Abstract

Both copper and zinc were mobilized during prehnite-pumpellyite facies metamorphism in the Keweenawan basalts of northern Michigan. Cu, averaging 70 ppm in basalts, was leached from epidotized or more highly metamorphosed rocks in the zone of dehydration; it was introduced into hydrated rocks where pumpellyite, prehnite, laumontite, and chlorite are the principal hydrous secondary phases. Native copper is a common associate of the metamorphic assemblage in such rocks. The chemical data suggest that fluids evolved by dehydration at depth may well have contained from 1,500 to 2,000 ppm Cu. These fluids percolated upward toward lower temperature levels where hydration was dominant and where the residual concentration of Cu-ions in the fluids could have reached high levels if precipitation had not occurred. The model supplies nearly five orders of magnitude more copper than has been produced by a century of mining in the Keweenaw Peninsula. The data suggest that enrichment or depletion of native copper should be spatially related to the distribution of certain hydrous minerals in the Keweenawan lava pile. Zn, which averages 100 ppm in unaltered olivine tholeiites, was leached from all highly altered rocks in permeable zones and removed by metamorphic fluids. Abundance of Ni was not appreciably affected by the metamorphic process except where there was dilution by void-filling secondary minerals.

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