HYDROTHERMAL FE AND SI OXYHYDROXIDE DEPOSITS FROM SOUTH-PACIFIC INTRAPLATE VOLCANOS AND EAST PACIFIC RISE AXIAL AND OFF-AXIAL REGIONS

Fe- and Si-rich oxyhydroxide deposits are among the most common hydrot hermal materials encountered on the submarine intraplate volcanoes of the Society, Austral, and Pitcairn regions and on accreting plate boun daries of the East Pacific near 21-degrees-N, 12-degrees 50' N, and 11 -degrees 30' N. Based on field observations and compositional variatio ns, the hydrothermal deposits are divided into four types: (1) Fe oxyh ydroxides forming chimneys and mounds, depleted in trace metals (Co Cu + Zn + Ni < 0.1 wt %), with a total Fe of 27 to 45 wt percent; (2) Fe oxyhydroxides associated with sulfides, enriched in trace metals (C o + Cu + Zn + Ni > 0.4-19%), with 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, with Si and Fe contents o f 7 to 20 percent and 20 to 30 percent, respectively; and (4) Si-Fe ox yhydroxides enriched in opaline silica, with high Si (>35%) and low Fe (<10%), and strongly depleted in trace metals. Although type 2 repres ents the alteration products of preexisting sulfides, the other Fe and Si oxyhydroxide deposits are primary low-temperature (<70-degrees-C) hydrothermal precipitates forming edifices, mounds, and flat-lying dep osits. These hydrothermal deposits have lower 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 b etween the intraplate and East Pacific Rise regions. Both the intrapla te and the EPR deposits are associated with pillow lava, sheet flows, hyaloclastites, and hydrothermally altered flows having zones of sulfi de and silicate mineralizations, although, up to now, no sulfide edifi ces have been found on the intraplate volcanoes of the South Pacific. Nevertheless, high-temperature solutions have percolated through the v olcanics, giving rise to sulfides (pyrite, chalcopyrite) and silicates (quartz, andradite, actinolite) in relatively high temperature (>250- degrees-C) mineralized zones. It is believed that the Fe and Si oxyhyd roxide deposits of types 1, 3, and 4 are formed during mixing between descending cold seawater and ascending hot hydrothermal fluids in a cr ustal environment.