Ch. 2— Resource Assessments and Expectations • 49 



because they have different hydrauhc behavior than 

 less dense materials they can become concentrated 

 into mineable deposits. 



In addition to hydraulic behavior, a number of 

 other factors influence the distribution and char- 

 acter of placer deposits on the continental shelf and 

 coastal areas. These factors include sources of the 

 minerals, mechanisms for their erosion and trans- 

 port, and processes of concentration and preserva- 

 tion of the deposits. 



While placer minerals can be derived from pre- 

 viously formed, consolidated, or unconsolidated 

 sedimentary deposits, their primary source is from 

 igneous and metamorphic rocks. Of these rocks, 

 those that had originally been enriched in heavy 

 minerals and were present in sufficiently large 

 volumes would provide a richer source of material 

 for forming valuable placer deposits. For example, 

 chromite and platinum- group metals occur in ultra- 

 mafic rocks such as dunite and peridotite, and the 

 proximity of such rocks to the coast would enhance 

 the possibility of finding chromite or platinum 

 placers. While small podiform peridotite deposits 

 are found from northern Vermont to Georgia, 

 ultramafic rocks are not overly common in the At- 

 lantic coastal region. Consequently, the prospects 

 for locating chromite or platinum placers in surfi- 

 cial sediments of the Atlantic shelf would be low. 

 Other rock types, such as high-grade metamorphic 

 rocks, would be a likely source of titanium minerals 

 such as rutile, and high-grade metamorphic rocks 

 are found throughout the Appalachians. Placer de- 

 posits are generally formed from minerals dispersed 

 in rock units, when great amounts of rock have been 

 reduced by weathering over very long periods of 

 time. 



Time is a factor in the formation of placer de- 

 posits in several respects. In addition to their 

 chemistry, the resistance of minerals to weather- 

 ing is time and climate dependent. In a geomor- 

 phologically mature environment where a broad 

 shelf is adjacent to a wide coastal plain of low re- 

 lief, such as the middle and southern Atlantic mar- 

 gin, the most resistant heavy minerals wUl be found 

 to dominate placer deposit composition. These 

 would include the chemically stable placer minerals 

 such as the precious metals, rutile, zircon, mona- 

 zite, and tourmaline. Less resistant heavy minerals, 

 such as amphiboles, garnets, and pyroxenes, which 



are more abundant in igneous rock, dominate 

 heavy mineral assemblages in more immature tec- 

 tonically active areas such as the Pacific coast. 

 These minerals are currently of less economic in- 

 terest. 



Placer deposits are frequently classified into three 

 groups based on their physical and hydraulic char- 

 acteristics. The first group is the heaviest minerals 

 such as gold, platinum, and cassiterite (tin oxide). 

 Because of their high specific gravities, which range 

 from 6.8 to 21, these minerals are deposited fairly 

 near their source rock and tend to concentrate in 

 stream channels. For gold and platinum, the me- 

 dian distance of transport is probably on the order 

 of 10 miles. ^ Heavy minerals with a lighter specific 

 gravity, in the range of 4.2 to 5.3, form the sec- 

 ond group and tend to concentrate in beach depos- 

 its; but they also can be found at considerable dis- 

 tances from shore in areas where sediments have 

 been worked and reworked through several ero- 

 sional and depositional cycles. Minerals of eco- 

 nomic importance in this group include chromite, 

 rutile, ilmenite, monazite, and zircon.' The third 

 group is the gemstones of which diamonds are the 

 major example. These are very resistant to weather- 

 ing, but are of relatively low specific gravity in the 

 range of 2.5 to 4.1. 



As a first step in assessing placer minerals re- 

 sources potential in the Canadian offshore, a set 

 of criteria was developed and the criteria were listed 

 according to their relative importance.'" A rank- 

 ing scheme was then adopted to assess the impli- 

 cations of each criterion with regard to the likeli- 

 hood of a placer occurring offshore (table 2-3). This 

 approach can be applied to the U.S. EEZ. 



"K.O. Emery and L.C. Noakes, "Economic Placer Deposits on 

 the Continental Shelf," United Nations Economic Commission for 

 Asia and the Far East, Technical Bulletin, vol. 1, 1968, pp. 95-111. 



'Rutile and ilmenite are major titanium minerals (along with leu- 

 coxene), and monazite is a source of yttrium and rare earth elements 

 which have many catalytic applications in addition to uses in metal- 

 lurgy, ceramics, electronics, nuclear engineering, and other areas. Zir- 

 con is used for facings on foundry molds, in ceramics and other refrac- 

 tory applications, and in several chemical products. Zircon is also 

 processed for zirconium and halfnium metal, which are used in nu- 

 clear components and other specialized applications in jet engines, 

 reentry vehicles, cutting tools, chemical processing equipment, and 

 superconducting magnets. 



'"P.B. Hale and P. McLaren, "A Preliminary Assessment of Un- 

 consolidated Mineral Resources in the Canadian Offshore," CIM 

 Bulletin, September 1984, p. 11. 



