U-PB ZIRCON AND RUTILE CHRONOLOGY OF ARCHEAN GREENSTONE FORMATION AND GOLD MINERALIZATION IN THE VAL DOR REGION, QUEBEC

U-Pb geochronology provides an absolute time framework for the evolution of the Sigma gold deposit and surrounding rocks at Val d'Or, southern Abitibi subprovince. The Bourlamaque batholith, the largest pluton in the area, gives a 2699.8 +/- 1.0 Ma U-Pb zircon age. This pluton cuts the Val d'Or Formation which hosts the mineralization. A U-Pb zircon age of 2704.9 +/- 1.1 Ma on a felsic volcanic rock, the Colombiere "rhyolite", 13 km east of the mine dates that formation. The gold-bearing quartz vein system at Sigma is hosted by andesites and two generations of porphyry intrusions, all metamorphosed to the greenschist facies. The oldest porphyry ("porphyritic diorite") shows the same deformation as the volcanic rocks, and has a 2703.7 +/- 2.5 Ma U-Pb zircon age. The porphyritic diorite and volcanic rocks are cut by feldspar-porphyry dykes which post-date regional folding and have a 2694.0 +/- 2.2 Ma U-Pb zircon age. Regional greenschist metamorphism has been dated directly, with a U-Pb date of 2684 +/- 7 Ma on rutile in the Colombiere "rhyolite". The mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals. Hydrothermal rutile, from an alteration halo around the veins in andesite, has a 2599 +/- 9 Ma U-Pb age. Textural evidence clearly indicates that the wall-rock alteration and vein filling are contemporaneous, and hence the vein system and gold mineralization appear to have developed at least 80 m.y. after the formation and metamorphism of host greenstones.