A thermodynamic and kinetic model is proposed to explain the formation of bornite, chalcopyrite, and magnetite during partial roasting of enargite-containing copper concentrates. The proposed mechanism involves solid-solid-gas and solid-gas reactions between chalcocite, pyrrhotite, gaseous sulfur, and oxygen. Bornite formation could be explained through a mechanism of simultaneous formation of bornite and chalcopyrite from Cu2S + FeS + gaseous sulfur. After the initial formation of a bornite/chalcopyrite layer, solid-state diffusion appears to control these reactions, with faster diffusion kinetics favoring the formation of chalcopyrite over that of bornite. The newly formed chalcopyrite reacts continuously with additional Cu2S to form bornite up to the complete disappearance of either the available Cu2S or FeS. Although thermodynamically the direct formation of bornite is more favorable than that of chalcopyrite, the lower diffusion kinetics of bornite formation could explain the preferential formation of chalcopyrite prior to the formation of bornite. Without enough chalcocite to form bornite by reaction with chalcopyrite, no bornite can be formed, leaving chalcopyrite as the only reaction product. In the presence of oxygen, chalcocite and pyrrhotite form magnetite, chalcopyrite, and SO2. The proposed model is thermodynamically consistent with the experimental results obtained in the laboratory.