Introduction - Metals and Society: An Introduction to Economic Geology - page 14

a b c

Map of the deposit Grade profile 3D modeling

Copper grade (%) cut-off grade:

2,0 4000 $ / ton

copper grade

1,8

0,25%
0,5%

1,6

0,25%
0,5%

1,4

1%

1,2

2% 1%

2%

1,0 8000 $ / ton

1234

radius (km)

0,8

0,6

20000$ / ton

0,4

0,2

1 km

radius (km) 0 1234

Fig. 1.6 Highly schematized plan of an ore deposit and variations in ore grade, showing how an
increase in ore price results in a decrease in the tenor limit which increases the volume of ore that
can be mined and the quantity of metal that can be recovered

are abundant in the Earth's crust and they are present in high concentrations in ores.
As a consequence their price is relatively low. Other metals are present in far lower
concentrations and their price is much higher.
In any deposit the ore type varies, from small areas of rich, high-grade ore to
larger areas with lower grades. The values shown in Fig. 1.6 a are the average grades
that are mined, a mixture of high and low grade ore. What is left in the ground after
mining is material, geologically very similar to the material that has been mined,
but simply containing a lower concentration of the ore metal, a concentration that is
below a certain threshold. This important parameter is called the cut-off grade. To
include sub-ore in the material being mined would lead to the operation becoming
unprofitable: the cost of mining would exceed the value of the recovered metal.
But what would happen if the metal price improves? It is evident that if the price
increases , the cut-off grade decreases because lower-grade material can then be
mined at a profit. As a consequence, the amount of mineable material in the deposit
increases.
The example discussed in Exercise 1.5 illustrates clearly how the amount of
recoverable metal depends on the price. Taking the argument further, if society
requires a commodity, and if no substitute can be found, then the price will increase
to the extent that low-grade accumulations of the commodity become ore. There
are, of courses, many limits and complications, but this type of argument leads us to
suggest that the resources of many or most metals will never be exhausted.

Box 1.5 Estimation of the Amount of Recoverable Ore as a Function
of Price and Cut-Off Grade
Figure 1.6 is a sketch of a hypothetical ore deposit: a rich, high-grade core is
surrounded by a much larger volume of lower grade material. Suppose that
the price of copper increases from $4,000 to $8,000 per ton, as it did during
the period 2004-2008, and that the increase led to a drop in the cut-off grade.
(continued)