Earth's 'gold kitchen' — why gold concentrates at subduction zones, and repeated mantle melting is the key

Why does gold concentrate along volcanic arcs at subduction zones? A team led by GEOMAR's Dr Christian Timm investigated this question using the Kermadec Arc in the South Pacific.

The team analysed 66 volcanic glass samples from the Kermadec Arc and adjacent Havre Trough. Volcanic glass forms when lava is quenched rapidly by seawater, preserving the original magma's chemical composition.

Gold concentrations several times higher than mid-ocean ridges

The analysis revealed that primitive glass samples from the Kermadec Arc have gold concentrations several times higher than mid-ocean ridge magmas. Up to 6 nanograms of gold per gram of rock were detected.

When we analysed these samples, we found that their gold concentrations are often several times higher than those of comparable magmas from mid-ocean ridges

— Dr Christian Timm (GEOMAR, marine geologist)

The key is repeated melting

Initially, the team assumed water released from the subducting slab directly controlled gold enrichment. But the data told a different story. Water facilitates mantle melting, but the key to high gold concentrations is repeated high-degree melting.

Our data show that water mainly facilitates mantle melting. The key factor for high gold concentrations is the high — and in part repeated — degree of melting

— Dr Christian Timm

Gold in the mantle is bound in sulphide minerals. At high degrees of melting, these minerals break down, releasing their gold completely into the melt. Not a single melting event, but multiple stages concentrate gold in the magma.

The first step in gold's life cycle

We are effectively looking at the first step in the life cycle of gold. It begins with the transfer of gold from the mantle into a melt that eventually forms volcanoes. The alchemy starts long before the metal reaches the surface

— Dr Christian Timm

For more on seafloor geology, see 'Football-pitch-sized coral reefs discovered off Uruguay.'

A note from the author: The phrase 'Earth's gold kitchen' is brilliantly catchy. The mechanism — gold concentrating as the mantle melts repeatedly — is like reducing a sauce.

That said, current concentrations are nowhere near mineable. Earth's alchemy operates on scales beyond human measure.