Time constraints on magmatism, mineralisation and metamorphism at the Falun base metal sulphide deposit, Sweden, using U–Pb geochronology on zircon and monazite

• Published in: Precambrian research 2016-06, Vol.278, p.52-68
• Main Authors: Kampmann, Tobias C., Stephens, Michael B., Ripa, Magnus, Hellström, Fredrik A., Majka, Jarosław
• Format: Article
• Language: English
• Subjects: Falun deposit; Fennoscandian Shield; Malmgeologi; Monazite; Ore Geology; SIMS U-Pb geochronology; Svecokarelian orogeny; Zircon
• ISSN: 0301-9268; 1872-7433; 1872-7433
• DOI: 10.1016/j.precamres.2016.03.011

•Polyphase, felsic magmatism at the Falun deposit occurred within a few million years.•Base metal sulphide mineralisation is constrained between 1894±3Ma and 1891±3Ma.•A major phase of metamorphic overprint is dated to 1831±8Ma and 1822±5Ma. U–Th–Pb (zircon and monazite) ion probe data have provided constraints on the timing of emplacement and metamorphism of magmatic rocks close to the Palaeoproterozoic, Falun base metal sulphide deposit in the Bergslagen lithotectonic unit, Fennoscandian Shield, Sweden, and, thereby the timing of mineralisation. Hydrothermal alteration and mineralisation at Falun are constrained to a short interval of several million years between a 207Pb/206Pb weighted average age of 1894±3Ma for a rhyolitic sub-volcanic rock in the felsic volcanic to sub-volcanic host rock suite, and a 207Pb/206Pb weighted average age of 1891±3Ma for a post-sulphide, porphyritic dacite dyke. Magmatism also included the emplacement of granite plutons with igneous crystallization ages of 1894±3, 1894±2Ma and 1893±3Ma. The felsic sub-volcanic to volcanic activity and the emplacement of dacite dykes and granite plutons overlap in age within their respective analytical uncertainties, indicating hydrothermal alteration and sulphide mineralisation inside a narrow time span of intense magmatic activity, and burial of the supracrustal rocks. Two distinct patchy and homogeneous metamorphic monazite types in a felsic volcanic rock around and hydrothermally altered rocks at the Falun deposit yield 207Pb/206Pb weighted average ages of 1831±8Ma and 1822±5Ma, respectively. These ages fall well within the temporal range of a younger 1.84–1.81Ga (M2) metamorphic episode during the 2.0–1.8Ga Svecokarelian orogeny, with the older episode (M1) inside the Bergslagen lithotectonic unit at around 1.86Ga. This shows the major influence of the M2 event in the north-western part of this unit, leading to a complete resetting of the U–Th–Pb isotope system in monazite.