TRANSMISSION ELECTRON MICROSCOPY (TEM) AND BULK CHEMICAL INVESTIGATIONS OF COPPER AND ASSOCIATED SILVER FROM MICHIGAN’S KEWEENAW PENINSULA: IMPLICATIONS FOR FORMATION AND ARCHAEOLOGICAL INTERPRETATIONS

Michigan’s Keweenaw Peninsula proudly hosts one of the world’s largest deposits of native copper. While mining has essentially ceased in the region, the area was extensively mined for more than a century. In addition, numerous Native American artifacts have been produced from copper in this region. Although much work has been done investigating the formation and occurrence of these deposits, little work was done analyzing details of the mineralogy of the copper. Whether or not mineralogical information can provide meaningful insight into both geological and archaeological interpretations of copper is not known. This study seeks to provide further insight into the mineralogy of both native copper and associated silver present within the region using Transmission Electron Microscopy (TEM) and inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Results suggest that the mineralogy of copper in these deposits is far more complex than previously believed, with similar micro- and nano-textures present in both the native copper and silver indicating similar mineralization patterns. Elemental mapping in Scanning Transmission Electron Microscopy (STEM) and bulk chemical analysis done on ICP-MS/OES shows variation in elemental impurities (Ca, Fe, Zr, Ni, Al, Ag, Au, Si, Mn, O) within the different samples from the region. Combined bulk chemistry and TEM work explains variation of chemical compositions in native copper commonly used in archaeological studies and may illustrate issues in the exact interpretation of sourcing copper artifacts. Results have the potential to provide some petrological constraints such a localized oxygen fugacity based on minerals interpreted to be nanoscale copper spinel phases observed in spherule textures and inclusions of Mn bearing minerals. The results of this investigation suggest that combined TEM and bulk chemical investigations of similar native metals worldwide may yield further insight and understanding of complexities in their formation and geologic history and for refining archaeological interpretations of copper.