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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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where Pb is the total amount of radiogenic lead produced by the decay of 238 U (99.276%
of modern uranium), 235 U (0.7196% of modern uranium) and 232 Th, and λ 1 , λ 2 , and λ 3 are
the decay constants for 238 U (0.000155125 Ma -1 ), 235 U (0.00098485 Ma -1 ), and 232 Th
(0.000049475 Ma -1 ), respectively. Results of such determinations have been shown to
yield valid ages when compared with ages derived from isotopic measurements (Bowles,
1990).
In order to obtain more accurate age estimates, counting times of 90 seconds were used
for Pb, longer than for other elements. Table 4 summarizes the chemical ages determined
from our microprobe analyses. Most Spruce Pine uraninites date approximately between
350 Ma and 400 Ma; the Pink mine yields the oldest (443 Ma) age. These results are in
good agreement with published U-Pb age dates for Spruce Pine district pegmatites. For
example, Aldrich and others (1958) report uraninite ages from the Chestnut Flats mine of
370-420 Ma, as compared to 378-411 Ma in this study. Uraninites from other Spruce
Pine localities have been dated between 340 Ma and 370 Ma (Rodgers, 1952), while
Alter and McColley (1942) reported ages ranging from 358 Ma to 382 Ma from a zoned
single crystal of uraninite from Mitchell County. Clarkeite from the Fanny Gouge mine
yields chemical ages of 360-375 Ma, comparable to those obtained from primary
uraninite and virtually identical to the clarkeite formation ages (360-380 Ma) determined
by Finch and Ewing (1997). These ages are consistent with late-stage pegmatite
crystallization of clarkeite as originally proposed by Ross and others (1931).
Several niobate-tantalate minerals containing significant amounts of uranium (5-20
weight percent UO 2 ) were identified in this study: samarskite, fergusonite, uranoan
microlite, and plumbopyrochlore. Our data (Table 3) indicate that Spruce Pine
samarskites are enriched in Y plus REE, so are classed as samarskite-(Y). We
consistently find very little Ca in samarskite-(Y), and therefore conclude that
calciosamarskite, although it has been reported from Mitchell County (Figure 4), is rare.
Instead, a trend from the Y-rich sample analyzed by Allen (1877) towards, but not quite
reaching, the ishikawaite field is apparent. Our data further suggest greater
compositional variability in pyrochlore-group minerals than heretofore recognized. The
only previously published analysis from the Spruce Pine district that is known to us is of
uranoan pyrochlore (hatchettolite of Allen, 1877; Frondel, 1958). Uranmicrolite has been
reported from the Sullins Wiseman mine (Wilson and McKenzie, 1985), but no published
analysis exists. We confirm the presence of microlite containing substantial uranium
(uranoan microlite), albeit not enough to be called uranmicrolite according to Hogarth
(1977). We have also identified a Pb-rich variety of pyrochlore, plumbopyrochlore
(Table 3; Figure 6), not previously reported from the Spruce Pine pegmatite district.
A similar assemblage of uranium-bearing niobate-tantalate minerals was recently
described from a pegmatite in the Inner Piedmont belt of northwestern South Carolina
(Warner and Fleisher, 2004). At this site (Maw Bridge pegmatite) samarskite-(Y) and
fergusonite occur together with the betafite member of the pyrochlore group. Table 5
compares the compositions of the above minerals and shows that there is a remarkable
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