Distribution and general characteristics

Where undiluted hydrothermal fluids mix with cold seawater at and near the seafloor, minerals precipitate out of solution to form chimney structures and other deposits. These deposits are mineralogically complex, containing sulphides (e.g. pyrite, marcasite, chalcopyrite, sphalerite, wurtzite, galena), sulphates (e.g. anhydrite, barite), silica and oxyhydroxides (e.g. Hannington etal., 1995  

Where these deposits are relatively large, such as at the TAG mound on the Mid-Atlantic Ridge, they resemble ancient volcanic-hosted massive sulphide (VMS) deposits like those presently found in ophiolites (Rona Scott, 1993 Humphris etal., 1995 Mills, 1995 Goulding etal., 1998 Scott, 1997). Some of the metals at hydrothermal vents are discharged as black smoke along with gases and other dissolved ions, and are expelled into plumes above and away from the vents (see Section 2.4). 

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