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hole records. That method can produce reliable estimates except in one important step:
Seedorff et al. contoured "isopleths" of the product of grade and thickness at each drill
hole. Grade and thickness are not correlative in the Carlsbad Potash Mining District, and
that method thus will induce error. The Seedorff et al. study did include maps of each ore
bed, which were useful in comparing this evaluation to theirs.
Selection of a computer program to calculate in-place volumes and grades.
There are a number of geology-oriented computer programs that could be used to determine
ore resources in-place based on drill hole intercepts. One of the more widely used
programs is MacGridzo marketed by RockWare Earth Science Software. This particular
program was selected because of its ease of use, the ability to readily perform
mathematical calculations on individual grids (cells), perform su=ations of selected
areas, etc. In addition, experience existed in applying the MacGridzo program to
determine ore reserves at a nearby potash mine. In that study there was the ability to
compare computer-generated data based on simple drill-hole information with actual
results from mining that reserve. This provided confidence in the MacGridzo program as
a useful tool in estimating potash reserves.
A separate spreadsheet program was used for inputting the drill-hole information.
and another software program to compile histograms and charts of output data generated
within the MacGridzo program. Figure 1 diagrams the methodology used to input,
calculate and output data using the MacGridzo program.
Brief description of the MacGridzo program. MacGridzo places a rectangular
grid over a set of randomly spaced information data points (in our case the selected ore
intercept information: depth. thickness, and grade). This increment of the hole data can
be considered the "Z" component. while the hole location provides the "X" and "Y" of a
two-dimensional array.
The program then calculates a unique value for the Z-component at the center of
each grid based on interpolation of the Z-component values of nearby holes. To accomplish
these calculations, the program can be set on either a "radial search" or simply a
method based on the nearest set of drill-hole information. The radial search mode was
used to avoid emphasis on a cluster of hole~: -.;;
Once the grid values are determined for any parameter such as grade or thickness,
they can be recalculated using simple algorithms. For example, the grade can be adjusted
for thickness to obtain the adjusted grade based on an increase in mining height. Similarly.
the toDS in that grid can be determined for the new thickness using a "tonnage factor".
In the case of langbeinire ores, the tonnage factor was also adjusted per grid based on the
percent langbeinire in the ore. An important capability is that the thickness values can be
assigned independent of the grade values. ThIs is a distinct advantage over the method
used by Seedorff et al. (1978). Another advantage of the MacGridzo program is contour
plots, whether it be grade, ratio of sylvite to langbeinite. thickness. products of thickness
and grade. or structure of the top or bottoms of an ore zone, that can be readily produced
to assist in visualization of the data files.
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