CHEMISTRY OF ORE-FORMING FLUIDS AND MINERAL FORMATION RATES IN AN ACTIVE HYDROTHERMAL SULFIDE DEPOSIT ON THE MID-ATLANTIC RIDGE

The chemical composition of diffuse effluent from the TAG hydrothermal mound, 26 degrees N Mid-Atlantic Ridge, directly demonstrates that mi xing of entrained seawater and high-temperature black smoker fluids le ads to the precipitation of sulfides, silica, and anhydrite and to the zone refining of metals, including Cu, Zn, U, and the rare earth elem ents in an actively forming sulfide ore body. Rates of mineral formati on deduced from these data are extremely rapid and are up to an order of magnitude greater for anhydrite (5-15 x 10(7) kg/yr) than for sulfi des (10(6)-10(7) kg/yr). Comparison with mineral inventories for TAG s uggests that virtually all of the anhydrite in the TAG mound is contem porary, and confirms that the sulfide precipitated episodically during the history of mound growth and that much of it has been lost by oxid ation and reworking.