Fe-andSi-rich oxyhydroxide deposits are among the most common hydrothermal materials encountered on the submarine intraplate volcanoes of the Society, Austral, and Pitcairn regions and on accreting plate boundaries of the East Pacific near 21 ø N, 12050 ' N, and 11ø30 • N. Based on field observations and compositional variations, the hydrothermal deposits are divided into four types: (1) Fe oxyhydroxides forming chimneys and mounds, depleted in trace metals (Co + Cu + Zn + Ni < 0.1 •vt %), •vith a total Fe of 27 to 45 •vt percent; (2) Fe oxyhydroxides associated •vith sulfides, enriched in trace metals (Co + Cu + Zn + Ni > 0.4-1%), •vith an iron content of 30 to 50 percent; (3) Fe-Si oxyhydroxides also forming chimneys and mounds enriched in clay (nontronite), depleted in trace metals, •vith Si and Fe contents of 7 to 20 percent and 20 to 30 percent, respectively; and (4) Si-Fe oxyhy&oxides enriched in opaline silica, •vith high Si (>35%) and lo•v Fe (<10%), and strongly depleted in trace metals. Although type 2 represents the alteration products of preexisting sulfides, the other Fe and Si oxyhydroxide deposits are primary lo•v-temperature (<70øC) hydrothermal precipitates forming edifices, mounds, and flat-lying deposits. These hydrothermal deposits have lo•ver total rare earth element (REE) contents (10-200 ppm) than their associated volcanics and have variable anomalies in Ce and Eu. There are no substantial differences in morphology and composition of the Fe and Si oxyhydroxide deposits bet•veen the intraplate and East Pacific Rise regions. Both the intraplate and the EPR deposits are associated with pillo•v lava, sheet flows, hyaloclastites, and hydrothermally altered flows having zones of sulfide and silicate mineralizations, although, up to no•v, no sulfide edifices have been found on the intraplate volcanoes of the South Pacific. Nevertheless, high-temperature solutions have percolated through the volcanics, giving rise to sulfides (pyrite, chalcopyrite) and silicates (quartz, andradite, actinolite) in relatively high temperature (>250øC) mineralized zones. It is believed that the Fe and Si oxyhydroxide deposits of types 1, 3, and 4 are formed during mixing bet•veen descending cold sea•vater and ascending hot hydrothermal fluids in a crustal environment.