The Sherridon Group, a stratigraphic unit of the Kisseynew gneiss belt, near Sherridon, Manitoba is typically dominated by quartz-rich gneisses. Discontinuous lenses of cordierite-anthophyllite rocks, the focus of the present study, occur within these quartz-biotite gneiss units. These cordierite-anthophyllite gneisses are extremely coarse-grained in places, and are spatially associated with barren (Star Lake) and sulfide-mineralized zones (Sherridon; Elken Lake). The present chemical composition of the Star Lake-Elken Lake gneisses indicates hydrothermal alteration of a basaltic progenitor producing chlorite-rich alteration pipes. This alteration involves a depletion in alkali and alkaline earth elements with an apparent enrichment in Al and Mg as a function of constant summation. These chemical modifications are the result of reactions between heated seawater and basaltic rocks of the crust. A modern analogue of the Sherridon Group environment might be the shelf and slope deposits of modern island arcs.