THE CONTOUR METHOD

Contouring of grade times width values is commonly used as an aid in exploration, to recognize and project mineralized trends.

A more rigorous application of the contour method can also be used for reserve estimation.

The contour method is useful for estimating reserves of planar mineralized bodies intersected by a number of drill holes. The contour method is a smoothing technique that allows the geologist to apply judgment about the variability of the mineralization within the plane of the mineralized body.

The technique is particularly effective in dealing with metal deposits with significant assay variability, such as gold deposits.

In this type of deposit, contouring limits the areal influence of local high metal values (“nugget effect”) by scaling the contours between adjacent drill holes. This is done instead of cutting high assays to an arbitrary value and projecting them halfway to adjacent drill holes.

General procedures for the contour method are described below, and an example with explanatory notes is illustrated in Figures 1 to 4.

Procedures

Prior to applying the contouring method of reserve estimation, it is necessary to confirm the reliability of the database, as for all other reserve estimation methods. In particular, it is necessary to confirm that drilling to outline the reserve is of sufficient density for the particular commodity and style of mineralization.

Although the contouring method is a two-dimensional approach, a critical preparatory step is understanding the three-dimensional geometry of the mineralized body. Geological continuity must be established by correlating rock units, structures and alteration zones, using a reasonable geological interpretation. This is generally done on sets of drill hole cross sections and level plans, with interpretation and correlation confirmed from section to section. Once the geologist is satisfied that the mineralization forms a discrete deposit rather than a seemingly random collection of assays, the reserve estimate can be continued.

Assay values and widths of each intersection are plotted on a longitudinal section or plan, including those with low or nil values. These values are then contoured to represent the variability of metal values and thicknesses within the mineralized body. Assay values are generally plotted as grade times thickness (GT) to reflect metal content. Separate maps are required for each variable included in the estimate.

The orientation of the section or plan must be as close to parallel as possible to the mineralized plane and is usually vertical or horizontal for convenience. For an idealized tabular deposit. the volume of a segment along the plane of the deposit is the same as the volume projected on the vertical longitudinal section. A different longitudinal section or plan must be constructed for each separate mineralized body, particularly if they overlap. It is confusing, even misleading, to plot intersections from different lenses on the same longitudinal section.

Limits of mineralization should be clearly depicted on the longitudinal view, either as a geological constraint (fault, contact, end of host unit, etc.) or by low grade or blank holes in the plane. Limits may also be imposed by a minimum cut-off grade boundary.

CONTOURING MUST REFLECT GEOLOGICAL INTERPRETATION

Areas between thickness contours are measured, multiplied by the weighted average thickness for each contour interval, and summed to obtain total volume of the mineralized body. This volume is then converted to tonnage using an appropriate density factor.

Similarly, the areas between metal value (GT) contours are measured, multiplied by the average GT value for each contour interval, and summed to obtain a measure of total metal content of the mineralized body. This total is then divided by total volume to obtain the average grade.

The way that the GT data are contoured affects the final results, since the contouring represents the distribution of metal values within the deposit. It is, therefore, important that the contouring reflect the geologists understanding of the distribution of the mineralization, including perceived linear trends and geological constraints, together with knowledge of the type and style of mineralization. Similarly, the way that thickness values are contoured affects the tonnage, and care must be taken that the contours represent the geometry of the deposit.

The contour intervals should be chosen to maintain a relatively even contour spacing. For some metals, such as those with highly skewed (“nuggety”) statistical distributions, a geometric rather than a linear contour interval is more appropriate. For example, Archean lode gold deposits may be better represented by using geometric or logarithmic contour intervals, whereas linear contour intervals may be more appropriate for massive base metal sulphide deposits. Final selection of the contour intervals will depend on an appreciation of the statistical distribution of assays for the deposit being contoured. The type of contour interval — geometric, linear or logarithmic — chosen for one parameter (thickness) will often, but not always, apply to the other parameters (grade and GT).

MULTIPLY THICKNESS AND GT VALUES BY SG

Where considerable variation in density or specific gravity (SG) is documented within a mineralized body, and sufficient data are available, drill hole intersections on the longitudinal view can be weighted by SG prior to contouring. That is, both thickness and GT values are multiplied by the SG for each drill hole intersection.

Contour Method Example

Figure 1 shows thickness data for a hypothetical deposit. Data points have been derived by converting drill hole data to vectors at right angles to the projection plane. In general, these vectors are derived by multiplying the drill hole intercepts with a set of trigonometric functions and applying a correction with respect to the plane of the projection.

The cutoff grade position is obtained by contouring grade data. The cutoff, four grams per tonne, was again selected hypothetically.

Volume of the deposit within the cutoff grade limit is obtained by measuring the areas of each contour interval, and multiplying by the average thickness for each interval. These are summed to obtain the total volume, which is converted to tonnage using the average specific gravity (SG).

Figure 2 shows Grade X Thickness (GT) for the same deposit. Geometric contour intervals are used in this case because of the skewed data distribution.

The area within each contour interval is measured within the cutoff limit of the mineralization. Each area is multiplied by the geometric average GT value for each contour interval. The average grade is obtained by dividing the total “metal content” by total volume.