In 1967, during my PhD program, I went on an economic geology trip with Dr. Bill Kelly and Dr. Stewart Turneaure to Upper Michigan, where, among other things, we descended into one of the last remaining active underground mines owned by the Calumet and Hecla Company. In fact, the last of their copper mines was abandoned in 1970; the influx of water into these deep mines was ultimately impossible to manage. While underground we were invited by the mine geologist to help ourselves and sample a 1.5 m X 1.5 m leaf of native copper, which projected out from the side of one of the tunnels. It was as thick as 10 cm, and thinned down to ~1 cm. The leaf of copper was a vein filling. The other graduate students pulled out their rock hammer and beat on the malleable copper, as had been done countless times (and to no avail) by previous visitors. I had been told in advance that this invitation might be extended, and thus was able to pull out of my knapsack a new hacksaw. As I began to saw deeeply into the copper (and it took quite an effort; tough stuff), the mine geologist freaked out for a moment and wanted me to halt, but then he recovered quickly and did the right thing, acknowledging that the invitation still stood. He recovered more and said to me, you’ve got the only piece ever taken from this copper leaf! You can see why I like this specimen. Years ago I sprayed it with lacquer to prevent it turning green. As of yet, I have never passed any electric current through it, though it is tempting.
Though much of the native copper in the deposits of northern Michigan occurred as vesicle fillings in basalt or as ‘cement’ in granule conglomerates, this native copper was part of a fracture filling, i.e., a vein of native copper. The atmospheric and ground-water chemical conditions that existed at that place and at that time were highly anomalous to what we experience today. What does it take, for example, to precipitate NATIVE copper, and not some silicate or carbonate or oxide of copper?
Age & Formation
Calumet Conglomerate is the host for the native copper, late Mesoproterozoic, ~ 1.1 Ga.