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Mineralogy and geochemistry of pegmatites on Mount Begbie, British Columbia Dixon, Andrea


The pegmatite field on Mount Begbie, near Revelstoke, B.C., is a Li-rich pegmatite field in the southern Canadian Cordillera that is relatively well-known among prospectors, but until now, under-studied by academics. Mapping the pegmatite field on the northeastern slope of the mountain revealed a dense population of S-type granitic pegmatites belonging to the LCT family with diverse ranges of mineralogy corresponding to barren, beryl-columbite, beryl-columbite-phosphate, and lepidolite-subtype compositions. Typically, the pegmatites are not strongly metamorphosed and show only rare foliation, meaning they primarily postdate the exhumation of the Thor-Odin Culmination that occurred during the Late Paleocene to Early Eocene. They are likely related to other granitic bodies (i.e., the Ladybird granite suite) resulting from the exhumation event rather than any other known intrusions in the Monashee complex. The dikes in the study area are elongate, with most at least 10 m long and are usually not more than one meter wide; the largest is approximately 520 m long and 10 m wide. Orientations of the pegmatites are dictated in part by conjugate shear planes developed in the host rock prior to the emplacement of the dikes; nearly all strike between 295° and 330° and have a subvertical dip. Fractionation is variable within the most primitive of the pegmatites and it may be controlled by localized fluid accumulation or the composition of the original source rock. Despite variability, fractionation within the pegmatite field increases from the southeast to the northwest, suggesting the source pluton may be located to the southeast of the study area at depth. Detailed examination of the minerals tourmaline, sekaninaite, beryl, rare-element oxides, and zircon as well as the mostly qualitative study of phosphates and other minerals provides insight into the geochemistry and mineralogy of the individual dikes. The results suggest that some of the primitive dikes are more fractionated than they appear and highlight other pegmatites as having unusual compositions for typical beryl-columbite and beryl-columbite-phosphate-subtype pegmatites. Elevated contents of Be and Li in sekaninaite and the presence of qitianlingite in the Mount Begbie pegmatite field are somewhat unique compared to other cordierite and rare-element oxide localities worldwide.

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