Sulfide formation and hydrothermal alteration of hemipelagic sediment in Middle Valley, Northern Juan de Fuca Ridge

Middle Valley is a sedimented fault-bounded rift valley at the northern end of the Juan de Fuca Ridge. Sediment thicknesses are highly variable and increase from the margins to 1500 m in the center. Within the Valley, eight mound-like features up to 60 m high and several hundred meters across were observed with SeaMARC II acoustic imagery and Seabeam bathymetry. Two areas displaying anomalously high heat-flow values were actively venting hydrothermal fluids from sulfide chimneys; one area coincides with a sulfide mound, and the other is associated with a flat sediment surface. Unaltered hemipelagic sediment in Middle Valley consists of quartz, plagioclase, amphibole, smectite, chlorite, illite and irregular mixed-layer clays. Hydrothermally altered sediment additionally contains authigenic calcite (concretions and cement), barite, gypsum, pyrite, amorphous silica, and Mg-rich silicates. The altered sediment is enriched in Ca, Fe, S, Si, Cr and As; other elements such as Mn, Cu, Pb and Zn, which are concentrated in oxidized surface layers, were most likely enriched by diagenetic processes.