Mineralogical deportment and Trace Element Geochemistry of REEs in the Blötberget Apatite-Iron-Oxide Deposit, Central Sweden

Abstract

This study employed a multi-analytical approach, combining Nordic Iron rare earth element (REE) data on a meter scale with Minalyze (Veracio) drill core scanner data on a millimetre scale, to identify the most enriched REE zones in the Blötberget iron oxide-apatite (IOA) deposit. The goal was to develop and apply a new method for identifying REE-enriched samples and extracting more information from existing, poorly constrained data, while reducing sampling time and improving cost efficiency. Three drill cores were sampled: one from the hanging wall, one from the footwall, and one from the iron ore, to investigate REE-hosting phases, textures, and compositional variations across the mineralised zones using X-ray fluorescence, SEM, and LA-ICP-MS. The results show significantly higher concentrations of heavy rare earth elements (HREEs) in apatite but lower light rare earth element (LREE) contents compared to those commonly reported in other Kiruna-type deposits. Clear compositional and textural differences exist between the structural domains: the footwall is dominated by allanite, a refractory REE host, while the hanging wall contains free-milling monazite and xenotime occurring as large, isolated grains. Apatite in the hanging wall is enriched in both LREE and HREE, including the highest HREE values measured, whereas footwall apatite is comparatively LREE-depleted. Eu anomalies support derivation from separate magma batches, with the footwall representing a more evolved intrusive protolith and the hanging wall a less evolved, subvolcanic equivalent. Textural evidence indicates that monazite postdates apatite, with apatite locally breaking down to form secondary REE phases during a late magmatic to hydrothermal stage. This remobilisation locally enriched the host rocks without obliterating the magmatic signature. By analogy with Kiruna, where secondary monazite and xenotime formed during multiple hydrothermal pulses between ~1.9 and 1.5 Ga, Blötberget may also preserve evidence of a prolonged or episodic hydrothermal history. From an economic perspective, the mineralogical contrast has direct implications: the hanging wall, with free-milling monazite and xenotime, is more favourable for REE extraction, whereas the allanite-rich footwall poses challenges for processing. Apatite across all zones is relatively low in deleterious elements compared to other IOAs, enhancing its potential as a phosphorus byproduct source. A comprehensive and unbiased core scan of Blötberget is recommended to evaluate whether the observed REE enrichments and mineralogical variations are localised features or representative of broader zones, and geochronological dating of monazite and xenotime will be critical to constrain the timing of REE enrichment relative to iron ore formation.