Sulphur’s Role in Gold Transport in Magmatic Fluids
Sulphur’s Role in Gold Transport in Magmatic Fluids
When tectonic plates collide and one sinks beneath the other, it generates magma rich in volatile substances like water, sulfur, and chlorine. As this magma ascends, it releases magmatic fluids containing sulfur and chlorine, which bind to metals like gold and copper, helping transport these metals toward the Earth's surface. The precise role of different sulfur forms in metal transport, however, has been difficult to study due to the extreme conditions of natural magmas, which are challenging to replicate in a laboratory setting. Now, a team from the University of Geneva (UNIGE) has shown that sulfur, in its bisulfide (HS-) form, is crucial for transporting gold in magmatic fluids. These findings were published in Nature Geoscience.
As two tectonic plates collide, the subducting plate sinks into the Earth's mantle, where it heats up and releases large amounts of water. This water lowers the mantle's melting point, allowing it to melt under high pressure and temperatures exceeding a thousand degrees Celsius, forming magma. Since magma is less dense than the surrounding mantle, it rises towards the Earth’s surface.
"Due to the drop in pressure, the rising magma becomes saturated with water-rich fluid, which then forms magmatic fluid bubbles, leaving a silicate melt behind," explains Stefan Farsang, a postdoctoral fellow at UNIGE and the study’s first author. These magmatic fluids contain water as well as dissolved volatile elements like sulfur and chlorine, which are essential in extracting metals such as gold and copper from the silicate melt and helping them move toward the surface.
Sulfur’s Multiple Forms
Sulfur can easily be reduced or oxidized, meaning it can gain or lose electrons—a process known as redox. The redox state of sulfur is crucial as it affects its ability to bind to other elements, such as metals. However, for over a decade, scientists have debated the redox state of sulfur in magmatic fluids and its role in mobilizing and transporting metals.
Zoltán Zajacz, an associate professor in the Department of Earth Sciences at UNIGE and coauthor of the study, explains, "A pivotal 2011 paper suggested that sulfur radicals (S3-) play this role. However, the methods used in that study had several limitations, especially in replicating the relevant magmatic pressure-temperature and redox conditions, which we have now overcome."
A Revolutionary Approach
The UNIGE team developed a new experimental setup in which a quartz cylinder and a liquid resembling a magmatic fluid were placed inside a sealed gold capsule. The capsule was then subjected to pressure and temperature conditions typical of magmas found in the Earth's upper crust. "Our setup allows for precise control of the redox conditions, something that was not possible before," adds Stefan Farsang.
During the experiments, the quartz cylinder fractured, allowing the synthetic magmatic fluid to enter. The quartz then trapped microscopic droplets of fluid similar to those found in nature, which could be analyzed at high temperature and pressure using Raman spectroscopy. Unlike previous spectroscopic experiments, which were conducted at temperatures up to 700 °C, the UNIGE team successfully raised the temperature to 875 °C, mimicking conditions found in natural magmas.