Mineralogical-geochemical features of corundum Miaskite-pegmatite from mine no. 210 (Ilmeny mountains, South Urals): preliminary results

Abstract

Miaskite-pegmatite of mine no. 210 exhibits an unusual mineral composition for the Ilmeny Mountains. It contains a signifcant amount of sapphire-like corundum (uncommon of nepheline-bearing pegmatites) and various Th-bearing minerals («thoro-aeschinite», pyrochlore, phosphates). Pegmatite has a zonal structure, in which the central nepheline and lateral feldspar zones difer in composition of rock-forming and accessory minerals. Corundum is found in all zones of pegmatite. Accessory minerals of the central, eastern, and western zones include (i) Ti-niobates (columbite, pyrochlore, srilankite), zircon, spinel and thorianite, (ii) columbite, pyrochlore and zircon, and (iii) columbite, «ilmenorutile», toro-aeschinite, monazite-La and Ce, and zircon, respectively. The calculations based on two-feldspar thermometer for diferent pegmatite zones show a decrease in temperature from the periphery toward the center, which is in agreement with the variability of mineral assemblages. Pegmatite is strongly altered, which is expressed in the formation of cancrinite and sodalite, hydration of pyrochlore of the central zone, and signifcant compositional changes of Ti-niobates in the lateral zones of pegmatite. The Rb-Sr age of corundum miaskite-pegmatite is ~275 Ma, but the Rb-Sr system is signifcantly destroyed. The initial 87Sr/86Sr(275) isotopic ratio and ?Nd(275) value of the mineral indicate its crustal formation conditions. The geochemistry of corundum points to its multistage crystallization. The data points on Fe vs. Ga / Mg and FeO – Cr2O3 – MgO – V2O3 vs. FeO + TiO2 + Ga2O3 plots correspond to both «magmatic» and «metasomatic» corundum.