Spruce Pine Mining District

The Spruce Pine Mining District is a swath of the valley of the North Toe River in the Blue Ridge Mountains of northwestern North Carolina. The area is mined for its mica, kaolin, quartz and feldspar.[1] Spruce Pine district is one of the largest suppliers of high-purity quartz, which is used in the manufacture of silicon for integrated circuits.[2] The district is named after the town of Spruce Pine, which is located in the middle of the region and is the hub of major mining activity there. The district is approximately 25 miles (40 km) long and 5 miles (8.0 km) wide.

Spruce Pine Mining District
Stratigraphic range: 450-470 Ma
Sample of the Spruce Pine Pegmatite
TypeGeological formation
Unit ofTugaloo Terrane
OverliesCranberry Gneiss
Thicknessup to 10,500 feet (3,200 m)
Location
LocationSpruce Pine, NC
Coordinates35.942922°N 82.08268°W / 35.942922; -82.08268
RegionAppalachian Mountains
CountryUnited States
Type section
Named forSpruce Pine, NC

Regional geology

The district is located within the Blue Ridge upland region, and is bounded on the eastern edge by the Blue Ridge Front.[1] The pegmatites intruded into the Ashe Formation. The pegmatites and country rock are part of the Spruce Pine thrust block, the highest thrust sheet of the Blue Ridge belt, the latter being part of the Tugaloo Terrane which was accreted to North America during the Taconic Orogeny.[2] A more refined age for the pegmatites within the district has been found through 40Ar/39Ar dating of micas. They have been dated to approximately 336 million years old.[3] The approximate age of the whole district was taken from Sm/Nd dating in the hornblendes, and was found to be from 470 to 450 million years old. This puts the oldest rocks in the district within the Ordovician period.[2] The master stream of the district is the North Toe River.[1]

Igneous Rock Units

Pegmatite

Major minerals include plagioclase, microcline, quartz, and muscovite. Perthitic textures on the microcline are possible.[1][4] Minor minerals include biotite and garnet.[1] The size of this unit ranges from inches to thousands of feet.[1] It will weather into light-colored soils.[1]

Dunite
Thin section of microcline from the Spruce Pine Pegmatite showing a perthitic texture

Minerals in this unit include talc, amphibole, chlorite, and chrysotile.[1] It will weather to a dark-colored, organic-poor soil.[1]

Metamorphic Rock Units

Quartzite

This is a relatively rare unit within the district. The mica and hornblende rock units grade into it. Only about five percent of the rock units within the district are composed of quartzite.[1]

Hornblende Rocks

Major minerals include hornblende, quartz, plagioclase, and andesine. Minor minerals include epidote, zoisite, garnet, and sphene. Metamorphic textures in these rocks can either be schistose or gneissic. They transition into the mica gneiss and schist units.[1]

Mica Gneiss and Mica Schist

Major minerals in both include mica, quartz, plagioclase, and muscovite. Minor minerals for both include garnet, ilmenite, epidote, and apatite.[1] The texture of the mica gneiss has homogenous mineral layers, with less foliation than the mica schist.[1] It weathers into a dark red soil.[1] The texture of the mica schist is made up of heterogenous layers of minerals. It weathers into a micaceous, brown soil that is distinct from the other soil types in the district.[1]

Cranberry Gneiss

The Cranberry Gneiss rock unit can be found interbedded with other units in the district.[1]

Significance in the mining industry

The district is known for producing multiple types of commercial materials. Ground micas are collected from the mining of other minerals in the district. Sheet mica also exists, but it is not mined.[1] Feldspars are another major mineral that is mined here.[1] Some companies that mine in the Spruce Pine district include: the Feldspar Corporation, Covia, K-T Feldspar, and US Gypsum.[5]

References
  1. Brobst, Donald Albert (1953). "Geology of the Plumtree area, Spruce Pine district, North Carolina". Open-File Report. doi:10.3133/ofr5326.
  2. Swanson, Samuel E.; Veal, William B. (2010-04-15). "Mineralogy and petrogenesis of pegmatites in the Spruce Pine District, North Carolina, USA". Journal of Geosciences. 55 (1): 27–42. doi:10.3190/jgeosci.062. ISSN 1802-6222.
  3. Tollo, Richard P. (2004-01-01). Proterozoic Tectonic Evolution of the Grenville Orogen in North America. Geological Society of America. ISBN 978-0-8137-1197-3.
  4. Sheets, Julia M. (1997). "Crystallographic Controls on the Alteration of Microcline Perthites from the Spruce Pine District, North Carolina". Clays and Clay Minerals. 45 (3): 404–417. doi:10.1346/CCMN.1997.0450310. ISSN 0009-8604. S2CID 129789582.
  5. Hawley, George (June 2011). "MICA". Mining Engineering. 63: 82–85 – via https://me.smenet.org/issueIndex.cfm?issueID=117. {{cite journal}}: External link in |via= (help)
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