SCANNING ELECTRON MICROSCOPY INVESTIGATION OF NATIVE COPPER SAMPLES FROM THE KEWEENAW PENINSULA REVEALS UNEXPECTED MINERAL DIVERSITY

The Keweenaw Peninsula in northern Michigan has long been known well before European contact for its extensive copper deposits and much of this material was in the form of native copper. Although bulk chemical investigations of Au, Ag, and As among other metals in native copper from the region are somewhat common, little work has been done on investigating the mineralogical impurities of the native copper. Details of the nature and overall diversity of mineralogical impurities in native copper from the Keweenaw are poorly understood.

Scanning electron microscopy (SEM) was used to investigate the textures in selected native copper samples from the Keweenaw Peninsula. Samples approximately 1 x 3 cm were carefully polished by hand and investigated using a Zeiss Supra 35 VP variable pressure instrument. Samples were investigated predominantly in variable pressure back-scatter mode.

For most sample material the Cu-rich matrix made up approximately 85% of the sample, with inclusions making up the other 15%. Amongst the Cu-rich matrix were many inclusions of dark grains that contained appreciable Al and Si. The inclusions varied in size from approximately 15μm in diameter to less than 2 μm in diameter. As expected silver was present, along with occurrences of anorthite and chrysocolla. Silver was observed as subeuhedral micrometer scale inclusions. The euhedral anorthite inclusions varied from approximately 680 μm in diameter to slightly smaller than 100 nm. Although As is commonly found in native copper in the region at concentrations of a few thousand parts per million, no discrete arsenic minerals were identified during SEM. SEM work is ongoing, and occurrences of As minerals have not been ruled out entirely.

These results indicate the samples investigated are mineralogically complex, containing appreciable amounts of discrete silver, anorthite, and chrysocolla. There is also apparent solid solution of Al and Si with Cu and complex textures occur. Results of this investigation suggest that native copper from the Keweenaw region may be more mineralogically diverse than previously recognized. The findings of this investigation warrant more detailed study using transmission electron microscopy. The diverse mineral assemblage may provide further constraints on the origin of the Keweenaw deposits