Chalcopyrite disease, consisting of a multitude of submicron- to micron-sized blebs of chalcopyrite concentrated along fractures or rims or in relatively iron-rich bands of sphalerite crystals, is a common ore texture which is critical to the understanding of the paragenesis of many ore deposits. It has now been synthesized in the laboratory by hydrothermal interaction of sphalerite with a hot copper-bearing fluid (Bourcier et al., 1984; Eldridge, 1984). Experimental conditions approximated those of the copper mineralization period of a kuroko orebody as set out by Ohmoto et al. (1983). Massive replacement of sphalerite by chalcocite, resembling supergene zone textures, occurred when starting solutions were highly supersaturated in copper with respect to copper sulfides. Chalcopyrite disease textures were formed only when starting solutions were close to equilibrium with chalcopyrite. The disease appears to have grown by the replacement of the FeS component of sphalerite, as suggested by Barton (1978), and the reaction governing the process is likely to be:Zn (sub 0.80) Fe (sub 0.20) S + 0.20Cu (super +) + 0.20H (super +) = 0.60ZnS + 0.20CuFeS 2 + 0.20Zn (super +2) + 0.10H 2 .This reaction has a small negative volume change, implying that the process is not self-limiting if Zn (super +2) and H 2 are continually removed from the system. The disease may even continue growth with additional iron from solution before attaining a positive delta V. Marcasite, intergrown with the sphalerite used in the experiment, was also partially replaced by chalcopyrite, perhaps in a process linked by recycling of the H 2 produced in the reaction above, according to the reaction:FeS 2 + Cu (super +) + 0.5H 2 = CuFeS 2 + H (super +) .It appears that the chalcopyrite disease represents the earliest stages of replacement of sphalerite by chalcopyrite and that the process may proceed to total replacement. The disease process, together with the chemically sympathetic replacement of iron sulfides by chalcopyrite, may explain how a kuroko-type orebody may have a stratigraphy that is the inverse of its paragenesis and how copper-rich cores of sea-floor, high-temperature, hydrothermal chimneys progress outward through the earlier formed zinc-rich rims. The overall replacement of iron and zinc sulfides by chalcopyrite may be represented by the reaction:FeS 2 + 2ZnS + Fe (super +2) + 2Cu (super +) = 2CuFeS 2 + 2Zn (super +2) .