Gold nanoparticles melt at lower temperatures than macroscopic gold

Gold, a mesmerizing precious metal, has captivated humans throughout history. It has been a symbol of opulence and prosperity in various civilizations, from the legendary wealth of Priam’s Treasure to the mythical city of El Dorado. Traditionally, it was believed that gold deposits formed when hot aqueous solution flows, known as hydrothermal fluids, transported dissolved metal until it accumulated in specific areas of the Earth’s upper crust. However, recent discoveries of gold nanoparticles within these mineral deposits have raised questions about the validity of the established model.

In a groundbreaking publication in Scientific Reports, the scientific community reengages in the debate surrounding the traditional models for gold transport in nature. This new study presents a revelation: gold nanoparticles, when exposed to hydrothermal fluids, exhibit the remarkable ability to melt and create gold nanomelts at temperatures lower than the melting point of macroscopic gold. These nanomelts, formed at temperatures below 500°C, could potentially remobilize gold in aqueous fluids more efficiently, facilitating the formation of economically significant accumulations.

Leading the research are experts from the Faculty of Earth Sciences and the Institute of Nanoscience and Nanotechnology (IN2UB) at the University of Barcelona, in collaboration with the Andalusian Institute of Earth Sciences (IACT-CSIC) and the Department of Mineralogy and Petrology at the University of Granada. The study also benefits from the involvement of the Scientific and Technological Centers of the UB (CCiTUB) and the Center for Scientific Instrumentation of the University of Granada (CIC). With their combined expertise, these institutions shed light on a new aspect of gold’s behavior in hydrothermal systems, challenging the established understanding of its transportation and deposition.

Gold, the most precious noble metal

In the 8th century, Jabir ibn Hayyan, a renowned alchemist of the Islamic world, documented the purification of gold and the extraction of pure mercury from cinnabar. Centuries later, in the 16th century, Georgii Agricolae’s work “De ortu & causis subterraneorum” shed light on the transportation of gold in the Earth’s upper crust. Agricolae’s research established that gold was dissolved in hydrothermal fluids, which are hot aqueous solutions ranging from 50ºC to 500ºC. The characteristics of these fluids depend on geological factors and their depth, typically several kilometers beneath the Earth’s surface.

Most of the world’s gold deposits have been formed through this mode of transportation. However, gold can also accumulate when primary gold deposits are exposed to the surface through tectonic processes and subsequently eroded. This erosion gives rise to the famous gold nuggets that prospectors discovered during the height of the gold rush along riverbanks.

Gold nanoparticles were first synthesized by John Turkevich in the 1950s, but it wasn’t until the early 1990s that they were observed in natural gold deposits. Specifically, these nanoparticles were found in bonanza-type deposits with high concentrations of gold in Nevada, United States. This discovery supported the hypothesis that gold could be transported as suspended nanoparticles in fluid, rather than being dissolved.

Professor Joaquín A. Proenza, from the UB Department of Mineralogy, Petrology, and Applied Geology, explains that hydrothermal gold deposits vary widely depending on various factors. The most significant deposit types globally include orogenic, Carlin, and epithermal gold deposits. However, studies characterizing the mineralizing fluids have revealed their limited ability to dissolve gold. Regardless of their nature, these fluids are insufficient to transport the large amounts of gold found in auriferous mineralizations, particularly those of the bonanza type.

Hence, Professor Proenza, a member of the Research Group on Mineral Resources for Energy Transition (MinResET), concludes that hydrothermal fluids alone cannot account for the formation of gold deposits. There must be alternative mechanisms at play to explain the existence of these highly gold-rich deposits.

When gold nanoparticles melt down

In a significant breakthrough, this study unveils the melting process of gold nanoparticles, shedding light on their formation and behavior. The research was conducted on gold-rich samples sourced from the Cu-Co-Ni-Au deposits in Cuba’s Habana-Matanzas region, where a substantial abundance of gold nanoparticles was observed. Diego Domínguez-Carretero from the University of Barcelona (UB), the first author of the article and a doctoral candidate under the guidance of Joaquín A. Proenza and Antonio García Casco from the University of Granada, explains the findings.

The study not only uncovers the entire formation process of gold nanoparticles but also reveals their release from the minerals in which they were embedded, their exposure to hydrothermal fluids leading to melting, and ultimately their remobilization through auriferous nanofusions that are immiscible with the hydrothermal fluid. This comprehensive understanding of the gold nanoparticle formation process represents a significant advancement in the field.

To achieve these results, the research team employed a combination of classical techniques such as optical microscopy and field emission scanning electron microscopy, along with innovative methodologies including hydro-separation, focused ion beam, and high-resolution transmission electron microscopy. These state-of-the-art analytical techniques were made available through collaborations with the CCiTUB, the Advanced Microscopy Laboratory at the University of Zaragoza, and the CIC.

Previously, the formation of gold nanomelts required the presence of additional elements such as bismuth (Bi), tellurium (Te), or antimony (Sb), which are not consistently found together with gold in many mineral deposits. However, this study marks the first time the sequential transformation of gold nanoparticles into gold nanocrystals has been observed without the need for these additional elements like Bi, Te, or Sb.

The authors emphasize that this paradigm shift in understanding the origin of gold offers a more realistic framework for establishing genetic models, which are vital for mineral deposit geologists aiming to comprehend the complex geological processes and factors influencing the formation of mineral deposits. These insights are instrumental in informing exploration campaigns for gold mining and other forms of mining, as genetic models serve as a foundation for identifying and locating new deposits.

The research team concludes that their work, which investigates the influence of metallic nanoparticles on the formation of critical mineral deposits, represents an innovative line of research at the national and European levels. They have collaborated with José María González Jiménez from the Andalusian Institute of Earth Sciences to pursue this cutting-edge research endeavor.

Source: University of Barcelona

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