A spheroidal skarn body at Gib Lake, Pontiac County, Quebec, occurs in phlogopite marble, and shows concentric zones several centimeters thick. The zonal sequence is (a) core of blue scapolite; (b) pargasite–albite intergrowth; (c) pure phlogopite; (d) serpentinized forsterite–dolomite marble. The minerals have been analyzed. A zoned skarn with a similar sequence was found 15 miles south on Calumet Island.Phase assemblages are consistent with upper amphibolite facies metamorphism. The zonal sequence is explained by reaction between a spheroidal quartzo-feldspathic mass and an enveloping dolomitic marble through radial diffusion of Mg (inward) and Si, Al (outward), establishing local equilibrium between adjacent phases. Mg, Al, and Si behaved as phase-determining inert components, Ca was probably in excess, while Na, K, H, Cl, C, F, S were perfectly mobile.