region. The Late Silurian—Devonian period is believed to be a highly significant timeframe for rare metal mineralization, particularly dominated by niobium elements, in the East Kunlun region.