Geoscience Reference
In-Depth Information
Fig. 14.38
Structural domains
for the Escondida Norte deposit.
Some estimated oxide bodies are
also shown (in
purple
)
Table 14.15
Oxide and sulfide indicators
Indicator
to be defined as mixed mineralization. If one of the indica-
tors is above 50 % and the other one below, then the block is
defined as either oxide or partial leach, depending on which
indicator is higher. Several comments are relevant:
1. The rules in Table
14.16
are subjective; although logical
and based on the most favorable likelihood for defining
the mineralization units, it is still imperfect, since the
indicator kriging model will tend to smooth out sharp
variations and geological transitions.
2. The indicators are treated independently. This is a limita-
tion of the method, since it is known that the presence of
sulfide and oxide minerals above the secondary enrich-
ment blanket are controlled by similar geological factors,
most importantly the position of the water table through
time. A better alternative may be to use multiple indica-
tor kriging (MIK). In MIK all indicators would be kriged
simultaneously and no decision regarding the sequencing
of estimation would be needed.
2
3. The comparison of the average of the estimated indicators
and the average resulting from a nearest-neighbor assign-
ment show an acceptable agreement with the exception of
one domain of the oxide indicator model.
Figure
14.39
shows a perspective view of the resulting oxide
mineralization model, along with the structural domains de-
fined. This example illustrates one possible methodology
for developing geologic models in circumstances where
(a) the drill hole geologic information is too sparse to be
Leach
Oxide
Partial
leach
Mixed
Oxide
0
1
0
1
Sulfide
0
0
1
1
Table 14.16
Rules for assigning mineralogy units to estimated blocks
Condition
Numeric
code
Mineralization
unit
IKOX < 0.5 y IKSUL < 0.5
1
Leach
IKOX ≥ 0.5 y IKSUL < 0.5
2
Oxide
IKOX < 0.5 y IKSUL ≥ 0.5
4
Partial leach
IKOX ≥ 0.5 y IKSUL ≥ 0.5
5
Mixed
oxide or sulfide minerals or as the proportion of the blocks
with oxide or sulfide mineralization.
For simplicity, a single mineralization unit code was as-
signed to each estimated block above the top of sulphides
(TOS) surface. Therefore, after the kriging runs were com-
pleted, a set of rules for assigning the final mineralization
unit to the block was enforced. Table
14.16
shows these rules
used to define the final mineralization unit in each block
above the TOS in the Escondida Norte deposit.
IKOX and IKSUL represent the interpolated value of the
oxide and the sulfide Indicators, respectively. If both values
are below 50 % (simultaneously), then the block is defined as
leached material. If both values are equal or above 50 %, then
the probability of the block having both (simultaneously)
oxides and sulfides minerals is high enough for that block
2
In subsequent years this modeling methodology was upgraded to a
full MIK estimation of the above-TOS units.
