The Late Proterozoic Seal Lake basin of Central Labrador contains some 250 native copper and copper sulfide showings within tholeiitic basalts and associated gray shales. These copper occurrences resemble those of the Keweenawan copper province in host-rock lithologies, stratigraphic configuration, constituent metals, and mineralogic zoning, even though no economic deposits have been discovered. It seems quite likely also that the mineralization in the two areas is similar in age and genesis. The resemblances are obscured to some extent by the postmineralization deformation in the Seal Lake area due to thrusting from the adjacent Grenville orogenic belt to the south.The present study concerns mainly the sulfide facies of copper mineralization in shales of the Adeline Island Formation, which bears many similarities to the well-documented mineralization of the basal Nonesuch shale at White Pine, Michigan. Detailed field mapping has now defined stratigraphic and structural relations of the Adeline Island Formation and permits identification of favorable beds for stratiform copper sulfide mineralization within a narrow stratigraphic interval.The mineralization occurs chiefly as chalcocite and bornite disseminated within host shales. At one locality vertical zoning of copper-iron sulfides, similar to that observed at White Pine, is preserved in relatively little-deformed beds. In general, however, the sulfides are locally redistributed in the highly deformed shales, and an important proportion of the copper sulfides occurs in fractures cutting mineralized zones.

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