白云鄂博矿床富含稀土流体成矿作用与精细年代学研究

Giant and famous Bayan Obo REE-Fe-Nb deposit is located at Inner Mongolia, north China. This project used SHRIMP and evaporation methods of the zircon U-Pb dating to identify the real ages of basement rocks and mineralization at Bayan Obo. The project also systematically studied the nature of REE daughter mineral-bearing and other type fluid inclusions in different ores and veins in Bayan Obo deposit. Those can provide new ideas to understand mineralizing processes at Bayan Obo. Zircon U-Pb ages have been determined for basement rocks. On the U-Pb concordia diagram, data of three samples yield upper intercept ages of 1948-1917 Ma. Thus, these basement rocks are of Paleoproterozoic. Five zircon fractions from a carbonatite dyke well define a discordia giving an upper-intercept model age of 2070 ± 33 Ma, and SHRIMP dating of the same sample also showed 1920 ± 25 Ma age. It is considered that a carbonatite - syenite - alkaline rock complex might have been developed 1900 Ma ago. Zircon U-Pb dating from fenite alteration rocks near "Wu" carbonatite dike is 1417±19Ma, indicating that different generation of the carbonatite magmatic activity existed. Based on the REE and trace element geochemistry, three types of the carbonatite dykes were found, they may correspond to the different evolutionary stages of the carbonatite magmatism. Fluid inclusions in Bayan Obo REE deposits can also be recognized as liquid-vapour, aqueous-carbonic and aqueous liquid-vapour-solid inclusions. REE minerals identified as daughters inclusions are special feature in the deposit. There exists a high temperature, high saline, alkaline-rich, CO2-rich and REE-rich fluid during REE mineralisation. These alkali-carbonatite fluids are capable of transporting huge amounts of REE and Fe. The interaction of the fluids with carbonate host rocks and fluid immiscibility are important in the precipitation of both REE minerals and Fe oxides. The transport and precipitation of REE can last to late stage fluid influx despite of low temperatures and salinities. Raman spectroscopy of rare-earth daughter minerals in fluid inclusions, combined with SEM-EDS analyses, shows that these minerals might be Bastnaesite and Cebaite. Fluid inclusions in skarn, caused by contacting metasomatism between Hercynian granite and H8 dolomite, in the Eastern Contacting Zoning at Bayan Obo show that inclusion types are CH4-rich and daughter mineral-bearing inclusions. K-feldspar identified as daughter minerals is special feature in the inclusions. There exist different fluid inclusion types in skarn and in REE-Nb-Fe deposit, which imply that the REE mineralization of Bayan Obo deposit was not directly related to the granite magmatism.

对白云鄂博地区不同类型矿石和脉体中新发现的含稀土子矿物的流体包裹体和该类包裹体的稳定同位素进行系统研究，弄清富含稀土流体的特征、来源、演化及其成矿作用；用二次离子质谱仪测定矿床中锆石和稀土矿物及流体包裹体中稀土子矿物等单颗粒矿物年龄，确定成矿时代。这些将为白云鄂博超大型矿床成因提出新的认识。