Page 44 - Introduction to Mineral Exploration

2: MINERALOGY OF ECONOMIC DEPOSITS 27

provide a photographic image of the surface. fundamental parameters used in the design
Other electrons penetrate to depths of 1–2 µm and optimization of processing plants. Gu
and excite the atoms of the mineral causing (2002) explained that the MLA system consists
them to give off characteristic X-radiation of a specially developed software package and a
which can be used to identify the elements standard modern SEM fitted with an energy
present and measure their amounts. With this dispersive spectrum (EDS) analyzer. The
chemical information and knowledge of some on-line program of the MLA software pack-
optical properties reasonable inferences can be age automatically controls the SEM, captures
made concerning the identity of minute grains sample images, performs necessary image ana-
and inclusions. In addition important element lysis (see Chapter 6), and acquires EDS X-ray
ratios such as Fe:Ni in pentlandite ((Fe,Ni) 9 S 8 ) spectra. Typically, 40–100 images (containing
and Sb:As in tetrahedrite ((Cu,Fe,Ag,Zn) 12 Sb 4 S 13 ) 4000–10,000 grains) are acquired for each
–tennantite ((Cu, Ag,Fe,Zn) 12 As 4 S 13 ), and small sample block and a dozen blocks (of 30 mm
amounts of possible byproducts and their diameter) are measured overnight. The MLA
mineralogical location can be determined. Use- off-line processing program transforms the raw
ful references are Goldstein et al. (1981), Reed image into quality sample images, from which
(1993), and Zussman (1977). most important minerals can be differentiated
The ion microprobe uses an ion rather an using modern image analysis methods.
electron source and measures the mass of
secondary ions from the sample rather than X-
rays. This technique allows the measurement Differential thermal analysis
of much lower concentrations of heavy ele- This method is principally used for clay and
ments than the electron microprobe and has clay-like minerals which undergo dehydration
been widely used in the search for the location and other changes on heating. Measurement
of gold and PGM in metallurgical testing. A of the differences in temperature between an
summary of the technique can be found in unknown specimen and reference material
Larocque and Cabri (1998). during heating allow for the determination of
the position and intensity of exothermic and
endothermic reactions. Comparison with the
Scanning electron microscopy (SEM)
behavior of known materials aids in the iden-
SEM is of great value in the three-dimensional tification of extremely fine-grained particles
examination of surfaces at magnifications from that are difficult to identify by other methods
×20 to 100,000. Textures and porosity can be (Hutchison 1974).
studied and, with an analytical facility, indi-
vidual grains can be analyzed and identified in
situ. Excellent microphotographs can be taken Autoradiography
and, with a tilting specimen stage, stereo- Radioactive minerals emit alpha and beta
graphic pairs can be produced. This equipment particles which can be recorded on photo-
is particularly valuable in the study of lime- graphic film or emulsions in contact with the
stones, sandstones, shales, clays, and placer minerals, thus revealing their location in a rock
materials. The method is discussed in Gold- or ore. In ores this technique often shows up
stein et al. (1981) and Tucker (1988). Recent the presence of ultra-fine-grained radioactive
developments such as the QEM (Quantitative material whose presence might otherwise go
Evaluation of Minerals)*SEM allows the auto- unrecorded. The technique is simple and cheap
matic quantification of mineral composition and suitable for use on hand specimens and
and size in a similar manner to point count thin and polished sections (Robinson 1952,
and image analysis for optical microscopes, Zussman 1977).
although the QEM*SEM is rapidly being over-
taken by the Mineral Liberation Analyser
(MLA) which has particular importance in Cathodoluminescence
applied mineralogy and metallurgical pro- Luminescence (fluorescence and phosphores-
cessing. Mineral Liberation Analyser data are cence) is common in the mineral kingdom.