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: (I) weathering profile, (II) magnesiocarbonatite, (III) phosphate-siderite-rich magnesio- and ferro-carbonatite (MF), (IV) phosphate-rich MF carbonatite, (V) phosphate-poor MF carbonatite, and (VI) calcio-carbonatite. The integration of visible near-infrared (VNIR) and thermal infrared (TIR) reflectance spectra reveal that the primary Nd-related absorption features occur ∼745 nm when hosted by phosphates (Zones II-IV), and shift to ∼740 nm when hosted by carbonates. Reassessment of the whole-rock geochemistry, based on the newly defined zones, reveals the gradual variation of λ 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 LREE/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, localised HREE enrichment occurred during late-stage interactions between MF carbonatite and pre-existing calcio-carbonatite.
Laukamp et al. (Wed,) studied this question.