A recent paper in the journal Science Advances sheds new light on how concentrations of metals used in renewable energy technologies can be transported from deep within the earth’s interior mantle by low-temperature, carbon-rich melts.
The article details how an international team led by Isra Ezad, a postdoctoral research fellow at Australia’s Macquarie University, carried out high-pressure and high-temperature experiments creating small amounts of molten carbonate material at conditions similar to those around 90 kilometres depth in the mantle, below the earth’s crust.
Their experiments showed carbonate melts can dissolve and carry a range of critical metals and compounds from surrounding rocks in the mantle—new information that may inform future metal prospecting.
“We knew that carbonate melts carried rare earth elements, but this research goes further,” Ezad said in a media statement. “We show this molten rock containing carbon takes up sulphur in its oxidized form, while also dissolving precious and base metals—‘green’ metals of the future—extracted from the mantle.”
Most of the rock that lies deep in the planet’s crust and below in the mantle is silicate in composition, like the lava that comes out of volcanoes.
However, a fraction of a percent of these deep rocks contain small amounts of carbon and water that cause them to melt at lower temperatures than other portions of the mantle.
These carbonate melts effectively dissolve and transport base metals like nickel, copper and cobalt; precious metals, including gold and silver, and oxidized sulphur, distilling these metals into potential deposits.
“Our findings suggest carbonate melts enriched in sulphur may be more widespread than previously thought, and can play an important role in concentrating metal deposits,” Ezad said.
To run their experiments, the researchers used two natural mantle compositions: a mica pyroxenite from western Uganda and a fertile spinel lherzolite from Cameroon.
Ezad explained that thicker continental crust regions tend to form in older inland regions of continents, where they can act as a sponge, sucking up carbon and water.
“Carbon-sulphur melts appear to dissolve and concentrate these metals within discrete mantle regions, moving them into shallower crustal depths, where dynamic chemical processes can lead to ore deposit formation,” the scientist pointed out.
In her view, this study indicates that tracking carbonate melts could give us a better understanding of large-scale metal redistribution and ore formation processes over earth’s history.
“As the world transitions away from fossil fuels to battery, wind and solar technologies, demand for these essential metals is skyrocketing, and it’s becoming harder to find reliable sources,” Ezad said. “These new data provide us with a mineral exploration space previously not considered for base and precious metals—deposits from carbonate melts.”
