Page 224 - Introduction to Mineral Exploration

10: EVALUATION TECHNIQUES 207

pipe, must obey the three times the maximum sible contribution to the total error. The two
particle size rule, i.e. the width, length, and techniques which follow are less rigorous, and
depth must be at least three times the maxi- more error prone, but less expensive.
mum particle size. This is defined as the largest
screen size which retains 5% of the material. Chip sampling
There are three hand sampling techniques, Chip samples are obtained by collecting rock
namely channel, chip, and grab sampling. particles chipped from a surface, either along a
line or over an area. In an established mine,
Channel sampling rock chips from blastholes are sampled using
In mineral exploration, a channel is cut in scoops, channels, or pipes pushed into the
an outcrop, usually the same diameter as the heap. With a large database of chip samples,
core being collected, to maintain the volume– statistical correlation between core and chip
variance relationships. It is cut using a hammer samples may establish a correction factor for
and chisel or a circular saw, across the strike of chip sampling results and thus reduce error.
the mineralisation (see Fig. 5.3). As the mater- Chip sampling is used also as an inexpensive
ial is cut it is allowed to fall to the floor on to reconnaissance tool to see if mineralisation
a plastic sheet or sample tray from which it is of sufficient interest to warrant the more
is collected and bagged. Samples are normally expensive channel sampling.
0.5–5 kg in weight, mostly 1–2 kg, and each is
rarely taken over 2 m or so in length, but Grab sampling
should match the core sample length. There In this case the samples of mineralized rock
is no point in oversampling and sample spac- are not taken in place, as are channel and chip
ing perhaps can be determined from the range samples, but consist of already broken mater-
(a) (section 10.4.1) derived from a study of a ial. Representative handfuls or shovelfuls of
semi-variogram. broken rock are picked at random at some con-
A mineral deposit can often be resolved venient location and these form the sample. It
into distinct and separate types of mineralisa- is a low cost and rapid method and best used
tion. Sampling these different types as separate where the mineralized rock has a low variance
entities rather than as one large sample can and mineralisation and waste break into part-
reduce natural variation and thus the variance, icles of about the same size. It is particularly
and keep sample weights to a minimum. This useful as a means of quality control of miner-
stratified sampling is also of value where the alisation at strategic sampling points such as
separate types require different mineral pro- stope outlets and in an open pit (Annels 1991).
cessing techniques due to variation in either
grain size or mineralogy. Such sample data Comparison of methods
are then more useful for mine planning. Such Bingham Canyon mine in the United States
samples can be recombined statistically into and the Palabora Mine in South Africa con-
one composite result for the whole of the ducted a series of tests whereby the entire
mineralisation. product from blasthole drilling was collected in
In well-jointed or well-bedded rocks collec- 30-degree segments from the full 360 degrees
tion by hammering the rock face presents dif- (C.P. Fish, personal communication). One
ficulties in that adjacent nonsample material blasthole may produce between 2 and 3 t. Each
will fall with the material being cut but this segment was sampled by the traditional means
must be rejected. Additionally, it is important described above, and compared to the remain-
that a representative collection of rock masses ing material in the segment after crushing to
is collected. Frequently, mineral values of in- 3 mm and reducing in a 16-segment rotary-
terest occur in altered rocks which may well divider. The results showed:
be more friable than adjacent, harder, unminer- 1 No segment ever gave the same assay as the
alized ground, and overcollection of this easily whole mass. The spatial variation was great
cut material will provide a biased sample. and there was no consistency in the way the
Channel sampling provides the best tech- material dispersed.
nique of delimiting and extracting a sample 2 Subsample analyses gave higher analytical
and, consequently, provides the smallest pos- results than the 100% mass. Scoop samples