113-1 Analysis of Muscovite From North Carolina Pegmatite Districts by Handheld Laser-Induced Breakdown Spectroscopy (LIBS)

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones (Posters)

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Abstract:

The compositional evolution of pegmatite magmas during fractional crystallization and concomitant compositional differentiation is well established. Muscovite carries geochemical fingerprint of pegmatite evolution, with alkali element (e.g., Li, Rb, Cs) abundances generally increasing and K/Rb ratios decreasing with increasing degree of melt fractionation. K/Rb-alkali elements systematics can provide a useful guide to judging the geochemical evolution and degree of fractionation of individual pegmatite bodies and pegmatite groups, which is important information in an exploration or prospect evaluation context. Granitic pegmatite districts in North Carolina include the Blue Ridge region Spruce Pine, Franklin-Sylva, and Cashiers Districts; and in the Piedmont region, the Shelby District and Carolina Tin-Spodumene Belt (CTSB). Most are mineralogically simple quartz-feldspars-muscovite pegmatites, with beryl, spodumene, and columbite-group minerals occurring in some evolved pegmatites. Pegmatite character was assessed by LIBS analysis of muscovite via element-specific calibration curves for Li, K, Zn, Ga, Rb, Sn and Cs. Blue Ridge region muscovite has Li contents from 0.0003 to 0.351 wt. % and K/Rb ratios between 397 to 28 compared to 0.0012 to 0.471 and 206 to 9 for the Piedmont region. In the Blue Ridge region, 107 samples from the Franklin-Sylva and Cashiers Districts are Li-poor muscovites, whereas 81 primitive to moderately fractionated pegmatites from the Spruce Pine District contain Li-poor to Li-rich muscovites. In the Piedmont region, 14 muscovite samples from primitive pegmatites of the Shelby District are Li-poor while 145 samples from spodumene-bearing and barren quartz-feldspar CTSB pegmatites are largely Li-rich muscovites. Li contents of muscovite across the CTSB vary from 0.018 to 0.471 wt. %, with spodumene-bearing pegmatites clearly distinguished by lower K/Rb ratios (x̄_SP = 17 ± 5 2σ vs. x̄_B = 38  ± 18 2σ). The highest Cs, Sn and Zn contents (0.519 wt.%, 0.362 wt.% and 0.356 wt.%, respectively) occur in muscovite from the spodumene-bearing CTSB pegmatites of the Piedmont region whereas Ga contents of muscovite (0.035 to 0.37 wt. %) are similar in the Blue Ridge and Piedmont regions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6029

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