Figure 1. Porphyry copper deposits.
(Taken from http://pubs.usgs.gov/fs/fs053-03/fs053-03.html)
You see, I always have a strong interest in Geology and
Geomorphology, and indeed I have been a tutor of the undergraduate
Environmental Chemistry course for some time! I read ‘Nature Geoscience’ regularly
and this month I have read a fascinating article about ‘porphyry copper deposit’.
[1]
Figure 2. Formation of porphyry copper deposits. Taken from Source [2].
The 'golden child' here is copper (should be called 'copper
child?), and these copper deposits indeed have high economical values. These
deposits are porphyritic, and intrusive (hence plutonic) in nature. The
positions of these deposits are often found near the convergent plate margins,
and above the subducting plate. The magmatic (hydrothermal) fluid first rises
through the continental crust. When the hydrothermal fluid rises, the rate of
its cooling increases gradually, as it passes through the area of the fractures
and intrusions. Mineralization leads to the formation of the porphyry copper
deposits (Figure 2). [2]
A key theme of the article is about tectonic uplift, which is
the 'lifting-up' of the surface of the mountain belts, in opposite to the
direction of gravity. One of the components is known as 'exhumation', which is
defined as 'the displacement of rocks with respect to surface'. Quantitatively,
the rate of exhumation is related to the rate of erosion or the removal of overburden
by tectonic processes. So, exhumation comes hand in hand with exogenic denudation
processes. Erosion will wear away the mountains, and so the metamorphic rocks
or reserves from below will become exposed at the surface.[3]
The Nature Geoscience article shows the relationship between
climate and the effects it may cause to the orogenic geomorphology. The
researches have studied Cenozoic porphyry copper deposits at convergent
tectonic settings. From various data, they have discovered that a higher precipitation,
which means a higher erosion rate by rain water, will lead to faster
exhumation. The consequence of rapid exhumation is the sparsely distributed
porphyry copper deposits, with a relatively younger age. By contrast, lower
precipitation, which corresponds to an arid region, will lead to slower
exhumation, and the copper deposits are more abundant and relatively aged.
Therefore, in a sense the age of the copper deposits is related to the
exhumation rates of the active orogens, and so this can serve as a 'sensor' to
uncover the geological phenomenon.
Upon the completion of this article, I have discovered that
it also has been covered on the web already. Well, that means this research is
really significant for many! [4]
By Ed Law
19/6/2015
Reference and sources:
1. A climate signal in exhumation patterns revealed by
porphyry copper deposits
Brian J. Yanites and Stephen E. Kesler
Nature Geoscience 2015, 8, 462–465.
doi:10.1038/ngeo2429
2. http://ns.umich.edu/new/releases/22883-a-climate-signal-in-the-global-distribution-of-copper-deposits
3. Surface uplift, uplift of rocks, and exhumation of rocks.
P. England, P. Molnar
Geology, 1990, 1173-1177.
4. http://www.indicoresources.com/s/CopperPorphyryDeposits.asp
