The Vikings obtained most of their iron from bog ore, as did the Saxons in England - I don't know if you would consider those examples of serious iron age civilizations, but it seemed to be suffient to provide for plenty of helmets, swords, mail hauberks, etc.
Producing iron requires many specialized skills, if the Vikings did abandon western Greenland for North America, I could see how that knowledge could easily be lost in a few generations - or even more quickly with the deaths of a few key people.
Doing smithing on the run is a different sort of business than rendering ore down to iron, and then doing the build up to implements.
Back in Scanderhoovia the Vikings had access to one of the world's great iron lodes ~ up in Newfoundland, you'll notice that even today modern men have NOT managed to build a major steel industry!
Doesn't mean they aren't trying, but a quick review of literature available on the internet reveals that it wasn't until the late 1800s that the major iron ore deposits in Newfoundland were FOUND and DEVELOPED to any degree.
Remember, in a society with, at best, wooden shovels, it is unlikely they'll find much iron ore under 20 feet of wind blown loess or other debris.