Hyperspectral and whole-rock geochemical analysis of the Mt Weld REE deposit: new insights into geological evolution and REE enrichment

Recent research on carbonatite-hosted rare earth element (REE) deposits has highlighted the diversity of REE-bearing minerals and their origins. This study investigates the mineralogy and geochemistry of a 1 km-long drill core intersecting unweathered carbonatite of the Mt Weld REE deposit in Western Australia, using hyperspectral reflectance spectroscopy and whole-rock geochemistry. Six distinct zones are identified within the central carbonatite: (1) weathering profile; (2) magnesiocarbonatite; (3) phosphate–siderite-rich magnesio- and ferrocarbonatite (MF); (4) phosphate-rich MF carbonatite; (5) phosphate-poor MF carbonatite; and (6) calciocarbonatite. The integration of visible near-infrared (VNIR) and thermal infrared (TIR) reflectance spectra reveal that the primary Nd-related absorption features occur at c . 745 nm when hosted by phosphates (Zones II–IV), and shift to c . 740 nm when hosted by carbonates. Reassessment of the whole-rock geochemistry, based on the newly defined zones, reveals the gradual variation of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>λ</mml:mi> </mml:math> values (a quantitative descriptor of REE patterns), reflecting the progressive magmatic fractionation from Mg-rich to Fe-rich carbonatite magma, with the decreasing of general light REE (LREE)/heavy REE (HREE) fractionation (La/Yb). The total concentration of REE is initially enriched in Mg-rich carbonatite but subsequently decreases as phosphorus concentrates within Fe-rich carbonatite. REE precipitate as monazite during the hydrothermal evolution of each magmatic stage, transitioning to REE-fluorocarbonates in the late-stage magma when REE and phosphorus become depleted. The MF carbonatite is interpreted to undergo multiple magmatic stages, each influenced by brine-melt and hydrothermal processes that control the distribution of LREE. In contrast, localized HREE enrichment occurred during late-stage interactions between MF carbonatite and pre-existing calciocarbonatite.