File:Inch-scale layered anorthosite-norite (Stillwater Complex, Neoarchean, 2.71 Ga; mine core dump, Stillwater Mine, Beartooth Mountains, Montana, USA) (31406477845).jpg
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editDescriptionInch-scale layered anorthosite-norite (Stillwater Complex, Neoarchean, 2.71 Ga; mine core dump, Stillwater Mine, Beartooth Mountains, Montana, USA) (31406477845).jpg |
Inch-scale layered anorthosite-norite from the Precambrian of Montana, USA. (3.4 cm across at its widest) 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, 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, but other lithologies also occur. The J-M Reef is the highest grade deposit known for platinum-group elements (PGEs). The sample shown above is not a platinum ore - it's part of a core taken decades ago during exploration studies by the Stillwater Mine. Notice the layering, from dark-speckled norite to light gray anorthosite to dark-speckled norite. Stratigraphy: Stillwater Complex, Neoarchean, 2.71 Ga Locality: exploratory core dump pile along the southern side of West Fork Stillwater Road (= Forest Service Road 2846), next to the Stillwater Mine's tailings pond, Stillwater Canyon, Beartooth Mountains, southern Montana, USA |
Date | |
Source | Inch-scale layered anorthosite-norite (Stillwater Complex, Neoarchean, 2.71 Ga; mine core dump, Stillwater Mine, Beartooth Mountains, Montana, USA) |
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@N05/31406477845 (archive). It was reviewed on 5 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
5 December 2019
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current | 06:16, 5 December 2019 | 1,715 × 2,375 (2.96 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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Date and time of data generation | 00:35, 4 December 2016 |
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Horizontal resolution | 180 dpi |
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File change date and time | 02:27, 4 December 2016 |
Y and C positioning | Co-sited |
Exif version | 2.21 |
Date and time of digitizing | 00:35, 4 December 2016 |
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Scene capture type | Portrait |
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Date metadata was last modified | 21:27, 3 December 2016 |
Unique ID of original document | 660632BF3155DD0CC8FD8417E9382D7A |