Mount Weld Rare Earth Element Deposit, Western Australia

In today's high-tech world, rare earth elements (REE) are indispensable. These seventeen unique metallic elements, including the lanthanides, yttrium, and scandium, possess diverse chemical, magnetic, and luminescent properties vital for industries ranging from electronics to clean energy technologies. As global demand for REEs soars, securing a diverse and sustainable supply chain outside traditional sources becomes critical. This is where Australia's Mount Weld Rare Earth Element deposit, steps onto the global stage.

The Discovery and Geological Story of Mount Weld

Located approximately 250 kilometers northeast of Kalgoorlie in Western Australia, the Mount Weld carbonatite complex was first discovered in 1966 through an airborne magnetics survey revealing a pronounced circular magnetic anomaly beneath surficial cover. Drilling confirmed the presence of a Paleoproterozoic carbonatite intrusion, dating back approximately 2.06 billion years, which intruded into much older Archean volcano-sedimentary sequences of the Yilgarn Craton. This carbonatite is part of a broader regional alkaline province, hinting at its deep, mantle-derived origins.

Location of the Mount Weld Rare Earth Element deposit.

Recent deep drilling has peeled back layers of geological history, revealing the fascinating internal architecture of the primary carbonatite itself. It consists of:

• A central magnesio- to ferrocarbonatite zone, approximately 600 meters in diameter, which is naturally enriched in light rare earth elements (LREE) but low in niobium. Primary REE mineralization here occurs as fine-grained monazite in fractures and voids, and as late magmatic synchysite/bastnäsite.
• A surrounding thick annulus (1-1.5 km wide) of calciocarbonatite, which is rich in niobium but contains lower concentrations of REEs.

This internal zonation, coupled with the presence of a wide fenite or glimmerite alteration halo around the outer margin, showcases a geological blueprint shared with other major carbonatite complexes globally

Mount Weld rare earth element deposit solid geology map showing the location of the carbonatite intrusion.

The Weathering Advantage: Supergene Enrichment at Mount Weld

What truly sets Mount Weld apart as a world-class REE deposit is its exceptionally thick weathering profile, known as laterite, which blankets the underlying carbonatite to depths exceeding 120 meters. This laterite developed during the Mesozoic to Cainozoic eras, following a Permian glaciation that scoured older weathering products.

The formation of this REE-rich laterite is a testament to intensive supergene enrichment processes. Key factors favoring this enrichment include:

• High fluid-to-rock ratios and long fluid residence times, which allowed REEs to be liberated from decomposing primary igneous minerals.
• Abundant complexing agents (such as fluoride, chloride, and carbonate anions) in groundwater solutions, which helped mobilize the REEs.
• Crucially, a wide range of pH and alkalinity conditions in the groundwater solutions influenced the stability of different REE complexes, leading to pronounced fractionation, separation, and deposition of LREE and HREE.
• LREEs (such as lanthanum and cerium) were typically deposited as plumbogummite-group minerals, rhabdophane, and secondary monazite in the upper and middle parts of the weathering profile.
• HREEs and yttrium remained stable in solutions under higher pH and alkalinity, moving deeper into the profile where they were preferentially incorporated into churchite and xenotime.

• The unique karst topography of the carbonatite, with a central topographic low, channeled groundwater flow, and associated decreasing pH conditions favored the mobilization and concentration of large amounts of REE, forming a highly REE-enriched laterite center.

This intricate weathering process resulted in an approximately 5-fold increase in REE concentrations from the primary carbonatite to the overlying paleoregolith. Remarkably, large-scale horizontal migration of REE and niobium during weathering was minimal, meaning the chemistry of the regolith largely mirrors that of the underlying fresh carbonatite.

Global Significance and Mining Outlook

Mount Weld is recognized as one of the largest and highest-grade REE deposits globally. It has been actively mined since 2011, with rare earth oxides concentrated on-site and then shipped to Malaysia for further refining. The deposit provides a critical supply of LREEs from its "Central Lanthanide Deposit" and also contains valuable HREEs in the "Duncan Deposit" area. Beyond REEs, Mount Weld also boasts significant resources of niobium, tantalum, zirconium, titanium, and phosphate.

The deposit presents unique characteristics compared to other major REE mines worldwide. Unlike some, where bastnäsite is the primary ore mineral, Mount Weld's exploited ore (in the regolith) is dominated by monazite. Furthermore, its primary carbonatite includes a large volume of unmineralized calciocarbonatite and exhibits distinct primitive isotopic signatures, differentiating it from other major deposits like Mountain Pass.

However, mining Mount Weld's riches is not without its challenges. The REE-bearing minerals are often extremely fine-grained and intimately intergrown at sub-micron scales, making extraction complex. A significant insight from recent research is that REEs are not only found in distinct REE minerals but also in common gangue minerals (like fluorapatite, iron and manganese oxyhydroxides, quartz, and ilmenite). These gangue minerals can host REEs both as nano- to micron-scale inclusions and within their crystal structures, at concentrations ranging from tens to thousands of parts per million. Considering that gangue can comprise up to 90% of the ore volume, this widespread distribution of REEs presents both a challenge and an opportunity for optimizing processing and recovery.

Mount Weld continues to be a strategic asset for Australia and the world, providing essential elements for modern technologies. Its complex geology and mineralogy also make it a living laboratory for understanding the intricate processes of supergene enrichment in carbonatites, guiding future exploration and innovation in the global REE industry.

Total magnetic map and cross sections over the Mount Weld Rare Earth Element Deposit, Western Australia.

Sources:

• ⁠Australian Mines Atlas⁠
• ⁠Mount Weld Deposit Summary Report⁠
• ⁠PorterGeo Database - Mount Weld⁠
• ⁠The primary geology of the Paleoproterozoic Mt Weld Carbonatite Complex, Western Australia⁠
• ⁠Mineralogy and Distribution of REE in Oxidised Ores of the Mount Weld Laterite Deposit, Western Australia
• Rare-earth element mineralisation within the Mt. Weld carbonatite laterite, Western Australia
• Complex REE systematics of carbonatites and weathering products from uniquely rich Mount Weld REE deposit, Western Australia
• Compositional variation in pyrochlores from the Mt Weld carbonatite laterite, Western Australia