File:Fault zone in inch-scale layered plagioclase feldspar & pyroxene (Stillwater Complex, Neoarchean, 2.71 Ga; near the Stillwater Mine, Beartooth Mountains, Montana, USA) 1.jpg
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English: Fault zone in inch-scale layered plagioclase feldspar & pyroxene in the Precambrian of Montana, USA.
Southern Montana’s Beartooth Mountains has one of the few platinum mines in North America. There, platinum and palladium are mined from the 2.71 billion-years-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 originated 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 mafic and ultramafic, intrusive igneous rocks. Common lithologies include gabbro, norite, harzburgite, anorthosite, troctolite, chromitite, pyroxenite, and dunite. Portions of the Stillwater have been metamorphosed. Olivine is the most commonly altered component - it's usually been metamorphosed to serpentine. The main platinum (Pt) and palladium (Pd) occurrence is in the Johns-Manville Reef (J-M Reef), an interval in the lower part of the Lower Banded Series. The Pt and 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 outcrop seen here does not have platinum ore. The layering consists of alternating bands of plagioclase feldspar and pyroxene. The layering was originally horizontal; Stillwater Complex rocks have been structurally tilted by mountain building. The band from lower left to upper right is a fault zone. The fault zone itself is displaced at center. Stratigraphy: Lower Banded Series, Stillwater Complex, Neoarchean, 2.71 Ga Locality: roadcut along West Fork Stillwater Road (= Forest Service Road 2846), near & above the Stillwater Mine, Beartooth Mountains, southern Montana, USA |
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| Source | https://www.flickr.com/photos/47445767@N05/52377667521/ |
| 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@N05/52377667521. It was reviewed on 2 November 2022 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 22:26, 2 November 2022 | | 4,000 × 3,000 (6.63 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/52377667521/ with UploadWizard |
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