Ch. 2— Resource Assessments and Expectations • 65 



suggests that, if crustal materiad remains close to 

 hydrothermal heat sources for a longer period of 

 time, it might become even more greatly enriched 

 through hydrothermal mineralization.^" In any 

 event, a complete series of hydrothermal phases can 

 be expected at slow-spreading centers, ranging from 

 high-temperature sulfides to low-temperature ox- 

 ides. The hydrothermal mineral phases include 

 massive, disseminated and stockwork sulfide depos- 

 its and stratiform oxides, hydroxides, and silicates. 



To account further for their differences, the 

 deeper seated heat sources at slow-spreading centers 

 can be inferred to favor development of leaky 

 hydrothermal systems leading to precipitation of 

 the sulfides beneath the seafloor. This inferrence, 

 however, cannot be verified until the deposits are 

 drilled extensively. 



Another view regarding the differences in poten- 

 tial for mineralization between fast- versus slow- 

 spreading ridge systems suggests that the extent of 

 hydrothermal activity and polymetaJlic sulfide depo- 

 sition along oceanic ridge systems is more a func- 

 tion of that particular segment's episodic magmatic 

 phase than the spreading rate of the ridge as a 

 whole. ^' According to this view, at any given time 

 a ridge segment with a medium or slow average 

 spreading rate may show active hydrothermal vent- 

 ing as extensive as that found along segments with 

 fast spreading rates. Thus, massive polymetallic sul- 

 fide deposits may be present along slow-spreading 

 ridge segments, but they probably would be sepa- 

 rated by greater time and distance intervals. 



Another factor, particularly on the Gorda Ridge, 

 is the amount of sediment cover. The 90-mile-long, 



sediment-filled Escanaba Trough at the southern 

 part of the Gorda Ridge is similar to the Guaymas 

 Basin in the Gulf of California, where hydrother- 

 mal sulfide mineralization has been found. The 

 amount of sediment entering an active spreading 

 center is critical to the formation and preservation 

 of the sulfide deposits. Too much material deliv- 

 ered during mineralization will dilute the sulfide 

 and reduce the economic value of the deposit. On 

 the other hand, an insufficient sediment flux can 

 result in eventual oxidation and degradation of the 

 unprotected deposit. 



Sulfide deposits and active hydrothermal dis- 

 charge zones have been found on the southern Juan 

 de Fuca Ridge beyond the 200-nautical-mile limit 

 of the EEZ. The Juan de Fuca Ridge is a medium- 

 rate spreading axis separating at the rate of 3 inches 

 per year. Zinc and silver-rich sulfides have been 

 dredged from two vent sites that lie less than a mOe 

 apart. Photographic information combined with ge- 

 ologic inference suggests a crude first-order esti- 

 mate of 500,000 tons of zinc and silver sulfides in 

 a 4-mile-long segment of the axial valley.'^ 



Although marine polymetallic sulfide deposits 

 may someday prove to be a potential resource in 

 their own right, the current value of oceanfloor sul- 

 fides lies in the scientific understanding of their for- 

 mation processes as well as their assistance in the 

 possible discovery of analogous deposits on land 

 (figure 2-8). Cyprus; Kidd Creek, Canada; and the 

 Kuroko District in Japan are all mining sites for 

 polymetallic sulfides, and all of these areas show 

 the presence of underlying oceanic crust. The key 

 to the past by studying the present is unraveling 

 the mechanisms by which this very important class 

 of minerals and ores were formed. 



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

 Red Sea, Adantic Ocean and Indian Ocean," p. 140. 



^'A. Malahoff, "Polymetallic Sulfides — A Renewable Marine Re- 

 source," Marine Mining: A New Beginning, Conference Proceed- 

 ings, ]u\y 18-21, 1982, Hilo, HI, State of Hawaii, sponsored by De- 

 partment of Planning and Economic Development, 1985, pp. 31-60. 



"R.A. Koski, W.R. Normark, and J.L. Morton, "Massive Sul- 

 fide Deposits on the Southern Juan de Fuca Ridge: Results of Inves- 

 tigations in the USGS Study Area, 1980-83," Marine Mining, vol. 

 4, No. 2, 1985, pp. 147-164. 



ALASKA REGION 



In southeastern Alaska, the coast is mountainous 

 and heavily glaciated. Glacial sediments cover 

 much of the shelf, which averages about 30 miles 

 in width. The Gulf of Alaska has a wide shelf that 



was mostly covered by glaciers during the Pleisto- 

 cene. The eastern coast of the Gulf is less moun- 

 tainous and lower than the steep western coast. The 

 source rocks in the region include a wide range of 



