Page 179 - Introduction to Mineral Exploration

162 C.J. MOON

the reader should consult Fletcher (1981, 1987) Isotopic analysis is not yet widely used in
and Thompson and Walsh (1989). exploration although some pilot studies, such
The differences between the methods shown as that of Gulson (1986), have been carried
are cost, the detection limits of analysis, speed out. The main reason for this is the difficulty
of analysis, and the need to take material into and cost of analysis. Although high resolution
solution. Most general analysis in developed ICP sourced mass spectrometers are finding
countries is carried out by inductively coupled their way into commercial laboratories and are
plasma emission spectrometry (ICP–ES), often the main hope for cheap analysis, they are not
in combination with inductively coupled yet routine.
plasma mass spectrometry (ICP–MS), or X-ray The choice of analytical method will aim
fluorescence (XRF). All three methods require at optimizing contrast of the main target ele-
highly sophisticated laboratories, pure chem- ment. For example, it is little use determining
icals, continuous, nonfluctuating power sup- the total amount of nickel in an ultramafic
plies, and readily available service personnel, rock when the majority of nickel is in olivine
features not always present in developing and the target sought is nickel sulfides. It
countries. In less sophisticated environments, would be better in this case to choose a reagent
high quality analysis can be provided by atomic which will mainly extract nickel from sulfides
absorption spectrophotometry (AAS), which and little from olivine. In soils and stream
was the most commonly used method in sediments, optimum contrast for base metals is
developed countries until about 1980. Another normally obtained by a strong acid (e.g. nitric +
method which is widely used in industry is hydrochloric acids) attack that does not dis-
neutron activation analysis (NAA) but its use solve all silicates, and an ICP–ES or AAS finish.
is restricted to countries with cheap nuclear Most rock analysis uses total analysis by XRF
reactor time, mainly Canada. or by ICP–ES/ICP–MS following a fusion or
Precious metals (gold and platinum group nitric–perchloric–hydrofluoric acid attack.
elements) have been extremely difficult to de-
termine accurately at background levels. The
boom in precious metal exploration has, how- 8.3 INTERPRETATION
ever, changed this and commercial laboratories
are able to offer inexpensive gold analysis at Once the analytical data have been received
geochemical levels (5 ppb to 1 ppm) using solv- from the laboratory and checked for precision
ent extraction and AAS, ICP–ES or alternat- and accuracy, the question of how the data is
ively NAA on solid samples. For evaluation the treated and interpreted needs to be addressed.
method of fire assay is still without equal: in As the data are likely to be multi-element and
this the precious metals are extracted into a there are likely to be a large number of samples
small button which is then separated from the this will involve the use of statistical analysis
slag and determined by AAS, ICP–ES, or ICP– on a computer. It is recommended that the data
MS. The analysis of precious metals is different are received from the analyst either in the form
from most major elements and base metals in of a floppy disk, CD-ROM, or over the Internet.
that large subsamples are preferred to overcome Re-entering data into a computer from a paper
the occurrence of gold as discrete grains. Typic- copy is expensive and almost certain to intro-
ally 30 or 50 g are taken in contrast to 0.25–1 g duce major errors. Normally the data will be
for base metals; in Australia, 8 kg are often transferred into an electronic database (see
leached with cyanide to provide better samp- section 9.1) to allow easy access or, in the case
ling statistics (further details in section 16.1.2). of small data sets stored in a spreadsheet.
Elements which occur as anionic species
are generally difficult to measure, especially
the chloride, bromide, and iodide ions which 8.3.1 Statistics
serve as some of the ore transporting ligands. The object of geochemical exploration is to
Although some of these elements can be define significant anomalies. In the simplest
determined by ICP–ES or XRF, the most useful case these are the highest values of the ele-
method is ion chromatography. ment sought but they could be an elemental