Lake Superior-type iron formation:

Iron formations which formed during Middle Precambrian time (about 2,000 million years ago) and were first described in the Lake Superior region. The type is characterized by interlayers (bands) of iron minerals and chert (quartz). At that time in the earths history, enough oxygen had accumulated in seawater that dissolved iron which had earlier weathered from the surrounding continent could be oxidized. The oxygen and iron combined to form hematite (Fe2O3), limonite (Fe2O3.2H2O) and iron carbonate (FeCO3). These iron minerals could no longer remain in solution and precipitated from the sea water in varying proportions with quartz, thus producing banded iron formation. These iron formations are abundant in the present day Lake Superior region and may be best known in Minnesota as the source of taconite on the Mesabi Iron range.