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 x 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 x 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 microstructural and compositional techniques.