I'm no pro, but reading about Roman history they had gigantic mines where they used slaves to extract resources. I think I remember reading that large portions of Spain were mines. In the book SPQR it states that they generated so much pollution from this that the ice in Greenland shows a noticeable increase in pollution around that time (100 BC-ish onwards).

They also conquered so much territory that I'm presuming they took swords/minerals from those regions. Those people would have had their own mines and the Romans now had access to them too.