Abstract

Mount Mica is a poorly zoned, sodic LCT-type pegmatite consisting dominantly of quartz, albite, and muscovite in the outer portion of the pegmatite. Micas are the principal K minerals throughout the pegmatite. Potassium feldspar and rare-element minerals such as lepidolite, elbaite, Cs-rich beryl, fluorapatite, and pollucite are restricted to the core zone of the pegmatite. The dearth of rare element (Li, F, Cs, and Rb) substitution in minerals of the outer portion of the pegmatite and the relatively low K/Rb ratio of K-feldspar in the core zone indicate that the overall degree of fractionation of the pegmatite is moderate. Aluminum-micas belong to the muscovite–polylithionite chemical trend, with a compositional gap between muscovite and trilithionite. Lithium is incorporated into the micas mainly via the Li3Al−1−2 substitution mechanism. Lithium-Al micas in the wall zone are chemically homogeneous, but abruptly evolve into higher Cs + Rb bearing Li-rich muscovite and lepidolite in the core zone. An abrupt change in grain size, texture, and chemical composition is evident in the micas from the core zone. Pods of fine- to coarse-grained lepidolite occur sporadically throughout the core zone. Muscovite crystals located adjacent to lepidolite pods or adjacent to or within miarolitic cavities may be overgrown by a coarse-grained mosaic of lepidolite crystals. Some of the overgrown muscovite crystals display interlayering on a microscopic μm scale. The interlayering suggests late-stage rapid crystallization along a diffusion-controlled crystal-liquid interface. The abrupt increase of the Cs, Rb, and F in K-feldspar, muscovite, and lepidolite and the occurrence of such highly evolved species as F-rich lepidolite, pollucite pods, elbaite tourmaline, Cs-rich beryl, and spodumene in miarolitic cavities in the core zone suggest that incompatible elements were retained in the residual fluid until their concentration was high enough to initiate crystallization of incompatible-rich mineral phases. The relatively low abundance of incompatible elements in the hanging-wall zone (HWZ) and footwall zone (FWZ) suggest that the fractionation process was very efficient in sweeping the incompatibles into the core zone of the pegmatite, producing proportionally small volumes inside the pegmatite with very high enrichment in incompatible elements.

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