Copper deposits - and why they are getting hard to find
It is not that copper is particularly rare. However, copper is one of those useful elements which has a huge variety of applications. Therefore demand for copper is steadily increasing. Unfortunately, the increase in demand has not been matched by an increase in mining of the ore. Indeed if anything, the production of copper is stable, and is likely to slow down in the near future. Of the 28 largest copper mines in the world, 21 have no potential for expansion. Indeed many large copper mines may well be exhausted at some time in the next five years.
There’s a lot of copper still in the earth. It is estimated that copper concentration in the crust is about 0.005 to 0.007%. Given the overall volume of Earth’s crust, that is a lot. If we could extract all the copper it would keep us going for millions of years at present levels of demand. But the reality is that most of that copper is at too low a concentration to be viable for mining. Most copper is mined or extracted as copper sulfide from large open pit mines in porphyry copper deposits that contain between 0.4 and 1% of copper. Anything below 0.3% is not economical even with the most modern extraction tools available.
The term 'porphyry' is a general expression for any igneous rock which contains large-grained crystals - for example quartz. But this term also describes rich localized metal ore deposits such as those of gold, copper, lead or zinc. The enrichment of ore with metals happens during different stages of cooling magma which creates porphyritic textures and also leads to the separation of dissolved metals into distinct zones.The metal enrichment happens in the porphyry itself. Therefore large low-grade copper ore bodies associated with porphyritic intrusive rocks can be referred to as copper porphyry deposits.
Some of those deposits are very large. For example the Great Copper Mountain was a mine in Falun, Sweden that was open for nearly a millennium before it finally closed in 1992. During that period it was the main producer of copper in Europe and when at peak production in the 17th century it produced two-thirds of Europe's copper .
However, finding such large deposits of copper is not an easy task, and all the really easy deposits have already been found. But recent studies by a group led by Cin-Ty Lee of the Department of Earth Science at Rice University in Texas may help to pinpoint the locations for new discoveries of large copper deposits.
When studying Earth's arc magmas, the group discovered large amounts of sulfides - minerals which contain reduced forms of sulfur bonded to metals like copper. Earth's arc magmas are thought to be the building blocks for continents. They are created in subduction zones where two tectonic plates slide one beneath the other. The subducting plate takes crust and sediments from the Earth’s surface deep into the mantle. In turn, this forces the hot mantle upwards from the Earth's interior. This is the principle by which arc magmas are formed.
Because sulfides are heavy and dense they sink to the lower part of arc magma, accumulating there instead of on the Earth's surface.
But, as Lee explains: 'If the continental arc grows thicker over time, the accumulated copper-bearing sulfides are driven deeper to where higher temperatures can re-melt these copper-rich dregs, releasing them to rejoin arc magmas.'
This is what the researchers, who recently published an article in the journal Science, (reference below) think happened in the Andes Mountains and in western North America where rich copper deposits have been found. Indeed Chile, the western United States and Peru are at present the world’s leading producers of copper.
Extrapolating from their data, the researchers suggest that there are some large yet undiscovered deposits of copper in Siberia, northern China, Mongolia and some parts of Australia.
If the group is correct and the new deposits are found, we may be able to postpone by some time the inevitable day when the earth runs out of minable copper. One good thing about copper is that it is highly reusable. Indeed an estimated 80% of all the copper ever mined is still in use today. So recycling could be called the major source of copper in the modern world.
C.-T. A. Lee, P. Luffi, E. J. Chin, R. Bouchet, R. Dasgupta, D. M. Morton, V. Le Roux, Q.-z. Yin, D. Jin. Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation. Science, 2012; 336, pp. 64-68
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