Lake Superior. Not Lake Michigan. Funny as hell. The shore of Lake Michigan is nearly all Limestone deposits. Yet just 80 miles north and to the west end of the UP, Its iron and copper and other rich minerals..gold, silver, zinc, etc.
On the Stoneington Peninsula, they have drilled and found oil. The water there is really sulferous and the guys that ran the thumper trucks back about 30 years ago said that there is oil under there. They found it, drilled it and capped it off and shut the hell up about it.
And you are right. The costs of digging that float copper out is not economical. Drives the mining companies nuts, I tell you.
I am gonna see if I can find the article about that mass of copper for you.
Here is a very poor article about it, but..
http://www.waymarking.com/waymarks/WMC7JD_Worlds_Largest_Glacial_Copper_282_Tons_Marquette_MI