The Sigma mine at Val d'Or, Quebec, in the Abitibi belt is typical of vein-type gold deposits in Archcan greenstone belts. Although the relative timing of gold mineralization with respect to magmatism, metamorphism, deformation, and hydrothermal events in the mine is well documented, precise isotopic age constraints have, until recently, been lacking. This paper presents the results of a 40 Ar/ 39 Ar dating study of the Sigma mine and of the Davidson-Tisdale deposit in the Abitibi belt. In the Sigma mine, dates on metamorphic amphibole indicate that the regional metamorphism, which predates the vein formation, occurred at ca. 2.68 Ga. Muscovite from the gold-bearing quartz veins yields a "plateau" date of 2579 + or - 9 Ma (2Sigma ), whereas biotite from the veins shows disturbed, saddle-shaped, age spectra. The mild disturbance of the biotite is attributed to later diabase dike emplacement, and a 40 Ar/ 39 Ar date on a dike indicates that this event is younger than 2.3 Ga. Thermal effects of the Grenville orogeny (1.2-1.0 Ga) may also be present. A muscovite separate from a gold-bearing quartz vein at the Davidson-Tisdale deposit yields a 2607 + or - 10 Ma (2Sigma ) plateau date. Although the amphibole and muscovite dates at the Sigma mine bracket the gold mineralization event at between ca. 2.68 and 2.58 Ga, a number of tectonothermal histories are possible for this time interval. It is concluded, based on all evidence, that the muscovite plateau dates are the time of vein and gold formation, which thus occurred in the Sigma mine ca. 100 m.y., and in the Davidson-Tisdale deposit ca. 70 m.y., after syn- to post-tectonic magmatism and regional metamorphism.

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