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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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these same minerals with variable (mm’s to m’s) grain size, and a massive quartz core.
When present, zoning is equally well displayed in the large and small pegmatites at
Spruce Pine. Fine-grained (aplitic) equigranular granitic rock is a rare part of the Spruce
Pine plutonic suite. Dikes of aplitic granite cross cut pegmatitic rocks and form irregular
elongate masses that parallel some of the pegmatite dikes. Mineral layering, so called
line rock occurs in some of the aplitic granite. Dick Jahns suggested, based on his years
of mapping in the Spruce Pine District, that small Spruce Pine pegmatites should be a
good model for crystallization of the Spruce Pine magmas (Jahns, personal
communication, 1978).
Textures in Spruce Pine pegmatites are dominated by magmatic features. The large grain
size and skeletal and dendritic crystal forms pictured in Maurice (1940) and Raymond
(1995, pp. 196, 225, 226, 232) and discussed by Swanson (1978a, 1978b, and Fenn,
1986) are all features of magmatic crystallization (London, 2008). Pegmatites also
contain, post-magmatic features, such as the exsolution and deformation in the feldspar,
related to post-magmatic recrystallization.
The purpose of this study is to determine the crystallization history of a Spruce Pine
pegmatite and to use this history to model crystallization of Spruce Pine magmas.
Despite a long mining history and a rich legacy of geologic information, relatively little is
known about the Spruce Pine pegmatites (London, 2008). Spruce Pine pegmatites are
mica pegmatites (Černý, 1991) and relatively little is know about this type of pegmatite.
This study adds to the general understanding of crystallization processes in mica
pegmatites, and specifically to crystallization of Spruce Pine magmas.
METHODS
A small pegmatite (LUF1) in the old Lawson United Feldspar quarry was selected for
study in 1978. The quarry is still active today (KT Feldspar) and the LUF1 pegmatite
and enclosing granite is long gone. The quarry is about 1.5 km north of the community
of Minpro in the Spruce Pine District (Figure 2). Feldspar is produced from a very coarse
grained garnet muscovite granitoid pluton that is typical of the Spruce Pine intrusions.
The quarry is located near the margin of the pluton and the granitoid rocks are foliated
parallel to the contacts with the country rock. A number of pegmatitic and aplitic dikes
and elongate pods oriented subparallel to the foliation of the host granitoid rock occur in
the quarry. Contacts between the pegmatitic and aplitic rocks and the host granitoid are
typically gradational, suggesting the granite was still partially molten when pegmatitic
and aplitic rocks crystallized. Some of the pegmatites show crude zoning with a core of
quartz (± K-feldspar).
The LUF1 dike was exposed on a steep working face of the open pit quarry, about 5
meters above the quarry floor. Access to the pegmatite was provided by debris at the
base of the quarry wall. The pegmatite had a sharp contact with the host granite and an
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