‘Fizzy pop’ process reveals copper-rich volcanoes

Identifying magmas that experience the same process that makes fizzy drinks ‘bubbly’ has been used by a team of University of Leicester geologists to predict whether a volcano’s magma is likely to be rich in copper.

They have developed a technique that can be used to predict whether an area surrounding a volcano is potentially a good site for future copper mining, a metal that is vital for the transition to green technologies.

The study, published in the journal Geology, is a proof of concept for a method that could be used by the mineral exploration industry to identify potential new sites. It was funded by the Central England NERC Training Alliance.

The technique takes advantage of a process in magma called fluid saturation that scientists have found is associated with the formation of copper deposits. When water saturates in a magma, it forms a separate fluid, in the same way that fizzy drinks become bubbly once no more CO₂ can be dissolved within them. This water has been shown to strip the magma of fluid-loving elements like chlorine that also take copper with them. These copper-rich fluids are what form mineral deposits.

This process of stripping the magma of its chlorine and copper is captured in a type of mineral called apatite which remains in the magma as it erupts, cools and crystalises. Apatite is a very common mineral found in many different environments, but it is particularly good at hosting elements like chlorine that bind to copper – making them ideal for ‘fingerprinting’ the environment they were formed in.

The researchers collected apatite samples from two volcanoes in the Philippines for this study: Pinatubo, which has reached fluid saturation, and Taal, which hasn’t. These were then sliced extremely thinly and examined using a scanning electron microscope, to look for the chemical ‘fingerprint’ of fluid saturation.

Lead author George Stonadge, a PhD researcher in the University of Leicester School of Geography, Geology and the Environment, said: “We’ve used this very innocuous mineral apatite, which many researchers neglected for years because it was so seemingly insignificant, and found that it has a certain chemical fingerprint that is very different depending on whether it comes from a magma that has experienced fluid saturation and a magma that hasn't”.

“We don't fully understand how these copper- and fluid-rich magmas form. But we know that there are certain characteristics to the magmas that mean it's more likely that you get a lot of copper forming in relation to them. My PhD has shown that this process of fluid saturation is very, very well recorded in apatite crystals from the copper deposits themselves”.

“Being able to quickly identify these fluid-saturated magmas will be really useful during the early stages of exploration as it will allow areas of potential economic value to be located quickly. By comparison with a lot of the current techniques used by the exploration industry, it is very, very cheap and very, very accessible.”

Dr Andrew Miles, George’s PhD supervisor, said: “The world needs more copper because of where we're going with green technology, and we already know where all the big copper deposits are on Earth. So if you've already found the easy ones, the next step is to try and find the ones that aren't so easy to find.”

“This new technique offers an exciting and cost-effective way of finding these harder-to-find copper deposits that is so important if we’re to meet our global targets for the green energy transition.”    

• ‘The volatile record of volcanic apatite and its implications for the formation of porphyry copper deposits’ is published in Geology, DOI: 10.1130/G51461.1, Link: https://doi.org/10.1130/G51461.1