Ch. 2— Resource Assessments and Expectations • 61 



Phosphorite Deposits 



The southern portion of the California border- 

 land is well known for marine phosphorite depos- 

 its. The deposits are located on the tops of the nu- 

 merous banks in areas relatively free of sediment. 

 The phosphorites are Miocene in age and are gen- 

 erally found in water depths between 100 and 1 ,300 

 feet. The deposits consist of sand, pebbles, biologi- 

 cal remains, and phosphorite nodules. Relatively 

 rich surficial nodule deposits averaging 27 percent 

 P2O5 are found on the Coronado, Thirty Mile and 

 Forty Mile banks, and west of San Diego. Estimates 

 based on available data on grade and extent of the 

 major deposits known in the region indicate a re- 

 source base of approximately 72 million tons of 

 phosphate nodules and 57 million tons of phosphatic 

 sands. *^ However, because assumptions were nec- 

 essary to derive these tonnages, these estimates 

 should be regarded as being within only an order 

 of magnitude of the actual resource potential of the 

 area. Further sampling and related investigations 

 are necessary to define the resource base more ac- 

 curately. 



Phosphorite deposits are also found further north 

 off central California at water depths of 3,300 to 

 4,600 feet. These deposits, located off Pescadero 

 Point, on Sur Knoll and Twin Knolls, range in 

 P2O5 content from 1 1 .5 to 31 percent, with an aver- 

 age content of 24 percent.** However, their patchy 

 distribution and occurrence at relatively great water 

 depths make them economically less attractive than 

 the deposits off the southern California shore. 



Polymetallic Sulfide Deposits 



"Polymetallic sulfide" is a popular term used to 

 describe the suites of intimately associated sulfide 

 minerals that have been found in geologically ac- 

 tive areas of the oceanfloor. The relatively recent 

 discovery of the seabed sulfide deposits was not an 

 accident. The discovery confirmed years of research 

 and suggestions regarding geological and geochem- 

 ical processes at the ocean ridges. Research related 



"H.D. Hess, "Preliminary Resource Assessment — Phosphorites 

 of the Southern California Borderland," Program Feasibility Docu- 

 ment — OCS Hard Minerals Leasing, app. 11, U.S. Department of 

 the Interior, 1978, p. 21. 



"H.T. Mullins and R.F. Rasch, "Sea-Floor Phosphorites along 

 the Central California Continental Margin," Economic Geology, vol. 

 80, 1985, pp. 696-715. 



to: 1) separation of oceanic plates, 2) magma up- 

 welling at the ocean ridges, 3) chemical evolution 

 of seawater, and 4) land-based ore deposits that 

 were once submarine, has contributed to and cul- 

 minated in hypotheses of seawater circulation and 

 mineral deposition at ocean spreading centers that 

 closely fit recent observations. Much of the current 

 interest in the marine polymetallic sulfides stems 

 from the dynamic nature of the processes of for- 

 mation and their role in hypotheses of the evolu- 

 tion of the Earth's crust. An understanding of the 

 conditions resulting in the formation of these ma- 

 rine sulfides allows geologists to better predict the 

 occurrence of other marine deposits and to better 

 understand the processes that formed similar ter- 

 restrial deposits. 



Geologic Considerations 



The Gorda Ridge and possibly part of the Juan 

 de Fuca Ridge (pending unsetded boundary claims) 

 are within the EEZ of the United States. They are 

 part of the seafloor spreading ridge system that ex- 

 tends over 40,000 miles through the world's oceans. 

 These spreading centers are areas where molten 

 rock (less dense than the solid, cold ocean crust) 

 rises to the seafloor from depth, as the plates move 

 apart. Plates move apart from one another at differ- 

 ent rates, ranging from 1 to 6 inches per year. 

 Limited evidence suggests that the relative rate of 

 spreading has an influence on the type, distribu- 

 tion, and nature of the hydrothermal deposits 

 formed, and that significant differences can be ex- 

 pected between slow-spreading centers and inter- 

 mediate- to fast-spreading centers.*^ 



The mineralization process involves the interac- 

 tion of ocean water with hot oceanic crust. Simply 

 stated, ocean water percolates downward through 

 fractures in the solid ocean crust. Heated at depth, 

 the water interacts with the rock, leaching metals. 

 Key to the creation of an ore deposit, the metals 

 become more concentrated in the percolating water 

 than they are in the surrounding rocks. The hot 

 (300 to 400 ° centigrade) metal-laden brine moves 

 upward and mixes with the cold ocean water, caus- 

 ing the metals to precipitate, forming sulfide min- 



'^P.A. Rona, "Hydrothermal Mineralization at Slow-Spreading 

 Centers: Red Sea, Atlantic Ocean, and Indian Ocean," Marine Min- 

 ing, vol. 5, No. 2, 1985, pp. 117-145. 



