Page 45 - Introduction to Mineral Exploration

28 A.M. EVANS

In cathodoluminescence the exciting radiation
is a beam of electrons and a helpful supple-
ment to this technique is ultraviolet fluores-
cence microscopy. Both techniques are used
on the microscopic scale to study transparent
minerals. Minerals with closely similar optical
properties or which are very fine-grained can be
readily differentiated by their different lumi-
nescent colors, e.g. calcite v. dolomite, feldspar
v. quartz, halite v. sylvite. Features not seen
in thin sections using white light may appear,
thin veins, fractures, authigenic overgrowths,
growth zones in grains, etc. A good description
of the apparatus required and the method itself
is given in Tucker (1988).
FIG. 2.1 A random section through a solid consisting
of a framework of spheres of equal size.
2.2.3 Quantitative analysis

Grain size and shape
Grain size measurement methods for loose
The recovery is the percentage of the total materials, e.g. gravels and sands, placer de-
metal or industrial mineral contained in the posits, or clays, vary, according to grain size,
ore that is recovered in the concentrate; a from calipers on the coarsest fragments,
recovery of 90% means that 90% of the metal through sieving and techniques using settling
in the ore passes into the concentrate and 10% velocities, to those dependent upon changes in
is lost in the tailings. It might be thought that if electrical resistance as particles are passed
one were to grind ores to a sufficiently fine through small electrolyte-filled orifices. These
grain size then complete separation of mineral methods are described in Tucker (1988) and
phases might occur to make 100% recovery other books on sedimentary petrography.
possible. In the present state of technology this Less direct methods have to be employed
is not the case, as most mineral processing with solid specimens because a polished or
techniques fail in the ultra-fine size range. thin section will only show random profiles
Small mineral grains and grains finely inter- through the grains (Fig. 2.1). Neither grain size,
grown with other minerals are difficult or nor shape, nor sorting can be measured dir-
impossible to recover in the processing plant, ectly from a polished or thin section and
and recovery may be poor. Recoveries from when the actual grains are of different sizes
primary (bedrock) tin deposits are tradition- microscopic measurements using micrometer
ally poor, ranging over 40–80% with an aver- oculars (Hutchison 1974) or other techniques
age around 65%, whereas recoveries from invariably overestimate the proportion of
copper ores usually lie in the range 80–90%. small grains present. This bias can be removed
Sometimes fine grain size and/or complex by stereological methods if the grains are of
intergrowths may preclude a mining opera- simple regular shapes (cubes, spheres, parall-
tion. The McArthur River deposit in the North- elepipeds, etc.). Otherwise stereological trans-
ern Territory of Australia contains 200 Mt formation is not possible and great care must
grading 10% zinc, 4% lead, 0.2% copper, and be taken in using grain size measurements
45 ppm silver with high grade sections run- taken from sectioned specimens.
ning up to 24% zinc and 12% lead. This An indication of grain shape can be obtained
enormous deposit of base metals remained by measuring a large number of intercepts
unworked from its discovery in 1956 until (lengths of randomly chosen grain diameters)
1995 because of the ultra-fine grain size and and analyzing these, e.g. the presence of a sub-
despite years of mineral processing research stantial number of very small intercepts would
on the ore. indicate that the grains were angular and the