Earth’s hidden “golden kitchen” may be operating far beneath the ocean floor. Island arcs are series of volcanoes that develop above subduction zones where one oceanic plate subducts beneath another, and are often unusually rich in gold. Scientists have long debated why these regions become so rich.
A new study led by Dr. Christian Timm, a marine geologist at the GEOMAR Kiel-Helmholtz Center for Marine Research, provides a deeper explanation.
“Our study shows that the melting hydrous mantle beneath the island arc is the primary driver of gold enrichment,” Tim says. “In these environments, the mantle behaves like a multi-stage dissolution system that concentrates gold in stages.”
Volcanic glass preserves ancient magma
To investigate how gold and other precious metals behave as the mantle melts beneath submarine subduction zones, researchers studied 66 volcanic glass samples taken from the ocean floor along the Kermadec Island arc and the adjacent Harbor Trough north of New Zealand.
Volcanic glass is formed when lava in water cools rapidly. This rapid cooling locks in much of the magma’s original chemical composition, giving scientists a valuable record of conditions deep under the ocean.
The most revealing sample was so-called primitive glass. These preserve the chemistry of the original magma before crystallization changes its composition.
“When we analyzed these samples, we found that their gold concentrations were several times higher than those in comparable magmas from mid-ocean ridges,” Tim said. “This raises an important question: What processes are involved in this enrichment?”
The researchers measured gold at extremely low concentrations and compared it to other chalcophilic (“sulfur-loving”) elements such as silver, copper, selenium, and platinum. Because these elements react in similar ways during melting, their chemical patterns can reveal what was happening inside the mantle.
Chemical clues indicating repeated melting
This result indicates that the mantle beneath the Kermadec Island arc melts at relatively high temperatures above the sulfide liquidus in the presence of water. At this point, the sulfide minerals may begin to fully decompose. Under these conditions, the magma maintains a silver to copper ratio similar to that found in the mantle.
The researchers also detected original gold concentrations of up to 6 nanograms per gram of rock. The amount may sound small, but it is unusually large for magma derived from the mantle. The samples also contained gold-to-copper ratios that far exceeded those measured in fertile mantle and pristine mid-ocean ridge basalts.
According to the researchers, these chemical signals are best explained by a mantle source that has already been depleted by previous melting and then melted again.
The main process concentrating gold in these magmas is likely to be advanced multistep dissolution of the water-rich, oxidized mantle.
Despite its high value, this rock does not contain enough gold to be mined commercially. Economically useful deposits require concentrations several orders of magnitude higher.
Water helps dissolve the mantle
The researchers initially thought that water released from the descending plate might directly control the amount of gold entering the magma. New data suggests a more complex process.
“We initially thought that water released from the subduction zone directly controlled gold enrichment,” Tim says. “However, our data show that water primarily drives mantle melting. The main driver of high gold concentrations is the high degree and, in part, repeated nature of the melting.”
Therefore, water is thought to primarily act as a trigger to help melt more widespread mantle rocks. The stronger and more repeated the melting, the more effectively the gold can be moved into the magma.
The chemical location of gold within the mantle is also important.
“Gold in the mantle is typically bound to sulfide minerals,” Tim explains. “At high levels of dissolution, these minerals break down and the gold is completely released into the melt.”
This means that gold can remain trapped during limited melting. However, when the dissolution becomes intense enough to destroy the sulfide minerals, the stored gold is released and enters the rising magma.
“Our results show that gold enrichment is not the result of a single dissolution event, but of multiple steps,” Tim added. “Only through repeated melting can the gold become highly concentrated in the magma.”
The beginning of gold’s geological journey
The discovery improves scientists’ understanding of gold-rich deposits associated with intraoceanic island arcs, such as the Kermadec arc. They show that repeated mantle melting, aided by water, has a strong effect on the amount of gold transported upward by magma.
The results also move some of the explanation for gold deposits deeper into the Earth. Although near-surface processes determine whether condensed deposits ultimately form, the chemical history of the mantle beneath the subduction zone can establish starting conditions long before magma rises.
The same mechanism may help explain why thermosulfidic water deposits along submarine island arcs often contain unusually large amounts of gold. These deposits form when hot, mineral-rich fluids circulate through volcanic regions beneath the ocean.
“The mechanisms we have identified may contribute to the elevated gold content observed in subduction zone hydrothermal systems,” Tim says. “However, this association requires further investigation.”
“We are effectively looking at the first step in the gold lifecycle,” Tim concludes. “It begins with the transfer of gold from the mantle into a melt that eventually forms volcanoes. Alchemy begins long before the metal reaches the surface.”
