592 



UNITED STATES MINERAL RESOURCES 



the Gold Mountain quartz veins in Tertiary tuffa- 

 ceous sediments, pyroclastics, flows, and associated 

 intrusives in the Republic district of Washington 

 have yielded about 71/2 million ounces of silver as a 

 byproduct of gold mining. Epithermal gold-quartz 

 veins in silicified zones in the Paleocene Swauk For- 

 mation at Wenatchee, Wash., have also yielded con- 

 siderable byproduct silver. 



Gold-bearing conglomerate deposits in the Wit- 

 vv^atersrand district, Transvaal, South Africa, have 

 yielded more gold than the total gold produced in 

 Alaska, California, and Colorado. Similar conglom- 

 erates have been mined in West Africa at Tarkwa 

 and Abosso. The fineness of the gold in tvi'o Wit- 

 watersrand mines ranges predominantly from 750 

 to 925 (Von Rahden, 1965, p. 983). 



All placer gold contains byproduct silver, although 

 the amounts in the gold are much smaller than in the 

 nearby lodes, and the total quantity per placer is 

 small. The placers on the vilest side of the Sierra 

 Nevada, and near Fairbanks, Alaska, and the Yukon, 

 Canada, however, have yielded appreciable quanti- 

 ties of silver. Future production of large amounts of 

 silver from gold placers is unlikely because of the 

 low grade of the remaining gravels and because of 

 environmental considerations. 



NICKEL AND MAGNETITE DEPOSITS 



About a million ounces of silver is produced an- 

 nually from the nickel ore bodies at Sudbury, On- 

 tario. (See chapter on "Nickel.") The silver occurs 

 in small amounts in chalcopyrite, and in galena of 

 the mineralogically more complex "oifset deposits," 

 which are probably hydrothermal. Production is ex- 

 pected to continue for many years. 



At Cornwall and Morgantown, Pa., silver is re- 

 covered as a byproduct, along with gold, copper, and 

 cobalt, from high-temperature hydrothermal magne- 

 tite deposits. (See chapter on "Iron.") The silver 

 occurs in small quantities in chalcopyrite that is 

 common in the ores. A small but steady output has 

 come from these deposits for many years. Although 

 Cornwall is approaching exhaustion, Morgantown is 

 a major new mine and is expected to continue 

 production. 



SILVER AS A MAJOR CONSTITUENT 



Mineral deposits in which silver is the main con- 

 stituent include a wide variety of types, and they 

 constituted some of the most productive deposits in 

 the notable districts that were mined in the 19th 

 century. They are no longer very productive except 

 in a few major districts. These silver-rich deposits 

 are (1) epithermal veins, lodes, and pipes, (2) epi- 



thermal disseminated and breccia deposits, (3) epi- 

 thermal silver-manganese deposits, (4) epithermal 

 silver-lead-zinc replacement deposits, (5) epither- 

 mal silver-copper-barite deposits, (6) mesothermal 

 silver-lead-zinc-copper deposits, (7) mesothermal 

 cobalt-silver, cobalt-uraninite-silver, and cobalt- 

 silver-zeolite deposits, (8) sandstone silver deposits, 

 and (9) sea-floor muds and hot-spring deposits. 



EPITHERMAL VEINS, LODES, AND PIPES 



Epithermal veins, lodes, and pipes form within a 

 few thousand feet of the surface, under slight load, 

 and generally in brittle rocks where open fractures 

 and cavities are abundant. They are formed by 

 deposition of silver-b3aring minerals from hot as- 

 cending solutions in areas of volcanic activity. The 

 presence of anomalous amounts of arsenic, anti- 

 mony, mercury, bismuth, tellurium, and selenium 

 are good geochemical indicators of favorable areas 

 for prospecting and exploration. They are dis- 

 tinguished from mesothermal and hypothermal 

 deposits by small grain size, open cavities, crustifica- 

 tion, banding, and comb structures. They most com- 

 monly occur in Tertiary volcanic flows or tuffs, 

 generally andesitic to rhyolitic in composition, or in 

 fault fissures between these and other rocks. Only 

 rarely do they occur in basalt, or in the underlying 

 or adjacent rocks of plutonic or sedimentary origin. 

 The common ore minerals are argentite, pyrargy- 

 rite, polybasite, and other complex silver sulfosalts, 

 argentiferous tetrahedrite, argentiferous galena, and 

 sphalerite. Most of the deposits contain gold, either 

 free or alloyed with silver as electrum, and base- 

 metal sulfides that are seldom of economic im- 

 portance. Many of the deposits contain cerargyrite 

 and native silver as secondary minerals in the zone 

 of oxidation. The gangue minerals include quartz, 

 and typically less abundant adularia, calcite, dolo- 

 mite, barite, and fluorite, and rarely, rhodochrosite 

 and rhodonite. Wallrock alteration is generally well 

 developed and marked by abundant kaolin, chlorite, 

 carbonate minerals, and pyrite, and locally by seri- 

 cite or zeolites. The ore bodies commonly occur as 

 shoots or chimneys of richly mineralized vein ma- 

 terial surrounded by lower grade ore or barren gan- 

 gue minerals. They are generally confined to fault 

 fissures and other fractures but may locally extend 

 into the wallrocks. The ore bodies range widely in 

 size and grade; some have been mined continuously 

 for strike lengths of several thousands of feet, and 

 have yielded ore that contained from a few ounces 

 to many thousands of ounces of silver per ton. Few 

 deposits are known to extend to depths of more than 



