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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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occupies the A site. The specific name given to samarskite minerals is based on the A-
site occupancy. Hanson and others (1999) proposed a three-fold subdivision: samarskite-
(Y), wherein the A site is dominated by Y+REE; ishikawaite, in which the A site is
dominantly U+Th: and calciosamarskite, which contains predominantly Ca in the A site.
A ternary diagram (Figure 4) can be employed to distinguish among these three members.
The samarskites from Spruce Pine analyzed in this study all plot in the samarskite-(Y)
field. The sample from the Polly Randolph mine (last analysis in Table 3) contains the
highest proportion of U+Th and plots very close to the ishikawaite field. The filled star
corresponds to the samarskite from Mitchell County analyzed by Allen (1877); it plots at
the yttrium-rich end of the samarskite-(Y) compositions found in this study. Also shown
in Figure 4 is the composition of calciosamarskite from Mitchell County (star) reported in
Hanson and others (1999). All of the samarskites we have analyzed contain very little Ca
and, consequently, are clearly distinct from the Ca-rich sample studied by Hanson and
others (1999).
Fergusonite
Fergusonite was identified in samples from the McKinney mine, where it occurs in
association with samarskite and a Ta-rich member of the pyrochlore group (Figure 5A),
and the W. W. Wiseman mine, where it is intergrown with a Pb-rich pyrochlore and
uraninite (Figure 5B). Fergusonite is Y- and REE-rich (see analyses #5,8,9 in Table 3).
Heavy REE such as Gd, Dy, Ho, and Er were found to have the highest concentrations.
Fergusonite is further characterized by high Nb 2 O 5 relative to Ta 2 O 5 , very low TiO 2
concentrations, and FeO below the limit of detectability. Uranium concentrations
(roughly 5 weight percent UO 2 ) are lower than those found in samarskite. The structural
formula for fergusonite is ABO 4 (Finch and Murakami, 1999), identical to that of
samarskite. Like samarskite, the B site is occupied by Nb, Ta and Ti. However, the A
site in fergusonite is larger, excluding smaller cations such as Fe and Mn that occur in
samarskite.
Pyrochlore group
Samples from the McKinney and W. W. Wiseman mines also contain niobate-tantalate
minerals belonging to the pyrochlore group. The latter are intergrown with fergusonite
and, in the case of the McKinney mine, samarskite (Figure 5). Three analyses are given
in Table 3 - analyses #3 and #4, both from the McKinney mine, are Ta-rich, while
analysis #10 (W. W. Wiseman mine) is Nb- and Pb-rich. In addition to their Ta-rich
nature, the McKinney mine pyrochlores contain appreciable CaO and TiO 2 and modest
amounts of Na 2 O, but little or no Y or REE. Most relevant to this study, UO 2
concentrations (approximately 10-15 weight percent) are comparable to samarskite. The
Pb-rich material from the W. W. Wiseman mine has lower UO 2 , but higher ThO 2 and
FeO; it also contains appreciable SiO 2 (> 3 weight percent).
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