About 5% of the Earth’s crust is composed of iron, the fourth most abundant element after aluminum, silicon, and oxygen, and the most abundant element in the ferride group (atomic numbers 22-28) - iron, titanium, vanadium, chromium, manganese, cobalt, and nickel. The ferride elements are strongly lithophile and form compounds with oxygen or oxyanions. Nickel and cobalt are strongly chalcophile and combine readily with sulphur and, to a lesser extent, with oxygen. Iron and manganese are both chalcophile and lithophile and commonly occur as oxide and sulphide minerals. Iron is also strongly siderophile, but occurrences of native iron are rarely found. Because of its chemical properties and abundance in the Earth, iron is widely distributed in rocks in silicate minerals and concentrated in oxide, sulphide, and carbonate minerals in most of the mineral deposits that occur in, or are associated with, metalliferous sediments or igneous rocks. Iron ore deposits are highly diversified and only the principal types of metallogenetic significance in Canada are considered here.

More than 95% of the iron ore resources of the world occur in iron-formation. The term iron-formation has been used for stratigraphic units of layered, bedded, or laminated rocks of all ages that contain 15% or more iron, in which the iron minerals are commonly interbanded with quartz, chert, and/or carbonate. The bedding of iron-formation generally conforms with the primary bedding in the associated sedimentary, volcanic, or metasedimentary rocks (James, 1954; Gross, 1959a, 1965; Brandt et al., 1972).