The Sullivan mine is the largest of several stratiform lead-zinc-silver deposits located near Kimberley in southeastern British Columbia. The ore is enclosed in beds of the Aldridge Formation (Purcell Supergroup), a sequence of middle Proterozoic metasediments of greenschist metamorphic grade.Recent experimental data on the systems Fe-Zn-S and Fe-As-S should provide a basis for evaluation of the P-T conditions of metamorphism of the ore. The composition of sphalerite was determined in samples representing all major ore-bearing stratigraphic horizons. The mean weight percent FeS of sphalerite in samples containing the assemblage sphalerite-pyrrhotite-pyrite is 13.0 (14.2 mole percent). If it is assumed that the sphalerite equilibrated with hexagonal pyrrhotite and pyrite, and has remained unchanged, this composition suggests a pressure of approximately 5 kilobars (Scott, 1973), well in excess of that which can be accounted for by present estimates of lithostatic load during the East Kootenay Orogeny. Analyses of sphalerite from the sphalerite-pyrrhotite assemblage indicate that sphalerite composition is essentially homogeneous if pyrrhotite is present and does not reflect the presence or absence of pyrite.Studies of the pyrrhotite using a magnetic colloid indicate that monoclinic and hexagonal pyrrhotite coexist and are both volumetrically significant, although a number of factors have influenced the extent of the inversion of hexagonal to monoclinic form. It appears that sphalerite in the Sullivan orebody has re-equilibrated in response, perhaps, to oxidizing conditions and cannot be used as a barometer of peak metamorphic conditions for this orebody.Arsenopyrite in samples of Sullivan ore were also analyzed using the electron microprobe. Arsenopyrite coexisting with pyrrhotite and pyrite in the footwall contained 32.3 atomic percent As and that in the hanging wall contained 33.0 atomic percent; these compositions indicate temperatures in the range of 400 degrees to 491 degrees C. Temperatures indicated by the fractionation of sulfur isotopes between sphalerite and galena in ten samples show large variation but average 300 degrees or 340 degrees C, depending on the experimental curve used in the determination. Oxygen isotope fractionation between quartz and magnetite suggests temperatures of 400 degrees to 560 degrees .