Polymetallic sulfide lenses occur in Ordovician-Silurian sequences of clastic and volcanic rocks of the western Cape Breton Highlands, Nova Scotia. The rocks have been affected by thrusting and metamorphism during late Silurian to Devonian Acadian deformation. The sulfides occur as highly sheared and recrystallized lenses concordant to bedding. Mineral compositions of equilibrium sphalerite - arsenopyrite - pyrrhotite - pyrite intergrowths provide a sliding-scale indicator of P-T-f(S 2 ) variations during metamorphism. The arsenopyrite contains between 33 to 29 atom % As, a range reflecting recrystallization within a changing thermal regime, at temperatures between 510 degrees and 300 degrees C. Pyrrhotite with up to 47.3 atom % Fe is recorded; however, most of the pyrrhotite is relatively sulfur-rich, likely owing to low-temperature inversion or partial oxidation. The dominant composition of sphalerite, between 13-14 mol % FeS, indicates intermediate-to high-pressure conditions. Pressure estimates of 5.5 to 6.9 kilobars are consistent with upper-greenschist metamorphic assemblages found in the surrounding rocks. Decreasing fugacity of sulfur accompanied retrogression in the sulfide lenses as a consequence of the buffering capacity of the coexisting pyrite and pyrrhotite.