File:Chromitite with serpentinite (Neoarchean; Red Lodge Chromite District, Beartooth Mountains, Montana, USA) 1.jpg

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English: Chromitite with serpentinite in the Precambrian of Montana, USA.

This rock is at an old chromite mine in southern Montana's Red Lodge Mining District, in the Beartooth Mountains. Active mining took place during the 1940s. Most of the old, small mines occur on the high plateaus or just below the eroded plateau edges.

Mines in the Red Lodge District targeted chromium ores in the form of black-colored chromitite rocks hosted in serpentinite and bastite (= serpentinized pyroxene). In this area, lenses, or pods, or fault-chopped stratiform bodies of serpentinite occur among high-grade metamorphic rocks such as quartzites, gneisses, migmatites, amphibolites, etc.

The rocks in the Beartooth Mountains are very old. They are principally Archean in age (2.5+ billion years old). The chromite-bearing serpentinite bodies of the Red Lodge District were originally chromite-bearing peridotites (dunites) of early Neoarchean age, or older. Granulite-grade to upper amphibolite-grade regional metamorphism at about 2.75 Ga altered the original dunites to serpentinites. The rocks of the area were also altered by greenschist-grade metamorphism between 1.6 to 1.8 billion years ago, during the late Paleoproterozoic.

The dark-colored areas of this sample are chromitite, a granular-textured rock rich in the mineral chromite (FeCr2O4 - iron chromium oxide). The greenish areas are serpentinite, a low- to high-grade metamorphic rock dominated by one or more serpentine-group minerals (Mg3Si2O5(OH)4 - magnesium hydroxy-silicate). Serpentinites range in color from light to dark green to black (sometimes whitish or bluish). They usually have a waxy feel and luster. Serpentinite forms by metamorphism of peridotite in the presence of water. Peridotite is an intrusive igneous rock rich in olivine ((Mg,Fe)2SiO4 - magnesium iron silicate).

Locality: old chromite mine, near the eroded edge of the Beartooth Plateau, southeastern side of Rock Creek Canyon, southwest of the town of Red Lodge, Red Lodge Chromite District (Red Lodge Mining District), Beartooth Mountains, southern Montana, USA (45° 02' 04.36" North latitude, 109° 24' 35.11" West longitude)
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Source https://www.flickr.com/photos/47445767@N05/52574716481/
Author James St. John

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This file is licensed under the Creative Commons Attribution 2.0 Generic license.
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/52574716481. It was reviewed on 20 December 2022 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

20 December 2022

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current13:42, 20 December 2022Thumbnail for version as of 13:42, 20 December 20224,000 × 2,475 (6.83 MB)Ser Amantio di Nicolao (talk | contribs)Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/52574716481/ with UploadWizard

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