Authigenic Tourmaline from the Middle Precambrian Belcher Group, Northwest Territories, Canada

ABSTRACT Authigenic green tourmaline occurs in sandstones of the Fairweather Formation as cavity fillings associated with both primary and secondary porosity, as a replacement of boron-enriched silt clasts, and in laminated mudstones. Boron appears to have been derived from volcanic rocks of the underlying Eskimo Formation, and also as a result of increased salinities in a broad supratidal-flat environment. The enrichment of boron in pore fluids was enhanced by the development of paleosols that are associated with the sandstones and mudstones. The presence of authigenic tourmaline in marine carbonate-sandstone-shale associations may be a useful indicator of evaporitic conditions or supratidal environments.