Page 47 - Introduction to Mineral Exploration

30 A.M. EVANS

improve their accuracy for that particular min-
eral deposit type. The American Geological
Institute Data Sheets (sheet 15.1) and various
books (Spock 1953, Thorpe & Brown 1985,
Tucker 1988, Barnes & Lisle 2003 and others)
have comparison charts to help the field geo- (a) (b)
logist in estimating percentage compositions
in hand specimens.

2.2.4 Economic significance of textures

Mineral interlocking
(c)
Ores are crushed during milling to liberate (d)
the various minerals from each other (section
2.2.3) and for concentration a valuable mineral
has to be reduced to less than its liberation size
in order to separate it from its surrounding
gangue. Crushing and grinding of rock is expen-
sive and if the grain size of a mineral is below
about 0.05 mm the cost may well be higher
than the value of the liberated constituents.
In addition there are lower limits to the degree
of milling possible dictated by the separation (e) (f)
processes to be employed because these are
most effective over certain grain size ranges: FIG. 2.2 (a)–(d) Grains from a mineral dressing plant.
e.g. magnetic separation, 0.02–2.5 mm; froth (a) Granular texture; black represents an ore
mineral, unornamented represents gangue (×0.6). (b)
flotation, 0.01–0.3 mm; electrostatic separa- A pyrite grain veined by chalcopyrite (black) (×177).
tion, 0.12–1.4 mm. (c) A sphalerite grain containing small rounded
In Fig. 2.2 a number of intergrowth patterns inclusions of chalcopyrite (black) (×133).
are illustrated. Further crushing of the granular (d) Pyrite grain coated with supergene chalcocite
textured grains in (a) will give good separation (black) (×233). (e) Grains of pyrrhotite with exsolved
of ore (black) from gangue – this is an ideal tex- granular pentlandite in the interstices and flame
ture from the processing point of view. In (b) exsolution bodies within the pyrrhotite (×57).
further crushing of the tiny pyrite grain veined (f) Exsolution blades of ilmenite in a magnetite
by chalcopyrite (black) is out of the question grain (×163).
and this copper will be lost to the tailings.
The chalcopyrite (black) occurring as spheroids
in sphalerite grains (c) is too small to be liber- ing to the amount by which they fall below
ated and will go as a copper loss into the zinc the contracted grade. Exsolution textures com-
concentrate. The grain of pyrite coated with monly devalue ores by locking up ore minerals
supergene chalcocite (black) in (d) will, during and by introducing impurities. In (e) the tiny
froth flotation, carry the pyrite as a diluting flame-shaped exsolution bodies of pentlandite
impurity into the copper concentrate. The (black) in the pyrrhotite grain will go with the
grain is too small for separation of the two pyrrhotite into the tailings and the ilmenite
minerals by crushing. It must be noted that bodies (black) in magnetite (f) are likewise too
the market price for a metal does not apply small to be liberated by further grinding and
fully or directly to concentrates. The purchase will contaminate the magnetite concentrate. If
terms quoted by a custom smelter are usually this magnetite is from an ilmenite orebody
based on a nominal concentrate grade and then these interlocked ilmenite bodies will be a
lower concentrate grades are penalized accord- titanium loss.