127°30'W) and southern ( SESCA site centered at latitude 40°45'N, 

 longitude 127°30'N) portions of the Escanaba Trough ( Koski et 

 al . , 1988). Active hot springs associated with a community of 

 vent animals were found at one of the volcanic centers in the 

 NESCA area by DSRV Alvin in 1988 (Campbell et al . , 1988b). 

 Additional dives are scheduled in 1988 with DSV Sea Cliff at the 

 two areas in the Escanaba Trough. 



Ten relatively large massive sulfide occurrences as wide as 

 200 m and numerous smaller occurrences were found in the NESCA 

 and SESCA areas. The sulfide occurrences are spatially 

 associated with the basaltic edifices at the volcanic centers 

 that intrude and uplift the sediment fill composed of terrigenous 

 turbidites (Morton et al . , 1987; Morton and Holmes, 1987). In 

 both areas the larger deposits occur at faults around the base of 

 sediment-capped hills composed of basaltic sheet and pillow lava 

 flows. The occurrences consist of constructional ledges and 

 mounds of massive sulfides tens of meters in diameter and several 

 meters high that are capped by occasional chimney-like structures 

 and surrounded by an apron of sulfide talus. The massive 

 sulfides typically contain higher contents of zinc (< 43 

 percent), lead (< 7.6 percent), and silver (< 700 ppm) than 

 massive sulfides at sediment-starved oceanic ridges, reflecting 

 additional provenance of these metals by dissolution from the 

 terrigenous sediments (Table 2; Koski et al . , 1988). Samples of 

 the massive sulfides are impregnated with asphaltic petroleum 

 formed by cracking of organic matter in the underlying sediments 

 by upwelling hydrothermal solutions (Koski and Kvenvolden, 1986). 

 The presence of intact small sulfide chimneys on sediment and 

 fresh pillow flows indicates recent hydrothermal activity. 

 Subsurface mineralization is evidenced by sulfide veinlets in 

 sediment cores and sulf ide-cemented sediment. The overall size 

 of the deposits present in the two areas of the southern section 

 of the Gorda Ridge depends on whether the distribution of 

 sulfides is limited to faults which penetrate the sediment or is 

 laterally extensive through the layers of sediment. This 

 question will require drilling to resolve. 



CONCLUSIONS 



The Mid-Atlantic Ridge and the Gorda Ridge have moved to 

 the forefront of research investigating hydrothermal and related 

 processes at oceanic ridges. The recent discovery of black 

 smokers, sizeable polymetallic sulfide deposits, and vent animals 

 at the Mid-Atlantic Ridge stimulates investigation of slow- 

 spreading oceanic ridges in general and the Gorda Ridge in 

 particular owing to its slow-spreading characteristics and 

 accessible location. The sediment-starved northern section and 

 possibly the central section of the Gorda Ridge are tectonically 

 and volcanically active settings favorable for hydrothermal 

 activity anticipated to be associated with mineralization and 

 vent animals at sites localized by the intersection of structural 



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