Geoscience Reference
In-Depth Information
Fig. 7.2 Caterpillar 992 front-end loader used at Cerro Vanguardia's
Osvaldo Diez vein. Cerro Vanguardia is a Gold-Silver deposit located
in the Patagonia Region of Southern Argentina (photo courtesy of Cerro
Vanguardia S.A.). Benches are 5 m high
Fig. 7.3 A hand-specimen approximately 3 inches in size showing
typical Porphyry Cu mineralization (Chrysocolla in Type-D veinlet,
courtesy of BHP Billiton)
sublevel caving methods. In a traditional cut-and-fill opera-
tion, with 5 m lifts, the SMU depends on the geometry of the
orebody, but usually is 5 × 5 × 5 m, assuming that the mine
can separate ore and waste from the stope.
The definition of an SMU is convenient for block mod-
eling, but does not realistically represent the extraction
process: shovels and loaders do not load cubes! Moreover,
individual SMUs cannot be selected independently although
the concept of an SMU assumes free selection. The actual
practice of ore and waste selection shows that the SMU con-
cept is a convenient approximation. Mining along boundar-
ies is generally more selective than the nominal SMU size
for the mine, and typically an isolated SMU-size pod of
waste or mineral will not be mined.
The photo in Fig. 7.3 is a hand specimen of typical Porphyry
Cu mineralization, where, within the solid rock mass, high-
grade veinlets of Chrysocolla (Cu mineralization) are seen. If
this mineralization was to be sampled on a very fine scale, the
dispersion of the Cu grades resulting from the laboratory as-
says could be represented by a distribution like the one shown
in Fig. 7.4 , top. If the sample volume taken were to be larger,
then there would be more mixing of material in any given
sample, thus the higher-grade veinlets being mixed with the
lower grade material surrounding them. In this case, a distribu-
tion like the one shown in Fig. 7.4 (bottom) may be obtained.
Note that the means of the distributions are the same
(grades are mass fractions and they scale up linearly, so that
the overall average is maintained), but the standard devia-
tion and coefficient of variation is smaller for the larger vol-
ume distribution. Also, the minimum and the maximum of
the distribution are closer to the overall mean. There is also
a general tendency for the larger-volume distribution to be
more symmetric than the original distribution.
Since mineralization is not homogeneous, mixing of dif-
ferent grade material always occurs. This is true for all types
of mineralization, and depends on the nature of the geologic
events that produced the mineralization. The presence of
mineralized veinlets, highly fractured zones or units, and
more or less permeable lithologies impact the amount of in-
ternal dilution to be expected.
Geologic Contact Dilution is defined as the dilution and
ore loss resulting from the extraction of material of different
geologic characteristics. This type of dilution can often be
accounted for when using sub-cells or partial blocks in the
definition of the resource block model (Chap. 3): the grades
and other characteristics of each geologic unit that comes
7.2
Types of Dilution and Ore Loss
There are several sources of dilution and ore loss. Dilution
and ore loss are always closely linked, and references to dilu-
tion include both cases. The main sources of dilution may be
classified into three different categories (Rossi 2002 ):
Internal Dilution or Change of Support is a consequence
of predicting resources at a different volume than the original
data (Parker 1980 ). The resource estimate requires a degree of
averaging within blocks and is generally modeled using the
volume-variance or change of support correction, as discussed
in detail in the next section. This mixture of material necessar-
ily includes high and low grade mineralization, which will be
more significant if the mineralization is less continuous. Also,
the larger the block size considered, the larger the amount of
mixing of mineralization or internal dilution.
 
Search WWH ::




Custom Search