Join us as we delve into the fascinating geological story of the Mount Weld carbonatite complex in Western Australia, home to one of the world's richest Rare Earth Element (REE) deposits. We'll explore its discovery, unique geological setting, the intricate processes of weathering that led to its extraordinary enrichment, and the diverse mineralogy that makes it a critical source for modern technologies.Discovery and Significance:The Mount Weld carbonatite was first discovered in 1967 following a regional aeromagnetic survey in 1966, which revealed a pronounced magnetic anomaly associated with a large circular structure hidden beneath alluvial sediments.Initially targeted for niobium, uranium, and phosphate, interest in Rare Earth Elements began in 1988.Mount Weld is now a world-class REE deposit, ranking among the highest-grade globally, with current proven ore reserves averaging 8.3% Total Rare Earth Oxides (TREO). It's one of the most strategically important REE mines outside China.Mining commenced in 2011, with concentrates shipped to Malaysia for refining into high-quality rare earth minerals.A Journey Through Time: Geological Setting & Formation:Located 250 km northeast of Kalgoorlie, Mount Weld is a Paleoproterozoic carbonatite intrusion within the Archean Yilgarn Craton's Eastern Goldfields Province.The carbonatite complex is generally described as a steeply plunging maar-type diatreme or cylindrical body, 3-4 km in diameter, intruding into volcano-sedimentary greenstones.Its age is well-constrained around 2.06 Ga (specifically 2056 ± 67 Ma based on monazite Th-Pb dating). This age coincides with a regional tectono-magmatic event that produced alkaline ultramafic-mafic igneous rocks and kimberlites in the area.The complex exhibits a distinct geological architecture: a central magnesio- to ferrocarbonatite core (~600 m diameter) rich in REE, surrounded by a broad (~1-1.5 km) calciocarbonatite annulus with higher niobium concentrations. This structure is strikingly similar to other major global carbonatite complexes like Ngualla and Mirima Hill.Primary REE mineralization within the fresh carbonatite is primarily found in the central magnesio- to ferrocarbonatite, with monazite and late magmatic REE fluorocarbonates (synchysite/bastnäsite). Experimental studies show REE solubilities in carbonatite melts can be high (up to >10 wt%), suggesting magmatic processes played a significant role in the initial REE enrichment.The Power of Weathering: Supergene Enrichment:The current economic REE resources are almost exclusively hosted within a thick lateritic regolith (weathering profile) overlying the carbonatite, ranging from 10 to over 120 meters in thickness.This weathering profile developed post-Permian glaciation and prior to Eocene lacustrine sediments, suggesting a formation period during the Late Mesozoic to Early Cenozoic.The extreme REE enrichment (up to 50% TREO in grab samples) is largely attributed to long-term leaching and redeposition by groundwater movement.The highest REE concentrations occur in a central topographic low of the laterite, indicating significant lateral REE mobility towards this "solution sink hole".Studies indicate a ~5x upgrade in REE (and Nb) concentrations from the primary carbonatite to the overlying paleoregolith, with minimal horizontal migration of ore elements on a complex scale during weathering. This means the regolith broadly reflects the underlying carbonatite's trace element signatureSources:Australian Mines AtlasMount Weld Deposit Summary ReportPorterGeo Database - Mount WeldThe primary geology of the Paleoproterozoic Mt Weld Carbonatite Complex, Western AustraliaMineralogy and Distribution of REE in Oxidised Ores of the Mount Weld Laterite Deposit, Western AustraliaRare-earth element mineralisation within the Mt. Weld carbonatite laterite, Western AustraliaDisclaimer:AI generated content created using Google's NotebookLM.

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