Copper: a world trade in 3000 BC?
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The chief ingredient for bronze is copper. The era around 3000 BC saw more than 500,000 tons of copper being mined in the so-called Upper Peninsula, in the American state of Michigan. The largest mine was on Isle Royale, an island in Lake Superior, near the Canadian border. Here, there are thousands of prehistoric copper pits, dug thousands of years ago by ancient peoples unknown. The Minong Belt on Isle Royale has a distance of one and three quarter miles in length and is nearly four hundred feet wide. The copper pits range ten to thirty feet deep with connecting tunnels; one archaeologist estimated that their digging would take the equivalent of 10,000 men working for 1000 years.
After two centuries of speculation, no-one has ever satisfactorily explained where the world’s purest copper might have gone. Extraction from Isle Royal began in 5300 BC, with some even claiming that it began as early as 6000 BC. Evidence for smelting is known to exist from “only” 4000 BC onwards.
http://www.philipcoppens.com/copper.html
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