The chemical composition of diffuse effluent from the TAG hydrothermal mound, 26°N Mid-Atlantic Ridge, directly demonstrates that mixing of entrained seawater and high-temperature black smoker fluids leads to the precipitation of sulfides, silica, and anhydrite and to the zone refining of metals, including Cu, Zn, U, and the rare earth elements in an actively forming sulfide ore body. Rates of mineral formation deduced from these data are extremely rapid and are up to an order of magnitude greater for anhydrite (5–15 × 107 kg/yr) than for sulfides (106–107 kg/yr). Comparison with mineral inventories for TAG suggests that virtually all of the anhydrite in the TAG mound is contemporary, and confirms that the sulfide precipitated episodically during the history of mound growth and that much of it has been lost by oxidation and reworking.

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