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ABSTRACT: Southern Juan De Fuca Ridge and Escanaba Trough, Gorda Ridge: Comparison of Hydrothermal Deposits in Sediment-Free and Sediment-Covered Ridge Settings

Laura M. Benninger, Randolph A. Koski, Robert A. Zierenberg

Southern Juan De Fuca Ridge (SJDF) is a low-relief, sediment-starved ridge axis that has a total opening rate of 6 cm/year and is characterized by lobate and brecciated sheet flows and pillows of MORB composition. Sulfide deposits form at ^sim2200 m water depth and are concentrated within a narrow graben centered within the ridge axial valley. Solitary and coalesced chimneys (0.25 to 12 m high) rise directly from the basalt basement and are composed predominantly of Zn sulfide accompanied by Fe and Cu-Fe sulfide and traces of Pb sulfide. Anhydrite occurs as a minor phase in some chimneys. Sulfide chimneys were formed by rapid-venting of high temperature (^sim285°C) fluids. These acidic fluids (pH ^sim3.5) are enriched in Cl, Na, and Ca and are depleted in Cu and Zn.< P>

In contrast to SJDF, Escanaba Trough (ET) is spreading at ^sim2.3 cm/year and has the high relief and axial graben morphology typical of the slow-spreading Mid-Atlantic Ridge. The floor of the axial valley is buried by up to 500 m of clay and terrigenous silt. Rare basement exposures reveal unbrecciated sheet flows and pillow basalts of MORB composition. Large chimney-topped sulfide mounds up to 20 m high and hundreds of meters in extent occur at ^sim3250 m water depth at the base of sediment hills; sulfide veins, small chimneys, and clastic deposits occur on, and within, the sediment between hills. Two distinct sulfide types occur at ET. Pyrrhotite-rich sulfide is enriched in Fe, Cu, and As and is associated with low-velocity venting of warm (<220° C) alkaline (pH ^sim5.4) fl ids that are currently depositing anhydrite and barite sinter deposits on top of the sulfide mounds. Polymetallic sulfide is enriched in Fe, Pb, Zn, As, Sb, Ag, Sn, and Bi and may be associated with higher-temperature, high-velocity venting. Barite is a major phase in many sulfide samples and occurs as massive chimneys in sediment and on sulfide mounds.

The differences in distribution, morphology, and composition of hydrothermal deposits at SJDF and ET appear to be related to differences in substrate permeability, structure, and lithology.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990