Page 46 - Introduction to Mineral Exploration

2: MINERALOGY OF ECONOMIC DEPOSITS 29

opposite would show that the grains lacked only ore mineral and can be picked out using
edges and were smooth and convex (Jones ultraviolet light.
1987). The volume percentage of ore and/or
deleterious minerals can be of crucial import-
ance in mineral processing and a knowledge
Modal analysis
of whether a wanted metal is present in one
Modal analysis produces an accurate represen- or several minerals. If the latter is the case,
tation of the distribution and volume percent their relative proportions may also be of great
of a given mineral in a thin or polished section. importance. Some examples of this are:
Two methods of analysis are normally used, 1 Gold ores – is all the gold present as native
namely: gold (free-milling gold) or is some in the form of
1 Area percentage. The surface area of mineral tellurides or enclosed by sulfides (refractory
grains of the same mineral are measured rela- gold)? Native gold is readily leached from
tive to the total surface area of the thin section, milled ores by cyanide solutions, but refractory
giving the areal proportions of each mineral gold resists leaching and has to be roasted (after
type. Since volumes in this situation are dir- concentration) before cyaniding or leached
ectly proportional to areas, these are also the under pressure, thereby increasing the cost of
volume percentages. the treatment and of course decreasing the
2 Point count. Each mineral occurrence along value of the ore.
a series of traverse line across a given thin sec- 2 Titanium ores in anorthosites will have
tion is counted. At least 2000 individual points significant amounts of titanium locked up in
must be counted for a statistically valid result. titaniferous magnetite, sphene, and augite from
The number of grains counted, the spacing which it is not recoverable.
between points, and successive traverse lines 3 The skarn iron orebody at Marmoraton,
is dependent on the mean grain size of the Ontario assayed on average 50% Fe, but only
sample. 37.5% (in magnetite) was recoverable, the rest
Modal analysis can be used to compare was locked in silicates. These and similar
rocks from different areas if there are only thin devaluing features are readily detected and
sections. No chemical analysis is required. quantified by microscopic investigations.
The work can be achieved manually using Valuations based on assays alone may be
a petrographic microscope. Modern optical grossly exaggerated.
microscopes and SEMs use image analysis (see If the chemical compositions of the minerals
Chapter 6) to count mineral grains and to cal- are known, dividing volume percentages by
culate areal proportions automatically (Jones mineral densities (and converting to percent)
1987, Sprigg 1987). Statistically representative provides the weight percentages of the min-
numbers of points are achieved routinely using erals, and by using Table 2.1 (or by calcula-
image analysis. tion) gives us an entirely independent way of
However, care must be taken with foliated, obtaining an estimate of the grade. This may
banded rocks which should only be sampled at be of value as a check on chemical or X-ray
right angles to the banding (Hutchison 1974). fluorescence assays.
Experience shows that porphyritic rocks are The mineral explorationist should train
difficult to count. Similarly, care must be taken him or herself to make visual modal estimates
to ensure that the total area of the sample is in the field using hand specimens and natural
larger than the maximum diameter of the exposures. By estimating the volume percent-
smallest grain size. Very coarse-grained rocks age of a metallic mineral such as chalcopyrite
such as pegmatites can be measured with a (often the only or principal copper mineral in a
grid drawn on transparent material and placed mineral deposit) and looking up the copper con-
on outcrop, joint, or mine surfaces. A similar tent in Table 2.1, a visual assay can be made.
technique can be used to visually estimate the When the first laboratory assays become avail-
grade of a mineral deposit where the sampler is able for a prospect under investigation explora-
sure that no ore mineral will be missed, e.g. tionists will have reference material with
tungsten deposits where the scheelite is the which they can compare their estimates and