QUANTITATIVE ANALYSIS OF LI, K, RB, AND CS IN RARE-ELEMENT PEGMATITE MUSCOVITE AND THE RAPID DETERMINATION OF PEGMATITE FRACTIONATION BY HANDHELD LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS)

LIBS is a form of optical emission spectroscopy that has the expansive capability to undertake rapid and simultaneous multi-element analysis of geological materials and is particularly suited to the analysis of the light elements (Z<20). Quantitative analysis by LIBS is possible because the intensity of light emission for an element during its transition from an excited state in the LIBS plasma is proportional to its concentration in the material being analyzed. The calibration curve approach directly relates the measured intensity of an elemental peak in the LIBS emission spectrum to standards of known composition for a similar type of matrix measured under the same analytical conditions. Calibration curves have been developed for Li, K, Rb, and Cs in micas analyzed by handheld LIBS for a suite of muscovites and lepidolites of known composition. Analytical variability and different approaches to spectral normalization were assessed during our calibration work These calibrations were applied to LIBS analysis of muscovite separates from pegmatites at a Li prospect in the Carolina Tin-Spodumene Belt (CSTB) in Gaston County, NC where barren pegmatites are characterized by feldspar, mica, garnet, and sometimes tourmaline, whereas fertile Li-enriched pegmatites are dominated by assemblages of spodumene, quartz, and feldspar. Lithium can substitute in the octahedral site of the muscovite structure via couple substitutions involving Si, Al and vacancies. Evolved muscovite compositions in rare-element pegmatites generally have low K/Rb ratios and high Li contents, with pegmatite compositional fractionation trends typically characterized by decreasing K/Rb with increasing Li content. Thus, elevated values of Li in muscovite can be a pathfinder for the presence of Li-bearing assemblages in rare-element pegmatites, with a threshold value of approximately 0.05 wt. % serving as a prospecting guide for spodumene-bearing pegmatites. Fertile and barren pegmatites at the CTSB prospect can be distinguished on the basis of their K/Rb-Li systematics from LIBS analysis of muscovite using our calibration.