Mineralization in rift-filling rocks began during diagenesis and continued through burial metamorphism and associated hydrothermal activity which altered primary minerals and filled open spaces with secondary minerals. Mauk and others (1992a, 1992b) presented evidence for two distinct stages of copper mineralization in the White Pine deposit. Mainstage copper sulfides and subordinate native copper, which make up the bulk of the mineralization, formed during diagenesis of the Nonesuch Shale. A second stage of native copper and subordinate copper sulfide mineralization was synchronous with thrust faulting and interpreted as contemporaneous with late compression of the rift.

In the Portage Lake Volcanics, native copper and native silver mineralization occurs with secondary minerals (typically K-feldspar, chlorite, calcite, prehnite, epidote, quartz, and calcite) in amygdules, interstices, and replacements in basalt, and as open-space filling in interflow sedimentary layers. The timing of native copper mineralization, estimated to be about 1060 Ma (Bornhorst and others, 1988) corresponds to faulting related to the late compression of the rift . It is probable that the initiation of faulting resulted in faults and fractures that created permeability pathways along which hot, copper-mineralizing fluids generated in hotter and deeper parts of the rift were able to move buoyantly upwards and be focused in dipping flow tops and sedimentary layers (Bornhorst, 1992). More than 5 billion kilograms of native copper and 0.5 million kilograms of native silver were produced from the Keweenaw Peninsula mining district from the 1840's until the 1960's.

Small, high-grade, basalt-hosted chalcocite deposits are confined to an area about 13 km long and 2 km wide within and near the base of the Portage Lake Volcanics at the eastern end of the Keweenaw Peninsula (Broderick, 1946). Mineralization occurs as roughly stratabound chalcocite in brecciated and amygdular basalt flow tops, and as chalcocite and very rarely native copper in veins and tension fractures in basalt and andesite dikes. The deposits contain a total of about 7 million tons of ore with an average grade of 2.3% Cu, and the largest of the deposits, 543S, is estimated to have minable reserves of 1.1 million tons grading 4% copper and is scheduled for development in 1995 (Northern Miner, v. 80, no. 41).