English: Iron ore from the Precambrian of Minnesota, USA. (~8.5 centimeters across at its widest)

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 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 rock seen here is iron ore from the former Pioneer Mine at Ely, a town ~east of Soudan. Active mining ceased long ago and the mining area is now flooded. The rock is mostly hematite; other observed minerals (on a crack surface) include magnetite, pyrite (often as cubic crystals), and limonite. Very small vugs are common.

The geology of the rocks at the Zenith Mine, which was adjacent to the Pioneer Mine, is described in detail in Machamer (1968). The iron ore rocks targeted by Ely's mines are not primary lithologies of the Soudan Iron-Formation - the original rocks were mostly jaspilite (e.g., see: www.flickr.com/photos/jsjgeology/18408512133). Iron ore bodies at the Pioneer and Zenith Mines formed by significant alteration and replacement of Soudan Iron-Formation rocks. Machamer (1968) concluded that Ely's iron ores formed principally by replacement as minerals crystallized from hypogene hydrothermal fluids moving upward through highly-fractured and brecciated iron formation rocks. The iron in the fluids was likely derived from high-grade metamorphism of greenstone and iron formation rocks that occur much deeper than the ore bodies themselves. The alteration/replacement event occurred some time during the Precambrian.

Stratigraphy: altered Soudan Iron-Formation member, Ely Greenstone, Vermilion Greenstone Belt, Neoarchean, ~2.722 Ga [= age of the original, unaltered rocks]

Locality: Pioneer Mine (now flooded & part of Miners Lake), northern side of the town of Ely, northeastern St. Louis County, northeastern Minnesota, USA

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References cited:

Machamer (1968) - Geology and origin of the iron ore deposits at the Zenith Mine, Vermilion District, Minnesota. Minnesota Geological Survey Special Publication Series SP-2. 56 pp.

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

Jirsa et al. (2010) - Geologic map of Minnesota, Precambrian bedrock geology, description of map units. Minnesota Geological Survey State Map Series S-22.