File:Sulfidic anorthosite (platinum-palladium ore) (Johns-Manville Reef, Stillwater Complex, Neoarchean, 2.71 Ga; Stillwater Mine, Beartooth Mountains, Montana, USA) 8.jpg
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Summary
| Description |
English: Sulfidic anorthosite from the Precambrian of Montana, USA.
Southern Montana’s Beartooth Mountains has one of only three platinum mines in North America. There, platinum and palladium are mined from the 2.71 billion-year-old Stillwater Complex, a classic example of an LLI (large, layered igneous province). LLIs are large intrusive bodies that display large-scale and small-scale layering, even including cross bedding, ripples, graded bedding, channelforms, and other sedimentary-like features. The Stillwater started out as a large subsurface mass of slowly cooling magma. As various minerals crystallized, they settled to the bottom of the magma chamber. This resulted in layering. Igneous rocks that formed this way have a cumulate texture. Currents in the still-liquid portions of the magma chamber produced the sedimentary structures mentioned above. Most of the Stillwater displays only large-scale layering. The rocks in the Stillwater are ultramafic & mafic intrusive igneous rocks. Common lithologies include gabbros, norites, harzburgites, anorthosites, troctolites, chromitites, pyroxenites, and dunites. Portions of the Stillwater have been metamorphosed. Olivine is the most commonly altered component - it is usually metamorphosed to serpentine. The main platinum & palladium occurrence is in the Johns-Manville Reef (J-M Reef), an interval in the lower part of the Lower Banded Series. There, the Pt & Pd occur in intercumulate sulfides, typically pyrrhotite (Fe1-xS) and chalcopyrite (CuFeS2). Platinum ores in the J-M Reef are principally sulfidic anorthosites (e.g., see this photo), but other lithologies also occur. The J-M Reef is the highest grade deposit known for platinum-group elements (PGEs). Anorthosite is an uncommon intrusive igneous rock with a coarsely-crystalline texture and dominated by grayish-colored plagioclase feldspar. The small patches of fool's gold in this sample are the pyrrhotite and chalcopyrite (~1 to 2% of the rock). Stratigraphy: Johns-Manville Reef, Lower Banded Series, Stillwater Complex, Neoarchean, 2.71 Ga Locality: Stillwater Mine, Mountain View area, southwest of Nye, Beartooth Mountains, southern Montana, USA |
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| Source | https://www.flickr.com/photos/47445767@N05/50424451423/ |
| 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@N05/50424451423. It was reviewed on 9 October 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| Date/Time | Thumbnail | Dimensions | User | Comment | |
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| current | 06:01, 9 October 2020 | | 2,271 × 2,080 (3.88 MB) | Ser Amantio di Nicolao | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/50424451423/ with UploadWizard |
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