A team from UNIGE is transforming our understanding of gold transport and ore deposit formation by investigating sulfur in magmatic fluids under extreme pressures and temperatures.
When one tectonic plate sinks beneath another, it creates magmas rich in volatile elements like water, sulfur, and chlorine. As these magmas rise, they release magmatic fluids. These fluids carry sulfur and chlorine, which bind with metals such as gold and copper, helping to transport them toward the Earth’s surface. Because the extreme conditions found in natural magmas are difficult to replicate in the lab, the exact role of sulfur in metal transport has long been a subject of debate.
A research team from the University of Geneva (UNIGE) has introduced an innovative approach to address this uncertainty. Their study reveals that sulfur, specifically in its bisulfide (HS⁻) form, plays a critical role in transporting gold within magmatic fluids. This breakthrough, published in Nature Geoscience, sheds new light on the processes that move precious metals through the Earth’s crust.
When two tectonic plates collide, the subducting plate plunges into the Earth’s mantle, heats up and releases large amounts of water. This water lowers the melting temperature of the mantle, which melts under high pressure and temperatures exceeding a thousand degrees Celsius to form magmas. As the liquid magma is less dense than the rest of the mantle, it migrates towards the Earth’s surface.
‘‘Due to the drop in pressure, magmas rising towards the Earth’s surface saturate a water-rich fluid, which is then released as magmatic fluid bubbles, leaving a silicate melt behind” explains Stefan Farsang, a postdoctoral fellow at the Department of Earth Sciences at UNIGE’s Faculty of Science and first author of the study. Magmatic fluids are therefore composed partly of water, but also of dissolved volatile elements such as sulfur and chlorine. These two elements are crucial because they extract gold, copper, and other metals from the silicate melt into the magmatic fluid, thus facilitating their migration towards the surface.
Several forms of sulfur
Sulfur can easily be reduced or oxidized, i.e. gain or lose electrons, a process known as redox. The redox state of sulfur is important because it affects its ability to bind to other elements, such as metals. However, one debate has divided the scientific community for more than a decade: what is the redox state of the sulfur present in the magmatic fluid that mobilizes and transports metals?
Zoltán Zajacz, associate professor in the Department of Earth Sciences at UNIGE’s Faculty of Science and coauthor of the study, explains: “A seminal paper in 2011 suggested that S3- sulfur radicals play this role. However, the experimental and analytical methods had several limitations, particularly when it came to reproducing relevant magmatic pressure-temperature and redox conditions, which we have now overcome.’’
Methodological revolution
The UNIGE team placed a quartz cylinder and a liquid with a composition similar to that of a magmatic fluid in a sealed gold capsule. The capsule was then put into a pressure vessel, which was then brought to pressure and temperature conditions characteristic of magmas emplaced in the Earth’s upper crust. ‘‘Above all, our setup facilitates flexible control of the redox conditions in the system, which wasn’t possible before,” adds Stefan Farsang.
During the experiments, the quartz cylinder is fractured, allowing the synthetic magmatic fluid to enter. The quartz then traps microscopic-sized droplets of fluid like those found in nature, and the form of sulfur in these can be analyzed at high temperatures and pressure by using lasers with an analytical technique known as Raman spectroscopy. While previous spectroscopic experiments were typically run up to 700 °C, the UNIGE team succeeded in raising the temperature to 875 °C characteristic of natural magmas.
Bisulphide as a transporter
The study shows that bisulphide (HS-), hydrogen sulfide (H₂S), and sulfur dioxide (SO₂) are the major sulfur species present in the experimental fluids at magmatic temperatures. The role of bisulphide in metal transport was already well documented in lower-temperature so-called hydrothermal fluids that originate from the higher-temperature magmatic fluids. However, bisulfite was thought to have very limited stability at magmatic temperatures. Thanks to their cutting-edge methodology, the UNIGE team was able to show that in magmatic fluids too, bisulphide is responsible for transporting most of the gold.
‘‘By carefully choosing our laser wavelengths, we also showed that in previous studies, the number of sulfur radicals in geologic fluids was severely overestimated and that the results of the 2011 study were in fact based on a measurement artifact, putting an end to this debate,’’ says Stefan Farsang. The conditions leading to the formation of important precious metal ore deposits have now been clarified. Since much of the world’s copper and gold production comes from deposits formed by magma-derived fluids, this study
may contribute to their exploration by opening up important perspectives for understanding their formation.
Reference: “Sulfur species and gold transport in arc magmatic fluids” by Stefan Farsang, and Zoltán Zajacz, 16 December 2024, Nature Geoscience.
DOI: 10.1038/s41561-024-01601-3
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