We present the results of quasi-equilibrium directional crystallization of melt of the composition (mol.%): Fe = 31.9, Ni = 1.7, Cu = 16.0, S = 50.4. The produced ingot consisted of the following zones: monosulfide solid solution mss + isocubanite icb (CuFe2S3) + Cu3Fe4S7 // icb + intermediate solid solution iss (~Cu2Fe3S5) + pentlandite // mixture of unidentified decay products of solid solution iss1 // chalcopyrite cp + putoranite put // cp + talnakhite tal + bornite bn. The results of chemical analysis of the ingot were used to construct the distribution curves of the components in the solid and to calculate the change in the composition of sulfide melt (L) during directional crystallization in four zones of the ingot. Phase reactions involving a melt have been established. It is shown that the Cu–Fe–Ni–S system has a two-phase equilibrium region L + icb*. A similar region was earlier found in the ternary Cu–Fe–S system. A crystallization zone of two intermediate solid solutions iss1 and iss2, which were earlier obtained by the common method of isothermal annealing and quenching, has been revealed. The peritectic character of crystallization of the phases icb*, iss1, and iss2 from the melt is established. The results obtained show the fundamental possibility of the existence of a new type of zoning in the formation of copper-nickel low-sulfur sulfide orebodies.