HYDROTHERMAL ALTERATION ASSOCIATED WITH MASSIVE SULFIDE DEPOSITS, MIDDLE VALLEY, NORTHERN JUAN-DE-FUCA RIDGE

Middle Valley is a sediment-covered rift near the northern extremity o f the Juan de Fuca Ridge. Hydrothermal fluids are presently being disc harged at two vent fields about 3 km apart, Bent Hill (BH) and the Are a of Active Venting (AAV). At the AAV, the hydrothermal alteration con sists of an inner, spatially restricted, higher-temperature (>120-degr ees-C) zone (Zone I) that consists of grey, moderately to strongly ind urated, and locally brecciated, fractured and veined sediment. Hydroth ermal minerals within Zone I include a Mg-rich smectite-group mineral, chlorite, amorphous silica, barite, pyrite and gypsum. Zone I is surr ounded by a more widespread, lower-temperature (<120-degrees-C) zone ( Zone II) that consists of moderately indurated, blue-green sediments t hat consist of a Mg-rich smectite-group mineral, carbonate, barite and minor pyrite. The zonal distribution of hydrothermal minerals is cont rolled by the chemical and physical evolution of high-temperature (up to 274-degrees-C) fluid as it migrated outward and upward from a centr al conduit, reacted with detrital and biogenic minerals, and mixed wit h downwelling seawater. This model of fluid migration is supported by 1) heat flow values and temperatures based on oxygen isotope fractiona tion in hydrothermal minerals, which decrease away from vent sites, 2) Sr-87/Sr-86 ratios for hydrothermal minerals that define a mixing lin e, from less radiogenic hydrothermal Sr near the vent sites to more ra diogenic Sr distal from vent sites; 3) deltaC-13 Values in carbonate c oncretions that range from moderately negative (about -15 parts per th ousand) in Zone I to highly negative (-30 parts per thousand) in Zone II; 4) deltaS-34 values in hydrothermal pyrite that are similarly zone d, from positive values (0.8 to 7.5 parts per thousand) in Zone I to h ighly negative values (-14.3 to -39.7 parts per thousand) in Zone II; 5) pore-water compositions that define a convex-upward hydrothermal ac idic plume enriched in Ca, Si and Ba, and depleted in Mg; and 6) strat abound zones of alteration associated with highly permeable sediment t hat suggest an along-strata control on permeability.