中文摘要：

安哥拉Bonga碳酸岩型烧绿石矿床是一个重要的铌矿床,烧绿石是碳酸岩中主要的含Nb矿物。Bonga碳酸岩中发育了5种类型（阶段）的烧绿石（Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ）。其中烧绿石Ⅰ为岩浆（原生）烧绿石,其余为热液交代形成的烧绿石。烧绿石Ⅰ与氟磷灰石共生,以富Nb贫Ta、Ca、Na占A位和F占满Y位为特征,烧绿石的结晶和Nb、Ta等高场强元素的含量受碳酸岩浆中挥发分F和H2O的控制。Bonga环带烧绿石发育,多为原生的岩浆震荡环带烧绿石遭受了后期低温热液作用改造而成,具有岩浆振荡环带结构和热液烧绿石的化学特征。高温的热液过程中岩浆烧绿石被烧绿石Ⅱ交代,以F和Na含量的降低,Ca含量基本不变,A空位急剧增多为特征。而在稍低的温度下,经热液作用交代形成了烧绿石Ⅳ（A空位高,其他占位元素少）。最晚阶段富Sr的烧绿石Ⅲ（Sr含量较高）和烧绿石Ⅴ形成于相对低温的热液作用过程中,与石英、菱锶矿等热液矿物共生,Sr可能来源于对岩浆碳酸盐矿物的热液交代作用。在整个热液作用过程中,Bonga的烧绿石可能存在以下元素替换机制：3Nb5＋＋2Ca2＋→2Si 4＋＋Fe3＋＋2（U,Th）4＋;2Nb5＋＋Na＋＋2Ca2＋→Si 4＋＋Fe3＋＋2（U,Th）4＋＋A□。在更低温、氧化的条件下,烧绿石蚀变形成大量的含铌金红石和少量的易解石,此过程中释放出Ce和Th等,形成独居石、氟碳钙铈矿、氟碳铈矿、Sr-REE磷酸盐矿物和方钍石等。此外,还发现了含铌针铁矿,显示Bonga烧绿石中的Nb在表生条件下发生了迁移。

英文摘要：

Pyrochlore is the major Nb-bearing mineral in Bonga carbonatite Nb deposit. Five types （stages） of pyrochlore （ I , Ⅱ , Ⅲ , Ⅳ, Ⅴ ） were identified in Bonga carbonatite. Type I pyrochlore is magmatic （primary） pyrochlore while other types were formed by hydrothermal metasomatism. Pyrochlore I is characterized by Nb-rich and Ta-poor, dominant occupancy of Ca and Na at the A site and full occupancy of F at the Y site. The crystallization and contents of high field-strength elements （Nb, Ta） of pyrochlore I were controlled by concentrations of volatiles in carbonatite magma, such as F and He O. Zoned pyrochlore was well developed in Bonga, which are primary magmatic oscillatory zoned pyrochlore modified by late low-temperature hydrothermal processes, these pyrochlores have the texture of magmatic oscillatory zoning but with chemical characteristics of hydrothermal pyrochlore. During high-temperature hydrothermal processes, pyrochlore can be replaced by pyrochlore Ⅱ with less contents of F, Na, and sharp increase of vacancies at the A site. On the condition of relatiVEly low-temperature, pyrochlore Ⅳ （with high A vacancies, low occupancy of cations at the A site） was formed by hydrothermal metasomatism. Low-temperature hydrothermal processes gave rise to pyrochlore Ⅲ （relatively high Sr content） and pyrochlore V （rich in Sr）, which are associated with hydrotherrnal minerals such as quartz, strontianite, etc. Strontium may be derived from hydrothermal leaching of rnagmatic carbonates. During the whole hydrothermal processes, the following substitution mechanisms in pyrochlore may have happened in Bonga： 3Nb5＋＋2Ca2＋→2Si 4＋＋Fe3＋＋2（U,Th）4＋;2Nb5＋＋Na＋＋2Ca2＋→Si 4＋＋Fe3＋＋2（U,Th）4＋＋A□ Pyroehlore can be replaced by Nb-rich rutile and minor aeschynite at lower- temperature and oxidizing conditions. During this process, cerium and thorium were released from pyrochlore, forming Ce-phosphates （e. g. monazite）, Ce-carbonates （e. g. h