Nav: Home

Copper-bottomed deposits

March 15, 2017

The world's most valuable copper deposits, known as porphyry deposits, originate from cooling magma. But how can we predict the size of these deposits? What factors govern the amount of copper present? Researchers at the University of Geneva (UNIGE), Switzerland, have studied over 100,000 combinations to establish the depth and number of years required for magma to produce a given amount of copper. The same scientists have also devised a model that can detect the quantity of copper held in a deposit by means of a simple factor analysis. The research, which is published in the journal Scientific Reports, will make it possible to estimate the potential for mining the metal before beginning any drilling. It is a model that will undoubtedly be of great benefit to mining companies.

Porphyry copper deposits account for 75% of natural copper worldwide. They are formed by magma chambers situated between 10 and 15 km beneath the Earth's surface. At this depth, the magma heats to around 900°C but when it comes into contact with the surrounding rock, it cools and crystallises. The water in the magma can then no longer be in solution: it forms bubbles that escape to the surface, carrying with them a substantial part of the copper originally contained in the magma. At a depth of around 2-3 km, the bubbles cool down in the porosities of the rocks, and precipitate the copper they contain as sulphide, creating deposits that may include from 1 to >200 million tons of copper. This explains why Massimo Chiaradia and Luca Caricchi, researchers in the earth sciences department in the faculty of science at UNIGE, were so keen to discover what dictates the amount of copper in a deposit and whether it was possible to anticipate its size.

More magma means more copper

The volume of magma determines the amount of copper, but under what conditions does the volume of the initial magma form? Chiaradia explains: "We used models that incorporate the depth and timescale at which the magma accumulates, the duration of the build-up that forms the deposit, the water content of the magma and the quantity of copper in the water. We then varied these parameters from a minimum to a maximum based on actual measurements." By modifying the parameters, the scientists obtained 100,000 simulations that they compared with the actual data available to them, which helped define the ideal conditions for the formation of a huge deposit. As Caricchi adds: "The optimum conditions for creating a magmatic system that results in the formation of a deposit of 30 to 240 million tons of copper is a depth of over 20 km and a continuous injection time of molten magma of over 2 million years."

In search of the ideal deposit

Magma contains water, copper and various other chemical components, including Strontium (Sr) and Yttrium (Y). We know that when the Sr divided by Y ratio is between 50 and 150 in the magma, there is a high probability of finding copper in the deposit. The researchers at UNIGE integrated this ratio into their new model and merged it with the estimated formation time for deposits. Other minerals are associated with copper in these deposits, which allows scientists to date them thanks to the natural decay of uranium into lead and rhenium into osmium. This enabled the scientists to establish the age, i.e. the birth, but also the length, i.e. the number of years, for forming a copper deposit, which can range from tens of thousands of years to two million years. "These two items of data -- the Sr / Y ratio and the duration of the formation -- meant we could design a table of probabilities for determining the amount of copper in the deposit under analysis", continues Chiaradia. Mining companies will be able to use this model to assess the size of a copper deposit at the initial research stage, before starting any significant drilling work. "Our model," says Caricchi, "which we have compared to real data, has an excellent match rate, and it can save an enormous amount of time and money during mining explorations."
-end-


Université de Genève

Related Magma Articles:

Volcanic crystals give a new view of magma
Volcanologists are gaining a new understanding of what's going on inside the magma reservoir that lies below an active volcano and they're finding a colder, more solid place than previously thought, according to new research published June 16 in the journal Science.
Thermal history of magma may help scientists hone in on volcanic eruption forecasts
A new study analyzed crystals of the mineral zircon -- zirconium silicate -- in magma from an eruption in the Taupo Volcanic Zone in New Zealand about 700 years ago to determine the magma's history.
Crystals once deep inside a volcano offer new view of magma, eruption timing
Volcanologists are gaining a better understanding of what's going on inside the magma reservoir that lies below New Zealand's Mount Tarawera volcano.
Forget the red hot blob: Volcanic zircon crystals give new view of magma
The classic red teardrop of magma underneath a volcano peak is too simplistic.
Deep magma reservoirs are key to volcanic 'super-eruptions', new research suggests
Large reservoirs of magma stored deep in the Earth's crust are key to producing some of the Earth's most powerful volcanic eruptions, new research has shown.
More Magma News and Magma Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Anthropomorphic
Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#534 Bacteria are Coming for Your OJ
What makes breakfast, breakfast? Well, according to every movie and TV show we've ever seen, a big glass of orange juice is basically required. But our morning grapefruit might be in danger. Why? Citrus greening, a bacteria carried by a bug, has infected 90% of the citrus groves in Florida. It's coming for your OJ. We'll talk with University of Maryland plant virologist Anne Simon about ways to stop the citrus killer, and with science writer and journalist Maryn McKenna about why throwing antibiotics at the problem is probably not the solution. Related links: A Review of the Citrus Greening...