SILVER 



587 



lodes from which it was derived. The difference has 

 generally been explained as loss of silver during 

 weathering, but Mertie (1940) suggested that in 

 part it may be due to a primary difference between 

 the gold in deep-seated parts of the lode and that 

 in the apical part from which the placer gold was 

 derived. 



In addition to silver alloyed with gold, appreciable 

 quantities of silver minerals occur in many gold 

 deposits. Such primary gold deposits grade in many 

 places into predominantly silver deposits, with an 

 increase in silver-bearing minerals such as native 

 silver, tetrahedrite, tennantite, argentite, hessite, 

 pearceite, polybasite, pyrargyrite, proustite, and 

 argentian galena. 



Hypogene manganese-silver deposits are widely 

 known. Most commonly they are silver-lead sulfide 

 deposits with associated manganosiderite, such as 

 at Leadville, Colo., or with associated rhodochrosite, 

 such as at Phillipsburg, Mont. D. F. Hewett and his 

 associates recently recognized another type of hypo- 

 gene manganese-silver deposit, which consists of 

 black silver- and manganese-bearing calcite or of 

 silver-bearing manganese oxides (Hewett, 1964, 

 1968 ; Hewett and others, 1963 ; Hewett and Radtke, 

 1967; Radtke and others, 1967; and Smith, 1970). 

 In the black calcites, the silver is in intergrowths 

 and inclusions of argentian chalcophanite (aurorite) , 

 argentian todorokite, and silver-bearing lead-manga- 

 nese oxides. The deposits on Treasure Hill, Hamilton, 

 Nev., have this mineralogy. 



TYPES OF DEPOSITS 



Types of silver deposits are described here under 

 two broad categories: mineral deposits with by- 

 product and coproduct silver and mineral deposits 

 with silver as a major constituent. Most of the free 

 world's silver comes from deposits of the first cate- 

 gory. 



BYPRODUCT SILVER 



Silver is an important byproduct in (1) porphyry 

 copper deposits, (2) copper-zinc-lead replacement 

 deposits and vein clusters, and (3) massive sulfide 

 deposits ; these three types of deposits account for 

 29 percent of domestic and 20 percent of Canadian 

 silver production. Other types of deposits in which 

 silver is important as a byproduct are (4) lead-zinc 

 replacement deposits, (5) Mississippi Valley- and 

 Alpine-type lead, zinc, and fluorspar deposits and 

 related deposits, (6) copper deposits in sandstones 

 and shales, (7) native copper deposits, (8) gold de- 

 posits in veins, conglomerates, and placers, and (9) 

 nickel and magnetite deposits. 



These deposits are discussed in this chapter pri- 

 marily insofar as they relate to production and 

 resources of silver; for more detailed information 

 the reader is referred to the respective chapters on 

 copper, zinc, lead, and gold. 



PORPHYRY COPPER DEPOSITS 



Porphyry copper deposits are a major source of 

 silver in the United States and supply approximately 

 20 percent of the domestic production. They are an 

 important source of silver in Chile and to a lesser 

 extent in northern Mexico. Porphyry copper de- 

 posits originally were defined as disseminated cop- 

 per mineralization in felsic porphyritic intrusive 

 bodies, but the term has been broadened generally 

 to include all large, low-grade epigenetic hypogene 

 copper, copper-molybdenum, and molybdenum de- 

 posits that can be mass-mined (Titley and Hicks, 

 1966; see also chapter on "Copper"). Although the 

 percentage of silver in the ore is very low — about 

 0.01-0.1 ounce of silver per ton of ore — the total 

 amount of silver that is recovered is very significant 

 because of the vast tonnages mined. The porphyry 

 molybdenum deposits, on the other hand, are vir- 

 tually free of silver except for trace amounts. Dur- 

 ing 1970, the Bingham open-pit mine in Utah, a 

 copper-molybdenum deposit, was the fifth largest 

 silver producer in the United States, with an output 

 of 2.6 million ounces of silver from 40 million tons 

 of ore, or 0.065 of an ounce of silver recovered per 

 ton of ore (Lindstrom, 1971). Arizona contains the 

 largest known concentration of porphyry copper 

 deposits. Fourteen such deposits average 0.06 of an 

 ounce of silver recovered per ton of ore (8 ounces 

 of silver per ton of copper). Known "predicted" re- 

 sources of porphyry copper deposits in Arizona are 

 estimated to be 80 million tons of copper (A. F. 

 Shride, oral commun., 1972) from which an esti- 

 mated 640 million ounces of silver can be recovered. 



In recent years, porphyry copper deposits that con- 

 tain silver have been found in Puerto Rico ; in Maine 

 near Jackman, along the Quebec border; in British 

 Columbia; and in Panama. The deposits in Puerto 

 Rico are relatively lean in silver and contain only a 

 few parts per million of silver per ton of ore (D. P. 

 Cox, oral commun., 1972). Most are not yet being 

 mined. Porphyry copper deposits and copper-molyb- 

 denum deposits are now in production in British 

 Columbia. Porphyry copper deposits in Chile annu- 

 ally yield about 31/0 million troy ounces of silver. 

 Similar deposits in Peru also produce silver. The 

 newly discovered porphyry copper deposits in Irian, 

 Bougainville, New Guinea, and Iran will probably 



