Concentrations of 0.19 + or - 0.02 and 0.55 + or - 0.08 mole percent FeS have been measured in sphalerite crystallized at 1 kbar in equilibrium with the chalcopyrite-pyrite-bornite sulfur fugacity buffer at temperatures of 271 degrees and 395 degrees C, respectively. These data yield a relationship of log C = --1353/T + 1.77, where C is measured in mole percent FeS, T is in degrees K, and the pressure is 1 kb. The effect of pressure on C is evaluated. Hence-forth, indications of equilibrium or disequilibrium crystallization and estimates of temperature of formation may be made for the assemblage sphalerite-chalcopyrite-pyrite-bornite at temperatures in the range 150 degrees to 450 degrees C.The composition of bornite in equilibrium with chalcopyrite and pyrite is found to be Cu (sub 5.24) Fe (sub 0.86) S (sub 4.00) at 271 degrees C and Cu (sub 5.02) Fe (sub 0.93) S (sub 4.00) at 395 degrees C. At these temperatures chalcopyrite and pyrite in the buffer assemblage do not deviate measurably from stoichiometry. Bornite and chalcopyrite are found to incorporate 0.03 to 0.18 and 0.00 to 0.43 atomic percent Zn in the range 197 degrees to 503 degrees C.

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