58 • Marine Minerals: Exploring Our New Ocean Frontier 



Precious Metals 



Placer deposits containing precious metals have 

 been found throughout the Pacific coastal region 

 both offshore and along modern day beaches (fig- 

 ure 2-6). In the south, streams in the southern Cali- 

 fornia borderland drain a coastal region of sand- 

 stone and mudstone marine sediments and granitic 

 intrusives. These source rocks do not offer much 

 hope of economically significant precious metal con- 

 centrations offshore, and fluvial placers have not 

 been important in this area. North of Point Con- 

 ception, gold placers have been worked and addi- 

 tional deposits might be found offshore. 



The most promising region along the Pacific 

 coast of the coterminous States is likely to be off 

 northern California and southern Oregon where 

 sediments from the Klamath Mountains are depos- 

 ited. The Klamath Mountains are excellent source 

 rocks containing, among other units, podiform 

 ultramafic intrusives, which are thought to be the 

 source of the platinum placers found in the region. 

 Gold-bearing diorite intrusives are also present and 

 provide economically interesting source rocks. Plati- 

 num and gold placers have both been mined from 

 beaches in the region. In some areas, small flecks 

 of gold appear in offshore surface sediments. 



Several small gold and platinum beach placers 

 have been mined on the coast of Washington from 

 deposits which may have been supplied by glacially 

 transported material from the north. The Olym- 

 pic Mountains are not particularly noted for their 

 ore mineralization, but gold and chromite-bearing 

 rocks are found in the Cascades. 



Two questions remain: do offshore deposits ex- 

 ist? and, if so, are they economic? For heavier 

 minerals such as gold or platinum, only very fine- 

 grained material is likely to be found offshore. Gold 

 is not uncommon on Pacific beaches from north- 

 ern California to Washington, but is often too fine- 

 grained and too dispersed to be economically re- 

 covered at present. However, some experts also ar- 

 gue that in areas undergoing both uplift and cyclic 

 glaciation and erosion, such as the shelf off south- 

 ern Oregon, there may be several cycles of retrain- 

 ment and progressive transport which could allow 

 even the coarser grains of the precious metals to 

 be transported some distance seaward on the shelf. ^^ 



"K.C. Bowman, "Evaluation of Heavy Mineral Concentrations 

 on the Southern Oregon Continental Shelf," Proceedings, Eighth An- 



Black Sand — Chromite Deposits 



Chromite-rich black sands are found in relict 

 beach deposits in uplifted marine terraces and in 

 modern beach deposits along the coast of southern 

 Oregon. The terrace deposits were actively mined 

 for their chromium content during World War II. 

 Remaining onshore deposits are not of current eco- 

 nomic interest. However, there are indications that 

 offshore deposits may be of future economic inter- 

 est. Geologic factors in the development of placer 

 deposits in relatively high-energy coastal regimes 

 offer clues to chromite resource expectations in the 

 EEZ. 



Geologic Considerations 



The ultimate source of chromite in the black 

 sands found along the Oregon coast of Coos and 

 Curry counties is the more or less serpentinized 

 ultramafic rock in the Klamath Mountains. How- 

 ever much of the chromite in the beach deposits 

 appears to have been reworked from Tertiary 

 sedimentary rocks. ^* Chromite eroded out of the 

 peridotites and serpentines of the Klamath Moun- 

 tains was deposited in Tertiary sediments. Changes 

 in sea level eroded these deposits and the chromite 

 was released again and concentrated into deposits 

 by wind, wave, and current action. These depos- 

 its have been uplifted and preserved in the present 

 terraces and beach deposits. 



This reworking through deposition, erosion, and 

 redeposition is an important consideration in the 

 formation of offshore placer deposits. Not only does 

 reworking allow for the accumulation of more min- 

 erals of economic value over time, but it also al- 

 lows the less resistant (and generally less valuable) 

 heavy minerals such as pyroxenes and amphiboles 

 to break down and thus not dilute or lower the 

 grade of the deposit. 



The river systems in the region were largely re- 

 sponsible for eroding and transporting the heavy 

 minerals from the Klamath Mountains. Once in 

 the marine environment, reworking of minerals was 

 enhanced during periods of continental glaciation 

 when the sea level fluctuated and the shoreline 



nual Conference, Marine Technology Society, 1972, pp. 237-253. 

 '*A.B. Griggs, "Chromite-Bearing Sands of the Southern Part of 

 the Coast of Oregon," U.S. Geological Survey Bulletin, 945-E, pp. 

 113-150. 



