浙东南吴小垟矿床晚白垩世铌成矿特征及其找矿指示意义

Late Cretaceous niobium mineralization in the Wuxiaoyang deposit,southeastern Zhejiang:Metallogenic characteristics and prospecting significance

稀有金属铌钽在我国华南存在多时代成矿特征,前人对其最晚成矿期晚白垩世成矿事件报道较少。本文以浙东南的吴小垟铌矿床为研究对象,结合野外调查、岩石学、矿物原位微区分析和同位素年代学等研究,确定矿床铌成矿岩体为细粒黑云母花岗岩,成岩成矿年龄分别为89.2±0.8 Ma和86.5±1.0 Ma,揭示了该区存在晚白垩世花岗岩侵位及其相关铌成矿事件。铌除了以独立矿物的形式赋存在铌铁矿、铌铁金红石、铅烧绿石中,还有部分铌赋存于黑云母中。在岩浆-热液作用下,云母类型发生了从铁云母→黑鳞云母→铁锂云母的变化。铁云母Nb含量最高可达1253×10^(-6),黑鳞云母-铁锂云母铌含量最高至794×10^(-6),黑云母极度富铌的特征,指示该花岗岩具有非常好的铌找矿潜力,富铌黑云母可以作为一种铌钽找矿指示性矿物。综合上述矿物学特征,得出吴小垟矿床存在两阶段铌成矿事件:岩浆期原生铌铁矿族矿物、原生富铌黑云母和铌铁金红石的形成;以及后期热液作用下铌从岩浆和原生含铌矿物中迁移出形成次生的含铌氧化物。

The rare mental niobium-tantalum has multi-epoch metallogenic characteristics in South of China,and the Late Cretaceous metallogenic event as the latest metallogenic stage is rarely reported by predecessors.This paper focuses on the Wuxiaoyang Niobium deposit in southeast of Zhejiang Province.By combining field investigation,petrology,in situ microzone analysis of minerals and isotopic chronology,we determined that the main body of niobium mineralization is fine-grained biotite granite.The diagenetic and metallogenic ages are 89.2±0.8 Ma and 86.5±1.0 Ma respectively,revealing an important Late Cretaceous granite emplacement and niobium mineralization event in this area.Niobium occurs in the form of independent minerals in columbite group mineral,rutile and plumbopyrchlore.In addition,part of niobium occurs in biotite.Under magma-hydrothermal interaction,the mica evolved from annite to protolithionite to zinnwaldite;the highest Nb-content of annite was 1253×10^(-6),and the highest Nbcontent of protolithionite and zinnwaldite was 794×10^(-6).As the extremely Nb-rich characteristics of biotite indicate that the deposit has a very well niobium prospecting potential,the Nb-rich biotite can be used as an indicator mineral for niobium-tantalum prospecting.Based on the above mineralogical characteristics,there are two stages of niobium mineralization events in this deposit:firstly,the formation of primary columbite group minerals,Nb-rich biotite and rutile in magmatic stage;secondly,niobium migrates from magma and primary Nb-bearing minerals to form secondary niobium-containing oxides under late hydrothermal action.