Geoscience Reference
In-Depth Information
Fig. 14.50
Schematic work flow,
Lince-EstefanÃa
were reblocked and assigned to the same blocks used in the
Resource Model. This allows for a direct comparison be-
tween the two models, and to assign an uncertainty model to
the predicted grade. Figure
14.50
is a schematic representa-
tion of the general work flow at Minera Michilla.
Five-meter down-the-hole composites were used for both
estimating and simulating the resources. The composites are
tagged according to the interpreted geologic units, and the
length is chosen due to the selectivity required in both the
underground and open pit mines. In the case of the open pit,
even though the nominal bench height is 10 m, the opera-
tion works by cleaning out areas of waste surrounding the
mantos, and sometimes mining partial, 5 m benches where
deemed necessary. Blast holes are sampled at half-benches,
that is, there are two 5 m blast hole sample for every hole
drilled to the 10 m bench. These 5 m blast hole data were
used in the open pit area (Lince) to improve the definition
of the variogram models and to better condition the simula-
tion in mined-out areas and close to the current topographic
surface at the time.
The SIS technique requires the definition of indicator
thresholds in order to discretize the original grade distribu-
tion. The definition of these indicators was identical for the
estimation and the simulation models, and included the fol-
lowing indicator classes: from 0.0 to 0.19 % TCu; from 0.2
to 0.49 % TCu; from 0.5 to 0.79 % TCu; from 0.8 to 0.99 %
TCu; from 1.0 to 1.19 % TCu; from 1.2 to 1.49 % TCu; from
1.5 to 1.99 % TCu; from 2.0 to 2.99 % TCu; from 3.0 to
4.99 % TCu; from 5.0 to 6.99 % TCu; from 7.0 to 9.99 %
TCu; and greater than 10.0 % TCu. The corresponding de-
clustered class means were obtained for each class, using the
median of the last class to avoid over-estimation of the high
grade portion (outlier control).
14.5.2
Developing the Model of Uncertainty
The geologic model includes the definition of a mineralized
envelope, defined at 0.1 % TCu. The purpose of the mineral-
ized envelope is to define the volume within which miner-
alization can exist; outside this volume, there is no mineral-
ization. In this sense, this is a geochemical-type boundary,
whose purpose is to avoid overestimation of the kriging pro-
cess into barren areas. This is necessary because mineral-
ized bodies can abruptly end due to a post-mineral fault, or
without any obvious reasons. This same envelope was used
as an external hard limit to define the overall volume of the
simulation model.
In addition, there are three major geologic units (GUs)
defined (in fact, lithological groupings), which are used to
define estimation domains at the time of grade estimation.
The units are volcanic breccias (generally mineralized), an-
desites (could be mineralized, but generally barren or poorly
mineralized mantos), and intrusives (including tectonic brec-
cias), which are mostly barren, but could occasionally pres-
ent significant mineralization.
