Sulfide-Sulfate Chimneys on the East Pacific Rise, 11° and 13° N Latitudes. Part I: Mineralogy and Paragenesis

Ursula M. Graham, Gregg J. S. Bluth, and Hiroshi Ohmoto

Abstract. Hydrothermal fluids up to ~350°C emerge from vent sites at 11° and 13°N latitudes, East Pacific Rise. Variations in the morphology, zoning and mineralogy of 27 chimneys from 11 vent sites can be represented by three active and one extinct-clogged chimney. Microscopic observation of 127 polished sections of these four chimneys has revealed four types each of marcasite, pyrite and wurtzite, and two types of sphalerite, which are distinct in their crystal habits. The consistency in the paragenetic sequence in the spatial distribution with respect to the exterior chimney wall of these Fe and Zn sulfides, as well as anhydrite, chalcopyrite and bornite in these four chimneys suggests that all chimneys in the 11 vent sites represent similar hydrothermal processes. The variation among the chimneys reflects simply the difference in the maturity (longevity of venting) and preservation of a complete (ideal) chimney, which is comprised of 8 mineralogical zones. Formation of zone-1 minerals (anhydrite with minor subhedral pyrite, and anhedral marcasite and wurtzite) takes place during mixing of hydrothermal fluids and cold seawater when the rate of mineral precipitation exceeds mineral dissolution by cold seawater. During outward growth of a chimney, zone-1 minerals are continuously dissolved by later hydrothermal fluids, and almost metasomatically transformed successively to zone 2 (mostly large colloform marcasite and wurtzite), zone 3 (mostly cubes of pyrite and colloform sphalerite), zone 4 (mostly bornite), and zone 5 (mostly chalcopyrite); some new minerals (e.g., cubes of pyrite, sphalerite, chalcopyrite) also precipitate directly from the hydrothermal fluids within the pore space created by the dissolution of zone-1 minerals. With continued venting, channel openings enlarge, and each sulfide zone tends to become wider and monomineralic; the width of the znhydrite-rich zone 1, however, becomes larger during early venting and then decreases with time. Clogging of a chimney occurs during the waning stage of hydrothermal activity, by precipitation of zone-7 and -8 minerals (wurtzite, marcasite and pyrite) along the inner chalcopyrite wall of zone 5, and recrystallization in time to form zone 6 (mostly pyrite and sphalerite). With decreasing hydrothermal activity, the chimney walls begin to dissolve from the outside by reaction with cold seawater. The observed paragenesis suggest that the rate of cooling and oxidation, as well as the metal-sulfur chemistry of hydrothermal fluids, are important parameters that control the chimney mineralogy. Quenching of H₂S-rich fluids, caused by rapid mixing with cold seawater, may result in precipitation of pyrrhotite and wurtzite, while anhydrite precipitates by incorporating SO₄^2- of local seawater. Anhydrite layers formed by such a process may play important roles both physically and chemically in the development of sulfide mineralogy within the chimney walls by creating environments that promote not only oxidation of hydrothermal H₂S to generate polysulfide species for FeS₂, but also recrystallization and replacement of earlier minerals in warmer conditions.