They should appear on old maps. It all depends on the chance, the formula counts from 1 to 75 randomly, and if 50 falls out and if the other conditions are favorable, then an artifact appears.
Initially, I had the idea of making them shouldn't be minable. They are not destructible. I wanted them to appear next to the ore, but I couldn’t realize it, so I allowed the player to dig them out and move them to the base. They can’t be created by yourself, at the early stage of the game they are very useful, while in the series or at the end, they are of little use.
With the old saves, I’ll think about how to do it in order to find an artifact on the map, because the main territory has already been explored, and also prohibit the extraction of artifacts to the player