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.