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 partially flooded pit shown above is an old iron mine - the Pillsbury Mine, as seen from railroad tracks on the eastern side of Rt. 73/Rt. 169 on the southwestern side of the town of Chisholm, Minnesota. The mine targeted high-grade iron ore in the Paleoproterozoic-aged Biwabik Iron-Formation. The reddish-brown walls on the left side of the pit are in-situ outcrops of iron formation.

Stratigraphy: Biwabik Iron-Formation, Paleoproterozoic, ~1.878 Ga