Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

ORIGIN 23
(percent)
Hiwassee schist -__ . _. . .._._. _________ 29.90
Nantahala slate ______________________ _____.____________. 41.18
The abundant alumina in these schists is probably
of sedimentary origin. The A1203 content of the
average shale is 15.4 percent, according to Clarke
(1924, p. 34). Kyanite schists are also present in
the region; their relations to the graphitic mica
schists are not discussed by Bayley, but it seems
probable that they were formed from aluminous
sedimentary rocks.
Kyanite is common in quartz veins and pegmatites
in some areas of kyanite schist and gneiss. For
reasons advanced below, we conclude that kyanite
in the quartz veins and pegmatites is later than
kyanite in the schists and gneisses, and that the
kyanite in the quartz veins and pegmatites was
formed from A1203 derived from the country rock.
However, other workers have proposed that hydrothermal
fluids from pegmatites or quartz veins have
had an important part in the formation of kyanite
in the micaceous schists and gneisses. Stuckey
(1932, 1935) has concluded that the kyanite in the
North Carolina schists was formed by metasomatic
replacement by hydrothermal solutions from the
quartz veins, pegmatites, and their parent magmas
after metamorphism of the host rock. Furcron (1950)
also thought that hot solutions derived from quartz
veins, pegmatites, and granitic bodies were instrumental
in formation of kyanite, but that the alumina
was derived from the aluminous country rock and
not from an igneous source.
In the Burnsville-Swannanoa area, North Carolina,
for example, it is most unlikely that alumina
in the kyanite schists could have been derived from
pegmatites and their parent magmas, because the
belt, 30 miles long, of kyanite schists and gneisses
does not coincide with the area of most abundant
pegmatite deposits, the Spruce Pine pegmatite district,
lying just east of the north end of this belt.
In many occurrences the kyanite crystals in the
country rock schist, gneiss, or quartzose rock
are generally conformable to the foliation, whereas
kyanite in the intruding quartz veins or pegmatites
is generally not oriented parallel to the foliation
(fig. 25). Thus, the kyanite in the quartz veins and
the pegmatites seems younger than the kyanite in
the host rock. The vein or pegmatite fluids may.
have been contaminated by A1203 from the country
rock. Or possibly both the alumina and the silica in
some kyanite-quartz veins have been derived from
aluminous country rock by means of metamorphic
differentiation, as was suggested by Read (1933)
for the origin of some quartz-kyanite veins in kyanite
schists in Unst, Shetland Islands.
The bulk of the evidence, therefore, seems to us
to point to the aluminous country rock as the source
of the alumina now in the kyanite in the schists
and gneisses and also in the pegmatites and quartz
veins. More detailed field, petrographic, and chemical
studies are needed in the large areas of kyanite
schist and gneiss in order to gather more evidence
on the source of the alumina and its behavior during
metamorphism.
The sillimanite schists of the Piedmont and Blue
Ridge belts consist chiefly of sillimanite-quartzbiotite-muscovite
schists that are commonly graph­
itic. They are intruded by granite bodies and
innumerable pegmatites. The problem of the source
of alumina in the sillimanite schists is the same
as that for the kyanite schists and gneisses. In the
Piedmont belt of North Carolina, Hunter and White
(1946) have suggested that sillimanite was formed
FIGURE 25. Quartz-kyanite vein in kyanite quartzite at north end of Clubb
Mountain, Lincoln County, N.C. Vein occurs in northwesternmost lens
shown on pi. 9. Note sharp contact at the top of the vein between vertically
oriented kyanite prisms and kyanite quartzite. The vein is practically
horizontal and the foliation in kyanite quartzite is parallel to the plane of
the photograph. Scale, 1 foot. Photograph by W. C. Overstreet.