铌钽矿研究进展和攀西地区铌钽矿成因初探

铌钽矿主要产出类型包括伟晶岩型、富Li-F花岗岩型、碱性侵入岩型、碳酸岩型及冲积砂矿型。前2种类型以钽为主,后3种则以铌占主导。铌和钽大多以铌钽独立矿物(铌铁矿、钽铁矿、细晶石、烧绿石等)呈浸染状分布于含矿岩石中,也有部分以类质同象的形式分布于云母、榍石、霓石、钛铁矿等矿物中。关于铌钽矿的富集机制,一些学者认为可由富F-Na和稀有金属(铌、钽等)的花岗质熔体经结晶分异作用形成;另一些学者则根据铌钽矿化与岩石的钠长石化、锂云母化等紧密共生的特点,认为铌钽的富集是岩浆期后流体交代早期形成的花岗岩所致。攀西(攀枝花-西昌)地区的铌钽矿床(化)基本上都是沿着断裂带分布,矿体赋存于印支期碱性岩脉(碱性正长伟晶岩)中,有少数存在于碱性花岗岩中,与区域上邻近的正长岩体及花岗岩体关系密切。其矿石矿物主要为烧绿石、褐钇铌矿等。初步推断,攀西地区的铌钽矿与二叠纪地幔柱活动有关。碱性的正长岩体及花岗岩体与广泛分布的峨眉山玄武岩、辉长岩均是地幔柱岩浆活动的产物,长英质岩体(包括正长岩体和花岗岩体)是富铌钽岩石的母岩体。碱性伟晶岩脉(如炉库和白草地区)是碱性岩浆逐步演化的产物,含矿的碱性花岗岩是花岗质岩浆分异演化的结果。此外,在该地区的铌钽矿床中,铌钽矿物几乎都富集在钠长石化发育的地段,说明后期的热液交代对铌钽的富集也起到了一定作用。因此,攀西地区铌钽的富集是岩浆结晶分异和岩浆期后热液交代共同作用的结果。

Advances in study of Nb-Ta ore deposits in Panxi area and tentative discussion on genesis of these ore deposits

Niobium and tantalum ore deposits are mainly hosted in pegmatites,Li-F-rich granites,alkali intrusive rocks,carbonatites and alluvial sandstones.The first two types of ore deposits are dominated by tantalum and the other ore deposits by niobium.Niobium and tantalum are present mainly as independent minerals(such as columbite,tantalite,microlite and pyrochlore) disseminated in the host rocks and occasionally isomorphously in other minerals(such as mica,titanite,aegirine and ilmenite).Two main enrichment mechanisms for Nb-Ta ores are suggested: ① fractional crystallization from granitic melt rich in F,Na and rare metals;② hydrothermal metasomatism of granite after magmatism.The Nb-Ta ore deposits in Panxi area(Panzhihua-Xichang) are mostly distributed along the fracture belt,and the ore bodies are hosted in alkali pegmatite veins and alkali granites,closely related to adjacent syenites and granites widely spread in this area.The main ore minerals are pyrochlore and fergusenite.Studies show that the metallogenesis might have been an event related to Permian Emeishan mantle plume.The felsic rocks(syenites and granites),probably the parent rocks of the ore deposits,were products of Permian Emeishan mantle plume similar to widespread Emeishan basalts and gabbros.The alkali pegmatite veins(e.g.,Luku and Baicao) were formed by fractional crystallization from alkali magma and ore-bearing alkali granites by differentiation of granitic magma.Besides,the hydrothermal metasomatism might have contributed to the concentration of niobium and tantalum,as evidenced by the fact that ore minerals are always present where albitization occurs.It is therefore considered that the formation of Nb-Ta ore deposits in Panxi area resulted from both fractional crystallization and hydrothermal metasomatism.