DISTURBANCE OF U-Pb IN ZIRCON BY Mo-W-MINERALISING MAGMATIC FLUIDS IN THE LATE SVECOFENNIAN BISPBERGS KLACK GRANITE, SWEDEN

The Bergslagen area of south-central Sweden is one of the most important Mo-W ore districts in northern Europe. For example, the skarn-hosted ores in the Yxsjöberg and Wigström mines were significant producers of tungsten during the 20th century, while the molybdenite-rich granites of Bispbergs Klack and Pingstaberg were subject to test mining during World War II. These ores formed in connection with Late Orogenic granitic magmatism in the Svecofennian Domain. The Bispbergs Klack granite is where A.F. Cronstedt and C.W. Scheele first discovered the minerals molybdenite and scheelite during the 18th century. Despite this historical notoriety, the age of this granite has not been previously determined. The ages of unmineralised Svecofennian Late Orogenic granites are well constrained to between 1830-1780 Ma, however conventional (TIMS) determinations from mineralised Late Svecofennian granites have yielded unrealistically young (1760-1740 Ma) ages. The deviation of ages between unmineralised and mineralised granites cannot be explained within the context of any accepted tectonic model for the region. To resolve this discrepancy, zircons from Bispbergs Klack were imaged using backscattered electron and cathodoluminescence techniques. This revealed complex patterns of inheritance, magmatic domains, and importantly, post-crystallisation alteration. As none of the Late Orogenic granites have experienced post-magmatic regional metamorphism, this alteration was likely caused by late-magmatic mineralising hydrothermal activity. Subsequent in situ analysis using the ion microprobe resulted in a discordant but reliable isochron age of 1794 ± 7 for the Bispbergs Klack granite, in agreement with ages of unmineralised granites. Our result shows that age discrepancies between unmineralised and mineralised granites are a consequence of disturbance of the U-Pb system in zircons by late magmatic mineralising fluids. This further implies that TIMS ages from mineralised granites in other areas should be regarded with caution.