The Cu-bearing hypogene mineralization at Tsumeb, South West Africa/Namibia, is interpreted to have resulted from the interaction of hot circulating Cu-rich saline solutions and host dolomites. Fluids attending formation of the main sulfide stage were warm (210–280°C) and moderately saline (6–12 wt.% NaCl equivalent). Phase relations within the CuFeAsS system suggest that the observed paragenetic sequence (chalcopyrite → bornite → chalcocite → enargite → tennantite) is consistent with a model calling for successive introduction of a single Cu-rich solution into a host dolomite. The Cu and As content of the solution decreased as it passed through the permeable dolomite precipitating CuAs-sulfides and sulfosalts. A concomitant increase in pH created by the dissolution and buffering effect of dolomite produced sufficient changes in the fluid chemistry to shift the stable sulfide mineral assemblage.