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High-temperature hydrothermal fluids exit from the sea floor on the East Pacific Rise at 21°N. Mixing of the hydrothermal fluids with seawater leads to the precipitation of sulfide and sulfate minerals. These precipitates may become dispersed in the water column or may aggregate to form chimney like structures and basal mounds.

Our study of the distribution and textures of minerals in chimney samples collected during 1979 suggests a relatively simple model of chimney growth. Such growth starts with the rapid formation of a highly permeable wall composed of anhydrite and minor sulfides. Vertical growth rates of 30 cm/day for new chimneys were observed in 1.981 after preexisting chimneys were razed. Mixing of seawater and hydrothermal fluid through this wall causes the precipitation of additional sulfides and sulfates, which further reduces the permeability of the chimney wall. The temperature of the hydrothermal fluid inside chimneys gradually increases, and the fluid composition approaches the composition of fluid in the chimney roots. The mineralogy of chimney interiors changes gradually in response to changes in fluid temperature and composition. As chimney exteriors become increasingly isolated from the interior hydrothermal fluid, their temperature drops and new mineral assemblages are produced by reaction of ambient seawater and the mineral assemblages of the chimney walls. Weathering assemblages similar to supergene sulfide assemblages can develop during reactions of this type.

The process of basal mound formation is less well understood than the process of chimney formation. Stockwork fracturing is characteristic of the footwall of many ophiolite-hosted massive sulfide deposits, which suggests that basal mound growth commences with intense localized fracturing of the sea floor. Fractures produced during such events provide conduits for rising hydrothermal fluids, and sulfide and sulfate mineral precipitates accumulate on the sea floor around the fractures and may form a basal mound. Chimney structures develop around larger openings, while smaller openings clog with precipitates to form a low-permeability crust on basal mounds; hydrothermal circulation beneath such a crust was discovered in 1981. The upper part of the mounds is a transition zone between low-temperature ambient seawater and the high-temperature fluid within the mound interior; this transition zone is probably zinc rich.

The geologic preservation of sulfide-sulfate deposits on the ocean floor requires burial rates that exceed the rate of oxidation and dissolution.

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