Geochemistry, Mineralogy and Ore Content of Alkaline Granite Magmatism of East Sayan Zone (On the Example of Zashikhinsky Deposit)

Abstract

The Zashikhinsky deposit (Irkutsk region) is confined to the Late Paleozoic East Sayan zone composed of rare-metal alkaline granites. In terms of differentiation degree and ore potential, the rare-metal granites are divided into three facies varieties: early facies involving microcline-albite amphibole-bearing granites, middle facies represented by leucocratic quartz-albite granites, and late facies including essentially pure albite varieties. Pure albite (albitites) is the most ore-bearing variety; REE contents can reach as high as 10000 ppm, in cases even 15440 ppm. In the plots of paired correlations of rock forming and trace element, all facies of the rare-metal granites define clear compositional trends that suggest their derivation from a single magma during its differentiation (from early facies to late ones). All facies of the alkaline granites of the massif show similar shapes of REE pattern and are different only in total REE contents. In addition, they also show similar REE spidergrams, which, however, differ only in small variations of anomalies sizes. Obtained geological, petrographic, and geochemical data suggest a common magmatic genesis of the rare-metal granites and albitites. Accessory minerals in rocks and ores are columbite, Nb-rutile, zircon, xenotime and gagaranite. It should be noted, that rocks of the complex show no metasomatic zoning and metasomatic alteration of minerals by other minerals, characteristic of metasomatic genesis.