Abstract: A thermodynamic prediction of the Gibbs free energy of formation (ΔGfo) of nukundamite (empirical composition Cu5.5FeS6.5) was made in order to specify whether the nukundamite + chalcopyrite or the bornite + pyrite assemblage is stable in the Cu-Fe-S system. The results of calculations using previously reported data of ΔGfo values of some Cu-Fe-sulphide minerals in equilibrium with nukundamite indicate that the total free energy of the nukundamite + chalcopyrite assemblage is appreciably higher than that of the bornite + pyrite assemblage in the temperature range 250–400°C. This means that nukundamite + chalcopyrite is a metastable assemblage under common ore-forming conditions.
The occurrence of nukundamite is not uncommon in the Fijian kuroko deposits in contrast to the Japanese kuroko deposits. A thermochemical treatment for this phenomenon leads to the interpretation that the black ore containing nukundamite in the Fijian deposit was formed under relatively highsulphidation and low-pH conditions. This suggestion is in good agreement with the present experimental result that the bornite + pyrite assemblage was produced in the temperature range 350–250°C by using near-neutral hydrothermal solutions.