584 



UNITED STATES MINERAL RESOURCES 



eral varieties of hoarded and reclaimed silver. The 

 U.S. Treasury stocks totaled 2.1 billion ounces in 

 1958, but have since been depleted by transfer to 

 stockpile, redemption of silver certificates, and sales. 

 This depletion of Treasury silver stocks has in- 

 creased our dependence upon newly mined silver, 

 liquidation of private holdings of silver, and re- 

 claimed silver. Much of the hoarded silver is in coins 

 that have much greater value to coin collectors and 

 speculators than their metal content, unless the price 

 of silver markedly increases. The supply gap of 

 newly mined silver to silver consumption is world- 

 wide, not just domestic, and a large increase in 

 reclaimed silver, release of hoarded silver, and in 

 silver substitutes will be necessary to fill the ex- 

 pected gap within the next decade unless new silver 

 resources are developed. To emphasize current silver 

 needs, the present annual consumption of silver in 

 photography alone is 10 million troy ounces greater 

 than the domestic production prior to the recent 

 decline in mining of western silver-lead ores. Even 

 assuming an increasing replacement of silver by 

 substitutes in the next 30 years, our needs for pho- 

 tography alone will probably increase by 2^/2 times 

 by the year 2000, to more than 100 million troy 

 ounces per year (Ageton, 1970, p. 734). 



EXPLOITATION 



Silver has become a major industrial metal and is 

 widely used in the arts. Figure 66 shows the annual 

 U.S. production, consumption, and average price 

 from 1860 to 1972. The more-than 100 million ounces 

 per year gap between U.S. production and consump- 

 tion is apparent even after the main use of silver for 

 coinage ceased. A smaller but similar gap exists be- 

 tween world production and consumption. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



Silver is a chalcophile element; that is, it will 

 combine with sulfur or related anions rather than 

 form silicate minerals. It has a valence of +1 and 

 an ionic radius of Ag+ 1.13 A. Thus silver is similar 

 in size and valence to copper (Cu+, 0.96 A) and may 

 substitute for it in various minerals. 



The crustal abundance of silver estimated to be 

 0.07 ppm (parts per million) (Vinogradov, 1962). It 

 is most abundant in basalt (0.1 ppm) and igneous 

 rocks of intermediate composition (andesite and 

 diorite, 0.07 ppm), and slightly less in ultramafic 

 rocks (0.05 ppm) and felsic granites (0.05 ppm). Ore 

 deposits in which silver is mined as a major com- 

 ponent are most often associated with igneous rocks 

 of intermediate composition. The silver-lead lime- 



stone replacement deposits that occur so extensively 

 in the Western United States are localized near in- 

 trusive bodies of quartz monzonite and granodiorite. 

 The rich epithermal silver deposits such as Tonopah 

 and Comstock in Nevada are associated with andesite 

 flows and shallow intrusives or with rhyolite, and 

 the rich silver-cobalt ores at Cobalt, Ontario, and 

 the silver ores at Gowganda, Ontario, and Kongs- 

 berg, Norway, are spatially and probably genetically 

 related to diabase intrusives. 



Of the world's total production of silver, about 

 75 percent is a byproduct or coproduct of base-metal 

 ores. In base-metal ores, silver generally has an 

 affinity for galena > chalcopyrite > sphalerite, and 

 the fractionation of silver between coexisting galena 

 and sphalerite ranges from 50:1 to 100:1. Galena is 

 commonly highly argentiferous. Electron micro- 

 probe work on Darwin, Calif., galenas by G. K. 

 Czamanske (oral commun., 1972) indicates approxi- 

 mately 0.2 percent (about 60 troy ounces per short 

 ton) can be present in galena with no microinclu- 

 sions. Experimental studies at elevated temperatures 

 have shown complete solid solutions in the systems 

 AgSbS2-2PbS (miargyrite-galena) , AgBiS2-2PbS 

 (matildite-galena) , and AgBiSz-AgMiSej (matildite- 

 selenium analog) ( Wernick, 1960 ; Van Hook, 1960) . 

 Natural galenas from Darwin, Calif., have been 

 analyzed that contained as much as 3.79 percent 

 silver and 7.86 percent bismuth (Hall, 1971, p. 122) . 

 Ag+ and Sb^+ or Bi^+ thus may form a coupled sub- 

 stitution for 2Pb^+ in galena at the elevated tem- 

 peratures of formation of mesothermal ore deposits 

 (about 350°C). Malakhov (1969) tabulated the anti- 

 mony and bismuth contents of 204 galenas and 

 found that low Sb:Bi ratios are characteristic of 

 high-temperature galena and that high ratios of 

 Sb:Bi are characteristic of low-temperature galena. 



In a few lead-zinc deposits, silver is concentrated 

 in sphalerite rather than galena. In the southeast 

 Missouri lead belt, sphalerite contains as much as 

 45 ounces of silver per ton, probably in solid solution 

 with sphalerite, whereas galena contains about 1 

 ounce of silver per ton (Hall and Heyl, 1968 ; Taylor 

 and Radtke, 1969) . The geochemical controls for the 

 rare concentration of silver in sphalerite rather than 

 galena are not yet understood. 



Silver commonly is greatly enriched in the zone 

 of oxidation, over that in the primary ore, which 

 accounts for many of the shallow bonanza silver 

 deposits that were mined during the latter part of 

 the last century. Only a small part of the primary 

 ore in these deposits was economically minable. 

 Morris and Lovering (1952, p. 698) found at the 

 Tintic Standard mine (Utah) that silver migrates 



