The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 159

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES

Demonstrated Probability range

Cumulative production

Measured Indicated Inferred Hypothetical Speculative

ECONOMIC

Reserves Inferred reserves

MARGINALLY
ECONOMIC

Marginal Reserves
Demonstrated subeconomic resources
Includes non-conventional and low-grade material

Inferred marginal reserves

SUBECONOMIC

Inferred subeconomic resources

Other occurences

Fig. 6.12 Principles of the mineral resource classification system (USGS, 1980)

Any true assessment of the mineral capital on Earth, should thus be based
on resources rather than on reserves or reserve base. However, as resources is the
most comprehensive classification of the aforementioned, information is often scarce,
inaccurate and/or incomplete as it can be seen in Table 6.10.
Estimates of resources are dynamic by definition as still too little is known about
the Earth's crust. Most of the deposits worked at present are close to the surface
with the deepest open-pit mine less than 1 km deep and the deepest underground
mine descending down 3.5 km (BGS, 2006). This is really no depth at all considering
that the average slice of the Earth's crust is some 40 km thick, meaning that only
approximately the outer one-tenth of the continental crust is of interest (Dunham,
1974).

6.8.2 Average mineral ore grades

For some time, many geologists assumed that the amount of less common metals
existing at different grades in the crust could be represented by lognormal or similar
unimodal frequency distributions. This assumption was first questioned by Skinner
(1976) for those metals that make up less than 0.1% of the Earth's crust. He
suggested that for such scarce metals the distribution might be bimodal and that
the small mode at higher grades would represent metal concentrations of non-silicate
minerals localised in mineral deposits (DeYoung and Singer, 1981) (see Fig. 6.13).
The original mathematical procedures correlated the tonnage of the ore with
its mean grade (Lasky, 1950; Cargill et al., 1980). Later, the fractal relationship
exhibited by a variety of natural processes, was proved to be better applicable to
mineral deposits, at least for mercury, copper and uranium in the U.S (Turcotte,
1986). This fractal relationship follows the expression of Eq. (6.3).

M c
M

3

x m
x c =

(6.3)

Where x m is the average concentration in the deposit; x c the concentration in
the Earth's crust; M the tonnage of the deposit; M c the tonnage of the piece of
land under consideration and F the fractal relationship to be determined.