GOLD 



267 



erals in placer deposits. Gold is soluble in hydro- 

 chloric acid in the presence of manganese dioxide 

 but is appreciably so only at concentrations of acid 

 so high as to occur only rarely in nature (Kraus- 

 kopf, 1967, p. 523-526). 



MINERALOGY 

 Gold occurs mainly as native metal, always alloyed 

 with variable amounts of silver and other metals, 

 mainly copper and iron. Other than native gold, the 

 only important gold minerals are the tellurides, com- 

 posed of gold or gold plus silver, copper, or lead com- 

 bined with tellurium; calaverite (AuTez) and syl- 

 vanite ([Au,Ag]Te2) are the most common of these 

 minerals. Other gold minerals are rare. Gold also 

 occurs in small amounts in many minerals, generally 

 less than 5 ppm in suliide minerals, and less than 1 

 ppm in other minerals; however, as much as 500 

 ppm has been reported from sphalerite (zinc sul- 

 fide) (Jones and Fleischer, 1969, p. 11) . 



TYPES OF GOLD DEPOSITS 

 Gold occurs in many and varied geologic environ- 

 ments, which are grouped into seven broad cate- 

 gories in this report. 



Historically, the most productive general types of 

 deposits have been gold-quartz lodes and a subcate- 

 gory of these, the epithermal ("bonanza") deposits 

 (mainly in the North American Cordilleran region) ; 

 geologically young placers; and ancient (fossil) 

 placers. Other types are marine placers; bedrock 

 deposits of disseminated gold, a type represented by 

 several recent discoveries in Nevada ; and deposits of 

 other metals or minerals which produce gold only as 

 a byproduct (Internat. Geol. Cong., 1930; Lindgren, 

 1933; Emmons, 1937; Cooke, 1946; Canadian Inst. 

 Mining and Metall., 1948 ; Bateman, 1950 ; Common- 

 wealth Mining and Metall. Cong., 6th, 1957; Com- 

 monwealth Mining and Metall. Cong., 8th, 1965 ; De 

 Kun, 1965 ; Koschmann and Bergendahl, 1968 ; Cana- 

 da Geol. Survey, 1970 ; Park and MacDiarmid, 1970 ; 

 Roberts and others, 1971) . 



GOU3-QUARTZ LODES 



Examples of gold-quartz lodes are Mother Lode- 

 Grass Valley, Calif.; Homestake, S. Dak.; Central 

 City, Colo. ; Alaska Juneau-Treadwell, Alaska ; Por- 

 cupine and Kirkland Lake-Larder Lake, Ontario, 

 Canada; Morro Velho, Minas Gerais, Brazil; Kolar, 

 Mysore, India; Kalgoorlie, Western Australia; Ben- 

 digo-Ballarat, Victoria, Australia; Ashanti, Ghana; 

 the Bulawayo and Hartley belts, Rhodesia; and the 

 Kilo and Moto districts, Zaire. This category com- 

 prises a wide variety of deposits, but these are essen- 



tially hydrothermal veins of quartz and gold that 

 either replace wallrock (Homestake) or fill open 

 spaces along fracture zones (Mother Lode). They 

 were formed at great depths below the earth's sur- 

 face and persist to depths of thousands of feet below 

 the present surface, and most of them are enclosed 

 in Precambrian rocks and are of Precambrian age. 

 Alteration of wallrocks ranges from slight (Home- 

 stake) to widespread and pervasive (Alaska Ju- 

 neau). The gold content of ores mined ranges from 

 about 0.3 to about 0.6 ounce per ton ; only rarely has 

 much ore containing more than 0.6 ounce per ton 

 been found, and conversely in only a few mines has 

 ore containing as little as 0.1 ounce per ton been 

 mined profitably. 



The size of the gold-quartz lodes varies greatly, 

 and data on individual deposits or mines are difficult 

 to obtain for many of the great gold-mining dis- 

 tricts. Many mines have produced at least 1 million 

 ounces (North Star and Empire mines. Grass Valley, 

 Calif.) ; an appreciable number have yielded 2-5 

 million ounces (Alaska Juneau; Brownhill-Oroya 

 and probably several other mines at Kalgoorlie, as 

 well as at Bendigo and Ballarat, Australia) ; a few 

 have produced 5-20 million ounces (Hollinger, Mcln- 

 tyre, and Dome mines. Porcupine, Ontario ; Kirkland 

 Lake zone, including the Lake Shore and other mines, 

 Kirkland Lake, Ontario; Kerr-Addison mine at 

 Larder Lake, Ontario; Morro Velho, Brazil) ; and at 

 least two have produced more than 20 million ounces 

 (Homestake, S. Dak., and Kolar, India). Homestake 

 is the most productive lode gold mine in the world 

 and has a total production of about 30 million ounces 

 since its discovery in 1876. It is of interest to note, 

 however, that the total output of the Homestake 

 mine is about equal to 1 year's production at present 

 rates from the Witwatersrand district (fossil plac- 

 er) of South Africa. 



The percentage of world gold production attribut- 

 able to these deposits is difficult to estimate. Most of 

 their production probably has come during the 20th 

 century, although some of them, particularly the 

 California deposits, were mined before 1900, and a 

 few, such as Kolar, India, may have been worked 

 2,000 years ago or more. These lode deposits have 

 accounted for perhaps 25 percent of the world's gold 

 production since 1900, and in 1969 they yielded 20- 

 25 percent of the world output. They have produced 

 about 25-30 percent of all the gold mined in the 

 United States, and since 1950 they have produced 

 nearly one-half of the U.S. gold output. 



Their future importance appears easier to assess. 

 Production from these deposits is fairly steady in 

 the United States, Brazil, and India ; is decreasing in 



