Economic viability is largely a question of competition between alternatives. As long as 60% iron ore is available there is little reason to focus on 30% iron ore and so on. Today 60% iron ore is only worth about 100$/ton.
Hypothetically, if we where down to 5% being the best option, then chances are your backyard would be just as viable as anywhere else. Plants will happily grow grow with up to about 55% iron in their soil. So in some place you could literally just use surface dirt as your “ore.”
Hypothetically, if we where down to 5% being the best option, then chances are your backyard would be just as viable as anywhere else. Plants will happily grow grow with up to about 55% iron in their soil. So in some place you could literally just use surface dirt as your “ore.”