Abstract

In the Chibougamau-Chapais area, a spatial relationship exists between Cu-Au mineralization contained in east-west-trending shear zones and northeast-striking faults. This has led many authors to propose a genetic link between the Cu-Au Opemisca deposits at Chapais, such as the Cooke mine the main Au producer, and the northeast-striking Gwillim Lake fault. Structural analysis at the Cooke mine shows that the gold mineralization is located in sulfide-rich quartz veins hosted by northwest-southeast and east-west brittle-ductile reverse-oblique shear zones at a high angle to the northeast faults. The mineralized shears were produced by a subhorizontal, north-south-directed shortening compatible with the Kenoran orogeny. The shears are offset by oblique northeast sinistral faults. These faults, as well as the regional Gwillim Lake and Chibougamau-Copper faults, are products of the same subhorizontal, north-south-directed shortening and regional strain regime.This study demonstrates that although the Gwillim Lake fault system crosscuts the mineralization found at the Cooke mine, it could still have played a key role in the development of dilatancy favoring the circulation of mineralizing fluids. In such a scenario, the Gwillim Lake fault is thought to be a product of a progressive Kenoran deformation event to which both the east-west and north-east faults or shears are related. However, crosscutting relationships and strain compatibility indicate that the Gwillim Lake fault formed slightly later than the gold-hosting shear zones in a progressive deformation event. Nonetheless, the presence of this northeast regional structure in the vicinity of gold-copper or copper-gold deposits is significant.

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