Page 267 - Introduction to Mineral Exploration

250 M.K.G. WHATELEY & B. SCOTT

that this may cause have been discussed and Annels (1991) covers this subject as well as
emphasis placed on the need to pay careful drilling, sampling, and resource estimation.
attention to sampling error. Discussions of the problems of resource defini-
Sample extraction must be random and tion are covered by the Australasian Joint Ore
preferably of equal volume, but as the samples Reserves Committee (the JORC code: JORC
may amount to several kilograms or even 2003), Annels (1991), and Whateley and Harvey
tonnes in weight there must be a systematic (1994). The JORC website hosts a number of
reduction of mass and grain size before ana- technical papers that explain the JORC code,
lysis can take place. Sample reducing systems its history and implementation. A very good
require thoughtful design and must be care- source for case histories is the volume of
fully cleaned between each operation. Gold- Edwards (2001).
and platinum-bearing samples present special An introductory text for geotechnical inves-
problems because their high density may lead tigations is written by West (1991), while
to their partial segregation from lighter gangue Hoek and Brown (1980) and Hoek and Bray
material during sample preparation. (1977) are still the most sought after books that
Samples may be acquired by pitting and describe the application of rock mechanics to
trenching and by various types of drilling. If underground and surface mines respectively.
coring methods are used then effective core Brassington (1988) gives a good description of
recovery is essential and the core logging must the basic fieldwork necessary to understand
be most methodically performed. Drilling con- the hydrogeology of an area.
tracts should be carefully thought out and
based on sound knowledge of the drilling con-
ditions in the area concerned. Drillholes devi- 10.8 APPENDIX: USE OF GY S SAMPLING
’
ate from a straight line and must be surveyed. FORMULA
Drilling patterns must be carefully planned and
the geologist in charge ready to modify them as 1 Introduction
results come to hand. These are worked examples of the calcula-
The analytical data derived from the above tion of the total variance (TE) of sphalerite
investigations are used in mineral resource and chalcopyrite mineralisation and the two-
and ore reserve estimations, and for grade cal- sided confidence of that mineralisation. These
culations. Classical statistical methods have are followed by a worked example of how to
only a limited application and geostatistical calculate the mass of a sample to be taken
methods are normally used. The exploration knowing the confidence limits.
sample collection work provides a great oppor- Using the short form of the Gy formula for
tunity to initiate geotechnical investigations. the fundamental variance (section 10.1.4):
The collection of geotechnical data will pro- 2
vide mining engineers with essential informa- S o (FE) = C/K
tion for the design of open pit and underground Taking K as 125,000:
workings. It will also provide a starting point
from which the hydrogeology of the target area S o (FE) = C/125,000
2
can be elucidated.
and the variance of the total error (TE) is esti-
mated as twice that of the fundamental error
10.7 FURTHER READING (FE). It is assumed that the liberation factor β
is constant and equals unity, which is a safe
Two textbooks which cover statistical analysis assumption.
of geological data are the eminently readable
Davis (2003) and Swan and Sandilands (1995), 2 Calculation of total variance (TE) and
while Isaaks and Srivastava (1989) give a very double-sided confidence interval: sphalerite.
readable account of geostatistics. The mathem- A prospect contains sphalerite mineralisation
atical details of geostatistical estimations are of mean grade 7.0% zinc in a gangue of density
−3
elaborated by Journel and Huijbregts (1978). 2.7 kg m .