A carbonatite-pyroxenite complex outcrops on Iron Island and other small islands in Lake Nipissing but is mostly hidden beneath the lake. However, many drill holes into the bottom of the lake as well as on the islands have enabled mapping of a large part of the complex. It is intruded into Precambrian granitic rocks, which are extensively fenitized, and partly overlain by Palaeozoic sandstones. The core of the complex is a dolomitic carbonatite which contains up to 60% of magnetite and hematite and some pyrite. Silico-carbonatites consisting principally of carbonate, biotite and phlogopite, apatite (up to 20%), Fe-Ti oxides, pyroxene, amphibole, andraditic garnet and uranian pyrochlore lie adjacent to the carbonatite and surround an area of ijolite which is strongly carbonated. The southern part of the complex consists mainly of carbonated pyroxenite and ijolitic rocks. The pyroxenites contain titanaugite and magnetite-rich zones as well as local enrichments in pyrite and pyrrhotine, apatite and sparse chalcopyrite. Fenites outcrop over most of Iron Island and are quartz-bearing in the north passing southwards into a sodic pyroxene syenitic fenite. All rock types are traversed by calcite veins, and veins with baryte, fluorite and hematite are abundant while various late lamprophyric and basic dykes were cut during drilling.