Abstract: Endoskarns formed where a swarm of diorite dykes have intruded calcite marble at Redcap Creek include an inner melilite-dominated, a wollastonite-dominated, and an outer massive tilleyite zone in contact with marble. The massive tilleyite is unusual in that it is coarse-grained (prisms 2–15 cm in length) and its deep grey colour contrasts with the lighter coloured varieties described elsewhere. A chemical analysis gives a formula close to ideal, with only minor substitution of Al, Ti, and Mg. Refined unit cell parameters are in close agreement with those quoted in the literature. The skarns have clearly formed by transport of Si, Mg, Fe, Al, and Ti from the igneous rocks, and Ca in the reverse direction from the marble. Activity diagrams derived from experimental data are most useful in interpreting the zonal sequence of endoskarns, and preliminary results suggest mass transfer at low Xco2 and temperature of the order of 800°C or higher for the formation of the massive tilleyite.