Geology Reference
In-Depth Information
altered wall rock
pyrite
quartz crystals
chalcopyrite
calcite
cavity
fine quartz
sharp vein wall
Figure 2.13
Sketch cross-section across a complex hydrothermal mineral vein.
in mineral content reflect the changing species that were being precipitated, as
time progressed, from the hot fluids permeating through the vein. A field sketch
of a complex mineral vein, as in Figure 2.13, is an excellent way of recording
the fine detail. Hydrothermal fluids permeating through joints/fractures are also
often responsible for the alteration of the host rocks on either side, involving,
for example chemical reactions that break down feldspars to softer minerals
such as clays. Hydrothermal systems and their associated vein networks can be
very important in mineral exploration, particularly of sulfides and PGEs (see
also Chapter 6).
2.5.2.2 Aplites
Aplites
are veins of fine-grained felsic (that is, generally light-coloured), crys-
talline materials, usually of a paler colour (white or pinkish) than their host
(Figure 2.14). Aplites probably represent a residual fraction of relatively silica-
rich magma which remains within the intrusion when the bulk of the magma
has crystallised. If this magma is then injected into the solidified parts of the
intrusion, it will cool rapidly and form the fine-grained veins dominated by
quartz and feldspar crystals seen in aplites. Aplites are particularly common in
granitoid intrusions where, to judge from their lack of chilled margins, they are
usually intruded into hot solidified material.
2.5.2.3 Pegmatites
Pegmatites
are veins of exceptionally coarse-grained, igneous-textured material
(Figure 2.15), usually incorporating the same minerals as their hosts, that is
quartz, feldspars, micas and occasionally amphiboles in granitic pegmatites.
Some may incorporate occasional large crystals of exotic minerals such as
apatite, tourmaline and beryl (see Table 3.4). Like aplites, pegmatites are thought
