Unlocking the journey of gold through magmatic fluids
When one plate sinks beneath another, it creates magma that is rich in volatiles such as water, sulfur and chlorine. As these magmas rise, they release magmatic fluid, in which sulfur and chlorine combine with metals such as gold and copper and transport these metals to the Earth’s surface. The exact role of different forms of sulfur in metal transport remains highly controversial, as the extreme conditions associated with natural magmas are difficult to reproduce in the laboratory. However, an innovative approach by a team at the University of Geneva (UNIGE) has shown that sulfur in its disulfide (HS–) form is critical for gold transport in magmatic fluids. These findings were published in natural geosciences.
When two tectonic plates collide, the subducting plate sinks into the Earth’s mantle, heating up and releasing large amounts of water. This water lowers the melting temperature of the Earth’s mantle, which melts to form magma at high pressures and temperatures of more than a thousand degrees Celsius. Because liquid magma is less dense than the rest of the Earth’s mantle, it migrates toward the Earth’s surface.
“As the pressure drops, magma rises toward the Earth’s surface, saturating it with water-rich fluids, which are then released in the form of bubbles of magmatic fluid, leaving behind silicic acid,” explained Stefan Farsang, a postdoc in the Department of Earth Sciences. Salt melt.
Several forms of sulfur
Sulfur can be easily reduced or oxidized, that is, gaining or losing electrons, a process called redox. The redox state of sulfur is important because it affects its ability to combine with other elements, such as metals. However, the scientific community has been debating it for more than a decade: What is the redox state of sulfur in the magmatic fluids that mobilize and transport metals?
Zoltán Zajacz, associate professor at the Department of Geosciences at UNIGE’s Faculty of Science and co-author of the study, explains: “A seminal paper in 2011 showed that S3-sulfide radicals play this role. However, the experimental and analytical methods have some limitations, Especially in reproducing the relevant magma pressure-temperature and redox conditions, which we have now overcome.
methodological revolution
The UNIGE team placed quartz cylinders and a liquid similar in composition to magmatic fluids into sealed gold capsules. The capsule is then placed into a pressure vessel, which is then exposed to pressure and temperature conditions unique to magma in Earth’s upper crust. “Most importantly, our setup facilitates flexible control of redox conditions in the system, which was not possible before,” adds Stefan Farsang.
During the experiment, the quartz cylinder fractured, allowing synthetic magmatic fluid to enter. The quartz then captures microscopic-sized fluid droplets like those found in nature, and the sulfur forms within them can be analyzed using laser and Raman spectroscopy techniques at high temperatures and pressures. While previous spectroscopic experiments typically operated at temperatures as high as 700 °C, the UNIGE team succeeded in raising the temperature of natural magma to 875 °C.
Disulfides as transporters
Studies have shown that disulfide (HS-), hydrogen sulfide (H2S) and sulfur dioxide (SO2) are the main sulfur species present in experimental fluids at magmatic temperatures. The role of disulfides in metal transport has been well documented in low-temperature so-called hydrothermal fluids derived from high-temperature magmatic fluids. However, disulfides are thought to have very limited stability at magmatic temperatures. Thanks to their cutting-edge method, the UNIGE team was able to demonstrate that disulfides are also responsible for transporting most of the gold in magmatic fluids.
“Through careful selection of laser wavelengths, we also showed that the levels of sulfur radicals in geofluids were significantly overestimated in previous studies, and that the results of the 2011 study were in fact based on measurement artefacts, thus putting an end to this. situation debate,” said Stéphane Farsan. The conditions for the formation of important precious metal deposits are now clear. Since most of the world’s copper and gold production comes from deposits formed by magma-derived fluids, this research could provide important insights into their formation and aid their exploration.
2024-12-16 18:00:31