Trace Elemental Analysis of Copper Inclusions in the Portage Lake Volcanics Series of the Keweenaw Peninsula, MI Using Particle-Induced X-ray Emission Spectroscopy (PIXE)
Dr. Graham Peaslee and Dr. Edward Hansen
The Portage Lake Volcanics is a mid-Proterozoic series made up of over 200 individual subaerial thoeiilitic basaltic flows, which contain extensive amygdaloidal and brecciated flow tops in addition to interflow conglomerates, hosting the secondary copper deposits that make the Keweenaw Peninsula region of Michigan famous. The intent of this study is to explore variations in the trace elemental composition of copper deposits in the area and explore possible geological implications of such variations. Thick sections of conglomerates and basalts containing copper inclusions from the Centennial, Calumet & Hecla, Phoenix, and Allouez mines, among others, were analyzed using particle-induced x-ray emission spectroscopy. PIXE was performed in the Hope College Ion Beam Analysis Laboratory using a 1.7MV Pelletron tandem particle accelerator. The ion beam was focused on copper inclusions and spectra were collected and analyzed using gupix software. Ratios of trace nickel, silver, and arsenic to copper were calculated and compared across different inclusions in a single sample and across samples from different geographic locations, formations, and mine depths. Results show some variation in elemental ratios within samples, and greater variation across samples from different mines, formations, and depths. Elemental compositions are likely subject to a variety of factors influencing local conditions in copper mineralization.
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