MINERALOGY AND GENESIS OF NUKHUNGOLSKY CORUNDUM OCCURRENCE (SOUTH-EASTERN SAYAN REGION)
The paper provides data on geology and mineral composition of the Nukhungolsky corundum occurrence, the largest one in East Siberia. It is located inside Kitoy series within the Kitoysky andalusite-sillimanite deposit (south-eastern Sayan region). Corundum occurs as small grains (lesser than 1 mm) in andalusite-bearing schists, in segregations among migmatites and orthotectites; but it forms large crystals in corundum-sillimanite-microcline muscovite-bearing metasomatites. In the whole, this corundum mineralization was formed through a complicated evolution: from Archean laterite weathering crusts, their subsequent metamorphism with formation of ilmenite-biotite-andalusite schists, processes of ultrametamorphism (granitization) with formation of migmatites and granites with the age 2483 Ma after alumina schists, and post-ultrametamorphic changes with appearance of large corundum crystals (up to 8 cm) in the muscovite-bearing corundum-sillimanite-microcline association. In these newly-formed corundum-bearing rocks the change of texture-structural and mineral characteristics of protolith is traced by development of different minerals and their generations. The minerals of the Nukhungolsky occurrence inherit specificity of their apolaterite substratum, which predetermine the typochemical features of corundum, andalusite, sillimanite, biotite, ilmenite, microcline and muscovite. They are enriched in non-typical compounds: SiO <sub>2</sub>, TiO <sub>2</sub>, FeO, MnO, MgO, CaO, Cr <sub>2</sub>O <sub>3</sub> and NiO. By its gemological properties the studied corundum belongs to the low-grade jeweler material.