The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 166

FIGURE 4.2
Ore Body, Host Rock and Cutoff
Grade

Ore Minerals

Gangue Minerals

The transition from the zone of
unusual high concentration of metals
to host rock with average concentra-
tion is often gradual or blurred. The
cutoff grade is an economic term and
marks the transition from ore to the
geological envelope of 'no commercial
value'.

Cutoff Grade (COG)

Ore Deposit
c COG

Host Rock Formation
Concentration of Ore
Minerals (c) Less Than
Cutoff Grade

Fault

Cutoff Grade - The Economic Defi nition of Ore

Only a few mineral deposits are commercially viable. For a mineral deposit to become an
ore deposit requires that the commercially valuable minerals can be extracted at a proi t.
The metal content of a mineral deposit is called the grade. This is generally expressed as a
percentage by weight, or in the case of precious metals, as ounces per ton. Higher mineral
prices allow lower metal concentrations to become economically attractive to mining. Most
iron ores need a grade of at least 50 percent of iron to attract interest in mining, whereas
economically attractive concentrations of gold are as low as 1/1000 of 1 percent ( Case 4.1 ).
Of course, the more concentrated the desired minerals, the more valuable the deposit. The
lower limit, however , is i xed primarily by economic considerations, but other aspects are
also important. The lower limit of grade is called the cutoff grade (COG) and it varies
according to metal prices, available mining technology, and the nature, size and location
of the mineral deposits. The cutoff grade is an economic term, and it marks the transition
from ore to waste rock in a given ore deposit (as illustrated in Figure 4.2 ). Waste rock is
dei ned as rock which does not contain an adequate percentage of ore minerals to be eco-
nomically valuable as a source of these minerals. The cutoff grade represents this thresh-
old between economic and uneconomic ore. Mine plans are drawn up with some prior
assumptions about cutoff grades. As the nature of the actual ore body is revealed during
mining, and in response to changes in mineral prices and available mining technology, the
cutoff grade of an ore will probably change over the life of the mine. The cutoff grade is
an operational criterion which is applied at the point of mining (Lane 1988). An increase
in metal prices may well turn past mine waste dumps into valuable ore resources.
It is not always possible to say exactly what the grade must be, or how much of the
given mineral must be present, for a substance in a given deposit to be considered an ore.
Two deposits may have the same size and identical grade, but one may be identii ed as an
ore and the other may not. Much depends on location, as illustrated in Case 4.2 .
Small, selectively mined deposits with a high gold-silver content and a high proi t
margin are often referred to as bonanza deposits, the focus of mining in the early days.
Bonanza deposits are few in number, and in the past their discoveries were the cause
of gold rushes (see the Chapter Five). Today's deposits are usually of large tonnage and

Most iron ores need a grade
of at least 50 percent of iron
to attract interest in mining,
whereas economically attractive
concentrations of gold are as low
as 1/1000 of 1 percent.

The cutoff grade is an economic
term, and it marks the transition
from ore to waste rock in a given
ore deposit.