12 



UNITED STATES MINERAL RESOURCES 



cannot be recovered now (for example, Blondel and 

 Lasky, 1956). 



The degree of certainty about the existence of 

 the materials is described by terms such as proved, 

 probable, and possible, the terms traditionally used 

 by industry, and measured, indicated, and inferred, 

 the terms devised during World War II by the Geo- 

 logical Survey and the Bureau of Mines to serve 

 better the broader purpose of national resource 

 appraisal. Usage of these degree-of-certainty terms 

 is by no means standard, but all their definitions 

 show that they refer only to deposits or structures 

 known to exist. 



Thus, one of the generally accepted definitions of 

 possible ore states that it is to apply to deposits 

 whose existence is known from at least one ex- 

 posure, and another definition refers to an ore body 

 sampled only on one side. The definition of inferred 

 reserves agreed to by the Survey and the Bureau 

 of Mines permits inclusion of completely concealed 

 deposits for which there is specific geologic evi- 

 dence and for which the specific location can be 

 described, but it makes no allowance for ore in un- 

 known structures of undiscovered districts. The 

 previous definitions of both sets of terms also link 

 them to deposits minable at a profit; the classifica- 

 tion system comprised of these terms has thus 

 neglected deposits that might become minable as 

 the result of technologic or economic developments. 



To remedy these defects, I have suggested that 

 existing terminology be expanded into the broader 

 framework shown in figure 4, in which degree of 

 certainty increases from right to left and feasi- 

 bility of economic recovery increases from bottom 

 to top. Either of the series of terms already used 

 to describe degree of certainty may be used with 

 reference to identified deposits and applied not only 

 to presently minable deposits but to others that 

 have been identified with the same degree of cer- 

 tainty. Feasibility-of-recovery categories are desig- 

 nated by the terms recoverable, paramarginal, and 

 submarginal. 



Paramarginal resources are defined here as those 

 that are recoverable at prices as much as 1.5 times 

 those prevailing now. (I am indebted to S. P. 

 Schweinfurth for suggesting the prefix para to indi- 

 cate that the materials described are not only those 

 just on the margin of economic recoverability, the 

 common economic meaning of the term marginal.) 

 At first thought this price factor may seem to be 

 unrealistic. The fact is, however, that prices of many 

 mineral commodities vary within such a range from 

 place to place at any given time, and a price elas- 



Degree of certainty 



Figure 4. — Classification of mineral reserves and resources. 

 Degree of certainty increases from right to left, and 

 feasibility of economic recovery increases from bottom to 

 top. 



ticity of this order of magnitude is not uncommon 

 for many commodities over a space of a few years 

 or even months, as shown by recent variations in 

 prices of copper, mercury, silver, sulfur, and coal. 

 Deposits in this category thus become commercially 

 available at price increases that can be borne with- 

 out serious economic effects, and chances are that 

 improvements in existing technology will make them 

 available at prices little or no higher than those 

 prevailing now. 



Over the longer period, we can expect that tech- 

 nologic advances will make it profitable to mine 

 resources that would be much too costly to produce 

 now, and, of course, that is the reason for trying to 

 take account of submarginal resources. Again, it 

 might seem ridiculous to consider resources that 

 cost two or three times more than those produced 

 now as having any future value at all. But keep in 

 mind, as one of many examples, that the cutoff 

 grade for copper has been reduced progressively not 

 just by a factor of two or three but by a factor of 

 10 since the turn of the century and by a factor of 

 about 250 over the history of mining. Many of the 

 fuels and minerals being produced today would once 

 have been classed as submarginal under this defini- 

 tion, and it is reasonable to believe that continued 

 technologic progress will create recoverable reserves 

 from this category. 



