Jaspilite from the Precambrian of Michigan, USA. (cut & polished surface)

Banded iron formations, or BIFs, are unusual, dense 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), and do not form today - they're “extinct”! Many specific varieties of iron formation are known, and some are given special rock names. For example, jaspilite is an attractive reddish & silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite.

Because of their age, most BIFs have been around long enough to have been subjected to one or more orogenic (mountain-building) events. As such, most BIFs are folded and/or metamorphosed to varying degrees.

BIFs are known from around the world, but some of the most famous & extensive BIF deposits are found in the vicinity of North America’s Lake Superior Basin. Many BIFs have economic concentrations of iron and are mined. BIFs are the most important variety of iron ore on Earth.

The BIF shown above is from the Ironwood Iron-Formation of northern Michigan. BIFs are relatively widespread in Michigan’s Upper Peninsula (UP). The Ironwood Iron-Formation outcrops in the western UP’s Gogebic Iron Range. Its layers have been steeply tilted by the Penokean Orogeny (~1.85 billion years) and by mid-continent rifting (~1.10 billion years). The rock shown above is a classic example of jaspilite. It consists of interbedded magnetite-rich layers (= dark gray) (Fe3O4) and bright red hematitic metachert layers (“jasper”). Minor quartz layers, with a little pyrite, are also present.

Stratigraphy: Ironwood Iron-Formation, upper Paleoproterozoic, ~1.874-1.878 Ga