云母:花岗岩-伟晶岩稀有金属成矿作用的重要标志矿物

云母是花岗岩、伟晶岩中的重要造岩矿物,不仅是整个岩浆阶段的结晶产物,而且也是热液过程的参与者。作为层状硅酸盐矿物,层间或八面体位置上可容纳锂、铷铯、锡、铌钽等稀有金属。本文结合前人研究积累和作者近年来的研究成果,阐述了云母作为一个重要的稀有金属成矿标志矿物的矿物学特征。铁锂云母-锂云母是稀有金属成矿作用中重要的锂矿物,同时云母中锂含量可以反映花岗岩的分异程度。铷、铯可以置换云母层间钾,在高演化花岗岩、伟晶岩中可以形成铷、铯为主的云母(既可以是锂云母系列,也可以是黑云母系列)。黑云母是稀有金属花岗岩中一个特殊的矿物。准铝质含锡花岗岩中黑云母锡含量可达100×10~(-6),其锡含量可以指示其锡成矿能力。稀有金属花岗岩中,常见的是铌钽氧化物矿物。但是最近研究发现,黑云母中铌可以超常富集(超过1000×10~(-6)),成为稀有金属花岗岩中最重要、甚至唯一的铌矿物,形成一种以富铌黑云母为特色的新类型稀有金属花岗岩,并可能代表了一种新型的潜在铌资源。基于云母在花岗岩中的重要性和结构的特殊性,今后要利用微区成分和结构分析技术,加强对云母中稀有金属晶体化学的研究,以及进一步揭示云母对稀有金属成矿的特殊重要意义。

Micas: Important indicators of granite-pegmatite-related rare-metal mineralization

Micas are a type of most essential rock-forming minerals in granites and pegmatites. They not only appear as products of crystallization of the granitic magmatism throughout its whole evolutionary process, but also are partly involved in the hydrothermal processes. As a type of layer silicates, the interlayer or octahedral structural sites of micas can accommodate rare-elements such as Li, Rb, Cs, Sn, Nb, and Ta to variable extents. In this paper, we will describe the mineralogical characteristics of the micas as a type of important indicative minerals for rare-metal mineralizations, based on both literature data and our proper recent results. The zinnwaldite-lepidolite is apparently an important lithium mineral of rare-metal mineralization, and lithium contents in micas may be regarded as an index of fractional degree of granitic magma. Rubidium and cesium can substitute interlayer potassium, and interestingly dominate over potassium to form specific micas (both lepidolite and biotite) in highly evolved granites and pegmatites. Biotite is a type of particular mica in rare-metal granites. It may contain 100×10^-6 level of tin in metaluminous tin granites, which may thus reflect ore potential of host granites. In rare-metal granites, Nb and Ta are generally hosted by Nb-Ta oxides. However, it is a new finding that biotite with extreme Nb enrichment (>1000×10^-6) may be the most important, and even the unique, Nb carrier in certain type of granites which may be considered as a new type of rare-metal granites, and possibly represent a new style of potential Nb resource. According to their special structural properties and high abundances in granites and pegmatites, micas deserve drawing more attention on their crystal-chemical significances in granite-related rare-metal mineralization and studying with extensive application of micro-structural and compositional techniques.