order of 1 kilometer wide centered at the axis and bounded by 

 marginal zones of active extension where the newly formed oceanic 

 crust is faulted and undergoes differential uplift. Specific 

 features of the Mid-Atlantic Ridge and most other slow-spreading 

 oceanic ridges is the presence of a well-defined rift valley with 

 widths up to about 30 km between crests of flanking rift 

 mountains and relief up to 3 km from rift valley floor to the 

 mountain crests (Fig. 4). A well-defined rift valley is absent 

 in intermediate-spreading oceanic ridges like the Juan de Fuca 

 Ridge and the Galapagos spreading center, and intermediate- to 

 fast-spreading ridges like the East Pacific Rise. 



Mineralization 



The TAG Hydrothermal field where the first hot black 

 smokers, polymetallic massive sulfide deposits and vent animals 

 in the Atlantic Ocean were discovered in 1985 is situated along a 

 linear segment of the rift valley of the Mid-Atlantic Ridge at 

 26°08'N, 44°49'W ( Rona et al., 1986a). The field occupies a 6 by 

 8 km area of the rift valley including the floor and the east 

 wall (Fig. 5). The area encompasses warm springs and low- 

 temperature mineral deposits on the east wall between depths of 

 2300 and 3300 m, and hot springs associated with vent animals and 

 high-temperature deposits on the lower portion of the east wall 

 and floor of the rift valley between depths of 3300 and 3900 m. 

 The low-temperature deposits comprise stratiform layered 

 manganese oxides and iron oxides, hydroxides and silicates in 

 patches up to tens of meters in diameter distributed along faults 

 trending subparallel to the ridge axis at mid-depth on the east 

 wall (Table 2; Rona et al . , 1984; Thompson et al . , 1985). 



The high-temperature deposits of the TAG Hydrothermal Field 

 are primarily massive sulfides that occur in mound shapes (Rona 

 et al., 1986a; Thompson et al., 1988). One mound is inactive and 

 is about 1500 m wide by 100 m high, rising from a depth of 3400 m 

 near the base of the east wall (Rona et al., 1986b). A second 

 mound is hydrothermally active and is about 250 m wide, rising 50 

 m from a depth of 3670 m at the juncture between the base of the 

 east wall and the floor of the rift valley (Fig. 5). Massive 

 sulfide samples recovered from the hydrothermally active mound 

 include copper-, iron- and zinc-rich varieties (Table 2) similar 

 to massive sulfides from Pacific sites. The copper-iron massive 

 sulfides from the active mound have relatively high content of 

 free native gold (16 ppm; 0.5 oz. per ton; Hannington et al., 

 1988). The gold is inferred to have been concentrated by a 

 secondary supergene process involving circulation of low- 

 temperature seawater through the massive sulfides initially 

 precipitated from high-temperature solutions, similar to the 

 process that has concentrated gold recovered from some ancient 

 massive sulfide deposits on land. This is the highest content of 

 gold and the first occurrence of secondary enrichment reported in 

 massive sulfide deposits anywhere on the seafloor. The discovery 



11 



