Geoscience Reference
In-Depth Information
Fig. 5.16
Crossplots of Ni-Fe and mlr(Ni)-mlr(Fe)
Fig. 5.17
Crossplots of Ni-Fe and ilr(Ni)-ilr(Fe)
The accumulation approach works well when there is
little or no correlation between grade and thickness. Also,
thickness should be true thickness, that is, the samples need
to be considered as true (normal to dip) mineralized thick-
nesses. It is worth noting that the accumulation and thickness
variables can be estimated using any technique, including
geostatistical methods.
The motivation for using accumulations is generally that,
for thin tabular or vein-type deposits, the mineralized inter-
cepts are of irregular length; there are very few intercepts (or
just one) per drill hole; and that grade variability is high, with
somtimes the thinner intercept having the highest grades. In
grade models and geologic resource estimates (David
1977
;
Glacken and Snowden
2001
). This is particularly true in the
case of sedimentary and stratabound-type deposits.
Estimating accumulation (grade-thickness (GT) and
thickness (T) variables) came about early in the history of
resource estimation. It has been applied mostly to narrow
orebodies, and initially as an adaptation of the polygonal
approach. Grade multiplied by thickness is proportional to
metal content, which is estimated independently from thick-
ness. The final grade is obtained at each point or block by
dividing the accumulation by the thickness.
