A type of sulfur that exists in subduction zones can promote gold to move closer to the Earth’s surface, helping humans exploit this rare metal.
Gold can move from the mantle to the Earth’s surface thanks to sulfur. Image: Wikimedia
A research team at the University of Michigan was able to find a new geological mechanism that creates gold ore on Earth. The new hypothesis combines numerical modeling with field observations, which could be used to support future gold mining, according to research results published in the journal Proceedings of the National Academy of Sciences.
Gold is one of the rarest metals on Earth. Most of the gold is in the planet’s mantle. Gold is often concentrated in volcanic rocks or magma above ground. The process by which gold is transferred in large quantities from the mantle to the Earth’s surface to form ore beds has long been a controversial issue. However, new research uncovers at least one mechanism to explain this process.
The secret may lie in sulfur. According to the research team, some types of sulfur form at high temperatures and pressures at depths of tens of kilometers below active volcanoes around the globe. Those sulfur species appear to draw gold out of the mantle and into magma, which is then carried to the Earth’s surface. The research team’s idea is based on the understanding that gold prefers to form bonds with 2-3 sulfur atoms if conditions permit. It is this gold-sulfur combination that is the basis for the thermodynamic model of gold ore formation because, because it is very inert, gold will remain in the mantle if not “forced” to move.
If exposed to sulfur-rich liquids, gold forms as above and becomes extremely mobile (especially in large areas of the mantle). After entering the magma, gold only needs time to move upward, leading to ore beds for humans to exploit and refine. One hot spot for this mechanism is the subduction zone, the area where one tectonic plate sinks beneath another. Subduction zones are often associated with the tectonic plate boundary between heavy oceanic crust and lighter continental crust. There, the Earth’s mantle is most likely to experience conditions that make it easiest for gold to reach the surface. As the tectonic plate sinks and then melts under the extreme conditions of the mantle, the process provides the perfect environment and materials such as a sulfur-rich liquid to produce gold-bearing magma.
“On every continent around the Pacific from New Zealand to Indonesia, the Philippines, Japan, Russia, Alaska, the western United States, and Canada, down to Chile, we have many active volcanoes,” Adam Simon , professor of Earth and environmental sciences at the University of Michigan, said. “All active volcanoes form above in or subduction zones. The process that leads to volcanic eruptions is also the process that forms gold ore.”