Granite Systems with Rare-Metal Pegmatites

Abstract

For most rare-metal pegmatite fields, two generations of granitic pegmatites are documented, namely, beryll-bearing (often with tantaloniobates and muscovite, which are inseparable from vein granites of the leucogranite complex), and Na-Li (Li, Ta, Cs, Be, and Sn) (LCT-pegmatites). The latter, in turn, subdivide into (i) a “multicomponent rare-metal type” with pegmatite zoning and main mineralization (large crystals of spodumene, tantalates, beryll, cassiterite, pollucite, petalite, and amblygonite) in the central parts of the respective bodies (Koktogai, Bernic Lake, Bikita, Karibib, Varutrask, Vishnyakovskoe, and others) and (ii) “albite-spodumene pegmatites” that compose multiple extended dikes that are devoid of pegmatite zoning and are grouped into fields of up to 10–15 km or more in length (Kings Mountain, Zavitinskoe, Gol’tsovoe, Kolmozero, Polmos-Tundrovoe, Tastyg, and others). For a number of deposits (e.g., Zavitinskoe, Vasin-Myl’k, and Shukbyul’), the occurrence of multicomponent rare-metal pegmatites in the “heads” of feeding dikes of “albite–spodumene pegmatites” is established. Any attempt to identify parent granites for “albite–spodumene pegmatites” is a priori futile, because these are not pegmatites, but granites, although specific ones (in fact, spodumene granites of the Allakha type). The purely terminological correction, granite instead of pegmatite, surprisingly has both scientific and forecasting implications. The scientific implications imply that, first, the problem about parent granites is solved and, second, the “pegmatite” status of “albite–spodumene pegmatites” no longer debated. These pegmatites correspond to the particular, spodumene–rare-metal–granite stage in the history of magmatism of certain pegmatite-bearing areas (the end of this stage was marked by the formation of true multicomponent pegmatites that are characteristic of only this stage). Regarding the forecast implications, it can be supposed that such multicomponent deposits as Koktogai, Bernic Lake, Bikita, Karibib, and Vishnyakovskoe are underlain by a suite of dikes comprised by spodumene-bearing rare-metal granites (i.e., an independent Li deposit), and multicomponent pegmatites are differentiates of these granites.