File:Uraninite-bearing pegmatitic granite (White Cap Pegmatite, Paleoproterozoic, ~1.7 Ga; White Cap Mine, Black Hills, South Dakota, USA) 1 (23507221899).jpg
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Uraninite-bearing pegmatitic granite in the Precambrian of South Dakota, USA. This loose rock sample is in the White Cap Mine in the Black Hills of South Dakota. The mine operated for several decades in the 1900s. It is developed in a pegmatitic granite intrusion and targeted the minerals beryl (for its beryllium), K-feldspar (for its "potash" - potassium), and mica. Pegmatitic granite is a felsic, crystalline, intrusive igneous rock with a pegmatitic texture - all or almost all the crystals are larger than 1 cm. The huge crystals of pegmatites are formed by cooling of water-rich magma, usually in the periphery of a larger pluton. The White Cap Pegmatite itself is one of about 24,000 pegmatite bodies in and around the Harney Peak Granite outcrop area of the Black Hills. The pegmatites are ~contemporaneous with the Harney Peak Granite, which dates to 1.695 to 1.715 Ga. The pegmatitic granite sample shown above is moderately radioactive. Cracking the sample revealed a decent-sized uraninite crystal. Uraninite is UO2 - uranium dioxide, which is nonmetallic-lustered and black in color. Pegmatites tend to have high concentrations of unusual minerals and rare elements. Rare elements don't readily fit into the molecular-level structures of quartz, feldspar, and mica, which are the most common minerals in granitic plutons. Such elements get concentrated in the last remnants of melt (magma). Such melt is also usually water-rich. Cooling of water-rich magma results in huge crystals - pegmatites. Beryl, tourmaline, lepidolite, spodumene, and uraninite are examples of ordinarily-uncommon to scarce minerals that are relatively common in many pegmatites. Geologic unit: White Cap Pegmatite, late Paleoproterozoic, ~1.7 Ga Locality: White Cap Mine (a.k.a. Mica King Mine; a.k.a. King Mica Mine), just south of Greyhound Gulch Road, ~1 to 1.25 air miles southeast of the town of Keystone, southeastern Black Hills, western South Dakota, USA (43° 52' 48.93" North latitude, 103° 24' 26.66" West longitude) More info. on the White Cap Mine & White Cap Pegmatite: Norton et al. (1964) - White Cap Mine. in: Geology and mineral deposits of some pegmatites in the southern Black Hills, South Dakota. United States Geological Survey Professional Paper 297: 337-341. (<a href="http://pubs.usgs.gov/pp/0297e/report.pdf" rel="nofollow">pubs.usgs.gov/pp/0297e/report.pdf</a>) |
| Date | |
| Source | Uraninite-bearing pegmatitic granite (White Cap Pegmatite, Paleoproterozoic, ~1.7 Ga; White Cap Mine, Black Hills, South Dakota, USA) 1 |
| 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@N05/23507221899. It was reviewed on 22 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 05:52, 22 October 2019 | | 3,256 × 2,636 (3.38 MB) | Rudolphous (talk | contribs) | Transferred from Flickr via #flickr2commons |
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