The Gunflint iron formation is part of a Late Precambrian sedimentary-volcanic series that overlies with major unconformity an ancient granite-greenstone pediment surface. Iron- and silica-bearing rocks of the Gunflint were deposited in a marginal basin that bordered open sea water in a region of volcanic activity. Volcanism played a dual role: it provided a steady source of iron- and silica-bearing solutions and induced cyclical fluctuations in the basin of deposition. This resulted in cyclical sedimentation of iron- and silica-bearing rocks.The fundamental sedimentary cycle consists of the following facies in ascending order: algal chert (thin), tuffaceous shale (thin) and greenalite taconite (thick); taconite facies grade laterally into banded chert-carbonate facies. Where fully developed, the formation contains two cyclical units designated respectively Lower and Upper Gunflint members. The sedimentary facies are described and environments of deposition discussed. The distribution of hematite in the formation indicates an oxidizing atmosphere during Gunflint time. Magnetite formed largely by thermal metamorphism accompanying diabase intrusions. Some magnetite which has no apparent metamorphic association may have formed by primary precipitation or diagenetic alteration.Greenalite is the most abundant iron silicate mineral in the Gunflint formation. Greenalite granules appear to have been formed by diagenetic adjustment of a primary hydrous silica-ferrous iron gel. Secondary amphiboles were formed by metamorphic recrystallization of greenalite. Illite is a common constituent of the shale facies and was formed by alteration of tuff fragments. Minnesotaite and stilpnomelane are quantitatively absent in the Gunflint formation in contrast to their abundance in the Biwabik formation of Minnesota. The relations suggest that minnesotaite and stilpnomelane are secondary metamorphic minerals rather than primary or diagenetic. Absence of abundant secondary silicates in the Gunflint may be related to the absence of hematite ore bodies.