8
Subdividing Large Laybacks
An initial set of six laybacks was defined based on nested
pits corresponding to several increasing copper prices. This
resulted in six laybacks with 4, 39, 274, 154, 200 and 93
million tons respectively. For the base plan, laybacks 1 and
2 were combined, layback 3 was split into four pieces, lay-
backs 4 and 5 were split into two pieces, resulting in ten
laybacks with 43, 71, 47, 45, 110, 82, 72, 58, 142 and 93
million tons respectively (Figure 9).
The subdivision was achieved by intersecting the first
set of nested pits with additional sets of nested pits which
only assigned value to blocks on the northeastern or south-
western halves of the deposit. The 2.8% value loss for
the larger laybacks is the result of slower access to higher
grade ore at the bottom of layback L3alt (Figure 10a) and
corresponding deferral of cash flows from years 1–4 into
years 6+ (Figure 10b). For different deposits with a more
dramatic distinction between stripping at the top and ore
zones at the bottom, subdivision of large laybacks would
be expected to have a larger value impact and minimize
or eliminate periods of negative cash flow by limiting the
duration and peak amount of stripping investment before a
layback’s ore is exposed.
Stockpiling
The no stockpile case only loses 2.5% of life-of-mine value
because the stockpiled ore has a small margin above break-
even and because of heavy discounting since most of the
stockpiled ore won’t be processed until the end of the mine
life. In addition to some NPV benefit, stockpiling may be
desirable for: resource preservation (12% more ounces pro-
duced vs no-stockpile case) providing a backup ore source
9a) Six laybacks based only on copper price nested pits
9b) Ten laybacks with NE/SW subdivision
Figure 9. Layback design alternatives: plan views (bench 20
slice) and NE/SW sections
10a) Vertical advance comparison
10b) Discounted cash flow profile comparison
Figure 10. Alternate laybacks comparison highlights: solid
lines=six laybacks dashed lines=base case ten laybacks
Subdividing Large Laybacks
An initial set of six laybacks was defined based on nested
pits corresponding to several increasing copper prices. This
resulted in six laybacks with 4, 39, 274, 154, 200 and 93
million tons respectively. For the base plan, laybacks 1 and
2 were combined, layback 3 was split into four pieces, lay-
backs 4 and 5 were split into two pieces, resulting in ten
laybacks with 43, 71, 47, 45, 110, 82, 72, 58, 142 and 93
million tons respectively (Figure 9).
The subdivision was achieved by intersecting the first
set of nested pits with additional sets of nested pits which
only assigned value to blocks on the northeastern or south-
western halves of the deposit. The 2.8% value loss for
the larger laybacks is the result of slower access to higher
grade ore at the bottom of layback L3alt (Figure 10a) and
corresponding deferral of cash flows from years 1–4 into
years 6+ (Figure 10b). For different deposits with a more
dramatic distinction between stripping at the top and ore
zones at the bottom, subdivision of large laybacks would
be expected to have a larger value impact and minimize
or eliminate periods of negative cash flow by limiting the
duration and peak amount of stripping investment before a
layback’s ore is exposed.
Stockpiling
The no stockpile case only loses 2.5% of life-of-mine value
because the stockpiled ore has a small margin above break-
even and because of heavy discounting since most of the
stockpiled ore won’t be processed until the end of the mine
life. In addition to some NPV benefit, stockpiling may be
desirable for: resource preservation (12% more ounces pro-
duced vs no-stockpile case) providing a backup ore source
9a) Six laybacks based only on copper price nested pits
9b) Ten laybacks with NE/SW subdivision
Figure 9. Layback design alternatives: plan views (bench 20
slice) and NE/SW sections
10a) Vertical advance comparison
10b) Discounted cash flow profile comparison
Figure 10. Alternate laybacks comparison highlights: solid
lines=six laybacks dashed lines=base case ten laybacks