Gold solubility in the CaO-SiO2-Al2O3-Fe2O3 slag system was measured at 1723 K under an oxidizing atmosphere. Gold solubility in the present slag system increased with increasing slag basicity, which was quantified by the Vee ratio (= CaO/SiO2), theoretical optical basicity, and activity of CaO. However, the effect of Fe2O3 and Al2O3 on gold solubility was negligible. From the thermodynamic assessment, it was found that gold was stabilized as the AuO− (aurate) complex ion, and thus the dissolution reaction into the slag was proposed. The aurate capacity was originally defined from the dissolution reaction. The iso-Au solubility contours were plotted in the CaO-Fe2O3-(SiO2 + Al2O3) pseudo-ternary diagram, from which the lower content of CaO can be proposed to be useful for higher recovery of gold (i.e., lower solubility) based on the thermodynamic view during pyrometallurgical processing of gold-containing E-waste materials. However, because the viscosity of the slag increases by decreasing the content of CaO, the operating window for best practice should be carefully proposed by considering the physicochemical properties of molten slag.