Seawater reacted with basaltic glass at 260°C and 500 bars under water-dominated conditions (50 : 1 water/rock ratio) efficiently leached and maintained heavy metals in solution. Cu, Zn, and Ba are transferred in significant proportions to the aqueous phase, while Fe and Mn attain concentrations of 45 and 20 ppm respectively as the basalt is completely made over to magnesian smectite. High metal solubility is a function of acidity maintained by large excess of dissolved Mg and equilibria with the alteration phase. Metal concentrations and relative proportions are consistent within limits required for metal-rich fluid which produced East Pacific Rise metalliferous sediments. Experiments mixing metal-bearing altered seawater and normal seawater were carried out as a qualitative indicator of sea-floor precipitation processes. Bulk composition of the precipitates are strongly influenced by mixing ratio. Precipitates range from silica-magnesium rich under low dilution by seawater to essentially pure ferric hydroxide under conditions of high dilution.