An experimental investigation of high-temperature seawater/basalt interactions has been conducted in order to better evaluate the geochemical and economic implications of hydrothermal circulation of seawater in the oceanic crust along active mid-ocean ridges. The results indicate that, as seawater reacts with basalt between 200°C and 500°C at 500-800 bars, the fluid tends to change from an oxygenated, slightly alkaline, Na�, Mg��, SO₄[superscipt =], Cl� solution to a reducing, acidic, Na�, Ca��, Cl� solution with Fe, Mn and Cu concentrations up to 1500, 190 and 0.3 ppm respectively. Silica concentrations in the fluid approach saturation values with respect to quartz at all temperatures; however, Al abundances remain very low (~0.5 ppm). Montmorillonite, nontronite, albite, tremolite-actinolite, chalcopyrite, pyrrhotite and hematite were the dominant alteration products formed. These data imply that large-scale circulation of seawater in the oceanic crust could account for the Al-deficient metalliferous sediments associated with mid-ocean ridges and be important in the genesis of certain Fe-Cu sulfide ore deposits. The process could also affect the geochemical budgets of certain elements and exert substantial control on the steady-state composition of seawater by removing excess Na and Mg and adding Ca, Si and H to the oceans
