Fractionation and Enrichment Patterns in White Mica from Li Pegmatites of the Wekusko Lake Pegmatite Field, Manitoba, Canada

Abstract

The Wekusko Lake pegmatite field in central Manitoba, Canada, is known for its multiple pegmatite dike occurrences, most remarkably its Li-rich pegmatites of economic importance. The Li-rich pegmatites from Wekusko Lake are the focus of this study and belong to the Green Bay group of the Wekusko Lake pegmatite field. These dikes were dated at ca. 1.78 Ga and were emplaced early during the D4 brittle–ductile deformational event. The results presented here describe in detail the pegmatite mineralogy, textures, and zonation of the Li-rich dikes of the Green Bay group, with emphasis on white mica chemistry. The aim of this study is to establish the nature and evolution of white micas from the Li-rich pegmatites of the Wekusko Lake pegmatite field in Manitoba. We aim to understand the differentiation mechanisms that allowed the high level of trace element enrichment observed in the white micas of the studied Li-rich pegmatites. Major and trace elements in white micas from a representative and well studied pegmatite dike were analyzed by electron microprobe and LA-ICP-MS. White mica compositions and textural evidence were used to define two different populations that seem to have been affected by magmatic processes (fractional crystallization) and a secondary episode of metasomatism. Fractional crystallization modeling of a granitic melt can explain some of the trace element enrichment, but extreme Cs enrichment cannot be explained via this mechanism. We interpret that many metasomatic white micas crystallized in boundary layers. Their compositions are controlled by the local melt composition, but aqueous fluids likely contributed to the development of the white mica textures. The substitution mechanisms at play depend on the type of trend and on the stage of evolution. Li enrichment without M2+ enrichment in metasomatic white mica grains is observed, and it is postulated that Fe3+ in white mica explains this behavior. The K/Rb ratio decreases in white micas with fractionation, whereas the concentrations of incompatible elements, such as Cs, Rb, Tl, Ta, and Li, increase. At Wekusko Lake, the Nb and Ta contents seem to be controlled in part by the presence of columbite group minerals and in part by crystallization in boundary layers.