Nizhnesayanskii (Lower Sayan, Beloziminskii)

The Nizhnesayanskii intrusion takes the form of an oval, vertical, tube-shaped body in which the rocks are concentrically zoned. The alkaline rocks cut schists, diabases and conglomerates of Riphean age. Within a radius of 500-800 m from the contact the country rocks are fenitized, and near the contact they are transformed into banded biotite-pyroxene-feldspar and biotite-albite rocks. The central part of the complex consists of carbonatite while around the periphery are areas of nepheline-pyroxene rocks and nepheline syenites. The oldest rocks are pyroxenites which are preserved as small, altered blocks among ijolites. They are composed of clinopyroxene with accessory titanomagnetite, perovskite and phlogopite. The blocks of pyroxenite have been nephelinized and are transformed around their margins to nepheline-pyroxene rocks of highly variable texture. The second phase forms a stock of ijolite in which the rocks vary from fine- to coarse-grained. They consist of aegirine-hedenbergite, nepheline and small amounts of schorlomite, biotite and calcite. The third phase of intrusion involved the emplacement of nepheline and cancrinite syenites over the whole area of the complex as numerous veins and dykes which vary in thickness from several centimetres to 10-20 m, and can be up to 300-400 m in length. The nepheline syenites are leucocratic, trachytic rocks of nepheline, microcline and biotite. The next phase is represented by dykes and pipes of picrite porphyry. The intrusive events closed with the emplacement of a complex stock of carbonatites. Three stages of carbonatite have been identified. The first stage comprises calcite carbonatites with phlogopite, diopside, forsterite and dysanalyte. The second stage also involves calcite carbonatites but with apatite, magnetite, phlogopite and pyrochlore. The third stage carbonatites are of dolomite-ankerite with amphibole, molybdenite, pyrite, sphalerite, galenite, parisite, bastnaesite, monazite witha little quartz, fluorite, strontianite and apatite. Bagdasarov (1981) describes, with illustrations, complex vermicular intergrowths involving forsterite, magnetite, andradite and calcite in early calcite carbonatites. Calcite from carbonatite with phlogopite, of the second carbonatite stage, gave d18Oo/oo values of +6.9 and d13Co/oo -6.3, and calcite from third stage amphibole carbonatite gave d18Oo/oo values of +7.3 (Kononova and Yashina, 1984). d34So/oo values for sulphides from urtite were +1.9 to 0.6, from syenite -0.8 to -1.2 and from carbonatite +2.9 to -4.2 (Grinenko et al., 1970). Compositions of rocks and minerals are available in monographs by Kononova (1976) and Pozharitskaya and Samoilov (1972).