Ra. Zierenberg et al., GENESIS OF MASSIVE SULFIDE DEPOSITS ON A SEDIMENT-COVERED SPREADING CENTER, ESCANABA TROUGH, SOUTHERN GORDA RIDGE, Economic geology and the bulletin of the Society of Economic Geologists, 88(8), 1993, pp. 2069-2098
The Escanaba trough is a sediment-filled axial valley in the slow-spre
ading (2.3 cm/yr) southern part of Gorda Ridge. The hemipelagic and tu
rbiditic sediment fill is 300 to > 1,200 m thick and was rapidly depos
ited during Pleistocene low stands of sea level. Local areas of excess
magmatism, relative to the rate of extension, form igneous centers a
few kilometers in diameter that are spaced at intervals of approximate
ly 15 km along the spreading axis. Sediment cover is thinner over thes
e igneous centers and the sedimentary sequence is disrupted by igneous
intrusions and faulting. The coexistence of tectonic extension with r
apid sediment deposition favors the formation of sheeted sills rather
than basalt flows that form the upper-most oceanic crust at sediment-f
ree spreading centers. Circular sediment hills as much as 1,200 m in d
iameter and 120 m high are interpreted as uplifted fault blocks above
laccolithic intrusions emplaced above the igneous centers. Massive sul
fide deposits that formed on the peripheries of these hills have surfa
ce exposures of greater than 100 m in at least one direction, but the
full dimensions of sulfide mineralization are poorly known. The sulfid
e deposits are composed predominantly of pyrrhotite with less abundant
isocubanite, chalcopyrite, sphalerite, arsenopyrite, and marcasite. B
arite-rich and polymetallic massive sulfide occur locally and have hig
her contents of Zn, Pb, Ag, As, Sb, and Sn than does pyrrhotite-rich m
assive sulfide. Polymetallic massive sulfide has low Au contents, but
barite-rich and pyrrhotite-rich massive sulfide samples are enriched i
n gold relative to most sediment-hosted massive sulfide deposits, aver
aging more than 1 g/t Au. Massive sulfide from the Escanaba trough is
enriched in group IV, V, and VI elements relative to deposits formed o
n sediment-free spreading centers due to interaction of hydrothermal f
luid with sediment. Further evidence of hydrothermal fluid-sediment in
teraction is provided by the alkali-rich nature of hydrothermal fluid
sampled from 220-degrees-C vents, the presence of sulfide samples cont
aining thermogenic hydrocarbon derived from terrigenous organic matter
in the sediment, and radiogenic Pb isotope ratios of massive sulfide.
Sulfide sulfur is derived from basaltic rocks and from seawater sulfa
te that is reduced by high-temperature reaction with iron silicates or
sedimentary organic matter. Sediment is extensively altered to clinoc
hlore by Mg metasomatism in localized mixing zones where seawater is d
rawn into the upper part of hydrothermal discharge zones. Shallow subs
urface deposition of sulfide is interpreted to be an important process
in the formation of the deposits, A geologic model of the hydrotherma
l circulation proposes that the heat to drive the hydrothermal circula
tion system is provided both by a regionally extensive sheeted sill co
mplex and by local laccolithic intrusions. Reaction of heated seawater
with basaltic rocks controls the initial composition of the hydrother
mal fluid, but interaction of the hydrothermal fluid with sediment in
the upflow zone alters the fluid chemistry and results in enrichment o
f the sulfide deposits in group IV, V, and VI metals. The geologic set
ting in an oceanic rift environment, associated lithologies such as mi
xed flyschlike sediment and tholeiitic basalt, and the composition (Fe
sulfide-dominant, Cu-Zn deposits with Cu, Zn >> Pb) of the Escanaba n
aba trough deposits generally are analogous to ancient massive sulfide
deposits that are classified as Besshi type. The difference in tecton
ic settings among the modern sediment-hosted deposits which formed in
open-ocean spreading centers and rifted continental margins and the co
ntrast in morphology and composition compared with many ancient sedime
nt-hosted deposits imply that Besshi-type deposits form within a multi
tude of ocean rift environments.