In the classification of reserves and resources recently adopted by the U.S. Geological Survey and the U.S. Bureau of Mines, several categories of reserves and resources are differentiated which are useful in reporting objectively the state of knowledge about the existence and recoverability of minerals and in establishing targets for exploration and technologic development. In this classification, the terms “reserves” and “resources” are used in the traditional sense. Reserves are considered to be identified deposits recoverable under existing economic and technologic conditions. Resources include undiscovered deposits of the same quality as reserves, as well as deposits presently unrecoverable for either economic or technologic reasons. These categories are further subdivided to indicate the degree of certainty of their existence and the feasibility of their recovery under present economic conditions. Rather than attempting to predict how much eventually will be found and produced—as in the concept of ultimate reserves or production—estimates in these terms indicate what we know and what we hypothesize or speculate about.
Two subcategories of measured reserves that would be a useful addition to this classification are “developed measured reserves” and “undeveloped measured reserves.”