Geoscience Reference
In-Depth Information
niques, but also present in reverse circulation holes and in, to
a lesser degree, existing diamond drill holes.
The under-representation of TCu grades in the drill holes
was explained by the loss of high grade chalcocite (copper
sulfide), sometimes present in non-crystalline form, and easy
to wash away during the drilling process. Shorter infill drill
holes were less likely to loose such material, and so were the
blast holes, because of their larger diameter, large numbers,
and also awareness of the problem. To improve the short-
term grade and tonnage estimates, it was important to in-
corporate the most recent production information and local
geologic mapping.
Another important requisite is that the quarterly model
be obtained in a short time, hopefully in two or three days of
work, and without requiring significant additional resources
other than those already available. An additional require-
ment is the company's goal: to obtain a model with ± 5 %
accuracy on a monthly basis for both copper grades and ton-
nages above economic cutoff.
The database used for the study and quarterly model up-
dates is the same 15 m composites database used to estimate
the long-term resource model. This included the more recent
infill holes, and also the addition of the current blast hole
database. The blast holes represent the grade of a full 15 m
bench.
ones correspond to the same definition of estimation do-
mains, but after updating for the quarterly model. The area
shown is the complete volume planned to be mined in this
bench in the period considered. Note that there is generally
good agreement between the two models of estimation do-
mains, although there are differences near contacts.
The TCu, SCu, Fe and As grades were estimated using
the same methodology as used in the long-term resource
model, i.e., ordinary kriging, and using the same kriging
plans. The data used was all data available, including blast
holes. The estimation was done on three different estimation
passes, which helped control the influence of each data type.
Blast holes were used only in the first pass, using the small-
est search neighborhood and more data restrictions before a
block could be estimated. This restricted the influence of the
more abundant blast hole data.
Figure 13.2 shows for the same area in bench 2845 the
estimated TCu grades for both the long-term and quarterly
block models. Grades are color-coded according to the leg-
end shown. Note that there are some differences which are
significant for short-term planning, and mostly near con-
tacts. The differences are both gains and losses. The quar-
terly model better delineates areas of high and low grades.
For example, observe at the northern tip of the area shown
(North of coordinate 108,000N) where the quarterly model
predicts a NW-trending higher-grade narrow structure higher
than 3 % TCu, and shown in orange. This high grade corridor
was not predicted by the long-term resource model.
Overall, results from the medium-term model are as ex-
pected. The use of infill drilling and blast hole increases the
grade and metal content of the reserve model, and also in-
creases its variability. The local definition of geology and
grade increases also the confidence level in the estimated
values. The Quarterly model is less smoothed than the long-
term model.
Figure 13.3 shows the comparison of the grade-tonnage
curves of the long-term (LT) and quarterly (QT) models.
Note how both models have very similar tonnages above
cutoff, but the QT model presents slightly higher grades for
most cutoffs. The cutoffs of interest are 0.7 % TCu (direct
mill feed) and 0.3 % TCu (marginal stockpile).
Figure 13.4 shows the grades for the two models by bench
averages, for the Quarterly period beginning February 2002.
Note that most benches have very similar estimated grades,
although there are some where the overall average is some-
what different. This is particularly the case for Bench 2845,
the grades shown also in Fig. 13.2 .
Figure 13.5 shows the comparison of the relative differ-
ences of monthly TCu grade averages of the LT and QT mod-
els for the three-month period beginning in February 2002.
They are compared to the conditional simulation reference
model, which was calibrated to production data. Negative
errors imply underestimation of TCu grades for the month.
13.2.2
Updating the Geologic Model
Since the production geology (bench, face, and blast hole
cuttings mapping) was done by a different group of geolo-
gists than those that map the exploration and infill drill holes,
a prior step of consolidating and homogenizing nomencla-
tures and coding was required.
The lithology, alteration, and mineralization type models
were updated from the existing geologic model (used to esti-
mate the long-term resource model) only within the volume
corresponding to the next three months of production. An
additional area surrounding this volume was also re-modeled
to allow the “tie-in” of the long-term geologic model with
the more detailed Short-term model. The updating of the
geologic model was done by modifying the existing inter-
pretation from the long-term resource model on plan view.
The polygons were adjusted bench by bench, from which
three-dimensional solids were created. It is not necessary to
apply the same level of detail as for the long-term model (see
Chap. 3), since the update is an adjustment of a prior inter-
pretation. If unexpected geologic features are encountered,
then it would be necessary to review the original geologic
interpretation.
Figure 13.1 shows an example of the resulting Total cop-
per (TCu) estimation domains for Bench 2845. The larger
blocks are the long-term resource model blocks, the smaller
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