Molybdenite Re-Os dating constrains gravitational collapse of the Sveconorwegian orogen, SW Scandinavia

peer reviewe

Age constraints on c. 1.9 Ga volcanism, basin evolution and mineralization at the world-class Zinkgruvan Zn-Pb-Ag(-Cu) deposit, Bergslagen, Sweden

We present improved age constraints for the world-class Zinkgruvan Zn-Pb-Ag and Cu deposit: one of the largest Zn deposits of the Fennoscandian shield, and one of the earliest large, basin-hosted Zn deposits that formed from oxidized saline brines. Secondary Ion Mass Spectrometry (SIMS) U-Pb dating on zircon is used to constrain at least two phases of c. 1.9 Ga volcanism in the Zinkgruvan area, separated by a period of fluvial sedimentation, all of which predated formation of the stratiform Zn-Pb-Ag mineralization. A 1908 ± 4 Ma age for a rhyolitic rock of the first volcanic phase is the oldest recorded U-Pb zircon age of a volcanic rock in the Bergslagen lithotectonic unit (BLU) where Zinkgruvan is located. Similarly, two identical ages of 1898 ± 5 Ma for volcanic rocks belonging to the second volcanic phase indicate that the local volcanic activity, which formed the stratigraphic footwall, ended earlier in the Zinkgruvan area than in other parts of the BLU, where intense explosive felsic volcanic and intrusive activity until c. 1891 Ma has been demonstrated. This, along with a hybrid siliciclastic-volcaniclastic (tuffitic) character of the Zinkgruvan ore host, confirms earlier interpretations that the Zinkgruvan deposit formed in an actively subsiding basin, distal to active volcanic centers in the BLU in the time range 1.90–1.89 Ga. Our results support models suggesting that basinal brine-driven hydrothermal systems in sedimentary basins distal to volcanic centers could form world-class Zn deposits as early as c. 1.90 Ga.Validerad;2023;Nivå 2;2023-07-06 (hanlid)</p

1.83-1.82 Ga formation of a juvenile volcanic arc - implications from U-Pb and Sm-Nd analyses of the Oskarshamn-Jönköping Belt, southeastern Sweden

The Oskarshamn-Jönköping Belt in southeastern Sweden is a geographically well-defined area comprising calc-alkaline intrusions and volcanic rocks together with units of coarse-grained clastic metasedimentary rocks. Deviating from the general composition is the Fröderyd Group with basalts of MORB character. The belt is surrounded by the 1.81-1.77 Ga Transscandinavian Igneous Belt. A conglomerate clast from the central part of the Oskarshamn-Jönköping Belt yielded a U-Pb zircon age of 1829±8. This age confirms the c. 1.83-1.82 Ga formation age of the belt. Sm-Nd whole rock analyses of various rock types throughout the Oskarshamn-Jönköping Belt show that no substantially older (&gt;100 m.y.) continental material contributed to these rocks. Furthermore, the high positive εNd values of many of the analysed rocks points to the depleted mantle as the main component of their source. The Sm-Nd analyses also suggest that while some of the felsic units were formed by a high degree of magmatic fractionation from a mantle derived melt others were formed as the result of remobilisation of older Svecofennian crust. The new U-Pb data together with previously published ages imply that the formation of the Oskarshamn-Jönköping Belt was a rather quick process, possibly completed in c. 10 m.y. The most likely model of genesis for the area, based on the new U-Pb and Sm-Nd data, is formation at a continental margin subduction zone to the present south-west of a slightly older Svecofennian continent. In such a model the Fröderyd Group represents either a fore-arc setting or a back-arc rift