MICROSTRUCTURAL CONTROL OF GOLD MINERALIZATION IN SHEAR-ZONE-HOSTED GOLD DEPOSITS: EXAMPLES FROM THE SUPERIOR PROVINCE OF NORTHWESTERN ONTARIO

Microstructural control of gold mineralization is significant to explaining the close association between gold deposits and major ductile shear zones. Musselwhite Mine and Hammond Reef are two examples of shear-zone-hosted gold deposits located in northwestern Ontario, in the Superior Province of the Canadian Shield. Both deposits lie within or adjacent to regional shear zones, and have been ductilely deformed during regional metamorphism. However the lithologies hosting the gold deposits are very different. At Musselwhite Mine the main ore-hosting lithology is metamorphosed banded iron formation whereas at Hammond Reef gold is found in metamorphosed tonalite. Despite the difference in rock types, microscopic analysis of three different gold-hosting lithologies from these deposits demonstrates close similarities in the microstructures which host gold and the relative timing of gold mineralization.

Gold-hosting microstructures for these deposits are most commonly related to heterogeneous ductile deformation, indicating structural control on mineralization at a microscopic scale. Gold commonly occurs in fractures in competent minerals such as garnet and pyrite. These gold-hosting fractures do not extend throughout the matrix but are restricted to the competent minerals. Gold is also associated with other microstructures, including strain shadows. Gold occurs as inclusions within metamorphic minerals at both Musselwhite Mine and Hammond Reef. Also gold commonly occurs on plane defects such as grain and subgrain boundaries.

In both deposits gold mineralization occurred during synchronous metamorphism and deformation. Inclusions of gold within metamorphic minerals indicate gold mineralization before or during metamorphism, while gold within fractures in metamorphic minerals as well as in strain shadows around metamorphic minerals indicates gold mineralization during or after deformation.

Close similarities between the relative timing of gold mineralization as well as the microstructures which host gold at these shear-zone-hosted gold deposits indicates a role for ductile deformation in controlling gold mineralization at a microscopic scale.