Hydrothermal deposits were sampled by submersible from the calderas and summit benches of two seamounts 20 and 30 km west of the East Pacific Rise at 21°N. The deposits document a wide variety of off-axis hydrothermal activity, including samples from the interiors of seafloor sulfide deposits.

In contrast to most mid-ocean ridge deposits, sulfide deposits on Green Seamount are rich in Si, Fe, and Cu, are poor in Zn, and contain abundant quartz. Sulfide chimneys grew by quenching of hydrothermal fluids and deposition of walls of colloform pyrite, marcasite, Zn sulfide, and minor galena, whereas coarse-grained pyrite and chalcopyrite were deposited at higher temperatures (>250 °C) in chimney interiors. As chimney walls grew outward into sea water, increased temperatures in the inner walls resulted in recrystallization to pyrite and deposition of pyrite and chalcopyrite in pore space. Cooling in the outer portions of chimneys caused deposition of opal at temperatures of about 80–170 °C. As chimneys coalesced and opal-rich material was heated in the interiors of sulfide edifices, opal was recrystallized to chalcedony and quartz. Faulting has exposed the interiors of sulfide deposits, where further recrystallization and deposition of chalcopyrite and quartz occurred at temperatures of 230–320 °C. Clogging of chimneys, changes in flow paths, and cooling of the deposits led to the deposition of late mixed-layer illite-smectite, iron sulfides, Fe-poor sphalerite, opal, and barite. The sulfide deposits formed over a time span of at least 140,000 to 70,000 yr ago, probably coincident with caldera and pit crater formation. The ages of the sulfide deposits provide constraints on the last volcanic and tectonic activity, and they also suggest that it took a maximum of ∼260,000 yr for the seamount to reach its present height.

Red Seamount is presently hydrothermally active, and deposits consisting predominantly of amorphous Fe-oxyhydroxide are forming on small pillow cones in the caldera at temperatures of 10–15 °C. Hydrothermal nontronite deposits capped by Mn-oxide crusts cover much of the southern summit bench of Red Seamount and formed at temperatures of around 30 °C. Iron-rich talc and sulfide grains in nontronite provide evidence of former high-temperature activity and suggest that a sulfide deposit is present nearby.

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