Specularite in the Precambrian of Minnesota, USA.

The Soudan Iron-Formation is a Neoarchean-aged banded iron formation (BIF) unit in the Vermilion Greenstone Belt of northeastern Minnesota, USA. Stratigraphically, the Soudan is considered to be the middle member of the Ely Greenstone. Metamorphosed pillow basalt successions occur above and below the iron-rich interval:

- upper member of the Ely Greenstone - Soudan Iron-Formation member - lower member of the Ely Greenstone

Radiometric dating of Ely Greenstone rocks (see Jahn & Murthy, 1975) indicates that the Soudan Iron-Formation is about 2.69 billion years old.

The Soudan Iron-Formation was deposited in a deep-water environment and is closely associated with several types of volcanic rocks formed in an ancient island arc setting. As such, the unit is classified as an Algoma-type iron-formation. Superior-type iron-formations were deposited in relatively shallow water, continental shelf settings.

The Soudan Iron-Formation is exposed in & around the town of Soudan, Minnesota at roadcuts, glacially-eroded outcrop knobs, surface mines, and underground mines. The best exposures are at Soudan Underground Mine State Park. The Soudan Mine targeted high-grade iron ore (specularite) for many decades and closed in the early 1960s. The site is now accessible to the public and underground tours are offered.

Observed iron-rich lithologies in the Soudan Iron-Formation include specularite, jaspilite BIF, and magnetite BIF.

The underground outcrop shown above is the ceiling of a stope with specularite, which is a high-grade iron ore and the mining target at the Soudan Mine. Specularite is a finely-crystalline to coarsely-crystalline metamorphic rock dominated by silvery-gray specular hematite (Fe2O3 - iron oxide). "Specular" means "mirror" - it refers to the shiny, intensely lustrous appearance of each specular hematite crystal flake. The brick red-colored material is merely hematite dust that covers and coats most of the specularite. The dust (referred to as "schmutz" by a tour guide) was generated by mining activities.

Specularite occurs as small to large, irregular-shaped masses within the jaspilite facies of the Soudan Iron-Formation. Studies have shown that the specularite formed by metamorphic alteration of jaspilite (= quartz-rich, banded iron formation), resulting in the leaching away of quartz/silica.

Stratigraphy: Soudan Iron-Formation member, Ely Greenstone, Vermilion Greenstone Belt, Neoarchean, ~2.69 Ga

Locality: ceiling of stope at the 27th level of the Soudan Mine, Soudan Underground Mine State Park, Soudan, northeastern Minnesota, USA

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