The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 158

Apart from a physical way of classifying mineral concentrations, there is also an
economical way of doing so. This is explained in the following section.
6.8.1 The economic classification of minerals
Concentrations of minerals can be classified as resources, reserves and reserve base,
depending on certain factors, as will be explained next.
The U.S. Bureau of Mines defines a resource as a concentration of naturally
occurring solid, liquid, or gaseous material in or on the Earth's crust in such a form
and amount that economic extraction of a commodity from that concentration is
currently or potentially feasible.
Reserve base is defined as that part of an identified resource 15 that meets speci-
fied minimum physical and chemical criteria related to current mining and produc-
tion practices, including grade, quality, thickness, and depth. Reserves, meanwhile,
are that part of the reserve base which could be economically extracted or pro-
duced at the time of determination. Reserve base and reserves are subdivided in
order of increasing confidence into demonstrated and inferred. The latter are esti-
mates based on an assumed continuity beyond indicated resources, for which there
is geological evidence. There may be no samples or measurements. Demonstrated
reserves are the sum of measured and indicated resources. Where one speaks of
measured resources, one refers to those materials whose quality and quantity have
been determined by quantitative data including appropriate analyses, from closely
spaced and geologically well-known sample sites, within a margin of error of less
than 20%.
Indicated resources are those in which the grade and or quality are computed
from information similar to that used for measured resources but the sites for in-
spection, sampling, measurement are farther apart or are otherwise less adequately
spaced.
Fig. 6.12 shows the mineral resources and reserves classification after McKelvey
(1972).
Additional geological information expands the known number of reserves. Like-
wise commodity prices and the development of more e cient technologies play an
important role, as previously unworkable grades become economically profitable.
Hence, neither reserve base, nor reserves are good indicators with which to assess
mineral capital. In fact, total world reserves of most mineral commodities are larger
now than at any time in the past (Highley et al., 2004) due to the reasons mentioned
in the previous paragraph. To give an example, once it became known that it was
economic to mine copper porphyry deposits for their ore in the early part of the
20th century, the world's known copper reserves and therefore resources, increased
by several hundred percent (Govett and Govett, 1974).
15 Identified resources: resources whose location, grade, quality and quantity are known or esti-
mated from specific geologic evidence.