ANALYSIS OF MUSCOVITE FROM THE SPRUCE PINE PEGMATITE DISTRICT BY LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS)

The Spruce Pine pegmatite district of western North Carolina encompasses one of the largest populations of granitic pegmatites in the southern Appalachian region. Most pegmatites here are mineralogically simple, consisting primarily of quartz, feldspars, and muscovite although rare-element mineralization in the form of beryl, columbite-group minerals, zircon and allanite is present but uncommon throughout much of the district.

LIBS is a form of atomic emission spectroscopy in which pulsed laser light of high energy is tightly focused to ablate a minute amount of a sample and create a plasma in which constituent elements can be detected and identified through spectral analysis of emitted light. LIBS is capable of quantitative analysis via the development of element-specific calibration curves. For this study, calibration curves were developed for Li, K, and Rb in muscovite on a SciAps Z-903 handheld analyzer and 60 individual measurements were averaged to produce a single composite spectrum for each location and then five such spectra acquired at different locations on the sample surface averaged to obtain the Li, K, and Rb abundance estimates from the elemental calibration curves.

The behavior of rare alkali elements in muscovite during the crystallization of pegmatite melts is well documented and in general, increases with increasing degree of fractionation. Muscovite from common pegmatites typically exhibits high K/Rb ratios that range mostly from 650 to 40 and Li-K/Rb systematics can provide a useful guide to the extent of geochemical evolution and degree of fractionation of individual pegmatite bodies and pegmatite groups, which is important information in an exploration or prospect evaluation context. LIBS analysis of Spruce Pine muscovite documents the primitive to moderately fractionated nature of pegmatites across the district.

Pegmatite	# of Analyses	Li (ppm)	K/Rb	Pegmatite	# of Analyses	Li (ppm)	K/Rb
Crabtree	9	140-1000	153-13	McKinney-Bon Ami	12	120-400	54-13
Hoot Owl	11	620-1680	51-15	Sibelco Hawkins	14	410-1530	276-31
Brushy Creek	15	70-460	68-31	8 other locations	1 each	80-670	93-26