That door is golden because it requires a boss key. The boss key is supposed to be in the middle golden room on the map, but under some circumstances the game decides to replace it with a regular key for some reason. I've also seen regular keys in that room disappear if you leave the room without picking them up, not sure if that applies to the boss key as well. Unless you actually missed that room you are probably stuck, with the only solutions being to edit your save file to add a boss key to your inventory or start the game over from scratch.