SILVER 



595 



1968). The district has yielded 1,747,000 ounces of 

 silver and 5,350 tons of copper, mostly from the 

 United Copper mine. The mines have been idle for 

 many years, and it is unlikely that mining vs^ill be 

 resumed under present or foreseeable conditions. 



MESOTHERMAL COBALT-SILVER, COBALT-URANINITE SILVER, 

 AND COBALT-SILVER-ZEOLITE DEPOSITS 



The cobalt-silver veins of Cobalt, Ontario, have 

 yielded about 500 million ounces of silver to date, 

 and several mines have been revived in recent years. 

 The deposits occur in thin, short, vertical veins in 

 diabase sills about 1,000 feet thick. The main min- 

 erals are native silver, cobalt, nickel and iron arsen- 

 ides, arsenic sulfides, dolomite, and calcite. Some 

 galena, chalcopyrite, pyrite, sphalerite, pyrrhotite, 

 and tetrahedrite are present. Typical high-grade ore 

 averages 10 percent silver, 9 percent cobalt, 6 per- 

 cent nickel, and 39 percent arsenic (Jones, 1953, p. 

 18). The origin of the silver veins is controversial. 

 Many believe the silver source was in the magma 

 that gave rise to the diabase. Others suggest the ore 

 minerals or metals vi^ere derived from sedimentary 

 rocks interbedded in the greenstones. 



Similar deposits on the east shore of Great Bear 

 Lake, northwestern Canada, contain native silver, 

 cobalt and nickel arsenides, chalcopyrite, some uran- 

 inite, and bismuth, in thin quartz veins. The princi- 

 pal economic mineral in some veins is uraninite, and 

 in others, native silver. Gangue is altered host rock, 

 quartz, carbonates, and hematite. Five stages of 

 mineralization have been recognized; pitchblende is 

 part of an early stage and native silver is part of the 

 latest stage. 



The Echo Bay mine, opened in 1964, has produced 

 nearly 16 million ounces of silver. In 1971, this mine 

 produced about 2.5 million ounces of silver and 

 650,000 pounds of copper. Mill heads average 70 

 ounces of silver per ton and 0.95 percent copper 

 (Northern Miner, Jan. 6, 1972). Four large masses, 

 largely of silver, that weighed 1,620, 800, 412, and 

 344 pounds were mined in August 1971 ; the largest 

 mass contained about 70 percent silver. 



Deposits somewhat similar to those at Great Bear 

 Lake were formerly quite productive at Kongsberg, 

 Norway; and at Andreasberg, Annaberg, and 

 Schneeberg in Saxony, Germany. These deposits con- 

 tain native silver, silver sulfides, cobalt, bismuth, 

 and nickel arsenide associated with lead and zinc 

 sulfides. Uraninite is much less abundant in these 

 deposits than at Great Bear Lake although locally 

 the veins are enriched in uraninite. Zeolites, barite, 

 fluorite, calcite, and ankerite are the main gangue 

 minerals. The veins at Schneeberg are ore bearing 



in metamorphic clay-slates but are impoverished in 

 the underlying granite. The veins are multistage: 

 the nickel-cobalt-bismuth ores are associated with an 

 early stage; uraninite with an intermediate stage; 

 and silver with a late stage. 



SANDSTONE SILVER DEPOSITS 



Silver chloride deposits disseminated in sandstone 

 occur in southern Utah at Silver Reef (Proctor, 

 1953). Small ones are also found to the southwest 

 near St. George. The mines, which were productive 

 for about 20 years in the late 19th century, yielded 

 nearly 8 million ounces of silver. 



The deposits are a silver-rich variant of the Colo- 

 rado Plateau sandstone uranium deposits and the 

 sandstone copper-silver deposits previously de- 

 scribed (Fischer, 1937). Most of the Silver Reef 

 deposits contain some copper and, locally, some 

 uranium and vanadium. The ore bodies are re- 

 stricted to the Silver Reef Sandstone Member (of 

 Proctor, 1953) of the Upper Triassic Chinle Forma- 

 tion and occur in blankets and lenses on the limbs 

 and nose of a major anticline. The ore bodies were 

 emplaced prior to anticlinal folding. They tend to be 

 richest in carbonaceous trash pockets and in cross- 

 bedded sandstone beds and lenticular channels. The 

 silver is mostly in cerargyrite, but it occurs locally as 

 native silver. Associated minerals are malachite, 

 azurite, and locally carnotite. Trace-element studies 

 indicate that silver bromides are probably also pres- 

 ent. The ore minerals are essentially invisible and 

 occur as micrograins cementing the sandstone 

 grains, and as thin gray crusts on lignite. Pyritic 

 concretions, now altered to limonite, locally are es- 

 pecially rich in silver. The ores were worked to the 

 water level and the mines closed. Little exploration 

 was done below the water table, although the geology 

 suggests that unoxidized ore, similar in grade to the 

 oxidized ore, should be present. Little search has 

 been made for similar ores elsewhere in the Colorado 

 Plateau. 



SEA-FLOOR MUDS AND HOT-SPRING DEPOSITS 



Mineralized sea-floor muds of the Red Sea contain 

 lead, zinc, and iron sulfides, and some silver in len- 

 ticular blankets in local basins (Degens and Ross, 

 1969). They are being deposited by heated brines 

 rising through the sea floor in the central part of the 

 Red Sea. The blankets, which are very large, are 

 examples of mineral deposits being deposited at 

 present by hydrothermal solutions. Such deposits 

 may ultimately form massive sulfide blankets such 

 as those at Meggen, West Germany. 



Near Niland, Calif., close to the Salton Sea, a very 



