Seblyavr

The complex cuts and fenitizes Archaean gneisses and is a downward tapering intrusion of 4x5 km2. Clinopyroxenites are abundant with a gradual transition to nepheline pyroxenites at the margins of the complex. The pyroxenites consist of diopside-augite, perovskite, titanomagnetite and phlogopite with subordinate amphibole, chlorite, calcite, actinolite and apatite. Amongst the pyroxenites are blocks 10-150 m in diameter of olivinites comprising olivine (Fo90), perovskite and titanomagnetite with minor phlogopite, calcite, serpentine, clinohumite and tremolite. The olivine blocks have marginal zones of olivine-pyroxene rocks. Schlieren of perovskite-titanomagnetite ores are associated with the ultramafic rocks which are also crossed by numerous ijolite veins consisting of nepheline and aegirine-augite with lesser amounts of hastingsite, biotite, schorlomite, titanite, magnetite, pyrite and chalcopyrite. The most abundant veins are of carbonatite and apatite-rich rocks (Lapin, 1979), which are arcuate and mainly concentrated in the central part of the complex and define a conical, layered body dipping inwards at 30-60°. They cut the silicate rocks. The carbonatite veins include dolomite-phlogopite types, which are massively textured and contain titanomagnetite and perovskite and subordinate ilmenite, apatite, zirconolite and baddeleyite; dolomite-amphibole carbonatite veins include richterite, tremolite, titanomagnetite and hedenbergite with minor phlogopite, apatite, zircon and pyrochlore. Bulakh and Ivannikov (1984) distinguish an older series comprising major calcite, phlogopite, actinolite and titanite with subordinate hedenbergite-diopside, alkaline amphibole, dolomite, titanomagnetite, iddingsite, serpentine, zirconolite, baddeleyite and zircon and calcite-dolomite and ankerite carbonatites containing baryte and strontianite with minor quartz, chlorite, pyrite, galena, sphalerite, ancylite, pyrochlore and zircon. A younger series consists of veins and pods of carbonate-baryte rocks. The apatite-rich veins, the complex mineralogy and mineral and rock chemistry of which is described in some detail by Lapin (1979), have sharp, discordent contacts but there are also areas of apatite-rich rocks which pass gradually into the enclosing rocks. The apatite-rich rocks, which are referred to as 'camaphorites' by Lapin, are phoscorites. Alkaline lamprophyres and alkaline syenites cut all the rocks of the complex described above. The surface of the complex is covered by a weathered crust in which apatite and vermiculite are concentrated.