The deposits are of three types. Lode-type deposits are deep, narrow, conformable lenses of massive hematite within steeply-dipping Precambrian iron formation. They are believed to be hypogene replacements of the iron formation, formed by hot fluids during metamorphism; they contain much less alumina and phosphorus than supergene ores, their structural relations suggest formation after deformation, and they have 'mudstone' envelopes that suggest argillic alteration. Crust-type deposits are relatively shallow bodies formed by supergene enrichment (leaching of silica). Derived-type deposits are products of mechanical and chemical weathering, transportation, and accumulation of iron from the iron formation. Examples of all three types occur in other countries.