Page 43 - Introduction to Mineral Exploration

26 A.M. EVANS

method may have to be used. Some primary tin such as Berry et al. (1983) or the more compre-
deposits contain appreciable amounts of topaz hensive method in Jones (1987).
which, because of its hardness, increases the The techniques of identifying minerals in
abrasion of crushing and grinding equipment, thin section are taught to all geologists and
thus raising the operating costs. in polished sections to most, and will not be
To summarize, the information that is described here. For polished section work the
required from a sample includes some, or all, of reader is referred to Craig and Vaughan (1994)
the following: (i) the grade of the economic and Ineson (1989), as well as the online manual
minerals; (ii) the bulk chemical composition; of Ixer and Duller (1998). Modern optical micro-
(iii) the minerals present; (iv) the proportions scopes have significantly increased resolution
of each of these and their chemical composi- and oil immersion is not often used in com-
tions; (v) their grain size; (vi) their textures and mercial laboratories. Simple microscope and
mineral locking patterns; (vii) any changes in scanning electron microscope (SEM) methods
these features from one part of an orebody to are usually all that is required to effectively
another. identify all the minerals in the samples. SEM
and other methods requiring sophisticated
equipment are discussed below.
2.2.1 Sampling
Mineralogical investigations will lose much of
their value if they are not based on systematic X-ray diffraction
and adequate sampling of all the material that X-ray diffraction is used to identify clay min-
might go through the processing plant, i.e. min- eral structure and properties, and for mineral
eralized material and host rock. The basics of analysis and mineral abundance measurements
sound sampling procedures are discussed in through spectroscopic sensing. Modern X-ray
Chapter 10. The material on which the miner- diffractometers can work well on solid speci-
alogist will have to work can vary from solid, mens, compacted powder pellets representing
coherent rock through rock fragments and whole rocks, or on a few grains on a smear
chips with accompanying fines to loose sand. mount. Multiple mounts can be automatically
Where there is considerable variation in the fed into the diffractometer.
size of particles in the sample it is advant- The rock sample is normally powdered and
ageous to screen (sieve) the sample to obtain packed into an aluminum holder. It is then
particles of roughly the same size, as these placed in the diffractometer and bombarded
screened fractions are much easier to sample with X-rays. The diffracted rays are collected
than the unsized material. by a detector and the information relayed to a
The mineralogist will normally subsample computer where it is converted to d-values
the primary samples obtained by geologists of specific intensities. This information can
from the prospect to produce a secondary then be shown graphically in the form of
sample, and this in turn may be further reduced a diffraction pattern or “diffractogram.” The
in bulk to provide the working sample using diffractograms from the unknown sample are
techniques discussed in Jones (1987) and recent then matched against a database of 70,000
technological innovations. recorded phases for mineral identification. The
latest instruments allow for rapid recognition
of the entire spectrum of the sample in minutes
2.2.2 Mineral identification
using a computer to match patterns and iden-
Initial investigations should be made using tify the minerals present.
the naked eye, the hand lens and a stereobino-
cular microscope to: (i) determine the ore types
present and (ii) select representative specimens Electron and ion probe microanalyzers
for thin and polished section preparation. At With this equipment a beam of high energy
2
this stage uncommon minerals may be iden- electrons is focused on to about 1–2 µm of the
tified in the hand specimen by using the deter- surface of a polished section or a polished thin
minative charts in mineralogical textbooks section. Some of the electrons are reflected and