184 



UNITED STATES MINERAL RESOURCES 



dance but lower than could be mined under present 

 economic and technological conditions. Subeconomic 

 resources may be either identified or hypothetical, 

 but inasmuch as data on grade and tonnage of sub- 

 economic deposits are rarely available, no distinction 

 is made between these categories. Subeconomic 

 speculative resources represent such a high degree 

 of uncertainty that they cannot be given numerical 

 values. The lower limit of copper content of depos- 

 its in the subeconomic category depends on one's 

 personal expectations of revolutionary advances in 

 mineral recovery and mining methods. Copper will 

 ultimately be mined at lower grades than most other 

 metals because recovery is facilitated by its tend- 

 ency to occur as minute sulfide grains rather than 

 in silicates. However, adverse effects on the en- 

 vironment and the magnitude of the labor and power 

 requirements mitigate against the creation of huge 

 open-pit mines in rocks containing only a few 

 hundred parts per million of copper. 



Table 41 gives some of the identified and hypo- 



Table 41. — Partial list of subeconomic (conditional) re- 

 sources, in millions of tons, of copper ' 



^ Conditional resources: Specific, identified mineral deposits whose con- 

 tained minerals are not profitably recoverable with exlstingr technology 

 and economic conditions. 



thetical subeconomic resources that may become 

 economically important in the future. 



In the United States, conditional resources in por- 

 phyry copper deposits are probably very large. 

 Lowell (1970, p. 69) predicted that average grades 

 in open-pit copper mines will decline to 0.25 percent 

 by the year 2000. In-place leaching of deposits of 

 even lower grade may soon become economical. In 

 addition, some large deposits such as at SafFord, 

 Ariz., are too deep or too irregularly distributed to 

 permit profitable mining at the present time but 

 may become minable in the near future. An estimate 

 of the hypothetical subeconomic resources in por- 

 phyry copper deposits averaging 0.25 percent cop- 

 per requires some knowledge of the frequency of 

 such deposits compared with deposits of minable 

 grade. Moreover, to predict the increase in reserves 

 of known deposits in response to a drop in average 

 ore grades requires some knowledge of the rate at 

 which copper content falls off with distance outward 



from boundaries of porphyry copper ore bodies. If 

 the slope of the curve of copper plotted against 

 distance outward is steep for most deposits, de- 

 creases in average grade will have little effect on 

 reserves. Inasmuch as these data are not available, 

 we are reduced to making the simple assumption 

 that there are at least as many deposits averaging 

 0.1-0.4 percent as there are deposits of currently 

 minable grade. The discovery and development of 

 this additional 7-8 billion tons of material in the 

 western porphyry copper provinces with average 

 grades of about 0.25 percent would yield about 20 

 million tons of additional copper. 



Large conditional resources in sedimentary cop- 

 per deposits in the United States are difficult to 

 evaluate. Geologically, sedimentary deposits pass 

 from economic ores to subeconomic resources when 

 the beds in which the copper minerals occur become 

 too thin or become separated by thick beds of barren 

 rock. Also, some beds may have engineering char- 

 acteristics that make mining dangerous or expen- 

 sive, or a steeply dipping copper-bearing bed may 

 reach depths that are beyond the reach of present- 

 day underground mining practice. Nevertheless, the 

 copper in certain sedimentary deposits is known to 

 be continuous at fairly constant grade for great 

 distances. 



The Nonesuch Shale at White Pine, Mich., is such 

 a deposit. W. S. White (oral commun., 1972) esti- 

 mated that 11 million tons of copper metal exists 

 in currently subeconomic mineralized beds. The 

 sedimentary copper deposits recently being explored 

 in the Belt Supergroup of Montana and Idaho prob- 

 ably contain large conditional resources, but no in- 

 formation is yet available on their minable reserves. 



The copper-nickel deposits in the Duluth Gabbro 

 Complex in Minnesota represent a large conditional 

 resource. P. K. Sims (oral commun., 1972) esti- 

 mated that 14 billion tons of mineralized rock exists 

 along the lower contact of the gabbro body. This 

 material has a lower cutoff of 0.25 percent and an 

 average grade of 0.58 percent combined copper 

 nickel. Assuming a 3 to 1 copper-nickel ratio, these 

 deposits contain slightly more than 60 million tons 

 of copper metal. Some of this copper is dessemi- 

 nated in bodies as large as 100 million tons and may 

 soon become amenable to mass-mining methods, ac- 

 cording to Sims. 



Summing up these large conditional resources 

 (table 41) gives a total of 111 million tons of copper 

 which may at some time become available in the 

 United States. 



Among the world's most important conditional 

 resources are those of the sea floor. These comprise 



