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The Spruce Pine Pegmatite Mining District
by Robert S. "Bo" Smith (updated November 30th, 2018)
Peer reviewed by Alex Glover, PG. -Former Head Geologist of the Feldspar Corporation of America (May, 2012)
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Spruce
Pine
Pegmatite
(smokey quartz, feldspar, and mica) |
Geologic History: The Spruce Pine Pegmatite formed as the result of the intrusion of granitic magma into the Ashe Metamorphic Suite (metamorphosed sedimentary rocks about 800 million years old). The intrusion of the magma occurred during the Devonian Period of the Paleozoic Era (about 380 million years ago). The cooling rate of the magma deep inside the Earth was very slow resulting very large sized crystals of the primary minerals in the granite rocks. This was a time in geologic history where the North American Plate collided with other plates of the Earth's crust resulting in the formation of The Appalachian Mountains. This mountain range was very high; higher than the American Rockies and perhaps even higher than the Himalayas. Subsequent cycles of metamorphism produced additional minerals in the pegmatite body. Cycles of weathering and erosion of the mountains removed tens of thousands of feet of rock material creating the piedmont and the Atlantic Coastal Plain and reducing the present Blue Ridge Mountains exposing the Spruce Pine Pegmatite.
Economic Uses: The Spruce Mine Pegmatite has been a major factor in the economic development in the area which is currently the world's primary producer of high quality silica (quartz) for use in the manufacture of semi conductors, fiber optics, and solar panel components. Feldspar is used in china, sanitary ware, and glass making. Mica was used by Native Americans in the area for thousands of years for ornamental purposes and as a trading item. Early colonists used mica for windows in their cabins. In the late 1890s, mica began to be used for furnace windows due to its transparency and high heat resistance. Due to its flexibility, it was used for curtain material which could be "rolled up" for coaches. Mica was an important mineral resource in the first half of the 20th century. Inventors of the first electrical equipment such as telephones, telegraph transmitters, and other electrical equipment used mica as an insulator. During World War II and in the 1950s, mica was used in artillery and aircraft gun sights and for use in vacuum tubes. During this period, mica was considered a strategic material and the price was supported by the federal government. Mica mining boomed until the mica was removed from the strategic materials list in 1958. Mica is still used in highly technical circuit boards for use in equipment such as electron microscopes and by NASA. It is also used as scrap or flake mica for a functional filler in joint compound mud where it serves as an anti-shrinking material. Mica is also currently used in cosmetics. Other minerals associated with the pegmatite; such as emerald, aquamarine, golden beryl, and garnet are of high interest to gem dealers and mineral collectors.
Minerals found in the Spruce Pine Pegmatite Mining District: The primary minerals of the pegmatite are feldspar, quartz, and mica. These three minerals are the most commercially important Spruce Pine Pegmatite minerals because of the purity and abundance of the minerals and because they occur in large crystalline masses (due to the very slow cooling rate of the magma). Several types of feldspar can be found in the pegmatites in the area; two types of potash feldspar; orthoclase and microcline (potassium aluminum silicate), sodium feldspar; albite (sodium aluminum silicate), and oligoclase (a type of plagioclase feldspar which is mostly a sodium feldspar with some calcium). Amazonite, a relatively rare green variety of microcline feldspar and Moonstone, a gemstone variety of orthoclase is found here but they are rare. The quartz, SiO2, is sometimes clear and colorless (var. rock crystal). But normally, it is translucent and usually has a gray color (var. smoky quartz) or white (milky quartz). The mica is a variety called Muscovite which is normally a silver to light brown color but is sometimes green and even a rum or ruby color but can appear almost black in book form. The percentage of the primary minerals in the Spruce Pine Pegmatite are 65% feldspar (35% plagioclase; albite and oligoclase and 30% potasium feldspar; mostly microcline and some orthoclase), 25% smoky quartz, and 10% muscovite mica. The order of crystallization of these minerals from the magma was; first plgioclase feldspar, next potasium feldspar (first microcline then orthoclase), muscovite mica, and lastly quartz. There are numerous other minerals found in association with the pegmatite which are of interest to mineral collectors. The most common of these minerals is garnet which is usually found as almandine. Another variety of garnet, rhodolite, is less common but is a pale rose to red ruby color is sought by gem and mineral collectors. Tourmaline, a complex silicate rock forming mineral is sometimes found as crystals of the black variety (Schorl) but is occasionally found in the dark green variety highly prized by collectors. Kyanite, a blue bladed mineral is sometimes found associated with tourmaline. Beryllium minerals; beryl and the mineral apatite are sometimes found as accessory minerals in the more alkaline pegmatite rocks. Beryl (hardness 71/2-8) is sought by collectors as a gemstone in several varieties; aquamarine (pale green to blue), goshenite (colorless), heliodore (yellow/gold color), and emerald (dark green). Apatite occurs as a pale beige to green color. The green variety of apatite can distinguished from varieties of beryl (although both occur as hexagonal crystals) due to its lesser hardness (5) and because it produces an orange color under short wave fluorescent light. Zoisite, the pink gemmy variety called Thulite, also occurs in the pegmatite and can be confused with garnet. Other accessory minerals found with the feldspar are blue and green Hyalite Opal, and Autunite as well as other uranium minerals. Several metallic minerals occur but are rare in the pegmatite. The Titanium minerals ilmenite and rutile and molybdenite (MoS2) have been found in association with quartz.To get more details and see examples of what miinerals can be found at some of the mines in the Spruce Pine Pining District, check out my website
Bo's Mine Tours
Other Interesting minerals found near but not part of the Spruce Pine Pegmatite: An iron mineral, magnetite was mined in the Cranberry area of Avery County from the Civil War days (used for rifle barrels) and during the early days of automobile manufacturing until the mines were closed in the 1930s . Sulfide metallic minerals; galena (lead sulfide), pyrite (iron sulfide), chalcopyrite (copper iron sulfide) and possibly argentite (silver sulfide) can be found in Avery and Caldwell counties. There are also stories of gold and platinum being found associated with these minerals. Early Spanish explorers came to the Spruce Pine area in the early 1600s and spent over a hundred years in the area looking for gold and gem minerals but did not find any. The first American gold rush took place in the 1830s-1850s in North Carolina from the Charlotte area to an area south of Marion. You can pan for gold in the once gold bearing gravel at the Thermal Valley Mine located on Rote 226 south. of Marion.