Essei

The Essei complex is located 30 km north of Essei lake and is the most southerly occurrence of the Maimecha-Kotui Province. The complex and an apatite deposit associated with it have been investigated by Porshnev and Suvalova (1970), Egorov (1991) and Kravchenko and Bagdasarov (1987). According to geophysical and exploration data the complex has an oval form and dimensions of 3.5x4.2 km. The country rocks are mainly limestones, including dolomites, which are cut by dolerite dykes. The complex has a symmetrical, concentrically zoned structure and is composed principally of carbonatite and rocks of the phoscorite series with jacupirangite, melteigite and ijolite occupying less than 0.7 km2. Relicts of olivinite have been encountered in jacupirangite and melteigite. The jacupirangite and melteigite, which appear to form a discontinuous, steeply dipping body, are cut by veins of phoscorite and carbonatite and are also known from xenoliths in the younger rocks. The main minerals of the jacupirangite and melteigite are clinopyroxene (up to 90%), nepheline, biotite, titanomagnetite, titanite and perovskite with minor olivine, aegirine-augite, apatite, melanite, K-feldspar, phlogopite, zeolites and cancrinite. The ijolites are composed of 50-60% nepheline, 40-50% pyroxene with titanomagnetite, biotite-phlogopite, melanite and accessory perovskite, titanite, apatite, cancrinite, zeolite and calcite. The rocks of the phoscorite series form two discontinuous belts in the northern central area of the complex. The more northerly belt comprises magnetite phoscorite rich in forsterite whereas the more southerly belt contains nelsonites. The principal minerals of the phoscorites are calcite, apatite, magnetite, forsterite and biotite with accessory baddeleyite and zircon. Panina and Shatskiy (1974a) studied primary melt and liquid-gas inclusions in apatite and forsterite from these rocks and inclusions in nepheline and pyroxene from silicate rocks (Panina and Shatskiy, 1974b). Dolomite carbonatite forms a central circular stock and a series of arcuate dykes and veins. It is composed of dolomite, apatite, magnetite and minor forsterite, aegirine-diopside, pyrrhotite and pyrochlore. In the very centre of this body the carbonatite is almost free of none-carbonate minerals. The dolomite stock is surrounded by a 0.5-1.5 km wide calcite carbonatite, in which calcite is the sole carbonate mineral; olivine, magnetite, apatite and phlogopite are also present in these carbonatites.