Phase Relations in the Cu-Fe-S, Cu-Ni-S, and Fe-Ni-S Systems

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Abstract
The phase relations in the Cu-Fe-S, Fe-Ni-S, and Cu-Ni-S systems were investigated by silica-tube quenching, differential thermal analysis, and high-temperature X-ray powder diffraction experiments. In addition, portions of the Cu-Fe-S and Fe-Ni-S systems were studied by gold-tube quenching and differential thermal analysis experiments under high confining pressures. At elevated temperatures extensive liquid immiscibility fields span the sulfur-rich region of each of the three systems, whereas homogeneous liquid fields dominate the phase relations in their central portions. The average composition of the Sudbury Cu-Fe-Ni sulfide ore, when projected onto the Cu-Fe-S plane, is accounted for above 860° C by a mixture of copper containing hexagonal pyrrhotite and copper-rich sulfide liquid. Thus at high temperatures a mechanism exists that may be responsible for certain copper-rich segregations observed in this type of ore. The minerals of the Cu-Ni-S system, with the rare exception of millerite, do not occur in Sudbury-type ores. Knowledge of the phase relations in this system is prerequisite, however, for systematic investigations of the complex Cu-Fe-Ni-S system. Applications of the phase relations in the Cu-Fe-S and Fe-Ni-S systems to typical ore assemblages show that extensive reequilibration took place among the sulfides after their initial deposition. The sulfides in Sudbury-type ores commonly have compositions and crystal structures that can be produced in the laboratory only at low temperatures.

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