Equilibration between sulfide liquid and olivine is expressed in terms of the exchange coefficient for Fe and Ni, KD = (XNiS/XFeS)sulfide_liquid/(XNiO/XFeO)olivine. The positive dependence of KD on Ni + Cu content of sulfide liquid, as well as on fO2, has been demonstrated experimentally and gives rise to a critical nonlinearity in the relationship between sulfide and olivine-saturated silicate liquid compositions. Measured KD values for olivine-sulfide pairs from disseminated magmatic sulfide ores at Betheno (Western Australia) and Mirabela (Brazil) are consistent with independent estimates of fO2, and fall within the range where the composition dependence of KD is strong. This effect has been modeled quantitatively, using an empirical best fit to available experimental data as a parameterization of the KD variability, and calculating the equilibrium distribution of Fe, Ni, and Mg between coexisting olivine, silicate melt, and sulfide liquid as a function of the silicate/sulfide mass ratio R. It is shown that the composition dependence of KD is a key factor in giving rise to extremely Ni rich sulfides as exemplified by Betheno and Mirabela, where coexisting olivine is also Ni rich. Highly Ni enriched sulfides with anomalously high Ni/Cu ratios may be more common in nature than is commonly recognized and do not need to be explained by hydrothermal alteration processes.