Geoscience Reference
In-Depth Information
Table 4.3
Estimation domains for total copper, Escondida 2001 resource model
Estimation
Domain
Mineralization
Lithology
Alteration
Structural
Domain
0
LIX (0)
ALL
ALL
ALL
OXIDE (1)
ALL
ALL
ALL
1
2
CUPRITE
ALL
ALL
ALL
3
PARTIAL LEACH
ALL
ALL
ALL
4
MIX
ALL
ALL
ALL
5
ALL
PC+TB
(Rhyolite+Tuffs)
ALL
ALL
6
6+9
(Cc+Py; Cc+Cv+Py)
ALL
QSA (1)
1+4
7
6+9+7+10+13+14
(Cc+Py; Cc+Cv+Py; Cc+Cpy+Py;
Cv+Py; Cc+Cv+Cpy+Py;
Cv+Cpy+Py)
ALL
SCC (2)
1+4
8
6+9
(Cc+Py; Cc+Cv+Py)
ALL
QSA (1)
3
9
6+9
(Cc+Py; Cc+Cv+Py)
ALL
SCC (2)
3
10
7+10+13+14
(Cc+Cpy+Py; Cv+Py
Cc+Cv+Cpy+Py; Cv+Cpy+Py)
ALL
QSA (1)
1+4
11
7+10+13+14
(Cc+Cpy+Py; Cv+Py
Cc+Cv+Cpy+Py; Cv+Cpy+Py)
ALL
QSA (1)
3
12
7+10+13+14
(Cc+Cpy+Py; Cv+Py
Cc+Cv+Cpy+Py; Cv+Cpy+Py)
ALL
SCC (2)
3
8+10+12
(Cpy+Py; Py; Bn+Cpy+Py)
ALL
K+B (3)
ALL
13
6+9+7+10+13+14
(Cc+Py; Cc+Cv+Py; Cc+Cpy+Py;
Cv+Py;Cc+Cv+Cpy+Py;
Cv+Cpy+Py)
ALL
ALL
2
14
to define the pairs, but a true three-dimensional method is
preferred to avoid directional biases. In this method, pairs
within pre-specified distances are found through a three-
dimensional search of nearby assay intervals belonging to a
different unit.
Figure
4.13
shows the grade averages at either side of
the contact between the Cc + Py and the Cc + Cpy + Py units
from the Escondida case study. Each point in the figure cor-
responds to the TCu average grouped at 2 m distance classes
from the contact. Despite the high variability in the averag-
es, the grade transition is smooth, from higher grades in the
Cc + Py unit to lower grades in the Cc + Cpy + Py unit, and as
would be expected from units that are defined as transitional
mineralogical assemblages. A trend could be modeled as a
function of distance from the contact.
Another example (Fig.
4.14
) shows that the profile of av-
erage TCu grades at the contact between the final estimation
Domains 6 and 7 at Escondida (see Table
4.3
) is hard. In this
case, the TCu grades change significantly crossing from one
unit to the other in a very short distance. Therefore, it is not
advisable to use composites from estimation Domain 7 to
estimate TCu grade in estimation Domain 6.
Considering stationary domains in the presence of soft
boundaries is often inappropriate. In general, soft boundaries
as the one shown in Fig.
4.13
are characterized by a non-
stationary behavior near the contact. The mean, variance or
covariance are not constant within a zone of influence of one
rock type into the other and their values depend on the loca-
tion relative to the boundary, as illustrated by Fig.
4.15
.
The correct reproduction of soft boundaries in resource
models improves dilution and mineral resource estimates.
The areas close to contacts are usually areas of higher uncer-
tainty, as shown by the abundance of red colors in Fig.
4.16
.
In the presence of complex contacts and multiple bound-
aries, it may be appropriate to model the non-stationary fea-
tures present in the local neighborhood. The non-stationary
features of the mean, variance, and covariance can be param-
etrized into a local model of coregionalization (Larrondo and
Deutsch 2005). Estimation of the grades can be performed
using a form of non-stationary cokriging (Chap. 8).
Trends within estimation domains are also common. In
certain circumstances, trends need to be explicitly modeled
or taken into account, particularly when simulating grade
distributions (Chap. 10). In other instances, such as grade es-
