Geology Reference
In-Depth Information
Figure 10.2
Arsenopyrite, chalcopyrite, stannite, quartz and apatite vein, Corn-
wall, UK.
Associated with mineralisation, the host rocks become altered by
hydrothermal fluids, the effects of which are usually more pervasive and exten-
sive than the mineralisation itself. Therefore, field occurrences of hydrothermal
alteration may provide a means of determining whether the region around or
within an igneous body might be mineralised. Alteration zones can form on a
small scale in the vicinity of veins and fractures infiltrated by hot circulating
fluids. Indeed, such alteration halos are regarded by economic geologists as the
'smoking gun' - clear proof of hydrothermal activity pointing to the likelihood
of mineralised deposits nearby. The nature of the alteration halo around and
inside the fracture can be instructive. Narrow veins and fractures are often
infilled with secondary clay minerals or green epidote. An extreme form of
hydrothermal alteration in granitic rocks is kaolinisation, a process whereby the
entire rock mass is altered to secondary clay minerals (kaolin). These kinds of
deposits, common in parts of Cornwall are of regional economic significance
where the clay is mined to make pottery. Halos surrounding hydrothermal
veins comprise phyllic and propylitic zones where fluid flow is accompanied
by dissolution-precipitation chemical reactions. These reactions can be complex
but commonly involve the dissolution of feldspar. Plagioclase is particularly
