Abstract
The Bandana dolomite marble is coarse-grained, apatite-clinochlore-dolomite
marble with non-mimectic, granoblastic texture. Chemical analysis indicates almost
pure dolomite (99.8 %), and exceedingly low silica, alumina and iron (<0.25 wt. %
total). Although extent (0.5 M tons estimated), and location of the deposit limits its
uses for dimension stone, the marble has been mined for monumental stone (post
1990). Enclosed within amphibolite facies (locally kyanite grade) metasedimentary
and metavolcanic rocks of the Ashe Metamorphic Suite, it is intruded along its
length by muscovite-class pegmatite possibly related to the Spruce Pine plutonic
suite. The marble-pegmatite contact is sharp, suggesting intrusion at depth.
Prograde decarbonation of the marble forms tremolite and diopside, talc is
absent or consumed pre-peak metamorphism. Pegmatite intrusion into Si-poor
dolomite marble results in bimetasomatic formation of a magnesian exoskarn. The
exoskarn is heterogeneous in composition, texture and spatial relationship with
respect to sample areas. Exoskarn envelope width ranges from 1 .0 cm to 0.5 m,
except for a large 3 m x 5 m outcrop of massive calcite-quartz-tremolite-diopside
skarn. Size is dependent on fracture porosity, silicate abundance in marble, and
pegmatite conditions. Compositional variance includes zonation with mono-
mineralic actinolite formation at the marble-pegmatite contact due to mobilization of
components, and local chemical equilibrium. Exoskarn phases include tremolite-
actinolite ± calcite ± quartz ± diopside ± apatite, and indicate contribution of Si,
Al, and volatiles from the pegmatite. Localized intercalation of quartz-mica schist
provides an Fe component as well as being a conduit for fluid infiltration during
retrograde conditions. Endoskarn formation is less obvious in the pegmatite,
indicated by desilicification in the pegmatite and diffusion of Ca from the marble.
Occurrence of scapolite and vesuvianite in one area indicates localized fluid flow
and interaction of quartz and albite-anorthite with halogens. A combination of
infiltration and diffusion processes helps explain the variance observed in the
skarn-types.
The marble has LREE > HREE, a moderately negative Eu anomaly (Eu/Eu*
avg = 0.499504), and Ba (<20ppm) and Sr (44-95ppm) are low. Stable isotope
results 5180 (1 8.57 - 23.77%o V-SMOW) and 513C (0.64 - 3.47%o V-PDB) are
consistent with a marine carbonate protolith.
Bandana Dolomite Marble, Mitchell County,
Spruce Pine District, North Carolina
Jason J. Millington, James A. Dockal , Michael S. Smith, and Paul A. Thayer
Department of Earth Sciences, University of North Carolina - Wilmington, Wilmington, NC, 28403
Introduction
The Bandana dolomite marble (BDM) crops out in rail and stream cuts on the
east side of the North Toe River along Lower Sink Hole Creek, Mitchell
County. The BDM is exposed in a rail cut 1 580 m northwest of the town of
Bandana in Mitchell County next to the North Toe River. The site is located on
property currently owned by Bud Phillips, and can be reached by traveling north
on N.C. 80 from Bandana for 640 m, turning left on Old Marble Mine Road and
following it for 620 m, then turning to the right and following a gated, unpaved
mining road for 690 m to the main marble outcrop. The exact location of the
marble is: latitude 35.981 3347, longitude 82.1 901 93. A muscovite-class
pegmatite dike, of leucogranodioritic composition crosscuts the marble. The
BDM is very limited in its extent; thickness for the two layers combined is ~30m
and length of strike ~500m, with discontinuous exposure. The intrusion also is
limited in size, but has complex geometry being both concordant and
discordant, in that it cross-cuts the marble and regional foliation while also
following the strike of the marble. Pegmatites within the Burnsville fault zone
have been dated at 377.3 ± 2.6 Ma (Trupe et al.2003) and the Chalk Mountain
pluton has been dated at 377.7 ± 2.5 Ma using ID-TIMS and U-Pb techniques
(Miller et al. 2006). This gives a minimum age for the marble. The basaltic
protolith for the AMS has a maximum age of -800 Ma (Abbott et al. 1 984). The
timing and conditions of the Spruce Pine plutonic suite (SPPS) suggest that the
thermal gradient between the BDM and the intruding pegmatite was low, shown
in the sharp, non-gradational contacts (figure 4) and small contact aureole in
the marble and the country rock. While the thermal gradient was low, the
chemical gradient was high due to the positioning of two distinctively different
lithologic types. Prograde and retrograde conditions favored formation of
endoskarn and magnesian exoskarn. Skarn terminolgy is according to Einaudi
1982, mineral symbols after Kretz 1983.
The BDM has been divided it into four different areas (SA1 , SA2, SA3, SA4)
based upon changes in mineralogy, texture, and spatial position. Sample
locations are shown on Figure 2, except for the natural exposure SA3 located
-300 m NE along strike of SA4. Mineralogical and textural differences can be
attributed to intrusion proximity, presence of intervening country rock,
secondary mineral formation, and heterogeneous nature of the pegmatite.
Bandana Dolomite Marble Site Map with Cross-Section
Figure 1 . Location and Thrust Sheet Map for Study Area
Map from Stewart et al. 1 997.
Geologic Setting
The study area is located in the southern portion of the Appalachian
orogenic belt, within the Blue Ridge physiographic province of western North
Carolina, near its border with Tennessee (figurel ). The Blue Ridge province
is bound on the east by the Brevard Fault zone and on the west by the Blue
Ridge fault systems (Butler 1 973). The Blue Ridge is a complex of four
crystalline thrust sheets, which are characterized by cataclastic fault or
mylonite zones at their borders (Adams et al.1 997). The Blue Ridge is a
geologically complex, polymetamorphic terrane. The sequence of
metamorphic grades is inverted, with the highest metamorphic grades found
in the structurally highest thrust sheet. The structurally highest of the four
thrust sheets is the Spruce Pine thrust sheet of Goldberg et al. (1 997), which
the Bandana dolomite marble is located within. Metavolcanic and
metasedimentary rocks of amphibolite facies (kyanite grade) dominate the
lithology of the Spruce Pine thrust sheet (Adams et al.1 997). This lithologic
package is called the Ashe Metamorphic Suite (AMS), which is composed of
the Ashe and Alligator Back formations (Adams et al. 1 997). The Ashe
Formation in the Spruce Pine area can best be described as interlayered
muscovite-biotite schist and gneiss, along with amphibole schist and gneiss,
quartz-feldspar gneiss, and minor dunite (Abbott et al. 1984) occurring as
small bodies and pods.
Figure 3. Sample area 1 facing SE
Sample area 4 is located to the east 45 m
along strike.
Figure 4. Sample area 2 showing
sharp, non-gradational contact
between marble and pegmatite.
Dolomite Marble Petrology
Marble assemblage is consistent between sample
areas and is composed of apatite + clinochlore +
calcite + dolomite + graphite. Calculated dolomite
content for the three whole-rock bulk chemistry
samples is 99.8%. Changes in the modal content
are observed at and confined to areas near
contacts and fractures where tremolite, diopside
and retrograde talc are formed. Silicate mineral
abundance usually increases rapidly from <1% to
100% (indicating skarn or pegmatite) over a short
distance (1 to 5 cm). Dolomite crystal size ranges
from 0.1 mm to 7 mm, and is dominantly polygonal
granoblastic (figure 5c) with no apparent preferred
orientation. Twinning (type III and IV) is common,
often in conjugate sets of 2 to 3 orientations,
sometimes occupying a majority of the crystal.
Mortar-like texture (figure 5a) is developed in
dolomite adjacent to retrograde actinolite exoskarn.
Textural changes are associated with second phase
partical content (silicates) and skarn formation.
Apatite occurs as 0.1 to 0.25 mm anhedral to
subhedral grains, equally distributed between
dolomite crystal boundaries, and within dolomite
crystals. SEM analysis of fluroapatite with back
scatter imaging showed no zonation. EDX analysis
revealed no substitution by cations other then Ca,
and a slight increase in F content from rim to core.
Calcite occurs near fractures and in association
with clinochlore (figure 5d), both have <1% modal
occurrence. Clinochlore occurs as 0.3 to 0.5 mm
laths, sometime as six-sided pseudo-hexagonal
platelets. Formulas were calculated, assuming 12
oxygens per formula, from SEM EDX analysis and
gave an average composition for clinochlore of:
(Mg 4.77 Ca
о
.04 Fe 0 ig) Al (Si 2.89 Al
^ц)
O10 (OH)8.
Graphite is rare (<1%) with a size range of 30-100
microns, which precludes it from modal counts.
Figure 5. Photomicrograph of Bandana dolomite
marble sample areas. Top (a): SA1 marble with mortar
texture. Middle Top (b): SA2 marble with polygonal
granoblastic Texture. Middle Bottom (c): SA3 with well-
developed polygonal granoblastic texture. Bottom (d):
Back-scatter electron image of calcite enclosing
clinochlore in dolomite. All photomicrographs (a, b, c)
taken under crossed polars.
Figure 6. Bandana area map
showing location of marble
(blue), pegmatites (red), and a
heavily altered ultramafic body
(green). The community of
Bandana is located in the
southeast corner of the map.
Symbols define strike and dip of
regional foliation
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