Iron is used in-universe to symbolize swords and killing (see for instance the phrase "taking the iron price"). Thus, it is no coincidence that the Faceless Men assassins deal in iron coins for some purpose, and that the fearsome Braavosi bank is called the "Iron Bank." Similarly, note that the Faceless Man calling himself the Alchemist says he can turn "iron into gold" (not the usual lead-to-gold), and since he kills using a poisoned gold coin, he actually did use gold for the purpose he would normally use iron.
"Iron into gold" referred to the Alchemist's offer to exchange gold for the Maester's iron key. Also, if "iron" did refer to killing people, and he killed Pate with a gold coin, he would have turned gold into iron, not vice versa.
"Iron into gold" referred to the Alchemist's offer to exchange gold for the Maester's iron key. Also, if "iron" did refer to killing people, and he killed Pate with a gold coin, he would have turned gold into iron, not vice versa.