Hot spring deposits on the East Pacific Rise at 21°N: preliminary description of mineralogy and genesis

Abstract

Hydrothermal sulfide-sulfate deposits were sampled from eight active and inactive vent sites along the East Pacific Rise at 21°N during the RISE expedition of April, 1979. The mineralogy of the samples has been determined by X-ray diffractometry, scanning electron microscopy, and X-ray energy dispersive analysis. Mounds of Zn, Fe, and Cu sulfides, dominated by sphalerite, pyrite, and lesser chalcopyrite, are topped by inactive and active chimneys, spouting fluids ≤350°C. The outer zones of active chimneys bear abundant anhydrite precipitated from heated ambient seawater, in addition to hydrothermal pyrite and sphalerite. Mg-hydroxysulfate-hydrate, a phase identified in seawater heating experiments, but previously not observed in nature, is intimately intergrown with anhydrite. Hottest chimneys contain massive chalcopyrite±bornite in their interior zones and belch fluids blackened by a presumably non-equilibrium assemblage of pyrrhotite plus minor sphalerite and pyrite. The early, outer walls of chimneys form from sulfates and the sulfide minerals in black smoke, but metastable pyrrhotite in outer zones is rapidly recrystallized to pyrite or marcasite. Reduction in the permeability of the outer walls permits a high-temperature (>∼250°C), low-pH environment within chimneys that enhances precipitation of Cu-Fe sulfides in the central zones. Cooler, worm-covered chimneys emit white fluids bearing particulates of amorphous silica, barite, and pyrite. Amorphous silica and barite are also widely associated with fossilized worm-tubes. Two inactive chimneys are filled with sphalerite, wurtzite, sulfur, pyrite, and marcasite. Anhydrite has been dissolved from these dead chimneys, and the sulfate assemblage is dominated by barite and alteration products such as jarosite and natrojarosite. Silicates other than amorphous silica are not abundant in these deposits, although talc forms in hot chimneys from seawater Mg and hydrothermal silica, and nontronite is found in sediments on the crest of the East Pacific Rise. Other accessory phases identified include copper-rich sulfides such as cubanite, chalcocite, covellite, and digenite; galena; Fe-oxyhydroxides, including goethite; and gypsum. Chimney debris accumulates to form basal mounds, and the mineralogical differences between mounds and chimneys are attributable to weathering of mounds. Mn-oxyhydroxides form crusts within a few meters of the vents, but are not coprecipitating with the sulfide/sulfate minerals.