铌钽分异富集成矿机制及铌钽矿物测试新技术研究进展

Research progress on the mechanism for the formation of Nb-Ta deposits by fractionation and enrichment and method development for columbite-tantalite analysis—a review

铌、钽作为稀有金属,其熔点高、密度大、耐高温、超导性等优异性能使其在人类社会多个领域具有不可替代性,也是近20年及未来全球都极为重视的战略资源。了解铌钽在不同岩浆系统中的分异富集及不同类型铌钽矿床的成矿机制是至关重要的。前人已有研究认为影响铌钽分异富集的主要因素有岩浆系统的碱性程度、熔体中水的含量、熔体温度,以及主要含铌、钽的矿物(铌钽铁矿、黑云母、多硅白云母等)的分离结晶和部分熔融过程中的结晶与熔融。此外,高浓度的助熔剂(F,Li, Be, P,H_2O)对铌钽分异富集也有不可忽视的作用。前人研究发现一些铌钽矿床表现出岩浆结晶分异成矿作用特征,但在铌钽矿化发育区域,常伴有强烈的钠长石化和锂云母化等交代作用现象,因此单一的岩浆结晶分异作用或热液交代作用都不能全面地解释复杂的铌钽成矿作用。铌钽分异富集成矿过程中,结晶分异和热液交代作用占比如何,哪个阶段更加富集铌和钽,这些问题有待进一步解决。此外,由于其高铀低普通铅的特性,铌铁矿族矿物也被广泛应用于LA-ICP-MS U-Pb定年,常用标样为Coltan139,其激光分辨率有待进一步提高以解决铌钽矿物复杂化学分带的激光原位定年问题。铌钽矿物Lu-Hf同位素工作开展较少,未来需继续开展同位素工作以示踪物源并指示铌钽分异富集成矿机制。

Rare metals niobium and tantalum are irreplaceable in many fields of human society due to their high melting point,high density,high-temperature resistance,superconductivity and other excellent properties.They are also strategic resources the world has attached great importance to in the last 20 years and beyond.It is very important to understand Nb-Ta fractionation and enrichment in different magmatic systems and the metallogenesis of different types of Nb-Ta deposits for Nb-Ta ore prospecting.Considered by previous studies the main factors affecting Nb-Ta fractionation and enrichment are the alkalinity of host magmas,water content in melts,melt temperature,fractional crystallization of main Nb-Ta-bearing minerals(columbite-tantalite,biotite,phengite,etc.)and crystallization-melting of the main minerals during partial melting.In addition,the high concentration of fluxing elements(F,Li,Be,P,H 2O)also plays an important role in Nb-Ta fractionation and enrichment.Previous studies have found Nb-Ta mineralization could be characterized by magmatic crystallization differentiation,in some cases,but was often associated spatially with strong albitization or lepidolization.Therefore,metallogenic models solely based on magmatic crystallization differentiation or hydrothermal metasomatism could not fully explain the complex mineralization of Nb and Ta.Other questions need to be further addressed include:What is the proportion of crystallization-differentiation to hydrothermal-metasomatism in the process of Nb-Ta fractionation and enrichment?And which stage is more enriched in Nb and Ta?Besides,as columbite-tantalite(coltan)is widely used in LA-ICP-MS U-Pb dating due to its high U and low common Pb contents,with Coltan139 commonly used as a reference standard,laser resolution needs to be further improved to achieve in-situ dating of complex chemical zoning in coltan.The Lu-Hf isotope analysis of coltan is less carried out.In future,coltan Lu-Hf isotope analysis should be developed to trace the Nb-Ta source and clarify the mechanism of Nb-Ta fractionation and enrichment.