GOLD 



269 



Placer deposits were easy to find and easy to mine ; 

 consequently they were discovered early and were 

 a very important source of gold in the ancient world 

 and indeed as late as the 19th century. Their con- 

 tribution to total world production cannot be esti- 

 mated closely, but it probably is between one-fourth 

 and one-third of the total, or somewhere between 

 750 million and 1 billion ounces. 



Since the beginning of gold mining in the United 

 States, placer deposits have yielded about 114 mil- 

 lion ounces of gold, or about 36 percent of all gold 

 mined. Of this amount, about 68 miUion ounces have 

 come from the Sierra Nevada placers of California — 

 about 53-54 million ounces from young placers and 

 14-15 million ounces from Tertiary placers (Merwin, 

 1968) . Other large U.S. placer deposits include those 

 of Fairbanks, Alaska (7.2 million oz) , Nome, Alaska 

 (3.5 miUion oz), Virginia City, Mont. (2.5 million 

 oz) , and Boise Basin, Idaho (2.3 million oz) (Kosch- 

 mann and Bergendahl, 1968). 



Production from placer mines has been declining 

 for many years, and at present the mines yield sig- 

 nificant amounts of gold only in the U.S.S.R. and 

 Colombia ; they probably account for 5-10 percent of 

 the world production. Placer gold production in the 

 United States in 1969 was only 25,418 ounces, a little 

 more than 1 percent of the total U.S. output. The 

 reasons for this decline are varied — ease of discovery 

 and exploitation and consequent early exhaustion, 

 increasing costs of operations, and, particularly in 

 the United States, environmental problems such as 

 water pollution and siltation. 



Placer deposits are not expected to be a significant 

 factor in future U.S. gold production. The country 

 has been so well explored that probably no large 

 deposits remain undiscovered. Moreover, regulations 

 dealing with stream pollution and water use will 

 probably become more stringent rather than less so. 

 Both these factors make it most unlikely that the 

 current downward trend in placer production will 

 be reversed. Worldwide, the situation appears simi- 

 lar to that in the United States. In Colombia, placer 

 production has been significant, but it has been de- 

 clining gradually and was less than 200,000 ounces 

 in 1969, or only about half the output of 10 years 

 ago. Little is known about production of placer gold 

 from the U.S.S.R. in recent years; perhaps 50 per- 

 cent of the total production of 290-350 million 

 ounces (estimated) has come from placers, but the 

 percentage derived from these deposits at present 

 is not known. Other formerly productive placers are 

 either virtually exhausted (Egypt, Spain, Australia, 

 Canada) or are being worked on a much reduced 

 scale (Brazil, Peru, Bolivia). 



ANCIENT (FOSSIL) PLACERS 



Examples of ancient (fossil) placers are the Wit- 

 watersrand district. Republic of South Africa; Tar- 

 kwa, Ghana ; and the Serra de Jacobina, Bahia, Bra- 

 zil. These deposits are placers that formed in the 

 geologically distant past (all examples are of Pre- 

 cambrian age) and have been lithified to conglom- 

 erate to become part of the bedrock ; they may thus 

 be termed "fossil placers" in the same sense as the 

 remains of organisms found in rocks are called 

 fossils. These conglomerates consist of small well- 

 rounded pebbles of quartz embedded in a matrix of 

 pyrite and micaceous minerals. In addition, they con- 

 tain a large suite of heavy and resistant minerals, 

 the most valuable of which are gold, uraninite, and 

 platinum-group metals. The conglomerates were laid 

 down originally on the earth's surface, but they 

 were buried and are now tilted to varying degrees. 

 The individual conglomerate beds generally are only 

 a few feet thick but have lateral extents measured 

 in miles and are known to reach depths of more 

 than 2 miles below the earth's surface. Their gold 

 content ranges from 0.2 to 0.8 ounce per ton; the 

 average grade of gold ore milled in the Witwaters- 

 rand district between 1893 and 1969 was about 0.35 

 ounce per ton (Chamber of Mines of South Africa, 

 1969, p. 60), at Tarkwa the ore averages 0.2 ounce 

 per ton (De Kun, 1965, p. 91), and at Jacobina the 

 gold content of ore mined is about 0.5 ounce per ton 

 (Gross, 1968, p. 272). 



Although the Jacobina deposits were mined as 

 early as the 17th century, this type of deposit did 

 not produce any appreciable part of the world's gold 

 until discovery of the Witwatersrand district in 

 1886. Since then this great district has become an 

 increasingly important source of gold, and since 

 1954 it has produced more than one-half of the 

 world's gold output exclusive of the U.S.S.R. It 

 seems safe to say that no other single mining dis- 

 trict has dominated the world output of a mineral 

 commodity so completely and for so long as the Rand 

 has dominated gold production; in 1970, for exam- 

 ple, 67 percent of the world's newly mined gold 

 came from Rand mines. 



The future importance of the fossil placer deposits 

 can hardly be overestimated. The very large re- 

 serves — about 600 million ounces according to sev- 

 eral recent estimates (Gold Producers Comm., 

 1967(7), p. 41; Kavanagh, 1968, p. 557; U.S. Bur. 

 Mines, 1972, p. 61) — of the Witwatersrand indicate 

 that the district will continue to be the principal 

 source of the world's gold for many years, even 

 though some dire predictions about the life of the 

 district, based on a fixed price of $35 per ounce for 



