52 • Marine Minerals: Exploring Our New Ocean Frontier 



concentrations of heavy minerals parallel to the 

 present shoreline off the Virginia coast in water 

 depths between 30 and 60 feet, hypothesized sources 

 from the Chesapeake Bay and the Delaware River.'* 

 The deposit was thought to be a possible ancient 

 strandline where the heavy minerals were concen- 

 trated by hydraulic fractionation. 



Bottom topography may be an important clue 

 to surface concentrations of heavy minerals. One 

 investigation off Smith Island near the mouth of 

 Chesapeake Bay found high concentrations of heavy 

 minerals on the surface of a layer of fine sand that 

 was distributed along the flanks of topographic 

 ridges. '^ However, coring data are needed to pro- 

 vide information on the vertical distribution of 

 placer minerals and on whether or not similar bur- 

 ied topography is preserved and contains similar 

 heavy mineral concentrations. 



Overall, the south Atlantic EEZ would be a 

 favorable prospective region for titanium placers, 

 based on maturity of heavy mineral assemblages, 

 although sediment cover is thinner and more patchy 

 than farther north. However, individual features 

 such as submerged sand ridges could contain con- 

 centrated deposits. 



As with sand and gravel, regional resource esti- 

 mates are probably not very useful since they are 

 based on gross generalizations. This caveat notwith- 

 standing, recent studies indicate that the average 

 heavy mineral content of sediments on the Atlan- 

 tic shelf is on the order of 2 percent, and that the 

 total volume of sand and gravel may be larger than 

 earlier estimates.'^ '^ These studies suggest that 

 whatever the total offshore resource base is esti- 

 mated to be, the southern Atlantic EEZ may hold 

 considerable promise for titanium placer deposits 

 of future interest, particularly in areas of paleo- 

 stream channels where there are major gaps in the 



'*B.K. Goodwin and J.B. Thomas, "Inner Shelf Sediments Off of 

 Chesapeake Bay III, Heavy Minerals," Special Scientific Report No. 

 68, Virginia Institute of Marine Science, 1973, p. 34. 



'^C.R. Berquist and C.H. Hobbs, "Assessment of Economic Heavy 

 Minerals of the Virginia Inner Continental Shelf," Virginia Division 

 of Mineral Resources Open-File Report 86-1, 1986, p. 17. 



"U.S. Department of the Interior, Program Feasibility Document: 

 DCS Hard Minerals Leasing, prepared for the Assistant Secretaries 

 of Energy and Minerals and Land and Water Resources by the OCS 

 Mining Policy Phase II Task Force, August 1979, Executive Sum- 

 mary, p. 40. 



"Grosz, Hathaway, and Esowitz, "Placer Deposits of Heavy 

 Minerals in AUantic Continental Shelf Sediments," p. 387. 



Trail Ridge formation (a major onshore titanium 

 sand deposit). In any event, only high-grade, acces- 

 sible deposits would be potentially attractive, and 

 the total heavy mineral assemblage would deter- 

 mine the economics of the deposit. 



Phosphorite Deposits 



Sedimentary deposits consisting primarily of 

 phosphate minerals are called phosphorites. The 

 principal component of marine phosphorites is car- 

 bonate fluorapatite. Marine phosphorites occur as 

 muds, sands, nodules, plates, and crusts, gener- 

 ally in water depths of less than 3,300 feet. Phos- 

 phatic minerals are also found as cement bonding 

 other detrital minerals. Marine phosphorite deposits 

 are related to areas of upwelling and high biopro- 

 ductivity on the continental shelves and upper 

 slopes, particularly in lower latitudes. 



Bedded phosphorite deposits of considerable areal 

 extent are of major economic importance in the 

 Southeastern United States. The bedded deposits 

 in the Southeastern United States are related to 

 multiple depositional sequences in response to 

 transgressive and regressive sea level changes.'^ 

 Major phosphate formation in this region began 

 about 20 million years ago during the Miocene. 

 Low-grade phosphate deposits are found in young- 

 er surficial sediments on the continental shelf, but 

 these are largely reworked from underlying units. 

 While these surface sediments are probably not of 

 economic interest, they may be important tracers 

 for Miocene deposits in the shallow subsurface. 



On the Atlantic shelf, the northernmost area of 

 interest for phosphate deposits is the Onslow Bay 

 area off North Carolina. (Concentrations of up to 

 19 percent phosphate have been reported in relict 

 sediments on Georges Bank, but these are unlikely 

 to be of economic interest.) In the Onslow Bay area, 

 the Pungo River Formation outcrops in an north- 

 east-southwest belt about 95 miles long by 15 to 

 30 miles wide and extends into the subsurface to 

 the east and southeast. The Pungo River Forma- 

 tion is a major sedimentary phosphorite unit un- 

 der the north-central coastal plain of North Caro- 



'=S.R. Riggs, D.W. Lewis, A.K. Scarborough, etal., "Cyclic Depo- 

 sition of Neogene Phosphorites in the Aurora Area, North Carolina, 

 and Their Possible Relationship to Global Sea-Level Fluctuations," 

 Southeastern Geology, vol. 23, No. 4, 1982, pp. 189-204. 



