The redistribution of copper during low-grade metamorphism of the Karmutsen Volcanics, Vancouver Island, British Columbia

Abstract

The Karmutsen Volcanics, exposed on northern Vancouver Island, are a sequence of chemically uniform tholeiitic basalts with a thickness greater than 5 km. Within the Karmutsen a stratigraphic succession from close-packed pillow lava to breccias to flows resulted from a transition from deep marine to subaerial eruption of the parent magmas. All lithologies have undergone prehnite-pumpellyite facies burial metamorphism which substantially redistributed Cu within the flows but not within the subaqueously erupted breccias or pillow lavas. There is no evidence for substantial metamorphic redistribution of Cr, Ni, Ti, Zn, and Mn, which vary only slightly in the three stratigraphic units. Like other transition elements, Cu abundance is confined to a restricted range (120-200 ppm) in the subaqueously erupted rocks. However, in the flows Cu is very unevenly distributed and is generally less abundant than in the subaqueously erupted rocks. Posteruptive redistribution of copper is evidenced by the fact that the present distribution of copper in the flows can be related to posteruptive mineral assemblages. Lowest copper abundances are in red flows rich in hematite which probably resulted from deuteric alteration. Highest copper abundances and most native copper occurrences are in green, pumpellyite-rich flows. The initial copper content of the flows is not known for certain but is likely to have been similar to that of the subaqueously erupted rocks which are in other respects chemically similar to the flows. It is believed that high f O 2 in low-grade metamorphic fluids led to the dissolution of chalcopyrite and sporadic precipitation of native copper in association with amygdular prehnite. Intense pumpellyitization of some flows locally lowered the f O 2 and possibly also raised the pH of the metamorphic solutions, thus favoring deposition of copper in these flows. Copper was not redistributed in the subaqueously erupted units because these lacked the appropriate deuteric Fe-Ti oxide assemblages.