The first question would be why haven't magi already discovered such things - is it possible to use Rego to refine ores to pure materials, or use Muto to transform away impurities?
The second one would be has this already been discovered elsewhere (note the claims for Persian alchemists to have refined arsenic before Albertus Magnus)?
If the metal has always been there, then this implies that creation is a vast mystery waiting for people to unlock it. If it hasn't, that implies more things are being added to creation - how is this done? Is there a special group of angels whose job is to add in the Creator's new designs?
Arsenic bearing compounds themselves will be well-known to magi as they form major red and yellow pigments used for illuminating manuscripts.