This paper describes some aspects of equilibria between gypsum-anhydrite, dolomite-calcite, and sphalerite-galena which are of interest in the interpretation of carbonate-hosted lead-zinc deposits. It is noted that the commonly observed replacement of sulfate by carbonate minerals cannot take place unless there is a "sink" for the hydrogen ions generated in this process. This sink is normally sulfate reduction to H 2 S or sulfur but could conceivably be some other reaction.Equilibrium reaction-path calculations show that reaction between organic carbon and calcium sulfate at 100 degrees to 150 degrees C in a saline brine containing Mg (super +2) leads to precipitation of dolomite (analogous to the white sparry dolomite of ore districts) if aqueous metals are not present and to precipitation of sulfides but dissolution of dolomite if metals are present. Reaction between metals and H 2 S in such a brine in the absence of simultaneous sulfate reduction results in much greater amounts of dolomite dissolution (7.5 times more under the model conditions). In the absence of a pH increase caused by some other reaction (such as silicate alteration), carbonates and sulfides cannot precipitate together. A generalized scheme for Mississippi Valley-type deposit models in terms of reservoirs and fluxes is also presented.