Petrology, geochemistry, and diamond potential of Christopher Island Formation lamprophyre dykes, Thirty Mile Lake area, central Churchill Province

A detailed petrologic and geochemical study of 1.84 GA lamprophyre (alkali minette) dykes of the Christopher Island Formation (CIF), central Churchill Province was completed to further characterize and contribute to the understanding of these Keewatin ultrapotassic rocks. The dykes consist of phlogopite-biotite and rare clinopyroxene phenocrysts contained within a matrix of the same, plus alkali feldspar, apatite, calcite, and autometasomatic alteration minerals. Mantle-derived, strongly zoned phlogopite-biotite megacrysts partially equilibrated with the magmas during ascent. The average whole-rock composition is typical of minettes; variations are controlled by crystal fractionation, magma mixing, and crustal contamination. Exceptional carrying capacity and high magma temperature are inferred from study of entrained xenoliths/-crysts. The diamond potential of the CIF is favourable: a sub-cratonic mantle has been tapped over a vast area by rapidly ascending magmas, the rocks have affinities to lamproites, and a diamondiferous member of the CIF has previously been found.