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

36 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES
to the west were recently mapped by Brown and
Sunderman (1954).
GENERAL GEOLOGY
The principal rocks in this part of the Piedmont
metamorphic belt include several varieties of mica
schist and gneiss, hornblende gneiss, and kyanite
quartzite, all of probable Precambrian age. Pegmatites
and quartz veins are abundant. Bodies of
granite occur to the north and east but not in the
vicinity of the kyanite deposits. Slate of Ordovician
age occurs in a narrow north-trending syncline at
Arvonia, a few miles north of the district; the
southern end of this syncline may extend into the
mapped area. Conglomerate, sandstone, shale, and
coal beds of Triassic age occur in a basin about
15 miles long just east of the northern part of the
mapped area. Diabase dikes of Triassic age cut all
rock formations in the region.
The rocks of pre-Triassic age have been highly
deformed and metamorphosed. The trends of the
rock formations and the foliation are variable because
of complex folding, but in general lie between
north and east. The grade of metamorphism is uniform,
and belongs to the amphibolite facies (Turner,
1948, p. 76-88), as exemplified by such assemblages
as biotite-oligoclase-quartz (with epidote-garnet in
places) and hornblende-oligoclase or andesinequartz
(with biotite-epidote-garnet locally).
Erosion since Triassic time has produced a gently
rolling- surface and deep weathering of the underlying
rocks. The highly resistant beds of kyanite
quartzite form ridges and knobs (Willis Mountain
and neighboring ridges, and Woods, Baker, and
Leigh Mountains) that rise 100 to 500 feet above
the general peneplain surface.
ROCK TYPES
STRATIGRAPHY
The dominant rocks in the mapped area are biotite
gneiss, hornblende gneiss, and muscovite-quartz
schist (pis. 2, 3). Kyanite quartzite is a distinctive
rock that forms conspicuous outcrops but underlies
only a small part of the entire district. The thickness
of kyanite quartzite is less than 40 feet at most
places, and exceeds 50 feet only at Willis and Baker
Mountains. Garnetiferous kyanite schist is exposed
beneath kyanite quartzite at several localities. Thinner
and less extensive rock units in the metamorphic
series are quartzite, ferruginous quartz rock,
and garnet-quartz rock. These rocks are probably
late Precambrian.
The graphitic schist exposed in the headwaters
of Whispering Creek (pi. 2) probably is in the southern
extension of the syncline of Arvonia slate of
Late Ordovician age. The Ordovician age for the
Arvonia slate was originally assigned to fossils
found in one of the slate quarries at Arvonia (Darton,
1892; Watson and Powell, 1911); Stose and
Stose (1948) have recently questioned this age determination,
and suggest that these slates may be
Silurian.
Igneous rocks intruding the metamorphic series
are pegmatite, biotite-quartz syenite, a few thin
dikes of aplite, and diabase. Small bodies of pegmatite
occur throughout the region; biotite-quartz syenite
is known only at three localities at the northern
end of the district. The pegmatites and syenite may
be Ordovician or later Paleozoic in age. Diabase dikes
of Triassic age cut the metamorphic rocks of the
mapped area and the Triassic sedimentary rocks to
the east. Quartz veins are widely distributed in the
metamorphic rocks; some veins contain abundant
tourmaline.
The key to the stratigraphic order of the metamorphic
series is found in a large anticline lying
east and northeast of Willis Mountain. Biotite
gneiss is the major rock type exposed in the core
of this anticline (herein called the Whispering
Creek anticline); kyanite quartzite lies on both
flanks of the fold (pi. 2). This thick sequence of
biotite gneiss, containing a few thin layers of hornblende
gneiss, is evidently older than the kyanite
quartzite. Biotite gneiss also lies beneath kyanite
quartzite in an anticline at Woods Mountain, and is
exposed to the north of (and stratigraphically beneath?)
kyanite quartzite at Baker Mountain (pi.
3). In the Whispering Creek anticline, a thin sequence
of biotite gneiss lies just above the kyanite
quartzite, and is overlain by a thick series of hornblende
gneiss. In the anticline at Woods Mountain,
hornblende gneiss appears to lie directly above kyanite
quartzite. In the Baker Mountain area, the
thickest sequence of hornblende gneiss seems to be
just below the kyanite quartzite horizon, but some
hornblende gneiss lies above the quartzite, and is
overlain by muscovite-quartz schist.
The similarity, though not strict identity, of the
apparent stratigraphic order in the different parts
of the district strongly suggests that the thick series
of biotite gneiss is the oldest formation. Kyanite
quartzite occurs in the upper part of the biotite
gneiss, and is overlain by hornblende gneiss in the
Willis Mountain-Woods Mountain area and by muscovite
schist in the Baker Mountain-Madisonville
area. This interpretation of the stratigraphic order
is shown diagrammatically in figure 28.