Verkhnesayanskii (Sredneziminskii)

The massif, oval in outline, cuts upper Proterozoic conglomerates, schists and sandstones. The country rocks are intensively fenitized up to 300-400 m from the contact zone. In the formationm of the massif four series have been distinguished: (1) pyroxenite, ijolite-melteigite, (2) nepheline and cancrinite syenites, (3) picrite porphyrite and alnoite and (4) carbonatites. Pyroxenites have been preserved as xenoliths among nepheline-pyroxene rocks and carbonatites. They are fine-grained rocks composed of clinopyroxene and Ti-Fe oxide minerals. Ijolites comprise nepheline and aegirine-hedenbergite; among minor minerals are biotite, schorlomite, microcline and calcite. Nepheline syenite forms dykes 1-30 m thick and 10-400 m long which are situated predominantly in the southern part of the complex cutting ijolites. They are coarse-grained massive rocks, which are occasionally porphyritic, with phenocrysts including K-feldspar, nepheline and pyroxene, while in some cancrinite syenites the cancrinite also forms phenocrysts. In the syenites minor and accessory minerals include biotite, apatite, titanite and zircon. Porphyritic picrite and alnoite are found as blocks among the carbonatites and also occur as small dykes in ijolite-melteigite. The porphyritic picrites consist of olivine and diopside in serpentine, and the alnoites comprise olivine, melilite, phlogopite and serpentine. Carbonatites make up the core of the complex as well as numerous veins and dykes. Within the main carbonatite body are many fragments of silicate rocks amongst which ijolite-melteigites are dominant. There are many types of early carbonatites including varieties with biotite, diopside, apatite, hatchettolite, pyrochlore, zircon, magnetite and pyrite; the later ankerite carbonatites include chlorite, quartz, baryte, strontianite, burkbankite, ancylite, parisite, thorite, monazite, sulphides and zeolites. Calcite from phlogopite-bearing calcite carbonatite (second stage) gave d18Oo/oo values of +6.6 and d13Co/oo -6.3, while amphibole-bearing calcite carbonatite (third stage) gave d18Oo/oo values of +6.4 and d13Co/oo -6.3; ankerite carbonatite of the fourth stage gave d18Oo/oo values from +13 to +14 (Kononova and Yashina, 1984). d34So/oo for sulphides from peralkaline syenites were +2.9 and from carbonatites +1.6 to -5.1 (Grinenko et al., 1970).