Koksharovskii

The Koksharovskii complex of alkaline and ultrabasic rocks is a faulted intrusion lying inside a complex tectonic block of Palaeozoic sediments. The complex has an area of 15 km2 and the structure is different in the eastern (Fig. 251) and southwestern (Fig. 252) parts. The complex can be divided into two phases the first of which, covering about 90% of the area, is of pyroxenites which are variably replaced by biotite and contain hornblende, ilmenite and titanomagnetite in variable amounts; sometimes they contain garnet and sulphides. Titanaugite and aegirine-augite jacupirangites are rather less abundant. The second phase of alkaline igneous activity commenced with the emplacement of veins and dykes of foyaite, which are not very numerous. These are medium- and coarse-grained rocks consisting of alkali feldspar and albite (50-70%), nepheline (20-40%), aegirine-augite, some arfvedsonite and biotite (5-10%) and accessory zircon, magnetite, titanite, apatite and ilmenite. The foyaites are cut by a widespread system of alkaline dykes which traverse both the pyroxenites and the volcanic and sedimentary country rocks. The most abundant of the dykes are lujavrites but rischorrites, mariupolites, cancrinite, zeolite and liebenerite syenites and syenite porphyries are also present. The lujavrites form zoned dykes 1.2 cm to 20-25 m thick which extend for up to 100-150 m. The central parts of the dykes are formed by even and medium-grained nepheline syenite containing nepheline (30-40%), feldspar (30-60%), aegirine (5-30%), biotite (1-5%), arfvedsonite, eudialyte, zirkelite, elpidite, lamprophyllite, titanite, apatite, Fe-Ti oxide minerals, zircon and pyrochlore. The outer parts of the dykes have a quenched texture and correspond compositionally to tinguaite. The intrusive sequence was continued by the introduction of veins, which are narrow and usually sinuous, of nepheline syenite pegmatites and syenite-aplites that cut pyroxenites and nepheline syenites. The igneous activity was completed by the emplacement of carbonatites. The carbonatite occupies an oval body of about 0.5x1 km at the eastern end of the complex (Fig. 251). It consists of calcite, up to 20% apatite, aegirine-augite, hastingsite, biotite, titanite, albite, alkali feldspar, Fe-Ti oxides and quartz. Apart from the apatite and titanite all the other minerals are intensively corroded by the calcite. Detailed information on the composition of rocks and minerals can be found in the paper by Zalishchak (1969).