Deformation of the Gowganda Formation, Matachewan area, Ontario, by post-Early Proterozoic reactivation of the Archean Larder Lake – Cadillac break, with implications for gold exploration

Abstract

Segments of the Larder Lake – Cadillac break (LLCB), an east–west-trending Archean shear zone in the southern Abitibi greenstone belt, are covered by Early Proterozoic sedimentary rocks of the Gowganda Formation. These rocks severely hamper exploration for Archean gold deposits associated with the LLCB. Along the parts of the LLCB covered by Gowganda Formation in the Matachewan area, Ontario, synsedimentary structures localized along the paleotopographic lineament of the LLCB and deformation structures due to post-Gowganda reactivation of the LLCB and related splay faults can be used to locate underlying Archean faults. Deformation structures in the Gowganda Formation are localized where the northeast–southwest-trending LLCB and associated splay faults are intersected by north–south-trending Archean faults. These Proterozoic structures comprise an array of right-stepping, en echelon folds with variably developed axial-planar cleavage, aligned along the trend of the northeast–southwest-trending basement faults, and linear zones of folds with associated axial-planar cleavage aligned along the trend of north–south-trending basement faults. Kinematic analysis of the structures in the Gowganda Formation indicates dominantly dextral strike-slip reactivation of northeast–southwest-trending faults, and dominantly reverse reactivation of north–south-trending faults. Reactivation of Archean faults may have occurred during the tectonic event that produced the Kapuskasing structure, and (or) during the Grenville orogeny. Past geochemical surveys conducted in one of these deformation zones within the Gowganda Formation in western Quebec indicate that Au can be remobilized from the Archean basement into the deformed Proterozoic rocks. Thus it is possible to delineate the Archean shear zones and test for the presence of associated gold mineralization in areas where the Archean faults are overlain by the Gowganda Formation.