Experiments were conducted at 375–400 °C, 500 bars to assess the role of redox and pH on Fe, Cu and H 2S concentrations at temperatures, pressures and total dissolved chloride concentrations applicable to the Trans-Atlantic Geo-Traverse (TAG) hydrothermal system at 26° N, Mid-Atlantic Ridge. The magnetite–bornite–anhydrite mineral assemblage imposed redox constraints, while pH was buffered with a 0.65 molal Na–K–Cl-bearing aqueous fluid coexisting with K-feldspar, muscovite and quartz. Results indicate a strong inverse correlation between dissolved Cu and H 2S (aq), whereas dissolved Fe is more sensitive to pH. Magnetite–bornite–anhydrite-fluid equilibria is characterized by unusually low mFe / mCu (molar) ratio and low H 2(aq)–H 2S (aq) activities. To form this mineral assemblage during subseafloor hydrothermal alteration, however, requires elevated fluid / rock mass ratios, such that the ferrous iron and sulfur in the rock become sufficiently depleted to render pyrite and chalcopyrite unstable. The relatively high Cu concentrations, the low H 2S (aq) and mFe / mCu ratios observed at TAG vent fluids are consistent with constraints imposed by magnetite–bornite–anhydrite-fluid equilibria at pH of approximately 4.9. Available chemical data confirm that the TAG hydrothermal system has experienced elevated fluid / rock mass ratios in keeping with the extent of mass transfer needed to generate oxidizing, low H 2S (aq) conditions.