Geology Reference
In-Depth Information
a. Unimodal
Grade
b. Bimodal
Grade
Fig. 6.13 Two possible relationships between ore grade and the metal, mineral, or energy content
of the resource base, according to Skinner (1976).
The aforementioned methodologies have the principal objective of determining
the tonnage of ore with grades above a specified value, providing thus a basis for
estimating ore reserves. They are however not useful for the quantitative ana-
lysis undertaken in this topic, given that they require information regarding the
ore grades and tonnage of existing deposits, which is in effect what the authors
themselves are trying to ascertain. In this respect, a more appropriate study was
undertaken by Cox and Singer (1992), in which a compendium of geological models
was presented from over 3,900 well-characterised deposits all over the world.
Accordingly, the authors calculated using Eq. (6.4) the weighted average grades
(x
m
), considering the average tonnage (M) and grade (x
m
) of the different deposits
containing each particular mineral as shown in Table 6.9.
R
M
0
x
m
dM
R
M
0
x
m
=
(6.4)
dM
Some of the values may appear to be quite low. Nevertheless, it must be pointed
out that they are averages. In most deposits various minerals are extracted as
byproducts, at a relatively low grade. It would simply not be cost-effective if they
were extracted alone.
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