安哥拉Bonga碳酸岩型铌矿床地质地球化学特征及成因

Geology,geochemistry and genesis of the Bonga carbonatite-type niobium deposit,Angola

安哥拉Bonga岩体是位于Paran-Angola-Etendeka火成岩省的一个碳酸岩杂岩体,呈孤立的岩塞状产于前寒武纪结晶基底的断裂交汇部位,环状和岩筒构造发育。在岩石组成上,发育一套正长岩+碳酸岩+霓长岩±碱玄质-响岩质喷发角砾岩组合;在矿物组合上,以方解石为主,伴生烧绿石、磷灰石、磁铁矿、萤石和稀土矿物。烧绿石作为岩体中主要的含铌矿物,富集形成了超大型岩浆成因碳酸岩型铌矿床。在化学组成上,主要为钙铁质碳酸岩,富集Sr、Ba、Mn、Nb、Th和LREE,亏损K、Ti和U;稀土元素呈轻稀土富集的右倾分布型式,无Ce、Eu异常。这些特征与世界上典型钙铁质火成碳酸岩一致。Zr/Hf、Y/Ho比值和Y含量反映岩体形成过程中有热液参与,属于一种熔体-流体过渡的高度演化的岩浆体系。Bonga铌矿床形成于岩浆期和热液期,早期形成的为红褐色自形氟钙和羟钙烧绿石,晚期为黄绿色半自形-它形空钙烧绿石。

The Bonga carbonatite complex in Angola belongs to the Parana-Angola-Etendeka Igneous Province. It crops out as an isolated circular plug in the Precambrian crystalline basement along the intersection of faults. Petrographically, the complex consists of syenite, carbonatite and fenite with some tephritic-phonolitic eruptive breccias. The carbonatite is mainly composed of calcite with minor pyrochlore, apatite, magnetite, fluorite and REE minerals. Enriched in pyrochlore, Bonga is a giant magmatic carhonatite-type niobium deposit. Chemically, the carbonatite belongs to sovite （Ca-carbonatite） and ferrocarbonatite, has high contents of Sr, Ba, Mn, Nb, Th and LREE, and is depleted in K, Ti and U with no Ce and Eu anormalies. These geochemical features indicate that the Bonga intrusion is similar to the other typical igneous Ca- and Fe-carbonatites in the world. Zr/Hf and Y/HOratios and normalized Y contents of the rocks suggest that the Bonga carbonatite belongs to a highly evolved magmatic system, transitional between pure melts and hydrothermal fluids. The Bonga niobium deposit formed during magmatic and hydrothermal periods. Reddish brown euhedral pyrochlore （fluroealciopyrochlore and hydroxycalciopyrochlore） crystalized during the early magmatic period, while formation of yellowish green sub-euhedral to anhedral pyrochlore （kenocalciopyrochlore） occurred later during the hydrothermal event.