Diamondiferous intrusive lamprophyric dykes have been identified North of Lake Superior, near Marathon, Ontario. The Madonna Dyke is a diamondiferous occurrence that lies within the Superior Province within a region host to multiple alkalic and carbonatitic complexes, many of which are linked to the intrusive activity resulting from the Midcontinent rifting event at 1.1 Ga.The Madonna Dyke is a hypabyssal rock with medium- to fine-grained phenocrysts of pseudomorphed olivine, spinel, pyroxene, and amphibole set in a dark green to black altered groundmass of mainly calcite after melilite, REE-poor apatite, phlogopite and spinel. Pseudomorphed olivine occurs as microphenocrysts, phenocrysts and rare macrocrysts replaced by serpentine, magnetite and calcite. A few fresh olivine macrocrysts show mantle compositions ranging from Fo91 to Fo92. Clinopyroxenes are aluminous diopside with Al2O3 ranging from 3.11 to 14.47 wt.%. Groundmass mica show kinoshitalite – phlogopite compositions with up to 4 wt.% BaO and 20.9 wt.% Al2O3. Spinel-group mineral compositions follow Magnetic Trend #2 – the Titanomagnetite Trend, where spinels range in composition from aluminous magnesian chromites to titanian magnesian chromites to titanian chromites to members of the ulvöspinel-magnetite series. Spinel-group minerals occur as red chromium spinel phenocrysts to macrocrysts with magnesium-rich cores and iron-rich rims, often associated with olivine phenocrysts and macrocrysts. They also occur as fine-grained opaque groundmass titanomagnetites with altered cores, and as reaction products forming a necklace texture around olivine. Atoll spinels are present. Although the Madonna Dyke shows some textural and petrogenetic features of kimberlites, the mineralogy, including the presence of calcite after melilite and amphibole, are analogous with an ultramafic lamprophyre of Alnöitic affinity.