ON THE GENESIS OF THE MALYSHEVSK BERYLLIUM-EMERALD DEPOSIT (MIDDLE URALS, RUSSIA)

Abstract

The Malyshevsk (Mariinsk) beryllium-emerald deposit is the most important object in the Izumrudnye Kopi ore field (the eastern slope of the Middle Urals). Its genetic type and formation conditions are specified after generalization of data concerning its geology and mineralogy. It is shown that intensive graniterelated hydrothermal processes in the host ultramafic rocks are dominated by the tectonic position of the ore field in the deep Susan fault at the contact between two crustal blocks, namely, the Murzinsk-Aduisk microcontinent and Rezhevsk volcanogenic depression. The ore field relates to the eastern exocontact of the Aduisk granite pluton (278-201 Ma), which was crystallized at the abyssal depth level (4-6 km). The uncommon association of rare metal pegmatites (Nb, Ta, and Be) with greisens (Be, Mo, Bi, and F) originated at a deep level in an environment of intensive tectonic movements. The greisens consist of desilicified facies (phlogopitic micacectus rocks), because the host rocks are ultramafic and basic. Veins consist mainly of plagioclase and, in addition to beryl, contain emerald, chrysoberyl, phenakite, and bromellite. The Izumrudnye Kopi ore field extends for 1100 m along strike with a width of 110-150 in and more than 500 in along dip. It contains two morphological types of ore bodies: beryl-plagioclase veins (0.25 wt % BeO) and emerald-bearing veinlet-metasomatic micaceous zones (0.089 wt % BeO). Both of these types relate to the same stage of the greisenization process. The latitudinal plagioclase-beryl veins occur in hard rock blocks, and meridional veinlet-metasomatic zones with emerald occur in plastic milonitic talc schists. The intensive postore tectonic movements strongly changed the initial layering and appearance of ore bodies, destroyed beryl crystals, and formed hydrothermal beryllium mineral assemblages. Despite prolonged mining the deposit remains an important source of emerald, juveline chrysoberyl (alexandrite), phenakite, and high quality collection samples. Beryl is a byproduct of ore sorting.