Geoscience Reference
In-Depth Information
Fig. 7.5 Sectional view of a de-
posit with a pseudo-stratigraphic
control. The lithology units are
represented by red (volcanic brec-
cias) and blue (andesites), with
cross cutting dykes (in purple ).
Blocks are 5 × 5 m and can be
used for scale; the vertical exten-
sion shown is about 800 m. The
block model (with sub-cells) is
overlaid on the geologic model;
supporting drill holes are not
shown. Courtesy of Minera Mich-
illa S.A., Chile
This unit-less factor provides an indication of how important
contact dilution may be. A ratio of 0.05 or higher generally
indicates high contact dilution, and is characteristic of vein-
type, skarns, or thin, tabular deposits, while values less than
0.01 correspond to bulk tonnage, massive, or porphyry type
deposits.
For massive deposits, contact dilution is generally a
local issue, since the bulk of the tonnage will be mined
away from contacts, and therefore its importance from a
global resource model may be limited. Still, it can impact
the positioning of a final pit wall or stope, as well as the
corresponding volume of waste that needs to be removed
to access the ore (mining strip ratios). It is a very different
case for skarn-type and small, narrow tabular or vein-type
deposits, where contact dilution may be the most conse-
quential type of dilution.
Figure 7.5 shows a cross section of a lithology model
for the Lince-Estefanía Cu deposit, with the corresponding
block model with sub-cells overlaid on the view. Notice how
the general stratigraphy is crosscut by intrusive dykes. Also
notice that, by virtue of the relative high contact surface area
to volume ratio, the impact of geologic contact dilution is
likely to be significant. The contact dilution can be incorpo-
rated into the block model using two alternative but concep-
tually similar techniques:
1. The sub-cell method, as shown in Fig. 7.5 , provides a
better definition of the geologic contacts. As discussed in
Chap. 3, these sub-cells are then re-blocked to the parent
block size of the model to provide the diluted grades and
maintaining the proportions of each geologic unit within
each block.
2. A direct calculation of the proportion (percentage) of each
unit within each block, storing the percentage of each unit
within the block.
The average grade of the block is expressed as the proportion-
weighted average of the grades of each individual geologic
unit within the block:
= n
V
i
(7.3)
Z
p i
· z
i = 1
where z v represents the block grade average, p i , i = 1,…,n ,
represent the percentage of total mass for each of the n geo-
logic units that may be present in the block, and z i represent
the grade of each individual unit within the block.
Another, less desirable option, is to empirically intro-
duce into the block model factors that penalize the grades of
blocks at or near contacts, according to pre-specified criteria.
This was done, for example, for one of the Escondida Mine's
resource models. In this method, if a contact between a high
grade and waste geologic zones passes through any given
block, the grade of that block is downgraded arbitrarily. The
limitations of this procedure are significant, since the factors
applied are empirical and global, as opposed to diluting ac-
cording to the locally estimated grades.
Another method that can be used to estimate dilution and
ore loss due to geologic contacts is to draw ore envelopes
around the mineralized zones, and then estimate an over-
break, or additional volume for mining. This can be done
on sections or benches, and provides an estimate of the total
grade and tonnage of material that will be recovered. A simi-
lar method is also used by mining engineers to estimate op-
erational dilution. The method is best suited for deposits with
well-defined ore zones with hard boundaries, such as vein
type or epithermal Au deposits.
Geologic contact dilution is quantified from the geologic
model. Thus, the local accuracy of the contact dilution esti-
mate depends on the quality of the geologic model.
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