Nizhne-Burul’Zaiskii

Nizhne-Burul’zaiskii covers 2.3 km x 220-250 m and intrudes marbled limestones of Upper Proterozoic age. From drilling it appears that the intrusion forms a vertical sheet and extends to at least 450 m. It was emplaced in two phases: gabbro, gabbro-pyroxenite and pyroxenite; and an urtite-jacupirangite series. Nepheline syenite veins are rather rare and there are some younger leucocratic granite dykes. Within the massif there is a distinct zonality. The periphery is composed of jacupirangites, which are gradually replaced towards the centre first by melteigites and then by ijolites. Within the ijolites urtite forms isolated bodies 225-350 m across. The rocks of the urtite-jacupirangite series are medium-grained and display trachytic textures. They consist of varying proportions of nepheline and clinopyroxene together with amphibole (hastingsite, hornblende), garnet, biotite, oligoclase, calcite, apatite, magnetite and some secondary minerals including zeolites. The clinopyroxenes are variable in composition and may be salite, fassaite or hedenbergite. The gabbros and gabbro-pyroxenites are usually considerably altered to amphibolite. The nepheline syenites which form veins are medium- to coarse-grained rocks composed of oligoclase (15-30%), orthoclase (40-45%), nepheline (20-25%), rare grains of aegirine-augite partly replaced by amphibole and garnet. Data on the chemical compositions of the rocks will be found in Andre’Ev et al (1969) and Sharackshinov (1975) and mineral compositions are in Andre’E va and Troneva (1986).