Introduction - Metals and Society: An Introduction to Economic Geology - page 15

The consequence is that a much larger amount of ore can be mined and the
amount of copper that can be recovered increases.
In the example, the radius of the zone that can be mined increases from 1.6
to 2.7 km as the cut-off grade drops from 1% to 0.5%. The tonnage of ore that
can be mined depends of the square of this distance (assuming that the
maximum depth of mining remains fixed at 1 km) and the volume increases
from 2.6 km 3 ( 1.6 2 1) to 7.3 km 3 ( 2.7 2 1). Taking into account the

lower grade of the newly recoverable ore (0.5 instead of 1%), the tonnage of
mined copper almost doubles, from (2.6

¼ ¼

10 9 10 7 to 4.9 0.01 )2.6

¼

10 7 t.

1.5.2 Nature of the Ore

Another factor that strongly influences the viability of a deposit is the nature of the
ore. Characteristics to be considered include the type of mineral, the grain size, and
the texture of the ore, all of which influence the cost of mining and the extraction of
the valuable commodity. The lowest extraction costs are for ores in which the
extracted element is only mechanically bound into its gangue (e.g. free-milling gold
ores or placer deposits); higher extraction costs are associated with ores in which
the element is chemically bound to sulfur or oxide (most base-metal ores) because it
takes more energy to break such chemical bonds than to mechanically liberate a
particle. The highest extraction costs are for ores in which the element is chemically
bound to silicates because these bonds are much stronger than metal-sulfur bonds.
Consider, for example, the two major types of nickel ore: magmatic and lateritic. In
the first, the ore mineral is sulfide (mainly pentlandite, (Fe,Ni) 9 S 8 ) whereas in
lateritic ore it is garnierite (a clay mineral) or goethite (Fe hydroxide). Each type
of ore has its advantages and disadvantages. The capital investment and the energy
required to extract Ni is much higher for the lateritic ores, a major disadvantage in
these days of increasing energy costs; on the other hand, the refinement of sulfide
ore produces vast amount of sulfur, only some of which can the sold as a by-
product.
The grain size and the hardness of the ore influence the cost of grinding it to the
fine powder that is fed into the refinery or smelter. Three Zn-Pb ( Cu) deposits in

Australia provide a striking example (Fig. 1.7 ). All have similar ore grades but the
Broken Hill deposit has been metamorphosed to granulite facies and its coarse ore
is very easy to process; Mt Isa is less metamorphosed and its finer-grained ore is less
attractive; and the virtually unmetamorphosed McArthur River ore is so fine that the
ore metals cannot be extracted from waste minerals by simple crushing.
Also to be mentioned in this category are minor elements that increase or
decrease the value of an ore. In many cases, the ore contains amounts of valuable
metals in concentrations that are below the normal cut-off grade, but if they are
extracted as a by-product during the recovery of the major ore metals they