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Digging Into the World of Copper

UI professor’s research reveals new patterns in how economically important copper deposits are distributed across the Earth

About 75 percent of the world’s copper comes from porphyry copper deposits. That’s three-quarters of the copper in your phone, your laptop and other high-tech devices that use the metal in their circuits.

New research at the University of Idaho and the University of Michigan unearths how porphyry copper deposits are distributed around the world.

Brian Yanites — an assistant professor in the UI Department of Geological Sciences — and Stephen Kesler — an emeritus professor in the Department of Earth and Environmental Sciences at Michigan — paired their areas of expertise to explore how the building of mountains and the climate in different regions of the world work together to determine the deposits’ sites. Their work was published in the online edition of Nature Geoscience on May 11.

Yanites is a geomorphologist, studying the Earth’s topography. Kesler is an economic geologist, studying the formation of deposits that can be mined for raw materials.

Porphyry copper deposits initially form beneath volcanoes, on average 2 kilometers below the surface. Over the course of millions or tens of millions of years, erosion exposes them.

Yanites and Kesler set out to determine whether the movement of the Earth’s tectonic plates alone shaped the rate at which porphyry copper deposits were exposed, or if climate also drove the erosion and mineral exposure.

“Geomorphology has always been trying to document whether climate has an influence on erosion. Actually quantifying and documenting that has been difficult,” Yanites says. “This was an attempt to look at it on a mountain-building timescale.”

They examined data on the age, depth and number of exposed porphyry copper deposits in regions around the world. When they compared these data to the regions’ climates, they noticed a pattern. The youngest deposits are found in areas of high rainfall, such as the tropics, indicating rapid erosion, and deposits are older in dry areas, indicating low rates of erosion.

They then counted the number of deposits in the different regions and found something striking: Where erosion is rapid, there were relatively few deposits, but locations with low erosion rates contain a high density of deposits. Such regions include the Atacama Desert in the Andes Mountains and the American Southwest — both places where porphyry copper mining is important to the economy.

The researchers developed a simple model to explain the relationship between erosion rate and the number of deposits in a region. Yanites compared it to a game on the TV show “American Gladiators,” where competitors had to run past tennis-ball launching cannons to reach their goal.

“The faster the competitors run, the lower the chance that they will get hit because they spend less time in the ‘danger zone,’” Yanites says. “Similarly, rock layers that spend less time in the porphyry production zone — due to rapidly eroding landscapes above — have less of a chance of getting injected with one of these valuable deposits.”

Whether this work will help companies find new porphyry copper deposits remains to be seen, as most of the deposits have likely already been found. But knowing how tectonic movement and climate can work together over millions of years to uncover deposits could help understand the distribution of other useful metals that form at specific depths in the Earth, such as epithermal gold.

“This is the first time that we’ve found a connection between geomorphology and economic geology,” Yanites says. “It’s exciting to think that erosion and the building of our mountain landscapes influences where society gets its resources from, and it’s another line of evidence of the importance of climate in the shape of the landscape.”

In addition to potential economic benefits, Yanites is excited that his and Kesler’s research also brings new understanding to how the Earth changes over time. Yanites encourages this big-picture knowledge in his students and values it in his research pursuits.

“You build a mountain range with tectonics, but climate can affect how you build that mountain range, too,” he says. “It’s important to understand how the Earth works as a system.”

Article by Tara Roberts for the University of Idaho

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