Geoscience Reference
In-Depth Information
.
Fig. 13.2
TCu grades for the
long-term and quarterly models,
Bench 2845, Escondida Mine
off grades may be complex and site specific. Many different
costs and variables may come into play. One possible defini-
tion of a processing (also called marginal or in-pit) cutoff
grade is:
of unknown true values for each block are plotted against the
corresponding estimated grades. The most important task in
grade control is to avoid as much as possible sending mate-
rial to the wrong destination.
Chapter 7.4 discussed the issue from the point of views
of the Information Effect, including perfect and imperfect
selection. In traditional geostatistical literature the term im-
perfect selection is used to signify that the decision is based
on estimates of grade, and without the knowledge of the true
values. Perfect selection is thus impossible, because we can
never know the true in-situ grades.
Another consideration is that free selection is impossible.
Ore and waste blocks cannot be selected independently of
each other during mining. This causes dilution and ore loss.
There are also other practical (operational) factors affecting
the decision, including how exactly the ore/waste markers
have been laid out in the extraction area; a certain amount of
unavoidable dilution (unplanned operational dilution); and
mistakes made at the time of extraction, including some as
simple as sending the loaded truck to the wrong destination.
(
)
c cc
+−
c
t
o
w
z
=
z
=
t
c
c
pr
pr
where c
t
is the unit treatment (milling) cost; c
o
is the unit ore
mining cost; c
w
is the unit waste mining cost; r is the metal
recovery factor; p is the unit metal price; and z
c
is the grade
that makes revenue nil. In this marginal cutoff equation,
costs such as General and Administration (G&A) and mining
costs are not considered, only the additional costs that may
exist when mining ore as opposed to waste. This cutoff grade
is applicable when the operation has already committed to
moving the material. The only remaining decision is whether
it is sent to the waste dumps, stockpiled, or processed.
Grade control attempts to minimize miss-classification.
The basic issue is shown in Figure
13.6
, where a scatterplot
