Introduction. The Biwabik Iron Formation is a layered sequence of iron-rich sedimentary rocks that was metamorphosed by intrusions of the Duluth Complex. The metamorphic recrystallization of iron-formation locally produced iron-rich amphiboles and other fibrous iron-silicate minerals.he nature of the intrusive contact between the gabbroic rocks of the Duluth Complex and the Biwabik Iron Formation is fairly well known from exposures in the Dunka Pit and from hundreds of drill holes that penetrated the footwall of the Duluth Complex.