As mentioned before, I concur that Solum is a decent place for iron. I visited 3 caves (randomly), and was able to walk away with about 45 iron. Be prepared to grapple alot. Said caves were under HUGE ice mountains. Aside from the 45 iron I walked away from, I have a few hundred pieces of coal, copper, and silver. Also, manage to accumulate about 15 or so pieces of Ancient Tech Fragment; and the super bonus 1 Ancient Tech Component. I did not have to dig through any mountain to get all of it. They were all surface veins in the caves that you can see, and you just have to dig to get it. That session took a few hours, but I was going at it slow because of all the lag I was getting. Having said that, I found it easier to find Copper, Silver, and Gold than Iron.
And, I am going to add another comment here; and I mean no offense to any of the players, just expressing my thoughts and my feelings about the rarity of iron, fragments and components. Your builds are breathtakingly beautiful, by the way. With that out of the way, every time I walk through a player owned tunnel in the mountain, I see some Ancient Tech Fragments, Ancient Tech Components, and Iron sealed away; I can't help but think what a waste.