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ABSTRACT: Volcanic and Sedimentary Controls on Hydrothermal Mineralization at Escanaba Trough, Southern Gorda Ridge

Janet L. Morton, Robert A. Zierenberg, Alice S. Davis, Roger P. Denlinger, Christopher G. Fox

Turbidite and hemipelagic sediment several hundred meters thick covers the southern two-thirds of the axial valley floor of Escanaba Trough. Recent igneous activity along the trough is confined to discrete volcanic edifices where sills intrude and disrupt the sediment layers and basalt flows are locally extruded at the seafloor. The volcanic edifices define three north-south-trending en echelon ridge segments. The uniform seismic character of the sedimentary fill consists of thin, highly reflective zones 2-10 m apart in the upper several tens of meters of sediment; these zones probably correspond to sand or coarse silt layers observed in gravity core samples. A seismically transparent zone from about 50 to 100 m depth may correspond to a thick sand interval, as encountere at similar depth at nearby DSDP site 35 (1969). Basalt compositions range from relatively primitive to moderately fractionated; lead isotopic compositions are more radiogenic than for other Gorda Ridge basalt samples, consistent with assimilation of a small amount of sediment into the basaltic melt.

Steep-sided hills of uplifted sediment about 1 km in diameter fist 50 to 100 m above the volcanic edifices. Mechanical modeling suggests that these hills formed by the inflation of basaltic sills at the sediment-basement interface, at which time sediment was assimilated by the magma. Sediment-hosted hydrothermal sulfide deposits several hundred meters across and several tens of meters high have developed along the base of the sediment hills, and smaller sulfide mounds and chimneys occur on the sediment-covered seafloor away from the hills. Disseminated sulfide in a sand-rich sediment layer was cored in one location. We suggest that two factors control the location of sulfide mineralization: (1) Faults that formed as the hills were uplifted provide the primary conduits for ascending hy rothermal fluids, localizing the seafloor sulfide deposition along the perimeters of the hills; and (2) silt and sand layers allow horizontal migration of hydrothermal fluids away from the fault zones and may feed the smaller deposits. Subsurface mineral deposition is probably occurring in these coarser sediment layers.

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