Ch. 5— Mining and At-Sea Processing Tectinologies • 197 



Figure 5-19.— Offshore Chromite Sands, 

 Oregon Continental Shelf 



Coos Bay 



Pacific 

 Ocean 



r 



Cape Oregon 



Scale 



6 

 Miles 



12 



More than 20% 



heavy minerals 



in sand 



SOURCE; Adapted from T. Parmenter and R. Bailey, The Oregon Ocean Book 

 (Salem, OR: Oregon Department of Land Conservation and Develop- 

 ment, 1985), p. 21. 



Operational and Geological Characteristics. — 



The site selected for this scenario Hes seaward of 

 the Rogue River, from 2 to 4 miles offshore. Water 

 depths in the vicinity of the mine site are between 

 150 and 300 feet. The main deposit is assumed to 

 be roughly 22 miles long by 6 miles wide and strad- 

 dles the boundary between State and Federal 

 waters. 



Summer waves, generally from the northwest, 

 are driven by strong onshore winds and range in 

 height from 2 to 10 feet. In winter, waves are 

 characteristically from the west or southwest and 

 average 3 to 20 feet. The most severe storms, which 

 occur from November through March, may occa- 

 sionally produce wave heights in excess of 60 feet. 

 The severity of the wave regime off the coast of Ore- 

 gon has been compared to that of the North Sea. 

 In addition to weather, a seasonal factor that may 

 affect mining activity is prior use of the area by sea 

 lions as a breeding ground and by salmon fisher- 

 men for sport and commercial fishing. 



Coastal terrace deposits between Coos Bay and 

 Bandon, north of the scenario site, are likely ana- 

 logs of potential continental shelf placers (see ch. 

 2). Most samples taken from these deposits have 

 contained from 6 percent to as much as 13 percent 

 chromite, usually concentrated in the bottom 3 to 

 15 feet of the stratigraphic section, although sam- 

 ples containing as much as 25 percent chromite 

 have been taken in some places.'^ 



This scenario assumes that offshore placers con- 

 tain similar grades of chromite and that the aver- 

 age grade is closer to 6 percent. Magnetic anomaly 

 studies associated with surface concentrations in the 

 scenario area suggest that the potential placer bodies 

 lie beneath a sediment overburden that ranges from 

 less than 3 feet to more than 100 feet thick. The 

 ore body thickness at the mining site is assumed 

 to be less than 25 feet. 



Mining Technology. — This scenario assumes 

 that the chromite placers are largely unconsolidated 

 deposits and that a trailing suction hopper dredge 

 similar to the one used in the titanium sands sce- 

 nario is applicable for mining. The dredge is 

 equipped with twin 3,400-horsepower suction 

 pumps, giving it a greater suction capacity than the 

 dredge used to mine titanium sands. 



Dredging in rough seas at depths ranging from 

 150 to 300 feet will require a special design; how- 

 ever, it is assumed this need wUl not present greater 

 technical problems or costs than, for example, 

 building dredges or pipe-laying vessels for the North 

 Sea. The dredge is similar in its other characteris- 

 tics to the hopper dredge described in the titanium 

 sands scenario. 



At-Sea Processing. — High volumes of ore can 

 be brought to the surface at relatively low cost, but 

 transporting the material to shore is costly. There- 

 fore, there is an incentive to enrich the ore as much 

 as economically and technically feasible prior to 

 transporting it to shore. This scenario assumes pri- 

 mary beneficiation at sea by a simple, low-cost proc- 

 ess of screening and gravity separation. The sys- 

 tem might incorporate devices such as cones, jigs, 

 spirals, or a very large sluice box. As in the titanium 



"LaVerne D. Kulm, College of Oceanography, Oregon State 

 University, OTA Workshop on Pacific Minerals, Newport, Oregon, 

 Nov. 20, 1986. 



