Reconnaissance investigation of the Cu-Fe-Zn-S system has been conducted into the phase equilibria of the geologically significant portion containing pyrrhotite, pyrite, bornite, chalcopyrite, cubanite, and sphalerite. Chalcopyrite-sphalerite relationships were emphasized because of their common association as intimately intergrown phases (interpreted as exsolution or replacement) in major Cu-Zn ore bodies. The investigation employed silica tube-type experiments in the temperature range 800°C to 500°C. Data indicate that solid solubility of ZnS in intermediate solid solution (a high temperature chalcopyrite-like phase) and of .CuS and FeS in sphalerite depend on temperature, sulfur content, and metal:metal ratios, Illustrations of temperature and sulfur controls are listed below:

CuS in sphalerite - temperature - 800°C, 11.74 mole percent 500°C, 3.36 mole percent sulfur content - excess - 800°C, decreases from 11.74 to 6.03 deficiency - 800°C, decreases from 11.74 to 5.98 ZnS in iss - temperature - 800°C, 22.73 mole percent 500°C, 4.67 mole percent sulfur content - excess - 800°C, decreases from 22.73 to 7.1 (with Fe)

Variation of Cu/Fe ratios in iss from 23.93/53.88 to 40.51/50.08 produces a change of ZnS solubility in iss from 22.73 mole. percent to 9.4 mole percent. It is concluded that intergrowths found in ores may result from reduction in temperature, change in sulfur activity or ordering in a crystal structure. The low levels of impurity found in natural sphalerites and chalcopyrites indicate their virtual total expulsion on cooling.