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

Linders, Wictor LU (2016) In Dissertations in Geology at Lund University GEOR02 20152
Department of Geology
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... (More)
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. (Less)
Popular Abstract (Swedish)
De sällsynta jordartsmetallerna är en grupp av grundämnen som används mycket i elektronik och som finns med på EUs lista över kritiska råvaror. Utanför Riddarhyttan i Bergslagen, Sverige, finns en känd mineralisering av sällsynta jordartsmetaller i den metavulkaniska berggrunden. Genom att ta prover och analysera mineral som monazit och zirkon med hjälp av en metod som kallas Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) går det att åldersbestämma berggrunden och på det viset fastställa en maximal ålder på mineraliseringen. Den här studien daterade fyra prov av metavulkaniter med mineralet zirkon till 1899 ± 4 miljoner år (Ma), 1910 ± 5 Ma, 1911 ± 6 Ma och 1916 ± 4 Ma. Ett prov med metavulkanit daterades med... (More)
De sällsynta jordartsmetallerna är en grupp av grundämnen som används mycket i elektronik och som finns med på EUs lista över kritiska råvaror. Utanför Riddarhyttan i Bergslagen, Sverige, finns en känd mineralisering av sällsynta jordartsmetaller i den metavulkaniska berggrunden. Genom att ta prover och analysera mineral som monazit och zirkon med hjälp av en metod som kallas Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) går det att åldersbestämma berggrunden och på det viset fastställa en maximal ålder på mineraliseringen. Den här studien daterade fyra prov av metavulkaniter med mineralet zirkon till 1899 ± 4 miljoner år (Ma), 1910 ± 5 Ma, 1911 ± 6 Ma och 1916 ± 4 Ma. Ett prov med metavulkanit daterades med monazit till 1824 ± 7 Ma, vilket kan jämföras med en närliggande granit som daterades till 1822 ± 10 Ma. Resultaten antyder att flera lokaler med metavulkaniter med mineraliseringar av sällsynta jordartsmetaller är 5 till 10 miljoner år äldre än tidigare dokumenterat i den här delen av Bergslagen. Monazit- och granitdateringarna överlappar, vilket indikerar att åtminstone delar av monaziterna som finns i metavulkaniterna kan ha (om-)kristalliserat till följd av värme och fluider från granitiska intrusioner under en period av regionalmetamorfos. Geokemiska undersökningar av metavulkaniterna indikerar att de härstammar från en bergartstyp som kallas ryolit och att geokemin ändrats under kraftig hydrothermal omvandling. (Less)
Please use this url to cite or link to this publication:
author
Linders, Wictor LU
supervisor
organization
course
GEOR02 20152
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Riddarhyttan, REE, Metavolcanic rocks, LA-ICP-MS, Zircon, Monazite.
publication/series
Dissertations in Geology at Lund University
report number
466
language
English
additional info
External supervisor: Erik Jonsson, the Geological Survey of Sweden (SGU)
id
8871832
date added to LUP
2016-04-29 14:16:16
date last changed
2016-04-29 14:16:16
@misc{8871832,
  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.},
  author       = {Linders, Wictor},
  keyword      = {Riddarhyttan,REE,Metavolcanic rocks,LA-ICP-MS,Zircon,Monazite.},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {U-Pb geochronology and geochemistry of host rocks to the Bastnäs-type REE mineralization in the Riddarhyttan area, west central Bergslagen, Sweden},
  year         = {2016},
}