File:Banded iron formation (Trommald Formation, Paleoproterozoic, ~1.87 Ga; Portsmouth Mine, Cuyuna North Range, Minnesota, USA) 2 (50539417332).jpg
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Banded iron formation from the Precambrian of Minnesota, USA Banded iron formations, or BIFs, are unusual, dense, marine sedimentary rocks consisting of alternating layers of iron-rich oxides and iron-rich silicates. Most BIFs are Late Archean to Proterozoic in age and do not form in today's oceans - 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. Seen here is a vintage hematite-magnetite BIF sample from Minnesota's Cuyuna North Range. All of the iron mines are inactive - the last mining operation shut down in the 1980s and the pits are now lakes. The target rocks in the Cuyuna North Range were BIFs in the Trommald Formation, which is correlative with the Biwabik Iron-Formation, the Ironwood Iron-Formation, and the Negaunee Iron-Formation. They all date to about 1.87 to 1.88 billion years. Stratigraphy: Trommald Formation, Animikie Group, Paleoproterozoic, ~1.87 Ga Locality: Portsmouth Mine, just north of the town of Crosby, southern Cuyuna North Range, east-central Crow Wing County, central Minnesota, USA (46° 29’ 19.85" North latitude, 93° 57’ 57.31" West longitude) |
| Date | |
| Source | Banded iron formation (Trommald Formation, Paleoproterozoic, ~1.87 Ga; Portsmouth Mine, Cuyuna North Range, Minnesota, USA) 2 |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/50539417332. It was reviewed on 3 November 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 03:18, 3 November 2020 | | 3,843 × 1,521 (4.85 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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