Geology Reference
In-Depth Information
10
MINERALISATION AND GEOTECHNICAL
PROPERTIES
10.1 Mineralisation and Key Minerals
Mineralisation associated with igneous rocks, and calc-alkaline suites in particu-
lar, is related to the emplacement of a hot magma body at a high level in the crust
which may initiate hydrothermal convection within both the cooling intrusion
via veins and fractures and the surrounding permeable country rocks (see discus-
sion of hydrothermal veins in Section 2.5.2). In some cases radiothermal heat
associated with K,U,Th-rich granites may stimulate hydrothermal convection
of meteoric groundwaters (that is, those derived by downwards-seeping rain-
water) long after the intrusion has solidified. Trace elements, particularly base
metals (iron, copper, lead, zinc), tin and tungsten, derived from the igneous
body and/or from leaching of the country rocks by hydrothermal fluids, may
be concentrated and precipitated when the fluids cool as disseminated oxide or
sulfide crystals in the igneous rock or as veins, exploiting a joint network that
may also invade the country rock. Disseminated mineralisation, usually most
prominant near the periphery of a porphyritic diorite or porphyritic granodiorite
intrusion, is termed a
porphyry mineral deposit
(notably the massive porphyry
copper deposits which are dominated by chalcopyrite and pyrite), and networks
of small veins, again usually concentrated near intrusion-country rock contact
zones, are known as
stockworks
. Veins usually contain one or more minerals
which are abundant but do not have commercial value, and are termed
gangue
minerals (for example, quartz, calcite, barytes) together with a lesser quantity
(perhaps a few percent) of metalliferous minerals (for example, Figure 10.2);
they may be simple (one generation of minerals) or complex comprising two
or more generations, the timing identified by cross-cutting relationships. When
mineralised veins are encountered in the field, the nature of the vein or fracture
infillings, both metalliferous and gangue crystals and their proportions should
be recorded. The width (aperture), orientation (dip and strike) and frequency
should be measured in the field. It is also important to note the characteristics
of the wall rocks, as explained below.
