Page 271 - Introduction to Mineral Exploration

254 B. SCOTT & M.K.G. WHATELEY

Market forecast mental economic specification. With a strip-
What will be the future demand and value for ping ratio of 4:1, 4 t of waste has to be removed
the minerals produced? This important factor for every 1 t of ore recovered; if the mining
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is considered separately below. costs were $US 2 t then the extraction cost
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per tonne of ore is $US10 t . Conversely, if,
Political forecast say, the stripping ratio were 8:1 at the same
If a lease agreement has been negotiated with mining cost then the extraction cost per tonne
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the local government how long is this govern- of ore would be $US 18 t .
ment likely to remain in power? Will a new 1 Open pit mining costs vary from about $US1
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government wish to either cancel or modify to $US3 t of rock extracted. An economy of
this agreement? (See also section 1.5.4.) scale is apparent in that the higher charge is for
production levels of lower than 10,000 tonnes
Financial return per day (tpd), while the lower cost refers to out-
The development of a mine is a form of capital puts exceeding 40,000 tpd of ore plus waste
investment. The main concern is for this in- rock.
vestment to produce a suitable level of return. 2 Underground mining is generally on a
The optimization of this financial return will smaller scale than open pit mining. An output
probably decide the number of years of produc- of 10,000 tpd would be modest for an open pit
tion. As a very general rule the minimum life of but large for an underground mine. This gener-
a mine should be 7–10 years. ally lower level of production is in part related
to the more selective nature of underground
extraction in that large tonnages of waste rock
Production rate
do not have to be removed in order to expose
A commonly used guide for the Average and extract ore. This is exemplified in under-
Annual Production Rate (AAPR) is: ground block caving. Northparkes Cu/Au mine
in Australia is one of the most productive
(ore reserve) . 075 underground mines in the world, producing
=
AAPR
. 65 5 Mt of ore per year (Chadwick 2002). Block
caving is designed as a low maintenance work-
This is a broad generalization but for ore re- ing environment with minimal operating costs
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serves of 20 Mt, the AAPR is 1.5 Mt a , and for and high productivities.
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10 Mt it is 0.9 Mt a . Factors relevant to the 3 Underground mining costs vary greatly de-
annual production rate include the following: pending upon the mining method employed.
In large orebodies where production is mech-
Type of mining anized, costs per tonne of ore extracted can be
The two basic types of mining are either under- comparable with those of an open pit. At the
ground (Fig. 11.1) or from the surface (open pit), other end of the scale, in narrow veins using
(Fig. 11.2). As production from the former can labor-intensive methods, costs can be as high
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be more than three times the cost of the latter, as $US30–40 t .
underground extraction may require at least 4 Generally underground mining costs are
three times the content of valuable components higher than those at the surface. However, as
in the mineralisation to meet this higher cost. the depth of an open pit increases, the ratio
This in turn implies a lower tonnage, but a of waste to ore mined becomes greater and
higher grade of ore reserve and a lower annual more waste rock per tonne of ore is extracted
production rate. (Fig. 11.2). This has the effect of increasing the
A basic characteristic of open pit mining is overall extraction cost until eventually it can
the surface removal of large tonnages of waste equal underground mining costs.
rock (overburden) in order to expose the ore for
extraction (Fig. 11.2). This removal of waste Scale of operations
rock and ore usually proceeds simultaneously The economies of scale mean that usually the
and the resultant stripping ratio of tonnage larger the annual output the lower the produc-
of waste removed to tonnage of ore is a funda- tion cost per tonne of mineralized rock mined.