GEOLOGY, MINERALOGY, AND CHEMISTRY OF SEDIMENT-HOSTED CLASTIC MASSIVESULFIDES IN SHALLOW CORES, MIDDLE VALLEY, NORTHERN JUAN-DE-FUCA-RIDGE

Middle Valley is a sediment-covered rift near the northern end of Juan de Fuca Ridge. Hydrothermal fluids are presently being discharged at two vent fields about 3 km apart, Bent Hill and the area of active ven ting. Bent Hill consists of a 60-m-high sediment mound, one 35-m-high inactive sulfide mound near its southern margin, and two hydrothermall y active 25-m-high sulfide mounds about 330 m farther south. An anhydr ite chimney near the summit of one mound is venting 265-degrees-C flui ds. The area of active venting is a 800- by 400-m area of high acousti c reflectivity that consists of several hydrothermal mounds with anhyd rite chimneys near the summit which are venting 184-degrees to 274-deg rees-C fluids. The hydrothermally active chimneys at both Bent Hill an d the area of active venting consist of anhydrite and Mg-rich silicate s with mitior pyrite, Cu-Fe sulfide, sphalerite, and galena. Hydrother mal discharge in these areas appears to be focused along extensional f aults. At the Bent Hill massive sulfide deposit, clastic sulfide layer s are interbedded with hydrothermally altered and unaltered hemipelagi c and turbiditic sediment along the flanks of the sulfide mound. Sulfi de clasts display textures commonly found in chimneys and consist of a n open interlocking network of pyrrhotite crystals with interstitial w urtzite, isocubanite, and chalcopyrite. The minerals forming this netw ork are variably replaced, veined, and cemented by sphalerite, pyrite, marcasite, magnetite, hematite, amorphous silica, hydrothermal clays, and barite. Zinc contents are highly variable and range up to 11.2 wt percent, with Cu up to 1.2 wt percent. Other metal contents are low: Pb ranges up to 2,600 ppm, Ag up to 45 ppm, and Au up to 371 ppb. Sulf ide textures and mineralogy suggest that the Bent Hill sulfide mound f ormed by the build-up and collapse of sulfide chimneys, the resediment ation of sulfide debris and the formation of clastic sulfide layers, a nd the infilling and replacement of clastic sulfides by hydrothermal f luids near vents. The quench assemblage pyrrhotite-wurtzite (now sphal erite)-isocubanite iii chimney clasts is consistent with high-temperat ure (>300-degrees-C) fluid discharge. This primary assemblage has been altered to pyrite, marcasite, sphalerite, and iron oxides by reaction with lower temperature and more oxidizing hydrothermal fluid and seaw ater. The most likely oxidant is sulfate in seawater entrained into th e sulfide mound. Sulfur isotope values (deltaS-34pyrrhotite = 8.1 part s per thousand; deltaS-34pyrite = 4.1 parts per thousand) that are con sistently more positive than basaltic sulfur support the addition of s eawater sulfur. Pb isotope values for the Bent Hill deposit that are t ransitional between midocean ridge basalt (MORB) and Middle Valley sed iments indicate that the sulfides probably formed from fluids which or iginated in the oceanic crust but which have been modified by reaction with lower temperature (<274-degrees-C) fluids generated in the sedim entary pile, similar to those now venting in Middle Valley.