It is well known that the Minoans, before the Romans had visited NA and had mines here. so it is not a big stretch to believe the Romans got here too. As the Romans were far more advanced than many later civilizations that followed and during the dark ages much of their technology and indeed the technology of humans was lost.
again even before the Romans and the Minoans Clovis man and descendants made it to NA. indeed the world was a well traveled land. do you think the Olmec just grew from seed?