The only time I've ever been able to build the Iron Works was when I purposely build an additional city after I could see all the Iron and Coal on the map. The two resources would just fit into a city boundary, but weren't being shared by the same city. So, I built myself a settler and started a new city where I needed it. I rushed a Temple then an Aqueduct, and added eleven workers to the city. Six turns after nothing was there, I had a size 12 city with 10 culture which now had Iron and Coal inside it's borders. Since I'd already terraformed the land around the other cities, I had an instant city producing a decent amount of shields. I think I rushed a factory as well before my borders expanded and then built the Iron Works after that. As someone else pointed out, once the Iron works was built, every single Wonder built after that time was in that city.