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

22 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES
ment of fluids. Therefore, "devolatilization" could
take place at lower temperatures in regional metamorphism
than in thermal metamorphism, and this
early removal of water may permit the recrystallization
of the hydrous aluminum silicates at relatively
low temperatures within the kyanite field,
resulting in the formation of kyanite. In contact
metamorphism, however, water may remain in the
system to higher temperatures, and the breakdown
of the hydrous aluminum silicates may be retarded
until temperatures are reached at which andalusite,
sillimanite, or mullite is crystallized.
A fundamental problem in understanding the
chemical processes that may have taken place during
formation of the Al2Si0 5 minerals is whether
all the aluminum was an original constituent of the
rock before metamorphism, or whether the relative
content of aluminum increased by some means during
metamorphism. If the aluminum content was
increased during metamorphism, was it by introduction
of aluminum, or were other constituents
removed to leave the rock relatively enriched in
aluminum? If aluminum was introduced by hydrothermal
fluids, was the aluminum of magmatic
origin or was it derived from aluminous country
rock, perhaps at considerable distance from its present
site ?
The apparent paragenetic relations of the Al2SiO5
minerals may be very misleading at some places.
The important chemical reactions involving aluminum
may have taken place before metamorphism, and
the records of these transformations then may have
been completely destroyed during metamorphism.
For example, a body of igneous rock may be leached
of practically all its constituents except A1203 and
Si02 by hydrothermal fluids associated with volcanism
and may later be recrystallized to kyanitequartz
rock by regional metamorphism. If none of
the original minerals or other primary features are
preserved, the kyanite-quartz rock may appear to
be a metamorphosed sediment. Or, very minor chemical
changes (see section on origin of quartzose
deposits, p. 25) could have occurred in the waning
stages of metamorphism, such as solution of kyanite
and its recrystallization in quartz veins or
slight replacement of kyanite by clay minerals, that
could be incorrectly interpreted as evidence of the
major process of origin.
Some of the fundamental problems of metamorphism,
such as metamorphic differentiation, diffusion,
and metasomatism, are involved in the chemical
processes that may operate in the formation of the
Al2Si0 5 minerals. A lengthy discussion of these
mechanisms, about which there is still much speculation,
is not practical here. These problems are well
reviewed in such recent texts as Turner (1948),
Turner and Verhoogen (1951), Ramberg (1952),
Earth (1952), and Fyfe and others (1958). Some
of the chemical problems of origin of the Southeastern
kyanite, sillimanite, and andalusite deposits
are discussed below.
ORIGIN OF SOUTHEASTERN DEPOSITS
Some deposits of the Al2Si05 minerals in the
Southeast appear to have been formed by the metamorphism
of aluminous sedimentary rocks, accompanied
by little chemical change during recrystallization.
In other deposits, the formation of the
Al2Si0 5 minerals probably was the result of
chemical changes of considerable magnitude.
KYANITE GROUP MINERALS IN MICACEOUS SCHISTS
AND GNEISSES, QUARTZ VEINS, AND (PEGMATITES
Considering first the kyanite-bearing schists and
gneisses that are widespread in certain areas, particularly
in the Blue Ridge, it seems to us that these
rocks have been formed by the recrystallization of
aluminous sedimentary rocks with a minimum
change in bulk composition during metamorphism.
However, we have little new evidence to offer on
this matter, because we have studied kyanite schists
in detail only in the Kings Mountain district, North
Carolina-South Carolina, and Farmville district,
Virginia, where they occur in only small quantity
in association with kyanite quartzite. Our conclusions
rest mainly upon the evaluation of information
gathered by others in the large areas of kyanitemica
schists and gneisses. These areas have been
mapped in detail only in Habersham and Rabun
Counties, Ga. (fig. 56), and at the north end of the
Burnsville-Swannanoa area, North Carolina (fig.
39). In the north Georgia area the pattern of distribution
of kyanite in thin persistent zones of
graphitic mica schist around the flanks of a large
dome clearly indicates that these kyanite-bearing
layers are metamorphosed aluminous sedimentary
rocks. The evidence is not so striking in the North
Carolina area, but the occurrence of kyanite gneiss
within a definite zone, 6 to 8 miles wide and over
30 miles long, seems to indicate that these rocks,
too, were originally aluminous sediments. In the
Tate quadrangle, Pickens and Cherokee Counties,
Ga., some of the schists contain much more alumina
than the average shale, as shown by the analyses
of two graphitic mica schists given by Bayley (1928,
p. 65):