Fold Friday: Michipicoten Banded Iron, Ontario, Canada

Folded banded iron formations within volcanic and sedimentary deposits in rocks several billions of years old.

Credit: Flickr / James St. John

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This series, Fold Friday, is exclusively for paid subscribers and offers an in-depth summary of some amazing folded formations I have found.

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Banded-iron formations are sedimentary geologic layers found ubiquitously on continents around the planet and are remnant geologic formations from the primordial Earth. Most banded-iron formations contain over 15% iron, are thinly laminated, and contain minerals chert, jasper, and quartz. The high iron content of these layers makes them a suitable, though low-grade, ore to mine iron.

Banded-iron formations are typically older than 1.7 billion years old and have not formed since Precambrian time. Because banded-iron formations have not formed in nearly two billion years, their origins are debated and special conditions on Earth are thought to be required to form these unique geologic layers.

Some propose that banded-iron formations are directly related to volcanic eruptions, which were much more common on Earth 2 billion years ago. Other origins for banded-iron layers are rhythmic deposition from solutions containing high levels of silica and iron, or oxidation of iron-rich sediments.

These unique layers are billions of years old, and thus typically heavily deformed and metamorphosed, making them an excellent specimen to highlight for Fold Friday.

The Michipicoten Group

Canada contains some of the oldest rocks on the planet (in some places, arguably the oldest), and contains many interesting layers to study the Earth’s ancient past.

The Michipicoten group in the Algoma District near Ontario, Canada offshore of the northeast corner of Lake Superior is Precambrian in age (beginning at Earth’s formation 4.5 billion years ago and spanning until ~540 million years ago), and contains layers of volcanic pyroclastic flows, sedimentary layers, and banded-iron formations. Here, geologic strata contain deposits from the eruption of andesite-basalt flows and pyroclastic deposits, violent volcanic eruptions, and large-scale hot-spring deposits.

The Michipicoten group formed from continuous depositions of volcanic eruptive deposits and sedimentary units. The banded iron formations are attributed to eruptive sequences from vents in the Earth’s surface where hot water and vapors are expelled (fumaroles). Iron is likely associated with subvolcanic origins and silica is a result of chemical leaching from volcanic rocks.