Approximately 400 kilometers northwest of Sydney, just south of Dubbo, one can find the Toongi deposit—a significant geological formation that dates back around 215 million years, resulting from ancient volcanic eruptions. This site is notably abundant in rare earth elements, comprising a group of 16 metallic elements that are crucial for contemporary technologies, ranging from electric vehicles to solar panels and mobile devices.

As efforts escalate to extract resources from this deposit, projections indicate that the demand for rare earths will reach unprecedented levels in the coming decades, presenting significant economic opportunities and challenges.

To enhance our understanding of these deposits, it is essential to investigate their formation processes. Our recent research concerning Australian volcanism, published in Nature Communications Earth and Environment, reveals how minute crystals formed within these volcanoes provide valuable insights into the genesis of rare earth deposits and guide our search for additional sources.

The Genesis of Rare Earths and Mantle Melting

The formation of rare earth element deposits originates from the partial melting of Earth’s mantle, a layer rich in iron and magnesium-bearing minerals that also contain trace amounts of rare earth elements. During the melting process, these elements readily transfer into the magma. Notably, the extent of melting influences the concentration of rare earth elements within the magma, a significant factor in their eventual accumulation.

As the magma ascends towards the Earth’s surface, it cools, and various minerals predominantly composed of oxygen, silicon, calcium, aluminum, magnesium, and iron begin to crystallize, resulting in a concentration of rare earth elements in the residual magma.

Geological Significance: From Greenland to New South Wales

In instances where magma solidifies within the crust, it can yield rocks enriched with critical metals, as exemplified by the Gardar Igneous Complex in Southern Greenland, which hosts multiple rare earth deposits. Conversely, in central New South Wales, magmas associated with rare earth elements erupted to the surface, collectively classified as the Benolong Volcanic Suite.

Central to this suite is the Toongi deposit, a remnant of an ancient volcanic plumbing system, comprising a “congealed” intrusion rich in critical metals. Given the rarity of magmas enriched in rare earth elements, very few sites globally are deemed viable for mining, emphasizing the importance of further research into the processes surrounding their formation.

Crystallography and Volcanic Insights

The mechanisms that transpire within a magma as it ascends from deep within the Earth reveal essential clues about volcanic activity, particularly through the analysis of crystalline structures. The mineral clinopyroxene serves as a remarkable record keeper of these processes, effectively documenting the conditions under which it crystallized.

In our research, we identified clinopyroxene crystals across various rocks in the Benolong Volcanic Suite, contrasting the composition of the mineralized Toongi intrusion with that of other rocks lacking rare earth enrichment. Our findings indicate two significant differences in composition, pointing to critical insights about the crystallization processes occurring within the Toongi deposit.

Our analysis revealed that clinopyroxene crystals from Toongi exhibit distinct characteristics, such as an hourglass-like internal structure indicative of rapid crystallization likely triggered by gas release. Conversely, non-mineralized samples showed no such signs of rapid formation.

This research enhances our understanding of how rare earth deposits form and guides us in identifying other volcanic systems that may harbor these vital resources, contributing to the global search for critical metals necessary for sustainable energy technologies.

Brenainn Simpson, PhD Candidate, The University of Queensland; Carl Spandler, Associate Professor, University of Adelaide, and Teresa Ubide, ARC Future Fellow and Associate Professor in Igneous Petrology/Volcanology, The University of Queensland

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