Graywacke was reacted with seawater and with a saturated NaCl brine containing Ca, Mg, and K, using a solid/liquid ratio of 1/10. The degree of reaction was minimal at 200 degrees C but significant at 350 degrees C. In both fluids anorthite, dolomite, and some illite and quartz, amounting to about 25 percent of the graywacke, were converted to smectite-chlorite and albite. Both fluids lost all their Mg and gained K, Ca, and CO 2 . Only the brine gained and maintained significant concentrations of heavy metals, indicating the importance of chloride complexing. Chloride concentration in seawater is apparently insufficient for extensive complexing under the experimental conditions.Chloride complexing, however, serves only to maintain metals in solution and prevents their reprecipitation as alteration products. Alteration involving production and consumption of hydrogen ions is required for the release of metals from the source matrix. In the experiments Mg metasomatism and dedolomitization provided the H (super +) for the process. Metal solubilization by the brine followed the order of abundance in the graywacke: Zn > Ni > Cu > Pb > Sb > Cd, suggesting a common matrix and no selective leaching effect. In other words, as a given increment of rock was altered, all of its component metals were released. Metal concentration in solution therefore was not limited by sulfide solubility and represents minimum values.Changes in the bulk composition of the graywacke were minimal, except for increases in Mg content, and the final rock would probably not be recognized as an altered source rock in the field.The fluids generated during the experiments would likely subject wall rock at the site of ore deposition to hydrolytic alteration, including potassium metasomatism.