A Thermodynamic Prediction on the Stability of the Nukundamite + Chalcopyrite and Bornite + Pyrite Assemblages

Shoji Kojima and Teiichi Ueno
Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Aoba 980, Sendai, Japan
Department of Earth Sciences, Fukuoka University of Education, Munakata 811-41, Fukuoka, Japan

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.

Keywords: nukundamite • chalcopyrite • bornite • pyrite • stability • kuroko deposits

Mineralogical Magazine; June 1994 v. 58; no. 391; p. 235-245; DOI: 10.1180/minmag.1994.058.391.06
© 1994, The Mineralogical Society
Mineralogical Society (www.minersoc.org)