Ch. 2— Resource Assessments and Expectations • 59 



retreated and advanced across the shelf at least four 

 times. During these glacial periods, high rainfall, 

 probable alpine glaciation in the higher Klamath 

 peaks, and increased stream gradients from lowered 

 base levels all contributed to accelerated erosion of 

 the source area. Concentrations of heavy opaque 

 minerals along the outer edge of the continental 

 shelf off southern Oregon demonstrate the trans- 

 port capacity of the pluvial- glacial streams during 

 low stands of the sea.^^ High discharge and low 

 stands of sea also allow for the formation of chan- 

 nel deposits on the shelf. During high interglacial 

 stands of the sea, estuarine entrapment of sediments 

 is a larger factor in the distribution of heavy min- 

 erals in the coastal environment. Each transgres- 

 sion and regression of the sea has the opportunity 

 to rework relict or previously formed deposits. Pres- 

 ervation of these deposits is related to changes in 

 the energy intensity of their environment. 



While most geologists agree that uplifted beach 

 terrace deposits and submerged offshore deposits 

 are secondary sources of resistant heavy minerals 

 in the formation of placer deposits, questions re- 

 main about which secondary source is more impor- 

 tant. Differing views on the progressive enrichment 

 of placer deposits have implications for locating con- 

 centrations of heavy minerals of economic value. 

 One view is that each sea-level transgression re- 

 works and concentrates on the shelf the heavy min- 

 erals laid down earlier, and any deposits produced 

 during the more recent transgression are likely to 

 be richer or more extensive than the raised terrace 

 deposits that served as secondary sources since their 

 emergence. This concentration effect would espe- 

 cially include those deposits now offshore which 

 could be enriched by a winnowing process that re- 

 moves the finer, lighter material, thereby concen- 

 trating the heavy minerals.^® The other view is that 

 offshore deposits are likely to be reworked as the 

 sea level rises and heavy mineral concentrations in 

 former beaches tend to move shoreward with the 

 transgressing shore zone so that then the modern 

 beaches would be richest in potentially economic 

 heavy minerals. In this view, offshore deposits 

 would be important secondary sources to the mod- 



ern beaches, and raised terraces would be the next 

 richest in heavy minerals.^' 



Prospects for Future Development 



The black sand deposits that were mined for 

 chromite in the past offer a clue as to the nature 

 of the deposits that might be found offshore. Dur- 

 ing World War II, approximately 450,000 tons of 

 crude sand averaging about 10 percent chromite 

 or 5 percent chromic oxide (Cr203) were produced. 

 This yielded about 52,000 tons of concentrate at 

 37 to 39 percent Cr203. The chromium to iron ra- 

 tio of the concentrate was 1.6:1. A number of in- 

 vestigators have examined other onshore deposits. 

 The upraised terraces near Bandon, Oregon, have 

 been assessed for their chromite content with the 

 aid of a drilling program. Over 2.1 million tons of 

 sand averaging 3 to 7 percent CrsOs is estimated 

 for this 15-mile area.'^ Deposit thicknesses range 

 from 1 to 20 feet, and associated minerals include 

 magnetite, ilmenite, garnet, and zircon. 



In a minerals availability appraisal of chromium, 

 the U.S. Bureau of Mines assessed the southwest 

 Oregon beach sands as having demonstrated re- 

 sources (reserve base) of 11,935,000 short tons of 

 mineralized material with a contained Cr203 con- 

 tent of 666,000 tons.^^ In the broader category of 

 identified resources, the Oregon beach sands con- 

 tain 50,454,000 tons of mineralized material with 

 a Cr203 content of 2,815,000 tons. None of the 

 beach sand material is ranked as reserves because 

 it is not economically recoverable at current prices. 

 If recovered, the demonstrated resources would 

 amount to a little over one year's current domes- 

 tic chromium consumption. 



Another indication of the nature of potential Ore- 

 gon offshore deposits comes from studies of coastal 

 terrace placers, modern beach deposits, and off- 

 shore current patterns. In general, longshore cur- 

 rents tend to concentrate heavy minerals along the 

 southern side of headlands. This concentration is 



"Ibid., p. 241. 



^'Bowman, Evaluation of Heavy Mineral Concentrations on the 

 Southern Oregon Continental Shelf, p. 243. 



^'Emery and Noakes, "Economic Placer Deposits on the Continental 

 Shelf," p. 107. 



^^Bowman, "Evaluation of Heavy Mineral Concentrations on the 

 Southern Oregon Continental Shelf," pp. 242-243. 



"J.F. Lemons, Jr., E.H. Boyle, Jr., and C.C. Kilgore, "Chro- 

 mium Availability — Domestic, A Minerals Availability System Ap- 

 praisal," U.S. Bureau of Mines Information Circular, IC 8895, 1982, 

 p. 4. 



