60 • Marine Minerals: Exploring Our New Ocean Frontier 



thought to be the result of differential seasonal long- 

 shore transport and shoreline orientation with re- 

 gard to storm swell approach and zones of deceler- 

 ating longshore currents. In addition, platform 

 gradient also influences the distribution of placer 

 sands, with steeper gradients increasing placer 

 thickness. Similarly, the formation of offshore 

 placer deposits would be determined by paleo- 

 shoreline position and geometry, platform gradient, 

 and paleo-current orientation.*" 



Bathymetric data indicate several wave-cut 

 benches left from former still stands of sea level. 

 Concentrations of heavy minerals that may be re- 

 lated to submerged beach deposits have been found 

 in water depths ranging from 60 to 490 feet. Sur- 

 face samples of these deposits have black sand con- 

 centrations of 10 to 30 percent or more, and some 

 are associated with magnetic anomalies indicating 

 a likelihood of black sand placers within sediment 

 thicknesses ranging from 3 to 115 feet. In addition, 

 gold is found in surface sediments in some of these 

 areas. These submerged features would be likely 

 prospects for high concentrations of chromite and 

 possibly for associated gold or platinum. 



Several Oregon offshore areas containing con- 

 centrations of chromite-bearing black sands in the 

 surface sediment have been mapped. These areas 

 range from less than 1 square mile to over 80 square 

 miles in areal extent, and they are found from Cape 

 Ferrelo north to the Coquille River, with the largest 

 area nearly 25 miles long, centered along the coast 

 off the Rogue River. If metal tenor (content) in- 

 creases with depth, as some investigators expect, 

 there may be considerable potential for economi- 

 cally interesting deposits offshore. Also depending 

 on the value of any associated heavy minerals, chro- 

 mite might be recovered either as the primary prod- 

 uct or as the byproduct of other minerals extraction. 



Other Heavy Minerals 



North of Point Conception in California, a few 

 small ultramafic bodies are found within coastal 

 drainage basins. Heavy mineral fractions in beach 

 and stream sediments are relatively high in titanium 



minerals associated with monazite and zircon, and 

 small quantities of chromite have been found. 

 Titanium minerals have been mined from beach 

 sands in this area in the past. 



The Klamath Mountains of southwestern Ore- 

 gon and northwestern California contain a com- 

 plex of sedimentary, metasedimentary, metavol- 

 canic, granitoid, and serpentinized ultramafic rocks 

 that are the source of most, if not all, of the heavy 

 minerals and free metals found on the continental 

 shelf in that region. In addition to metallic gold, 

 platinum metals, and chromite discussed previ- 

 ously, these minerals include ilmenite, magnetite, 

 garnet, and zircon. Abrasion during erosion and 

 transport of these minerals is minimal, and they 

 are generally resistant to chemical weathering. 



Another area of interest for heavy mineral placer 

 deposits is off the mouth of the Columbia River. 

 The Columbia River drains a large and geologi- 

 cally diverse region and its sediments dominate the 

 coastal areas of northern Oregon and southern 

 Washington. A large concentration of titanium-rich 

 black sand has been reported on the shelf south of 

 the Columbia River.*' Sand from this deposit has 

 been found to average about 5 percent ilmenite and 

 10 to 15 percent magnetite. Several other smaller 

 areas on the Oregon shelf containing high heavy 

 mineral concentrations lie seaward of or adjacent 

 to river systems. Estimates of heavy mineral con- 

 tent on the Oregon shelf suggest a potential of sev- 

 eral million tons each of ilmenite, rutile, and zir- 

 con.*^ 



Chromite, ilmenite, and magnetite cire also found 

 in heavy mineral placers on the Washington coast. 

 Five areas on the Washington shelf contain ano- 

 malously high concentrations of heavy minerals. 

 Three areas south of the Hoh River and off Gray's 

 Harbor are at depths of 60 to 1 70 feet and prob- 

 ably represent beach deposits formed during low 

 stands of the sea. Two more areas are near the 

 mouth of the Columbia River. 



*»C.D. Peterson, G.W. Gleeson, and N. Wetzel, "Stratigraphic 

 Development, Mineral Sources, and Preservation of Marine Placers 

 from Pleistocene Terraces in Southern Oregon, USA," Sedimentary 

 Geology, in press. 



*'R.L. Phillips, "Heavy Minerals and Bedrock Minerals on the 

 Continental Shelf off Washington, Oregon, and California," Program 

 Feasibility Document — OCS Hard Minerals Leasing, app. B, U.S. 

 Department of the Interior, 1979, pp. 14-17. 



"Beauchahip and Cruickshank, "Placer Minerals on the U.S. Con- 

 tinental Shelves — Opportunity for Development," p. 700. 



