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 Hull-Rust-Mahoning Mine shown above is an active iron mine in Minnesota's Mesabi Iron Range. Multiple mines have operated at this site and have since merged into one giant operation, which is currently a pit ~5 miles long and ~1.5 miles wide. Mining started in the mid-1890s and continues today. A significant percentage of America's iron needs during World War I and World War II were met by production from this single mine.

The target rocks at the Hull-Rust-Mahoning Mine are taconites in the Precambrian-aged Biwabik Iron-Formation, which is the iron ore producing unit throughout the Mesabi Iron Range. Early mining operations focused on the weathered zone of the Biwabik Iron-Formation, which is rich in earthy hematite and low in silicates. Modern mining produces taconite as iron ore, which is relatively rich in silicates but still has significant hematite content.

The southern wall of the mine is shown above. A north-south geologic cross-section through this mine that is on display at the public overlook seems to indicate that this wall is composed of the "natural ore", which is the local mining term for the high-grade iron ore that is dominated by earthy hematite and has had most of the silicates leached away by weathering.

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