Geoscience Reference
In-Depth Information
4.3
Case Study: Estimation Domains
Definition for the Escondida Mine
A similar process of developing new variables for lithol-
ogy and alteration was completed. The grouping of initial
mapped elements resulted in three alteration codes, QSA,
SCC, and K-B (white, green and potassic-biotite alteration,
respectively), and three lithologies: porphyry, andesite, and
rhyolite.
With respect to lithology, the following characteristics are
noted:
•  Tuffs were grouped with Rhyolite, (PC).
•  The following codes were ignored due to lack of spatial 
representation: Dacites, Gravels, Tectonic Breccias, Undif-
ferentiated Porphyry (9), Diorites, and Pebble Dykes.
•  Hydrothermal  Breccias  and  Igneous  Breccias  were 
grouped with the main Escondida Porphyry unit.
With respect to alteration, the following groupings were made:
•  A new code QSA was formed grouping all Quartz, Seric-
ite, and Clays (Sericite, Clays, Silicified, and Advanced
Argillic). This also known as white alteration.
•  Similarly, a new SCC code was formed by grouping Pro-
pilitic, Sericite-Chlorite-Clays, K-S Transition in Por-
phyry, Silicified in Andesites, and Silicified in Porphyry.
This is sometimes referred to as “green alteration” because
of the presence of chlorite. Propilitic alteration is very dif-
ferent from the other components of this SCC grouping
described. However, it is deemed pertinent here because
there are very few intervals coded as propilitic alteration.
Normally, in other porphyry deposits, propilitic alteration
is observed as a halo on the outskirts of the deposit, and
would be advisable to model it separately.
•  A third alteration K-B was formed by grouping Potassic 
(K) and Biotite alterations (B).
•  The fresh, unaltered rock is volumetrically unimportant 
and was ignored.
With the simplification of the original codes, completed by
Escondida geologists, the basic elements of the geological
model have been defined, and combinations of these ele-
ments define the initial set of estimation domains.
Five structural domains were identified based on observa-
tions in the pit and drill hole data. At Escondida, like most
mineral deposits, structures control the spatial distribution
of TCu grades in different areas of the deposit. Figure 4.1
shows the domains and the current pit projection as modeled
by structural geologists. Domain 5 (to the West of the depos-
it-bounding Ferrocarril fault, in brown) was not considered,
since there is no evidence of mineralization. The basic build-
ing blocks for defining the estimation domains were defined
with the remaining four structural domains.
The process of defining estimation domains is best illustrat-
ed with an example. The following has been taken from a
definition of Total Copper (TCu) estimation domains at BHP
Billiton's Escondida copper deposit. It is reproduced here
courtesy of BHP Billiton, Base Metals Division.
Not all the aspects of a given geologic variable are valid
or useful at the time of defining estimation domains. The
first step in the process is to define those aspects that will
be considered. This initial selection of important geologic
attributes should be decided by the geologists who know the
deposit well. An understanding of how geologic variables
may impact resource estimation is also required.
The definition of estimation domains at Escondida was
greatly assisted by the operating mine. The open pit afforded
the opportunity for confirmation by direct observation of the
assumed relationships described by the drill hole data. At Es-
condida, the geologic variables considered were mineraliza-
tion type, alteration, lithology, and structural domains.
In the case of mineralization types, all high enrichment
mineralization (HE1, HE2, and HE3) would be modeled as
below Top of Sulfides (TDS) and above Top of Chalcopy-
rite (TDCpy). It was shown through statistical and additional
chemical analyses that the Covelite-Pyrite (Cv + Py) unit has
the statistical and spatial characteristics of low enrichment
mineralization. Also the Chalcocite-Chalcopyrite-Pyrite
(Cc + Cpy + Py) unit has characteristics of high enrichment
mineralization, particularly for higher benches where the
proportion of Cc in this unit is more significant. This is to
be expected, since these mineral assemblages are transitions
from higher to lower enrichment mineralization.
Following similar reasoning for all alteration, lithology
and structure categories, the original codes in the database
were translated into a simplified version, and are shown in
Table 4.1 . The most important characteristics of the resulting
mineralization codes are the following:
•  All  mineralization  with  some  cuprite  described  was 
grouped into a single code (Cuprite, Cuprite + Ox,
Cuprite + Mx, and Cuprite + Cc + Py into Cuprite). This is
because Cu cannot be recovered from cuprite using the
existing processing facilities, and is detrimental to the
overall Cu recovery in a flotation plant.
•  High Enrichment is defined as of Cc + Py and Cc + Cv + Py.
•  Low  Enrichment  groups  the  units  Cc + Cpy + Py, 
Cc + Cv + Cpy + Py, Cv + Cpy + Py, and Cv + Py.
•  All primary mineralization is lumped into one category 
(Py, Cpy + Py, and Bn + Cpy + Py), because, at the time of
the study, the bulk of the processed ore will come from
enriched mineralization units.
•  All  other  elements  used  are  the  original  codes:  Leach 
(code 0), green Oxides (code 1), Partial Leach (code 4),
and Mixed (code 5).
4.3.1
Exploratory Data Analysis of the Initial
Database
The database consisted of 2,140 drill holes with 215,681 as-
says, lithology, mineralization, and alteration records. Histo-
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