Phoenicians in north america would explain what happened to all the missing copper in north america.
Huge amounts of copper ore where excavated in unknown times, but the natives didn't do it. It's against their religion and they didn't had the tools and expertise for it. Ofcourse the riddle remains: where they brought the copper to! Maybe it was aliens afterall.
I've read about the PreColumbian copper ore extraction (having grown up in Michigan), and I'd not be too surprised if those operations served the needs of some kind of trade; however, while there is no accounting for the estimated quantity extracted, it may just not have been found. There's not much reason to think that trade wasn't as extensive in the Americas as it was elsewhere, particularly since I'm always willing to consider transoceanic traffic.
That said, I'd say the windows of time to be considered would be periods when the climate was warmest, when the Arctic melted for navigation (which happened during the Medieval Warming, and the previous warming period was even warmer), permitting access to Hudson's Bay and the rivers flowing into it, and thus down into Lake Superior (the mines are in the Upper Peninsula of Michigan).