The Mount Mather Creek sodalite-bearing carbohydrothermal breccia dyke is located ~35 km northwest of Golden, British Columbia, Canada, within the Foreland Belt on the eastern side of the Mississippian-to-Devonian British Columbia Alkaline Province. The dyke occurs in situ on the western side of the Mount Mather Creek gully, cropping out over a distance of ~80 m with a thickness of up to 10 m, intruding a syncline of Middle and Upper Cambrian Chancellor Group carbonate rocks. To date, no parental alkaline complex has been found adjacent or proximal to the dyke. The breccia dyke is flow-banded and matrix-supported and consists of host rock clasts, medium- to coarse-grained, anhedral-to-poikilitic carbonate-rich syenite segregations that occur as veins and pods, and fine-grained banded sodalite plus carbonate fragments in a carbonate-rich matrix. The sodalite-carbonate segregations consist dominantly of sodalite, ferroan dolomite, calcite and microcline, with an extensive suite of trace minerals enriched in rare earth elements (REE), Na, Ba and Sr, including albite, analcime, ancylite-(Ce), chabazite-Na, fluorapatite, baryte, barytocalcite, cancrinite, galena, goethite, gonnardite, harmotome, edingtonite, a potentially new Mg-bearing edingtonite-like mineral, natrolite, nordstrandite, pyrite, quintinite and sphalerite. Alteration of the primary silicate–carbonate assemblage has resulted in a secondary assemblage of hydrothermal or carbohydrothermal REE–Ba–Sr–Na phases including albite, analcime, cancrinite, gonnardite and nordstrandite after sodalite, together with complex intergrowths of ancylite-(Ce), barytocalcite, edingtonite, and a potentially-new Mg-bearing edingtonite-like mineral. Remobilisation of Ba and Sr from barytocalcite resulted in crystallisation of late-stage baryte and Sr-rich calcite. Mount Mather Creek is only the fifth Canadian occurrence of the Ba-zeolite, edingtonite, and the second Canadian occurrence of the rare layered double hydroxide quintinite. The Mount Mather Creek breccia is a carbohydrothermal deposit, the product of a low temperature (<450°C), highly evolved, alkaline, SiO2-undersaturated, Na–Ba–REE–Cl-rich, residual carbonated silicate melt whose parental origins remain unknown.