Distribution and Composition of Massive Sulfide Deposits at Escanaba Trough, Southern Gorda Ridge

Abstract

Massive sulfide deposits were photographed and sampled at two sites (southern or SESCA and northern or NESCA) on the sediment-covered Escanaba Trough segment of Gorda Ridge, a slow-spreading midocean ridge located offshore of northern California within the U.S. Exclusive Economic Zone. Massive sulfide ledges, mounds, and chimneys in various stages of oxidation and erosion are spatially associated with the margins of small (1 km wide, 100 m high) sediment hills at SESCA and NESCA, and with faulted sediment on the north flank of a volcanic hill at NESCA. The sediment hills are uplifted above volcanic edifices that rise through sediment filling the axial valley. Two types of massive sulfide have been dredged from Escanaba Trough. Porous aggregates of pyrrhotite-rich sulfide contain small amounts of intergrown isocubanite, chalcopyrite, sphalerite, and loellingite. In some samples, pyrrhotite is partly to completely replaced by marcasite. Pyrrhotite-rich massive sulfide generally contains low amounts of base and precious metals, but barite encrustations on some of the samples are enriched in Zn, Pb, Ag, and Au. This sulfide type is largely derived from mound and talus deposits associated with the sediment hills at SESCA and NESCA. Zoned polymetallic sulfide samples from a single dredge at the NESCA site comprise the second massive sulfide type. These polymetallic samples are fragments from the wall of a sulfide structure with a large fluid channelway. Proceeding from the inner to outer wall, the mineral zonation includes isocubanite, sphalerite, arsenopyrite, galena, tetrahedrite, a Pb-Sb suifosalt, and argentite. Polymetallic sulfide samples are enriched in Pb, As, Sb, Ag, Hg, and Sn relative to massive sulfide recovered from other spreading axes. Other dredge samples include sulfide-cemented sediment-clast breccia and sediment containing abundant interstitial galena and hydrocarbon. The composition of massive sulfide from both areas indicates extensive interaction between hydrothermal fluids and the sediment in Escanaba Trough. Pyrrhotite-rich sulfide probably forms above sites of diffuse, sluggish discharge, whereas polymetallic sulfide structures formed in response to focused, high-velocity discharge at a “black smoker” type of vent.