Interstitial water samples were obtained from two sites in the Escanaba Trough, off the coast of Northern California. Chloride concentrations in the drill site most affected by the presence of hydrothermal fluids, show values above and below average sea water. These profiles can be understood best in terms of supercritical phase separation processes occurring at depth in the hydrothermal system at the base of the sediment column. Phase separation is associated with phase segregation in which low chloride fluids exit the hydrothermal zone through sandy horizons, and the fluids with elevated chloride emanate into the bottom waters through cracks and fissures. Although values of delta3He indicate the importance of interactions with basalts, the increased values of the Br/Cl ratio, as well as the presence of iodide, indicate that pore fluid-sediment interactions also must have played an important role. In this paper, we investigate the halide systematics of the pore fluids of both a low heat-flow reference site (Site 1037) and of a hydrothermal site (Site 1038).
Copyright: Water-Rock Interaction, Arehart & Hulston (eds)© 1998
Balkema, Rotterdam, ISBN 90 5410 942. 4
Reprinted with permission from A.A. Balkema Publishers (A.A.Balkema, P.O.Box 1675, Rotterdam, Netherlands)
James, R.H., Duckworth, R.C., Palmer, M.R., and the ODP Leg 169 Shipboard Scient ific Party, 1998. Drilling of sediment-hosted massive sulphide deposits at the Middle Valley and Escanaba Trough spreading centres: ODP Leg 169. In Mills, R.A., and Harrison, K. (Eds.), Modern Ocean Floor Processes and the Geological Record, Spec. Publ. Geol. Soc. London, 148:177-199.
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 crosscutting 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.
Reprinted with permission from the Geological Society of London.