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A thermodynamic prediction on the stability of the nukundamite + chalcopyrite and bornite + pyrite assemblages

Published online by Cambridge University Press:  05 July 2018

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

nukundamitechalcopyritebornitepyritestabilitykuroko deposits
Type
Mineralogy
Information
Mineralogical Magazine , Volume 58 , Issue 391 , June 1994 , pp. 235 - 245
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1994

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