Abstract
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)
Abstract
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.