116
To assign a mineral resource to an Indicated category, the geological and grade continuity
must be established directly from available data. Spatial distribution of the data should allow
confident interpretation of the geological framework. It should also reasonably reflect the
continuity of the mineralization. For this purpose, there are a number of factors that can be
applied by QPs for resource classification:
• Data quality
• Structural setting
• Geological continuity
• Drill hole spacing
• Grade continuity, often based on variogram analysis
• Applied cut-off for resource reporting
• Data used to estimate a block:
− Number of drill holes
− Number of samples
− Number of octants
− Average distance to informing composites
The most often discussed and used component for resource classification is drill hole spacing. This is evident from NI 43-101 reports the authors reviewed. Management of exploration
companies often ask the authors what drill hole spacing would be necessary for allocation of
resources to at least an Indicated category. The answer is never straightforward. Allocation to
an Indicated category is subjective—assignment to the category is often open to interpretation and potential modifications.
Fortunately, there are geostatistical tools that can make the assignment to an Indicated category less subjective. This paper focusses on two primary groups of methods: (1) those aimed
at a general assessment of drill hole spacing and may be applicable at early stages in a project;
and (2) those aimed at local assessments of drill spacing while accounting for local grade variability which may be more relevant for intermediate to advanced studies. Finally, an approach
to design the location of infill drill holes for the purposes of classification upgrading is proposed. This may be important as a project advances towards feasibility and the focus of an
infill drill campaign may be to strategically drill in areas to upgrade Inferred areas to an Indicated category. The examples presented in this paper are based on a low grade gold deposit in
Nevada, US. For this presentation the grades have been modified by a constant factor.
The compiled methods and procedures laid out in this paper were specifically chosen for
their practicality and accessibility to the general mining practitioner. The authors recognize
that other, more advanced approaches have been proposed; however, many of them are not
considered standard in the industry at this time.
2 GENERAL ASSESSMENT OF DRILL HOLE SPACING
One of the simplest tools often used for an assessment of drill hole spacing for the Indicated
category is an indicator variogram (Journel, 1983; Journel, 1985). Drill hole composite assay
data are converted to 0/1 indicators based on a predefined cut-off, similar to the cut-off used
for resource reporting. If a composite assay is higher or equal to the cut-off it is converted to
a value of one. Otherwise the assay is given a value of zero. A modelled range of continuity
from a variogram calculated on the indicators is the first pass assessment of necessary drill
hole spacing for the Indicated category. It is usually applied at early stages of an exploration
project. A 2/3 of the modelled range of continuity is often given for the drill hole spacing.
For example, as shown in Figure 1(a), the range of continuity is approximately 250 ft, which
indicates drill hole spacing of approximately 160 ft.
Another, less known, tool is based on a technique called “p-gram” (Srivastava, 2004;
Leuangthong and Nowak, 2015). To construct p-grams, composites above a predefined cut-off
grade are coded with an indicator variable. Using a process similar to indicator variography,
To assign a mineral resource to an Indicated category, the geological and grade continuity
must be established directly from available data. Spatial distribution of the data should allow
confident interpretation of the geological framework. It should also reasonably reflect the
continuity of the mineralization. For this purpose, there are a number of factors that can be
applied by QPs for resource classification:
• Data quality
• Structural setting
• Geological continuity
• Drill hole spacing
• Grade continuity, often based on variogram analysis
• Applied cut-off for resource reporting
• Data used to estimate a block:
− Number of drill holes
− Number of samples
− Number of octants
− Average distance to informing composites
The most often discussed and used component for resource classification is drill hole spacing. This is evident from NI 43-101 reports the authors reviewed. Management of exploration
companies often ask the authors what drill hole spacing would be necessary for allocation of
resources to at least an Indicated category. The answer is never straightforward. Allocation to
an Indicated category is subjective—assignment to the category is often open to interpretation and potential modifications.
Fortunately, there are geostatistical tools that can make the assignment to an Indicated category less subjective. This paper focusses on two primary groups of methods: (1) those aimed
at a general assessment of drill hole spacing and may be applicable at early stages in a project;
and (2) those aimed at local assessments of drill spacing while accounting for local grade variability which may be more relevant for intermediate to advanced studies. Finally, an approach
to design the location of infill drill holes for the purposes of classification upgrading is proposed. This may be important as a project advances towards feasibility and the focus of an
infill drill campaign may be to strategically drill in areas to upgrade Inferred areas to an Indicated category. The examples presented in this paper are based on a low grade gold deposit in
Nevada, US. For this presentation the grades have been modified by a constant factor.
The compiled methods and procedures laid out in this paper were specifically chosen for
their practicality and accessibility to the general mining practitioner. The authors recognize
that other, more advanced approaches have been proposed; however, many of them are not
considered standard in the industry at this time.
2 GENERAL ASSESSMENT OF DRILL HOLE SPACING
One of the simplest tools often used for an assessment of drill hole spacing for the Indicated
category is an indicator variogram (Journel, 1983; Journel, 1985). Drill hole composite assay
data are converted to 0/1 indicators based on a predefined cut-off, similar to the cut-off used
for resource reporting. If a composite assay is higher or equal to the cut-off it is converted to
a value of one. Otherwise the assay is given a value of zero. A modelled range of continuity
from a variogram calculated on the indicators is the first pass assessment of necessary drill
hole spacing for the Indicated category. It is usually applied at early stages of an exploration
project. A 2/3 of the modelled range of continuity is often given for the drill hole spacing.
For example, as shown in Figure 1(a), the range of continuity is approximately 250 ft, which
indicates drill hole spacing of approximately 160 ft.
Another, less known, tool is based on a technique called “p-gram” (Srivastava, 2004;
Leuangthong and Nowak, 2015). To construct p-grams, composites above a predefined cut-off
grade are coded with an indicator variable. Using a process similar to indicator variography,