Hyperspectral EDS-WDS-CL Characterization of Metasomatized Zr-Nb-REE-bearing Phonolites from Victoria, Australia

The Gallows Hill Phonolite is a Triassic, fine-grained subvolcanic sill outcropping within the Upper Devonian Mansfield Basin, in Victoria (Australia), containing elevated concentrations of Zr-Nb-RE elements (∼0.26-0.28 wt%), and was recently assessed for critical mineral potential by the Geological Survey of Victoria (Andrews et al., 2025). This study found the bulk rock chemistry is homogeneous, however fine-grained mineralogy which is very complex was observed warranting further study. Here, we performed hyperspectral X-ray and cathodoluminescence (CL) mapping in an electron probe microanalyzer (EPMA), coupled with an automated cluster analysis combined with quantitative analysis of clustered X-ray spectra of discrete phases. This approach identified and quantified over 35 phases from Gallows Hill phonolite, including several unexpected rare phases. A standards-based energy dispersive X-ray spectrometry (EDS) quantification method measured compositions in agreement with expected mineral stoichiometry, with elemental detection limits in the range of <∼20-1,000 ppm, depending on phase abundance, and proved reliable even for challenging mineral species, such as Zr-Nb-Ti and multi-rare earth element (REE)-bearing minerals. The mineral identification and clustering procedure revealed compositional variations across different minerals, as well as distinct CL signals, indicating that the rock samples recorded overprinting of the original mineralogy. Our results show that the rocks' textures indicate late-stage magmatic crystallization of Zr-Nb-Ti silicate phases together with REE-mineralogy like worldwide reported alkali igneous Zr-Nb-REE deposits (analogous Toongi-Dubbo Zr-Nb-Hf-REE deposit in New South Wales, Australia). Secondary overprints altered the rocks, leading to the dissolution of feldspathoids, as well as remobilization and locally redepositioning of various Zr-Nb-REE phases.