CRUSTAL ABUNDANCE OF ELEMENTS, AND MINERAL RESERVES AND RESOURCES 



25 



68 million tons were called reserve, then the ratio 

 would be reduced to about 36. 



Using the information in table 3, I have modified 

 McKelvey's original diagram relating domestic re- 

 serves to crustal abundance (fig. 6). Domestic re- 



tc 10- 



ABUNDANCE (-4) IN CONTINENTAL CRUST, IN PERCENT 



Figure 6. — Domestic reserves of elements compared to their 

 abundance in the, earth's crust. Modified from McKelvey 

 (1960). 



serves of some metals have increased and new 

 element-abundance numbers for continental crust 

 are available since McKelvey first published this 

 diagram in 1960. Because actual reserves of lead 

 and molybdenum now equal 2.7 AxlO'°, potential 

 recoverable resources of the other elements also 

 should approach this line. Note that the factor is 2.7 

 on this diagram and 2.45 on the table of abundance, 

 mass, and reserves. This is because reserve tonnages 

 on this figure are calculated in short tons to con- 

 form with McKelvey's original work. The 2.45 fac- 

 tor applies for the metric system. Of course, this 

 line will continue to shift as additional reserves are 

 discovered, but it does give us some broad measure 

 of what we might expect. 



It is a simple fact, but often overlooked, that we 

 cannot expect to find reserves or resources of the 

 least abundant elements in the same magnitude as 

 the more abundant elements. Thus, in a crude way, 

 because copper is about 20,000 times more abundant 



than gold in the earth's crust, we can expect that 

 recoverable resources of copper should be about 

 20,000 times greater than recoverable resources of 

 gold. This simple relationship, of course, is modified 

 by the inherent geochemical nature of each element. 



If we accept the abundance-reserve relationship, 

 it follows that i?=2.45xl0'' is a minimum total re- 

 source estimate because the relationship is based 

 upon currently recoverable resources and does not 

 include resources whose feasibility of economic re- 

 covery is not established. Total resource estimates 

 of an element that vary substantially from this 

 formula probably are caused by geologic factors 

 and the inherent geochemical nature of the element. 

 However, the discovery of Carlin-type gold depos- 

 its, Spor Mountain beryllium deposits, and Precam- 

 brian Belt Supergroup copper deposits, all in the 

 last decade, indicate that we need to critically ex- 

 amine our criteria for where and how to look for 

 mineral deposits. Certainly, significant mineral de- 

 posits remain undiscovered because exploration 

 efforts are commonly confined to the classic en- 

 vironments of ore deposition. 



As analytical techniques improve, as our under- 

 standing of geochemical processes improves, and as 

 exploration methods improve so that we can con- 

 fidently explore and examine the crust to a rea- 

 sonable depth, the reserve-resource-abundance rela- 

 tion should become more closely defined. Perhaps 

 then we can determine whether ore deposits are 

 chiefly formed by different ore-forming processes 

 in a uniform crust or because of differences in 

 original composition of crustal segments. 



REFERENCES CITED 



Goldschmidt, V. M., 1964, Geochemistry: Oxford, Clarendon 

 Press, 730 p. 



Lee, Tan, and Yao, Chi-lung, 1970, Abundance of chemical 

 elements in the earth's crust and its major tectonic 

 units: Internat. Geology Rev., v. 12, no. 7, p. 778-786. 



McKelvey, V. E., 1960, Relation of reserves of the elements 

 to their crustal abundance : Am. Jour. Sci., v. 258-A, 

 (Bradley volume) p. 234-241. 



1972, Mineral resource estimates and public policy: 



Am. Scientist, v. 60, no. 1, p. 32-40; reprinted in this 

 volume. 



U.S. Bureau Mines, 1970, Mineral facts and problems, 1970: 

 U.S. Bur. Mines Bull. 650, 1291 p. 



Vinogradov, A. P., 1962, Average contents of chemical ele- 

 ments in the principal types of igneous rocks of the 

 earth's crust: Geochemistry, no. 7, p. 641-664. 



