Archaeologists tell us the roughly 5000 open pit mines running for a hundred miles on the north shore of Upper Michigan were worked from aproximately 3000 - 1200 BC to extract anywhere from 100 - 500 thousand pounds (or some texts say tons) of essentially pure copper. Such a quantity was certainly not seen by Joliet and Marquette when they explored the Mississippi valley and discovered the aboriginal Americans who posessed copper trinkets. This is enough tonnage to support a proper Bronze Age.
We are further told by a combination of archaeologists and ancient historians that the Trojan war is dated anywhere from 1290 to 1135BC; the same time is given for the fall of the Hittite empire.
The way I put all this together is: either the Greeks or Hittites, or both, or possibly a third party "Arms Merchant" society, was sailing across the Atlantic Ocean for almost 2000 years taking copper from the south shore of Lake Superior and building an eastern Mediterranean culture on it.
The Greeks and Hittites then went to war at Troy and were so mutually devastated that they had no further need for or perhaps interest in Bronze armor and war weapons; or were too economically smashed to make such a voyage, so the fleet stoppped sailing. In a single generation of inactivity they could forget how to get here, making future travel during for example the Roman era, impossible unless lost knowledge was relearned and lost mines rediscovered.
If Mediterranean people freighted this much metal from upper Michigan to the Atlantic I see three sensible roads. They could have barged it down the St Lawrence with a portage around Niagra falls. Until the St Lawrence Seaway was dynamited in the 1950s there was also a rough stretch on the lower river which may have required a detour.
The second road is the Mississippi river.
The third possibility is to travel entirely overland with pack trains of horses.
I would guess each of these was tried.
American Indians have said they had horses before the Spaniards brought them to the Americas at the time of Columbus. And yet Zoologists tell us the American wild horses all come originally from Europe. This may be an indicator in one way or another.
Mark Twain tells us how trecherous the Mississippi river was in his day with sand bars shifting during the night and logs coming down the river from a tree being washed out of the bank upstream. If a barge was upset when full of copper ingots in either the Mississippi river or in the St Lawrence from an unsuccesful attempt at running the lower rapids, I would like to make a search for the cargo with a metal detector. In the Mississippi valley especially, it would be a monumentally big job because the river has changed course so much over the years.