Richard Knott,2 Yves Fouquet,3 José Honnorez,2 Sven Petersen,4 and Marcel Bohn3


Mineralogical, textural, chemical, and isotopic features of a vertical section through the active Trans-Atlantic Geotraverse (TAG) hydrothermal mound reveal the nature of subsurface mineralization. The multistage growth and evolution of the TAG mound occurs by the following processes: (1) near-surface (<10 m depth) hydrothermal precipitation of porous Fe-Cu-Zn sulfide and Si-Fe-oxyhydroxides; (2) modification of surface material within the mound (>20 m depth) by sequential overgrowth, recrystallization and mineral dissolution; (3) hydrothermal mineralization within the mound, forming Fe-Cu sulfides, anhydrite and quartz; and (4) alteration and mineralization of basalt basement beneath the mound. During the long history of hydrothermal activity, these processes have driven the TAG mound toward a mineralogy dominated by pyrite and depleted in Cu, Zn, and trace elements. The basement beneath the mound is ultimately altered to pyrite-quartz.

Sulfur-isotope composition of sulfides in the range +4.4‰ to +8.9‰ requires a deep hydrothermal source with elevated delta34S to generate an end-member fluid with estimated delta34S of +5.5‰. Vein-related sulfide mineralization is isotopically light, whereas sulfide disseminated in altered basalt is isotopically heavy. The systematic variations between sulfide generations and a general increase with depth are a result of sulfate reduction in a shallow seawater-hydrothermal circulation system developed around the hydrothermal feeder zone. This generates hydrothermal fluid and sulfide mineralization with a maximum delta34S of +8.9‰. Mixing between this shallow circulated fluid and the end-member hydrothermal component would explain the variations of up to 3‰ observed between different sulfide generations in the mound.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Institut de Géologie, Université Louis Pasteur, 1 rue Blessig, Strasbourg 67084 Cedex, France. rknott@globalnet.co.uk
3IFREMER, Centre de Brest, DRO/GM BP 70, 29280 Plouzane, France.
4Lehrstuhl für Lagerstättenlehre, Institut für Mineralogie, Technische Universität Bergakademie Freiberg, Brennhausgasse 14, D-09596 Freiberg, Federal Republic of Germany.