TRANSMISSION ELECTRON MICROSCOPY (TEM) INVESTIGATION OF SILVER FROM NATIVE COPPER/SILVER COMPOSITE NUGGETS FROM THE QUINCY STAMP MILL, KEWEENAW PENINSULA, NORTHERN MICHIGAN

Northern Michigan’s Keweenaw Peninsula is a well-known locality for native copper, with a history rich in mining through the late 1960’s. While mining in this area has essentially ceased, there remains a great interest in the mineralogy of the region. A previous TEM investigation into the mineralogical complexity of native copper from the Keweenaw Peninsula revealed an unexpected diversity in nanotextures. This current investigation focuses on native copper/silver composite nuggets from the Quincy Stamp Mill to determine if they possess the same complexity observed in copper from the previous study. Preliminary findings suggest that this is indeed the case. The native silver material has regions of cohesive crystals that are on average approximately 0.5 µm in diameter, which are dominated by Ag but also contain trace amounts of Zr. However, the dominant pervasive texture in the native silver is round to oblong spherules that are approximately 5 to 10 nm in diameter. This texture is strikingly similar to the spherule textures found in native copper nugget material from previous work. The spherules also contain extensive impurities of Cu, Si, Mn, O, Fe, Ca, and trace amounts of Zr, Au and Hg. Au appears to exclusively occur in the spherule texture. Tentatively the impurities are interpreted as a mechanism that may inhibit crystal growth. Further investigation is still necessary to fully assess the complexity of the textures and mineralogy present in the native copper/silver composite nuggets. The exact sequence of mineralization is unclear but variation in oxygen fugacity and impurities occur in both copper and silver from the region. However the textural and geochemical similarities observed thus far between silver and copper materials from the region indicate similar mineralization processes occurred at the nanoscale. This work suggests TEM investigations of native metals from this and other regions are warranted.