Petrogenesis of Extremely LithiumRich Spodumene Aplites of the Tastyg Deposit, Sangilen Highland, Tyva Republic

The magmatic origin of Lirich spodumene gran� ites and pegmatites is confirmed by geological data, finds of melt inclusions in the minerals of these rocks, and, additionally, for homogenous veins, by the simi� larity of their average bulk compositions to that of the Lisaturated granitic eutectoid melt (1, 2). Being alu� minous leucocratic granitoids in composition, spo� dumene pegmatites are typically characterized by sig� nificant predominance of normative albite over ortho� clase and, therefore, are called albite-spodumene pegmatites. The high lithium contents and "albite" evolution trend make them similar to Li-F raremetal granites. This resemblance is debatable (3), since the shift of the minimum of a granite melt toward the albite apex for lithium-fluorine granites is supposedly caused by the increase in size of the quartz crystalliza� tion field under the influence of fluorine. However, this model is inappropriate for spodumene pegmatites, which differ in the extremely low contents of fluorine and often other volatile components (B, P, H2O). The study of spodumene aplites and pegmatites of the Tastyg deposit, which were formed under lowered activity of H2O, F, and B, revealed for the first time an unusual trend of their differentiation toward petalite composition, which resulted in extremely high lithium contents in these granitoids (4, 5). The Tastyg lithium deposit is situated within the sublatitudinal belt of raremetal pegmatites in the southern part of the Sangilen highland (Fig. 1). This region is part of the Tuva-Mongolia microcontinent within the Caledonian fold system of the southern framing of the Siberian Platform. Vein swarm of the Tastyg deposit consists of several tens of closely spaced subvertical dikes from 1 to 20 m thick and up to 1 km long, which bear prominent evidence of intrusive ori� gin (Fig. 2). They intrude a thick sequence of Protero� zoic limestones, being confined to the intersection zone of steeply dipping faults. It is suggested that these veins are genetically related to the Early Paleozoic granites, though direct contacts with granite massifs were not found. The dikes are characterized by weakly zoned (homogenous) internal structure and consist mainly (up to 80 vol %) of finegrained granitic rocks. Major minerals are represented by variable proportions of quartz, feldspars, and spodumene (the only Li car� rier). This indicates that the vein bodies were formed in an instable tectonic setting, which prevented intra� chamber differentiation of melts. In some dikes, aplites contain contactparallel bands and lenses of pegmatites of the same mineral composition. Aplites