Zhidoy

This arcuate massif has an area of only 0.85 km2 and is situated in the Onot graben, which is formed of Riphean schists and sandstones. It is composed predominantly (90%) of perovskite and ilmenite pyroxenites. Ijolite dykes, feldspar ijolites, feldspathoid (nepheline, cancrinite) syenites and carbonatites cut the pyroxenites, among which perovskite pyroxenites dominate. These are medium-grained rocks with distinct trachytic textures in which pyroxene phenocrysts may comprise 10-50% of the rock. They consist of clinopyroxene (66%), a small amount of hornblende (4%), biotite, titanite, Ti-Fe oxide minerals (15%), apatite (7%), perovskite (6.5%) and ilmenite (1%). Pyrrhotite, pentlandite, chalcopyrite and pyrite are accessories. Ilmenite pyroxenites are fine-grained rocks with massive and sometimes layered structures. They contain clinopyroxene (72%), a small amount of hornblende, biotite and titanite, Ti-Fe oxide minerals (10%), ilmenite (7.5%) and apatite (6%). Accessory minerals include sulphides and occasionally perovskite. Ijolite-melteigites form dykes up to 20 m thick. They are medium-grained rocks comprising clinopyroxene (50%), nepheline (7-25%), Ti-Fe oxide minerals, perovskite, apatite, biotite and calcite. Feldspathic ijolite is a black, fine-grained rock which forms a dyke about 0.5 m thick consisting of aegirine, nepheline and anorthoclase. Feldspathoidal syenites occur as dykes and veins and are composed of anorthoclase (54%), nepheline (16%), cancrinite (16%) and aegirine (12%), a little biotite and albite and accessory zircon, titanite, fluorite, clinozoisite, calcite and baddeleyite; columbite also occurs. Carbonatites are found as veins 0.2-5 m thick. They are calcite carbonatites with biotite, apatite, sulphides and pyrochlore. Further information on rock and mineral compositions is available in Konev (1970).