Sills up to 20 m thick intrude Lower and Middle Devonian sediments at Sextant Rapids on the Abitibi River and 3 km downstream at Coral Rapids where a 30 m thick dyke also outcrops. The Sextant Rapids rock consists of serpentinized olivine, augite, a little biotite, calcite, fluorite, magnetite and thomsonite. The sill at Coral Rapids is similar but olivine phenocrysts are fresh and probable melilite occurs in the groundmass. The dyke is composite with an earlier lamprophyric phase petrographically similar to the sill, and a later phase referred to as kimberlitic but comprising serpentinized clinopyroxene, phlogopite, zeolite, 10% calcite, magnetite, ilmenite and a fine grained altered groundmass. Numerous small xenoliths include granite and basalts in which possible melilite is found.