Abstract

The Swedish part of the Fennoscandian Shield hosts a variety of rare earth element (REE) deposits, including magmatic to magmatic-hydrothermal types. This paper focuses on the origin of the Olserum-Djupedal REE-phosphate mineralisation located in the sparsely studied Västervik region, SE Sweden. Here, mineralisation occurs in three main areas, Olserum, Djupedal and Bersummen. Primary hydrothermal REE mineralisation formed at high temperatures (about 600 °C), leading to precipitation of monazite-(Ce), xenotime-(Y), fluorapatite and minor (Y,REE,U,Fe)-(Nb,Ta)-oxides in veins and vein zones dominated by biotite, amphibole, magnetite and quartz. The veins are hosted primarily by metasedimentary rocks present close to, or within, the contact aureole of a local 1.8 Ga ferroan alkali feldspar granite pluton, but also occur within in the chemically most primitive granite in the outermost part of that pluton. In the Djupedal area, REE-mineralised metasedimentary bodies are extensively migmatised, with migmatisation post-dating the main stage of mineralisation. In the Olserum and Bersummen areas, the REE-bearing veins are cross-cut by abundant pegmatitic to granitic dykes. The field relationships demonstrate a protracted magmatic evolution of the granitic pluton and a clear spatial and temporal relationship of the REE mineralisation to the granite.The major and trace element chemistry of ore-associated biotite and magnetite support genetic links between all mineralised areas. Biotite mineral chemistry data further demonstrate a distinct chemical trend from metasediment-hosted ore-associated biotite distal to the major contact of the granite to the biotite in the granite-hosted veins. This trend is characterised by a systematic decrease in Mg and Na and a coupled increase in Fe and Ti with proximity to the granite-hosted veins. The halogen compositions of ore-associated biotite indicate elevated contents of HCl and HF in the primary REE mineralising fluid. Calculated log(fHF/fHCl) values in the Olserum area suggest a constant ratio of about −1 at temperatures of 650–550 °C during the evolution of the primary hydrothermal system. In the Djupedal and Bersummen areas, the fluid locally equilibrated at lower log(fHF/fHCl) values down to −2. High Na contents in ore-associated biotite and amphibole, and the abundance of primary ore-associated biotite indicate a K- and Na-rich character of the primary REE mineralising fluid and suggest initial high-temperature K-Na metasomatism. With subsequent cooling of the system, the fluid evolved locally to more Ca-rich compositions as indicated by the presence of the Ca-rich minerals allanite-(Ce) and uvitic tourmaline and by the significant calcic alteration of monazite-(Ce). The later Ca-rich stages were probably coeval with low to medium-high temperature (200–500 °C) Na-Ca metasomatism variably affecting the granite and the wall rocks, producing distinct white quartz-plagioclase rocks.All observations and data lead us to discard the prevailing model that the REE mineralisation in the Olserum-Djupedal district represents assimilated and remobilised former heavy mineral-rich beds. Instead, we propose that the primary REE mineralisation formed by granite-derived fluids enriched in REE and P that were expelled early during the evolution of a local granitic pluton. The REE mineralisation developed primarily in the contact aureole of this granite and represents the product of a high temperature contact metamorphic-hydrothermal mineralising system. The REE mineralisation probably formed synchronously with K-Na and subsequent Na-Ca metasomatism affecting the granite and the wall rocks. The later Na-Ca metasomatic stage is probably related to a regional Na ± Ca metasomatic and associated U ± REE mineralising system operating concurrently with granitic magmatism at c. 1.8 Ga in the Västervik region. This highlights the potential for discovering hitherto unknown REE deposits and for the reappraisal of already known deposits in this part of the Fennoscandian Shield.

Highlights

  • The emplacement of granitic plutons into the Earth’s continental crust causes a significant heat input to the immediate wall rocks, inducing contact metamorphism and circulation of hot fluids

  • We propose that the primary rare earth element (REE) mineralisation formed by granite-derived fluids enriched in REE and P that were expelled early during the evolution of a local granitic pluton

  • Metasomatism is in most cases essential to attain oregrade concentrations, as aptly shown by rare earth element (REE) deposits associated with fenitisation of the wall rocks during the emplacement of carbonatitic or silica-undersaturated intrusions (e.g., Kresten and Morogan, 1986; Morogan and Woolley, 1988; Morogan, 1989; Sjöqvist et al, 2017; Elliott et al, 2018)

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Summary

Introduction

The emplacement of granitic plutons into the Earth’s continental crust causes a significant heat input to the immediate wall rocks, inducing contact metamorphism and circulation of hot fluids. The fluids associated with such an environment can be magmatic fluids released from the crystallising pluton, pore fluids in wall rocks, metamorphic fluids produced by devolatilisation in the adjacent wall rocks, and meteoric water that is convected into hydrothermal systems during later cooling of the pluton (Hansen, 1995; Kesler, 2005; Ingebritsen and Appold, 2012) If these fluids transport significant concentrations of metals, either by extracting them from the magma via an exsolving fluid phase, or due to interaction with the surrounding wall rocks, and if the fluids can be focused into distinct structural pathways or zones, they may form primary magmatic-hydrothermal deposits such as for example porphyry Cu deposits, as well as related metasomatic skarn deposits (Hedenquist and Lowenstern, 1994; Cox, 2005; Heinrich and Candela, 2013). It is essential to study how the REE behave in hydrothermal REE mineralising systems that show a clear spatial relation to granitic (sensu lato) plutons and their associated metasomatic or contact aureoles

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