The basic explanation that applies to all your metals is magic and metals don't interact well. This is a standard modern fantasy setting. The iron and metals in the world absorb magic or render it useless. More natural settings still have magic (e.g. a forest) but its greatly reduced as more iron cities are built and more forests are cut down.
Now there are different ways to go about achieve this aim, and to have special effects with all your fair-folk. You can either setup a system, like the periodic table that defines the interactions between the metal elements and magic (e.g. Iron has 2 electrons on the outer shell, which cause it to do X. It has 4 shells which means it effects this area of the body). This has an advantage of being a set system, which means it becomes hard to poke holes once you have defined it. Like, why can't gold act as a neurotoxin. Well according to the laws of my world, it will do X, Y and Z as I have defined.
The other way would be to assign an effect to each element, and then develop a handwavium solution based on magic. So in my world, Gold is a very good conductor of magic and will actively draw it away from a persons body eventually killing them if they don't let go. Silver is a neurotoxin because it bonds with the leylines in a fairfolks body and drains mana away from it. This issue is good if you have a restricted metal system (e.g. Only iron, copper, gold and silver exist in your world). Once you expand it out, you run into issues with metals that have very similar properties, but then have completely different effects on a persons body. e.g. Iron and Cobalt. You run into a loop of if X can do Y, why can't Z do Y, but because you have defined it arbitrarily your system is just, because I want it to do that.
So if your going to keep asking more questions based on different metallic elements, I would suggest creating a system instead of asking a new question for each element you have.
But to satisfy your question,
Silver seeks out the leylines in a fairfolks body and absorbs the mana, destroying the leyline.
Gold is a natural battery/sink for mana and will drain/store any mana it comes in contact with
Copper seeks out the leylines in a fairfolks body and inhibits the passage of mana