Earthy hematite (iron ore) from the Precambrian of Minnesota, USA. (public display, Minnesota Discovery Center, Chisholm, Minnesota, USA)

Banded iron formations, or BIFs, are unusual, dense, marine sedimentary rocks consisting of alternating layers of iron-rich oxides and iron-rich silicates. Most BIFs are Proterozoic in age, although some are Late Archean. They do not form in today's oceans - they're an example of an “extinct” rock type. Many specific varieties of iron formation are known. The most attractive type is jaspilite, which is a reddish & silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite. Most BIFs have been subjected to one or more orogenic (mountain-building) events, resulting in folding and/or metamorphism. BIFs are known from around the world, but some of the most famous & extensive deposits are found in the vicinity of North America’s Lake Superior Basin. Many BIFs have economic concentrations of iron and are mined as iron ores.

The rocks shown above are high-grade iron ores from an iron mine in Minnesota's Mesabi Iron Range. Mines in the area target rocks of the Biwabik Iron-Formation, which is dominated by taconite and cherty taconite. Early mining operations exploited high-grade iron ore in the form of earthy hematite and limonite in the weathered zone in the surface and near-surface portions of the Biwabik Iron-Formation. The weathered zone formed as groundwater leached out silica from taconite rocks, resulting in a deposit rich in iron oxide and low in quartz. The Mesabi Iron Range's weathered zone iron ores are essentially mined out. Modern iron mines target the lower-grade taconite as iron ore.

Stratigraphy: weathered zone in the Biwabik Iron-Formation, Paleoproterozoic, ~1.878 Ga