Geochemical and spectroscopic investigation of apatite in the Siilinjärvi carbonatite complex: Keys to understanding apatite forming processes and assessing potential for rare earth elements

• Main Authors: Decrée, Sophie, Savolainen, Mikko, Mercadier, Julien, Debaille, Vinciane, Höhn, Stefan, Frimmel, Hartwig H.E.E., Baele, Jean-Marc
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: Technologie de l'environnement; contrôle de la pollution; Géochimie; Pétrologie; Environnement et pollution; Archean; Carbonatite-related ore deposits; Cathodoluminescence; Fennoscandian shield; LA-ICPMS; LIBS; Raman spectroscopy; Rare earth elements; Fennoscandian
• Online Access: http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/313656; https://dipot.ulb.ac.be/dspace/bitstream/2013/313656/3/Decreeetal2020.pdf

The Siilinjärvi phosphate deposit (Finland) is hosted by an Archean carbonatite complex. The main body is composed of glimmerite, carbonatite and combinations thereof. It is surrounded by a well-developed fenitization zone. Almost all the rocks pertaining to the glimmerite-carbonatite series are considered for exploitation of phosphate. New petrological and in-situ geochemical as well as spectroscopic data obtained by cathodoluminescence, Raman and laser-induced breakdown spectroscopy make it possible to constrain the genesis and evolution of apatite through time. Apatite in the glimmerite-carbonatite series formed by igneous processes. An increase in rare earth elements (REE) content during apatite deposition can be explained by re-equilibration of early apatite (via sub-solidus diffusion at the magmatic stage) with a fresh carbonatitic magma enriched in these elements. This late carbonatite emplacement has been known as a major contributor to the overall P and REE endowment of the system and is likely connected to fenitization and alkali-rich fluids. These fluids - enriched in REE - would have interacted with apatite in the fenite, resulting in an increase in REE content through coupled dissolution–reprecipitation processes. Finally, a marked decrease in LREE is observed in apatite hosted by fenite. It highlights the alteration of apatite by a REE-poor fluid during a late-magmatic/hydrothermal stage. Regarding the potential for REE exploitation, geochemical data combined with an estimation of the reserves indicate a sub-economic potential of REE to be exploited as by-products of phosphate mining. Spectroscopic analyses further provide helpful data for exploration, by determining the P and REE distribution and the enrichment in carbonatite and within apatite. SCOPUS: ar.j info:eu-repo/semantics/published