Geoscience Reference
In-Depth Information
network of cavities and fractures that existed in the rocks. Because of the chemistry
of the rocks deep down, many different chemical elements were brought to the upper
levels and crystallized there to form new and valuable minerals, or caused alterations
of the earlier solid rocks.
FIG
45.
Simple diagram of a slice through the Earth's upper levels, showing how the tem-
perature patterns around a granite body have been responsible for the distribution of
minerals containing the more important chemical elements.
The granites probably solidified in the Earth at temperatures of about 850 °C, and
most of the mineralisation happened at rather lower temperatures as the rocks cooled
(Fig. 45). Tin, wolfram, arsenic and copper minerals formed at between 500 and 300
°C, whereas silver, lead, zinc, uranium, nickel and cobalt minerals formed at between
300 and 200 °C, and iron minerals between 200 and 50 °C.
The tin of Cornwall was a major reason why some of the early inhabitants of
mainland Europe were interested in Britain. In fact there is evidence that tin minerals
were being gathered here more than 3,000 years ago, during the Bronze Age. In those
days, much of the material was collected from young sands and gravels derived from
the weathering and erosion of the mineral-bearing rock, unlike later times when mining
techniques were developed to extract tin directly from the bedrock.
Some granite areas contain much more mineralisation than others, and the range
of minerals and chemical elements that are present varies greatly. This depends on the
temperature of the granite emplacement and the chemistry of the fluids accompanying
and following the granite. The Land's End and Carnmenellis granites are particularly
rich in tin, and it is around these granites, in areas near to St Ives, Camborne, Redruth
and Helston, that most of the mining has been concentrated. The remains of this mining
are often clear to see (Fig. 46), but the presence of the minerals themselves does not
generally influence the natural scenery.
