ABSTRACT
The Crabtree pegmatite in the Spruce Pine district of North Carolina is a beryl (emerald, green beryl, aquamarine
and yellow beryl) and tourmaline-bearing granitic pegmatite. It is texturally unzoned with a weakly developed
differentiation of the slightly coarser grained segregations and aplitic units, and variously assimilated xenoliths.
Brown to black tourmaline is common in the pegmatite margin, in random sunburst patches in the pegmatite and as
radiating sprays from the xenoliths. Emerald and green beryl occurs in the biotite and chlorite selvage and the
pegmatite margin, whereas aquamarine and yellow beryl occurs in the pegmatite core. Green and purple fluorite,
white mica, almandine-spessartine composition garnet and xenotime are accessory minerals. It has been
sporadically mined for gem-quality beryl from 1894 to the late 1970’s and has produced a number of good quality
emeralds, one of which was 70 carats. The pegmatite intrudes at the boundary between garnet-titanite-zoisite
bearing hornblende-plagioclase gneiss (blastomylonite) and plagioclase-quartz-biotite-white mica porphyroclastic
gneiss. These have been hydrothermally altered adjacent to the pegmatite and associated aplite introducing
chlorite and calcite at the expense of biotite, garnet, hornblende, titanite and zoisite.
Emerald hosts abundant, randomly oriented, tabular and rounded quartz and plagioclase inclusions, some of
which contain biotite-phlogopite microlites. Green beryl occurs as subhedral to euhedral crystals with biotite and
white mica inclusions and has fewer quartz and plagioclase inclusions than the emerald. Inclusion-free aquamarine
is subhedral with minor fractures while euhedral hexagonal prisms of yellow beryl have thin fractures and minor
inclusions of quartz and plagioclase.
MgO and Na20 are higher in the emerald than in the green and yellow beryl whereas Fe203 predominates in
aquamarine. Emerald contains significantly higher amounts of Cr (1564 ppm) and Cs (1066 ppm) than do the other
beryl varieties. V, Rb and Ni are higher in the emerald than in the yellow beryl and aquamarine while Zn
concentrations are the highest in the aquamarine (140 ppm).
The tourmaline is intermediate between dravite-schorl and has elevated F concentration with values reaching 0.8
apfu in the pegmatite margin. Mg in tourmaline in the exocontacts and the margin is high (up to 2.1 apfu) while Fe
is highest in the xenolith tourmaline (2.0 apfu). Cr in the tourmaline tends to have high concentrations in the
exocontacts, reaching 0.14 apfu. The Crabtree pegmatite represents an albitic magma having Be,
В
and F that
intruded a Cr enriched protolith to produce emerald.
GEOGRAPHIC AND GEOLOGIC SETTING
The Crabtree pegmatite is located 2.5 km northwest of Little Switzerland, NC and 7.5 km southwest of Spruce Pine,
NC (Figure 1). The area lies within the eastern Blue Ridge Province of the southern Appalachians. Various schist,
gneiss, and mylonite-series rocks of the Ashe Metamorphic Suite (AMS) underlie the area. These were
metamorphosed to amphibolite grade during the early Paleozoic and then intruded by peraluminous granitic plutons,
dikes, sills and over a thousand pegmatite bodies of the Spruce Pine plutonic suite during the Devonian. The region
was subjected to transpressional deformation during the Alleghanian, forming a stack of crystalline thrust sheets; the
area in which the Crabtree is situated is a part of the Gossan Lead block of this stack (Figure 2). This was initiated
under greenschist facies conditions during the Pennsylvanian and concluded under brittle conditions during the
Permian.
The Crabtree may or may not be a member of the Spruce Pine plutonic suite. It differs from the pegmatite bodies of
the Spruce Pine by its lack of significant amounts muscovite and the presence of tourmaline, beryl (especially
emerald), and fluorite. The Crabtree pegmatite intruded hornblende-plagioclase gneiss and possibly biotite-white
mica-plagioclase-quartz gneiss. The principal minerals of the Crabtree are plagioclase, K-feldspar, cordierite, and
quartz with accessory beryl, biotite, fluorite, tourmaline, and xenotime.
Figure 1. Location map
for the Big Crabtree
Emerald Mine. Mine site
is located at the large
cross-pick, other (but not
all) pegmatite mine sites
located at the smaller
cross-picks. Dotted line
indicates general position
of a topographic
lineament that may be a
shear zone. Map
constructed from portions
of the Micaville (M),
Spruce Pine (SP), Little
Switzerland (LS) and
Celo (C) 7.5 minute
quadrangle topographic
maps.
Figure 2. Thrust blocks of the Blue
Ridge Belt. The Crabtree pegmatite
intrudes the Gossan Lead block, which
is comprised of the Ashe and Alligator
Back Metamorphic Suites. Fault block
terminology after Raymond (1998).
Unfortunately the contact between the pegmatite and the rocks of the AMS is not exposed at the mine site. This is due
to the presence of an extensive zone of hydrothermal alteration that seems to postdate the emplacement of the
pegmatite and the presence of a shear zone that post dates the hydrothermal alteration. This shear zone was subjected
to a later episode of hydrothermal alteration. The first episode of hydrothermal alteration resulted in modification of the
rocks of the AMS and portions of the pegmatite forming a granofels that is characterized by the presence of chlorite,
possibly formed at the expense of biotite and K-feldspar of the pegmatite and the calcic amphiboles of the AMS. Also
present in the granofels, in addition to the unreacted minerals of the pegmatite and AMS, are actinolite, calcite, and
sillimanite. The area affected by this hydrothermal alteration is extensive, ranging well beyond the mine site. However, it
was not all encompassing as large areas of relatively unaltered AMS interfinger with the granofels.
The granofels is in contact with the Crabtree pegmatite along its west side (Figure 3). Both the granofels and the
pegmatite were cut by a fault or shear zone. The evidence for this is indirect and consists of the presence of a
pronounced north-south lineament that traverses the mine site and the presence, on the mine dump, of mylonite-series
rock derived from the pegmatite and granofels. This rock displays brittlely deformed K-feldspar and quartz no
cordierite, no chlorite of type found in granofels, little plagioclase, and contains a foliated matrix of aphanitic sericite,
chlorite, and epidote. The quartz displays a new crystal growth, which ‘heals’ the fracturing suggesting a later stage of
hydothermal alteration that may have also have generated the epidote and converted the cordierite to pinite. The shear
has apparently juxtaposed a portion of the unaltered AMS adjacent to the pegmatite at the mine site giving the
impression that the pegmatite body lies in contact with typical AMS rock, specifically biotite-white mica quartz schist
(Figure 3).
Beryl (Aquamarine, Emerald, Green and Yellow) and
Tourmaline of the Crabtree Pegmatite, Spruce Pine District,
Mitchell County, North Carolina
Christine Tappen1, Michael S. Smith2 and James A. Dockal2
department of Earth and Planetary Sciences, American Museum of Natural History,
Central Park West at 79th St., New York, NY 10024 and department of Earth Sciences,
University of North Carolina - Wilmington, Wilmington, NC, 28403
MINING HISTORY
The Crabtree (or Big Crabtree) mine, one of the three known emerald-bearing, pegmatite-related deposits in North Carolina, was discovered in
1894 by J. L. Rorison and D. A. Bowman who developed the property (Figure 4; Kunz, 1907; Sinkankas, 1989). Later, the American Gem and
Pearl Company (Tiffany Corporation) of New York City sporadically mined by in 1919 and 1935. In 1937, the mineral rights were sold to the
Crabtree Mining Corporation which operated the mine until 1957. In 1957, the Little Switzerland Emerald Mines, Inc. opened the spoil piles to
visitors for collection. Small scale mining occurred throughout the 1960’s to 1980’s. By this time the majority of material from the Crabtree mine
was used in the local tourist oriented ‘gem mining’ business. However, by the late 1980’s, groundwater accumulation, inadequate pumping
facilities, and environmental concerns resulted in suspension of underground mining. An attempt to acquire more bulk emerald-bearing
samples by blasting resulted in the suspension of any further mining ventures in 1993 - 1994 (Figure 5). Currently, 6 acres containing the mine
are held by the Mineral City Mining Co., with one of the owners holding another 160 acres adjoining.
Big Crabtree Emerald Mine
Little Switzerland, NC
p
Pegmatite: K-feldspar. plagioclase.
cordierite. quartz, tourmaline, beryl
and xenotime.
\
' ' - .
/
‘
gf
Granofels: k-feldspar. plagioclase.
cordierite, quartz, actinolite.
chlorite, magnetite.
bg
Mica schist, biotite plagioclase gneiss,
and hornblende plagioclase gneiss of
the Ash Metamorphic Suite; slightly
chloritic. mostly saprolitic on the
surface.
35
Foliation
Mesoscopic Fold Axis
Reported depth of
workings is XII meters.
A
spring
•
О
N
Tape and Compass Map
by Janies A. Dockal
September 1997
Figure 4. Mine workings and
buildings at the Crabtree
emerald mine around 1898.
From Kunz (1907) North
Carolina Geological and
Economic Survey, Bulletin
no. 12, Plate X.
Figure 5. Water filled shaft
and mine spoil debris at the
Crabtree emerald mine in
2005. View is to the south.
View in Figure 4 is
apparently to the north.
Figure 3. Mine map of the Crabtree emerald mine as it
appeared in 1997. Rock outcroppings are colored pink (P)
for the areas of the main pegmatite body, green (gf) for
areas of granofels or hydrothermally altered country rock
and pegmatite, and purple (bg) for areas of undifferentiated
and slightly altered country rock that is probably of the Ashe
Metamorphic Suite. Uncolored areas are soil, tailings, and
other mine related debris. Pegmatite is not observed in
direct contact with the country rock, but mine dump samples
suggest that the pegmatite may be separated from the
country rock on the east side by a brittle shear zone. Drawn
by J. A. Dockal.
Figure 6. Exposure of
Crabtree pegmatite.
Hanging wall (to west)
behind Christine. See
Figure 3 for orientation.
Type 1 Xenoliths
Type 2 Xenoliths
Type 3 Xenoliths
Aplitic Units
Pegmatite Margin
and Biotite Schist
Alteration Zone
Figure 7. Schematic diagram of the Crabtree granitic pegmatite in map view. After mining
excavation, the surface exposure is 5.4 m in length and 3.5 m in width. The pegmatite has a
north-south strike with a 55° SE dip (based on the contact with the granofels, see Figure 3).
The contact of the pegmatite with the granofels at the western boundary is sharp and
irregularly curved (Figure 8). At this contact, there are two magma-induced, metasomatic
alteration assemblages known as exocontacts or selvages. The chlorite granofels alteration
zone grades into a biotite selvage inward toward the pegmatite. In addition, three
mineralogically and texturally distinct biotite-white mica xenoliths are found within the
pegmatite and represent various degrees of progressive assimilation. Modified from Tappan
and Smith (2003).
Figure 8. Closeup of tourmaline-
biotite-emerald exocontacts at margin
of Crabtree pegmatite (see Figure 3
for location).