Input from the deep: Hot vents and cold seeps

Abstract

The discovery of black smokers, massive sulfides and vent biota at the crest of the East Pacific Rise at 21°N in 1979 (Francheteau et al. 1979; Spiess et al. 1980) confirmed that the formation of new oceanic crust through seafloor spreading is intimately associated with hydrothermal venting and the formation of metallic mineral deposits at the seafloor. The 350°C hydrothermal fluids discharging from the black smoker chimneys at this site at a water depth of about 2,600 m continuously precipitate metal sulfides in response to mixing of the high-temperature hydrothermal fluids with ambient seawater. Seawater which penetrates deeply into the oceanic crust at seafloor spreading centers is being converted to a hydrothermal fluid with low pH, low Eh, and high temperature during water-rock interaction above a high-level magma chamber. This fluid is capable of leaching large amounts of metals and other elements from the rocks. Metal sulfides, including pyrite, sphalerite, and chalcopyrite which are precipitated from the hydrothermal fluids, gradually accumulate at and just below the seafloor where they can form large sulfide deposits. The TAG hydrothermal mound at the Mid-Atlantic Ridge at 26°N, for example, has a diameter of about 200 m and a height above seafloor of about 50 m. Drilling during Leg 158 of the Ocean Drilling Program indicated that the deposit contains 2.7 Mt of massive sulfide, containing 2 wt.% Cu, and 1.2 Mt of stockwork mineralization extending 125 m below the seafloor and containing 1 wt.% Cu (Hannington et al. 1998). The deposit is capped by a large black smoker complex with as many as 100 black smoker vents at a temperature of more than 360°C.