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

ECONOMIC CONSIDERATIONS 33
reau of Mines (table 11) indicate that the coarse
sillimanite crystals are easily separable, but that
very fine grinding would be necessary to liberate
the fine fibrous sillimanite from quartz. This difficulty
is a handicap to the potential economic value
of the sillimanite quartzite deposits.
Sillimanite in micaceous schist or gneiss is commonly
finely fibrous and intergrown with fine white
mica; in some places it also occurs as coarse crystals
or in small nodular aggregates of fibrous crystals.
Beneficiation tests have shown that a good sillimanite
concentrate can be made of the coarser sillimanite
in some deposits; however, this concentrate
may have a high iron-oxide content and
acid leaching may be necessary to remove the iron
(Hudson, 1946; Teague, 1950). It is often difficult
to make a satisfactory sillimanite concentrate from
rock containing sillimanite that is intimately intergrown
with white mica (Hudson, 1946; Hash and
Van Horn, 1951). To separate the sillimanite from
the mica, very fine grinding may be necessary; in
some concentrates, the percentage recovery of sillimanite
may be very low. Careful beneficiation tests
would be essential in the evaluation of a sillimanite
deposit.
Structure. The sillimanite quartzite deposits of
the Kings Mountain district seem to be metamorphosed
sedimentary rocks that have undergone a
higher degree of metamorphism than the nearby
deposits of kyanite quartzite. The sillimanite quartzite
bodies range from lenses a few hundred feet
in length to layers several thousand feet in length;
their thickness is generally less than 10 feet, but
reaches 50 feet in places. Parts of some lenses are
practically barren of sillimanite. The longer deposits
have curved trends due to folding and some
of the shorter lenticular bodies are in curved zones
(pi. 7); the lenses are probably the remnants of a
once-continuous bed or beds that have become separated
by close folding and shearing.
Sillimanite schist and gneiss also seem to be metamorphosed
aluminous sedimentary rocks. Alternating
layers of sillimanite-rich schist and sillimanitepoor
schist occur in extensive linear zones or large
irregular areas. The nature of distribution of sillimanite
schist has been demonstrated very well in
the Shelby quadrangle, North Carolina. (Yates, Overstreet,
and Griffitts, written communication,
1954), where it forms large curved zones, as much
as several miles wide, that are associated with biotite
schist. These curved bodies of sillimanite schist
are evidently highly folded and perhaps also faulted.
Probably similar complex structures exist in the
sillimanite schist of other areas. Some sillimanite
schist layers and zones in the Warne-Sylva belt of
western North Carolina extend for miles with little
deviation in strike (Hash and Van Horn, 1951). Silllike
bodies of granitic rock cut both sillimanite and
biotite schists in the Shelby quadrangle. Pegmatite
and granite are widespread in other areas of sillimanite
schist. In some places where float of sillimanite
schist is abundant on the surface, trenching
or drilling has shown that granite or schist that is
lean or barren in sillimanite occurs at depth. The
sillimanite schist fragments have evidently accumulated
as float on the surface because of high resistance
to weathering. The absence of sillimanite
schist beneath such areas may be the result of lateral
movement of sillimanite float in the soil, or it
may be due to complex structural conditions.
ANDAL.USITE DEPOSITS
In the Southeast, andalusite occurs as a minor constituent
of schist and quartz veins in some areas,
it is present in some sillimanite quartzite in the
Kings Mountain district (where it is locally more
abundant than the sillimanite), and it is rather common
in some pyrophyllite-bearing quartzose deposits
of replacement origin. This last type of deposit seems
to be the most promising as a potential source of
andalusite. Other high-alumina minerals corundum,
diaspore, topaz, kyanite, and pyrophyllite
commonly accompany andalusite in these quartzose
replacement deposits. Information on the content
of high-alumina minerals in these deposits is available
only for the Brewer deposit, South Carolina.
Drill-hole samples taken there by the Bureau of
Mines show that 70 out of 231 core sections contain
over 15 percent combined topaz, andalusite, and
kyanite (Peyton and Lynch, 1953, p. 7). Because of
the similar physical properties of most of the highalumina
minerals, it may prove difficult to separate
andalusite from some of the other high-alumina minerals
by ore-dressing methods. However, separation
of andalusite would probably not be necessary because
the minerals are similar in chemical composition.
Some andalusite-pyrophyllite-quartz deposits
are lenticular or roughly circular in surface outline,
having diameters of several hundred feet. No information
is available on their structure, but it
seems probable that they are pipelike bodies that
may have considerable vertical extent. Other deposits
are a thousand feet or more long; they may
be tabular veinlike bodies.