by Professor Richard Herrington, Head of the Department of Earth Sciences
The world needs copper – we all need copper. It carries the electricity and hot water in our homes through cables and pipes. It is part of all the electrical appliances we use at home and in industry – an essential ingredient in any low-carbon economy. The sources and security of supply of copper are important in economic terms and of great interest for government policy and business strategy.
Every person in the UK uses around 8kg of copper per year. Worldwide usage exceeds 24 million tonnes annually and, whilst around 41% of European copper needs are met by recycling, the demands of growing economies like China and India mean that 75% of this usage is met by mined metal. Copper can’t be grown and simply recycling what we have already extracted won’t keep pace with demands.
More than 75% of the copper we mine comes from porphyry copper deposits. These deposits, derived from porphyry magmas, are the largest sources of copper and molybdenum on the planet – and they also yield a significant part of the world’s gold supply. These deposits are increasingly hard to find: most that lie within a few hundred metres of the surface have already been discovered and exploited. Geologists looking for new deposits must therefore use the surface signals of buried deposits to guide the search for new ore.
A new paper published in the scientific journal Nature Geoscience by the author and colleagues (Porphyry copper enrichment linked to excess aluminium in plagioclase) proposes that such a signal can be found in the aluminium content of crystals in solidified magmas associated with nearby porphyry deposits.
Porphyry copper deposits can be huge and may carry billions of tonnes of ore but typically contain only 0.5–1.5% copper. The metals occur as sulfide minerals – in veins or disseminations – within a shallowly intruded magma body that forms at the base of volcanoes in active tectonic regions such as the Andes of South America. Rapid cooling of these magma bodies forms an igneous rock with a texture composed of both large and small crystals that geologists call ‘porphyritic’. It is this characteristic rock texture which gives rise to the name – porphyry copper deposit.
Current theory suggests that copper, the element sulphur and associated metals become concentrated in super-heated water-rich fluids that are released from crystallising porphyry magmas. Copper and other metals combine with the sulfur in the fluid to form sulfide minerals, which are then trapped as disseminations or veins within the rock. Most of the known porphyry copper deposits occur in places such as the Andes and were found at or close to the surface by geologists using fairly routine discovery tools. However, as the shallow deposits become exhausted, we need to use more subtle surface clues to find where new deposits that are hidden at depth, improving the probability of discovery.
This new work uses electron microprobe analysis (a relatively cheap analytical method available in many geoscience laboratories) to measure the aluminium content of the abundant silicate mineral plagioclase (which constitutes up to 70% of ‘porphyry’ magmatic rocks). The level of aluminium content is thought to make it possible to distinguish the rocks likely to be related to large copper deposits from those rocks which are not.
The reason why aluminium is thought to signal copper deposits is that ‘excess’ aluminium entering into the mineral structure of plagioclase is linked to high water content in the magma. Water-rich magmas are more likely to form the super-heated water-rich fluids that form porphyry copper deposits from magmas.
The paper argues that this excess aluminium prevents copper in the magma from being incorporated into the growing mineral crystals, leaving more copper to be taken up by the super-heated magmatic water that goes on to form the copper deposits.
The work also suggests that ore-forming events in such systems might occur as a result of to very well defined pulses of fluid release: this could be investigated by analysis of the plagioclase crystals. However, more data is needed to confirm if this could be used as a reliable technique to be used for copper prospecting, but it joins a string of new theories that have blossomed in the last 3 years helping unravel how porphyry copper deposits form.
These theories have been scrutinised in a news and views article by Jeremy Richards from the University of Alberta in the same issue of Nature Geoscience. These exciting new theories and the discussion that is generated by them are changing the way we go about exploring for these giant hidden copper deposits.
Jamie J. Wilkinson, Zhaoshan Changa, David R. Cookea, Michael J. Bakera, Clara C. Wilkinson, Shaun Inglisa, Huayong Chena and Bruce Gemmella (2015) The chlorite proximitor: A new tool for detecting porphyry ore deposits. Journal of Geochemical Exploration, 152, 10–26 doi:10.1016/j.gexplo.2015.01.005