SILVER 



591 



smaller quantity of silver comes from southwestern 

 Oklahoma deposits. 



Lenticular, bedded, and roll deposits of dissemi- 

 nated chalcocite and pyrite, that locally contain 

 uranium and vanadium minerals, occur in sand- 

 stones, mostly of Mesozoic age, in eastern New 

 Mexico, southeastern Colorado, and in the Colorado 

 Plateau province of Colorado, New Mexico, Utah, 

 and Arizona (Fischer, 1937). Similar deposits are 

 known in northeastern Utah, southern Idaho, and 

 northeastern Pennsylvania. Most of these deposits 

 are lean in silver, but contain small quantities in 

 ores that have been mined for their copper content. 

 Silver might be recovered from such ores in the 

 future. 



Disseminated chalcocite, bornite, covellite, and 

 chalcopyrite, locally with a little native copper and 

 silver, occur in beds, veins, breccia zones, and along 

 dikes in sandstones of Triassic age in Connecticut, 

 New Jersey, Pennsylvania, Maryland, and Virginia, 

 and in the Cashin mine in southwestern Colorado 

 (Emmons, 1906), and in nearby deposits in Utah. 



The existence of possible economic concentrations 

 of copper in beds of the Precambrian Belt Super- 

 group has been generally recognized only recently. 

 (See chapter on "Copper.") One deposit has been 

 developed to the point where it could be mined when 

 desired, and other prospects are in various stages of 

 exploration. These deposits are said to average about 

 1 percent copper and to contain about 2 ounces of 

 silver per ton for each percent of copper. Much work 

 remains to be done before any realistic forecast can 

 be made of the silver resources of the Belt Basin. 

 They obviously could constitute an important re- 

 source of both silver and copper. Company explora- 

 tion geologists stated (A. E. Weissenbom, oral 

 commun., 1971) that the expected size of individual 

 deposits may be as much as 60-100 million tons. 



NATIVE COPPER DEPOSITS 



Very large deposits of native copper and asso- 

 ciated native silver occur in amygdaloidal basalt 

 flows and conglomerates on the Keweenaw Penin- 

 sula, northern Michigan; probably they were de- 

 posited by heated chloride-rich brines that rose 

 updip from beneath the present location of Lake 

 Superior (White, 1968). The deposits have been 

 productive from prehistoric times until 1970, when 

 the last mines were closed after having yielded 

 nearly 5.4 million tons of copper and a very sub- 

 stantial quantity of silver. Most of the silver pro- 

 duction was not recorded, however, because it was 

 retained in the "Lake copper" produced, which con- 

 tains as much as 8 ounces of silver per ton. The 



remaining resources in this district are very large, 

 but mostly low grade, and are awaiting better metal 

 prices and improved mining and recovery methods. 



GOLD DEPOSITS IN VEINS, CONGLOMERATES, AND PLACERS 



Silver is alloyed with gold in all precious-metal 

 deposits. The amount of silver is indicated by the 

 fineness of the gold, which is the proportion of gold 

 to silver expressed in parts per thousand (thus, for 

 example, "900 fine" means a gold to silver ratio of 

 9:1, or 10 percent silver). Mesothermal and hypo- 

 thermal gold-quartz veins generally have more gold 

 than silver, whereas the epithermal precious-metal 

 and the shale and sandstone deposits have more sil- 

 ver than gold. 



Gold-quartz veins are found extensively in meta- 

 morphic terranes around the world: in Western 

 United States and Alaska ; in the Precambrian shield 

 of Canada and the United States; at Victoria, Au- 

 stralia; and in the Precambrian areas of South 

 Africa, the Ural Mountains, the Austrian Alps, 

 Brazil, India, and Siberia. 



The most extensive gold-quartz veins in the United 

 States are in California along the belt of Mesozoic 

 granitic rocks. The most productive was the Mother 

 Lode Belt, which extends for a distance of 130 miles 

 on the west side of the Sierra Nevada batholith in 

 a folded belt of metamorphosed Paleozoic and Meso- 

 zoic volcanic and sedimentary rocks. The Mother 

 Lode is a vein system with individual veins as much 

 as 1 to 2 miles long; some veins have been mined 

 5,000 to 10,000 feet down the dip. 



The gold-quartz veins in the Kirkland Lake and 

 Porcupine districts in the Canadian Shield have 

 many similarities to the Mother Lode veins. They 

 occur in tightly folded Keewatin andesite and basalt 

 flows, and the mineralogy of the veins is simple. The 

 silver to gold ratios range from 0.11 to 0.25. 



The Homestake mine. South Dakota, produced 

 approximately 28 million ounces of gold and 6I/2 

 million ounces of silver between 1878 and 1965, a 

 gross of nearly $1 billion at today's prices (A. L. 

 Slaughter, in Ridge, 1968, p. 1437). The ore occurs 

 as a replacement of isoclinally folded Precambrian 

 Homestake Formation (Hosted and Wright, 1923), 

 and consists of several percent of pyrrhotite, arseno- 

 pyrite, and pyrite in a calcite-quartz-chlorite-anker- 

 ite gangue. 



Other important deposits of gold-quartz veins in 

 the United States that have yielded byproduct silver 

 include those in the Colorado mineral belt, the Appa- 

 lachian Piedmont gold belts in the southeastern 

 United States, the Neihart district, Montana, Mogol- 

 lon district. New Mexico, Wallapai district, Arizona, 

 the London vein system. Alma district, Colorado, and 



