Archean gold-bearing quartz veins at the Sigma Mine, Abitibi greenstone belt, Quebec; Part I, Geologic relations and formation of the vein system

Abstract

The Sigma deposit consists of an Archean gold-bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite, both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes. The veins cut all rock types and are composed chiefly of quartz and tourmaline, with minor amounts of carbonates, pyrite, chlorite, scheelite, and free gold. They were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes. The veins were not significantly affected by any subsequent metamorphism or tectonism.Two principal types of veins occur in the mine; subvertical veins and subhorizontal veins. They are contemporaneous and cogenetic. Subvertical veins occur within steeply dipping, east-west-trending ductile shear zones along which reverse subvertical displacements have taken place. The veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears. Subhorizontal veins occupy extensional fractures which developed between ductile shear zones, preferentially in the more competent host rocks. These veins were formed by one or several episodes of open-space filling.Geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones. A structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation: the Z-axis trending north-south and horizontal, the Y-axis plunging 10 degrees west, and the X-axis plunging 80 degrees east. The deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north-south-trending compression postdating regional deformation.