1. PETROLOGY OF HYDROTHERMAL
MINERALIZATION:
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ABSTRACTMineralogical, 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. |
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