Synnyrites-New Complex Alumina-Potassic Raw Material

Abstract

The paper is devoted to synnyrites-a high alumina-potassic (17-21% K2O, 22-24% Al2O3) silicate raw material, the main resources of which are concentrated in Siberia and territorially confined to the Baikal-Amur railway. Synnyrites consist of K-feldspar (65-75 vol.%), kalsilite (15-25%), nepheline (up to 10%) and ferromagnesian minerals (biotite, some -times, garnet and pyroxene, 1-5%). Kalsilite and K-feldspar are in maximum potassic. Nepheline contains up to 25-35mol.% of kalsilite mineral; mica is to magnesian (100MgO/MgO+FeO>25 mol.%) low-titanian biotite. Garnet corresponds to low alumina melanite. Pyroxene is a member of isomorphic series of diopside-hedenbergite-aegirine. Salic minerals are mainly present as symplectic intergrowths (dactilotypic, poikilitic, pseudoleucite texture), seldom as individual idiomorphic grains. Large synnyritiferous plutons were formed in Paleozoic and Mesozoic ages during tectono-magmatic activity of older consolidated blocks of the Earth's crust within Baikal-Stanovoy rifting system where potassic alkaline-basaltoid magmatism is widespread. The plutons are large layered ethmoliths. They are made up of two differentiated series : lower melano-, meso cratic series composed of mica pyroxenites, shonkinites, pulaskites and upper leucocratic series represented by nepheline and pseudoleucite syenites, synnyrites. Synnyrites are mainly concentrated in apical and upper of the layered plutons. From the bottom to the top, of the plutons the amounts of K2O, Al2O3 and SiO2 increase and the ferromangnesian mineral contents decrease in the rocks. The formation of synnyrite-bearing plutons is the result of slow crystallization of an initial alkaline-basaltoid melt at a substantial depth and widely manifested processes of intrachamber differentiation and fractionation. It is supposed that synnyrites were formed due to segregation of leucite in the top of magmatic chamber and further transformation of leucite into kalsilite and K-feldspar. A few methods of using and processing synnyrites are suggested : 1) after mechanical enrichment, the ore may be used as chlorine-free low-concentrated potassic fertilizer for a long time ; 2) low-fer-riferous K-feldspar and kalsilite concen -trates, potassic alum may be obtained from synnyrites by mechanical enrichment, flotation or acidic decomposition; 3) high concentrated chlorine-free potassic fertilizers, alumina, fine dispersed silica and others may be produced from synnyrites by deep chemical processing with virtually no wastes. Introduction Synnyrites were firstly found by A. Ya. ZHIDKOV (1962,1963) within the Synnyr alkaline massif in northern Baikalian region. These unique intrusive rock-ores are composed mainly of sym plectic K-feldspar-kalsilite intergrowths and are characterized by a very high alkali content at very low Na2O / K2O. ratio (1:12 in the average). The synnyrite varieties of highest quality contain (in wt.%) up to 17-20 K2O, 22-24 Al2O3, 56-58 SiO2. At present four synnyritiferous plutons (Synnyr, Sakun, Murun, Yaksha) have been revealed in the territory of Russian Federation (KOSTYUK et al., 1990; ZHIDKOV and SMYSLOV, 1982). The rocks close to synnyrites in chemical and modal composition were also found in other alkaline massifs of the world : Lomam, SE Aldan, Russia (VAVILOV et al., 1986), Khibini, Kola Peninsula, Russia (BORUTSKII et al., 1974), Batbjerg, East Green land (GITTINS et al., 1983), Selavik Lake, Western Alaska (MILLER, 1972), Verkhnii Dunkeldyk, East Pamir, Tajikistan (DMITRIEV, 1976). Synnyrite deposits genetically pertain to large layered plutons in which a limited or entire rock spectrum from ultrabasic-basic to alkaline variet ies is involved. The well-known deposits of syn nyrites are situated in Siberia and located within the zone of the Baikal-Amur railway (Fig. 1). These deposits are comparable with large salt ba sins in potassium reserves and large bauxitiferous provinces of the world in alumina reserves. The high contents of potassium and alumina in rocks in combination with their large reserves and with good technological development for extraction ore favorable for synnyrites to be considered as new complex alumina-potassium raw material (DANTSIG et al., 1984). Received on January 28, 1994, accepted on May 10, 1994 * Institute of Mineralogy & Petrography, Siberian Branch of Russian Science Academy, Universitetsky pr. 3, No vosibirsk 90, 630090, Russia