The anhydrite veins formed in hydrothermally altered basalts. The S isotope data indicate that the anhydrite formed from fluids similar to sea-water sulphate.-K.A.R.

Anhydrite occurs in veins in hydrothermally altered basalts recovered from Hole 504B during Leg 83 of the Deep Sea Drilling Project. Sulfur isotopic data indicate that the anhydrites formed from fluids with sulfur isotopic compositions similar to seawater sulfate. Anhydrite probably formed as a pulse of relatively unreacted seawater was heated when it entered a relatively hot hydrothermal system containing evolved fluids. Reheating and continued evolution of the system followed anhydrite deposition. Preservation of anhydrite in Hole 504B was probably favored by the high temperatures and by the low permeability that resulted from the sealing of cracks with secondary minerals. Evidence also indicates that anhydrite was partly replaced by laumontite and prehnite at relatively high temperatures, and possibly by calcite at lower temperatures.

The sulphides occur in a pillow-dyke transition zone. Transmitted and reflected light microscopy, XRD and probe analyses were used to decipher the complex paragenesis; minerals present include quartz, albite, sphene, chlorite, clays, actinolite, pyrite, epidote, sphalerite, chalcopyrite, galena, calcite and zeolites.-K.A.R.