Geoscience Reference
In-Depth Information
these situations, multiplying the grade with the thickness
will result in a variable with lower coefficient of variation
(variability), and thus easier to estimate without having to
risk over-spreading grade. Thickness, while sometimes vari-
able, will generally be a smoothly varying attribute in space,
and thus even easier to estimate.
Deposits in which grade-thickness variables have been
used include tabular and vein-type gold, platinum reefs, ul-
tramafic deposits (chromite, for example), some nickel later-
ites, and uranium roll-front type deposits.
Consider an operation that has both reverse circulation
(RC) and diamond drill holes (DDH) in its resource data-
base. For a 1 m interval, a typical RC drill hole sample will
represent close to 50 kg, considering that a 5.25 inch diam-
eter hole is drilled. The corresponding HQ-size DDH, for the
same 1 m length, will represent close to 28 kg. Although the
two sample weights are quite different, the difference and its
impact is negligible. This is because the sample is (a) usually
composited, and (b) multiple composites are used to estimate
blocks. The blocks may be as small as 5 × 5 × 5 m (for small-
er, selective deposits), equivalent perhaps to 350 metric tons,
and up to 25 × 25 × 15 m or larger blocks for massive depos-
its, equivalent to almost 25,000 tons considering an in-situ
bulk density of 2.65 t/m 3 . This will depend on the specific
resource modeling case.
Further justification for compositing is that small scale
assays may be highly variable, which can be mitigated by
compositing. Compositing to an appropriate length will
show less variability making the corresponding geostatisti-
cal analysis, including variography, more robust.
In particular, compositing has a dramatic affect on the
nugget effect, that is, the completely random portion of the
variability. The decrease in the nugget variance will be in-
versely proportional to the degree of compositing.
The composited dataset is important to the overall qual-
ity of the resource model. Several decisions must be made
in practice, including whether or not to use composites;
the most appropriate length; the compositing method used;
whether to truncate the composites at geologic boundaries;
how to handle missing intervals within the composite (with
no assay information); and the minimum acceptable com-
posite length.
5.6
Compositing and Outliers
5.6.1
Drill Hole Composites
The original grade values (assays) in the database are usually
averaged to pre-specified lengths: a procedure referred to as
compositing. This is not strictly a requirement for resource
estimation; however, the homogenization of the data scale
or support and correction for incompletely sampled intervals
motivates compositing in almost all cases. Most resource es-
timation software assumes the data are at a constant support.
Compositing also incorporates a certain amount of dilu-
tion into the raw data prior to estimation or simulation. The
mining operation is expected to work at a certain level of
selectivity that is larger than the scale of the raw assays. In
the case of open pits, selectivity in the vertical dimension
is generally fixed by the bench height. In the case of under-
ground mines, selectivity is a function of the mining meth-
od. The height of the lift or slice in a cut-and-fill or similar
method determines the selectivity in the vertical direction.
The composite length may be made equal to the bench or lift
height to put data at the same vertical support as the mine
selectivity.
The composite is typically calculated by a length-weight-
ed average and may also be weighted by specific gravity
and core recovery. Compositing can be done to obtain a
representative value for ore body intersections, lithological
or metallurgical composites, regular length down-the-hole
composites, bench composites or section composites, high
grade composites, or minimum length and grade composites.
Each of these types of composites are produced for dif-
ferent purposes and in different situations. Regular length or
bench composites are most common in resource estimation.
There are geostatistcal models that can provide for mixtures
of support sizes in the original data, but estimation software
almost always assumes the data are of constant support.
Drill holes of different diameters are commonly used in
the same deposit. There will also be partial composites at the
end of drill holes or at hard geologic contacts. In practice,
slight differences in support size will have little effect on the
final resource estimate.
5.6.2
Composite Lengths and Methods
The composite length chosen is commonly a function of
the anticipated mine selectivity. Shorter composite lengths
can be used to increase the statistical population available
for variography and estimation; however, the final vario-
grams should be based on the composites that are going into
estimation. Shorter composites permit more accurate repre-
sentation of geological contacts.
Compositing can be done using two basic methods: down-
the-hole, or by bench. Bench composites are common in
open pit mines with nearly vertical drilling. The method in-
volves defining the top and bottom elevations of each bench
and then compositing all sample intervals that fall within
those elevations. The mid-bench elevation is assigned as the
centroid of the composite which is assumed to be vertical.
Although convenient, the bench compositing method
commonly used in open pits has some shortcomings. If the
drill hole is inclined, the actual composited length will be
 
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