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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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Granitic and metamorphic rocks of the Spruce Pine area are part of the Spruce Pine thrust
sheet, the structurally highest thrust plate in this part of the Blue Ridge. Ash Formation
schists and gneisses preserve a polyphase metamorphic history (Figure 1) in mineral
assemblages and structures (Butler, 1973, 1991; Abbott and Raymond, 1984; Adams et
al., 1995). Peak metamorphism was assumed to be middle to upper amphibolite facies
(Butler, 1973; Abbott and Raymond, 1984), but detailed mapping in the Spruce Pine
thrust sheet revealed an earlier eclogite facies metamorphic event locally preserved near
the base of the thrust sheet (Adams et al., 1995; Adams and Trupe, 1997). The earliest
age determination on the Spruce Pine pegmatites was done on uraninite and yielded an
age of 440 Ma (Aldrich et al., 1958) (Figure 1). Warner (this volume) shows the
complex mineralogy of the uranium oxide minerals in the Spruce Pine pegmatites and
this complexity makes the early U-Pb dates on uraninite questionable. Whole rock Rb-Sr
ages for two Spruce Pine pegmatites are 404 and 392 Ma (Kish, 1983, 1989) while
Johnson (et al., 2001) report a U-Pb zircon age of 377 Ma for a Spruce Pine pluton.
Spruce Pine magmas were intruded near the end of the peak metamorphism (middle to
upper amphibolite facies, dates on garnet and hornblende at 379-472 Ma, Goldberg and
Dallmeyer,1997) (Figure 1).
Spruce Pine granitic rocks are referred to as white rock or alaskite based on their low
mafic mineral content. Spruce Pine granitic rocks contain more plagioclase than K-
feldspar and lesser amounts of quartz and are best called granodiorite or quartz
monzonite, depending on the classification used. Common accessory minerals include
white mica (muscovite), biotite, and garnet along with minor amounts of epidote, apatite,
zircon, and various U minerals (see Warner this volume). The granitic rocks are
peraluminous, S-type granitic rocks.
Spruce Pine Granitoid Rocks
Granitic rocks of the Spruce Pine plutonic suite are coarse-grained, light-colored white
mica granite and granodiorite that contain more plagioclase than K-feldspar. Common
accessory minerals include garnet and epidote and, less commonly, biotite. Less
common are rare U-oxide minerals (see Warner, this volume), beryl (including the green
variety, emerald) and apatite. Spruce pine granitic rocks are characterized by larger
grains of quartz, feldspar, white mica and garnet surrounded by a thin fine-grained matrix
of these same phases resulting in a mortar structure. The fine-grained matrix is often just
a few grains wide, but is common around many of the larger grains. Some feldspar and
mica grains show evidence of deformation (bent and broken twin and cleavage planes).
Large quartz grains have undulose extinction. Variations in grain size produces a
foliation in some of the granitic rocks that is roughly parallel to the regional foliation in
the country rocks (London, 2008).
Mining of feldspar and mica from Spruce Pine granitic rocks started over 100 years ago
and continues today (Olson, 1944; Brobst, 1962; Lesure, 1968). Native Americans
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