U-Pb geochronology and geochemistry of host rocks to the Bastnäs-type REE mineralization in the Riddarhyttan area, west central Bergslagen, Sweden

Abstract: The geochronology of felsic metavolcanic rocks hosting the Bastnäs-type rare earth element (REE) mineralization in the Riddarhyttan area in the Bergslagen region, Sweden, was investigated using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Th-Pb on zircon and monazite. U-Pb data from four host rock samples yield dates of 1899 ± 4 Ma (MSWD=2.2), 1910 ± 5 Ma (MSWD=3.2), 1911 ± 6 Ma (MSWD=3.2) and 1916 ± 4 Ma (MSWD=1.4). These dates are interpreted as igneous crystallization ages and indicate that several localities with felsic metavolcanic rocks hosting the REE-mineralization are 5 to 10 Myr older than previously known in this part of the Bergslagen region. These dates together with available published geochronology data for metavolcanics in Bergslagen suggest that magmatism might either have formed a continuum or that it was episodic between 1920 to 1876 Ma. A 1824 ± 7 Ma (MSWD=1.2) date for a population of monazite in one of the felsic metavolcanic rocks is temporally correlated with a nearby granite, where variably discordant zircon yield a 1822 ± 10 Ma (MSWD=1.7) date. The monazite and granite dates are overlapping, suggesting that at least part of the monazites in the felsic metavolcanic rocks might have crystallized or recrystallized due to heat and fluids from emplacement and crystallization of granite intrusions during the presumed late Svecokarelian phase (c. 1.85-1.78 Ga) of regional metamorphism and ductile deformation. Further geochronological work is required to assess the temporal and geological relationship with the 1820 Ma granite magmatism and the monazite (re-)crystallization. The geochemical characteristics of the metavolcanic rocks are assessed using whole rock (WR) data in conjunction with mass-balance calculations based on the isocon method. The results imply that the felsic metavolcanic rocks hosting the REE-mineralization are mostly consisting of a variably altered rhyolite precursor. WR chemistry data show that geochemical differences among the samples are not the result of fractional crystallization processes. REE patterns are similar to those presented by numerous authors for the Svecofennian volcanic and subvolcanic suite of intrusive rocks (1.91-1.89 Ga) and as stated by earlier workers comparable with those found in a normal to mature, active continental margin. Isocon analyses, using Al2O3, Zr and Hf as immobile elements, indicate that few of the geochemical signals in the felsic metavolcanic rocks are of primary, igneous, origin and instead significant mass gain ranging from +27% to 139% suggests that multiple elements have been gained (e.g. SiO2; MgO, K2O; Co; Eu and Fe2O3) or lost (Pb; Ni; Zn; Na2O; CaO; Th and U) during chemical alteration processes.