Geology Reference
In-Depth Information
susceptible, especially their calcitic (An-rich) cores. Feldspar pseudomorphs,
mixtures of clay minerals, albite and chlorite can sometimes be identified with
a hand lens.
Common forms that may be encountered and recognised in field samples are:
1.
silicification
, in which quartz overgrowths and replacements of existing
minerals occurs;
2.
kaolinisation
,or
argillic alteration
- the
alteration
of
feldspar
to
clay
minerals;
3.
greisenisation
- the alteration to alkali feldspar to secondary white mica,
sericite;
4.
propylitic alteration
, in which plagioclase feldspar alters to epidote mafic
minerals to chlorite;
5.
potassium silicate alteration
, in which plagioclase feldspar and amphibole
alter to K-feldspar and biotite mica.
For example, an outcrop of greisenised granite with disseminated pyrite
(FeS
2
) may indicate that deposits of chalcopyrite (CuFeS
2
) are not far away.
Figure 10.3 illustrates, in cross-section, the way in which both wall-rock
alteration (for example, from K - silicate to greisenisation to kaolinisation or
propylitic alteration) and the nature of the metalliferous minerals precipitated
(for example, from chalcopyrite to pyrite for Cu-Fe bearing veins) both
change with falling fluid temperatures and with distance from the centre of a
mineralised stockwork. Thus, concentric zones of alteration and mineralisation
may be recognised in field studies on a wide range of scales. Table 10.1 lists
the properties of some of the more common metalliferous minerals that might
be found often in association with calc-alkaline igneous intrusions. Careful
examination of the Table 10.1 shows that most of these metallic minerals can
be distinguished from one another using colour and hardness criteria.
10.2 Mineralisation in Layered Mafic Intrusions
Many layered intrusions contain chromite-rich seams (chromitites), which host
the majority of the world's Cr reserves and may contain significant platinum
group element (PGE) mineralisation. Examples with economic deposits include
the Bushveld, South Africa and Stillwater Complex, Montana, United States,
though
many
others
have
occurrences
of
chromities
(for
example,
Rum,
Scotland).
The largest example of a mafic-ultramafic layered intrusion associated
with such economic deposits is the Bushveld Complex, shown by the map
in Figure 10.4: the intrusion is about 450 km in diameter and about 9 km
in thickness. At the margins of the Bushveld, where it is emplaced into
