Page 96 - Introduction to Mineral Exploration

5: FROM PROSPECT TO PREFEASIBILITY 79

alternation of color and texture giving rise to
the term “live,” in contrast to ironstones of
nonsulfide origin that show little variation and
are known as “dead” ironstones. Ironstones
also preserve chemical characteristics of their
parent rock, although these can be consider-
ably modified due to the leaching of mobile
elements.
The recognition of weathered sulfides over-
lying base metal or gold deposits and the
prediction of subsurface grade is therefore of
extreme importance. This particularly applies
in areas of laterite development. These areas,
such as much of Western Australia, have been
stable for long periods of geological time, all
rocks are deeply weathered, and the percent-
age of ironstones that overlie nonbase metal
sulfides (known rather loosely as false gossans)
is large. In Western Australia the main tech-
niques for investigating these are (Butt &
Zeegers 1992):
1 visual description of weathered rocks;
2 examination for relic textures;
3 chemical analysis.
Visual recognition requires experience and
a good knowledge of primary rock textures.
Visual recognition can also be supplemented
by chemical analysis for trace and major ele-
ments. Hallberg (1984) showed that Zr-TiO 2
plots are extremely useful in confirming rock
types. Both elements are immobile and easy
FIG. 5.4 Gossan (above adit) overlying sulfides and cheap to determine at the levels required.
exposed in the pit wall of the San Miguel deposit, They may be supplemented by Cr determina-
Rio Tinto, southern Spain.
tions in ultrabasic areas, although Cr is more
mobile than the other two elements.
Relic textures can sometimes be observed
or secondary minerals. For example, copper in hand specimen but this is usually supple-
sulfides oxidize to secondary minerals that mented by microscopic examination using
have distinctive green or blue colors such as either a binocular microscope or a petrographic
malachite and azurite. Metals such as lead and microscope and an impregnated thin section.
zinc normally form white secondary minerals In a classic study of relic textures, Blanchard
in carbonate areas that are not easily distin- (1968) described the textures resulting from the
guished on color grounds from the host carbon- weathering of sulfides. Major types are shown
ates. A fuller list is given in Table 5.3. Besides in Fig. 5.5. Although boxwork textures are dia-
providing ions to form secondary minerals, sul- gnostic they are, unfortunately, not present in
fides often leave recognizable traces of their every gossan overlying base metal sulfides and
presence in the form of the spaces that they textural examination must be supplemented
occupied. These spaces are relic textures, by chemical analysis.
often known as boxworks from their distinct- The choice of material to be sampled and the
ive shapes, and are frequently infilled with interpretation need to be carefully scrutinized.
limonite and goethite. Ironstones overlying Andrew (1978) recommends taking 20 samples
sulfides have a varied appearance with much as representative of each gossan and using a