Interbedded specularite-quartzite from the Precambrian of Minnesota, USA.

The Soudan Iron-Formation is a Neoarchean-aged, metamorphosed 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 and Jirsa et al., 2010) indicates that the Soudan Iron-Formation is about 2.722 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 above sample is part of a small monument at the Tower Train Museum in the town of Tower, Minnesota. The silvery-gray layers are specular hematite (micaceous hematite) and the purplish-gray layers are quartzite.

Stratigraphy: Soudan Iron-Formation member, Ely Greenstone, Vermilion Greenstone Belt, Neoarchean, ~2.722 Ga

Locality: monument at Tower Train Museum, Tower, northeastern Minnesota, USA

References cited:

Jahn & Murthy (1975) - Rb-Sr ages of the Archean rocks from the Vermilion District, northeastern Minnesota. Geochimica et Cosmochimica Acta 39: 1679-1689.