SILVER 



593 



2,000 feet below the surface, and most are confined 

 to less than 1,000 feet below the outcrop. 



The most famous epithermal silver deposit in the 

 United States, and perhaps the world, is the Com- 

 stock Lode in western Nevada (Becker, 1882; Bas- 

 tin, 1922; Gianella, 1936; Thompson, 1956). This 

 great deposit yielded nearly a half billion dollars in 

 gold and silver from ore bodies along a strike length 

 of about 3 miles and to a depth of more than 2,000 

 feet. Another notable epithermal vein deposit is the 

 Hornsilver, in the San Francisco district of Utah 

 (Butler, 1913). This giant vein of silver-bearing 

 galena, sphalerite, argentite, pyrargyrite, and chal- 

 copyrite lies in a large fault between Tertiary 

 quartz-latite flows on the east and lower Paleozoic 

 limestone on the west. The vein was rich in silver, 

 lead, and gold where partly oxidized near the sur- 

 face; as in many epithermal veins, the productive 

 part extended to only about 1,000 feet. Other notable 

 epithermal vein deposits in the United States are 

 those at Tonopah (Nolan, 1935) and Austin (Ross, 

 1953), Nev. ; Mojave (Tucker, 1935) and Randsburg 

 (HuHn, 1925), CaHf . ; and the Mogollon district. 

 New Mexico (Anderson, 1957). Many of the rich 

 silver deposits of Mexico and the Andes belong to 

 this type. The eastern part of the Harz vein dis- 

 trict in West Germany contains epithermal silver- 

 lead veins with a wide variety of silver minerals. 



Epithermal precious- and base-metal veins and 

 lodes in Tertiary andesitic breccia of the San Juan 

 Mountains of Colorado contain both silver and gold. 

 Major silver-producing mines are still active in the 

 Telluride-Sneffels district, near Silverton, and at 

 Creede, where the host rocks are mostly felsic vol- 

 canic rocks. Most of the deposits in the San Juan 

 Mountains are within or near the margins of giant 

 calderas (Steven, 1968). At Creede, a new vein sys- 

 tem recently opened up by the Homestake Mining 

 Co. contains an unusual native silver and copper sul- 

 fide ore. Farther north this vein probably grades 

 into very coarse grained, vuggy, argentiferous 

 galena, sphalerite, and copper minerals similar to 

 the Holy Moses, OK, and Amethyst veins to the east 

 (Steven and Ratte, 1965) . 



In the Silver Cliif and Querida districts of south- 

 central Colorado, breccia pipes in rhyolite are ce- 

 mented by rich crusts of galena, sphalerite, various 

 silver minerals, free gold, and gold tellurides. The 

 best known deposits are those of the Bassick and 

 Geyser mines. Nearby is the Bull Domingo pipe with 

 mineralogically similar ore that cements a Tertiary 

 breccia formed in Precambrian rocks. The deposits 

 are small, but locally rich. Near Silver Cliff, a vol- 

 canic basin is covered by a blanket of altered rhyo- 



lite with many small intersecting silver veins. 

 Beneath part of the district is a large, low-grade, 

 epithermal porphyry-type silver-lead-zinc-copper 

 deposit which is a resource for the future. 



EPITHERMAL DISSEMINATED AND BRECCIA DEPOSITS 



The large deposit at Calico, in southeastern Cali- 

 fornia (Wright and others, 1953), has yielded about 

 $20 million in silver from barite veins along faults 

 and from rich pockets of cerargyrite, embolite, and 

 native silver in shattered volcanic rocks and lake 

 beds of Miocene age. Few of the deposits extend to 

 depths greater than 200 feet. Exploration in 1966 

 indicated a disseminated silver deposit containing 

 many millions of tons of low-grade silver-bearing 

 material in faulted and brecciated sedimentary rocks 

 of late Miocene age along the Calico fault zone. 



The Monitor district, California, also has low- 

 grade disseminated silver in braided fissure zones in 

 altered andesite of Miocene-Pliocene age. The silver 

 occurs as argentite, polybasite, and pyrargyrite. 

 Similar deposits are reported in Nevada and Arizona. 



Silver-antimony-lead-bearing epithermal jasper- 

 oid reefs that replace limestone were mined in the 

 Taylor and Success districts east of Ely, Nev. 

 (Drewes, 1967). Similar ores, of silver, occur in 

 jasperoid mantos in the Vipont district in the north- 

 west corner of Utah. Many other small deposits of 

 this type are known in Colorado, Utah, and Nevada. 



EPITHERMAL SILVER-MANGANESE DEPOSITS 



Silver-rich manganese carbonate, manganiferous 

 black calcite, and manganese oxide deposits, occur- 

 ring as veins in layered volcanic rocks and as re- 

 placement deposits in limestone, were briefly pro- 

 ductive in the United States. The best known dis- 

 tricts are Lake Valley, N. Mex. ; Pioche, Tybo, and 

 White Pine, Nev. ; Escalante, southwestern Utah ; 

 Tombstone and Aquila, Ariz. ; Silver Cliff, Colo. ; and 

 Modoc, Calif. In these and numerous other similar 

 districts in the west, the silver mineral mined was 

 cerargyrite that occurred in small near-surface de- 

 posits ; the hypogene silver minerals were not deter- 

 mined in most deposits (Hewett and others, 1965; 

 Hewett and Radtke, 1967) . The most recently devel- 

 oped deposit of this type is in the Escalante district 

 near Modena, Utah. Here a large tonnage of 10- to 

 15-ounce silver-manganese ore has been developed in 

 a large vein of manganese oxides, fluorite, black cal- 

 cite, vanadinite, pyromorphite, descloizite, and 

 wulfenite; the vein, in rhyolite, extends below the 

 shallow water table without sulfides. In the White 

 Pine district, Nevada, the cerargyite ores are worked 

 out, but aurorite, argentian todorokite, and hydrous 



