Borgoiskii

The Borgoi intrusion, with an area of about 2.6 km2, is the largest manifestation of nepheline syenites in the region of the Dzhida River. In the north the intrusion is in contact with extrusive rocks and Recent sediments and in the south with metamorphic diorite. The nepheline syenites grade into syenites, but usually the transition zone does not exceed 10 cm in width. Nepheline syenites with only moderate nepheline contents of about 4-5%, are the most widespread rocks of the intrusion. Albitization is widely developed and mariupolite zones are found accompanying pegmatites of nepheline syenite. Zones of syenite replaced by muscovite are encountered in some places. Within the nepheline syenites xenoliths of biotite schist, and relicts of metamorphosed extrusive rocks, as well as chains of lens-like bodies of syenite porphyry, tinguaite and solvsbergite have been identified. The solvsbergites consist of alkali feldspar (30-75%), albite (20-49%), aegirine (0-13%), hornblende (0-16%) and biotite (0-7%); sodalite, muscovite, fluorite, lavenite, apatite and titanite are also present. The tinguaite is usually present as dykes which have a porphyritic texture with large alkali feldspar phenocrysts. The groundmass of these rocks contains nepheline (7%), albite (42%0, aegirine (4%), biotite (6%) and hornblende. Small blocks, up to 30 cm across, of ijolite have been found and consist of aegirine 49%, nepheline 43%, albite 4%, alkali feldspar 2%, biotite, titanite, fluorite and apatite. Thermometry on gas-fluid inclusions in nepheline in nepheline syenite indicates temperatures varying from 80 to 780ºC (Panina, 1972).