James Cook University, Australia

Massive sulphide deposits actively forming from hydrothermal systems within sedimented environments have been drilled during Ocean Drilling Program Leg 169 at two locations along the Juan de Fuca/Gorda spreading centres. The Bent Hill Massive Sulphide and Ore Drilling Program deposits. Middle Valley, include iron- and zinc-rich massive and semi-massive sulphides underlain by a well-developed feeder zone characterized by sulphide impregnations and cross-cutting copper-rich veins. Ridge-parallel normal faulting is probably involved in providing high-permeability pathways for focused discharge at the seafloor, and this is a key element in creating these large ore deposits. In strong contrast, massive sulphide recovered from the Central Hill hydrothermal site, Escanaba Trough, suggests mineralization forms only a thin (5-15 m) veneer over the sediment sequence. Interstitial waters recovered from this area have chlorinities both significantly higher and lower than seawater. The only way to explain this variation is that the fluids contain a hydrothermal component which has undergone supercritical phase separation at depth. Diffuse discharge of hydrothermal fluids through the sediments evidently precludes the formation of a large ore deposit in this area.