Geoscience Reference
In-Depth Information
Table 3.1
Lithology mapped, logged, and modeled, October 2001 Resource Model (used with permission from BHP Billiton)
LITHOLOGY
Mapped/Logged codes
Modeled (Oct 2001 Model)
Feldspar Porphyry (Escondida Porphyry)
PF
Modeled as such
Rhyolite
PC
Modeled as such
Undifferentiated Porphyry
PU
Modeled either as PF or AN depending on its spatial location
Andesites
AN
Modeled as such
Igneous Breccias
BI
Modeled as Breccias (single unit)
Hydrothermal Breccias
BH
Modeled as Breccias (single unit)
Tectonic Breccias
BT
Modeled as Breccias (single unit)
Gravel
GR
Modeled as such
Late Dacites
DT
Absorbed by the major unit that contains it
Diorites
DR
Absorbed by the major unit that contains it
Tuffs
TB
Absorbed by the major unit that contains it
Pebble Dykes
PD
Absorbed by the major unit that contains it
encountered problem is that information related to a drill
hole (geologic mapping; topographic survey of the drill
hole collars; down the hole survey of the drill hole incli-
nations; laboratory assay certificates, etc.) is disorganized
and misplaced. It may be located in different file cabinets,
different offices, or in different parts of the world. This
results in a high probability of losing costly information.
The recommended solution is to keep individual binders
(one per drill hole) with all its relevant information, and
a backup in a different location. Another common over-
sight is improper backup procedures for computerized in-
formation.
g. Determination and modeling of the mineralization con-
trols. Lithology, alteration, mineralization, structural,
and other relevant information must be analyzed and in-
terpreted. This data must be maintained even if not all
these variables result in interpretable mineralization con-
trols. This process should utilize a combination of field
observations, plausible genetic theories, and should make
extensive use of statistical tools (see Chaps. 2 and 4) to
identify geologic controls. The process is iterative and
should be started as soon as there is enough information
to statistically describe relationships between grades and
geology.
h. Development of a geologic model that adequately cap-
tures the mineralization controls for estimation domains
and grade estimation. This is in addition to the working
geologic model used for exploration.
i. Effective presentation and communication of the model
should be considered an essential part of the work itself
(Peters
1978
). The use of visualization tools such as
three-dimensional models, two-dimensional cross sec-
tions and plan views are essential. Appropriate scales
typically range from 1:200 to 1:1,000. Plots should show
color-coded drill hole information (geology and assay),
working or final polygons representing the interpreted
geologic variables, and topographic and/or bedrock sur-
faces. All drill holes should be properly identified. A
plan view at the top of the plot showing the drill hole
trace is also convenient if showing a cross or longitudi-
nal section. Three-dimensional visualization tools should
be routinely used both for validation and presentation
purposes.
Tables
3.1
,
3.2
, and
3.3
show an example taken from the Es-
condida mine in northern Chile operated by BHP Billiton for
lithology, alteration and mineralization type variables. The
tables show the variables mapped, logged, and then modeled
as of 2001.
There can be several reasons why a given unit is not mod-
eled. For example, Dacites, Diorites, Pebble Dykes, and
Tuffs are absorbed into the major unit that surrounds them
at the time of modeling, because their spatial extent is not
significant compared to the scale of mining (Table
3.1
).
Undifferentiated Porphyry is either a transition between an
Andesitic-type rock and the feldspar porphyry, or it is logged
as such because the sample is too altered or broken up to
be properly recognized. In either case, Undifferentiated Por-
phyry is usually located along the Andesite—Escondida Por-
phyry contact; therefore, it is assimilated to one or the other
according to which one is closest.
The mineralization types (Table
3.2
) are considered the
most important mineralization controls. More of the mapped
and logged units are actually modeled. Also, different min-
eralization types are routed to different processing plants.
Oxide mineralization is recovered using an acid leaching
process with a solvent extraction and electro-winning (SX-
EW) recovery plant, while sulphide mineralization (high
enrichment, low enrichment, and primary mineralization)
is processed in a floatation plant. The units with cuprite are
modeled together because they are quite small.
More grouping is done for alteration zones because they
are more difficult to accurately map. There are a number of
transitional units with mixtures of different alteration events,
which complicate their mapping and modeling. Therefore,
the tendency of geologists is to model only the major units,
see Table
3.3
.
This example shows that some variables are mapped and
logged, but not necessarily modeled. This example is spe-
