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

Abstract

a b s t r a c t 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 207 Pb/ 206 Pb weighted average age of 1894 ± 3 Ma for a rhyolitic sub-volcanic rock in the felsic volcanic to sub-volcanic host rock suite, and a 207 Pb/ 206 Pb weighted average age of 1891 ± 3 Ma for a post-sulphide, porphyritic dacite dyke. Magmatism also included the emplacement of granite plutons with igneous crystallization ages of 1894 ± 3, 1894 ± 2 Ma and 1893 ± 3 Ma. 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 207 Pb/ 206 Pb weighted average ages of 1831 ± 8 Ma and 1822 ± 5 Ma, respectively. These ages fall well within the temporal range of a younger 1.84-1.81 Ga (M 2 ) metamorphic episode during the 2.0-1.8 Ga Svecokarelian orogeny, with the older episode (M 1 ) inside the Bergslagen lithotectonic unit at around 1.86 Ga. This shows the major influence of the M 2 event in the north-western part of this unit, leading to a complete resetting of the U-Th-Pb isotope system in monazite.

Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen Ore District, Fennoscandian Shield, Sweden

“…The amphibolites also form isolated intrusions elsewhere in the Falun inlier. The field relationships and crystallization ages suggest emplacement during a late stage of, or after, the hydrothermal alteration event, but prior to metamorphism and deformation (Kampmann et al, 2016a).…”

“…SIMS U-Pb (zircon) ages of the volcanic and subvolcanic rocks, the felsic dikes, and the GDG plutonic rocks overlap in age within their respective analytical uncertainties at around 1894 Ma, indicating hydrothermal alteration and sulfide mineralization within a narrow time span of <10 m.y. when intense magmatic activity and burial of the supracrustal rocks prevailed (Kampmann et al, 2016a). Volcaniclastic rocks in the Falun inlier include fine-grained, volcanic silt-to sandstone and pumice breccia (Table 1).…”

“…Dikes and granitoid rock, diorite, and gabbro intrusive rocks: The base metal mineralization at Falun was intruded by feldspar-phyric dacite dikes, spatially associated with mafic dikes, which are now metamorphosed to metadacite and amphibolite, respectively (Geijer, 1917;Koark, 1962;Kampmann et al, 2016a;Fig. 2).…”

“…Metamorphosed mafic to intermediate and felsic granitoid rock, diorite, and gabbro intrusive rocks show no textural or mineralogical evidence of hydrothermal alteration, with the exception of some sericitization of feldspar in the rocks Large et al (2001). (Kampmann et al, 2016a). Five mafic to intermediate samples are classified as gabbro to diorite in a TAS plutonic rock classification diagram ( Fig.…”

“…A new structural model for the Falun deposit, involving sheath folding during the Svecokarelian orogeny (Kampmann et al, 2016b), in combination with new geochronological data from the deposit, indicating a short time interval (<10 m.y.) between felsic volcanism and mineralization (Kampmann et al, 2016a), provide the framework for this study. The current work is focused on the character of the hydrothermal system and ore formation at the Falun deposit with the following aims: (1) determine the nature, zonation, and intensity of hydrothermal alteration associated with Falun by masschange calculations; (2) identify premetamorphic alteration mineral assemblages; and (3) constrain the geologic conditions during the mineralizing event, which have implications for ore formation, later structural modification, and exploration.…”