A Microstructural study of Musselwhite Mine and Hammond Reef shear-zone-hosted gold deposits

Abstract

Musselwhite Mine and Hammond Reef are shear-zone-hosted gold deposits located in Northwestern Ontario, in the Western Superior Province of the Canadian Shield. A detailed microscopic investigation of three gold-hosting lithologies from Musselwhite Mine and Hammond Reef demonstrate close similarities in the microstructures which host gold and the relative timing of gold mineralization. The gold deposit at Musselwhite Mine is hosted by metamorphosed banded iron formation, while Hammond Reef is hosted by metamorphosed tonalite. Despite the difference in rock types, Musselwhite Mine and Hammond Reef are similar in that that they are located proximal to regional shear zones, have undergone regional metamorphism and are dominated by ductile deformation. Although these two deposits are hosted by completely different lithologies the microstructures which host gold are very similar, indicating structural control on mineralization at a microscopic scale. The most common gold-hosting microstructures in these gold deposits result from heterogeneous deformation. Gold mineralization commonly occurs in fractures in competent minerals such as garnet and pyrite in every lithology in this study. These gold-hosting fractures do not extend throughout the matrix but are restricted to the competent minerals because the competent minerals are resistant to the ductile deformation around them. Gold mineralization is also associated with other deformation-induced microstructures, such as strain shadows. Gold mineralization is hosted by metamorphic minerals at both Musselwhite Mine and Hammond Reef. Gold inclusions occur at Musselwhite Mine in such metamorphic minerals as grunerite and garnet and at Hammond Reef gold inclusions occur in metamorphic muscovite. Also gold mineralization commonly occurs on plane defects, for example on grain boundaries. Relative timing of gold mineralization is shown to have occurred during ongoing metamorphism and deformation. Inclusions within metamorphic minerals indicate that gold mineralization must have occurred before or during metamorphism, while gold mineralization associated with deformational features indicate gold mineralization to have occurred during or after deformation. This thesis demonstrates close similarities between the relative timing of gold mineralization as well as the microstructures which host gold at Musselwhite Mine and Hammond Reef.