New discoveries shed light on finding valuable 'green' metals

Research led by Macquarie University sheds new light on how concentrations of metals used in renewable energy technologies can be transported from deep in the Earth's inner crust by low-temperature, carbon-rich melt.

Findings Published This week in the magazine Scientific advances Help global efforts to find these valuable raw materials.

A postdoctoral researcher at Macquarie University's School of Natural Sciences, Dr. An international team led by Isra Ezad conducted high-pressure and high-temperature experiments to create small amounts of molten carbonate material in conditions similar to those in the mantle 90 kilometers deep, below the Earth's crust.

Their experiments showed that carbonate melting can dissolve and transport a range of important metals and compounds from surrounding rocks in the mantle – new information that could inform future metal prospecting.

„We knew that carbonate melts carry rare earth elements, but this research goes further,” says Dr. Ezzat.

„This carbon-rich molten rock takes up sulfur in its oxidized form, while precious and base metals—the 'green' metals of the future—are extracted from the mantle.”

Deep in the Earth's crust, most of the rocks in the lower crust are silicates in composition, similar to the lava that erupts from volcanoes.

However, a small proportion (a fraction of one percent) of these deeper rocks contain small amounts of carbon and water, which melt at lower temperatures than other parts of the mantle.

This carbonate effectively dissolves base metals (including nickel, copper, and cobalt), precious metals (including gold and silver), and oxidized sulfur, filtering these metals into viable deposits.

„Our findings suggest that sulphur-enriched carbonate melts may be more widespread than previously thought, and may play an important role in the enrichment of metal deposits,” says Dr Essad.

The researchers used two natural mantle compounds: a mica pyroxenite from western Uganda and a rich spinel lherzolite from Cameroon.

Thick continental crust forms in the old hinterlands of the continents, where they act as sponges, absorbing carbon and water, Dr. Ezzat says.

„Carbon-sulfur melting dissolves and concentrates these metals within distinct crustal regions, moving them to shallower crustal depths where dynamic chemical processes lead to the formation of ore deposits,” says Dr. Essad.

Dr Essad says the study indicates that monitoring carbonate melting can give us a better understanding of large-scale metal redistribution and ore formation processes throughout Earth's history.

„As the world transitions from fossil fuels to battery, wind and solar technology, the demand for these essential metals is rising and reliable sources are becoming harder to find,” says Dr Essad.

„These new data provide us with a previously unconsidered mineral exploration space for base and precious metals—carbonate melt deposits,” he says.