‘Goldilocks zone’ the key to metal discoveries

SensOre

UK and Australian researchers have come to understand the specific temperature that allows some metals to rise into Earth’s crust, aiding exploration and discovery.

The research, published in Nature Communications, was performed in collaboration between the University of Western Australia (UWA), the University of Leicester and Cardiff University, in partnership with BHP.

The team focussed on a gap in research concerning metals such as copper, gold, nickel and iron, and how they are transferred from the Earth’s mantle up into the Earth’s crust.

The crust forms the outermost part of Earth and measures between zero to 100 kilometres deep. The mantle begins about 30 kilometres from the Earth’s surface and is about 3000 kilometres deep.

As the deepest mine on Earth – AngloGold Ashanti’s Mponeng gold mine in South Africa – has only reached about four kilometres deep, it hasn’t yet become possible to mine metals from the Earth’s mantle.

For this reason, the research team found it important to understand where these metals – key to the decarbonisation of many technologies – might appear in the crust.

Professor Marco Fiorentini, from UWA’s Centre of Exploration Targeting in the School of Earth Sciences, said the metals’ subterranean transportation was dependent on fluctuations in temperature.

“Fortunately, every now and then nature does most of the hard work for us,” Fiorentini said.

“At times, green metals are extracted from the deep mantle and transported into the overlying shallower crust through the formation and build-up of large volcanic edifices.”

The research found that a “Goldilocks” temperature of about 1100°C to 1200°C was ideal for copper-gold rich sulphides to liquify and rise through the mantle into the crust.

Fiorenti said the volcanic edifices were picky about when they allow the metals to pass through.

“If the temperature is either too hot or too cold, these valves remain shut and metals cannot pass through,” he said.

“And at various stages the metals can be lost on the way, becoming trapped at the interface between the mantle and the crust, before they can be found and mined.”

The paper acknowledged the importance of this finding for exploration companies, as they can analyse rocks on Earth’s surface to understand where this Goldilocks temperature may have occurred below.

“This mechanism provides key insights into the formation of porphyry-epithermal Cu-Au deposits and could aid in better predicting their location during exploration targeting if tracers of this lower crustal process can be identified in upper crustal magmatic rocks,” the paper stated.

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