268 



UNITED STATES MINERAL RESOURCES 



Canada and Australia; and does not appear to be 

 rising appreciably in any country. Although several 

 new mines opened in Canada during the last 10 

 years, and other new deposits are likely to be found 

 there and elsewhere, the general trend of output is 

 downward. 



All the examples cited except Alaska Juneau- 

 Treadwell, Mother Lode-Grass Valley, and Bendigo- 

 Ballarat are in Precambrian rocks, mainly metamor- 

 phosed volcanic and sedimentary rocks of the green- 

 stone-granite association, and many of the Siberian 

 lode gold deposits are also in Precambrian rocks. 

 Precambrian terrane appears to be favorable for this 

 type of gold deposit, and it is likely that most new 

 finds will be in areas of Precambrian rocks, such as 

 the Canadian, Brazil-Guyanan, and African-Arabian 

 shields. 



EPITHERMAL ("BONANZA") DEPOSITS 



Examples of the "bonanza" deposits of the North 

 American Cordillera are Goldfield, Virginia City, 

 and Tonopah, Nev. ; Cripple Creek, Telluride, Silver- 

 ton, and Ouray, Colo. ; and El Oro, Pachuca, and 

 Guanajuato, Mexico. Geologically similar deposits 

 occur in Transylvania (Rumania), Nicaragua, the 

 Philippines, New Zealand, and Japan. These deposits 

 are hydrothermal veins of quartz, carbonate miner- 

 als, barite, and fluorite containing gold or gold 

 tellurides and variable but commonly large amounts 

 of silver relative to gold. They were emplaced mainly 

 by filling of open spaces, and most of them are in 

 highly altered volcanic rocks of Tertiary age. They 

 were formed within a few thousand feet of the pres- 

 ent surface and persist to depths of only a few 

 thousand feet at most, generally much less. The gold 

 content of ore mined is extremely variable ; some ore 

 at Goldfield contained 20 ounces of gold per ton, but 

 most ore in these deposits probably contained 0.5-1 

 ounce of gold per ton, somewhat more than the gold- 

 quartz lodes. 



The "bonanza" deposits, although they generally 

 persist only to relatively shallow depths, may con- 

 tain very large amounts of gold and silver. Cripple 

 Creek, Colo., produced about 19 million ounces of 

 gold, and the Comstock district at Virginia City, 

 Nev., produced 8.5 million ounces, as well as more 

 than 200 million ounces of silver. Other sizable de- 

 posits include Goldfield, Nev. (4.2 million oz) ; Tellu- 

 ride, Colo. (4 million oz plus more than 63 million 

 oz of silver) ; Tonopah, Nev. (1.9 million oz plus 

 more than 100 million oz of silver) ; Waihi (4 mil- 

 lion oz) and Thames (1.5 million oz) in the Hauraki 

 region, North Island, New Zealand; and El Oro, 

 Mexico (2 million oz). 



Grold deposits of this general type probably have 

 been mined for thousands of years, first in Egypt 

 and the Balkans. In the United States their exploita- 

 tion dates from around 1860, and from then until 

 the 1920's they were a major source of gold. Four 

 deposits alone — Cripple Creek, Virginia City, Gold- 

 field, and Telluride — have produced more than one- 

 tenth of all United States gold, and from 1860 to 

 1920 they accounted for nearly one-fifth of United 

 States gold production. 



Very little gold is produced at present from "bo- 

 nanza" deposits, and practically none, in the United 

 States. Despite much effort in the United States 

 during the past half century, no new deposit or sig- 

 nificant extension of a known deposit has been dis- 

 covered. Inasmuch as these deposits are probably 

 the most easily discoverable type of bedrock gold 

 deposit, it appears unlikely that they will be a 

 significant factor in future gold production. 



YOUNG PLACERS 



Examples of young placers are deposits along the 

 American, Feather, and Yuba Rivers in the Sierra 

 Nevada of California ; along Alder Gulch at Virginia 

 City, Mont.; on the Yukon River at Fairbanks, 

 Alaska, and on and near the beach at Nome, Alaska ; 

 at Klondike in the Yukon Territory of Canada; in 

 the upper Lena, Yenisey, and Amur Rivers in Si- 

 beria ; and along the east flank of the Andes Moun- 

 tains in Bolivia, Peru, Ecuador, and Colombia. The 

 Tertiary placers of the Sierra Nevada and the geo- 

 logically similar placers at Ballarat, Australia, as 

 well as the Upper Cretaceous to lower Tertiary auri- 

 ferous conglomerates of northwestern Wyoming are 

 also placed in this group; they are older and more 

 consolidated than the typical young placers, but they 

 are more similar to the young than to the fossil 

 placers. 



The young placer deposits are composed primarily 

 of unconsolidated or semiconsolidated sand and 

 gravel that contain very small amounts of native 

 gold and other heavy minerals. Most are stream 

 deposits and occur along present stream valleys or 

 on benches or terraces of preexisting streams. Much 

 less commonly, they are beach deposits, as at Nome, 

 Alaska, or are residual deposits, such as those in 

 the saprolite (deeply weathered rock) of the south- 

 eastern United States. The deposits contain from a 

 few cents to several dollars worth of gold per cubic 

 yard (1 cu yd of gravel weighs about 1.5 tons, and 

 a very rich placer deposit would thus contain only 

 a few grams of gold per ton of gravel) . The largest 

 placers yielded several million ounces of gold; most 

 placers, however, were much smaller. 



