GENESIS OF MASSIVE SULFIDE DEPOSITS ON A SEDIMENT-COVERED SPREADING CENTER, ESCANABA TROUGH, SOUTHERN GORDA RIDGE

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.