Geoscience Reference
In-Depth Information
Fig. 3.17
Straightening function
3.4.3
Block Models
Block models describe the three-dimensional volumes with
(relatively) small-sized parallelepipeds. Block models are
convenient tools for mine evaluation, resource estimation
and mine planning, including pit or stope optimization, and
mine scheduling. The vast majority of mineral resource es-
timates are obtained using block models. There are excep-
tions, particularly in early exploration stages and where no
computerized modeling techniques are used. Preliminary
estimates can be obtained by manual calculations, typically
on cross sections, where areas of influence are drawn on
paper, and then projected in between sections. For each of
these areas, an estimated grade is obtained by a weighted
arithmetic average which, along with the dimensions of the
area and an assumed specific gravity, can be used to esti-
mate the tonnage and grade of each section (Stone and Dunn
1995; Sinclair and Blackwell
2002
).
The geometry of the block model depends on the charac-
teristics of the deposit, the geological features being mod-
eled, and mine planning requirements, such as equipment
size and type to be used by the operation. Block size and
geometry is an important decision in resource modeling.
Two-dimensional models may be appropriate for strati-
graphically controlled sedimentary or evaporitic deposits
including coal, heavy sands deposits, oil sands, some ura-
nium deposits, and some industrial minerals, such as nitrates,
iodine, and boron mineral deposits. Typical applications of
two-dimensional models relate to modeling surfaces such as
topography and surfaces that define different geologic char-
acteristics. Sometimes block models are defined as a stacked
set of two-dimensional models in the presence of sequences
of mineralized strata.
Three-dimensional grids are used to model massive de-
posits such as porphyry copper deposits, massive sulphide
deposits, skarn, vein-types, and other types of tabular and
a
b
Fig. 3.18
a
Normal fault transform
b
Reverse fault transform leads to
multiple Z values at some locations
sedimentary or pseudo-sedimentary deposits with significant
development in the third dimension.
3.4.4
Block Size
The block size should be decided based on the drill hole data
spacing and other engineering considerations. Larger blocks
are easier to estimate than smaller blocks in the sense that
the predicted grades are more likely to be close to the actual
grade of the block. On the other hand, too large a block size
is not useful for pit optimization and mine planning. Typical
