Ore Deposits and Plate Techtonics:
Pegmaties in the Spruce Pine District, North Carolina:
By: Patrick Cassidy
Geosci 451
Pegmatites are highly valuable igneous intrusions that are often found at
subduction zones. When a tectonic plate sinks collides into another and one gets
forced underneath the other. When the subducting plate plunges closer to the
earths center it becomes heated and eventually melts forming a magma body that
will then begin to rise. From this magma body smaller magmatic intrusions will rise
through cracks in the rock and this is what eventually become pegmatites.
Pegmatites are unique in the way that they cool very slowly which is what allows for
the massive crystals that make these formations so valuable. Normal plutonic rocks
cool after around 10,000 years and form crystals around 5 mm while pegmatite
rocks cool after 2.5 million years that allows crystals to form at 1 m.
Spruce Pine is a district in North Carolina that is historically known for its
strong mining background. It is located in the western part of the North Carolina in
the Blue Ridge mountain range that is also rich in minerals. The first people who
mined in the area were the Native Americans who were mining mica at least as early
as the 1500s for decorations and for trading. In addition to the native the Spanish
that had come over mined the area in search of gold and silver. It is in the east that
the first precious metals were mined before the California got attention for the large
amounts of gold that could be found in throughout the west. This area has had over
714 mining prospects including thousands of pegmatites and over 400 mines as of
1962. It is now a major producer of high quality quartz, mica, and feldspar that has
made its mark on the international level. The quartz from this area is known as
some of the highest quality in the world and is sold for around $50,000 per tonne.
This very high quality quartz is what Silicon Valley and other electronics companies
run on.
What formed this mineral sweet spot was a tectonic event that occurred
about 380 million years ago. At this time in the Paleozoic era the African plate was
approaching the eastern North American plate in a collision that forced the ocean
plate to subduct below the eastern part of what is now the United States. This
collision is what caused the Appalachian mountain chain that travels the length of
the eastern United States. The ocean plate sank miles below the surface becoming
heated to around 2000 degrees Celsius. Once the rock was heated it moved through
the cracks the rock and moved towards the surface. It is estimated that it took
approximately 100 million years, which is what caused Spruce Pine to have some of
the largest feldspar and mica crystal in the world. What caused the extremely slow
cooling process is due to them being formed at a deep depth and remained insulated
by the large plutonic bodies that are also in the area. The ore bodies in the area are
capable of being 1000 feet wide and 2000 feet in length allowing for extremely
economical deposits. In addition to the size of the deposits the grain size ranges
from about 0.5 inches to 15 feet adding to their value.
The term used for the type of pegmatite formed in the Spruce Pine district is
Alaskite. The term is defined as a holocrystaline granular plutonic rock high in
quartz and alkali feldspar that contain little to no mafic minerals that allows for the
purity that the area is known for. In addition the areas pegmatites are considered to
be in the muscovite rare element class. This means that it is a granitic/ rare element
rich pegmatite that intrudes into a mica rich bedrock. The pegmatites in the area are
estimated to contain 25% quartz, 15% muscovite, 20% microcline, and 40%
oligoclase all of which are mineable materials. Throughout the Carolinas there are
many mineral deposits that bring large amounts of money into the states.
In 1937 the USGS estimated that $15,000,000 was annually brought in from
mining and by 1985 over $700,000,000 worth was mined on an annual basis. Spruce
Pine has been a major producer of Feldspar since 1917 and is the leading producer
in the states producing 491,000 tons in 1985. Feldspar is widely used throughout
the world as a component in the glass and ceramics industry. It is estimated that
North Carolina produces 42% of the Feldspar produced in the United States valued
at slightly over $18,000,000. North Carolina is also the leading producer of the scrap
mica in the U.S. producing 80 tons in 1985. Mica can be used for many different uses
from electronics to fire proofing items. Beyond this is what the area is famous for
which is the high quality quartz. There is not enough information to accurately
figure out how much quartz comes out of the area yearly but its purity is more
important being one of the most expensive in the world and being one if not the top
producer for high end silicon chips.
The Spruce Pine district is one of the oldest and mining areas in the
United States containing some extremely unique minerals. Although there is new
technology coming out where it is possible to create lab made high quality quartz it
is not economically feasible to compete with the quality of quartz being mined. The
geology of North Carolina created a very slow cooling process that allowed for the
massive crystals to form that the area is known for. Beyond just the quartz the area
is a major contender in the ceramics and glass industry being one of the largest
producers of feldspar in the world as well as being a major producer of scrap and
sheet mica.
References
Brobst, Donald A. "Geology of the Spruce Pine District Avery, Yancy, and Mitchell
Counties North Carolina." Geological Survey Bulletin 1122-a (1962): A1-24.
Print.
Horton, J. Wright., and Victor A. Zullo. The Geology of the Carolinas: Carolina
Geological Society Fiftieth Anniversary Volume. Knoxville: University of
Tennessee, 1991. Print.
London, David. Pegmatites. [Ottawa, Ontario?]: Mineralogical Association of Canada,
2008. Print.
"MicaStatistics and Information." USGS Minerals Information: Mica. N.p., n.d. Web. 30
Nov. 2012.
<http://minerals.usgs.gov/minerals/pubs/commodity/mica/index.html>.
Nelson, Sue. "Silicon Valley's Secret Recipe." BBC News. BBC, 08 Feb. 2009. Web. 2
Nov. 2012. <http://news.bbc.co.uk/2/hi/8178580.stm>.
Pohl, Walter. Economic Geology: Principles and Practice. Oxford: Wiley-Blackwell,
2011. Print.
"The Spruce Pine Mining District." The Spruce Pine Mining District. N.p., n.d. Web. 24
Nov. 2012. <http://www.mitchellcounty.com/festival/spminingdistrict.html>.