Prairie Lake is a circular complex covering 8.8 km2 and with a strong aeromagnetic expression (Ferguson, 1971). Exposure is poor but it has been trenched and drilled. The principal rock types are ijolites into which are intruded arcuate bands of carbonatite, more especially near the margins. The ijolites are very variable and pass into mixed carbonate-silicate rocks. Melteigitic varieties contain magnetite and biotite and these pass into more leucocratic rocks, with wollastonite, melanite, calcite and some alkali feldspar. Pseudoleucites, some with polygonal outlines, occur in urtites and the petrography and mineral analyses of these are given by Watkinson (1973). Pegmatitic segregations may contain up to 50% wollastonite, the rest being predominantly nepheline. A striking orbicular ijolite occurs on the eastern side of the complex with orbs 2-3 cm in diameter. There is minor aegirine syenite. The carbonatites are principally calcitic but there is local dolomite and late ferruginous varieties. Pyrochlore, sometimes uraniferous, apatite, pyrite and brookite are present. The complex is emplaced into granitic Archaean rocks which are fenitized adjacent to the contacts. Mica and pyroxene-bearing lamprophyres and micro-ijolite dykes occur within the intrusion.