270 



UNITED STATES MINERAL RESOURCES 



gold, have been made during the last 5 years (Worid 

 Mining, 1967). No fossil placer gold district even 

 approaching the size of the Rand has been discov- 

 ered anywhere in the world, but it seems at least 

 possible that some of the ancient shield areas — 

 Canadian, Brazil-Guyanan, or African-Arabian, in 

 particular — may contain other extensive gold-bear- 

 ing conglomerates. Indeed, geologically similar ura- 

 nium-bearing conglomerates were discovered at Blind 

 River (or Elliot Lake) , Ontario, Canada, in 1949, but 

 these contain only minor amounts of gold (Roscoe, 

 1957 ; Bateman, 1958) . No fossil placer deposits have 

 been found in the United States, and extensive field- 

 work done in recent years indicates that none are 

 likely to be exposed at the present surface. 



MARINE PLACERS 



Examples of marine placer deposits that have 

 yielded significant production are lacking. Sediment 

 on the ocean floor in several parts of the world is 

 known to contain small amounts of gold. Most of 

 this gold was derived from the land, transported by 

 streams to the ocean basins, and deposited with 

 clastic sediments on the ocean floor. Some appears 

 to have been derived from the reworking of gold- 

 bearing rocks and sediments on the sea floor. None 

 of these deposits is being mined at present, and the 

 only reported production of gold from a marine 

 placer is a small amount recovered as a byproduct 

 of tin dredging in Malaysia. 



The largest known gold-bearing area on the sea 

 floor is in the northern Bering Sea, where concen- 

 trations of 30-100 ppb have been found over a very 

 broad area including Norton Sound along the south 

 edge of Seward Peninsula in Alaska (U.S. Geol. Sur- 

 vey, 1968, p. A79; 1969, p. 30). On the continental 

 shelf off Oregon and northern California, concentra- 

 tions of gold ranging from 10 to 390 ppb have been 

 found in several areas (Moore and Silver, 1968). 

 These deposits are believed to have formed at or 

 near sea level during a lower stand of the sea. 



Marine placer deposits of tin are exploited on a 

 large scale in Malaysia, Thailand, and Indonesia, but 

 these placers contain only negligible amounts of 

 gold; a small production of gold is reported from 

 West Malaysia. Beach sand deposits of the titanium 

 minerals ilmenite and rutile in Australia, and simi- 

 lar deposits of ilmenite in Ceylon, also are mined 

 on a large scale, but these apparently contain no 

 recoverable gold. 



The future importance of marine gold placers such 

 as those of the Bering Sea is difficult to assess but 

 would appear to be minor. The volume of material 

 available is large, but the gold content is very low, 



and operating conditions are severe. 



DEPOSITS OF DISSEMINATED GOLD 



Examples of deposits of gold disseminated in bed- 

 rock are Carlin (Hardie, 1966), Cortez (Wells and 

 others, 1969) , Getchell (Erickson and others, 1964) , 

 and Gold Acres (Wrucke and others, 1968), all in 

 Nevada (Roberts and others, 1971). These appear to 

 be a new major type of gold occurrence, although 

 deposits geologically somewhat similar to them have 

 long been known, such as Mercur, Utah (Butler, and 

 others, 1920, p. 382-395), and Bald Mountain, Dead- 

 wood, and others, South Dakota (Irving, 1904, p. 

 111-162; Connolly and O'Harra, 1929, p. 142-171). 

 All these deposits were brought into production 

 after 1935, and Cortez was discovered in 1966. The 

 deposits consist of very fine grained gold (0.01-10 

 microns, or 0.00001-0.010 mm) disseminated in silty 

 and carbonaceous dolomitic limestone; at both Car- 

 lin and Cortez the host rock is of Silurian age. Gold 

 is accompanied by silica, a little pyrite and other 

 sulfide minerals, and barite. The ore was formed by 

 hydrothermal replacement of the host rock. It con- 

 tains about 0.3 ounce of gold per ton and very little 

 silver, generally less than 10 percent of the amount 

 of gold, as well as a characteristic suite of trace 

 elements — arsenic, antimony, mercury, and, less 

 commonly, tungsten. Some of the deposits are of 

 Tertiary age, and others may be slightly older; the 

 Carlin deposit has not been dated closely but pre- 

 sumably is within the same age range. 



Because only a few deposits of disseminated gold 

 have been discovered, little is known about the 

 amount of gold this type of deposit may contain. 

 Both the Carlin and the Cortez deposits contain more 

 than 1 million ounces of gold, and the Getchell mine 

 has yielded nearly 0.5 million ounces. 



These deposits have produced only a tiny fraction 

 of U.S. or world gold; from 1936 to 1969, they 

 yielded slightly more than 2 million ounces, less than 

 3 percent of U.S. production during that period. 

 However, they have become an increasingly impor- 

 tant contributor to United States output, and in 

 1969 the Carlin and Cortez mines alone accounted 

 for about 22 percent of U.S. production. 



The future importance of this type of deposit is 

 difiicult to assess on a worldwide basis, but for the 

 United States it is likely to be great. Because of the 

 very fine grained nature of their gold, these deposits 

 went largely unrecognized during the great wave of 

 prospecting in the last half of the 19th century and 

 the early 20th century, and the deposits that were 

 discovered were too low grade to be attractive. The 

 discovery of two major deposits, Carlin and Cortez, 



