272 



UNITED STATES MINERAL RESOURCES 



speculative resources cannot be estimated as accu- 

 rately as reserves, but these resources are probably 

 large compared with current U.S. gold production 

 or consumption but small compared with total world 

 reserves. The largest potential resources of gold in 

 the United States are in Tertiary or Cretaceous 

 placer deposits or auriferous conglomerates, fol- 

 lowed in decreasing order of magnitude by dissemi- 

 nated, lode, "bonanza," and other placer deposits. 



The Tertiary-Cretaceous placer deposits of the 

 United States represent a very large resource of 

 gold, but at the present time they are not economic 

 to mine. The auriferous conglomerates of north- 

 western Wyoming contain 35-200 ppb gold in a 

 volume exceeding 50 cubic miles, and they probably 

 contain several hundred million ounces of gold (6- 

 10 yr production from the Witwatersrand) (Ant- 

 weiler and Love, 1967). The Tertiary placers of the 

 Sierra Nevada have an estimated 3-4 billion cubic 

 yards of gravel containing 20-30 million ounces of 

 gold (Merwin, 1968, p. 2) . In both areas, formidable 

 and, for the foreseeable future, probably insurmount- 

 able technologic and legal problems militate against 

 utilization of these resources. 



Deposits of disseminated gold — the so-called 

 Carlin-type deposits — probably contain the second 

 largest domestic resources of gold and almost cer- 

 tainly the largest undiscovered resources of grade 

 high enough to be mined at the present price of 

 gold or even at a substantially higher price. The 

 two largest deposits were discovered as late as the 

 1960's, and others may well have been missed by 

 earlier prospectors. The area of possible occurrence 

 of favorable host rocks, principally in north-central 

 Nevada, is large, and the known deposits contain 

 enough gold — one-half million ounces or more 

 apiece — to make discovery of others rewarding. 



Undiscovered resources of gold undoubtedly re- 

 main to be found in the lode- and "bonanza"-type 

 deposits, but the amount is probably small compared 

 with that in the disseminated deposits. The lode and 

 "bonanza" deposits have been sought for many 

 years, and most of them discoverable with present 

 exploration techniques probably have been found. 

 The lode deposits occur principally in rocks of Pre- 

 cambrian age, rocks which have a complex geologic 

 history. Each year our knowledge of the geology of 

 these rocks is improving, and this in time should 

 lead to development of guides that will increase 

 chances for discovery of new deposits. For the 

 "bonanza"-type deposits, a better understanding of 

 the geochemistry of the extensive alteration halo 

 that envelops these deposits might provide a useful 

 tool for recognizing the outer fringes of the halo 



over a concealed deposit. However, much effort has 

 been expended toward these ends in recent years, 

 but so far no new discoveries have been reported. 



Gold resources of young placer deposits must be 

 small ; these deposits are easy to find and have been 

 extensively mined, and the likelihood that any 

 young placer of significant size remains undiscovered 

 is very slight. Moreover, the increasingly strict legal 

 restraints on placer mining because of water pollu- 

 tion problems inherent in this kind of mining are 

 discouraging to further prospecting. 



Inasmuch as gold may be recoverable even at very 

 low concentrations, materials containing gold in 

 amounts near crustal abundance should perhaps be 

 considered as potential future resources. Such ma- 

 terials would include, for instance, sea floor sedi- 

 ment such as in the Bering Sea, or sea water itself, 

 both of which presumably contain enormous quanti- 

 ties of gold. However, the concentration of gold in 

 both is far too low for economic recoverability at 

 present, and in fact, the average concentration of 

 gold in sea water is so minute as to exclude it from 

 serious consideration as a potential source in the 

 foreseeable future. (At a medium concentration of 

 0.03 ppb, and with gold at $70 per oz, the gold in a 

 metric ton of sea water would be worth $.0000639; 

 expressed in another way, the amount of sea water 

 that would have to be processed to extract 1 oz of 

 gold, assuming a concentration of 0.03 ppb and 

 complete recovery, would be approximately 1 million 

 m^ — enough water to flood two football fields to a 

 depth of 100 m.) 



Not enough is known about uneconomic and un- 

 discovered gold resources outside the United States 

 to permit an estimate of their magnitude at this 

 time. The one figure shown in table 51 (col. C) for 

 gold resources outside the United States represents 

 mainly deposits that are considered reserves by other 

 authorities. It seems clear, however, as noted pre- 

 viously, that the most favorable areas for discovery 

 of new deposits of the two types that historically 

 have been the most productive — the gold-quartz 

 lodes and the fossil placers — are those of Precam- 

 brian rocks, particularly rocks in the approximate 

 age range of 1.6-2.7 billion years. Rocks in this age 

 group are hosts for all the major Precambrian gold 

 deposits of the world — Witwatersrand, Homestake, 

 Kolar, Porcupine, Kirkland Lake-Larder Lake, Kal- 

 goorlie, Tarkwa, and probably Morro Velho. Pre- 

 cambrian rocks constitute the Canadian, Brazilian- 

 Guyana, African-Arabian, Australian, and Indian 

 shields and are exposed over extensive areas in 

 Siberia. In the United States, Precambrian rocks 

 older than about 1.6 billion years are not widely 



