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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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irregular quartz core (Figure 3). Width of the dike was variable, but averaged about 1 m.
K-feldspar from the dike is light pink, plagioclase is white, quartz is dark gray, muscovite
is greenish-gray, garnet is dark red, and epidote is dark green. Individual feldspar
crystals exceed 40 cm in length. Crystals of muscovite and graphic intergrowths of K-
feldspar and quartz were distributed around the quartz core (Figure 3). Graphic textures
were not found in feldspar immediately adjacent to the quartz core (Figure 3). Spacing
between the rods of the quartz in the graphic texture increased toward the quartz core
until the quartz formed individual grains not intergrown with the feldspar to form the
graphic texture.
Modal Analysis
Modal analysis of the pegmatite was done using photographs of the pegmatite and a 5 cm
grid. Mineralogy of each grid point was checked by taking the photographs and grid to
the outcrop. Over 1500 points were analyzed representing an area of almost 4 square
meters. The technique worked well for the coarser grained feldspars, muscovite, and
quartz core. The modal analysis could not distinguish fine-grained minerals (garnet,
quartz in graphic intergrowths) and underestimated these phases in the pegmatite. Modal
composition of the pegmatite was (in volume percent): 57.8% plagioclase, 33.6 % K-
feldspar, 7.5 % quartz, and 1.1 % muscovite. These are reasonable estimates of the
relative proportions of feldspar and muscovite, but underestimate the fine-grained quartz
and garnet. The coarse scale of the modal analysis could easily have underestimated the
fine-grained quartz content by a factor of two or three. The pegmatite is a granite
(verging on granodiorite) based on the proportion of feldspars (Streckeisen, 1976).
Several chemical analyses of Spruce Pine pegmatites are available in the literature
(Olson, 1944; Parker, 1952; Burnham, 1967), all in the granite to granodiorite range.
Modal analysis of individual, coarse-grained feldspar crystals was done on thin sections
from several samples. Exsolved and host feldspars were determined using a point
spacing of 0.3 mm. Modal proportions of feldspars, combined with the composition of
the feldspar phases were combined to estimate the pre-exsolution composition of the
feldspar (Table 1).
Samples
Large hand samples were collected from various positions within the pegmatite and from
the immediately adjacent granitic rocks during 1978. Mining activity within the quarry
subsequently removed the pegmatite and host granitic rock. Many of the hand samples
contained portions of very coarse-grained feldspar crystals, but an effort was made to
include other mineral phases in each sample. Polished thin sections were prepared when
the grain size was small enough to provide a representative sample on a thin section
scale. Very coarse-grained samples were sub-sampled by hand picking and microprobe
analyses were done on grain mounts.
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