SILVER 



599 



been done, as in north-central Wisconsin, some im- 

 portant discoveries have been made. 



During the 1960's, a large, low-grade disseminated 

 silver deposit vs^as explored by drilling and trenching 

 in faulted areas in the Miocene Barstow Formation 

 and in the formation at Burcham in southern Cali- 

 fornia. The exploration vi^ork indicates the potential 

 for large, lov^^-grade silver deposits that could be 

 mined at lovi^ cost in the vicinity of the former 

 bonanza epithermal deposits. Such deposits may 

 occur in other places in the West and they constitute 

 a potential for large-scale mining during periods of 

 good silver prices. 



The sandstone silver deposits at Silver Reef, Utah, 

 were v^^orked only to the ground-water table. If they 

 are variants of the Colorado Plateau uranium de- 

 posits, as the geology indicates, the deposits could 

 change into primary silver sulfide ore below the 

 water table. The mineralogy and form of this hypo- 

 thetical primary ore is unknown, but if it is similar 

 to Plateau uranium ores, it should be in blankets and 

 rolls, should be similar in grade to the ore above the 

 water table, and should be concentrated in fluvial 

 channels near pyrite concretions and carbonaceous 

 trash pockets. 



The discovery of many mixed base- and precious- 

 metal massive sulfide deposits in Ireland in the 

 1960's indicates a strong possibility that similar 

 ores occur in the almost identical geologic setting 

 along trend in Scotland and northern England. Simi- 

 larly, if continental drift and ocean-floor spreading 

 is a valid concept, Newfoundland should have many 

 more lead-zinc-silver massive sulfides than have 

 already been found. Boulders of massive sulfide ore 

 are common in the glacial drift derived from the 

 interior of the island. 



The work of Hewett and his associates (Hewett, 

 1968 ; Hewett and others, 1965 ; Hewett and Radtke, 

 1967) on primary epithermal manganese-calcite and 

 manganese-silver deposits has indicated the likeli- 

 hood of finding silver ores under many of the west- 

 ern (and some Appalachian) epithermal manganese 

 oxide deposits. Similarly the Walton, Nova Scotia, 

 barite deposits change to silver-copper-barite ores at 

 depth. This change indicates that all eastern Tri- 

 assic barite, as well as all copper and manganese 

 deposits in the United States, should be reexamined 

 with a similar possibihty in mind. 



Another potential source of silver and base metals 

 is the mineralized sediments that occur in thermal 

 deeps, such as the Red Sea basin (Degens and Ross, 

 1969). According to Bischoff and Manheim (1969, 

 p. 535), the uppermost 10 meters of the Atlantic 

 Deep in the Red Sea contains 50 million tons of 



mineral-bearing sediment averaging 1.6 ounces of 

 silver, 3.4 percent lead, 1.3 percent copper, and 0.10 

 percent zinc. Old basins that had similar hot-spring 

 activity, such as the Mississippi River embayment of 

 the Coastal Plain and the Salton Sea region in the 

 Gulf of California, should be reexamined for possi- 

 bilities of similar deposits. 



PROBLEMS FOR RESEARCH 



Without question, major new resources of silver 

 can be located and developed in the United States if 

 the need increases, if prices for silver and other 

 metals improve, and if the general industrial climate 

 is favorable for the production of silver. The present 

 decline in silver production will probably continue 

 for years, however, until new, more efficient mining 

 methods and cleaner recovery methods are developed. 

 The smelters that have closed, or are about to close, 

 are old and obsolete, and are almost impossible to 

 bring up to present environmental-control standards. 

 New wet methods and bacterial recovery methods 

 may be part of the answer to cleaner recovery meth- 

 ods, if the silver is recovered along with base metals, 

 but some of the answer must be in new smelters in 

 which controls for fumes and ash are built in when 

 the plant is constructed. A tall stack to catch the 

 dust and disperse the fumes is no longer a suflScient 

 control. 



The recent emphasis in mining methods has been 

 on mass production from huge open pits that are 

 largely permanently destructive to large acreages of 

 land in the West, even if partly reclaimed. Yet some 

 of the largest, most efficient, lowest cost mines in the 

 country are underground operations, which if prop- 

 erly planned have much fewer permanent environ- 

 mental problems. Further improvements and break- 

 throughs are needed in techniques of underground 

 mining, and the surface plants should be made at- 

 tractive and relatively permanent, as they are in 

 most European countries and in Japan. 



The mineralogy and accepted natural occurrences 

 of silver should be reevaluated. Silver minerals are 

 notably diflScult to identify in the field and by the 

 older mineralogic methods. In part, this is because 

 they commonly occur in microscopic-size crusts, 

 grains, and inclusions. Silver-free and silver-rich 

 types of sphalerite, galena, tetrahedrite, and copper 

 sulfides look alike. Analysis is commonly the only 

 way to determine the presence of silver, and sensi- 

 tive methods like atomic absorption or the electron 

 microprobe must be used to detect trace amounts. 



Traditionally, ores rich in galena, chalcopyrite, 

 tetrahedrite, and mixed complex sulfide and sulfosalt 

 ores are analyzed for silver, whereas sphalerite, 



