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

ORIGIN 27
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springs and solfataras has resulted in end products
that may consist of porous masses of silica, or silica,
alumina, and titania. It must be realized that considerable
amounts of silicon and aluminum may actually
be removed in the leaching process; on the
other hand, silicon and (or) aluminum may be
deposited by the altering solutions under some
circumstances.
This type of rock alteration may have been active
in the formation of some of the Southeastern deposits
of aluminous minerals in quartzose rocks.
Similar deposits of pyrophyllite, alunite, and quartz
in Mesozoic volcanic rocks at Kyuquot Sound, British
Columbia, were thought by Clapp (1915) to
have been formed by solfataric processes. Stuckey
(1928) states that hydrothermal activity may have
taken place at shallow depths in the Southeastern
deposits; however, he favors the view that pyrophyllite
was formed by hydrothermal solutions
from granitic magmas at intermediate temperatures
and pressures after metamorphism of the volcanic
rocks. D. E. White (written communication,
1956) is doubtful that this type of alumina-silica
deposit could be formed through surficial leaching
by acid spring waters, with acid derived by oxidation
of H 2S with atmospheric oxygen. He points out
that deposits formed in this way are generally
small, are restricted to, or near, the water table,
have a different paragenesis (aluminous minerals
formed before silica), and are not known to have
undergone appreciable fluorine fixation. White considers
that the deeper alumina-silica deposits of the
type described by Burbank (which contains the
fluorine-bearing aluminum silicate zunyite) and
Levering are formed by hypogene acid leaching
that probably does not involve atmospheric oxygen;
he agrees that it may be possible that the aluminasilica
replacement deposits of the Southeast have
formed by similar processes.
The environment of formation of these replacement
deposits of quartz, pyrophyllite, kyanite, andalusite,
and topaz in the Southeast remains a problem.
Some kyanite-quartz deposits may be masses
of volcanic rock that were first altered by solfataric
activity and then metamorphosed to the present
mineral assemblage. Other deposits, such as some
andalusite-pyrophyllite-quartz deposits, for example,
might represent the deeper zones of extensive
solfataric centers. Possibly in some places the rock
alteration was contamporaneous with mild metamorphism
of the volcanic rocks.
Deposits of kyanite-quartz rock and sillimanitequartz
rock that seem to be metamorphosed sediments
might have several possible relationships to a
volcanic environment. These deposits probably were
formed by the metamorphism of beds of sandy clay
or clayey sand, which might have been formed in
several ways. Some volcanic rocks are readily
weathered to very clayey soils under certain conditions;
some volcanic soils from Java, Sumatra, and
Hawaii have been found to carry free alumina
(Clarke, 1924, p. 502), and the ultimate product of
thorough weathering can be clay or bauxite. This
material then might be eroded and transported by
streams to a site of deposition, such as a lake basin.
The parent sediment of kyanite quartzite may have
been a mixture of (a) quartz sand derived from
siliceous residue of hydrothermal origin or from
some quartz-bearing rock, and (b) transported clay
that has been derived from weathered basalt
(Farmville district) or from more silicic volcanic
rocks (Kings Mountain district). High-alumina
clays of probable sedimentary origin are associated
with volcanic rocks at several places in the
Pacific Coast States. Some geologists have regarded
these clays as being of volcanic origin, but
Alien (1946) has given evidence for the sedimentary
origin of the following clay deposits: lone
clay, California; Cowlitz clay deposit, Washington;
and the Hobart Butte and Molalla clay deposits in
Oregon.
Sedimentary deposits of clay and quartz sand in
a volcanic terrane might also be formed by the erosion
of hydrothermal silica and clay from a solfataric
center and deposition of the material in a
restricted basin that was uncontaminated by other
sediments. In this way, alumina-silica deposits of
sedimentary origin might occur in the same region
as alumina-silica deposits of hydrothermal origin.
The basic feature of both these processes hydrothermal
leaching and weathering is the removal
of practically all constituents except alumina and
silica; a certain proportion of alumina and silica
might be removed, too. Titania is also a final product
of both processes, which doubtless explains the
widespread occurrence of small amounts of rutile
in the deposits of Al2Si0 5 minerals in quartzose
rocks.
ORIGIN OF HIGH-ALUMINA QUARTZOSE DEPOSITS IN OTHER
REGIONS
Various modes of origin have been ascribed to
quartzose deposits of high-alumina minerals in other
parts of the world. In the Picuris Range, N. Mex.
(Montgomery, 1953), beds of kyanite quartzite and
sillimanite quartzite, ranging from a few feet to