The Winston Lake mine is located 145 km northeast of Thunder Bay, Ontario, in the Archean Superior Province. Although the orebody is classified as a typical volcanogenic massive sulfide deposit, it differs in the fact that it was intruded by a large gabbro sill prior to regional metamorphism. Presumably, the orebody experienced high T, low P contact metamorphism. Based on the alteration assemblage of anthophyllite-cordierite-sillimanite-staurolite, the regional metamorphism that followed reached the sillimanite zone of the amphibolite facies. The silicate assemblages suggest a temperature range for metamorphism of approximately 610-640°C and pressures of 2.2 to 5 kbars, assuming that P(H2O) = P(Total) and that cordierite contains magnesium.
Sphalerite in 38 samples from seven drill-holes through the Winston Lake orebody and the adjacent Pick Lake zone was analyzed on a scanning electron microscope. The samples are separated into three occurrences: sphalerite-pyrrhotite-pyrite equilibrium assemblages, sphalerite-pyrrhotite contacts, and sphalerite encapsulated in pyrite. The compositions of sphalerite from the triple junctions and sphalerite-pyrrhotite contacts are similar, yielding 11.04 ± 0.67 (P = 8.82) and 11.54 ± 0.87 mole % FeS. Sphalerite enclosed in pyrite fell into three compositional ranges: 5 - 6 mole % FeS indicating sphalerite-pyrite equilibrium, approximately 11 mole % FeS indicating sphalerite-pyrrhotite-pyrite equilibrium, and a single sample yielding 15.2 mole % FeS (P = 4.36 kb).
Sphalerite compositions indicate that any effects of contact metamorphism has been obscured by regional metamorphism. The relative lack of monoclinic pyrrhotite formation indicates that retrograde alteration is not responsible for the high calculated pressures obtained from the majority of the sphalerite compositions. It is suggested that equilibrium of sulfides at peak pressure of metamorphism may have been attained before peak temperatures as represented by the silicate assemblages and thus account for the discrepancy in pressure between the sphalerite geobarometer and the silicates.