Thorium : A Natural Byproduct of Rare Earth Extraction
Thorium is produced as a byproduct of monazite during rare earth extraction — two strategic resources derived from the same mineral, in the same deposits, at the same time [1]. This co-occurrence is explained by deep magmatic processes that concentrate these elements in the same types of rocks and minerals — opening up new perspectives for the mining industry and the energy transition.
These two families of critical resources not only share the same deposits, but also a central role in the technologies that will power the global energy transition. This article explores the geochemical mechanisms behind this co-occurrence, and what it concretely means for mineral exploration.
Thorium: An Actinide with Singular Nuclear Potential
Thorium (Th) is a naturally radioactive chemical element belonging to the actinide family. Present in the Earth's crust at concentrations higher than those of uranium, it is primarily found in minerals such as monazite. Unlike uranium-235, which is fissile, thorium-232 is not directly fissile: it is fertile, meaning it must first absorb a neutron to transform into uranium-233 (U-233), a fissile isotope capable of sustaining a nuclear chain reaction. This characteristic fuels interest in thorium fuel cycles in certain advanced reactor designs, where it is envisioned as a potential alternative to conventional nuclear fuels.
Properties and Nuclear Potential
Research on thorium reactors is currently ongoing at the experimental and pilot stages. Major programs in Asia, particularly in China, highlight the advantages of this approach in terms of nuclear non-proliferation, CO2 emission reduction, and decreased radiotoxicity of waste [1]. Furthermore, thorium reactors produce less long-lived nuclear waste than conventional fuels.
Although this technology remains at the stage of technological and regulatory development, it is attracting growing interest internationally. If public and private investments continue, thorium could, over the longer term, constitute an energetically and economically viable option [1]. It is precisely in this context of long-term anticipation that the question of its extraction — and its co-occurrence with rare earths — takes on its full significance.
Rare Earths: Critical Elements with Explosive Demand
Rare Earths Elements (REE) are at the heart of the global energy transition. Understanding which ones are found alongside thorium, and why their demand is surging, helps better grasp the strategic interest of their geological co-occurrence.
Rare Earths: Strongly Growing Demand
The pressure on global rare earth supply is considerable. Demand for magnetic rare earths is expected to rise from 59 kilotonnes (kt) in 2022 to 176 kt in 2035, driven by the combined growth of electric vehicles and wind power [3]. These magnetic rare earths represent approximately 30% of total REE volume, but capture more than 80% of their market value [3].
Beyond magnets, overall demand for rare earths across all categories is expected to increase by 400 to 600% in the coming decades, and neodymium in particular is projected to outpace available supply by 250% as early as 2030 [4]. These figures illustrate the scale of the supply challenge. This challenge is all the more acute given that currently, between 90 and 98% of global REE production is controlled by China (78%) and a few other countries [1], exposing many Western economies to significant strategic dependency.
Which Rare Earths Are Associated with Thorium?
Thorium is frequently associated with rare earths because they concentrate together in certain minerals and geological environments -- particularly monazite, a phosphate that incorporates both rare earths and thorium [8]. In practice, monazite typically contains 55 to 60% rare earth oxides and 5 to 10% thorium oxide (ThO2) [6], which explains why thorium anomalies are often used as an exploration indicator for potential rare earth mineralizations. In other types of deposits, notably certain carbonatitic systems, rare earths can also be hosted by fluorocarbonates such as bastnäsite; and since these environments concentrate "incompatible" elements, they can also be associated with notable thorium grades depending on the geological context [5].
The Geological Link: Why Do Thorium and Rare Earths Coexist?
The simultaneous presence of thorium and rare earths in the same deposits results from deep geochemical processes that concentrate these elements in very specific magmatic environments. This is one of the fundamental principles of critical mineral geology.
Carbonatites and Alkaline Complexes: Preferred Environments
The most significant rare earth deposits globally are often associated with two types of geological formations: carbonatites (igneous rocks composed primarily of carbonate minerals) and alkaline intrusions (magmatic rocks rich in sodium and potassium). Both environments share a key characteristic: they naturally concentrate incompatible elements.
Specifically, the magmatic differentiation processes that concentrate rare earths also generate minerals containing other incompatible elements, including thorium and uranium. This is why abnormally high thorium grades are frequently observed in REE-rich carbonatites and alkaline intrusions. Deposits associated with these two types of formations occur in the same environments due to their spatial association, their shared enrichment in incompatible elements, and similarities in their genesis [5].
From a structural perspective, these formations tend to be located in stable continental tectonic units — shields, cratons, crystalline blocks — generally associated with intracontinental rift and fault systems [5]. This geological reality explains why certain regions of the world, including the Canadian Shield, present particularly interesting exploration potential for both types of resources.
Monazite: The Key Mineral of Co-occurrence
At the heart of this geochemical relationship lies a discrete yet critically important phosphate mineral: monazite. Its typical composition makes it a natural vehicle for the thorium-rare earth co-occurrence, with 55 to 60% rare earth oxides, 24 to 29% phosphate (P2O5), 0.2 to 0.4% uranium oxide (U3O8), and 5 to 10% thorium oxide (ThO2) [6]. In certain commercial deposits, the thorium content can reach 12% ThO2, and in exceptional cases, up to 20-30% [7].
This composition makes monazite a doubly valuable mineral from an exploration standpoint. Indeed, elevated thorium concentrations in a study area serve as an indicator of potential rare earth mineralization, since these two groups of elements are geochemically similar and are systematically found in the same types of deposits [8]. For geologists, thorium anomalies detected during geophysical or geochemical surveys thus constitute a relevant exploration signal.
Thorium, a Byproduct of Rare Earth Extraction
This geological relationship has a direct consequence for the industrial sector: thorium is currently produced as a byproduct of monazite, which is itself extracted to recover rare earths destined for alternative energy sources such as wind and solar power [1]. In other words, monazite is mined for its rare earths — not for its thorium [9].
This means that surplus quantities of thorium recovered during REE extraction, which do not find an immediate commercial outlet, must be stored in specially designed facilities [1]. As a result, as global demand for rare earths increases, additional quantities of thorium will inevitably be produced in parallel — a reality that reinforces the importance of developing commercial or energy-related uses for this actinide.
Opportunities and Advantages of This Co-occurrence
The geological co-occurrence of thorium and rare earths opens concrete prospects for the mining industry and for diversifying critical mineral supply chains.
From an economic standpoint, the joint production of thorium and rare earths within a single mining project allows extraction infrastructure to be shared, thereby reducing unit costs and improving overall profitability [1]. Moreover, the fact that these elements are rarely found in easily exploitable concentrations reinforces the economic appeal of deposits where both naturally coexist [10]: a single site can generate two distinct sources of value.
Contribution to the Energy Transition
The relationship between thorium and rare earths is, ultimately, written into the very chemistry of the Earth. This geological link illustrates well that the energy transition does not rely on isolated resources, but on deeply interconnected mineral value chains. Understanding these associations makes it possible to better guide exploration and plan more responsible deposit development. The challenge for economies like Canada is precisely to transform this natural co-occurrence into a strategic advantage: contributing both to today's green technologies through rare earths, and to tomorrow's energy solutions through the thorium nuclear pathway.
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References
[1] Society for Mining, Metallurgy & Exploration. "Thorium as a Byproduct of Rare Earth Element Production." SME Technical Briefings, 2024. SME, https://www.smenet.org/What-We-Do/Technical-Briefings/Thorium-as-a-Byproduct-of-Rare-Earth-Element-Produ.
[2] OECD NEA "Perspectives on the Use of Thorium in the Nuclear Fuel Cycle", 2015. Nuclear Energy Agency, https://www.oecd-nea.org/upload/docs/application/pdf/2019-12/7228-thorium-es.pdf.
[3] McKinsey & Company. "Powering the Energy Transition's Motor: Circular Rare Earth Elements." McKinsey Insights, 2025. McKinsey & Company, https://www.mckinsey.com/industries/metals-and-mining/our-insights/powering-the-energy-transitions-motor-circular-rare-earth-elements.
[4] Mining International. "Rare Earth Elements' Role in the Energy Transition." Mining International, 2024. https://www.mining-international.org/rare-earth-elements-role-in-the-energy-transition/.
[5] Verplanck, Philip L., et al. "A Deposit Model for Carbonatite and Peralkaline Intrusion-Related Rare Earth Element Deposits." USGS Open-File Report 2011-1256, 2011. U.S. Geological Survey, https://pubs.usgs.gov/of/2011/1256/report/OF11-1256.pdf.
[6] Kim, Jimin, et al. "Radiological Assessment of Monazite." Scientific Reports, vol. 13, 2023, article 15572. Nature, https://www.nature.com/articles/s41598-023-42287-8.
[7] "Monazite." Wikipedia, Wikimedia Foundation. https://en.wikipedia.org/wiki/Monazite.
[8] Belmont Resources. "Cracking the Stone: Rare Earths and Thorium." Belmont Resources, 2023. https://belmontresources.com/crackingstone-rare-earths/.
[9] U.S. Geological Survey. "Thorium." Mineral Commodity Summaries 2025, 2025. USGS, https://pubs.usgs.gov/periodicals/mcs2025/mcs2025-thorium.pdf.
[10] Columbia Climate School. "The Energy Transition Will Need More Rare Earth Elements. Can We Secure Them Sustainably?" Columbia Climate School News, Apr. 5, 2023. https://news.climate.columbia.edu/2023/04/05/the-energy-transition-will-need-more-rare-earth-elements-can-we-secure-them-sustainably/.
