Gold, coveted for its rarity and value, has long intrigued scientists due to a fundamental mystery: how does it migrate from the depths of the Earth's mantle to accessible surface deposits?
In a significant breakthrough, an international team of researchers led by the University of Michigan has developed an innovative thermodynamic model that sheds light on this process. Published in the Proceedings of the National Academy of Sciences, the findings redefine our understanding of the geological processes related to this precious metal.
The study pinpoints sulfur as the key player in transporting gold from the mantle to magma. At depths of 50 to 80 kilometers, extreme pressure and temperature conditions allow sulfur to interact with gold, forming a highly mobile chemical compound known as gold-trisulfide. This complex can traverse magma and ascend toward the Earth's crust.
Professor Adam Simon, a co-author of the study, explained, "Pure gold is chemically inert in the mantle and does not tend to mobilize. However, when it interacts with sulfur-rich fluids, it combines to form a complex that can travel through magma, facilitating its transfer to more accessible zones." This discovery not only deepens our understanding of gold chemistry but also elucidates why certain geological environments are more conducive to significant gold deposits.
The researchers' model emphasizes subduction zones, areas where one tectonic plate sinks beneath another. These volcanically active regions generate sulfur-rich fluids that interact with magma, enriching surface layers with gold. Simon noted, "The processes that fuel volcanic eruptions are the same that generate gold deposits." Regions along the Pacific Ring of Fire, including Indonesia, Japan, Alaska, and the Andes, are particularly relevant.
The described mechanism also highlights how partial melting of the subducted plate releases sulfur-rich fluids, creating conditions favorable for the formation of gold-trisulfide and its subsequent migration to the surface, where it solidifies into exploitable deposits.
To validate their hypothesis, the team conducted laboratory experiments simulating the extreme conditions of the Earth's mantle, meticulously controlling pressure and temperature to replicate chemical interactions in magma. Based on these experiments, they developed a robust thermodynamic model explaining how specific mantle conditions facilitate the formation and movement of the gold-trisulfide complex.
This approach not only enhances the understanding of mantle chemistry but also has practical applications in designing mining exploration strategies. Simon added, "This combination of experimentation and modeling provides us with a powerful tool to identify regions with high potential for gold deposits." The scientific article, titled Mantle oxidation by sulfur drives the formation of giant gold deposits in subduction zones, includes contributions from experts in China, Switzerland, Australia, and France. Beyond resolving a long-standing scientific debate, the results open new avenues for mining, enabling a more efficient and sustainable approach to identifying and exploiting gold resources.
This study establishes a clear link between tectonic and volcanic processes and the formation of mineral deposits, solidifying its status as a reference in the field of economic geology. With this newfound understanding, mining exploration may benefit from more precise tools, focusing on subduction regions where conditions for gold formation are most favorable.