Abstract

The Alaska Juneau lode gold deposit is hosted by a series of polydeformed Permian to Late Triassic volcanic, pelitic, volcaniclastic, and mafic intrusive rocks. Rocks in the mine area have been sheared and metamorphosed to greenschist grade. Interpretation of rock fabrics indicates several generations of ductile and brittle deformation. Prior to mineralization, reverse shear occurred along northwest-striking and northeast-dipping ductile shear zones. Mineralization consists of Eocene auriferous quartz–carbonate veins, which cut the regional metamorphic fabrics. Mineralization was followed by reverse right-lateral shear along northwest-trending ductile–brittle shear zones. Two northwest-striking and steeply dipping vein sets host the bulk of the ore. Orientation of carbonate fibers within the quartz veins were used to determine the deformation regime that existed during mineralization. Plunge of the fibers indicate that down-to-the-northeast extension occurred synchronous with mineralization. Structural data support a model whereby the Alaska Juneau deposit formed after the peak of ductile deformation during a period of local extension. Localization of veins to areas of infolded phyllite and gabbro suggests that competency contrasts within host rocks enhanced vein emplacement. Veining may have been facilitated by a change from a contractional to a transpressive deformational regime which may have led to local extension and fluid migration to favorable deposition sites.

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