62 • Marine Minerals: Exploring Our New Ocean Frontier 



Figure 2-7.— Formation of Marine 

 Polymetailic Sulfide Deposits 



<^ Axis of -^ 

 ♦-\spreading__ 



>§so?T^ 



Pacific 

 plate 



The Juan de Fuca and Gorda Ridges are active spreading 

 centers off the coasts of Washington, Oregon, and California. 

 Polymetailic sulfides are formed at spreading centers, where 

 seawater heated by magma circulates through the rocl<s of 

 the seafloor dissolving many minerals and depositing 

 massive sulfide bodies containing zinc, copper, iron, lead, 

 cadmium, and silver. Such sulfide deposits have been found 

 on the Juan de Fuca Ridge v\/ithin the FEZ of Canada and on 

 the Gorda Ridge within the U.S. FEZ. 



SOURCES: Office of Tecfinology Assessment, 1987; M.A. Cfiamp, W.P Dillon, and 

 D.G. Howell, "Non-Llving EEZ Resources; Minerals, Oil, and Gas," 

 Oceanus, volume 27, number 4, winter 1984/85. 



erals along cracks, crusts on the oceanfloor, or 

 chimneys or stacks (figure 2-7). High temperatures 

 suggest Uttle or no mixing with cold seawater be- 

 fore the solutions exit through the vents. 



The degree to which the ore solution is diluted 

 in the subsurface depends on the porosity or frac- 

 turing of the near surface rock and also determines 

 the final exit temperature and composition of the 

 hydrothermal fluids. The fluid ranges from man- 

 ganese-rich in extreme dilution to iron-dominated 

 at intermediate dilution levels to sulfide deposition 

 when little dilution occurs. In support of these ob- 

 servations, investigators have found sulfide depo- 

 sition at the vents with manganese oxide deposits 

 farther away from the seawater-hydrothermal fluid 

 interface.*^ Iron oxides are often found in associa- 

 tion with, but at a distance from, the active vent 

 and sulfide mineralization. 



An important control on the location of hydro- 

 thermal mineralization, either beneath or on the 

 seafloor at a spreading center, is whether the sub- 

 seafloor hydrothermal convection system is leaky 

 or tight. In leaky high-intensity hydrothermal sys- 

 tems, seawater penetrates downward through frac- 

 tures in the crust and mixes with upweUing primary 

 hydrothermal solutions, causing precipitation of dis- 

 seminated, stockwork, and possibly massive copper- 

 iron-zinc sulfides beneath the seafloor. Dilute, low- 

 temperature solutions depleted in metals discharge 

 through vents to precipitate stratiform iron and 

 manganese oxides, hydroxide, and silicate depos- 

 its on the seafloor and suspended particulate mat- 

 ter enriched in iron and manganese in the water 

 column.*^ In tight, high-intensity hydrotliermal sys- 

 tems, primary hydrothermal solutions undergo neg- 

 ligible mixing with normal seawater beneath the 

 seafloor and discharge through vents to precipitate 

 massive copper-iron-zinc sulfide deposits on the 

 seafloor and suspended particulate matter enriched 

 in various metals in the water column. 



"East Pacific Rise Study Group, "Crustal Processes of the Mid- 

 Ocean Ridge," Science, vol. 213, July 3, 1981, pp. 31-40. 



"Rona, "Hydrothermal Mineralization at Slow-Spreading Centers: 

 Red Sea, Atlantic Ocean, and Indian Ocean," p. 123. 



