Abstract

The Chedeville field of granitic pegmatites extends over ca. 1200 m along a SW-NE trend in the southeastern part of the Saint-Sylvestre granite, northern Massif Central, France. The occurrence of lepidolite as the main lithium mineral, and the geochemical characteristics of the pegmatites, enriched in Li, Rb, Cs, Ta, and Sn, point to an affiliation with the rare-element class, complex type, lepidolite subtype. Vein-like pegmatite bodies have a well-developed internal structure, with a coarse upper zone, a fine-grained banded zone, an aplitic zone, and a lower layered zone with vertical wedge-shaped crystals of feldspar. Flow banding is common, and a repetition of the zonation provides evidence of multiple batches of magma. Metasomatism at the margins between units or of enclaves leads to the development of lepidolite-rich rocks. The geochemical characteristics of associated aplites match those of other occurrences of rare-metal granite at several localities in the northern Massif Central. However, Nb-Ta mineral characteristics are different from those of the granitic rocks. Uranmicrolite is restricted to the purple lepidolite zones, and columbite-tantalite is more Mn-rich than in granites. The Mn/(Fe+Mn) value actually varies over a narrow range (+ or -0.01) within each sample, but the variation between different bodies of pegmatite (from 0.90 to 0.96 and 0.99) again suggests multiple intrusions. Metasomatic units (accounting for less than 5% of the bodies) are characterized by a lower Nb/Ta in the whole rock and a higher Mn/Fe in the columbite-tantalite than in corresponding aplitic parts. Such rocks may be strongly enriched in Zr, Hf, and Th, suggesting a significant mobility of these elements at the metasomatic stage.

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