The modern mining industry in America found success in extracting copper from Michigan starting in the 1840's. Many of those mines in Keweenaw Peninsula and Isle Royale were set up at the locations previously established centuries before. There were about 5000 mine pits on Isle Royale mostly in the ten to twenty foot depth range. The estimates on the copper extracted from those mines centuries ago range as high as 500,000 to 750,000 tons.
The debate is over who originally mined the copper out of Keweenaw Peninsula and Isle Royale. The majority believe that Native Americans mined the area starting about 5000 BC. There have been copper artifacts found among several Native American cultures. Sheets of copper had been used for trade along with gold and silver. Yet the small amount of copper found to date is such a small fraction of what was mined there is speculation it went elsewhere.
There is one theory that the Minoans of Crete had discovered the copper deposits and either mined it themselves or forced the locals to be their labor. The Minoans had been traders and made their way by ship to other countries outside their island nation. Had the Minoans discovered and mined the copper it would have given them the resources to spark the Bronze Age which began on Crete around 2700 BC. The gap from the start of the Michigan mining to the start of the Bronze age may have been due to having minimal luck in finding huge deposits in the early going and their mining practices likely improved over time.