GOLD 



273 



exposed. Their main areas of outcrop are in the 

 eastern ranges of the Rocky Mountains in New 

 Mexico, Colorado, Wyoming, and Montana; in the 

 Black Hills in South Dakota; and in northern Michi- 

 gan, Wisconsin, and Minnesota around the west end 

 of Lake Superior. Smaller areas are scattered 

 through Arizona, southern Nevada, and southeast- 

 ern California. 



PROSPECTING TECHNIQUES 



Because of the nature of gold — its high density, 

 its resistance to tarnish and hence its high visibility, 

 and its distinctive color — the most useful field tool 

 in the search for gold deposits has been the gold pan, 

 with which gold can be recognized immediately if it 

 is in large enough particles. The gold pan is still a 

 useful implement, but it is unreliable for detecting 

 gold tellurides or extremely fine grained gold such 

 as characterizes the Carlin-type deposits. For meas- 

 urement of unseen gold in rock samples, the fire 

 assay has until recently been the standard method. 

 However, the fire assay is both costly and time con- 

 suming and has been supplanted to a large degree 

 by atomic absorption spectrophotometry, a rapid 

 and economical method that can detect as little as 

 20 ppb gold in a sample. This method has made pos- 

 sible the routine analysis of large numbers of sam- 

 ples and thus has greatly facilitated exploration for 

 gold. 



PROBLEMS FOR RESEARCH 



Early in 1966, the U.S. Geological Survey under- 

 took a large research program, the Heavy Metals 

 Program, to tackle many of the geologic problems 

 related to gold. This program is no longer in exist- 

 tence, but research is being continued on many im- 

 portant problems as part of the Geological Survey's 

 mineral-resource program. 



A principal geologic problem related to U.S. gold 

 deposits is the geologic and geochemical nature of 

 the disseminated (Carlin-type) deposits. Inasmuch 

 as these deposits probably represent the largest rela- 

 tively high grade gold resource of the United States, 

 further research on them would appear to have a 

 high possible payoff. Much is already known about 

 the geologic setting and geochemical makeup of 

 these deposits, and they are the subject of continu- 

 ing research. Nevertheless, no new discoveries have 

 been reported since Cortez, Nev., in 1966. Is some 

 vital clue perhaps being overlooked? 



The greatest gold-producing district in the world 

 is the Witwatersrand. Counterparts of the Precam- 

 brian gold-uranium placer deposits of the Rand seem 

 to be very rare, and none even remotely comparable 



in productivity has been found. What critical com- 

 bination of geologic factors was operative in the 

 formation of these deposits? If this question could 

 be answered satisfactorily, would it be possible to 

 predict where and when other such combinations 

 might have occurred in the geologic past? 



Very large amounts of gold must have been eroded 

 from the Precambrian deposits of the Canadian 

 Shield in Ontario and Quebec, yet very little gold 

 has beei; found in younger rocks overlying the shield 

 along its southern edge even though they were ex- 

 tensively sampled in the Heavy Metals Program. 

 Are there other more likely places where this gold 

 might have been concentrated? 



The high concentration of major gold deposits in 

 rocks of intermediate Precambrian age (2,700- 

 1,600 million years) has already been noted. Many 

 of these deposits are in rocks of, or related to, the 

 so-called greenstone-granite association (districts in 

 Ontario and Quebec, Canada; Kalgoorlie, Australia; 

 Kolar, India; Rhodesia). Do the auriferous green- 

 stone-granite terranes have any geochemical peculi- 

 arities that would distinguish them from lithologic- 

 ally similar but barren terranes? 



Broad alteration halos surround "bonanza"-type 

 gold deposits. Much is being done to characterize 

 these halos chemically and to determine the chemical 

 and mineralogic changes that occur during the for- 

 mation of a halo and its subsequent weathering. Will 

 it be possible to distinguish a halo associated with 

 an ore deposit from one in barren rock? 



Gold deposits occur in mountain ranges at or near 

 enough to the surface to be detected with existing 

 exploration tools. But what of these deposits that 

 undoubtedly occur in adjacent valleys beneath a 

 thick cover of younger sediment? Can exploration 

 tools be developed to detect them? Measurement of 

 the mercury content of air, which is higher over 

 certain types of ore deposits, holds promise. Are 

 there other elements that might be useful? 



A group of related problems concerns the distri- 

 bution of gold in rocks. Considerable information is 

 available on the gold content of various rock types 

 and minerals and of bedrock ores, but many gaps in 

 knowledge remain. For instance, there seem to be 

 three points, or relatively narrow ranges of points, 

 in the spectrum of gold concentrations: (1) crustal 

 abundance of 0.003-0.004 ppm; (2) gold content of 

 the lowest grade gold ore mined in modern times 

 (Alaska Juneau mine, average grade of ore only 

 about 0.04 oz per ton, or a little more than 1 ppm) 

 and of the porphyry copper deposits, which yield 

 gold as a byproduct (0.001-0.018 oz per ton, or 

 0.04-0.6 ppm) ; and (3) gold content of ore being 



