The Atlantic Continental Shelf (ACS) includes an area of about 3.91 × 105 km2 and probably contains 3-7 × 1011 m3 of sand and gravel which may locally contain commercially exploitable placer deposits of heavy minerals. Preliminary results of our studies indicate an average of 2.0 percent heavy minerals in these deposits. This contrasts with previous estimates of 0.16 percent heavy minerals in Shelf sand and gravel. In addition, earlier estimates that show a heavy-mineral volume of 1.3 × 109 m3 contained within the sands and gravels are almost certainly low.
Economically valuable heavy-mineral assemblages are prominent in areas on the Shelf. This assemblage consists of titanium oxide minerals, zirconium/ hafnium, and rare-earth-bearing minerals as well as foundry and abrasive minerals and precious metals. Because global. onshore reserves of placer titanium minerals may fall short of demand in as few as 20 years, offshore research in placers will become more important.
U.S. Geological Survey (USGS) research on the nature and distribution of heavy-mineral deposits indicates a high potential for titanium minerals in the central and southern Atlantic Shelf regions. A large number of vibracore and surface grab samples, currently being studied for their heavy-minera1 content, will provide three-dimensional data on such deposits. Further exploration will require a method that overcomes our inability to see or sample effectively titanium deposits buried in the sediments of the continental shelves. New geophysical methods, such as Induced Polarization (IP), have a demonstrated potential to assess rapidly such resources in situ.
President Reagan's declaration of the Exclusive Economic Zone in 1983 provided an impetus to define the hard mineral resources contained within ocean floor sediments and rocks. This declaration amplified the potential for exploiting placer deposits in continental shelf sediments as future sources for strategic and critical mineral commodities.
The United States is dependent on imports from Australia, South Africa, and a number of other countries for about 80 percent of its ilmenite, about 60 percent of its rutile, and virtually all of its monazite (U.S. Bureau of Mines, 1985). Near-shore marine sand deposits and beach-complex sediments including beach, dune, inlet, washover fan, and barrier environments in the Southeastern United States, are important domestic sources of ilmenite (44–70 percent Ti02), leucoxene (altered ilmenite containing up to 90 percent Ti02), rutile (93-96 percent Ti02), zircon (zirconium-hafnium silicate), and monazite (a thorium-rich rare earth phosphate mineral), as well as a number of other heavy-mineral species of economic interest (Thoenen and Warne, 1949; Pirkle and others, 1977; Force and others, 1982). Other commercial-grade deposits are present in modern and ancient shoreline deposits in the Southeastern United States; however, their relatively small size, geographic distribution, and competing land use sometimes preclude commercial exploitation.
Deposits of heavy minerals also have been mined in the coastal plain of New Jersey (Markewicz and others, 1958). These deposits, however, a least partially of fluviodeltaic origin, rather than of beach-complex origin as the deposits in the Southeastern United States are (Garnar, 1978).