Wd. Goodfellow et Jm. Franklin, GEOLOGY, MINERALOGY, AND CHEMISTRY OF SEDIMENT-HOSTED CLASTIC MASSIVESULFIDES IN SHALLOW CORES, MIDDLE VALLEY, NORTHERN JUAN-DE-FUCA-RIDGE, Economic geology and the bulletin of the Society of Economic Geologists, 88(8), 1993, pp. 2037-2068
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.