Sixteen K-Ar isotopic ages are reported from eleven mineral deposits in southern Norway. Clay mineral separate fractions of altered wall rocks and suitable vein material have been analyzed from old mines in the Kongsberg area, the Lassedalen fluorite prospect, and scattered deposits within the Oslo igneous province. In concluding that the isotopically determined "thermal event" recorded in the potassium-rich clay mineral assemblages is that resulting from metasomatism and mineralization, the Permian period has been delineated for the emplacement of this varied suite of ores. The analyses distinguish two groups of deposits having mean ages of 233 + or - 3 m.y. and 265 + or - 3 m.y. respectively. The older group of ages is related to the Kongsberg silver mining field where epigenetic vein deposits in Precambrian rocks are distant from the supposed parent igneous material. The second and younger group of ages comprises mainly contact and dike controlled deposits within the Oslo igneous province, as well as the Lassedalen fluorite deposit and a "silver vein" in the Kongsberg area.

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